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

Geochemistry, Geophysics, Geosystems     Full-text available via subscription   (21 followers)
Geophysical Research Letters     Full-text available via subscription   (41 followers)
Global Biogeochemical Cycles     Full-text available via subscription   (3 followers)
Journal of Advances in Modeling Earth Systems     Open Access   (2 followers)
Journal of Geophysical Research : Atmospheres     Partially Free   (18 followers)
Journal of Geophysical Research : Biogeosciences     Full-text available via subscription   (5 followers)
Journal of Geophysical Research : Earth Surface     Partially Free   (22 followers)
Journal of Geophysical Research : Oceans     Partially Free   (14 followers)
Journal of Geophysical Research : Planets     Full-text available via subscription   (12 followers)
Journal of Geophysical Research : Solid Earth     Full-text available via subscription   (21 followers)
Journal of Geophysical Research : Space Physics     Full-text available via subscription   (13 followers)
Paleoceanography     Full-text available via subscription   (4 followers)
Radio Science     Full-text available via subscription   (3 followers)
Reviews of Geophysics     Full-text available via subscription   (17 followers)
Space Weather     Full-text available via subscription   (3 followers)
Tectonics     Full-text available via subscription   (7 followers)
Water Resources Research     Full-text available via subscription   (83 followers)
Geochemistry, Geophysics, Geosystems    [23 followers]  Follow    
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     ISSN (Online) 1525-2027
     Published by American Geophysical Union (AGU) Homepage  [17 journals]   [SJR: 2.156]   [H-I: 61]
  • Strain field analysis on Montserrat (W.I.) as tool for assessing permeable
           flow paths in the magmatic system of Soufrière Hills Volcano
    • Authors: Stefanie Hautmann; Fred Witham, Thomas Christopher, Paul Cole, Alan T. Linde, I. Selwyn Sacks, R. Stephen J. Sparks
      Pages: n/a - n/a
      Abstract: Strain dilatometers have been operated on the volcanic island of Montserrat (West Indies) for more than a decade and have proven to be a powerful technique to approach short‐term dynamics in the deformational field in response to pressure changes in the magmatic system of the andesitic dome‐building Soufrière Hills Volcano (SHV). We here demonstrate that magmatic activity in each of the different segments of the SHV magmatic system (shallow dyke‐conduit, upper and lower magma chambers) generates a characteristic strain pattern that allows the identification of operating sources in the plumbing system based on a simple scheme of amplitude ratios. We use this method to evaluate strain data from selected Vulcanian explosions and gas emission events that occurred at SHV between 2003 and 2012. Our results show that the events were initiated by a short phase of contraction of either one or both magma chambers and a simultaneous inflation of the shallow feeder system. The initial phase of the events usually lasted only tens to hundreds of seconds before the explosion/gas emission started and the system recovered. The short duration of this process points at rapid transport of fluids rather than magma ascent to generate the pressure changes. We suggest the propagation of tensile hydraulic fractures as viable mechanism to provide a pathway for fluid migration in the magmatic system at the observed time scale. Fluid mobilization was initiated by a sudden destabilization of large pockets of already segregated fluid in the magma chambers. Our study demonstrates that geodetic observables can provide unprecedented insights into complex dynamic processes within a magmatic system commonly assessed by theoretical modeling and petrologic observations.
      PubDate: 2014-03-21T10:56:51.172834-05:
      DOI: 10.1002/2013GC005087
  • Geochemistry of volcanic glasses from the Louisville Seamount Trail (IODP
           Expedition 330): Implications for eruption environments and mantle melting
    • Authors: Alexander R.L. Nichols; Christoph Beier, Philipp A. Brandl, Stefan H. Krumm
      Pages: n/a - n/a
      Abstract: Volcanic glasses recovered from four guyots during drilling along the Louisville Seamount Trail, southwest Pacific, have been analyzed for major, trace and volatile elements (H2O, CO2, S, Cl, F), and oxygen isotopes. Compared to other oceanic island settings they are geochemically homogeneous, providing no evidence of the tholeiitic stage that characterizes Hawai'i. The degrees and depth of partial melting remained constant over 1 – 3 Ma represented by the drill holes, and along‐chain over several million years. The only exception is Hadar Guyot with compositions that suggest small degree preferential melting of an enriched source, possibly because it erupted on the oldest and thickest lithosphere. Incompatible element enriched glass from late‐stage volcaniclastics imply lower degrees of melting as the volcanoes moved off the melting anomaly. Volcaniclastic glasses from throughout the igneous basement are degassed suggesting generation during shallow submarine eruptions (
      PubDate: 2014-03-21T04:39:06.102389-05:
      DOI: 10.1002/2013GC005086
  • Regional controls on magma ascent and storage in volcanic arcs
    • Authors: Estelle Chaussard; Falk Amelung
      Pages: n/a - n/a
      Abstract: Understanding the controls for magma ascent and storage depth is important for volcanic hazard assessment. Regional differences in the depth of magma storage between volcanic arcs suggest that the settings of subduction zones and of overriding plates influence how magma ascends through the crust. Here we use a compilation of data for 70 volcanoes in 15 volcanic regions to better understand the geodynamic controls on magma storage. We describe the subduction system, which consists of the subducting slab, the mantle wedge and the upper plate with 12 parameters encompassing the kinematics of the subduction, the structure and geometry of the slab, the timing of the subduction, the thermal structure of the slab, the upper‐plate crustal structure, its stress regimes, and its thermal structure. We find that the magma reservoir depths correlate with the upper‐plate crustal structure and with the stress regimes. Shallow reservoirs (25 km) than in thick crust (> 45 km). Similarly, shallow magma reservoirs are 33 to 69% more common in extensional and strike slip stress regimes that in compressional regimes. This illustrates the effect of buoyancy for magma ascent as well as the importance of stress and preexisting structures.
      PubDate: 2014-03-21T04:39:04.140411-05:
      DOI: 10.1002/2013GC005216
  • Paraguay‐Araguaia Belt Conductivity Anomaly: A fundamental tectonic
           boundary in South American Platform imaged by electromagnetic induction
    • Authors: M. S. Bologna; A. L. Padilha, M. B. Pádua, Í. Vitorello, F. H. Chamalaun
      Pages: n/a - n/a
      Abstract: We discovered and mapped a 1200 km‐long zone of enhanced electrical conductivity through a large array of magnetometers, over a narrow corridor parallel to the Paraguay and Araguaia belts which surround the Amazon craton. The anomaly has been further studied using magnetotelluric measurements made along a 300 km profile crossing the Paraguay belt; modeling shows the anomaly at this location as a very strong 100 km‐wide upper‐crust to mid‐crust conductor, located under a region where surficial rocks show an increase in deformation and metamorphism within the belt. From the high conductivity of the anomalous structure, its observed geometry and surface rock exposures, the most likely interpretation of its source is graphitized biogenic material in metasediments, now deeply underthrust in a Neoproterozoic or Early Cambrian suture zone. This result strongly supports the hypothesis that the collision between the Amazon plate and the western Paraná block has probably closed an ocean in the Paraguay belt region, contrary to former propositions of an ensialic evolution for this belt. Although the precise age of this ocean closure have been a matter of debate, our finding significantly constrains the tectonic setting of West Gondwana amalgamation involving the Amazon plate and its surrounding blocks.
      PubDate: 2014-03-20T14:17:26.927009-05:
      DOI: 10.1002/2013GC004970
  • Issue Information
    • Pages: i - i
      PubDate: 2014-03-20T13:56:16.905349-05:
      DOI: 10.1002/ggge.20318
  • Modified expression for bulb‐tracer depletion: Effect on argon
           dating standards
    • Authors: Robert J. Fleck; Andrew T. Calvert
      Pages: n/a - n/a
      Abstract: 40Ar/39Ar geochronology depends critically on well calibrated standards, often traceable to first‐principles K‐Ar age calibrations using bulb‐tracer systems. Tracer systems also provide precise standards for noble‐gas studies and interlaboratory calibration. The exponential expression long‐used for calculating isotope tracer concentrations in K‐Ar age dating and calibration of 40Ar/39Ar age standards may provide a close approximation of those values, but is not correct. Appropriate equations are derived that accurately describe the depletion of tracer reservoirs and concentrations of sequential tracers. The true form of the expression is a power law, not exponential, and a similar expression was presented by Miiller (2006, J. Res. Natl. Inst. Stand. Technol., 111(5), 335–360). Evaluation of the expressions demonstrates that systematic error introduced through use of the exponential approximation may be substantial where reservoir volumes are small and resulting depletion constants are large. Traditional use of large reservoir to tracer volumes and the resulting small depletion constants have kept errors well less than experimental uncertainties in most previous K‐Ar and calibration studies. Use of the proper expression, however, permits use of volumes appropriate to the problems addressed.
