ALBERT

Description: To study how an impacting plume modifies the mantle lithosphere, we analyzed the microstructures and crystal preferred orientations (CPO) of 29 peridotites and 37 pyroxenites that sample the mantle root of the Ontong Java Plateau (OJP) from 60 to 120 km depth. The peridotites show a strong compositional variability, but homogeneous coarse-granular to tabular microstructures, except for those equilibrated at the shallowest and deepest depths, which are porphyroclastic. All peridotites have clear olivine CPO, with dominant fiber-[010] patterns. Low intragranular misorientations and straight grain boundaries in olivine suggest that, above 100 km depth, annealing often followed deformation. Calculated density and P-wave velocities of the peridotites decrease weakly with depth. S-wave velocities decrease faster, resulting in increasing Vp/Vs ratio with depth. Calculated densities and seismic velocity profiles are consistent with those estimated for normal mantle compositions under a cold oceanic geotherm. Enrichment in pyroxenites may further increase seismic velocities. The calculated seismic properties cannot therefore explain the low S-waves velocities predicted by Rayleigh wave tomography and ScS data in the mantle beneath the OJP. Calculated P- and S-wave anisotropy is variable (2-12%). It is higher on average in the deeper section of the lithosphere. Because olivine has dominantly [010]-fiber CPO patterns, if foliations are horizontal, vertically propagating S-waves and Rayleigh waves will sample very weak anisotropy in the OJP mantle lithosphere. Moreover, if the orientation of the lineation changes with depth, the anisotropy-induced contrast in seismic properties might produce an intralithospheric reflector marking the stratification of the OJP mantle root.

Description: The quantification of heat and mass flow between deep reservoirs and the surface is important for understanding magmatic and hydrothermal systems. Here, we use high-resolution measurement of carbon dioxide flux (ϕCO 2 ) and heat flow at the surface to characterize the mass (CO 2 and steam) and heat released to the atmosphere from two magma-hydrothermal systems. Our soil gas and heat flow surveys at Rotokawa and White Island in the Taupō Volcanic Zone, New Zealand, include over 3,000 direct measurements of ϕCO 2 and soil temperature and 60 carbon isotopic values on soil gases. Carbon dioxide flux was separated into background and magmatic/hydrothermal populations based on the measured values and isotopic characterization. Total CO 2 emission rates (ΣCO 2 ) of 441 ± 84 t d -1 and 124 ± 18 t d -1 were calculated for Rotokawa (2.9 km 2 ) and for the crater floor at White Island (0.3 km 2 ), respectively. The total CO 2 emissions differ from previously published values by +386 t d -1 at Rotokawa and +25 t d -1 at White Island, demonstrating that earlier research underestimated emissions by 700% (Rotokawa) and 25% (White Island). These differences suggest that soil CO 2 emissions facilitate more robust estimates of the thermal energy and mass flux in geothermal systems than traditional approaches. Combining the magmatic/hydrothermal-sourced CO 2 emission (constrained using stable isotopes) with reservoir H 2 O:CO 2 mass ratios and the enthalpy of evaporation, the surface expression of thermal energy release for the Rotokawa hydrothermal system (226 MW t ) is 10 times greater than the White Island crater floor (22.5 MW t ).

Description: To better constrain the mechanical behavior of sediments accreted to accretionary prism, we conducted triaxial mechanical tests on natural samples from the Miura-Boso paleo-accretionary prism (Japan) in drained conditions with confining pressures up to 200 MPa as well as post-experiments P-wave velocity (V p ) measurements. During experiments, deformation is principally non-coaxial and accommodated by two successive modes of deformation, both associated with strain-hardening and velocity-strengthening behavior: (1) compaction-assisted shearing, distributed in a several mm-wide shear zone and (2) faulting, localized within a few tens of µm-wide, dilatant fault zone. Deformation is also associated with (1) a decrease in Young's modulus all over the tests, (2) anomalously low V p in the deformed samples compared to their porosity and (3) an increase in sensitivity of V p to effective pressure. We interpret this evolution of the poroelastic properties of the material as reflecting the progressive breakage of intergrain cement and the formation of microcracks along with macroscopic deformation. When applied to natural conditions, these results suggest that the deformation style (localized vs distributed) of shallow (z 〈 a few km) sediments is mainly controlled by the variations in stress/strain rate during the seismic cycle and is therefore independent of the porosity of sediments. Finally, we show that the effect of strain, through cement breakage and microcracks formation, may lower V p for effective pressure up to 40 MPa. As a consequence, the low V p anomalies observed in Nankai accretionary prisms by seismic imaging between 2 and 4km depth [ Kitajima and Saffer , 2012] could reflect sediment deformation rather than porosity anomalies. This article is protected by copyright. All rights reserved.

Description: This study presents a new thermodynamic model for the calculation of phase relations during the melting of anhydrous spinel lherzolite at pressures of 1–2.5 GPa. The model is based on the total energy minimization algorithm for calculating phase equilibria within multicomponent systems and the thermodynamic configuration of Ueki and Iwamori [2013]. The model is based on a SiO 2 – Al 2 O 3 – FeO–Fe 3 O 4 –MgO–CaO system that includes silicate melt, olivine, clinopyroxene, orthopyroxene, and spinel as possible phases. The molar Gibbs free energy of the melt phase is modeled quasi-empirically, and the thermodynamic parameters for silicate melt end-member components are calibrated with a polybaric calibration database. The temperatures and pressures used in this newly compiled calibration dataset are 1230–1600 ∘ C and 0.9–3 GPa, corresponding to the stability range of spinel lherzolite. The modeling undertaken during this study reproduces the general features of experimentally determined melting phase relations of spinel lherzolite at 1–2.5 GPa, including the solidus temperature, the melt composition, the chemical reaction during melting and the degree of melting. This new thermodynamic modeling also reproduces phase relations of various bulk compositions from fertile to deplete spinel lherzolite and can be used in the modeling of polybaric mantle melting within various natural settings. Comparing the results derived from this new modeling with those produced using previous models indicates that the new approach outlined here, involving a combination of total energy minimization and the direct calibration of melt thermodynamic parameters at pressure and temperature conditions corresponding to mantle melting with a relatively simple melt thermodynamic equation, can accurately model polybaric melting phase relations. This article is protected by copyright. All rights reserved.

Description: Knowledge on the behavior of Mg isotopes during metamorphic dehydration is the prerequisite for applying Mg isotopes as tracers for crustal recycling. Here, we report Mg isotopic compositions of metapelites from the Onawa contact aureole, Maine. Except one sample, all metapelites across the aureole, from the wall-rock regional metamorphic rocks to the partially melted rocks adjacent to the pluton, have similar Mg isotopic compositions (δ 26 Mg = -0.09 to +0.12‰). This observation indicates limited Mg isotope fractionation during metamorphic dehydration and fluid-rock interaction, due to the low Mg concentration in fluids relative to rocks. Our results suggest that Mg isotopic compositions of metapelites can record those of their protoliths and, hence, recycled clastic sedimentary materials may preserve their low-temperature Mg isotopic signatures through subduction zones. Therefore, Mg isotopes may serve as new tracers for crustal recycling, for example, tracing components experienced weathering cycles within granite sources.

Description: Magnetic anomaly identifications underpin plate tectonic reconstructions and form the primary dataset from which age of the oceanic lithosphere and seafloor spreading regimes in the ocean basins can be determined. Although these identifications are an invaluable resource, their usefulness to the wider scientific community has been limited due to the lack of a central community infrastructure to organize, host and update these interpretations. We have developed an open-source, community-driven online infrastructure as a repository for quality-checked magnetic anomaly identifications from all ocean basins. We provide a global sample dataset that comprises 96,733 individually picked magnetic anomaly identifications organized by ocean basin and publication reference, and provide accompanying Hellinger-format files, where available. Our infrastructure is designed to facilitate research in plate tectonic reconstructions or research that relies on an assessment of plate reconstructions, for both experts and non-experts alike. To further enhance the existing repository and strengthen its value, we encourage others in the community to contribute to this effort.

Description: We present an efficient method for high-volume heavy mineral separation from clay-rich rocks using an ultrasonic probe. The ultrasonic clay separator (UCS) is an easily constructed device that allows for the recovery of high-density minerals, as small as 10 microns, with a minimum of sample preparation. Heavy mineral recovery from clay-rich material with the UCS yields a greater number of small (〈100 micron) grains and approximately double the amount of material from that of gravity settling and decanting. Despite development with heavy mineral recovery in mind, the UCS should suitable for recovering small grain size geologic materials from flocculating clay-rich material.

Description: In the detachment mode of slow seafloor spreading, convex-upward detachment faults take up a high proportion of the plate separation velocity exposing gabbro and serpentinized peridotite on the seafloor. Large, long-lived hydrothermal systems such as TAG are situated off-axis and may be controlled by fluid flow up a detachment fault, with the source of magmatic heat being as deep as 7 kmbsf. The consequences of such deep circulation for the evolution of fluid temperature and salinity have not previously been investigated. Microthermometry on fluid inclusions trapped in diabase, gabbro and trondjhemite, recovered at the Atlantis Massif Oceanic Core Complex (30° N, mid-Atlantic Ridge), reveals evidence for magmatic exsolution, phase separation, and mixing between hydrothermal fluids and previously phase separated fluids. Four types of fluid inclusions were identified, ranging in salinity from 1.4 wt.% NaCl to 35 wt.% NaCl, although the most common inclusions have salinities close to seawater (3.4 wt.% NaCl). Homogenization temperatures range from 160 to 〉400 °C, with the highest temperatures in hypersaline inclusions trapped in trondjhemite and the lowest temperatures in low salinity inclusions trapped in quartz veins. The fluid history of the Atlantis Massif is interpreted in the context of published thermochronometric data from the Massif, and a comparison with the inferred circulation pattern beneath the TAG hydrothermal field, to better constrain the pressure temperature conditions of trapping and when in the history of exhumation of the rocks sampled by IODP Hole U1309D fluids have been trapped.

Description: We study the elastic wave speed structure of the crust and the uppermost mantle in western Tibet using P and S-wave arrival times from regional earthquakes recorded by a temporary seismic network. We relocate the earthquakes, and subsequently invert travel time residuals for 3D distributions of wave speed. Resolution tests with a variety of input structures are used to verify the reliability of our results. The crust beneath western Tibet has low P-wave speed (5.9 - 6.3 km/s) throughout its nearly 80 km thickness, with lower values in this range concentrated within the Lhasa block. Beneath the Himalaya wave speeds are higher. Southern and western limits of the slow material beneath the Tibetan Plateau correlate with the Karakoram fault, and dip beneath the plateau at ~40° angle. We find no evidence of a sub-horizontal low velocity zone in the crust. In the uppermost mantle we find a long and narrow region of fast (up to 8.4 km/s) P-wave speed extending from the Karakoram fault in NE direction, and crossing the Bangong-Nujiang suture. In a north-south cross-section, the distribution of relatively fast P-wave speed suggests a ramp-flat geometry consistent with India underthrusting the Tibetan Plateau at least as far as 32.5°N. A plausible interpretation of the upper mantle fast feature is the formation of eclogite from the mafic lower-crustal material of India after it is underthrust beneath Tibet. Notably, in western Tibet this process only takes place in a narrow region.

Description: We have employed shear wave splitting techniques to image anisotropy beneath the D'Entrecasteaux Islands, in southeastern Papua New Guinea. Our results provide a detailed picture of the extending continent that lies immediately ahead of a propagating mid-ocean ridge tip; we image the transition from continental to oceanic extension. A dense shear wave splitting dataset from a 2010-11 passive-source seismic deployment is analyzed using single- and multi-channel methods. Splitting delay times of 1-1.5 s are observed and fast axes of anisotropy trending N-S, parallel to rifting direction, predominate the results. This trend is linked to lattice-preferred orientation of olivine, primarily in the shallow convecting mantle, driven by up to 200 km of N-S continental extension ahead of the westward-propagating Woodlark Rift. This pattern differs from several other continental rifts that evince rift-strike-parallel fast axes and is evident despite the complex recent tectonic history. We contend that across most of this rift, the unusually high rate and magnitude of extension has been sufficient to produce a regime change to a mid-ocean-ridge-like mantle fabric. Stations in the south of our array show more complex splitting that might be related to melt or to complex inherited structure at the edge of the extended region.

Description: From April 2010 through February 2011, CO 2 flux surveys were performed on Lake Rotomahana, New Zealand. The area has been hydrothermally active with fumaroles and sublacustrine hydrothermal activity before and since the eruption of Mt Tarawera in 1886. The total CO 2 emission from the lake calculated by sequential Gaussian simulation is 549 ± 72 t day -1 . Two different mechanisms of degassing, diffusion through the water-air interface and bubbling, are distinguished using a graphical statistical approach. The carbon dioxide budget calculated for the lake confirms that the main source of CO 2 to the atmosphere is by diffusion covering 94.5 % of the lake area (mean CO 2 flux 25 g m -2 day -1 ) and to a lesser extent, bubbling (mean CO 2 flux 1297 g m -2 day -1 ). Mapping of the CO 2 flux over the entire lake, including over lakefloor vents detected during the survey, correlates with eruption craters formed during the 1886 eruption. These surveys also follow regional tectonic patterns present in the southeastern sector of Lake Rotomahana suggesting a deep magmatic source (~ 10 km) for CO 2 and different pathways for the gas to escape to the surface. The values of δ 13 C CO2 (-2.88 and -2.39 ‰) confirm the magmatic origin of CO 2 .

Description: Regional-scale geologic structures characteristic of mantle lithosphere within cratons found in continent interiors are interpreted using geo-registered diverse data sets from the Slave craton of northwest Canada. We developed and applied a new method for mapping seismic discontinuities in three dimensions using multi-year observations at sparse, individual broadband receivers. New, fully 3-D conductivity models used all available magnetotelluric data. Discontinuity surfaces and conductivity models were geo-registered with previously published P-wave and surface wave velocity models to confirm first-order structures such as a mid-lithosphere discontinuity. Our 3-D model to 400 km depth was calibrated by ‘drill hole’ observations derived from xenolith suites extracted from kimberlites. A number of new structural discontinuities emerge from direct comparison of co-registered data sets and models. Importantly we distinguish primary mantle layers from secondary features related to younger metasomatism. Sub-horizontal Slave craton layers with tapered, wedge-shaped margins indicate construction of the craton core at 2.7 Ga by underthrusting and flat stacking of lithosphere. Mapping of conductivity and metasomatism in 3-D, the latter inferred via mineral recrystallization and resetting of isotopic ages in xenoliths, indicates overprinting of the primary layered structures. The observed distribution of relatively conductive mantle at 100–200 km depths is consistent with pervasive metasomatism; vertical ‘chimneys’ reaching to crustal depths in locations where kimberlites erupted or where Au mineralization is known.

Description: Understanding the nature of the crust has long been a goal for seismologists when imaging the Earth. This is particularly true in volcanic regions where imaging melt storage and migration can have important implications for the size and nature of an eruption. Receiver functions and the H- κ stacking (H κ ) technique are often used to constrain crustal thickness (H) and the ratio of P- to S-wave velocities ( κ ). In this paper I show that it is essential to consider anisotropy when performing H κ . I show that in a medium with horizontally transverse isotropy a strong variation in κ with back azimuth is present which characterises the anisotropic medium. In a vertically transverse isotropic medium no variation in κ with back azimuth is observed, but κ is increased across all back azimuths. Thus, estimates of κ are more difficult to relate to composition than previously thought. I extend these models to melt induced anisotropy and show that similar patterns are observed, but with more significant variations and increases in κ . Based on these observations I develop a new anisotropic H- κ stacking technique which inverts H κ data for melt fraction, aspect ratio and orientation of melt inclusions. I apply this to data for the Afar Depression and show that melt is stored in interconnected stacked sills in the lower crust, which likely supply the recent volcanic eruptions and dike intrusions. This new technique can be applied to any anisotropic medium where it can provide constraints on the average crustal anisotropy.

