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

Geochemistry, Geophysics, Geosystems     Full-text available via subscription   (Followers: 21, SJR: 2.156, h-index: 61)
Geophysical Research Letters     Full-text available via subscription   (Followers: 46, SJR: 2.668, h-index: 142)
Global Biogeochemical Cycles     Full-text available via subscription   (Followers: 5, SJR: 2.4, h-index: 109)
J. of Advances in Modeling Earth Systems     Open Access   (Followers: 2, SJR: 0.126, h-index: 2)
J. of Geophysical Research : Atmospheres     Partially Free   (Followers: 21)
J. of Geophysical Research : Biogeosciences     Full-text available via subscription   (Followers: 7)
J. of Geophysical Research : Earth Surface     Partially Free   (Followers: 23)
J. of Geophysical Research : Oceans     Partially Free   (Followers: 16)
J. of Geophysical Research : Planets     Full-text available via subscription   (Followers: 13)
J. of Geophysical Research : Solid Earth     Full-text available via subscription   (Followers: 23)
J. of Geophysical Research : Space Physics     Full-text available via subscription   (Followers: 15)
Paleoceanography     Full-text available via subscription   (Followers: 4, SJR: 2.16, h-index: 82)
Radio Science     Full-text available via subscription   (Followers: 3, SJR: 0.527, h-index: 47)
Reviews of Geophysics     Full-text available via subscription   (Followers: 19, SJR: 8.837, h-index: 87)
Space Weather     Full-text available via subscription   (Followers: 3, SJR: 0.496, h-index: 16)
Tectonics     Full-text available via subscription   (Followers: 7, SJR: 2.16, h-index: 79)
Water Resources Research     Full-text available via subscription   (Followers: 166, SJR: 1.769, h-index: 110)
Journal Cover Geochemistry, Geophysics, Geosystems
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     ISSN (Online) 1525-2027
     Published by American Geophysical Union (AGU) Homepage  [17 journals]   [SJR: 2.156]   [H-I: 61]
  • A Cenozoic uplift history of Mexico and its surroundings from longitudinal
           river profiles
    • Authors: Simon N. Stephenson; Gareth G. Roberts, Mark J. Hoggard, Alexander C. Whittaker
      Pages: n/a - n/a
      Abstract: Geodynamic models of mantle convection predict that Mexico and western North America share a history of dynamic support. We calculate admittance between gravity and topography, which indicates that the elastic thickness of the plate in Mexico is 11 km and in western North America it is 12 km. Admittance at wavelengths > 500 km in these regions suggests that topography is partly supported by sub‐crustal processes. These results corroborate estimates of residual topography from isostatic calculations and suggest that the amount of North American topography supported by the mantle may exceed 1 km. The Cenozoic history of magmatism, sedimentary flux, thermochronometric denudation estimates and uplifted marine terraces imply that North American lithosphere was uplifted and eroded during the last 30 Ma. We jointly invert 533 Mexican and North American longitudinal river profiles to reconstruct a continent‐scale rock uplift rate history. Uplift rate is permitted to vary in space and time. Erosional parameters are calibrated using incision rate data in southwest Mexico and the Colorado Plateau. Calculated rock uplift rates were 0.15–0.2 mm/yr between 25–10 Ma. Central Mexico experienced the highest uplift rates. Central and southern Mexico continued to uplift at 0.1 mm/yr until recent times. This uplift history is corroborated by independent constraints. We predict clastic flux to the Gulf of Mexico and compare it to independent estimates. We tentatively suggest that the loop between uplift, erosion and deposition can be closed here. Mexico's staged uplift history suggests that its dynamic support has changed during the last 30 Ma. This article is protected by copyright. All rights reserved.
      PubDate: 2014-11-17T09:30:55.873118-05:
      DOI: 10.1002/2014GC005425
       
  • Sensitivity experiments on true polar wander
    • Authors: Marianne Greff‐Lefftz; Jean Besse
      Pages: n/a - n/a
      Abstract: Using sensitivity experiments based on the position of subductions and of superplumes, we derive models for the temporal evolution of 3D mass anomalies in the mantle and compute the associated inertia perturbations and polar wander. We show that although the large length‐scale mantle dynamics during the Earth's history may have been dominated by coupled supercontinent‐superplume cycles, subductions alone are sufficient to trigger major True Polar Wander (TPW) episodes, or rotation of the whole lithosphere and mantle with respect to the earth's rotation axis. We present two examples. We speculate that the distribution of continents with respect to the equator on the Earth's surface is driven by episodic subductions during the Wilson cycle: alternating fast subduction girdles around continents and upwellings during the divergence phases, with both reduced or stopped subductions activity around continents and moderate inter‐continental subductions during the convergence phases, lead to successive equatorial or polar distributions of continents, both configurations being separated by strong episodes of TPW. Finally, using plate reconstructions and geologic maps, over the period 1100–720 Ma, the period of amalgamation and destruction of the Rodinia supercontinent, we explain with our model the observed large eastward/westward and poleward/equatorward motions of the rotation axis. This article is protected by copyright. All rights reserved.
      PubDate: 2014-11-17T07:37:03.048088-05:
      DOI: 10.1002/2014GC005504
       
  • Lifting the cover of the cauldron: Convection in hot planets
    • Authors: Yanick Ricard; Stéphane Labrosse, Fabien Dubuffet
      Pages: n/a - n/a
      Abstract: Convection models of planetary mantles do not usually include a specific treatment of near‐surface dynamics. In all situations where surface dynamics is faster than internal dynamics, the lateral transport of material at the surface forbids the construction of a topography that could balance the internal convective stresses. This is the case if intense erosion erases the topography highs and fills in the depressions or if magma is transported through the lithosphere and spreads at the surface at large distances. In these cases, the usual boundary condition of numerical simulations, that the vertical velocity cancels at the surface should be replaced by a condition where the vertical flux on top of the convective mantle equilibrates that allowed by the surface dynamics. We show that this new boundary condition leads to the direct transport of heat to the surface and changes the internal convection that evolves toward a heat‐pipe pattern. We discuss the transition between this extreme situation where heat is transported to the surface to the usual situation where heat diffuses through the lithosphere. This mechanism is much more efficient to cool a planet and might be the major cooling mechanism of young planets. Even the modest effect of material transport by erosion on Earth is not without effect on mantle convection and should affect the heat flow budget of our planet. This article is protected by copyright. All rights reserved.
      PubDate: 2014-11-17T07:34:55.511295-05:
      DOI: 10.1002/2014GC005556
       
  • Large‐scale mechanical buckle fold development and the initiation of
           tensile fractures
    • Authors: Andreas Eckert; Peter Connolly, Xiaolong Liu
      Pages: n/a - n/a
      Abstract: Tensile failure associated with buckle folding is commonly associated to the distribution of outer arc extension but has also been observed on fold limbs. This study investigates whether tensile stresses and associated failure can be explained by the process of buckling under realistic in‐situ stress conditions. A 2D plane strain finite element modeling approach is used to study single‐layer buckle folds with a Maxwell visco‐elastic rheology. A variety of material parameters are considered and their influence on the initiation of tensile stresses during the various stages of deformation is analyzed. It is concluded that the buckling process determines the strain distribution within the fold layer but is not solely responsible for the initiation of tensile stresses. The modeling results show that tensile stresses are most dependent on the permeability, viscosity and overburden thickness. Low permeability (
      PubDate: 2014-11-14T04:21:45.079039-05:
      DOI: 10.1002/2014GC005502
       
  • Rock magnetic properties and paleomagnetic behavior of Neogene marine
           sediments from northern Chile
    • Authors: Claudio A. Tapia; Gary S. Wilson
      Pages: n/a - n/a
      Abstract: Neogene sediments from La Portada and Bahia Inglesa formations located at Mejillones Peninsula and Caldera Port, Northern Chile carry multicomponent magnetizations which can lead to mis‐ identification of the characteristic paleomagnetic vector and consequently, errors in chronostratigraphic and structural interpretation of the strata. Here we present a complete rock magnetic and paleomagnetic behavior study of Neogene sediments from La Portada and Bahia Inglesa formations. Isothermal remanent magnetization, thermal‐magnetic susceptibility, magnetic hysteresis and X ray diffractometer experiments are combined with paleomagnetic, alternating field and thermal demagnetization data to determine magnetization history. Remanent magnetizations generally comprises three components: a low blocking temperature (Tb) component, below 150∘C, close to the present day field in direction, interpreted as a thermo‐ viscous component; an intermediate Tb component, between 150∘‐290∘C, overprinted by the low and the high Tb components, considered as the survival of the characteristic detrital magnetization; and a high Tb component, above 290∘C, thought to be of diagenetic origin. Detrital magnetite, titanomagnetite and low titanium maghemite are identified as the main carrier of the magnetic remanence. Rock magnetic results of a minor group of samples at both locations, detected the presence of a high‐coercivity mineral, possibly hematite, interpreted to carry the high Tb component and to represent oxidation of minerals in post‐depositional processes.
      PubDate: 2014-11-11T01:21:56.43228-05:0
      DOI: 10.1002/2014GC005336
       
  • Crustal structure beneath the Rif Cordillera, North Morocco, from the
           RIFSIS wide‐angle reflection seismic experiment
    • Authors: Alba Gil; Josep Gallart, Jordi Diaz, Ramon Carbonell, Montserrat Torne, Alan Levander, Mimoun Harnafi
      Pages: n/a - n/a
      Abstract: The different geodynamic models proposed since the late 90's to account for the complex evolution of the Gibraltar Arc System lack definite constraints on the crustal structure of the Rif orogen. Here we present the first well‐resolved P‐wave velocity crustal models of the Rif cordillera and its southern continuation towards the Atlas made using controlled‐source seismic data. Two 300+ km‐long wide‐angle reflection profiles crossed the Rif along NS and EW trends. The profiles recorded simultaneously 5 land explosions of 1Tn each using ~850 high frequency seismometers. The crustal structure revealed from 2D forward modeling delineates a complex, laterally‐varying crustal structure below the Rif domains. The most surprising feature, seen on both profiles, is a ~50 km deep crustal root localized beneath the External Rif. To the east, the crust thins rapidly by 20 km across the Nekkor fault, indicating that the fault is a crustal scale feature. On the NS profile the crust thins more gradually to 40 km thickness beneath Middle Atlas and 42 km beneath the Betics. These new seismic results are in overall agreement with regional trends of Bouguer gravity and are consistent with recent receiver function estimates of crustal thickness. The complex crustal structure of the Rif orogen in the Gibraltar Arc is a consequence of the Miocene collision between the Iberian and African plates. Both the abrupt change in crustal thickness at the Nekkor fault and the unexpectedly deep Rif crustal root can be attributed to interaction of the subducting Alboran slab with the North African passive margin at late Oligocene‐early Miocene times.
      PubDate: 2014-11-11T01:21:30.519946-05:
      DOI: 10.1002/2014GC005485
       
  • Changing tectonic controls on the long‐term carbon cycle from
           Mesozoic to present
    • Authors: Benjamin Mills; Stuart J. Daines, Timothy M. Lenton
      Pages: n/a - n/a
      Abstract: Tectonic drivers of degassing and weathering processes are key long‐term controls on atmospheric CO2. However, there is considerable debate over the changing relative importance of different carbon sources and sinks. Existing geochemical models have tended to rely on indirect methods to derive tectonic drivers, such as inversion of the seawater87Sr/86Sr curve to estimate uplift or continental basalt area. Here we use improving geologic data to update the representation of tectonic drivers in the COPSE biogeochemical model. The resulting model distinguishes CO2 sinks from terrestrial granite weathering, total basalt weathering and seafloor alteration. It also distinguishes CO2 sources from subduction zone metamorphism and from igneous intrusions. We reconstruct terrestrial basaltic area from data on the extent of large igneous provinces, and use their volume to estimate their contribution to degassing. We adopt a recently‐published reconstruction of subduction‐related degassing, and relate seafloor weathering to ocean crust creation rate. Revised degassing alone tends to produce unrealistically high CO2, but this is counteracted by the inclusion of seafloor alteration and global basalt weathering, producing a good overall fit to Mesozoic‐Cenozoic proxy CO2 estimates and a good fit to 87Sr/86Sr data. The model predicts that seafloor alteration and terrestrial weathering made similar contributions to CO2 removal through the Triassic and Jurassic, after which terrestrial weathering increased and seafloor weathering declined. We predict that basalts made a greater contribution to silicate weathering than granites through the Mesozoic, before the contribution of basalt weathering declined over the Cenozoic due to decreasing global basaltic area.
      PubDate: 2014-11-11T00:53:59.284964-05:
      DOI: 10.1002/2014GC005530
       
  • The Iceland Basin excursion: Age, duration and excursion field geometry
    • Authors: J.E.T. Channell
      Pages: n/a - n/a
      Abstract: The Iceland Basin geomagnetic excursion coincided with the marine isotope stage (MIS) 6/7 boundary. The age and duration of the excursion, at seven North Atlantic sites with sufficient isotope data, are estimated by matching marine isotope stage (MIS) 7a‐c to a calibrated template. Two criteria for defining the excursion, virtual geomagnetic pole (VGP) latitudes
      PubDate: 2014-11-11T00:41:01.939089-05:
      DOI: 10.1002/2014GC005564
       
  • FORCulator: A micromagnetic tool for simulating first‐order reversal
           curve diagrams
    • Authors: Richard J. Harrison; Ioan Lascu
      Pages: n/a - n/a
      Abstract: We describe a method for simulating first‐order reversal curve (FORC) diagrams of interacting single‐domain particles. Magnetostatic interactions are calculated in real space, allowing simulations to be performed for particle ensembles with arbitrary geometry. For weakly interacting uniaxial particles, the equilibrium magnetization at each field step is obtained by direct solution of the Stoner‐Wohlfarth model, assuming a quasi‐static distribution of interaction fields. For all other cases, the equilibrium magnetization is calculated using an approximate iterated solution to the Landau‐Lifshitz‐Gilbert equation. Multithreading is employed to allow multiple curves to be computed simultaneously, enabling FORC diagrams to be simulated in reasonable time using a standard desktop computer. Statistical averaging and post processing lead to simulated FORC diagrams that are comparable to their experimental counterparts. The method is applied to several geometries of relevance to rock and environmental magnetism, including densely packed random clusters and partially collapsed chains. The method forms the basis of FORCulator, a freely available software tool with graphical user interface that will enable FORC simulations to become a routine part of rock magnetic studies.
      PubDate: 2014-11-10T22:39:09.987862-05:
      DOI: 10.1002/2014GC005582
       
