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Journal Cover   Geochemistry, Geophysics, Geosystems
  [SJR: 2.56]   [H-I: 69]   [25 followers]  Follow
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   ISSN (Online) 1525-2027
   Published by American Geophysical Union (AGU) Homepage  [17 journals]
  • Issue Information
    • PubDate: 2015-05-19T07:57:09.330758-05:
      DOI: 10.1002/ggge.20560
  • Strain decoupling reveals variable seismogenic risk in SE Japan (Nankai
    • Authors: James Van Tuyl; Tiago M. Alves, Gregory F. Moore
      Abstract: The determination of in situ stress states is vital in understanding the behavior of faults and subsequent seismogenesis of accretionary prisms. In this paper, a high quality 3D seismic volume is used to map the depth of the extensional‐compressional decoupling (ECD) boundary in the accretionary prism of Nankai, with the prior knowledge that strike‐slip and compressional stresses occur deeper than 1250 meters below seafloor (mbsf) in the Kumano Basin, changing to extension towards the seafloor. A total of 1108 faults from the accretionary prism are analyzed to estimate paleostresses via fault inversion and slip tendency techniques. A key result is this paper is that the ECD boundary can be used as a proxy to identify active structures on accretionary prisms as its depth depends on: a) local tectonic uplift in areas adjacent to active faults, and b) on the thickness of sediment accumulated above active thrust anticlines. The depth of the ECD boundary ranges from 0 to ∼650 mbsf, being notably shallower than in the Kumano Basin. In Nankai, frontal regions of the imbricate thrust zone, and the megasplay fault zone, reveal the shallower ECD depths and correlate with the regions where faulting is most active. As a corollary, this work confirms that estimates of stress state variability based on the analysis of 3D seismic data are vital to understand the behavior of faults and potential seismogenic regions on convergent margins. This article is protected by copyright. All rights reserved.
      PubDate: 2015-05-18T04:01:39.566209-05:
      DOI: 10.1002/2015GC005778
  • Microstructural evolution of gas hydrates in sedimentary matrices observed
           with synchrotron X‐ray computed tomographic microscopy
    • Authors: Marwen Chaouachi; Andrzej Falenty, Kathleen Sell, Frieder Enzmann, Michael Kersten, David Haberthür, Werner F. Kuhs
      Abstract: The formation process of gas hydrates in sedimentary matrices is of crucial importance for the physical and transport properties of the resulting aggregates. This process has never been observed in‐situ at sub‐micron resolution. Here, we report on synchrotron‐based micro‐tomographic studies by which the nucleation and growth processes of gas hydrate were observed at 276 K in various sedimentary matrices such as natural quartz (with and without admixtures of montmorillonite type clay) or glass beads with different surface properties, at varying water saturation. Both juvenile water and metastably gas‐enriched water obtained from gas hydrate decomposition was used. Xenon gas was employed to enhance the density contrast between gas hydrate and the fluid phases involved. The nucleation sites can be easily identified and the various growth patterns are clearly established. In sediments under‐saturated with juvenile water, nucleation starts at the water‐gas interface resulting in an initially several micrometer thick gas hydrate film; further growth proceeds to form isometric single crystals of 10‐20 μm size. The growth of gas hydrate from gas‐enriched water follows a different pattern, via the nucleation in the bulk of liquid producing polyhedral single crystals. A striking feature in both cases is the systematic appearance of a fluid phase film of up to several µm thickness between gas hydrates and the surface of the quartz grains. These micro‐structural findings are relevant for future efforts of quantitative rock physics modeling of gas hydrates in sedimentary matrices and explain the anomalous attenuation of seismic/sonic waves. This article is protected by copyright. All rights reserved.
      PubDate: 2015-05-15T10:12:37.704314-05:
      DOI: 10.1002/2015GC005811
  • Determination of low B/Ca ratios in carbonates using ICP‐QQQ
    • Authors: Silvia Diez Fernández; Jorge Ruiz Encinar, Alfredo Sanz‐Medel, Kirsten Isensee, Heather M. Stoll
      Abstract: The very low B/Ca ratios characteristic of some natural biogenic carbonates, are of interest for research in ocean acidification but represent an analytical challenge. We describe a method using a novel instrument configuration (ICP‐QQQ), for which we are not aware of any previously published geological applications, and for coccoliths, a sample type unique in its low B content and organic phases. Detection limits as low as 0.41 µmol/mol were achieved. Isobaric interferences, out of the reach even for SF‐ICP‐MS, can be solved using this instrument, which permits the safe measurement of the lowest abundance Ca isotope (46Ca). This allows maximizing the B concentration measured (matrix concentration up to 800 ppm Ca) while maintaining both B and Ca signals in counting mode. More significantly for low B samples, the ICP‐QQQ is also able to overcome the interference of the ubiquitous 12C tail on the 11B mass, which otherwise leads to significant overestimates at very low B concentrations. This could be a reason for the significantly lower B/Ca ratios observed for the low B content interlaboratory calibration standards (Carrara and OKA), while matching for the high B content standards was good. Finally, results obtained in the analysis of coccoliths grown in laboratory culture seems to corroborate that SIMS analysis of the samples mounted in Indium leads also to B/Ca overestimates due to porosity effects, as previously observed using LA‐ICP‐MS. This approach also permits the interference‐free measurement of P/Ca and S/Ca ratios, which could be used as indicators of the complete removal of the organic matter from the samples. This article is protected by copyright. All rights reserved.
      PubDate: 2015-05-15T10:12:26.445729-05:
      DOI: 10.1002/2015GC005817
  • Contrasting styles of (U)HP rock exhumation along the Cenozoic
           Adria‐Europe plate boundary (Western Alps, Calabria, Corsica)
    • Authors: Marco G. Malusà; Claudio Faccenna, Suzanne L. Baldwin, Paul G. Fitzgerald, Federico Rossetti, Maria Laura Balestrieri, Martin Danišík, Alessandro Ellero, Giuseppe Ottria, Claudia Piromallo
      Abstract: Since the first discovery of ultra‐high pressure (UHP) rocks 30 years ago in the Western Alps, the mechanisms for exhumation of (U)HP terranes worldwide are still debated. In the Western Mediterranean, the presently accepted model of synconvergent exhumation (e.g., the channel‐flow model) is in conflict with parts of the geologic record. We synthesize regional geologic data and present alternative exhumation mechanisms that consider the role of divergence within subduction zones. These mechanisms, i.e., (i) the motion of the upper plate away from the trench and (ii) the rollback of the lower plate, are discussed in detail with particular reference to the Cenozoic Adria‐Europe plate boundary, and along three different transects (Western Alps, Calabria‐Sardinia, and Corsica ‐ Northern Apennines). In the Western Alps, (U)HP rocks were exhumed from the greatest depth at the rear of the accretionary wedge during motion of the upper plate away from the trench. Exhumation was extremely fast, and associated with very low geothermal gradients. In Calabria, HP rocks were exhumed from shallower depths and at lower rates during rollback of the Adriatic plate, with repeated exhumation pulses progressively younging towards the foreland. Both mechanisms were active to create boundary divergence along the Corsica – Northern Apennines transect, where European southeastward subduction was progressively replaced along‐strike by Adriatic northwestward subduction. The tectonic scenario depicted for the Western Alps trench during Eocene exhumation of (U)HP rocks correlates well with present day eastern Papua New Guinea, which is presented as a modern analogue of the Paleogene Adria‐Europe plate boundary. This article is protected by copyright. All rights reserved.
      PubDate: 2015-05-15T10:12:18.794389-05:
      DOI: 10.1002/2015GC005767
  • Dynamics of intraoceanic subduction initiation: 2. Suprasubduction zone
           ophiolite formation and metamorphic sole exhumation in context of absolute
           plate motions
    • Authors: Douwe J.J. van Hinsbergen; Kalijn Peters, Marco Maffione, Wim Spakman, Carl Guilmette, Cedric Thieulot, Oliver Plümper, Derya Gürer, Fraukje M. Brouwer, Ercan Aldanmaz, Nuretdin Kaymakci
      Abstract: Analyzing subduction initiation is key for understanding the coupling between plate tectonics and underlying mantle. Here we focus on supra‐subduction zone (SSZ) ophiolites and how their formation links to intra‐oceanic subduction initiation in an absolute plate motion frame. SSZ ophiolites form the majority of exposed oceanic lithosphere fragments and are widely recognized to have formed during intra‐oceanic subduction initiation. Structural, petrological, geochemical, and plate kinematic constraints on their kinematic evolution show that SSZ crust forms at forearc spreading centers at the expense of a mantle wedge, thereby flattening the nascent slab. This leads to the typical inverted pressure gradients found in metamorphic soles that form at the subduction plate contact below and during SSZ crust crystallization. Former spreading centers are preserved in forearcs when subduction initiates along transform faults or off‐ridge oceanic detachments. We show how these are reactivated when subduction initiates in the absolute plate motion direction of the inverting weakness zone. Upon inception of slab‐pull due to e.g. eclogitization, the sole is separated from the slab, remains welded to the thinned overriding plate lithosphere and can become intruded by mafic dikes upon asthenospheric influx into the mantle wedge. We propound that most ophiolites thus formed under special geodynamic circumstances and may not be representative of normal oceanic crust. Our study highlights how far‐field geodynamic processes and absolute plate motions may force intra‐oceanic subduction initiation as key towards advancing our understanding of the entire plate tectonic cycle. This article is protected by copyright. All rights reserved.
