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Publisher: American Geophysical Union (AGU)   (Total: 17 journals)

Geochemistry, Geophysics, Geosystems     Full-text available via subscription   (Followers: 21, SJR: 2.156, h-index: 61)
Geophysical Research Letters     Full-text available via subscription   (Followers: 46, SJR: 2.668, h-index: 142)
Global Biogeochemical Cycles     Full-text available via subscription   (Followers: 5, SJR: 2.4, h-index: 109)
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J. of Geophysical Research : Earth Surface     Partially Free   (Followers: 22)
J. of Geophysical Research : Oceans     Partially Free   (Followers: 16)
J. of Geophysical Research : Planets     Full-text available via subscription   (Followers: 13)
J. of Geophysical Research : Solid Earth     Full-text available via subscription   (Followers: 23)
J. of Geophysical Research : Space Physics     Full-text available via subscription   (Followers: 15)
Paleoceanography     Full-text available via subscription   (Followers: 4, SJR: 2.16, h-index: 82)
Radio Science     Full-text available via subscription   (Followers: 3, SJR: 0.527, h-index: 47)
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Tectonics     Full-text available via subscription   (Followers: 7, SJR: 2.16, h-index: 79)
Water Resources Research     Full-text available via subscription   (Followers: 156, SJR: 1.769, h-index: 110)
Journal Cover Geochemistry, Geophysics, Geosystems
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     ISSN (Online) 1525-2027
     Published by American Geophysical Union (AGU) Homepage  [17 journals]   [SJR: 2.156]   [H-I: 61]
  • Segmentation and eruptive activity along the East Pacific Rise at
           16°N, in relation with the nearby Mathematician hot spot
    • Authors: M. Le Saout; A. Deschamps, S. A. Soule, P. Gente
      Pages: n/a - n/a
      Abstract: The 16°N segment of the East Pacific Rise is the most over‐inflated and shallowest of this fast‐spreading ridge, in relation with an important magma flux due to the proximity of the Mathematician hotspot. Here, we analyze the detailed morphology of the axial dome and of the Axial Summit Trough (AST), the lava morphology and the geometry of fissures and faults, in regard to the attributes of the magma chamber beneath and of the nearby hotspot. The data used are 1‐meter‐resolution bathymetry combined with seafloor photos and videos. At the dome summit, the AST is highly segmented by ten 3rd and 4th ‐order discontinuities over a distance of 30 km. Often, two contiguous and synchronous ASTs coexist. Such a configuration implies a wide (1100 m minimum) zone of diking. The existence of contiguous ASTs, their mobility, their general en echelon arrangement accommodating the bow shape of the axial dome toward the hotspot, plus the existence of a second magma lens under the western half of the summit plateau, clearly reflect the influence of the hotspot on the organization of the spreading system. The different ASTs exhibit contrasted widths and depths. We suggest that narrow ASTs reflect an intense volcanic activity that produces eruptions covering the tectonic features and partially filling the ASTs. AST widening and deepening would indicate a decrease in volcanic activity but with continued dike intrusions at the origin of abundant sets of fissures and faults that are not masked by volcanic deposits.
      PubDate: 2014-10-21T11:01:18.682752-05:
      DOI: 10.1002/2014GC005560
  • Incorporating 3‐D parent nuclide zonation for apatite 4He/3He
           thermochronometry: An example from the Appalachian Mountains
    • Authors: Matthew Fox; Ryan E. McKeon, David L. Shuster
      Pages: n/a - n/a
      Abstract: The ability to constrain km‐scale exhumation with apatite 4He/3He thermochronometry is well established and the technique has been applied to a range of tectonic and geomorphic problems. However, multiple sources of uncertainty in specific crystal characteristics limit the applicability of the method, especially when geologic problems require identifying small perturbations in a cooling path. Here we present new 4He/3He thermochronometric data from the Appalachian Mountains, which indicate significant parent nuclide zonation in an apatite crystal. Using LA‐ICPMS measurements of U and Th in the same crystal, we design a 3‐D model of the crystal to explore the effects of intra‐crystal variability in radiation damage accumulation. We describe a numerical approach to solve the 3‐D production‐diffusion equation. Using our numerical model and a previously determined time temperature path for this part of the Appalachians, we find excellent agreement between predicted and observed 4He/3He spectra. Our results confirm this time‐temperature path and highlight that for complex U and Th zonation patterns, 3‐D numerical models are required to infer an accurate time‐temperature history. In addition, our results provide independent and novel evidence for a radiation damage control on diffusivity. The ability to exploit intra‐crystal differences in 4He diffusivity (i.e., temperature sensitivity) greatly increases the potential to infer complex thermal histories.
      PubDate: 2014-10-20T03:44:39.218353-05:
      DOI: 10.1002/2014GC005464
  • Assessment of relative Ti, Ta, and Nb (TITAN) enrichments in ocean island
    • Authors: Bradley J. Peters; James M.D. Day
      Pages: n/a - n/a
      Abstract: The sensitivity of trace element concentrations to processes governing solid‐melt interactions has made them valuable tools for tracing the effects of partial melting, fractional crystallization, metasomatism and similar processes on the composition of a parental melt. Recent studies of ocean island basalts (OIB) have sought to correlate Ti, Ta and Nb (TITAN) anomalies to isotopic tracers, such as 3He/4He and 187Os/188Os ratios, which may trace primordial deep mantle sources. A new compilation of global OIB trace element abundance data indicates that positive TITAN anomalies, though statistically pervasive features of OIB, may not be compositional features of their mantle sources. OIB show a range of Ti (Ti/Ti*=0.28‐2.35), Ta (Ta/Ta*=0.11‐93.4) and Nb (Nb/Nb*=0.13‐17.8) anomalies that show negligible correlations with 3He/4He ratios, indicating that TITAN anomalies are not derived from the less‐degassed mantle source traced by high‐3He/4He. Positive TITAN anomalies can be modelled using variable degrees (0.1‐10%) of non‐modal batch partial melting of garnet‐spinel lherzolite at temperatures and pressures considered typical for OIB petrogenesis, and subjecting this partial melt to fractional crystallization and assimilation of mid‐ocean ridge basalt‐like crust (AFC). Correlations of TITAN anomalies with modal abundances of olivine and clinopyroxene in porphyritic Canary Islands lavas provide empirical support for this process and indicate that high abundances of these phases in OIB may create misleading trace element anomalies on primitive mantle‐normalized spider diagrams. Because partial melting and AFC are common to all mantle‐derived magmas, caution should be used when attributing TITAN anomalies to direct sampling of recycled or deep mantle sources by hotspots.
      PubDate: 2014-10-20T03:44:37.277039-05:
      DOI: 10.1002/2014GC005506
  • Issue Information
    • Pages: i - i
      PubDate: 2014-10-16T14:10:17.046689-05:
      DOI: 10.1002/ggge.20325
  • Segmentation of plate coupling, fate of subduction fluids, and modes of
           arc magmatism in Cascadia, inferred from magnetotelluric resistivity
    • Authors: Philip E. Wannamaker; Rob L. Evans, Paul A. Bedrosian, Martyn J. Unsworth, Virginie Maris, R Shane McGary
      Pages: n/a - n/a
      Abstract: Five magnetotelluric (MT) profiles have been acquired across the Cascadia subduction system and transformed using 2D and 3D non‐linear inversion to yield electrical resistivity cross sections to depths of ˜200 km. Distinct changes in plate coupling, subduction fluid evolution, and modes of arc magmatism along the length of Cascadia are clearly expressed in the resistivity structure. Relatively high resistivities under the coasts of northern and southern Cascadia correlate with elevated degrees of inferred plate locking, and suggest fluid‐ and sediment‐deficient conditions. In contrast, the north‐central Oregon coastal structure is quite conductive from the plate interface to shallow depths offshore, correlating with poor plate locking and the possible presence of subducted sediments. Low‐resistivity fluidized zones develop at slab depths of 35‐40 km starting ˜100 km west of the arc on all profiles, and are interpreted to represent prograde metamorphic fluid release from the subducting slab. The fluids rise to forearc Moho levels, and sometimes shallower, as the arc is approached. The zones begin close to clusters of low frequency earthquakes, suggesting fluid controls on the transition to steady sliding. Under the northern and southern Cascadia arc segments, low upper mantle resistivities are consistent with flux melting above the slab plus possible deep convective backarc upwelling toward the arc. In central Cascadia, extensional deformation is interpreted to segregate upper mantle melts leading to underplating and low resistivities at Moho to lower crustal levels below the arc and near backarc. The low‐ to high‐temperature mantle wedge transition lies slightly trenchward of the arc.
