ALBERT

Description: Identical high precision U-Pb baddeleyite ages, together with paleomagnetic and geochemical data, on mafic dykes occurring over an area of 140,000 km2, define a Paleoproterozoic giant dyke swarm at ca. 2.367 Ga in the Dharwar craton, south India, referred to here as the Dharwar giant dyke swarm. All six U-Pb ages on these dykes are identical within error and suggest emplacement of this swarm within a geologically short time span of ∼5 Myr. A systematic southward progression in the trend of dykes from N48°E to N90°E, defines a fan angle of about 40° with convergence to a focal point about 300 km west of the present-day Dharwar craton boundary, resulting in a spectacular radiating dyke swarm extending across the entire eastern Dharwar craton. The large areal extent, radiating dyke pattern and short duration imply a mantle plume origin for the Dharwar giant dyke swarm. Despite their large areal distribution, all dykes in this swarm are geochemically coherent and have similar primitive mantle-normalized trace element patterns and rare earth element characteristics. Although the NE part of the swarm is magnetically overprinted, a remanence survives that has the same direction as primary magnetizations from dykes in the southern part of the swarm.

Description: Surface displacements solutions of elastic deformation around an inflating magma chamber generally assume that the associated internal overpressure is limited by the bedrock tensile strength. When considering stress equilibrium in the bedrock adjacent to a spherical or infinitely long cylinder, the gravity body force actually resists tensile failure, thus leading to a much larger pressure threshold. And when considering a Coulomb failure criterion, analytical and numerical models predict that shear failure develops instead of tensile failure. Here, three numerical codes are used to compare elasto-plastic solutions of surface displacements and patterns of failure in plane-strain. Shear failure propagates independently from the surface downward, then from the chamber walls upwards, and finally the two plasticized domains connect. Another test with internal underpressure (simulating source deflation) fits standard solutions from tunneling engineering. The effect of pore fluid pressures is also explored. In case of lithostatic fluid pore pressure in the bedrock, the gravity effect cancels out, and tensile failure is enabled for an overpressure close to the tensile strength. Coupled hydromechanical models in undrained conditions indicate that the initial bedrock porosity modifies the evolution of fluid pressure, volumetric strain and effective normal stress, and consequently also the pressure threshold for the onset of failure. We show that a bedrock of low porosity is more prone to fail than a bedrock of high porosity. In summary, our elasto-plastic and hydromechanical models illustrate the contexts for either tensile or shear failure around magmatic bodies, at the same time complementing and delimiting predictions deduced from elasticity.

Description: In this paper we look at changes of the statistics of the stationary explosive process at a basaltic volcano as a proxy for departures from thermodynamic equilibrium conditions in the shallow plumbing system. Specifically, we investigate the explosion process of Stromboli volcano that occurred during 2002–2003, 2006–2007 and 2010–2011. The first two periods were characterized by eruptions with significant lava effusion and strong paroxysmal events, while the last one shows persistent explosive activity accompanied by minor episodes of lava flow. We use three-component seismic data recorded by broadband stations operating on the volcano and, for 2007 and 2010–2011 cases, strainmeter data from a Sacks-Evertson borehole dilatometer. For each time interval we study the explosive process by looking at the inter-occurrence times and at the amplitude distribution. Moreover, we analyze its waveforms, spectral content and polarization properties. In all three cases we find sharp increases of the explosion rate associated with swarms. Swarms are characterized by quasi-monochromatic seismic events with frequency peak close to about 3 Hz, higher amplitude than the usual explosions and variability coefficient of the inter-occurrence times close to 0.5. In correspondence to the swarms, we also observe negative variations in the strain signals, which indicate a depressurization in the shallow plumbing system. This depressurization emerges clearly from the data collected during 2010–2011, whereas it is less sharp for the 2007 episode, and has been estimated in about 105 Pa. From the polarization analysis we infer that this depressurization affects the upper 0.3–0.8 km of the plumbing system.

Description: Convergent plate boundaries accommodate intraplate displacement within a ∼100–1000 m thick shear zone. Marine geophysicists typically define this zone, the subduction channel (SC), as the sedimentary layer between the downgoing oceanic crust and the base of the upper plate. Geologists and modelers, instead, perceive the SC as a specific type of shear zone. The original theory of SCs was developed when the net accretion of marine sediments to the forearc was thought to typify a convergent margin. While erosive margins were briefly mentioned, their mechanics were not discussed in any detail. We now realize that subduction erosion is taking place at roughly half of the modern subduction margins. Here we review and revise the theory of erosive SCs (1) to unify this concept across disciplines, focusing on the meaning of the channel's boundaries; (2) to redefine the portions of the forearc included in the SC concept; and (3) to better idealize this dynamic system where material supply to the channel, fluid content, and the heterogeneity of deformation all influence the SC's upper and lower boundaries. Migration of the channel boundaries controls the downdip variation of tectonic mechanisms that shape the margin. Within the shallow,

Description: The abundances of 60 elements in 616 Ocean Floor Basaltic (OFB) glasses from the Abyssal Volcanic Glass Data File (AVGDF) of the Smithsonian Institution have been determined by laser-ablation (LA)-ICP-MS and electron microprobe analysis (EMPA). The elements analyzed include all 28 of the refractory lithophile elements, which provide the framework for establishing the geochemical behavior and source abundances of volatile, chalcophile and siderophile elements. In addition to the traditionally analyzed elements (rare earth elements (REE), high field strength elements (HFSE), large ion lithophile elements (LILE) and first row transition elements (FRTE)), we report analyses for lesser-analyzed elements (Li, Be, Ga, Ge, As, Se, Mo, Ag, Cd, In, Sn, Sb, W, Tl and Bi). The precision of the method for most elements is between 2 and 4%, one standard deviation, although ratios of elements determined simultaneously are more precise (e.g., REE, Zr/Hf). Subsets of 329 glasses were analyzed by electron microprobe for S and 154 glasses for Cl. The results define a representative trace element geochemistry of OFB, against which local variations resulting from differences in basalt petrogenesis in a range of tectonic settings or different styles of magmatic differentiation may be compared.

Description: Several factors influence the topography of the overriding plate in a subduction zone: mantle wedge flow caused by motion of the descending slab, lithosphere elasticity, and interaction between the solid lithosphere and the viscous asthenosphere. This paper presents the results of new subduction modeling that incorporates both elastic/elastoplastic deformation in the overriding plate and viscous deformation in the asthenosphere. The effects of fluid-solid coupling and interaction were considered using an arbitrary Lagrangian-Eulerian technique that solves for the continuous deformation of fluid flow in contact with the rigid plate. Modeling results show that thicker lithosphere, which is harder to deform, contributes to the development of broadly shallower back-arc basins. As lithospheric strength decreases, its ability to withstand outside force (fluid load) resulting from flow in the mantle wedge also declines, leading to the development of deep and narrow basins. Large-scale deformation of the lithosphere modifies both the mantle wedge geometry and the wedge flow pattern, which in turn increases the fluid load and thereby enhances topographic variation. Generally, the effect of interaction is enhanced with lithospheric deformation. The elastoplastic model also leads to a weaker lithosphere and a correspondingly larger deflection in the lithosphere. Similarly to previous studies, back-arc basins are deeper when mantle viscosity is higher. For a 50 km thick elastic lithosphere, the lithosphere-asthenosphere interaction is negligible when the asthenospheric viscosity is less than 4 × 1019 Pa s. Increasing the rate of subduction or decreasing the dip angle will also lead to the development of deeper basins.

Description: We make use of observations on orogenic strain accumulation and deformation partitioning in the Central Andes to explore the backarc strength evolution at the lithospheric scale. In plan view, the Altiplano-Puna plateaux experienced rapid initial increase of surface area undergoing active deformation during the Cenozoic. Beyond the maximum lateral extent reached around 10–15 Ma (40–50% of entire proto-Andes undergoing deformation) at 10–20% total strain, rapid localization initiated at the eastern flank of the Altiplano (Inter- and Subandean thrust belt) but not at the Puna latitude. Localization was associated with a significant increase in bulk shortening rate. Average fault slip rates equally increased by an order of magnitude following a protracted period of stable average rates. Estimates of strength evolution based on force balance calculations and critical wedge analysis suggest significant backarc weakening driving this change after the Middle Miocene. Strain accumulation led to localization and weakening with development of a detachment propagating through crust and upper mantle. We find that lithosphere-scale failure resulting from strain weakening beyond a critical strain threshold (c. 20%) and fault coalescence with formation of a weak detachment in shales (effective coefficient of friction 〈 0.1) plays a key role in the evolution of the Andes. Strain-related lithosphere weakening appears to dominate over the impact of external forcing mechanisms, such as variations of plate convergence, mantle-assisted processes, or erosion. Comparison of these orogen-scale observations with experimental rock rheology indicates substantial similarity of deformation behavior with similar weakening thresholds across a wide range of scales.

Description: Indonesia is arguably one of the tectonically most complex regions on Earth today due to its location at the junction of several major tectonic plates and its long history of collision and accretion. It is thus an ideal location to study the interaction between subducting plates and mantle convection. Seismic anisotropy can serve as a diagnostic tool for identifying various subsurface deformational processes, such as mantle flow, for example. Here, we present novel shear wave splitting results across the Indonesian region. Using three different shear phases (local S, SKS, and downgoing S) to improve spatial resolution of anisotropic fabrics allows us to distinguish several deformational features. For example, the block rotation history of Borneo is reflected in coast-parallel fast directions, which we attribute to fossil anisotropy. Furthermore, we are able to unravel the mantle flow pattern in the Sulawesi and Banda region: We detect toroidal flow around the Celebes Sea slab, oblique corner flow in the Banda wedge, and sub-slab mantle flow around the arcuate Banda slab. We present evidence for deep, sub-520 km anisotropy at the Java subduction zone. In the Sumatran backarc, we measure trench-perpendicular fast orientations, which we assume to be due to mantle flow beneath the overriding Eurasian plate. These observations will allow to test ideas of, for example, slab–mantle coupling in subduction regions.

Description: We present the interpretation of newly acquired high-quality industry-standard deep seismic reflection and swath bathymetry data to provide insight into the structural style and evolution of the Mentawai Fault Zone (MFZ). The MFZ lies along the boundary between the accretionary wedge and the proposed continental backstop. This zone exhibits arcuate ridges on the seafloor, convex toward the east. Beneath these ridges the structures developed as landward-vergent imbricated backthrusts in the inner part of the accretionary wedge and higher-angle backthrusts that deformed the forearc basin sediments. In the forearc high, anticlines were developed due to the seaward-vergent forearc high thrusts originating in the accretionary wedge. The imbricated backthrusts may have initiated during the Early-Middle Miocene contemporaneously with the slide and back-rotation of forearc high thrusts. In the Late Miocene, the higher-angle backthrusts were initiated. Continuous contraction induced the frontal higher-angle backthrusts and formed a fold-thrust belt toward the east during the Pliocene. The folds and thrusts were disturbed by diapirs and mud volcanoes. Backthrusting and fold-thrust belts developed in the MFZ may explain the compressional features observed at the boundary between the accretionary wedge and continental backstop along the southern Sumatra margin. The backthrusts along the MFZ are waning in activity and hence the risk of a large earthquake and associated tsunami at the present time should be small.

Description: This article introduces the software Melange, a 3D lattice-particle hybrid model. The software was specifically designed in order to simulate ductile visco-elasto-plastic deformation and can be used to study tectonic processes in the lithosphere from the micro to the macro scale. Melange is under an open source license. The code takes both relevant yield mechanisms for the deformation of lithospheric material into account: dynamic brittle failure and ductile creep, where ductile creep is modeled as viscoelasticity. The software considers effects of the local geology, of the inherent disorder of geomaterials, of rheological layering of the lithosphere and applies repulsion when the material fractures. Driving forces are the externally applied strain and the gravitational load. Melange applies an elastically isotropic regular 3D lattice with HCP geometry and next-nearest neighbor interactions. Young's modulus, viscosity, material density and system size can be freely chosen. Poisson's ratio is restricted to values

Description: The Yakima Folds (YF) comprise anticlines above reverse faults cutting flows of the Miocene Columbia River Basalt Group of central Washington State. The YF are bisected by the ~1100-km-long Olympic-Wallowa Lineament (OWL), which is an alignment of topographic features including known faults. There is considerable debate about the origin and earthquake potential of both the YF and OWL, which lie near six major dams and a large nuclear waste storage site. Here I show that the trends of the faults forming the YF relative to the OWL match remarkably well the trends of the principal stress directions at the end of a vertical strike-slip fault. This comparison and the termination of some YF against the OWL are consistent with the YF initially forming as splay faults caused by an along-strike decrease in the amount of strike-slip on the OWL. The hypothesis is that the YF faults initially developed as splay faults in the early to mid Miocene under NNW-oriented principal compressive stress, but the anticlines subsequently grew with thrust motion after the principal compressive stress direction rotated to N-S or NNE after the mid-Miocene. A seismic profile across one of the YF anticlines shows folding at about 7 km depth, indicating deformation of sub-basalt strata. The seismic profile and the hypothesized relationship between the YF and the OWL suggest that the structures are connected in the middle or lower crust, and that the faults forming the YF are large-scale splay faults associated with a major strike-slip fault system.

Description: Numerical thermo-chemical mantle convection simulations in a spherical annulus geometry with self-consistently calculated mineralogy and mineral physics are used to predict detailed deep mantle seismic structures, particularly local radial profiles of shear wave velocity (Vs) and bulk sound velocity (Vb). The predicted structures are compared to seismological observations and used to guide the interpretation of seismic observations and to test the model. The mantle composition is described as a mixture of MORB (Mid-Oceanic-Ridge-Basalt) and harzburgitic end-members in the Na2O-CaO-FeO-MgO-Al2O3-SiO2 system. To assess the influence of chemical variability, four different sets of end-member compositions are evaluated. Results confirm that the post-perovskite (pPv) phase causes anti-correlated S wave and bulk sound velocities in the deep mantle, due to pPv being fast in Vs but slow in bulk sound velocity. Local 1-D seismic profiles display great lateral variability, and often have multiple discontinuities in the deep mantle due to MORB layers in folded slabs, with a positive Vs anomaly and negative bulk sound anomaly, or the perovskite-pPv phase transition. The pPv transition is not visible inside piles of segregated MORB because of the high temperature and small velocity contrast of pPv in MORB. Piles of segregated MORB are seismically slow in both Vs and bulk sound despite being expected to be fast in Vs, because they are hotter than the surrounding material. Anelasticity has a significant influence on profiles of Vs only in the lower thermal boundary layer, which corresponds to below 2600 to 2800 km depth depending on region, where temperatures are higher than the extrapolated adiabat. These results indicate the importance of using a joint geodynamical-mineralogical approach to predict and aid in the interpretation of deep mantle seismic structure, because interpretations based on seismology and mineral physics alone may be misleading and do not capture the strong lateral variability in 1-D structure obtained here: for example, multiple reflections arising from folded slabs and the precise balance between thermal and compositional influences on seismic structure.

Description: Oceanic core complexes at slow-spreading ridges represent the uplifted footwalls of large-offset ‘detachment’ faults that initiate at steep dips, and rotate to flatten via a ‘rolling hinge’ mechanism in response to flexural unloading. A key question is whether oceanic core complex development is accommodated entirely by displacement on the detachment fault zone, or if significant internal deformation of the footwall occurs during flexure and rotation. We investigate this by constraining the internal architecture of the Atlantis Massif oceanic core complex (Mid-Atlantic Ridge, 30°N) using Formation MicroScanner borehole wall images and cores from the 1416 m-deep Integrated Ocean Drilling Program Hole U1309D. Two distinct sets of structures are observed. N-S-striking, E-dipping structures dominating the upper 385 m are interpreted as a brittle to semi-brittle zone of fracturing in the footwall. Structures with this geometry occur down to 750 m below seafloor, suggesting that the detachment damage zone extends deep into the footwall. The nature of this deformation is, however, enigmatic: several cataclastic shear zones with reverse geometry in their current orientations may be rotated extensional faults or relate to shortening at the base of the flexing beam of a very weak footwall. By contrast, E-W-striking, N- and S-dipping structures dominate the lowermost kilometer of the borehole. They likely represent conjugate fractures formed in the hanging wall of a late, E-W normal fault zone, separating gabbroic rocks of the central dome of Atlantis Massif from serpentinized peridotite to the south, responsible for post-detachment uplift of the southern margin of the massif.

