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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: 42, SJR: 2.668, h-index: 142)
Global Biogeochemical Cycles     Full-text available via subscription   (Followers: 3, 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: 14)
J. of Geophysical Research : Planets     Full-text available via subscription   (Followers: 12)
J. of Geophysical Research : Solid Earth     Full-text available via subscription   (Followers: 22)
J. of Geophysical Research : Space Physics     Full-text available via subscription   (Followers: 13)
Paleoceanography     Full-text available via subscription   (Followers: 4, SJR: 2.16, h-index: 82)
Radio Science     Full-text available via subscription   (Followers: 2, 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: 123, 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]
  • Late Pleistocene stratigraphy of IODP Site U1396 and compiled chronology
           offshore of south and south west Montserrat, Lesser Antilles
    • Authors: Deborah Wall‐Palmer; Maya Coussens, Peter J. Talling, Martin Jutzeler, Michael Cassidy, Isabelle Marchant, Martin R. Palmer, Sebastian F. L. Watt, Christopher W. Smart, Jodie K. Fisher, Malcolm B. Hart, Andrew Fraass, Jessica Trofimovs, Anne Le Friant, Osamu Ishizuka, Tatsuya Adachi, Mohammed Aljahdali, Georges Boudon, Christoph Breitkreuz, Daisuke Endo, Akihiko Fujinawa, Robert Hatfield, Matthew J. Hornbach, Kyoko Kataoka, Sara Lafuerza, Fukashi Maeno, Michael Manga, Michael Martinez‐Colon, Molly McCanta, Sally Morgan, Takeshi Saito, Angela L. Slagle, Adam J. Stinton, K. S. V. Subramanyam, Yoshihiko Tamura, Benoit Villemant, Fei Wang
      Pages: n/a - n/a
      Abstract: Marine sediments around volcanic islands contain an archive of volcaniclastic deposits, which can be used to reconstruct the volcanic history of an area. Such records hold many advantages over often incomplete terrestrial data sets. This includes the potential for precise and continuous dating of intervening sediment packages, which allow a correlatable and temporally constrained stratigraphic framework to be constructed across multiple marine sediment cores. Here we discuss a marine record of eruptive and mass‐wasting events spanning ∼250 ka offshore of Montserrat, using new data from IODP Expedition 340, as well as previously collected cores. By using a combination of high‐resolution oxygen isotope stratigraphy, AMS radiocarbon dating, biostratigraphy of foraminifera and calcareous nannofossils, and clast componentry, we identify five major events at Soufriere Hills volcano since 250 ka. Lateral correlations of these events across sediment cores collected offshore of the south and south west of Montserrat have improved our understanding of the timing, extent and associations between events in this area. Correlations reveal that powerful and potentially erosive density‐currents traveled at least 33 km offshore and demonstrate that marine deposits, produced by eruption‐fed and mass‐wasting events on volcanic islands, are heterogeneous in their spatial distribution. Thus, multiple drilling/coring sites are needed to reconstruct the full chronostratigraphy of volcanic islands. This multidisciplinary study will be vital to interpreting the chaotic records of submarine landslides at other sites drilled during Expedition 340 and provides a framework that can be applied to the stratigraphic analysis of sediments surrounding other volcanic islands.
      PubDate: 2014-07-25T11:12:51.967581-05:
      DOI: 10.1002/2014GC005402
  • Low‐frequency earthquakes reveal punctuated slow slip on the deep
           extent of the Alpine Fault, New Zealand
    • Authors: Calum J. Chamberlain; David R. Shelly, John Townend, Tim A. Stern
      Pages: n/a - n/a
      Abstract: We present the first evidence of low‐frequency earthquakes (LFEs) associated with the deep extension of the transpressional Alpine Fault beneath the central Southern Alps of New Zealand. Our database comprises a temporally continuous 36 month‐long catalog of 8760 LFEs within 14 families. To generate this catalog, we first identify 14 primary template LFEs within known periods of seismic tremor and use these templates to detect similar events in an iterative stacking and cross‐correlation routine. The hypocentres of 12 of the 14 LFE families lie within 10 km of the inferred location of the Alpine Fault at depths of approximately 20–30 km, in a zone of high P‐wave attenuation, low P‐wave speeds, and high seismic reflectivity. The LFE catalog consists of persistent, discrete events punctuated by swarm‐like bursts of activity associated with previously and newly identified tremor periods. The magnitudes of the LFEs range between ML – 0.8 and ML 1.8, with an average of ML 0.5. We find that the frequency‐magnitude distribution of the LFE catalog both as a whole and within individual families is not consistent with a power law, but that individual families' frequency‐amplitude distributions approximate an exponential relationship, suggestive of a characteristic length‐scale of failure. We interpret this LFE activity to represent quasi‐continuous slip on the deep extent of the Alpine Fault, with LFEs highlighting asperities within an otherwise steadily creeping region of the fault.
      PubDate: 2014-07-25T11:12:20.430978-05:
      DOI: 10.1002/2014GC005436
  • Evidence of an upper mantle seismic anomaly opposing the Cocos slab
           beneath the Isthmus of Tehuantepec, Mexico
    • Authors: YoungHee Kim; Hobin Lim, Meghan S. Miller, Fred Pearce, Robert W. Clayton
      Pages: n/a - n/a
      Abstract: Subduction of the Cocos plate beneath southern Mexico is characterized by several unusual features, such as a discontinuous volcanic arc, unusual arc chemistry, and anomalously low topography of Tehuantepec Isthmus. Recent seismic images from both receiver functions and seismic tomography suggest that there may be an additional, opposing structure dipping to the southwest from the Gulf of Mexico, and these images have been previously explained by a southwest‐dipping slab. However, standard models of the Caribbean tectonic history do not support this interpretation. To better define the Cocos slab's structure and the possible existence of a structure dipping in the opposite direction, dense seismic data across southern Mexico are used to form high‐resolution seismic images, based on the 2‐D generalized radon transform method, and to relocate regional earthquakes. Our images show the Cocos plate dipping at 30° to the northeast encounters the anomaly that is dipping in the opposite sense at ∼150 km depth. Relocated seismicity clearly delineates a Wadati‐Benioff zone that marks the subducting Cocos plate. A cluster of seismicity also appears at ∼150 km depth which may be related to the subduction of the Tehuantepec ridge and/or to the imaged seismic structure with opposite polarity.
      PubDate: 2014-07-25T11:04:43.127986-05:
      DOI: 10.1002/2014GC005320
  • Composition and flux of explosive gas release at LUSI mud volcano (East
           Java, Indonesia)
    • Authors: Loÿc Vanderkluysen; Michael R. Burton, Amanda B. Clarke, Hilairy E. Hartnett, Jean‐François Smekens
      Pages: n/a - n/a
      Abstract: The LUSI mud volcano has been erupting since May 2006 in the densely populated Sidoarjo regency (East Java, Indonesia), forcing the evacuation of 40,000 people and destroying industry, farmland, and over 10,000 homes. Mud extrusion rates of 180,000 m3 d−1 were measured in the first few months of the eruption, decreasing to a loosely documented 4000 m for methane and approximately 600 m for carbon dioxide; however, the mass fractions of these gases are insufficient to explain the observed dynamics. Rather, the primary driver of the cyclic bubble‐bursting activity is decompressional boiling of water, which initiates a few tens of meters below the surface, setting up slug flow in the upper conduit. Our measured gas flux and conceptual model lead to a corresponding upper‐bound estimate for the mud‐water mass flux of 105 m3 d−1.
      PubDate: 2014-07-24T14:56:14.927819-05:
      DOI: 10.1002/2014GC005275
  • Normal faulting sequence in the Pumqu‐Xainza Rift constrained by
           InSAR and teleseismic body‐wave seismology
    • Authors: Hua Wang; John R. Elliott, Timothy J. Craig, Tim J. Wright, Jing Liu‐Zeng, Andrew Hooper
      Pages: n/a - n/a
      Abstract: Normal faulting earthquakes play an important role in the deformation of continents, and pose significant seismic hazard, yet important questions remain about their mechanics. We use InSAR and body‐wave seismology to compute dislocation models and centroid moment solutions for four normal‐faulting earthquakes (Mw 5.7–6.2) that occurred in the Pumqu‐Xainza Rift (PXR), southern Tibet, a region where low‐angle normal faulting has previously been inferred. We also use the fault locations and slip to investigate the correlation between earthquakes and surface topography, and to calculate stress interactions between the earthquakes. The InSAR and body‐wave models give consistent focal mechanisms except for the magnitude of the 1996 event, which may be overestimated due to postseismic deformation in the long‐interval interferograms. We calculate the static stress changes due to coseismic slip and find that the 1993 event was too distant to cause triggering of the later events, but that the 1998 event pair occurred in regions of increased Coulomb stress resulting from the 1996 event. All the fault planes found here dip at 40–60°, reinforcing the absence in observations for low‐angle normal faulting earthquakes (dip 
      PubDate: 2014-07-24T14:55:17.643005-05:
      DOI: 10.1002/2014GC005369
  • Sea level and climate forcing of the Sr isotope composition of late
           Miocene Mediterranean marine basins
    • Authors: T. F. Schildgen; D. Cosentino, G. Frijia, F. Castorina, F. Ö. Dudas, A. Iadanza, G. Sampalmieri, P. Cipollari, A. Caruso, S. A. Bowring, M. R. Strecker
      Pages: n/a - n/a
      Abstract: Sr isotope records from marginal marine basins track the mixing between seawater and local continental runoff, potentially recording the effects of sea level, tectonic, and climate forcing in marine fossils and sediments. Our 110 new 87Sr/86Sr analyses on oyster and foraminifera samples from six late Miocene stratigraphic sections in southern Turkey, Crete, and Sicily show that 87Sr/86Sr fell below global seawater values in the basins several million years before the Messinian Salinity Crisis, coinciding with tectonic uplift and basin shallowing. 87Sr/86Sr from more centrally located basins (away from the Mediterranean coast) drop below global seawater values only during the Messinian Salinity Crisis. In addition to this general trend, 55 new 87Sr/86Sr analyses from the astronomically tuned Lower Evaporites in the central Apennines (Italy) allow us to explore the effect of glacio‐eustatic sea level and precipitation changes on 87Sr/86Sr. Most variation in our data can be explained by changes in sea level, with greatest negative excursions from global seawater values occurring during relative sea level lowstands, which generally coincided with arid conditions in the Mediterranean realm. We suggest that this greater sensitivity to lowered sea level compared with higher runoff could relate to the inverse relationship between Sr concentration and river discharge. Variations in the residence time of groundwater within the karst terrain of the circum‐Mediterranean region during arid and wet phases may help to explain the single (robust) occurrence of a negative excursion during a sea level highstand, but this explanation remains speculative without more detailed paleoclimatic data for the region.
      PubDate: 2014-07-24T14:52:43.557557-05:
      DOI: 10.1002/2014GC005332
  • 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
  • Reactive halogens (BrO and OClO) detected in the plume of Soufrière
           Hills Volcano during an eruption hiatus
    • Authors: Amy Donovan; Vitchko Tsanev, Clive Oppenheimer, Marie Edmonds
      Pages: n/a - n/a
      Abstract: Volcanic plumes are sites of dynamic chemistry involving halogen gases. Here we present new data on the relative abundances of SO2, BrO and OClO gases emitted from Soufrière Hills Volcano (SHV). They were collected during an eruptive hiatus but during sustained degassing at this halogen‐rich volcano. By comparison with data from a previous study during an eruptive phase (Bobrowski et al, 2003) and application of the data and modelling of Villemant et al. (2008), we suggest that, after consideration of errors, either the rate of HBr conversion to BrO is variable, ranging from ~30% to ~15%, and/or the relative partitioning of Cl and Br into the gas phase from the melt changes according to eruptive activity. We examine the potential implications of this for fluid‐melt partitioning, and compare our results with data from the experimental literature. Our work contributes towards understanding the controls on the BrO/SO2 ratio for volcano monitoring purposes; the changes in plume chemistry with regard to bromine at the onset of lava extrusion may be large and rapid. OClO was detected in the plume at SHV for the first time. This species has only previously been detected in emissions from Mount Etna (Bobrowski et al., 2007, using ground‐based methods) and from Puyehue Cordon Caulle (Theys et al., 2014, using satellite‐based methods). No HCHO or NOy species were detected in the spectra.