  • The affects of alteration and porosity on seismic velocities in oceanic
           basalts and diabases
    • Authors: R. L. Carlson
      Pages: n/a - n/a
      Abstract: Seismic velocities in the lavas that cap normal oceanic crust are affected by both crack porosity and alteration of the primary mineral phases, chiefly to clays. Porosity accounts for 75‐80% of the velocity variation in sonic log velocities in the lava sections of Holes 504B and 1256D, but the effect of alteration on the properties of the basalts has not been assessed. In this analysis, the grain velocities in basalt and diabase samples are estimated from an empirical linear relationship between grain density and the P‐wave modulus. The theoretical velocity in fresh, zero‐porosity basalt or diabase is 6.96±0.07 km/sec. Grain velocities in the diabase samples are statistically indistinguishable from the theoretical velocity, and show no variation with depth; alteration does not significantly affect the velocities in the diabase samples from Hole 504B. This result is consistent with previous analyses, which demonstrated that velocities in the dikes are controlled by crack porosity. In basalt lab samples, alteration reduces the average sample grain velocity to 6.74±0.02 km/sec; cracks at the sample scale further reduce the velocity to 5.86±0.03 km/sec, and large‐scale cracks in the lavas reduce the average in situ velocity to 5.2±0.3 km/sec. Cracks account for nearly 90% of the difference between seismic (in situ) velocities and the theoretical velocity in the unaltered solid material. Basalt grain velocities show a small, but significant systematic increase with depth; the influence of alteration decreases with depth in the lavas, reaching near zero at the base of the lavas in Holes 504B and 1256D.
      PubDate: 2014-11-08T11:26:30.580818-05:
      DOI: 10.1002/2014GC005537
       
  • A joint Monte Carlo analysis of seafloor compliance, Rayleigh wave
           dispersion, and receiver functions at ocean bottom seismic stations
           offshore New Zealand
    • Authors: Justin S. Ball; Anne F. Sheehan, Joshua C. Stachnik, Fan‐Chi Lin, John A. Collins
      Pages: n/a - n/a
      Abstract: Teleseismic body‐wave imaging techniques such as receiver function analysis can be notoriously difficult to employ on ocean‐bottom seismic data due largely to multiple reverberations within the water and low‐velocity sediments. In lieu of suppressing this coherently scattered noise in ocean‐bottom receiver functions, these site effects can be modeled in conjunction with shear velocity information from seafloor compliance and surface wave dispersion measurements to discern crustal structure. A novel technique to estimate 1D crustal shear‐velocity profiles from these data using Monte Carlo sampling is presented here. We find that seafloor compliance inversions and P‐S conversions observed in the receiver functions provide complimentary constraints on sediment velocity and thickness. Incoherent noise in receiver functions from the MOANA ocean bottom seismic experiment limit the accuracy of the practical analysis at crustal scales, but synthetic recovery tests and comparison with independent unconstrained nonlinear optimization results affirm the utility of this technique in principle.
      PubDate: 2014-11-08T11:15:56.574721-05:
      DOI: 10.1002/2014GC005412
       
  • Regional‐scale input of dispersed and discrete volcanic ash to the
           Izu‐Bonin and Mariana subduction zones
    • Authors: Rachel P. Scudder; Richard W. Murray, Julie C. Schindlbeck, Steffen Kutterolf, Folkmar Hauff, Claire C. McKinley
      Pages: n/a - n/a
      Abstract: We have geochemically and statistically characterized bulk marine sediment and ash layers at Ocean Drilling Program Site 1149 (Izu‐Bonin Arc) and Deep Sea Drilling Project Site 52 (Mariana Arc), and have quantified that multiple dispersed ash sources collectively comprise ~30‐35% of the hemipelagic sediment mass entering the Izu‐Bonin‐Mariana subduction system. Multivariate statistical analyses indicate that the bulk sediment at Site 1149 is a mixture of Chinese Loess, a second compositionally distinct eolian source, a dispersed mafic ash, and a dispersed felsic ash. We interpret the source of these ashes as respectively being basalt from the Izu‐Bonin Front Arc (IBFA) and rhyolite from the Honshu Arc. Sr‐, Nd‐, and Pb isotopic analyses of the bulk sediment are consistent with the chemical/statistical‐based interpretations. Comparison of the mass accumulation rate of the dispersed ash component to discrete ash layer parameters (thickness, sedimentation rate, and number of layers) suggests that eruption frequency, rather than eruption size, drives the dispersed ash record. At Site 52, the geochemistry and statistical modeling indicates that Chinese Loess, IBFA, dispersed BNN (boninite from Izu‐Bonin), and a dispersed felsic ash of unknown origin are the sources. At Site 1149 the ash layers and the dispersed ash are compositionally coupled, whereas at Site 52 they are decoupled in that there are no boninite layers, yet boninite is dispersed within the sediment. Changes in the volcanic and eolian inputs through time indicate strong arc‐ and climate‐related controls.
      PubDate: 2014-11-08T00:37:27.408732-05:
      DOI: 10.1002/2014GC005561
       
  • Curie temperatures of titanomagnetite in ignimbrites: Effects of
           emplacement temperatures, cooling rates, exsolution, and cation ordering
    • Authors: Mike Jackson; Julie A. Bowles
      Pages: n/a - n/a
      Abstract: Pumices, ashes and tuffs from Mt. St. Helens and from Novarupta contain two principal forms of titanomagnetite: homogeneous grains with Curie temperatures in the range 350‐500°C; and oxyexsolved grains with similar bulk composition, containing ilmenite lamellae, and having Curie temperatures above 500°C. Thermomagnetic analyses and isothermal annealing experiments in combination with stratigraphic settings and thermal models show that emplacement temperatures and cooling history may have affected the relative proportions of homogeneous and exsolved grains, and have clearly had a strong influence on the Curie temperature of the homogeneous phase. The exsolved grains are most common where emplacement temperatures exceeded 600°C, and in laboratory experiments, heating to over 600°C in air causes the homogeneous titanomagnetites to oxyexsolve rapidly. Where emplacement temperatures were lower, Curie temperatures of the homogeneous grains are systematically related to overburden thickness and cooling timescales, and thermomagnetic curves are generally irreversible, with lower Curie temperatures measured during cooling, but little or no change is observed in room‐temperature susceptibility. We interpret this irreversible behavior as reflecting variations in the degree of cation ordering in the titanomagnetites, although we cannot conclusively rule out an alternative interpretation involving fine‐scale subsolvus unmixing. Short‐range ordering within the octahedral sites may play a key role in the observed phenomena. Changes in the Curie temperature have important implications for the acquisition, stabilization and retention of natural remanence, and may in some cases enable quantification of the emplacement temperatures or cooling rates of volcanic units containing homogeneous titanomagnetites.
      PubDate: 2014-11-06T18:45:01.929005-05:
      DOI: 10.1002/2014GC005527
       
  • Microstructures, composition, and seismic properties of the Ontong Java
           Plateau mantle root
    • Authors: Andréa Tommasi; Akira Ishikawa
      Pages: n/a - n/a
      Abstract: To study how an impacting plume modifies the mantle lithosphere, we analyzed the microstructures and crystal preferred orientations (CPO) of 29 peridotites and 37 pyroxenites that sample the mantle root of the Ontong Java Plateau (OJP) from 60 to 120 km depth. The peridotites show a strong compositional variability, but homogeneous coarse‐granular to tabular microstructures, except for those equilibrated at the shallowest and deepest depths, which are porphyroclastic. All peridotites have clear olivine CPO, with dominant fiber‐[010] patterns. Low intragranular misorientations and straight grain boundaries in olivine suggest that, above 100 km depth, annealing often followed deformation. Calculated density and P‐wave velocities of the peridotites decrease weakly with depth. S‐wave velocities decrease faster, resulting in increasing Vp/Vs ratio with depth. Calculated densities and seismic velocity profiles are consistent with those estimated for normal mantle compositions under a cold oceanic geotherm. Enrichment in pyroxenites may further increase seismic velocities. The calculated seismic properties cannot therefore explain the low S‐waves velocities predicted by Rayleigh wave tomography and ScS data in the mantle beneath the OJP. Calculated P‐ and S‐wave anisotropy is variable (2‐12%). It is higher on average in the deeper section of the lithosphere. Because olivine has dominantly [010]‐fiber CPO patterns, if foliations are horizontal, vertically propagating S‐waves and Rayleigh waves will sample very weak anisotropy in the OJP mantle lithosphere. Moreover, if the orientation of the lineation changes with depth, the anisotropy‐induced contrast in seismic properties might produce an intralithospheric reflector marking the stratification of the OJP mantle root.
      PubDate: 2014-11-05T08:08:11.184537-05:
      DOI: 10.1002/2014GC005452
       
  • Soil CO2 emissions as a proxy for heat and mass flow assessment,
           Taupō Volcanic Zone, New Zealand
    • Authors: Bloomberg S; Werner C, Rissmann C, Mazot A, Horton T, Gravley D, Kennedy B, Oze C.
      Pages: n/a - n/a
      Abstract: The quantification of heat and mass flow between deep reservoirs and the surface is important for understanding magmatic and hydrothermal systems. Here, we use high‐resolution measurement of carbon dioxide flux (ϕCO2) and heat flow at the surface to characterize the mass (CO2 and steam) and heat released to the atmosphere from two magma‐hydrothermal systems. Our soil gas and heat flow surveys at Rotokawa and White Island in the Taupō Volcanic Zone, New Zealand, include over 3,000 direct measurements of ϕCO2 and soil temperature and 60 carbon isotopic values on soil gases. Carbon dioxide flux was separated into background and magmatic/hydrothermal populations based on the measured values and isotopic characterization. Total CO2 emission rates (ΣCO2) of 441 ± 84 t d‐1 and 124 ± 18 t d‐1 were calculated for Rotokawa (2.9 km2) and for the crater floor at White Island (0.3 km2), respectively. The total CO2 emissions differ from previously published values by +386 t d‐1 at Rotokawa and +25 t d‐1 at White Island, demonstrating that earlier research underestimated emissions by 700% (Rotokawa) and 25% (White Island). These differences suggest that soil CO2 emissions facilitate more robust estimates of the thermal energy and mass flux in geothermal systems than traditional approaches. Combining the magmatic/hydrothermal‐sourced CO2 emission (constrained using stable isotopes) with reservoir H2O:CO2 mass ratios and the enthalpy of evaporation, the surface expression of thermal energy release for the Rotokawa hydrothermal system (226 MWt) is 10 times greater than the White Island crater floor (22.5 MWt).
      PubDate: 2014-11-05T01:46:56.453817-05:
      DOI: 10.1002/2014GC005327
       
  • The role of elasticity in slab bending
    • Authors: Loic Fourel; Saskia Goes, Gabriele Morra
      Pages: n/a - n/a
      Abstract: Previous studies showed that plate rheology exerts a dominant control on the shape and velocity of subducting plates. Here, we perform a systematic investigation of the role of elasticity in slab bending, using fully dynamic 2D models where an elastic, viscoelastic or viscoelastoplastic plate subducts freely into a purely viscous mantle. We derive a scaling relationship between the bending radius of viscoelastic slabs and the Deborah number, De, which is the ratio of Maxwell time over deformation time. We show that De controls the ratio of elastically stored energy over viscously dissipated energy and find that at De >‐2, substantially less energy is required to bend a viscoelastic slab to the same shape as a purely viscous slab with the same intrinsic viscosity. Elastically stored energy at higher De favours retreating modes of subduction via unbending, while trench advance only occurs for some cases with De < 10‐2. We estimate the apparent Deborah numbers of natural subduction zones and find values ranging from 10‐3 to > 1, where most zones have low De < 10‐2, but a few young plates have De > 0.1. Slabs with De < 10‐2 either have very low viscosities or they may be yielding, in which case our De estimates may be underestimated by up to an order of magnitude potentially pointing towards a significant role of elasticity in ~60% of the subduction zones. In support of such a role of elasticity in subduction, we find that increasing De correlates with increasing proportion of larger seismic events in both instrumental and historic catalogues.
      PubDate: 2014-11-01T08:19:57.555072-05:
      DOI: 10.1002/2014GC005535
       
  • Influence of late Pleistocene glaciations on the hydrogeology of the
           continental shelf offshore Massachusetts, USA
    • Authors: Jacob Siegel; Mark Person, Brandon Dugan, Denis Cohen, Daniel Lizarralde, Carl Gable
      Pages: n/a - n/a
      Abstract: Multiple late Pleistocene glaciations that extended onto the continental shelf offshore Massachusetts, USA may have emplaced as much as 100 km3 of freshwater (salinity less than 5 ppt) in continental shelf sediments. To estimate the volume and extent of offshore freshwater, we developed a three‐dimensional, variable‐density model that couples fluid flow and heat and solute transport for the continental shelf offshore Massachusetts. The stratigraphy for our model is based on high‐resolution, multi‐channel seismic data. The model incorporates the last 3 Ma of climate history by prescribing boundary conditions of sea‐level change and ice sheet extent and thickness. We incorporate new estimates of the maximum extent of a late Pleistocene ice sheet to near the shelf‐slope break. Model results indicate that this late Pleistocene ice sheet was responsible for much of the emplaced freshwater. We predict that the current freshwater distribution may reach depths of up to 500 m below sea level and up to 30 km beyond Martha's Vineyard. The freshwater distribution is strongly dependent on the three‐dimensional stratigraphy and ice‐sheet history. Our predictions improve our understanding of the distribution of offshore freshwater, a potential non‐renewable resource for coastal communities along recently glaciated margins.
      PubDate: 2014-10-30T04:12:52.645546-05:
      DOI: 10.1002/2014GC005569
       
  • Imaging of CO2 bubble plumes above an erupting submarine volcano, NW
           Rota‐1, Mariana Arc
    • Authors: William W. Chadwick; Susan G. Merle, Nathaniel J. Buck, J. William Lavelle, Joseph A. Resing, Vicki Ferrini
      Pages: n/a - n/a
      Abstract: NW Rota‐1 is a submarine volcano in the Mariana volcanic arc located ~100 km north of Guam. Underwater explosive eruptions driven by magmatic gases were first witnessed there in 2004 and continued until at least 2010. During a March 2010 expedition, visual observations documented continuous but variable eruptive activity at multiple vents at ~560 m depth. Some vents released CO2 bubbles passively and continuously, while others released CO2 during stronger but intermittent explosive bursts. Plumes of CO2 bubbles in the water column over the volcano were imaged by an EM122 (12 kHz) multibeam sonar system. Throughout the 2010 expedition numerous passes were made over the eruptive vents with the ship to document the temporal variability of the bubble plumes and relate them to the eruptive activity on the seafloor, as recorded by an in situ hydrophone and visual observations. Analysis of the EM122 mid‐water dataset shows: (1) bubble plumes were present on every pass over the summit and they rose 200‐400 m above the vents but dissolved before they reached the ocean surface, (2) bubble plume deflection direction and distance correlate well with ocean current direction and velocity determined from the ship's acoustic doppler current profiler, (3) bubble plume heights and volumes were variable over time and correlate with eruptive intensity as measured by the in situ hydrophone. This study shows that mid‐water multibeam‐sonar data can be used to characterize the level of eruptive activity and its temporal variability at a shallow submarine volcano with robust CO2 output.
      PubDate: 2014-10-30T03:57:55.269164-05:
      DOI: 10.1002/2014GC005543
       
