ISTerre 2014

Résumé: Previous studies have linked biogeophysical signatures to the presence of iron minerals resulting from distinct biophysicochemical processes. Utilizing geophysical methods as a proxy of such biophysicochemical processes requires an understanding of the geophysical signature of the different iron minerals. Laboratory experiments were conducted to investigate the complex conductivity and magnetic susceptibility signatures of five iron minerals disseminated in saturated porous media under variable iron mineral content and grain size. Both pyrite and magnetite show high quadrature and inphase conductivities compared to hematite, goethite, and siderite, whereas magnetite was the highly magnetic mineral dominating the magnetic susceptibility measurements. The quadrature conductivity spectra of both pyrite and magnetite exhibit a well-defined characteristic relaxation peak below 10 kHz, not observed with the other iron minerals. The quadrature conductivity and magnetic susceptibility of individual and a mixture of iron minerals are dominated and linearly proportional to the mass fraction of the highly conductive (pyrite and magnetite) and magnetic (magnetite) iron minerals, respectively. The quadrature conductivity magnitude increased with decreasing grain size diameter of magnetite and pyrite with a progressive shift of the characteristic relaxation peak toward higher frequencies. The quadrature conductivity response of a mixture of different grain sizes of iron minerals is shown to be additive, whereas magnetic susceptibility measurements were insensitive to the variation in grain size diameters (1-0.075 mm). The integration of complex conductivity and magnetic susceptibility measurements can therefore provide a complimentary tool for the successful investigation of in situ biophysicochemical processes resulting in biotransformation or secondary iron mineral precipitation.

Résumé: Terrestrial cosmogenic nuclide concentrations in sediment are used to quantify mean denudation rates in catchments. This article explores the differences between the Be-10 concentration in fine (sand) and in coarse (1-3 or 5-10 cm pebbles) river sediment. Sand and pebbles were sampled at four locations in the Huasco Valley, in the arid Chilean Andes. Sand has Be-10 concentrations between 4.8 and 8.3.10(5) at g(-1), while pebbles have smaller concentrations between 2.2 and 3.3.10(5) at g(-1). It appears that the different concentrations, systematically measured between sand and pebbles, are the result of different denudation rates, linked with the geomorphologic processes that originated them. We propose that the Be-10 concentrations in sand are determined by the mean denudation rate of all of the geomorphologic processes taking place in the catchment, including debris flow processes as well as slower processes such as hill slope diffusion. In contrast, the concentrations in pebbles are probably related to debris flows occurring in steep slopes. The mean denudation rates calculated in the catchment are between 30 and 50 m/Myr, while the denudation rates associated with debris flow are between 59 and 81 m/Myr. These denudation rates are consistent with those calculated using different methods, such as measuring eroded volumes. (C) 2013 Elsevier B.V. All rights reserved.

Résumé: We use wavefield decomposition methods (time domain cross correlation and frequency domain multiple-signal classification) to analyze seismic data recorded by a dense, small-aperture array located 2 km West of Arenal volcano, Costa Rica, and operated during 2.5 days. The recorded wavefield is dominated by harmonic tremor and includes also spasmodic tremor and long-period (LP) events. We find that the initial stages of LP events are characterized by three different wave arrivals. These arrivals propagate with similar back azimuths pointing to the volcano summit (similar to 80 degrees N) and increasing apparent slowness of 0.4, 1.1, and 1.7 s/km. Spasmodic tremors cannot be regarded as coherent signals. On the contrary, harmonic tremors are highly coherent, characterized by the stability of the apparent slowness vector estimates. Apparent slowness lays in the range 1-2 s/km. Back azimuths point in the general direction of the volcano but with a large variability (40-120 degrees N). Nevertheless, there are long-term variations and evidences of multiple simultaneous components in the harmonic tremor wavefield. These observations suggest that LP events and tremor are generated in a shallow source area near the volcano summit, although they do not share exactly the same source region or source processes. The tremor source is located in the shallowest part of the plumbing system, beneath the lava crust. This dynamic region is subject to complex fluctuations of the physical conditions. Degassing events at different locations of this region might generate variable seismic radiation patterns. The effects of topography and heterogeneous shallow structure of the volcano may amplify these variations and produce the wide directional span observed for volcanic tremor. On the other hand, the LP source seems to be more repeatable. LP events are likely triggered by fragmentation of the fluid flow in a slightly deeper portion of the volcanic conduits.

Résumé: A picture of the upper crustal structure of the Irpinia active faults system in southern Italy was obtained by combining new geological evidences, lithological properties, and microseismicity distribution. P and S wave velocity models indicate high V-P/V-S and low V(P)xV(S) values, suggesting fluid accumulation within a similar to 15 kmwide rock volume where intensemicroseismicity is located. The 1980 Irpinia, M-s 6.9, earthquake nucleated within the same fault-bounded volume. We suggest that concentration of background seismicity is mainly controlled by high pore fluid pressure. Its increase in fluid-filled cracks around major faults leads to earthquakes' nucleation. Seismic pumping along major faults carries fluids through the conduit system represented by the intensely fractured damage zone. Conversely, the cross-fault barrier behavior of the low-permeability fault core leads to pore fluid pressures building up within the fault-bounded block, thus producing a positive feedback triggering earthquake nucleation within the volume, which behaves as an “earthquake reservoir.”

Résumé: Disimilatory iron-reducing bacteria play an important role in the reduction of Fe(hydr)oxides and the production of secondary solid-iron mineral phases that can have magnetic properties. Magnetic susceptibility can therefore play an important role in identifying zones where microbial-mediated iron reduction is occurring. We investigated the magnetic susceptibility variations in a hydrocarbon-contaminated aquifer where methanogenesis and iron reduction are the main biogeochemical processes. Our objectives are to (1) determine the variability of magnetic susceptibility, (2) determine the hydrobiogeochemical controls on the magnetic susceptibility variability, and (3) evaluate the use of magnetic susceptibility as a viable technique for identifying zones where the coupling of iron and organic carbon cycling is occurring. Magnetic susceptibility data were acquired down 11 boreholes within contaminated and uncontaminated locations. We show that magnetic susceptibility values for boreholes within the free phase plume are higher than values for boreholes within the dissolved phase plume and background. Magnetic susceptibility values are highest within the zone of water table fluctuation with peaks predominantly occurring at the highest water table marks and are also coincident with high concentrations of dissolved Fe(II) and organic carbon content, suggesting that the zone of water table fluctuation is most biologically active. High magnetic susceptibility values within the vadose zone above the free phase plume are coincident with a zone of methane depletion suggesting aerobic or anaerobic oxidation of methane coupled to iron reduction. Our results suggest that magnetic susceptibility can be used as a viable tool in iron and carbon cycling studies. Key Points <list list-type=“bulleted” id=“jgrg20157-list-0001”> <list-item id=“jgrg20157-li-0001”>Microbial mediated iron-reduction can result in the precipitation of magnetite <list-item id=“jgrg20157-li-0002”>Higher magnetic susceptibility occurs within zones of iron-reduction <list-item id=“jgrg20157-li-0003”>Magnetic susceptibility is a viable tool in iron and carbon cycling studies

Résumé: The Beni Bousera peridotite massif and its metamorphic surrounding rocks have been analyzed by the fission track (FT) method. The aim was to determine the cooling and uplift history of these mantle and associated crustal rocks after the last major metamorphic event that dates back to the Lower Miocene-Upper Oligocene time (similar to 22-24 Ma). The zircon FT analyses give an average cooling-i.e., below 320 A degrees C-age of similar to 19.5 Ma. In addition, the apatite FT data give an average cooling-i.e., below 110 A degrees C-age of similar to 15.5 Ma. Taking into account the thermal properties of the different thermochronological systems used in this work, we have estimated a rate of cooling close to 50 A degrees C/Ma. This cooling rate constrains a denudation rate of about similar to 2 mm year(-1) from 20 to 15 Ma. These results are similar to those determined in the Ronda peridotite massif of the Betic Cordilleras documenting that some ultrabasic massifs of the internal zones of the two segments of the Gibraltar Arc have a similar evolution. However, Burdigalian sediments occur along the Betic segment (Alozaina area, western Betic segment) unconformably overlying peridotite. At this site, ultramafic rock was exposed to weathering at ages ranging from 20.43 to 15.97 Ma. Since the Beni Bousera peridotite was still at depth until 15.5 Ma, we infer that no simple age projection from massif to massif is possible along the Gibraltar Arc. Moreover, the confined fission track lengths data reveal that a light warming (similar to 100 A degrees C) has reheated the massif during the Late Miocene before the Pliocene-Quaternary tectonic uplift.

Résumé: In this study we investigate the relationship between the dips of seismic reflectors, which are used to define sequence boundaries, and the orientation (dip and dip direction) of bedding surfaces at core scales. Sequence boundaries from seismic data and lithostratigraphic boundaries from cores and logs are compared with the bedding orientations measured on borehole images of Miocene siliciclastic sediments at Integrated Ocean Drilling Program Expedition 313 Site M28. It is not surprising that bedding orientations show huge variations at scales that are too small to be detected on seismic profiles. However, changes of orientation defined as rotation between two successive intervals match the depths of approximately half of the seismic sequence boundaries. While they do not match boundaries between lithostratigraphic units, changes of orientation frequently correlate with maxima and minima in the gamma ray signal, suggesting that they are related to changes in depositional processes rather than to changes in lithology. This study suggests for the first time that bedding attitudes can be used as a stratigraphic tool at various scales from bed to bed across depth intervals of tens of meters.

Résumé: In this work, infrared transmission spectra (2-25 μm range, 5000-400 cm(-1)) of 40 carbonaceous chondrites were analyzed (21 CMs, 5 CVs, 6 CRs, 3 CKs, 3 C2s and 2 Cis). All these meteorite groups are known to have experienced significant aqueous alteration (except the CKs). These IR measurements provide information about the parent body processes experienced, as well as spectra for comparison with observations of Solar System small bodies and possibly with astronomical observations of accretion and debris disks. This study reveals that each meteorite group appears to have specific signatures in the measured IR spectral range. In the case of the Cl and CM groups, results show a variability in the shape of the silicate features that can be related to the evolution of the mineralogy with increasing extent of aqueous alteration extent as described by several authors with other techniques. This evolution of the silicate feature can be seen in the variation in the relative intensities of olivine and phyllosilicate IR features. The variability in the silicate features is correlated with the intensity of an -OH related absorption at 3-p,m, which can be used for the classification of the meteorites according to the level of hydration. Interestingly, in the case of CM chondrites, for which the mineralogy is expected to be dominated by phyllosilicates (serpentine mostly), the shape of the silicate absorption resembles that of an amorphous silicate, with a broad and symmetric 10(-1) μm band, unlike terrestrial phyllosilicates. The CV and CI( groups have IR spectra that are dominated by olivine absorption. From this feature, it is possible to determine average Mg numbers for the olivine. For the CVs, the olivine Mg numbers appear to decrease in the order Kaba-Grosnaja-Vigarano-Mokoia-Allende. This trend is likely related to the long duration of metamorphism experienced by these samples and the chemical re-equilibration between chondritic components. In the case of CK chondrites, the inferred bulk Mg# of olivine is 67 ( 1), and no variation is observed between the three studied samples, which is likely related to their high degree of equilibration. The 6 CR chondrites show the most variability in their IR spectra, from CM-like spectra in the case of the CR1 Grosvenor Mountains (GRO) 95577 to CV-like spectra for Roberts Massif (RBT) 04133 and Graves Nunataks (GRA) 06100 (one of them being most probably misclassified). Spectra of the remaining CRs show mixtures of various silicate component. Finally, these spectra can be used for comparison with emission spectra of fine-grained asteroid surfaces and dust-rich cometary tails. In the case of Tempel 1, the only group of CC that matches the observed feature around 10-pm region is the CR group. The spectral comparison shows some striking similarities between CRs and Tempel 1 dust. A genetic link between CR2 and comets is not proven, but mineralogical similarities are suggested from the IR spectra. (C) 2013 Elsevier Inc. All rights reserved.

Résumé: In this study, rare earth element (REE) distribution has been investigated in a weathering profile from central Madagascar. Combination of bulk rock geochemical data (elements and isotopes) with mineral characterization reveals a remarkable evolution of the REE abundances and REE-minerals in the vertical weathering profile. In the fresh tonalite (bedrock), REE + Y concentrations are typical of granitoids (299-363 ppm) and the main REE-minerals are allanite and chevkinite. In the C-horizon (saprolite), primary REE-minerals disappear and REEs are transported via fluid to precipitate rhabdophane group minerals in cracks and pores. The presence of sulfate ligands, produced by sulfide oxidation, may be responsible for the REE speciation, as suggested by the composition of the secondary REE-minerals. Rhabdophane group minerals contain up to 9 wt% SO3 and 7 wt% CaO, indicating a mixture between rhabdophane sensu stricto, (REE) PO4 center dot H2O, and tristamite, (Ca,U,Fe-(III))(PO4,SO4)center dot 2H(2)O. Due to intense Ca-leaching, rhabdophane disappears and Al-phosphates (alunite-jarosite group) are found in the soil. Cerianite ((CeO2)-O-(IV)) also precipitates in the B-horizon of the soil. Mass transfer calculations based on immobile Ti indicate significant REE leaching in A-horizon with preferential leaching of the heavy REE. REEs accumulate partly in the B-horizon. The uniform Nd isotope compositions and the constant proportion of immobile elements do not reveal external input. In the B-horizon, total REE + Y reach 2194 ppm with high Ce concentrations (1638 ppm; 9*Ce-bedrock) compared to other REE (3-4*REEbedrock). Tetravalent Ce state is dominant in the B-horizon and requires oxidizing conditions that likely account for the accumulation of redox-sensitive elements in B-horizon (e.g., Mn, Fe, Co). Under oxidizing conditions, cerianite precipitation causes a Ce fractionation from other trivalent REE. In comparison to the ion adsorption clay of southern China, preferential heavy REE enrichment was not observed in the weathering profile. Another remarkable peculiarity of the studied profile is the occurrence of Gd2SO6 grains. The discovery of this new mineral demonstrates that a natural process exists that that can fractionate REE to such an extent to produce a pure gadolinium end-member mineral. An understanding of such a mechanisms is crucial for the REE geochemistry of low temperature alteration processes as well as for the formation of REE ore deposits or industrial processing. (C) 2013 Elsevier Ltd. All rights reserved.

Résumé: The eastern limestone cliff of Mount Neron (French Alps) was the theater for two medium-size rockfalls between summer and winter 2011. On 14 August 2011, a similar to 2000 m(3) rock compartment detached from the cliff, fell 100 m below and propagated down the slope. Although most of the fallen rocks deposited on the upper part of the slope, some blocks of about 15 m in size were stopped by a ditch and an earthen barrier after a run-out of 800 m. An unstable overhanging similar to 2600 m(3) compartment remained attached to the cliff and was blasted on 13 December 2011. During this artificially triggered event, 7 blocks reached the same ditch, with volumes ranging from 0.8 to 12 m(3). A semi-permanent seismic array located about 2.5 km from the site recorded the two events, providing a unique opportunity to understand and to compare the seismic phases generated during natural and artificially triggered rockfalls. Both events have signal duration of similar to 100 s with comparable maximum amplitudes recorded at large distances (computed local magnitude of 1.14 and 1.05, respectively), most of the energy lying below 20 Hz. Remote sensing techniques (photogrammetry and lidar) were employed before and after the provoked rockfall, allowing the volume and fracturing to be characterized. This event was filmed by two video cameras, and the generated ground motions were recorded using two temporary 3C seismic sensors and three seismic arrays deployed at the slope toe. Videos and seismogram processing provided estimates of the propagation velocity during the successive rockfall phases, which ranges from 12 to 30 m s(-1). The main seismic phases were obtained from combined video and seismic signal analyses. The two most energetic phases are related to the ground impact of fallen material after free fall, and to individual rock block impacts into the ditch and the earthen barrier. These two phases are characterized by similar low-frequency content but show very different particle motions. The discrete element technique allowed reproducing the key features of the rockfall dynamics, yielding propagation velocities compatible with experimental observations.

Résumé: Continuous recordings of ambient seismic noise across large seismic arrays allows a new type of processing using the cross-correlation technique on broadband data. We propose to apply double beamforming (DBF) to cross correlations to extract a particular wave component of the reconstructed signals. We focus here on the extraction of the surface waves to measure phase velocity variations with great accuracy. DBF acts as a spatial filter between two distant subarrays after cross correlation of the wavefield between each single receiver pair. During the DBF process, horizontal slowness and azimuth are used to select the wavefront on both subarray sides. DBF increases the signal-to-noise ratio, which improves the extraction of the dispersive wave packets. This combination of cross correlation and DBF is used on the Transportable Array (USArray), for the central U.S. region. A standard model of surface wave propagation is constructed from a combination of the DBF and cross correlations at different offsets and for different frequency bands. The perturbation (phase shift) between each beam and the standard model is inverted. High-resolution maps of the phase velocity of Rayleigh and Love waves are then constructed. Finally, the addition of azimuthal information provided by DBF is discussed, to construct curved rays that replace the classical great-circle path assumption. Key Points <list list-type=“bulleted”> <list-item id=“jgrb50487-li-0001”>Extraction of surface wave from cross correlation using double beamforming <list-item id=“jgrb50487-li-0002”>Phase velocity measurement and inversion <list-item id=“jgrb50487-li-0003”>Measurement and addition of azimuthal information in inversion

Résumé: To investigate the crystallization of pyroxene in spinifex-textured komatiites, we undertook a series of experiments in which compositions in the CaO-MgO-Al2O3-SiO2 CMAS system were cooled rapidly in a thermal gradient. Cooling rates were generally between 5 and 10 degrees C h(-1), but some runs were made at 100-200 degrees C h(-1); thermal gradients were between 10 and 20 degrees C cm(-1). These conditions reproduced those at various depths in the crust of komatiite lava flow. The starting composition was chosen to have pigeonite on the liquidus, and most of the experimental charges crystallized zoned pigeonite-diopside crystals like those in komatiite lavas. An intriguing aspect of the experimental results was their lack of reproducibility. Some experiments crystallized forsterite, whereas others that were run under similar conditions crystallized two pyroxenes and no forsterite; some experiments were totally glassy, but others crystallized entirely to pyroxene. The degree of supercooling at the onset of pyroxene crystallization was variable, from less than 25 degrees C to more than 110 degrees C. We attribute these results to the difficulty of nucleation of pyroxene under the conditions of the experiments. In some cases forsterite crystallized metastably and modified the liquid composition to inhibit pyroxene crystallization; in others no nucleation took place until a large degree of supercooling was achieved, and then pyroxene crystallized rapidly. Pigeonite crystallized under a wide range of conditions, at cooling rates from 3 to 100 degrees C h(-1). The notion that this mineral only forms at low cooling rates is not correct.

Résumé: Earth's surface topography is generated by tectonically induced variations in crustal thickness combined with erosion and, to a lesser degree, by vertical stresses caused by convection in the underlying mantle. Rock hardness and resistance to erosion are also commonly thought to influence topography because hard rocks, such as granites and basalts, usually form topographic highs in the landscape. Here we use analytical and numerical models to simulate the erosion-induced isostatic rebound of rocks. We find that the isostatic rebound that accompanies erosion causes denser rocks to occupy higher elevations in the landscape, thereby creating topographic relief that is proportional to surface rock density differences rather than rock hardness. We quantify this effect, taking into account the flexural strength of the continental lithosphere. We show that in a steady-state erosional setting, density-dependent isostatic rebound can cause the densest rocks to be exhumed at double the rate of surrounding, less-dense rocks and has a stronger effect than typical rock hardness variations. The results imply that denser rock formations should erode faster and therefore be characterized by younger thermochronological ages. Thermochronological data sets from the Kinabalu granite in Borneo and the Shakhdara-Alichur gneiss domes in Pamir confirm this counter-intuitive result. Our findings imply that lateral variations in surface rock density have significant control on the shaping of the large-scale features of Earth's surface.