      PubDate: 2015-05-15T10:11:58.145754-05:
      DOI: 10.1002/2015GC005745
  • Water input and water release from the subducting Nazca Plate along
           southern Central Chile (33°S–46°S)
    • Authors: David Völker; Michael Stipp
      Abstract: The age of the subducting Nazca Plate off Chile increases northwards from 0 Ma at the Chile Triple Junction (46°S) to 37 Ma at the latitude of Valparaíso (32°S). Age‐related variations in the thermal state of the subducting plate impact on (a) the water influx to the subduction zone, as well as on (b) the volumes of water that are released under the continental forearc or, alternatively, carried beyond the arc. Southern Central Chile is an ideal setting to study this effect, because other factors for the subduction zone water budget appear constant. We determine the water influx by calculating the crustal water uptake and by modeling the upper mantle serpentinization at the outer rise of the Chile Trench. The water release under forearc and arc is determined by coupling FEM thermal models of the subducting plate with stability fields of water‐releasing mineral reactions for upper and lower crust and hydrated mantle. Results show that both the influx of water stored in, and the outflux of water released from upper crust, lower crust and mantle vary drastically over segment boundaries. In particular, the oldest and coldest segments carry roughly twice as much water into the subduction zone as the youngest and hottest segments, but their release flux to the forearc is only about one fourth of the latter. This high variability over a subduction zone of 
      PubDate: 2015-05-12T17:53:41.488248-05:
      DOI: 10.1002/2015GC005766
  • Long‐term deformation driven by small ambient tectonic stresses and
           strong oscillating tidal within Enceladus with analogy to rock behavior
           near the San Andreas Fault
    • Authors: Norman H. Sleep
      Abstract: Strong tidal stresses brought much of the icy shell of Enceladus into frictional failure at past times of high orbital eccentricity. The frictional behavior of shallow terrestrial rock exposed to repeated episodes of strong seismic waves provides analogy. Frictional failure produces cracks that lower the shear modulus. Seismic regolith develops where the shear modulus increases linearly with depth. Imposed peak strains barely cause frictional failure within self‐organized regolith. With regard to Enceladus, eccentricity could continue to build up in the past since little anelastic strain and hence tidal dissipation occurred within the self‐organized regolith and within the underlying cold ice. A frictional instability analogous to the formation of weak major faults on the Earth likely occurred once the regolith was many kilometers thick. The effective coefficient of friction dropped to low levels along major faults within the deep cold ice. Tidal dissipation on these faults heated the ice starting thermal convention within the South Polar Terrain. Once thermal buoyancy produced stresses, the oscillating stresses from tides nonlinearly enhanced the rate of tectonic convection. Warm ice that dissipates tides now exists within Enceladus. The eccentricity will likely decrease and the object will then freeze. This article is protected by copyright. All rights reserved.
      PubDate: 2015-05-06T20:25:09.900742-05:
      DOI: 10.1002/2015GC005725
  • Nitrogen isotope ratios of nitrate and N* anomalies in the subtropical
           South Pacific
    • Authors: Chisato Yoshikawa; Akiko Makabe, Takuhei Shiozaki, Sakae Toyoda, Osamu Yoshida, Ken Furuya, Naohiro Yoshida
      Abstract: Nitrogen isotopic ratios of nitrate (δ15N–NO3−) were analyzed above 1000 m water depth along 17°S in the subtropical South Pacific during the revisit WOCE P21 cruise in 2009. The δ15N–NO3− and N* values were as high as 17‰ and as low as −18 μmol N L−1, respectively, at depths around 250 m east of 115°W, but were as low as 5‰ and as high as +1 μmol N L−1, respectively, in subsurface waters west of 170°W. The relationships among NO3− concentrations, N* values, δ15N–NO3− values, and oxygen and nitrite concentrations suggest that a few samples east of 90°W were from suboxic and nitrite‐accumulated conditions and were possibly affected by in situ water column denitrification. Most of the high‐δ15N–NO3− and negative‐N* waters were probably generated by mixing between Subantarctic Mode Water from the Southern Ocean and Oxygen Deficit Zone Water from the eastern tropical South Pacific, with remineralization of organic matter occurring during transportation. Moreover, the relationship between δ15N–NO3− and N* values, as well as Trichodesmium abundances and size‐specific nitrogen fixation rates at the surface, suggest that the low‐δ15N–NO3− and positive‐N* subsurface waters between 160°E and 170°W were generated by the input of remineralized particles created by in situ nitrogen fixation, mainly by Trichodesmium spp. Therefore, the δ15N values of sediments in this region are expected to reveal past changes in nitrogen fixation or denitrification rates in the subtropical South Pacific. This article is protected by copyright. All rights reserved.
      PubDate: 2015-05-05T09:03:34.38037-05:0
      DOI: 10.1002/2014GC005678
  • A seamless multitrack multitemporal InSAR algorithm
    • Authors: Manoochehr Shirzaei
      Abstract: The zones of overlap between adjacent Synthetic Aperture Radar (SAR) satellite tracks are illuminated twice more frequently than elsewhere in the SAR scene. Here, an alternative approach is presented to combine the overlapping segments of SAR images acquired at adjacent tracks and generate accurate and high spatiotemporal resolution map of the surface deformation field. To this end, a new approach is developed to unify the datums. Effects due to the difference in look angle between two overlapping tracks and atmospheric delay are estimated and removed using Kalman and wavelet based filters. This approach is first tested at Hawaii Island, where tracks 200 and 429 of Envisat C‐band satellite overlap over the Kilauea south flank. The obtained time series improves the temporal sampling rate by a factor of two and comparison with GPS time series demonstrates that the presented method accurately measures the nonlinear deformation field. The advantages of this method are further demonstrated by combining SAR data sets acquired by Envisat C‐band and ALOS L‐band satellites over the San Francisco Bay Area, California. The validation test shows that the seamless combination of C‐ and L‐Band time series accurately measures the surface deformation at higher resolution. This article is protected by copyright. All rights reserved.
      PubDate: 2015-05-04T03:29:04.061215-05:
      DOI: 10.1002/2015GC005759
  • The competing effects of sulfide saturation versus degassing on the
           behavior of the chalcophile elements during the differentiation of hydrous
    • Authors: Frances E. Jenner; Erik H. Hauri, Emma S. Bullock, Stephan König, Richard J. Arculus, John A. Mavrogenes, Nicole Mikkelson, Charlotte Goddard
      Abstract: There is a lack of consensus regarding the roles of sulfide saturation versus volatile degassing on the partitioning of Cu and Ag during differentiation and eruption of convergent margin magmas. Because of their oxidized character, volatile‐rich magmas from the Eastern Manus Backarc Basin (EMBB) only reach sulfide saturation following magnetite‐driven reduction of the melt: the so‐called “magnetite crisis”. If sulfide saturation typically precedes volatile saturation, the magnetite crisis will limit the proportion of Cu and Ag that can partition from the melt into an exsolving volatile‐rich phase, which may contribute to the sporadic occurrence of magmatic‐hydrothermal ore deposits at convergent margins. However, it is unclear whether the magnetite crisis is a common or rare event during differentiation of volatile‐rich magmas. We report major and trace element data for submarine volcanic glasses from the Tonga Arc‐proximal Valu Fa Ridge (VFR; SW Pacific). Cu‐Se‐Ag systematics of samples erupting at the southern VFR suggest magnetite fractionation triggered sulfide saturation. The similarity in chalcophile element systematics of the southern VFR and EMBB samples is unlikely to be coincidental, and may indicate the magnetite crisis is a common event during differentiation of hydrous melts. However, unlike many convergent margin magmas, it is unlikely the evolving VFR and EMBB were saturated in a S‐bearing volatile phase prior to magnetite fractionation. Hence, the metal‐depleting magnetite crisis may be restricted to backarc basin magmas that do not degas volatiles prior to magnetite fractionation and potentially convergent margin magmas fractionating at high‐pressures in the continental crust. This article is protected by copyright. All rights reserved.
      PubDate: 2015-04-30T02:41:45.079762-05:
      DOI: 10.1002/2014GC005670
  • Plume‐cratonic lithosphere interaction recorded by water and other
           trace elements in peridotite xenoliths from the Labait volcano, Tanzania
    • Authors: Hejiu Hui; Anne H. Peslier, Roberta L. Rudnick, Antonio Simonetti, Clive R. Neal
      Abstract: Water and other trace element concentrations in olivine (1‐39 ppm H2O), orthopyroxene (10‐150 ppm H2O), and clinopyroxene (16‐340 ppm H2O) of mantle xenoliths from the Labait volcano, located on the edge of the Tanzanian craton along the eastern branch of the East African Rift, record melting and subsequent refertilization by plume magmas in a stratified lithosphere. These water contents are at the lower end of the range observed in other cratonic mantle lithospheres. Despite correlations between water content and indices of melting in orthopyroxene from the shallow peridotites, and in both olivine and orthopyroxene from the deep peridotites, water concentrations are too high for the peridotites to be simple residues. Instead, the Labait water contents are best explained as reflecting interaction between residual peridotite with a melt having relatively low water content (
      PubDate: 2015-04-29T11:02:15.997231-05:
      DOI: 10.1002/2015GC005779
  • Early sedimentation and deformation in the Kumano forearc basin linked
           with Nankai accretionary prism evolution, southwest Japan
    • Authors: Sebastian G. Ramirez; Sean S. P. S. Gulick, Nicholas W. Hayman
      Abstract: Early forearc basin sedimentation and subsidence appear to be linked with convergent margin evolution, yet they do not follow predictable spatiotemporal patterns. In the Kumano basin of southwest Japan 3D seismic mapping of major unconformities, combined with biostratigraphic age constraints from two Integrated Ocean Drilling Program (IODP) drill sites illustrates the development of several tectonostratigraphic packages during early evolution and initial creation of accommodation space in the forearc basin. Between ∼3.8 Ma and ∼2.06 Ma a series of slope basins developed between thrust‐anticlines oriented along a plate convergence‐perpendicular axis. A thin and sheet‐like interval, likely slope‐cover, unconformably overlies the slope basin sediments. Deposition of this interval at IODP Site C0009 lasted from ∼2.06 Ma until between ∼1.24 Ma and ∼1.34 Ma and was partly coeval with the early upper Kumano basin sedimentary packages. Landward, postdating the slope‐cover sediments, a third sedimentary package was deposited before ∼0.9 Ma. This section correlates with an interval rich in terrigenous material (e.g. woody debris), indicating a possible distinct source or sediment routing system. Our work supports the idea that early forearc basin sediments may be deposited in an actively deforming outer wedge environment and that the outer‐to‐inner wedge transition of the lower forearc basin is likely a multi‐stage process. The unsteady spatiotemporal nature of inner‐outer wedge coupling may lead to erratic stratigraphic patterns, such as have been observed in forearc basins worldwide. This article is protected by copyright. All rights reserved.
