Éditeur(s) : Elsevier Science Bv
Résumé : Here we present a new geochemical study of Lopevi volcano, one the most active volcanoes in the Vanuatu island arc. We focus on the temporally well-defined sequence of lava flows emitted since 1960, and for the first time, on pre-1960 volcanic products, including high-MgO basalts and felsic andesites, the most evolved lavas sampled so far on this island. This work reports the first Pb and Hf isotopic study of lavas from Lopevi island. These lavas display correlations between differentiation indexes such as SiO2 content and isotopic ratios. The felsic andesites extend the known correlations with both the least (Sr-Pb) and the most (Nd-Hf) radiogenic isotopic compositions on the island. Our results confirm that the rising magma interacted with the sub-arc crust. Assimilation-Fractional Crystallization (AFC) quantitative modeling of trace element ratios and isotopic compositions requires 1% and 10% of assimilated partial melts of a mafic oceanic crust to account for the pre- and post-1960 lavas, respectively. The post-1960 lavas differ from the former lavas emitted similar to 20 years earlier by enrichments in fluid mobile elements (K, Ba, Rb ... ), Th, and Light Rare Earth Elements (LREE). We ascribe these features to slight variations in the metasomatic agent added to the sub-arc mantle and ultimately derived from the subducted lithosphere. However, the contrasting time scales involved in subducted lithosphere dehydration and magma genesis, relative to the time elapsed between eruptions of the two lava series, suggest that two different portions of mantle which have undergone slightly different metasomatism, gave birth to the Lopevi lavas. These distinct magmas are still present beneath the volcano.
Journal Of Volcanology And Geothermal Research (0377-0273) (Elsevier Science Bv), 2013-08 , Vol. 264 , P. 72-84

Droits : 2013 Elsevier B.V. All rights reserved.
http://archimer.ifremer.fr/doc/00167/27801/26223.pdf
DOI:10.1016/j.jvolgeores.2013.07.005
http://archimer.ifremer.fr/doc/00167/27801/

Éditeur(s) : HAL CCSD
Résumé : International audience
Metamorphic soles correspond to m to 500 m thick highly strained metamorphic rock units found beneath myloniticbanded peridotites at the base of large-scale ophiolites, as exemplified in Oman. Metamorphic soles aremainly composed of metabasalts deriving from the downgoing oceanic lithosphere and metamorphosed up togranulite-facies conditions by heat transfer from the mantle wedge. Pressure–temperature peak conditions areusually estimated at 1.00.2 GPa and 800100ºC. The absence of HP–LT metamorphism overprint implies thatmetamorphic soles have been formed and exhumed during subduction infancy. In this view, metamorphic soleswere strongly deformed during their accretion to the mantle wedge (corresponding, now, to the base of the ophiolite).Therefore, metamorphic soles and banded peridotites are direct witnesses of the dynamics of early subductionzones, in terms of thermal structure, fluid migration and rheology evolution across the nascent slab interface.Based on fieldwork and EBSD analyses, we present a detailed (micro–) structural study performed on samplescoming from the Sumeini window, the better-preserved cross-section of the metamorphic sole of Oman. Largedifferences are found in the deformation (CPO, grain size, aspect ratio) of clinopyroxene, amphibole and plagioclase,related to mineralogical changes linked with the distance to the peridotite contact (e.g., hardening due to theappearance of garnet and clinopyroxene).To model the incipient slab interface in laboratory, we carried out 5 hydrostatic annealing and simple-shear experimentson Griggs solid-medium apparatus. Deformation experiments were conducted at axial strain rates of 10-6s-1. Fine-grained amphibolite was synthetized by adding 1 wt.% water to a (Mid-Ocean Ridge) basalt powder asa proxy for the metamorphic sole (amphibole + plagioclase + clinopyroxene garnet assemblage). To synthetizegarnet, 2 experiments were carried out in hydrostatic conditions and with deformation at 800C with confiningpressure of 2 GPa. Another simple-shear experiment has been carried out at 800C and 1 GPa with fined-grainednatural garnet. With the aim of mimicking the early slab interface (between the metamorphic sole and bandedperidotites at the base of the ophiolite), 2 simple-shear deformation experiments with 2 layers have been carriedout at 800C and confining pressure of 1 GPa. The bottom layer was made of hydrated basalt powder and the toplayer was made of olivine.Fined-grained garnet-free amphibolite is significantly weaker than dunite but the appearance of harder mineralsin the amphibolite (garnet and clinopyroxene) has major implications on its rheological evolution. These resultsallow liking field observations of strain localization at the interface to the metamorphic sole formation
European Geosciences Union General Assembly 2016
Vienne, Austria

