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

Geochemistry, Geophysics, Geosystems     Full-text available via subscription   (Followers: 21, SJR: 2.156, h-index: 61)
Geophysical Research Letters     Full-text available via subscription   (Followers: 46, SJR: 2.668, h-index: 142)
Global Biogeochemical Cycles     Full-text available via subscription   (Followers: 3, SJR: 2.4, h-index: 109)
J. of Advances in Modeling Earth Systems     Open Access   (Followers: 2, SJR: 0.126, h-index: 2)
J. of Geophysical Research : Atmospheres     Partially Free   (Followers: 20)
J. of Geophysical Research : Biogeosciences     Full-text available via subscription   (Followers: 5)
J. of Geophysical Research : Earth Surface     Partially Free   (Followers: 22)
J. of Geophysical Research : Oceans     Partially Free   (Followers: 15)
J. of Geophysical Research : Planets     Full-text available via subscription   (Followers: 13)
J. of Geophysical Research : Solid Earth     Full-text available via subscription   (Followers: 23)
J. of Geophysical Research : Space Physics     Full-text available via subscription   (Followers: 15)
Paleoceanography     Full-text available via subscription   (Followers: 4, SJR: 2.16, h-index: 82)
Radio Science     Full-text available via subscription   (Followers: 2, SJR: 0.527, h-index: 47)
Reviews of Geophysics     Full-text available via subscription   (Followers: 19, SJR: 8.837, h-index: 87)
Space Weather     Full-text available via subscription   (Followers: 3, SJR: 0.496, h-index: 16)
Tectonics     Full-text available via subscription   (Followers: 7, SJR: 2.16, h-index: 79)
Water Resources Research     Full-text available via subscription   (Followers: 147, 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]
  • The early middle Miocene subduction complex of the Louisiade Archipelago,
           southern margin of the Woodlark Rift
    • Authors: Laura E. Webb; Suzanne L. Baldwin, Paul G. Fitzgerald
      Pages: n/a - n/a
      Abstract: Field, microstructural and 40Ar/39Ar thermochronologic data from the Louisiade Archipelago, the southern rifted margin of the Woodlark Basin in SE Papua New Guinea, document an accretionary wedge that formed during Early–Middle Miocene N‐dipping subduction of the Australian margin and transpression along the Australian–Pacific plate boundary. Metasedimentary rocks of the Calvados Schist and the metagabbros that intrude them were metamorphosed at up to greenschist‐facies conditions. Three tectonic foliations (S1–S3) are present and F1–F3 fold hinges plunge ESE or WNW, parallel to mineral, stretching, and intersection lineations. Fold vergence is dominantly to the SW, and top‐to‐the‐SW thrusting of ultramafic rocks over the Calvados Schist is documented locally on Rossel Island. The data suggest progressive deformation associated with NNE–SSW shortening and ESE–WNW extension via dissolution‐precipitation creep and, more locally, dislocation creep. 40Ar/39Ar step‐heating analyses of three white mica separates yield Middle Miocene plateau or plateau‐like segments that are affected by variable Pliocene argon loss and are interpreted as syntectonic mica growth during metamorphism and deformation followed by partial resetting just prior to the onset of seafloor spreading in the Woodlark Basin. A ca. 12 Ma 40Ar/39Ar age from a dacite sill of the Panarora Volcanics provides a minimum age constraint for the termination of northward subduction and a maximum age for cross‐cutting brittle strike‐slip faults. These data are critical to constraining the subduction–exhumation history of the world's youngest high–ultrahigh‐pressure terrane and further support analogies between SE Papua New Guinea and the Early Oligocene Western Alps.
      PubDate: 2014-09-18T05:08:08.803665-05:
      DOI: 10.1002/2014GC005500
       
  • Absolute plate motions and regional subduction evolution
    • Authors: M.V. Chertova; W. Spakman, A.P. van den Berg, D.J.J. van Hinsbergen
      Pages: n/a - n/a
      Abstract: We investigate the influence of absolute plate motion on regional 3‐D evolution of subduction using numerical thermo‐mechanical modeling. Building on our previous work (Chertova et al. 2014), we explore the potential impact of four different absolute plate motion frames on subduction evolution in the western Mediterranean region during the last 35My. One frame is data‐based and derived from the global moving hotspot reference frame (GMHRF) of Doubrovine et al. [2012] and three are invented frames: a motion frame in which the African plate motion is twice that in the GMHRF, and two frames in which either the African plate or the Iberian continent is assumed fixed to the mantle. The relative Africa‐Iberia convergent is the same in all frames. All motion frames result in distinctly different 3‐D subduction evolution showing a critical dependence of slab morphology evolution on absolute plate motion. We attribute this to slab dragging through the mantle forced by the absolute motion of the subducting plate, which causes additional viscous resistance affecting subduction evolution. We observed a strong correlation between increase in northward Africa motion and decrease in the speed of westward slab rollback along the African margin. We relate this to increased mantle resistance against slab dragging providing new insight into propagation and dynamics of subduction transform edge propagator (STEP) faults. Our results demonstrate a large sensitivity of 3‐D slab evolution to the absolute motion of the subducting plate, which inversely suggests that detailed modeling of natural subduction may provide novel constraints on absolute plate motions.
      PubDate: 2014-09-17T02:18:00.229793-05:
      DOI: 10.1002/2014GC005494
       
  • Issue Information
    • Pages: i - i
      PubDate: 2014-09-16T09:47:28.448979-05:
      DOI: 10.1002/ggge.20324
       
  • Geochemical insights into the role of metasomatic hornblendite in
           generating alkali basalts
    • Authors: Li‐Qun Dai; Zi‐Fu Zhao, Yong‐Fei Zheng
      Pages: n/a - n/a
      Abstract: Experimental petrology suggested the role of hornblendite in generating alkali basalt. This mechanism is confirmed by an integrated study of major‐trace elements and radiogenic isotopes for Mesozoic alkali basalts from the Qinling orogen in China. The alkali basalts have high contents of MgO (4.8‐11.1 wt.%, Mg# = 47‐69), Na2O+K2O (2.9‐5.4 wt.%), TiO2 (2.0‐3.1 wt.%) but low content of SiO2 (41.4‐49.6 wt.%), which are generally silica‐undersaturated with normative minerals of nepheline and olivine. They exhibit OIB‐like trace element distribution patterns, with enrichment of LILE and LREE but no depletion of HFSE relative to the primitive mantle. They also show relatively depleted Sr‐Nd‐Hf isotope compositions, with low initial 87Sr/86Sr ratios of 0.7028 to 0.7058, positive εNd(t) values of 4.0 to 9.8 and εHf(t) values of 8.8 to 13.5 for whole‐rock, and positive εHf(t) values of 5.2 to 16.4 for zircon. Such element and isotope features indicate their origination from the juvenile subcontinental lithospheric mantle (SCLM) source with involvement of crustal components. The alkali basalts generally have high K2O/Na2O ratios, and high K2O and TiO2 contents, suggesting their derivation from partial melting of hornblendite‐rich mantle lithology. They also exhibit variable K/La and Ti/La ratios that are correlated with (La/Yb)N ratios, indicating a geochemical heterogeneity of the SCLM source. Taken together, all the above geochemical features can be accounted for by partial melting of a hornblendite‐rich SCLM source. The hornblendite would be generated by reaction of the juvenile SCLM wedge peridotite with felsic melts derived from subducted Palaeotethyan oceanic crust at the slab‐mantle interface in the subduction channel. Therefore, orogenic alkali basalts record recycling of the subducted fossil oceanic crust, and the metasomatic hornblendite is an important lithology in local SCLM domains above fossil subduction channels.
      PubDate: 2014-09-13T10:56:50.614212-05:
      DOI: 10.1002/2014GC005486
       
  • The dynamics of global change at the Paleocene‐Eocene thermal
           maximum: A data‐model comparison
    • Authors: Timothy J. Bralower; Katrin J. Meissner, Kaitlin Alexander, Deborah J. Thomas
      Pages: n/a - n/a
      Abstract: We integrate published stable isotopic, chemical, mineralogical and biotic data from the onset of the Paleocene Eocene thermal maximum (PETM) at Site 690, Maud Rise in the Southern Ocean. The integrated dataset documents a sequence of environmental steps including warming of the ocean from the surface downwards, and modification of its thermal and nutrient structure, acidification of the deep ocean and the onset of continental weathering. The age of the events with respect to the onset of the PETM is calibrated with three different age models. The relative and absolute timing of the steps are compared with simulated temperature, salinity, calcite saturation, and dissolved PO4 and O2, at different depths in the ocean, generated with the UVic Earth System Climate Model of intermediate complexity. The simulation supports the top to bottom transfer of heat and carbon, and generally agrees with age models in terms of the durations of leads and lags in temperature, C‐isotope and biotic responses. Moreover, the simulation shows that stratification increased and the nutricline strengthened at the onset of the PETM. These environmental changes explain the abundance of deep dwelling nannoplankton and foraminifera during the early part of the event. The modelled calcite saturation is consistent with a harsh deep sea habitat at the time of the benthic foraminiferal extinction.
      PubDate: 2014-09-13T10:47:00.561488-05:
      DOI: 10.1002/2014GC005474
       
  • Estimating shallow water sound power levels and mitigation radii for the
           R/V Marcus G. Langseth using an 8 km long MCS streamer
    • Authors: Timothy J. Crone; Maya Tolstoy, Helene Carton
      Pages: n/a - n/a
      Abstract: For seismic surveys in shallow‐water environments the complexity of local geology and seafloor topography can make it difficult to accurately predict associated sound levels and establish appropriate mitigation radii required to ensure the safety of local marine protected species. This is primarily because necessary detailed information regarding the local seafloor topography and subseafloor geology is often unavailable before a survey begins. One potential solution to this problem is to measure received levels using the ship's multi‐channel seismic (MCS) streamer \citep{Diebold2010}, which could allow for the dynamic real‐time determination of sound levels and mitigation radii while a survey is underway. We analyze {\em R/V Langseth} streamer data collected on the shelf and slope near the Washington coast during the Cascadia Open‐Access Seismic Transects (COAST) and Ridge2Trench projects to measure received levels up to a distance of approximately 8 km from the sound source array. We establish methods to filter, clean, and process streamer data to accurately determine received power levels and confidently establish mitigation radii. We show that in shallow water measured power levels can fluctuate due to the influence of seafloor topographic features, but that the use of the streamer for the establishment of dynamic mitigation radii is feasible and should be further pursued. The establishment of mitigation radii based on local conditions may help maximize the safety of marine protected species while also maximizing the ability of researchers to conduct seismic studies.
      PubDate: 2014-09-10T10:19:16.997535-05:
      DOI: 10.1002/2014GC005420
       
