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Geophysical Research Letters     Full-text available via subscription   (Followers: 84, SJR: 3.323, h-index: 185)
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J. of Geophysical Research : Solid Earth     Full-text available via subscription   (Followers: 32)
J. of Geophysical Research : Space Physics     Full-text available via subscription   (Followers: 38)
Paleoceanography     Full-text available via subscription   (Followers: 4, SJR: 3.067, h-index: 100)
Radio Science     Full-text available via subscription   (Followers: 34, SJR: 1.072, h-index: 59)
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Tectonics     Full-text available via subscription   (Followers: 9, SJR: 2.628, h-index: 96)
Water Resources Research     Full-text available via subscription   (Followers: 68, SJR: 2.661, h-index: 144)
Journal Cover Geochemistry, Geophysics, Geosystems
  [SJR: 2.439]   [H-I: 91]   [25 followers]  Follow
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   ISSN (Online) 1525-2027
   Published by AGU Homepage  [17 journals]
  • Thermochronological evidence of a low‐angle, midcrustal detachment plane
           beneath the central South Island, New Zealand
    • Abstract: Oblique continental convergence and uplift in the Southern Alps, New Zealand is largely accommodated by dextral transpression on the Alpine Fault. However, towards the south of the orogen the Alpine Fault becomes increasingly strike‐slip, despite evidence for high exhumation rates in the Pacific plate. Here, we present 41 new apatite and zircon fission‐track ages to investigate the role of the southern Alpine Fault in Pacific plate exhumation since the Miocene. Through development of a new, maximum likelihood fission‐track age calculation method (to overcome extremely low (< 0.1 ppm) 238U concentrations in apatites) we estimate the width of the fully reset apatite zone (ages 
      PubDate: 2016-09-21T11:10:35.52932-05:0
      DOI: 10.1002/2016GC006402
  • Effect of melt composition on crustal carbonate assimilation: Implications
           for the transition from calcite consumption to skarnification and
           associated CO2 degassing
    • Authors: L. B. Carter; R. Dasgupta
      Abstract: Skarns are residue of relatively low‐temperature magma‐induced decarbonation in the crust largely associated with silicic plutons. Mafic magmatic intrusions are also capable of releasing excess CO2 due to carbonate assimilation. However, the effect of mafic to silicic melt evolution on the decarbonation processes, in addition to temperature controls on carbonate‐intrusive magmatic systems, particularly at continental arcs, remains unclear. In this study, experiments performed in a piston cylinder apparatus at mid‐crustal depth (0.5 GPa) at supersolidus temperatures (900 to 1200°C) document calcite interaction with andesite and dacite melts at equilibrium under closed‐system conditions at calcite saturation in a 1:1 melt‐calcite ratio by weight. With increasing silica content in the starting melt, at similar melt fractions and identical pressure, assimilation decreases drastically (≤65% andesite‐calcite to ≤18% dacite‐calcite). In conjunction, the CaO/SiO2 ratio in melts resulting from calcite assimilation in andesitic starting material is >1, but ≤0.3 in those formed from dacite‐calcite interaction. With increasing silica‐content in the starting melt skarn mineralogy, particularly wollastonite, increases in modal abundance while diopsidic clinopyroxene decreases slightly. More CO2 is released with andesite‐calcite reaction (≤2.9 × 1011 g/y) than with more skarn‐like dacite‐calcite interaction (≤8.1 × 1010 g/y, at one volcano assuming respective calcite‐free‐superliquidus conditions and a magma flux of 1012 g/y). Our experimental results thus suggest that calcite assimilation in more mafic magmas may have first degassed a significant amount of crustal carbon before the melt evolves to more silicic compositions, producing skarn. Crustal decarbonation in long‐lived magmatic systems may hence deliver significant albeit diminishing amounts of carbon to the atmosphere and contribute to long‐term climate change. This article is protected by copyright. All rights reserved.
      PubDate: 2016-09-20T18:50:21.881601-05:
      DOI: 10.1002/2016GC006444
  • Experimental constraints on the relationship between clay abundance, clay
           fabric, and frictional behavior for the Central Deforming Zone of the San
           Andreas Fault
    • Authors: Jasmaria Wojatschke; Marco M. Scuderi, Laurence N. Warr, Brett M. Carpenter, Demian Saffer, Chris Marone
      Abstract: The presence of smectite (saponite) in fault gouge from the Central Deforming Zone of the San Andreas Fault at Parkfield, CA has been linked to low mechanical strength and aseismic slip. However, the precise relationship between clay mineral structure, fabric development, fault strength, and the stability of frictional sliding is not well understood. We address these questions through the integration of laboratory friction tests and FIB‐SEM analysis of fault rock recovered from the San Andreas Fault Observatory at Depth (SAFOD) borehole. Intact fault rock was compared with experimentally sheared fault gouge and different proportions of either quartz clasts or SAFOD clasts extracted from the sample. Nano‐textural measurements show the development of localized clay particle alignment along shear folia developed within synthetic gouges; such slip planes have multiples of random distribution (MRD) values of 3.0‐4.9. The MRD values measured are higher than previous estimates (MRD 1.5) that show lower degrees of shear localization and clay alignment averaged over larger volumes. The intact fault rock exhibits less well‐developed nano‐clay fabrics than the experimentally sheared materials, and MRD values decrease with smectite content. We show that the abundance, strength, and shape of clasts all influence fabric evolution via strain localization: quartz clasts yield more strongly developed clay fabrics than serpentine‐dominated SAFOD clasts. Our results suggest that: 1) both clay abundance and the development of nano‐scale fabrics play a role in fault zone weakening and 2) aseismic creep is promoted by slip along clay shears with >20 wt% smectite content and MRD values ≥2.7. This article is protected by copyright. All rights reserved.
      PubDate: 2016-09-19T18:55:33.539017-05:
      DOI: 10.1002/2016GC006500
  • Tidally controlled gas bubble emissions: A comprehensive study using
           long‐term monitoring data from the NEPTUNE cabled observatory offshore
           Vancouver Island
    • Abstract: Long‐term monitoring over one year revealed high temporal variability of gas emissions at a cold seep in 1250 m water depth offshore Vancouver Island, British Columbia. Data from the North East Pacific Time series Underwater Networked Experiment observatory operated by Ocean Networks Canada were used. The site is equipped with a 260 kHz Imagenex sonar collecting hourly data, conductivity‐temperature‐depth sensors, bottom pressure recorders, current meter, and an ocean bottom seismograph. This enables correlation of the data and analyzing trigger mechanisms and regulating criteria of gas discharge activity. Three periods of gas emission activity were observed: (a) short activity phases of few hours lasting several months, (b) alternating activity and inactivity of up to several day‐long phases each, and (c) a period of several weeks of permanent activity. These periods can neither be explained by oceanographic conditions nor initiated by earthquakes. However, we found a clear correlation of gas emission with bottom pressure changes controlled by tides. Gas bubbles start emanating during decreasing tidal pressure. Tidally induced pressure changes also influence the subbottom fluid system by shifting the methane solubility resulting in exsolution of gas during falling tides. These pressure changes affect the equilibrium of forces allowing free gas in sediments to emanate into the water column at decreased hydrostatic load. We propose a model for the fluid system at the seep, fueled by a constant sub‐surface methane flux and a frequent tidally controlled discharge of gas bubbles into the ocean, transferable to other gas emission sites in the world's oceans. This article is protected by copyright. All rights reserved.
      PubDate: 2016-09-15T08:30:51.74655-05:0
      DOI: 10.1002/2016GC006528
  • Large‐scale tectonic cycles in Europe revealed by distinct Pb
           isotope provinces
    • Abstract: Lead isotopic systematics of U‐poor minerals, such as sulfides and feldspars, can provide unique insights into the origin and evolution of continents because these minerals ‘freeze in' the Pb isotopic composition of the crust during major tectonothermal events, allowing the history of a continent to be told through Pb isotopes. Lead model ages constrain the timing of crust formation while time‐integrated U/Pb, Th/Pb, and Th/U ratios shed light onto key geochemical processes associated with continent formation. Using ∼6800 Pb isotope measurements of primarily lead ores and minor K‐feldspar, we mapped out the Pb isotope systematics across Europe and the Mediterranean. Lead model ages define spatially distinct age provinces, consistent with major tectonic events ranging from the Paleozoic to the Proterozoic and latest Archean. However, the regions defined by time‐integrated U/Pb and Th/Pb ratios cut across the boundaries of age provinces, with high U/Pb systematics characterizing most of southern Europe. Magmatic influx, followed by segregation of dense sulfide‐rich mafic cumulates, resulted in foundering of U‐ and Th‐poor lower crust, thereby changing the bulk composition of the continental crust and leading to distinct time‐integrated U‐Th/Pb provinces. We show that the tectonic assembly of small crustal fragments leaves the crust largely undifferentiated, whereas the formation of supercontinents results in fundamental changes in the composition of the crust, identifiable in time and space by means of Pb isotope systematics. Observations based on Pb isotopes open up a new perspective on possible relationships between crustal thickness and geodynamic processes, in particular the role of crustal foundering into the mantle and the mechanisms responsible for the existence of cratons. This article is protected by copyright. All rights reserved.
      PubDate: 2016-09-15T08:25:33.51387-05:0
      DOI: 10.1002/2016GC006524
  • Unusually low 234Th in a hydrothermal effluent plume over the Southwest
           Indian Ridge
    • Authors: Weifeng Yang; Xinxing Zhang, Min Chen, Yusheng Qiu
      Abstract: Particle dynamics in hydrothermal plumes are crucial to understanding the cycling of carbon and trace elements in the global oceans, but this subject area has been poorly studied. We investigated radioactive 234Th in a hydrothermal plume of a recently discovered vent over the Southwest Indian Ridge (SWIR). Above the plume, total 234Th was in equilibrium with 238U, showing the typical characteristic of general deep water. However, there was a 234Th deficit within the plume, with 234Th/238U ratios in the 0.77‐0.91 range. Particulate 234Th accounted for ∼10% of the total 234Th, contrasting with 4% in the overlying water. On average, the scavenging and removal rates of 234Th were 17.5±2.5 dpm m−3 d−1 and 11.8±2.5 dpm m−3 d−1. The residence time of dissolved 234Th (avg. 108±8 d) was much higher than particulate 234Th (avg. 19±1 d), indicating that scavenging of 234Th from dissolved to particulate phase dominated its residence timescale. Particulate organic carbon (POC) increased 15% within the plume, compared with the overlying water. Combining the removal of 234Th and the ratio of POC to particulate 234Th, the POC removal flux was 9.3±0.6 mmol m−2 d−1. Similarly, 2.2±0.6 mmol m−2 of particulate nitrogen (PN) was removed per day from the plume. The magnitude of POC and PN removal implied an important role of the hydrothermal plume in delivering organic matter to the seafloor. This study thus reveals the different particle dynamic characteristics within the hydrothermal plume over the SWIR compared to the ordinary deep oceans. This article is protected by copyright. All rights reserved.
      PubDate: 2016-09-15T08:25:29.751443-05:
      DOI: 10.1002/2016GC006580
  • Brucite chimney formation and carbonate alteration at the Shinkai Seep
           Field, a serpentinite‐hosted vent system in the Southern Mariana Forearc
    • Authors: T. Okumura; Y. Ohara, R. J. Stern, T. Yamanaka, Y. Onishi, H. Watanabe, C. Chen, S. H. Bloomer, I. Pujana, S. Sakai, T. Ishii, K. Takai
      Abstract: Brucite‐carbonate chimneys were discovered from the deepest known (∼5700 m depth) serpentinite‐hosted ecosystem – the Shinkai Seep Field (SSF) in the southern Mariana forearc. Textural observations and geochemical analysis reveal three types (I‐III) of chimneys formed by the precipitation and dissolution of constitutive minerals. Type I chimneys are bright white to light yellow, have a spiky crystalline and wrinkled surface with microbial mat and contain more brucite; these formed as a result of rapid precipitation under high fluid discharge conditions. Type II chimneys exhibit white to dull brown coloration, tuberous textures like vascular bundles, and are covered with grayish microbial mats and dense colonies of Phyllochaetopterus. This type of chimney is characterized by inner brucite‐rich and outer carbonate rich zones and is thought to have precipitated from lower fluid discharge conditions than type I chimneys. Type III chimneys are ivory colored, have surface depressions and lack living microbial mats or animals. This type of chimney mainly consists of carbonate, and is in a dissolution stage. Stable carbon isotope compositions of carbonates in the two types (I and II) of active chimneys are extremely 13C‐enriched (up to +24.1‰), which may reflect biological 12C consumption under extremely low dissolved inorganic carbon concentration in alkaline fluid. Type III chimneys have 13C compositions indicating re‐equilibration with seawater. Our findings demonstrate for the first time that carbonate chimneys can be form below carbonate compensation depth and provide new insights about linked geologic, hydrologic, and biological processes of the global deep‐sea serpentinite‐hosted vent systems. This article is protected by copyright. All rights reserved.
      PubDate: 2016-09-10T10:21:17.262518-05:
      DOI: 10.1002/2016GC006449
  • Dependence of seismic coupling on normal fault style along the Northern
           Mid‐Atlantic Ridge
    • Abstract: While normal faults are essential in shaping the seafloor formed at slow‐spreading mid‐ocean ridges, information on their behavior on short (seismic cycle) time scales is limited. Here we combine catalogs of hydro‐acoustically and teleseismically recorded earthquakes to characterize the state of seismic coupling along the Northern Mid‐Atlantic Ridge (MAR) between 12 and 35ºN. Along this portion of the MAR axis, tectonic extension is either taken up by steep conjugate faults that outline well‐defined ridge‐parallel abyssal hills, or dominantly by a large‐offset detachment fault on one side of the axis.We investigate variations in seismicity and seismic moment release rates across thirty ridge sections that can be clearly characterized either as abyssal hill or detachment bearing. We find that detachment‐bearing sections are associated with significantly greater seismicity and moment release rates than abyssal hill bearing sections, but show variability that may reflect the along‐axis extent of individual detachment faults. Overall, the measured seismic moment release rates fail to account for the long‐term fault slip rates. This apparent seismic deficit could indicate a mixed‐mode of fault slip where earthquakes only account for ∼10–30% of offset build‐up at abyssal hill faults, while the rest is accommodated by some form of transient aseismic creep. We find this seismic coupling fraction to be significantly greater (∼40–60%) at individual detachment systems, which is somewhat at odds with the common inference that detachment faults can sustain long‐lived localized strain because they are weak. We therefore propose alternative interpretations for seismic coupling based on dynamic friction theory. This article is protected by copyright. All rights reserved.
      PubDate: 2016-09-10T10:05:32.020298-05:
      DOI: 10.1002/2016GC006460
  • Lithium isotopic systematics of submarine vent fluids from arc and
           back‐arc hydrothermal systems in the western Pacific
    • Authors: Daisuke Araoka; Yoshiro Nishio, Toshitaka Gamo, Kyoko Yamaoka, Hodaka Kawahata
      Abstract: The Li concentration and isotopic composition (δ7Li) in submarine vent fluids are important for oceanic Li budget and potentially useful for investigating hydrothermal systems deep under the seafloor because hydrothermal vent fluids are highly enriched in Li relative to seawater. Although Li isotopic geochemistry has been studied at mid‐ocean‐ridge (MOR) hydrothermal sites, in arc and back‐arc settings Li isotopic composition has not been systematically investigated. Here, we determined the δ7Li and 87Sr/86Sr values of 11 end‐member fluids from 5 arc and back‐arc hydrothermal systems in the western Pacific and examined Li behavior during high‐temperature water–rock interactions in different geological settings. In sediment‐starved hydrothermal systems (Manus Basin, Izu‐Bonin Arc, Mariana Trough, and North Fiji Basin), the Li concentrations (0.23–1.30 mmol/kg) and δ7Li values (+4.3‰ to +7.2‰) of the end‐member fluids are explained mainly by dissolution‐precipitation model during high‐temperature seawater–rock interactions at steady state. Low Li concentrations are attributable to temperature‐related apportioning of Li in rock into the fluid phase and phase separation process. Small variation in Li among MOR sites is probably caused by low‐temperature alteration process by diffusive hydrothermal fluids under the seafloor. In contrast, the highest Li concentrations (3.40 − 5.98 mmol/kg) and lowest δ7Li values (+1.6‰ to +2.4‰) of end‐member fluids from the Okinawa Trough demonstrate that the Li is predominantly derived from marine sediments. The variation of Li in sediment‐hosted sites can be explained by the differences in degree of hydrothermal fluid–sediment interactions associated with the thickness of the marine sediment overlying these hydrothermal sites. This article is protected by copyright. All rights reserved.