Description: A Bayesian network approach has been developed to estimate uncertainties for gridded bathymetry models that store grids of large regional or worldwide bathymetry. A previous technique to estimate uncertainty for historical data uses Monte Carlo simulations to estimate uncertainty using soundings data, but it is infeasible to implement for gridded bathymetry models. The Bayesian network is designed to estimate uncertainty using gridded bathymetry and a distribution of navigation uncertainties for the survey data used to create the grid as inputs. This paper discusses how the Monte Carlo technique is used to train the network and how to implement it to estimate gridded uncertainty. A demonstration of this approach is presented that uses the data from the original paper for the Monte Carlo technique. Both Monte Carlo and Bayesian network estimators provide higher uncertainty with bottom slope. The Bayesian network estimate is generally greater than the Monte Carlo approach, which is by design for navigation safety, but is within an order of magnitude agreement.

Description: Oxygen isotope values of diatom silica (δ18Odiatom) are increasingly used as paleoclimate proxies; however, the magnitude and timing of post-mortem alteration of δ18Odiatom values has been unclear. In freshwater diatom silica from a human-made pond in northern New Mexico, post-mortem alteration of δ18Odiatom values occurs within one year of sediment burial. Diatom silica collected antemortem has an average δ18Odiatom value of 21.5‰ (VSMOW, σ = 1.3, n = 23), whereas diatom silica from two sediment cores from the same freshwater environment records significantly higher δ18Odiatom values (28.9‰, σ = 0.8, n = 13). The difference in ante- and post-mortem δ18Odiatom values indicate post-mortem alteration of diatom silica oxygen that results in a 〉7‰ increase in δ18O values. This study demonstrates that δ18Odiatom values reach mature values within 0.5 years of frustule death. Initial diatom δ18O values that record growing conditions are rapidly overwritten during silica maturation, and the mature diatom δ18O values approach isotopic equilibrium for quartz-water. The rapid post-mortem alteration of diatom δ18O values explains much of the disparate data regarding silica-water fractionation for diatom silica and has a profound effect on the use of diatom silica δ18O values as a paleoclimate proxy in lacustrine and marine environments.

Description: The Amsterdam–St Paul (ASP) oceanic plateau results from the interaction between the ASP hot spot and the Southeast Indian ridge. A volcanic chain, named the Chain of the Dead Poets (CDP), lies to its northward tip and is related to the hot spot intraplate activity. The ASP plateau and CDP study reveals that ASP plume composition is inherited from oceanic crust and pelagic sediments recycled in the mantle through a 1.5 Ga subduction process. The ASP plateau lavas have a composition (major and trace elements and Sr-Nd-Pb-Hf isotopes) reflecting the interaction between ASP plume and the Indian MORB mantle, including some clear DUPAL input. The Indian upper mantle below ASP plateau is heterogeneous and made of a depleted mantle with lower continental crust (LCC) fragments probably delaminated during the Gondwana break-up. The lower continental crust is one of the possible reservoirs for the DUPAL anomaly origin that our data support. The range of magnitude of each end-member required in ASP plateau samples is (1) 45% to 75% of ASP plume and (2) 25% to 55% of Indian DM within 0% to a maximum of 6% of LCC layers included within. The three end-members involved (plume, upper mantle and lower continental crust) and their mixing in different proportions enhances the geochemical variability in the plateau lavas. Consequently, the apparent composition homogeneity of Amsterdam Island, an aerial summit of the plateau, may result from the presence of intermediate magmatic chambers into the plateau structure.

Description: Grain-size, terrigenous element and rock magnetic remanence data of Quaternary marine sediments retrieved at the NW African continental margin off Gambia (gravity core GeoB 13602–1, 13°32.71′N, 17°50.96′W) were jointly analyzed by end-member (EM) unmixing methods to distinguish and budget past terrigenous fluxes. We compare and cross-validate the identified single-parameter EM systems and develop a numerical strategy to calculate associated multiparameter EM properties. One aeolian and two fluvial EMs were found. The aeolian EM is much coarser than the fluvial EMs and is associated with a lower goethite/hematite ratio, a higher relative concentration of magnetite and lower Al/Si and Fe/K ratios. Accumulation rates and grain sizes of the fluvial sediment appear to be primarily constrained by shore distance (i.e., sea level fluctuations) and to a lesser extent by changes in hinterland precipitation. High dust fluxes occurred during the Last Glacial Maximum (LGM) and during Heinrich Stadials (HS) while the fluvial input remained unchanged. Our approach reveals that the LGM dust fluxes were ∼7 times higher than today's. However, by far the highest dust accumulation occurred during HS 1 (∼300 g m−2 yr −1), when dust fluxes were ∼80 fold higher than today. Such numbers have not yet been reported for NW Africa, and emphasize strikingly different environmental conditions during HSs. They suggest that deflation rate and areal extent of HSs dust sources were much larger due to retreating vegetation covers. Beyond its regional and temporal scope, this study develops new, in principle, generally applicable strategies for multimethod end-member interpretation, validation and flux budgeting calibration.

Description: The D'Entrecasteaux Island gneiss domes, Papua New Guinea, expose the world's youngest ultrahigh-pressure rocks (5–8 Ma), which were exhumed nearly isothermally to the mid-crust at rates of cm yr−1. Because quartz grains are internally distorted by deformation bands and have strongly interlobate grain boundary microstructures indicative of rapid grain-boundary migration, we infer that Ti content in the quartz was frozen in by dynamic recrystallization that accompanied the continued deformation of the gneisses as they were exhumed through the crust toward the surface. We infer that average Ti content of an assemblage of quartz grains in a sample depends on the ratio of cooling rate to dynamic recrystallization rate. An increase in the former preserves and captures a larger fraction of relatively high-Ti, high-temperature grains, whereas an increase in the latter removes such relict grains and replaces them with a newer set of lower-Ti, low-temperature recrystallized grains. Based on this concept, we use Ti-in-quartz as a tool to infer domains of differing relative cooling rate across four D'Entrecasteaux Island domes. Ti concentrations in quartz grains from 89 samples of quartzofeldspathic gneiss and eclogite increase inwardly away from the dome margins (2.5–20 ppm) toward the center of the domes (20–〉100 ppm). From this, we infer that the most rapidly cooled rocks were exhumed near the center of the domes. Our data reinforces structurally based arguments that the symmetric domes were emplaced vertically into the crust as diapirs, rather than laterally exhumed as a result of large-magnitude slip on dome-bounding detachment faults.

Description: Detailed, quantitative electron backscatter diffraction (EBSD) analysis of polycrystalline diamond rocks (diamondites) provides insights into their formation and significance. The fine grain size of diamondites is usually attributed to rapid crystallization. However, EBSD reveals significant intragrain bending, distinct low-angle boundaries and straight to highly irregular high-angle grain boundaries. Highly deformed grains may be in contact with others showing little or no deformation. These features are typical for crystal plastic deformation, in which differential stress generates dislocations according to the dominant slip systems in diamond. Dislocations accumulate to form subgrain boundaries, with grain-size reduction by rotation of subgrains, nucleation and growth of new grains, and migration of grain boundaries. Such features are seen in other polygranular materials such as deformed metals, and quartzite deformed at high temperatures. During this process, interaction with fluids produced interstitial garnets with compositions different from those of the primary inclusions. Oscillatory CL zoning in diamonds developed through diffusion along subgrain boundaries, rather than being a primary growth feature. Diamondites are thus not simply products of primary crystallization, but may be strongly deformed, recrystallized and modified. Integrated EBSD, CL and in situ chemical and isotopic analysis provide the spatial control and new insights into mantle processes.

Description: Measurements of excess 230Th (230Thxs) have proved to be a useful tool in constraining changes in sedimentation rate, and improving our understanding of the fluxes of other components into marine sediments. To obtain the initial activity of 230Thxs (230Thxs0) in sediment: the total measured 230Th must be corrected for the presence of 230Th associated with detrital minerals, for ingrowth from uranium-bearing authigenic phases and then also corrected for the decay of 230Thxs since deposition. We describe a number of improvements in the way these corrections are applied to obtain more accurate determinations of 230Thxs0. We present a new method for the determination of a local estimate for the detrital 238U/232Th activity ratio; suggest more appropriate values for the isotopic composition of authigenic uranium; and question the assumption of secular equilibrium in detrital material. We also present a new, freely-available MATLAB® script called ‘XSage’ that can calculate 230Thxs0, from user-supplied datasets of uranium and thorium isotope activities from sedimentary samples following the theoretical approach described. ‘XSage’ can determine variations in sedimentation rate between stratigraphic horizons of known age and thus produce high-resolution age models. Using a Monte Carlo approach, the program calculates uncertainties for these age models and on the durations of intervals between tie-points. An example of the application of the XSage program using a previously published record is provided.

Description: Synthetic polycrystalline samples of Fe-Ti oxides (titanomagnetite, Tmtss; ilmenite-hematitess, Ilmss; pseudobrookitess, Psbss) are very sensitive to changes in the redox conditions at high temperatures, either during synthesis experiments or during thermomagnetic measurements. For instance, exposure to air for a few seconds at the end of a synthesis run at 1300°C of a Tmtss-Ilmss sample produces surficial oxidation down to a depth of some 100 μm. This oxidation zone is well visible on backscattered electron images of polished sections through the sample pellet. It is characterized by so-called trellis “oxyexsolution” textures, i.e., fine lamellae of Ilmss within the Tmtss crystals and lamellae of Psbss within the Ilmss crystals. In this oxidation zone the newly grown Ilmss lamellae and the surrounding Tmtss are more Fe rich than the original crystals. The presence of trellis textures in the crystals of both coexisting phases, Tmtss and Ilmss, show that only short-scaled elemental transport within the crystals was involved and that equilibrium was not attained. Even though the oxidation zone is very narrow, the imprint of the new Tmtss compositions is well recognizable in temperature-dependent magnetic susceptibility curves. In temperature-dependent saturation magnetization (MS-T) curves, however, the contribution of more Fe-rich Tmtss from the oxidation zone can be easily overseen. However, surficial oxidation of Tmtss does occur during MS-T measurements with a variable field translation balance, apparently in relation with insufficient Ar flowing around the sample. Further examples of rapid surficial oxidation of Fe-Ti oxide samples are also discussed.

Description: Records of seawater chemistry help constrain temporal variations in geochemical processes that impact the global carbon cycle and climate through Earth's history. Here we reconstruct Cenozoic seawater Sr/Ca (Sr/Casw) using fossil Conus and turritellid gastropod Sr/Ca. Combined with an oxygen isotope paleotemperature record from the same samples, the gastropod record suggests that Sr/Casw was slightly higher in the Eocene (∼11.4 ± 3 mmol/mol) than today (∼8.54 mmol/mol) and remained relatively stable from the mid- to late Cenozoic. We compare our gastropod Cenozoic Sr/Casw record with a published turritellid gastropod Sr/Casw record and other published biogenic (benthic foraminifera, fossil fish teeth) and inorganic precipitate (calcite veins) Sr/Casw records. Once the uncertainties with our gastropod-derived Sr/Casw are taken into account the Sr/Casw record agrees reasonably well with biogenic Sr/Casw records. Assuming a seawater [Ca] history derived from marine evaporite inclusions, all biogenic-based Sr/Casw reconstructions imply decreasing seawater [Sr] through the Cenozoic, whereas the calcite vein Sr/Casw reconstruction implies increasing [Sr] through the Cenozoic. We apply a simple geochemical model to examine the implications of divergence among these seawater [Sr] reconstructions and suggest that the interpretation and uncertainties associated with the gastropod and calcite vein proxies need to be revisited. Used in conjunction with records of carbonate depositional fluxes, our favored seawater Sr/Ca scenarios point to a significant increase in the proportion of aragonite versus calcite deposition in shelf sediments from the Middle Miocene, coincident with the proliferation of coral reefs. We propose that this occurred at least 10 million years after the seawater Mg/Ca threshold was passed, and was instead aided by declining levels of atmospheric carbon dioxide.

Description: Apatite triple dating (fission track, U-Pb and U-Th/He techniques) has been applied to detrital grains from the sedimentary core drilled during the ANDRILL 2A project, which documents the Miocene history of the Victoria Land Basin (western Ross Sea). High-temperature cooling ages show two main clusters (about 30 and 500 Ma) whereas most of low-temperature data are late Oligocene-Early Miocene in age. These latter data are related to the exhumation of the Transantarctic Mountains south of the Discovery Accommodation Zone. Comparison between low-temperature ages suggests that the Transantarctic Mountains have been in a phase of post-orogenic decay since at least 30 Ma. The Oligocene U-Pb data demonstrate the presence of a volcanic event well before the McMurdo volcanic group, whose onset is commonly places at 19 Ma. The location of the volcanic centers is unknown, but they could be below the Ross Ice Shelf south of drilling site. As a whole, these data indicate a major flow of sediments from south to north with only minor contributions from nearby outlet glaciers of the East Antarctic Ice Sheet.

Description: The Ganos fault is the westernmost segment of the North Anatolian Fault that experienced the Mw = 7.4 earthquake of 9 August 1912. The earthquake revealed 45-km-long of surface ruptures inland, trending N70°E, and 5.5 m of maximum right lateral offset near Güzelköy. The long-term deformation of the fault is clearly expressed by several pull-apart basins and sag ponds, pressure and shutter ridges and offset streams. In parallel with detailed geomorphologic investigations, we measured co-seismic and cumulative displacements along the fault, and selected the Güzelköy site for paleoseismology. A microtopographic survey at the site yields 10.5 ± 0.5 m and 35.4 ± 1.5 m cumulative lateral offsets of stream channels and geomorphologic features. Seven paleoseismic parallel and cross-fault trenches document successive faulting events and provide the timing of past earthquakes on the Ganos fault segment. Radiocarbon dating of successive colluvial wedges in trench T1, and the fresh scarplet above (probably 1912 surface rupture) indicate the occurrence of three faulting events since the 14th century. Parallel trenches (3, 5, 6 and 7) expose paleo-channels and show a cumulative right-lateral offset of 16.5 ± 1.5 m next to the fault, and 21.3 ± 1.5 m total channel deflection. Radiocarbon dating of past channel units and fault scarp-related colluvial deposits imply an average 17 +/− 5 mm/year slip rate and 323 ± 142 years recurrence interval of large earthquakes during the last 1000 years on the Ganos fault. The succession of past faulting events and inferred slip rate west of the Marmara Sea provide more constraint on the long-term faulting behavior in the seismic gap of the North Anatolian Fault and may contribute to a better seismic hazard assessment in the Istanbul region.

Description: In order to evaluate the influence of diagenetic and post-sampling processes on the stable oxygen and carbon isotope compositions of biogenic carbonates, we conducted a multiproxy study of organic-rich sediments from the eastern Pacific oxygen minimum zone. Core MD02-2520, which was retrieved from the Gulf of Tehuantepec (Mexico), has seasonal laminations and covers the last 40 kyr. Together with the presence of gypsum crystals and inorganic calcite aggregates, the occurrence of large excursions in the stable oxygen and carbon isotope records of both planktonic and benthic foraminifera (as large as +3‰ in δ18O and −5‰ in δ13C) point to significant secondary transformations. Storage-related gypsum precipitation was ruled out since it implies sulfide reoxidation by oxygen that triggers biogenic calcite dissolution, which proved to be of minor importance here. Instead, precipitation of authigenic calcite during early diagenesis appears to be the most likely process responsible for the observed isotopic excursions. The δ13C composition for inorganic calcite aggregates (−5 to −7‰) suggests a major contribution from anaerobic oxidation of organic matter. The δ34S composition for gypsum crystals (−10 to +15‰) suggests a major contribution from anaerobic reoxidation of authigenic sulfides, potentially involving reactions with metal oxides and sulfur disproportionation. A minor part of the gypsum might possibly have formed as a result of local pore water salinity increases induced by gas hydrate formation.