  • Early Cenozoic exhumation in the Bendeleben Mountains (Seward Peninsula,
           Alaska) constrained by (U‐Th)/He thermochronology and thermal
           modeling
    • Authors: Kalin T. McDannell; Jaime Toro, Jeremy K. Hourigan, Daniel Harris
      Pages: n/a - n/a
      Abstract: In the Bendeleben Mountains, Seward Peninsula, mid‐Cretaceous granites are exposed in an uplifted block bounded on its south side by an E‐W striking normal fault. The Bendeleben fault has well‐preserved scarps 4‐7 m in height that offset Holocene moraines. Seismic activity, young normal faulting, and Quaternary basaltic volcanism are all evidence of active extension. South of the Bendeleben fault, there is a 3‐4 km deep basin. Fifteen apatite (U‐Th)/He ages from granitic samples of the footwall yield an Eocene weighted mean age of 41.3±4.8 Ma. Biotite 40Ar/39Ar ages from the country rock of the Bendeleben pluton are 81‐83 Ma. In spite of the young fault scarps, Pecube and HeFTy t‐T modeling results illustrate that rapid exhumation of the Bendeleben Mountains occurred in the Late Cretaceous‐Eocene and slowed since the Oligocene. A weak age‐elevation relationship of apatite He ages and a lack of correlation between age and distance from the fault indicate that exhumation was accomplished with minimal block rotation on a steeply dipping, long‐lived normal fault. Timing of extension in the Seward Peninsula can be correlated with deformation in the offshore Hope Basin where seismic reflection lines document Early Tertiary large‐magnitude normal faulting followed by minor post‐Miocene reactivation. The faulting observed in the Bendeleben Mountains is part of an extensional system that spans a large portion of the Bering Strait region. The tectonic model proposed in previous studies suggests that clockwise rotation of the Bering block relative to North America is the cause of extensional deformation in western Alaska.
      PubDate: 2014-09-09T19:29:23.694789-05:
      DOI: 10.1002/2014GC005424
       
  • Provenance of the late quaternary sediments in the Andaman Sea:
           Implications for monsoon variability and ocean circulation
    • Authors: Neeraj Awasthi; Jyotiranjan S. Ray, Ashutosh K. Singh, Shraddha T. Band, Vinai K. Rai
      Pages: n/a - n/a
      Abstract: We present a geochemical and Sr‐Nd isotopic study on a sediment core collected from the Andaman Sea in an attempt to reconstruct the Late Quaternary weathering and erosion patterns in the watersheds of the river systems of Myanmar and understand their controlling factors. Age control is based on nine radiocarbon dates and δ18O stratigraphy. The rate of sedimentation was strongly controlled by fluctuations of the monsoon. We identify three major sediment provenances: (1) the Irrawaddy catchment, (2) the western slopes of the Indo‐Burman‐Arakan (IBA) mountain ranges and the Andaman Islands and (3) the catchments of Salween and Sittang and the Bengal shelf, with the first two contributing 30 to 60% of the material. Enhanced contributions from juvenile sources and corresponding positive shifts of δ18O are observed at seven time periods (11‐14, 20‐23, 36, 45, 53, 57 and 62 ka) of which five are synchronous with cooling of the northern hemisphere ‐ suggesting a link between the changes in sediment provenances and the shifting of the locus of the summer monsoon, southwards from the Himalayas, without substantial reduction in intensity. Our data, and that from other cores in the region suggest that an eastward moving surface current disperses sediments, derived from the Bengal shelf and western margin of Myanmar, from the eastern Bay of Bengal into the western Andaman Sea and that its strength has increased since the LGM. The existence of this current during the LGM implies that the Andaman Sea and the Bay of Bengal were well connected during the last glacial period.
      PubDate: 2014-09-09T19:24:15.008141-05:
      DOI: 10.1002/2014GC005462
       
  • Sulfur isotope evolution in sulfide ores from Western Alps: Assessing the
           influence of subduction‐related metamorphism
    • Authors: Fabio Giacometti; Katy A. Evans, Gisella Rebay, John Cliff, Andrew G. Tomkins, Piergiorgio Rossetti, Gloria Vaggelli, David T. Adams
      Pages: n/a - n/a
      Abstract: Sulfides entering subduction zones can play an important role in the release of sulfur and metals to the mantle wedge and contribute to the formation of volcanic arc‐associated ores. Fractionation of stable sulfur isotopes recorded by sulfides during metamorphism can provide evidence of fluid‐rock interactions during metamorphism and give insights on sulfur mobilization. A detailed micro‐textural and geochemical study was performed on mineralized samples from two ocean floor‐related sulfide deposits (Servette and Beth‐Ghinivert) in high‐pressure units of the Italian Western Alps, which underwent different metamorphic evolutions. The combination of micro‐textural investigations with δ34S values from in situ ion‐probe analyses within individual pyrite and chalcopyrite grains allowed an evaluation of the effectiveness of metamorphism in modifying the isotopic record and mobilizing sulfur and metals and have insights on fluid circulation within the slab. Textures and isotopic compositions inherited from the protolith are recorded at Beth‐Ghinivert, where limited metamorphic re‐crystallization is attributed to limited interaction with metamorphic fluids. Isotopic modification by metamorphic processes occurred only at the sub‐millimeter scale at Servette, where local interactions with infiltrating hydrothermal fluid are recorded by metamorphic grains. Notwithstanding the differences recorded by the two deposits, neither underwent intensive isotopic re‐equilibration or records evidence of intense fluid‐rock interaction and S mobilization during metamorphism. Therefore, subducted sulfide deposits dominated by pyrite and chalcopyrite are unlikely to release significant quantities of sulfur to the mantle wedge and to arc magmatism sources at metamorphic grades below the lower eclogite facies.
      PubDate: 2014-09-09T19:20:54.650317-05:
      DOI: 10.1002/2014GC005459
       
  • Three‐dimensional flow in the subslab mantle
    • Authors: Karen Paczkowski; Laurent G.J. Montési, Maureen D. Long, Christopher J. Thissen
      Pages: n/a - n/a
      Abstract: Three‐dimensional models of mantle flow at subduction zones make it possible to explain the common occurrence of trench‐parallel sub‐slab seismic anisotropy. Sub‐slab flow becomes inherently three‐dimensional when slab‐driven flow interacts with a wide variety of ambient background mantle flow conditions. This interaction depends on slab geometries, mechanical coupling parameters, and lower mantle viscosities. Deflection of sub‐slab mantle flow is a robust feature for all model parameters and geometries as the slab acts as an obstruction to the ambient, background mantle flow. Background mantle flow can become trench‐perpendicular or trench‐parallel sub‐slab flow depending on whether the ambient background mantle flow is deflected beneath the bottom of the slab or towards the edge of the slab. The first case is especially prominent in models with short slabs that do not penetrate into the lower mantle. The second case is especially prominent in models with long, steep slabs. The results are also highly sensitive to the amount of mechanical coupling between the subducting plate and the mantle beneath it. High levels of coupling create a boundary layer of trench‐perpendicular entrained flow, pushing the deflection due to the obstructing slab away from the slab. We compare our sub‐slab flow model predictions with a global set of seismic anisotropy fast directions in the sub‐slab mantle, and find generally good agreement between the anisotropy observations (dominantly trench‐parallel or trench‐perpendicular) and the mantle flow directions predicted for decoupled systems.
      PubDate: 2014-09-09T19:19:48.303475-05:
      DOI: 10.1002/2014GC005441
       
  • Tracking the exhumation of a pliocene (U)HP terrane: U‐Pb and
           trace‐element constraints from zircon, D'Entrecasteaux Islands,
           Papua New Guinea
    • Authors: Joel W. DesOrmeau; Stacia M. Gordon, Timothy A. Little, Samuel A. Bowring
      Pages: n/a - n/a
      Abstract: Domal structures within the D'Entrecasteaux Islands of eastern Papua New Guinea expose ultrahigh‐pressure (UHP) Pliocene (5.6–4.6 Ma) eclogites and evidence for partial melting. To better interpret the (U)HP exhumation history, U‐Pb geochronology and trace‐element abundances were determined in zircon from variably deformed host gneiss and crystallized melt (leucosomes, sills, dikes, and plutons) from the Goodenough and Normanby Domes by ID‐TIMS (isotope‐dilution thermal ionization mass spectrometry) and ICP‐MS (inductively coupled plasma mass spectrometry), respectively, to constrain the timing of melt crystallization and deformation relative to UHP metamorphism. Zircons extracted from orthogneiss and deformed granodiorite sills of Normanby Dome, located ~40 km southeast of the UHP eclogite, record HP metamorphism from 5.66 ± 0.02 Ma to 5.04 ± 0.07 Ma and melt crystallization at ca. 4.1 Ma. Strongly deformed, layer‐parallel leucosomes from Goodenough Dome, ~20 km northwest of the UHP eclogite, began to crystallize by 3.85 ± 0.02 Ma. These dates indicate that melt crystallization began in the Goodenough and Normanby Domes within ~0.75 m.y. of (U)HP metamorphism. The ID‐TIMS dates from the orthogneiss and crystallized melt show that exhumation and cooling of the (U)HP rocks in the PNG terrane began first in the east, within Normanby Dome, then to the west, in the Goodenough Dome ~ 1 m.y. later, and finally the middle dome rocks, exposed within the Mailolo Dome, cooled ~ 2 m.y. after exhumation of Normanby Dome. All domes reveal synchronous crystallization of late, non‐deformed melts and final extension‐driven exhumation by 1.82 ± 0.03 Ma.
      PubDate: 2014-09-05T11:34:36.272146-05:
      DOI: 10.1002/2014GC005396
       
  • A family of repeating low‐frequency earthquakes at the downdip edge
           of tremor and slip
    • Authors: Justin R Sweet; Kenneth C Creager, Heidi Houston
      Pages: n/a - n/a
      Abstract: We analyze an isolated Low‐Frequency Earthquake (LFE) family located at the downdip edge of the main episodic tremor and slip (ETS) zone beneath western Washington State. The 9000 individual LFEs from this repeating family cluster into 198 swarms that recur roughly every week. Cumulative LFE seismic moment for each swarm correlates strongly with the time until the next swarm, suggesting that these LFE swarms are time‐predictable. Precise double‐difference relative locations for 700 individual LFEs within this family show a distribution that is approximately 2 km long and 500 m wide, elongated parallel to the relative plate convergence direction. The distribution of locations (
      PubDate: 2014-09-02T04:22:15.428687-05:
      DOI: 10.1002/2014GC005449
       