      PubDate: 2016-09-10T10:05:23.387838-05:
      DOI: 10.1002/2016GC006355
  • Ultra‐high Sensitivity Moment Magnetometry of Geological Samples
           Using Magnetic Microscopy
    • Authors: Eduardo A. Lima; Benjamin P. Weiss
      Abstract: Useful paleomagnetic information is expected to be recorded by samples with moments up to three orders of magnitude below the detection limit of standard superconducting rock magnetometers. Such samples are now detectable using recently developed magnetic microscopes, which map the magnetic fields above room‐temperature samples with unprecedented spatial resolutions and field sensitivities. However, realizing this potential requires the development of techniques for retrieving sample moments from magnetic microscopy data. With this goal, we developed a technique for uniquely obtaining the net magnetic moment of geological samples from magnetic microscopy maps of unresolved or nearly unresolved magnetization. This technique is particularly powerful for analyzing small, weakly magnetized samples such as meteoritic chondrules and terrestrial silicate crystals like zircons. We validated this technique by applying it to field maps generated from synthetic sources and also to field maps measured using a superconducting quantum interference device (SQUID) microscope above geological samples with moments down to 10−15 Am2. For the most magnetic rock samples, the net moments estimated from the SQUID microscope data are within error of independent moment measurements acquired using lower sensitivity standard rock magnetometers. In addition to its superior moment sensitivity, SQUID microscope net moment magnetometry also enables the identification and isolation of magnetic contamination and background sources, which is critical for improving accuracy in paleomagnetic studies of weakly magnetic samples. This article is protected by copyright. All rights reserved.
      PubDate: 2016-09-08T04:05:42.038885-05:
      DOI: 10.1002/2016GC006487
  • A comparison of Thellier‐type and multispecimen paleointensity
           determinations on Pleistocene and historical lava flows from Lanzarote
           (Canary Islands, Spain)
    • Abstract: Sixteen Miocene, Pleistocene and historic lava flows have been sampled in Lanzarote (Canary Islands) for paleointensity analysis with both the Coe and multispecimen methods. Besides obtaining new data, the main goal of the study was the comparison of paleointensity results determined with two different techniques. Characteristic Remanent Magnetization (ChRM) directions were obtained in 15 flows and 12 were chosen for paleointensity determination. In Thellier‐type experiments a selection of reliable paleointensity determinations (43 of 78 studied samples) was performed using sets of criteria of different stringency, trying to relate the quality of results to the strictness of the chosen criteria. Uncorrected and fraction and domain‐state corrected multispecimen paleointensity results were obtained in all flows. Results with the Coe method on historical flows either agree with the expected values or show moderately lower ones, but multispecimen determinations display a large deviation from the expected result in one case. No relation can be detected between correct or anomalous results and paleointensity determination quality or rock‐magnetic properties. However, results on historical flows suggest that agreement between both methods could be a good indicator of correct determinations. Comparison of results obtained with both methods on seven Pleistocene flows yields an excellent agreement in four and disagreements in three cases. Pleistocene determinations were only accepted if either results from both methods agreed or a result was based on a sufficiently large number (n>4) of individual Thellier type determinations. In most Pleistocene flows a VADM around 5·1022Am2 was observed, although two flows displayed higher values around 9·1022Am2. This article is protected by copyright. All rights reserved.
      PubDate: 2016-09-02T10:50:29.385384-05:
      DOI: 10.1002/2016GC006396
  • The role of symbiotic algae in the formation of the coral polyp skeleton:
           3‐D morphological study based on X‐ray micro‐computed tomography
    • Authors: Shinya Iwasaki; Mayuri Inoue, Atsushi Suzuki, Osamu Sasaki, Harumasa Kano, Akira Iguchi, Kazuhiko Sakai, Hodaka Kawahata
      Abstract: Symbiotic algae of primary polyps play an important role in calcification of coral skeletons. However, the function of the symbiotic algae, including the way they influence the physical features of their host skeleton under various conditions, is not well understood. We used X‐ray micro‐computed tomography to observe skeletal shape characteristics in symbiotic and aposymbiotic primary polyps of Acropora digitifera that were cultured at various temperature and pCO2 levels (temperature 27, 29, 33 °C; pCO2 400, 800, 1000 µatm). Symbiotic polyps had a basal plate with a well‐developed folding margin supporting the branched skeleton, whereas aposymbiotic ones did not. The features of the folding margin suggest that it might be the initial growth stage of the epitheca. In addition, three‐dimensional (3‐D) morphological measurements made by X‐ray micro‐computed tomography show that the branched skeletons of symbiotic primary polyps were taller than those of aposymbiotic ones, suggesting that zooxanthellae in coral primary polyps play a critical role in the height growth of skeletal branches. Furthermore, results of the temperature‐ and pCO2‐controlled experiments suggest that global warming might greatly affect the activity of zooxanthellae, whereas ocean acidification might reduce calcification by damaging the coral host itself. Our findings provide new knowledge about the role of zooxanthellae in coral calcification. This article is protected by copyright. All rights reserved.
      PubDate: 2016-08-31T20:55:22.97819-05:0
      DOI: 10.1002/2016GC006536
  • Synchrotron X‐ray computed microtomography study on gas hydrate
           decomposition in a sedimentary matrix
    • Abstract: In‐situ synchrotron X‐ray computed microtomography with sub‐micrometer voxel size was used to study the decomposition of gas hydrates in a sedimentary matrix. Xenon‐hydrate was used instead of methane hydrate to enhance the absorption contrast. The microstructural features of the decomposition process were elucidated indicating that the decomposition starts at the hydrate‐gas interface; it does not proceed at the contacts with quartz grains. Melt water accumulates at retreating hydrate surface. The decomposition is not homogeneous and the decomposition rates depend on the distance of the hydrate surface to the gas phase indicating a diffusion‐limitation of the gas transport through the water phase. Gas is found to be metastably enriched in the water phase with a concentration decreasing away from the hydrate‐water interface. The initial decomposition process facilitates redistribution of fluid phases in the pore space and local re‐formation of gas hydrates. The observations allow also rationalizing earlier conjectures from experiments with low spatial resolutions and suggest that the hydrate‐sediment assemblies remain intact until the hydrate spacers between sediment grains finally collapse; possible effects on mechanical stability and permeability are discussed. The resulting time resolved characteristics of gas hydrate decomposition and the influence of melt water on the reaction rate are of importance for a suggested gas recovery from marine sediments by depressurization. This article is protected by copyright. All rights reserved.
      PubDate: 2016-08-29T09:25:24.163631-05:
      DOI: 10.1002/2016GC006521
  • Determining the flux of methane into Hudson Canyon at the edge of methane
           clathrate hydrate stability
    • Authors: Alexander Weinstein; Luis Navarrete, Carolyn Ruppel, Thomas C. Weber, Mihai Leonte, Matthias Kellermann, Eleanor Arrington, David L. Valentine, Mary I. Scranton, John D. Kessler
      Abstract: Methane seeps were investigated in Hudson Canyon, the largest shelf‐break canyon on the northern US Atlantic Margin. The seeps investigated are located at or updip of the nominal limit of methane clathrate hydrate stability. The acoustic identification of bubble streams was used to guide water column sampling in a 32 km2 region within the canyon's thalweg. By incorporating measurements of dissolved methane concentration with methane oxidation rates and current velocity into a steady‐state box model, the total emission of methane to the water column in this region was estimated to be 12 kmol methane per day (range: 6 – 24 kmol methane per day). These analyses suggest this methane is largely retained inside the canyon walls below 300 m water depth, and that it is aerobically oxidized to near completion within the larger extent of Hudson Canyon. Based on estimated methane emissions and measured oxidation rates, the oxidation of this methane to dissolved CO2 is expected to have minimal influences on seawater pH. This article is protected by copyright. All rights reserved.
      PubDate: 2016-08-29T09:20:23.123763-05:
      DOI: 10.1002/2016GC006421
  • Mass transfer in the lower crust: Evidence for incipient melt assisted
           flow along grain boundaries in the deep arc granulites of Fiordland, New
    • Authors: Catherine A. Stuart; Sandra Piazolo, Nathan R. Daczko
      Abstract: Knowledge of mass transfer is critical in improving our understanding of crustal evolution, however mass transfer mechanisms are debated, especially in arc environments. The Pembroke Granulite is a gabbroic gneiss, passively exhumed from depths of > 45 km from the arc root of Fiordland, New Zealand. Here, enstatite and diopside grains are replaced by coronas of pargasite and quartz, which may be asymmetric, recording hydration of the gabbroic gneiss. The coronas contain microstructures indicative of the former presence of melt, supported by pseudosection modeling consistent with the reaction having occurred near the solidus of the rock (630–710°C, 8.8–12.4kbar). Homogeneous mineral chemistry in reaction products indicates an open system, despite limited metasomatism at the hand sample scale. We propose the partial replacement microstructures are a result of a reaction involving an externally‐derived hydrous, silicate melt and the relatively anhydrous, high‐grade assemblage. Trace element mapping reveals a correlation between reaction microstructure development and bands of high‐Sr plagioclase, recording pathways of the reactant melt along grain boundaries. Replacement microstructures record pathways of diffuse porous melt flow at a kilometer scale within the lower crust, which was assisted by small proportions of incipient melt providing a permeable network. This work recognizes melt flux through the lower crust in the absence of significant metasomatism, which may be more common than is currently recognized. As similar microstructures are found elsewhere within the exposed Fiordland lower crustal arc rocks, mass transfer of melt by diffuse porous flow may have fluxed an area > 10,000 km2. This article is protected by copyright. All rights reserved.
      PubDate: 2016-08-29T09:20:22.299032-05:
      DOI: 10.1002/2015GC006236
  • An experimental study of the role of subsurface plumbing on geothermal
    • Abstract: In order to better understand the diverse discharge styles and eruption intervals observed at geothermal features, we performed three series of laboratory experiments with differing plumbing geometries. A single, straight conduit that connects a hot water bath (flask) to a vent (funnel) can originate geyser‐like periodic eruptions, continuous discharge like a boiling spring, and fumarole‐like steam discharge, depending on the conduit length and radius. The balance between the heat loss from the conduit walls and the heat supplied from the bottom determines whether and where water can condense which in turn controls discharge style. Next, we connected the conduit to a cold water reservoir through a branch, simulating the inflow from an external water source. Colder water located at a higher place than a branching point can flow into the conduit to stop the boiling in the flask, controlling the periodicity of the eruption. When an additional branch is connected to a second cold water reservoir, the two cold reservoirs can interact. Our experiments show that branching allows new processes to occur, such as recharge of colder water and escape of steam from side channels, leading to greater variation in discharge styles and eruption intervals. This model is consistent with the fact that eruption duration is not controlled by emptying reservoirs. We show how differences in plumbing geometries can explain various discharge styles and eruption intervals observed in El Tatio, Chile and Yellowstone, USA. This article is protected by copyright. All rights reserved.
      PubDate: 2016-08-25T19:20:35.86327-05:0
      DOI: 10.1002/2016GC006472
  • Automated paleomagnetic and rock magnetic data acquisition with an
           in‐line horizontal ‘2G' system
    • Authors: Tom A.T. Mullender; Thomas Frederichs, Christian Hilgenfeldt, Lennart V. de Groot, Karl Fabian, Mark J. Dekkers
      Abstract: Today's paleomagnetic and magnetic proxy studies involve processing of large sample collections while simultaneously demanding high quality data and high reproducibility. Here we describe a fully automated interface based on a commercial horizontal pass‐through ‘2G' DC‐SQUID magnetometer. This system is operational at the universities of Bremen (Germany) and Utrecht (Netherlands) since 1998 and 2006, respectively, while a system is currently being built at NGU Trondheim (Norway). The magnetometers are equipped with ‘in‐line' alternating field (AF) demagnetization, a direct‐current bias field coil along the co‐axial AF demagnetization coil for the acquisition of anhysteretic remanent magnetization (ARM) and a long pulse‐field coil for the acquisition of isothermal remanent magnetization (IRM). Samples are contained in dedicated low magnetization perspex holders that are manipulated by a pneumatic pick‐and‐place‐unit. Upon desire samples can be measured in several positions considerably enhancing data quality in particular for magnetically weak samples. In the Bremen system, the peak of the IRM pulse fields is actively measured which reduces the discrepancy between the set field and the field that is actually applied. Techniques for quantifying and removing gyroremanent overprints and for measuring the viscosity of IRM further extend the range of applications of the system. Typically c. 300 paleomagnetic samples can be AF demagnetized per week (15 levels) in the three‐position protocol. The versatility of the system is illustrated by several examples of paleomagnetic and rock magnetic data processing. This article is protected by copyright. All rights reserved.
      PubDate: 2016-08-19T09:45:32.50452-05:0
      DOI: 10.1002/2016GC006436
  • Strain localization in polycrystalline material with second phase
           particles: Numerical modeling with application to ice mixtures
    • Authors: D. Cyprych; S. Brune, S. Piazolo, J. Quinteros
      Abstract: We use a centimeter‐scale 2D numerical model to investigate the effect of the presence of a second phase with various volume percent, shape and orientation on strain localization in a visco‐elastic matrix. In addition, the evolution of bulk rheological behavior of aggregates during uniaxial compression is analyzed. The rheological effect of dynamic recrystallization processes in the matrix is reproduced by viscous strain softening. We show that the presence of hard particles strengthens the aggregate, but also causes strain localization and the formation of ductile shear zones in the matrix. The presence of soft particles weakens the aggregate, while strain localizes within the particles and matrix between particles. The shape and the orientation of second phases controls the orientation, geometry and connectivity of ductile shear zones. We propose an analytical scaling method that translates the bulk stress measurements of our 2D simulations to 3D experiments. Comparing our model to the laboratory uniaxial compression experiments on ice cylinders with hard second phases allows the analysis of transient and steady‐state strain distribution in ice matrix, and strain partitioning between ice and second phases through empirical calibration of viscous softening parameters. We find that the ice matrix in two‐phase aggregates accommodates more strain than the applied bulk strain, while at faster strain rates some of the load is transferred into hard particles. Our study illustrates that dynamic recrystallization processes in the matrix are markedly influenced by the presence of a second phase. This article is protected by copyright. All rights reserved.
      PubDate: 2016-08-19T09:40:32.130934-05:
      DOI: 10.1002/2016GC006471
  • Radial anisotropy beneath northeast Tibet, implications for lithosphere
           deformation at a restraining bend in the Kunlun fault and its vicinity
    • Authors: Lun Li; Aibing Li, Michael A. Murphy, Yuanyuan V. Fu
      Abstract: Three‐dimensional shear wave velocity and radial anisotropy models of the crust and upper mantle beneath the NE Tibetan plateau are constructed from new measurements of Love wave dispersions (20‐77s) and previously obtained Rayleigh wave dispersions (20‐87s) using a two‐plane‐wave method. The mid‐lower crust is characterized with positive anisotropy (VSH>VSV) with large strength beneath the Qinling and Qilian Mountains and relative weak values beneath the Anyemaqen Mountain. The large positive anisotropy can be explained by horizontal alignment of anisotropic minerals in the mid‐lower crust due to crustal flow. The mantle lithosphere above 90 km is largely isotropic while weak positive anisotropy appears beneath 90 km, which probably marks the lithosphere‐asthenosphere boundary (LAB). A low shear wave velocity anomaly and relatively negative radial anisotropy are imaged in the entire lithosphere beneath the restraining bend in the eastern Kunlun fault, consistent with a weak lithosphere experiencing vertical thickening under horizontal compression. The asthenosphere at the restraining bend is characterized by significant low velocity and positive radial anisotropy, reflecting that the asthenosphere here is probably hotter, has more melts, and deforms more easily than the surrounding region. We propose that the lithosphere at the restraining bend was vertically thickened and subsequently delaminated locally, and induced asthenosphere upwelling. This model explains the observations of velocity and anisotropy anomalies in the lithosphere and asthenosphere as well as geological observations of rapid rock uplift at the restraining bend of the Kunlun fault. This article is protected by copyright. All rights reserved.
      PubDate: 2016-08-19T09:40:29.281344-05:
      DOI: 10.1002/2016GC006366
  • Shear wave anisotropy in northwestern South America and its link to the
           Caribbean and Nazca subduction geodynamics
    • Abstract: To investigate the subduction dynamics in northwestern South America, we measured SKS and slab‐related local S splitting at 38 seismic stations. Comparison between the delay times of both phases shows that most of the SKS splitting is due to entrained mantle flow beneath the subducting Nazca and Caribbean slabs. On the other hand, the fast polarizations of local S‐waves are consistently aligned with regional faults, which implies the existence of a lithosphere‐confined anisotropy in the overriding plate, and that the mantle wedge is not contributing significantly to the splitting. Also, we identified a clear change in SKS fast directions at the trace of the Caldas Tear (∼5°N), which represents a variation in the subduction style. To the north of ∼5°N fast directions are consistently parallel to the flat subduction of the Caribbean plate‐Panama arc beneath South America, while to the south fast polarizations are subparallel to the Nazca‐South America subduction direction. A new change in the SKS splitting pattern is detected at ∼2.8ºN, which is related to another variation in the subduction geometry marked by the presence of a lithosphere‐scale tearing structure, named here as Malpelo Tear; in this region, NE‐SW‐oriented SKS fast directions are consistent with the general dip direction of the underthrusting of the Carnegie Ridge beneath South America. Further inland, this NE‐SW‐trending mantle flow continues beneath the Eastern Cordillera of Colombia and Merida Andes of Venezuela. Finally, our results suggest that the sub‐slab mantle flow in northwestern South America is strongly controlled by the presence of lithospheric tearing structures. This article is protected by copyright. All rights reserved.
