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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: 41, SJR: 2.668, h-index: 142)
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J. of Geophysical Research : Planets     Full-text available via subscription   (Followers: 12)
J. of Geophysical Research : Solid Earth     Full-text available via subscription   (Followers: 21)
J. of Geophysical Research : Space Physics     Full-text available via subscription   (Followers: 13)
Paleoceanography     Full-text available via subscription   (Followers: 4, SJR: 2.16, h-index: 82)
Radio Science     Full-text available via subscription   (Followers: 2, SJR: 0.527, h-index: 47)
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Tectonics     Full-text available via subscription   (Followers: 7, SJR: 2.16, h-index: 79)
Water Resources Research     Full-text available via subscription   (Followers: 123, 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]
  • Paleointensity of the geomagnetic field in the Late Cretaceous and
           earliest Paleogene obtained from drill cores of the Louisville seamount
    • Authors: Toshitsugu Yamazaki; Yuhji Yamamoto
      Pages: n/a - n/a
      Abstract: Information on the strength of the geomagnetic field is important for understanding the behavior of the geodynamo. Polarity reversal frequency increases toward the Cenozoic since the end of the Cretaceous Normal Superchron. Accumulating reliable paleointensity data in this time interval is still desired for elucidating the possibility of a link between reversal frequency and paleointensity. We conducted a paleointensity study on 288 samples from four seamounts from ~74 to ~50 Ma in age that constitute the Louisville seamount trail. Based on thermomagnetic curves and scanning electron microscopy, we first excluded 134 samples that showed evidence for low‐temperature oxidation. We then applied the Tsunakawa‐Shaw method with low‐temperature demagnetization and double heating after rejecting some samples that do not preserve stable primary remanence, and 52 successful results were obtained. An average virtual axial dipole moment (VADM) of 3.75±1.52 (x1022 Am2) was obtained at Site U1372 on Canopus seamount (~74 Ma) based on 16 independent measurements. Sites U1373 and U1374 on Rigil seamount (~70 Ma) yielded a mean of 3.79±1.40 (n=16), and Site U1376 on Burton seamount (~65 Ma) yielded a mean of 3.70±1.37 (n=8). These VADMs are similar to the long‐term average of the last 200 m.y. by Tauxe et al. [2013] and the mean of the last 5 m.y. by Yamamoto and Tsunakawa [2005]. The volcanic sequence drilled at Site U1372 probably belongs to Chron C33n of 5.59 m.y. long, whereas others belong to shorter chrons. Our results support no discernible relationship between polarity length and paleointensity.
      PubDate: 2014-05-31T12:33:01.683138-05:
      DOI: 10.1002/2014GC005298
  • Compensation of the Meyer‐Neldel Compensation Law for H diffusion in
    • Authors: Alan G. Jones
      Pages: n/a - n/a
      Abstract: The Meyer‐Neldel Rule (MNR), or compensation law, linearly relates the pre‐exponent term to the logarithm of the excitation enthalpy for any process that is thermally driven in an Arrhenian manner, and MNR fits can be used to calibrate and validate laboratory experimental results. Both robust least‐squares linear regressions and non‐robust regressions on selected subsets for individual minerals with sufficient experimental data demonstrate that hydrogen diffusion in minerals obeys the MNR with differing MNR intercepts and gradients depending on the mineral. In particular, nominally anhydrous mantle minerals have very distinct and different MNR parameters compared to hydrous and crustal minerals, with garnet proving to be an outlier lying inbetween the two. Furthermore, the variations of the estimated intercepts and gradients of the various MNRs are not random, but remarkably they themselves fall on a striking linear trend. This observation, if more broadly true, has profound implications for materials sciences and understanding of solid state physics, as it implies that the compensation rule is itself compensated.
      PubDate: 2014-05-31T06:25:10.934818-05:
      DOI: 10.1002/2014GC005261
  • Helium isotopic textures in Earth's upper mantle
    • Authors: David W. Graham; Barry B. Hanan, Christophe Hémond, Janne Blichert‐Toft, Francis Albarède
      Pages: n/a - n/a
      Abstract: We report 3He/4He for 150 mid‐ocean ridge basalt (MORB) glasses from the Southeast Indian Ridge (SEIR). Between 81°E and 101°E 3He/4He varies from 7.5 to 10.2 RA, encompassing more than half the MORB range away from ocean island hot spots. Abrupt transitions are present and in one case the full range occurs over ∼10 km. Melting of lithologically heterogeneous mantle containing a few percent garnet pyroxenite or eclogite leads to lower 3He/4He, while 3He/4He above ∼9 RA likely indicates melting of pyroxenite‐free or eclogite‐free mantle. Patterns in the length scales of variability represent a description of helium isotopic texture. We utilize four complementary methods of spectral analysis to evaluate this texture, including periodogram, redfit, multitaper method, and continuous wavelet transform. Long‐wavelength lobes with prominent power at 1000 and 500 km are present in all treatments, similar to hot spot‐type spectra in Atlantic periodograms. The densely sampled region of the SEIR considered separately shows significant power at ∼100 and ∼30–40 km, the latter scale resembling heterogeneity in the bimodal distribution of Hf and Pb isotopes in the same sample suite. Wavelet transform coherence reveals that 3He/4He varies in‐phase with axial depth along the SEIR at ∼1000 km length scale, suggesting a coupling between melt production, 3He/4He and regional variations in mantle temperature. Collectively, our results show that the length scales of MORB 3He/4He variability are dominantly controlled by folding and stretching of heterogeneities during regional (∼1000 km) and mesoscale (∼100 km) mantle flow, and by sampling during the partial melting process (∼30 km).
      PubDate: 2014-05-30T10:29:39.804677-05:
      DOI: 10.1002/2014GC005264
  • Confirmation of progressive plate motion during the Midcontinent Rift's
           early magmatic stage from the Osler Volcanic Group, Ontario, Canada
    • Authors: Nicholas L. Swanson‐Hysell; Angus A. Vaughan, Monica R. Mustain, Kristofer E. Asp
      Pages: n/a - n/a
      Abstract: As the supercontinent Rodinia was assembling ca. 1.1 billion years ago, there was extensive magmatism on at least five Proterozoic continents including the development of the North American Midcontinent Rift. New paleomagnetic data from 84 lava flows of the Osler Volcanic Group of the Midcontinent Rift reveal that there was a significant and progressive decrease in inclination between the initiation of extrusive volcanism in the region (ca. 1110 Ma) and ca. 1105 ± 2 Ma (during the “early stage” of rift development). Paleomagnetic poles can be calculated for the lower portion of the reversed Osler Volcanic Group (40.9°N, 218.6°E, A95 = 4.8°, N = 30) and the upper portion of the reversed Osler Volcanic Group (42.5°N, 201.6°E, A95 = 3.7°, N = 59; this pole can be assigned the age of ca. 1105 ± 2 Ma). This result is a positive test of the hypothesis that there was significant plate motion during the early stage of rift development. In addition to being a time of widespread volcanism on Laurentia and other continents, this interval of the late Mesoproterozoic was characterized by rapid paleogeographic change.
      PubDate: 2014-05-30T10:22:52.30609-05:0
      DOI: 10.1002/2013GC005180
  • Effects of the core‐shell structure on the magnetic properties of
           partially oxidized magnetite grains: Experimental and micromagnetic
    • Authors: Kunpeng Ge; Wyn Williams, Qingsong Liu, Yongjae Yu
      Pages: n/a - n/a
      Abstract: The relationship between magnetic hysteresis parameters and the degree of oxidation of ultrafine magnetite particles is examined by both experimental measurements (distributed particle assemblage with median grain size of ∼80 nm and standard deviation 0.43) and micromagnetic simulations (single particles from 40 nm to 140 nm). Experimental results show that both coercivity (Bc) and the ratio of saturation remanence to saturation magnetization (Mrs/Ms) increase slowly, as the oxidation parameter z increases from 0 to ∼0.9. Thereafter, both parameters decrease sharply as magnetite becomes completely oxidized to maghemite. Numerical simulations of hysteresis loop and microstructure using a micromagnetic model with a core‐shell geometry (a stoichiometric core surrounded by an oxidized shell) show three categories of behavior for magnetic grains during oxidation. First, the coercivity of SD particles decreases as oxidation proceeds, but their remanence magnetization remains in a uniform state. Second, for PSD sized particles near the critical SD boundary (80 nm to 100 nm), the initial vortex domain structure changes to a SD as oxidation occurs and returns to a vortex state upon complete maghemitization, resulting in an initial rise and then fall of Bc and Mrs. Finally, larger PSD grains remain a vortex state throughout the maghemitization, with less variations of Bc and Mrs. The predicted magnetic properties exhibit good agreement with experimental observations and suggest that the domain arrangement is likely to be dominated by a core‐shell structure with strong exchange coupling at their interface. Overall, the partially oxidized magnetite in SD‐PSD range can reliably record palaeomagnetic signals.
      PubDate: 2014-05-30T10:17:33.514051-05:
      DOI: 10.1002/2014GC005265
  • Contrasting subduction structures within the Philippine Sea plate: Hydrous
           oceanic crust and anhydrous volcanic arc crust
    • Authors: Ryuta Arai; Takaya Iwasaki, Hiroshi Sato, Susumu Abe, Naoshi Hirata
      Pages: n/a - n/a
      Abstract: We show contrasting subduction structures within the Philippine Sea plate inferred from active‐source wide‐angle reflection data. Previous studies showed that large‐amplitude reflections from the slab are observed in southwest Japan and indicated that a thin low‐velocity layer with a high fluid content is formed along the top of the subducting oceanic crust. On the contrary, we found that the slab reflections have smaller amplitudes in the Izu collision zone, central Japan, where the Izu‐Bonin volcanic arc has been colliding/subducting, suggesting that such a low‐velocity layer does not exist beneath the collision zone. This structural difference is also supported by P‐wave and S‐wave velocity anomalies by passive‐source tomography and electrical conductivity, and correlates with the regional distribution of deep tremors and intraslab earthquakes, both of which are induced by dehydration processes within the downgoing slab. Based on these comparisons, we suggest that the original structure of the incoming plate controls the contrasting subducting systems: typical oceanic plate absorbs water by hydrothermal circulation at spreading centers and/or seawater infiltration at outer rises, whereas volcanic arc crust consumes a large amount of hydrous minerals for melt production and metamorphoses to more stable, anhydrous forms before subduction.
      PubDate: 2014-05-30T10:12:04.984117-05:
      DOI: 10.1002/2014GC005321
  • A uniformly processed data set of SKS shear wave splitting measurements: A
           global investigation of upper mantle anisotropy beneath seismic stations
    • Authors: J. Walpole; J. Wookey, G. Masters, J. M. Kendall
      Pages: n/a - n/a
      Abstract: Anisotropy in the Earth's upper mantle is a signature of past and present deformation. Here we present a new data set of ∼50,000 uniformly processed SKS shear wave splitting measurements that probe upper mantle anisotropy beneath seismic stations in the frequency band 0.02–0.1 Hz. The data set consists of measurements obtained at ∼2000 seismic stations from ∼2000 events. We identify several stations characterized by an apparent absence of shear wave splitting (so‐called “null stations”). Station‐averaged measurements are obtained by stacking shear wave splitting error surfaces. The stacked data set shows excellent agreement with a compilation of previous SKS measurements. The average amount of splitting beneath seismic stations (after error surface stacking) is 0.8 s, slightly lower than that found previously by vectorial averaging of non‐null measurement splitting parameters. The data set disagrees, however, with an azimuthally anisotropic surface wave tomography model (DKP2005), suggesting that caution should be exercised when using such models for geodynamic interpretation, especially in continental regions. Studying our data set in detail, we find evidence that flow in the asthenosphere exerts partial control over SKS splitting in orogenic regions globally. In the active orogenic environment of the western USA, where we have the densest coverage, our data suggest that shallow asthenospheric flow is guided by a wall of thick lithosphere to the east.
      PubDate: 2014-05-30T09:57:37.485951-05:
      DOI: 10.1002/2014GC005278
  • Tectonic and magmatic control of hydrothermal activity along the
           slow‐spreading Central Indian Ridge, 8°S–17°S
    • Authors: Juwon Son; Sang‐Joon Pak, Jonguk Kim, Edward T. Baker, Ok‐Rye You, Seung‐Kyu Son, Jai‐Woon Moon
      Pages: n/a - n/a
      Abstract: The complex geology and expansive axial valleys typical of slow‐spreading ridges makes evaluating their hydrothermal activity a challenge. This challenge has gone largely unmet, as the most undersampled MOR type for hydrothermal activity is slow spreading (20–55 mm/yr). Here we report the first systematic hydrothermal plume survey conducted on the Central Indian Ridge (CIR, 8°S–17°S), the most extensive such survey yet conducted on a slow‐spreading ridge. Using a combined CTD/Miniature Autonomous Plume Recorder (MAPR) package, we used 118 vertical casts along seven segments of the CIR (∼700 km of ridge length) to estimate the frequency of hydrothermal activity. Evidence for hydrothermal activity (particle and methane plumes) was found on each of the seven spreading segments, with most plumes found between 3000 and 3500 m, generally
      PubDate: 2014-05-30T09:55:37.251719-05:
      DOI: 10.1002/2013GC005206
  • Kinematics of the western Caribbean: Collision of the Cocos Ridge and
           upper plate deformation
    • Authors: Daisuke Kobayashi; Peter LaFemina, Halldór Geirsson, Eric Chichaco, Antonio A. Abrego, Hector Mora, Eduardo Camacho
      Pages: n/a - n/a
      Abstract: Subduction of the Cocos plate and collision of the Cocos Ridge have profound effects on the kinematics of the western Caribbean, including crustal shortening, segmentation of the overriding plate, and tectonic escape of the Central American fore arc (CAFA). Tectonic models of the Panama Region (PR) have ranged from a rigid block to a deforming plate boundary zone. Recent expansion of GPS networks in Panama, Costa Rica, and Colombia makes it possible to constrain the kinematics of the PR. We present an improved kinematic block model for the western Caribbean, using this improved GPS network to test a suite of tectonic models describing the kinematics of this region. The best fit model predicts an Euler vector for the counterclockwise rotation of the CAFA relative to the Caribbean plate at 89.10°W, 7.74°N, 1.193° Ma−1, which is expressed as northwest‐directed relative block rates of 11.3 ± 1.0–16.5 ± 1.1 mm a−1 from northern Costa Rica to Guatemala. This model also predicts high coupling along the Nicoya and Osa segments of the Middle American subduction zone. Our models demonstrate that the PR acts as a single tectonic block, the Panama block, with a predicted Euler vector of 107.65°W, 26.50°N, 0.133° Ma−1. This rotation manifests as northeast migration of the Panama block at rates of 6.9 ± 4.0–7.8 ± 4.8 mm a−1 from southern Costa Rica to eastern Panama. We interpret this motion as tectonic escape from Cocos Ridge collision, redirected by collision with the North Andes block, which migrates to the northwest at 12.2 ± 1.2 mm a−1.
