for Journals by Title or ISSN
for Articles by Keywords
help

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

Geochemistry, Geophysics, Geosystems     Full-text available via subscription   (Followers: 24, SJR: 2.56, h-index: 69)
Geophysical Research Letters     Full-text available via subscription   (Followers: 52, SJR: 3.493, h-index: 157)
Global Biogeochemical Cycles     Full-text available via subscription   (Followers: 5, SJR: 3.239, h-index: 119)
J. of Advances in Modeling Earth Systems     Open Access   (Followers: 2, SJR: 1.944, h-index: 7)
J. of Geophysical Research : Atmospheres     Partially Free   (Followers: 22)
J. of Geophysical Research : Biogeosciences     Full-text available via subscription   (Followers: 6)
J. of Geophysical Research : Earth Surface     Partially Free   (Followers: 24)
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: 26)
J. of Geophysical Research : Space Physics     Full-text available via subscription   (Followers: 15)
Paleoceanography     Full-text available via subscription   (Followers: 4, SJR: 3.22, h-index: 88)
Radio Science     Full-text available via subscription   (Followers: 3, SJR: 0.959, h-index: 51)
Reviews of Geophysics     Full-text available via subscription   (Followers: 20, SJR: 9.68, h-index: 94)
Space Weather     Full-text available via subscription   (Followers: 3, SJR: 1.319, h-index: 19)
Tectonics     Full-text available via subscription   (Followers: 9, SJR: 2.748, h-index: 85)
Water Resources Research     Full-text available via subscription   (Followers: 201, SJR: 2.189, h-index: 121)
Journal Cover   Geochemistry, Geophysics, Geosystems
  [SJR: 2.56]   [H-I: 69]   [26 followers]  Follow
    
   Full-text available via subscription Subscription journal
   ISSN (Online) 1525-2027
   Published by American Geophysical Union (AGU) Homepage  [17 journals]
  • Episodic Holocene eruption of the Salton Buttes rhyolites, California,
           from paleomagnetic, U‐Th, and Ar/Ar dating
    • Authors: Heather M. Wright; Jorge A. Vazquez, Duane E. Champion, Andrew T. Calvert, Margaret T. Mangan, Mark Stelten, Kari M. Cooper, Charles Herzig, Alexander Schriener
      Pages: n/a - n/a
      Abstract: In the Salton Trough, CA, five rhyolite domes form the Salton Buttes: Mullet Island, Obsidian Butte, Rock Hill, and North and South Red Hills, from oldest to youngest. Results presented here include 40Ar/39Ar anorthoclase ages, 238U‐230Th zircon crystallization ages, and comparison of remanent paleomagnetic directions with the secular variation curve, which indicate that all domes are Holocene. 238U‐230Th zircon crystallization ages are more precise but within uncertainty of 40Ar/39Ar anorthoclase ages, suggesting that zircon crystallization proceeded until shortly before eruption in all cases except one. Remanent paleomagnetic directions require three eruption periods: (1) Mullet Island, (2) Obsidian Butte, (3) Rock Hill, North Red Hill, and South Red Hill. Borehole cuttings logs document up to two shallow tephra layers. North and South Red Hill likely erupted within 100 years of each other, with a combined 238U‐230Th zircon isochron age of: 2.83 ± 0.60 ka (2 sigma); paleomagnetic evidence suggests this age predates eruption by hundreds of years (1800 cal BP). Rock Hill erupted closely in time to these eruptions. The Obsidian Butte 238U‐230Th isochron age (2.86 ± 0.96 ka) is nearly identical to the combined Red Hill age, but its Virtual Geomagnetic Pole position suggests a slightly older age. The age of aphyric Mullet Island dome is the least well constrained: zircon crystals are resorbed and the paleomagnetic direction is most distinct; possible Mullet Island ages include ca. 2300, 5900, 6900, and 7700 cal BP. Our results constrain the duration of Salton Buttes volcanism to between ca. 5900 and 500 years. This article is protected by copyright. All rights reserved.
      PubDate: 2015-03-20T06:13:13.923188-05:
      DOI: 10.1002/2015GC005714
       
  • The impact of splay faults on fluid flow, solute transport, and pore
           pressure distribution in subduction zones: A case study offshore the
           Nicoya Peninsula, Costa Rica
    • Authors: Rachel M. Lauer; Demian M. Saffer
      Pages: n/a - n/a
      Abstract: Observations of seafloor seeps on the continental slope of many subduction zones illustrate that splay faults represent a primary hydraulic connection to the plate boundary at depth, carry deeply sourced fluids to the seafloor, and are in some cases associated with mud volcanoes. However, the role of these structures in forearc hydrogeology remains poorly quantified. We use a 2‐D numerical model that simulates coupled fluid flow and solute transport driven by fluid sources from tectonically driven compaction and smectite transformation to investigate the effects of permeable splay faults on solute transport and pore pressure distribution. We focus on the Nicoya margin of Costa Rica as a case study, where previous modeling and field studies constrain flow rates, thermal structure, and margin geology. In our simulations, splay faults accommodate up to 33% of the total dewatering flux, primarily along faults that outcrop within 25‐km of the trench. The distribution and fate of dehydration‐derived fluids is strongly dependent on thermal structure, which determines the locus of smectite transformation. In simulations of a cold end‐member margin, smectite transformation initiates 30‐km from the trench, and 64% of the dehydration‐derived fluids are intercepted by splay faults and carried to the middle and upper slope, rather than exiting at the trench. For a warm end‐member, smectite transformation initiates 7‐km from the trench, and the associated fluids are primarily transmitted to the trench via the décollement (50%), and faults intercept only 21% of these fluids. For a wide range of splay fault permeabilities, simulated fluid pressures are near lithostatic where the faults intersect overlying slope sediments, providing a viable mechanism for the formation of mud volcanoes. This article is protected by copyright. All rights reserved.
      PubDate: 2015-03-20T05:21:37.912904-05:
      DOI: 10.1002/2014GC005638
       
  • Issue Information
    • Pages: i - iv
      PubDate: 2015-03-18T16:23:23.226291-05:
      DOI: 10.1002/ggge.20558
       
  • A global model for cave ventilation and seasonal bias in speleothem
           paleoclimate records
    • Authors: Eric W. James; Jay L. Banner, Benjamin Hardt
      Pages: n/a - n/a
      Abstract: Cave calcite deposits (speleothems) provide long and continuous records of paleoenvironmental conditions in terrestrial settings. Typical environmental proxy measurements include speleothem growth rate and variations in elemental and isotope geochemistry. Commonly the assumption is made that speleothems grow continuously and at a constant rate throughout the year. However, seasonal variation of growth rate may be the rule in many caves. Here we apply observations of modern calcite growth and cave‐air CO2 concentrations and a model of factors controlling cave ventilation to construct a global model predicting where cave calcite growth may be seasonal. Previous models and measurements of calcite precipitation in caves demonstrate the retardation of speleothem growth by high cave‐air CO2. Elevated CO2 is commonly dissipated by ventilation driven by density differences between cave and surface air. Seasonal cycles in atmospheric temperature, pressure and humidity commonly drive these density contrasts. Modeling these changes latitudinally and globally indicates a geographic control on seasonal cave ventilation and thus on a principal controlling factor of speleothem growth. The model predicts that given constant water, calcium and CO2 inputs, speleothems from temperate to boreal continental regions commonly accumulate more calcite in the cool season and less or none in the warm season. These models predict that proxies from temperate to boreal speleothems may be seasonally biased due to seasonal ventilation, whereas tropical and maritime records should not. This article is protected by copyright. All rights reserved.
      PubDate: 2015-03-16T21:41:20.318093-05:
      DOI: 10.1002/2014GC005658
       
  • Natural remanent magnetization acquisition in bioturbated sediment:
           General theory and implications for relative paleointensity
           reconstructions
    • Authors: R. Egli; X. Zhao
      Pages: n/a - n/a
      Abstract: We present a general theory for the acquisition of natural remanent magnetizations (NRM) in sediment under the influence of (a) magnetic torques, (b) randomizing torques, and (c) torques resulting from interaction forces. Dynamic equilibrium between (a) and (b) in the water column and at the sediment‐water interface generates a detrital remanent magnetization (DRM), while much stronger randomizing torques may be provided by bioturbation inside the mixed layer. These generate a so‐called mixed remanent magnetization (MRM), which is stabilized by mechanical interaction forces. During the time required to cross the surface mixed layer, DRM is lost and MRM is acquired at a rate that depends on bioturbation intensity. Both processes are governed by a MRM lock‐in function. The final NRM intensity is controlled mainly by a single parameter γ that is defined as the product of rotational diffusion and mixed layer thickness, divided by sedimentation rate. This parameter defines three regimes: (1) slow mixing (γ  10) with MRM acquisition and full DRM randomization, and (3) intermediate mixing. Because the acquisition efficiency of DRM is larger than that of MRM, NRM intensity is particularly sensitive to γ in case of mixed regimes, generating variable NRM acquisition efficiencies. This model explains (1) lock‐in delays that can be matched with empirical reconstructions from paleomagnetic records, (2) the existence of small lock‐in depths that lead to DRM preservation, (3) specific NRM acquisition efficiencies of magnetofossil‐rich sediments, and (4) some relative paleointensity artifacts. This article is protected by copyright. All rights reserved.
      PubDate: 2015-03-16T21:40:50.193541-05:
      DOI: 10.1002/2014GC005672
       
  • Gravity anomalies, crustal structure, and seismicity at subduction zones:
           1. Seafloor roughness and subducting relief
    • Authors: Dan Bassett; Anthony B. Watts
      Pages: n/a - n/a
      Abstract: An ensemble averaging technique is used to remove the long‐wavelength topography and gravity field from subduction zones. >200 residual bathymetric and gravimetric anomalies are interpreted within forearcs, many of which are attributed to the tectonic structure of the subducting plate. The residual‐gravimetric expression of subducting fracture zones extends >200 km landward of the trench‐axis. The bathymetric expression of subducting seamounts with height ≥1 km and area ≥500 km2 (N=36), and aseismic ridges (N>10), is largest near the trench (within 70 km) and above shallow subducting slab depths (SLAB1.0
      PubDate: 2015-03-14T11:10:03.004997-05:
      DOI: 10.1002/2014GC005684
       