  • Understanding which parameters control shallow ascent of silicic effusive
           magma
    • Authors: Mark E. Thomas; Jurgen W. Neuberg
      Pages: n/a - n/a
      Abstract: The estimation of the magma ascent rate is key to predicting volcanic activity and relies on the understanding of how strongly the ascent rate is controlled by different magmatic parameters. Linking potential changes of such parameters to monitoring data is an essential step to be able to use these data as a predictive tool. We present the results of a suite of conduit flow models Soufrière that assess the influence of individual model parameters such as the magmatic water content, temperature or bulk magma composition on the magma flow in the conduit during an extrusive dome eruption. By systematically varying these parameters we assess their relative importance to changes in ascent rate. We show that variability in the rate of low frequency seismicity, assumed to correlate directly with the rate of magma movement, can be used as an indicator for changes in ascent rate and, therefore, eruptive activity. The results indicate that conduit diameter and excess pressure in the magma chamber are amongst the dominant controlling variables, but the single most important parameter is the volatile content (assumed as only water). Modelling this parameter in the range of reported values causes changes in the calculated ascent velocities of up to 800%.
      PubDate: 2014-10-30T02:38:09.834649-05:
      DOI: 10.1002/2014GC005529
       
  • The flow structure of jets from transient sources and implications for
           modeling short‐duration explosive volcanic eruptions
    • Authors: K. N. Chojnicki; A. B. Clarke, R. J. Adrian, J.C. Phillips
      Pages: n/a - n/a
      Abstract: We used laboratory experiments to examine the rise process in neutrally‐buoyant jets that resulted from an unsteady supply of momentum, a condition that defines plumes from discrete Vulcanian‐ and Strombolian‐style eruptions. We simultaneously measured the analog‐jet discharge rate (the supply rate of momentum) and the analog‐jet internal velocity distribution (a consequence of momentum transport and dilution). Then, we examined the changes in the analog‐jet velocity distribution over time to assess the impact of the supply‐rate variations on the momentum‐driven rise dynamics. We found that the analogue jet velocity distribution changes significantly and quickly as the supply rate varied, such that the whole‐field distribution at any instant differed considerably from the time‐average. We also found that entrainment varied in space and over time with instantaneous entrainment coefficient values ranging from 0 to 0.93 in an individual unsteady jet. Consequently, we conclude that supply rate variations exert first‐order control over jet dynamics, and therefore cannot be neglected in models without compromising their capability to predict large‐scale eruption behavior. These findings emphasize the fundamental differences between unsteady and steady jet dynamics, and show clearly that: (i) variations in source momentum flux directly control the dynamics of the resulting flow; and (ii) impulsive flows driven by sources of varying flux cannot reasonably be approximated by quasi‐steady flow models. New modeling approaches capable of describing the time‐dependent properties of transient volcanic eruption plumes are needed before their trajectory, dilution and stability can be reliably computed for hazards management.
      PubDate: 2014-10-30T02:35:21.635712-05:
      DOI: 10.1002/2014GC005471
       
  • Understanding a submarine eruption through time series hydrothermal plume
           sampling of dissolved and particulate constituents: West Mata,
           2008–2012
    • Authors: Tamara Baumberger; Marvin D. Lilley, Joseph A. Resing, John E. Lupton, Edward T. Baker, David A. Butterfield, Eric J. Olson, Gretchen L. Früh‐Green
      Pages: n/a - n/a
      Abstract: Four cruises between 2008 and 2012 monitored the continuing eruption of West Mata volcano in the NE Lau Basin as it produced plumes of chemically altered water above its summit. Although large enrichments in 3He, CO2, Fe, and Mn were observed in the plumes, the most notable enrichment was that of H2, which reached concentrations as high as 14843 nM. Strongly enriched H2 concentrations in the water column result from reactions between seawater or magmatic water and extremely hot rocks. In 2008, the observation of elevated H2 concentrations in the water column above West Mata pointed to vigorous ongoing eruptions at the volcano's summit. The eruption was confirmed by visual observations made by the ROV Jason 2 in 2009 and demonstrated that H2 measurements are a vital instrument to detect ongoing volcanic eruptions at the seafloor. Elevated H2 in 2010 showed that the eruption was ongoing, although at a reduced level given a maximum H2 concentration of 4410 nM. In 2012, H2 levels in the water column declined significantly, to a maximum of only 7 nM, consistent with visual observations from the Quest‐4000 ROV that found no evidence of an ongoing volcanic eruption. Methane behaved independently of other measured gases and its concentrations in the hydrothermal plume were very low. We attribute its minimal enrichments to a mixture of mantle carbon reduced to CH4 and biological CH4 from diffuse flow sites. This study demonstrates that ongoing submarine volcanic eruptions are characterized by high dissolved H2 concentrations present in the overlying water column.
      PubDate: 2014-10-30T00:42:49.816508-05:
      DOI: 10.1002/2014GC005460
       
  • Anthropophile elements in river sediments: Overview from the Seine River,
           France
    • Authors: Jiu‐Bin Chen; Jérôme Gaillardet, Julien Bouchez, Pascale Louvat, Yi‐Na Wang
      Pages: n/a - n/a
      Abstract: In contrast to larger river systems that drain relatively pristine basins, little is known about the sediment geochemistry of rivers impacted by intense human activities. In this paper, we present a systematic investigation of the anthropogenic overprints on element geochemistry in sediments of the human‐impacted Seine River, France. Most elements are fractionated by grain size, as shown by the comparison between suspended particulate matter (SPM) and riverbank deposits (RBD). The RBD are particularly coarse and enriched in carbonates and heavy minerals and thus in elements such as Ba, Ca, Cr, Hf, Mg, Na, REEs, Sr, Ti, Th, and Zr. Although the enrichment/depletion pattern of some elements (e.g. K, REEs, Zr, etc.) can largely be explained by a binary mixture between two sources, other elements such as Ag, Bi, Cr, Cd, Co, Cu, Fe, Mo, Ni, Pb, Sb, Sn, W and Zn in SPM in Paris show that a third end‐member having anthropogenic characteristics is needed to account for their enrichment at low water stage. These “anthropophile” elements, with high enrichment factors (EF) relative to the upper continental crust (UCC), display a progressive enrichment downstream and different geochemical behaviors with respect to the hydrodynamic conditions (e.g. grain size) compared to elements having mainly a natural origin. Our findings emphasize the need for systematic studies of these anthropophile elements in other human‐impacted rivers using geochemical normalization techniques, and stress the importance of studying the chemical variability associated with hydrodynamic conditions when characterizing riverine element geochemistry and assessing their flux to the ocean.
      PubDate: 2014-10-30T00:42:43.72233-05:0
      DOI: 10.1002/2014GC005516
       
  • Rayleigh wave phase velocities in the Atlantic upper mantle
    • Authors: Esther K. James; Colleen A. Dalton, James B. Gaherty
      Pages: n/a - n/a
      Abstract: Phase velocity in the period range 30‐130 seconds is measured for approximately 10,000 fundamental‐mode Rayleigh waves traversing the Atlantic basin. In order to isolate the signal of the oceanic upper mantle, paths with >30% of their length through continental upper mantle are excluded. The lateral distribution of Rayleigh wave phase velocity in the Atlantic upper mantle is explored with two approaches. One, phase velocity is allowed to vary only as a function of seafloor age. Two, a general two‐dimensional parameterization is utilized in order to capture perturbations to age‐dependent structure. In both scenarios, phase velocity shows a strong dependence on seafloor age at all periods, with higher velocity associated with older seafloor. Removing age‐dependent velocity from the 2‐D phase‐velocity maps highlights areas of anomalously low velocity, almost all of which are proximal to locations of hotspot volcanism. The age‐dependent phase velocities for the Atlantic are not consistent with a half‐space cooling model and are best explained by a plate cooling model with thickness of 75 km and mantle temperature of 1400oC. In contrast, age‐dependent phase velocities for the Pacific basin determined by Nishimura and Forsyth [1989] can be fit reasonably well by a half‐space cooling model with mantle temperature approximately 50oC warmer than the Atlantic. Comparison of Rayleigh wave phase velocity and fractionation‐corrected Na concentrations in mid‐ocean‐ridge basalts erupted at 87 axial ridge segments reveals a positive correlation coefficient that increases with period, as expected if along‐ridge variations in mantle potential temperature are controlling both quantities.
      PubDate: 2014-10-27T11:51:07.780232-05:
      DOI: 10.1002/2014GC005518
       
  • Apparent timing and duration of the Matuyama‐Brunhes geomagnetic
           reversal in Chinese loess
    • Authors: Hui Zhao; Xiaoke Qiang, Youbin Sun
      Pages: n/a - n/a
      Abstract: The Matuyama‐Brunhes (MB) geomagnetic reversal in Chinese loess has been studied extensively as an important boundary for land‐ocean stratigraphic and paleoclimatic correlations. However, the apparent timing and duration of the MB boundary remains controversial in Chinese loess deposits due to its inconsistent stratigraphic position and the uncertain chronologies. Here we synthesized high‐resolution paleomagnetic records from four loess sequences in the central Chinese Loess Plateau and synchronized the loess‐paleosol chronology by matching the grain size variations to orbitally‐tuned grain size time series. The synthesized paleomagnetic results reveal consistent features of the MB transition in Chinese loess, including the stratigraphic position (L8/S8 transition), timing (˜808‐826 ka), duration (˜14‐16 ka), and rapid directional oscillations. Compared with the MB transition in marine records (770‐775 ka), the timing of the MB transition is relatively older and longer in Chinese loess, due to a complex interplay between different remanence acquisition mechanisms which occurred during the course of post‐depositional physical and chemical processes.
      PubDate: 2014-10-27T11:41:23.690395-05:
      DOI: 10.1002/2014GC005497
       
  • Volatile cycling of H2O, CO2, F, and Cl in the HIMU mantle: A new window
           provided by melt inclusions from oceanic hot spot lavas at Mangaia, Cook
           Islands
    • Authors: Rita A. Cabral; Matthew G. Jackson, Kenneth T. Koga, Estelle F. Rose‐Koga, Erik H. Hauri, Martin J. Whitehouse, Allison A. Price, James M.D. Day, Nobumichi Shimizu, Katherine A. Kelley
      Pages: n/a - n/a
      Abstract: Mangaia hosts the most radiogenic Pb‐isotopic compositions observed in ocean island basalts and represents the HIMU (high μ = 238U/204Pb) mantle end‐member, thought to result from recycled oceanic crust. Complete geochemical characterization of the HIMU mantle end‐member has been inhibited due to a lack of deep submarine glass samples from HIMU localities. We homogenized olivine‐hosted melt inclusions separated from Mangaia lavas and the resulting glassy inclusions made possible the first volatile abundances to be obtained from the HIMU mantle end‐member. We also report major and trace element abundances and Pb‐isotopic ratios on the inclusions, which have HIMU isotopic fingerprints. We evaluate the samples for processes that could modify the volatile and trace element abundances post‐mantle melting, including diffusive Fe and H2O‐loss, degassing, and assimilation. H2O/Ce ratios vary from 119 to 245 in the most pristine Mangaia inclusions; excluding an inclusion that shows evidence for assimilation, the primary magmatic H2O/Ce ratios vary up to ~200, and are consistent with significant dehydration of oceanic crust during subduction and long‐term storage in the mantle. CO2 concentrations range up to 2346 ppm CO2 in the inclusions. Relatively high CO2 in the inclusions, combined with previous observations of carbonate blebs in other Mangaia melt inclusions, highlight the importance of CO2 for the generation of the HIMU mantle. F/Nd ratios in the inclusions (30 ± 9; 2σ standard deviation) are higher than the canonical ratio observed in oceanic lavas, and Cl/K ratios (0.079 ± 0.028) fall in the range of pristine mantle (0.02‐0.08).
      PubDate: 2014-10-25T03:01:20.280804-05:
      DOI: 10.1002/2014GC005473
       
  • The role of viscoelasticity in subducting plates
    • Authors: R. J. Farrington; L.‐N. Moresi, F.A. Capitanio
      Pages: n/a - n/a
      Abstract: Subduction of tectonic plates into Earth's mantle occurs when one plate bends beneath another at convergent plate boundaries. The characteristic time of deformation at these convergent boundaries approximates the Maxwell relaxation time for olivine at lithospheric temperatures and pressures, it is therefore by definition a viscoelastic process. While this is widely acknowledged, the large‐scale features of subduction can, and have been, successfully reproduced assuming the plate deforms by a viscous mechanism alone. However the energy rates and stress profile within convergent margins are influenced by viscoelastic deformation. In this study viscoelastic stresses have been systematically introduced into numerical models of free subduction, using both the viscosity and shear modulus to control the Maxwell relaxation time. The introduction of an elastic deformation mechanism into subduction models produces deviations in both the stress profile and energy rates within the subduction hinge when compared to viscous only models. These variations result in an apparent viscosity that is variable throughout the length of the plate, decreasing upon approach and increasing upon leaving the hinge. At realistic Earth parameters, we show that viscoelastic stresses have a minor effect on morphology yet are less dissipative at depth and result in an energy transfer between the energy stored during bending and the energy released during unbending. We conclude that elasticity is important during both bending and unbending within the slab hinge with the resulting stress loading and energy profile indicating that slabs maintain larger deformation rates at smaller stresses during bending, retaining their strength during unbending at depth.
      PubDate: 2014-10-25T02:55:32.019261-05:
      DOI: 10.1002/2014GC005507
       
  • Spatial extent and degree of oxygen depletion in the deep
           proto‐North Atlantic basin during Oceanic Anoxic Event 2
    • Authors: Niels A.G.M. van Helmond; Itzel Ruvalcaba Baroni, Appy Sluijs, Jaap S. Sinninghe Damsté, Caroline P. Slomp
      Pages: n/a - n/a
      Abstract: Massive organic matter burial due to widespread ocean anoxia across the Cenomanian/Turonian boundary event (~94 Ma), resulted in a major perturbation of the global carbon cycle: the so‐called Oceanic Anoxic Event 2 (OAE2). The characteristics and spatial distribution of the OAE2 deposits that formed in the deep basin of the proto‐North Atlantic remain poorly described, however. Here, we present proxy data of redox sensitive (trace) elements (e.g., Mo, Fe/Al, Corg/Ptot and Mn) for OAE2 sediments from five Deep Sea Drilling Project and Ocean Drilling Program sites located in the deep proto‐North Atlantic basin. Our results highlight that bottom waters in the entire deep proto‐North Atlantic were anoxic during most of OAE2. Furthermore, regressions of Mo with total organic carbon content (TOC), previously shown to document the degree of water mass restriction, confirm that the water circulation in the proto‐North Atlantic basin was severely restricted during OAE2. Comparison of these values to Mo/TOC ratios in the present‐day Black Sea suggest a renewal frequency of the deep proto‐North Atlantic water mass of between 0.5 and 4 ka, compared to a maximum of ~200 years for the present‐day northern Atlantic. The Plenus Cold Event, a cooler episode during the early stages of OAE2 hypothesized to be caused by declining pCO2 due to extensive burial of organic matter, appears to have led to temporary re‐oxygenation of the bottom water in the deep proto‐North Atlantic basin during OAE2.
      PubDate: 2014-10-23T09:30:29.293047-05:
      DOI: 10.1002/2014GC005528
       
