for Journals by Title or ISSN
for Articles by Keywords

Publisher: AGU   (Total: 17 journals)   [Sort by number of followers]

Showing 1 - 17 of 17 Journals sorted alphabetically
Geochemistry, Geophysics, Geosystems     Full-text available via subscription   (Followers: 24, SJR: 2.56, h-index: 69)
Geophysical Research Letters     Full-text available via subscription   (Followers: 74, SJR: 3.493, h-index: 157)
Global Biogeochemical Cycles     Full-text available via subscription   (Followers: 8, SJR: 3.239, h-index: 119)
J. of Advances in Modeling Earth Systems     Open Access   (Followers: 3, SJR: 1.944, h-index: 7)
J. of Geophysical Research : Atmospheres     Partially Free   (Followers: 66)
J. of Geophysical Research : Biogeosciences     Full-text available via subscription   (Followers: 12)
J. of Geophysical Research : Earth Surface     Partially Free   (Followers: 33)
J. of Geophysical Research : Oceans     Partially Free   (Followers: 28)
J. of Geophysical Research : Planets     Full-text available via subscription   (Followers: 26)
J. of Geophysical Research : Solid Earth     Full-text available via subscription   (Followers: 32)
J. of Geophysical Research : Space Physics     Full-text available via subscription   (Followers: 27)
Paleoceanography     Full-text available via subscription   (Followers: 4, SJR: 3.22, h-index: 88)
Radio Science     Full-text available via subscription   (Followers: 16, SJR: 0.959, h-index: 51)
Reviews of Geophysics     Full-text available via subscription   (Followers: 25, SJR: 9.68, h-index: 94)
Space Weather     Full-text available via subscription   (Followers: 8, SJR: 1.319, h-index: 19)
Tectonics     Full-text available via subscription   (Followers: 8, SJR: 2.748, h-index: 85)
Water Resources Research     Full-text available via subscription   (Followers: 71, SJR: 2.189, h-index: 121)
Journal Cover Geochemistry, Geophysics, Geosystems
  [SJR: 2.56]   [H-I: 69]   [24 followers]  Follow
   Full-text available via subscription Subscription journal
   ISSN (Online) 1525-2027
   Published by AGU Homepage  [17 journals]
  • Origin of geochemical mantle components: Role of subduction filter
    • Abstract: We quantitatively explore element redistribution at subduction zones using numerical mass balance models to evaluate the roles of the subduction zone filter in the Earth's geochemical cycle. Our models of slab residues after arc magma genesis differ from previous ones by being internally consistent with geodynamic models of modern arcs that successfully explain arc magma genesis, and include element fluxes from the dehydration/melting of each underlying slab component. We assume that the mantle potential temperature (Tp) was 1400–1650°C at 3.5–1.7 Ga and gradually decreased to 1300–1350°C today. Hot subduction zones with Tp ∼1650°C have a thermal structure like modern SW Japan where high‐Mg andesite is formed that is chemically like continental crust. After 2.5–1.7 Gyr of storage in the mantle, the residual igneous oceanic crust from hot subduction zones can evolve isotopically to the HIMU mantle component, the residual base of the mantle wedge to EMI, the residual sediment becomes an essential part of EMII, and the residual top of the mantle wedge can become the subcontinental lithosphere component. The Common or Focal Zone component is a stable mixture of the first three residues occasionally mixed with early depleted mantle. Slab residue that recycled earlier (∼2.5 Ga) form the DUPAL anomaly in the southern hemisphere, whereas residues of more recent recycling (∼1.7 Ga) underlie the northern hemisphere. These ages correspond to major continental crust forming events. The east‐west heterogeneity of the depleted upper mantle involves sub continental mantle except in the Pacific. This article is protected by copyright. All rights reserved.
      PubDate: 2016-07-18T03:52:52.856613-05:
      DOI: 10.1002/2016GC006343
  • Age and compositional data of zircon from sepiolite drilling mud to
           identify contamination of ocean drilling samples
    • Authors: Graham D. M. Andrews; Axel K. Schmitt, Cathy J. Busby, Sarah R. Brown, Peter Blum, Janet. C. Harvey
      Abstract: Zircon extracted from drilled oceanic rocks is increasingly used to answer geologic questions related to igneous and sedimentary sequences. Recent zircon studies using samples obtained from marine drill cores revealed that drilling muds used in the coring process may contaminate the samples. The JOIDES Resolution Science Operator of the International Ocean Discovery Program has been using two types of clays, sepiolite and attapulgite, which both have salt water viscosifier properties able to create a gel‐like slurry that carries drill cuttings out of the holes several hundred meters deep. The dominantly used drilling mud is sepiolite originating from southwestern Nevada, USA. This sepiolite contains abundant zircon crystals with U‐Pb ages ranging from 1.89 to 2889 Ma and continental trace element, δ18O, and εHf isotopic compositions. A dominant population of 11 to 16 Ma zircons in sepiolite drilling mud makes identification of contamination in drilled Neogene successions particularly challenging. Interpretation of zircon analyses related to ocean drilling should be cautious of zircon ages in violation of independently constrained age models and that have age populations overlapping those in the sepiolite. Because individual geochronologic and geochemical characteristics lack absolute discriminatory power, it is recommended to comprehensively analyze all dated zircon crystals from cores exposed to drill mud for trace element, δ18O, and εHf isotopic compositions. Zircon analyzed in situ (i.e., in petrographic sections) are assumed to be trustworthy. This article is protected by copyright. All rights reserved.
      PubDate: 2016-07-15T04:16:02.603628-05:
      DOI: 10.1002/2016GC006397
  • Water‐rich and volatile‐undersaturated magmas at Hekla
           volcano, Iceland
    • Abstract: Olivine‐hosted melt inclusions from four eruptions at Hekla volcano in Iceland were analyzed for their dissolved H2O, CO2, S and Cl contents. A positive correlation between the repose interval, magmatic evolution and volatile contents of magmas is revealed. H2O is the dominant volatile species; it behaves as an incompatible component, increasing in concentration over time as a result of fractional crystallization in the magma. The full suite of H2O contents ranges from a low of 0.80 wt. % in basaltic andesites to a maximum of 5.67 wt. % in rhyolites. Decreasing H2O/K2O at fixed major element compositions suggests that syn‐eruptive degassing reduces H2O contents significantly. Hekla magmas are CO2 poor, with very low concentrations present only in the most evolved compositions (∼20‐30 ppm or less). The decrease in S content from basaltic andesite to rhyolite demonstrates that sulfide saturation is attained when the melt composition reaches basaltic andesite, resulting in the precipitation of pyrrhotite. Low CO2/Nb ratios suggest that vapor saturation is most likely reached during an early period of cooling and solidification in the crust. Fresh injections of mafic magma interact with previously solidified intrusives, producing new melts that are volatile‐undersaturated. Vapor saturation pressures obtained using the most volatile‐rich melt inclusions suggest the presence of a magma chamber at a minimum depth of ∼7 km. This is in agreement with geophysical observations from recent small‐volume eruptions, but given the possibility of volatile‐undersaturated melts, some of the magmas may reside at greater depths. This article is protected by copyright. All rights reserved.
      PubDate: 2016-07-14T09:35:42.980275-05:
      DOI: 10.1002/2016GC006336
  • The water retention curve and relative permeability for gas production
    • Authors: Nariman Mahabadi; Sheng Dai, Yongkoo Seol, Tae Sup Yun, Jaewon Jang
      Abstract: The water retention curve and relative permeability are critical to predict gas and water production from hydrate‐bearing sediments. However, values for key parameters that characterize gas and water flows during hydrate dissociation have not been identified due to experimental challenges. This study utilizes the combined techniques of micro‐focus X‐ray computed tomography (CT) and pore‐network model simulation to identify proper values for those key parameters, such as gas entry pressure, residual water saturation, and curve fitting values. Hydrates with various saturation and morphology are realized in the pore‐network that was extracted from micron‐resolution CT images of sediments recovered from the hydrate deposit at the Mallik site, and then the processes of gas invasion, hydrate dissociation, gas expansion, and gas and water permeability are simulated. Results show that greater hydrate saturation in sediments lead to higher gas entry pressure, higher residual water saturation, and steeper water retention curve. An increase in hydrate saturation decreases gas permeability but has marginal effects on water permeability in sediments with uniformly distributed hydrate. Hydrate morphology has more significant impacts than hydrate saturation on relative permeability. Sediments with heterogeneously distributed hydrate tend to result in lower residual water saturation and higher gas and water permeability. In this sense, the Brooks‐Corey model that uses two fitting parameters individually for gas and water permeability properly capture the effect of hydrate saturation and morphology on gas and water flows in hydrate‐bearing sediments. This article is protected by copyright. All rights reserved.
      PubDate: 2016-07-14T09:35:36.375488-05:
      DOI: 10.1002/2016GC006372
  • Representing anisotropic subduction zones with isotropic velocity models:
           A characterization of the problem and some steps on a possible path
    • Authors: M. J. Bezada; M. Faccenda, D. R. Toomey
      Abstract: Despite the widely known fact that mantle flow in and around subduction zones produces the development of considerable seismic anisotropy, most P‐wave tomography efforts still rely on the assumption of isotropy. In this study, we explore the potential effects of erroneous assumption on tomographic images and explore an alternative approach. We conduct a series of synthetic tomography tests based on a geodynamic simulation of subduction and rollback. The simulation results provide a self‐consistent distribution of isotropic (thermal) anomalies and seismic anisotropy which we use to calculate synthetic delay times for a number of realistic and hypothetical event distributions. We find that anisotropy‐induced artifacts are abundant and significant for teleseismic, local and mixed event distributions. The occurrence of artifacts is not reduced, and indeed can be exacerbated, by increasing richness in ray‐path azimuths and incidence angles. The artifacts that we observe are, in all cases, important enough to significantly impact the interpretation of the images. We test an approach based on prescribing the anisotropy field as an a priori constraint and find that even coarse approximations to the true anisotropy field produce useful results. Using approximate anisotropy fields can result in reduced RMS misfit to the travel time delays and reduced abundance and severity of imaging artifacts. We propose that the use of anisotropy fields derived from geodynamic modeling and constrained by seismic observables may constitute a viable alternative to isotropic tomography that does not require the inversion for anisotropy parameters in each node of the model. This article is protected by copyright. All rights reserved.
      PubDate: 2016-07-14T09:26:33.943968-05:
      DOI: 10.1002/2016GC006507
  • Late‐Pleistocene precipitation δ18O interpolated across the
           global landmass
    • Authors: Scott Jasechko
      Abstract: Global water cycles, ecosystem assemblages and weathering rates were impacted by the ∼4°C of global warming that took place over the course of the last glacial termination. Fossil groundwaters can be useful indicators of late‐Pleistocene precipitation isotope compositions, which, in turn, can help to test hypotheses about the drivers and impacts of glacial‐interglacial climate changes. Here, a global catalog of 128 fossil groundwater records is used to interpolate late‐Pleistocene precipitation δ18O across the global landmass. The interpolated data show that extratropical late‐Pleistocene terrestrial precipitation was near uniformly depleted in 18O relative to the late Holocene. By contrast, tropical δ18O responses to deglacial warming diverged; late‐Pleistocene δ18O was higher‐than‐modern across India and South China but lower‐than‐modern throughout much of northern and southern Africa. Groundwaters that recharged beneath large northern hemisphere ice sheets have different Holocene‐Pleistocene δ18O relationships than paleowaters that recharged subaerially, potentially aiding reconstructions of englacial transport in paleo ice sheets. Global terrestrial late‐Pleistocene precipitation δ18O maps may help to map 3D groundwater age distributions, constrain Pleistocene mammal movements, and better understand glacial climate dynamics. This article is protected by copyright. All rights reserved.
      PubDate: 2016-07-13T10:43:20.289536-05:
      DOI: 10.1002/2016GC006400
  • Controls on the distribution of deep‐sea sediments
    • Abstract: Deep‐sea sediments represent the largest geological deposit on Earth and provide a record of our planet's response to conditions at the sea surface from where the bulk of material originates. We use a machine learning method to analyze how the distribution of 14,400 deep‐sea sediment sample lithologies is connected to bathymetry and surface oceanographic parameters. Our probabilistic Gaussian process classifier shows that the geographic occurrence of five major lithologies in the world's ocean can be predicted using just three parameters. Sea‐surface salinity and temperature provide a major control for the growth and composition of plankton and specific ranges are also associated with the influx of non‐aerosol terrigenous material into the ocean, while bathymetry is an important parameter for discriminating the occurrence of calcareous sediment, clay and coarse lithogenous sediment from each other. We find that calcareous and siliceous oozes are not linked to high surface productivity. Diatom and radiolarian oozes are associated with low salinities at the surface but with discrete ranges of temperatures reflecting the diversity of planktonic species in different climatic zones. Biosiliceous sediments cannot be used to infer paleodepth, but are good indicators of paleotemperature and paleosalinity. Our analysis provides a new framework for constraining paleosurface ocean environments from the geological record of deep‐sea sediments. It shows that small shifts in salinity and temperature significantly affect the lithology of seafloor sediment. As deep‐sea sediments represent the largest carbon sink on Earth these shifts need to be considered in the context of global ocean warming. This article is protected by copyright. All rights reserved.
      PubDate: 2016-07-13T10:43:12.571597-05:
      DOI: 10.1002/2016GC006428
  • An integrated natural remanent magnetization acquisition model for the
           Matuyama‐Brunhes reversal recorded by the Chinese loess
    • Authors: Chunsheng Jin; Qingsong Liu, Pengxiang Hu, Zhaoxia Jiang, Cange Li, Peng Han, Huihui Yang, Wentian Liang
      Abstract: Geomagnetic polarity reversal boundaries are key isochronous chronological controls for the long Chinese loess sequences, and further facilitate paleoclimatic correlation between Chinese loess and marine sediments. However, owing to complexity of post‐depositional remanent magnetization (pDRM) acquisition processes related to variable dust sedimentary environments on the Chinese Loess Plateau (CLP), there is a long‐standing dispute concerning the downward shift of the pDRM recorded in Chinese loess. In this study, after careful stratigraphic correlation of representative climatic tie points and the Matuyama‐Brunhes boundaries (MBB) in the Xifeng, Luochuan, and Mangshan loess sections with different pedogenic environments, the downward shift of the pDRM is semi‐quantitatively estimated and the acquisition model for the loess natural remanent magnetization (NRM) is discussed. The measured MB transition zone has been affected by the surficial mixing layer (SML) and remagnetization. Paleoprecipitation is suggested to be the dominant factor controlling the pDRM acquisition processes. Rainfall‐controlled leaching would restrict the efficiency of the characterized remanent magnetization carriers aligning along the ancient geomagnetic field. We conclude that the MBB in the central CLP with moderate paleoprecipitation could be considered as an isochronous chronological control after moderate upward adjustment. A convincing case can then be made to correlate L8/S8 to MIS 18/19. This article is protected by copyright. All rights reserved.
      PubDate: 2016-07-12T08:15:50.932254-05:
      DOI: 10.1002/2016GC006407
  • Density and P‐wave velocity structure beneath the Paraná
           Magmatic Province: Refertilization of an ancient lithospheric mantle
    • Authors: Carlos Chaves; Naomi Ussami, Jeroen Ritsema
      Abstract: We estimate density and P‐wave velocity perturbations in the mantle beneath the southeastern South America plate from geoid anomalies and P‐wave traveltime residuals to constrain the structure of the lithosphere underneath the Paraná Magmatic Province (PMP) and conterminous geological provinces. Our analysis shows a consistent correlation between density and velocity anomalies. The P‐wave speed and density are 1% and 15 kg/m3 lower, respectively, in the upper mantle under the Late Cretaceous to Cenozoic alkaline provinces, except beneath the Goiás Alkaline Province (GAP), where density (+20 kg/m3) and velocity (+0.5%) are relatively high. Underneath the PMP, the density is higher by about 50 kg/m3 in the north and 25 kg/m3 in the south, to a depth of 250 − 300 km. These values correlate with high‐velocity perturbations of +0.5% and +0.3%, respectively. Profiles of density perturbation versus depth in the upper mantle are different for the PMP and the adjacent Archean São Francisco (SFC) and Amazonian (AC) cratons. The Paleoproterozoic PMP basement has a high‐density root. The density is relatively low in the SFC and AC lithospheres. A reduction of density is a typical characteristic of chemically depleted Archean cratons. A more fertile Proterozoic and Phanerozoic subcontinental lithospheric mantle has a higher density, as deduced from density estimates of mantle xenoliths of different ages and composition. In conjunction with Re‐Os isotopic studies of the PMP basalts, chemical and isotopic analyses of peridodite xenoliths from the GAP in the northern PMP, and electromagnetic induction experiments of the PMP lithosphere, our density and P‐wave speed models suggest that the densification of the PMP lithosphere and flood basalt generation are related to mantle refertilization. Metasomatic refertilization resulted from the introduction of asthenospheric components from the mantle wedge above Proterozoic subduction zones, which surrounded the Paraná lithosphere. The high‐density PMP lithosphere is presently gravitationally unstable and prone to delamination. This article is protected by copyright. All rights reserved.