  • Evaluating the utility of B/Ca ratios in planktic foraminifera as a proxy
           for the carbonate system: A case study of Globigerinoides ruber
    • Authors: Michael J. Henehan; Gavin L. Foster, James W. B. Rae, Katherine C. Prentice, Jonathan Erez, Helen C. Bostock, Brittney J. Marshall, Paul A. Wilson
      Pages: n/a - n/a
      Abstract: B/Ca ratios in foraminifera have attracted considerable scientific attention as a proxy for past ocean carbonate system. However, the carbonate system controls on B/Ca ratios are not straightforward, with Δ[CO32−] ([CO32−]in situ – [CO32−]at saturation) correlating best with B/Ca ratios in benthic foraminifera, rather than pH,B(OH)4−HCO3−, or B(OH)4−DIC (as a simple model of boron speciation in seawater and incorporation into CaCO3 would predict). Furthermore, culture experiments have shown that in planktic foraminifera properties such as salinity and [B]sw can have profound effects on B/Ca ratios beyond those predicted by simple partition coefficients. Here, we investigate the controls on B/Ca ratios in G. ruber via a combination of culture experiments and core‐top measurements, and add to a growing body of evidence that suggests B/Ca ratios in symbiont‐bearing foraminiferal carbonate are not a straightforward proxy for past seawater carbonate system conditions. We find that, while B/Ca ratios in culture experiments co‐vary with pH, in open ocean sediments this relationship is not seen. In fact, our B/Ca data correlate best with [PO43‐] (a previously undocumented association) and in most regions, salinity. These findings might suggest a precipitation rate or crystallographic control on boron incorporation into foraminiferal calcite. Regardless, our results underscore the need for caution when attempting to interpret B/Ca records in terms of the ocean carbonate system, at the very least in the case of mixed‐layer planktic foraminifera. This article is protected by copyright. All rights reserved.
      PubDate: 2015-03-13T21:01:24.640984-05:
      DOI: 10.1002/2014GC005514
       
  • Tracking the Late Jurassic apparent (or true) polar shift in U‐Pb
           dated kimberlites from Cratonic North America (superior province of
           Canada)
    • Authors: Dennis V. Kent; Bruce A. Kjarsgaard, Jeffrey S. Gee, Giovanni Muttoni, Larry M. Heaman
      Pages: n/a - n/a
      Abstract: Different versions of a composite apparent polar wander (APW) path of variably selected global poles assembled and averaged in North American coordinates using plate reconstructions show either a smooth progression or a large (∼30°) gap in mean paleopoles in the Late Jurassic, between about 160 Ma and 145 Ma. In an effort to further examine this issue, we sampled accessible outcrops/subcrops of kimberlites associated with high precision U‐Pb perovskite ages in the Timiskaming area of Ontario, Canada. The 154.9±2.4 Ma Peddie kimberlite yields a stable normal polarity magnetization that is coaxial within less than 5° of the reverse polarity magnetization of the 157.5±1.2 Ma Triple B kimberlite. The combined ∼156 Ma Triple B and Peddie pole (75.5°N, 189.5°E, A95=2.8°) lies about midway between igneous poles from North America nearest in age (169 Ma Moat volcanics and the 146 Ma Ithaca kimberlites) showing that the polar motion was at a relatively steady yet rapid (∼1.5°/Myr) pace. A similar large rapid polar swing has been recognized in the Middle to Late Jurassic APW path for Adria‐Africa and Iran‐Eurasia, suggesting a major mass redistribution. One possibility is that slab breakoff and subduction reversal along the western margin of the Americas triggered an episode of true polar wander. This article is protected by copyright. All rights reserved.
      PubDate: 2015-03-13T21:01:10.899923-05:
      DOI: 10.1002/2015GC005734
       
  • Reconciling subduction dynamics during Tethys closure with
           large‐scale Asian tectonics: Insights from numerical modeling
    • Authors: F.A. Capitanio; A. Replumaz, N. Riel
      Pages: n/a - n/a
      Abstract: We use three‐dimensional numerical models to investigate the relation between subduction dynamics and large‐scale tectonics of continent interiors. The models show how the balance between forces at the plate margins such as subduction, ridge push and far‐field forces, controls the coupled plate margins and interiors evolution. Removal of part of the slab by lithospheric break‐off during subduction destabilizes the convergent margin, forcing migration of the subduction zone, whereas in the upper plate large‐scale lateral extrusion, rotations and back‐arc stretching ensue. When external forces are modeled, such as ridge push and far‐field forces, indentation increases, with large collisional margin advance and thickening in the upper plate. The balance between margin and external forces leads to similar convergent margin evolutions, whereas major differences occur in the upper plate interiors. Here, three strain regimes are found: large‐scale extrusion, extrusion and thickening along the collisional margin, and thickening only, when negligible far‐field forces, ridge push, and larger far‐field forces, respectively, add to the subduction dynamics. The extrusion tectonics develop a strong asymmetry towards the oceanic margin driven by large‐scale subduction, with no need of pre‐existing heterogeneities in the upper plate. Because the slab break‐off perturbation is transient, the ensuing plate tectonics is time‐dependent. The modeled deformation and its evolution are remarkably similar to the Cenozoic Asian tectonics, explaining large‐scale lithospheric faulting and thickening, and coupling of indentation, extrusion and extension along the Asian convergent margin as a result of large‐scale subduction process. This article is protected by copyright. All rights reserved.
      PubDate: 2015-03-12T07:03:19.001821-05:
      DOI: 10.1002/2014GC005660
       
  • Grain‐size dynamics beneath mid‐ocean ridges: Implications for
           permeability and melt extraction
    • Authors: Andrew J. Turner; Richard F. Katz, Mark D. Behn
      Pages: n/a - n/a
      Abstract: Grain size is an important control on mantle viscosity and permeability, but is difficult or impossible to measure in situ. We construct a two‐dimensional, single phase model for the steady‐state mean grain size beneath a mid‐ocean ridge. The mantle rheology is modelled as a composite of diffusion creep, dislocation creep, dislocation accommodated grain boundary sliding, and a plastic stress limiter. The mean grain size is calculated by the paleowattmeter relationship of Austin and Evans [2007]. We investigate the sensitivity of our model to global variations in grain growth exponent, potential temperature, spreading‐rate, and mantle hydration. We interpret the mean grain‐size field in terms of its permeability to melt transport. The permeability structure due to mean grain size may be approximated as a high permeability region beneath a low permeability region. The transition between high and low permeability regions occurs across a boundary that is steeply inclined toward the ridge axis. We hypothesise that such a permeability structure generated from the variability of the mean grain size may focus melt towards the ridge axis, analogous to Sparks and Parmentier [1991]‐type focusing. This focusing may, in turn, constrain the region where significant melt fractions are observed by seismic or magnetotelluric surveys. This interpretation of melt focusing via the grain‐size permeability structure is consistent with MT observation of the asthenosphere beneath the East Pacific Rise [Baba et al., 2006; Key et al., 2013]. This article is protected by copyright. All rights reserved.
      PubDate: 2015-03-11T01:40:46.036718-05:
      DOI: 10.1002/2014GC005692
       
  • An upper‐mantle seismic discontinuity beneath the Galápagos
           Archipelago and its implications for studies of the
           lithosphere‐asthenosphere boundary
    • Authors: Joseph S. Byrnes; Emilie E. E. Hooft, Douglas R. Toomey, Darwin R. Villagómez, Dennis J. Geist, Sean C. Solomon
      Pages: n/a - n/a
      Abstract: An upper‐mantle seismic discontinuity (the Gutenberg or G discontinuity), at which shear wave velocity decreases with depth, has been mapped from S‐to‐p conversions in radial receiver functions recorded across the Galápagos Archipelago. The mean depth of the discontinuity is 91 ± 8km beneath the southeastern archipelago and 72 ± 5km beneath surrounding regions. The discontinuity appears deeper beneath the portion of the Nazca plate that we infer passed over the Galápagos mantle plume than elsewhere in the region. We equate the depth of the G discontinuity to the maximum depth extent of anhydrous melting, which forms an overlying layer of dehydrated and depleted mantle. We attribute areas of shallow discontinuity depth to the formation of the dehydrated layer near the Galápagos Spreading Center and areas of greater discontinuity depth to its modification over a mantle plume with an excess temperature of 115 ± 30 °C. The G discontinuity lies within a high‐seismic‐velocity anomaly that we conclude forms by partial dehydration and a gradual but steady increase in seismic velocity with decreasing depth after upwelling mantle first encounters the solidus for volatile‐bearing mantle material. At the depth of the solidus for anhydrous mantle material, removal of remaining water creates a sharp increase in velocity with decreasing depth; this discontinuity may also mark a site of melt accumulation. Results from seismic imaging, the compositions of Galápagos lavas, and rare‐earth‐element concentrations across the archipelago require that mantle upwelling and partial melting occur over a broad region within the dehydrated and depleted layer. We conclude that the G discontinuity beneath the archipelago does not mark the boundary between rigid lithosphere and convecting asthenosphere. This article is protected by copyright. All rights reserved.
      PubDate: 2015-03-02T15:17:43.168769-05:
      DOI: 10.1002/2014GC005694
       