Résumé: Here, we report analyses by synchrotron X-ray fluorescence microscopy of the elemental composition of eight candidate impact features extracted from the Stardust Interstellar Dust Collector (SIDC). Six of the features were unambiguous tracks, and two were crater-like features. Five of the tracks are so-called “midnight” tracks-that is, they had trajectories consistent with an origin either in the interstellar dust stream or as secondaries from impacts on the Sample Return Capsule (SRC). In a companion paper reporting synchrotron X-ray diffraction analyses of ISPE candidates, we show that two of these particles contain natural crystalline materials: the terminal particle of track 30 contains olivine and spinel, and the terminal particle of track 34 contains olivine. Here, we show that the terminal particle of track 30, Orion, shows elemental abundances, normalized to Fe, that are close to CI values, and a complex, fine-grained structure. The terminal particle of track 34, Hylabrook, shows abundances that deviate strongly from CI, but shows little fine structure and is nearly homogenous. The terminal particles of other midnight tracks, 29 and 37, had heavy element abundances below detection threshold. A third, track 28, showed a composition inconsistent with an extraterrestrial origin, but also inconsistent with known spacecraft materials. A sixth track, with a trajectory consistent with secondary ejecta from an impact on one of the spacecraft solar panels, contains abundant Ce and Zn. This is consistent with the known composition of the glass covering the solar panel. Neither crater-like feature is likely to be associated with extraterrestrial materials. We also analyzed blank aerogel samples to characterize background and variability between aerogel tiles. We found significant differences in contamination levels and compositions, emphasizing the need for local background subtraction for accurate quantification.

Résumé: Imaging tectonic faults with small vertical offsets in argillites (clay rock) using geophysical methods is challenging. In the context of deep radioactive waste disposals, the presence of such faults has to be assessed because they can modify the rock-confining properties. In the Tournemire Experimental Platform (TEP, Aveyron, France), fault zones with small vertical offsets and complex shape have been identified from underground works. However, 3D high-resolution surface seismic methods have limitations in this context that led us to consider the detection and characterization of the faults directly from underground works. We investigated the potential of seismic fullwave-form inversion (FWI) applied in a transmission configuration to image the clay rock medium in a horizontal plane between galleries and compared it with first-arrival traveltime tomography (FATT). Our objective was to characterize seismic velocities of a block of argillites crossed by a subvertical fault zone with a small vertical offset. The specific measurement configuration allowed us to neglect the influence of the galleries on the wave propagation and to simplify the problem by considering a 2D isotropic horizontal imaging domain. Our FWI scheme relied on a robust adaptation of early arrival waveform tomography. The results obtained with FATT and FWI were in accordance, and both correlated with the geologic observations from the gallery walls and boreholes. We found that even though various simplifications was done in the inversion scheme and only a part of the data was used, FWI allowed us to get higher resolution images than FATT, and it was especially less sensitive to the incomplete illumination because it also used diffracted energy. Our results highlighted the complexity of the fault zone, showing a complex interaction of the main fault system with a secondary system composed of decimetric fractures associated with the presence of water.

Résumé: We report the quantitative characterization by synchrotron soft X-ray spectroscopy of 31 potential impact features in the aerogel capture medium of the Stardust Interstellar Dust Collector. Samples were analyzed in aerogel by acquiring high spatial resolution maps and high energy-resolution spectra of major rock-forming elements Mg, Al, Si, Fe, and others. We developed diagnostic screening tests to reject spacecraft secondary ejecta and terrestrial contaminants from further consideration as interstellar dust candidates. The results support an extraterrestrial origin for three interstellar candidates: I1043,1,30 (Orion) is a 3 pg particle with Mg-spinel, forsterite, and an iron-bearing phase. I1047,1,34 (Hylabrook) is a 4 pg particle comprising an olivine core surrounded by low-density, amorphous Mg-silicate and amorphous Fe, Cr, and Mn phases. I1003,1,40 (Sorok) has the track morphology of a high-speed impact, but contains no detectable residue that is convincingly distinguishable from the background aerogel. Twenty-two samples with an anthropogenic origin were rejected, including four secondary ejecta from impacts on the Stardust spacecraft aft solar panels, nine ejecta from secondary impacts on the Stardust Sample Return Capsule, and nine contaminants lacking evidence of an impact. Other samples in the collection included I1029,1,6, which contained surviving solar system impactor material. Four samples remained ambiguous: I1006,2,18, I1044,2,32, and I1092,2,38 were too dense for analysis, and we did not detect an intact projectile in I1044,3,33. We detected no radiation effects from the synchrotron soft X-ray analyses; however, we recorded the effects of synchrotron hard X-ray radiation on I1043,1,30 and I1047,1,34.

Résumé: We present a reconstruction of the mean axisymmetric azimuthal and meridional flows in the Derviche Tourneur Sodium installation in Grenoble liquid sodium experiment. The experimental device sets a spherical Couette flow enclosed between two concentric spherical shells where the inner sphere holds a strong dipolar magnet, which acts as a magnetic propeller when rotated. Measurements of the mean velocity, mean induced magnetic field, and mean electric potentials have been acquired inside and outside the fluid for an inner sphere rotation rate of 9 Hz (Rm similar or equal to 28). Using the induction equation to relate all measured quantities to the mean flow, we develop a nonlinear least-squares inversion procedure to reconstruct a fully coherent solution of the mean velocity field. We also include in our inversion the response of the fluid layer to the nonaxisymmetric time-dependent magnetic field that results from deviations of the imposed magnetic field from an axial dipole. The mean azimuthal velocity field we obtain shows superrotation in an inner region close to the inner sphere where the Lorentz force dominates, which contrasts with an outer geostrophic region governed by the Coriolis force, but where the magnetic torque remains the driver. The meridional circulation is strongly hindered by the presence of both the Lorentz and the Coriolis forces. Nevertheless, it contributes to a significant part of the induced magnetic energy. Our approach sets the scene for evaluating the contribution of velocity and magnetic fluctuations to the mean magnetic field, a key question for dynamo mechanisms.

Résumé: The neoformation of chlorite and K-white mica in fault rocks from two main faults of the central Catalan Coastal Ranges, the VallSs and the Hospital faults, has allowed us to constrain the P-T conditions during fault evolution using thermodynamic modeling. Crystallization of M1 and M2 muscovite and microcline occured as result of deuteric alteration during the exhumation of the pluton (290 A degrees C > T > 370 A degrees C) in the Permian. After that, three tectonic events have been distinguished. The first tectonic event, attributed to the Mesozoic rifting, is characterized by precipitation of M3 and M4 phengite together with chlorite and calcite C1 at temperatures between 190 and 310 A degrees C. The second tectonic event attributed to the Paleogene compression has only been identified in the Hospital fault with precipitation of low-temperature calcite C2. The shortcut produced during inversion of the VallSs fault was probably the responsible for the lack of neoformed minerals within this fault. Finally, the third tectonic event, which is related to the Neogene extension, is characterized in the VallSs fault by a new generation of chlorite, associated with calcite C4 and laumontite, formed at temperatures between 125 and 190 A degrees C in the absence of K-white mica. Differently, the Hospital fault is characterized by the precipitation of calcite C3 during the syn-rift stage at temperatures around 150 A degrees C and by low-temperature fluids precipitating calcites C5, C6 and PC1 during the post-rift stage. During the two extensional events (Mesozoic and Neogene), faults acted as conduits for hot fluids producing anomalous high geothermal gradients (50 A degrees C/km minimum).

Résumé: Landslides accommodate slow, aseismic slip and fast, seismic rupture, which are sensitive to fluid pressures and rock frictional properties. The study of strain partitioning in the Sechilienne landslide (France) provides a unique insight into this sensitivity. Here we show with hydromechanical modeling that a significant part of the observed landslide motions and associated seismicity may be caused by poroelastic strain below the landslide, induced by groundwater table variations. In the unstable volume near the surface, calculated strain and rupture may be controlled by stress transfer and friction weakening above the phreatic zone and reproduce well high-motion zone characteristics measured by geodesy and geophysics. The key model parameters are friction weakening and the position of groundwater level, which is sufficiently constrained by field data to support the physical validity of the model. These results are of importance for the understanding of surface strain evolution under weak forcing. Key Points <list list-type=“bulleted” id=“grl51228-list-0001”> <list-item id=“grl51228-li-0001”>Deep fluid pressures can activate stable or slowly moving giant landslides <list-item id=“grl51228-li-0002”>No significant stressing is needed to explain seismic and geodetic data <list-item id=“grl51228-li-0003”>Implications for the partitioning of seismic and aseismic sliding

Résumé: Super-eruptions are extremely rare events. Indeed, the global frequency of explosive volcanic eruptions is inversely proportional to the volume of magma released in a single event(1,2). The rate of magma supply, mechanical properties of the crust and magma, and tectonic regime are known to play a role in controlling eruption frequency and magnitude(3-7), but their relative contributions have not been quantified. Here we use a thermomechanical numerical model of magma injection into Earth's crust and Monte Carlo simulations to explore the factors controlling the recurrence rates of eruptions of different magnitudes. We find that the rate of magma supply to the upper crust controls the volume of a single eruption. The time interval between magma injections into the subvolcanic reservoir, at a constant magma-supply rate, determines the duration of the magmatic activity that precedes eruptions. Our simulations reproduce the observed relationship between eruption volume and magma chamber residence times and replicate the observed correlation between erupted volumes and caldera dimensions(8,9). We also find that magma buoyancy is key to triggering super-eruptions, whereas pressurization associated with magma injection is responsible for relatively small and frequent eruptions. Our findings help improve our ability to decipher the long-term activity patterns of volcanic systems.

Résumé: Cross-correlation and cross-spectral time delays often exhibit strong outliers due to ambiguities or cycle jumps in the correlation function. Their number increases when signal-to-noise, signal similarity or spectral bandwidth decreases. Such outliers heavily determine the time-delay probability density function and the results of further computations (e.g. double-difference location and tomography) using these time delays. In the present research we expressed cross-correlation as a function of the squared difference between signal amplitudes and show that they are closely related. We used this difference as a cost function whose minimum is reached when signals are aligned. Ambiguities may be removed in this function by using a priori information. We propose using the traveltime difference as a priori time-delay information. By modelling the probability density function of the traveltime difference by a Cauchy distribution and the probability density function of the data (differences of seismic signal amplitudes) by a Laplace distribution we were able to find explicitly the time-delay a posteriori probability density function. The location of the maximum of this a posteriori probability density function is the maximum a posteriori time-delay estimation for earthquake signals. Using this estimation to calculate time delays for earthquakes on the south flank of Kilauea statistically improved the cross-correlation time-delay estimation for these data and resulted in successful double-difference relocation for an increased number of earthquakes. This robust time-delay estimation improves the spatiotemporal resolution of seismicity rates in the south flank of Kilauea.

Résumé: We analyse the scaling and distribution of average dynamic source properties (fracture energy, static, dynamic and apparent stress drops) using 31 kinematic inversion models from 21 crustal earthquakes. Shear-stress histories are computed by solving the elastodynamic equations while imposing the slip velocity of a kinematic source model as a boundary condition on the fault plane. This is achieved using a 3-D finite difference method in which the rupture kinematics are modelled with the staggered-grid-split-node fault representation method of Dalguer & Day. Dynamic parameters are then estimated from the calculated stress-slip curves and averaged over the fault plane. Our results indicate that fracture energy, static, dynamic and apparent stress drops tend to increase with magnitude. The epistemic uncertainty due to uncertainties in kinematic inversions remains small (phi similar to 0.1 in log(10) units), showing that kinematic source models provide robust information to analyse the distribution of average dynamic source parameters. The proposed scaling relations may be useful to constrain friction law parameters in spontaneous dynamic rupture calculations for earthquake source studies, and physics-based near-source ground-motion prediction for seismic hazard and risk mitigation.

Résumé: Large-scale planetary or stellar magnetic fields generated by a dynamo effect are mostly attributed to flows forced by buoyancy forces in electrically conducting fluid layers. However, these large-scale fields may also be controlled by tides, as previously suggested for the star tau-boo, Mars, or the early Moon. By simulating a small local patch of a rotating fluid, Barker & Lithwick have recently shown that tides can drive small-scale dynamos by exciting a hydrodynamic instability, the so-called elliptical (or tidal) instability. By performing global magnetohydrodynamic simulations of a rotating spherical fluid body, we investigate if this instability can also drive the observed large-scale magnetic fields. We are thus interested in the dynamo threshold and the generated magnetic field in order to test if such a mechanism is relevant for planets and stars. Rather than solving the problem in a geometry deformed by tides, we consider a spherical fluid body and add a body force to mimic the tidal deformation in the bulk of the fluid. This allows us to use an efficient spectral code to solve the magnetohydrodynamic problem. We first compare the hydrodynamic results with theoretical asymptotic results and numerical results obtained in a truly deformed ellipsoid, which confirms the presence of elliptical instability. We then perform magnetohydrodynamic simulations and investigate the dynamo capability of the flow. Kinematic and self-consistent dynamos are finally simulated, showing that the elliptical instability is capable of generating a dipole-dominated large-scale magnetic field in global simulations of a fluid rotating sphere.

Résumé: The Tungurahua volcano, in Ecuador, has been experiencing a substantial activity period since 1999, with several eruptions, including those of 2006 and 2008. We use a persistent scatterers approach to analyze a time series of Envisat synthetic aperture radar (SAR) data over the period 2003-2009, to investigate surface deformation in the region of the volcano. We measure a continuous large-scale uplift with a maximum line of sight displacement rate of about 8 mm/yr, which is the first evidence of a sustained inflation in the Andes for an active volcano encompassing several eruptions. We model this signal as magma emplacement in a permanent storage zone at 11.5 km below sea level, with a net inflow rate of 7 million m(3)/yr. The paroxysmal eruptions in 2006 and 2008 did not seem to disrupt this long-term signal. However, we observe significant deformation during the 2006 eruption consistent with an additional intrusion of 4.5 million m(3) of magma.

Résumé: P-to-S receiver functions (PRFs) from the Polar Earth Observing Network (POLENET) GPS and seismic leg of POLENET spanning West Antarctica and the Transantarctic Mountains deployment of seismographic stations provide new estimates of crustal thickness across West Antarctica, including the West Antarctic Rift System (WARS), Marie Byrd Land (MBL) dome, and the Transantarctic Mountains (TAM) margin. We show that complications arising from ice sheet multiples can be effectively managed and further information concerning low-velocity subglacial sediment thickness may be determined, via top-down utilization of synthetic receiver function models. We combine shallow structure constraints with the response of deeper layers using a regularized Markov chain Monte Carlo methodology to constrain bulk crustal properties. Crustal thickness estimates range from 17.04 km at Fishtail Point in the western WARS to 455 km at Lonewolf Nunataks in the TAM. Symmetric regions of crustal thinning observed in a transect deployment across the West Antarctic Ice Sheet correlate with deep subice basins, consistent with pure shear crustal necking under past localized extension. Subglacial sediment deposit thicknesses generally correlate with trough/dome expectations, with the thickest inferred subice low-velocity sediment estimated as approximate to 0.4 km within the Bentley Subglacial Trench. Inverted PRFs from this study and other published crustal estimates are combined with ambient noise surface wave constraints to generate a crustal thickness map for West Antarctica south of 75 degrees S. Observations are consistent with isostatic crustal compensation across the central WARS but indicate significant mantle compensation across the TAM, Ellsworth Block, MBL dome, and eastern and western sectors of thinnest WARS crust, consistent with low density and likely dynamic, low-viscosity high-temperature mantle. Key Points <list list-type=“bulleted”> <list-item id=“jgrb50441-li-0001”>Crustal structure of West Antarctica <list-item id=“jgrb50441-li-0002”>Receiver functions over ice sheets <list-item id=“jgrb50441-li-0003”>MCMC inversion coupled with surface waves

Résumé: Knowledge of the average composition of the upper continental crust is crucial to establish not only how it formed but also when. While well constrained averages have been suggested for its major and trace element composition, no values exist for its Nd and Hf isotopic compositions even though radiogenic isotopic systems provide valuable information on its average model age. Here we present Nd and Hf isotopic data determined on a large number of loess deposits from several continents. We demonstrate that these deposits have very uniform Nd and Hf isotopic compositions. We obtain an average Nd isotopic composition that is similar to previous estimates for the upper continental crust (epsilon(Nd) = -10.3 +/- 1.2 (1 sigma)) and we establish a new Hf average value at epsilon(Hf) = -13.2 +/- 2 (1 sigma). This average falls on the “Terrestrial Array”, demonstrating that the two parent-daughter ratios are not decoupled during crust formation. Trace element data acquired on the same set of samples allow us to calculate an average Sm-147/Nd-144 ratio for the upper continental crust: 0.1193 +/- 0.0026, a value slightly higher than previous estimates. Based on the relationship between Sm/Nd and Nd isotopes, we estimate the average Nd extraction age of upper continental crust from the depleted mantle at T-DM(Nd) = 1.82 +/- 0.07 Ga. This model age is entirely consistent with previous suggestions made for example by Goldstein et al. (1984). Assuming that for each individual sample, the Hf model age cannot be younger than the Nd model age, our new Nd-Hf isotopic data provide a value for the very poorly known Lu-176/Hf-177 ratio of the upper crust. Our estimate is Lu-176/Hf-177 = 0.0125 +/- 0.0018, a value significantly lower than commonly used values (0.0150-0.0159; Griffin et al., 2002; Goodge and Vervoort, 2006; Hawkesworth et al., 2010) but higher than Rudnick and Gao's (2003) estimate of 0.0083. The impact of our new Lu-176/Hf-177 ratio on crustal model ages of zircon populations is not simple to evaluate but the Hf model ages calculated with this new Lu/Hf ratio could be younger by up to 500 Ma. (C) 2013 Elsevier B.V. All rights reserved.

Résumé: The lithospheric structures beneath the Pyrenees, which holds the key to settle long-standing controversies regarding the opening of the Bay of Biscay and the formation of the Pyrenees, are still poorly known. The temporary PYROPE and IBERARRAY experiments have recently filled a strong deficit of seismological stations in this part of western Europe, offering a new and unique opportunity to image crustal and mantle structures with unprecedented resolution. Here we report the results of the first tomographic study of the Pyrenees relying on this rich data set. The important aspects of our tomographic study are the precision of both absolute and relative traveltime measurements obtained by a nonlinear simulated annealing waveform fit and the detailed crustal model that has been constructed to compute accurate crustal corrections. Beneath the Massif Central, the most prominent feature is a widespread slow anomaly that reflects a strong thermal anomaly resulting from the thinning of the lithosphere and upwelling of the asthenosphere. Our tomographic images clearly exclude scenarios involving subduction of oceanic lithosphere beneath the Pyrenees. In contrast, they reveal the segmentation of lithospheric structures, mainly by two major lithospheric faults, the Toulouse fault in the central Pyrenees and the Pamplona fault in the western Pyrenees. These inherited Hercynian faults were reactivated during the Cretaceous rifting of the Aquitaine and Iberian margins and during the Cenozoic Alpine convergence. Therefore, the Pyrenees can be seen as resulting from the tectonic inversion of a segmented continental rift that was buried by subduction beneath the European plate.

