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  Subjects -> PHYSICS (Total: 780 journals)
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PHYSICS (565 journals)            First | 1 2 3 4 5 6 | Last

COSPAR Colloquia Series     Full-text available via subscription   (Followers: 1)
Cryogenics     Hybrid Journal   (Followers: 19)
Current Applied Physics     Full-text available via subscription   (Followers: 4)
Diamond and Related Materials     Hybrid Journal   (Followers: 12)
Differential Equations and Nonlinear Mechanics     Open Access   (Followers: 4)
Doklady Physics     Hybrid Journal   (Followers: 1)
Dynamical Properties of Solids     Full-text available via subscription  
ECS Journal of Solid State Science and Technology     Full-text available via subscription   (Followers: 1)
Egyptian Journal of Remote Sensing and Space Science     Open Access   (Followers: 5)
EJNMMI Physics     Open Access  
Embedded Systems Letters, IEEE     Hybrid Journal   (Followers: 23)
Energy Procedia     Open Access   (Followers: 4)
Engineering Failure Analysis     Hybrid Journal   (Followers: 41)
Engineering Fracture Mechanics     Hybrid Journal   (Followers: 20)
Environmental Fluid Mechanics     Hybrid Journal   (Followers: 2)
EPJ Nonlinear Biomedical Physics     Open Access  
EPJ Quantum Technology     Open Access  
EPJ Techniques and Instrumentation     Full-text available via subscription  
EPJ Web of Conferences     Open Access  
European Journal of Physics     Full-text available via subscription   (Followers: 5)
European Journal of Physics Education     Open Access   (Followers: 4)
European Physical Journal - Applied Physics     Full-text available via subscription   (Followers: 5)
European Physical Journal C     Hybrid Journal  
Europhysics News     Open Access   (Followers: 1)
Experimental Mechanics     Hybrid Journal   (Followers: 18)
Experimental Methods in the Physical Sciences     Full-text available via subscription  
Experimental Techniques     Hybrid Journal   (Followers: 46)
Exploration Geophysics     Hybrid Journal   (Followers: 4)
Few-Body Systems     Hybrid Journal  
Fire and Materials     Hybrid Journal   (Followers: 5)
Flexible Services and Manufacturing Journal     Hybrid Journal   (Followers: 1)
Fluctuation and Noise Letters     Hybrid Journal   (Followers: 1)
Fluid Dynamics     Hybrid Journal   (Followers: 5)
Fortschritte der Physik/Progress of Physics     Hybrid Journal  
Frontiers in Physics     Open Access   (Followers: 2)
Frontiers of Materials Science     Hybrid Journal   (Followers: 4)
Frontiers of Physics     Hybrid Journal   (Followers: 1)
Fusion Engineering and Design     Hybrid Journal   (Followers: 9)
Geochemistry, Geophysics, Geosystems     Full-text available via subscription   (Followers: 25)
Geografiska Annaler, Series A: Physical Geography     Hybrid Journal   (Followers: 4)
Geophysical Research Letters     Full-text available via subscription   (Followers: 53)
Geoscience and Remote Sensing, IEEE Transactions on     Hybrid Journal   (Followers: 21)
Glass Physics and Chemistry     Hybrid Journal   (Followers: 2)
Granular Matter     Hybrid Journal   (Followers: 2)
Graphs and Combinatorics     Hybrid Journal   (Followers: 7)
Gravitation and Cosmology     Hybrid Journal  
Handbook of Geophysical Exploration: Seismic Exploration     Full-text available via subscription  
Handbook of Metal Physics     Full-text available via subscription  
Handbook of Surface Science     Full-text available via subscription   (Followers: 3)
Handbook of Thermal Analysis and Calorimetry     Full-text available via subscription  
Haptics, IEEE Transactions on     Hybrid Journal   (Followers: 4)
Heat Transfer - Asian Research     Hybrid Journal   (Followers: 8)
High Energy Density Physics     Hybrid Journal   (Followers: 2)
High Pressure Research: An International Journal     Hybrid Journal   (Followers: 1)
IEEE Journal of Quantum Electronics     Hybrid Journal   (Followers: 16)
IEEE Signal Processing Magazine     Full-text available via subscription   (Followers: 36)
IET Optoelectronics     Hybrid Journal   (Followers: 3)
Il Colle di Galileo     Open Access  
Imaging Science Journal     Hybrid Journal   (Followers: 2)
Indian Journal of Biochemistry and Biophysics (IJBB)     Open Access   (Followers: 4)
Indian Journal of Physics     Hybrid Journal   (Followers: 4)
Indian Journal of Pure & Applied Physics (IJPAP)     Open Access   (Followers: 9)
Indian Journal of Radio & Space Physics (IJRSP)     Open Access   (Followers: 6)
Industrial Electronics, IEEE Transactions on     Hybrid Journal   (Followers: 13)
Industry Applications, IEEE Transactions on     Hybrid Journal   (Followers: 4)
Infinite Dimensional Analysis, Quantum Probability and Related Topics     Hybrid Journal  
InfraMatics     Open Access  
Infrared Physics & Technology     Hybrid Journal  
Intelligent Transportation Systems Magazine, IEEE     Full-text available via subscription   (Followers: 2)
Intermetallics     Hybrid Journal   (Followers: 8)
International Applied Mechanics     Hybrid Journal   (Followers: 2)
International Geophysics     Full-text available via subscription   (Followers: 4)
International Heat Treatment and Surface Engineering     Hybrid Journal   (Followers: 2)
International Journal for Computational Methods in Engineering Science and Mechanics     Hybrid Journal   (Followers: 10)
International Journal for Ion Mobility Spectrometry     Hybrid Journal   (Followers: 1)
International Journal for Simulation and Multidisciplinary Design Optimization     Full-text available via subscription   (Followers: 1)
International Journal of Abrasive Technology     Hybrid Journal   (Followers: 2)
International Journal of Aeroacoustics     Full-text available via subscription   (Followers: 7)
International Journal of Applied Electronics in Physics & Robotics     Open Access   (Followers: 1)
International Journal of Astronomy and Astrophysics     Open Access   (Followers: 3)
International Journal of Computational Materials Science and Surface Engineering     Hybrid Journal   (Followers: 8)
International Journal of Damage Mechanics     Hybrid Journal   (Followers: 5)
International Journal of Fatigue     Hybrid Journal   (Followers: 11)
International Journal of Fracture     Hybrid Journal   (Followers: 9)
International Journal of Geometric Methods in Modern Physics     Hybrid Journal   (Followers: 1)
International Journal of Geophysics     Open Access   (Followers: 4)
International Journal of Heat and Fluid Flow     Hybrid Journal   (Followers: 17)
International Journal of Low Radiation     Hybrid Journal  
International Journal of Low-Carbon Technologies     Open Access   (Followers: 1)
International Journal of Mass Spectrometry     Hybrid Journal   (Followers: 13)
International Journal of Material Forming     Hybrid Journal   (Followers: 2)
International Journal of Materials and Product Technology     Hybrid Journal   (Followers: 4)
International Journal of Mechanical Sciences     Hybrid Journal   (Followers: 6)
International Journal of Mechanics and Materials in Design     Hybrid Journal   (Followers: 5)
International Journal of Medical Physics, Clinical Engineering and Radiation Oncology     Open Access   (Followers: 4)
International Journal of Micro-Nano Scale Transport     Full-text available via subscription   (Followers: 2)
International Journal of Microstructure and Materials Properties     Hybrid Journal   (Followers: 7)
International Journal of Microwave Science and Technology     Open Access   (Followers: 2)
International Journal of Modeling, Simulation, and Scientific Computing     Hybrid Journal   (Followers: 1)
International Journal of Modern Physics A     Hybrid Journal   (Followers: 1)

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Journal Cover   Geochemistry, Geophysics, Geosystems
  [SJR: 2.56]   [H-I: 69]   [25 followers]  Follow
    
   Full-text available via subscription Subscription journal
   ISSN (Online) 1525-2027
   Published by American Geophysical Union (AGU) Homepage  [17 journals]
  • Lithospheric shear wave velocity and radial anisotropy beneath the
           northern part of North China from surface wave dispersion analysis
    • Authors: Yuanyuan V. Fu; Yuan Gao, Aibing Li, Yutao Shi
      Abstract: Rayleigh and Love wave phase velocities in the northern part of the North China are obtained from ambient noise tomography in the period range of 8 to 35 s and two plane wave earthquake tomography at periods of 20 to 91 s using data recorded at 222 broadband seismic stations from the temporary North China Seismic Array and permanent China Digital Seismic Array. The dispersion curves of Rayleigh and Love wave from 8 to 91 s are jointly inverted for the 3‐D shear wave structure and radial anisotropy in the lithosphere to 140 km depth. Distinct seismic structure are observed from the Fenhe Graben and Taihang Mountain to North China Basin. The North China Basin from the lower crust to the depth of 140 km is characterized by high velocity anomaly, reflecting mafic intrusion and residual materials after the extraction of melt, and by strong radial anisotropy with Vsh > Vsv implying horizontal layering of intrusion and alignment of minerals due to vigorous extensional deformation and subsequent thermal annealing. However, low velocity anomaly and positive radial anisotropy are observed in the Fenhe Graben and Taihang Mountain, suggesting the presence of partial melt in the lithosphere due to the mantle upwelling and horizontal flow pull. This article is protected by copyright. All rights reserved.
      PubDate: 2015-06-19T17:51:39.82515-05:0
      DOI: 10.1002/2015GC005825
       
  • Nonlinear attenuation from the interaction between different types of
           seismic waves and interaction of seismic waves with shallow ambient
           tectonic stress
    • Authors: Norman H. Sleep; Nori Nakata
      Abstract: Strong seismic waves bring rock into frictional failure at the uppermost few hundred meters. Numerous small fractures slip with the cumulative effect of anelastic strain and nonlinear attenuation; these fractures should not distinguish between remote sources of stress. Still, frictional failure criteria are not evident especially when seismic waves change the normal traction on fractures. We identify three earthquakes as examples where consideration of interaction among dynamic stresses from different wave types and ambient tectonic stress provides theoretical predictions of nonlinear attenuation that are potentially testable with single station seismograms. For example, because Rayleigh waves produce shallow horizontal dynamic tension and compression, frictional failure should preferentially occur on the tensile half‐cycle if no shallow tectonic stress is present and on the compressional half‐cycle if the tectonic stress is already near thrust‐faulting failure. We observed neither effect on records from the 2011 Mw 9.0 Great Tohoku earthquake. However, Rayleigh waves from this event appear to have brought rock beneath MYGH05 station into frictional failure at ∼10 m depth and thus suppressed high‐frequency S‐waves. The tensile half‐cycle of high frequency P‐waves reduced normal traction on horizontal planes beneath station IWTH25 during the 2008 Mw 6.9 Iwate‐Miyagi earthquake, weakening the rock in shear and suppressing high‐frequency S‐waves. The near‐field velocity pulse from the 1992 Mw 7.3 Landers earthquake brought the uppermost few hundred meters of granite beneath Lucerne station into frictional failure, suppressing high frequency S‐waves. These mildly positive examples support the reality of nonlinear wave interaction, warranting study future strong ground motions. This article is protected by copyright. All rights reserved.
      PubDate: 2015-06-19T17:51:00.336745-05:
      DOI: 10.1002/2015GC005832
       
  • Mantle helium along the Newport‐Inglewood fault zone, Los Angeles
           basin, California—A leaking paleo‐subduction zone
    • Authors: J. R. Boles; G. Garven, H. Camacho, J. E. Lupton
      Abstract: Mantle helium is a significant component of the helium gas from deep oil wells along the Newport‐Inglewood fault zone (NIFZ) in the Los Angeles (LA) basin. Helium isotope ratios are as high as 5.3 Ra (Ra=3He/4He ratio of air) indicating 66% mantle contribution, (assuming R/Ra = 8 for mantle), and most values are higher than 1.0 Ra. Other samples from basin margin faults and from within the basin have much lower values (R/Ra 
      PubDate: 2015-06-19T17:50:43.830667-05:
      DOI: 10.1002/2015GC005951
       
  • Issue Information
    • PubDate: 2015-06-16T08:53:33.961031-05:
      DOI: 10.1002/ggge.20561
       
  • A community benchmark for viscoplastic thermal convection in a 2‐D
           square box
    • Authors: N. Tosi; C. Stein, L. Noack, C. Hüttig, P. Maierová, H. Samuel, D. R. Davies, C. R. Wilson, S. C. Kramer, C. Thieulot, A. Glerum, M. Fraters, W. Spakman, A. Rozel, P. J. Tackley
      Abstract: Numerical simulations of thermal convection in the Earth's mantle often employ a pseudo‐plastic rheology in order to mimic the plate‐like behavior of the lithosphere. Yet the benchmark tests available in the literature are largely based on simple linear rheologies in which the viscosity is either assumed to be constant or weakly dependent on temperature. Here we present a suite of simple tests based on non‐linear rheologies featuring temperature‐, pressure‐, and strain rate dependent viscosity. Eleven different codes based on the finite volume, finite element, or spectral methods have been used to run five benchmark cases leading to stagnant lid, mobile lid, and periodic convection in a 2‐D square box. For two of these cases, we also show resolution tests from all contributing codes. In addition, we present a bifurcation analysis, describing the transition from a mobile lid regime to a periodic regime, and from a periodic regime to a stagnant lid regime, as a function of the yield stress. At a resolution of around 100 cells or elements in both vertical and horizontal directions, all codes reproduce the required diagnostic quantities with a discrepancy of at most ∼ 3% in the presence of both linear and non‐linear rheologies. Furthermore they consistently predict the critical value of the yield stress at which the transition between different regimes occurs. As the most recent mantle convection codes can handle a number of different geometries within a single solution framework, this benchmark will also prove useful when validating viscoplastic thermal convection simulations in such geometries. This article is protected by copyright. All rights reserved.
      PubDate: 2015-06-15T11:26:14.973399-05:
      DOI: 10.1002/2015GC005807
       