      PubDate: 2015-04-25T02:15:52.030777-05:
      DOI: 10.1002/2014GC005643
  • Water circulation and global mantle dynamics: Insight from numerical
    • Authors: Takashi Nakagawa; Tomoeki Nakakuki, Hikaru Iwamori
      Abstract: We investigate water circulation and its dynamical effects on global‐scale mantle dynamics in numerical thermo‐chemical mantle convection simulations. Both dehydration–hydration processes and dehydration melting are included. We also assume the rheological properties of hydrous minerals and density reduction caused by hydrous minerals. Heat transfer due to mantle convection seems to be enhanced more effectively than water cycling in the mantle convection system when reasonable water dependence of viscosity is assumed, due to effective slab dehydration at shallow depths. Water still affects significantly the global dynamics by weakening the near‐surface oceanic crust and lithosphere, enhancing the activity of surface plate motion compared to dry mantle case. As a result, including hydrous minerals, the more viscous mantle is expected with several orders of magnitude compared to the dry mantle. The average water content in the whole mantle is regulated by the dehydration‐hydration process. The large‐scale thermo‐chemical anomalies, as is observed in the deep mantle, is found when a large density contrast between basaltic material and ambient mantle is assumed (4 to 5%), comparable to mineral physics measurements. Through this study, the effects of hydrous minerals in mantle dynamics are very important for interpreting the observational constraints on mantle convection. This article is protected by copyright. All rights reserved.
      PubDate: 2015-04-24T15:17:30.565885-05:
      DOI: 10.1002/2014GC005701
  • Mantle plume capture anchoring and outflow during Galápagos
           plume‐ridge interaction
    • Authors: S.A. Gibson; D. Geist, M.A. Richards
      Abstract: Compositions of basalts erupted between the main zone of Galápagos plume upwelling and adjacent Galápagos Spreading Centre (GSC) provide important constraints on dynamic processes involved in transfer of deep‐mantle‐sourced material to mid‐ocean ridges. We examine recent basalts from central and northeast Galápagos including some that have less radiogenic Sr, Nd and Pb isotopic compositions than plume influenced basalts (E‐MORB) from the nearby ridge. We show that the location of E‐MORB, greatest crustal thickness and elevated topography on the GSC correlates with a confined zone of low‐velocity, high‐temperature mantle connecting the plume stem and ridge at depths of ∼100 km. At this site on the ridge, plume‐driven upwelling involving deep melting of partially‐dehydrated, recycled ancient oceanic crust, plus plate‐limited shallow melting of anhydrous peridotite, generate E‐MORB and larger amounts of melt than elsewhere on the GSC. The first order control on plume stem to ridge flow is rheological rather than gravitational, and strongly influenced by flow regimes initiated when the plume was on axis (> 5 Ma). During subsequent northeast ridge migration material upwelling in the plume stem appears to have remained “anchored” to a contact point on the GSC. This deep, confined NE plume stem‐to‐ridge flow occurs via a network of melt channels, embedded within the normal spreading and advection of plume material beneath the Nazca plate, and coincides with locations of historic volcanism. Our observations require a more dynamically complex model than proposed by most studies, which rely on radial solid‐state outflow of heterogeneous plume material to the ridge. This article is protected by copyright. All rights reserved.
      PubDate: 2015-04-24T15:11:13.871683-05:
      DOI: 10.1002/2015GC005723
  • Provenance of plumes in global convection models
    • Authors: Rakib Hassan; Nicolas Flament, Michael Gurnis, Dan J. Bower, Dietmar Müller
      Abstract: In global convection models constrained by plume motions and subduction history over the last 230 Myr, plumes emerge preferentially from the edges of thermochemical structures that resemble present‐day Large Low Shear Velocity Provinces (LLSVPs) beneath Africa and the Pacific Ocean. It has been argued that Large Igneous Provinces (LIPs) erupting since 200 Ma may originate from plumes that emerged from the edges of the LLSVPs and numerical models have been devised to validate this hypothesis. Although qualitative assessments that are broadly in agreement with this hypothesis have been derived from numerical models, a quantitative assessment has been lacking. We present a novel plume detection scheme and derive Monte Carlo‐based statistical correlations of model plume eruption sites and reconstructed LIP eruption sites. We show that models with a chemically anomalous lower mantle are highly correlated to reconstructed LIP eruption sites, whereas the confidence level obtained for a model with purely thermal plumes falls just short of 95%. A network of embayments separated by steep ridges form in the deep lower mantle in models with a chemically anomalous lower mantle. Plumes become anchored to the peaks of the chemical ridges and the network of ridges acts as a floating anchor, adjusting to slab push forces through time. The network of ridges imposes a characteristic separation between conduits that can extend into the interior of the thermochemical structures. This may explain the observed clustering of reconstructed LIP eruption sites that mostly but not exclusively occur around the present‐day LLSVPs. This article is protected by copyright. All rights reserved.
      PubDate: 2015-04-24T15:06:22.42153-05:0
      DOI: 10.1002/2015GC005751
  • On the temporal evolution of long‐wavelength mantle structure of the
           Earth since the early Paleozoic
    • Authors: Shijie Zhong; Maxwell L. Rudolph
      Abstract: The seismic structure of the Earth's lower mantle is characterized by a dominantly degree‐2 pattern with the African and Pacific large low shear velocity provinces (i.e., LLSVP) that are separated by circum‐Pacific seismically fast anomalies. It is important to understand the origin of such a degree‐2 mantle structure and its temporal evolution. In this study, we investigated the effects of plate motion history and mantle viscosity on the temporal evolution of the lower mantle structure since the early Paleozoic by formulating 3‐D spherical shell models of thermochemical convection. For convection models with realistic mantle viscosity and no initial structure, it takes about ∼50 Myrs to develop dominantly degree‐2 lower mantle structure using the published plate motion models for the last either 120 Ma or 250 Ma. However, it takes longer time to develop the mantle structure for more viscous mantle. While the circum‐Pangea subduction in plate motion history models promotes the formation of degree‐2 mantle structure, the published pre‐Pangea plate motions before 330 Ma produce relatively cold lower mantle in the African hemisphere and significant degree‐1 structure in the early Pangea (∼300 Ma) or later times, even if the lower mantle has an initially degree‐2 structure and a viscosity as high as 1023 Pas. This suggests that the African LLSVP may not be stationary since the early Paleozoic. With the published plate motion models and lower mantle viscosity of 1022 Pas, our mantle convection models suggest that the present‐day degree‐2 mantle structure may have largely been formed by ∼200 Ma. This article is protected by copyright. All rights reserved.
      PubDate: 2015-04-24T05:54:37.501933-05:
      DOI: 10.1002/2015GC005782
  • The insular shelves of the Faial–Pico Ridge (Azores archipelago): A
           morphological record of its evolution
    • Authors: R. Quartau; J. Madeira, N. C. Mitchell, F. Tempera, P. F. Silva, F. Brandão
      Abstract: Shelves surrounding reefless volcanic ocean islands are formed by surf erosion of their slopes during changing sea levels. Post‐erosional lava flows, if abundant, can cross the coastal cliffs and fill partially or completely the accommodation space left by erosion. In this study, multibeam bathymetry, high‐resolution seismic reflection profiles and sediment samples are used to characterize the morphology of the insular shelves adjacent to Faial and Pico islands. The data shows offshore fresh lava flow morphologies, as well as an irregular basement beneath shelf sedimentary bodies and reduced shelf width adjacent to older volcanic edifices. These observations suggest that the shelves have been significantly filled by volcanic progradation and can thus be classified as “rejuvenated”. Despite the general volcanic infilling of the shelves around the islands, most of their edges are below the depth of the Last Glacial Maximum, revealing that that at least parts of the island have subsided after the shelves formed by surf erosion. Prograding lava deltas reached the shelf edge in some areas triggering small slope failures, locally decreasing the shelf width and depth of their edges. These areas can represent a significant risk for the local population; hence their identification can be useful for hazard assessment and contribute to wiser land‐use planning. Shelf morphology, subaerial geomorphology, magnetic anomalies and crustal structure data were also interpreted to reconstruct the longer term combined onshore and offshore evolution of the Faial‐Pico ridge. The subaerial emergence of this ridge is apparently older than previously thought, i.e., before ∼850 ka. This article is protected by copyright. All rights reserved.