  • Structural heterogeneity of the midcrust adjacent to the central Alpine
           Fault, New Zealand: Inferences from seismic tomography and seismicity
           between Harihari and Ross
    • Authors: Sandra Bourguignon; Stephen Bannister, C. Mark Henderson, John Townend, Haijiang Zhang
      Pages: n/a - n/a
      Abstract: Determining the rates and distributions of microseismicity near major faults at different points in the seismic cycle is a crucial step towards understanding plate boundary seismogenesis. We analyse data from temporary seismic arrays spanning the central section of the Alpine Fault, New Zealand, using double‐difference seismic tomography. This portion of the fault last ruptured in a large earthquake in 1717 AD and is now late in its typical 330‐year cycle of Mw∼8 earthquakes. Seismicity varies systematically with distance from the Alpine Fault: (1) directly beneath the fault trace, earthquakes are sparse and largely confined to the footwall at depths of 4–11km; (2) at distances of 0–9km southeast of the trace, seismicity is similarly sparse and shallower than 8km; (3) at distances of 9–20km southeast of the fault trace, earthquakes are much more prevalent and shallower than 7km. Hypocentre lineations here are subparallel to faults mapped near the Main Divide of the Southern Alps, confirming that those faults are active. The region of enhanced seismicity is associated with the highest topography and a high‐velocity tongue doming at 3–5km depth. The low‐seismicity zone adjacent to the Alpine Fault trace is associated with Vp and Vs values at mid‐crustal depths about 8% and 6% lower than further southeast. We interpret lateral variations in seismicity rate to reflect patterns of horizontal strain rate superimposed on heterogeneous crustal structure, and the variations in seismicity cutoff depth to be controlled by temperature and permeability structure variations. This article is protected by copyright. All rights reserved.
      PubDate: 2015-02-28T00:48:31.190045-05:
      DOI: 10.1002/2014GC005702
       
  • Authigenesis of magnetic minerals in gas hydrate‐bearing sediments
           in the Nankai Trough, offshore Japan
    • Authors: Myriam Kars; Kazuto Kodama
      Pages: n/a - n/a
      Abstract: Gas hydrate occurrence is one of the possible mechanisms invoked for iron sulfide formation. A high resolution rock magnetic study was conducted in IODP Expedition 316 Hole C0008C located in the Megasplay Fault Zone of the Nankai Trough, offshore Japan. In this particular zone, no bottom simulating reflectors (BSR), indicating the base of the gas hydrate stability field, have been identified. 218 Pleistocene samples were collected from 70 to 110 m CSF in order to document the changes in the concentration, grain size and rock magnetic parameters of magnetic minerals, through the gas hydrate‐bearing horizons. Two different populations of magnetic grains are recognized in the pseudo‐single domain range. Three types of magnetic mineral assemblages are identified: iron oxides (magnetite), ferrimagnetic iron sulfides (greigite and pyrrhotite) and their mixture. Greigite and pyrrhotite are authigenic and constitute six layers, called IS1 to IS6. IS1, IS3, IS4 and IS6 are associated with pore water anomalies, suggesting the occurrence of gas hydrates and anoxic conditions. IS2 and IS5 are probable gas hydrates horizons, although there is no independent data to confirm it. The remaining intervals are mainly composed of detrital iron oxides and paramagnetic iron sulfides. Two scenarios based on different diagenetic stages are proposed to explain the variations in the magnetic properties and mineralogy over the studied interval. The results suggest that rock magnetism appears useful to better constrain the gas hydrate distribution in Hole C0008C, and counterbalances the low resolution of pore water analyses and the absence of a BSR. This article is protected by copyright. All rights reserved.
      PubDate: 2015-02-26T09:17:32.219882-05:
      DOI: 10.1002/2014GC005614
       
  • Hydrous upwelling across the mantle transition zone beneath the Afar
           Triple Junction
    • Authors: D.A. Thompson; J.O.S. Hammond, J‐M. Kendall, G.W. Stuart, G.R. Helffrich, D. Keir, A. Ayele, B. Goitom
      Pages: n/a - n/a
      Abstract: The mechanisms that drive the upwelling of chemical heterogeneity from the lower to upper mantle (e.g. thermal vs. compositional buoyancy) are key to our understanding of whole mantle convective processes. We address these issues through a receiver function study on new seismic data from recent deployments located on the Afar Triple Junction, a location associated with deep mantle upwelling. The detailed images of upper mantle and mantle transition zone structure illuminate features that give insights into the nature of upwelling from the deep Earth. A seismic low velocity layer directly above the mantle transition zone, interpreted as a stable melt layer, along with a prominent 520 km discontinuity suggest the presence of a hydrous upwelling. A relatively uniform transition zone thickness across the region suggests a weak thermal anomaly (
      PubDate: 2015-02-26T08:44:17.569043-05:
      DOI: 10.1002/2014GC005648
       
  • New mafic magma refilling a quiescent volcano: Evidence from
           He‐Ne‐Ar isotopes during the 2011–2012 unrest at
           Santorini, Greece
    • Authors: A.L. Rizzo; F. Barberi, M.L. Carapezza, A. Di Piazza, L. Francalanci, F. Sortino, W. D'Alessandro
      Pages: n/a - n/a
      Abstract: In 2011‐2012 Santorini was characterized by seismic‐geodetic‐geochemical unrest, which was unprecedented since the most‐recent eruption occurred in 1950 and led to fear an eruption was imminent. This unrest offered a chance for investigating the processes leading to volcanic reactivation and the compositional characteristics of involved magma. We have thus analyzed the He‐Ne‐Ar‐isotope composition of fluid inclusions in olivines and clinopyroxenes from cumulate mafic enclaves hosted in cogenetic dacitic lavas of the 1570–1573 and 1925–1928 eruptions of Nea Kameni. These unique data on Aegean volcanism were compared with those of gases collected in quiescent periods and during the unrest. The 3He/4He‐ratios (3.1–4.0Ra) are significantly lower than the typical arc‐volcano values (R/Ra∼7–8), suggesting the occurrence of magma contamination in Santorini plumbing system, which would further modify the 3He/4He‐ratio of parental magmas generated in the local metasomatized mantle. The 3He/4He‐values of enclaves (3.1–3.6Ra) are comparable to those measured in gases during quiescent periods, confirming that enclaves reflect the He‐isotope signature of magma residing at shallow depths and feeding passive degassing. A significant increase in soil CO2 flux from Nea Kameni and anomalous compositional variations in the fumaroles were identified during the unrest, accordingly with previous studies. Simultaneously, 3He/4He‐ratios up to 4.0Ra were also measured, demonstrating that the unrest was due to the intrusion into the shallow plumbing system of a more‐primitive 3He‐rich magma, which is even volatile richer and less contaminated than mafic magma erupted as enclaves. This new intrusion did not however trigger an eruption. This article is protected by copyright. All rights reserved.
      PubDate: 2015-02-26T07:57:47.210549-05:
      DOI: 10.1002/2014GC005653
       
  • Emplacement of submarine lava flow fields: A geomorphological model from
           the Niños eruption at the Galápagos Spreading Center
    • Authors: J. Timothy McClinton; Scott M. White
      Pages: n/a - n/a
      Abstract: In the absence of any direct observations of an active submarine eruption at a mid‐ocean ridge (MOR), our understanding of volcanic processes there is based on the interpretation of eruptive products. Submarine lava flow morphology serves as a primary indicator of eruption and emplacement processes; however, there is typically a lack of visual observations and bathymetric data at a scale and extent relevant to submarine lava flows, which display meter‐ to sub‐meter‐scale morphological variability. In this paper, we merge submersible‐based visual observations with high‐resolution multibeam bathymetry collected by an autonomous underwater vehicle (AUV) and examine the fine‐scale geomorphology of Niños, a submarine lava flow field at the Galápagos Spreading Center (GSC).We identify separate morphological facies (i.e. morphofacies) within the lava flow field, each having distinct patterns of lava flow morphology and volcanic structures. The spatial and stratigraphic arrangement of morphofacies suggests that they were emplaced sequentially as the eruption progressed, implying that the Niños eruption consisted of at least three eruptive phases. We estimate eruption parameters and develop a chronological model that describes the construction of the Niños lava flow field. An initial phase with high effusion rates emplaced sheet flows, then an intermediate phase emplaced a platform of lobate lavas, and then an extended final phase with low effusion rates emplaced a discontinuous row of pillow lava domes. We then compare this model to mapped lava flow fields at other MORs. Despite disparities in scale, the morphological similarities of volcanic features at MORs with different spreading rates suggest common emplacement processes that are primarily controlled by local magma supply. This article is protected by copyright. All rights reserved.
      PubDate: 2015-02-25T05:01:44.587778-05:
      DOI: 10.1002/2014GC005632
       
  • Observations of pore‐scale growth patterns of carbon dioxide hydrate
           using X‐ray computed microtomography
    • Authors: Xuan Hien Ta; Tae Sup Yun, Balasingam Muhunthan, Tae‐Hyuk Kwon
      Pages: n/a - n/a
      Abstract: Natural and artificial gas hydrates with internal pores of nano‐ to centimeters and weak grain‐cementation have been widely reported, while the detailed formation process of grain‐cementing hydrates remains poorly identified. Pore‐scale morphology of carbon dioxide (CO2) hydrate formed in a partially brine‐saturated porous medium was investigated via X‐ray computed microtomography (X‐ray CMT). Emphasis is placed on the pore‐scale growth patterns of gas hydrate, including the growth of dendritic hydrate crystals on pre‐formed hydrate and water‐wetted grains, porous nature of the hydrate phase, volume expansion of more than 200% during the water‐to‐hydrate phase transformation, preference of unfrozen water wetting hydrophilic minerals, and the relevance to a weak cementation effect on macro‐scale physical properties. The presented pore‐scale morphology and growth patterns of gas hydrate are expected in natural sediment settings where free gas is available for hydrate formation, such as active gas vents, gas seeps, mud volcanoes, permafrost gas hydrate provinces, and CO2 injected formation for the sake of geologic carbon storage; and in laboratory hydrate samples synthesized from partially brine‐saturated sediments or formed from water‐gas interfaces. This article is protected by copyright. All rights reserved.
      PubDate: 2015-02-24T02:21:30.431266-05:
      DOI: 10.1002/2014GC005675
       