insu-01309394
https://hal-insu.archives-ouvertes.fr/insu-01309394

Éditeur(s) : Elsevier
Résumé : In biological modelling of the coastal phytoplankton dynamics, the light attenuation coefficient is often expressed as a function of the concentrations of chlorophyll and mineral suspended particulate matter (SPM). In order to estimate the relationship between these parameters over the continental shelf of the northern Bay of Biscay, a set of in situ data has been gathered for the period 1998-2003 when SeaWiFS imagery is available. These data comprise surface measurements of the concentrations of total SPM, chlorophyll, and irradiance profiles from which is derived the attenuation coefficient of the photosynthetically available radiation, K-PAR. The performance of the IFREMER look-up table used to retrieve the chlorophyll concentration from the SeaWiFS radiance is evaluated on this new set of data. The quality of the estimated chlorophyll concentration is assessed from a comparison of the variograms of the in situ and satellite-derived chlorophyll concentrations. Once the chlorophyll concentration is determined, the non living SPM, which is defined as the SPM not related to the dead or alive endogenous phytoplankton, is estimated from the radiance at 555 nm by inverting a semi-analytic model. This method provides realistic estimations of concentrations of chlorophyll and SPM over the continental shelf all over the year. Finally, a relationship, based on non living SPM and chlorophyll, is proposed to estimate K-PAR on the continental shelf of the Bay of Biscay. The same formula is applied to non living SPM and chlorophyll concentrations, observed in situ or derived from SeaWiFS radiance.
Remote Sensing of Environment (0034-4257) (Elsevier), 2005-03 , Vol. 95 , N. 1 , P. 29-46

Droits : 2004 Elsevier Inc. All rights reserved
http://archimer.ifremer.fr/doc/2005/publication-1172.pdf
DOI:10.1016/j.rse.2004.11.007
http://archimer.ifremer.fr/doc/00000/1172/

Éditeur(s) : HAL CCSD
Résumé : The presence of obstacles met in natural environment by the flow, influences the dispersion of air pollutants which it carries. This thesis, based on spatiotemporal micro-meteorological measurements in tropical region, aims at studying the dispersion of compounds emitted by the ecosystem mangrove according to the weather conditions. At a daily scale, the meteorological analysis reveals, besides the presence of Trade Winds, the existence of thermal flows of which nocturnal thermodynamic parameters have been defined. The field observation has shown that the weak intensity of the wind disadvantages the dispersion of compounds emitted during the night. At a finer scale, the existence of a spectral gap separating large scale phenomena of those related to turbulence has been demonstrated. These last ones were treated by spectral and statistical approaches to quantify the turbulent fluxes of momentum and sensible heat during the night, for an averaged duration of 13 minutes. The stratification of the surface layer has been considered and determined by Monin-Obukhov similarity theory. The measurements acquired at night and the application of Kolmogorov theory allowed us to deduct the dissipation rate of energy and the size of the vortices at 5.5 meters in height. This work has also led us, by gas chromatography coupled to mass spectrometry, to identify, in mangrove, sulphurated volatil organic compounds, organohalogen, BTEX, ... All these results will later serve to feed dispersion models related to the studied ecosystem.
La présence d'obstacles rencontrés en milieu naturel par l'écoulement, influence la dispersion des polluants qu'il transporte. Cette thèse, fondée sur des mesures micro-météorologiques spatio-temporelles en région tropicale, vise à étudier la dispersion de composés émis par l'écosystème mangrove en fonction des conditions météorologiques. A l'échelle journalière, l'analyse météorologique révèle, outre la présence des Alizés, l'existence d'écoulements thermiques dont des paramètres thermodynamiques nocturnes ont été définis. L'observation terrain nous a montré que la faible intensité du vent défavorise la dispersion des composés émis durant la nuit. A une échelle plus fine, l'existence d'un trou spectral séparant les phénomènes de grande échelle de ceux liés à la turbulence a été mise en évidence. Ces derniers ont été traités par des approches spectrale et statistique pour quantifier les flux turbulents de quantité de mouvement et de chaleur sensible, pendant la nuit, pour une durée de moyenne de l'ordre de 13 minutes. L'état de stratification de la couche de surface a été pris en compte et déterminé par la théorie de similitude de Monin-Obukhov. Les mesures nocturnes effectuées et l'application de la théorie de Kolmogorov ont permis de déduire les taux de dissipation de l'énergie et la taille des tourbillons à 5,5 m d'altitude. Ce travail a également conduit, par l'analyse chromatographique en phase gazeuse couplée à la spectrométrie de masse, à l'identification, en mangrove, de composés organiques volatils soufrés, des organohalogénés, des BTEX, ... L'ensemble de ces résultats servira ultérieurement à alimenter les modèles de dispersion spécifiques liés aux écosystèmes étudiés.
https://tel.archives-ouvertes.fr/tel-00665882

Droits : info:eu-repo/semantics/OpenAccess
NNT : 11AGUY0438
tel-00665882
https://tel.archives-ouvertes.fr/tel-00665882
https://tel.archives-ouvertes.fr/tel-00665882/document
https://tel.archives-ouvertes.fr/tel-00665882/file/ThA_seCdAlexis.pdf

Éditeur(s) : HAL CCSD Springer Verlag
Résumé : International audience
Chromitite pods in the Mayarí-Cristal ophiolitic massif (eastern Cuba) were formed in the Late Cretaceous when island arc tholeiites and MORB-like back-arc basin basalts reacted with residual mantle peridotites and generated chromite-rich bodies enclosed in dunite envelopes. Platinum-group minerals (PGM) in the podiform chromitites exhibit important Os-isotope heterogeneities at the kilometric, hand sample and thin section scales. 187Os/188Os calculated at the time of chromitite crystallization (~90 Ma) ranges between 0.1185 and 0.1295 (γOs = −7.1 to +1.6, relative to enstatite chondrite), and all but one PGM have subchondritic 187Os/188Os. Grains in a single hand sample have initial 187Os/188Os that spans from 0.1185 to 0.1274, and in one thin section it varies between 0.1185 and 0.1232 in two PGM included in chromite which are only several millimeters apart. As the Os budget of a single micrometric grain derives from a mantle region that was at least several m3 in size, the variable Os isotopic composition of PGM in the Mayarí-Cristal chromitites probably reflects the heterogeneity of their mantle sources on the 10-100 m scale. Our results show that this heterogeneity was not erased by pooling and mingling of individual melt batches during chromitite crystallization but was transferred to the ore deposits on mineral scale. The distribution of the Os model ages calculated for PGM shows four main peaks, at ~100, 500, 750 and 1,000 Ma. These variable Os model ages reflect the presence of different depleted domains in the oceanic (Pacific-related) upper mantle of the Greater Antilles paleo-subduction zone. The concordance between the age of crystallization of the Mayarí-Cristal chromitites and the most recent peak of the Os model age distribution in PGM supports that Os in several grains was derived from fertile domains of the upper mantle, whose bulk Os isotopic composition is best approximated by that of enstatite chondrites; on the other hand, most PGM are crystallized by melts that tapped highly refractory mantle sources.
ISSN: 0010-7999