  • Regional study of the archean to proterozoic crust at the Sudbury Neutrino
           Observatory (SNO+), Ontario: Predicting the geoneutrino flux
    • Authors: Yu Huang; Virginia Strati, Fabio Mantovani, Steven B. Shirey, William F. McDonough
      Pages: n/a - n/a
      Abstract: The SNO+ detector that is currently under construction in Ontario, Canada, will be a new kiloton scale liquid scintillation detector with the capability of recording geoneutrino events that can be used to constrain the strength of the Earth's radiogenic power, and in turn, to test compositional models of the bulk silicate Earth (BSE). We constructed a detailed 3‐D model of the regional crust centered at SNO+ from compiled geological, geophysical and geochemical information. Crustal cross sections obtained from refraction and reflection seismic surveys were used to characterize the crust and assign uncertainties to its structure. The average Moho depth in the study area is 42.3 ± 2.6 km. The upper crust was divided into seven dominant lithologic units on the basis of regional geology. The abundances of U and Th and their uncertainties in each upper crustal lithologic unit were determined from analyses of representative outcrop samples. The average chemical compositions of the middle and lower crust beneath the SNO+ region were determined by coupling local seismic velocity profiles with a global compilation of the chemical compositions of amphibolite and granulite facies rocks. Monte Carlo simulations were used to predict the geoneutrino signal originating from the regional crust at SNO+ and to track asymmetrical uncertainties of U and Th abundances. The total regional crust contribution of the geoneutrino signal at SNO+ is predicted to be TNU (a Terrestrial Neutrino Unit is one geoneutrino event per 1032 target protons per year), with the Huronian Supergroup near SNO+ dominantly contributing TNU to this total. Future systematically sampling of this regional unit and denser seismic surveys will better model its composition and structure, and thus reduce the uncertainty on geoneutrino signal at SNO+. The bulk crustal geoneutrino signal at SNO+ is estimated to be TNU, which is lower than that predicted in a global scale reference model that uses an average composition of the global upper continental crust [Huang et al., 2013], due to the fact that Archean to Proterozoic Canadian Shield has lower U and Th concentrations. Finally, without accounting for uncertainties on the signal from continental lithospheric mantle and convecting mantle, the total geoneutrino signal at SNO+ is predicted to be TNU
      PubDate: 2014-09-02T03:28:11.190576-05:
      DOI: 10.1002/2014GC005397
       
  • A geodetic plate motion and global strain rate model
    • Authors: Corné Kreemer; Geoffrey Blewitt, Elliot C. Klein
      Pages: n/a - n/a
      Abstract: We present a new global model of plate motions and strain rates in plate boundary zones constrained by horizontal geodetic velocities. This Global Strain Rate Model (GSRM v.2.1) is a vast improvement over its predecessor both in terms of amount of data input as in an increase in spatial model resolution by factor of ˜2.5 in areas with dense data coverage. We determined 6739 velocities from time‐series of (mostly) continuous GPS measurements; i.e., by far the largest global velocity solution to date. We transformed 15772 velocities from 233 (mostly) published studies onto our core solution to obtain 22511 velocities in the same reference frame. Care is taken to not use velocities from stations (or time periods) that are affected by transient phenomena; i.e., this data‐set consists of velocities best representing the interseismic plate velocity. About 14% of the Earth is allowed to deform in 145,086 deforming grid cells (0.25º longitude by 0.2º latitude in dimension). The remainder of the Earth’s surface is modeled as rigid spherical caps representing 50 tectonic plates. For 36 plates we present new GPS‐derived angular velocities. For all the plates that can be compared with the most recent geologic plate motion model, we find that the difference in angular velocity is significant. The rigid‐body rotations are used as boundary conditions in the strain rate calculations. The strain rate field is modeled using the Haines and Holt method, which uses splines to obtain an self‐consistent interpolated velocity gradient tensor field, from which strain rates, vorticity rates, and expected velocities are derived. We also present expected faulting orientations in areas with significant vorticity, and update the no‐net rotation reference frame associated with our global velocity gradient field. Finally, we present a global map of recurrence times for Mw=7.5 characteristic earthquakes.
      PubDate: 2014-08-30T09:03:17.45883-05:0
      DOI: 10.1002/2014GC005407
       
  • Source‐related variables for the description of the oceanic carbon
           system
    • Authors: G. Walin; J. Hieronymus, J. Nycander
      Pages: n/a - n/a
      Abstract: The oceanic carbon system is commonly described in terms of the two state variables total carbon, DIC, and alkalinity, Alk. Here we suggest the use of alternative source adapted state variables, Acidic Carbon, AC and Basic Carbon, BC, defined by and related to (DIC, Alk) with a simple linear transformation. (AC, BC) can be interpreted as representing respectively the supply to the system of carbon dioxide and dissolved carbonate, keeping in mind that supply of hydrogen ions acts to transform from basic carbon to acidic carbon. Accordingly these variables tell us how much carbon dioxide or dissolved carbonate we actually have in the water, despite the fact that the major part of the carbon resides in bicarbonate ions. We claim that using these source related variables as a compliment to the traditional variables, offers a number of advantages in the formulation of continuity equations, as well as in the interpretation of observations and modelling results. The traditional definition of alkalinity is related to a measuring procedure rather than to the supply of material to the system. Here we demonstrate that alkalinity, though defined in the traditional way, may be interpreted in terms of sources and sinks acting on the system. In the case of ocean water this amounts to twice the supply of dissolved carbonate minus the net supply of free hydrogen ions. We argue that this interpretation is a useful complement to the traditional definition. Every process that affects the state of the carbon system may be quantified in terms of supply of carbon dioxide, Fa, carbonate ions, Fb, or hydrogen ions, E.
      PubDate: 2014-08-27T11:35:54.719499-05:
      DOI: 10.1002/2014GC005383
       
  • The magnitude distribution of dynamically triggered earthquakes
    • Authors: Stephen Hernandez; Emily E. Brodsky, Nicholas J. van der Elst
      Pages: n/a - n/a
      Abstract: Large dynamic strains carried by seismic waves are known to trigger seismicity far from their source region. It is unknown, however, whether surface waves trigger only small earthquakes, or whether they can also trigger large earthquakes. To partially address this question, we evaluate whether current data can distinguish between the magnitude distribution of triggered and untriggered small earthquakes. We use a mixing model approach in which total seismicity is decomposed into 2 classes: “triggered” events initiated or advanced by far‐field dynamic strains, and “untriggered” spontaneous events consisting of everything else. The b‐value of a mixed data set, bMIX, is decomposed into a weighted sum of b‐values of its constituent components, bT and bU. We utilize the previously observed relationship between triggering rate and dynamic strain amplitude to identify the fraction of triggered events in populations of earthquakes and then invert for bT. For Californian seismicity, data are consistent with a single‐parameter Gutenberg‐Richter hypothesis governing the magnitudes of both triggered and untriggered earthquakes.
      PubDate: 2014-08-27T11:18:34.072837-05:
      DOI: 10.1002/2014GC005404
       
  • Integrated geophysical‐petrological modeling of
           lithosphere‐asthenosphere boundary in central Tibet using
           electromagnetic and seismic data
    • Authors: Jan Vozar; Alan G. Jones, Javier Fullea, Matthew R. Agius, Sergei Lebedev, Florian Le Pape, Wenbo Wei
      Pages: n/a - n/a
      Abstract: We undertake a petrologically‐driven approach to jointly model magnetotelluric (MT) and seismic surface wave dispersion (SW) data from central Tibet, constrained by topographic height. The approach derives realistic temperature and pressure distributions within the upper mantle and characterizes mineral assemblages of given bulk chemical compositions as well as water content. This allows us to define a bulk geophysical model of the upper mantle based on laboratory and xenolith data for the most relevant mantle mineral assemblages and to derive corresponding predicted geophysical observables. One‐dimensional deep resistivity models were derived for two groups of MT stations. One group, located in the Lhasa Terrane, shows the existence of an electrically conductive upper‐mantle layer and shallower conductive upper‐mantle layer for the other group, located in the Qiangtang Terrane. The subsequent one‐dimensional integrated petrological‐geophysical modeling suggests a lithosphere‐asthenosphere boundary (LAB) at a depth of 80‐120 km with a dry lithosphere for the Qiangtang Terrane. In contrast, for the Lhasa Terrane the LAB is located at about 180 km but the presence of a small amount of water in the lithospheric mantle (
      PubDate: 2014-08-27T11:15:38.339924-05:
      DOI: 10.1002/2014GC005365
       
  • Modeling ash fall distribution from a Yellowstone supereruption
    • Authors: Larry G. Mastin; Alexa R. Van Eaton, Jacob B. Lowenstern
      Pages: n/a - n/a
      Abstract: We used the volcanic ash transport and dispersion model Ash3d to estimate the distribution of ashfall that would result from a modern‐day Plinian supereruption at Yellowstone volcano. The simulations required modifying Ash3d to consider growth of a continent‐scale umbrella cloud and its interaction with ambient wind fields. We simulated eruptions lasting 3 days, 1 week, and 1 month, each producing 330 km3 of volcanic ash, dense‐rock equivalent (DRE). Results demonstrate that radial expansion of the umbrella cloud is capable of driving ash upwind (westward) and crosswind (N‐S) in excess of 1500 km, producing more‐or‐less radially symmetric isopachs that are only secondarily modified by ambient wind. Deposit thicknesses are decimeters to meters in the northern Rocky Mountains, centimeters to decimeters in the northern Midwest, and millimeters to centimeters on the East, West, and Gulf Coasts. Umbrella cloud growth may explain the extremely widespread dispersal of the ∼640 ka and 2.1 Ma Yellowstone tephra deposits in the eastern Pacific, northeastern California, southern California, and South Texas.
      PubDate: 2014-08-27T08:58:22.250117-05:
      DOI: 10.1002/2014GC005469
       