  • Mid‐mantle anisotropy in subduction zones and deep water transport
    • Authors: Andy Nowacki; J.‐Michael Kendall, James Wookey, Asher Pemberton
      Pages: n/a - n/a
      Abstract: The Earth's transition zone has until recently been assumed to be seismically isotropic. Increasingly, however, evidence suggests that ordering of material over seismic wavelengths occurs there, but it is unclear what causes this. We use the method of source‐side shear wave splitting to examine the anisotropy surrounding earthquakes deeper than 200km in slabs around the globe. We find significant amounts of splitting (≤ 2.4s), confirming that the transition zone is anisotropic here. However, there is no decrease in the amount of splitting with depth, as would be the case for a metastable tongue of olivine which thins with depth, suggesting this is not the cause. The amount of splitting does not appear to be consistent with processes in the ambient mantle, such as lattice preferred orientation development in wadsleyite, ringwoodite or MgSiO3‐perovskite. We invert for the orientation of several mechanisms‐subject to uncertainties in mineralogy and deformation‐and the best fit is given by up‐dip flattening in a style of anisotropy common to hydrous phases and layered inclusions. We suggest that highly anisotropic hydrous phases or hydrated layering is a likely cause of anisotropy within the slab, implying significant water transport from the surface down to at least 660 km depth. This article is protected by copyright. All rights reserved.
      PubDate: 2015-02-24T01:49:32.368-05:00
      DOI: 10.1002/2014GC005667
       
  • Untangling boulder dislodgement in storms and tsunamis: Is it possible
           with simple theories?
    • Authors: R. Weiss; P. Diplas
      Pages: n/a - n/a
      Abstract: Boulders can move during storms and tsunamis. It is difficult to find a simple method to distinguish boulders moved by tsunami waves from those moved during storms in the field. In this contribution, we explore boulder dislodgement by storm and tsunami waves by solving an adapted version of Newton's Second Law of Motion in polar coordinates and defining a critical position for boulder dislodgement. We find that the boulder dislodgement is not only a function of the causative wave, but also of the roughness in the vicinity of the boulder and the slope angle. We employ the amplitude of storm and tsunami waves to dislodge boulders of given masses to evaluate if boulder dislodgement in storms can be untangled from boulder transport in tsunamis. As the main result of our numerical experiments, we find a significant difference between storm and tsunami waves to dislodge the same boulder for large masses and large roughness values. This allows us to conclude that simple theories are applicable to answer the questions asked in the title, but we argue only if they contain a critical dislodgement condition like the one presented here. This article is protected by copyright. All rights reserved.
      PubDate: 2015-02-24T00:43:12.953584-05:
      DOI: 10.1002/2014GC005682
       
  • Paleointensity determinations from the Etendeka province, Namibia, support
           a low magnetic field strength leading up to the Cretaceous normal
           superchron
    • Authors: Sarah C. Dodd; Adrian R. Muxworthy, Conall Mac Niocaill
      Pages: n/a - n/a
      Abstract: Paleointensity estimates provide much needed information on field generation within Earth's core and upon the convective processes at work within the mantle. We present new paleointensity estimates from the early Cretaceous Etendeka large igneous province in Namibia (~135Ma) which add to the sparse southern hemisphere dataset. The Early Cretaceous marks an important change in the Earth's magnetic field from a state of rapid polarity reversals, to one of long‐term stability associated with the onset of the Cretaceous Normal Superchron at c. 121Ma. Paleointensity determinations, using the IZZI protocol, were carried out on a total of 172 specimens from 14 sites encompassing the exposed stratigraphy of the Etendeka province. Numerous checks of data reliability were considered before results were accepted, including partial thermoremanent magnetisation (pTRM) checks and pTRM tail checks, hysteresis properties, thermomagnetic analyses, observations under reflected light, and changes to room‐temperature susceptibility during the experiments. Following these checks a total of 64 individual samples from 5 sites were considered to provide reliable paleointensity determinations. These results were combined to provide site mean data with an overall average virtual dipole moment (VDM) for the study of 2.5 ± 1.0 x 1022 Am2. This value equates to approximately 30% of present Earth's field and, when considered alongside existing studies, suggests that Earth's field strength was low in the time leading up to the Cretaceous normal superchron. This article is protected by copyright. All rights reserved.
      PubDate: 2015-02-19T08:21:18.653493-05:
      DOI: 10.1002/2014GC005707
       
  • Viscosity measurements of crystallizing andesite from Tungurahua volcano
           (Ecuador)
    • Authors: Magdalena Oryaëlle Chevrel; Corrado Cimarelli, Lea deBiasi, Jonathan Hanson, Yan Lavallée, Fabio Arzilli, Donald B. Dingwell
      Pages: n/a - n/a
      Abstract: Viscosity has been determined during isothermal crystallization of an andesite from Tungurahua volcano (Ecuador). Viscosity was continuously recorded using the concentric cylinder method and employing a Pt‐sheathed alumina spindle at 1 bar and from 1400 ºC to sub‐liquidus temperatures to track rheological changes during crystallization. The disposable spindle was not extracted from the sample but rather left in the sample during quenching thus preserving an undisturbed textural configuration of the crystals. The inspection of products quenched during the crystallization process reveals evidence for heterogeneous crystal nucleation at the spindle and near the crucible wall, as well as crystal alignment in the flow field. At the end of the crystallization, defined when viscosity is constant, plagioclase is homogeneously distributed throughout the crucible (with the single exception of experiment performed at the lowest temperature). In this experiments, the crystallization kinetics appear to be strongly affected by the stirring conditions of the viscosity determinations. A TTT (Time‐Temperature‐Transformation) diagram illustrating the crystallization “nose” for this andesite under stirring conditions and at ambient pressure has been constructed. We further note that at a given crystal content and distribution, the high aspect ratio of the acicular plagioclase yields a shear‐thinning rheology at crystal contents as low as 13 vol.%, and that the relative viscosity is higher than predicted from existing viscosity models. These viscosity experiments hold the potential for delivering insights into the relative influences of the cooling path, undercooling and deformation on crystallization kinetics and resultant crystal morphologies, as well as their impact on magmatic viscosity. This article is protected by copyright. All rights reserved.
      PubDate: 2015-02-19T08:13:41.83061-05:0
      DOI: 10.1002/2014GC005661
       
  • Historical bathymetric charts and the evolution of Santorini submarine
           volcano, Greece
    • Authors: A. B. Watts; P. Nomikou, J. D. P. Moore, M. M. Parks, M. Alexandri
      Pages: n/a - n/a
      Abstract: Historical bathymetric charts are a potential resource for better understanding the dynamics of the seafloor and the role of active processes, such as submarine volcanism. The British Admiralty, for example, have been involved in lead line measurements of seafloor depth since the early 1790s. Here, we report on an analysis of historical charts in the region of Santorini volcano, Greece. Repeat lead line surveys in 1848, late 1866 and 1925‐1928 as well as multibeam swath bathymetry surveys in 2001 and 2006 have been used to document changes in seafloor depth. These data reveal that the flanks of the Kameni Islands, a dacitic dome complex in the caldera center, have shallowed by up to ~175 m and deepened by up to ~80 m since 1848. The largest shallowing occurred between the late 1866 and 1925‐1928 surveys and the largest deepening occurred during the 1925‐1928 and 2001 and 2006 surveys. The shallowing is attributed to the emplacement of lavas during effusive eruptions in both 1866‐1870 and 1925‐1928 at rates of up to 0.18 and 0.05 km3 a‐1 respectively. The deepening is attributed to a load‐induced viscoelastic stress relaxation following the 1866‐1870 and 1925‐1928 lava eruptions. The elastic thickness and viscosity that best fits the observed deepening are 1.0 km and ~1016 Pa s respectively. This parameter pair, which is consistent with the predictions of a shallow magma chamber thermal model, explains both the amplitude and wavelength of the historical bathymetric data and the present day rate of subsidence inferred from InSAR analysis. This article is protected by copyright. All rights reserved.
      PubDate: 2015-02-19T08:13:29.44729-05:0
      DOI: 10.1002/2014GC005679
       
  • Episodic and multistaged gravitational instability of cratonic lithosphere
           and its implications for reactivation of the North China Craton
    • Authors: Yongming Wang; Jinshui Huang, Shijie Zhong
      Pages: n/a - n/a
      Abstract: Archean cratons are the most stable tectonic units and their lithospheric mantle is chemically depleted and buoyant relative to the underlying mantle. The chemical depletion leads to high viscosity that maintains the long‐term stability of cratons. However, the eastern part of the North China Craton (~1200 km in horizontal length scale) had been extensively reactivated and modified over a time scale of ~100 Myrs in the Mesozoic and Cenozoic. While the causes for the weakening of the North China Craton, a necessary condition for its reactivation, are still in debate, we investigate gravitational instability of compositionally buoyant lithosphere, by computing 2‐D thermochemical convection models with different buoyancy number, lithospheric viscosity and rheology. We find that the gravitational instability of cratonic lithosphere can happen over a larger range of buoyancy numbers with non‐Newtonian rheology, but lithospheric instability with Newtonian rheology only happens with relatively small buoyancy numbers. For cratonic lithosphere with non‐Newtonian rheology and relatively weak temperature‐dependent viscosity, the instability starts in the cold, shallow part of the lithosphere and has small horizontal length scale (
      PubDate: 2015-02-19T08:11:52.304994-05:
      DOI: 10.1002/2014GC005681
       
  • Application of the cBΩ model to the calculation of diffusion
           parameters of Si in silicates
    • Authors: Baohua Zhang; Shuangming Shan
      Pages: n/a - n/a
      Abstract: Silicon diffusion in major mantle minerals plays an important role in understanding a number of physical and chemical processes in the Earth's interior. Inspection of existing experimental data reveals linear compensation law between the pre‐exponential factors and the activation energies for Si diffusion in various minerals by focusing on those of geophysical interest. On the basis of the observed compensation relationship, here, we propose a thermodynamic model, the so‐called cBΩ model that interconnects point defect parameters with the bulk properties to reproduce the Si self‐diffusion coefficients in different rock‐forming minerals. When the uncertainties are considered, the predicted results show that the temperature and pressure dependences of self‐diffusion coefficients concur with existing experimental data and theoretical calculations in most cases. This article is protected by copyright. All rights reserved.
      PubDate: 2015-02-12T19:27:21.282047-05:
      DOI: 10.1002/2014GC005551
       