      PubDate: 2016-07-11T10:46:06.535257-05:
      DOI: 10.1002/2016GC006369
  • Automated cleaning of foraminifera tests before Mg/Ca analysis using a
           pipette robot
    • Abstract: The molar ratio of magnesium to calcium (Mg/Ca) in foraminiferal calcite is a widely used proxy for reconstructing past seawater temperatures. Thorough cleaning of tests is required before analysis to remove contaminant phases such as clay and organic matter. We have adapted a commercial pipette robot to automate an established cleaning procedure, the “Mg‐cleaning” protocol of Barker et al. [2003]. Efficiency of the automated 9‐step method was assessed through monitoring Al/Ca of trial samples (GeoB4420‐2 core‐catcher). Planktonic foraminifera Globigerinoides ruber, Globigerinoides sacculifer and Neogloboquadrina dutertrei from this sample gave Mg/Ca consistent with the habitat range of the three species, and 40 ‐ 60% sample recovery after cleaning. Comparison between manually cleaned and robot cleaned samples of G. ruber [white] from a sediment core (GeoB16602) showed good correspondence between the two methods for Mg/Ca (r =0.93, p
      PubDate: 2016-07-11T10:45:36.478556-05:
      DOI: 10.1002/2016GC006422
  • A distinct source and differentiation history for Kolumbo submarine
           volcano, Santorini volcanic field, Aegean arc
    • Authors: Martijn Klaver; Steven Carey, Paraskevi Nomikou, Ingrid Smet, Athanasios Godelitsas, Pieter Vroon
      Abstract: This study reports the first detailed geochemical characterization of Kolumbo submarine volcano in order to investigate the role of source heterogeneity in controlling geochemical variability within the Santorini volcanic field in the central Aegean arc. Kolumbo, situated 15 km to the northeast of Santorini, last erupted in 1650 AD and is thus closely associated with the Santorini volcanic system in space and time. Samples taken by remotely‐operated vehicle that were analyzed for major element, trace element and Sr‐Nd‐Hf‐Pb isotope composition include the 1650 AD and underlying K2 rhyolitic, enclave‐bearing pumices that are nearly identical in composition (73 wt. % SiO2, 4.2 wt. % K2O). Lava bodies exposed in the crater and enclaves are basalts to andesites (52‐60 wt. % SiO2). Biotite and amphibole are common phenocryst phases, in contrast with the typically anhydrous mineral assemblages of Santorini. The strong geochemical signature of amphibole fractionation and the assimilation of lower crustal basement in the petrogenesis of the Kolumbo magmas indicates that Kolumbo and Santorini underwent different crustal differentiation histories and that their crustal magmatic systems are unrelated. Moreover, the Kolumbo samples are derived from a distinct, more enriched mantle source that is characterized by high Nb/Yb (>3) and low 206Pb/204Pb (
      PubDate: 2016-07-11T10:40:26.89916-05:0
      DOI: 10.1002/2016GC006398
  • Multi‐scale approach to (micro)porosity quantification in
           continental spring carbonate facies Case study from the Cakmak quarry
           (Denizli, Turkey)
    • Abstract: Carbonate spring deposits gained renewed interest as potential contributors to subsurface reservoirs and as continental archives of environmental changes. In contrast to their fabrics, petrophysical characteristics – and especially the importance of microporosity (< 1µm) – are less understood. This study presents the combination of advanced petrophysical and imaging techniques to investigate the pore network characteristics of three, common and widespread spring carbonate facies, as exposed in the Pleistocene Cakmak quarry (Denizli, Turkey): the extended Pond, the dipping crystalline Proximal Slope Facies and the draping Apron and Channel Facies deposits formed by encrustation of biological substrate. Integrating mercury injection capillary pressure, bulk and diffusion Nuclear Magnetic Resonance (NMR), NMR profiling and Brunauer–Emmett–Teller (BET) measurements with microscopy and micro‐computer tomography (µ‐CT), shows that NMR T2 distributions systematically display a single group of micro‐sized pore bodies, making up between 6 and 33% of the pore space (average NMR T2 cut‐off value: 62 ms). Micropore bodies are systematically located within cloudy crystal cores of granular and dendritic textures in all facies. The investigated properties therefore do not reveal differences in micropore size or shape with respect to more or less biology‐associated facies. The pore network of the travertine facies is distinctive in terms of (i) the percentage of microporosity, (ii) the connectivity of micropores with meso‐ to macropores, and (ii) the degree of heterogeneity at micro‐ and macroscale. Results show that an approach involving different NMR experiments provided the most complete view on the 3D pore network especially when microporosity and –connectivity are of interest. This article is protected by copyright. All rights reserved.
      PubDate: 2016-07-01T03:31:11.223346-05:
      DOI: 10.1002/2016GC006382
  • Helium Release During Shale Deformation: Experimental Validation
    • Authors: Stephen J. Bauer; W. Payton Gardner, Jason Heath
      Abstract: This work describes initial experimental results of helium tracer release monitoring during deformation of shale. Naturally occurring radiogenic 4He is present in high concentration in most shales. During rock deformation, accumulated helium could be released as fractures are created and new transport pathways are created. We present the results of an experimental study in which confined reservoir shale samples, cored parallel and perpendicular to bedding, which were initially saturated with helium to simulate reservoir conditions, are subjected to triaxial compressive deformation. During the deformation experiment, differential stress, axial and radial strains are systematically tracked. Release of helium is dynamically measured using a helium mass spectrometer leak detector. Helium released during deformation is observable at the laboratory scale and the release is tightly coupled to the shale deformation. These first measurements of dynamic helium release from rocks undergoing deformation show that helium provides information on the evolution of microstructure as a function of changes in stress and strain. This article is protected by copyright. All rights reserved.
      PubDate: 2016-07-01T03:31:06.4738-05:00
      DOI: 10.1002/2016GC006352
  • Magnetic signatures of serpentinization at ophiolite complexes
    • Abstract: We compare magnetic properties of 58 variably serpentinized peridotites from three ophiolite complexes (Pindos, Greece; Oman; Chenaillet, France) and the mid‐Atlantic ridge near the Kane fracture zone (MARK). The Pindos and Oman sites show low susceptibility and remanence (K
      PubDate: 2016-07-01T03:30:46.299299-05:
      DOI: 10.1002/2016GC006321
  • A computationally efficient spectral method for modeling core dynamics
    • Authors: P. Marti; M. A. Calkins, K. Julien
      Abstract: An efficient, spectral numerical method is presented for solving problems in a spherical shell geometry that employs spherical harmonics in the angular dimensions and Chebyshev polynomials in the radial direction. We exploit the three‐term recurrence relation for Chebyshev polynomials that renders all matrices sparse in spectral space. This approach is significantly more efficient than the collocation approach and is generalizable to both the Galerkin and tau methodologies for enforcing boundary conditions. The sparsity of the matrices reduces the computational complexity of the linear solution of implicit‐explicit timestepping schemes to $O(N)$operations, compared to O(N2) operations for a collocation method. The method is illustrated by considering several example problems of important dynamical processes in the Earth's liquid outer core. Results are presented from both fully nonlinear, time‐dependent numerical simulations and eigenvalue problems arising from the investigation of the onset of convection and the inertial wave spectrum. We compare the explicit and implicit temporal discretization of the Coriolis force; the latter becomes computationally feasible given the sparsity of the differential operators. We find that implicit treatment of the Coriolis force allows for significantly larger timestep sizes compared to explicit algorithms; for hydrodynamic and dynamo problems at an Ekman number of E = 10−5, timestep sizes can be increased by a factor of 3 to 16 times that of the explicit algorithm, depending on the order of the timestepping scheme. The implementation with explicit Coriolis force scales well to at least 2048 cores, while the implicit implementation scales to 512 cores. This article is protected by copyright. All rights reserved.
      PubDate: 2016-07-01T03:30:36.321858-05:
      DOI: 10.1002/2016GC006438
  • Interactions between active faulting, volcanism, and sedimentary processes
           at an island arc: Insights from Les Saintes channel, Lesser Antilles arc
    • Authors: F. Leclerc; N. Feuillet, C. Deplus
      Abstract: New high‐resolution marine geophysical data allow to characterize a large normal fault system in the Lesser Antilles arc, and to investigate the interactions between active faulting, volcanism, sedimentary and mass wasting processes. Les Saintes fault system is composed of several normal faults that form a 30 km wide half‐graben accommodating NE‐SW extension. It is bounded by the Roseau fault, responsible for the destructive Mw6.3 21/11/2004 earthquake. The Roseau fault has been identified from the island of Basse‐Terre to Dominica. It is thus 40 km long, and it could generate Mw7 earthquakes in the future. Several submarine volcanoes are also recognized. We show that the fault system initiated after the main volcanic construction and subsequently controls the emission of volcanic products. The system propagates southward through damage zones. At the tip of the damage zones, several volcanic cones were recently emplaced probably due to fissures opening in an area of stress increase. A two‐way interaction is observed between active faulting and sedimentary processes. The faults control the development of the main turbiditic system made of kilometer wide canyons, as well as the location of sediment ponding. In turn, erosion and sedimentation prevent scarp growth at the seafloor. Faulting also enhances mass‐wasting processes. Since its initiation, the fault system has consequently modified the morphologic evolution of the arc through perturbation of the sedimentary processes and localization of the more recent volcanic activity. This article is protected by copyright. All rights reserved.
      PubDate: 2016-06-30T03:50:45.197572-05:
      DOI: 10.1002/2016GC006337
  • Volcano Deformation Survey over the Northern and Central Andes with ALOS
           InSAR Time Series
    • Authors: Anieri M. Morales Rivera; Falk Amelung, Patricia Mothes
      Abstract: We use ALOS‐1 Interferometric Synthetic Aperture Radar data spanning the period of 2007‐2011 to obtain time‐dependent ground deformation data over all of the volcanoes in Colombia, Ecuador and Peru. We detect deformation on or near the proximity of Galeras, Reventador, Tungurahua, Guagua Pichincha, Sangay, and Cerro Auquihuato volcanoes, uncovering previously undocumented deformation in the latter three. Deformation is attributed to changes in pressurization of the volcanic systems (Galeras, Tungurahua, Guagua Pichincha, and Cerro Auquihuato), subsidence associated with flow deposits (Reventador), and flank creep (Sangay). Our models suggest that the pressure sources are located at depths of ∼1 to 6 km from the surface, indicating that the measurable deformation within our data is restricted to shallow magma chambers and hydrothermal systems. This article is protected by copyright. All rights reserved.
      PubDate: 2016-06-23T03:51:00.732361-05:
      DOI: 10.1002/2016GC006393
  • Microearthquake activity, lithospheric structure and deformation modes at
           an amagmatic ultraslow spreading Southwest Indian Ridge segment
    • Authors: Florian Schmid; Vera Schlindwein
      Abstract: While nascent oceanic lithosphere at slow‐ to fast spreading mid‐ocean ridges (MOR) is relatively well studied, much less is known about the lithospheric structure and properties at ultraslow MORs. Here we present microearthquake data from a one‐year ocean bottom seismometer deployment at the amagmatic, Oblique Supersegment of the ultraslow spreading Southwest Indian Ridge. A refraction seismic experiment was performed to constrain upper lithosphere P‐velocities and results were used to construct a 1D‐velocity model for earthquake location. Earthquake foci were located individually and subsequently relocated relative to each other to sharpen the image of seismically active structures. Frequent earthquake activity extends to 31 km beneath the sea floor, indicating an exceptionally thick brittle lithosphere and an undulating brittle‐ductile transition that implies significant variations in the along‐axis thermal structure of the lithosphere. We observe a strong relation between petrology, microseismicity distribution and topography along the ridge axis: Peridotite dominated areas associate with deepest hypocenters, vast volumes of lithosphere that deforms aseismically as a consequence of alteration and the deepest axial rift valley. Areas of basalt exposure correspond to shallower hypocenters, shallower and more rugged axial seafloor. Focal mechanisms deviate from pure extension and are spatially variable. Earthquakes form an undulating band of background seismicity and do not delineate discrete detachment faults as common on slow spreading ridges. Instead, the seismicity band sharply terminates to the south, immediately beneath the rift boundary. Considering the deep alteration, large steep boundary faults might be present but are entirely aseismic. This article is protected by copyright. All rights reserved.
      PubDate: 2016-06-23T03:50:42.14911-05:0
      DOI: 10.1002/2016GC006271
  • Hydrostatic pressure effect on magnetic hysteresis parameters of
           pseudo‐single‐domain magnetite
    • Authors: Masahiko Sato; Yuhji Yamamoto, Takashi Nishioka, Kazuto Kodama, Nobutatsu Mochizuki, Hideo Tsunakawa
      Abstract: This paper reports the first in situ magnetic hysteresis measurements of pseudo‐single‐domain (PSD) magnetite under high pressure up to 1 GPa. The magnetic hysteresis measurements of stoichiometric PSD magnetite samples under hydrostatic pressure were carried out using a piston‐cylinder high‐pressure cell, and the pressure dependence of the hysteresis parameters of PSD magnetite was calculated from the hysteresis curves. It was found that coercivity (Bc) increases with increasing pressure as a quadratic function up to 1 GPa by ∼90%, which is different from the pressure dependences of Bc of multidomain and single‐domain magnetites. Coercivity of remanence also increases as a quadratic function, and saturation remanence (Mrs) increases with pressure up to 0.5 GPa by ∼20% until reaching saturation. In contrast, saturation magnetization is constant up to 1 GPa. The approximate demagnetizing factor calculated from the ratio Bc/Mrs increases with increasing pressure, suggesting that the number of lamellar domains increases with increasing pressure. The number of lamellar domains and domain wall width are theoretically estimated to increase under high pressure due to the changes in magnetostriction, elastic, and magnetocrystalline anisotropy constants, and these changes in magnetic domain structure should relate to the changes in the magnetic properties of PSD magnetite. This article is protected by copyright. All rights reserved.
      PubDate: 2016-06-23T03:45:54.097589-05:
      DOI: 10.1002/2016GC006406
  • Geochemical records from loess deposits in Japan over the last 210 kyr:
           Lithogenic source changes and paleoclimatic indications
    • Abstract: The generation, transport, and accumulation of tropospheric dust have changed with the paleoclimatic changes of the Quaternary Period. Such dust has accumulated in Japan ∼3000 km leeward of the source deserts in China. We analyzed the fractions of windblown fine quartz and bulk major and trace elements, and Sr–Nd–Pb isotopic compositions of loess in SW Japan deposited over the past 210 kyr. The results indicated extensive accumulation of tropospheric dust mixed with tephra fragments derived from the nearby Daisen volcano. The accumulation rate of fine quartz and selected elemental/isotopic compositions can be used as climatic proxies that reflect greater accumulation of dust in times of colder climate. Chemical indices for weathering show enhanced effects of weathering during times of warmer climate. The trace element compositions of the loess deposits are surprisingly similar to those of the Chinese loess, hemipelagic sediments in the Sea of Japan and the western Pacific Ocean, and the distal Chinese dust found in Canada. This similarity indicates that the loess dust shares major fractions of these fine‐grained sediments, and that geochemical fractionation during the transport was limited. The Sr–Nd–Pb isotope compositions of the SW Japan loess indicate an origin predominantly in the Gobi Desert. The high‐latitude Pacific sediments and high‐latitude dust in Canada also show the same signature. However, isotopic compositions of samples from the southern Chinese loess plateau and mid‐ to low‐latitude Pacific sediments are largely derived from the Taklimakan desert, which indicates different delivery pathways of the tropospheric dusts. This article is protected by copyright. All rights reserved.
      PubDate: 2016-06-22T18:55:40.388058-05:
      DOI: 10.1002/2016GC006322
  • Applications of detrital geochronology and thermochronology from glacial
           deposits to the Paleozoic and Mesozoic thermal history of the Ross
           Embayment, Antarctica
    • Authors: Bethany Welke; Kathy Licht, Andrea Hennessy, Sidney Hemming, Elizabeth Pierce Davis, Christine Kassab
      Abstract: Till from moraines at the heads of six major outlet glaciers in the Transantarctic Mountains (TAM) and from till beneath three West Antarctic ice streams have a ubiquitous zircon U‐Pb age population spanning the time of the Ross/Pan‐African Orogenies (610‐475 Ma). Geo‐ and thermochronology of detrital minerals in these Antarctic glacial tills reveal two different thermal histories for the central and southern TAM. Double‐dating of the zircons reveals a geographically widespread (U‐Th)/He (ZHe) population of 180‐130 Ma in most of the till samples. Sandstone outcrops at Shackleton Glacier, and three Beacon Supergroup sandstone clasts from three moraines, have ZHe ages that fall entirely within this range. The similar population and proximity of many of the till samples to Beacon outcrops lead us to suggest that this extensive ZHe population in the tills is derived from Beacon Supergroup rocks and reflects the thermal response of the Beacon Basin to the breakup of Gondwana. A second population of older (>200 Ma) ZHe ages in tills at the head of Byrd, Nimrod and Reedy Glaciers. For the tills at the head of the Nimrod and Byrd Glacier, integrating the double‐dated zircon results with 40Ar/39Ar of hornblende, muscovite and biotite, and U‐Pb and (U‐Th‐Sm)/He double‐dates on apatite yields a typical pattern of early rapid orogenic cooling (∼4‐10˚C/my) 590‐475 Ma after the emplacement of the Granite Harbour Intrusives. Low temperature thermochronometers at these sites yield variable but quite old ages (ZHe 480‐70 Ma and AHe 200‐70 Ma) that require a long history at low temperature. This article is protected by copyright. All rights reserved.
      PubDate: 2016-06-22T03:40:29.030063-05:
      DOI: 10.1002/2015GC005941
  • A new Bayesian Event Tree tool to track and quantify volcanic unrest and
           its application to Kawah Ijen volcano
    • Authors: Roberto Tonini; Laura Sandri, Dmitri Rouwet, Corentin Caudron, Warner Marzocchi, Pak Suparjan
      Abstract: Although most of volcanic hazard studies focus on magmatic eruptions, volcanic hazardous events can also occur when no migration of magma can be recognized. Examples are tectonic and hydrothermal unrest that may lead to phreatic eruptions. Recent events (e.g. Ontake eruption on September 2014) have demonstrated that phreatic eruptions are still hard to forecast, despite being potentially very hazardous. For these reasons, it is of paramount importance to identify indicators that define the condition of non‐magmatic unrest, in particular for hydrothermal systems. Often, this type of unrest is driven by movement of fluids, requiring alternative monitoring setups, beyond the classical seismic‐geodetic‐geochemical architectures. Here we present a new version of the probabilistic BET (Bayesian Event Tree) model, specifically developed to include the forecasting of non‐magmatic unrest and related hazards. The structure of the new event tree differs from the previous schemes by adding a specific branch to detail non‐magmatic unrest outcomes. A further goal of this work consists in providing a user‐friendly, open‐access, and straightforward tool to handle the probabilistic forecast and visualize the results as possible support during a volcanic crisis. The new event tree and tool are here applied to Kawah Ijen stratovolcano, Indonesia, as exemplificative application. In particular, the tool is set on the basis of monitoring data for the learning period 2000‐2010, and is then blindly applied to the test period 2010‐2012, during which significant unrest phases occurred. This article is protected by copyright. All rights reserved.
      PubDate: 2016-06-21T05:40:26.566073-05:
      DOI: 10.1002/2016GC006327
  • Hydraulic properties of samples retrieved from the Wenchuan earthquake
           fault scientific drilling project hole‐1 (WFSD‐1) and the
           surface rupture zone: Implications for coseismic slip weakening and fault
    • Abstract: In this study, we report the hydraulic properties of samples recovered from the first borehole of the Wenchuan earthquake Fault Scientific Drilling and from outcrops associated with the surface rupture zone of the 2008 Wenchuan earthquake. Compositional and microstructural analyses have also been performed on selected samples. Using the pore pressure oscillation method, the permeability measurements show that 1) fault gouge samples have low permeabilities, decreasing from 2 × 10−18 m2 at an effective pressure (Pe) of 10 MPa (equivalent to an in‐situ depth of 600 m) to 9 × 10−21 m2 at 155 MPa. 2) Intact and cemented samples are impermeable with permeabilities less than 2 × 10−20 m2 at 10 MPa. 3) Fractured samples have variable permeabilities, ranging from 3 × 10−15 to 1 × 10−20 m2 at 10 MPa, and are most insensitive to changes in the effective pressure. 4) Granitic cataclasites have a moderate permeability at low pressure (i.e. 10−16 – 10−17 m2 at 10 MPa); which decreases rapidly with increasing Pe. Hydraulic conduction of the fault is believed to be influenced by the permeability of the fractures developed, which is controlled by the density, aperture and/or connectivity of the fractures. Microstructural and compositional analyses of the samples indicate that the fault zone heals through chemically‐mediated fracture closure related to mineral precipitation, possibly assisted by pressure solution of stressed fracture asperities. Although other weakening mechanisms remain possible, our lab measurements combined with numerical modeling reveal that thermal/thermochemical pressurization, perhaps leading to gouge fluidization, played an important role in the dynamic weakening of the Wenchuan earthquake, at least in the study area. This article is protected by copyright. All rights reserved.