hal-00822742
https://hal.archives-ouvertes.fr/hal-00822742
DOI : 10.1007/s00410-010-0575-2

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
In the Oman ophiolite, the horizon where the melt lens pinched during drifting away from the ocean ridge axis has been identified. Starting in the Root Zone of the Sheeted Dike Complex (RZSDC) located above this horizon, 18 Sections down to the upper gabbros unit have been mapped in great detail, in selected areas of the southern massifs of this ophiolite. They are complemented by 133 sites, located throughout the entire ophiolite, where the transition from the RZSDC to the uppermost foliated gabbros is well exposed. Altogether, half the sites and 11 cross sections display, within a few tens of meters beneath the RZSDC, a magmatic foliation which is parallel to the overlying sheeted dikes. In the other sites and cross sections, the gabbro foliation is either flat-lying or steep but not parallel to the sheeted dikes. Compared to the RZSDC isotropic ophitic gabbros where clinopyroxene is interstitial between plagioclase laths, in the topmost steeply foliated gabbros, clinopyroxene is idiomorphic, becoming rapidly granular and tabular down section by recrystallization and peripheral alteration to hornblende. Moving down from these uppermost gabbros and over one hundred meters, the steep foliation becomes stronger and the poorly recovered top gabbros grade into the recrystallized, granular gabbros of the gabbro unit. These repeated observations indicate to ascribe these gabbros to subsidence of a compacting mush from the floor of the melt lens into the underlying, main magma chamber. The topmost gabbros beneath RZSDC, which were expelled from the melt lens by drifting very soon after settling on the melt lens floor, display in plagioclase a spectacular zoning pointing to a fast cooling. Moving downwards, stronger foliation, increased compaction and recovery-recrystallization are explained by the time spent by the subsiding mush inside the magma chamber increasing by one order of magnitude (similar to 50 to 500 yr) over this vertical distance. This field-based study brings compelling field evidence supporting the former models of subsidence which were based on the assumption that the mush that settled onto the floor of the melt lens is sucked downwards during drifting of the crust away from the ridge axis.
ISSN: 0012-821X

hal-00420903
https://hal.archives-ouvertes.fr/hal-00420903
DOI : 10.1016/j.epsl.2009.04.012

Éditeur(s) : HAL CCSD American Geophysical Union
Résumé : We study the Africa-Iberia plate boundary in the vicinity of Gibraltar. Numerous models have been proposed for that region throughout the last decades, proposing mechanisms that range widely from continental delamination, convective removal, to subduction of oceanic lithosphere. To better constrain upper-mantle structure under the region, we study waveforms of P-waves that traverse the Alboran Sea region between Spain and Morocco. These show dispersive behavior, which, together with early arrival times, confirms the presence of an anomalous upper mantle structure under the Alboran Sea. The dispersion is consistent with that expected from subducted lithosphere. Waveforms of body waves therefore provide a way to better constrain the elusive mantle structure and dynamics of the Alboran Sea region
ISSN: 0094-8276

hal-00407670
https://hal.archives-ouvertes.fr/hal-00407670
DOI : 10.1029/2007GL030964

Éditeur(s) : HAL CCSD
Résumé : In the bay of Aigues-Mortes, the managers of the co astal cities try to preserve the socio-economic and patrimonial stakes against the coastal hazards, such as contamination, storm surge and erosion. In this context, a massive beach nourishment of sand (1 million of m3 for 10km of coastline) was made at the beginning of year 2008 to reduce erosion. The REPBAM project (REchargements et Panaches sédiment aires dans la Baie d'Aigues-Mortes) aims to qualify and quantify by in-situ measurements and numerical modelling: 1. the geomorphological evolution of this modified coast and 2. the impact of the sediment's resuspension on the coastal ecosystems, in particular the one of the Posidonia oceanica seagrass. A numerical modelling "wave/current/sediment" is first implemented for December 2008. Nested and curvilinear grids allow a very high resolution for the beach area but also include the inner shelf where the seagrasses are located. Hydrodynamic and topo-bathymetric measurements were acquired from January to May 2012. Model and measurements results show the same trends during storms: strong currents, seaward migrationof the innerbar, sediment resuspension and turbid on the inner shelf.
Coastal Dynamics 2013
Arcachon, France