      PubDate: 2014-03-19T08:35:06.914566-05:
      DOI: 10.1002/2013GC005205
  • Laboratory formation of noncementing hydrates in sandy sediments
    • Authors: Jeong‐Hoon Choi; Sheng Dai, Jong‐Ho Cha, Yongkoo Seol
      Pages: n/a - n/a
      Abstract: Natural hydrate‐bearing sediment (HBS) predominantly exists in non‐cementing habit, and its limited availability for use in laboratory studies demands a time‐effective and repeatable laboratory process for forming representative samples with natural accumulation habit. This study reports on a three‐step laboratory process for forming non‐cementing methane hydrate in sandy sediments: (1) initial HBS formation under excess‐gas conditions; (2) slow saline water (5wt % CaCl2) injection under strictly controlled PT conditions; and (3) a temperature warming/cooling cycle. Changes in compressional wave velocity (Vp) of sediment, as well as pressure‐temperature (P‐T) condition, were monitored throughout the tests. The evolution of Vp, in good agreement with rock physics model calculations, suggested that the transition from cementing hydrate into non‐cementing hydrate occurs during saline injection as well as temperature warming/cooling cycle. The proposed process appeared to be an efficient and consistent substitute for the existing methods, to form non‐cementing hydrate habit in sandy sediments.
      PubDate: 2014-03-19T08:34:40.858647-05:
      DOI: 10.1002/2014GC005287
  • Relationship between the Cascadia forearc mantle wedge, nonvolcanic
           tremor, and the downdip limit of seismogenic rupture
    • Authors: Patricia A. McCrory; Roy D. Hyndman, J. Luke Blair
      Pages: n/a - n/a
      Abstract: Great earthquakes anticipated on the Cascadia subduction fault can potentially rupture beyond the geodetically and thermally inferred locked zone to the depths of episodic tremor and slip (ETS) or to the even deeper forearc mantle corner (FMC). To evaluate these extreme rupture limits, we map the FMC from southern Vancouver Island to central Oregon by combining published seismic velocity structures with a model of the Juan de Fuca plate. These data indicate that the FMC is somewhat shallower beneath Vancouver Island (36–38 km) and Oregon (35–40 km) and deeper beneath Washington (41–43 km). The updip edge of tremor follows the same general pattern, overlying a slightly shallower Juan de Fuca plate beneath Vancouver Island and Oregon (˜30 km) and a deeper plate beneath Washington (˜35 km). Similar to the Nankai subduction zone, the best constrained FMC depths correlate with the center of the tremor band suggesting that ETS is controlled by conditions near the FMC rather than directly by temperature or pressure. Unlike Nankai, a gap as wide as 70 km exists between the downdip limit of the inferred locked zone and the FMC. This gap also encompasses a ˜50 km wide gap between the inferred locked zones and the updip limit of tremor. The separation of these features offers a natural laboratory for determining the key controls on downdip rupture limits.
      PubDate: 2014-03-19T08:34:14.225511-05:
      DOI: 10.1002/2013GC005144
  • Strength of slab inferred from the seismic tomography and geologic history
           around the Japanese Islands
    • Authors: Satoru Honda
      Pages: n/a - n/a
      Abstract: I analyze the seismic tomography around the Japanese Islands to elucidate the present slab morphology. A simple conversion from the high speed to the temperature anomaly is done based on the recent study of the conversion coefficient from the velocity to the thermal anomaly and on the study of the relation between the seismicity and the temperature in the slab. I find recognizable amount of cold temperature anomalies under the subducting slab. This suggests a rather continuous slab throughout the mantle consistent with the results of many recent numerical models. However, there still is a gap in the slab or diluted slab under the transition zone. To understand this, I construct a simple half‐kinematic model of subduction zone in which a Byerlee's type yield stress and the depth independent yield stress are assumed. Taking into account the geologic history around the Japanese Islands, I find that the slab morphology similar to the tomographic image is obtained when the yield stress is O(100 MPa). The present study shows that the detailed studies of seismic tomography and the tectonic history of the surveyed area can provide the constraints on the slab dynamics.
      PubDate: 2014-03-19T08:04:54.365856-05:
      DOI: 10.1002/2014GC005225
  • Crustal thickness and Moho character of the fast‐spreading East
           Pacific Rise from 9°42′N to 9°57′N from
           poststack‐migrated 3‐D MCS data
    • Authors: Omid Aghaei; Mladen R. Nedimović, Helene Carton, Suzanne M. Carbotte, J. Pablo Canales, John C. Mutter
      Pages: n/a - n/a
      Abstract: We computed crustal thickness (5740 ± 270 m) and mapped Moho reflection character using 3‐D seismic data covering 658 km2 of the fast‐spreading East Pacific Rise (EPR) from 9°42′N to 9°57′N. Moho reflections are imaged within ∼87% of the study area. Average crustal thickness varies little between large sections of the study area suggesting regionally uniform crustal production in the last ∼180 Ka. However, individual crustal thickness measurements differ by as much as 1.75 km indicating that the mantle melt delivery has not been uniform. Third‐order, but not fourth‐order ridge discontinuities are associated with changes in the Moho reflection character and/or near‐axis crustal thickness. This suggests that the third‐order segmentation is governed by melt distribution processes within the uppermost mantle while the fourth‐order ridge segmentation arises from midcrustal to upper‐crustal processes. In this light, we assign fourth‐order ridge discontinuity status to the debated ridge segment boundary at ∼9°45′N and third‐order status at ∼9°51.5′N to the ridge segment boundary previously interpreted as a fourth‐order discontinuity. Our seismic results also suggest that the mechanism of lower‐crustal accretion varies along the investigated section of the EPR but that the volume of melt delivered to the crust is mostly uniform. More efficient mantle melt extraction is inferred within the southern half of our survey area with greater proportion of the lower crust accreted from the axial magma lens than that for the northern half. This south‐to‐north variation in the crustal accretion style may be caused by interaction between the melt sources for the ridge and the Lamont seamounts.
      PubDate: 2014-03-18T13:24:07.464179-05:
      DOI: 10.1002/2013GC005069
  • Influence of combined primordial layering and recycled MORB on the coupled
           thermal evolution of Earth's mantle and core
    • Authors: Takashi Nakagawa; Paul J. Tackley
      Pages: n/a - n/a
      Abstract: A thermo‐chemical mantle convection model with both primordial compositional layering and recycling of mid‐ocean ridge basalt (MORB) coupled to a parameterized core heat balance model is used to investigate how the thermo‐chemical evolution of the mantle affects the thermal history of the core including primordial material proposed by early Earth hypotheses. The viscosity formulation has been improved from our previous works. The amount of MORB that accumulates above the CMB is strongly dependent on effective Rayleigh number, such that more accumulates at higher Ra (lower viscosity), but a continuous layer of MORB is not obtained here. With initial primordial layering, large‐scale thermo‐chemical anomalies are found in the deep mantle, which are generated mainly by the primordial material with small amount of segregated basaltic material on top of it, localized in the hot upwelling region. A successful core evolution can only be obtained when initial primordial layering is present. In conclusion, primordial material above the CMB originated from early mantle differentiation might be needed to construct a realistic model of a coupled mantle and core evolution. However, in the current study, the convective vigor is lower than realistic and we only consider the case that primordial material is denser than MORB.
      PubDate: 2014-03-18T13:23:32.696056-05:
      DOI: 10.1002/2013GC005128
  • Geochemical and isotopic study of a plutonic suite and related early
           volcanic sequences in the southern Mariana forearc
    • Authors: Julie A. Johnson; Rosemary Hickey‐Vargas, Patricia Fryer, Vincent Salters, Mark K. Reagan
      Pages: n/a - n/a
      Abstract: The forearc of the southern Mariana arc preserves igneous suites formed during the initiation of subduction between the Pacific and Philippine Sea plates about 50 Ma ago. We have studied rare suites of gabbroic to tonalitic plutonic rocks dredged from two locations in the Mariana forearc by cruise by University of Hawai'i cruise KK81‐06‐26. Comparison of the chemical and isotopic (Sr, Nd, Pb, and Hf) characteristics of these rocks with well‐studied volcanics from the forearc reveals that the plutonics from dredge RD63 and RD64 are chemically related to boninites erupted at 48–43 Ma. This is the first report of boninite‐like plutonics in the southern Mariana trench. These suites have trace element characteristics consistent with island arc settings (U/Th: 0.58–1.44, Nb/La: 0.18–0.79) and other features uniquely connected with boninites: TiO2  25. RD63 plutonics resemble nearby boninite volcanics and were likely derived from differentiated boninite magma with 58% SiO2, forming gabbro by crystal accumulation, diorite and quartz diorite by crystallization, and tonalite by crystallization and/or partial melting. The RD64 suite (gabbro through tonalite) may have had a more depleted magma source and formed by accumulation and crystallization only. Although the physical dimensions of the plutonic body are unknown, the relationship with boninites indicates that felsic intrusives can form during early stages of island arc development. Such rocks could form part of midcrustal low‐velocity layers detected in arc crust by seismic studies. Tonalites similar to those studied here are also found in some ophiolites.