      PubDate: 2014-10-14T12:02:11.596984-05:
      DOI: 10.1002/2014GC005509
  • Deep water recycling through time
    • Authors: Valentina Magni; Pierre Bouilhol, Jeroen van Hunen
      Pages: n/a - n/a
      Abstract: We investigate the dehydration processes in subduction zones and their implications for the water cycle throughout Earth's history. We use a numerical tool that combines thermo‐mechanical models with a thermodynamic database to examine slab dehydration for present‐day and early Earth settings and its consequences for the deep water recycling. We investigate the reactions responsible for releasing water from the crust and the hydrated lithospheric mantle and how they change with subduction velocity (vs), slab age (a) and mantle temperature (Tm). Our results show that faster slabs dehydrate over a wide area: they start dehydrating shallower and they carry water deeper into the mantle. We parameterize the amount of water that can be carried deep into the mantle, W (x105 kg/m2), as a function of vs (cm/yr), a (Myrs), and Tm (°C): W=1.06υs+0.14α−0.023Tm+17 We generally observe that a 1) 100°C increase in the mantle temperature, or 2) ~15 Myr decrease of plate age, or 3) decrease in subduction velocity of ~2 cm/yr all have the same effect on the amount of water retained in the slab at depth, corresponding to a decrease of ~2.2x105 kg/m2 of H2O. We estimate that for present‐day conditions ~26% of the global influx water, or 7x108 Tg/Myr of H2O, is recycled into the mantle. Using a realistic distribution of subduction parameters, we illustrate that deep water recycling might still be possible in early Earth conditions, although its efficiency would generally decrease. Indeed, 0.5‐3.7x108 Tg/Myr of H2O could still be recycled in the mantle at 2.8 Ga.
      PubDate: 2014-10-14T12:02:04.52594-05:0
      DOI: 10.1002/2014GC005525
  • Shallow methane hydrate system controls ongoing, downslope sediment
           transport in a low‐velocity active submarine landslide complex,
           Hikurangi Margin, New Zealand
    • Authors: Joshu J. Mountjoy; Ingo Pecher, Stuart Henrys, Gareth Crutchley, Philip M. Barnes, Andreia Plaza‐Faverola
      Pages: n/a - n/a
      Abstract: Morphological and seismic data from a submarine landslide complex east of New Zealand indicate flow‐like deformation within gas hydrate‐bearing sediment. This “creeping” deformation occurs immediately downslope of where the base of gas hydrate stability reaches the seafloor, suggesting involvement of gas hydrates. We present evidence that, contrary to conventional views, gas hydrates can directly destabilize the seafloor. Three mechanisms could explain how the shallow gas hydrate system could control these landslides. 1) Gas hydrate dissociation could result in excess pore pressure within the upper reaches of the landslide. 2) Overpressure below low‐permeability gas hydrate‐bearing sediments could cause hydrofracturing in the gas hydrate zone valving excess pore pressure into the landslide body. 3) Gas hydrate‐bearing sediment could exhibit time‐dependent plastic deformation enabling glacial‐style deformation. We favor the final hypothesis, that the landslides are actually creeping seafloor glaciers. The viability of rheologically controlled deformation of a hydrate sediment mix is supported by recent laboratory observations of time‐dependent deformation behavior of gas‐hydrate‐bearing sands. The controlling hydrate is likely to be strongly dependent on formation controls and inter‐sediment hydrate morphology. Our results constitute a paradigm shift for evaluating the effect of gas hydrates on seafloor strength which, given the widespread occurrence of gas hydrates in the submarine environment, may require a re‐evaluation of slope stability following future climate‐forced variation in bottom water temperature.
      PubDate: 2014-10-14T02:58:27.905801-05:
      DOI: 10.1002/2014GC005379
  • Lucky Strike seamount: Implications for the emplacement and rifting of
    • Authors: J. Escartín; A. Soule, M. Cannat, D. J. Fornari, D. Düşünür, R. Garcia
      Pages: n/a - n/a
      Abstract: The history of emplacement, tectonic evolution, and dismemberment of a central volcano within the rift valley of the slow‐spreading Mid‐Atlantic Ridge at the Lucky Strike Segment is deduced using near‐bottom sidescan sonar imagery and visual observations. Volcano emplacement is rapid (
      PubDate: 2014-10-14T01:04:00.438503-05:
      DOI: 10.1002/2014GC005477
  • Eruptive history and magmatic stability of Erebus volcano, Antarctica:
           Insights from englacial tephra
    • Authors: Nels A. Iverson; Philip R. Kyle, Nelia W. Dunbar, William C. McIntosh, Nicholas J.G. Pearce
      Pages: n/a - n/a
      Abstract: A tephrostratigraphy of the active Antarctic volcano, Mt. Erebus, was determined from englacial tephra on the ice‐covered flanks of Erebus and an adjacent volcano. The tephra are used to reconstruct the eruptive history and magmatic evolution of Erebus. More fine grained and blocky particles define tephra formed in phreatomagmatic eruptions and larger fluidal shards are characteristic of magmatic eruptions and in some cases both eruptive types are identified in a single mixed tephra. The eruptions forming the mixed tephra likely started as phreatomagmatic eruptions which transitioned into Strombolian eruptions as the non‐magmatic water source was exhausted. We reconstructed the eruptive history of Erebus using the tephra layers stratigraphic position, 40Ar/39Ar ages, shard morphology and grain size. Major and trace element analyses of individual glass shards were measured by electron probe microanalysis and LA‐ICP‐MS. Trachybasalt, trachyte and phonolite tephra were identified. All phonolitic tephra are Erebus‐derived with compositions similar to volcanic bombs erupted from Erebus over the past 40 years. The tephra show that Erebus magma has not significantly changed for 40ka. The uniformity of the glass chemical composition implies that the phonolite magma has crystallized in the same manner without change throughout the late Quaternary, suggesting long‐term stability of the Erebus magmatic system. Trachyte and trachybasalt tephra were likely erupted from Marie Byrd Land and the McMurdo Sound area, respectively. The trachytic tephra can be regionally correlated, and could provide an important time‐stratigraphic marker in Antarctic ice cores.
      PubDate: 2014-10-08T04:25:27.372547-05:
      DOI: 10.1002/2014GC005435
  • Quantifying temporal variations in landslide‐driven sediment
           production by reconstructing paleolandscapes using tephrochronology and
           lidar: Waipaoa River, New Zealand
    • Authors: Corina Cerovski‐Darriau; Joshua J. Roering, Michael Marden, Alan S. Palmer, Eric L. Bilderback
      Pages: n/a - n/a
      Abstract: Hillslope response to climate‐driven fluvial incision controls sediment export and relief generation in most mountainous settings. Following the shift to a warmer, wetter climate after the Last Glacial Maximum (LGM) (˜18 ka), the Waipaoa River (New Zealand) rapidly incised up to 120 meters, leaving perched, low‐relief hillslopes unadjusted to that base level fall. In the Mangataikapua—a 16.5 km2 tributary principally comprised of weak mélange—pervasive post‐LGM landslides responded to >50 m of fluvial incision by sculpting and denuding >99% of the catchment. By reconstructing LGM and younger paleosurfaces from tephra identified by electron microprobe analysis (EMPA) and lidar‐derived surface roughness, we estimate the volume, timing, and distribution of hillslope destabilization in the Mangataikapua and the relative contribution of landslide‐prone terrain to post‐LGM landscape evolution. We calculate volume change between four paleosurfaces constrained by tephra age (Rerewhakaaitu, 17.5 ka; Rotoma, 9.4 ka; Whakatane, 5.5 ka; and Waimihia, 3.4 ka). From the paleosurface reconstructions, we calculate the total post‐LGM hillslope sediment contribution from the Mangataikapua catchment to be 0.5 ± 0.06 (s.d.) km3, which equates to a sub‐catchment averaged erosion rate of ˜1.6 mm yr‐1. This is double the previous hillslope volume when normalized by study area, demonstrating that landslide‐prone catchments disproportionately contribute to the terrestrial post‐LGM sediment budget. Finally, we observe particularly rapid post‐Waimihia erosion rates, likely impacted by human settlement.
      PubDate: 2014-10-08T03:52:44.066793-05:
      DOI: 10.1002/2014GC005467
  • Enhanced recycling during oceanic anoxic event 2 in the proto‐North
    • Authors: I. Ruvalcaba Baroni; I. Tsandev, C. P. Slomp
      Pages: n/a - n/a
      Abstract: Evidence from sediment core records and model studies suggest that increased nutrient supply played a key role in the initiation of the Cenomanian‐Turonian oceanic anoxic event 2 (OAE2; 94 Ma). However, the relative roles of nitrogen (N) and phosphorus (P) availability in controlling primary productivity during the event are not fully understood. Here, we expand an existing multi‐box model of the coupled cycles of P, carbon and oxygen in the proto‐North Atlantic by adding the marine N cycle. With the updated version of the model, we test the hypothesis that enhanced availability of P can fuel N2‐fixation, increase primary productivity and drive large parts of the proto‐North Atlantic to anoxia during OAE2. In a sensitivity analysis, we demonstrate that N dynamics in the proto‐North Atlantic respond strongly to variations in oxygen and P supply from the Pacific Ocean and to changes in circulation. The implemented N cycle weakly modifies the carbon cycle, implying that P was the major nutrient controlling primary productivity during OAE2. Our model suggests that both N2‐fixation and upwelling of recycled were enhanced during OAE2 and that N2‐fixation was the major source of N in the proto‐North Atlantic. Denitrification was more important in the water column than in sediments, with high rates in the open ocean and in the Western Interior. High P inputs in the proto‐North Atlantic led to widespread N2‐fixation, which more than compensated for the loss of N through denitrification. As a consequence, rates of primary productivity and organic carbon burial were high.
