Éditeur(s) : Copernicus Gesellschaft Mbh
Résumé : During the GWADASEIS cruise (Lesser Antilles volcanic arc, February-March 2009) a very high resolution (VHR) seismic-reflection survey was performed in order to constrain Late Quaternary to Present faulting. The profiles we obtained evidence frequent "ponding" of reworked sediments in the deepest areas, similar to the deposition of Mediterranean "homogenites". These bodies are acoustically transparent (few ms t.w.t. thick) and are often deposited on the hanging walls of dominantly normal faults, at the base of scarps. Their thickness appears sufficient to compensate (i.e. bury) co-seismic scarps between successive earthquakes, resulting in a flat and horizontal sea floor through time. In a selected area (offshore Montserrat and Nevis islands), piston coring (4 to 7m long) was dedicated to a sedimentological analysis of the most recent of these particular layers. It corresponds to non-stratified homogenous calcareous silty sand (reworked calcareous plankton and minor volcanoclastics). This layer can be up to 2 m thick, and overlies fine-grained hemipelagites. The upper centimeters of the latter represent the normal RedOx water/sediment interface. Pb-210 and Cs-137 activities lack in the massive sands, while a normal profile of unsupported Pb-210 decrease is observed in the hemipelagite below, together with a Cs-137 peak corresponding to the Atmospheric Nuclear Experiments (1962). The RedOx level was thus capped by a recent instantaneous major sedimentary event considered as post-1970 AD; candidate seismic events to explain this sedimentary deposits are either the 16 March 1985 earthquake or the 8 October 1974 one (M-w = 6.3 and M-w = 7.4, respectively). This leads to consider that the syntectonic sedimentation in this area is not continuous but results from accumulation of thick homogenites deposited after the earthquakes (as observed in the following weeks after Haiti January 2010 event, McHugh et al., 2011). The existence of such deposits suggests that, in the area of study, vertical throw likely results from cumulated effects of separated earthquakes rather than from aseismic creep. Examination of VHR profiles shows that all major co-seismic offsets are recorded in the fault growth sequence and that co-seismic offsets can be precisely estimated. By using a sedimentation rate deduced from Pb-210 decrease curve (0.5 mmyr(-1)) and taking into account minor reworking events detected in cores, we show that the Redonda system may have been responsible for five > M-6 events during the last 34 000 yr. The approach presented in this work differs from fault activity analyses using displaced sets of isochronous surfaces and postulating co-seismic offsets. Combining VHR seismic imagery and coring we can decipher co-seismic vs. slow continuous displacement, and thus actually estimate the amplitude and the time distribution of major co-seismic offsets.
Natural Hazards And Earth System Sciences (1561-8633) (Copernicus Gesellschaft Mbh), 2012 , Vol. 12 , N. 5 , P. 1755-1767

Droits : Author(s) 2012. CC Attribution 3.0 License.
http://archimer.ifremer.fr/doc/00089/20040/21990.pdf
DOI:10.5194/nhess-12-1755-2012
http://archimer.ifremer.fr/doc/00089/20040/

Éditeur(s) : Université des Antilles Études caribéennes
Résumé : Les îles de la Caraïbe orientale sont particulièrement exposées aux éruptions volcaniques. L’ensemble des Petites Antilles comprend en effet une vingtaine de volcans actifs, aux styles éruptifs parmi les plus dangereux et destructeurs. La définition de facteurs de vulnérabilité autorise des comparaisons et un classement des îles en fonction de leur niveau d’exposition. Cette approche permet d’attirer l’attention sur la nécessité d’une gestion proactive des risques volcaniques en contexte insulaire.
The Eastern Caribbean islands are particularly exposed to volcanic eruptions. The Lesser Antilles arc indeed includes a score of active volcanoes, whose eruptive styles are among the most dangerous and destructive. The definition of vulnerability factors leads to some comparisons and a classification of the islands according to their level of exposure. This approach draws the attention to the imperative need for a proactive management of the volcanic risks in this insular context.
Caraïbes

Droits : info:eu-repo/semantics/openAccess
urn:doi:10.4000/etudescaribeennes.316
http://journals.openedition.org/etudescaribeennes/316