      PubDate: 2014-05-29T15:20:35.172637-05:
      DOI: 10.1002/2014GC005234
  • Reply to the comment by Granier on “Early Aptian paleoenvironmental
           evolution of the Bab Basin at the southern Neo‐Tethys margin:
           Response to global carbon‐cycle perturbations across Ocean Anoxic
           Event 1a”
    • Authors: Kazuyuki Yamamoto; Masatoshi Ishibashi, Hideko Takayanagi, Yoshihiro Asahara, Tokiyuki Sato, Hiroshi Nishi, Yasufumi Iryu
      Pages: n/a - n/a
      PubDate: 2014-05-29T15:20:32.699595-05:
      DOI: 10.1002/2014GC005390
  • Comment on “Early Aptian paleoenvironmental evolution of the Bab
           Basin at the southern Neo‐Tethys margin: Response to global
           carbon‐cycle perturbations across Ocean Anoxic Event 1a” by K.
           Yamamoto et al.
    • Authors: Bruno R. C. Granier
      Pages: n/a - n/a
      PubDate: 2014-05-29T15:18:18.589328-05:
      DOI: 10.1002/2014GC005350
  • Acoustic estimates of methane gas flux from the seabed in a 6000 km2
           region in the Northern Gulf of Mexico
    • Authors: Thomas C. Weber; Larry Mayer, Kevin Jerram, Jonathan Beaudoin, Yuri Rzhanov, Dave Lovalvo
      Pages: n/a - n/a
      Abstract: Seeps of free methane gas escaping the seabed can be found throughout the ocean basins. To understand the role of methane gas seeps in the global carbon cycle—including both gas added to the atmosphere and that which is dissolved and potentially oxidized in the ocean volume—it is important to quantify the amount of methane escaping the seabed. Few large‐scale mapping projects of natural methane seeps have been undertaken, however, and even among these, quantitative estimates of flux are rare. Here we use acoustic mapping techniques to survey 357 natural methane seeps in a large region (6000 km2) of the northern Gulf of Mexico and outline a general approach for methane seep mapping using a combination of multibeam and split‐beam echo sounders. Using additional measurements collected with a remotely operated vehicle (ROV) together with the acoustic mapping results, we estimate the total gas flux within the 6000 km2 region to be between 0.0013 and 0.16 Tg/yr, or between 0.003 and 0.3% of the current estimates for global seabed methane seepage rates.
      PubDate: 2014-05-29T14:30:44.344588-05:
      DOI: 10.1002/2014GC005271
  • Thermogenic methane injection via bubble transport into the upper Arctic
           Ocean from the hydrate‐charged Vestnesa Ridge, Svalbard
    • Authors: Andrew J. Smith; Jürgen Mienert, Stefan Bünz, Jens Greinert
      Pages: n/a - n/a
      Abstract: We use new gas‐hydrate geochemistry analyses, echosounder data, and three‐dimensional P‐Cable seismic data to study a gas‐hydrate and free‐gas system in 1200 m water depth at the Vestnesa Ridge offshore NW Svalbard. Geochemical measurements of gas from hydrates collected at the ridge revealed a thermogenic source. The presence of thermogenic gas and temperatures of ∼3.3°C result in a shallow top of the hydrate stability zone (THSZ) at ∼340 m below sea level (mbsl). Therefore, hydrate‐skinned gas bubbles, which inhibit gas‐dissolution processes, are thermodynamically stable to this shallow water depth. This was confirmed by hydroacoustic observations of flares in 2010 and 2012 reaching water depths between 210 and 480 mbsl. At the seafloor, bubbles are released from acoustically transparent zones in the seismic data, which we interpret as regions where free gas is migrating through the hydrate stability zone (HSZ). These intrusions result in vertical variations in the base of the HSZ (BHSZ) of up to ∼150 m, possibly making the shallow hydrate reservoir more susceptible to warming. Such Arctic gas‐hydrate and free‐gas systems are important because of their potential role in climate change and in fueling marine life, but remain largely understudied due to limited data coverage in seasonally ice‐covered Arctic environments.
      PubDate: 2014-05-29T14:18:54.738474-05:
      DOI: 10.1002/2013GC005179
  • Sensitivity of sediment geochemical proxies to coring location and corer
           type in a large lake: Implications for paleolimnological reconstruction
    • Authors: Yue Han Lu; Philip A. Meyers, John A. Robbins, Brian J. Eadie, Nathan Hawley, Kang Hyeun Ji
      Pages: n/a - n/a
      Abstract: We compared a suite of geochemical proxies in sediment cores collected in 1982, 1988, 1991, and 2003 from sites near the depocenter of Lake Erie to evaluate the reliability of paleoenvironmental reconstructions derived from lacustrine sediments. Our proxies included the concentrations and carbon isotopic compositions of organic and inorganic carbon (TOC, CaCO3, δ13Corg, and δ13CCaCO3), augmented by organic C to total N ratios (Corg:Ntot), δ15N, and carbonate δ18O values (δ18OCaCO3). The three coring sites were clustered within 12 km; two types of corers—a Box corer and a Benthos gravity corer—were used for the 1991 sampling campaign. The variance of most proxies was accounted for not only by temporal environmental changes but also by coring locations and corer type, indicating that sediment spatial heterogeneity and differences in sediment recovery due to the use of different corers also played a part in determining the geochemical compositions of these cores. The TOC, δ13Corg, and δ13CCaCO3 values showed decadal temporal patterns that were consistent between the multiple sampling campaigns. In contrast, the δ15N, Corg:Ntot, CaCO3, and δ18OCaCO3 exhibited across‐core differences in their temporal variations, making it difficult to extract consistent environment information from different cores. Our findings suggest that in addition to temporal environmental changes, high‐resolution paleolimnological reconstruction is sensitive to many factors that could include spatial sediment heterogeneity, discontinuous sedimentation processes, bioturbation, sediment dating uncertainty, and artifacts associated with analytical and coring procedures. Therefore, multiple‐core sampling and analysis are important in reliably reconstructing environmental changes, particularly for large, heterogeneous lacustrine basins.
      PubDate: 2014-05-29T14:04:19.389657-05:
      DOI: 10.1002/2013GC004989
  • Precessional control of Sr ratios in marginal basins during the Messinian
           Salinity Crisis?
    • Authors: R. P. M. Topper; S. Lugli, V. Manzi, M. Roveri, P. Th. Meijer
      Pages: n/a - n/a
      Abstract: Based on 87Sr/86Sr data of the Primary Lower Gypsum (PLG) deposits in the Vena del Gesso basin—a marginal basin of the Mediterranean during the Messinian Salinity Crisis—a correlation between 87Sr/86Sr values and precessional forcing has recently been proposed but not yet confirmed. In this study, a box model is set up to represent the Miocene Mediterranean deep basin and a connected marginal basin. Measurements of 87Sr/86Sr in the Vena del Gesso and estimated salinity extrema are used to constrain model results. In an extensive analysis with this model, we assess whether coeval 87Sr/86Sr and salinity fluctuations could have been forced by precession‐driven changes in the fresh water budget. A comprehensive set of the controlling parameters is examined to assess the conditions under which precession‐driven 87Sr/86Sr variations occur and to determine the most likely setting for PLG formation. Model results show that precession‐driven 87Sr/86Sr and salinity fluctuations in marginal basins are produced in settings within a large range of marginal basin sizes, riverine strontium characteristics, amplitudes of precessional fresh water budget variation, and average fresh water budgets of both the marginal and deep basin. PLG deposition most likely occurred when the Atlantic‐Mediterranean connection was restricted, and the average fresh water budget in the Mediterranean was significantly less negative than at present day. Considering the large range of settings in which salinities and 87Sr/86Sr fluctuate on a precessional timescale, 87Sr/86Sr variations are expected to be a common feature in PLG deposits in marginal basins of the Mediterranean.
      PubDate: 2014-05-29T13:57:09.087183-05:
      DOI: 10.1002/2013GC005192
  • Isotope and trace element insights into heterogeneity of sub‐ridge
    • Authors: Soumen Mallick; Henry J. B. Dick, Afi Sachi‐Kocher, Vincent J.M. Salters
      Pages: n/a - n/a
      Abstract: Geochemical data for abyssal peridotites are used to determine the relationship to mid‐ocean ridge basalts from several locations at ridge segments on the SW Indian Ridge (SWIR), the Mid‐Cayman‐Rise (MCR), and the Mid Atlantic Ridge (MAR). Based on chemical and petrological criteria peridotites are categorized as being either dominantly impregnated with melt or being residual after recent melting. Those that are considered impregnated with melt also have isotopic compositions similar to the basalts indicating impregnation by a melt aggregate. At the SWIR and MCR residual peridotite Nd‐isotopic compositions partly overlap the Nd‐isotopic compositions of the basalts but extend to more radiogenic compositions. The differences between peridotite and basalt Nd‐isotopic compositions can be explained by incorporating a low‐solidus component with enriched isotopic signature in the sub‐ridge mantle: a component that is preferentially sampled by the basalts. At the MAR, peridotites and associated basalts have overlapping Nd‐isotopic compositions, suggesting a more homogeneous MORB mantle. The combined chemistry and petrography indicates a complex history with several depletion and enrichment events. The MCR data indicates that a low‐solidus component can be a ubiquitous component of the asthenosphere. Residual abyssal peridotites from limited geographic areas also show significant chemical variations that could be associated with initial mantle heterogeneities related to events predating the ridge‐melting event. Sm‐Nd model ages for possible earlier depletion events suggest these could be as old as 2.4 Ga.
      PubDate: 2014-05-28T08:30:25.55461-05:0
      DOI: 10.1002/2014GC005314
  • Differentiating flow, melt, or fossil seismic anisotropy beneath Ethiopia
    • Authors: J. O. S. Hammond; J.‐M. Kendall, J. Wookey, G. W. Stuart, D. Keir, A. Ayele
      Pages: n/a - n/a
      Abstract: Ethiopia is a region where continental rifting gives way to oceanic spreading. Yet the role that pre‐existing lithospheric structure, melt, mantle flow, or active upwellings may play in this process is debated. Measurements of seismic anisotropy are often used to attempt to understand the contribution that these mechanisms may play. In this study, we use new data in Afar, Ethiopia along with legacy data across Ethiopia, Djibouti, and Yemen to obtain estimates of mantle anisotropy using SKS‐wave splitting. We show that two layers of anisotropy exist, and we directly invert for these. We show that fossil anisotropy with fast directions oriented northeast‐southwest may be preserved in the lithosphere away from the rift. Beneath the Main Ethiopian Rift and parts of Afar, anisotropy due to shear segregated melt along sharp changes in lithospheric thickness dominates the shear‐wave splitting signal in the mantle. Beneath Afar, away from regions with significant lithospheric topography, melt pockets associated with the crustal and uppermost mantle magma storage dominate the signal in localized regions. In general, little anisotropy is seen in the uppermost mantle beneath Afar suggesting melt retains no preferential alignment. These results show the important role melt plays in weakening the lithosphere and imply that as rifting evolves passive upwelling sustains extension. A dominant northeast‐southwest anisotropic fast direction is observed in a deeper layer across all of Ethiopia. This suggests that a conduit like plume is lacking beneath Afar today, rather a broad flow from the southwest dominates flow in the upper mantle.
      PubDate: 2014-05-27T13:22:03.590402-05:
      DOI: 10.1002/2013GC005185
  • A fictitious domain method for lithosphere‐asthenosphere
           interaction: Application to periodic slab folding in the upper mantle
    • Authors: Nestor G. Cerpa; Riad Hassani, Muriel Gerbault, Jean‐Herve Prévost
      Pages: n/a - n/a
      Abstract: We present a new approach for the lithosphere‐asthenosphere interaction in subduction zones. The lithosphere is modeled as a Maxwell viscoelastic body sinking in the viscous asthenosphere. Both domains are discretized by the finite element method, and we use a staggered coupling method. The interaction is provided by a nonmatching interface method called the fictitious domain method. We describe a simplified formulation of this numerical technique and present 2‐D examples and benchmarks. We aim at studying the effect of mantle viscosity on the cyclicity of slab folding at the 660 km depth transition zone. Such cyclicity has previously been shown to occur depending on the kinematics of both the overriding and subducting plates, in analog and numerical models that approximate the 660 km depth transition zone as an impenetrable barrier. Here we applied far‐field plate velocities corresponding to those of the South‐American and Nazca plates at present. Our models show that the viscosity of the asthenosphere impacts on folding cyclicity and consequently on the slab's dip as well as the stress regime of the overriding plate. Values of the mantle viscosity between 3 and 5 × 1020 Pa s are found to produce cycles similar to those reported for the Andes, which are of the order of 30–40 Myr (based on magmatism and sedimentological records). Moreover, we discuss the episodic development of horizontal subduction induced by cyclic folding and, hence, propose a new explanation for episodes of flat subduction under the South‐American plate.
      PubDate: 2014-05-27T13:19:20.51371-05:0
      DOI: 10.1002/2014GC005241
  • A novel framework for quantifying past methane recycling by
           Sphagnum‐methanotroph symbiosis using carbon and hydrogen isotope
           ratios of leaf wax biomarkers
    • Authors: Jonathan E. Nichols; Peter D. F. Isles, Dorothy M. Peteet
      Pages: n/a - n/a
      Abstract: The concentration of atmospheric methane is strongly linked to variations in Earth's climate. Currently, we can directly reconstruct the total atmospheric concentration of methane, but not individual terms of the methane cycle. Northern wetlands, dominated by Sphagnum, are an important contributor of atmospheric methane, and we seek to understand the methane cycle in these systems. We present a novel method for quantifying the proportion of carbon Sphagnum assimilates from its methanotrophic symbionts using stable isotope ratios of leaf‐wax biomarkers. Carbon isotope ratios of Sphagnum compounds are determined by two competing influences, water content and the isotope ratio of source carbon. We disentangled these effects using a combined hydrogen and carbon isotope approach. We constrained Sphagnum water content using the contrast between the hydrogen isotope ratios of Sphagnum and vascular plant biomarkers. We then used Sphagnum water content to calculate the carbon isotope ratio of Sphagnum's carbon pool. Using a mass balance equation, we calculated the proportion of recycled methane contributed to the Sphagnum carbon pool, “PRM.” We quantified PRM in peat monoliths from three microhabitats in the Mer Bleue peatland complex. Modern studies have shown that water table depth and vegetation have strong influences on the peatland methane cycle on instrumental time scales. With this new approach, δ13C of Sphagnum compounds are now a useful tool for investigating the relationships among hydrology, vegetation, and methanotrophy in Sphagnum peatlands over the time scales of entire peatland sediment records, vital to our understanding of the global carbon cycle through the Late Glacial and Holocene.