  • Source and magma mixing processes in continental subduction factory:
           Geochemical evidence from postcollisional mafic igneous rocks in the Dabie
           orogen
    • Authors: Li‐Qun Dai; Zi‐Fu Zhao, Yong‐Fei Zheng, Juan Zhang
      Pages: n/a - n/a
      Abstract: Postcollisional mafic igneous rocks commonly exhibit petrological and geochemical heterogeneities, but their origin still remains enigmatic. While source mixing is substantial due to the crust‐mantle interaction during continental collision, magma mixing is also significant during postcollisional magmatism. The two processes are illustrated by Early Cretaceous mafic igneous rocks in the Dabie orogen. These mafic rocks show arc‐like trace element distribution patterns and enriched Sr‐Nd‐Pb isotope compositions, indicating their origination from enriched mantle sources. They have variable whole‐rock εNd(t) values of ‐17.6 to ‐5.2 and zircon εHf(t) values of ‐29.0 to ‐7.7, pointing to source heterogeneities. Such whole‐rock geochemical features are interpreted by the source mixing through melt‐peridotite reaction in the continental subduction channel. Clinopyroxene and plagioclase megacrystals show complex textural and compositional variations, recording three stages of mineral crystallization during magma evolution. Cpx‐1 core has low Cr and Ni but high Ba, Rb and K, indicating its crystallization from a mafic melt (Melt 1) derived from partial melting of hydrous peridotite rich in phlogopite. Cpx‐1 mantle and Cpx‐2 exhibit significantly high Cr, Ni and Al2O3 but low Rb and Ba, suggesting their crystallization from pyroxenite‐derived mafic melt (Melt 2). Whole‐rock initial 87Sr/86Sr ratios of gabbro lies between those of Pl‐1core (crystallized from Melt 1) and Pl‐1 mantle and Pl‐2 core (crystallized from Melt 2), providing isotopic evidence for magma mixing between Melt 1 and Melt 2. Taken together, a heterogeneously enriched mantle source would be generated by the source mixing due to reaction of the overlying subcontinental lithospheric mantle wedge peridotite with felsic melts derived from partial melting of different rocks of the deeply subducted continental crust during the continental collision. The magma mixing would occur between mafic melts that were derived from partial melting of the heterogeneously metasomatic mantle domains in the postcollisional stage. As a consequence, the source and magma mixing processes in the continental subduction factory are responsible for the significant variations in the whole‐rock and mineral geochemistries of postcollisional mafic igneous rocks. This article is protected by copyright. All rights reserved.
      PubDate: 2015-02-12T19:14:01.071431-05:
      DOI: 10.1002/2014GC005620
       
  • Germanium/silicon of the Ediacaran‐Cambrian Laobao cherts:
           Implications for the bedded chert formation and paleoenvironment
           interpretations
    • Authors: Lin Dong; Bing Shen, Cin‐Ty A. Lee, Xu‐jie Shu, Yang Peng, Yuanlin Sun, Zhuanhong Tang, Hong Rong, Xianguo Lang, Haoran Ma, Fan Yang, Wen Guo
      Pages: n/a - n/a
      Abstract: Sedimentary strata of the terminal Ediacaran (635–542 Ma) to early Cambrian (542–488 Ma) Laobao‐Liuchapo bedded cherts in the South China Block include the Ediacaran Oxidation Event and the Cambrian explosion. Understanding the origin and depositional environment of the bedded cherts may provide insight into how the Earth's surface environment changed between the Proterozoic and Phanerozoic. We measured major and trace element compositions and Ge/Si ratios of the Laobao cherts from northern Guangxi Province. The Laobao cherts were deposited in the deep basinal environment of the South China Block. We show that the composition of the Laobao cherts is determined by a mixture of four components: quartz, clay, carbonate, and pyrite/iron‐oxide. The quartz component is the dominant component of the Laobao cherts. The maximum estimated Ge/Si of the quartz component is between 0.4 and 0.5 μmol/mol, which is close to the Ge/Si of modern seawater and biogenic silica but one order of magnitude lower than that of hydrothermal fluids. These Ge/Si systematics suggest that normal seawater rather than mid‐ocean ridge hydrothermal fluids is the primary Si source for the Laobao cherts. The Ge/Si of the clay component varies between 1 and 10 μmol/mol, which is comparable to the Ge/Si of typical marine clays, but 10 to 100 times lower than that of chert nodules from early Ediacaran beds (the Doushantuo Formation) predating the terminal Ediacaran Labao cherts studied here. Our observations indicate that the clay component Ge/Si ratio decreased from the early Ediacaran to the late Ediacaran. We speculate that high Ge/Si ratios in clays reflect the preferential chelation of Ge by dissolved organic compounds adsorbed onto clays. If so, this suggests that the decrease in Ge/Si ratio of the clay component in the Ediacaran signifies a decrease in the total dissolved organic carbon content of seawater towards the Ediacaran‐Cambrian transition, consistent with oxidation of the oceans during the late Ediacaran. Finally, the seawater origin of the Laobao cherts also suggests that replacement of carbonate may not be the primary cause for bedded chert formation. Instead, direct precipitation from seawater or early diagenetic silicification of calcareous sediments, perhaps due to the emergence of Si‐accumulation bacteria, may have been responsible for the bedded Laobao‐Liuchapo chert formation in South China Block. This article is protected by copyright. All rights reserved.
      PubDate: 2015-02-12T07:01:06.744745-05:
      DOI: 10.1002/2014GC005595
       
  • Origin and evolution of the Kolbeinsey Ridge and Iceland Plateau,
           N‐Atlantic
    • Authors: Bryndís Brandsdóttir; Emilie E. E. Hooft, Rolf Mjelde, Yoshio Murai
      Pages: n/a - n/a
      Abstract: Variations in crustal structure along the 700 km long KRISE7 refraction/reflection and gravity profile, straddling 66.5°N across the Iceland Shelf, Iceland Plateau and western Norway Basin confirm that extinct spreading centers coexisted with the now extinct Ægir Ridge prior to the initiation of the Kolbeinsey Ridge at 26 Ma. The western 300 km of the profile, across the Iceland shelf, formed by rifting at the Kolbeinsey Ridge, whereas the eastern 400 km, across the Iceland Plateau and the western Norway Basin, formed by earlier rifting, possibly containing slivers of older oceanic or continental crust rifted off the central E‐Greenland margin along with the Jan Mayen Ridge. Crustal thickness increases gradually across the Iceland shelf, from 12–13 km near the Kolbeinsey Ridge to 24–28 km near the eastern shelf edge, decreasing abruptly across the shelf edge, to 12–13 km. The Iceland Plateau has crustal thickness ranging from 12–15 km decreasing to 5–8 km across the western Norway Basin and 4–5 km at the Ægir Ridge. We suggest that high‐velocity lower crustal domes and corresponding gravity highs across the Iceland plateau mark the location of extinct rift axes, that coexisted with the Ægir Ridge. Similar lower crustal domes are associated with the currently active rift segments within Iceland and the Kolbeinsey Ridge. This article is protected by copyright. All rights reserved.
      PubDate: 2015-02-10T08:41:44.373421-05:
      DOI: 10.1002/2014GC005540
       
  • The carbon‐isotope signature of ultramafic xenoliths from the
           Hyblean Plateau (southeast Sicily, Italy): Evidence of mantle
           heterogeneity
    • Authors: Alessandra Correale; Antonio Paonita, Andrea Rizzo, Fausto Grassa, Mauro Martelli
      Pages: n/a - n/a
      Abstract: We investigated the carbon isotope composition of mantle source beneath the Hyblean Plateau (southeast Sicily, Italy) by studying CO2 in fluid inclusions from ultramafic xenoliths recovered in some Miocene diatremes. In order to constrain the processes influencing the isotopic marker of carbon we combined δ13CCO2 results with information about noble gases (He and Ar) obtained in a previous investigation of the same products. Although Ar/CO2 and He/Ar ratios provide evidence of Rayleigh‐type fractional degassing, the isotopic geochemistry of carbon is poorly influenced by this process. Mixing related to metasomatic processes where MORB‐type pyroxenitic melts permeate a peridotite mantle probably contaminated by crustal fluids inherited from a fossil subduction can explain the measured δ13C and CO2/3He variations, ranging from −4‰ to −2‰ and from 109 to 1010, respectively. Simple mass‐balance calculations highlighted that the Hyblean peridotite source was mainly contaminated by the carbonate source, being carbonate and organic matter present at a ratio that varied within the range from 7:1 to 4:1. This article is protected by copyright. All rights reserved.
      PubDate: 2015-02-10T03:19:05.035556-05:
      DOI: 10.1002/2014GC005656
       
  • Stress drops for intermediate‐depth intraslab earthquakes beneath
           Hokkaido, northern Japan: Differences between the subducting oceanic crust
           and mantle events
    • Authors: Saeko Kita; Kei Katsumata
      Pages: n/a - n/a
      Abstract: Spatial variations in the stress drop for 1726 intermediate‐depth intraslab earthquakes were examined in the subducting Pacific plate beneath Hokkaido, using precisely relocated hypocenters, the corner frequencies of events, and detailed determined geometry of the upper interface of the Pacific plate. The results show that median stress drop for intraslab earthquakes generally increases with an increase in depth from ∼10 to 157 Mpa at depths of 70 to 300 km. More specifically, median stress drops for events in the oceanic crust decrease (9.9 to 6.8 MPa) at depths of 70 to 120km and increase (6.8 to 17 MPa) at depths of 120 to 170 km, whereas median stress drop for events in the oceanic mantle decrease (21.6 to 14.0 MPa) at depths of 70 to 170km, where the geometry of the Pacific plate is well determined. The increase in stress drop with depth in the oceanic crust at depths of 120–170 km, for which several studies have shown an increase in velocity, can be explained by an increase in the velocity and a decrease in the water content due to the phase boundary with dehydration in the oceanic crust. Stress drops for events in the oceanic mantle were larger than those for events in the oceanic crust at depths of 70–120 km. Differences in both the rigidity of the rock types and in the rupture mechanisms for events between the oceanic crust and mantle could be causes for the stress drop differences within a slab. This article is protected by copyright. All rights reserved.
      PubDate: 2015-02-05T02:02:07.678454-05:
      DOI: 10.1002/2014GC005603
       
