Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
An active seismic experiment has been conducted across the southern Ryukyu margin east of Taiwan over the whole trench-arc-backarc system in May 2009. Twenty-four ocean bottom seismometers (OBS) were deployed from the Ryukyu trench to the southern Okinawa trough over the Ryukyu arc and forearc. Wide angle seismic data were recorded by the OBS array while coincident reflection seismic data were acquired using a 6 km long streamer and a 6600 cubic inch seismic airgun array. Results from tomographic inversion of 21091 travel time picks along this line allowed us to image crustal structures of the Ryukyu margin down to a depth of 25 km. The transect has been designed to provide a better seismic velocity structure of the subduction zone in a highly deformed area that has produced an M8 earthquake in 1920. The line crosses a seismic cluster of earthquakes which source mechanisms are still poorly understood. The subducting oceanic crust of the Huatung Basin is about 5-6 km thick. The underlying mantle exhibits low seismic velocities around 7.8 km/s suggesting some hydrothermal alterations or alteration of the upper mantle through faults generated by the flexure of the subducting plate as it enters the subduction. Low velocities, up to 4.5 km/s, associated with the accretionary wedge are well imaged from the trench back to the Nanao forearc. A major result concerns the abrupt termination of the buttress at the rear of the accretionary wedge. Despite the low resolution of the tomographic inversion near the subduction interface, several lines of evidence supporting the presence of a low velocity zone beneath the toe of the forearc buttress could be established. The Moho beneath the Ryukyu non-volcanic arc is located at a depth around 25 km depth.
ISSN: 0040-1951

hal-00795573
https://hal.archives-ouvertes.fr/hal-00795573
DOI : 10.1016/j.tecto.2011.10.010

Éditeur(s) : HAL CCSD Elsevier
Résumé : We present field data from segmented normal faults having particular displacement and overlapping geometries that may be related to down-dip branching of the fault segments. Based on a 3-D numerical modeling study of computed displacement and stress fields around different geometries of down-dip branched normal fault segments, we show that the bends of fault surfaces that coalesce at depth exert a significant influence on (I) the displacement distribution on the fault segment branches and (2) the quasi-static stress fields around the relay zones. The asymmetry in the displacement profiles and low fault interaction at relay zones modeled are consistent with the fault segment geometries observed in field data. As an application, we model the geometry and interaction of the Vallo di Diano normal fault which is segmented at the Earth's surface and which produced an earthquake of magnitude 6.4 along a single fault surface at depth. Numerical modeling of the segmented fault produces an asymmetric displacement profile and low shear stress at the relay zone consistent with the profile and fault interaction inferred from the held. We conclude that asymmetry of the displacement profile and large overlap/separation ratio of the unlinked relay zone can be indicators of the presence of a continuous fault surface underneath. These geometrical attributes are therefore important to consider in the probabilistic analysis of seismic hazard along segmented normal faults.
ISSN: 0191-8141

hal-00411161
https://hal.archives-ouvertes.fr/hal-00411161
DOI : 10.1016/j.jsg.2008.07.005

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
Constraining mantle deformation beneath plate boundaries where plates interact with each other, such as beneath active or halted mountain belts, is a particularly important objective of "mantle tectonics" that may bring a depth extent to the Earth's surface observation. Such mantle deformation can be mapped at scale lengths of several tens of kilometers through the analysis of seismological data and particularly by mapping seismic anisotropy from the splitting analysis of vertically-propagating SKS waves that largely reflect the strain-induced crystal preferred orientations of the rock-forming minerals within the upper mantle. In the present study, we analyse data from approximately 50 broadband seismic stations covering the Western Alps and we provide a coherent picture of upper mantle anisotropy beneath the belt. The large-scale anisotropy pattern is characterized by fast split directions that closely follow the trend of the belt. Moreover, the maximum anisotropy magnitude is not located beneath the internal zones of the belt but instead beneath external units. All suggests that the anisotropy is likely dominated by sublithospheric mantle deformation. We propose that the observed anisotropy pattern can be explained by recent or active mantle flow around the Eurasian slab presently plunging beneath the inner parts of the Alps.
ISSN: 0012-821X

hal-00617828
https://hal.archives-ouvertes.fr/hal-00617828
https://hal.archives-ouvertes.fr/hal-00617828/document
https://hal.archives-ouvertes.fr/hal-00617828/file/Barruol_SKS_Alpes_EPSL2011_hal.pdf
DOI : 10.1016/j.epsl.2010.12.040