      PubDate: 2014-10-08T03:36:57.126731-05:
      DOI: 10.1002/2014GC005453
  • Carbon cycling and burial in New Zealand's fjords
    • Authors: Jessica L. Hinojosa; Christopher M. Moy, Claudine H. Stirling, Gary S. Wilson, Timothy I. Eglinton
      Pages: n/a - n/a
      Abstract: Understanding carbon cycling in continental margin settings is critical for constraining the global carbon cycle. Here we apply a multiproxy geochemical approach to evaluate regional carbon cycle dynamics in six New Zealand fjords. Using carbon and nitrogen concentrations and isotopes, lipid biomarkers, and redox‐sensitive element concentrations, we show that the New Zealand fjords have carbon‐rich surface sediments in basins that promote long‐term storage (i.e., semi‐restricted basins with sediment accumulation rates of up to 3 mm yr‐1). Using δ13C distributions to develop a mixing model, we find that organic carbon in fjord sediments is well‐mixed from marine and terrestrial sources in down‐fjord gradients. This is driven by high regional precipitation rates of >6 m yr‐1, which promote carbon accumulation in fjord basins through terrestrial runoff. In addition, we have identified at least two euxinic sub‐basins, based on uranium, molybdenum, iron, and cadmium enrichment, that contain >7% organic carbon. Because the strength and position of the Southern Hemisphere westerly winds control precipitation and fjord circulation, carbon delivery and storage in the region are intimately linked to westerly wind variability. We estimate that the fjord region (759 km2) may be exporting up to 1.4 x 107 kgC yr‐1, outpacing other types of continental margins in rates of carbon burial by up to three orders of magnitude.
      PubDate: 2014-10-08T03:36:46.239793-05:
      DOI: 10.1002/2014GC005433
  • Flow dynamics of Nankai Trough submarine landslide inferred from internal
           deformation using magnetic fabric
    • Authors: Toshiya Kanamatsu; Kiichiro Kawamura, Michael Strasser, Beth Novak, Yujin Kitamura
      Pages: n/a - n/a
      Abstract: Submarine landslide deposits in an active subduction zone were investigated by Integrated Ocean Drilling Program Expedition 333 as the “Nankai Trough Submarine Landslides History”. The expedition recovered a Pleistocene to Holocene sequence of stacked mass‐transport deposits at Site C0018, located within a slope basin on the footwall of the megasplay fault in the Nankai Trough off the Kii Peninsula, southwest Japan. Six mass‐transport deposit units intercalated with coherent intervals were recovered from the upper 190‐m of the drilled succession. Anisotropy of magnetic susceptibility variations within mass‐transport deposit units was used to ascertain their rheology. Shape parameters and magnetic fabric orientation reveal inhomogeneity through the sequences, indicating that different compaction and shear occurred within individual units. The upper intervals of younger units generally represents a magnetic fabric formed under vertical compression. However, the lower intervals involve magnetic fabrics indicating lateral shear with in‐depth gradual change. In the older mass‐transport deposit, a distribution of magnetic foliation forms tightly folded strata, indicating different sliding patterns from the younger mass‐transport deposit units. Using available paleomagnetic data, the shear directions of basal intervals of units are reoriented, producing two different sliding orientations. Variation in the flow type and supply route is interpreted to reflect the slope condition (e.g. slope gradient and susceptibility to ground motion), which has been controlled by local tectonic evolution of the accretionary wedge.
      PubDate: 2014-10-07T02:38:27.074011-05:
      DOI: 10.1002/2014GC005409
  • Eruptive modes and hiatus of volcanism at West Mata seamount, NE Lau
           basin: 1996–2012
    • Authors: Robert W. Embley; Susan G. Merle, Edward T. Baker, Kenneth H. Rubin, John E. Lupton, Joseph A. Resing, Robert P. Dziak, Marvin D. Lilley, William W. Chadwick, T. Shank, Ron Greene, Sharon L. Walker, Joseph Haxel, Eric Olson, Tamara Baumberger
      Pages: n/a - n/a
      Abstract: We present multiple lines of evidence for years‐ to decade‐long changes in the location and character of volcanic activity at West Mata seamount in the NE Lau Basin over a 16 year period, and a hiatus in summit eruptions from early 2011 through at least September 2012. Boninite lava and pyroclasts were observed erupting from its summit in 2009 and hydroacoustic data from a succession of hydrophones moored nearby show near‐continuous eruptive activity from January 2009 to early 2011. Successive differencing of seven multibeam bathymetric surveys of the volcano made in the 1996‐2012 period reveal a pattern of extended constructional volcanism on the summit and northwest flank punctuated by eruptions along the volcano's WSW rift zone (WSWRZ). Away from the summit, the volumetrically largest eruption during the observational period occurred between May 2010 and November 2011 at ~2920 m depth near the base of the WSWRZ. The (nearly) equally long ENE rift zone did not experience any volcanic activity during the 1996‐2012 period. The cessation of summit volcanism recorded on the moored hydrophone was accompanied or followed by the formation of a small summit crater and a landslide on the eastern flank. Water column sensors, analysis of gas samples in the overlying hydrothermal plume and dives with a remotely operated vehicle in September 2012 confirmed that the summit eruption had ceased. Based on the historical eruption rates calculated using the bathymetric differencing technique, the volcano could be as young as several thousand years.
      PubDate: 2014-10-06T04:51:47.313417-05:
      DOI: 10.1002/2014GC005387
  • Deconvolution of continuous paleomagnetic data from pass‐through
           magnetometer: A new algorithm to restore geomagnetic and environmental
           information based on realistic optimization
    • Authors: Hirokuni Oda; Chuang Xuan
      Pages: n/a - n/a
      Abstract: The development of pass‐through superconducting rock magnetometers (SRM) has greatly promoted collection of paleomagnetic data from continuous long‐core samples. The output of pass‐through measurement is smoothed and distorted due to convolution of magnetization with the magnetometer sensor response. Although several studies could restore high‐resolution paleomagnetic signal through deconvolution of pass‐through measurement, difficulties in accurately measuring the magnetometer sensor response have hindered the application of deconvolution. We acquired reliable sensor response of an SRM at the Oregon State University based on repeated measurements of a precisely fabricated magnetic point source. In addition, we present an improved deconvolution algorithm based on Akaike's Bayesian Information Criterion (ABIC) minimization, incorporating new parameters to account for errors in sample measurement position and length. The new algorithm was tested using synthetic data constructed by convolving “true” paleomagnetic signal containing an “excursion” with the sensor response. Realistic noise was added to the synthetic measurement using Monte Carlo method based on measurement noise distribution acquired from 200 repeated measurements of a u‐channel sample. Deconvolution of 1000 synthetic measurements with realistic noise closely resembles the “true” magnetization, and successfully restored fine‐scale magnetization variations including the “excursion”. Our analyses show that inaccuracy in sample measurement position and length significantly affects deconvolution estimation, and can be resolved using the new deconvolution algorithm. Optimized deconvolution of 20 repeated measurements of a u‐channel sample yielded highly consistent deconvolution results and estimates of error in sample measurement position and length, demonstrating the reliability of the new deconvolution algorithm for real pass‐through measurements.
      PubDate: 2014-10-04T06:47:20.553198-05:
      DOI: 10.1002/2014GC005513
  • The early middle Miocene subduction complex of the Louisiade Archipelago,
           southern margin of the Woodlark Rift
    • Authors: Laura E. Webb; Suzanne L. Baldwin, Paul G. Fitzgerald
      Pages: n/a - n/a
      Abstract: Field, microstructural and 40Ar/39Ar thermochronologic data from the Louisiade Archipelago, the southern rifted margin of the Woodlark Basin in SE Papua New Guinea, document an accretionary wedge that formed during Early–Middle Miocene N‐dipping subduction of the Australian margin and transpression along the Australian–Pacific plate boundary. Metasedimentary rocks of the Calvados Schist and the metagabbros that intrude them were metamorphosed at up to greenschist‐facies conditions. Three tectonic foliations (S1–S3) are present and F1–F3 fold hinges plunge ESE or WNW, parallel to mineral, stretching, and intersection lineations. Fold vergence is dominantly to the SW, and top‐to‐the‐SW thrusting of ultramafic rocks over the Calvados Schist is documented locally on Rossel Island. The data suggest progressive deformation associated with NNE–SSW shortening and ESE–WNW extension via dissolution‐precipitation creep and, more locally, dislocation creep. 40Ar/39Ar step‐heating analyses of three white mica separates yield Middle Miocene plateau or plateau‐like segments that are affected by variable Pliocene argon loss and are interpreted as syntectonic mica growth during metamorphism and deformation followed by partial resetting just prior to the onset of seafloor spreading in the Woodlark Basin. A ca. 12 Ma 40Ar/39Ar age from a dacite sill of the Panarora Volcanics provides a minimum age constraint for the termination of northward subduction and a maximum age for cross‐cutting brittle strike‐slip faults. These data are critical to constraining the subduction–exhumation history of the world's youngest high–ultrahigh‐pressure terrane and further support analogies between SE Papua New Guinea and the Early Oligocene Western Alps.