Résumé: The topography of the core-mantle boundary (CMB) is directly linked to the dynamics of both the mantle and the outer core, although it is poorly constrained and understood. Recent studies have produced topography models with mutual agreement up to degree 2. Abroad-band waveform inversion strategy is introduced and applied here, with relatively low computational cost and based on a first-order Born approximation. Its performance is validated using synthetic waveforms calculated in theoretical earth models that include different topography patterns with varying lateral wavelengths, from 600 to 2500 km, and magnitudes (similar to 10 km peak-to-peak). The source-receiver geometry focuses mainly on the P-diff, PKP, PcP and ScS phases. The results show that PKP branches, PcP and ScS generally perform well and in a similar fashion, while P-diff yields unsatisfactory results. We investigate also how 3-D mantle correction influences the output models, and find that despite the disturbance introduced, the models recovered do not appear to be biased, provided that the 3-D model is correct. Using cross-correlated traveltimes, we derive new topography models from both P and S waves. The static corrections used to remove the mantle effect are likely to affect the inversion, compromising the agreement between models derived from P and S data. By modelling traveltime residuals starting from sensitivity kernels, we show how the simultaneous use of volumetric and boundary kernels can reduce the bias coming from mantle structures. The joint inversion approach should be the only reliable method to invert for CMB topography using absolute cross-correlation traveltimes.

Résumé: The middle Oxfordian warming climate and sea-level rise initiated the development of vast carbonate platforms in some western European basins. At the same time, however, siliciclastics and siliceous sponges dominated certain marginal areas of the Iberian ramp. There, siliciclastic input was particularly prominent during the latest Oxfordian and may have been related to a global sea-level fall, synsedimentary tectonic activity, or humid climatic conditions in the hinterland. Field analyses and computer modelling have been previously used to determine the factors that controlled sedimentation. However, it is still unclear if the specific conditions that prevailed during the latest Oxfordian were due to eustasy, tectonics or climate, and when precisely they occurred. Here, we document major changes in sedimentological, micropalaeontological, and mineralogical records on the Iberian ramp during this interval. Detailed sedimentary facies and palynofacies analyses combined with sequence-stratigraphic and cyclostratigraphic analyses of the Ricla Barranco section enable the establishment of a highresolution time frame. Based on the quartz and mica percentage fluctuations, one large- and seven small-scale sequences are defined. The large-scale sequence boundaries correlate with third-order sequence boundaries Ox 8 and Kim 1 defined by Hardenbol et al. (1998). The large-scale maximum-flooding surface corresponds to the base of the most calcareous interval and to the maximum abundance of marine phytoplankton and opaque, equidimensional phytoclasts. The small-scale sequences correspond to the 100-kyr orbital eccentricity cycle. Calcareous nannofossils and clay minerals were used as palaeoclimatic proxies. Nannofossil abundances and fluxes are lower in the upper part than in the lower part of the interval studied, suggesting a decrease in seasurface trophic conditions, also shown by an increase in the relative abundance of oligotrophic taxa. This upper part is also characterised by an increase in smectite, which coincides with the base of the large-scale highstand deposit, and is interpreted as reflecting the establishment of dry conditions. A first increase in smectite occurs in the lower part of the succession, and coincides with high percentages of quartz and mica. This latter mineralogical assemblage is interpreted as recording the onset of the Late Jurassic to Early Cretaceous rifting stage, which occurred just before the Planula-Galar ammonite subzone transition. The present study points out a return toward optimum conditions for carbonate sedimentation only 300 kyr after the prominent increase in siliciclastic input due to tectonic activity. The recovery of carbonate production was accompanied by a global sea-level rise and by decreasing rainfall on nearby land. (c) 2014 Elsevier B.V. All rights reserved.

Résumé: Continental margins receive natural and anthropogenic trace elements (TEs) from direct atmospheric deposition of aerosols onto the sea surface and from advection of riverine suspended particles and/or resuspended sediments from the continental shelf/slope. When the margin is incised by submarine canyons, as for example in the Northwestern Mediterranean Sea, most of these particles are preferentially transferred via these topographic features towards their final repositories in the abyssal plain. The Gulf of Lions (GoL) shelf receives the largest particulate riverine input to the Western Mediterranean, with its associated chemical contaminants originating from the industrialized and urbanized Rhone Valley. Sediment samples (grabs, cores and moored traps) collected in the Cap de Creus (CdC) Canyon and its adjacent areas at the Southwestern exit of the GoL were analyzed to explore the origin, dispersion, transfer and accumulation of a suite of TEs (Ag, Cd, Co, Cr, Cu, Ni, Pb, Zn and V) from the GoL shelf to the adjacent continental rise. Distributions of Cu, Cr, Ni, Pb, Zn and V in the surface sediments of the shelf confirm their terrigenous origin in association with clay minerals, whereas Ag and Cd are more associated with organic matter (OM). All these TEs are anthropogenically enriched in the Rhone prodelta sediments. Anthropogenic influence remains clearly discernible in the GoL shelf surface sediments for Ag, Pb and Zn. Hydrodynamical resuspension and sorting of shelf sediments occur at the head of the CdC Canyon during dense shelf-water cascading events. During these events, the material collected in moored sediment traps contains a higher coarse carbonate fraction slightly impoverished in TEs compared to the clays of the nepheloid layer and the organically-rich particles deposited before and at the end of the cascading period. Upper and middle canyon sediments are characterized by high sedimentation rates (similar to 0.2 cm yr(-1)) of fine clay material. Conversely, sediments from the lower continental slope and rise exhibit low sedimentation rates (similar to 0.06 cm yr(-1)) and receive carbonaceous planktonic detritus from the water column. At the lower continental slope, coarse material includes foraminifers and pteropods, whereas at the continental rise finer planktonic-derived material is more abundant. Both in the CdC Canyon and in its adjacent lower continental slope/rise sediments, Co, Cu, Cr, Ni and V are associated with clay, whereas Ag, Cu and Pb are preferentially associated with OM. Cadmium, Cr, and Zn are also associated with OM in canyon sediments. Carbonaceous plankton appears to be especially efficient for scavenging Ag, whereas, Cr, V, Zn and Pb are diluted by biogenic carbonates. An authigenic Mn fraction is enriched with Co and Ni. Lead and Zn concentration levels and vertical profile patterns, along with Pb stable isotopic ratios, indicate that significant parts of Pb and Zn are of anthropogenic origin. A sediment chronology based on Pb-210 dating reveals that Pb anthropization, mainly from gasoline additives, culminated between 1960 and 1980, being the current concentrations >40% lower than 30 years ago. A similar distribution is observed for Zn, which originates mainly from combustion processes; but the reduction of Zn contamination amounts to only 20% during the same period. The largest anthropogenic Pb accumulation occurs in the middle part of CdC Canyon, with an inventory of 200 μg cm(-2). At the most distal part of the continental rise anthropogenic Pb accumulation within the first similar to 10 cm below the surface sediment is estimated around 10 μg cm(-2), which is similar to the direct atmospheric deposition estimate. (C) 2014 Elsevier B.V. All rights reserved.

Résumé: Both climatic and tectonic processes affect bedrock erosion and exhumation in convergent orogens, but determining their respective influence is difficult. A requisite first step is to quantify long-term (similar to 10(6)year) erosion rates within an orogen. In the Himalaya, past studies suggest long-term erosion rates varied in space and time along the range front, resulting in numerous tectonic models to explain the observed erosion rate distribution. Here, we invert a large data set of new and existing thermochronological ages to determine both long-term exhumation rates and the kinematics of Neogene tectonic activity in the eastern Himalaya in Bhutan. New data include 31 apatite and five zircon (U-Th)/He ages, and 49 apatite and 16 zircon fission-track ages along two north-south oriented transects across the orogen in western and eastern Bhutan. Data inversion was performed using a modified version of the 3-D thermokinematic model Pecube, with parameter ranges defined by available geochronologic, metamorphic, structural, and geophysical data. Among several important observations, our three main conclusions are as follows: (1) Thermochronologic ages do not spatially correlate with surface traces of major fault zones but appear to reflect the geometry of the underlying Main Himalayan Thrust; (2) our data are compatible with a strong tectonic influence, involving a variably dipping Main Himalayan Thrust geometry and steady state topography; and (3) erosion rates have remained constant in western Bhutan over the last similar to 10Ma, while a significant decrease occurred at similar to 6Ma in eastern Bhutan, which we partially attribute to convergence partitioning into uplift of the Shillong Plateau.

Résumé: The lithosphere-asthenosphere boundary (LAB) beneath the continents is a key interface in plate tectonics, yet its nature remains elusive. A partial melt layer has been advocated to explain its geophysical characteristics. However, the main counter-argument is that such a layer cannot be stable as melts should rise through the lithosphere. Density measurements of volatile-containing alkali basalts taken as a proxy for LAB melts show that they are neutrally buoyant at the pressure (P)-temperature (T) conditions of the LAB under continents. Complementary X-ray diffraction and Raman data provide structural insights on melt compaction mechanisms. Basalts generated below the lithosphere may thus be gravitationally trapped and accumulate over time. Their presence provides answers to key questions on continental lithosphere geodynamics, and in particular on cratonic keels stability. This buoyancy trap would cease to exist at higher mantle T such as those relevant of the Archean, and as such, could be linked to the onset of plate tectonics. (C) 2014 Elsevier B.V. All rights reserved.

Résumé: The present study aims primarily at refining a structure model for interlayer cations and H2O molecules in tri-hydrated (3W) smectite (d(001) = 18-19 angstrom). The <2 μm fraction of the SWy-2 source clay (low-charge montmorillonite) was saturated by Mg2+, Ca2+, Ba2+, or Na cations, before collection of X-ray diffraction (XRD) patterns at 98% relative humidity. Experimental d(001) values derived for the essentially homogeneous 3W hydrates provided volume constraints for Grand Canonical Monte Carlo (GCMC) simulations. Computed atomic density distribution of interlayer species were used in turn to calculate XRD intensities of 00l reflections. The agreement between calculated and experimental 00l intensities allowed validating the GCMC results of both interlayer H2O content and distribution of interlayer species (cations and H2O molecules). Computed atomic density profiles do not correspond to the usual model of three discrete planes of H2O molecules but rather exhibit two sharp planes of H2O molecules wetting the clay surfaces (at similar to 2.7 angstrom from the clay layer surface). Additional H2O molecules belong to cation hydration shells or define a poorly organized ensemble filling internal voids. This alternative model suggests that the stability of the 3W hydrate results from the dual interaction of some H2O molecules with interlayer cation, through their second hydration shell; and with the 2:1 clay surface. Computed atomic density profiles were approximated to propose an interlayer structure model for 3W smectite. This simplified model includes two sets of two planes (symmetrical relative to the interlayer mid-plane) for H2O molecules and one set for interlayer cations. This model allows reproducing experimental XRD patterns for the different samples investigated and thus represents a valid set of parameters for routine quantitative analysis of XRD profiles in an effort to determine smectite reactivity close to water-saturated conditions. Implications of such studies are crucial to provide experimental constraints on the behavior of the main vector of element transfer under conditions common in surficial environments and prevailing in waste repositories. In addition, the present study provides an experimental validation of structure models derived from the widely used ClayFF model, and thus allows its use to predict the fate of water in clayey systems close to water-saturated conditions.

Résumé: Serpentinites are an important component of the oceanic lithosphere that formed at slow to ultra-slow spreading centers. Serpentine could thus be one of the most abundant hydrous minerals recycled into the mantle in subduction zones. Prograde metamorphism in subducted serpentinites is characterized by the destabilization of lizardite into antigorite, and then into secondary olivine. The nature of the fluid released during these phase transitions is controlled by redox reactions and can be inferred from oxidation state of Fe in serpentine minerals. We used bulk rock analyses, magnetic measurements, SEM observations and μXANES spectroscopy to establish the evolution of Fe2O3Tot(BR) and magnetite content in serpentinite and Fe oxidation state in serpentine minerals from ridge to subduction settings. At mid-ocean ridges, during the alteration of peridotite into serpentinite, iron is mainly redistributed between magnetite and oceanic serpentine (usually lizardite). The Fe3+/Fe-Total ratio in lizardite and the modal percentage of magnetite progressively increase with the degree of local serpentinization to maxima of about 0.8 and 7 wt%, respectively, in fully serpentinized peridotites. During subduction, the Fe2O3Tot(BR) of serpentinite remains constant (similar to 7-10 wt%, depending on the initial Fe content of the peridotite) while the modal percentage of magnetite decreases to less than 2% in eclogite facies rocks. The Fe3+/Fe-Total ratio in serpentine also decreases down to 0.2-0.4 in antigorite at eclogite facies. Our results show that, in the first 70 km of subduction, the transition from lizardite to antigorite is accompanied by a reduction of Fe in bulk rock samples and in serpentine minerals. This redox reaction might be coupled with the oxidation of reduced oceanic phases such as sulfides, and the formation of oxidized fluids (e.g. SOx, H2O, COx). At greater depths, the beginning of antigorite dehydration leads to an increase of Fe3+/Fe-Total in relict antigorite, in agreement with the preferential partitioning of ferric iron into serpentine rather than into olivine. (C) 2014 Elsevier B.V. All rights reserved.

Résumé: The depth-dependent attenuation of the secondary cosmic-ray particle flux due to snow cover and its effects on production rates of cosmogenic nuclides constitutes a potential source of uncertainty for studies conducted in regions characterized by frequent seasonal snow burial. Recent experimental and numerical modelling studies have yielded new constraints on the effect of hydrogen-rich media on the production rates of cosmogenic nuclides by low- and high-energy neutrons (<10(-3) MeV and >10(2) MeV, respectively). Here we present long-term neutron-detector monitoring data from a natural setting that we use to quantify the effect of snow cover on the attenuation of fast neutrons (0.1-10 MeV), which are responsible for the production of Ne-21 from Mg and Cl-36 from K. We use data measured between July 2001 and May 2008 at seven stations located throughout the Ecrins-Pelvoux massif (French Western Alps) and its surroundings, at elevations ranging from 200 to 2500 m a.s.l. From the cosmic-ray fluxes recorded during summer, when snow is absent, we infer an apparent attenuation length of 148 g cm(-2) in the atmosphere at a latitude of similar to 45 degrees N and for altitudes ranging from similar to 200 to 2500 m a.s.l. Using snow water-equivalent (SWE) values obtained through snow-coring campaigns that overlap in time the neutron monitoring for five stations, we show that fast neutrons are much more strongly attenuated in snow than predicted by a conventional mass-shielding formulation and the attenuation length estimated in the atmosphere. We suggest that such strong attenuation results from boundary effects at the atmosphere/snow interface induced by the high efficiency of water as a neutron moderator. Finally, we propose an empirical model that allows calculating snow-shielding correction factors as a function of SWE for studies using Ne-21 and Cl-36 analyses in Mg- and K-rich minerals, respectively. This empirical model is of interest for studies with a focus on cosmic-ray exposure dating, particularly if the target rocks are made up of mafic to ultramafic units where seasonal snow-cover is a common phenomenon. (C) 2014 Elsevier B.V. All rights reserved.

Résumé: We have used the Artificial Neural Network method (ANN) for the derivation of physically sound, easy-to-handle, predictive ground-motion models from a subset of the Reference database for Seismic ground-motion prediction in Europe (RESORCE). Only shallow earthquakes (depth smaller than 25 km) and recordings corresponding to stations with measured properties have been selected. Five input parameters were selected: the moment magnitude , the Joyner-Boore distance , the focal mechanism, the hypocentral depth, and the site proxy . A feed-forward ANN type is used, with one 5-neuron hidden layer, and an output layer grouping all the considered ground motion parameters, i.e., peak ground acceleration (PGA), peak ground velocity (PGV) and 5 %-damped pseudo-spectral acceleration (PSA) at 62 periods from 0.01 to 4 s. A procedure similar to the random-effects approach was developed to provide between and within event standard deviations. The total standard deviation () varies between 0.298 and 0.378 (log unit) depending on the period, with between-event and within-event variabilities in the range 0.149-0.190 and 0.258-0.327, respectively. Those values prove comparable to those of conventional GMPEs. Despite the absence of any a priori assumption on the functional dependence, our results exhibit a number of physically sound features: magnitude scaling of the distance dependency, near-fault saturation distance increasing with magnitude, amplification on soft soils and even indications for nonlinear effects in softer soils.

Résumé: X-ray absorption near-edge structure (XANES) spectroscopy and spectromicroscopy have been extensively used to characterize biominerals. Using either Ca or C spectra, unique information has been obtained regarding amorphous biominerals and nanocrystal orientations. Building on these results, we demonstrate that recording XANES spectra of calcium carbonate at the oxygen K-edge enables polarization-dependent imaging contrast (PIC) mapping with unprecedented contrast, signal-to-noise ratio, and magnification. O and Ca spectra are presented for six calcium carbonate minerals: aragonite, calcite, vaterite, monohydrocalcite, and both hydrated and anhydrous amorphous calcium carbonate. The crystalline minerals reveal excellent agreement of the extent and direction of polarization dependences in simulated and experimental XANES spectra due to X-ray linear dichroism. This effect is particularly strong for aragonite, calcite, and vaterite. In natural biominerals, oxygen PIC-mapping generated high-magnification maps of unprecedented clarity from nacre and prismatic structures and their interface in Mytilus californianus shells. These maps revealed blocky aragonite crystals at the nacre-prismatic boundary and the narrowest calcite needle-prisms. In the tunic spicules of Herdmania momus, O PIC-mapping revealed the size and arrangement of some of the largest vaterite single crystals known. O spectroscopy therefore enables the simultaneous measurement of chemical and orientational information in CaCO3 biominerals and is thus a powerful means for analyzing these and other complex materials. As described here, PIC-mapping and spectroscopy at the O K-edge are methods for gathering valuable data that can be carried out using spectromicroscopy beamlines at most synchrotrons without the expense of additional equipment.

Résumé: We introduce a method for imaging the earthquake source dynamics from the inversion of ground motion records based on a parallel genetic algorithm. The source model follows an elliptical patch approach and uses the staggered-grid split-node method to simulate the earthquake dynamics. A statistical analysis is used to estimate errors in both inverted and derived source parameters. Synthetic inversion tests reveal that the average rupture speed (V-r), the rupture area, and the stress drop () may be determined with formal errors of similar to 30%, similar to 12%, and similar to 10%, respectively. In contrast, derived parameters such as the radiated energy (E-r), the radiation efficiency ((r)), and the fracture energy (G) have larger errors, around similar to 70%, similar to 40%, and similar to 25%, respectively. We applied the method to the M-w 6.5 intermediate-depth (62km) normal-faulting earthquake of 11 December 2011 in Guerrero, Mexico. Inferred values of =29.26.2MPa and (r)=0.260.1 are significantly higher and lower, respectively, than those of typical subduction thrust events. Fracture energy is large so that more than 73% of the available potential energy for the dynamic process of faulting was deposited in the focal region (i.e., G=(14.43.5)x10(14)J), producing a slow rupture process (V-r/V-S=0.470.09) despite the relatively high energy radiation (E-r=(0.54 +/- 0.31)x10(15)J) and energy-moment ratio (E-r/M-0=5.7x10(-5)). It is interesting to point out that such a slow and inefficient rupture along with the large stress drop in a small focal region are features also observed in both the 1994 deep Bolivian earthquake and the seismicity of the intermediate-depth Bucaramanga nest. Key Points <list list-type=“bulleted” id=“jgrb50865-list-0001”><list-item id=“jgrb50865-li-0001”>New method for imaging the earthquakes source dynamics<list-item id=“jgrb50865-li-0002”>Fundamental source parameters of an intermediate-depth Mexican earthquake<list-item id=“jgrb50865-li-0003”>Slow rupture, low radiation efficiency, high stress drop, and small rupture area

Résumé: This article presents comparisons among the five ground-motion models described in other articles within this special issue, in terms of data selection criteria, characteristics of the models and predicted peak ground and response spectral accelerations. Comparisons are also made with predictions from the Next Generation Attenuation (NGA) models to which the models presented here have similarities (e.g. a common master database has been used) but also differences (e.g. some models in this issue are nonparametric). As a result of the differing data selection criteria and derivation techniques the predicted median ground motions show considerable differences (up to a factor of two for certain scenarios), particularly for magnitudes and distances close to or beyond the range of the available observations. The predicted influence of style-of-faulting shows much variation among models whereas site amplification factors are more similar, with peak amplification at around 1s. These differences are greater than those among predictions from the NGA models. The models for aleatory variability (sigma), however, are similar and suggest that ground-motion variability from this region is slightly higher than that predicted by the NGA models, based primarily on data from California and Taiwan.