Éditeur(s) : HAL CCSD
Résumé : National audience
The Lesser Antilles subduction zone has produced no recent strong thrust earthquakes, making it difficult to quantify the seismic hazard from such events. The Lesser Antilles arc has a low subduction rate and an accretionary wedge that is very wide at its southern end. To investigate the effect of the wedge on seismogenesis, numerical models of forearc thermal structure were constructed along six transects perpendicular to the arc in order to determine the thermally predicted width of the seismogenic zone. The geometry of each section is constrained by published seismic profiles and crustal models derived from gravity and seismic data and by earthquake hypocenters at depth. A major constraint on the deep part of the model is that mantle temperature beneath the volcanic arc should achieve a temperature of 1,100°C to generate partial melts. Predicted surface heat flow is compared to the available heat flow observations. Thermal modeling results indicate a systematic southward increase in the width of the seismogenic zone, more than doubling in width from north to south and corresponding to a dramatic southward increase in forearc width (distance from the arc to the deformation front of the accretionary wedge). The minimum width of the seismogenic zone (distance between the intersections of the subduction interface with the 150°C and 350°C isotherms) increases from about 80 km, north of 16°N, to 230 km, at 13°N. The maximum width (between the 100°C and 450°C isotherms) ranges from about 150 km in the north to up to 320 km in the south. This large variation in the width of the seismogenic zone is a consequence of the increasing width of the accretionary wedge to the south, caused by the increased thickness of sediment on the subducting plate. There is good agreement between the thermally predicted seismogenic limits and the sparse distribution of recorded thrust earthquakes, which are observed only in the northern portion of the arc. Possible scenarios for mega-thrust earthquakes are discussed. Depending on the segment length (along-strike) of the rupture plane, the occurrence of an event of magnitude 8-9 cannot be excluded.
Bulletin Société Géologique de France

insu-00814056
https://hal-insu.archives-ouvertes.fr/insu-00814056
DOI : 10.2113/gssgfbull.184.1-2.47

Éditeur(s) : HAL CCSD Elsevier
Résumé : Carbon emission at volcanic centers requires a constant balance between output (mostly by volcanism, either at plate boundaries or intraplate) and input (mostly at trench settings) of carbon from and to the Earth's mantle. The form of carbon that resides in the mantle is controlled by depth (pressure) and oxygen fugacity, the latter in turn depending on the depth and the concentration of iron in the mantle. In the shallow, lithospheric mantle, carbon is likely to be present in the oxidized form of CO2 (except under cratons where carbon is reduced to graphite or diamond). Below approximately 90 km, in the asthenosphere, the oxidized form of carbon is carbonate, either mineral or melt, depending on the thermal regime. At depths greater than approximately 150 km, the asthenospheric mantle is too reducing for carbon to stay in its oxidized form and only diamond is present, unless there is sufficient hydrogen to form reduced C–H fluids. Hence, the region located in the depth range of 90 to 150 km deep is where carbonatitic melts can most likely be produced and impregnate the surrounding mantle through metasomatism. The upper bound of this region is called the carbonate ledge. This limit prevents carbonate (either solid or molten) from ascending because of degassing and CO2 liberation. The lower bound is a redox front where redox melting (that is, melting caused by oxidation) may take place in an ascending portion of carbon-containing mantle. Carbonatite eruptions and presence of carbonate mineral inclusions in deep-seated diamonds provide evidence that these boundaries can be trespassed in some cases.An analysis of the experimental data that has bearing on silicate melting in the presence of carbon further shows that the carbonate ledge is a melting curve with a negative or flat Clapeyron (dP/dT) slope. In the carbonated ultrabasic (peridotite) systems, the carbonate ledge is located between ~ 2–3 GPa. The ledge divides the pressure–temperature space into a region of low-pressure silicate melt production, and a high-pressure region where carbonatites can be produced. Carbonatitic melts in equilibrium with mantle peridotite have compositions close to dolomitic (approximately equal amounts of Ca and Mg) with a general trend of becoming markedly more magnesian with increasing pressure. Calcic carbonatites may be stable at pressures < 2 GPa if clinopyroxene is absent. At mantle transition zone pressure range, there seems to be a melting temperature decrease (negative fusion slope), which may be caused by the stabilization of majoritic garnet.The carbonated basic (broadly eclogitic) system is more complex than the peridotitic one, because of the strong control of bulk silicate composition on melting temperatures, and hence, on melt composition. In carbonated eclogite systems, we propose that the effect of bulk composition upon all observed features can perhaps be related to silica super-saturation, or lack thereof. In some cases, high calcium (> 80 mol% CaCO3) melts can be produced, making the melting of carbonated eclogites an appealing scenario for the genesis of calcio-carbonatites in the Earth's mantle. Comparison with modeled pressure–temperature paths of subducted oceanic lithosphere shows that fusion of carbonated eclogite at depths shallower than 200 km should be expected for hot (Cascadian-type) subduction thermal regimes. On the other hand, in the case of cooler thermal regimes (Honshu-type, for instance), subducted carbonates may be stable to greater depths in Earth at trench settings, depending on the bulk composition of the system.Furthermore, high-pressure experiments show evidence of a continuum among carbonatitic, kimberlitic, melilititic, and basaltic liquids, for increasing melting degree of carbonated peridotite. This continuum has not been documented in the case of fusion of carbonated eclogite. It may be present, however, when certain sediments are fused, although the silicate melts are granitic to rhyodacitic instead of being kimberlitic in composition. Additional high-pressure work on phase relations in the simple binary system CaCO3–MgCO3 and specific focus on oxide solubility in the vapor phase have the potential to further clarify phase relations on complex silicate–carbonate systems at mantle conditions.
ISSN: 0009-2541

hal-01277770
https://hal.archives-ouvertes.fr/hal-01277770
DOI : 10.1016/j.chemgeo.2015.05.018