Description: A Fortran 90 program to visualize data on the Yin-Yang grid system is developed. The purpose of this study is to provide simulation researchers with a source code as a starting point of their own custom-made visualization tools. A basic but sufficiently diverse set of visualization methods are implemented using a Fortran 90 binding for OpenGL for scalar and vector fields defined or simulated on the Yin-Yang grid.

Description: Twelve submarine mud volcanoes (MV) in the Kumano forearc basin within the Nankai Trough subduction zone were investigated for hydrocarbon origins and fluid dynamics. Gas hydrates diagnostic for methane concentrations exceeding solubilities were recovered from MVs 2, 4, 5, and 10. Molecular ratios (C 1 /C 2 〈250) and stable carbon isotopic compositions (δ 13 C-CH 4 〉−40‰ V-PDB) indicate that hydrate-bound hydrocarbons (HCs) at MVs 2, 4, and 10 are derived from thermal cracking of organic matter. Considering thermal gradients at the nearby IODP Sites C0009 and C0002, the likely formation depth of such HCs ranges between 2,300 and 4,300 m below seafloor (mbsf). With respect to basin sediment thickness and the minimum distance to the top of the plate boundary thrust we propose that the majority of HCs fueling the MVs is derived from sediments of the Cretaceous to Tertiary Shimanto belt below Plio-/Pleistocene to recent basin sediments. With respect to sizes and appearances hydrates are suggested to be relicts of higher MV activity in the past, although the sporadic presence of vesicomyid clams at MV 2 showed that fluid migration is sufficient to nourish chemosynthesis-based organisms in places. Distributions of dissolved methane at MVs 3, 4, 5, and 8 pointed at fluid supply through one or few MV conduits and effective methane oxidation in the immediate subsurface. The aged nature of the hydrates suggests that the major portion of methane immediately below the top of the methane-containing sediment interval is fueled by current hydrate dissolution rather than active migration from greater depth.

Description: 40 Ar/ 39 Ar geochronology depends critically on well calibrated standards, often traceable to first-principles K-Ar age calibrations using bulb-tracer systems. Tracer systems also provide precise standards for noble-gas studies and interlaboratory calibration. The exponential expression long-used for calculating isotope tracer concentrations in K-Ar age dating and calibration of 40 Ar/ 39 Ar age standards may provide a close approximation of those values, but is not correct. Appropriate equations are derived that accurately describe the depletion of tracer reservoirs and concentrations of sequential tracers. The true form of the expression is a power law, not exponential, and a similar expression was presented by Miiller (2006, J. Res. Natl. Inst. Stand. Technol., 111 (5), 335–360). Evaluation of the expressions demonstrates that systematic error introduced through use of the exponential approximation may be substantial where reservoir volumes are small and resulting depletion constants are large. Traditional use of large reservoir to tracer volumes and the resulting small depletion constants have kept errors well less than experimental uncertainties in most previous K-Ar and calibration studies. Use of the proper expression, however, permits use of volumes appropriate to the problems addressed.

Description: Natural hydrate-bearing sediment (HBS) predominantly exists in non-cementing habit, and its limited availability for use in laboratory studies demands a time-effective and repeatable laboratory process for forming representative samples with natural accumulation habit. This study reports on a three-step laboratory process for forming non-cementing methane hydrate in sandy sediments: (1) initial HBS formation under excess-gas conditions; (2) slow saline water (5wt % CaCl 2 ) injection under strictly controlled PT conditions; and (3) a temperature warming/cooling cycle. Changes in compressional wave velocity ( V p ) of sediment, as well as pressure-temperature (P-T) condition, were monitored throughout the tests. The evolution of V p , in good agreement with rock physics model calculations, suggested that the transition from cementing hydrate into non-cementing hydrate occurs during saline injection as well as temperature warming/cooling cycle. The proposed process appeared to be an efficient and consistent substitute for the existing methods, to form non-cementing hydrate habit in sandy sediments.

Description: Great earthquakes anticipated on the Cascadia subduction fault can potentially rupture beyond the geodetically and thermally inferred locked zone to the depths of episodic tremor and slip (ETS) or to the even deeper forearc mantle corner (FMC). To evaluate these extreme rupture limits, we map the FMC from southern Vancouver Island to central Oregon by combining published seismic velocity structures with a model of the Juan de Fuca plate. These data indicate that the FMC is somewhat shallower beneath Vancouver Island (36–38 km) and Oregon (35–40 km) and deeper beneath Washington (41–43 km). The updip edge of tremor follows the same general pattern, overlying a slightly shallower Juan de Fuca plate beneath Vancouver Island and Oregon (˜30 km) and a deeper plate beneath Washington (˜35 km). Similar to the Nankai subduction zone, the best constrained FMC depths correlate with the center of the tremor band suggesting that ETS is controlled by conditions near the FMC rather than directly by temperature or pressure. Unlike Nankai, a gap as wide as 70 km exists between the downdip limit of the inferred locked zone and the FMC. This gap also encompasses a ˜50 km wide gap between the inferred locked zones and the updip limit of tremor. The separation of these features offers a natural laboratory for determining the key controls on downdip rupture limits.

Description: During eruptions onto low slopes, basaltic Pahoehoe lava can form thin lobes that progressively coalesce and inflate to many times their original thickness, due to a steady injection of magma beneath brittle and viscoelastic layers of cooled lava that develop sufficient strength to retain the flow. Inflated lava flows forming tumuli and pressure ridges have been reported in different kinds of environments, such as at contemporary subaerial Hawaiian-type volcanoes in Hawaii, La Réunion and Iceland, in continental environments (states of Oregon, Idaho, Washington), and in the deep sea at Juan de Fuca Ridge, the Galapagos spreading center, and at the East Pacific Rise (this study). These lava have all undergone inflation processes, yet they display highly contrasting morphologies that correlate with their depositional environment, the most striking difference being the presence of water. Lava that have inflated in subaerial environments display inflation structures with morphologies that significantly differ from subaqueous lava emplaced in the deep sea, lakes, and rivers. Their height is 2 to 3 times smaller and their length being 10 to 15 times shorter. Based on heat diffusion equation, we demonstrate that more efficient cooling of a lava flow in water leads to the rapid development of thicker (by 25%) cooled layer at the flow surface, which has greater yield strength to counteract its internal hydrostatic pressure than in subaerial environments, thus limiting lava breakouts to form new lobes, hence promoting inflation. Buoyancy also increases the ability of a lava to inflate by 60%. Together, these differences can account for the observed variations in the thickness and extent of subaerial and subaqueous inflated lava flows.

Description: I analyze the seismic tomography around the Japanese Islands to elucidate the present slab morphology. A simple conversion from the high speed to the temperature anomaly is done based on the recent study of the conversion coefficient from the velocity to the thermal anomaly and on the study of the relation between the seismicity and the temperature in the slab. I find recognizable amount of cold temperature anomalies under the subducting slab. This suggests a rather continuous slab throughout the mantle consistent with the results of many recent numerical models. However, there still is a gap in the slab or diluted slab under the transition zone. To understand this, I construct a simple half-kinematic model of subduction zone in which a Byerlee's type yield stress and the depth independent yield stress are assumed. Taking into account the geologic history around the Japanese Islands, I find that the slab morphology similar to the tomographic image is obtained when the yield stress is O(100 MPa). The present study shows that the detailed studies of seismic tomography and the tectonic history of the surveyed area can provide the constraints on the slab dynamics.

Description: As the supercontinent Rodinia was assembling ca. 1.1 billion years ago, there was extensive magmatism on at least five Proterozoic continents including the development of the North American Midcontinent Rift. New paleomagnetic data from 84 lava flows of the Osler Volcanic Group of the Midcontinent Rift reveal that there was a significant and progressive decrease in inclination between the initiation of extrusive volcanism in the region (ca. 1110 Ma) and ca. 1105 ± 2 Ma (the “early stage” of rift development). Paleomagnetic poles can be calculated for the lower portion of the reversed Osler Volcanic Group (40.9°N, 218.6 °E, A 95 =4.8°, N=30) and the upper portion of the reversed Osler Volcanic Group (42.5°N, 201.6 °E, A 95 =3.7°, N=59; this pole can be assigned the age of ca. 1105 ± 2 Ma). This result is a positive test of the hypothesis that there was significant plate motion during the early stage of rift development. In addition to being a time of widespread volcanism on Laurentia and other continents, this interval of the late Mesoproterozoic was characterized by rapid paleogeographic change.

Description: The relationship between magnetic hysteresis parameters and the degree of oxidation of ultrafine magnetite particles is examined by both experimental measurements (distributed particle assemblage with median grain size of ~80 nm and standard deviation 0.43) and micromagnetic simulations (single particles from 40 nm to 140 nm). Experimental results show that both coercivity (B c ) and the ratio of saturation remanence to saturation magnetization (M rs /M s ) increase slowly, as the oxidation parameter z increases from 0 to ~0.9. Thereafter both parameters decrease sharply as magnetite becomes completely oxidized to maghemite. Numerical simulations of hysteresis loop and microstructure using a micromagnetic model with a core-shell geometry (a stoichiometric core surrounded by an oxidized shell) show three categories of behavior for magnetic grains during oxidation. Firstly, the coercivity of SD particles decreases as oxidation proceeds, but their remanence magnetization remains in a uniform state. Secondly, for PSD sized particles near the critical SD boundary (80 nm to 100 nm), the initial vortex domain structure changes to a SD as oxidation occurs and returns to a vortex state upon complete maghemitization, resulting in an initial rise and then fall of B c and M rs . Finally, larger PSD grains remain a vortex state throughout the maghemitization, with less variations of B c and M rs . The predicted magnetic properties exhibit good agreement with experimental observations and suggest that the domain arrangement is likely to be dominated by a core-shell structure with strong exchange coupling at their interface. Overall, the partially oxidized magnetite in SD-PSD range can reliably record palaeomagnetic signals.

Description: We report 3 He/ 4 He for 150 mid-ocean ridge basalt (MORB) glasses from the Southeast Indian Ridge (SEIR). Between 81°-101°E 3 He/ 4 He varies from 7.5 to 10.2 R A , encompassing more than half the MORB range away from ocean island hotspots. Abrupt transitions are present and in one case the full range occurs over ~10 km. Melting of lithologically heterogeneous mantle containing a few percent garnet pyroxenite or eclogite leads to lower 3 He/ 4 He, while 3 He/ 4 He above ~9 R A likely indicates melting of pyroxenite-free or eclogite-free mantle. Patterns in the length scales of variability represent a description of helium isotopic texture. We utilize four complementary methods of spectral analysis to evaluate this texture, including Periodogram, Redfit, Multi-Taper Method and Continuous Wavelet Transform. Long-wavelength lobes with prominent power at 1000 km and 500 km are present in all treatments, similar to hotspot-type spectra in Atlantic periodograms. The densely sampled region of the SEIR considered separately shows significant power at ~100 km and ~30-40 km, the latter scale resembling heterogeneity in the bimodal distribution of Hf and Pb isotopes in the same sample suite. Wavelet transform coherence reveals that 3 He/ 4 He varies in-phase with axial depth along the SEIR at ~1000 km length scale, suggesting a coupling between melt production, 3 He/ 4 He and regional variations in mantle temperature. Collectively, our results show that the length scales of MORB 3 He/ 4 He variability are dominantly controlled by folding and stretching of heterogeneities during regional (~1000 km) and mesoscale (~100 km) mantle flow, and by sampling during the partial melting process (~30 km).

Description: Reconstructing convective flow in the Earth's mantle is a crucial issue for a diversity of disciplines, from seismology to sedimentology. The common and fundamental limitation of these reconstructions based on geodynamic modelling is the unknown initial conditions. Because of the chaotic nature of convection in the Earth's mantle, errors in initial conditions grow exponentially with time and limit forecasting and hindcasting abilities. In this work we estimate for the first time the limit of predictability of Earth's mantle convection. Following the twin experiment method, we compute the Lyapunov time (i.e. e-folding time) for state-of-the art 3D spherical convection models, varying rheology and Rayleigh number. Our most Earth-like and optimistic solution gives a Lyapunov time of 136±13 My. Rough estimates of the uncertainties in best guessed initial conditions are around 5%, leading to a limit of predictability for mantle convection of 95 My. Our results suggest that error growth could produce unrealistic convective structures over timescales shorter than that of Pangea dispersal.

Description: Low-magnitude earthquake swarms (M L ≤ 2.8), consisting of up to 47 events of similar waveforms, have been observed repeatedly in the central Southern Alps, a rapidly uplifting orogen bounded by the transpressive Alpine Fault in the South Island of New Zealand. We compare nine background swarms recorded between November 2008 and April 2010 with five delayed-triggered swarms that occurred after the M W 7.8 Dusky Sound and the M W 7.1 Darfield (Canterbury) earthquakes. The two types of swarms are similar in terms of the magnitudes, depths, focal mechanisms, and inter-event times of the constituent microearthquakes, and appear to both involve the rupture of steeply-dipping faults in highly fractured crust in a 10 km × 12 km area in the centre of the SAMBA network. The delayed-triggered swarms occurred at similar epicentral distances (c.4.5× the rupture length of the mainshocks) to the Dusky Sound and Darfield earthquakes, commenced shortly after the passage of the surface waves, continued for ~5 and ~2 days, respectively, and were followed by a ≥2 day-long quiescent period, which may suggest clock-advanced of faults in their failure-cycle. Triggering thresholds of ≥0.01 MPa proposed elsewhere are similar to the dynamic stress changes computed for the Southern Alps (≥0.09 MPa). However, as 98% of the locatable triggered events occurred several hours after the surface waves had passed, the dynamic stress changes associated with the surface waves themselves are unlikely to have triggered the earthquakes directly. Instead, we suggest that the locations and delays of the triggered swarms are more consistent with triggering by pore pressure diffusion.

Description: The isotropic and anisotropic P -wave velocity structure of the upper oceanic crust on the Endeavour Segment of the Juan de Fuca Ridge is studied using refracted travel time data collected by an active-source, three-dimensional tomography experiment. The isotropic velocity structure is characterized by low crustal velocities in the overlapping spreading centers (OSCs) at the segment ends. These low velocities are indicative of pervasive tectonic fracturing and persist off-axis, recording the history of ridge propagation. Near the segment center, velocities within the upper 1 km show ridge-parallel bands with low velocities on the outer flanks of topographic highs. These features are consistent with localized thickening of the volcanic extrusive layer from eruptions extending outside of the axial valley that flow down the fault-tilted blocks that form the abyssal hill topography. On-axis velocities are generally relatively high beneath the hydrothermal vent fields likely due to the infilling of porosity by mineral precipitation. Lower velocities are observed beneath the most vigorous vent fields in a seismically active region above the axial magma chamber and may reflect increased fracturing and higher temperatures. Seismic anisotropy is high on-axis but decreases substantially off-axis over 5-10 km (0.2-0.4 Ma). This decrease coincides with an increase in seismic velocities resolved at depths ≥1 km and is attributed to the infilling of cracks by mineral precipitation associated with near-axis hydrothermal circulation. The orientation of the fast-axis of anisotropy is ridge-parallel near the segment center but curves near the segment ends reflecting the tectonic fabric within the OSCs.

Description: Chemical and isotopic signatures for black shales serve as potential proxies for reconstruction of paleoenvironmental conditions. Here we bring Rock-Eval, major and trace element and Re-Os isotopic data together to examine the environmental record at the Cambrian-Ordovician Global Stratotype Section and Point ( GSSP ) at Green Point in western Newfoundland, Canada. The Green Point shales are oil mature and contain Type-II organic material of marine origin. A Re-Os isochron for the shales provides the first radiometric age for shale deposition at the GSSP at 484 ± 16 Ma, with an initial 187 Os/ 188 Os ratio of 0.74 ± 0.05 (Model 3 age; MSWD = 21; n =13; σ uncertainties). Factor analysis of the geochemical dataset shows association of most trace elements with total organic carbon (TOC) and S contents, ensuring an authigenic origin for most elements and hence, their validity for evaluating the paleo-redox state. Relatively high enrichment factors for redox-sensitive elements ( e.g . Re, U and Mo) compared to average shale, but comparatively low enrichment compared to modern Black Sea sediments, suggest deposition in anoxic, but not euxinic waters. Comparison of Lower Ordovician shale geochemistry datasets at a global scale leads us to suggest that anoxic conditions and warm oceanic regimes were restricted to the margins of Laurentia and Baltica whereas depositional basins with colder waters ( e.g . Avalonia and Gondwana) were less reducing. These outcomes underscore the important role of paleogeography in regulating oceanic conditions and marine life.

