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

Geochemistry, Geophysics, Geosystems     Full-text available via subscription   (Followers: 26, SJR: 2.56, h-index: 69)
Geophysical Research Letters     Full-text available via subscription   (Followers: 54, SJR: 3.493, h-index: 157)
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J. of Geophysical Research : Solid Earth     Full-text available via subscription   (Followers: 26)
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Paleoceanography     Full-text available via subscription   (Followers: 3, SJR: 3.22, h-index: 88)
Radio Science     Full-text available via subscription   (Followers: 3, SJR: 0.959, h-index: 51)
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Tectonics     Full-text available via subscription   (Followers: 9, SJR: 2.748, h-index: 85)
Water Resources Research     Full-text available via subscription   (Followers: 102, SJR: 2.189, h-index: 121)
Journal Cover   Geochemistry, Geophysics, Geosystems
  [SJR: 2.56]   [H-I: 69]   [26 followers]  Follow
   Full-text available via subscription Subscription journal
   ISSN (Online) 1525-2027
   Published by American Geophysical Union (AGU) Homepage  [17 journals]
  • Visualizing tephra deposits and sedimentary processes in the marine
           environment: The potential of X‐ray microtomography
    • Authors: Adam J. Griggs; Siwan M. Davies, Peter M. Abbott, Mark Coleman, Adrian P. Palmer, Tine L. Rasmussen, Richard Johnston
      Abstract: Localised tephra deposition in marine sequences is the product of many complex primary and secondary depositional processes. These can significantly influence the potential applicability of tephra deposits as isochronous marker horizons and current techniques, used in isolation, may be insufficient to fully unravel these processes. Here, we demonstrate the innovative application of X‐ray microtomography (µCT) to successfully identify tephra deposits preserved within marine sediments and use these parameters to reconstruct their internal three‐dimensional structure. Three‐dimensional visualizations and animations of tephra dispersal in the sediment permits a more thorough assessment of post‐depositional processes revealing a number of complex micro‐sedimentological features that are not revealed by conventional methods. These features include bioturbation burrows and horizontally discontinuous tephra packages, which have important ramifications for the stratigraphic placement of the isochron in a sedimentary sequence. Our results demonstrate the potential for utilising rigorous two‐ and three‐dimensional micro‐sedimentological analysis of the ichnofabric to enhance and support the use of tephra deposits as isochronous marker horizons and to identify the stratigraphic position that best reflects the primary fallout of ash. The application also provides an exceptional insight into the style and rate of sedimentation processes and permits an assessment of the stratigraphic integrity of a tephra deposit. We discuss the possibility of applying these µCT methods to the identification of cryptotephras within various palaeoclimatic sequences and to enhance our understanding of marine sedimentation processes. This article is protected by copyright. All rights reserved.
      PubDate: 2015-11-20T10:46:19.51505-05:0
      DOI: 10.1002/2015GC006073
  • Submarine and subaerial lavas in the West Antarctic Rift System: Temporal
           record of shifting magma source components from the lithosphere and
    • Abstract: The petrogenesis of Cenozoic alkaline magmas in the West Antarctic Rift System (WARS) remains controversial, with competing models highlighting the roles of decompression melting due to passive rifting, active plume upwelling in the asthenosphere, and flux melting of a lithospheric mantle metasomatized by subduction. In this study, seamounts sampled in the Terror Rift region of the Ross Sea provide the first geochemical information from submarine lavas in the Ross Embayment in order to evaluate melting models. Together with subaerial samples from Franklin Island, Beaufort Island, and Mt. Melbourne in Northern Victoria Land (NVL), these Ross Sea lavas exhibit ocean island basalt (OIB)‐like trace element signatures and isotopic affinities for the C or FOZO mantle endmember. Major‐oxide compositions are consistent with the presence of multiple recycled lithologies in the mantle source region(s), including pyroxenite and volatile‐rich lithologies such as amphibole‐bearing, metasomatized peridotite. We interpret these observations as evidence that ongoing tectonomagmatic activity in the WARS is facilitated by melting of subduction‐modified mantle generated during 550 – 100 Ma subduction along the paleo‐Pacific margin of Gondwana. Following ingrowth of radiogenic daughter isotopes in high‐µ (U/Pb) domains, Cenozoic extension triggered decompression melting of easily fusible, hydrated metasomes. This multistage magma generation model attempts to reconcile geochemical observations with increasing geophysical evidence that the broad seismic low‐velocity anomaly imaged beneath West Antarctica and most of the Southern Ocean may be in part a compositional structure inherited from previous active margin tectonics. This article is protected by copyright. All rights reserved.
      PubDate: 2015-11-20T10:37:15.137957-05:
      DOI: 10.1002/2015GC006076
  • Diking‐induced moderate‐magnitude earthquakes on a youthful
           rift border fault: The 2002 Nyiragongo‐Kalehe sequence, D.R. Congo
    • Authors: C. Wauthier; B. Smets, D. Keir
      Abstract: On 24 October 2002, a Mw 6.2 earthquake occurred in the central part of the Lake Kivu basin, Western Branch of the East African Rift. This is the largest event recorded in the Lake Kivu area since 1900. An integrated analysis of radar interferometry (InSAR), seismic and geological data, demonstrates that the earthquake occurred due to normal‐slip motion on a major pre‐existing east‐dipping rift border fault. A Coulomb stress analysis suggests that diking events, such as the January 2002 dike intrusion, could promote faulting on the western border faults of the rift in the central part of Lake Kivu. We thus interpret that dike‐induced stress changes can cause moderate to large‐magnitude earthquakes on major border faults during continental rifting. Continental extension processes appear complex in the Lake Kivu basin, requiring the use of a hybrid model of strain accommodation and partitioning in the East African Rift. This article is protected by copyright. All rights reserved.
      PubDate: 2015-11-20T10:37:02.542431-05:
      DOI: 10.1002/2015GC006110
  • Coupling changes in densities and porosity to fluid pressure variations in
           reactive porous fluid flow: Local thermodynamic equilibrium
    • Authors: Benjamin Malvoisin; Yury Yu. Podladchikov, Johannes C. Vrijmoed
      Abstract: Mineralogical reactions which generate or consume fluids play a key role during fluid flow in porous media. Such reactions are linked to changes in density, porosity, permeability and fluid pressure which influence fluid flow and rock deformation. To understand such a coupled system equations were derived from mass conservation and local thermodynamic equilibrium. The presented mass conservative modelling approach describes the relationships between evolving fluid pressure, porosity, fluid and solid density, and devolatilization reactions in multi‐component systems with solid solutions. This first step serves as a framework for future models including aqueous speciation and transport. The complexity of univariant and multi‐variant reactions is treated by calculating look‐up tables from thermodynamic equilibrium calculations. Simplified cases were also investigated to understand previously studied formulations. For non‐deforming systems or systems divided into phases of constant density the equations can be reduced to porosity wave equations with addition of a reactive term taking the volume change of reaction into account. For closed systems an expression for the volume change of reaction and the associated pressure increase can be obtained. The key equations were solved numerically for the case of devolatilization of three different rock types that may enter a subduction zone. Reactions with positive Clapeyron slope lead to increase in porosity and permeability with decreasing fluid pressure resulting in sharp fluid pressure gradients around a negative pressure anomaly. The opposite trend is obtained for reactions having a negative Clapeyron slope during which sharp fluid pressure gradients were only generated around a positive pressure anomaly. Coupling of reaction with elastic deformation induces a more efficient fluid flow for reactions with negative Clapeyron slope than for reactions with positive Clapeyron slope. This article is protected by copyright. All rights reserved.
      PubDate: 2015-11-20T10:36:38.123727-05:
      DOI: 10.1002/2015GC006019
  • Vesiculation in rhyolite at low H2O contents: A thermodynamic model
    • Abstract: We present experimental data on the thermodynamics and kinetics of bubble nucleation and growth in weakly H2O‐oversaturated rhyolitic melts. The high‐temperature (900‐1100°C) experiments involve heating of rhyolitic obsidian from Hrafntinnuhryggur, Krafla, Iceland to above their glass transition temperature (Tg ∼ 690°C) at 0.1 MPa for times of 0.25‐24 hours. During experiments, the rhyolite cores increase in volume as H2O vapour‐filled bubbles nucleate and expand. The extent of vesiculation, as tracked by porosity, is mapped in temperature‐time (T‐t) space. At constant temperature and for a characteristic dwell time, the rhyolite cores achieve a maximum volume where the T‐t conditions reach thermochemical equilibrium. For each T‐t snapshot of vesiculation, we use 3D analysis of X‐ray computed tomographic (XCT) images of the quenched cores to obtain the bubble number density (BND) and bubble size distribution (BSD). BNDs for the experimental cores are insensitive to T and t, indicating a single nucleation event. All BSDs converge to a common distribution, independent of T, melt viscosity (η), or initial degree of saturation, suggesting a common growth process. We use these data to calibrate an empirical model for predicting the rates and amounts of vesiculation in rhyolitic melts as a function of η and thermochemical affinity (A): two computable parameters that are dependent on T, pressure and H2O content. The model reproduces the experimental dataset and data from the literature to within experimental error, and has application to natural volcanic systems where bubble formation and growth are not diffusion limited (e.g., lavas, domes, ignimbrites, conduit infill). This article is protected by copyright. All rights reserved.
      PubDate: 2015-11-20T10:36:21.889149-05:
      DOI: 10.1002/2015GC006024
  • Structure of the mantle beneath the Alboran basin from magnetotelluric
    • Abstract: We present results of marine MT acquisition in the Alboran sea that also incorporates previously acquired land MT from southern Spain into our analysis. The marine data show complex MT response functions with strong distortion due to seafloor topography and the coastline, but inclusion of high resolution topography and bathymetry and a seismically defined sediment unit into a 3D inversion model has allowed us to image the structure in the underlying mantle. The resulting resistivity model is broadly consistent with a geodynamic scenario that includes subduction of an eastward trending plate beneath Gibraltar, which plunges nearly vertically beneath the Alboran. Our model contains three primary features of interest: a resistive body beneath the central Alboran, which extends to a depth of ∼150 km. At this depth, the mantle resistivity decreases to values of ∼100 Ohm‐m, slightly higher than those seen in typical asthenosphere at the same depth. This transition suggests a change in slab properties with depth, perhaps reflecting a change in the nature of the seafloor subducted in the past. Two conductive features in our model suggest the presence of fluids released by the subducting slab or a small amount of partial melt in the upper mantle (or both). Of these, the one in the center of the Alboran basin, in the uppermost‐mantle (20‐30km depth) beneath Neogene volcanics and west of the termination of the Nekkor Fault, is consistent with geochemical models, which infer highly thinned lithosphere and shallow melting in order to explain the petrology of seafloor volcanics. This article is protected by copyright. All rights reserved.
      PubDate: 2015-11-14T03:25:06.058303-05:
      DOI: 10.1002/2015GC006100
  • Formation of andesite melts and Ca‐rich plagioclase in the submarine
           Monowai Volcanic System, Kermadec Arc
    • Authors: Fabian Kemner; Karsten M. Haase, Christoph Beier, Stefan Krumm, Philipp A. Brandl
      Abstract: Andesites are typical rocks of island arcs and may either form by fractional crystallization processes or by mixing between a mafic and a felsic magma. Here, we present new petrographic and geochemical data from lavas of the submarine Monowai volcanic system in the northern Kermadec island arc that display a continuous range in composition from basalt to andesite. Using petrology, major, trace and volatile element data we show that basaltic magmas mostly evolve to andesitic magmas by fractional crystallization. Our thermobarometric calculations indicate that the formation of the large caldera is related to eruption of basaltic‐andesitic to andesitic magmas from a magma reservoir in the deeper crust. Small variations in trace element ratios between the caldera and the large active cone imply a homogeneous mantle source. Contrastingly, resurgent dome melts of the caldera stagnated at shallower depths, are more depleted and show a stronger subduction input than the other edifices. The Monowai basaltic glasses contain less than 1 wt.% H2O and follow typical tholeiitic fractionation trends. High‐An plagioclase crystals observed in the Monowai lavas likely reflect mixing of H2O‐saturated melt batches with hot and dry tholeiitic, decompression melt batches. The result is a relatively H2O‐poor mafic magma at Monowai implying that partial melting of the mantle wedge is only partly due to the volatile flux and that adiabatic melting may play a significant role in the formation of the parental melts of the Monowai volcanic system and possibly other arc volcanoes. This article is protected by copyright. All rights reserved.
      PubDate: 2015-11-05T03:48:10.846402-05:
      DOI: 10.1002/2015GC005884
  • Anisotropic low‐velocity lower crust beneath the northeastern margin
           of Tibetan Plateau: Evidence for crustal channel flow
    • Authors: Xuzhang Shen; Xiaohui Yuan, Junsheng Ren
      Abstract: Detailed seismic structure in the crust beneath the northeastern margin of Tibetan Plateau was revealed by receiver functions of a regional permanent seismic network. At most stations, negative P‐to‐S converted phases can be detected in the radial receiver functions, prior to the Moho phases, indicating low velocities in the mid‐lower crust. Prominent azimuthal variations in the transverse receiver functions with polarity reversal suggest azimuthal anisotropy in the crust. We used time variations of the P‐to‐S converted phases at the Moho in the radial receiver functions and the azimuth‐weighted stacking of transverse receiver functions to determine the fast direction and magnitude of anisotropy. The low‐velocity mid‐lower crust with the coherent azimuthal anisotropy in the northeastern margin of Tibetan Plateau is consistent with the lower‐crustal channel flow model. This article is protected by copyright. All rights reserved.
      PubDate: 2015-11-05T03:47:44.514968-05:
      DOI: 10.1002/2015GC005952
  • Constraints on the evolution of crustal flow beneath northern Tibet
    • Authors: Florian Le Pape; Alan G. Jones, Martyn J. Unsworth, Jan Vozar, Wenbo Wei, Jin Sheng, Gaofeng Ye, Jianen Jing, Hao Dong, Letian Zhang, Chengliang Xie
      Abstract: Crustal flow is an important tectonic process active in continent‐continent collisions and which may be significant in the development of convergent plate boundaries. In this study, the results from multi‐dimensional electrical conductivity modeling have been combined with laboratory studies of the rheology of partially molten rocks to characterize the rheological behavior of the middle‐to‐lower crust of both the Songpan‐Ganzi and Kunlun terranes in the northern Tibetan Plateau. Two different methods are adopted to develop constraints on melt fraction, temperature and crustal flow velocity in the study area. The estimates of these parameters are then used to evaluate whether crustal flow can occur on the northern margin of the Tibetan plateau. In the Songpan‐Ganzi crust, all conditions are satisfied for topography‐driven channel flow to be dominant, with partial melt not being required for flow at temperature above 1000°C. Further north, the Kunlun fault defines the southern boundary of a transition zone between the Tibetan plateau and the Qaidam basin. Constrained by the estimated melt fractions, it is shown that channel injection across the fault requires temperatures close to 900°C. The composition of igneous rocks found at the surface confirm those conditions are met for the southern Kunlun ranges. To the north, the Qaidam basin is characterized by colder crust that may reflect an earlier stage in the channel injection process. In the study area at least 10% of the eastward directed Tibetan crustal flow could be deflected northwards across the Kunlun Fault and injected into the transition zone defining the northern margin of the Tibetan plateau. This article is protected by copyright. All rights reserved.
      PubDate: 2015-11-05T03:47:14.054077-05:
      DOI: 10.1002/2015GC005828
  • The meaning of mid‐lithospheric discontinuities: A case study in the
           northern U.S. craton
    • Authors: Emily Hopper; Karen M. Fischer
      Abstract: Converted wave imaging has revealed significant negative velocity gradients, often termed mid‐lithospheric discontinuities, within the thick, high velocity mantle beneath cratons. In this study we investigate the origins and implications of these structures with high resolution imaging of mantle discontinuities beneath the Archean Wyoming, Superior and Medicine Hat and Proterozoic Yavapai and Trans‐Hudson terranes. Sp phases from 872 temporary and permanent broadband stations, including the EarthScope Transportable Array, were migrated into three‐dimensional common conversion point stacks. Four classes of discontinuities were observed. 1) A widespread, near‐flat negative velocity gradient occurs largely at 70‐90 km depth beneath both Archean and Proterozoic cratons. This structure is consistent with the top of a frozen‐in layer of volatile‐rich melt. 2) Dipping negative velocity gradients are observed between 85‐200 km depth. The clearest examples occur at the suture zones between accreted Paleoproterozoic Yavapai arc terranes and the Wyoming and Superior cratons. These interfaces could represent remnant subducting slabs, and together with eclogite in xenoliths, indicate that subduction‐related processes likely contributed to cratonic mantle growth. 3) Sporadic positive velocity gradients exist near the base of the lithospheric mantle, perhaps due to laterally‐variable compositional layering. In contrast to off‐craton regions, clear Sp phases are typically not seen at lithosphere‐asthenosphere boundary depths beneath Archean and Proterozoic terranes, consistent with a purely thermal contrast between cratonic mantle lithosphere and asthenosphere. This article is protected by copyright. All rights reserved.
      PubDate: 2015-11-05T03:43:18.433534-05:
      DOI: 10.1002/2015GC006030
  • Bivalve shell horizons in seafloor pockmarks of the last
           glacial‐interglacial transition suggest a 1000 years of methane
           emissions in the Arctic Ocean
    • Abstract: We studied discrete bivalve shell horizons in two gravity cores from seafloor pockmarks on the Vestnesa Ridge (ca. 1200 m water depth), western Svalbard (79° 00' N, 06° 55' W) to provide insight into the temporal and spatial dynamics of seabed methane seeps. The shell beds, dominated by two genera of the family Vesicomyidae: Phreagena s.l. and Isorropodon sp., were 20‐30cm thick and centered at 250‐400cm deep in the cores. The carbon isotope composition of inorganic (δ13C from ‐13.02‰ to +2.36‰) and organic (δ13C from ‐29.28‰ to ‐21.33‰) shell material and a two‐end member mixing model indicate that these taxa derived between 8% and 43% of their nutrition from chemosynthetic bacteria. In addition, negative δ13C values for planktonic foraminifera (‐6.7‰ to ‐3.1‰), micritic concretions identified as methane‐derived authigenic carbonates, and pyrite encrusted fossil worm tubes at the shell horizons indicate a sustained paleo‐methane seep environment. Combining sedimentation rates with 14C ages for bivalve material from the shell horizons, we estimate the horizons persisted for about 1000 years between approximately 17,707 to 16,680 yrs. BP (corrected). The seepage event over a 1000‐year time interval was most likely associated with regional stress‐related faulting and the subsequent release of over‐pressurized fluids. This article is protected by copyright. All rights reserved.
