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Geochemistry, Geophysics, Geosystems

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ISSN (Online) 1525-2027
Published by American Geophysical Union (AGU)

[17 journals]

The P and S wave velocity structure of the mantle beneath eastern Africa and the African superplume anomaly
- Authors: Gabriel D. Mulibo; Andrew A. Nyblade
  Pages: n/a - n/a
  Abstract: P and S relative arrival time residuals from teleseismic earthquakes recorded on over 60 temporary AfricaArray broadband seismic stations deployed in Uganda, Tanzania and Zambia between 2007 and 2011 have been inverted, together with relative arrival time residuals from earthquakes recorded by previous deployments, for a tomographic image of mantle wave speed variations extending to a depth of 1200 km beneath eastern Africa. The image shows a low wave speed anomaly (LWA) well developed at shallow depths (100‐200 km) beneath the Eastern and Western branches of the Cenozoic East African rift system and northwestern Zambia, and a fast wave speed anomaly at depths ≤ 350 km beneath the central and northern parts of the East African Plateau and the eastern and central parts of Zambia. At depths ≥350 km the LWA is most prominent under the central and southern parts of the East African Plateau and dips to the southwest beneath northern Zambia, extending to a depth of at least 900 km. The amplitude of the LWA is consistent with a ~150‐300 K thermal perturbation, and its depth extent indicates that the African superplume, originally identified as a lower mantle anomaly, is likely a whole mantle structure. A superplume extending from the core‐mantle boundary to the surface implies an origin for the Cenozoic extension, volcanism and plateau uplift in eastern Africa rooted in the dynamics of the lower mantle.
  PubDate: 2013-06-12T01:36:10.647404-05:
  DOI: 10.1002/ggge.20150
Synchronization between tides and sustained oscillations of the hydrothermal system of Campi Flegrei (Italy)
- Authors: E. De Lauro; S. De Martino, M. Falanga, S. Petrosino
  Pages: n/a - n/a
  Abstract: Long continuous seismic data recorded at five broadband seismic stations during 2006 at Campi Flegrei caldera have been analyzed. Introducing a coarse‐grained method, we evaluate the time evolution of amplitude and polarization of the seismic noise in the frequency band common to Long‐Period events. The series are modulated on tidal time scales: the root‐mean square is basically dominated by solar contribution, while the azimuth of the polarization vector shows lunar diurnal and semidiurnal constituents. In addition, we find that in the frequency band common to Long‐Period events the azimuths are polarized towards a specific area, suggesting that these persistent oscillations can be induced by the activity of the shallow geothermal reservoir.
  PubDate: 2013-06-12T01:35:34.765249-05:
  DOI: 10.1002/ggge.20149
Characterization of hyperalkaline fluids produced by low temperature serpentinization of mantle peridotites in the Oman and Ligurian ophiolites
- Authors: Valérie Chavagnac; Christophe Monnin, Georges Ceuleneer, Cédric Boulart, Guilhem Hoareau
  Pages: n/a - n/a
  Abstract: A regional survey of alkaline springs in Oman and Ligurian ophiolites shows that the alkaline water compositions significantly vary from one ophiolite to the other and within the same ophiolite. The first order correlation between the Na (and K) and Cl concentrations points to fluid compositions only partly due to evaporation. The scatter around the evaporation line implies that Na and Cl may not be conservative during the alteration of the ultramafic rocks. Mg is almost entirely depleted at pH > 10.5 as a result of serpentine formation within the ultramafic body and of brucite (and minor hydrotalcite) precipitation at the springs. Ca accumulates in the high pH fluids and is consumed by Ca‐carbonate formation at the springs, by mixing with river waters or by the CO2 supply from the atmosphere. Thermodynamic calculations show that brucite saturation is reached at pH values around 10.5 which triggers major changes in the water composition. The waters evolve from a quartz‐saturated low pH continental environment to a brucite‐dominated high pH serpentinizing system at low temperature. The highest water salinities are found in springs located along the basal thrust plane of the ophiolite. The highest Al concentrations are found in some springs located on the crustal side of the mantle/crust boundary. This poses the question of the hydrologic pathways and of the role of the mineralogical composition of the altered formations.
  PubDate: 2013-06-12T01:35:23.169094-05:
  DOI: 10.1002/ggge.20147
Mineralogical assemblages forming at hyper‐alkaline warm springs hosted on ultramafic rocks: A case study of Oman and Ligurian ophiolites
- Authors: Valérie Chavagnac; Georges Ceuleneer, Christophe Monnin, Benjamin Lansac, Guilhem Hoareau, Cédric Boulart
  Pages: n/a - n/a
  Abstract: We report on the mineralogical assemblages found in the hyper‐alkaline springs hosted on Liguria and Oman ophiolites based on exhaustive XRD and SEM analyses. In Liguria, hyper‐alkaline springs produce a thin brownish calcite precipitate that covers the bedrock due to the concomitant atmospheric CO2 uptake and neutralization of the hyper‐alkaline waters. No brucite and portlandite minerals are observed. The discharge of alkaline waters in Oman ophiolite forms white‐orange precipitates. Calcium carbonate minerals (calcite and/or aragonite) are the most abundant and ubiquitous precipitates and are produced by the same mechanism as in Liguria. This process is observed as a thin surface crust made of rhombohedral calcite. Morphological features of aragonite vary from needle‐, bouquet‐, dumbbell‐, spheroidal‐like habitus according to the origin of carbon, the temperature and the ionic composition of the hyper‐alkaline springs, and the biochemical and organic compounds. Brucite is observed both at hyper‐alkaline springs located at the thrust plane and at the paleo‐Moho. The varying mixing proportions between the surface run‐off waters and the hyper‐alkaline ones control brucite precipitation. The Layered Double Hydroxide minerals occur solely in vicinity of hyper‐alkaline springs emerging within the bedded gabbros. Finally, the dominant mineralogical associations we found in Oman (Ca‐bearing carbonates and brucite) in a serpentinizing environment driven by the meteoric waters are surprisingly the same as those observed at the Lost City hydrothermal site in a totally marine environment.
  PubDate: 2013-06-12T01:35:21.053571-05:
  DOI: 10.1002/ggge.20146
Simplified mantle architecture and distribution of radiogenic power
- Authors: Ricardo Arevalo; William F. McDonough, Andreas Stracke, Matthias Willbold, Thomas J. Ireland, Richard J. Walker
  Pages: n/a - n/a
  Abstract: The mantle reservoir that represents the source region of ocean island basalts (OIB) and feeds hotspot volcanism is predicted to contain 160 ± 20 (2σm) ng/g Th, a heat‐producing element. This critical model composition indicates that the OIB source region (OSR) comprises a significant amount of recycled oceanic crust and constitutes 19+3−2 (2σm)% of the mantle by mass. The mass fraction of this reservoir supports a mantle architecture with a basal thermochemical layering at an average depth of 2000 ± 100 (2σm) km or two thermochemical piles that extend up to midmantle levels. The hotspot source described here generates 10 pW/kg of radiogenic heat and supplies 7.3 TW to the planet's total surface heat flux. Given that the silicate portion of the Earth produces some 20.4 TW of radiogenic power, with 7.2 TW derived from the continental crust, the mantle source responsible for mid‐ocean ridge volcanism provides only 5.9 TW of radiogenic power (or 5x more radiogenic heat than the source of mid‐ocean ridges, thus contributing to the energetics that drive mantle convection and potentially the formation of long‐lived plumes via bottom heating of the modern mantle. The potential for a sequestered or unsampled mantle reservoir would impact the relative mass fractions of the source regions of OIB and mid‐ocean ridge volcanism but not the compositional model of the OSR presented here.
  PubDate: 2013-06-08T05:07:01.497261-05:
  DOI: 10.1002/ggge.20152
The 2004 Aceh‐Andaman Earthquake: early clay dehydration controls shallow seismic rupture
- Authors: Jacob Geersen; Lisa McNeill, Timothy J. Henstock, Christoph Gaedicke
  Pages: n/a - n/a
  Abstract: The physical state of the shallow plate‐boundary fault governs the updip extent of seismic rupture during powerful subduction zone earthquakes and thus on a first order impacts on the tsunamigenic hazard of such events. During the 2004 Mw 9.2 Aceh‐Andaman Earthquake seismic rupture extended unusually far seaward below the accretionary prism causing the disastrous Indian Ocean Tsunami. Here we show that the formation of a strong bulk sediment section and a high fluid‐pressured predécollement, that likely enabled the 2004 rupture to reach the shallow plate‐boundary, result from thermally controlled diagenetic processes in the upper oceanic basement and overlying sediments. Thickening of the sediment section to >2 km ~160 km seaward of the subduction zone increases temperatures at the sediment basement interface and triggers mineral transformation and dehydration (e.g. smectite–illite) prior to subduction. The liberated fluids migrate into a layer that likely host high porosity and permeability and that is unique to the 2004 rupture area where they generate a distinct overpressured predécollement. Clay mineral transformation further supports processes of semi‐lithification, induration of sediments, and coupled with compaction dewatering all amplified by the thick sediment section together strengthens the bulk sediments. Farther south, where the 2005 Sumatra Earthquake did not include similar shallow rupture, sediment thickness on the oceanic plate is significantly smaller. Therefore, similar diagenetic processes occur later and deeper in the subduction zone. Hence we propose that shallow seismic rupture during the 2004 earthquake is primarily controlled by the thickness and composition of oceanic plate sediments.
  PubDate: 2013-06-07T05:54:57.757331-05:
  DOI: 10.1002/ggge.20193
Interaction between magma intrusion and flank dynamics at Mt. Etna in 2008, imaged by integrated dense GPS and DInSAR data
- Authors: Alessandro Bonforte; Francesco Guglielmino, Giuseppe Puglisi
  Pages: n/a - n/a
  Abstract: Global Positioning System (GPS) and Differential Interferometric Synthetic Aperture Radar (DInSAR) data, collected from July 2007 to July 2008 on Mt. Etna, are analysed to define the dynamics preceding and accompanying the onset of the eruption on 13 May 2008. Short and long‐term comparisons have been made on both GPS and radar data, covering similar time windows. Thanks to the availability of three GPS surveys the year before the eruption onset, an increase in the seawards movement of the NE flank of the volcano has been detected in the few months before the dike intrusion. The GPS ground deformation pattern also shows a slight inflation centred on the western side of the volcano in the pre‐eruptive long‐term comparison (from July 2007 to May 2008). The GPS has been integrated with DInSAR data by the SISTEM approach, to take advantage of the different methodologies and provide high spatial sampling of the 3D ground displacement pattern. We inverted the SISTEM results in order to model the pressure source causing the observed pre‐eruptive inflation. The subsequent emplacement of the eruptive dike was imaged by two GPS surveys carried out on a dense network over the uppermost part of the volcano on May 6 and 13, i.e. a few days before and a few hours after the beginning of the eruption. We inverted this comparison to define the position, geometry and kinematics of the dike. The dike intrusion was also imaged by DInSAR data with temporal baselines of 2‐3 months, which confirm strong displacements localized on the summit area, rapidly decreasing towards the middle flanks of the volcano, as detected by very short‐term GPS data; furthermore, the comparison between DInSAR and GPS data highlighted the presence of a depressurizing source localized beneath the upper south‐western area, acting just after the dike intrusion. Finally, the long period (one year) GPS and DInSAR data were integrated by SISTEM in order to finely depict the 3D ground deformation pattern with the highest spatial resolution. The long‐period data allowed the complex kinematics of the volcano to be finely imaged and highlighting the interaction between flank dynamics and magma injection.
  PubDate: 2013-06-07T05:42:35.808674-05:
  DOI: 10.1002/ggge.20190
Quaternary channels within the Northeast German Basin and their relevance on Double Diffusive Convective transport processes: Constraints from 3D thermohaline numerical simulations
- Authors: Björn Onno Kaiser; Mauro Cacace, Magdalena Scheck‐Wenderoth
  Pages: n/a - n/a
  Abstract: The internal geological structure of the Northeast German Basin (NEGB) is affected by intense salt diapirism and by the presence of several stratified aquifer complexes of regional relevance. The shallow Quaternary to late Tertiary freshwater aquifer is separated from the underlying Mesozoic saline aquifers by an embedded Tertiary clay enriched aquitard (Rupelian Aquitard). An important feature of this aquitard is that hydraulic connections between the upper and lower aquifers do exist in areas where the Rupelian Aquitard is missing (hydrogeological windows). Three‐dimensional thermohaline numerical simulations are carried out to investigate the effects of such hydrogeological windows in the Rupelian Aquitard on the resulting groundwater, temperature and salinity distributions. Numerical results suggest that hydrogeological windows act as preferential domains of hydraulic interconnectivity between the different aquifers at depth, and enable vigorous heat and mass transport which causes a mixing of warm and saline groundwater with cold and less saline groundwater within both aquifers. In areas where the Rupelian Aquitard confines the Mesozoic aquifer, dissolved solutes from major salt structures are transported laterally giving rise to plumes of variable salinity content ranging from few hundreds of meters to several tens of kilometers. Furthermore, destabilizing thermal buoyancy forces may overwhelm counteracting stabilizing salinity induced forces offside of salt domes. This may result in buoyant upward groundwater flow transporting heat and mass to shallower levels within the same Mesozoic Aquifer.
  PubDate: 2013-06-07T05:39:51.805036-05:
  DOI: 10.1002/ggge.20192
Quantifying and understanding the uncertainty of atmospheric CO2 concentrations determined from calcic paleosols
- Authors: D.O. Breecker
  Pages: n/a - n/a
  Abstract: A computer program (PBUQ) that uses Monte Carlo simulations to propagate uncertainty through regression equations and the equation for the paleosol carbonate CO2 paleobarometer is presented. PBUQ includes options for all of the common approaches to determining values for input variables and incorporates several recent advancements relevant to determining values for soil‐respired CO2 concentrations, δ13C values of respired CO2, δ13C values of atmospheric CO2 and temperatures of soil carbonate formation. PBUQ is intended to improve confidence in paleoatmospheric CO2 research by helping researchers draw statistically significant conclusions. PBUQ can also be used to attribute and partition error among various sources and thereby advance this technique. Sensitivity analysis indicates that S(z) is the largest source of uncertainty for most paleosols and that uncertainty is minimized for soils in which CO2 is an evenly balanced mixture between soil‐derived and atmospheric components. Evenly balanced mixtures are most likely for paleosols formed in deserts and for weakly‐developed paleosols. Development of proxies for soil‐respired CO2 concentrations and δ13C values of soil‐respired CO2 specifically for such soils is perhaps the most crucial next step for improving this technique. Currently, calcic paleosols are best used to test the significance of trends and/or differences among time slices in paleoatmospheric CO2 concentration. Application to quantifying Earth System Sensitivity will require large scale averaging of determinations from individual paleosols and/or reduced uncertainty associated with input variables.
  PubDate: 2013-06-06T11:47:28.232918-05:
  DOI: 10.1002/ggge.20189
Sr‐Nd‐Hf‐Pb isotope and trace element evidence for the origin of alkalic basalts in the Garibaldi Belt, northern Cascade arc
- Authors: Emily K. Mullen; Dominique Weis
  Pages: n/a - n/a
  Abstract: In the Garibaldi Belt, the northern segment of the Cascade arc, basalts at Bridge River Cones, Salal Glacier, and Mt. Meager (BSM volcanic centers) are alkalic, atypical for an arc setting. Subduction signatures are negligible or absent from primitive alkalic basalts from Salal Glacier and Bridge River, while altered oceanic crust may have contributed a minimal amount of fluid at Mt. Meager. More evolved BSM basalts display trace element signatures considered typical of arc lavas, but this is a consequence of deep crustal assimilation rather than primary input from the subducted slab. Primary BSM basalts represent 3‐8% melts that segregated from enriched garnet lherzolite at significantly higher temperatures and pressures (70‐105 km) than calc‐alkaline Cascade arc basalts. The BSM mantle source is significantly more incompatible element‐enriched than the depleted mantle tapped by calc‐alkaline Cascade arc basalts. The BSM basalts are also isotopically distinct from calc‐alkaline Cascade arc basalts, more similar to MORB and intraplate basalts of the NE Pacific and NW North America. The relatively deep, hot, and geochemically distinct mantle source for BSM basalts is consistent with upwelling asthenosphere. The BSM volcanic centers are close to the projected trace of the Nootka fault, which forms the boundary between the subducting Juan de Fuca plate and the near‐stagnant Explorer plate. A gap or attenuated zone between the plates may promote upwelling of enriched asthenosphere that undergoes low‐degree decompression melting to generate alkalic basalts that are essentially free of slab input yet occur in an arc setting.
  PubDate: 2013-06-06T11:47:20.996715-05:
  DOI: 10.1002/ggge.20191
Major and trace element composition of the high 3He/4He mantle: Implications for the composition of a non‐chonditic Earth
- Authors: Matthew G. Jackson; A. Mark Jellinek
  Pages: n/a - n/a
  Abstract: The bulk composition of the silicate portion of the Earth (BSE) has long been assumed to be tied to chondrites, in which refractory, lithophile elements like Sm and Nd exist in chondritic relative abundances. However, the 142Nd/144Nd ratios of modern terrestrial samples are 18±5 ppm higher than the ordinary‐chondrite reservoir, and this challenges the traditional BSE model. Here we investigate a hypothesis that this terrestrial 142Nd excess is related to a Sm/Nd ratio 6% higher than chondritic. This Sm/Nd ratio yields a superchondritic 143Nd/144Nd (~0.5130) similar to that identified in the highest 3He/4He mantle reservoir, and we argue that this reservoir represents the BSE composition for lithophile elements. We develop a compositional model for BSE in which the elevated Sm/Nd requires a shift of 143Nd/144Nd from 0.51263 (chondritic) to 0.51300. The new BSE composition is depleted in highly incompatible elements, including K, relative to the chondrite‐based BSE, and offers a solution the “missing” 40Ar paradox. This BSE compositional model requires that >83% of the mantle is depleted to form continental crust. It also implies a ~30% reduction in BSE U, Th and K, and therefore in the current rate of radiogenic heating and, thus, a proportional increase in the heat flow delivered to surface by plate tectonics. We explore thermal history models including effects related to a newly recognized evolution in the style of plate tectonics over Earth history: The lower radiogenic heat production may delay the onset of core convection and dynamo action to as late as 3.5 Gyr.
  PubDate: 2013-06-05T23:56:44.758709-05:
  DOI: 10.1002/ggge.20188
Cemented mounds and hydrothermal sediments on the detachment surface at kane megamullion: A new manifestation of hydrothermal venting
- Authors: Brian E. Tucholke; Susan E. Humphris, Henry J. B. Dick
  Pages: n/a - n/a
  Abstract: Long‐lived detachment faults are now known to be important in tectonic evolution of slow‐spreading mid‐ocean ridges, and there is increasing evidence that fluid flow plays a critical role in development of detachment systems. Here we document a new manifestation of low‐temperature hydrothermal venting associated with the detachment fault that formed Kane Megamullion ~3.3‐2.1 m.y. ago in the western rift‐valley wall of the Mid‐Atlantic Ridge. Hydrothermal effects on the detachment surface include 1) cemented mounds of igneous rock and chalk debris containing hydrothermal Mn oxides and Fe oxyhydroxides, and 2) layered deposits of similar Fe‐Mn minerals ± interbedded chalks. Mounds are roughly conical, ~1‐10 meters high, and contain primarily basalts with lesser gabbro, serpentinite, and polymict breccia. The layered Fe‐Mn‐rich sediments are flat‐bedded to contorted and locally are buckled into low‐relief linear or polygonal ridges. We propose that the mounds formed where hydrothermal fluids discharged through the detachment hanging wall near the active fault trace. Hydrothermal precipitates cemented hanging‐wall debris and welded it to the footwall, and this debris persisted as mounds as the footwall was exhumed and surrounding unconsolidated material sloughed off the sloping detachment surface. Some of the layered Fe‐Mn‐rich deposits may have precipitated from fluids discharging from the hanging‐wall vents, but they also precipitated from low‐temperature fluids venting from the exposed footwall through overlying chalks. Observed natural disturbance and abnormally thin hydrogenous Fe‐Mn crusts on some contorted, hydrothermal Fe‐Mn‐rich chalks on ~2.7 Ma crust suggest diffuse venting that is geologically recent. Results of this study imply that there are significant fluid pathways through all parts of detachment systems and that low‐temperature venting through fractured detachment footwalls may continue for several million years off‐axis.
  PubDate: 2013-06-05T23:56:42.370393-05:
  DOI: 10.1002/ggge.20186
Petrological Constraints on Melt Generation Beneath the Asal Rift (Djibouti) using Quaternary basalts
- Authors: Paul Pinzuti; Eric Humler, Isabelle Manighetti, Yves Gaudemer
  Pages: n/a - n/a
  Abstract: The temporal evolution of the mantle melting processes in the Asal Rift is evaluated from the chemical composition of 56 new lava flows sampled along 10 km of the rift axis and 9 km off‐axis (i.e., erupted within the last 620 ky). Petrological and primary geochemical results show that most of the samples of the inner floor of the Asal Rift are affected by plagioclase accumulation. Trace element ratios and major element compositions corrected for mineral accumulation and crystallization show a symmetric pattern relative to the rift axis and preserved a clear signal of mantle melting depth variations. While FeO, Fe8.0, Zr/Y and (Dy/Yb)N decrease from the rift shoulders to the rift axis, SiO2, Na/Ti, Lu/Hf increase and Na2O and Na8.0 are constant across the rift. These variations are qualitatively consistent with shallow melting beneath the rift axis and deeper melting for off‐axis lava flows. Na8.0 and Fe8.0 contents show that beneath the rift axis, melting paths are shallow, from 81 ± 4 km to 43 ± 5 km. These melting paths are consistent with adiabatic melting in normal‐temperature fertile asthenosphere, beneath an extensively thinned mantle lithosphere. On the contrary, melting on the rift shoulders (from 107 ± 7 km to 67 ± 8 km) occurred beneath thicker lithosphere, requiring a mantle solidus temperature 100 ± 40 °C hotter. In this geodynamic environment, the calculated rate of lithospheric thinning appears to be 4.0 ± 2.0 cm yr‐1, a value close to the mean spreading rate (2.9 ± 0.2 cm yr‐1) over the last 620 ky.
  PubDate: 2013-06-05T23:52:54.583043-05:
  DOI: 10.1002/ggge.20187
Mylonitic deformation at the Kane oceanic core complex: Implications for the rheological behavior of oceanic detachment faults
- Authors: Lars N. Hansen; Michael J. Cheadle, Barbara E. John, Susan M. Swapp, Henry J. B. Dick, Brian E. Tucholke, Maurice A. Tivey
  Pages: n/a - n/a
  Abstract: The depth extent, strength, and composition of oceanic detachment faults remain poorly understood because the grade of deformation‐related fabrics varies widely among sampled oceanic core complexes. We address this issue by analyzing fault rocks collected from the Kane oceanic core complex at 23°30′N on the Mid‐Atlantic Ridge. A portion of the sample suite was collected from a younger fault scarp that cuts the detachment surface and exposes the interior of the most prominent dome. The style of deformation was assessed as a function of proximity to the detachment surface, revealing a ~450‐m‐thick zone of high‐temperature mylonitization overprinted by a ~200‐m‐thick zone of brittle deformation. Geothermometry of deformed gabbros demonstrates that crystal‐plastic deformation occurred at temperatures >700°C. Analysis of the morphology of the complex in conjunction with recent thermochronology suggests that deformation initiated at depths of ~7 km. Thus we suggest the detachment system extended into or below the brittle‐plastic transition (BPT). Microstructural evidence suggests that gabbros and peridotites with high‐temperature fabrics were dominantly deforming by dislocation‐accommodated processes and diffusion creep. Recrystallized‐grain‐size piezometry yields differential stresses consistent with those predicted by dry‐plagioclase flow laws. The temperature and stress at the BPT determined from laboratory‐derived constitutive models agree well with the lowest temperatures and highest stresses estimated from gabbro mylonites. We suggest that the variation in abundance of mylonites among oceanic core complexes can be explained by variation in the depth of the BPT, which depends to a first order on the thermal structure and water content of newly forming oceanic lithosphere.
  PubDate: 2013-06-03T23:17:04.196173-05:
  DOI: 10.1002/ggge.20184
Study of the western edge of the African Large Low Shear Velocity Province
- Authors: Daoyuan Sun; Meghan S. Miller
  Pages: n/a - n/a
  Abstract: It is well established that there is a large low shear velocity province (LLSVP) in the lowermost mantle beneath Africa, extending from beneath the southeastern Atlantic Ocean to the southwestern Indian Ocean. The detailed 3D geometry of the LLSVP is crucial to the understanding of how this prominent lower mantle feature developed and evolved. Most studies have concentrated on mapping the southern and eastern edges of African LLSVP using the Kaapvaal array at South Africa. Here, we use data from recently deployed arrays in the western Mediterranean to study its northwestern edge and evaluate the sharpness of its boundaries. Travel time and waveform modeling of S and SKS phases suggest the existence of sharp low velocity anomalies in the lowermost mantle beneath the eastern Atlantic Ocean, which agrees with global tomography models. However, the S‐SKS differential times vary up to 6 sec across the array. To match the large travel time variations and waveforms, the existence of a slow velocity structure with a sharp top is required. The structure has a 3.5% reduction in shear wave velocity and a height of ˜600 km above the core‐mantle boundary, which is lower in topography than the southern and eastern part of the African LLSVP. Further 3D synthetic waveforms and modeling calculations demonstrate the existence of these sharp boundaries for the northwestern portion of the African LLSVP. The strong lateral variation in both travel times and waveforms suggests that this part of the African LLSVP may be as complex as the mid‐Pacific LLSVP. To explain the observed sharp top and large dome‐like structure, a thermo‐chemical origin of the African LLSVP is supported.
  PubDate: 2013-06-03T06:19:29.848248-05:
  DOI: 10.1002/ggge.20185
Explosive expansion of a slowly‐decompressed magma analogue evidence for delayed bubble nucleation
- Authors: Eleonora Rivalta; Karen Pascal, Jeremy Phillips, Alessandro Bonaccorso
  Pages: n/a - n/a
  Abstract: While ascending in the plumbing system of volcanoes, magma undergoes decompression at rates spanning several orders of magnitude and set by a number of factors internal and external to the volcano. Slow decompression generally results in an effusive or mildly explosive expansion of the magma, but counterexamples of sudden switches from effusive to explosive eruptive behaviour have been documented at various volcanoes worldwide. The mechanisms involved in this behavior are currently debated, in particular for basaltic magmas. Here, we explore the interplay between decompression rate and vesiculation vigour by decompressing a magma analogue obtained by dissolving pine resin into acetone in varying proportions. Analogue experiments allow direct observations of the processes of bubble nucleation and growth, flow dynamics and fragmentation that is not currently possible with magmatic systems. Our mixtures contain solid particles and upon decompression, nucleation of acetone bubbles is observed. We find that mixtures with a high acetone content, containing smaller and fewer solid particles, experience strong supersaturation and fragment under very slow decompressions, despite having low viscosity, while mixtures with lower acetone content, with more and larger solid particles, degas efficiently without fragmentation. We interpret our results in terms of delayed bubble nucleation due to a lack of efficient nucleation sites. We discuss how a similar mechanism might induce violent, explosive expansion in volatile‐rich and poorly crystalline low‐silica magmas, by analogy with the behaviour of rhyolitic magmas.