      PubDate: 2016-06-20T19:10:51.100029-05:
      DOI: 10.1002/2016GC006376
  • Geodetically constrained models of viscoelastic stress transfer and
           earthquake triggering along the North Anatolian fault
    • Authors: Phoebe M.R. DeVries; Plamen G. Krastev, Brendan J. Meade
      Abstract: Over the past 80 years, 8 MW>6.7 strike‐slip earthquakes west of 40º longitude have ruptured the North Anatolian fault (NAF) from east to west. The series began with the 1939 Erzincan earthquake in eastern Turkey, and the most recent 1999 MW=7.4 Izmit earthquake extended the pattern of ruptures into the Sea of Marmara in western Turkey. The mean time between seismic events in this westward progression is 8.5±11 years (67% confidence interval), much greater than the timescale of seismic wave propagation (seconds to minutes). The delayed triggering of these earthquakes may be explained by the propagation of earthquake‐generated diffusive viscoelastic fronts within the upper mantle that slowly increase the Coulomb failure stress change (ΔCFS) at adjacent hypocenters. Here we develop three‐dimensional stress transfer models with an elastic upper crust coupled to a viscoelastic Burgers rheology mantle. Both the Maxwell (ηM=4x1018‐1x1019 Pa·s) and Kelvin (ηK=1x1018‐1x1019 Pa·s) viscosities are constrained by studies of geodetic observations before and after the 1999 Izmit earthquake. We combine this geodetically constrained rheological model with the observed sequence of large earthquakes since 1939 to calculate the time evolution of ΔCFS changes along the North Anatolian fault due to viscoelastic stress transfer. Apparent threshold values of mean ΔCFS at which the earthquakes in the eight decade sequence occur are between ∼0.02 to ∼3.15 MPa and may exceed the magnitude of static ΔCFS values by as much as 177%. By 2023, we infer that the mean time‐dependent stress change along the northern NAF strand in the Marmara Sea near Istanbul, which may have previously ruptured in 1766, may reach the mean apparent time‐dependent stress thresholds of the previous NAF earthquakes. This article is protected by copyright. All rights reserved.
      PubDate: 2016-06-20T10:50:39.394223-05:
      DOI: 10.1002/2016GC006313
  • The last 2 Myr of accretionary wedge construction in the central Hikurangi
           margin (North Island, New Zealand): Insights from structural modeling
    • Abstract: Three depth‐converted and geologically interpreted seismic profiles provide a clear image of the offshore outer accretionary wedge associated with oblique subduction of the Pacific Plate beneath the central Hikurangi margin. Plio‐Quaternary turbidites deposited over the pelagic cover sequence of the Hikurangi Plateau have been accreted to the margin by imbrication along E‐verging thrust faults that propagated up‐section from the plate boundary décollement. Growth stratigraphy of piggy‐back basins and thrusting of progressively younger horizons trace the eastward advance of the leading thrust front over c. 60 km in the last 2 Myr. Moderate internal shortening of fault‐bounded blocks typically 4‐8 km wide reflects rapid creation of thrust faults, with some early formed faults undergoing out‐of‐sequence reactivation to maintain critical wedge taper. Multi‐stage structural restorations show that forward progression of shortening involves: (1) initial development of a c. 10‐25 km wide “proto‐thrust” zone, comprising conjugate sets of moderately to steeply dipping low‐displacement (c. 10‐100 m) reverse faults; and (2) growth of thrust faults that exploit some of the early proto‐thrust faults and propagate up‐section with progressive break‐through of folds localized above the fault tips. The youngest, still unbreached folds deform the present‐day seafloor. Progressive retro‐deformations show that macroscopic thrust faults and folds account for less than 50% of the margin‐perpendicular shortening imposed by plate convergence. Arguably, significant fractions of the missing components can be attributed to meso‐ and microscopic scale layer‐parallel shortening within the wedge, in the proto‐thrust zones, and in the outer décollement zone. This article is protected by copyright. All rights reserved.
      PubDate: 2016-06-17T18:26:21.433175-05:
      DOI: 10.1002/2016GC006341
  • Causes of unrest at silicic calderas in the East African Rift: New
           constraints from InSAR and soil‐gas chemistry at Aluto volcano,
    • Authors: William Hutchison; Juliet Biggs, Tamsin A. Mather, David M. Pyle, Elias Lewi, Gezahegn Yirgu, Stefano Caliro, Giovanni Chiodini, Laura E. Clor, Tobias P. Fischer
      Abstract: Restless silicic calderas present major geological hazards, and yet many also host significant untapped geothermal resources. In East Africa this poses a major challenge, although the calderas are largely unmonitored their geothermal resources could provide substantial economic benefits to the region. Understanding what causes unrest at these volcanoes is vital for weighing up the opportunities against the potential risks. Here we bring together new field and remote sensing observations to evaluate causes of ground deformation at Aluto, a restless silicic volcano located in the Main Ethiopian Rift (MER). Interferometric Synthetic Aperture Radar (InSAR) data reveal the temporal and spatial characteristics of a ground deformation episode that took place between 2008 and 2010. Deformation time‐series reveal pulses of accelerating uplift that transition to gradual long‐term subsidence, and analytical models support inflation source depths of ∼5 km. Gases escaping along the major fault zone of Aluto show high CO2 flux, and a clear magmatic carbon signature (CO2–δ13C of −4.2 to −4.5 ‰). This provides compelling evidence that the magmatic and hydrothermal reservoirs of the complex are physically connected. We suggest that a coupled magmatic‐hydrothermal system can explain the uplift‐subsidence signals. We hypothesize that magmatic fluid injection and/or intrusion in the cap of the magmatic reservoir drives edifice wide inflation while subsequent deflation is related to magmatic degassing and depressurization of the hydrothermal system. These new constraints on the plumbing of Aluto yield important insights into the behaviour of rift volcanic systems and will be crucial for interpreting future patterns of unrest. This article is protected by copyright. All rights reserved.
      PubDate: 2016-06-17T18:25:54.895516-05:
      DOI: 10.1002/2016GC006395
  • Organochemical characteristics of carbonaceous materials as indicators of
           heat recorded on an ancient plate‐subduction fault
    • Authors: S. Kaneki; T. Hirono, H. Mukoyoshi, Y. Sampei, M. Ikehara
      Abstract: Coseismic shear stress and slip distance affect subduction‐related earthquake processes. They need to be understood to evaluate the earthquake's mechanism, and the tsunami generation potential near trenches. The amount of frictional heat generated depends on the shear stress and slip distance, which are therefore able to be derived from the temperature recorded in the fault. Here we developed a new temperature proxy for carbonaceous materials by performing spectroscopic, thermogravimetric, and organic elemental analyses in conjunction with heating experiments. We found marked anomalies in the infrared and Raman spectra and atomic compositions of carbonaceous materials retrieved from the slip zone of an ancient megasplay fault in the Cretaceous Shimanto accretionary complex, Japan: the infrared spectra show extinction of aliphatic C–H bonding and very weak aromatic C=C bonding, and the Raman spectra show a slightly elevated ratio of disordered band intensity to graphitic band intensity and relatively low H/O and O/C ratios. These correlate well with the spectral features of host‐rock carbonaceous materials after heating to 600°C. Thus, we conclude that the slip zone experienced a temperature of 600°C during a past earthquake event, indicating coseismic slip of 2–9 m, which could have generated a large tsunami if the ruptures propagated to the seafloor. This article is protected by copyright. All rights reserved.
      PubDate: 2016-06-17T18:20:38.48975-05:0
      DOI: 10.1002/2016GC006368
  • Seasonal changes in magnetic parameters of sediments with changing redox
           conditions in Hiroshima Bay, Japan
    • Authors: Noriko Kawamura; Yuka Amano, Naoto Ishikawa
      Abstract: To describe and interpret the relationship between spatial and seasonal changes in the sedimentary environment of nearshore sediments and their magnetic properties, magnetic and geochemical analyses were performed on sediment samples from three stations in Hiroshima Bay, Japan. Vertical stratification of the water column in the bay changes throughout the year, and magnetic hysteresis parameters and mineralogy in the bay sediments vary in response to changes in redox conditions of bottom waters. Magnetite and hematite are present year‐round at all stations. The presence of maghemitized magnetite is inferred at a station located at the entrance to the bay. Greigite is recognized at all stations in September 2011 but is not found at the entrance to the bay when water column stratification is disturbed from October 2011. The presence of maghemite and goethite is inferred at two stations in the inner bay when the sedimentary environment is oxic. The remanent coercivity/coercivity ratio (Hcr/Hc) also varies, both spatially and temporally, which reflects changes in magnetic mineralogy. Increased of Hcr/Hc values are likely to be caused by goethite and/or maghemite formation when water column stratification is disturbed and the seafloor is oxic. Concentration‐dependent magnetic parameters do not respond to seasonal changes in the redox conditions of bottom waters. Reaction times and/or changes in chemical and physical conditions may be insufficient to affect these parameters in the sediments of Hiroshima Bay. This article is protected by copyright. All rights reserved.
      PubDate: 2016-06-17T18:14:04.667729-05:
      DOI: 10.1002/2016GC006347
  • Deformation in the mantle wedge associated with Laramide flat‐slab
    • Authors: Whitney M. Behr; Douglas Smith
      Abstract: Laramide crustal deformation in the Rocky Mountains of the west‐central United States is often considered to relate to a narrow segment of shallow subduction of the Farallon slab, but there is no consensus as to how deformation along the slab–mantle lithosphere interface was accommodated. Here we investigate deformation in mantle rocks associated with hydration and shear above the flat slab at its contact with the base of the North American plate. The rocks we focus on are deformed, hydrated, ultramafic inclusions hosted within diatremes of the Navajo Volcanic Field in the central Colorado Plateau that erupted during the waning stages of the Laramide orogeny. We document a range of deformation textures, including granular peridotites, porphyroclastic peridotites, mylonites, and cataclasites, which we interpret to reflect different proximities to a slab–mantle‐interface shear zone. Mineral assemblages and chemistries constrain deformation to hydrous conditions in the temperature range ∼550‐750°C. Despite the presence of hydrous phyllosilicates in modal percentages of up to 30%, deformation was dominated by dislocation creep in olivine. The mylonites exhibit an uncommon lattice preferred orientation (LPO) in olivine, known as B‐type LPO in which the a‐axes are aligned perpendicular to the flow direction. The low temperature, hydrated setting in which these fabrics formed are consistent with laboratory experiments that indicate B‐type LPOs form under conditions of high stress and high water contents; furthermore, the mantle wedge context of these LPOs is consistent with observations of trench‐parallel anisotropy in the mantle wedge above many modern subduction zones. Differential stress magnitudes in the mylonitic rocks estimated using paleopiezometry range from 290 and 444 MPa, and calculated effective viscosities using a wet olivine flow law are on the order of 1019‐1023 Pa s. The high stress magnitudes, high effective viscosities and high strains recorded in these rocks are consistent with models that invoke significant basal shear tractions as contributing to Laramide uplift and contraction in the continental interior. This article is protected by copyright. All rights reserved.
      PubDate: 2016-06-17T18:13:49.674399-05:
      DOI: 10.1002/2016GC006361
  • The relationship between eruptive activity, flank collapse, and sea level
           at volcanic islands: A long‐term (>1 Ma) record offshore
           Montserrat, Lesser Antilles
    • Abstract: Hole U1395B, drilled southeast of Montserrat during Integrated Ocean Drilling Program Expedition 340, provides a long (>1 Ma) and detailed record of eruptive and mass‐wasting events (>130 discrete events). This record can be used to explore the temporal evolution in volcanic activity and landslides at an arc volcano. Analysis of tephra fall and volcaniclastic turbidite deposits in the drill cores reveals three heightened periods of volcanic activity on the island of Montserrat (∼930 ka to ∼900 ka, ∼810 ka to ∼760 ka, and ∼190 ka to ∼120 ka) that coincide with periods of increased volcano instability and mass‐wasting. The youngest of these periods marks the peak in activity at the Soufrière Hills volcano. The largest flank collapse of this volcano (∼130 ka) occurred towards the end of this period, and two younger landslides also occurred during a period of relatively elevated volcanism. These three landslides represent the only large (>0.3 km3) flank collapses of the Soufrière Hills edifice, and their timing also coincides with periods of rapid sea‐level rise (>5 m/ka). Available age data from other island arc volcanoes suggests a general correlation between the timing of large landslides and periods of rapid sea‐level rise, but this is not observed for volcanoes in intra‐plate ocean settings. We thus infer that rapid sea‐level rise may modulate the timing of collapse at island arc volcanoes, but not in larger ocean‐island settings. This article is protected by copyright. All rights reserved.
      PubDate: 2016-06-17T03:00:33.919227-05:
      DOI: 10.1002/2015GC006053
  • Magma plumbing for the 2014–2015 Holuhraun eruption, Iceland
    • Abstract: The 2014‐15 Holuhraun eruption on Iceland was located within the Askja fissure swarm, but was accompanied by caldera subsidence in the Bárðarbunga central volcano 45 km to the southwest. Geophysical monitoring of the eruption identified a seismic swarm that migrated from Bárðarbunga to the Holuhraun eruption site over the course of 2 weeks. In order to better understand this lateral connection between Bárðarbunga and Holuhraun, we present mineral textures and compositions, mineral‐melt‐equilibrium calculations, whole rock and trace element data, and oxygen‐isotope ratios for selected Holuhraun samples. The Holuhraun lavas are compositionally similar to recorded historical eruptions from the Bárðarbunga volcanic system, but are distinct from the historical eruption products of the nearby Askja system. Thermobarometry calculations indicate a polybaric magma plumbing system for the Holuhraun eruption, wherein clinopyroxene and plagioclase crystallized at average depths of ∼ 17 km and ∼ 5 km, respectively. Crystal resorption textures and oxygen isotope variations imply that this multi‐level plumbing system facilitated magma mixing and assimilation of low‐δ18O Icelandic crust prior to eruption. In conjunction with the existing geophysical evidence for lateral migration, our results support a model of initial vertical magma ascent within the Bárðarbunga plumbing system followed by lateral transport of aggregated magma batches within the upper crust to the Holuhraun eruption site. This article is protected by copyright. All rights reserved.
      PubDate: 2016-06-16T03:41:27.752798-05:
      DOI: 10.1002/2016GC006317
  • Quantifying melt production and degassing rate at mid‐ocean ridges
           from global mantle convection models with plate motion history
    • Authors: Mingming Li; Benjamin Black, Shijie Zhong, Michael Manga, Maxwell L. Rudolph, Peter Olson
      Abstract: The Earth's surface volcanism exerts first‐order controls on the composition of the atmosphere and the climate. On Earth, the majority of surface volcanism occurs at mid‐ocean ridges. In this study, based on the dependence of melt fraction on temperature, pressure and composition, we compute melt production and degassing rate at mid‐ocean ridges from three dimensional global mantle convection models with plate motion history as the surface velocity boundary condition. By incorporating melting in global mantle convection models, we connect deep mantle convection to surface volcanism, with deep and shallow mantle processes internally consistent. We compare two methods to compute melt production: a tracer method and an Eulerian method. Our results show that melt production at mid‐ocean ridges is mainly controlled by surface plate motion history, and that changes in plate tectonic motion, including plate reorganizations, may lead to significant deviation of melt production from the expected scaling with seafloor production rate. We also find a good correlation between melt production and degassing rate beneath mid‐ocean ridges. The calculated global melt production and CO2 degassing rate at mid‐ocean ridges varies by as much as a factor of 3 over the past 200 Myr. We show that mid‐ocean ridge melt production and degassing rate would be much larger in the Cretaceous, and reached maximum values at ∼150‐120 Ma. Our results raise the possibility that warmer climate in the Cretaceous could be due in part to high magmatic productivity and correspondingly high outgassing rates at mid‐ocean ridges during that time. This article is protected by copyright. All rights reserved.
      PubDate: 2016-06-16T03:41:15.946194-05:
      DOI: 10.1002/2016GC006439
  • Slab mantle dehydrates beneath Kamchatka: yet recycles water into the deep
    • Abstract: The subduction of hydrated slab mantle is the most important and yet weakly constrained factor in the quantification of the Earth's deep geologic water cycle. The most critical unknowns are the initial hydration state and the dehydration behavior of the subducted oceanic mantle. Here we present a combined thermomechanical, thermodynamic and geochemical model of the Kamchatka subduction zone that indicates significant dehydration of subducted slab mantle beneath Kamchatka. Evidence for the subduction of hydrated oceanic mantle comes from across‐arc trends of boron concentrations and isotopic compositions in arc volcanic rocks. Our thermodynamic‐geochemical models successfully predict the complex geochemical patterns and the spatial distribution of arc volcanoes in Kamchatka assuming the subduction of hydrated oceanic mantle. Our results show that water content and dehydration behavior of the slab mantle beneath Kamchatka can be directly linked to compositional features in arc volcanic rocks. Depending on hydration depth of the slab mantle, our models yield water recycling rates between 1.1 x 103 and 7.4 x 103 TgMa−1km−1 corresponding to values between 0.75 x 106 and 5.2 x 106 TgMa−1 for the entire Kamchatkan subduction zone. These values are up to one order of magnitude lower than previous estimates for Kamchatka, but clearly show that subducted hydrated slab mantle significantly contributes to the water budget in the Kamchatkan subduction zone. This article is protected by copyright. All rights reserved.
      PubDate: 2016-06-16T03:41:00.555718-05:
      DOI: 10.1002/2016GC006335
  • Asteroid bombardment and the core of Theia as possible sources for the
           Earth's late veneer component
    • Authors: Norman H. Sleep
      Abstract: The silicate Earth contains Pt‐group elements in roughly chondritic relative ratios, but with absolute concentrations
      PubDate: 2016-06-15T10:33:28.619465-05:
      DOI: 10.1002/2016GC006305
  • S‐to‐P heterogeneity ratio in the lower mantle and
           thermo‐chemical implications
    • Authors: Andrea Tesoniero; Fabio Cammarano, Lapo Boschi
      Abstract: We evaluate the thermo‐chemical state of the lower mantle by analysing the differences in the pattern of heterogeneity between shear and compressional velocity variations and the S‐to‐P heterogeneity ratio (RS/P = δlnVS/δlnVP) as mapped in our model SPani and in alternative joint models. Robust structural differences between VP and VS evidence the presence of compositional heterogeneity within the two Large Low Shear Velocity Provinces (LLSVPs). We find also an increasing decorrelation with depth that can be associated with compositional layering of the LLSVPs. In addition, our model shows heterogeneity in the transition zone and mid mantle by complex morphology of subducting slabs and further differences between VP and VS that point to an unexpected heterogeneous lower mantle. Precise estimates of compositional heterogeneities are not yet affordable because of the difficulty to provide quantitative measure of RS/P, making it difficult to use this ratio to evaluate chemical heterogeneity. For instance, RS/P global median value () drops from ∼2.8 to ∼1.9, at 2500km depth when the VP component of SPani is replaced by a VP model resulting from a differently regularized inversion and obtaining an equally good data fit. An increase of 20% of the SPani VP anomalies also drastically reduces without significantly degrading the data fit. Noise in model parameters also leads to overestimate RS/P in the two LLSVPs as we show with synthetic tests. Additional mineral physics uncertainties for compositional effects on RS/P and for the conversion of δlnVS and δlnVP into density further complicates a precise chemical interpretation. This article is protected by copyright. All rights reserved.
