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

Geochemistry, Geophysics, Geosystems     Full-text available via subscription   (Followers: 25, SJR: 2.56, h-index: 69)
Geophysical Research Letters     Full-text available via subscription   (Followers: 53, SJR: 3.493, h-index: 157)
Global Biogeochemical Cycles     Full-text available via subscription   (Followers: 5, SJR: 3.239, h-index: 119)
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J. of Geophysical Research : Atmospheres     Partially Free   (Followers: 22)
J. of Geophysical Research : Biogeosciences     Full-text available via subscription   (Followers: 6)
J. of Geophysical Research : Earth Surface     Partially Free   (Followers: 24)
J. of Geophysical Research : Oceans     Partially Free   (Followers: 14)
J. of Geophysical Research : Planets     Full-text available via subscription   (Followers: 13)
J. of Geophysical Research : Solid Earth     Full-text available via subscription   (Followers: 26)
J. of Geophysical Research : Space Physics     Full-text available via subscription   (Followers: 15)
Paleoceanography     Full-text available via subscription   (Followers: 3, SJR: 3.22, h-index: 88)
Radio Science     Full-text available via subscription   (Followers: 3, SJR: 0.959, h-index: 51)
Reviews of Geophysics     Full-text available via subscription   (Followers: 20, SJR: 9.68, h-index: 94)
Space Weather     Full-text available via subscription   (Followers: 3, SJR: 1.319, h-index: 19)
Tectonics     Full-text available via subscription   (Followers: 9, SJR: 2.748, h-index: 85)
Water Resources Research     Full-text available via subscription   (Followers: 82, SJR: 2.189, h-index: 121)
Journal Cover   Geochemistry, Geophysics, Geosystems
  [SJR: 2.56]   [H-I: 69]   [25 followers]  Follow
   Full-text available via subscription Subscription journal
   ISSN (Online) 1525-2027
   Published by American Geophysical Union (AGU) Homepage  [17 journals]
  • A Bayesian, multivariate calibration for Globigerinoides ruber Mg/Ca
    • Authors: D. Khider; G. Huerta, C. Jackson, L.D. Stott, J. Emile‐Geay
      Abstract: The use of Mg/Ca in marine carbonates as a paleothermometer has been challenged by observations that implicate salinity as a contributing influence on Mg incorporation into biotic calcite and that dissolution at the sea‐floor alters the original Mg/Ca. Yet, these factors have not yet been incorporated into a single calibration model. We introduce a new Bayesian calibration for Globigerinoides ruber Mg/Ca based on 186 globally‐distributed core top samples, which explicitly takes into account the effect of temperature, salinity, and dissolution on this proxy. Our reported temperature, salinity, and dissolution (here expressed as deep‐water ΔCO32‐) sensitivities are (±2σ) 8.7±0.9%/°C, 3.9±1.2%/psu, and 3.3±1.3%/μ−1 below a critical threshold of 21 μmol/kg, in good agreement with previous culturing and core‐top studies. We then perform a sensitivity experiment on a published record from the western tropical Pacific to investigate the bias introduced by these secondary influences on the interpretation of past temperature variability. This experiment highlights the potential for misinterpretations of past oceanographic changes when the secondary influences of salinity and dissolution are not accounted for. Multi‐proxy approaches could potentially help deconvolve the contributing influences but this awaits better characterization of the spatio‐temporal relationship between salinity and δ18Osw over millennial and orbital timescales. This article is protected by copyright. All rights reserved.
      PubDate: 2015-07-22T03:58:44.749955-05:
      DOI: 10.1002/2015GC005844
  • Geology, sulphide geochemistry, and supercritical venting at the Beebe
           Hydrothermal Vent Field, Cayman Trough
    • Authors: Alexander P. Webber; Stephen Roberts, Bramley J. Murton, Matthew R.S. Hodgkinson
      Abstract: The Beebe Vent Field (BVF) is the world's deepest known hydrothermal system, at 4960m below sea level. Located on the Mid‐Cayman Spreading Centre, Caribbean, the BVF hosts high temperature (∼401°C) ‘black smoker' vents that build Cu, Zn and Au‐rich sulphide mounds and chimneys. The BVF is highly gold‐rich, with Au values up to 93 ppm and an average Au:Ag ratio of 0.15. Gold precipitation is directly associated with diffuse flow through ‘beehive' chimneys. Significant mass‐wasting of sulphide material at the BVF, accompanied by changes in metal content, results in metaliferous talus and sediment deposits. Situated on very thin (2‐3km thick) oceanic crust, at an ultraslow spreading centre, the hydrothermal system circulates fluids to a depth of ∼1.8km in a basement that is likely to include a mixture of both mafic and ultramafic lithologies. We suggest hydrothermal interaction with chalcophile‐bearing sulphides in the mantle rocks, together with precipitation of Au in beehive chimney structures, has resulted in the formation of a Au‐rich volcanogenic massive sulphide (VMS) deposit. With its spatial distribution of deposit materials and metal contents, the BVF represents a modern day analogue for basalt hosted, Au‐rich VMS systems. This article is protected by copyright. All rights reserved.
      PubDate: 2015-07-22T03:40:30.387821-05:
      DOI: 10.1002/2015GC005879
  • Water‐rich bending faults at the Middle America Trench
    • Authors: Samer Naif; Kerry Key, Steven Constable, Rob L. Evans
      Abstract: The portion of the Central American margin that encompasses Nicaragua is considered to represent an end‐member system where multiple lines of evidence point to a substantial flux of subducted fluids. The seafloor spreading fabric of the incoming Cocos plate is oriented parallel to the trench such that flexural bending at the outer rise optimally reactivates a dense network of normal faults that extend several kilometers into the upper mantle. Bending faults are thought to provide fluid pathways that lead to serpentinization of the upper mantle. While geophysical anomalies detected beneath the outer rise have been interpreted as broad crustal and upper mantle hydration, no observational evidence exists to confirm that bending faults behave as fluid pathways. Here, we use seafloor electromagnetic data collected across the Middle America Trench (MAT) offshore of Nicaragua to create a comprehensive electrical resistivity image that illuminates the infiltration of seawater along bending faults. We quantify porosity from the resistivity with Archie's law and find that our estimates for the abyssal plain oceanic crust are in good agreement with independent observations. As the Cocos crust traverses the outer rise, the porosity of the dikes and gabbros progressively increase from 2.7% and 0.7% to 4.8% and 1.7%, peaking within 20 km of the trench axis. We conclude that the intrusive crust subducts twice as much pore water as previously thought, significantly raising the flux of fluid to the seismogenic zone and the mantle wedge. This article is protected by copyright. All rights reserved.
      PubDate: 2015-07-21T08:23:13.310045-05:
      DOI: 10.1002/2015GC005927
  • Eduction, extension, and exhumation of ultrahigh‐pressure rocks in
           metamorphic core complexes due to subduction initiation
    • Authors: Kenni Dinesen Petersen; W. Roger Buck
      Abstract: The controversy over the exhumation of ultra‐high pressure (UHP) rocks centers on whether it involves rising of pieces of crust detached from subducted continental lithosphere or an entire subducted plate that undergoes “eduction”, i.e. reverse subduction. We present a new thermomechanical model of continental subduction showing that these apparently contrasting mechanisms can occur together: Crust subducted deep enough is heated and weakened, causing limited diapiric rise, while crust subducted to shallower depths retains strength and is exhumed only by eduction. The model also shows for the first time how eduction followed by seafloor spreading can occur in a zone of regional convergence. This occurs spontaneously when subduction of buoyant crust causes a subduction zone to “lock up” in one place causing a new subduction zone to form in another. The model is consistent with many features of the youngest region of UHP rock exhumation on earth: the D'Entrecasteaux Islands. UHP exhumation and the amount of regional extension, as well as the seismic structure around the islands, can be explained by eduction. Ductile flow fabrics, seen on the islands, would result from exhumation of the most deeply subducted crust heated enough to undergo partial melting. Reversal of motion on the north‐dipping continental subduction zone, required by this model, was likely triggered by initiation of the New Britain Trench, as suggested previously. Our model implies that the crust of Goodenough Basin, south of the islands, was exhumed by eduction in the last 5 Ma and this hypothesis can be tested by drilling. This article is protected by copyright. All rights reserved.
      PubDate: 2015-07-17T03:28:24.269522-05:
      DOI: 10.1002/2015GC005847
  • Reply to comment by Ivan Pineda‐Velasco, Tai T. Nguyen, Hiroshi
           Kitagawa, and Eizo Nakamura on “Diverse magmatic effects of
           subducting a hot slab in SW Japan: Results from forward modeling”
    • Authors: Jun‐Ichi Kimura; Takashi Miyazaki, Stefanov Vaglarov Bogdan, Satoru Haraguchi, Qing Chang, James B Gill
      Abstract: The Comment by Pineda‐Velasco et al. [2015] examined Pb isotope data presented by Kimura et al. [2014]. The authors' points are that (1) there is uncertainty in the analytical results of Kimura et al. [2014] due to the effect of mass fractionation, and therefore, (2) the interpretations of Kimura et al. [2014] based on the extent of crustal assimilation and the estimated Pb isotopic composition of the crustal component are erroneous. In response to the Comment, we report a flaw in the original paper that the samples from the Aono, Daisen, and Kannabe regions were analyzed using conventional TIMS methods and all other samples were analyzed using thallium‐spiked multi‐collector inductively‐coupled‐plasma mass spectrometry (TS‐MC‐ICP‐MS). We have re‐analyzed the sample powders from Karasugasen, Daisen, and Aono using TS‐MC‐ICP‐MS. Our new results showed considerable overlap with the data in Pineda‐Velasco et al. [2015]. Therefore, the isotopic trends shown by the conventional TIMS in Kimura et al. [2014] were analytical artifacts from mass bias. We conclude that the crustal assimilation proposed by Kimura et al. [2014] was erroneous in terms of Pb isotopes, nevertheless some crustal assimilation in the Karasugasen lava is evident from the chemical zoning of hornblende phenocrysts. Although the original Pb isotope argument for crustal contamination was wrong, the ABS4 modeling is unaffected because of no to subtle changes in estimated mantle source compositions in their ABS4 model. This article is protected by copyright. All rights reserved.
      PubDate: 2015-07-17T02:22:07.781949-05:
      DOI: 10.1002/2015GC005984
  • Comment on “Diverse magmatic effects of subducting a hot slab in SW
           Japan: Results from forward modeling'' by J.‐I. Kimura et al.
    • Authors: Ivan Pineda‐Velasco; Tai T. Nguyen, Hiroshi Kitagawa, Eizo Nakamura
      PubDate: 2015-07-16T04:33:10.722884-05:
      DOI: 10.1002/2015GC005914
  • Varying styles of magmatic strain accommodation across the East African
    • Authors: James D. Muirhead; Simon A. Kattenhorn, Nicolas Le Corvec
      Abstract: Observations of active dike intrusions provide present day snapshots of the magmatic contribution to continental rifting. However, unravelling the contributions of upper crustal dikes over the timescale of continental rift evolution is a significant challenge. To address this issue, we analyzed the morphologies and alignments of >1,500 volcanic cones to infer the distribution and trends of upper crustal dikes in various rift basins across the East African Rift (EAR). Cone lineament data reveal along‐axis variations in the distribution and geometries of dike intrusions as a result of changing tectono‐magmatic conditions. In younger (
      PubDate: 2015-07-16T04:28:13.092083-05:
      DOI: 10.1002/2015GC005918
  • River geochemistry, chemical weathering, and atmospheric CO2 consumption
           rates in the Virunga Volcanic Province (East Africa)
    • Authors: Charles M. Balagizi; François Darchambeau, Steven Bouillon, Mathieu M. Yalire, Thibault Lambert, Alberto V. Borges
      Abstract: We report a water chemistry data set from 13 rivers of the Virunga Volcanic Province (VVP) (Democratic Republic of Congo), sampled between December 2010 and February 2013. Most parameters showed no pronounced seasonal variation, whereas their spatial variation suggests a strong control by lithology, soil type, slope and vegetation. High total suspended matter (289‐1467 mg L−1) was recorded in rivers in the Lake Kivu catchment, indicating high soil erodibility, partly as a consequence of deforestation and farming activities. Dissolved and particulate organic carbon (DOC and POC) were lower in rivers from lava fields, and higher in non‐volcanic sub‐catchments. Stable carbon isotope signatures (δ13C) of POC and DOC mean δ13C of ‐22.5 and ‐23.5 ‰, respectively), are the first data to be reported for the highland of the Congo River basin and showed a much higher C4 contribution than in lowland areas. Rivers of the VVP were net sources of CH4 to the atmosphere (4‐ 5052 nmol L−1). Most rivers show N2O concentrations close to equilibrium, but some rivers showed high N2O concentrations related to denitrification in groundwaters. δ13C signatures of dissolved inorganic carbon suggested magmatic CO2 inputs to aquifers/soil, which could have contributed to increase basalt weathering rates. This magmatic CO2‐mediated basalt weathering strongly contributed to the high major cation concentrations and total alkalinity. Thus, chemical weathering (39.0 ‐2779.9 t km−2 yr−1) and atmospheric CO2 consumption (0.4‐ 37.0 x 106 mol km−2 yr−1) rates were higher than previously reported in literature for basaltic terrains. This article is protected by copyright. All rights reserved.