  PubDate: 2013-06-03T04:40:52.193916-05:
  DOI: 10.1002/ggge.20183
In situ stress and pore pressure in the Kumano Forearc Basin, offshore SW Honshu from downhole measurements during riser drilling
- Authors: D. M. Saffer; P. B. Flemings, D. Boutt, M.‐L. Doan, T. Ito, L. McNeill, T. Byrne, M. Conin, W. Lin, Y. Kano, E. Araki, N. Eguchi, S. Toczko
  Pages: n/a - n/a
  Abstract: In situ stress and pore pressure are key parameters governing rock deformation, yet direct measurements of these quantities are rare. During Integrated Ocean Drilling Program (IODP) Expedition #319, we drilled through a forearc basin at the Nankai subduction zone and into the underlying accretionary prism. We used the Modular Formation Dynamics Tester tool (MDT) for the first time in IODP to measure in situ minimum stress, pore pressure, and permeability at 11 depths between 729.9 and 1533.9 mbsf. Leak‐off testing at 708.6 mbsf conducted as part of drilling operations provided a second measurement of minimum stress. The MDT campaign included nine single‐probe (SP) tests to measure permeability and in situ pore pressure and two dual‐packer (DP) tests to measure minimum principal stress. Permeabilities defined from the SP tests range from 6.53 × 10−17 to 4.23 × 10−14 m2. Pore fluid pressures are near hydrostatic throughout the section despite rapid sedimentation. This is consistent with the measured hydraulic diffusivity of the sediments and suggests that the forearc basin should not trap overpressures within the upper plate of the subduction zone. Minimum principal stresses are consistently lower than the vertical stress. We estimate the maximum horizontal stress from wellbore failures at the leak‐off test and shallow MDT DP test depths. The results indicate a normal or strike‐slip stress regime, consistent with the observation of abundant active normal faults in the seaward‐most part of the basin, and a general decrease in fault activity in the vicinity of Site C0009.
  PubDate: 2013-05-29T11:28:34.475405-05:
  DOI: 10.1029/2012GC004525
Global sediment thickness dataset updated for the Australian‐Antarctic Southern Ocean
- Authors: Joanne Whittaker; Alexey Goncharov, Simon Williams, R. Dietmar Müller, German Leitchenkov
  Pages: n/a - n/a
  Abstract: We present a new, 5‐minute sediment thickness grid for the Australian‐Antarctic region (60°–155°E, 30°–70°S). New seismic reflection and refraction data have been used to add detail to the conjugate Australian and Antarctic margins and intervening ocean floor where regional sediment thickness patterns were poorly known previously. On the margins, sediment thickness estimates were computed from velocity‐depth functions from sonobuoy/refraction velocity solutions ground‐truthed against seismic reflection data. For the Southeast Indian Ridge abyssal plains, sediment thickness contours from Geli et al. (2007) were used. The new regional minimum sediment thickness grid was combined with the global NGDC sediment grid (Divins, 2004) to create an updated global grid. Even using the minimum estimates, sediment accumulations on the extended Australian and Antarctic continental margins are 2 km thicker across large regions and up to 9 km thicker in the Ceduna Basin compared to the global NGDC compilation of sediment thickness data.
  PubDate: 2013-05-29T00:02:29.568453-05:
  DOI: 10.1002/ggge.20181
Can a sheet‐like low‐velocity region form an elongated LIP?
- Authors: Atsuko Namiki; Kenta Sueyoshi, Nozomu Takeuchi
  Pages: n/a - n/a
  Abstract: High‐resolution tomography of the lower mantle has revealed the existence of another chemically distinct region with low‐velocity and a sheet‐like structure beneath the western Pacific. On the other hand, Large Igneous Provinces (LIPs) sometimes have elongated shapes. If a sheet‐like upwelling reaches the Earth's surface while maintaining its shape, an elongated LIP may form. In order to test this hypothesis, we perform a series of experiments and investigate the stability of a buoyant sheet. The experimental results show that the buoyant fluid accumulates at the top of the sheet to form a buoyant cylinder. The gravitational instability divides the cylinder into several plume heads. We develop a model to explain the growth of the buoyant cylinder and the time scale until instability begins. Our model shows that a thin sheet‐like upwelling with a width of 200 km, a small density difference from the ambient mantle, 10 kg m‐3, and a high supply rate of buoyant fluid, 0.1 m yr‐1, can reach the Earth's surface while maintaining its shape. We thus infer that LIPs with an elongated shape can be generated by sheet‐like upwellings. The width of the observed sheet‐like low‐velocity region beneath the western Pacific is 500 km and is marginally sufficient to form an elongated LIP.
  PubDate: 2013-05-28T23:56:40.645252-05:
  DOI: 10.1002/ggge.20182
Observations of the volume flux of a seafloor hydrothermal plume using an acoustic imaging sonar
- Authors: G. Xu; D. R. Jackson, K. G. Bemis, P. A. Rona
  Pages: n/a - n/a
  Abstract: We present a 26‐day time series (October 2010) of physical properties (volume flux, flow velocity, expansion rate) of a vigorous deep‐sea hydrothermal plume measured using our Cabled Observatory Vent Imaging Sonar (COVIS), which is connected to the NEPTUNE Canada Cabled Observatory at the Main Endeavour Field on the Juan de Fuca Ridge. COVIS quantitatively monitors the initial buoyant rise of the plume from ~ 5 m to ~ 15 m above the vents. The time series exhibits temporal variations of the plume vertical volume flux (1.93‐5.09 m3/s), centerline vertical velocity component (0.11‐0.24 m/s) and expansion rate (0.082‐0.21 m/m); these variations have major spectral peaks at semi‐diurnal (~ 2 cycle/day) and inertial oscillation (~ 1.5 cycle/day) frequencies. The plume expansion rates (average ~ 0.14 m/m) are inversely proportional to the plume centerline vertical velocity component (coefficient of determination R2 ~ 0.5). This inverse proportionality, as well as the semi‐diurnal frequency, indicates interaction between the plume and ambient ocean currents consistent with an entrainment of ambient seawater that increases with the magnitude of ambient currents. The inertial oscillations observed in the time series provide evidence for the influence of surface storms on the hydrodynamics of hydrothermal plumes.
  PubDate: 2013-05-25T00:03:03.296161-05:
  DOI: 10.1002/ggge.20177
A low‐relief shield volcano origin for the South Kaua'i Swell
- Authors: Garrett Ito; Michael O. Garcia, John R. Smith, Brian Taylor, Ashton Flinders, Brian Jicha, Seiko Yamasaki, Dominique Weis, Lisa Swinnard, Chuck Blay
  Pages: n/a - n/a
  Abstract: The South Kaua'i Swell (SKS) is a 110 km x 80 km ovoid bathymetric feature that stands >2 km high and abuts the southern flank of the island of Kaua'i. The origin of the SKS was investigated using multibeam bathymetry and acoustic backscatter, gravity data, radiometric ages, and geochemistry of rock samples. Most of the SKS rock samples are tholeiitic in composition with ages of 3.9‐5.4 Ma indicating they were derived from shield volcanism. The ages and compositions of the SKS rocks partially overlap with those of the nearby Ni'ihau, Kaua'i and West Ka'ena volcano complexes. The SKS was originally described as a landslide; however, this interpretation is problematic given the ovoid shape of SKS, its relatively smooth, flat‐to‐convex surface, and the lack of an obvious source region that could accommodate what would be one of Earth's most voluminous (6 x 103 km3) landslides. The morphology, size, and the surrounding gravity anomaly are more consistent with the SKS being a low‐relief shield volcano, which was partially covered with a small volume of landside debris from south Kaua'i and later with some secondary volcanic seamounts. A shield origin would imply that Hawaiian and possibly other hotspot shield volcanoes can take on a wider variety of forms than is commonly thought, ranging from tall island‐building shields, to smaller edifices such as Ka'ena Ridge and Mahukona, to even lower‐relief volcanoes represented by the SKS and possibly the South West O'ahu Volcanic Field.
  PubDate: 2013-05-24T23:53:26.041317-05:
  DOI: 10.1002/ggge.20159
Sediment‐enriched adakitic magmas from the Daisen volcanic field, Southwest Japan
- Authors: Maureen Feineman; Takuya Moriguti, Tetsuya Yokoyama, Sakiko Terui, Eizo Nakamura
  Pages: n/a - n/a
  Abstract: The Quaternary Southwest Japan Arc is a product of subduction of the hot, young Philippine Sea Plate beneath the Eurasian Continental Plate. The magmas erupted from the Southwest Japan Arc belong to a category of magmas commonly referred to as “adakites” or “adakitic magmas”. These magmas show trace element evidence for interaction with garnet at depth, and may be associated with partial melting of subducted altered oceanic crust. Also found throughout the southern Sea of Japan region are alkali basalts with little apparent connection to the subduction zone. We have determined major element, trace element, and Sr, Nd, Pb, and U‐Th isotopic compositions for a bimodal suite of lavas erupted at the Daisen volcanic field in the Southwest Japan Arc. These magmas consist of mildly alkaline basalts and a calcalkaline intermediate suite, separated by a ~10 wt. % silica gap. The intermediate magmas show trace element and isotopic evidence for interaction with garnet, consistent with partial melting of the hot, young (~20 Ma) Philippine Sea Plate. The Daisen intermediate magmas are distinct from other adakitic magmas in their radiogenic isotopic characteristics, consistent with a significant contribution (~25%) from subducted Nankai Trough sediments. Our data suggest that the basalts erupted at the Daisen volcanic field are not parental to the intermediate magmas, and contain a small contribution of EM1‐like mantle common in Sea of Japan alkali basalts but not apparent in the Daisen intermediate magmas.
  PubDate: 2013-05-24T23:52:33.92264-05:0
  DOI: 10.1002/ggge.20176
In search of long term hemispheric asymmetry in the geomagnetic field: Results from high northern latitudes
- Authors: G. Cromwell; L. Tauxe, H. Staudigel, C.G. Constable, A.A.P. Koppers, R‐B. Pedersen
  Pages: n/a - n/a
  Abstract: Investigations of the behavior of the geomagnetic field on geological time‐scales rely on globally distributed datasets from dated lava flows. We present the first suitable data from the Arctic region, comprising 37 paleomagnetic directions from Jan Mayen 71°N, 0.2‐461 ka) and Spitsbergen (79°N, 1‐9.2 Ma) and 5 paleointensity results. Dispersion of the Arctic virtual geomagnetic poles over the last 2 Ma (27.3±4.0°) is significantly lower than that from published Antarctic datasets (32.1 ± 5.0°). Arctic average virtual axial dipole moment (76.8 ± 24.3 ZAm2) is high in comparison with the same time interval (34.8 ± 8.2 ZAm2), although the data are still too sparse in the Arctic to be definitive. These data support a long‐lived hemispheric asymmetry of the magnetic field, contrasting higher, more stable fields in the north with lower average strength and more variable field directions in the south. Such features require significant non‐axial‐dipole contributions over 105‐106 years.
  PubDate: 2013-05-24T23:47:39.692296-05:
  DOI: 10.1002/ggge.20174
Bimodal volcanism of the high lava plains and Northwestern Basin and Range of Oregon: The distribution and tectonic implications of age‐progressive rhyolites
- Authors: Mark T. Ford; Anita L. Grunder, Robert A. Duncan
  Pages: n/a - n/a
  Abstract: Multiple episodes of Oligocene and younger silicic volcanism are represented in the high lava plateau of central and southeastern Oregon. From 12 Ma to Recent, volcanism is strongly bimodal with nearly equal volumes of basalt and rhyolite. It is characterized by moderate to high silica (SiO2 >72 wt. %) rhyolitic tuffs and domes that are younger to the west, and widespread, tholeiitic basalts that show no temporal pattern. We report 18 new 40Ar/39 Ar incremental heating ages on rhyolites, and establish that the timing of the age‐progressive rhyolites is decoupled from basaltic pulses. This work expands on that of previous workers by clearly linking the High Lava Plains (HLP) and Northwestern‐most Basin and Range (NWBR) rhyolite volcanism into a single age‐progressive trend. The spatial‐temporal relationship of the rhyolite outcrops and regional tectonics indicate that subsidence due to increasingly dense crust creates large, primarily sediment‐filled basins within the more volcanically active HLP. The west‐northwest age progression in rhyolitic volcanism is counter to the trend expected for a quasi‐stationary mantle upwelling relative to North American plate motion. We attribute the rhyolitic age progression to mantle upwelling in response to slab rollback and steepening, and this is consistent with mantle anisotropy under the region and analog slab rollback models. This removes the necessity of deep mantle plume involvement. Laboratory experimental studies indicate that the geometry of the downgoing slab can focus upwelling or asthenospheric counterflow into a constricted band, resulting in greater volcanic volumes in the HLP as compared to the NWBR.
  PubDate: 2013-05-24T23:43:48.795888-05:
  DOI: 10.1002/ggge.20175
Creep events at the brittle ductile transition
- Authors: Luc L. Lavier; Richard A. Bennett, Ravindra Duddu
  Pages: n/a - n/a
  Abstract: We present an analytic formulation to model creep events at the transition between brittle behavior in the crust and viscous behavior in ductile shear zones. We assume that creep events at the brittle ductile transition (BDT) are triggered by slip on optimally oriented fractures or network of fractures filled with weak ductile material. These events are expressed as transient flow in ductile shear zones likely aided by the release of crustal fluids. We show that the creep in the shear zone can be modeled as the motion of a forced damped oscillator composed of a brittle viscoelastic crust, a ductile shear zone and a creeping zone of fractures at the BDT. The time scale of the events varies between seconds to thousands of years depending on the viscous, elastic and brittle‐plastic properties of the fractured BDT, the shear zone and the crust. The nature of the events depends on the aspect ratio, γ of the shear zone thickness, Hw to the length of the fractured zone, w. We find that thick shear zones with small fractures at the BDT are stiff and generate creep oscillations. Thin shear zones with well‐connected fractures over a large width have very small stiffness and are well lubricated. They generate slow creep events or steady creep event. The former are similar to transient slip events and the latter to creep at the far field tectonic rates. The viscosity of the shear zone, ηw enhances lubrication if it is small and stiffness if it is large.
  PubDate: 2013-05-24T23:41:50.348675-05:
  DOI: 10.1002/ggge.20178
Early‐stage rifting of the northern Tyrrhenian Sea Basin: Results from a combined wide‐angle and multichannel seismic study
- Authors: Moeller S; Grevemeyer I, Ranero C.R, Berndt C, Klaeschen D, Sallares V, Zitellini N, de Franco R.
  Pages: n/a - n/a
  Abstract: Extension of the continental lithosphere leads to the formation of rift basins and ultimately may create passive continental margins. The mechanisms that operate during the early‐stage of crustal extension are still intensely debated. We present the results from coincident multichannel seismic and wide‐angle seismic profiles that transect across the northern Tyrrhenian Sea basin. The profiles cross the Corsica Basin (France) to the Latium Margin (Italy) where the early‐rift stage of the basin is well preserved. We found two domains, each with a distinct tectonic style, heat‐flow and crustal thickness. One domain is the Corsica Basin in the west that formed before the main rift‐phase of the northern Tyrrhenian Sea opening (~8‐4 Ma). The second domain is rifted continental crust characterized by tilted blocks and half‐graben structures in the central region and at the Latium Margin. These two domains are separated by a deep (~10 km) sedimentary complex of the eastern portion of the Corsica Basin. Travel‐time tomography of wide‐angle seismic data reveals the crustal architecture and a sub‐horizontal 15‐17 ± 1 km deep Moho discontinuity under the basin. To estimate the amount of horizontal extension we have identified the pre‐, syn‐, and post‐tectonic sedimentary units and calculated the relative displacement of faults. We found that major faults initiated at angles of 45‐50° and that the rifted domain is horizontally stretched by a factor of β=1.3 (~8‐10 mm/a). The crust has been thinned from ~24 to 17 km indicating a similar amount of extension (~30%). The transect represents one of the best imaged early‐rifts and implies that the formation of crustal‐scale detachments, or long‐lived low‐angle normal faults, is not a general feature that controls the rift initiation of continental crust. Other young rift basins, like the Gulf of Corinth, the Suez‐Rift or Lake Baikal, display features resembling the northern Tyrrhenian Basin, suggesting that half graben formations and distributed homogeneous crustal thinning are a common feature during rift initiation.
  PubDate: 2013-05-24T23:40:10.764002-05:
  DOI: 10.1002/ggge.20180
Buoyancy and localizing properties of continental mantle lithosphere: Insights from thermomechanical models of the eastern Gulf of Aden
- Authors: L. Watremez; E. Burov, E. d'Acremont, S. Leroy, B. Huet, L. Le Pourhiet, N. Bellahsen
  Pages: n/a - n/a
  Abstract: Physical properties of the mantle lithosphere have a strong influence on the rifting processes and rifted structures. In particular, in context of rifting, two of these properties have been overlooked: (1) Mohr‐Coulomb plasticity (localizing pressure‐dependent) may not be valid at mantle depths as opposed to non‐localizing pressure‐independent plasticity (hereafter, perfect plasticity), and (2) lithosphere buoyancy can vary, depending on the petrological composition of the mantle. Focussing on the Arabian plate, we show that the lithosphere may be negatively buoyant. We use thermomechanical modeling to investigate the importance of mantle rheology and composition on the formation of a passive margin, ocean‐continent transition (OCT) and oceanic basin. We compare the results of this parametric study to observations in the eastern Gulf of Aden (heat‐flow, refraction seismics and topography) and show that (1) mantle lithosphere rheology controls the margin geometry and timing of the rifting; (2) lithosphere buoyancy has a large impact on the seafloor depth and the timing of partial melting; and (3) a perfectly plastic mantle lithosphere 20 kg m−3 denser than the asthenosphere best fits with observed elevation in the Gulf of Aden. Finally, thermomechanical models suggest that partial melting can occur in the mantle during the Arabian crustal break‐up. We postulate that the produced melt could then infiltrate through the remnant continental mantle lithosphere, reach the surface and generate oceanic crust. This is in agreement with the observed narrow OCT composed of exhumed continental mantle intruded by volcanic rocks in the eastern Gulf of Aden.
  PubDate: 2013-05-24T23:27:13.660509-05:
  DOI: 10.1002/ggge.20179
Active deformation in old oceanic lithosphere and significance for earthquake hazard: Seismic imaging of the Coral Patch Ridge area and neighboring abyssal plains (SW Iberian Margin)
- Authors: Sara Martínez‐Loriente; Eulàlia Gràcia, Rafael Bartolome, Valentí Sallarès, Hector Perea, Claudio Lo Iacono, Christopher Connors, Dirk Klaeschen, Pedro Terrinha, Juan José Dañobeitia, Nevio Zitellini
  Pages: n/a - n/a
  Abstract: Recently acquired high‐resolution multichannel seismic profiles together with bathymetric and sub‐bottom profiler data from the external part of the Gulf of Cadiz (Iberia‐Africa plate boundary) reveal active deformation involving old (Mesozoic) oceanic lithosphere. This area is located 180 km offshore the SW Iberian Peninsula and embraces the prominent NE‐SW trending Coral Patch Ridge, and part of the surronding deep Horseshoe and Seine abyssal plains. E‐W trending dextral strike‐slip faults showing surface deformation of flower‐like structures predominate in the Horseshoe Abyssal Plain, whereas NE‐SW trending compressive structures prevail in the Coral Patch Ridge and Seine Hills. Although the Coral Patch Ridge region is characterized by subdued seismic activity, the area is not free from seismic hazard. Most of the newly mapped faults correspond to active blind thrusts and strike‐slip faults that are able to generate large magnitude earthquakes (Mw 7.2 to 8.4). This may represent a significant earthquake and tsunami hazard that has been overlooked so far.
  PubDate: 2013-05-22T03:16:44.355335-05:
  DOI: 10.1002/ggge.20173
Oceanographic variability in the South Pacific Convergence Zone region over the last 210 years from multi‐site coral Sr/Ca records
- Authors: Henry C. Wu; Braddock K. Linsley, Emilie P. Dassié, Benedetto Schiraldi, Peter B. deMenocal
  Pages: n/a - n/a
  Abstract: In the South Pacific Convergence Zone (SPCZ), the variability in a sub‐seasonally resolved microatoll Porites colony Sr/Ca record from Tonga and a previously published high‐resolution record from Fiji are strongly influenced by sea surface temperature (SST) over the calibration period from 1981 to 2004 (R2 = 0.67–0.68). However, the Sr/Ca‐derived SST correlation to instrumental SST decreases back in time. The lower frequency secular trend (~1°C) and decadal‐scale (~2–3°C) modes in Sr/Ca‐derived SST are almost two times larger than that observed in instrumental SST. The coral Sr/Ca records suggest that local effects on SST generate larger amplitude variability than gridded SST products indicate. Reconstructed δ18O of seawater (δ18Osw) at these sites correlate with instrumental sea surface salinity (SSS; r = 0.64–0.67) but not local precipitation (r = −0.10 to −0.22) demonstrating that the advection and mixing of different salinity water masses may be the predominant control on δ18Osw in this region. The Sr/Ca records indicate SST warming over the last 100 years and appears to be related to the expansion of the western Pacific warm pool (WPWP) including an increasing rate of expansion in the last ~20 years. The reconstructed δ18Osw over the last 100 years also shows surface water freshening across the SPCZ. The warming and freshening of the surface ocean in our study area suggests that the SPCZ has been shifting (expanding) southeast, possibly related to the southward shift and intensification of the South Pacific gyre over the last 50 years in response to strengthened westerly winds.
  PubDate: 2013-05-17T16:15:43.27612-05:0
  DOI: 10.1029/2012GC004293
Variability of internal frontal bore breaking above Opouawe Bank methane seep area (New Zealand)
- Authors: Hans van Haren; Jens Greinert
  Pages: n/a - n/a
  Abstract: Large internal wave breaking is observed exceeding a vertical array of 61 high‐resolution temperature sensors at 1 m intervals between 7 and 67 m above the bottom. The array was moored for 5 days at 969 m of Opouawe Bank, New Zealand, a known methane seep area. As breaking internal waves dominate sediment resuspension above sloping topography in other ocean areas, they are expected to also influence methane transport. Despite being visible in single beam echosounder data, indications for turbulence due to rising gas bubbles are not found in the present 1‐Hz sampled temperature records. Likely, the mooring was too far away from the very localized bubble release spot. Instead, the temperature sensors show detailed internal wave – turbulence transitions. Every tidal cycle, a solibore (a frontal turbulent bore with a train of trailing solitary waves) changes shape and intensity. These solibores are highly turbulent and they restratify the bottom boundary layer, thereby maintaining efficient mixing. Details of different turbulent bore developments are discussed. Averaged over a few tidal cycles and over the sensors range, mean vertical eddy diffusivity amounts to 3±1×10‐3 m2 s‐1 and mean turbulent kinetic energy dissipation to 1.6±0.7×10‐7 W kg‐1, with variations over four orders of magnitude. Such turbulence will affect the distribution of dissolved methane and other geochemical species in the lower 100‐150 m above the bottom and their release from the bottom. The above mean values are remarkably similar to those found at various other sites in the NE Atlantic Ocean.
  PubDate: 2013-05-16T11:21:40.170599-05:
  DOI: 10.1002/ggge.20170
Subduction and slab breakoff controls on Asian Indentation tectonics and Himalayan Western Syntaxis formation
- Authors: Fabio A. Capitanio; Anne Replumaz
  Pages: n/a - n/a
  Abstract: We test the link between large‐scale Asian continent deformations and Indian slab subduction and breakoff during convergence by means of three‐dimensional numerical models of the subducting‐upper plates system. We find that the subduction of the buoyant continent results in the reduction of convergence velocity comparable to that observed in the Indian motions, yet the upper plate deformation remains accommodated in a narrow belt along a straight margin. Comparable rates are measured when the subducting slab breakoff is modeled, although the convergent margin deforms and curves markedly, with large underplating contiguous to ongoing subduction, similar to what observed along the Himalayan range. The models support the interpretation of the Himalayan Western Syntaxis evolution, the progressive curvature of the Indian margin and the underthrusting as a consequence of the Indian slab breakoff. The modeled slab detachment is followed by short‐lived large stresses in the upper plate interiors, propagating at large distance from the margin with a trend similar to several major Asian lithospheric faults. Such localised stress has likely provided the conditions for the formation of the Central Asian intra‐continental faulting, the Bangong‐Red River and the Altyn Tagh faults, that followed successive Indian slab breakoff episodes. Continent subduction and breakoff during India‐Asia convergence offer an explanation for the different deformation mechanisms as the long‐lived understhrusting and the episodic lithospheric faulting in the Asian continent and their link to deep processes.
  PubDate: 2013-05-16T11:20:48.361467-05:
  DOI: 10.1002/ggge.20171
The effect of particle size on the rheology of liquid‐solid mixtures with application to lava flows: Results from analogue experiments
- Authors: P. Del Gaudio; G. Ventura, J. Taddeucci
  Pages: n/a - n/a
  Abstract: We investigate the effect of crystal size on the rheology of basaltic magmas by means of a rheometer and suspensions of silicon oil with natural magmatic crystals of variable size (from 63 μm to 0.5 mm) and volume fraction φ (from 0.03 to 0.6). At constant φ, finer suspensions display higher viscosities than coarser ones. Shear thinning (flow index n0.1‐0.2 and is more pronounced (stronger departure from the Newtonian behavior) in finer suspensions. Maximum packing and average crystal size displays a non‐linear, positive correlation, while yield stress develops at φ >0.2‐0.3 irrespective of the crystal size. We incorporate our results into physical models for flow of lava and show that, with respect to lava flows containing coarser crystals, those with smaller crystals are expected to: 1) flow at lower velocity, 2) have a lower velocity gradient, and 3) be more prone to develop a region of plug flow. Our experimental results explain the observation that phenocryst‐bearing and microlite‐bearing lavas at Etna volcano (Italy) show smooth pahoehoe and rough aa' surfaces, respectively.
  PubDate: 2013-05-16T11:20:31.525782-05:
  DOI: 10.1002/ggge.20172
The spatial and temporal evolution of strain during the separation of Australia and Antarctica
- Authors: Philip Ball; Graeme Eagles, Cynthia Ebinger, Ken McClay, Jennifer Totterdell
  Pages: n/a - n/a
  Abstract: A re‐evaluation of existing onshore and offshore gravity, magnetic, seismic reflection, and well data from the Australo‐Antarctic margins suggests that magmatism and along‐strike lithospheric heterogeneities have influenced the localization of initial rifting. The 3D crustal architecture of the Australian and Antarctic margins, which formed during multiple rifting episodes spanning ~80 My, reveal local asymmetries along strike. Rift structures from the broad, late Jurassic (165‐145 Ma) rift zone are partially overprinted by a narrower, mid‐to‐late Cretaceous rift zone ~100 Ma, which evolved in highly extended crust. This late‐stage rift zone is located within a region of heterogeneous crust with faults that cut late syn‐rift strata, interpreted as a continent ocean transition zone. This late stage transitional rift is populated by seismically identified rift‐parallel basement highs and intra‐crustal bodies with corresponding positive Bouguer gravity and magnetic anomalies. These undrilled features can be interpreted as exposures of exhumed mantle rocks, lower crustal rocks and/or as discrete magmatic bodies. Our results suggest that strain across an initially broad Australo‐Antarctic rift system (165‐145 Ma) migrated to a narrow rift zone with some magmatism at 100‐83 Ma. Breakup did not occur until ~53 Ma within the eastern Bight‐Wilkes and Otway‐Adélie margin sectors, suggesting a west to east propagation of seafloor spreading. The prolonged eastwards propagation of seafloor spreading processes and the increased asymmetry of the Australian‐Antarctic margins coincides with a change from rift‐perpendicular to oblique rifting processes, which in turn coincide with along‐ strike variations in cratonic to Palaeozoic lithosphere.