      PubDate: 2014-03-17T10:32:54.535447-05:
      DOI: 10.1002/2013GC005053
  • Effects of mineral composition and heterogeneity on the reservoir quality
           evolution with CO2 intrusion
    • Authors: Leilei Yang; Tianfu Xu, Bing Yang, Hailong Tian, Hongwu Lei
      Pages: n/a - n/a
      Abstract: During the diagenesis of rocks in a sedimentary basin, an intrusion of CO2 could cause dissolution or/and precipitation of the surrounding rocks. As a result, the reservoir quality may be altered. The mineral composition and heterogeneity have pronounced impacts on the geochemical reaction and reservoir quality evolution. A numerical simulation method is employed to investigate the influences of primary mineral on the diagenesis. Based on the measured data from the Songliao Basin, a total of 26 two‐dimensional models with different mineral composition are set up. To mimic the regionally heterogeneous distribution of mineral composition, the Monte Carlo method is employed. A CO2 gas reservoir of magma intrusion origin, located in the Songliao Basin, northeastern China, is selected for the present study. The reservoir is an ideal site for investigating the impact of mineral heterogeneity on diagenesis after the CO2 intrusion. In this reservoir, with the presence of high‐pressure CO2, the mineral heterogeneity causes significant dissolution and precipitation of minerals, which decreases the reservoir porosity and degrades the reservoir quality. The geochemical reactions caused by different mineral composition vary widely. The mineral heterogeneity causes similar distributions of the geochemical reactions and reservoir quality evolution. Dominant secondary minerals are dawsonite in the early diagenesis and ankerite in the late stage. The diagenetic sequences modeled by our numerical simulations reproduce the petrographic and geochemical data well.
      PubDate: 2014-03-17T10:32:16.470276-05:
      DOI: 10.1002/2013GC005157
  • Plate rotation during continental collision and its relationship with the
           exhumation of UHP metamorphic terranes: Application to the Norwegian
    • Authors: A. D. Bottrill; J. Hunen, S. J. Cuthbert, H.K. Brueckner, M. B. Allen
      Pages: n/a - n/a
      Abstract: Lateral variation and asynchronous onset of collision during the convergence of continents can significantly affect the burial and exhumation of subducted continental crust. Here, we use 3D numerical models for continental collision to discuss how deep burial and exhumation of high and ultrahigh pressure metamorphic (HP/UHP) rocks are enhanced by diachronous collision and the resulting rotation of the colliding plates. Rotation during collision locally favours eduction, the inversion of the subduction, and may explain the discontinuous distribution of ultra‐high pressure (UHP) terranes along collision zones. For example the terminal (Scandian) collision of Baltica and Laurentia, which formed the Scandinavian Caledonides, resulted in the exhumation of only one large HP/UHP terrane, the Western Gneiss Complex (WGC), near the southern end of the collision zone. Rotation of the subducting Baltica plate during collision may provide an explanation for this distribution. We explore this hypothesis by comparing orthogonal and diachronous collision models and conclude that a diachronous collision can transport continental material up to 60 km deeper, and heat material up to 300 °C hotter, than an orthogonal collision. Our diachronous collision model predicts that subducted continental margin material returns to the surface only in the region where collision initiated. The diachronous collision model is consistent with petrological and geochonological observations from the Western Gneiss Complex and makes predictions for the general evolution of the Scandinavian Caledonides. We propose the collision between Laurentia and Baltica started at the southern end of the collisional zone, and propagated northward. This asymmetric geometry resulted in the counter clockwise rotation of Baltica with respect to Laurentia, consistent with paleomagnetic data from other studies. Our model may have applications to other orogens with regional UHP terranes, such as the Dabie Shan and Papua New Guinea cases, where block rotation during exhumation has also been recorded.
      PubDate: 2014-03-17T01:54:57.518519-05:
      DOI: 10.1002/2014GC005253
  • Central Cascadia subduction zone creep
    • Authors: Gina M. Schmalzle; Robert McCaffrey, Kenneth C. Creager
      Pages: n/a - n/a
      Abstract: Central Cascadia between 43‐46°N has reduced interseismic uplift and coseismic subsidence from multiple thrust earthquakes suggesting elevated persistent fault creep in this section of the subduction zone. We estimate subduction thrust 'decade‐scale' locking and crustal block rotations from three‐component continuous Global Positioning System (GPS) time series from 1997 to 2013, as well as 80‐year tide gauge and leveling‐derived uplift rates. Geodetic observations indicate coastal central Oregon is rising at a slower rate than coastal Washington, southern Oregon and northern California. Modeled locking distributions suggest a wide locking transition zone that extends inland under central Oregon. Paleoseismic records of multiple great earthquakes along Cascadia indicate less subsidence in central Oregon. The Cascade thrust under central Oregon may be partially creeping for at least 6500 years (the length of the paleoseismic record) reducing interseismic uplift and resulting in reduced coseismic subsidence. Large accretions of an Eocene age basalt (Siletzia terrane) between 43‐46°N may be less permeable compared to surrounding terranes, potentially increasing pore fluid pressures along the fault interface resulting in a wide zone of persistent fault creep. In a separate inversion, three‐component GPS time series from July 1, 2005 to January 1, 2011 are used to estimate upper plate deformation, locking between slow slip events (SSEs), slip from 16 SSEs and an earthquake mechanism. Cumulative SSEs and tectonic tremor are weakest between 43‐46°N where partial fault creep is increased and Siletzia terrane is thick, suggesting that surrounding rock properties may influence the mode of slip.
      PubDate: 2014-03-14T16:14:30.199189-05:
      DOI: 10.1002/2013GC005172
  • Abrupt climate‐induced changes in carbonate burial in the Arabian
           Sea: Causes and consequences
    • Authors: Pothuri Divakar Naidu; Arun Deo Singh, Raja Ganeshram, Shivranjan Kumar Bharti
      Pages: n/a - n/a
      Abstract: [1] We present high‐resolution records of aragonite contents and pteropods abundance in two sediment cores (SK 17 and MD 76‐131) within the Oxygen Minimum Zone (OMZ) of the eastern Arabian Sea. We show large increases in aragonite contents during glacial and particularly during stadials (Heinrich Events). Using aragonite content, pteropods abundance, organic carbon percentage and abundance of fertile (eutrophic) species of planktonic foraminifer we demonstrate that aragonite contents in the eastern Arabian Sea primarily reflects preservation linked to the deepening of Aragonite Compensation Depth (ACD) in the Arabian Sea. We show that these aragonite preservation events correspond with time equivalents of Henrich Events when Arabian Sea experienced large declines in monsoon driven productivity and greater penetration of Antarctica Intermediate Water (AAIW). Thus, pteropod preservation in the Arabian Sea appears to be linked to rapid climate change through atmospheric and oceanic teleconnections. We suggest that the role of aragonite carbonate production and burial in margins and the resultant CO2 climate feedback to rapid climate changes remains poorly constrained.
      PubDate: 2014-03-14T06:07:42.596561-05:
      DOI: 10.1002/2013GC005065
  • Constraining melt geometries beneath the Afar Depression, Ethiopia from
           teleseismic receiver functions: The anisotropic H‐κ stacking
    • Authors: J. O. S. Hammond
      Pages: n/a - n/a
      Abstract: Understanding the nature of the crust has long been a goal for seismologists when imaging the Earth. This is particularly true in volcanic regions where imaging melt storage and migration can have important implications for the size and nature of an eruption. Receiver functions and the H‐κ stacking (Hκ) technique are often used to constrain crustal thickness (H) and the ratio of P‐ to S‐wave velocities (κ). In this paper I show that it is essential to consider anisotropy when performing Hκ. I show that in a medium with horizontally transverse isotropy a strong variation in κ with back azimuth is present which characterises the anisotropic medium. In a vertically transverse isotropic medium no variation in κ with back azimuth is observed, but κ is increased across all back azimuths. Thus, estimates of κ are more difficult to relate to composition than previously thought. I extend these models to melt induced anisotropy and show that similar patterns are observed, but with more significant variations and increases in κ. Based on these observations I develop a new anisotropic H‐κ stacking technique which inverts Hκ data for melt fraction, aspect ratio and orientation of melt inclusions. I apply this to data for the Afar Depression and show that melt is stored in interconnected stacked sills in the lower crust, which likely supply the recent volcanic eruptions and dike intrusions. This new technique can be applied to any anisotropic medium where it can provide constraints on the average crustal anisotropy.
      PubDate: 2014-03-12T12:41:50.130235-05:
      DOI: 10.1002/2013GC005186
  • A Fortran visualization program for spherical data on a Yin‐Yang
    • Authors: Masato Yoshida; Akira Kageyama
      Pages: n/a - n/a
      Abstract: A Fortran 90 program to visualize data on the Yin‐Yang grid system is developed. The purpose of this study is to provide simulation researchers with a source code as a starting point of their own custom‐made visualization tools. A basic but sufficiently diverse set of visualization methods are implemented using a Fortran 90 binding for OpenGL for scalar and vector fields defined or simulated on the Yin‐Yang grid.