      PubDate: 2014-09-18T05:08:08.803665-05:
      DOI: 10.1002/2014GC005500
  • Absolute plate motions and regional subduction evolution
    • Authors: M.V. Chertova; W. Spakman, A.P. van den Berg, D.J.J. van Hinsbergen
      Pages: n/a - n/a
      Abstract: We investigate the influence of absolute plate motion on regional 3‐D evolution of subduction using numerical thermo‐mechanical modeling. Building on our previous work (Chertova et al. 2014), we explore the potential impact of four different absolute plate motion frames on subduction evolution in the western Mediterranean region during the last 35My. One frame is data‐based and derived from the global moving hotspot reference frame (GMHRF) of Doubrovine et al. [2012] and three are invented frames: a motion frame in which the African plate motion is twice that in the GMHRF, and two frames in which either the African plate or the Iberian continent is assumed fixed to the mantle. The relative Africa‐Iberia convergent is the same in all frames. All motion frames result in distinctly different 3‐D subduction evolution showing a critical dependence of slab morphology evolution on absolute plate motion. We attribute this to slab dragging through the mantle forced by the absolute motion of the subducting plate, which causes additional viscous resistance affecting subduction evolution. We observed a strong correlation between increase in northward Africa motion and decrease in the speed of westward slab rollback along the African margin. We relate this to increased mantle resistance against slab dragging providing new insight into propagation and dynamics of subduction transform edge propagator (STEP) faults. Our results demonstrate a large sensitivity of 3‐D slab evolution to the absolute motion of the subducting plate, which inversely suggests that detailed modeling of natural subduction may provide novel constraints on absolute plate motions.
      PubDate: 2014-09-17T02:18:00.229793-05:
      DOI: 10.1002/2014GC005494
  • Geochemical insights into the role of metasomatic hornblendite in
           generating alkali basalts
    • Authors: Li‐Qun Dai; Zi‐Fu Zhao, Yong‐Fei Zheng
      Pages: n/a - n/a
      Abstract: Experimental petrology suggested the role of hornblendite in generating alkali basalt. This mechanism is confirmed by an integrated study of major‐trace elements and radiogenic isotopes for Mesozoic alkali basalts from the Qinling orogen in China. The alkali basalts have high contents of MgO (4.8‐11.1 wt.%, Mg# = 47‐69), Na2O+K2O (2.9‐5.4 wt.%), TiO2 (2.0‐3.1 wt.%) but low content of SiO2 (41.4‐49.6 wt.%), which are generally silica‐undersaturated with normative minerals of nepheline and olivine. They exhibit OIB‐like trace element distribution patterns, with enrichment of LILE and LREE but no depletion of HFSE relative to the primitive mantle. They also show relatively depleted Sr‐Nd‐Hf isotope compositions, with low initial 87Sr/86Sr ratios of 0.7028 to 0.7058, positive εNd(t) values of 4.0 to 9.8 and εHf(t) values of 8.8 to 13.5 for whole‐rock, and positive εHf(t) values of 5.2 to 16.4 for zircon. Such element and isotope features indicate their origination from the juvenile subcontinental lithospheric mantle (SCLM) source with involvement of crustal components. The alkali basalts generally have high K2O/Na2O ratios, and high K2O and TiO2 contents, suggesting their derivation from partial melting of hornblendite‐rich mantle lithology. They also exhibit variable K/La and Ti/La ratios that are correlated with (La/Yb)N ratios, indicating a geochemical heterogeneity of the SCLM source. Taken together, all the above geochemical features can be accounted for by partial melting of a hornblendite‐rich SCLM source. The hornblendite would be generated by reaction of the juvenile SCLM wedge peridotite with felsic melts derived from subducted Palaeotethyan oceanic crust at the slab‐mantle interface in the subduction channel. Therefore, orogenic alkali basalts record recycling of the subducted fossil oceanic crust, and the metasomatic hornblendite is an important lithology in local SCLM domains above fossil subduction channels.
      PubDate: 2014-09-13T10:56:50.614212-05:
      DOI: 10.1002/2014GC005486
  • The dynamics of global change at the Paleocene‐Eocene thermal
           maximum: A data‐model comparison
    • Authors: Timothy J. Bralower; Katrin J. Meissner, Kaitlin Alexander, Deborah J. Thomas
      Pages: n/a - n/a
      Abstract: We integrate published stable isotopic, chemical, mineralogical and biotic data from the onset of the Paleocene Eocene thermal maximum (PETM) at Site 690, Maud Rise in the Southern Ocean. The integrated dataset documents a sequence of environmental steps including warming of the ocean from the surface downwards, and modification of its thermal and nutrient structure, acidification of the deep ocean and the onset of continental weathering. The age of the events with respect to the onset of the PETM is calibrated with three different age models. The relative and absolute timing of the steps are compared with simulated temperature, salinity, calcite saturation, and dissolved PO4 and O2, at different depths in the ocean, generated with the UVic Earth System Climate Model of intermediate complexity. The simulation supports the top to bottom transfer of heat and carbon, and generally agrees with age models in terms of the durations of leads and lags in temperature, C‐isotope and biotic responses. Moreover, the simulation shows that stratification increased and the nutricline strengthened at the onset of the PETM. These environmental changes explain the abundance of deep dwelling nannoplankton and foraminifera during the early part of the event. The modelled calcite saturation is consistent with a harsh deep sea habitat at the time of the benthic foraminiferal extinction.
      PubDate: 2014-09-13T10:47:00.561488-05:
      DOI: 10.1002/2014GC005474
  • Estimating shallow water sound power levels and mitigation radii for the
           R/V Marcus G. Langseth using an 8 km long MCS streamer
    • Authors: Timothy J. Crone; Maya Tolstoy, Helene Carton
      Pages: n/a - n/a
      Abstract: For seismic surveys in shallow‐water environments the complexity of local geology and seafloor topography can make it difficult to accurately predict associated sound levels and establish appropriate mitigation radii required to ensure the safety of local marine protected species. This is primarily because necessary detailed information regarding the local seafloor topography and subseafloor geology is often unavailable before a survey begins. One potential solution to this problem is to measure received levels using the ship's multi‐channel seismic (MCS) streamer \citep{Diebold2010}, which could allow for the dynamic real‐time determination of sound levels and mitigation radii while a survey is underway. We analyze {\em R/V Langseth} streamer data collected on the shelf and slope near the Washington coast during the Cascadia Open‐Access Seismic Transects (COAST) and Ridge2Trench projects to measure received levels up to a distance of approximately 8 km from the sound source array. We establish methods to filter, clean, and process streamer data to accurately determine received power levels and confidently establish mitigation radii. We show that in shallow water measured power levels can fluctuate due to the influence of seafloor topographic features, but that the use of the streamer for the establishment of dynamic mitigation radii is feasible and should be further pursued. The establishment of mitigation radii based on local conditions may help maximize the safety of marine protected species while also maximizing the ability of researchers to conduct seismic studies.
      PubDate: 2014-09-10T10:19:16.997535-05:
      DOI: 10.1002/2014GC005420
  • Early Cenozoic exhumation in the Bendeleben Mountains (Seward Peninsula,
           Alaska) constrained by (U‐Th)/He thermochronology and thermal
    • Authors: Kalin T. McDannell; Jaime Toro, Jeremy K. Hourigan, Daniel Harris
      Pages: n/a - n/a
      Abstract: In the Bendeleben Mountains, Seward Peninsula, mid‐Cretaceous granites are exposed in an uplifted block bounded on its south side by an E‐W striking normal fault. The Bendeleben fault has well‐preserved scarps 4‐7 m in height that offset Holocene moraines. Seismic activity, young normal faulting, and Quaternary basaltic volcanism are all evidence of active extension. South of the Bendeleben fault, there is a 3‐4 km deep basin. Fifteen apatite (U‐Th)/He ages from granitic samples of the footwall yield an Eocene weighted mean age of 41.3±4.8 Ma. Biotite 40Ar/39Ar ages from the country rock of the Bendeleben pluton are 81‐83 Ma. In spite of the young fault scarps, Pecube and HeFTy t‐T modeling results illustrate that rapid exhumation of the Bendeleben Mountains occurred in the Late Cretaceous‐Eocene and slowed since the Oligocene. A weak age‐elevation relationship of apatite He ages and a lack of correlation between age and distance from the fault indicate that exhumation was accomplished with minimal block rotation on a steeply dipping, long‐lived normal fault. Timing of extension in the Seward Peninsula can be correlated with deformation in the offshore Hope Basin where seismic reflection lines document Early Tertiary large‐magnitude normal faulting followed by minor post‐Miocene reactivation. The faulting observed in the Bendeleben Mountains is part of an extensional system that spans a large portion of the Bering Strait region. The tectonic model proposed in previous studies suggests that clockwise rotation of the Bering block relative to North America is the cause of extensional deformation in western Alaska.