Éditeur(s) : HAL CCSD Elsevier
Résumé : Primary arc melts may form through fluxed or adiabatic decompression melting in the mantle wedge, or via a combination of both processes. Major limitations to our understanding of the formation of primary arc melts stem from the fact that most arc lavas are aggregated blends of individual magma batches, further modified by differentiation processes in the sub-arc mantle lithosphere and overlying crust. Primary melt generation is thus masked by these types of second-stage processes. Magma-hosted peridotites sampled as xenoliths in subduction zone magmas are possible remnants of sub-arc mantle and magma generation processes, but are rarely sampled in active arcs. Published studies have emphasised the predominantly harzburgitic lithologies with particularly high modal orthopyroxene in these xenoliths; the former characteristic reflects the refractory nature of these materials consequent to extensive melt depletion of a lherzolitic protolith whereas the latter feature requires additional explanation.Here we present major and minor element data for pristine, mantle-derived, lava-hosted spinel-bearing harzburgite and dunite xenoliths and associated primitive melts from the active Kamchatka and Bismarck arcs. We show that these peridotite suites, and other mantle xenoliths sampled in circum-Pacific arcs, are a distinctive peridotite type not found in other tectonic settings, and are melting residues from hydrous melting of silica-enriched mantle sources. We explore the ability of experimental studies allied with mantle melting parameterisations (pMELTS, Petrolog3) to reproduce the compositions of these arc peridotites, and present a protolith (‘hybrid mantle wedge’) composition that satisfies the available constraints. The composition of peridotite xenoliths recovered from erupted arc magmas plausibly requires their formation initially via interaction of slab-derived components with refractory mantle prior to or during the formation of primary arc melts. The liquid compositions extracted from these hybrid sources are higher in normative quartz and hypersthene (i.e., they have a more silica-saturated character) in comparison with basalts derived from prior melt-depleted asthenospheric mantle beneath ridges. These primary arc melts range from silica-rich picrite to boninite and high-Mg basaltic andesite along a residual spinel harzburgite cotectic. Silica enrichment in the mantle sources of arc-related, subalkaline picrite-boninite-andesite suites coupled with the amount of water and depth of melting, are important for the formation of medium-Fe (‘calc-alkaline’) andesite-dacite-rhyolite suites, key lithologies forming the continental crust.
ISSN: 0016-7037

hal-01553520
https://hal.archives-ouvertes.fr/hal-01553520
DOI : 10.1016/j.gca.2016.09.030

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
I use GPS interseismic velocities and classic 2D elastic half-space models with a screw dislocation to estimate the long-term fault slip rate, locking depth, and the offset between the surface fault trace and the location of the dislocation below the seismogenic zone for 13 segments along 8 major strike-slip faults. Using deduced strike-slip rates and the position of the dislocation to normalize the interseismic velocities to facilitate comparison of spatial patterns of deformation, I show that no substantial differences can be detected, ruling out a large asymmetry in interseismic velocities across the 8 faults used in this study. Only the Carrizo Plain segment of the San Andreas Fault shows a significant asymmetry that cannot be explained by shifting the position of the dislocation at depth relative to the fault trace. However, the resulting perturbation is less than 10% of total strike-slip rate. Fault traces are usually curved, defining a concave side. When the dislocation at depth is significantly offset from the fault trace, the shift is always toward the block on the concave side of the fault trace. This suggests that the fault zone in the lower crust may develop a simpler geometry more consistent with relative motion across the fault than its upper seismogenic part constrained by the structural complexity of the brittle crust. Since the faults used in this study are at different times in their interseismic period, comparing the interseismic velocity fields across them allows identification of possible variations of the interseismic velocities with time. When normalized by slip rate and dislocation location, all the faults show the same interseismic strain with no significant differences between deduced locking depths. These comparisons suggest that if temporal variations occur as suggested by some dynamic earthquake cycle models, they are small and below the accuracy of the available geodetic measurements.
ISSN: 0040-1951

hal-01171753
https://hal.archives-ouvertes.fr/hal-01171753
DOI : 10.1016/j.tecto.2015.01.013