      PubDate: 2015-07-16T04:27:14.648344-05:
      DOI: 10.1002/2015GC005999
  • LA‐ICPMS Ba/Ca analyses of planktic foraminifera from the Bay of
           Bengal: Implications for late Pleistocene orbital control on monsoon
           freshwater flux
    • Authors: David Evans; Rehemat Bhatia, Heather Stoll, Wolfgang Müller
      Abstract: Indian Summer Monsoon (ISM) indices are characterised by large secular variation during both glacials and interglacials. Although much information about palaeo‐monsoon intensity is derived from such indicies, current datasets do not relate simply to precipitation. In order to directly constrain the variability of ISM freshwater flux to the Bay of Bengal, we report Ba/Ca LA‐ICPMS data of the surface‐dwelling foraminifera G. ruber from core RC12‐343 (central Bay of Bengal) between 68‐47 ka. Planktic foraminifera Ba/Ca directly relates to seawater Ba/Ca, in turn principally controlled by freshwater flux. Our foraminifera‐derived Ba/Casw record for the central Bay of Bengal is highly coherent with that derived from δ18O measurements of the same material, implying that these reconstructions are not significantly biased by potential shifts in δ18Ofreshwater. Validating this method allows us to produce a freshwater stack for the last 80 ka for the Bay of Bengal, enabling the orbital controls on ISM precipitation to be examined for the first time. The highest freshwater flux in the last 80 ka was ∼3× larger than present and occurred during the early‐mid Holocene. We show that the orbital timing of this record is best explained by a combination of factors with a weighting of ∼45% given to the 30° N‐equator mean summer insolation gradient and ∼55% given to the variability of May‐July insolation at the equator and June‐July insolation at 30° N. These processes are consistent with current mechanistic understanding of ISM forcings and demonstrate a dominant orbital control on monsoon precipitation amount on millennial‐timescales. This article is protected by copyright. All rights reserved.
      PubDate: 2015-07-16T04:26:23.921548-05:
      DOI: 10.1002/2015GC005822
  • Issue Information
    • PubDate: 2015-07-15T03:28:12.520321-05:
      DOI: 10.1002/ggge.20562
  • Crystallographic preferred orientations may develop in nanocrystalline
           materials on fault planes due to surface energy interactions
    • Authors: Virginia G. Toy; Thomas M. Mitchell, Anthony Druiventak, Richard Wirth
      Abstract: A layer of substantially non‐crystalline material, composed of partially annealed nanopowder with local melt, was experimentally generated by comminution during ∼1.5 mm total slip at ∼2.5x10−6 ms−1, Pconf ∼ 0.5 GPa, and 450°C or 600°C, on sawcut surfaces in novaculite. The partially annealed nanopowder comprises angular grains mostly 5‐200 nm diameter in a variably dense packing arrangement. A sharp transition from wall rock to partially annealed nanopowder illustrates the nanopowder effectively localizes shear, consistent with generation of nanoparticles during initial fragmentation, not by progressive grain size reduction. Dislocation densities in nanopowder grains or immediate wall rock are not significantly high, but there are planar plastic defects spaced at 5‐200 nm parallel to the host quartz grain's basal plane. We propose these plastic defects developed into through‐going fractures to generate nanocrystals. The partially annealed nanopowder has a crystallographic preferred orientation (CPO) that we hypothesise developed due to surface energy interactions to maximize coincident site lattices (CSL) during annealing. This mechanism may also have generated CPO recently described in micro/nanocrystalline calcite fault gouges. This article is protected by copyright. All rights reserved.
      PubDate: 2015-07-14T10:01:35.047042-05:
      DOI: 10.1002/2015GC005857
  • Effects of chemical composition, water, and temperature on physical
           properties of continental crust
    • Authors: Mattia Guerri; Fabio Cammarano, James A.D. Connolly
      Abstract: We explore the influence of major elements chemistry and H2O‐content on the density and seismic velocity of crustal rocks by computing stable and metastable crustal mineralogy and elastic properties as a function of pressure and temperature (P‐T). Proposed average compositions of continental crust result in significantly different properties, for example a difference in computed density of ∼ 4% is obtained at a given P‐T. Phase transformations affect crustal properties at the point that crustal seismic discontinuities can be explained with mineral reactions rather than chemical stratification. H2O, even if introduced in small amount in the chemical system, has an effect on physical properties comparable to that attributed to variations in major elements composition. Thermodynamical relationships between physical properties differ significantly from commonly used empirical relationships. Density models obtained by inverting CRUST 1.0 compressional wave velocity [Laske et al., 2013] are different from CRUST 1.0 density and translate into variations in isostatic topography and gravitational field that ranges ±600 m and ±150 mGal respectively. Inferred temperatures are higher than reference geotherms in the upper crust and in the deeper portions of thick orogenic crust, consistently with presence of metastable rocks. Our results highlight interconnections/dependencies among chemistry, pressure, temperature, seismic velocities and density that need to be addressed to better understand the crustal thermo‐chemical state. This article is protected by copyright. All rights reserved.
      PubDate: 2015-07-14T08:04:31.82752-05:0
      DOI: 10.1002/2015GC005819
  • Spatiotemporal variations of the slow slip event between 2008 and 2013 in
           the southcentral Alaska subduction zone
    • Authors: Yuning Fu; Zhen Liu, Jeffrey T. Freymueller
      Abstract: We apply a Kalman filter based time‐dependent slip inversion method to model a long‐term Slow Slip Event (SSE) in the southcentral Alaska subduction zone from 2008 to 2013. This event occurred downdip of the asperity that ruptured in the 1964 earthquake, the same part of plate interface that slipped during a previous SSE between 1998 and 2001. Most of the slip deficit that accumulated during the steady period between 2001 and 2008 (8 years total) in the SSE source region was released by this SSE. Our results indicate both lateral and down dip propagation during this event. The SSE started at the end of 2008 at the upper section of the slip patch, and gradually propagated to the east and to the deeper part of the interface. Our results indicate no connection between this SSE in Upper Cook Inlet and another SSE in Lower Cook Inlet that started in 2010. Analysis of the earthquake catalog in the southcentral Alaska subduction zone shows a clear increase in seismicity associated with the 2008‐2013 SSE. With the data from a newly available continuous GPS site, we now can better constrain the start time of the 1998‐2001 SSE as ∼1998.58. This article is protected by copyright. All rights reserved.
      PubDate: 2015-07-14T08:04:23.37089-05:0
      DOI: 10.1002/2015GC005904
  • Profiling planktonic foraminiferal crust formation
    • Authors: Juliane Steinhardt; Lennart J. de Nooijer, Geert‐Jan A. Brummer, Gert‐Jan Reichart
      Abstract: Planktonic foraminifera migrate vertically through the water column during their life, thereby growing and calcifying over a range of depth‐associated conditions. Some species form a calcite veneer, crust or cortex at the end of their lifecycle. This additional calcite layer may vary in structure, composition and thickness, potentially accounting for most of their total shell mass and thereby dominating the element and isotope signature of the whole shell. Here we apply laser ablation ICP‐MS depth profiling to assess variability in thickness and Mg/Ca composition of shell walls of three encrusting species derived from sediment traps. Compositionally, Mg/Ca is significantly lower in the crusts of Neogloboquadrina dutertrei and Globorotalia scitula, as well as in the cortex of Pulleniatina obliquiloculata, independent of the species‐specific Mg/Ca of their lamellar calcite shell. Wall thickness accounts for nearly half of the total thickness in both crustal species and nearly a third in cortical P. obliquiloculata, regardless of their initial shell wall thickness. Crust thickness and crustal Mg/Ca decreases towards the younger chambers in N. dutertrei and to a lesser extent, also in G. scitula. In contrast, the cortex of P. obliquiloculata shows a nearly constant thickness and uniform Mg/Ca through the complete chamber wall. Patterns in thickness and Mg/Ca of the crust indicate that temperature is not the dominant factor controlling crust formation. Instead, we present a depth‐resolved model explaining compositional differences within individuals and between successive chambers as well as compositional heterogeneity of the crust and lamellar calcite in all three species studied here. This article is protected by copyright. All rights reserved.
      PubDate: 2015-07-14T07:26:03.966367-05:
      DOI: 10.1002/2015GC005752
  • The Manihiki Plateau—A multistage volcanic emplacement history
    • Authors: R. Pietsch; G. Uenzelmann‐Neben
      Abstract: The formation history of the Manihiki Plateau, a Large Igneous Province, is poorly understood. New high resolution seismic reflection data across the High Plateau, the largest edifice of the Manihiki Plateau, provides evidence for multistage magmatic emplacement. Improved data quality allows for an identification of an earlier volcanic phase, the initial formation phase (>125 Ma), in addition to the previously known volcanic formation phases: the expansion phase (125‐116) formerly called main‐phase and the secondary volcanic phase (100‐65 Ma). This enhances the understanding of the emplacement scenario. An intrabasement reflection band IB1 reveals the end of initial volcanic formation and forms the nucleus of the High Plateau. This feature provides indications that it continued beyond the Manihiki Scarp and thus supports the hypothesis of an extension of the Manihiki Plateau to the East during the initial formation and expansion phases. The expansion phase is characterized by massive volcanic outpourings leveling and extending the basement throughout the High Plateau and the neighboring Western Plateaus, which in contrast shows massive tectonic alteration. Extrusion centers formed within the secondary volcanic phase (ending ∼65 Ma) are mainly concentrated along the margins of the High Plateau, suggesting the magmatic sources shifted from those being related to the initial emplacement and expansion phases of the High Plateau to induced volcanism at the tectonically altered margins. This article is protected by copyright. All rights reserved.
      PubDate: 2015-07-14T07:25:43.009957-05:
      DOI: 10.1002/2015GC005852
  • Particle dynamics in the rising plume at Piccard Hydrothermal Field,
           Mid‐Cayman Rise
    • Authors: M. L. Estapa; J. A. Breier, C. R. German
      Abstract: Processes active in rising hydrothermal plumes, such as precipitation, particle aggregation, and biological growth, affect particle size distributions and can exert important influences on the biogeochemical impact of submarine venting of iron to the oceans and their sediments. However, observations of particle size distribution within these systems, to date, are both limited and conflicting. In a novel buoyant hydrothermal plume study at the recently discovered high‐temperature (398°C) Piccard Hydrothermal Field, Mid‐Cayman Rise, we report optical measurements of particle size distributions (PSDs). We describe the plume PSD in terms of a simple, power‐law model commonly used in studies of upper‐ and coastal ocean particle dynamics. Observed PSD slopes, derived from spectral beam attenuation and laser diffraction measurements, are among the highest found to date anywhere in the ocean and ranged from 2.9 to 8.5. Beam attenuation at 650 nm ranged from near zero to a rarely‐observed maximum of 192 m−1 at 3.5 m above the vent. We did not find large (>100 μm) particles that would settle rapidly to the sediments. Instead, beam attenuation was well‐correlated to total iron, suggesting the first‐order importance of particle dilution, rather than precipitation or dissolution, in the rising plume at Piccard. Our observations at Piccard caution against the assumption of rapid deposition of hydrothermal, particulate metal fluxes, and illustrate the need for more particle size and composition measurements across a broader range of sites, globally. This article is protected by copyright. All rights reserved.
      PubDate: 2015-07-14T07:25:40.4561-05:00
      DOI: 10.1002/2015GC005831
  • Imaging continental breakup using teleseismic body waves: The Woodlark
           Rift, Papua New Guinea
    • Authors: Zachary Eilon; Geoffrey A Abers, James B Gaherty, Ge Jin
      Abstract: This study images the upper mantle beneath the D'Entrecasteax Islands, Papua New Guinea, providing insight into mantle deformation beneath a highly rifted continent adjacent to propagating spreading centers. Differential travel times from P‐ and S‐wave teleseisms recorded during the 2010‐2011 CDPapua passive seismic experiment are used to invert for separate VP and VS velocity models of the continental rift. A low‐velocity structure marks the E‐W axis of the rift, correlating with the thinnest crust, high heat flow, and a linear trend of volcanoes. This slow region extends 250 km along strike from the oceanic spreading centers, demonstrating significant mantle extension ahead of seafloor breakup. The rift remains narrow to depth indicating localization of extension, perhaps as a result of mantle hydration. A high‐VP structure at depths of 90‐120 km beneath the north of the array is more than 6.5% faster than the rift axis and contains well‐located intermediate depth earthquakes. These independent observations place firm constraints on the lateral thermal contrast at depth between the rift axis and cold lithosphere to the north that may be related to recent subduction, although the polarity of subduction cannot be resolved. This geometry is gravitationally unstable; downwelling or small‐scale convection could have facilitated rifting and rapid lithospheric removal, although this may require a wet mantle to be realistic on the required timescales. The high‐V structure agrees with the maximum P,T conditions recorded by young ultra‐high pressure rocks exposed on the rift axis and may be implicated in their genesis. This article is protected by copyright. All rights reserved.