      PubDate: 2014-07-23T02:05:44.622788-05:
      DOI: 10.1002/2014GC005419
  • Extent and distribution of aseismic slip on the Ismetpaşa segment of
           the North Anatolian Fault (Turkey) from Persistent Scatterer InSAR
    • Authors: Esra Cetin; Ziyadin Cakir, Mustapha Meghraoui, Semih Ergintav, Ahmet M. Akoglu
      Pages: n/a - n/a
      Abstract: We use the Persistent Scatterer InSAR (PSI) technique with elastic dislocation models and geology along the creeping section of the North Anatolian Fault (NAF) at Ismetpaşa, to map and deduce the velocity field and the aseismic slip distribution. Revealing the spatiotemporal nature of the creep helped us associate the creep with potential lithological controls, hence providing a new perspective to better understand the underlying causes and mechanisms. The PSI analysis of Envisat ASAR images between 2003 and 2010 reveals a clear picture of surface creep along the fault and a new interseismic velocity field transitioning gradually between the creeping and the locked fault sections. The creep rate is found to fluctuate along a 100 km long section of the fault in a manner similar to that along the Hayward fault, reaching a maximum of ∼20 ± 2 mm/yr, close to the far field plate velocity (∼25 ± 1.5 mm/yr). At Ismetpaşa, it is in the range of 8 ± 2 mm/yr, consistent with the previous geodetic observations. The creeping section appears to extend 30 km further east than those previously reported. Modeling of the PSI data reveals a heterogeneous creep distribution at depth with two main patches confined mostly to the uppermost 5 km portion of the seismogenic crust, releasing annually 6.2 × 1016 Nm (Mw = 5.1) geodetic moment. Our analysis combined with previous studies suggests that creep might have commenced as postseismic deformation following the 1944 earthquake and has evolved to stable fault creep with time. There is a correlation between aseismic surface creep and the geology along the fault as it is in major part associated to rocks with low frictional strength such as the andesitic‐basaltic, limestone, and serpentine bodies within the fault zone.
      PubDate: 2014-07-22T10:24:55.958357-05:
      DOI: 10.1002/2014GC005307
  • Along‐axis hydrothermal flow at the axis of slow spreading
           Mid‐Ocean Ridges: Insights from numerical models of the Lucky Strike
           vent field (MAR)
    • Authors: Fabrice J. Fontaine; Mathilde Cannat, Javier Escartin, Wayne C. Crawford
      Pages: n/a - n/a
      Abstract: The processes and efficiency of hydrothermal heat extraction along the axis of mid‐ocean ridges are controlled by lithospheric thermal and permeability structures. Hydrothermal circulation models based on the structure of fast and intermediate spreading ridges predict that hydrothermal cell organization and vent site distribution are primarily controlled by the thermodynamics of high‐temperature mid‐ocean ridge hydrothermal fluids. Using recent constraints on shallow structure at the slow spreading Lucky Strike segment along the Mid‐Atlantic Ridge, we present a physical model of hydrothermal cooling that incorporates the specificities of a magma‐rich slow spreading environment. Using three‐dimensional numerical models, we show that, in contrast to the aforementioned models, the subsurface flow at Lucky Strike is primarily controlled by across‐axis permeability variations. Models with across‐axis permeability gradients produce along‐axis oriented hydrothermal cells and an alternating pattern of heat extraction highs and lows that match the distribution of microseismic clusters recorded at the Lucky Strike axial volcano. The flow is also influenced by temperature gradients at the base of the permeable hydrothermal domain. Although our models are based on the structure and seismicity of the Lucky Strike segment, across‐axis permeability gradients are also likely to occur at faster spreading ridges and these results may also have important implications for the cooling of young crust at fast and intermediate spreading centers.
      PubDate: 2014-07-22T10:24:45.538763-05:
      DOI: 10.1002/2014GC005372
  • Three‐dimensional inversion of seafloor magnetotelluric data
           collected in the Philippine Sea and the western margin of the northwest
           Pacific Ocean
    • Authors: Noriko Tada; Kiyoshi Baba, Hisashi Utada
      Pages: n/a - n/a
      Abstract: We report a result of three‐dimensional (3‐D) upper mantle electrical conductivity inversion of seafloor magnetotelluric data. We used existing data at 25 sites in the Philippine Sea and the western margin of the Pacific Ocean. In order to obtain a reliable model by 3‐D inversion, we evaluated the large and small‐scale topographic effects. We also conducted a comprehensive search of the one‐dimensional (1‐D) profiles of the study area in order to determine the best initial and prior models. A two‐phase inversion method was applied so that the error floors for the diagonal and off‐diagonal elements of the impedance tensor could be separately controlled. Through this first attempt at inverting real data, we obtained basic knowledge about tuning the inversion parameters and conditions. We also proposed a procedure to evaluate the reliability of the 3‐D conductivity anomalies imaged by the inversion by conducting checkerboard and sensitivity tests. After the iterations converged, 13 distinct anomalies were found in the inverted 3‐D conductivity model; four conductive and two resistive anomalies were confirmed to be resolved enough by the data through the checkerboard test. Then the sensitivity tests were conducted to quantify how each anomaly was required by the observed data, and we confirmed that the intensities of three conductive anomalies and one resistive anomaly were statistically significant. This paper presented an example of possible approach in 3‐D seafloor electromagnetic inversion procedure for imaging reliable electrical conductivity structure of the oceanic mantle, which will be useful in understanding dynamics and evolution of solid Earth.
      PubDate: 2014-07-22T10:24:35.311913-05:
      DOI: 10.1002/2014GC005421
  • Atmospheric transport of mineral dust from the Indo‐Gangetic Plain:
           Temporal variability, acid processing, and iron solubility
    • Authors: Bikkina Srinivas; M. M. Sarin, R. Rengarajan
      Pages: n/a - n/a
      Abstract: Atmospheric transport of chemical constituents from the Indo‐Gangetic Plain (IGP) to the Bay of Bengal is a conspicuous seasonal feature that occurs during the late NE‐monsoon (December‐March). With this perspective, aerosol composition and abundance of mineral dust have been studied during November 2009 ‐ March 2010 from a sampling site (Kharagpur: 22.3 N, 87.3E) in the IGP, representing the atmospheric outflow to the Bay of Bengal. The chemical composition of PM2.5 suggests the dominance of nss‐SO42‐ (6.9 – 24.3 µg m‐3); whereas the abundance of mineral dust varied from 3 to 18 µg m‐3. The concentration of aerosol iron (FeTot) and its fractional solubility (Fews % = Fews/FeTot *100, where Fews is the water‐soluble fraction of FeTot) varied from 60 to 1144 ng m‐3 and from 6.7 to 26.5 %, respectively. A striking similarity in the temporal variability of total inorganic acidity (TIA = NO3‐ + nss‐SO42‐) and Fews (%) provides evidence for acid processing of mineral dust (alluvium) during atmospheric transport from the IGP. The contribution of TIA to water‐soluble inorganic species [(nss‐SO42‐ + NO3‐)/ΣWSIS], mass ratios of Ca/Al and Fe/Al, abundance of dust (%) and Fews (%) in the IGP‐outflow are similar to the aerosol composition over the Bay of Bengal. With the rapid increase in anthropogenic activities over south and south‐east Asia, the enhanced fractional solubility of aerosol iron (attributed to acid processing of mineral dust) has implications to further increase the air‐sea deposition of Fe to the surface ocean.
      PubDate: 2014-07-22T02:09:59.622471-05:
      DOI: 10.1002/2014GC005395
  • Evolution of stress and fault patterns in oblique rift systems: 3‐D
           numerical lithospheric‐scale experiments from rift to breakup
    • Authors: Sascha Brune
      Pages: n/a - n/a
      Abstract: Rifting involves complex normal fault systems that are controlled by extension direction, reactivation of pre‐rift structures, sedimentation, and dyke dynamics. The relative impact of these factors on the observed fault pattern, however, is difficult to deduce from field‐based studies alone. This study provides insight in crustal stress patterns and fault orientations by employing a laterally homogeneous, 3D rift setup with constant extension velocity. The presented numerical forward experiments cover the whole spectrum of oblique extension. They are conducted using an elasto‐visco‐plastic finite element model and involve crustal and mantle layers accounting for self‐consistent necking of the lithosphere. Despite recent advances, 3D numerical experiments still require relatively coarse resolution so that individual faults are poorly resolved. This issue is addressed by applying a post‐processing method that identifies the stress regime and preferred fault azimuth at each surface element. The simple model setup results in a surprising variety of fault orientations that are solely caused by the three‐dimensionality of oblique rift systems. Depending on rift obliquity, these orientations can be grouped in terms of rift‐parallel, extension‐orthogonal, and intermediate normal fault directions as well as strike‐slip faults. While results compare well with analog rift models of low to moderate obliquity, new insight is gained in advanced rift stages and highly oblique settings. Individual fault populations are activated in a characteristic multi‐phase evolution driven by lateral density variations of the evolving rift system. In natural rift systems this pattern might be modified by additional heterogeneities, surface processes and dyke dynamics.
      PubDate: 2014-07-22T01:10:50.108671-05:
      DOI: 10.1002/2014GC005446
  • Robust coupled fluid‐particle simulation scheme in Stokes‐flow
           regime: Toward the geodynamic simulation including granular media
    • Authors: Mikito Furuichi; Daisuke Nishiura
      Pages: n/a - n/a
      Abstract: We present a simulation scheme for solving high‐viscosity fluid and particle dynamics in a coupled computational fluid dynamics and discrete element method (CFD‐DEM) framework. This simulation scheme is intended to be used for geodynamical magmatic studies such as crystal settling at the melting roof of a magma chamber. The high‐viscosity fluid is treated by the Stokes‐flow approximation, where the fluid interacts with particles via the drag force in a cell‐averaged manner. The particles are tracked with contact forces by DEM. To efficiently solve such Stokes‐DEM coupled equations, we propose two key techniques. One is formulation of particle motion without the inertial term, allowing a larger time step at higher viscosities. The other is a semi‐implicit treatment of the cell‐averaged particle velocity in the fluid equation to stabilize the calculation. We simulate the settling particles in strongly viscous fluids in three dimensions and compare the results with the experimental and theoretical results. Our solution strategy is found to be robust and successfully captures the collective behavior of the particles. The simulation method presented here will be useful in various fields interested in long‐term dynamics of high‐viscosity granular media.
      PubDate: 2014-07-17T11:07:30.809469-05:
      DOI: 10.1002/2014GC005281
  • Crustal and upper mantle structure beneath south‐western margin of
           the Arabian Peninsula from teleseismic tomography
    • Authors: Félicie Korostelev; Clémence Basuyau, Sylvie Leroy, Christel Tiberi, Abdulhakim Ahmed, Graham W. Stuart, Derek Keir, Frédérique Rolandone, Ismail Ganad, Khaled Khanbari, Lapo Boschi
      Pages: n/a - n/a
      Abstract: We image the lithospheric and upper asthenospheric structure of western continental Yemen with 24 broadband stations to evaluate the role of the Afar plume on the evolution of the continental margin and its extent eastward along the Gulf of Aden. We use teleseismic tomography to compute relative P wave velocity variations in south‐western Yemen down to 300 km depth. Published receiver function analysis suggest a dramatic and localized thinning of the crust in the vicinity of the Red Sea and the Gulf of Aden, consistent with the velocity structure that we retrieve in our model. The mantle part of the model is dominated by the presence of a low‐velocity anomaly in which we infer partial melting just below thick Oligocene flood basalts and recent off‐axis volcanic events (from 15 Ma to present). This low‐velocity anomaly could correspond to an abnormally hot mantle and could be responsible for dynamic topography and recent magmatism in western Yemen. Our new P wave velocity model beneath western Yemen suggests the young rift flank volcanoes beneath margins and on the flanks of the Red Sea rift are caused by focused small‐scale diapiric upwelling from a broad region of hot mantle beneath the area. Our work shows that relatively hot mantle, along with partial melting of the mantle, can persist beneath rifted margins after breakup has occurred.
      PubDate: 2014-07-17T10:54:05.416847-05:
      DOI: 10.1002/2014GC005316
  • Multi‐scale convection in a geodynamo simulation with uniform heat
           flux along the outer boundary
    • Authors: Hiroaki Matsui; Eric King, Bruce Buffett
      Pages: n/a - n/a
      Abstract: It is generally expected that Earth's magnetic field, which is generated by convecting liquid metal within its core, will substantially alter that convection through the action of Lorentz forces. In most dynamo simulations, however, Lorentz forces do very little to change convective flow, which is predominantly fine‐scaled. An important exception to this observation is in dynamo models that employ uniform heat flux boundary conditions, rather than the usual uniform temperature conditions, in which multi‐scale convection is observed. We investigate the combined influence of thermal boundary conditions and magnetic fields using four simulations: two dynamos and two non‐magnetic models, with either uniform temperature or heat flux fixed at the outer boundary. Of the four, only the fixed‐heat‐flux dynamo simulation produces multi‐scale convective flow patterns. Comparison between the models suggests that the fixed‐flux dynamo generates large patches of strong azimuthal magnetic field that suppress small‐scale convective motions. By allowing temperature to vary along the outer boundary, the fixed‐flux dynamo generates stronger azimuthal flow and, in turn, stronger magnetic field, and the resulting Lorentz forces alter the nature of convective flow. Extrapolation of the analyses presented here suggests that magnetic fields may also suppress small‐scale convection in the Earth's core.