  • Continental breakup and UHP rock exhumation in action: GPS results from
           the Woodlark Rift, Papua New Guinea
    • Authors: Laura M. Wallace; Susan Ellis, Tim Little, Paul Tregoning, Neville Palmer, Robert Rosa, Richard Stanaway, John Oa, Edwin Nidkombu, John Kwazi
      Pages: n/a - n/a
      Abstract: We show results from a network of campaign Global Positioning System (GPS) sites in the Woodlark Rift, southeastern Papua New Guinea, in a transition from seafloor spreading to continental rifting. GPS velocities indicate anticlockwise rotation (at 2‐2.7 º/Myr, relative to Australia) of crustal blocks north of the rift, producing 10‐15 mm/yr of extension in the continental rift, increasing to 20‐40 mm/yr of seafloor spreading at the Woodlark Spreading Center. Extension in the continental rift is distributed among multiple structures. These data demonstrate that low‐angle normal faults in the continents, such as the Mai'iu Fault, can slip at high rates nearing 10 mm/yr. Extensional deformation observed in the D'Entrecasteaux Islands, the site of the world's only actively exhuming Ultra‐High Pressure (UHP) rock terrane, supports the idea that extensional processes play a critical role in UHP rock exhumation. GPS data do not require significant interseismic coupling on faults in the region, suggesting that much of the deformation may be aseismic. Westward transfer of deformation from the Woodlark Spreading Center to the main plate boundary fault in the continental rift (the Mai'iu fault) is accommodated by clockwise rotation of a tectonic block beneath Goodenough Bay, and by dextral strike‐slip on transfer faults within (and surrounding) Normanby Island. Contemporary extension rates in the Woodlark Spreading Center are 30‐50% slower than those from seafloor spreading‐derived magnetic anomalies. The 0.5 Ma‐present seafloor spreading estimates for the Woodlark Basin may be overestimated, and a re‐evaluation of these data in the context of the GPS rates is warranted.
      PubDate: 2014-10-23T06:35:48.361968-05:
      DOI: 10.1002/2014GC005458
       
  • Segmentation and eruptive activity along the East Pacific Rise at
           16°N, in relation with the nearby Mathematician hot spot
    • Authors: M. Le Saout; A. Deschamps, S. A. Soule, P. Gente
      Pages: n/a - n/a
      Abstract: The 16°N segment of the East Pacific Rise is the most over‐inflated and shallowest of this fast‐spreading ridge, in relation with an important magma flux due to the proximity of the Mathematician hotspot. Here, we analyze the detailed morphology of the axial dome and of the Axial Summit Trough (AST), the lava morphology and the geometry of fissures and faults, in regard to the attributes of the magma chamber beneath and of the nearby hotspot. The data used are 1‐meter‐resolution bathymetry combined with seafloor photos and videos. At the dome summit, the AST is highly segmented by ten 3rd and 4th ‐order discontinuities over a distance of 30 km. Often, two contiguous and synchronous ASTs coexist. Such a configuration implies a wide (1100 m minimum) zone of diking. The existence of contiguous ASTs, their mobility, their general en echelon arrangement accommodating the bow shape of the axial dome toward the hotspot, plus the existence of a second magma lens under the western half of the summit plateau, clearly reflect the influence of the hotspot on the organization of the spreading system. The different ASTs exhibit contrasted widths and depths. We suggest that narrow ASTs reflect an intense volcanic activity that produces eruptions covering the tectonic features and partially filling the ASTs. AST widening and deepening would indicate a decrease in volcanic activity but with continued dike intrusions at the origin of abundant sets of fissures and faults that are not masked by volcanic deposits.
      PubDate: 2014-10-21T11:01:18.682752-05:
      DOI: 10.1002/2014GC005560
       
  • Incorporating 3‐D parent nuclide zonation for apatite 4He/3He
           thermochronometry: An example from the Appalachian Mountains
    • Authors: Matthew Fox; Ryan E. McKeon, David L. Shuster
      Pages: n/a - n/a
      Abstract: The ability to constrain km‐scale exhumation with apatite 4He/3He thermochronometry is well established and the technique has been applied to a range of tectonic and geomorphic problems. However, multiple sources of uncertainty in specific crystal characteristics limit the applicability of the method, especially when geologic problems require identifying small perturbations in a cooling path. Here we present new 4He/3He thermochronometric data from the Appalachian Mountains, which indicate significant parent nuclide zonation in an apatite crystal. Using LA‐ICPMS measurements of U and Th in the same crystal, we design a 3‐D model of the crystal to explore the effects of intra‐crystal variability in radiation damage accumulation. We describe a numerical approach to solve the 3‐D production‐diffusion equation. Using our numerical model and a previously determined time temperature path for this part of the Appalachians, we find excellent agreement between predicted and observed 4He/3He spectra. Our results confirm this time‐temperature path and highlight that for complex U and Th zonation patterns, 3‐D numerical models are required to infer an accurate time‐temperature history. In addition, our results provide independent and novel evidence for a radiation damage control on diffusivity. The ability to exploit intra‐crystal differences in 4He diffusivity (i.e., temperature sensitivity) greatly increases the potential to infer complex thermal histories.
      PubDate: 2014-10-20T03:44:39.218353-05:
      DOI: 10.1002/2014GC005464
       
  • Assessment of relative Ti, Ta, and Nb (TITAN) enrichments in ocean island
           basalts
    • Authors: Bradley J. Peters; James M.D. Day
      Pages: n/a - n/a
      Abstract: The sensitivity of trace element concentrations to processes governing solid‐melt interactions has made them valuable tools for tracing the effects of partial melting, fractional crystallization, metasomatism and similar processes on the composition of a parental melt. Recent studies of ocean island basalts (OIB) have sought to correlate Ti, Ta and Nb (TITAN) anomalies to isotopic tracers, such as 3He/4He and 187Os/188Os ratios, which may trace primordial deep mantle sources. A new compilation of global OIB trace element abundance data indicates that positive TITAN anomalies, though statistically pervasive features of OIB, may not be compositional features of their mantle sources. OIB show a range of Ti (Ti/Ti*=0.28‐2.35), Ta (Ta/Ta*=0.11‐93.4) and Nb (Nb/Nb*=0.13‐17.8) anomalies that show negligible correlations with 3He/4He ratios, indicating that TITAN anomalies are not derived from the less‐degassed mantle source traced by high‐3He/4He. Positive TITAN anomalies can be modelled using variable degrees (0.1‐10%) of non‐modal batch partial melting of garnet‐spinel lherzolite at temperatures and pressures considered typical for OIB petrogenesis, and subjecting this partial melt to fractional crystallization and assimilation of mid‐ocean ridge basalt‐like crust (AFC). Correlations of TITAN anomalies with modal abundances of olivine and clinopyroxene in porphyritic Canary Islands lavas provide empirical support for this process and indicate that high abundances of these phases in OIB may create misleading trace element anomalies on primitive mantle‐normalized spider diagrams. Because partial melting and AFC are common to all mantle‐derived magmas, caution should be used when attributing TITAN anomalies to direct sampling of recycled or deep mantle sources by hotspots.
      PubDate: 2014-10-20T03:44:37.277039-05:
      DOI: 10.1002/2014GC005506
       
  • Issue Information
    • Pages: i - i
      PubDate: 2014-10-16T14:10:17.046689-05:
      DOI: 10.1002/ggge.20325
       
  • Segmentation of plate coupling, fate of subduction fluids, and modes of
           arc magmatism in Cascadia, inferred from magnetotelluric resistivity
    • Authors: Philip E. Wannamaker; Rob L. Evans, Paul A. Bedrosian, Martyn J. Unsworth, Virginie Maris, R Shane McGary
      Pages: n/a - n/a
      Abstract: Five magnetotelluric (MT) profiles have been acquired across the Cascadia subduction system and transformed using 2D and 3D non‐linear inversion to yield electrical resistivity cross sections to depths of ˜200 km. Distinct changes in plate coupling, subduction fluid evolution, and modes of arc magmatism along the length of Cascadia are clearly expressed in the resistivity structure. Relatively high resistivities under the coasts of northern and southern Cascadia correlate with elevated degrees of inferred plate locking, and suggest fluid‐ and sediment‐deficient conditions. In contrast, the north‐central Oregon coastal structure is quite conductive from the plate interface to shallow depths offshore, correlating with poor plate locking and the possible presence of subducted sediments. Low‐resistivity fluidized zones develop at slab depths of 35‐40 km starting ˜100 km west of the arc on all profiles, and are interpreted to represent prograde metamorphic fluid release from the subducting slab. The fluids rise to forearc Moho levels, and sometimes shallower, as the arc is approached. The zones begin close to clusters of low frequency earthquakes, suggesting fluid controls on the transition to steady sliding. Under the northern and southern Cascadia arc segments, low upper mantle resistivities are consistent with flux melting above the slab plus possible deep convective backarc upwelling toward the arc. In central Cascadia, extensional deformation is interpreted to segregate upper mantle melts leading to underplating and low resistivities at Moho to lower crustal levels below the arc and near backarc. The low‐ to high‐temperature mantle wedge transition lies slightly trenchward of the arc.
      PubDate: 2014-10-14T12:02:11.596984-05:
      DOI: 10.1002/2014GC005509
       
  • Deep water recycling through time
    • Authors: Valentina Magni; Pierre Bouilhol, Jeroen van Hunen
      Pages: n/a - n/a
      Abstract: We investigate the dehydration processes in subduction zones and their implications for the water cycle throughout Earth's history. We use a numerical tool that combines thermo‐mechanical models with a thermodynamic database to examine slab dehydration for present‐day and early Earth settings and its consequences for the deep water recycling. We investigate the reactions responsible for releasing water from the crust and the hydrated lithospheric mantle and how they change with subduction velocity (vs), slab age (a) and mantle temperature (Tm). Our results show that faster slabs dehydrate over a wide area: they start dehydrating shallower and they carry water deeper into the mantle. We parameterize the amount of water that can be carried deep into the mantle, W (x105 kg/m2), as a function of vs (cm/yr), a (Myrs), and Tm (°C): W=1.06υs+0.14α−0.023Tm+17 We generally observe that a 1) 100°C increase in the mantle temperature, or 2) ~15 Myr decrease of plate age, or 3) decrease in subduction velocity of ~2 cm/yr all have the same effect on the amount of water retained in the slab at depth, corresponding to a decrease of ~2.2x105 kg/m2 of H2O. We estimate that for present‐day conditions ~26% of the global influx water, or 7x108 Tg/Myr of H2O, is recycled into the mantle. Using a realistic distribution of subduction parameters, we illustrate that deep water recycling might still be possible in early Earth conditions, although its efficiency would generally decrease. Indeed, 0.5‐3.7x108 Tg/Myr of H2O could still be recycled in the mantle at 2.8 Ga.
      PubDate: 2014-10-14T12:02:04.52594-05:0
      DOI: 10.1002/2014GC005525
       
  • Shallow methane hydrate system controls ongoing, downslope sediment
           transport in a low‐velocity active submarine landslide complex,
           Hikurangi Margin, New Zealand
    • Authors: Joshu J. Mountjoy; Ingo Pecher, Stuart Henrys, Gareth Crutchley, Philip M. Barnes, Andreia Plaza‐Faverola
      Pages: n/a - n/a
      Abstract: Morphological and seismic data from a submarine landslide complex east of New Zealand indicate flow‐like deformation within gas hydrate‐bearing sediment. This “creeping” deformation occurs immediately downslope of where the base of gas hydrate stability reaches the seafloor, suggesting involvement of gas hydrates. We present evidence that, contrary to conventional views, gas hydrates can directly destabilize the seafloor. Three mechanisms could explain how the shallow gas hydrate system could control these landslides. 1) Gas hydrate dissociation could result in excess pore pressure within the upper reaches of the landslide. 2) Overpressure below low‐permeability gas hydrate‐bearing sediments could cause hydrofracturing in the gas hydrate zone valving excess pore pressure into the landslide body. 3) Gas hydrate‐bearing sediment could exhibit time‐dependent plastic deformation enabling glacial‐style deformation. We favor the final hypothesis, that the landslides are actually creeping seafloor glaciers. The viability of rheologically controlled deformation of a hydrate sediment mix is supported by recent laboratory observations of time‐dependent deformation behavior of gas‐hydrate‐bearing sands. The controlling hydrate is likely to be strongly dependent on formation controls and inter‐sediment hydrate morphology. Our results constitute a paradigm shift for evaluating the effect of gas hydrates on seafloor strength which, given the widespread occurrence of gas hydrates in the submarine environment, may require a re‐evaluation of slope stability following future climate‐forced variation in bottom water temperature.
      PubDate: 2014-10-14T02:58:27.905801-05:
      DOI: 10.1002/2014GC005379
       
  • Lucky Strike seamount: Implications for the emplacement and rifting of
           
    • Authors: J. Escartín; A. Soule, M. Cannat, D. J. Fornari, D. Düşünür, R. Garcia
      Pages: n/a - n/a
      Abstract: The history of emplacement, tectonic evolution, and dismemberment of a central volcano within the rift valley of the slow‐spreading Mid‐Atlantic Ridge at the Lucky Strike Segment is deduced using near‐bottom sidescan sonar imagery and visual observations. Volcano emplacement is rapid (
      PubDate: 2014-10-14T01:04:00.438503-05:
      DOI: 10.1002/2014GC005477
       
  • Eruptive history and magmatic stability of Erebus volcano, Antarctica:
           Insights from englacial tephra
    • Authors: Nels A. Iverson; Philip R. Kyle, Nelia W. Dunbar, William C. McIntosh, Nicholas J.G. Pearce
      Pages: n/a - n/a
      Abstract: A tephrostratigraphy of the active Antarctic volcano, Mt. Erebus, was determined from englacial tephra on the ice‐covered flanks of Erebus and an adjacent volcano. The tephra are used to reconstruct the eruptive history and magmatic evolution of Erebus. More fine grained and blocky particles define tephra formed in phreatomagmatic eruptions and larger fluidal shards are characteristic of magmatic eruptions and in some cases both eruptive types are identified in a single mixed tephra. The eruptions forming the mixed tephra likely started as phreatomagmatic eruptions which transitioned into Strombolian eruptions as the non‐magmatic water source was exhausted. We reconstructed the eruptive history of Erebus using the tephra layers stratigraphic position, 40Ar/39Ar ages, shard morphology and grain size. Major and trace element analyses of individual glass shards were measured by electron probe microanalysis and LA‐ICP‐MS. Trachybasalt, trachyte and phonolite tephra were identified. All phonolitic tephra are Erebus‐derived with compositions similar to volcanic bombs erupted from Erebus over the past 40 years. The tephra show that Erebus magma has not significantly changed for 40ka. The uniformity of the glass chemical composition implies that the phonolite magma has crystallized in the same manner without change throughout the late Quaternary, suggesting long‐term stability of the Erebus magmatic system. Trachyte and trachybasalt tephra were likely erupted from Marie Byrd Land and the McMurdo Sound area, respectively. The trachytic tephra can be regionally correlated, and could provide an important time‐stratigraphic marker in Antarctic ice cores.
      PubDate: 2014-10-08T04:25:27.372547-05:
      DOI: 10.1002/2014GC005435
       