Éditeur(s) : American Geophysical Union
Résumé : Magmatism associated with subducting plate edges or slab tears has been suggested in the southern Okinawa Trough. The cross back-arc volcanic trail, which consists of a cluster of about 70 seamounts, is located above a Ryukyu slab tear lying along the 123.3°E meridian. In November 2003, more than 3300 earthquakes recorded in this area by 15 ocean bottom seismometers and surrounding land stations during a period of 12 days were used to determine the three-dimensional Vp and Vs velocity structures and Vp/Vs ratios. A mantle inflow characterized by low Vp and Vs and high Vp/Vs passing through the slab tear is imaged. The fluid and/or melt component is rising obliquely from the slab tear in the directions of the cross back-arc volcanic trail, the northern slope of the southern Okinawa Trough and to north of Iriomote Island. The asthenospheric intake is also imaged by an inclined chip-like high Vp/Vs and low Vp and Vs body dipping northerly, which might be linked to the slab retreat. West of the slab tear, most of the earthquakes are located around low Vp and Vs and high Vp/Vs bodies, which suggests that the seismicity is related to magmatic and/or fluid activities. East of it, earthquakes are concentrated in an area characterized by high Vp and Vs velocities and low Vp/Vs, suggesting that the magma chamber is absent beneath the axial part of the trough and that normal faulting is the main factor controlling the seismicity.
Journal of Geophysical Research ( JGR ) - Solid earth (0148-0227) (American Geophysical Union), 2007-08 , Vol. 112 , N. B8 , P. NIL_13-NIL_32

Droits : 2007 AGU
http://archimer.ifremer.fr/doc/2007/publication-3032.pdf
DOI:10.1029/2006JB004703
http://archimer.ifremer.fr/doc/00000/3032/

Éditeur(s) : HAL CCSD Société géologique de France
Résumé : International audience
In this paper we present and discuss new investigations performed on both the magmatic basement and the sedimentary formations of La Désirade. We report structural and sedimentary evidences for several episodes of deformation and displacement occurring prior to the present day tectonics. The main faults, respectively N130 ± 10°, N040 ± 10° and N090 ± 10°, previously considered as marker of the current tectonic regime corresponds to reactivated tectonic structures developed first during late Cretaceous compression and second during Pliocene to early Pleistocene extension. We demonstrate also the importance of late Pliocene-early Pleistocene and middle-late Pleistocene vertical movements in this part of the Lesser Antilles fore-arc as well as the role of compressive tectonics in the over thickened character of the arc basement in the Guadeloupe archipelago
EISSN: 1777-5817

hal-00830329
https://hal.archives-ouvertes.fr/hal-00830329
DOI : 10.2113/gssgfbull.184.1-2.21

Éditeur(s) : HAL CCSD
Résumé : International audience
Timor Island was at time considered as an example of "accretionary prism" linked to the collision between the Australian block and the Banda arc. However, its geological evolution is more complex. Five main superimposed structural units are distinguished in West Timor. The today structure is the result of three main tectonic events that occurred during the Late Oligocene, Late Early Pliocene and Late Pliocene-Early Pleistocene times, respectively. Our field investigations in the 1990 to 2000 decade completed with geochemical analyses and K-Ar datings (Jurassic and Miocene ages) of magmatism allow to precise the geodynamic evolution of Timor that can be summarized as follows: a first block was detached from Gondwana (unit 2) and drifted to the Asiatic margin until the Late Oligocene when it collided with the Asiatic active margin (unit 3). Then, the new block formed by both 2 and 3 units drifted to the south during the Miocene and the Early Pliocene until it collided with the Australian margin (ASM), by the Late Early Pliocene. Then, the Australian and Timor blocks moved together towards the North-North East during the Late Pliocene until they collided with the Banda fore-arc (unit 4). Later on (Pleistocene), Timor island was capped by the "autochthon" (unit 5) and then on (Quaternary?) by the Banda volcanic arc northward thrusted over the South Banda basin. Taking in consideration its close relationships with both the Australian plate and the Eurasian one. Timor may be considered as a key area for building this geodynamical scenario of Indonesia.
Bulletin Société Géologique de France

insu-00933548
https://hal-insu.archives-ouvertes.fr/insu-00933548
DOI : 10.2113/gssgfbull.184.6.569

Éditeur(s) : HAL CCSD Wiley-Blackwell
Résumé : International audience
Garnet growth in high-pressure, mafic garnet granulites formed by dehydration melting of hornblende-gabbronorite protoliths in the Jijal complex (Kohistan palaeo-island arc complex, north Pakistan) was investigated through a microstructural EBSD-SEM and HRTEM study. Composite samples preserve a sharp transition in which the low-pressure precursor is replaced by garnet through a millimetre-sized reaction front. A magmatic foliation in the gabbronorite is defined by mafic-rich layering, with an associated magmatic lineation defined by the shape-preferred orientation (SPO) of mafic clusters composed of orthopyroxene (Opx), clinopyroxene (Cpx), amphibole (Amp) and oxides. The shape of the reaction front is convoluted and oblique to the magmatic layering. Opx, Amp and, to a lesser extent, Cpx show a strong lattice-preferred orientation (LPO) characterized by an alignment of [001] axes parallel to the magmatic lineation in the precursor hornblende-gabbronorite. Product garnet (Grt) also displays a strong LPO. Two of the four < 111 > axes are within the magmatic foliation plane and the density maximum is subparallel to the precursor magmatic lineation. The crystallographic relationship < 111 >(Grt)//[001](Opx,Cpx,Amp) deduced from the LPO was confirmed by TEM observations. The sharp and discontinuous modal and compositional variations observed at the reaction front attest to the kinetic inhibition of prograde solid-state reactions predicted by equilibrium-phase diagrams. The P-T field for the equilibration of Jijal garnet granulites shows that the reaction affinities are 5-10 kJ mol.(-1) for the Grt-in reaction and 0-5 kJ mol.(-1) for the Opx-out reaction. Petrographic and textural observations indicate that garnet first nucleated on amphibole at the rims of mafic clusters; this topotactic replacement resulted in a strong LPO of garnet. Once the amphibole was consumed in the reaction, the parallelism of [001] axes of the mafic-phase reactants favoured the growth of garnet crystals with similar orientations over a pyroxene substrate. These aggregates eventually sintered into single-crystal garnet. In the absence of deformation, the orientation of mafic precursor phases conditioned the nucleation site and the crystallographic orientation of garnet because of topotaxial transformation reactions and homoepitaxial growth of garnet during the formation of high-pressure, mafic garnet-granulite after low-pressure mafic protoliths.
ISSN: 0263-4929