  • Variations of the lithospheric strength and elastic thickness in North
           America
    • Authors: Magdala Tesauro; Mikhail K. Kaban, Walter D. Mooney
      Abstract: We evaluate the effect of temperature variations on strength and effective elastic thickness (Te) of the lithosphere of the North American (NA) continent. To this purpose, we use two thermal models that are corrected for compositional variations and anelasticity effects in the upper mantle. These thermal models are obtained from a joint inversion of gravity data and two recent seismic tomography models (NA07 and SL2013sv). The crustal rheology was defined using NACr14, the most recent NA crustal model. This model specifies seismic velocities and thickness for a three‐layer model of the crystalline crust. Strength in the lithosphere and in the crust has similar distributions, indicating that local geotherms play a dominant role in determining strength rather than crustal composition. A pronounced contrast is present in strength between cratonic and off‐cratonic regions. Lithospheric strength in the off‐cratonic regions is prevalently localized within the crust and Te shows low values (150 km). In contrast to previous results, our models indicate that Phanerozoic regions located close to the edge of the cratons, as the Appalachians, are characterized by low strength. We also find that locally weak zones exist within the cratons (e.g., beneath the intracratonic Illinois Basin and Midcontinent rift). Seismic tomography models NA07 and SL2013sv differ mainly in some peripheral parts of the cratons, as the Proterozoic Canadian Platform, the Grenville and the western part of the Yavapai‐Mazatzal province, where the integrated strength for the model NA07 is ten times larger than in model SL2013sv due to a temperature difference (>200˚C) in the uppermost mantle. The differences in Te between the two models are less pronounced. In both models, Proterozoic regions reactivated by Meso‐Cenozoic tectonics (e.g., Rocky Mountains and the Mississippi Embayment) are characterized by a weak lithosphere due to the absence of the mechanically strong part of the mantle lithospheric layer. Intraplate earthquakes are distributed along the edges of the cratons, demonstrating that tectonic stress accumulates there, while the cores of the cratons remain undeformed. In both models intraplate earthquakes occur in weak lithosphere (∼0.5 x10^13 Pa s, Te ∼15 km) or near the edges of strong cratonic blocks, characterized by pronounced contrasts of strength and Te. This article is protected by copyright. All rights reserved.
      PubDate: 2015-06-11T11:18:36.596803-05:
      DOI: 10.1002/2015GC005937
       
  • What happens to soil organic carbon as coastal marsh ecosystems change in
           response to increasing salinity? An exploration using ramped pyrolysis
           
    • Authors: Elizabeth K Williams; Brad E Rosenheim
      Abstract: Coastal wetlands store vast amounts of organic carbon, globally, and are becoming increasingly vulnerable to the effects of anthropogenic sea‐level rise. To understand the effect of sea‐level rise on organic carbon fate and preservation in this global sink, it is necessary to characterize differences in the biogeochemical stability of coastal wetland soil organic carbon (SOC). Here, we use ramped pyrolysis/oxidation decomposition characteristics as proxies for SOC stability to understand the fate of carbon storage in coastal wetlands comprising the Mississippi River deltaic plain, undergoing rapid rates of local sea level rise. Soils from three wetland types (fresh, brackish, and salt marshes) along a salinity gradient were subjected to ramped pyrolysis analysis to evaluate decomposition characteristics related to thermochemical stability of SOC. At equivalent soil depths, we observed that fresh marsh SOC was more stable than brackish and salt marsh SOC. Depth, isotopic, elemental, and chemical compositions, bulk density, and water content of SOC all exhibited different relationships with SOC stability across the marsh salinity gradient, indicative of different controls on SOC stability within each marsh type. The differences in stability imply stronger preservation potential of fresh marsh soil carbon, compared to that of salt and brackish marshes. Considering projected marsh ecosystem responses to sea‐level rise, these observed stability differences are important in planning and implementing coastal wetland carbon‐focused remediation and improving climate model feedbacks with the carbon cycle. Specifically, our results imply that ecosystem changes associated with sea‐level rise will initiate the accumulation of less stable carbon in coastal wetlands. This article is protected by copyright. All rights reserved.
      PubDate: 2015-06-11T11:18:21.635345-05:
      DOI: 10.1002/2015GC005839
       
  • Miocene climate change on the Chinese Loess Plateau: Possible links to the
           growth of the northern Tibetan Plateau and global cooling
    • Authors: Youbin Sun; Long Ma, Jan Bloemendal, Steven Clemens, Xiaoke Qiang, Zhisheng An
      Abstract: The evolution of the Asian monsoon‐arid environmental system during the Cenozoic was closely related to the growth of the Himalayan‐Tibetan Plateau and global climate change. However, due to inconsistencies in paleoclimatic reconstructions and to various constraints on the timing of the growth of the Tibetan Plateau, the relative impacts of regional uplift and global cooling on Asian climate change remain controversial. Here we investigate the mineralogical composition of a Miocene Red Clay deposit on the western Chinese Loess Plateau in order to infer changes in chemical weathering and monsoon intensity. Variations of four mineralogical ratios (chlorite/quartz, illite/quartz, calcite/quartz, protodolomite/quartz) reveal that the summer monsoon intensity was relatively strong during the early Miocene (23.5‐18.5 Ma), weakened gradually until ∼9.5 Ma, and strengthened again in the late Miocene. We synthesized previously published thermochronological data from the northeastern Tibetan Plateau and surrounding mountains, and the results suggest that two phases of the rapid growth of northern Tibet occurred around 24‐17 Ma and 13‐7 Ma. Comparison of paleoclimatic proxies and thermochronological data suggests that the gradual weakening of the summer monsoon intensity from 18.5 to 9.5 Ma paralleled global cooling, whereas two intervals of strengthened monsoon in the early and late Miocene were possibly related to the rapid growth of northern Tibet. Our combination of paleoenvironmental proxies and thermochronological data reveals possible links between Miocene Asian monsoon evolution, phased growth of the Tibetan Plateau, and global climate change, and confirms the interconnection of geodynamic and atmospheric processes in the geological past. This article is protected by copyright. All rights reserved.
      PubDate: 2015-06-11T11:18:05.962522-05:
      DOI: 10.1002/2015GC005750
       
  • Lithospheric structure of the Texas‐Gulf of Mexico passive margin
           from surface wave dispersion and migrated Ps receiver functions
    • Authors: Mohit Agrawal; Jay Pulliam, Mrinal K. Sen, Harold Gurrola
      Abstract: The seismic velocity structure beneath Texas Gulf Coastal Plain (GCP) is imaged by migrating Ps receiver functions with a seismic velocity model found by fitting surface wave dispersion. We use seismic data from a linear array of 22 broadband stations, spaced 16‐20 km apart. A Common Conversion Point (CCP) stacking technique is applied to earthquake data to improve the S/N ratios of receiver functions. Using an incorrect velocity model for depth migration of a stacked CCP image may produce an inaccurate image of the subsurface. To find sufficiently accurate P‐ and S‐velocity models, we first apply a nonlinear modeling technique to fit Rayleigh wave group velocity dispersion via Very Fast Simulated Annealing. Vs ranges from 1.5 km/s in shallow layers of the GCP in to 4.5 km/s beneath the Llano uplift and just outboard of the Balcones Fault Zone (BFZ). The BFZ is characterized by slow velocities that persist to nearly 100 km depth. In the stacked image, the largest‐amplitude positive‐polarity event ranges from the surface, at the Llano uplift, to a maximum depth of ∼ 16 km beneath Matagorda Island. We attribute this event to the sediment‐basement contact, which is expected to produce a large impedance contrast. Another large‐amplitude and positive‐polarity event at ∼35 km depth, which likely marks the Moho, disappears outboard of the Luling Fault Zone. The disappearance of the Moho beneath the GCP may be due to serpentinization of the upper mantle, which would reduce the impedance contrast between the lower crust and upper mantle dramatically. This article is protected by copyright. All rights reserved.
      PubDate: 2015-06-11T11:17:52.13368-05:0
      DOI: 10.1002/2015GC005803
       
  • New insights into the nature of debris‐avalanche deposits offshore
           Montserrat using Remotely Operated Vehicles (ROVs)
    • Authors: S.F.L. Watt; M. Jutzeler, P.J. Talling, S.N. Carey, R.S.J. Sparks, M. Tucker, A.J. Stinton, J.K. Fisher, D. Wall‐Palmer, V. Hühnerbach, S.G. Moreton
      Abstract: Submarine landslide deposits have been mapped around many volcanic islands, but interpretations of their structure, composition and emplacement are hindered by the challenges of investigating deposits directly. Here, we report on detailed observations of four landslide deposits around Montserrat collected by Remotely Operated Vehicles, integrating direct imagery and sampling with sediment‐core and geophysical data. These complementary approaches enable a more comprehensive view of large‐scale mass wasting processes around island‐arc volcanoes than has been achievable previously. The most recent landslide occurred at 11.5–14 ka (Deposit 1; 1.7 km3) and formed a radially‐spreading hummocky deposit that is morphologically similar to many subaerial debris‐avalanche deposits. Hummocks comprise angular lava and hydrothermally‐altered fragments, implying a deep‐seated, central subaerial collapse, inferred to have removed a major proportion of lavas from an eruptive period that now has little representation in the subaerial volcanic record. A larger landslide (Deposit 2; 10 km3) occurred at ∼130 ka and transported intact fragments of the volcanic edifice, up to 900 m across and over 100 m high. These fragments were rafted within the landslide, and are best exposed near the margins of the deposit. The largest block preserves a primary stratigraphy of subaerial volcanic breccias, of which the lower parts are encased in hemipelagic mud eroded from the seafloor. Landslide deposits south of Montserrat (Deposits 3 and 5) indicate the wide variety of debris‐avalanche source lithologies around volcanic islands. Deposit 5 originated on the shallow submerged shelf, rather than the terrestrial volcanic edifice, and is dominated by carbonate debris. This article is protected by copyright. All rights reserved.
      PubDate: 2015-06-11T11:17:43.063172-05:
      DOI: 10.1002/2015GC005781
       
  • Crustal‐scale degassing due to magma system destabilization and
           magma‐gas decoupling at Soufrière Hills Volcano, Montserrat
    • Authors: T.E. Christopher; J. Blundy, K. Cashman, P. Cole, M. Edmonds, P.J. Smith, R.S.J. Sparks, A. Stinton
      Abstract: Activity since 1995 at Soufrière Hills Volcano (SHV), Montserrat has alternated between andesite lava extrusion and quiescence, which are well‐correlated with seismicity and ground deformation cycles. Large variations in SO2 flux do not correlate with these alternations, but high and low HCl/SO2 characterising lava dome extrusion and quiescent periods respectively. Since lava extrusion ceased (February 2010) steady SO2 emissions have continued at an average rate of 374 tonnes/day (± 140 t/d), and incandescent fumaroles (temperatures up to 610oC) on the dome have not changed position or cooled. Occasional short bursts (over several hours) of higher (∼ 10x) SO2 flux have been accompanied by swarms of volcano‐tectonic earthquakes. Strain data from these bursts indicate activation of the magma system to depths up to 10 km. SO2 emissions since 1995 greatly exceed the amounts that could be derived from 1.1 km3 of erupted andesite, and indicating extensive partitioning of sulfur into a vapour phase, as well as efficient decoupling and outgassing of sulfur‐rich gases from the magma. These observations are consistent with a vertically‐extensive, crustal magmatic mush beneath SHV. Three states of the magmatic system are postulated to control degassing. During dormant periods (103 to 104 years) magmatic vapour and melts separate as layers from the mush and decouple from each other. In periods of unrest (years) without eruption, melt and fluid layers become unstable, ascend and can amalgamate. Major destabilisation of the mush system leads to eruption, characterised by magma mixing and release of volatiles with different ages, compositions and sources. This article is protected by copyright. All rights reserved.
      PubDate: 2015-06-11T11:17:24.359999-05:
      DOI: 10.1002/2015GC005791
       
  • Lithospheric architecture beneath Hudson Bay
    • Authors: Robert W. Porritt; Meghan S. Miller, Fiona A. Darbyshire
      Abstract: Hudson Bay overlies some of the thickest Precambrian lithosphere on Earth, whose internal structures contain important clues to the earliest workings of plate formation. The terminal collision, the Trans‐Hudson Orogen, brought together the Western Churchill craton to the northwest and the Superior craton to the southeast. These two Archean cratons along with the Paleo‐Proterozoic Trans‐Hudson internides, form the core of the North American craton. We use S to P converted wave imaging and absolute shear velocity information from a joint inversion of P to S receiver functions, new ambient noise derived phase velocities, and teleseismic phase velocities to investigate this region and determine both the thickness of the lithosphere and the presence of internal discontinuities. The lithosphere under central Hudson Bay approaches ∼350 km thick but is thinner (∼200‐250 km) around the periphery of the Bay. Furthermore, the amplitude of the LAB conversion from the S receiver functions is unusually large for a craton, suggesting a large thermal contrast across the LAB, which we interpret as direct evidence of the thermal insulation effect of continents on the asthenosphere. Within the lithosphere, mid‐lithospheric discontinuities, significantly shallower than the base of the lithosphere, are often imaged, suggesting the mechanisms that form these layers are common. Lacking time‐history information, we infer that these discontinuities reflect re‐activation of formation structures during deformation of the craton. This article is protected by copyright. All rights reserved.
      PubDate: 2015-06-11T11:16:40.884185-05:
      DOI: 10.1002/2015GC005845
       
  • Magmatic recharge in continental flood basalts: Insights from the Chifeng
           igneous province in Inner Mongolia
    • Authors: Xun Yu; Cin‐Ty A. Lee, Li‐Hui Chen, Gang Zeng
      Abstract: Eruptive sequences can be used as windows into the thermal and chemical evolution of magma chambers. We examined a continuous vertical section of the Baichahe basalt flow associated with the late Cenozoic Chifeng flood basalt in Inner Mongolia, North China. From oldest to youngest, MgO increases, K2O, light rare earths and other incompatible elements decrease, and Nb/La and radiogenic Pb isotopic ratios increase, all of which indicate increasing primitiveness and decreasing contribution of crustal contamination with time. The variable Pb isotope and incompatible element ratios require a component of crustal contamination, most likely of a lower crustal component (unradiogenic Pb, and low Ce/Pb) in the earliest lavas. Fractional crystallization can explain some of the elemental systematics, but alone cannot explain variable incompatible element ratios and Pb isotopes, nor the temporal trend to more primitive compositions. Crustal assimilation with or without fractional crystallization also cannot explain all the elemental systematics. We find instead that recharge by a primitive magma, in combination with fractional crystallization and decreasing rates of crustal assimilation, is needed to explain the observed geochemical systematics. Our observations suggest that the delivery of fresh basalt to the magma chamber must increase at rates faster than the crust can be assimilated or that the rates of crustal assimilation must decrease. However, progressive addition of primitive magma should heat up the crust and lead to more crustal assimilation. We suggest that during the initial stages of forming a magma chamber, the magma cools and develops an outer crystalline rind of mafic to ultramafic cumulates. This results in a thickening non‐convecting chemical boundary layer, which serves to insulate the magma chamber from further assimilation of crust and cooling, the latter resulting in the reduction of crystallization rates and the buffering of magma compositions at more primitive compositions. We show that certain segments of other large igneous provinces also display an evolution towards more primitive magmas with time, indicating that magmatic recharge may be a common feature of basaltic magma chambers. This article is protected by copyright. All rights reserved.
      PubDate: 2015-06-09T00:55:33.881344-05:
      DOI: 10.1002/2015GC005805
       