      PubDate: 2014-07-17T04:08:41.726933-05:
      DOI: 10.1002/2014GC005432
  • Correlated patterns in hydrothermal plume distribution and apparent
           magmatic budget along 2500 km of the Southeast Indian Ridge
    • Authors: Edward T. Baker; Christophe Hémond, Anne Briais, Marcia Maia, Daniel S. Scheirer, Sharon L. Walker, Tingting Wang, Yongshun John Chen
      Pages: n/a - n/a
      Abstract: Multiple geological processes affect the distribution of hydrothermal venting along a mid‐ocean ridge. Deciphering the role of a specific process is often frustrated by simultaneous changes in other influences. Here we take advantage of the almost constant spreading rate (65‐71 mm/yr) along 2500 km of the Southeast Indian Ridge (SEIR) between 77°‐99°E to examine the spatial density of hydrothermal venting relative to regional and segment‐scale changes in the apparent magmatic budget. We use 227 vertical profiles of light backscatter and (on 41 profiles) oxidation‐reduction potential along 27 1st‐ and 2nd‐order ridge segments on and adjacent to the Amsterdam‐St. Paul (ASP) Plateau to map ph, the fraction of casts detecting a plume. At the regional scale, venting on the five segments crossing the magma‐thickened hotspot plateau is almost entirely suppressed (ph = 0.02). Conversely, the combined ph (0.34) from all other segments follows the global trend of ph versus spreading rate. Off the ASP Plateau, multi‐segment trends in ph track trends in the regional axial depth, high where regional depth increases and low where it decreases. At the individual segment scale, a robust correlation between ph and cross‐axis inflation for 1st‐order segments shows that different magmatic budgets among 1st‐order segments are expressed as different levels of hydrothermal spatial density. This correlation is absent among 2nd‐order segments. Eighty‐five percent of the plumes occur in eight clusters totaling ~350 km. We hypothesize that these clusters are a minimum estimate of the length of axial melt lenses underlying this section of the SEIR.
      PubDate: 2014-07-17T04:04:55.240659-05:
      DOI: 10.1002/2014GC005344
  • Effect of latent heat of freezing on crustal generation at low spreading
    • Authors: Norman H. Sleep; Jessica M. Warren
      Pages: n/a - n/a
      Abstract: Lithospheric structure changes at low spreading rates (
      PubDate: 2014-07-16T04:17:54.238951-05:
      DOI: 10.1002/2014GC005423
  • Insights into magmatic processes and hydrothermal alteration of in situ
           superfast spreading ocean crust at ODP/IODP Site 1256 from a cluster
           analysis of rock magnetic properties
    • Authors: Mark J. Dekkers; David Heslop, Emilio Herrero‐Bervera, Gary Acton, David Krasa
      Pages: n/a - n/a
      Abstract: We analyze magnetic properties from Ocean Drilling Program (ODP)/Integrated ODP (IODP) Hole 1256D (6°44.1' N, 91°56.1' W) on the Cocos Plate in ~15.2 Ma oceanic crust generated by superfast seafloor spreading, the only drill hole that has sampled all three oceanic crust layers in a tectonically undisturbed setting. Fuzzy c‐means cluster analysis and non‐linear mapping are utilized to study down‐hole trends in the ratio of the saturation remanent magnetization and the saturation magnetization, the coercive force, the ratio of the remanent coercive force and coercive force, the low‐field magnetic susceptibility, and the Curie temperature, to evaluate the effects of magmatic and hydrothermal processes on magnetic properties. A statistically robust five‐cluster solution separates the data predominantly into three clusters that express increasing hydrothermal alteration of the lavas, which differ from two distinct clusters mainly representing the dikes and gabbros. Extensive alteration can obliterate magnetic property differences between lavas, dikes, and gabbros. The imprint of thermochemical alteration on the iron‐titanium oxides is only partially related to the porosity of the rocks. Thus, the analysis complements interpretation based on electrofacies analysis. All clusters display rock magnetic characteristics compatible with an ability to retain a stable natural remanent magnetization suggesting that the entire sampled sequence of ocean crust can contribute to marine magnetic anomalies. Paleointensity determination is difficult because of the propensity of oxy‐exsolution during laboratory heating and/or the presence of intergrowths. The upper part of the extrusive sequence, the granoblastic dikes, and moderately altered gabbros may contain a comparatively uncontaminated thermoremanent magnetization.
      PubDate: 2014-07-16T04:16:57.890206-05:
      DOI: 10.1002/2014GC005343
  • Dynamics of lithospheric thinning and mantle melting by edge‐driven
           convection: Application to Moroccan Atlas mountains
    • Authors: Lars Kaislaniemi; Jeroen van Hunen
      Pages: n/a - n/a
      Abstract: Edge‐driven convection (EDC) forms in the upper mantle at locations of lithosphere thickness gradients, e.g. craton edges. In this study we show how the traditional style of EDC, a convection cell governed by the cold downwelling below an edge alternates with another style of EDC, in which the convection cell forms as a secondary feature with a hot asthenospheric shear flow from underneath the thicker lithosphere. These alternating EDC styles produce episodic lithosphere erosion and decompression melting. Three‐dimensional models of EDC show that convection rolls form perpendicular to the thickness gradient at the lithosphere‐asthenosphere boundary. Stagnant‐lid convection scaling laws are used to gain further insight in the underlying physical processes. Application of our models to the Moroccan Atlas mountains region shows that the combination of these two styles of EDC can reproduce many of the observations from the Atlas mountains, including two distinct periods of Cenozoic volcanism, a semi‐continuous corridor of thinned lithosphere under the Atlas mountains, and piecewise delamination of the lithosphere. A very good match between observations and numerical models is found for the lithosphere thicknesses across the study area, amounts of melts produced, and the length of the quiet gap in between volcanic episodes show quantitative match to observations.
      PubDate: 2014-07-16T04:16:56.447272-05:
      DOI: 10.1002/2014GC005414
  • Formation and geomorphologic history of the Lonar impact crater deduced
           from in situ cosmogenic 10Be and 26Al
    • Authors: Atsunori Nakamura; Yusuke Yokoyama, Yasuhito Sekine, Kazuhisa Goto, Goro Komatsu, P. Senthil Kumar, Hiroyuki Matsuzaki, Ichiro Kaneoka, Takafumi Matsui
      Pages: n/a - n/a
      Abstract: The Lonar impact crater is one of a few craters on Earth formed directly in basalt, providing a unique opportunity to study an analogue for crater degradation processes on Mars. Here we present surface 10Be and 26Al exposure dates in order to determine the age and geomorphic evolution of Lonar crater. Together with a 14C age of pre‐impact soil, we obtain a crater age of 37.5 ± 5.0 ka, which contrasts with a recently reported and apparently older 40Ar/39Ar age (570 ± 47 ka). This suggests that the 40Ar/39Ar age may have been affected by inherited radiogenic 40Ar (40Ar*inherited) in the impact glass. The spatial distribution of surface exposure ages of Lonar crater differs from that for Barringer crater, indicating Lonar crater rim is actively eroding. Our new chronology provides a unique opportunity to compare the geomorphological history of the two craters, which have similar ages and diameters, but are located in different climate and geologic settings.
      PubDate: 2014-07-16T04:02:08.927998-05:
      DOI: 10.1002/2014GC005376
  • Seafloor basalt alteration and chemical change in the ultra thinly
           sedimented South Pacific
    • Authors: Guo‐Liang Zhang; Christopher Smith‐Duque
      Pages: n/a - n/a
      Abstract: Determining the relationship between ocean floor basalt alteration and sedimentation is fundamental to understanding how oceanic crust evolves with time. Ocean floor basalts recovered at IODP Sites U1365 (~100 Ma) and U1368 (~13.5 Ma) in the South Pacific have been subjected to remarkably low sedimentation rates (0.71 to 1.1 m·Myr‐1, respectively). We report detailed petrographic and geochemical analysis of basalt cores from these sites in order to investigate what impact sediment insulation has on seafloor alteration beyond 10‐15 Myr of ocean crust formation. Both sites exhibit low temperature (
      PubDate: 2014-07-15T03:46:27.20656-05:0
      DOI: 10.1002/2013GC005141
  • Skeletal oxygen and carbon isotope compositions of Acropora coral primary
           polyps experimentally cultured at different temperatures
    • Authors: Kozue Nishida; Kei Ishikawa, Akira Iguchi, Yasuaki Tanaka, Mizuho Sato, Toyoho Ishimura, Mayuri Inoue, Takashi Nakamura, Kazuhiko Sakai, Atsushi Suzuki
      Pages: n/a - n/a
      Abstract: We investigated temperature and growth‐rate dependency of skeletal oxygen and carbon isotopes in primary polyps of Acropora digitifera (Scleractinia: Acroporidae) by culturing them at 20, 23, 27, or 31°C. Calcification was most rapid at 27 and 31°C. We obtained a δ18O‐temperature relationship (−0.18‰ °C−1) consistent with reported ranges for Porites, indicating that juvenile Acropora polyps can be used for temperature reconstruction. A growth‐rate dependency of skeletal isotopes was detected in the experimental polyps cultured at lower water temperatures, when the skeletal growth rate of these polyps was also low. The estimated upper calcification flux limit for a kinetic isotope effect to be observed in the δ18O‐growth rate relationship (∼0.4–0.7 g CaCO3 cm−2 yr−1) was similar to the calcification flux in Porites corresponding to a linear extension rate of 5 mm yr−1, the maximum rate at which the kinetic isotope effect is evident. This result suggests that the calcification flux can be used as a measure of growth rate‐related isotope fractionation, that is, the kinetic isotope effect, in corals of different genera and at different growth stages.
      PubDate: 2014-07-14T11:57:09.778277-05:
      DOI: 10.1002/2014GC005322
  • Active alkaline traps to determine acidic‐gas ratios in volcanic
           plumes: Sampling techniques and analytical methods
    • Authors: J. Wittmer; N. Bobrowski, M. Liotta, G. Giuffrida, S. Calabrese, U. Platt
      Pages: n/a - n/a
      Abstract: In situ measurements have been the basis for monitoring volcanic gas emissions for many years and—being complemented by remote sensing techniques—still play an important role to date. Concerning in situ techniques for sampling a dilute plume, an increase in accuracy and a reduction of detection limits are still necessary for most gases (e.g., CO2, SO2, HCl, HF, HBr, HI). In this work, the Raschig‐Tube technique (RT) is modified and utilized for application on volcanic plumes. The theoretical and experimental absorption properties of the RT and the Drechsel bottle (DB) setups are characterized and both are applied simultaneously to the well‐established Filter packs technique (FP) in the field (on Stromboli Island and Mount Etna). The comparison points out that FPs are the most practical to apply but the results are error‐prone compared to RT and DB, whereas the RT results in up to 13 times higher analyte concentrations than the DB in the same sampling time. An optimization of the analytical procedure, including sample pretreatment and analysis by titration, Ion Chromatography, and Inductively Coupled Plasma Mass Spectrometry, led to a comprehensive data set covering a wide range of compounds. In particular, less abundant species were quantified more accurately and iodine was detected for the first time in Stromboli's plume. Simultaneously applying Multiaxis Differential Optical Absorption Spectroscopy (MAX‐DOAS) the chemical transformation of emitted bromide into bromine monoxide (BrO) from Stromboli and Etna was determined to 3–6% and 7%, respectively, within less than 5 min after the gas release from the active vents.
      PubDate: 2014-07-14T11:57:07.061102-05:
      DOI: 10.1002/2013GC005133
  • Consolidation state of incoming sediments to the Nankai Trough subduction
           zone: Implications for sediment deformation and properties
    • Authors: Hiroko Kitajima; Demian M. Saffer
      Pages: n/a - n/a
      Abstract: The hydromechanical properties of accreted and underthrust sediments are key parameters controlling the mechanics of earthquakes and the development of fluid pressure in subduction zones. We conducted consolidation tests on sediments from the Philippine Sea Plate (PSP) in the Nankai Trough to understand the consolidation state and hydraulic properties of the incoming sediment section before its incorporation into the subduction zone. We used mudstone and sandstone cores sampled from the Integrated Ocean Drilling Program Nankai Trough Seismogenic Zone Experiment at two reference sites (Site C0011 located on a basement low; and Site C0012 located on a basement high). Our experimental results indicate that most of the mudstone samples are normally consolidated or overconsolidated, with overconsolidation ratios (OCR) ranging from 0.89 to 2.52 at Site C0011 and 0.86 to 3.85 at Site C0012. Higher OCR values at Site C0012, at least at shallow depths, are likely caused by erosional unloading. This implies that Site C0011 may serve as a better geotechnical reference site. We also find that mudstones accreted along the frontal thrust are severely overconsolidated relative to coeval mudstones at Site C0011, which likely reflects enhanced consolidation due to increased horizontal tectonic stress. Sandstones in the incoming section on the PSP exhibit 2–3 orders of magnitude higher in situ permeability than the mudstones, and the siliciclastic sandstone we tested maintains a high permeability at stresses up to at least 70 MPa, suggesting that the sandstones may act as important pathways for drainage or pore pressure translation from depths of several kilometers.