      PubDate: 2014-05-27T11:58:01.696608-05:
      DOI: 10.1002/2014GC005242
  • Large volume submarine ignimbrites in the Shikoku Basin: An example for
           explosive volcanism in the Western Pacific during the Late Miocene
    • Authors: Steffen Kutterolf; Julie C. Schindlbeck, Rachel P. Scudder, Richard W. Murray, Kevin T. Pickering, Armin Freundt, Shasa Labanieh, Ken Heydolph, Sanny Saito, Hajime Naruse, Michael B. Underwood, Huaichun Wu
      Pages: n/a - n/a
      Abstract: During IODP Expedition 322, an interval of Late Miocene (7.6 to ∼9.1 Ma) tuffaceous and volcaniclastic sandstones was discovered in the Shikoku Basin (Site C0011B), Nankai region. This interval consists of bioturbated silty claystone including four 1–7 m thick interbeds of tuffaceous sandstones (TST) containing 57–82% (by volume) pyroclasts. We use major and trace element glass compositions, as well as radiogenic isotope compositions, to show that the tuffaceous sandstones beds derived from single eruptive events, and that the majority (TST 1, 2, 3a) came from different eruptions from a similar source region, which we have identified to be the Japanese mainland, 350 km away. In particular, diagnostic trace element ratios (e.g., Th/La, Sm/La, Rb/Hf, Th/Nb, and U/Th) and isotopic data indicate a marked contribution from a mantle source beneath continental crust, which is most consistent with a Japanese mainland source and likely excludes the Izu‐Bonin island arc and back arc as a source region for the younger TST beds. Nevertheless, some of the chemical data measured on the oldest sandstone bed (TST 3b, Unit IIb) show affinity to or can clearly be attributed to an Izu‐Bonin composition. While we cannot completely exclude the possibility that all TST beds derived from unknown and exotic Izu‐Bonin source(s), the collected lines of evidence are most consistent with an origin from the paleo‐Honshu arc for TST 1 through 3a. We therefore suggest the former collision zone between the Izu‐Bonin arc and Honshu paleo‐arc as the most likely region where the eruptive products entered the ocean, also concurrent with nearby (∼200 km) possible Miocene source areas for the tuffaceous sandstones at the paleo‐NE‐Honshu arc. Estimating the distribution area of the tuffaceous sandstones in the Miocene between this source region and the ∼350 km distant Expedition 322, using bathymetric constraints, we calculate that the sandstone beds represent minimum erupted magma volumes between ∼1 and 17 km3 (Dense Rock Equivalent (DRE)). We conclude that several large volume eruptions occurred during the Late Miocene time next to the collision zone of paleo‐Honshu and Izu‐Bonin arc and covered the entire Philippine Sea plate with meter thick, sheet‐like pyroclastic deposits that are now subducted in the Nankai subduction zone.
      PubDate: 2014-05-27T11:46:02.562291-05:
      DOI: 10.1002/2014GC005263
  • Serpentinization of mantle‐derived peridotites at mid‐ocean
           ridges: Mesh texture development in the context of tectonic exhumation
    • Authors: Stéphane Rouméjon; Mathilde Cannat
      Pages: n/a - n/a
      Abstract: At slow spreading ridges, axial detachment faults exhume mantle‐derived peridotites and hydrothermal alteration causes serpentinization in a domain extending more than 1 km next to the fault. At the microscopic scale, serpentinization progresses from a microfracture network toward the center of olivine relicts and forms a mesh texture. We present a petrographic study (SEM, EBSD and Raman) of the serpentine mesh texture in a set of 278 abyssal serpentinized peridotites from the Mid‐Atlantic and Southwest Indian ridges. We show that serpentinization initiated along two intersecting sets of microfractures that have consistent orientations at the sample scale, and in at least one studied location, at the 100m‐scale. We propose that these microfractures formed in fresh peridotites due to combined thermal and tectonic stresses and subsequently served as channels for serpentinizing fluids. Additional reaction‐induced cracks developed for serpentinization extents < 20%. The resulting microfracture network has a typical spacing of ˜60 µm but most serpentinization occurs next to a subset of these microfractures that define mesh cells 100‐400 µm in size. Apparent mesh rim thickness is on average 33±19 µm corresponding to serpentinization extents of 70‐80%. Published laboratory experiments suggest that mesh rims formation could be completed in a few years (i.e. quasi‐instantaneous at the plate tectonic timescale). The depth and extent of the serpentinization domain in the detachment fault's footwall are probably variable in time and space and as a result we expect that the serpentine mesh‐texture at slow spreading ridges forms at variable rates with a spatially heterogeneous distribution.
      PubDate: 2014-05-23T12:06:56.491658-05:
      DOI: 10.1002/2013GC005148
  • Issue Information
    • Pages: i - i
      PubDate: 2014-05-22T13:45:02.780014-05:
      DOI: 10.1002/ggge.20320
  • The morphology of insular shelves as a key for understanding the
           geological evolution of volcanic islands: Insights from Terceira Island
    • Authors: R. Quartau; A. Hipólito, C. Romagnoli, D. Casalbore, J. Madeira, F. Tempera, C. Roque, F. L. Chiocci
      Pages: n/a - n/a
      Abstract: Shelves from volcanic ocean islands result from the competition between two main processes, wave erosion that forms and enlarges them and volcanic progradation that reduces their dimension. In places where erosion dominates over volcanism, shelf width can be used as a proxy for the relative age of the subaerial volcanic edifices and reconstruction of their extents prior to erosion can be achieved. In this study, new multibeam bathymetry and high‐resolution seismic reflection profiles are exploited to characterize the morphology of the insular shelves adjacent to each volcanic edifice of Terceira Island in order to improve the understanding of its evolution. Subaerial morphological and geological/stratigraphic data were also used to establish the connection between the onshore and offshore evolution. Shelf width contiguous to each main volcanic edifice is consistent with the known subaerial geological history of the island; most of the older edifices have wider shelves than younger ones. The shelf edge proved to be a very useful indicator in revealing the original extent of each volcanic edifice in plan view. Its depth was also used to reconstruct vertical movements, showing that older edifices like Serra do Cume‐Ribeirinha, Guilherme Moniz, and Pico Alto have subsided while more recent ones have not. The morphology of the shelf (namely the absence/presence of fresh lava flow morphologies and several types of erosional, depositional, and tectonic features) integrated with the analysis of the coastline morphology allowed us to better constrain previous geological interpretations of the island evolution.
      PubDate: 2014-05-22T13:27:30.665503-05:
      DOI: 10.1002/2014GC005248
  • Louisville Seamount Chain: Petrogenetic processes and geochemical
           evolution of the mantle source
    • Authors: Loÿc Vanderkluysen; John J. Mahoney, Anthony A. P. Koppers, Christoph Beier, Marcel Regelous, Jeffrey S. Gee, Peter F. Lonsdale
      Pages: n/a - n/a
      Abstract: The Louisville Seamount Chain is a ˜4300 km long chain of submarine volcanoes in the southwestern Pacific that spans an age range comparable to that of the Hawaiian‐Emperor chain and is commonly thought to represent a hotspot track. Dredging in 2006 recovered igneous rocks from 33 stations on 22 seamounts covering some 49 Myr of the chain's history. All samples are alkalic, similar to previous dredge and drill samples, providing no evidence for a Hawaiian‐type tholeiitic shield‐volcano stage. Major and trace element variations appear to be predominantly controlled by small but variable extents of fractional crystallization and by partial melting. Isotopic values define only a narrow range, in agreement with a surprising long‐term source homogeneity – relative to the length‐scale of melting – and overlap with proposed fields for the “C” and “FOZO” mantle end‐members. Trace element and isotope geochemistry is uncorrelated with either seamount age or lithospheric thickness at the time of volcanism, except for a small number of lavas from the westernmost Louisville Seamounts built on young (
      PubDate: 2014-05-20T04:21:40.705679-05:
      DOI: 10.1002/2014GC005288
  • Ta'u and Ofu/Olosega Volcanoes: The “Twin Sisters” of Samoa,
           their P, T, X Melting Regime, and Global Implications
    • Authors: Stanley R. Hart; Matthew G. Jackson
      Pages: n/a - n/a
      Abstract: The Samoan islands of Ta'u and Ofu/Olosega (Ofol hereafter) are single shield volcanoes that have erupted alkali basalt for the past 70 ky and 440 ky respectively. They are 20 km apart, and are the easternmost sub‐aerial expressions of the Samoan plume. The isotopic data for these islands is published; we report here the first major and trace element data for Ofol. The two islands are similar isotopically and in trace elements. Their high 3He/4He marks them as being a FOZO mantle end‐member. By comparing data from both volcanoes, we test the efficacy of melting models in constraining the mantle compositions and their P‐T of melting. We show that the mantle sources are similar, with spidergrams that peak at Ta (4x BSE), and Lu ~ 0.5x BSE. Melts and mantle sources both have Lu/Hf ratios that are too low to support measured 176Hf/177Hf ratios, pointing to a young enrichment event in the mantle source. Degrees of melting are 6.5% for Ta'u and 5.2% for Ofol. P‐ T of melting show a wide, overlapping range, but define a precise array; average values are 1475°C ‐ 77 km for Ta'u, and 1550°C ‐ 110 km for Ofol. The deepest P‐T estimate is 4.2 GPa and 1550°C. The P‐T array is either a melting adiabat, or a mixing line of melts equilibrated at various depths. Kinetic modeling shows melt re‐equilibration will be likely for ascent velocities (m/yr) less than 40/(conduit radius in cm)2. P‐T estimates for melting may typically be minimum values.
      PubDate: 2014-05-16T04:33:01.478645-05:
      DOI: 10.1002/2013GC005221
  • Reply to comment by Hu et al. on “Holocene evolution in weathering
           and erosion patterns in the Pearl River delta"
    • Authors: Peter D. Clift
      Pages: n/a - n/a
      PubDate: 2014-05-16T03:56:53.074639-05:
      DOI: 10.1002/2014GC005371
  • The intensity of the geomagnetic field from 2.4 Ga old Indian dykes
    • Authors: Jean‐Pierre Valet; Jean Besse, Anil Kumar, Sayoob Vadakke‐Chanat, Edouard Philippe
      Pages: n/a - n/a
      Abstract: Precambrian paleomagnetic records from dyke swarms provide a unique source of information regarding the Archean geomagnetic field and more specifically the average field strength produced by the early dynamo. We sampled 16 paleomagnetic sites from the Dharwar giant dyke swarm in southern India which was emplaced between 2.365 and 2.368 Ga. Despite taking great care in selecting locations exempt of any geological disturbance, only two of these sites provided primary directions with very steep inclinations and therefore were emplaced in close proximity to a magnetic pole. Paleointensity experiments were conducted on a subset of samples from the dyke margins. The characteristic magnetization is carried by single domain magnetite grains with a very narrow range of unblocking temperatures inferred from the sharp decrease by at least 75% of their remanence above 520°C. The paleointensity results indicate an average low field of 9.2 +/‐ 7 μT, consistent with reported values from Canadian dyke swarms for the same period. These results combined with the Thellier‐Thellier determinations obtained so far for the Precambrian suggest that a low field period prevailed from circa 2.3 to 1.8 Ga, while the preceeding and following time intervals are characterized by significantly stronger paleointensities. Although this suite of episodes is not fully incompatible with previous models for the long‐term evolution of the geodynamo, it is tempting to make the link with the recent suggestion of an early dynamo sustained within a conductive magma layer at the base of the mantle from 3.5 to 2.5 Ga which progressively declined until convection became sufficiently efficient to reactivate a strong dynamo process within the earth's liquid core.
      PubDate: 2014-05-16T03:49:26.04125-05:0
      DOI: 10.1002/2014GC005296
  • Three dimensional electrical structure of the crust and upper mantle in
           Ordos block and adjacent area: Evidence of regional lithospheric
    • Authors: Hao Dong; Wenbo Wei, Gaofeng Ye, Sheng Jin, Alan G. Jones, Jianen Jing, Letian Zhang, Chengliang Xie, Fan Zhang, Hui Wang
      Pages: n/a - n/a
      Abstract: Long period magnetotelluric (MT) data from project SINOPROBE were acquired and modeled, using three‐dimensional (3D) MT inversion, to study the electrical structure of Ordos Block, a component of the North China Craton. For the first time a high resolution 3D resistivity model of the lithosphere is defined for the region. Contrary to what would be expected for a stable cratonic block, a prominent lithospheric conductive complex is revealed extending from the upper mantle to the mid‐to‐lower crust beneath the northern part of Ordos. Correlating well with results of seismic studies, the evidence from our independent magnetotelluric data supports regional modification of the lithosphere under the north Ordos and lithosphere thinning beneath Hetao Graben. The abnormally conductive structure may result from upwelling of mantle material in mid‐to‐late Mesozoic beneath the northern margin of the Ordos block.
      PubDate: 2014-05-16T03:49:24.53125-05:0
      DOI: 10.1002/2014GC005270
  • Relative water and gas permeability for gas production from
           hydrate‐bearing sediments
    • Authors: Nariman Mahabadi; Jaewon Jang
      Pages: n/a - n/a
      Abstract: Relative water and gas permeability equations are important for estimating gas and water production from hydrate‐bearing sediments. However, experimental or numerical study to determine fitting parameters of those equations is not available in the literature. In this study, a pore‐network model is developed to simulate gas expansion and calculate relative water and gas permeability. Based on the simulation results, fitting parameters for modified Stone equation are suggested for a distributed hydrate system where initial hydrate saturations range from Sh=0.1 to 0.6. The suggested fitting parameter for relative water permeability is nw≈2.4 regardless of initial hydrate saturation while the suggested fitting parameter for relative gas permeability is increased from ng=1.8 for Sh=0.1 to ng=3.5 for Sh=0.6. Results are relevant to other systems that experience gas exsolution such as pockmark formation due to sea level change, CO2 gas formation during geological CO2 sequestration, and gas bubble accumulation near the downstream of dams.
      PubDate: 2014-05-16T03:49:15.358815-05:
      DOI: 10.1002/2014GC005331
  • Subslab anisotropy beneath the Sumatra and circum‐Pacific subduction
           zones from source‐side shear wave splitting observations
    • Authors: Colton Lynner; Maureen D. Long
      Pages: n/a - n/a
      Abstract: Understanding the dynamics of subduction is critical to our overall understanding of plate tectonics and the solid earth system. Observations of seismic anisotropy can yield constraints on deformation patterns in the mantle surrounding subducting slabs, providing a tool for studying subduction dynamics. While many observations of seismic anisotropy have been made in subduction systems, our understanding of the mantle beneath subducting slabs remains tenuous due to the difficulty of constraining anisotropy in the sub‐slab region. Recently, the source‐side shear wave splitting technique has been refined and applied to several subduction systems worldwide, making accurate and direct measurements of sub‐slab anisotropy feasible and offering unprecedented spatial and depth coverage in the sub‐slab mantle. Here we present source‐side shear wave splitting measurements for the Central America, Alaska‐Aleutians, Sumatra, Ryukyu, and Izu‐Bonin‐Japan‐Kurile subduction systems. We find that measured fast splitting directions in these regions generally fall into two broad categories, aligning either with the strike of the trench or with the motion of the subducting slab relative to the overriding plate. Trench parallel fast splitting directions dominate beneath the Izu‐Bonin, Japan, and southern Kurile slabs and part of the Sumatra system, while fast directions that parallel the motion of the downgoing plate dominate in the Ryukyu, Central America, northern Kurile, western Sumatra, and Alaska‐Aleutian regions. We find that plate motion parallel fast splitting directions in the sub‐slab mantle are more common than previously thought. We observe a correlation between fast direction and age of the subducting lithosphere; older lithosphere (> 95 Ma) is associated with trench parallel splitting while younger lithosphere (< 95 Ma) is associated with plate motion parallel fast splitting directions. Finally, we observe source‐side splitting for deep earthquakes (transition zone depths) beneath Japan and Sumatra, suggesting the presence of anisotropy at mid‐mantle depths beneath these regions.