Description: Identification of methane sources controlling hydrate distribution and concentrations in continental margins remains a major challenge in gas hydrate research. Lack of deep fluid samples and high quality regional scale seismic reflection data may lead to underestimation of the significance of fluid escape from subducting and compacting sediments in the global inventory of methane reaching the hydrate zone, the water column and the atmosphere. The distribution of concentrated hydrate zones in relation to focused fluid flow across the southern Hikurangi subduction margin was investigated using high quality, long offset (10 km streamer), pre-stack depth migrated multichannel seismic data. Analysis of low P wave velocity zones, bright-reverse polarity reflections and dim-amplitude anomalies reveals pathways for gas escape and zones of gas accumulation. The study shows the structural and stratigraphic settings of three main areas of concentrated hydrates: (1) the Opouawe Bank, dominated by focused periodic fluid input along thrust faults sustaining dynamic hydrate concentrations and gas chimneys development; (2) the frontal anticline, with a basal set of proto-thrusts controlling permeability for fluids from deeply buried and subducted sediments sustaining hydrate concentrations at the crest of the anticline; and (3) the Hikurangi Channel, with buried sand dominated channels hosting significant amounts of gas beneath the base of the hydrate zone. In sand dominated channels gas injection into the hydrate zone favors highly concentrated hydrate accumulations. The evolution of fluid expulsion controlling hydrate formation offshore southern Hikurangi is described in stages during which different methane sources (in situ, buried and thermogenic) have been dominant.

Description: The abundance and distribution of dissolved and particulate Rhenium (Re) has been measured in several rivers draining the Himalaya and Peninsular India, from their origin to outflow into the Bay of Bengal and the Arabian Sea. The large data set resulting from this study on rivers flowing through a variety of lithologies e.g., the crystallines and sediments of the Himalaya, Deccan basalts, Vindhyan sediments and the Indian shield significantly enhances our understanding of the aqueous geochemistry of Re and also constrains its sources to rivers and fluxes to the sea. The concentration of dissolved Re in rivers of the Himalaya and the Peninsular India shows wide range; 1.4 to 72.7 pmol/kg (mean 7.8 pmol/kg) and 0.5 to 122 pmol/kg (mean 15 pmol/kg) respectively. The discharge weighted average annual flux of dissolved Re transported by the rivers from these regions are ∼5800 and ∼15,700 mol/year respectively. The major source of dissolved Re, as determined from inter-element associations, is black shales for the Himalayan rivers and pyrites in basalts for the east flowing Deccan rivers. In addition, there are evidences of considerable anthropogenic supply of Re to some of the rivers that have very high Re concentrations. Estimates of anthropogenic supply based on their Re/K ratios suggest that this source accounts for most of the Re in the Peninsular rivers, particularly the Godavari. The annual flux of anthropogenic Re transported by the Peninsular rivers is ∼14,600 mol, most of which is from the Godavari. This anthropogenic flux accounts for ∼70% of the total Re supply by the Indian rivers to the adjacent seas and 3.4% of the global riverine flux to the oceans. The global average, pre-anthropogenic (natural) concentration of dissolved Re in rivers is estimated to be ∼3 pmol/kg based on Re−K correlation. This value is much lower than the contemporary average determined from the measured concentrations and earlier estimate of natural Re based on Re−SO4 link.

Description: We present a three-dimensional finite element thermomechanical model idealizing the complex deformation processes associated with evolution of the San Andreas Fault system (SAFS) in northern and central California over the past 20 Myr. More specifically, we investigate the mechanisms responsible for the eastward (landward) migration of the San Andreas plate boundary over time, a process that has largely determined the evolution and present structure of SAFS. Two possible mechanisms had been previously suggested. One mechanism suggests that the Pacific plate first cools and captures uprising mantle in the slab window, subsequently causing accretion of the continental crustal blocks. An alternative scenario attributes accretion to the capture of plate fragments (microplates) stalled in the ceased Farallon-North America subduction zone. Here we test both these scenarios numerically using a recently developed lithospheric-scale code, SLIM3D, that employs free surface, nonlinear temperature- and stress-dependent elastoviscoplastic rheology and allows for self-generation of faults. Modeling suggests that microplate capture is the primary driving mechanism for the eastward migration of the plate boundary, while the slab window cooling mechanism alone is incapable of explaining this phenomenon. We also show that the system evolves to the present day structure of SAFS only if the coefficient of friction at mature faults is low (0.08 for the best fit model). Thus, our model provides an independent constraint supporting the “weak fault in a strong crust” hypothesis for SAFS.

Description: The Kuril Trench subduction zone is one of the most seismogenic regions, where underthrust earthquakes with M 〉 8 recur along the trench. The seismic gap between the source areas of the 1973 Nemuro-oki and 2003 Tokachi-oki earthquakes, which are typical underthrust earthquakes faulting with rupture velocities of ∼3 km/s, has been ruptured by the 1952 Tokachi-oki earthquake. The seismic gap has also slipped incidental to neighboring asperities. The difference in slip pattern on the plate interface generally appears as a spatial difference in seismic structure on the plate interface, such as a reflectivity of the plate interface. We estimated the crustal velocity structure and analyzed the reflectivity of the plate interface to investigate the physical properties of the plate interface by performing an air gun–ocean bottom seismometer experiment on the along-trench profile across the seismic gap. Strong reflections from the plate interface were observed in the 1952 Tokachi-oki source area including the seismic gap, rather than in the 1973 Nemuro-oki source area. The strong reflectivity of the plate interface in such the seismic gap with an incidental slip suggests that a slip pattern in the corresponding seismic gap would be conditionally stable. The coupling condition in the source areas of the eastern part of the source area of the 1952 earthquake is different from that in source areas of typical underthrust earthquakes, such as the 2003 Tokachi-oki and 1973 Nemuro-oki earthquakes. Our results suggest that the 1952 Tokachi-oki earthquake was a complex earthquake with the characteristic of a tsunami earthquake.

Description: Mapping and sampling of 18 eruptive units in two study areas along the Galápagos Spreading Center (GSC) provide insight into how magma supply affects mid-ocean ridge (MOR) volcanic eruptions. The two study areas have similar spreading rates (53 versus 55 mm/yr), but differ by 30% in the time-averaged rate of magma supply (0.3 × 106 versus 0.4 × 106 m3/yr/km). Detailed geologic maps of each study area incorporate observations of flow contacts and sediment thickness, in addition to sample petrology, geomagnetic paleointensity, and inferences from high-resolution bathymetry data. At the lower-magma-supply study area, eruptions typically produce irregularly shaped clusters of pillow mounds with total eruptive volumes ranging from 0.09 to 1.3 km3. At the higher-magma-supply study area, lava morphologies characteristic of higher effusion rates are more common, eruptions typically occur along elongated fissures, and eruptive volumes are an order of magnitude smaller (0.002–0.13 km3). At this site, glass MgO contents (2.7–8.4 wt. %) and corresponding liquidus temperatures are lower on average, and more variable, than those at the lower-magma-supply study area (6.2–9.1 wt. % MgO). The differences in eruptive volume, lava temperature, morphology, and inferred eruption rates observed between the two areas along the GSC are similar to those that have previously been related to variable spreading rates on the global MOR system. Importantly, the documentation of multiple sequences of eruptions at each study area, representing hundreds to thousands of years, provides constraints on the variability in eruptive style at a given magma supply and spreading rate.

Description: Settled indoor and outdoor dusts in urban environment represent an important source of secondary pollution. Magnetic characteristics of the settled dust from six cities in Bulgaria are explored, allowing comparison on a national (country) scale. Monthly variations of the mass-specific magnetic susceptibilities (χindoor) and (χoutdoor) and calculated dust loading rates for a period of 17 months do not show seasonal variability, probably due to the dominant role of traffic-related emissions and soil-derived particles in the settled dust. The main magnetic mineral is magnetite, present as spherules and irregular particles of pseudo-single-domain grain sizes. Systematically lower remanence coercivities are obtained for outdoor dusts when compared with the corresponding indoor samples, implying that penetration of smaller particles of ambient origin indoors is the main source of the indoor dust. Mean yearly values of the ratio (χindoor/χoutdoor) for each city show statistically significant correlation with mortality due to cardiovascular diseases. This ratio reveals the source- and site-specific importance of the anthropogenically derived toxicogenic fraction. Heavy metal content of the settled dust is related to the contribution from several pollution sources (soil-derived, combustion and industrial), discriminated through analysis of principal components. SEM/EDX analyses reveal abundant presence of anthopogenic Fe-containing spherules, irregular particles and diesel exhaust conglomerates. High molecular weight polyaromatic hydrocarbons (PAH) dominate the total PAH content of the outdoor dust samples. The observed linear correlation between total PAH content, coercivity of remanence and the ratio Mrs/χ suggest either adsorption of PAHs on iron oxide particles and especially magnetite, or emission related increase in total PAH concentration along with a decrease of effective magnetic grain size of the accompanying magnetic fraction.

Description: In order to reveal the potential effects of early diagenesis on magnetic minerals in deep-sea sediments, we studied early diagenetic zones and magnetic mineral characteristics of Lower Pliocene hemipelagic sediment samples from IODP Sites U1305, U1306, and U1307 on Eirik Drift, Labrador Sea. All samples analyzed were unlithified silty clay sediments recovered by a piston corer from depths down to 200 meters composite depth (mcd). Based on shipboard interstitial-water geochemistry, we divided the sediment column from each site into six early diagenetic zones. Magnetite (Fe3O4) was present at all analyzed depths, whereas maghemite (γFe2O3) was found only above the iron reduction zone. We attribute this to associated changes in interstitial redox conditions, which induced preferential dissolution of maghemitized surfaces on magnetite grains. Mineral magnetic results indicate a general down-hole change in mean grain size of magnetic minerals. At Site U1307, which has relatively low organic carbon contents, the diagenetic zones occur at greater depths than at the other studies sites. This suggests that interstitial oxygen levels at this site remained high enough to degrade organic matter through oxic bacterial activity, and that detrital magnetic minerals have been preserved even at depth.

Description: The Nihewan Basin (40°N) in North China is a rich source of Early Pleistocene Paleolithic sites and thus a key area for studying early human evolution in high-latitude (from an early human perspective) East Asia. Here a high-resolution magnetostratigraphic investigation is carried out on a fluvio-lacustrine section in the northeastern Nihewan Basin, which contains the Feiliang and Lanpo Paleolithic sites. Paleomagnetic results suggest that this section records the lower portion of the Brunhes polarity chron and the upper Matuyama polarity chron. Furthermore, the Jaramillo polarity subchron and seven of the nine validated geomagnetic excursions within the Matuyama polarity chron are identified, including the Kamikatsura, Santa Rosa, Intra-Jaramillo, Cobb Mountain, Bjorn, Gardar and Gilsa excursions. The Feiliang artifact layer is located just at the bottom of the Cobb Mountain excursion, thus its age is estimated to be ∼1.2 Ma. The Lanpo artifact layer appears to be coeval with the Gilsa excursion, yielding an estimated age of ∼1.6 Ma. This study provides new evidence for the presence of early humans in North China before 1.5 Ma and documents the powerful role of geomagnetic excursions: they provide valuable age control points for ongoing efforts to date the early Paleolithic sites.

Description: Following the release of global continental effective elastic thickness (Te) maps obtained using different approaches, we now have the opportunity to provide better constraints on Te. We improve previous estimates of Te derived from thermo-rheological models of lithospheric strength (or Ter) using new equations that consider variations of the Young's Modulus in the lithosphere. These new values are quantitatively compared with those obtained from an inverse approach (or Tei) based on a comparison of the spectral coherence between topography and gravity anomalies with the flexural response of an equivalent elastic plate to loading. The two models show in general a good agreement, having equal means (at the 95% significance level) in about half of the continental areas. In other regions Tei exceeds Ter in about 65% of the data points, showing that Tei provides an upper bound on Te. The two data sets have a similar range, but demonstrate different distributions. Ter has a bimodal distribution, with the two peaks representative of the cratons and of the areas outside of them. In contrast, Tei has more uniform distribution without predominant peaks. Our models show higher similarities in the Meso-Cenozoic orogens than in the Archaean and Proterozoic shields and platforms, due to the methods employed. For the regions with the most robust determinations of Ter and Tei, the relationship between them is close to linear. The results of this work can be used for further studies on the mechanical properties of the lithosphere.

Description: Mélange rocks outcrop widely in the central and western frontal sectors of the Betic Cordillera, as in many other collisional orogens where they form part of the accretionary wedges. Extrusion of Triassic plastic clays and evaporites was favored by the progressive accretion of the Betic External Zones, mixing rocks of different provenance and forming a synorogenic frontal mélange unit. MT data coupled with gravity data are a valid combined methodology to characterize the geometry of these mélange units, since the characterization of plastic rocks geometry is usually uncertain using seismic techniques. The results correlate well with known geological features (sedimentary basins, calcareous ranges, evaporitic rocks) and reveal the deep geometry. A resistive body, slightly dipping toward the SE, points to the continuity of the Iberian Massif below the Guadalquivir basin (2°) and the External Zones (6–8°). To the south, gravity models show the Iberian continental crust subducting below the Internal Zones with a roughly 20–35° slope. The main conductive bodies are related to the location of evaporitic rocks involved in the frontal mélange. They overlie the Iberian Massif and, southwards, the frontal Jurassic and Cretaceous limestone sequences of the External and Median Subbetics. In this setting, thick plastic rock units placed above the foreland could act as a lubricant facilitating continental subduction, and being progressively accreted toward a frontal mélange.

Description: The Pacific Northwest (PNW) has a complex tectonic history and over the past ∼17 Ma has played host to several major episodes of intraplate volcanism. These events include the Steens/Columbia River flood basalts (CRB) and the striking spatiotemporal trends of the Yellowstone/Snake River Plain (Y/SRP) and High Lava Plains (HLP) regions. Several different models have been proposed to explain these features, which variously invoke the putative Yellowstone plume, rollback and steepening of the Cascadia slab, extensional processes in the lithosphere, or a combination of these. Here we integrate seismologic, geodynamic, geochemical, and petrologic results from the multidisciplinary HLP project and associated analyses of EarthScope USArray seismic data to propose a conceptual model for post-20 Ma mantle dynamics beneath the PNW and the relationships between mantle flow and surface tectonomagmatic activity. This model invokes rollback subduction as the main driver for mantle flow beneath the PNW beginning at ∼20 Ma. A major pulse of upwelling due to slab rollback and upper plate extension and consequent melting produced the Steens/CRB volcanism, and continuing trench migration enabled mantle upwelling and hot, shallow melting beneath the HLP. An additional buoyant mantle upwelling is required to explain the Y/SRP volcanism, but subduction-related processes may well have played a primary role in controlling its timing and location, and this upwelling likely continues today in some form. This conceptual model makes predictions that are broadly consistent with seismic observations, geodynamic modeling experiments, and petrologic and geochemical constraints.