      PubDate: 2015-07-03T09:50:57.704778-05:
      DOI: 10.1002/2015GC005835
  • Effects of inherited cores and magmatic overgrowths on zircon
           (U‐Th)/He ages and age‐eU trends from Greater Himalayan
           sequence rocks, Mt. Everest region, Tibet
    • Authors: D.A. Orme; P.W. Reiners, J.K. Hourigan, B. Carrapa
      Abstract: Previous constraints on the timing and rate of exhumation of the footwall of the South Tibetan detachment system (STDS) north of Mt. Everest suggest rapid Miocene cooling from ∼ 700°C to 120°C between ca. 14‐17 Ma. However, twenty‐five new single grain zircon He ages from leucogranites intruding Greater Himalayan Sequence rocks in the footwall of the STDS are between 9.9 to 15 Ma, with weighted mean ages between 10 and 12 Ma. Zircon grains exhibit a positive correlation between age and effective uranium (eU). Laser ablation zircon U‐Pb geochronology, detailed SEM observations and laser ablation depth‐profiling of these zircons reveal low‐eU 0.5‐2.5 Ga inherited cores overgrown by high‐eU 17‐22 Ma rims. This intragranular zonation produces ages as much as 32% too young when a standard alpha‐ejection correction assuming uniform eU distribution is applied. Modeling of the effects of varying rim thickness and rim eU concentration on the bulk grain eU and alpha‐ejection correction suggests that zonation also exerts the primary control on the form of the age‐eU correlation observed. Application of grain‐specific zonation‐dependent age corrections to our data yields zircon He ages between 14‐17 Ma, in agreement with AFT and 40Ar/39Ar ages. Growth of magmatic rims followed by cooling to 
      PubDate: 2015-07-02T07:54:12.871204-05:
      DOI: 10.1002/2015GC005818
  • Thermal dependency of shell growth, microstructure, and stable isotopes in
           laboratory‐reared Scapharca broughtonii (Mollusca: Bivalvia)
    • Authors: Kozue Nishida; Atsushi Suzuki, Ryosuke Isono, Masahiro Hayashi, Yusuke Watanabe, Yuzo Yamamoto, Takahiro Irie, Yukihiro Nojiri, Chiharu Mori, Mizuho Sato, Kei Sato, Takenori Sasaki
      Abstract: We experimentally examined the growth, microstructure, and chemistry of shells of the bloody clam, Scapharca broughtonii (Mollusca: Bivalvia), reared at five temperatures (13, 17, 21, 25, and 29°C) with a constant pCO2 condition (∼450 ppm). In this species, the exterior side of the shell is characterized by a composite prismatic structure, on the interior side it has a crossed lamellar structure on the interior surface. We previously found a negative correlation between temperature and the relative thickness of the composite prismatic structure in field‐collected specimens. In the reared specimens, the relationship curve between temperature and the growth increment of the composite prismatic structure was humped‐shaped, with a maximum at 17°C, which was compatible with the results obtained in the field‐collected specimens. In contrast, the thickness of the crossed lamellar structure was constant over the temperature range tested. These results suggest that the composite prismatic structure principally accounts for the thermal dependency of shell growth, and this inference was supported by the finding that shell growth rates were significantly correlated with the thickness of the composite prismatic structure. We also found a negative relationship between the rearing temperature and δ18O of the shell margin, in close quantitative agreement with previous reports. The findings presented here will contribute to the improved age determination of fossil and recent clams based on seasonal microstructural records. This article is protected by copyright. All rights reserved.
      PubDate: 2015-07-02T07:19:22.934613-05:
      DOI: 10.1002/2014GC005634
  • Age of the Lava Creek supereruption and magma chamber assembly from
           combined 40Ar/39Ar and U‐Pb dating of sanidine and zircon crystals
    • Authors: Naomi E. Matthews; Jorge A. Vazquez, Andrew T. Calvert
      Abstract: The last supereruption from the Yellowstone Plateau formed Yellowstone caldera and ejected the >1000 km3 of rhyolite that composes the Lava Creek Tuff. Tephra from the Lava Creek eruption is a key Quaternary chronostratigraphic marker, in particular for dating the deposition of mid‐Pleistocene glacial and pluvial deposits in western North America. To resolve the timing of eruption and crystallization history for the Lava Creek magma, we performed (1) 40Ar‐39Ar dating of single sanidine crystals to delimit eruption age and (2) ion microprobe U‐Pb and trace‐element analyses of the crystal faces and interiors of single zircons to date the interval of zircon crystallization and characterize magmatic evolution. Sanidines from the two informal members composing Lava Creek Tuff yield a preferred 40Ar/39Ar isochron date of 631.3 ± 4.3 ka. Crystal faces on zircons from both members yield a weighted mean 206Pb/238U date of 626.5 ± 5.8 ka, and have trace element concentrations that vary with the eruptive stratigraphy. Zircon interiors yield a mean 206Pb/238U date of 659.8 ± 5.5 ka, and reveal reverse and/or oscillatory zoning of trace element concentrations, with many crystals containing high U concentration cores that likely grew from highly evolved melt. The occurrence of distal Lava Creek tephra in stratigraphic sequences marking the marine isotope stage 16—15 transition supports the apparent eruption age of ca. 631 ka. The combined results reveal that Lava Creek zircons record episodic heating, renewed crystallization, and an overall up‐temperature evolution for Yellowstone's subvolcanic reservoir in the 103—104 interval before eruption. This article is protected by copyright. All rights reserved.
      PubDate: 2015-07-02T06:36:59.453925-05:
      DOI: 10.1002/2015GC005881
  • Shock wave synthesis of amino acids from solutions of ammonium formate and
           ammonium bicarbonate
    • Authors: Chizuka Suzuki; Yoshihiro Furukawa, Toshimori Sekine, Hiromoto Nakazawa, Takeshi Kakegawa
      Abstract: The emergence of life's building blocks, such as amino acids and nucleobases, on the prebiotic Earth was a critical step for the beginning of life. Reduced species with low mass, such as ammonia, amines, or carboxylic acids, are potential precursors for these building blocks of life. These precursors may have been provided to the prebiotic ocean by carbonaceous chondrites and chemical reactions related to meteorite impacts on the early Earth. The impact of extraterrestrial objects on Earth occurred more frequently during this period than at present. Such impacts generated shock waves in the ocean, which have the potential to progress chemical reactions to form the building blocks of life from reduced species. To simulate shock‐induced reactions in the prebiotic ocean, we conducted shock‐recovery experiments on ammonium bicarbonate solution and ammonium formate solution at impact velocities ranging from 0.51 to 0.92 km/s. In the products from the ammonium formate solution, several amino acids (glycine, alanine, ß‐alanine, and sarcosine) and aliphatic amines (methylamine, ethylamine, propylamine, and butylamine) were detected, although yields were less than 0.1 mol% of the formic acid reactant. From the ammonium bicarbonate solution, smaller amounts of glycine, methylamine, ethylamine, and propylamine were formed. The impact velocities used in this study represent minimum cases because natural meteorite impacts typically have higher velocities and longer durations. Our results therefore suggest that shock waves could have been involved in forming life's building blocks in the ocean of prebiotic Earth, and potentially in aquifers of other planets, satellites, and asteroids. This article is protected by copyright. All rights reserved.
      PubDate: 2015-07-02T04:31:41.68426-05:0
      DOI: 10.1002/2015GC005783
  • Appreciation of peer reviewers for 2014
    • Authors: Thorsten W. Becker; Jay Bass, Janne Blichert‐Toft, Lou Derry, Cin‐Ty Lee, James Tyburczy, Derek Vance, Yusuke Yokoyama
      Abstract: The editorial and scientific publishing process relies on the sustained work of volunteer reviewers, and evaluating the inter‐disciplinary and broad interest papers published in G‐Cubed can be a particular challenge. As editors and associated editors, we are therefore hugely appreciative of the efforts of our reviewers, and would like to thank and acknowledge them in this editorial. G‐Cubed published 257 manuscripts out of 431 submissions in 2014, and for this we were able to rely on the efforts of 710 dedicated reviewers. Their names are listed below, in italics those 27 who provided three or more reviews. A big thank you from the G‐Cubed team!. This article is protected by copyright. All rights reserved.
      PubDate: 2015-07-01T10:36:24.123782-05:
      DOI: 10.1002/2015GC005982
  • Lithospheric shear wave velocity and radial anisotropy beneath the
           northern part of North China from surface wave dispersion analysis
    • Authors: Yuanyuan V. Fu; Yuan Gao, Aibing Li, Yutao Shi
      Abstract: Rayleigh and Love wave phase velocities in the northern part of the North China are obtained from ambient noise tomography in the period range of 8 to 35 s and two plane wave earthquake tomography at periods of 20 to 91 s using data recorded at 222 broadband seismic stations from the temporary North China Seismic Array and permanent China Digital Seismic Array. The dispersion curves of Rayleigh and Love wave from 8 to 91 s are jointly inverted for the 3‐D shear wave structure and radial anisotropy in the lithosphere to 140 km depth. Distinct seismic structure are observed from the Fenhe Graben and Taihang Mountain to North China Basin. The North China Basin from the lower crust to the depth of 140 km is characterized by high velocity anomaly, reflecting mafic intrusion and residual materials after the extraction of melt, and by strong radial anisotropy with Vsh > Vsv implying horizontal layering of intrusion and alignment of minerals due to vigorous extensional deformation and subsequent thermal annealing. However, low velocity anomaly and positive radial anisotropy are observed in the Fenhe Graben and Taihang Mountain, suggesting the presence of partial melt in the lithosphere due to the mantle upwelling and horizontal flow pull. This article is protected by copyright. All rights reserved.
      PubDate: 2015-06-19T17:51:39.82515-05:0
      DOI: 10.1002/2015GC005825
  • Nonlinear attenuation from the interaction between different types of
           seismic waves and interaction of seismic waves with shallow ambient
           tectonic stress
    • Authors: Norman H. Sleep; Nori Nakata
      Abstract: Strong seismic waves bring rock into frictional failure at the uppermost few hundred meters. Numerous small fractures slip with the cumulative effect of anelastic strain and nonlinear attenuation; these fractures should not distinguish between remote sources of stress. Still, frictional failure criteria are not evident especially when seismic waves change the normal traction on fractures. We identify three earthquakes as examples where consideration of interaction among dynamic stresses from different wave types and ambient tectonic stress provides theoretical predictions of nonlinear attenuation that are potentially testable with single station seismograms. For example, because Rayleigh waves produce shallow horizontal dynamic tension and compression, frictional failure should preferentially occur on the tensile half‐cycle if no shallow tectonic stress is present and on the compressional half‐cycle if the tectonic stress is already near thrust‐faulting failure. We observed neither effect on records from the 2011 Mw 9.0 Great Tohoku earthquake. However, Rayleigh waves from this event appear to have brought rock beneath MYGH05 station into frictional failure at ∼10 m depth and thus suppressed high‐frequency S‐waves. The tensile half‐cycle of high frequency P‐waves reduced normal traction on horizontal planes beneath station IWTH25 during the 2008 Mw 6.9 Iwate‐Miyagi earthquake, weakening the rock in shear and suppressing high‐frequency S‐waves. The near‐field velocity pulse from the 1992 Mw 7.3 Landers earthquake brought the uppermost few hundred meters of granite beneath Lucerne station into frictional failure, suppressing high frequency S‐waves. These mildly positive examples support the reality of nonlinear wave interaction, warranting study future strong ground motions. This article is protected by copyright. All rights reserved.
      PubDate: 2015-06-19T17:51:00.336745-05:
      DOI: 10.1002/2015GC005832
  • Mantle helium along the Newport‐Inglewood fault zone, Los Angeles
           basin, California—A leaking paleo‐subduction zone
    • Authors: J. R. Boles; G. Garven, H. Camacho, J. E. Lupton
      Abstract: Mantle helium is a significant component of the helium gas from deep oil wells along the Newport‐Inglewood fault zone (NIFZ) in the Los Angeles (LA) basin. Helium isotope ratios are as high as 5.3 Ra (Ra=3He/4He ratio of air) indicating 66% mantle contribution, (assuming R/Ra = 8 for mantle), and most values are higher than 1.0 Ra. Other samples from basin margin faults and from within the basin have much lower values (R/Ra 
      PubDate: 2015-06-19T17:50:43.830667-05:
      DOI: 10.1002/2015GC005951
  • A community benchmark for viscoplastic thermal convection in a 2‐D
           square box
    • Authors: N. Tosi; C. Stein, L. Noack, C. Hüttig, P. Maierová, H. Samuel, D. R. Davies, C. R. Wilson, S. C. Kramer, C. Thieulot, A. Glerum, M. Fraters, W. Spakman, A. Rozel, P. J. Tackley
      Abstract: Numerical simulations of thermal convection in the Earth's mantle often employ a pseudo‐plastic rheology in order to mimic the plate‐like behavior of the lithosphere. Yet the benchmark tests available in the literature are largely based on simple linear rheologies in which the viscosity is either assumed to be constant or weakly dependent on temperature. Here we present a suite of simple tests based on non‐linear rheologies featuring temperature‐, pressure‐, and strain rate dependent viscosity. Eleven different codes based on the finite volume, finite element, or spectral methods have been used to run five benchmark cases leading to stagnant lid, mobile lid, and periodic convection in a 2‐D square box. For two of these cases, we also show resolution tests from all contributing codes. In addition, we present a bifurcation analysis, describing the transition from a mobile lid regime to a periodic regime, and from a periodic regime to a stagnant lid regime, as a function of the yield stress. At a resolution of around 100 cells or elements in both vertical and horizontal directions, all codes reproduce the required diagnostic quantities with a discrepancy of at most ∼ 3% in the presence of both linear and non‐linear rheologies. Furthermore they consistently predict the critical value of the yield stress at which the transition between different regimes occurs. As the most recent mantle convection codes can handle a number of different geometries within a single solution framework, this benchmark will also prove useful when validating viscoplastic thermal convection simulations in such geometries. This article is protected by copyright. All rights reserved.