      PubDate: 2016-08-19T09:40:25.436529-05:
      DOI: 10.1002/2016GC006323
  • Permeability of oceanic crustal rock samples from IODP Hole 1256D
    • Authors: L.A. Gilbert; M.L. Bona
      Abstract: Permeability is an important parameter of oceanic crust: it controls hydrothermal circulation and influences the exchange of heat and chemicals between seawater and the crust. Using the most complete section of intact, in situ normal oceanic crust, this paper presents the first permeability measurements of samples from Integrated Ocean Drilling Program (IODP) Hole 1256D in a relatively undisturbed section through lavas, dikes, and into gabbros. At in situ pressures, saturated gabbro from Hole 1256D is about half as permeable as basalt (2.4 × 10−20 m2 and 4.0 × 10−20 m2, respectively). Although fresh basalt samples have higher permeabilities, the basalts at Hole 1256D contain saponite, an alteration mineral which drastically reduces permeability. These measurements represent an opportunity for comparison to models that predict permeability at IODP Hole 1256D. Similar to model predictions, sample permeability generally decreases with depth. However, even after applying the scaling rule, models predict higher permeabilities than exhibited by the samples, suggesting large‐scale cracks still control permeability in the 15 My old crust at Hole 1256D. This article is protected by copyright. All rights reserved.
      PubDate: 2016-08-17T10:30:27.704821-05:
      DOI: 10.1002/2016GC006467
  • Horizontal compressive stress regime on the northern Cascadia margin
           inferred from borehole breakouts
    • Authors: M. Riedel; A. Malinverno, K. Wang, D. Goldberg, G. Guerin
      Abstract: During Integrated Ocean Drilling Program Expedition 311 five boreholes were drilled across the accretionary prism of the northern Cascadia subduction zone. Logging‐while‐drilling borehole images are utilized to determine breakout orientations to define maximum horizontal compressive stress orientations. Additionally, wireline logging data at two of these sites and from Site 889 of Ocean Drilling Program Leg 146 are used to define breakouts from differences in the aperture of caliper arms. At most sites, the maximum horizontal compressive stress SHmax is margin‐normal, consistent with plate convergence. Deviations from this trend reflect local structural perturbations. Our results do not constrain stress magnitudes. If the margin‐normal compressional stress is greater than the vertical stress, the margin‐normal SHmax direction we observe may reflect current locking of a velocity‐weakening shallow megathrust and thus potential for trench‐breaching, tsunamigenic rupture in a future megathrust earthquake. This article is protected by copyright. All rights reserved.
      PubDate: 2016-08-17T10:30:27.475412-05:
      DOI: 10.1002/2016GC006443
  • Large fluctuations of shallow seas in low‐lying Southeast Asia
           driven by mantle flow
    • Abstract: The Sundaland continental promontory, as the core of Southeast Asia, is one of the lowest lying continental regions, with half of the continental area presently inundated by a shallow sea. The role of mantle convection in driving long‐wavelength topography and vertical motion of the lithosphere in this region has often been ignored when interpreting regional stratigraphy, including a widespread Late Cretaceous‐Eocene unconformity, despite a consensus that Southeast Asia is presently situated over a large‐amplitude, dynamic topography low resulting from long‐term post‐Pangea subduction. We use forward numerical models to link mantle flow with surface tectonics, and compare predicted trends of dynamic topography with eustasy and regional paleogeography to determine the influence of mantle convection on regional basin histories. A Late Cretaceous collision of Gondwana‐derived terranes with Sundaland choked the active margin, leading to slab breakoff and a ∼10‐15 Myr‐long subduction hiatus. Slab breakoff likely resulted in several hundred meters of dynamic uplift and emergence of Sundaland between ∼80 and 60 Ma, and may explain the absence of a Late Cretaceous‐Eocene sedimentary record. Renewed subduction from ∼60 Ma reinitiated dynamic subsidence of Sundaland, leading to submergence from ∼40 Ma despite falling long‐term global sea levels. Our results highlight a complete ‘down‐up‐down' dynamic topography cycle experienced by Sundaland, with transient dynamic topography manifesting as a major regional unconformity in sedimentary basins. This article is protected by copyright. All rights reserved.
      PubDate: 2016-08-12T10:10:26.185406-05:
      DOI: 10.1002/2016GC006434
  • Segment‐scale variations in seafloor volcanic and tectonic processes
           from multibeam sonar imaging, mid‐Atlantic Ridge Rainbow region
    • Authors: Deborah E. Eason; Robert A. Dunn, J. Pablo Canales, Robert Sohn
      Abstract: Along‐axis variations in melt supply and thermal structure can lead to significant variations in the mode of crustal accretion at mid‐ocean ridges. We examine variations in seafloor volcanic and tectonic processes on the scale of individual ridge segments in a region of the slow‐spreading Mid‐Atlantic Ridge (35º45'‐36º35'N) centered on the Rainbow non‐transform discontinuity (NTD). We use multibeam sonar backscatter amplitude data, taking advantage of multifold and multi‐directional coverage from the MARINER geophysical study to create a gridded compilation of seafloor reflectivity, and interpret the sonar image within the context of other data to examine seafloor properties and identify volcanic flow fields and tectonic features. Along the spreading segments, differences in volcanic productivity, faulting, eruption style and frequency correlate with inferred magma supply. Regions of low magma supply are associated with more widely spaced faults, and larger volcanic flow fields that are more easily identified in the backscatter image. Identified flow fields with the highest backscatter occur near the ends of ridge segments. Their relatively smooth topography contrasts with the more hummocky, cone‐dominated terrain that dominates most of the neovolcanic zone. Patches of seafloor with high, moderately high, and low backscatter intensity across the Rainbow massif are spatially correlated with observations of basalt, gabbro and serpentinized peridotite, and sediment respectively. Large detachment faults have repeatedly formed along the inside corners of the Rainbow NTD, producing a series of oceanic core complexes along the wake of the NTD. A new detachment fault is currently forming in the ridge segment just north of the now inactive Rainbow massif. This article is protected by copyright. All rights reserved.
      PubDate: 2016-08-09T04:32:15.970129-05:
      DOI: 10.1002/2016GC006433
  • Reconstruction of hydrographic changes in the southern Norwegian Sea
           during the past 135 kyr and the impact of different foraminiferal Mg/Ca
           cleaning protocols
    • Authors: Mohamed M. Ezat; Tine L. Rasmussen, Jeroen Groeneveld
      Abstract: The shallow subsurface hydrography in the southern Norwegian Sea during the past 135,000 years was investigated using parallel measurements of Mg/Ca and δ18O in shells of the planktic foraminiferal species Neogloboquadrina pachyderma. Two cleaning methods were applied prior to Mg/Ca analysis, ‘Mg Cleaning' and ‘full cleaning' methods. Different results were obtained, which are most likely due to a more efficient removal of Mn‐contaminant coatings of the shells, when the ‘full cleaning' procedure was applied. We further combined Mg/Ca and B/Ca from the ‘full cleaning' method with δ18O values to constrain the calcification temperature and seawater‐δ18O (δ18Osw) in the past. During Heinrich Stadial (HS)1 (∼18.5–15 ka) N. pachyderma constituted >80% of the planktic foraminiferal population, δ18Osw decreased by ∼1.5‰, and shallow subsurface temperature increased by ∼1.5–3°C, suggesting strong stratification in the upper water column and a possible subsurface inflow of Atlantic water below a well‐developed halocline during the calcification seasons of N. pachyderma. Similar decreases in δ18Osw are also recorded for other Heinrich stadials (HS2, ‐3, ‐4, ‐6 and ‐11). Our temperature estimates confirm previous observations of the delay of the last interglacial ‘Eemian' warm peak in the Eastern Nordic Seas compared to the North Atlantic, and a late warming coinciding with the summer insolation minimum at 60°N. In addition, relatively high values of δ18Osw during the early Eemian suggest a shallow subsurface inflow of Atlantic water below a thin layer of Arctic surface water. This article is protected by copyright. All rights reserved.
      PubDate: 2016-08-09T03:43:59.106088-05:
      DOI: 10.1002/2016GC006325
  • Climatic evolution of the central equatorial Pacific since the Last
           Glacial Maximum
    • Authors: Inah Seo; Yuri Lee, Yong Il Lee, Chan Min Yoo, Kiseong Hyeong
      Abstract: This paper investigates paleoceanographic changes at a central equatorial Pacific site (6°40′N, 177°28′W) since the last glacial maximum using planktic foraminifera assemblages, together with the oxygen isotope (δ18O) and Mg/Ca compositions of three species (Globigerinoides sacculifer, Pulleniatina obliquiloculata, and Globorotalia tumida) that dwell in the mixed layer, upper thermocline, and lower thermocline, respectively. While the Mg/Ca‐derived temperatures of the mixed layer and lower thermocline varied within a narrow range from 18 ka onwards, the upper thermocline temperature increased by as much as 3°C during the last deglaciation (18–12 ka) with a simultaneous decrease of δ18O. These changes are best explained by an enhanced mixing of the upper ocean and a reduced habitat depth separation between P. obliquiloculata and G. sacculifer during the 18–12 ka interval. The planktic foraminifera assemblage during the same period resembles modern composition at subtropical central Pacific sites that are strongly influenced by the northeasterly Trades and North Equatorial Current (NEC). We suggest that the study site, presently under the control of the Intertropical Convergence Zone (ITCZ)–North Equatorial Countercurrent, had been influenced by the northeasterly Trades and NEC during the 18–12 ka interval. This interpretation is consistent with previous documentation of a more southerly location of the ITCZ during two Northern Hemisphere cooling events; the Heinrich Stadial 1 and the Younger Dryas, and implies that the mean annual position of the ITCZ was located south of the study site, by at least 2° of latitude. This article is protected by copyright. All rights reserved.
      PubDate: 2016-08-08T10:12:28.360306-05:
      DOI: 10.1002/2016GC006371
  • Interpretation of the provenance of small‐scale heterogeneity as
           documented in a single eruptive unit from Mt. Jefferson, Central Oregon
    • Authors: Gokce Ustunisik; Matthew W. Loewen, Roger L. Nielsen, Frank J. Tepley
      Abstract: Mt. Jefferson is a large composite volcano located in the central Oregon Cascades that has erupted a diverse compositional suite of lavas from basalt to rhyodacite (50‐72 wt. % SiO2). Individual eruptive units contain multiple populations of plagioclase, and a variety of mafic textural/mineralogical components often preserved as large cm‐ to mm‐sized enclaves.Understanding the processes active in any volcanic center requires that we document the products of those processes. In this contribution we documented the small scale compositional diversity within a single eruptive unit at Mt. Jefferson, the Whitewater Creek andesite, in order to answer three questions: (1) what are the characteristics and scale of diversity in a single eruptive unit, (2) what is the provenance of the observed components, and (3) how does that observed small scale diversity relate to the larger scale diversity observed between other flows at Mt. Jefferson.Our analyses are based on major, trace element concentrations for phenocrysts and melt inclusions from a single eruptive unit, the Whitewater Creek andesite which is one of the most heterogeneous units erupted at Mt. Jefferson. We have identified at least four distinct components present at the cm scale. These components, identified on the basis of their mineralogy and composition, include three mafic (two pyroxene+plagioclase; plagioclase‐hornblende and olivine‐orthopyroxene) and one silicic component (dacitic groundmass).To understand the relationship between the observed textural components and the magma types erupted at Mt Jefferson, equilibrium liquid compositions were calculated from the phenocryst compositions. The range exhibited by those calculated liquids covers much of the entire range represented by the lavas at Mt. Jefferson. However, it is difficult to directly connect them to specific magma types observed at Mt. Jefferson. We attribute our inability to directly link the textural components to the known magma types to post mixing diffusive re‐equilibration during storage and transport. These results suggest that great care should be taken in interpretation of whole rock data. This article is protected by copyright. All rights reserved.
      PubDate: 2016-08-08T10:12:25.259193-05:
      DOI: 10.1002/2016GC006297
  • Correction algorithm for on‐line continuous flow δ13C and δ18O
           carbonate and cellulose stable isotope analyses
    • Authors: M. N. Evans; K. J. Selmer, B. T. Breeden, A. S. Lopatka, R. E. Plummer
      Abstract: We describe an algorithm to correct for scale compression, runtime drift, and amplitude effects in carbonate and cellulose oxygen and carbon isotopic analyses made on two online continuous flow isotope ratio mass spectrometry (CF‐IRMS) systems using gas chromatographic (GC) separation. We validate the algorithm by correcting measurements of samples of known isotopic composition which are not used to estimate the corrections. For carbonate δ13C (δ18O) data, median precision of validation estimates for two reference materials and two calibrated working standards is 0.05% (0.07%); median bias is 0.04% (0.02%) over a range of 49.2% (24.3%). For α‐cellulose δ13C (δ18O) data, median precision of validation estimates for one reference material and five working standards is 0.11% (0.27%); median bias is 0.13% (‐0.10%) over a range of 16.1% (19.1%). These results are within the 5th‐95th percentile range of subsequent routine runtime validation exercises in which one working standard is used to calibrate the other. Analysis of the relative importance of correction steps suggests that drift and scale‐compression corrections are most reliable and valuable. If validation precisions are not already small, routine cross‐validated precision estimates are improved by up to 50% (80%). The results suggest that correction for systematic error may enable these particular CF‐IRMS systems to produce δ13C and δ18O carbonate and cellulose isotopic analyses with higher validated precision, accuracy and throughput than is typically reported for these systems. The correction scheme may be used in support of replication‐intensive research projects in paleoclimatology and other data‐intensive applications within the geosciences. This article is protected by copyright. All rights reserved.
      PubDate: 2016-08-08T05:18:17.18164-05:0
      DOI: 10.1002/2016GC006469
  • The effect of bimineralic composition on extensional processes at
           lithospheric scale
    • Authors: Suzon Jammes; Luc L. Lavier
      Abstract: We investigate how lithospheric scale compositional heterogeneities affect kilometric scale deformation processes. To this end, we perform numerical experiments of lithospheric extension in which we vary the Moho temperature and the mineralic composition of the mantle and the crust. In both the crust and the mantle we use a explicit bimineralic composition by randomly distributing two mineral phases in the materials. Comparison of our models to simulations using an implicit bimineralic composite (one average viscous flow laws for a two‐phase aggregate) crust and mantle demonstrates that an explicit bimineralic composition assimilated to heterogeneities succeeds in explaining observations related to the formation of rifted margins such a: 1) the absence of a sharp deformation zone at the brittle ductile transition (BDT), 2) the initiation of the rifting process as a wide delocalized rift system with multiple normal faults dipping in both directions; 3) the development of anastomosing shear zones in the middle/lower crust and the upper lithospheric mantle similar to the crustal scale anastomosing patterns observed in the field or in seismic data; 4) the preservation of undeformed lenses of material leading to lithospheric scale boudinage structure and resulting in the formation of continental ribbons as observed along the Iberian‐Newfoundland margin. This article is protected by copyright. All rights reserved.
      PubDate: 2016-08-03T03:00:44.189795-05:
      DOI: 10.1002/2016GC006399
  • Estimating the magnetization distribution within rectangular rock samples
    • Authors: A. L. A. Reis; V. C. Oliveira, E. Yokoyama, A. C. Bruno, J. M. B. Pereira
      Abstract: Over the last decades, scanning magnetic microscopy techniques have been increasingly used in paleomagnetism and rock magnetism. Different from standard paleomagnetic magnetometers, scanning magnetic microscopes produce high‐resolution maps of the vertical component of the magnetic induction field (flux density) on a plane located over the sample. These high‐resolution magnetic maps can be used for estimating the magnetization distribution within a rock sample by inversion. Previous studies have estimated the magnetization distribution within rock samples by inverting the magnetic data measured on a single plane above the sample. Here we present a new spatial domain method for inverting the magnetic induction measured on four planes around the sample in order to retrieve its internal magnetization distribution. We have presumed that the internal magnetization distribution of the sample varies along one of its axes. Our method approximates the sample geometry by an interpretation model composed of a one‐dimensional array of juxtaposed rectangular prisms with uniform magnetization. The Cartesian components of the magnetization vector within each rectangular prism are the parameters to be estimated by solving a linear inverse problem. Our method automatically deals with the averaging of the measured magnetic data due to the finite size of the magnetic sensor, preventing the application of a deconvolution before the inversion. Tests with synthetic data show the performance of our method in retrieving complex magnetization distributions even in the presence of magnetization heterogeneities. Moreover, they show the advantage of inverting the magnetic data on four planes around the sample adn how this new acquisition scheme improves the estimated magnetization distribution within the rock sample. We have also applied our method to invert experimentally measured magnetic data produced by a highly‐magnetized synthetic sample that was manufactured in the laboratory. The results show that, even in the presence of apparent position noise, our method was able to retrieve the magnetization distribution consistent with the isothermal remanent magnetization induced in the sample. This article is protected by copyright. All rights reserved.