      PubDate: 2016-06-13T03:25:36.859967-05:
      DOI: 10.1002/2016GC006293
  • Overriding plate, mantle wedge, slab, and sub‐slab contributions to
           seismic anisotropy beneath the northern Central Andean Plateau
    • Authors: Maureen D. Long; C. Berk Biryol, Caroline M. Eakin, Susan L. Beck, Lara S. Wagner, George Zandt, Estella Minaya, Hernando Tavera
      Abstract: The Central Andean Plateau, the second‐highest plateau on Earth, overlies the subduction of the Nazca Plate beneath the central portion of South America. The origin of the high topography remains poorly understood, and this puzzle is intimately tied to unanswered questions about processes in the upper mantle, including possible removal of the overriding plate lithosphere and interaction with the flow field that results from the driving forces associated with subduction. Observations of seismic anisotropy can provide important constraints on mantle flow geometry in subduction systems. The interpretation of seismic anisotropy measurements in subduction settings can be challenging, however, because different parts of the subduction system may contribute, including the overriding plate, the mantle wedge above the slab, the slab itself, and the deep upper mantle beneath the slab. Here we present measurements of shear wave splitting for core phases (SKS, SKKS, PKS, and sSKS), local S, and source‐side teleseismic S phases that sample the upper mantle beneath southern Peru and northern Bolivia, relying mostly on data from the CAUGHT experiment. We find evidence for seismic anisotropy within most portions of the subduction system, although the overriding plate itself likely makes only a small contribution to the observed delay times. Average fast orientations generally trend roughly trench‐parallel to trench‐oblique, contradicting predictions from the simplest two‐dimensional flow models and olivine fabric scenarios. Our measurements suggest complex, layered anisotropy beneath the northern portion of the Central Andean Plateau, with significant departures from a two‐dimensional mantle flow regime. This article is protected by copyright. All rights reserved.
      PubDate: 2016-06-09T18:50:59.171753-05:
      DOI: 10.1002/2016GC006316
  • Improving the method of low‐temperature anisotropy of magnetic
           susceptibility (LT‐AMS) measurements in air
    • Authors: R. Issachar; T. Levi, V. Lyakhovsky, S. Marco, R. Weinberger
      Abstract: This study examines the limitations of the method of low‐temperature anisotropy of magnetic susceptibility (LT‐AMS) measurements in air and presents technical improvements that significantly reduce the instrumental drift and measurement errors. We analyzed the temperature profile of porous chalk core after cooling in liquid nitrogen and found that the average temperature of the sample during the LT‐AMS measurement in air is higher than 77K and close to 92K. This analysis indicates that the susceptibility of the paramagnetic minerals are amplified by a factor ∼3.2 relative to that of room temperature AMS (RT‐AMS). In addition, it was found that liquid nitrogen was absorbed in the samples during immersing and contributed diamagnetic component of ∼‐9 × 10−6 SI to the total mean susceptibility. We showed that silicone sheet placed around and at the bottom of the measuring coil is an effective thermal protection, preventing instrument drift by the cold sample. In this way, the measuring errors of LT‐AMS reduced to the level of RT‐AMS, allowing accurate comparison with standard AMS measurements. We examined the applicability of the LT‐AMS measurements on chalk samples that consist
      PubDate: 2016-06-08T10:45:27.662176-05:
      DOI: 10.1002/2016GC006339
  • On edge melting under the Colorado Plateau margin
    • Abstract: Asthenosphere beneath the relatively thin lithosphere of the Basin and Range province appears to be juxtaposed in step‐like fashion against the Colorado Plateau's thick lithospheric keel. Primary to near‐primary basalts are found above this edge, in the San Francisco‐Morman Mountain volcanic fields, north central Arizona, western USA. We show that at least two distinct peridotite‐dominated mantle end‐members contributed to the origin of the basalts. One has paired Nd and Hf isotopic characteristics that cluster near the mantle array and trace element patterns as expected for melts generated in the asthenosphere, possibly in the presence of garnet. The second has isotopic compositions displaced above the εHf ‐ εNd mantle array which, together with its trace element characteristics, indicate contributions from hydrogenous sediments and/or melt (carbonatite or silicate)‐related metasomatism. Melt equilibration temperatures obtained from Si‐ and Mg‐thermobarometry are mostly 1340‐1425°C and account for the effects of water (assumed to be 2 wt.%) and estimated CO2 (variable). Melt equilibration depths cluster at the inferred location of the lithosphere‐asthenosphere boundary at ∼70‐75 km beneath the southwestern margin of the Colorado Plateau but scatter to somewhat greater values (∼100 km). Melt generation may have initiated in or below the garnet‐spinel facies transition zone and continued as mantle and/or melts upwelled, assimilating and sometimes equilibrating with shallower contaminated mantle, until melts were finally extracted. This article is protected by copyright. All rights reserved.
      PubDate: 2016-06-06T10:20:37.942555-05:
      DOI: 10.1002/2016GC006349
  • Seismological structure of the 1.8 Ga Trans‐Hudson Orogen of North
    • Authors: Amy Gilligan; Ian D. Bastow, Fiona A. Darbyshire
      Abstract: Precambrian tectonic processes are debated: what was the nature and scale of orogenic events on the younger, hotter, and more ductile Earth? Northern Hudson Bay records the Paleoproterozoic collision between the Western Churchill and Superior plates ‐ the ∼1.8 Ga Trans‐Hudson Orogeny (THO) ‐ and is an ideal locality to study Precambrian tectonic structure. Integrated field, geochronological, and thermobarometric studies suggest that the THO was comparable to the present‐day Himalayan‐Karakoram‐Tibet Orogen (HKTO). However, detailed understanding of the deep crustal architecture of the THO, and how it compares to that of the evolving HKTO, is lacking. The joint inversion of receiver functions and surface wave data provides new Moho depth estimates and shear velocity models for the crust and uppermost mantle of the THO. Most of the Archean crust is relatively thin (∼39 km) and structurally simple, with a sharp Moho; upper crustal wavespeed variations are attributed to post‐formation events. However, the Quebec‐Baffin segment of the THO has a deeper Moho (∼45 km) and a more complex crustal structure. Observations show some similarity to recent models, computed using the same methods, of the HKTO crust. Based on Moho character, present‐day crustal thickness, and metamorphic grade, we support the view that southern Baffin Island experienced thickening during the THO of a similar magnitude and width to present‐day Tibet. Fast seismic velocities at >10 km below southern Baffin Island may be the result of partial eclogitization of the lower crust during the THO, as is currently thought to be happening in Tibet. This article is protected by copyright. All rights reserved.
      PubDate: 2016-06-03T03:30:37.237104-05:
      DOI: 10.1002/2016GC006419
  • Microstructural and geochemical constraints on the evolution of deep arc
    • Authors: Emily J. Chin; Vincent Soustelle, Greg Hirth, Alberto Saal, Seth C. Kruckenberg, John Eiler
      Abstract: Mantle xenoliths from the Sierra Nevada, California, USA sampled a vertical column (60 – 120 km) of lithosphere that formed during Mesozoic continental arc magmatism. This lithosphere experienced an anti‐clockwise P‐T‐t path resulting in rapid cooling that effectively “quenched in” features inherited from earlier high‐temperature conditions. Here, we combine new mineral chemistry data (water, trace element, and major element concentrations) with mineral crystallographic preferred orientations (CPO) to investigate the relationship between melt infiltration and deformation. The peridotites record a refertilization trend with increasing depth, starting from shallow, coarse‐protogranular, less‐melt infiltrated spinel peridotite with strong, orthorhombic olivine CPO to deep, fine‐porphyroclastic, highly melt‐infiltrated garnet peridotite with weak, axial‐[010] olivine CPO. In contrast to the observed axial‐[010] CPOs, subgrain boundary orientations and misorientation axes suggest the dominant activation of the [100](001) slip system, suggesting deformation under moderately hydrous conditions. After accounting for effects of subsolidus cooling, we see coherent trends between mineral trace element abundance and water content, indicating that melt infiltration led to an increase in water content of the peridotites. However, measured olivine and pyroxene water contents in all peridotites (5 – 10 wt ppm and 30 – 500 wt ppm, respectively) are lower than that required to promote dominant [100](001) slip system observed in both natural and experimental samples. These results suggest that deformation occurred earlier along the P‐T path, probably during or shortly after hydrous melt infiltration. Subsequent rapid cooling at 90 Ma led to water loss from olivine (owing to decreased solubility at low temperature), leaving behind a deep arc lithosphere that remained viscously coupled to the Farallon slab until the opening of the slab window in the Late Cenozoic. This article is protected by copyright. All rights reserved.
      PubDate: 2016-06-03T03:30:31.963135-05:
      DOI: 10.1002/2015GC006156
  • Helium as a tracer for fluids released from Juan de Fuca lithosphere
           beneath the Cascadia fore arc
    • Authors: P. A. McCrory; J. E. Constantz, A. G. Hunt, J. L. Blair
      Abstract: Helium isotopic ratios (3He/4He) observed in 25 mineral springs and wells above the Cascadia forearc provide a marker for fluids derived from Juan de Fuca lithosphere. This exploratory study documents a significant component of mantle‐derived helium within forearc springs and wells, and in turn, documents variability in helium enrichment across the Cascadia forearc. Sample sites arcward of the forearc mantle corner generally yield significantly higher ratios (∼1.2–4.2 RA) than those seaward of the corner (∼0.03–0.7 RA). 3He detected above the inner forearc mantle wedge may represent a mixture of both oceanic lithosphere and forearc mantle sources, whereas 3He detected seaward of the forearc mantle corner likely has only an oceanic source. The highest ratios in the Cascadia forearc coincide with slab depths (∼40–45 km) where metamorphic dehydration of young oceanic lithosphere is expected to release significant fluid and where tectonic tremor occurs, whereas little fluid is expected to be released from the slab depths (∼ 25–30 km) beneath sites seaward of the corner. These observations provide independent evidence that tremor is associated with deep fluids, and further suggest that high pore pressures associated with tremor may serve to keep fractures open for 3He migration through the ductile upper mantle and lower crust. This article is protected by copyright. All rights reserved.
      PubDate: 2016-06-02T10:32:42.921464-05:
      DOI: 10.1002/2015GC006198
  • Evolution of the northern Argentine margin during the Cenozoic controlled
           by bottom current dynamics and gravitational processes
    • Abstract: A detailed reflection seismic investigation on sediment deposition at the northern Argentine margin (37°S to 42°S) resolves major modifications in oceanographic circulation during the Cenozoic, which resulted from variations in both climatic and tectonic processes. After an extensive erosional period following the onset of glaciation of Antartica at ∼34 Ma, which affected all water depth levels, a buried elongated mounded drift within the continental shelf was shaped by bottom current activity during the Miocene. This may represent the earliest deposits of the Malvinas Current that branches from the Antartic Circumpolar Current and today is part of a complex shallow water circulation system known as the Brazil‐Malvinas confluence. At the same time a major terrace grew to its present form on the upper slope indicating that a precursor of Antarctic Intermediate Water was also part of the Brazil‐Malvinas confluence. After another major erosional phase inferred from a seismic unconformity at ∼6 Ma, sheeted drifts, mounded drifts and sediment waves formed at the continental rise during the Pliocene/Pleistocene. These extensive contourite deposits are diagnostic for a steady north setting bottom flow at the depth level of todays Antarctic Bottomwater. Evidence for downslope transport mainly stems from the presence of buried turbidites and canyon related depocenters. These features can be related to Andean uplift during the Eocene and to the activation of the canyon system during the Pliocene. This article is protected by copyright. All rights reserved.
      PubDate: 2016-06-02T10:32:39.945939-05:
      DOI: 10.1002/2015GC006232
  • Ridge‐spotting: A new test for Pacific absolute plate motion models
    • Abstract: Relative plate motions provide high‐resolution descriptions of motions of plates relative to other plates. Yet geodynamically, motions of plates relative to the mantle are required since such motions can be attributed to forces (e.g., slab pull, ridge push) acting upon the plates. Various reference frames have been proposed, such as the hotspot reference frame, to link plate motions to a mantle framework. Unfortunately, both accuracy and precision of absolute plate motion models lag behind those of relative plate motion models. Consequently, it is paramount to use relative plate motions in improving our understanding of absolute plate motions. A new technique called “ridge‐spotting” combines absolute and relative plate motions and examines the viability of proposed absolute plate motion models. We test the method on six published Pacific absolute plate motions models, including fixed and moving hotspot models as well as a geodynamically‐derived model. Ridge‐spotting reconstructs the Pacific‐Farallon and Pacific‐Antarctica ridge systems over the last 80 Myr. All six absolute plate motion models predict large amounts of northward migration and monotonic clockwise rotation for the Pacific‐Farallon ridge. A geodynamic implication of our ridge migration predictions is that the suggestion that the Pacific‐Farallon ridge may have been pinned by a large mantle upwelling is not supported. Unexpected or erratic ridge behaviors may be tied to limitations in the models themselves or (for Indo‐Atlantic models) discrepancies in the plate circuits used to project models into the Pacific realm. Ridge‐spotting is promising and will be extended to include more plates and other ocean basins. This article is protected by copyright. All rights reserved.
      PubDate: 2016-05-31T10:37:42.832924-05:
      DOI: 10.1002/2016GC006404
  • Increased rates of large magnitude explosive eruptions in Japan in the
           Late Neogene and Quaternary
    • Authors: S.H. Mahony; R.S.J. Sparks, L.M. Wallace, S.L. Engwell, E.M. Scourse, N.H. Barnard, J. Kandlbauer, S.K. Brown
      Abstract: Tephra layers in marine sediment cores from scientific ocean drilling largely record high‐magnitude silicic explosive eruptions in the Japan arc for up to the last 20 million years. Analysis of the thickness variation with distance of 180 tephra layers from a global dataset suggests that the majority of the visible tephra layers used in this study are the products of caldera‐forming eruptions with magnitude (M) >6, considering their distances at the respective drilling sites to their likely volcanic sources. Frequency of visible tephra layers in cores indicates a marked increase in rates of large magnitude explosive eruptions at ∼8 Ma, 6–4 Ma and further increase after ∼2 Ma. These changes are attributed to major changes in tectonic plate interactions. Lower rates of large magnitude explosive volcanism in the Miocene are related to a strike‐slip dominated boundary (and temporary cessation or deceleration of subduction) between the Philippine Sea Plate and southwest Japan, combined with the possibility that much of the arc in northern Japan was submerged beneath sea level partly due to previous tectonic extension of Northern Honshu related to formation of the Sea of Japan. Changes in plate motions and subduction dynamics during the ∼8 Ma to present period led to (1) increased arc‐normal subduction in southwest Japan (and resumption of arc volcanism) and (2) shift from extension to compression of the upper plate in northeast Japan, leading to uplift, crustal thickening and favourable conditions for accumulation of the large volumes of silicic magma needed for explosive caldera‐forming eruptions. This article is protected by copyright. All rights reserved.
      PubDate: 2016-05-31T10:37:06.888646-05:
      DOI: 10.1002/2016GC006362
  • Late Cretaceous intraplate silicic volcanic rocks from the Lake Chad
           region: An extension of the Cameroon volcanic line
    • Abstract: Silicic volcanic rocks at Hadjer el Khamis, near Lake Chad, are considered to be an extension of the Cameroon volcanic line (CVL) but their petrogenetic association is uncertain. The silicic rocks are divided into peraluminous and peralkaline groups with both rock types chemically similar to within‐plate granitoids. In situ U/Pb zircon dating yielded a mean 206Pb/238U age of 74.4 ± 1.3 Ma indicating the magmas erupted ∼10 million years before the next oldest CVL rocks (i.e. ∼66 Ma). The Sr isotopes (i.e. ISr = 0.7021 to 0.7037) show a relatively wide range but the Nd isotopes (i.e. 143Nd/144Ndi = 0.51268 to 0.51271) are uniform and indicate that the rocks were derived from a moderately depleted mantle source. Thermodynamic modeling show that the silicic rocks likely formed by fractional crystallization of a mafic parental magma but that the peraluminous rocks were affected by low temperature alteration processes. The silicic rocks are isotopically similar to Late Cretaceous basalts identified within the Late Cretaceous basins (i.e. 143Nd/144Ndi = 0.51245 to 0.51285) of Chad than the uncontaminated CVL rocks (i.e. 143Nd/144Ndi = 0.51270 to 0.51300). The age and isotopic compositions suggest the silicic volcanic rocks of the Lake Chad region are related to Late Cretaceous extensional volcanism in the Termit basin. It is unlikely that the silicic volcanic rocks are petrogenetically related to the CVL but it is possible that, in both cases, magmatism was structurally controlled by suture zones that formed during the opening of the Central Atlantic Ocean and/or the Pan‐African Orogeny. This article is protected by copyright. All rights reserved.
      PubDate: 2016-05-31T05:15:27.874702-05:
      DOI: 10.1002/2016GC006298
  • Influence of temperature, pressure, and oxygen fugacity on the electrical
           conductivity of dry eclogite, and geophysical implications
    • Authors: Lidong Dai; Haiying Hu, Heping Li, Lei Wu, Keshi Hui, Jianjun Jiang, Wenqing Sun
      Abstract: The electrical conductivity of eclogite was measured at temperatures of 873–1173 K and pressures of 1.0–3.0 GPa within a frequency range of 0.1–106 Hz using a YJ‐3000t multianvil press and Solartron‐1260 impedance/gain‐phase analyzer. Three solid‐state oxygen buffers (Cu + CuO, Ni + NiO, and Mo + MoO2) were employed to control the oxygen fugacity. Experimental results indicate that the electrical conductivity of the samples tended to increase with increasing temperature, conforming to an Arrhenius relation. Under the control of a Cu + CuO oxygen buffer, the electrical conductivity of the eclogite decreased with a rise in pressure, and its corresponding activation volume and activation energy at atmospheric pressure were calculated as −2.51 ± 0.29 cm3/mole and 0.86 ± 0.12 eV, respectively. At 2.0 GPa, the electrical conductivity of the eclogite increased with increasing oxygen fugacity, and the pre‐exponential factor increased while the activation enthalpy decreased. The observed positive exponential factor for the dependence of electrical conductivity on oxygen fugacity, as well as the negative activation volume, confirm that the hopping of small polarons is the dominant conduction mechanism in eclogite at high temperatures and pressures. Our results suggest that the electrical conductivity of dry eclogite under various redox conditions cannot explain the high anomalies in conductivity under stable mid–lower continental crust and under the Dabie–Sulu ultrahigh‐pressure metamorphic belt of eastern China. This article is protected by copyright. All rights reserved.