      PubDate: 2015-06-15T11:26:14.973399-05:
      DOI: 10.1002/2015GC005807
  • Variations of the lithospheric strength and elastic thickness in North
    • Authors: Magdala Tesauro; Mikhail K. Kaban, Walter D. Mooney
      Abstract: We evaluate the effect of temperature variations on strength and effective elastic thickness (Te) of the lithosphere of the North American (NA) continent. To this purpose, we use two thermal models that are corrected for compositional variations and anelasticity effects in the upper mantle. These thermal models are obtained from a joint inversion of gravity data and two recent seismic tomography models (NA07 and SL2013sv). The crustal rheology was defined using NACr14, the most recent NA crustal model. This model specifies seismic velocities and thickness for a three‐layer model of the crystalline crust. Strength in the lithosphere and in the crust has similar distributions, indicating that local geotherms play a dominant role in determining strength rather than crustal composition. A pronounced contrast is present in strength between cratonic and off‐cratonic regions. Lithospheric strength in the off‐cratonic regions is prevalently localized within the crust and Te shows low values (150 km). In contrast to previous results, our models indicate that Phanerozoic regions located close to the edge of the cratons, as the Appalachians, are characterized by low strength. We also find that locally weak zones exist within the cratons (e.g., beneath the intracratonic Illinois Basin and Midcontinent rift). Seismic tomography models NA07 and SL2013sv differ mainly in some peripheral parts of the cratons, as the Proterozoic Canadian Platform, the Grenville and the western part of the Yavapai‐Mazatzal province, where the integrated strength for the model NA07 is ten times larger than in model SL2013sv due to a temperature difference (>200˚C) in the uppermost mantle. The differences in Te between the two models are less pronounced. In both models, Proterozoic regions reactivated by Meso‐Cenozoic tectonics (e.g., Rocky Mountains and the Mississippi Embayment) are characterized by a weak lithosphere due to the absence of the mechanically strong part of the mantle lithospheric layer. Intraplate earthquakes are distributed along the edges of the cratons, demonstrating that tectonic stress accumulates there, while the cores of the cratons remain undeformed. In both models intraplate earthquakes occur in weak lithosphere (∼0.5 x10^13 Pa s, Te ∼15 km) or near the edges of strong cratonic blocks, characterized by pronounced contrasts of strength and Te. This article is protected by copyright. All rights reserved.
      PubDate: 2015-06-11T11:18:36.596803-05:
      DOI: 10.1002/2015GC005937
  • What happens to soil organic carbon as coastal marsh ecosystems change in
           response to increasing salinity? An exploration using ramped pyrolysis
    • Authors: Elizabeth K Williams; Brad E Rosenheim
      Abstract: Coastal wetlands store vast amounts of organic carbon, globally, and are becoming increasingly vulnerable to the effects of anthropogenic sea‐level rise. To understand the effect of sea‐level rise on organic carbon fate and preservation in this global sink, it is necessary to characterize differences in the biogeochemical stability of coastal wetland soil organic carbon (SOC). Here, we use ramped pyrolysis/oxidation decomposition characteristics as proxies for SOC stability to understand the fate of carbon storage in coastal wetlands comprising the Mississippi River deltaic plain, undergoing rapid rates of local sea level rise. Soils from three wetland types (fresh, brackish, and salt marshes) along a salinity gradient were subjected to ramped pyrolysis analysis to evaluate decomposition characteristics related to thermochemical stability of SOC. At equivalent soil depths, we observed that fresh marsh SOC was more stable than brackish and salt marsh SOC. Depth, isotopic, elemental, and chemical compositions, bulk density, and water content of SOC all exhibited different relationships with SOC stability across the marsh salinity gradient, indicative of different controls on SOC stability within each marsh type. The differences in stability imply stronger preservation potential of fresh marsh soil carbon, compared to that of salt and brackish marshes. Considering projected marsh ecosystem responses to sea‐level rise, these observed stability differences are important in planning and implementing coastal wetland carbon‐focused remediation and improving climate model feedbacks with the carbon cycle. Specifically, our results imply that ecosystem changes associated with sea‐level rise will initiate the accumulation of less stable carbon in coastal wetlands. This article is protected by copyright. All rights reserved.
      PubDate: 2015-06-11T11:18:21.635345-05:
      DOI: 10.1002/2015GC005839
  • Miocene climate change on the Chinese Loess Plateau: Possible links to the
           growth of the northern Tibetan Plateau and global cooling
    • Authors: Youbin Sun; Long Ma, Jan Bloemendal, Steven Clemens, Xiaoke Qiang, Zhisheng An
      Abstract: The evolution of the Asian monsoon‐arid environmental system during the Cenozoic was closely related to the growth of the Himalayan‐Tibetan Plateau and global climate change. However, due to inconsistencies in paleoclimatic reconstructions and to various constraints on the timing of the growth of the Tibetan Plateau, the relative impacts of regional uplift and global cooling on Asian climate change remain controversial. Here we investigate the mineralogical composition of a Miocene Red Clay deposit on the western Chinese Loess Plateau in order to infer changes in chemical weathering and monsoon intensity. Variations of four mineralogical ratios (chlorite/quartz, illite/quartz, calcite/quartz, protodolomite/quartz) reveal that the summer monsoon intensity was relatively strong during the early Miocene (23.5‐18.5 Ma), weakened gradually until ∼9.5 Ma, and strengthened again in the late Miocene. We synthesized previously published thermochronological data from the northeastern Tibetan Plateau and surrounding mountains, and the results suggest that two phases of the rapid growth of northern Tibet occurred around 24‐17 Ma and 13‐7 Ma. Comparison of paleoclimatic proxies and thermochronological data suggests that the gradual weakening of the summer monsoon intensity from 18.5 to 9.5 Ma paralleled global cooling, whereas two intervals of strengthened monsoon in the early and late Miocene were possibly related to the rapid growth of northern Tibet. Our combination of paleoenvironmental proxies and thermochronological data reveals possible links between Miocene Asian monsoon evolution, phased growth of the Tibetan Plateau, and global climate change, and confirms the interconnection of geodynamic and atmospheric processes in the geological past. This article is protected by copyright. All rights reserved.
      PubDate: 2015-06-11T11:18:05.962522-05:
      DOI: 10.1002/2015GC005750
  • Lithospheric structure of the Texas‐Gulf of Mexico passive margin
           from surface wave dispersion and migrated Ps receiver functions
    • Authors: Mohit Agrawal; Jay Pulliam, Mrinal K. Sen, Harold Gurrola
      Abstract: The seismic velocity structure beneath Texas Gulf Coastal Plain (GCP) is imaged by migrating Ps receiver functions with a seismic velocity model found by fitting surface wave dispersion. We use seismic data from a linear array of 22 broadband stations, spaced 16‐20 km apart. A Common Conversion Point (CCP) stacking technique is applied to earthquake data to improve the S/N ratios of receiver functions. Using an incorrect velocity model for depth migration of a stacked CCP image may produce an inaccurate image of the subsurface. To find sufficiently accurate P‐ and S‐velocity models, we first apply a nonlinear modeling technique to fit Rayleigh wave group velocity dispersion via Very Fast Simulated Annealing. Vs ranges from 1.5 km/s in shallow layers of the GCP in to 4.5 km/s beneath the Llano uplift and just outboard of the Balcones Fault Zone (BFZ). The BFZ is characterized by slow velocities that persist to nearly 100 km depth. In the stacked image, the largest‐amplitude positive‐polarity event ranges from the surface, at the Llano uplift, to a maximum depth of ∼ 16 km beneath Matagorda Island. We attribute this event to the sediment‐basement contact, which is expected to produce a large impedance contrast. Another large‐amplitude and positive‐polarity event at ∼35 km depth, which likely marks the Moho, disappears outboard of the Luling Fault Zone. The disappearance of the Moho beneath the GCP may be due to serpentinization of the upper mantle, which would reduce the impedance contrast between the lower crust and upper mantle dramatically. This article is protected by copyright. All rights reserved.
      PubDate: 2015-06-11T11:17:52.13368-05:0
      DOI: 10.1002/2015GC005803
  • New insights into the nature of debris‐avalanche deposits offshore
           Montserrat using Remotely Operated Vehicles (ROVs)
    • Authors: S.F.L. Watt; M. Jutzeler, P.J. Talling, S.N. Carey, R.S.J. Sparks, M. Tucker, A.J. Stinton, J.K. Fisher, D. Wall‐Palmer, V. Hühnerbach, S.G. Moreton
      Abstract: Submarine landslide deposits have been mapped around many volcanic islands, but interpretations of their structure, composition and emplacement are hindered by the challenges of investigating deposits directly. Here, we report on detailed observations of four landslide deposits around Montserrat collected by Remotely Operated Vehicles, integrating direct imagery and sampling with sediment‐core and geophysical data. These complementary approaches enable a more comprehensive view of large‐scale mass wasting processes around island‐arc volcanoes than has been achievable previously. The most recent landslide occurred at 11.5–14 ka (Deposit 1; 1.7 km3) and formed a radially‐spreading hummocky deposit that is morphologically similar to many subaerial debris‐avalanche deposits. Hummocks comprise angular lava and hydrothermally‐altered fragments, implying a deep‐seated, central subaerial collapse, inferred to have removed a major proportion of lavas from an eruptive period that now has little representation in the subaerial volcanic record. A larger landslide (Deposit 2; 10 km3) occurred at ∼130 ka and transported intact fragments of the volcanic edifice, up to 900 m across and over 100 m high. These fragments were rafted within the landslide, and are best exposed near the margins of the deposit. The largest block preserves a primary stratigraphy of subaerial volcanic breccias, of which the lower parts are encased in hemipelagic mud eroded from the seafloor. Landslide deposits south of Montserrat (Deposits 3 and 5) indicate the wide variety of debris‐avalanche source lithologies around volcanic islands. Deposit 5 originated on the shallow submerged shelf, rather than the terrestrial volcanic edifice, and is dominated by carbonate debris. This article is protected by copyright. All rights reserved.
      PubDate: 2015-06-11T11:17:43.063172-05:
      DOI: 10.1002/2015GC005781
  • Crustal‐scale degassing due to magma system destabilization and
           magma‐gas decoupling at Soufrière Hills Volcano, Montserrat
    • Authors: T.E. Christopher; J. Blundy, K. Cashman, P. Cole, M. Edmonds, P.J. Smith, R.S.J. Sparks, A. Stinton
      Abstract: Activity since 1995 at Soufrière Hills Volcano (SHV), Montserrat has alternated between andesite lava extrusion and quiescence, which are well‐correlated with seismicity and ground deformation cycles. Large variations in SO2 flux do not correlate with these alternations, but high and low HCl/SO2 characterising lava dome extrusion and quiescent periods respectively. Since lava extrusion ceased (February 2010) steady SO2 emissions have continued at an average rate of 374 tonnes/day (± 140 t/d), and incandescent fumaroles (temperatures up to 610oC) on the dome have not changed position or cooled. Occasional short bursts (over several hours) of higher (∼ 10x) SO2 flux have been accompanied by swarms of volcano‐tectonic earthquakes. Strain data from these bursts indicate activation of the magma system to depths up to 10 km. SO2 emissions since 1995 greatly exceed the amounts that could be derived from 1.1 km3 of erupted andesite, and indicating extensive partitioning of sulfur into a vapour phase, as well as efficient decoupling and outgassing of sulfur‐rich gases from the magma. These observations are consistent with a vertically‐extensive, crustal magmatic mush beneath SHV. Three states of the magmatic system are postulated to control degassing. During dormant periods (103 to 104 years) magmatic vapour and melts separate as layers from the mush and decouple from each other. In periods of unrest (years) without eruption, melt and fluid layers become unstable, ascend and can amalgamate. Major destabilisation of the mush system leads to eruption, characterised by magma mixing and release of volatiles with different ages, compositions and sources. This article is protected by copyright. All rights reserved.
      PubDate: 2015-06-11T11:17:24.359999-05:
      DOI: 10.1002/2015GC005791
  • Lithospheric architecture beneath Hudson Bay
    • Authors: Robert W. Porritt; Meghan S. Miller, Fiona A. Darbyshire
      Abstract: Hudson Bay overlies some of the thickest Precambrian lithosphere on Earth, whose internal structures contain important clues to the earliest workings of plate formation. The terminal collision, the Trans‐Hudson Orogen, brought together the Western Churchill craton to the northwest and the Superior craton to the southeast. These two Archean cratons along with the Paleo‐Proterozoic Trans‐Hudson internides, form the core of the North American craton. We use S to P converted wave imaging and absolute shear velocity information from a joint inversion of P to S receiver functions, new ambient noise derived phase velocities, and teleseismic phase velocities to investigate this region and determine both the thickness of the lithosphere and the presence of internal discontinuities. The lithosphere under central Hudson Bay approaches ∼350 km thick but is thinner (∼200‐250 km) around the periphery of the Bay. Furthermore, the amplitude of the LAB conversion from the S receiver functions is unusually large for a craton, suggesting a large thermal contrast across the LAB, which we interpret as direct evidence of the thermal insulation effect of continents on the asthenosphere. Within the lithosphere, mid‐lithospheric discontinuities, significantly shallower than the base of the lithosphere, are often imaged, suggesting the mechanisms that form these layers are common. Lacking time‐history information, we infer that these discontinuities reflect re‐activation of formation structures during deformation of the craton. This article is protected by copyright. All rights reserved.