      PubDate: 2015-04-15T10:57:56.793069-05:
      DOI: 10.1002/2015GC005733
  • Magnetic and geochemical characterization of Andosols developed on basalts
           in the Massif Central, France
    • Authors: Hana Grison; Eduard Petrovsky, Sarka Stejskalova, Ales Kapicka
      Abstract: Identification of Andosols is primarily based upon the content of their colloidal constituents – clay and metal‐humus complexes – and on the determining of andic properties. This needs time and cost‐consuming geochemical analyses. Our primary aim of this study is to describe the magnetic and geochemical properties of soils rich in iron oxides derived from strongly magnetic volcanic basement (in this case Andosols). Secondary aim is to explore links between magnetic and chemical parameters of Andic soils with respect to genesis factors: parent material age, precipitation and thickness of the soil profile. Six pedons of andic properties, developed on basaltic lavas, were analysed down to parent rock by a set of magnetic and geochemical methods. Magnetic data of soil and rock samples reflect the type, concentration and particle‐size distribution of ferrimagnetic minerals. Geochemical data include soil reaction (pH in H2O), cation exchange capacity, organic carbon and different forms of extractable iron and aluminum content. Our results suggest the following: (1) Magnetic measurements of low‐field mass‐specific magnetic susceptibility can be a reliable indicator for estimating andic properties, and in combination with thermomagnetic curves may be suitable for discriminating between alu‐andic and sil‐andic subtypes. (2) In the studied Andosols, strong relationships were found between (a) magnetic grain‐size parameters, precipitation and exchangeable bases; (b) concentration of ferrimagnetic particles and degree of crystallization of free iron; and (c) parameters reflecting changes in magneto‐mineralogy and soil genesis (parent material age + soil depth). This article is protected by copyright. All rights reserved.
      PubDate: 2015-04-15T10:44:12.802248-05:
      DOI: 10.1002/2015GC005716
  • Abbot Ice Shelf, structure of the Amundsen Sea continental margin and the
           southern boundary of the Bellingshausen Plate seaward of West Antarctica
    • Authors: James R. Cochran; Kirsty J. Tinto, Robin E. Bell
      Abstract: Inversion of NASA Operation IceBridge airborne gravity over the Abbot Ice Shelf in West Antarctica for sub‐ice bathymetry defines an extensional terrain made up of east‐west trending rift basins formed during the early stages of Antarctica/Zealandia rifting. Extension is minor, as rifting jumped north of Thurston Island early in the rifting process. The Amundsen Sea Embayment continental shelf west of the rifted terrain is underlain by a deeper, more extensive sedimentary basin also formed during rifting between Antarctica and Zealandia. A well‐defined boundary zone separates the mildly extended Abbot extensional terrain from the deeper Amundsen Embayment shelf basin. The shelf basin has an extension factor, β, of 1.5‐1.7 with 80‐100 km of extension occurring across an area now 250 km wide. Following this extension, rifting centered north of the present shelf edge and proceeded to continental rupture. Since then, the Amundsen Embayment continental shelf appears to have been tectonically quiescent and shaped by subsidence, sedimentation and the advance and retreat of the West Antarctic Ice Sheet. The Bellingshausen Plate was located seaward of the Amundsen Sea margin prior to incorporation into the Antarctic Plate at about 62 Ma. During the latter part of its independent existence, Bellingshausen plate motion had a clockwise rotational component relative to Antarctica producing convergence across the north‐south trending Bellingshausen Gravity Anomaly structure at 94°W and compressive deformation on the continental slope between 94°W and 102°W. Farther west, the relative motion was extensional along an east‐west trending zone occupied by the Marie Byrd Seamounts. This article is protected by copyright. All rights reserved.
      PubDate: 2015-04-15T10:43:16.07419-05:0
      DOI: 10.1002/2014GC005570
  • Geochemical variations in Japan Sea back‐arc basin basalts formed by
           high‐temperature adiabatic melting of mantle metasomatized by
           sediment subduction components
    • Authors: Yuka Hirahara; Jun‐Ichi Kimura, Ryoko Senda, Takashi Miyazaki, Hiroshi Kawabata, Toshiro Takahashi, Qing Chang, Vaglarov S. Bogdan, Takeshi Sato, Shuichi Kodaira
      Abstract: The Yamato Basin in the Japan Sea is a back‐arc basin characterized by basaltic oceanic crust that is twice as thick as typical oceanic crust. Two types of ocean floor basalts, formed during the opening of the Japan Sea in the Middle Miocene, were recovered from the Yamato Basin during Ocean Drilling Program Legs 127/128. These can be considered as depleted (D‐type) and enriched (E‐type) basalts based on their incompatible trace element and Sr–Nd–Pb–Hf isotopic compositions. Both types of basalts plot along a common mixing array drawn between depleted mantle and slab sediment represented by a sand‐rich turbidite on the Pacific Plate in the NE Japan forearc. The depleted nature of the D‐type basalts suggests that the slab sediment component is nil to minor relative to the dominant mantle component, whereas the enrichment of all incompatible elements in the E‐type basalts was likely caused by a large contribution of bulk slab sediment in the source. The results of forward model calculations using adiabatic melting of a hydrous mantle with sediment flux indicate that the melting conditions of the source mantle for the D‐type basalts are deeper and hotter than those for the E‐type basalts, which appear to have formed under conditions hotter than those of normal mid‐oceanic ridge basalts (MORB). These results suggest that the thicker oceanic crust was formed by greater degrees of melting of a hydrous metasomatized mantle source at unusually high mantle potential temperature during the opening of the Japan Sea. This article is protected by copyright. All rights reserved.
      PubDate: 2015-04-15T10:42:48.440545-05:
      DOI: 10.1002/2015GC005720
  • Basalt Volatile Fluctuations During Continental Rifting: An Example From
           the Rio Grande Rift, USA
    • Authors: Michael C. Rowe; John C. Lassiter, Kathleen Goff
      Abstract: Hydration and metasomatism of the lithospheric mantle potentially influences both the magmatic and tectonic evolution of southwestern North America. Prior studies have suggested that volatile enrichments to the mantle underlying western North America resulted from shallow subduction of the Farallon Plate during the Laramide (∼74‐40 Ma). This study examines temporal and spatial variations in volatile elements (H2O, Cl, F, S) determined from olivine‐ and orthopyroxene‐hosted melt inclusions along and across the Rio Grande Rift, the easternmost extent of Laramide shallow subduction. Maximum chlorine enrichments are observed in the southern rift with a Cl/Nb of ∼210 and reduce with time to MORB‐OIB levels (∼5‐17). Measured water abundances are
      PubDate: 2015-04-14T14:10:47.507959-05:
      DOI: 10.1002/2014GC005649
  • Estimates of future warming‐induced methane emissions from hydrate
           offshore West Svalbard for a range of climate models
    • Authors: Héctor Marín‐Moreno; Timothy A. Minshull, Graham K. Westbrook, Bablu Sinha
      Abstract: Methane hydrate close to the hydrate stability limit in seafloor sediment could represent an important source of methane to the oceans and atmosphere as the oceans warm. We investigate the extent to which patterns of past and future ocean‐temperature fluctuations influence hydrate stability in a region offshore West Svalbard where active gas venting has been observed. We model the transient behavior of the gas hydrate stability zone at 400‐500 m water depth (mwd) in response to past temperature changes inferred from historical measurements and proxy data and we model future changes predicted by seven climate models and two climate‐forcing scenarios (Representative Concentration Pathways RCPs 2.6 and 8.5). We show that over the past 2000 yr, a combination of annual and decadal temperature fluctuations could have triggered multiple hydrate‐sourced methane emissions from seabed shallower than 400 mwd during episodes when the multi‐decadal average temperature was similar to that over the last century (∼2.6°C). These temperature fluctuations can explain current methane emissions at 400 mwd, but decades to centuries of ocean warming are required to generate emissions in water deeper than 420 m. In the venting area, future methane emissions are relatively insensitive to the choice of climate model and RCP scenario until 2050 yr, but are more sensitive to the RCP scenario after 2050 yr. By 2100 CE, we estimate an ocean uptake of 97‐1050 TgC from marine Arctic hydrate‐sourced methane emissions, which is 0.06‐0.67% of the ocean uptake from anthropogenic CO2 emissions for the period 1750‐2011. This article is protected by copyright. All rights reserved.
      PubDate: 2015-04-10T01:19:54.574729-05:
      DOI: 10.1002/2015GC005737
  • Investigation of the records of earthquake slip in carbonaceous materials
           from the Taiwan Chelungpu fault by means of infrared and Raman
    • Authors: Tetsuro Hirono; Yuka Maekawa, Hikaru Yabuta
      Abstract: To understand the mechanism of fault lubrication during the 1999 Taiwan Chi‐Chi earthquake, we developed a new temperature proxy for carbonaceous materials by using infrared and Raman spectroscopies together with heating and friction experiments. We found marked anomalies in the infrared and Raman spectra of carbonaceous materials retrieved from the primary slip zone of the earthquake: the infrared spectra exhibited very weak aliphatic CH2 and CH3 peaks and aromatic C=C absorbance peaks, and the Raman spectra exhibited very weak disordered and graphitic bands and a high ratio of disordered band area to graphitic band area. Those weak peaks and bands and the band area ratio were reproduced by heating carbonaceous materials from the nearby host rock to 700 °C. These results suggest that the frictional heat in the slip zone reached approximately 700 °C. We characterized the host rock's carbonaceous materials by means of elemental analysis, pyrolysis–gas chromatography–mass spectrometry, and simultaneous thermogravimetry–differential scanning calorimetry and found that the H/C and O/C ratios were 0.108 and 0.400, respectively (which are close to the ratios for lignin) and that the volatile fraction was as high as 48 wt %. The pyrolysates obtained by heating from 100 to 400 °C were dominated by phenols, fatty alcohols, and n‐alkanes. When the residue from pyrolysis at 100–400 °C was rapidly heated to 700 °C, the resulting pyrolysate was dominated by phenols, aromatic compounds, heterocyclic compounds, and n‐alkenes. This information suggests that changes in the infrared and Raman spectra with increasing temperature may have been due to decomposition and aromatization reactions during pyrolysis. Rapid heating during earthquake slip may promote reactions of carbonaceous materials that are different from the reactions that occur during long‐term metamorphism. This article is protected by copyright. All rights reserved.