  • PRIMELT3 MEGA.xlsm software for primary magma calculation: Peridotite
           primary magma MgO contents from the liquidus to the solidus
    • Authors: C. Herzberg; P.D. Asimow
      Pages: n/a - n/a
      Abstract: An upgrade of the PRIMELT algorithm [Herzberg and O'Hara, 2002] for calculating primary magma composition is given together with its implementation in PRIMELT3 MEGA.xlsm software. It supersedes PRIMELT2.xls [Herzberg and Asimow, 2008] in correcting minor mistakes in melt fraction and computed Ni content of olivine, it identifies residuum mineralogy, and it provides a thorough analysis of uncertainties in mantle potential temperature and olivine liquidus temperature. The uncertainty analysis was made tractable by the computation of olivine liquidus temperatures as functions of pressure and partial melt MgO content between the liquidus and solidus. We present a computed anhydrous peridotite solidus in T‐P space using relations amongst MgO, T and P along the solidus; it compares well with experiments on the solidus. Results of the application of PRIMELT3 to a wide range of basalts shows that the mantle sources of ocean islands and large igneous provinces were hotter than oceanic spreading centers, consistent with earlier studies and expectations of the mantle plume model. This article is protected by copyright. All rights reserved.
      PubDate: 2015-02-05T01:41:00.653967-05:
      DOI: 10.1002/2014GC005631
       
  • A comprehensive interpretative model of slow slip events on Mt. Etna's
           eastern flank
    • Authors: Mario Mattia; Valentina Bruno, Tommaso Caltabiano, Andrea Cannata, Flavio Cannavò, Walter D'Alessandro, Giuseppe Di Grazia, Cinzia Federico, Salvatore Giammanco, Alessandro La Spina, Marco Liuzzo, Manfredi Longo, Carmelo Monaco, Domenico Patanè, Giuseppe Salerno
      Pages: n/a - n/a
      Abstract: Starting off from a review of previous literature on kinematic models of the unstable eastern flank of Mt. Etna, we propose a new model. The model is based on our analysis of a large quantity of multidisciplinary data deriving from an extensive and diverse network of INGV monitoring devices deployed along the slopes of the volcano. Our analysis had a twofold objective: first, investigating the origin of the recently observed slow‐slip events on the eastern flank of Mt. Etna; and secondly, defining a general kinematic model for the instability of this area of the volcano. To this end, we investigated the 2008 to 2013 period using data collected from different geochemical, geodetic and seismic networks, integrated with the tectonic and geologic features of the volcano and including the volcanic activity during the observation period. The complex correlations between the large quantities of multidisciplinary data have given us the opportunity to infer, as outlined in this work, that the fluids of volcanic origin and their interrelationship with aquifers, tectonic and morphological features play a dominant role in the large scale instability of the eastern flank of Mt. Etna. Furthermore, we suggest that changes in the strain distribution due to volcanic inflation/deflation cycles are closely connected to changes in shallow depth fluid circulation. Finally, we propose a general framework for both the short and long term modelling of the large flank displacements observed. This article is protected by copyright. All rights reserved.
      PubDate: 2015-02-04T22:29:31.764933-05:
      DOI: 10.1002/2014GC005585
       
  • The emergence of volcanic oceanic islands on a slow‐moving plate:
           The example of Madeira Island, NE Atlantic
    • Authors: Ricardo S. Ramalho; António Brum da Silveira, Paulo E. Fonseca, José Madeira, Michael Cosca, Mário Cachão, Maria M. Fonseca, Susana N. Prada
      Pages: n/a - n/a
      Abstract: The transition from seamount to oceanic island typically involves surtseyan volcanism. However, the geological record at many islands in the NE Atlantic ‐ all located within the slow‐moving Nubian plate ‐ does not exhibit evidence for an emergent surtseyan phase but rather an erosive unconformity between the submarine basement and the overlying subaerial shield sequences. This suggests that the transition between seamount and island may frequently occur by a relative fall of sea level through uplift, eustatic changes, or a combination of both, and may not involve summit volcanism. In this study we explore the consequences for island evolutionary models using Madeira Island (Portugal) as a case‐study. We have examined the geologic record at Madeira using a combination of detailed fieldwork, biostratigraphy, and 40Ar/39Ar geochronology in order to document the mode, timing, and duration of edifice emergence above sea level. Our study confirms that Madeira's subaerial shield volcano was built upon the eroded remains of an uplifted seamount, with shallow marine sediments found between the two eruptive sequences and presently located at 320–430 m above sea level. This study reveals that Madeira emerged around 7.0–5.6 Ma essentially through an uplift process and before volcanic activity resumed to form the subaerial shield volcano. Basal intrusions are a likely uplift mechanism, and their emplacement is possibly enhanced by the slow‐motion of the Nubian plate relative to the source of partial melting. Alternating uplift and subsidence episodes suggest that island edifice growth may be governed by competing dominantly volcanic and dominantly intrusive processes. This article is protected by copyright. All rights reserved.
      PubDate: 2015-02-04T02:50:03.936193-05:
      DOI: 10.1002/2014GC005657
       
  • Generic along‐strike segmentation of Afar normal faults, East
           Africa: Implications on fault growth and stress heterogeneity on
           seismogenic fault planes
    • Authors: I. Manighetti; C. Caulet, L. De Barros, C. Perrin, F. Cappa, Y. Gaudemer
      Pages: n/a - n/a
      Abstract: Understanding how natural faults are segmented along their length can provide useful insights into fault growth processes, stress distribution on fault planes, and earthquake dynamics. We use cumulative displacement profiles to analyze the two largest‐scales of segmentation of ∼900 normal faults in Afar, East Africa. We build upon a prior study by Manighetti et al. (2009) and develop a new signal processing method aimed at recovering the number, position, displacement and length of both the major (i.e., longest) and the subordinate, secondary segments within the faults. Regardless of their length, age, geographic location, total displacement and slip rate, 90% of the faults contain two to five major segments, whereas more than 70% of these major segments are divided into two to four secondary segments. In each hierarchical rank of fault segmentation, most segments have a similar proportional length, whereas the number of segments slightly decreases with fault structural maturity. The along‐strike segmentation of the Afar faults is thus generic at its two largest scales. We summarize published fault segment data on 42 normal, reverse and strike‐slip faults worldwide, and find a similar number (two to five) of major and secondary segments across the population. We suggest a fault growth scenario that might account for the generic large‐scale segmentation of faults. The observation of a generic segmentation suggests that seismogenic fault planes are punctuated with a deterministic number of large stress concentrations, which are likely to control the initiation, arrest and hence extent and magnitude of earthquake ruptures. This article is protected by copyright. All rights reserved.
      PubDate: 2015-02-04T02:11:22.275014-05:
      DOI: 10.1002/2014GC005691
       
  • Re‐Os isotope and platinum group elements of a Focal Zone mantle
           source, Louisville Seamounts Chain, Pacific Ocean
    • Authors: Maria Luisa G. Tejada; Takeshi Hanyu, Akira Ishikawa, Ryoko Senda, Katsuhiko Suzuki, Godfrey Fitton, Rebecca Williams
      Pages: n/a - n/a
      Abstract: The Louisville Seamount Chain (LSC) is, besides the Hawaiian‐Emperor Chain, one of the longest‐lived hotspot traces. We report here the first Re‐Os isotope and platinum group element (PGE) data for Canopus, Rigil, and Burton Guyots along the chain, which were drilled during IODP Expedition 330. The LSC basalts possess (187Os/188Os)i = 0.1245‐0.1314 that are remarkably homogeneous and do not vary with age. A Re‐Os isochron age of 64.9 ± 3.2 Ma was obtained for Burton seamount (the youngest of the three seamounts drilled), consistent with 40Ar‐39Ar data. Isochron‐derived initial 187Os/188Os ratio of 0.1272 ± 0.0008, together with data for olivines (0.1271‐0.1275), are within the estimated primitive mantle values. This (187Os/188Os)i range is similar to those of Rarotonga (0.124‐0.139) and Samoan shield (0.1276‐0.1313) basalts and lower than those of Cook‐Austral (0.136‐0.155) and Hawaiian shield (0.1283‐0.1578) basalts, suggesting little or no recycled component in the LSC mantle source. The PGE data of LSC basalts are distinct from those of oceanic lower crust. Variation in PGE patterns can be largely explained by different low degrees of melting under sulfide‐saturated conditions of the same relatively fertile mantle source, consistent with their primitive mantle‐like Os and primordial Ne isotope signatures. The PGE patterns and the low 187Os/188Os composition of LSC basalts contrast with those of Ontong Java Plateau (OJP) tholeiites. We conclude that the Re‐Os isotope and PGE composition of LSC basalts reflect a relatively pure deep‐sourced common mantle sampled by some ocean island basalts but is not discernible in the composition of OJP tholeiites. This article is protected by copyright. All rights reserved.
      PubDate: 2015-02-03T02:05:27.300155-05:
      DOI: 10.1002/2014GC005629
       
  • Intraplate volcanism due to convective instability of stagnant slabs in
           the Mantle Transition Zone
    • Authors: Matthew H. Motoki; Maxim D. Ballmer
      Pages: n/a - n/a
      Abstract: The study of volcanism can further our understanding of Earth's mantle processes and composition. Continental intraplate volcanism commonly occurs above subducted slabs that stagnate in the Mantle Transition Zone (MTZ), such as in Europe, eastern China, and western North America. Here, we use two‐dimensional numerical models to explore the evolution of stagnant slabs in the MTZ and their potential to sustain mantle upwellings that can support volcanism. We find that weak slabs may go convectively unstable within tens of Myr. Upwellings rise out of the relatively warm underbelly of the slab, are entrained by ambient‐mantle flow and reach the base of the lithosphere. The first and most vigorous upwellings rise adjacent to lateral heterogeneity within the slab. Ultimately, convective instability also acts to separate the compositional components of the slab, harzburgite and eclogite, from each other with harzburgite rising into the upper mantle and eclogite sinking into the lower mantle. Such a physical filtering process may sustain a long‐term compositional gradient across the MTZ. This article is protected by copyright. All rights reserved.
      PubDate: 2015-01-30T00:12:54.355354-05:
      DOI: 10.1002/2014GC005608
       