hal-00411323
https://hal.archives-ouvertes.fr/hal-00411323
DOI : 10.1111/J.1525-1314.2008.00789.x

Éditeur(s) : Gauthier-Villars
Résumé : The Barbados Ridge composes the major part of the forearc region of the Lesser Antilles magmatic arc. The toe of the ridge is almost certainly the locus of modern accretion of sediment from the subducting Atlantic seafloor. The pre-Pleistocene geology of the island of Barbados, which exposes the crest of the Barbados Ridge, is best explained by accretion, implying that the ridge as a whole is an accretionary prism. Important elements of the island geology in this interpretation are the occurrence of much if not all of the pre-Pleistocene rocks in fault-bounded packets and the juxtaposition of age-overlapping abyssal pelagite and continent-derived fan deposits in such packets. The orientation of tectonic structures on Barbados, however, implies that the Paleogene subduction that created the older part of the prism occurred in a differently configured zone from the present one.
Oceanologica Acta, Special issue (0399-1784) (Gauthier-Villars), 1981

Droits : info:eu-repo/semantics/openAccess
http://archimer.ifremer.fr/doc/00245/35674/34182.pdf
http://archimer.ifremer.fr/doc/00245/35674/

Éditeur(s) : HAL CCSD Société géologique de France
Résumé : International audience
In this paper, we provide the first structural map of Les Saintes archipelago (Guadeloupe, Lesser Antilles). The finite strain pattern displays four families of fault systems characterized by their statistical structural orientations: N000-N020, N050-N070, N090-N110 and N130-N140 trending fault systems. Our onshore results thus underline a fault network much more complex than the one depicted by the previous offshore geophysical investigations around Les Saintes archipelago, which show only N120-N150 trending system. According to the available K-Ar dating of the volcanic rocks and the relative chronology of the faults defined in the field, we determine the deformation history in Les Saintes islands since the last 3 Ma. The four highlighted trending fault systems are already active since the Pliocene and are consistent with the present-day extensional tectonics in the Guadeloupe archipelago compatible with the reactiva-tion of inherited structures at the active arc scale. We interpret the tectonic evolution of Les Saintes islands as the result of interplay between subduction of aseismic ridges (Tiburon and Barracuda ridges) and oblique convergence. Furthermore , we recognized an exhumed geothermal paleo-system in Terre-de-Haut island which is a good analogue of the present-day active Bouillante geothermal system. Its duration is estimated at 400 k.y. during the Pliocene.
EISSN: 1777-5817

hal-01625778
https://hal.univ-antilles.fr/hal-01625778
https://hal.univ-antilles.fr/hal-01625778/document
https://hal.univ-antilles.fr/hal-01625778/file/VeratIEtAl_Final.pdf
DOI : 10.2113/gssgfbull.187.1.3