Description: Dissolved platinum concentrations of eleven large pristine river systems in East Asia (∼200 samples) were determined to better constrain the oceanic platinum budget. Most samples had concentrations less than 1.4 pM; relatively high concentrations up to 5.8 pM were measured in only approximately 6% of the samples. The median Pt concentrations of the individual river systems had only a small range, from 0.18 pM (Duman) to 0.63 pM (Huang He), and the difference in Pt yield mainly resulted from the difference in runoff. The rivers draining the eastern Tibetan Plateau – the Salween, Mekong, Chang Jiang (Yangtze), Hong (Red), and Huang He (Yellow) – had higher Pt yield than the rivers of the Russian Far East – the Amur, Lena, Yana, Indigirka, and Kolyma. If the discharge-weighted mean Pt concentration of our samples (0.36 pM) is extrapolated globally, the estimated riverine flux of dissolved Pt to the ocean is 13 × 103 mol y−1. Based on this riverine flux, the estimated oceanic residence time of Pt is 24 ± 10 kyrs. A 50% release and 50% uptake of Pt in estuaries would modify this to 16 kyrs and 45 kyrs, respectively.

Description: Magnetic iron minerals are widespread and indicative sediment constituents in estuarine, coastal and shelf systems. We combine environmental magnetic, sedimentological and numerical methods to identify magnetite-enriched placer-like zones in a complex coastal system and delineate their formation mechanisms. Magnetic susceptibility and remanence measurements on 245 surficial sediment samples collected in and around Tauranga Harbour, the largest barrier-enclosed tidal estuary of New Zealand, reveal several discrete enrichment zones controlled by local hydrodynamic conditions. Active magnetite enrichment takes place in tidal channels, which feed into two coast-parallel nearshore magnetite-enriched belts centered at water depths of 6–10 m and 10–20 m. A close correlation between magnetite content and magnetic grain size was found, where higher susceptibility values are associated within coarser magnetic crystal sizes. Two key mechanisms for magnetite enrichment are identified. First, tide-induced residual currents primarily enable magnetite enrichment within the estuarine channel network. A coast-parallel, fine sand magnetite enrichment belt in water depths of less than 10 m along the barrier island has a strong decrease in magnetite content away from the southern tidal inlet and is apparently related to active coast-parallel transport combined with mobilizing surf zone processes. A second, less pronounced, but more uniform magnetite enrichment belt at 10–20 m water depth is composed of non-mobile, medium-coarse-grained relict sands, which have been reworked during post-glacial sea level transgression. We demonstrate the potential of magnetic methods to reveal and differentiate coastal magnetite enrichment patterns and investigate their formative mechanisms.

Description: We have produced common conversion point (CCP) stacked Ps and Sp receiver function image volumes of the Moho and lithosphere-asthenosphere boundary (LAB) beneath the western United States using Transportable Array data. The large image volumes and the diversity of tectonic environments they encompass allow us to investigate evolution of these structural discontinuities. The Moho is a nearly continuous topographic surface, whereas the LAB is not and the seismic images show a more complex expression. The first order change in LAB depth in the western U.S. occurs along the Cordilleran hingeline, the former Laurasian passive margin along the southwestern Precambrian North American terranes. The LAB is about 50% deeper to the east of the hingeline than to the west, with most of the increase in LAB thickness being in the mantle lithosphere. We infer that the Moho and the LAB are Late Mesozoic or Cenozoic everywhere west of the hingeline, modified during Farallon subduction and its aftermath. Between the hingeline and the Rocky Mountain Front, the LAB, and to a lesser extent the Moho, have been partially reset during the Cenozoic by processes that continue today. Seismicity and recent volcanism in the interior of the western U.S. are concentrated along gradients in crustal and/or lithospheric thickness, for example the hingeline, and the eastern edge of the coastal volcanic-magmatic terranes. To us this suggests that lateral gradients in integrated lithospheric strength focus deformation. Similarly, areas conjectured to be the sites of convective downwellings and associated volcanism are located along gradients in regional lithosphere thickness.

Description: The Logatchev hydrothermal field at 14°45′N on the MAR is characterized by gas plumes that are enriched in methane and helium compared to the oceanic background. We investigated CH4 concentration and δ13C together with δ3He in the water column of that region. These data and turbidity measurements indicate that apart from the known vent fields, another vent site exists northeast of the vent field Logatchev 1. The distribution of methane and 3He concentrations along two sections were used in combination with current measurements from lowered acoustic Doppler current profilers (LADCP) to calculate the horizontal plume fluxes of these gases. According to these examinations 0.02 μmol s−1 of 3He and 0.21 mol s−1 of methane are transported in a plume that flows into a southward direction in the central part of the valley. Based on 3He measurements of vent fluid (22 ± 6 pM), we estimate a total vent flux in this region of about 900 L s−1 and a total flux of CH4 of 3.2 mol s−1.

Description: Heating or cooling can lead to high stresses in rocks due to the different thermal-elastic properties of minerals. In the upper 4 km of the crust, such internal stresses might cause fracturing. Yet it is unclear if thermal elasticity contributes significantly to critical stresses and failure deeper in Earth's continental crust, where ductile creep causes stress relaxation. We combined a heating experiment conducted in a Synchrotron microtomograph (Advanced Photon Source, USA) with numerical simulations to calculate the grain-scale stress field in granite generated by slow burial. We find that deviatoric stresses 〉100 MPa can be stored during burial, with relaxation times from 100's to 1000's ka, even in the ductile crust. Hence, grain-scale thermal-elastic stresses may serve as nuclei for instabilities, thus rendering the continental crust close to criticality.

Description: Stable isotope dendroclimatology using α-cellulose has unique potential to deliver multimillennial-scale, sub-annually resolved, terrestrial climate records. However, lengthy processing and analytical methods often preclude such reconstructions. Variants of the Brendel extraction method have reduced these limitations, providing fast, easy methods of isolating α-cellulose in some species. Here, we investigate application of Standard Brendel (SBrendel) variants to resinous soft-woods by treating samples of kauri (Agathis australis), ponderosa pine (Pinus ponderosa) and huon pine (Lagarastrobus franklinii), varying reaction vessel, temperature, boiling time and reagent volume. Numerous samples were visibly ‘under-processed’ and Fourier Transform infrared spectroscopic (FTIR) investigation showed absorption peaks at 1520 cm−1 and ∼1600 cm−1 in those fibers suggesting residual lignin and retained resin respectively. Replicate analyses of all samples processed at high temperature yielded consistent δ13C and δ18O despite color and spectral variations. Spectra and isotopic data revealed that α-cellulose δ13C can be altered during processing, most likely due to chemical contamination from insufficient acetone removal, but is not systematically affected by methodological variation. Reagent amount, temperature and extraction time all influence δ18O, however, and our results demonstrate that different species may require different processing methods. FTIR prior to isotopic analysis is a fast and cost effective way to determine α-cellulose extract purity. Furthermore, a systematic isotopic test such as we present here can also determine sensitivity of isotopic values to methodological variables. Without these tests, isotopic variability introduced by the method could obscure or ‘create’ climatic signals within a data set.

Description: Trace element ratios Mg/Ca, Sr/Ca, and Ba/Ca are readily measured in speleothems and may be closely related to hydrological balance, enhancing paleoclimate information inferred from stable isotope measurements. We develop a model which simulates the variation in dripwater chemistry resulting from variable degree of water-rock interaction and prior calcite precipitation (PCP), with the latter process depending both on drip interval and drip oversaturation with respect to CaCO3. Partition coefficients between speleothem and dripwater are dependent on temperature for Mg and on speleothem growth rate for Sr and Ba, as observed in laboratory experiments. The drip oversaturation state, regulated both by cave pCO2 and the dilution and soil karst dissolution processes, strongly affects stalagmite trace element concentrations by modulating the extent of PCP and speleothem growth rates. Application of an inverse model confirms that seasonal CO2 cycles can explain the uncorrelated seasonal cycles in Mg/Ca and Sr/Ca observed in our speleothem records from NW Spain for which high CO2 coincides with dry season. In absence of seasonal variations in drip interval, cycles in cave pCO2 can produce seasonal covariation in Sr/Ca, Mg/Ca and Ba/Ca. In long time series (104 yr) where seasonal sampling resolution is not obtained in stalagmites, a change from dominance of summer to winter rainfall can shift the season of strongest stalagmite deposition to one of lower mean CO2 and hence greater PCP and higher Mg/Ca, Sr/Ca and Ba/Ca ratios. Caves best suited to record a dominantly water balance signal, such as mean drip intervals, are those with minimal seasonal variation in cave pCO2.

Description: The acoustic scintillation method is applied to the investigation and monitoring of a vigorous hydrothermal plume from Dante within the Main Endeavour vent field (MEF) in the Endeavour Ridge segment. A 40 day time series of the plume's vertical velocity and temperature fluctuations provides a unique opportunity to study deep sea plume dynamics in a tidally varying horizontal cross flow. An integral plume model that takes into account ambient stratification and horizontal cross flows is established from the conservation equations of mass, momentum and density deficit. Using a linear additive entrainment velocity in the model (E = αUm + βU⊥) that is a function of both the plume relative axial velocity (Um) and the relative ambient flow perpendicular to the plume (U⊥) gives consistent results to the experimental data, suggesting entrainment coefficients α = 0.1 and β = 0.6. Also from the integral model, the plume height in a horizontal cross flow (Ua) is shown to scale as 1.8B1/3Ua−1/3N−2/3 for 0.01 ≤ Ua ≤ 0.1 m/s where B is the initial buoyancy transport and N is the ambient stratification, both of which are assumed constant.

Description: Outcrops of deeply derived ultramafic rocks and gabbros are widespread along slow spreading ridges where they are exposed in the footwall of detachment faults. We report on the microstructural and petrological characteristics of a large number of samples from ultramafic exposures in the walls of the Mid-Atlantic Ridge (MAR) axial valley at three distinct locations at lat. 13°N and 14°45′N. One of these locations corresponds to the footwall beneath a corrugated paleo-fault surface. Bearing in mind that dredging and ROV sampling may not preserve the most fragile lithologies (fault gouges), this study allows us to document a sequence of deformation, and the magmatic and hydrothermal history recorded in the footwall within a few hundred meters of the axial detachment fault. At the three sampled locations, we find that tremolitic amphiboles have localized deformation in the ultramafic rocks prior to the onset of serpentinization. We interpret these tremolites as hydrothermal alteration products after evolved gabbroic rocks intruded into the peridotites. We also document two types of brittle deformation in the ultramafic rocks, which we infer could produce the sustained low magnitude seismicity recorded at ridge axis detachment faults. The first type of brittle deformation affects fresh peridotite and is associated with the injection of the evolved gabbroic melts, and the second type affects serpentinized peridotites and is associated with the injection of Si-rich hydrothermal fluids that promote talc crystallization, leading to strain localization in thin talc shear zones. We also observed chlorite + serpentine shear zones but did not identify samples with serpentine-only shear zones. Although the proportion of magmatic injections in the ultramafic rocks is variable, these characteristics are found at each investigated location and are therefore proposed as fundamental components of the deformation in the footwall of the detachment faults associated with denudation of mantle-derived rocks at the MAR.

Description: Improving estimates of past ocean temperatures is paramount to our understanding of ocean circulation and its role in climate change. Magnesium/calcium (Mg/Ca) ratios of carapaces of the benthic ostracod genus Krithe were determined from new, globally distributed core top samples from the Norwegian Sea, Cape Hatteras shelf, Gulf of Mexico, Sulawesi Margin (Indonesia), New Zealand shelf, Ceara Rise, and the North Atlantic. A linear regression of the Krithe Mg/Ca ratios and bottom water temperature (BWT) reveals a significant correlation for locations where temperature during carapace calcification was above ∼3°C, which can be described by the equation Mg/Ca = (0.972 ± 0.152) * BWT + (7.948 ± 1.103) consistent with previous North Atlantic calibrations. Deviations from the global calibration line below ∼3°C follow the same pattern observed for benthic foraminifera, suggesting that the incorporation of magnesium into ostracodal calcite may be secondarily controlled by changes in carbonate ion concentration. Therefore, we propose a linear regression that describes the relationship between magnesium incorporation, temperature, and carbonate saturation for low temperatures (

Description: This study summarizes organic carbon isotope (δ13C) and total organic carbon (TOC) data from a series of tests undertaken to provide an appropriate methodology for pre-analysis treatment of mudstones from an Upper Carboniferous sedimentary succession, in order to develop a consistent preparation procedure. The main treatments involved removing both inorganic carbonate and hydrocarbons (which might be extraneous) before δ13C and TOC analysis. The results show that decarbonating using hydrochloric acid causes significant reduction in δ13C and total carbon (TC) of the bulk material due to the removal of inorganic carbonate. These changes are most pronounced where soluble calcium carbonate (rather than Ca-Mg-Fe carbonate) is present. Deoiled samples show only slightly higher mean δ13C where visible bitumen was extracted from the bulk sample. Moreover, the isotopic signatures of the extracts are closely correlated to those of their respective bulk samples, suggesting that small yields of hydrocarbons were generated in situ with no isotopic fractionation. In addition, further δ13C and TC analyses were performed on samples where mixing of oil-based drilling mud with brecciated core material had been undertaken. Brecciated mudstone material did not display distinct isotopic signals compared to the surrounding fine-grained material. Overall we show that the most accurate assessment of bulk organic carbon isotopes and concentration in these samples can be achieved through decarbonating the material prior to measurement via the ‘rinse method’. However, our results support recent findings that pre-analysis acid treatments can cause variable and unpredictable errors in δ13C and TOC values. We believe that, despite these uncertainties, the findings presented here can be applied to paleoenvironmental studies on organic matter contained within sedimentary rocks over a range of geological ages and compositions.

Description: The Woodlark Basin is one of the rare places on earth where the transition from continental breakup to seafloor spreading can be observed. The potential juxtaposition of continental rocks, a large magmatic heat source, crustal-scale faulting, and hydrothermal circulation has made the Woodlark Basin a prime target for seafloor mineral exploration. However, over the past 20 years, only two locations of active hydrothermalism had been found. In 2009 we surveyed 435 km of the spreading axis for the presence of hydrothermal plumes. Only one additional plume was found, bringing the total number of plumes known over 520 km of ridge axis to only 3, much less than at ridges with similar spreading rates globally. Particularly the western half of the basin (280 km of axis) is apparently devoid of high temperature plumes despite having thick crust and a presumably high magmatic budget. This paucity of hydrothermal activity may be related to the peculiar tectonic setting at Woodlark, where repeated ridge jumps and a re-location of the rotation pole both lead to axial magmatism being more widely distributed than at many other, more mature and stable mid-ocean ridges. These factors could inhibit the development of both a stable magmatic heat source and the deeply penetrating faults needed to create long-lived hydrothermal systems. We conclude that large seafloor massive sulfide deposits, potential targets for seafloor mineral exploration, will probably not be present along the spreading axis of the Woodlark Basin, especially in its younger, western portion.