  • Origins of felsic magmas in Japanese subduction zone: Geochemical
           
    • Authors: Jun‐Ichi Kimura; Yoshitaka Nagahashi, Yasufumi Satoguchi, Qing Chang
      Abstract: Dacitic to rhyolitic glass shards from 80 widespread tephras erupted during the past 5 Mys from calderas in Kyushu, and SW, central, and NE Japan were analyzed. Laser ablation inductively coupled plasma mass spectrometry was used to determine 10 major and 33 trace elements and 207Pb/206Pb–208Pb/206Pb isotope ratios. The tephras were classified into three major geochemical types and their source rocks were identified as plutonic, sedimentary, and intermediate amphibolite rocks in the upper crust. A few tephras from SW Japan were identified as adakite and alkali rhyolite and were regarded to have originated from slab melt and mantle melt, respectively. The Pb isotope ratios of the tephras are comparable to those of the intermediate lavas in the source areas but are different from the basalts in these areas. The crustal assimilants for the intermediate lavas were largely from crustal melts and are represented by the rhyolitic tephras. A large heat source is required for forming large volumes of felsic crustal melts and is usually supplied by the mantle via basalt. Hydrous arc basalt formed by cold slab subduction is voluminous, and its heat transfer with high water content may have melted crustal rocks leading to effective felsic magma production. Coincidence of basalt and felsic magma activities shown by this study suggests caldera‐forming eruptions are ultimately the effect of a mantle‐driven cause. This article is protected by copyright. All rights reserved.
      PubDate: 2015-06-05T03:05:22.453762-05:
      DOI: 10.1002/2015GC005854
       
  • Tectonic evolution of 200 km of Mid‐Atlantic Ridge over 10 million
           years—Interplay of volcanism and faulting
    • Authors: Johnson R Cann; Deborah K. Smith, Javier Escartin, Hans Schouten
      Abstract: We reconstruct the history of the mode of accretion of an area of the Mid‐Atlantic Ridge south of the Kane fracture zone using bathymetric morphology. The area includes 200 km of the spreading axis and reaches to 10 Ma on either side. We distinguish three tectonic styles: 1) volcanic construction with eruption and intrusion of magma coupled with minor faulting, 2) extended terrain with abundant large‐offset faults, 3) detachment faulting marked by extension on single long‐lived faults. Over 40% of the seafloor is made of extended terrain and detachment faults. The area includes products of seven spreading segments. The spreading axis has had detachment faulting or extended terrain on one or both sides for 70% of the last 10 Ma. In some parts of the area, regions of detachment faulting and extended terrain lie close to segment boundaries. Regions of detachment faulting initiated at 10 Ma close to the adjacent fracture zones to the north and south, and then expanded away from them. We discuss the complex evidence from gravity, seismic surveys and bathymetry for the role of magma supply in generating tectonic style. Overall we conclude that input of magma at the spreading axis has a general control on the development of detachment faulting, but the relationship is not strong. Other factors may include a positive feedback that stabilizes detachment faulting at the expense of volcanic extension, perhaps through the lubrication of active detachment faults by the formation of low friction materials (talc, serpentine) on detachment fault surfaces. This article is protected by copyright. All rights reserved.
      PubDate: 2015-06-05T02:59:43.509141-05:
      DOI: 10.1002/2015GC005797
       
  • Semiautomatic fracture zone tracking
    • Authors: Paul Wessel; Kara J. Matthews, R. Dietmar Müller, Aline Mazzoni, Joanne M. Whittaker, Robert Myhill, Michael T. Chandler
      Abstract: Oceanic fracture zone traces are widely used in studies of seafloor morphology and plate kinematics. Satellite altimetry missions have resulted in high‐resolution gravity maps in which all major fracture zones and other tectonic fabric can be identified, and numerous scientists have digitized such lineaments. We have initiated a community effort to maintain low‐cost infrastructure that allows seafloor fabric lineaments to be stored, accessed and updated. A key improvement over past efforts is our processing software (released as a GMT5 supplement) that allows for semi‐automatic corrections to previously digitized fracture zone traces given improved gridded data sets. Here we report on our seafloor fabric processing tools, which complement our database of seafloor fabric lineations, magnetic anomaly identifications and plate kinematic models. This article is protected by copyright. All rights reserved.
      PubDate: 2015-06-05T02:59:06.663321-05:
      DOI: 10.1002/2015GC005853
       
  • Imaging evidence for Hubbard Glacier advances and retreats since the last
           glacial maximum in Yakutat and Disenchantment Bays, Alaska
    • Authors: Julie M. Zurbuchen; Sean P.S. Gulick, Maureen A.L. Walton, John A. Goff
      Abstract: High‐resolution 2‐D multi‐channel seismic data, collected during the 2012 UTIG‐USGS National Earthquake Hazards Reduction Program survey of Disenchantment and Yakutat Bays in southeast Alaska, provide insight into their glacial history. These data show evidence of two unconformities, appearing in the form of channels, and are interpreted to be advance pathways for Hubbard Glacier. The youngest observable channel, thought to have culminated near the main phase of the Little Ice Age (LIA), is imaged in Disenchantment Bay and ends at a terminal moraine near Blizhni Point. An older channel, thought to be from an advance that culminated in the early phase of the LIA, extends from Disenchantment Bay into the northeastern edge of Yakutat Bay, turning southward at Knight Island and terminating on the southeastern edge of Yakutat Bay. Our interpretation is that Hubbard Glacier has repeatedly advanced around the east side of Yakutat Bay in Knight Island Channel, possibly due to the presence of Malaspina Glacier cutting off access to central Yakutat Bay during times of mutual advance. We observe two distinct erosional surfaces and retreat sequences of Hubbard Glacier in Yakutat Bay, supporting the hypothesis that minor glacial advances in fjords do not erode all prior sediment accumulations. Interpretation of chaotic seismic facies between these two unconformities suggests that Hubbard Glacier exhibits rapid retreats and that Disenchantment Bay is subject to numerous episodes of outburst flooding and morainal bank collapse. These findings also suggest that tidewater glaciers preferentially reoccupy the same channels in bay and marine settings during advances. This article is protected by copyright. All rights reserved.
      PubDate: 2015-06-03T03:32:18.700587-05:
      DOI: 10.1002/2015GC005815
       
  • Pore water geochemistry at two seismogenic areas in the Sea of Marmara
    • Authors: Livio Ruffine; Yoan Germain, Alina Polonia, Alexis de Prunelé, Claire Croguennec, Jean‐Pierre Donval, Mathilde Pitel‐Roudaut, Emmanuel Ponzevera, Jean‐Claude Caprais, Christophe Brandily, Claire Bollinger, Céline Grall, Louis Géli, Luca Gasperini
      Abstract: Within the Sea of Marmara, the highly active North Anatolian Fault (NAF) is responsible for major earthquakes (Mw >=7), and acts as a pathway for fluid migration from deep sources to the seafloor. This work reports on pore water geochemistry from three sediment cores collected in the gulfs of Izmit and Gemlik, along the Northern and the Middle strands of the NAF, respectively. The resulting dataset shows that anaerobic oxidation of methane (AOM) is the major process responsible for sulfate depletion in the shallow sediment. In the Gulf of Gemlik, depth concentration profiles of both sulfate and alkalinity exhibit a kink‐type profile. The Sulfate Methane Transition Zone (SMTZ) is located at moderate depth in the area. In the Gulf of Izmit, the low concentrations observed near the seawater‐sediment interface for sulfate, calcium, strontium and magnesium results from rapid geochemical processes, AOM and carbonate precipitation, occurring in the uppermost part of the sedimentary column and sustained by free methane accumulation. Barite dissolution and carbonate recrystallization have also been identified at deeper depth at the easternmost basin of the Gulf of Izmit. This is supported by the profile of the strontium isotope ratios (87Sr/86Sr) as a function of depth which exhibits negative anomalies compared to the modern seawater value. The strontium isotopic signature also shows that these carbonates had precipitated during the reconnection of the Sea of Marmara with the Mediterranean Sea. Finally, a first attempt to interpret the sulfate profiles observed in the light of the seismic activity at both sites is presented. We propose the hypothesis that seismic activity in the areas is responsible for the transient sulfate profile, and that the very shallow SMTZ depths observed in the Gulf of Izmit is likely due to episodic release of significant amount of methane. This article is protected by copyright. All rights reserved.
      PubDate: 2015-06-03T03:30:03.089073-05:
      DOI: 10.1002/2015GC005798
       
  • Hydrogen and 40Ar/39Ar isotope evidence for multiple and protracted
           paleofluid flow events within the long‐lived North Anatolian
           Keirogen (Turkey)
    • Authors: Austin Boles; Ben van der Pluijm, Andreas Mulch, Halim Mutlu, I. Tonguç Uysal, Laurence N. Warr
      Abstract: We present a new approach to identifying the source and age of paleofluids associated with low‐temperature deformation in the brittle crust, using hydrogen isotopic compositions (δD) and 40Ar/39Ar geochronology of authigenic illite in clay gouge‐bearing fault zones. The procedure involves grain size separation, polytype modeling, and isotopic analysis, creating a mixing line that is used to extrapolate to δD and age of pure authigenic and detrital material. We use this method on samples collected along the surface trace of today's North Anatolian Fault (NAF). δD values of the authigenic illite population, obtained by extrapolation, are ‐89±3‰, ‐90±2‰, and ‐97±2‰ (VSMOW) for samples KSL, RES4‐1, and G1G2, respectively. These correspond to δD fluid values of ‐62‰ to ‐85‰ for the temperature range of 125°C ±25°, indistinguishable from present‐day precipitation values. δD values of the detrital illite population are ‐45±13‰, ‐60±6‰, and ‐64±6‰ for samples KSL, G1G2, and RES4‐1, respectively. Corresponding δD fluid values at 300°C are ‐26 to ‐45‰ and match values from adjacent metamorphic terranes. Corresponding clay gouge ages are 41.4 ±3.4 Ma (authigenic) and 95.8±7.7 Ma (detrital) for sample G2 and 24.6 ±1.6 Ma (authigenic) and 96.5±3.8 Ma (detrital) for sample RES4‐1, demonstrating a long history of meteoric fluid infiltration in the area. We conclude that today's NAF incorporated pre‐existing, weak clay‐rich rocks that represent earlier mineralizing fluid events. The samples preserve at least three fluid flow pulses since the Eocene and indicate that meteoric fluid has been circulating in the upper crust in the North Anatolian Keirogen since that time. This article is protected by copyright. All rights reserved.
      PubDate: 2015-06-03T02:55:08.097335-05:
      DOI: 10.1002/2015GC005810
       
  • Probing the transition between seismically coupled and decoupled segments
           along an ancient subduction interface
    • Authors: Samuel Angiboust; Josephine Kirsch, Onno Oncken, Johannes Glodny, Patrick Monié, Erik Rybacki
      Abstract: The transition zone at the downdip end of seismic coupling along subduction interfaces is often the site of megathrust earthquake nucleation and concentrated postseismic afterslip, as well as the focus site of episodic tremor and slip features. Exhumed remnants of the former Alpine subduction zone found in the Swiss Alps allow analyzing fluid and deformation processes near the transition zone region (30‐40 km paleodepth). The Dent Blanche Thrust (DBT) is a lower blueschist‐facies shear zone interpreted as a fossilized subduction interface where granitic mylonites overlie a metamorphosed accretionary wedge. We report field observations from the DBT region where multiple, several tens of meters‐thick foliated cataclastic networks are interlayered within the basal DBT mylonites. Petrological results and microstructural observations indicate that the various cataclasis events took place at near peak metamorphic conditions (400‐500°C, 1.1‐1.3 GPa) during subduction of the Tethyan seafloor in Eocene times (42‐48 Ma). Some of these networks exhibit mutual cross‐cutting relationships between mylonites, foliated cataclasites and vein systems indicating mutual overprinting between brittle deformation and ductile creep. Whole‐rock chemical compositions, in situ 40Ar‐39Ar age data of recrystallized phengite, and Sr isotopic signatures reveal that DBT rocks also underwent multiple hydrofracturing and metasomatic events via the infiltration of fluids mainly derived from the oceanic metasediments underneath the DBT. From the rock fabrics we infer strain rate fluctuations of several orders of magnitude beyond subduction strain rates (c. 10−12s−1) accompanied by fluctuation of supra‐lithostatic and quasi‐lithostatic fluid pressures (1≥λ>0.95). DBT brittle‐plastic deformation switches highlight the diversity of deformation processes and fluid‐rock interactions in the transition zone region of the subduction interface. This article is protected by copyright. All rights reserved.
      PubDate: 2015-06-03T02:54:53.732295-05:
      DOI: 10.1002/2015GC005776
       
  • Magmatic water contents determined through clinopyroxene: Examples from
           the Western Canary Islands, Spain
    • Authors: Franz A. Weis; Henrik Skogby, Valentin R. Troll, Frances M. Deegan, Börje Dahrén
      Abstract: Water is a key parameter in magma genesis, magma evolution, and resulting eruption styles, because it controls the density, the viscosity, as well as the melting and crystallization behavior of a melt. The parental water content of a magma is usually measured through melt inclusions in minerals such as olivine, a method which may be hampered, however, by the lack of melt inclusions suitable for analysis, or post‐entrapment changes in their water content. An alternative way to reconstruct the water content of a magma is to use nominally anhydrous minerals (NAMs), such as pyroxene, which take up low concentrations of hydrogen as a function of the magma's water content. During magma degassing and eruption, however, NAMs may dehydrate. We therefore tested a method to reconstruct the water contents of dehydrated clinopyroxene phenocrysts from the Western Canary islands (n=28) through re‐hydration experiments followed by infrared and Mössbauer spectroscopy. Employing currently available crystal/melt partitioning data, the results of the experiments were used to calculate parental water contents of 0.71 ±0.07 to 1.49 ±0.15 wt. % H2O for Western Canary magmas during clinopyroxene crystallization at upper mantle conditions. This H2O range is in agreement with calculated water contents using plagioclase‐liquid‐hygrometry, and with previously published data for mafic lavas from the Canary Islands and comparable ocean island systems elsewhere. Utilizing NAMs in combination with hydrogen treatment can therefore serve as a proxy for pre‐eruptive H2O contents, which we anticipate becoming a useful method applicable to mafic rocks where pyroxene is the main phenocryst phase. This article is protected by copyright. All rights reserved.
      PubDate: 2015-06-03T02:54:40.735061-05:
      DOI: 10.1002/2015GC005800
       