  PubDate: 2013-05-14T04:22:01.675335-05:
  DOI: 10.1002/ggge.20160
Mass‐transport‐dominated sedimentation in a foreland basin, the hidaka trough, Northern Japan
- Authors: Atsushi Noda; Taqumi TuZino, Masato Joshima, Shusaku Goto
  Pages: n/a - n/a
  Abstract: Mass transport is an important process of sediment redistribution from shallow to deep sea basins. It is vital to understand this process for disaster prevention and protection of economic interests in coastal and offshore areas. We describe mass‐transport‐dominated sedimentation in an active foreland basin, the Hidaka Trough, which developed from collision between the northeastern Japan arc and the Kuril arc. The basin is deformed by east‐west compression associated with large, frequent earthquakes. The trough is filled with thick (>4.5 km) sediments, ranging from coal‐bearing Cretaceous terrestrial strata to modern diatomaceous hemipelagic mud and volcanic ash. Bottom‐simulating reflectors and the distribution of mud volcanoes, pockmarks, and acoustic wipe‐out zones on the seismic records suggest the presence of subsurface gases in the sediments. The basin features stacked mass transport deposits (MTDs), but no channel‐levee systems have developed. The MTDs are relatively thin (
  PubDate: 2013-05-14T04:20:28.703494-05:
  DOI: 10.1002/ggge.20169
Holocene evolution in weathering and erosion patterns in the Pearl river delta
- Authors: Dengke Hu; Peter D. Clift, Philipp Böning, Robyn Hannigan, Stephen Hillier, Jerzy Blusztajn, Shiming Wan, Dorian Q. Fuller
  Pages: n/a - n/a
  Abstract: Sediments in the Pearl River delta have the potential to record the weathering response of this river basin to climate change since 9.5 ka, most notably weakening of the Asian monsoon since the Early Holocene (~8 ka). Cores from the Pearl River delta show a clear temporal evolution of weathering intensity, as measured by K/Al, K/Rb and clay mineralogy, that shows deposition of less weathered sediment at a time of weakening monsoon rainfall in the Early‐Mid Holocene (6.0–2.5 ka). This may reflect an immediate response to a less humid climate, or more likely reduced reworking of older deposits from river terraces as the monsoon weakened. Human settlement of the Pearl River basin may have had a major impact on landscape and erosion as a result of the establishment of widespread agriculture. After around 2.5 ka weathering intensity sharply increased, despite limited change in the monsoon, but at a time when anthropogenic pollutants (e.g., Cu, Zn and Pb) increased and when the flora of the basin changed. 87Sr/86Sr co‐varies with these other proxies but is also partly influenced by the presence of carbonate. The sediments in the modern Pearl River are even more weathered than the youngest material from the delta cores. We infer that the spread of farming into the Pearl River basin around 2.7 ka was followed by a widespread reworking of old, weathered soils after 2.5 ka, and large‐scale disruption of the river system that was advanced by 2.0 ka.
  PubDate: 2013-05-14T04:17:54.582813-05:
  DOI: 10.1002/ggge.20166
Temporal variability of in situ methane concentrations in gas hydrate‐bearing sediments near Bullseye Vent, Northern Cascadia margin
- Authors: Laura Lapham; Rachel Wilson, Michael Riedel, Charles K. Paull, M. Elizabeth Holmes
  Pages: n/a - n/a
  Abstract: To assess the temporal variability in the methane fluxes from marine sediments that overly gas hydrate bearing sediments and the factors that might control its rate, in situ methane concentrations were measured near Bullseye Vent on the Northern Cascadia continental margin. A long‐term sampling device collected overlying water and pore‐fluid samples from 25 cm above seafloor, at the sediment‐water interface, and 7 cmbsf (cm below seafloor) over a 9‐month period (August 2009‐May 2010). These samples provide a record at ~4 day resolution of in situ methane, ethane, propane, sulfate, and chloride concentrations, as well as stable carbon isotope ratios of methane (δ13C‐CH4) and dissolved inorganic carbon (δ13C ‐DIC). We show that pore‐fluids near the sediment‐water interface are saturated or super‐saturated with respect to methane (~80 mM) and the methane flux from the seabed is variable over time. We hypothesized that regional seismic activity controlled this variable CH4 flux in the Northern Cascadia continental margin setting. However, we found no direct correlation between earthquakes and CH4 flux. We also posited alternative controls on CH4 flux variability, such as storms, regional oceanography and microbial activity. Again, no direct correlation was seen. This study takes first steps towards exploring which physical factors play a role in methane flux from hydrate‐bearing sediments.
  PubDate: 2013-05-14T04:09:26.005472-05:
  DOI: 10.1002/ggge.20167
Relating sulfate and methane dynamics to geology: The accretionary prism offshore SW Taiwan
- Authors: Pei‐Chuan Chuang; Andrew W. Dale, Klaus Wallmann, Matthias Haeckel, Tsanyao Frank Yang, Nai‐Chen Chen, Hsiao‐Chi Chen, Hsuan‐Wen Chen, Saulwood Lin, Chih‐Hsien Sun, Chen‐Feng You, Chorng‐Shern Horng, Yunshuen Wang, San‐Hsiung Chung
  Pages: n/a - n/a
  Abstract: [1] Geochemical data (CH4, SO42–, I–, Cl–, particulate organic carbon (POC), δ13C–CH4 and δ13C–CO2) are presented from the upper 30 m of marine sediment on a tectonic submarine accretionary wedge offshore southwest Taiwan. The sampling stations covered three ridges (Tai–Nan, Yung–An and Good Weather), each characterized by bottom simulating reflectors, acoustic turbidity and different types of faulting and anticlines. Sulfate and iodide concentrations varied little from seawater‐like values in the upper 1‐3 m of sediment at all stations; a feature which is consistent with irrigation of seawater by gas bubbles rising through the soft surface sediments. Below this depth, sulfate was rapidly consumed within 5 to 10 m by anaerobic oxidation of methane (AOM) at the sulfate‐methane transition. Carbon isotopic data imply a mainly biogenic methane source. A numerical transport‐reaction model was used to identify the supply pathways of methane and estimate turnover rates at the three ridges. Methane gas ascending from deep layers, facilitated by thrusts and faults, was by far the dominant term in the methane budget at all sites. Differences in the proximity of the sampling sites to the faults and anticlines mainly accounted for the variability in gas fluxes and AOM rates. By comparison, methane produced in situ by POC degradation within the modeled sediment column was unimportant. This study demonstrates that the geochemical trends in the continental margins offshore SW Taiwan are closely related to the different geological settings.
  PubDate: 2013-05-13T08:35:28.848083-05:
  DOI: 10.1002/ggge.20168
LA‐ICP‐MS depth profiling perspective on cleaning protocols for elemental analyses in planktic foraminifers
- Authors: Lael Vetter; Howard J. Spero, Ann D. Russell, Jennifer S. Fehrenbacher
  Pages: n/a - n/a
  Abstract: Measurements of trace metal ratios in foraminiferal calcite are routinely used to reconstruct paleoceanographic conditions. Analyses using solution‐based inductively coupled plasma mass spectrometry (ICP‐MS) require dissolution of the entire foraminifer shell. The potential exists for contamination from adherent clays, mineralized coatings, and other diagenetic components that confound the biogenic trace metal signal. We present results from a cleaning experiment on fossil specimens of the planktic foraminifer Orbulina universa that were cracked into several shell fragments and subjected to different cleaning protocols. We use LA‐ICP‐MS depth profiling to evaluate the effects of reductive, oxidative, and chelating (DTPA) cleaning protocols on shell Mg/Ca and Ba/Ca ratios. Using the natural pattern of intrashell Mg/Ca heterogeneity exhibited by O. universa, we demonstrate that reductive and oxidative cleaning can dissolve shell calcite from available surfaces, although intrashell Mg/Ca minima and maxima are unaffected. High‐resolution depth profiles can be used to identify areas of heterogeneous intrashell Ba/Ca, which can be excluded from computations of whole‐shell Ba/Ca. The size and density of shell pores plays a major role in the degree of contamination from sedimentary material. We demonstrate an approach for computing whole‐shell Me/Ca ratios from LA‐ICP‐MS depth profiles that accounts for potential contamination and diagenetic overprinting.
  PubDate: 2013-05-08T03:53:20.772868-05:
  DOI: 10.1002/ggge.20163
Geomagnetic field intensity variations in Western Europe over the past 1100 years
- Authors: A. Genevey; Y. Gallet, E. Thébault, S. Jesset, M. Le Goff
  Pages: n/a - n/a
  Abstract: Ten archeointensity results have been obtained from brick and ceramic fragments collected in France and precisely dated to between the 10th and the 18th centuries. Intensity experiments were performed using the Triaxe protocol taking into account cooling rate and thermoremanent magnetization anisotropy effects. Together with our previous results from France and Belgium, we computed a geomagnetic field intensity variation curve for Western Europe covering the past 1100 years. This curve is characterized by a general decreasing trend at the millennial timescale punctuated by three short intensity peaks, during the 12th century, around 1350‐1400 AD and ~1600 AD. A similar evolution but with smoother variations due to data scatter is also observed in Western Europe and to a lesser extent in Eastern Europe when all available archeointensity data fulfilling quality criteria are used. Comparison of our archeointensity variation curve with the climatic record derived from fluctuations in length of the Swiss glaciers shows a good temporal concordance between all geomagnetic field intensity maxima detected in Western Europe over the past millennium and cooling episodes. A comparison is further discussed between these intensity maxima and episodes of low rates of 14C production. A common pattern of variations between both records is recognized between the middle of the 10th and of the beginning of 18th centuries. If significant, such coincidences suggest a dual geomagnetic and solar origin for the century‐scale climate and radionuclide production variations during at least the past millennium.
  PubDate: 2013-05-06T23:12:05.75606-05:0
  DOI: 10.1002/ggge.20165
ESR signal intensity and crystallinity of quartz from gobi and sandy deserts in East Asia and implication for tracing Asian dust provenance
- Authors: Youbin Sun; Hongyun Chen, Ryuji Tada, Dominik Weiss, Min Lin, Shin Toyoda, Yan Yan, Yuko Isozaki
  Pages: n/a - n/a
  Abstract: Electron Spin Resonance (ESR) signal intensity and Crystallinity Index (CI) of fine‐ (16 μm) quartz were measured in surface samples from the Taklimakan desert in western China, the Badain Juran, Tengger and Mu Us deserts in northern China, and the Gobi desert in southern Mongolia to evaluate whether these geophysical parameters can serve as reliable provenance tracers of Asian dust. The results indicate that spatial variability of both ESR signal intensity and CI is evident within the Taklimakan deserts and the Mongolian Gobi, but less significant in the three deserts of northern China. Coarse‐grained quartz from the Mongolian Gobi and northern China deserts can be differentiated from the Taklimakan desert using the ESR signal intensity. Fine‐grained quartz originating from three major Asian dust sources, i.e., the Gobi‐sandy deserts in western China, northern China and southern Mongolia, can be distinguished effectively using the combination of ESR and CI signals. Our results suggest that ESR signal intensity and CI can discriminate the sources of fine‐grained quartz better than coarse‐grained quartz, providing an effective approach to trace the provenance of fine‐grained dust deposition on the land and in the ocean.
  PubDate: 2013-05-06T23:05:50.063378-05:
  DOI: 10.1002/ggge.20162
Structural and petrophysical characterization of the upper basement crustal section at ODP/IODP site 1256 (East Pacific Ocean)
- Authors: Fontana Emanuele; Tartarotti Paola
  Pages: n/a - n/a
  Abstract: New ocean crust is constantly being formed from mid‐ocean ridge axis. Voluminous flows of lava are emplaced also away from the ridge axis, adding off‐axis crustal layers to the crustal pile. Research on deep‐sea is of great importance to better understand the mechanisms and the nature of the crust forming and evolution. In this work we decipher the first stages of the post‐magmatic evolution of an intact volcanic section from the upper oceanic crust at ODP/IODP Site 1256 (Eastern Pacific Ocean). Using for the first time an innovative core‐log integration technique to match direct (core‐related) and indirect (borehole‐related) data by depth‐shifting and reorienting individual core pieces recovered by drilling, we are able to identify the clusterization of structures and physical properties within distinct downdeep “strong” and “weak” lava zones, reflecting the cooling and tectonic evolution of lavas rather than lithological variations. We define the evolution of the structural zones that typically affect lava flows: colonnades and entablature zones, studying an off‐axis lava flow encountered in present‐day upper ocean crust. For the same off‐axis flow we are also able to suggest the lava flow direction (NW – SE) and its relationships with the paleo‐ridge axis. Despite the environmental difficulties in the study of the subseafloor under deep water and using only one‐dimensional data deriving from ocean drilling, this work shows how an array of diverse data can be integrated into a coherent interpretation of lava flow history obtaining detailed information on the mechanism of submarine lava emplacement and flow.
  PubDate: 2013-05-06T22:51:31.377847-05:
  DOI: 10.1002/ggge.20161
A combined vacuum crushing and sieving (CVCS) system designed to determine noble gas paleo‐temperatures from stalagmite samples
- Authors: Nadia Vogel; Matthias S. Brennwald, Dominik Fleitmann, Rainer Wieler, Colin Maden, Andreas Süsli, Rolf Kipfer
  Pages: n/a - n/a
  Abstract: This paper presents a novel extraction device for water and noble gases from speleothem samples for noble gas paleo‐temperature determination. The “combined vacuum crushing and sieving (CVCS) system” was designed to reduce the atmospheric noble gas contents from air inclusions in speleothem samples by up to two orders of magnitude without adsorbing atmospheric noble gases onto the freshly produced grain surfaces, a process that had often hampered noble gas temperature (NGT) determination in the past. We also present the results from first performance tests of the CVCS system processing stalagmite samples grown at a known temperature. This temperature is reliably reproduced by the NGTs derived from Ar, Kr, and Xe extracted from the samples. The CVCS system is, therefore, suitable for routine determinations of accurate NGTs. In combination with stalagmite dating, these NGTs will allow reconstructing past regional temperature evolutions, and also support the interpretation of the often complex stable isotope records preserved in the stalagmites' calcite.
  PubDate: 2013-05-06T22:42:21.464882-05:
  DOI: 10.1002/ggge.20164
Reconstructing lava flow emplacement processes at the hotspot‐affected Galápagos spreading center, 95°W and 92°W
- Authors: Tim McClinton; Scott M. White, Alice Colman, John M. Sinton
  Pages: n/a - n/a
  Abstract: Volcanic eruptions at mid‐ocean ridges (MORs) control the permeability, internal structure, and architecture of oceanic crust, thus establishing the foundation for the evolution of the ocean basins. To better understand the emplacement of submarine lava flows at MORs, we have integrated submersible‐based geologic mapping with remote sensing techniques to characterize the lava flow morphology within previously mapped lava flow fields produced during single eruptive episodes at the Galápagos Spreading Center (GSC). Detailed attributes describing the surface geometry and texture of the lava flows have been extracted from high‐resolution sonar data and combined with geo‐referenced visual observations from submersible dives and camera tows; based on signatures contained in these data, a fuzzy logic‐based classification algorithm categorized lava flow morphology as pillows, lobates, or sheets. The resulting digital thematic maps offer an unprecedented view of GSC lava morphology, collectively covering 77km2 of ridge axis terrain at a resolution of 2m x 2m. Error assessments with independent visual reference data indicate approximately 90% agreement, comparable to subaerial classification studies. The digital lava morphology maps enable quantitative, spatially comprehensive measurements of the abundance and distribution of lava morphologies over large areas of seafloor and within individual eruptive units. A comparison of lava flow fields mapped at lower‐ and higher‐magma‐supply settings (95° and 92°W, respectively) indicates that effusion rates increase along with magma supply and independent of spreading rate at the GSC, although a complete range of eruptive behavior exists at each setting.
  PubDate: 2013-05-05T23:08:00.866457-05:
  DOI: 10.1002/ggge.20157
Arrhenius rheology vs. frank‐kamenetskii rheology ‐ implications for mantle dynamics
- Authors: C. Stein; U. Hansen
  Pages: n/a - n/a
  Abstract: The viscosity of planetary mantle material is strongly temperature‐dependent. This dependence is described by an Arrhenius law. But for the realistic viscosity contrast that appears over the depth of the mantle, strong gradients in the upper thermal boundary layer occur. These strong gradients are not realisable in numerical models. Therefore the Frank‐Kamenetskii approximation, leading to a linearised exponential viscosity function, is commonly used. Much research on the plate‐mantle system has been done applying the Frank‐Kamenetskii rheology. The question though still arises, if these results can be reproduced when using the Arrhenius law. Here it has to be kept in mind, that in numerical models the realistic viscosity contrast appearing over the mantle can neither be coped with by the Arrhenius nor the Frank‐Kamenetskii approach. Thus the computational aspects to date only allow the taking into account viscosity contrasts being moderate as compared to realistic values and extrapolations to planet‐like values have to be made for both rheologies. Comparing results obtained by the commonly used forms of the Arrhenius and the Frank‐Kamenetskii approach, we observe the same change in flow behaviour from mobile‐lid to stagnant‐lid convection. The differences are only of quantitative nature: In the stagnant‐lid regime, some Arrhenius formulations lead to a thinner top boundary layer which results in values of lithospheric thicknesses being more realistic. Here, it has to be noted that different forms of the Arrhenius law have been used which differ among themselves. When properly scaled, the differences between the Frank‐Kamenetskii and Arrhenius rheology can, however, be strongly diminished. To understand general features in the plate tectonics‐mantle convection system an additional stress dependence of the viscosity has to be considered. The discrepancies between the various viscosity formulations are then of even less importance, because for an Earth‐like convection regime, the top viscosity is more strongly influenced by the stress dependence rather than by the temperature dependence.
  PubDate: 2013-05-05T22:58:57.523038-05:
  DOI: 10.1002/ggge.20158
Dynamics of outer rise faulting in oceanic‐continental subduction systems
- Authors: John B. Naliboff; Magali I. Billen, Taras Gerya, Jessie Saunders
  Pages: n/a - n/a
  Abstract: During subduction, bending of downgoing oceanic lithosphere gives rise to normal faulting due to the extensional stress state generated in the upper plate. As deformation patterns inherently reflect a material's state of stress and rheology, extensive global observations of outer rise faulting patterns and subduction dynamics provide a unique opportunity to examine the factors controlling outer rise deformation. Despite a wide range of observed oceanic plate ages, convergence rates and slab pull magnitudes across modern subduction systems, however, measured outer rise faulting patterns show effectively no correlation to variations in these parameters. This lack of correlation may reflect that outer rise faulting patterns are strongly sensitive to all of these parameters, are dependent on additional parameters such as downgoing‐overriding plate coupling or that existing faulting measurements require additional analysis. In order to provide a basis for future analysis of outer rise faulting patterns, we build on previous thermo‐mechanical numerical models of outer rise deformation and explore the relationship between outer rise faulting patterns, subduction dynamics and brittle rheology in an oceanic‐continental subduction system. Analysis of time‐averaged outer rise faulting patterns indicates that downgoing plate age and velocity, downgoing‐overriding plate coupling and slab pull all significantly affect faulting patterns, while variations in brittle rheology have a significantly smaller impact. These relationships reflect that the sensitivity of outer‐rise faulting patterns to the frictional properties of the oceanic crust and mantle is small compared to variations in the overall stress state and deformation rate of subduction systems. In order to gain additional insight into the origin outer rise faulting patterns, future numerical studies should focus on specific regions in order to place constraints on the structure of the downgoing plate and dynamics of the subduction system.
  PubDate: 2013-05-03T02:59:15.559073-05:
  DOI: 10.1002/ggge.20155
Shear strength of sediments approaching subduction in the nankai trough, Japan as constraints on forearc mechanics
- Authors: Matt J. Ikari; Andre Hüpers, Achim J. Kopf
  Pages: n/a - n/a
  Abstract: The mechanical behavior of the plate boundary fault zone is of paramount importance in subduction zones, because it controls megathrust earthquake nucleation and propagation as well as the structural style of the forearc. In the Nankai area along the NanTroSEIZE (Kumano) drilling transect offshore SW Japan, a heterogeneous sedimentary sequence overlying the oceanic crust enters the subduction zone. In order to predict how variations in lithology, and thus mechanical properties, affect the formation and evolution of the plate boundary fault, we conducted laboratory tests measuring the shear strengths of sediments approaching the trench covering each major lithological sedimentary unit. We observe that shear strength increases non‐linearly with depth, such that the (apparent) coefficient of friction decreases. In combination with a critical taper analysis, the results imply that the plate boundary position is located on the main frontal thrust. Further landward, the plate boundary is expected to step down into progressively lower stratigraphic units, assisted by moderately elevated pore pressures. As seismogenic depths are approached, the décollement may further step down to lower volcaniclastic or pelagic strata but this requires specific overpressure conditions. High taper angle and elevated strengths in the toe region may be local features restricted to the Kumano transect.
  PubDate: 2013-05-03T02:58:41.575894-05:
  DOI: 10.1002/ggge.20156
A core‐top study of dissolution effect on B/Ca in Globigerinoides sacculifer from the tropical Atlantic: Potential bias for paleo‐reconstruction of seawater carbonate chemistry
- Authors: R. Coadic; F. Bassinot, D. Dissard, E. Douville, M. Greaves, E. Michel
  Pages: n/a - n/a
  Abstract: It has been recently shown that B/Ca in planktonic foraminiferal calcite can be used as a proxy for seawater pH. Based on the study of surface sediments (multi‐cores) retrieved along a depth transect on the Sierra Leone Rise (Eastern Equatorial Atlantic), we document the decrease of B/Ca and Mg/Ca of Globigerinoides sacculifer shells with increasing water depth and dissolution. This effect of dissolution on B/Ca may potentially represent a severe bias for paleo‐pH reconstructions using this species. Samples of G. sacculifer were analyzed independently at two laboratories for B/Ca and Mg/Ca. Both sets of results show a systematic decrease of B/Ca and Mg/Ca along the depth transect, with an overall loss of ~14 µmol/mol (~15%) for B/Ca and of ~0.7 mmol/mol (~21%) for Mg/Ca between the shallowest (2640 m) and the deepest (4950 m) sites. Because of this dissolution effect, surface water pH reconstructed from B/Ca of G. sacculifer decreases by ~0.11 units between the shallowest site and the deepest site, a magnitude similar to the expected glacial/interglacial surface water pH changes.
  PubDate: 2013-04-29T11:51:18.502225-05:
  DOI: 10.1029/2012GC004296
Corrrection to: Deformation associated with the denudation of mantle‐derived rocks at the Mid‐Atlantic Ridge 13°‐15°N: the role of magmatic injections and hydrothermal alteration
- Authors: Suzanne Picazo; Mathilde Cannat, Adélie Delacour, Javier Escartín, Stéphane Rouméjon, Serguei Silantyev
  Pages: n/a - n/a
  PubDate: 2013-04-27T12:46:35.887309-05:
  DOI: 10.1002/ggge.20154
The 1874‐76 volcano‐tectonic episode at Askja, North Iceland: Lateral flow revisited
- Authors: Margaret E. Hartley; Thor Thordarson
  Pages: n/a - n/a
  Abstract: The Askja volcanic system, North Iceland, experienced a volcano‐tectonic episode between 1874 and 1876, the climax of which was a rhyolitic, phreatoplinian to Plinian eruption at Askja central volcano on 28th‐29th March 1875. Fissure eruptions also occurred in 1875, producing the Nýjahraun lava, 45‐65 km north of Askja. The Nýjahraun basalt is indistinguishable, in terms of whole‐rock major elements, from the small‐volume basaltic eruptions that took place at Askja in the early 20th century. It has been suggested that all of these basalts originated from a shallow magma chamber beneath Askja, with the Nýjahraun eruptions being fed by northward‐propagating lateral dykes. It has also been conjectured that the Holuhraun lava, located at the southern tip of the Askja volcanic system 15‐25 km south of Askja, was connected with the 1874‐76 Askja volcano‐tectonic episode. We re‐examine these interpretations in light of new whole‐rock, glass and melt inclusion analyses from samples collected along the length of the Askja volcanic system. Glasses from Nýjahraun and the Askja 20th century eruptions are geochemically distinct. We suggest that the Askja 20th century basalts mixed with evolved melts in the crust, while the Nýjahraun magma evolved without such interactions. The Holuhraun basalt is more similar to lavas erupted on the Bárðarbunga‐Veiðivötn volcanic system than to postglacial basalts from Askja, indicating that particular geochemical signatures are not necessarily confined to the tectonic or structural surface expression of single volcanic systems. This has important implications for the identification and delineation of individual volcanic systems beneath the northwest sector of Vatnajökull.
  PubDate: 2013-04-27T01:43:01.354939-05:
  DOI: 10.1002/ggge.20151
Testing inverse kinematic models of paleocrustal thickness in extensional systems with high‐resolution forward thermo‐mechanical models
- Authors: Erik A. Kneller; Markus Albertz, Garry D. Karner, Christopher A. Johnson
  Pages: n/a - n/a
  Abstract: Reconstructing continental paleocrustal thickness is important for estimating tectonic accommodation, constraining three‐dimensional basin geometry during early rifting phases of extensional margins and predicting the distribution of thick crustal sills that may block the global ocean and create restricted basins. We test an inverse kinematic method for modeling paleocrustal thickness by inverting the final bulk crustal structure produced from high‐resolution thermo‐mechanical models of lithospheric extension. The inverse kinematic method assumes pure shear, includes simple rules based on geodynamic models and field observations and requires displacement boundary conditions and the prescription of a transition from rigid to non‐rigid deformation. The inverse pure‐shear method produces a history of bulk crustal thickness that closely matches the forward models provided that the width of the rift zone is narrow during the later phases of continental extension when crust undergoes hyper‐extension. We also observe that the width and surface trace of large‐scale shear zones observed in the thermo‐mechanical models coincide with inflection points and large gradients in inverted non‐rigid velocity field. Our results demonstrate that if displacement boundary conditions can be constrained and the transition from rigid to non‐rigid deformation defines a narrow rift zone during hyper‐extension then relatively simple kinematic rules can be used to invert present‐day bulk crustal structure for paleocrustal thickness, bulk lateral strain and aspects of upper crustal shear zone geometry from extensional systems with non‐linear rheology, structures dominated by simple shear in the upper crust, depth‐dependent extension and asymmetric crustal thinning.
  PubDate: 2013-04-26T04:42:15.242283-05:
  DOI: 10.1002/ggge.20153
Apparent overconsolidation of mudstones in the Kumano Basin of southwest Japan: Implications for fluid pressure and fluid flow within a forearc setting
- Authors: Junhua Guo; Michael B. Underwood, William J. Likos, Demian M. Saffer
  Pages: n/a - n/a
  Abstract: The Kumano Basin is located in the Nankai Trough subduction zone of southwest Japan. During the past 1.6 million years, approximately 800 meters of sandy turbidites and hemipelagic mud were deposited near the distal edge of the forearc basin, at Site C0002 of the Integrated Ocean Drilling Program. Constant‐rate‐of‐strain consolidation tests yield estimates of in situ permeability that range from 2.6 × 10−17 m2 to 2.5 × 10−18 m2; overconsolidation ratios range from 1.7 to 2.6, and values of the compression index range from 0.39 to 0.78. Several processes contributed to the apparent overconsolidation. Strata dip toward land, and pore fluids probably migrate up‐dip and vent along a bathymetric notch near the seaward edge of the basin. Efficient lateral drainage through sandy turbidites has kept pore pressures within interbeds of mudstone at (or close to) hydrostatic. In addition, alteration of dispersed volcanic glass, precipitation of authigenic clay minerals, and collapse of random grain fabric has probably strengthened the bonding among grains. Cementation is particularly likely within the lower basin (unit III), where values of porosity remain anomalously high. If fluid overpressures (and underconsolidation) exist anywhere within the basin, the most likely loci are where sandy turbidites terminate against impermeable mudstones along landward‐dipping on‐lap surfaces. Those types of on‐lap geometries, in addition to structural closures, might provide promising targets for oil/gas accumulation in other forearc basins, particularly where petroleum source rocks have been buried to the optimal depths of catagenesis.