      PubDate: 2016-08-03T03:00:37.200727-05:
      DOI: 10.1002/2016GC006329
  • Experimental constraints on the serpentinization rate of fore‐arc
           peridotites: Implications for the upwelling condition of the
           slab‐derived fluid
    • Authors: T. Nakatani; M. Nakamura
      Abstract: To constrain the water circulation in subduction zones, the hydration rates of peridotites were investigated experimentally in fore‐arc mantle conditions. Experiments were conducted at 400–580°C and 1.3 and 1.8 GPa, where antigorite is expected to form as a stable serpentine phase. Crushed powders of olivine ± orthopyroxene and orthopyroxene + clinopyroxene were reacted with 15 wt% distilled water for 4–19 days. The synthesized serpentine varieties were lizardite and aluminous lizardite (Al‐lizardite) in all experimental conditions except those of 1.8 GPa and 580°C in the olivine + orthopyroxene system, in which antigorite was formed. In the olivine + orthopyroxene system, the reactions were interface‐controlled except for the reaction at 400°C, which was transport‐controlled. The corresponding reaction rates were 7.0 × 10−12−1.5 × 10−11 m s−1 at 500–580°C and 7.5 × 10−16 m2 s−1 at 400°C for the interface‐ and transport‐controlled reactions, respectively. Based on a simple reaction‐transport model including these hydration rates, we infer that penetration of the slab‐derived fluid all the way through a water‐unsaturated fore‐arc mantle is allowed only when focused flow occurs with a spacing larger than 77–229 km in hot subduction zones such as Nankai and Cascadia. However, the necessary spacing is only 2.3–4.6 m in intermediate‐temperature subduction zones such as Kyushu and Costa Rica. These calculations imply that fluid leakage in hot subduction zones may occur after the fore‐arc mantle is totally hydrated, whereas in intermediate‐temperature subduction zones, leakage through a water‐unsaturated fore‐arc mantle may be facilitated. This article is protected by copyright. All rights reserved.
      PubDate: 2016-08-03T03:00:30.475233-05:
      DOI: 10.1002/2016GC006295
  • Marine silicate weathering in the anoxic sediment of the Ulleung Basin:
           Evidence and consequences
    • Abstract: Marine silicate weathering (MSiW) in anoxic sediments has been recently shown to be a significant sink for CO2 generated by methanogenesis. Independently, the roles of clay dehydration (illitization) in producing water and driving upward fluid advection have been well established in deep marine sediments, but to date the K+ source required for the reaction has not been established. Here we present chemical and strontium isotope properties of pore fluids from seven cores in the Ulleung Basin, which show radiogenic 87Sr/86Sr values (up to ∼0.71045), very high alkalinity values (maximum ∼130 mM), and enrichment in H4SiO4, Na+, K+, and Mg2+, consistent with MSiW. This reaction consumes CO2, generates alkalinity, and acts as a K+ source for illitization; water released from MSiW‐supported illitization drives upward fluid flow. Our results highlight the importance of MSiW along continental margins and its underappreciated role in carbon cycling, silicate diagenesis, and hydrogeology of marine systems. This article is protected by copyright. All rights reserved.
      PubDate: 2016-08-03T03:00:25.481052-05:
      DOI: 10.1002/2016GC006356
  • Cenozoic volcanism in the Bohemian Massif in the context of P and
    • Abstract: New high‐resolution tomographic models of P‐ and S‐wave isotropic‐velocity perturbations for the Bohemian upper mantle are estimated from carefully pre‐processed travel‐time residuals of teleseismic P, PKP and S waves recorded during the BOHEMA passive seismic experiment. The new data resolve anomalies with scale lengths 30‐50 km. The models address whether a small mantle plume in the western Bohemian Massif is responsible for this geodynamically active region in central Europe, as expressed in recurrent earthquake swarms. Velocity‐perturbations of the P‐ and S‐wave models show similar features, though their resolutions are different. No model resolves a narrow sub‐vertical low‐velocity anomaly, which would validate the ‘baby‐plume' concept. The new tomographic inferences complement previous studies of the upper mantle beneath the Bohemian Massif, in a broader context of the European Cenozoic Rift System (ECRIS) and of other Variscan Massifs in Europe. The low‐velocity perturbations beneath the Eger Rift, observed in about 200km‐broad zone, agree with shear‐velocity models from full‐waveform inversion, which also did not identify a mantle plume beneath the ECRIS. Boundaries between mantle domains of three tectonic units that comprise the region, determined from studies of seismic anisotropy, represent weak zones in the otherwise rigid continental mantle lithosphere. In the past, such zones could have channeled upwelling of hot mantle material, which on its way could have modified the mantle domain boundaries and locally thinned the lithosphere. This article is protected by copyright. All rights reserved.
      PubDate: 2016-08-02T10:20:42.594672-05:
      DOI: 10.1002/2016GC006318
  • Acquisition of detrital magnetization in four turbidites
    • Abstract: Turbiditic events are mostly avoided in paleomagnetic studies and therefore their remanence and magnetic properties are poorly described. Turbidites are exempt of bioturbation and potentially provide pertinent information about depositional remanence. We studied four quaternary turbidites of different origins in marine sediment cores. Upward fining of both magnetic and sedimentary fractions indicates that coarser grains reached the bottom first. We observe a progressive shallowing of the magnetic inclinations between the upper and bottom layers of the turbidites that increases with the size of the events and obeys a simple linear scaling law. Measurements of magnetic anisotropy suggest that hydrodynamic conditions prevailing during deposition seem to be dominant for the alignment of the magnetic grains. We suggest that small spherical grains are randomly oriented with zero resultant magnetization in presence of strong turbulent conditions, while the alignment of elongated grains is constrained by the competition between gravity and magnetic forces. A possible scenario is that under turbulent conditions they tend to rest at the bottom with their long axes parallel to the sediment surface and therefore with shallow inclinations, whereas weakly turbulent conditions like during the smallest (26 cm thick) event do not disturb the magnetic alignment and therefore do not generate inclination shallowing. This article is protected by copyright. All rights reserved.
      PubDate: 2016-07-22T10:30:26.590322-05:
      DOI: 10.1002/2016GC006378
  • Mass budget partitioning during explosive eruptions: Insights from the
           2006 paroxysm of Tungurahua volcano, Ecuador
    • Abstract: How and how much the mass of juvenile magma is split between vent‐derived tephra, PDC deposits and lavas (i.e. mass partition) is related to eruption dynamics and style. Estimating such mass partitioning budgets may reveal important for hazard evaluation purposes. We calculated the volume of each product emplaced during the August 2006 paroxysmal eruption of Tungurahua volcano (Ecuador) and converted it into masses using high‐resolution grainsize, componentry and density data. This dataset is one of the first complete descriptions of mass partitioning associated with a VEI 3 andesitic event. The scoria fall deposit, near‐vent agglutinate and lava flow include 28, 16 and 12 wt. % of the erupted juvenile mass, respectively. Much (44 wt. %) of the juvenile material fed Pyroclastic Density Currents (i.e dense flows, dilute surges and co‐PDC plumes), highlighting that tephra fall deposits do not depict adequately the size and fragmentation processes of moderate PDC‐forming event. The main parameters controlling the mass partitioning are the type of magmatic fragmentation, conditions of magma ascent, and crater area topography. Comparisons of our dataset with other PDC‐forming eruptions of different style and magma composition suggest that moderate andesitic eruptions are more prone to produce PDCs, in proportions, than any other eruption type. This finding may be explained by the relatively low magmatic fragmentation efficiency of moderate andesitic eruptions. These mass partitioning data reveal important trends that may be critical for hazard assessment, notably at frequently active andesitic edifices. This article is protected by copyright. All rights reserved.
      PubDate: 2016-07-22T10:25:29.406909-05:
      DOI: 10.1002/2016GC006431
  • Recognizing magnetostratigraphy in overprinted and altered marine
           sediments: Challenges and solutions from IODP Site U1437
    • Authors: Robert J. Musgrave; Myriam Kars
      Abstract: Core disturbance, drilling overprints, post‐depositional acquisition of remanence, authigenic growth of magnetic iron sulfides, and alteration all contribute challenges to recognizing the primary magnetostratigraphy in marine sediments. We address these issues in a sequence of tuffaceous muds and volcaniclastics at International Ocean Discovery Program Site U1437 and produce the longest continuous magnetic polarity stratigraphy in the history of scientific ocean drilling. Remanence measurements were filtered to remove intervals affected by fluidization, plastic sediment disturbance, and core biscuiting. Drilling overprints are concentrated in the disturbed annulus surrounding intact core material. Bioturbation was limited to a vertical extent of at most 15 cm. Changes in sediment color, stiffness, and magnetic hysteresis all suggest that remanence was locked in within a few meters of the sediment–water interface. We did not observe any systematic offset between magnetostratigraphic and biostratigraphic datums. Authigenic growth of greigite, in response to both initial sulfate reduction in the upper 50 m of the sediment column and to deeper resupply of sulfate, has led to magnetic overprinting. Anomalous polarity artefacts, extending
      PubDate: 2016-07-22T10:25:23.667718-05:
      DOI: 10.1002/2016GC006386
  • The profile of the rare earth elements in the Canada Basin, Arctic Ocean
    • Authors: Jon Yang; Brian Haley
      Abstract: We analyzed the dissolved rare earth element (REE) content of three water column profiles (two shelf sites and one deep basin site) in the Canada Basin in order to better constrain the behavior of REEs in the Arctic Ocean. Dissolved concentrations of the REEs in the surface are 1.3 to 1.9 times higher than deep water (>500 m) concentrations, which are constant with depth (La: 19‐23 pM, Nd: 14‐17 pM, Yb: 4.0‐4.3 pM). The dominant source of REEs to the surface waters of the Canada Basin is most likely Pacific water flowing through the Bering Strait and Chukchi Sea and/or the Mackenzie River. Dissolved REEs in the intermediate and deep waters are constant and appear to behave conservatively, allowing us to investigate this aspect of REE behavior in the oceans. Calculated deep ocean residence times of the REEs in the Canada Basin range from 450‐700 years and match the age of these waters. We postulate that these values are likely applicable to global deep ocean reservoirs and that observed deviations from this conservative value can help to constrain non‐conservative processes acting on the REEs. This article is protected by copyright. All rights reserved.
      PubDate: 2016-07-22T10:20:22.515526-05:
      DOI: 10.1002/2016GC006412
  • Significance of sediment reverberations on receiver functions of broadband
           OBS data: Comments on Olugboji et al. (2016) “Nature of the Seismic
           Lithosphere‐Asthenosphere Boundary within Normal Oceanic Mantle from
           High‐Resolution Receiver Functions”
    • Authors: Hitoshi Kawakatsu; Yuki Abe
      PubDate: 2016-07-20T10:25:57.221221-05:
      DOI: 10.1002/2016GC006418
  • Reply to comment by Kawakatsu and Abe on “Nature of the seismic
           lithosphere‐asthenosphere boundary within normal oceanic mantle from
           high‐resolution receiver functions”
    • Abstract: Kawakatsu and Abe [2016] have highlighted the potential complicating effect of sediment reverberations on the analysis and interpretation of crust and mantle phases inferred from receiver functions analyzed from ocean‐bottom seismograms. In their comment, they identify resonant peaks in the power spectrum at one of the stations, T06, in the analysis of [Olugboji et al., 2016], and demonstrate with synthetic modeling how sediment‐induced resonances can cause instability in the recovered receiver‐function (RF) traces. They also request a detailed explanation of how LQT rotation is conducted, and why its use leads to stable receiver functions in the analysis of Olugboji et al. [2016].We welcome this query as an opportunity to highlight certain technical aspects of the data‐analysis procedures used in Olugboji et al [2016]. Our methods derive partly from methods recommended by previous studies of receiver functions estimated from seismic seafloor data [Bostock and Trehu, 2012; Janiszewski and Abers, 2015; Audet, 2016], particularly the use of the modal wavefield decomposition [e.g., Reading et al, 2003]) (which we approximated by the LQT rotation) to suppress reverberation signals in the overlying water column [Bostock and Trehu, 2012]. This article is protected by copyright. All rights reserved.
      PubDate: 2016-07-20T10:20:30.643996-05:
      DOI: 10.1002/2016GC006453
  • Origin of geochemical mantle components: Role of subduction filter
    • Abstract: We quantitatively explore element redistribution at subduction zones using numerical mass balance models to evaluate the roles of the subduction zone filter in the Earth's geochemical cycle. Our models of slab residues after arc magma genesis differ from previous ones by being internally consistent with geodynamic models of modern arcs that successfully explain arc magma genesis, and include element fluxes from the dehydration/melting of each underlying slab component. We assume that the mantle potential temperature (Tp) was 1400–1650°C at 3.5–1.7 Ga and gradually decreased to 1300–1350°C today. Hot subduction zones with Tp ∼1650°C have a thermal structure like modern SW Japan where high‐Mg andesite is formed that is chemically like continental crust. After 2.5–1.7 Gyr of storage in the mantle, the residual igneous oceanic crust from hot subduction zones can evolve isotopically to the HIMU mantle component, the residual base of the mantle wedge to EMI, the residual sediment becomes an essential part of EMII, and the residual top of the mantle wedge can become the subcontinental lithosphere component. The Common or Focal Zone component is a stable mixture of the first three residues occasionally mixed with early depleted mantle. Slab residue that recycled earlier (∼2.5 Ga) form the DUPAL anomaly in the southern hemisphere, whereas residues of more recent recycling (∼1.7 Ga) underlie the northern hemisphere. These ages correspond to major continental crust forming events. The east‐west heterogeneity of the depleted upper mantle involves sub continental mantle except in the Pacific. This article is protected by copyright. All rights reserved.
      PubDate: 2016-07-18T03:52:52.856613-05:
      DOI: 10.1002/2016GC006343
  • Age and compositional data of zircon from sepiolite drilling mud to
           identify contamination of ocean drilling samples
    • Authors: Graham D. M. Andrews; Axel K. Schmitt, Cathy J. Busby, Sarah R. Brown, Peter Blum, Janet. C. Harvey
      Abstract: Zircon extracted from drilled oceanic rocks is increasingly used to answer geologic questions related to igneous and sedimentary sequences. Recent zircon studies using samples obtained from marine drill cores revealed that drilling muds used in the coring process may contaminate the samples. The JOIDES Resolution Science Operator of the International Ocean Discovery Program has been using two types of clays, sepiolite and attapulgite, which both have salt water viscosifier properties able to create a gel‐like slurry that carries drill cuttings out of the holes several hundred meters deep. The dominantly used drilling mud is sepiolite originating from southwestern Nevada, USA. This sepiolite contains abundant zircon crystals with U‐Pb ages ranging from 1.89 to 2889 Ma and continental trace element, δ18O, and εHf isotopic compositions. A dominant population of 11 to 16 Ma zircons in sepiolite drilling mud makes identification of contamination in drilled Neogene successions particularly challenging. Interpretation of zircon analyses related to ocean drilling should be cautious of zircon ages in violation of independently constrained age models and that have age populations overlapping those in the sepiolite. Because individual geochronologic and geochemical characteristics lack absolute discriminatory power, it is recommended to comprehensively analyze all dated zircon crystals from cores exposed to drill mud for trace element, δ18O, and εHf isotopic compositions. Zircon analyzed in situ (i.e., in petrographic sections) are assumed to be trustworthy. This article is protected by copyright. All rights reserved.