  • Trace elements in quartz shed light on sediment provenance
    • Authors: Nicholas D. Tailby; E. Bruce Watson, Michael R. Ackerson
      Abstract: Quartz is one of the most common minerals on the surface of the earth, and is a primary rock‐forming mineral across the rock cycle. These two factors make quartz an obvious target for sediment provenance studies. Observations from experimental and natural samples demonstrate that the trace‐element content of quartz often reflects the conditions of quartz formation. When quartz is weathered from its primary crystallization setting (i.e., quartz from a granitoid) it can retain many chemical signatures of formation throughout the sedimentation processes. These geochemical signatures can be used to understand the primary source of individual quartz grains within a sediment. Here we present a case study from the Bega River catchment to demonstrate that quartz grains in sediments at the mouth of the Bega River are sourced from granitoids within the drainage basin. Data presented here also indicate that a portion of the beach sediment is also derived from either – (i) sedimentary rocks within the basin or; (ii) mixing with sediments at the mouth of the river. The Bega River catchment was selected for this study because it is both small and has a well‐constrained bedrock lithology, making it an ideal location to test the utility of this provenance technique. However, quartz trace‐element provenance has broad applications to modern and ancient sediments and can be used in lieu of, or in conjunction with, other provenance techniques to elucidate sediment transport through time. This article is protected by copyright. All rights reserved.
      PubDate: 2015-05-28T10:32:13.527838-05:
      DOI: 10.1002/2015GC005896
       
  • Effects of simple acid leaching of crushed and powdered geological
           materials on high‐precision Pb isotope analyses
    • Authors: Erin Todd; Andreas Stracke, Erik E. Scherer
      Abstract: We present new results of simple acid leaching experiments on the Pb isotope composition of USGS standard reference material powders and on ocean island basalt whole rock splits and powders. Rock samples were leached with cold 6N HCl in an ultrasonic bath, then on a hot plate, and washed with ultrapure H2O before sample digestion in HF‐HNO3 and chromatographic purification of Pb. Lead isotope analyses were measured by Tl‐doped MC‐ICPMS. Intra‐ and inter‐session analytical reproducibilities of repeated analyses of both synthetic Pb solutions and Pb from single digests of chemically processed natural samples were generally better than 100 ppm (2 S.D.). The comparison of leached and unleached samples shows that leaching consistently removes variable amounts of contaminants that differ in Pb isotopic composition for different starting materials. For repeated digests of a single sample, analyses of leached samples reproduce better than those of unleached ones, confirming that leaching effectively removes most of the heterogeneously distributed extraneous Pb. Nevertheless, the external reproducibility of leached samples is still up to an order of magnitude worse than that of Pb solution standards (∼100 ppm). More complex leaching methods employed by earlier studies yield Pb isotope ratios within error of those produced by our method and at similar levels of reproducibility, demonstrating that our simple leaching method is as effective as more complex leaching techniques. Therefore, any Pb isotope heterogeneity among multiple leached digests of samples in excess of the external reproducibility is attributed to inherent isotopic heterogeneity of the sample. The external precision of ∼100 ppm (2 S.D.) achieved for Pb isotope ratio determination by Tl‐doped MC‐ICPMS is thus sufficient for most rocks. The full advantage of the most precise Pb isotope analytical methods is only realized in cases where the natural isotopic heterogeneity among samples in a studied suite is substantially below 100 ppm. This article is protected by copyright. All rights reserved.
      PubDate: 2015-05-28T10:32:02.269031-05:
      DOI: 10.1002/2015GC005804
       
  • Multiscale postseismic behavior on a mega‐thrust: the 2012 Nicoya
           earthquake, Costa Rica
    • Authors: Rocco Malservisi; Susan Y. Schwartz, Nicholas Voss, Marino Protti, Victor Gonzalez, Timothy H. Dixon, Yan Jiang, Andy V. Newman, Jacob Richardson, Jacob I. Walter, Denis Voyenko
      Abstract: The Nicoya peninsula in northwest Costa Rica overlies a section of the subduction megathrust along the Middle America Trench. On September 5, 2012, a moment magnitude 7.6 megathrust earthquake occurred beneath a dense network of continuous GPS and seismic stations. Many of the GPS stations recorded the event at high rate, 1 Hz or better. We analyze the temporal and spatial evolution of surface deformation after the earthquake. Our results show that the main rupture was followed by significant afterslip within the first 3 hours following the main event. The behavior of the surface displacement can be represented by relaxation processes with three characteristic times: 7, 70 and more than 400 days. We assume that the long relaxation time corresponds to viscoelastic relaxation and the intermediate relaxation time corresponds to afterslip on the main fault. The short relaxation time may represent a combination of rapid afterslip, poroelastic adjustment in the upper crust, or other processes. During the first few months that followed the earthquake, afterslip likely released a significant amount of slip deficit still present following the coseismic rupture, in particular up‐dip of the rupture. Afterslip seems to be bounded up‐dip by regions affected by slow slip events prior to the earthquake, suggesting that the two processes are influenced by different frictional properties. This article is protected by copyright. All rights reserved.
      PubDate: 2015-05-28T10:31:54.598781-05:
      DOI: 10.1002/2015GC005794
       
  • Paleomagnetic evidence for vertical‐axis rotations of crustal blocks
           in the Woodlark Rift, SE Papua New Guinea: Miocene to Present‐Day
           kinematics in one of the world's most rapidly extending plate boundary
           zones
    • Authors: Elizabeth A. Cairns; Timothy A. Little, Gillian M. Turner, Laura M. Wallace, Susan Ellis
      Abstract: The continental Woodlark Rift, in SE Papua New Guinea lies west of a propagating oceanic spreading centre in the Woodlark Basin, and is currently one of few places on Earth where active continental breakup is thought to be occurring. Here, north‐south extension is localized on a few major normal faults. We determined characteristic remanent magnetization (ChRM) components from demagnetization profiles of >300 individual specimens. From these, 157 components contribute to paleomagnetic directions for six formations. We compare Early Miocene (∼20 Ma) to Late Pliocene (3.0 ± 0.5) ChRM mean directions, at four localities, with contemporaneous expected field directions corresponding to the Australian Plate. Time‐varying finite rotations from Cape Vogel Peninsula (28‐12°) suggest anticlockwise rotation had begun by ∼15 Ma. This rotation may have been accompanied by rifting, ∼7 Ma earlier than previously inferred. Furthermore that early extension may have occurred south of the present rift, and that deformation later migrated north of the Peninsula. Pliocene vertical‐axis rotations are consistent with GPS‐determined plate motions, suggesting that contemporary rift kinematics were established by ∼3 Ma. Finite anticlockwise rotation (10.1 ± 7.6°) in the Amphlett Islands is accordant with sea floor spreading in the Woodlark Basin, suggesting this locality has seen the full Woodlark plate motion since 3 Ma. Clockwise rotation of the Goodenough Bay Block (‐6.5 ± 11.2°) since the Late Miocene has accomplished transfer of deformation between major extensional corridors, and an especially rapid local rotation (‐16.3 ± 9.5°) in NW Normanby Island may suggest an incipient dextral transfer fault. This article is protected by copyright. All rights reserved.
      PubDate: 2015-05-28T10:31:40.868853-05:
      DOI: 10.1002/2015GC005788
       
  • Late Miocene to Recent formation of the Aure‐Moresby
           fold‐thrust belt and foreland basin as a consequence of Woodlark
           microplate rotation, Papua New Guinea
    • Authors: Bryan Ott; Paul Mann
      Abstract: The Aure‐Moresby fold‐thrust belt and Aure‐Moresby foreland basin is located in the eastern Gulf of Papua (GOP), Papua New Guinea (PNG) and formed during late Miocene‐Recent time as the result of large‐scale, counterclockwise rotation of the 355,000 km2 Woodlark microplate. To document the structure, stratigraphy and age of convergent deformation along the poorly studied, western edge of the rotating Woodlark microplate, we integrate results of 2,538 km of previously unpublished 2D seismic reflection data with onshore geologic and GPS studies from previous workers. The late Miocene Aure‐Moresby fold‐thrust belt is a 400‐km‐long, northwest‐trending fold‐belt exposed onshore in Papua New Guinea that plunges to the southeast, where continuous folds and northeast‐dipping thrusts can be imaged in the subsurface for more than 250 km. The arcuate trend of the Aure‐Moresby fold‐thrust belt along the southwestern coast and offshore areas of the Papuan peninsula parallels the shape of the adjacent, offshore Aure‐Moresby foreland basin and the strike of the transpressional segment of the left‐lateral Owen‐Stanley zone (OSFZ) passing along the center of Papuan peninsula. As the OSFZ becomes more transtensional east of 148°E, folds of the Aure‐Moresby fold‐thrust belt along southern coast of the peninsula become less prominent, and the adjacent Aure‐Moresby foreland basin transitions into an undeformed Cenozoic passive margin setting. These observations of convergent an left‐lateral deformation along is the Aure‐Moresby fold‐thrust belt are consistent with: 1) counterclockwise rotation of the Woodlark microplate known from regional GPS studies; 2) coeval opening of the Woodlark basin along its southern edge in the late Miocene; and 3) rapid subduction at the New Britain trench along its northern edge. The kinematics of the rotating Woodlark microplate are driven by slab pull forces acting on the actively, subducting northern edge of the microplate. This article is protected by copyright. All rights reserved.
      PubDate: 2015-05-28T10:31:12.256541-05:
      DOI: 10.1002/2014GC005668
       
  • High‐resolution magnetics reveal the deep structure of a
           volcanic‐arc related basalt‐hosted hydrothermal site
           (Palinuro, Tyrrhenian Sea)
    • Authors: Florent Szitkar; Sven Petersen, Fabio Caratori Tontini, Luca Cocchi
      Abstract: High‐resolution magnetic surveys have been acquired over the partially sedimented Palinuro massive sulfide deposits in the Aeolian volcanic arc, Tyrrhenian Sea. Surveys flown close to the seafloor using an autonomous underwater vehicle (AUV) show that the volcanic arc‐related basalt‐hosted hydrothermal site is associated with zones of lower magnetization. This observation reflects the alteration of basalt affected by hydrothermal circulation and/or the progressive accumulation of a non‐magnetic deposit made of hydrothermal and volcaniclastic material and/or a thermal demagnetization of titanomagnetite due to the upwelling of hot fluids. To discriminate among these inferences, estimate the shape of the non‐magnetic deposit and the characteristics of the underlying altered area – the stockwork – we use high‐resolution vector magnetic data acquired by the AUV Abyss (GEOMAR) above a crater‐shaped depression hosting a weakly active hydrothermal site. Our study unveils a relatively small non‐magnetic deposit accumulated at the bottom of the depression and locked between the surrounding volcanic cones. Thermal demagnetization is unlikely but the stockwork extends beyond the limits of the non‐magnetic deposit, forming lobe‐shaped zones believed to be a consequence of older volcanic episodes having contributed in generating the cones. This article is protected by copyright. All rights reserved.
      PubDate: 2015-05-21T18:43:59.25731-05:0
      DOI: 10.1002/2015GC005769
       
  • Advantages of a conservative velocity interpolation (CVI) scheme for
           particle‐in‐cell methods with application in geodynamic
           modelling
    • Authors: Hongliang Wang; Roberto Agrusta, Jeroen van Hunen
      Abstract: The particle‐in‐cell method is generally considered a flexible and robust method to model the geodynamic problems with chemical heterogeneity. However, velocity interpolation from grid points to particle locations is often performed without considering the divergence of the velocity field, which can lead to significant particle dispersion or clustering if those particles move through regions of strong velocity gradients. This may ultimately result in cells void of particles, which, if left untreated, may, in turn, lead to numerical inaccuracies. Here, we apply a two‐dimensional conservative velocity interpolation scheme (CVI) to steady state and time‐dependent flow fields with strong velocity gradients (e.g. due to large local viscosity variation), and derive and apply the three‐dimensional equivalent. We show that the introduction of CVI significantly reduces the dispersion and clustering of particles in both steady‐state and time‐dependent flow problems, and maintains a locally steady number of particles, without the need for ad‐hoc remedies such as very high initial particle densities or re‐seeding during the calculation. We illustrate that this method provides a significant improvement to particle distributions in common geodynamic modelling problems such as subduction zones or lithosphere‐asthenosphere boundary dynamics. This article is protected by copyright. All rights reserved.
      PubDate: 2015-05-21T18:37:42.42154-05:0
      DOI: 10.1002/2015GC005824
       
  • Did phosphorus derived from the weathering of large igneous provinces
           fertilize the Neoproterozoic ocean?
    • Authors: Forrest Horton
      Abstract: Primary productivity and organic carbon burial rates in the Precambrian were highly sensitive to fluxes of phosphorus (P) from the weathering of continental crust. Large igneous provinces (LIPs)—containing substantial P and highly susceptible to chemical weathering—occurred regularly during the breakup of the Rodinia supercontinent, and flood basalts probably covered 3.7–7.4 × 106 km2 at a time when a low‐latitude continental configuration expedited weathering. Assuming chemical weathering liberated much of the P contained in the flood basalts, an estimated 1–4 × 1017 mol of biologically available P entered the ocean from LIPs between 900 Ma and 500 Ma. Especially voluminous LIP magmatism began at ∼850 Ma and culminated with the Franklin Province at 720 Ma, after which an estimated bioavailable P flux from flood basalts of 1–5 × 109 mol/yr may have been sustained for millions of years, elevating primary production and organic carbon burial rates. P enrichment of LIP magmas prior to eruption could have contributed to efficient reactive P delivery to the ocean: liquid–crystal fractionation beneath thick cratonic lithosphere and the incorporation of metasomatic P potentially enriched Neoproterozoic LIP magmas more than anytime previously. Thus, a unique convergence of tectonic conditions—supercontinent breakup, voluminous mafic volcanism containing abundant P, and a low‐latitude continental configuration—may have facilitated an unprecedented flux of bioavailable P to the ocean that was capable of triggering oxidation of the ocean‐atmosphere system and enabling accelerated biologic diversification. This article is protected by copyright. All rights reserved.
      PubDate: 2015-05-21T18:34:47.500841-05:
      DOI: 10.1002/2015GC005792
       
  • Pressure disequilibria induced by rapid valve closure in noble gas
           extraction lines
    • Authors: Leah E. Morgan; Brett Davidheiser‐Kroll
      Abstract: Pressure disequilibria during rapid valve closures can affect calculated molar quantities for a range of gas abundance measurements (e.g. K‐Ar geochronology, (U‐Th)/He geochronology, noble gas cosmogenic chronology). Modeling indicates this effect in a system with a 10 L reservoir reaches a bias of 1% before 1,000 pipette aliquants have been removed from the system, and a bias of 10% before 10,000 aliquants. Herein we explore the causes and effects of this problem, which is the result of volume changes during valve closure. We also present a solution in the form of an electro‐pneumatic pressure regulator that can precisely control valve motion. This solution reduces the effect to ca. 0.3% even after 10,000 aliquants have been removed from a 10 L reservoir. This article is protected by copyright. All rights reserved.
      PubDate: 2015-05-21T18:34:34.705107-05:
      DOI: 10.1002/2015GC005823
       