Résumé : For several years the fishermen of the West Indies have directed their activities towards new fishing grounds, notably the bathyales zones adjacent to island plateaux up to approximately 1000 metres in depth. Explorations earned out since 1989 have shown that diverse crustacean populations could be exploited. However, populations and exploitable stocks arc not well known. The use of crustaceans traps as a fishing technique permits the capture of shellfish, shrimps and prawns of which at least six species could have commercial interest. The main objective of this work is do draw up an inventory of the principal commercial species as well as the accompanying species belonging to the subordres Decapoda macrura, anomura, brachyura, and to the orders Isopoda and Amphipoda. It is intended for fishermen and other socio-professionals of the fishing industry. It should enable them to identify and name these previously unexploited varieties of crustacea to facilitate their sale in local markets.
Depuis quelques années, des pécheurs antillais orientent leurs activités vers de nouveaux secteurs de pèche, notamment les zones bathyales adjacentes aux plateaux insulaires, jusque vers 1000 mètres de profondeur. Des campagnes exploratoires menées depuis 1989. on montré que diverses populations carcinologiques pouvaient l'aire l'objet d'une exploitation. Toutefois, populations et stocks exploitables ne sont pas bien connus. L'emploi du casier comme technique de pêche, permet la capture de crustacés: crevettes et langoustines, dont au moins 6 espèces peuvent avoir un intérêt commercial. Ce travail nomenclatural a pour premier objectif de dresser l'inventaire ces principales espèces commerciales ainsi que des espèces accompagnatrices appartenant aux infra-ordres des Décapodes macroures, anomoures, brachyoures et aux ordres des Isopodes et des Amphipodes. Il est destine aux pécheurs et aux autres personnes des catégories socioprofessionnelles de la filière pêche. II devrait leur permettre d'identifier et de nommer ces produits nouveaux pour faciliter leur vente sur les marchés locaux.

Droits : info:eu-repo/semantics/openAccess
http://archimer.ifremer.fr/doc/1993/rapport-992.pdf
http://archimer.ifremer.fr/doc/00000/992/

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
Reef positions record the interaction between eustasy and tectonics, and have been used worldwide to characterize vertical deformations of upper-plates at different time-scales and constrain the seismic behavior of megathrusts. Along the Lesser Antilles volcanic arc, high-resolution marine geophysical data were collected on the 2–20 km wide eastern Martinique reef platform to reconstruct its stratigraphic and morphologic history, and understand the influence of local normal faulting, volcanism and plate-scale subduction processes on Holocene and Late Pleistocene reef development. The subsiding Martinique platform's stratigraphy is composed of multiple superimposed sea-level highstand deposits separated by subaerial exposure surfaces of sea-level low stands. The carbonate platform consists of two laterally-extensive carbonate units (unit U2 overlying unit U3) that extend to the platform edge to a depth of − 95 m MSL (mean sea level), and form two morphologic terraces, M2 and M3 respectively. The landward portion of unit U2 is partially overlain between 0 and − 60 m MSL by the living reef tract U1. The current reef is composed of a landward fringing reef, a lagoon and a seaward barrier reef, the latter forming a double-bank barrier around the Caravelle Peninsula. In near-shore multi-channel seismic profiles, a distinct reflector at ~− 35 m MSL, probably a subaerial exposure surface E1, separates the reef sequence formed during the last transgression from a Pleistocene fossil reef tract forming unit U2. Offshore of Mount Pelée volcano (Late Pleistocene), the Holocene reef did not develop above unit U2, whose upper surface is incised by channels and apparent sinkholes. During the Holocene transgression, the possibility of excessive turbidity due to volcanic activity may have inhibited reef development in this area. The un-dated unit U2 probably developed 120–130 ka ago during the last interglacial (MIS 5.5) + 6 m MSL highstand as thick, extensive reefs deposited all along the Lesser Antilles arc. Due to subsidence, MIS 5.5 reefs are not represented by onshore facies, except along the southern Sainte Anne Peninsula where normal faulting and uplift balances island-scale subsidence. Based on unit U2's present elevation and assuming an MIS 5.5 age and + 6 m MSL sea level, Martinique has subsided at maximum 0.3 m/ky, likely due to subduction processes that question the coupling state of the megathrust.
ISSN: 0025-3227

hal-01307036
https://hal.archives-ouvertes.fr/hal-01307036
DOI : 10.1016/j.margeo.2015.08.001

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 Archaeopress
Résumé : International audience
The first formative occupation of the West Indies was by continental populations, moving up from their place of origin in the Orinoco Basin, shortly before our era. In Martinique these groups, attached to the cedrosan saladoid sub-series, have been well studied and for a considerable time. It is upon this work that we have attempted to build an overall analysis of this phase of the prehistory of the Islands. The research drew upon the results of unpublished field work. This led to a more precise understanding of the nature and constitution of these archeological strata and, in turn, of the organisation of the Amerindian villages and their management of the island space. A review of their artisanal production ( ceramics, stone tools, personal adornment) has given us a closer understanding of these populations. It also provides the basis for a future paleo-ethnographic study. Further, an analysis of the origin of the lithic materials has enabled us to place the ancient saladoid occupation of Martinique both in the geography of the West Indian Arc and in the Carribean Basin as a whole. Progressively a picture was thus put together of pioneer agro-ceramicist groups characterised by a high degree of organisation and by the spiritual concepts expressed in their artisanal production.
Les premières occupations formatives des Antilles sont liées à la migration, peu avant le début de notre ère, de populations continentales originaires du bassin de l'Orénoque. En Martinique, l'étude de ces groupes, rattachés à la sous-série saladoïde cedrosane, a une histoire riche et ancienne. C'est fort de ces acquis que nous avons tenté de réaliser une analyse globale de cette phase de la préhistoire des Antilles. Notre recherche s'appuie sur l'analyse des résultats d'un ensemble d'opérations de terrain inédites. Les informations ainsi obtenues nous permettent d'étudier la nature et le mode de constitution de ces gisements archéologiques. On commence alors à distinguer l'organisation de ces villages amérindiens et le mode de gestion de l'espace insulaire qui s'y rattache. L'analyse de différents types de productions artisanales (céramique, outillage en pierre, parure) nous permet d'accéder à une connaissance plus intime de ces populations. Elle offre aussi les moyens nécessaires à la réalisation d'une future étude palethnographique. Par ailleurs, l'analyse de l'origine géographique des matières premières lithiques nous permet de réintégrer l'occupation saladoïde ancienne de la Martinique dans l'espace géographique qui est le sien!: l'arc antillais et plus largement le bassin caraïbe. Ainsi, se dégage progressivement l'image de groupes pionniers agro-céramistes caractérisés par un très haut degré d'organisation et par la matérialisation permanente de leurs conceptions spirituelles au travers de leur production artisanale.
https://hal.univ-antilles.fr/hal-00966882