Description: The outcome of paleointensity experiments largely depends on the rock-magnetic properties of the samples. To assess the relation between volcanic emplacement processes and rock-magnetic properties we sampled a vertical transect in a ˜6 m thick inflated lava flow at Hawaii, emplaced in ˜588 AD. Its rock-magnetic properties vary as function of distance from the flow top; the observations can be correlated to the typical cooling rate profile for such a flow. The top and to a lesser extent the bottom parts of the flow cooled faster and reveal a composition of ˜TM60 in which the magnetic remanence is carried by fine-grained titanomagnetites, relatively rich in titanium, with associated low Curie and unblocking temperatures. The titanomagnetite in the slower cooled central part of the flow is unmixed into the magnetite and ülvospinel end-members as evidenced by scanning electron microscope observation. The remanence is carried by coarse-grained magnetite lamella (˜TM0) with high Curie and unblocking temperatures. The calibrated pseudo-Thellier results that can be accepted yield an average paleointensity of 44.1 ± 2.4 μT. This is in good agreement with the paleointensity results obtained using the thermal IZZI-Thellier technique (41.6 ± 7.4 μT) and a recently proposed record for Hawaii. We therefore suggest that the chance of obtaining a reliable paleointensity from a particular cooling unit can be increased by sampling lavas at multiple levels at different distances from the top of the flow combined with careful preliminary testing of the rock-magnetic properties.

Description: Volcanic glasses recovered from four guyots during drilling along the Louisville Seamount Trail, southwest Pacific, have been analyzed for major, trace and volatile elements (H 2 O, CO 2 , S, Cl, F), and oxygen isotopes. Compared to other oceanic island settings they are geochemically homogeneous, providing no evidence of the tholeiitic stage that characterizes Hawai'i. The degrees and depth of partial melting remained constant over 1 – 3 Ma represented by the drill holes, and along-chain over several million years. The only exception is Hadar Guyot with compositions that suggest small degree preferential melting of an enriched source, possibly because it erupted on the oldest and thickest lithosphere. Incompatible element enriched glass from late-stage volcaniclastics imply lower degrees of melting as the volcanoes moved off the melting anomaly. Volcaniclastic glasses from throughout the igneous basement are degassed suggesting generation during shallow submarine eruptions (〈20 mbsl) or as subaerial flows entered the sea. Drill depths may no longer reflect relative age due to post-quench downslope movement. Higher volatile contents in late-stage volcaniclastics indicate submarine eruptions at 118 – 258 mbsl and subsidence of the edifices below sea level by the time they erupted, or generation in flank eruptions. Glass from intrusion margins suggest emplacement ~100 m below the surface. The required uplift to achieve these paleo-quench depths and the subsequent subsidence to reach their current depths exceed that expected for normal oceanic lithosphere, consistent with the Louisville melting anomaly being 〈100 °C hotter than normal asthenosphere at 50 – 70 Ma when the guyots were erupted.

Description: Understanding the controls for magma ascent and storage depth is important for volcanic hazard assessment. Regional differences in the depth of magma storage between volcanic arcs suggest that the settings of subduction zones and of overriding plates influence how magma ascends through the crust. Here we use a compilation of data for 70 volcanoes in 15 volcanic regions to better understand the geodynamic controls on magma storage. We describe the subduction system, which consists of the subducting slab, the mantle wedge and the upper plate with 12 parameters encompassing the kinematics of the subduction, the structure and geometry of the slab, the timing of the subduction, the thermal structure of the slab, the upper-plate crustal structure, its stress regimes, and its thermal structure. We find that the magma reservoir depths correlate with the upper-plate crustal structure and with the stress regimes. Shallow reservoirs (〈5 km depths) are 52% more common in young Tertiary crust than in old Precambrian crust and 42 % more common in thin crust (〉25 km) than in thick crust (〉 45 km). Similarly, shallow magma reservoirs are 33 to 69% more common in extensional and strike slip stress regimes that in compressional regimes. This illustrates the effect of buoyancy for magma ascent as well as the importance of stress and preexisting structures.

Description: We present a shear wave splitting (SWS) database for the western and central United States as part of a lasting effort to build a uniform SWS database for the entire North America. The SWS measurements were obtained by minimizing the energy on the transverse component of the PKS, SKKS, and SKS phases. Each of the individual measurements was visually checked to ensure quality. This version of the database contains 16105 pairs of splitting parameters. The data used to generate the parameters were recorded by 1774 digital broadband seismic stations over the period of 1989-2012, and represented all the available data from both permanent and portable seismic networks archived at the Incorporated Research Institutions for Seismology Data Management Center in the area of 26.00 ∘ N to 50.00 ∘ N and 125.00 ∘ W to 90.00 ∘ W. About 10000 pairs of the measurements were from the 1092 USArray Transportable Array stations. The results show that approximately 2/3 of the fast orientations are within 30 ∘ from the absolute plate motion (APM) direction of the North American plate, and most of the largest departures with the APM are located along the eastern boundary of the western US orogenic zone and in the central Great Basins. The splitting times observed in the western US are larger than, and those in the central US are comparable with the global average of 1.0 s. The uniform database has an unprecedented spatial coverage and can be used for various investigations of the structure and dynamics of the Earth.

Description: Shelves from volcanic ocean islands result from the competition between two main processes, wave erosion that forms and enlarges them and volcanic progradation that reduces their dimension. In places where erosion dominates over volcanism, shelf width can be used as a proxy for the relative age of the subaerial volcanic edifices and reconstruction of their extents prior to erosion can be achieved. In this study new multibeam bathymetry and high-resolution seismic reflection profiles are exploited to characterize the morphology of the insular shelves adjacent to each volcanic edifice of Terceira Island in order to improve the understanding of its evolution. Subaerial morphological and geological/stratigraphic data were also used to establish the connection between the onshore and offshore evolution. Shelf width contiguous to each main volcanic edifice is consistent with the known subaerial geological history of the island; most of the older edifices have wider shelves than younger ones. The shelf edge proved to be a very useful indicator in revealing the original extent of each volcanic edifice in plan-view. Its depth was also used to reconstruct vertical movements, showing that older edifices like Serra do Cume–Ribeirinha, Guilherme Moniz and Pico Alto have subsided whilst more recent ones have not. The morphology of the shelf (namely the absence/presence of fresh lava flow morphologies and several types of erosional, depositional and tectonic features) integrated with the analysis of the coastline morphology allowed us to better constrain previous geological interpretations of the island evolution.

Description: Both silicate melts and aqueous fluids are thought to play critical roles in the chemical differentiation of the Earth's crust and mantle. Yet their relative effects are poorly constrained. We have addressed this issue by measuring partition coefficients for 50 trace and minor elements in experimentally produced aqueous fluids, co-existing basanite melts and peridotite minerals. The experiments were conducted at 1.0-4.0 GPa and 950-1200 °C in single capsules containing (either 40 or 50 wt. %) H 2 O and trace element enriched basanite glass. This allowed run products to be easily identified and analysed by a combination of electron microprobe and LAM-ICP-MS. Fluid and melt compositions were re-constructed from mass-balances and published solubility data for H 2 O in silicate melts. Relative to the basanite melt, the solutes from H 2 O-fluids are enriched in SiO 2 , alkalis, Ba and Pb, but depleted in FeO, MgO, CaO and REE. With increasing pressure the mutual solubility of fluids and melts increases rapidly with complete miscibility between H 2 O and basanitic melts occurring between 3.0 and 4.0 GPa at 1100 °C. Although LREE are favoured over HREE in the fluid phase, they are less soluble than the HFSE (Nb, Ta, Zr, Hf and Ti). Thus the relative depletions of HFSE that are characteristic of arc magmas must be due to a residual phase that concentrates HFSE (e.g. rutile). Otherwise H 2 O-fluids have the capacity to impart many of the geochemical characteristics that distinguish some rocks and melts from the deep mantle lithosphere (e.g. MARID and lamproites).

Description: We describe the aftershocks of a Mw 7.4 intraplate normal-faulting earthquake that occurred 150 km east Ogasawara (Bonin) Islands, Japan, on December 21, 2010. It occurred beneath the outer trench-slope of the Izu-Ogasawara trench, where the Pacific plate subducts beneath the Philippine Sea plate. Aftershock observations using ocean bottom seismographs (OBS) began soon after the earthquake and multi-channel seismic reflection surveys were conducted across the aftershock area. Aftershocks were distributed in a NW-SE belt 140 km long, oblique to the N-S trench axis. They formed three subparallel lineations along a fracture zone in the Pacific plate. The OBS observations combined with data from stations on Chichi-jima and Haha-jima Islands revealed a migration of the aftershock activity. The first hour, which likely outlines the mainshock rupture, was limited to an 80-km-long area in the central part of the subsequent aftershock area. The first hour activity occurred mainly around, and appears to have been influenced by, nearby large seamounts and oceanic plateau, such as the Ogasawara Plateau and the Uyeda Ridge. Over the following days, the aftershocks expanded beyond or into these seamounts and plateau. The aftershock distribution and migration suggest that crustal heterogeneities related to a fracture zone and large seamounts and oceanic plateau in the incoming Pacific plate affected the rupture of the mainshock. Such pre-existing structures may influence intraplate normal-faulting earthquakes in other regions of plate flexure prior to subduction.

Description: Variations in seismic velocities are essential in developing a better understanding of continental plate tectonics. Fortunately, the USArray has provided an excellent set of regional phases from the recent M5.6 Oklahoma earthquake (2011/11/06, Table 1) that can be used for such studies. Its strike-slip mechanism produced an extraordinary set of tangential recordings extending to the northern edge of the USArray. The crossover of the crustal slow S to the faster S n phase is well observed. S m S has a critical distance of around 2° and its first multiple, SmS 2 , reaches critical angle near a distance of about 4°, and so on, until S m S n merges with the stronger crustal Love waves. These waveforms are modeled in the period band of 2—100 s by assuming a simple three-layer crust and a two-layer mantle, which allows a grid-search approach. Our results favor a 15-km-thick low-velocity zone (LVZ) in the lower crust with an average shear velocity of less than 3.6 km/s. The short-period Lg waves (S-waves, at periods of 0.5—2 s) travel with velocities near 3.5 km/s and decay with distance faster than high-frequency S n (〉5.0 Hz) which travels at a velocity of 4.6 km/s and persists to large distances. Although these short-period waveforms are not modeled, their amplitude and travel times can be explained by adding a small velocity jump just below the Moho with essentially no attenuation. P n is equally strong but is complicated by the interference produced by the depth phase sP, but well modeled. The P-velocities appear normal with no definitive LVZ. While these observations of S n and P n are common beneath most cratons, the lower crustal LVZ appears to be anomalous and maybe indicative of hydrous processes, possibly caused by the descending Farallon slab.

Description: We have performed reaction experiments between 1, 4 and 5 wt.% CO 2 -bearing MORB-eclogite (recycled oceanic crust)–derived low degree andesitic partial melt and fertile peridotite at 1375 °C, 3 GPa for infiltrating melt fractions of 25% and 33% by weight. We observe that the reacted melts are alkalic with degree of alkalinity or Si-undersaturation increasing with increasing CO 2 content in reacting melt. Consequently, an andesite evolves through basanite to nephelinite owing to greater drawdown of SiO 2 from melt and enhanced precipitation of orthopyroxene in residue. We have developed an empirical model to predict reacted melt composition as a function of reacting andesite fraction and source CO 2 concentration. Using our model, we have quantified the mutual proportions of equilibrated melt from andesite-peridotite (+ CO 2 ) hybridization and subsequent peridotite (± CO 2 )-derived melt required to produce the major element composition of various ocean island basalts. Our model can thus be applied to characterize the source of ocean islands from primary alkalic lava composition. Accordingly, we determined that average HIMU source requires 20 wt.% of MORB-eclogite-derived melt relative to peridotite containing 0.4 wt.% CO 2 and subsequent contribution of 40% of volatile-free peridotite partial melt. We demonstrate that mantle hybridization by eclogite melt-peridotite (± CO 2 ) reaction in the system can produce high MgO (〉15 wt.%) basaltic melts at mantle potential temperature ( T P ) of 1350 °C. Therefore, currently used thermometers to estimate T P using MgO content of primary alkalic melts need to be revised, with corrections for melt-rock reaction in a heterogeneous mantle as well as presence of CO 2 .

Description: We obtain scaling relationships for nonlinear attenuation of S-waves and Love waves within sedimentary basins to assist numerical modeling. These relationships constrain the past peak ground velocity (PGV) of strong 3-4 s Love waves from San Andreas events within Greater Los Angeles, as well as the maximum PGV of future waves that can propagate without strong nonlinear attenuation. During each event, the shaking episode cracks the stiff, shallow rock. Over multiple events this repeated damage in the upper few hundred meters leads to self-organization of the shear modulus. Dynamic strain is PGV divided by phase velocity and dynamic stress is strain times the shear modulus. The frictional yield stress is proportional to depth times the effective coefficient of friction. At the eventual quasi-steady self-organized state, the shear modulus increases linearly with depth allowing inference of past typical PGV where rock over the damaged depth range barely reaches frictional failure. Still greater future PGV would cause frictional failure throughout the damaged zone, nonlinearly attenuating the wave. Assuming self-organization has taken place, estimated maximum past PGV within Greater Los Angeles basins is 0.4 to 2.6 m s -1 . The upper part of this range includes regions of accumulating sediments with low S-wave velocity that may have not yet compacted, rather than having been damaged by strong shaking. Published numerical models indicate that strong Love waves from the San Andreas Fault pass through Whittier Narrows. Within this corridor, deep drawdown of the water table from its currently shallow and pre-industrial levels would nearly double PGV of Love waves reaching Downtown Los Angeles.

Description: Paleomagnetic studies of dyke swarms from the Variscan belt of Europe can be used to reconstruct internal post-orogenic rotations within the fold belt. Here, we present paleomagnetic data from 13 late Variscan dykes from Sardinia ranging in age from 298 ± 5Ma to 270 ± 10Ma. The dykes can be grouped on the basis of their different directions in strike in a northern, a central-eastern and a south-eastern province. Paleomagnetic component directions have been obtained using thermal and alternating field demagnetization techniques, which give reproducible results. The paleomagnetic mean directions differ significantly between northern Sardinia and south-eastern and central-eastern Sardinia, the latter two regions yielding statistically similar paleomagnetic mean directions. These results indicate that Sardinia fragmented into two, arguably three, crustal blocks after emplacement of the dykes, which experienced differential relative rotations, as is also indicated by the differences in overall strike directions. The determination of timing, sense and magnitude of these rotations has major implications for the reconstruction of the geodynamic evolution of the region in post-Carboniferous times. We argue that the observed block rotations occurred during the Permian as the result of post-Variscan intra-Pangea mobility possibly related to the transformation of an Early Permian Pangea B to a Late Permian Pangea A.