      PubDate: 2015-11-05T03:43:08.147595-05:
      DOI: 10.1002/2015GC005980
  • Frictional properties of gabbro at conditions corresponding to slow slip
           events in subduction zones
    • Authors: E. K. Mitchell; Y. Fialko, K. M. Brown
      Abstract: We conducted a series of experiments to explore the rate and state frictional properties of gabbro at conditions thought to be representative of slow slip events (SSEs) in subduction zones. The experiments were conducted using a heated direct shear apparatus. We tested both solid and simulated gouge samples at low effective normal stress (5‐30 MPa) over a broad range of temperatures (20–600°C) under dry and hydrated conditions. In tests performed on dry solid samples we observed stable sliding at low temperatures (20–150°C), stick‐slip at high temperatures (350–600°C) and a transitional “episodic slow slip” behavior at intermediate temperatures (200–300°C). In tests performed on dry gouge samples we observed stable sliding at all temperatures. Under hydrated conditions, the gouge samples exhibited episodic slow slip and stick‐slip behavior at temperatures between 300–500°C. Our results show a decrease in the rate parameter (a– b) with temperature for both solid and gouge samples; friction transitions from velocity‐strengthening to velocity‐weakening at temperature of about 150°C for both solid and gouge samples. We do not observe transition to velocity‐strengthening friction at the high end of the tested temperature range. Our results suggest that the occurrence of slow slip events and the downdip limit of the seismogenic zone on subduction megathrusts cannot be solely explained by the temperature dependence of frictional properties of gabbro. Further experimental studies are needed to evaluate the effects of water fugacity and compositional heterogeneity (e.g., the presence of phyllosilicates) on frictional stability of subduction megathrusts. This article is protected by copyright. All rights reserved.
      PubDate: 2015-10-29T18:48:57.060118-05:
      DOI: 10.1002/2015GC006093
  • Constraints on shear velocity in the cratonic upper mantle from Rayleigh
           wave phase velocity
    • Authors: Aaron C. Hirsch; Colleen A. Dalton, Jeroen Ritsema
      Abstract: Seismic models provide constraints on the thermal and chemical properties of the cratonic upper mantle. Depth profiles of shear velocity from global and regional studies contain positive velocity gradients in the uppermost mantle and often lack a low‐velocity zone, features that are difficult to reconcile with the temperature structures inferred from surface heat‐flow data and mantle‐xenolith thermobarometry. Furthermore, the magnitude and shape of the velocity profiles vary between different studies, impacting the inferences drawn about mantle temperature and composition. In this study, forward modeling is used to identify the suite of one‐dimensional shear‐velocity profiles that are consistent with phase‐velocity observations made for Rayleigh waves traversing Precambrian cratons. Two approaches to the generation of 1‐D models are considered. First, depth profiles of shear velocity are predicted from thermal models of the cratonic upper mantle that correspond to a range of assumed values of mantle potential temperature, surface heat flow, and radiogenic heat production in the lithosphere. Second, shear velocity depth profiles are randomly generated. In both cases, Rayleigh wave phase velocity is calculated from the Earth models, and acceptable models are identified on the basis of comparison to observed phase velocity. The results show that it is difficult but not impossible to find acceptable Earth models that contain a low‐velocity zone in the upper mantle, and that temperature structures that are consistent with constraints from mantle xenoliths yield phase‐velocity predictions lower than observed. For most acceptable randomly generated Earth models, shear velocity merges with the global average at approximately 300 km. This article is protected by copyright. All rights reserved.
      PubDate: 2015-10-29T18:47:36.993761-05:
      DOI: 10.1002/2015GC006066
  • The importance of a Ni correction with ion counter in the double spike
           analysis of Fe isotope compositions using a 57Fe/58Fe double spike
    • Authors: V.A. Finlayson; J.G. Konter, L. Ma
      Abstract: We present a new method capable of measuring iron isotope ratios of igneous materials to high precision by multicollector inductively coupled plasma mass spectrometry (MC‐ICP‐MS) using a 57Fe‐58Fe double spike. After sample purification, near‐baseline signal levels of nickel are still present in the sample solution, acting as an isobaric interference on 58 amu. To correct for the interference, the minor 60Ni isotope is monitored and used to subtract a proportional 58Ni signal from the total 58 amu beam. The 60Ni signal is difficult to precisely measure on the Faraday detector due to Johnson noise occurring at similar magnitude. This noise‐dominated signal is subtracted from the total 58 amu beam, and its error amplified during the double spike correction. Placing the 60Ni beam on an ion counter produces a more precise measurement, resulting in a near‐threefold improvement in δ56Fe reproducibility, from ±0.145‰ when measured on Faraday to 0.052‰. Faraday detectors quantify the 60Ni signal poorly, and fail to discern the transient 20Ne40Ar interference visible on the ion counter, which is likely responsible for poor reproducibility. Another consideration is instrumental stability (defined herein as drift in peak center mass), which affects high resolution analyses. Analyses experiencing large drift relative to bracketing standards often yield non‐replicating data. Based on this, we present a quantitative outlier detection method capable of detecting drift‐affected data. After outlier rejection, long‐term precision on individual runs of our secondary standard improves to ±0.046‰. Averaging 3‐4 analyses further improves precision to 0.019‰, allowing distinction between ultramafic minerals. This article is protected by copyright. All rights reserved.
      PubDate: 2015-10-29T18:35:57.47474-05:0
      DOI: 10.1002/2015GC006012
  • Magmatic consequences of the transition from orthogonal to oblique
           subduction in Panama
    • Authors: Tyrone O. Rooney; Kristin D. Morell, Paulo Hidalgo, Pastora Fraceschi
      Abstract: The closure of the Central American Seaway is linked with tectonic and magmatic processes that have controlled the evolution of the Isthmus of Panama. We focus on the terminal stages of arc activity in the Central Panama region, and present new geochemical data from ∼9 Ma explosive silicic volcanism preserved in three syngenetic tuff beds from the Gatun Formation. The magmatic evolution of the Gatun Formation is controlled by a series of magma mushes where pyroxene is the dominant early forming mafic mineral, with amphibole appearing only relatively late in the fractionation sequence. Our data shows Gatun lavas exhibit a strong subduction signature, consistent with plate reconstruction models showing arc‐normal subduction from Costa Rica to Panama pre‐8.5 Ma. However, large ion lithophile elements are depleted in the Gatun Formation in comparison to other regional suites, indicative of a lower flux of subduction fluid to the Gatun Formation mantle source, which is explained by a shift towards magma generation by decompression following the collision of the arc with South America. Oblique subduction commencing ∼8.5 Ma resulted in the shutdown of normal arc activity throughout Panama. We interpret subsequent regional Quaternary adakitic volcanism as a response to this oblique subduction. The now more refractory mantle wedge required greater fluid flux in order to melt. The resultant volatile‐rich melts were more prone to deep fractionation of amphibole and garnet cumulates forming adakites. Deep fractionation was potentially enhanced by changing stress regimes on the upper‐plate caused by oblique subduction. This article is protected by copyright. All rights reserved.
      PubDate: 2015-10-26T18:48:58.45387-05:0
      DOI: 10.1002/2015GC006150
  • Movement of the intertropical convergence zone during the
           mid‐Pleistocene transition and the response of atmospheric and
           surface ocean circulations in the central equatorial Pacific
    • Authors: Inah Seo; Yong Il Lee, Wonnyon Kim, Chan Min Yoo, Kiseong Hyeong
      Abstract: This paper investigates the causes of a brief, but prominent, cooling episode (1.1–0.8 Ma) that occurred in the equatorial upwelling region of the Atlantic and Pacific during the mid‐Pleistocene Transition (MPT) using temporal changes in dust provenance, regional hydrology, and surface productivity recorded in a deep‐sea sediment core from the central equatorial Pacific. The 87Sr/86Sr and ɛNd values of the inorganic silicate fraction indicate deposition of dust from Australia and Central/South America before 0.8 Ma, but a gradual increase in Asian dust deposition after 0.8 Ma. The change in dust provenance was accompanied by an increased dust flux and a decrease in surface productivity and salinity. These changes can be explained by the southward movement of the Intertropical Convergence Zone (ITCZ) and North Equatorial Counter Current (NECC) and the direct influence of these features on the site after 0.8 Ma. Our results, together with previously published Atlantic data, suggest the northward position of the ITCZ between 1.1 and 0.9 Ma, and the southward position thereafter. The meridional movement of the ITCZ is in phase with the cooling and warming trend in upwelling regions in the equatorial Pacific and Atlantic, which suggests strengthening of southeast trades relative to its northern counterpart between 1.1 and 0.9 Ma as a plausible cause of this brief cooling event. The southward movement of the ITCZ from 0.9 to 0.8 Ma indicates more significant cooling in the Northern Hemisphere (NH) than in the Southern Hemisphere, which is supportive of the interpretation that the NH ice sheet expanded significantly and stabilized after 0.9 Ma. This article is protected by copyright. All rights reserved.
      PubDate: 2015-10-26T18:38:14.859126-05:
      DOI: 10.1002/2015GC006077
  • Optimization of end‐members used in multiple linear regression
           geochemical mixing models
    • Authors: Ann G. Dunlea; Richard W. Murray
      Abstract: Tracking marine sediment provenance (e.g., of dust, ash, hydrothermal material, etc.) provides insight into contemporary ocean processes and helps construct paleoceanographic records. In a simple system with only a few end‐members that can be easily quantified by a unique chemical or isotopic signal, chemical ratios and normative calculations can help quantify the flux of sediment from the few sources. In a more complex system (e.g., each element comes from multiple sources), more sophisticated mixing models are required. MATLAB codes published in Pisias et al. (2013) solidified the foundation for application of a constrained least square (CLS) multiple linear regression technique that can use many elements and several end‐members in a mixing model. However, rigorous sensitivity testing to check the robustness of the CLS model is time‐ and labor intensive. MATLAB codes provided in this paper reduce the time and labor involved and facilitate finding a robust and stable CLS model. By quickly comparing the goodness of fit between thousands of different end member combinations, users are able to identify trends in the results that reveal the CLS solution uniqueness and the end‐member composition precision required for a good fit. Users can also rapidly check that they have the appropriate number and type of end‐members in their model. In the end, these codes improve the user's confidence that the final CLS model(s) they select are the most reliable solutions. These advantages are demonstrated by application of the codes in two case studies of well‐studied datasets (Nazca Plate, South Pacific Gyre). This article is protected by copyright. All rights reserved.
      PubDate: 2015-10-26T18:37:23.72648-05:0
      DOI: 10.1002/2015GC006132
  • The 2‐D‐X‐ray diffraction as a tool for the rapid,
           nondestructive detection of low calcite quantities in aragonitic corals
    • Abstract: Paleoclimate reconstructions based on reef corals require precise detection of diagenetic alteration. Secondary calcite can significantly affect paleotemperature reconstructions at very low amounts of ∼ 1%. X‐ray powder diffraction is routinely used to detect diagenetic calcite in aragonitic corals. This procedure has its limitations as single powder samples might not represent the entire coral heterogeneity. A conventional and a 2D X‐ray diffractometer were calibrated with gravimetric powder standards of high‐ and low‐magnesium calcite (0.3% to 25% calcite). Calcite contents 
      PubDate: 2015-10-26T10:39:35.053196-05:
      DOI: 10.1002/2015GC006009
  • A random‐walk algorithm for modeling lithospheric density and the
           role of body forces in the evolution of the Midcontinent Rift
    • Authors: Will Levandowski; Oliver Boyd, Rich Briggs, Ryan Gold
      Abstract: This paper develops a Monte Carlo algorithm for extracting three‐dimensional lithospheric density models from geophysical data. Empirical scaling relationships between velocity and density create a 3D starting density model, which is then iteratively refined until it reproduces observed gravity and topography. This approach permits deviations from uniform crustal velocity‐density scaling, which provide insight into crustal lithology and prevent spurious mapping of crustal anomalies into the mantle. We test this algorithm on the Proterozoic Midcontinent Rift (MCR), north‐central U.S. The MCR provides a challenge because it hosts a gravity high overlying low shear‐wave velocity crust in a generally flat region. Our initial density estimates are derived from a seismic velocity/crustal thickness model based on joint inversion of surface‐wave dispersion and receiver functions. By adjusting these estimates to reproduce gravity and topography, we generate a lithospheric‐scale model that reveals dense middle crust and eclogitized lowermost crust within the rift. Mantle lithospheric density beneath the MCR is not anomalous, consistent with geochemical evidence that lithospheric mantle was not the primary source of rift‐related magmas and suggesting that extension occurred in response to far‐field stress rather than a hot mantle plume. Similarly, the subsequent inversion of normal faults resulted from changing far‐field stress that exploited not only warm, recently faulted crust but also a gravitational potential energy low in the MCR. The success of this density modeling algorithm in the face of such apparently contradictory geophysical properties suggests that it may be applicable to a variety of tectonic and geodynamic problems. This article is protected by copyright. All rights reserved.
      PubDate: 2015-10-23T03:54:27.800404-05:
      DOI: 10.1002/2015GC005961
  • Reactive transport in a partially molten system with binary solid solution
    • Authors: Jacob S. Jordan; Marc A. Hesse
      Abstract: Melt extraction from the Earth's mantle through high‐porosity channels is required to explain the composition of the oceanic crust. Feedbacks from reactive melt transport are thought to localize melt into a network of high‐porosity channels. Recent studies invoke lithological heterogeneities in the Earth's mantle to seed the localization of partial melts. Therefore, it is necessary to understand the reaction fronts that form as melt flow across the lithological interface between the heterogeneity and the ambient mantle. Here, we present a chromatographic analysis of reactive melt transport across lithological boundaries, using the theory of hyperbolic conservation laws. This is an extension of linear trace element chromatography to the coupling of major elements and energy transport. Our analysis allows the prediction of the non‐linear feedbacks that arise in reactive melt transport due to changes in porosity. This study considers the special case of a partially molten porous medium with binary solid solution. As melt traverses a lithological contact, binary solid solution leads to the formation of a reacted zone between an advancing reaction front and the initial contact. The analysis also shows that the behavior of a fertile heterogeneity depends on its absolute concentration, in addition to compositional differences between itself and the refractory background. We present a regime diagram that predicts if melt emanating from a fertile heterogeneity localizes into high porosity channels or develops a zero porosity shell. The theoretical framework presented here provides a useful tool for understanding non‐linear feedbacks in reactive melt transport, because it can be extended to more complex and realistic phase behaviors. This article is protected by copyright. All rights reserved.
      PubDate: 2015-10-23T03:54:04.897884-05:
      DOI: 10.1002/2015GC005956
  • Evidence and models for lower crustal flow beneath the Galápagos
    • Abstract: The volcanic Galápagos Islands are constructed upon a broad platform, with their active westernmost islands marking the current position of the hotspot. Built upon young oceanic lithosphere (
      PubDate: 2015-10-23T03:50:51.133605-05:
      DOI: 10.1002/2015GC006136
  • San Andreas Fault dip, peninsular ranges mafic lower‐crust and
           partial melt in the Salton Trough, Southern California, from
           ambient‐noise tomography
    • Authors: Shahar Barak; Simon L. Klemperer, Jesse F. Lawrence
      Abstract: We use ambient‐noise tomography to improve CVM‐H11.9, a community velocity model of southern California. Our new 3D shear‐velocity model with 0.05°x0.05° lateral and 1‐km vertical blocks reveals new structure beneath the San Andreas Fault (SAF), Peninsular Ranges batholith (PRB), southern Sierra Nevada batholith (SNB) and the Salton Trough (ST). We use four years of data recorded on 849 broadband stations, vastly more than previous studies and including our own broadband Salton Seismic Imaging Project, a 40‐station transect across the ST, as well as other campaign stations in both Mexico and the U.S.A. Mean lower‐crust and upper‐mantle wavespeeds (3.6 km/s @ 20km, 4.2 km/s @ 40km) are low by global standards. Across the SAF, southeast of San Gorgonio Pass, we observe vertical to steeply‐dipping lateral velocity contrasts that extend beneath the Moho. Beneath the western PRB and westernmost southern SNB, we observe relatively high shear‐velocities (≥ 3.8 km/s) in the lower crust that we interpret as the mafic roots of the overlying arc. Relatively high‐velocity upper‐mantle (up to ∼4.5 km/s) may be part of the intact arc, or possibly a remnant of the Farallon plate. Beneath the ST, we observe zones of low shear‐velocity in the lower crust and upper mantle which permit up to ∼4.5% melt in the lower crust and up to ∼6% melt in the upper mantle, depending on the assumed composition and pore geometry. Our results preclude the existence of older continental crust beneath the ST and support the creation of new crust beneath the ST. This article is protected by copyright. All rights reserved.
      PubDate: 2015-10-19T04:19:14.057887-05:
      DOI: 10.1002/2015GC005970
  • Focusing of upward fluid migration beneath the arc: Effect of mineral
           grain size variation in the mantle wedge
    • Authors: Ikuko Wada; Mark D. Behn
      Abstract: We use numerical models to investigate the effects of mineral grain size variation on fluid migration in the mantle wedge at subduction zones and on the location of the volcanic arc. Previous coupled thermal‐grain size evolution (T‐GSE) models predict small grain size (< 1 mm) in the corner flow of the mantle wedge, a down‐dip grain size increase by ∼two orders of magnitude along the base of the mantle wedge, and finer grain size in the mantle wedge for colder‐slab subduction zones. We integrate these T‐GSE modeling results with a fluid migration model, in which permeability depends on grain size, and fluid flow through a moving mantle matrix is driven by fluid buoyancy and dynamic pressure gradients induced by mantle flow. Our modeling results indicate that fluids introduced along the base of the mantle wedge beneath the forearc are initially dragged down‐dip by corner flow due to the small grain size and low permeability immediately above the slab. As grain size increases with depth, permeability increases, resulting in upward fluid migration. Fluids released beneath the arc and the backarc are also initially dragged down‐dip, but typically are not transported as far laterally before they begin to travel upward. As the fluids rise through the backarc mantle wedge, they become deflected towards the trench due to the effect of mantle inflow. The combination of down‐dip migration in the forearc and trench‐ward migration in the backarc results in pathways that focus fluids to the arc. This article is protected by copyright. All rights reserved.