      PubDate: 2014-09-09T19:29:23.694789-05:
      DOI: 10.1002/2014GC005424
  • Provenance of the late quaternary sediments in the Andaman Sea:
           Implications for monsoon variability and ocean circulation
    • Authors: Neeraj Awasthi; Jyotiranjan S. Ray, Ashutosh K. Singh, Shraddha T. Band, Vinai K. Rai
      Pages: n/a - n/a
      Abstract: We present a geochemical and Sr‐Nd isotopic study on a sediment core collected from the Andaman Sea in an attempt to reconstruct the Late Quaternary weathering and erosion patterns in the watersheds of the river systems of Myanmar and understand their controlling factors. Age control is based on nine radiocarbon dates and δ18O stratigraphy. The rate of sedimentation was strongly controlled by fluctuations of the monsoon. We identify three major sediment provenances: (1) the Irrawaddy catchment, (2) the western slopes of the Indo‐Burman‐Arakan (IBA) mountain ranges and the Andaman Islands and (3) the catchments of Salween and Sittang and the Bengal shelf, with the first two contributing 30 to 60% of the material. Enhanced contributions from juvenile sources and corresponding positive shifts of δ18O are observed at seven time periods (11‐14, 20‐23, 36, 45, 53, 57 and 62 ka) of which five are synchronous with cooling of the northern hemisphere ‐ suggesting a link between the changes in sediment provenances and the shifting of the locus of the summer monsoon, southwards from the Himalayas, without substantial reduction in intensity. Our data, and that from other cores in the region suggest that an eastward moving surface current disperses sediments, derived from the Bengal shelf and western margin of Myanmar, from the eastern Bay of Bengal into the western Andaman Sea and that its strength has increased since the LGM. The existence of this current during the LGM implies that the Andaman Sea and the Bay of Bengal were well connected during the last glacial period.
      PubDate: 2014-09-09T19:24:15.008141-05:
      DOI: 10.1002/2014GC005462
  • Sulfur isotope evolution in sulfide ores from Western Alps: Assessing the
           influence of subduction‐related metamorphism
    • Authors: Fabio Giacometti; Katy A. Evans, Gisella Rebay, John Cliff, Andrew G. Tomkins, Piergiorgio Rossetti, Gloria Vaggelli, David T. Adams
      Pages: n/a - n/a
      Abstract: Sulfides entering subduction zones can play an important role in the release of sulfur and metals to the mantle wedge and contribute to the formation of volcanic arc‐associated ores. Fractionation of stable sulfur isotopes recorded by sulfides during metamorphism can provide evidence of fluid‐rock interactions during metamorphism and give insights on sulfur mobilization. A detailed micro‐textural and geochemical study was performed on mineralized samples from two ocean floor‐related sulfide deposits (Servette and Beth‐Ghinivert) in high‐pressure units of the Italian Western Alps, which underwent different metamorphic evolutions. The combination of micro‐textural investigations with δ34S values from in situ ion‐probe analyses within individual pyrite and chalcopyrite grains allowed an evaluation of the effectiveness of metamorphism in modifying the isotopic record and mobilizing sulfur and metals and have insights on fluid circulation within the slab. Textures and isotopic compositions inherited from the protolith are recorded at Beth‐Ghinivert, where limited metamorphic re‐crystallization is attributed to limited interaction with metamorphic fluids. Isotopic modification by metamorphic processes occurred only at the sub‐millimeter scale at Servette, where local interactions with infiltrating hydrothermal fluid are recorded by metamorphic grains. Notwithstanding the differences recorded by the two deposits, neither underwent intensive isotopic re‐equilibration or records evidence of intense fluid‐rock interaction and S mobilization during metamorphism. Therefore, subducted sulfide deposits dominated by pyrite and chalcopyrite are unlikely to release significant quantities of sulfur to the mantle wedge and to arc magmatism sources at metamorphic grades below the lower eclogite facies.
      PubDate: 2014-09-09T19:20:54.650317-05:
      DOI: 10.1002/2014GC005459
  • Three‐dimensional flow in the subslab mantle
    • Authors: Karen Paczkowski; Laurent G.J. Montési, Maureen D. Long, Christopher J. Thissen
      Pages: n/a - n/a
      Abstract: Three‐dimensional models of mantle flow at subduction zones make it possible to explain the common occurrence of trench‐parallel sub‐slab seismic anisotropy. Sub‐slab flow becomes inherently three‐dimensional when slab‐driven flow interacts with a wide variety of ambient background mantle flow conditions. This interaction depends on slab geometries, mechanical coupling parameters, and lower mantle viscosities. Deflection of sub‐slab mantle flow is a robust feature for all model parameters and geometries as the slab acts as an obstruction to the ambient, background mantle flow. Background mantle flow can become trench‐perpendicular or trench‐parallel sub‐slab flow depending on whether the ambient background mantle flow is deflected beneath the bottom of the slab or towards the edge of the slab. The first case is especially prominent in models with short slabs that do not penetrate into the lower mantle. The second case is especially prominent in models with long, steep slabs. The results are also highly sensitive to the amount of mechanical coupling between the subducting plate and the mantle beneath it. High levels of coupling create a boundary layer of trench‐perpendicular entrained flow, pushing the deflection due to the obstructing slab away from the slab. We compare our sub‐slab flow model predictions with a global set of seismic anisotropy fast directions in the sub‐slab mantle, and find generally good agreement between the anisotropy observations (dominantly trench‐parallel or trench‐perpendicular) and the mantle flow directions predicted for decoupled systems.
      PubDate: 2014-09-09T19:19:48.303475-05:
      DOI: 10.1002/2014GC005441
  • Tracking the exhumation of a pliocene (U)HP terrane: U‐Pb and
           trace‐element constraints from zircon, D'Entrecasteaux Islands,
           Papua New Guinea
    • Authors: Joel W. DesOrmeau; Stacia M. Gordon, Timothy A. Little, Samuel A. Bowring
      Pages: n/a - n/a
      Abstract: Domal structures within the D'Entrecasteaux Islands of eastern Papua New Guinea expose ultrahigh‐pressure (UHP) Pliocene (5.6–4.6 Ma) eclogites and evidence for partial melting. To better interpret the (U)HP exhumation history, U‐Pb geochronology and trace‐element abundances were determined in zircon from variably deformed host gneiss and crystallized melt (leucosomes, sills, dikes, and plutons) from the Goodenough and Normanby Domes by ID‐TIMS (isotope‐dilution thermal ionization mass spectrometry) and ICP‐MS (inductively coupled plasma mass spectrometry), respectively, to constrain the timing of melt crystallization and deformation relative to UHP metamorphism. Zircons extracted from orthogneiss and deformed granodiorite sills of Normanby Dome, located ~40 km southeast of the UHP eclogite, record HP metamorphism from 5.66 ± 0.02 Ma to 5.04 ± 0.07 Ma and melt crystallization at ca. 4.1 Ma. Strongly deformed, layer‐parallel leucosomes from Goodenough Dome, ~20 km northwest of the UHP eclogite, began to crystallize by 3.85 ± 0.02 Ma. These dates indicate that melt crystallization began in the Goodenough and Normanby Domes within ~0.75 m.y. of (U)HP metamorphism. The ID‐TIMS dates from the orthogneiss and crystallized melt show that exhumation and cooling of the (U)HP rocks in the PNG terrane began first in the east, within Normanby Dome, then to the west, in the Goodenough Dome ~ 1 m.y. later, and finally the middle dome rocks, exposed within the Mailolo Dome, cooled ~ 2 m.y. after exhumation of Normanby Dome. All domes reveal synchronous crystallization of late, non‐deformed melts and final extension‐driven exhumation by 1.82 ± 0.03 Ma.