      PubDate: 2015-06-11T11:16:40.884185-05:
      DOI: 10.1002/2015GC005845
  • Magmatic recharge in continental flood basalts: Insights from the Chifeng
           igneous province in Inner Mongolia
    • Authors: Xun Yu; Cin‐Ty A. Lee, Li‐Hui Chen, Gang Zeng
      Abstract: Eruptive sequences can be used as windows into the thermal and chemical evolution of magma chambers. We examined a continuous vertical section of the Baichahe basalt flow associated with the late Cenozoic Chifeng flood basalt in Inner Mongolia, North China. From oldest to youngest, MgO increases, K2O, light rare earths and other incompatible elements decrease, and Nb/La and radiogenic Pb isotopic ratios increase, all of which indicate increasing primitiveness and decreasing contribution of crustal contamination with time. The variable Pb isotope and incompatible element ratios require a component of crustal contamination, most likely of a lower crustal component (unradiogenic Pb, and low Ce/Pb) in the earliest lavas. Fractional crystallization can explain some of the elemental systematics, but alone cannot explain variable incompatible element ratios and Pb isotopes, nor the temporal trend to more primitive compositions. Crustal assimilation with or without fractional crystallization also cannot explain all the elemental systematics. We find instead that recharge by a primitive magma, in combination with fractional crystallization and decreasing rates of crustal assimilation, is needed to explain the observed geochemical systematics. Our observations suggest that the delivery of fresh basalt to the magma chamber must increase at rates faster than the crust can be assimilated or that the rates of crustal assimilation must decrease. However, progressive addition of primitive magma should heat up the crust and lead to more crustal assimilation. We suggest that during the initial stages of forming a magma chamber, the magma cools and develops an outer crystalline rind of mafic to ultramafic cumulates. This results in a thickening non‐convecting chemical boundary layer, which serves to insulate the magma chamber from further assimilation of crust and cooling, the latter resulting in the reduction of crystallization rates and the buffering of magma compositions at more primitive compositions. We show that certain segments of other large igneous provinces also display an evolution towards more primitive magmas with time, indicating that magmatic recharge may be a common feature of basaltic magma chambers. This article is protected by copyright. All rights reserved.
      PubDate: 2015-06-09T00:55:33.881344-05:
      DOI: 10.1002/2015GC005805
  • Origins of felsic magmas in Japanese subduction zone: Geochemical
    • Authors: Jun‐Ichi Kimura; Yoshitaka Nagahashi, Yasufumi Satoguchi, Qing Chang
      Abstract: Dacitic to rhyolitic glass shards from 80 widespread tephras erupted during the past 5 Mys from calderas in Kyushu, and SW, central, and NE Japan were analyzed. Laser ablation inductively coupled plasma mass spectrometry was used to determine 10 major and 33 trace elements and 207Pb/206Pb–208Pb/206Pb isotope ratios. The tephras were classified into three major geochemical types and their source rocks were identified as plutonic, sedimentary, and intermediate amphibolite rocks in the upper crust. A few tephras from SW Japan were identified as adakite and alkali rhyolite and were regarded to have originated from slab melt and mantle melt, respectively. The Pb isotope ratios of the tephras are comparable to those of the intermediate lavas in the source areas but are different from the basalts in these areas. The crustal assimilants for the intermediate lavas were largely from crustal melts and are represented by the rhyolitic tephras. A large heat source is required for forming large volumes of felsic crustal melts and is usually supplied by the mantle via basalt. Hydrous arc basalt formed by cold slab subduction is voluminous, and its heat transfer with high water content may have melted crustal rocks leading to effective felsic magma production. Coincidence of basalt and felsic magma activities shown by this study suggests caldera‐forming eruptions are ultimately the effect of a mantle‐driven cause. This article is protected by copyright. All rights reserved.
      PubDate: 2015-06-05T03:05:22.453762-05:
      DOI: 10.1002/2015GC005854
  • Tectonic evolution of 200 km of Mid‐Atlantic Ridge over 10 million
           years—Interplay of volcanism and faulting
    • Authors: Johnson R Cann; Deborah K. Smith, Javier Escartin, Hans Schouten
      Abstract: We reconstruct the history of the mode of accretion of an area of the Mid‐Atlantic Ridge south of the Kane fracture zone using bathymetric morphology. The area includes 200 km of the spreading axis and reaches to 10 Ma on either side. We distinguish three tectonic styles: 1) volcanic construction with eruption and intrusion of magma coupled with minor faulting, 2) extended terrain with abundant large‐offset faults, 3) detachment faulting marked by extension on single long‐lived faults. Over 40% of the seafloor is made of extended terrain and detachment faults. The area includes products of seven spreading segments. The spreading axis has had detachment faulting or extended terrain on one or both sides for 70% of the last 10 Ma. In some parts of the area, regions of detachment faulting and extended terrain lie close to segment boundaries. Regions of detachment faulting initiated at 10 Ma close to the adjacent fracture zones to the north and south, and then expanded away from them. We discuss the complex evidence from gravity, seismic surveys and bathymetry for the role of magma supply in generating tectonic style. Overall we conclude that input of magma at the spreading axis has a general control on the development of detachment faulting, but the relationship is not strong. Other factors may include a positive feedback that stabilizes detachment faulting at the expense of volcanic extension, perhaps through the lubrication of active detachment faults by the formation of low friction materials (talc, serpentine) on detachment fault surfaces. This article is protected by copyright. All rights reserved.
      PubDate: 2015-06-05T02:59:43.509141-05:
      DOI: 10.1002/2015GC005797
  • Semiautomatic fracture zone tracking
    • Authors: Paul Wessel; Kara J. Matthews, R. Dietmar Müller, Aline Mazzoni, Joanne M. Whittaker, Robert Myhill, Michael T. Chandler
      Abstract: Oceanic fracture zone traces are widely used in studies of seafloor morphology and plate kinematics. Satellite altimetry missions have resulted in high‐resolution gravity maps in which all major fracture zones and other tectonic fabric can be identified, and numerous scientists have digitized such lineaments. We have initiated a community effort to maintain low‐cost infrastructure that allows seafloor fabric lineaments to be stored, accessed and updated. A key improvement over past efforts is our processing software (released as a GMT5 supplement) that allows for semi‐automatic corrections to previously digitized fracture zone traces given improved gridded data sets. Here we report on our seafloor fabric processing tools, which complement our database of seafloor fabric lineations, magnetic anomaly identifications and plate kinematic models. This article is protected by copyright. All rights reserved.
      PubDate: 2015-06-05T02:59:06.663321-05:
      DOI: 10.1002/2015GC005853
  • Imaging evidence for Hubbard Glacier advances and retreats since the last
           glacial maximum in Yakutat and Disenchantment Bays, Alaska
    • Authors: Julie M. Zurbuchen; Sean P.S. Gulick, Maureen A.L. Walton, John A. Goff
      Abstract: High‐resolution 2‐D multi‐channel seismic data, collected during the 2012 UTIG‐USGS National Earthquake Hazards Reduction Program survey of Disenchantment and Yakutat Bays in southeast Alaska, provide insight into their glacial history. These data show evidence of two unconformities, appearing in the form of channels, and are interpreted to be advance pathways for Hubbard Glacier. The youngest observable channel, thought to have culminated near the main phase of the Little Ice Age (LIA), is imaged in Disenchantment Bay and ends at a terminal moraine near Blizhni Point. An older channel, thought to be from an advance that culminated in the early phase of the LIA, extends from Disenchantment Bay into the northeastern edge of Yakutat Bay, turning southward at Knight Island and terminating on the southeastern edge of Yakutat Bay. Our interpretation is that Hubbard Glacier has repeatedly advanced around the east side of Yakutat Bay in Knight Island Channel, possibly due to the presence of Malaspina Glacier cutting off access to central Yakutat Bay during times of mutual advance. We observe two distinct erosional surfaces and retreat sequences of Hubbard Glacier in Yakutat Bay, supporting the hypothesis that minor glacial advances in fjords do not erode all prior sediment accumulations. Interpretation of chaotic seismic facies between these two unconformities suggests that Hubbard Glacier exhibits rapid retreats and that Disenchantment Bay is subject to numerous episodes of outburst flooding and morainal bank collapse. These findings also suggest that tidewater glaciers preferentially reoccupy the same channels in bay and marine settings during advances. This article is protected by copyright. All rights reserved.
      PubDate: 2015-06-03T03:32:18.700587-05:
      DOI: 10.1002/2015GC005815
  • Pore water geochemistry at two seismogenic areas in the Sea of Marmara
    • Authors: Livio Ruffine; Yoan Germain, Alina Polonia, Alexis de Prunelé, Claire Croguennec, Jean‐Pierre Donval, Mathilde Pitel‐Roudaut, Emmanuel Ponzevera, Jean‐Claude Caprais, Christophe Brandily, Claire Bollinger, Céline Grall, Louis Géli, Luca Gasperini
      Abstract: Within the Sea of Marmara, the highly active North Anatolian Fault (NAF) is responsible for major earthquakes (Mw >=7), and acts as a pathway for fluid migration from deep sources to the seafloor. This work reports on pore water geochemistry from three sediment cores collected in the gulfs of Izmit and Gemlik, along the Northern and the Middle strands of the NAF, respectively. The resulting dataset shows that anaerobic oxidation of methane (AOM) is the major process responsible for sulfate depletion in the shallow sediment. In the Gulf of Gemlik, depth concentration profiles of both sulfate and alkalinity exhibit a kink‐type profile. The Sulfate Methane Transition Zone (SMTZ) is located at moderate depth in the area. In the Gulf of Izmit, the low concentrations observed near the seawater‐sediment interface for sulfate, calcium, strontium and magnesium results from rapid geochemical processes, AOM and carbonate precipitation, occurring in the uppermost part of the sedimentary column and sustained by free methane accumulation. Barite dissolution and carbonate recrystallization have also been identified at deeper depth at the easternmost basin of the Gulf of Izmit. This is supported by the profile of the strontium isotope ratios (87Sr/86Sr) as a function of depth which exhibits negative anomalies compared to the modern seawater value. The strontium isotopic signature also shows that these carbonates had precipitated during the reconnection of the Sea of Marmara with the Mediterranean Sea. Finally, a first attempt to interpret the sulfate profiles observed in the light of the seismic activity at both sites is presented. We propose the hypothesis that seismic activity in the areas is responsible for the transient sulfate profile, and that the very shallow SMTZ depths observed in the Gulf of Izmit is likely due to episodic release of significant amount of methane. This article is protected by copyright. All rights reserved.
      PubDate: 2015-06-03T03:30:03.089073-05:
      DOI: 10.1002/2015GC005798
  • Hydrogen and 40Ar/39Ar isotope evidence for multiple and protracted
           paleofluid flow events within the long‐lived North Anatolian
           Keirogen (Turkey)
    • Authors: Austin Boles; Ben van der Pluijm, Andreas Mulch, Halim Mutlu, I. Tonguç Uysal, Laurence N. Warr
      Abstract: We present a new approach to identifying the source and age of paleofluids associated with low‐temperature deformation in the brittle crust, using hydrogen isotopic compositions (δD) and 40Ar/39Ar geochronology of authigenic illite in clay gouge‐bearing fault zones. The procedure involves grain size separation, polytype modeling, and isotopic analysis, creating a mixing line that is used to extrapolate to δD and age of pure authigenic and detrital material. We use this method on samples collected along the surface trace of today's North Anatolian Fault (NAF). δD values of the authigenic illite population, obtained by extrapolation, are ‐89±3‰, ‐90±2‰, and ‐97±2‰ (VSMOW) for samples KSL, RES4‐1, and G1G2, respectively. These correspond to δD fluid values of ‐62‰ to ‐85‰ for the temperature range of 125°C ±25°, indistinguishable from present‐day precipitation values. δD values of the detrital illite population are ‐45±13‰, ‐60±6‰, and ‐64±6‰ for samples KSL, G1G2, and RES4‐1, respectively. Corresponding δD fluid values at 300°C are ‐26 to ‐45‰ and match values from adjacent metamorphic terranes. Corresponding clay gouge ages are 41.4 ±3.4 Ma (authigenic) and 95.8±7.7 Ma (detrital) for sample G2 and 24.6 ±1.6 Ma (authigenic) and 96.5±3.8 Ma (detrital) for sample RES4‐1, demonstrating a long history of meteoric fluid infiltration in the area. We conclude that today's NAF incorporated pre‐existing, weak clay‐rich rocks that represent earlier mineralizing fluid events. The samples preserve at least three fluid flow pulses since the Eocene and indicate that meteoric fluid has been circulating in the upper crust in the North Anatolian Keirogen since that time. This article is protected by copyright. All rights reserved.