      PubDate: 2016-07-15T04:16:02.603628-05:
      DOI: 10.1002/2016GC006397
  • Water‐rich and volatile‐undersaturated magmas at Hekla
           volcano, Iceland
    • Abstract: Olivine‐hosted melt inclusions from four eruptions at Hekla volcano in Iceland were analyzed for their dissolved H2O, CO2, S and Cl contents. A positive correlation between the repose interval, magmatic evolution and volatile contents of magmas is revealed. H2O is the dominant volatile species; it behaves as an incompatible component, increasing in concentration over time as a result of fractional crystallization in the magma. The full suite of H2O contents ranges from a low of 0.80 wt. % in basaltic andesites to a maximum of 5.67 wt. % in rhyolites. Decreasing H2O/K2O at fixed major element compositions suggests that syn‐eruptive degassing reduces H2O contents significantly. Hekla magmas are CO2 poor, with very low concentrations present only in the most evolved compositions (∼20‐30 ppm or less). The decrease in S content from basaltic andesite to rhyolite demonstrates that sulfide saturation is attained when the melt composition reaches basaltic andesite, resulting in the precipitation of pyrrhotite. Low CO2/Nb ratios suggest that vapor saturation is most likely reached during an early period of cooling and solidification in the crust. Fresh injections of mafic magma interact with previously solidified intrusives, producing new melts that are volatile‐undersaturated. Vapor saturation pressures obtained using the most volatile‐rich melt inclusions suggest the presence of a magma chamber at a minimum depth of ∼7 km. This is in agreement with geophysical observations from recent small‐volume eruptions, but given the possibility of volatile‐undersaturated melts, some of the magmas may reside at greater depths. This article is protected by copyright. All rights reserved.
      PubDate: 2016-07-14T09:35:42.980275-05:
      DOI: 10.1002/2016GC006336
  • The water retention curve and relative permeability for gas production
           from hydrate‐bearing sediments: Pore‐network model simulation
    • Authors: Nariman Mahabadi; Sheng Dai, Yongkoo Seol, Tae Sup Yun, Jaewon Jang
      Abstract: The water retention curve and relative permeability are critical to predict gas and water production from hydrate‐bearing sediments. However, values for key parameters that characterize gas and water flows during hydrate dissociation have not been identified due to experimental challenges. This study utilizes the combined techniques of micro‐focus X‐ray computed tomography (CT) and pore‐network model simulation to identify proper values for those key parameters, such as gas entry pressure, residual water saturation, and curve fitting values. Hydrates with various saturation and morphology are realized in the pore‐network that was extracted from micron‐resolution CT images of sediments recovered from the hydrate deposit at the Mallik site, and then the processes of gas invasion, hydrate dissociation, gas expansion, and gas and water permeability are simulated. Results show that greater hydrate saturation in sediments lead to higher gas entry pressure, higher residual water saturation, and steeper water retention curve. An increase in hydrate saturation decreases gas permeability but has marginal effects on water permeability in sediments with uniformly distributed hydrate. Hydrate morphology has more significant impacts than hydrate saturation on relative permeability. Sediments with heterogeneously distributed hydrate tend to result in lower residual water saturation and higher gas and water permeability. In this sense, the Brooks‐Corey model that uses two fitting parameters individually for gas and water permeability properly capture the effect of hydrate saturation and morphology on gas and water flows in hydrate‐bearing sediments. This article is protected by copyright. All rights reserved.
      PubDate: 2016-07-14T09:35:36.375488-05:
      DOI: 10.1002/2016GC006372
  • Representing anisotropic subduction zones with isotropic velocity models:
           A characterization of the problem and some steps on a possible path
    • Authors: M. J. Bezada; M. Faccenda, D. R. Toomey
      Abstract: Despite the widely known fact that mantle flow in and around subduction zones produces the development of considerable seismic anisotropy, most P‐wave tomography efforts still rely on the assumption of isotropy. In this study, we explore the potential effects of erroneous assumption on tomographic images and explore an alternative approach. We conduct a series of synthetic tomography tests based on a geodynamic simulation of subduction and rollback. The simulation results provide a self‐consistent distribution of isotropic (thermal) anomalies and seismic anisotropy which we use to calculate synthetic delay times for a number of realistic and hypothetical event distributions. We find that anisotropy‐induced artifacts are abundant and significant for teleseismic, local and mixed event distributions. The occurrence of artifacts is not reduced, and indeed can be exacerbated, by increasing richness in ray‐path azimuths and incidence angles. The artifacts that we observe are, in all cases, important enough to significantly impact the interpretation of the images. We test an approach based on prescribing the anisotropy field as an a priori constraint and find that even coarse approximations to the true anisotropy field produce useful results. Using approximate anisotropy fields can result in reduced RMS misfit to the travel time delays and reduced abundance and severity of imaging artifacts. We propose that the use of anisotropy fields derived from geodynamic modeling and constrained by seismic observables may constitute a viable alternative to isotropic tomography that does not require the inversion for anisotropy parameters in each node of the model. This article is protected by copyright. All rights reserved.
      PubDate: 2016-07-14T09:26:33.943968-05:
      DOI: 10.1002/2016GC006507
  • Late‐Pleistocene precipitation δ18O interpolated across the
           global landmass
    • Authors: Scott Jasechko
      Abstract: Global water cycles, ecosystem assemblages and weathering rates were impacted by the ∼4°C of global warming that took place over the course of the last glacial termination. Fossil groundwaters can be useful indicators of late‐Pleistocene precipitation isotope compositions, which, in turn, can help to test hypotheses about the drivers and impacts of glacial‐interglacial climate changes. Here, a global catalog of 128 fossil groundwater records is used to interpolate late‐Pleistocene precipitation δ18O across the global landmass. The interpolated data show that extratropical late‐Pleistocene terrestrial precipitation was near uniformly depleted in 18O relative to the late Holocene. By contrast, tropical δ18O responses to deglacial warming diverged; late‐Pleistocene δ18O was higher‐than‐modern across India and South China but lower‐than‐modern throughout much of northern and southern Africa. Groundwaters that recharged beneath large northern hemisphere ice sheets have different Holocene‐Pleistocene δ18O relationships than paleowaters that recharged subaerially, potentially aiding reconstructions of englacial transport in paleo ice sheets. Global terrestrial late‐Pleistocene precipitation δ18O maps may help to map 3D groundwater age distributions, constrain Pleistocene mammal movements, and better understand glacial climate dynamics. This article is protected by copyright. All rights reserved.
      PubDate: 2016-07-13T10:43:20.289536-05:
      DOI: 10.1002/2016GC006400
  • Controls on the distribution of deep‐sea sediments
    • Abstract: Deep‐sea sediments represent the largest geological deposit on Earth and provide a record of our planet's response to conditions at the sea surface from where the bulk of material originates. We use a machine learning method to analyze how the distribution of 14,400 deep‐sea sediment sample lithologies is connected to bathymetry and surface oceanographic parameters. Our probabilistic Gaussian process classifier shows that the geographic occurrence of five major lithologies in the world's ocean can be predicted using just three parameters. Sea‐surface salinity and temperature provide a major control for the growth and composition of plankton and specific ranges are also associated with the influx of non‐aerosol terrigenous material into the ocean, while bathymetry is an important parameter for discriminating the occurrence of calcareous sediment, clay and coarse lithogenous sediment from each other. We find that calcareous and siliceous oozes are not linked to high surface productivity. Diatom and radiolarian oozes are associated with low salinities at the surface but with discrete ranges of temperatures reflecting the diversity of planktonic species in different climatic zones. Biosiliceous sediments cannot be used to infer paleodepth, but are good indicators of paleotemperature and paleosalinity. Our analysis provides a new framework for constraining paleosurface ocean environments from the geological record of deep‐sea sediments. It shows that small shifts in salinity and temperature significantly affect the lithology of seafloor sediment. As deep‐sea sediments represent the largest carbon sink on Earth these shifts need to be considered in the context of global ocean warming. This article is protected by copyright. All rights reserved.
      PubDate: 2016-07-13T10:43:12.571597-05:
      DOI: 10.1002/2016GC006428
  • An integrated natural remanent magnetization acquisition model for the
           Matuyama‐Brunhes reversal recorded by the Chinese loess
    • Authors: Chunsheng Jin; Qingsong Liu, Pengxiang Hu, Zhaoxia Jiang, Cange Li, Peng Han, Huihui Yang, Wentian Liang
      Abstract: Geomagnetic polarity reversal boundaries are key isochronous chronological controls for the long Chinese loess sequences, and further facilitate paleoclimatic correlation between Chinese loess and marine sediments. However, owing to complexity of post‐depositional remanent magnetization (pDRM) acquisition processes related to variable dust sedimentary environments on the Chinese Loess Plateau (CLP), there is a long‐standing dispute concerning the downward shift of the pDRM recorded in Chinese loess. In this study, after careful stratigraphic correlation of representative climatic tie points and the Matuyama‐Brunhes boundaries (MBB) in the Xifeng, Luochuan, and Mangshan loess sections with different pedogenic environments, the downward shift of the pDRM is semi‐quantitatively estimated and the acquisition model for the loess natural remanent magnetization (NRM) is discussed. The measured MB transition zone has been affected by the surficial mixing layer (SML) and remagnetization. Paleoprecipitation is suggested to be the dominant factor controlling the pDRM acquisition processes. Rainfall‐controlled leaching would restrict the efficiency of the characterized remanent magnetization carriers aligning along the ancient geomagnetic field. We conclude that the MBB in the central CLP with moderate paleoprecipitation could be considered as an isochronous chronological control after moderate upward adjustment. A convincing case can then be made to correlate L8/S8 to MIS 18/19. This article is protected by copyright. All rights reserved.
      PubDate: 2016-07-12T08:15:50.932254-05:
      DOI: 10.1002/2016GC006407
  • Density and P‐wave velocity structure beneath the Paraná Magmatic
           Province: Refertilization of an ancient lithospheric mantle
    • Authors: Carlos Chaves; Naomi Ussami, Jeroen Ritsema
      Abstract: We estimate density and P‐wave velocity perturbations in the mantle beneath the southeastern South America plate from geoid anomalies and P‐wave traveltime residuals to constrain the structure of the lithosphere underneath the Paraná Magmatic Province (PMP) and conterminous geological provinces. Our analysis shows a consistent correlation between density and velocity anomalies. The P‐wave speed and density are 1% and 15 kg/m3 lower, respectively, in the upper mantle under the Late Cretaceous to Cenozoic alkaline provinces, except beneath the Goiás Alkaline Province (GAP), where density (+20 kg/m3) and velocity (+0.5%) are relatively high. Underneath the PMP, the density is higher by about 50 kg/m3 in the north and 25 kg/m3 in the south, to a depth of 250 − 300 km. These values correlate with high‐velocity perturbations of +0.5% and +0.3%, respectively.Profiles of density perturbation versus depth in the upper mantle are different for the PMP and the adjacent Archean São Francisco (SFC) and Amazonian (AC) cratons. The Paleoproterozoic PMP basement has a high‐density root. The density is relatively low in the SFC and AC lithospheres. A reduction of density is a typical characteristic of chemically depleted Archean cratons. A more fertile Proterozoic and Phanerozoic subcontinental lithospheric mantle has a higher density, as deduced from density estimates of mantle xenoliths of different ages and composition.In conjunction with Re‐Os isotopic studies of the PMP basalts, chemical and isotopic analyses of peridodite xenoliths from the GAP in the northern PMP, and electromagnetic induction experiments of the PMP lithosphere, our density and P‐wave speed models suggest that the densification of the PMP lithosphere and flood basalt generation are related to mantle refertilization. Metasomatic refertilization resulted from the introduction of asthenospheric components from the mantle wedge above Proterozoic subduction zones, which surrounded the Paraná lithosphere. The high‐density PMP lithosphere is presently gravitationally unstable and prone to delamination. This article is protected by copyright. All rights reserved.
      PubDate: 2016-07-11T10:46:06.535257-05:
      DOI: 10.1002/2016GC006369
  • Automated cleaning of foraminifera tests before Mg/Ca analysis using a
           pipette robot
    • Abstract: The molar ratio of magnesium to calcium (Mg/Ca) in foraminiferal calcite is a widely used proxy for reconstructing past seawater temperatures. Thorough cleaning of tests is required before analysis to remove contaminant phases such as clay and organic matter. We have adapted a commercial pipette robot to automate an established cleaning procedure, the “Mg‐cleaning” protocol of Barker et al. [2003]. Efficiency of the automated 9‐step method was assessed through monitoring Al/Ca of trial samples (GeoB4420‐2 core‐catcher). Planktonic foraminifera Globigerinoides ruber, Globigerinoides sacculifer and Neogloboquadrina dutertrei from this sample gave Mg/Ca consistent with the habitat range of the three species, and 40 ‐ 60% sample recovery after cleaning. Comparison between manually cleaned and robot cleaned samples of G. ruber [white] from a sediment core (GeoB16602) showed good correspondence between the two methods for Mg/Ca (r =0.93, p
      PubDate: 2016-07-11T10:45:36.478556-05:
      DOI: 10.1002/2016GC006422
  • A distinct source and differentiation history for Kolumbo submarine
           volcano, Santorini volcanic field, Aegean arc
    • Authors: Martijn Klaver; Steven Carey, Paraskevi Nomikou, Ingrid Smet, Athanasios Godelitsas, Pieter Vroon
      Abstract: This study reports the first detailed geochemical characterization of Kolumbo submarine volcano in order to investigate the role of source heterogeneity in controlling geochemical variability within the Santorini volcanic field in the central Aegean arc. Kolumbo, situated 15 km to the northeast of Santorini, last erupted in 1650 AD and is thus closely associated with the Santorini volcanic system in space and time. Samples taken by remotely‐operated vehicle that were analyzed for major element, trace element and Sr‐Nd‐Hf‐Pb isotope composition include the 1650 AD and underlying K2 rhyolitic, enclave‐bearing pumices that are nearly identical in composition (73 wt. % SiO2, 4.2 wt. % K2O). Lava bodies exposed in the crater and enclaves are basalts to andesites (52‐60 wt. % SiO2). Biotite and amphibole are common phenocryst phases, in contrast with the typically anhydrous mineral assemblages of Santorini. The strong geochemical signature of amphibole fractionation and the assimilation of lower crustal basement in the petrogenesis of the Kolumbo magmas indicates that Kolumbo and Santorini underwent different crustal differentiation histories and that their crustal magmatic systems are unrelated. Moreover, the Kolumbo samples are derived from a distinct, more enriched mantle source that is characterized by high Nb/Yb (>3) and low 206Pb/204Pb (
      PubDate: 2016-07-11T10:40:26.89916-05:0
      DOI: 10.1002/2016GC006398
  • Multi‐scale approach to (micro)porosity quantification in continental
           spring carbonate facies Case study from the Cakmak quarry (Denizli,
    • Abstract: Carbonate spring deposits gained renewed interest as potential contributors to subsurface reservoirs and as continental archives of environmental changes. In contrast to their fabrics, petrophysical characteristics – and especially the importance of microporosity (< 1µm) – are less understood. This study presents the combination of advanced petrophysical and imaging techniques to investigate the pore network characteristics of three, common and widespread spring carbonate facies, as exposed in the Pleistocene Cakmak quarry (Denizli, Turkey): the extended Pond, the dipping crystalline Proximal Slope Facies and the draping Apron and Channel Facies deposits formed by encrustation of biological substrate. Integrating mercury injection capillary pressure, bulk and diffusion Nuclear Magnetic Resonance (NMR), NMR profiling and Brunauer–Emmett–Teller (BET) measurements with microscopy and micro‐computer tomography (µ‐CT), shows that NMR T2 distributions systematically display a single group of micro‐sized pore bodies, making up between 6 and 33% of the pore space (average NMR T2 cut‐off value: 62 ms). Micropore bodies are systematically located within cloudy crystal cores of granular and dendritic textures in all facies. The investigated properties therefore do not reveal differences in micropore size or shape with respect to more or less biology‐associated facies. The pore network of the travertine facies is distinctive in terms of (i) the percentage of microporosity, (ii) the connectivity of micropores with meso‐ to macropores, and (ii) the degree of heterogeneity at micro‐ and macroscale. Results show that an approach involving different NMR experiments provided the most complete view on the 3D pore network especially when microporosity and –connectivity are of interest. This article is protected by copyright. All rights reserved.
      PubDate: 2016-07-01T03:31:11.223346-05:
      DOI: 10.1002/2016GC006382
  • Helium Release During Shale Deformation: Experimental Validation
    • Authors: Stephen J. Bauer; W. Payton Gardner, Jason Heath
      Abstract: This work describes initial experimental results of helium tracer release monitoring during deformation of shale. Naturally occurring radiogenic 4He is present in high concentration in most shales. During rock deformation, accumulated helium could be released as fractures are created and new transport pathways are created. We present the results of an experimental study in which confined reservoir shale samples, cored parallel and perpendicular to bedding, which were initially saturated with helium to simulate reservoir conditions, are subjected to triaxial compressive deformation. During the deformation experiment, differential stress, axial and radial strains are systematically tracked. Release of helium is dynamically measured using a helium mass spectrometer leak detector. Helium released during deformation is observable at the laboratory scale and the release is tightly coupled to the shale deformation. These first measurements of dynamic helium release from rocks undergoing deformation show that helium provides information on the evolution of microstructure as a function of changes in stress and strain. This article is protected by copyright. All rights reserved.