      PubDate: 2014-03-12T06:52:17.436225-05:
      DOI: 10.1002/2013GC004967
  • Lithospheric architecture of the Slave craton, northwest Canada, as
           determined from an interdisciplinary 3‐D model
    • Authors: D.B. Snyder; M.J. Hillier, B.A. Kjarsgaard, E.A. de Kemp, J.A. Craven
      Pages: n/a - n/a
      Abstract: Regional‐scale geologic structures characteristic of mantle lithosphere within cratons found in continent interiors are interpreted using geo‐registered diverse data sets from the Slave craton of northwest Canada. We developed and applied a new method for mapping seismic discontinuities in three dimensions using multi‐year observations at sparse, individual broadband receivers. New, fully 3‐D conductivity models used all available magnetotelluric data. Discontinuity surfaces and conductivity models were geo‐registered with previously published P‐wave and surface wave velocity models to confirm first‐order structures such as a mid‐lithosphere discontinuity. Our 3‐D model to 400 km depth was calibrated by ‘drill hole’ observations derived from xenolith suites extracted from kimberlites. A number of new structural discontinuities emerge from direct comparison of co‐registered data sets and models. Importantly we distinguish primary mantle layers from secondary features related to younger metasomatism. Sub‐horizontal Slave craton layers with tapered, wedge‐shaped margins indicate construction of the craton core at 2.7 Ga by underthrusting and flat stacking of lithosphere. Mapping of conductivity and metasomatism in 3‐D, the latter inferred via mineral recrystallization and resetting of isotopic ages in xenoliths, indicates overprinting of the primary layered structures. The observed distribution of relatively conductive mantle at 100–200 km depths is consistent with pervasive metasomatism; vertical ‘chimneys’ reaching to crustal depths in locations where kimberlites erupted or where Au mineralization is known.
      PubDate: 2014-03-11T11:01:47.824392-05:
      DOI: 10.1002/2013GC005168
  • Titanium concentration in quartz as a record of multiple deformation
           mechanisms in an extensional shear zone
    • Authors: William O. Nachlas; Donna L. Whitney, Christian Teyssier, Brian Bagley, Andreas Mulch
      Pages: n/a - n/a
      Abstract: Results of high precision analysis of Ti concentration ([Ti]) in quartz representing different recrystallization microstructures in a suite of progressively deformed quartzite mylonites show the effect of recrystallization on distribution of Ti in quartz. Petrographic observations and ion microprobe analysis reveals three texturally and geochemically distinct quartz microstructures in mylonites: (1) cores of recrystallized quartz ribbons preserve the highest [Ti] and are interpreted to have recrystallized via grain boundary migration recrystallization, (2) recrystallized rims and quartz grain margins preserve a lower and more variable [Ti] and are interpreted to reflect the combined influence of subgrain rotation and bulging recrystallization, and (3) neocrystallized quartz precipitated in dilatancy sites has low (~1 ppm) [Ti]. Muscovite in non‐mylonitic quartzite (at the base of the sampling traverse) is compositionally zoned, whereas muscovite in mylonitic quartzite shows a progressive decreasing in zoning with higher strain. Three‐dimensional phase distribution mapping using X‐ray computed tomography analysis of rock hand samples reveals that Ti‐bearing accessory phases are less abundant and more dispersed in higher strained mylonites compared to non‐mylonitic quartzite. This study demonstrates the influence of deformation and dynamic recrystallization on Ti substitution in quartz and evaluates the Ti buffering capacity of aqueous fluids (meteoric vs. metamorphic/magmatic) as well as the distribution and reactivity of Ti‐bearing accessory phases in a deforming quartzite. Results of this study suggest that Ti‐in‐quartz thermobarometry of deformed quartz is a sensitive technique for resolving the multi‐stage history of quartz deformation and recrystallization in crustal shear zones.
      PubDate: 2014-03-11T11:01:21.91265-05:0
      DOI: 10.1002/2013GC005200
  • Rock uplift and erosion rate history of the Bergell Intrusion from the
           inversion of low temperature thermochronometric data
    • Authors: Matthew Fox; Rebecca Reverman, Frédéric Herman, Maria G. Fellin, Pietro Sternai, Sean D. Willett
      Pages: n/a - n/a
      Abstract: The Bergell Intrusion (European Alps) was one of the first locations where thermochronometry was used to resolve changes in erosion rate, yet, relating these changes to variations in climate or in local tectonics remains challenging. One approach that enables changes in erosion rate to be related to changes in climate or rock uplift rate is to utilize landscape evolution models, as topographic response to these forcing parameters is unique. Furthermore, low temperature thermochronometric systems have the potential to resolve topography through time and thus topographic response. We present new (U–Th)/He data for samples collected across 2km of relief from the Bergell. The ages range from ~2 Ma to ~16 Ma and define an age elevation with an apparent exhumation rate of 0.1 km/Myr. In order to infer erosion rates, we use a thermo‐kinematic model to solve the heat equation in the crust, track material points through time and predict thermochronometric data. Paleo‐topography and erosion rate are parameterized using the stream power model. We find that rock uplift rates were 0.4 km/Myr from ~25 Ma to ~20 Ma and subsequently decreased to 0.05 km/Myr. This results in a gradual decrease in erosion rate from rates of 0.4 to 0.1 km/Myr. A recent increase in rock uplift rate at ~4 Ma to ~0.6 km/Myr is required to explain the youngest ages and high topographic relief.
      PubDate: 2014-03-08T05:40:28.003518-05:
      DOI: 10.1002/2013GC005224
  • Ongoing lithospheric removal in the western Mediterranean: Evidence from
           Ps receiver functions and thermobarometry of Neogene basalts (PICASSO
    • Authors: Sally Thurner; Imma Palomeras, Alan Levander, Ramon Carbonell, Cin‐Ty Lee
      Pages: n/a - n/a
      Abstract: The western Mediterranean tectonic system consists of the Betic Mountains in southern Spain and the Rif Mountains in northern Morocco curved around the back‐arc extensional Alboran basin. Multiple tectonic models have been developed to explain the coeval compressional and extensional tectonic processes that have affected the western Mediterranean since the Oligocene. In order to provide constraints on these evolutionary models, we use Ps teleseismic receiver functions (RF), thermobarometric analyses of post‐Oligocene basalts, and previous teleseismic tomography images to investigate the lithospheric structure of the region. Ps RFs were calculated using seismic data from 239 broadband seismic stations in southern Iberia and northern Morocco and thermobarometric analysis was performed on 19 volcanic samples distributed throughout the region. The RF images reveal a highly variable Moho depth (~25 km to ~55 km), as well as a strong positive, subMoho horizon between ~45 and ~80 km depth beneath the central Betic and Rif Mountains, which we interpret to be the top of the previously imaged Alboran Sea slab. Thermobarometric constraints from magmas in the eastern Betics and Rif indicate mantle melting depths between 40‐60 km, typical of melting depths beneath mid‐oceanic ridges where little to no lithosphere exists. Together, the RF and thermobarometric data suggest ongoing and recent slab detachment resulting from delamination of the continental lithosphere.
      PubDate: 2014-03-06T13:35:28.348244-05:
      DOI: 10.1002/2013GC005124
  • The effects of secondary mineral formation on Coe‐type
           paleointensity determinations: Theory and simulation
    • Authors: Xiangyu Zhao; Qingsong Liu, Greig A. Paterson, Huafeng Qin, Shuhui Cai, Yongjae Yu, Rixiang Zhu
      Pages: n/a - n/a
      Abstract: Thellier‐type experiments are the most widely applied approaches for determining the absolute paleointensities of Earth's magnetic field. One major problem, however, is that specimens are prone to thermal alteration due to the intensive thermal treatment during experiments. Linear Arai plots with acceptable partial thermal remanent magnetization (pTRM) checks have been considered as evidence for the absence of or negligible effects of thermal alteration and as reliable indicators of high‐quality paleointensity estimates. However, by simulating the Coe variant of the Thellier method on assemblages of single domain (SD) magnetite particles, it is demonstrated that new magnetic minerals, which form during thermal treatments, can result in linear, concave‐up, or concave‐down Arai plots depending on the magnetic properties of both the primary and secondary magnetic phases. Among this range of behavior, pseudo‐ideal Arai plots, which are linear with acceptable pTRM check statistics, would lead to paleointensity underestimates. It is further demonstrated that pTRM checks are proportional to the degree of underestimate with a magnetic granulometry dependency for SD particles. Due to the complexity of this dependency, pTRM check statistics are only comparable when specimens have similar magnetic properties. This suggests that a universal threshold for pTRM check statistics is not likely to be effective. Since the criteria of linearity and low pTRM check statistics are insufficient to guarantee the fidelity of the estimates auxiliary rock magnetic methods such as temperature dependent hysteresis parameters and anhysteretic remanent magnetization are highly recommended to identify the presence of alteration.