  PubDate: 2013-04-25T10:56:01.203155-05:
  DOI: 10.1029/2012GC004204
Measuring the Curie temperature
- Authors: K. Fabian; V. P. Shcherbakov, S. A. McEnroe
  Pages: n/a - n/a
  Abstract: Curie point temperatures (TC) of natural and synthetic magnetic materials are commonly determined in rock magnetism by several measurement methods that can be mutually incompatible and may lead to inconsistent results. Here the common evaluation routines for high‐temperature magnetization and magnetic initial susceptibility curves are analyzed and revised based on Landau's theory of second‐order phase transitions. It is confirmed that in high‐field magnetization curves TC corresponds to the inflection point, below the temperature of maximum curvature or the double‐tangent intersection point. At least four different physical processes contribute to the initial magnetic susceptibility near the ordering temperature. They include variation of saturation magnetization, superparamagnetic behavior, magnetization rotation, and magnetic domain wall motion. Because each of these processes may influence the apparent position of TC, initial susceptibility and high‐field curves can yield deviating estimates of TC. A new procedure is proposed to efficiently determine the temperature variation of several magnetic parameters on a vibrating‐sample magnetometer, by repeatedly measuring quarter‐hysteresis loops during a single heating cycle. This procedure takes measurements during the inevitable waiting time necessary for thermal equilibration of the sample, whereby it is not slower than the commonly performed measurements on a Curie balance. However, it returns saturation magnetization, saturation remanence, high‐field and low‐field slopes, and other parameters as a function of temperature, which provide independent information about TC and other sample properties.
  PubDate: 2013-04-24T15:31:52.854991-05:
  DOI: 10.1029/2012GC004440
The influence of water on rheology and strain localization in the lower continental crust
- Authors: A. J. Getsinger; G. Hirth, H. Stünitz, E. T. Goergen
  Pages: n/a - n/a
  Abstract: We investigated deformation processes within a lower crustal shear zone exposed in gabbros from Arnøya, Norway. Over a distance of ~1 meter the gabbro progresses from nominally undeformed to highly sheared where it is adjacent to a hydrous pegmatite. With increasing proximity to the pegmatite, there is a significant increase in the abundance of amphibole and zoisite (which form at the expense of pyroxene and calcic plagioclase) and a slight increase in the strength of plagioclase lattice preferred orientation, but there is little change in recrystallized plagioclase grain size. Phase diagrams, the presence of hydrous reaction products, and deformation mechanism maps all indicate that the water activity (aH2O) during deformation must have been high (~1) in the sheared gabbro compared to the non‐hydrated, surrounding host gabbro. These observations indicate that fluid intrusion into mafic lower crust initiates syn‐deformational, water‐consuming reactions, creating a rheological contrast between wet and dry lithologies that promotes strain localization. Thus, deformation of lower continental crust can be accommodated in highly localized zones of enhanced fluid infiltration. These results provide an example of how fluid weakens lower continental crust lithologies at high pressures and temperatures.
  PubDate: 2013-04-23T03:50:44.904729-05:
  DOI: 10.1002/ggge.20148
Micromagnetics and magnetomineralogy of ultrafine magnetite inclusions in the Modipe Gabbro
- Authors: A. R. Muxworthy; M. E. Evans
  Pages: n/a - n/a
  Abstract: Iron oxide inclusions in pyroxene crystals from the Modipe Gabbro, Botswana, have been studied to determine their recording fidelity. Hysteresis parameters, first‐order reversal curves, isothermal remanent magnetization acquisition curves, and thermomagnetic data all indicate that the iron oxide occurs as stoichiometric magnetite in the form of single‐domain and/or small pseudo single‐domain particles. Using distributions for the grain shape and nearest‐neighbor spacings determined from optical micrographs as input, a first‐order reversal curve (FORC) diagram was simulated using a numerical micromagnetic model. The simulated FORC diagram was found to closely match the measured FORC distributions. Analysis of the interaction fields in the model found that the standard deviation of the interaction field distribution was 2.7 mT compared to a bulk coercivity of ~20 mT, suggesting that the majority of particles are unlikely to be affected by magnetostatic interactions. Thellier analysis of the samples induced with an initial laboratory thermoremanence yielded near‐perfect behavior. It is suggested that the Modipe Gabbro dated at 2784.0 ± 1.5 (2σ) Ma is potentially an ideal recorder of the geomagnetic field.
  PubDate: 2013-04-18T14:09:55.275465-05:
  DOI: 10.1029/2012GC004445
Magma flow between summit and Pu'u 'Ō'ō at Kīlauea Volcano, Hawai'i
- Authors: C. P. Montagna; H. M. Gonnermann
  Pages: n/a - n/a
  Abstract: Volcanic eruptions are often accompanied by spatio‐temporal migration of ground deformation, a consequence of pressure changes within magma reservoirs and pathways. We modeled the propagation of pressure variations through the east rift zone of Kīlauea Volcano, Hawai'i, caused by magma withdrawal during the early eruptive episodes (1983‐85) of the ongoing Pu'u 'Ō'ō‐Kupaianaha eruption. Eruptive activity at the Pu'u 'Ō'ō vent was typically accompanied by abrupt deflation that lasted for several hours and was followed by a sudden onset of gradual inflation, once the eruptive episode had ended. Similar patterns of deflation and inflation were recorded at Kīlauea's summit, approximately 15 km to the north‐west, albeit with time delays of hours. These delay times can be reproduced by modeling the spatio‐temporal changes in magma pressure and flow rate within an elastic‐walled dike that traverses Kīlauea's east rift zone. Key parameters that affect the behavior of the magma‐dike system are the dike dimensions, the elasticity of the wall rock, the magma viscosity, and to a lesser degree the magnitude and duration of the pressure variations themselves. Combinations of these parameters define a transport efficiency and a pressure diffusivity, which vary somewhat from episode to episode, resulting in variations in delay times. The observed variations in transport efficiency are most easily explained by small, localized changes to the geometry of the magma pathway.
  PubDate: 2013-04-15T06:01:59.631975-05:
  DOI: 10.1002/ggge.20145
A new seismogeodetic approach applied to GPS and accelerometer observations of the 2012 Brawley seismic swarm: Implications for earthquake early warning
- Authors: Jianghui Geng, Yehuda Bock, Diego Melgar, Brendan W. Crowell, Jennifer S. Haase, Cecil H; Ida M
  Pages: n/a - n/a
  Abstract: The 26 August 2012 Brawley seismic swarm of hundreds of events ranging from M1.4 to M5.5 in the Salton Trough, California provides a unique data set to investigate a new seismogeodetic approach that combines Global Positioning System (GPS) and accelerometer observations to estimate displacement and velocity waveforms. First in simulated real‐time mode, we analyzed 1‐5 Hz GPS data collected by 17 stations fully encircling the swarm zone at near‐source distances up to about 40 km using precise point positioning with ambiguity resolution (PPP‐AR). We used a reference network of North American GPS stations well outside the region of deformation to estimate fractional‐cycle biases and satellite clock parameters, which were then combined with ultra‐rapid orbits from the International GNSS Service to estimate positions during the Brawley seismic swarm. Next, we estimated seismogeodetic displacements and velocities from GPS phase and pseudorange observations and 100‐200 Hz accelerations collected at three pairs of GPS and seismic stations in close proximity using a new tightly‐coupled Kalman filter approach as an extension of the PPP‐AR process. We can clearly discern body waves in the velocity waveforms, including P‐wave arrivals not detectable with the GPS‐only approach for earthquake magnitudes as low as Mw 4.6 and significant static offsets for magnitudes as low as Mw 5.4. Our study shows that GPS networks upgraded with strong motion accelerometers can provide new information for improved understanding of the earthquake rupture process and be of critical value in creating a robust early warning system for any earthquake of societal significance.
  PubDate: 2013-04-15T06:01:45.505883-05:
  DOI: 10.1002/ggge.20144
A rootless Rockies– Support and lithospheric structure of the Colorado Rocky mountains Inferred from CREST and TA seismic data
- Authors: Steven M. Hansen; Ken G. Dueker, Josh C. Stachnik, Richard C. Aster, Karl E. Karlstrom
  Pages: n/a - n/a
  Abstract: Support for the Colorado high topography is resolved using seismic data from the Colorado Rocky Mountain (CRM) Experiment and Seismic Transects (CREST). The average crustal thickness, derived from P wave receiver function imaging, is 48 km. However, a negative correlation between Moho depth and elevation is observed which negates Airy‐Heiskanen isostasy. Shallow Moho (< 45 km depth) is found beneath some of the highest elevations and therefore, the CRM are rootless. Deep Moho (45–51 km) regions indicate structure inherited from the Proterozoic assembly of the continent. Shear wave velocities from surface wave tomography are mapped to density employing empirical velocity‐to‐density relations in the crust and mantle temperature modeling. Predicted elastic plate flexure and gravity fields derived from the density model agree with observed long‐wavelength topography and Bouguer gravity. Therefore, low density crust and mantle are sufficient to support much of the CRM topography. Centers of Oligocene volcanism, e.g., the San Juan Mountains, display reduced crustal thicknesses and lowest average crustal velocities suggesting magmatic modification strongly influenced modern lithospheric structure and topography. Mantle velocities span 4.02–4.64 km/s at 73–123 km depth, and peak lateral temperature variations of 600° C are inferred. S wave receiver function imaging suggests that the lithosphere‐asthenosphere boundary is 100–150 km deep beneath the Colorado Plateau, and 150–200 km deep beneath the High Plains. A region of negative arrivals beneath the CRM above 100 km depth correlates with low mantle velocities and is interpreted as thermally modified/metasomatised lithosphere resulting from Cenozoic volcanism.
  PubDate: 2013-04-15T06:01:32.275566-05:
  DOI: 10.1002/ggge.20143
Paleointensity results from the Late‐Archaean Modipe Gabbro of Botswana
- Authors: A. R. Muxworthy; M. E. Evans, S. J. Scourfield, J. G. King
  Pages: n/a - n/a
  Abstract: Rock‐magnetic and Thellier‐Thellier‐Coe paleointensity results are reported for a new collection of samples from the 2.78 Ga Modipe Gabbro of Botswana. The magnetic properties are very favorable, leading to an unusually high success rate and a well‐constrained result of 36‐40 μT (95% confidence interval). We discuss the long‐standing problem of allowing for the enormous differences between the natural and laboratory cooling rates, and apply a temperature‐dependent correction derived from first‐order reversal curve (FORC) data. Whereas pure single‐domain (SD) corrections often lead to quite large decreases in paleointensity estimates (sometimes exceeding 50%), we find a modest increase of about 10%. The Earth's Magnetic Dipole moment in the late Archaean is thereby estimated to have been ~6x1022 Am2.
  PubDate: 2013-04-15T06:01:20.424656-05:
  DOI: 10.1002/ggge.20142
Comparison and calibration of non‐heating paleointensity methods: A case study using dusty olivine
- Authors: Sophie‐Charlotte L.L. Lappe; Joshua M. Feinberg, Adrian Muxworthy, Richard J. Harrison
  Pages: n/a - n/a
  Abstract: We present a comparative study of non‐heating paleointensity methods, with the aim of determining the optimum method for obtaining paleointensities from “dusty olivine” in chondritic meteorites. The REM method, whereby thermoremanent magnetization (TRM) is normalized by saturation isothermal remanent magnetization (SIRM), is shown to ‘over normalize’ TRM in dusty olivine due to the transformation of stable single‐vortex (SV) states to metastable single‐domain (SD) states in a saturating field. The problem of over normalization is reduced in the REMc and REM' methods, which more effectively isolate the high‐coercivity stable SD component of remanence. A calibration factor of f = 1600 (1000 < f < 2900) is derived for the REM' method. Anhysteric remanent magnetization (ARM) is shown to be a near perfect analogue of TRM in the stable SD component of dusty olivine. ARM normalization of the high‐coercivity (100‐150 mT) remanence with a calibration factor fARM = 0.91 (0.7 < fARM < 1.2) yields paleofield estimates within ± 30% of the actual field values for SD dominated samples. A Preisach method for simulating TRM acquisition using information extracted from first‐order reversal curve (FORC) diagrams is shown to work well for SD dominated samples, but fails when there is a large proportion of SV remanence carriers. The failure occurs because (i) SV states are not properly incorporated into the Preisach distribution of remanence carriers, and (ii) the acquisition of TRM by SV states is not properly modelled by the underlying SD thermal relaxation theory.
  PubDate: 2013-04-15T06:01:18.985258-05:
  DOI: 10.1002/ggge.20141
Interaction of microseisms with crustal heterogeneity: A case study from the San Jacinto fault zone area
- Authors: G. Hillers; Y. Ben‐Zion, M. Landès, M. Campillo
  Pages: n/a - n/a
  Abstract: We perform a multi‐component analysis to evaluate the validity and limits of noise imaging in the San Jacinto fault zone (SJFZ) area. Estimates of noise propagation and scattering length scales in the area are combined with a noise‐correlation based analysis of variability of noise constituents, excitation regions, and propagation patterns. We evaluate the quality of correlation‐phase and ‐amplitude imaging of tectonic features in the context of observed noise properties. Statistical properties of a regional high‐resolution 3D velocity model indicate that propagation of double‐frequency microseism Rayleigh waves is sensitive to medium heterogeneity in the southern California plate boundary area. The analysis of noise correlation functions constructed from records of a regional seismic network suggests stable excitation of microseisms along the southern California coastline. The proximity to the source region together with randomization properties of the heterogeneous medium govern the scattered yet anisotropic character of the wave field. Insignificant travel time errors resulting from the associated imperfect reconstruction of inter‐station Green's function estimates allow the resolution of a velocity contrast across the SJFZ from noise correlations. However, attenuation estimates are biased by the anisotropic propagation directions. The interaction of the ambient surface wave field with medium heterogeneity facilitates imaging of the velocity structure, but the inversion of the amplitude pattern is limited since it is dominated by wave field instead of medium properties.
  PubDate: 2013-04-15T06:01:10.212543-05:
  DOI: 10.1002/ggge.20140
Turbidite record of frequency and source of large volume (>100 km3) Canary Island landslides in the last 1.5Ma: implications for landslide triggers and geohazards
- Authors: J.E. Hunt; R.B. Wynn, P.J. Talling, D.G. Masson
  Pages: n/a - n/a
  Abstract: During the last two decades numerous studies have focused on resolving the landslide histories of the Canary Islands. Issues surrounding the preservation and dating of onshore and proximal submarine landslide deposits precludes accurate determination of event ages. However, submarine landslides often disaggregate and generate sediment gravity flows. Volcaniclastic turbidites sampled from Madeira Abyssal Plain piston cores represent a record of eight large‐volume failures from the Western Canary Islands in the last 1.5 Ma. During this time there is a mean recurrence rate of 200 ka, while the islands of El Hierro and Tenerife have individual landslide recurrences of 500 ka and 330 ka, respectively. Deposits from the 15 ka El Golfo landslide from El Hierro and 165 ka Icod landslide from Tenerife are examined. This study also identifies potential deposits associated with the Orotava (535 ka), Güímar (850 ka) and Rogues de García landslides (1.2 Ma) from Tenerife, El Julan (540 ka) and El Tiñor (1.05 Ma) landslides from El Hierro, and the Cumbre Nueva landslide (485 ka) from La Palma. Seven of eight landslides occurred during major deglaciations or subsequent interglacial periods, which represent 55% of the time. However, all of the studied landslides occur during or at the end of periods of protracted island volcanism, which generally represent 60% of the island histories. Although climate may precondition failures, it is suggested that volcanism presents a more viable preconditioning and trigger mechanism for Canary Island landslides.
  PubDate: 2013-04-15T06:01:04.19445-05:0
  DOI: 10.1002/ggge.20139
Multistage collapse of eight Western Canary Island landslides in the last 1.5 Ma: Sedimentological and geochemical evidence from subunits in submarine flow deposits
- Authors: J.E. Hunt; R.B. Wynn, P.J. Talling, D.G. Masson
  Pages: n/a - n/a
  Abstract: Volcaniclastic turbidites in the Madeira Abyssal Plain provide a record of major landslides from the Western Canary Islands in the last 1.5 Ma. These volcaniclastic turbidites are composed of multiple fining‐upwards turbidite sands, known as subunits. The subunits indicate that the landslides responsible for the sediment gravity flows occurred in multiple stages. The subunits cannot result from flow reflection or splitting because the compositions of volcanic glasses from each individual subunit in an event bed are subtly different. This indicates that each subunit represents a discrete failure as part of a multistage landslide. This has significant implications for geohazard assessments, as multistage failures reduce the magnitude of the associated tsunami. The multistage failure mechanism reduces individual landslide volumes from up to 350 km3 to less than 100 km3. Thus although multistage failure ultimately reduce the potential landslide and tsunami threat, the landslide events may still generate significant tsunamis close to source.
  PubDate: 2013-04-15T06:00:52.866008-05:
  DOI: 10.1002/ggge.20138
Geomagnetic jerks as chaotic fluctuations of the Earth's magnetic field
- Authors: E. Qamili; A. De Santis, A. Isac, M. Mandea, B. Duka, A. Simonyan
  Pages: n/a - n/a
  Abstract: The geomagnetic field is chaotic and can be characterized by a mean exponential time scale after which it is no longer predictable. It is also ergodic, so time analyses can substitute the more difficult phase space analyses. Taking advantage of these two properties of the Earth's magnetic field, a scheme of processing global geomagnetic models in time is presented, to estimate fluctuations of the time scale τ. Here considering that the capability to predict the geomagnetic field is reduced over periods of geomagnetic jerks, we propose a method to detect these events over a long time span. This approach considers that epochs characterized by relative minima of fluctuations in time scale τ, i.e., those periods when a geomagnetic field is less predictable, are possible jerk occurrence dates. We analyze the last 400 years of the geomagnetic field (covered by the Gufm1 model) to detect minima of fluctuations, i.e., epochs characterized by low values of the time scale. Most of the well known jerks are confirmed through this method and a few others have been suggested. Finally, we also identify some short periods when the field is less chaotic (more predictable) than usual, naming these periods as steady state geomagnetic regime, to underline their opposite behavior with respect to jerks.
  PubDate: 2013-04-12T15:46:51.543775-05:
  DOI: 10.1029/2012GC004398
SAHKE geophysical transect reveals crustal and subduction zone structure at the southern Hikurangi margin, New Zealand
- Authors: S. Henrys; A. Wech, R. Sutherland, T. Stern, M. Savage, H. Sato, K. Mochizuki, T. Iwasaki, D. Okaya, A. Seward, B. Tozer, J. Townend, E. Kurashimo, T. Iidaka, T. Ishiyama
  Pages: n/a - n/a
  Abstract: The Seismic Array HiKurangi Experiment (SAHKE) investigated the structure of the forearc and subduction plate boundary beneath the southern North Island along a 350 km transect. Tomographic inversion of first‐arrival travel times was used to derive a 15‐20 km deep P‐wave image of the crust. The refracted phases and migrated reflection events image subducting slab geometry and crustal structure. In the west, Australian Plate Moho depth decreases westward across the Taranaki Fault system from 35 to ~28‐30 km. In the east, subducted Pacific Plate oceanic crust is recognised to have a positive velocity gradient, but becomes less distinct beneath the Tararua Ranges, where the interface increases in dip at about 15 km depth from 15°. This bend in the subducted plate is associated with vertical clusters in seismicity, splay fault branching, and low‐velocity high‐attenuation material that we interpret to be an underplated subduction sedimentary channel. We infer that a step down in the decollément transfers slip on the plate interface at the top of a subduction channel to the oceanic crust and drives local uplift of the Tararua Ranges. Reflections from the Wairarapa Fault show that it is listric and soles into the top of underplated sediments, which in turn abut the Moho of the over‐riding plate at ~32 km depth, near the downdip end of the strongly locked zone. The change in dip of the Hikurangi subduction interface is spatially correlated with the transition from geodetically determined locked to unlocked areas of the plate interface.
  PubDate: 2013-04-09T04:35:37.485859-05:
  DOI: 10.1002/ggge.20136
Hydrous magmatism triggered by assimilation of hydrothermally altered rocks in fossil oceanic crust (Northern Oman ophiolite)
- Authors: L. France; B. Ildefonse, J. Koepke
  Pages: n/a - n/a
  Abstract: Mid‐ocean ridges magmatism is by and large considered to be mostly dry. Nevertheless, numerous works in the last decade have shown that a hydrous component is likely to be involved in ocean ridges magmas genesis and / or evolution. The petrology and geochemistry of peculiar coarse grained gabbros sampled in the upper part of the gabbroic sequence from the Northern Oman ophiolite (Wadi Rajmi) provide information on the origin and fate of hydrous melts in fast spreading oceanic settings. Uncommon crystallization sequences for oceanic settings (clinopyroxene crystallizing before plagioclase), extreme mineral compositions (plagioclase An% up to 99, and clinopyroxene Mg# up to 96), and the presence of magmatic amphibole, imply the presence of a high water activity during crystallization. Various petrological and geochemical constraints point to hydration resulting from the recycling of hydrothermal fluids. This recycling event may have occurred at the top of the axial magma chamber where assimilation of anatectic hydrous melts is recurrent along mid‐ocean ridges, or close to segments ends where fresh magma intrudes previously hydrothermally altered crust. In ophiolitic settings, hydration and remelting of hydrothermally altered rocks producing hydrous melts may also occur during the obduction process. Although dry magmatism dominates oceanic magmatism, the dynamic behavior of fast spreading ocean ridge magma chambers has the potential to produce the observed hydrous melts (either in ophiolites or at spreading centers), which are thus part of the general mid‐ocean ridges lineage.
  PubDate: 2013-04-09T04:35:21.690416-05:
  DOI: 10.1002/ggge.20137
Quantification and restoration of extensional deformation along the Western Iberia and Newfoundland rifted margins
- Authors: Emilie Sutra; Gianreto Manatschal, Geoffroy Mohn, Patrick Unternehr
  Pages: n/a - n/a
  Abstract: Many recent papers describe the structure of the Iberia and Newfoundland rifted margins, however, none of them propose kinematic restorations of the complete rift system in order to quantify the amount of extension necessary to exhume mantle and initiate seafloor spreading. In our study, we use two pairs of cross sections considered as conjugate lines: one across the Galicia Bank‐Flemish Cap and the other across the Southern Iberia Abyssal Plain‐Flemish Pass. Both transects have been imaged by reflection and refraction seismic methods and have been drilled during ODP Legs 103, 149, 173 and 210. Drilling penetrated parts of the rift stratigraphy and the underlying basement. The cross sections can therefore be considered as the best‐documented conjugate transects across present‐day hyper‐extended, magma‐poor rifted margins. The aim of this paper is three fold: 1) provide a detailed description of the crustal architecture of the two conjugate sections; 2) define the extensional structures and their ages; and 3) quantify the amount of strain and strain rate accommodated along these lines. This paper proposes a quantitative description of extension along the Iberia‐Newfoundland rift system and discusses limitations and problems in quantifying extensional deformation along hyper‐extended rifted margins.
  PubDate: 2013-04-02T17:01:56.989388-05:
  DOI: 10.1002/ggge.20135
Recent Volcanic Accretion at 9–10°N East Pacific Rise as Resolved by Combined Geochemical and Geological Observations
- Authors: C. L. Waters; K. W. W. Sims, S. A. Soule, J. Blichert‐Toft, N. W. Dunbar, T. Plank, J. Prytulak, R. A. Sohn, M. A. Tivey
  Pages: n/a - n/a
  Abstract: The ridge crest at 9–10°N East Pacific Rise (EPR) is dominated by overlapping lava flows that have overflowed the axial summit trough and flowed off‐axis, forming a shingle‐patterned terrain up to ~2–4 km on either side of the axial summit trough. In this study, we employ 230Th‐226Ra dating methods, in conjunction with geochemistry and seafloor geological observations, in an effort to discern the stratigraphic relationships between adjacent flows. We measured major and trace elements and 87Sr/86Sr, 143Nd/144Nd, 176Hf/177Hf, and 238U‐230Th‐226Ra for lava glass samples collected from several flow units up to ~2 km away from the axial summit trough on the ridge crest at 9°50’N EPR. Statistical analysis of the 238U‐230Th‐226Ra data indicates that all but one measured sample from these flows cannot be resolved from the zero‐age population; thus, we cannot confidently assign model ages to samples for discerning stratigraphic relationships among flows. However, because groups of samples can be distinguished based on similarities in geochemical compositions, particularly incompatible element abundances with high precision‐normalized variability such as U and Th, and because the range of compositions is much greater than that represented by samples from the 1991–92 and 2005–06 eruptions, we suggest that the dive samples represent six to ten eruptive units despite indistinguishable model ages. Geochemical variability between individual flows with similar ages requires relatively rapid changes in parental melt composition over the past ~2 ka, and this likely reflects variations in the relative mixing proportions of depleted and enriched melts derived from a heterogeneous mantle source.
  PubDate: 2013-04-01T11:34:52.067478-05:
  DOI: 10.1002/ggge.20134
Sulfide geochronology along the endeavour segment of the Juan de Fuca Ridge
- Authors: John W. Jamieson; Mark D. Hannington, David A. Clague, Deborah S. Kelley, John R. Delaney, James F. Holden, Margaret K. Tivey, Linda E. Kimpe
  Pages: n/a - n/a
  Abstract: Forty‐nine hydrothermal sulfide‐sulfate rock samples from the Endeavour Segment of the Juan de Fuca Ridge, northeastern Pacific Ocean, were dated by measuring the decay of 226Ra (half‐life of 1,600 years) in hydrothermal barite to provide a history of hydrothermal venting at the site over the past 6,000 years. This dating method is effective for samples ranging in age from ~200 to 20,000 years old and effectively bridges an age gap between shorter‐ and longer‐lived U‐series dating techniques for hydrothermal deposits. Results show that hydrothermal venting at the active High Rise, Sasquatch and Main Endeavour fields began at least 850, 1,450 and 2,300 years ago, respectively. Barite ages of other, inactive deposits on the axial valley floor, are between ~1,200 and ~2,200 years old, indicating past widespread hydrothermal venting outside of the currently active vent fields. Samples from the half‐graben on the eastern slope of the axial valley range in age from ~1,700 to ~2,925 years, and a single sample from outside the axial valley, near the westernmost valley fault scarp is 5,853 ± 205 years old. The spatial relationship between hydrothermal venting and normal faulting suggests a temporal relationship, with progressive younging of sulfide deposits from the edges of the axial valley towards the center of the rift. These relationships are consistent with the inward migration of normal faulting towards the center of the valley over time and a minimum age of onset of hydrothermal activity in this region of 5,850 years.
  PubDate: 2013-04-01T11:34:38.119027-05:
  DOI: 10.1002/ggge.20133
The oxygen isotope composition of Earth's oldest rocks and evidence of a terrestrial magma ocean
- Authors: D. Rumble; S. Bowring, T. Iizuka, T. Komiya, A. Lepland, M. T. Rosing, Y. Ueno
  Pages: n/a - n/a
  Abstract: Analysis of Hadean and Archean rocks for 16O‐17O‐18O isotopes demonstrates that the Terrestrial Mass Fractionation Line of oxygen isotopes has had the same slope and intercept for at least the past 4.0 and probably for as long as 4.2 billion years. The homogenization of oxygen isotopes required to produce such a long‐lived consistency was most easily established by mixing in a terrestrial magma ocean. The measured identical oxygen isotope mass fractionation lines for Earth and Moon suggest that oxygen isotope reservoirs of both bodies were homogenized at the same time during a giant moon‐forming impact. But other sources of heat for global melting cannot be excluded such as bolide impacts during early accretion of proto‐Earth, the decay of short‐lived radioactive isotopes, or the energy released during segregation of core from mantle.
  PubDate: 2013-03-29T07:40:35.549861-05:
  DOI: 10.1002/ggge.20128
Genesis of Cenozoic low‐Ca alkaline basalts in the Nanjing basaltic field, eastern China: the case for mantle xenolith‐magma interaction
- Authors: Gang Zeng; Li‐Hui Chen, Sen‐Lin Hu, Xi‐Sheng Xu, Liang‐Feng Yang
  Pages: n/a - n/a
  Abstract: Although peridotite xenoliths are common in alkaline basalts, it is still unclear whether the chemical compositions of their host rocks have been affected by these mantle fragments, and, if so, what processes are involved in this alteration of the host basalts. Here, we document a kind of xenolith‐rich alkaline basalts from the Nanjing basaltic field, eastern China. These basalts contain lower concentrations of CaO (4.1–7.8 wt.%) and Sc (3.3–17.8 ppm), and have lower Ca/Al (0.3–0.6) and higher Na/Ti ratios (2.8–11.2) than other Cenozoic basalts in this area. These xenoliths‐rich basalts show good correlations between elemental ratios (e.g., Lu/Hf and Ca/Al) and εHf values, which are indicative of mixing of two distinct components during the genesis of the magmas that formed these basalts: a high‐εHf endmember (with low Lu/Hf and Ca/Al ratios) and the primitive melt‐related low‐εHf endmember. In addition, peridotite xenoliths hosted in these basalts have distinct core–mantle textures, with the margins having higher modal olivine abundances (70%) than the xenolith cores (52%). Within the xenolith margins, some orthopyroxenes are enclosed in the olivines, and all clinopyroxenes are sponge‐textured. These sponge‐textured clinopyroxenes have higher CaO and Sc concentrations, higher Ca/Al ratios, and lower Na/Ti ratios than clinopyroxenes within the cores of the xenoliths, suggesting that the xenoliths underwent low‐pressure melting within the host magma. This indicates that xenolith‐rich magmas within the study area were contaminated during ascent by melts derived from mantle xenoliths within the magmas, transforming the magmas into the low‐Ca alkaline basalts.