  • Dynamics of intra‐oceanic subduction initiation, part 1: Oceanic
           detachment fault inversion and the formation of forearc ophiolites
    • Authors: Marco Maffione; Cedric Thieulot, Douwe J.J. van Hinsbergen, Antony Morris, Oliver Plümper, Wim Spakman
      Abstract: Subduction initiation is a critical link in the plate tectonic cycle. Intra‐oceanic subduction zones can form along transform faults and fracture zones, but how subduction nucleates parallel to mid‐ocean ridges, as in e.g. the Neotethys Ocean during the Jurassic, remains a matter of debate. In recent years, extensional detachment faults have been widely documented adjacent to slow‐ and ultraslow‐spreading ridges where they cut across the oceanic lithosphere. These structures are extremely weak due to widespread occurrence of serpentine and talc resulting from hydrothermal alteration, and can therefore effectively localize deformation. Here, we show geochemical, tectonic, and paleomagnetic evidence from the Jurassic ophiolites of Albania and Greece for a subduction zone formed in the western Neotethys parallel to a spreading ridge along an oceanic detachment fault. With 2‐D numerical modeling exploring the evolution of a detachment‐ridge system experiencing compression, we show that serpentinized detachments are always weaker than spreading ridges. We conclude that, owing to their extreme weakness, oceanic detachments can effectively localize deformation under perpendicular far‐field forcing, providing ideal conditions to nucleate new subduction zones parallel and close to (or at) spreading ridges. Direct implication of this, is that resumed magmatic activity in the forearc during subduction initiation can yield widespread accretion of supra‐subduction zone ophiolites at or close to the paleoridge. Our new model casts the enigmatic origin of regionally extensive ophiolite belts in a novel geodynamic context, and calls for future research on three‐dimensional modeling of subduction initiation and how upper plate extension is associated with that. This article is protected by copyright. All rights reserved.
      PubDate: 2015-05-21T18:19:11.86839-05:0
      DOI: 10.1002/2015GC005746
       
  • Multi‐parametric study of the February‐April 2013 paroxysmal
           phase of Mt. Etna New South‐East crater
    • Authors: Letizia Spampinato; Mariangela Sciotto, Andrea Cannata, Flavio Cannavò, Alessandro La Spina, Mimmo Palano, Giuseppe G. Salerno, Eugenio Privitera, Tommaso Caltabiano
      Abstract: Between January 2011 and April 2013, Mt. Etna's eruptive activity consisted of episodic intra‐crater strombolian explosions and paroxysms from Bocca Nuova, Voragine, and the New South‐East (NSEC) summit craters, respectively. Eruptions from NSEC consisted of initial increasing strombolian activity and lava flow output, passing to short‐lasting lava fountaining. In this study we present seismic, infrasound, radiometric, plume SO2 and HCl fluxes and geodetic data collected by the INGV monitoring system between May 2012 and April 2013. The multi‐parametric approach enabled characterization of NSEC eruptive activity at both daily and monthly time scales and tracking of magma movement within Mt. Etna's plumbing system. While seismic, infrasound and radiometric signals give insight on the energy and features of the 13 paroxysms fed by NSEC, SO2 and halogen fluxes shed light on the likely mechanisms triggering the eruptive phenomena. GPS data provided clear evidence of pressurization of Mt. Etna's plumbing system from May 2012 to middle February 2013 and depressurization during the February‐April 2013 eruptive activity. Taking into account geochemical data, we propose that the paroxysms' sequence represented the climax of a waxing‐waning phase of degassing that had started as early as December 2012, and eventually ended in April 2013. Integration of the multidisciplinary observations suggests that the February‐April 2013 eruptive activity reflects a phase of release of a volatile‐rich batch of magma that had been stored in the shallow volcano plumbing system at least four months before, and with the majority of gas released between February and March 2013. This article is protected by copyright. All rights reserved.
      PubDate: 2015-05-21T18:18:40.583587-05:
      DOI: 10.1002/2015GC005795
       
  • Experimental mixtures of superparamagnetic and single domain magnetite
           with respect to Day‐Dunlop plots
    • Authors: Monika Kumari; Ann M Hirt, Rene Uebe, Dirk Schüler, Éva Tompa, Mihály Pósfai, Wolfram Lorenz, Fredrik Ahrentorp, Christian Jonasson, Christer Johansson
      Abstract: Day‐Dunlop plots are widely used in paleomagnetic and environmental studies as a tool to determine the magnetic domain state of magnetite, i.e., superparamagnetic (SP), stable single‐domain (SD), pseudo‐single‐domain (PSD), multidomain (MD) and their mixtures. The few experimental studies that have examined hysteresis properties of SD‐SP mixtures of magnetite found that the ratios of saturation remanent magnetization to saturation magnetization and the coercivity of remanence to coercivity are low, when compared to expected theoretical mixing trends based on Langevin theory. This study reexamines Day‐Dunlop plots using experimentally controlled mixtures of SD and SP magnetite grains. End members include magnetotactic bacteria (MSR‐1) as the SD source, and a commercial ferrofluid or magnetotactic bacteria (ΔA12) as the SP source. Each SP‐component was added incrementally to a SD sample. Experimental results from these mixing series show that the magnetization and coercivity ratios are lower than the theoretical prediction for bulk SP magnetic size. Although steric repulsion was present between the particles we cannot rule out interaction in the ferrofluid for higher concentrations. The SP bacteria are non‐interacting as the magnetite was enclosed by an organic bi‐lipid membrane. Our results demonstrate that the magnetization and coercivity ratios of SD‐SP mixtures can lie in the PSD range, and that an unambiguous interpretation of particle size can only be made with information about the magnetic properties of the end members. This article is protected by copyright. All rights reserved.
      PubDate: 2015-05-21T18:18:24.000597-05:
      DOI: 10.1002/2015GC005744
       
  • Geochemical heterogeneities in magma beneath Mount Etna recorded by
           2001‐2006 melt inclusions
    • Authors: Federica Schiavi; Alberto Rosciglione, Hiroshi Kitagawa, Katsura Kobayashi, Eizo Nakamura, Pasquale Mario Nuccio, Luisa Ottolini, Antonio Paonita, Riccardo Vannucci
      Abstract: We present a geochemical study on olivine‐ and clinopyroxene‐hosted melt inclusions (MIs) from 2001‐2006 Etna basaltic lavas and pyroclastites. Three MI suites are distinguished on the basis of trace element fingerprinting. Type‐1 MIs (from 2001 Upper South and 2002 Northeast vents) share their trace element signature with low‐K lavas erupted before 1971. Critical trace element ratios (e.g., K/La, Ba/Nb), along with Pb isotope data of Type‐1 MIs provide evidence for a heterogeneous mantle source resulting from mixing of three end‐members with geochemical and isotopic characteristics of EM2, DMM and HIMU components. Type‐1 MIs composition does not support involvement of subduction‐related components. Type‐2 (from 2001 Lower and 2002 South vents) and Type‐3 (2004 eruption) MIs reveal “ghost plagioclase signatures”, namely lower concentrations in strongly incompatible elements, and positive Sr, Ba and Eu anomalies. Both Type‐1 and Type‐2 MIs occur in 2006 olivines, which highlight the occurrence of mixing between Type‐1 and Type‐2 end‐members. Type‐2/Type‐3 MIs testify to en‐route processes (plagioclase assimilation and volatile fluxing) peculiar for “deep dike fed” eruptions. The latter are strongly controlled by tectonics or flank instability that occasionally promote upraise of undegassed, more radiogenic primitive magma, which may interact with plagioclase‐rich crystal mush/cumulates before erupting. Type‐2/Type‐3 MIs approach the less radiogenic Pb isotopic composition of plagioclase from prehistoric lavas, thus suggesting geochemical overprinting of present‐day melts by older products released from distinct mantle sources. Our study emphasizes that MIs microanalysis offers new insights on both source characteristics and en‐route processes, allowing to a link between melt composition and magma dynamics. This article is protected by copyright. All rights reserved.
      PubDate: 2015-05-21T18:18:13.685358-05:
      DOI: 10.1002/2015GC005786
       
  • Strain decoupling reveals variable seismogenic risk in SE Japan (Nankai
           Trough)
    • Authors: James Van Tuyl; Tiago M. Alves, Gregory F. Moore
      Abstract: The determination of in situ stress states is vital in understanding the behavior of faults and subsequent seismogenesis of accretionary prisms. In this paper, a high quality 3D seismic volume is used to map the depth of the extensional‐compressional decoupling (ECD) boundary in the accretionary prism of Nankai, with the prior knowledge that strike‐slip and compressional stresses occur deeper than 1250 meters below seafloor (mbsf) in the Kumano Basin, changing to extension towards the seafloor. A total of 1108 faults from the accretionary prism are analyzed to estimate paleostresses via fault inversion and slip tendency techniques. A key result is this paper is that the ECD boundary can be used as a proxy to identify active structures on accretionary prisms as its depth depends on: a) local tectonic uplift in areas adjacent to active faults, and b) on the thickness of sediment accumulated above active thrust anticlines. The depth of the ECD boundary ranges from 0 to ∼650 mbsf, being notably shallower than in the Kumano Basin. In Nankai, frontal regions of the imbricate thrust zone, and the megasplay fault zone, reveal the shallower ECD depths and correlate with the regions where faulting is most active. As a corollary, this work confirms that estimates of stress state variability based on the analysis of 3D seismic data are vital to understand the behavior of faults and potential seismogenic regions on convergent margins. This article is protected by copyright. All rights reserved.
      PubDate: 2015-05-18T04:01:39.566209-05:
      DOI: 10.1002/2015GC005778
       
  • Microstructural evolution of gas hydrates in sedimentary matrices observed
           with synchrotron X‐ray computed tomographic microscopy
    • Authors: Marwen Chaouachi; Andrzej Falenty, Kathleen Sell, Frieder Enzmann, Michael Kersten, David Haberthür, Werner F. Kuhs
      Abstract: The formation process of gas hydrates in sedimentary matrices is of crucial importance for the physical and transport properties of the resulting aggregates. This process has never been observed in‐situ at sub‐micron resolution. Here, we report on synchrotron‐based micro‐tomographic studies by which the nucleation and growth processes of gas hydrate were observed at 276 K in various sedimentary matrices such as natural quartz (with and without admixtures of montmorillonite type clay) or glass beads with different surface properties, at varying water saturation. Both juvenile water and metastably gas‐enriched water obtained from gas hydrate decomposition was used. Xenon gas was employed to enhance the density contrast between gas hydrate and the fluid phases involved. The nucleation sites can be easily identified and the various growth patterns are clearly established. In sediments under‐saturated with juvenile water, nucleation starts at the water‐gas interface resulting in an initially several micrometer thick gas hydrate film; further growth proceeds to form isometric single crystals of 10‐20 μm size. The growth of gas hydrate from gas‐enriched water follows a different pattern, via the nucleation in the bulk of liquid producing polyhedral single crystals. A striking feature in both cases is the systematic appearance of a fluid phase film of up to several µm thickness between gas hydrates and the surface of the quartz grains. These micro‐structural findings are relevant for future efforts of quantitative rock physics modeling of gas hydrates in sedimentary matrices and explain the anomalous attenuation of seismic/sonic waves. This article is protected by copyright. All rights reserved.
      PubDate: 2015-05-15T10:12:37.704314-05:
      DOI: 10.1002/2015GC005811
       
  • Determination of low B/Ca ratios in carbonates using ICP‐QQQ
    • Authors: Silvia Diez Fernández; Jorge Ruiz Encinar, Alfredo Sanz‐Medel, Kirsten Isensee, Heather M. Stoll
      Abstract: The very low B/Ca ratios characteristic of some natural biogenic carbonates, are of interest for research in ocean acidification but represent an analytical challenge. We describe a method using a novel instrument configuration (ICP‐QQQ), for which we are not aware of any previously published geological applications, and for coccoliths, a sample type unique in its low B content and organic phases. Detection limits as low as 0.41 µmol/mol were achieved. Isobaric interferences, out of the reach even for SF‐ICP‐MS, can be solved using this instrument, which permits the safe measurement of the lowest abundance Ca isotope (46Ca). This allows maximizing the B concentration measured (matrix concentration up to 800 ppm Ca) while maintaining both B and Ca signals in counting mode. More significantly for low B samples, the ICP‐QQQ is also able to overcome the interference of the ubiquitous 12C tail on the 11B mass, which otherwise leads to significant overestimates at very low B concentrations. This could be a reason for the significantly lower B/Ca ratios observed for the low B content interlaboratory calibration standards (Carrara and OKA), while matching for the high B content standards was good. Finally, results obtained in the analysis of coccoliths grown in laboratory culture seems to corroborate that SIMS analysis of the samples mounted in Indium leads also to B/Ca overestimates due to porosity effects, as previously observed using LA‐ICP‐MS. This approach also permits the interference‐free measurement of P/Ca and S/Ca ratios, which could be used as indicators of the complete removal of the organic matter from the samples. This article is protected by copyright. All rights reserved.
      PubDate: 2015-05-15T10:12:26.445729-05:
      DOI: 10.1002/2015GC005817
       
  • Contrasting styles of (U)HP rock exhumation along the Cenozoic
           Adria‐Europe plate boundary (Western Alps, Calabria, Corsica)
    • Authors: Marco G. Malusà; Claudio Faccenna, Suzanne L. Baldwin, Paul G. Fitzgerald, Federico Rossetti, Maria Laura Balestrieri, Martin Danišík, Alessandro Ellero, Giuseppe Ottria, Claudia Piromallo
      Abstract: Since the first discovery of ultra‐high pressure (UHP) rocks 30 years ago in the Western Alps, the mechanisms for exhumation of (U)HP terranes worldwide are still debated. In the Western Mediterranean, the presently accepted model of synconvergent exhumation (e.g., the channel‐flow model) is in conflict with parts of the geologic record. We synthesize regional geologic data and present alternative exhumation mechanisms that consider the role of divergence within subduction zones. These mechanisms, i.e., (i) the motion of the upper plate away from the trench and (ii) the rollback of the lower plate, are discussed in detail with particular reference to the Cenozoic Adria‐Europe plate boundary, and along three different transects (Western Alps, Calabria‐Sardinia, and Corsica ‐ Northern Apennines). In the Western Alps, (U)HP rocks were exhumed from the greatest depth at the rear of the accretionary wedge during motion of the upper plate away from the trench. Exhumation was extremely fast, and associated with very low geothermal gradients. In Calabria, HP rocks were exhumed from shallower depths and at lower rates during rollback of the Adriatic plate, with repeated exhumation pulses progressively younging towards the foreland. Both mechanisms were active to create boundary divergence along the Corsica – Northern Apennines transect, where European southeastward subduction was progressively replaced along‐strike by Adriatic northwestward subduction. The tectonic scenario depicted for the Western Alps trench during Eocene exhumation of (U)HP rocks correlates well with present day eastern Papua New Guinea, which is presented as a modern analogue of the Paleogene Adria‐Europe plate boundary. This article is protected by copyright. All rights reserved.
      PubDate: 2015-05-15T10:12:18.794389-05:
      DOI: 10.1002/2015GC005767
       