Résumé: Over the past two decades, geophysical observations have shown that earthquakes can trigger other earthquakes, raising the possibility that earthquake interaction plays an important role in the earth's deformation. We analyze here a storm of earthquakes in Greece and show that their interaction provides remarkable insight into the mechanics of one of the fastest deforming continental region in the world. A rupture of the African slab initiates a cascade of large earthquakes and a long episode of slow slip marking the downward plunge of the slab, the concomitant rollback of the subduction, and the subsequent detachment of southern Greece from the Eurasian plate. Intense crustal deformation, indicative of the resulting plate stretching, follows. This slow deformation which spreads in a few months over more than 500km lasts similar to 3years and triggers earthquakes. The observations also show that the retreat of the African subduction is the motor of the Aegean deformation.

Résumé: The reliability of ab initio methods to predict accurate thermodynamic properties and coordination geometries of mercury-thiolate complexes was examined with calculations at various levels of theory. The second-order Moller-Plesset perturbation theory (MP2) method in connection with the Stuttgart-Dresden-Bonn relativistic effective core potentials and the related correlation consistent valence basis set gives optimized Hg(RS) (n) model structures in good agreement with experimental data. Differences in thermodynamic stability among various models can be estimated with chemical precision using single-point energy calculation at the CCSD(T) level of theory performed on the MP2-optimized structures. This computational scheme was applied next to calculate the stability of aqueous linear (two coordinated), trigonal, and tetrahedral mercury-thiolate complexes. In alkaline solutions, the difference in complexation Gibbs free energy between the most stable (trigonal) and the less stable (tetrahedral) model complexes formed with free ligands is only -4.7 kcal mol(-1). At neutral pH, the linear coordination is most stable. When the thiol ligands are structurally associated, as in biological systems, the trigonal coordination is most stable from pH 4.8 to 10.6. The relative stabilities of the three Hg-(RS) (n) bonding configurations reported herein can be further modified in biological environment by Hg-induced folding of proteins.

Résumé: We present a new method that can be used to quantitatively evaluate the consistency between balanced section restorations and thermochronological data sets from orogenic belts. We have applied our method to a crustal-scale area-balanced cross-section restoration along a profile in the Central Pyrenees. This restoration is well constrained and supported by a wide variety of geological and geophysical data. Moreover, an extensive thermochronological data set has been collected independently in the area. We use the structural-kinematic software 2D-Move to constrain a set of velocity fields that describes the kinematics of the Central Pyrenees. Using these velocity fields as input for the thermokinematic code PECUBE, we derive predictions of the thermal history and a range of thermochronometric ages for the modeled area. We find that the kinematic history of the belt as inferred from section balancing is in good agreement with the published thermochronological data. High-temperature (zircon fission-track and K-feldspar Ar-Ar) data constrain the thermal structure of the belt as well as the timing of underplating. Low-temperature (apatite fission-track and (U-Th)/He) data require late synorogenic sedimentary burial of the southern flank of the Pyrenees between late Eocene (40Ma) to late Miocene (9Ma) times, consistent with previous studies, and imply that no such burial occurred on the northern flank.

Résumé: The NASA Stardust spacecraft exposed an aerogel collector to the interstellar dust passing through the solar system. We performed X-ray fluorescence element mapping and abundance measurements, for elements 19 <= Z <= 30, on six “interstellar candidates,” potential interstellar impacts identified by Stardust@Home and extracted for analyses in picokeystones. One, I1044,3,33, showed no element hot-spots within the designated search area. However, we identified a nearby surface feature, consistent with the impact of a weak, high-speed particle having an approximately chondritic (CI) element abundance pattern, except for factor-of-ten enrichments in K and Zn and an S depletion. This hot-spot, containing approximately 10 fg of Fe, corresponds to an approximately 350 nm chondritic particle, small enough to be missed by Stardust@Home, indicating that other techniques may be necessary to identify all interstellar candidates. Only one interstellar candidate, I1004,1,2, showed a track. The terminal particle has large enrichments in S, Ti, Cr, Mn, Ni, Cu, and Zn relative to Fe-normalized CI values. It has high Al/Fe, but does not match the Ni/Fe range measured for samples of Al-deck material from the Stardust sample return capsule, which was within the field-of-view of the interstellar collector. A third interstellar candidate, I1075,1,25, showed an Al-rich surface feature that has a composition generally consistent with the Al-deck material, suggesting that it is a secondary particle. The other three interstellar candidates, I1001,1,16, I1001,2,17, and I1044,2,32, showed no impact features or tracks, but allowed assessment of submicron contamination in this aerogel, including Fe hot-spots having CI-like Ni/Fe ratios, complicating the search for CI-like interstellar/interplanetary dust.

Résumé: The Iranian plateau is a flat similar to 1.5-2 km high plateau thought to result from the collision between the Arabian and Eurasian plates since similar to 30 +/- 5 Ma, and may represent a young analogue to the so far better studied Tibetan plateau. In order to constrain the exhumation history of the internal Zagros and of the Iranian plateau, we herein present apatite (U-Th)/He (AHe) and apatite (AFT) and zircon fission-track (ZFT) data on plutonic rocks from the Sanandaj-Sirjan Zone (SSZ), Urumieh-Dokhtar magmatic arc (UDMA), Central Iran and Kopet Dagh. Thermochronologic data show that the SSZ was exhumed early in the collision process (essentially before 25-20 Ma), with a likely acceleration of cooling during the late Eocene, from 0.04 to 03 mm/year. Results suggest that cooling of the internal Zagros migrated from the SSZ to the UDMA during a more mature stage of the continental collision, after similar to 17 Ma (i.e., coeval with the outward propagation of deformation and topography fronts in the external Zagros). Constant exhumation rates in the UDMA (similar to 0.3 mm/year) suggest that no significant variation of erosion rates occurred since the onset of continental collision. In Central Iran, the overlap of ZFT, AFT and AHe ages from gneissic samples points to rapid cooling during the late Eocene (similar to 42 degrees C/Myr), which is consistent with previous reports on the formation of Eocene metamorphic core-complexes. (C) 2014 Elsevier B.V. All rights reserved.

Résumé: Studies of low-frequency earthquakes (LFEs) have focused on detecting events within previously identified tectonic tremor. However, the principal LFE detection tools of matched-filter searches are intrinsically incapable of detecting events that have not already been characterized previously as a template event. In this study, we therefore focus on generating the largest number possible of LFE templates by uniformly applying a recently developed LFE template detection method to a 2.5 yearlong data set in Guerrero, Mexico. Using each of the detected templates in a matched-filter search, we then form event families that each represents a single source. We finally develop simple, empirical statistics to select the event families that represent LFEs. Our resulting catalog contains 1120 unique LFE sources and a total of 1,849,486 detected LFEs over the 2.5 yearlong data set. The locations of the LFE sources are then divided into subcatalogs based on their distance from the subduction trench. Considering each LFE as a small unit of slip along the subduction interface, we observe discrete episodes of LFE activity in the region associated with large slow-slip events; this is in direct contrast to the near-continuous activity observed 35 km farther downdip within the previously identified LFE/tremor sweet spot. Key Points<list list-type=“bulleted”><list-item id=“jgrb50853-li-0001”>Unprecedented systematic analysis of low-frequency earthquakes<list-item id=“jgrb50853-li-0002”>Precise details of two different low-frequency earthquake activity regimes<list-item id=“jgrb50853-li-0003”>Evolution of low-frequency earthquake activity during a strong slow-slip event

Résumé: Analysis of magmatic and sedimentary rocks of several large igneous provinces has demonstrated that the release of gas during plutonic-metamorphic processes may be linked to global climate change and mass extinctions. Aguablanca, one of the largest Cu-Ni-PGE deposits in Europe, formed during the Variscan orogeny when a mafic magma intruded limestones and shales, creating a contact aureole composed of marble, skarn and hornfels. Our petrological and geochemical investigation of the aureole provides evidence that a combination of the two processes led to the formation of the ore deposit: The assimilation of terrigenous sediments supplied S to the magma while the assimilation of carbonates changed the oxygen fugacity and decreased the solubility of sulfur in the magma. The metamorphic assemblages in the contact aureole are directly related to heterogeneity of the protolith and particularly to the original proportions of calcite and clay. We modeled carbon dioxide degassing during contact metamorphism and showed that pure limestone is relatively unproductive because of its high reaction temperature. The presence of clay, however, leads to the formation of calc-silicates and significantly enhances CO2 degassing. Our estimations suggest that degassing of the Aguablanca contact aureole released about 74.8 Mt of CO2, a relatively low volume that we attribute to the composition of the host rock, mainly a pure limestone. A far larger volume of carbon dioxide was emitted by the contact metamorphism of dolostones in the contact aureole of Panzhihua (part of Emeishan large igneous province, SW China). We propose that the level of emission of carbon dioxide depends strongly on the nature of the protolith and has to be considered when predicting environmental impact during the emplacement of large igneous provinces.

Résumé: River sediments naturally sample and average large areas of eroded continental crust. They are ideal targets not only for provenance studies based on isotopic compositions, but also can be used to establish average continental crust isotopic values. In large fluvial systems, however, mineral sorting processes significantly modify the mineralogy, and thus the geochemistry of the transported sediments. We still do not know, in any quantitative way, to what extent mineral sorting affects and fractionates the isotopic compositions of river sediments. Here, we focus on this issue and try to decipher the role of each mineral species in the bulk isotopic compositions of bedloads and suspended loads sampled at the outflow of the Ganga River that drains the Himalayan mountain range. We analyzed Nd, Hf, Sr, and Pb isotopic compositions as well as trace element contents of a large number of pure mineral fractions (K-feldspar, plagioclase, muscovite, biotite, magnetite, zircon, titanite, apatite, monazite/allanite, amphibole, epidote, garnet, carbonate and clay) separated from bedload and bank sediments. We combine these data with mineral proportions typical of the Ganga sediments to perform Monte Carlo simulations that quantify the contributions of individual mineral species to the Nd, Hf, Sr, and Pb isotopic budgets of bedloads and suspended loads. The isotopic systematics of river sediments are buffered by very few minerals. Despite their extremely low proportions in sediments, zircon and monazite/allanite control Hf and Nd isotopes, respectively. Feldspars, epidote, and carbonate buffer the Sr isotopic budget while clay, feldspars, and heavy minerals dominate Pb isotopes. Hafnium, Sr, and Pb isotopic differences between bedloads and suspended loads are well explained by their different mineral compositions. This confirms that Hf, Sr and Pb isotopic compositions of sediments are strongly biased by mineral sorting processes during fluvial transport; hence they do not always constitute good proxies for provenance studies. In addition, we anticipate that fractionation of the isotopic systems continues at the river/ocean interface to deliver sediments to the deep ocean that are not necessarily similar to their crustal precursors, creating a systematic bias between the compositions of crustal sources and oceanic sediments. (C) 2013 Elsevier B. V. All rights reserved.

Résumé: The Earth's topography is shaped by surface processes that operate on various scales. In particular, river processes control landscape dynamics over large length scales, whereas hillslope processes control the dynamics over smaller length scales. This scale separation challenges numerical treatments of landscape evolution that use space discretization. Large grid spacing cannot account for the dynamics of water divides that control drainage area competition, and erosion rate and slope distribution. Small grid spacing that properly accounts for divide dynamics is computationally inefficient when studying large domains. Here we propose a new approach for landscape evolution modeling that couples irregular grid-based numerical solutions for the large-scale fluvial dynamics and continuum-based analytical solutions for the small-scale fluvial and hillslope dynamics. The new approach is implemented in the landscape evolution model DAC (divide and capture). The geometrical and topological characteristics of DAC's landscapes show compatibility with those of natural landscapes. A comparative study shows that, even with large grid spacing, DAC predictions fit well an analytical solution for divide migration in the presence of horizontal advection of topography. In addition, DAC is used to study some outstanding problems in landscape evolution. (i) The time to steady-state is investigated and simulations show that steady-state requires much more time to achieve than predicted by fixed area calculations, due to divides migration and persistent reorganization of low-order streams. (ii) Large-scale stream captures in a strike-slip environment are studied and show a distinct pattern of erosion rates that can be used to identify recent capture events. (iii) Three tectono-climatic mechanisms that can lead to asymmetric mountains are studied. Each of the mechanisms produces a distinct morphology and erosion rate distribution. Application to the Southern Alps of New Zealand suggests that tectonic advection, precipitation gradients and non-uniform tectonic uplift act together to shape the first-order topography of this mountain range. Copyright (c) 2014 John Wiley & Sons, Ltd.

Résumé: We use continuous GPS measurements from 31 stations in southern Mexico to model coseismic slip and post-seismic deformation from the 2012 March 20 M-w = 7.5 Ometepec earthquake, the first large thrust earthquake to occur below central Mexico during the modern GPS era. Coseismic offsets ranging from similar to 280 mm near the epicentre to 5 mm or less at sites far from the epicentre are fit best by a rupture focused between similar to 15 and 35 km depth, consistent with an independent seismological estimate. The corresponding geodetic moment of 1.4 x 10(20) N center dot m is within 10 per cent of two independent seismic estimates. Transient post-seismic motion recorded by GPS sites as far as 300 km from the rupture has a different horizontal deformation gradient and opposite sense of vertical motion than do the coseismic offsets. A forward model of viscoelastic relaxation as a result of our new coseismic slip solution incorrectly predicts uplift in areas where post-seismic subsidence was recorded and indicates that viscoelastic deformation was no more than a few per cent of the measured post-seismic deformation. The deformation within 6 months of the earthquake was thus strongly dominated by fault afterslip. The post-seismic GPS time-series are well fit as logarithmically decaying fault afterslip on an area of the subduction interface up to 10 times larger than the earthquake rupture zone, extending as far as 220 km inland. Afterslip had a cumulative geodetic moment of 2.0 x 10(20) N center dot m, similar to 40 per cent larger than the Ometepec earthquake. Tests for the shallow and deep limits for the afterslip require that it included much of the earthquake rupture zone as well as regions of the subduction interface where slow slip events and non-volcanic tremor have been recorded and areas even farther downdip on the flat interface. Widespread afterslip below much of central Mexico suggests that most of the nearly flat subduction interface in this region is conditionally stable and thus contributes measurable transient deformation to large areas of Mexico south of and in the volcanic belt.

Résumé: We present laboratory experimental results demonstrating that librational forcing of an ellipsoidal container of water can produce intense motions through the mechanism of a libration driven elliptical instability (LDEI). These libration studies are conducted using an ellipsoidal acrylic container filled with water. A particle image velocimetry method is used to measure the 2D velocity field in the equatorial plane over hundreds libration cycles for a fixed Ekman number, E = 2 x 10(-5). In doing so, we recover the libration induced base flow and a time averaged zonal flow. Further, we show that LDEI in non-axisymmetric container geometries is capable of driving both intermittent and saturated turbulent motions in the bulk fluid. Additionally, we measure the growth rate and amplitude of the LDEI induced excited flow in a fully ellipsoidal container at more extreme parameters than previously studied [Noir et al., “Experimental study of libration-driven flows in nonaxisymmetric containers,” Phys. Earth Planet. Inter. 204-205, 1 (2012); Cebron et al., Phys. Fluids 24, 061703, “Libration driven elliptical instability,” (2012)]. Excitation of bulk filling turbulence by librational forcing provides a mechanism for transferring rotational energy into turbulent fluid motion and thus can play an important role in the thermal evolution, interior dynamics, and magneto-hydrodynamics of librating bodies, as appear to be common in solar system settings [e.g., Comstock and Bills, “A solar system survey of forced librations in longitude,” J. Geophys. Res. Planets 108, 1 (2003)]. (C) 2014 AIP Publishing LLC.

Résumé: Subseafloor environments preserved in Archean greenstone belts provide an analogue for investigating potential subsurface habitats on Mars. The c. 3.5-3.4 Ga pillow lava metabasalts of the mid-Archean Barberton greenstone belt, South Africa, have been argued to contain the earliest evidence for microbial subseafloor life. This includes candidate trace fossils in the form of titanite microtextures, and sulfur isotopic signatures of pyrite preserved in metabasaltic glass of the c. 3.472 Ga Hooggenoeg Formation. It has been contended that similar microtextures in altered martian basalts may represent potential extraterrestrial biosignatures of microbe-fluid-rock interaction. But despite numerous studies describing these putative early traces of life, a detailed metamorphic characterization of the microtextures and their host alteration conditions in the ancient pillow lava metabasites is lacking. Here, we present a new nondestructive technique with which to study the in situ metamorphic alteration conditions associated with potential biosignatures in mafic-ultramafic rocks of the Hooggenoeg Formation. Our approach combines quantitative microscale compositional mapping by electron microprobe with inverse thermodynamic modeling to derive low-temperature chlorite crystallization conditions. We found that the titanite microtextures formed under subgreenschist to greenschist facies conditions. Two chlorite temperature groups were identified in the maps surrounding the titanite microtextures and record peak metamorphic conditions at 315 +/- 40 degrees C (\documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepacka ge{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepa ckage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document}$ ${ \rm XFe}{ ( { \rm chlorite} ) }^{3 + }$$\end{document}=25-34%) and lower-temperature chlorite veins/microdomains at T=210 +/- 40 degrees C (lower \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackag e{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepac kage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document}$ ${ \rm XFe}{ ( { \rm chlorite} ) }^{3 + }$$\end{document}=40-45%). These results provide the first metamorphic constraints in textural context on the Barberton titanite microtextures and thereby improve our understanding of the local preservation conditions of these potential biosignatures. We suggest that this approach may prove to be an important tool in future studies to assess the biogenicity of these earliest candidate traces of life on Earth. Furthermore, we propose that this mapping approach could also be used to investigate altered mafic-ultramafic extraterrestrial samples containing candidate biosignatures. Key Words: Biosignatures on early Earth and Mars-Mid-Archean metabasalts and serpentinites-Mapping low-temperature alteration-Chlorite-Phyllosilicates. Astrobiology 14, xxx-xxx.

Résumé: We report here the first inventory of mercury (Hg) contamination in the ecosystem of Vientiane city, a representative emerging city bordering the Mekong River. Total Hg (THg) concentration in soil and sediments of both contrasting non-urbanized (wetland, rice paddy, Mekong River) and urbanized areas (wastewater canal and associated wastewater irrigated wetland) was low (8 to 101 ng g(-1)), reflecting the essentially non-industrialized status of the capital. Dissolved THg (2.2 +/- 2.2 ng l(-1)) and monomethyl-mercury (MMHg) concentrations (0.06 +/- 0.09 ng l(-1)) were also low in both surface and soil or sediment pore water with higher concentrations (up to 9 ng g(-1), THg and 0.53 ng g(-1) MMHg) measured in wetland and rice paddy suboxic waters. Dissolved organic carbon was identified as the main carrier in surface water for dissolved Hg transport towards the Mekong River. The measurement of low THg concentrations in rice (6.4 +/- 1.0 ng g(-1)) and fish (51 +/- 40 ng g(-1)) sampled in rice paddies and in the Mekong River confirmed the pristine state of the ecosystem of Vientiane area. Based on these data, we evaluated a low environmental Hg-attributable health risk for the local population whose diet relies on both fish and rice. (C) 2014 Elsevier B.V. All rights reserved.