      PubDate: 2014-07-14T11:24:07.52904-05:0
      DOI: 10.1002/2014GC005360
  • Long‐term (17 Ma) turbidite record of the timing and frequency of
           large flank collapses of the Canary Islands
    • Authors: J. E. Hunt; P.J. Talling, M.A. Clare, I. Jarvis, R.B. Wynn
      Pages: n/a - n/a
      Abstract: Volcaniclastic turbidites on the Madeira Abyssal Plain provide a record of large‐volume volcanic island flank collapses from the Canary Islands. This long‐term record spans 17 Ma, and comprises one hundred and twenty‐five volcaniclastic beds. Determining the timing, provenance and volumes of these turbidites provides key information about the occurrence of mass wasting from the Canary Islands, especially the western islands of Tenerife, La Palma and El Hierro. These turbidite records demonstrate that landslides often coincide with protracted periods of volcanic edifice growth, suggesting that loading of the volcanic edifices may be a key preconditioning factor for landslide triggers. Furthermore, the last large‐volume failures from Tenerife coincide with explosive volcanism at the end of eruptive cycles. Many large‐volume Canary Island landslides also occurred during periods of warmer and wetter climates associated with sea‐level rise and subsequent highstand. However, these turbidites are not serially dependent and any association with climate or sea level change is not statistically significant.
      PubDate: 2014-07-14T04:40:40.247968-05:
      DOI: 10.1002/2014GC005232
  • Local and regional trends in Plio‐Pleistocene δ18O records from
           benthic foraminifera
    • Authors: David B. Bell; Simon J. A. Jung, Dick Kroon, Lucas J. Lourens, David A. Hodell
      Pages: n/a - n/a
      Abstract: We present new orbital‐resolution Pliocene‐Pleistocene benthic stable oxygen isotope (δ18Ob) records from Ocean Drilling Program Sites 1264 and 1267, from Walvis Ridge in the Southeast Atlantic. We compare long‐term (>250 kyr) inter‐basin δ18Ob‐gradients between Pacific and North Atlantic regional stacks, as well as intra‐ and inter‐basin gradients from the perspective of Walvis Ridge. The δ18Ob values from Sites 1264 and 1267 are almost always higher than deep North Atlantic and Pacific sites, with large gradients (>0.5 ‰) emerging abruptly at ˜2.4 Ma and persisting until ˜1.3 Ma. From this, we infer the presence of a new water mass, which resulted from the influence of dense, 18O enriched Nordic Sea overflow waters via the abyssal East Atlantic. Meanwhile, long‐term average δ18Ob values in the North Atlantic appear to have remained within 0‐0.25 ‰ lower than in the Pacific. However, the magnitude of this difference is sensitive to the inclusion of records from the equatorial West Atlantic. These results, together with constraints based on temperature, salinity and density, suggest an influence of the seawater δ18O (δ18OSW) versus salinity relationship of source waters on δ18Ob values within the Atlantic. In particular, the abrupt emergence at ˜2.4 Ma of higher δ18Ob values at Sites 1264 and 1267, relative to North Atlantic records, appears to require a low latitude surface water δ18OSW signal. This implies a connection between northward heat transport and deep‐water export into the abyssal East Atlantic. Hence, our results have implications for the interpretation of δ18Ob records and highlight the potential for δ18Obto constrain deep‐Atlantic water mass sources and pathways during the Plio‐Pleistocene.
      PubDate: 2014-07-14T04:15:02.08147-05:0
      DOI: 10.1002/2014GC005297
  • Anisotropy of magnetic susceptibility in natural olivine single crystals
    • Authors: Andrea R. Biedermann; Thomas Pettke, Eric Reusser, Ann M. Hirt
      Pages: n/a - n/a
      Abstract: Mantle flow dynamics can cause preferential alignment of olivine crystals that results in anisotropy of physical properties. To interpret anisotropy in mantle rocks, it is necessary to understand the anisotropy of olivine single crystals. We determined anisotropy of magnetic susceptibility (AMS) for natural olivine crystals. High‐field AMS allows for the isolation of the anisotropy due to olivine alone. The orientations of the principal susceptibility axes are related to the olivine's crystallographic structure as soon as it contains > 3 wt.% FeO. The maximum susceptibility is parallel to the c‐axis both at room temperature (RT) and at 77 K. The orientation of the minimum axis at RT depends on iron content; it is generally parallel to the a‐axis in crystals with 3 – 5 wt.% FeO, and along b in samples with 6 – 10 wt.% FeO. The AMS ellipsoid is prolate and the standard deviatoric susceptibility, k', is on the order of 8*10‐10 m3/kg for the samples with < 1wt.% FeO, and ranges from 3.1*10‐9 m3/kg to 5.7*10‐9 m3/kg for samples with 3‐10 wt.% FeO. At 77 K, the minimum susceptibility is along b, independent of iron content. The shape of the AMS ellipsoid is prolate for samples with < 5 wt.% FeO, but can be prolate or oblate for higher iron content. The degree of anisotropy increases at 77 K with p77' = 7.1 ± 0.5. The results from this study will allow AMS fabrics to be used as a proxy for olivine texture in ultramafic rocks with high olivine content.
      PubDate: 2014-07-12T06:30:47.442401-05:
      DOI: 10.1002/2014GC005386
  • Deciphering bottom current velocity and paleoclimate signals from
           contourite deposits in the Gulf of Cádiz during the last 140 kyr: An
           inorganic geochemical approach
    • Authors: André Bahr; Francisco J. Jiménez‐Espejo, Nada Kolasinac, Patrick Grunert, F. Javier Hernández‐Molina, Ursula Röhl, Antje H.L. Voelker, Carlota Escutia, Dorrik A.V. Stow, David Hodell, Carlos A. Alvarez‐Zarikian
      Pages: n/a - n/a
      Abstract: Contourites in the Gulf of Cádiz (GC) preserve a unique archive of Mediterranean Outflow Water (MOW) variability over the past 5.3 Ma. In our study we investigate the potential of geochemical data obtained by XRF scanning to decipher bottom current processes and paleoclimatic evolution at two different sites drilled during IODP Expedition 339 through contourites in the northern GC: Site U1387, which is bathed by the upper MOW core, and Site U1389, located more proximal to the Strait of Gibraltar. The lack of major downslope transport during the Pleistocene makes both locations ideally suited for our study. The results indicate that the Zr/Al ratio, representing the relative enrichment of heavy minerals (zircon) over less dense alumnosilicates under fast bottom current flow, is the most useful indicator for a semi‐quantitative assessment of current velocity. Although most elements are biased by current‐related processes, the bromine (Br) record, representing organic content, preserves the most pristine climate signal rather independent of grain‐size changes. Hence, Br can be used for chronostratigraphy and site‐to‐site correlation in addition to stable isotope stratigraphy. Based on these findings we reconstructed MOW variability for Marine Isotope Stages (MIS) 1 to 5 using the Zr/Al ratio from Site U1387. The results reveal abrupt, millennial‐scale variations of MOW strength during Greenland Stadials (GS) and Interstadials (GI) with strong MOW during GS and glacial Terminations and a complex behavior during Heinrich Stadials. Millennial‐scale variability persisting during periods of poorly expressed GS/GI cyclicities implies a strong internal oscillation of the Mediterranean/North Atlantic climate system.
      PubDate: 2014-07-12T06:30:38.270555-05:
      DOI: 10.1002/2014GC005356
  • 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
  • Approximately 1.78 Ga mafic dykes in the Lüliang Complex, North China
           Craton: Zircon ages and Lu‐Hf isotopes, geochemistry, and
    • Authors: Xi Wang; Wenbin Zhu, Meng Luo, Xingmin Ren, Xiang Cui
      Pages: n/a - n/a
      Abstract: Mafic dyke swarms are excellent time markers and paleo‐stress indicators. Numerous late Paleoproterozoic mafic dykes are exposed throughout the Trans‐North China Orogen (TNCO). Most of these dykes trend NW‐SE or NNW‐SSE, nearly parallel to the orogen, while a series of E‐W‐trending mafic dykes are restricted in the Lüliang and southern Taihang areas in the central segment of the TNCO. These dykes were mostly considered to be linked with break‐up of the supercontinent Columbia previously. In this study, sixteen mafic dykes were investigated in the Lüliang Complex. Zircon LA‐ICP‐MS dating of four samples yields magmatic crystallization ages of 1.78‐1.79 Ga. These dykes belong to the tholeiite series and consist of basalt, basaltic andesite and andesite. They are enriched in LREE and LILE and depleted in HFSE, and have negative zircon εHf(t) values of ‐1.7 to ‐12.2. The E‐W‐trending mafic dykes show similar geochemical and isotopic features compare to the NW‐SE‐trending dykes in other complexes. They were most likely originated from a lithospheric mantle metasomatised by subduction‐related fluids and later emplaced along extensional fractures in a post‐collisional setting. NW‐SE‐trending fractures were formed due to gravitational collapse and thinning of the lithosphere. E‐W‐trending fractures in the central segment of the orogen constitute a transverse accommodation belt to equilibrate the different amounts of extension between the northern and southern TNCO. The impact of the post‐orogenic extension might have continued to ca. 1680 Ma as evidenced by the presence of abundant ca. 1750‐1680 Ma anorthosite‐ gabbro‐ mangerite‐ rapakivi granite suites (AMCG‐like) occurring in the northern NCC.
      PubDate: 2014-07-12T06:29:57.781042-05:
      DOI: 10.1002/2014GC005378
  • Variable remanence acquisition efficiency in sediments containing biogenic
           and detrital magnetites: Implications for relative paleointensity signal
    • Authors: Tingping Ouyang; David Heslop, Andrew P. Roberts, Chengjing Tian, Zhaoyu Zhu, Yan Qiu, Xuechao Peng
      Pages: n/a - n/a
      Abstract: Widespread geological preservation of biogenic magnetite makes it important to assess how such particles contribute to sedimentary paleomagnetic signals. We studied a sediment core from the South China Sea that passes the strict empirical criteria for magnetic “uniformity” used in relative paleointensity studies. Such assessments are based routinely on bulk magnetic parameters that often fail to enable identification of mixed magnetic mineral assemblages. Using techniques that enable component‐specific magnetic mineral identification, we find that biogenic and detrital magnetites occur in approximately equal concentrations within the studied sediments. We analyzed normalized remanence signals associated with the two magnetite components to assess whether co‐occurring biogenic and detrital magnetites record geomagnetic information in the same way and with the same efficiency. Paleomagnetic directions for the two components have no phase lag, which suggests that the biogenic and detrital magnetites acquired their magnetizations at equivalent times. However, we find that the biogenic magnetite is generally 2–4 times more efficient as the detrital magnetite in contributing to the natural remanent magnetization (NRM) despite their approximately equal magnetic contributions. Variations in the concentration and efficiency of remanence acquisition of the two components suggest that a significant part of the NRM is controlled by nongeomagnetic factors that will affect relative paleointensity recording. We recommend that methods suited to the detection of variable recording efficiency associated with biogenic and detrital magnetites should be used on a routine basis in relative paleointensity studies.
      PubDate: 2014-07-12T06:23:39.394157-05:
      DOI: 10.1002/2014GC005301
  • Pb isotope compositions of detrital K‐feldspar grains in the
           upper‐middle Yangtze River system: Implications for sediment
           provenance and drainage evolution
    • Authors: Zengjie Zhang; Shane Tyrrell, Chang'an Li, J. Stephen Daly, Xilin Sun, Qiwen Li
      Pages: n/a - n/a
      Abstract: The upper‐middle Yangtze River drains the Qiangtang Block, the Songpan‐Ganzi, the Yangtze Craton, and the Qinling‐Dabie orogenic belt. These tectonic units have been shown to have heterogeneous Pb isotopic compositions, which allow this isotope system to be used as a sediment provenance tool. In this study we have employed laser ablation multiple collector inductively coupled plasma mass spectrometry (LA‐MC‐ICPMS) to measure Pb isotope compositions of sand‐sized K‐feldspar grains from the upper‐middle Yangtze River. Data are presented from four major tributaries: Yalongjiang, Minjiang, Jialingjiang, and Hanjiang, as well as from the main Yangtze River near Yichang. A portion of K‐feldspar grains in the Yalongjiang shows an ultraradiogenic character (206Pb/204Pb > 20), which is unique in the upper‐middle Yangtze. Moreover, these ultraradiogenic grains were transported as far as Yichang, just downstream from the Three Gorges, suggesting that the Pb‐in‐K‐feldspar method could be applied to the sediments within the Jianghan Basin to date the formation of the Three Gorges. Pb isotopic data from Yichang indicate that erosion in the Longmen Shan and neighboring regions is more important than the Jinshajiang in supplying sediment. The grains in Hanoi Basin have little overlap with the Songpan‐Ganzi, but show a good match with the Yangtze Craton in its range of lower 206Pb/204Pb ratios. These observations support the idea that the “Middle Yangtze” used to be a tributary of the paleo‐Red River and that there has been no drainage linking the Songpan‐Ganzi and the Red River since the Eocene.
      PubDate: 2014-07-12T06:23:36.759815-05:
      DOI: 10.1002/2014GC005391
  • Why are the δ13Corg values in Phanerozoic black shales more negative
           than in modern marine organic matter?