  PubDate: 2013-03-29T07:40:20.30873-05:0
  DOI: 10.1002/ggge.20127
Microstructures and quartz lattice‐preferred orientations in the eclogite‐bearing migmatitic gneisses of the D' Entrecasteaux Islands, Papua New Guinea
- Authors: T. A. Little; B. R. Hacker, S. J. Brownlee, G. Seward
  Pages: n/a - n/a
  Abstract: Lattice preferred orientations (LPO) of quartz in gneiss domes of the D' Entrecasteaux Islands, Woodlark Rift shed insight into exhumation of the world's youngest (~5–7 Ma) eclogite‐bearing terrane at cm/yr rates. We focus on deformation that affected the terrane as it transited between lower crustal depths and the surface, including: 1) grain‐scale deformation mechanisms; and 2) style of flow and mode of emplacement of the domes. Electron‐backscatter diffraction (EBSD) was used to analyze microstructure and LPOs of 37 quartzofeldspathic gneiss samples that enclose meter‐scale mafic blocks preserving original eclogite‐facies assemblages. During exhumation of the UHP terrane, gneisses were retrogressed in the amphibolite‐facies at lower crustal depths. The LPOs change from dome cores to carapaces, consistent withdecreasing deformational temperatures. In the relatively chilled outer carapaces of the domes, the quartz LPOs consist of mostly crossed‐girdle [c]‐axis patterns, with some cleft‐girdle and small‐circle LPOs, and record dislocation creep accommodated by mixed‐ slip. In the cores of the migmatitic domes, a chessboard pattern of subgrains is common, and quartz LPOs primarily record prism‐[c] slip, probably at >630 °C. Other microstructures indicate recovery by high‐temperature grain‐boundary migration. Grain‐boundary mobility was anisotropic, leading to strong grain‐shape fabrics oblique to foliation, but not obviously relatable to shear sense. Evidence for melt‐present deformation is abundant, and microstructures (including partially dissolved feldspar grains) indicate some deformation by fluid‐assisted grain‐boundary diffusion creep. LPO's in carapace rocks are symmetrical, recording flow that was dominantly coaxial. We interpret the gneiss domes to have been emplaced into the rift as partially molten diapirs.
  PubDate: 2013-03-27T17:31:55.116318-05:
  DOI: 10.1002/ggge.20132
Seawater recharge into oceanic crust: IODP Exp 327 site U1363 grizzly bare outcrop
- Authors: C. Geoffrey Wheat; Samuel M. Hulme, Andrew T. Fisher, Beth N. Orcutt, Keir Becker
  Pages: n/a - n/a
  Abstract: Systematic differences in sediment thermal and pore water chemical profiles from Integrated Ocean Drilling Program Site U1363 document mixing and reaction within the basaltic crust adjacent to Grizzly Bare outcrop, asite of hydrothermal recharge into 3.6 My‐old basaltic crust. A transect of seven holes was drilled ~50 m to ~750 m away from the base of the outcrop. Temperatures at the sediment‐basement interface increase from ~6 °C to >30 °C with increasing distance from the outcrop, and heat flow is suppressed within several hundred meters from the outcrop. Calculated fluid compositions at the sediment‐basement interface are generally explained by mixing between bottom seawater and altered crustal basement fluids, with a composition similar but not identical to fluids from seeps at Baby Bare outcrop, located ~45 km to the northeast. Reactions within upper basement and overlying sediment affect a variety of ions (Mn, Fe, Mo, Si, PO43‐, V, and U) and δ13DIC, indicating a diagenetic influence and diffusive exchange with overlying sediment pore waters. The apparent 14C age of basal pore fluids is much older than bottom seawater. Collectively, these results are consistent with seawater recharge at Grizzly Bare outcrop; however, there are strong gradients in fluid composition within 50 m of the outcrop, providing evidence for complex flow paths and vigorous mixing of young, recently recharged seawater with much older, more reacted basement fluid. The proximity of these altered fluids to the edge of the outcrop raise the possibility for fluid seepage from the outcrop in addition to seawater recharge.
  PubDate: 2013-03-27T17:31:45.121005-05:
  DOI: 10.1002/ggge.20131
Natural seepage on the continental slope to the east of Mississippi Canyon in the northern Gulf of Mexico
- Authors: Asrarur Rahman Talukder; Andrew Ross, Emma Crooke, Charlotte Stalvies, Christine Trefry, Xiubin Qi, David Fuentes, Stephane Armand, Andrew Revill
  Pages: n/a - n/a
  Abstract: From June 5th to September 15th 2010, a multidisciplinary marine survey was undertaken onboard the M/V Ryan Chouest in the region of the BP Deepwater Horizon incident site in the Gulf of Mexico. The primary objective of the survey was the continuous monitoring of hydrocarbon abundance from sea surface down to a maximum depth of 120 m. Compound abundances were inferred using a hydrocarbon sensor array with associated vertical cast system. In order to better understand the potential inputs from natural seepage in the vicinity of the spill, a Simrad EK 60 high resolution split beam echo‐sounder, operated at 38 kHz, was included in the survey between July 7th and September 15th 2010. During this period three fields of natural seeps characterised by hydro‐acoustic flares were studied in detail. These seep fields are at water depths of ca. 430 m, 880 m and 1370 m. They are associated with extensive cold seep systems. In particular the area around Seep Field 1 (the vicinity of Deepwater Horizon) seems to present a vast area of active natural seepages in the Gulf of Mexico. The repeat surveys at two of the fields suggested that the cold seep systems here were active, with expulsions of hydrocarbons into the water column, at least during the periods of our acoustic surveys. Multiple lines of evidence gathered during the survey indicated that the observed hydro‐acoustic flares at the three fields identified consisted of oily bubble streams of gases of thermogenic origin. However, direct observation and sampling are required to reveal the precise nature of the flares. In the deep water Gulf of Mexico, the formation of a hydrate rim around bubbles seems to be a very important mechanism for the long transport of methane and oil in the water column.
  PubDate: 2013-03-27T17:31:35.529157-05:
  DOI: 10.1002/ggge.20130
A reference Earth model for the heat producing elements and associated geoneutrino flux
- Authors: Yu Huang; Viacheslav Chubakov, Fabio Mantovani, Roberta L. Rudnick, William F. McDonough
  Pages: n/a - n/a
  Abstract: The recent geoneutrino experimental results from KamLAND and Borexino detectors reveal the usefulness of analyzing the Earth's geoneutrino flux, as it provides a constraint on the strength of the radiogenic heat power and this, in turn, provides a test of compositional models of the bulk silicate Earth (BSE). This flux is dependent on the amount and distribution of heat producing elements (HPEs: U, Th and K) in the Earth's interior. We have developed a geophysically‐based, three‐dimensional global reference model for the abundances and distributions of HPEs in the BSE. The structure and composition of the outermost portion of the Earth, the crust and underlying lithospheric mantle, is detailed in the reference model, this portion of the Earth has the greatest influence on the geoneutrino fluxes. The reference model combines three existing geophysical models of the global crust and yields an average crustal thickness of 34.4 ± 4.1 km in the continents and 8.0 ± 2.7 km in the oceans, and a total mass (in 1022 kg) of oceanic, continental and bulk crust is 0.67 ± 0.23, 2.06 ± 0.25 and 2.73 ± 0.48, respectively. In situ seismic velocity provided by CRUST 2.0 allows us to estimate the average composition of the deep continental crust by using new and updated compositional databases for amphibolite and granulite facies rocks in combination with laboratory ultrasonic velocities measurements. An updated xenolithicperidotite database is used to represent the average composition of continental lithospheric mantle. Monte Carlo simulation is used to predict the geoneutrino flux at 16 selected locations and to track the asymmetrical uncertainties of radiogenic heat power due to the log‐normal distributions of HPE concentrations in crustal rocks.
  PubDate: 2013-03-27T17:31:28.335536-05:
  DOI: 10.1002/ggge.20129
Distinct lateral variations of upper mantle anisotropy beneath eastern China revealed by shear‐wave splitting
- Authors: Hui Huang; Mingjie Xu, Liangshu Wang, Zhouchuan Huang, Pan Wang, Ning Mi, Hua Li, Dayong Yu
  Pages: n/a - n/a
  Abstract: A dense seismic array consisting of 28 temporary stations was deployed to study the crustal and upper‐mantle deformations beneath eastern China. We measured the splitting parameters in the crust and mantle by analyzing P‐to‐S phases converted at the Moho discontinuity (called PmS phases) and the core‐mantle boundary (i.e. core phases), respectively. The splitting parameters of core phases are retrieved at most stations while that of the PmS phases are retrieved at only a few stations. Distinct lateral variations of the fast polarization directions analyzed with the core phases are found in different tectonic blocks in eastern China. The delay times in the mantle and crust are moderately large (~1 s) and averagely smaller than 0.3 s, respectively. By the Fresnel‐zone analysis, the laterally variant lithospheric anisotropy is revealed between the two sub‐blocks (Southeast China Orogenic Belt and Yangtze Craton) of the eastern South China Block. In contrast, in the southeastern North China Craton, the anisotropy in a relatively deep layer contributes to the splitting observations.
  PubDate: 2013-03-26T07:45:53.853265-05:
  DOI: 10.1002/ggge.20126
Seismic properties of the Kohistan oceanic arc root: insights from laboratory measurements and thermodynamic modelling
- Authors: B. S. G. Almqvist; J.‐P. Burg, J. Berger, L. Burlini
  Pages: n/a - n/a
  Abstract: P wave velocities (Vp) have been measured in the laboratory and calculated using thermodynamic modelling, for 7 representative rock samples from the lower crust to mantle section of the Kohistan paleo‐island arc. Lower crustal rocks comprise plagioclase‐rich gabbro, garnet‐bearing gabbro and hornblendite; mantle rocks comprise garnetite, pyroxenite, websterite and dunite. Measurements were performed at confining pressures up to 0.5 GPa and temperatures up to 1000 °C. Vp were also calculated using rock major element chemistry with the Perple_X software package. Calculated Vp match closely the laboratory measurements. At depths representative for the arc root, Vp of upper mantle rocks vary from 7.7 – 8.1 km/s, whereas the lower crustal rocks have velocities between 6.9 – 7.5 km/s. P wave anisotropy is small, with exceptions of sheared gabbros. Measured and calculated seismic properties are consistent with, and complement a growing database of published seismic properties from the Kohistan arc. In the light of such data, we discuss seismic imaging of present‐day island arcs. Intermediate Vp (7.4–7.7 km/s) in arc roots can be explained by pyroxenites and garnet‐bearing mafic rocks. Strong seismic reflectors may be related to garnetites (8.0 – 8.2 km/s).
  PubDate: 2013-03-25T16:26:54.874089-05:
  DOI: 10.1002/ggge.20125
Effect of CO2 Hydrate Formation on Seismic Wave Velocities of Fine‐grained Sediments
- Authors: Hak‐Sung Kim; Gye‐Chun Cho, Tae‐Hyuk Kwon
  Pages: n/a - n/a
  Abstract: This study examines the effect of gas hydrate formation on seismic wave velocities of fine‐grained sediments. Synthesis of gas hydrates in fine‐grained sediments has proved to be challenging, and how hydrate formation would affect the seismic wave velocities and stiffness of clay‐rich sediments has not yet been fully understood. In this study, CO2 hydrate was synthesized in remolded and partially water‐saturated clayey silt sediments that were originally cored from a hydrate occurrence region in the Ulleung Basin, East Sea, offshore Korea. After achieving excess water conditions, compressional wave and shear wave velocities were measured for different hydrate saturations and under different vertical effective stresses. The results reveal that the compressional wave velocity VP and shear wave velocity VS increase and the stress‐dependency of VP and VS decreases as the hydrate saturation SH increases from 0% to ~60%. In particular, the VS‒SH trend lies between the grain‐cementing model and the load‐bearing model, suggesting that gas hydrate formation in clayey silt sediments causes weak cementation from a hydrate saturation less than ~28%. The weak cementation in fine‐grained sediments can be explained by the breakage of hydrate bonds that are cementing grains during sediment compression and/or the innate weakness in bonding between hydrate crystals and fine mineral grains owing to the presence of unfrozen water films on clay mineral surfaces. In addition, it is found that at low SH the cementation effect on VP is masked by the high stiffness of pore‐filling phases, but it becomes pronounced at SH greater than 47%.
  PubDate: 2013-03-21T14:07:18.717279-05:
  DOI: 10.1002/ggge.20102
New views on ‘old’ carbon in the Amazon river: Insight from the source of organic carbon eroded from the Peruvian Andes
- Authors: K.E. Clark; R. G. Hilton, A.J. West, Y. Malhi, D.R. Grocke, C.L. Bryant, P.L. Ascough, A. Robles C, M. New
  Pages: n/a - n/a
  Abstract: Mountain rivers play a key role in the delivery of particulate organic carbon (POC) to large river systems and the ocean. Due to the extent of its drainage area and runoff, the Amazon River is one of Earth's most important biogeochemical systems. However the source of POC eroded from the humid region of the Eastern Andes and the input of fossil POC from sedimentary rocks (POCfossil) remains poorly constrained. Here we collected suspended sediments from the Kosñipata River during flood events to better characterise Andean POC, measuring the nitrogen to organic carbon ratio (N/C), stable carbon isotopes (δ13Corg) and radiocarbon (Δ14Corg). Δ14Corgvalues ranged from ‐711‰ to ‐15‰ and significant linear trends between Δ14Corg,N/C andδ13Corg suggested that this reflects the mixing of POCfossil with very young organic matter (Δ14Corg ~ 50‰) from the terrestrial biosphere (POCnon‐fossil). Using N/C and Δ14Corg in an end member mixing analysis, we quantify the fraction of POCfossil (to within 0.1) and find that it contributes a constant proportion of the suspended sediment mass (0.37 ± 0.03%) and up to 80% of total POC. In contrast, the relative contribution of POCnon‐fossil was variable, being most important during the rising limb and peak discharges of flood‐events. The new data shed light on published measurements of ‘old’ POC (low Δ14Corg) in Andean‐fed tributaries of the Amazon River, with their Δ14Corg and δ13Corg values consistent with variable addition of POCfossil. The findings suggest a greater persistence of Andean POC in the lowland Amazon than previously recognised.
  PubDate: 2013-03-21T10:26:41.759032-05:
  DOI: 10.1002/ggge.20122
Evaluating upper versus lower crustal extension through structural reconstructions and subsidence analysis of basins adjacent to the D’ Entrecasteaux Islands, eastern Papua New Guinea
- Authors: Guy Fitz; Paul Mann
  Pages: n/a - n/a
  Abstract: The D’ Entrecasteaux Island (DEI) gneiss domes are fault‐bounded domes with ~2.5 km of relief exposing ultrahigh‐pressure (UHP) and high‐pressure (HP) metamorphic gneisses and migmatites exhumed in an Oligocene‐Miocene arc‐continent collision and subduction zone subject to Late Miocene to Recent continental extension. Multi‐channel seismic (MCS) reflection data and well data show the Trobriand basin formed as a forearc basin caused by southward Miocene subduction at the Trobriand trench. Subduction slowed at ~8 Ma as the margin transitioned to an extensional tectonic environment. Since then, the Trobriand basin has subsided 1–2.5 km as a broad sag basin with few normal faults deforming the basin fill. South of the DEI, the Good enough rift basin developed after extension began (~8 Ma) as the hanging‐wall of the north‐dipping Owen‐Stanley normal fault that bounds the basin's southern margin. The lack of upper crustal extension accompanying subsidence in the Trobriand and Good enough basins suggests depth‐dependent lithospheric extension since 8 Ma has accompanied uplift of the DEI gneiss domes. Structural reconstructions of seismic profiles show 2.3 to 13.4 km of basin extension in the upper crust, while syn‐rift basin subsidence values indicate at least 20.7 to 23.6 km of extension occurred in the entire crust since ~8 Ma. Results indicating thinning is preferentially accommodated in the lower crust surrounding the DEI are used to constrain a schematic model of uplift of the DEI domes involving vertical exhumation of buoyant, post‐orogenic lower crust, far‐field extension from slab rollback, and an inverted two‐layer crustal density structure.
  PubDate: 2013-03-21T10:26:35.462877-05:
  DOI: 10.1002/ggge.20123
Mafic injection as a trigger for felsic magmatism – a numerical study
- Authors: M. Schubert; T. Driesner, T. V. Gerya, P. Ulmer
  Pages: n/a - n/a
  Abstract: The origin of crustal‐scale silicic magmatism remains a matter of debate, and notable uncertainty exists concerning the physical mechanisms that drive ascent and emplacement of felsic magmas in upper crustal regions. A 2D numerical model demonstrates that injection of mantle‐derived mafic magma into a partially molten hot zone in the lower crust can drive felsic magma ascent and intrusion into upper crustal levels. The injection of mafic magma induces over pressure in the reservoir, which increases crustal stresses and triggers development of brittle/plastic shear zones, and can drive significant surface uplift. The emerging topography causes a non‐uniform over pressure distribution in the reservoir and can trigger felsic magma ascent along crustal shear zones. Based on systematic numerical experiments we investigate the influence of crustal strength and injection rate. The initial upper crustal strength controls the degree of crustal faulting and surface uplift and, therefore, whether felsic magma ascent can be initiated or not. The final upper crustal strength influences the depth and final style of felsic intrusion. The injection rate of mafic magma determines the time scale of overpressure growth and surface uplift stage. In contrast, the duration of the subsequent felsic ascent and intrusion emplacement stages remains nearly constant. Our results imply that mafic underplating and intrusion into the lower crust may not only be a prime control for the generation of felsic magmas in the lower crust, but may also be an important physical driving mechanism for felsic magma ascent and intrusion into upper crustal levels.
  PubDate: 2013-03-21T10:26:26.982923-05:
  DOI: 10.1002/ggge.20124
Rapid eruption of the Ningwu volcanics in eastern China: Response to cretaceous subduction of the pacific plate
- Authors: Yan‐Jie Tang; Hong‐Fu Zhang, Ji‐Feng Ying, Ben‐Xun Su, Xian‐Hua Li, M. Santosh
  Pages: n/a - n/a
  Abstract: The relationship between lithospheric evolution of eastern Eurasia and subduction of the Pacific plate has long been debated. However, the timing and implications of subduction on the tectonics of eastern China are not well constrained. Here, we present new zircon U‐Pb ages and Hf isotopes, elemental and Sr‐Nd‐Pb isotopic data on Cretaceous volcanic rocks from the Ningwu basin, eastern China to further address this issue. Our age data reveal rapid eruption of the volcanic rocks within a short duration from 133 to 130 Ma. The rocks, mostly characterized by shoshonitic and high‐K calc‐alkaline signatures, display light rare earth element and Pb enrichment, Nb, Ta and Ti depletion, highly radiogenic Sr‐Pb isotopic ratios and variable εHf(t) (+1.8 to −10), suggesting derivation from an enriched lithospheric mantle metasomatized by marine sediments. The early lavas (133.3 ± 1.1 Ma) show stronger subducted‐related signatures than the late lavas (130.1 ± 1.0 Ma), which we interpret to reflect consumption of a significant volume of fusible subducted components in the early melting phase. The large εHf(t) variation of late lavas suggests greater involvement of asthenospheric melts and lower crust in their petrogenesis. The youngest age (130 Ma) appears to coincide with an inferred change in the direction of Pacific‐Eurasia convergence, manifested as a change from extension to transpression in eastern China. The narrow window of eruption may signify a rapid change of the tectonic regime in the Early Cretaceous.
  PubDate: 2013-03-21T08:31:26.042361-05:
  DOI: 10.1002/ggge.20121
Revised Tectonic Evolution of the Eastern Indian Ocean
- Authors: Joanne M. Whittaker; Simon E. Williams, R. Dietmar Müller
  Pages: n/a - n/a
  Abstract: Published plate tectonic models for the Australian‐Antarctic plate pair imply geologically improbable scenarios at either, or both, ends of the Cretaceous rift and spreading system. Controversy also exists around the location of and motion at the plate boundary extending west of Australia‐Antarctica, through the Kerguelen Plateau region. We present a plate tectonic model of relative motions among India, Australia and Antarctica from the onset of continental rifting to the establishment of rapid seafloor spreading, at ~43 Ma. The model conforms to a wide range of geological/geophysical evidence and reconstructs the formation of both the western Kerguelen region and the eastern Tasman region. The incorporation of spatio‐temporally continuous plate boundaries reveals the presence of a plate boundary beneath the contiguous Central Kerguelen Plateau and Broken Ridge for ~65 million years.To investigate the relationship between the plate boundary system and the Kerguelen plume we test three alternative absolute reference frames. Using a fixed hotspot reference frame, the Indian Ocean mid‐ocean ridge system remains within 500 km of the Kerguelen plume, while the proximity of the plate boundaries and the plume is more variable with a moving hotspot reference frame. Proximity between the plume, plate boundaries, and the Central Kerguelen Plateau/Broken Ridge for ~65Myrs, suggests that these specific features were not formed by a single, short‐lived (5‐10 Myr) pulse of magmatic activity, but rather bya ~25 Myr period of relatively high magma flux followed by ~40 Myr period of lower volume magmatic activity, an interpretation not excluded by the relatively sparse dredge and drill ages.
  PubDate: 2013-03-19T10:01:36.954209-05:
  DOI: 10.1002/ggge.20120
Characteristics of magma‐driven hydrothermal systems at oceanic spreading centers
- Authors: Robert P. Lowell; Aida Farough, Joshua Hoover, Kylin Cummings
  Pages: n/a - n/a
  Abstract: We use one‐ and two‐limb single‐pass models to determine vent field characteristics such as mass flow rate Q, bulk permeability in the discharge zone kd, thickness of the conductive boundary layer at the base of the system δ, magma replenishment rate, and residence time in the discharge zone. Data on vent temperature, vent field area, heat output, and the surface area and depth of the sub‐axial magma chamber (AMC) constrain the models. The results give Q ~ 100 kg/s, kd ~ 10‐13 m2 and δ ~ 10 m, essentially independent of spreading rate, and detailed characteristics of the AMC. In addition, we find no correlation between heat output at individual vent fields and spreading rate or depth to the AMC. We conclude that high‐temperature hydrothermal systems are driven by local magma supply rates in excess of that needed for steady state crustal production and that crustal permeability enables hydrothermal circulation to tap magmatic heat regardless of AMC depth. Using data on partitioning of heat flow between focused and diffuse flow, we find that that 80‐90% of the hydrothermal heat output is derived from high‐temperature fluid, even though much of the heat output discharges as low‐temperature fluid. In some cases, diffuse flow fluids may exhibit considerable conductive cooling or heating. By assuming conservative mixing of diffuse flow fluids at EPR 9°50′N, we find that most transport of metals such as Fe and Mn occurs in diffuse flow and that CO2, H2, and CH4 are taken up by microbial activity.
  PubDate: 2013-03-18T15:47:46.99943-05:0
  DOI: 10.1002/ggge.20109
Asian dust input in the western Philippine Sea: Evidence from radiogenic Sr and Nd isotopes
- Authors: Fuqing Jiang; Martin Frank, Tiegang Li, Tianyu Chen, Zhaokai Xu, Anchun Li
  Pages: n/a - n/a
  Abstract: The radiogenic strontium (Sr) and neodymium (Nd) isotope compositions of the detrital fraction of surface and subsurface sediments have been determined to trace sediment provenance and contributions from Asian dust off the east coast of Luzon Islands in the western Philippine Sea. The Sr and Nd isotope compositions have been very homogenous near the east coast of the Luzon Islands during the latest Quaternary yielding relatively least radiogenic Sr (87Sr/86Sr = 0.70453 to 0.70491) and more radiogenic Nd isotope compositions (εNd(0) = +5.3 to +5.5). These isotope compositions are similar to Luzon rocks and show that these sediments were mainly derived from the Luzon Islands. In contrast, the Sr and Nd isotope compositions of sediments on the Benham Rise and in the Philippine Basin are markedly different in that they are characterized by overall more variable and more radiogenic Sr isotope compositions (87Sr/86Sr = 0.70452 to 0.70723) and less radiogenic Nd isotope compositions (εNd(0) = ‐5.3 to +2.4). The Sr isotope composition in the Huatung Basin is intermediate between those of the east coast of Luzon and Benham Rise, but shows the least radiogenic Nd isotope compositions. The data are consistent with a two end‐member mixing relationship between Luzon volcanic rocks and eolian dust from the Asian continent, which is characterized by highly radiogenic Sr and unradiogenic Nd isotope compositions. The results show that Asian continental dust contributes about 10‐50% of the lithogenic fraction of the sediments on Benham Rise in the western Philippine Sea, which offers the potentials to reconstruct the climatic evolution of eastern Asia from these sediments and compare this information to the records from the central and northern Pacific.
  PubDate: 2013-03-18T15:47:45.887864-05:
  DOI: 10.1002/ggge.20116
Using ammonium pore water profiles to assess stoichiometry of deep remineralization processes in methanogenic continental margin sediments
- Authors: David J. Burdige; Tomoko Komada
  Pages: n/a - n/a
  Abstract: In many continental margin sediments, a deep reaction zone exists which is separated from remineralization processes near the sediment surface. Here, methane diffuses upwards to a depth where it is oxidized by downwardly diffusing sulfate. However, the methane sources that drive this anaerobic oxidation of methane (AOM)in the sulfate‐methane transition zone (SMT) may vary among sites. In particular these sources can be thought of as: “internal” sources from in situ methanogenesis (regardless of where it occurs in the sediment column) that is ultimately coupled to organic matter deposition and burial; and “external” sources such as hydrocarbon reservoirs derived from ancient source rocks, or deeply‐buried gas hydrates, both of which are decoupled from contemporaneous organic carbon deposition at the sediment surface. Using a modeling approach we examine the relationship between different methane sources and pore water sulfate, methane, dissolved inorganic carbon (DIC), and ammonium profiles. We show that pore water ammonium profiles through the SMT represent an independent “tracer” of remineralization processes occurring in deep sediments that complement information obtained from profiles of solutes directly associated with AOM and carbonate precipitation, i.e., DIC, methane, and sulfate. Pore water DIC profiles also show an inflection point in the SMT based on the type of deep methane source and the presence/absence of accompanying upward DIC fluxes. With these results we present a conceptual framework which illustrates how shallow pore water profiles from continental margin settings can be used to obtain important information about remineralization processes and methane sources in deep sediments.