Éditeur(s) : HAL CCSD World Scientific
Résumé : The present paper presents a laboratory experiment of rip current circulations over a moveable bed. The rip current characteristics over four distinct beach morphologies, exhibiting more or less developed nature-like bar-rip morphology,were investigated. For each video run, the same off shore shore-normal waves were generated by the wave maker with the same mean water level in order to study the sensitivity of the rip current characteristics as a function of the beach morphology only. In each case, a 1-hour video run was used to track a large number (~30) of drifters released within the surf zone. Image coordinates were then rectifie d to still water level Cartesian coordinates to compute drifter velocities, mean characteristics and surf zone rete ntion rates. Results show the presence of classic rip current patterns with counter-rotating cells and a relatively narrowoffshore-directed jet with, for three of the situations, a reasonably symmetric shape. Non-surprisingly, it was found that rip current intensity increases with increasing relative depth of the rip channel. The wave-driven circulations were strongly unstable. Computed standard deviation in fl ow intensity and direction provides high resolution information on the spatial variability of the rip current insta bilities with, for instance, highly-pulsating and weakly directionally variable offshore-directed flow in the rip channel. Conversely to what was previously hypothesized in the literature, there was hardly trace of vortices being shed offs hore and drifters exiting the surf zone compartment were not systematically caught by a pulsating jet. The cause for drifter exiting the semi-enclosed surf zone compartment remains, however, elusive and deserves further investigations. The computed surf zone retention rates (~90%) were of the order of those previously observed in the field, with no clear relationship with the mean rip current velocity or relative depth of the rip channel. Further video-runs will have to be analyzed to explore potential explanations
Proceedings of Coastal Dynamics 2009 Impacts of Human Activities on Dynamic Coastal Processes

ISBN : 978-981-4282-46
hal-00854534
https://hal.archives-ouvertes.fr/hal-00854534

Éditeur(s) : Gauthier-Villars
Résumé : Relatively little is known of the fauna found on rock substrates in the deep ocean. As a result of the Franco-British deep diving cruise Cyaporc in 1986 using the submersible Cyana we present observations of a highly diverse sponge, cnidarian and echinoderm fauna occurring between 1 800 and 3 000 m on the steep slopes to the west of the Porcupine Bank and Goban Spur. This fauna is dominated. by suspension feeders and appears to show a vertical zonation corresponding to water mass structure. The highest biomass and variety of echinoderms are found between 2 100 and 2 600 m bathed in northward flowing North East Atlantic Deep Water. We present also additional observations of species known from previous sampling programmes but rarely photographed in their natural environment.
Oceanologica Acta (0399-1784) (Gauthier-Villars), 1992 , Vol. 15 , N. 2 , P. 211-226

Droits : info:eu-repo/semantics/openAccess
http://archimer.ifremer.fr/doc/00100/21154/18773.pdf
http://archimer.ifremer.fr/doc/00100/21154/

Éditeur(s) : HAL CCSD Nature Publishing Group
Résumé : International audience
Temperature and fluid pressure conditions control rock deformation and mineralization on geological faults, and hence the distribution of earthquakes1. Typical intraplate continental crust has hydrostatic fluid pressure and a near-surface thermal gradient of 31 ± 15 degrees Celsius per kilometre2, 3. At temperatures above 300–450 degrees Celsius, usually found at depths greater than 10–15 kilometres, the intra-crystalline plasticity of quartz and feldspar relieves stress by aseismic creep and earthquakes are infrequent. Hydrothermal conditions control the stability of mineral phases and hence frictional–mechanical processes associated with earthquake rupture cycles, but there are few temperature and fluid pressure data from active plate-bounding faults. Here we report results from a borehole drilled into the upper part of the Alpine Fault, which is late in its cycle of stress accumulation and expected to rupture in a magnitude 8 earthquake in the coming decades4, 5. The borehole (depth 893 metres) revealed a pore fluid pressure gradient exceeding 9 ± 1 per cent above hydrostatic levels and an average geothermal gradient of 125 ± 55 degrees Celsius per kilometre within the hanging wall of the fault. These extreme hydrothermal conditions result from rapid fault movement, which transports rock and heat from depth, and topographically driven fluid movement that concentrates heat into valleys. Shear heating may occur within the fault but is not required to explain our observations. Our data and models show that highly anomalous fluid pressure and temperature gradients in the upper part of the seismogenic zone can be created by positive feedbacks between processes of fault slip, rock fracturing and alteration, and landscape development at plate-bounding faults.
ISSN: 0028-0836

hal-01685757
https://hal.archives-ouvertes.fr/hal-01685757
DOI : 10.1038/nature22355

Éditeur(s) : Pêche thonière et dispositifs de concentration de poissons, Caribbean-Martinique, 15-19 Oct 1999
Résumé : Since 1980 a programme of Fish Aggregation Device (fad) deployment has been carried out in the Cook Islands. Government has deployed all fads between 1 to 3 nautical miles from shore at depths ranging from 800 to 1,600 metres. The fads have undergone various designs with varying degrees of success. The average fad lifespan during the early 1980s was nine months; however, it increased to 18 months, with some fads in operation in excess of 30 months. Each fad cost between nz$ 7,000 and nz$ 9,000 to build and deploy. Since their introduction, fad have been widely accepted as a very effective apparatus in coastal small-scale fishing activities, specifically for pelagic species. Judging by the progressive behaviour of local fishing communities, there is sufficient reason to believe that fads have created productive fishing zones for Cook Islands fishermen to enable them to supply the ever-increasing local market for fresh fish. fads are an integral part of local fishing communities and provide a reasonable form of income to the increasing fishing community.