      PubDate: 2014-05-15T15:37:23.400413-05:
      DOI: 10.1002/2014GC005239
  • A new bathymetric compilation for the South Orkney Islands, Antarctic
           Peninsula (49°–39°W to 64°–59°S): Insights
           into the glacial development of the continental shelf
    • Authors: William A. Dickens; Alastair G.C. Graham, James A. Smith, Julian A. Dowdeswell, Robert D. Larter, Claus‐Dieter Hillenbrand, Phil N. Trathan, Jan Erik Arndt, Gerhard Kuhn
      Pages: n/a - n/a
      Abstract: We present a new, high resolution (300 m) bathymetric grid of the South Orkney Islands and surrounding continental shelf, northeast of the Antarctic Peninsula. The new grid, derived from a compilation of marine echo‐sounding data, improves previous regional bathymetric representations and helps to visualise the morphology of the shelf in unrivalled detail. The compilation forms important baseline information for a range of scientific applications and end users including oceanographers, glacial modellers, biologists and geologists. In particular, due to our limited understanding of glacial history in this region, the bathymetry provides the first detailed insights into past glacial regimes. The continental shelf is dominated by seven glacially eroded troughs, marking the pathways of glacial outlets that once drained a former ice cap centred on the South Orkney Islands. During previous glacial periods, grounded ice extended to the shelf edge north of the islands. A large, ~250 km long sediment depocentre, interpreted as a maximum former ice limit of one or more Cenozoic glaciations, suggests that ice was only grounded to the ~300‐350 m contour in the south. Hypsometric analyses support this interpretation, indicating that a significant proportion of the shelf has been unaffected by glacial erosion. Using these observations, we propose a preliminary ice cap reconstruction for maximum glaciation of the South Orkney plateau, suggesting an ice coverage of about ~19,000 km2. The timing of maximum ice extent, number of past advances and pattern of subsequent deglaciation(s) remain uncertain and will require further targeted marine geological and geophysical investigations to resolve.
      PubDate: 2014-05-15T15:34:43.437029-05:
      DOI: 10.1002/2014GC005323
  • Along‐arc variation in the 3D thermal structure around the junction
           between the Japan and Kurile arcs
    • Authors: Manabu Morishige; Peter E. Keken
      Pages: n/a - n/a
      Abstract: The thermal structure in subduction zones has a strong influence on seismogenesis and arc volcanism. Traditional 2D models have been used to provide reasonable agreement between models and observations, but in a number of cases clear 3D effects are present. One such case is in the Northern Japan subduction system. At the junction between Japan and Kurile arcs surface heat flow and the occurrence of intermediate‐depth seismicity is different than in the Tohoku and Hokkaido regions. We investigate the effects of 3D slab geometry and a local deepening of slab‐mantle decoupling depth on the thermal structure in this region based on 3D finite element approach. We find that both effects produce the along‐arc variation of slab surface temperature, which could reach ˜100°C. The warmer region arises through 3D effects of thermal conduction and the colder region arises through localized slow incoming flow in the case where 3D slab geometry is taken into account. 3D flow arises where a local deepening of slab‐mantle decoupling depth is assumed, which leads to both warmer and colder regions. The effects on surface heat flow are small. While intermediate‐depth seismicity in the subducted crust is suggested to be controlled by temperature‐dependent phase transitions the predicted changes in thermal structure are not sufficient to cause the observed deepening of seismicity. This suggests the thermal structure of this subduction zone may be more strongly influenced by time‐dependent deformation of the overriding crust and slab.
      PubDate: 2014-05-15T13:23:21.415228-05:
      DOI: 10.1002/2014GC005394
  • Comment on “Holocene evolution in weathering and erosion patterns in
           the Pearl River delta” by Hu et al.
    • Authors: Jianguo Liu; Wen Yan, Zhong Chen, Han Chen, Jun Lu
      Pages: n/a - n/a
      PubDate: 2014-05-15T12:38:32.31985-05:0
      DOI: 10.1002/2013GC005202
  • Geochemical evidence for active tropical serpentinization in the Santa
           Elena Ophiolite, Costa Rica: An analog of a humid early Earth?
    • Authors: Ricardo Sánchez‐Murillo; Esteban Gazel, Esther M. Schwarzenbach, Melitza Crespo‐Medina, Matthew O. Schrenk, Jan Boll, Ben C. Gill
      Pages: n/a - n/a
      Abstract: Serpentinization is a planetary process that has important consequences on geochemical cycles, supporting microbial activity through the formation of H2 and CH4 and having the potential to sequester atmospheric CO2. We present geochemical evidence of active serpentinization in the Santa Elena Ophiolite, Costa Rica which is sustained by peridotites with a degree of serpentinization less than 50% with no evidence of an internal heat source. Average spring water temperatures are 29.1°C. Two hyperalkaline spring systems were discovered, with a spring fluid pH up to 11.18. The fluids are characterized by low Mg (1.0–5.9 mg/L) and K (1.0–5.5 mg/L) and relative high Ca (29–167 mg/L), Na (16–27 mg/L), Cl (26–29 mg/L), hydroxide (41–63 mg/L), and carbonate (31–49 mg/L). Active CH4 (24.3% v/v) vents coupled with carbonate deposits ( δ13CCO2 =−27 to −14‰; δ18OCO2 =−17 to − 6‰) also provide evidence for active serpentinization and carbonation. Isotope ratios of the alkaline fluids (δ18O = −7.9‰, δ2H = −51.4‰) and groundwater (δ18O = −7.6‰; δ2H = −48.0‰) suggests that, during base flow recession, springs are fed by groundwater circulation. Methanogenic Archaea, which comprises a relatively high percentage of the 16S rRNA gene tag sequences, suggests that biological methanogenesis may play a significant role in the system. Santa Elena's extreme varying weather results in a scenario that could be of significant importance for (a) improving the knowledge of conditions on a humid early Earth or Mars that had periodic changes in water supply, (b) revealing new insights on serpentinizing solute transport, and (c) modeling hydrogeochemical responses as a function of recharge.
      PubDate: 2014-05-14T15:26:30.827726-05:
      DOI: 10.1002/2013GC005213
  • Eruptive timing and 200‐year episodicity at 92°W on the
           hotspot‐influenced Galapagos Spreading Center derived from
           geomagnetic paleointensity
    • Authors: Julie A. Bowles; Alice Colman, J. Timothy McClinton, John M. Sinton, Scott M. White, Kenneth H. Rubin
      Pages: n/a - n/a
      Abstract: Eruptive timing in mid‐ocean ridge systems is relatively poorly constrained, despite being an important variable in our understanding of many mid‐ocean ridge processes, including volcanic construction; magma recharge, flux, and storage; and the stability of hydrothermal systems and biological communities. Only a handful of absolute eruption chronologies exist, yet they are essential in understanding how eruptive timing varies with important controlling variables. To construct an eruptive history at one location on the Galápagos Spreading Center, we present age determinations derived from geomagnetic paleointensity. To aid interpretation of the paleointensity data, we also present results from on‐bottom magnetic anomaly measurements and forward modeling of topographic‐induced magnetic anomalies. Anomalies may lead to a 1‐2 µT bias in flow‐mean paleointensities, which does not significantly affect the overall interpretation. Paleointensity results for the three youngest sampled units are indistinguishable, consistent with the flows being emplaced in relatively rapid succession. Comparisons with models of geomagnetic field behavior suggest these flows were erupted sometime in the past 100‐200 years. The fourth sampled unit has a significantly higher paleointensity, consistent with an age of roughly 400 yrs. The possible bias in paleointensity data allows for ages as young as ˜50 years for the youngest three flows and 200‐400 years for the oldest flow. This age distribution demonstrates an episodicity in the emplacement of the largest flows at this location, with a 200‐300 year period of relative quiescence between emplacement of the oldest unit and the three youngest units.
      PubDate: 2014-05-12T02:10:56.332776-05:
      DOI: 10.1002/2014GC005315
  • P wave radial anisotropy tomography of the upper mantle beneath the North
           China Craton
    • Authors: Jian Wang; Huohua Wu, Dapeng Zhao
      Pages: n/a - n/a
      Abstract: We present the first P wave radial anisotropy tomography of the crust and upper mantle beneath the North China Craton (NCC), determined using a large number of high‐quality arrival‐time data of local earthquakes and teleseismic events. Our results show a prominent high‐velocity (high‐V) anomaly down to ˜250 km depth beneath the Ordos block, a high‐V anomaly in the mantle transition zone beneath the eastern NCC, and a low‐velocity (low‐V) anomaly down to ˜300 km depth beneath the Trans‐North China Orogen (TNCO). The Ordos block exhibits significant negative radial anisotropy (i.e., vertical Vp > horizontal Vp), suggesting that its cratonic lithosphere has kept the frozen‐in anisotropy formed by vertical growth via high‐degree melting mantle plume in the early Earth. Prominent low‐V anomalies with positive radial anisotropy (i.e., horizontal Vp > vertical Vp) exist beneath the Qilian and Qaidam blocks down to ˜400 km depth, suggesting that the horizontal material flow resulting from the Tibetan Plateau is blocked by the Ordos thick lithosphere. Beneath the eastern NCC, high‐V anomalies with negative radial anisotropy exist in the upper mantle, possibly reflecting sinking remains of the Archean cratonic lithosphere. A high‐V anomaly with positive radial anisotropy is revealed in the mantle transition zone under the eastern NCC, which may reflect the stagnant Pacific slab.
      PubDate: 2014-05-11T21:14:36.984129-05:
      DOI: 10.1002/2014GC005279
  • High‐resolution 40Ar/39Ar dating using a mechanical sample transfer
           system combined with a high‐temperature cell for step heating
           experiments and a multicollector ARGUS noble gas mass spectrometer
    • Authors: Jörg A. Pfänder; Blanka Sperner, Lothar Ratschbacher, Albrecht Fischer, Martin Meyer, Martin Leistner, Helmut Schaeben
      Pages: n/a - n/a
      Abstract: 40Ar/39Ar dating of young (5×10‐16 mol 36Ar is better than 0.5 – 1.0 ‰ (1σ, n=4–8). We illustrate the system performance by 40Ar/39Ar dating of whole‐rock samples and mineral separates from the Oman ophiolite as well as from the Siebengebirge, Heldburg, and Rhön volcanic provinces in Central Germany.
      PubDate: 2014-05-10T03:11:41.702309-05:
      DOI: 10.1002/2014GC005289
  • Combined 147,146Sm‐143,142Nd constraints on the longevity and
           residence time of early terrestrial crust
    • Authors: Antoine S.G. Roth; Bernard Bourdon, Stephen J. Mojzsis, John F. Rudge, Martin Guitreau, Janne Blichert‐Toft
      Pages: n/a - n/a
      Abstract: Primordial silicate differentiation controlled the composition of Earth's oldest crust. Inherited 142Nd anomalies in Archean rocks are vestiges of the mantle‐crust differentiation before ca. 4300 Ma. Here, we report new whole‐rock 147,146Sm‐143,142Nd data for the Acasta Gneiss Complex (AGC; Northwest Territories, Canada). Our 147Sm‐143Nd data combined with literature data define an age of 3371±141 Ma (2 SD) and yield an initial ε143Nd of ‐5.6±2.1. These results are at odds with the Acasta zircon U‐Pb record, which comprises emplacement ages of 3920‐3960 Ma. Ten of our thirteen samples show 142Nd deficits of ‐9.6±4.8 ppm (2 SD) relative to the modern Earth. The discrepancy between 142Nd anomalies and a mid‐Archean 147Sm‐143Nd age can be reconciled with Nd isotope reequilibration of the AGC during metamorphic perturbations at ca. 3400 Ma. A model age of ca. 4310 Ma is derived for the early‐enrichment of the Acasta source. Two compositional end‐members can be identified: a felsic component with 142Nd/144Nd identical to the modern Earth, and a mafic component with 142Nd/144Nd as low as ‐14.1 ppm. The ca. 4310 Ma AGC source is ~200 Myr younger than those estimated for Nuvvuagittuq (northern Québec) and Isua (Itsaq Gneiss Complex, West Greenland). The AGC does not have the same decoupled Nd‐Hf isotope systematics as these other two terranes, which have been attributed to the crystallization of an early magma ocean. The Acasta signature rather is ascribed to the formation of Hadean crust that was preserved for several hundred Myr. Its longevity can be linked to 142Nd evolution in the mantle and does not require slow mantle stirring times nor modification of its convective mode.
      PubDate: 2014-05-07T13:04:46.747321-05:
      DOI: 10.1002/2014GC005313
  • Hydrocarbon seepage and its sources at mud volcanoes of the Kumano forearc
           basin, Nankai Trough subduction zone
    • Authors: Thomas Pape; Patrizia Geprägs, Sebastian Hammerschmidt, Paul Wintersteller, Jiangong Wei, Timo Fleischmann, Gerhard Bohrmann, Achim J. Kopf
      Pages: n/a - n/a
      Abstract: Twelve submarine mud volcanoes (MV) in the Kumano forearc basin within the Nankai Trough subduction zone were investigated for hydrocarbon origins and fluid dynamics. Gas hydrates diagnostic for methane concentrations exceeding solubilities were recovered from MVs 2, 4, 5, and 10. Molecular ratios (C1/C2 −40‰ V‐PDB) indicate that hydrate‐bound hydrocarbons (HCs) at MVs 2, 4, and 10 are derived from thermal cracking of organic matter. Considering thermal gradients at the nearby IODP Sites C0009 and C0002, the likely formation depth of such HCs ranges between 2,300 and 4,300 m below seafloor (mbsf). With respect to basin sediment thickness and the minimum distance to the top of the plate boundary thrust we propose that the majority of HCs fueling the MVs is derived from sediments of the Cretaceous to Tertiary Shimanto belt below Plio‐/Pleistocene to recent basin sediments. With respect to sizes and appearances hydrates are suggested to be relicts of higher MV activity in the past, although the sporadic presence of vesicomyid clams at MV 2 showed that fluid migration is sufficient to nourish chemosynthesis‐based organisms in places. Distributions of dissolved methane at MVs 3, 4, 5, and 8 pointed at fluid supply through one or few MV conduits and effective methane oxidation in the immediate subsurface. The aged nature of the hydrates suggests that the major portion of methane immediately below the top of the methane‐containing sediment interval is fueled by current hydrate dissolution rather than active migration from greater depth.
      PubDate: 2014-05-03T04:56:03.459714-05:
      DOI: 10.1002/2013GC005057
  • Characterizing magnetofossils from first‐order reversal curve
           central ridge signatures
    • Authors: David Heslop; Andrew P. Roberts, Liao Chang
      Pages: n/a - n/a
      Abstract: The central ridge structure of a first‐order reversal curve (FORC) distribution is indicative of uniaxial noninteracting single domain magnetic particles, which provides the opportunity to identify and characterize biogenic magnetic mineral remains (magnetofossils) in sediments. Recent studies have shown that magnetofossils are widespread in the geological record and that they carry useful environmental information and contribute to paleomagnetic recording, which makes it essential to quantify how these biogenic components contribute to the magnetic properties of sediments. We present results from six sedimentary sequences whose magnetic mineral assemblages contain a significant magnetofossil contribution. Using principal component analysis, we find that the central ridge properties exhibit both intra‐ and inter‐sequence variability that may be ascribed to external environmental factors. While samples from individual sediment sequences tend to cluster together, there is a continuum of inter‐sequence behavior that appears to be related to a variety of magnetofossil properties. We demonstrate the complexity of biogenic magnetic components in sedimentary environments, but also the power and potential of FORC central ridges for understanding magnetic mixtures and unraveling environmental information.