      PubDate: 2016-05-30T03:41:42.900612-05:
      DOI: 10.1002/2016GC006282
  • Achieving comparable uncertainty estimates with Kalman filters or linear
           smoothers for bathymetry data
    • Authors: Brian S. Bourgeois; Paul A. Elmore, William E. Avera, Samantha J. Zambo
      Abstract: This paper examines and contrasts two estimation methods, Kalman filtering and linear smoothing, for creating interpolated data products from bathymetry measurements. Using targeted examples, we demonstrate previously obscured behavior showing the dependence of linear smoothers on the spatial arrangement of the measurements, yielding markedly different estimation results that the Kalman filter. For bathymetry data, we have modified the variance estimates from both the Kalman filter and linear smoothers to obtain comparable estimators for dense data. These comparable estimators produce uncertainty estimates that are statistically insignificant via hypothesis testing. Achieving comparable estimation is accomplished by applying the ‘propagated uncertainty' concept and a numerical realization of Tobler's principle to the measurement data prior to the computation of the estimate. We show new mathematical derivations for these modifications. In addition, we show test results with a) synthetic data and b) gridded bathymetry in the area of the Scripps and La Jolla Canyons. Our 10‐fold cross‐validation for case b) show that the modified equations create comparable uncertainty for both gridding algorithms with null hypothesis acceptance rates of greater than 99.95% of the data points. In contrast, bilinear interpolation has ten times the amount of rejection. We then discuss how the uncertainty estimators are, in principle, applicable to interpolate geophysical data other than bathymetry. This article is protected by copyright. All rights reserved.
      PubDate: 2016-05-30T03:30:34.172602-05:
      DOI: 10.1002/2015GC006239
  • Magmatic and tectonic extension at the Chile Ridge: Evidence for mantle
           controls on ridge segmentation
    • Abstract: We use data from an extensive multibeam bathymetry survey of the Chile Ridge to study tectonomagmatic processes at the ridge axis. Specifically, we investigate how abyssal hills evolve from axial faults, how variations in magmatic extension influence morphology and faulting along the spreading axis, and how these variations correlate with ridge segmentation. The bathymetry data are used to estimate the fraction of plate separation accommodated by normal faulting, and the remaining fraction of extension, M, is attributed primarily to magmatic accretion. Results show that M ranges from 0.85 to 0.96, systematically increasing from first‐ and second‐order ridge segment offsets towards segment centers as the depth of ridge axis shoals relative to the flanking highs of the axial valley. Fault spacing, however, does not correlate with ridge geometry, morphology, or M along the Chile Ridge, which suggests the observed increase in tectonic strain toward segment ends is achieved through increased slip on approximately equally spaced faults. Variations in M along the segments follow variations in petrologic indicators of mantle melt fraction, both showing a preferred length scale of 50+/‐20 km that persists even along much longer ridge segments. In comparison, mean M and axial relief fail to show significant correlations with distance offsetting the segments. These two findings suggest a form of magmatic segmentation that is partially decoupled from the geometry of the plate boundary. We hypothesize this magmatic segmentation arises from cells of buoyantly upwelling mantle that influence tectonic segmentation from the mantle, up. This article is protected by copyright. All rights reserved.
      PubDate: 2016-05-27T18:50:35.031417-05:
      DOI: 10.1002/2016GC006380
  • Shock‐induced deformation phenomena in magnetite and their
           consequences on magnetic properties
    • Abstract: This study investigates the effects of shock waves on magnetic and microstructural behavior of multidomain magnetite from a magnetite‐bearing ore, experimentally shocked to pressures of 5, 10, 20 and 30 GPa. Changes in apparent crystallite size and lattice parameter were determined by X‐ray diffraction, and grain fragmentation and defect accumulation were studied by scanning and transmission electron microscopy. Magnetic properties were characterized by low‐temperature saturation isothermal remanent magnetization (SIRM), susceptibility measurements around the Verwey transition as well as by hysteresis parameters at room temperature. It is established, that the shock‐induced refinement of magnetic domains from MD to SD‐PSD range is a result of cooperative processes including brittle fragmentation of magnetite grains, plastic deformation with shear bands and twins as well as structural disordering in form of molten grains and amorphous nano‐clusters. Up to 10 GPa, a decrease of coherent crystallite size, lattice parameter, saturation magnetization and magnetic susceptibility, and an increase in coercivity, SIRM, width of Verwey transition is mostly associated with brittle grain fragmentation. Starting from 20 GPa, a slight recovery is documented in all magnetic and non‐magnetic parameters. In particular, the recovery in SIRM is correlated with an increase of the lattice constant. The recovery effect is associated with the increasing influence of shock heating/annealing at high shock pressures. The strong decrease of magnetic susceptibility at 30 GPa is interpreted as a result of strong lattice damage and distortion. Our results unravel the microstructural mechanisms behind the loss of magnetization and the modification of magnetic properties of magnetite and contribute to our understanding of shock‐induced magnetic phenomena in impacted rocks on earth and in meteorites. This article is protected by copyright. All rights reserved.
      PubDate: 2016-05-27T18:50:28.915998-05:
      DOI: 10.1002/2016GC006338
  • South Atlantic intermediate water advances into the North‐east
           Atlantic with reduced Atlantic meridional overturning circulation during
           the last glacial period
    • Abstract: The Nd isotopic composition (εNd) of seawater and cold‐water coral (CWC) samples from the Gulf of Cádiz and the Alboran Sea, at a depth of 280‐827 m were investigated in order to constrain mid‐depth water mass dynamics within the Gulf of Cádiz over the past 40 ka. εNd of glacial and Holocene CWC from the Alboran Sea and the northern Gulf of Cádiz reveals relatively constant values (‐8.6 to ‐9.0 and ‐9.5 to ‐10.4, respectively). Such values are similar to those of the surrounding present‐day mid‐depth waters from the Mediterranean Outflow Water (MOW; εNd ∼ ‐9.4) and Mediterranean Sea Water (MSW; εNd ∼ ‐9.9). In contrast, glacial εNd values for CWC collected at thermocline depth (550‐827 m) in the southern Gulf of Cádiz display a higher average value (‐8.9±0.4) compared to the present‐day value (‐11.7±0.3). This implies a higher relative contribution of water masses of Mediterranean (MSW) or South Atlantic origin (East Antarctic Intermediate Water, EAAIW). Our study has produced the first evidence of significant radiogenic εNd values (∼ ‐8) at 19, 23‐24 and 27 ka, which are coeval with increasing iceberg discharges and a weakening of Atlantic Meridional Overturning Circulation (AMOC). Since MOW εNd values remained stable during the last glacial period, it is suggested that these radiogenic εNd values most likely reflect an enhanced northward propagation of glacial EAAIW into the eastern Atlantic Basin. This article is protected by copyright. All rights reserved.
      PubDate: 2016-05-23T03:40:47.223456-05:
      DOI: 10.1002/2016GC006281
  • Crustal anisotropy from tectonic tremor in Guerrero, Mexico
    • Abstract: We present new shear wave anisotropy measurements in the continental crust in northern Guerrero obtained from tectonic tremor. Measurements of crustal anisotropy had not been performed in this area due to the lack of seismicity. However, tectonic tremor activity is abundant and offers an opportunity to determine anisotropy parameters. Polarization and splitting analyses were performed using broadband three‐component seismograms. Results show that splitting times range between 0.07 and 0.36 s. These values are similar to the splitting magnitudes typically observed in the continental crust. The state of stress in the continental crust was investigated by inverting focal mechanisms determined in this study, and also from previous structural geology studies. Unfortunately, no stress measurements were possible in the area where tectonic tremor occurs. It was determined that, to the south of the study area, near the Pacific coast, and to the north, in the volcanic arc, the maximum compressive stress shows a general E‐W trend. The fast polarization directions are oriented NE‐SW and are oblique to the observed maximum compressive stress surrounding the study area. Thus, the relationship between the maximum compressive stress and the observed anisotropic pattern cannot be conclusively established. Several factors such as non‐linear strain in the continental crust as a result of Slow Slip Events, variations of pore fluid pressure, deep crustal mineralogy, and/or upper crust foliations and schistosity could be inducing the observed anisotropy pattern. In general, the fast axes tend to parallel the regional Laramide and Tertiary folds‐and‐thrusts which strike NNE‐SSW. This system of folds‐and‐thrusts is highly foliated in low‐grade schist and seems likely to control the anisotropic structure observed within the tectonic tremor region in Guerrero. This article is protected by copyright. All rights reserved.
      PubDate: 2016-05-23T03:40:26.76716-05:0
      DOI: 10.1002/2016GC006358
  • Melt‐rock interactions and fabric development of peridotites from
           North Pond in the Kane area, mid‐Atlantic ridge: Implications of
           microstructural and petrological analyses of peridotite samples from IODP
           Hole U1382A
    • Abstract: North Pond is an isolated sedimentary pond on the western flank of the Kane area along the Mid‐Atlantic Ridge. Drill‐hole U1382A of IODP Expedition 336 recovered peridotite and gabbro samples from a sedimentary breccia layer in the pond, from which we collected six fresh peridotite samples. The peridotite samples came from the southern slope of the North Pond where an oceanic core complex is currently exposed. The samples were classified as spinel harzburgite, plagioclase‐bearing harzburgite, and a vein‐bearing peridotite that contains tiny gabbroic veins. No obvious macroscopic shear deformation related to the formation of a detachment fault was observed. The spinel harzburgite with a protogranular texture was classified as refractory peridotite. The degree of partial melting of the spinel harzburgite is estimated to be ∼17%, and melt depletion would have occurred at high temperatures in the uppermost mantle beneath the spreading axis. The progressive melt–rock interactions between the depleted spinel harzburgite and the percolating melts of Normal‐Mid Ocean Ridge Basalt (N‐MORB) produced the plagioclase‐bearing harzburgite and the vein‐bearing peridotite at relatively low temperatures. This implies that the subsequent refertilization occurred in an extinct spreading segment of the North Pond after spreading at the axis. Olivine fabrics in the spinel and plagioclase‐bearing harzburgites are of types AG, A, and D, suggesting the remnants of a mantle flow regime beneath the spreading axis. The initial olivine fabrics appear to have been preserved despite the later melt–rock interactions. The peridotite samples noted above preserve evidence of mantle flow and melt–rock interactions beneath a spreading ridge that formed at ∼8 Ma. This article is protected by copyright. All rights reserved.
      PubDate: 2016-05-21T02:50:39.505736-05:
      DOI: 10.1002/2016GC006429
  • Increased precipitation and weathering across the Paleocene‐Eocene
           thermal maximum in central China
    • Authors: Zuoling Chen; Zhongli Ding, Shiling Yang, Chunxia Zhang, Xu Wang
      Abstract: Global warming during the Paleocene‐Eocene Thermal Maximum (PETM) ∼55.5 million years ago (Ma) was associated with a massive release of carbon to the ocean‐atmosphere system, as evidenced by a prominent negative carbon isotope excursion (CIE) and widespread dissolution of marine carbonates. The paleohydrologic response to the PETM warming has been studied worldwide; however, relevant records of environmental perturbation in Asia are lacking so far. Here we extend the record of this event in central China, a subtropical paleosetting, through geochemical and mineralogical analyses of lacustrine sediments. Geochemical indicators of authigenic carbonates – including molar Mg/Ca and Sr/Ca ratios – suggest an overall increased precipitation across the PETM, compatible with the disappearance of authigenic dolomite and the appearance of kaolinite in the strata. The relatively humid conditions persisted long after the carbon‐cycle perturbation had stopped, implying that the transient hyper‐greenhouse warming might have forced the regional climate system into a new climate state that was not easily reversed. Additionally, a gradual increase in chemical index of alteration (CIA) and the appearance of kaolinite are associated with the PETM, indicating an intensified silicate weathering and pedogenesis in the watershed in response to warmer and more humid climate. Our results corroborate the theory that an accelerated continental chemical weathering served as a negative feedback to sequester carbon and lower the atmospheric greenhouse‐gas levels during the PETM. This article is protected by copyright. All rights reserved.
      PubDate: 2016-05-21T02:45:35.564379-05:
      DOI: 10.1002/2016GC006333
  • A mass balance and isostasy model: Exploring the interplay between
           magmatism, deformation, and surface erosion in continental arcs using
           central Sierra Nevada as a case study
    • Authors: Wenrong Cao; Scott Paterson
      Abstract: A one‐dimensional mass balance and isostasy model is used to explore the feedbacks between magmatism, deformation and surface erosion and how they together affect crustal thickness, elevation, and exhumation in a continental arc. The model is applied to central Sierra Nevada in California by parameterizing magma volume and deformational strain. The simulations capture the first‐order Mesozoic‐Cenozoic histories of crustal thickness, elevation and erosion including moderate Triassic crustal thickening and Jurassic crustal thinning followed by a strong Cretaceous crustal thickening, the latter resulting in a 60‐70‐km‐thick crust plus a 20‐km‐thick arc eclogitic root, and a ∼5 km elevation in the Late Cretaceous. The contribution of contractional deformation to the crustal thickening is twice that of the magmatism. The contribution to elevation from magmatism is dampened by the formation of an eclogitic root. Erosion rate increases with the magnitude of crustal thickening (by magmatism and deformation) but its peak rate always lags behind the peak rate of thickening. We propose that thickened crust initially promotes magma generation by downward transport of materials to the magma source region, which may eventually jam the mantle wedge affecting the retro‐arc underthrusting process and reducing arc magmatism. This article is protected by copyright. All rights reserved.
      PubDate: 2016-05-18T11:22:34.751684-05:
      DOI: 10.1002/2015GC006229
  • The coupled 182W‐142Nd record of early terrestrial mantle
    • Abstract: New Sm‐Nd, Lu‐Hf, Hf‐W, and Re‐Os isotope data, in combination with highly siderophile element (HSE, including Re, Os, Ir, Ru, Pt, Pd) and W abundances, are reported for the 3.55 Ga Schapenburg komatiites, South Africa. The Schapenburg komatiites define a Re‐Os isochron with an age of 3550±87 Ma and initial γ187Os = +3.7±0.2 (2SD). The absolute HSE abundances in the mantle source of the Schapenburg komatiite system are estimated to be only 29±5% of those in the present‐day bulk silicate Earth (BSE). The komatiites were derived from mantle enriched in the decay products of the long‐lived 147Sm and 176Lu nuclides (initial ε143Nd = +2.4±0.1, ε176Hf = +5.7±0.3, 2SD). By contrast, the komatiites are depleted, relative to the modern mantle, in 142Nd and 182W (μ182W = ‐8.4±4.5, μ142Nd = ‐4.9±2.8, 2SD). These results constitute the first observation in terrestrial rocks of coupled depletions in 142Nd and 182W. Such isotopic depletions require derivation of the komatiites from a mantle domain that formed within the first ∼30 Ma of Solar System history and was initially geochemically enriched in highly incompatible trace elements as a result of crystal‐liquid fractionation in an early magma ocean. This mantle domain further must have experienced subsequent melt depletion, after 182Hf had gone extinct, to account for the observed initial excesses in 143Nd and 176Hf. The survival of early‐formed 182W and 142Nd anomalies in the mantle until at least 3.55 Ga indicates that the products of early planetary differentiation survived both later planetary accretion and convective mantle mixing during the Hadean. This work moreover renders unlikely that variable late accretion, by itself, can account for all of the observed W isotope variations in Archean rocks. This article is protected by copyright. All rights reserved.
      PubDate: 2016-05-18T11:22:02.99476-05:0
      DOI: 10.1002/2016GC006324
  • Velocity structure near IODP Hole U1309D, Atlantis Massif from waveform
           inversion of streamer data and borehole measurements
    • Authors: Alistair J. Harding; Adrien F. Arnulf, Donna K. Blackman
      Abstract: Seismic full waveform inversion (FWI) is a promising method for determining the detailed velocity structure of the igneous oceanic crust, especially for locations such as the Mid‐Atlantic Ridge with significant lateral heterogeneity and seafloor topography. We examine the accuracy of FWI by inverting, after downward continuation to datum just above the seafloor, a multi‐channel seismic (MCS) profile from Atlantis Massif oceanic core complex at 30° N that passes close to Integrated Ocean Drilling Program (IODP) Hole U1309D and comparing the results against borehole measurements and existing on‐bottom refraction data. The comparisons include the results of IODP Expedition 340T, which extended the sonic logging and vertical seismic profiling to the bottom of the borehole at 1400 m below seafloor. Compared to travel time tomography, the refinement in velocity and velocity gradient produced by FWI significantly improves the overall match to the borehole measurements, and allows the multilevel pattern of deformation and alteration of the detachment footwall seen in Hole U1309D to be extrapolated across the Central Dome. Prestack depth migration of the profile using the FWI velocities reveals the top and edges of the high‐velocity, gabbroic core of the massif. It also indicates that the comparatively uniform gabbroic rocks drilled at Hole U1309D extend to ∼2.5 km below seafloor but overlie an extended, ∼2 km thick, mantle transition zone. This article is protected by copyright. All rights reserved.
      PubDate: 2016-05-11T19:11:05.875862-05:
      DOI: 10.1002/2016GC006312
  • Crustal structure and extension mode in the northwestern margin of the
           South China Sea
    • Authors: Jinwei Gao; Shiguo Wu, Kirk McIntosh, Lijun Mi, Zheng Liu, George Spence
      Abstract: Combining multi‐channel seismic reflection and gravity modeling, this study has investigated the crustal structure of the northwestern South China Sea margin. These data constrain a hyper‐extended crustal area bounded by basin‐bounding faults corresponding to an aborted rift below the Xisha Trough with a sub‐parallel fossil ridge in the adjacent Northwest Sub‐basin. The thinnest crust is located in the Xisha Trough, where it is remnant lower crust with a thickness of less than 3 km. Gravity modeling also revealed a hyper‐extended crust across the Xisha Trough. The post‐rift magmatism is well developed and more active in the Xisha Trough and farther southeast than on the northwestern continental margin of the South China Sea; and the magmatic intrusion/extrusion was relatively active during the rifting of Xisha Trough and the Northwest Sub‐basin. A narrow continent‐ocean transition zone with a width of ∼ 65 km bounded seawards by a volcanic buried seamount is characterized by crustal thinning, rift depression, low gravity anomaly and the termination of the break‐up unconformity seismic reflection. The aborted rift near the continental margin means that there may be no obvious detachment fault like that in the Iberia‐Newfoundland type margin. The symmetric rift, extreme hyper‐extended continental crust and hotter mantle materials indicate that continental crust underwent stretching phase (pure‐shear deformation), thinning phase and breakup followed by onset of seafloor spreading and the mantle‐lithosphere may break up before crustal‐necking in the northwestern South China Sea margin. This article is protected by copyright. All rights reserved.