Droits : info:eu-repo/semantics/openAccess
http://archimer.ifremer.fr/doc/00042/15323/12654.pdf
http://archimer.ifremer.fr/doc/00042/15323/

Éditeur(s) : HAL CCSD Elsevier
Résumé : This study analyzes the records of several trace metals sensitive to redox conditions in continental shelf sediments off Concepcion, Chile (36 degrees S). The continental margin off Concepcion (36 S; 73 W) lies beneath an important upwelling center characterized by high primary production rates and, consequently, high fluxes of organic matter. In spring and summer, this material settles to the seafloor where it decays, producing periods of very low oxygen content in bottom waters (<1 mL L-1). In addition, an oxygen minimum zone develops at similar to 100-400 m water depth, where dissolved oxygen levels are <0.5 mL L-1. This situation changes during strong El Nino events, when dissolved oxygen at the bottom increases drastically (>1 mL L-1).;The goals of this study were to determine the input of trace metals to the sediment and to decipher how this information can be used to reveal variations in primary productivity or bottom oxygenation. Gravity cores collected at two stations - VG06-2 over the mid-shelf station (88 m water depth, upper boundary of the oxygen minimum zone) and VG06-3 over the outer shelf (120 m water depth, within the oxygen minimum zone) - were sampled for high resolution profiles (1 cm) of trace metals, biogenic opal, stable isotopes, and total organic carbon. The results suggest that the variability in the trace metal distribution on the continental shelf off Concepcion is determined by redox conditions and the organic carbon flux to the bottom. Some sections of the sediment cores from the outer shelf showed appreciable authigenic enrichment of U, Cd, and Mo (EF: 5-10, 2-5, and 10-16 respectively) along with heavier values of delta N-15, suggesting periods of suboxic conditions. During these periods, fluxes of organic material to the bottom were higher, as indicated by elevated TOC and opal contents. Alternating periods of higher and lower trace metal contents were not observed mid-shelf as they were on the outer shelf. Rather, the mid-shelf samples showed authigenic enrichment of U, Cd, and Mo (EF: 1-6, 4-5, and 10-20, respectively) throughout the core except in a 10-cm-thick gray layer. In general, authigenic enrichment of U. Mo, and Cd occurred at both sites, coincident with olive green layers in the cores. These layers were associated with periods of elevated primary productivity and suboxic conditions. Such periods did not seem to last as long as the oxygenated periods, which had higher inputs of refractive detrital material, coincident with the occurrence of distinct gray sediment layers.
ISSN: 0079-6611

hal-00682159
https://hal.archives-ouvertes.fr/hal-00682159
DOI : 10.1016/j.pocean.2011.07.010

Éditeur(s) : SéretB. & J.-Y. Sire, eds Paris: Soc. Fr. lchtyol. & IRD, 2000: 29-45
Résumé : Forty one manned submersible dives were carried out during 2 cruises in the Bay of Biscay covering a range of depth from about 100 to 2100 m. Chondrichthyan species were observed in most of the dives, some of them were easily identified and their behaviour could be studied. Twenty-two chondrichthyan species (9 sharks, 8 rays and 5 chimaeras) were recorded. Shark species were always seen active except Scyliorhinus canicula and Galeus melastomus. Some representatives were found motionless on the bottom, sometimes forming groups for the first species. Chimaeras appeared to strongly react to the presence of the submersible. The density of chondrichtyes and their relative frequency in the total fish assemblage were estimated, the results are globally consistent with former knowledge and the differences with results from trawl surveys are discussed. The observations presented here suggest peak of Chimaera monstrosa abundance at upper slope depth (200-500 m) that was not documented formerly for that area.
Quarante et une plongées en submersible habité ont été effectuées lors de 2 campagnes dans le Golfe de Gascogne par des profondeurs de 100 à 2100 m environ. Des Chondrichtyens ont été observés au cours de la plupart des plongées, 22 espèces (9 requins, 8 raies et 5 chimères) ont été identifiées et leurs comportements ont pu être étudiés. Les requins étaient toujours actifs sauf Scyliorhinus canicula et Galeus melastomus dont certains représentants ont été trouvés immobiles sur le fond, parfois en groupe pour la première espèce. Les chimères se sont montrées très réactives à la présence du submersible. Les densités et fréquences relatives pour l'ensemble des Chondrichtyens ont été estimées, les résultats sont globalement cohérents avec les estimations antérieures; les différences avec les estimations issues de chalutages sont discutées. Les observations présentées ici suggèrent qu'il existe un pic d'abondance de Chimaera monstrosa à la profondeur de la pente supérieure (200-500 m). Ce pic n'avait pas été observé précédemment dans cette zone.