      PubDate: 2014-09-05T11:34:36.272146-05:
      DOI: 10.1002/2014GC005396
  • A family of repeating low‐frequency earthquakes at the downdip edge
           of tremor and slip
    • Authors: Justin R Sweet; Kenneth C Creager, Heidi Houston
      Pages: n/a - n/a
      Abstract: We analyze an isolated Low‐Frequency Earthquake (LFE) family located at the downdip edge of the main episodic tremor and slip (ETS) zone beneath western Washington State. The 9000 individual LFEs from this repeating family cluster into 198 swarms that recur roughly every week. Cumulative LFE seismic moment for each swarm correlates strongly with the time until the next swarm, suggesting that these LFE swarms are time‐predictable. Precise double‐difference relative locations for 700 individual LFEs within this family show a distribution that is approximately 2 km long and 500 m wide, elongated parallel to the relative plate convergence direction. The distribution of locations (
      PubDate: 2014-09-02T04:22:15.428687-05:
      DOI: 10.1002/2014GC005449
  • Regional study of the archean to proterozoic crust at the Sudbury Neutrino
           Observatory (SNO+), Ontario: Predicting the geoneutrino flux
    • Authors: Yu Huang; Virginia Strati, Fabio Mantovani, Steven B. Shirey, William F. McDonough
      Pages: n/a - n/a
      Abstract: The SNO+ detector that is currently under construction in Ontario, Canada, will be a new kiloton scale liquid scintillation detector with the capability of recording geoneutrino events that can be used to constrain the strength of the Earth's radiogenic power, and in turn, to test compositional models of the bulk silicate Earth (BSE). We constructed a detailed 3‐D model of the regional crust centered at SNO+ from compiled geological, geophysical and geochemical information. Crustal cross sections obtained from refraction and reflection seismic surveys were used to characterize the crust and assign uncertainties to its structure. The average Moho depth in the study area is 42.3 ± 2.6 km. The upper crust was divided into seven dominant lithologic units on the basis of regional geology. The abundances of U and Th and their uncertainties in each upper crustal lithologic unit were determined from analyses of representative outcrop samples. The average chemical compositions of the middle and lower crust beneath the SNO+ region were determined by coupling local seismic velocity profiles with a global compilation of the chemical compositions of amphibolite and granulite facies rocks. Monte Carlo simulations were used to predict the geoneutrino signal originating from the regional crust at SNO+ and to track asymmetrical uncertainties of U and Th abundances. The total regional crust contribution of the geoneutrino signal at SNO+ is predicted to be TNU (a Terrestrial Neutrino Unit is one geoneutrino event per 1032 target protons per year), with the Huronian Supergroup near SNO+ dominantly contributing TNU to this total. Future systematically sampling of this regional unit and denser seismic surveys will better model its composition and structure, and thus reduce the uncertainty on geoneutrino signal at SNO+. The bulk crustal geoneutrino signal at SNO+ is estimated to be TNU, which is lower than that predicted in a global scale reference model that uses an average composition of the global upper continental crust [Huang et al., 2013], due to the fact that Archean to Proterozoic Canadian Shield has lower U and Th concentrations. Finally, without accounting for uncertainties on the signal from continental lithospheric mantle and convecting mantle, the total geoneutrino signal at SNO+ is predicted to be TNU
      PubDate: 2014-09-02T03:28:11.190576-05:
      DOI: 10.1002/2014GC005397
  • A geodetic plate motion and global strain rate model
    • Authors: Corné Kreemer; Geoffrey Blewitt, Elliot C. Klein
      Pages: n/a - n/a
      Abstract: We present a new global model of plate motions and strain rates in plate boundary zones constrained by horizontal geodetic velocities. This Global Strain Rate Model (GSRM v.2.1) is a vast improvement over its predecessor both in terms of amount of data input as in an increase in spatial model resolution by factor of ˜2.5 in areas with dense data coverage. We determined 6739 velocities from time‐series of (mostly) continuous GPS measurements; i.e., by far the largest global velocity solution to date. We transformed 15772 velocities from 233 (mostly) published studies onto our core solution to obtain 22511 velocities in the same reference frame. Care is taken to not use velocities from stations (or time periods) that are affected by transient phenomena; i.e., this data‐set consists of velocities best representing the interseismic plate velocity. About 14% of the Earth is allowed to deform in 145,086 deforming grid cells (0.25º longitude by 0.2º latitude in dimension). The remainder of the Earth’s surface is modeled as rigid spherical caps representing 50 tectonic plates. For 36 plates we present new GPS‐derived angular velocities. For all the plates that can be compared with the most recent geologic plate motion model, we find that the difference in angular velocity is significant. The rigid‐body rotations are used as boundary conditions in the strain rate calculations. The strain rate field is modeled using the Haines and Holt method, which uses splines to obtain an self‐consistent interpolated velocity gradient tensor field, from which strain rates, vorticity rates, and expected velocities are derived. We also present expected faulting orientations in areas with significant vorticity, and update the no‐net rotation reference frame associated with our global velocity gradient field. Finally, we present a global map of recurrence times for Mw=7.5 characteristic earthquakes.
      PubDate: 2014-08-30T09:03:17.45883-05:0
      DOI: 10.1002/2014GC005407
  • Source‐related variables for the description of the oceanic carbon
    • Authors: G. Walin; J. Hieronymus, J. Nycander
      Pages: n/a - n/a
      Abstract: The oceanic carbon system is commonly described in terms of the two state variables total carbon, DIC, and alkalinity, Alk. Here we suggest the use of alternative source adapted state variables, Acidic Carbon, AC and Basic Carbon, BC, defined by and related to (DIC, Alk) with a simple linear transformation. (AC, BC) can be interpreted as representing respectively the supply to the system of carbon dioxide and dissolved carbonate, keeping in mind that supply of hydrogen ions acts to transform from basic carbon to acidic carbon. Accordingly these variables tell us how much carbon dioxide or dissolved carbonate we actually have in the water, despite the fact that the major part of the carbon resides in bicarbonate ions. We claim that using these source related variables as a compliment to the traditional variables, offers a number of advantages in the formulation of continuity equations, as well as in the interpretation of observations and modelling results. The traditional definition of alkalinity is related to a measuring procedure rather than to the supply of material to the system. Here we demonstrate that alkalinity, though defined in the traditional way, may be interpreted in terms of sources and sinks acting on the system. In the case of ocean water this amounts to twice the supply of dissolved carbonate minus the net supply of free hydrogen ions. We argue that this interpretation is a useful complement to the traditional definition. Every process that affects the state of the carbon system may be quantified in terms of supply of carbon dioxide, Fa, carbonate ions, Fb, or hydrogen ions, E.
      PubDate: 2014-08-27T11:35:54.719499-05:
      DOI: 10.1002/2014GC005383
  • The magnitude distribution of dynamically triggered earthquakes
    • Authors: Stephen Hernandez; Emily E. Brodsky, Nicholas J. van der Elst
      Pages: n/a - n/a
      Abstract: Large dynamic strains carried by seismic waves are known to trigger seismicity far from their source region. It is unknown, however, whether surface waves trigger only small earthquakes, or whether they can also trigger large earthquakes. To partially address this question, we evaluate whether current data can distinguish between the magnitude distribution of triggered and untriggered small earthquakes. We use a mixing model approach in which total seismicity is decomposed into 2 classes: “triggered” events initiated or advanced by far‐field dynamic strains, and “untriggered” spontaneous events consisting of everything else. The b‐value of a mixed data set, bMIX, is decomposed into a weighted sum of b‐values of its constituent components, bT and bU. We utilize the previously observed relationship between triggering rate and dynamic strain amplitude to identify the fraction of triggered events in populations of earthquakes and then invert for bT. For Californian seismicity, data are consistent with a single‐parameter Gutenberg‐Richter hypothesis governing the magnitudes of both triggered and untriggered earthquakes.
      PubDate: 2014-08-27T11:18:34.072837-05:
      DOI: 10.1002/2014GC005404
  • Integrated geophysical‐petrological modeling of
           lithosphere‐asthenosphere boundary in central Tibet using
           electromagnetic and seismic data
    • Authors: Jan Vozar; Alan G. Jones, Javier Fullea, Matthew R. Agius, Sergei Lebedev, Florian Le Pape, Wenbo Wei
      Pages: n/a - n/a
      Abstract: We undertake a petrologically‐driven approach to jointly model magnetotelluric (MT) and seismic surface wave dispersion (SW) data from central Tibet, constrained by topographic height. The approach derives realistic temperature and pressure distributions within the upper mantle and characterizes mineral assemblages of given bulk chemical compositions as well as water content. This allows us to define a bulk geophysical model of the upper mantle based on laboratory and xenolith data for the most relevant mantle mineral assemblages and to derive corresponding predicted geophysical observables. One‐dimensional deep resistivity models were derived for two groups of MT stations. One group, located in the Lhasa Terrane, shows the existence of an electrically conductive upper‐mantle layer and shallower conductive upper‐mantle layer for the other group, located in the Qiangtang Terrane. The subsequent one‐dimensional integrated petrological‐geophysical modeling suggests a lithosphere‐asthenosphere boundary (LAB) at a depth of 80‐120 km with a dry lithosphere for the Qiangtang Terrane. In contrast, for the Lhasa Terrane the LAB is located at about 180 km but the presence of a small amount of water in the lithospheric mantle (
      PubDate: 2014-08-27T11:15:38.339924-05:
      DOI: 10.1002/2014GC005365
  • Cenozoic epeirogeny of the Arabian Peninsula from drainage modeling
    • Authors: J. W. P. Wilson; G. G. Roberts, M. J. Hoggard, N. J. White
      Pages: n/a - n/a
      Abstract: It is generally accepted that the Arabian Peninsula has been uplifted by sub‐crustal processes. Positive residual depth anomalies from oceanic crust in the Red Sea and in the Gulf of Aden suggest that a region surrounding this peninsula is dynamically supported. Admittance calculations, surface wave tomography studies, and receiver function analyses all imply that regional topography is generated and maintained by some combination of mantle convective circulation and lithospheric thickness changes. Despite these significant advances, the spatial and temporal uplift rate history of the Arabian Peninsula is not well known. Here, we show that a regional uplift rate history can be obtained by jointly inverting 225 longitudinal river profiles that drain this peninsula. Our strategy assumes that shapes of individual river profiles are controlled by uplift rate history and moderated by erosional processes. We used local measurements of incision rate to calibrate the relevant erosional parameters. In our inverse algorithm, uplift rate is permitted to vary smoothly as a function of space and time but upstream drainage area remains invariant. We also assume that knickzone migration is not lithologically controlled. Implications of these important assumptions have been investigated. Our results suggest that the Arabian Peninsula underwent two phase asymmetric uplift during the last 20–30 Ma at rates of 0.05–0.1 mm/yr– 1. The southwestern flank of the peninsula has been uplifted by 1.5–2.5 km. Regional stratigraphic constraints, the age and composition of volcanism, paleosol formation, incised peneplains, emergent marine terraces, and thermochronometric measurements corroborate our calculated patterns of uplift. Progessive development of three domal swells along the western margin of the peninsula is consistent with localized upwelling of hot asthenospheric mantle.