      PubDate: 2014-03-06T06:07:08.992537-05:
      DOI: 10.1002/2013GC005165
  • BurnMan: A lower mantle mineral physics toolkit
    • Authors: Sanne Cottaar; Timo Heister, Ian Rose, Cayman Unterborn
      Pages: n/a - n/a
      Abstract: We present BurnMan, an open‐source mineral physics toolbox to determine elastic properties for specified compositions in the lower mantle by solving an Equation of State (EoS). The toolbox, written in Python, can be used to evaluate seismic velocities of new mineral physics data or geodynamic models, and as the forward model in inversions for mantle composition. The user can define the composition from a list of minerals provided for the lower mantle or easily include their own. BurnMan provides choices in methodology, both for the EoS and for the multi‐phase averaging scheme. The results can be visually or quantitatively compared to observed seismic models. Example user scripts show how to go through these steps. This paper includes several examples realized with BurnMan: First, we benchmark the computations to check for correctness. Second, we exemplify two pitfalls in EoS modeling: using a different EoS than the one used to derive the mineral physical parameters or using an incorrect averaging scheme. Both pitfalls have led to incorrect conclusions on lower mantle composition and temperature in the literature. We further illustrate that fitting elastic velocities separately or jointly leads to different Mg/Si ratios for the lower mantle. However, we find that, within mineral physical uncertainties, a pyrolitic composition can match PREM very well. Finally, we find that uncertainties on specific input parameters result in a considerable amount of variation in both magnitude and gradient of the seismic velocities.
      PubDate: 2014-03-06T00:54:48.503326-05:
      DOI: 10.1002/2013GC005122
  • Dynamic lithosphere within the Great Basin
    • Authors: Ryan C. Porter; Matthew J. Fouch, Nicholas C. Schmerr
      Pages: n/a - n/a
      Abstract: To place new constraints on the short‐term, broad‐scale lithospheric evolution of plate interiors, we utilize broadband seismic data from the Great Basin region of the Western United States to produce high‐resolution images of the crust and upper mantle. Our results suggest that parts of the Great Basin lithosphere has been removed, likely via inflow of hot asthenosphere as subduction of the Farallon spreading center occurred and the region extended. In our proposed model, fragments of thermal lithosphere removed by this process were gravitationally unstable and subsequently sank into the underlying mantle, leaving behind less dense, stronger, chemically‐depleted lithosphere. This destabilization process promotes volcanism, deformation, and the reworking of continental lithosphere inboard from plate margins. Our results provide evidence for a new mechanism of lithospheric evolution that is likely common and significant in post‐subduction tectonic settings.
      PubDate: 2014-03-05T22:46:08.420857-05:
      DOI: 10.1002/2013GC005151
  • On improving the selection of Thellier‐type paleointensity data
    • Authors: Greig A. Paterson; Lisa Tauxe, Andrew J. Biggin, Ron Shaar, Lori C. Jonestrask
      Pages: n/a - n/a
      Abstract: The selection of paleointensity data is a challenging, but essential step for establishing data reliability. There is, however, no consensus as to how best to quantify paleointensity data and which data selection processes are most effective. To address these issues, we begin to lay the foundations for a more unified and theoretically justified approach to the selection of paleointensity data. We present a new compilation of standard definitions for paleointensity statistics to help remove ambiguities in their calculation. We also compile the largest‐to‐date data set of raw paleointensity data from historical locations and laboratory control experiments with which to test the effectiveness of commonly used sets of selection criteria. Although most currently used criteria are capable of increasing the proportion of accurate results accepted, criteria that are better at excluding inaccurate results tend to perform poorly at including accurate results and vice versa. In the extreme case, one widely used set of criteria, which is used by default in the ThellierTool software (v4.22), excludes so many accurate results that it is often statistically indistinguishable from randomly selecting data. We demonstrate that, when modified according to recent single domain paleointensity predictions, criteria sets that are no better than a random selector can produce statistically significant increases in the acceptance of accurate results and represent effective selection criteria. The use of such theoretically derived modifications places the selection of paleointensity data on a more justifiable theoretical foundation and we encourage the use of the modified criteria over their original forms.
      PubDate: 2014-03-05T22:05:18.734724-05:
      DOI: 10.1002/2013GC005135
  • Crustal thickness and velocity structure across the Moroccan Atlas from
    • Authors: P Ayarza; R. Carbonell, A. Teixell, I. Palomeras, D. Martí, A. Kchikach, M. Harnafi, A. Levander, J. Gallart, M.L. Arboleya, J. Alcalde, M. Charroud, M. Amrhar
      Pages: n/a - n/a
      Abstract: The crustal structure and topography of the Moho boundary beneath the Atlas Mountains of Morocco has been constrained by a controlled source, wide‐angle seismic reflection transect: the SIMA experiment. This paper presents the first results of this project, consisting of an almost 700 km long, high‐resolution seismic profile acquired from the Sahara craton across the High and the Middle Atlas and the Rif Mountains. The interpretation of this seismic dataset is based on forward modeling by raytracing, and has resulted in a detailed crustal structure and velocity model for the Atlas Mountains. Results indicate that the High Atlas features a moderate crustal thickness, with the Moho located at a minimum depth of 35 km to the S and at around 31 km to the N, in the Middle Atlas. Upper crustal shortening is resolved at depth through a crustal root where the Saharan crust underthrusts the northern Moroccan crust. This feature defines a lower crust imbrication that, locally, places the Moho boundary at ~40‐41 km depth in the northern part of the High Atlas. The P‐wave velocity model is characterized by relatively low velocities, mostly in the lower crust and upper mantle, when compared to other active orogens and continental regions. These low deep crustal velocities together with other geophysical observables such as conductivity estimates derived from MT measurements, moderate Bouguer gravity anomaly, high heat flow, and surface exposures of recent alkaline volcanism lead to a model where partial melts are currently emplaced at deep crustal levels and in the upper mantle. The resulting model supports the existence of a mantle upwelling as mechanism that would contribute significantly to sustain the High Atlas topography. However, the detailed Moho geometry deduced in this work should lead to a revision of the exact geometry and position of this mantle feature and will require new modeling efforts.
      PubDate: 2014-03-05T22:02:56.854131-05:
      DOI: 10.1002/2013GC005164
  • Deep plume origin of the Louisville hotspot: Noble gas evidence
    • Authors: Takeshi Hanyu
      Pages: n/a - n/a
      Abstract: Noble gas compositions have been reported for basaltic core samples from Louisville seamounts recovered during IODP Expedition 330. The in‐vacuum crushing techniques were employed to extract noble gases from fresh olivine phenocrysts and submarine glasses with ages between 50 and 74 Ma. Stepwise crushing tests confirmed the extraction of magmatic noble gases from the olivine samples with minimal release of posteruption radiogenic nuclides; however, this was not always the case for the glass samples. The 3He/4He ratios of the studied samples range from a value similar to those of mid‐ocean ridge basalts (MORB) to slightly elevated ratios up to 10.6 Ra. These ratios are not as high as those observed in other ocean island basalts, suggesting that the Louisville mantle plume was weak or the samples represent late‐stage magmatic activity of the seamounts. However, two Louisville seamount basalts exhibit a primordial Ne isotopic signature that can be clearly discriminated from MORB Ne ratios. The He and Ne isotopic compositions of the Louisville seamount basalts can be explained by the mixing of less degassed mantle and depleted upper mantle with different He/Ne ratios. The presence of the less degassed mantle component in the source of the Louisville seamounts documents a deep origin of their mantle plume.
      PubDate: 2014-03-04T14:35:45.356502-05:
      DOI: 10.1002/2013GC005085
  • Geomagnetic field intensity determination from Pleistocene trachytic lava
           flows in Jeju Geopark
    • Authors: Doohee Jeong; Yongjae Yu, Qingsong Liu, Zhaoxia Jiang, Gi Won Koh, Dong‐Chan Koh
      Pages: n/a - n/a
      Abstract: A composite of 28 trachytic lava flows were recovered from the Jeju Geopark Drilling Project (JGDP) in Jeju Geopark, one of the new seven wonders of Nature declared by UNESCO in 2011. Each trachytic lava flow has a tendency to increase in magnetic grain size from the rapidly cooled brecciated margin and vesicle streaked zone downward into the massive crystalline flow interiors. The brecciated margin and vesicle streaked zone of individual trachytic lava flow contains exclusively fine‐grained magnetite as inclusions in plagioclase. High‐fidelity paleointensity determinations were obtained from 26 (out of 224 examined) samples from JGDP cores. Temporal variation of virtual axial dipole moments (VADMs) calculated from the absolute paleointensity estimates follows the trend of sint‐800 data for the interval from ∼80 to ∼360 ka. High VADM from flow 21 possibly represents real intensity peak, as previously recognized high VADM in Japan at ∼336 ka, in Trans‐Mexican volcanism ∼339, and in Hawaii ∼340–350 ka. Perhaps such a strong magnetic intensity near ∼325–350 ka might be smoothed out in relative paleointensity records.