Description: High-resolution magnetic surveys acquired near the seafloor show that active basalt-hosted hydrothermal sites are associated with zones of lower magnetization. This observation may reflect the thermal demagnetization of a hot hydrothermal zone, the alteration of basalt affected by hydrothermal circulation, and/or the presence of thick, non-magnetic hydrothermal deposits. In order to discriminate among these inferences, we acquired vector magnetic data 50 m above inactive hydrothermal site Krasnov using the Remotely-Operated Vehicle (ROV) Victor . This deep hydrothermal site, located 7 km east of the Mid-Atlantic Ridge (MAR) axis at 16°38'N, is dissected by major normal faults and shows no evidence of recent hydrothermal activity. It is therefore a perfect target for investigating the magnetic signature of an inactive basalt-hosted hydrothermal site. Krasnov exhibits a strong negative magnetic anomaly, which implies that the lower magnetization observed at basalt-hosted hydrothermal sites is not a transient effect associated to hydrothermal activity, but remains after activity ceases. Thermal demagnetization plays only a secondary role, if any, in the observed magnetic low. Forward models suggest that both the non-magnetic hydrothermal deposits and an altered zone of demagnetized basalt are required to account for the observed magnetic low. The permanence of this magnetic signature makes it a useful tool to explore mid-ocean ridges and detect inactive hydrothermal sites.

Description: The ferrimagnetic mineral greigite (cubic Fe 3 S 4 ) is well known as an intracellular biomineralization product in magnetic bacteria and as a widely occurring authigenic mineral in anoxic sediments. Due to the lack of suitable single-crystal specimens, the magnetic anisotropy parameters of greigite have remained poorly constrained, to the point where not even the easy axis of magnetization is known. Here we report on an effort to determine the anisotropy parameters on the basis of ferromagnetic resonance (FMR) powder spectroscopy on hydrothermally synthesized, chemically pure greigite microcrystals dispersed in a nonmagnetic matrix. In terms of easy axis orientations, the FMR data are consistent with 〈111〉 or 〈100〉, or less likely, a more general 〈uv0〉 type. With a g factor of 2.09, the anisotropy field is about 90 mT and in some samples may reach 125 mT, compared to 30 mT for cubic magnetite. This confirms the dominating role of cubic anisotropy on the magnetic properties of greigite, which we show to be responsible for large SIRM/k values. K 1 is in the range -15 … - 23 J/m 3 (〈111〉) or +10 … +15 kJ/m 3 (〈100〉), yielding upper limits of 44 or 34 nm for the superparamagnetic grain size, respectively.

Description: We present multiple lines of evidence for years- to decade-long changes in the location and character of volcanic activity at West Mata seamount in the NE Lau Basin over a 16 year period, and a hiatus in summit eruptions from early 2011 through at least September 2012. Boninite lava and pyroclasts were observed erupting from its summit in 2009 and hydroacoustic data from a succession of hydrophones moored nearby show near-continuous eruptive activity from January 2009 to early 2011. Successive differencing of seven multibeam bathymetric surveys of the volcano made in the 1996-2012 period reveal a pattern of extended constructional volcanism on the summit and northwest flank punctuated by eruptions along the volcano's WSW rift zone (WSWRZ). Away from the summit, the volumetrically largest eruption during the observational period occurred between May 2010 and November 2011 at ~2920 m depth near the base of the WSWRZ. The (nearly) equally long ENE rift zone did not experience any volcanic activity during the 1996-2012 period. The cessation of summit volcanism recorded on the moored hydrophone was accompanied or followed by the formation of a small summit crater and a landslide on the eastern flank. Water column sensors, analysis of gas samples in the overlying hydrothermal plume and dives with a remotely operated vehicle in September 2012 confirmed that the summit eruption had ceased. Based on the historical eruption rates calculated using the bathymetric differencing technique, the volcano could be as young as several thousand years.

Description: Understanding carbon cycling in continental margin settings is critical for constraining the global carbon cycle. Here we apply a multiproxy geochemical approach to evaluate regional carbon cycle dynamics in six New Zealand fjords. Using carbon and nitrogen concentrations and isotopes, lipid biomarkers, and redox-sensitive element concentrations, we show that the New Zealand fjords have carbon-rich surface sediments in basins that promote long-term storage (i.e., semi-restricted basins with sediment accumulation rates of up to 3 mm yr -1 ). Using δ 13 C distributions to develop a mixing model, we find that organic carbon in fjord sediments is well-mixed from marine and terrestrial sources in down-fjord gradients. This is driven by high regional precipitation rates of 〉6 m yr -1 , which promote carbon accumulation in fjord basins through terrestrial runoff. In addition, we have identified at least two euxinic sub-basins, based on uranium, molybdenum, iron, and cadmium enrichment, that contain 〉7% organic carbon. Because the strength and position of the Southern Hemisphere westerly winds control precipitation and fjord circulation, carbon delivery and storage in the region are intimately linked to westerly wind variability. We estimate that the fjord region (759 km 2 ) may be exporting up to 1.4 x 10 7 kgC yr -1 , outpacing other types of continental margins in rates of carbon burial by up to three orders of magnitude.

Description: Evidence from sediment core records and model studies suggest that increased nutrient supply played a key role in the initiation of the Cenomanian-Turonian oceanic anoxic event 2 (OAE2; 94 Ma). However, the relative roles of nitrogen (N) and phosphorus (P) availability in controlling primary productivity during the event are not fully understood. Here, we expand an existing multi-box model of the coupled cycles of P, carbon and oxygen in the proto-North Atlantic by adding the marine N cycle. With the updated version of the model, we test the hypothesis that enhanced availability of P can fuel N 2 -fixation, increase primary productivity and drive large parts of the proto-North Atlantic to anoxia during OAE2. In a sensitivity analysis, we demonstrate that N dynamics in the proto-North Atlantic respond strongly to variations in oxygen and P supply from the Pacific Ocean and to changes in circulation. The implemented N cycle weakly modifies the carbon cycle, implying that P was the major nutrient controlling primary productivity during OAE2. Our model suggests that both N 2 -fixation and upwelling of recycled were enhanced during OAE2 and that N 2 -fixation was the major source of N in the proto-North Atlantic. Denitrification was more important in the water column than in sediments, with high rates in the open ocean and in the Western Interior. High P inputs in the proto-North Atlantic led to widespread N 2 -fixation, which more than compensated for the loss of N through denitrification. As a consequence, rates of primary productivity and organic carbon burial were high.

Description: The development of pass-through superconducting rock magnetometers (SRM) has greatly promoted collection of paleomagnetic data from continuous long-core samples. The output of pass-through measurement is smoothed and distorted due to convolution of magnetization with the magnetometer sensor response. Although several studies could restore high-resolution paleomagnetic signal through deconvolution of pass-through measurement, difficulties in accurately measuring the magnetometer sensor response have hindered the application of deconvolution. We acquired reliable sensor response of an SRM at the Oregon State University based on repeated measurements of a precisely fabricated magnetic point source. In addition, we present an improved deconvolution algorithm based on Akaike's Bayesian Information Criterion (ABIC) minimization, incorporating new parameters to account for errors in sample measurement position and length. The new algorithm was tested using synthetic data constructed by convolving “true” paleomagnetic signal containing an “excursion” with the sensor response. Realistic noise was added to the synthetic measurement using Monte Carlo method based on measurement noise distribution acquired from 200 repeated measurements of a u-channel sample. Deconvolution of 1000 synthetic measurements with realistic noise closely resembles the “true” magnetization, and successfully restored fine-scale magnetization variations including the “excursion”. Our analyses show that inaccuracy in sample measurement position and length significantly affects deconvolution estimation, and can be resolved using the new deconvolution algorithm. Optimized deconvolution of 20 repeated measurements of a u-channel sample yielded highly consistent deconvolution results and estimates of error in sample measurement position and length, demonstrating the reliability of the new deconvolution algorithm for real pass-through measurements.

Description: Submarine landslide deposits in an active subduction zone were investigated by Integrated Ocean Drilling Program Expedition 333 as the “Nankai Trough Submarine Landslides History”. The expedition recovered a Pleistocene to Holocene sequence of stacked mass-transport deposits at Site C0018, located within a slope basin on the footwall of the megasplay fault in the Nankai Trough off the Kii Peninsula, southwest Japan. Six mass-transport deposit units intercalated with coherent intervals were recovered from the upper 190-m of the drilled succession. Anisotropy of magnetic susceptibility variations within mass-transport deposit units was used to ascertain their rheology. Shape parameters and magnetic fabric orientation reveal inhomogeneity through the sequences, indicating that different compaction and shear occurred within individual units. The upper intervals of younger units generally represents a magnetic fabric formed under vertical compression. However, the lower intervals involve magnetic fabrics indicating lateral shear with in-depth gradual change. In the older mass-transport deposit, a distribution of magnetic foliation forms tightly folded strata, indicating different sliding patterns from the younger mass-transport deposit units. Using available paleomagnetic data, the shear directions of basal intervals of units are reoriented, producing two different sliding orientations. Variation in the flow type and supply route is interpreted to reflect the slope condition (e.g. slope gradient and susceptibility to ground motion), which has been controlled by local tectonic evolution of the accretionary wedge.

Description: A tephrostratigraphy of the active Antarctic volcano, Mt. Erebus, was determined from englacial tephra on the ice-covered flanks of Erebus and an adjacent volcano. The tephra are used to reconstruct the eruptive history and magmatic evolution of Erebus. More fine grained and blocky particles define tephra formed in phreatomagmatic eruptions and larger fluidal shards are characteristic of magmatic eruptions and in some cases both eruptive types are identified in a single mixed tephra. The eruptions forming the mixed tephra likely started as phreatomagmatic eruptions which transitioned into Strombolian eruptions as the non-magmatic water source was exhausted. We reconstructed the eruptive history of Erebus using the tephra layers stratigraphic position, 40 Ar/ 39 Ar ages, shard morphology and grain size. Major and trace element analyses of individual glass shards were measured by electron probe microanalysis and LA-ICP-MS. Trachybasalt, trachyte and phonolite tephra were identified. All phonolitic tephra are Erebus-derived with compositions similar to volcanic bombs erupted from Erebus over the past 40 years. The tephra show that Erebus magma has not significantly changed for 40ka. The uniformity of the glass chemical composition implies that the phonolite magma has crystallized in the same manner without change throughout the late Quaternary, suggesting long-term stability of the Erebus magmatic system. Trachyte and trachybasalt tephra were likely erupted from Marie Byrd Land and the McMurdo Sound area, respectively. The trachytic tephra can be regionally correlated, and could provide an important time-stratigraphic marker in Antarctic ice cores.

Description: Hillslope response to climate-driven fluvial incision controls sediment export and relief generation in most mountainous settings. Following the shift to a warmer, wetter climate after the Last Glacial Maximum (LGM) (˜18 ka), the Waipaoa River (New Zealand) rapidly incised up to 120 meters, leaving perched, low-relief hillslopes unadjusted to that base level fall. In the Mangataikapua—a 16.5 km 2 tributary principally comprised of weak mélange—pervasive post-LGM landslides responded to 〉50 m of fluvial incision by sculpting and denuding 〉99% of the catchment. By reconstructing LGM and younger paleosurfaces from tephra identified by electron microprobe analysis (EMPA) and lidar-derived surface roughness, we estimate the volume, timing, and distribution of hillslope destabilization in the Mangataikapua and the relative contribution of landslide-prone terrain to post-LGM landscape evolution. We calculate volume change between four paleosurfaces constrained by tephra age (Rerewhakaaitu, 17.5 ka; Rotoma, 9.4 ka; Whakatane, 5.5 ka; and Waimihia, 3.4 ka). From the paleosurface reconstructions, we calculate the total post-LGM hillslope sediment contribution from the Mangataikapua catchment to be 0.5 ± 0.06 (s.d.) km 3 , which equates to a sub-catchment averaged erosion rate of ˜1.6 mm yr -1 . This is double the previous hillslope volume when normalized by study area, demonstrating that landslide-prone catchments disproportionately contribute to the terrestrial post-LGM sediment budget. Finally, we observe particularly rapid post-Waimihia erosion rates, likely impacted by human settlement.

Description: We present a new global model of plate motions and strain rates in plate boundary zones constrained by horizontal geodetic velocities. This Global Strain Rate Model (GSRM v.2.1) is a vast improvement over its predecessor both in terms of amount of data input as in an increase in spatial model resolution by factor of ˜2.5 in areas with dense data coverage. We determined 6739 velocities from time-series of (mostly) continuous GPS measurements; i.e., by far the largest global velocity solution to date. We transformed 15772 velocities from 233 (mostly) published studies onto our core solution to obtain 22511 velocities in the same reference frame. Care is taken to not use velocities from stations (or time periods) that are affected by transient phenomena; i.e., this data-set consists of velocities best representing the interseismic plate velocity. About 14% of the Earth is allowed to deform in 145,086 deforming grid cells (0.25º longitude by 0.2º latitude in dimension). The remainder of the Earth’s surface is modeled as rigid spherical caps representing 50 tectonic plates. For 36 plates we present new GPS-derived angular velocities. For all the plates that can be compared with the most recent geologic plate motion model, we find that the difference in angular velocity is significant. The rigid-body rotations are used as boundary conditions in the strain rate calculations. The strain rate field is modeled using the Haines and Holt method, which uses splines to obtain an self-consistent interpolated velocity gradient tensor field, from which strain rates, vorticity rates, and expected velocities are derived. We also present expected faulting orientations in areas with significant vorticity, and update the no-net rotation reference frame associated with our global velocity gradient field. Finally, we present a global map of recurrence times for M w =7.5 characteristic earthquakes.

Description: We used laboratory experiments to examine the rise process in neutrally-buoyant jets that resulted from an unsteady supply of momentum, a condition that defines plumes from discrete Vulcanian- and Strombolian-style eruptions. We simultaneously measured the analog-jet discharge rate (the supply rate of momentum) and the analog-jet internal velocity distribution (a consequence of momentum transport and dilution). Then, we examined the changes in the analog-jet velocity distribution over time to assess the impact of the supply-rate variations on the momentum-driven rise dynamics. We found that the analogue jet velocity distribution changes significantly and quickly as the supply rate varied, such that the whole-field distribution at any instant differed considerably from the time-average. We also found that entrainment varied in space and over time with instantaneous entrainment coefficient values ranging from 0 to 0.93 in an individual unsteady jet. Consequently, we conclude that supply rate variations exert first-order control over jet dynamics, and therefore cannot be neglected in models without compromising their capability to predict large-scale eruption behavior. These findings emphasize the fundamental differences between unsteady and steady jet dynamics, and show clearly that: (i) variations in source momentum flux directly control the dynamics of the resulting flow; and (ii) impulsive flows driven by sources of varying flux cannot reasonably be approximated by quasi-steady flow models. New modeling approaches capable of describing the time-dependent properties of transient volcanic eruption plumes are needed before their trajectory, dilution and stability can be reliably computed for hazards management.

Description: In contrast to larger river systems that drain relatively pristine basins, little is known about the sediment geochemistry of rivers impacted by intense human activities. In this paper, we present a systematic investigation of the anthropogenic overprints on element geochemistry in sediments of the human-impacted Seine River, France. Most elements are fractionated by grain size, as shown by the comparison between suspended particulate matter (SPM) and riverbank deposits (RBD). The RBD are particularly coarse and enriched in carbonates and heavy minerals and thus in elements such as Ba, Ca, Cr, Hf, Mg, Na, REEs, Sr, Ti, Th, and Zr. Although the enrichment/depletion pattern of some elements ( e.g . K, REEs, Zr, etc.) can largely be explained by a binary mixture between two sources, other elements such as Ag, Bi, Cr, Cd, Co, Cu, Fe, Mo, Ni, Pb, Sb, Sn, W and Zn in SPM in Paris show that a third end-member having anthropogenic characteristics is needed to account for their enrichment at low water stage. These “anthropophile” elements, with high enrichment factors (EF) relative to the upper continental crust (UCC), display a progressive enrichment downstream and different geochemical behaviors with respect to the hydrodynamic conditions ( e.g . grain size) compared to elements having mainly a natural origin. Our findings emphasize the need for systematic studies of these anthropophile elements in other human-impacted rivers using geochemical normalization techniques, and stress the importance of studying the chemical variability associated with hydrodynamic conditions when characterizing riverine element geochemistry and assessing their flux to the ocean.