      PubDate: 2014-05-02T04:00:21.89239-05:0
      DOI: 10.1002/2014GC005291
  • 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
  • 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
  • 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
  • Seismic mountain building: Landslides associated with the 2008 Wenchuan
           earthquake in the context of a generalized model for earthquake volume
    • Authors: Gen Li; A. Joshua West, Alexander L. Densmore, Zhangdong Jin, Robert N. Parker, Robert G. Hilton
      Pages: 833 - 844
      Abstract: Here we assess earthquake volume balance and the growth of mountains in the context of a new landslide inventory for the Mw 7.9 Wenchuan earthquake in central China. Coseismic landslides were mapped from high‐resolution remote imagery using an automated algorithm and manual delineation, which allow us to distinguish clustered landslides that can bias landslide volume calculations. Employing a power‐law landslide area‐volume relation, we find that the volume of landslide‐associated mass wasting (∼2.8 + 0.9/−0.7 km3) is lower than previously estimated (∼5.7–15.2 km3) and comparable to the volume of rock uplift (∼2.6 ± 1.2 km3) during the Wenchuan earthquake. If fluvial evacuation removes landslide debris within the earthquake cycle, then the volume addition from coseismic uplift will be effectively offset by landslide erosion. If all earthquakes in the region followed this volume budget pattern, the efficient counteraction of coseismic rock uplift raises a fundamental question about how earthquakes build mountainous topography. To provide a framework for addressing this question, we explore a group of scaling relations to assess earthquake volume balance. We predict coseismic uplift volumes for thrust‐fault earthquakes based on geophysical models for coseismic surface deformation and relations between fault rupture parameters and moment magnitude, Mw. By coupling this scaling relation with landslide volume‐Mw scaling, we obtain an earthquake volume balance relation in terms of moment magnitude Mw, which is consistent with the revised Wenchuan landslide volumes and observations from the 1999 Chi‐Chi earthquake in Taiwan. Incorporating the Gutenburg‐Richter frequency‐Mw relation, we use this volume balance to derive an analytical expression for crustal thickening from coseismic deformation based on an index of seismic intensity over a defined area. This model yields reasonable rates of crustal thickening from coseismic deformation (e.g., ∼0.1–0.5 km Ma−1 in tectonically active convergent settings), and implies that moderate magnitude earthquakes (Mw ≈ 6–7) are likely responsible for most of the coseismic contribution to rock uplift because of their smaller landslide‐associated volume reduction. Our first‐order model does not consider a range of factors (e.g., lithology, climate conditions, epicentral depth, and tectonic setting), nor does it account for viscoelastic effects or isostatic responses to erosion, and there are important large uncertainties on the scaling relationships used to quantify coseismic deformation. Nevertheless, our study provides a conceptual framework and invites more rigorous modeling of seismic mountain building.
      PubDate: 2014-04-11T13:53:16.4403-05:00
      DOI: 10.1002/2013GC005067
  • Bayesian noise‐reduction in Arabia/Somalia and Nubia/Arabia finite
           rotations since ∼20 Ma: Implications for Nubia/Somalia relative
    • Authors: Giampiero Iaffaldano; Rhys Hawkins, Malcolm Sambridge
      Pages: 845 - 854
      Abstract: Knowledge of Nubia/Somalia relative motion since the Early Neogene is of particular importance in the Earth Sciences, because it (i) impacts on inferences on African dynamic topography; and (ii) allows us to link plate kinematics within the Indian realm with those within the Atlantic basin. The contemporary Nubia/Somalia motion is well known from geodetic observations. Precise estimates of the past‐3.2‐Myr average motion are also available from paleo‐magnetic observations. However, little is known of the Nubia/Somalia motion prior to ∼3.2 Ma, chiefly because the Southwest Indian Ridge spread slowly, posing a challenge to precisely identify magnetic lineations. This also makes the few observations available particularly prone to noise. Here we reconstruct Nubia/Somalia relative motions since ∼20 Ma from the alternative plate‐circuit Nubia‐Arabia‐Somalia. We resort to trans‐dimensional hierarchical Bayesian Inference, which has proved effective in reducing finite‐rotation noise, to unravel the Arabia/Somalia and Arabia/Nubia motions. We combine the resulting kinematics to reconstruct the Nubia/Somalia relative motion since ∼20 Ma. We verify the validity of the approach by comparing our reconstruction with the available record for the past ∼3.2 Myr, obtained through Antarctica. Results indicate that prior to ∼11 Ma the total motion between Nubia and Somalia was faster than today. Furthermore, it featured a significant strike‐slip component along the Nubia/Somalia boundary. It is only since ∼11 Ma that Nubia diverges away from Somalia at slower rates, comparable to the present‐day one. Kinematic changes of some 20% might have occurred in the period leading to the present‐day, but plate‐motion steadiness is also warranted within the uncertainties.
      PubDate: 2014-04-02T15:23:11.032709-05:
      DOI: 10.1002/2013GC005089
  • Empirical evidence for the fractionation of carbon isotopes between
           diamond and iron carbide from the Earth's mantle
    • Authors: S. Mikhail; C. Guillermier, I. A. Franchi, A. D. Beard, K. Crispin, A. B. Verchovsky, A. P. Jones, H. J. Milledge
      Pages: 855 - 866
      Abstract: We have studied two samples of mantle diamond containing iron carbide inclusions from Jagersfontein kimberlite, South Africa. Syngenetic crystal growth is inferred using morphological characteristics. These samples provide an opportunity to investigate the isotopic partitioning of 13C in a terrestrial natural high‐pressure and high‐temperature (HPHT) system. The difference for the δ13C values between the diamond and coexisting iron carbide averaged 7.2 ± 1.3‰. These data are consistent with available data from the literature showing iron carbide to be 13C‐depleted relative to elemental carbon (i.e., diamond). We infer that the minerals formed by crystallization of diamond and iron carbide at HPHT in the mantle beneath the Kaapvaal Craton. It is unclear whether crystallization occurred in subcratonic or sublithospheric mantle; in addition, the source of the iron is also enigmatic. Nonetheless, textural coherence between diamond and iron carbide resulted in isotopic partitioning of 13C between these two phases. These data suggest that significant isotopic fractionation of 13C/12C (Δ13C up to >7‰) can occur at HPHT in the terrestrial diamond stability field. We note that under reducing conditions at or below the iron‐iron wustite redox buffer in a cratonic or deep mantle environment in Earth, the cogenesis of carbide and diamond may produce reservoirs of 13C‐depleted carbon that have conventionally been interpreted as crustal in origin. Finally, the large Δ13C for diamond‐iron carbide shown here demonstrates Δ13C for silicate‐metallic melts is a parameter that needs to be constrained to better determine the abundance of carbon within the Earth's metallic core.
      PubDate: 2014-04-02T15:24:28.322624-05:
      DOI: 10.1002/2013GC005138
  • Constraints on a shallow offshore gas environment determined by a
           multidisciplinary geophysical approach: The Malin Sea, NW Ireland
    • Authors: Xavier Garcia; Xavier Monteys, Rob L. Evans, Michal Szpak
      Pages: 867 - 885
      Abstract: During the Irish National Seabed Survey (INSS) in 2003, a gas related pockmark field was discovered and extensively mapped in the Malin Shelf region (NW Ireland). In summer 2006, additional complementary data involving core sample analysis, multibeam and single‐beam backscatter classification, and a marine controlled‐source electromagnetic survey were obtained in specific locations.This multidisciplinary approach allowed us to map the upper 20 m of the seabed in an unprecedented way and to correlate the main geophysical parameters with the geological properties of the seabed. The EM data provide us with information about sediment conductivity, which can be used as a proxy for porosity and also to identify the presence of fluid and fluid migration pathways. We conclude that, as a whole, the central part of the Malin basin is characterized by higher conductivities, which we interpret as a lithological change. Within the basin several areas are characterized by conductive anomalies associated with fluid flow processes and potentially the presence of microbial activity, as suggested by previous work. Pockmark structures show a characteristic electrical signature, with high‐conductivity anomalies on the edges and less conductive, homogeneous interiors with several high‐conductivity anomalies, potentially associated with gas‐driven microbial activity.
      PubDate: 2014-04-02T15:37:12.260568-05:
      DOI: 10.1002/2013GC005108
  • Geochemistry of the Cambrian Sirius Passet Lagerstätte, Northern
    • Authors: Ange Le Boudec; Jon Ineson, Minik Rosing, Lasse Døssing, François Martineau, Christophe Lécuyer, Francis Albarède
      Pages: 886 - 904
      Abstract: The lower Cambrian Sirius Passet Lagerstätte (∼518 Ma) consists of mudstones about 8 m thick located in the Franklinian Basin of North Greenland. We analyzed major and trace elements plus the S, C, Cu, Fe, Zn, and Mo isotope compositions. Factor analysis allowed the lithology of the mudstone to be broken down into variable proportions of two inputs, a dry felsic component and a hydrous mafic component (smectite or chlorite). Zircons U‐Pb ages indicate multiple sources, the local Proterozoic basement of Northern Greenland (1250–2400 Ma) and Pan‐African felsic magmas (620–650 Ma) from across the Iapetus ocean. Diagenesis involved the reduction of Fe, S, and Mo from seawater and pyritization. The Sirius Passet Lagerstätte formed in oxygen‐starved muds inhibiting degradation of organic matter underneath a well‐ventilated water column. The chemistry of the samples, their very fine grain size, their apparent lack of graded bedding, and the age of zircons suggest that the Lagerstätte may represent wind‐blown dust deposited on the continental slope.
      PubDate: 2014-04-02T15:37:22.920556-05:
      DOI: 10.1002/2013GC005068
  • Model evaluation of the coherence of a common source water oxygen isotopic
           signal recorded by tree‐ring cellulose and speleothem calcite
    • Authors: S. R. Managave
      Pages: 905 - 922
      Abstract: Oxygen isotopic compositions (δ18O) of tree rings and speleothems inherit δ18O of precipitation (δ18Oppt) and are used to reconstruct past variations in the amount of precipitation or temperature. Integration of δ18O records of both the proxies for a multiproxy climate reconstruction necessitates understanding of the extent to which the initial δ18Oppt is modified in both the proxies before getting preserved in them. A forward modeling approach, assuming both the proxies have access to a common source water δ18O signal, is employed to assess the expected maximum correlation between the δ18O records of the two proxies on interannual time scale and the factors influencing the correlation. Variations in the leaf temperature, relative humidity, δ18O of atmospheric water vapor introduce variations in δ18O of tree‐ring cellulose, while the variations in the cave temperature and oxygen isotopic equilibrium/disequilibrium conditions between calcite and drip water influence δ18O of speleothem calcite. Variation introduced by these factors in δ18O of individual proxies as compared to the variation in δ18Oppt is crucial in deciding the correlation between δ18O records of the two proxies. When the former is higher than or comparable with the latter, finding one to one correspondence between the δ18O records of the two proxies could be challenging. Modeling exercise revealed a favorable climatic condition at Jagdalpur (19.08°N, 82.03°E), a potential tropical location for tree ring and speleothem‐based climate reconstruction, for inheriting a common source water δ18O signal in both the proxies. δ18O records of tree ring and annually resolved speleothem from Jagdalpur could be integrated for the multiproxy climate reconstruction.
      PubDate: 2014-04-02T15:37:27.511982-05:
      DOI: 10.1002/2013GC004983
  • Magnetic properties of variably serpentinized peridotites and their
           implication for the evolution of oceanic core complexes
    • Authors: Marco Maffione; Antony Morris, Oliver Plümper, Douwe J. J. van Hinsbergen
      Pages: 923 - 944
      Abstract: Serpentinization of ultramafic rocks during hydrothermal alteration at mid‐ocean ridges profoundly changes the physical, chemical, rheological, and magnetic properties of the oceanic lithosphere. There is renewed interest in this process following the discovery of widespread exposures of serpentinized mantle on the seafloor in slow spreading oceans. Unroofing of mantle rocks in these settings is achieved by displacement along oceanic detachment faults, which eventually results in structures known as oceanic core complexes (OCCs). However, we have limited understanding of the mechanisms of serpentinization at the seafloor and in particular their relationship with the evolution of OCCs. Since magnetite is a direct product of serpentinization, the magnetic properties of variably serpentinized peridotites can provide unique insights into these mechanisms and their evolution in the oceanic lithosphere. Here we present new results from an integrated, rock magnetic, paleomagnetic, and petrological study of variably serpentinized peridotites from the first fossil OCC recognized in an ophiolite. Integration with existing data from mid‐ocean ridge‐related abyssal peridotites recovered from several scientific ocean drilling sites yields the first magnetic database from peridotites extending across the complete range (0–100%) of degrees of serpentinization. Variations in a range of magnetic parameters with serpentinization, and associated paleomagnetic data, provide: (i) key constraints on the mechanism(s) of serpentinization at mid‐ocean ridges; (ii) insights on the potential for serpentinized peridotites to contribute to marine magnetic anomalies; and (iii) evidence that leads to a new conceptual model for the evolution of serpentinization and related remanence acquisition at OCCs.
      PubDate: 2014-04-02T15:40:35.734937-05:
      DOI: 10.1002/2013GC004993
  • Background and delayed‐triggered swarms in the central Southern
           Alps, South Island, New Zealand
    • Authors: C. M. Boese; K. M. Jacobs, E. G. C. Smith, T. A. Stern, J. Townend
      Pages: 945 - 964
      Abstract: Low‐magnitude earthquake swarms (ML ≤ 2.8), consisting of up to 47 events of similar waveforms, have been observed repeatedly in the central Southern Alps, a rapidly uplifting orogen bounded by the transpressive Alpine Fault in the South Island of New Zealand. We compare nine background swarms recorded between November 2008 and April 2010 with five delayed‐triggered swarms that occurred after the MW 7.8 Dusky Sound and the MW 7.1 Darfield (Canterbury) earthquakes. The two types of swarms are similar in terms of the magnitudes, depths, focal mechanisms, and interevent times of the constituent microearthquakes, and appear to both involve the rupture of steeply dipping faults in highly fractured crust in a 10 km × 12 km area in the center of the SAMBA network. The delayed‐triggered swarms occurred at similar epicentral distances (c. 4.5× the rupture length of the mainshocks) to the Dusky Sound and Darfield earthquakes, commenced shortly after the passage of the surface waves, continued for ∼5 and ∼2 days, respectively, and were followed in each case by a ≥2 day long quiescent period, which may suggest clock‐advanced of faults in their failure‐cycle. Triggering thresholds of ≥0.01 MPa proposed elsewhere are similar to the dynamic stress changes computed for the Southern Alps (≥0.09 MPa). However, as 98% of the locatable triggered events occurred several hours after the surface waves had passed, the dynamic stress changes associated with the surface waves themselves are unlikely to have triggered the earthquakes directly. Instead, we suggest that the locations and delays of the triggered swarms are more consistent with triggering by pore pressure diffusion.
      PubDate: 2014-04-02T15:48:42.884782-05:
      DOI: 10.1002/2013GC005171
  • Natural surface rebound of the Bangkok plain and aquifer characterization
           by persistent scatterer interferometry
    • Authors: Kazuya Ishitsuka; Yo Fukushima, Takeshi Tsuji, Yasuhiro Yamada, Toshifumi Matsuoka, Pham Huy Giao
      Pages: 965 - 974
      Abstract: We estimated recent surface displacements around Bangkok by means of persistent scatterer interferometry with ALOS/PALSAR images acquired from November 2007 to December 2010. Land subsidence due to excessive groundwater pumping has been reported in this region. However, we detected ground surface uplift around the mega‐city, along with seasonal surface displacement, with high spatial resolution. We then discriminated long‐term natural rebound and seasonal displacement by fitting exponential and sinusoidal functions to displacement time‐series, and mapped their spatial distributions. This mapping allowed us to infer that the second and third shallowest aquifers are laterally continuous, whereas the shallowest aquifer has lateral discontinuities. The temporal decay rate of the long‐term rebound might reflect spatial changes of the Chao Phraya River watershed or the magnitude of the preceding groundwater extraction. We demonstrated that our method of decomposing the displacement time series into different spatial and temporal patterns is useful for understanding aquifer connectivity and the elastic response pattern in an aquifer system.