ISBN : 1 84171 649 9
hal-00966882
https://hal.univ-antilles.fr/hal-00966882

Éditeur(s) : HAL CCSD Société géologique de France
Résumé : International audience
New geodynamical data from West Africa bring consistent informations on the pre-Mesozoic reconstruction within a large area running from the western Sahara to the Colombian cordillera. These new data support a Neoproterozoic Ocean (WANO) between the Amazonian (AMC) and West African (WAC) cratons previously to the Iapetus and Rheic oceans. We delineate 31 blocs detached from the surrounding three continents: NAC (North American Craton), AMC and WAC. 7 came from the WAC margin, 7 from the NAC, 6 from the AMC and 11 from an intermediate volcano sedimentary domain (COB) built on a 1200-1000 Ma oceanic crust. These imbricated blocks formed a tight mosaic by the Hercynian/Alleghanian tectonic event which gave way to the Pangea super-continent. But, during the Atlantic Ocean opening these blocks began to move. They were separated by new oceanic basins. However, previously to the Pangea, blocks from the COB domain formed two sets of garlands located on the northwestern Gondwana margin. The northern one moved to the North until the Silurian to collide the NAC (Taconic tectonic event) meanwhile the southern one remains on the Gondwana margin. All together were gathered by the Carboniferous/Permian time. Then, the framework for the opening of the Atlantic Ocean was not totally disconnected from the "Variscan" collage and many variscan weakness zones were re-used as initial breaking zones. Beyond this tectonic impact, the pre-mesozoic assemblage allows us to compare this "Caribbean" island arc with another one: the Indonesian "Banda" arc. Thus, West Africa is a geological key area for correlations between the Caribbean, the Appalachian, the Brazilian "Nordeste" and the West European domains and for the understanding of the Atlantic Ocean opening process.
EISSN: 1777-5817

hal-00842747
https://hal.archives-ouvertes.fr/hal-00842747
DOI : 10.2113/gssgfbull.184.1-2.5

Éditeur(s) : HAL CCSD European Geosciences Union
Résumé : International audience
During the GWADASEIS cruise (Lesser Antilles volcanic arc, February-March 2009) a very high resolution (VHR) seismic-reflection survey was performed in order to constrain Late Quaternary to Present faulting. The profiles we obtained evidence frequent "ponding" of reworked sediments in the deepest areas, similar to the deposition of Mediterranean "homogenites". These bodies are acoustically transparent (few ms t.w.t. thick) and are often deposited on the hanging walls of dominantly normal faults, at the base of scarps. Their thickness appears sufficient to compensate (i.e. bury) co-seismic scarps between successive earthquakes, resulting in a flat and horizontal sea floor through time. In a selected area (offshore Montserrat and Nevis islands), piston coring (4 to 7 m long) was dedicated to a sedimentological analysis of the most recent of these particular layers. It corresponds to non-stratified homogenous calcareous silty sand (reworked calcareous plankton and minor volcanoclastics). This layer can be up to 2 m thick, and overlies finegrained hemipelagites. The upper centimeters of the latter represent the normal RedOx water/sediment interface. 210Pb and 137Cs activities lack in the massive sands, while a normal profile of unsupported 210Pb decrease is observed in the hemipelagite below, together with a 137Cs peak correspond- ing to the Atmospheric Nuclear Experiments (1962). The RedOx level was thus capped by a recent instantaneous major sedimentary event considered as post-1970 AD; candidate seismic events to explain this sedimentary deposits are either the 16 March 1985 earthquake or the 8 October 1974 one (Mw=6.3 and Mw=7.4, respectively). This leads to consider that the syntectonic sedimentation in this area is not continuous but results from accumulation of thick homogenites deposited after the earthquakes (as observed in the following weeks after Haiti January 2010 event, McHugh et al., 2011). The existence of such deposits suggests that, in the area of study, vertical throw likely results from cumulated effects of separated earthquakes rather than from aseismic creep. Examination of VHR profiles shows that all major co-seismic offsets are recorded in the fault growth sequence and that co-seismic offsets can be precisely estimated. By using a sedimentation rate deduced from 210Pb decrease curve (0.5 mm yr−1) and taking into account minor reworking events detected in cores, we show that the Redonda system may have been responsible for five >M6 events during the last 34 000 yr. The approach presented in this work differs from fault activity analyses using displaced sets of isochronous surfaces and postulating co-seismic offsets. Combining VHR seismic imagery and coring we can decipher co-seismic vs. slow continuous displacement, and thus actually estimate the amplitude and the time distribution of major co-seismic offsets.
ISSN: 1561-8633