      PubDate: 2016-07-01T03:31:06.4738-05:00
      DOI: 10.1002/2016GC006352
  • Magnetic signatures of serpentinization at ophiolite complexes
    • Abstract: We compare magnetic properties of 58 variably serpentinized peridotites from three ophiolite complexes (Pindos, Greece; Oman; Chenaillet, France) and the mid‐Atlantic ridge near the Kane fracture zone (MARK). The Pindos and Oman sites show low susceptibility and remanence (K
      PubDate: 2016-07-01T03:30:46.299299-05:
      DOI: 10.1002/2016GC006321
  • A computationally efficient spectral method for modeling core dynamics
    • Authors: P. Marti; M. A. Calkins, K. Julien
      Abstract: An efficient, spectral numerical method is presented for solving problems in a spherical shell geometry that employs spherical harmonics in the angular dimensions and Chebyshev polynomials in the radial direction. We exploit the three‐term recurrence relation for Chebyshev polynomials that renders all matrices sparse in spectral space. This approach is significantly more efficient than the collocation approach and is generalizable to both the Galerkin and tau methodologies for enforcing boundary conditions. The sparsity of the matrices reduces the computational complexity of the linear solution of implicit‐explicit timestepping schemes to $O(N)$operations, compared to O(N2) operations for a collocation method. The method is illustrated by considering several example problems of important dynamical processes in the Earth's liquid outer core. Results are presented from both fully nonlinear, time‐dependent numerical simulations and eigenvalue problems arising from the investigation of the onset of convection and the inertial wave spectrum. We compare the explicit and implicit temporal discretization of the Coriolis force; the latter becomes computationally feasible given the sparsity of the differential operators. We find that implicit treatment of the Coriolis force allows for significantly larger timestep sizes compared to explicit algorithms; for hydrodynamic and dynamo problems at an Ekman number of E = 10−5, timestep sizes can be increased by a factor of 3 to 16 times that of the explicit algorithm, depending on the order of the timestepping scheme. The implementation with explicit Coriolis force scales well to at least 2048 cores, while the implicit implementation scales to 512 cores. This article is protected by copyright. All rights reserved.
      PubDate: 2016-07-01T03:30:36.321858-05:
      DOI: 10.1002/2016GC006438
  • Interactions between active faulting, volcanism, and sedimentary processes
           at an island arc: Insights from Les Saintes channel, Lesser Antilles arc
    • Authors: F. Leclerc; N. Feuillet, C. Deplus
      Abstract: New high‐resolution marine geophysical data allow to characterize a large normal fault system in the Lesser Antilles arc, and to investigate the interactions between active faulting, volcanism, sedimentary and mass wasting processes. Les Saintes fault system is composed of several normal faults that form a 30 km wide half‐graben accommodating NE‐SW extension. It is bounded by the Roseau fault, responsible for the destructive Mw6.3 21/11/2004 earthquake. The Roseau fault has been identified from the island of Basse‐Terre to Dominica. It is thus 40 km long, and it could generate Mw7 earthquakes in the future. Several submarine volcanoes are also recognized. We show that the fault system initiated after the main volcanic construction and subsequently controls the emission of volcanic products. The system propagates southward through damage zones. At the tip of the damage zones, several volcanic cones were recently emplaced probably due to fissures opening in an area of stress increase. A two‐way interaction is observed between active faulting and sedimentary processes. The faults control the development of the main turbiditic system made of kilometer wide canyons, as well as the location of sediment ponding. In turn, erosion and sedimentation prevent scarp growth at the seafloor. Faulting also enhances mass‐wasting processes. Since its initiation, the fault system has consequently modified the morphologic evolution of the arc through perturbation of the sedimentary processes and localization of the more recent volcanic activity. This article is protected by copyright. All rights reserved.
      PubDate: 2016-06-30T03:50:45.197572-05:
      DOI: 10.1002/2016GC006337
  • Volcano Deformation Survey over the Northern and Central Andes with ALOS
           InSAR Time Series
    • Authors: Anieri M. Morales Rivera; Falk Amelung, Patricia Mothes
      Abstract: We use ALOS‐1 Interferometric Synthetic Aperture Radar data spanning the period of 2007‐2011 to obtain time‐dependent ground deformation data over all of the volcanoes in Colombia, Ecuador and Peru. We detect deformation on or near the proximity of Galeras, Reventador, Tungurahua, Guagua Pichincha, Sangay, and Cerro Auquihuato volcanoes, uncovering previously undocumented deformation in the latter three. Deformation is attributed to changes in pressurization of the volcanic systems (Galeras, Tungurahua, Guagua Pichincha, and Cerro Auquihuato), subsidence associated with flow deposits (Reventador), and flank creep (Sangay). Our models suggest that the pressure sources are located at depths of ∼1 to 6 km from the surface, indicating that the measurable deformation within our data is restricted to shallow magma chambers and hydrothermal systems. This article is protected by copyright. All rights reserved.
      PubDate: 2016-06-23T03:51:00.732361-05:
      DOI: 10.1002/2016GC006393
  • Microearthquake activity, lithospheric structure and deformation modes at
           an amagmatic ultraslow spreading Southwest Indian Ridge segment
    • Authors: Florian Schmid; Vera Schlindwein
      Abstract: While nascent oceanic lithosphere at slow‐ to fast spreading mid‐ocean ridges (MOR) is relatively well studied, much less is known about the lithospheric structure and properties at ultraslow MORs. Here we present microearthquake data from a one‐year ocean bottom seismometer deployment at the amagmatic, Oblique Supersegment of the ultraslow spreading Southwest Indian Ridge. A refraction seismic experiment was performed to constrain upper lithosphere P‐velocities and results were used to construct a 1D‐velocity model for earthquake location. Earthquake foci were located individually and subsequently relocated relative to each other to sharpen the image of seismically active structures. Frequent earthquake activity extends to 31 km beneath the sea floor, indicating an exceptionally thick brittle lithosphere and an undulating brittle‐ductile transition that implies significant variations in the along‐axis thermal structure of the lithosphere. We observe a strong relation between petrology, microseismicity distribution and topography along the ridge axis: Peridotite dominated areas associate with deepest hypocenters, vast volumes of lithosphere that deforms aseismically as a consequence of alteration and the deepest axial rift valley. Areas of basalt exposure correspond to shallower hypocenters, shallower and more rugged axial seafloor. Focal mechanisms deviate from pure extension and are spatially variable. Earthquakes form an undulating band of background seismicity and do not delineate discrete detachment faults as common on slow spreading ridges. Instead, the seismicity band sharply terminates to the south, immediately beneath the rift boundary. Considering the deep alteration, large steep boundary faults might be present but are entirely aseismic. This article is protected by copyright. All rights reserved.
      PubDate: 2016-06-23T03:50:42.14911-05:0
      DOI: 10.1002/2016GC006271
  • Hydrostatic pressure effect on magnetic hysteresis parameters of
           pseudo‐single‐domain magnetite
    • Authors: Masahiko Sato; Yuhji Yamamoto, Takashi Nishioka, Kazuto Kodama, Nobutatsu Mochizuki, Hideo Tsunakawa
      Abstract: This paper reports the first in situ magnetic hysteresis measurements of pseudo‐single‐domain (PSD) magnetite under high pressure up to 1 GPa. The magnetic hysteresis measurements of stoichiometric PSD magnetite samples under hydrostatic pressure were carried out using a piston‐cylinder high‐pressure cell, and the pressure dependence of the hysteresis parameters of PSD magnetite was calculated from the hysteresis curves. It was found that coercivity (Bc) increases with increasing pressure as a quadratic function up to 1 GPa by ∼90%, which is different from the pressure dependences of Bc of multidomain and single‐domain magnetites. Coercivity of remanence also increases as a quadratic function, and saturation remanence (Mrs) increases with pressure up to 0.5 GPa by ∼20% until reaching saturation. In contrast, saturation magnetization is constant up to 1 GPa. The approximate demagnetizing factor calculated from the ratio Bc/Mrs increases with increasing pressure, suggesting that the number of lamellar domains increases with increasing pressure. The number of lamellar domains and domain wall width are theoretically estimated to increase under high pressure due to the changes in magnetostriction, elastic, and magnetocrystalline anisotropy constants, and these changes in magnetic domain structure should relate to the changes in the magnetic properties of PSD magnetite. This article is protected by copyright. All rights reserved.
      PubDate: 2016-06-23T03:45:54.097589-05:
      DOI: 10.1002/2016GC006406
  • Geochemical records from loess deposits in Japan over the last 210 kyr:
           Lithogenic source changes and paleoclimatic indications
    • Abstract: The generation, transport, and accumulation of tropospheric dust have changed with the paleoclimatic changes of the Quaternary Period. Such dust has accumulated in Japan ∼3000 km leeward of the source deserts in China. We analyzed the fractions of windblown fine quartz and bulk major and trace elements, and Sr–Nd–Pb isotopic compositions of loess in SW Japan deposited over the past 210 kyr. The results indicated extensive accumulation of tropospheric dust mixed with tephra fragments derived from the nearby Daisen volcano. The accumulation rate of fine quartz and selected elemental/isotopic compositions can be used as climatic proxies that reflect greater accumulation of dust in times of colder climate. Chemical indices for weathering show enhanced effects of weathering during times of warmer climate. The trace element compositions of the loess deposits are surprisingly similar to those of the Chinese loess, hemipelagic sediments in the Sea of Japan and the western Pacific Ocean, and the distal Chinese dust found in Canada. This similarity indicates that the loess dust shares major fractions of these fine‐grained sediments, and that geochemical fractionation during the transport was limited. The Sr–Nd–Pb isotope compositions of the SW Japan loess indicate an origin predominantly in the Gobi Desert. The high‐latitude Pacific sediments and high‐latitude dust in Canada also show the same signature. However, isotopic compositions of samples from the southern Chinese loess plateau and mid‐ to low‐latitude Pacific sediments are largely derived from the Taklimakan desert, which indicates different delivery pathways of the tropospheric dusts. This article is protected by copyright. All rights reserved.
      PubDate: 2016-06-22T18:55:40.388058-05:
      DOI: 10.1002/2016GC006322
  • Applications of detrital geochronology and thermochronology from glacial
           deposits to the Paleozoic and Mesozoic thermal history of the Ross
           Embayment, Antarctica
    • Authors: Bethany Welke; Kathy Licht, Andrea Hennessy, Sidney Hemming, Elizabeth Pierce Davis, Christine Kassab
      Abstract: Till from moraines at the heads of six major outlet glaciers in the Transantarctic Mountains (TAM) and from till beneath three West Antarctic ice streams have a ubiquitous zircon U‐Pb age population spanning the time of the Ross/Pan‐African Orogenies (610‐475 Ma). Geo‐ and thermochronology of detrital minerals in these Antarctic glacial tills reveal two different thermal histories for the central and southern TAM. Double‐dating of the zircons reveals a geographically widespread (U‐Th)/He (ZHe) population of 180‐130 Ma in most of the till samples. Sandstone outcrops at Shackleton Glacier, and three Beacon Supergroup sandstone clasts from three moraines, have ZHe ages that fall entirely within this range. The similar population and proximity of many of the till samples to Beacon outcrops lead us to suggest that this extensive ZHe population in the tills is derived from Beacon Supergroup rocks and reflects the thermal response of the Beacon Basin to the breakup of Gondwana. A second population of older (>200 Ma) ZHe ages in tills at the head of Byrd, Nimrod and Reedy Glaciers. For the tills at the head of the Nimrod and Byrd Glacier, integrating the double‐dated zircon results with 40Ar/39Ar of hornblende, muscovite and biotite, and U‐Pb and (U‐Th‐Sm)/He double‐dates on apatite yields a typical pattern of early rapid orogenic cooling (∼4‐10˚C/my) 590‐475 Ma after the emplacement of the Granite Harbour Intrusives. Low temperature thermochronometers at these sites yield variable but quite old ages (ZHe 480‐70 Ma and AHe 200‐70 Ma) that require a long history at low temperature. This article is protected by copyright. All rights reserved.
      PubDate: 2016-06-22T03:40:29.030063-05:
      DOI: 10.1002/2015GC005941
  • A new Bayesian Event Tree tool to track and quantify volcanic unrest and
           its application to Kawah Ijen volcano
    • Authors: Roberto Tonini; Laura Sandri, Dmitri Rouwet, Corentin Caudron, Warner Marzocchi, Pak Suparjan
      Abstract: Although most of volcanic hazard studies focus on magmatic eruptions, volcanic hazardous events can also occur when no migration of magma can be recognized. Examples are tectonic and hydrothermal unrest that may lead to phreatic eruptions. Recent events (e.g. Ontake eruption on September 2014) have demonstrated that phreatic eruptions are still hard to forecast, despite being potentially very hazardous. For these reasons, it is of paramount importance to identify indicators that define the condition of non‐magmatic unrest, in particular for hydrothermal systems. Often, this type of unrest is driven by movement of fluids, requiring alternative monitoring setups, beyond the classical seismic‐geodetic‐geochemical architectures. Here we present a new version of the probabilistic BET (Bayesian Event Tree) model, specifically developed to include the forecasting of non‐magmatic unrest and related hazards. The structure of the new event tree differs from the previous schemes by adding a specific branch to detail non‐magmatic unrest outcomes. A further goal of this work consists in providing a user‐friendly, open‐access, and straightforward tool to handle the probabilistic forecast and visualize the results as possible support during a volcanic crisis. The new event tree and tool are here applied to Kawah Ijen stratovolcano, Indonesia, as exemplificative application. In particular, the tool is set on the basis of monitoring data for the learning period 2000‐2010, and is then blindly applied to the test period 2010‐2012, during which significant unrest phases occurred. This article is protected by copyright. All rights reserved.
      PubDate: 2016-06-21T05:40:26.566073-05:
      DOI: 10.1002/2016GC006327
  • Hydraulic properties of samples retrieved from the Wenchuan earthquake
           fault scientific drilling project hole‐1 (WFSD‐1) and the surface
    • Abstract: In this study, we report the hydraulic properties of samples recovered from the first borehole of the Wenchuan earthquake Fault Scientific Drilling and from outcrops associated with the surface rupture zone of the 2008 Wenchuan earthquake. Compositional and microstructural analyses have also been performed on selected samples. Using the pore pressure oscillation method, the permeability measurements show that 1) fault gouge samples have low permeabilities, decreasing from 2 × 10−18 m2 at an effective pressure (Pe) of 10 MPa (equivalent to an in‐situ depth of 600 m) to 9 × 10−21 m2 at 155 MPa. 2) Intact and cemented samples are impermeable with permeabilities less than 2 × 10−20 m2 at 10 MPa. 3) Fractured samples have variable permeabilities, ranging from 3 × 10−15 to 1 × 10−20 m2 at 10 MPa, and are most insensitive to changes in the effective pressure. 4) Granitic cataclasites have a moderate permeability at low pressure (i.e. 10−16 – 10−17 m2 at 10 MPa); which decreases rapidly with increasing Pe. Hydraulic conduction of the fault is believed to be influenced by the permeability of the fractures developed, which is controlled by the density, aperture and/or connectivity of the fractures. Microstructural and compositional analyses of the samples indicate that the fault zone heals through chemically‐mediated fracture closure related to mineral precipitation, possibly assisted by pressure solution of stressed fracture asperities. Although other weakening mechanisms remain possible, our lab measurements combined with numerical modeling reveal that thermal/thermochemical pressurization, perhaps leading to gouge fluidization, played an important role in the dynamic weakening of the Wenchuan earthquake, at least in the study area. This article is protected by copyright. All rights reserved.
      PubDate: 2016-06-20T19:10:51.100029-05:
      DOI: 10.1002/2016GC006376
  • Geodetically constrained models of viscoelastic stress transfer and
           earthquake triggering along the North Anatolian fault
    • Authors: Phoebe M.R. DeVries; Plamen G. Krastev, Brendan J. Meade
      Abstract: Over the past 80 years, 8 MW>6.7 strike‐slip earthquakes west of 40º longitude have ruptured the North Anatolian fault (NAF) from east to west. The series began with the 1939 Erzincan earthquake in eastern Turkey, and the most recent 1999 MW=7.4 Izmit earthquake extended the pattern of ruptures into the Sea of Marmara in western Turkey. The mean time between seismic events in this westward progression is 8.5±11 years (67% confidence interval), much greater than the timescale of seismic wave propagation (seconds to minutes). The delayed triggering of these earthquakes may be explained by the propagation of earthquake‐generated diffusive viscoelastic fronts within the upper mantle that slowly increase the Coulomb failure stress change (ΔCFS) at adjacent hypocenters. Here we develop three‐dimensional stress transfer models with an elastic upper crust coupled to a viscoelastic Burgers rheology mantle. Both the Maxwell (ηM=4x1018‐1x1019 Pa·s) and Kelvin (ηK=1x1018‐1x1019 Pa·s) viscosities are constrained by studies of geodetic observations before and after the 1999 Izmit earthquake. We combine this geodetically constrained rheological model with the observed sequence of large earthquakes since 1939 to calculate the time evolution of ΔCFS changes along the North Anatolian fault due to viscoelastic stress transfer. Apparent threshold values of mean ΔCFS at which the earthquakes in the eight decade sequence occur are between ∼0.02 to ∼3.15 MPa and may exceed the magnitude of static ΔCFS values by as much as 177%. By 2023, we infer that the mean time‐dependent stress change along the northern NAF strand in the Marmara Sea near Istanbul, which may have previously ruptured in 1766, may reach the mean apparent time‐dependent stress thresholds of the previous NAF earthquakes. This article is protected by copyright. All rights reserved.