Description: Shatsky Rise consists of thick (∼30 km maximum) basaltic crust with various geochemical compositions. Geochemistry data indicate that four magma types exist on the plateau; namely normal, low-Ti, high-Nb, and U1349 types. The normal type is the most abundant in volume and appears on all three large edifices of the plateau: Tamu, Ori, and Shirshov massifs. Composition of the normal type is similar to normal mid-ocean ridge basalt (N-MORB) composition, but with slight relative enrichment of the more incompatible elements. The low-Ti type is distinguished from the normal type basalt by slightly lower Ti content at a given MgO. Composition of the high-Nb type is characterized by distinctively high contents of incompatible trace elements. U1349 type basalts are composed of more primitive and depleted compositions compared with the others. The normal type basalts constitute ∼94% of the lava units of the oldest Tamu Massif and non-normal types (i.e., the other three types) basalts comprise ∼57% on the younger Ori Massif, implying that geochemical compositions may have become heterogeneous with time. Petrological examination demonstrates that compositions of the normal-, low-Ti-, and high-Nb-type basalts evolved through fractional crystallization of olivine, plagioclase, and augite in shallow magma chambers (

Description: We obtained areal variations of crustal thickness, magnetic intensity, and degree of melting of the sub-axial upwelling mantle at Thetis and Nereus Deeps, the two northernmost axial segments of initial oceanic crustal accretion in the Red Sea, where Arabia is separating from Africa. The initial emplacement of oceanic crust occurred at South Thetis and Central Nereus roughly ∼2.2 and ∼2 Ma, respectively, and is taking place today in the northern Thetis and southern Nereus tips. Basaltic glasses major and trace element composition suggests a rift-to-drift transition marked by magmatic activity with typical MORB signature, with no contamination by continental lithosphere, but with slight differences in mantle source composition and/or potential temperature between Thetis and Nereus. Eruption rate, spreading rate, magnetic intensity, crustal thickness and degree of mantle melting were highest at both Thetis and Nereus in the very initial phases of oceanic crust accretion, immediately after continental breakup, probably due to fast mantle upwelling enhanced by an initially strong horizontal thermal gradient. This is consistent with a rift model where the lower continental lithosphere has been replaced by upwelling asthenosphere before continental rupturing, implying depth-dependent extension due to decoupling between the upper and lower lithosphere with mantle-lithosphere-necking breakup before crustal-necking breakup. Independent along-axis centers of upwelling form at the rifting stage just before oceanic crust accretion, with buoyancy-driven convection within a hot, low viscosity asthenosphere. Each initial axial cell taps a different asthenospheric source and serves as nucleus for axial propagation of oceanic accretion, resulting in linear segments of spreading.

Description: Sandstone injectites form by up or down-section flow of a mobilized sand slurry through fractures in overlying rock. They act as reservoirs and high-permeability conduits through lower permeability rock in hydrocarbon systems. The Yellow Bank Creek Complex, Santa Cruz County, California is the largest known exposure of a sandstone injectite in the world. The complex contains granular textures that record processes of sand slurry flow, multiple pore fluids, and dewatering after emplacement. The injection was initially mobilized from a source containing both water and hydrocarbons. The water-sand slurry reached emplacement depth first, due to lower fluid viscosity. As the sand slurry emplaced, the transition from slurry flow to pore water percolation occurred. This transition resulted in preferred flow channels ∼6 mm wide in which sand grains were weakly aligned (laminae). The hydrocarbon-sand slurry intruded the dewatering sands and locally deformed the laminae. Compaction of the injectite deposit and pore fluid escape caused spaced compaction bands and dewatering pipes which created convolutions of the laminae. The hydrocarbon-rich sand slurry is preserved today as dolomite-cemented sand with oil inclusions. The laminae in this injectite are easily detected due to preferential iron oxide-cementation of the well-aligned sand laminae, and lack of cement in the alternating laminae. Subtle textures like these may develop during sand flow and be present but difficult to detect in other settings. They may explain permeability anisotropy in other sand deposits.

Description: Measurements of electrical conductivity of “slightly damp” mantle minerals from different laboratories are inconsistent, requiring geophysicists to make choices between them when interpreting their electrical observations. These choices lead to dramatically different conclusions about the amount of water in the mantle, resulting in conflicting conclusions regarding rheological conditions; this impacts on our understanding of mantle convection, among other processes. To attempt to reconcile these differences, we test the laboratory-derived proton conduction models by choosing the simplest petrological scenario possible – cratonic lithosphere – from two locations in southern Africa where we have the most complete knowledge. We compare and contrast the models with field observations of electrical conductivity and of the amount of water in olivine and show that none of the models for proton conduction in olivine proposed by three laboratories are consistent with the field observations. We derive statistically model parameters of the general proton conduction equation that satisfy the observations. The pre-exponent dry proton conduction term (σ0) and the activation enthalpy (ΔHwet) are derived with tight bounds, and are both within the broader 2σ errors of the different laboratory measurements. The two other terms used by the experimentalists, one to describe proton hopping (exponent r on pre-exponent water content Cw) and the other to describe H2O concentration-dependent activation enthalpy (term αCw1/3 added to the activation energy), are less well defined and further field geophysical and petrological observations are required, especially in regions of higher temperature and higher water content.

Description: 146Sm decays to 142Nd with a relatively short half-life (∼68 Ma). The 142Nd/144Nd of modern terrestrial mantle-derived lavas is 18 ± 5 ppm higher than the chondrite reservoir. The difference in 142Nd/144Nd between Earth and chondrites likely owes to Sm/Nd ratios 6% higher in the accessible Earth that arose within the first 30 million years following accretion. In order to constrain the early history of the mantle domains sampled by ocean island basalts (OIB) and mid-ocean ridge basalts (MORB), we present high-precision 142Nd/144Nd measurements on 11 different lavas from five hot spots, and one lava each from the Indian and Atlantic ridges. The lavas examined in this study bracket much of the known Sr-Nd-Pb-He isotopic variability the in mantle. These data complement existing high-precision 142Nd/144Nd data on MORB and OIB lavas. In agreement with previous studies, we find that MORB and OIB lavas examined for high-precision 142Nd/144Nd exhibit ratios that are indistinguishable from the terrestrial standard and are 15–20 ppm higher than the average obtained for ordinary and enstatite chondrites. The uniform, superchondritic 142Nd/144Nd data in OIB and MORB are consistent with derivation from a common, early formed (

Description: The global warming impact of substituting natural gas for coal and oil is currently in debate. We address this question here by comparing the reduction of greenhouse warming that would result from substituting gas for coal and some oil to the reduction which could be achieved by instead substituting zero carbon energy sources. We show that substitution of natural gas reduces global warming by 40% of that which could be attained by the substitution of zero carbon energy sources. At methane leakage rates that are ∼1% of production, which is similar to today's probable leakage rate of ∼1.5% of production, the 40% benefit is realized as gas substitution occurs. For short transitions the leakage rate must be more than 10 to 15% of production for gas substitution not to reduce warming, and for longer transitions the leakage must be much greater. But even if the leakage was so high that the substitution was not of immediate benefit, the 40%-of-zero-carbon benefit would be realized shortly after methane emissions ceased because methane is removed quickly from the atmosphere whereas CO2 is not. The benefits of substitution are unaffected by heat exchange to the ocean. CO2 emissions are the key to anthropogenic climate change, and substituting gas reduces them by 40% of that possible by conversion to zero carbon energy sources. Gas substitution also reduces the rate at which zero carbon energy sources must eventually be introduced.

Description: The scale and geometry of chemical and isotopic heterogeneities in the source of plumes have important scientific implications on the nature, composition and origin of plumes and on the dynamics of mantle mixing over time. Here, we address these issues through the study of Marquesas Islands, one of the Archipelagoes in Polynesia. We present new Sr, Nd, Pb, Hf isotopes as well as trace element data on lavas from several Marquesas Islands and demonstrate that this archipelago consists of two adjacent and distinct rows of islands with significantly different isotopic compositions. For the entire 5.5 Ma construction period, the northern islands, hereafter called the Ua Huka group, has had systematically higher 87Sr/86Sr and lower 206Pb/204Pb ratios than the southern Fatu Hiva group at any given 143Nd/144Nd value. The shape and curvature of mixing arrays preclude the ambient depleted MORB mantle as one of the mixing end-members. We believe therefore that the entire isotopic heterogeneity originates in the plume itself. We suggest that the two Marquesas isotopic stripes originate from partial melting of two adjacent filaments contained in small plumes or “plumelets” that came from a large dome structure located deep in the mantle under Polynesia. Low-degree partial melting under Marquesas and other “weak” Polynesian hot spot chains (Pitcairn-Gambier, Austral-Cook, Society) sample small areas of the dome and preserve source heterogeneities. In contrast, more productive hot spots build up large islands such as Big Island in Hawaii or Réunion Island, and the higher degrees of melting blur the isotopic variability of the plume source.

Description: The joint application of different seismological techniques for seismic noise analysis, and the results of a volcanological and morphostructural survey, have allowed us to obtain a detailed and well constrained image of the shallow crustal structure of the Solfatara volcano (Campi Flegrei caldera, Italy). Horizontal-to-vertical spectral ratios, inversion of surface wave dispersion curves and polarization analysis provided resonance frequencies and peak amplitudes, shear wave velocity profiles and polarization pattern of coherent ambient noise. These results, combined in a unique framework, indicate that the volcanic edifice is characterized by lateral and vertical discontinuities and heterogeneities in terms of shear wave velocity, lithological contrasts and structural setting. The interpretation of the seismological results, with the volcanological and morphostructural constraints, supports the hypothesis that the volcano has been characterized by a complex and intense activity, with the alternation of constructive and destructive phases, during which magmatic and phreatomagmatic explosions built a complex tuff-cone, later reworked by atmospheric agents and altered by hydrothermal activity. The differences in the velocity structure between the central and eastern parts of the crater have been interpreted as resulting from a possible eastward migration of the eruptive vent along the deformational features affecting the area, and to the presence of viscous lava and lithified tuff bodies within the feeding conduits, which are buried under a covering of reworked materials of variable thickness. The observed fault and fracture systems, partially inherited from regional structural setting and exhumed during volcanism and ground deformation episodes also seems to strongly control wave propagation, affecting the noise polarization properties.

Description: The Mugi mélange in the Shimanto accretionary complex, southwest Japan, records faulting and fluid flow patterns at the updip limit of the seismogenic region of the Nankai subduction zone. To characterize the origin and behavior of syn-tectonic fluids, we investigated the carbon, oxygen, and strontium isotopic compositions, and rare earth element (REE) patterns of syn-tectonic calcite within veins along fault zones in the mélange, as well as the Sr isotopic compositions and REE patterns of surrounding host rocks. With the exception of intra-basalt veins formed prior to subduction, the δ13C values of veins range from −10‰ to −19‰, suggesting a mixed carbon source (i.e., marine carbonate and organic matter). The vein-forming fluids have positive oxygen isotopic compositions (+2‰ to +9‰ (SMOW)) and high 87Sr/86Sr values (0.70794–0.70850), suggesting that the source was rock-buffered fluids affected both by terrigenous sediments and altered oceanic crust. The veins found in filling the fault zone associated with tectonic underplating have different REE patterns to those of the other veins, implying a difference in physicochemical processes affecting the fault zone near the subduction megathrust.

Description: There has been much debate as to whether 210Pb-226Ra disequilibria in young volcanic rocks result from partial melting, cumulate interaction or magma degassing. Here we present new data from basalts erupted in 1978 from Ardoukoba volcano in the Asal Rift. The (210Pb/226Ra)t ratios are very low (0.2 to 0.6) and appear to correlate negatively with (226Ra/230Th). Invariant (230Th/238U) and (231Pa/235U) ratios require similar melting rates, porosities, and extents for all parental magmas. Thus, the range in (226Ra/230Th), which is negatively correlated with Th concentration, reflects fractional crystallization over millennia after the magmas were emplaced into the crust. This precludes the 210Pb deficits from resulting from partial melting. Instead, the 210Pb deficits must have formed subsequent to magma differentiation and are interpreted to reflect several decades of magma degassing. Many young basalts erupted in a variety of tectonic settings are similarly depleted in 210Pb with respect to 226Ra, suggesting that they continuously degas over a period of a few to several decades, perhaps reflecting the time required to rise to the surface from deeper reservoirs. In some of these basalts, gas accumulation leads to the shallowest, most evolved, and earliest erupting magmas having the highest (210Pb/226Ra) ratios and sometimes 210Pb excesses.

Description: Broadband data from the Meso-America Subduction Experiment (MASE) line in central Mexico were used to image the subducted Cocos plate and the overriding continental lithosphere beneath central Mexico using a generalized radon transform based migration. Our images provide insight into the process of subducting relatively young oceanic lithosphere and its complex geometry beneath continental North America. The converted and reverberated phase image shows complete horizontal tectonic underplating of the Cocos oceanic lithosphere beneath the North American continental lithosphere, with a clear image of a very thin low-velocity oceanic crust (7–8 km) which dips at 15–20 degrees at Acapulco then flattens approximately 300 km from the Middle America Trench. Farther inland the slab then appears to abruptly change from nearly horizontal to a steeply dipping geometry of approximately 75 degrees underneath the Trans-Mexican Volcanic Belt (TMVB). Where the slab bends underneath the TMVB, the migrated image depicts the transition from subducted oceanic Moho to continental Moho at ∼230 km from the coast, neither of which were clearly resolved in previous seismic images. The deeper seismic structure beneath the TMVB shows a prominent negative discontinuity (fast-to-slow) at ∼65–75 km within the upper mantle. This feature, which spans horizontally beneath the arc (∼100 km), may delineate the top of a layer of ponded partial melt.

Description: Located adjacent to the NE Pacific convergent boundary, Cascadia Basin has a global impact well beyond its small geographic size. Composed of young oceanic crust formed at the Juan de Fuca Ridge, igneous rocks underlying the basin are partially insulated from cooling of their initial heat of formation by a thick layer of pelagic and turbidite sediments derived from the adjacent North American margin. The igneous seafloor is eventually consumed at the Cascadia subduction zone, where interactions between the approaching oceanic crust and the North American continental margin are partially controlled by the thermal environment. Within Cascadia Basin, basement topographic relief varies dramatically, and sediments have a wide range of thickness and physical properties. This variation produces regional differences in heat flow and basement temperatures for seafloor even of similar age. Previous studies proposed a north-south thermal gradient within Cascadia Basin, with high geothermal flux and crustal temperatures measured in the heavily sedimented northern portion near Vancouver Island and lower than average heat flux and basement temperatures predicted for the central and southern portions of the basin. If confirmed, this prediction has implications for processes associated with the Cascadia subduction zone, including the location of the “locked zone” of the megathrust fault. Although existing archival geophysical data in the central and southern basin are sparse, nonuniformly distributed, and derived from a wide range of historical sources, a substantial N-S geothermal gradient appears to be confirmed by our present compilation of combined water column and heat flow measurements.

Description: Volcanic eruptions inject sulfur into the atmosphere mainly in form of SO2 and H2S. The ratio of these species (H2S/SO2), which is usually used as the mirror of the oxidation state of the source magma, varies significantly according to the type of activity, tectonic setting etc. This study aims to investigate the role of the hot core of plinian and sub-plinian volcanic plumes (T 〉 600°C) in sulfur speciation based on the thermodynamic equilibrium in this temperature range. We consider the hot core as a box model in which 1000°C magmatic gas and 25°C ambient air are mixed and show that it functions as a hot oxidizing reactor for S species. Processes inside the hot core usually decrease the H2S content of the system but can either increase or decrease SO2 depending on initial oxidation state. Thus the SO2 injected into the atmosphere is not essentially generated directly from the magma but it can be produced in the hot core as the result of H2S oxidation. Besides, volcanic cloud compositions do not mirror the source conditions. Considering three types of tectonic settings (convergent plate, divergent plate and hot spot) we propose that H2S emission is more likely under reduced conditions in divergent plate and hot spot volcanic settings.

Description: The Lau Backarc Basin (S.W. Pacific) hosts numerous spreading centers and rifts, including the Rochambeau Rifts (RR), Northwest Lau Spreading Center (NWLSC), and Central Lau Spreading Center (CLSC). Samples from the NWLSC, RR and CLSC show no evidence for a subduction-derived component in their mantle source regions or evidence for S loss during eruption. The contents of S in glasses from the NWLSC and many from the CLSC and the RR are lower than MORB at a given FeOTOT, indicating melts were initially sulfide-undersaturated. During differentiation, the decrease in Cu and Ag contents at ∼7 wt% MgO and the concomitant change in chalcophile element ratios marks the onset of sulfide saturation. The initially sulfide-undersaturated compositions of samples from the NWLSC are attributed to partial melting at pressures higher than parental MORB. The NWLSC and some of the CLSC and RR samples are strikingly enriched in Cu and Ag compared with MORB. This is a characteristic shared by basalts generated in many plume-related tectonic settings. The only plume-related samples that appear to be sulfide-saturated during differentiation and plot within the MORB array are alkaline basalts from the nearby Samoan islands. RR and CLSC basalts have a range in Cu contents, which can be explained by variable mixing between a high-Cu NWLSC-type melt with low-Cu sources from the Samoan plume (RR) and MORB-type mantle (CLSC). The RR alone of these three suites have markedly positive Pb, As, Tl and subtle Mo anomalies, possibly related to assimilation of old, hydrothermally altered, Vitiaz Arc crust.