      PubDate: 2016-05-11T19:10:52.081787-05:
      DOI: 10.1002/2016GC006247
  • Satellite‐derived surface temperature and in situ measurement at
           Solfatara of Pozzuoli (Naples, Italy)
    • Authors: M. Silvestri; C. Cardellini, G. Chiodini, M.F. Buongiorno
      Abstract: Ground thermal anomalies in volcanic‐hydrothermal systems, where the outflow of hot fluids gives rise to fumarolic fields, soil degassing and hot soils, have, up to now, rarely been investigated by using satellite. Here we report a comparison between surface temperature derived by satellite data and a large dataset of measured soil temperatures and CO2 fluxes for a volcanic‐hydrothermal system, the Solfatara of Pozzuoli (Campi Flegrei, Italy). Surface temperatures derived from ASTER (Advanced Spaceborne Thermal Emission and Reflection Radiometer) data are compared with soil temperatures and CO2 fluxes from four surveys performed in 2003, 2010 and in 2014. The good match between the spatial distributions of computed and measured temperatures suggests the adequacy of satellite data to describe the Solfatara thermal anomaly, while the correspondence between temperatures and CO2 fluxes, evidences the link between degassing and heating processes. The ASTER derived surface temperatures (14 ‐ 37°C) are coherent with those measured in the soil (10 ‐ 97°C at 10 cm depth), considering the effect of the thermal gradients which characterize the degassing area of Solfatara. This study shows that satellite data can be a very powerful tool with which to study surface thermal anomalies, and can provide a supplementary tool to monitor thermal evolution of restless volcanoes. This article is protected by copyright. All rights reserved.
      PubDate: 2016-05-11T19:10:42.620696-05:
      DOI: 10.1002/2015GC006195
  • Anisotropic tomography of the European lithospheric structure from surface
           wave studies
    • Abstract: We present continental‐scale seismic isotropic and anisotropic imaging of shear wave upper‐mantle structure of tectonically diversified terranes creating the European continent. Taking into account the 36‐200 s period range of surface waves enables us to model the deep subcontinental structure at different vertical scale‐lengths down to 300 km. After very strict quality selection criteria, we have obtained phase wavespeeds at different periods for fundamental Rayleigh and Love modes from about 9000 three‐component seismograms. Dispersion measurements are performed by using Fourier‐domain waveform inversion technique named 'roller‐coaster‐type' algorithm. We used the reference model with a varying average crustal structure for each source‐station path. That procedure led to significant improvement of the quality and number of phase wavespeed dispersion measurements compared to the common approach of using a reference model with one average crustal structure. Surface wave dispersion data are inverted at depth for retrieving isotropy and anisotropy parameters. The fast axis directions related to azimuthal anisotropy at different depths constitute a rich database for geodynamical interpretations. Shear wave anomalies of the horizontal dimension larger than 200 km are imaged in our models. They correlate with tectonic provinces of varying age‐provenance. Different anisotropy patterns are observed along the most distinctive feature on our maps ‐ the bordering zone between the Palaeozoic and Precambrian Europe. We discuss the depth changes of the lithosphere‐asthenosphere boundary along the profiles crossing the chosen tectonic units of different origin and age: Fennoscandia, East European Craton, Anatolia, Mediterranean subduction zones. Within the flat and stable cratonic lithosphere, we find traces of the mid‐lithospheric discontinuity. This article is protected by copyright. All rights reserved.
      PubDate: 2016-05-11T19:10:30.155427-05:
      DOI: 10.1002/2015GC006243
  • Heat flow along the Costa Rica seismogenesis project drilling transect:
           Implications for hydrothermal and seismic processes
    • Authors: Bridget Hass; Robert N. Harris
      Abstract: Heat flow analysis of the Costa Rica convergent margin is carried out for seven sites drilled during Integrated Ocean Drilling Program (IODP) Expeditions 334 and 344 as part of the Costa Rica Seismogenesis Project (CRISP). These expeditions are designed to better understand erosional subduction zones. Heat flow measurements were made to improve estimates of the thermal structure of this erosive margin and are located on the incoming plate, toe, lower, middle, and upper slopes of the margin. Heat flow values corrected for the effects of seafloor bathymetry and sedimentation are on average 15% higher than uncorrected values and range from approximately 158 to 200 mW/m2 on the incoming plate to values of approximately 50 mW/m2 on the middle and upper slopes of the margin. These values are consistent with previous estimates of heat flow showing a landward decrease in heat flow consistent with the subduction the Cocos plate. Preferred thermal models of the shallow subduction zone successfully predicting observed values of heat flow incorporate fluid flow within the upper oceanic aquifer have an uppermost permeability of 10−9.5 m2 and a plate boundary effective coefficient of friction of 0.06. These models suggest that temperatures on the subduction thrust reach 100° C at distances between 30 and 35 km landward of the deformation front. The updip limit of seismicity, as defined by aftershocks events of ML 1‐4 recorded following the Mw 6.9 Quepos earthquake, occurs at 25 km landward of the deformation front at temperatures cooler than the 100‐150°C typically predicted. This article is protected by copyright. All rights reserved.
      PubDate: 2016-05-10T04:00:25.768933-05:
      DOI: 10.1002/2016GC006314
  • Appreciation of peer reviewers for 2015
    • 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 252 manuscripts out of 472 submissions in 2015, and for this we were able to rely on the efforts of 712 dedicated reviewers. Their names are listed below, in italics those 41 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: 2016-05-10T03:51:18.427814-05:
      DOI: 10.1002/2016GC006426
  • Shear wave splitting analyses in Tian Shan: Geodynamic implications of
           complex seismic anisotropy
    • Authors: Solomon G. Cherie; Stephen S. Gao, Kelly H. Liu, Ahmed A. Elsheikh, Fansheng Kong, Cory A. Reed, Bin B. Yang
      Abstract: The Tian Shan is a tectonically complex intracontinental orogenic belt situated between the Tarim Basin and the Kazakh Shield. The vast majority of the previous SWS measurements were presented as station averages, which are only valid when the anisotropy structure can be approximated by a single layer of anisotropy with a horizontal axis of symmetry, i.e., a model of simple anisotropy. A variety of anisotropy‐forming hypotheses have been proposed based on the station‐averaged measurements. In this study, we measure the splitting parameters at 25 stations that recorded high‐quality data from a wide back‐azimuthal range for the purpose of identifying and characterizing complex anisotropy. Among the 25 stations, 15 of them show systematic azimuthal variations in the observed splitting parameters with a 90° periodicity that is consistent with a model of 2‐layered anisotropy. The fast orientations of the upper layer range from 50° to 90° measured clockwise from the north, which are subparallel to the strike of the orogenic belt, and the splitting times are between 0.9‐1.9 s. The corresponding values for the lower layer are ‐45° to ‐85° and 1.2‐2.2 s, respectively. The remaining 10 stations demonstrate azimuthally invariant splitting parameters with strike‐parallel fast orientations, and can be represented by a single layer of anisotropy with a horizontal axis of symmetry. We propose that the strike‐parallel anisotropy is caused by lithospheric shortening, and anisotropy in the lower layer is associated with WNW‐ward flow of asthenospheric material sandwiched between the subducting Tarim lithosphere and the thick Kazakh lithospheric root. This article is protected by copyright. All rights reserved.
      PubDate: 2016-05-10T03:50:33.100491-05:
      DOI: 10.1002/2016GC006269
  • Volatile element isotopes of submarine hydrothermal mineral deposits in
           the western Pacific
    • Abstract: The abundance and isotopic compositions of volatile elements trapped in fluid inclusions of submarine hydrothermal mineral deposits in Western Pacific subduction zones (Okinawa Trough, Izu‐Bonin arc, Mariana Trough, and Lau Basin) and in Kuroko ores in northeastern Japan are presented. The helium isotopic compositions corrected for air contribution of the Okinawa and Mariana troughs, ranging 4.49Ra‐7.68Ra are lower than those of the Izu‐Bonin and Lau Basin, 7.62Ra‐8.91 Ra. This characteristic might reflect the differences in regional tectonic setting. The Okinawa and Mariana troughs are related to back‐arc spreading with strong graben sedimentary signature, whereas the Izu‐Bonin arc is associated with island arc magmatism. The arc contribution to the Lau Basin volcanism is significantly strong, even though it is assigned to back‐arc spreading. Nitrogen isotopes can also be explained by a similar hypothesis, whereas argon and carbon isotopes cannot be used to discriminate tectonic setting. δ13C–CO2/3He and δ15N–N2/36Ar diagrams elucidate the source of carbon and nitrogen. The MOR‐type mantle contributions to carbon are mostly smaller in the Okinawa and Mariana troughs (ranging 0.06‐8.9% with the average of 2.4%) than in the Izu‐Bonin and Lau Basin (2.1‐25% with the average of 7.7%). The sedimentary contributions to nitrogen are larger in the Okinawa and Mariana troughs (11‐65% with the average of 35%) than in the Izu‐Bonin and Lau Basin (4‐24% with the average of 15%), and the Kuroko samples agree well with the latter. Carbon and nitrogen fluxes are again higher in Okinawa trough than in Izu‐Bonin arc. This article is protected by copyright. All rights reserved.
      PubDate: 2016-05-10T03:50:31.56083-05:0
      DOI: 10.1002/2016GC006360
  • Contrasting sediment melt and fluid signatures for magma components in the
           Aeolian Arc: Implications for numerical modeling of subduction systems
    • Authors: Denis Zamboni; Esteban Gazel, Jeffrey G. Ryan, Claudia Cannatelli, Federico Lucchi, Zachary D. Atlas, Jarek Trela, Sarah E. Mazza, Benedetto De Vivo
      Abstract: The complex geodynamic evolution of Aeolian Arc in the southern Tyrrhenian Sea resulted in melts with some of the most pronounced along the arc geochemical variation in incompatible trace elements and radiogenic isotopes worldwide, likely reflecting variations in arc magma source components. Here we elucidate the effects of subducted components on magma sources along different sections of the Aeolian Arc by evaluating systematics of elements depleted in the upper mantle but enriched in the subducting slab, focusing on a new set of B, Be, As, and Li measurements. Based on our new results, we suggest that both hydrous fluids and silicate melts were involved in element transport from the subducting slab to the mantle wedge. Hydrous fluids strongly influence the chemical composition of lavas in the central arc (Salina) while a melt component from subducted sediments probably plays a key role in metasomatic reactions in the mantle wedge below the peripheral islands (Stromboli). We also noted similarities in subducting components between the Aeolian Archipelago, the Phlegrean Fields and other volcanic arcs/arc segments around the world (e.g. Sunda, Cascades, Mexican Volcanic Belt). We suggest that the presence of melt components in all these locations resulted from an increase in the mantle wedge temperature by inflow of hot asthenospheric material from tears/windows in the slab or from around the edges of the sinking slab. This article is protected by copyright. All rights reserved.
      PubDate: 2016-05-10T03:50:24.212227-05:
      DOI: 10.1002/2016GC006301
  • New insights into lithology and hydrogeology of the northern Newark Rift
    • Authors: N. V. Zakharova; D. S. Goldberg, P. E. Olsen, D. V. Kent, S. Morgan, Q. Yang, M. Stute, J. Matter
      Abstract: The marginal facies of the Triassic rift basins in the eastern United States are poorly documented, particularly on the hinge or hanging wall margins. This study presents a lithological description and multi‐scale petrophysical analysis of basement rocks, overlying marginal facies of the early synrift strata, and the basal contact of the Palisade Sill that were drilled and cored in the northeastern part of the Newark Basin, near its terminus. The expression of the Stockton Formation differs from that in the central basin in having thinner layers, with uncertain temporal relationship to the type area. The bottom 50 m is lithologically distinct with brick‐red to dark‐purple mudstones and sandstones, abundant gypsum‐filled fractures, and a thin zone with anomalously high uranium concentration, not associated with organic‐rich mudstones as other occurrences in the basin. The crystalline basement is apparently Fordham gneiss, overlain by a thin sandstone layer and a dark‐purple hydrophilic mudstone. Despite the abundance of coarse‐grained strata and multiple sets of tectonic fractures, hydraulically transmissive zones are sparse, and do not uniquely correlate to fracture and/or matrix characteristics. Enhanced transmissivity may exist along intrusion boundaries due to enhanced thermal fracturing, but more hydraulic data are needed to verify it. Comparison of petrophysical data in two boreholes ∼210 m apart shows no direct correlation of individual lithological units and their hydraulic properties, although the overall formation characteristics are similar. The results highlight challenges for outcrop correlation at the marginal edges of the rift basins and estimating reservoir properties of these heterogeneous formations. This article is protected by copyright. All rights reserved.
      PubDate: 2016-05-09T18:43:01.615822-05:
      DOI: 10.1002/2015GC006240
  • Testing for supply‐limited and kinetic‐limited chemical
           erosion in field measurements of regolith production and chemical
    • Authors: Ken L. Ferrier; Clifford S. Riebe, W. Jesse Hahm
      Abstract: Chemical erosion contributes solutes to oceans, influencing atmospheric CO2 and thus global climate via the greenhouse effect. Quantifying how chemical erosion rates vary with climate and tectonics is therefore vital to understanding feedbacks that have maintained Earth's environment within a habitable range over geologic time. If chemical erosion rates are strongly influenced by the availability of fresh minerals for dissolution, then there should be strong connections between climate, which is modulated by chemical erosion, and tectonic uplift, which supplies fresh minerals to Earth's surface. This condition, referred to as supply‐limited chemical erosion, implies strong tectonic control of chemical erosion rates. It differs from kinetic‐limited chemical erosion, in which dissolution kinetics and thus climatic factors are the dominant regulators of chemical erosion rates. Here we present a statistical method for determining whether chemical erosion of silicate‐rich bedrock is supply‐limited or kinetic‐limited, as an approach for revealing the relative importance of tectonics and climate in Earth's silicate weathering thermostat. We applied this method to published datasets of mineral supply rates and regolith chemical depletion, and were unable to reject the null hypothesis that chemical erosion is supply‐limited in eight of sixteen cases. In seven of the remaining eight cases, we found behavior that is closer to supply‐limited than kinetic‐limited, suggesting that tectonics may often dominate over climate in regulating chemical erosion rates. However, statistical power analysis shows that new measurements across a wider range of supply rates are needed to help quantify feedbacks between climate and tectonics in Earth's long‐term climatic evolution. This article is protected by copyright. All rights reserved.
      PubDate: 2016-05-09T18:30:37.51818-05:0
      DOI: 10.1002/2016GC006273
  • Pore water chemistry reveals gradients in mineral transformation across a
           model basaltic hillslope
    • Authors: Michael Pohlmann; Katerina Dontsova, Robert Root, Joaquin Ruiz, Peter Troch, Jon Chorover
      Abstract: The extent of weathering incongruency during soil formation from rock controls local carbon and nutrient cycling in ecosystems, as well as the evolution of hydrologic flow paths. Prior studies of basalt weathering, including those that have quantified the dynamics of well‐mixed, bench‐scale laboratory reactors or characterized the structure and integrated response of field systems, indicate a strong influence of system scale on weathering rate and trajectory. For example, integrated catchment response tends to produce lower weathering rates than do well mixed reactors, but the mechanisms underlying these disparities remain unclear. Here we present pore water geochemistry and physical sensor data gathered during two controlled rainfall‐runoff events on a large‐scale convergent model hillslope mantled with 1 m uniform depth of granular basaltic porous media. The dense sampler and sensor array (1,488 sampler and sensors embedded in 330 m3 of basalt) showed that rainfall‐induced dissolution of basaltic glass produced supersaturation of pore waters with respect to multiple secondary phase solids including allophane, gibbsite, ferrihydrite, birnessite and calcite. The spatial distribution of saturation state was heterogeneous, suggesting an accumulation of solutes leading to precipitation of secondary solids along hydrologic flow paths. Rapid dissolution of primary silicates was widespread throughout the entire hillslope, irrespective of up‐gradient flowpath length. However, coherent spatial variations in solution chemistry and saturation indices were observed in depth profiles and between distinct topographic regions of the hillslope. Colloids (60 ‐ 2000 nm) enriched in iron (Fe), aluminum (Al), and phosphorus (P) were mobile in soil pore waters. This article is protected by copyright. All rights reserved.
      PubDate: 2016-05-09T18:30:33.835731-05:
      DOI: 10.1002/2016GC006270
  • On the solvability of incompressible Stokes with viscoplastic rheologies
           in geodynamics
    • Authors: Marc Spiegelman; Dave A. May, Cian R. Wilson
      Abstract: Plasticity/failure is an essential ingredient in geodynamics models as earth materials cannot sustain unbounded stresses. However, many questions remain as to appropriate models of plasticity as well as effective solvers for these strongly non‐linear systems. Here we present some simplified model problems designed to elucidate many of the issues involved for the description and solution of viscoplastic problems as currently used in geodynamic modeling. We consider compression and extension of a viscoplastic layer overlying an isoviscous layer and introduce a single plastic yield criterion which includes the most commonly used viscoplasticity models: von Mises, depth‐dependent von Mises and Drucker‐Prager. We show that for all rheologies considered, successive substitution schemes (aka Picard iteration) often stall at large values of the non‐linear residual, producing spurious solutions. However, combined Picard‐Newton schemes can be effective for rheologies that are independent of the dynamic pressure. Difficulties arise when solving incompressible Stokes problems for rheologies that depend on the dynamic pressure such as Drucker‐Prager viscoplasticity. Analysis suggests that incompressible Stokes can become ill‐posed when the dependence of the deviatoric stress tensor on dynamic pressure (i.e. ∂τ/∂p′ ) becomes large. We demonstrate empirically that, in these cases, Newton solvers can fail by introducing spurious shear bands and discuss the consequence of interpreting the results of non‐converged computations. Even for problems where solvers converge, Drucker‐Prager viscoplasticity can produce dynamic pressures that deviate significantly from lithostatic and both the velocity and pressure fields should be evaluated to determine whether solutions are geologically reasonable. This article is protected by copyright. All rights reserved.
      PubDate: 2016-05-09T18:30:31.834373-05:
      DOI: 10.1002/2015GC006228
  • Origin and dynamics of depositionary subduction margins
    • Authors: Paola Vannucchi; Jason P. Morgan, Eli A. Silver, Jared W. Kluesner
      Abstract: Here we propose a new framework for forearc evolution that focuses on the potential feedbacks between subduction tectonics, sedimentation, and geomorphology that take place during an extreme event of subduction erosion. These feedbacks can lead to the creation of a”depositionary forearc”, a forearc structure that extends the traditional division of forearcs into accretionary or erosive subduction margins by demonstrating a mode of rapid basin accretion during an erosive event at a subduction margin. A depositionary mode of forearc evolution occurs when terrigenous sediments are deposited directly on the forearc while it is being removed from below by subduction erosion. In the most extreme case, an entire forearc can be removed by a single subduction erosion event followed by depositionary replacement without involving transfer of sediments from the incoming plate. We need to further recognize that subduction forearcs are often shaped by interactions between slow, long‐term processes and sudden extreme events reflecting the sudden influences of large‐scale morphological variations in the incoming plate. Both types of processes contribute to the large‐scale architecture of the forearc, with extreme events associated with a replacive depositionary mode that rapidly creates sections of a typical forearc margin. The persistent upward diversion of the megathrust is likely to affect its geometry, frictional nature, and hydrogeology. Therefore, the stresses along the fault and individual earthquake rupture characteristics are also expected to be more variable in these erosive systems than in systems with long‐lived megathrust surfaces. This article is protected by copyright. All rights reserved.