  • Cenozoic epeirogeny of the Arabian Peninsula from drainage modeling
    • Authors: J. W. P. Wilson; G. G. Roberts, M. J. Hoggard, N. J. White
      Pages: n/a - n/a
      Abstract: It is generally accepted that the Arabian Peninsula has been uplifted by sub‐crustal processes. Positive residual depth anomalies from oceanic crust in the Red Sea and in the Gulf of Aden suggest that a region surrounding this peninsula is dynamically supported. Admittance calculations, surface wave tomography studies, and receiver function analyses all imply that regional topography is generated and maintained by some combination of mantle convective circulation and lithospheric thickness changes. Despite these significant advances, the spatial and temporal uplift rate history of the Arabian Peninsula is not well known. Here, we show that a regional uplift rate history can be obtained by jointly inverting 225 longitudinal river profiles that drain this peninsula. Our strategy assumes that shapes of individual river profiles are controlled by uplift rate history and moderated by erosional processes. We used local measurements of incision rate to calibrate the relevant erosional parameters. In our inverse algorithm, uplift rate is permitted to vary smoothly as a function of space and time but upstream drainage area remains invariant. We also assume that knickzone migration is not lithologically controlled. Implications of these important assumptions have been investigated. Our results suggest that the Arabian Peninsula underwent two phase asymmetric uplift during the last 20–30 Ma at rates of 0.05–0.1 mm/yr– 1. The southwestern flank of the peninsula has been uplifted by 1.5–2.5 km. Regional stratigraphic constraints, the age and composition of volcanism, paleosol formation, incised peneplains, emergent marine terraces, and thermochronometric measurements corroborate our calculated patterns of uplift. Progessive development of three domal swells along the western margin of the peninsula is consistent with localized upwelling of hot asthenospheric mantle.
      PubDate: 2014-08-27T00:02:26.109071-05:
      DOI: 10.1002/2014GC005283
       
  • Tectonics, topography, and river system transition in East Tibet: Insights
           from the sedimentary record in Taiwan
    • Authors: Qing Lan; Yi Yan, Chi‐Yue Huang, Peter D. Clift, Xuejie Li, Wenhuang Chen, Xingchang Zhang, Mengming Yu
      Pages: n/a - n/a
      Abstract: [1] The Cenozoic in East Asia is marked by major changes in tectonics, landscapes and river systems, although the timing and nature of such changes remains disputed. We investigate the geochemistry and neodymium isotope character of Cenozoic mudstones spanning the breakup‐unconformity in the Western Foothills of Taiwan in order to constrain erosion and drainage development in southern China during the opening of the South China Sea. The La/Lu, Eu/Eu*, Th/Sc, Th/La, Cr/Th and εNd values in these rocks show an abrupt change between ca. 31 Ma and 25 Ma. Generally the higher εNd values in sediments deposited prior to 31 Ma indicate erosion from Phanerozoic granitic sources exposed in coastal South China, whereas the lower εNd values suggest that the main sources had evolved to inland southern China by ca. 25 Ma. The SHRIMP U‐Pb ages of zircons from a tuff, together with biostratigraphy data constrain the breakup‐unconformity to be between ca. 39 and 33 Ma, suggesting that the seafloor spreading in the South China Sea commenced before ca. 33 Ma. This is significantly older than most of the oceanic crust preserved in the deeper part of the basin. Diachronous westward younging of the breakup‐unconformities and provenance changes of basins are consistent with seafloor spreading propagating from east to west. Initial spreading of the South China Sea prior to ca. 33 Ma corresponds to tectonic adjustment in East Asia, including extrusion of the Indochina block and the rotation and eastward retreat of the subducting Pacific Plate.
      PubDate: 2014-08-26T05:14:49.41221-05:0
      DOI: 10.1002/2014GC005310
       
  • The relationship between seismicity and fault structure on the Discovery
           transform fault, East Pacific Rise
    • Authors: Monica Wolfson‐Schwehr; Margaret S. Boettcher, Jeffrey J. McGuire, John A. Collins
      Pages: n/a - n/a
      Abstract: There is a global seismic moment deficit on mid‐ocean ridge transform faults, and the largest earthquakes on these faults do not rupture the full fault area. We explore the influence of physical fault structure, including step‐overs in the fault trace, on the seismic behavior of the Discovery transform fault, 4S on the East Pacific Rise. One year of microseismicity recorded during a 2008 ocean bottom seismograph deployment (24,377 0 ≤ ML ≤ 4.6 earthquakes) and 24 years of Mw ≥ 5.4 earthquakes obtained from the Global Centroid Moment Tensor catalog, are correlated with surface fault structure delineated from high‐resolution multibeam bathymetry. Each of the 15 5.4 ≤ Mw ≤ 6.0 earthquakes that occurred on Discovery between January 1, 1990 ‐ April 1, 2014 was relocated into one of five distinct rupture patches using a teleseismic surface wave cross‐correlation technique. Microseismicity was relocated using the HypoDD relocation algorithm. The western fault segment of Discovery (DW) is composed of three zones of varying structure and seismic behavior: a zone with no large events and abundant microseismicity, a fully coupled zone with large earthquakes, and a complex zone with multiple fault strands and abundant seismicity. In general, microseismicity is reduced within the patches defined by the large, repeating earthquakes. While the extent of the large rupture patches on DW correlates with physical features in the bathymetry, step‐overs in the primary fault trace are not observed at patch boundaries, suggesting along‐strike heterogeneity in fault zone properties controls the size and location of the large events.
      PubDate: 2014-08-21T11:18:08.798583-05:
      DOI: 10.1002/2014GC005445
       
  • Local and regional trends in Plio‐Pleistocene δ18O records from
           benthic foraminifera
    • Authors: David B. Bell; Simon J. A. Jung, Dick Kroon, Lucas J. Lourens, David A. Hodell
      Pages: n/a - n/a
      Abstract: We present new orbital‐resolution Pliocene‐Pleistocene benthic stable oxygen isotope (δ18Ob) records from Ocean Drilling Program Sites 1264 and 1267, from Walvis Ridge in the Southeast Atlantic. We compare long‐term (>250 kyr) interbasin δ18Ob‐gradients between Pacific and North Atlantic regional stacks, as well as intra and interbasin gradients from the perspective of Walvis Ridge. The δ18Ob values from Sites 1264 and 1267 are almost always higher than deep North Atlantic and Pacific sites, with large gradients (>0.5‰) emerging abruptly at ∼2.4 Ma and persisting until ∼1.3 Ma. From this, we infer the presence of a new water mass, which resulted from the influence of dense, 18O‐enriched Nordic sea overflow waters via the abyssal East Atlantic. Meanwhile, long‐term average δ18Ob values in the North Atlantic appear to have remained within 0–0.25‰ lower than in the Pacific. However, the magnitude of this difference is sensitive to the inclusion of records from the equatorial West Atlantic. These results, together with constraints based on temperature, salinity, and density, suggest an influence of the seawater δ18O (δ18OSW) versus salinity relationship of source waters on δ18Ob values within the Atlantic. In particular, the abrupt emergence at ∼2.4 Ma of higher δ18Ob values at Sites 1264 and 1267, relative to North Atlantic records, appears to require a low‐latitude surface water δ18OSW signal. This implies a connection between northward heat transport and deep water export into the abyssal East Atlantic. Hence, our results have implications for the interpretation of δ18Ob records and highlight the potential for δ18Ob to constrain deep Atlantic water mass sources and pathways during the Plio‐Pleistocene.
      PubDate: 2014-08-20T10:19:00.159808-05:
      DOI: 10.1002/2014GC005297
       
  • Long‐term (17 Ma) turbidite record of the timing and frequency of
           large flank collapses of the Canary Islands
    • Authors: J. E. Hunt; P. J. Talling, M. A. Clare, I. Jarvis, R. B. Wynn
      Pages: n/a - n/a
      Abstract: Volcaniclastic turbidites on the Madeira Abyssal Plain provide a record of large‐volume volcanic island flank collapses from the Canary Islands. This long‐term record spans 17 Ma, and comprises 125 volcaniclastic beds. Determining the timing, provenance and volumes of these turbidites provides key information about the occurrence of mass wasting from the Canary Islands, especially the western islands of Tenerife, La Palma and El Hierro. These turbidite records demonstrate that landslides often coincide with protracted periods of volcanic edifice growth, suggesting that loading of the volcanic edifices may be a key preconditioning factor for landslide triggers. Furthermore, the last large‐volume failures from Tenerife coincide with explosive volcanism at the end of eruptive cycles. Many large‐volume Canary Island landslides also occurred during periods of warmer and wetter climates associated with sea‐level rise and subsequent highstand. However, these turbidites are not serially dependent and any association with climate or sea level change is not statistically significant.
      PubDate: 2014-08-20T10:16:22.147685-05:
      DOI: 10.1002/2014GC005232
       
  • Formation and geomorphologic history of the Lonar impact crater deduced
           from in situ cosmogenic 10Be and 26Al
    • Authors: Atsunori Nakamura; Yusuke Yokoyama, Yasuhito Sekine, Kazuhisa Goto, Goro Komatsu, P. Senthil Kumar, Hiroyuki Matsuzaki, Ichiro Kaneoka, Takafumi Matsui
      Pages: n/a - n/a
      Abstract: The Lonar impact crater is one of a few craters on Earth formed directly in basalt, providing a unique opportunity to study an analog for crater degradation processes on Mars. Here we present surface 10Be and 26Al exposure dates in order to determine the age and geomorphic evolution of Lonar crater. Together with a 14C age of preimpact soil, we obtain a crater age of 37.5 ± 5.0 ka, which contrasts with a recently reported and apparently older 40Ar/39Ar age (570 ± 47 ka). This suggests that the 40Ar/39Ar age may have been affected by inherited radiogenic 40Ar (40Ar*inherited) in the impact glass. The spatial distribution of surface exposure ages of Lonar crater differs from that for Barringer crater, indicating Lonar crater rim is actively eroding. Our new chronology provides a unique opportunity to compare the geomorphological history of the two craters, which have similar ages and diameters, but are located in different climate and geologic settings.
      PubDate: 2014-08-19T14:13:06.558942-05:
      DOI: 10.1002/2014GC005376
       
  • History and dynamics of net rotation of the mantle and lithosphere
    • Authors: M. L. Rudolph; S.J. Zhong
      Pages: n/a - n/a
      Abstract: The net rotation of Earth's lithosphere with respect to the underlying mantle is the longest‐wavelength component of toroidal flow in the mantle and is sensitive to both mantle buoyancy structure and lateral viscosity variations. The lithospheric net rotation in the geologic past implied by plate reconstructions using a hotspot reference frame for the past 100 Myr is up to five times greater than the present‐day rate of lithospheric net rotation. We explore the role of lateral viscosity variations associated with subcontinental keels in producing the lithospheric net rotation for the geologic past and find that the introduction of subcontinental keels improves the agreement between modeled net rotation and the net rotation present in the plate reconstructions for the past 25 Myr. However, our models with continental keels produce at most 0.16°/Myr of differential rotation between the lithosphere and lower mantle for present‐day, and explaining the most rapid rates of lithospheric net rotation during the Cretaceous and Paleogene remains challenging. This suggests the need for either an additional mechanism for generating lithospheric net rotation, or an adjustment to the absolute mantle reference frame relative to which plate motions are specified.
      PubDate: 2014-08-18T22:36:58.979803-05:
      DOI: 10.1002/2014GC005457
       