Droits : info:eu-repo/semantics/openAccess
http://archimer.ifremer.fr/doc/2000/acte-2571.pdf
http://archimer.ifremer.fr/doc/00000/2571/

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
Excess gas phase in magmas erupting explosively is well known worldwide. However, the origin of this gas phase, in excess of what can be dissolved in the erupting magma at depth, and the rate of gas accumulation is less well defined. The 2010 mildly explosive eruption at Eyjafjallajökull, Iceland, produced mingled tephra of benmoreitic and trachytic composition whereas alkali basalt was emitted during preceding flank eruption. Tephra of the first explosive phase are composed of three glass types, alkaline rhyolite, mixed benmoreite, and basalt, which suggests that the basaltic magma intruded a pre-existing rhyolitic magma chamber, and ultimately triggered the eruption. The mixed benmoreitic tephra (erupted on 15 and 17 April 2010) had large 210Po in excess of 210Pb View the MathML source[(Po210/Pb210)0=1.88] at the time of eruption, and possibly a small 210Pb excess over its parent 226Ra. In contrast, the preceding flank eruption produced basalt with View the MathML source(Po210)0=0, upon eruption, and the final trachyte had lost most of its 210Po during open-system degassing. The 210Po excess in the first erupted benmoreites is interpreted to result from 210Po degassing of basaltic magma and the accumulation of 210Po-enriched gas, either in the upper part of the basaltic intrusion, below the rhyolite–basalt interface, or in the pre-existing residual rhyolitic magma chamber. From a simple model of radon and polonium accumulation in the rhyolitic reservoir, the ratio of the mass of basalt magma degassing over the mass of magma accumulating the excess gas decreased from 20 to 15 over 2 days, implying zoned magma reservoir, with the uppermost and gas-richest part erupting first. The duration of pre-eruptive gas accumulation in this model is approximately one year. This corresponds closely to the initiation of a seismic swarm beneath Eyjafjallajökull, early June 2009, which was the first pre-eruptive signal detected. The coincidence between initiation of gas accumulation at relatively shallow depth and deeper seismicity strongly suggests that the excess gas phase originated from a basalt magma batch intruded at depth, and that this gas phase reached the surface approximately a year later.
ISSN: 0012-821X

hal-01217142
https://hal.archives-ouvertes.fr/hal-01217142
DOI : 10.1016/j.epsl.2015.06.054

Éditeur(s) : Elsevier
Résumé : In situ microbialites occurring in reef rocks dredged between 80 and 130 in water depth on the modern fore-reef slopes of Tahiti and the Marquesas islands yield ages ranging from 17,100 2900 to 4410 2250 years BP, suggesting that they played a prominent role during the last deglacial sea level rise. Microbialites developed in both shallow and deep water depositional environments where they characterize various zones of the reef tracts (reef crests, upper reef slopes, deep fore-reef slopes), reflecting contrasting scenarios of microbialite development involving ''reefal microbialites'' in shallow-water settings and ''slope microbialites'' that formed in environments deeper than 1020 in and extending down to more than 100 m. Reefal microbialites correspond to a late stage of encrustation of the dead parts of coral colonies, or more commonly, of related encrusting organisms (red algae and foraminifers), thus forming surface crusts. Slope microbialites generally form the ultimate stage of a biological succession indicating a deepening sequence, whereby shallow water corals and associated encrusting organisms are replaced by deeper water assemblages of red algae and foraminifers before microbialite growth. The precipitation of phosphatic-iron-manganese crusts and the deposition of planktonic micritic limestones on the microbialites characterize a deepening-upward sequence. The widespread development of microbialites in reef sequences from the Last Deglaciation characterizes a period of environmental degradation consequential oil the rapid sea-level rise and abrupt climatic changes of that time. The reported biological succession reflects changes in water quality, and especially an increase in nutrients. In shallow-water settings, increased alkalinity and nutrient availability in interstitial waters were related to surface fluxes and terrestrial groundwater seepage while slope environments were exposed to continuous upwelling of nutrient-rich deeper waters during the last deglacial sea level rise. The age differences between corals and overlying slope microbialites range from 1600 to 8400 years, based on high-precision U-series age measurements of both corals and microbialites, and indicates that a significant time (several thousand years) elapsed between the development of the coralgal frameworks and the growth of slope microbialite crusts. Microbialites cannot be considered as part of the drowning event some 14,000 years ago that resulted in the demise of reef frameworks in the 90-110 m present depth range, but are Substantially younger. (c) 2006 Elsevier B.V. All rights reserved.
Sedimentary Geology (0037-0738) (Elsevier), 2006-03 , Vol. 185 , N. 3-4 , P. 277-295

Droits : 2006 Elsevier B.V. All rights reserved.
http://archimer.ifremer.fr/doc/2006/publication-4204.pdf
DOI:10.1016/j.sedgeo.2005.12.018
http://archimer.ifremer.fr/doc/00000/4204/