      PubDate: 2015-10-19T04:16:29.354119-05:
      DOI: 10.1002/2015GC005950
  • How partial melting affects small‐scale convection in a
           plume‐fed sublithospheric layer beneath fast‐moving plates
    • Authors: Roberto Agrusta; Andrea Tommasi, Diane Arcay, Alicia Gonzalez, Taras Gerya
      Abstract: Numerical models show that small‐scale convection (SSC) occurring atop a mantle plume is a plausible mechanism to rejuvenate the lithosphere. The triggering of SSC depends on the density contrast and on the rheology of the unstable layer underlying the stagnant upper part of the thermal boundary layer (TBL). Both properties may be changed by partial melting. We analyze, using 2D numerical simulations, how partial melting influences the dynamics of time‐dependent SSC instabilities and the resulting thermo‐mechanical rejuvenation of an oceanic plate moving atop of a plume. Our simulations show a complex behavior, with acceleration, no change, or delay of the SSC onset, due to competing effects of the latent heat of partial melting, which cools the plume material, and of the buoyancy increase associated with both melt retention and depletion of residue following melt extraction. The melt‐induced viscosity reduction is too localized to affect significantly SSC dynamics. Faster SSC triggering is promoted for low melting degrees (low plume temperature anomalies, thick lithosphere, or fast moving plates), which limit both the temperature reduction due to latent heating and the accumulation of depleted buoyant residue to the upper part of the unstable layer. In contrast, high partial melting degrees lead to a strong temperate decrease due to latent heat of melting and development of a thick depleted layer within the sublithospheric convecting layer, which delays the development of gravitational instabilities. Despite differences in SSC dynamics, the thinning of the lithosphere is not significantly enhanced relatively to simulations that neglect partial melting. This article is protected by copyright. All rights reserved.
      PubDate: 2015-10-15T10:16:31.211912-05:
      DOI: 10.1002/2015GC005967
  • Hypoxia‐driven variations in iron and manganese shuttling in the
           Baltic Sea over the past 8 kyr
    • Authors: Conny Lenz; Tom Jilbert, Daniel J. Conley, Caroline P. Slomp
      Abstract: The Baltic Sea has experienced three major intervals of bottom water hypoxia following the intrusion of seawater ca. 8 kyrs ago. These intervals occurred during the Holocene Thermal Maximum (HTM), Medieval Climate Anomaly (MCA) and during recent decades. Here, we show that sequestration of both Fe and Mn in Baltic Sea sediments generally increases with water depth, and we attribute this to shelf‐to‐basin transfer (“shuttling”) of Fe and Mn. Burial of Mn in slope and basin sediments was enhanced following the lake‐brackish/marine transition at the beginning of the hypoxic interval during the HTM. During hypoxic intervals, shelf‐to‐basin transfer of Fe was generally enhanced but that of Mn was reduced. However, intensification of hypoxia within hypoxic intervals led to decreased burial of both Mn and Fe in deep basin sediments. This implies a non‐linearity in shelf Fe release upon expanding hypoxia with initial enhanced Fe release relative to oxic conditions followed by increased retention in shelf sediments, likely in the form of iron sulfide minerals. For Mn, extended hypoxia leads to more limited sequestration as Mn carbonate in deep basin sediments, presumably because of more rapid reduction of Mn oxides formed after inflows and subsequent escape of dissolved Mn to the overlying water. Our Fe records suggest that modern Baltic Sea hypoxia is more widespread than in the past. Furthermore, hypoxia‐driven variations in shelf‐to‐basin transfer of Fe may have impacted the dynamics of P and sulfide in the Baltic Sea thus providing potential feedbacks on the further development of hypoxia. This article is protected by copyright. All rights reserved.
      PubDate: 2015-10-15T10:14:47.10025-05:0
      DOI: 10.1002/2015GC005960
  • Identifying cryptotephra units using correlated rapid, nondestructive
           methods: VSWIR spectroscopy, X‐ray fluorescence, and magnetic
    • Authors: Molly C. McCanta; Robert G. Hatfield, Bradley J. Thomson, Simon J. Hook, Elizabeth Fisher
      Abstract: Understanding the frequency, magnitude, and nature of explosive volcanic eruptions is essential for hazard planning and risk mitigation. Terrestrial stratigraphic tephra records can be patchy and incomplete due to subsequent erosion and burial processes. In contrast the marine sedimentary record commonly preserves a more complete historical record of volcanic activity as individual events are archived within continually accumulating background sediments. While larger tephra layers are often identifiable by changes in sediment color and/or texture, smaller fallout layers may also be present that are not visible to the naked eye. These cryptotephra are commonly more difficult to identify and often require time‐consuming and destructive point counting, petrography, and microscopy work. Here we present several rapid, non‐destructive, and quantitative core‐scanning methodologies (magnetic susceptibility, visible to shortwave infrared spectroscopy, and XRF core scanning) which, when combined, can be used to identify the presence of increased volcaniclastic components (interpreted to be cryptotephra) in the sedimentary record. We develop a new spectral parameter (BDI1000VIS) that exploits the absorption of the 1 µm near‐infrared band in tephra. Using pre‐determined mixtures BDI1000VIS can accurately identify tephra layers in concentrations >15–20%. When applied to the upper ∼270 kyr record of IODP core U1396C from the Caribbean Sea, and verified by traditional point counting, twenty‐nine potential cryptotephra layers were identified as originating from eruptions of the Lesser Antilles Volcanic Arc. Application of these methods in future coring endeavors can be used to minimize the need for physical disaggregation of valuable drill core material and allow for near‐real time recognition of tephra units, both visible and cryptotephra. This article is protected by copyright. All rights reserved.
      PubDate: 2015-10-14T09:56:49.771912-05:
      DOI: 10.1002/2015GC005913
  • Paleomagnetism of Miocene volcanics on Sao Tome: Paleosecular variation at
           the Equator and a comparison to its latitudinal dependence over the last 5
    • Authors: N.D. Opdyke; D.V. Kent, D.A. Foster, K. Huang
      Abstract: A collection was made in January 2009 of 10 oriented samples from each of 54 sites in lavas on Sao Tome Island (nominal location 0.3º N, 6.5º E). Some sites were affected by lightning leaving a total of 42 sites for analysis of paleosecular variation. Overall magnetic properties were excellent (highly stable magnetizations carried by pseudo‐single domain magnetite). After principal component analysis of progressive alternating field demagnetization data for the samples, twenty‐two sites had normal polarity magnetizations (D = 0. 6º, I = ‐8.3º, α95 = 4.3º, κ = 53.1) and 20 had reverse magnetizations (D = 176.0º, I = 4.2º, α95 = 7.3º, κ = 20.8); the directions are within 5° of antiparallel, yielding a positive reversal test. The combined data set of 42 site mean virtual geomagnetic poles converted to common (normal) polarity yields a pole position at 86.0°N, 211.5ºE, A95=3.1º. Ar/Ar and K/Ar dating reveals that these rocks are Miocene in age (∼5–11 Ma), old enough to allow northerly plate motion to help explain the slightly far‐sided pole position. The between‐site dispersion in virtual geomagnetic poles was estimated as the angular standard deviation, Sb, and equaled 11.4° with 95% confidence interval between 9.9° and 13.4°. This article is protected by copyright. All rights reserved.
      PubDate: 2015-10-14T09:47:56.087677-05:
      DOI: 10.1002/2015GC005901
  • Issue Information
    • PubDate: 2015-10-14T05:55:33.377088-05:
      DOI: 10.1002/ggge.20565
  • FFluid circulation and carbonate vein precipitation in the footwall of an
           oceanic core complex, Ocean Drilling Program Site 175, Mid‐Atlantic
    • Abstract: Carbonate veins recovered from the mafic/ultramafic footwall of an oceanic detachment fault on the Mid‐Atlantic Ridge record multiple episodes of fluid movement through the detachment and secondary faults. High‐temperature (∼75‐175°C) calcite veins with elevated REE contents and strong positive Eu‐anomalies record the mixing of up‐welling hydrothermal fluids with infiltrating seawater. Carbonate precipitation is most prominent in olivine‐rich troctolite, which also display a much higher degree of greenschist and sub‐greenschist alteration relative to gabbro and diabase. Low‐temperature calcite and aragonite veins likely precipitated from oxidizing seawater that infiltrated the detachment fault and/or within secondary faults late or post footwall denudation. Oxygen and carbon isotopes lie on a mixing line between seawater and Logatchev‐like hydrothermal fluids, but precipitation temperatures are cooler than would be expected for isenthalpic mixing, suggesting conductive cooling during upward flow. There is no depth dependence of vein precipitation temperature, indicating effective cooling of the footwall via seawater infiltration through fault zones. One sample contains textural evidence of low‐temperature, seawater‐signature veins being cut by high‐temperature, hydrothermal‐signature veins. This indicates temporal variability in the fluid mixing, possibly caused by deformation‐induced porosity changes or dike intrusion. The strong correlation between carbonate precipitation and olivine‐rich troctolites suggests that the presence of unaltered olivine is a key requirement for carbonate precipitation from seawater and hydrothermal fluids. Our results also suggest that calcite‐talc alteration of troctolites may be a more efficient CO2 trap than serpentinized peridotite. This article is protected by copyright. All rights reserved.
      PubDate: 2015-10-13T10:27:23.730246-05:
      DOI: 10.1002/2015GC006041
  • A new bathymetry of the Northeast Greenland continental shelf: constraints
           on glacial and other processe
    • Authors: Jan Erik Arndt; Wilfried Jokat, Boris Dorschel, Reidun Myklebust, Julian A. Dowdeswell, Jeffrey Evans
      Abstract: A new digital bathymetric model (DBM) for the Northeast Greenland (NEG) continental shelf (74°N to 81°N) is presented. The DBM has a grid cell size of 250 m x 250 m and incorporates bathymetric data from 30 multibeam cruises, more than 20 singlebeam cruises and first reflector depths from industrial seismic lines. The new DBM substantially improves the bathymetry compared to older models. The DBM not only allows a better delineation of previously known seafloor morphology but, in addition, reveals the presence of previously unmapped morphological features including glacially‐derived troughs, fjords, grounding‐zone wedges and lateral moraines. These submarine landforms are used to infer the past extent and ice‐flow dynamics of the Greenland Ice Sheet during the last full‐glacial period of the Quaternary and subsequent ice retreat across the continental shelf. The DBM reveals cross‐shelf bathymetric troughs that may enable the inflow of warm Atlantic water masses across the shelf, driving enhanced basal melting of the marine‐terminating outlet glaciers draining the ice sheet to the coast in Northeast Greenland. Knolls, sinks and hummocky seafloor on the middle shelf are also suggested to be related to salt diapirism. North‐south orientated elongate depressions are identified that probably relate to ice‐marginal processes in combination with erosion caused by the East Greenland Current. A single guyot‐like peak has been discovered and is interpreted to have been produced during a volcanic event approximately 55 Ma ago. This article is protected by copyright. All rights reserved.
      PubDate: 2015-10-13T10:26:57.700236-05:
      DOI: 10.1002/2015GC005931
  • Overriding plate deformation and variability of fore‐arc deformation
           during subduction: Insight from geodynamic models and application to the
           Calabria subduction zone
    • Abstract: In nature, subducting slabs and overriding plate segments bordering subduction zones are generally embedded within larger plates. Such large plates can impose far‐field boundary conditions that influence the style of subduction and overriding plate deformation. Here we present dynamic laboratory models of progressive subduction in three‐dimensional space, in which the far‐field boundary conditions at the trailing edges of the subducting plate (SP) and overriding plate (OP) are varied. Four configurations are presented: Free (both plates free), SP‐Fixed, OP‐Fixed and SP‐OP‐Fixed. We investigate their impact on the kinematics and dynamics of subduction, particularly focusing on overriding plate deformation. The results indicate that the variation in far‐field boundary conditions has an influence on the slab geometry, subduction partitioning and trench migration partitioning. Our models also indicate that in natural (narrow) subduction zones, assuming a homogeneous overriding plate, the formation of backarc basins (e.g., Tyrrhenian Sea, Aegean Sea, Scotia Sea) is generally expected to occur at a comparable location (250‐700 km from the trench), irrespective of the boundary condition. In addition, our models indicate that the style of forearc deformation (shortening or extension) is influenced by the mobility of the overriding plate through controlling the force normal to the subduction zone interface (trench suction). Our geodynamic model that uses the SP‐OP‐Fixed setup is comparable to the Calabria subduction zone with respect to subduction kinematics, slab geometry, trench curvature and accretionary configuration. Furthermore, the model can explain backarc and forearc extension at the Calabria subduction zone since the latest Middle Miocene as a consequence of subduction of the narrow Calabrian slab and the immobility of the subducting African plate and overriding Eurasian plate. This setting induced strong trench suction, driving forearc extension, and forced subduction to be accommodated almost entirely by slab rollback (not trenchward subducting plate motion), while trench retreat was accommodated almost entirely by backarc and forearc extension (not trenchward overriding plate motion), comparable to our SP‐OP‐Fixed model. This article is protected by copyright. All rights reserved.
      PubDate: 2015-10-09T17:49:16.025742-05:
      DOI: 10.1002/2015GC005958
  • Chromium isotope signature during continental crust subduction recorded in
           metamorphic rocks
    • Abstract: The chromium isotope compositions of 27 metamorphic mafic rocks with varying metamorphic degrees from eastern China were systematically measured to investigate the Cr isotope behavior during continental crust subduction. The Cr isotope compositions of all samples studied were Bulk Silicate Earth (BSE)‐like, with δ53CrNIST979 of greenschists, amphibolites and eclogites ranging from ‐0.06‰ to ‐0.17‰, ‐0.05‰ to ‐0.27‰, and ‐0.01‰ to ‐0.24‰, respectively. The lack of resolvable isotopic variability among the metamorphic rocks from different metamorphic zones indicated that no systematic Cr isotope fractionation was associated with the degree of metamorphism. However, the Cr isotopic variability among homologous samples may have reflected effects induced by metamorphic dehydration with a change of redox state, rather than protolith heterogeneity (i.e., magma differentiation). In addition, the differences in δ53Cr (Δ53CrCpx‐Gt) between co‐existing clinopyroxene (Cpx) and garnet (Gt) from two garnet pyroxenites were 0.06‰ and 0.34‰, indicating that significant inter‐mineral Cr isotope disequilibria could occur during metamorphism. To provide a basis for comparison with metamorphic rocks and to provide further constraints on the potential Cr isotope heterogeneity in the mantle and the protolith of some metamorphic rocks, we analyzed mantle‐derived chromites and the associated peridotites from Luobusa, and we obtained the following general order: chromite‐free peridotites (‐0.21‰ to ‐0.11‰) 
      PubDate: 2015-10-09T17:38:10.191851-05:
      DOI: 10.1002/2015GC005944
  • Crust and upper mantle structure associated with extension in the Woodlark
           Rift, Papua New Guinea from Rayleigh‐wave tomography
    • Authors: Ge Jin; James B. Gaherty, Geoff Abers, Younghee Kim, Zachary Eilon, W. Roger Buck
      Abstract: The Woodlark seafloor spreading center is propagating westwards into the Australian plate near the D'Entrecasteaux Islands (DI), Papua New Guinea, generating an active transition zone from continental rifting to seafloor spreading. From March 2010 to July 2011, we deployed 31 inland and 8 off‐shore broadband seismic stations around the DI region, to explore the dynamic processes of the lithosphere extension and the exhumation of the high‐pressure terranes exposed on those islands. We measure the multi‐band (10‐60 s) Rayleigh‐wave phase velocities from both ambient‐noise and earthquake signals. These measurements are then inverted for a three‐dimensional shear‐velocity model for the crust and upper mantle. The results indicate that the lithosphere extension is localized near the rift axis beneath the DI, with a shear‐velocity structure in the upper mantle that is similar to mid‐ocean ridges. Beneath the Kiribisi Basin west of DI, an ultra‐slow shear‐velocity anomaly ($\sim$4.0 km/s) is observed at shallow mantle depth (30‐60 km), which can be interpreted either by the presence of excess partial melt due to slow melt extraction, or by the existence of felsic crustal material buried to mantle depth and not yet exhumed. This article is protected by copyright. All rights reserved.
      PubDate: 2015-10-09T17:37:57.563888-05:
      DOI: 10.1002/2015GC005840
  • Playing jigsaw with Large Igneous Provinces—A plate tectonic
           reconstruction of Ontong Java Nui, West Pacific
    • Abstract: The three largest Large Igneous Provinces (LIP) of the western Pacific – Ontong Java, Manihiki and Hikurangi plateaus – were emplaced during the Cretaceous Normal Superchron and show strong similarities in their geochemistry and petrology. The plate tectonic relationship between those LIPs, herein referred to as Ontong Java Nui, is uncertain, but a joined emplacement was proposed by Taylor [2006]. Since this hypothesis is still highly debated and struggles to explain features such as the strong differences in crustal thickness between the different plateaus, we revisited the joined emplacement of Ontong Java Nui in light of new data from the Manihiki Plateau. By evaluating seismic refraction/wide‐angle reflection data along with seismic reflection records of the margins of the proposed “Super”‐LIP, a detailed scenario for the emplacement and the initial phase of break‐up has been developed. The LIP is a result of an interaction of the arriving plume head with the Phoenix‐Pacific spreading ridge in the early Cretaceous. The break‐up of the LIP shows a complicated interplay between multiple micro‐plates and tectonic forces such as rifting, shearing and rotation. Our plate kinematic model of the western Pacific incorporates new evidence from the break‐up margins of the LIPs, the tectonic fabric of the seafloor as well as previously published tectonic concepts such as the rotation of the LIPs. The updated rotation poles of the western Pacific allow a detailed plate tectonic reconstruction of the region during the Cretaceous Normal Superchron and highlights the important role of LIPs in the plate tectonic framework. This article is protected by copyright. All rights reserved.