      PubDate: 2014-04-02T15:42:34.897882-05:
      DOI: 10.1002/2013GC005154
  • Piecewise delamination of Moroccan lithosphere from beneath the Atlas
    • Authors: M. J. Bezada; E. D. Humphreys, J.M. Davila, R. Carbonell, M. Harnafi, I. Palomeras, A. Levander
      Pages: 975 - 985
      Abstract: The elevation of the intracontinental Atlas Mountains of Morocco and surrounding regions requires a mantle component of buoyancy, and there is consensus that this buoyancy results from an abnormally thin lithosphere. Lithospheric delamination under the Atlas Mountains and thermal erosion caused by upwelling mantle have each been suggested as thinning mechanisms. We use seismic tomography to image the upper mantle of Morocco. Our imaging resolves the location and shape of lithospheric cavities and of delaminated lithosphere ∼400 km beneath the Middle Atlas. We propose discontinuous delamination of an intrinsically unstable Atlas lithosphere, enabled by the presence of anomalously hot mantle, as a mechanism for producing the imaged structures. The Atlas lithosphere was made unstable by a combination of tectonic shortening and eclogite loading during Mesozoic rifting and Cenozoic magmatism. The presence of hot mantle sourced from regional upwellings in northern Africa or the Canary Islands enhanced the instability of this lithosphere. Flow around the retreating Alboran slab focused upwelling mantle under the Middle Atlas, which we infer to be the site of the most recent delamination. The Atlas Mountains of Morocco stand as an example of large‐scale lithospheric loss in a mildly contractional orogen.
      PubDate: 2014-04-09T15:02:24.874831-05:
      DOI: 10.1002/2013GC005059
  • Evidence for a broadly distributed Samoan‐plume signature in the
           northern Lau and North Fiji Basins
    • Authors: Allison A. Price; Matthew G. Jackson, Janne Blichert‐Toft, Paul S. Hall, John M. Sinton, Mark D. Kurz, Jerzy Blusztajn
      Pages: 986 - 1008
      Abstract: Geochemical enrichment of lavas in the northern Lau Basin may reflect the influx of Samoan‐plume mantle into the region. We report major and trace element abundances and He‐Sr‐Nd‐Hf‐Pb‐isotopic measurements for 23 submarine volcanic glasses covering 10 locations in the northern Lau and North Fiji Basins, and for three samples from Wallis Island, which lies between Samoa and the Lau Basin. These data extend the western limit of geochemical observations in the Basins and improve the resolution of North‐South variations in isotopic ratios. The Samoan hot spot track runs along the length of the northern trace of the Lau and North Fiji Basins. We find evidence for a Samoan‐plume component in lavas as far West as South Pandora Ridge (SPR), North Fiji Basin. Isotopic signatures in SPR samples are similar to those found in Samoan Upolu shield lavas, but show a slight shift toward MORB‐like compositions. We explain the origin of the enriched signatures by a model in which Samoan‐plume material and ambient depleted mantle undergo decompression melting during upwelling after transiting from beneath the thick Pacific lithosphere to beneath the thin lithosphere in the northern Lau and North Fiji Basins. Other lavas found in the region with highly depleted isotopic signatures may represent isolated pockets of depleted mantle in the basins that evaded this enrichment process. We further find that mixing between the two components in our model, a variably degassed high‐3He/4He Samoan component and depleted MORB, can explain the diversity among geochemical data from the northern Lau Basin.
      PubDate: 2014-04-11T15:55:31.175069-05:
      DOI: 10.1002/2013GC005061
  • Detection of overpressure zones and a statistical model for pore pressure
           estimation from well logs in the Krishna‐Godavari Basin, India
    • Authors: Dip Kumar Singha; Rima Chatterjee
      Pages: 1009 - 1020
      Abstract: Abnormally high pressures, measured by repeat formation tester (RFT) and detected by well log data from 10 wells in the Krishna‐Godavari (K‐G) Basin, occur in the Vadaparru Shale of Miocene and Raghavapuram Shale of Early Cretaceous age. Overpressures generated by disequilibrium compaction, and pore pressures have been estimated using the conventional Eaton sonic equation with an exponent of 3.0. The observed abnormal pore pressure gradient ranges from 11.85 to 13.10 MPa/km, whereas fracture pressure gradient varies from 17.40 to 19.78 MPa/km. The magnitude of vertical stress (Sv) has a gradient from 21.00 to 23.10 MPa/km. The minimum horizontal principal stress (Sh) magnitude is found to vary from 64 to 77% of the Sv in normally pressured to overpressured sediments. A multiple linear regression model with a squared multiple correlation coefficient (R2) of 0.94 is proposed for pore pressure prediction from gamma ray, density and sonic logs to focus on efficient drilling operations and to prevent borehole instability. The statistical model has been calibrated with the RFT data from five wells covering about 3400 sq. km area of the onshore K‐G Basin. The model predicted pore pressure values are in close agreement with the actual RFT data for another four wells including a well in the offshore K‐G Basin. Hence, the proposed regression model may be useful for predicting pore pressure from other well logs in the K‐G Basin.
      PubDate: 2014-04-11T15:55:22.958326-05:
      DOI: 10.1002/2013GC005162
  • Re‐Os age and depositional environment for black shales from the
           Cambrian‐Ordovician boundary, Green Point, western Newfoundland
    • Authors: Gyana Ranjan Tripathy; Judith L. Hannah, Holly J. Stein, Gang Yang
      Pages: 1021 - 1037
      Abstract: Chemical and isotopic signatures for black shales serve as potential proxies for reconstruction of paleoenvironmental conditions. Here we bring Rock‐Eval, major and trace element and Re‐Os isotopic data together to examine the environmental record at the Cambrian‐Ordovician Global Stratotype Section and Point (GSSP) at Green Point in western Newfoundland, Canada. The Green Point shales are oil mature and contain Type II organic material of marine origin. A Re‐Os isochron for the shales provides the first radiometric age for shale deposition at the GSSP at 484 ± 16 Ma, with an initial 187Os/188Os ratio of 0.74 ± 0.05 (Model 3 age; MSWD = 21; n = 13; 2σ uncertainties). Factor analysis of the geochemical data set shows association of most trace elements with total organic carbon (TOC) and S contents, ensuring an authigenic origin for most elements and hence, their validity for evaluating the paleo‐redox state. Relatively high‐enrichment factors for redox‐sensitive elements (e.g., Re, U, and Mo) compared to average shale, but comparatively low enrichment compared to modern Black Sea sediments, suggest deposition in anoxic, but not euxinic waters. Comparison of Lower Ordovician shale geochemistry data sets at a global scale leads us to suggest that anoxic conditions and warm oceanic regimes were restricted to the margins of Laurentia and Baltica, whereas depositional basins with colder waters (e.g., Avalonia and Gondwana) were less reducing. These outcomes underscore the important role of paleogeography in regulating oceanic conditions and marine life.
      PubDate: 2014-04-11T15:33:10.17992-05:0
      DOI: 10.1002/2013GC005217
  • Deep‐sea redox across the Paleocene‐Eocene thermal maximum
    • Authors: Cecily Pälike; Margaret L. Delaney, James C. Zachos
      Pages: 1038 - 1053
      Abstract: Large amounts of 13C‐depleted carbon were released to the oceans and atmosphere during a period of abrupt global warming at the Paleocene‐Eocene thermal maximum (PETM) (∼55 Ma). Investigations of qualitative sedimentologic and paleontologic redox proxies such as bioturbation and benthic assemblages from pelagic and hemipelagic sections suggest transient reductions in bottom water oxygen during this interval, possibly on a global scale. Here, we present bulk sediment manganese (Mn) and uranium (U) enrichment factors (EF) in Atlantic and Pacific deep‐sea cores to constrain relative paleoredox changes across the PETM. Mn EF range from 1 to 9 in Atlantic sites, 1 to 35 in Southern Ocean sites, and are at crustal averages (EF = 1) in Pacific sites. U EF range from 1 to 5 in Atlantic sites, 1 to 90 in Southern Ocean sites, and are at crustal averages in Pacific sites. Our results indicate suboxic conditions prior to, during, and in the recovery from the PETM at intermediate depth sites in the Atlantic and Southern Ocean while the Pacific sites remained relatively oxygenated. The difference in oxygenation between the Atlantic and Pacific sites leads us to suggest the source for isotopically light carbon release during the PETM was in the Atlantic.
      PubDate: 2014-04-11T15:31:48.344446-05:
      DOI: 10.1002/2013GC005074
  • Physics of crustal fracturing and chert dike formation triggered by
           asteroid impact, ∼3.26 Ga, Barberton greenstone belt, South Africa
    • Authors: Norman H. Sleep; Donald R. Lowe
      Pages: 1054 - 1070
      Abstract: Archean asteroid impacts, reflected in the presence of spherule beds in the 3.2–3.5 Ga Barberton greenstone belt (BGB), South Africa, generated extreme seismic waves. Spherule bed S2 provides a field example. It locally lies at the contact between the Onverwacht and Fig Tree Groups in the BGB, which formed as a result of the impact of asteroid (possibly 50 km diameter). Scaling calculations indicate that very strong seismic waves traveled several crater diameters from the impact site, where they widely damaged Onverwacht rocks over much of the BGB. Lithified sediments near the top of the Onverwacht Group failed with opening‐mode fractures. The underlying volcanic sequence then failed with normal faults and opening‐mode fractures. Surficial unlithified sediments liquefied and behaved as a fluid. These liquefied sediments and some impact‐produced spherules‐filled near‐surface fractures, today represented by swarms of chert dikes. Strong impact‐related tsunamis then swept the seafloor. P waves and Rayleigh waves from the impact greatly exceeded the amplitudes of typical earthquake waves. The duration of extreme shaking was also far longer, probably hundreds of seconds, than that from strong earthquakes. Dynamic strains of ∼10−3 occurred from the surface and downward throughout the lithosphere. Shaking weakened the Onverwacht volcanic edifice and the surface layers locally moved downhill from gravity accommodated by faults and open‐mode fractures. Coast‐parallel opening‐mode fractures on the fore‐arc coast of Chile, formed as a result of megathrust events, are the closest modern analogs. It is even conceivable that dynamic stresses throughout the lithosphere initiated subduction beneath the Onverwacht rocks.
      PubDate: 2014-04-14T10:30:19.976763-05:
      DOI: 10.1002/2014GC005229
  • Magnetic properties and paleointensities as function of depth in a
           Hawaiian lava flow
    • Authors: Lennart V. de Groot; Mark J. Dekkers, Martijn Visscher, Geertje W. ter Maat
      Pages: 1096 - 1112
      Abstract: The outcome of paleointensity experiments largely depends on the rock‐magnetic properties of the samples. To assess the relation between volcanic emplacement processes and rock‐magnetic properties, we sampled a vertical transect in a ∼6 m thick inflated lava flow at Hawaii, emplaced in ∼588 AD. Its rock‐magnetic properties vary as function of distance from the flow top; the observations can be correlated to the typical cooling rate profile for such a flow. The top and to a lesser extent the bottom parts of the flow cooled faster and reveal a composition of ∼TM60 in which the magnetic remanence is carried by fine‐grained titanomagnetites, relatively rich in titanium, with associated low Curie and unblocking temperatures. The titanomagnetite in the slower cooled central part of the flow is unmixed into the magnetite and ülvospinel end‐members as evidenced by scanning electron microscope observation. The remanence is carried by coarse‐grained magnetite lamella (∼TM0) with high Curie and unblocking temperatures. The calibrated pseudo‐Thellier results that can be accepted yield an average paleointensity of 44.1 ± 2.4 μT. This is in good agreement with the paleointensity results obtained using the thermal IZZI‐Thellier technique (41.6 ± 7.4 μT) and a recently proposed record for Hawaii. We therefore suggest that the chance of obtaining a reliable paleointensity from a particular cooling unit can be increased by sampling lavas at multiple levels at different distances from the top of the flow combined with careful preliminary testing of the rock‐magnetic properties.
      PubDate: 2014-04-22T17:00:43.742827-05:
      DOI: 10.1002/2013GC005094
  • Regional controls on volcano seismicity along the Aleutian arc
    • Authors: Helena Buurman; Christopher J. Nye, Michael E. West, Cheryl Cameron
      Pages: 1147 - 1163
      Abstract: We identify patterns in volcano seismicity along the Aleutian arc using nearly 10 years of seismic data recorded at 46 volcanoes. The volcanoes in the central portion of the arc—those located from Aniakchak to Okmok—are associated with significantly more seismicity at depths below 15 km. We also examine the median weight percent SiO2 compositions of the seismically monitored volcanoes by compiling published geochemical data. We find that the transition between felsic volcanism in the east to more mafic volcanism in the west occurs in the same region where the depth distribution of volcanic earthquakes changes. Since deep volcanic earthquakes are often thought to be generated by the ascent of magma through the deep crust (i.e., depths > 15 km), our results suggest that magma ascent is more prolific in the central part of the arc compared to the western and eastern regions. This observation is in agreement with the location of the largest and most historically active volcanoes in the Aleutian arc, which are found in same region that generates abundant deep volcano seismicity. We propose two models to explain these apparent variations in magmatic flux: (1) a stress‐based model, in which subduction obliquity and the collision of the Yakutat block affect the stress regime in the upper plate, inhibiting the rise of magma in eastern and western regions of the arc and (2) a melt‐based model, where more magma is generated in the central region of the arc through increased H2O in the downgoing slab via water‐laden sediments and subducting fracture zones.
      PubDate: 2014-04-22T15:19:37.4503-05:00
      DOI: 10.1002/2013GC005101
  • Fluid evolution in an Oceanic Core Complex: A fluid inclusion study from
           IODP hole U1309 D—Atlantis Massif, 30°N, Mid‐Atlantic
    • Authors: Teddy Castelain; Andrew M. McCaig, Robert A. Cliff
      Pages: 1193 - 1214
      Abstract: In the detachment mode of slow seafloor spreading, convex‐upward detachment faults take up a high proportion of the plate separation velocity exposing gabbro and serpentinized peridotite on the seafloor. Large, long‐lived hydrothermal systems such as TAG are situated off axis and may be controlled by fluid flow up a detachment fault, with the source of magmatic heat being as deep as 7 kmbsf. The consequences of such deep circulation for the evolution of fluid temperature and salinity have not previously been investigated. Microthermometry on fluid inclusions trapped in diabase, gabbro, and trondjhemite, recovered at the Atlantis Massif Oceanic Core Complex (30°N, Mid‐Atlantic Ridge), reveals evidence for magmatic exsolution, phase separation, and mixing between hydrothermal fluids and previously phase‐separated fluids. Four types of fluid inclusions were identified, ranging in salinity from 1.4 to 35 wt % NaCl, although the most common inclusions have salinities close to seawater (3.4 wt % NaCl). Homogenization temperatures range from 160 to >400°C, with the highest temperatures in hypersaline inclusions trapped in trondjhemite and the lowest temperatures in low‐salinity inclusions trapped in quartz veins. The fluid history of the Atlantis Massif is interpreted in the context of published thermochronometric data from the Massif, and a comparison with the inferred circulation pattern beneath the TAG hydrothermal field, to better constrain the pressure temperature conditions of trapping and when in the history of exhumation of the rocks sampled by IODP Hole U1309D fluids have been trapped.