  PubDate: 2013-03-18T15:47:42.441077-05:
  DOI: 10.1002/ggge.20117
Analysis of Normal Fault Populations in the Kumano Forearc Basin,Nankai Trough, Japan: 1 Multiple Orientations and Generations ofFaults from 3‐D Coherency Mapping
- Authors: G. F. Moore; B. B. Boston, A. F. Sacks, D. M. Saffer
  Pages: n/a - n/a
  Abstract: Analyses of normal faults in the Kumano forearc basin of the Nankai Trough reveal multiple normal fault populations in a region generally thought to be under compression. Most fault shave offsets of less than 20 m, dips of 60–70° and show no growth structures, indicating that the faults were active for short periods of time. The oldest generation of faults is older than ~0.9 Ma and strikes ~050–060°. The next oldest faults strike ~160–170°, are older than 0.44 Ma and are related to local uplift along the western edge of the region. The youngest faults cut the seafloor; shallow faults near the SE margin of the basin curve from ~100° in the middle of the survey area to ~145° at the SE corner of the area. The pattern of the two youngest fault populations is consistent with the regional stress pattern (maximum horizontal stresssubparallel to the trench). Orientations of older fault populations are caused by uplift of the underlying accretionary prism, implying that the forearc basin region is not as stable as previously thought. Reconstruction of displacements on the youngest faults shows that the overall horizontal extension is less than 2%, concentrated near the seaward edge of the basin. The active normal faults distributed throughout the basin support the idea that the horizontal stress parallel to the plate convergence direction does not reach the critical stress to activate or form thrust faults and produce horizontal shortening within the shallow portion of the inner wedge.
  PubDate: 2013-03-18T15:47:40.073349-05:
  DOI: 10.1002/ggge.20119
Constraints on core‐mantle boundary topography from normal mode splitting
- Authors: Gaia Soldati; Paula Koelemeijer, Lapo Boschi, Arwen Deuss
  Pages: n/a - n/a
  Abstract: The frequencies of Earth's normal modes are split by rotation, ellipticity, and internal structure of the Earth. Thus, models of mantle heterogeneity and discontinuity topography generate splitting that may be tested against observations. We insert maps of core‐mantle boundary (CMB) topography, which are derived via either a purely seismic or a joint tomographic/geodynamic inversion of body waves data, on top of tomographic model S20RTS. We then calculate synthetic splitting functions for normal modes that have been shown to be sensitive to CMB topography and compare these to observed normal mode splitting data. The CMB topography maps obtained via geodynamically‐constrained tomography fit normal mode data better than purely seismic maps, in particular when the geodynamic constraint also accounts for the presence of post‐perovskite in the D" region. We test the significance of the reduction in misfit using the concept of observability which suggests that normal modes are able to observe the difference between the different CMB topography maps. In addition, we find that the statistical significance, assessed by checking what fraction of 1000 randomly generated CMB models achieve a comparatively good fit as the desired model, is higher than 92 % for degree 2 and 98 % for all degrees. In summary, we have identified a model of CMB topography that fits body wave data and improves, at least to some extent, the fit to normal mode data, and is coherent with the large‐scale pattern of deep mantle heterogeneity expected on the basis of convection modelling.
  PubDate: 2013-03-18T08:46:34.009269-05:
  DOI: 10.1002/ggge.20115
Analysis of Normal Fault Populations in the Kumano Forearc Basin, Nankai Trough, Japan: 2. Principal Axes of Stress and Strain from Inversion of Fault Orientations
- Authors: Alison Sacks; Demian M. Saffer, Donald Fisher
  Pages: n/a - n/a
  Abstract: We use a high‐resolution 3D seismic survey to map a population of recent normal faults within the Kumano Basin of the Nankai subduction zone, in order to quantify patterns of strain and stress state over the last 0.44 Myr. We identify distinct fault populations that define three phases of extension. Phases 1 and 2 comprise NW‐SE striking faults located along the western basin edge and in the northwestern portion of the study area, respectively. The NE‐SW striking faults of Phase 3 comprise the largest population, and extend ~20 km landward from the basin's seaward edge. Phase 2 faults typically terminate within a few reflectors of the seafloor, whereas most Phase 3 faults form seafloor scarps. Inversion of the fault populations documents NE‐SW extension during Phases 1 and 2, and NW‐SE extension during Phase 3, consistent with both core scale structures and horizontal stress orientations defined at Integrated Ocean Drilling Program (IODP) boreholes. Slip on Phase 3 faults accommodates strain of up to ~1‐2%, concentrated near the basin's seaward edge. Inversion for a best‐fit stress tensor yields a subvertical σ1 and sub‐horizontal σ2 and σ3 for all faulting phases. We find that during Phase 3 in most portions of the basin, σ2 = σ3 (SHmax = Shmin), reflecting widely varying fault strikes. This contrasts with distinct SHmax and Shmin magnitudes inferred from IODP borehole data; these observations may be reconciled if the orientation of maximum horizontal stress fluctuates due to variation of subduction parallel compression through the seismic cycle.
  PubDate: 2013-03-18T08:46:32.963799-05:
  DOI: 10.1002/ggge.20118
Lead isotope provinciality of central North Pacific Deep Water over the Cenozoic
- Authors: Tian‐Yu Chen; Hong‐Fei Ling, Rong Hu, Martin Frank, Shao‐Yong Jiang
  Pages: n/a - n/a
  Abstract: Understanding the pre‐anthropogenic Pb cycle of central North Pacific deep water has attracted a lot of attention in recent years, partly because of its unique geographical location in that it is a remote gyre system characterized by high dust fluxes and sluggish overturning circulation. However, the factors controlling Pb isotope evolution in this area over the Cenozoic are still controversial and various mechanisms have been proposed in previous studies. Here we report new Pb and Nd isotope time series of 4 ferromanganese crusts (2 from the western Pacific near the Mariana arc and 2 from the central Pacific). Together with previously published records we discuss for the first time the significance of a persistent and systematic Pb isotopic provinciality recorded by central North Pacific crusts over the Cenozoic. We propose that globally well mixed stratosphere volcanic aerosols could contribute Pb but have not been the major factors controlling the Pb isotope distribution in the central North Pacific over time. Island arc input (and probably enhanced hydrothermal input between about 45 and 20 Ma) likely controlled the Pb isotope provinciality and evolution prior to ~20 Ma, when coeval Pb isotope records in different crusts showed large differences and atmospheric silicate dust flux was extremely low. After the Eocene, in particular after 20 Ma, Asian dust input has become an isotopically resolvable source, while island arc‐derived Pb has remained important to balance the dust input and to produce the observed Pb isotope distribution in the central North Pacific during this period.
  PubDate: 2013-03-18T08:46:29.190821-05:
  DOI: 10.1002/ggge.20114
The Palos Verdes Anticlinorium along the Los Angeles, California coast: Implications for underlying thrust faulting.
- Authors: Christopher C. Sorlien; Leonardo Seeber, Bruce P. Luyendyk, Michael A. Fisher, Ray W. Sliter, William R. Normark, Kris G. Broderick
  Pages: n/a - n/a
  Abstract: The Palos Verdes anticlinorium (PVA) is a 70 km‐long, NW‐trending Pliocene‐Quaternary transpressional high that separates the onshore Los Angeles basin from the offshore San Pedro basin. The PVA southwest limb underlies a 70 km‐long SW‐facing seafloor escarpment. This escarpment is separated into two segments by a 6 km right step. It overlies two levels of NE‐dipping low‐angle thrust faults. These faults project beneath Los Angeles towards known SW‐verging blind thrust faults and may be the same regional faults. These conclusions are based on a detailed geometric representation constructed from a regional analysis of 5,000 km of multichannel seismic reflection profiles, logs and paleontology from 19 wells, published sea floor geology and multibeam bathymetric data. Patterns of sedimentation and erosion indicate that folding of PVA initiated during Pliocene time and propagated southward, with folding of part of the southern PVA occurring during the Quaternary. Structural relief continues to grow with regional long‐term rock uplift of the PVA crest and ongoing subsidence in the adjacent basins. The low‐angle NE‐dipping faults beneath the PVA must be late‐Quaternary active in order to maintain hanging‐wall rock uplift above subsiding footwalls. Structural geometry requires that the underlying faults have slipped at 1.1 to 1.6 mm/year over Quaternary time.
  PubDate: 2013-03-15T09:24:18.030841-05:
  DOI: 10.1002/ggge.20112
Evidence that low‐temperature oceanic hydrothermal systems play an important role in the silicate‐carbonate weathering cycle and long‐term climate regulation
- Authors: Laurence A. Coogan; Kathryn M. Gillis
  Pages: n/a - n/a
  Abstract: The feedbacks between changes in atmospheric CO2 levels, climate and CO2 drawdown into rocks are incompletely understood. In particular, the role of the upper oceanic crust in this long‐term carbon cycling is debated. Here, a simple model for the precipitation of calcite in the upper oceanic crust is developed with the aim of understanding why late Mesozoic upper oceanic crust contains several times higher CO2 concentrations (~2.5 wt%) than Cenozoic upper oceanic crust (~0.5 wt%). The modeling shows that neither heating of seawater, nor leaching of Ca from the rock with charge balance maintained by Mg uptake by the rock, can lead to >0.2 wt% CO2 uptake by the oceanic crust. Alkalinity production during fluid‐rock reaction in the crust allows substantially more CO2 to be taken up by the crust in calcite, and is consistent with changes in the major element composition of late Mesozoic upper oceanic crust due to hydrothermal alteration. The higher CO2 content of late Mesozoic than Cenozoic upper oceanic crust thus requires greater alkalinity production by fluid‐rock reactions in the late Mesozoic. This may have been due to higher bottom water temperature and/or seawater having a different composition leading to different secondary minerals forming in the late Mesozoic. Irrespective of the mechanism, the negative feedback on atmospheric CO2 levels provided by enhanced hydrothermal CO2 consumption in the late Mesozoic was of similar magnitude to that from continental weathering.
  PubDate: 2013-03-15T09:24:15.563084-05:
  DOI: 10.1002/ggge.20113
Correction to “Temporal and crustal effects on differentiation of tholeiite to calcalkaline and ferro‐trachytic suites, High Lava Plains, Oregon, USA”
- Authors: Martin J. Streck; Anita L. Grunder
  Pages: n/a - n/a
  PubDate: 2013-03-14T13:08:02.596227-05:
  DOI: 10.1002/ggge.20099
Geochemical insight into differences in the physical structures and dynamics of two adjacent maar lakes at Mt. Vulture volcano (southern Italy)
- Authors: A. Caracausi; M. Nicolosi, P.M. Nuccio, R. Favara, M. Paternoster, A. Rosciglione
  Pages: n/a - n/a
  Abstract: We report on the first geochemical investigation of the Monticchio maar lakes (Mt. Vulture volcano, southern Italy) covering an annual cycle that aimed at understanding the characteristic features of the physical structures and dynamics of the two lakes. We provide the first detailed description of the lakes based on high‐resolution CTD profiles, chemical and isotopic (H and O) compositions of the water, and the amounts of dissolved gases (e.g., He, Ar, CH4 and CO2). The combined data set reveals that the two lakes, which are separated by less than 200 m, exhibit different dynamics: one is a meromictic lake, where the waters are rich in biogenic and mantle‐derived gases, while the other is a monomictic lake, which exhibits complete turnover of the water in winter and the release of dissolved gases. Our data strongly suggest that the differences in the dynamics of the two lakes are due to different density profiles affected by dissolved solutes, mainly Fe, which is strongly enriched in the deep water of the meromictic lake.A conceptual model of water balance was constructed based on the correlation between the chemical composition of the water and the hydrogen isotopic signature. Gas‐rich groundwaters that feed both of the lakes and evaporation processes subsequently modify the water chemistry of the lakes.Our data highlight that no further potential hazardous accumulation of lethal gases is expected at the Monticchio lakes. Nevertheless, geochemical monitoring is needed to prevent the possibility of vigorous gas releases that have previously occurred in historical time.
  PubDate: 2013-03-13T18:33:24.894078-05:
  DOI: 10.1002/ggge.20111
Depositional architecture, provenance, and tectonic/eustatic modulation of Miocene submarine fans in the Shikoku basin: results from NanTroSEIZE
- Authors: Kevin T. Pickering; Michael B. Underwood, Sanny Saito, Hajime Naruse, Steffen Kutterolf, Rachel Scudder, Jin‐Oh Park, Gregory F. Moore, Angela Slagle
  Pages: n/a - n/a
  Abstract: Seismo‐stratigraphy, coring and LWD during IODP Expeditions 319, 322, and 333 (Sites C0011 / C0012) show three Miocene submarine fans in the NE Shikoku Basin, with broadly coeval deposits at ODP Site 1177 and DSDP Site 297 (NW Shikoku Basin). The sediment dispersal patterns have major implications for paleogeographies at that time. The oldest, finer‐grained (Kyushu) fan has sheet‐like geometry; quartz‐rich flows were fed mostly from an ancestral landmass in the East China Sea. During prolonged hemipelagic mud deposition at C0011‐C0012 (~ 12.2 to 9.1 Ma), sand supply continued at Sites 1177 and 297. Sand delivery to much of the Shikoku Basin halted during a phase of sinistral strike‐slip to oblique plate motion, after which the Daiichi Zenisu Fan (~ 9.1 to 8.0 Ma) was fed by submarine channels. The youngest fan (Daini Zenisu; ~ 8.0 to 7.6 Ma) has sheet‐like geometry with thick‐bedded, coarse‐grained pumiceous sandstones. The pumice fragments were fed from a mixed provenance that included the collision zone of the Izu‐Bonin and Honshu arcs. The shift from channelized to sheet‐like flows was favored by renewal of relatively rapid northward subduction, which accentuated the trench as a bathymetric depression. Increased sand supply appears to correlate with long‐term eustatic lowstands of sea level. The stratigraphic position and 3‐D geometry of the sandbodies has important implications for subduction‐related processes, including the potential for focused fluid flow and fluid overpressures above and below the plate‐boundary fault: in sheet‐like sands, pathways for fluid flow have greater horizontal permeability compared with channel sands.
  PubDate: 2013-03-13T07:53:18.17327-05:0
  DOI: 10.1002/ggge.20107
Waning magmatic activity along the Southern Explorer Ridge revealed through fault restoration of rift topography
- Authors: A. Deschamps; M. A. Tivey, W. W. Chadwick, R. W. Embley
  Pages: n/a - n/a
  Abstract: We combine high‐resolution bathymetry acquired using the Autonomous Underwater Vehicle ABE with digital seafloor imagery collected using the Remotely Operated Vehicle ROPOS across the axial valley of the Southern Explorer Ridge (SER) to infer the recent volcanic and tectonic processes. The SER is an intermediate spreading ridge located in the northeast Pacific. It hosts the Magic Mountain hydrothermal vent. We reconstruct the unfaulted seafloor terrain at SER based on calculations of the vertical displacement field and fault parameters. The vertical changes between the initial and the restored topographies reflect the integrated effects of volcanism and tectonism on relief‐forming processes over the last 11000‐14000 yr. The restored topography indicates that the axial morphology evolved from a smooth constructional dome >500 m in diameter, to a fault‐bounded graben, ~500 m ‐wide and 30‐70 m deep. This evolution has been accompanied by changes in eruptive rate, with deposition of voluminous lobate and sheet flows when the SER had a domed morphology, and limited‐extent low‐effusion rate pillow eruptions during graben development. Most of the faults shaping the present axial valley post‐date the construction of the dome. Our study supports a model of cyclic volcanism at the SER with periods of effusive eruptions flooding the axial rift, centered on the broad plateau at the summit of the ridge, followed by a decrease in eruptive activity and a subsequent dominance of tectonic processes, with minor low‐effusion rate eruptions confined to the axial graben. The asymmetric shape of the axial graben supports an increasing role of extensional processes, with a component of simple shear in the spreading processes.
  PubDate: 2013-03-12T18:58:21.811465-05:
  DOI: 10.1002/ggge.20110
Obliquity and long eccentricity pacing of the middle Miocene climate transition
- Authors: Jun Tian; Mei Yang, Mitchell W. Lyle, Roy Wilkens, Julia K. Shackford
  Pages: n/a - n/a
  Abstract: The middle Miocene East Antarctic ice sheet expansion (EAIE), which is indicated by an abrupt ~1‰ increase in global benthic foraminiferal 18O at ~13.8 Ma, marks the middle Miocene climate transition (MMCT) and has been related to astronomically modulated changes in the global carbon cycle. Here we present high‐resolution (3‐4 kyr) benthic foraminiferal 18O and 13C records from IODP Site U1337 in the central equatorial Pacific, which spans the period 12.2‐15.8 Ma. The isotopic records clearly demonstrate significant imprints from periodic variations in the Earth's orbital parameters, particularly the obliquity (40 kyr) and the long eccentricity (400 kyr) cycles. While the benthic 18O and 13C exhibit nearly identical amplitudes for glacial‐interglacial cycles from 15.8 to 12.2 Ma, the long‐term trends in the benthic 18O and 13C had started to reverse after the beginning of the EAIE. Within the 400 kyr band, the benthic ‐18O and 13C display a constant phase relationship between 15.8 and 12.2 Ma. At the 41 kyr band, however, a phase reversal reaching >180 degrees between ‐18O and 13C occurs from 13.8 Ma to 14.0 Ma during the period of the EAIE. A similar phase relationship of benthic foraminiferal ‐18O and 13C at the 400 kyr band and the 41 kyr band is also observed at ODP site 1146 from the northern South China Sea. This phase jump occurs when the long‐term trends in 18O and 13C split, suggesting a decoupling of the global ice volume and ocean carbon reservoir changes during the middle Miocene.
  PubDate: 2013-03-11T15:57:34.3001-05:00
  DOI: 10.1002/ggge.20108
Early Eocene – to – middle Miocene cooling and aridification of East Antarctica
- Authors: S. Passchier; S.M. Bohaty, F. Jiménez‐Espejo, J. Pross, U. Röhl, T. Flierdt, C. Escutia, H. Brinkhuis
  Pages: n/a - n/a
  Abstract: Few high‐latitude terrestrial records document the timing and nature of the Cenozoic “Greenhouse” to “Icehouse” transition. Here we exploit the bulk geochemistry of marine siliciclastic sediments from drillcores on Antarctica's continental margin to extract a unique semi‐quantitative temperature and precipitation record for the Eocene – to – mid–Miocene (~54–13 Ma). Alkaline elements are strongly enriched in the detrital mineral fraction in fine‐grained siliciclastic marine sediments and only occur as trace metals in the biogenic fraction. Hence, terrestrial climofunctions similar to the chemical index of alteration (CIA) can be applied to the alkaline major element geochemistry of marine sediments on continental margins in order to reconstruct changes in precipitation and temperature. We validate this approach by comparison with published paleotemperature and precipitation records derived from fossil wood, leaves and pollen, and find remarkable agreement, despite uncertainties in the calibrations of the different proxies. A long‐term cooling on the order of ≥8 °C is observed between the Early Eocene Climatic Optimum (~54–52 Ma) and the middle Miocene (~15–13 Ma) with the onset of transient cooling episodes in the middle Eocene at ~46–45 Ma. High‐latitude stratigraphic records currently exhibit insufficient temporal resolution to reconstruct continental aridity and inferred ice‐sheet development during the mid– to – late Eocene (~45–37 Ma). However, we find an abrupt aridification of East Antarctica near the Eocene‐Oligocene transition (~34 Ma), which suggests that ice coverage influenced high‐latitude atmospheric circulation patterns through albedo effects from the earliest Oligocene onward.
  PubDate: 2013-03-11T15:57:33.029398-05:
  DOI: 10.1002/ggge.20106
Temporal trends in the vent fluid iron and sulfide chemistry following the 2005, 2006 eruptions at East Pacific Rise, 9º50’N
- Authors: Mustafa Yücel; George W. Luther
  Pages: n/a - n/a
  Abstract: The chemistry of the vent fluids that emanate to the seafloor undergoes dramatic changes after volcanic eruptions. Data on these changes are still few, but the best‐studied example is the East Pacific Rise (EPR) at 9º50’N, where after the 1991 and 1992 eruptions the temporal evolution of the vent fluid chemistry was documented. The area underwent another eruption sequence during late 2005 / early 2006 and here we show that a similar evolution is recurring in iron and sulfide contents of the high‐temperature fluids sampled in June 2006, January 2007 and June 2008. The vents have had increasing dissolved iron and decreasing acid‐volatile sulfide (AVS, free sulfide plus FeS) concentrations with an order of magnitude variation. In addition, chromium reducible sulfides (mainly pyrite) also had fivefold decreasing concentrations over the three years. Our results confirm a pattern that was noted only once before for 9º50’N EPR, and emphasize the dramatic yearly variability in the concentrations of iron‐sulfur species emanating from vents.
  PubDate: 2013-03-07T11:53:27.382858-05:
  DOI: 10.1002/ggge.20088
In‐situ re‐deposition of trace metals mobilized by CO2‐charged brines
- Authors: M. Wigley; N. Kampman, H. J. Chapman, B. Dubacq, M. J. Bickle
  Pages: n/a - n/a
  Abstract: Mobilization of contaminants by CO2‐charged brines is one concern relating to injection of CO2 as part of carbon capture and storage projects. This study monitors the mobility of trace metals in an exhumed CO2‐charged aquifer near the town of Green River, Utah (USA), where CO2‐charged brines have bleached red sandstones, and concentrated trace metals at the bleaching reaction front. Mass balance calculations on the trace metal enrichments are used to calculate time‐integrated fluid fluxes and show that a significant fraction of the metals mobilized by the CO2‐rich brines are re‐deposited locally. A sequential extraction procedure on metal‐enriched samples shows that these metals are incorporated into secondary carbonate and oxide phases which have been shown to grow at the CO2‐promoted bleaching reaction front. We argue that whilst CO2‐charged brines are capable of mobilizing trace metals, local metal re‐deposition implies that the potential for contamination of overlying freshwater aquifers is low.
  PubDate: 2013-03-01T12:41:32.500037-05:
  DOI: 10.1002/ggge.20104
The influence of non‐uniform ambient noise on crustal tomography in Europe
- Authors: P. Basini; T. Nissen‐Meyer, L. Boschi, E. Casarotti, J. Verbeke, O. Schenk, D. Giardini
  Pages: n/a - n/a
  Abstract: Ambient‐noise seismology is of great relevance to high‐resolution crustal imaging, thanks to the unprecedented dense data coverage it affords in regions of little seismicity. Under the assumption of uniformly distributed noise sources, it has been used to extract the Greenâ€™s function between two receivers. We determine the imprint of this assumption by means of wave propagation and adjoint methods in realistic 3D Earth models. In this context, we quantify the sensitivity of ambient‐noise cross correlations from central Europe with respect to noise‐source locations and shear wavespeed structure. We use ambient noise recorded over one year at 196 stations, resulting in a database of 864 cross‐correlations. Our mesh is built upon a combined crustal and 3D tomographic model. We simulate synthetic ambient‐noise cross‐correlations in different frequency bands using a 3D spectral‐element method. Traveltime cross‐correlation measurements in these different frequency bands define the misfit between synthetics and observations as a basis to compute sensitivity kernels using the adjoint method. We perform a comprehensive analysis varying geographic station and noise‐source distributions around the European seas. The deterministic sensitivity analysis allows for estimating where the starting crustal model shows better accordance with our dataset and gain insight into the distribution of noise sources in the European region. This highlights the potential importance to consider localized noise distributions for tomographic imaging and forms the basis of a tomographic inversion in which the distribution of noise sources may be treated as a free parameter similar to earthquake tomography.
  PubDate: 2013-03-01T12:41:10.734453-05:
  DOI: 10.1002/ggge.20081
Prediction of Silicate Melt Viscosity from Electrical Conductivity: A Model and its Geophysical Implications
- Authors: Anne Pommier; Rob L. Evans, Kerry Key, James A. Tyburczy, Stephen Mackwell, Jimmy Elsenbeck
  Pages: n/a - n/a
  Abstract: Our knowledge of magma dynamics would be improved if geophysical data could be used to infer rheological constraints in melt‐bearing zones. Geophysical images of the Earth's interior provide frozen snapshots of a dynamical system. However, knowledge of a rheological parameter such as viscosity would constrain the time dependent dynamics of melt bearing zones. We propose a model that relates melt viscosity to electrical conductivity for naturally‐occurring melt compositions (including H2O) and temperature. Based on laboratory measurements of melt conductivity and viscosity, our model provides a rheological dimension to the interpretation of electromagnetic anomalies caused by melt and partially molten rocks (melt fraction ~ >0.7).
  PubDate: 2013-02-27T16:04:48.936583-05:
  DOI: 10.1002/ggge.20103
Termination of a six‐year ridge‐spreading event observed using a seafloor seismic network on the Endeavour Segment, Juan de Fuca Ridge
- Authors: Robert T. Weekly; William S.D. Wilcock, Emilie E.E. Hooft, Douglas R. Toomey, Paul R. McGill, Debra S. Stakes
  Pages: n/a - n/a
  Abstract: We present automatically determined epicenters and magnitudes for 36,523 earthquakes recorded along the Endeavour segment between August 2003 and October 2006 using a local ocean‐bottom seismometer (OBS) network. The catalog is dominated by two swarm sequences in January and February 2005 in the vicinity of the Endeavour overlapping spreading center, which included earthquakes in West Valley, the northern portion of the Endeavour segment, southwest Endeavour Valley and the Endeavour vent fields. These swarms are attributed to volcanism including a dike intrusion on the northern Endeavour in February 2005 and smaller diking events on the propagating tip of the West Valley segment in both swarms. The dike on the northern Endeavour propagated to the south, which is inconsistent with magma sourced from the axial magma chamber beneath the elevated central portion of the segment. Following the swarms, seismic activity on the Endeavour segment decreased on average to ~15% of pre‐swarm values and almost ceased at the segment ends. We infer that a six‐year non‐eruptive event that started with a swarm in 1999 and finished with the 2005 swarms ruptured the entire segment and relieved plate‐spreading stresses. The inferred coupling between the 1999 and 2005 events, the observation of extensive precursory activity prior to the 2005 swarms, and the interaction between seismically active regions during the swarms is consistent with static triggering with delays influenced by viscoelastic relaxation, hydraulic diffusion and magma withdrawal and replenishment.
  PubDate: 2013-02-27T16:04:36.4182-05:00
  DOI: 10.1002/ggge.20105
Unmixing of stable isotope signals using single specimen δ18O analyses
- Authors: J.C. Wit; G.J. Reichart, G.M. Ganssen
  Pages: n/a - n/a
  Abstract: The resolution at which foraminiferal stable isotopes are applied in paleo‐environmental studies is ever increasing, resulting in continuous sampling of sediment cores. The resolution of such continuously sampled records depends on the rate of sedimentation of foraminiferal shells in its relation to the intensity of bioturbation. Bioturbation essentially mixes sediment layers of different age, altering the primary climate signal, thereby impacting the accuracy of both the timing and magnitude of reconstructed climate changes. A new approach to assess and correct the impact of bioturbation is investigated here, based on the δ18O of individual specimens of planktonic foraminifera Globorotalia inflata from a series of boxcore samples in the Eastern North Atlantic. Average δ18O values decrease southward from 1.62 to 1.07 ‰ with the exception of site T86‐11 (1.35 ‰). The δ18O distribution of each station can be fitted with a uni‐ to polymodal distribution. A non‐unimodal distribution strongly suggests admixing of bioturbated individuals. Quantification of these distributions allows deconvolving the original and bioturbated signals and subsequently provides a correction for bioturbation.
  PubDate: 2013-02-27T16:04:27.611846-05:
  DOI: 10.1002/ggge.20101
Effects of permeability fields on fluid, heat, and oxygen isotope transport in extensional detachment systems
- Authors: Raphaël Gottardi; Po‐Hao Kao, Martin O. Saar, Christian Teyssier
  Pages: n/a - n/a
  Abstract: Field studies of Cordilleran metamorphic core complexes indicate that meteoric fluids permeated the upper crust down to the detachment shear zone and interacted with highly deformed and recrystallized (mylonitic) rocks. The presence of fluids in the brittle/ductile transition zone is recorded in the oxygen and hydrogen stable isotope compositions of the mylonites and may play an important role in the thermomechanical evolution of the detachment shear zone. Geochemical data show that fluid flow in the brittle upper crust is primarily controlled by the large‐scale fault‐zone architecture.We conduct continuum‐scale (i.e., large‐scale, partial‐bounceback) lattice‐Boltzman fluid, heat, and oxygen isotope transport simulations of an idealized cross‐section of a metamorphic core complex. The simulations investigate the effects of crust and fault permeability fields as well as buoyancy driven flow on two‐way coupled fluid and heat transfer and resultant exchange of oxygen isotopes between meteoric fluid and rock. Results show that fluid migration to mid‐ to lower‐crustal levels is fault controlled and depends primarily on the permeability contrast between the fault zone and the crustal rocks. High fault/crust permeability ratios lead to channelized flow in the fault and shear zones while lower ratios allow leakage of the fluids from the fault into the crust. Buoyancy affects mainly flow patterns (more upward directed) and, to a lesser extent, temperature distributions (disturbance of the geothermal field by ~25 °C). Channelized fluid flow in the shear zone leads to strong vertical and horizontal thermal gradients, comparable to field observations. The oxygen isotope results show δ18O depletion concentrated along the fault and shear zones, similar to field data.