Résumé: The temporal evolution and geochemical evolution of Marquesas hotspot volcanoes have often been interpreted with reference to the Hawaiian model, where a tholeiitic shield-building stage is followed by an alkali basaltic post-shield stage, followed after a 0.4 to 2.5 Myr long quiescence period, by a rejuvenated basanitic/nephelinitic stage. Here we discuss geochemical data on 110 Marquesas lavas also dated using the unspiked K-40-Ar-40 method on separated groundmass (including 45 new ages measured on the southern islands of Hiva Oa, Motane, Tahuata and Fatu Hiva). Sample locations were positioned on detailed geological maps to determine their shield or post-shield position with respect to the caldera collapse event(s), without taking into account their geochemical features. A rather regular decrease of the ages towards SE, consistent with the Pacific plate motion, is observed from Eiao (5.52 Ma) to Fatu Hiva (1.11 Ma), and rejuvenated basanitic volcanism occurs only in Ua Huka (1.15-0.76 Ma). The occurrence of intermediate and evolved lavas is restricted to the post-caldera stage, with the exception of Eiao island. However, many other features of the Marquesas chain are rather atypical with respect to those of Hawaii. Although Marquesas shields are tholeiitic, several of them (Eiao, Tahuata) contain interbedded alkali basaltic and basanitic flows. Moreover, post-shield volcanoes are either alkali basalts (Ua Huka), tholeiites (Hiva Oa, Tahuata, Fatu Hiva) or both (Nuku Hiva). This feature is consistent with the temporal continuity of the two stages and the usually short length of the post-shield period (<0.2 Myr). In a given island, the trace element and isotopic compositions of shield and post-shield lavas overlap, although both display large variations. The sources of alkali basalts and basanites are more enriched than those of the contemporaneous tholeiites. These specific features support the hypothesis of an extremely heterogeneous Marquesas plume. The “weak” character of this plume led to low partial melting degrees, which in turn resulted in the preservation in the basaltic magmas of geochemical features inherited from small-size source heterogeneities. (C) 2013 Published by Elsevier B.V.

Résumé: Mountain glaciers represent one of the largest repositories of fresh water in alpine regions globally. However, little is known about the processes by which water moves through these systems. Analysis of data from a deployment of seismometers located near an ice marginal lake in the Swiss Alps reveals, for the first time, that harmonic tremor occurs within mountain glaciers and that individual icequakes at the glacier base can exhibit harmonic properties. These observations suggest that there is a complex network of fluid-induced fracture processes at the glacier base. Modeling changes in the observed harmonic frequencies indicates that the spectral characteristics of seismic data can provide important information about hydraulic fracture geometry and fluid pressure at depth, leading to important insights into subglacial hydrologic processes. Future modeling of these processes may lead to improved glacial outburst flood hazard predictions.

Résumé: We present new methods for short-term earthquake forecasting that employ space, time, and magnitude kernels to smooth seismicity. These methods are purely statistical and rely on very few assumptions about seismicity. In particular, we do not use Omori-Utsu law, and only one of our two new models assumes a Gutenberg-Richter law to model the magnitude distribution; the second model estimates the magnitude distribution nonparametrically with kernels. We employ adaptive kernels of variable bandwidths to estimate seismicity in space, time, and magnitude bins. To project rates over short time scales into the future, we simply assume persistence, that is, a constant rate over short time windows. The resulting forecasts from the two new kernel models are compared with those of the epidemic-type aftershock sequence (ETAS) model generated by Werner et al. (2011). Although our new methods are simpler and require fewer parameters than ETAS, the obtained probability gains are surprisingly close. Nonetheless, ETAS performs significantly better in most comparisons, and the kernel model with a Gutenberg-Richter law attains larger gains than the kernel model that nonparametrically estimates the magnitude distribution. Finally, we show that combining ETAS and kernel model forecasts, by simply averaging the expected rate in each bin, can provide greater predictive skill than ETAS or the kernel models can achieve individually.

Résumé: Low enthalpy geothermal systems exploited with ground source heat pumps or groundwater heat pumps present many advantages within the context of sustainable energy use. Designing, monitoring and controlling such systems requires the measurement of spatially distributed temperature fields and the knowledge of the parameters governing groundwater flow (permeability and specific storage) and heat transport (thermal conductivity and volumetric thermal capacity). Such data are often scarce or not available. In recent years, the ability of electrical resistivity tomography (ERT), self-potential method (SP) and distributed temperature sensing (DTS) to monitor spatially and temporally temperature changes in the subsurface has been investigated. We review the recent advances in using these three methods for this type of shallow applications. A special focus is made regarding the petrophysical relationships and on underlying assumptions generally needed for a quantitative interpretation of these geophysical data. We show that those geophysical methods are mature to be used within the context of temperature monitoring and that a combination of them may be the best choice regarding control and validation issues.

Résumé: Using seismic noise based monitoring techniques we find that seismic velocity variations (dv/v) observed with the borehole array of the Taiwan Chelungpu-fault Drilling Project (TCDP) are controlled by strong precipitation events associated with the Madden-Julian Oscillation (MJO), a dynamic intraseasonal atmospheric pattern in the tropical atmosphere. High-frequency noise (>1 Hz) excited by steady anthropogenic activity in the vicinity of the TCDP allows daily resolution of dv/v time series. Relatively large fluid discharge properties control the equilibration of the ground water table and hence seismic velocities on time scales smaller than the average precipitation recurrence interval. This leads to the observed synchronous 50-80 day periodicity in dv/v and rainfall records in addition to the dominant annual component. Further evidence for the governing role of hydraulic properties is inferred from the similarity of observed dv/v timing, amplitude, and recovery properties with dv/v synthetics generated by a combined model of ground water table changes and diffusive propagation of seismic energy. The lapse time (tau) dependent increase of dv/v amplitudes is controlled by the sensitivity of the diffuse wave field sampled at 1100 m depth to shallower water level fluctuations. The significant vertical offset between stations and water level explains the direct tau dependence which is opposite to the trend previously inferred from measurements at the surface. (C) 2014 Elsevier B.V. All rights reserved.

Résumé: We provide empirical evidence for the impact of surface processes on the structure of the present-day foreland fold-and-thrust belt of the Himalaya. We have reconstructed and analyzed ten balanced cross sections distributed along the entire length of the Himalayan arc. Here, we focus on the Siwalik Group, which represents the deformed part of the foreland basin and consists of synorogenic Middle Miocene to Pleistocene sediments that form the youngest and frontal part of the Himalayan orogen. We make two important observations: (1) a distinct west-to-east increase in strain and strain rate correlates with plate convergence rates, and (2) belt morphology is inversely correlated with rainfall amount. According to the predictions of the critical taper model, an eastward increase in convergence rate would induce higher rates of material accretion. Thus, the Himalayan fold-and-thrust belt should widen eastward, yet we have observed the opposite. However, higher annual rainfall amounts and specific stream power appear to favor a narrower belt. Thus, we suggest that the morphology of the Himalayan foreland fold-and-thrust belt is controlled primarily by surface processes, in accordance with the critical taper model.

Résumé: Fast uplift and exhumation of the Himalaya and Tibet and fast subsidence in the foreland basin portray the primary Neogene evolution of the Indian-Eurasian collision zone. We relate these events to the relative northward drift of India over its own slab. Our mantle-flow model derived from seismic tomography shows that dynamic topography over the southward-folded Indian slab explains the modern location of the foreland depocenter. Back in time, our model suggests that the stretched Indian slab detached from the Indian plate during the indentation of the Eurasian plate, and remained stationary underneath the northward-drifting Indian continent. We model the associated southward migration of the dynamic deflection of the topography and show that subsidence has amounted to similar to 6000 m in the foreland basin since 15 Ma, while the dynamic surface uplift of the Himalaya amounted to similar to 1000 m during the early Miocene. While competing with other processes, transient dynamic topography may thus explain, to a large extent, both the uplift history of the Himalaya and subsidence of its foreland basin, and should not be ignored.

Résumé: A series of 16 vulcanian explosions occurred at Ubinas volcano between May 24 and June 14, 2009. The intervals between explosions were from 2.1 h to more than 6 days (mean interval, 33 h). Considering only the first nine explosions, the average time interval was 7.8 h. Most of the explosions occurred after a short time interval (<8 h) and had low energy, which suggests that the refilling time was not sufficient for large accumulation of gas. A tremor episode followed 75% of the explosions, which coincided with pulses of ash emission. The durations of the tremors following the explosions were longer for the two highest energy explosions. To better understand the physical processes associated with these eruptive events, we localized the sources of explosions using two seismic antennas that were composed of three-component 10 and 12 sensors. We used the high-resolution MUSIC-3C algorithm to estimate the slowness vector for the first waves that composed the explosion signals recorded by the two antennas assuming propagation in a homogeneous medium. The initial part of the explosions was dominated by two frequencies, at 1.1 Hz and 1.5 Hz, for which we identified two separated sources located at 4810 m and 3890 m +/- 390 altitude, respectively. The position of these two sources was the same for the full 16 explosions. This implies the reproduction of similar mechanisms in the conduit. Based on the eruptive mechanisms proposed for other volcanoes of the same type, we interpret the position of these two sources as the limits of the conduit portion that was involved in the fragmentation process. Seismic data and ground deformation recorded simultaneously less than 2 km from the crater showed a decompression movement 2 s prior to each explosion. This movement can be interpreted as gas leakage at the level of the cap before its destruction. The pressure drop generated in the conduit could be the cause of the fragmentation process that propagated deeper. Based on these observations, we interpret the position of the highest source as the part of the conduit under the cap, and the deeper source as the limit of the fragmentation zone. (C) 2013 Elsevier B.V. All rights reserved.

Résumé: The pore structure of magmatic rocks records processes operating during magma solidification and cooling. It has first order effects on the petrophysical properties of the magmatic rocks, and also influences mass transfer and mineral reactions during subsequent metamorphism or weathering. Here, the pore space characteristics of an andesitic sill intrusion were determined by multiscale resolution computed X-ray microtomography (mu-CT), and the 3D structure was used for transport modeling. Unaltered andesite has a power law distribution of pore volumes over a range of five orders of magnitude. The probability distribution function (PDF) scales with the inverse square of the pore volume (V), PDF proportional to V-2. This scaling behavior is attributed to the coalescence of pores at crystal-melt boundaries. Large pores are concentrated on the outer margins of amphibole and plagioclase phenocrystals. Incipient weathering of the andesite is associated with preferential growth of weathering products in the largest pores. This can be explained by a model in which diffusion of external components into the porous andesite is controlled by a random network of grain boundaries and/or microfractures. This network preferentially links the larger pores to the system boundaries and it is the major fluid transport pathway, confining incipient weathering into a small fraction of the rock volume only. (C) 2014 Elsevier B.V. All rights reserved.

Résumé: We reconstruct the history of denudation and landscape evolution of the northern East- Brazilian continental margin using apatite fission-track thermochronology and thermal history modeling. This part of the Brazilian Atlantic margin is morphologically characterized by inland and coastal plateaus surrounding a wide low-lying inland region, the Sertaneja Depression. The apatite fission track ages and mean track lengths vary from 39 +/- 4 to 350 +/- 57 Ma and from 10.0 +/- 0.3 to 14.2 +/- 0.2 μm, respectively, implying a protracted history of spatially variable denudation since the Permian at relatively low rates (<50 m My(-1)). The Sertaneja Depression and inland plateaus record Permian-Early Jurassic (300-180 Ma) denudation that precedes rifting of the margin by > 60 Myrs. In contrast, the coastal regions record up to 2.5 km of Late Jurassic-Early Cretaceous (150-120 Ma) denudation, coeval with rifting of the margin. The samples from elevated coastal regions, the Borborema Plateau and the Mantiqueira Range, record cooling from temperatures above 120 degrees C since the Late Cretaceous extending to the Cenozoic. We interpret this denudation as related to post-rift uplift of these parts of the margin, possibly resulting from compressional stresses transmitted from the Andes and/or magmatism at that time. Several samples from these areas also record accelerated Neogene (<30 Ma) cooling, which may record landscape response to a change from a tropical to a more erosive semi-arid climate during this time. The inferred denudation history is consistent with the offshore sedimentary record, but not with evolutionary scenarios inferred from the recognition of “planation surfaces” on the margin. The denudation history of the northeastern Brazilian margin implies a control of pre-, syn- and post-rift tectonic and climatic events on landscape evolution. (C) 2014 Elsevier Ltd. All rights reserved.

Résumé: A possible risk of geomechanical nature related to deep injection of CO2 is the shear reactivation of faults, hence potentially leading to the creation of new leakage pathways and eventually inducing earthquakes felt at the surface. Current practices to evaluate fault stability in the domain of CO2 storage still remain limited regarding two issues: 1. Faults are complex and heterogeneous geological systems, which do not correspond to discrete surfaces as already postulated by many authors. Reservoir-scale faults in a priori low-deformed reservoirs targeted for CO2 storage can present high complex architecture, which might influence the hydro-mechanical behaviour of the fault system; 2. Chemical interactions (dissolution and precipitation processes, chemically-induced weakening, etc.) between CO2-enriched brine and the minerals constituting the fault zone can affect the mechanical stability and the transport properties of the faulted/fractured system. The research project FISIC (www.anr-fisic.fr, funded by the French National research Agency) intends to overcome those limitations by accurately modelling the hydro-chemo-mechanical complexity of a fault zone. The main goal is to improve the stability analysis of a fault both undertaking pressure increase and alteration due to the presence of an acidic fluid. The progress of this research project is presented here.

Résumé: The interaction of an As-rich natural pyrite (FeS2.08As0.043) with aqueous Se(IV) was investigated as a function of pH, ferrous iron concentration, and reaction time. Arsenic is often the most abundant minor constituent of natural pyrite, and is believed to substitute for S in the pyrite structure. EXAFS measurements confirmed the presence of AsS dianion group, with arsenic in the same local configuration as in the arsenopyrite. Speciation studies indicated that Se(O) was the unique reduction product in the pH range 5.05-8.65 over a reaction period of >1 month, while trace amounts of FeSeO3 might be formed at pH >= 6.10. At pH > 6.07, the formation of Fe(III)-(oxyhydr)oxide is kinetically favored, and it consumed nearly all the aqueous iron, including the extra added Fe2+, thereby inhibiting the formation of the thermodynamically most stable product: FeSe2. After oxidation by Se(IV), the occurrence of surface S-0, significant aqueous sulfur deficit, and excessive leaching of arsenic in solution, indicate the preferential release of As impurity via arsenopyrite oxidation. The data suggest that the polysulfide-elemental sulfur pathway, which prevails in acid-soluble metal sulfides, is an important pathway in the oxidation of As-rich pyrite, in addition to the thiosulfate pathway for acid-insoluble pyrite. Control experiments on As-free natural pyrite further support this mechanism. This study confirms the potential of reductive precipitation to attenuate the mobility of Se in the environment and demonstrates that minor elements commonly present in natural pyrite can play a significant role on its dissolution pathway. (C) 2014 Elsevier Ltd. All rights reserved.

Résumé: Traditional methods used to invert gravity data are generally based on smoothness as a regularizer and the time-lapse inversion of gravity data is traditionally based on sequential inversion. A smooth density model can be in contradiction with the known geology of the target and sequential time-lapse inversion may lead to the presence of artefacts in the sequence of tomograms due to the propagation of errors from the initial dataset and its associated tomogram. We propose a deterministic time-lapse algorithm to invert a sequence of gravity data combining two features: an image focusing technique and the use of a time-dependent regularizer using an Active Time Constrained (ATC) approach. These two features are included directly in the objective function to minimize. The ATC inversion of time-lapse gravity data is efficient in filtering out noise-contaminated data as long as the noise is not correlated over time. Our approach can also be used to incorporate prior information regarding the density model we want to retrieve. The forward solver is based on a commercial finite element solver with a high flexibility in meshing irregular domains, a feature that is important to include realistic topography from digital elevation maps, for instance, and to describe the density distribution of geological structures with complex geometries. We benchmark the accuracy of the forward modelling code using an analytical expression and test the effectiveness of the focusing algorithm. We show the advantage of our approach in the case of the water flooding of an oil reservoir in order to detect and monitor the position of the oil-water encroachment front. We also test the model against synthetic data describing the evolution of a hydrothermal system feed by a magmatic source and the collapse of a mine. In all these cases, the approach we follow is successful in monitoring density changes.

Résumé: A simple and reproducible analog experiment was used to simulate fracture formation in a low-permeability elastic solid during internal fluid/gas production, with the objective of developing a better understanding of the mechanisms that control the dynamics of fracturing, fracture opening and closing, and fluid transport. In the experiment, nucleation, propagation, and coalescence of fractures within an elastic gelatin matrix, confined in a Hele-Shaw cell, occurred due to CO2 production via fermentation of sugar, and it was monitored by optical means. We first quantified how a fracture network develops, and then how intermittent fluid transport is controlled by the dynamics of opening and closing of fractures. The gas escape dynamics exhibited three characteristic behaviors: (1) Quasiperiodic release of gas with a characteristic frequency that depends on the gas production rate but not on the system size. (2) A 1/f power spectrum for the fluctuations in the total open fracture area over an intermediate range of frequencies (f), which we attribute to collective effects caused by interaction between fractures in the drainage network. (3) A 1/f(2) power spectrum was observed at high frequencies, which can be explained by the characteristic behavior of single fractures.

Résumé: The present study shows that the presence of Li, Sb, As and B as trace elements significantly influences textural properties such as particle size distribution, morphology and specific surface area of chrysotile synthesized under hydrothermal conditions (P = 8.2 MPa, T = 300 degrees C and high-alkaline pH (13.5)). Conversely, traces of Cs did not have any textural effect under these conditions. Furthermore, chrysotile nanotubes size and morphology depend strongly on the element considered. Indeed, large chrysotile with cylinder in cylinder morphology (outer diameter up to 50 nm) precipitated in the presence of Li, Sb and As. This implies lower specific surface area (124-160 m(2) g(-1)) compared to undoped chrysotile (184 m(2) g(-1) with about 14 nm in width). The presence of boron favors the precipitation of thin fibrous nanotubes similar to undoped chrysotile in width, but significantly longer, with tubes length that can reach three microns in length. In this case, the specific surface area increase slightly from 184 to 196 m(2)/g. The solid-liquid partition coefficient for each investigated trace element was determined using Langmuir equation. This well-tubular geo-material can be used as a model to better understand the effects of trace elements on the precipitation of minerals that are relevant in Earth systems (e.g., serpentinization processes) and societal applications (e.g., asbestos toxicity and CO2 sequestration). (C) 2013 Elsevier Inc. All rights reserved.