      PubDate: 2014-04-23T12:32:11.359528-05:
      DOI: 10.1002/2013GC004975
  • Zircon growth in (U)HP quartzo‐feldspathic host gneisses exhumed in
           the Woodlark Rift of Papua New Guinea
    • Authors: N. A. Zirakparvar; S. L. Baldwin, A. K. Schmitt
      Pages: 1258 - 1282
      Abstract: To understand zircon behavior as a function of bulk composition and metamorphic grade in the world's youngest (U)HP terrane, we report U‐Pb SIMS spot‐mode and depth‐profiling analyses for quartzo‐feldspathic gneisses. Zircons from two gneisses contain Cretaceous inherited cores, with μm sized metamorphic rims requiring depth profiling for reliable dating. Linear regression of the common‐Pb uncorrected data for rims yield 206Pb/238U ages of 2.89 ± 0.29 Ma and 2.77 ± 0.99 Ma (concordia intercept ages at 95% confidence). The older age is within two‐sigma error of previously reported 206Pb/238U ages on zircons from mafic eclogite within the gneiss, indicating that rims formed on inherited cores within host gneisses during eclogite facies metamorphism. At the (U)HP locality zircons from host gneiss lack inheritance and yield a 206Pb/238U age of 3.66 ± 0.13 Ma. These results are younger than previously reported 206Pb/238U ages on zircons from coesite eclogite within the gneiss, but are within error of the youngest reported LA‐ICP‐MS 206Pb/238U zircon ages on retrogressed mafic eclogite. We also report intragrain geochemical heterogeneity, indicated by zircon Hf, Y, and Ti variations in depth profiles which suggest chemical disequilibrium over the interval of zircon growth. Collectively, these results indicate that zircon recrystallization and new growth of zircon rims on relict grains occurred during eclogite facies metamorphism and during subsequent retrogression, but not at (U)HP conditions. Comparison between results from PNG and other (U)HP terranes bolsters previous suggestions that the PNG (U)HP terrane evolved rapidly.
      PubDate: 2014-04-25T12:43:03.244896-05:
      DOI: 10.1002/2013GC004964
  • Magma flow directions in the sheeted dike complex at superfast spreading
           mid‐ocean ridges: Insights from IODP Hole 1256D, Eastern Pacific
    • Authors: Eugenio E. Veloso; Nicholas W. Hayman, Ryo Anma, Masako Tominaga, Rodrigo T. González, Toshitsugu Yamazaki, Natalia Astudillo
      Pages: 1283 - 1295
      Abstract: Integrated Ocean Drilling Program (IODP) Hole 1256D successfully sampled a complete section of an intact oceanic crustal sheeted dike complex (SDC) (from 1061 to 1320 meters below seafloor; mbsf) on a 15 Ma old Cocos Plate. A series of rock magnetic measurements were carried out to understand the magmatic processes that accreted this end‐member, superfast‐spread (200 mm/yr full rate) oceanic crust. Results indicate that main ferromagnetic minerals are predominantly pseudo single‐domain (titano)magnetite crystals, responsible for both anisotropy of magnetic susceptibility (AMS) and magnetic remanence signals. AMS fabrics were reoriented into a geographic reference frame using magnetic remanence data, and corrected for a counterclockwise rotation of the Cocos Plate relative to the East Pacific Rise (EPR) ca. 15 Ma. Corrected AMS fabrics were then compared with the orientations of chilled margins previously obtained from Formation MicroScanner (FMS) images of the SDC at Hole 1256D. For some samples taken from close to dike margins, a dike‐normal orientation of the minimum AMS axes (Kmin) of prolate AMS ellipsoids mean that the long axis (Kmax) can be used to infer magma flow directions. Subvertical Kmin orientations in the interior of the dikes, however, may have required settling or compaction of the magma shortly after intrusion, thus rearranging the AMS fabric. Despite this orientation of Kmin axes, orientation of Kmax axes indicate a rather constant subhorizontal paleo‐flow direction, suggesting that magmas most probably traveled to the surface considerable distances from source regions within the EPR system.
      PubDate: 2014-04-25T12:27:10.815193-05:
      DOI: 10.1002/2013GC004957
  • Upper crustal seismic structure of the Endeavour segment, Juan de Fuca
           Ridge from traveltime tomography: Implications for oceanic crustal
    • Authors: Robert T. Weekly; William S. D. Wilcock, Douglas R. Toomey, Emilie E. E. Hooft, Eunyoung Kim
      Pages: 1296 - 1315
      Abstract: The isotropic and anisotropic P wave velocity structure of the upper oceanic crust on the Endeavour segment of the Juan de Fuca Ridge is studied using refracted traveltime data collected by an active‐source, three‐dimensional tomography experiment. The isotropic velocity structure is characterized by low crustal velocities in the overlapping spreading centers (OSCs) at the segment ends. These low velocities are indicative of pervasive tectonic fracturing and persist off axis, recording the history of ridge propagation. Near the segment center, velocities within the upper 1 km show ridge‐parallel bands with low velocities on the outer flanks of topographic highs. These features are consistent with localized thickening of the volcanic extrusive layer from eruptions extending outside of the axial valley that flow down the fault‐tilted blocks that form the abyssal hill topography. On‐axis velocities are generally relatively high beneath the hydrothermal vent fields likely due to the infilling of porosity by mineral precipitation. Lower velocities are observed beneath the most vigorous vent fields in a seismically active region above the axial magma chamber and may reflect increased fracturing and higher temperatures. Seismic anisotropy is high on‐axis but decreases substantially off axis over 5–10 km (0.2–0.4 Ma). This decrease coincides with an increase in seismic velocities resolved at depths ≥1 km and is attributed to the infilling of cracks by mineral precipitation associated with near‐axis hydrothermal circulation. The orientation of the fast‐axis of anisotropy is ridge‐parallel near the segment center but curves near the segment ends reflecting the tectonic fabric within the OSCs.
      PubDate: 2014-04-25T12:38:07.791868-05:
      DOI: 10.1002/2013GC005159
  • Lithospheric waveguide beneath the Midwestern United States; massive
           low‐velocity zone in the lower crust
    • Authors: Risheng Chu; Don Helmberger
      Pages: 1348 - 1362
      Abstract: Variations in seismic velocities are essential in developing a better understanding of continental plate tectonics. Fortunately, the USArray has provided an excellent set of regional phases from the recent M5.6 Oklahoma earthquake (6 November 2011, Table 1) that can be used for such studies. Its strike‐slip mechanism produced an extraordinary set of tangential recordings extending to the northern edge of the USArray. The crossover of the crustal slow S to the faster Sn phase is well observed. SmS has a critical distance of around 2° and its first multiple, SmS2, reaches critical angle near a distance of about 4°, and so on, until SmSn merges with the stronger crustal Love waves. These waveforms are modeled in the period band of 2–100 s by assuming a simple three‐layer crust and a two‐layer mantle, which allows a grid‐search approach. Our results favor a 15 km thick low‐velocity zone (LVZ) in the lower crust with an average shear velocity of less than 3.6 km/s. The short‐period Lg waves (S waves, at periods of 0.5–2 s) travel with velocities near 3.5 km/s and decay with distance faster than high‐frequency Sn (>5.0 Hz) which travels at a velocity of 4.6 km/s and persists to large distances. Although these short‐period waveforms are not modeled, their amplitude and travel times can be explained by adding a small velocity jump just below the Moho with essentially no attenuation. Pn is equally strong but is complicated by the interference produced by the depth phase sP, but well modeled. The P velocities appear normal with no definitive LVZ. While these observations of Sn and Pn are common beneath most cratons, the lower crustal LVZ appears to be anomalous and maybe indicative of hydrous processes, possibly caused by the descending Farallon slab.
      PubDate: 2014-04-25T13:03:09.872769-05:
      DOI: 10.1002/2013GC004914
  • Distribution and migration of aftershocks of the 2010 Mw 7.4 Ogasawara
           Islands intraplate normal‐faulting earthquake related to a fracture
           zone in the Pacific plate
    • Authors: Koichiro Obana; Tsutomu Takahashi, Tetsuo No, Yuka Kaiho, Shuichi Kodaira, Mikiya Yamashita, Takeshi Sato, Takeshi Nakamura
      Pages: 1363 - 1373
      Abstract: We describe the aftershocks of a Mw 7.4 intraplate normal‐faulting earthquake that occurred 150 km east Ogasawara (Bonin) Islands, Japan, on 21 December 2010. It occurred beneath the outer trench slope of the Izu‐Ogasawara trench, where the Pacific plate subducts beneath the Philippine Sea plate. Aftershock observations using ocean bottom seismographs (OBSs) began soon after the earthquake and multichannel seismic reflection surveys were conducted across the aftershock area. Aftershocks were distributed in a NW‐SE belt 140 km long, oblique to the N‐S trench axis. They formed three subparallel lineations along a fracture zone in the Pacific plate. The OBS observations combined with data from stations on Chichi‐jima and Haha‐jima Islands revealed a migration of the aftershock activity. The first hour, which likely outlines the main shock rupture, was limited to an 80 km long area in the central part of the subsequent aftershock area. The first hour activity occurred mainly around, and appears to have been influenced by, nearby large seamounts and oceanic plateau, such as the Ogasawara Plateau and the Uyeda Ridge. Over the following days, the aftershocks expanded beyond or into these seamounts and plateau. The aftershock distribution and migration suggest that crustal heterogeneities related to a fracture zone and large seamounts and oceanic plateau in the incoming Pacific plate affected the rupture of the main shock. Such preexisting structures may influence intraplate normal‐faulting earthquakes in other regions of plate flexure prior to subduction.
      PubDate: 2014-04-25T12:57:55.53618-05:0
      DOI: 10.1002/2014GC005246
  • Nonlinear attenuation of S‐waves and Love waves within ambient rock
    • Authors: Norman H. Sleep; Brittany A. Erickson
      Pages: 1419 - 1440
      Abstract: We obtain scaling relationships for nonlinear attenuation of S‐waves and Love waves within sedimentary basins to assist numerical modeling. These relationships constrain the past peak ground velocity (PGV) of strong 3–4 s Love waves from San Andreas events within Greater Los Angeles, as well as the maximum PGV of future waves that can propagate without strong nonlinear attenuation. During each event, the shaking episode cracks the stiff, shallow rock. Over multiple events, this repeated damage in the upper few hundred meters leads to self‐organization of the shear modulus. Dynamic strain is PGV divided by phase velocity, and dynamic stress is strain times the shear modulus. The frictional yield stress is proportional to depth times the effective coefficient of friction. At the eventual quasi‐steady self‐organized state, the shear modulus increases linearly with depth allowing inference of past typical PGV where rock over the damaged depth range barely reaches frictional failure. Still greater future PGV would cause frictional failure throughout the damaged zone, nonlinearly attenuating the wave. Assuming self‐organization has taken place, estimated maximum past PGV within Greater Los Angeles Basins is 0.4–2.6 m s−1. The upper part of this range includes regions of accumulating sediments with low S‐wave velocity that may have not yet compacted, rather than having been damaged by strong shaking. Published numerical models indicate that strong Love waves from the San Andreas Fault pass through Whittier Narrows. Within this corridor, deep drawdown of the water table from its currently shallow and preindustrial levels would nearly double PGV of Love waves reaching Downtown Los Angeles.
      PubDate: 2014-04-29T13:01:12.43661-05:0
      DOI: 10.1002/2014GC005250
  • What causes low magnetization at basalt‐hosted hydrothermal
           sites? Insights from inactive site Krasnov (MAR 16°38′N)
    • Authors: Florent Szitkar; Jérôme Dyment, Yujin Choi, Yves Fouquet
      Pages: 1441 - 1451
      Abstract: High‐resolution magnetic surveys acquired near the seafloor show that active basalt‐hosted hydrothermal sites are associated with zones of lower magnetization. This observation may reflect the thermal demagnetization of a hot hydrothermal zone, the alteration of basalt affected by hydrothermal circulation, and/or the presence of thick, nonmagnetic hydrothermal deposits. In order to discriminate among these inferences, we acquired vector magnetic data 50 m above inactive hydrothermal site Krasnov using the Remotely Operated Vehicle (ROV) Victor. This deep hydrothermal site, located 7 km east of the Mid‐Atlantic Ridge (MAR) axis at 16°38′N, is dissected by major normal faults and shows no evidence of recent hydrothermal activity. It is therefore a perfect target for investigating the magnetic signature of an inactive basalt‐hosted hydrothermal site. Krasnov exhibits a strong negative magnetic anomaly, which implies that the lower magnetization observed at basalt‐hosted hydrothermal sites is not a transient effect associated with hydrothermal activity, but remains after activity ceases. Thermal demagnetization plays only a secondary role, if any, in the observed magnetic low. Forward models suggest that both the nonmagnetic hydrothermal deposits and an altered zone of demagnetized basalt are required to account for the observed magnetic low. The permanence of this magnetic signature makes it a useful tool to explore midocean ridges and detect inactive hydrothermal sites.
      PubDate: 2014-04-29T12:47:40.543527-05:
      DOI: 10.1002/2014GC005284
  • Experimental determination of carbonate‐associated sulfate δ34S
           in planktonic foraminifera shells
    • Authors: Guillaume Paris; Jennifer S. Fehrenbacher, Alex L. Sessions, Howard J. Spero, Jess F. Adkins
      Pages: 1452 - 1461
      Abstract: Understanding the coupling of oxygen, carbon, and sulfur cycles in the past is critical for reconstructing the history of biogeochemical cycles, paleoclimatic variations, and oceanic chemistry. The abundance of sulfur isotopes (δ34S) in sulfate from ancient marine carbonates, or carbonate‐associated sulfate (CAS), is commonly used, along with other archives (mainly evaporites and barite), to estimate the δ34S of seawater throughout Earth history. Analyses of CAS from hand‐picked foraminifera are potentially valuable because this group of organisms is used in numerous paleoceanographic studies. They could provide coupled, high‐resolution records of δ13C, δ18O, and δ34S isotopic changes directly linked to orbitally tuned records of climate change through the Cenozoic. Such measurements have not previously been possible due to limitations of sensitivity in conventional IRMS‐based techniques. However, the recent development of CAS analysis by multicollector inductively coupled plasma mass spectrometry (MC‐ICP‐MS) now allows us to work on samples containing just a few nmol of sulfur with accuracy for δ34S values approaching 0.1‰ and, consequently, to analyze hand‐picked samples of foraminifera shells. Here we report the results of culture experiments with the planktonic species Orbulina universa, that establish a shell:seawater δ34S calibration for future applications to the fossil record. Our new method uses
      PubDate: 2014-04-29T12:39:41.065115-05:
      DOI: 10.1002/2014GC005295
  • Interpreting bryophyte stable carbon isotope composition: Plants as
           temporal and spatial climate recorders
    • Authors: Jessica Royles; Aline B. Horwath, Howard Griffiths
      Pages: 1462 - 1475
      Abstract: Bryophytes are unable to control tissue water content although physiological adaptations allow growth in a wide range of habitats. Carbon isotope signals in two mosses (Syntrichia ruralis and Chorisodontium aciphyllum) and two liverworts (Conocephalum conicum and Marchantia polymorpha), whether instantaneous (real time, Δ13C), or organic matter (as δ13COM), provide an assimilation‐weighted summary of bryophyte environmental adaptations. In mosses, δ13COM is within the measured range of Δ13C values, which suggests that other proxies, such as compound‐specific organic signals, will be representative of historical photosynthetic and growth conditions. The liverworts were photosynthetically active over a wider range of relative water contents (RWC) than the mosses. There was a consistent 5‰ offset between Δ13C values in C. conicum and M. polymorpha, suggestive of greater diffusion limitation in the latter. Analysis of a C. aciphyllum moss‐peat core showed the isotopic composition over the past 200 years reflects recent anthropogenic CO2 emissions. Once corrected for source‐CO2 inputs, the seasonally integrated Δ13COM between 1350 and 2000 A.D. varied by 1.5‰ compared with potential range of the 12‰ measured experimentally, demonstrating the relatively narrow range of conditions under which the majority of net assimilation takes place. Carbon isotope discrimination also varies spatially, with a 4‰ shift in epiphytic bryophyte organic matter found between lowland Amazonia and upper montane tropical cloud forest in the Peruvian Andes, associated with increased diffusion limitation.