    • Authors: Philip A. Meyers
      Pages: n/a - n/a
      Abstract: The δ13Corg values of Phanerozoic black shales average −27‰, whereas those of modern marine organic matter average −20‰. The black shale isotopic values mimic those of continental organic matter, yet their organic geochemical properties mandate that they contain predominantly marine organic matter. Hypotheses that proposed to explain the low δ13C values of black shales include diagenetic losses of isotopically heavier organic matter components, releases of isotopically light carbon from methane clathrates or extensive magmatic events, greater photosynthetic discrimination against 13C during times of higher atmospheric pCO2, and greenhouse climate stratification of the surface ocean that magnified photic zone recycling of isotopically light organic matter. Although the last possibility seems contrary to the vertical mixing that leads to the high productivity of modern oceanic upwelling systems, it is consistent with the strongly stratified conditions that accompanied deposition of the organic carbon‐rich Pliocene‐Pleistocene sapropels of the Mediterranean Sea. Because most Phanerozoic black shales contain evidence of photic zone anoxia similar to the sapropels, well‐developed surface stratification of the oceans was likely involved in their formation. Existence of isotopically light land plant organic matter during several episodes of extensive magmatism that accompanied black shale deposition implies massive release of mantle CO2 that added to the greenhouse conditions that favored oceanic stratification. The 13C depletion common to most Phanerozoic black shales apparently resulted from a greenhouse climate associated with elevated atmospheric pCO2 that led to a strongly stratified ocean and photic zone recycling of organic matter in, augmented by magmatic CO2 releases.
      PubDate: 2014-07-12T06:23:34.09141-05:0
      DOI: 10.1002/2014GC005305
  • In situ geochemistry of Lower Paleozoic dolomites in the northwestern
           Tarim basin: Implications for the nature, origin, and evolution of
           diagenetic fluids
    • Authors: Wei Zhang; Ping Guan, Xing Jian, Fan Feng, Caineng Zou
      Pages: n/a - n/a
      Abstract: Lower Paleozoic sedimentary rocks in the northwestern Tarim basin were strongly altered by complicated geofluids, which resulted in the occurrence of various diagenetic minerals (e.g., dolomite). Here, in situ major, trace, and rare earth element geochemistry of Lower Ordovician diagenetic dolomite grains as well as petrography were performed to unravel the geochemical features, the nature, and origin of the diagenetic fluids. The results indicate that different geochemical information can be detected within a single sample, even within a single dolomite grain. Five generations of diagenetic dolomite have been identified based on geochemical signatures, resulting from four distinct types of diagenetic fluids: (1) HREE enrichment (PAAS‐normalized), low ΣREE, no Eu anomaly, low Mn, Ba, moderate Fe, and high Sr contents are probably due to early burial dolomitizing fluids; (2) MREE enrichment, high ΣREE, high Mn, Fe, and low Sr content are likely to be associated with Devonian deep‐circulating crustal hydrothermal fluids; (3) flat or LREE enrichment pattern with obviously positive Eu anomaly is inferred to be linked to Permian magmatic hydrothermal fluids; and (4) flat REE pattern, moderate ΣREE, no Eu anomaly, low Mn, Ba, moderate Fe, and high Sr contents are probably due to late burial dolomitizing fluids. The significances of in situ method demonstrated in this study, compared with the whole rock analysis, include not only contamination‐free analysis but also unraveling the internal geochemical variation within a single sample or a mineral grain. Thus, for the geochemical study of complicated diagenetic process, in situ method should be preferentially considered.
      PubDate: 2014-07-10T15:39:15.080718-05:
      DOI: 10.1002/2013GC005194
  • Interplate seismicity at the CRISP drilling site: The 2002 Mw 6.4 Osa
           Earthquake at the southeastern end of the Middle America Trench
    • Authors: Ivonne G. Arroyo; Ingo Grevemeyer, Cesar R. Ranero, Roland von Huene
      Pages: n/a - n/a
      Abstract: We investigate potential relations between variations in seafloor relief and age of the incoming plate and interplate seismicity. Westward from Osa Peninsula in Costa Rica a major change in the character of the incoming Cocos Plate is displayed by abrupt lateral variations in seafloor depth and thermal structure. Here, a Mw 6.4 thrust earthquake was followed by three aftershock clusters in June 2002. Initial relocations indicate that the main shock occurred fairly trenchward of most large earthquakes along the Middle America Trench off central Costa Rica. The earthquake sequence occurred while a temporary network of OBH and land stations ~80 km to the northwest were deployed. By adding readings from permanent local stations, we obtain uncommon P wave coverage of a large subduction zone earthquake. We relocate this catalog using a non‐linear probabilistic approach within both, a 1‐D and a 3‐D P wave velocity models. The main shock occurred ~25 km from the trench and probably along the plate interface at 5 to 10 km depth. We analyze teleseismic data to further constrain the rupture process of the main shock. The best depth estimates indicate that most of the seismic energy was radiated at shallow depth below the continental slope, supporting the nucleation of the Osa earthquake at ~6 km depth. The location and depth coincide with the plate boundary imaged in pre‐stack depth‐migrated reflection lines shot near the nucleation area. Aftershocks propagated downdip to the area of a 1999 Mw 6.9 sequence and partially overlapped it. The results indicate that underthrusting of the young and buoyant Cocos Ridge has created conditions for interplate seismogenesis shallower and closer to the trench axis than elsewhere along the central Costa Rica margin.
      PubDate: 2014-07-10T10:56:32.451961-05:
      DOI: 10.1002/2014GC005359
  • Controlled‐atmosphere thermal demagnetization and paleointensity
           analyses of extraterrestrial rocks
    • Authors: Clément Suavet; Benjamin P. Weiss, Timothy L. Grove
      Pages: n/a - n/a
      Abstract: We describe an apparatus to conduct thermal demagnetization of extraterrestrial rocks in a controlled atmosphere appropriate for a wide range of oxygen fugacities within the stability domain of iron. Thermal demagnetization and Thellier‐Thellier paleointensity experiments on lunar basalt synthetic analogs show that the controlled atmosphere prevents oxidation of magnetic carriers. When combined with multidomain paleointensity techniques, this opens the possibility of highly accurate thermal demagnetization and paleointensity measurements on rocks from the Moon and asteroids.
      PubDate: 2014-07-01T04:21:33.167587-05:
      DOI: 10.1002/2013GC005215
  • The role of magmatically driven lithospheric thickening on arc front
    • Authors: L. Karlstrom; C.‐T. A. Lee, M. Manga
      Pages: n/a - n/a
      Abstract: Volcanic activity at convergent plate margins is localized along lineaments of active volcanoes that focus rising magma generated within the mantle below. In many arcs worldwide, particularly continental arcs, the volcanic front migrates away from the interface of subduction (the trench) over millions of years, reflecting coevolving surface forcing, tectonics, crustal magma transport, and mantle flow. Here we show that extraction of melt from arc mantle and subsequent magmatic thickening of overlying crust and lithosphere can drive volcanic front migration. These processes are consistent with geochemical trends, such as increasing La/Yb, which show that increasing depths of differentiation correlate with arc front migration in continental arcs. Such thickening truncates the underlying mantle flow field, squeezing hot mantle wedge and the melting focus away from the trench while progressively decreasing the volume of melt generated. However, if magmatic thickening is balanced by tectonic extension in the upper plate, a steady crustal thickness is achieved that results in a more stationary arc front with long‐lived mantle melting. This appears to be the case for some island arcs. Thus, in combination with tectonic modulation of crustal thickness, magmatic thickening provides a self consistent model for volcanic arc front migration and the composition of arc magmas.
      PubDate: 2014-06-30T15:42:58.026231-05:
      DOI: 10.1002/2014GC005355
  • The Cobb hotspot: HIMU‐DMM mixing and melting controlled by a
           progressively thinning lithospheric lid
    • Authors: John Chadwick; Randall Keller, George Kamenov, Gene Yogodzinski, John Lupton
      Pages: n/a - n/a
      Abstract: The Cobb Seamount Chain in the northeast Pacific basin records the composition of the Cobb hotspot for the past 33 Myr, as the migrating Juan de Fuca Ridge approached and ultimately overran it ca. 0.5 Myr ago. In this first comprehensive geochemical study of the Cobb chain, major and trace element compositions and Sr, Nd, Pb, and Hf isotopic ratios were measured for whole‐rock samples from throughout the chain, and He isotopes were acquired for olivine phenocrysts from one seamount. Trace element modeling indicates increased melting along the chain over time, with progressively more depleted lavas as the ridge approached the hotspot. The isotopic data reveal the first evidence of the high µ (µ = 238U/204Pb) (HIMU) mantle component in the north Pacific basin, and are consistent with a progressively decreasing mixing proportion of HIMU melts relative to those from depleted mid‐ocean ridge basalt mantle (DMM) in the chain over time. Decreasing lithospheric thickness over the Cobb hotspot due to the approach of the migrating Juan de Fuca ridge allowed adiabatic melting to continue to shallower depths, leading to increased melt fractions of the refractory DMM component in the hotspot and more depleted and MORB‐like lavas in the younger Cobb seamounts.
      PubDate: 2014-06-26T12:02:57.987204-05:
      DOI: 10.1002/2014GC005334
  • Issue Information
    • Pages: i - i
      PubDate: 2014-06-13T10:53:58.981225-05:
      DOI: 10.1002/ggge.20321
  • Helium isotopic textures in Earth's upper mantle
    • Authors: David W. Graham; Barry B. Hanan, Christophe Hémond, Janne Blichert‐Toft, Francis Albarède
      Pages: n/a - n/a
      Abstract: We report 3He/4He for 150 mid‐ocean ridge basalt (MORB) glasses from the Southeast Indian Ridge (SEIR). Between 81°E and 101°E 3He/4He varies from 7.5 to 10.2 RA, encompassing more than half the MORB range away from ocean island hot spots. Abrupt transitions are present and in one case the full range occurs over ∼10 km. Melting of lithologically heterogeneous mantle containing a few percent garnet pyroxenite or eclogite leads to lower 3He/4He, while 3He/4He above ∼9 RA likely indicates melting of pyroxenite‐free or eclogite‐free mantle. Patterns in the length scales of variability represent a description of helium isotopic texture. We utilize four complementary methods of spectral analysis to evaluate this texture, including periodogram, redfit, multitaper method, and continuous wavelet transform. Long‐wavelength lobes with prominent power at 1000 and 500 km are present in all treatments, similar to hot spot‐type spectra in Atlantic periodograms. The densely sampled region of the SEIR considered separately shows significant power at ∼100 and ∼30–40 km, the latter scale resembling heterogeneity in the bimodal distribution of Hf and Pb isotopes in the same sample suite. Wavelet transform coherence reveals that 3He/4He varies in‐phase with axial depth along the SEIR at ∼1000 km length scale, suggesting a coupling between melt production, 3He/4He and regional variations in mantle temperature. Collectively, our results show that the length scales of MORB 3He/4He variability are dominantly controlled by folding and stretching of heterogeneities during regional (∼1000 km) and mesoscale (∼100 km) mantle flow, and by sampling during the partial melting process (∼30 km).
      PubDate: 2014-05-30T10:29:39.804677-05:
      DOI: 10.1002/2014GC005264
  • Confirmation of progressive plate motion during the Midcontinent Rift's
           early magmatic stage from the Osler Volcanic Group, Ontario, Canada
    • Authors: Nicholas L. Swanson‐Hysell; Angus A. Vaughan, Monica R. Mustain, Kristofer E. Asp
      Pages: n/a - n/a
      Abstract: As the supercontinent Rodinia was assembling ca. 1.1 billion years ago, there was extensive magmatism on at least five Proterozoic continents including the development of the North American Midcontinent Rift. New paleomagnetic data from 84 lava flows of the Osler Volcanic Group of the Midcontinent Rift reveal that there was a significant and progressive decrease in inclination between the initiation of extrusive volcanism in the region (ca. 1110 Ma) and ca. 1105 ± 2 Ma (during the “early stage” of rift development). Paleomagnetic poles can be calculated for the lower portion of the reversed Osler Volcanic Group (40.9°N, 218.6°E, A95 = 4.8°, N = 30) and the upper portion of the reversed Osler Volcanic Group (42.5°N, 201.6°E, A95 = 3.7°, N = 59; this pole can be assigned the age of ca. 1105 ± 2 Ma). This result is a positive test of the hypothesis that there was significant plate motion during the early stage of rift development. In addition to being a time of widespread volcanism on Laurentia and other continents, this interval of the late Mesoproterozoic was characterized by rapid paleogeographic change.