Résumé: Aims. Three-dimensional (3D) maps of the Galactic interstellar matter (ISM) are a potential tool of wide use, but accurate and detailed maps are still lacking. One of the ways to construct the maps is to invert individual distance-limited ISM measurements, a method we have applied here to measurements of stellar color excess in the optical. Methods. We assembled color excess data together with the associated parallax or photometric distances to constitute a catalog of similar or equal to 23 000 sightlines for stars within 2.5 kpc. The photometric data are taken from Stromgren catalogs, the Geneva photometric database, and the Geneva-Copenhagen survey. We also included extinctions derived towards open clusters. We applied an inversion method based on a regularized Bayesian approach to this color excess dataset, a method previously used for mapping at closer distances. Results. We show the dust spatial distribution resulting from the inversion by means of planar cuts through the differential opacity 3D distribution, and by means of 2D maps of the integrated opacity from the Sun up to various distances. The mapping assigns locations to the nearby dense clouds and represents their distribution at the spatial resolution that is allowed by the dataset properties, i.e. similar or equal to 10 pc close to the Sun and increasing to similar or equal to 100 pc beyond 1 kpc. Biases toward nearby and/or weakly extincted stars make this dataset particularly appropriate to mapping the local and neighboring cavities and to locating faint, extended nearby clouds, which are both goals that are difficult or impossible with other mapping methods. The new maps reveal a similar or equal to 1 kpc wide empty region in the third quadrant in the continuation of the so-called CMa tunnel of the Local Cavity, a cavity that we identify as the Superbubble GSH238+00+09 detected in radio emission maps and that is found to be bounded by the Orion and Vela clouds. The maps also show an extended narrower tunnel in the opposite direction (l similar or equal to 70 degrees) that also extends the Local Bubble further and together with it forms a conspicuous cavity bounded by the main Lup, Sco, Oph, Aql, Lac, Cep, and Tau clouds and OB associations. This chain of cavities and surrounding dense regions constitute the first computed representation of the well known Gould belt/Lindblad ring structures. Finally, almost all off-plane faint features that appear in 2D dust maps have a counterpart in the 3D maps, providing the dust distribution in nearby tenuous clouds.

Résumé: XMapTools is a MATLAB(C)-based graphical user interface program for electron microprobe X-ray image processing, which can be used to estimate the pressure-temperature conditions of crystallization of minerals in metamorphic rocks. This program (available online at http://www.xmaptools.com) provides a method to standardize raw electron microprobe data and includes functions to calculate the oxide weight percent compositions for various minerals. A set of external functions is provided to calculate structural formulae from the standardized analyses as well as to estimate pressure-temperature conditions of crystallization, using empirical and semi-empirical thermobarometers from the literature. Two graphical user interface modules, Chem2D and Triplot3D, are used to plot mineral compositions into binary and ternary diagrams. As an example, the software is used to study a high-pressure Himalayan eclogite sample from the Stak massif in Pakistan. The high-pressure paragenesis consisting of omphacite and garnet has been retrogressed to a symplectitic assemblage of amphibole, plagioclase and clinopyroxene. Mineral compositions corresponding to 165,000 analyses yield estimates for the eclogitic pressure-temperature retrograde path from 25 kbar to 9 kbar. Corresponding pressuretemperature maps were plotted and used to interpret the link between the equilibrium conditions of crystallization and the symplectitic microstructures. This example illustrates the usefulness of XMapTools for studying variations of the chemical composition of minerals and for retrieving information on metamorphic conditions on a microscale, towards computation of continuous pressure-temperature-and relative time path in zoned metamorphic minerals not affected by post-crystallization diffusion. (C) 2013 Elsevier Ltd. All rights reserved.

Résumé: We present a methodology to model the spatio-temporal variations of microbarom detections at a global scale. Our model combines the source term resulting from the non-linear ocean-wave interaction and a simplified description of the long-range infrasound propagation through the stratospheric waveguide. We compare model predictions with observations at infrasound stations of the International Monitoring System between 2008 and 2009. Our results show a first-order consistency between the observed and modelled trends of microbarom backazimuth detections for most stations. Taking into account stratospheric wind effect on the infrasound propagation systematically improves the fit between the observations the model predictions. However, correctly predicting patterns of weekly variation of detections turns out to be more challenging and would require further improving the source and the propagation models. Shortterm and regional quantitative comparisons could then be carried out based on the metrics developed in this study.

Résumé: The impact of engineered nanomaterials on plants, which act as a major point of entry of contaminants into trophic chains, is little documented. The foliar pathway is even less known than the soil-root pathway. However, significant inputs of nanoparticles (NPs) on plant foliage may be expected due to deposition of atmospheric particles or application of NP-containing pesticides. The uptake of Ag-NPs in the crop species Lactuca sativa after foliar exposure and their possible biotransformation and phytotoxic effects were studied. In addition to chemical analyses and ecotoxicological tests, micro X-ray fluorescence, micro X-ray absorption spectroscopy, time of flight secondary ion mass spectrometry and electron microscopy were used to localize and determine the speciation of Ag at sub-micrometer resolution. Although no sign of phytotoxicity was observed, Ag was effectively trapped on lettuce leaves and a thorough washing did not decrease Ag content significantly. We provide first evidence for the entrapment of Ag-NPs by the cuticle and penetration in the leaf tissue through stomata, for the diffusion of Ag in leaf tissues, and oxidation of Ag-NPs and complexation of Ag+ by thiol-containing molecules. Such type of information is crucial for better assessing the risk associated to Ag-NP containing products. (C) 2013 Elsevier B.V. All rights reserved.

Résumé: Full waveform inversion of ground-penetrating radar data is an emerging technique for the quantitative, high-resolution imaging of the near subsurface. Here, we present a 2-D frequency-domain full waveform inversion for the simultaneous reconstruction of the dielectric permittivity and of the electrical conductivity. The inverse problem is solved with a quasi-Newton optimization scheme, where the influence of the Hessian is approximated by the L-BFGS-B algorithm. This formulation can be considered to be fully multiparameter since it enables to update permittivity and conductivity values within the same descent step, provided we define scales of measurement through a reference permittivity, a reference conductivity, and an additional scaling factor. Numerical experiments on a benchmark from the literature demonstrate that the inversion is very sensitive to the parameter scaling, despite the consideration of the approximated Hessian that should correct for parameter dimensionalities. A proper scaling should respect the natural sensitivity of the misfit function and give priority to the parameter that has the most impact on the data (the permittivity, in our case). We also investigate the behaviour of the inversion with respect to frequency sampling, considering the selected frequencies either simultaneously or sequentially. As the relative imprint of permittivity and conductivity in the data varies with frequency, the simultaneous reconstruction of both parameters takes a significant benefit from broad frequency bandwidth data, so that simultaneous or cumulative strategies should be favoured. We illustrate our scaling approach with a realistic synthetic example for the imaging of a complex subsurface from on-ground multioffset data. Considering data acquired only from the ground surface increases the ill-posedness of the inverse problem and leads to a strong indetermination of the less-constrained conductivity parameters. A Tikhonov regularization can prevent the creation of high-wavenumber artifacts in the conductivity model that compensate for erroneous low-wavenumber structures, thus enabling to select model solutions. We propose a workflow for multiparameter imaging involving both parameter scaling and regularization. Optimal combinations of scaling factors and regularization weights can be identified by seeking regularization levels that exhibit a clear minimum of final data misfit with respect to parameter scaling. We confirm this workflow by inverting noise-contaminated synthetic data. In a surface-to-surface acquisition configuration, we have been able to reconstruct an accurate permittivity structure and a smooth version of the conductivity distribution, based entirely on the analysis of the data misfit with respect to parameter scaling, for different regularization levels.

Résumé: The Kromberg Formation (ca. 3432 Ma) in the Barberton Greenstone Belt, South Africa, contains well-preserved chert beds at the tops of turbidite deposits. At the interface, siltstone, which consist of Kfeldspar, K-mica, microquartz with minor lithic fragments and heavy minerals, grades into chert, which consists of microquartz and minor K-mica (<15%). K-feldspars show preserved twins typical of microcline, orthoclase and sanidine. Based on the heterogeneity of the clastic fraction (i.e. shape, size, nature), the lack of in situ metasomatic features (i.e. crystal overgrowths, silica replacement) and the continuity of the siliceous matrix through the siltstone-to-chert transition, we argue that (1) the clastic particles are detrital, (2) some were altered and metasomatized at their source, (3) in situ metasomatism was limited to minor seritization of K-feldspars, and (4) the silica is of primary origin and precipitated from ambient marine fluids. Our petrographic observations reinforce the model advocated by Rouchon and Orberger (2008) and Rouchon et al. (2009) for chert deposited in clastic-rich setting and we favor a formation of both the siltstones and cherts as chemico-clastic sediments. We argue for the contemporaneous deposition of clastic grains from turbiditity currents and precipitation of silica on phyllosilicate reactive surfaces, both in the water column and at the sediment water interface. As the rate of clastic sedimentation declined, the accumulation of silica flocs on suspended phyllosilicates first accompanied, then replaced the deposition of detrital grains, to form a siliceous ooze at the seafloor. Contrary to current interpretations for detritus-rich cherts, which invoke a secondary origin via Si- and K-metasomatism, we propose that the present model prevailed in a variety of Archean settings where fine-grained sediments were deposited. The composition of both the siltstone and chert reflects mainly the environment in which they formed. They are interpreted as mixtures of two main components: (1) silica, which contains extremely low concentrations of trace elements and contributes only SiO2 to the bulk composition, and (2) another phase that dominates the trace element composition. Here, K-mica and K-feldspar control the chemical signal and reflect a felsic source to the turbidites (i.e. Hooggenoeg dacites and volcaniclastics). (C) 2014 Elsevier B.V. All rights reserved.

Résumé: We used the ambient noise cross-correlation and stretching methods to calculate variations in seismic velocities in the region of Volcan de Colima, Mexico. More than 15 years of continuous records were processed, producing long time series of velocity variations related to volcanic activity, meteorological effects, and earthquakes. Velocity variations associated with eruptive activity are tenuous, which probably reflects the open state of the volcano during the study period. Fifteen events among 26 regional tectonic earthquakes produced sharp, temporary decreases in seismic velocities, which then recovered progressively following a linear trend as a function of the logarithm of time. For the 15 events, the amplitude of the perturbation increased almost linearly with the logarithm of the amplitude of the seismic waves that shook the edifice. The most dramatic apparent velocity variation was a drop of up to 2.6% during the nearby M7.4 Tecoman earthquake in 2003. In order to locate the perturbation in the horizontal plane we applied an inverse method based on the radiative transfer approximation. We also used an original approach based on the frequency dependence of velocity variations to estimate the depth of the perturbation. Our results show that the velocity variation was well localized in the shallow layers (< 800 m) of the volcano, with almost no variations occurring outside the edifice. We discuss several possible interpretations and conclude that the most plausible explanation for the velocity decreases is the nonlinear elastic behavior of the granular volcanic material and its mechanical softening induced by transient strains.

Résumé: Interfacial free energies often control heterogeneous nucleation of calcium carbonate (CaCO3) on mineral surfaces. Here we report an in situ experimental study of CaCO3 nucleation on mica (muscovite) and quartz, which allows us to obtain the interfacial energies governing heterogeneous nucleation. In situ grazing incidence small-angle X-ray scattering (GISAXS) was used to measure nucleation rates at different supersaturations. The rates were incorporated into classical nucleation theory to calculate the effective interfacial energies (alpha'). Ex situ Raman spectroscopy identified both calcite and vaterite as CaCO3 polymorphs; however, vaterite is the most probable heterogeneous nuclei mineral phase. The alpha' was 24 mJ/m(2) for the vaterite mica system and 32 mJ/m(2) for the vaterite quartz system. The smaller alpha' of the CaCO3 mica system led to smaller particles and often higher particle densities on mica. A contributing factor affecting alpha' in our system was the smaller structural mismatch between CaCO3 and mica compared to that between CaCO3 and quartz. The extent of hydrophilicity and the surface charge could not explain the observed CaCO3 nucleation trend on mica and quartz. The findings of this study provide new thermodynamic parameters for subsurface reactive transport modeling and contribute to our understanding of mechanisms where CaCO3 formation on surfaces is of concern.

Résumé: We present the first density model of Stromboli volcano (Aeolian Islands, Italy) obtained by simultaneously inverting land-based (543) and sea-surface (327) relative gravity data. Modern positioning technology, a 1 x 1 m digital elevation model, and a 15 x 15 m bathymetric model made it possible to obtain a detailed 3-D density model through an iteratively reweighted smoothness-constrained least-squares inversion that explained the land-based gravity data to 0.09 mGal and the sea-surface data to 5 mGal. Our inverse formulation avoids introducing any assumptions about density magnitudes. At 125 m depth from the land surface, the inferred mean density of the island is 2380 kg m(-3), with corresponding 2.5 and 97.5 percentiles of 2200 and 2530 kg m-3. This density range covers the rock densities of new and previously published samples of Paleostromboli I, Vancori, Neostromboli and San Bartolo lava flows. High-density anomalies in the central and southern part of the island can be related to two main degassing faults crossing the island (N41 and NM) that are interpreted as preferential regions of dyke intrusions. In addition, two low-density anomalies are found in the northeastern part and in the summit area of the island. These anomalies seem to be geographically related with past paroxysmal explosive phreato-magmatic events that have played important roles in the evolution of Stromboli Island by forming the Scari caldera and the Neostromboli crater, respectively. (C) 2014 Elsevier B.V. All rights reserved.

Résumé: The interaction of Se(IV) with natural pyrrhotite was investigated at pH conditions ranging from acidic to nearly neutral. The results indicate that the reduction rate can be described in terms of a pseudo-first order reaction. At pH similar to 4.0 to similar to 5.0, the rate decreased with increasing pH. Unexpectedly, at pH similar to 5.0, the rate increased with increasing reaction time. This response was also observed at pH similar to 6.0. Two different reaction mechanisms were proposed to explain pyrrhotite oxidation by Se(IV). Because pyrrhotite is acid-soluble and can be attacked by both Fe3+ and protons, direct reduction by the released aqueous sulfide dominates the reaction at low pH, whereas the cyclic oxidation of aqueous Fe2+ adsorbed on pyrrhotite surfaces becomes predominant at high pH. Phosphate, which can be irreversibly bound to Fe3+ intermediates even under acidic conditions, can significantly decrease the reaction rate by an order of magnitude at pH similar to 4.5. In contrast to the thermodynamic calculations and the predicted prevalence of FeSe based on previous reports of aqueous Se(IV) reduction by synthetic mackinawite or troilite, only Se(0) was observed as the reaction product in this study. This observation confirmed that a slow reaction favors the formation of Se(0) rather than iron selenides. (C) 2014 Elsevier B.V. All rights reserved.

Résumé: The lithospheric architecture of the Pyrenees is still uncertain and highly debated. Here, we provide new constraints from a high-resolution 3-D S-wave velocity model of the Pyrenees and the adjacent foreland basins. This model is obtained from ambient noise tomography on records of temporary and permanent seismic arrays installed in southwestern France and northern Spain. We first computed group velocity maps for Rayleigh waves in the 5 to 55 s period range using noise correlation stacks at 1500-8500 station pairs. As the crust is very heterogeneous, poor results were obtained using a single starting model in a linearized inversion of group velocity dispersion curves for the shear wave structure. We therefore built a starting model for each grid node by full exploration of the model space. The resulting 3-D shear wave velocity model is compared to data from previous geophysical studies as a validation test. Despite the poor sensitivity of surface waves to seismic discontinuities, the geometry of the top of the basement and the Moho depth are retrieved well, except along the Cantabrian coast. Major reflectors of the ECORS deep seismic sounding profiles in the central and western Pyrenees coincide with sharp velocity gradients in our velocity model. We retrieve the difference between the thicker Iberian crust and the thinner European crust, the presence of low-velocity material of the Iberian crust underthrust beneath the European crust in the central Pyrenees, and the structural dissymmetry between the South Pyrenean Zone and the North Pyrenean Zone at the shallow crustal level. In the Labourd-Maul,on-Arzacq region (western Pyrenees), there is a high S-wave velocity anomaly at 20-30 km in depth, which might explain the positive Bouguer anomaly of the Labourd Massif. This high-velocity lower crust, which is also detected beneath the Parentis area, might be an imprint of the Albian-Aptian rifting phase. The southeastern part of the Massif Central has an unusual velocity structure, with a very shallow Moho (21-25 km) above an uppermost mantle with anomalously low shear wave velocity.

Résumé: We present transient streaming potential data collected during falling-head permeameter tests performed on samples of two sands with different physical and chemical properties. The objective of the work is to estimate hydraulic conductivity (K) and the electrokinetic coupling coefficient (Cl) of the sand samples. A semi-empirical model based on the falling-head permeameter flow model and electrokinetic coupling is used to analyze the streaming potential data and to estimate K and Cl. The values of K estimated from head data are used to validate the streaming potential method. Estimates of K from streaming potential data closely match those obtained from the associated head data, with less than 10% deviation. The electrokinetic coupling coefficient was estimated from streaming potential vs. (1) time and (2) head data for both sands. The results indicate that, within limits of experimental error, the values of Cl estimated by the two methods are essentially the same. The results of this work demonstrate that a temporal record of the streaming potential response in falling-head permeameter tests can be used to estimate both K and Cl. They further indicate the potential for using transient streaming potential data as a proxy for hydraulic head in hydrogeology applications.

Résumé: A series of San Carlos olivine aggregates, sintered at high pressure and high temperature, with two different porosities (around 1 and 10%) and grain sizes (1-5 μm and 0-38 μm) were reacted at 300 degrees C and 500 bars in the presence of pure water. The reaction progress was monitored magnetically and the composition and distribution of the reaction products were analyzed at the end of each experiment. Brucite formation mainly occurred at the aggregate surface as a result of both congruent olivine dissolution and aqueous Mg and Si buffering by the reaction products, i.e. brucite and lizardite. The measured reaction progress did not exceed 2.6% after 290 d, which strongly contrasts with previous studies performed on San Carlos olivine powders (i.e., isolated grains in aqueous solution). Hence, limited water transport through the intergranular region of the aggregate drastically decreased the olivine surface area accessible to water and thus slowed down the whole serpentinization process. When extrapolated to peridotite relevant olivine grain sizes, our experimental results indicate that the water diffusion rate will become so slow that the first layer of primary minerals exposed to water within a mesh structure must fully react before the next mineral layer starts reacting (“layer by layer” mechanism). This type of reaction-transport mechanism is obviously not consistent with the micro-scale serpentine distribution in the mesh of oceanic peridotite samples, therefore additional water transport pathways are required. Cracks formed under extensional thermal stresses are good candidates since, in comparison to grain boundary or reaction-induced fractures, they are wide enough to drastically enhance water transport in oceanic peridotites and therefore account for the observed textures. The 'layer by layer' mechanism inferred here can only set a lower time bound for serpentinization completion. Assuming a mesh size of 1 mm and an initial grain size of 100 pm and considering a temperature ranging from 100 to 300 degrees C with permanent water saturation, completion should take place within 100-1000 yr. Surprisingly, this duration represents only 1 to 10% of the estimated timescale of the natural serpentinization process, emphasizing the central role played by water availability in the natural reaction process. (C) 2014 Elsevier B.V. All rights reserved.

Résumé: Principal component analysis (PCA) is a multivariate data analysis approach commonly used in X-ray absorption spectroscopy to estimate the number of pure compounds in multicomponent mixtures. This approach seeks to describe a large number of multicomponent spectra as weighted sums of a smaller number of component spectra. These component spectra are in turn considered to be linear combinations of the spectra from the actual species present in the system from which the experimental spectra were taken. The dimension of the experimental dataset is given by the number of meaningful abstract components, as estimated by the cascade or variance of the eigenvalues (EVs), the factor indicator function (IND), or the F-test on reduced EVs. It is shown on synthetic and real spectral mixtures that the performance of the IND and F-test critically depends on the amount of noise in the data, and may result in considerable underestimation or overestimation of the number of components even for a signal-to-noise (s/n) ratio of the order of 80 (sigma = 20) in a XANES dataset. For a given s/n ratio, the accuracy of the component recovery from a random mixture depends on the size of the dataset and number of components, which is not known in advance, and deteriorates for larger datasets because the analysis picks up more noise components. The scree plot of the EVs for the components yields one or two values close to the significant number of components, but the result can be ambiguous and its uncertainty is unknown. A new estimator, NSS-stat, which includes the experimental error to XANES data analysis, is introduced and tested. It is shown that NSS-stat produces superior results compared with the three traditional forms of PCA-based component-number estimation. A graphical user-friendly interface for the calculation of EVs, IND, F-test and NSS-stat from a XANES dataset has been developed under LabVIEW for Windows and is supplied in the supporting information. Its possible application to EXAFS data is discussed, and several XANES and EXAFS datasets are also included for download.