      PubDate: 2014-03-01T23:31:31.425027-05:
      DOI: 10.1002/2013GC005028
  • Correction to “Extreme crustal thinning in the Bay of Biscay and the
           Western Pyrenees: From observations to modeling”
    • Authors: Suzon Jammes; Patricia Persaud, Luc Lavier, Gianreto Manatschal
      Pages: n/a - n/a
      PubDate: 2014-03-01T17:32:52.11764-05:0
      DOI: 10.1002/2013GC00505
  • Anisotropy beneath a highly extended continental rift
    • Authors: Zachary Eilon; Geoffrey A. Abers, Ge Jin, James B. Gaherty
      Pages: n/a - n/a
      Abstract: We have employed shear wave splitting techniques to image anisotropy beneath the D'Entrecasteaux Islands, in southeastern Papua New Guinea. Our results provide a detailed picture of the extending continent that lies immediately ahead of a propagating mid‐ocean ridge tip; we image the transition from continental to oceanic extension. A dense shear wave splitting dataset from a 2010‐11 passive‐source seismic deployment is analyzed using single‐ and multi‐channel methods. Splitting delay times of 1‐1.5 s are observed and fast axes of anisotropy trending N‐S, parallel to rifting direction, predominate the results. This trend is linked to lattice‐preferred orientation of olivine, primarily in the shallow convecting mantle, driven by up to 200 km of N‐S continental extension ahead of the westward‐propagating Woodlark Rift. This pattern differs from several other continental rifts that evince rift‐strike‐parallel fast axes and is evident despite the complex recent tectonic history. We contend that across most of this rift, the unusually high rate and magnitude of extension has been sufficient to produce a regime change to a mid‐ocean‐ridge‐like mantle fabric. Stations in the south of our array show more complex splitting that might be related to melt or to complex inherited structure at the edge of the extended region.
      PubDate: 2014-01-17T22:24:28.560912-05:
      DOI: 10.1002/2013GC005092
  • CO2 discharge from the bottom of volcanic Lake Rotomahana, New Zealand
    • Authors: Agnes Mazot; Florian M. Schwandner, Bruce Christenson, Cornel E.J. Ronde, Salvatore Inguaggiato, Brad Scott, Duncan Graham, Karen Britten, J. Keeman, Karine Tan
      Pages: n/a - n/a
      Abstract: From April 2010 through February 2011, CO2 flux surveys were performed on Lake Rotomahana, New Zealand. The area has been hydrothermally active with fumaroles and sublacustrine hydrothermal activity before and since the eruption of Mt Tarawera in 1886. The total CO2 emission from the lake calculated by sequential Gaussian simulation is 549 ± 72 t day‐1. Two different mechanisms of degassing, diffusion through the water‐air interface and bubbling, are distinguished using a graphical statistical approach. The carbon dioxide budget calculated for the lake confirms that the main source of CO2 to the atmosphere is by diffusion covering 94.5 % of the lake area (mean CO2 flux 25 g m‐2 day‐1) and to a lesser extent, bubbling (mean CO2 flux 1297 g m‐2 day‐1). Mapping of the CO2 flux over the entire lake, including over lakefloor vents detected during the survey, correlates with eruption craters formed during the 1886 eruption. These surveys also follow regional tectonic patterns present in the southeastern sector of Lake Rotomahana suggesting a deep magmatic source (~ 10 km) for CO2 and different pathways for the gas to escape to the surface. The values of δ13CCO2 (‐2.88 and ‐2.39 ‰) confirm the magmatic origin of CO2.
      PubDate: 2014-01-16T21:04:35.373435-05:
      DOI: 10.1002/2013GC004945
  • Crustal and uppermost mantle structure beneath western Tibet using seismic
           traveltime tomography
    • Authors: Ayda Shokoohi Razi; Vadim Levin, Steven W. Roecker, Guo‐chin Dino Huang
      Pages: n/a - n/a
      Abstract: We study the elastic wave speed structure of the crust and the uppermost mantle in western Tibet using P and S‐wave arrival times from regional earthquakes recorded by a temporary seismic network. We relocate the earthquakes, and subsequently invert travel time residuals for 3D distributions of wave speed. Resolution tests with a variety of input structures are used to verify the reliability of our results. The crust beneath western Tibet has low P‐wave speed (5.9 ‐ 6.3 km/s) throughout its nearly 80 km thickness, with lower values in this range concentrated within the Lhasa block. Beneath the Himalaya wave speeds are higher. Southern and western limits of the slow material beneath the Tibetan Plateau correlate with the Karakoram fault, and dip beneath the plateau at ~40° angle. We find no evidence of a sub‐horizontal low velocity zone in the crust. In the uppermost mantle we find a long and narrow region of fast (up to 8.4 km/s) P‐wave speed extending from the Karakoram fault in NE direction, and crossing the Bangong‐Nujiang suture. In a north‐south cross‐section, the distribution of relatively fast P‐wave speed suggests a ramp‐flat geometry consistent with India underthrusting the Tibetan Plateau at least as far as 32.5°N. A plausible interpretation of the upper mantle fast feature is the formation of eclogite from the mafic lower‐crustal material of India after it is underthrust beneath Tibet. Notably, in western Tibet this process only takes place in a narrow region.
      PubDate: 2014-01-15T20:19:15.850963-05:
      DOI: 10.1002/2013GC005143
  • An ultrasonic method for isolating nonclay components from clay‐rich
    • Authors: Gregory D. Hoke; Mark D. Schmitz, Samuel A Bowring
      Pages: n/a - n/a
      Abstract: We present an efficient method for high‐volume heavy mineral separation from clay‐rich rocks using an ultrasonic probe. The ultrasonic clay separator (UCS) is an easily constructed device that allows for the recovery of high‐density minerals, as small as 10 microns, with a minimum of sample preparation. Heavy mineral recovery from clay‐rich material with the UCS yields a greater number of small (
      PubDate: 2014-01-15T08:00:47.108459-05:
      DOI: 10.1002/2013GC005125
  • Limited magnesium isotope fractionation during metamorphic dehydration in
           metapelites from the Onawa contact aureole, Maine
    • Authors: Wang‐Ye Li; Fang‐Zhen Teng, Boswell A. Wing, Yilin Xiao
      Pages: n/a - n/a
      Abstract: Knowledge on the behavior of Mg isotopes during metamorphic dehydration is the prerequisite for applying Mg isotopes as tracers for crustal recycling. Here, we report Mg isotopic compositions of metapelites from the Onawa contact aureole, Maine. Except one sample, all metapelites across the aureole, from the wall‐rock regional metamorphic rocks to the partially melted rocks adjacent to the pluton, have similar Mg isotopic compositions (δ26Mg = ‐0.09 to +0.12‰). This observation indicates limited Mg isotope fractionation during metamorphic dehydration and fluid‐rock interaction, due to the low Mg concentration in fluids relative to rocks. Our results suggest that Mg isotopic compositions of metapelites can record those of their protoliths and, hence, recycled clastic sedimentary materials may preserve their low‐temperature Mg isotopic signatures through subduction zones. Therefore, Mg isotopes may serve as new tracers for crustal recycling, for example, tracing components experienced weathering cycles within granite sources.
      PubDate: 2014-01-11T13:32:14.972875-05:
      DOI: 10.1002/2013GC004992
  • Continental scale body wave tomography of India: Evidence for attrition
           and preservation of lithospheric roots
    • Authors: Arun Singh; J.‐P. Mercier, M. Ravi Kumar, D. Srinagesh, R. K. Chadha
      Pages: n/a - n/a
      Abstract: We assemble P and S waveforms of 2301 teleseismic earthquakes registered at 413 broadband seismic stations spanning the Indian plate from the southern tip of India to the Himalayan collision belt and generate an accurate data set of 52050 P and 30423 S arrival times through the multi‐channel cross correlation approach. These travel times are then inverted to obtain 3D P and S velocity structure of the subcontinent at a 2°x2° lateral resolution. The heterogeneous nature of the Indian lithospheric mantle revealed in this study suggests that the lithospheric roots are not uniformly thick on a regional scale. The key cratonic segments of the Indian shield are characterized by pockets of high velocity anomalies (~3% ) at shallow depths (
      PubDate: 2014-01-09T10:29:15.588542-05:
      DOI: 10.1002/2013GC005056
  • Accuracy and precision of 88Sr/86Sr and 87Sr/86Sr measurements by
           MC‐ICPMS compromised by high barium concentrations
    • Authors: Howie D. Scher; Elizabeth M. Griffith, Wayne P. Buckley
      Pages: n/a - n/a
      Abstract: Barite (BaSO4) is a widely distributed mineral that incorporates strontium (Sr) during formation. Mass dependent fractionation of Sr isotopes occurs during abiotic precipitation of barite and formation of barite associated with biological processes (e.g., bacterial sulfide oxidation). Sr isotopes in barite can provide provenance information as well as potentially reconstruct sample formation conditions (e.g., saturation state, temperature, biotic vs. abiotic). Incomplete separation of Ba from Sr has complicated measurements of Sr isotopes by MC‐ICPMS (de Souza, G. F., B. C. Reynolds, M. Kiczka, and B. Bourdon (2010), Evidence for mass‐dependent isotopic fractionation of strontium in a glaciated granitic watershed, Geochimica et Cosmochimica Acta, 74(9), 2596‐2614). In this study we tested the effects of Ba in Sr sample solutions and modified extraction chromatography of Sr using Eichrom Sr Spec (Eichrom Technologies LLC, USA) resin to enable rapid, accurate and precise measurements of 88Sr/86Sr and 87Sr/86Sr ratios from Ba‐rich matrices. Sr isotope ratios of sample solutions doped with Ba were statistically indistinguishable from Ba‐free sample solutions below 1 ppm Ba. Deviations in both 87Sr/86Sr and δ88/86Sr occurred above 1 ppm Ba. An updated extraction chromatography method tested with barite and Ba‐doped seawater produces Sr sample solutions containing 10 to 100 ppb levels of Ba. The practice of Zr spiking for external mass‐discrimination correction of 88Sr/86Sr ratios was also evaluated, and it was confirmed that variable Zr levels do not have adverse effects on the accuracy and precision of 87Sr/86Sr ratios in the Zr concentration range required to produce accurate δ88/86Sr values.