      PubDate: 2015-04-09T02:34:31.598074-05:
      DOI: 10.1002/2014GC005622
  • Occurrence of greigite in the Pliocene sediments of Lake Qinghai, China,
           and its paleoenvironmental and paleomagnetic implications
    • Authors: FU Chaofeng; Jan Bloemendal, Qiang Xiaoke, Mimi J. Hill, AN ZhiSheng
      Abstract: Lake Qinghai in North China is the largest interior plateau lake in Central Asia and is climatically sensitive. An almost continuous 626‐m long sediment core was drilled in an in‐filled part of the southern lake basin of Lake Qinghai. The magnetic susceptibility record reveals the presence of two distinct peaks within an interval of fine‐grained lacustrine sediments of Lower Pliocene age. We selected a depth interval of approximately 40 m spanning the magnetic susceptibility peaks for detailed rock magnetic and geochemical analyses in order to identify the magnetic mineralogy responsible and to assess its possible paleoenvironmental and paleomagnetic implications. Rock magnetic, X‐ray diffraction analysis, scanning electron microscopy (SEM) and energy dispersive analysis of X‐ray (EDAX) analyses reveal that the main magnetic carrier is greigite (Fe3S4). The greigite is of early diagenetic origin and formed in an interval of high lake level and inferred relatively warm, humid climate. The greigite‐enriched zones are separated by an interval of relatively high total sulfur and organic carbon content and we infer that in the adjacent greigite‐bearing zones the lower concentrations of sulfur and organic carbon, and high levels of reactive iron, arrested the process of pyritization resulting in the preservation of the greigite on a timescale of several million years. The greigite zones contain narrow intervals of normally‐magnetized sediments which may be previously unrecognized cryptochrons within the Gilbert Chron, or alternatively they may reflect the continued formation of greigite long after the age of deposition of the surrounding sediment matrix. This article is protected by copyright. All rights reserved.
      PubDate: 2015-04-09T02:33:28.330578-05:
      DOI: 10.1002/2014GC005677
  • Quantifying near‐field and off‐fault deformation patterns of
           the 1992 Mw 7.3 Landers earthquake
    • Authors: Christopher W.D. Milliner; James F. Dolan, James Hollingsworth, Sebastien Leprince, Francois Ayoub, Charles Sammis
      Abstract: Co‐seismic surface deformation in large earthquakes is typically measured using field mapping and with a range of geodetic methods (e.g., InSAR, lidar differencing, and GPS). Current methods, however, either fail to capture patterns of near‐field co‐seismic surface deformation or lack pre‐event data. Consequently, the characteristics of off‐fault deformation and the parameters that control it remain poorly understood. We develop a standardized method to fully measure the surface, near‐field, co‐seismic deformation patterns at high‐resolution using the COSI‐Corr program by correlating pairs of aerial photographs taken before and after the 1992 Mw 7.3 Landers earthquake. COSI‐Corr offers the advantage of measuring displacement across the entire zone of surface deformation and over a wider aperture than that available to field geologists. For the Landers earthquake, our measured displacements are systematically larger than the field measurements, indicating the presence of off‐fault deformation. We show that 46 % of the total surface displacement occurred as off‐fault deformation, over a mean deformation width of 154 m. The magnitude and width of off‐fault deformation along the rupture is primarily controlled by the macroscopic structural complexity of the fault system, with a weak correlation with the type of near‐surface materials through which the rupture propagated. Both the magnitude and width of distributed deformation are largest in stepovers, bends, and at the southern termination of the surface rupture. We find that slip along the surface rupture exhibits a consistent degree of variability at all observable length scales and that the slip distribution is self‐affine fractal with dimension of 1.56. This article is protected by copyright. All rights reserved.
      PubDate: 2015-04-07T06:22:20.782611-05:
      DOI: 10.1002/2014GC005693
  • What processes control the chemical compositions of arc front
    • Authors: Stephen J Turner; Charles H Langmuir
      Abstract: Arc front stratovolcanoes have global chemical systematics that constrain processes at convergent margins. Positive correlations exist for arc averages among “fluid mobile,” “high field strength,” and “large ion lithophile” elements. 143Nd/144Nd and 87Sr/86Sr from rear‐arc lavas lacking subduction signature align with the oceanic “mantle array,” and correlate with arc front 143Nd/144Nd. Most chemical parameters (but not isotopes) also correlate well with crustal thickness and slightly less well with the slab thermal parameter, but not with the depth of the slab or model slab surface temperatures. Successful models of arc volcanism should account for these global regularities. Two distinct models can quantitatively account for the observations—different extents of melting of the mantle wedge caused by variations in wedge thermal structure, or varying contributions from the subducting slab owing to variations in the slab thermal structure. The wedge melting model has constant contributions from ocean crust, sediment and mantle wedge to lavas globally, while the slab model varies slab contributions with slab temperature. The wedge melting model fit improves by incorporating convergence rate and slab dip, which should affect the wedge thermal structure; the slab model is not supported by a similar analysis. The wedge model also more easily accommodates the isotope data. The two models predict different primary H2O contents, with large variations in H2O for the wedge model, and relatively constant H2O for the slab model. An evaluation of the effects of varying sediment compositions on arc lavas will benefit from considering the very different consequences of the two models. This article is protected by copyright. All rights reserved.
      PubDate: 2015-04-07T05:31:15.394429-05:
      DOI: 10.1002/2014GC005633
  • A protocol for variable‐resolution first‐order reversal curve
           (FORC) measurements
    • Authors: Xiang Zhao; David Heslop, Andrew P. Roberts
      Abstract: High‐resolution first‐order reversal curve (FORC) diagrams are being increasingly used in rock and environmental magnetism, including for detection of biomagnetic signals in sediments. Resolution can be a major barrier to obtaining high‐quality FORC diagrams and time‐consuming measurements that employ small field steps are necessary to resolve the finest features of a FORC distribution. We present a new experimental protocol with irregularly spaced field steps that allow different parts of a FORC diagram to be measured at different resolutions. Larger numbers of measurements can, therefore, be made in key regions of a FORC distribution to resolve diagnostic features at higher resolution. Specification of the field steps in the irregular measurement grid is based on major hysteresis properties; no a priori knowledge concerning the underlying FORC distribution is required. FORC diagrams obtained with conventional measurements and with our new measurement protocol give consistent results. Because of its variable resolution, the irregular protocol provides a clear representation of fine‐scale features produced by quasi‐reversible superparamagnetic and non‐interacting single‐domain particles. Although the proposed irregular measurement protocol is not as efficient at suppressing noise as recently developed post‐processing techniques (e.g., VARIFORC, Egli [2013]), it enables efficient high‐resolution analysis for relatively strongly magnetized samples where measurement noise is not detrimental to FORC distribution estimation. This article is protected by copyright. All rights reserved.
      PubDate: 2015-04-07T04:58:19.327094-05:
      DOI: 10.1002/2014GC005680
  • Development of topography in 3‐D continental collision models
    • Authors: A. E. Pusok; Boris J. P. Kaus
      Abstract: Understanding the formation and evolution of high mountain belts, such as the Himalayas and the adjacent Tibetan Plateau, has been the focus of many tectonic and numerical models. Here, we employ 3D numerical simulations to investigate the role that subduction, collision and indentation play on lithosphere dynamics at convergent margins, and to analyze the conditions under which large topographic plateaus can form in an integrated lithospheric and upper‐mantle scale model. Distinct dynamics are obtained for the oceanic subduction side (trench retreat, slab roll‐back) and the continental‐collision side (trench advance, slab detachment, topographic uplift, lateral extrusion). We show that slab‐pull alone is insufficient to generate high topography in the upper‐plate, and that external forcing and the presence of strong blocks such as the Tarim Basin are necessary to create and shape anomalously high topographic fronts and plateaus. Moreover, scaling is used to predict four different modes of surface expression in continental‐collision models: (I)‐low‐amplitude homogenous shortening, (II)‐high‐amplitude homogenous shortening, (III)‐Alpine‐type topography with topographic front and low plateau, and (IV)‐Tibet‐Himalaya‐type topography with topographic front and high plateau. Results of semi‐analytical models suggest that the Argand number governs the formation of high topographic fronts, while the amplitude of plateaus is controlled by the initial buoyancy ratio of the upper plate. Applying these results to natural examples, we show that the Alps belong to regime (III), the Himalaya‐Tibet to regime (IV), whereas the Andes‐Altiplano fall at the boundary between regimes (III)‐(IV). This article is protected by copyright. All rights reserved.