Éditeur(s) : HAL CCSD Oxford University Press (OUP)
Résumé : International audience
We report new structural, microstructural, petrological, and major- and trace-element data on ultramafic rocks from the Kondyor zoned ultramafic complex in Far-East Russia. The ultramafic rocks are subdivided into three subconcentric lithologies, from core to rim: (1) a metasomatic domain where generally phlogopite-rich dykes pervasively intrude dunite; (2) a main dunite core; (3) a pyroxenite rim. The ultramafic rocks have nearly vertical contacts with the surrounding Archaean basement (gneisses, quartzites and marbles) and hornfelsed Riphean sediments. The hornfelsed sediments show a relatively steep ( 60), outward dipping layering, which rapidly flattens to horizontal away from the inner contact. Although the Riphean sediments define a dome-like structure, the inward, shallow dipping foliation of the dunites indicates a synformal structure. Detailed petro-structural investigations indicate that the Kondyor dunites were deformed by solid-state flow under asthenospheric mantle conditions. The outward textural change from coarse- to fine-grained equigranular dunite and the outward-increasing abundance of subgrains and recrystallized olivine grains suggest dynamic recrystallization while fluid circulation was channelized within the core metasomatic zone, with a decreasing melt fraction from core to rim, and also suggest that solid-state deformation induced grain-size reduction towards the cooling border of the Kondyor massif. Based on their geochemistry, the dunites are interpreted as mantle rocks strongly affected by reaction with melts similar to the JurassicCretaceous Aldan Shield lamproites. Rim pyroxenites were formed by a melt-consuming peritectic reaction, implying the existence of at least a small, conductive thermal gradient around the dunite body while the latter was still at near-solidus temperature conditions. This suggests that the zoned structure of Kondyor was initiated at mantle depths, most probably within the subcontinental lithosphere. Upon cooling, the lamproitic melts were progressively focused in the central part of the massif and drained into vein conduits where they reacted with the wall-rock dunite. Two-dimensional numerical modelling based on finite-differences with a marker-in-cell technique incorporates temperature-dependent rheologies for both molten and non-molten host rocks. The modelling consolidates the structural, petrological and geochemical interpretations, which show that the dunites represent the synformal, flat-lying apex of an asthenospheric mantle diapir, triggered by fluid pressure channelized in the core, which nearly reached the Earths surface. We conclude that translithospheric mantle diapirism is an important mode of mass transfer in the Earth.
ISSN: 0022-3530

hal-00424508
https://hal.archives-ouvertes.fr/hal-00424508
DOI : 10.1093/petrology/egn083

Éditeur(s) : Gauthier-Villars
Résumé : During RV Gaveshani cruise No. 235 (August-September, 1993), particulate matter was collected from euphotic and deeper waters in the Bay of Bengal and analysed for biogenic silica (BSi), lithogenic silica (LSi) and particulate organic carbon (POC). Mean depth profiles of BSi showed a minimum in the surface waters and a maximum in the middle depths of the euphotic zone. The deepening of BSi maxima from north to south coincides with the deepening of the thermocline and thickening of the surface layer. In coastal waters, the contribution of BSi to total particulate silica was high (> 80 %); in offshore waters it was 60-70 %. BSi/POC (0.086) and LSi/POC (0.023) ratios show that particulate matter consists mostly of biogenic material, while the BSi/LSi (1.61) ratio revealed the dominance of siliceous phytoplankton in the euphotic zone. High concentrations of BSi (0.49 mu mol l(-1)) and LSi (1.89 mu mol l(-1)) in deeper waters are due to a high sedimentation rate, the resuspension of bottom sediments and the flow of large amounts of riverine siliceous material.
Oceanologica Acta (0399-1784) (Gauthier-Villars), 1997 , Vol. 20 , N. 3 , P. 493-500

Droits : info:eu-repo/semantics/openAccess
http://archimer.ifremer.fr/doc/00093/20459/18131.pdf
http://archimer.ifremer.fr/doc/00093/20459/