      PubDate: 2016-05-09T10:56:48.345631-05:
      DOI: 10.1002/2016GC006259
  • Teleseismic P wave tomography and mantle dynamics beneath eastern Tibet
    • Authors: Jianshe Lei; Dapeng Zhao
      Abstract: We determined a new 3‐D P‐wave velocity model of the upper mantle beneath eastern Tibet using 112,613 high‐quality arrival‐time data collected from teleseismic seismograms recorded by a new portable seismic array in Yunnan and permanent networks in southwestern China. Our results provide new insights into the mantle structure and dynamics of eastern Tibet. High‐velocity (high‐V) anomalies are revealed down to 200 km depth under the Sichuan basin and the Ordos and Alashan blocks. Low‐velocity (low‐V) anomalies are imaged in the upper mantle under the Kunlun‐Qilian and Qinling fold zones, and the Songpan‐Ganzi, Qiangtang, Lhasa and Chuan‐Dian diamond blocks, suggesting that eastward moving low‐V materials are extruded to eastern China after the obstruction by the Sichuan basin, and the Ordos and Alashan blocks. Furthermore, the extent and thickness of these low‐V anomalies are correlated with the surface topography, suggesting that the uplift of eastern Tibet could be partially related to these low‐V materials having a higher temperature and strong positive buoyancy. In the mantle transition zone (MTZ), broad high‐V anomalies are visible from the Burma arc northward to the Kunlun fault and eastward to the Xiaojiang fault, and they are connected upward with the Wadati‐Benioff seismic zone. These results suggest that the subducted Indian slab has traveled horizontally for a long distance after it descended into the MTZ, and return corner flow and deep slab dehydration have contributed to forming the low‐V anomalies in the big mantle wedge. Our results shed new light on the dynamics of the eastern Tibetan plateau. This article is protected by copyright. All rights reserved.
      PubDate: 2016-05-05T03:51:03.696487-05:
      DOI: 10.1002/2016GC006262
  • Andesitic crustal growth via mélange partial melting: Evidence from
           Early Cretaceous arc dioritic/andesitic rocks in southern Qiangtang,
           central Tibet
    • Abstract: Deciphering the petrogenesis of andesitic/dioritic rocks is fundamental to understanding the formation of the continental crust. Here we present detailed petrology, geochronology, major and trace element, Sr–Nd–Hf–O isotope data for the Early Cretaceous (ca. 122 Ma) dioritic rocks in the Bizha area in southern Qiangtang, Tibet. The dioritic rocks are characterized by large ion lithophile elements, Pb and light rare earth elements but depletion of high field strength elements with slightly enriched and variable εNd(t) values of ‐0.01 to ‐3.31 and initial 87Sr/86Sr isotopic ratios of 0.7053 to 0.7062. They also have variable magmatic zircon Hf‐O isotope compositions (εHf(t) = ‐5.3 to +3.6 and δ18O = +7.3 to +9.5 ‰). Combined with contemporary andesitic lavas in southern Qiangtang, we suggest that the intermediate magmatic rocks in this area were most probably derived by partial melting of a subduction mélange, which is a mixture of mid‐oceanic ridge basalts (MORBs), sediments and mantle wedge peridotites, formed along the interface between the subducted slab and the overlying mantle wedge in a subduction channel before ∼ 124 Ma. The mélange diapir melting was triggered by the asthenospheric upwelling and hot corner flow caused by roll‐back of the northward subducted Bangong‐Nujiang oceanic slab during the Early Cretaceous. The Early Cretaceous intermediate magmatic rocks in southern Qiangtang have an overall continental crust‐like andesitic composition. Therefore, partial melting of mélange provides an important support for the generation of andesitic magmas in continental arcs and the “andesite model” for crustal growth. This article is protected by copyright. All rights reserved.
      PubDate: 2016-05-04T01:04:10.456927-05:
      DOI: 10.1002/2016GC006248
  • Magnetic record of deglaciation using FORC‐PCA, sortable‐silt
           grain size, and magnetic excursion at 26 ka, from the Rockall Trough (NE
    • Authors: J.E.T. Channell; R.J. Harrison, I. Lascu, I. N. McCave, F.D. Hibbert, W.E.N. Austin
      Abstract: Core MD04‐2822 from the Rockall Trough has apparent sedimentation rates of ∼ 1 m/kyr during the last deglaciation (Termination I). Component magnetization directions indicate a magnetic excursion at 16.3 m depth in the core, corresponding to an age of 26.5 ka, implying an excursion duration of ∼350 years. Across Termination I, the mean grain size of sortable silt implies reduced bottom‐current velocity in the Younger Dryas and Heinrich Stadial (HS)‐1A, and increased velocities during the Bølling‐Allerød warm period. Standard bulk magnetic parameters imply fining of magnetic grain size from the mid‐Younger Dryas (∼12 ka) until ∼ 8 ka. First‐order reversal curves (FORCs) were analyzed using ridge extraction to differentiate single domain (SD) from background (detrital) components. Principal component analysis (FORC‐PCA) was then used to discriminate three end members corresponding to SD, pseudo‐single domain (PSD) and multidomain (MD) magnetite. The fining of bulk magnetic grain size from 12 to 8 ka is due to reduction in concentration of detrital (PSD+MD) magnetite, superimposed on a relatively uniform concentration of SD magnetite produced by magnetotactic bacteria. The decrease in PSD+MD magnetite concentration from 12 to 8 ka is synchronized with increase in benthic δ13C, and with major (∼70 m) regional sea‐level rise, and may therefore be related to detrital sources on the shelf that had reduced influence as sea level rose, and to bottom‐water reorganization as Northern Source Water (NSW) replaced Southern Source Water (SSW). This article is protected by copyright. All rights reserved.
      PubDate: 2016-05-02T18:52:20.557649-05:
      DOI: 10.1002/2016GC006300
  • Density and seismic velocity of hydrous melts under crustal and upper
           mantle conditions
    • Authors: Kenta Ueki; Hikaru Iwamori
      Abstract: We present a new model for calculating the density of hydrous silicate melts as a function of P, T, H2O concentration, and melt composition. We optimize Vpr,Tr,∂V/∂T,∂V/∂P,∂V2/∂T∂P, and K' of H2O end‐member components in hydrous silicate melts, as well as K' of anhydrous silicate melts, using previously reported experimental results. The parameter set for H2O end‐member component in silicate melt optimized in this study is internally consistent with the parameter values for the properties of anhydrous silicate melt reported by Lange and Carmichael [1987; 1990]. The model calculation developed in this study reproduces the experimentally determined densities of various hydrous melts, and can be used to calculate the relationships between pressures, temperatures, and H2O concentrations of various hydrous melts from ultramafic to felsic compositions at pressures of 0–4.29 GPa. Using the new parameter set, we investigate the effects of H2O content on the seismic velocity of hydrous melts, as well as seismic velocities in partially molten regions of subduction zones. The results show that water content in silicate melt plays a key role in determining seismic velocity structure, and therefore must be taken into account when interpreting seismic tomography. This article is protected by copyright. All rights reserved.
      PubDate: 2016-05-02T18:48:08.518349-05:
      DOI: 10.1002/2015GC006242
  • A new protocol for three‐axis static alternating field
           demagnetization of rocks
    • Authors: David R. Finn; Robert S. Coe
      Abstract: Static three‐axis alternating field (AF) demagnetization is the most common method regularly implemented for removing magnetic components of rock samples. This method is so widely used that one of its main limitations, the acquisition of gyroremanence (GRM), is often not accounted for or even discussed. The presence of GRM likely interferes more than is recognized in accurate determination of the most stable remanence. The accepted method proposed by Dankers and Zjiderveld (1981) for excluding GRM affected measurements requires nearly triple the amount of lab work, and by consequence, is almost never regularly implemented on large batches of samples. Here, we present a laboratory procedure and subsequent analysis (SI method) that removes the effects of GRM in static AF demagnetization without requiring extra laboratory work. This paper, therefore, describes a new standard protocol for efficient static AF demagnetization of rocks. This article is protected by copyright. All rights reserved.
      PubDate: 2016-05-02T18:45:48.988599-05:
      DOI: 10.1002/2015GC006178
  • Heat flow, strong near‐fault seismic waves, and near‐fault
           tectonics on the central San Andreas fault
    • Authors: Norman H. Sleep
      Abstract: The main San Andreas Fault strikes subparallel to compressional folds and thrust faults. Its fault normal traction is on average a factor of γ=1+2μthr(1+μthr2+μthr), where µthris the coefficient of friction for thrust faults, times the effective lithostatic pressure. A useful upper limit for µthr of 0.6 (where γ is 3.12) is obtained from the lack of heat flow anomalies by considering off‐fault convergence at a rate of 1 mm/yr for 10 km across strike. If the fault‐normal traction is in fact this high, the well‐known heat‐flow constraint of average stresses of 10‐20 MPa during strike‐slip on the main fault becomes more severe. Only a few percent of the total slip during earthquakes can occur at the peak stress before dynamic mechanisms weaken the fault. The spatial dimension of the high‐stress rupture‐tip zone is ∼10 m for γ = 3.12 and, for comparison, ∼100 m for γ = 1. High dynamic stresses during shaking occur within these distances of the fault plane. In terms of scalars, fine‐scale tectonic stresses cannot exceed the difference between failure stress and dynamic stress. Plate scale slip causes stresses to build up near geometrical irregularities of the fault plane. Strong dynamic stresses near the rupture tip facilitate anelastic deformation with the net effects of relaxing the local deviatoric tectonic stress and accommodating deformation around the irregularities. There also is a mild tendency for near‐fault material to extrude upward. Slip on minor thrust faults causes the normal traction on the main fault to be spatially variable. This article is protected by copyright. All rights reserved.
      PubDate: 2016-04-27T19:00:32.937246-05:
      DOI: 10.1002/2016GC006280
  • PmagPy: Software package for paleomagnetic data analysis and a bridge to
           the Magnetics Information Consortium (MagIC) Database
    • Abstract: The Magnetics Information Consortium (MagIC) database provides an archive with a flexible data model for paleomagnetic and rock magnetic data. The PmagPy software package is a cross‐platform and open‐source set of tools written in Python for the analysis of paleomagnetic data that serves as one interface to MagIC, accommodating various levels of user expertise. PmagPy facilitates thorough documentation of sampling, measurements, datasets, visualization, and interpretation of paleomagnetic and rock magnetic experimental data. Although not the only route into the MagIC database, PmagPy makes preparation of newly published datasets for contribution to MagIC as a byproduct of normal data analysis and allows manipulation as well as reanalysis of datasets downloaded from MagIC with a single software package. The graphical user interface (GUI), Pmag GUI enables use of much of PmagPy's functionality, but the full capabilities of PmagPy extend well beyond that. Over 400 programs and functions can be called from the command line interface mode, or from within the interactive Jupyter notebooks. Use of PmagPy within a notebook allows for documentation of the workflow from the laboratory to the production of each published figure or data table, making research results fully reproducible. The PmagPy design and its development using GitHub accommodates extensions to its capabilities through development of new tools by the user community. Here we describe the PmagPy software package and illustrate the power of data discovery and re‐use through a re‐analysis of published paleointensity data which illustrates how the effectiveness of selection criteria can be tested. This article is protected by copyright. All rights reserved.
      PubDate: 2016-04-27T18:55:34.525654-05:
      DOI: 10.1002/2016GC006307
  • Mantle control of the geodynamo: Consequences of top‐down regulation
    • Authors: Peter Olson
      Abstract: The mantle global circulation, including deep subduction and lower mantle superplumes, exerts first‐order controls on the evolution of the core, the history of the geodynamo, and the structure of the geomagnetic field. Mantle global circulation models that include realistic plate motions, deep subduction, and compositional heterogeneity similar to the observed large low seismic velocity provinces in the lower mantle predict that the present‐day global average heat flux at the core‐mantle boundary (CMB) exceeds 85 mW m−2. This is sufficient to drive the present‐day geodynamo by thermochemical convection and implies a very young inner core, with inner core nucleation between 400 and 1000 Ma. The mantle global circulation also generates spatially heterogeneous heat flux at the CMB, with peak‐to‐peak lateral variations exceeding 100 mW m−2. Such extreme lateral variability in CMB heat flux, in conjunction with the high thermal conductivity of the core, implies that the liquid outer core is thermally unstable beneath the high seismic velocity regions in the lower mantle but thermally stable beneath the large low seismic velocity provinces. Numerical dynamo simulations show how this pattern of heterogeneous boundary heat flux affects flow in the outer core, producing localized circulation patterns beneath the CMB tied to the mantle heterogeneity and long‐lived deviations from axial symmetry in the geomagnetic field. This article is protected by copyright. All rights reserved.
      PubDate: 2016-04-25T09:52:07.798165-05:
      DOI: 10.1002/2016GC006334
  • Fractionation of Cu and Mo isotopes caused by vapor‐liquid
           partitioning, evidence from the Dahutang W‐Cu‐Mo deposit
    • Authors: Junming Yao; Ryan Mathur, Weidong Sun, Weile Song, Huayong Chen, Laurence Mutti, Xinkui Xiang, Xiaohong Luo
      Abstract: The study presents δ65Cu and δ97Mo isotope values from cogenetic chalcopyrite and molybdenite found in veins and breccias of the Dahutang W‐Cu‐Mo ore field in China. The samples span a 3 to 4 Km range. Both isotopes show a significant degree of fractionation. Cu isotope values in the chalcopyrite range from ‐0.31‰ to +1.48‰, and Mo isotope values in the molybdenite range from ‐0.03‰ to +1.06‰. For the cogenetic sulfide veined samples, a negative slope relationship exists between δ65Cu and δ97Mo values, which suggest a similar fluid history. Rayleigh distillation models the vein samples' change in isotope values. The breccia samples do not fall on the trend, thus indicating a different source mineralization event. Measured fluid inclusion and δD and δ18O data from cogenetic quartz indicate changes in temperature, and mixing of fluids do not appear to cause the isotopic shifts measure. Related equilibrium processes associated with the partitioning of metal between the vapor‐fluid in the hydrothermal system could be the probable cause for the relationship seen between the two isotope systems. This article is protected by copyright. All rights reserved.
      PubDate: 2016-04-22T08:46:03.292767-05:
      DOI: 10.1002/2016GC006328
  • Inference of the viscosity structure and mantle conditions beneath the
           Central Nevada Seismic Belt from combined postseismic and lake unloading
    • Authors: Haylee Dickinson; Andrew M. Freed, Christopher Andronicos
      Abstract: We test whether a single depth‐dependent Newtonian viscosity structure can be found to explain measured surface deformation in Western Nevada from two separate loading events: tectonic loading from a series of 7 historic earthquakes in the Central Nevada Seismic Belt and non‐tectonic loading from the formation and evaporation of co‐located Pleistocene‐aged Lake Lahontan. Rheologic studies are generally plagued with non‐uniqueness issues due to the limitations of observational constraints. Here, we reduce non‐uniqueness by solving for a single rheologic structure that can simultaneously satisfy all observational constraints associated with all events. Model results suggest that Western Nevada is underlain by a strong lower crust (order 1020 Pa s), a relatively weak mantle (order 5 x 1018 Pa s) from 40 to 80 km, and a much weaker mantle (order 1018 Pa s) below 80 km. We would thus place the mechanical lithosphere/asthenosphere boundary (LAB) at 40 km depth. Thermal modeling of conductive geothermal gradients, combined with melting curves calculated for enriched and depleted mantle compositions suggest that the viscosity decrease at 40 km depth (the LAB) is associated with the onset of wet melting of mantle lithosphere hydrated by past subduction and is about 10 km shallower than the inferred transition from conduction to convection. This article is protected by copyright. All rights reserved.
      PubDate: 2016-04-22T05:02:27.928779-05:
      DOI: 10.1002/2015GC006207
  • Exhumation of the Panama basement complex and basins: Implications for the
           closure of the cCentral American seaway
    • Abstract: The emergence of the Central American isthmus occurred episodically from Eocene to Pliocene time and was caused by a series of tectonic and volcanic processes. Results from zircon U‐Pb geochronology, zircon (U‐Th)/He (ZHe) and apatite (U‐Th)/He (AHe) thermochronology, and zircon Lu‐Hf isotopic data from sedimentary (sandstones and recent river sands) and plutonic rocks from the Azuero Peninsula and Central Panama document the exhumation and uplift history of the Panamanian basement complex. Our data supports previous paleobotanical and thermochronological studies that suggest that by middle Eocene time some areas of Central Panama and Azuero Peninsula were exposed above sea level as a series of islands surrounded by shallow open marine waters. The Gatuncillo, Cobachón and Tonosí formations were deposited during this partial emergence. Transtension in the Oligocene‐early Miocene produced various pull‐apart basins (e.g., the Canal Basin) and local uplift that exhumed the Eocene strata (Gatuncillo and Cobachón formations). This event probably reduced circulation between the Pacific Ocean and the Caribbean Sea. The Tonosí Formation records late Miocene to Pleistocene cooling and exhumation, which may be related to uplift above the subducting Coiba Ridge. These results suggest that the emergence of the Isthmus of Panama followed a series of diachronous events that led to the final closure of the Central American seaway. This article is protected by copyright. All rights reserved.
      PubDate: 2016-04-20T18:05:47.60936-05:0
      DOI: 10.1002/2016GC006289
  • Correction of interstitial water changes in calibration methods applied to
           XRF core‐scanning major elements in long sediment cores: Case study
           from the South China Sea
    • Authors: Quan Chen; Catherine Kissel, Aline Govin, Zhifei Liu, Xin Xie
      Abstract: Fast and non‐destructive X‐ray fluorescence (XRF) core scanning provides high‐resolution element data that are widely used in paleoclimate studies. However, various matrix and specimen effects prevent the use of semi‐quantitative raw XRF core‐scanning intensities for robust paleoenvironmental interpretations. We present here a case study of a 50.8‐m‐long piston Core MD12‐3432 retrieved from the northern South China Sea. The absorption effect of interstitial water is identified as the major source of deviations between XRF core‐scanning intensities and measured element concentrations. The existing two calibration methods, i.e. normalized median‐scaled calibration (NMS) and multivariate log‐ratio calibration (MLC), are tested with this sequence after the application of water absorption correction. The results indicate that an improvement is still required to appropriately correct the influence of downcore changes in interstitial water content in the long sediment core. Consequently, we implement a new polynomial water content correction in NMS and MLC methods, referred as NPS and P_MLC calibrations. Results calibrated by these two improved methods indicate that the influence of downcore water content changes is now appropriately corrected. We therefore recommend either of the two methods to be applied for robust paleoenvironmental interpretations of major elements measured by XRF‐scanning in long sediment sequences with significant downcore interstitial water content changes. This article is protected by copyright. All rights reserved.