      PubDate: 2015-10-09T17:37:32.172637-05:
      DOI: 10.1002/2015GC006036
  • Magnetostratigraphic constraints on the age of the lower Beaufort Group,
           western Karoo Basin, South Africa, and a critical analysis of existing
           U‐Pb geochronological data
    • Authors: E. Tohver; L. Lanci, A. Wilson, J. Hansma, S. Flint
      Abstract: We studied 3 partially overlapping sections with a composite thickness of ∼600 m in the upper Permian fluvial siltstones and fine‐grained sandstones of the Abrahamskraal Formation, the basal unit of the Beaufort Group, in the Karoo Basin of Western Cape Province, South Africa. Paleomagnetic analysis reveals three components of Natural Remanent Magnetization (NRM). Heating to ∼180°C removes a remanent magnetization parallel to the present‐day field, which is interpreted as a viscous overprint. An intermediate unblocking temperature component is removed by heating to 450°C; this direction is always of normal polarity and is identical to a regional overprint imparted during the Early Jurassic emplacement of the Karoo Large Igneous Province. A high temperature component isolated above 450°C is of dual polarity, and is interpreted as primary on the basis of a positive reversals test. The virtual geomagnetic pole position for the Abrahamskraal Formation computed from the average high temperature characteristic remanent magnetization direction is in agreement with the late Permian directions for stable Gondwana and with previous results from the lowermost Abrahamskraal Formation and Waterford Formation at the Ouberg Pass section. The predominantly normal polarity of this magnetization is in agreement with either a middle‐late Lopingian age (ca. 254‐256 Ma) or a late Guadalupian age (ca. 262 Ma) according to the Global Geomagnetic Polarity Timescale. We integrate these new results with existing magnetostratigraphic, biostratigraphic, and geochronological results from the Karoo Basin, with particular emphasis on the controversy over zircon age data reported from the underlying Ecca Group. This article is protected by copyright. All rights reserved.
      PubDate: 2015-10-06T10:46:54.062141-05:
      DOI: 10.1002/2015GC005930
  • Receiver function imaging of the mantle transition zone beneath the South
           China Block
    • Abstract: Upper mantle discontinuities are influenced by convection‐related thermal heterogeneities arising in complex geodynamic settings. Slab roll‐back of the Pacific plate and mantle upwelling in the Meso‐Cenozoic caused the extension and spreading of continental segments in the South China Block leading to profound variations of the local temperature conditions. We processed 201 teleseismic events beneath 87 stations in the Hainan, Guangdong, and Fujian provinces in the South China Block, and extracted 4172 high‐quality receiver functions. We imaged the topography of the local mantle discontinuities by using phase‐weighted common conversion point stacking of the receiver functions, which effectively improves the P‐to‐S converted phases. We found that the average depths of the discontinuities at 410 and 660 km depth are 414 km and 657 km, respectively, while no clearly defined discontinuity at 520 km depth was detected. We mapped the thickness of the mantle transition zone (MTZ), which can reflect temperature and/or compositional heterogeneities as well as the presence of water, and discussed possible geodynamic implications. In particular, we found that the MTZ beneath the Leizhou Peninsula in the Hainan province is 42 km thinner than average. This scenario suggests that the Hainan plume is responsible for positive temperature anomalies between ∼270 and 380 K and between ∼200 and 240 K at the 660 and 410 km discontinuities, respectively. We also observed a prominent uplifting of the 660 km boundary beneath the coast regions that may be indicative of lateral flow of the Hainan plume. This article is protected by copyright. All rights reserved.
      PubDate: 2015-10-06T10:17:20.209373-05:
      DOI: 10.1002/2015GC005978
  • Phanerozoic surface history of southern Peninsular India from apatite
           (U‐Th‐Sm)/He data
    • Abstract: Quantifying bedrock cooling history is crucial for understanding the long‐term landform evolution across passive margins and its control onto the sediment routing system. To constrain the low‐temperature cooling history and its relationships to the Phanerozoic tectonic events of southern Peninsular India, we present new apatite (U‐Th‐Sm)/He analyses of 39 Precambrian basement samples. The new AHe ages range from 38.1 ± 6.8 to 364.2 ± 44.6 Ma: they are younger than 50 Ma in the Palghat Gap region and older than 200 Ma in the interior of the Deccan Plateau. Thermal modeling based on apatite (U‐Th‐Sm)/He data indicates enhanced cooling and exhumation in the interior of the Deccan Plateau by Permian‐Triassic times followed by gradual cooling up to the Present. This discrete episode of Permian‐Triassic cooling is associated with continental extension that preceded the Early Jurassic breakup of Gondwana. Bedrock cooling and exhumation on the southeastern and southern limits of the Deccan Plateau was likely accomplished by Late Cretaceous drainage reorganization. The distribution of old (> 200 Ma) AHe ages over the > 2600 m high Nilgiri Plateau reflects very low erosion/exhumation rates and adds to examples of long‐lived post‐orogenic topography. The relatively younger AHe ages from the ∼30 km wide low mountain pass (Palghat Gap) within the Western Ghat Mountains attest for intense Late Cenozoic erosion likely facilitated by the erodible lithological backbone of the Neoproterozoic shear zone. AHe ages across the western coastal plain challenge the widely hold notion of ∼3 km of post breakup isostatic rebound in response to erosion of the margin. Instead, the new AHe data are more compatible with less than 1‐1.5 km of crustal denudation along the coastal strip. This article is protected by copyright. All rights reserved.
      PubDate: 2015-10-05T19:05:36.094681-05:
      DOI: 10.1002/2015GC005977
  • Oligo‐Miocene magnetostratigraphy and magnetic anisotropy of the
           Baxbulak section from the Pamir‐Tian Shan convergence zone
    • Authors: Zihua Tang; Xinxin Dong, Xu Wang, Zhongli Ding
      Abstract: As the northernmost part of the Indo‐Eurasian collision belt, the Pamir‐Tian Shan convergence zone (PTCZ) is a strategic location for understanding intracontinental deformation. Here we present a magnetostratigraphic investigation of a continuous section from the Baxbulak region, to better constrain regional tectonic history. Rock magnetic analyses indicate that hematite and magnetite are the main carriers of characteristic remanent magnetization. The resulting polarity sequence allows a distinct correlation to the geomagnetic polarity time scale, showing that the section spans the interval of 29.1‐20.7 Ma. Rock magnetic results further suggest that paramagnetic and antiferromagnetic minerals dominantly contribute to anisotropy of magnetic susceptibility (AMS) of the sequence. Thus, the AMS would indicate the preferred orientations of the mineral grains that are sensitive to tectonic strain. At around 26 Ma, the grouped principal minimum perpendicular to the bedding diverts to a girdle distribution in a N‐S direction, demonstrating the overprint of tectonic fabric to previous weakly deformed sedimentary fabric. This would be interpreted as a marked increase in tectonic strain, consistent with various evidence from the Pamir and the neighboring basin that show the Pamir began to migrate northward. Moreover, the coincident changes in distribution of AMS principal axes, in both direction and magnitude, are comparable to the regional counterclockwise rotations observed from paleomagnetic data, likely related to orogenesis. This article is protected by copyright. All rights reserved.
      PubDate: 2015-10-05T03:55:05.625985-05:
      DOI: 10.1002/2015GC005965
  • Analysis of bubble plume distributions to evaluate methane hydrate
           decomposition on the continental slope
    • Authors: H. Paul Johnson; Una K. Miller, Marie S. Salmi, Evan A. Solomon
      Abstract: Cascadia margin sediments contain a rich reservoir of carbon derived both from terrestrial input and sea surface productivity. A portion of this carbon exists as solid gas hydrate within sediment pore spaces which previous studies have shown to be a methane reservoir of substantial size on both the Vancouver Island and Oregon portions of the Cascadia margin. Multi‐channel seismic reflection profiles on the Cascadia margin show the widespread presence of Bottom Simulating Reflectors (BSRs) within the sediment column, indicating the gas hydrate reservoir extends from the deformation front at 3000 meters depth to the upper limit of gas hydrate stability near 500 meters water depth. In this study, we compile an inventory of methane bubble plume sites on the Cascadia margin identified in investigations carried out for a range of interdisciplinary goals that also include sites volunteered by commercial fishermen. High plume density anomalies are associated with both the continental shelf (
      PubDate: 2015-10-01T11:31:04.138528-05:
      DOI: 10.1002/2015GC005955
  • A high‐order numerical study of reactive dissolution in an upwelling
           heterogeneous mantle: 2. Effect of shear deformation
    • Authors: Conroy Baltzell; E.M. Parmentier, Yan Liang, Seshu Tirupathi
      Abstract: High‐porosity dunite channels produced by orthopyroxene dissolution may provide pathways for orthopyroxene‐undersaturated melt generated in the deep mantle to reach shallower depth without extensive chemical reequilibration with surrounding mantle. Previous studies have considered these high porosity channels and melt localization in the presence of a uniform upwelling mantle flow through the process of melt‐rock reaction as well as shear deformation; but not both simultaneously. In this Part II of a numerical study of high‐porosity melt and dunite channel formation during reactive dissolution, we considered the effect of shear deformation on channel distribution and channel geometry in an upwelling and viscously compacting mantle column. We formulated a high‐order numerical experiment using conditions similar to those in Part I, but with an additional prescribed horizontal shearing component in the solid matrix, as could be present in flowing mantle beneath spreading centers. Our focus was to examine orthopyroxene dissolution to determine the behavior of dunite formation and its interaction with melt flow field, by varying the upwelling and shear rate, orthopyroxene solubility gradient, and domain height. Introduction of shearing tilts the developing dunite, causing asymmetry in the orthopyroxene gradient between the dunite channels and the surrounding harzburgite. The downwind gradient is sharp, nearly discontinuous; whereas the upwind gradient is more gradual. For higher shear rates, a wave‐like pattern of alternating high and low porosity bands form on the downwind side of the channel. The band spacing increases with increasing shear rate, relative melt flow, and orthopyroxene solubility gradient, whereas the band angle is independent of solubility gradient and increases with increasing shear rate and decreasing relative melt flow rate. Such features could be observable in the field and provide evidence for mantle shearing. Standing wave‐like patterns of melt fraction also develop on the downwind side with possible implications for the interpretation of seismic velocities in upwelling mantle. This article is protected by copyright. All rights reserved.
      PubDate: 2015-10-01T11:30:42.237561-05:
      DOI: 10.1002/2015GC006038
  • Anisotropic upper crust above the aftershock zone of the 2013 Ms 7.0
           Lushan earthquake from the shear wave splitting analysis
    • Authors: Ying Liu; Haijiang Zhang, Xin Zhang, Shunping Pei, Meijian An, Shuwen Dong
      Abstract: We have conducted a systematic shear wave splitting analysis using 1000 selected aftershocks with M >2 from the 2013 Ms 7.0 Lushan earthquake along the Longmenshan fault system in southwest China. Polarization directions of fast shear waves show a bimodal distribution with one dominant direction approximately parallel to the fault strike and the other close to the regional maximum horizontal compressive stress direction. This indicates that in this area mechanisms causing crustal seismic anisotropy are both stress induced and fault zone structure controlled. Delay times between fast and slow shear waves do not show a clear trend of increase for deeper events, suggesting the anisotropic zone is mostly above the aftershocks, which are generally located below 8 km. We further applied a shear wave splitting tomography method to measured delay times to characterize the spatial distribution of seismic anisotropy. The three‐dimensional anisotropic percentage model shows strong anisotropy above 8 km but low anisotropy below it. The mainshock slip zone and its aftershocks are associated with very low or negligible anisotropy and high velocity, indicating that the zones with high anisotropy and low velocity are mechanically weak and it is difficult for stress to accumulate there. The main and back reverse fault zones are associated with high anisotropic anomalies above ∼8 km, likely caused by shear fabric or microfractures aligned parallel to the fault zone. This article is protected by copyright. All rights reserved.
      PubDate: 2015-10-01T10:58:01.462905-05:
      DOI: 10.1002/2015GC005972
  • Geomagnetic paleointensity in historical pyroclastic density currents:
           Testing the effects of emplacement temperature and postemplacement
    • Authors: Julie A. Bowles; Jeffrey S. Gee, Mike J. Jackson, Margaret S. Avery
      Abstract: Thellier‐type paleointensity experiments were conducted on welded ash matrix or pumice from the 1912 Novarupta (NV) and 1980 Mt. St. Helens (MSH) pyroclastic density currents (PDCs) with the intention of evaluating their suitability for geomagnetic paleointensity studies. PDCs are common worldwide, but can have complicated thermal and alteration histories. We attempt to address the role that emplacement temperature and post‐emplacement hydrothermal alteration may play in non‐ideal paleointensity behavior of PDCs. Results demonstrate two types of non‐ideal behavior: unstable remanence in multi‐domain (MD) titanomagnetite, and non‐ideal behavior linked to fumarolic and vapor phase alteration. Emplacement temperature indirectly influences MSH results by controlling the fraction of homogenous MD vs. oxyexsolved pseudo‐single domain titanomagnetite. NV samples are more directly influenced by vapor phase alteration. The majority of NV samples show distinct two‐slope behavior in the natural remanent magnetization – partial thermal remanent magnetization plots. We interpret this to arise from a (thermo)chemical remanent magnetization associated with vapor phase alteration, and samples with high water content (>0.75% loss on ignition) generate paleointensities that deviate most strongly from the true value. We find that PDCs can be productively used for paleointensity, but that – as with all paleointensity studies – care should be taken in identifying potential post‐emplacement alteration below the Curie temperature, and that large, welded flows may be more alteration‐prone. One advantage in using PDCs is that they typically have greater areal (spatial) exposure than a basalt flow, allowing for more extensive sampling and better assessment of errors and uncertainty. This article is protected by copyright. All rights reserved.
      PubDate: 2015-10-01T10:57:44.952264-05:
      DOI: 10.1002/2015GC005910
  • Neodymium isotopic composition in foraminifera and authigenic phases of
           South China Sea sediments: Implications for the hydrology of the North
           Pacific Ocean over the past 25 kyr
    • Abstract: εNd and normalized Rare Earth Elements (REE) patterns of benthic and planktonic foraminifera and Fe‐Mn coatings precipitated on sediments have been investigated for the South China Sea (SCS) to (1) assess the reliability of the extraction of past seawater εNd in the SCS and to (2) reconstruct past hydrological changes during the last 25 kyr. Reductively cleaned mono‐specific planktonic foraminifera (G. ruber) and mixed benthic foraminifera in core‐top sediments from 1500 to 2400 m display similar εNd values to those of the modern Pacific Deep Water (εNd of ‐3.9 ∼ ‐4.4). Furthermore, the εNd of the reductive cleaning solutions shows similar εNd values to ones obtained on cleaned foraminifera. Combined with PAAS‐normalized REE patterns, these results confirm that the oxidative and reductive cleaning procedure applied to foraminifera does not totally remove all of the Fe‐Mn coating and that εNd values yielded by cleaned planktonic foraminifera retain the εNd imprint of the bottom and/or pore water. εNd values obtained from a leaching procedure carried out on the bulk non‐decarbonated sediments are comparable to the εNd values of the modern PDW, whereas a similar leaching procedure applied to decarbonated sediments reveals a bias due to contamination with Nd deriving from lithogenic particles. In core MD05‐2904, seawater εNd, reconstructed from planktonic foraminifera, indicates that the last glacial period is characterized by lower εNd (‐5.2±0.2 to ‐6.4±0.3) than the late Holocene (‐4.1±0.2). Assuming that Nd input from river does not change strongly the εNd of the PDW of the northern SCS, these εNd variations suggest a higher relative proportions of southern source water in the deep‐water of the western subtropical Pacific Ocean during the last glacial period. This article is protected by copyright. All rights reserved.
      PubDate: 2015-10-01T10:57:29.589346-05:
      DOI: 10.1002/2015GC005871
  • Tectonic slicing of subducting oceanic crust along plate interfaces:
           Numerical modeling
    • Authors: J.B. Ruh; L. Le Pourhiet, Ph. Agard, E. Burov, T. Gerya
      Abstract: Multikilometer‐sized slivers of high‐pressure low‐temperature metamorphic oceanic crust and mantle are observed in many mountain belts. These blueschist and eclogite units were detached from the descending plate during subduction. Large‐scale thermo‐mechanical numerical models based on finite difference marker‐in‐cell staggered grid technique are implemented to investigate slicing processes that lead to the detachment of oceanic slivers and their exhumation before the onset of the continental collision phase. In particular, we investigate the role of the serpentinized sub‐crustal slab mantle in the mechanisms of shallow and deep crustal slicing. Results show that spatially homogeneous serpentinization of the sub‐Moho slab mantle leads to complete accretion of oceanic crust within the accretionary wedge. Spatially discontinuous serpentinization of the slab mantle in form of unconnected patches can lead to shallow slicing of the oceanic crust below the accretionary wedge and to its deep slicing at mantle depths depending on the patch length, slab angle, convergence velocity and continental geothermal gradient. P‐T paths obtained in this study are compared to natural examples of shallow slicing of the Crescent Terrane below Vancouver Island and deeply sliced crust of the Lago Superiore and Saas‐Zermatt units in the Western Alps. This article is protected by copyright. All rights reserved.
      PubDate: 2015-09-29T11:20:04.550608-05:
      DOI: 10.1002/2015GC005998
  • An efficient and general approach for implementing thermodynamic
           phase‐equilibria information in geophysical and geodynamic studies
    • Authors: Juan Carlos Afonso; Sergio Zlotnik, Pedro Diez
      Abstract: We present a flexible, general and efficient approach for implementing thermodynamic phase equilibria information (in the form of sets of physical parameters) into geophysical and geodynamic studies. The approach is based on tensor rank decomposition methods, which transform the original multi‐dimensional discrete information into a separated representation that contains significantly fewer terms, thus drastically reducing the amount of information to be stored in memory during a numerical simulation or geophysical inversion. Accordingly, the amount and resolution of the thermodynamic information that can be used in a simulation or inversion increases substantially. In addition, the method is independent of the actual software used to obtain the primary thermodynamic information, and therefore it can be used in conjunction with any thermodynamic modeling program and/or database. Also, the errors associated with the decomposition procedure are readily controlled by the user, depending on her/his actual needs (e.g. preliminary runs vs full resolution runs). We illustrate the benefits, generality and applicability of our approach with several examples of practical interest for both geodynamic modeling and geophysical inversion/modeling. Our results demonstrate that the proposed method is a competitive and attractive candidate for implementing thermodynamic constraints into a broad range of geophysical and geodynamic studies. MATLAB implementations of the method and examples are provided as supplementary material and can be downloaded from the journal's website. This article is protected by copyright. All rights reserved.