Description: NW Rota-1 is a submarine volcano in the Mariana volcanic arc located ~100 km north of Guam. Underwater explosive eruptions driven by magmatic gases were first witnessed there in 2004 and continued until at least 2010. During a March 2010 expedition, visual observations documented continuous but variable eruptive activity at multiple vents at ~560 m depth. Some vents released CO 2 bubbles passively and continuously, while others released CO 2 during stronger but intermittent explosive bursts. Plumes of CO 2 bubbles in the water column over the volcano were imaged by an EM122 (12 kHz) multibeam sonar system. Throughout the 2010 expedition numerous passes were made over the eruptive vents with the ship to document the temporal variability of the bubble plumes and relate them to the eruptive activity on the seafloor, as recorded by an in situ hydrophone and visual observations. Analysis of the EM122 mid-water dataset shows: (1) bubble plumes were present on every pass over the summit and they rose 200-400 m above the vents but dissolved before they reached the ocean surface, (2) bubble plume deflection direction and distance correlate well with ocean current direction and velocity determined from the ship's acoustic doppler current profiler, (3) bubble plume heights and volumes were variable over time and correlate with eruptive intensity as measured by the in situ hydrophone. This study shows that mid-water multibeam-sonar data can be used to characterize the level of eruptive activity and its temporal variability at a shallow submarine volcano with robust CO 2 output.

Description: Mangaia hosts the most radiogenic Pb-isotopic compositions observed in ocean island basalts and represents the HIMU (high μ = 238 U/ 204 Pb) mantle end-member, thought to result from recycled oceanic crust. Complete geochemical characterization of the HIMU mantle end-member has been inhibited due to a lack of deep submarine glass samples from HIMU localities. We homogenized olivine-hosted melt inclusions separated from Mangaia lavas and the resulting glassy inclusions made possible the first volatile abundances to be obtained from the HIMU mantle end-member. We also report major and trace element abundances and Pb-isotopic ratios on the inclusions, which have HIMU isotopic fingerprints. We evaluate the samples for processes that could modify the volatile and trace element abundances post-mantle melting, including diffusive Fe and H 2 O-loss, degassing, and assimilation. H 2 O/Ce ratios vary from 119 to 245 in the most pristine Mangaia inclusions; excluding an inclusion that shows evidence for assimilation, the primary magmatic H 2 O/Ce ratios vary up to ~200, and are consistent with significant dehydration of oceanic crust during subduction and long-term storage in the mantle. CO 2 concentrations range up to 2346 ppm CO 2 in the inclusions. Relatively high CO 2 in the inclusions, combined with previous observations of carbonate blebs in other Mangaia melt inclusions, highlight the importance of CO 2 for the generation of the HIMU mantle. F/Nd ratios in the inclusions (30 ± 9; 2σ standard deviation) are higher than the canonical ratio observed in oceanic lavas, and Cl/K ratios (0.079 ± 0.028) fall in the range of pristine mantle (0.02-0.08).

Description: Previous studies showed that plate rheology exerts a dominant control on the shape and velocity of subducting plates. Here, we perform a systematic investigation of the role of elasticity in slab bending, using fully dynamic 2D models where an elastic, viscoelastic or viscoelastoplastic plate subducts freely into a purely viscous mantle. We derive a scaling relationship between the bending radius of viscoelastic slabs and the Deborah number, De, which is the ratio of Maxwell time over deformation time. We show that De controls the ratio of elastically stored energy over viscously dissipated energy and find that at De 〉 - 2 , substantially less energy is required to bend a viscoelastic slab to the same shape as a purely viscous slab with the same intrinsic viscosity. Elastically stored energy at higher De favours retreating modes of subduction via unbending, while trench advance only occurs for some cases with De 〈 10 -2 . We estimate the apparent Deborah numbers of natural subduction zones and find values ranging from 10 - 3 to 〉 1, where most zones have low De 〈 10 -2 , but a few young plates have De 〉 0.1. Slabs with De 〈 10 -2 either have very low viscosities or they may be yielding, in which case our De estimates may be underestimated by up to an order of magnitude potentially pointing towards a significant role of elasticity in ~60% of the subduction zones. In support of such a role of elasticity in subduction, we find that increasing De correlates with increasing proportion of larger seismic events in both instrumental and historic catalogues.

Description: The 16°N segment of the East Pacific Rise is the most over-inflated and shallowest of this fast-spreading ridge, in relation with an important magma flux due to the proximity of the Mathematician hotspot. Here, we analyze the detailed morphology of the axial dome and of the Axial Summit Trough (AST), the lava morphology and the geometry of fissures and faults, in regard to the attributes of the magma chamber beneath and of the nearby hotspot. The data used are 1-meter-resolution bathymetry combined with seafloor photos and videos. At the dome summit, the AST is highly segmented by ten 3 rd and 4th -order discontinuities over a distance of 30 km. Often, two contiguous and synchronous ASTs coexist. Such a configuration implies a wide (1100 m minimum) zone of diking. The existence of contiguous ASTs, their mobility, their general en echelon arrangement accommodating the bow shape of the axial dome toward the hotspot, plus the existence of a second magma lens under the western half of the summit plateau, clearly reflect the influence of the hotspot on the organization of the spreading system. The different ASTs exhibit contrasted widths and depths. We suggest that narrow ASTs reflect an intense volcanic activity that produces eruptions covering the tectonic features and partially filling the ASTs. AST widening and deepening would indicate a decrease in volcanic activity but with continued dike intrusions at the origin of abundant sets of fissures and faults that are not masked by volcanic deposits.

Description: Massive organic matter burial due to widespread ocean anoxia across the Cenomanian/Turonian boundary event (~94 Ma), resulted in a major perturbation of the global carbon cycle: the so-called Oceanic Anoxic Event 2 (OAE2). The characteristics and spatial distribution of the OAE2 deposits that formed in the deep basin of the proto-North Atlantic remain poorly described, however. Here, we present proxy data of redox sensitive (trace) elements (e.g., Mo, Fe/Al, C org /P tot and Mn) for OAE2 sediments from five Deep Sea Drilling Project and Ocean Drilling Program sites located in the deep proto-North Atlantic basin. Our results highlight that bottom waters in the entire deep proto-North Atlantic were anoxic during most of OAE2. Furthermore, regressions of Mo with total organic carbon content (TOC), previously shown to document the degree of water mass restriction, confirm that the water circulation in the proto-North Atlantic basin was severely restricted during OAE2. Comparison of these values to Mo/TOC ratios in the present-day Black Sea suggest a renewal frequency of the deep proto-North Atlantic water mass of between 0.5 and 4 ka, compared to a maximum of ~200 years for the present-day northern Atlantic. The Plenus Cold Event, a cooler episode during the early stages of OAE2 hypothesized to be caused by declining p CO 2 due to extensive burial of organic matter, appears to have led to temporary re-oxygenation of the bottom water in the deep proto-North Atlantic basin during OAE2.

Description: We show results from a network of campaign Global Positioning System (GPS) sites in the Woodlark Rift, southeastern Papua New Guinea, in a transition from seafloor spreading to continental rifting. GPS velocities indicate anticlockwise rotation (at 2-2.7 º/Myr, relative to Australia) of crustal blocks north of the rift, producing 10-15 mm/yr of extension in the continental rift, increasing to 20-40 mm/yr of seafloor spreading at the Woodlark Spreading Center. Extension in the continental rift is distributed among multiple structures. These data demonstrate that low-angle normal faults in the continents, such as the Mai'iu Fault, can slip at high rates nearing 10 mm/yr. Extensional deformation observed in the D'Entrecasteaux Islands, the site of the world's only actively exhuming Ultra-High Pressure (UHP) rock terrane, supports the idea that extensional processes play a critical role in UHP rock exhumation. GPS data do not require significant interseismic coupling on faults in the region, suggesting that much of the deformation may be aseismic. Westward transfer of deformation from the Woodlark Spreading Center to the main plate boundary fault in the continental rift (the Mai'iu fault) is accommodated by clockwise rotation of a tectonic block beneath Goodenough Bay, and by dextral strike-slip on transfer faults within (and surrounding) Normanby Island. Contemporary extension rates in the Woodlark Spreading Center are 30-50% slower than those from seafloor spreading-derived magnetic anomalies. The 0.5 Ma-present seafloor spreading estimates for the Woodlark Basin may be overestimated, and a re-evaluation of these data in the context of the GPS rates is warranted.

Description: Four cruises between 2008 and 2012 monitored the continuing eruption of West Mata volcano in the NE Lau Basin as it produced plumes of chemically altered water above its summit. Although large enrichments in 3 He, CO 2, Fe, and Mn were observed in the plumes, the most notable enrichment was that of H 2 , which reached concentrations as high as 14843 nM. Strongly enriched H 2 concentrations in the water column result from reactions between seawater or magmatic water and extremely hot rocks. In 2008, the observation of elevated H 2 concentrations in the water column above West Mata pointed to vigorous ongoing eruptions at the volcano's summit. The eruption was confirmed by visual observations made by the ROV Jason 2 in 2009 and demonstrated that H 2 measurements are a vital instrument to detect ongoing volcanic eruptions at the seafloor. Elevated H 2 in 2010 showed that the eruption was ongoing, although at a reduced level given a maximum H 2 concentration of 4410 nM. In 2012, H 2 levels in the water column declined significantly, to a maximum of only 7 nM, consistent with visual observations from the Quest-4000 ROV that found no evidence of an ongoing volcanic eruption. Methane behaved independently of other measured gases and its concentrations in the hydrothermal plume were very low. We attribute its minimal enrichments to a mixture of mantle carbon reduced to CH 4 and biological CH 4 from diffuse flow sites. This study demonstrates that ongoing submarine volcanic eruptions are characterized by high dissolved H 2 concentrations present in the overlying water column.

Description: Subduction of tectonic plates into Earth's mantle occurs when one plate bends beneath another at convergent plate boundaries. The characteristic time of deformation at these convergent boundaries approximates the Maxwell relaxation time for olivine at lithospheric temperatures and pressures, it is therefore by definition a viscoelastic process. While this is widely acknowledged, the large-scale features of subduction can, and have been, successfully reproduced assuming the plate deforms by a viscous mechanism alone. However the energy rates and stress profile within convergent margins are influenced by viscoelastic deformation. In this study viscoelastic stresses have been systematically introduced into numerical models of free subduction, using both the viscosity and shear modulus to control the Maxwell relaxation time. The introduction of an elastic deformation mechanism into subduction models produces deviations in both the stress profile and energy rates within the subduction hinge when compared to viscous only models. These variations result in an apparent viscosity that is variable throughout the length of the plate, decreasing upon approach and increasing upon leaving the hinge. At realistic Earth parameters, we show that viscoelastic stresses have a minor effect on morphology yet are less dissipative at depth and result in an energy transfer between the energy stored during bending and the energy released during unbending. We conclude that elasticity is important during both bending and unbending within the slab hinge with the resulting stress loading and energy profile indicating that slabs maintain larger deformation rates at smaller stresses during bending, retaining their strength during unbending at depth.

Description: Phase velocity in the period range 30-130 seconds is measured for approximately 10,000 fundamental-mode Rayleigh waves traversing the Atlantic basin. In order to isolate the signal of the oceanic upper mantle, paths with 〉30% of their length through continental upper mantle are excluded. The lateral distribution of Rayleigh wave phase velocity in the Atlantic upper mantle is explored with two approaches. One, phase velocity is allowed to vary only as a function of seafloor age. Two, a general two-dimensional parameterization is utilized in order to capture perturbations to age-dependent structure. In both scenarios, phase velocity shows a strong dependence on seafloor age at all periods, with higher velocity associated with older seafloor. Removing age-dependent velocity from the 2-D phase-velocity maps highlights areas of anomalously low velocity, almost all of which are proximal to locations of hotspot volcanism. The age-dependent phase velocities for the Atlantic are not consistent with a half-space cooling model and are best explained by a plate cooling model with thickness of 75 km and mantle temperature of 1400 o C. In contrast, age-dependent phase velocities for the Pacific basin determined by Nishimura and Forsyth [1989] can be fit reasonably well by a half-space cooling model with mantle temperature approximately 50 o C warmer than the Atlantic. Comparison of Rayleigh wave phase velocity and fractionation-corrected Na concentrations in mid-ocean-ridge basalts erupted at 87 axial ridge segments reveals a positive correlation coefficient that increases with period, as expected if along-ridge variations in mantle potential temperature are controlling both quantities.

Description: The Matuyama-Brunhes (MB) geomagnetic reversal in Chinese loess has been studied extensively as an important boundary for land-ocean stratigraphic and paleoclimatic correlations. However, the apparent timing and duration of the MB boundary remains controversial in Chinese loess deposits due to its inconsistent stratigraphic position and the uncertain chronologies. Here we synthesized high-resolution paleomagnetic records from four loess sequences in the central Chinese Loess Plateau and synchronized the loess-paleosol chronology by matching the grain size variations to orbitally-tuned grain size time series. The synthesized paleomagnetic results reveal consistent features of the MB transition in Chinese loess, including the stratigraphic position (L 8 /S 8 transition), timing (˜808-826 ka), duration (˜14-16 ka), and rapid directional oscillations. Compared with the MB transition in marine records (770-775 ka), the timing of the MB transition is relatively older and longer in Chinese loess, due to a complex interplay between different remanence acquisition mechanisms which occurred during the course of post-depositional physical and chemical processes.

Description: Multiple late Pleistocene glaciations that extended onto the continental shelf offshore Massachusetts, USA may have emplaced as much as 100 km 3 of freshwater (salinity less than 5 ppt) in continental shelf sediments. To estimate the volume and extent of offshore freshwater, we developed a three-dimensional, variable-density model that couples fluid flow and heat and solute transport for the continental shelf offshore Massachusetts. The stratigraphy for our model is based on high-resolution, multi-channel seismic data. The model incorporates the last 3 Ma of climate history by prescribing boundary conditions of sea-level change and ice sheet extent and thickness. We incorporate new estimates of the maximum extent of a late Pleistocene ice sheet to near the shelf-slope break. Model results indicate that this late Pleistocene ice sheet was responsible for much of the emplaced freshwater. We predict that the current freshwater distribution may reach depths of up to 500 m below sea level and up to 30 km beyond Martha's Vineyard. The freshwater distribution is strongly dependent on the three-dimensional stratigraphy and ice-sheet history. Our predictions improve our understanding of the distribution of offshore freshwater, a potential non-renewable resource for coastal communities along recently glaciated margins.

Description: The estimation of the magma ascent rate is key to predicting volcanic activity and relies on the understanding of how strongly the ascent rate is controlled by different magmatic parameters. Linking potential changes of such parameters to monitoring data is an essential step to be able to use these data as a predictive tool. We present the results of a suite of conduit flow models Soufrière that assess the influence of individual model parameters such as the magmatic water content, temperature or bulk magma composition on the magma flow in the conduit during an extrusive dome eruption. By systematically varying these parameters we assess their relative importance to changes in ascent rate. We show that variability in the rate of low frequency seismicity, assumed to correlate directly with the rate of magma movement, can be used as an indicator for changes in ascent rate and, therefore, eruptive activity. The results indicate that conduit diameter and excess pressure in the magma chamber are amongst the dominant controlling variables, but the single most important parameter is the volatile content (assumed as only water). Modelling this parameter in the range of reported values causes changes in the calculated ascent velocities of up to 800%.