hal-00908082
https://hal.archives-ouvertes.fr/hal-00908082

Éditeur(s) : HAL CCSD Société nationale de protection de la nature
Résumé : International audience
The island of Moorea in French Polynesia is representative of a relatively well preserved small tropical oceanic island but both its terrestrial and marine ecosystems are currently under strong anthropogenic pressures. The terrestrial ecosystems are facing an increasing urbanization, and the destruction and modification of their natural habitats by cultivation and forestry plantations, accidental or intentional fires, and particularly by the invasion of alien plant and animal species. Coastal areas and reef ecosystems are threatened by shoreline artificialization, sedimentation due to soil erosion, increasing uses of marine and lagoon resources and frequency of coral bleaching events. A significant erosion of terrestrial and marine biodiversity is observed on the island since the last decades. In the context of the Land Management Plan (PGA) and Marine Area Management Plan (PGEM) recently set up in the island by local authorities, scientific surveys and inventories were made in 2006 in order to assess the status of terrestrial biodiversity (vascular plants) and marine biodiversity (reef fishes) and compare areas with different anthropogenic pressures. Studies conducted on the flora and the vegetation at low and middle elevation along altitudinal transects show that the plant invasion process is related to climatic and topographic factors, and species autoecology. The marine study aims to compare the protected areas with control areas in order to test the effectiveness of protection measures and to propose a streamlined monitoring protocol. A methodological study in both terrestrial and marine ecosystems was conducted in order to propose biodiversity indicators which are crucial for decision making in conservation and sustainable development of island biodiversity. We used Geographic Information System (GIS) as a fast and intuitive tool for mapping indicators and their variability. This methodological approach developed on the island of Moorea could be used as a model for other tropical islands
Moorea est une petite île océanique tropicale de la Polynésie française relativement bien préservée, mais dont les écosystèmes terrestres et marins sont actuellement soumis à de fortes pressions anthropiques. Le milieu terrestre est confronté à une urbanisation croissante, à la destruction ou à la modification des habitats naturels pour l’extension des cultures ou des plantations forestières, aux incendies volontaires ou accidentels, et particulièrement à l’invasion d’espèces végétales et animales introduites. Les zones côtières et les récifs coralliens sont menacés par de multiples perturbations telles que l’artificialisation des lignes de rivage, l’apport de sédiments dus à l’érosion des sols, l’augmentation des usages des ressources marines et lagonaires et de la fréquence des phénomènes de blanchissement du corail. Une érosion sensible de la biodiversité terrestre et marine de l’île est observée depuis les dernières décennies. Dans le cadre du Plan Général d’Aménagement (PGA) et du Plan de Gestion de l’Espace Maritime (PGEM) mis en place par les autorités locales, des inventaires scientifiques ont été réalisés en 2006 dans le but d’évaluer l’état de la biodiversité terrestre (plantes vasculaires) et marine (poissons récifaux) et de comparer des zones soumises à différentes perturbations. Les études réalisées sur la flore et la végétation de basse et moyenne altitude le long de transects altitudinaux montrent des processus d’invasion végétale différents en relation avec les facteurs climatiques, topographiques et autoécologiques des espèces. Les études en milieu marin visent à comparer des aires protégées à des aires témoins afin de suivre l’efficacité des mesures de gestion. La réflexion méthodologique menée parallèlement sur les deux écosystèmes a pour but d’illustrer différents indices de biodiversité nécessaires à la prise de décision en matière de conservation ou de développement durable de la biodiversité insulaire et de réaliser un état de référence pour établir un diagnostic sur l’état de la biodiversité. La représentation cartographique des indices à l’aide d’un SIG, illustrée dans cette étude avec la richesse spécifique terrestre et marine, permet une lecture intuitive et rapide de la variabilité de cet indice dont la restitution est basée sur une standardisation du nombre d’espèces. Cette approche méthodologique développée sur l’île de Moorea pourrait servir de modèle pour d’autres îles tropicales
EISSN: 2429-6422

hal-01306285
https://hal-univ-perp.archives-ouvertes.fr/hal-01306285

Éditeur(s) : HAL CCSD Wiley-Blackwell
Résumé : International audience
New petrographic and microstructural observations, mineral equilibria modelling and U/Pb (monazite) geochronological studies were carried out to investigate the relationships between deformation and metamorphism across the Rehamna massif (Moroccan Variscan belt). In this area, typical Barrovian (muscovite to staurolite) zones developed in Cambrian to Carboniferous metasedimentary rocks that are distributed around a dome-like structure. First assemblages are characterized by the presence of locally preserved andalusite, followed by prograde evolution culminating at 6 kbar and 620 °C in the structurally deepest staurolite zone rocks. This Barrovian sequence was subsequently uplifted to supracrustal levels, heterogeneously reworked at greenschist facies conditions, which was followed locally by static growth of andalusite, indicating heating to 2.5–4 kbar and 530–570 °C. The 206Pb/238U monazite age of 298.3 ± 4.1 Ma is interpreted as minimum age of peak metamorphic conditions, whereas the ages of 275.8 ± 1.7 Ma and 277.0 ± 1.1 Ma date decompression and heating at low pressure, in agreement with previous dating of Permian granitoids intruding the Rehamna massif. The prograde metamorphism occurred during thickening and associated horizontal flow in the deeper crust (S1 horizontal schistosity). The horizontally disposed metamorphic zones were subsequently uplifted by a regional scale antiform during ongoing N–S compression. The re-heating of the massif follows late massive E–W shortening, refolding and retrograde shearing of all previous fabrics coevally with regionally important intrusions of Permian granitoids. We argue that metamorphic evolution of the Rehamna massif occurred several hundred kilometres from the convergent plate boundaries in the interior of continental Gondwanan plate. The tectonometamorphic history of the Rehamna massif is put into Palaeozoic plate tectonic perspective and Late Carboniferous reactivation of (Devonian)–Early Carboniferous basins formed during stretching of the north Gondwana margin and formation of the Palaeotethys Ocean. The inherited heat budget of these magma-rich basins plays a role in the preferential location of this intracontinental orogen. It is shown that rapid transition from lithospheric stretching to compression is characterized by specific HT type of Barrovian metamorphism, which markedly differs from similar Barrovian sequences along Palaeozoic plate boundaries reported from Variscan Europe.
ISSN: 0263-4929

hal-01468009
https://hal.archives-ouvertes.fr/hal-01468009
DOI : 10.1111/jmg.12214