      PubDate: 2015-09-23T09:48:56.706727-05:
      DOI: 10.1002/2015GC006031
  • Architecture of North Atlantic contourite drifts modified by transient
           circulation of the Icelandic mantle plume
    • Abstract: Overflow of Northern Component Water, the precursor of North Atlantic Deep Water, appears to have varied during Neogene times. It has been suggested that this variation is moderated by transient behavior of the Icelandic mantle plume, which has influenced North Atlantic bathymetry through time. Thus pathways and intensities of bottom currents that control deposition of contourite drifts could be affected by mantle processes. Here, we present regional seismic reflection profiles that cross sedimentary accumulations (Björn, Gardar, Eirik and Hatton Drifts). Prominent reflections were mapped and calibrated using a combination of boreholes and legacy seismic profiles. Interpreted seismic profiles were used to reconstruct solid sedimentation rates. Björn Drift began to accumulate in late Miocene times. Its average sedimentation rate decreased at ∼2.5 Ma and increased again at ∼0.75 Ma. In contrast, Eirik Drift started to accumulate in early Miocene times. Its average sedimentation rate increased at ∼5.5 Ma and decreased at ∼2.2 Ma. In both cases, there is a good correlation between sedimentation rates, inferred Northern Component Water overflow, and the variation of Icelandic plume temperature independently obtained from the geometry of diachronous V‐shaped ridges. Between 5.5 and 2.5 Ma, the plume cooled, which probably caused subsidence of the Greenland‐Iceland‐Scotland Ridge, allowing drift accumulation to increase. When the plume became hotter at 2.5 Ma, drift accumulation rate fell. We infer that deep‐water current strength is modulated by fluctuating dynamic support of the Greenland‐Scotland Ridge. Our results highlight the potential link between mantle convective processes and ocean circulation. This article is protected by copyright. All rights reserved.
      PubDate: 2015-09-23T09:46:44.751972-05:
      DOI: 10.1002/2015GC005947
  • Petrofabric development during experimental partial melting and
           recrystallization of a mica‐schist analogue
    • Authors: Bjarne S. G. Almqvist; Andrea R. Biedermann, Iwona Klonowska, Santanu Misra
      Abstract: Magnetic properties and the anisotropy of magnetic susceptibility (AMS) present promising methods to track mineral orientation and petrofabric in rocks that have undergone partial melting. In order to better understand the source of the magnetic signal in these types of rocks, the interpretation of field observations may be integrated with laboratory experiments, designed to re‐create conditions of partial melting. A set of experiments is presented in this study, where synthetic foliated quartz‐muscovite aggregates undergo partial melting at 300 MPa hydrostatic confining pressure and 750°C. Magnetic properties and AMS are measured before and after partial melting. Prior to partial melting, the synthetic aggregate shows a compaction‐related oblate magnetic fabric, dominated by paramagnetic muscovite that contains small amounts of iron. Post‐experiment samples show neoblasts that crystallize from incongruent melt reactions. Most notably for the magnetic fabric, the breakdown of muscovite results in growth of secondary phases of Fe‐bearing spinel and biotite. Isothermal remanence acquisition and temperature‐dependence of susceptibility indicate that the spinel is magnetite. The degree of magnetic anisotropy reduces significantly after partial melting, but notably the orientation of the principal axes of susceptibility mimic the AMS of the original quartz‐muscovite aggregate. Additionally, the post‐experiment samples show a relationship between the amount of sample shortening (compaction) and the degree of magnetic anisotropy and susceptibility ellipsoid shape factor. These results suggest that petrofabrics in rocks that undergo partial melting at near hydrostatic pressure conditions may in part be inherited, or mimic, the original petrofabric of a sedimentary or metasedimentary rock. This article is protected by copyright. All rights reserved.
      PubDate: 2015-09-23T09:43:29.13753-05:0
      DOI: 10.1002/2015GC005962
  • Tremor‐genic slow slip regions may be deeper and warmer and may slip
           slower than nontremor‐genic regions
    • Abstract: Slow slip events (SSEs) are observed worldwide and often coincide with tectonic tremor. Notable examples of SSEs lacking observed tectonic tremor, however, occur beneath Kīlauea Volcano, Hawaii, the Boso Peninsula, Japan, {near San Juan Bautista on the San Andreas Fault, California, and recently in Central Ecuador. These SSEs are similar to other worldwide SSEs in many ways (e.g., size or duration), but lack the concurrent tectonic tremor observed elsewhere; instead they trigger swarms of regular earthquakes. We investigate the physical conditions that may distinguish these non‐tremor‐genic SSEs from those associated with tectonic tremor including: slip velocity, pressure, temperature, fluids and fault asperities, although we cannot eliminate the possibility that tectonic tremor may be obscured in highly attenuating regions. Slip velocities of SSEs at Kīlauea Volcano (∼10−6 m/s) and Boso Peninsula (∼10−7 m/s) are among the fastest SSEs worldwide. Kīlauea Volcano, the Boso Peninsula and Central Ecuador are also among the shallowest SSEs worldwide, and thus have lower confining pressures and cooler temperatures in their respective slow slip zones. {Fluids also likely contribute to tremor generation, and no corresponding zone of high vp/vs has been noted at Kīlauea or Boso. We suggest that the relatively faster slip velocities at Kīlauea Volcano and the Boso Peninsula result from specific physical conditions that may also be responsible for triggering swarms of regular earthquakes adjacent to the slow slip, while different conditions produce slower SSE velocities elsewhere and trigger tectonic tremor. This article is protected by copyright. All rights reserved.
      PubDate: 2015-09-21T11:59:57.677185-05:
      DOI: 10.1002/2015GC005895
  • Which is the better proxy for paleo‐current strength:
           Sortable‐silt mean size (SS‐) or sortable‐silt mean
           grain diameter (d‐SS)? A case study from the Nordic Seas
    • Authors: Andrea D. Tegzes; Eystein Jansen, Richard J. Telford
      Abstract: The coarseness of the 10‐63 µm terrigenous silt (i.e. sortable silt) fraction tends to vary independently of sediment supply in current‐sorted muds in the world's oceans, with coarser sediments representing relatively greater near‐bottom flow speeds. Traditionally the coarseness of this size fraction is described using an index called sortable‐silt mean size (SS‐), which is an arithmetic average calculated from the differential volume or mass distribution of grains within the 10‐63 µm terrigenous silt fraction, where the relative weights of the individual size bins become increasingly disproportionate, with respect to the actual number of grains within those size bins, towards the coarse end of the size range. This not only increases the absolute value of the apparent “mean size” within the 10‐63 μm terrigenous silt fraction, but it may also affect the apparent pattern of relative changes in the coarseness of the sortable‐silt fraction along the core. In addition, it makes SS‐ more prone to biases due to, for example, analytical errors. Here we present a detailed analysis of grain‐size distributions over three selected Holocene time intervals from two complementary sediment cores (JM97‐948/2A, MD95‐2011), extracted from the center of a high‐accumulation area along the flow path of the main branch of the Atlantic Inflow into the Nordic Seas and show that differential‐number‐based statistics, which likely better describes variations in the actual coarseness of the sortable‐silt fraction, may provide a more robust alternative to SS‐. This article is protected by copyright. All rights reserved.
      PubDate: 2015-09-16T11:06:52.292795-05:
      DOI: 10.1002/2014GC005655
  • Teleseismic P wave spectra from USArray and implications for upper mantle
           attenuation and scattering
    • Authors: Samantha Cafferky; Brandon Schmandt
      Abstract: Teleseismic P wave amplitude spectra from deep earthquakes recorded by USArray are inverted for maps of upper mantle Δt* for multiple frequency bands within 0.08–2 Hz. All frequency bands show high Δt* regions in the southwestern U.S., southern Rocky Mountains and Appalachian margin. Low Δt* is more common across the cratonic interior. Inversions with narrower frequency bands yield similar patterns, but greater Δt* magnitudes. Even the two standard deviation Δt* magnitude for the widest band is ∼2–7 times greater than predicted by global QS tomography or an anelastic olivine thermal model, suggesting that much of the Δt* signal is non‐thermal in origin. Non‐thermal contributions are further indicated by only a moderate correlation between Δt* and P travel times. Some geographic variations, such as high Δt* in parts of the cratonic interior with high mantle velocities and low heat flow, demonstrate that the influence of temperature is regionally overwhelmed. Transverse spectra are used to investigate the importance of scattering because they would receive no P energy in the absence of 3‐D heterogeneity or anisotropy. Transverse to vertical (T/Z) spectral ratios for stations with high Δt* are higher and exhibit steeper increases with frequency compared to T/Z spectra for low Δt* stations. The large magnitude of Δt* estimates and the T/Z spectra are consistent with major contributions to Δt* from scattering. A weak positive correlation between intrinsic attenuation and apparent attenuation due to scattering may contribute to Δt* magnitude and the moderate correlation of Δt* with travel times. This article is protected by copyright. All rights reserved.
      PubDate: 2015-09-15T03:08:21.060734-05:
      DOI: 10.1002/2015GC005993
  • Missing western half of the Pacific Plate: Geochemical nature of the
           Izanagi–Pacific Ridge interaction with a stationary boundary between
           the Indian and Pacific mantles
    • Abstract: The source mantle of the basaltic ocean crust on the western half of the Pacific Plate was examined using Pb–Nd–Hf isotopes. The results showed that the subducted Izanagi–Pacific Ridge (IPR) formed from both Pacific (180–∼80 Ma) and Indian (∼80–70 Ma) mantles. The western Pacific Plate becomes younger westward and is thought to have formed from the IPR. The ridge was subducted along the Kurile–Japan–Nankai–Ryukyu (KJNR) Trench at 60–55 Ma and leading edge of the Pacific Plate is currently stagnated in the mantle transition zone. Conversely, the entire eastern half of the Pacific Plate, formed from isotopically distinct Pacific mantle along the East Pacific Rise and the Juan de Fuca Ridge, largely remains on the seafloor. The subducted IPR is inaccessible; therefore, questions regarding which mantle might be responsible for the formation of the western half of the Pacific Plate remain controversial. Knowing the source of the IPR basalts provides insight into the Indian–Pacific mantle boundary before the Cenozoic. Isotopic compositions of the basalts from borehole cores (165–130 Ma) in the western Pacific show that the surface oceanic crust is of Pacific mantle origin. However, the accreted ocean floor basalts (∼80–70 Ma) in the accretionary prism along the KJNR Trench have Indian mantle signatures. This indicates the younger western Pacific Plate of IPR origin formed partly from Indian mantle and that the Indian–Pacific mantle boundary has been stationary in the western Pacific at least since the Cretaceous. This article is protected by copyright. All rights reserved.
      PubDate: 2015-09-14T03:54:03.683285-05:
      DOI: 10.1002/2015GC005911
  • Glacioeustasy, Meteoric Diagenesis, and the Carbon Cycle During the
    • Authors: Blake Dyer; Adam C. Maloof, John A. Higgins
      Abstract: Mid‐Carboniferous carbonates in the western United States have undergone Pleistocene Bahamas‐style meteoric diagenesis that may be associated with expanding late Paleozoic ice sheets. Fourteen stratigraphic sections from carbonate platforms illustrate the regional distribution and variable intensity of physical and chemical diagenesis just below the mid‐Carboniferous unconformity. Each section contains top‐negative carbon isotope excursions that terminate in regional exposure surfaces that are associated with some combination of karst towers, desiccation cracks, fabric destructive recrystallization, or extensive root systems. The timing of the diagenesis is synchronous with similarly‐scaled top‐negative carbon isotope excursions observed by others in England, Kazakhstan, and China. The mass flux of negative carbon required to generate similar isotopic profiles across the areal extent of middle Carboniferous platform carbonates is a significant component of the global carbon cycle. We present a simple carbon box model to illustrate that the δ13C of dissolved inorganic carbon in the ocean could be elevated by ∼1.4 textperthousand∼as isotopically lower carbon from the terrestrial organic weathering reacts with exposed platforms before reaching the ocean and atmosphere. These results represent an improvement on global biogeochemical models that have struggled to provide a congruent solution to the high δ13C of the late Paleozoic icehouse. This article is protected by copyright. All rights reserved.
      PubDate: 2015-09-14T03:53:16.26003-05:0
      DOI: 10.1002/2015GC006002
  • Deep subsurface carbon cycling in the Nankai Trough (Japan) –
           evidence of tectonically induced stimulation of a deep microbial
    • Authors: N. Riedinger; M. Strasser, R.N. Harris, G. Klockgether, T.W. Lyons, E. Screaton
      Abstract: The abundance of microbial life and the sources of energy necessary for deep subsurface microbial communities remain enigmatic. Here we investigate deep microbial processes and their potential relationships to tectonic events in sediments from the Nankai Trough offshore Japan, drilled and sampled during IODP (Integrated Ocean Drilling Program) Expedition 316. Observed methane isotope profiles indicate that microbially mediated methane production occurs at Sites C0006 and C0007 in sediments below ∼450 meters below seafloor (mbsf) and ∼425 mbsf, respectively. The active carbon cycling in these deep subsurface sediments is likely related to the highly dynamic tectonic regime at Nankai Trough. We propose that transient increases in temperature have re‐stimulated organic matter degradation at these distinct depths and explore several candidate processes for transient heating. Our favored hypothesis is frictional heating associated with earthquakes. In concert with transient heating leading to the reactivation of recalcitrant organic matter, the heterogeneous sedimentary system provides niches for microbial life. The newly available/accessible organic carbon compounds fuel the microbial community – resulting in an onset of methanogenesis several hundred meters below the seafloor. This process is captured in the methane C‐isotope signal, showing the efficacy of methane C‐isotopes for delineating locations of active microbial processes in deeply buried sediments. Additionally, simple model approaches applied to observed chemical pore water profiles can potentially constrain timing relationships, which can then be linked to causative tectonic events. Our results suggest the occurrence of slip‐to‐the‐trench earthquake(s) 200‐400 year ago, which could relate to historical earthquakes (1707 Hoei and/or 1605 Keicho earthquakes). This article is protected by copyright. All rights reserved.
      PubDate: 2015-09-10T02:29:13.531219-05:
      DOI: 10.1002/2015GC006050
  • Erosion patterns and mantle sources of topographic change across the
           southern African Plateau derived from the shallow and deep records of
    • Authors: Jessica R. Stanley; Rebecca M. Flowers, David R. Bell
      Abstract: Flow in the sub‐lithospheric mantle is increasingly invoked as a mechanism to explain both modern and past surface topography, but the importance of this phenomenon and its influence at different localities are debated. Southern Africa is an elevated continental shield proposed to represent dynamically supported topography. However, this region is also characterized by a complex lithospheric architecture variably affected by Cretaceous heating, thinning, and metasomatic alteration. We used apatite (U‐Th)/He thermochronometry on fifteen Cretaceous kimberlites from an ∼600 km long transect across the Kaapvaal Craton, combined with information from xenoliths in these pipes, to determine the plateau interior erosion history. The goal was to determine the relationships with lithospheric modification patterns and thereby better isolate the sub‐lithospheric contribution to elevation. The results document a wave of erosion from west to east across the craton from ∼120 Ma to
      PubDate: 2015-09-07T07:58:25.560615-05:
      DOI: 10.1002/2015GC005969
  • Inter‐laboratory comparison of magnesium isotopic compositions of 12
           felsic to ultramafic igneous rock standards analyzed by MC‐ICPMS
    • Abstract: To evaluate the inter‐laboratory mass bias for high‐precision stable Mg isotopic analysis of natural materials, a suite of silicate standards ranging in composition from felsic to ultramafic were analyzed in five laboratories by using three types of multi‐collector inductively coupled plasma mass spectrometer (MC‐ICPMS). Magnesium isotopic compositions from all labs were in agreement for most rocks within quoted uncertainties but are significantly (up to 0.3‰ in 26Mg/24Mg, > 4 times of uncertainties) different for some mafic samples. The inter‐laboratory mass bias does not correlate with matrix element/Mg ratios and the mechanism for producing it is uncertain but very likely arises from column chemistry. Our results suggest that standards with different matrices are needed to calibrate the efficiency of column chemistry and caution should be taken when dealing with samples with complicated matrices. Well‐calibrated standards with matrix elements matching samples should be used to reduce the inter‐laboratory mass bias. This article is protected by copyright. All rights reserved.
      PubDate: 2015-09-07T07:25:29.016917-05:
      DOI: 10.1002/2015GC005939
  • Late Miocene to recent plate tectonic history of the southern central
           America convergent margin
    • Authors: K. D. Morell
      Abstract: New plate reconstructions constrain the tectonic evolution of the subducting Cocos and Nazca plates across the southern Central American subduction zone from late Miocene to Recent. Because of the strong relationships between lower and upper (Caribbean) plate dynamics along this margin, these constraints have wide‐ranging implications for the timing and growth of upper plate deformation and volcanism in southern Central America. The reconstructions outline three important events in the Neogene history of this margin: 1) the coeval development of the Panama Triple Junction with the initiation of oblique subduction of the Nazca plate at ∼8.5 Ma; 2) the initiation of seamount and rough crust subduction beginning at ∼3‐4 Ma; and 3) Cocos Ridge subduction from ∼2‐3 Ma. A comparison of these events with independent geologic, geomorphic, volcanic, and stratigraphic datasets reveals that the timing, rates and origin of subducting crust directly impacted the Neogene growth of upper plate deformation and volcanism in southern Central America. These analyses constrain the timing, geometry and causes of a number of significant tectonic and volcanic processes, including rapid Plio‐Quaternary arc‐forearc contraction due to Cocos Ridge subduction, the detachment of the Panama microplate at ∼1‐3 Ma, and the late Miocene cessation of mantle‐wedge‐derived volcanism across ∼300 km of the subduction zone. This article is protected by copyright. All rights reserved.