      PubDate: 2014-08-27T00:02:26.109071-05:
      DOI: 10.1002/2014GC005283
  • Tectonics, topography, and river system transition in East Tibet: Insights
           from the sedimentary record in Taiwan
    • Authors: Qing Lan; Yi Yan, Chi‐Yue Huang, Peter D. Clift, Xuejie Li, Wenhuang Chen, Xingchang Zhang, Mengming Yu
      Pages: n/a - n/a
      Abstract: [1] The Cenozoic in East Asia is marked by major changes in tectonics, landscapes and river systems, although the timing and nature of such changes remains disputed. We investigate the geochemistry and neodymium isotope character of Cenozoic mudstones spanning the breakup‐unconformity in the Western Foothills of Taiwan in order to constrain erosion and drainage development in southern China during the opening of the South China Sea. The La/Lu, Eu/Eu*, Th/Sc, Th/La, Cr/Th and εNd values in these rocks show an abrupt change between ca. 31 Ma and 25 Ma. Generally the higher εNd values in sediments deposited prior to 31 Ma indicate erosion from Phanerozoic granitic sources exposed in coastal South China, whereas the lower εNd values suggest that the main sources had evolved to inland southern China by ca. 25 Ma. The SHRIMP U‐Pb ages of zircons from a tuff, together with biostratigraphy data constrain the breakup‐unconformity to be between ca. 39 and 33 Ma, suggesting that the seafloor spreading in the South China Sea commenced before ca. 33 Ma. This is significantly older than most of the oceanic crust preserved in the deeper part of the basin. Diachronous westward younging of the breakup‐unconformities and provenance changes of basins are consistent with seafloor spreading propagating from east to west. Initial spreading of the South China Sea prior to ca. 33 Ma corresponds to tectonic adjustment in East Asia, including extrusion of the Indochina block and the rotation and eastward retreat of the subducting Pacific Plate.
      PubDate: 2014-08-26T05:14:49.41221-05:0
      DOI: 10.1002/2014GC005310
  • The relationship between seismicity and fault structure on the Discovery
           transform fault, East Pacific Rise
    • Authors: Monica Wolfson‐Schwehr; Margaret S. Boettcher, Jeffrey J. McGuire, John A. Collins
      Pages: n/a - n/a
      Abstract: There is a global seismic moment deficit on mid‐ocean ridge transform faults, and the largest earthquakes on these faults do not rupture the full fault area. We explore the influence of physical fault structure, including step‐overs in the fault trace, on the seismic behavior of the Discovery transform fault, 4S on the East Pacific Rise. One year of microseismicity recorded during a 2008 ocean bottom seismograph deployment (24,377 0 ≤ ML ≤ 4.6 earthquakes) and 24 years of Mw ≥ 5.4 earthquakes obtained from the Global Centroid Moment Tensor catalog, are correlated with surface fault structure delineated from high‐resolution multibeam bathymetry. Each of the 15 5.4 ≤ Mw ≤ 6.0 earthquakes that occurred on Discovery between January 1, 1990 ‐ April 1, 2014 was relocated into one of five distinct rupture patches using a teleseismic surface wave cross‐correlation technique. Microseismicity was relocated using the HypoDD relocation algorithm. The western fault segment of Discovery (DW) is composed of three zones of varying structure and seismic behavior: a zone with no large events and abundant microseismicity, a fully coupled zone with large earthquakes, and a complex zone with multiple fault strands and abundant seismicity. In general, microseismicity is reduced within the patches defined by the large, repeating earthquakes. While the extent of the large rupture patches on DW correlates with physical features in the bathymetry, step‐overs in the primary fault trace are not observed at patch boundaries, suggesting along‐strike heterogeneity in fault zone properties controls the size and location of the large events.
      PubDate: 2014-08-21T11:18:08.798583-05:
      DOI: 10.1002/2014GC005445
  • History and dynamics of net rotation of the mantle and lithosphere
    • Authors: M. L. Rudolph; S.J. Zhong
      Pages: n/a - n/a
      Abstract: The net rotation of Earth's lithosphere with respect to the underlying mantle is the longest‐wavelength component of toroidal flow in the mantle and is sensitive to both mantle buoyancy structure and lateral viscosity variations. The lithospheric net rotation in the geologic past implied by plate reconstructions using a hotspot reference frame for the past 100 Myr is up to five times greater than the present‐day rate of lithospheric net rotation. We explore the role of lateral viscosity variations associated with subcontinental keels in producing the lithospheric net rotation for the geologic past and find that the introduction of subcontinental keels improves the agreement between modeled net rotation and the net rotation present in the plate reconstructions for the past 25 Myr. However, our models with continental keels produce at most 0.16°/Myr of differential rotation between the lithosphere and lower mantle for present‐day, and explaining the most rapid rates of lithospheric net rotation during the Cretaceous and Paleogene remains challenging. This suggests the need for either an additional mechanism for generating lithospheric net rotation, or an adjustment to the absolute mantle reference frame relative to which plate motions are specified.
      PubDate: 2014-08-18T22:36:58.979803-05:
      DOI: 10.1002/2014GC005457
  • Ambient tectonic stress as fragile geological feature
    • Authors: Norman H. Sleep
      Pages: n/a - n/a
      Abstract: Strong seismic waves produce frictional failure within shallow pervasively cracked rocks. Distributed failure preferentially relaxes ambient tectonic stresses, providing a fragility measure of past strong shaking. Relaxation of the regional fault‐normal compression appears to have occurred within granite from 768 m down to ~1000‐1600 m depth at the Pilot Hole near Parkfield, California. Subsequent movements on the main fault have imposed strike‐slip stress within the relaxed region. Peak ground velocities of ~2 m s‐1 are inferred for infrequent (few 1000 yr recurrence) past earthquakes from stress relaxation within the granite and from the variation of S‐wave velocity with depth in the overlying sandstone. Conversely, frequent strong shaking in slowly deforming regions relaxes shallow ambient tectonic stress. This situation is expected beneath Whittier Narrows, where strong Love waves from numerous San Andreas events repeatedly produced nonlinear behavior.
      PubDate: 2014-08-18T03:21:00.697367-05:
      DOI: 10.1002/2014GC005426
  • Reconciling mantle attenuation‐temperature relationships from
           seismology, petrology, and laboratory measurements
    • Authors: GA Abers; KM Fischer, G Hirth, DA Wiens, T Plank, BK Holtzman, C McCarthy, E. Gazel
      Pages: n/a - n/a
      Abstract: Seismic attenuation measurements provide a powerful tool for sampling mantle properties. Laboratory experiments provide calibrations at seismic frequencies and mantle temperatures for dry melt‐free rocks, but require ~102 ‐ 103 extrapolations in grain size to mantle conditions; also, the effects of water and melt are not well understood. At the same time, body‐wave attenuation measured from dense broadband arrays provides reliable estimates of shear‐wave attenuation (QS‐1), affording an opportunity for calibration. We reanalyze seismic datasets that sample arc and back‐arc mantle, in Central America, the Marianas and the Lau basin, confirming very high attenuation (QS ~ 25‐80) at 1 Hz and depths of 50‐100 km. At each of these sites, independent petrological studies constrain the temperature and water content where basaltic magmas last equilibrated with the mantle, 1300‐1450°C. The QS measurements correlate inversely with the petrologically‐inferred temperatures, as expected. However, dry attenuation models predict QS too high by a factor of 1.5‐5. Modifying models to include effects of H2O and rheology‐dependent grain size shows that the effects of water‐enhanced dissipation and water‐enhanced grain growth nearly cancel, so H2O effects are modest. Therefore high H2O in the arc source region cannot explain the low QS, nor in the backarc where lavas show modest water content. Most likely the high attenuation reflects the presence of melt, and some models of melt effects come close to reproducing observations. Overall, body‐wave QS can be reconciled with petrologic and laboratory inferences of mantle conditions if melt has a strong influence beneath arcs and backarcs.
      PubDate: 2014-08-16T01:24:55.445493-05:
      DOI: 10.1002/2014GC005444
  • Identification and environmental interpretation of diagenetic and biogenic
           greigite in sediments: A lesson from the Messinian Black Sea
    • Authors: Liao Chang; Iuliana Vasiliev, Christiaan van Baak, Wout Krijgsman, Mark J. Dekkers, Andrew P. Roberts, John D. Fitz Gerald, Annelies van Hoesel, Michael Winklhofer
      Pages: n/a - n/a
      Abstract: Greigite (Fe3S4) is a widespread authigenic magnetic mineral in anoxic sediments, and is also commonly biosynthesized by magnetotactic bacteria in aqueous environments. While the presence of fossilized bacterial magnetite (Fe3O4) has now been widely demonstrated, the preservation of greigite magnetofossils in the geological record is only poorly constrained. Here, we investigate Mio‐Pliocene sediments of the former Black Sea to test whether we can detect greigite magnetofossils, and to unravel potential environmental controls on greigite formation. Our magnetic analyses and transmission electron microscope (TEM) observations indicate the presence of both diagenetic and bacterial greigite, and suggest a potentially widespread preservation of greigite magnetofossils in ancient sediments, which has important implications for assessing the reliability of paleomagnetic records carried by greigite. TEM‐based chemical and structural analyses also indicate the common presence of nickel‐substituted diagenetic iron sulfide crystals with a ferrimagnetic greigite structure. In addition, our cyclostratigraphic framework allows correlation of magnetic properties of Messinian former Black Sea sediments (Taman Peninsula, Russia) to global climate records. Diagenetic greigite enhancements appear to be climatically controlled, with greigite mainly occurring in warm/wet periods. Diagenetic greigite formation can be explained by variations in terrigenous inputs and dissolved pore water sulfate concentrations in different sedimentary environments. Our analysis demonstrates the usefulness of greigite for studying long‐term climate variability in anoxic environments.