Description: Timing is crucial to understanding the causes and consequences of events in Earth history. The calibration of geological time relies heavily on the accuracy of radioisotopic and astronomical dating. Uncertainties in the computations of Earth's orbital parameters and in radioisotopic dating have hampered the construction of a reliable astronomically calibrated time scale beyond 40 Ma. Attempts to construct a robust astronomically tuned time scale for the early Paleogene by integrating radioisotopic and astronomical dating are only partially consistent. Here, using the new La2010 and La2011 orbital solutions, we present the first accurate astronomically calibrated time scale for the early Paleogene (47–65 Ma) uniquely based on astronomical tuning and thus independent of the radioisotopic determination of the Fish Canyon standard. Comparison with geological data confirms the stability of the new La2011 solution back to ∼54 Ma. Subsequent anchoring of floating chronologies to the La2011 solution using the very long eccentricity nodes provides an absolute age of 55.530 ± 0.05 Ma for the onset of the Paleocene/Eocene Thermal Maximum (PETM), 54.850 ± 0.05 Ma for the early Eocene ash −17, and 65.250 ± 0.06 Ma for the K/Pg boundary. The new astrochronology presented here indicates that the intercalibration and synchronization of U/Pb and 40Ar/39Ar radioisotopic geochronology is much more challenging than previously thought.

Description: Empirical studies with earthquake catalogs suggest that large events (M 〉 5) are rarely triggered in significant numbers by passing surface waves at remote distances from main shocks. Triggered, small (M 〈 5) earthquakes are routinely associated with the passage of surface waves from large (M 〉 7) main shocks. Since large earthquakes involve larger rupture areas, we study the spatial and temporal characteristics of dynamic stress change for clues. Using a 3D finite element method, we model the complete wavefield from the 2002 M = 7.9 Denali earthquake recorded near the Wasatch Front in Utah, where details about triggered seismicity are known. In particular, we load our model with a displacement seismogram to acquire a time series of the stress change tensor and model failure of a representative normal fault based on these stress changes. We note that the stress-change regime varies rapidly between favoring strike-slip, thrust, and normal faulting, with durations lasting ∼1–4 s. We find that these stress regimes usually affect only some fraction of a fault surface at any given time. Stress amplitudes also vary, meaning that ideal conditions for triggering are short-lived and spatially limited. Stress conditions can also rapidly reverse to regimes that inhibit slip. Given these stressing conditions, we conclude that it may be difficult for a larger rupture area to experience the temporally and spatially coherent stress change necessary to develop into a large magnitude earthquake.

Description: Subduction dynamics is strongly dependent on the geometry and rheology of the subducting slab and adjacent plates, as well as on the induced mantle flow driven by the evolution of tectonic configurations along subduction zones. However, these processes, and the associated plate tectonic driving forces, are difficult to study using time-dependent 3-dimensional computer simulations due to limitations in computing resources. We investigate these phenomena with a novel numerical approach, using BEM-Earth, a Stokes flow solver based on the Boundary Element Method (BEM) with a Fast-Multipole (FM) implementation. The initial BEM-Earth model configurations self-consistently determine the evolution of the entire lithosphere-mantle system without imposing additional constraints in a whole-Earth spherical setting. We find that models without an overriding plate overestimate trench retreat by 65% in a 20 m.y. model run. Also, higher viscosity overriding plates are associated with higher velocity subducting slabs, analogue to faster oceanic plates subducting beneath more rigid continental lithosphere. In our models poloidal flows dominate the coupling between the down-going and overriding plates, with trench-orthogonal length variations in overriding plates inducing flows at least ∼2× stronger than trench-parallel width variations. However, deformation in the overriding plate is related to its length and width, with narrower and longer plates extending more than wider and shorter plates.

Description: The upper Yangtze River flows southward on the southeastern Tibet characterizing by uniquely low and continuous relief. The river makes a sharp turn at Shigu, heading northeast, and forms the first bend of the Yangtze River. Many previous studies assumed southward flow of the ancestral Yangtze River from Shigu to the South China Sea. However, field evidence of southward flow of the paleo-Yangtze is lacking. In this paper we report our identification, based on detrital zircon U-Pb age distributions, of a range of fluvial sands left by the paleo-Yangtze in Tongdian, Madeng and Nanjian basins. Cosmogenic 10Be and 26Al burial dating provides burial ages for these fluvial sands from 1.7 to over 8.7 Ma. Rerouting of the Yangtze River therefore occurred within the last 1.7 Ma, postdating the major uplift of the central Tibet. We attribute the rerouting of the Yangtze River to response to activation of the Dali fault system, and in a larger scale, initiation of crustal deformation by clockwise rotation around eastern Himalayan syntaxis 2–4 Ma ago. Reorganization of the Yangtze drainage pattern most likely reflects regional uplift and displacement due to lower crust flowing beneath major faults in the southeastern Tibet and Yunnan.

Description: Low-Ca boninites (LCB) are arc-related magmatic rocks enriched in large ion lithophile elements (LILE), light rare earth elements (LREE), Zr and Hf relative to medium to heavy REE (MREE-HREE). These signatures are commonly attributed to a unique slab-derived agent that metasomatized a depleted mantle source but their origin in such an agent remains enigmatic. We report andesite-hosted refractory spinel harzburgite xenoliths from the Kamchatka arc, which contain a series of orthopyroxene-rich veins; these veins range in thickness, the contents of clinopyroxene and amphibole and degrees of reaction with the host. Vein minerals in reaction zones with host harzburgites show progressive depletion in MREE-HREE at constant Zr-Hf and develop patterns (U-shaped REE with Zr-Hf spikes) that mimic those of LCB. Major and trace element modeling suggests that these veins (1) formed from a high-temperature (≥1400°C), MgO-rich (∼30 wt.%) and silicic (∼54 wt.% SiO2) initial melt, strongly depleted in Al2O3, TiO2 and alkalis and (2) record fractionation of the initial melt to form hydrous liquids; the initial melt was equilibrated with metasomatized Kamchatka harzburgites and was similar in trace element abundances to island arc tholeiite (LREE-depleted to flat LREE-MREE patterns, low Nb and Ta but no significant Zr-Hf anomalies, high LILE). We argue that primary magmas of LCB formed by fluid-fluxed (LILE-rich and [Nb, Ta]-depleted fluid) melting of a cpx-free, highly refractory (≥30% melt extraction) harzburgitic source similar to arc peridotites from Kamchatka and elsewhere. This peculiar source may explain the distinctive major element features of LCB primary magmas. We propose that the primary magmas of LCB develop their characteristic trace element patterns through fractionation and reaction with refractory peridotites in the mantle wedge. Slab-related components alone may explain the high LILE in LCB but not their distinctive REE patterns and positive Zr-Hf anomalies.

Description: Here we report the first measurements of the H2O content of magmas and mantle xenoliths from the Big Pine Volcanic Field (BPVF), California, in order to constrain the melting process in the mantle, and the role of asthenospheric and lithospheric sources in this westernmost region of the Basin and Range Province, western USA. Melt inclusions trapped in primitive olivines (Fo82–90) record surprisingly high H2O contents (1.5 to 3.0 wt.%), while lithospheric mantle xenoliths record low H2O concentrations (whole rock 500 ka, to cooler (∼1220°C) and shallower melting (∼1 GPa) conditions in younger magmas. The estimated depth of melting correlates strongly with some trace element ratios in the magmas (e.g., Ce/Pb, Ba/La), with deeper melts having values closer to upper mantle asthenosphere values, and shallower melts having values more typical of subduction zone magmas. This geochemical stratification is consistent with seismic observations of a shallow lithosphere-asthenosphere boundary (∼55 km depth). Combined trace element and cryoscopic melting models yield self-consistent estimates for the degree of melting (∼5%) and source H2O concentration (∼1000 ppm). We suggest two possible geodynamic models to explain small-scale convection necessary for magma generation. The first is related to the Isabella seismic anomaly, either a remnant of the Farallon Plate or foundered lithosphere. The second scenario is related to slow extension of the lithosphere.

Description: Ilopango Caldera was formed ∼1810 years ago by eruption of the Tierra Blanca Joven (TBJ) dacite (70 km3) in central El Salvador. A subsequent eruption in 1880 produced a cluster of dacite domes in the center of Lago Ilopango that contain olive-bearing enclaves of basaltic andesite. The purpose of this study is to use trace element, isotope, and U-series data from the TBJ and 1880 eruptions to assess petrogenesis and the timescale of magma storage. We find that although the range of trace element data in the TBJ dacite can be reproduced by simple crystal fractionation of a plagioclase- and amphibole-rich mineral assemblage, the 87Sr/86Sr and 207Pb/204Pb data suggest that the 1880 basaltic andesite enclave has a different source than the dacites. This is consistent with U-series data that show the TBJ dacites have lower (230Th/232Th) than the 1880 basaltic andesite enclave (1.5 versus 1.6, respectively). All Ilopango rocks have 230Th excesses, and the range in (238U/232Th) of the TBJ dacites can be modeled by crystal fractionation of a mineral assemblage including accessory zircon and allanite from a magma that is similar in composition to the enclave. Mineral isochrons yield crystallization ages of

Description: Here we report new data on the sulfur isotopic compositions (δ34S) of fumarolic and plume gases collected at Mount Etna volcano during 2008–2009. While low-temperature fumaroles are affected by postmagmatic processes that modify the pristine isotopic signature, high-temperature and plume gases allow establishment of a δ34S range of ∼0 ± 1‰ for magmatic SO2. We compared our data with those from S dissolved in primitive melt inclusions from 2002 lava and in whole rocks that erupted during the past two thousand years. Such a comparison revealed that δ34S is systematically lower for magmatic gases than for sulfur dissolved in the melt. We modeled how isotopic fractionation due to magma degassing process may vary δ34S value in both the melt and gaseous phases. This modeling required assessment of the fractionation factor (αgas-melt). The most recent measurements on the oxidation state of sulfur in basaltic melt inclusions indicate that nearly all S is dissolved as sulfate (S6+), which would be possible in oxidized magmatic systems (ΔNNO ≥ 1). Under these conditions the exsolved gaseous phase is depleted with respect to the melt and the proposed model fits both gas and melt data, and constrains the Etnean magmatic δ34S to 1.0 ± 1.5‰. It is remarkable that the assessed redox conditions—which are significantly more oxidizing than previously thought—are able to explain why the dominant sulfur species measured in the Etnean plume is SO2.

Description: One of the most widely used numerical modeling techniques in geodynamics to study the evolution of geomaterials is the “marker-and-cell” technique. In such methods the material lithology is represented by Lagrangian particles (markers), while the continuum equations are solved on a background mesh. Significant research has been devoted to improving the efficiency and scalability of these numerical methods to enable high-resolution simulations to be performed on modest computational resources. In contrast, little attention has been given to developing visualization techniques suitable for interrogation high-resolution 3D particle data sets. We describe an efficient algorithm for performing a volume reconstruction of the lithology field defined via particles (code available upon request from the author). The algorithm generates an Approximate Voronoi Diagram (AVD) which transforms particle data sets into a cell-based, volumetric data set. The volumetric representation enables cross sections of the material configuration to be constructed efficiently and unambiguously, thereby enabling the interior material structure of the simulation results to be analyzed. Examples from geodynamic simulations are used to demonstrate visual results possible using this visualization technique. Performance comparisons are made between existing implementations of exact and approximate Voronoi diagrams. Overall, the AVD developed herein is found to be extremely competitive as a visualizing tool for massive particle data sets as it is extremely efficient, has low memory requirements and can be trivially used in a distributed memory computing environment.

Description: The Northwest Lau Backarc Basin, consisting of the Northwest Lau Spreading Center (NWLSC) and the Rochambeau Rifts (RR), is unique in having elevated 3He/4He ratios (up to 28 Ra) in the erupted lavas, clearly indicating a hot spot or ocean island basalt (OIB)-type signature. This OIB-type helium signature does not appear in any other part of the Lau Basin. Water column plume surveys conducted in 2008 and 2010 identified several sites of active hydrothermal discharge along the NWLSC-RR and showed that the incidence of hydrothermal activity is high, consistent with the high spreading rate of ∼100 mm/year. Hydrocasts into the Central Caldera and Southern Caldera of the NWLSC detected elevated 3He/4He (δ3He = 55% and 100%, respectively), trace metals (TMn, TFe), and suspended particles, indicating localized hydrothermal venting at these two sites. Hydrocasts along the northern rift zone of the NWLSC also had excess δ3He, TMn, and suspended particles suggesting additional sites of hydrothermal activity. The RR are dominated by Lobster Caldera, a large volcano with four radiating rift zones. Hydrocasts into Lobster Caldera in 2008 detected high δ3He (up to 239%) and suspended particle and TMn signals, indicating active venting within the caldera. A repeat survey of Lobster in 2010 confirmed the site was still active two years later. Plumes at Lobster Caldera and Central Caldera have end-member 3He/4He ratios of 19 Ra and 11 Ra, respectively, confirming that hot spot-type helium is also present in the hydrothermal fluids.

Description: Anomalous volcanism and tectonics between near-ridge mantle plumes and mid-ocean ridges provide important insights into the mechanics of plume-lithosphere interaction. We present new observations and analysis of multibeam, side scan sonar, sub-bottom chirp, and total magnetic field data collected during the R/V Melville FLAMINGO cruise (MV1007; May–June, 2010) to the Northern Galápagos Volcanic Province (NGVP), the region between the Galápagos Archipelago and the Galápagos Spreading Center (GSC) on the Nazca Plate, and to the region east of the Galápagos Transform Fault (GTF) on the Cocos Plate. The NGVP exhibits pervasive off-axis volcanism related to the nearby Galápagos hot spot, which has dominated the tectonic evolution of the region. Observations indicate that ∼94% of the excess volcanism in our survey area occurs on the Nazca Plate in three volcanic lineaments. Identified faults in the NGVP are consistent with normal ridge spreading except for those within a ∼60 km wide swath of transform-oblique faults centered on the GTF. These transform-oblique faults are sub-parallel to the elongation direction of larger lineament volcanoes, suggesting that lineament formation is influenced by the lithospheric stress field. We evaluate current models for lineament formation using existing and new observations as well as numerical models of mantle upwelling and melting. The data support a model where the lithospheric stress field controls the location of volcanism along the lineaments while several processes likely supply melt to these eruptions. Synthetic magnetic models and an inversion for crustal magnetization are used to determine the tectonic history of the study area. Results are consistent with creation of the GTF by two southward ridge jumps, part of a series of jumps that have maintained a plume-ridge separation distance of 145 km to 215 km since ∼5 Ma.

Description: The active subduction of the young Philippine Sea (PHS) plate and the old Pacific plate has resulted in significant seismic heterogeneity and anisotropy in Southwest (SW) Japan. In this work we determined a detailed 3-D P wave anisotropic tomography of the crust and upper mantle beneath SW Japan using ∼540,000 P wave arrival times from 5,249 local earthquakes recorded by 1095 stations. The PHS slab is imaged clearly as a high-velocity (high-V) anomaly which exhibits considerable lateral variations. Significant low-velocity (low-V) anomalies are revealed above and below the PHS slab. The low-V anomalies above the PHS slab may reflect the upwelling flow in the mantle wedge and the PHS slab dehydration, and they form the source zone of the arc volcanoes in SW Japan. The low-V zones under the PHS slab may reflect the upwelling flow in the big mantle wedge above the Pacific slab. The anisotropy in the crust and upper mantle is complex. In Kyushu, the P wave fast velocity direction (FVD) is generally trench-normal in the mantle wedge under the back-arc, which is consistent with the corner flow driven by the PHS slab subduction. The FVD is trench-parallel in the subducting PHS slab under Kyushu. We think that the intraslab seismicity is a potential indicator to the slab anisotropy. That is, the PHS slab with seismicity has kept its original fossil anisotropy formed at the mid-ocean ridge, while the aseismic PHS slab has reproduced the anisotropy according to its current deformation.