Description: Pumices, ashes and tuffs from Mt. St. Helens and from Novarupta contain two principal forms of titanomagnetite: homogeneous grains with Curie temperatures in the range 350-500°C; and oxyexsolved grains with similar bulk composition, containing ilmenite lamellae, and having Curie temperatures above 500°C. Thermomagnetic analyses and isothermal annealing experiments in combination with stratigraphic settings and thermal models show that emplacement temperatures and cooling history may have affected the relative proportions of homogeneous and exsolved grains, and have clearly had a strong influence on the Curie temperature of the homogeneous phase. The exsolved grains are most common where emplacement temperatures exceeded 600°C, and in laboratory experiments, heating to over 600°C in air causes the homogeneous titanomagnetites to oxyexsolve rapidly. Where emplacement temperatures were lower, Curie temperatures of the homogeneous grains are systematically related to overburden thickness and cooling timescales, and thermomagnetic curves are generally irreversible, with lower Curie temperatures measured during cooling, but little or no change is observed in room-temperature susceptibility. We interpret this irreversible behavior as reflecting variations in the degree of cation ordering in the titanomagnetites, although we cannot conclusively rule out an alternative interpretation involving fine-scale subsolvus unmixing. Short-range ordering within the octahedral sites may play a key role in the observed phenomena. Changes in the Curie temperature have important implications for the acquisition, stabilization and retention of natural remanence, and may in some cases enable quantification of the emplacement temperatures or cooling rates of volcanic units containing homogeneous titanomagnetites.

Description: Teleseismic body-wave imaging techniques such as receiver function analysis can be notoriously difficult to employ on ocean-bottom seismic data due largely to multiple reverberations within the water and low-velocity sediments. In lieu of suppressing this coherently scattered noise in ocean-bottom receiver functions, these site effects can be modeled in conjunction with shear velocity information from seafloor compliance and surface wave dispersion measurements to discern crustal structure. A novel technique to estimate 1D crustal shear-velocity profiles from these data using Monte Carlo sampling is presented here. We find that seafloor compliance inversions and P-S conversions observed in the receiver functions provide complimentary constraints on sediment velocity and thickness. Incoherent noise in receiver functions from the MOANA ocean bottom seismic experiment limit the accuracy of the practical analysis at crustal scales, but synthetic recovery tests and comparison with independent unconstrained nonlinear optimization results affirm the utility of this technique in principle.

Description: We have geochemically and statistically characterized bulk marine sediment and ash layers at Ocean Drilling Program Site 1149 (Izu-Bonin Arc) and Deep Sea Drilling Project Site 52 (Mariana Arc), and have quantified that multiple dispersed ash sources collectively comprise ~30-35% of the hemipelagic sediment mass entering the Izu-Bonin-Mariana subduction system. Multivariate statistical analyses indicate that the bulk sediment at Site 1149 is a mixture of Chinese Loess, a second compositionally distinct eolian source, a dispersed mafic ash, and a dispersed felsic ash. We interpret the source of these ashes as respectively being basalt from the Izu-Bonin Front Arc (IBFA) and rhyolite from the Honshu Arc. Sr-, Nd-, and Pb isotopic analyses of the bulk sediment are consistent with the chemical/statistical-based interpretations. Comparison of the mass accumulation rate of the dispersed ash component to discrete ash layer parameters (thickness, sedimentation rate, and number of layers) suggests that eruption frequency, rather than eruption size, drives the dispersed ash record. At Site 52, the geochemistry and statistical modeling indicates that Chinese Loess, IBFA, dispersed BNN (boninite from Izu-Bonin), and a dispersed felsic ash of unknown origin are the sources. At Site 1149 the ash layers and the dispersed ash are compositionally coupled, whereas at Site 52 they are decoupled in that there are no boninite layers, yet boninite is dispersed within the sediment. Changes in the volcanic and eolian inputs through time indicate strong arc- and climate-related controls.

Description: Seismic velocities in the lavas that cap normal oceanic crust are affected by both crack porosity and alteration of the primary mineral phases, chiefly to clays. Porosity accounts for 75-80% of the velocity variation in sonic log velocities in the lava sections of Holes 504B and 1256D, but the effect of alteration on the properties of the basalts has not been assessed. In this analysis, the grain velocities in basalt and diabase samples are estimated from an empirical linear relationship between grain density and the P-wave modulus. The theoretical velocity in fresh, zero-porosity basalt or diabase is 6.96±0.07 km/sec. Grain velocities in the diabase samples are statistically indistinguishable from the theoretical velocity, and show no variation with depth; alteration does not significantly affect the velocities in the diabase samples from Hole 504B. This result is consistent with previous analyses, which demonstrated that velocities in the dikes are controlled by crack porosity. In basalt lab samples, alteration reduces the average sample grain velocity to 6.74±0.02 km/sec; cracks at the sample scale further reduce the velocity to 5.86±0.03 km/sec, and large-scale cracks in the lavas reduce the average in situ velocity to 5.2±0.3 km/sec. Cracks account for nearly 90% of the difference between seismic (in situ) velocities and the theoretical velocity in the unaltered solid material. Basalt grain velocities show a small, but significant systematic increase with depth; the influence of alteration decreases with depth in the lavas, reaching near zero at the base of the lavas in Holes 504B and 1256D.

Description: The oceanic carbon system is commonly described in terms of the two state variables total carbon, DIC, and alkalinity, Alk. Here we suggest the use of alternative source adapted state variables, Acidic Carbon, AC and Basic Carbon, BC, defined by and related to (DIC, Alk) with a simple linear transformation. (AC, BC) can be interpreted as representing respectively the supply to the system of carbon dioxide and dissolved carbonate, keeping in mind that supply of hydrogen ions acts to transform from basic carbon to acidic carbon. Accordingly these variables tell us how much carbon dioxide or dissolved carbonate we actually have in the water, despite the fact that the major part of the carbon resides in bicarbonate ions. We claim that using these source related variables as a compliment to the traditional variables, offers a number of advantages in the formulation of continuity equations, as well as in the interpretation of observations and modelling results. The traditional definition of alkalinity is related to a measuring procedure rather than to the supply of material to the system. Here we demonstrate that alkalinity, though defined in the traditional way, may be interpreted in terms of sources and sinks acting on the system. In the case of ocean water this amounts to twice the supply of dissolved carbonate minus the net supply of free hydrogen ions. We argue that this interpretation is a useful complement to the traditional definition. Every process that affects the state of the carbon system may be quantified in terms of supply of carbon dioxide, F a , carbonate ions, F b , or hydrogen ions, E .

Description: Large dynamic strains carried by seismic waves are known to trigger seismicity far from their source region. It is unknown, however, whether surface waves trigger only small earthquakes, or whether they can also trigger large earthquakes. To partially address this question, we evaluate whether current data can distinguish between the magnitude distribution of triggered and untriggered small earthquakes. We use a mixing model approach in which total seismicity is decomposed into 2 classes: “triggered” events initiated or advanced by far-field dynamic strains, and “untriggered” spontaneous events consisting of everything else. The b -value of a mixed data set, b MIX , is decomposed into a weighted sum of b -values of its constituent components, b T and b U . We utilize the previously observed relationship between triggering rate and dynamic strain amplitude to identify the fraction of triggered events in populations of earthquakes and then invert for b T . For Californian seismicity, data are consistent with a single-parameter Gutenberg-Richter hypothesis governing the magnitudes of both triggered and untriggered earthquakes.

Description: We undertake a petrologically-driven approach to jointly model magnetotelluric (MT) and seismic surface wave dispersion (SW) data from central Tibet, constrained by topographic height. The approach derives realistic temperature and pressure distributions within the upper mantle and characterizes mineral assemblages of given bulk chemical compositions as well as water content. This allows us to define a bulk geophysical model of the upper mantle based on laboratory and xenolith data for the most relevant mantle mineral assemblages and to derive corresponding predicted geophysical observables. One-dimensional deep resistivity models were derived for two groups of MT stations. One group, located in the Lhasa Terrane, shows the existence of an electrically conductive upper-mantle layer and shallower conductive upper-mantle layer for the other group, located in the Qiangtang Terrane. The subsequent one-dimensional integrated petrological-geophysical modeling suggests a lithosphere-asthenosphere boundary (LAB) at a depth of 80-120 km with a dry lithosphere for the Qiangtang Terrane. In contrast, for the Lhasa Terrane the LAB is located at about 180 km but the presence of a small amount of water in the lithospheric mantle (〈0.02 wt%) is required to fit the MT responses. Our results suggest two different lithospheric configurations beneath southern and central Tibetan Plateau. The model for the Lhasa Terrane implies under-thrusting of a moderately wet Indian plate. The model for the Qiangtang Terrane shows relatively thick and conductive crust and implies a thin and dry Tibetan lithosphere.

Description: It is generally accepted that the Arabian Peninsula has been uplifted by sub-crustal processes. Positive residual depth anomalies from oceanic crust in the Red Sea and in the Gulf of Aden suggest that a region surrounding this peninsula is dynamically supported. Admittance calculations, surface wave tomography studies, and receiver function analyses all imply that regional topography is generated and maintained by some combination of mantle convective circulation and lithospheric thickness changes. Despite these significant advances, the spatial and temporal uplift rate history of the Arabian Peninsula is not well known. Here, we show that a regional uplift rate history can be obtained by jointly inverting 225 longitudinal river profiles that drain this peninsula. Our strategy assumes that shapes of individual river profiles are controlled by uplift rate history and moderated by erosional processes. We used local measurements of incision rate to calibrate the relevant erosional parameters. In our inverse algorithm, uplift rate is permitted to vary smoothly as a function of space and time but upstream drainage area remains invariant. We also assume that knickzone migration is not lithologically controlled. Implications of these important assumptions have been investigated. Our results suggest that the Arabian Peninsula underwent two phase asymmetric uplift during the last 20–30 Ma at rates of 0.05–0.1 mm/yr – 1 . The southwestern flank of the peninsula has been uplifted by 1.5–2.5 km. Regional stratigraphic constraints, the age and composition of volcanism, paleosol formation, incised peneplains, emergent marine terraces, and thermochronometric measurements corroborate our calculated patterns of uplift. Progessive development of three domal swells along the western margin of the peninsula is consistent with localized upwelling of hot asthenospheric mantle.

Description: We used the volcanic ash transport and dispersion model Ash3d to estimate the distribution of ashfall that would result from a modern-day Plinian supereruption at Yellowstone volcano. The simulations required modifying Ash3d to consider growth of a continent-scale umbrella cloud and its interaction with ambient wind fields. We simulated eruptions lasting 3 days, 1 week, and 1 month, each producing 330 km 3 of volcanic ash, dense-rock equivalent (DRE). Results demonstrate that radial expansion of the umbrella cloud is capable of driving ash upwind (westward) and crosswind (N-S) in excess of 1500 km, producing more-or-less radially symmetric isopachs that are only secondarily modified by ambient wind. Deposit thicknesses are decimeters to meters in the northern Rocky Mountains, centimeters to decimeters in the northern Midwest, and millimeters to centimeters on the East, West, and Gulf Coasts. Umbrella cloud growth may explain the extremely widespread dispersal of the ∼640 ka and 2.1 Ma Yellowstone tephra deposits in the eastern Pacific, northeastern California, southern California, and South Texas.

Description: Field, microstructural and 40 Ar/ 39 Ar thermochronologic data from the Louisiade Archipelago, the southern rifted margin of the Woodlark Basin in SE Papua New Guinea, document an accretionary wedge that formed during Early–Middle Miocene N-dipping subduction of the Australian margin and transpression along the Australian–Pacific plate boundary. Metasedimentary rocks of the Calvados Schist and the metagabbros that intrude them were metamorphosed at up to greenschist-facies conditions. Three tectonic foliations (S 1 –S 3 ) are present and F 1 –F 3 fold hinges plunge ESE or WNW, parallel to mineral, stretching, and intersection lineations. Fold vergence is dominantly to the SW, and top-to-the-SW thrusting of ultramafic rocks over the Calvados Schist is documented locally on Rossel Island. The data suggest progressive deformation associated with NNE–SSW shortening and ESE–WNW extension via dissolution-precipitation creep and, more locally, dislocation creep. 40 Ar/ 39 Ar step-heating analyses of three white mica separates yield Middle Miocene plateau or plateau-like segments that are affected by variable Pliocene argon loss and are interpreted as syntectonic mica growth during metamorphism and deformation followed by partial resetting just prior to the onset of seafloor spreading in the Woodlark Basin. A ca. 12 Ma 40 Ar/ 39 Ar age from a dacite sill of the Panarora Volcanics provides a minimum age constraint for the termination of northward subduction and a maximum age for cross-cutting brittle strike-slip faults. These data are critical to constraining the subduction–exhumation history of the world's youngest high–ultrahigh-pressure terrane and further support analogies between SE Papua New Guinea and the Early Oligocene Western Alps.

Description: [1] The Cenozoic in East Asia is marked by major changes in tectonics, landscapes and river systems, although the timing and nature of such changes remains disputed. We investigate the geochemistry and neodymium isotope character of Cenozoic mudstones spanning the breakup-unconformity in the Western Foothills of Taiwan in order to constrain erosion and drainage development in southern China during the opening of the South China Sea. The La/Lu, Eu/Eu*, Th/Sc, Th/La, Cr/Th and εNd values in these rocks show an abrupt change between ca. 31 Ma and 25 Ma. Generally the higher εNd values in sediments deposited prior to 31 Ma indicate erosion from Phanerozoic granitic sources exposed in coastal South China, whereas the lower εNd values suggest that the main sources had evolved to inland southern China by ca. 25 Ma. The SHRIMP U-Pb ages of zircons from a tuff, together with biostratigraphy data constrain the breakup-unconformity to be between ca. 39 and 33 Ma, suggesting that the seafloor spreading in the South China Sea commenced before ca. 33 Ma. This is significantly older than most of the oceanic crust preserved in the deeper part of the basin. Diachronous westward younging of the breakup-unconformities and provenance changes of basins are consistent with seafloor spreading propagating from east to west. Initial spreading of the South China Sea prior to ca. 33 Ma corresponds to tectonic adjustment in East Asia, including extrusion of the Indochina block and the rotation and eastward retreat of the subducting Pacific Plate.

Description: Although prior work suggests that a mantle plume is associated with Cenozoic rifting and volcanism in West Antarctica, the existence of a plume remains conjectural. Here, we use P-wave receiver functions (PRFs) from the Antarctic POLENET array to estimate mantle transition zone thickness, which is sensitive to temperature perturbations, throughout previously unstudied parts of West Antarctica. We obtain over 8000 high-quality PRFs using an iterative, time-domain deconvolution method filtered with a Gaussian-width of 0.5 and 1.0, corresponding to frequencies less than ∼0.24 Hz and ∼0.48 Hz, respectively. Single-station and common conversion point stacks, migrated to depth using the AK135 velocity model, indicate that mantle transition zone thickness throughout most of West Antarctica does not differ significantly from the global average, except in two locations; one small region exhibits a vertically thinned (210±15km) transition zone beneath the Ruppert Coast of Marie Byrd Land and another laterally broader region shows slight, vertical thinning (225±25km) beneath the Bentley Subglacial Trench. We also observe the 520 discontinuity and a prominent negative peak above the mantle transition zone throughout much of West Antarctica. These results suggest that the mantle transition zone may be hotter than average in two places, possibly due to upwelling from the lower mantle, but not broadly across West Antarctica. Furthermore, we propose that the transition zone may be hydrated due to 〉100 million years of subduction beneath the region during the early Mesozoic. This article is protected by copyright. All rights reserved.

Description: The ability to constrain km-scale exhumation with apatite 4 He/ 3 He thermochronometry is well established and the technique has been applied to a range of tectonic and geomorphic problems. However, multiple sources of uncertainty in specific crystal characteristics limit the applicability of the method, especially when geologic problems require identifying small perturbations in a cooling path. Here we present new 4 He/ 3 He thermochronometric data from the Appalachian Mountains, which indicate significant parent nuclide zonation in an apatite crystal. Using LA-ICPMS measurements of U and Th in the same crystal, we design a 3-D model of the crystal to explore the effects of intra-crystal variability in radiation damage accumulation. We describe a numerical approach to solve the 3-D production-diffusion equation. Using our numerical model and a previously determined time temperature path for this part of the Appalachians, we find excellent agreement between predicted and observed 4 He/ 3 He spectra. Our results confirm this time-temperature path and highlight that for complex U and Th zonation patterns, 3-D numerical models are required to infer an accurate time-temperature history. In addition, our results provide independent and novel evidence for a radiation damage control on diffusivity. The ability to exploit intra-crystal differences in 4 He diffusivity (i.e., temperature sensitivity) greatly increases the potential to infer complex thermal histories.