      PubDate: 2014-08-16T01:23:37.2059-05:00
      DOI: 10.1002/2014GC005411
  • Nd and Sr isotope compositions of different phases of surface sediments in
           the South Pacific: Extraction of seawater signatures, boundary exchange,
           and detrital/dust provenance
    • Authors: Mario Molina‐Kescher; Martin Frank, Ed C. Hathorne
      Pages: n/a - n/a
      Abstract: The radiogenic isotope composition of neodymium (Nd) and strontium (Sr) are useful tools to investigate present and past oceanic circulation or input of terrigenous material. We present Nd and Sr isotope compositions extracted from different sedimentary phases, including early diagenetic Fe‐Mn coatings, ‘unclean' foraminiferal shells, fossil fish teeth, and detritus of marine surface sediments (core‐tops) covering the entire mid‐latitude South Pacific. Comparison of detrital Nd isotope compositions to deep‐water values from the same locations suggest that ‘boundary exchange' has little influence on the Nd isotope composition of western South Pacific seawater. Concentrations of Rare Earth Elements (REE) and Al/Ca ratios of ‘unclean' planktonic foraminifera suggest that this phase is a reliable recorder of seawater Nd isotope composition. The signatures obtained from fish teeth and ‘non‐decarbonated' leachates of bulk sediment Fe‐Mn oxyhydroxide coatings also agree with ‘unclean' foraminifera. Direct comparison of Nd isotope compositions extracted using these methods with seawater Nd isotope compositions is complicated by the low accumulation rates yielding radiocarbon ages of up to 24 kyrs, thus mixing the signal of different ocean circulation modes. This suggests that different past seawater Nd isotope compositions have been integrated in authigenic sediments from regions with low sedimentation rates. Combined detrital Nd and Sr isotope signatures indicate a dominant role of the Westerly winds transporting lithogenic material from South New Zealand and Southeastern Australia to the open South Pacific. The proportion of this material decreases towards the east, where supply from the Andes increases and contributions from Antarctica cannot be ruled out.
      PubDate: 2014-08-05T02:43:27.981471-05:
      DOI: 10.1002/2014GC005443
  • Metasomatized ancient lithospheric mantle beneath the young Zealandia
           microcontinent and its role in HIMU‐like intraplate magmatism
    • Authors: JM Scott; TE Waight, QHA van der Meer, JM Palin, AF Cooper, C Münker
      Pages: n/a - n/a
      Abstract: There has been long debate on the asthenospheric versus lithospheric source for numerous intraplate basalts with ocean island basalt (OIB) and high time‐integrated U/Pb (HIMU)‐like source signatures that have erupted through the Zealandia continental crust. Analysis of 157 spinel facies peridotitic mantle xenoliths from 25 localities across Zealandia permits the first comprehensive regional description of the sub‐continental lithospheric mantle (SCLM) and insights into whether it could be a source to the intraplate basalts. Contrary to previous assumptions, the Oligocene‐Miocene Zealandia SCLM is highly heterogeneous. It is composed of a refractory craton‐like domain (West Otago) adjacent to several moderately fertile domains (East Otago, North Otago, Auckland Islands). Each domain has an early history decoupled from the overlying Carboniferous and younger continental crust, and each domain has undergone varying degrees of depletion followed by re‐enrichment. Clinopyroxene grains reveal trace element characteristics (low Ti/Eu, high Th/U) consistent with enrichment through reaction with carbonatite. This metasomatic overprint has a composition that closely matches HIMU in Sr, Pb + Nd isotopes. However, clinopyroxene Hf isotopes are in part highly radiogenic and decoupled from the other isotope systems, and also mostly more radiogenic than the intraplate basalts. If the studied spinel facies xenoliths are representative of the thin Zealandia SCLM, the melting of garnet facies lithosphere could only be the intraplate basalt source if it had a less radiogenic Hf‐Nd isotope composition than the investigated spinel facies, or was mixed asthenospheric‐derived melts containing less radiogenic Hf.
      PubDate: 2014-08-02T03:49:45.763415-05:
      DOI: 10.1002/2014GC005300
  • Persistent uplift of the Lazufre volcanic complex (Central Andes): New
           insights from PCAIM inversion of InSAR time series and GPS data
    • Authors: D. Remy; J.L Froger, H Perfettini, S. Bonvalot, G. Gabalda, F. Albino, V. Cayol, D. Legrand, M. De Saint Blanquat
      Pages: n/a - n/a
      Abstract: We reanalyzed the surface Displacements observed at the Lazufre Volcanic Complex in the Southern Andean Central Volcanic Zone using GPS measurements made between 2006 and 2008 and a large InSAR dataset. We performed a detailed spatiotemporal analysis of the displacements using a Principal Component Analysis Inversion Method (PCAIM). The PCAIM reveals a source with no significant changes in shape and dimension and with a remarkably linear strength increase over the whole period of observation (i.e. 2003 to 2010). Then we used a three‐dimensional Mixed Boundary Element Method (MBEM) to invert the first component of surface displacement as obtained from PCAIM. We explored a continuum of geometries from a shallow elliptic crack to a deep massive truncated elliptical cone that could represent a sill or a large magma chamber, respectively. The best models indicate a large flat‐topped source with a roof area between 40 and 670 km2 and a depth of between 2 and 14 km below ground surface. Lastly, on the basis of the limited data available for the thermo‐mechanical structure of the crust in the Southern Andean Central Volcanic Zone we consider some possible scenarios to explain the spatial and temporal pattern of displacements at Lazufre.
      PubDate: 2014-07-31T03:17:28.850604-05:
      DOI: 10.1002/2014GC005370
  • Coring disturbances in IODP piston cores with implications for offshore
           record of volcanic events and the Missoula megafloods
    • Authors: Martin Jutzeler; James D. L. White, Peter J. Talling, Molly McCanta, Sally Morgan, Anne Le Friant, Osamu Ishizuka
      Pages: n/a - n/a
      Abstract: Piston cores collected from IODP drilling platforms (and its predecessors) provide the best long‐term geological and climatic record of marine sediments worldwide. Coring disturbances affecting the original sediment texture have been recognized since the early days of coring, and include deformation resulting from shear of sediment against the core barrel, basal flow‐in due to partial stroke, loss of stratigraphy, fall‐in, sediment loss through core catchers, and structures formed during core recovery and on‐deck transport. The most severe disturbances occur in non‐cohesive (sandy) facies, which are particularly common in volcanogenic environments and submarine fans. Although all of these types of coring disturbances have been recognized previously, our contribution is novel because it provides an easily accessible summary of methods for their identification. This contribution gives two specific examples on the importance of these coring disturbances. We show how suck‐in of sediments during coring artificially created very thick volcaniclastic sand layers in cores offshore Montserrat and Martinique (Lesser Antilles). We then analyze very thick, structureless sand layers from the Escanaba Trough inferred to be a record of the Missoula mega‐floods. These sand layers tend to coincide with the base of core sections, and their facies suggest coring disturbance by basal flow‐in, destroying the original structure and texture of the beds. We conclude by outlining and supporting IODP‐led initiatives to further reduce and identify coring disturbances, and acknowledge their recent successes in drilling challenging sand‐rich settings, such as during IODP Expedition 340.
      PubDate: 2014-07-24T04:22:23.637509-05:
      DOI: 10.1002/2014GC005447
  • Tectonic structure, lithology, and hydrothermal signature of the Rainbow
           massif (Mid‐Atlantic Ridge 36°14’N)
    • Authors: Muriel Andreani; Javier Escartin, Adélie Delacour, Benoit Ildefonse, Marguerite Godard, Jérôme Dyment, Anthony E. Fallick, Yves Fouquet
      Pages: n/a - n/a
      Abstract: Rainbow is a dome‐shaped massif at the 36°14’N non‐transform offset along the Mid‐Atlantic Ridge. It hosts three ultramafic‐hosted hydrothermal sites: Rainbow is active and high‐temperature; Clamstone and Ghost City are fossil and low‐temperature. The MoMARDREAM cruises (2007, 2008) presented here provided extensive rock sampling throughout the massif that constrains the geological setting of hydrothermal activity. The lithology is heterogeneous with abundant serpentinites surrounding gabbros, troctolites, chromitites, plagiogranites, and basalts. We propose that a W‐dipping detachment fault, now inactive, uplifted the massif and exhumed these deep‐seated rocks. Present‐day deformation is accommodated by SSW‐NNE faults and fissures, consistent with oblique teleseismic focal mechanisms and stress rotation across the discontinuity. Faults localize fluid flow and control the location of fossil and active hydrothermal fields that appear to be ephemeral and lacking in spatio‐temporal progression. Markers of high‐temperature hydrothermal activity (˜350°C) are restricted to some samples from the active field while a more diffuse, lower‐temperature hydrothermal activity (
      PubDate: 2014-07-12T06:30:09.246195-05:
      DOI: 10.1002/2014GC005269
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