  • Temperature and velocity measurements of a rising thermal plume
    • Authors: Neil Cagney; William H. Newsome, Carolina Lithgow‐Bertelloni, Aline Cotel, Stanley R. Hart, John A. Whitehead
      Pages: n/a - n/a
      Abstract: The three‐dimensional velocity and temperature fields surrounding an isolated thermal plume in a fluid with temperature‐dependent viscosity are measured using Particle‐Image Velocimetry and thermochromatic liquid‐crystals, respectively. The experimental conditions are relevant to a plume rising through the mantle. It is shown that while the velocity and the isotherm surrounding the plume can be used to visualise the plume, they do not reveal the finer details of its structure. However, by computing the Finite‐Time Lyapunov Exponent fields from the velocity measurements, the material lines of the flow can be found, which clearly identify the shape of the plume head and characterise the behaviour of the flow along the plume stem. It is shown that the vast majority of the material in the plume head has undergone significant stretching and originates from a wide region very low in the fluid domain, which is proposed as a contributing factor to the small‐scale isotopic variability observed in ocean‐island basalt regions. Finally, the Finite‐Time Lyapunov Exponent fields are used to calculate the steady‐state rise velocity of the thermal plume, which is found to scale linearly with the Rayleigh number, in contrast to some previous work. The possible cause and the significance of these conflicting results are discussed, and it is suggested that the scaling relationship may be affected by the temperature‐dependence of the fluid viscosity in the current work. This article is protected by copyright. All rights reserved.
      PubDate: 2015-01-30T00:11:47.106618-05:
      DOI: 10.1002/2014GC005576
       
  • Along‐axis variation in crustal thickness at the ultraslow spreading
           Southwest Indian Ridge (50°E) from a wide‐angle seismic
           experiment
    • Authors: Xiongwei Niu; Aiguo Ruan, Jiabiao Li, T. A. Minshull, Daniel Sauter, Zhenli Wu, Xuelin Qiu, Minghui Zhao, Y. John Chen, Satish Singh
      Pages: n/a - n/a
      Abstract: The Southwest Indian Ridge (SWIR) is characterized by an ultraslow spreading rate, thin crust and extensive outcrops of serpentinized peridotite. Previous studies have used geochemical and geophysical data to suggest the presence of a thicker crust at the central and shallowest portions of the SWIR, from the Prince Edward (35°30′E) to the Gallieni (52°20′E) fracture zones. Here we present a new analysis of wide‐angle seismic data along the ridge 49°17′E ‐ 50°49′E. Our main conclusions are as follows: 1) we find an oceanic layer 2 of roughly constant thickness and steep velocity gradient, underlain by a layer 3 with variable thickness and low velocity gradient; 2) the crustal thickness varies from c. 5 km beneath non‐transform discontinuities (NTDs) up to c. 10km beneath a segment center; 3) the melt supply is focused in segment centers despite a small NTD between adjacent segments; 4) the presence of a normal upper mantle velocity indicates that no serpentinization occurs beneath this thick crust. Our observation of thick crust at an ultra‐slow spreading ridge adds further complexity to relationships between crustal thickness and spreading rate, and supports previous suggestions that the extent of mantle melting is not a simple function of spreading rate, and that mantle temperature or chemistry (or both) must vary significantly along‐axis. This article is protected by copyright. All rights reserved.
      PubDate: 2015-01-28T10:18:35.457422-05:
      DOI: 10.1002/2014GC005645
       
  • South Asian monsoon history over the past 60 kyr recorded by radiogenic
           isotopes and clay mineral assemblages in the Andaman Sea
    • Authors: Sajid Ali; Ed C. Hathorne, Martin Frank, Daniel Gebregiorgis, Karl Stattegger, Roland Stumpf, Steffen Kutterolf, Joel E. Johnson, Liviu Giosan
      Pages: n/a - n/a
      Abstract: The Late Quaternary variability of the South Asian (or Indian) monsoon has been linked with glacial‐interglacial and millennial scale climatic changes but past rainfall intensity in the river catchments draining into the Andaman Sea remains poorly constrained. Here, we use radiogenic Sr, Nd, and Pb isotope compositions of the detrital clay‐size fraction and clay mineral assemblages obtained from sediment core NGHP Site 17 in the Andaman Sea to reconstruct the variability of the South Asian monsoon during the past 60 kyr. Over this time interval εNd values changed little, generally oscillating between −7.3 and −5.3 and the Pb isotope signatures are essentially invariable, which is in contrast to a record located further northeast in the Andaman Sea. This indicates that the source of the detrital clays did not change significantly during the last glacial and deglaciation suggesting the monsoon was spatially stable. The most likely source region is the Irrawaddy river catchment including the Indo‐Burman Ranges with a possible minor contribution from the Andaman Islands. High smectite/(illite+chlorite) ratios (up to 14), as well as low 87Sr/86Sr ratios (0.711) for the Holocene period indicate enhanced chemical weathering and a stronger South Asian monsoon compared to marine oxygen isotope stages 2 and 3. Short, smectite‐poor intervals exhibit markedly radiogenic Sr isotope compositions and document weakening of the South Asian monsoon, which may have been linked to short‐term northern Atlantic climate variability on millennial time scales. This article is protected by copyright. All rights reserved.
      PubDate: 2015-01-28T08:47:14.833205-05:
      DOI: 10.1002/2014GC005586
       
  • Split‐beam echo sounder observations of natural methane seep
           variability in the northern Gulf of Mexico
    • Authors: Kevin Jerram; Thomas C. Weber, Jonathan Beaudoin
      Pages: n/a - n/a
      Abstract: A method for positioning and characterizing plumes of bubbles from marine gas seeps using an 18‐kHz scientific split‐beam echosounder (SBES) was developed and applied to acoustic observations of plumes of presumed methane gas bubbles originating at approximately 1400m depth in the northern Gulf of Mexico. A total of 161 plume observations from 27 repeat surveys were grouped by proximity into 35 clusters of gas vent positions on the seafloor. Profiles of acoustic target strength per vertical meter of plume height were calculated with compensation for both the SBES beam pattern and the geometry of plume ensonification. These profiles were used as indicators of the relative fluxes and fates of gas bubbles acoustically observable at 18kHz and showed significant variability between repeat observations at time intervals of 1 hour to 7.5 months. Active gas venting was observed during approximately one third of the survey passes at each cluster. While gas flux is not estimated directly in this study owing to lack of bubble size distribution data, repeat surveys at active seep sites showed variations in acoustic response that suggest relative changes in gas flux of up to one order of magnitude over time scales of hours. The minimum depths of acoustic plume observations at 18kHz averaged 875m and frequently coincided with increased amplitudes of acoustic returns in layers of biological scatterers, suggesting acoustic masking of the gas bubble plumes in these layers. Minimum plume depth estimates were limited by the SBES field of view in only five instances. This article is protected by copyright. All rights reserved.
      PubDate: 2015-01-28T07:08:40.20173-05:0
      DOI: 10.1002/2014GC005429
       
  • P and S velocity tomography of the Mariana subduction system from a
           combined land‐sea seismic deployment
    • Authors: Mitchell Barklage; Douglas A. Wiens, James A. Conder, Sara Pozgay, Hajime Shiobara, Hiroko Sugioka
      Pages: n/a - n/a
      Abstract: Seismic imaging provides an opportunity to constrain mantle wedge processes associated with subduction, volatile transport, arc volcanism, and backarc spreading. We investigate the seismic velocity structure of the upper mantle across the Central Mariana subduction system using data from the 2003‐2004 Mariana Subduction Factory Imaging Experiment, an 11‐month deployment consisting of 20 broadband seismic stations installed on islands and 58 semi‐broadband ocean bottom seismographs. We determine the three dimensional VP and VP/VS structure using over 25,000 local and over 2000 teleseismic arrival times. The mantle wedge is characterized by slow velocity and high VP/VS beneath the forearc, an inclined zone of slow velocity underlying the volcanic front, and a strong region of slow velocity beneath the backarc spreading center. The slow velocities are strongest at depths of 20‐30km in the forearc, 60‐70km beneath the volcanic arc, and 20‐30km beneath the spreading center. The forearc slow velocity anomalies occur beneath Big Blue seamount and are interpreted as resulting from mantle serpentinization. The depths of the maximum velocity anomalies beneath the arc and backarc are nearly identical to previous estimates of the final equilibrium depths of mantle melts from thermobarometry, strongly indicating that the low velocity zones delineate regions of melt production in the mantle. The arc and backarc melt production regions are well separated at shallow depths, but may be connected at depths greater than 80km. This article is protected by copyright. All rights reserved.
      PubDate: 2015-01-28T07:02:20.25662-05:0
      DOI: 10.1002/2014GC005627
       
  • The evolution of MORB and plume mantle volatile budgets: Constraints from
           fission Xe isotopes in Southwest Indian Ridge basalts
    • Authors: Rita Parai; Sujoy Mukhopadhyay
      Pages: n/a - n/a
      Abstract: We present high‐precision measurements of the fission isotopes of xenon (Xe) in basalts from the Southwest Indian Ridge (SWIR) between 16 and 25°E. Corrections for syn‐ to post‐eruptive atmospheric contamination yield the Xe isotopic compositions of SWIR mantle sources. We solve for the proportions of mantle Xe derived from the primordial mantle Xe budget, recycling of atmospheric Xe, decay of short‐lived 129I, fission of extinct 244Pu and fission of extant 238U. Xe isotope systematics evident in SWIR basalts and other mantle‐derived samples provide new insights into the integrated history of mantle source degassing and regassing. We find that recycled atmospheric Xe dominates the Xe inventories of the SWIR Western and Eastern Orthogonal Supersegment mantle sources (∼80‐90% of 132Xe is recycled in origin), consistent with results from studies of plume‐influenced basalts from Iceland (Mukhopadhyay, 2012) and the Rochambeau Rift (Pető et al., 2013). While significant regassing of the mantle is evident, we also find differences in the extent of degassing of the MORB and plume sources. MORB sources are consistently characterized by a lower fraction of fission Xe derived from Pu‐fission, indicating a greater extent of degassing relative to the plume source. The prevalence of recycled atmospheric Xe in mantle sources indicates incorporation of depleted recycled material even into mantle sources with primitive He and Ne isotopic compositions. Consequently, depleted lithophile isotopic compositions in mantle sources with primitive He and Ne cannot be interpreted as evidence for a non‐chondritic bulk silicate Earth. This article is protected by copyright. All rights reserved.
      PubDate: 2015-01-28T05:54:42.092539-05:
      DOI: 10.1002/2014GC005566
       