  • Quantifying temporal variations in landslide‐driven sediment
           production by reconstructing paleolandscapes using tephrochronology and
           lidar: Waipaoa River, New Zealand
    • Authors: Corina Cerovski‐Darriau; Joshua J. Roering, Michael Marden, Alan S. Palmer, Eric L. Bilderback
      Pages: n/a - n/a
      Abstract: Hillslope response to climate‐driven fluvial incision controls sediment export and relief generation in most mountainous settings. Following the shift to a warmer, wetter climate after the Last Glacial Maximum (LGM) (˜18 ka), the Waipaoa River (New Zealand) rapidly incised up to 120 meters, leaving perched, low‐relief hillslopes unadjusted to that base level fall. In the Mangataikapua—a 16.5 km2 tributary principally comprised of weak mélange—pervasive post‐LGM landslides responded to >50 m of fluvial incision by sculpting and denuding >99% of the catchment. By reconstructing LGM and younger paleosurfaces from tephra identified by electron microprobe analysis (EMPA) and lidar‐derived surface roughness, we estimate the volume, timing, and distribution of hillslope destabilization in the Mangataikapua and the relative contribution of landslide‐prone terrain to post‐LGM landscape evolution. We calculate volume change between four paleosurfaces constrained by tephra age (Rerewhakaaitu, 17.5 ka; Rotoma, 9.4 ka; Whakatane, 5.5 ka; and Waimihia, 3.4 ka). From the paleosurface reconstructions, we calculate the total post‐LGM hillslope sediment contribution from the Mangataikapua catchment to be 0.5 ± 0.06 (s.d.) km3, which equates to a sub‐catchment averaged erosion rate of ˜1.6 mm yr‐1. This is double the previous hillslope volume when normalized by study area, demonstrating that landslide‐prone catchments disproportionately contribute to the terrestrial post‐LGM sediment budget. Finally, we observe particularly rapid post‐Waimihia erosion rates, likely impacted by human settlement.
      PubDate: 2014-10-08T03:52:44.066793-05:
      DOI: 10.1002/2014GC005467
       
  • Enhanced recycling during oceanic anoxic event 2 in the proto‐North
           Atlantic
    • Authors: I. Ruvalcaba Baroni; I. Tsandev, C. P. Slomp
      Pages: n/a - n/a
      Abstract: Evidence from sediment core records and model studies suggest that increased nutrient supply played a key role in the initiation of the Cenomanian‐Turonian oceanic anoxic event 2 (OAE2; 94 Ma). However, the relative roles of nitrogen (N) and phosphorus (P) availability in controlling primary productivity during the event are not fully understood. Here, we expand an existing multi‐box model of the coupled cycles of P, carbon and oxygen in the proto‐North Atlantic by adding the marine N cycle. With the updated version of the model, we test the hypothesis that enhanced availability of P can fuel N2‐fixation, increase primary productivity and drive large parts of the proto‐North Atlantic to anoxia during OAE2. In a sensitivity analysis, we demonstrate that N dynamics in the proto‐North Atlantic respond strongly to variations in oxygen and P supply from the Pacific Ocean and to changes in circulation. The implemented N cycle weakly modifies the carbon cycle, implying that P was the major nutrient controlling primary productivity during OAE2. Our model suggests that both N2‐fixation and upwelling of recycled were enhanced during OAE2 and that N2‐fixation was the major source of N in the proto‐North Atlantic. Denitrification was more important in the water column than in sediments, with high rates in the open ocean and in the Western Interior. High P inputs in the proto‐North Atlantic led to widespread N2‐fixation, which more than compensated for the loss of N through denitrification. As a consequence, rates of primary productivity and organic carbon burial were high.
      PubDate: 2014-10-08T03:36:57.126731-05:
      DOI: 10.1002/2014GC005453
       
  • Carbon cycling and burial in New Zealand's fjords
    • Authors: Jessica L. Hinojosa; Christopher M. Moy, Claudine H. Stirling, Gary S. Wilson, Timothy I. Eglinton
      Pages: n/a - n/a
      Abstract: Understanding carbon cycling in continental margin settings is critical for constraining the global carbon cycle. Here we apply a multiproxy geochemical approach to evaluate regional carbon cycle dynamics in six New Zealand fjords. Using carbon and nitrogen concentrations and isotopes, lipid biomarkers, and redox‐sensitive element concentrations, we show that the New Zealand fjords have carbon‐rich surface sediments in basins that promote long‐term storage (i.e., semi‐restricted basins with sediment accumulation rates of up to 3 mm yr‐1). Using δ13C distributions to develop a mixing model, we find that organic carbon in fjord sediments is well‐mixed from marine and terrestrial sources in down‐fjord gradients. This is driven by high regional precipitation rates of >6 m yr‐1, which promote carbon accumulation in fjord basins through terrestrial runoff. In addition, we have identified at least two euxinic sub‐basins, based on uranium, molybdenum, iron, and cadmium enrichment, that contain >7% organic carbon. Because the strength and position of the Southern Hemisphere westerly winds control precipitation and fjord circulation, carbon delivery and storage in the region are intimately linked to westerly wind variability. We estimate that the fjord region (759 km2) may be exporting up to 1.4 x 107 kgC yr‐1, outpacing other types of continental margins in rates of carbon burial by up to three orders of magnitude.
      PubDate: 2014-10-08T03:36:46.239793-05:
      DOI: 10.1002/2014GC005433
       
  • Flow dynamics of Nankai Trough submarine landslide inferred from internal
           deformation using magnetic fabric
    • Authors: Toshiya Kanamatsu; Kiichiro Kawamura, Michael Strasser, Beth Novak, Yujin Kitamura
      Pages: n/a - n/a
      Abstract: Submarine landslide deposits in an active subduction zone were investigated by Integrated Ocean Drilling Program Expedition 333 as the “Nankai Trough Submarine Landslides History”. The expedition recovered a Pleistocene to Holocene sequence of stacked mass‐transport deposits at Site C0018, located within a slope basin on the footwall of the megasplay fault in the Nankai Trough off the Kii Peninsula, southwest Japan. Six mass‐transport deposit units intercalated with coherent intervals were recovered from the upper 190‐m of the drilled succession. Anisotropy of magnetic susceptibility variations within mass‐transport deposit units was used to ascertain their rheology. Shape parameters and magnetic fabric orientation reveal inhomogeneity through the sequences, indicating that different compaction and shear occurred within individual units. The upper intervals of younger units generally represents a magnetic fabric formed under vertical compression. However, the lower intervals involve magnetic fabrics indicating lateral shear with in‐depth gradual change. In the older mass‐transport deposit, a distribution of magnetic foliation forms tightly folded strata, indicating different sliding patterns from the younger mass‐transport deposit units. Using available paleomagnetic data, the shear directions of basal intervals of units are reoriented, producing two different sliding orientations. Variation in the flow type and supply route is interpreted to reflect the slope condition (e.g. slope gradient and susceptibility to ground motion), which has been controlled by local tectonic evolution of the accretionary wedge.
      PubDate: 2014-10-07T02:38:27.074011-05:
      DOI: 10.1002/2014GC005409
       
  • Eruptive modes and hiatus of volcanism at West Mata seamount, NE Lau
           basin: 1996–2012
    • Authors: Robert W. Embley; Susan G. Merle, Edward T. Baker, Kenneth H. Rubin, John E. Lupton, Joseph A. Resing, Robert P. Dziak, Marvin D. Lilley, William W. Chadwick, T. Shank, Ron Greene, Sharon L. Walker, Joseph Haxel, Eric Olson, Tamara Baumberger
      Pages: n/a - n/a
      Abstract: We present multiple lines of evidence for years‐ to decade‐long changes in the location and character of volcanic activity at West Mata seamount in the NE Lau Basin over a 16 year period, and a hiatus in summit eruptions from early 2011 through at least September 2012. Boninite lava and pyroclasts were observed erupting from its summit in 2009 and hydroacoustic data from a succession of hydrophones moored nearby show near‐continuous eruptive activity from January 2009 to early 2011. Successive differencing of seven multibeam bathymetric surveys of the volcano made in the 1996‐2012 period reveal a pattern of extended constructional volcanism on the summit and northwest flank punctuated by eruptions along the volcano's WSW rift zone (WSWRZ). Away from the summit, the volumetrically largest eruption during the observational period occurred between May 2010 and November 2011 at ~2920 m depth near the base of the WSWRZ. The (nearly) equally long ENE rift zone did not experience any volcanic activity during the 1996‐2012 period. The cessation of summit volcanism recorded on the moored hydrophone was accompanied or followed by the formation of a small summit crater and a landslide on the eastern flank. Water column sensors, analysis of gas samples in the overlying hydrothermal plume and dives with a remotely operated vehicle in September 2012 confirmed that the summit eruption had ceased. Based on the historical eruption rates calculated using the bathymetric differencing technique, the volcano could be as young as several thousand years.
      PubDate: 2014-10-06T04:51:47.313417-05:
      DOI: 10.1002/2014GC005387
       
  • Deconvolution of continuous paleomagnetic data from pass‐through
           magnetometer: A new algorithm to restore geomagnetic and environmental
           information based on realistic optimization
    • Authors: Hirokuni Oda; Chuang Xuan
      Pages: n/a - n/a
      Abstract: The development of pass‐through superconducting rock magnetometers (SRM) has greatly promoted collection of paleomagnetic data from continuous long‐core samples. The output of pass‐through measurement is smoothed and distorted due to convolution of magnetization with the magnetometer sensor response. Although several studies could restore high‐resolution paleomagnetic signal through deconvolution of pass‐through measurement, difficulties in accurately measuring the magnetometer sensor response have hindered the application of deconvolution. We acquired reliable sensor response of an SRM at the Oregon State University based on repeated measurements of a precisely fabricated magnetic point source. In addition, we present an improved deconvolution algorithm based on Akaike's Bayesian Information Criterion (ABIC) minimization, incorporating new parameters to account for errors in sample measurement position and length. The new algorithm was tested using synthetic data constructed by convolving “true” paleomagnetic signal containing an “excursion” with the sensor response. Realistic noise was added to the synthetic measurement using Monte Carlo method based on measurement noise distribution acquired from 200 repeated measurements of a u‐channel sample. Deconvolution of 1000 synthetic measurements with realistic noise closely resembles the “true” magnetization, and successfully restored fine‐scale magnetization variations including the “excursion”. Our analyses show that inaccuracy in sample measurement position and length significantly affects deconvolution estimation, and can be resolved using the new deconvolution algorithm. Optimized deconvolution of 20 repeated measurements of a u‐channel sample yielded highly consistent deconvolution results and estimates of error in sample measurement position and length, demonstrating the reliability of the new deconvolution algorithm for real pass‐through measurements.
      PubDate: 2014-10-04T06:47:20.553198-05:
      DOI: 10.1002/2014GC005513
       
  • The early middle Miocene subduction complex of the Louisiade Archipelago,
           southern margin of the Woodlark Rift
    • Authors: Laura E. Webb; Suzanne L. Baldwin, Paul G. Fitzgerald
      Pages: n/a - n/a
      Abstract: Field, microstructural and 40Ar/39Ar thermochronologic data from the Louisiade Archipelago, the southern rifted margin of the Woodlark Basin in SE Papua New Guinea, document an accretionary wedge that formed during Early–Middle Miocene N‐dipping subduction of the Australian margin and transpression along the Australian–Pacific plate boundary. Metasedimentary rocks of the Calvados Schist and the metagabbros that intrude them were metamorphosed at up to greenschist‐facies conditions. Three tectonic foliations (S1–S3) are present and F1–F3 fold hinges plunge ESE or WNW, parallel to mineral, stretching, and intersection lineations. Fold vergence is dominantly to the SW, and top‐to‐the‐SW thrusting of ultramafic rocks over the Calvados Schist is documented locally on Rossel Island. The data suggest progressive deformation associated with NNE–SSW shortening and ESE–WNW extension via dissolution‐precipitation creep and, more locally, dislocation creep. 40Ar/39Ar step‐heating analyses of three white mica separates yield Middle Miocene plateau or plateau‐like segments that are affected by variable Pliocene argon loss and are interpreted as syntectonic mica growth during metamorphism and deformation followed by partial resetting just prior to the onset of seafloor spreading in the Woodlark Basin. A ca. 12 Ma 40Ar/39Ar age from a dacite sill of the Panarora Volcanics provides a minimum age constraint for the termination of northward subduction and a maximum age for cross‐cutting brittle strike‐slip faults. These data are critical to constraining the subduction–exhumation history of the world's youngest high–ultrahigh‐pressure terrane and further support analogies between SE Papua New Guinea and the Early Oligocene Western Alps.
      PubDate: 2014-09-18T05:08:08.803665-05:
      DOI: 10.1002/2014GC005500
       
  • Absolute plate motions and regional subduction evolution
    • Authors: M.V. Chertova; W. Spakman, A.P. van den Berg, D.J.J. van Hinsbergen
      Pages: n/a - n/a
      Abstract: We investigate the influence of absolute plate motion on regional 3‐D evolution of subduction using numerical thermo‐mechanical modeling. Building on our previous work (Chertova et al. 2014), we explore the potential impact of four different absolute plate motion frames on subduction evolution in the western Mediterranean region during the last 35My. One frame is data‐based and derived from the global moving hotspot reference frame (GMHRF) of Doubrovine et al. [2012] and three are invented frames: a motion frame in which the African plate motion is twice that in the GMHRF, and two frames in which either the African plate or the Iberian continent is assumed fixed to the mantle. The relative Africa‐Iberia convergent is the same in all frames. All motion frames result in distinctly different 3‐D subduction evolution showing a critical dependence of slab morphology evolution on absolute plate motion. We attribute this to slab dragging through the mantle forced by the absolute motion of the subducting plate, which causes additional viscous resistance affecting subduction evolution. We observed a strong correlation between increase in northward Africa motion and decrease in the speed of westward slab rollback along the African margin. We relate this to increased mantle resistance against slab dragging providing new insight into propagation and dynamics of subduction transform edge propagator (STEP) faults. Our results demonstrate a large sensitivity of 3‐D slab evolution to the absolute motion of the subducting plate, which inversely suggests that detailed modeling of natural subduction may provide novel constraints on absolute plate motions.
      PubDate: 2014-09-17T02:18:00.229793-05:
      DOI: 10.1002/2014GC005494
       