  • Ambient tectonic stress as fragile geological feature
    • Authors: Norman H. Sleep
      Pages: n/a - n/a
      Abstract: Strong seismic waves produce frictional failure within shallow pervasively cracked rocks. Distributed failure preferentially relaxes ambient tectonic stresses, providing a fragility measure of past strong shaking. Relaxation of the regional fault‐normal compression appears to have occurred within granite from 768 m down to ~1000‐1600 m depth at the Pilot Hole near Parkfield, California. Subsequent movements on the main fault have imposed strike‐slip stress within the relaxed region. Peak ground velocities of ~2 m s‐1 are inferred for infrequent (few 1000 yr recurrence) past earthquakes from stress relaxation within the granite and from the variation of S‐wave velocity with depth in the overlying sandstone. Conversely, frequent strong shaking in slowly deforming regions relaxes shallow ambient tectonic stress. This situation is expected beneath Whittier Narrows, where strong Love waves from numerous San Andreas events repeatedly produced nonlinear behavior.
      PubDate: 2014-08-18T03:21:00.697367-05:
      DOI: 10.1002/2014GC005426
       
  • Reconciling mantle attenuation‐temperature relationships from
           seismology, petrology, and laboratory measurements
    • Authors: GA Abers; KM Fischer, G Hirth, DA Wiens, T Plank, BK Holtzman, C McCarthy, E. Gazel
      Pages: n/a - n/a
      Abstract: Seismic attenuation measurements provide a powerful tool for sampling mantle properties. Laboratory experiments provide calibrations at seismic frequencies and mantle temperatures for dry melt‐free rocks, but require ~102 ‐ 103 extrapolations in grain size to mantle conditions; also, the effects of water and melt are not well understood. At the same time, body‐wave attenuation measured from dense broadband arrays provides reliable estimates of shear‐wave attenuation (QS‐1), affording an opportunity for calibration. We reanalyze seismic datasets that sample arc and back‐arc mantle, in Central America, the Marianas and the Lau basin, confirming very high attenuation (QS ~ 25‐80) at 1 Hz and depths of 50‐100 km. At each of these sites, independent petrological studies constrain the temperature and water content where basaltic magmas last equilibrated with the mantle, 1300‐1450°C. The QS measurements correlate inversely with the petrologically‐inferred temperatures, as expected. However, dry attenuation models predict QS too high by a factor of 1.5‐5. Modifying models to include effects of H2O and rheology‐dependent grain size shows that the effects of water‐enhanced dissipation and water‐enhanced grain growth nearly cancel, so H2O effects are modest. Therefore high H2O in the arc source region cannot explain the low QS, nor in the backarc where lavas show modest water content. Most likely the high attenuation reflects the presence of melt, and some models of melt effects come close to reproducing observations. Overall, body‐wave QS can be reconciled with petrologic and laboratory inferences of mantle conditions if melt has a strong influence beneath arcs and backarcs.
      PubDate: 2014-08-16T01:24:55.445493-05:
      DOI: 10.1002/2014GC005444
       
  • Identification and environmental interpretation of diagenetic and biogenic
           greigite in sediments: A lesson from the Messinian Black Sea
    • Authors: Liao Chang; Iuliana Vasiliev, Christiaan van Baak, Wout Krijgsman, Mark J. Dekkers, Andrew P. Roberts, John D. Fitz Gerald, Annelies van Hoesel, Michael Winklhofer
      Pages: n/a - n/a
      Abstract: Greigite (Fe3S4) is a widespread authigenic magnetic mineral in anoxic sediments, and is also commonly biosynthesized by magnetotactic bacteria in aqueous environments. While the presence of fossilized bacterial magnetite (Fe3O4) has now been widely demonstrated, the preservation of greigite magnetofossils in the geological record is only poorly constrained. Here, we investigate Mio‐Pliocene sediments of the former Black Sea to test whether we can detect greigite magnetofossils, and to unravel potential environmental controls on greigite formation. Our magnetic analyses and transmission electron microscope (TEM) observations indicate the presence of both diagenetic and bacterial greigite, and suggest a potentially widespread preservation of greigite magnetofossils in ancient sediments, which has important implications for assessing the reliability of paleomagnetic records carried by greigite. TEM‐based chemical and structural analyses also indicate the common presence of nickel‐substituted diagenetic iron sulfide crystals with a ferrimagnetic greigite structure. In addition, our cyclostratigraphic framework allows correlation of magnetic properties of Messinian former Black Sea sediments (Taman Peninsula, Russia) to global climate records. Diagenetic greigite enhancements appear to be climatically controlled, with greigite mainly occurring in warm/wet periods. Diagenetic greigite formation can be explained by variations in terrigenous inputs and dissolved pore water sulfate concentrations in different sedimentary environments. Our analysis demonstrates the usefulness of greigite for studying long‐term climate variability in anoxic environments.
      PubDate: 2014-08-16T01:23:37.2059-05:00
      DOI: 10.1002/2014GC005411
       
  • Deciphering bottom current velocity and paleoclimate signals from
           contourite deposits in the Gulf of Cádiz during the last 140 kyr: An
           inorganic geochemical approach
    • Authors: André Bahr; Francisco J. Jiménez‐Espejo, Nada Kolasinac, Patrick Grunert, F. Javier Hernández‐Molina, Ursula Röhl, Antje H. L. Voelker, Carlota Escutia, Dorrik A. V. Stow, David Hodell, Carlos A. Alvarez‐Zarikian
      Pages: n/a - n/a
      Abstract: Contourites in the Gulf of Cádiz (GC) preserve a unique archive of Mediterranean Outflow Water (MOW) variability over the past 5.3 Ma. In our study, we investigate the potential of geochemical data obtained by XRF scanning to decipher bottom current processes and paleoclimatic evolution at two different sites drilled during IODP Expedition 339 through contourites in the northern GC: Site U1387, which is bathed by the upper MOW core, and Site U1389, located more proximal to the Strait of Gibraltar. The lack of major downslope transport during the Pleistocene makes both locations ideally suited for our study. The results indicate that the Zr/Al ratio, representing the relative enrichment of heavy minerals (zircon) over less dense alumnosilicates under fast bottom current flow, is the most useful indicator for a semiquantitative assessment of current velocity. Although most elements are biased by current‐related processes, the bromine (Br) record, representing organic content, preserves the most pristine climate signal rather independent of grain‐size changes. Hence, Br can be used for chronostratigraphy and site‐to‐site correlation in addition to stable isotope stratigraphy. Based on these findings, we reconstructed MOW variability for Marine Isotope Stages (MIS) 1–5 using the Zr/Al ratio from Site U1387. The results reveal abrupt, millennial‐scale variations of MOW strength during Greenland Stadials (GS) and Interstadials (GI) with strong MOW during GS and glacial Terminations and a complex behavior during Heinrich Stadials. Millennial‐scale variability persisting during periods of poorly expressed GS/GI cyclicities implies a strong internal oscillation of the Mediterranean/North Atlantic climate system.
      PubDate: 2014-08-14T13:03:56.17845-05:0
      DOI: 10.1002/2014GC005356
       
  • Stable isotopes of surface water across the Longmenshan margin of the
           eastern Tibetan Plateau
    • Authors: Qiang Xu; Gregory D. Hoke, Jing Liu‐Zeng, Lin Ding, Wei Wang, Yang Yang
      Pages: n/a - n/a
      Abstract: Characterization of the stable isotope compositions (δ18O and δD) of modern‐day surface waters traversing mountain ranges and bordering continental plateaus is important for refining climate models and establishing modern isotope‐elevation gradients along mountain ranges. The Longmenshan margin of the Tibetan Plateau is a steep, 4 km topographic front situated near the boundaries between westerlies and Asian monsoon moisture sources, and is previously unexplored with respect to the variation in water isotopic composition with elevation. This study reports stable isotope data from 101 water samples collected from streams, springs, and ponds along the Min River (Minjiang) watershed, which traverses the Longmenshan margin. Local meteoric water lines, d‐excess values, and surrounding precipitation and river water datasets suggest that precipitation across the Longmenshan margin is dominated by the East Asian summer monsoon. The increase in d‐excess values with increasing catchment elevation breaks down as local moisture recycling becomes important at elevations > 3 km a.s.l.. Along the Min River, however, the δ18Ow and δDw values decrease with increasing catchment elevation, which fit second order polynomial curves and are well approximated by a simple Rayleigh fractionation processes. The temperature‐corrected oxygen values in authigenic carbonates from the Pleistocene Zoige Basin, north of the Min River watershed, yield elevations equivalent to present by the oxygen isotope‐elevation relationship of the Longmenshan margin.
      PubDate: 2014-08-05T02:46:03.461866-05:
      DOI: 10.1002/2014GC005252
       
  • Nd and Sr isotope compositions of different phases of surface sediments in
           the South Pacific: Extraction of seawater signatures, boundary exchange,
           and detrital/dust provenance
    • Authors: Mario Molina‐Kescher; Martin Frank, Ed C. Hathorne
      Pages: n/a - n/a
      Abstract: The radiogenic isotope composition of neodymium (Nd) and strontium (Sr) are useful tools to investigate present and past oceanic circulation or input of terrigenous material. We present Nd and Sr isotope compositions extracted from different sedimentary phases, including early diagenetic Fe‐Mn coatings, ‘unclean' foraminiferal shells, fossil fish teeth, and detritus of marine surface sediments (core‐tops) covering the entire mid‐latitude South Pacific. Comparison of detrital Nd isotope compositions to deep‐water values from the same locations suggest that ‘boundary exchange' has little influence on the Nd isotope composition of western South Pacific seawater. Concentrations of Rare Earth Elements (REE) and Al/Ca ratios of ‘unclean' planktonic foraminifera suggest that this phase is a reliable recorder of seawater Nd isotope composition. The signatures obtained from fish teeth and ‘non‐decarbonated' leachates of bulk sediment Fe‐Mn oxyhydroxide coatings also agree with ‘unclean' foraminifera. Direct comparison of Nd isotope compositions extracted using these methods with seawater Nd isotope compositions is complicated by the low accumulation rates yielding radiocarbon ages of up to 24 kyrs, thus mixing the signal of different ocean circulation modes. This suggests that different past seawater Nd isotope compositions have been integrated in authigenic sediments from regions with low sedimentation rates. Combined detrital Nd and Sr isotope signatures indicate a dominant role of the Westerly winds transporting lithogenic material from South New Zealand and Southeastern Australia to the open South Pacific. The proportion of this material decreases towards the east, where supply from the Andes increases and contributions from Antarctica cannot be ruled out.
      PubDate: 2014-08-05T02:43:27.981471-05:
      DOI: 10.1002/2014GC005443
       