insu-00844581
https://hal-insu.archives-ouvertes.fr/insu-00844581
https://hal-insu.archives-ouvertes.fr/insu-00844581/document
https://hal-insu.archives-ouvertes.fr/insu-00844581/file/nhess-12-1755-2012.pdf
DOI : 10.5194/nhess-12-1755-2012

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
The Souttoufide belt is situated between the High-Atlas in the north, the Mauritanides in the south and the West African Craton as well as the Tindouf basin in the east. Thus, this belt is a part of the West African fold belt system which surrounds the West African Craton. Outcrops occur only in three areas, while the rest of the Souttoufide belt is covered by Meso- to Cenozoic sediments. These areas are, from the south to the north, the Adrar Souttouf Massif, the Smara Zemmour area, and the western part of the Anti-Atlas. Because of the continuing processing and numerous publications, the Anti-Atlas cannot be dealt with in detail. Fundamental geological work in both of the other areas was mainly done during the second third of the 20th century, followed by several decades of inactivity. Numerous studies were done in the Adrar Souttouf massif during the last decade, while the Smara-Zemmour area still remains poorly investigated. Most of them were focused on geochronological analyses and geochemistry. Accordingly, the obtained data led to a new interpretation. The Adrar Souttouf is interpreted as a Pan-African belt exhibiting a Neoproterozoic island arc reworked by the Variscan collision between the North American Craton (NAC) and the West African Craton (WAC). The Smara-Zemmour area is considered as the external part of this Variscan belt thrusted onto the Palaeozoic Tindouf basin. The western Anti-Atlas as considered as a Pan-African active margin reworked during the Variscan orogen. Together, these three areas, lead to the hypothesis of a Late Pan-African belt capped by the Early and Middle Palaeozoic covers and then affected by the Middle Carboniferous tectonic event (330 Ma) related to the Pangaean assembly. Correlations with adjacent belts (Mauritanides and Anti-Atlas) are considered.
ISSN: 0899-5362

hal-01382815
https://hal.archives-ouvertes.fr/hal-01382815
DOI : 10.1016/j.jafrearsci.2015.04.016

Éditeur(s) : HAL CCSD Elsevier
Résumé : Kyanite and zoisite eclogites from the Limousin area in the Variscan French Massif Central (FMC) have been formed by deep subduction to 100 km depth at peak pressure-temperature conditions around 660 degrees C and 2.9 GPa. They belong to a thin tectonic unit comprising ophiolitic bodies devoid of HP-UHP markers and forming a transported suture zone now separating the two main lithotectonic units of the FMC with contrasted tectono-metamorphic features (namely the lower and upper allochthons). Composition of kyanite-eclogites is similar to supra-subduction-zone basalts found in back-arc basins or island arcs (LREE depleted, negative Nb-Ta anomalies, epsilon Nd: +6 to + 9). Zoisite eclogites have the major-element fingerprint of ultrabasic iron-rich plagioclase cumulates common in continental intrusions or forming Fe-Ti oceanic gabbros. The distribution of immobile trace-elements is however more comparable to alkaline Si-undersaturated rocks, but the Sr-Nd isotopic composition (Sr-87/Sr-86: 0.706; epsilon Nd: -3 to -5) suggests the intervention of an old radiogenic crustal component during their petrogenesis. The chemical composition of hydrous zoisite eclogites does not correspond closely to a common magmatic precursor. It is thus suggested that, by analogy with well exposed worldwide HP-UHP terranes, at least a part of the trace-element signature and the isotopic compositions are controlled by channelled fluid-rock interaction at UHP conditions, leading to localised segregation of zoisite that hosts nearly all Sr and LREE budgets of the bulk eclogite. Sr-Nd isotopic composition of these samples is thus controlled by zoisite and probably represents the isotopic composition of the fluid phase present during UHP recrystallisation. Zircons extracted from a zoisite eclogite were dated in-situ by LA-ICP-MS. Crystallisation of the magmatic precursor is bracketed between 489 and 475 Ma and the UHP event is dated at 412 +/- 10 Ma, in agreement with published ages for the Eo-Variscan HP stage in the FMC. A late resetting of the U-Pb isotopic system at 382 +/- 7 Ma is related to an anatectic high pressure-medium pressure event, well known in the Limousin area. The oceanic eclogites dated in this study are tentatively linked to the Galicia-South Brittany Lower Paleozoic Ocean; they show evidence of the existence of an intra-oceanic subduction zone at the Lower Ordovician and of partial closure of this oceanic domain by ocean-continent subduction during Early Devonian.
ISSN: 0024-4937