      PubDate: 2015-06-03T02:55:08.097335-05:
      DOI: 10.1002/2015GC005810
  • Probing the transition between seismically coupled and decoupled segments
           along an ancient subduction interface
    • Authors: Samuel Angiboust; Josephine Kirsch, Onno Oncken, Johannes Glodny, Patrick Monié, Erik Rybacki
      Abstract: The transition zone at the downdip end of seismic coupling along subduction interfaces is often the site of megathrust earthquake nucleation and concentrated postseismic afterslip, as well as the focus site of episodic tremor and slip features. Exhumed remnants of the former Alpine subduction zone found in the Swiss Alps allow analyzing fluid and deformation processes near the transition zone region (30‐40 km paleodepth). The Dent Blanche Thrust (DBT) is a lower blueschist‐facies shear zone interpreted as a fossilized subduction interface where granitic mylonites overlie a metamorphosed accretionary wedge. We report field observations from the DBT region where multiple, several tens of meters‐thick foliated cataclastic networks are interlayered within the basal DBT mylonites. Petrological results and microstructural observations indicate that the various cataclasis events took place at near peak metamorphic conditions (400‐500°C, 1.1‐1.3 GPa) during subduction of the Tethyan seafloor in Eocene times (42‐48 Ma). Some of these networks exhibit mutual cross‐cutting relationships between mylonites, foliated cataclasites and vein systems indicating mutual overprinting between brittle deformation and ductile creep. Whole‐rock chemical compositions, in situ 40Ar‐39Ar age data of recrystallized phengite, and Sr isotopic signatures reveal that DBT rocks also underwent multiple hydrofracturing and metasomatic events via the infiltration of fluids mainly derived from the oceanic metasediments underneath the DBT. From the rock fabrics we infer strain rate fluctuations of several orders of magnitude beyond subduction strain rates (c. 10−12s−1) accompanied by fluctuation of supra‐lithostatic and quasi‐lithostatic fluid pressures (1≥λ>0.95). DBT brittle‐plastic deformation switches highlight the diversity of deformation processes and fluid‐rock interactions in the transition zone region of the subduction interface. This article is protected by copyright. All rights reserved.
      PubDate: 2015-06-03T02:54:53.732295-05:
      DOI: 10.1002/2015GC005776
  • Magmatic water contents determined through clinopyroxene: Examples from
           the Western Canary Islands, Spain
    • Authors: Franz A. Weis; Henrik Skogby, Valentin R. Troll, Frances M. Deegan, Börje Dahrén
      Abstract: Water is a key parameter in magma genesis, magma evolution, and resulting eruption styles, because it controls the density, the viscosity, as well as the melting and crystallization behavior of a melt. The parental water content of a magma is usually measured through melt inclusions in minerals such as olivine, a method which may be hampered, however, by the lack of melt inclusions suitable for analysis, or post‐entrapment changes in their water content. An alternative way to reconstruct the water content of a magma is to use nominally anhydrous minerals (NAMs), such as pyroxene, which take up low concentrations of hydrogen as a function of the magma's water content. During magma degassing and eruption, however, NAMs may dehydrate. We therefore tested a method to reconstruct the water contents of dehydrated clinopyroxene phenocrysts from the Western Canary islands (n=28) through re‐hydration experiments followed by infrared and Mössbauer spectroscopy. Employing currently available crystal/melt partitioning data, the results of the experiments were used to calculate parental water contents of 0.71 ±0.07 to 1.49 ±0.15 wt. % H2O for Western Canary magmas during clinopyroxene crystallization at upper mantle conditions. This H2O range is in agreement with calculated water contents using plagioclase‐liquid‐hygrometry, and with previously published data for mafic lavas from the Canary Islands and comparable ocean island systems elsewhere. Utilizing NAMs in combination with hydrogen treatment can therefore serve as a proxy for pre‐eruptive H2O contents, which we anticipate becoming a useful method applicable to mafic rocks where pyroxene is the main phenocryst phase. This article is protected by copyright. All rights reserved.
      PubDate: 2015-06-03T02:54:40.735061-05:
      DOI: 10.1002/2015GC005800
  • Trace elements in quartz shed light on sediment provenance
    • Authors: Nicholas D. Tailby; E. Bruce Watson, Michael R. Ackerson
      Abstract: Quartz is one of the most common minerals on the surface of the earth, and is a primary rock‐forming mineral across the rock cycle. These two factors make quartz an obvious target for sediment provenance studies. Observations from experimental and natural samples demonstrate that the trace‐element content of quartz often reflects the conditions of quartz formation. When quartz is weathered from its primary crystallization setting (i.e., quartz from a granitoid) it can retain many chemical signatures of formation throughout the sedimentation processes. These geochemical signatures can be used to understand the primary source of individual quartz grains within a sediment. Here we present a case study from the Bega River catchment to demonstrate that quartz grains in sediments at the mouth of the Bega River are sourced from granitoids within the drainage basin. Data presented here also indicate that a portion of the beach sediment is also derived from either – (i) sedimentary rocks within the basin or; (ii) mixing with sediments at the mouth of the river. The Bega River catchment was selected for this study because it is both small and has a well‐constrained bedrock lithology, making it an ideal location to test the utility of this provenance technique. However, quartz trace‐element provenance has broad applications to modern and ancient sediments and can be used in lieu of, or in conjunction with, other provenance techniques to elucidate sediment transport through time. This article is protected by copyright. All rights reserved.
      PubDate: 2015-05-28T10:32:13.527838-05:
      DOI: 10.1002/2015GC005896
  • Effects of simple acid leaching of crushed and powdered geological
           materials on high‐precision Pb isotope analyses
    • Authors: Erin Todd; Andreas Stracke, Erik E. Scherer
      Abstract: We present new results of simple acid leaching experiments on the Pb isotope composition of USGS standard reference material powders and on ocean island basalt whole rock splits and powders. Rock samples were leached with cold 6N HCl in an ultrasonic bath, then on a hot plate, and washed with ultrapure H2O before sample digestion in HF‐HNO3 and chromatographic purification of Pb. Lead isotope analyses were measured by Tl‐doped MC‐ICPMS. Intra‐ and inter‐session analytical reproducibilities of repeated analyses of both synthetic Pb solutions and Pb from single digests of chemically processed natural samples were generally better than 100 ppm (2 S.D.). The comparison of leached and unleached samples shows that leaching consistently removes variable amounts of contaminants that differ in Pb isotopic composition for different starting materials. For repeated digests of a single sample, analyses of leached samples reproduce better than those of unleached ones, confirming that leaching effectively removes most of the heterogeneously distributed extraneous Pb. Nevertheless, the external reproducibility of leached samples is still up to an order of magnitude worse than that of Pb solution standards (∼100 ppm). More complex leaching methods employed by earlier studies yield Pb isotope ratios within error of those produced by our method and at similar levels of reproducibility, demonstrating that our simple leaching method is as effective as more complex leaching techniques. Therefore, any Pb isotope heterogeneity among multiple leached digests of samples in excess of the external reproducibility is attributed to inherent isotopic heterogeneity of the sample. The external precision of ∼100 ppm (2 S.D.) achieved for Pb isotope ratio determination by Tl‐doped MC‐ICPMS is thus sufficient for most rocks. The full advantage of the most precise Pb isotope analytical methods is only realized in cases where the natural isotopic heterogeneity among samples in a studied suite is substantially below 100 ppm. This article is protected by copyright. All rights reserved.
      PubDate: 2015-05-28T10:32:02.269031-05:
      DOI: 10.1002/2015GC005804
  • Multiscale postseismic behavior on a mega‐thrust: the 2012 Nicoya
           earthquake, Costa Rica
    • Authors: Rocco Malservisi; Susan Y. Schwartz, Nicholas Voss, Marino Protti, Victor Gonzalez, Timothy H. Dixon, Yan Jiang, Andy V. Newman, Jacob Richardson, Jacob I. Walter, Denis Voyenko
      Abstract: The Nicoya peninsula in northwest Costa Rica overlies a section of the subduction megathrust along the Middle America Trench. On September 5, 2012, a moment magnitude 7.6 megathrust earthquake occurred beneath a dense network of continuous GPS and seismic stations. Many of the GPS stations recorded the event at high rate, 1 Hz or better. We analyze the temporal and spatial evolution of surface deformation after the earthquake. Our results show that the main rupture was followed by significant afterslip within the first 3 hours following the main event. The behavior of the surface displacement can be represented by relaxation processes with three characteristic times: 7, 70 and more than 400 days. We assume that the long relaxation time corresponds to viscoelastic relaxation and the intermediate relaxation time corresponds to afterslip on the main fault. The short relaxation time may represent a combination of rapid afterslip, poroelastic adjustment in the upper crust, or other processes. During the first few months that followed the earthquake, afterslip likely released a significant amount of slip deficit still present following the coseismic rupture, in particular up‐dip of the rupture. Afterslip seems to be bounded up‐dip by regions affected by slow slip events prior to the earthquake, suggesting that the two processes are influenced by different frictional properties. This article is protected by copyright. All rights reserved.
      PubDate: 2015-05-28T10:31:54.598781-05:
      DOI: 10.1002/2015GC005794
  • Paleomagnetic evidence for vertical‐axis rotations of crustal blocks
           in the Woodlark Rift, SE Papua New Guinea: Miocene to Present‐Day
           kinematics in one of the world's most rapidly extending plate boundary
    • Authors: Elizabeth A. Cairns; Timothy A. Little, Gillian M. Turner, Laura M. Wallace, Susan Ellis
      Abstract: The continental Woodlark Rift, in SE Papua New Guinea lies west of a propagating oceanic spreading centre in the Woodlark Basin, and is currently one of few places on Earth where active continental breakup is thought to be occurring. Here, north‐south extension is localized on a few major normal faults. We determined characteristic remanent magnetization (ChRM) components from demagnetization profiles of >300 individual specimens. From these, 157 components contribute to paleomagnetic directions for six formations. We compare Early Miocene (∼20 Ma) to Late Pliocene (3.0 ± 0.5) ChRM mean directions, at four localities, with contemporaneous expected field directions corresponding to the Australian Plate. Time‐varying finite rotations from Cape Vogel Peninsula (28‐12°) suggest anticlockwise rotation had begun by ∼15 Ma. This rotation may have been accompanied by rifting, ∼7 Ma earlier than previously inferred. Furthermore that early extension may have occurred south of the present rift, and that deformation later migrated north of the Peninsula. Pliocene vertical‐axis rotations are consistent with GPS‐determined plate motions, suggesting that contemporary rift kinematics were established by ∼3 Ma. Finite anticlockwise rotation (10.1 ± 7.6°) in the Amphlett Islands is accordant with sea floor spreading in the Woodlark Basin, suggesting this locality has seen the full Woodlark plate motion since 3 Ma. Clockwise rotation of the Goodenough Bay Block (‐6.5 ± 11.2°) since the Late Miocene has accomplished transfer of deformation between major extensional corridors, and an especially rapid local rotation (‐16.3 ± 9.5°) in NW Normanby Island may suggest an incipient dextral transfer fault. This article is protected by copyright. All rights reserved.
      PubDate: 2015-05-28T10:31:40.868853-05:
      DOI: 10.1002/2015GC005788
  • Late Miocene to Recent formation of the Aure‐Moresby
           fold‐thrust belt and foreland basin as a consequence of Woodlark
           microplate rotation, Papua New Guinea
    • Authors: Bryan Ott; Paul Mann
      Abstract: The Aure‐Moresby fold‐thrust belt and Aure‐Moresby foreland basin is located in the eastern Gulf of Papua (GOP), Papua New Guinea (PNG) and formed during late Miocene‐Recent time as the result of large‐scale, counterclockwise rotation of the 355,000 km2 Woodlark microplate. To document the structure, stratigraphy and age of convergent deformation along the poorly studied, western edge of the rotating Woodlark microplate, we integrate results of 2,538 km of previously unpublished 2D seismic reflection data with onshore geologic and GPS studies from previous workers. The late Miocene Aure‐Moresby fold‐thrust belt is a 400‐km‐long, northwest‐trending fold‐belt exposed onshore in Papua New Guinea that plunges to the southeast, where continuous folds and northeast‐dipping thrusts can be imaged in the subsurface for more than 250 km. The arcuate trend of the Aure‐Moresby fold‐thrust belt along the southwestern coast and offshore areas of the Papuan peninsula parallels the shape of the adjacent, offshore Aure‐Moresby foreland basin and the strike of the transpressional segment of the left‐lateral Owen‐Stanley zone (OSFZ) passing along the center of Papuan peninsula. As the OSFZ becomes more transtensional east of 148°E, folds of the Aure‐Moresby fold‐thrust belt along southern coast of the peninsula become less prominent, and the adjacent Aure‐Moresby foreland basin transitions into an undeformed Cenozoic passive margin setting. These observations of convergent an left‐lateral deformation along is the Aure‐Moresby fold‐thrust belt are consistent with: 1) counterclockwise rotation of the Woodlark microplate known from regional GPS studies; 2) coeval opening of the Woodlark basin along its southern edge in the late Miocene; and 3) rapid subduction at the New Britain trench along its northern edge. The kinematics of the rotating Woodlark microplate are driven by slab pull forces acting on the actively, subducting northern edge of the microplate. This article is protected by copyright. All rights reserved.