  • Dynamics of intraoceanic subduction initiation: 2. Suprasubduction zone
           ophiolite formation and metamorphic sole exhumation in context of absolute
           plate motions
    • Authors: Douwe J.J. van Hinsbergen; Kalijn Peters, Marco Maffione, Wim Spakman, Carl Guilmette, Cedric Thieulot, Oliver Plümper, Derya Gürer, Fraukje M. Brouwer, Ercan Aldanmaz, Nuretdin Kaymakci
      Abstract: Analyzing subduction initiation is key for understanding the coupling between plate tectonics and underlying mantle. Here we focus on supra‐subduction zone (SSZ) ophiolites and how their formation links to intra‐oceanic subduction initiation in an absolute plate motion frame. SSZ ophiolites form the majority of exposed oceanic lithosphere fragments and are widely recognized to have formed during intra‐oceanic subduction initiation. Structural, petrological, geochemical, and plate kinematic constraints on their kinematic evolution show that SSZ crust forms at forearc spreading centers at the expense of a mantle wedge, thereby flattening the nascent slab. This leads to the typical inverted pressure gradients found in metamorphic soles that form at the subduction plate contact below and during SSZ crust crystallization. Former spreading centers are preserved in forearcs when subduction initiates along transform faults or off‐ridge oceanic detachments. We show how these are reactivated when subduction initiates in the absolute plate motion direction of the inverting weakness zone. Upon inception of slab‐pull due to e.g. eclogitization, the sole is separated from the slab, remains welded to the thinned overriding plate lithosphere and can become intruded by mafic dikes upon asthenospheric influx into the mantle wedge. We propound that most ophiolites thus formed under special geodynamic circumstances and may not be representative of normal oceanic crust. Our study highlights how far‐field geodynamic processes and absolute plate motions may force intra‐oceanic subduction initiation as key towards advancing our understanding of the entire plate tectonic cycle. This article is protected by copyright. All rights reserved.
      PubDate: 2015-05-15T10:11:58.145754-05:
      DOI: 10.1002/2015GC005745
       
  • Water input and water release from the subducting Nazca Plate along
           southern Central Chile (33°S–46°S)
    • Authors: David Völker; Michael Stipp
      Abstract: The age of the subducting Nazca Plate off Chile increases northwards from 0 Ma at the Chile Triple Junction (46°S) to 37 Ma at the latitude of Valparaíso (32°S). Age‐related variations in the thermal state of the subducting plate impact on (a) the water influx to the subduction zone, as well as on (b) the volumes of water that are released under the continental forearc or, alternatively, carried beyond the arc. Southern Central Chile is an ideal setting to study this effect, because other factors for the subduction zone water budget appear constant. We determine the water influx by calculating the crustal water uptake and by modeling the upper mantle serpentinization at the outer rise of the Chile Trench. The water release under forearc and arc is determined by coupling FEM thermal models of the subducting plate with stability fields of water‐releasing mineral reactions for upper and lower crust and hydrated mantle. Results show that both the influx of water stored in, and the outflux of water released from upper crust, lower crust and mantle vary drastically over segment boundaries. In particular, the oldest and coldest segments carry roughly twice as much water into the subduction zone as the youngest and hottest segments, but their release flux to the forearc is only about one fourth of the latter. This high variability over a subduction zone of 
      PubDate: 2015-05-12T17:53:41.488248-05:
      DOI: 10.1002/2015GC005766
       
  • Long‐term deformation driven by small ambient tectonic stresses and
           strong oscillating tidal within Enceladus with analogy to rock behavior
           near the San Andreas Fault
    • Authors: Norman H. Sleep
      Abstract: Strong tidal stresses brought much of the icy shell of Enceladus into frictional failure at past times of high orbital eccentricity. The frictional behavior of shallow terrestrial rock exposed to repeated episodes of strong seismic waves provides analogy. Frictional failure produces cracks that lower the shear modulus. Seismic regolith develops where the shear modulus increases linearly with depth. Imposed peak strains barely cause frictional failure within self‐organized regolith. With regard to Enceladus, eccentricity could continue to build up in the past since little anelastic strain and hence tidal dissipation occurred within the self‐organized regolith and within the underlying cold ice. A frictional instability analogous to the formation of weak major faults on the Earth likely occurred once the regolith was many kilometers thick. The effective coefficient of friction dropped to low levels along major faults within the deep cold ice. Tidal dissipation on these faults heated the ice starting thermal convention within the South Polar Terrain. Once thermal buoyancy produced stresses, the oscillating stresses from tides nonlinearly enhanced the rate of tectonic convection. Warm ice that dissipates tides now exists within Enceladus. The eccentricity will likely decrease and the object will then freeze. This article is protected by copyright. All rights reserved.
      PubDate: 2015-05-06T20:25:09.900742-05:
      DOI: 10.1002/2015GC005725
       
  • Nitrogen isotope ratios of nitrate and N* anomalies in the subtropical
           South Pacific
    • Authors: Chisato Yoshikawa; Akiko Makabe, Takuhei Shiozaki, Sakae Toyoda, Osamu Yoshida, Ken Furuya, Naohiro Yoshida
      Abstract: Nitrogen isotopic ratios of nitrate (δ15N–NO3−) were analyzed above 1000 m water depth along 17°S in the subtropical South Pacific during the revisit WOCE P21 cruise in 2009. The δ15N–NO3− and N* values were as high as 17‰ and as low as −18 μmol N L−1, respectively, at depths around 250 m east of 115°W, but were as low as 5‰ and as high as +1 μmol N L−1, respectively, in subsurface waters west of 170°W. The relationships among NO3− concentrations, N* values, δ15N–NO3− values, and oxygen and nitrite concentrations suggest that a few samples east of 90°W were from suboxic and nitrite‐accumulated conditions and were possibly affected by in situ water column denitrification. Most of the high‐δ15N–NO3− and negative‐N* waters were probably generated by mixing between Subantarctic Mode Water from the Southern Ocean and Oxygen Deficit Zone Water from the eastern tropical South Pacific, with remineralization of organic matter occurring during transportation. Moreover, the relationship between δ15N–NO3− and N* values, as well as Trichodesmium abundances and size‐specific nitrogen fixation rates at the surface, suggest that the low‐δ15N–NO3− and positive‐N* subsurface waters between 160°E and 170°W were generated by the input of remineralized particles created by in situ nitrogen fixation, mainly by Trichodesmium spp. Therefore, the δ15N values of sediments in this region are expected to reveal past changes in nitrogen fixation or denitrification rates in the subtropical South Pacific. This article is protected by copyright. All rights reserved.
      PubDate: 2015-05-05T09:03:34.38037-05:0
      DOI: 10.1002/2014GC005678
       
  • A seamless multitrack multitemporal InSAR algorithm
    • Authors: Manoochehr Shirzaei
      Abstract: The zones of overlap between adjacent Synthetic Aperture Radar (SAR) satellite tracks are illuminated twice more frequently than elsewhere in the SAR scene. Here, an alternative approach is presented to combine the overlapping segments of SAR images acquired at adjacent tracks and generate accurate and high spatiotemporal resolution map of the surface deformation field. To this end, a new approach is developed to unify the datums. Effects due to the difference in look angle between two overlapping tracks and atmospheric delay are estimated and removed using Kalman and wavelet based filters. This approach is first tested at Hawaii Island, where tracks 200 and 429 of Envisat C‐band satellite overlap over the Kilauea south flank. The obtained time series improves the temporal sampling rate by a factor of two and comparison with GPS time series demonstrates that the presented method accurately measures the nonlinear deformation field. The advantages of this method are further demonstrated by combining SAR data sets acquired by Envisat C‐band and ALOS L‐band satellites over the San Francisco Bay Area, California. The validation test shows that the seamless combination of C‐ and L‐Band time series accurately measures the surface deformation at higher resolution. This article is protected by copyright. All rights reserved.
      PubDate: 2015-05-04T03:29:04.061215-05:
      DOI: 10.1002/2015GC005759
       
  • The competing effects of sulfide saturation versus degassing on the
           behavior of the chalcophile elements during the differentiation of hydrous
           melts
    • Authors: Frances E. Jenner; Erik H. Hauri, Emma S. Bullock, Stephan König, Richard J. Arculus, John A. Mavrogenes, Nicole Mikkelson, Charlotte Goddard
      Abstract: There is a lack of consensus regarding the roles of sulfide saturation versus volatile degassing on the partitioning of Cu and Ag during differentiation and eruption of convergent margin magmas. Because of their oxidized character, volatile‐rich magmas from the Eastern Manus Backarc Basin (EMBB) only reach sulfide saturation following magnetite‐driven reduction of the melt: the so‐called “magnetite crisis”. If sulfide saturation typically precedes volatile saturation, the magnetite crisis will limit the proportion of Cu and Ag that can partition from the melt into an exsolving volatile‐rich phase, which may contribute to the sporadic occurrence of magmatic‐hydrothermal ore deposits at convergent margins. However, it is unclear whether the magnetite crisis is a common or rare event during differentiation of volatile‐rich magmas. We report major and trace element data for submarine volcanic glasses from the Tonga Arc‐proximal Valu Fa Ridge (VFR; SW Pacific). Cu‐Se‐Ag systematics of samples erupting at the southern VFR suggest magnetite fractionation triggered sulfide saturation. The similarity in chalcophile element systematics of the southern VFR and EMBB samples is unlikely to be coincidental, and may indicate the magnetite crisis is a common event during differentiation of hydrous melts. However, unlike many convergent margin magmas, it is unlikely the evolving VFR and EMBB were saturated in a S‐bearing volatile phase prior to magnetite fractionation. Hence, the metal‐depleting magnetite crisis may be restricted to backarc basin magmas that do not degas volatiles prior to magnetite fractionation and potentially convergent margin magmas fractionating at high‐pressures in the continental crust. This article is protected by copyright. All rights reserved.
      PubDate: 2015-04-30T02:41:45.079762-05:
      DOI: 10.1002/2014GC005670
       
  • Plume‐cratonic lithosphere interaction recorded by water and other
           trace elements in peridotite xenoliths from the Labait volcano, Tanzania
    • Authors: Hejiu Hui; Anne H. Peslier, Roberta L. Rudnick, Antonio Simonetti, Clive R. Neal
      Abstract: Water and other trace element concentrations in olivine (1‐39 ppm H2O), orthopyroxene (10‐150 ppm H2O), and clinopyroxene (16‐340 ppm H2O) of mantle xenoliths from the Labait volcano, located on the edge of the Tanzanian craton along the eastern branch of the East African Rift, record melting and subsequent refertilization by plume magmas in a stratified lithosphere. These water contents are at the lower end of the range observed in other cratonic mantle lithospheres. Despite correlations between water content and indices of melting in orthopyroxene from the shallow peridotites, and in both olivine and orthopyroxene from the deep peridotites, water concentrations are too high for the peridotites to be simple residues. Instead, the Labait water contents are best explained as reflecting interaction between residual peridotite with a melt having relatively low water content (
      PubDate: 2015-04-29T11:02:15.997231-05:
      DOI: 10.1002/2015GC005779
       
  • Early sedimentation and deformation in the Kumano forearc basin linked
           with Nankai accretionary prism evolution, southwest Japan
    • Authors: Sebastian G. Ramirez; Sean S. P. S. Gulick, Nicholas W. Hayman
      Abstract: Early forearc basin sedimentation and subsidence appear to be linked with convergent margin evolution, yet they do not follow predictable spatiotemporal patterns. In the Kumano basin of southwest Japan 3D seismic mapping of major unconformities, combined with biostratigraphic age constraints from two Integrated Ocean Drilling Program (IODP) drill sites illustrates the development of several tectonostratigraphic packages during early evolution and initial creation of accommodation space in the forearc basin. Between ∼3.8 Ma and ∼2.06 Ma a series of slope basins developed between thrust‐anticlines oriented along a plate convergence‐perpendicular axis. A thin and sheet‐like interval, likely slope‐cover, unconformably overlies the slope basin sediments. Deposition of this interval at IODP Site C0009 lasted from ∼2.06 Ma until between ∼1.24 Ma and ∼1.34 Ma and was partly coeval with the early upper Kumano basin sedimentary packages. Landward, postdating the slope‐cover sediments, a third sedimentary package was deposited before ∼0.9 Ma. This section correlates with an interval rich in terrigenous material (e.g. woody debris), indicating a possible distinct source or sediment routing system. Our work supports the idea that early forearc basin sediments may be deposited in an actively deforming outer wedge environment and that the outer‐to‐inner wedge transition of the lower forearc basin is likely a multi‐stage process. The unsteady spatiotemporal nature of inner‐outer wedge coupling may lead to erratic stratigraphic patterns, such as have been observed in forearc basins worldwide. This article is protected by copyright. All rights reserved.
      PubDate: 2015-04-25T02:15:52.030777-05:
      DOI: 10.1002/2014GC005643
       
  • Water circulation and global mantle dynamics: Insight from numerical
           modeling
    • Authors: Takashi Nakagawa; Tomoeki Nakakuki, Hikaru Iwamori
      Abstract: We investigate water circulation and its dynamical effects on global‐scale mantle dynamics in numerical thermo‐chemical mantle convection simulations. Both dehydration–hydration processes and dehydration melting are included. We also assume the rheological properties of hydrous minerals and density reduction caused by hydrous minerals. Heat transfer due to mantle convection seems to be enhanced more effectively than water cycling in the mantle convection system when reasonable water dependence of viscosity is assumed, due to effective slab dehydration at shallow depths. Water still affects significantly the global dynamics by weakening the near‐surface oceanic crust and lithosphere, enhancing the activity of surface plate motion compared to dry mantle case. As a result, including hydrous minerals, the more viscous mantle is expected with several orders of magnitude compared to the dry mantle. The average water content in the whole mantle is regulated by the dehydration‐hydration process. The large‐scale thermo‐chemical anomalies, as is observed in the deep mantle, is found when a large density contrast between basaltic material and ambient mantle is assumed (4 to 5%), comparable to mineral physics measurements. Through this study, the effects of hydrous minerals in mantle dynamics are very important for interpreting the observational constraints on mantle convection. This article is protected by copyright. All rights reserved.
      PubDate: 2015-04-24T15:17:30.565885-05:
      DOI: 10.1002/2014GC005701
       