      PubDate: 2015-04-06T10:53:20.148008-05:
      DOI: 10.1002/2015GC005732
  • Water in Hawaiian peridotite minerals: A case for a dry metasomatized
           oceanic mantle lithosphere
    • Authors: Anne H. Peslier; Michael Bizimis
      Abstract: The distribution of water concentrations in the upper mantle has drastic influence on its melting, rheology, and electrical and thermal conductivities and yet is primarily known indirectly from analyses of OIB and MORB. Here, actual mantle samples, eight peridotite xenoliths from Salt Lake Crater (SLC) and one from Pali in Oahu in Hawaii were analyzed by FTIR. Water contents of orthopyroxene, clinopyroxene and the highest measured in olivine are 116‐222, 246‐442, and 10‐26 ppm weight H2O respectively. Although pyroxene water contents correlate with indices of partial melting, they are too high to be explained by simple melting modeling. Mantle‐melt interaction modeling reproduces best the SLC data. These peridotites represent depleted oceanic mantle older than the Pacific lithosphere that has been refertilized by nephelinite melts containing
      PubDate: 2015-04-06T10:35:52.190899-05:
      DOI: 10.1002/2015GC005780
  • The electrical structure of the central Pacific upper mantle constrained
           by the NoMelt experiment
    • Authors: Emily Sarafian; Rob. L. Evans, John Collins, Jimmy Elsenbeck, Glenn Gaetani, James B. Gaherty, Greg Hirth, Daniel Lizarralde
      Abstract: The NoMelt Experiment imaged the mantle beneath 70 Ma Pacific seafloor with the aim of understanding the transition from the lithosphere to the underlying convecting asthenosphere. Seafloor magnetotelluric data from four stations were analyzed using 2‐D regularized inverse modeling. The preferred electrical model for the region contains an 80 km thick resistive (>103 Ωm) lithosphere with a less resistive (∼50 Ωm) underlying asthenosphere. The preferred model is isotropic and lacks a highly conductive (≤10 Ωm) layer under the resistive lithosphere that would be indicative of partial melt. We first examine temperature profiles that are consistent with the observed conductivity profile. Our profile is consistent with a mantle adiabat ranging from 0.3 °C/km‐0.5 °C/km. A choice of the higher adiabatic gradient means that the observed conductivity can be explained solely by temperature. In contrast, a 0.3 °C/km adiabat requires an additional mechanism to explain the observed conductivity profile. Of the plausible mechanisms, H2O, in the form of hydrogen dissolved in olivine, is the most likely explanation for this additional conductivity. Our profile is consistent with a mostly dry lithosphere to 80 km depth, with bulk H2O contents increasing to between 25 and 400 ppm by weight in the asthenosphere with specific values dependent on the choice of laboratory dataset of hydrous olivine conductivity and the value of mantle oxygen fugacity. The estimated H2O contents support the theory that the rheological lithosphere is a result of dehydration during melting at a mid‐ocean ridge with the asthenosphere remaining partially hydrated and weakened as a result. This article is protected by copyright. All rights reserved.
      PubDate: 2015-03-28T08:38:09.515254-05:
      DOI: 10.1002/2014GC005709
  • Tracking the weathering of basalts on Mars using lithium isotope
           fractionation models
    • Authors: Alberto G. Fairén; Elisabeth L. Adams, Carolina Gil, Luis Gago‐Duport, Esther R. Uceda, Steven W. Squyres, J. Alexis P. Rodríguez, Alfonso F. Davila, Christopher P. McKay
      Abstract: Lithium (Li), the lightest of the alkali elements, has geochemical properties that include high aqueous solubility (Li is the most fluid mobile element) and high relative abundance in basalt‐forming minerals (values ranking between 0.2 and 12 ppm). Li isotopes are particularly subject to fractionation because the two stable isotopes of lithium – 7Li and 6Li – have a large relative mass difference (∼15%) that results in significant fractionation between water and solid phases. The extent of Li isotope fractionation during aqueous alteration of basalt depends on the dissolution rate of primary minerals – the source of Li – and on the precipitation kinetics leading to formation of secondary phases. Consequently, a detailed analysis of Li isotopic ratios in both solution and secondary mineral lattices could provide clues about past martian weathering conditions, including weathering extent, temperature, pH, supersaturation and evaporation rate of the initial solutions in contact with basalt rocks. In this paper we discuss ways in which martian aqueous processes could have lead to Li isotope fractionation. We show that Li isotopic data obtained by future exploration of Mars could be relevant to highlighting different processes of Li isotopic fractionation in the past, and therefore to understanding basalt weathering and environmental conditions early in the planet's history. This article is protected by copyright. All rights reserved.
      PubDate: 2015-03-28T08:18:22.479653-05:
      DOI: 10.1002/2015GC005748
  • An open source Bayesian Monte Carlo isotope mixing model with applications
           in Earth surface processes
    • Authors: Carli A. Arendt; Sarah M. Aciego, Eric A. Hetland
      Abstract: The implementation of isotopic tracers as constraints on source contributions has become increasingly relevant to understanding Earth surface processes. Interpretation of these isotopic tracers has become more accessible with the development of Bayesian Monte Carlo (BMC) mixing models, which allow uncertainty in mixing end‐members and provide methodology for systems with multi‐component mixing. This study presents an open source multiple isotope BMC mixing model that is applicable to Earth surface environments with sources exhibiting distinct end‐member isotopic signatures. Our model is first applied to new δ18O and δD measurements from the Athabasca Glacier, which showed expected seasonal melt evolution trends and vigorously assessed the statistical relevance of the resulting fraction estimations. To highlight the broad applicability of our model to a variety of Earth surface environments and relevant isotopic systems we expand our model to two additional case studies: deriving melt sources from δ18O, δD and 222Rn measurements of Greenland Ice Sheet bulk water samples and assessing nutrient sources from ϵNd and 87Sr/86Sr measurements of Hawaiian soil cores. The model produces results for the Greenland Ice Sheet and Hawaiian soil datasets that are consistent with the originally published fractional contribution estimates. The advantage of this method is that it quantifies the error induced by variability in the end‐member compositions, unrealized by the models previously applied to the above case studies. Results from all three case studies demonstrate the broad applicability of this statistical BMC isotopic mixing model for estimating source contribution fractions in a variety of Earth surface systems. This article is protected by copyright. All rights reserved.
      PubDate: 2015-03-27T03:36:25.743217-05:
      DOI: 10.1002/2014GC005683
  • The thinning of subcontinental lithosphere: The roles of plume impact and
           metasomatic weakening
    • Authors: Hongliang Wang; Jeroen van Hunen, D. Graham Pearson
      Abstract: Geologically rapid (10s of Myr) partial removal of thick continental lithosphere is evident beneath Precambrian terranes such as North China craton, southern Africa and the North Atlantic Craton, and has been linked with thermo‐mechanical erosion by mantle plumes. We performed numerical experiments with realistic viscosities to test this hypothesis and constrain the most important parameters that influence cratonic lithosphere erosion. Our models indicate that the thermo‐mechanical erosion by a plume impact on typical Archean lithospheric mantle is unlikely to be more effective than long‐term erosion from normal plate‐mantle interaction. Therefore, unmodified cratonic roots that have been stable for billions of years will not be significantly disrupted by the erosion of a plume event. However, the buoyancy and strength of highly depleted continental roots can be modified by fluid‐melt metasomatism, and our models show this is essential for the thinning of originally stable continental roots. The long‐term but punctuated history of metasomatic enrichment beneath ancient continents makes this mode of weakening very likely. The effect of the plume impact is to speed up the erosion significantly and help the removal of the lithospheric root to occur within 10s of Myrs if affected by metasomatic weakening. This article is protected by copyright. All rights reserved.
      PubDate: 2015-03-27T03:36:14.766436-05:
      DOI: 10.1002/2015GC005784
  • Azimuthal anisotropy beneath north central Africa from shear wave
           splitting analyses
    • Authors: Awad A. Lemnifi; Kelly H. Liu, Stephen S. Gao, Cory A. Reed, Ahmed A. Elsheikh, Youqiang Yu, Abdala A. Elmelade
      Abstract: This study represents the first multi‐station investigation of azimuthal anisotropy beneath the interior of north central Africa, including Libya and adjacent regions, using shear wave splitting (SWS) analysis. Data used in the study include recently available broadband seismic data obtained from 15 stations managed by the Libyan Center for Remote Sensing and Space Science, and those from 5 other stations at which data are publicly accessible. A total of 583 pairs of high‐quality SWS measurements utilizing the PKS, SKKS, and SKS phases demonstrate primarily N‐S fast orientations with an average splitting delay time of approximately 1.2 s. An absence of periodic azimuthal variation of the observed splitting parameters indicates the presence of simple anisotropy, and lack of correlation between surficial features and the splitting parameters suggests that the origin of the observed anisotropy is primarily asthenospheric. This conclusion is enhanced by non‐periodic azimuthal variation of the splitting parameters observed at one of the stations located near the boundary of areas with different anisotropic properties. We interpret the observed anisotropy to be the consequence of northward movement of the African plate relative to the asthenosphere toward the Hellenic and Calabrian subduction zones. Local variance in fast orientations may be attributable to flow deflection by the northern edge of the African continental root. The observations provide critical and previously lacking constraints on mantle dynamic models in the vicinity of the convergent boundary between the African and Eurasian plates. This article is protected by copyright. All rights reserved.