      PubDate: 2015-05-28T10:31:12.256541-05:
      DOI: 10.1002/2014GC005668
  • High‐resolution magnetics reveal the deep structure of a
           volcanic‐arc related basalt‐hosted hydrothermal site
           (Palinuro, Tyrrhenian Sea)
    • Authors: Florent Szitkar; Sven Petersen, Fabio Caratori Tontini, Luca Cocchi
      Abstract: High‐resolution magnetic surveys have been acquired over the partially sedimented Palinuro massive sulfide deposits in the Aeolian volcanic arc, Tyrrhenian Sea. Surveys flown close to the seafloor using an autonomous underwater vehicle (AUV) show that the volcanic arc‐related basalt‐hosted hydrothermal site is associated with zones of lower magnetization. This observation reflects the alteration of basalt affected by hydrothermal circulation and/or the progressive accumulation of a non‐magnetic deposit made of hydrothermal and volcaniclastic material and/or a thermal demagnetization of titanomagnetite due to the upwelling of hot fluids. To discriminate among these inferences, estimate the shape of the non‐magnetic deposit and the characteristics of the underlying altered area – the stockwork – we use high‐resolution vector magnetic data acquired by the AUV Abyss (GEOMAR) above a crater‐shaped depression hosting a weakly active hydrothermal site. Our study unveils a relatively small non‐magnetic deposit accumulated at the bottom of the depression and locked between the surrounding volcanic cones. Thermal demagnetization is unlikely but the stockwork extends beyond the limits of the non‐magnetic deposit, forming lobe‐shaped zones believed to be a consequence of older volcanic episodes having contributed in generating the cones. This article is protected by copyright. All rights reserved.
      PubDate: 2015-05-21T18:43:59.25731-05:0
      DOI: 10.1002/2015GC005769
  • Advantages of a conservative velocity interpolation (CVI) scheme for
           particle‐in‐cell methods with application in geodynamic
    • Authors: Hongliang Wang; Roberto Agrusta, Jeroen van Hunen
      Abstract: The particle‐in‐cell method is generally considered a flexible and robust method to model the geodynamic problems with chemical heterogeneity. However, velocity interpolation from grid points to particle locations is often performed without considering the divergence of the velocity field, which can lead to significant particle dispersion or clustering if those particles move through regions of strong velocity gradients. This may ultimately result in cells void of particles, which, if left untreated, may, in turn, lead to numerical inaccuracies. Here, we apply a two‐dimensional conservative velocity interpolation scheme (CVI) to steady state and time‐dependent flow fields with strong velocity gradients (e.g. due to large local viscosity variation), and derive and apply the three‐dimensional equivalent. We show that the introduction of CVI significantly reduces the dispersion and clustering of particles in both steady‐state and time‐dependent flow problems, and maintains a locally steady number of particles, without the need for ad‐hoc remedies such as very high initial particle densities or re‐seeding during the calculation. We illustrate that this method provides a significant improvement to particle distributions in common geodynamic modelling problems such as subduction zones or lithosphere‐asthenosphere boundary dynamics. This article is protected by copyright. All rights reserved.
      PubDate: 2015-05-21T18:37:42.42154-05:0
      DOI: 10.1002/2015GC005824
  • Did phosphorus derived from the weathering of large igneous provinces
           fertilize the Neoproterozoic ocean?
    • Authors: Forrest Horton
      Abstract: Primary productivity and organic carbon burial rates in the Precambrian were highly sensitive to fluxes of phosphorus (P) from the weathering of continental crust. Large igneous provinces (LIPs)—containing substantial P and highly susceptible to chemical weathering—occurred regularly during the breakup of the Rodinia supercontinent, and flood basalts probably covered 3.7–7.4 × 106 km2 at a time when a low‐latitude continental configuration expedited weathering. Assuming chemical weathering liberated much of the P contained in the flood basalts, an estimated 1–4 × 1017 mol of biologically available P entered the ocean from LIPs between 900 Ma and 500 Ma. Especially voluminous LIP magmatism began at ∼850 Ma and culminated with the Franklin Province at 720 Ma, after which an estimated bioavailable P flux from flood basalts of 1–5 × 109 mol/yr may have been sustained for millions of years, elevating primary production and organic carbon burial rates. P enrichment of LIP magmas prior to eruption could have contributed to efficient reactive P delivery to the ocean: liquid–crystal fractionation beneath thick cratonic lithosphere and the incorporation of metasomatic P potentially enriched Neoproterozoic LIP magmas more than anytime previously. Thus, a unique convergence of tectonic conditions—supercontinent breakup, voluminous mafic volcanism containing abundant P, and a low‐latitude continental configuration—may have facilitated an unprecedented flux of bioavailable P to the ocean that was capable of triggering oxidation of the ocean‐atmosphere system and enabling accelerated biologic diversification. This article is protected by copyright. All rights reserved.
      PubDate: 2015-05-21T18:34:47.500841-05:
      DOI: 10.1002/2015GC005792
  • Pressure disequilibria induced by rapid valve closure in noble gas
           extraction lines
    • Authors: Leah E. Morgan; Brett Davidheiser‐Kroll
      Abstract: Pressure disequilibria during rapid valve closures can affect calculated molar quantities for a range of gas abundance measurements (e.g. K‐Ar geochronology, (U‐Th)/He geochronology, noble gas cosmogenic chronology). Modeling indicates this effect in a system with a 10 L reservoir reaches a bias of 1% before 1,000 pipette aliquants have been removed from the system, and a bias of 10% before 10,000 aliquants. Herein we explore the causes and effects of this problem, which is the result of volume changes during valve closure. We also present a solution in the form of an electro‐pneumatic pressure regulator that can precisely control valve motion. This solution reduces the effect to ca. 0.3% even after 10,000 aliquants have been removed from a 10 L reservoir. This article is protected by copyright. All rights reserved.
      PubDate: 2015-05-21T18:34:34.705107-05:
      DOI: 10.1002/2015GC005823
  • Dynamics of intra‐oceanic subduction initiation, part 1: Oceanic
           detachment fault inversion and the formation of forearc ophiolites
    • Authors: Marco Maffione; Cedric Thieulot, Douwe J.J. van Hinsbergen, Antony Morris, Oliver Plümper, Wim Spakman
      Abstract: Subduction initiation is a critical link in the plate tectonic cycle. Intra‐oceanic subduction zones can form along transform faults and fracture zones, but how subduction nucleates parallel to mid‐ocean ridges, as in e.g. the Neotethys Ocean during the Jurassic, remains a matter of debate. In recent years, extensional detachment faults have been widely documented adjacent to slow‐ and ultraslow‐spreading ridges where they cut across the oceanic lithosphere. These structures are extremely weak due to widespread occurrence of serpentine and talc resulting from hydrothermal alteration, and can therefore effectively localize deformation. Here, we show geochemical, tectonic, and paleomagnetic evidence from the Jurassic ophiolites of Albania and Greece for a subduction zone formed in the western Neotethys parallel to a spreading ridge along an oceanic detachment fault. With 2‐D numerical modeling exploring the evolution of a detachment‐ridge system experiencing compression, we show that serpentinized detachments are always weaker than spreading ridges. We conclude that, owing to their extreme weakness, oceanic detachments can effectively localize deformation under perpendicular far‐field forcing, providing ideal conditions to nucleate new subduction zones parallel and close to (or at) spreading ridges. Direct implication of this, is that resumed magmatic activity in the forearc during subduction initiation can yield widespread accretion of supra‐subduction zone ophiolites at or close to the paleoridge. Our new model casts the enigmatic origin of regionally extensive ophiolite belts in a novel geodynamic context, and calls for future research on three‐dimensional modeling of subduction initiation and how upper plate extension is associated with that. This article is protected by copyright. All rights reserved.
      PubDate: 2015-05-21T18:19:11.86839-05:0
      DOI: 10.1002/2015GC005746
  • Multi‐parametric study of the February‐April 2013 paroxysmal
           phase of Mt. Etna New South‐East crater
    • Authors: Letizia Spampinato; Mariangela Sciotto, Andrea Cannata, Flavio Cannavò, Alessandro La Spina, Mimmo Palano, Giuseppe G. Salerno, Eugenio Privitera, Tommaso Caltabiano
      Abstract: Between January 2011 and April 2013, Mt. Etna's eruptive activity consisted of episodic intra‐crater strombolian explosions and paroxysms from Bocca Nuova, Voragine, and the New South‐East (NSEC) summit craters, respectively. Eruptions from NSEC consisted of initial increasing strombolian activity and lava flow output, passing to short‐lasting lava fountaining. In this study we present seismic, infrasound, radiometric, plume SO2 and HCl fluxes and geodetic data collected by the INGV monitoring system between May 2012 and April 2013. The multi‐parametric approach enabled characterization of NSEC eruptive activity at both daily and monthly time scales and tracking of magma movement within Mt. Etna's plumbing system. While seismic, infrasound and radiometric signals give insight on the energy and features of the 13 paroxysms fed by NSEC, SO2 and halogen fluxes shed light on the likely mechanisms triggering the eruptive phenomena. GPS data provided clear evidence of pressurization of Mt. Etna's plumbing system from May 2012 to middle February 2013 and depressurization during the February‐April 2013 eruptive activity. Taking into account geochemical data, we propose that the paroxysms' sequence represented the climax of a waxing‐waning phase of degassing that had started as early as December 2012, and eventually ended in April 2013. Integration of the multidisciplinary observations suggests that the February‐April 2013 eruptive activity reflects a phase of release of a volatile‐rich batch of magma that had been stored in the shallow volcano plumbing system at least four months before, and with the majority of gas released between February and March 2013. This article is protected by copyright. All rights reserved.
      PubDate: 2015-05-21T18:18:40.583587-05:
      DOI: 10.1002/2015GC005795
  • Experimental mixtures of superparamagnetic and single domain magnetite
           with respect to Day‐Dunlop plots
    • Authors: Monika Kumari; Ann M Hirt, Rene Uebe, Dirk Schüler, Éva Tompa, Mihály Pósfai, Wolfram Lorenz, Fredrik Ahrentorp, Christian Jonasson, Christer Johansson
      Abstract: Day‐Dunlop plots are widely used in paleomagnetic and environmental studies as a tool to determine the magnetic domain state of magnetite, i.e., superparamagnetic (SP), stable single‐domain (SD), pseudo‐single‐domain (PSD), multidomain (MD) and their mixtures. The few experimental studies that have examined hysteresis properties of SD‐SP mixtures of magnetite found that the ratios of saturation remanent magnetization to saturation magnetization and the coercivity of remanence to coercivity are low, when compared to expected theoretical mixing trends based on Langevin theory. This study reexamines Day‐Dunlop plots using experimentally controlled mixtures of SD and SP magnetite grains. End members include magnetotactic bacteria (MSR‐1) as the SD source, and a commercial ferrofluid or magnetotactic bacteria (ΔA12) as the SP source. Each SP‐component was added incrementally to a SD sample. Experimental results from these mixing series show that the magnetization and coercivity ratios are lower than the theoretical prediction for bulk SP magnetic size. Although steric repulsion was present between the particles we cannot rule out interaction in the ferrofluid for higher concentrations. The SP bacteria are non‐interacting as the magnetite was enclosed by an organic bi‐lipid membrane. Our results demonstrate that the magnetization and coercivity ratios of SD‐SP mixtures can lie in the PSD range, and that an unambiguous interpretation of particle size can only be made with information about the magnetic properties of the end members. This article is protected by copyright. All rights reserved.
      PubDate: 2015-05-21T18:18:24.000597-05:
      DOI: 10.1002/2015GC005744
  • Geochemical heterogeneities in magma beneath Mount Etna recorded by
           2001‐2006 melt inclusions
    • Authors: Federica Schiavi; Alberto Rosciglione, Hiroshi Kitagawa, Katsura Kobayashi, Eizo Nakamura, Pasquale Mario Nuccio, Luisa Ottolini, Antonio Paonita, Riccardo Vannucci
      Abstract: We present a geochemical study on olivine‐ and clinopyroxene‐hosted melt inclusions (MIs) from 2001‐2006 Etna basaltic lavas and pyroclastites. Three MI suites are distinguished on the basis of trace element fingerprinting. Type‐1 MIs (from 2001 Upper South and 2002 Northeast vents) share their trace element signature with low‐K lavas erupted before 1971. Critical trace element ratios (e.g., K/La, Ba/Nb), along with Pb isotope data of Type‐1 MIs provide evidence for a heterogeneous mantle source resulting from mixing of three end‐members with geochemical and isotopic characteristics of EM2, DMM and HIMU components. Type‐1 MIs composition does not support involvement of subduction‐related components. Type‐2 (from 2001 Lower and 2002 South vents) and Type‐3 (2004 eruption) MIs reveal “ghost plagioclase signatures”, namely lower concentrations in strongly incompatible elements, and positive Sr, Ba and Eu anomalies. Both Type‐1 and Type‐2 MIs occur in 2006 olivines, which highlight the occurrence of mixing between Type‐1 and Type‐2 end‐members. Type‐2/Type‐3 MIs testify to en‐route processes (plagioclase assimilation and volatile fluxing) peculiar for “deep dike fed” eruptions. The latter are strongly controlled by tectonics or flank instability that occasionally promote upraise of undegassed, more radiogenic primitive magma, which may interact with plagioclase‐rich crystal mush/cumulates before erupting. Type‐2/Type‐3 MIs approach the less radiogenic Pb isotopic composition of plagioclase from prehistoric lavas, thus suggesting geochemical overprinting of present‐day melts by older products released from distinct mantle sources. Our study emphasizes that MIs microanalysis offers new insights on both source characteristics and en‐route processes, allowing to a link between melt composition and magma dynamics. This article is protected by copyright. All rights reserved.