      PubDate: 2014-05-30T10:22:52.30609-05:0
      DOI: 10.1002/2013GC005180
  • Effects of the core‐shell structure on the magnetic properties of
           partially oxidized magnetite grains: Experimental and micromagnetic
    • Authors: Kunpeng Ge; Wyn Williams, Qingsong Liu, Yongjae Yu
      Pages: n/a - n/a
      Abstract: The relationship between magnetic hysteresis parameters and the degree of oxidation of ultrafine magnetite particles is examined by both experimental measurements (distributed particle assemblage with median grain size of ∼80 nm and standard deviation 0.43) and micromagnetic simulations (single particles from 40 nm to 140 nm). Experimental results show that both coercivity (Bc) and the ratio of saturation remanence to saturation magnetization (Mrs/Ms) increase slowly, as the oxidation parameter z increases from 0 to ∼0.9. Thereafter, both parameters decrease sharply as magnetite becomes completely oxidized to maghemite. Numerical simulations of hysteresis loop and microstructure using a micromagnetic model with a core‐shell geometry (a stoichiometric core surrounded by an oxidized shell) show three categories of behavior for magnetic grains during oxidation. First, the coercivity of SD particles decreases as oxidation proceeds, but their remanence magnetization remains in a uniform state. Second, for PSD sized particles near the critical SD boundary (80 nm to 100 nm), the initial vortex domain structure changes to a SD as oxidation occurs and returns to a vortex state upon complete maghemitization, resulting in an initial rise and then fall of Bc and Mrs. Finally, larger PSD grains remain a vortex state throughout the maghemitization, with less variations of Bc and Mrs. The predicted magnetic properties exhibit good agreement with experimental observations and suggest that the domain arrangement is likely to be dominated by a core‐shell structure with strong exchange coupling at their interface. Overall, the partially oxidized magnetite in SD‐PSD range can reliably record palaeomagnetic signals.
      PubDate: 2014-05-30T10:17:33.514051-05:
      DOI: 10.1002/2014GC005265
  • Contrasting subduction structures within the Philippine Sea plate: Hydrous
           oceanic crust and anhydrous volcanic arc crust
    • Authors: Ryuta Arai; Takaya Iwasaki, Hiroshi Sato, Susumu Abe, Naoshi Hirata
      Pages: n/a - n/a
      Abstract: We show contrasting subduction structures within the Philippine Sea plate inferred from active‐source wide‐angle reflection data. Previous studies showed that large‐amplitude reflections from the slab are observed in southwest Japan and indicated that a thin low‐velocity layer with a high fluid content is formed along the top of the subducting oceanic crust. On the contrary, we found that the slab reflections have smaller amplitudes in the Izu collision zone, central Japan, where the Izu‐Bonin volcanic arc has been colliding/subducting, suggesting that such a low‐velocity layer does not exist beneath the collision zone. This structural difference is also supported by P‐wave and S‐wave velocity anomalies by passive‐source tomography and electrical conductivity, and correlates with the regional distribution of deep tremors and intraslab earthquakes, both of which are induced by dehydration processes within the downgoing slab. Based on these comparisons, we suggest that the original structure of the incoming plate controls the contrasting subducting systems: typical oceanic plate absorbs water by hydrothermal circulation at spreading centers and/or seawater infiltration at outer rises, whereas volcanic arc crust consumes a large amount of hydrous minerals for melt production and metamorphoses to more stable, anhydrous forms before subduction.
      PubDate: 2014-05-30T10:12:04.984117-05:
      DOI: 10.1002/2014GC005321
  • A uniformly processed data set of SKS shear wave splitting measurements: A
           global investigation of upper mantle anisotropy beneath seismic stations
    • Authors: J. Walpole; J. Wookey, G. Masters, J. M. Kendall
      Pages: n/a - n/a
      Abstract: Anisotropy in the Earth's upper mantle is a signature of past and present deformation. Here we present a new data set of ∼50,000 uniformly processed SKS shear wave splitting measurements that probe upper mantle anisotropy beneath seismic stations in the frequency band 0.02–0.1 Hz. The data set consists of measurements obtained at ∼2000 seismic stations from ∼2000 events. We identify several stations characterized by an apparent absence of shear wave splitting (so‐called “null stations”). Station‐averaged measurements are obtained by stacking shear wave splitting error surfaces. The stacked data set shows excellent agreement with a compilation of previous SKS measurements. The average amount of splitting beneath seismic stations (after error surface stacking) is 0.8 s, slightly lower than that found previously by vectorial averaging of non‐null measurement splitting parameters. The data set disagrees, however, with an azimuthally anisotropic surface wave tomography model (DKP2005), suggesting that caution should be exercised when using such models for geodynamic interpretation, especially in continental regions. Studying our data set in detail, we find evidence that flow in the asthenosphere exerts partial control over SKS splitting in orogenic regions globally. In the active orogenic environment of the western USA, where we have the densest coverage, our data suggest that shallow asthenospheric flow is guided by a wall of thick lithosphere to the east.
      PubDate: 2014-05-30T09:57:37.485951-05:
      DOI: 10.1002/2014GC005278
  • Tectonic and magmatic control of hydrothermal activity along the
           slow‐spreading Central Indian Ridge, 8°S–17°S
    • Authors: Juwon Son; Sang‐Joon Pak, Jonguk Kim, Edward T. Baker, Ok‐Rye You, Seung‐Kyu Son, Jai‐Woon Moon
      Pages: n/a - n/a
      Abstract: The complex geology and expansive axial valleys typical of slow‐spreading ridges makes evaluating their hydrothermal activity a challenge. This challenge has gone largely unmet, as the most undersampled MOR type for hydrothermal activity is slow spreading (20–55 mm/yr). Here we report the first systematic hydrothermal plume survey conducted on the Central Indian Ridge (CIR, 8°S–17°S), the most extensive such survey yet conducted on a slow‐spreading ridge. Using a combined CTD/Miniature Autonomous Plume Recorder (MAPR) package, we used 118 vertical casts along seven segments of the CIR (∼700 km of ridge length) to estimate the frequency of hydrothermal activity. Evidence for hydrothermal activity (particle and methane plumes) was found on each of the seven spreading segments, with most plumes found between 3000 and 3500 m, generally
      PubDate: 2014-05-30T09:55:37.251719-05:
      DOI: 10.1002/2013GC005206
  • Kinematics of the western Caribbean: Collision of the Cocos Ridge and
           upper plate deformation
    • Authors: Daisuke Kobayashi; Peter LaFemina, Halldór Geirsson, Eric Chichaco, Antonio A. Abrego, Hector Mora, Eduardo Camacho
      Pages: n/a - n/a
      Abstract: Subduction of the Cocos plate and collision of the Cocos Ridge have profound effects on the kinematics of the western Caribbean, including crustal shortening, segmentation of the overriding plate, and tectonic escape of the Central American fore arc (CAFA). Tectonic models of the Panama Region (PR) have ranged from a rigid block to a deforming plate boundary zone. Recent expansion of GPS networks in Panama, Costa Rica, and Colombia makes it possible to constrain the kinematics of the PR. We present an improved kinematic block model for the western Caribbean, using this improved GPS network to test a suite of tectonic models describing the kinematics of this region. The best fit model predicts an Euler vector for the counterclockwise rotation of the CAFA relative to the Caribbean plate at 89.10°W, 7.74°N, 1.193° Ma−1, which is expressed as northwest‐directed relative block rates of 11.3 ± 1.0–16.5 ± 1.1 mm a−1 from northern Costa Rica to Guatemala. This model also predicts high coupling along the Nicoya and Osa segments of the Middle American subduction zone. Our models demonstrate that the PR acts as a single tectonic block, the Panama block, with a predicted Euler vector of 107.65°W, 26.50°N, 0.133° Ma−1. This rotation manifests as northeast migration of the Panama block at rates of 6.9 ± 4.0–7.8 ± 4.8 mm a−1 from southern Costa Rica to eastern Panama. We interpret this motion as tectonic escape from Cocos Ridge collision, redirected by collision with the North Andes block, which migrates to the northwest at 12.2 ± 1.2 mm a−1.
      PubDate: 2014-05-29T15:20:35.172637-05:
      DOI: 10.1002/2014GC005234
  • Reply to the comment by Granier on “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”
    • Authors: Kazuyuki Yamamoto; Masatoshi Ishibashi, Hideko Takayanagi, Yoshihiro Asahara, Tokiyuki Sato, Hiroshi Nishi, Yasufumi Iryu
      Pages: n/a - n/a
      PubDate: 2014-05-29T15:20:32.699595-05:
      DOI: 10.1002/2014GC005390
  • Comment on “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” by K.
           Yamamoto et al.
    • Authors: Bruno R. C. Granier
      Pages: n/a - n/a
      PubDate: 2014-05-29T15:18:18.589328-05:
      DOI: 10.1002/2014GC005350
  • Acoustic estimates of methane gas flux from the seabed in a 6000 km2
           region in the Northern Gulf of Mexico
    • Authors: Thomas C. Weber; Larry Mayer, Kevin Jerram, Jonathan Beaudoin, Yuri Rzhanov, Dave Lovalvo
      Pages: n/a - n/a
      Abstract: Seeps of free methane gas escaping the seabed can be found throughout the ocean basins. To understand the role of methane gas seeps in the global carbon cycle—including both gas added to the atmosphere and that which is dissolved and potentially oxidized in the ocean volume—it is important to quantify the amount of methane escaping the seabed. Few large‐scale mapping projects of natural methane seeps have been undertaken, however, and even among these, quantitative estimates of flux are rare. Here we use acoustic mapping techniques to survey 357 natural methane seeps in a large region (6000 km2) of the northern Gulf of Mexico and outline a general approach for methane seep mapping using a combination of multibeam and split‐beam echo sounders. Using additional measurements collected with a remotely operated vehicle (ROV) together with the acoustic mapping results, we estimate the total gas flux within the 6000 km2 region to be between 0.0013 and 0.16 Tg/yr, or between 0.003 and 0.3% of the current estimates for global seabed methane seepage rates.
      PubDate: 2014-05-29T14:30:44.344588-05:
      DOI: 10.1002/2014GC005271
  • Thermogenic methane injection via bubble transport into the upper Arctic
           Ocean from the hydrate‐charged Vestnesa Ridge, Svalbard
    • Authors: Andrew J. Smith; Jürgen Mienert, Stefan Bünz, Jens Greinert
      Pages: n/a - n/a
      Abstract: We use new gas‐hydrate geochemistry analyses, echosounder data, and three‐dimensional P‐Cable seismic data to study a gas‐hydrate and free‐gas system in 1200 m water depth at the Vestnesa Ridge offshore NW Svalbard. Geochemical measurements of gas from hydrates collected at the ridge revealed a thermogenic source. The presence of thermogenic gas and temperatures of ∼3.3°C result in a shallow top of the hydrate stability zone (THSZ) at ∼340 m below sea level (mbsl). Therefore, hydrate‐skinned gas bubbles, which inhibit gas‐dissolution processes, are thermodynamically stable to this shallow water depth. This was confirmed by hydroacoustic observations of flares in 2010 and 2012 reaching water depths between 210 and 480 mbsl. At the seafloor, bubbles are released from acoustically transparent zones in the seismic data, which we interpret as regions where free gas is migrating through the hydrate stability zone (HSZ). These intrusions result in vertical variations in the base of the HSZ (BHSZ) of up to ∼150 m, possibly making the shallow hydrate reservoir more susceptible to warming. Such Arctic gas‐hydrate and free‐gas systems are important because of their potential role in climate change and in fueling marine life, but remain largely understudied due to limited data coverage in seasonally ice‐covered Arctic environments.
      PubDate: 2014-05-29T14:18:54.738474-05:
      DOI: 10.1002/2013GC005179
  • Sensitivity of sediment geochemical proxies to coring location and corer
           type in a large lake: Implications for paleolimnological reconstruction
    • Authors: Yue Han Lu; Philip A. Meyers, John A. Robbins, Brian J. Eadie, Nathan Hawley, Kang Hyeun Ji
      Pages: n/a - n/a
      Abstract: We compared a suite of geochemical proxies in sediment cores collected in 1982, 1988, 1991, and 2003 from sites near the depocenter of Lake Erie to evaluate the reliability of paleoenvironmental reconstructions derived from lacustrine sediments. Our proxies included the concentrations and carbon isotopic compositions of organic and inorganic carbon (TOC, CaCO3, δ13Corg, and δ13CCaCO3), augmented by organic C to total N ratios (Corg:Ntot), δ15N, and carbonate δ18O values (δ18OCaCO3). The three coring sites were clustered within 12 km; two types of corers—a Box corer and a Benthos gravity corer—were used for the 1991 sampling campaign. The variance of most proxies was accounted for not only by temporal environmental changes but also by coring locations and corer type, indicating that sediment spatial heterogeneity and differences in sediment recovery due to the use of different corers also played a part in determining the geochemical compositions of these cores. The TOC, δ13Corg, and δ13CCaCO3 values showed decadal temporal patterns that were consistent between the multiple sampling campaigns. In contrast, the δ15N, Corg:Ntot, CaCO3, and δ18OCaCO3 exhibited across‐core differences in their temporal variations, making it difficult to extract consistent environment information from different cores. Our findings suggest that in addition to temporal environmental changes, high‐resolution paleolimnological reconstruction is sensitive to many factors that could include spatial sediment heterogeneity, discontinuous sedimentation processes, bioturbation, sediment dating uncertainty, and artifacts associated with analytical and coring procedures. Therefore, multiple‐core sampling and analysis are important in reliably reconstructing environmental changes, particularly for large, heterogeneous lacustrine basins.
      PubDate: 2014-05-29T14:04:19.389657-05:
      DOI: 10.1002/2013GC004989
  • Precessional control of Sr ratios in marginal basins during the Messinian
           Salinity Crisis?