Description: The process of data selection in paleointensity studies is an essential step to ensure data fidelity. There is, however, no consensus as to the best approach to consistently select data with most studies using arbitrarily defined thresholds for selection. We present a new numerical model that simulates the variability of paleointensity data from hypothetical ideal samples acquiring a thermoremanent magnetization (TRM) by incorporating experimental noise, which has been constrained using over 75,000 data measurements. Using Monte Carlo analyses, we investigate the behavior of simulated data and characterize the distributions of parameters typically used to select paleointensity data. We use the 95th percentiles of the distributions to define thresholds for the maximum likely parameter values that can result from experimental noise. These represent values below which we cannot distinguish non-ideal behavior from noise. We find that a number of parameters are highly sensitive to noise and laboratory field strength (e.g., partial TRM, pTRM, check CDRAT and pTRM tail check δt*); this sensitivity may diminish their ability to identify non-ideal behavior. The fractional (f) dependence of some parameters and the proportion of inaccurate results provide justification for f ≥ 0.35 when selecting data from both Thellier-Thellier and Coe protocol experiments. The manifestation of noise in the original Thellier method, however, is different to that of methods that use zero-field heating steps. This suggests that the data selection procedure for the Thellier method should be different, but it also suggests that, contrary to previous analyses, the accuracy and scatter of results from this method are more sensitive to noise than methods that use zero-field heating steps. The general approach taken here is shown to be a powerful means of understanding the behavior of selection parameters and has the potential to be extended to models incorporating non-ideal behavior resulting from alteration and multidomain grains.

Description: We develop, validate, and apply a new strategy for estimating parameters in a geophysical model from interferometric synthetic aperture radar (InSAR) measurements. The observable quantity is a particular component of the deformation gradient tensor, defined as the derivative of the change in range with respect to the easting coordinate. This range change gradient is derived from wrapped phase data by a quadtree resampling procedure. Since the range change gradient is a continuous function of position, the strategy avoids the pitfalls associated with phase unwrapping techniques. To quantify the misfit between the observed and modeled values of the range gradient, the objective function calculates the cost as the absolute value of their difference, averaged over all samples. To minimize the objective function, we use a simulated annealing algorithm. This algorithm requires several thousand evaluations of the fitting function to find the optimum solution: the estimate of the model parameters that produces the lowest value of cost. For computational efficiency, we approximate the fitting function using a Taylor series. The simulated annealing algorithm then evaluates the approximate and fast version of the fitting function. After performing these two steps several times, the scheme converges, typically in a few iterations. We apply the strategy to Krafla central volcano in Iceland. Using a data set composed of eight interferometric pairs acquired by the ERS-1 and ERS-2 satellites over a 6-year interval between 1993 and 1999, we estimate the four parameters in a Mogi model. Results suggest a source at 4.98 ± 0.21 km depth and a deflation rate that decays exponentially over the interval, in agreement with previous studies.

Description: Speleothems have been regarded as ideal archives for recording variations of the Earth's magnetic field because they crystallize rapidly and are seldom modified by post-depositional processes, and because they can be precisely dated. Magnetic fabric research on speleothems has potential to reveal details of their interior structure and the distribution of ferrimagnetic minerals, which would benefit investigations of geomagnetic field behavior. Two stalagmites (HS4 and WD1) from caves in the middle reaches of the Yangtze River were collected for magnetic fabric study. The anisotropy of magnetic susceptibility (AMS) of the stalagmites is mainly controlled by calcite crystals and the directions of the minimum axes of AMS ellipsoids indicate that their crystallographic orientations are roughly perpendicular to the growth laminae. Anisotropy of the isothermal remanent magnetization (AIRM) indicates that the distributions of ferrimagnetic minerals in the stalagmites are not correlated with stalagmite growth laminae. Mean directions of the maximum principal axes of AIRM (R1 axes) for WD1 are close to that of the natural remanent magnetization, which suggests that the orientation of ferrimagnetic minerals in this stalagmite are likely to have been controlled by the geomagnetic field.

Description: A three-dimensional P wave velocity model of the crust and upper mantle down to 400-km depth beneath eastern Tibet is obtained using many temporary seismic stations of the ASCENT project and the Namche Barwa Broadband Seismic Network. We collected 16,508 arrival times of P, Pn and Pg phases from 573 local and regional earthquakes and 7,450 P wave arrivals from seismograms of 435 teleseismic events. Our high-quality data set enables us to reconstruct the 3-D velocity structure under eastern Tibet in more detail than the previous studies. In the shallow depth, our results show that the low velocity zones are not interconnected well (no wide-spread low velocity zones), which may reflect the complex pattern of material flowing in the study region. Below the Moho, we find that the Indian lithospheric mantle underthrusts sub-horizontally under eastern Tibet, and the extent of the northward advancing Indian lithosphere decreases from west to east. In the north, the Asian lithospheric mantle is detected under the vicinity of the Qaidam Basin. Between the Indian and Asian lithospheric mantles, there is an obvious low-velocity anomaly which may reflect an upwelling mantle diapir.

Description: In the 2011 off the Pacific coast of Tohoku Earthquake, groundwater pressure changes were observed in and around the Mizunami Underground Research Laboratory (MIU) in Central Japan, where two vertical shafts and horizontal research galleries are excavated in the granitic rock mass. Coseismic changes of groundwater pressure are believed to correspond to crustal dilation/contraction induced by earthquakes. In this study we calculated volumetric strain changes due to the Tohoku Earthquake based on previously reported fault slip models. The calculation indicates approximately 2 × 10−7 of dilational strain around the MIU. Based on the strain sensitivities calculated from tidal responses at the monitoring boreholes, the dilation corresponds to drawdowns of several tens of centimeters, and is almost the same as the drawdown observed in the boreholes at distances greater than 1 km from the MIU. In contrast, rapid elevation of groundwater pressures associated with the earthquake was observed in the boreholes within the 500 m vicinity of the MIU. The anomalous elevation is explained by a temporary recovery of the drawdown due to excavation of the shafts and a unique permeability increase induced by the coseismic dilation of heterogeneous local geological structures such as impervious faults controlling the hydrogeological environment.

Description: Major intracontinental strike-slip faults tend to mark boundaries between lithospheric blocks of contrasting mechanical properties along much of their length. Both crustal and mantle heterogeneities can form such boundaries, but the role of crustal versus mantle strength contrasts for localizing strain sufficiently to generate major faults remains unclear. Using the crustal velocity field observed through the Global Positioning System (GPS) in the epicentral area of the M7.2 2010 El Mayor-Cucapah earthquake, Baja California, we find that transient deformation observed after the event is anomalously small in areas of relatively high seismic velocity in the shallow upper mantle (∼50 km depth). This pattern is best explained with a laterally heterogeneous viscoelastic structure that mimics the seismic structure. The mantle of the Southern Colorado River Desert (SCRD) and Peninsular Ranges (PR), which bound the fault system to its east and west, respectively, have anomalously high viscosity and seismic velocity. We hypothesize that compared with the rest of the San Andreas fault (SAF) system to its north, the strike-slip fault system in northern Baja California is narrow because of the presence of the PR and SCRD high-viscosity regions which bound it.

Description: The relative heat carried by diffuse versus discrete venting of hydrothermal fluids at mid-ocean ridges is poorly constrained and likely varies among vent sites. Estimates of the proportion of heat carried by diffuse flow range from 0% to 100% of the total axial heat flux. Here, we present an approach that integrates imagery, video, and temperature measurements to accurately estimate this partitioning at a single vent site, Tour Eiffel in the Lucky Strike hydrothermal field along the Mid-Atlantic Ridge. Fluid temperatures, photographic mosaics of the vent site, and video sequences of fluid flow were acquired during the Bathyluck'09 cruise (Fall, 2009) and the Momarsat'10 cruise (Summer, 2010) to the Lucky Strike hydrothermal field by the ROV Victor6000 aboard the French research vessel the “Pourquoi Pas”? (IFREMER, France). We use two optical methods to calculate the velocities of imaged hydrothermal fluids: (1) for diffuse venting, Diffuse Flow Velocimetry tracks the displacement of refractive index anomalies through time, and (2) for discrete jets, Particle Image Velocimetry tracks eddies by cross-correlation of pixel intensities between subsequent images. To circumvent video blurring associated with rapid velocities at vent orifices, exit velocities at discrete vents are calculated from the best fit of the observed velocity field to a model of a steady state turbulent plume where we vary the model vent radius and fluid exit velocity. Our results yield vertical velocities of diffuse effluent between 0.9 cm s−1 and 11.1 cm s−1 for fluid temperatures between 3°C and 33.5°C above that of ambient seawater, and exit velocities of discrete jets between 22 cm s−1 and 119 cm s−1 for fluid temperatures between 200°C and 301°C above ambient seawater. Using the calculated fluid velocities, temperature measurements, and photo mosaics of the actively venting areas, we calculate a heat flux due to diffuse venting from thin fractures of 3.15 ± 2.22 MW, discrete venting of 1.07 ± 0.66 MW, and, by incorporating previous estimates of diffuse heat flux density from Tour Eiffel, diffuse flux from the main sulfide mound of ∼15.6 MW. We estimate that the total integrated heat flux from the Tour Eiffel site is 19.82 ± 2.88 MW and that the ratio of diffuse to discrete heat flux is ∼18. We discuss the implication of these results for the characterization of different vent sites within Lucky Strike and in the context of a compilation of all available measurements of the ratio of diffuse to discrete heat flux.

Description: Here we demonstrate with a study of the Lucky Strike hydrothermal field that image mosaicing over large seafloor areas is feasible with new image processing techniques, and that repeated surveys allow temporal studies of active processes. Lucky Strike mosaics, generated from 〉56,000 images acquired in 1996, 2006, 2008 and 2009, reveal the distribution and types of diffuse outflow throughout the field, and their association with high-temperature vents. In detail, the zones of outflow are largely controlled by faults, and we suggest that the spatial clustering of active zones likely reflects the geometry of the underlying plumbing system. Imagery also provides constraints on temporal variability at two time-scales. First, based upon changes in individual outflow features identified in mosaics acquired in different years, we document a general decline of diffuse outflow throughout the vent field over time-scales up to 13 years. Second, the image mosaics reveal broad patches of seafloor that we interpret as fossil outflow zones, owing to their association with extinct chimneys and hydrothermal deposits. These areas encompass the entire region of present-day hydrothermal activity, suggesting that the plumbing system has persisted over long periods of time, loosely constrained to hundreds to thousands of years. The coupling of mosaic interpretation and available field measurements allow us to independently estimate the heat flux of the Lucky Strike system at ∼200 to 1000 MW, with 75% to 〉90% of this flux taken up by diffuse hydrothermal outflow. Based on these heat flux estimates, we propose that the temporal decline of the system at short and long time scales may be explained by the progressive cooling of the AMC, without replenishment. The results at Lucky Strike demonstrate that repeated image surveys can be routinely performed to characterize and study the temporal variability of a broad range of vent sites hosting active processes (e.g., cold seeps, hydrothermal fields, gas outflows, etc.), allowing a better understanding of fluid flow dynamics from the sub-seafloor, and a quantification of fluxes.

Description: Topographic development inboard of the continental margin is a predicted response to ridge subduction. New thermochronology results from the western Alaska Range document ridge subduction related orogenesis. K-feldspar thermochronology (KFAT) of bedrock samples from the Tordrillo Mountains in the western Alaska Range complement existing U-Pb, 40Ar/39Ar and AFT (apatite fission track) data to provide constraints on Paleocene pluton emplacement, and cooling as well as Late Eocene to Miocene vertical movements and exhumation along fault-bounded blocks. Based on the KFAT analysis we infer rapid exhumation-related cooling during the Eocene in the Tordrillo Mountains. Our KFAT cooling ages are coeval with deposition of clastic sediments in the Cook Inlet, Matanuska Valley and Tanana basins, which reflect high-energy depositional environments. The Tordrillo Mountains KFAT cooling ages are also the same as cooling ages in the Iliamna Lake region, the Kichatna Mountains of the western Alaska Range, and Mt. Logan in the Wrangell-St. Elias Mountains, thus rapid cooling at this time encompasses a broad region inboard of, and parallel to, the continental margin extending for several hundred kilometers. We infer these cooling events and deposition of clastic rocks are related to thermal effects that track the eastward passage of a slab window in Paleocene-Eocene time related to the subduction of the proposed Resurrection-Kula spreading ridge. In addition, we conclude that the reconstructed KFATmax negative age-elevation relationship is likely related to a long period of decreasing relief in the Tordrillo Mountains.

Description: The Pliocene epoch is considered the most recent analog of modern warming because CO2 levels were similar to the present. To explore the carbonate minerals formed in the warmer Pliocene epoch, we studied two continuous sections of the Red Clay Formation on the Chinese Loess Plateau (CLP) by X-ray Diffraction (XRD), Fourier Transform Infrared Spectroscopy (FTIR), Scanning Electron Microscopy (SEM) and Stable Isotope Mass Spectrometry. The Red Clay Formation on the CLP exhibits diagnostic FTIR absorption features of calcite and protodolomite. This allowed quantification of the two carbonate minerals by the FTIR method. Using the FTIR method we estimate the average concentration of protodolomite in Bajiazui is 3.6% whereas the Duanjiapo section is 6.0%. Protodolomite occurrence is more consistent and the concentration is higher from ∼6.5–4.2 Ma B.P. and decreases markedly from 4.2–2.6 Ma B.P. Red Clay protodolomite is depleted in both δ13CPDB and δ18OPDB, ranging from −4.1‰ to −10.4 and from −6.7‰ to −11.6, respectively, and has a slightly higher δ18O value than the calcites. SEM observations show that Red Clay protodolomite is composed of euhedral rhombic crystals that range from 1–20 μm in diameter, grow into the soil voids and coexist with authigenic calcite and palygorskite. These observations imply that the protodolomite grew in situ and is authigenic from pedogenesis. Dolomitization in the Red Clay sequence appears to be the result of overcoming kinetic barriers. We propose that in the Red Clay a warm climate with seasonal dry conditions leads to the formation of calcrete from soil pore waters thereby enriching the pore solutions with respect to Mg2+ and significantly increasing the Mg/Ca ratio bringing about the formation of protodolomite.

Description: We describe a geophysical study of oceanic core complexes (OCC) and surrounding seafloor on the Mid-Atlantic Ridge at 13°N–14°N and off-axis to ∼1.9 Myr. Data include a detailed, deep-towed side scan sonar, magnetic field and bathymetry survey, supplemented by concurrent sea-surface bathymetry, magnetic field and gravity measurements. Using side scan and bathymetry, we infer areas and relative ages of seafloor volcanism, revealing a complex pattern of melt accretion across the median valley including close to its walls. We estimate tectonic and magmatic extension throughout the area, and find that average tectonic extension since chron 2 on plates containing OCCs is up to three times that on their conjugates. Deep-towed magnetic data reveal asymmetric spreading (faster on OCC-containing plates) and crustal magnetization that is highly heterogeneous on a scale of ∼5 km, suggesting that exhumed domes of OCCs have highly variable lithologies, perhaps comprising both serpentinized peridotite and gabbro. Improved fits to magnetic data are provided by models incorporating ∼45°of OCC footwall rotation. An axial zone of normal magnetization, of presumed Brunhes epoch, has highly variable width and amplitude, with parts of the ridge axis displaying very low or apparently reversed magnetization. Gravity requires that OCCs have dense cores capped by lower density zones several kilometers thick. Gravity data indicate longer term patterns of crustal thickness and melt distribution that are broadly consistent with numerical models of OCC formation and show that waxing magmatism may terminate OCCs.