Éditeur(s) : HAL CCSD American Geophysical Union
Résumé : International audience
Based on observations from both modem convergent margins and sandbox modeling, we examine the possible conditions favoring frontal accretion and/or frontal and basal tectonic erosion. Mean characteristic parameters (!.t, !.t* • and [) are used to discuss the mechanical stability of 28 transects across the frontal part of convergent margins where the Coulomb theory is applicable. Natural observations reveal that "typical accretionary wedges" are characterized by low tapers with smooth surface slope and subducting plate, low convergence rates and thick trench sediment, while "nonaccretionary wedges" display large tapers with irregular surface slopes and rough subducting plate, high convergence rates and almost no trench fill. Sandbox experiments were performed to illustrate the effects of seamounts/ridges in the subduction zone on the deformation of an accretionary wedge. These experiments show that a wedge of sand is first trapped and pushed in front of the seamount which acts as a moving bulldozer. This is followed by a tunnelling effect of the subducting seamount through the frontal wedge material, which results in considerable sand reworking. At an advanced subduction stage, the d6collement jumps back from a high level in the wedge to its former basal position. We conclude that a high trench sedimentation rate relative to the convergence rate leads to frontal accretion. In contrast, several conditions may favor tectonic erosion of the upper plate. First, oceanic features, such as grabens, seamounts or ridges, may trap upper plate material during their subduction process. Second, destabilization of the upper plate material by internal fluid overpressuring causing hydrofracturing is probably another important mechanism.
ISSN: 0148-0227

hal-01261600
https://hal.archives-ouvertes.fr/hal-01261600
https://hal.archives-ouvertes.fr/hal-01261600/document
https://hal.archives-ouvertes.fr/hal-01261600/file/lallemandJGR1994.pdf

Éditeur(s) : HAL CCSD Oxford University Press (OUP)
Résumé : International audience
Mantle-derived xenoliths hosted by melilitite lavas from In Teria (Ahaggar, SE Algeria) include garnet and spinel peridotites, pyroxenite and phlogopite megacrysts. The spinel and garnet peridotites record an early deformation event, which formed porphyroclastic microstructures and olivine crystal preferred orientations, followed by static infiltration of hydrous alkaline melts. This metasomatic stage (stage 1) is characterized by the crystallization of phlogopite in the garnet and spinel peridotites, amphibole in the spinel peridotites and clinopyroxene in the garnet peridotite, which record chemical equilibration with an alkaline silicate melt. These early events were largely overprinted by carbonatitic metasomatism (stage 2), which is observed only in the spinel peridotites. Spinel peridotite major and trace element compositions, as well as the compositions of newly formed minerals, are characteristic of interaction with carbonate melt, associated with strong enrichment in incompatible trace elements in clinopyroxene. This second stage was followed by crystallization of pyroxenites (stage 3) in vein conduits, probably segregated from alkaline melts. We propose a scenario in which the different metasomatic imprints record successive stages of interaction between lithospheric mantle and sublithospheric melts throughout the Cenozoic. In Sr–Nd isotope space, the host melilitites and several xenoliths are clustered and plot close to the HIMU mantle end-member. However, some peridotite xenoliths are shifted towards more radiogenic 87Sr/86Sr values. In 207Pb/204Pb–206Pb/204Pb and 208Pb/204Pb–06Pb/204Pb space the In Teria samples define a relatively large domain characterized by high 206Pb/204Pb and 208Pb/204Pb, consistent with a contribution of an HIMU component, considered to represent a sublithospheric signature. The highest 87Sr/86Sr values are comparable with those ascribed to the EM1 mantle end-member, representing the signature of the lower continental lithosphere, and are probably inherited from the pre-metasomatic lithospheric mantle beneath In Teria. Numerical modelling of porous percolation of melt of sublithospheric origin through an EM1-like lithospheric mantle protolith reproduces the In Teria peridotite compositions, using moderately sub-chondritic Sr/Nd values for the peridotite (e.g. In Teria garnet peridotite) and moderately super-chondritic Sr/Nd values in the melt (approximately ocean island basalt values). A few spinel peridotites require a component characterized by a 143Nd/144Nd signature higher than both the EM1 end-member and the local Ahaggar basalts; the 208Pb/204Pb compositions of several samples point to a component with a depleted mid-ocean ridge basalt (MORB) mantle (DMM) signature. Thus the lithospheric mantle beneath In Teria probably did not have a uniform EM1 signature before the onset of metasomatism; it included a DMM peridotite component as well as some peridotites with elevated 143Nd/144Nd values recording long-term LREE depletion.
ISSN: 0022-3530

hal-01356781
https://hal.archives-ouvertes.fr/hal-01356781
DOI : 10.1093/petrology/egw009