  • Geochemical insights into the role of metasomatic hornblendite in
           generating alkali basalts
    • Authors: Li‐Qun Dai; Zi‐Fu Zhao, Yong‐Fei Zheng
      Pages: n/a - n/a
      Abstract: Experimental petrology suggested the role of hornblendite in generating alkali basalt. This mechanism is confirmed by an integrated study of major‐trace elements and radiogenic isotopes for Mesozoic alkali basalts from the Qinling orogen in China. The alkali basalts have high contents of MgO (4.8‐11.1 wt.%, Mg# = 47‐69), Na2O+K2O (2.9‐5.4 wt.%), TiO2 (2.0‐3.1 wt.%) but low content of SiO2 (41.4‐49.6 wt.%), which are generally silica‐undersaturated with normative minerals of nepheline and olivine. They exhibit OIB‐like trace element distribution patterns, with enrichment of LILE and LREE but no depletion of HFSE relative to the primitive mantle. They also show relatively depleted Sr‐Nd‐Hf isotope compositions, with low initial 87Sr/86Sr ratios of 0.7028 to 0.7058, positive εNd(t) values of 4.0 to 9.8 and εHf(t) values of 8.8 to 13.5 for whole‐rock, and positive εHf(t) values of 5.2 to 16.4 for zircon. Such element and isotope features indicate their origination from the juvenile subcontinental lithospheric mantle (SCLM) source with involvement of crustal components. The alkali basalts generally have high K2O/Na2O ratios, and high K2O and TiO2 contents, suggesting their derivation from partial melting of hornblendite‐rich mantle lithology. They also exhibit variable K/La and Ti/La ratios that are correlated with (La/Yb)N ratios, indicating a geochemical heterogeneity of the SCLM source. Taken together, all the above geochemical features can be accounted for by partial melting of a hornblendite‐rich SCLM source. The hornblendite would be generated by reaction of the juvenile SCLM wedge peridotite with felsic melts derived from subducted Palaeotethyan oceanic crust at the slab‐mantle interface in the subduction channel. Therefore, orogenic alkali basalts record recycling of the subducted fossil oceanic crust, and the metasomatic hornblendite is an important lithology in local SCLM domains above fossil subduction channels.
      PubDate: 2014-09-13T10:56:50.614212-05:
      DOI: 10.1002/2014GC005486
       
  • The dynamics of global change at the Paleocene‐Eocene thermal
           maximum: A data‐model comparison
    • Authors: Timothy J. Bralower; Katrin J. Meissner, Kaitlin Alexander, Deborah J. Thomas
      Pages: n/a - n/a
      Abstract: We integrate published stable isotopic, chemical, mineralogical and biotic data from the onset of the Paleocene Eocene thermal maximum (PETM) at Site 690, Maud Rise in the Southern Ocean. The integrated dataset documents a sequence of environmental steps including warming of the ocean from the surface downwards, and modification of its thermal and nutrient structure, acidification of the deep ocean and the onset of continental weathering. The age of the events with respect to the onset of the PETM is calibrated with three different age models. The relative and absolute timing of the steps are compared with simulated temperature, salinity, calcite saturation, and dissolved PO4 and O2, at different depths in the ocean, generated with the UVic Earth System Climate Model of intermediate complexity. The simulation supports the top to bottom transfer of heat and carbon, and generally agrees with age models in terms of the durations of leads and lags in temperature, C‐isotope and biotic responses. Moreover, the simulation shows that stratification increased and the nutricline strengthened at the onset of the PETM. These environmental changes explain the abundance of deep dwelling nannoplankton and foraminifera during the early part of the event. The modelled calcite saturation is consistent with a harsh deep sea habitat at the time of the benthic foraminiferal extinction.
      PubDate: 2014-09-13T10:47:00.561488-05:
      DOI: 10.1002/2014GC005474
       
  • Estimating shallow water sound power levels and mitigation radii for the
           R/V Marcus G. Langseth using an 8 km long MCS streamer
    • Authors: Timothy J. Crone; Maya Tolstoy, Helene Carton
      Pages: n/a - n/a
      Abstract: For seismic surveys in shallow‐water environments the complexity of local geology and seafloor topography can make it difficult to accurately predict associated sound levels and establish appropriate mitigation radii required to ensure the safety of local marine protected species. This is primarily because necessary detailed information regarding the local seafloor topography and subseafloor geology is often unavailable before a survey begins. One potential solution to this problem is to measure received levels using the ship's multi‐channel seismic (MCS) streamer \citep{Diebold2010}, which could allow for the dynamic real‐time determination of sound levels and mitigation radii while a survey is underway. We analyze {\em R/V Langseth} streamer data collected on the shelf and slope near the Washington coast during the Cascadia Open‐Access Seismic Transects (COAST) and Ridge2Trench projects to measure received levels up to a distance of approximately 8 km from the sound source array. We establish methods to filter, clean, and process streamer data to accurately determine received power levels and confidently establish mitigation radii. We show that in shallow water measured power levels can fluctuate due to the influence of seafloor topographic features, but that the use of the streamer for the establishment of dynamic mitigation radii is feasible and should be further pursued. The establishment of mitigation radii based on local conditions may help maximize the safety of marine protected species while also maximizing the ability of researchers to conduct seismic studies.
      PubDate: 2014-09-10T10:19:16.997535-05:
      DOI: 10.1002/2014GC005420
       
  • Early Cenozoic exhumation in the Bendeleben Mountains (Seward Peninsula,
           Alaska) constrained by (U‐Th)/He thermochronology and thermal
           modeling
    • Authors: Kalin T. McDannell; Jaime Toro, Jeremy K. Hourigan, Daniel Harris
      Pages: n/a - n/a
      Abstract: In the Bendeleben Mountains, Seward Peninsula, mid‐Cretaceous granites are exposed in an uplifted block bounded on its south side by an E‐W striking normal fault. The Bendeleben fault has well‐preserved scarps 4‐7 m in height that offset Holocene moraines. Seismic activity, young normal faulting, and Quaternary basaltic volcanism are all evidence of active extension. South of the Bendeleben fault, there is a 3‐4 km deep basin. Fifteen apatite (U‐Th)/He ages from granitic samples of the footwall yield an Eocene weighted mean age of 41.3±4.8 Ma. Biotite 40Ar/39Ar ages from the country rock of the Bendeleben pluton are 81‐83 Ma. In spite of the young fault scarps, Pecube and HeFTy t‐T modeling results illustrate that rapid exhumation of the Bendeleben Mountains occurred in the Late Cretaceous‐Eocene and slowed since the Oligocene. A weak age‐elevation relationship of apatite He ages and a lack of correlation between age and distance from the fault indicate that exhumation was accomplished with minimal block rotation on a steeply dipping, long‐lived normal fault. Timing of extension in the Seward Peninsula can be correlated with deformation in the offshore Hope Basin where seismic reflection lines document Early Tertiary large‐magnitude normal faulting followed by minor post‐Miocene reactivation. The faulting observed in the Bendeleben Mountains is part of an extensional system that spans a large portion of the Bering Strait region. The tectonic model proposed in previous studies suggests that clockwise rotation of the Bering block relative to North America is the cause of extensional deformation in western Alaska.
      PubDate: 2014-09-09T19:29:23.694789-05:
      DOI: 10.1002/2014GC005424
       
  • Provenance of the late quaternary sediments in the Andaman Sea:
           Implications for monsoon variability and ocean circulation
    • Authors: Neeraj Awasthi; Jyotiranjan S. Ray, Ashutosh K. Singh, Shraddha T. Band, Vinai K. Rai
      Pages: n/a - n/a
      Abstract: We present a geochemical and Sr‐Nd isotopic study on a sediment core collected from the Andaman Sea in an attempt to reconstruct the Late Quaternary weathering and erosion patterns in the watersheds of the river systems of Myanmar and understand their controlling factors. Age control is based on nine radiocarbon dates and δ18O stratigraphy. The rate of sedimentation was strongly controlled by fluctuations of the monsoon. We identify three major sediment provenances: (1) the Irrawaddy catchment, (2) the western slopes of the Indo‐Burman‐Arakan (IBA) mountain ranges and the Andaman Islands and (3) the catchments of Salween and Sittang and the Bengal shelf, with the first two contributing 30 to 60% of the material. Enhanced contributions from juvenile sources and corresponding positive shifts of δ18O are observed at seven time periods (11‐14, 20‐23, 36, 45, 53, 57 and 62 ka) of which five are synchronous with cooling of the northern hemisphere ‐ suggesting a link between the changes in sediment provenances and the shifting of the locus of the summer monsoon, southwards from the Himalayas, without substantial reduction in intensity. Our data, and that from other cores in the region suggest that an eastward moving surface current disperses sediments, derived from the Bengal shelf and western margin of Myanmar, from the eastern Bay of Bengal into the western Andaman Sea and that its strength has increased since the LGM. The existence of this current during the LGM implies that the Andaman Sea and the Bay of Bengal were well connected during the last glacial period.
      PubDate: 2014-09-09T19:24:15.008141-05:
      DOI: 10.1002/2014GC005462
       
  • Sulfur isotope evolution in sulfide ores from Western Alps: Assessing the
           influence of subduction‐related metamorphism
    • Authors: Fabio Giacometti; Katy A. Evans, Gisella Rebay, John Cliff, Andrew G. Tomkins, Piergiorgio Rossetti, Gloria Vaggelli, David T. Adams
      Pages: n/a - n/a
      Abstract: Sulfides entering subduction zones can play an important role in the release of sulfur and metals to the mantle wedge and contribute to the formation of volcanic arc‐associated ores. Fractionation of stable sulfur isotopes recorded by sulfides during metamorphism can provide evidence of fluid‐rock interactions during metamorphism and give insights on sulfur mobilization. A detailed micro‐textural and geochemical study was performed on mineralized samples from two ocean floor‐related sulfide deposits (Servette and Beth‐Ghinivert) in high‐pressure units of the Italian Western Alps, which underwent different metamorphic evolutions. The combination of micro‐textural investigations with δ34S values from in situ ion‐probe analyses within individual pyrite and chalcopyrite grains allowed an evaluation of the effectiveness of metamorphism in modifying the isotopic record and mobilizing sulfur and metals and have insights on fluid circulation within the slab. Textures and isotopic compositions inherited from the protolith are recorded at Beth‐Ghinivert, where limited metamorphic re‐crystallization is attributed to limited interaction with metamorphic fluids. Isotopic modification by metamorphic processes occurred only at the sub‐millimeter scale at Servette, where local interactions with infiltrating hydrothermal fluid are recorded by metamorphic grains. Notwithstanding the differences recorded by the two deposits, neither underwent intensive isotopic re‐equilibration or records evidence of intense fluid‐rock interaction and S mobilization during metamorphism. Therefore, subducted sulfide deposits dominated by pyrite and chalcopyrite are unlikely to release significant quantities of sulfur to the mantle wedge and to arc magmatism sources at metamorphic grades below the lower eclogite facies.
      PubDate: 2014-09-09T19:20:54.650317-05:
      DOI: 10.1002/2014GC005459
       
  • Three‐dimensional flow in the subslab mantle
    • Authors: Karen Paczkowski; Laurent G.J. Montési, Maureen D. Long, Christopher J. Thissen
      Pages: n/a - n/a
      Abstract: Three‐dimensional models of mantle flow at subduction zones make it possible to explain the common occurrence of trench‐parallel sub‐slab seismic anisotropy. Sub‐slab flow becomes inherently three‐dimensional when slab‐driven flow interacts with a wide variety of ambient background mantle flow conditions. This interaction depends on slab geometries, mechanical coupling parameters, and lower mantle viscosities. Deflection of sub‐slab mantle flow is a robust feature for all model parameters and geometries as the slab acts as an obstruction to the ambient, background mantle flow. Background mantle flow can become trench‐perpendicular or trench‐parallel sub‐slab flow depending on whether the ambient background mantle flow is deflected beneath the bottom of the slab or towards the edge of the slab. The first case is especially prominent in models with short slabs that do not penetrate into the lower mantle. The second case is especially prominent in models with long, steep slabs. The results are also highly sensitive to the amount of mechanical coupling between the subducting plate and the mantle beneath it. High levels of coupling create a boundary layer of trench‐perpendicular entrained flow, pushing the deflection due to the obstructing slab away from the slab. We compare our sub‐slab flow model predictions with a global set of seismic anisotropy fast directions in the sub‐slab mantle, and find generally good agreement between the anisotropy observations (dominantly trench‐parallel or trench‐perpendicular) and the mantle flow directions predicted for decoupled systems.
      PubDate: 2014-09-09T19:19:48.303475-05:
      DOI: 10.1002/2014GC005441
       
  • Tracking the exhumation of a pliocene (U)HP terrane: U‐Pb and
           trace‐element constraints from zircon, D'Entrecasteaux Islands,
           Papua New Guinea
    • Authors: Joel W. DesOrmeau; Stacia M. Gordon, Timothy A. Little, Samuel A. Bowring
      Pages: n/a - n/a
      Abstract: Domal structures within the D'Entrecasteaux Islands of eastern Papua New Guinea expose ultrahigh‐pressure (UHP) Pliocene (5.6–4.6 Ma) eclogites and evidence for partial melting. To better interpret the (U)HP exhumation history, U‐Pb geochronology and trace‐element abundances were determined in zircon from variably deformed host gneiss and crystallized melt (leucosomes, sills, dikes, and plutons) from the Goodenough and Normanby Domes by ID‐TIMS (isotope‐dilution thermal ionization mass spectrometry) and ICP‐MS (inductively coupled plasma mass spectrometry), respectively, to constrain the timing of melt crystallization and deformation relative to UHP metamorphism. Zircons extracted from orthogneiss and deformed granodiorite sills of Normanby Dome, located ~40 km southeast of the UHP eclogite, record HP metamorphism from 5.66 ± 0.02 Ma to 5.04 ± 0.07 Ma and melt crystallization at ca. 4.1 Ma. Strongly deformed, layer‐parallel leucosomes from Goodenough Dome, ~20 km northwest of the UHP eclogite, began to crystallize by 3.85 ± 0.02 Ma. These dates indicate that melt crystallization began in the Goodenough and Normanby Domes within ~0.75 m.y. of (U)HP metamorphism. The ID‐TIMS dates from the orthogneiss and crystallized melt show that exhumation and cooling of the (U)HP rocks in the PNG terrane began first in the east, within Normanby Dome, then to the west, in the Goodenough Dome ~ 1 m.y. later, and finally the middle dome rocks, exposed within the Mailolo Dome, cooled ~ 2 m.y. after exhumation of Normanby Dome. All domes reveal synchronous crystallization of late, non‐deformed melts and final extension‐driven exhumation by 1.82 ± 0.03 Ma.
      PubDate: 2014-09-05T11:34:36.272146-05:
      DOI: 10.1002/2014GC005396
       