Description: Two-dimensional pre-stack depth migrated seismic reflection data, gravity and velocity models are used to assess the nature and origin of a prominent, buried ridge, the Porcupine Median Volcanic Ridge (PMVR) within the Porcupine Basin, offshore Ireland. The debate on the origin of the PMVR during the past 30 years has followed the evolution of the concept of continental margin genesis. In this paper, the origin of the ridge is evaluated on the basis of the internal geometry and velocity structure, revealed by the seismic data. Implication of the presence of these type of ridges in hyper-extensional rifted margins is discussed and compared with other margins. The analysis indicates that the ridge is an extrusive volcanic ridge, probably tholeiitic in composition, constructed by stacked hyaloclastite deltas and topped by carbonate platforms. The results invalidate previously proposed models involving highly rotated fault blocks and the serpentinite mud volcanism. The extension magnitude analysis suggests a highly stretched setting where limited mantle serpentinization may have occurred, but the architecture and velocity of the PMVR demonstrates that it is made of lower velocity materials than serpentinite. During the opening of the North Atlantic, the PMVR represents the northern time-equivalent magmatic event expressed along the Newfoundland-Iberia-Galicia, recorded by the J anomaly that originate from Cretaceous volcanic deposits.

Description: Hot spot volcanoes are commonly constructed in a characteristic sequence of stages. After the volumetrically dominant shield stage, a protracted period of quiescence ends with a final stage of activity: rejuvenated volcanism. The mechanism responsible for generating rejuvenated volcanism is not generally agreed upon. New data obtained for samples 200 m down-section in a deeply incised canyon on Savai‘i (Samoa) are unusually enriched isotopically and indicate a relatively voluminous rejuvenated stage compared to other intraplate volcanoes. Using a modified flexural model originally proposed for Hawai‘i, we suggest that the location of Samoa near the Tonga Trench terminus causes plate flexure resulting in upward flow of the shallow mantle driving partial melting. In particular, subduction-related plate bending in the Samoan region may cause a larger flexural amplitude than generated by volcanic loading in Hawai‘i. The larger amplitude may explain the larger volume of rejuvenated melt in Samoa, constrained by our new data. Moreover, we argue that Sr-Nd-Pb-Os-He-Ne isotopes in Samoan rejuvenated lavas are all consistent with sampling of a lithospheric component that is characterized by a metasomatic imprint from the Pacific Plate's earlier passage over the Rarotonga hot spot. Furthermore, temperature estimates for the melts suggest a drop in temperature during the predicted shallower melting due to flexural uplift, compared to the conditions during shield volcanism. Thus, flexural bending and metasomatism of the Samoan lithosphere may have generated the voluminous and geochemically distinct Samoan rejuvenated lavas, implying the lithosphere may play an important role during this stage in non- Hawaiian hot spots.

Description: At oceanic margins, syn-convergent exhumation, subduction erosion, and inter-plate coupling are intimately related, but ample questions remain concerning their interaction and individual mechanisms. To analyze these interactions for a thick-skinned, visco-elastic wedge, we focus on properly modeling stresses, energies, and topographies at the inter-plate and wedge bounding interfaces using a Coulomb frictional contact algorithm. In this innovative plane-strain, free surface, Lagrangian finite element model, fault dynamics is modulated by retreating subduction. Subduction is dynamically driven by slab-pull due to a slab sinking in a semi-analytic, computationally favorable approximation of three-dimensional induced mantle flow. Nodal trajectories show that continuous underthrusting of a slab induces a steady state corner flow through forced underplating and subsequent trenchward extrusion due to gravitational spreading. This flow pattern confirms early-proposed models of syn-orogenic deep-seated rock exhumation propelled by coexisting extension and continuous shortening at depth. A distinct reduction in upward flowing material and accompanying decrease of exhumation velocities, to millimeters per year as observed in nature, is induced by a diversion of orogenic wedge material toward the mantle once a subduction channel is formed. The key parameter affecting model evolution and spontaneous formation of a subduction channel is basal friction, which modulates the amount of erosion. However, formation of a subduction channel entrance needs to be ensured through the deformability of the overriding plate, which is influenced by applied pressure at the overriding plate tip and material properties. The down dragging of the overriding plate is sufficient above a threshold inter-plate shear stress of about 2–7 MPa.

Description: Ferrihydrite is an important iron oxyhydroxide for earth and environmental sciences, biology, and technology. Nevertheless, its mineral structure remains a matter of debate. The stumbling block is whether a significant amount of tetrahedrally coordinated iron is present. Here we present the first X-ray magnetic circular dichroïsm (XMCD) measurements performed on a well characterized synthetic sample of 6-line ferrihydrite, at both K and L2,3 energy edges of iron. XMCD results demonstrate unambiguously the presence of tetrahedrally coordinated Fe(III) in the mineral structure, in quantities compatible with the latest extended X-ray absorption fine structure (EXAFS) analyses suggesting a concentration of 20–30%. Moreover, we find an antiferromagnetic coupling between tetrahedral and octahedral sublattices, with the octahedral sublattice parallel to the external magnetic field.

Description: We have analyzed four sediment cores from the Southern Indian Ocean (ODP sites 757, 758, 1135 and 762) with high carbonate content, in order to reconstruct the neodymium isotopic composition (εNd) of ancient intermediate South Indian seawater from Late Cretaceous (90 Ma) to Early Eocene (40 Ma). The εNd variations are highly consistent and exhibit reproducible patterns over a very large geographic area, confirming the seawater origin of the signal. Combining geochemical constraints with paleogeographic reconstructions, we highlight the respective roles of (1) large-scale tectonic events, (2) continental weathering from surrounding Precambrian terrains (90–65 Ma), (3) oceanic circulation changes (50–40 Ma) and, possibly, (4) local volcanism of the ultra-fast spreading South East Indian Ridge (SEIR) (60–50 Ma) on the Nd isotopic composition of South Indian seawater. Between 60 Ma and 50 Ma, the regional Nd isotopic variations closely mimic changes in SEIR spreading rate. We suggest that the Nd isotopic composition of seawater could be influenced by Nd of volcanic origin in the vicinity of ultra-fast spreading ridges (〉13 cm/yr). The India-Asia collision closed the Equatorial Seaway between Asia and India and drastically changed oceanic circulation patterns in the Indian Ocean: warm and more radiogenic Pacific equatorial seawater was diverted to the South by the East Indian coast. A stronger mixing of this Pacific seawater with South Indian seawater would explain the rapid shift of εNd from 50 Ma (−11) to 40 Ma (−8).

Description: We present new 3-D tomographic models of VP, VS and VP/VS ratio anomalies in the upper mantle beneath EC and adjacent areas. This data was collected and interpreted with the goal of clarifying geodynamic processes that caused spatially variable event histories throughout Eastern China (EC) during Mesozoic to Cenozoic time. The tomographic images were constructed by inverting body wave travel-times recorded at ∼1300 stations within the upgraded China National Seismic Network, and 9 temporary arrays. Resolution tests for different depths and the featured velocity anomalies verify that the tomographic images capture the velocity heterogeneities in the upper mantle to depths of 700 km. The salient features of VP, VS and VP/VS ratio anomalies can be clearly identified. These include strong multiscale heterogeneities occupying the upper mantle beneath EC and differences in the spatial scale of anomalies found beneath northern and southern areas of EC. These features demonstrate a degree of spatial variability in the geodynamic evolution of EC. We propose two mechanisms to explain these patterns. First, the western front of the subducted slab may have imparted greater horizontal compressional stress in areas where it impinged further eastward into EC. These areas would experience stronger convection and an altered stress regime in the upper mantle, creating significant thermal anomalies beneath the South China Block (SCB) relative to the eastern North China Craton (NCC). Second, differing thermal states and viscosities for the eastern NCC and the Cathaysia Block (CaB) resulted in differing responses to regional deformation. The Archean hinterland of the eastern NCC specifically has a colder thermal state and higher viscosity, and therefore exhibits only small-scale heterogeneities due to the effect of shear localization. The Neoproterozoic CaB has a relatively warm thermal state with lower viscosity, and thus deformed more continuously.

Description: Geophysical data acquired using R/V Polarstern constrain the structure and age of the rifted oceanic margin of West Antarctica. West of the Antipodes Fracture Zone, the 145 km wide continent–ocean transition zone (COTZ) of the Marie Byrd Land sector resembles a typical magma-poor margin. New gravity and seismic reflection data indicates initial continental crust of thickness 24 km, that was stretched 90 km. Farther east, the Bellingshausen sector is broad and complex with abundant evidence for volcanism, the COTZ is ∼670 km wide, and the nature of crust within the COTZ is uncertain. Margin extension is estimated to be 106–304 km in this sector. Seafloor magnetic anomalies adjacent to Marie Byrd Land near the Pahemo Fracture Zone indicate full-spreading rate during c33–c31 (80–68 Myr) of 60 mm yr−1, increasing to 74 mm yr−1 at c27 (62 Myr), and then dropping to 22 mm yr−1 by c22 (50 Myr). Spreading rates were lower to the west. Extrapolation towards the continental margin indicates initial oceanic crust formation at around c34y (84 Myr). Subsequent motion of the Bellingshausen plate relative to Antarctica (84–62 Myr) took place east of the Antipodes Fracture Zone at rates

Description: Lava flow mapping is both an essential component of volcano monitoring and a valuable tool for investigating lava flow behavior. Although maps are traditionally created through field surveys, remote sensing allows an extraordinary view of active lava flows while avoiding the difficulties of mapping on location. Synthetic aperture radar (SAR) imagery, in particular, can detect changes in a flow field by comparing two images collected at different times with SAR coherence. New lava flows radically alter the scattering properties of the surface, making the radar signal decorrelated in SAR coherence images. We describe a new technique, SAR Coherence Mapping (SCM), to map lava flows automatically from coherence images independent of look angle or satellite path. We use this approach to map lava flow emplacement during the Pu‘u ‘Ō‘ō-Kupaianaha eruption at Kīlauea, Hawai‘i. The resulting flow maps correspond well with field mapping and better resolve the internal structure of surface flows, as well as the locations of active flow paths. However, the SCM technique is only moderately successful at mapping flows that enter vegetation, which is also often decorrelated between successive SAR images. Along with measurements of planform morphology, we are able to show that the length of time a flow stays decorrelated after initial emplacement is linearly related to the flow thickness. Finally, we use interferograms obtained after flow surfaces become correlated to show that persistent decorrelation is caused by post-emplacement flow subsidence.

Description: Beneath mid-ocean ridges, magma is thought to rise through a network of high porosity channels that form by reactive flow. Partial mantle melts travel rapidly through these channels to the surface, and retain the geochemical signature of their source rock. Global analyses of mid-ocean ridge lavas indicates that the mantle is chemically heterogeneous, but the consequences of this heterogeneity for reactive porous flow remain unclear. Using numerical models of coupled magma/mantle dynamics, we investigate the relationships between mantle heterogeneity, melting, and magmatic channelization. The models are based on conservation mass, momentum, energy, and composition in a system with two phases and two thermodynamic components in local thermodynamic equilibrium. One of these components is more fusible than the other. In this context, we find that heterogeneities enriched in the more fusible component can nucleate magmatic channels. To understand this result we consider an expression for the melting rate derived from the conservation principles. This expression quantifies the relationship of decompression, reactive flow, and thermal diffusion to the melting rate. With it, we assess their relative importance in the ambient mantle, channels, and enriched heterogeneities. In our models, heat diffuses into fertile channels and powers melting, in combination with reactive flow. These results suggest that thermal diffusion influences the dynamics of magmatic channelization.

Description: The volcanic succession on Montserrat provides an opportunity to examine the magmatic evolution of island arc volcanism over a ∼2.5 Ma period, extending from the andesites of the Silver Hills center, to the currently active Soufrière Hills volcano (February 2010). Here we present high-precision double-spike Pb isotope data, combined with trace element and Sr-Nd isotope data throughout this period of Montserrat's volcanic evolution. We demonstrate that each volcanic center; South Soufrière Hills, Soufrière Hills, Centre Hills and Silver Hills, can be clearly discriminated using trace element and isotopic parameters. Variations in these parameters suggest there have been systematic and episodic changes in the subduction input. The SSH center, in particular, has a greater slab fluid signature, as indicated by low Ce/Pb, but less sediment addition than the other volcanic centers, which have higher Th/Ce. Pb isotope data from Montserrat fall along two trends, the Silver Hills, Centre Hills and Soufrière Hills lie on a general trend of the Lesser Antilles volcanics, whereas SSH volcanics define a separate trend. The Soufrière Hills and SSH volcanic centers were erupted at approximately the same time, but retain distinctive isotopic signatures, suggesting that the SSH magmas have a different source to the other volcanic centers. We hypothesize that this rapid magmatic source change is controlled by the regional transtensional regime, which allowed the SSH magma to be extracted from a shallower source. The Pb isotopes indicate an interplay between subduction derived components and a MORB-like mantle wedge influenced by a Galapagos plume-like source.

Description: The Hawaii Scientific Drilling Project recovered core from a 3.5 km deep hole from the flank of Mauna Kea volcano, providing a long, essentially continuous record of the volcano's physical and petrologic development that has been used to infer the chemical and physical characteristics of the Hawaiian mantle plume. Determining a precise accumulation rate via 40Ar/39Ar dating of the shield-stage tholeiites, which constitute 95–98% of the volcano's volume is challenging. We applied 40Ar/39Ar dating using laser- and furnace-heating in two laboratories (Berkeley and Curtin) to samples of two lava flows from deep in the core (∼3.3 km). All determinations yield concordant isochron ages, ranging from 612 ± 159 to 871 ± 302 ka (2σ; with P ≥ 0.90). The combined data yield an age of 681 ± 120 ka (P = 0.77) for pillow lavas near the bottom of the core. This new age, when regressed with 40Ar/39Ar isochron ages previously obtained for tholeiites higher in the core, defines a constant accumulation rate of 8.4 ± 2.6 m/ka that can be used to interpolate the ages of the tholeiites in the HSDP core with a mean uncertainty of about ±83 ka. For example at ∼3300 mbsl, the age of 664 ± 83 ka estimated from the regression diverges at the 95% confidence level from the age of 550 ka obtained from the numerical model of DePaolo and Stolper (1996). The new data have implications for the timescale of the growth of Hawaiian volcanoes, the paleomagnetic record in the core, and the dynamics of the Hawaiian mantle plume.

Description: Here we present the first four dimensional (time and three dimensional space resolved) experiment on a strongly deformed geological material. Results show that even complicated microstructures with large continuous and discontinuous changes in crystallographic orientation can be resolved quantitatively. The details that can be resolved are unprecedented and therefore the presented technique promises to become influential in a wide range of geoscientific investigations. Grain and subgrain scale processes are fundamental to mineral deformation and associated Earth Dynamics, and time resolved observation of these processes is vital for establishing an in-depth understanding of the latter. However, until recently, in situ experiments were restricted to observations of two dimensional surfaces. We compared experimental results from two dynamic, in situ annealing experiments on a single halite crystal; a 2D experiment conducted inside the scanning electron microscope and a 3D X-ray diffraction experiment. This allowed us to evaluate the possible effects of the free surface on grain and subgrain processes. The extent to which surface effects cause experimental artifacts in 2D studies has long been questioned. Our study shows that, although the nature of recovery processes are the same, the area swept by subgrain boundaries is up to 5 times larger in the volume than observed on the surface. We suggest this discrepancy is due to enhanced drag force on subgrain boundaries by thermal surface grooving. Our results show that while it is problematic to derive absolute mobilities from 2D experiments, derived relative mobilities between boundaries with different misorientation angles can be used.