      PubDate: 2015-03-27T03:36:03.520542-05:
      DOI: 10.1002/2014GC005706
  • Helium isotope, C/3He, and Ba‐Nb‐Ti signatures in the northern
           Lau Basin: Distinguishing arc, back‐arc, and hot spot affinities
    • Authors: John Lupton; Ken H. Rubin, Richard Arculus, Marvin Lilley, David Butterfield, Joseph Resing, Edward Baker, Robert Embley
      Abstract: The northern Lau Basin hosts a complicated pattern of volcanism, including Tofua Arc volcanoes, several back‐arc spreading centers and individual “rear‐arc” volcanoes not associated with these structures. Elevated 3He/4He ratios in lavas of the NW Lau Spreading Center suggest the influence of a mantle plume, possibly from Samoa. We show that lavas from mid‐ocean ridges, volcanic arcs, and hotspots occupy distinct, non‐overlapping fields in a 3He/4He vs. C/3He plot. Applied to the northern Lau Basin, this approach shows that most of Lau back‐arc spreading systems have mid‐ocean ridge 3He/4He ‐ C/3He characteristics, except the NW Lau spreading center, which has 3He/4He ‐ C/3He similar to “high 3He” hotspots such as Loihi, Kilauea, and Yellowstone, but with slightly lower C/3He. Niua seamount, on the northern extension of the Tofua Arc, falls squarely in the arc field. All of the NE Lau rear‐arc volcanoes, including the recently erupting West Mata, also have arc‐like 3He/4He ‐ C/3He characteristics. Ba‐Nb‐Ti contents of the lavas, which are more traditional trace element indicators of mantle source enrichment, depletion and subduction input, likewise indicate arc and hot spot influences in the lavas of the Northern Lau Basin, but in a more ambiguous fashion because of a complex prior history. This verifies that 3He/4He ‐ C/3He systematics are useful for differentiating between mid‐ocean ridge, arc, and hotspot affinities in submarine volcanic systems, that all three of these affinities are expressed in the northern Lau Basin, and provides additional support for the Samoan plume influence in the region. This article is protected by copyright. All rights reserved.
      PubDate: 2015-03-21T09:13:28.854924-05:
      DOI: 10.1002/2014GC005625
  • Gravity anomalies, crustal structure, and seismicity at subduction zones:
           2. Interrelationships between fore‐arc structure and seismogenic
    • Authors: Dan Bassett; Anthony B. Watts
      Abstract: An ensemble averaging technique is used to remove the long‐wavelength topography and gravity field associated with subduction zones. Short‐wavelength residual anomalies are attributed to the tectonic structure of subducting and overthrusting plates. A paired (positive‐negative) forearc anomaly is observed consisting of a long (>1000 km), linear, trench‐parallel ridge landward of the deep‐sea‐terrace basin. Ridges have amplitudes of 1500‐3000 m and 160‐240 mGal, wavelengths of 150‐200 km, and high gravity anomaly to topography ratios (50‐75 mGal km−1). The ridge crests correlate with the down‐dip limit of coseismic slip and strong interplate coupling and in Cascadia, the up‐dip limit of tremor epicenters. The ridge crest may be interpreted as defining the boundary between the velocity‐weakening and seismogenic region of the subduction interface and the down‐dip frictional transition zone. In Tonga‐Kermadec, the Kuril Islands and Chile landward ridges are associated with extinct volcanic arcs. Paired anomalies are attributed to the preferential subduction erosion of the outer‐forearc and a spatially varying combination of a) lower‐crustal underplating beneath the inner forearc, b) the transformation of interseismic strain into permanent geologic strain via faulting, folding or buckling of the inner‐forearc, and c) the relative trenchward migration of extinct volcanic arcs in regions operating with a net crustal deficit. Along‐strike transitions in forearc morphology and seismogenic behavior are related to pre‐existing crustal structure of subducting and overthrusting plates. Forearcs have the added potential of recording the time‐integrated response of the upper‐plate to subduction processes, and forearc structure should be considered in tandem with seismological observations. This article is protected by copyright. All rights reserved.
      PubDate: 2015-03-21T08:32:39.658955-05:
      DOI: 10.1002/2014GC005685
  • Episodic Holocene eruption of the Salton Buttes rhyolites, California,
           from paleomagnetic, U‐Th, and Ar/Ar dating
    • Authors: Heather M. Wright; Jorge A. Vazquez, Duane E. Champion, Andrew T. Calvert, Margaret T. Mangan, Mark Stelten, Kari M. Cooper, Charles Herzig, Alexander Schriener
      Abstract: In the Salton Trough, CA, five rhyolite domes form the Salton Buttes: Mullet Island, Obsidian Butte, Rock Hill, and North and South Red Hills, from oldest to youngest. Results presented here include 40Ar/39Ar anorthoclase ages, 238U‐230Th zircon crystallization ages, and comparison of remanent paleomagnetic directions with the secular variation curve, which indicate that all domes are Holocene. 238U‐230Th zircon crystallization ages are more precise but within uncertainty of 40Ar/39Ar anorthoclase ages, suggesting that zircon crystallization proceeded until shortly before eruption in all cases except one. Remanent paleomagnetic directions require three eruption periods: (1) Mullet Island, (2) Obsidian Butte, (3) Rock Hill, North Red Hill, and South Red Hill. Borehole cuttings logs document up to two shallow tephra layers. North and South Red Hill likely erupted within 100 years of each other, with a combined 238U‐230Th zircon isochron age of: 2.83 ± 0.60 ka (2 sigma); paleomagnetic evidence suggests this age predates eruption by hundreds of years (1800 cal BP). Rock Hill erupted closely in time to these eruptions. The Obsidian Butte 238U‐230Th isochron age (2.86 ± 0.96 ka) is nearly identical to the combined Red Hill age, but its Virtual Geomagnetic Pole position suggests a slightly older age. The age of aphyric Mullet Island dome is the least well constrained: zircon crystals are resorbed and the paleomagnetic direction is most distinct; possible Mullet Island ages include ca. 2300, 5900, 6900, and 7700 cal BP. Our results constrain the duration of Salton Buttes volcanism to between ca. 5900 and 500 years. This article is protected by copyright. All rights reserved.
      PubDate: 2015-03-20T06:13:13.923188-05:
      DOI: 10.1002/2015GC005714
  • The impact of splay faults on fluid flow, solute transport, and pore
           pressure distribution in subduction zones: A case study offshore the
           Nicoya Peninsula, Costa Rica
    • Authors: Rachel M. Lauer; Demian M. Saffer
      Abstract: Observations of seafloor seeps on the continental slope of many subduction zones illustrate that splay faults represent a primary hydraulic connection to the plate boundary at depth, carry deeply sourced fluids to the seafloor, and are in some cases associated with mud volcanoes. However, the role of these structures in forearc hydrogeology remains poorly quantified. We use a 2‐D numerical model that simulates coupled fluid flow and solute transport driven by fluid sources from tectonically driven compaction and smectite transformation to investigate the effects of permeable splay faults on solute transport and pore pressure distribution. We focus on the Nicoya margin of Costa Rica as a case study, where previous modeling and field studies constrain flow rates, thermal structure, and margin geology. In our simulations, splay faults accommodate up to 33% of the total dewatering flux, primarily along faults that outcrop within 25‐km of the trench. The distribution and fate of dehydration‐derived fluids is strongly dependent on thermal structure, which determines the locus of smectite transformation. In simulations of a cold end‐member margin, smectite transformation initiates 30‐km from the trench, and 64% of the dehydration‐derived fluids are intercepted by splay faults and carried to the middle and upper slope, rather than exiting at the trench. For a warm end‐member, smectite transformation initiates 7‐km from the trench, and the associated fluids are primarily transmitted to the trench via the décollement (50%), and faults intercept only 21% of these fluids. For a wide range of splay fault permeabilities, simulated fluid pressures are near lithostatic where the faults intersect overlying slope sediments, providing a viable mechanism for the formation of mud volcanoes. This article is protected by copyright. All rights reserved.
      PubDate: 2015-03-20T05:21:37.912904-05:
      DOI: 10.1002/2014GC005638
  • A global model for cave ventilation and seasonal bias in speleothem
           paleoclimate records
    • Authors: Eric W. James; Jay L. Banner, Benjamin Hardt
      Abstract: Cave calcite deposits (speleothems) provide long and continuous records of paleoenvironmental conditions in terrestrial settings. Typical environmental proxy measurements include speleothem growth rate and variations in elemental and isotope geochemistry. Commonly the assumption is made that speleothems grow continuously and at a constant rate throughout the year. However, seasonal variation of growth rate may be the rule in many caves. Here we apply observations of modern calcite growth and cave‐air CO2 concentrations and a model of factors controlling cave ventilation to construct a global model predicting where cave calcite growth may be seasonal. Previous models and measurements of calcite precipitation in caves demonstrate the retardation of speleothem growth by high cave‐air CO2. Elevated CO2 is commonly dissipated by ventilation driven by density differences between cave and surface air. Seasonal cycles in atmospheric temperature, pressure and humidity commonly drive these density contrasts. Modeling these changes latitudinally and globally indicates a geographic control on seasonal cave ventilation and thus on a principal controlling factor of speleothem growth. The model predicts that given constant water, calcium and CO2 inputs, speleothems from temperate to boreal continental regions commonly accumulate more calcite in the cool season and less or none in the warm season. These models predict that proxies from temperate to boreal speleothems may be seasonally biased due to seasonal ventilation, whereas tropical and maritime records should not. This article is protected by copyright. All rights reserved.