Éditeur(s) : Elsevier Sci Ltd
Résumé : A multi-scale global hindcast of ocean waves is presented that covers the years 1994-2012, based on recently published parameterizations for wind sea and swell dissipation [Ardhuin, F., Rogers, E., Babanin, A., Filipot, J.-F., Magne, R., Roland, A., van der Westhuysen, A., Queffeulou, P., Lefevre, J.-M., Aouf, L., Collard, F., 2010. Semi-empirical dissipation source functions for wind-wave models: Part I. Definition, calibration and validation. J. Phys. Oceanogr. 40 (9), 1917-1941]. Results from this hindcast include traditional wave parameters, like the significant wave height and mean periods, and we particularly consider the accuracy of the results for phenomenal sea states, with significant heights above 14 m. Using unbiased winds, there is no evidence of a bias in wave heights even for this very high range. Various spectral moments were also validated, including the surface Stokes drift and mean square slopes that are relevant for wave-current interactions modelling and remote sensing, and also spectra of seismic noise sources. The estimation of these parameters is made more accurate by the new wave growth and dissipation parameterizations. Associated air-sea fluxes of momentum and energy are significantly different from what is obtained with the WAM-Cycle 4 parameterization, with a roughness that is practically a function of wind speed only. That particular output of the model does not appear very realistic and will require future adjustments of the generation and dissipation parameterizations.
Ocean Modelling (1463-5003) (Elsevier Sci Ltd), 2013-10 , Vol. 70 , P. 174-188

Droits : 2012 Elsevier Ltd. All rights reserved.
http://archimer.ifremer.fr/doc/00155/26582/25477.pdf
DOI:10.1016/j.ocemod.2012.12.001
http://archimer.ifremer.fr/doc/00155/26582/

Résumé : Reconnus dans toute la zone intertropicale comme outils d'aménagement incontournables des petites pêches côtières, les DCP (Dispositifs de Concentration de Poissons) ont été implantés autour de La Réunion à partir de 1988 et gérés collectivement par le Comité Régional des Pêches. Ces DCP côtiers, ancrés à quelques milles des ports de pêche ont permis un redéploiement d'une grande partie de l'effort de pêche vers les espèces pélagiques du large, diminuant ainsi la pression exercée sur les espèces récifales côtières. La présente étude a pour objectifs de mettre à jour les connaissances techniques et économiques sur le "système DCP", autour des pêcheurs, de la pêche sur DCP et des DCP eux-mêmes. Une enquête des usages et des perceptions des pêcheurs vis-à-vis des DCP ancrés a été réalisée auprès de plus de la moitié des patrons de la petite pêche réunionnaise. Les principaux types d’usages et de stratégies mettent en évidence la forte polyvalence des pêcheurs, qui pratiquent quasiment tous la pêche sur DCP dont une majorité (51%) montrant un degré de dépendance élevé vis-à-vis de l’outil. Les DCP sont plébiscité par l’ensemble des pêcheurs, au titre de l’aménagement durable de la zone côtière, pour éviter report de la pression de pêche sur les ressources démersales. La pêche plaisance sur DCP est pointée du doigt, comme un facteur important de perturbation du système (conflits sur DCP, non-respect de la règlementation, etc…). Des sorties en mer ont rapporté les activités de pêche de façon détaillée : description des opérations de pêche, variabilité spatiale de la productivité de chaque DCP, apports des DCP ancrés dans la compétitivité économique des entreprises. Mais avec 25 marées seulement et le parc de DCP à moitié en place, seule la méthode et de premiers résultats ont pu être obtenus Ces sorties en mer doivent être poursuivies. L'exploitation des données 2002-2012 du Comité Régional des Pêches de La Réunion sur sa gestion du parc de DCP permet une description précise des évènements et des coûts concernant les DCP : poses, entretiens, pertes, etc. Il en ressort que la durée de vie moyenne des DCP ne dépasse guère 14 mois, avec une grande disparité géographique (durées de vie plus faibles au nord et à l’est), et un délai de repose conséquent de 5 mois1/2 en moyenne. L'analyse des coûts de maintenance et de main d'œuvre met en lumière la nécessité de stabiliser les financements sur un budget de 70 à 80 k€ /an pour permettre un taux optimum de remplissage du parc (> 80 %). Des perspectives d’amélioration technologique des dispositifs sont également présentées, tant sur la ligne de mouillage que sur le choix des flotteurs. Le modèle intermédiaire PLK à chapelet renforcé testé à la Réunion en mai 2012 sur 5 DCP semble être un compromis intéressant pour la Réunion à court et moyen terme pour les DCP ancrés les plus au large. Outre la poursuite des sorties en mer pour le suivi halieutique détaillé il convient maintenant d'engager une réflexion globale sur la gouvernance du "système DCP" à La Réunion (financement, gestion). Les enjeux sont essentiels pour le secteur de la petite pêche artisanale réunionnaise.