      PubDate: 2015-09-07T03:49:35.114864-05:
      DOI: 10.1002/2015GC005971
  • Quantitative estimates of Asian dust input to the western Philippine Sea
           in the mid‐late quaternary and its potential significance for
    • Authors: Zhaokai Xu; Tiegang Li, Peter D. Clift, Dhongil Lim, Shiming Wan, Hongjin Chen, Zheng Tang, Fuqing Jiang, Zhifang Xiong
      Abstract: We present a new high‐resolution multiproxy data set of Sr‐Nd isotopes, rare earth element, soluble iron, and total organic carbon data from International Marine Global Change Study Core MD06‐3047 located in the western Philippine Sea. We integrate our new data with published clay mineralogy, rare earth element chemistry, thermocline depth, and δ13C differences between benthic and planktonic foraminifera, in order to quantitatively constrain Asian dust input to the basin. We explore the relationship between Philippine Sea and high‐latitude Pacific eolian fluxes, as well as its significance for marine productivity and atmospheric CO2 during the mid–late Quaternary. Three different indices indicate that Asian dust contributes between ∼15% and ∼50% to the detrital fraction of the sediments. Eolian dust flux in Core MD06‐3047 is similar to that in the polar Southern Pacific sediment. Coherent changes for most dust flux maximum/minimum indicate that dust generation in interhemispheric source areas might have a common response to climatic variation over the mid–late Quaternary. Furthermore, we note relatively good coherence between Asian dust input, soluble iron concentration, local marine productivity, and even global atmospheric CO2 concentration over the entire study interval. This suggests that dust‐borne iron fertilization of marine phytoplankton might have been a periodic process operating at glacial/interglacial time scales over the past 700 ka. We suggest that strengthening of the biological pump in the Philippine Sea, and elsewhere in the tropical Western Pacific during the mid–late Quaternary glacial periods may contribute to the lowering of atmospheric CO2 concentrations during ice ages. This article is protected by copyright. All rights reserved.
      PubDate: 2015-09-07T03:48:54.877371-05:
      DOI: 10.1002/2015GC005929
  • Toward the reconciliation of seismological and petrological perspectives
           on oceanic lithosphere heterogeneity
    • Authors: B.L.N. Kennett; T. Furumura
      Abstract: The character of the high‐frequency seismic phases Po and So, observed after propagation for long distances in the oceanic lithosphere, requires the presence of scattering from complex structure in 3‐D. Current models use stochastic representations of seismic structure in the oceanic lithosphere. The observations are compatible with quasi‐laminate features with horizontal correlation length around 10 km and vertical correlation length 0.5 km, with a uniform level of about 2% variation through the full thickness of the lithosphere. Such structures are difficult to explain with petrological models, which would favour stronger heterogeneity at the base of the lithosphere associated with underplating from frozen melts. Petrological evidence mostly points to smaller‐scale features than suggested by seismology. The models from the different fields have been derived independently, with various levels of simplification. Fortunately it is possible to gently modify the seismological model towards stronger basal heterogeneity, but there remains a need for some quasi‐laminate structure throughout the mantle component of the oceanic lithosphere. The new models help to bridge the gulf between the different viewpoints, but ambiguities remain. This article is protected by copyright. All rights reserved.
      PubDate: 2015-09-07T02:54:57.567973-05:
      DOI: 10.1002/2015GC006017
  • Active mud volcanoes on the continental slope of the Canadian Beaufort Sea
    • Abstract: Morphologic features, 600 to 1,100 m across and elevated up to 30 m above the surrounding seafloor, interpreted to be mud volcanoes were investigated on the continental slope in the Beaufort Sea in the Canadian Arctic. Sediment cores, detailed mapping with an autonomous underwater vehicle and exploration with a remotely operated vehicle show that these are young and actively forming features experiencing ongoing eruptions. Biogenic methane and low‐chloride, sodium‐bicarbonate‐rich waters are extruded with warm sediment that accumulates to form cones and low relief circular plateaus. The chemical and isotopic compositions of the ascending water indicate that a mixture of meteoric water, seawater, and water from clay dehydration has played a significant role in the evolution of these fluids. The venting methane supports extensive siboglinid tubeworms communities and forms some gas hydrates within the near seafloor. We believe that these are the first documented living chemosynthetic biological communities in the continental slope of the western Arctic Ocean. This article is protected by copyright. All rights reserved.
      PubDate: 2015-09-04T18:15:54.741342-05:
      DOI: 10.1002/2015GC005928
  • Influence of plate tectonic mode on the coupled thermochemical evolution
           of Earth's mantle and core
    • Authors: Takashi Nakagawa; Paul J. Tackley
      Abstract: We investigate the influence of tectonic mode on the thermo‐chemical evolution of simulated mantle convection coupled to a parameterized core cooling model. The tectonic mode is controlled by varying the friction coefficient for brittle behavior, producing the three tectonic modes mobile lid (plate tectonics), stagnant lid and episodic lid. The resulting compositional structure of the deep mantle is strongly dependent on tectonic mode, with episodic lid resulting in a thick layer of subducted basalt in the deep mantle, whereas mobile lid produces only isolated piles and stagnant lid no basaltic layering. The tectonic mode is established early on, with subduction initiating at around 60 Myr from the initial state in mobile and episodic cases, triggered by the arrival of plumes at the base of the lithosphere. Crustal production assists subduction initiation, increasing the critical friction coefficient. The tectonic mode has a strong effect on core evolution via its influence on deep mantle structure; episodic cases in which a thick layer of basalt builds up experience less core heat flow and cooling and a failed geodynamo. Thus, a continuous mobile lid mode existing from early times matches Earth's mantle structure and core evolution better than an episodic mode characterized by large‐scale flushing (overturn) events. This article is protected by copyright. All rights reserved.
      PubDate: 2015-09-04T18:15:42.170868-05:
      DOI: 10.1002/2015GC005996
  • The three‐dimensional construction of the Rae craton, central Canada
    • Authors: David B. Snyder; James A. Craven, Mark Pilkington, Michael J. Hillier
      Abstract: Reconstruction of the 3‐dimensional tectonic assembly of early continents, first as Archean cratons and then Proterozoic shields, remains poorly understood. In this paper, all readily available geophysical and geochemical data are assembled in a 3‐D model with the most accurate bedrock geology in order to understand better the geometry of major structures within the Rae craton of central Canada. Analysis of geophysical observations of gravity and seismic wave speed variations revealed several lithospheric‐scale discontinuities in physical properties. Where these discontinuities project upward to correlate with mapped upper crustal geological structures, the discontinuities can be interpreted as shear zones. Radiometric dating of xenoliths provides estimates of rock types and ages at depth beneath sparse kimberlite occurrences. These ages can also be correlated to surface rocks. The 3.6–2.6 Ga Rae craton comprises at least three smaller continental terranes, that ‘cratonized' during a granitic bloom. Cratonization probably represents final differentiation of early crust into a relatively homogeneous, uniformly thin (35–42 km), tonalite‐trondhjemite‐granodiorite crust with pyroxenite layers near the Moho. The peak thermo‐tectonic event at 1.86–1.7 Ga was associated with the Hudsonian orogeny that assembled several cratons and lesser continental blocks into the Canadian Shield using a number of southeast‐dipping megathrusts. This orogeny metasomatized, mineralized and recrystallized mantle and lower crustal rocks, apparently making them more conductive by introducing or concentrating sulfides or graphite. Little evidence exists of thin slabs similar to modern oceanic lithosphere in this Precambrian construction history whereas underthrusting and wedging of continental lithosphere is inferred from multiple dipping discontinuities. This article is protected by copyright. All rights reserved.
      PubDate: 2015-09-03T11:34:39.854031-05:
      DOI: 10.1002/2015GC005957
  • Supercontinental inheritance and its influence on supercontinental
           breakup: The central Atlantic magmatic province and the break up of Pangea
    • Authors: Lisa Whalen; Esteban Gazel, Christopher Vidito, John Puffer, Michael Bizimis, William Henika, Mark J. Caddick
      Abstract: The Central Atlantic Magmatic Province (CAMP) is the large igneous province (LIP) that coincides with the breakup of the supercontinent Pangea. Major and trace element data, Sr‐Nd‐Pb radiogenic isotopes, and high‐precision olivine chemistry were collected on primitive CAMP dikes from Virginia (VA). These new samples were used in conjunction with a global CAMP dataset to elucidate different mechanisms for supercontinent breakup and LIP formation. In the Eastern North America Margin, CAMP flows are found primarily in rift basins that can be divided into northern or southern groups based on different tectonic evolution, rifting histories, and supercontinental inheritance. Geochemical signatures of CAMP suggest an upper mantle source modified by subduction processes. We propose that the greater number of accretionary events or metasomatism by sediment melts as opposed to fluids on the northern versus the southern Laurentian margin during the formation of Pangea led to different subduction‐related signatures in the mantle source of the northern versus southern CAMP lavas. CAMP samples have elevated Ni and low Ca in olivine phenocrysts indicating a significant pyroxenite component in the source, interpreted here as a result from subduction metasomatism. Different collisional styles during the Alleghanian orogeny in the North and South may have led to the diachroneity of the rifting of Pangea. Furthermore, due to a low angle of subduction, the Rheic plate may have underplated the lithosphere, then delaminated thus triggering both the breakup of Pangea and the formation of CAMP. This article is protected by copyright. All rights reserved.
      PubDate: 2015-09-03T11:34:23.730804-05:
      DOI: 10.1002/2015GC005885
  • Modeling olivine CPO evolution with complex deformation
           histories—Implications for the interpretation of seismic anisotropy
           in the mantle
    • Authors: Yuval Boneh; Luiz F.G. Morales, Edouard Kaminski, Philip Skemer
      Abstract: Relating seismic anisotropy to mantle flow requires detailed understanding of the development and evolution of olivine crystallographic preferred orientation (CPO). Recent experimental and field studies have shown that olivine CPO evolution depends strongly on the integrated deformation history, which may lead to differences in how the corresponding seismic anisotropy should be interpreted. In this study, two widely‐used numerical models for CPO evolution – D‐Rex and VPSC – are evaluated to further examine the effect of deformation history on olivine texture and seismic anisotropy. Building on previous experimental work, models are initiated with several different CPOs to simulate unique deformation histories. Significantly, models initiated with a pre‐existing CPO evolve differently than the CPOs generated without pre‐existing texture. Moreover, the CPO in each model evolves differently as a function of strain. Numerical simulations are compared to laboratory experiments by Boneh and Skemer (2014). In general, the D‐Rex and VPSC models are able to reproduce the experimentally observed CPOs, although the models significantly over‐estimate the strength of the CPO and in some instances produce different CPO from what is observed experimentally. Based on comparison with experiments, recommended parameters for D‐Rex are: M* = 10, λ* = 5, and χ = 0.3, and for VPSC: α = 10 ‐ 100. Numerical modeling confirms that CPO evolution in olivine is highly sensitive to the details of the initial CPO, even at strains greater than 2. These observations imply that there is a long transient interval of CPO realignment which must be considered carefully in the modeling or interpretation of seismic anisotropy in complex tectonic settings. This article is protected by copyright. All rights reserved.
      PubDate: 2015-09-01T11:15:15.675919-05:
      DOI: 10.1002/2015GC005964
  • A new method for calibrating a boron isotope paleo‐pH proxy using
           massive Porites corals
    • Authors: Kaoru Kubota; Yusuke Yokoyama, Tsuyoshi Ishikawa, Atsushi Suzuki
      Abstract: The boron isotope ratio (δ11B) of marine biogenic carbonates can reconstruct pH and pCO2 of seawater, and potentially CO2 concentration in the atmosphere. To date, δ11B‐pHSW calibration has been proposed via culturing experiments, where calcifying organisms are cultured under artificially acidified seawater. However, in scleractinian corals, reconstructed pH values using culture‐based calibrations do not agree well with actual observations of seawater CO2 chemistry. Thus, another approach is needed to establish a more reliable calibration method. In this study, we established field‐based calibrations for Chichijima and Tahiti, both located in subtropical gyres where surface seawater is close to CO2 equilibrium. We suggest a new approach to calibration of δ11B‐pH in which the long‐term δ11B variation of massive Porites corals is compared with the decreasing pH trend (i.e., ocean acidification) that has occurred since the Industrial Revolution. This calibration will offer a new avenue for studying seawater CO2 chemistry using coral δ11B in diverse settings, such as upwelling regions, coral reefs, and coastal areas. This article is protected by copyright. All rights reserved.
      PubDate: 2015-09-01T10:49:25.705661-05:
      DOI: 10.1002/2015GC005975
  • Characterization and petrological constraints of the
           mid‐lithospheric discontinuity
    • Abstract: Within continental lithosphere, widespread seismic evidence suggests a sharp discontinuous downward decrease in seismic velocity at 60‐160 km depth. This mid‐lithospheric discontinuity (MLD) may be due to anisotropy, melt, hydration, and/or mantle metasomatism. We survey global seismologic observations of the MLD, including observed depths, velocity contrasts, gradients, and locales across multiple seismic techniques. The MLD is primarily found in regions of thick continental lithosphere and is a decrease in seismic shear velocity (2‐7% over 10‐20 km) at 60‐160 km depth, the majority of observations clustering at 80‐100 km. Of xenoliths in online databases, 25% of amphibole‐bearing xenoliths, 90% of phlogopite‐bearing xenoliths, and none of carbonate‐bearing xenoliths were formed at pressures associated with these depth (2‐5 GPa). We used Perple_X modeling to evaluate the elastic moduli and densities of multiple petrologies to test if the MLD is a layer of crystallized melt. The fractional addition of 5‐10% phlogopite, 10‐15% carbonate, or 45‐100% pyroxenite produce a 2‐7% velocity decrease. We postulate this layer of crystallized melt would originate at active margins of continents and crystallize in place as the lithosphere cools. The concentration of mildly‐incompatible elements (Y, Ho, Er, Yb, and Lu) in xenoliths near the MLD is consistent with higher degrees of melting. Thus, we postulate that the MLD is the seismological signature of a chemical interface related to the paleo‐intersection of a volatile‐rich solidus and progressively cooling lithosphere. Furthermore, the MLD may represent a remnant chemical tracer of the lithosphere‐asthenosphere boundary (LAB) from when the lithosphere was active and young. This article is protected by copyright. All rights reserved.
      PubDate: 2015-09-01T10:47:21.517926-05:
      DOI: 10.1002/2015GC005943
  • Improving shallow‐water carbonate chemostratigraphy by means of
           rudist bivalve sclerochemistry
    • Authors: S. Huck; U. Heimhofer
      Abstract: Deep‐time shallow marine carbonate platforms record distinct biotic responses to climatic and environmental stressors. Unfortunately, precise temporal assignment of these biotic responses is often problematical due to poor biostratigraphic control and/or a significant diagenetic overprint of the neritic bulk carbonate chemostratigraphic inventory. An accurate stratigraphic framework is essential to better understand the causal relation between biotic events recorded by carbonate platforms and environmental changes that, for instance, culminated in mass extinction events or prolonged episodes of oceanic anoxia. Here, we provide an integrated carbon and strontium‐isotope stratigraphy of the Early Cretaceous subtropical Provence carbonate platform in SE France that is based solely on pristine low‐Mg calcite from rudist bivalves. Carbon‐isotope data of geochemically screened rudist fragments enabled reconstruction of a characteristic Barremian pattern including the Mid‐Barremian Event (MBE) that allowed for a precise correlation with stratigraphically well‐constrained Tethyan shallow‐water and hemipelagic reference sections. In order to evaluate ontogenetic carbon‐isotope changes and the overall variability of the shell‐derived carbon‐isotope data, numerous sclerochronological carbon‐isotope profiles of individual large rudist shells are presented. Strontium‐isotope stratigraphy supports the carbon‐isotope based age of the studied sections, but also provides unequivocal evidence for a major hiatus in the depositional record covering large parts of the Late Barremian. In contrast to biostratigraphic and bulk carbonate chemostratigraphic archives, the here established chronostratigraphy of carbonate platform evolution in the southern Provence region demonstrates a two‐fold resurgence of rudist‐rich carbonate platform production during the Early Aptian and arguably the latest Early Aptian. This article is protected by copyright. All rights reserved.
      PubDate: 2015-08-27T03:59:39.319544-05:
      DOI: 10.1002/2015GC005988
  • Ventilation and dissolved oxygen cycle in Lake Superior: Insights from a
           numerical model
    • Authors: Katsumi Matsumoto; Kathy S. Tokos, Chad Gregory
      Abstract: Ventilation and dissolved oxygen in Lake Superior are key factors that determine the fate of various natural and anthropogenic inputs to the lake. We employ an idealized age tracer and biogeochemical tracers in a realistically configured numerical model of Lake Superior to characterize its ventilation and dissolved O2 cycle. Our results indicate that Lake Superior is preferentially ventilated over rough bathymetry and that spring overturning following a very cold winter does not completely ventilate the lake interior. While this is unexpected for a dimictic lake, no part of the lake remains isolated from the atmosphere for more than 300 days. Our results also show that Lake Superior's oxygen cycle is dominated by solubility changes; as a result, the expected relationship between biological consumption of dissolved O2 and ventilation age does not manifest. This article is protected by copyright. All rights reserved.