    • Authors: R. P. M. Topper; S. Lugli, V. Manzi, M. Roveri, P. Th. Meijer
      Pages: n/a - n/a
      Abstract: Based on 87Sr/86Sr data of the Primary Lower Gypsum (PLG) deposits in the Vena del Gesso basin—a marginal basin of the Mediterranean during the Messinian Salinity Crisis—a correlation between 87Sr/86Sr values and precessional forcing has recently been proposed but not yet confirmed. In this study, a box model is set up to represent the Miocene Mediterranean deep basin and a connected marginal basin. Measurements of 87Sr/86Sr in the Vena del Gesso and estimated salinity extrema are used to constrain model results. In an extensive analysis with this model, we assess whether coeval 87Sr/86Sr and salinity fluctuations could have been forced by precession‐driven changes in the fresh water budget. A comprehensive set of the controlling parameters is examined to assess the conditions under which precession‐driven 87Sr/86Sr variations occur and to determine the most likely setting for PLG formation. Model results show that precession‐driven 87Sr/86Sr and salinity fluctuations in marginal basins are produced in settings within a large range of marginal basin sizes, riverine strontium characteristics, amplitudes of precessional fresh water budget variation, and average fresh water budgets of both the marginal and deep basin. PLG deposition most likely occurred when the Atlantic‐Mediterranean connection was restricted, and the average fresh water budget in the Mediterranean was significantly less negative than at present day. Considering the large range of settings in which salinities and 87Sr/86Sr fluctuate on a precessional timescale, 87Sr/86Sr variations are expected to be a common feature in PLG deposits in marginal basins of the Mediterranean.
      PubDate: 2014-05-29T13:57:09.087183-05:
      DOI: 10.1002/2013GC005192
  • Differentiating flow, melt, or fossil seismic anisotropy beneath Ethiopia
    • Authors: J. O. S. Hammond; J.‐M. Kendall, J. Wookey, G. W. Stuart, D. Keir, A. Ayele
      Pages: n/a - n/a
      Abstract: Ethiopia is a region where continental rifting gives way to oceanic spreading. Yet the role that pre‐existing lithospheric structure, melt, mantle flow, or active upwellings may play in this process is debated. Measurements of seismic anisotropy are often used to attempt to understand the contribution that these mechanisms may play. In this study, we use new data in Afar, Ethiopia along with legacy data across Ethiopia, Djibouti, and Yemen to obtain estimates of mantle anisotropy using SKS‐wave splitting. We show that two layers of anisotropy exist, and we directly invert for these. We show that fossil anisotropy with fast directions oriented northeast‐southwest may be preserved in the lithosphere away from the rift. Beneath the Main Ethiopian Rift and parts of Afar, anisotropy due to shear segregated melt along sharp changes in lithospheric thickness dominates the shear‐wave splitting signal in the mantle. Beneath Afar, away from regions with significant lithospheric topography, melt pockets associated with the crustal and uppermost mantle magma storage dominate the signal in localized regions. In general, little anisotropy is seen in the uppermost mantle beneath Afar suggesting melt retains no preferential alignment. These results show the important role melt plays in weakening the lithosphere and imply that as rifting evolves passive upwelling sustains extension. A dominant northeast‐southwest anisotropic fast direction is observed in a deeper layer across all of Ethiopia. This suggests that a conduit like plume is lacking beneath Afar today, rather a broad flow from the southwest dominates flow in the upper mantle.
      PubDate: 2014-05-27T13:22:03.590402-05:
      DOI: 10.1002/2013GC005185
  • A fictitious domain method for lithosphere‐asthenosphere
           interaction: Application to periodic slab folding in the upper mantle
    • Authors: Nestor G. Cerpa; Riad Hassani, Muriel Gerbault, Jean‐Herve Prévost
      Pages: n/a - n/a
      Abstract: We present a new approach for the lithosphere‐asthenosphere interaction in subduction zones. The lithosphere is modeled as a Maxwell viscoelastic body sinking in the viscous asthenosphere. Both domains are discretized by the finite element method, and we use a staggered coupling method. The interaction is provided by a nonmatching interface method called the fictitious domain method. We describe a simplified formulation of this numerical technique and present 2‐D examples and benchmarks. We aim at studying the effect of mantle viscosity on the cyclicity of slab folding at the 660 km depth transition zone. Such cyclicity has previously been shown to occur depending on the kinematics of both the overriding and subducting plates, in analog and numerical models that approximate the 660 km depth transition zone as an impenetrable barrier. Here we applied far‐field plate velocities corresponding to those of the South‐American and Nazca plates at present. Our models show that the viscosity of the asthenosphere impacts on folding cyclicity and consequently on the slab's dip as well as the stress regime of the overriding plate. Values of the mantle viscosity between 3 and 5 × 1020 Pa s are found to produce cycles similar to those reported for the Andes, which are of the order of 30–40 Myr (based on magmatism and sedimentological records). Moreover, we discuss the episodic development of horizontal subduction induced by cyclic folding and, hence, propose a new explanation for episodes of flat subduction under the South‐American plate.
      PubDate: 2014-05-27T13:19:20.51371-05:0
      DOI: 10.1002/2014GC005241
  • A novel framework for quantifying past methane recycling by
           Sphagnum‐methanotroph symbiosis using carbon and hydrogen isotope
           ratios of leaf wax biomarkers
    • Authors: Jonathan E. Nichols; Peter D. F. Isles, Dorothy M. Peteet
      Pages: n/a - n/a
      Abstract: The concentration of atmospheric methane is strongly linked to variations in Earth's climate. Currently, we can directly reconstruct the total atmospheric concentration of methane, but not individual terms of the methane cycle. Northern wetlands, dominated by Sphagnum, are an important contributor of atmospheric methane, and we seek to understand the methane cycle in these systems. We present a novel method for quantifying the proportion of carbon Sphagnum assimilates from its methanotrophic symbionts using stable isotope ratios of leaf‐wax biomarkers. Carbon isotope ratios of Sphagnum compounds are determined by two competing influences, water content and the isotope ratio of source carbon. We disentangled these effects using a combined hydrogen and carbon isotope approach. We constrained Sphagnum water content using the contrast between the hydrogen isotope ratios of Sphagnum and vascular plant biomarkers. We then used Sphagnum water content to calculate the carbon isotope ratio of Sphagnum's carbon pool. Using a mass balance equation, we calculated the proportion of recycled methane contributed to the Sphagnum carbon pool, “PRM.” We quantified PRM in peat monoliths from three microhabitats in the Mer Bleue peatland complex. Modern studies have shown that water table depth and vegetation have strong influences on the peatland methane cycle on instrumental time scales. With this new approach, δ13C of Sphagnum compounds are now a useful tool for investigating the relationships among hydrology, vegetation, and methanotrophy in Sphagnum peatlands over the time scales of entire peatland sediment records, vital to our understanding of the global carbon cycle through the Late Glacial and Holocene.
      PubDate: 2014-05-27T11:58:01.696608-05:
      DOI: 10.1002/2014GC005242
  • Large volume submarine ignimbrites in the Shikoku Basin: An example for
           explosive volcanism in the Western Pacific during the Late Miocene
    • Authors: Steffen Kutterolf; Julie C. Schindlbeck, Rachel P. Scudder, Richard W. Murray, Kevin T. Pickering, Armin Freundt, Shasa Labanieh, Ken Heydolph, Sanny Saito, Hajime Naruse, Michael B. Underwood, Huaichun Wu
      Pages: n/a - n/a
      Abstract: During IODP Expedition 322, an interval of Late Miocene (7.6 to ∼9.1 Ma) tuffaceous and volcaniclastic sandstones was discovered in the Shikoku Basin (Site C0011B), Nankai region. This interval consists of bioturbated silty claystone including four 1–7 m thick interbeds of tuffaceous sandstones (TST) containing 57–82% (by volume) pyroclasts. We use major and trace element glass compositions, as well as radiogenic isotope compositions, to show that the tuffaceous sandstones beds derived from single eruptive events, and that the majority (TST 1, 2, 3a) came from different eruptions from a similar source region, which we have identified to be the Japanese mainland, 350 km away. In particular, diagnostic trace element ratios (e.g., Th/La, Sm/La, Rb/Hf, Th/Nb, and U/Th) and isotopic data indicate a marked contribution from a mantle source beneath continental crust, which is most consistent with a Japanese mainland source and likely excludes the Izu‐Bonin island arc and back arc as a source region for the younger TST beds. Nevertheless, some of the chemical data measured on the oldest sandstone bed (TST 3b, Unit IIb) show affinity to or can clearly be attributed to an Izu‐Bonin composition. While we cannot completely exclude the possibility that all TST beds derived from unknown and exotic Izu‐Bonin source(s), the collected lines of evidence are most consistent with an origin from the paleo‐Honshu arc for TST 1 through 3a. We therefore suggest the former collision zone between the Izu‐Bonin arc and Honshu paleo‐arc as the most likely region where the eruptive products entered the ocean, also concurrent with nearby (∼200 km) possible Miocene source areas for the tuffaceous sandstones at the paleo‐NE‐Honshu arc. Estimating the distribution area of the tuffaceous sandstones in the Miocene between this source region and the ∼350 km distant Expedition 322, using bathymetric constraints, we calculate that the sandstone beds represent minimum erupted magma volumes between ∼1 and 17 km3 (Dense Rock Equivalent (DRE)). We conclude that several large volume eruptions occurred during the Late Miocene time next to the collision zone of paleo‐Honshu and Izu‐Bonin arc and covered the entire Philippine Sea plate with meter thick, sheet‐like pyroclastic deposits that are now subducted in the Nankai subduction zone.
      PubDate: 2014-05-27T11:46:02.562291-05:
      DOI: 10.1002/2014GC005263
  • The morphology of insular shelves as a key for understanding the
           geological evolution of volcanic islands: Insights from Terceira Island
    • Authors: R. Quartau; A. Hipólito, C. Romagnoli, D. Casalbore, J. Madeira, F. Tempera, C. Roque, F. L. Chiocci
      Pages: n/a - n/a
      Abstract: Shelves from volcanic ocean islands result from the competition between two main processes, wave erosion that forms and enlarges them and volcanic progradation that reduces their dimension. In places where erosion dominates over volcanism, shelf width can be used as a proxy for the relative age of the subaerial volcanic edifices and reconstruction of their extents prior to erosion can be achieved. In this study, new multibeam bathymetry and high‐resolution seismic reflection profiles are exploited to characterize the morphology of the insular shelves adjacent to each volcanic edifice of Terceira Island in order to improve the understanding of its evolution. Subaerial morphological and geological/stratigraphic data were also used to establish the connection between the onshore and offshore evolution. Shelf width contiguous to each main volcanic edifice is consistent with the known subaerial geological history of the island; most of the older edifices have wider shelves than younger ones. The shelf edge proved to be a very useful indicator in revealing the original extent of each volcanic edifice in plan view. Its depth was also used to reconstruct vertical movements, showing that older edifices like Serra do Cume‐Ribeirinha, Guilherme Moniz, and Pico Alto have subsided while more recent ones have not. The morphology of the shelf (namely the absence/presence of fresh lava flow morphologies and several types of erosional, depositional, and tectonic features) integrated with the analysis of the coastline morphology allowed us to better constrain previous geological interpretations of the island evolution.
      PubDate: 2014-05-22T13:27:30.665503-05:
      DOI: 10.1002/2014GC005248
  • Reply to comment by Hu et al. on “Holocene evolution in weathering
           and erosion patterns in the Pearl River delta"
    • Authors: Peter D. Clift
      Pages: n/a - n/a
      PubDate: 2014-05-16T03:56:53.074639-05:
      DOI: 10.1002/2014GC005371
  • Geochemical evidence for active tropical serpentinization in the Santa
           Elena Ophiolite, Costa Rica: An analog of a humid early Earth?
    • Authors: Ricardo Sánchez‐Murillo; Esteban Gazel, Esther M. Schwarzenbach, Melitza Crespo‐Medina, Matthew O. Schrenk, Jan Boll, Ben C. Gill
      Pages: n/a - n/a
      Abstract: Serpentinization is a planetary process that has important consequences on geochemical cycles, supporting microbial activity through the formation of H2 and CH4 and having the potential to sequester atmospheric CO2. We present geochemical evidence of active serpentinization in the Santa Elena Ophiolite, Costa Rica which is sustained by peridotites with a degree of serpentinization less than 50% with no evidence of an internal heat source. Average spring water temperatures are 29.1°C. Two hyperalkaline spring systems were discovered, with a spring fluid pH up to 11.18. The fluids are characterized by low Mg (1.0–5.9 mg/L) and K (1.0–5.5 mg/L) and relative high Ca (29–167 mg/L), Na (16–27 mg/L), Cl (26–29 mg/L), hydroxide (41–63 mg/L), and carbonate (31–49 mg/L). Active CH4 (24.3% v/v) vents coupled with carbonate deposits ( δ13CCO2 =−27 to −14‰; δ18OCO2 =−17 to − 6‰) also provide evidence for active serpentinization and carbonation. Isotope ratios of the alkaline fluids (δ18O = −7.9‰, δ2H = −51.4‰) and groundwater (δ18O = −7.6‰; δ2H = −48.0‰) suggests that, during base flow recession, springs are fed by groundwater circulation. Methanogenic Archaea, which comprises a relatively high percentage of the 16S rRNA gene tag sequences, suggests that biological methanogenesis may play a significant role in the system. Santa Elena's extreme varying weather results in a scenario that could be of significant importance for (a) improving the knowledge of conditions on a humid early Earth or Mars that had periodic changes in water supply, (b) revealing new insights on serpentinizing solute transport, and (c) modeling hydrogeochemical responses as a function of recharge.