  PubDate: 2013-02-27T16:04:17.977995-05:
  DOI: 10.1002/ggge.20100
Rare earth element concentrations and Nd isotopes in the Southeast Pacific Ocean
- Abstract: Three vertical profiles of rare earth element concentrations and Nd isotopic compositions have been measured in the remote southeast Pacific Ocean. The three stations represent contrasting environments: the oligotrophic center of the gyre (station GYR), the “transition zone” east of the South Tropical Front (station EGY), and the Peru‐Chile upwelling marked by a pronounced oxygen minimum (station UPX). Rare earth concentrations display nutrient like vertical profiles except at UPX where surface waters are enriched. At this station Nd isotopic compositions are clearly more radiogenic than in the open ocean, suggesting that boundary exchange process is releasing lithogenic rare earth element from the volcanic Andes. Unexpected radiogenic values (εNd reaching –3.7) are also observed at 2000 m at station GYR in the Upper Circumpolar Deep Water that commonly have εNd values around –6. Exchange processes related to hydrothermal activity are suspected to produce this increase in εNd in the vicinity of the East Pacific Rise. These results provide some guidance for higher resolution studies planned in this region by the international GEOTRACES program.
Decadal volcanic deformation in the Central Andes Volcanic Zone revealed by InSAR time series
- Abstract: Decadal trends of volcanic deformation in the Central Andes Volcanic Zone are identified with Interferometric Synthetic Aperture Radar (InSAR) stacks and time series velocity maps covering an area 19–27°S and 66–69°W. We combine over 750 ERS and Envisat interferograms from 2 descending and 3 ascending tracks. These tracks cover 100,000 km2 and span 1992 to 2011. Our analysis extends observations at Cerro Blanco, Uturuncu, and Lazufre volcanic centers and uncovers two previously undocumented deformation centers; Cerro Overo in Northern Chile, and Putana Volcano in Southwest Bolivia. Cerro Overo exhibits a transition from steady −0.4 cm/yr deflation to 0.5 cm/yr inflation over several years. Putana Volcano underwent a short‐lived episode of uplift between 13 September 2009 and 31 January 2010, with a maximum uplift of 4.0 cm. Cerro Blanco continues −1.0 cm/yr deflation since 1995. Uplift at Lazufre began between 1997–2000 and has gradually accelerated to 3.5 cm/yr since 2005. Uturuncu volcano continues 1.0 cm/yr monotonic uplift since 1992, and shows evidence for a broad moat of subsidence surrounding the uplifting region. Four of the nine deformation events in the CVZ are not obviously associated with a particular volcanic edifice. Furthermore, there is significant spatial and temporal variability of these deformation events within a small geographic area.
Seismic stratigraphic evidence for glacial expanse during glacial maxima in the Yakutat Bay Region, Gulf of Alaska
- Abstract: Modern shelf bathymetry bordering the Gulf of Alaska exhibits shelf‐crossing sea valleys that suggest focused pathways for ice flow during glacial conditions. Using an integrated seismic data set between the present Yakutat and Alsek Sea Valleys, we investigate the glacial stratigraphic record in order to improve our understanding of the regional glacial system during maximum glacial conditions. Our investigations reveal four glacial unconformities, of which, the latter two are overlain by sediment packages 100 s of m thick. We suggest that these unconformities are indicative of ice advance phases during the Little Ice Age (LIA), the Last Glacial Maxima (LGM), and two pre‐LGM advances with glacial retreat sequences preserved from the youngest two. The advances were dominated by ice expanse from the Malaspina Glacial system and Alsek River districts rather than the Hubbard Glacial system and only show distinctive morainal bank development near the shelf edge and near the mouth of, or within, modern bays, fjords, or river valleys. This observation strongly supports rapid and continuous retreat from glacial maxima conditions during climatic warming. All, but the inferred LIA sequence, transgress the shelf and exhibit concentrated erosion in overdeepened troughs, analogous to cross‐shelf troughs similar to those observed on other high‐latitude glaciated shelves. The two Alaskan troughs discussed here may be end‐member examples of high‐sediment flux systems due to the temperate glacial setting combined with an actively exhuming orogen.
Upper mantle seismic structure beneath central East Antarctica from body wave tomography: Implications for the origin of the Gamburtsev Subglacial Mountains.
- Abstract: [1] The Gamburtsev Subglacial Mountains (GSM), located near the center of East Antarctica, are the highest feature within the East Antarctic highlands and have been investigated seismically for the first time during the 2007/2008 International Polar Year by the Gamburtsev Mountains Seismic Experiment. Using data from a network of 26 broadband seismic stations and body wave tomography, the P and S wave velocity structure of the upper mantle beneath the GSM and adjacent regions has been examined. Tomographic images produced from teleseismic P and S phases reveal several large‐scale, small amplitude anomalies (δVp = 1.0%, δVs = 2.0%) in the upper 250 km of the mantle. The lateral distributions of these large‐scale anomalies are similar in both the P and S wave velocity models and resolution tests show that they are well resolved. Velocity anomalies indicate slower, thinner lithosphere beneath the likely Meso‐ or Neoproterozoic Polar Subglacial Basin and faster, thicker lithosphere beneath the likely Archean/Paleoproterozoic East Antarctic highlands. Within the region of faster, thicker lithosphere, a lower amplitude (δVp = 0.5%, δVs = 1.0%) slow to fast velocity pattern is observed beneath the western flank of the GSM, suggesting a suture between two lithospheric blocks possibly of similar age. These findings point to a Precambrian origin for the high topography of the GSM, corroborating other studies invoking a long‐lived highland landscape in central East Antarctica, as opposed to uplift caused by Permian/Cretaceous rifting or Cenozoic magmatism. The longevity of the GSM makes them geologically unusual, however, plausible analogues exist, such as the 550 Ma Petermann Ranges in central Australia. Additional uplift may have occurred by the reactivation of pre‐existing faults, for example during the Carboniferous‐Permian collision of Gondwana and Laurussia.
Influence of rotation on the metal rain in a Hadean magma ocean
- Abstract: Today it is widely accepted that during its early evolution the Earth experienced a magma ocean that covered most its surface. The separation of metal from silicate was much facilitated in the environment of such a magma ocean. The differentiation mechanism is known as the ‘metal‐rain scenario’. Our study will focus on the settling dynamics of these metal droplets.Because of the low viscosity of molten silicate and a higher rotation period of the Earth at that time the rotation has a potentially strong influence on the dynamics of the magma ocean. We use numerical 3D fluid simulations to analyze the combined effect of strong rotation and convection on the settling of the iron droplets.We show that the influence of rotation on the settling depends on the latitude. At the poles the influence of rotation is only marginal. At the equator, three different scenarios can be distinguished. First, at low rotation rates, the particles form a dense layer at the bottom. Second, for higher rotation rates the particles stay mostly suspended and layers form in the temperature field. Third, at higher rotation rates the particles form a ribbon‐like structure in the middle of the box.The influence of rotation on the iron droplets may lead to a scenario where part of the iron is kept in the mantle instead of transported to the core. This would have a strong influence on the later states of the differentiation process and the amountof siderophile elements in the mantel.
High Resolution Estimate for the Depositional Duration of the Triassic Latemar Platform: A New Magnetostratigraphy and Magnetic Susceptibility Cyclostratigraphy from Basinal Sediments at Rio Sacuz, Italy
- Abstract: A new magnetostratigraphyand magnetic susceptibility cyclostratigraphyfrom Middle Triassic basinal sediments at Rio Sacuz, northern Italy, suggests a depositional duration of ~1myr formost of the670 m carbonate Latemar Platform, giving a high resolution estimate for the duration for the Latemar's deposition. The new magnetostratigraphy from Rio Sacuz clarifies the ambiguities in a previous magnetostratigraphic study of the Latemar caused by lightning strikeremagnetizations.Our Rio Sacuzstudy was unaffected by lightning. Using thermal demagnetization, we show a N‐R‐N‐R sequence at the Latemar‐correlated locality of Rio Sacuz. With a polarity interval duration of ~0.25‐0.5 myr for the Middle Triassic, this gives a depositional duration of ~1‐2 myr. Rock magnetic experiments show that the magnetic carrier is magnetite, suggesting a primary, depositional‐ageremanence .Measurement of magnetic susceptibility at one meter intervals from Rio Sacuz reveals eight short eccentricity cycles (~95‐125 kyr) bundled into slightly more than two 405 kyr long eccentricity cycles indicating a more precise duration for the Rio Sacuz section of 0.8 to 1 myr. The shorter depositional duration required by this study forces a sub‐Milankovitch time scaleof 1‐2 kyrfor the Latemar platform's meter‐scalerhythmic bedding and provides strong evidence for non‐orbitally driven climate change in the Middle Triassic.
Confounding effects of coral growth and high SST variability on skeletal Sr/Ca: implications for coral paleothermometry
- Abstract: Massive corals offer continuous records of climate locked within their skeleton, with the most commonly applied paleo‐thermometer being Sr/Ca. Recently, however, problems with Sr/Ca thermometry indicate that the intrinsic variance of single‐core Sr/Ca time series differs between cores. Here, we compare the Sr/Ca records and growth parameters of two Porites lutea colonies sampled from the same reef zone, 0.72 km apart, with two gridded SST datasets, ERSST and HadISST, off NE Madagascar. Specifically, we address seasonal and interannual variability as well as trend differences between records over the same 43 year period. The two gridded SST datasets showed strong seasonality and weak positive ENSO anomalies on a slow 43‐year warming trend at significantly different rates. Both the coral Sr/Ca records showed the same clear seasonality and similar amplitudes in SST. However, on interannual timescales they displayed diverging 43‐year Sr/Ca trends and opposite responses to weak ENSO anomalies. Moreover, their growth response also differed as one coral showed increasing extension/calcification rates and Sr/Ca ratios (cooling) over the 43 years, while the other coral showed decreasing extension/calcification rates and Sr/Ca ratios (warming). Further, during positive ENSO events the calcification rates of the two corals were negatively correlated, while skeletal density anomalies were opposite. Possible explanations to why these corals are so different may be related to the corals growth response to SST changes. The growth response of individual corals to increasing SST seems to be opposite, which in turn are likely related to biological factors. Consequently, coral growth responses explain much of the inter‐colony Sr/Ca variability.
Changes inplanktic and benthic foraminifer assemblages in the Gulf of Lions, off South France: Response to climate and sea level change from MIS 6 to MIS 11
- Abstract: A multidisciplinary study involving micropaleontological and geochemical tools was carried out in borehole PRGL 1‐4 (Promess 1), with the aim of reconstructing the impact of climate change and sea level variation between 133 ka and 406 ka in the upper slope of the Gulf of Lions. We used factor analysis to obtain three main benthic assemblages related to eutrophic, mesotrophic and oxygenated environments; planktic foraminifers were grouped as warm‐water and cold‐turbulent species. These results were compared with records of CaCO3 and major and trace elements (Al, Ca, K, Sr) as well as the C/N ratio of organic matter. Power and cross‐spectral analysis showed a straightforward relationship between precession minima and thermal stratification of the water column as well as the occurrence of eutrophic bottom conditions during lowstand periods and mesotrophic environments at times of highstand. These eutrophic‐mesotrophic oscillations, usually driven by global eustatic change, also involved regional variations in CaCO3 source to this environment.During periods of precession maxima, enhancement of northwesterly winds increased primary productivity by mixing, enhancing the percentage of cold‐turbulent species in the water column and the proportion of oxygenated benthic species on the bottom. During interglacial stages, these events were recorded by lower biogenic carbonate at the expense of higher silicate‐related components most likely due to a higher supply from Pyrenees rivers. The record of oxygenated benthic species can be a good proxy to monitor past changes in Winter Intermediate Water dynamics driven by northwesterly winds.
Upper‐mantle fabrics beneath the Northern Apennines revealed by seismic anisotropy
- Abstract: We image anisotropic structure of the upper mantle beneath the Northern Apennines based on body‐wave data collected during the RETREAT experiment (2003‐2006). Joint analysis of anisotropic parameters evaluated from independent data sets – teleseismic P‐wave travel times and shear‐wave splitting – allows us to identify regions of different fabrics both in the mantle lithosphere and in the sub‐lithospheric mantle. We recognize three regions – the Tyrrhenian, Adriatic and Transition – with their own anisotropic characteristics. The slab‐parallel flow prevails in the sub‐lithospheric mantle beneath the thin Tyrrhenian plate, while nearly slab‐perpendicular orientation of the high velocities dominates on the Adriatic side of the region. The asthenospheric‐flow pattern excludes a simple corner‐flow model to fit the fabric of the upper mantle in the syn‐convergent extensional tectonics and suggests the end of the subduction roll‐back. We map fossil anisotropy with inclined symmetry axes within two domains of the thick continental Adriatic lithosphere. We estimate the lithosphere thickness of the Tyrrhenian and Adriatic plates at ~50 km and ~80 km, respectively, the latter being subducted down to no more than ~200 km with indications of inherited frozen‐in anisotropic fabric. If a potential detachment at the northern end of the Apennine slab exists then it would have to be narrow and in its initial stage. Synthetic tests of the well‐known trade‐off between isotropic heterogeneity and evaluated anisotropic parameters, along with combining independent data sets, document a sufficient separation of both effects.
Velocity‐conductivity relations for cratonic lithosphere and their application: Example of Southern Africa
- Abstract: Seismic velocity is a function of bulk vibrational properties of the media whereas electrical resistivity is most often a function of transport properties of an interconnected minor phase. In the absence of a minor conducting phase then the two should be inter‐relatable, primarily due to their sensitivity to temperature variation. We develop expressions between shear wave velocity and resistivity for varying temperature, composition and water content based on knowledge from two kimberlite fields; Jagersfontein (Kaapvaal Craton) and Gibeon (Rehoboth Terrane). We test the expressions through comparison between a new high‐resolution regional seismic model, derived from surface wave inversion of earthquake data from Africa and the surrounding regions, and a new electrical image from magnetotelluric (MT) data recorded in SAMTEX (Southern African Magnetotelluric Experiment). The data‐defined robust linear regression between the two is found to be statistically identical to the laboratory‐defined expression for 40 wt ppm water in olivine. Cluster analysis defines five clusters that are all geographically spatially distinct and tectonically relate to (i) fast, cold and variably‐wet Kaapvaal Craton, (ii) fast and wet central Botswana, (iii) slow, warm and wet Rehoboth Terrane, (iv) moderately fast, cold and very dry southernmost Angola Craton, and (v) slow, warm and somewhat dry Damara Belt. From the linear regression expression and the MT image we obtain predicted seismic velocity at 100 km and compare it with that from seismic observations. The differences between the two demonstrates that the linear relationship between Vs and resistivity is appropriate for over 80% of Southern Africa. Finally, using the regressions for varying water content, we infer water content in olivine across Southern Africa.
Modes, tempo and spatial variability of Cenozoic cratonic denudation: The West African example
- Abstract: Quantifying long‐term erosion of tropical shields is crucial to constraining the role of lateritic regolith covers as prominent sinks and sources of CO2 and sediments in the context of long‐term Cenozoic climate change. It is also a key to understanding long‐term landform evolution processes operating over most of the continental surface and their control onto the sediment routing system. We study the surface evolution of West Africa over three erosion periods (~ 45‐24, ~ 24‐11 and ~ 11‐0 Ma) recorded by relicts of 3 sub‐continental scale lateritic paleolandsurfaces whose age is bracketed by 39Ar/40Ar dating of lateritic K‐Mn oxides. Denudation depths and rates compiled from 380 field stations show that despite heterogeneities confined to early‐inherited reliefs, the sub‐region underwent low and homogeneous denudation (~ 2‐20 m Ma‐1) over most of its surface whatever the considered time interval. This homogeneity is further documented by a worldwide compilation of cratonic denudation rates, over long‐term, intermediate and modern Cenozoic time scales (100 – 107 yr). These results allow defining a steady‐state cratonic denudation regime that is weathering‐limited i.e., controlled by the thickness of the (lateritic) regolith available for stripping. Steady‐state cratonic denudation regimes are enabled by maintained compartmentalization of the base levels between river knick points controlled by relief inheritance. Under such regimes, lowering of base levels and their fossilization are primarily imposed by long‐term eustatic sea level fall and climate rather than by epeirogeny. The expression of steady‐state cratonic denudation regimes in clastic sedimentary fluxes remains to be investigated.
Early Aptian paleoenvironmental evolution of the Bab Basin at the southern Neo‐Tethys margin: Response to global carbon‐cycle perturbations across Ocean Anoxic Event 1a
- Abstract: Lower Aptian carbonates in the Bab Basin at the southern Neo‐Tethys margin record significant environmental changes across Oceanic Anoxic Event 1a (OAE1a). A long‐lasting negative shift of carbon‐isotope ratios (δ13C) associated with a distinct decrease in oxygen‐isotope ratios (δ18O) in orbitolinid‐rich carbonates characterizes the onset of OAE1a (Livello Selli), supporting a hypothesis that a long‐lasting volcanic CO2 emission is a main cause of OAE1a, inducing global warming. A bloom of microencrusters (Lithocodium and Bacinella) across the proto‐Bab Basin occurred synchronously at the beginning of the subsequent positive δ13C excursion, responding to the global carbon‐cycle perturbations. The carbonates, formed during the OAE1a, show higher strontium‐isotope ratios (87Sr/86Sr) compared with those of global seawater; this was likely caused by a local influx of isotopically heavier strontium, along with nutrients, into the proto‐Bab Basin. These biotic proliferations were triggered by an increased nutrient supply induced by intensified continental weathering due to the global warming suggested by the increase in δ18O values. Spatial variations in the δ13C values among sites in the Bab Basin and its surrounding platform are related to local environmental factors, such as the degree of mixing of basin water with ocean water and local removal of 12C by metabolic activity at the platform‐top. The δ13C profile of the studied core indicates global removal of organic carbon of OAE1a began during the early stage of the second‐order transgression and lasted until the early stage of the highstand after the OAE1a. The Livello Selli corresponds to the early stage of this transgression.
Low‐volume intraplate volcanism in the Early/Middle Jurassic Pacific basin documented by accreted sequences in Costa Rica
- Abstract: Countless seamounts occur on Earth that can provide important constraints on intraplate volcanism and plate tectonics in the oceans, yet their nature and origin remain poorly known due to difficulties in investigating the deep ocean. We present here new lithostratigraphic, age and geochemical data from Lower/Middle Jurassic and Lower Cretaceous sequences in the Santa Rosa accretionary complex, Costa Rica, which offer a valuable opportunity to study a small‐sized seamount from a subducted plate segment of the Pacific basin. The seamount is characterized by very unusual lithostratigraphic sequences with sills of potassic alkaline basalt emplaced within thick beds of radiolarite, basaltic breccia and hyaloclastite. An integration of new geochemical, biochronological and geochronological data with lithostratigraphic observations suggests that the seamount formed ~175 Ma ago on thick oceanic crust away from subduction zones and mid‐ocean ridges. This seamount travelled ~65 Ma in the Pacific before accretion. It resembles lithologically and compositionally “petit‐spot” volcanoes found off Japan, which form in response to plate flexure near subduction zones. Also, the composition of the sills and lava flows in the accreted seamount closely resembles that of potassic alkaline basalts produced by lithosphere cracking along the Line Islands chain. We hypothesize based on these observations, petrological constraints and formation of the accreted seamount coeval with the early stages of development of the Pacific plate that the seamount formed by extraction of small volumes of melt from the base of the lithosphere in response to propagating fractures at the scale of the Pacific basin.
Structures of the oceanic lithosphere‐asthenosphere boundary: mineral physics modeling and seismological signatures
- Abstract: We explore possible models for the seismological signature of the oceanic lithosphere‐asthenosphere boundary (LAB) using the latest mineral‐physics observations. The key features that need to be explained by any viable model include (1) a sharp (
Investigating δ13C and ∆14C within Mytilus californianus shells as proxies of upwelling intensity
- Abstract: Along the west coast of North America, climate and marine productivity is affected by seasonal to interannual changes in coastal upwelling. Our understanding of upwelling variability in the past is limited by the short duration of instrumental records. Changes in upwelling intensity are expected to further affect the 13C/12C (δ13C) and radiocarbon (∆14C) content of marine dissolved inorganic carbon (DIC) due to variable mixing of old, upwelled seawater into surface waters. If seasonal variations in the δ13C of DIC are recorded in marine bivalve shells, they may provide valuable information about the extent of upwelling in the past. Comparison of modern Mytilus californianus shells from South California with a 5‐year time series of coastal seawater ∆14C and δ13C allows an assessment of the suitability of Mytilus shell ∆14C and δ13C as proxies of upwelling intensity. We show that both absolute values and the seasonal range in ∆14C are preserved in shell ∆14C allowing its use as an indicator of upwelling intensity. Interpretation of shell δ13C is more problematic, with the δ13C of shell carbonate lower than seawater DIC by variable amounts (ranging from 0.5‐1.5 ‰) due to the incorporation of metabolic carbon. The spatial and temporal variability observed in specimens that grew during the severe El Niño event of 1997‐1998 demonstrates how a transect of shells spanning the western North American coastline can be used to reconstruct large‐scale patterns of seawater ∆14C variability for specific intervals of interest, such as those associated with ENSO‐type phenomena.
Tidal Modulation of Continuous Non‐Volcanic Seismic Tremor in the Chile Triple Junction Region
- Abstract: We located continuous seismic tremorwith coherent amplitude wavetrainsin the Chile Ridge subduction region (~46.5°S)in two clusters north and south of the Chonos Archipelago, between the Chile trench and the North Patagonian forearc. Tremor persisted from December 2004 to February 2007 (the entire period of the Chile Ridge Subduction Project temporary seismic deployment), and lasted > 17 hours on six occasions. Tremor in the more active southern cluster reached a maximum duration of 48 hours, and we observed no more than 3 continuous days without tremor activity. The cluster locations coincide with the surface projections of subducted transform faultsformed atthe Chile ridge. We also detected simultaneous, co‐located low‐frequency microearthquakes with well‐defined impulsive waves within the tremor signals distributed from the surface to 40 km depth, suggesting tremors and earthquakes are part of the same process. The periodicity of tremor duration is strongly correlated with semi‐diurnal, diurnal, and long period tides, M2, N2, K1, O1, P1, and Mm (12.421, 12.000, 23.934, 25.819, 24.066 hrs and 27.555 days, respectively). We found a significant correlation between tremor occurrence and earth tides when tidal stressis calculated forthe slip plane of a right‐lateral strike‐slip fault withstrikeN950E, which isnear parallelto subducted transform faults (N780E) of the Chile Ridge,indicatingthat the very small stresses resulting from the combination of ocean loading and solid earth tides (~1kPa) are sufficient to facilitate or suppress tremor production; tremors occur when shear stressesare maximum and wane or are low whenshear stresses are minimum.
Annual Variation of Coastal Uplift in Greenland as an Indicator of Variable and Accelerating Ice Mass Loss
- Abstract: Seasonal melting of the coastal part of the Greenland ice sheet is investigated using GPS vertical displacement data from coastal stations, combined with data on atmospheric and ocean temperatures. Using a high pass filter and cubic spline models, we estimate five variables describing seasonal uplift, a proxy for proximal mass loss, including duration of the melt season and the amount of summer uplift. Our analysis shows both temporal and spatial variations of uplift. Southern coastal Greenland experienced anomalously large uplift in summer 2010, implying significant melting that year. However, the northwest coast did not experience significant change in uplift at that time. Our data suggest that a combination of warm summer air temperature and warm sub‐surface ocean water temperature drove the large mass losses in 2010. Using the uplift pattern of 2008 ‐ 2010, and comparing to atmospheric data and ocean water temperature data, we show that warm Irminger water exerted significant influence on coastal melting in southeastern, southern and southwestern Greenland, reaching about 69ºN in 2010. North of this, Irminger water did not exert significant influence, in effect defining the northward limit of the sub‐polar gyre for that year. Thus, short term variability in the coastal GPS uplift signal can be used to infer an oceanographic parameter that has a critical influence on Greenland ice sheet health.
High‐resolution 3D numerical modeling of thrust wedges: Influence of décollement strength on transfer zones
- Abstract: The mechanics and dynamics of thin‐skinned compressible thrust wedges with prescribed offsets in the backstop, i.e. transfer zones, are investigated using a three‐dimensional finite difference numerical model with a visco‐brittle/plastic rheology. The main questions addressed are: (i) What is the influence of the initial length of the backstop offset and (ii) what is the effect of the frictional strength of the main décollement on the structural evolution of the brittle wedges along such transfer zones' Results show that the shorter the backstop offset, the earlier these two thrust planes connect, forming a curved frontal thrust along the entire width of the model. Younger, in‐sequence thrusts are formed parallel to this curved shape. Long backstop offsets produce strongly curved thrust faults around the indenting corner. Simulations with a weak basal friction evolve towards almost linear frontal thrusts orthogonal to the bulk shortening direction. Increased basal drag in models with a strong décollement favours propagation of the backstop offset into a transfer zone up to the frontal thrust. These simulations revealed that surface tapers of the wedge in front of the backstop promontory are larger than what the critical wedge theory predicts, whereas the tapers on the other side of the transfer zone are smaller than analytical values. This difference is amplified with increasing length of the backstop offset and/or strength of the décollement. Modelled surface elevation schemes reproduce well the topographic patterns of natural orogenic systems such as the topographic low along the Minab‐Zendan transform/transfer fault between the Zagros and Makran.
Crustal structure across the Costa Rican Volcanic Arc
- Abstract: Island arcs are proposed to be essential building blocks for the crustal growth of continents, however island arcs and continents are fundamentally different in bulk composition: mafic and felsic respectively. The substrate upon which arcs are built (oceanic crust vs. large igneous province) may have a strong influence on crustal genesis. We present results from an across‐arc wide‐angle seismic survey of the Costa Rican volcanic front, which test the hypothesis that juvenile continental crust is actively forming at this location. Travel‐time tomography constrains velocities in the upper arc to a depth of ~15 km where average velocities are
LABORATORY OBSERVATION OF ACOUSTIC FLUIDIZATION IN GRANULAR FAULT GOUGE AND IMPLICATIONS FOR DYNAMIC WEAKENING OF EARTHQUAKE FAULTS
- Abstract: Several lines of evidence, including remote triggering of earthquakes and modulation of seismic tremor by Earth tides, suggest that faults weaken when subject to shaking and dynamic stresses associated with the passage of seismic waves. However, the origin of such dynamic weakening is poorly understood. Here, we explore the role of acoustic resonance for dynamic fault weakening using laboratory measurements. Experiments were conducted using a split Hopkinson pressure bar assembly, with dynamic stressing via impact loading. Samples were composed of crushed rock particles from mine tailings with a particle size distribution similar to that found in natural fault gouge. We used pulse shaper techniques and carefully evaluated dynamic stresses recorded at the front and rear of the sample to ensure that dynamic force balance was satisfied. Our experiments document acoustic‐induced fluidization and dramatic dynamic weakening. Frictional strength and elastic modulus of simulated fault gouge are reduced by a factor of five to tenvia acoustic fluidization. We find a threshold acoustic pressure for fluidization that varies systematically with gouge zone properties. Our observations could help explain dynamic fault weakening and triggering of earthquake fault slip by dynamic stressing.