Description: The sensitivity of trace element concentrations to processes governing solid-melt interactions has made them valuable tools for tracing the effects of partial melting, fractional crystallization, metasomatism and similar processes on the composition of a parental melt. Recent studies of ocean island basalts (OIB) have sought to correlate Ti, Ta and Nb (TITAN) anomalies to isotopic tracers, such as 3 He/ 4 He and 187 Os/ 188 Os ratios, which may trace primordial deep mantle sources. A new compilation of global OIB trace element abundance data indicates that positive TITAN anomalies, though statistically pervasive features of OIB, may not be compositional features of their mantle sources. OIB show a range of Ti (Ti/Ti*=0.28-2.35), Ta (Ta/Ta*=0.11-93.4) and Nb (Nb/Nb*=0.13-17.8) anomalies that show negligible correlations with 3 He/ 4 He ratios, indicating that TITAN anomalies are not derived from the less-degassed mantle source traced by high- 3 He/ 4 He. Positive TITAN anomalies can be modelled using variable degrees (0.1-10%) of non-modal batch partial melting of garnet-spinel lherzolite at temperatures and pressures considered typical for OIB petrogenesis, and subjecting this partial melt to fractional crystallization and assimilation of mid-ocean ridge basalt-like crust (AFC). Correlations of TITAN anomalies with modal abundances of olivine and clinopyroxene in porphyritic Canary Islands lavas provide empirical support for this process and indicate that high abundances of these phases in OIB may create misleading trace element anomalies on primitive mantle-normalized spider diagrams. Because partial melting and AFC are common to all mantle-derived magmas, caution should be used when attributing TITAN anomalies to direct sampling of recycled or deep mantle sources by hotspots.

Description: Granitoids are widespread in Precambrian terranes as well as the Phanerozoic orogenic belts, but they have garnered little attention in paleomagnetic studies, because granitoids often contain abundant coarse-grained, magnetically unstable oxides. In this study, the first example of tiny, needle-shaped, exsolved oxides in plagioclase in a Paleoarchean granitoid is reported. The magnetic properties of single plagioclase crystals with the exsolved oxide inclusions have been studied to determine their paleomagnetic recording fidelity. Demagnetization experiments and hysteresis parameters indicate that the oxide inclusions are near stoichiometric magnetite and magnetically very stable. First-order reversal curve (FORC) diagrams reveal negligible magnetostatic interactions. Minimal interactions are also reflected by very efficient acquisition of anhysteretic remanent magnetization. Single plagioclase crystals exhibit strong magnetic remanence anisotropies, which require corrections to their paleodirectional and paleointensity data. Nonetheless, quantitative consideration of anisotropy tensors of the single plagioclase crystals indicates that the bias can be mitigated by properly averaging data from a few tens of single crystals. From the nonlinear thermoremanence acquisition of the plagioclase crystals, we estimate that the plagioclase crystals can reconstruct paleointensity up to 50 μ T. Local metamorphic condition suggests that those magnetite may carry remanence of ca. 3.2 to 3.3 Ga. We suggest that exsolved magnetite in granitoids is potentially a suitable target for the study of the early history of the geomagnetic field, and prompt detailed microscopic investigations as well as paleomagnetic tests to constrain the age of remanence. This article is protected by copyright. All rights reserved.

Description: We present new laser ablation ICPMS trace element concentration data for 28 elements in 97 mid ocean ridge basalt (MORB) glasses that cover all major spreading centers as well as Tl concentration data for all mineral phases in five lherzolites from the Lherz massif, France. The ratio between the elements thallium (Tl) and cerium (Ce) is nearly constant in MORB, providing evidence that the depleted MORB mantle (DMM) has uniform Ce/Tl. Lherzolite mineral data reveal that sulfides are heterogeneous and contain between 23 and 430ng/g of Tl while all other minerals contain Tl below the analytical detection limit of ∼1ng/g. We argue that Tl in MORB is controlled by residual sulfide during mantle melting. To investigate the observed relationship between Tl and Ce, we conduct models of fractional mantle melting, which show that the constant Ce/Tl in MORB is only reproduced if the ratio between clinopyroxene and sulfide in the upper mantle varies by less than 10%. In addition, the rate of melting for these two phases must be nearly identical as otherwise melt depletion and refertilization processes would lead to Ce/Tl fractionation. These model results allow us to establish a relationship for the sulfur content of DMM: [S] DMM = SCSS × M cpx ⁄R cpx , where SCSS is the sulfur concentration of a silicate melt at sulfide saturation, R cpx is the melt reaction coefficient and M cpx is the modal abundance of clinopyroxene in the DMM. Using this equation, we calculate that the average upper mantle sulfur concentration is 195±45μg/g. This article is protected by copyright. All rights reserved.

Description: Domal structures within the D'Entrecasteaux Islands of eastern Papua New Guinea expose ultrahigh-pressure (UHP) Pliocene (5.6–4.6 Ma) eclogites and evidence for partial melting. To better interpret the (U)HP exhumation history, U-Pb geochronology and trace-element abundances were determined in zircon from variably deformed host gneiss and crystallized melt (leucosomes, sills, dikes, and plutons) from the Goodenough and Normanby Domes by ID-TIMS (isotope-dilution thermal ionization mass spectrometry) and ICP-MS (inductively coupled plasma mass spectrometry), respectively, to constrain the timing of melt crystallization and deformation relative to UHP metamorphism. Zircons extracted from orthogneiss and deformed granodiorite sills of Normanby Dome, located ~40 km southeast of the UHP eclogite, record HP metamorphism from 5.66 ± 0.02 Ma to 5.04 ± 0.07 Ma and melt crystallization at ca. 4.1 Ma. Strongly deformed, layer-parallel leucosomes from Goodenough Dome, ~20 km northwest of the UHP eclogite, began to crystallize by 3.85 ± 0.02 Ma. These dates indicate that melt crystallization began in the Goodenough and Normanby Domes within ~0.75 m.y. of (U)HP metamorphism. The ID-TIMS dates from the orthogneiss and crystallized melt show that exhumation and cooling of the (U)HP rocks in the PNG terrane began first in the east, within Normanby Dome, then to the west, in the Goodenough Dome ~ 1 m.y. later, and finally the middle dome rocks, exposed within the Mailolo Dome, cooled ~ 2 m.y. after exhumation of Normanby Dome. All domes reveal synchronous crystallization of late, non-deformed melts and final extension-driven exhumation by 1.82 ± 0.03 Ma.

Description: For seismic surveys in shallow-water environments the complexity of local geology and seafloor topography can make it difficult to accurately predict associated sound levels and establish appropriate mitigation radii required to ensure the safety of local marine protected species. This is primarily because necessary detailed information regarding the local seafloor topography and subseafloor geology is often unavailable before a survey begins. One potential solution to this problem is to measure received levels using the ship's multi-channel seismic (MCS) streamer \citep{Diebold2010}, which could allow for the dynamic real-time determination of sound levels and mitigation radii while a survey is underway. We analyze {\em R/V Langseth} streamer data collected on the shelf and slope near the Washington coast during the Cascadia Open-Access Seismic Transects (COAST) and Ridge2Trench projects to measure received levels up to a distance of approximately 8 km from the sound source array. We establish methods to filter, clean, and process streamer data to accurately determine received power levels and confidently establish mitigation radii. We show that in shallow water measured power levels can fluctuate due to the influence of seafloor topographic features, but that the use of the streamer for the establishment of dynamic mitigation radii is feasible and should be further pursued. The establishment of mitigation radii based on local conditions may help maximize the safety of marine protected species while also maximizing the ability of researchers to conduct seismic studies.

Description: In the Bendeleben Mountains, Seward Peninsula, mid-Cretaceous granites are exposed in an uplifted block bounded on its south side by an E-W striking normal fault. The Bendeleben fault has well-preserved scarps 4-7 m in height that offset Holocene moraines. Seismic activity, young normal faulting, and Quaternary basaltic volcanism are all evidence of active extension. South of the Bendeleben fault, there is a 3-4 km deep basin. Fifteen apatite (U-Th)/He ages from granitic samples of the footwall yield an Eocene weighted mean age of 41.3±4.8 Ma. Biotite 40 Ar/ 39 Ar ages from the country rock of the Bendeleben pluton are 81-83 Ma. In spite of the young fault scarps, Pecube and HeFTy t-T modeling results illustrate that rapid exhumation of the Bendeleben Mountains occurred in the Late Cretaceous-Eocene and slowed since the Oligocene. A weak age-elevation relationship of apatite He ages and a lack of correlation between age and distance from the fault indicate that exhumation was accomplished with minimal block rotation on a steeply dipping, long-lived normal fault. Timing of extension in the Seward Peninsula can be correlated with deformation in the offshore Hope Basin where seismic reflection lines document Early Tertiary large-magnitude normal faulting followed by minor post-Miocene reactivation. The faulting observed in the Bendeleben Mountains is part of an extensional system that spans a large portion of the Bering Strait region. The tectonic model proposed in previous studies suggests that clockwise rotation of the Bering block relative to North America is the cause of extensional deformation in western Alaska.

Description: We present a geochemical and Sr-Nd isotopic study on a sediment core collected from the Andaman Sea in an attempt to reconstruct the Late Quaternary weathering and erosion patterns in the watersheds of the river systems of Myanmar and understand their controlling factors. Age control is based on nine radiocarbon dates and δ18O stratigraphy. The rate of sedimentation was strongly controlled by fluctuations of the monsoon. We identify three major sediment provenances: (1) the Irrawaddy catchment, (2) the western slopes of the Indo-Burman-Arakan (IBA) mountain ranges and the Andaman Islands and (3) the catchments of Salween and Sittang and the Bengal shelf, with the first two contributing 30 to 60% of the material. Enhanced contributions from juvenile sources and corresponding positive shifts of δ18O are observed at seven time periods (11-14, 20-23, 36, 45, 53, 57 and 62 ka) of which five are synchronous with cooling of the northern hemisphere - suggesting a link between the changes in sediment provenances and the shifting of the locus of the summer monsoon, southwards from the Himalayas, without substantial reduction in intensity. Our data, and that from other cores in the region suggest that an eastward moving surface current disperses sediments, derived from the Bengal shelf and western margin of Myanmar, from the eastern Bay of Bengal into the western Andaman Sea and that its strength has increased since the LGM. The existence of this current during the LGM implies that the Andaman Sea and the Bay of Bengal were well connected during the last glacial period.

Description: Sulfides entering subduction zones can play an important role in the release of sulfur and metals to the mantle wedge and contribute to the formation of volcanic arc-associated ores. Fractionation of stable sulfur isotopes recorded by sulfides during metamorphism can provide evidence of fluid-rock interactions during metamorphism and give insights on sulfur mobilization. A detailed micro-textural and geochemical study was performed on mineralized samples from two ocean floor-related sulfide deposits (Servette and Beth-Ghinivert) in high-pressure units of the Italian Western Alps, which underwent different metamorphic evolutions. The combination of micro-textural investigations with δ 34 S values from in situ ion-probe analyses within individual pyrite and chalcopyrite grains allowed an evaluation of the effectiveness of metamorphism in modifying the isotopic record and mobilizing sulfur and metals and have insights on fluid circulation within the slab. Textures and isotopic compositions inherited from the protolith are recorded at Beth-Ghinivert, where limited metamorphic re-crystallization is attributed to limited interaction with metamorphic fluids. Isotopic modification by metamorphic processes occurred only at the sub-millimeter scale at Servette, where local interactions with infiltrating hydrothermal fluid are recorded by metamorphic grains. Notwithstanding the differences recorded by the two deposits, neither underwent intensive isotopic re-equilibration or records evidence of intense fluid-rock interaction and S mobilization during metamorphism. Therefore, subducted sulfide deposits dominated by pyrite and chalcopyrite are unlikely to release significant quantities of sulfur to the mantle wedge and to arc magmatism sources at metamorphic grades below the lower eclogite facies.

Description: Three-dimensional models of mantle flow at subduction zones make it possible to explain the common occurrence of trench-parallel sub-slab seismic anisotropy. Sub-slab flow becomes inherently three-dimensional when slab-driven flow interacts with a wide variety of ambient background mantle flow conditions. This interaction depends on slab geometries, mechanical coupling parameters, and lower mantle viscosities. Deflection of sub-slab mantle flow is a robust feature for all model parameters and geometries as the slab acts as an obstruction to the ambient, background mantle flow. Background mantle flow can become trench-perpendicular or trench-parallel sub-slab flow depending on whether the ambient background mantle flow is deflected beneath the bottom of the slab or towards the edge of the slab. The first case is especially prominent in models with short slabs that do not penetrate into the lower mantle. The second case is especially prominent in models with long, steep slabs. The results are also highly sensitive to the amount of mechanical coupling between the subducting plate and the mantle beneath it. High levels of coupling create a boundary layer of trench-perpendicular entrained flow, pushing the deflection due to the obstructing slab away from the slab. We compare our sub-slab flow model predictions with a global set of seismic anisotropy fast directions in the sub-slab mantle, and find generally good agreement between the anisotropy observations (dominantly trench-parallel or trench-perpendicular) and the mantle flow directions predicted for decoupled systems.

Description: The deep magmatic processes in volcanic arcs are often poorly understood. We analyze the shear wave velocity (V S ) distribution in the crust and uppermost mantle below Mount Rainier, in the Cascades arc, resolving the main velocity contrasts based on converted phases within P coda via source normalization or receiver function (RF) analysis. To alleviate the trade-off between depth and velocity we use long period phase velocities (25-100s) obtained from earthquake surface waves, and at shorter period (7-21s) use seismic noise cross-correlograms. We use a transdimensional Bayesian scheme to explore the model space (V S in each layer, number of interfaces and their respective depths, level of noise on data). We apply this tool to 15 broadband stations from permanent and EarthScope temporary stations. Most results fall into 2 groups with distinctive properties. Stations east of the arc (Group I) have comparatively slower middle-to-lower crust (V S =3.4-3.8km/s at 25km depth), a sharp Moho and faster uppermost mantle (V S =4.2-4.4km/s). Stations in the arc (Group II) have a faster lower crust (V S =3.7-4km/s) overlying a slower uppermost mantle (V S =4.0-4.3km/s), yielding a weak Moho. Lower crustal velocities east of the arc (Group I) most likely represent ancient subduction mélanges mapped nearby. The lower crust for Group II ranges from intermediate to felsic. We propose that intermediate-felsic to felsic rocks represent the pre-arc basement, while intermediate composition indicates the mushy andesitic crustal magmatic system plus solidified intrusion along the volcanic conduits. We interpret the slow upper mantle as partial melt. This article is protected by copyright. All rights reserved.

Description: The thermodynamic modeling software MELTS is a powerful tool for investigating crystallization and melting in natural magmatic systems. Rhyolite-MELTS is a recalibration of MELTS that better captures the evolution of silicic magmas in the upper crust. The current interface of rhyolite-MELTS, while flexible, can be somewhat cumbersome for the novice. We present a new interface that uses web services consumed by a VBA backend in Microsoft Excel © . The interface is contained within a macro-enabled workbook, where the user can insert the model input information and initiate computations that are executed on a central server at OFM Research. Results of simple calculations are shown immediately within the interface itself. It is also possible to combine a sequence of calculations into an evolutionary path; the user can input starting and ending temperatures and pressures, temperature and pressure steps, and the prevailing oxidation conditions. The program shows partial updates at every step of the computations; at the conclusion of the calculations, a series of data sheets and diagrams are created in a separate workbook, which can be saved independently of the interface. Additionally, the user can specify a grid of temperatures and pressures and calculate a phase diagram showing the conditions at which different phases are present. The interface can be used to apply the rhyolite-MELTS geobarometer. We demonstrate applications of the interface using an example early-erupted Bishop Tuff composition. The interface is simple to use and flexible, but it requires an internet connection. The interface is distributed for free from http://melts.ofm-research.org . This article is protected by copyright. All rights reserved.