  • Eruptive and tectonic history of the Endeavour segment, Juan de Fuca
           Ridge, based on AUV mapping data and lava flow ages
    • Authors: David A. Clague; Brian M. Dreyer, Jennifer B. Paduan, Julie F. Martin, David W. Caress, James B. Gill, Deborah S. Kelley, Hans Thomas, Ryan A. Portner, John R. Delaney, Thomas P. Guilderson, Mary L. McGann
      Pages: n/a - n/a
      Abstract: High‐resolution bathymetric surveys from autonomous underwater vehicles ABE and D. Allan B. were merged to create a co‐registered map of 71.7 km2 of the Endeavour Segment of the Juan de Fuca Ridge. Radiocarbon dating of foraminifera in cores from three dives of remotely operated vehicle Doc Ricketts provide minimum eruption ages for 40 lava flows that are combined with the bathymetric data to outline the eruptive and tectonic history. The ages range from Modern to 10,700 marine‐calibrated years before present (yr BP). During a robust magmatic phase from >10,700 yr BP to ˜4300 yr BP, flows erupted from an axial high and many flowed >5 km down the flanks; some partly buried adjacent valleys. Axial magma chambers (AMCs) may have been wider than today to supply dike intrusions over a 2‐km‐wide axial zone. Summit Seamount formed by ˜4770 yr BP and was subsequently dismembered during a period of extension with little volcanism starting ˜4300 yr BP. This tectonic phase with only rare volcanic eruptions lasted until ˜2300 yr BP and may have resulted in near‐solidification of the AMCs. The axial graben formed by crustal extension during this period of low magmatic activity. Infrequent eruptions occurred on the flanks between 2620‐1760 yr BP and within the axial graben since ˜1750 yr BP. This most recent phase of limited volcanic and intense hydrothermal activity that began ˜2300 yr BP defines a hydrothermal phase of ridge development that coincides with the present‐day 1‐km wide AMCs and overlying hydrothermal vent fields.
      PubDate: 2014-08-02T05:36:58.085282-05:
      DOI: 10.1002/2014GC005415
       
  • Metasomatized ancient lithospheric mantle beneath the young Zealandia
           microcontinent and its role in HIMU‐like intraplate magmatism
    • Authors: JM Scott; TE Waight, QHA van der Meer, JM Palin, AF Cooper, C Münker
      Pages: n/a - n/a
      Abstract: There has been long debate on the asthenospheric versus lithospheric source for numerous intraplate basalts with ocean island basalt (OIB) and high time‐integrated U/Pb (HIMU)‐like source signatures that have erupted through the Zealandia continental crust. Analysis of 157 spinel facies peridotitic mantle xenoliths from 25 localities across Zealandia permits the first comprehensive regional description of the sub‐continental lithospheric mantle (SCLM) and insights into whether it could be a source to the intraplate basalts. Contrary to previous assumptions, the Oligocene‐Miocene Zealandia SCLM is highly heterogeneous. It is composed of a refractory craton‐like domain (West Otago) adjacent to several moderately fertile domains (East Otago, North Otago, Auckland Islands). Each domain has an early history decoupled from the overlying Carboniferous and younger continental crust, and each domain has undergone varying degrees of depletion followed by re‐enrichment. Clinopyroxene grains reveal trace element characteristics (low Ti/Eu, high Th/U) consistent with enrichment through reaction with carbonatite. This metasomatic overprint has a composition that closely matches HIMU in Sr, Pb + Nd isotopes. However, clinopyroxene Hf isotopes are in part highly radiogenic and decoupled from the other isotope systems, and also mostly more radiogenic than the intraplate basalts. If the studied spinel facies xenoliths are representative of the thin Zealandia SCLM, the melting of garnet facies lithosphere could only be the intraplate basalt source if it had a less radiogenic Hf‐Nd isotope composition than the investigated spinel facies, or was mixed asthenospheric‐derived melts containing less radiogenic Hf.
      PubDate: 2014-08-02T03:49:45.763415-05:
      DOI: 10.1002/2014GC005300
       
  • Correlation of cycles in lava lake motion and degassing at Erebus volcano,
           Antarctica
    • Authors: Nial Peters; Clive Oppenheimer, Drea Rae Killingsworth, Jed Frechette, Philip Kyle
      Pages: n/a - n/a
      Abstract: Several studies at Erebus volcano have recorded pulsatory behaviour in many of the observable properties of its active lava lake. A strong correlation between the variations in surface speed of the lake and the composition of gas emitted has previously been noted. While previous studies have shown that the SO2 flux and the surface elevation exhibit pulsatory behaviour with a similar period to that of the surface speed and gas composition, suggesting they are linked, a lack of overlap between the different measurements has prevented direct comparisons from being made. Using high time‐resolution measurements of surface elevation, surface speed, gas composition and SO2 flux we demonstrate for the first time an unambiguous link between the cyclic behaviour in each of these properties. We also show that the variation in gas composition may be explained by a subtle change in oxygen fugacity. The cycles are found to be in‐phase with each other, with a small but consistent lag of 1–3 min between the peaks in surface elevation and surface speed. Explosive events are found to have no observable effect on the pulsatory behaviour beyond the ~5 min period required for lake refill. The close correspondences between the varying lake surface motion, gas flux and composition, and modelled oxygen fugacity suggest strong links between magma degassing, redox change and the fluid dynamics of the shallow magmatic system.
      PubDate: 2014-07-31T03:55:49.446131-05:
      DOI: 10.1002/2014GC005399
       
  • Alteration of volcaniclastic deposits at Minna Bluff: Geochemical insights
           on mineralizing environment and climate during the Late Miocene in
           Antarctica
    • Authors: Joanne V. Antibus; Kurt S. Panter, Thomas I. Wilch, Nelia Dunbar, William McIntosh, Aradhna Tripati, Ilya Bindeman, Jerzy Blusztajn
      Pages: n/a - n/a
      Abstract: Secondary minerals in volcaniclastic deposits at Minna Bluff, a 45‐km‐long peninsula in the Ross Sea, are used to infer processes of alteration and environmental conditions in the Late Miocene. Glassy volcaniclastic deposits are altered and contain phillipsite and chabazite, low‐ to high‐Mg carbonates, chalcedony and clay. The δ18O of carbonates and chalcedony is variable, ranging from ‐0.50 to 21.53‰ and 0.68 to 10.37‰, respectively, and δD for chalcedony is light (‐187.8 to ‐220.6‰), corresponding to Antarctic meteoric water. A mean carbonate 87Sr/86Sr ratio of 0.70327 ±0.0009 (1σ, n = 12) is comparable to lava and suggests fresh water, as opposed to seawater, caused the alteration. Minerals were precipitated at elevated temperatures based on quartz‐calcite (91º and 104ºC) equilibrium, carbonate 13C‐18O (Δ47 = 5º to 43ºC) thermometry, and stability of zeolites in geothermal systems (>10º to ~100ºC). The alteration was a result of isolated, ephemeral events involving the exchange between heated meteoric water and glass during or soon after the formation of each deposit. Near surface evaporative distillation can explain 18O‐enriched compositions for some Mg‐rich carbonates and chalcedony. The δ18Owater calculated for carbonates (‐15.8 to ‐22.9‰) reveals a broad change, becoming heavier between ~12 and ~7 Ma, consistent with a warming climate. These findings are independently corroborated by the interpretation of Late Miocene sedimentary sequences recovered from nearby sediment cores. However, in contrast to a cold‐based thermal regime proposed for ice flow at core sites, wet‐based conditions prevailed at Minna Bluff; a likely consequence of high heat flow associated with an active magma system.
      PubDate: 2014-07-31T03:51:50.370357-05:
      DOI: 10.1002/2014GC005422
       
  • Persistent uplift of the Lazufre volcanic complex (Central Andes): New
           insights from PCAIM inversion of InSAR time series and GPS data
    • Authors: D. Remy; J.L Froger, H Perfettini, S. Bonvalot, G. Gabalda, F. Albino, V. Cayol, D. Legrand, M. De Saint Blanquat
      Pages: n/a - n/a
      Abstract: We reanalyzed the surface Displacements observed at the Lazufre Volcanic Complex in the Southern Andean Central Volcanic Zone using GPS measurements made between 2006 and 2008 and a large InSAR dataset. We performed a detailed spatiotemporal analysis of the displacements using a Principal Component Analysis Inversion Method (PCAIM). The PCAIM reveals a source with no significant changes in shape and dimension and with a remarkably linear strength increase over the whole period of observation (i.e. 2003 to 2010). Then we used a three‐dimensional Mixed Boundary Element Method (MBEM) to invert the first component of surface displacement as obtained from PCAIM. We explored a continuum of geometries from a shallow elliptic crack to a deep massive truncated elliptical cone that could represent a sill or a large magma chamber, respectively. The best models indicate a large flat‐topped source with a roof area between 40 and 670 km2 and a depth of between 2 and 14 km below ground surface. Lastly, on the basis of the limited data available for the thermo‐mechanical structure of the crust in the Southern Andean Central Volcanic Zone we consider some possible scenarios to explain the spatial and temporal pattern of displacements at Lazufre.
      PubDate: 2014-07-31T03:17:28.850604-05:
      DOI: 10.1002/2014GC005370
       
  • Influences of surface processes on fold growth during 3‐D detachment
           folding
    • Authors: M. Collignon; B.J.P. Kaus, D.A. May, N. Fernandez
      Pages: n/a - n/a
      Abstract: In order to understand the interactions between surface processes and multilayer folding systems, we here present fully coupled three‐dimensional numerical simulations. The mechanical model represents a sedimentary cover with internal weak layers, detached over a much weaker basal layer representing salt or evaporites. Applying compression in one direction results in a series of three‐dimensional buckle folds, of which the topographic expression consists of anticlines and synclines. This topography is modified through time by mass redistribution, which is achieved by a combination of fluvial and hillslope erosion, as well as deposition, and which can in return influence the subsequent deformation. Model results show that surface processes do not have a significant influence on folding patterns and aspect ratio of the folds. Nevertheless, erosion reduces the amount of shortening required to initiate folding and increases the exhumation rates. Increased sedimentation in the synclines contributes to this effect by amplifying the fold growth rate by gravity. The main contribution of surface processes is rather due to their ability to strongly modify the initial topography and hence the initial random noise, prior to deformation. If larger initial random noise is present, folds amplify faster, which is consistent with previous detachment folding theory. Variations in thickness of the sedimentary cover (in one or two directions) also have a significant influence on the folding pattern, resulting in linear, large aspect ratio folds.
      PubDate: 2014-07-31T03:17:23.993107-05:
      DOI: 10.1002/2014GC005450
       