      PubDate: 2014-04-29T12:38:15.195148-05:
      DOI: 10.1002/2013GC005169
  • Off‐fault long‐term damage: A condition to account for
           generic, triangular earthquake slip profiles
    • Authors: Frédéric Cappa; Clément Perrin, Isabelle Manighetti, Elodie Delor
      Pages: 1476 - 1493
      Abstract: Natural earthquake slip profiles have a generic triangular shape which the available rupture dynamics models fail to reproduce. Long‐term faults are embedded in long‐damaged crustal material, and the properties of the long‐term damage vary both across and along the faults. We examine the effects of the predamaged state of the medium on the earthquake slip distributions. We simulate long‐term damage by the decrease in the elastic modulus of the medium around the fault. We model the dynamic crack‐like rupture of a slip‐weakening planar, right‐lateral strike‐slip fault, and search which geometries and elastic properties of the long‐term damage produce a triangular slip profile on the rupture. We find that such a profile is produced only when a laterally heterogeneous preexisting damage zone surrounds the ruptured fault. The highest on‐fault slip develops in the most compliant region of the damage zone, and not necessarily above the earthquake hypocenter. The coseismic slip decreases in zones of stiffer damage. The amount of coseismic slip dissipated in the damage zone is large, at least 25–40% of maximum on‐fault slip, and can occur over large distances from the fault. Our study thus emphasizes that off‐fault preexisting damage should be considered for an accurate description of earthquake ruptures. It also motivates a reformulation of the available earthquake source inversion models since most of them do not include the inelastic deformations that occur in the near field of the earthquake ruptures.
      PubDate: 2014-04-29T12:36:46.368038-05:
      DOI: 10.1002/2013GC005182
  • Calculation of water‐bearing primary basalt and estimation of source
           mantle conditions beneath arcs: PRIMACALC2 model for WINDOWS
    • Authors: Jun‐Ichi Kimura; Alexey A. Ariskin
      Pages: 1494 - 1514
      Abstract: We present a new method for estimating the composition of water‐bearing primary arc basalt and its source mantle conditions. The PRIMACALC2 model uses a thermodynamic fractional crystallization model COMAGMAT3.72 and runs with an Excel macro to examine the mantle equilibrium and trace element calculations of a primary basalt. COMAGMAT3.72 calculates magma fractionation in 0–10 kb at various compositions, pressure, oxygen fugacity, and water content, but is only applicable for forward calculations. PRIMACALC2 first calculates the provisional composition of a primary basalt from an observed magma. The basalt composition is then calculated by COMAGMAT3.72 for crystallization. Differences in elemental concentrations between observed and the closest‐match calculated magmas are then adjusted in the primary basalt. Further iteration continues until the calculated magma composition converges with the observed magma, resulting in the primary basalt composition. Once the fitting is satisfied, back calculations of trace elements are made using stepwise addition of fractionated minerals. Mantle equilibrium of the primary basalt is tested using the Fo‐NiO relationship of olivine in equilibrium with the primary basalt, and thus with the source mantle. Source mantle pressure, temperature, and degree of melting are estimated using petrogenetic grids based on experimental data obtained in anhydrous systems. Mantle melting temperature in a hydrous system is computed by adjusting T with a parameterization for a water‐bearing system. PRIMACALC2 can be used either in dry or water‐bearing arc magmas and is also applicable to mid‐ocean ridge basalts and nonalkalic ocean island basalts.
      PubDate: 2014-04-29T13:33:28.575627-05:
      DOI: 10.1002/2014GC005329
  • Effect of variable CO2 on eclogite‐derived andesite and lherzolite
           reaction at 3 GPa—Implications for mantle source characteristics of
           alkalic ocean island basalts
    • Authors: Ananya Mallik; Rajdeep Dasgupta
      Pages: 1533 - 1557
      Abstract: We have performed reaction experiments between 1, 4, and 5 wt % CO2‐bearing MORB‐eclogite (recycled oceanic crust)‐derived low‐degree andesitic partial melt and fertile peridotite at 1375°C, 3 GPa for infiltrating melt fractions of 25% and 33% by weight. We observe that the reacted melts are alkalic with degree of alkalinity or Si undersaturation increasing with increasing CO2 content in reacting melt. Consequently, an andesite evolves through basanite to nephelinite owing to greater drawdown of SiO2 from melt and enhanced precipitation of orthopyroxene in residue. We have developed an empirical model to predict reacted melt composition as a function of reacting andesite fraction and source CO2 concentration. Using our model, we have quantified the mutual proportions of equilibrated melt from andesite‐peridotite (+ CO2) hybridization and subsequent peridotite (± CO2)‐derived melt required to produce the major element composition of various ocean island basalts. Our model can thus be applied to characterize the source of ocean islands from primary alkalic lava composition. Accordingly, we determined that average HIMU source requires 24 wt % of MORB‐eclogite‐derived melt relative to peridotite containing 2 wt % CO2 and subsequent contribution of 45% of volatile‐free peridotite partial melt. We demonstrate that mantle hybridization by eclogite melt‐peridotite (± CO2) reaction in the system can produce high MgO (>15 wt %) basaltic melts at mantle potential temperature (TP) of 1350°C. Therefore, currently used thermometers to estimate TP using MgO content of primary alkalic melts need to be revised, with corrections for melt‐rock reaction in a heterogeneous mantle as well as presence of CO2.
      PubDate: 2014-04-30T11:12:52.532546-05:
      DOI: 10.1002/2014GC005251
  • On the magnetocrystalline anisotropy of greigite (Fe3S4)
    • Authors: Michael Winklhofer; Liao Chang, Stephan H. K. Eder
      Pages: 1558 - 1579
      Abstract: The ferrimagnetic mineral greigite (cubic Fe3S4) is well known as an intracellular biomineralization product in magnetic bacteria and as a widely occurring authigenic mineral in anoxic sediments. Due to the lack of suitable single‐crystal specimens, the magnetic anisotropy parameters of greigite have remained poorly constrained, to the point where not even the easy axis of magnetization is known. Here we report on an effort to determine the anisotropy parameters on the basis of ferromagnetic resonance (FMR) powder spectroscopy on hydrothermally synthesized, chemically pure greigite microcrystals dispersed in a nonmagnetic matrix. In terms of easy axis orientations, the FMR data are consistent with or , or less likely, a more general type. With a g factor of 2.09, the anisotropy field is about 90 mT and in some samples may reach 125 mT, compared to 30 mT for cubic magnetite. This confirms the dominating role of cubic anisotropy on the magnetic properties of greigite, which we show to be responsible for large SIRM/k values. K1 is in the range −15 … −23 J/m3 () or +10 … +15 kJ/m3 (), yielding upper limits of 44 or 34 nm for the superparamagnetic grain size, respectively.
      PubDate: 2014-04-30T11:33:03.830709-05:
      DOI: 10.1002/2013GC005121
  • Hydrothermal heat mining in an incoming oceanic plate due to aquifer
           thickening: Explaining the high heat flow anomaly observed around the
           Japan Trench
    • Authors: Yoshifumi Kawada; Makoto Yamano, Nobukazu Seama
      Pages: 1580 - 1599
      Abstract: To explain the origin of a high heat flow anomaly observed within 150 km seaward of the Japan Trench, we construct a thermal model for an oceanic plate prior to subduction that includes the effect of hydrothermal circulation within a high‐permeability aquifer in its uppermost part. The model includes the effects of aquifer thickening, which is expected to occur near subduction zones where plate bending prior to subduction causes fracturing and faulting within the oceanic plate. Using typical parameter values for the Japan Trench, we find that hydrothermal circulation in the thickening aquifer mines heat from the underlying basement and can account for the observed high heat flow anomaly. The ratio of heat supply below the aquifer as a result of aquifer thickening to the inverse of the thermal resistance of the sediment layer is a control parameter for the system. As long as the aquifer permeability is higher than ∼10−13 m2, a typical value for the uppermost part of the oceanic plate, variations in other details of the hydrothermal circulation such as the exact value of the aquifer permeability and the size of the convection cells do not significantly change model results. Despite its strong influence on seafloor heat flow seaward of the trench, this hydrothermal heat mining does not affect significantly the thermal structure of the subducted oceanic plate. This finding indicates that surface heat flow anomaly around the trench may not correspond to temperature anomaly within the subducted oceanic plate and the megathrust seismogenic zone.
      PubDate: 2014-04-30T13:08:00.741328-05:
      DOI: 10.1002/2014GC005285
  • An investigation of the magnetic carriers and demagnetization
           characteristics of the Gulang loess section, northwestern Chinese Loess
    • Authors: Ting Chen; Xiaoke Qiang, Hui Zhao, Youbin Sun
      Pages: 1600 - 1616
      Abstract: We report the results of a rock magnetic and paleomagnetic investigation of unit L9 of the Gulang (GL) loess section, in the northwestern Chinese Loess Plateau (CLP). We demonstrate that the dominant magnetic remanence carriers are coarse‐grained pseudo single‐domain/multidomain magnetite particles, and hematite. The application of either thermal (THD) or alternating‐field demagnetization (AFD) yielded several normal polarity intervals interbedded with intervals of reversed polarity, but which cannot be correlated with standard geomagnetic polarity time scales. A greater number of normal polarity intervals, however, were evident after THD than after AFD. We propose that the normal polarities are either the result of overprinting of a viscous remanent magnetization (VRM) carried by coarse magnetite particles on the original depositional remanent magnetization (DRM); or that they represent genuine geomagnetic excursions (such as the Kamikatsura and Santa Rosa excursions) which are difficult to identify. In addition, we propose that in the case of the GL loess section, the effect of thermal demagnetization tails of a VRM is a possible reason why THD treatment yielded a greater number of normal polarity intervals than does AFD. However, reversed polarities obtained by either THD or AFD may be credible for sedimentary intervals with a relatively high content of hematite particles which preserve the primary DRM. Although numerous studies have proposed that THD provides more reliable paleomagnetic directions for loess sediments from the CLP, our results suggest that careful attention needs to be paid to the type of demagnetization method chosen, especially in the case of coarse‐grained loess deposits.
      PubDate: 2014-04-30T13:16:57.436033-05:
      DOI: 10.1002/2014GC005268
  • Determination of B/Ca of natural carbonates by HR‐ICP‐MS
    • Authors: Sambuddha Misra; Mervyn Greaves, Robert Owen, Joanna Kerr, Aurora C. Elmore, Henry Elderfield
      Pages: 1617 - 1628
      Abstract: We report a new method for HR‐ICP‐MS based accurate and precise B/Ca determination from low mass natural carbonates (≤5 µg CaCO3), utilizing a mixed acid matrix (0.1 M HNO3 and 0.3 M HF) and accurate matrix matching technique. Our procedural B/Ca blank of 2.0 ± 1.0 µmol/mol, internal precision ≤1.0%, average within run external precision ≤4.0% (2σ), and rapid sample analysis (60 samples/day) make the method well suited for routine measurements. Established methods of B/Ca determination require ≥65 µg CaCO3 to achieve a comparable external precision of 3.5% (2σ). We report a B/Ca detection limit of 2 µmol/mol compared to ≥10 µmol/mol for previous methods, a fivefold improvement. The method presented here can determine a wide range of B/Ca (9.0–250 µmol/mol) in mass limited samples with considerable tolerance for matrix matching efficiency (≤±30%). The long‐term reproducibility of B/Ca measured on Cambridge in‐house consistency standards containing
      PubDate: 2014-04-02T15:38:54.670708-05:
      DOI: 10.1002/2013GC005049
  • Community infrastructure and repository for marine magnetic
    • Authors: Maria Seton; Joanne M. Whittaker, Paul Wessel, R. Dietmar Müller, Charles DeMets, Sergey Merkouriev, Steve Cande, Carmen Gaina, Graeme Eagles, Roi Granot, Joann Stock, Nicky Wright, Simon E. Williams
      Pages: 1629 - 1641
      Abstract: Magnetic anomaly identifications underpin plate tectonic reconstructions and form the primary data set from which the age of the oceanic lithosphere and seafloor spreading regimes in the ocean basins can be determined. Although these identifications are an invaluable resource, their usefulness to the wider scientific community has been limited due to the lack of a central community infrastructure to organize, host, and update these interpretations. We have developed an open‐source, community‐driven online infrastructure as a repository for quality‐checked magnetic anomaly identifications from all ocean basins. We provide a global sample data set that comprises 96,733 individually picked magnetic anomaly identifications organized by ocean basin and publication reference, and provide accompanying Hellinger‐format files, where available. Our infrastructure is designed to facilitate research in plate tectonic reconstructions or research that relies on an assessment of plate reconstructions, for both experts and nonexperts alike. To further enhance the existing repository and strengthen its value, we encourage others in the community to contribute to this effort.
      PubDate: 2014-04-11T14:13:05.611028-05:
      DOI: 10.1002/2013GC005176
  • REDBACK: Open‐source software for efficient noise‐reduction in
           plate kinematic reconstructions
    • Authors: G. Iaffaldano; R. Hawkins, T. Bodin, M. Sambridge
      Pages: 1663 - 1670
      Abstract: Knowledge of past plate motions derived from ocean‐floor finite rotations is an important asset of the Earth Sciences, because it allows linking a variety of shallow‐rooted and deep‐rooted geological processes. Efforts have recently been taken toward inferring finite rotations at the unprecedented temporal resolution of 1 Myr or less, and more data are anticipated in the near future. These reconstructions, like any data set, feature a degree of noise that compromises significantly our ability to make geodynamical inferences. Bayesian Inference has been recently shown to be effective in reducing the impact of noise on plate kinematics inferred from high‐temporal‐resolution finite‐rotation data sets. We describe REDBACK, an open‐source software that implements transdimensional hierarchical Bayesian Inference for efficient noise‐reduction in plate kinematic reconstructions. Algorithm details are described and illustrated by means of a synthetic test.
      PubDate: 2014-04-30T13:05:53.346416-05:
      DOI: 10.1002/2014GC005309
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