Constraining clay hydration state and its role in active fault systems
- Abstract: To understand the role of hydrated clay minerals in active fault systems, a humidity chamber connected to an X‐ray diffractometer was used to determine the adsorption of water onto and/or into the crystal structure of smectite. This new type of analysis was done under specific temperature and humidity conditions, using powdered clay size fractions (< 2 micron) of rock samples from the San Andreas Fault (USA) and the Nankai Trough (Japan). Pressure cannot be controlled, but does not significantly affect clay swelling at shallow conditions. Air‐ dried samples show a discrete smectite phase that swells after traditional ethylene glycolation to an interlayer distance of 1.5 and 1.7 nm. Using the humidity chamber, however, the samples show a shorter interlayer distance, between 1.09 and 1.54 nm. Based on our analysis we show that (i) ethylene glycol overestimates the size of the interlayer space, and therefore water content, so is a crude maximum only, (ii) interlayer swelling occurs in smectite clay minerals at all temperatures between 25‐95 °C, and (iii) particle orientation increases with increasing humidity, indicating a higher mobility of smectite from interlayer hydration. Detailed characterization of the hydration state of smectite under original conditions is critical for understanding of clay‐fluid interaction, the mechanical behavior during fault displacements, and fluid budgets at depth. We propose that humidity chamber experiments should be the new standard procedure to constrain swelling characteristics of natural and synthetic clay minerals.
An Adaptive Staggered Grid Finite Difference Method for Modeling Geodynamic Stokes Flows with Strongly Variable Viscosity
- Abstract: Here we describe a new staggered grid formulation for discretizing incompressible Stokes flow which has been specifically designed for use on adaptive quad‐tree type meshes. The key to our new adaptive staggered grid (ASG) stencil is in the form of the stress conservative finite difference constraints which are enforced at the “hanging” velocity nodes between resolution transitions within the mesh. The new ASG discretization maintains a compact stencil, thus preserving the sparsity within the matrix which both minimizes the computational cost and enables the discrete system to be efficiently solved via sparse direct factorizations or iterative methods. We demonstrate numerically that the ASG stencil is (i) stable and does not produce spurious pressure oscillations across regions of grid refinement which intersect discontinuous viscosity structures and (ii) possesses the same order of accuracy as theclassical non‐adaptive staggered grid discretization.Several pragmatic error indicators that are used to drive adaptivity are introduced in order to demonstrate the superior performance of the ASG stencil over traditional non‐adaptive grid approaches. Furthermore, to demonstrate the potential of this new methodology, we present geodynamic examples of both lithospheric and planetary scales models.
How do “ghost transients” from past earthquakes affect GPS slip rate estimates on southern California faults'
- Abstract: In this study, we investigate the extent to which viscoelastic velocity perturbations (or “ghost transients”) from individual fault segments can affect elastic block model‐based inferences of fault slip rates from GPS velocity fields. We focus on the southern California GPS velocity field, exploring the effects of known, large earthquakes for two end‐member rheological structures. Our approach is to compute, at each GPS site, the velocity perturbation relative to a cycle average for earthquake cycles on particular fault segments. We then correct the SCEC CMM4.0 velocity field for this perturbation and invert the corrected field for fault slip rates. We find that if asthenosphere viscosities are low (3 × 1018 Pa s) the current GPS velocity field is significantly perturbed by viscoelastic earthquake cycle effects associated with the San Andreas Fault segment that last ruptured in 1857 (Mw = 7.9). Correcting the GPS velocity field for this perturbation (or “ghost transient”) adds about 5 mm/a to the SAF slip rate along the Mojave and San Bernardino segments. The GPS velocity perturbations due to large earthquakes on the Garlock Fault (most recently, events in the early 1600's) and the White Wolf Fault (most recently, the Mw = 7.3 1952 Kern County earthquake) are smaller and do not influence block‐model inverted fault slip rates. This suggests that either the large discrepancy between geodetic and geologic slip rates for the Garlock Fault is not due to a ghost transient, or that unmodeled transients from recent Mojave earthquakes may influence the GPS velocity field.
High Resolution ROV Mapping of a Slow‐Spreading Ridge: Mid‐Atlantic Ridge 45˚N
- Abstract: Axial Volcanic Ridges (AVRs) are found on most slow spreading mid‐ocean ridges and are thought to be the main locus of volcanism there. In this study we present high‐resolution mapping of a typical, well‐defined AVR on the Mid‐Atlantic Ridge at 45°N. The AVR is characterised by ‘hummocky terrain’, composed typically of pillowed or elongate pillowed flanks with pillowed or lobate lava flow summits, often with small haystacks sitting on their highest points. The AVR is surrounded by several areas of ‘flat seafloor’, composed of lobate and sheet lava flows. The spatial and morphological differences between these areas indicate different eruption processes operating on and off the AVR. Volcanic fissures are found all around and on the AVR, although those with the greatest horizontal displacement are found on the ridge crest and flat seafloor. Clusters of fissures may represent volcanic vents. Extremely detailed comparisons of sediment coverage and examination of contact relations around the AVR suggest that many of the areas of flat seafloor are a similar age or younger than the hummocky terrain of the AVR. Additionally, all the lavas surveyed have similar degrees of sediment cover, suggesting that the AVR was either built or resurfaced in the same 50 ka timeframe as the flat seafloor.
Seismic anisotropy around subduction zones: Insights from three‐dimensional modeling of upper mantle deformation and SKS splitting calculations
- Abstract: Inferring the circulation of the mantle around subducting plates from surface measurements of shear wave splitting patterns remains to date elusive. To assist the interpretation of the seismic signal and its relation with the mantle circulation pattern, we present a new methodology to compute the seismic anisotropy directly from the flow in the upper mantle of 3‐D numerical models of Earth‐like subduction. This computational strategy accounts for the non‐steady‐state evolution of subduction zones yielding mantle fabrics that are more consistent with the deformation history than previously considered. In the subduction models, a strong mantle fabric develops throughout the upper mantle with a magnitude of the anisotropy that is proportional to the amount of subduction and is independent of the subduction rate. The sub‐slab upper mantle is characterized by two domains with different fabrics: at shallow depth, the mantle entrained with the subducting slab develops trench‐perpendicular directed anisotropy due to simple shear deformation, while in the deeper mantle, slab rollback induces pure shear deformation causing trench‐parallel extension and fast seismic directions. Subducting plate advance favors the development of the fabric in the entrained mantle domain, while slab retreat increases the trench‐parallel anisotropy in the deeper upper mantle. In the deeper domain, the strength of the fabric is proportional to the horizontal divergence of the flow and weakens from the slab edges toward the center. As such, strong trench‐parallel anisotropy forms below retreating and relatively narrow slabs or at the margins of wider plates. The synthetic SKS splitting patterns calculated in the fore arc are controlled by the magnitude of the anisotropy in the upper domain, with trench‐perpendicular fast azimuths in the center of large plates and trench parallel toward the plate edges. Instead, above relatively narrow, retreating slabs (≤600 km and low subduction partitioning ratio (SPR)), azimuths are trench parallel due to the strong anisotropy in the lower sub‐slab domain. In all models, the anisotropy in the back arc and on the sides of the subducting plate is, respectively, trench perpendicular and sub‐parallel to the return flow at depth. Results from our regional scale models may help to infer the flow and composition of the upper mantle by comparison with the wide range of subduction zones seismic data observed globally.
Composition of plume‐influenced mid‐ocean ridge lavas and glasses from the Mid‐Atlantic Ridge, East Pacific Rise, Galápagos Spreading Center, and Gulf of Aden
- Abstract: The global mid‐ocean ridge system is peppered with localities where mantle plumes impinge on oceanic spreading centers. Here, we present new, high resolution and high precision data for 40 trace elements in 573 samples of variably plume‐influenced mid‐ocean ridge basalts from the Mid‐Atlantic ridge, the Easter Microplate and Salas y Gomez seamounts, the Galápagos spreading center, and the Gulf of Aden, in addition to previously unpublished major element and isotopic data for these regions. Included in the data set are the unconventional trace elements Mo, Cd, Sn, Sb, W, and Tl, which are not commonly reported by most geochemical studies. We show variations in the ratios Mo/Ce, Cd/Dy, Sn/Sm, Sb/Ce, W/U, and Rb/Tl, which are expected not to fractionate significantly during melting or crystallization, as a function of proximity to plume‐related features on these ridges. The Cd/Dy and Sn/Sm ratios show little variation with plume proximity, although higher Cd/Dy may signal increases in the role of garnet in the mantle source beneath some plumes. Globally, the Rb/Tl ratio closely approximates the La/SmN ratio, and thus provides a sensitive tracer of enriched mantle domains. The W/U ratio is not elevated at plume centers, but we find significant enrichments in W/U, and to a lesser extent the Mo/Ce and Sb/Ce ratios, at mid‐ocean ridges proximal to plumes. Such enrichments may provide evidence of far‐field entrainment of lower mantle material that has interacted with the core by deeply‐rooted, upwelling mantle plumes.
Rock‐magnetic artifacts on long‐term relative paleointensity variations in sediments
- Abstract: Long‐term changes of geomagnetic field intensity, including possible dependence on lengths of polarity intervals, provide fundamentally important information for understanding the geodynamo. A positive correlation between paleointensity and polarity interval length was previously suggested from an Oligocene (ca. 23–34 Ma) relative paleointensity record at Deep Sea Drilling Program Site 522 in the Atlantic Ocean, which is the only continuous paleointensity data set published so far for this age interval. We have conducted a paleomagnetic study of Eocene to Oligocene sediments at three sites in the eastern equatorial Pacific Ocean. Our objectives include revisiting the issue of the paleointensity‐polarity length correlation. Magnetic properties of the sediments meet the frequently used criteria for reliable relative paleointensity estimation. Although short‐wavelength normalized remanence intensity fluctuations associated with polarity boundaries and possible geomagnetic excursions agree among the three sites, long‐term changes are inconsistent. Apparent positive correlation between normalized intensity and polarity length was observed, but the normalized intensity has an obvious anti‐correlation with the ratio of anhysteretic remanent magnetization (ARM) to isothermal remanent magnetization (IRM), which is mainly controlled by the relative abundance of biogenic and terrigenous magnetic minerals. Furthermore, the normalized intensity correlates with sedimentation rate. These facts indicate a lithological contamination on the normalized intensity records. The dependence on ARM/IRM and sedimentation rate is also evident at Site 522. It is inferred that variations in sedimentation rate and the relative abundance of biogenic magnetite on depositional remanent magnetization acquisition efficiency may not be well compensated by the normalization. It is therefore premature to conclude that stronger geomagnetic fields were recorded during longer polarity intervals from currently available normalized intensity records.
High resolution imaging of the melt distribution in partially molten upper mantle rocks: evidence for wetted two‐grain boundaries
- Abstract: We determine the 3‐D melt geometry of partially molten samples of dunite containing 1.6 and 3.6 volume % of basaltic melt that were held in a piston cylinder apparatus at upper mantle conditions for 430 hours. Our approach involves serial sectioning and high‐resolution field emission SEM imaging. Resolution is such that melt pockets approaching ~30 nm in size were resolved while covering an area of ~300 by 230 micron. The principal result of this study is to show that thin layers (typically 100 nm or less in thickness) between adjacent grains observed in 2‐D images persist with depth, and are therefore wetted two‐grain boundaries. Melt geometries most closely resembling triple junction tubules of the isotropic equilibrium model occur at all three grain edges, but are small compared to larger pockets. The wetted grain boundaries at a dihedral angle >0° for this system are inferred to be due to slow expulsion of melt from dynamically reorganizing grain boundaries during steady state grain growth. The attenuation peak observed in forced torsional oscillation experiments on similar samples ( [18]) is likely related to the wetted grain boundaries. Grain growth, driven by surface energy reduction, occurs also at the larger grain sizes expected for the mantle. This suggests the presence of wetted grain boundaries and significant velocity reduction and attenuation in partially molten upper mantle, as observed for example in back‐arc basins.
Phase transitions of harzburgite and buckled slab under eastern China
- Abstract: Phase relations in harzburgite have been determined between 14 and 24 GPa and 1473 and 1673 K. At 1673 K, harzburgite transformed to wadsleyite + garnet + clinopyroxene below 19 GPa and decomposed into an assemblage of ringwoodite + garnet + stishovite above 20 GPa. Certain amounts of akimotoite were produced at still higher pressures (22–23 GPa). Finally perovskite and magnesiowüstite were found to coexist with garnet at 24.2 GPa. Compositions of all the phases were analyzed and elemental partitioning coefficients were determined among co‐existing phases. Combining our experimental data with available thermoelastic properties of major minerals in the earth's mantle, we modeled the velocity and density signatures of the stagnated oceanic slabs in the mantle transition zone (MTZ) under eastern China, based on kinematic slab thermal structure analysis. We examined two end‐member slab models: a conventional straight slab with deformation thickening, and an undulated slab with an oscillating wavelength of 200 km. We found that an undulated (buckled) slab model yields velocity anomalies (about 1–2% for Vp) that are consistent with seismic tomography models, taking into account of low‐pass filtering effects in seismic tomography studies. On the other hand, straight slab models yield velocity anomalies that are too high comparing with seismic tomography models. Our models provide important constraints on the thermal structure, mineralogy, composition, density, and velocities of slab materials in the MTZ.
Depths and Temperatures of <10.5 Ma Mantle Melting and the Lithosphere‐Asthenosphere Boundary below southern Oregon and northern California
- Abstract: Plagioclase and spinel lherzolite thermometry and barometry is applied to an extensive geochemical dataset of young (
Use of cosmogenic 129I to constrain numerical models 1 of fluid flow in marine sediments: Application to the 2 Blake Ridge Hydrate Province
- Abstract: Concentrations of cosmogenic iodine, 129I, in the pore fluid of marine sediments often indicate that the pore fluid is much older than the host sediment, even when vertical flow due to sediment compaction is taken into account. Old pore fluid has been used in previous studies to argue for pervasive upward fluid flow and a deep methane source for hydrate deposits. Alternatively, old pore fluid age may reflect more complex flow patterns. We use a two‐dimensional numerical transport model to account for the effects of topography and fractures on pore fluid pathlines when sediment permeability is anisotropic. We find that fluid focusing can cause significant lateral migration as well as regions where downward flow reverses direction and returns toward the seafloor. Longer pathlines can produce pore fluid ages much older than that expected with a one‐dimensional compaction model. For steady‐state models with geometry representative of Blake Ridge (USA), a well‐studied hydrate province, we find pore fluid ages beneath regions of topography and within fractured zones that are up to 70 Ma old. Our results suggest that the measurements of 129I/127I reflect a mixture of new and old pore fluid. However, old pore fluid need not originate at great depths. Methane within pore fluids can travel laterally several kilometers, implying an extensive source region around the deposit. This type of focusing should aid hydrate formation beneath topographic highs. © 2013 American Geophysical Union. All rights reserved
Apparent overconsolidation of mudstones in the Kumano Basin of southwest Japan: Implications for fluid pressure and fluid flow within a forearc setting
- Abstract: The Kumano Basin is located in the Nankai Trough subduction zone of southwest Japan. During the past 1.6 million years, approximately 800 meters of sandy turbidites and hemipelagic mud were deposited near the distal edge of the forearc basin, at Site C0002 of the Integrated Ocean Drilling Program. Constant‐rate‐of‐strain consolidation tests yield estimates of in situ permeability that range from 2.6 x 10‐17 m2 to 2.5 x 10‐18 m2; overconsolidation ratios range from 1.7 to 2.6, and values of the compression index range from 0.39 to 0.78. Several processes contributed to the apparent overconsolidation. Strata dip toward land, and pore fluids probably migrate up‐dip and vent along a bathymetric notch near the seaward edge of the basin. Efficient lateral drainage through sandy turbidites has kept pore pressures within interbeds of mudstone at (or close to) hydrostatic. In addition, alteration of dispersed volcanic glass, precipitation of authigenic clay minerals, and collapse of random grain fabric has probably strengthened the bonding among grains. Cementation is particularly likely within the lower basin (unit III), where values of porosity remain anomalously high. If fluid overpressures (and underconsolidation) exist anywhere within the basin, the most likely loci are where sandy turbidites terminate against impermeable mudstones along landward‐dipping on‐lap surfaces. Those types of on‐lap geometries, in addition to structural closures, might provide promising targets for oil/gas accumulation in other forearc basins, particularly where petroleum source rocks have been buried to the optimal depths of catagenesis.
Mantle composition controls the development of an Oceanic Core Complex
- Abstract: The thickness and continuity of oceanic crust is variable. Slow‐spreading ridge segments often contain areas of ‘amagmatic’ or tectonic extension, exposing areas of lower‐crust and upper‐mantle, and having little or no recent volcanism. These are interspersed with areas of ‘normal’ volcanic crust generated by ‘robust’ magmatic accretion. Tectonic spreading is accommodated by displacement on low‐angle extensional detachment faults, forming Oceanic Core Complexes. Although ‘amagmatic’ extension appears to be common at slow spreading rates, little is known about the mechanisms that drive the transition from magmatic spreading. Here, we report results from a detailed study of the Mid‐Atlantic Ridge (13°N‐14°N) and show, paradoxically, that despite the presence of several Core Complexes, melt production remains similar along the present‐day spreading axis, which erupts homogeneous ‘normal’ mid‐ocean ridge basalt. However, melt production during formation of the older crust off‐axis was derived from substantially lower degrees of melting of a heterogeneous mantle. During this magmatically restricted phase, melt production was limited by source composition. Small volumes of an enriched basalt (M1) were produced, derived from low‐fraction melts of enriched compositional heterogeneities embedded in an otherwise compositionally depleted upper‐mantle, which, in turn, erupted low‐fraction incompatible‐element‐poor basalts (M2). As a consequence of low magma flux, the crust was thin and insufficient to fully accommodate seafloor spreading. Faulting of this thin crust resulted in the development of detachment faults and the formation of OCCs. Thus we propose that periods of low melt production, resulting directly from depleted, heterogeneous mantle drives the transition from magmatic to amagmatic spreading. © 2013 American Geophysical Union. All rights reserved.
Seasonal moisture sources and the isotopic composition of precipitation, rivers and carbonates across the Andes at 32.5 ‐ 35.5°S
- Abstract: Constraining the influence of different moisture sources across the flanks of mountain ranges is important for understanding tectonic, geomorphic and paleoclimate problems at geologic timescales, as well as evaluating climate change and water resources on human timescales. The stable isotope compositions of stream waters and precipitation are an ideal tool for this task. This study reports the results of a two‐year monthly precipitation sampling campaign on the eastern flank of the Andes in the Mendoza Province of Argentina, which began in September 2008, in the Mendoza Province of Argentina. A total of 104 precipitation samples spanning some 2500 m of relief from 9 sites were analyzed for δD and δ18O. In addition, 81 samples from Andean rivers collected on both sides of the range in 2002 and 2007 were analyzed. We employ a Rayleigh isotope fractionation modeling approach to explore spatial and temporal variations in precipitation and river water compositions. The results indicate that precipitation on the eastern slopes of the Andes at ~33°S, at elevations above 2 km, is largely derived from a westerly, Pacific‐source component and a mixture of easterly and westerly sources below 2 km. Further South at ~35°S, river water compositions exhibit a strong winter influence. At 33°S, rivers have an isotopic minimum of ~‐18‰, across the core of the range, which has an average elevation of 4000 m, and are topographically offset from similar isotopic values of precipitation by +1000 m. Comparison of precipitation and river water data with temperature‐corrected δ18O estimates from pedogenic carbonates illustrates that carbonates capture the range of variability observed in modern precipitation and Rayleigh fractionation models. © 2013 American Geophysical Union. All rights reserved.
Stochastic modelling of the Earth's magnetic field: inversion for covariances over the observatory era
- Abstract: Inferring the core dynamics responsible for the observed geomagnetic secular variation requires knowledge of the magnetic field at the core mantle boundary together with its associated model covariances. However, all currently available field models have been built using regularization conditions, which force the expansions in the spatial and time domains to converge, but also hinders the calculation of reliable second order statistics. To tackle this issue, we propose a stochastic approach that integrates, through time covariance functions, some prior information on the time evolution of the geomagnetic field. We consider the time series of spherical harmonic coefficients as realizations of a continuous and differentiable stochastic process. Our specific choice of process, such that it is not twice differentiable, mainly relies on two properties of magnetic observatory records (time spectra, existence of geomagnetic jerks). In addition, the required characteristic times for the low degree coefficients are obtained from available models of the magnetic field and its secular variation based on satellite data. We construct the new family COV‐OBS of field models spanning the observatory and satellite era of 1840–2010. These models include the external dipole and permit sharper time changes of the internal field compared to previous regularized reconstructions. The a posteriori covariance matrix displays correlations in both space and time, which should be accounted for through the secular variation error model in core flow inversions and geomagnetic data assimilation studies. © 2013 American Geophysical Union. All rights reserved.
In Situ Stress and Pore Pressure in the Kumano Forearc Basin, offshore SW Honshu from Down‐hole Measurements During Riser Drilling
- Abstract: In situ stress and pore pressure are key parameters governing rock strength, yet direct measurements of these quantities are rare. During Integrated Ocean Drilling Program (IODP) Expedition #319, we drilled through a forearc basin at the Nankai subduction zone, and into the underlying accretionary prism. We used the Modular Formation Dynamics Tester tool (MDT) for the first time in IODP, to measure in situ minimum stress, pore pressure, and permeability at 11 depths between 729.9‐1533.9 mbsf. Leak‐off testing at 708.6 mbsf conducted as part of drilling operations provided a second measurement of minimum stress. The MDT campaign included 9 single probe (SP) tests to measure permeability and in situ pore pressure, and 2 dual packer (DP) tests to measure minimum principal stress. Permeabilities defined from the SP tests range from 6.53 x 10‐17 ‐ 4.23 x 10‐14 m2. Pore fluid pressures are near hydrostatic throughout the section, despite rapid sedimentation. This is consistent with the measured hydraulic diffusivity of the sediments, and suggests that the forearc basin should not trap overpressures within the upper plate of the subduction zone. Minimum principal stresses are consistently lower than the vertical stress. We estimate the maximum horizontal stress from wellbore failures at the leakoff test and shallow MDT DP test depths. The results indicate a normal or strike‐slip stress regime, consistent with the observation of abundant active normal faults in the seaward‐most part of the basin, and a general decrease in the fault activity in the vicinity of Site C0009.
The mode of deformation in the orogenic mid‐crust revealed by seismic attribute analysis
- Abstract: The processes and the structures within the middle and the lower crust are intimately related to the evolution of orogenies, but more detailed knowledge is needed of these deep crustal processes. Seismic reflection data, chiefly displayed in amplitude, are commonly used to interpret deep crustal structures. The strongest amplitude events are probably related to lithological changes and do not necessarily correspond to geological structures, tectonic boundaries, or deformation fabrics. Furthermore, the detailed structures and the strain distribution between the interpreted structural boundaries remain obscure. Here, we show an example of how seismic attributes, combined with the seismic facies interpretation technique, can be used to enhance 2D seismic reflection data from the Palaeoproterozoic Svecofennian crust of southern Finland, to reveal unprecedentedly detailed information about the deformation fabrics within the mid‐crust of a collisional orogen. The images are plausibly interpreted to show that the extension/lateral flow of the orogenic middle and lower crust was mainly accommodated by km‐scale S‐C′ structures. The structures form penetrative deformation fabrics which are correlated with outcrop observations. The successful enhancement of the seismic data confirms the ductile extension affecting hot orogenic crust, and gives new information about the strain distribution of the regional, syn‐ to late‐orogenic deformation. The seismic attribute method and the seismic facies interpretational approach described in this paper should be applicable to other seismic datasets from the crystalline basement.
Refining benthic foraminiferal Mg/Ca‐temperature calibrations using core‐tops from the western tropical Atlantic: Implication for paleotemperature estimation
- Abstract: Benthic foraminiferal Mg/Ca has been shown to have great potential as a proxy for reconstructing deep water temperatures. However, the exact relationship between Mg uptake in benthic foraminifera and temperature is still ambiguous and further exploration and refinement is much needed to reduce uncertainties associated with the method. Here, we present new core‐top Mg/Ca data from benthic foraminiferal species from the lower part of the thermocline in the western tropical Atlantic (northern Brazilian margin). This area is unusual in that the changes in carbonate chemistry along the transect are very small, making it an ideal region for isolating and studying the role of temperature in the incorporation of Mg into the benthic shells. Our esults show that benthic foraminiferal Mg/Ca largely reflects temperature in this area. Our data are combined with previously published data to produce new and improved Mg/Ca calibration equations for a number of benthic foraminiferal species within the Atlantic Ocean. Our study provides the first C. wuellerstorfi Mg/Ca data for the 4 to 6 °C temperature range and indicates that C. wuellerstorfi Mg/Ca is strongly controlled by temperature. As a result, the newly established C. wuellerstorfi calibration over the entire 0‐6 °C temperature range is significantly improved with respect to previous published C. wuellerstorfi calibrations limited to the coldest part between 0 and 4 °C. Other benthic species (Cibicidoides kullenbergi, Globocassidulina subglobosa, Uvigerina peregrina and Oridorsalis umbonatus) have also been studied, although these results are less conclusive.
Re‐evaluating genetic models for porphyry Mo mineralization at Questa, New Mexico: Implications for ore deposition following silicic ignimbrite eruption
- Abstract: The Questa porphyry Mo deposit in New Mexico provides a unique opportunity to study the relationship between pluton assembly and mineralization in a long‐lived volcanic field. Magmatism along the caldera margin initiated at ~ 25.20 Ma and continued for ~ 770 ka. During this time, the emplacement of mineralizing intrusions progressed westward and culminated in the assembly of the Questa Mo deposit between 24.76 Ma and 24.50 Ma. Molybdenite Re/Os geochronology shows that mineralization occurred in multiple pulses without thermal resetting of the chronometer. Because most of the molybdenite samples used in this study are from previous fluid inclusion studies, we treat Re/Os molybdenite as a new thermochronometer. Molybdenite Re/Os ages are integrated with zircon U/Pb ages to evaluate the cooling histories within the Mo deposit. This study suggests that individual cycles of magma emplacement and mineralization cooled rapidly. In contrast to prior genetic models for the Questa Mo deposit, these data show that the mineralizing intrusions were generated via rapid melt generation, separation, and intrusion into the shallow crust without involvement in a long‐lived magma chamber. It is proposed that the anomalously high magma flux event associated with ignimbrite eruption transferred materials (Mo, volatiles) from the upper mantle necessary for immediately subsequent mineralization. Partial melting and scavenging within a deep‐crustal hybridized zone generated Mo‐rich magma that ascended to form the Questa deposit. Moreover, this hypothesis predicts an important connection between caldera‐forming systems and porphyry‐style mineralization that could be incorporated into future exploration models.
High resolution bathymetry reveals contrasting landslide activity shaping the walls of the Mid‐Atlantic Ridge axial valley
- Abstract: Axial valleys are found along most slow‐spreading mid‐ocean ridges and are one of the most prominent topographic features on Earth. In this paper, we present the first deep‐tow swath bathymetry for the axial valley walls of the Mid‐Atlantic Ridge. These data allow us to analyze axial valley wall morphology with a very high resolution (0.5 to 1 m compared to ≥ 50 m for shipboard multibeam bathymetry), revealing the role played by landslides. Slow‐spreading ridge axial valleys also commonly expose mantle‐derived serpentinized peridotites in the footwalls of large offset normal faults (detachments). In our map of the Ashadze area (lat. 13°N), ultramafic outcrops have an average slope of 18° and behave as sliding deformable rock masses, with little fragmentation. By contrast, the basaltic seafloor in the Krasnov area (lat. 16°38’N) has an average slope of 32° and the erosion of the steep basaltic rock faces leads to extensive fragmentation, forming debris with morphologies consistent with non‐cohesive granular flow. Comparison with laboratory experiments suggests that the repose angle for this basaltic debris is > 25°. We discuss the interplay between the normal faults that bound the axial valley and the observed mass wasting processes. We propose that, along axial valley walls where serpentinized peridotites are exposed by detachment faults, mass wasting results in average slopes ≤ 20°, even in places where the emergence angle of the detachment is larger.