Résumé: Foreshock activity sometimes precedes the occurrence of large earthquakes, but the nature of this seismicity is still debated, and whether it marks transient deformation and/or slip nucleation is still unclear. We here study at the worldwide scale how foreshock occurrence affects the postseismic phase and find a significant positive correlation between foreshock and aftershock activities: earthquakes preceded by accelerating seismicity rates produce 40% more aftershocks on average, and the length of the aftershock zone after 20 days is 20% larger. These observations cannot be reproduced by standard earthquake clustering models that predict the accelerating pattern of foreshock occurrence but not its impact on aftershock activity. This strongly suggests that slow deformation transients, possibly related to episodic creep, could initiate prior to the main shock and extend past the coseismic phase, resulting in compound ruptures that include a very long period (up to tens of days) component.

Résumé: Short lead(II) alkanoates, from propionate to heptanoate, show a very intricate and reversible thermal behaviour, presenting crystalline phases and three different glass states (regular or amorphous, liquid crystal and rotator glasses) with different degrees of ordering depending on the alkyl chain length. A thorough thermal study was carried out in order to study the different phases and to analyze the thermodynamic parameters. The crystal structures of the compounds were solved by X-ray diffraction, showing similar arrangements of the 2D molecular stacking. PDF analyses of the local order in the glass structures showed shorter first neighbour lead-lead interatomic distances than in the crystalline structures. This allows establishment of a direct relationship between the structure and optical properties. Luminescence properties are, in fact, impressively enhanced in the glass states, passing from weak fluorescence at 77 K in the crystal phase to strong phosphorescence in the frozen glasses, which persists at room temperature. The high variability and the structure-property relationship described here pave the way for the design of materials with varied luminescence properties based on fine-tuning of their local structure.

Résumé: Volcanic eruptions are driven by the growth of gas bubbles in magma. Bubbles grow when dissolved volatile species, principally water, diffuse through the silicate melt and exsolve at the bubble wall. On rapid cooling, the melt quenches to glass, preserving the spatial distribution of water concentration around the bubbles (now vesicles), offering a window into pre-eruptive conditions. We measure the water distribution around vesicles in experimentally-vesiculated samples, with high spatial resolution. We find that, contrary to expectation, water concentration increases towards vesicles, indicating that water is resorbed from bubbles during cooling; textural evidence suggests that resorption occurs largely before the melt solidifies. Speciation data indicate that the molecular water distribution records resorption, whilst the hydroxyl distribution records earlier decompressive growth. Our results challenge the emerging paradigm that resorption indicates fluctuating pressure conditions, and lay the foundations for a new tool for reconstructing the eruptive history of natural volcanic products. (C) 2014 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/3.0/).

Résumé: Constraining the timing of initial collision is critical for understanding how long-term plate convergence is accommodated at collisional plate boundaries. Reevaluation of the age of collision from orogenic thermal evolution requires distinguishing the onset of submarine continental accretion from earlier oceanic subduction and subsequent subaerial orogenic evolution. We present new thermo-chronological constraints from the first age-elevation relationship transect in Taiwan and zircon and apatite fission-track ages from sediments in the western foreland. Our data reveal the onset of cooling from at least 7.1 +/- 1.3 Ma, at a minimum rate of 21 degrees C/m.y., in the submarine sedimentary wedge followed by a marked acceleration of subaerial exhumation after ca. 3.2 +/- 0.6 Ma at an average rate of 1.7 km/m.y. Our data reflect the effect of margin architecture as Taiwan evolved from submarine accretion of the distal extremely thinned continental margin to crustal thickening of the proximal margin and orogenic development.

Résumé: When applied to wave propagation modeling in anisotropic media, Perfectly Matched Layers (PML) exhibit instabilities. Incoming waves are amplified instead of being absorbed. Overcoming this difficulty is crucial as in many seismic imaging applications, accounting accurately for the subsurface anisotropy is mandatory. In this study, we present the SMART layer method as an alternative to PML approach. This method is based on the decomposition of the wavefield into components propagating inward and outward the domain of interest. Only outgoing components are damped. We show that for elastic and acoustic wave propagation in Transverse Isotropic media, the SMART layer is unconditionally dissipative: no amplification of the wavefield is possible. The SMART layers are not perfectly matched, therefore less accurate than conventional PML. However, a reasonable increase of the layer size yields an accuracy similar to PML. Finally, we illustrate that the selective damping strategy on which is based the SMART method can prevent the generation of spurious S-waves by embedding the source in a small zone where only S-waves are damped. (C) 2014 Elsevier Inc. All rights reserved.

Résumé: Groundwater flow in karst includes exchange of water between large fractures, conduits, and the surrounding porous matrix, which impacts both water quality and quantity. Electrical resistivity tomography combined with end-member mixing analysis (EMMA) and numerical flow and transport modeling was used to study mixing of karst conduit and matrix waters to understand spatial and temporal patterns of mixing during high flow and base flow conditions. To our knowledge, this is the first time EMMA and synthetic geophysical simulations have been combined. Here we interpret an 8 week time-lapse electrical resistivity data set to assess groundwater-surface mixing. We simulate flow between the karst conduits and the porous matrix to determine fractions of water recharged to conduits that has mixed with groundwater stored in the pore space of the matrix using a flow and transport model in a synthetic time-lapse resistivity inversion. Comparing the field and synthetic inversions, our results enable us to estimate exchange dynamics, spatial mixing, and flow conditions. Results showed that mixing occurred at a volumetric flux of 56 m(3)/d with a dispersivity around 1.69 m during the geophysical experiment. For these conditions, it was determined that conduit water composition ranged from 75% groundwater during base flow conditions to less than 50% groundwater in high flow conditions. Though subject to some uncertainties, the time-lapse inversion process provides a means to predict changing hydrologic conditions, leading to mixing of surface water and ground water and thus changes to water quantity and quality, as well as potential for water-rock reactions, in a semiconfined, sink-rise system. Key Points <list id=“wrcr20812-list-0001” list-type=“bulleted”> <list-item id=“wrcr20812-li-0001”>ERT and EMMA are used to determine karst groundwater surface water mixing <list-item id=“wrcr20812-li-0002”>Time-lapse inversion of ERT detects changing hydrologic conditions <list-item id=“wrcr20812-li-0003”>Characterization of karst mixing with field and synthetic inversions

Résumé: In the present study, we demonstrate that ordered dolomite can be precipitated via simultaneous dissolution of calcite and magnesite under hydrothermal conditions (from 100 to 200 degrees C). The temperature and high-carbonate alkalinity have significantly copromoted the dolomite formation. For example, when high-purity water was initially used as interacting fluid, only a small proportion of disordered dolomite was identified at 200 degrees C from XRD patterns and FESEM observations. Conversely, a higher proportion of ordered dolomite, i.e., clear identification of superstructure ordering reflections in XRD patterns, was determined when high-carbonate alkalinity solution was initially used in our system at the same durations of reaction. For this latter case, the dolomite formation is favorable therefrom 100 degrees C and two kinetic steps were identified: (1) protodolomite formation after about 5 days of reaction, characterized by rounded submicrometric particles from FESEM observations and by the absence of superstructure ordering reflections at 22.02 (101), 35.32 (015), 43.80 (021), etc. 2 theta in XRD patterns; (2) protodolomite to dolomite transformation, probably produced by a coupled dissolution-recrystallization process. Herein, the activation energy was estimated to be 29 kJ/mol by using a conventional Arrhenius linear equation. This study provides new experimental conditions to which dolomite could be formed in hydrothermal systems. Temperature and carbonate alkalinity are particularly key physicochemical parameters to promote dolomite precipitation in abiotic systems.

Résumé: This study presents a depth inversion of Scholte wave group and phase velocity maps obtained from cross-correlation of 6.5 hr of noise data from the Valhall Life of Field Seismic network. More than 2 600 000 vertical-vertical component cross-correlations are computed from the 2320 available sensors, turning each sensor into a virtual source emitting Scholte waves. We used a traditional straight-ray surface wave tomography to compute the group velocity map. The phase velocity maps have been computed using the Eikonal tomography method. The inversion of these maps in depth are done with the Neighbourhood Algorithm. To reduce the number of free parameters to invert, geological a priori information are used to propose a power-law 1-D velocity profile parametrization extended with a gaussian high-velocity layer where needed. These parametrizations allowed us to create a high-resolution 3-D S-wave model of the first 600 m of the Valhall subsurface and to precise the locations of geological structures at depth. These results would have important implication for shear wave statics and monitoring of seafloor subsidence due to oil extraction. The 3-D model could also be a good candidate for a starting model used in full-waveform inversions.

Résumé: The Curiosity rover has analyzed abundant light-toned fracture-fill material within the Yellowknife Bay sedimentary deposits. The ChemCam instrument, coupled with Mastcam and ChemCam/Remote Micro Imager images, was able to demonstrate that these fracture fills consist of calcium sulfate veins, many of which appear to be hydrated at a level expected for gypsum and bassanite. Anhydrite is locally present and is found in a location characterized by a nodular texture. An intricate assemblage of veins crosses the sediments, which were likely formed by precipitation from fluids circulating through fractures. The presence of veins throughout the entire similar to 5 m thick Yellowknife Bay sediments suggests that this process occurred well after sedimentation and cementation/lithification of those sediments. The sulfur-rich fluids may have originated in previously precipitated sulfate-rich layers, either before the deposition of the Sheepbed mudstones or from unrelated units such as the sulfates at the base of Mount Sharp. The occurrence of these veins after the episodes of deposition of fluvial sediments at the surface suggests persistent aqueous activity in relatively nonacidic conditions.

Résumé: We report here the first inventory of mercury (Hg) contamination in the region of Kedougou, Senegal, where mercury (Hg) is used for gold amalgamation in artisanal small-scale gold mining (ASGM) activities. Sediment cores were sampled during the dry and wet seasons, and at different locations along the Gambia River in the Kedougou region to evaluate the spatio-temporal trends of Hg distribution related to ASGM activities. The sediment samples were analysed for total mercury and trace element concentrations (Cr, Cu, Zn, Pb, As, and Sc). The results highlight high concentrations of total Hg (reaching up to 9.9 mg kg(-1)) in sediment cores sampled in the vicinity of ASGM operations, with the highest values reported for the dry season. The calculation of the Hg Enrichment Factor (EF) using Sc as refractory element confirms high enrichments around active sites of gold mining being proportional to the density of miners and the duration of mining activities. The Hg spatial distribution reveals a very local contamination and a limited downstream dispersion of the contaminant. The high Hg concentrations obtained at ASGM sites largely exceed the Sediment Quality Guidelines (SQGs) and the Probable effect concentration (PEC) for the Protection of Aquatic Life recommendation. Our results demonstrate that the ASGM activities discharged large amounts of Hg into the Gambia River ecosystem and that the accumulation of Hg in sediments may represent a significant human health risk and a potential source of toxicity for aquatic organisms. (c) 2014 Elsevier B.V. All rights reserved.

Résumé: Along the western margin of South America, plate convergence is accommodated by slip on the subduction interface and deformation of the overriding continent(1-6). In Chile(1-4), Bolivia(6), Ecuador and Colombia(5, 7), continental deformation occurs mostly through the motion of discrete domains, hundreds to thousands of kilometres in scale. These continental slivers are wedged between the Nazca and stable South American plates. Here we use geodetic data to identify another large continental sliver in Peru that is about 300-400 km wide and 1,500 km long, which we call the Inca Sliver. We show that movement of the slivers parallel to the subduction trench is controlled by the obliquity of plate convergence and is linked to prominent features of the Andes Mountains. For example, the Altiplano is located at the boundary of converging slivers at the concave bend of the central Andes, and the extending Gulf of Guayaquil is located at the boundary of diverging slivers at the convex bend of the northern Andes. Motion of a few large continental slivers therefore controls the present-day deformation of nearly the entire Andes mountain range. We also show that a 1,000-km-long section of the plate interface in northern Peru and southern Ecuador slips predominantly aseismically, a behaviour that contrasts with the highly seismic neighbouring segments. The primary characteristics of this low-coupled segment are shared by similar to 20% of the subduction zones in the eastern Pacific Rim.

Résumé: We analyse the statistics of phase fluctuations of seismic signals obtained from a temporary small aperture array deployed on a volcano in the French Auvergne. We demonstrate that the phase field satisfies Circular Gaussian statistics. We then determine the scattering mean free path of Rayleigh waves from the spatial phase decoherence. This phenomenon, observed for diffuse wavefields, is found to yield a good approximation of the scattering mean free path. Contrary to the amplitude, spatial phase decoherence is free from absorption effects and provides direct access to the scattering mean free path.

Résumé: The anisotropic quantum Heisenberg model with Curie-Weiss-type interactions is studied analytically in several variants of the microcanonical ensemble. (Non)equivalence of microcanonical and canonical ensembles is investigated by studying the concavity properties of entropies. The microcanonical entropy is obtained as a function of the energy and the magnetization vector in the thermodynamic limit. Since, for this model, is uniquely determined by , the same information can be encoded either in or . Although these two entropies correspond to the same physical setting of fixed and , their concavity properties differ. The entropy , describing the model at fixed total energy and in a homogeneous external magnetic field of arbitrary direction, is obtained by reduction from the nonconcave entropy . In doing so, concavity, and therefore equivalence of ensembles, is restored. has nonanalyticities on surfaces of co-dimension 1 in the -space. Projecting these surfaces into lower-dimensional phase diagrams, we observe that the resulting phase transition lines are situated in the positive-temperature region for some parameter values, and in the negative-temperature region for others. In the canonical setting of a system coupled to a heat bath of positive temperatures, the nonanalyticities in the microcanonical negative-temperature region cannot be observed, and this leads to a situation of effective nonequivalence even when formal equivalence holds.

Résumé: The computational burden of frequency-domain full-waveform inversion (FWI) of wide-aperture fixed-spread data is conventionally reduced by limiting the inversion to a few discrete frequencies. In this framework, frequency-domain seismic modeling is performed efficiently for multiple sources by solving the linear system resulting from the discretization of the time-harmonic wave equation with the massively parallel sparse direct solver. Frequency-domain seismic modeling based on the sparse direct solver (DSFDM) requires specific design finite-difference stencils of compact support to minimize the computational cost of the lower-upper decomposition of the impedance matrix in terms of memory demand and floating-point operations. A straightforward adaptation of such finite-difference stencil, originally developed for the (isotropic) acoustic-wave equation, is proposed to introduce vertical transverse isotropy (VTI) in the modeling without any extra computational cost. The method relies on a fourth-order wave equation, which is decomposed as the sum of a second-order elliptic-wave equation plus an anellipticity correction term. The stiffness matrix of the elliptic-wave equation is easily built from the isotropic stiffness matrix by multiplying its coefficients with factors that depend on Thomsen's parameters, whereas the anelliptic term is discretized with a parsimonious second-order staggered-grid stencil. Validation of DSFDM against finite-difference time-domain modeling performed in various synthetic models shows that a discretization rule of four grid points per minimum wavelength provides accurate DSFDM solutions. Moreover, comparison between real data from the Valhall field and DSFDM solutions computed in a smooth VTI subsurface model supports that the method can be used as a fast and accurate modeling engine to perform multiparameter VTI FWI of fixed-spread data in the viscoacoustic approximation.

Résumé: Trace elements and organic carbon inputs to the Mediterranean Sea from an urbanised area (Marseille City) were studied and characterised during flood events. Inputs were brought to the sea by two small coastal rivers whose waters were mixed together and also with treated wastewaters (TWW) just before discharge. The monitoring of the rivers during flood events showed the high temporal dynamics of water flow, suspended particulate matter (SPM), organic carbon and trace metals concentrations, typical of small coastal Mediterranean rivers and requiring an appropriate sampling strategy. Dissolved/particulate partition coefficient (log Kd) in rivers during floods remained quasi-constant for a given trace element, but differed from one element to another according to their affinity toward particles. Because of high SPM concentrations, trace elements were mainly brought to the sea during floods as particles, despite a weaker affinity for particles when compared to baseflow conditions for all studied elements but Pb. If the contribution of TWW dominated the elements baseflow discharge to the coastal zone, rivers outweighed during floods. When discharged to the sea, most trace elements underwent partial desorption in the salinity gradient, especially at highest salinity. Laboratory desorption experiment results were consistent with field data and showed slower desorption kinetics than in baseflow conditions, suggesting that trace elements desorption rates from particles are slower than sedimentation rates. With regard to heavy particles, it results in a potential impact of the sediment on benthic organisms and a possible further desorption after sediment resuspension events. With regard to light particles, it results to a possible additional desorption during offshore transport. (C) 2014 Elsevier B.V. All rights reserved.

Résumé: Many coasts feature sequences of Quaternary and Neogene shorelines that are shaped by a combination of sea-level oscillations and tectonics. We compiled a global synthesis of sea-level changes for the following highstands: MIS 1, MIS 3, MIS 5e and MIS 11. Also, we date the apparent onset of sequences of paleoshorelines either from published data or tentatively extrapolating an age for the uppermost, purported oldest shoreline in each sequence. Including the most documented MIS 5e benchmark, we identify 926 sequences out of which 185 also feature Holocene shorelines. Six areas are identified where elevations of the MIS 3 shorelines are known, and 31 feature elevation data for MIS 11 shorelines. Genetic relationships to regional geodynamics are further explored based on the elevations of the MIS 5e benchmark. Mean apparent uplift rates range from 0.01 0.01 mm/yr (hotspots) to 1.47 0.08 mm/yr (continental collision). Passive margins appear as ubiquitously uplifting, while tectonic segmentation is more important on active margins. From the literature and our extrapolations, we infer ages for the onset of formation for -180 coastal sequences. Sea level fingerprinting on coastal sequences started at least during mid Miocene and locally as early as Eocene. Whether due to the changes in the bulk volume of seawater or to the temporal variations in the shape of ocean basins, estimates of eustasy fail to explain the magnitude of the apparent sea level drop. Thus, vertical ground motion is invoked, and we interpret the longlasting development of those paleoshore sequences as the imprint of glacial cycles on globally uplifted margins in response to continental compression. The geomorphological expression of the sequences matches the amplitude and frequency of glacial cyclicity. From middle Pleistocene to present-day, moderately fast (100,000 yrs) oscillating sea levels favor the development of well identified strandlines that are distinct from one another. Pliocene and Lower Pleistocene strandlines associated with faster cyclicity (40,000 yrs) are more compact and easily merge into rasas, whereas older Cenozoic low-frequency eustatic changes generally led to widespread flat-lying coastal plains.(c) 2014 Elsevier B.V. All rights reserved.

Résumé: We model seismic and aseismic slip on the Japan megathrust in the area of the M(w)9.0, 2011 Tohoku-Oki earthquake based on daily time series from 400 GPS stations of the GEONET network and campaign measurements of six sea floor displacements. The coseismic and postseismic slip distributions are inverted simultaneously using principal component analysis-based inversion method (PCAIM). Exploring a wide range of boundary conditions and regularization constraints, we found the coseismic slip distribution to be quite compact with a peak slip between 30 and 50 m near the trench. Our model shows deep afterslip fringing the downdip edge of the coseismic rupture but also a dominant zone of shallow afterslip. Afterslip over the first 279 days following the main shock represents about 40% of the coseismic moment. We compare the coseismic and postseismic models with an interseismic coupling model derived from inland and sea bottom measurements determined in a self-consistent manner. Assuming that seismic and aseismic slip had to match the long-term slip rate along the megathrust, the recurrence time of Mw9.0 earthquakes is estimated to 100-300 years, while historical and paleotsunami records suggest a return period more of the order of 1000 years. The discrepancy is smaller if the shallower portion of the megathrust is assumed to produce both aseismic slip, as the afterslip model suggests, and seismic slip during occasional large tsunamigenic earthquakes.