  • Antarctic icebergs: A significant natural ocean sound source in the
           Southern Hemisphere
    • Authors: Haru Matsumoto; DelWayne R. Bohnenstiehl, Jean Tournadre, Robert P. Dziak, Joseph H. Haxel, T‐K A. Lau, Matt Fowler, Sigrid A. Salo
      Pages: n/a - n/a
      Abstract: In late 2007, two massive icebergs, C19a and B15a, drifted into open water and slowly disintegrated in the southernmost Pacific Ocean. Archived acoustic records show that the high‐intensity underwater sounds accompanying this breakup increased ocean noise levels at mid‐to‐equatorial latitudes over a period of ˜1.5 years. More typically, seasonal variations in ocean noise, which are characterized by austral summer‐highs and winter‐lows, appear to be modulated by the annual cycle of Antarctic iceberg drift and subsequent disintegration. This seasonal pattern is observed in all three Oceans of the Southern Hemisphere. The life cycle of Antarctic icebergs affects not only marine ecosystem but also the sound environment in far‐reaching areas and must be accounted for in any effort to isolate anthropogenic or climate‐induced noise contributions to the ocean soundscape.
      PubDate: 2014-07-30T04:30:46.434119-05:
      DOI: 10.1002/2014GC005454
       
  • Coring disturbances in IODP piston cores with implications for offshore
           record of volcanic events and the Missoula megafloods
    • Authors: Martin Jutzeler; James D. L. White, Peter J. Talling, Molly McCanta, Sally Morgan, Anne Le Friant, Osamu Ishizuka
      Pages: n/a - n/a
      Abstract: Piston cores collected from IODP drilling platforms (and its predecessors) provide the best long‐term geological and climatic record of marine sediments worldwide. Coring disturbances affecting the original sediment texture have been recognized since the early days of coring, and include deformation resulting from shear of sediment against the core barrel, basal flow‐in due to partial stroke, loss of stratigraphy, fall‐in, sediment loss through core catchers, and structures formed during core recovery and on‐deck transport. The most severe disturbances occur in non‐cohesive (sandy) facies, which are particularly common in volcanogenic environments and submarine fans. Although all of these types of coring disturbances have been recognized previously, our contribution is novel because it provides an easily accessible summary of methods for their identification. This contribution gives two specific examples on the importance of these coring disturbances. We show how suck‐in of sediments during coring artificially created very thick volcaniclastic sand layers in cores offshore Montserrat and Martinique (Lesser Antilles). We then analyze very thick, structureless sand layers from the Escanaba Trough inferred to be a record of the Missoula mega‐floods. These sand layers tend to coincide with the base of core sections, and their facies suggest coring disturbance by basal flow‐in, destroying the original structure and texture of the beds. We conclude by outlining and supporting IODP‐led initiatives to further reduce and identify coring disturbances, and acknowledge their recent successes in drilling challenging sand‐rich settings, such as during IODP Expedition 340.
      PubDate: 2014-07-24T04:22:23.637509-05:
      DOI: 10.1002/2014GC005447
       
  • Reactive halogens (BrO and OClO) detected in the plume of Soufrière
           Hills Volcano during an eruption hiatus
    • Authors: Amy Donovan; Vitchko Tsanev, Clive Oppenheimer, Marie Edmonds
      Pages: n/a - n/a
      Abstract: Volcanic plumes are sites of dynamic chemistry involving halogen gases. Here we present new data on the relative abundances of SO2, BrO and OClO gases emitted from Soufrière Hills Volcano (SHV). They were collected during an eruptive hiatus but during sustained degassing at this halogen‐rich volcano. By comparison with data from a previous study during an eruptive phase (Bobrowski et al, 2003) and application of the data and modelling of Villemant et al. (2008), we suggest that, after consideration of errors, either the rate of HBr conversion to BrO is variable, ranging from ~30% to ~15%, and/or the relative partitioning of Cl and Br into the gas phase from the melt changes according to eruptive activity. We examine the potential implications of this for fluid‐melt partitioning, and compare our results with data from the experimental literature. Our work contributes towards understanding the controls on the BrO/SO2 ratio for volcano monitoring purposes; the changes in plume chemistry with regard to bromine at the onset of lava extrusion may be large and rapid. OClO was detected in the plume at SHV for the first time. This species has only previously been detected in emissions from Mount Etna (Bobrowski et al., 2007, using ground‐based methods) and from Puyehue Cordon Caulle (Theys et al., 2014, using satellite‐based methods). No HCHO or NOy species were detected in the spectra.
      PubDate: 2014-07-23T02:05:44.622788-05:
      DOI: 10.1002/2014GC005419
       
  • Atmospheric transport of mineral dust from the Indo‐Gangetic Plain:
           Temporal variability, acid processing, and iron solubility
    • Authors: Bikkina Srinivas; M. M. Sarin, R. Rengarajan
      Pages: n/a - n/a
      Abstract: Atmospheric transport of chemical constituents from the Indo‐Gangetic Plain (IGP) to the Bay of Bengal is a conspicuous seasonal feature that occurs during the late NE‐monsoon (December‐March). With this perspective, aerosol composition and abundance of mineral dust have been studied during November 2009 ‐ March 2010 from a sampling site (Kharagpur: 22.3 N, 87.3E) in the IGP, representing the atmospheric outflow to the Bay of Bengal. The chemical composition of PM2.5 suggests the dominance of nss‐SO42‐ (6.9 – 24.3 µg m‐3); whereas the abundance of mineral dust varied from 3 to 18 µg m‐3. The concentration of aerosol iron (FeTot) and its fractional solubility (Fews % = Fews/FeTot *100, where Fews is the water‐soluble fraction of FeTot) varied from 60 to 1144 ng m‐3 and from 6.7 to 26.5 %, respectively. A striking similarity in the temporal variability of total inorganic acidity (TIA = NO3‐ + nss‐SO42‐) and Fews (%) provides evidence for acid processing of mineral dust (alluvium) during atmospheric transport from the IGP. The contribution of TIA to water‐soluble inorganic species [(nss‐SO42‐ + NO3‐)/ΣWSIS], mass ratios of Ca/Al and Fe/Al, abundance of dust (%) and Fews (%) in the IGP‐outflow are similar to the aerosol composition over the Bay of Bengal. With the rapid increase in anthropogenic activities over south and south‐east Asia, the enhanced fractional solubility of aerosol iron (attributed to acid processing of mineral dust) has implications to further increase the air‐sea deposition of Fe to the surface ocean.
      PubDate: 2014-07-22T02:09:59.622471-05:
      DOI: 10.1002/2014GC005395
       
  • Evolution of stress and fault patterns in oblique rift systems: 3‐D
           numerical lithospheric‐scale experiments from rift to breakup
    • Authors: Sascha Brune
      Pages: n/a - n/a
      Abstract: Rifting involves complex normal fault systems that are controlled by extension direction, reactivation of pre‐rift structures, sedimentation, and dyke dynamics. The relative impact of these factors on the observed fault pattern, however, is difficult to deduce from field‐based studies alone. This study provides insight in crustal stress patterns and fault orientations by employing a laterally homogeneous, 3D rift setup with constant extension velocity. The presented numerical forward experiments cover the whole spectrum of oblique extension. They are conducted using an elasto‐visco‐plastic finite element model and involve crustal and mantle layers accounting for self‐consistent necking of the lithosphere. Despite recent advances, 3D numerical experiments still require relatively coarse resolution so that individual faults are poorly resolved. This issue is addressed by applying a post‐processing method that identifies the stress regime and preferred fault azimuth at each surface element. The simple model setup results in a surprising variety of fault orientations that are solely caused by the three‐dimensionality of oblique rift systems. Depending on rift obliquity, these orientations can be grouped in terms of rift‐parallel, extension‐orthogonal, and intermediate normal fault directions as well as strike‐slip faults. While results compare well with analog rift models of low to moderate obliquity, new insight is gained in advanced rift stages and highly oblique settings. Individual fault populations are activated in a characteristic multi‐phase evolution driven by lateral density variations of the evolving rift system. In natural rift systems this pattern might be modified by additional heterogeneities, surface processes and dyke dynamics.
      PubDate: 2014-07-22T01:10:50.108671-05:
      DOI: 10.1002/2014GC005446
       
  • Multi‐scale convection in a geodynamo simulation with uniform heat
           flux along the outer boundary
    • Authors: Hiroaki Matsui; Eric King, Bruce Buffett
      Pages: n/a - n/a
      Abstract: It is generally expected that Earth's magnetic field, which is generated by convecting liquid metal within its core, will substantially alter that convection through the action of Lorentz forces. In most dynamo simulations, however, Lorentz forces do very little to change convective flow, which is predominantly fine‐scaled. An important exception to this observation is in dynamo models that employ uniform heat flux boundary conditions, rather than the usual uniform temperature conditions, in which multi‐scale convection is observed. We investigate the combined influence of thermal boundary conditions and magnetic fields using four simulations: two dynamos and two non‐magnetic models, with either uniform temperature or heat flux fixed at the outer boundary. Of the four, only the fixed‐heat‐flux dynamo simulation produces multi‐scale convective flow patterns. Comparison between the models suggests that the fixed‐flux dynamo generates large patches of strong azimuthal magnetic field that suppress small‐scale convective motions. By allowing temperature to vary along the outer boundary, the fixed‐flux dynamo generates stronger azimuthal flow and, in turn, stronger magnetic field, and the resulting Lorentz forces alter the nature of convective flow. Extrapolation of the analyses presented here suggests that magnetic fields may also suppress small‐scale convection in the Earth's core.
      PubDate: 2014-07-17T04:08:41.726933-05:
      DOI: 10.1002/2014GC005432
       