Description: Combined analyses of deep tow magnetic anomalies and International Ocean Discovery Program Expedition 349 cores show that initial seafloor spreading started around 33 Ma in the northeastern South China Sea (SCS), but varied slightly by 1-2 myr along the northern continent-ocean boundary (COB). A southward ridge jump of ∼ 20km occurred around 23.6 Ma in the East Subbasin; this timing also slightly varied along the ridge and was coeval to the onset of seafloor spreading in the Southwest Subbasin, which propagated for about 400km southwestward from ∼23.6 Ma to ∼ 21.5 Ma. The terminal age of seafloor spreading is ∼15 Ma in the East Subbasin and ∼16 Ma in the Southwest Subbasin. The full spreading rate in the East Subbasin varied largely from ∼20 to ∼80km/myr, but mostly decreased with time except for the period between ∼26.0 Ma and the ridge jump (∼23.6 Ma), within which the rate was the fastest at ∼70km/myr on average. The spreading rates are not correlated, in most cases, to magnetic anomaly amplitudes that reflect basement magnetization contrasts. Shipboard magnetic measurements reveal at least one magnetic reversal in the top 100m of basaltic layers, in addition to large vertical intensity variations. These complexities are caused by late-stage lava flows that are magnetized in a different polarity from the primary basaltic layer emplaced during the main phase of crustal accretion. Deep tow magnetic modeling also reveals this smearing in basement magnetizations by incorporating a contamination coefficient of 0.5, which partly alleviates the problem of assuming a magnetic blocking model of constant thickness and uniform magnetization. The primary contribution to magnetic anomalies of the SCS is not in the top 100m of the igneous basement. This article is protected by copyright. All rights reserved.

Description: Erupting magma often contains crystals over a wide range of sizes and shapes, potentially affecting magma viscosity over many orders of magnitude. A robust relation between viscosity and the modality of crystal sizes and shapes remains lacking, principally because of the dimensional complexity and size of the governing parameter space. We have performed a suite of shear viscosity measurements on liquid-particle suspensions of dynamical similarity to crystal-bearing magma. Our experiments encompass five suspension types, each consisting of unique mixtures of two different particle sizes and shapes. The experiments span two orthogonal subspaces of particle concentration, as well as particle size and shape for each suspension type, thereby providing insight into the topology of parameter space. For each suspension type, we determined the dry maximum packing fraction and measured shear rates across a range of applied shear stresses. The results were fitted using a Herschel-Bulkley model and augment existing predictive capabilities. We demonstrate that our results are consistent with previous work, including friction-based constitutive laws for granular materials. We conclude that predictions for ascent rates of crystal-rich magmas must take the shear-rate dependence of viscosity into account. Shear-rate dependence depends first and foremost on the volume fraction of crystals, relative to the maximum packing fraction, which in turn depends on crystal size and shape distribution. This article is protected by copyright. All rights reserved.

Description: Comparison of 398 fault offsets measured by visual analysis of WorldView high-resolution satellite imagery with deformation maps produced by COSI-Corr sub-pixel image correlation of Landsat-8 and SPOT5 imagery reveals significant complexity and distributed deformation along the 2013 M w 7.7 Balochistan, Pakistan earthquake. Average slip along the main trace of the fault was 4.2 m, with local maximum offsets up to 11.4 m. Comparison of slip measured from offset geomorphic features, which record localized slip along the main strand of the fault, to the total displacement across the entire width of the surface deformation zone from COSI-Corr reveals ~45 % off-fault deformation. Whereas previous studies have shown that the structural maturity of the fault exerts a primary control on the total percentage of off-fault surface deformation, large along-strike variations in the percentage of strain localization observed in the 2013 rupture imply the influence of important secondary controls. One such possible secondary control is the type of near-surface material through which the rupture propagated. We therefore compared the percentage off-fault deformation to the type of material (bedrock, old alluvium, young alluvium) at the surface and the distance of the fault to the nearest bedrock outcrop (a proxy for sediment thickness along this hybrid strike-slip/reverse slip fault). We find significantly more off-fault deformation in younger and/or thicker sediments. Accounting for and predicting such off-fault deformation patterns has important implications for the interpretation of geologic slip rates, especially for their use in probabilistic seismic hazard assessments, the behavior of near-surface materials during coseismic deformation, and the future development of microzonation protocols for the built environment. This article is protected by copyright. All rights reserved.

Description: As several modeling studies indicate, the structural expression and dynamic behavior of orogenic mountain belts is dictated not only by their rheological properties or by far-field tectonic motion, but also by the efficiency of erosion and sedimentation acting on its surface. Until recently, numerical investigations have been mainly limited to 2D studies because of the high computational cost required by 3D models. Here, we have efficiently coupled the landscape evolution model Cascade with the 3D thermo-mechanically coupled tectonics code FANTOM. Details of the coupling algorithms between both codes are given. We present results of numerical experiments designed to study the response of viscous-plastic crustal materials subjected to convergence and to surface processes including both erosion and sedimentation. In particular, we focus on the equilibration of both the tectonic structures and on the surface morphology of the orogen. We show that increasing the efficiency of fluvial erosion increases the frontal thrust angle, which in turn decreases the width of the orogen. In addition, the maximum summit elevation of the orogen during transient evolution is significantly higher in those models showcasing surface processes than those that do not. This illustrates the strong coupling between tectonic and surface processes. We also demonstrate that an along-strike gradient of erosion efficiency can have a major impact upon the landscape morphology and the tectonic structure and deformation of the orogen, in both the across- and along-strike directions. Overall, our results suggest that surface processes, by enhancing localization of deformation, can act as a positive forcing to topographic building. This article is protected by copyright. All rights reserved.

Description: Foulden Maar is a highly resolved maar lake deposit from the South Island of New Zealand comprising laminated diatomite punctuated by numerous diatomaceous turbidites. Basaltic clasts found in debris flow deposits near the base of the cored sedimentary sequence yielded two new 40 Ar/ 39 Ar dates of 24.51 ± 0.24 Ma and 23.38 ±. 24 Ma (2σ). The younger date agrees within error with a previously published 40 Ar/ 39 Ar date of 23.17 ± 0.19 Ma from a basaltic dyke adjacent to the maar crater. The diatomite is inferred to have been deposited over several tens of thousands of years in the latest Oligocene/earliest Miocene, and may have been coeval with the period of rapid glaciation and subsequent deglaciation of Antarctica known as the Mi-1 event. Sediment magnetic properties and SEM measurements indicate that the magnetic signal is dominated by pseudo-single domain pyrrhotite. The most likely source of detrital pyrrhotite is schist country rock fragments from the inferred tephra ring created by the phreatomagmatic eruption that formed the maar. Variations in magnetic mineral concentration indicate a decrease in erosional input throughout the depositional period, suggesting long-term (tens of thousands of years) environmental change in New Zealand in the latest Oligocene/earliest Miocene. This article is protected by copyright. All rights reserved.

Description: It has long been proposed that water incorporation in olivine has dramatic effects on the upper mantle properties, affecting large scale geodynamics and triggering high electrical conductivity. But the laboratory-based laws of olivine electrical conductivity predict contrasting effects of water, precluding the interpretation of geophysical data in term of mantle hydration. We review the experimental measurements of hydrous olivine conductivity and conclude that most of data are consistent when errors in samples water contents are considered. We report a new law calibrated on the largest database of measurements on hydrous olivine oriented single crystals and polycrystals. It fits most of measurements within uncertainties, and is compatible with most of geophysical data within petrological constraints on mantle olivine hydration. The isotropic conductivity (S/m) is where C H 2 O is the water concentration in olivine (wt.ppm) and T the temperature (K). The conductivity anisotropy of hydrous olivine might be higher than dry olivine, but preferential orientation should produce moderate anisotropy (~0–0.8 log unit). In the oceanic mantle, the enhancement of olivine conductivity is limited to ~1 log unit in the maximum range of mantle olivine water concentrations (0–500 wt.ppm). Strongest enhancements are expected in colder regions, like cratonic lithospheres and subduction settings. High conductivities in melt-free mantle require great depths and high water concentrations in olivine (〉0.1 S/m at 〉250 km and 〉200 wt.ppm). Thus, the hydration of olivine appears unlikely to produce the highest conductivities of the upper mantle. This article is protected by copyright. All rights reserved.

Description: ABSTRACT Rather than indicating formation/peak temperature, oxygen isotope fractionations preserved in mineral assemblages of slowly cooled plutonic and metamorphic rocks yield apparent equilibrium temperatures ( T ae ). The isotopic fractionations and T ae values deliver information about cooling history, as the extent of diffusive exchange of oxygen isotopes during cooling is controlled by the cooling time scale or cooling rate. Despite that several models, such as the Fast Grain Boundary (FGB) model, have been developed to simulate oxygen isotope exchange between coexisting minerals during cooling, extraction of cooling rate remains far from straightforward. On the other hand, there is a well-defined quantitative relationship between the Dodson closure temperature ( T c ) and the cooling rate, but T c cannot be directly measured. Based on simulation results of existing models for a variety of rock systems, including open systems (with an infinite fluid reservoir), closed systems (with negligible fluid participation) and semi-open systems (with moderate fluid participation), this study demonstrates that T ae of the mineral pair with the largest equilibrium isotope fractionation (PLEIF) is always bounded by their T c values, regardless of how mineral proportions vary or how significant a role fluid has played in isotopic exchange. If the two T c values happen to be similar, T ae will serve as a good approximation of both T c , provided that the equilibrium fractionation factor has been precisely determined as a function of temperature. One such pair is quartz-magnetite. By contrast, a mineral pair with similar T c but relatively small fractionation is susceptible to the disturbance from other minerals, hence does not always have T ae confined within their T c range. The relationship of T ae - T c correspondence for PLEIF with similar T c can be used to constrain either cooling rate (i.e., as a speedometry method) or oxygen isotope diffusivity if one of them has been independently determined. In the latter case, the inferred oxygen diffusivity may be an index of water fugacity (i.e., as a hygrometry method) when compared with experimental diffusivity values measured under different fluid conditions. This article is protected by copyright. All rights reserved.

Description: Here we present volatile, major and trace element concentrations of 64 olivine-hosted melt inclusions from the Lucky Strike segment on the mid-Atlantic ridge. Lucky Strike is one of two locations where a crustal melt lens has been seismically imaged on a slow-spreading ridge. Vapor-saturation pressures, calculated from CO 2 and H 2 O contents of Lucky Strike melt inclusions, range from approximately 300–3000 bars, corresponding to depths of 0.5–9.9 km below the seafloor. Approximately 50% of the melt inclusions record crystallization depths of 3–4 km, corresponding to the seismically-imaged melt lens depth, while an additional ~35% crystallize at depths 〉 4 km. This indicates that while crystallization is focused within the melt lens, significant crystallization also occurs in the lower crust and/or upper mantle. The melt inclusions span a range of major and trace element concentrations from normal to enriched basalts. Trace element ratios at all depths are heterogeneous, suggesting that melts are not efficiently homogenized in the mantle or crust, despite the presence of a melt lens. This is consistent with the transient nature of magma chambers proposed for slower-spreading ridges. To investigate the petrogenesis of the melt inclusion compositions, we compare the measured trace element compositions to theoretical melting calculations that consider variations in the melting geometry and heterogeneities in the mantle source. The full range of compositions can be produced by slight variations in the proportion of an Azores plume and depleted upper mantle components and changes in the total extent of melting. This article is protected by copyright. All rights reserved.

Description: Within the precession band, an inter-hemispheric anti-phase pattern in the tropical hydro-climate is supported by many paleo-records, and optimally explained by the forcing of precessional insolation change. However, scenarios within the western equatorial Pacific (WEP), which plays the role of the ascending center of atmospheric convection, remain poorly determined. In this study, a marine sediment core from the Halmahera Sea, East Indonesia, was analyzed with high-resolution XRF scanning, quantitative discrete XRF and ICP-AES/MS measurements. The terrigenous fractions in this core are constrained by their trace elemental characteristics to be locally sourced from Halmahera Island, and hence reflect variations in the local riverine runoff and precipitation. On this basis, a continuous record of precipitation changes of the western equatorial Pacific was reconstructed with multi-decadal resolution over the last ~240 ka, using an age model established by the correlation between an adjusted ice volume model and benthic δ 18 O constrained by 14 C dating. The records of terrigenous input show a dominant ~23 kyr periodicity with a 90°~100° phase lag to the boreal summer (i.e., in-phase with the boreal autumn) insolation change. This pattern can be explained by the variability in the convective activity over the WEP, which might be primarily controlled by precessional changes in the El Niño and Southern Oscillation (ENSO) system. A dynamic linkage is implied between the precessional variations in the convective activity in the WEP and the East Asian and Australia-Indonesian summer monsoons (EASM and AISM), in the sense of their distinct stable phase relationship to precession. This article is protected by copyright. All rights reserved.

Description: The Tibetan Himalaya represents the northernmost continental unit of the Indian plate that collided with Asia in the Cenozoic. Paleomagnetic studies on the Tibetan Himalaya can help constrain the dimension and paleogeography of ‘Greater India', the Indian plate lithosphere that subducted and underthrusted below Asia after initial collision. Here, we present a paleomagnetic investigation of a Jurassic (limestones) and Lower Cretaceous (volcaniclastic sandstones) section of the Tibetan Himalaya. The limestones yielded positive fold test, showing a pre-folding origin of the isolated remanent magnetizations. Detailed paleomagnetic analyses, rock magnetic tests, end-member modeling of acquisition curves of isothermal remanent magnetization, and petrographic investigation reveal that the magnetic carrier of the Jurassic limestones is authigenic magnetite, whereas the dominant magnetic carrier of the Lower Cretaceous volcaniclastic sandstones is detrital magnetite. Our observations lead us to conclude that the Jurassic limestones record a prefolding remagnetization, whereas the Lower Cretaceous volcaniclastic sandstones retain a primary remanence. The volcaniclastic sandstones yield an Early Cretaceous paleolatitude of 55.5°S [52.5°S, 58.6°S] for the Tibetan Himalaya, suggesting it was part of the Indian plate at that time. The size of ‘Greater India' during Jurassic time cannot be estimated from these limestones. Instead, a paleolatitude of the Tibetan Himalaya of 23.8°S [21.8°S, 26.1°S] during the remagnetization process is suggested. It is likely that the remagnetization, caused by the oxidation of early diagenetic pyrite to magnetite, was induced during 103-83 Ma or 77-67 Ma. The inferred paleolatitudes at these two time intervals imply very different tectonic consequences for the Tibetan Himalaya. This article is protected by copyright. All rights reserved.

Description: ABSTRACT Morphological and seismic data from a submarine landslide complex east of New Zealand indicate flow-like deformation within gas hydrate-bearing sediment. This “creeping” deformation occurs immediately downslope of where the base of gas hydrate stability reaches the seafloor, suggesting involvement of gas hydrates. We present evidence that, contrary to conventional views, gas hydrates can directly destabilize the seafloor. Three mechanisms could explain how the shallow gas hydrate system could control these landslides. 1) Gas hydrate dissociation could result in excess pore pressure within the upper reaches of the landslide. 2) Overpressure below low-permeability gas hydrate-bearing sediments could cause hydrofracturing in the gas hydrate zone valving excess pore pressure into the landslide body. 3) Gas hydrate-bearing sediment could exhibit time-dependent plastic deformation enabling glacial-style deformation. We favor the final hypothesis, that the landslides are actually creeping seafloor glaciers. The viability of rheologically controlled deformation of a hydrate sediment mix is supported by recent laboratory observations of time-dependent deformation behavior of gas-hydrate-bearing sands. The controlling hydrate is likely to be strongly dependent on formation controls and inter-sediment hydrate morphology. Our results constitute a paradigm shift for evaluating the effect of gas hydrates on seafloor strength which, given the widespread occurrence of gas hydrates in the submarine environment, may require a re-evaluation of slope stability following future climate-forced variation in bottom water temperature.