      PubDate: 2016-06-20T10:50:39.394223-05:
      DOI: 10.1002/2016GC006313
  • The last 2 Myr of accretionary wedge construction in the central Hikurangi
           margin (North Island, New Zealand): Insights from structural modeling
    • Abstract: Three depth‐converted and geologically interpreted seismic profiles provide a clear image of the offshore outer accretionary wedge associated with oblique subduction of the Pacific Plate beneath the central Hikurangi margin. Plio‐Quaternary turbidites deposited over the pelagic cover sequence of the Hikurangi Plateau have been accreted to the margin by imbrication along E‐verging thrust faults that propagated up‐section from the plate boundary décollement. Growth stratigraphy of piggy‐back basins and thrusting of progressively younger horizons trace the eastward advance of the leading thrust front over c. 60 km in the last 2 Myr. Moderate internal shortening of fault‐bounded blocks typically 4‐8 km wide reflects rapid creation of thrust faults, with some early formed faults undergoing out‐of‐sequence reactivation to maintain critical wedge taper. Multi‐stage structural restorations show that forward progression of shortening involves: (1) initial development of a c. 10‐25 km wide “proto‐thrust” zone, comprising conjugate sets of moderately to steeply dipping low‐displacement (c. 10‐100 m) reverse faults; and (2) growth of thrust faults that exploit some of the early proto‐thrust faults and propagate up‐section with progressive break‐through of folds localized above the fault tips. The youngest, still unbreached folds deform the present‐day seafloor. Progressive retro‐deformations show that macroscopic thrust faults and folds account for less than 50% of the margin‐perpendicular shortening imposed by plate convergence. Arguably, significant fractions of the missing components can be attributed to meso‐ and microscopic scale layer‐parallel shortening within the wedge, in the proto‐thrust zones, and in the outer décollement zone. This article is protected by copyright. All rights reserved.
      PubDate: 2016-06-17T18:26:21.433175-05:
      DOI: 10.1002/2016GC006341
  • Causes of unrest at silicic calderas in the East African Rift: New
           constraints from InSAR and soil‐gas chemistry at Aluto volcano, Ethiopia
    • Authors: William Hutchison; Juliet Biggs, Tamsin A. Mather, David M. Pyle, Elias Lewi, Gezahegn Yirgu, Stefano Caliro, Giovanni Chiodini, Laura E. Clor, Tobias P. Fischer
      Abstract: Restless silicic calderas present major geological hazards, and yet many also host significant untapped geothermal resources. In East Africa this poses a major challenge, although the calderas are largely unmonitored their geothermal resources could provide substantial economic benefits to the region. Understanding what causes unrest at these volcanoes is vital for weighing up the opportunities against the potential risks. Here we bring together new field and remote sensing observations to evaluate causes of ground deformation at Aluto, a restless silicic volcano located in the Main Ethiopian Rift (MER). Interferometric Synthetic Aperture Radar (InSAR) data reveal the temporal and spatial characteristics of a ground deformation episode that took place between 2008 and 2010. Deformation time‐series reveal pulses of accelerating uplift that transition to gradual long‐term subsidence, and analytical models support inflation source depths of ∼5 km. Gases escaping along the major fault zone of Aluto show high CO2 flux, and a clear magmatic carbon signature (CO2–δ13C of −4.2 to −4.5 ‰). This provides compelling evidence that the magmatic and hydrothermal reservoirs of the complex are physically connected. We suggest that a coupled magmatic‐hydrothermal system can explain the uplift‐subsidence signals. We hypothesize that magmatic fluid injection and/or intrusion in the cap of the magmatic reservoir drives edifice wide inflation while subsequent deflation is related to magmatic degassing and depressurization of the hydrothermal system. These new constraints on the plumbing of Aluto yield important insights into the behaviour of rift volcanic systems and will be crucial for interpreting future patterns of unrest. This article is protected by copyright. All rights reserved.
      PubDate: 2016-06-17T18:25:54.895516-05:
      DOI: 10.1002/2016GC006395
  • Organochemical characteristics of carbonaceous materials as indicators of
           heat recorded on an ancient plate‐subduction fault
    • Authors: S. Kaneki; T. Hirono, H. Mukoyoshi, Y. Sampei, M. Ikehara
      Abstract: Coseismic shear stress and slip distance affect subduction‐related earthquake processes. They need to be understood to evaluate the earthquake's mechanism, and the tsunami generation potential near trenches. The amount of frictional heat generated depends on the shear stress and slip distance, which are therefore able to be derived from the temperature recorded in the fault. Here we developed a new temperature proxy for carbonaceous materials by performing spectroscopic, thermogravimetric, and organic elemental analyses in conjunction with heating experiments. We found marked anomalies in the infrared and Raman spectra and atomic compositions of carbonaceous materials retrieved from the slip zone of an ancient megasplay fault in the Cretaceous Shimanto accretionary complex, Japan: the infrared spectra show extinction of aliphatic C–H bonding and very weak aromatic C=C bonding, and the Raman spectra show a slightly elevated ratio of disordered band intensity to graphitic band intensity and relatively low H/O and O/C ratios. These correlate well with the spectral features of host‐rock carbonaceous materials after heating to 600°C. Thus, we conclude that the slip zone experienced a temperature of 600°C during a past earthquake event, indicating coseismic slip of 2–9 m, which could have generated a large tsunami if the ruptures propagated to the seafloor. This article is protected by copyright. All rights reserved.
      PubDate: 2016-06-17T18:20:38.48975-05:0
      DOI: 10.1002/2016GC006368
  • Seasonal changes in magnetic parameters of sediments with changing redox
           conditions in Hiroshima Bay, Japan
    • Authors: Noriko Kawamura; Yuka Amano, Naoto Ishikawa
      Abstract: To describe and interpret the relationship between spatial and seasonal changes in the sedimentary environment of nearshore sediments and their magnetic properties, magnetic and geochemical analyses were performed on sediment samples from three stations in Hiroshima Bay, Japan. Vertical stratification of the water column in the bay changes throughout the year, and magnetic hysteresis parameters and mineralogy in the bay sediments vary in response to changes in redox conditions of bottom waters. Magnetite and hematite are present year‐round at all stations. The presence of maghemitized magnetite is inferred at a station located at the entrance to the bay. Greigite is recognized at all stations in September 2011 but is not found at the entrance to the bay when water column stratification is disturbed from October 2011. The presence of maghemite and goethite is inferred at two stations in the inner bay when the sedimentary environment is oxic. The remanent coercivity/coercivity ratio (Hcr/Hc) also varies, both spatially and temporally, which reflects changes in magnetic mineralogy. Increased of Hcr/Hc values are likely to be caused by goethite and/or maghemite formation when water column stratification is disturbed and the seafloor is oxic. Concentration‐dependent magnetic parameters do not respond to seasonal changes in the redox conditions of bottom waters. Reaction times and/or changes in chemical and physical conditions may be insufficient to affect these parameters in the sediments of Hiroshima Bay. This article is protected by copyright. All rights reserved.
      PubDate: 2016-06-17T18:14:04.667729-05:
      DOI: 10.1002/2016GC006347
  • Deformation in the mantle wedge associated with Laramide flat‐slab
    • Authors: Whitney M. Behr; Douglas Smith
      Abstract: Laramide crustal deformation in the Rocky Mountains of the west‐central United States is often considered to relate to a narrow segment of shallow subduction of the Farallon slab, but there is no consensus as to how deformation along the slab–mantle lithosphere interface was accommodated. Here we investigate deformation in mantle rocks associated with hydration and shear above the flat slab at its contact with the base of the North American plate. The rocks we focus on are deformed, hydrated, ultramafic inclusions hosted within diatremes of the Navajo Volcanic Field in the central Colorado Plateau that erupted during the waning stages of the Laramide orogeny. We document a range of deformation textures, including granular peridotites, porphyroclastic peridotites, mylonites, and cataclasites, which we interpret to reflect different proximities to a slab–mantle‐interface shear zone. Mineral assemblages and chemistries constrain deformation to hydrous conditions in the temperature range ∼550‐750°C. Despite the presence of hydrous phyllosilicates in modal percentages of up to 30%, deformation was dominated by dislocation creep in olivine. The mylonites exhibit an uncommon lattice preferred orientation (LPO) in olivine, known as B‐type LPO in which the a‐axes are aligned perpendicular to the flow direction. The low temperature, hydrated setting in which these fabrics formed are consistent with laboratory experiments that indicate B‐type LPOs form under conditions of high stress and high water contents; furthermore, the mantle wedge context of these LPOs is consistent with observations of trench‐parallel anisotropy in the mantle wedge above many modern subduction zones. Differential stress magnitudes in the mylonitic rocks estimated using paleopiezometry range from 290 and 444 MPa, and calculated effective viscosities using a wet olivine flow law are on the order of 1019‐1023 Pa s. The high stress magnitudes, high effective viscosities and high strains recorded in these rocks are consistent with models that invoke significant basal shear tractions as contributing to Laramide uplift and contraction in the continental interior. This article is protected by copyright. All rights reserved.
      PubDate: 2016-06-17T18:13:49.674399-05:
      DOI: 10.1002/2016GC006361
  • The relationship between eruptive activity, flank collapse, and sea level
           at volcanic islands: A long‐term (>1 Ma) record offshore Montserrat,
           Lesser Antilles
    • Abstract: Hole U1395B, drilled southeast of Montserrat during Integrated Ocean Drilling Program Expedition 340, provides a long (>1 Ma) and detailed record of eruptive and mass‐wasting events (>130 discrete events). This record can be used to explore the temporal evolution in volcanic activity and landslides at an arc volcano. Analysis of tephra fall and volcaniclastic turbidite deposits in the drill cores reveals three heightened periods of volcanic activity on the island of Montserrat (∼930 ka to ∼900 ka, ∼810 ka to ∼760 ka, and ∼190 ka to ∼120 ka) that coincide with periods of increased volcano instability and mass‐wasting. The youngest of these periods marks the peak in activity at the Soufrière Hills volcano. The largest flank collapse of this volcano (∼130 ka) occurred towards the end of this period, and two younger landslides also occurred during a period of relatively elevated volcanism. These three landslides represent the only large (>0.3 km3) flank collapses of the Soufrière Hills edifice, and their timing also coincides with periods of rapid sea‐level rise (>5 m/ka). Available age data from other island arc volcanoes suggests a general correlation between the timing of large landslides and periods of rapid sea‐level rise, but this is not observed for volcanoes in intra‐plate ocean settings. We thus infer that rapid sea‐level rise may modulate the timing of collapse at island arc volcanoes, but not in larger ocean‐island settings. This article is protected by copyright. All rights reserved.
      PubDate: 2016-06-17T03:00:33.919227-05:
      DOI: 10.1002/2015GC006053
  • Magma plumbing for the 2014–2015 Holuhraun eruption, Iceland
    • Abstract: The 2014‐15 Holuhraun eruption on Iceland was located within the Askja fissure swarm, but was accompanied by caldera subsidence in the Bárðarbunga central volcano 45 km to the southwest. Geophysical monitoring of the eruption identified a seismic swarm that migrated from Bárðarbunga to the Holuhraun eruption site over the course of 2 weeks. In order to better understand this lateral connection between Bárðarbunga and Holuhraun, we present mineral textures and compositions, mineral‐melt‐equilibrium calculations, whole rock and trace element data, and oxygen‐isotope ratios for selected Holuhraun samples. The Holuhraun lavas are compositionally similar to recorded historical eruptions from the Bárðarbunga volcanic system, but are distinct from the historical eruption products of the nearby Askja system. Thermobarometry calculations indicate a polybaric magma plumbing system for the Holuhraun eruption, wherein clinopyroxene and plagioclase crystallized at average depths of ∼ 17 km and ∼ 5 km, respectively. Crystal resorption textures and oxygen isotope variations imply that this multi‐level plumbing system facilitated magma mixing and assimilation of low‐δ18O Icelandic crust prior to eruption. In conjunction with the existing geophysical evidence for lateral migration, our results support a model of initial vertical magma ascent within the Bárðarbunga plumbing system followed by lateral transport of aggregated magma batches within the upper crust to the Holuhraun eruption site. This article is protected by copyright. All rights reserved.
      PubDate: 2016-06-16T03:41:27.752798-05:
      DOI: 10.1002/2016GC006317
  • Quantifying melt production and degassing rate at mid‐ocean ridges from
           global mantle convection models with plate motion history
    • Authors: Mingming Li; Benjamin Black, Shijie Zhong, Michael Manga, Maxwell L. Rudolph, Peter Olson
      Abstract: The Earth's surface volcanism exerts first‐order controls on the composition of the atmosphere and the climate. On Earth, the majority of surface volcanism occurs at mid‐ocean ridges. In this study, based on the dependence of melt fraction on temperature, pressure and composition, we compute melt production and degassing rate at mid‐ocean ridges from three dimensional global mantle convection models with plate motion history as the surface velocity boundary condition. By incorporating melting in global mantle convection models, we connect deep mantle convection to surface volcanism, with deep and shallow mantle processes internally consistent. We compare two methods to compute melt production: a tracer method and an Eulerian method. Our results show that melt production at mid‐ocean ridges is mainly controlled by surface plate motion history, and that changes in plate tectonic motion, including plate reorganizations, may lead to significant deviation of melt production from the expected scaling with seafloor production rate. We also find a good correlation between melt production and degassing rate beneath mid‐ocean ridges. The calculated global melt production and CO2 degassing rate at mid‐ocean ridges varies by as much as a factor of 3 over the past 200 Myr. We show that mid‐ocean ridge melt production and degassing rate would be much larger in the Cretaceous, and reached maximum values at ∼150‐120 Ma. Our results raise the possibility that warmer climate in the Cretaceous could be due in part to high magmatic productivity and correspondingly high outgassing rates at mid‐ocean ridges during that time. This article is protected by copyright. All rights reserved.
      PubDate: 2016-06-16T03:41:15.946194-05:
      DOI: 10.1002/2016GC006439
  • Slab mantle dehydrates beneath Kamchatka: yet recycles water into the deep
    • Abstract: The subduction of hydrated slab mantle is the most important and yet weakly constrained factor in the quantification of the Earth's deep geologic water cycle. The most critical unknowns are the initial hydration state and the dehydration behavior of the subducted oceanic mantle. Here we present a combined thermomechanical, thermodynamic and geochemical model of the Kamchatka subduction zone that indicates significant dehydration of subducted slab mantle beneath Kamchatka.Evidence for the subduction of hydrated oceanic mantle comes from across‐arc trends of boron concentrations and isotopic compositions in arc volcanic rocks. Our thermodynamic‐geochemical models successfully predict the complex geochemical patterns and the spatial distribution of arc volcanoes in Kamchatka assuming the subduction of hydrated oceanic mantle. Our results show that water content and dehydration behavior of the slab mantle beneath Kamchatka can be directly linked to compositional features in arc volcanic rocks.Depending on hydration depth of the slab mantle, our models yield water recycling rates between 1.1 x 103 and 7.4 x 103 TgMa−1km−1 corresponding to values between 0.75 x 106 and 5.2 x 106 TgMa−1 for the entire Kamchatkan subduction zone. These values are up to one order of magnitude lower than previous estimates for Kamchatka, but clearly show that subducted hydrated slab mantle significantly contributes to the water budget in the Kamchatkan subduction zone. This article is protected by copyright. All rights reserved.