      PubDate: 2014-01-09T10:06:55.575938-05:
      DOI: 10.1002/2013GC005134
  • Reconciling different equations for proton conduction using the
           Meyer‐Neldel compensation rule
    • Authors: Alan G. Jones
      Pages: n/a - n/a
      Abstract: Proton conduction in nominally anhydrous minerals is the likely explanation for moderate values of electrical resistivity observed in the lithospheric and sub‐lithospheric mantle. However, results from the various laboratories making the controlled measurements on mantle minerals, predominantly olivine, are not in agreement with one another. Importantly, the groups use different formalisms to fit their experimental data. In this paper we show that neither of the two formalisms employed by the various laboratories is consistent with the Meyer‐Neldel Rule (MNR), or Compensation Law, by which the pre‐exponent term of the Arrhenian equation is linearly related to the activation energy term. We also demonstrate why the formalism of Karato and colleagues can be used at low water contents (100 wt ppm and below), whereas at higher water contents (above 300 wt ppm), the formalism of Yoshino and Poe's labs needs to be employed. A new MNR self‐consistent formalism is presented that is applicable over all water contents. MNR consistency appears to operate for most processes that can be described by an Arrhenius equation, so its adoption through and MNR consistent formalism is highly recommended when fitting experimental observations.
      PubDate: 2014-01-08T15:06:18.789273-05:
      DOI: 10.1002/2013GC004911
  • Parameterizations of calcic clinopyroxene—Melt trace element
           partition coefficients
    • Authors: Jean H. Bédard
      Pages: 303 - 336
      Abstract: Semiempirical parameterizations of the values and trends of variation of Nernst trace element partition coefficient data for Ca‐rich clinopyroxene (cpx/liquidD) are provided, applicable mainly to common terrestrial magmatic suites. cpx/liquidD data for most trace elements show significant variability which cannot be neglected when modeling melting and crystallization. The influence of pressure on cpx/liquidD is strong for many elements, particularly Na and Sr, which increase as pressure rises, and most high‐field strength elements, which decrease with increasing pressure. Most cpx/liquidD values increase as temperature decreases, as wt % melt MgO, MgO# (MgO/MgO+FeOtotal), CaO, and FeO contents drop, as cpx molar Mg# (Mg/Mg+Fetotal) decreases, and as wt % melt SiO2 and Na2O+K2O increase. No clear trends are seen for variations of cpx/liquidD against melt H2O. For mafic melts, many elements show well‐defined trends of cpx/liquidD increase as the clinopyroxene tetrahedral Al content (cpx Aliv) increases. Many cpx/liquidD are well correlated against cpx/liquidDTi, and many “near‐neighbor” elements show good cpx/liquidD intercorrelations (e.g., Zr‐Hf, U‐Th, Nb‐Ta, La‐Ce, Yb‐Lu). Cpx/liquidD profiles calculated from these parameterizations can constrain changes of D values during melting or crystallization. Cpx/liquidD for the rare earth elements were fit to the lattice strain model to derive fits that can reproduce the cpx/liquidDREE profile shapes (REE = rare earth elements). These fits indicate that cpx/liquidDREE for melts more evolved than picritic basalts cannot be modeled assuming that all REE are in octahedral coordination in a single M2 site, but also require sixfold partitioning into an M1 site for Lu‐Yb‐Tm‐Er.
      PubDate: 2014-02-06T14:23:49.774195-05:
      DOI: 10.1002/2013GC005112
  • Geothermobarometric history of subduction recorded by quartz inclusions in
    • Authors: Kyle T. Ashley; Mark J. Caddick, Matthew J. Steele‐MacInnis, Robert J. Bodnar, Besim Dragovic
      Pages: 350 - 360
      Abstract: Burial histories of subduction zone rocks are often difficult to accurately constrain, owing to a lack of robust mineral geobarometers applicable to high pressure mineral assemblages. Knowledge of the depth‐histories of subduction is, however, required for our understanding of global geochemical cycles, subduction‐related seismicity, and the evolution of destructive tectonic boundaries. The high spatial resolution of quartz inclusion geobarometry can be used to determine pressure evolution during metamorphic growth of individual garnet crystals. Quartz inclusions in garnet from Sifnos, Greece, preserve such a record of the pressure of garnet growth, allowing detailed reconstruction of the metamorphic evolution of these rocks. Pressure‐dependent Raman spectra of quartz inclusions were combined with elastic modeling to infer the conditions at which they were trapped during garnet growth. All measured inclusions suggest that garnet growth occurred between 19 and 20.5 kbars, with little evidence for significant pressure variation during the garnet growth interval, which is interpreted to record ∼100°C of heating. Coupled with thermometry and geochronology, these results show that early, cold burial was followed by a phase of rapid heating, which immediately preceded exhumation. Garnet growth occurred primarily during this heating phase.
      PubDate: 2014-02-06T14:23:53.037013-05:
      DOI: 10.1002/2013GC005106
  • Comparable hydrogen isotopic fractionation of plant leaf wax
           n‐alkanoic acids in arid and humid subtropical ecosystems
    • Authors: Li Gao; Mei Zheng, Matthew Fraser, Yongsong Huang
      Pages: 361 - 373
      Abstract: Leaf wax hydrogen isotope proxies have been widely used to reconstruct past hydrological changes. However, published reconstructions have given little consideration for the potentially variable hydrogen isotopic fractionation relative to precipitation (εwax‐p) under different climate and environmental settings. Chief among various potential factors controlling fractionation is relative humidity, which is known to strongly affect oxygen isotopic ratios of plant cellulose, but its effect on hydrogen isotopic fractionation of leaf waxes is still ambiguous. Analyses of lake surface sediments and individual modern plants have provided valuable information on the variability of εwax‐p, but both approaches have significant limitations. Here, we present an alternative method to obtain the integrated, time‐resolved ecosystem‐level εwax‐p values, by analyzing modern aerosol samples collected weekly from arid (Arizona lowlands) and humid subtropical (Atlanta, Georgia) environments during the main growth season. Because aerosol samples mainly reflect regional leaf wax resources, the extreme contrast in the hydroclimate and associated vegetation assemblages between our study sites allows us to rigorously assess the impact of relative humidity and associated vegetation assemblages on leaf wax hydrogen isotopic fractionation. We show there is only minor difference (mostly
      PubDate: 2014-02-06T11:35:20.142985-05:
      DOI: 10.1002/2013GC005015
  • Deep crustal earthquakes in North Tanzania, East Africa: Interplay between
           tectonic and magmatic processes in an incipient rift
    • Authors: Julie Albaric; Jacques Déverchère, Julie Perrot, Andrey Jakovlev, Anne Deschamps
      Pages: 374 - 394
      Abstract: In this study, we explore the origin of lower crustal seismicity and the factors controlling rift propagation using seismological data recorded within the youngest part of the East African Rift System, the North Tanzanian Divergence (NTD). Most earthquakes below Lake Manyara occur at depth ranging between 20 and 40 km and have a swarm‐like distribution. Focal mechanisms of 26 events indicate a combination of strike‐slip and normal faulting involving Archaean basement structures and forming a relay zone. The derived local stress regime is transtensive and the minimum principal stress is oriented N110°E. Crustal seismic tomography reveals low‐velocity anomalies below the rifted basins in the NTD, interpreted as localized thermomechanical perturbations promoting fluid release and subsequent seismicity in the lower crust. SKS splitting analysis in the NTD indicates seismic anisotropy beneath 17 stations most likely due to aligned magma lenses and/or dikes beneath the rift and to the lithospheric fabrics. Our results favor a strain pattern intermediate between purely mechanical and purely magmatic. We suggest that melt products arising from a large asthenospheric thermal anomaly enhance lithospheric weakening and facilitate faulting and creeping on critically oriented inherited structures of the Precambrian lower crust. Although the crust is unlikely weakened at a point comparable to other parts of the East African Rift System, this deep‐seated thermomechanical process is efficient enough to allow slow rift propagation within the eastern Tanzanian cratonic edge.