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
In many orogens, high-pressure (HP) metamorphic rocks such as eclogite occur as lenses in quartzofeldspathic gneiss that equilibrated at much lower pressures. The pressure–temperature–time (P–T–t) history of eclogite relative to host gneiss provides information about mechanisms and timescales of exhumation of orogenic crust. The Montagne Noire of the southern Massif Central, France, is an eclogite–bearing gneiss (migmatite) dome located at the orogen-foreland transition of the Variscan belt. Results of our study show that it contains the youngest eclogite in the orogen, similar in age to migmatite and granite that crystallized under low-pressure conditions. P–T conditions for an exceptionally unaltered eclogite from the central Montagne Noire were estimated using a pseudosection supplemented by garnet–clinopyroxene and Zr-in-rutile thermometry. Results indicate peak P∼1.4 GPaP∼1.4 GPa and View the MathML sourceT∼725°C for Mg-rich garnet rim (50 mol% pyrope) + omphacite (36 mol% jadeite) + rutile + quartz. U–Pb geochronology (LA-ICP-MS) of 16 zoned zircon grains yielded ∼360 Ma (4 cores) and ∼315 Ma (12 rims and cores). Rare earth element abundances determined by LA-ICP-MS for dated zircon are consistent with crystallization of ∼315 Ma zircon under garnet-stable, plagioclase-unstable conditions that we interpret to indicate high pressure; in contrast, the ∼360 Ma zircon core corresponds to crystallization under lower pressure plagioclase-stable conditions. Based on garnet zoning and inclusion suites, rutile textures and Zr zoning, P–T results, and zircon petrochronology, we interpret the ∼315 Ma date as the age of eclogite-facies metamorphism that only slightly preceded dome formation and crystallization at 315–300 Ma. This age relation indicates that eclogite formation at high pressure and migmatite dome emplacement at low pressure were closely spaced in time. We propose that collapse-driven material transfer from the hot orogen to the cool foreland resulted in thickening of the orogen edge, leading to eclogite facies metamorphism of the deep crust. Soon after, the low-viscosity partially molten crust flowed from the plateau toward the foreland, incorporating and exhuming eclogite. The P–T–t history of the Montagne Noire eclogite shows that some dome material ascended from >40 km depth to shallow crustal levels, likely in a single decompression event, and that migmatite domes are therefore very efficient at exhuming the deep crust.
ISSN: 0012-821X

hal-01243499
https://hal.archives-ouvertes.fr/hal-01243499
DOI : 10.1016/j.epsl.2015.08.026

Éditeur(s) : HAL CCSD AGU and the Geochemical Society
Résumé : International audience
New magnetotelluric and receiver transfer function studies provide insights from the upper to the lower crust of the eastern Betic Cordillera, which is deformed by large folds, normal faults, and a major transcurrent left-lateral fault, the Carboneras Fault Zone (CFZ). Receiver function analysis determines a NNW dipping Moho reaching 20 degrees that increases in depth, from 20 km south of the CFZ up to 34 km in the Sierra de Los Filabres. In addition, seismic discontinuities determined in the upper crust are interpreted as major contacts between metamorphic complexes that are detached and folded. The MT inversion model reveals a conductive zone, also representing a crustal seismic discontinuity, associated with the Alpujarride/Nevado-Filabride contact and fitting the N vergent geometry of the Sierra Alhamilla antiform. A small flexure at Moho coincides with the CFZ, as revealed by the Bouguer anomaly trend, in agreement with the receiver function results. Moreover, the Bahr strike and tipper angle at the stations placed closest to the CFZ clearly reveal the continuity of the CFZ at least down to approximately 15 km in depth, crossing all the detected crustal discontinuities up to the Moho. The lack of a clear Moho offset associated with the Carboneras Fault supports the idea that some large strike-slip faults tend to accommodate the deformation by a broadening fault zone at lower crustal levels. Its nucleation could occur at the base of a thin crust, where melting processes critically reduced the lithospheric strength during the late Miocene, to then propagate upward, reaching the topographic surface. Northward, the lithosphere comprised moderately larger strength, and the crustal discontinuities favored the development of larger folds with kilometric amplitude instead of strike-slip faults since the late Miocene.
ISSN: 1525-2027

hal-00671024
https://hal.archives-ouvertes.fr/hal-00671024
DOI : 10.1029/2010GC003274

Éditeur(s) : HAL CCSD AGU and the Geochemical Society
Résumé : International audience
Based on global earthquake catalogs, the hypocenters, nodal planes, and seismic moments of worldwide subduction plate interface earthquakes were extracted for the period between 1900 and 2007. Assuming that the seismogenic zone coincides with the distribution of 5.5 <= M < 7 earthquakes, the subduction interface seismogenic zones were mapped for 80% of the trench systems and characterized with geometrical and mechanical parameters. Using this database, correlations were isolated between significant parameters to identify cause-effect relationships. Empirical laws obtained in previous studies were revisited in light of this more complete, accurate, and uniform description of the subduction interface seismogenic zone. The seismogenic zone was usually found to end in a fore-arc mantle, rather than at a Moho depth. The subduction velocity was the first-order controlling parameter for variations in the physical characteristics of plate interfaces, determining both the geometry and mechanical behavior. As such, the fast subduction zones and cold slabs were associated with large and steep plate interfaces, which, in turn, had large seismic rates. The subduction velocity could not account for the potential earthquake magnitude diversity that was observed along the trenches. Events with M-w >= 8.5 preferentially occurred in the vicinity of slab edges, where the upper plate was continental and the back-arc strain was neutral. This observation was interpreted in terms of compressive normal stresses along the plate interface. Large lateral ruptures should be promoted in neutral subduction zones due to moderate compressive stresses along the plate interface that allow the rupture to propagate laterally.
ISSN: 1525-2027

hal-00617681
https://hal.archives-ouvertes.fr/hal-00617681
https://hal.archives-ouvertes.fr/hal-00617681/document
https://hal.archives-ouvertes.fr/hal-00617681/file/heuretG32011.pdf
DOI : 10.1029/2010GC003230