      PubDate: 2016-06-16T03:41:00.555718-05:
      DOI: 10.1002/2016GC006335
  • Asteroid bombardment and the core of Theia as possible sources for the
           Earth's late veneer component
    • Authors: Norman H. Sleep
      Abstract: The silicate Earth contains Pt‐group elements in roughly chondritic relative ratios, but with absolute concentrations
      PubDate: 2016-06-15T10:33:28.619465-05:
      DOI: 10.1002/2016GC006305
  • S‐to‐P heterogeneity ratio in the lower mantle and
           thermo‐chemical implications
    • Authors: Andrea Tesoniero; Fabio Cammarano, Lapo Boschi
      Abstract: We evaluate the thermo‐chemical state of the lower mantle by analysing the differences in the pattern of heterogeneity between shear and compressional velocity variations and the S‐to‐P heterogeneity ratio (RS/P = δlnVS/δlnVP) as mapped in our model SPani and in alternative joint models. Robust structural differences between VP and VS evidence the presence of compositional heterogeneity within the two Large Low Shear Velocity Provinces (LLSVPs). We find also an increasing decorrelation with depth that can be associated with compositional layering of the LLSVPs. In addition, our model shows heterogeneity in the transition zone and mid mantle by complex morphology of subducting slabs and further differences between VP and VS that point to an unexpected heterogeneous lower mantle. Precise estimates of compositional heterogeneities are not yet affordable because of the difficulty to provide quantitative measure of RS/P, making it difficult to use this ratio to evaluate chemical heterogeneity. For instance, RS/P global median value () drops from ∼2.8 to ∼1.9, at 2500km depth when the VP component of SPani is replaced by a VP model resulting from a differently regularized inversion and obtaining an equally good data fit. An increase of 20% of the SPani VP anomalies also drastically reduces without significantly degrading the data fit. Noise in model parameters also leads to overestimate RS/P in the two LLSVPs as we show with synthetic tests. Additional mineral physics uncertainties for compositional effects on RS/P and for the conversion of δlnVS and δlnVP into density further complicates a precise chemical interpretation. This article is protected by copyright. All rights reserved.
      PubDate: 2016-06-13T03:25:36.859967-05:
      DOI: 10.1002/2016GC006293
  • Overriding plate, mantle wedge, slab, and sub‐slab contributions to
           seismic anisotropy beneath the northern Central Andean Plateau
    • Authors: Maureen D. Long; C. Berk Biryol, Caroline M. Eakin, Susan L. Beck, Lara S. Wagner, George Zandt, Estella Minaya, Hernando Tavera
      Abstract: The Central Andean Plateau, the second‐highest plateau on Earth, overlies the subduction of the Nazca Plate beneath the central portion of South America. The origin of the high topography remains poorly understood, and this puzzle is intimately tied to unanswered questions about processes in the upper mantle, including possible removal of the overriding plate lithosphere and interaction with the flow field that results from the driving forces associated with subduction. Observations of seismic anisotropy can provide important constraints on mantle flow geometry in subduction systems. The interpretation of seismic anisotropy measurements in subduction settings can be challenging, however, because different parts of the subduction system may contribute, including the overriding plate, the mantle wedge above the slab, the slab itself, and the deep upper mantle beneath the slab. Here we present measurements of shear wave splitting for core phases (SKS, SKKS, PKS, and sSKS), local S, and source‐side teleseismic S phases that sample the upper mantle beneath southern Peru and northern Bolivia, relying mostly on data from the CAUGHT experiment. We find evidence for seismic anisotropy within most portions of the subduction system, although the overriding plate itself likely makes only a small contribution to the observed delay times. Average fast orientations generally trend roughly trench‐parallel to trench‐oblique, contradicting predictions from the simplest two‐dimensional flow models and olivine fabric scenarios. Our measurements suggest complex, layered anisotropy beneath the northern portion of the Central Andean Plateau, with significant departures from a two‐dimensional mantle flow regime. This article is protected by copyright. All rights reserved.
      PubDate: 2016-06-09T18:50:59.171753-05:
      DOI: 10.1002/2016GC006316
  • Improving the method of low‐temperature anisotropy of magnetic
           susceptibility (LT‐AMS) measurements in air
    • Authors: R. Issachar; T. Levi, V. Lyakhovsky, S. Marco, R. Weinberger
      Abstract: This study examines the limitations of the method of low‐temperature anisotropy of magnetic susceptibility (LT‐AMS) measurements in air and presents technical improvements that significantly reduce the instrumental drift and measurement errors. We analyzed the temperature profile of porous chalk core after cooling in liquid nitrogen and found that the average temperature of the sample during the LT‐AMS measurement in air is higher than 77K and close to 92K. This analysis indicates that the susceptibility of the paramagnetic minerals are amplified by a factor ∼3.2 relative to that of room temperature AMS (RT‐AMS). In addition, it was found that liquid nitrogen was absorbed in the samples during immersing and contributed diamagnetic component of ∼‐9 × 10−6 SI to the total mean susceptibility. We showed that silicone sheet placed around and at the bottom of the measuring coil is an effective thermal protection, preventing instrument drift by the cold sample. In this way, the measuring errors of LT‐AMS reduced to the level of RT‐AMS, allowing accurate comparison with standard AMS measurements. We examined the applicability of the LT‐AMS measurements on chalk samples that consist
      PubDate: 2016-06-08T10:45:27.662176-05:
      DOI: 10.1002/2016GC006339
  • On edge melting under the Colorado Plateau margin
    • Abstract: Asthenosphere beneath the relatively thin lithosphere of the Basin and Range province appears to be juxtaposed in step‐like fashion against the Colorado Plateau's thick lithospheric keel. Primary to near‐primary basalts are found above this edge, in the San Francisco‐Morman Mountain volcanic fields, north central Arizona, western USA. We show that at least two distinct peridotite‐dominated mantle end‐members contributed to the origin of the basalts. One has paired Nd and Hf isotopic characteristics that cluster near the mantle array and trace element patterns as expected for melts generated in the asthenosphere, possibly in the presence of garnet. The second has isotopic compositions displaced above the εHf ‐ εNd mantle array which, together with its trace element characteristics, indicate contributions from hydrogenous sediments and/or melt (carbonatite or silicate)‐related metasomatism. Melt equilibration temperatures obtained from Si‐ and Mg‐thermobarometry are mostly 1340‐1425°C and account for the effects of water (assumed to be 2 wt.%) and estimated CO2 (variable). Melt equilibration depths cluster at the inferred location of the lithosphere‐asthenosphere boundary at ∼70‐75 km beneath the southwestern margin of the Colorado Plateau but scatter to somewhat greater values (∼100 km). Melt generation may have initiated in or below the garnet‐spinel facies transition zone and continued as mantle and/or melts upwelled, assimilating and sometimes equilibrating with shallower contaminated mantle, until melts were finally extracted. This article is protected by copyright. All rights reserved.
      PubDate: 2016-06-06T10:20:37.942555-05:
      DOI: 10.1002/2016GC006349
  • Microstructural and geochemical constraints on the evolution of deep arc
    • Authors: Emily J. Chin; Vincent Soustelle, Greg Hirth, Alberto Saal, Seth C. Kruckenberg, John Eiler
      Abstract: Mantle xenoliths from the Sierra Nevada, California, USA sampled a vertical column (60 – 120 km) of lithosphere that formed during Mesozoic continental arc magmatism. This lithosphere experienced an anti‐clockwise P‐T‐t path resulting in rapid cooling that effectively “quenched in” features inherited from earlier high‐temperature conditions. Here, we combine new mineral chemistry data (water, trace element, and major element concentrations) with mineral crystallographic preferred orientations (CPO) to investigate the relationship between melt infiltration and deformation. The peridotites record a refertilization trend with increasing depth, starting from shallow, coarse‐protogranular, less‐melt infiltrated spinel peridotite with strong, orthorhombic olivine CPO to deep, fine‐porphyroclastic, highly melt‐infiltrated garnet peridotite with weak, axial‐[010] olivine CPO. In contrast to the observed axial‐[010] CPOs, subgrain boundary orientations and misorientation axes suggest the dominant activation of the [100](001) slip system, suggesting deformation under moderately hydrous conditions. After accounting for effects of subsolidus cooling, we see coherent trends between mineral trace element abundance and water content, indicating that melt infiltration led to an increase in water content of the peridotites. However, measured olivine and pyroxene water contents in all peridotites (5 – 10 wt ppm and 30 – 500 wt ppm, respectively) are lower than that required to promote dominant [100](001) slip system observed in both natural and experimental samples. These results suggest that deformation occurred earlier along the P‐T path, probably during or shortly after hydrous melt infiltration. Subsequent rapid cooling at 90 Ma led to water loss from olivine (owing to decreased solubility at low temperature), leaving behind a deep arc lithosphere that remained viscously coupled to the Farallon slab until the opening of the slab window in the Late Cenozoic. This article is protected by copyright. All rights reserved.
      PubDate: 2016-06-03T03:30:31.963135-05:
      DOI: 10.1002/2015GC006156
  • Evolution of the northern Argentine margin during the Cenozoic controlled
           by bottom current dynamics and gravitational processes
    • Abstract: A detailed reflection seismic investigation on sediment deposition at the northern Argentine margin (37°S to 42°S) resolves major modifications in oceanographic circulation during the Cenozoic, which resulted from variations in both climatic and tectonic processes. After an extensive erosional period following the onset of glaciation of Antartica at ∼34 Ma, which affected all water depth levels, a buried elongated mounded drift within the continental shelf was shaped by bottom current activity during the Miocene. This may represent the earliest deposits of the Malvinas Current that branches from the Antartic Circumpolar Current and today is part of a complex shallow water circulation system known as the Brazil‐Malvinas confluence. At the same time a major terrace grew to its present form on the upper slope indicating that a precursor of Antarctic Intermediate Water was also part of the Brazil‐Malvinas confluence. After another major erosional phase inferred from a seismic unconformity at ∼6 Ma, sheeted drifts, mounded drifts and sediment waves formed at the continental rise during the Pliocene/Pleistocene. These extensive contourite deposits are diagnostic for a steady north setting bottom flow at the depth level of todays Antarctic Bottomwater. Evidence for downslope transport mainly stems from the presence of buried turbidites and canyon related depocenters. These features can be related to Andean uplift during the Eocene and to the activation of the canyon system during the Pliocene. This article is protected by copyright. All rights reserved.
      PubDate: 2016-06-02T10:32:39.945939-05:
      DOI: 10.1002/2015GC006232
  • Increased rates of large magnitude explosive eruptions in Japan in the
           Late Neogene and Quaternary
    • Authors: S.H. Mahony; R.S.J. Sparks, L.M. Wallace, S.L. Engwell, E.M. Scourse, N.H. Barnard, J. Kandlbauer, S.K. Brown
      Abstract: Tephra layers in marine sediment cores from scientific ocean drilling largely record high‐magnitude silicic explosive eruptions in the Japan arc for up to the last 20 million years. Analysis of the thickness variation with distance of 180 tephra layers from a global dataset suggests that the majority of the visible tephra layers used in this study are the products of caldera‐forming eruptions with magnitude (M) >6, considering their distances at the respective drilling sites to their likely volcanic sources. Frequency of visible tephra layers in cores indicates a marked increase in rates of large magnitude explosive eruptions at ∼8 Ma, 6–4 Ma and further increase after ∼2 Ma. These changes are attributed to major changes in tectonic plate interactions. Lower rates of large magnitude explosive volcanism in the Miocene are related to a strike‐slip dominated boundary (and temporary cessation or deceleration of subduction) between the Philippine Sea Plate and southwest Japan, combined with the possibility that much of the arc in northern Japan was submerged beneath sea level partly due to previous tectonic extension of Northern Honshu related to formation of the Sea of Japan. Changes in plate motions and subduction dynamics during the ∼8 Ma to present period led to (1) increased arc‐normal subduction in southwest Japan (and resumption of arc volcanism) and (2) shift from extension to compression of the upper plate in northeast Japan, leading to uplift, crustal thickening and favourable conditions for accumulation of the large volumes of silicic magma needed for explosive caldera‐forming eruptions. This article is protected by copyright. All rights reserved.
      PubDate: 2016-05-31T10:37:06.888646-05:
      DOI: 10.1002/2016GC006362
  • Late Cretaceous intraplate silicic volcanic rocks from the Lake Chad
           region: An extension of the Cameroon volcanic line
    • Abstract: Silicic volcanic rocks at Hadjer el Khamis, near Lake Chad, are considered to be an extension of the Cameroon volcanic line (CVL) but their petrogenetic association is uncertain. The silicic rocks are divided into peraluminous and peralkaline groups with both rock types chemically similar to within‐plate granitoids. In situ U/Pb zircon dating yielded a mean 206Pb/238U age of 74.4 ± 1.3 Ma indicating the magmas erupted ∼10 million years before the next oldest CVL rocks (i.e. ∼66 Ma). The Sr isotopes (i.e. ISr = 0.7021 to 0.7037) show a relatively wide range but the Nd isotopes (i.e. 143Nd/144Ndi = 0.51268 to 0.51271) are uniform and indicate that the rocks were derived from a moderately depleted mantle source. Thermodynamic modeling show that the silicic rocks likely formed by fractional crystallization of a mafic parental magma but that the peraluminous rocks were affected by low temperature alteration processes. The silicic rocks are isotopically similar to Late Cretaceous basalts identified within the Late Cretaceous basins (i.e. 143Nd/144Ndi = 0.51245 to 0.51285) of Chad than the uncontaminated CVL rocks (i.e. 143Nd/144Ndi = 0.51270 to 0.51300). The age and isotopic compositions suggest the silicic volcanic rocks of the Lake Chad region are related to Late Cretaceous extensional volcanism in the Termit basin. It is unlikely that the silicic volcanic rocks are petrogenetically related to the CVL but it is possible that, in both cases, magmatism was structurally controlled by suture zones that formed during the opening of the Central Atlantic Ocean and/or the Pan‐African Orogeny. This article is protected by copyright. All rights reserved.
      PubDate: 2016-05-31T05:15:27.874702-05:
      DOI: 10.1002/2016GC006298
  • Achieving comparable uncertainty estimates with Kalman filters or linear
           smoothers for bathymetry data
    • Authors: Brian S. Bourgeois; Paul A. Elmore, William E. Avera, Samantha J. Zambo
      Abstract: This paper examines and contrasts two estimation methods, Kalman filtering and linear smoothing, for creating interpolated data products from bathymetry measurements. Using targeted examples, we demonstrate previously obscured behavior showing the dependence of linear smoothers on the spatial arrangement of the measurements, yielding markedly different estimation results that the Kalman filter. For bathymetry data, we have modified the variance estimates from both the Kalman filter and linear smoothers to obtain comparable estimators for dense data. These comparable estimators produce uncertainty estimates that are statistically insignificant via hypothesis testing. Achieving comparable estimation is accomplished by applying the ‘propagated uncertainty' concept and a numerical realization of Tobler's principle to the measurement data prior to the computation of the estimate. We show new mathematical derivations for these modifications. In addition, we show test results with a) synthetic data and b) gridded bathymetry in the area of the Scripps and La Jolla Canyons. Our 10‐fold cross‐validation for case b) show that the modified equations create comparable uncertainty for both gridding algorithms with null hypothesis acceptance rates of greater than 99.95% of the data points. In contrast, bilinear interpolation has ten times the amount of rejection. We then discuss how the uncertainty estimators are, in principle, applicable to interpolate geophysical data other than bathymetry. This article is protected by copyright. All rights reserved.
      PubDate: 2016-05-30T03:30:34.172602-05:
      DOI: 10.1002/2015GC006239
  • Algorithms and software for U‐Pb geochronology by LA‐ICPMS
    • Authors: Noah McLean; James Bowring, George Gehrels
      Abstract: The past fifteen years have produced numerous innovations in geochronology, including experimental methods, instrumentation, and software that are revolutionizing the acquisition and application of geochronological data. For example, exciting advances are being driven by Laser‐Ablation ICP Mass Spectrometry (LA‐ICPMS), which allows for rapid determination of U‐Th‐Pb ages with 10s of μm‐scale spatial resolution. This method has become the most commonly applied tool for dating zircons, constraining a host of geological problems. The LA‐ICPMS community is now faced with archiving these data with associated analytical results and, more importantly, ensuring that data meet the highest standards for precision and accuracy and that inter‐laboratory biases are minimized. However, there is little consensus with regard to analytical strategies and data reduction protocols for LA‐ICPMS geochronology. The result is systematic inter‐laboratory bias and both under‐ and over‐estimation of uncertainties on calculated dates that, in turn, decrease the value of data in repositories such as EarthChem, which archives data and analytical results from participating laboratories. We present free open source software software that implements new algorithms for evaluating and resolving many of these discrepancies. This solution is the result of a collaborative effort to extend the U‐Pb_Redux software for the ID‐TIMS community to the LA‐ICPMS community. Now named ET_Redux, our new software automates the analytical and scientific workflows of data acquisition, statistical filtering, data analysis and interpretation, publication, community‐based archiving, and the compilation and comparison of data from different laboratories to support collaborative science. This article is protected by copyright. All rights reserved.
      PubDate: 2016-03-03T20:07:37.488273-05:
      DOI: 10.1002/2015GC006097
  • Issue Information
    • Pages: 2465 - 2466
      PubDate: 2016-08-13T03:57:45.615155-05:
      DOI: 10.1002/ggge.20834
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