hal-00512835
https://hal.archives-ouvertes.fr/hal-00512835
DOI : 10.1016/j.lithos.2010.05.013

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
We present an integrated study of geochemistry, petrofabrics and seismic properties of strongly sheared eclogites from the Chinese Continental Scientific Drilling (CCSD) project in the Sulu ultrahigh-pressure (UHP) metamorphic terrane, eastern China. First, geochemical data characterize diverse protoliths of the studied eclogites. The positive Eu- and Sr-anomalies, negative Nb anomaly and flat portion of heavy rare earth elements in coarse-grained rutile eclogites (samples B270 and B295) suggest a cumulate origin in the continental crust, whereas the negative Nb anomaly and enrichment of light rare earth elements in retrograde eclogites (samples B504, B15 and B19) imply an origin of continental basalts or island arc basalts. Second, P-wave velocities (V-p) of three typical eclogite samples were measured under confining pressures up to 500 MPa and temperatures to 700 degrees C. At 500 MPa and room temperature, the mean V-p reaches 8.50-8.53 km/s in samples B270 and B295 but drops to 7.86 km/s in sample B504, and the P-wave anisotropy changes from 1.7-2.7% to 5.5%, respectively. The pressure and temperature derivatives of V-p are larger in the retrograde eclogite than in fresh ones. Third, the electron backscatter diffraction (EBSD) measurements of the eclogites reveal random crystal preferred orientation (CPO) of garnet and pronounced CPO of omphacite, which is characterized by a strong concentration of [001]-axes sub-parallel to the lineation and of (010)-poles perpendicular to the foliation. The asymmetric CPO of omphacite in sample B270 recorded a top-to-the-south shear event during subduction of the Yangtze plate. The calculated fastest V-p is generally sub-parallel to the lineation, but a different deformation environment during exhumation could form second-order variations in omphacite CPO and affect the V-p distribution in eclogites (e.g., the fastest V-p is at similar to 35 degrees from the foliation in sample 13295). Comparison between measured and calculated seismic properties indicates that the CPO of omphacite controls the seismic anisotropy of eclogites at high pressure, and compositional layering and retrograde minerals will increase the anisotropy. Calculated P-wave velocities agree well with velocities measured at 500 MPa and room temperature for fresh eclogites, but much higher than those of retrograde eclogite. As a case study, the laboratory-derived V-p-P and V-p-T relationships were used to estimate P-wave velocities of eclogites and peridotites beneath the Western Superior Province, Canada. The results indicate that besides the fabric-induced anisotropy, the direction dependence of pressure and temperature derivatives of V-p can significantly increase seismic anisotropy of eclogites with depth, which results in eclogites being an important candidate for the seismic anisotropy in the upper mantle. Due to their very high density and velocity, garnet-rich eclogites within peridotite could be detected in seismic reflections in subduction zones.
ISSN: 0040-1951

hal-00463512
https://hal.archives-ouvertes.fr/hal-00463512
DOI : 10.1016/j.tecto.2008.09.027

Éditeur(s) : HAL CCSD National Academy of Sciences
Résumé : International audience
The Isua Supracrustal Belt, Greenland, of Early Archean age (3.81-3.70 Ga) represents the oldest crustal segment on Earth. Its complex lithology comprises an ophiolite-like unit and volcanic rocks reminiscent of boninites, which tie Isua supracrustals to an island arc environment. We here present zinc (Zn) isotope compositions measured on serpentinites and other rocks from the Isua supracrustal sequence and on serpentinites from modern ophiolites, midocean ridges, and the Mariana forearc. In stark contrast to modern midocean ridge and ophiolite serpentinites, Zn in Isua and Mariana serpentinites is markedly depleted in heavy isotopes with respect to the igneous average. Based on recent results of Zn isotope fractionation between coexisting species in solution, the Isua serpentinites were permeated by carbonate- rich, high- pH hydrothermal solutions at medium temperature (100-300 degrees C). Zinc isotopes therefore stand out as a pH meter for fossil hydrothermal solutions. The geochemical features of the Isua fluids resemble the interstitial fluids sampled in the mud volcano serpentinites of the Mariana forearc. The reduced character and the high pH inferred for these fluids make Archean serpentine mud volcanoes a particularly favorable setting for the early stabilization of amino acids.
ISSN: 0027-8424

insu-00674714
https://hal-insu.archives-ouvertes.fr/insu-00674714
DOI : 10.1073/pnas.1108061108