  • New Geophysical Constraints on a Failed Subduction Initiation: The
           Structure and Potential Evolution of the Gagua Ridge and Huatung Basin
    • Authors: Daniel H. Eakin; Kirk D. McIntosh, H. J. A. Van Avendonk, Luc Lavier
      Pages: n/a - n/a
      Abstract: We modeled the velocity structure of the Huatung Basin and Gagua Ridge using offshore wide‐angle seismic data along four ∼E‐W transects. These transects are accompanied by several multichannel seismic reflection (MCS) profiles that highlight the shallow deformation in this area east of Taiwan. Although it is agreed that the Gagua Ridge was the product of a transient compressional episode in the past, relatively few data have been collected that reveal the deeper structure resulting from this enigmatic process. The velocity models show evidence for normal, to thin, oceanic crustal thicknesses in the Huatung Basin and West Philippine Basin. Moho reflections from the associated MCS profiles confirm the thickness observed in the velocity models. The velocity models indicate significant crustal thickening associated with the Gagua Ridge, to 12‐18km along its entire length. Most importantly, the two central velocity models also show a significant asymmetry in the crustal thickening suggesting a westward underthrusting of >20km of WPB oceanic crust beneath that of the Huatung Basin. This geometry is extremely unexpected given interpretations that indicate the Huatung Basin could be significantly older than the West Philippine Basin (Early Cretaceous vs. Eocene). Our observations, along with recent geophysical data concerning the age of the Huatung Basin, indicate that the Gagua Ridge was the result of a failed subduction event during the Miocene that may have existed simultaneously and for a short time, competed with the Manila subduction zone to the west in accommodating convergence between the Eurasia and Philippine Sea plates. In this scenario, the present day Gagua Ridge represents a snapshot of a failed subduction initiation preserved in the geologic record. This article is protected by copyright. All rights reserved.
      PubDate: 2015-01-22T16:07:13.152925-05:
      DOI: 10.1002/2014GC005548
       
  • Time scales of intraoceanic arc magmatism from combined U‐Th and
           (U‐Th)/He zircon geochronology of Dominica, Lesser Antilles
    • Authors: Howe T.M; Schmitt A.K, Lindsay J.M, Shane P, Stockli D.F.
      Pages: n/a - n/a
      Abstract: The island of Dominica, located in the intra‐oceanic Lesser Antilles arc, has produced a series of intermediate (mostly andesitic) lava domes and ignimbrites since the early Pleistocene. (U‐Th)/He eruption ages from centres across the island range from ca. 3 ka to ca. 770 ka, with at least 10 eruptions occurring in the last 80 ka. Three eruptions occurred near the southern tip of Dominica (Plat Pays Volcanic Complex) in the past 15 ka alone. Zircon U‐Th ages from individual centres range from near‐eruption to secular equilibrium implicating protracted storage and recycling of zircons within the crust. Overlapping zircon crystallization peaks within deposits from geographically separated vents (up to 40km apart) indicate that magma associated with separate volcanic edifices crystallized zircon contemporaneously. Two lava domes from the southern sector of the island display exclusively young zircon rim ages (
      PubDate: 2015-01-20T17:39:17.582388-05:
      DOI: 10.1002/2014GC005636
       
  • Intraplate volcanism at the edges of the Colorado Plateau sustained by a
           combination of triggered edge‐driven convection and
           shear‐driven upwelling
    • Authors: Maxim D. Ballmer; Clinton P. Conrad, Eugene I. Smith, Racheal Johnsen
      Pages: n/a - n/a
      Abstract: Although volcanism in the southwestern United States has been studied extensively, its origin remains controversial. Various mechanisms such as mantle plumes, upwelling in response to slab sinking, and small‐scale convective processes have been proposed, but have not been evaluated within the context of rapidly shearing asthenosphere that is thought to underlie this region. Using geodynamic models that include this shear, we here explore spatio‐temporal patterns of mantle melting and volcanism near the Colorado Plateau. We show that the presence of viscosity heterogeneity within an environment of asthenospheric shearing can give rise to decompression melting along the margins of the Colorado Plateau. Our models indicate that eastward shear flow can advect pockets of anomalously low viscosity toward the edges of thickened lithosphere beneath the plateau, where they can induce decompression melting in two ways. First, the arrival of the pockets critically changes the effective viscosity near the plateau to trigger small‐scale edge‐driven convection. Second, they can excite shear‐driven upwelling (SDU), in which horizontal shear flow becomes redirected upward as it is focused within the low‐viscosity pocket. We find that a combination of “triggered” edge‐driven convection and SDU can explain volcanism along the margins of the Colorado Plateau, its encroachment toward the plateau's southwestern edge, and the association of volcanism with slow seismic anomalies in the asthenosphere. Geographic patterns of intraplate volcanism in regions of vigorous asthenospheric shearing may thus directly mirror viscosity heterogeneity of the sublithospheric mantle. This article is protected by copyright. All rights reserved.
      PubDate: 2015-01-20T17:38:31.131061-05:
      DOI: 10.1002/2014GC005641
       
  • Slip‐rate‐dependent melt extraction at oceanic transform
           faults
    • Authors: Hailong Bai; Laurent G. J. Montési
      Pages: n/a - n/a
      Abstract: Crustal thickness differences between oceanic transform faults and associated mid‐ocean ridges may be explained by melt migration and extraction processes. Slow‐slipping transform faults exhibit more positive gravity anomalies than the adjacent spreading centers, indicating relative thin crust in the transform domain, whereas at intermediate‐ and fast‐spreading ridges, transform faults are characterized by more negative gravity anomalies than the adjacent spreading centers, indicating thick crust in the transform domain. We present numerical models reproducing these observations and infer that melt can be extracted at fast‐slipping transforms, but not at slow‐slipping ones. Melt extraction is modeled as a three‐step process [Montési et al., 2011]. 1) Melt moves vertically through buoyancy‐driven porous flow enhanced by sub‐vertical dissolution channels. 2) Melt accumulates in and travels along a decompaction channel lining a low‐permeability barrier at the base of the thermal boundary layer. 3) Melt is extracted to the surface when it enters a melt extraction zone. A melt extraction width of 2 – 4km and a melt extraction depth of 15 – 20km are needed to fit the tectonic damages associated with oceanic plate boundaries that reach into the upper mantle. Our conclusions are supported by the different degrees of magmatic activities exhibited at fast‐ and slow‐slipping transforms as reflected in geological features, geochemical signals and seismic behaviors. We also constrain that the maximum lateral distance of crust‐level dike propagation is about 50 to 70km. This article is protected by copyright. All rights reserved.
      PubDate: 2015-01-20T17:37:51.280279-05:
      DOI: 10.1002/2014GC005579
       
  • Submarine record of volcanic island construction and collapse in the
           Lesser Antilles arc: First scientific drilling of submarine volcanic
           island landslides by IODP Expedition 340
    • Authors: Le Friant A; Ishizuka O, Boudon G, Palmer M.R, Talling P.J, Villemant B, Adachi T, Aljahdali M, Breitkreuz C, Brunet M, Caron B, Coussens M, Deplus C, Endo D, Feuillet N, Fraas A.J, Fujinawa A, Hart M.B, Hatfield R.G, Hornbach M, Jutzeler M, Kataoka K. S, Komorowski J‐C, Lebas E, Lafuerza S, Maeno F, Manga M, Martínez‐Colón M, McCanta M, Morgan S, Saito T, Slagle A, Sparks S, Stinton A, Stroncik N, Subramanyam K. S.V, Tamura Y, Trofimovs J, Voight B, Wall‐Palmer D, Wang F, Watt S.F.L.
      Pages: n/a - n/a
      Abstract: IODP Expedition 340 successfully drilled a series of sites offshore Montserrat, Martinique and Dominica in the Lesser Antilles from March to April 2012. These are among the few drill sites gathered around volcanic islands, and the first scientific drilling of large and likely tsunamigenic volcanic island‐arc landslide deposits. These cores provide evidence and tests of previous hypotheses for the composition and origin of those deposits. Sites U1394, U1399, and U1400 that penetrated landslide deposits recovered exclusively seafloor‐sediment, comprising mainly turbidites and hemipelagic deposits, and lacked debris avalanche deposits. This supports the concepts that i/ volcanic debris avalanches tend to stop at the slope break, and ii/ widespread and voluminous failures of pre‐existing low‐gradient seafloor sediment can be triggered by initial emplacement of material from the volcano. Offshore Martinique (U1399 and 1400), the landslide deposits comprised blocks of parallel strata that were tilted or micro‐faulted, sometimes separated by intervals of homogenized sediment (intense shearing), while Site U1394 offshore Montserrat penetrated a flat‐lying block of intact strata. The most likely mechanism for generating these large‐scale seafloor‐sediment failures appears to be propagation of a decollement from proximal areas loaded and incised by a volcanic debris avalanche. These results have implications for the magnitude of tsunami generation. Under some conditions, volcanic island landslide deposits comprised of mainly seafloor sediment will tend to form smaller magnitude tsunamis than equivalent volumes of subaerial block‐rich mass flows rapidly entering water. Expedition 340 also successfully drilled sites to access the undisturbed record of eruption fallout layers intercalated with marine sediment which provide an outstanding high‐resolution dataset to analyze eruption and landslides cycles, improve understanding of magmatic evolution as well as offshore sedimentation processes. This article is protected by copyright. All rights reserved.
      PubDate: 2015-01-06T00:41:56.426968-05:
      DOI: 10.1002/2014GC005652
       
 
 
JournalTOCs
School of Mathematical and Computer Sciences
Heriot-Watt University
Edinburgh, EH14 4AS, UK
Email: journaltocs@hw.ac.uk
Tel: +00 44 (0)131 4513762
Fax: +00 44 (0)131 4513327
 
About JournalTOCs
API
Help
News (blog, publications)
JournalTOCs on Twitter   JournalTOCs on Facebook

JournalTOCs © 2009-2015