      PubDate: 2014-05-14T15:26:30.827726-05:
      DOI: 10.1002/2013GC005213
  • Lithospheric architecture of the Slave craton, northwest Canada, as
           determined from an interdisciplinary 3‐D model
    • Authors: D.B. Snyder; M.J. Hillier, B.A. Kjarsgaard, E.A. de Kemp, J.A. Craven
      Pages: n/a - n/a
      Abstract: Regional‐scale geologic structures characteristic of mantle lithosphere within cratons found in continent interiors are interpreted using geo‐registered diverse data sets from the Slave craton of northwest Canada. We developed and applied a new method for mapping seismic discontinuities in three dimensions using multi‐year observations at sparse, individual broadband receivers. New, fully 3‐D conductivity models used all available magnetotelluric data. Discontinuity surfaces and conductivity models were geo‐registered with previously published P‐wave and surface wave velocity models to confirm first‐order structures such as a mid‐lithosphere discontinuity. Our 3‐D model to 400 km depth was calibrated by ‘drill hole’ observations derived from xenolith suites extracted from kimberlites. A number of new structural discontinuities emerge from direct comparison of co‐registered data sets and models. Importantly we distinguish primary mantle layers from secondary features related to younger metasomatism. Sub‐horizontal Slave craton layers with tapered, wedge‐shaped margins indicate construction of the craton core at 2.7 Ga by underthrusting and flat stacking of lithosphere. Mapping of conductivity and metasomatism in 3‐D, the latter inferred via mineral recrystallization and resetting of isotopic ages in xenoliths, indicates overprinting of the primary layered structures. The observed distribution of relatively conductive mantle at 100–200 km depths is consistent with pervasive metasomatism; vertical ‘chimneys’ reaching to crustal depths in locations where kimberlites erupted or where Au mineralization is known.
      PubDate: 2014-03-11T11:01:47.824392-05:
      DOI: 10.1002/2013GC005168
  • Paleomagnetic data from Late Paleozoic dykes of Sardinia: Evidence for
           block rotations and implications for the intra‐Pangea megashear
    • Authors: K. Aubele; V. Bachtadse, G. Muttoni, A. Ronchi
      Pages: 1684 - 1697
      Abstract: Paleomagnetic studies of dyke swarms from the Variscan belt of Europe can be used to reconstruct internal postorogenic rotations within the fold belt. Here we present paleomagnetic data from 13 late Variscan dykes from Sardinia ranging in age from 298 ± 5 to 270 ± 10 Ma. The dykes can be grouped on the basis of their different directions in strike in a northern, a central‐eastern and a south‐eastern province. Paleomagnetic component directions have been obtained using thermal and alternating field demagnetization techniques, which give reproducible results. The paleomagnetic mean directions differ significantly between northern Sardinia and south‐eastern and central‐eastern Sardinia, the latter two regions yielding statistically similar paleomagnetic mean directions. These results indicate that Sardinia fragmented into two, arguably three, crustal blocks after emplacement of the dykes, which experienced differential relative rotations, as is also indicated by the differences in overall strike directions. The determination of timing, sense, and magnitude of these rotations has major implications for the reconstruction of the geodynamic evolution of the region in post‐Carboniferous times. We argue that the observed block rotations occurred during the Permian as the result of post‐Variscan intra‐Pangea mobility possibly related to the transformation of an Early Permian Pangea B to a Late Permian Pangea A.
      PubDate: 2014-05-02T13:45:17.718029-05:
      DOI: 10.1002/2014GC005325
  • Crustal thickness and velocity structure across the Moroccan Atlas from
    • Authors: P. Ayarza; R. Carbonell, A. Teixell, I. Palomeras, D. Martí, A. Kchikach, M. Harnafi, A. Levander, J. Gallart, M. L. Arboleya, J. Alcalde, M. Fernández, M. Charroud, M. Amrhar
      Pages: 1698 - 1717
      Abstract: The crustal structure and topography of the Moho boundary beneath the Atlas Mountains of Morocco has been constrained by a controlled source, wide‐angle seismic reflection transect: the SIMA experiment. This paper presents the first results of this project, consisting of an almost 700 km long, high‐resolution seismic profile acquired from the Sahara craton across the High and the Middle Atlas and the Rif Mountains. The interpretation of this seismic data set is based on forward modeling by raytracing, and has resulted in a detailed crustal structure and velocity model for the Atlas Mountains. Results indicate that the High Atlas features a moderate crustal thickness, with the Moho located at a minimum depth of 35 km to the S and at around 31 km to the N, in the Middle Atlas. Upper crustal shortening is resolved at depth through a crustal root where the Saharan crust underthrusts the northern Moroccan crust. This feature defines a lower crust imbrication that, locally, places the Moho boundary at ∼40–41 km depth in the northern part of the High Atlas. The P‐wave velocity model is characterized by relatively low velocities, mostly in the lower crust and upper mantle, when compared to other active orogens and continental regions. These low deep crustal velocities together with other geophysical observables such as conductivity estimates derived from MT measurements, moderate Bouguer gravity anomaly, high heat flow, and surface exposures of recent alkaline volcanism lead to a model where partial melts are currently emplaced at deep crustal levels and in the upper mantle. The resulting model supports the existence of a mantle upwelling as mechanism that would contribute significantly to sustain the High Atlas topography. However, the detailed Moho geometry deduced in this work should lead to a revision of the exact geometry and position of this mantle feature and will require new modeling efforts.
      PubDate: 2014-05-07T15:04:07.947886-05:
      DOI: 10.1002/2013GC005164
  • Geochemistry of volcanic glasses from the Louisville Seamount Trail (IODP
           Expedition 330): Implications for eruption environments and mantle melting
    • Authors: Alexander R. L. Nichols; Christoph Beier, Philipp A. Brandl, David M. Buchs, Stefan H. Krumm
      Pages: 1718 - 1738
      Abstract: Volcanic glasses recovered from four guyots during drilling along the Louisville Seamount Trail, southwest Pacific, have been analyzed for major, trace, and volatile elements (H2O, CO2, S, and Cl), and oxygen isotopes. Compared to other oceanic island settings, they are geochemically homogeneous, providing no evidence of the tholeiitic stage that characterizes Hawai'i. The degrees and depth of partial melting remained constant over 1–3 Ma represented by the drill holes, and along‐chain over several million years. The only exception is Hadar Guyot with compositions that suggest small degree preferential melting of an enriched source, possibly because it erupted on the oldest and thickest lithosphere. Incompatible element enriched glass from late‐stage volcaniclastics implies lower degrees of melting as the volcanoes moved off the melting anomaly. Volcaniclastic glasses from throughout the igneous basement are degassed suggesting generation during shallow submarine eruptions (
      PubDate: 2014-05-07T15:06:34.896536-05:
      DOI: 10.1002/2013GC005086
  • Interaction of subducted slabs with the mantle transition‐zone: A
           regime diagram from 2‐D thermo‐mechanical models with a mobile
           trench and an overriding plate
    • Authors: F. Garel; S. Goes, D. R. Davies, J. H. Davies, S. C. Kramer, C. R. Wilson
      Pages: 1739 - 1765
      Abstract: Transition zone slab deformation influences Earth's thermal, chemical, and tectonic evolution. However, the mechanisms responsible for the wide range of imaged slab morphologies remain debated. Here we use 2‐D thermo‐mechanical models with a mobile trench, an overriding plate, a temperature and stress‐dependent rheology, and a 10, 30, or 100‐fold increase in lower mantle viscosity, to investigate the effect of initial subducting and overriding‐plate ages on slab‐transition zone interaction. Four subduction styles emerge: (i) a “vertical folding” mode, with a quasi‐stationary trench, near‐vertical subduction, and buckling/folding at depth (VF); (ii) slabs that induce mild trench retreat, which are flattened/“horizontally deflected” and stagnate at the upper‐lower mantle interface (HD); (iii) inclined slabs, which result from rapid sinking and strong trench retreat (ISR); (iv) a two‐stage mode, displaying backward‐bent and subsequently inclined slabs, with late trench retreat (BIR). Transitions from regime (i) to (iii) occur with increasing subducting plate age (i.e., buoyancy and strength). Regime (iv) develops for old (strong) subducting and overriding plates. We find that the interplay between trench motion and slab deformation at depth dictates the subduction style, both being controlled by slab strength, which is consistent with predictions from previous compositional subduction models. However, due to feedbacks between deformation, sinking rate, temperature, and slab strength, the subducting plate buoyancy, overriding plate strength, and upper‐lower mantle viscosity jump are also important controls in thermo‐mechanical subduction. For intermediate upper‐lower mantle viscosity jumps (×30), our regimes reproduce the diverse range of seismically imaged slab morphologies.
      PubDate: 2014-05-12T10:26:18.433392-05:
      DOI: 10.1002/2014GC005257
  • Plate rotation during continental collision and its relationship with the
           exhumation of UHP metamorphic terranes: Application to the Norwegian
    • Authors: A. D. Bottrill; J. van Hunen, S. J. Cuthbert, H. K. Brueckner, M. B. Allen
      Pages: 1766 - 1782
      Abstract: Lateral variation and asynchronous onset of collision during the convergence of continents can significantly affect the burial and exhumation of subducted continental crust. Here we use 3‐D numerical models for continental collision to discuss how deep burial and exhumation of high and ultrahigh pressure metamorphic (HP/UHP) rocks are enhanced by diachronous collision and the resulting rotation of the colliding plates. Rotation during collision locally favors eduction, the inversion of the subduction, and may explain the discontinuous distribution of ultra‐high pressure (UHP) terranes along collision zones. For example, the terminal (Scandian) collision of Baltica and Laurentia, which formed the Scandinavian Caledonides, resulted in the exhumation of only one large HP/UHP terrane, the Western Gneiss Complex (WGC), near the southern end of the collision zone. Rotation of the subducting Baltica plate during collision may provide an explanation for this distribution. We explore this hypothesis by comparing orthogonal and diachronous collision models and conclude that a diachronous collision can transport continental material up to 60 km deeper, and heat material up to 300°C hotter, than an orthogonal collision. Our diachronous collision model predicts that subducted continental margin material returns to the surface only in the region where collision initiated. The diachronous collision model is consistent with petrological and geochonological observations from the WGC and makes predictions for the general evolution of the Scandinavian Caledonides. We propose the collision between Laurentia and Baltica started at the southern end of the collisional zone, and propagated northward. This asymmetric geometry resulted in the counter clockwise rotation of Baltica with respect to Laurentia, consistent with paleomagnetic data from other studies. Our model may have applications to other orogens with regional UHP terranes, such as the Dabie Shan and Papua New Guinea cases, where block rotation during exhumation has also been recorded.
      PubDate: 2014-05-14T15:20:57.877713-05:
      DOI: 10.1002/2014GC005253
  • A uniform database of teleseismic shear wave splitting measurements for
           the western and central United States
    • Authors: Kelly H. Liu; Ahmed Elsheikh, Awad Lemnifi, Uranbaigal Purevsuren, Melissa Ray, Hesham Refayee, Bin B. Yang, Youqiang Yu, Stephen S. Gao
      Pages: 2075 - 2085
      Abstract: We present a shear wave splitting (SWS) database for the western and central United States as part of a lasting effort to build a uniform SWS database for the entire North America. The SWS measurements were obtained by minimizing the energy on the transverse component of the PKS, SKKS, and SKS phases. Each of the individual measurements was visually checked to ensure quality. This version of the database contains 16,105 pairs of splitting parameters. The data used to generate the parameters were recorded by 1774 digital broadband seismic stations over the period of 1989–2012, and represented all the available data from both permanent and portable seismic networks archived at the Incorporated Research Institutions for Seismology Data Management Center in the area of 26.00°N to 50.00°N and 125.00°W to 90.00°W. About 10,000 pairs of the measurements were from the 1092 USArray Transportable Array stations. The results show that approximately 2/3 of the fast orientations are within 30° from the absolute plate motion (APM) direction of the North American plate, and most of the largest departures with the APM are located along the eastern boundary of the western US orogenic zone and in the central Great Basins. The splitting times observed in the western US are larger than, and those in the central US are comparable with the global average of 1.0 s. The uniform database has an unprecedented spatial coverage and can be used for various investigations of the structure and dynamics of the Earth.
      PubDate: 2014-05-12T10:11:36.264344-05:
      DOI: 10.1002/2014GC005267
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