Éditeur(s) : HAL CCSD Copernicus Publications on behalf of the European Geosciences Union
Résumé : International audience
During the GWADASEIS cruise (Lesser Antilles volcanic arc, February–March 2009) a very high resolution (VHR) seismic-reflection survey was performed in order to constrain Late Quaternary to Present faulting. The profiles we obtained evidence frequent " ponding " of reworked sediments in the deepest areas, similar to the deposition of Mediterranean " homogenites ". These bodies are acoustically transparent (few ms t.w.t. thick) and are often deposited on the hanging walls of dominantly normal faults, at the base of scarps. Their thickness appears sufficient to compensate (i.e. bury) co-seismic scarps between successive earthquakes, resulting in a flat and horizontal sea floor through time. In a selected area (offshore Montserrat and Nevis islands), piston coring (4 to 7 m long) was dedicated to a sedimento-logical analysis of the most recent of these particular layers. It corresponds to non-stratified homogenous calcareous silty sand (reworked calcareous plankton and minor volcanoclas-tics). This layer can be up to 2 m thick, and overlies fine-grained hemipelagites. The upper centimeters of the latter represent the normal RedOx water/sediment interface. 210 Pb and 137 Cs activities lack in the massive sands, while a normal profile of unsupported 210 Pb decrease is observed in the hemipelagite below, together with a 137 Cs peak corresponding to the Atmospheric Nuclear Experiments (1962). The RedOx level was thus capped by a recent instantaneous major sedimentary event considered as post-1970 AD; candidate seismic events to explain this sedimentary deposits are either the 16 March 1985 earthquake or the 8 October 1974 one (M w = 6.3 and M w = 7.4, respectively). This leads to consider that the syntectonic sedimentation in this area is not continuous but results from accumulation of thick homogen-ites deposited after the earthquakes (as observed in the following weeks after Haiti January 2010 event, McHugh et al., 2011). The existence of such deposits suggests that, in the area of study, vertical throw likely results from cu-mulated effects of separated earthquakes rather than from aseismic creep. Examination of VHR profiles shows that all major co-seismic offsets are recorded in the fault growth sequence and that co-seismic offsets can be precisely estimated. By using a sedimentation rate deduced from 210 Pb decrease curve (0.5 mm yr −1) and taking into account minor reworking events detected in cores, we show that the Redonda system may have been responsible for five > M 6 events during the last 34 000 yr. The approach presented in this work differs from fault activity analyses using displaced sets of isochronous surfaces and postulating co-seismic offsets. Combining VHR seismic imagery and coring we can decipher co-seismic vs. slow continuous displacement, and thus actually estimate the amplitude and the time distribution of major co-seismic offsets.
Natural Hazards and Earth System Science

insu-01585969
https://hal-insu.archives-ouvertes.fr/insu-01585969
https://hal-insu.archives-ouvertes.fr/insu-01585969/document
https://hal-insu.archives-ouvertes.fr/insu-01585969/file/nhess-12-1755-2012-beck.pdf
DOI : 10.5194/nhess-12-1755-2012

Éditeur(s) : HAL CCSD
Résumé : International audience
This work presents a synthesis of the present-day knowledge on both emerged and submerged carbonate platforms of the Guadeloupe archipelago. Onshore and offshore data acquired during the KaShallow project are presented. Since the early Zanclean, some isolated platforms developed onto a Jurassic to Tortonian basement which displayed reliefs inherited from a major extensional tectonic episode dated from −8 to −5 Ma. Marie-Galante, Grande Terre and the southern shelf of Grande-Terre show strong similarities with Zanclean to Gelasian red-algal facies upward changing into early Calabrian coral facies. A major erosional unconformity (SB2) was recognized and allows to distinguish two early Calabrian formations (Calcaires à Agaricia Formation and Calcaires à Acropora Formation). The final emersion of these platforms is late Calabrian. The La Désirade platform displays Zanclean to early Piacenzian red-algal facies upward changing into coral facies. This platform emerged as soon as in the late Piacenzian. The island subsided again in relation with later extensional tectonic episode and early Calabrian reef platform then unconformably deposited. The Colombie bank deposits comprise Pliocene basinal deposits overlain by late Calabrian coral reefs. The Flandre bank might have emerged synchronously with La Désirade but was drowned only in the late Pleistocene-Holocene interval. Four extensional tectonic episodes have been identified in the late Miocene-Recent interval. Their effects depend on the location of the platforms within the fore-arc.
Bulletin Société Géologique de France

insu-00816463
https://hal-insu.archives-ouvertes.fr/insu-00816463
DOI : 10.2113/gssgfbull.184.1-2.99