      PubDate: 2015-08-27T03:50:06.364321-05:
      DOI: 10.1002/2015GC005916
  • Continent scale strike‐slip on a low‐angle fault beneath New
           Zealand's Southern Alps: Implications for crustal thickening in oblique
           collision zones
    • Abstract: New Zealand's Southern Alps lie adjacent to the continent‐scale dextral strike‐slip Alpine Fault, on the boundary between the Pacific and Australian plates. We show with a simple 2‐D model of crustal balancing that the observed crustal root and erosion (expressed as equivalent crustal shortening) is up to twice that predicted by the orthogonal plate convergence since ∼11 Ma, and even since ∼23 Ma when the Alpine Fault formed. We consider two explanations for this, involving a strong component of motion along the length of the plate‐boundary zone. Geophysical data indicate that the Alpine Fault has a listric geometry, flattening at mid‐crustal levels, and has accommodated sideways underthrusting of Australian plate crust beneath Pacific plate crust. The geometry of the crustal root, together with plate reconstructions, require the underthrust crust to be the hyper‐extended part of an asymmetric rift system which formed over 500 km farther south during the Eocene – the narrow remnant part today forms the western margin of the Campbell Plateau. At ∼10 Ma, the hyper‐extended margin underwent shallow subduction in the Puysegur subduction zone, and then was dragged over 300 km along the length of the Southern Alps beneath a low angle (
      PubDate: 2015-08-22T01:08:10.434771-05:
      DOI: 10.1002/2015GC005990
  • Deeply dredged submarine HIMU glasses from the Tuvalu Islands, Polynesia:
           Implications for volatile budgets of recycled oceanic crust
    • Abstract: Ocean island basalts (OIB) with extremely radiogenic Pb‐isotopic signatures are melts of a mantle component called HIMU (high µ, high 238U/204Pb). Until now, deeply‐dredged submarine HIMU glasses have not been available, which has inhibited complete geochemical (in particular, volatile element) characterization of the HIMU mantle. We report major, trace and volatile element abundances in a suite of deeply‐dredged glasses from the Tuvalu Islands. Three Tuvalu glasses with the most extreme HIMU signatures have F/Nd ratios (35.6±3.6) that are higher than the ratio (∼21) for global OIB and MORB, consistent with elevated F/Nd ratios in endmember HIMU Mangaia melt inclusions. The Tuvalu glasses with the most extreme HIMU composition have Cl/K (0.11 to 0.12), Br/Cl (0.0024) and I/Cl (5‐6 × 10−5) ratios that preclude significant assimilation of seawater‐derived Cl. The new HIMU glasses that are least degassed for H2O have low H2O/Ce ratios (75‐84), similar to ratios identified in endmember OIB glasses with EM1 and EM2 signatures, but significantly lower than H2O/Ce ratios (119 to 245) previously measured in melt inclusions from Mangaia. CO2‐H2O equilibrium solubility models suggest that these HIMU glasses (recovered in two different dredges at 2,500 to 3,600 meters water depth) have eruption pressures of 295 to 400 bars. We argue that degassing is unlikely to significantly reduce the primary melt H2O. Thus, the lower H2O/Ce in the HIMU Tuvalu glasses is a mantle signature. We explore oceanic crust recycling as the origin of the low H2O/Ce (∼50 to 80) in the EM1, EM2 and HIMU mantle domains. This article is protected by copyright. All rights reserved.
      PubDate: 2015-08-21T03:16:24.19131-05:0
      DOI: 10.1002/2015GC005966
  • Evolution of permeability across the transition from brittle failure to
           cataclastic flow in porous siltstone
    • Authors: Marco M. Scuderi; Hiroko Kitajima, Brett M. Carpenter, Demian M. Saffer, Chris Marone
      Abstract: Porous sedimentary rocks fail in a variety of modes ranging from localized, brittle deformation to pervasive, cataclastic flow. To improve our understanding of this transition and its affect on fluid flow and permeability, we investigated the mechanical behavior of a siltstone unit within the Marcellus Formation, PA USA, characterized by an initial porosity ranging from 41 to 45%. We explored both hydrostatic loading paths (σ1=σ2=σ3) and triaxial loading paths (σ1>σ2=σ3) while maintaining constant effective pressure (Pe=Pc‐Pp). Samples were deformed with an axial displacement rate of 0.1 μm/s (strain rate of 2x10−6 s−1). Changes in pore water volume were monitored (drained conditions) to measure the evolution of porosity. Permeability was measured at several stages of each experiment. Under hydrostatic loading, we find the onset of macroscropic grain crushing (P*) at 39 MPa. Triaxial loading experiments show a transition from brittle behavior with shear localization and compaction to cataclastic‐flow as confining pressure increases. When samples fail by shear localization, permeability decreases abruptly without significant changes in porosity. Conversely, for cataclastic deformation, permeability reduction is associated with significant porosity reduction. Post‐experiment observation of brittle samples show localized shear zones characterized by grain comminution. Our data show how zones of shear localization can act as barriers to fluid flow and thus modify the hydrological and mechanical properties of the surrounding rocks. Our results have important implications for deformation behavior and permeability evolution in sedimentary systems, and in particular where the stress field is influenced by injection or pumping. This article is protected by copyright. All rights reserved.
      PubDate: 2015-08-20T11:12:37.356814-05:
      DOI: 10.1002/2015GC005932
  • A 1400 year environmental magnetic record from varved sediments of Lake
           Xiaolongwan (northeast China) reflecting natural and anthropogenic soil
    • Authors: Youliang Su; Guoqiang Chu, Qingsong Liu, Zhaoxia Jiang, Xing Gao, Torsten Haberzettl
      Abstract: Lake sediments can provide high‐quality information about human activities. In this study, we investigate a sediment core from Lake Xiaolongwan using magnetic and geochemical methods. The dominant magnetic minerals of this sediment core are stable single domain (SSD) and superparamagnetic (SP) magnetite particles. The increasing amount of SP particles reflected by the rise of magnetic susceptibility and frequency dependent magnetic susceptibility since AD 1500 can be attributed to an increasing influx in pedogenic soil, which is related to a regional‐scale increase in the intensity of human activity in Northeastern China. This extends the timing of human activities, which is independent from climate changes and its effects on local ecosystems in Northeastern China significantly. This article is protected by copyright. All rights reserved.
      PubDate: 2015-08-20T01:38:45.352313-05:
      DOI: 10.1002/2015GC005880
  • First hydrothermal discoveries on the Australian‐Antarctic Ridge:
           Discharge sites, plume chemistry, and vent organisms
    • Abstract: The Australian‐Antarctic Ridge (AAR) is one of the largest unexplored regions of the global mid‐ocean ridge system. Here, we report a multi‐year effort to locate and characterize hydrothermal activity on two 1st‐order segments of the AAR: KR1 and KR2. To locate vent sites on each segment, we used profiles collected by Miniature Autonomous Plume Recorders on rock corers during R/V Araon cruises in March and December of 2011. Optical and oxidation‐reduction‐potential anomalies indicate multiple active sites on both segments. Seven profiles on KR2 found 3 sites, each separated by ∼25 km. Forty profiles on KR1 identified 13 sites, some within a few km of each other. The spatial density of hydrothermal activity along KR1 and KR2 (plume incidence of 0.34) is consistent with the global trend for a spreading rate of ∼70 mm/yr. The densest area of hydrothermal activity, named “Mujin”, occurred along the 20‐km‐long inflated section near the segment center of KR1. Continuous plume surveys conducted in January‐February of 2013 on R/V Araon found CH4/3He (1‐15 × 106) and CH4/Mn (0.01‐0.5) ratios in the plume samples, consistent with a basaltic‐hosted system and typical of ridges with intermediate spreading rates. Additionally, some of the plume samples exhibited slightly higher ratios of H2/3He and Fe/Mn than others, suggesting that those plumes are supported by a younger hydrothermal system that may have experienced a recent eruption. The Mujin‐field was populated by Kiwa crabs and seven‐armed Paulasterias starfish previously recorded on the East Scotia Ridge, raising the possibility of circum‐Antarctic biogeographic connections of vent fauna. This article is protected by copyright. All rights reserved.
      PubDate: 2015-08-18T18:33:52.560033-05:
      DOI: 10.1002/2015GC005926
  • Magnetic unmixing of first‐order reversal curve diagrams using
           principal component analysis
    • Authors: Ioan Lascu; Richard J. Harrison, Yuting Li, Joy R. Muraszko, James E. T. Channell, Alexander M. Piotrowski, David A. Hodell
      Abstract: We describe a quantitative magnetic unmixing method based on principal component analysis (PCA) of first‐order reversal curve (FORC) diagrams. For PCA we resample FORC distributions on grids that capture diagnostic signatures of single‐domain (SD), pseudo‐single‐domain (PSD), and multi‐domain (MD) magnetite, as well as of minerals such as hematite. Individual FORC diagrams are recast as linear combinations of end‐member (EM) FORC diagrams, located at user‐defined positions in PCA space. The EM selection is guided by constraints derived from physical modeling and imposed by data scatter. We investigate temporal variations of two EMs in bulk North Atlantic sediment cores collected from the Rockall Trough and the Iberian Continental Margin. Sediments from each site contain a mixture of magnetosomes and granulometrically distinct detrital magnetite. We also quantify the spatial variation of three EM components (a coarse silt‐sized MD component, a fine silt‐sized PSD component, and a mixed clay‐sized component containing both SD magnetite and hematite) in surficial sediments along the flow path of the North Atlantic Deep Water (NADW). These samples were separated into granulometric fractions, which helped constrain EM definition. PCA‐based unmixing reveals systematic variations in EM relative abundance as a function of distance along NADW flow. Finally, we apply PCA to the combined dataset of Rockall Trough and NADW sediments, which can be recast as a four‐EM mixture, providing enhanced discrimination between components. Our method forms the foundation of a general solution to the problem of unmixing multi‐component magnetic mixtures, a fundamental task of rock magnetic studies. This article is protected by copyright. All rights reserved.
      PubDate: 2015-08-14T03:36:41.971052-05:
      DOI: 10.1002/2015GC005909
  • Intraplate volcanism of the western Pacific: New insights from geological
           and geophysical observations in the Pigafetta Basin
    • Authors: Timothy J. Stadler; Masako Tominaga
      Abstract: Understanding intraplate volcanism is a key to deciphering the Earth's magmatic history. One of the largest intraplate volcanic events occurred during the mid Cretaceous, roughly 75 to 125 Ma in the western Pacific. To investigate the origin and effects of this volcanism on various Earth systems, we present the first comprehensive study of volcanism in the Pigafetta Basin using seismic surveys, magnetic and gravity modeling, and Ocean Drilling Program drill core and well log data from Site 801. Our results show that intraplate volcanism in the Pigafetta Basin coincides with the rest of the western Pacific seamount provinces, supporting the previously suggested plumelets scenario for the origin of intraplate volcanism during the mid Cretaceous volcanic events. We also discover that the late stage volcanism does not overprint the remanant magnetization acquired by the Jurassic ocean crust in the Pigafetta Basin, and hence, marine magnetic anomalies recorded in the Jurassic basement are preserved. Also, the formerly identified Rough Smooth Boundary (RSB) is indistinguishable from any other rough‐smooth topographic boundaries throughout the survey area suggesting that the RSB is unlikely to be a Cretaceous sill‐Jurassic basement boundary. Lastly, the apparent ages and spatial distribution of volcanic features suggests a dynamic history of hydrothermal circulation in the Pigafetta Basin, indicating that hydrothermal circulation was ongoing well past 100 Ma. This article is protected by copyright. All rights reserved.
      PubDate: 2015-08-12T03:42:07.870133-05:
      DOI: 10.1002/2015GC005873
  • Exploring the mineralogical heterogeneities of the Louisville Seamount
    • Authors: Michael J. Dorais
      Abstract: Diopside phenocrysts of the Louisville Seamount Trail show an increase in Ti, Al, and Na with decreasing Mg/(Mg+Fe) as is typical for clinopyroxene in alkalic basalts. Chondrite‐normalized REE patterns of calculated liquids from LA‐ICPMS analyses are comparable to whole‐rock and glass values. Exceptions are clinopyroxene crystals from the Rigil Seamount, the second oldest seamount drilled at the northern end of the chain. Some crystals from this site are strongly zoned with distinct compositional boundaries between cores and mantles. The cores have high Mg/(Mg+Fe) and low Al and Ti concentrations compared to the mantles and phenocrysts. Major element, clinopyroxene discrimination diagrams indicate that the clinopyroxene mantles and phenocrysts crystallized from alkalic basalts. In contrast, the Mg‐rich cores have tholeiitic affinities. The REE abundances of the cores are similar to that of clinopyroxene from transitional tholeiites of the Kerguelen Archipelago. Calculated liquid La/Yb values for the cores have ratios that are similar to transitional tholeiites in Hawaii, whereas the mantles have higher La/Yb values similar to Hawaiian alkalic basalts. The major and trace element compositions of clinopyroxene cores from the Rigil seamount suggest that a transitional tholeiitic magma was present, but no evidence for Hawaiian shield‐type tholeiites was found. Plagioclase crystals from the Rigil seamount have 86Sr/86Sr from 0.70306 to 0.70363, within the range of FOZO. The transitional tholeiitic signature of the Mg‐rich clinopyroxene cores probably did not have a distinct source compared to other Louisville magmas, but more likely indicates a higher degree of partial melting (2‐5%) of that FOZO source. This article is protected by copyright. All rights reserved.
      PubDate: 2015-08-12T03:41:44.858747-05:
      DOI: 10.1002/2015GC005917
  • Tracing mantle‐reacted fluids in magma‐poor rifted margins:
           The example of Alpine Tethyan rifted margins
    • Authors: Victor Hugo; G. Pinto, Gianreto Manatschal, Anne Marie Karpoff, Adriano Viana
      Abstract: The thinning of the crust and the exhumation of subcontinental mantle in magma‐poor rifted margins is accompanied by a series of extensional detachment faults. We show that exhumation along these detachments is intimately related to migration of fluids leading to changes in mineralogy and chemistry of the mantle, crustal and sedimentary rocks. Using field observation and analytical methods, we investigate the role of fluids in the fossil distal margins of the Alpine Tethys. Using Cr‐Ni‐V, Fe and Mn as tracers, we show that fluids used detachment faults as pathways and interacted with the overlying crust and sediments. These observations allow us to discuss when, where and how this interaction happened during the formation of the rifted margin. The results show that: (i) serpentinization of mantle rocks during their exhumation results in the depletion of elements and migration of mantle‐reacted fluids that are channeled along active detachment system; (ii) in earlier‐stages, these fluids affected the overlying syn‐tectonic sediments by direct migration from the underlying detachments; (iii) in later‐stages, these fluids arrived at the seafloor, were introduced into, or “polluted” the seawater and were absorbed by post‐tectonic sediments. We conclude that a significant amount of serpentinization occurred underneath the hyperextended continental crust, and that the mantle‐reacted fluids might have modified the chemical composition of the sediments and seawater. We propose that the chemical signature of serpentinization related to mantle exhumation is recorded in the sediments and may serve as a proxy to date serpentinization and mantle exhumation at present‐day magma‐poor rifted margins. This article is protected by copyright. All rights reserved.
      PubDate: 2015-08-12T03:40:26.956968-05:
      DOI: 10.1002/2015GC005830
  • Geochemistry of river‐borne clays entering the East China Sea
           indicates two contrasting types of weathering and sediment transport
    • Authors: Lei Bi; Shouye Yang, Chao Li, Yulong Guo, Quan Wang, James T. Liu, Ping Yin
      Abstract: The East China Sea is characterized by wide continental shelf receiving a huge input of terrigenous matter from both large rivers and mountainous rivers, which makes it an ideal natural laboratory for studying sediment source‐to‐sink transport processes. This paper presents mineralogical and geochemical data of the clays and bulk sediments from the rivers entering the East China Sea, aiming to investigate the general driving mechanism of silicate weathering and sediment transport processes in East Asian continental margin. Two types of river systems, tectonically‐stable continental rivers and tectonically‐active mountainous rivers, co‐exist in East Asia. As the direct weathering products, clays can better reflect the silicate weathering regimes within the two river systems. Provenance rock types are not the dominant factor causing silicate weathering intensity difference existed in the East Asian rivers. The silicate weathering intensity of tectonically‐stable river basins is primarily driven by monsoon climate, and the sediment transfer is relatively slow because of natural trapping process and increasing damming effect. The geochemistry of these river‐borne sediments can thus indicate paleo‐weathering intensities in East Asian continent. In contrast, silicate weathering intensity in tectonically‐active mountainous rivers is greatly limited by strong physical erosion despite the high temperature and highest monsoon rainfall. The factors controlling silicate weathering in tectonically‐active catchments are complex and thus, it should be prudent to use river sediment records to decipher paleoclimate change. These two different silicate weathering regimes and sediment transport processes are manifestations of the landscape evolution and overall dominate the sedimentation in Asian continental margin. This article is protected by copyright. All rights reserved.
      PubDate: 2015-08-12T03:39:40.424822-05:
      DOI: 10.1002/2015GC005867
  • Heterogeneous and asymmetric crustal accretion: New constraints from
           multibeam bathymetry and potential field data from the Rainbow area of the
           Mid‐Atlantic Ridge (36°15′N)
    • Authors: M. Paulatto; J. P. Canales, R. A. Dunn, R. A. Sohn
      Abstract: At slow‐spreading mid‐ocean ridges, crustal accretion style can vary significantly along and across ridge segments. In magma‐poor regions, seafloor spreading can be accommodated largely by tectonic processes, however, the internal structure and formation mechanism of such highly tectonized crust are not fully understood. We analyze multi‐beam bathymetry and potential field data from the Rainbow area of the Mid‐Atlantic Ridge (35º40'N‐37º40'N), a section of the ridge that shows diverse accretion styles. We identify volcanic, tectonized and sedimented terrain and measure exposed fault area to estimate the tectonic strain, T, and the fraction of magmatic accretion, M. Estimated T values range from 0.2‐0.4 on ridge segments to 0.6‐0.8 at the Rainbow non‐transform discontinuity (NTD). At segment ends T is asymmetric, reflecting asymmetries in accretion rate, topography and faulting between inside and outside offset corners. Detachment faults have formed preferentially at inside corners, where tectonic strain is higher. We identify at least two oceanic core complexes on the fossil trace of the NTD, in addition to the Rainbow massif, which occupies the offset today. A gravity high and low magnetization suggest that the Rainbow massif, which hosts a high‐temperature hydrothermal system, was uplifted by a west dipping detachment fault. Asymmetric plate ages indicate localization of tectonic strain at the inside corners and migration of the detachment towards and across the ridge axis, which may have caused emplacement of magma into the footwall. Hydrothermal circulation and heat extraction is possibly favored by increased permeability generated by fracturing of the footwall and deep‐penetrating second‐generation faults. This article is protected by copyright. All rights reserved.