  • Mantle plume capture anchoring and outflow during Galápagos
           plume‐ridge interaction
    • Authors: S.A. Gibson; D. Geist, M.A. Richards
      Abstract: Compositions of basalts erupted between the main zone of Galápagos plume upwelling and adjacent Galápagos Spreading Centre (GSC) provide important constraints on dynamic processes involved in transfer of deep‐mantle‐sourced material to mid‐ocean ridges. We examine recent basalts from central and northeast Galápagos including some that have less radiogenic Sr, Nd and Pb isotopic compositions than plume influenced basalts (E‐MORB) from the nearby ridge. We show that the location of E‐MORB, greatest crustal thickness and elevated topography on the GSC correlates with a confined zone of low‐velocity, high‐temperature mantle connecting the plume stem and ridge at depths of ∼100 km. At this site on the ridge, plume‐driven upwelling involving deep melting of partially‐dehydrated, recycled ancient oceanic crust, plus plate‐limited shallow melting of anhydrous peridotite, generate E‐MORB and larger amounts of melt than elsewhere on the GSC. The first order control on plume stem to ridge flow is rheological rather than gravitational, and strongly influenced by flow regimes initiated when the plume was on axis (> 5 Ma). During subsequent northeast ridge migration material upwelling in the plume stem appears to have remained “anchored” to a contact point on the GSC. This deep, confined NE plume stem‐to‐ridge flow occurs via a network of melt channels, embedded within the normal spreading and advection of plume material beneath the Nazca plate, and coincides with locations of historic volcanism. Our observations require a more dynamically complex model than proposed by most studies, which rely on radial solid‐state outflow of heterogeneous plume material to the ridge. This article is protected by copyright. All rights reserved.
      PubDate: 2015-04-24T15:11:13.871683-05:
      DOI: 10.1002/2015GC005723
       
  • Provenance of plumes in global convection models
    • Authors: Rakib Hassan; Nicolas Flament, Michael Gurnis, Dan J. Bower, Dietmar Müller
      Abstract: In global convection models constrained by plume motions and subduction history over the last 230 Myr, plumes emerge preferentially from the edges of thermochemical structures that resemble present‐day Large Low Shear Velocity Provinces (LLSVPs) beneath Africa and the Pacific Ocean. It has been argued that Large Igneous Provinces (LIPs) erupting since 200 Ma may originate from plumes that emerged from the edges of the LLSVPs and numerical models have been devised to validate this hypothesis. Although qualitative assessments that are broadly in agreement with this hypothesis have been derived from numerical models, a quantitative assessment has been lacking. We present a novel plume detection scheme and derive Monte Carlo‐based statistical correlations of model plume eruption sites and reconstructed LIP eruption sites. We show that models with a chemically anomalous lower mantle are highly correlated to reconstructed LIP eruption sites, whereas the confidence level obtained for a model with purely thermal plumes falls just short of 95%. A network of embayments separated by steep ridges form in the deep lower mantle in models with a chemically anomalous lower mantle. Plumes become anchored to the peaks of the chemical ridges and the network of ridges acts as a floating anchor, adjusting to slab push forces through time. The network of ridges imposes a characteristic separation between conduits that can extend into the interior of the thermochemical structures. This may explain the observed clustering of reconstructed LIP eruption sites that mostly but not exclusively occur around the present‐day LLSVPs. This article is protected by copyright. All rights reserved.
      PubDate: 2015-04-24T15:06:22.42153-05:0
      DOI: 10.1002/2015GC005751
       
  • On the temporal evolution of long‐wavelength mantle structure of the
           Earth since the early Paleozoic
    • Authors: Shijie Zhong; Maxwell L. Rudolph
      Abstract: The seismic structure of the Earth's lower mantle is characterized by a dominantly degree‐2 pattern with the African and Pacific large low shear velocity provinces (i.e., LLSVP) that are separated by circum‐Pacific seismically fast anomalies. It is important to understand the origin of such a degree‐2 mantle structure and its temporal evolution. In this study, we investigated the effects of plate motion history and mantle viscosity on the temporal evolution of the lower mantle structure since the early Paleozoic by formulating 3‐D spherical shell models of thermochemical convection. For convection models with realistic mantle viscosity and no initial structure, it takes about ∼50 Myrs to develop dominantly degree‐2 lower mantle structure using the published plate motion models for the last either 120 Ma or 250 Ma. However, it takes longer time to develop the mantle structure for more viscous mantle. While the circum‐Pangea subduction in plate motion history models promotes the formation of degree‐2 mantle structure, the published pre‐Pangea plate motions before 330 Ma produce relatively cold lower mantle in the African hemisphere and significant degree‐1 structure in the early Pangea (∼300 Ma) or later times, even if the lower mantle has an initially degree‐2 structure and a viscosity as high as 1023 Pas. This suggests that the African LLSVP may not be stationary since the early Paleozoic. With the published plate motion models and lower mantle viscosity of 1022 Pas, our mantle convection models suggest that the present‐day degree‐2 mantle structure may have largely been formed by ∼200 Ma. This article is protected by copyright. All rights reserved.
      PubDate: 2015-04-24T05:54:37.501933-05:
      DOI: 10.1002/2015GC005782
       
  • The insular shelves of the Faial–Pico Ridge (Azores archipelago): A
           morphological record of its evolution
    • Authors: R. Quartau; J. Madeira, N. C. Mitchell, F. Tempera, P. F. Silva, F. Brandão
      Abstract: Shelves surrounding reefless volcanic ocean islands are formed by surf erosion of their slopes during changing sea levels. Post‐erosional lava flows, if abundant, can cross the coastal cliffs and fill partially or completely the accommodation space left by erosion. In this study, multibeam bathymetry, high‐resolution seismic reflection profiles and sediment samples are used to characterize the morphology of the insular shelves adjacent to Faial and Pico islands. The data shows offshore fresh lava flow morphologies, as well as an irregular basement beneath shelf sedimentary bodies and reduced shelf width adjacent to older volcanic edifices. These observations suggest that the shelves have been significantly filled by volcanic progradation and can thus be classified as “rejuvenated”. Despite the general volcanic infilling of the shelves around the islands, most of their edges are below the depth of the Last Glacial Maximum, revealing that that at least parts of the island have subsided after the shelves formed by surf erosion. Prograding lava deltas reached the shelf edge in some areas triggering small slope failures, locally decreasing the shelf width and depth of their edges. These areas can represent a significant risk for the local population; hence their identification can be useful for hazard assessment and contribute to wiser land‐use planning. Shelf morphology, subaerial geomorphology, magnetic anomalies and crustal structure data were also interpreted to reconstruct the longer term combined onshore and offshore evolution of the Faial‐Pico ridge. The subaerial emergence of this ridge is apparently older than previously thought, i.e., before ∼850 ka. This article is protected by copyright. All rights reserved.
      PubDate: 2015-04-15T10:57:56.793069-05:
      DOI: 10.1002/2015GC005733
       
  • Magnetic and geochemical characterization of Andosols developed on basalts
           in the Massif Central, France
    • Authors: Hana Grison; Eduard Petrovsky, Sarka Stejskalova, Ales Kapicka
      Abstract: Identification of Andosols is primarily based upon the content of their colloidal constituents – clay and metal‐humus complexes – and on the determining of andic properties. This needs time and cost‐consuming geochemical analyses. Our primary aim of this study is to describe the magnetic and geochemical properties of soils rich in iron oxides derived from strongly magnetic volcanic basement (in this case Andosols). Secondary aim is to explore links between magnetic and chemical parameters of Andic soils with respect to genesis factors: parent material age, precipitation and thickness of the soil profile. Six pedons of andic properties, developed on basaltic lavas, were analysed down to parent rock by a set of magnetic and geochemical methods. Magnetic data of soil and rock samples reflect the type, concentration and particle‐size distribution of ferrimagnetic minerals. Geochemical data include soil reaction (pH in H2O), cation exchange capacity, organic carbon and different forms of extractable iron and aluminum content. Our results suggest the following: (1) Magnetic measurements of low‐field mass‐specific magnetic susceptibility can be a reliable indicator for estimating andic properties, and in combination with thermomagnetic curves may be suitable for discriminating between alu‐andic and sil‐andic subtypes. (2) In the studied Andosols, strong relationships were found between (a) magnetic grain‐size parameters, precipitation and exchangeable bases; (b) concentration of ferrimagnetic particles and degree of crystallization of free iron; and (c) parameters reflecting changes in magneto‐mineralogy and soil genesis (parent material age + soil depth). This article is protected by copyright. All rights reserved.
      PubDate: 2015-04-15T10:44:12.802248-05:
      DOI: 10.1002/2015GC005716
       
  • Abbot Ice Shelf, structure of the Amundsen Sea continental margin and the
           southern boundary of the Bellingshausen Plate seaward of West Antarctica
    • Authors: James R. Cochran; Kirsty J. Tinto, Robin E. Bell
      Abstract: Inversion of NASA Operation IceBridge airborne gravity over the Abbot Ice Shelf in West Antarctica for sub‐ice bathymetry defines an extensional terrain made up of east‐west trending rift basins formed during the early stages of Antarctica/Zealandia rifting. Extension is minor, as rifting jumped north of Thurston Island early in the rifting process. The Amundsen Sea Embayment continental shelf west of the rifted terrain is underlain by a deeper, more extensive sedimentary basin also formed during rifting between Antarctica and Zealandia. A well‐defined boundary zone separates the mildly extended Abbot extensional terrain from the deeper Amundsen Embayment shelf basin. The shelf basin has an extension factor, β, of 1.5‐1.7 with 80‐100 km of extension occurring across an area now 250 km wide. Following this extension, rifting centered north of the present shelf edge and proceeded to continental rupture. Since then, the Amundsen Embayment continental shelf appears to have been tectonically quiescent and shaped by subsidence, sedimentation and the advance and retreat of the West Antarctic Ice Sheet. The Bellingshausen Plate was located seaward of the Amundsen Sea margin prior to incorporation into the Antarctic Plate at about 62 Ma. During the latter part of its independent existence, Bellingshausen plate motion had a clockwise rotational component relative to Antarctica producing convergence across the north‐south trending Bellingshausen Gravity Anomaly structure at 94°W and compressive deformation on the continental slope between 94°W and 102°W. Farther west, the relative motion was extensional along an east‐west trending zone occupied by the Marie Byrd Seamounts. This article is protected by copyright. All rights reserved.
      PubDate: 2015-04-15T10:43:16.07419-05:0
      DOI: 10.1002/2014GC005570
       
  • Geochemical variations in Japan Sea back‐arc basin basalts formed by
           high‐temperature adiabatic melting of mantle metasomatized by
           sediment subduction components
    • Authors: Yuka Hirahara; Jun‐Ichi Kimura, Ryoko Senda, Takashi Miyazaki, Hiroshi Kawabata, Toshiro Takahashi, Qing Chang, Vaglarov S. Bogdan, Takeshi Sato, Shuichi Kodaira
      Abstract: The Yamato Basin in the Japan Sea is a back‐arc basin characterized by basaltic oceanic crust that is twice as thick as typical oceanic crust. Two types of ocean floor basalts, formed during the opening of the Japan Sea in the Middle Miocene, were recovered from the Yamato Basin during Ocean Drilling Program Legs 127/128. These can be considered as depleted (D‐type) and enriched (E‐type) basalts based on their incompatible trace element and Sr–Nd–Pb–Hf isotopic compositions. Both types of basalts plot along a common mixing array drawn between depleted mantle and slab sediment represented by a sand‐rich turbidite on the Pacific Plate in the NE Japan forearc. The depleted nature of the D‐type basalts suggests that the slab sediment component is nil to minor relative to the dominant mantle component, whereas the enrichment of all incompatible elements in the E‐type basalts was likely caused by a large contribution of bulk slab sediment in the source. The results of forward model calculations using adiabatic melting of a hydrous mantle with sediment flux indicate that the melting conditions of the source mantle for the D‐type basalts are deeper and hotter than those for the E‐type basalts, which appear to have formed under conditions hotter than those of normal mid‐oceanic ridge basalts (MORB). These results suggest that the thicker oceanic crust was formed by greater degrees of melting of a hydrous metasomatized mantle source at unusually high mantle potential temperature during the opening of the Japan Sea. This article is protected by copyright. All rights reserved.
      PubDate: 2015-04-15T10:42:48.440545-05:
      DOI: 10.1002/2015GC005720
       
  • Basalt Volatile Fluctuations During Continental Rifting: An Example From
           the Rio Grande Rift, USA
    • Authors: Michael C. Rowe; John C. Lassiter, Kathleen Goff
      Abstract: Hydration and metasomatism of the lithospheric mantle potentially influences both the magmatic and tectonic evolution of southwestern North America. Prior studies have suggested that volatile enrichments to the mantle underlying western North America resulted from shallow subduction of the Farallon Plate during the Laramide (∼74‐40 Ma). This study examines temporal and spatial variations in volatile elements (H2O, Cl, F, S) determined from olivine‐ and orthopyroxene‐hosted melt inclusions along and across the Rio Grande Rift, the easternmost extent of Laramide shallow subduction. Maximum chlorine enrichments are observed in the southern rift with a Cl/Nb of ∼210 and reduce with time to MORB‐OIB levels (∼5‐17). Measured water abundances are
      PubDate: 2015-04-14T14:10:47.507959-05:
      DOI: 10.1002/2014GC005649
       
  • Estimates of future warming‐induced methane emissions from hydrate
           offshore West Svalbard for a range of climate models
    • Authors: Héctor Marín‐Moreno; Timothy A. Minshull, Graham K. Westbrook, Bablu Sinha
      Abstract: Methane hydrate close to the hydrate stability limit in seafloor sediment could represent an important source of methane to the oceans and atmosphere as the oceans warm. We investigate the extent to which patterns of past and future ocean‐temperature fluctuations influence hydrate stability in a region offshore West Svalbard where active gas venting has been observed. We model the transient behavior of the gas hydrate stability zone at 400‐500 m water depth (mwd) in response to past temperature changes inferred from historical measurements and proxy data and we model future changes predicted by seven climate models and two climate‐forcing scenarios (Representative Concentration Pathways RCPs 2.6 and 8.5). We show that over the past 2000 yr, a combination of annual and decadal temperature fluctuations could have triggered multiple hydrate‐sourced methane emissions from seabed shallower than 400 mwd during episodes when the multi‐decadal average temperature was similar to that over the last century (∼2.6°C). These temperature fluctuations can explain current methane emissions at 400 mwd, but decades to centuries of ocean warming are required to generate emissions in water deeper than 420 m. In the venting area, future methane emissions are relatively insensitive to the choice of climate model and RCP scenario until 2050 yr, but are more sensitive to the RCP scenario after 2050 yr. By 2100 CE, we estimate an ocean uptake of 97‐1050 TgC from marine Arctic hydrate‐sourced methane emissions, which is 0.06‐0.67% of the ocean uptake from anthropogenic CO2 emissions for the period 1750‐2011. This article is protected by copyright. All rights reserved.
      PubDate: 2015-04-10T01:19:54.574729-05:
      DOI: 10.1002/2015GC005737
       