Résumé: We refer to periods of subduction strain accumulation beneath the Mentawai Islands, Sumatra, as supercycles, because each culminates in a series of partial ruptures of the megathrust in its final decades. The finale of the previous supercycle comprised two giant earthquakes in 1797 and 1833 and whatever happened in between. This behavior between two great ruptures has implications for how the megathrust will behave between its more recent partial failure, during the M-w 8.4 earthquake of 2007, and subsequent large ruptures. We synthesize previously published coral microatoll records and a large new coral data set to constrain not only these two giant ruptures but also the intervening interseismic megathrust behavior. We present detailed maps of coseismic uplift during the two earthquakes and of interseismic deformation during the periods 1755-1833 and 1950-2000, as well as models of the corresponding slip and coupling on the underlying megathrust. The large magnitudes we derive (M-w 8.6-8.8 for 1797 and M-w 8.8-8.9 for 1833) confirm that the 2007 earthquakes released only a fraction of the moment released during the previous rupture sequence. Whereas megathrust behavior leading up to the 1797 and 2007 earthquakes was similar and comparatively simple, behavior between 1797 and 1833 was markedly different and more complex: several patches of the megathrust became weakly coupled following the 1797 earthquake. We conclude that while major earthquakes generally do not involve rupture of the entire Mentawai segment, they may significantly change the state of coupling on the megathrust for decades to follow, influencing the progression of subsequent ruptures.

Résumé: We report on theoretical predictions of the decorrelation and phase-shift of coda waves induced by local changes in multiple scattering media. Using the multiple scattering formalism, we show that both expressions (decorrelation and phase-shift) involve a same sensitivity kernel based on the intensity transport in the medium. We compare the kernels based on the diffusion approximation with the ones based on the radiative transfer approximation, showing that the latter is more accurate at short times or for changes located close to the source or the receiver. We also perform a series of numerical simulations of wave propagation (finite differences) to validate our models in different configurations.

Résumé: In this paper, 3 years of surface displacement measurements obtained by space-borne synthetic aperture radar (SAR) observations are presented over the Argentiere glacier in the Mont-Blanc massif, France. This temperate glacier is instrumented by a network of four Global Positioning System (GPS) stations used as ground truth. Thirty-eight pairs of descending and ascending high-resolution TerraSAR-X (TSX) acquisitions covering the study region are used to derive displacement fields at 11-day intervals in spring and summer 2009 and summer 2011. The combination of ascending and descending pairs acquired over the same period allows 3-D displacement fields to be inverted. Our SAR analysis quantifies displacement rates from 10 cm/day at the altitude of 2600 m to 30 cm/day at the altitude of 1800 m. Time series of SAR displacement results are compared with in situ GPS measurements of a continuous station set up at the altitude of 2441 m. Both data sources present displacement of the same order of magnitude with an average value of 20 cm/day in 3-D and show intra-seasonal variabilities, with fast accelerations over short time intervals.

Résumé: The NASA Stardust mission used silica aerogel slabs to slowly decelerate and capture impinging cosmic dust particles for return to Earth. During this process, impact tracks are generated along the trajectory of the particle into the aerogel. It is believed that the morphology and dimensions of these tracks, together with the state of captured grains at track termini, may be linked to the size, velocity, and density of the impacting cosmic dust grain. Here, we present the results of laboratory hypervelocity impact experiments, during which cosmic dust analog particles (diameters of between 0.2 and 0.4 μm), composed of olivine, orthopyroxene, or an organic polymer, were accelerated onto Stardust flight-spare low-density (approximately 0.01 g cm(-3)) silica aerogel. The impact velocities (3-21 km s(-1)) were chosen to simulate the range of velocities expected during Stardust's interstellar dust (ISD) collection phases. Track lengths and widths, together with the success of particle capture, are analyzed as functions of impact velocity and particle composition, density, and size. Captured terminal particles from low-density organic projectiles become undetectable at lower velocities than those from similarly sized, denser mineral particles, which are still detectable (although substantially altered by the impact process) at 15 km s(-1). The survival of these terminal particles, together with the track dimensions obtained during low impact speed capture of small grains in the laboratory, indicates that two of the three best Stardust candidate extraterrestrial grains were actually captured at speeds much lower than predicted. Track length and diameters are, in general, more sensitive to impact velocities than previously expected, which makes tracks of particles with diameters of 0.4 μm and below hard to identify at low capture speeds (<10 km s(-1)). Therefore, although captured intact, the majority of the interstellar dust grains returned to Earth by Stardust remain to be found.

Résumé: Phytomanagement could be a viable alternative in areas polluted with wastes from chromium-using industries. This study investigated the ability of Silene vulgaris to take up Cr(III) and Cr(VI) with special attention on the mechanism used by this species to tolerate high doses of Cr(VI). Plants were grown semihydroponically with different concentrations of either Cr(III) or Cr(VI) . A combination of synchroton X-ray spectroscopic techniques, scanning electron and light microscopy and infrared spectroscopy were used to determine the distribution and speciation of Cr. S. vulgaris accumulated more Cr when grown with Cr(VI) resulting in an overall reduction in biomass. Starch accumulation in leaves may be attributed to an impartment between carbon utilization and assimilation resulted from stunted plant growth but not the complete inhibition of photosynthesis indicating that S. vulgaris possess tolerance mechanisms that allows it to survive in Cr(VI) rich environments. These primary tolerance mechanisms are (a) the total reduction of Cr(VI) to Cr(III) in the rhizosphere or just after uptake in the fine lateral root tips and (b) chelation of Cr(III) to the cell wall both of which reduce metal interference with critical cell functions., These mechanisms make S. vulgaris suitable for in situ remediation of Cr polluted soils.

Résumé: We reanalyzed the surface displacements observed at the Lazufre volcanic complex in the Southern Andean Central Volcanic Zone using GPS measurements made between 2006 and 2008 and a large InSAR data set. We performed a detailed spatiotemporal analysis of the displacements using a principal component analysis inversion method (PCAIM). The PCAIM reveals a source with no significant changes in shape and dimension and with a remarkably linear strength increase over the whole period of observation (i.e., 2003-2010). Then we used a three-dimensional mixed boundary element method (MBEM) to invert the first component of surface displacement as obtained from PCAIM. We explored a continuum of geometries from a shallow elliptic crack to a deep massive truncated elliptical cone that could represent a sill or a large magma chamber, respectively. The best models indicate a large flat-topped source with a roof area between 40 and 670 km(2) and a depth of between 2 and 14 km below ground surface. Lastly, on the basis of the limited data available for the thermomechanical structure of the crust in the Southern Andean Central Volcanic Zone, we consider some possible scenarios to explain the spatial and temporal pattern of displacements at Lazufre.

Résumé: We present an analysis of the M-w = 5.3 earthquake that occurred in the Southeast Indian Ridge on 2010 February 11 using USArray data. The epicentre of this event is antipodal to the USArray, providing us with an opportunity to observe in details the antipodal focusing of seismic waves in space and time. We compare the observed signals with synthetic seismograms computed for a spherically symmetric earth model (PREM). A beamforming analysis is performed over the different seismic phases detected at antipodal distances. Direct spatial snapshots of the signals and the beamforming results show that the focusing is well predicted for the first P-wave phases such as PKP or PP. However, converted phases (SKSP, PPS) show a deviation of the energy focusing to the south, likely caused by the Earth's heterogeneity. Focusing of multiple S-wave phases strongly deteriorates and is barely observable.

Résumé: The seismoelectric method is based on the interpretation of the electrical field associated with the conversion of mechanical to electromagnetic energy during the propagation of seismic waves in heterogeneous porous media. We propose an extension of a poroacoustic model that takes into account fluid flow and the effect of saturation. This model is coupled with an electrokinetic model accounting for the effect of saturation and in agreement with available experimental data in sands and carbonate rocks. We also developed new scaling laws for the permeability, the streaming potential coupling coefficient and the capillary entry pressure of porous media. The theory is developed for frequencies much below the critical frequency at which inertial effects starts to dominate in the Navier-Stokes equation (>10 kHz). The equations used to compute the propagation of the P waves and the seismoelectric effect in unsaturated condition are solved with finite elements using triangular meshing. We demonstrate the usefulness of a recently developed technique, seismoelectric beamforming, to localize saturation fronts by focusing seismic waves and looking at the resulting seismoelectric conversions. This method is applied to a cross-hole problem showing how a saturation front characterized by a drop in the electrical conductivity and compressibility is responsible for seismoelectric conversions. These conversions can be used, in turn, to determine the position of the front over time.

Résumé: Creep processes may relax part of the tectonic stresses in active faults, either by continuous or episodic processes. The aim of this study is to obtain a better understanding of these creep mechanisms and the manner in which they change in time and space. Results are presented from microstructural studies of natural samples collected from San Andreas Fault Observatory at Depth borehole drilled through the San Andreas Fault, which reveal the chronology of the deformation within three domain types. (i) A relatively undeformed zone of the host rock reflects the first step of the deformation process with fracturing and grain indentations showing the coupling between fracturing and pressure solution. (ii) Shear deformation development that associates fracturing and solution cleavage processes leads to profound changes in rock composition and behavior with two types of development depending on the ratio between the amount of dissolution and deposition: abundant mineral precipitation strengthens some zones while pervasive dissolution weakens some others, (iii) zones with mainly dissolution trended toward the present-day creeping zones thanks to both the passive concentration of phyllosilicates and their metamorphic transformation into soft minerals such as saponite. This study shows how interactions between brittle and viscous mechanisms lead to widespread transformation of the rocks and how a shear zone may evolve from a zone prone to earthquakes and postseismic creep to a zone of steady state creep. In parallel, the authors discuss how the creeping mechanism, mainly controlled by the very low friction of the saponite in the first 3-4km depth, may evolve with depth.

Résumé: The El Oro metamorphic province of SW Ecuador is a composite massif made of juxtaposed terranes of both continental and oceanic affinity that has been located in a fore-arc position since Late Paleozoic times. Various geochemical, geochronological, and metamorphic studies have been undertaken on the El Oro metamorphic province, providing an understanding of the origin and age of the distinct units. However, the internal structures and geodynamic evolution of this area remain poorly understood. Our structural analysis and thermal modeling in the El Oro metamorphic province show that this fore-arc zone underwent four main geological events. (1) During Triassic times (230-225Ma), the emplacement of the Piedras gabbroic unit at crustal-root level (similar to 9kbar) triggered partial melting of the metasedimentary sequence under an E-W extensional regime at pressure-temperature conditions ranging from 4.5 to 8.5kbar and from 650 to 900 degrees C for the migmatitic unit. (2) At 226Ma, the tectonic underplating of the Arenillas-Panupali oceanic unit (9kbar and 300 degrees C) thermally sealed the fore-arc region. (3) Around the Jurassic-Cretaceous boundary, the shift from trench-normal to trench-parallel subduction triggered the exhumation and underplating of the high-pressure, oceanic Raspas Ophiolitic Complex (18kbar and 600 degrees C) beneath the El Oro Group (130-120Ma). This was followed by the opening of a NE-SW pull-apart basin, which tilted the massif along an E-W subhorizontal axis (110Ma). (4) In Late Cretaceous times, an N-S compressional event generated heterogeneous deformation due to the presence of the Cretaceous Celica volcanic arc, which acted as a buttress and predominantly affected the central and eastern part of the massif.

Résumé: Filamentous, multicellular bacteria of the Desulfobulbaceae family form a biogeobattery in marine sediments by mediating an electric coupling between sulfide oxidation in deeper anoxic layers and oxygen reduction at the sediment surface. The electric fields generated in such biogeobatteries were determined with microelectrodes and correlated well with geochemical indicators of microbial electrogenic activity in the top 2 cm of the sediment. The electric fields collapsed within less than a minute when oxygen was removed and reestablished when oxygen was reintroduced, demonstrating a direct coupling between aerobic microbial activity and presence of electric fields. Modeling of the electric potential distribution was used to quantify and localize electron sources and sinks. Most anodic oxidation was confined to the lower part of the anoxic zone with nondetectable free sulfide, suggesting that iron sulfide was the main sulfide source and had already been depleted in the upper part of the anoxic zone. The flow of electrons from anoxic sediment layers fuelled up to 80% of the aerobic activity of the sediment. Responses of the electric field to the presence and absence of nitrate, nitrite, or nitrous oxide in the overlying water readily demonstrated potentials for cathodic reduction of nitrite or nitric oxide and possibly nitrate but not nitrous oxide.

Résumé: We use ambient noise cross-correlations to monitor small but reliable changes in seismic velocities and to analyse non-volcanic tremor (NVT) intensities during the slow slip event (SSE) that occurred in 2009 and 2010 in Guerrero. We test the sensitivity of the seismic velocity to strain variations in absence of strong motions. The 2009-2010 SSE presents a complex slip sequence with two subevents occurring in two different portions of the fault. From a seismic array of 59 seismometers, installed in small antennas, we detect a velocity drop with maximum amplitude at the time of the first subevent. We analyse the velocity change at different period bands and observe that the velocity perturbation associated with the SSE maximizes for periods longer than 12 s. Then a linearized inversion of the velocity change measured at different period bands is applied in order to determine the depth of the portion of the crust affected by this perturbation. No velocity change in the first 10 km is detected. Below, the velocity perturbation increases with depth, affecting the middle and lower crust. Finally, we compute the transient deformation produced by the SSE in an elastic model using the slip evolution recovered from the inversion of continuous GPS. The comparison between the velocity changes and the deformation suggests that the velocity change is correlated with the strain rate rather than with the strain. This result is similar to what was observed during the 2006 SSE in the same region and suggests a non-linear behaviour of the crust. The velocity changes can be interpreted together with other observables such as NVTs. During the 2009-2010 SSE we measure NVT activity using continuous seismic records filtered between 2 and 8 Hz. We observe a correlation between velocity changes (for period band greater than 14 s) and tremor activity whereas no correlation exists between velocity changes and seismic noise energy measured at long periods. These observations suggest that both seismic velocity change and NVT can be used as indication of transient deformation at depth.

Résumé: A possible risk of geomechanical nature related to deep injection of CO2 is the shear reactivation of faults, hence potentially leading to the creation of new leakage pathways and eventually inducing earthquakes felt at the surface. Current practices to evaluate fault stability in the domain of CO2 storage still remain limited regarding two issues: 1. Faults are complex and heterogeneous geological systems, which do not correspond to discrete surfaces as already postulated by many authors. Reservoir-scale faults in a priori low-deformed reservoirs targeted for CO2 storage can present high complex architecture, which might influence the hydro-mechanical behaviour of the fault system; 2. Chemical interactions (dissolution and precipitation processes, chemically-induced weakening, etc.) between CO2-enriched brine and the minerals constituting the fault zone can affect the mechanical stability and the transport properties of the faulted/fractured system. The research project FISIC (www.anr-fisic.fr, funded by the French National research Agency) intends to overcome those limitations by accurately modelling the hydro-chemo-mechanical complexity of a fault zone. The main goal is to improve the stability analysis of a fault both undertaking pressure increase and alteration due to the presence of an acidic fluid. The progress of this research project is presented here.

Résumé: We develop a new technique for monitoring temporal changes in fault zone environments based on cross-correlation of earthquake waveforms recorded by pairs of stations. The method is applied to waveforms of similar to 10 000 earthquakes observed during 100 d around the 1999 M 7.1 Duzce mainshock by a station located in the core damage zone of the North Anatolian Fault and a nearby station. To overcome clock problems, the correlation functions are realigned on a dominant peak. Consequently, the analysis focuses on measurements of coherency rather than traveltimes, and is associated with correlation coefficient of groups of events with a reference wavelet. Examination of coherency in different frequency bands reveals clear changes at a narrow band centred around 0.8 Hz. The results show a rapid drop of similar to 1-2 per cent of the coherency at the time of the Duzce event followed by gradual recovery with several prominent oscillations over 4 d. The observed changes likely reflect evolution of permeability and fluid motion in the core damage zone of the North Anatolian Fault. Compared to noise correlation processing, our analysis of earthquake waveform correlation (i) benefits from high level of coherence with short duration recorded signals, (ii) has considerably finer temporal sampling of fault dynamics after mainshocks than is possible with noise correlation, (iii) uses the coherence level to track property variations, which may be more robust than traveltime fluctuations in the coda of noise correlations. Studies utilizing both earthquake and noise waveforms at multiple pairs of stations across fault damage zones can improve significantly the understanding of fault zone processes.

Résumé: Structural-change detection and characterization, or structural-health monitoring, is generally based on modal analysis, for detection, localization, and quantification of changes in structure. Classical methods combine both variations in frequencies and mode shapes, which require accurate and spatially distributed measurements. In this study, the detection and localization of a local perturbation are assessed by analysis of frequency changes (in the fundamental mode and overtones) that are combined with a perturbation-based linear inverse method and a deconvolution process. This perturbation method is applied first to a bending beam with the change considered as a local perturbation of the Young's modulus, using a one-dimensional finite-element model for modal analysis. Localization is successful, even for extended and multiple changes. In a second step, the method is numerically tested under ambient-noise vibration from the beam support with local changes that are shifted step by step along the beam. The frequency values are revealed using the random decrement technique that is applied to the time-evolving vibrations recorded by one sensor at the free extremity of the beam. Finally, the inversion method is experimentally demonstrated at the laboratory scale with data recorded at the free end of a Plexiglas beam attached to a metallic support. (C) 2014 Acoustical Society of America.

Résumé: We demonstrate the experimental realization of a multiresonant metamaterial for Lamb waves, i.e., elastic waves propagating in plates. The metamaterial effect comes from the resonances of long aluminum rods that are attached to an aluminum plate. Using time-dependent measurements, we experimentally prove that this metamaterial exhibits wide band gaps as well as sub-and suprawavelength modes for both a periodic and a random arrangement of the resonators. The dispersion curve inside the metamaterial is predicted through hybridizations between flexural and compressional resonances in the rods and slow and fast Lamb modes in the plate. We finally underline how the various degrees of freedom of such system paves the way to the design of metamaterials for the control of Lamb waves in unprecedented ways.

Résumé: The geochemical processes of subduction modulate the recycling of Earth's surface materials into the mantle. Many key trace elements and isotopic tracers are strongly fractionated by subduction, leading to the distinctive chemical signatures of arc lavas and the development of chemically unique, near-surface mantle reservoirs, while other species largely pass through subduction zones back into the deep mantle. Differential recycling of trace elements through subduction processes leads to chemical heterogeneities within the mantle and to profound chemical fractionations between the mantle and crustal rocks.
This article reviews the current understanding of the thermal structures, petrologic makeup, and metamorphic and melting processes within subducting plates. The systematics of volcanic arc lavas, the primary chemical outflux of subduction, and how this outflux and those at shallower depths fractionate the composition of downgoing plates and return mobile species to the surface are examined. The fate of the species that predominantly pass through the subduction filter into the deep mantle and how this may lead over time to the development of chemically distinctive mantle reservoirs are discussed. With constraints from the systematics of two trace elements (boron, which does not pass through the subduction filter, and beryllium, which is largely returned to the deep mantle), an order of recycling efficiency for key trace elements during subduction is proposed.