Droits : 2012 Ifremer, CRPM La réunion, PARETO Ecoconsult
http://archimer.ifremer.fr/doc/00109/22027/19679.pdf
http://archimer.ifremer.fr/doc/00109/22027/19680.pdf
http://archimer.ifremer.fr/doc/00109/22027/

Éditeur(s) : HAL CCSD American Geophysical Union
Résumé : International audience
We propose in this paper an original method for quantifying the material losses at subduction zones mainly based on vertical movements of the active margins. We first test the method, using isostatic and elastic models, applying it to 15 different well-constrained convergent margins. The main conclusion is that most of them are presently close to the isostatic equilibrium. Consequently, we consider as a first hypothesis that such was also the case in the past and that erosion or accretion are natural mechanisms restoring the equilibrium in response to a perturbation. Then, we apply this model to compute the geometry of the palcoslab knowing the palcotopography of the margin. The comparison between present and past ocean margin lithospheric wedge geometries provides estimates of the removed material that constituted the initial wedge. Two cases were studied for palcoreconstructions: Japan and Peru. The results show that a considerable amount of material was removed during the Neogene along both margins. A detailed interpretation of the Neogene history of the Japan margin is proposed on the basis of our results.
ISSN: 0148-0227

hal-01261647
https://hal.archives-ouvertes.fr/hal-01261647
https://hal.archives-ouvertes.fr/hal-01261647/document
https://hal.archives-ouvertes.fr/hal-01261647/file/lallemanJGR1992.pdf
DOI : 10.1029/91JB02342

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
Subduction processes play a major role in plate tectonics and the subsequent geological evolution of Earth. This special issue focuses on ongoing research in subduction dynamics to a large extent (oceanic subduction, continental subduction, obduction…) for both past and active subduction zones and into mountain building processes and the early evolution of orogens. It puts together various approaches combining geophysics (imaging of subduction zones), petrology/geochemistry (metamorphic analysis of HP-UHP rocks, fluid geochemistry and magmatic signal, geochronology), seismology and geodesy (present-day evolution of subduction zones, active tectonics), structural geology (structure and evolution of mountain belts), and numerical modelling to provide a full spectrum of tools that can be used to constrain the nature and evolution of subduction processes and orogeny. Studies presented in this special issue range from the long-term (orogenic cycle) to short-term (seismic cycle).
ISSN: 0264-3707

insu-01287985
https://hal-insu.archives-ouvertes.fr/insu-01287985
https://hal-insu.archives-ouvertes.fr/insu-01287985/document
https://hal-insu.archives-ouvertes.fr/insu-01287985/file/rolland-2016.pdf
DOI : 10.1016/j.jog.2016.03.006

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
The crust of the Paleoproterozoic West African Craton in northern Ghana is dominantly made of Tonalite-Trondhjemite-Granodiorite (TTG) suites, low-K to high-K calc-alkaline granites, with minor LILE-enriched diorites, and chemically related lavas. Results of zircon U-Pb dating indicate that all these different rock types were formed together over a prolonged period of ca. 100 Ma, between 2.21 and 2.11 Ga. Zircon Hf isotope compositions of five samples are supra-chondritic (εHft from +1 to +6) indicating that the granitoids derive from the reworking of juvenile crustal components as old as 2.60 Ga. Geochemical data show that the TTGs were derived from the reworking of a low-K mafic crust, while biotite granites formed by the reworking of older felsic crustal rocks, and diorites derived from the melting of LILE-enriched mantle sources.The combined datasets support a three-step model for the formation of the crust of the West African craton in northern Ghana: (1) a juvenile, mafic proto-crust was extracted from the depleted mantle between 2.60 and 2.30 Ga, in an oceanic environment remote from any pre-existing continental nucleus; (2) the mafic proto-crust was reworked in several primitive magmatic arcs or in accreted oceanic plateaux. Crustal reworking initially produced minor felsic magmatism between 2.35 and 2.21 Ga, followed by the 2.21–2.15 Ga emplacement of low- to medium-P TTG and K-rich calc-alkaline granitoid suites, and was coeval with the formation of an enriched mantle source. (3) Various proto-continental terranes collided during the Eburnean orogeny between 2.14 and 2.11 Ga, triggering intense reworking of the composite crust and mantle reservoirs formed in the previous two steps, i.e. remnants of the juvenile mafic crust, older TTGs and biotite granites, and previously enriched mantle sources. The emplacement of high-P TTGs, granites and LILE-enriched diorites resulted from lower crustal heating in response to the delamination of the lower crust and lithospheric mantle. The Eburnean collision therefore marks the final stages of a 200–500 Ma long period of juvenile continental crust formation and stabilisation. The datasets further reveal that the Paleoproterozoic West African Craton comprises a minimum of two distinct crustal blocks, with north-western Ghana lying at their interface.
ISSN: 0301-9268

hal-01467999
https://hal.archives-ouvertes.fr/hal-01467999
DOI : 10.1016/j.precamres.2016.10.011

Éditeur(s) : Amer Geophysical Union
Résumé : We quantify, analyze, and characterize the frequency-dependent microseismic noise recorded by worldwide distributed seismic stations. Microseismic noise is generated through the interaction of ocean waves. It is the strongest ambient noise, and it is observed everywhere on Earth. We introduce a new approach which permits us to detect polarized signals in the time-frequency domain and which we use to characterize the microseismic noise. We analyze 7 years of continuous seismograms from the global GEOSCOPE network. Microseisms are dominated by Rayleigh waves, and we therefore focus on elliptically polarized signals. The polarized signals are detected in the time-frequency domain through a degree of polarization measure. We design polarization spectra and show that microseismic noise is more strongly polarized than noise in other frequency bands. This property is used to measure the directions of the polarized noise at individual stations as a function of time and frequency. Seasonal variations are found for the back azimuths and for the number of polarized signals at many stations. We show that the back azimuth directions are robust measurements that point toward the source areas computed from ocean wave models.
Geochemistry Geophysics Geosystems (1525-2027) (Amer Geophysical Union), 2011-07 , Vol. 12 , N. Q07014 , P. 14 p.