  • A family of repeating low‐frequency earthquakes at the downdip edge
           of tremor and slip
    • Authors: Justin R Sweet; Kenneth C Creager, Heidi Houston
      Pages: n/a - n/a
      Abstract: We analyze an isolated Low‐Frequency Earthquake (LFE) family located at the downdip edge of the main episodic tremor and slip (ETS) zone beneath western Washington State. The 9000 individual LFEs from this repeating family cluster into 198 swarms that recur roughly every week. Cumulative LFE seismic moment for each swarm correlates strongly with the time until the next swarm, suggesting that these LFE swarms are time‐predictable. Precise double‐difference relative locations for 700 individual LFEs within this family show a distribution that is approximately 2 km long and 500 m wide, elongated parallel to the relative plate convergence direction. The distribution of locations (
      PubDate: 2014-09-02T04:22:15.428687-05:
      DOI: 10.1002/2014GC005449
       
  • Regional study of the archean to proterozoic crust at the Sudbury Neutrino
           Observatory (SNO+), Ontario: Predicting the geoneutrino flux
    • Authors: Yu Huang; Virginia Strati, Fabio Mantovani, Steven B. Shirey, William F. McDonough
      Pages: n/a - n/a
      Abstract: The SNO+ detector that is currently under construction in Ontario, Canada, will be a new kiloton scale liquid scintillation detector with the capability of recording geoneutrino events that can be used to constrain the strength of the Earth's radiogenic power, and in turn, to test compositional models of the bulk silicate Earth (BSE). We constructed a detailed 3‐D model of the regional crust centered at SNO+ from compiled geological, geophysical and geochemical information. Crustal cross sections obtained from refraction and reflection seismic surveys were used to characterize the crust and assign uncertainties to its structure. The average Moho depth in the study area is 42.3 ± 2.6 km. The upper crust was divided into seven dominant lithologic units on the basis of regional geology. The abundances of U and Th and their uncertainties in each upper crustal lithologic unit were determined from analyses of representative outcrop samples. The average chemical compositions of the middle and lower crust beneath the SNO+ region were determined by coupling local seismic velocity profiles with a global compilation of the chemical compositions of amphibolite and granulite facies rocks. Monte Carlo simulations were used to predict the geoneutrino signal originating from the regional crust at SNO+ and to track asymmetrical uncertainties of U and Th abundances. The total regional crust contribution of the geoneutrino signal at SNO+ is predicted to be TNU (a Terrestrial Neutrino Unit is one geoneutrino event per 1032 target protons per year), with the Huronian Supergroup near SNO+ dominantly contributing TNU to this total. Future systematically sampling of this regional unit and denser seismic surveys will better model its composition and structure, and thus reduce the uncertainty on geoneutrino signal at SNO+. The bulk crustal geoneutrino signal at SNO+ is estimated to be TNU, which is lower than that predicted in a global scale reference model that uses an average composition of the global upper continental crust [Huang et al., 2013], due to the fact that Archean to Proterozoic Canadian Shield has lower U and Th concentrations. Finally, without accounting for uncertainties on the signal from continental lithospheric mantle and convecting mantle, the total geoneutrino signal at SNO+ is predicted to be TNU
      PubDate: 2014-09-02T03:28:11.190576-05:
      DOI: 10.1002/2014GC005397
       
  • A geodetic plate motion and global strain rate model
    • Authors: Corné Kreemer; Geoffrey Blewitt, Elliot C. Klein
      Pages: n/a - n/a
      Abstract: We present a new global model of plate motions and strain rates in plate boundary zones constrained by horizontal geodetic velocities. This Global Strain Rate Model (GSRM v.2.1) is a vast improvement over its predecessor both in terms of amount of data input as in an increase in spatial model resolution by factor of ˜2.5 in areas with dense data coverage. We determined 6739 velocities from time‐series of (mostly) continuous GPS measurements; i.e., by far the largest global velocity solution to date. We transformed 15772 velocities from 233 (mostly) published studies onto our core solution to obtain 22511 velocities in the same reference frame. Care is taken to not use velocities from stations (or time periods) that are affected by transient phenomena; i.e., this data‐set consists of velocities best representing the interseismic plate velocity. About 14% of the Earth is allowed to deform in 145,086 deforming grid cells (0.25º longitude by 0.2º latitude in dimension). The remainder of the Earth’s surface is modeled as rigid spherical caps representing 50 tectonic plates. For 36 plates we present new GPS‐derived angular velocities. For all the plates that can be compared with the most recent geologic plate motion model, we find that the difference in angular velocity is significant. The rigid‐body rotations are used as boundary conditions in the strain rate calculations. The strain rate field is modeled using the Haines and Holt method, which uses splines to obtain an self‐consistent interpolated velocity gradient tensor field, from which strain rates, vorticity rates, and expected velocities are derived. We also present expected faulting orientations in areas with significant vorticity, and update the no‐net rotation reference frame associated with our global velocity gradient field. Finally, we present a global map of recurrence times for Mw=7.5 characteristic earthquakes.
      PubDate: 2014-08-30T09:03:17.45883-05:0
      DOI: 10.1002/2014GC005407
       
  • Source‐related variables for the description of the oceanic carbon
           system
    • Authors: G. Walin; J. Hieronymus, J. Nycander
      Pages: n/a - n/a
      Abstract: The oceanic carbon system is commonly described in terms of the two state variables total carbon, DIC, and alkalinity, Alk. Here we suggest the use of alternative source adapted state variables, Acidic Carbon, AC and Basic Carbon, BC, defined by and related to (DIC, Alk) with a simple linear transformation. (AC, BC) can be interpreted as representing respectively the supply to the system of carbon dioxide and dissolved carbonate, keeping in mind that supply of hydrogen ions acts to transform from basic carbon to acidic carbon. Accordingly these variables tell us how much carbon dioxide or dissolved carbonate we actually have in the water, despite the fact that the major part of the carbon resides in bicarbonate ions. We claim that using these source related variables as a compliment to the traditional variables, offers a number of advantages in the formulation of continuity equations, as well as in the interpretation of observations and modelling results. The traditional definition of alkalinity is related to a measuring procedure rather than to the supply of material to the system. Here we demonstrate that alkalinity, though defined in the traditional way, may be interpreted in terms of sources and sinks acting on the system. In the case of ocean water this amounts to twice the supply of dissolved carbonate minus the net supply of free hydrogen ions. We argue that this interpretation is a useful complement to the traditional definition. Every process that affects the state of the carbon system may be quantified in terms of supply of carbon dioxide, Fa, carbonate ions, Fb, or hydrogen ions, E.
      PubDate: 2014-08-27T11:35:54.719499-05:
      DOI: 10.1002/2014GC005383
       
  • The magnitude distribution of dynamically triggered earthquakes
    • Authors: Stephen Hernandez; Emily E. Brodsky, Nicholas J. van der Elst
      Pages: n/a - n/a
      Abstract: Large dynamic strains carried by seismic waves are known to trigger seismicity far from their source region. It is unknown, however, whether surface waves trigger only small earthquakes, or whether they can also trigger large earthquakes. To partially address this question, we evaluate whether current data can distinguish between the magnitude distribution of triggered and untriggered small earthquakes. We use a mixing model approach in which total seismicity is decomposed into 2 classes: “triggered” events initiated or advanced by far‐field dynamic strains, and “untriggered” spontaneous events consisting of everything else. The b‐value of a mixed data set, bMIX, is decomposed into a weighted sum of b‐values of its constituent components, bT and bU. We utilize the previously observed relationship between triggering rate and dynamic strain amplitude to identify the fraction of triggered events in populations of earthquakes and then invert for bT. For Californian seismicity, data are consistent with a single‐parameter Gutenberg‐Richter hypothesis governing the magnitudes of both triggered and untriggered earthquakes.
      PubDate: 2014-08-27T11:18:34.072837-05:
      DOI: 10.1002/2014GC005404
       
  • Integrated geophysical‐petrological modeling of
           lithosphere‐asthenosphere boundary in central Tibet using
           electromagnetic and seismic data
    • Authors: Jan Vozar; Alan G. Jones, Javier Fullea, Matthew R. Agius, Sergei Lebedev, Florian Le Pape, Wenbo Wei
      Pages: n/a - n/a
      Abstract: We undertake a petrologically‐driven approach to jointly model magnetotelluric (MT) and seismic surface wave dispersion (SW) data from central Tibet, constrained by topographic height. The approach derives realistic temperature and pressure distributions within the upper mantle and characterizes mineral assemblages of given bulk chemical compositions as well as water content. This allows us to define a bulk geophysical model of the upper mantle based on laboratory and xenolith data for the most relevant mantle mineral assemblages and to derive corresponding predicted geophysical observables. One‐dimensional deep resistivity models were derived for two groups of MT stations. One group, located in the Lhasa Terrane, shows the existence of an electrically conductive upper‐mantle layer and shallower conductive upper‐mantle layer for the other group, located in the Qiangtang Terrane. The subsequent one‐dimensional integrated petrological‐geophysical modeling suggests a lithosphere‐asthenosphere boundary (LAB) at a depth of 80‐120 km with a dry lithosphere for the Qiangtang Terrane. In contrast, for the Lhasa Terrane the LAB is located at about 180 km but the presence of a small amount of water in the lithospheric mantle (
      PubDate: 2014-08-27T11:15:38.339924-05:
      DOI: 10.1002/2014GC005365
       
  • Cenozoic epeirogeny of the Arabian Peninsula from drainage modeling
    • Authors: J. W. P. Wilson; G. G. Roberts, M. J. Hoggard, N. J. White
      Pages: n/a - n/a
      Abstract: It is generally accepted that the Arabian Peninsula has been uplifted by sub‐crustal processes. Positive residual depth anomalies from oceanic crust in the Red Sea and in the Gulf of Aden suggest that a region surrounding this peninsula is dynamically supported. Admittance calculations, surface wave tomography studies, and receiver function analyses all imply that regional topography is generated and maintained by some combination of mantle convective circulation and lithospheric thickness changes. Despite these significant advances, the spatial and temporal uplift rate history of the Arabian Peninsula is not well known. Here, we show that a regional uplift rate history can be obtained by jointly inverting 225 longitudinal river profiles that drain this peninsula. Our strategy assumes that shapes of individual river profiles are controlled by uplift rate history and moderated by erosional processes. We used local measurements of incision rate to calibrate the relevant erosional parameters. In our inverse algorithm, uplift rate is permitted to vary smoothly as a function of space and time but upstream drainage area remains invariant. We also assume that knickzone migration is not lithologically controlled. Implications of these important assumptions have been investigated. Our results suggest that the Arabian Peninsula underwent two phase asymmetric uplift during the last 20–30 Ma at rates of 0.05–0.1 mm/yr– 1. The southwestern flank of the peninsula has been uplifted by 1.5–2.5 km. Regional stratigraphic constraints, the age and composition of volcanism, paleosol formation, incised peneplains, emergent marine terraces, and thermochronometric measurements corroborate our calculated patterns of uplift. Progessive development of three domal swells along the western margin of the peninsula is consistent with localized upwelling of hot asthenospheric mantle.
      PubDate: 2014-08-27T00:02:26.109071-05:
      DOI: 10.1002/2014GC005283
       
  • Tectonics, topography, and river system transition in East Tibet: Insights
           from the sedimentary record in Taiwan
    • Authors: Qing Lan; Yi Yan, Chi‐Yue Huang, Peter D. Clift, Xuejie Li, Wenhuang Chen, Xingchang Zhang, Mengming Yu
      Pages: n/a - n/a
      Abstract: [1] The Cenozoic in East Asia is marked by major changes in tectonics, landscapes and river systems, although the timing and nature of such changes remains disputed. We investigate the geochemistry and neodymium isotope character of Cenozoic mudstones spanning the breakup‐unconformity in the Western Foothills of Taiwan in order to constrain erosion and drainage development in southern China during the opening of the South China Sea. The La/Lu, Eu/Eu*, Th/Sc, Th/La, Cr/Th and εNd values in these rocks show an abrupt change between ca. 31 Ma and 25 Ma. Generally the higher εNd values in sediments deposited prior to 31 Ma indicate erosion from Phanerozoic granitic sources exposed in coastal South China, whereas the lower εNd values suggest that the main sources had evolved to inland southern China by ca. 25 Ma. The SHRIMP U‐Pb ages of zircons from a tuff, together with biostratigraphy data constrain the breakup‐unconformity to be between ca. 39 and 33 Ma, suggesting that the seafloor spreading in the South China Sea commenced before ca. 33 Ma. This is significantly older than most of the oceanic crust preserved in the deeper part of the basin. Diachronous westward younging of the breakup‐unconformities and provenance changes of basins are consistent with seafloor spreading propagating from east to west. Initial spreading of the South China Sea prior to ca. 33 Ma corresponds to tectonic adjustment in East Asia, including extrusion of the Indochina block and the rotation and eastward retreat of the subducting Pacific Plate.
      PubDate: 2014-08-26T05:14:49.41221-05:0
      DOI: 10.1002/2014GC005310
       
  • The relationship between seismicity and fault structure on the Discovery
           transform fault, East Pacific Rise
    • Authors: Monica Wolfson‐Schwehr; Margaret S. Boettcher, Jeffrey J. McGuire, John A. Collins
      Pages: n/a - n/a
      Abstract: There is a global seismic moment deficit on mid‐ocean ridge transform faults, and the largest earthquakes on these faults do not rupture the full fault area. We explore the influence of physical fault structure, including step‐overs in the fault trace, on the seismic behavior of the Discovery transform fault, 4S on the East Pacific Rise. One year of microseismicity recorded during a 2008 ocean bottom seismograph deployment (24,377 0 ≤ ML ≤ 4.6 earthquakes) and 24 years of Mw ≥ 5.4 earthquakes obtained from the Global Centroid Moment Tensor catalog, are correlated with surface fault structure delineated from high‐resolution multibeam bathymetry. Each of the 15 5.4 ≤ Mw ≤ 6.0 earthquakes that occurred on Discovery between January 1, 1990 ‐ April 1, 2014 was relocated into one of five distinct rupture patches using a teleseismic surface wave cross‐correlation technique. Microseismicity was relocated using the HypoDD relocation algorithm. The western fault segment of Discovery (DW) is composed of three zones of varying structure and seismic behavior: a zone with no large events and abundant microseismicity, a fully coupled zone with large earthquakes, and a complex zone with multiple fault strands and abundant seismicity. In general, microseismicity is reduced within the patches defined by the large, repeating earthquakes. While the extent of the large rupture patches on DW correlates with physical features in the bathymetry, step‐overs in the primary fault trace are not observed at patch boundaries, suggesting along‐strike heterogeneity in fault zone properties controls the size and location of the large events.
      PubDate: 2014-08-21T11:18:08.798583-05:
      DOI: 10.1002/2014GC005445
       