      PubDate: 2015-08-10T03:51:23.495754-05:
      DOI: 10.1002/2015GC005743
  • Influence of cratonic lithosphere on the formation and evolution of flat
           slabs: Insights from 3‐D time‐dependent modeling
    • Abstract: Several mechanisms have been suggested for the formation of flat slabs including buoyant features on the subducting plate, trenchward motion and thermal or cratonic structure of the overriding plate. Analysis of episodes of flat subduction indicate that not all flat slabs can be attributed to only one of these mechanisms and it is likely that multiple mechanisms work together to create the necessary conditions for flat slab subduction. In this study we examine the role of localized regions of cratonic lithosphere in the overriding plate in the formation and evolution of flat slabs. We explicitly build on previous models, by using time‐dependent simulations with three‐dimensional variation in overriding plate structure. We find that there are two modes of flat subduction: permanent underplating occurs when the slab is more buoyant (shorter or younger), while transient flattening occurs when there is more negative buoyancy (longer or older slabs). Our models show how regions of the slab adjacent to the sub‐cratonic flat portion continue to pull the slab into the mantle leading to highly contorted slab shapes with apparent slab gaps beneath the craton. These results show how the interpretation of seismic images of subduction zones can be complicated by the occurrence of either permanent or transient flattening of the slab, and how the signature of a recent flat slab episode may persist as the slab resumes normal subduction. Our models suggest that permanent underplating of slabs may preferentially occur below thick and cold lithosphere providing a built‐in mechanism for regeneration of cratons. This article is protected by copyright. All rights reserved.
      PubDate: 2015-08-07T04:33:07.19802-05:0
      DOI: 10.1002/2015GC005940
  • Coupled C‐S‐Fe geochemistry in a rapidly accumulating marine
           sedimentary system: Diagenetic and depositional implications
    • Authors: A. Peketi; A. Mazumdar, H.M. Joao, D. J. Patil, A. Usapkar, P. Dewangan
      Abstract: In the present study, we have investigated the C‐S‐Fe systematics in a sediment core (MD161‐13) from the Krishna‐Godavari (K‐G) basin, Bay of Bengal. The core covers the late Holocene period with high overall sedimentation rate of ∼573 cm ky−1. Pore fluid chemical analyses indicate that the depth of the present sulfate methane transition zone (SMTZ) is at ∼6 mbsf. The (ΔTA+ΔCa+ΔMg)/ΔSO42‐ ratios suggest that both organoclastic degradation and anaerobic oxidation of methane (AOM) drive sulfate reduction at the study site. The positive correlation between total organic carbon content (TOC) and chromium reducible sulfur (CRS) content indicates marked influence of organoclastic sulfate reduction on sulfidization. Coupled occurrence of 34S enriched iron sulfide (pyrite) with 12C enriched authigenic carbonate zones are the possible records of paleo‐sulfate methane transition zones where AOM driven focused sulfate reduction was likely fueled by sustained high methane flux from underlying gas rich zone. Aluminium normalized poorly reactive iron (FePR/Al) and La/Yb ratios suggest increasing contribution from Deccan basalts relative to that of Archean‐ Proterozoic granitic complex in sediment flux of Krishna‐Godavari basin during the last 4 ky. This article is protected by copyright. All rights reserved.
      PubDate: 2015-08-07T04:31:25.214918-05:
      DOI: 10.1002/2015GC005754
  • Effect of maghemization on the magnetic properties of nonstoichiometric
           pseudosingle‐domain magnetite particles
    • Abstract: The effect of maghemization on the magnetic properties of magnetite (Fe3O4) grains in the pseudo‐single‐domain (PSD) size range is investigated as a function of annealing temperature. X‐ray diffraction and transmission electron microscopy confirms the precursor grains as Fe3O4 ranging from ∼ 150 nm to ∼ 250 nm in diameter, whilst Mössbauer spectrometry suggests the grains are initially near‐stoichiometric. The Fe3O4 grains are heated to increasing reaction temperatures of 120 – 220 ºC to investigate their oxidation to maghemite (γ‐Fe2O3). High‐angle annular dark field imaging and localized electron energy‐loss spectroscopy reveals slightly oxidized Fe3O4 grains, heated to 140 ºC, exhibit higher oxygen content at the surface. Off‐axis electron holography allows for construction of magnetic induction maps of individual Fe3O4 and γ‐Fe2O3 grains, revealing their PSD (vortex) nature, which is supported by magnetic hysteresis measurements, including first‐order reversal curve analysis. The coercivity of the grains is shown to increase with reaction temperature up to 180 ºC, but subsequently decreases after heating above 200 ºC; this magnetic behavior is attributed to the growth of a γ‐Fe2O3 shell with magnetic properties distinct from the Fe3O4 core. It is suggested there is exchange coupling between these separate components that results in a vortex state with reduced vorticity. Once fully oxidized to γ‐Fe2O3, the domain states revert back to vortices with slightly reduced coercivity. It is argued that due to a core/shell coupling mechanism during maghemization, the directional magnetic information will still be correct, however, the intensity information will not be retained. This article is protected by copyright. All rights reserved.
      PubDate: 2015-08-06T11:12:26.282034-05:
      DOI: 10.1002/2015GC005858
  • S wave splitting in the offshore South Island, New Zealand: Insights into
           plate‐boundary deformation
    • Authors: S. C. Karalliyadda; M. K. Savage, A. Sheehan, J. Collins, D. Zietlow, A. Shelley
      Abstract: Local and regional S‐wave splitting in the offshore South Island of the New Zealand plate‐boundary zone provides constraints on the spatial and depth extent of the anisotropic structure with an enhanced resolution relative to land‐based and SKS studies. The combined analysis of offshore and land measurements using splitting tomography suggests plate‐boundary shear dominates in the central and northern South Island. The width of this shear zone in the central South Island is about 200 km, but is complicated by stress‐controlled anisotropy at shallow levels. In northern South Island, a broader (>200 km) zone of plate‐boundary parallel anisotropy is associated with the transitional faulting between the Alpine fault and Hikurangi subduction and the Hikurangi subduction zone itself. These results suggest S‐phases of deep events (∼ 90 km) in the central South Island are sensitive to plate‐boundary derived NE‐SW aligned anisotropic media in the upper‐lithosphere, supporting a “thin viscous sheet” deformation model. This article is protected by copyright. All rights reserved.
      PubDate: 2015-08-06T11:12:08.057655-05:
      DOI: 10.1002/2015GC005882
  • Wettability measurement under high P‐T conditions using X‐ray
           imaging with application to the brine‐supercritical CO2 system
    • Authors: Kuldeep Chaudhary; Eric J. Guiltinan, M. Bayani Cardenas, Jessica A. Maisano, Richard A. Ketcham, Philip C. Bennett
      Abstract: We present a new method for measuring wettability or contact angle of minerals at reservoir pressure‐temperature conditions using high‐resolution X‐ray computed tomography (HRXCT) and radiography. In this method, a capillary or a narrow slot is constructed from a mineral or a rock sample of interest wherein two fluids are allowed to form an interface that is imaged using X‐rays. After some validation measurements at room pressure‐temperature conditions, we illustrate this method by measuring the contact angle of CO2–brine on quartz, muscovite, shale, borosilicate glass, polytetrafluoroethylene (PTFE or Teflon), and polyether ether ketone (PEEK) surfaces at 60‐71°C and 13.8 – 22.8 MPa. At reservoir conditions, PTFE and PEEK surfaces were found to be CO2–wet with contact angles of 140° and 127°, respectively. Quartz and muscovite were found to be water–wet with contact angles of 26° and 58°, respectively under similar conditions. Borosilicate glass–air–brine at room conditions showed strong water‐wet characteristics with a contact angle of 9°, whereas borosilicate glass‐CO2–brine at 13.8 MPa and 60°C showed a decrease in its water‐wetness with contact angle of 54°. This method provides a new application for X‐ray imaging and an alternative to other methods. This article is protected by copyright. All rights reserved.
      PubDate: 2015-08-06T11:09:33.875019-05:
      DOI: 10.1002/2015GC005936
  • Late Pleistocene ages for the most recent volcanism and
           glacial‐pluvial deposits at Big Pine volcanic field, California,
           USA, from cosmogenic 36Cl dating
    • Authors: J.A. Vazquez; J. M. Woolford
      Abstract: The Big Pine volcanic field is one of several Quaternary volcanic fields that poses a potential volcanic hazard along the tectonically active Owens Valley of east‐central California, and whose lavas are interbedded with deposits from Pleistocene glaciations in the Sierra Nevada range. Previous geochronology indicates an ∼1.2 Ma history of volcanism, but the eruption ages and distribution of volcanic products associated with the most‐recent eruptions are poorly resolved. To delimit the timing and products of the youngest volcanism, we combine field mapping and cosmogenic 36Cl dating of basaltic lava flows in the area where lavas with youthful morphology and well‐preserved flow structures are concentrated. Field mapping and petrology reveal approximately fifteen vents and six principal flow units with variable geochemical composition and mineralogy. Cosmogenic 36Cl exposure ages for lava flow units from the top, middle, and bottom of the volcanic stratigraphy indicate eruptions at ca. 17 ka, 27 ka, and 40 ka, revealing several different and previously unrecognized episodes of late Pleistocene volcanism. Olivine to plagioclase‐pyroxene phyric basalt erupted from several vents during the most recent episode of volcanism at ca. 17 ka, and produced a lava flow field covering ∼ 35 km2. The late Pleistocene 36Cl exposure ages indicate that moraine and pluvial shoreline deposits that overly or modify the youngest Big Pine lavas reflect Tioga stage glaciation in the Sierra Nevada and the shore of Owens Lake during the last glacial cycle. This article is protected by copyright. All rights reserved.
      PubDate: 2015-08-01T10:44:20.049941-05:
      DOI: 10.1002/2015GC005889
  • Propagation of back‐arc extension into the arc lithosphere in the
           southern New Hebrides volcanic arc
    • Authors: M. Patriat; J. Collot, M. Fabre, L. Danyushevsky, S. Meffre, T. Falloon, P. Rouillard, B. Pelletier, M. Roach, M. Fournier
      Abstract: New geophysical data acquired during three expeditions of the R/V Southern Surveyor in the southern part of the North Fiji Basin allow us to characterize the deformation of the upper plate at the southern termination of the New Hebrides subduction zone, where it bends eastward along the Hunter Ridge. Unlike the northern end of the Tonga subduction zone, on the other side of the North Fiji Basin, the 90° bend does not correspond to the transition from a subduction zone to a transform fault, but it is due to the progressive retreat of the New Hebrides trench. The subduction trench retreat is accommodated in the upper plate by the migration toward the southwest of the New Hebrides arc and toward the south of the Hunter Ridge, so that the direction of convergence remains everywhere orthogonal to the trench. In the back‐arc domain, the active deformation is characterised by propagation of the back‐arc spreading ridge into the Hunter volcanic arc. The N‐S spreading axis propagates southward and penetrates in the arc, where it connects to a sinistral strike‐slip zone via an oblique rift. The collision of the Loyalty Ridge with the New Hebrides arc, less than two million years ago, likely initiated this deformation pattern and the fragmentation of the upper plate. In this particular geodynamic setting, with an oceanic lithosphere subducting beneath a highly sheared volcanic arc, a wide range of primitive subduction‐related magmas has been produced including adakites, island arc tholeiites, back‐arc basin basalts, and medium‐K subduction‐related lavas. This article is protected by copyright. All rights reserved.
      PubDate: 2015-08-01T10:43:49.688434-05:
      DOI: 10.1002/2015GC005717
  • Multiple mantle upwellings in the transition zone beneath the northern
           East African Rift System from relative P wave travel time tomography
    • Authors: Chiara Civiero; James O. S. Hammond, Saskia Goes, Stewart Fishwick, Abdulhakim Ahmed, Atalay Ayele, Cecile Doubre, Berhe Goitom, Derek Keir, Michael Kendall, Sylvie Leroy, Ghebrebrhan Ogubazghi, Georg Rumpker, Graham W. Stuart
      Abstract: Mantle plumes and consequent plate extension have been invoked as the likely cause of East African Rift volcanism. However, the nature of mantle upwelling is debated, with proposed configurations ranging from a single broad plume connected to the large low‐shear‐velocity province beneath Southern Africa, the so‐called African Superplume, to multiple lower‐mantle sources along the rift. We present a new P‐wave travel‐time tomography model below the northern East‐African, Red Sea and Gulf of Aden rifts and surrounding areas. Data are from stations that span an area from Madagascar to Saudi Arabia. The aperture of the integrated dataset allows us to image structures of ∼100 km length scale down to depths of 700‐800 km beneath the study region. Our images provide evidence of two clusters of low‐velocity structures consisting of features with diameter of 100‐200 km that extend through the transition zone, the first beneath Afar and a second just west of the Main Ethiopian Rift, a region with off‐rift volcanism. Considering seismic sensitivity to temperature, we interpret these features as upwellings with excess temperatures of 100±50 K. The scale of the upwellings is smaller than expected for lower mantle plume sources. This, together with the change in pattern of the low‐velocity anomalies across the base of the transition zone, suggests that ponding or flow of deep‐plume material below the transition zone may be spawning these upper‐mantle upwellings. This article is protected by copyright. All rights reserved.
      PubDate: 2015-08-01T10:42:37.541849-05:
      DOI: 10.1002/2015GC005948
  • A Bayesian, multivariate calibration for Globigerinoides ruber Mg/Ca
    • Abstract: The use of Mg/Ca in marine carbonates as a paleothermometer has been challenged by observations that implicate salinity as a contributing influence on Mg incorporation into biotic calcite and that dissolution at the sea‐floor alters the original Mg/Ca. Yet, these factors have not yet been incorporated into a single calibration model. We introduce a new Bayesian calibration for Globigerinoides ruber Mg/Ca based on 186 globally‐distributed core top samples, which explicitly takes into account the effect of temperature, salinity, and dissolution on this proxy. Our reported temperature, salinity, and dissolution (here expressed as deep‐water ΔCO32‐) sensitivities are (±2σ) 8.7±0.9%/°C, 3.9±1.2%/psu, and 3.3±1.3%/μ−1 below a critical threshold of 21 μmol/kg, in good agreement with previous culturing and core‐top studies. We then perform a sensitivity experiment on a published record from the western tropical Pacific to investigate the bias introduced by these secondary influences on the interpretation of past temperature variability. This experiment highlights the potential for misinterpretations of past oceanographic changes when the secondary influences of salinity and dissolution are not accounted for. Multi‐proxy approaches could potentially help deconvolve the contributing influences but this awaits better characterization of the spatio‐temporal relationship between salinity and δ18Osw over millennial and orbital timescales. This article is protected by copyright. All rights reserved.
      PubDate: 2015-07-22T03:58:44.749955-05:
      DOI: 10.1002/2015GC005844
  • Reply to comment by Ivan Pineda‐Velasco, Tai T. Nguyen, Hiroshi
           Kitagawa, and Eizo Nakamura on “Diverse magmatic effects of
           subducting a hot slab in SW Japan: Results from forward modeling”
    • Abstract: The Comment by Pineda‐Velasco et al. [2015] examined Pb isotope data presented by Kimura et al. [2014]. The authors' points are that (1) there is uncertainty in the analytical results of Kimura et al. [2014] due to the effect of mass fractionation, and therefore, (2) the interpretations of Kimura et al. [2014] based on the extent of crustal assimilation and the estimated Pb isotopic composition of the crustal component are erroneous. In response to the Comment, we report a flaw in the original paper that the samples from the Aono, Daisen, and Kannabe regions were analyzed using conventional TIMS methods and all other samples were analyzed using thallium‐spiked multi‐collector inductively‐coupled‐plasma mass spectrometry (TS‐MC‐ICP‐MS). We have re‐analyzed the sample powders from Karasugasen, Daisen, and Aono using TS‐MC‐ICP‐MS. Our new results showed considerable overlap with the data in Pineda‐Velasco et al. [2015]. Therefore, the isotopic trends shown by the conventional TIMS in Kimura et al. [2014] were analytical artifacts from mass bias. We conclude that the crustal assimilation proposed by Kimura et al. [2014] was erroneous in terms of Pb isotopes, nevertheless some crustal assimilation in the Karasugasen lava is evident from the chemical zoning of hornblende phenocrysts. Although the original Pb isotope argument for crustal contamination was wrong, the ABS4 modeling is unaffected because of no to subtle changes in estimated mantle source compositions in their ABS4 model. This article is protected by copyright. All rights reserved.
      PubDate: 2015-07-17T02:22:07.781949-05:
      DOI: 10.1002/2015GC005984
  • Comment on “Diverse magmatic effects of subducting a hot slab in SW
           Japan: Results from forward modeling'' by J.‐I. Kimura et al.
    • PubDate: 2015-07-16T04:33:10.722884-05:
      DOI: 10.1002/2015GC005914
  • Crustal‐scale degassing due to magma system destabilization and
           magma‐gas decoupling at Soufrière Hills Volcano, Montserrat
    • Authors: T.E. Christopher; J. Blundy, K. Cashman, P. Cole, M. Edmonds, P.J. Smith, R.S.J. Sparks, A. Stinton
      Abstract: Activity since 1995 at Soufrière Hills Volcano (SHV), Montserrat has alternated between andesite lava extrusion and quiescence, which are well‐correlated with seismicity and ground deformation cycles. Large variations in SO2 flux do not correlate with these alternations, but high and low HCl/SO2 characterising lava dome extrusion and quiescent periods respectively. Since lava extrusion ceased (February 2010) steady SO2 emissions have continued at an average rate of 374 tonnes/day (± 140 t/d), and incandescent fumaroles (temperatures up to 610oC) on the dome have not changed position or cooled. Occasional short bursts (over several hours) of higher (∼ 10x) SO2 flux have been accompanied by swarms of volcano‐tectonic earthquakes. Strain data from these bursts indicate activation of the magma system to depths up to 10 km. SO2 emissions since 1995 greatly exceed the amounts that could be derived from 1.1 km3 of erupted andesite, and indicating extensive partitioning of sulfur into a vapour phase, as well as efficient decoupling and outgassing of sulfur‐rich gases from the magma. These observations are consistent with a vertically‐extensive, crustal magmatic mush beneath SHV. Three states of the magmatic system are postulated to control degassing. During dormant periods (103 to 104 years) magmatic vapour and melts separate as layers from the mush and decouple from each other. In periods of unrest (years) without eruption, melt and fluid layers become unstable, ascend and can amalgamate. Major destabilisation of the mush system leads to eruption, characterised by magma mixing and release of volatiles with different ages, compositions and sources. This article is protected by copyright. All rights reserved.
      PubDate: 2015-06-11T11:17:24.359999-05:
      DOI: 10.1002/2015GC005791
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