Droits : 2011. American Geophysical Union. All Rights Reserved.
http://archimer.ifremer.fr/doc/00041/15219/12713.pdf
DOI:10.1029/2011GC003661
http://archimer.ifremer.fr/doc/00041/15219/

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
The New Caledonia ophiolite (Peridotite Nappe) is dominated by mantle lithologies, composed of forearc-related refractory harzburgites and minor lherzolites in both the spinel and plagioclase facies.In this study, a comprehensive geochemical data set (major, trace elements and Sr–Nd isotopes) is used to constrain the mantle evolution of the lherzolites and their relationships with the basalts from the Poya Terrane, which tectonically underlies the mantle rocks. The majority of the lherzolites are low-strain porphyroclastic tectonites. They likely record an asthenospheric origin followed by re-equilibration at lithospheric conditions, as supported by geothermometric estimates (T = 1100–940 °C and 920–890 °C for porphyroclastic and neoblastic spinel-facies assemblages, respectively). Olivine composition (Fo = 88.5–90.0 mol%), spinel Cr# ([molar 100 • Cr/(Cr + Al)] = 13–17) and relatively high amounts (7–8 vol%) of Al2O3- and Na2O-rich clinopyroxene (up to 0.5 and 6.5 wt.%, respectively) indicate a moderately depleted geochemical signature for the spinel lherzolites. Bulk rock and clinopyroxene rare earth elements (REE) patterns display a typical abyssal-type signature, i.e. steeply plunging LREE accompanied by nearly flat HREE to MREE. Clinopyroxene REE compositions of the spinel lherzolites may be reproduced by small amounts of fractional melting of a garnet lherzolite precursor (~ 4%), followed by 4%–5% melting in the spinel peridotite field. The plagioclase lherzolites show melt impregnation microstructures, Cr- and Ti-rich spinels and incompatible trace element enrichments (REE, Ti, Y, and Zr) in bulk rocks and clinopyroxenes. Impregnation modelling for these elements suggests that the plagioclase lherzolites originated from residual spinel lherzolites by entrapment of highly depleted (non-aggregated) MORB melt fractions in the shallow oceanic lithosphere. Nd isotope compositions of the investigated peridotites are consistent with derivation from an asthenospheric mantle source that experienced a recent MORB-producing depletion event. This evolution was most likely accomplished in a spreading ridge. However, geochemical trace element modelling and Nd isotopes do not support a genetic mantle–crust link between the lherzolites and enriched-MOR-type basalts from the Poya Terrane.
ISSN: 0024-4937

hal-01412440
https://hal.archives-ouvertes.fr/hal-01412440
DOI : 10.1016/j.lithos.2016.04.030

Éditeur(s) : HAL CCSD Oxford University Press (OUP)
Résumé : International audience
Anisotropy of low-field magnetic susceptibility (AMS) of basaltic lava flows can give some clues about post-emplacement tilting occurring in volcanic sequences. Such a study has been carried out on a sequence of 19 successive lava flows from Kerguelen Archipelago. Surprisingly, two different patterns were observed. The first one - herein called normal fabric - is consistent with the lava-flow direction inferred from geological observations, whereas the second one observed for about 70 per cent of the analysed samples - herein called abnormal fabric - appears to be unrelated to the shear history of the lava flows during their emplacements. These abnormal fabrics are not strictly inverse fabric but seem to have a more complex origin. The unusual nature of these abnormal fabrics has been confirmed by lattice preferred orientation of plagioclases obtained from high-resolution electron backscattered diffraction measurements. The aim of this study is to propose a physical interpretation of these fabrics. A first step in our understanding was to clearly identify the nature and the magnetic properties of the Fe-Ti oxides assemblage. Thus, we present a comprehensive rock magnetic analysis relying on low- and high-temperature thermomagnetic curves (K-T), crossover diagrams, first-order reversal curves, ore-microscopy, Mössbauer measurements and electron backscattered images. We found that in the present case the abnormal fabrics are probably linked to a mixture of prevailing single domain (SD) strongly interacting grains population and a subsidiary multidomain grains population. SD grains are identified as small magnetite crystallized within the interstitial glass in globular aggregates along the silicate framework. Then a question arises: to what extent such SD interacting grains in globular aggregates can affect the magnetic susceptibility and its anisotropy? To answer this question, we developed a physical model in which the total energy density of an SD grain with an uniaxial anisotropy is minimized. In this model, the distribution function of the direction of the magnetic interaction field is comprised between two boundary states. It is either isotropic, which formally corresponds to the thermal demagnetized state, or it is ordered, which formally corresponds to the Alternative Field demagnetized state. We show that in both cases the degree of anisotropy decreases as the interaction field increases. Thus, we conclude that the abnormal fabric encountered in this study can be simply explained by strong magnetostatic interactions that would explain the large scattering of AMS often observed in basaltic lava flows.
ISSN: 0956-540X

hal-00716449
https://hal.archives-ouvertes.fr/hal-00716449
https://hal.archives-ouvertes.fr/hal-00716449/document
https://hal.archives-ouvertes.fr/hal-00716449/file/gji-S-12-HAL.pdf
DOI : 10.1111/J.1365-246X.2012.05421.X