      PubDate: 2016-04-15T19:00:51.312838-05:
      DOI: 10.1002/2016GC006320
  • A microphysical interpretation of rate‐dependent and
           state‐dependent friction for fault gouge
    • Authors: Matt J. Ikari; Brett M. Carpenter, Chris Marone
      Abstract: The evolution of fault strength during the seismic cycle plays a key role in the mode of fault slip, nature of earthquake stress drop, and earthquake nucleation. Laboratory‐based rate‐ and state‐dependent friction (RSF) laws can describe changes in fault strength during slip, but the connections between fault strength and the mechanisms that dictate the mode of failure, from aseismic creep to earthquake rupture, remain poorly understood. The empirical nature of RSF laws remains a drawback to their application in nature. Here, we analyze an extensive friction data set of constitutive parameters with the goal of illuminating the microphysical processes controlling RSF. We document robust relationships between: (1) the initial value of sliding (or kinetic) friction, (2) RSF parameters, and (3) the time rates of frictional strengthening (aging). We derive a microphysical model based on asperity contact mechanics and show that these relationships are dictated by: (1) an activation energy that controls the rate of asperity growth by plastic creep, and (2) an inverse relationship between material hardness and the activation volume of plastic deformation. Collectively, our results illuminate the physics expressed by the RSF parameters, and which describe the absolute value of frictional strength and its dependence on time and slip rate. Moreover, we demonstrate that seismogenic fault behavior may be dictated by the interplay between grain properties and ambient conditions controlling the local shear strength of grain‐scale asperity contacts. This article is protected by copyright. All rights reserved.
      PubDate: 2016-04-12T20:31:28.922989-05:
      DOI: 10.1002/2016GC006286
  • Magnetic minerals in three Asian rivers draining into the South China Sea:
           Pearl, Red, and Mekong Rivers
    • Authors: Catherine Kissel; Zhifei Liu, Jinhua Li, Camille Wandres
      Abstract: The use of the marine sedimentary magnetic properties as tracers for changes in precipitation rate and in oceanic water masses transport and exchanges, implies to identify and to characterize the different sources of the detrital fraction. This is of particular importance in closed and/or marginal seas such as the South China Sea. We report on the magnetic properties of sedimentary samples collected in three main Asian rivers draining into the South China Sea: the Pearl, Red and Mekong Rivers. The geological formations as well as the present climatic conditions are different from one catchment to another. The entire set of performed magnetic analyses (low‐field magnetic susceptibility, ARM acquisition and decay, IRM acquisition and decay, back‐field acquisition, thermal demagnetization of 3‐axes IRM, hysteresis parameters, FORC diagrams and low‐temperature magnetic measurements) allow us to identify the magnetic mineralogy and the grain size distribution when magnetite is dominant. Some degree of variability is observed in each basin, illustrating different parent rocks and degree of weathering. On average it appears that the Pearl River is rich in magnetite along the main stream while the Mekong River is rich in hematite. The Red River is a mixture of the two. Compared to clay mineral assemblages and major element contents previously determined on the same samples, these new findings indicate that the magnetic fraction brings complementary information of great interest for environmental reconstructions based on marine sediments from the South China Sea. This article is protected by copyright. All rights reserved.
      PubDate: 2016-04-08T13:04:24.552368-05:
      DOI: 10.1002/2016GC006283
  • Isotope dilution analysis of Ca and Zr in apatite and zircon
           (U‐Th)/He chronometry
    • Authors: William R. Guenthner; Peter W. Reiners, Uttam Chowdhury
      Abstract: Because radiation damage influences He diffusivity, correlations between (U‐Th)/He ages and effective uranium (eU, eU=U+0.235×Th) concentrations of single apatite and zircon grains are important for understanding thermal histories. Here we describe a method for quantifying eU concentrations in apatite and zircon grains using isotope dilution ICP‐MS measurements of Zr and Ca and stoichiometry of zircon (ZrSiO4) and apatite (Ca5(PO4)3F) to obtain grain masses. Combined with independent U and Th measurements, these yield eU concentrations not based on the traditional morphologic measurements and assumptions. Additional benefits of this method include correct identification of an apatite or zircon and volume estimates for crystal shards. In some cases, this method gives eU concentrations consistent with those calculated with the morphologic approach, but often significant differences are observed between concentrations calculated from the two methods. Differences in eU concentrations for our apatite grains are greater and less than those morphology estimates, and the majority are between 0.7 and 31%. With the exception of two grains, all of our zircon grains have differences between 3 and 34% less than morphology estimates. These differences could result from incorrect grain width measurements, mischaracterized grain shape, or incorrect volume calculations of the pure mineral phase due to inclusions. These morphologic errors—combined with evidence for the accuracy of our isotope dilution method from analyses of reference materials—suggest that eU concentrations calculated from morphology may often be significantly inaccurate. Finally, we demonstrate that differences between the two measurements of eU cause age‐eU correlation variations for representative thermal histories. This article is protected by copyright. All rights reserved.
      PubDate: 2016-04-07T17:46:17.861797-05:
      DOI: 10.1002/2016GC006311
  • New analytic solutions for modeling vertical gravity gradient anomalies
    • Abstract: Modern processing of satellite altimetry for use in marine gravimetry involves computing the along‐track slopes of observed sea‐surface heights, projecting them into east‐west and north‐south deflection of the vertical grids, and using Laplace's equation to algebraically obtain a grid of the vertical gravity gradient (VGG). The VGG grid is then integrated via overlapping, flat Earth Fourier transforms to yield a free‐air anomaly grid. Because of this integration and associated edge effects, the VGG grid retains more short‐wavelength information (e.g., fracture zone and seamount signatures) that is of particular importance for plate tectonic investigations. While modeling of gravity anomalies over arbitrary bodies has long been a standard undertaking, similar modeling of VGG anomalies over oceanic features is not commonplace yet. Here, we derive analytic solutions for VGG anomalies over simple bodies and arbitrary 2‐D and 3‐D sources. We demonstrate their usability in determining mass excess and deficiency across the Mendocino fracture zone (a 2‐D feature) and find the best bulk density estimate for Jasper seamount (a 3‐D feature). The methodologies used herein are implemented in the Generic Mapping Tools, available from This article is protected by copyright. All rights reserved.
      PubDate: 2016-04-06T14:55:49.112418-05:
      DOI: 10.1002/2016GC006263
  • Early Miocene subduction in the western Mediterranean: Constraints from
           Rb‐Sr multimineral isochron geochronology
    • Authors: Kory L. Kirchner; Whitney M. Behr, Staci Loewy, Daniel F. Stockli
      Abstract: The Betic Cordillera of southern Spain is a complex orogen formed in the context of convergence between Africa and Iberia from the Mesozoic to the present. The internal zone of the orogen includes three tectonic complexes, two of which have been subducted to high pressure conditions, then exhumed back to the surface during subsequent extension. Subduction in the structurally lower complex, known as the Nevado‐Filabride Complex (NFC), has been a topic of debate for several years due to conflicting geochronological data. Here we use multi‐mineral isochron 87Rb/86Sr dating on carefully selected mineral samples from high pressure metamorphic rocks in the NFC to better constrain the timing of high pressure metamorphism and subduction in the region. Out of five samples analyzed, statistically valid multi‐mineral isochrons were obtained for one eclogite and two schists, yielding ages of 20.1+/‐1.1 Ma, 16.0+/‐0.3 Ma, and 13.3+/‐1.3 Ma, respectively. Despite that the other two eclogite samples appeared to preserve prograde mineral assemblages, low 87Rb/86Sr ratios in white mica precluded precise age calculations. These new ages are in close agreement with previously published Lu‐Hf ages on garnet and U‐Pb ages on metamorphic zircon overgrowths for the same rocks, but are substantially younger than published data from the 40Ar/39Ar technique. Combined with recently published tomographic images of slab structure beneath the Alboran Sea, the new ages support a tectonic model in which subduction occurred both prior to the Miocene and during the early to mid‐Miocene, but that it was punctuated in time by a pulse of extensional exhumation in the early Miocene associated with lithospheric delamination and/or slab tearing. This article is protected by copyright. All rights reserved.
      PubDate: 2016-04-01T01:56:58.593725-05:
      DOI: 10.1002/2015GC006208
  • Whole planet coupling between climate, mantle, and core: Implications for
           rocky planet evolution
    • Authors: Bradford J. Foley; Peter E. Driscoll
      Abstract: Earth's climate, mantle, and core interact over geologic timescales. Climate influences whether plate tectonics can take place on a planet, with cool climates being favorable for plate tectonics because they enhance stresses in the lithosphere, suppress plate boundary annealing, and promote hydration and weakening of the lithosphere. Plate tectonics plays a vital role in the long‐term carbon cycle, which helps to maintain a temperate climate. Plate tectonics provides long‐term cooling of the core, which is vital for generating a magnetic field, and the magnetic field is capable of shielding atmospheric volatiles from the solar wind. Coupling between climate, mantle, and core can potentially explain the divergent evolution of Earth and Venus. As Venus lies too close to the sun for liquid water to exist, there is no long‐term carbon cycle and thus an extremely hot climate. Therefore plate tectonics cannot operate and a long‐lived core dynamo cannot be sustained due to insufficient core cooling. On planets within the habitable zone where liquid water is possible, a wide range of evolutionary scenarios can take place depending on initial atmospheric composition, bulk volatile content, or the timing of when plate tectonics initiates, among other factors. Many of these evolutionary trajectories would render the planet uninhabitable. However, there is still significant uncertainty over the nature of the coupling between climate, mantle, and core. Future work is needed to constrain potential evolutionary scenarios and the likelihood of an Earth‐like evolution. This article is protected by copyright. All rights reserved.
      PubDate: 2016-03-24T03:31:19.396868-05:
      DOI: 10.1002/2015GC006210
  • Surface heat flow measurements from the East Siberian continental slope
           and southern Lomonosov Ridge, Arctic Ocean
    • Authors: Matt O'Regan; Pedro Preto, Christian Stranne, Martin Jakobsson, Andrey Koshurnikov
      Abstract: Surface heat flow data in the Arctic Ocean is needed to assess hydrocarbon and methane hydrate distributions, and provide constraints into the tectonic origins and nature of underlying crust. However, across broad areas of the Arctic, few published measurements exist. This is true for the outer continental shelf and slope of the East Siberian Sea, and the adjoining deep water ridges and basins. Here we present 21 new surface heat flow measurements from this region of the Arctic Ocean. On the Southern Lomonosov Ridge, the average measured heat flow, uncorrected for effects of sedimentation and topography, is 57 ± 4 mW/m2 (n=4). On the outer continental shelf and slope of the East Siberian Sea (ESS), The average is 57 ± 10 mW/m2 (n=16). An anomalously high heat flow of 203 ± 28 mW/m2 was measured at a single station in the Herald Canyon. With the exception of this high heat flow, the new data from the ESS is consistent with predictions for thermally equilibrated lithosphere of continental origin that was last affected by thermo‐tectonic processes in the Cretaceous to early Cenozoic. Variability within the data likely arises from differences in radiogenic heat production within the continental crust and overlying sediments. This can be further explored by comparing the data with geophysical constraints on sediment and crustal thicknesses. This article is protected by copyright. All rights reserved.
      PubDate: 2016-03-22T03:30:55.999517-05:
      DOI: 10.1002/2016GC006284
  • Geochemical evidence in the northeast Lau Basin for subduction of the
           Cook‐Austral volcanic chain in the Tonga Trench
    • Abstract: Lau Basin basalts host an array of geochemical signatures that suggest incorporation of enriched mantle source material often associated with intraplate hotspots, but the origin of these signatures remain uncertain. Geochemical signatures associated with mantle material entrained from the nearby Samoan hotspot are present in northwest Lau Basin lavas, and subducted seamounts from the Louisville hotspot track may contribute geochemical signatures to the Tonga Arc. However, lavas in the northeast Lau Basin (NELB) have unique enriched geochemical signatures that cannot be related to these hotspots, but can be attributed to the subduction of seamounts associated with the Cook‐Austral volcanic lineament. Here we present geochemical data on a new suite of NELB lavas—ranging in 40Ar/39Ar age from 1.3 Ma to 0.365 ka—that have extreme signatures of geochemical enrichment, including lavas with the highest 206Pb/204Pb (19.580) and among the lowest 143Nd/144Nd (0.512697) encountered in the Lau Basin to date. These signatures are linked to the canonical EM1 (enriched mantle 1) and HIMU (high‐μ= 238U/204Pb) mantle end‐members, respectively. Using a plate reconstruction model, we show that older portions of the traces of two of the Cook‐Austral hotspots that contributed volcanism to the Cook‐Austral volcanic lineament—the Rarotonga and Rurutu hotspots—were potentially subducted in the Tonga Trench beneath the NELB. The geochemical signatures of the Rarotonga, Rurutu, and Samoan hotspots provide a compelling match to the extreme geochemical components observed in the new NELB lavas. This article is protected by copyright. All rights reserved.
      PubDate: 2016-03-22T03:30:54.116557-05:
      DOI: 10.1002/2015GC006237
  • Algorithms and software for U‐Pb geochronology by LA‐ICPMS
    • Authors: Noah McLean; James Bowring, George Gehrels
      Abstract: The past fifteen years have produced numerous innovations in geochronology, including experimental methods, instrumentation, and software that are revolutionizing the acquisition and application of geochronological data. For example, exciting advances are being driven by Laser‐Ablation ICP Mass Spectrometry (LA‐ICPMS), which allows for rapid determination of U‐Th‐Pb ages with 10s of μm‐scale spatial resolution. This method has become the most commonly applied tool for dating zircons, constraining a host of geological problems. The LA‐ICPMS community is now faced with archiving these data with associated analytical results and, more importantly, ensuring that data meet the highest standards for precision and accuracy and that inter‐laboratory biases are minimized. However, there is little consensus with regard to analytical strategies and data reduction protocols for LA‐ICPMS geochronology. The result is systematic inter‐laboratory bias and both under‐ and over‐estimation of uncertainties on calculated dates that, in turn, decrease the value of data in repositories such as EarthChem, which archives data and analytical results from participating laboratories. We present free open source software software that implements new algorithms for evaluating and resolving many of these discrepancies. This solution is the result of a collaborative effort to extend the U‐Pb_Redux software for the ID‐TIMS community to the LA‐ICPMS community. Now named ET_Redux, our new software automates the analytical and scientific workflows of data acquisition, statistical filtering, data analysis and interpretation, publication, community‐based archiving, and the compilation and comparison of data from different laboratories to support collaborative science. This article is protected by copyright. All rights reserved.
      PubDate: 2016-03-03T20:07:37.488273-05:
      DOI: 10.1002/2015GC006097
  • Variations in timing of lithospheric failure on terrestrial planets due to
           chaotic nature of mantle convection
    • Authors: Teresa Wong; Viatcheslav S. Solomatov
      Abstract: We perform numerical simulations of lithospheric failure in the stagnant lid regime of temperature‐dependent viscosity convection, using the yield stress approach. We find that the time of failure can vary significantly for the same values of the controlling parameters due to the chaotic nature of the convective system. The general trend of the dependence of the time of lithospheric failure on the yield stress can be explained by treating lithospheric failure as a type of Rayleigh‐Taylor instability. This study suggests that it is important to address not only the question of whether plate tectonics can occur on a planet but also when it would occur if conditions are favorable. This article is protected by copyright. All rights reserved.
      PubDate: 2016-03-01T19:17:03.895603-05:
      DOI: 10.1002/2015GC006158
  • Performance benchmarks for a next generation numerical dynamo model
    • Abstract: Numerical simulations of the geodynamo have successfully represented many observable characteristics of the geomagnetic field, yielding insight into the fundamental processes that generate magnetic fields in the Earth's core. Because of limited spatial resolution, however, the diffusivities in numerical dynamo models are much larger than those in the Earth's core, and consequently, questions remain about how realistic these models are. The typical strategy used to address this issue has been to continue to increase the resolution of these quasi‐laminar models with increasing computational resources, thus pushing them toward more realistic parameter regimes. We assess which methods are most promising for the next generation of supercomputers, which will offer access to O(106) processor cores for large problems. Here we report performance and accuracy benchmarks from 15 dynamo codes that employ a range of numerical and parallelization methods. Computational performance is assessed on the basis of weak and strong scaling behavior up to 16,384 processor cores. Extrapolations of our weak scaling results indicate that dynamo codes that employ two‐ or three‐dimensional domain decompositions can perform efficiently on up to ∼106 processor cores, paving the way for more realistic simulations in the next model generation. This article is protected by copyright. All rights reserved.
      PubDate: 2016-02-12T08:20:31.344886-05:
      DOI: 10.1002/2015GC006159
  • Surface Slip during Large Owens Valley Earthquakes
    • Abstract: The 1872 Owens Valley earthquake is the third largest known historical earthquake in California. Relatively sparse field data and a complex rupture trace, however, inhibited attempts to fully resolve the slip distribution and reconcile the total moment release. We present a new, comprehensive record of surface slip based on lidar and field investigation, documenting 162 new measurements of laterally and vertically displaced landforms for 1872 and prehistoric Owens Valley earthquakes. Our lidar analysis uses a newly developed analytical tool to measure fault slip based on cross‐correlation of sub‐linear topographic features and produce a uniquely shaped probability density function (PDF) for each measurement. Stacking PDFs along strike to form cumulative offset probability distribution plots (COPDs) highlights common values corresponding to single‐ and multiple‐event displacements. Lateral offsets for 1872 vary systematically from ∼1.0–6.0 m and average 3.3 ± 1.1 m. Vertical offsets are predominantly east‐side down between ∼0.1–2.4 m, with a mean of 0.8 ± 0.5 m. The average lateral‐to‐vertical ratio compiled at specific sites is ∼6:1. Summing displacements across sub‐parallel, overlapping ruptures implies a maximum of 7–11 m and net average of 4.4 ± 1.5 m, corresponding to a geologic Mw ∼7.5 for the 1872 event. We attribute progressively higher‐offset lateral COPD peaks at 7.1 ± 2.0 m, 12.8 ± 1.5 m, and 16.6 ± 1.4 m to three earlier large surface ruptures. Evaluating cumulative displacements in context with previously dated landforms in Owens Valley suggests relatively modest rates of fault slip, averaging between ∼0.6–1.6 mm/yr (1σ) over the late Quaternary. This article is protected by copyright. All rights reserved.
      PubDate: 2016-01-08T19:13:33.397692-05:
      DOI: 10.1002/2015GC006033
  • Issue Information
    • Pages: 1567 - 1568
      PubDate: 2016-06-22T00:57:15.196379-05:
      DOI: 10.1002/ggge.20832
School of Mathematical and Computer Sciences
Heriot-Watt University
Edinburgh, EH14 4AS, UK
Tel: +00 44 (0)131 4513762
Fax: +00 44 (0)131 4513327
Home (Search)
Subjects A-Z
Publishers A-Z
Your IP address:
About JournalTOCs
News (blog, publications)
JournalTOCs on Twitter   JournalTOCs on Facebook

JournalTOCs © 2009-2016