      PubDate: 2015-05-21T18:18:13.685358-05:
      DOI: 10.1002/2015GC005786
  • Strain decoupling reveals variable seismogenic risk in SE Japan (Nankai
    • Authors: James Van Tuyl; Tiago M. Alves, Gregory F. Moore
      Abstract: The determination of in situ stress states is vital in understanding the behavior of faults and subsequent seismogenesis of accretionary prisms. In this paper, a high quality 3D seismic volume is used to map the depth of the extensional‐compressional decoupling (ECD) boundary in the accretionary prism of Nankai, with the prior knowledge that strike‐slip and compressional stresses occur deeper than 1250 meters below seafloor (mbsf) in the Kumano Basin, changing to extension towards the seafloor. A total of 1108 faults from the accretionary prism are analyzed to estimate paleostresses via fault inversion and slip tendency techniques. A key result is this paper is that the ECD boundary can be used as a proxy to identify active structures on accretionary prisms as its depth depends on: a) local tectonic uplift in areas adjacent to active faults, and b) on the thickness of sediment accumulated above active thrust anticlines. The depth of the ECD boundary ranges from 0 to ∼650 mbsf, being notably shallower than in the Kumano Basin. In Nankai, frontal regions of the imbricate thrust zone, and the megasplay fault zone, reveal the shallower ECD depths and correlate with the regions where faulting is most active. As a corollary, this work confirms that estimates of stress state variability based on the analysis of 3D seismic data are vital to understand the behavior of faults and potential seismogenic regions on convergent margins. This article is protected by copyright. All rights reserved.
      PubDate: 2015-05-18T04:01:39.566209-05:
      DOI: 10.1002/2015GC005778
  • Microstructural evolution of gas hydrates in sedimentary matrices observed
           with synchrotron X‐ray computed tomographic microscopy
    • Authors: Marwen Chaouachi; Andrzej Falenty, Kathleen Sell, Frieder Enzmann, Michael Kersten, David Haberthür, Werner F. Kuhs
      Abstract: The formation process of gas hydrates in sedimentary matrices is of crucial importance for the physical and transport properties of the resulting aggregates. This process has never been observed in‐situ at sub‐micron resolution. Here, we report on synchrotron‐based micro‐tomographic studies by which the nucleation and growth processes of gas hydrate were observed at 276 K in various sedimentary matrices such as natural quartz (with and without admixtures of montmorillonite type clay) or glass beads with different surface properties, at varying water saturation. Both juvenile water and metastably gas‐enriched water obtained from gas hydrate decomposition was used. Xenon gas was employed to enhance the density contrast between gas hydrate and the fluid phases involved. The nucleation sites can be easily identified and the various growth patterns are clearly established. In sediments under‐saturated with juvenile water, nucleation starts at the water‐gas interface resulting in an initially several micrometer thick gas hydrate film; further growth proceeds to form isometric single crystals of 10‐20 μm size. The growth of gas hydrate from gas‐enriched water follows a different pattern, via the nucleation in the bulk of liquid producing polyhedral single crystals. A striking feature in both cases is the systematic appearance of a fluid phase film of up to several µm thickness between gas hydrates and the surface of the quartz grains. These micro‐structural findings are relevant for future efforts of quantitative rock physics modeling of gas hydrates in sedimentary matrices and explain the anomalous attenuation of seismic/sonic waves. This article is protected by copyright. All rights reserved.
      PubDate: 2015-05-15T10:12:37.704314-05:
      DOI: 10.1002/2015GC005811
  • Determination of low B/Ca ratios in carbonates using ICP‐QQQ
    • Authors: Silvia Diez Fernández; Jorge Ruiz Encinar, Alfredo Sanz‐Medel, Kirsten Isensee, Heather M. Stoll
      Abstract: The very low B/Ca ratios characteristic of some natural biogenic carbonates, are of interest for research in ocean acidification but represent an analytical challenge. We describe a method using a novel instrument configuration (ICP‐QQQ), for which we are not aware of any previously published geological applications, and for coccoliths, a sample type unique in its low B content and organic phases. Detection limits as low as 0.41 µmol/mol were achieved. Isobaric interferences, out of the reach even for SF‐ICP‐MS, can be solved using this instrument, which permits the safe measurement of the lowest abundance Ca isotope (46Ca). This allows maximizing the B concentration measured (matrix concentration up to 800 ppm Ca) while maintaining both B and Ca signals in counting mode. More significantly for low B samples, the ICP‐QQQ is also able to overcome the interference of the ubiquitous 12C tail on the 11B mass, which otherwise leads to significant overestimates at very low B concentrations. This could be a reason for the significantly lower B/Ca ratios observed for the low B content interlaboratory calibration standards (Carrara and OKA), while matching for the high B content standards was good. Finally, results obtained in the analysis of coccoliths grown in laboratory culture seems to corroborate that SIMS analysis of the samples mounted in Indium leads also to B/Ca overestimates due to porosity effects, as previously observed using LA‐ICP‐MS. This approach also permits the interference‐free measurement of P/Ca and S/Ca ratios, which could be used as indicators of the complete removal of the organic matter from the samples. This article is protected by copyright. All rights reserved.
      PubDate: 2015-05-15T10:12:26.445729-05:
      DOI: 10.1002/2015GC005817
  • Contrasting styles of (U)HP rock exhumation along the Cenozoic
           Adria‐Europe plate boundary (Western Alps, Calabria, Corsica)
    • Authors: Marco G. Malusà; Claudio Faccenna, Suzanne L. Baldwin, Paul G. Fitzgerald, Federico Rossetti, Maria Laura Balestrieri, Martin Danišík, Alessandro Ellero, Giuseppe Ottria, Claudia Piromallo
      Abstract: Since the first discovery of ultra‐high pressure (UHP) rocks 30 years ago in the Western Alps, the mechanisms for exhumation of (U)HP terranes worldwide are still debated. In the Western Mediterranean, the presently accepted model of synconvergent exhumation (e.g., the channel‐flow model) is in conflict with parts of the geologic record. We synthesize regional geologic data and present alternative exhumation mechanisms that consider the role of divergence within subduction zones. These mechanisms, i.e., (i) the motion of the upper plate away from the trench and (ii) the rollback of the lower plate, are discussed in detail with particular reference to the Cenozoic Adria‐Europe plate boundary, and along three different transects (Western Alps, Calabria‐Sardinia, and Corsica ‐ Northern Apennines). In the Western Alps, (U)HP rocks were exhumed from the greatest depth at the rear of the accretionary wedge during motion of the upper plate away from the trench. Exhumation was extremely fast, and associated with very low geothermal gradients. In Calabria, HP rocks were exhumed from shallower depths and at lower rates during rollback of the Adriatic plate, with repeated exhumation pulses progressively younging towards the foreland. Both mechanisms were active to create boundary divergence along the Corsica – Northern Apennines transect, where European southeastward subduction was progressively replaced along‐strike by Adriatic northwestward subduction. The tectonic scenario depicted for the Western Alps trench during Eocene exhumation of (U)HP rocks correlates well with present day eastern Papua New Guinea, which is presented as a modern analogue of the Paleogene Adria‐Europe plate boundary. This article is protected by copyright. All rights reserved.
      PubDate: 2015-05-15T10:12:18.794389-05:
      DOI: 10.1002/2015GC005767
  • Dynamics of intraoceanic subduction initiation: 2. Suprasubduction zone
           ophiolite formation and metamorphic sole exhumation in context of absolute
           plate motions
    • Authors: Douwe J.J. van Hinsbergen; Kalijn Peters, Marco Maffione, Wim Spakman, Carl Guilmette, Cedric Thieulot, Oliver Plümper, Derya Gürer, Fraukje M. Brouwer, Ercan Aldanmaz, Nuretdin Kaymakci
      Abstract: Analyzing subduction initiation is key for understanding the coupling between plate tectonics and underlying mantle. Here we focus on supra‐subduction zone (SSZ) ophiolites and how their formation links to intra‐oceanic subduction initiation in an absolute plate motion frame. SSZ ophiolites form the majority of exposed oceanic lithosphere fragments and are widely recognized to have formed during intra‐oceanic subduction initiation. Structural, petrological, geochemical, and plate kinematic constraints on their kinematic evolution show that SSZ crust forms at forearc spreading centers at the expense of a mantle wedge, thereby flattening the nascent slab. This leads to the typical inverted pressure gradients found in metamorphic soles that form at the subduction plate contact below and during SSZ crust crystallization. Former spreading centers are preserved in forearcs when subduction initiates along transform faults or off‐ridge oceanic detachments. We show how these are reactivated when subduction initiates in the absolute plate motion direction of the inverting weakness zone. Upon inception of slab‐pull due to e.g. eclogitization, the sole is separated from the slab, remains welded to the thinned overriding plate lithosphere and can become intruded by mafic dikes upon asthenospheric influx into the mantle wedge. We propound that most ophiolites thus formed under special geodynamic circumstances and may not be representative of normal oceanic crust. Our study highlights how far‐field geodynamic processes and absolute plate motions may force intra‐oceanic subduction initiation as key towards advancing our understanding of the entire plate tectonic cycle. This article is protected by copyright. All rights reserved.
      PubDate: 2015-05-15T10:11:58.145754-05:
      DOI: 10.1002/2015GC005745
  • Water input and water release from the subducting Nazca Plate along
           southern Central Chile (33°S–46°S)
    • Authors: David Völker; Michael Stipp
      Abstract: The age of the subducting Nazca Plate off Chile increases northwards from 0 Ma at the Chile Triple Junction (46°S) to 37 Ma at the latitude of Valparaíso (32°S). Age‐related variations in the thermal state of the subducting plate impact on (a) the water influx to the subduction zone, as well as on (b) the volumes of water that are released under the continental forearc or, alternatively, carried beyond the arc. Southern Central Chile is an ideal setting to study this effect, because other factors for the subduction zone water budget appear constant. We determine the water influx by calculating the crustal water uptake and by modeling the upper mantle serpentinization at the outer rise of the Chile Trench. The water release under forearc and arc is determined by coupling FEM thermal models of the subducting plate with stability fields of water‐releasing mineral reactions for upper and lower crust and hydrated mantle. Results show that both the influx of water stored in, and the outflux of water released from upper crust, lower crust and mantle vary drastically over segment boundaries. In particular, the oldest and coldest segments carry roughly twice as much water into the subduction zone as the youngest and hottest segments, but their release flux to the forearc is only about one fourth of the latter. This high variability over a subduction zone of 
      PubDate: 2015-05-12T17:53:41.488248-05:
      DOI: 10.1002/2015GC005766
  • Plume‐cratonic lithosphere interaction recorded by water and other
           trace elements in peridotite xenoliths from the Labait volcano, Tanzania
    • Authors: Hejiu Hui; Anne H. Peslier, Roberta L. Rudnick, Antonio Simonetti, Clive R. Neal
      Abstract: Water and other trace element concentrations in olivine (1‐39 ppm H2O), orthopyroxene (10‐150 ppm H2O), and clinopyroxene (16‐340 ppm H2O) of mantle xenoliths from the Labait volcano, located on the edge of the Tanzanian craton along the eastern branch of the East African Rift, record melting and subsequent refertilization by plume magmas in a stratified lithosphere. These water contents are at the lower end of the range observed in other cratonic mantle lithospheres. Despite correlations between water content and indices of melting in orthopyroxene from the shallow peridotites, and in both olivine and orthopyroxene from the deep peridotites, water concentrations are too high for the peridotites to be simple residues. Instead, the Labait water contents are best explained as reflecting interaction between residual peridotite with a melt having relatively low water content (
      PubDate: 2015-04-29T11:02:15.997231-05:
      DOI: 10.1002/2015GC005779
  • What processes control the chemical compositions of arc front
    • Authors: Stephen J Turner; Charles H Langmuir
      Abstract: Arc front stratovolcanoes have global chemical systematics that constrain processes at convergent margins. Positive correlations exist for arc averages among “fluid mobile,” “high field strength,” and “large ion lithophile” elements. 143Nd/144Nd and 87Sr/86Sr from rear‐arc lavas lacking subduction signature align with the oceanic “mantle array,” and correlate with arc front 143Nd/144Nd. Most chemical parameters (but not isotopes) also correlate well with crustal thickness and slightly less well with the slab thermal parameter, but not with the depth of the slab or model slab surface temperatures. Successful models of arc volcanism should account for these global regularities. Two distinct models can quantitatively account for the observations—different extents of melting of the mantle wedge caused by variations in wedge thermal structure, or varying contributions from the subducting slab owing to variations in the slab thermal structure. The wedge melting model has constant contributions from ocean crust, sediment and mantle wedge to lavas globally, while the slab model varies slab contributions with slab temperature. The wedge melting model fit improves by incorporating convergence rate and slab dip, which should affect the wedge thermal structure; the slab model is not supported by a similar analysis. The wedge model also more easily accommodates the isotope data. The two models predict different primary H2O contents, with large variations in H2O for the wedge model, and relatively constant H2O for the slab model. An evaluation of the effects of varying sediment compositions on arc lavas will benefit from considering the very different consequences of the two models. This article is protected by copyright. All rights reserved.
      PubDate: 2015-04-07T05:31:15.394429-05:
      DOI: 10.1002/2014GC005633
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