  • Investigation of the records of earthquake slip in carbonaceous materials
           from the Taiwan Chelungpu fault by means of infrared and Raman
           spectroscopies
    • Authors: Tetsuro Hirono; Yuka Maekawa, Hikaru Yabuta
      Abstract: To understand the mechanism of fault lubrication during the 1999 Taiwan Chi‐Chi earthquake, we developed a new temperature proxy for carbonaceous materials by using infrared and Raman spectroscopies together with heating and friction experiments. We found marked anomalies in the infrared and Raman spectra of carbonaceous materials retrieved from the primary slip zone of the earthquake: the infrared spectra exhibited very weak aliphatic CH2 and CH3 peaks and aromatic C=C absorbance peaks, and the Raman spectra exhibited very weak disordered and graphitic bands and a high ratio of disordered band area to graphitic band area. Those weak peaks and bands and the band area ratio were reproduced by heating carbonaceous materials from the nearby host rock to 700 °C. These results suggest that the frictional heat in the slip zone reached approximately 700 °C. We characterized the host rock's carbonaceous materials by means of elemental analysis, pyrolysis–gas chromatography–mass spectrometry, and simultaneous thermogravimetry–differential scanning calorimetry and found that the H/C and O/C ratios were 0.108 and 0.400, respectively (which are close to the ratios for lignin) and that the volatile fraction was as high as 48 wt %. The pyrolysates obtained by heating from 100 to 400 °C were dominated by phenols, fatty alcohols, and n‐alkanes. When the residue from pyrolysis at 100–400 °C was rapidly heated to 700 °C, the resulting pyrolysate was dominated by phenols, aromatic compounds, heterocyclic compounds, and n‐alkenes. This information suggests that changes in the infrared and Raman spectra with increasing temperature may have been due to decomposition and aromatization reactions during pyrolysis. Rapid heating during earthquake slip may promote reactions of carbonaceous materials that are different from the reactions that occur during long‐term metamorphism. This article is protected by copyright. All rights reserved.
      PubDate: 2015-04-09T02:34:31.598074-05:
      DOI: 10.1002/2014GC005622
       
  • Occurrence of greigite in the Pliocene sediments of Lake Qinghai, China,
           and its paleoenvironmental and paleomagnetic implications
    • Authors: FU Chaofeng; Jan Bloemendal, Qiang Xiaoke, Mimi J. Hill, AN ZhiSheng
      Abstract: Lake Qinghai in North China is the largest interior plateau lake in Central Asia and is climatically sensitive. An almost continuous 626‐m long sediment core was drilled in an in‐filled part of the southern lake basin of Lake Qinghai. The magnetic susceptibility record reveals the presence of two distinct peaks within an interval of fine‐grained lacustrine sediments of Lower Pliocene age. We selected a depth interval of approximately 40 m spanning the magnetic susceptibility peaks for detailed rock magnetic and geochemical analyses in order to identify the magnetic mineralogy responsible and to assess its possible paleoenvironmental and paleomagnetic implications. Rock magnetic, X‐ray diffraction analysis, scanning electron microscopy (SEM) and energy dispersive analysis of X‐ray (EDAX) analyses reveal that the main magnetic carrier is greigite (Fe3S4). The greigite is of early diagenetic origin and formed in an interval of high lake level and inferred relatively warm, humid climate. The greigite‐enriched zones are separated by an interval of relatively high total sulfur and organic carbon content and we infer that in the adjacent greigite‐bearing zones the lower concentrations of sulfur and organic carbon, and high levels of reactive iron, arrested the process of pyritization resulting in the preservation of the greigite on a timescale of several million years. The greigite zones contain narrow intervals of normally‐magnetized sediments which may be previously unrecognized cryptochrons within the Gilbert Chron, or alternatively they may reflect the continued formation of greigite long after the age of deposition of the surrounding sediment matrix. This article is protected by copyright. All rights reserved.
      PubDate: 2015-04-09T02:33:28.330578-05:
      DOI: 10.1002/2014GC005677
       
  • Quantifying near‐field and off‐fault deformation patterns of
           the 1992 Mw 7.3 Landers earthquake
    • Authors: Christopher W.D. Milliner; James F. Dolan, James Hollingsworth, Sebastien Leprince, Francois Ayoub, Charles Sammis
      Abstract: Co‐seismic surface deformation in large earthquakes is typically measured using field mapping and with a range of geodetic methods (e.g., InSAR, lidar differencing, and GPS). Current methods, however, either fail to capture patterns of near‐field co‐seismic surface deformation or lack pre‐event data. Consequently, the characteristics of off‐fault deformation and the parameters that control it remain poorly understood. We develop a standardized method to fully measure the surface, near‐field, co‐seismic deformation patterns at high‐resolution using the COSI‐Corr program by correlating pairs of aerial photographs taken before and after the 1992 Mw 7.3 Landers earthquake. COSI‐Corr offers the advantage of measuring displacement across the entire zone of surface deformation and over a wider aperture than that available to field geologists. For the Landers earthquake, our measured displacements are systematically larger than the field measurements, indicating the presence of off‐fault deformation. We show that 46 % of the total surface displacement occurred as off‐fault deformation, over a mean deformation width of 154 m. The magnitude and width of off‐fault deformation along the rupture is primarily controlled by the macroscopic structural complexity of the fault system, with a weak correlation with the type of near‐surface materials through which the rupture propagated. Both the magnitude and width of distributed deformation are largest in stepovers, bends, and at the southern termination of the surface rupture. We find that slip along the surface rupture exhibits a consistent degree of variability at all observable length scales and that the slip distribution is self‐affine fractal with dimension of 1.56. This article is protected by copyright. All rights reserved.
      PubDate: 2015-04-07T06:22:20.782611-05:
      DOI: 10.1002/2014GC005693
       
  • What processes control the chemical compositions of arc front
           stratovolcanoes?
    • Authors: Stephen J Turner; Charles H Langmuir
      Abstract: Arc front stratovolcanoes have global chemical systematics that constrain processes at convergent margins. Positive correlations exist for arc averages among “fluid mobile,” “high field strength,” and “large ion lithophile” elements. 143Nd/144Nd and 87Sr/86Sr from rear‐arc lavas lacking subduction signature align with the oceanic “mantle array,” and correlate with arc front 143Nd/144Nd. Most chemical parameters (but not isotopes) also correlate well with crustal thickness and slightly less well with the slab thermal parameter, but not with the depth of the slab or model slab surface temperatures. Successful models of arc volcanism should account for these global regularities. Two distinct models can quantitatively account for the observations—different extents of melting of the mantle wedge caused by variations in wedge thermal structure, or varying contributions from the subducting slab owing to variations in the slab thermal structure. The wedge melting model has constant contributions from ocean crust, sediment and mantle wedge to lavas globally, while the slab model varies slab contributions with slab temperature. The wedge melting model fit improves by incorporating convergence rate and slab dip, which should affect the wedge thermal structure; the slab model is not supported by a similar analysis. The wedge model also more easily accommodates the isotope data. The two models predict different primary H2O contents, with large variations in H2O for the wedge model, and relatively constant H2O for the slab model. An evaluation of the effects of varying sediment compositions on arc lavas will benefit from considering the very different consequences of the two models. This article is protected by copyright. All rights reserved.
      PubDate: 2015-04-07T05:31:15.394429-05:
      DOI: 10.1002/2014GC005633
       
  • A protocol for variable‐resolution first‐order reversal curve
           (FORC) measurements
    • Authors: Xiang Zhao; David Heslop, Andrew P. Roberts
      Abstract: High‐resolution first‐order reversal curve (FORC) diagrams are being increasingly used in rock and environmental magnetism, including for detection of biomagnetic signals in sediments. Resolution can be a major barrier to obtaining high‐quality FORC diagrams and time‐consuming measurements that employ small field steps are necessary to resolve the finest features of a FORC distribution. We present a new experimental protocol with irregularly spaced field steps that allow different parts of a FORC diagram to be measured at different resolutions. Larger numbers of measurements can, therefore, be made in key regions of a FORC distribution to resolve diagnostic features at higher resolution. Specification of the field steps in the irregular measurement grid is based on major hysteresis properties; no a priori knowledge concerning the underlying FORC distribution is required. FORC diagrams obtained with conventional measurements and with our new measurement protocol give consistent results. Because of its variable resolution, the irregular protocol provides a clear representation of fine‐scale features produced by quasi‐reversible superparamagnetic and non‐interacting single‐domain particles. Although the proposed irregular measurement protocol is not as efficient at suppressing noise as recently developed post‐processing techniques (e.g., VARIFORC, Egli [2013]), it enables efficient high‐resolution analysis for relatively strongly magnetized samples where measurement noise is not detrimental to FORC distribution estimation. This article is protected by copyright. All rights reserved.
      PubDate: 2015-04-07T04:58:19.327094-05:
      DOI: 10.1002/2014GC005680
       
  • Development of topography in 3‐D continental collision models
    • Authors: A. E. Pusok; Boris J. P. Kaus
      Abstract: Understanding the formation and evolution of high mountain belts, such as the Himalayas and the adjacent Tibetan Plateau, has been the focus of many tectonic and numerical models. Here, we employ 3D numerical simulations to investigate the role that subduction, collision and indentation play on lithosphere dynamics at convergent margins, and to analyze the conditions under which large topographic plateaus can form in an integrated lithospheric and upper‐mantle scale model. Distinct dynamics are obtained for the oceanic subduction side (trench retreat, slab roll‐back) and the continental‐collision side (trench advance, slab detachment, topographic uplift, lateral extrusion). We show that slab‐pull alone is insufficient to generate high topography in the upper‐plate, and that external forcing and the presence of strong blocks such as the Tarim Basin are necessary to create and shape anomalously high topographic fronts and plateaus. Moreover, scaling is used to predict four different modes of surface expression in continental‐collision models: (I)‐low‐amplitude homogenous shortening, (II)‐high‐amplitude homogenous shortening, (III)‐Alpine‐type topography with topographic front and low plateau, and (IV)‐Tibet‐Himalaya‐type topography with topographic front and high plateau. Results of semi‐analytical models suggest that the Argand number governs the formation of high topographic fronts, while the amplitude of plateaus is controlled by the initial buoyancy ratio of the upper plate. Applying these results to natural examples, we show that the Alps belong to regime (III), the Himalaya‐Tibet to regime (IV), whereas the Andes‐Altiplano fall at the boundary between regimes (III)‐(IV). This article is protected by copyright. All rights reserved.
      PubDate: 2015-04-06T10:53:20.148008-05:
      DOI: 10.1002/2015GC005732
       
  • An open source Bayesian Monte Carlo isotope mixing model with applications
           in Earth surface processes
    • Authors: Carli A. Arendt; Sarah M. Aciego, Eric A. Hetland
      Abstract: The implementation of isotopic tracers as constraints on source contributions has become increasingly relevant to understanding Earth surface processes. Interpretation of these isotopic tracers has become more accessible with the development of Bayesian Monte Carlo (BMC) mixing models, which allow uncertainty in mixing end‐members and provide methodology for systems with multi‐component mixing. This study presents an open source multiple isotope BMC mixing model that is applicable to Earth surface environments with sources exhibiting distinct end‐member isotopic signatures. Our model is first applied to new δ18O and δD measurements from the Athabasca Glacier, which showed expected seasonal melt evolution trends and vigorously assessed the statistical relevance of the resulting fraction estimations. To highlight the broad applicability of our model to a variety of Earth surface environments and relevant isotopic systems we expand our model to two additional case studies: deriving melt sources from δ18O, δD and 222Rn measurements of Greenland Ice Sheet bulk water samples and assessing nutrient sources from ϵNd and 87Sr/86Sr measurements of Hawaiian soil cores. The model produces results for the Greenland Ice Sheet and Hawaiian soil datasets that are consistent with the originally published fractional contribution estimates. The advantage of this method is that it quantifies the error induced by variability in the end‐member compositions, unrealized by the models previously applied to the above case studies. Results from all three case studies demonstrate the broad applicability of this statistical BMC isotopic mixing model for estimating source contribution fractions in a variety of Earth surface systems. This article is protected by copyright. All rights reserved.
      PubDate: 2015-03-27T03:36:25.743217-05:
      DOI: 10.1002/2014GC005683
       
  • Gravity anomalies, crustal structure, and seismicity at subduction zones:
           2. Interrelationships between fore‐arc structure and seismogenic
           behavior
    • Authors: Dan Bassett; Anthony B. Watts
      Abstract: An ensemble averaging technique is used to remove the long‐wavelength topography and gravity field associated with subduction zones. Short‐wavelength residual anomalies are attributed to the tectonic structure of subducting and overthrusting plates. A paired (positive‐negative) forearc anomaly is observed consisting of a long (>1000 km), linear, trench‐parallel ridge landward of the deep‐sea‐terrace basin. Ridges have amplitudes of 1500‐3000 m and 160‐240 mGal, wavelengths of 150‐200 km, and high gravity anomaly to topography ratios (50‐75 mGal km−1). The ridge crests correlate with the down‐dip limit of coseismic slip and strong interplate coupling and in Cascadia, the up‐dip limit of tremor epicenters. The ridge crest may be interpreted as defining the boundary between the velocity‐weakening and seismogenic region of the subduction interface and the down‐dip frictional transition zone. In Tonga‐Kermadec, the Kuril Islands and Chile landward ridges are associated with extinct volcanic arcs. Paired anomalies are attributed to the preferential subduction erosion of the outer‐forearc and a spatially varying combination of a) lower‐crustal underplating beneath the inner forearc, b) the transformation of interseismic strain into permanent geologic strain via faulting, folding or buckling of the inner‐forearc, and c) the relative trenchward migration of extinct volcanic arcs in regions operating with a net crustal deficit. Along‐strike transitions in forearc morphology and seismogenic behavior are related to pre‐existing crustal structure of subducting and overthrusting plates. Forearcs have the added potential of recording the time‐integrated response of the upper‐plate to subduction processes, and forearc structure should be considered in tandem with seismological observations. This article is protected by copyright. All rights reserved.
      PubDate: 2015-03-21T08:32:39.658955-05:
      DOI: 10.1002/2014GC005685
       
  • Gravity anomalies, crustal structure, and seismicity at subduction zones:
           1. Seafloor roughness and subducting relief
    • Authors: Dan Bassett; Anthony B. Watts
      Abstract: An ensemble averaging technique is used to remove the long‐wavelength topography and gravity field from subduction zones. >200 residual bathymetric and gravimetric anomalies are interpreted within forearcs, many of which are attributed to the tectonic structure of the subducting plate. The residual‐gravimetric expression of subducting fracture zones extends >200 km landward of the trench‐axis. The bathymetric expression of subducting seamounts with height ≥1 km and area ≥500 km2 (N=36), and aseismic ridges (N>10), is largest near the trench (within 70 km) and above shallow subducting slab depths (SLAB1.0
      PubDate: 2015-03-14T11:10:03.004997-05:
      DOI: 10.1002/2014GC005684
       
 
 
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