      PubDate: 2015-03-16T21:41:20.318093-05:
      DOI: 10.1002/2014GC005658
  • Natural remanent magnetization acquisition in bioturbated sediment:
           General theory and implications for relative paleointensity
    • Authors: R. Egli; X. Zhao
      Abstract: We present a general theory for the acquisition of natural remanent magnetizations (NRM) in sediment under the influence of (a) magnetic torques, (b) randomizing torques, and (c) torques resulting from interaction forces. Dynamic equilibrium between (a) and (b) in the water column and at the sediment‐water interface generates a detrital remanent magnetization (DRM), while much stronger randomizing torques may be provided by bioturbation inside the mixed layer. These generate a so‐called mixed remanent magnetization (MRM), which is stabilized by mechanical interaction forces. During the time required to cross the surface mixed layer, DRM is lost and MRM is acquired at a rate that depends on bioturbation intensity. Both processes are governed by a MRM lock‐in function. The final NRM intensity is controlled mainly by a single parameter γ that is defined as the product of rotational diffusion and mixed layer thickness, divided by sedimentation rate. This parameter defines three regimes: (1) slow mixing (γ  10) with MRM acquisition and full DRM randomization, and (3) intermediate mixing. Because the acquisition efficiency of DRM is larger than that of MRM, NRM intensity is particularly sensitive to γ in case of mixed regimes, generating variable NRM acquisition efficiencies. This model explains (1) lock‐in delays that can be matched with empirical reconstructions from paleomagnetic records, (2) the existence of small lock‐in depths that lead to DRM preservation, (3) specific NRM acquisition efficiencies of magnetofossil‐rich sediments, and (4) some relative paleointensity artifacts. This article is protected by copyright. All rights reserved.
      PubDate: 2015-03-16T21:40:50.193541-05:
      DOI: 10.1002/2014GC005672
  • Gravity anomalies, crustal structure, and seismicity at subduction zones:
           1. Seafloor roughness and subducting relief
    • Authors: Dan Bassett; Anthony B. Watts
      Abstract: An ensemble averaging technique is used to remove the long‐wavelength topography and gravity field from subduction zones. >200 residual bathymetric and gravimetric anomalies are interpreted within forearcs, many of which are attributed to the tectonic structure of the subducting plate. The residual‐gravimetric expression of subducting fracture zones extends >200 km landward of the trench‐axis. The bathymetric expression of subducting seamounts with height ≥1 km and area ≥500 km2 (N=36), and aseismic ridges (N>10), is largest near the trench (within 70 km) and above shallow subducting slab depths (SLAB1.0
      PubDate: 2015-03-14T11:10:03.004997-05:
      DOI: 10.1002/2014GC005684
  • Evaluating the utility of B/Ca ratios in planktic foraminifera as a proxy
           for the carbonate system: A case study of Globigerinoides ruber
    • Authors: Michael J. Henehan; Gavin L. Foster, James W. B. Rae, Katherine C. Prentice, Jonathan Erez, Helen C. Bostock, Brittney J. Marshall, Paul A. Wilson
      Abstract: B/Ca ratios in foraminifera have attracted considerable scientific attention as a proxy for past ocean carbonate system. However, the carbonate system controls on B/Ca ratios are not straightforward, with Δ[CO32−] ([CO32−]in situ – [CO32−]at saturation) correlating best with B/Ca ratios in benthic foraminifera, rather than pH,B(OH)4−HCO3−, or B(OH)4−DIC (as a simple model of boron speciation in seawater and incorporation into CaCO3 would predict). Furthermore, culture experiments have shown that in planktic foraminifera properties such as salinity and [B]sw can have profound effects on B/Ca ratios beyond those predicted by simple partition coefficients. Here, we investigate the controls on B/Ca ratios in G. ruber via a combination of culture experiments and core‐top measurements, and add to a growing body of evidence that suggests B/Ca ratios in symbiont‐bearing foraminiferal carbonate are not a straightforward proxy for past seawater carbonate system conditions. We find that, while B/Ca ratios in culture experiments co‐vary with pH, in open ocean sediments this relationship is not seen. In fact, our B/Ca data correlate best with [PO43‐] (a previously undocumented association) and in most regions, salinity. These findings might suggest a precipitation rate or crystallographic control on boron incorporation into foraminiferal calcite. Regardless, our results underscore the need for caution when attempting to interpret B/Ca records in terms of the ocean carbonate system, at the very least in the case of mixed‐layer planktic foraminifera. This article is protected by copyright. All rights reserved.
      PubDate: 2015-03-13T21:01:24.640984-05:
      DOI: 10.1002/2014GC005514
  • Tracking the Late Jurassic apparent (or true) polar shift in U‐Pb
           dated kimberlites from Cratonic North America (superior province of
    • Authors: Dennis V. Kent; Bruce A. Kjarsgaard, Jeffrey S. Gee, Giovanni Muttoni, Larry M. Heaman
      Abstract: Different versions of a composite apparent polar wander (APW) path of variably selected global poles assembled and averaged in North American coordinates using plate reconstructions show either a smooth progression or a large (∼30°) gap in mean paleopoles in the Late Jurassic, between about 160 Ma and 145 Ma. In an effort to further examine this issue, we sampled accessible outcrops/subcrops of kimberlites associated with high precision U‐Pb perovskite ages in the Timiskaming area of Ontario, Canada. The 154.9±2.4 Ma Peddie kimberlite yields a stable normal polarity magnetization that is coaxial within less than 5° of the reverse polarity magnetization of the 157.5±1.2 Ma Triple B kimberlite. The combined ∼156 Ma Triple B and Peddie pole (75.5°N, 189.5°E, A95=2.8°) lies about midway between igneous poles from North America nearest in age (169 Ma Moat volcanics and the 146 Ma Ithaca kimberlites) showing that the polar motion was at a relatively steady yet rapid (∼1.5°/Myr) pace. A similar large rapid polar swing has been recognized in the Middle to Late Jurassic APW path for Adria‐Africa and Iran‐Eurasia, suggesting a major mass redistribution. One possibility is that slab breakoff and subduction reversal along the western margin of the Americas triggered an episode of true polar wander. This article is protected by copyright. All rights reserved.
      PubDate: 2015-03-13T21:01:10.899923-05:
      DOI: 10.1002/2015GC005734
  • An upper‐mantle seismic discontinuity beneath the Galápagos
           Archipelago and its implications for studies of the
           lithosphere‐asthenosphere boundary
    • Authors: Joseph S. Byrnes; Emilie E. E. Hooft, Douglas R. Toomey, Darwin R. Villagómez, Dennis J. Geist, Sean C. Solomon
      Abstract: An upper‐mantle seismic discontinuity (the Gutenberg or G discontinuity), at which shear wave velocity decreases with depth, has been mapped from S‐to‐p conversions in radial receiver functions recorded across the Galápagos Archipelago. The mean depth of the discontinuity is 91 ± 8km beneath the southeastern archipelago and 72 ± 5km beneath surrounding regions. The discontinuity appears deeper beneath the portion of the Nazca plate that we infer passed over the Galápagos mantle plume than elsewhere in the region. We equate the depth of the G discontinuity to the maximum depth extent of anhydrous melting, which forms an overlying layer of dehydrated and depleted mantle. We attribute areas of shallow discontinuity depth to the formation of the dehydrated layer near the Galápagos Spreading Center and areas of greater discontinuity depth to its modification over a mantle plume with an excess temperature of 115 ± 30 °C. The G discontinuity lies within a high‐seismic‐velocity anomaly that we conclude forms by partial dehydration and a gradual but steady increase in seismic velocity with decreasing depth after upwelling mantle first encounters the solidus for volatile‐bearing mantle material. At the depth of the solidus for anhydrous mantle material, removal of remaining water creates a sharp increase in velocity with decreasing depth; this discontinuity may also mark a site of melt accumulation. Results from seismic imaging, the compositions of Galápagos lavas, and rare‐earth‐element concentrations across the archipelago require that mantle upwelling and partial melting occur over a broad region within the dehydrated and depleted layer. We conclude that the G discontinuity beneath the archipelago does not mark the boundary between rigid lithosphere and convecting asthenosphere. This article is protected by copyright. All rights reserved.
      PubDate: 2015-03-02T15:17:43.168769-05:
      DOI: 10.1002/2014GC005694
  • Structural heterogeneity of the midcrust adjacent to the central Alpine
           Fault, New Zealand: Inferences from seismic tomography and seismicity
           between Harihari and Ross
    • Authors: Sandra Bourguignon; Stephen Bannister, C. Mark Henderson, John Townend, Haijiang Zhang
      Abstract: Determining the rates and distributions of microseismicity near major faults at different points in the seismic cycle is a crucial step towards understanding plate boundary seismogenesis. We analyse data from temporary seismic arrays spanning the central section of the Alpine Fault, New Zealand, using double‐difference seismic tomography. This portion of the fault last ruptured in a large earthquake in 1717 AD and is now late in its typical 330‐year cycle of Mw∼8 earthquakes. Seismicity varies systematically with distance from the Alpine Fault: (1) directly beneath the fault trace, earthquakes are sparse and largely confined to the footwall at depths of 4–11km; (2) at distances of 0–9km southeast of the trace, seismicity is similarly sparse and shallower than 8km; (3) at distances of 9–20km southeast of the fault trace, earthquakes are much more prevalent and shallower than 7km. Hypocentre lineations here are subparallel to faults mapped near the Main Divide of the Southern Alps, confirming that those faults are active. The region of enhanced seismicity is associated with the highest topography and a high‐velocity tongue doming at 3–5km depth. The low‐seismicity zone adjacent to the Alpine Fault trace is associated with Vp and Vs values at mid‐crustal depths about 8% and 6% lower than further southeast. We interpret lateral variations in seismicity rate to reflect patterns of horizontal strain rate superimposed on heterogeneous crustal structure, and the variations in seismicity cutoff depth to be controlled by temperature and permeability structure variations. This article is protected by copyright. All rights reserved.
      PubDate: 2015-02-28T00:48:31.190045-05:
      DOI: 10.1002/2014GC005702
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