  • Correlated patterns in hydrothermal plume distribution and apparent
           magmatic budget along 2500 km of the Southeast Indian Ridge
    • Authors: Edward T. Baker; Christophe Hémond, Anne Briais, Marcia Maia, Daniel S. Scheirer, Sharon L. Walker, Tingting Wang, Yongshun John Chen
      Pages: n/a - n/a
      Abstract: Multiple geological processes affect the distribution of hydrothermal venting along a mid‐ocean ridge. Deciphering the role of a specific process is often frustrated by simultaneous changes in other influences. Here we take advantage of the almost constant spreading rate (65‐71 mm/yr) along 2500 km of the Southeast Indian Ridge (SEIR) between 77°‐99°E to examine the spatial density of hydrothermal venting relative to regional and segment‐scale changes in the apparent magmatic budget. We use 227 vertical profiles of light backscatter and (on 41 profiles) oxidation‐reduction potential along 27 1st‐ and 2nd‐order ridge segments on and adjacent to the Amsterdam‐St. Paul (ASP) Plateau to map ph, the fraction of casts detecting a plume. At the regional scale, venting on the five segments crossing the magma‐thickened hotspot plateau is almost entirely suppressed (ph = 0.02). Conversely, the combined ph (0.34) from all other segments follows the global trend of ph versus spreading rate. Off the ASP Plateau, multi‐segment trends in ph track trends in the regional axial depth, high where regional depth increases and low where it decreases. At the individual segment scale, a robust correlation between ph and cross‐axis inflation for 1st‐order segments shows that different magmatic budgets among 1st‐order segments are expressed as different levels of hydrothermal spatial density. This correlation is absent among 2nd‐order segments. Eighty‐five percent of the plumes occur in eight clusters totaling ~350 km. We hypothesize that these clusters are a minimum estimate of the length of axial melt lenses underlying this section of the SEIR.
      PubDate: 2014-07-17T04:04:55.240659-05:
      DOI: 10.1002/2014GC005344
       
  • Effect of latent heat of freezing on crustal generation at low spreading
           rates
    • Authors: Norman H. Sleep; Jessica M. Warren
      Pages: n/a - n/a
      Abstract: Lithospheric structure changes at low spreading rates (
      PubDate: 2014-07-16T04:17:54.238951-05:
      DOI: 10.1002/2014GC005423
       
  • Insights into magmatic processes and hydrothermal alteration of in situ
           superfast spreading ocean crust at ODP/IODP Site 1256 from a cluster
           analysis of rock magnetic properties
    • Authors: Mark J. Dekkers; David Heslop, Emilio Herrero‐Bervera, Gary Acton, David Krasa
      Pages: n/a - n/a
      Abstract: We analyze magnetic properties from Ocean Drilling Program (ODP)/Integrated ODP (IODP) Hole 1256D (6°44.1' N, 91°56.1' W) on the Cocos Plate in ~15.2 Ma oceanic crust generated by superfast seafloor spreading, the only drill hole that has sampled all three oceanic crust layers in a tectonically undisturbed setting. Fuzzy c‐means cluster analysis and non‐linear mapping are utilized to study down‐hole trends in the ratio of the saturation remanent magnetization and the saturation magnetization, the coercive force, the ratio of the remanent coercive force and coercive force, the low‐field magnetic susceptibility, and the Curie temperature, to evaluate the effects of magmatic and hydrothermal processes on magnetic properties. A statistically robust five‐cluster solution separates the data predominantly into three clusters that express increasing hydrothermal alteration of the lavas, which differ from two distinct clusters mainly representing the dikes and gabbros. Extensive alteration can obliterate magnetic property differences between lavas, dikes, and gabbros. The imprint of thermochemical alteration on the iron‐titanium oxides is only partially related to the porosity of the rocks. Thus, the analysis complements interpretation based on electrofacies analysis. All clusters display rock magnetic characteristics compatible with an ability to retain a stable natural remanent magnetization suggesting that the entire sampled sequence of ocean crust can contribute to marine magnetic anomalies. Paleointensity determination is difficult because of the propensity of oxy‐exsolution during laboratory heating and/or the presence of intergrowths. The upper part of the extrusive sequence, the granoblastic dikes, and moderately altered gabbros may contain a comparatively uncontaminated thermoremanent magnetization.
      PubDate: 2014-07-16T04:16:57.890206-05:
      DOI: 10.1002/2014GC005343
       
  • Dynamics of lithospheric thinning and mantle melting by edge‐driven
           convection: Application to Moroccan Atlas mountains
    • Authors: Lars Kaislaniemi; Jeroen van Hunen
      Pages: n/a - n/a
      Abstract: Edge‐driven convection (EDC) forms in the upper mantle at locations of lithosphere thickness gradients, e.g. craton edges. In this study we show how the traditional style of EDC, a convection cell governed by the cold downwelling below an edge alternates with another style of EDC, in which the convection cell forms as a secondary feature with a hot asthenospheric shear flow from underneath the thicker lithosphere. These alternating EDC styles produce episodic lithosphere erosion and decompression melting. Three‐dimensional models of EDC show that convection rolls form perpendicular to the thickness gradient at the lithosphere‐asthenosphere boundary. Stagnant‐lid convection scaling laws are used to gain further insight in the underlying physical processes. Application of our models to the Moroccan Atlas mountains region shows that the combination of these two styles of EDC can reproduce many of the observations from the Atlas mountains, including two distinct periods of Cenozoic volcanism, a semi‐continuous corridor of thinned lithosphere under the Atlas mountains, and piecewise delamination of the lithosphere. A very good match between observations and numerical models is found for the lithosphere thicknesses across the study area, amounts of melts produced, and the length of the quiet gap in between volcanic episodes show quantitative match to observations.
      PubDate: 2014-07-16T04:16:56.447272-05:
      DOI: 10.1002/2014GC005414
       
  • Tectonic structure, lithology, and hydrothermal signature of the Rainbow
           massif (Mid‐Atlantic Ridge 36°14’N)
    • Authors: Muriel Andreani; Javier Escartin, Adélie Delacour, Benoit Ildefonse, Marguerite Godard, Jérôme Dyment, Anthony E. Fallick, Yves Fouquet
      Pages: n/a - n/a
      Abstract: Rainbow is a dome‐shaped massif at the 36°14’N non‐transform offset along the Mid‐Atlantic Ridge. It hosts three ultramafic‐hosted hydrothermal sites: Rainbow is active and high‐temperature; Clamstone and Ghost City are fossil and low‐temperature. The MoMARDREAM cruises (2007, 2008) presented here provided extensive rock sampling throughout the massif that constrains the geological setting of hydrothermal activity. The lithology is heterogeneous with abundant serpentinites surrounding gabbros, troctolites, chromitites, plagiogranites, and basalts. We propose that a W‐dipping detachment fault, now inactive, uplifted the massif and exhumed these deep‐seated rocks. Present‐day deformation is accommodated by SSW‐NNE faults and fissures, consistent with oblique teleseismic focal mechanisms and stress rotation across the discontinuity. Faults localize fluid flow and control the location of fossil and active hydrothermal fields that appear to be ephemeral and lacking in spatio‐temporal progression. Markers of high‐temperature hydrothermal activity (˜350°C) are restricted to some samples from the active field while a more diffuse, lower‐temperature hydrothermal activity (
      PubDate: 2014-07-12T06:30:09.246195-05:
      DOI: 10.1002/2014GC005269
       
  • Approximately 1.78 Ga mafic dykes in the Lüliang Complex, North China
           Craton: Zircon ages and Lu‐Hf isotopes, geochemistry, and
           implications
    • Authors: Xi Wang; Wenbin Zhu, Meng Luo, Xingmin Ren, Xiang Cui
      Pages: n/a - n/a
      Abstract: Mafic dyke swarms are excellent time markers and paleo‐stress indicators. Numerous late Paleoproterozoic mafic dykes are exposed throughout the Trans‐North China Orogen (TNCO). Most of these dykes trend NW‐SE or NNW‐SSE, nearly parallel to the orogen, while a series of E‐W‐trending mafic dykes are restricted in the Lüliang and southern Taihang areas in the central segment of the TNCO. These dykes were mostly considered to be linked with break‐up of the supercontinent Columbia previously. In this study, sixteen mafic dykes were investigated in the Lüliang Complex. Zircon LA‐ICP‐MS dating of four samples yields magmatic crystallization ages of 1.78‐1.79 Ga. These dykes belong to the tholeiite series and consist of basalt, basaltic andesite and andesite. They are enriched in LREE and LILE and depleted in HFSE, and have negative zircon εHf(t) values of ‐1.7 to ‐12.2. The E‐W‐trending mafic dykes show similar geochemical and isotopic features compare to the NW‐SE‐trending dykes in other complexes. They were most likely originated from a lithospheric mantle metasomatised by subduction‐related fluids and later emplaced along extensional fractures in a post‐collisional setting. NW‐SE‐trending fractures were formed due to gravitational collapse and thinning of the lithosphere. E‐W‐trending fractures in the central segment of the orogen constitute a transverse accommodation belt to equilibrate the different amounts of extension between the northern and southern TNCO. The impact of the post‐orogenic extension might have continued to ca. 1680 Ma as evidenced by the presence of abundant ca. 1750‐1680 Ma anorthosite‐ gabbro‐ mangerite‐ rapakivi granite suites (AMCG‐like) occurring in the northern NCC.
      PubDate: 2014-07-12T06:29:57.781042-05:
      DOI: 10.1002/2014GC005378
       
  • The Cobb hotspot: HIMU‐DMM mixing and melting controlled by a
           progressively thinning lithospheric lid
    • Authors: John Chadwick; Randall Keller, George Kamenov, Gene Yogodzinski, John Lupton
      Pages: n/a - n/a
      Abstract: The Cobb Seamount Chain in the northeast Pacific basin records the composition of the Cobb hotspot for the past 33 Myr, as the migrating Juan de Fuca Ridge approached and ultimately overran it ca. 0.5 Myr ago. In this first comprehensive geochemical study of the Cobb chain, major and trace element compositions and Sr, Nd, Pb, and Hf isotopic ratios were measured for whole‐rock samples from throughout the chain, and He isotopes were acquired for olivine phenocrysts from one seamount. Trace element modeling indicates increased melting along the chain over time, with progressively more depleted lavas as the ridge approached the hotspot. The isotopic data reveal the first evidence of the high µ (µ = 238U/204Pb) (HIMU) mantle component in the north Pacific basin, and are consistent with a progressively decreasing mixing proportion of HIMU melts relative to those from depleted mid‐ocean ridge basalt mantle (DMM) in the chain over time. Decreasing lithospheric thickness over the Cobb hotspot due to the approach of the migrating Juan de Fuca ridge allowed adiabatic melting to continue to shallower depths, leading to increased melt fractions of the refractory DMM component in the hotspot and more depleted and MORB‐like lavas in the younger Cobb seamounts.
      PubDate: 2014-06-26T12:02:57.987204-05:
      DOI: 10.1002/2014GC005334
       
 
 
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