  • History and dynamics of net rotation of the mantle and lithosphere
    • Authors: M. L. Rudolph; S.J. Zhong
      Pages: n/a - n/a
      Abstract: The net rotation of Earth's lithosphere with respect to the underlying mantle is the longest‐wavelength component of toroidal flow in the mantle and is sensitive to both mantle buoyancy structure and lateral viscosity variations. The lithospheric net rotation in the geologic past implied by plate reconstructions using a hotspot reference frame for the past 100 Myr is up to five times greater than the present‐day rate of lithospheric net rotation. We explore the role of lateral viscosity variations associated with subcontinental keels in producing the lithospheric net rotation for the geologic past and find that the introduction of subcontinental keels improves the agreement between modeled net rotation and the net rotation present in the plate reconstructions for the past 25 Myr. However, our models with continental keels produce at most 0.16°/Myr of differential rotation between the lithosphere and lower mantle for present‐day, and explaining the most rapid rates of lithospheric net rotation during the Cretaceous and Paleogene remains challenging. This suggests the need for either an additional mechanism for generating lithospheric net rotation, or an adjustment to the absolute mantle reference frame relative to which plate motions are specified.
      PubDate: 2014-08-18T22:36:58.979803-05:
      DOI: 10.1002/2014GC005457
       
  • Ambient tectonic stress as fragile geological feature
    • Authors: Norman H. Sleep
      Pages: n/a - n/a
      Abstract: Strong seismic waves produce frictional failure within shallow pervasively cracked rocks. Distributed failure preferentially relaxes ambient tectonic stresses, providing a fragility measure of past strong shaking. Relaxation of the regional fault‐normal compression appears to have occurred within granite from 768 m down to ~1000‐1600 m depth at the Pilot Hole near Parkfield, California. Subsequent movements on the main fault have imposed strike‐slip stress within the relaxed region. Peak ground velocities of ~2 m s‐1 are inferred for infrequent (few 1000 yr recurrence) past earthquakes from stress relaxation within the granite and from the variation of S‐wave velocity with depth in the overlying sandstone. Conversely, frequent strong shaking in slowly deforming regions relaxes shallow ambient tectonic stress. This situation is expected beneath Whittier Narrows, where strong Love waves from numerous San Andreas events repeatedly produced nonlinear behavior.
      PubDate: 2014-08-18T03:21:00.697367-05:
      DOI: 10.1002/2014GC005426
       
  • Reconciling mantle attenuation‐temperature relationships from
           seismology, petrology, and laboratory measurements
    • Authors: GA Abers; KM Fischer, G Hirth, DA Wiens, T Plank, BK Holtzman, C McCarthy, E. Gazel
      Pages: n/a - n/a
      Abstract: Seismic attenuation measurements provide a powerful tool for sampling mantle properties. Laboratory experiments provide calibrations at seismic frequencies and mantle temperatures for dry melt‐free rocks, but require ~102 ‐ 103 extrapolations in grain size to mantle conditions; also, the effects of water and melt are not well understood. At the same time, body‐wave attenuation measured from dense broadband arrays provides reliable estimates of shear‐wave attenuation (QS‐1), affording an opportunity for calibration. We reanalyze seismic datasets that sample arc and back‐arc mantle, in Central America, the Marianas and the Lau basin, confirming very high attenuation (QS ~ 25‐80) at 1 Hz and depths of 50‐100 km. At each of these sites, independent petrological studies constrain the temperature and water content where basaltic magmas last equilibrated with the mantle, 1300‐1450°C. The QS measurements correlate inversely with the petrologically‐inferred temperatures, as expected. However, dry attenuation models predict QS too high by a factor of 1.5‐5. Modifying models to include effects of H2O and rheology‐dependent grain size shows that the effects of water‐enhanced dissipation and water‐enhanced grain growth nearly cancel, so H2O effects are modest. Therefore high H2O in the arc source region cannot explain the low QS, nor in the backarc where lavas show modest water content. Most likely the high attenuation reflects the presence of melt, and some models of melt effects come close to reproducing observations. Overall, body‐wave QS can be reconciled with petrologic and laboratory inferences of mantle conditions if melt has a strong influence beneath arcs and backarcs.
      PubDate: 2014-08-16T01:24:55.445493-05:
      DOI: 10.1002/2014GC005444
       
  • Identification and environmental interpretation of diagenetic and biogenic
           greigite in sediments: A lesson from the Messinian Black Sea
    • Authors: Liao Chang; Iuliana Vasiliev, Christiaan van Baak, Wout Krijgsman, Mark J. Dekkers, Andrew P. Roberts, John D. Fitz Gerald, Annelies van Hoesel, Michael Winklhofer
      Pages: n/a - n/a
      Abstract: Greigite (Fe3S4) is a widespread authigenic magnetic mineral in anoxic sediments, and is also commonly biosynthesized by magnetotactic bacteria in aqueous environments. While the presence of fossilized bacterial magnetite (Fe3O4) has now been widely demonstrated, the preservation of greigite magnetofossils in the geological record is only poorly constrained. Here, we investigate Mio‐Pliocene sediments of the former Black Sea to test whether we can detect greigite magnetofossils, and to unravel potential environmental controls on greigite formation. Our magnetic analyses and transmission electron microscope (TEM) observations indicate the presence of both diagenetic and bacterial greigite, and suggest a potentially widespread preservation of greigite magnetofossils in ancient sediments, which has important implications for assessing the reliability of paleomagnetic records carried by greigite. TEM‐based chemical and structural analyses also indicate the common presence of nickel‐substituted diagenetic iron sulfide crystals with a ferrimagnetic greigite structure. In addition, our cyclostratigraphic framework allows correlation of magnetic properties of Messinian former Black Sea sediments (Taman Peninsula, Russia) to global climate records. Diagenetic greigite enhancements appear to be climatically controlled, with greigite mainly occurring in warm/wet periods. Diagenetic greigite formation can be explained by variations in terrigenous inputs and dissolved pore water sulfate concentrations in different sedimentary environments. Our analysis demonstrates the usefulness of greigite for studying long‐term climate variability in anoxic environments.
      PubDate: 2014-08-16T01:23:37.2059-05:00
      DOI: 10.1002/2014GC005411
       
  • Nd and Sr isotope compositions of different phases of surface sediments in
           the South Pacific: Extraction of seawater signatures, boundary exchange,
           and detrital/dust provenance
    • Authors: Mario Molina‐Kescher; Martin Frank, Ed C. Hathorne
      Pages: n/a - n/a
      Abstract: The radiogenic isotope composition of neodymium (Nd) and strontium (Sr) are useful tools to investigate present and past oceanic circulation or input of terrigenous material. We present Nd and Sr isotope compositions extracted from different sedimentary phases, including early diagenetic Fe‐Mn coatings, ‘unclean' foraminiferal shells, fossil fish teeth, and detritus of marine surface sediments (core‐tops) covering the entire mid‐latitude South Pacific. Comparison of detrital Nd isotope compositions to deep‐water values from the same locations suggest that ‘boundary exchange' has little influence on the Nd isotope composition of western South Pacific seawater. Concentrations of Rare Earth Elements (REE) and Al/Ca ratios of ‘unclean' planktonic foraminifera suggest that this phase is a reliable recorder of seawater Nd isotope composition. The signatures obtained from fish teeth and ‘non‐decarbonated' leachates of bulk sediment Fe‐Mn oxyhydroxide coatings also agree with ‘unclean' foraminifera. Direct comparison of Nd isotope compositions extracted using these methods with seawater Nd isotope compositions is complicated by the low accumulation rates yielding radiocarbon ages of up to 24 kyrs, thus mixing the signal of different ocean circulation modes. This suggests that different past seawater Nd isotope compositions have been integrated in authigenic sediments from regions with low sedimentation rates. Combined detrital Nd and Sr isotope signatures indicate a dominant role of the Westerly winds transporting lithogenic material from South New Zealand and Southeastern Australia to the open South Pacific. The proportion of this material decreases towards the east, where supply from the Andes increases and contributions from Antarctica cannot be ruled out.
      PubDate: 2014-08-05T02:43:27.981471-05:
      DOI: 10.1002/2014GC005443
       
  • Metasomatized ancient lithospheric mantle beneath the young Zealandia
           microcontinent and its role in HIMU‐like intraplate magmatism
    • Authors: JM Scott; TE Waight, QHA van der Meer, JM Palin, AF Cooper, C Münker
      Pages: n/a - n/a
      Abstract: There has been long debate on the asthenospheric versus lithospheric source for numerous intraplate basalts with ocean island basalt (OIB) and high time‐integrated U/Pb (HIMU)‐like source signatures that have erupted through the Zealandia continental crust. Analysis of 157 spinel facies peridotitic mantle xenoliths from 25 localities across Zealandia permits the first comprehensive regional description of the sub‐continental lithospheric mantle (SCLM) and insights into whether it could be a source to the intraplate basalts. Contrary to previous assumptions, the Oligocene‐Miocene Zealandia SCLM is highly heterogeneous. It is composed of a refractory craton‐like domain (West Otago) adjacent to several moderately fertile domains (East Otago, North Otago, Auckland Islands). Each domain has an early history decoupled from the overlying Carboniferous and younger continental crust, and each domain has undergone varying degrees of depletion followed by re‐enrichment. Clinopyroxene grains reveal trace element characteristics (low Ti/Eu, high Th/U) consistent with enrichment through reaction with carbonatite. This metasomatic overprint has a composition that closely matches HIMU in Sr, Pb + Nd isotopes. However, clinopyroxene Hf isotopes are in part highly radiogenic and decoupled from the other isotope systems, and also mostly more radiogenic than the intraplate basalts. If the studied spinel facies xenoliths are representative of the thin Zealandia SCLM, the melting of garnet facies lithosphere could only be the intraplate basalt source if it had a less radiogenic Hf‐Nd isotope composition than the investigated spinel facies, or was mixed asthenospheric‐derived melts containing less radiogenic Hf.
      PubDate: 2014-08-02T03:49:45.763415-05:
      DOI: 10.1002/2014GC005300
       
  • Persistent uplift of the Lazufre volcanic complex (Central Andes): New
           insights from PCAIM inversion of InSAR time series and GPS data
    • Authors: D. Remy; J.L Froger, H Perfettini, S. Bonvalot, G. Gabalda, F. Albino, V. Cayol, D. Legrand, M. De Saint Blanquat
      Pages: n/a - n/a
      Abstract: We reanalyzed the surface Displacements observed at the Lazufre Volcanic Complex in the Southern Andean Central Volcanic Zone using GPS measurements made between 2006 and 2008 and a large InSAR dataset. We performed a detailed spatiotemporal analysis of the displacements using a Principal Component Analysis Inversion Method (PCAIM). The PCAIM reveals a source with no significant changes in shape and dimension and with a remarkably linear strength increase over the whole period of observation (i.e. 2003 to 2010). Then we used a three‐dimensional Mixed Boundary Element Method (MBEM) to invert the first component of surface displacement as obtained from PCAIM. We explored a continuum of geometries from a shallow elliptic crack to a deep massive truncated elliptical cone that could represent a sill or a large magma chamber, respectively. The best models indicate a large flat‐topped source with a roof area between 40 and 670 km2 and a depth of between 2 and 14 km below ground surface. Lastly, on the basis of the limited data available for the thermo‐mechanical structure of the crust in the Southern Andean Central Volcanic Zone we consider some possible scenarios to explain the spatial and temporal pattern of displacements at Lazufre.
      PubDate: 2014-07-31T03:17:28.850604-05:
      DOI: 10.1002/2014GC005370
       
  • Coring disturbances in IODP piston cores with implications for offshore
           record of volcanic events and the Missoula megafloods
    • Authors: Martin Jutzeler; James D. L. White, Peter J. Talling, Molly McCanta, Sally Morgan, Anne Le Friant, Osamu Ishizuka
      Pages: n/a - n/a
      Abstract: Piston cores collected from IODP drilling platforms (and its predecessors) provide the best long‐term geological and climatic record of marine sediments worldwide. Coring disturbances affecting the original sediment texture have been recognized since the early days of coring, and include deformation resulting from shear of sediment against the core barrel, basal flow‐in due to partial stroke, loss of stratigraphy, fall‐in, sediment loss through core catchers, and structures formed during core recovery and on‐deck transport. The most severe disturbances occur in non‐cohesive (sandy) facies, which are particularly common in volcanogenic environments and submarine fans. Although all of these types of coring disturbances have been recognized previously, our contribution is novel because it provides an easily accessible summary of methods for their identification. This contribution gives two specific examples on the importance of these coring disturbances. We show how suck‐in of sediments during coring artificially created very thick volcaniclastic sand layers in cores offshore Montserrat and Martinique (Lesser Antilles). We then analyze very thick, structureless sand layers from the Escanaba Trough inferred to be a record of the Missoula mega‐floods. These sand layers tend to coincide with the base of core sections, and their facies suggest coring disturbance by basal flow‐in, destroying the original structure and texture of the beds. We conclude by outlining and supporting IODP‐led initiatives to further reduce and identify coring disturbances, and acknowledge their recent successes in drilling challenging sand‐rich settings, such as during IODP Expedition 340.
      PubDate: 2014-07-24T04:22:23.637509-05:
      DOI: 10.1002/2014GC005447
       
  • Tectonic structure, lithology, and hydrothermal signature of the Rainbow
           massif (Mid‐Atlantic Ridge 36°14’N)
    • Authors: Muriel Andreani; Javier Escartin, Adélie Delacour, Benoit Ildefonse, Marguerite Godard, Jérôme Dyment, Anthony E. Fallick, Yves Fouquet
      Pages: n/a - n/a
      Abstract: Rainbow is a dome‐shaped massif at the 36°14’N non‐transform offset along the Mid‐Atlantic Ridge. It hosts three ultramafic‐hosted hydrothermal sites: Rainbow is active and high‐temperature; Clamstone and Ghost City are fossil and low‐temperature. The MoMARDREAM cruises (2007, 2008) presented here provided extensive rock sampling throughout the massif that constrains the geological setting of hydrothermal activity. The lithology is heterogeneous with abundant serpentinites surrounding gabbros, troctolites, chromitites, plagiogranites, and basalts. We propose that a W‐dipping detachment fault, now inactive, uplifted the massif and exhumed these deep‐seated rocks. Present‐day deformation is accommodated by SSW‐NNE faults and fissures, consistent with oblique teleseismic focal mechanisms and stress rotation across the discontinuity. Faults localize fluid flow and control the location of fossil and active hydrothermal fields that appear to be ephemeral and lacking in spatio‐temporal progression. Markers of high‐temperature hydrothermal activity (˜350°C) are restricted to some samples from the active field while a more diffuse, lower‐temperature hydrothermal activity (
      PubDate: 2014-07-12T06:30:09.246195-05:
      DOI: 10.1002/2014GC005269
       
 
 
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