      PubDate: 2014-02-06T14:25:50.848634-05:
      DOI: 10.1002/2013GC005027
  • The late Pliocene Benguela upwelling status revisited by means of multiple
           temperature proxies
    • Authors: Guillaume Leduc; Dieter Garbe‐Schönberg, Marcus Regenberg, Camille Contoux, Johan Etourneau, Ralph Schneider
      Pages: 475 - 491
      Abstract: As compared to the late Pleistocene, Alkenone‐based sea surface temperature (SST) in the Benguela region revealed relatively warm and stable SST recorded between ∼3.5 and 2.0 Ma, and coincide with a period of increasing biological productivity as revealed by increasing deposition of biogenic opal. We assess how the hydrological patterns recorded in SST proxies are embedded in the geological record by performing a proxy‐proxy comparison. We used Laser‐Ablation Inductively Coupled Plasma‐Mass Spectrometry to measure the Mg/Ca on the planktonic foraminifera species Globigerina bulloides, allowing in situ measurements of Mg/Ca on individual foraminiferal tests. Mg/Ca‐derived temperatures provide much colder temperatures than alkenone‐derived SST by up to 10°C. We build a scenario involving contrasting sensitivities of paleothermometers upon the annual cycle, namely alkenones preferentially capturing SST when the surface ocean is warmer than the mean‐annual average SST, and G. bulloides capturing SST when upwelling intensifies. Multichamber analysis also suggests that G. bulloides migrates below the sea surface while calcifying its last chambers prior to gametogenesis, allowing the extraction of both surface and subsurface temperature from Mg/Ca measured on different chambers. The range of temperatures recorded between our multiple SST proxies is supported by the range of temperatures simulated with a general circulation model when different seasons, different water depth and different orbital configurations occurring during the late Pliocene are considered. A greater seasonal cycle in SST during the Pliocene can account for alkenone and Mg/Ca‐derived temperature contrast, pointing to a radically different mode of upwelling activity in the Benguela region compared to today.
      PubDate: 2014-03-01T20:31:32.243546-05:
      DOI: 10.1002/2013GC004940
  • Submarine gas seepage in a mixed contractional and shear deformation
           regime: Cases from the Hikurangi oblique‐subduction margin
    • Authors: Andreia Plaza‐Faverola; Ingo Pecher, Gareth Crutchley, Philip M. Barnes, Stefan Bünz, Thomas Golding, Dirk Klaeschen, Cord Papenberg, Joerg Bialas
      Pages: n/a - n/a
      Abstract: Gas seepage from marine sediments has implications for understanding feedbacks between the global carbon reservoir, seabed ecology and climate change. Although the relationship between hydrates, gas chimneys and seafloor seepage is well established, the nature of fluid sources and plumbing mechanisms controlling fluid escape into the hydrate zone and up to the seafloor remain one of the least understood components of fluid migration systems. In this study we present the analysis of new three‐dimensional high‐resolution seismic data acquired to investigate fluid migration systems sustaining active seafloor seepage at Omakere Ridge, on the Hikurangi subduction margin, New Zealand. The analysis reveals at high resolution, complex overprinting fault structures (i.e. protothrusts, normal faults from flexural extension, and shallow (
      PubDate: 2013-12-28T04:16:33.275405-05:
      DOI: 10.1002/2013GC005082
  • Water mass exchange between the Nordic Seas and the Arctic Ocean on
           millennial time scale during MIS 4–2
    • Authors: Tine L. Rasmussen; Erik Thomsen, Tove Nielsen
      Pages: n/a - n/a
      Abstract: The climate of the last glaciation c. 65,000–25,000 years ago was interrupted by about 15 abrupt temperature fluctuations, the so‐called Dansgaard‐Oeschger events consisting of warm interstadials and cold stadials recorded in Greenland ice cores. The largest fluctuations occur in the North Atlantic region, but they have been registered over the most of the world. The events are linked to changes in deep water formation in the Nordic Seas and North Atlantic, disrupting the thermohaline circulation. Yet, Dansgaard‐Oeschger events have so far not been recorded north of the convection areas in the central Nordic Seas, and it is not known if they affected the water exchange between the Nordic Seas and the Arctic Ocean. In this study, we analyze core JM05‐31GC from the northern Fram Strait at the very entrance to the Arctic Ocean. The core contains sediments from marine isotope stages (MIS) 4–2. The results show millennial time scale shifts in all the investigated proxies including the distribution of planktonic and benthic foraminifera, planktonic and benthic oxygen and carbon isotopes and several sedimentological parameters. In JM05‐31GC, the interstadials are characterized by relatively high surface and low bottom water temperatures, low content of IRD and well‐ventilated bottom water. Stadials are characterized by presence of icebergs, decreasing surface water and increasing bottom water temperatures due to increased inflow of Atlantic subsurface water. Ventilation decreased during Heinrich events and most stadials. The results show that the Dansgaard‐Oeschger events strongly affected the water exchange between the Nordic Seas and the Arctic Ocean.
      PubDate: 2013-12-28T04:15:04.619334-05:
      DOI: 10.1002/2013GC005020
  • Time constraints for the Yellow River traversing the Sanmen Gorge
    • Authors: Ping Kong; Jun Jia, Yong Zheng
      Pages: n/a - n/a
      Abstract: The Yellow River is the second longest river in China, a total course of 5464 km. The Yellow River traverses 30 gorges within its main course and the last one is the Sanmen Gorge. The timing of cut‐through the Sanmen Gorge indicates the final integration of the Yellow River. Previous geological surveys identify a paleolake in the Weihe Basin located west of the Sanmen Gorge; the lake was drained when the gorge was cut through. Using cosmogenic nuclide burial dating, we obtain ages of transition from lake to fluvial environments in the Weihe Basin and formation of fluvial terraces within the Sanmen Gorge. The timing of cut‐through the Sanmen Gorge is constrained to 1.3–1.5 Ma. Zircon U‐Pb age distributions further suggest that the Weihe, the largest tributary of the Yellow River running from west to east along the northern section of the Qinling Mountains, primarily flowed over the Sanmen Gorge, followed by the upper and middle Yellow River traversing 1.3–1.4 Ma ago. It appears that formation and integration of the Yellow River occurred in a short duration in the early Pleistocene, probably related to global climate changes.
      PubDate: 2013-12-28T04:12:58.293961-05:
      DOI: 10.1002/2013GC004912
  • Post‐orogenic rigid behavior of the eastern Songpan‐Ganze
           terrane: Insights from low‐temperature thermochronology and
           implications for intracontinental deformation in central Asia
    • Authors: Yuntao Tian; Barry P. Kohn, Shengbiao Hu, Andrew J. W. Gleadow
      Pages: n/a - n/a
      Abstract: The Songpan‐Ganze terrane (SGT), formed by Early Mesozoic closure of the Paleo‐Tethys Ocean, occupies a large area of the central‐eastern Tibetan Plateau. Late Mesozoic and Cenozoic strike‐slip deformation has been identified in the surrounding terranes and faults (e.g. western Qinling, Altyn Tagh and Kunlun faults); however, the coeval evolution of the SGT has not been well explored. We report apatite fission‐track and apatite and zircon (U‐Th)/He data from a >7 km deep borehole and outcrop samples covering an area of >150 × 150 km in the eastern SGT. Thermal history modeling suggests a distinct phase of Late Jurassic ‐ Early Cretaceous (~150‐100 Ma) cooling, followed by a prolonged stage of slow cooling, for all samples despite of their differences in depositional age (Mid ‐ Late Triassic time) and locality within a large area. The ubiquitous Late Jurassic ‐ Early Cretaceous cooling implies little differential deformation in the eastern SGT, and is best explained by regional rock uplift resulting from the transpressional strain field created by the contemporaneous Lhasa ‐ Qiangtang collision to the south. Projecting the contemporaneous deformation surrounding the SGT onto an Early Cretaceous paleogeographic terrane reconstruction, results in a new tectonic model. The model relates the Lhasa ‐ Qiangtang collision to 10s‐100s km of shearing along the Altyn Tagh and Kunlun faults, which transferred strain into central Asia (e.g. Qinling‐Dabie orogen). Results of this study suggest a rigid behavior for the eastern SGT, and highlight the important role of crustal strength discontinuities in accommodating and transferring crustal deformation.
      PubDate: 2013-12-23T09:01:37.385633-05:
      DOI: 10.1002/2013GC004951
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