Éditeur(s) : HAL CCSD Société géologique de France
Résumé : New radiolarian assemblages of Late Jurassic age were extracted from seven chert localities of the basement comp ex of La Desirade Island (Guadeloupe), the oldest geological unit of the Lesser Antilles arc and the eastern Caribbean region. These fauna range in age from Kimmeridgian or early Tithonian to early or early late Tithonian, showing that La Desirade cherts are not all coeval, contrary to what was established by previous studies. Furthermore, the close spatial association of cherts and volcanic elements composed of basaltic flows and volcaniclastics suggests a mixed setting of hydrothermal siliceous deposits and biogenous sedimentation consistent with an arc-related oceanic setting rather than an ophiolite-type oceanic crust. As the Youngest age is obtained to the northwest of the basement complex where as older data are found to the southeast, it also Suggests that La Desirade basement complex may have undergone some structural stacking consistent with an accretion-type geodynamic getting. Our biochronological data provide a preliminury try tectonic model of an eastward subduction zone for Lesser Antilles arc basement emplacement during the Mesozois.
EISSN: 1777-5817

hal-00445252
https://hal.archives-ouvertes.fr/hal-00445252
DOI : 10.2113/gssgfbull.180.5.399

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 AGU and the Geochemical Society
Résumé : International audience
Peridotite xenoliths brought up to the surface by the volcanism of the Kerguelen Islands represent a mantle that has been affected by a high degree of partial melting followed by intense melt percolation above the Kerguelen plume. These xenoliths are therefore particularly suitable to investigate effects of melt-rock interaction on crystallographic fabrics (lattice-preferred orientation (LPO)) of peridotite minerals and on the LPO-induced seismic properties of peridotites above a mantle plume. We have studied a suite of 16 ultramafic samples representative of different degrees of partial melting and magma-rock interaction among which the protogranular harzburgites are the least metasomatised xenoliths and dunites are the ultimate stage of metasomatism. Olivine LPO is characterized by high concentration of [010] axes perpendicular to the foliation and [100] axes close to the lineation or distributed in the foliation plane in harzburgites, whereas the high concentration of [100] axes is parallel to the lineation and [010] axes is perpendicular to the assumed foliation in dunites. Olivine LPO in harzburgites is interpreted as being due to a deformation regime in axial compression or transpression. The fabric strength of olivine decreases progressively from protogranular to poikilitic harzburgites and finally to dunites, for which it remains nevertheless significant (J index 3.8). Seismic properties calculated from LPO of minerals indicate that metasomatism at higher melt/rock ratio lowers the P wave velocities. The most significant difference between harzburgites and dunites corresponds to the distribution of S wave anisotropy. Harzburgites display the maximum of anisotropy within the foliation plane and the minimum of anisotropy perpendicular to the foliation plane, whereas the lowest anisotropy is parallel to the lineation for dunites. These modifications of seismic properties as a result of metasomatic processes may induce seismic heterogeneities in the mantle above the Kerguelen plume. In addition, assuming a lithospheric mantle primarily harzburgitic and structured with a horizontal foliation, the seismic properties calculated for the Kerguelen xenoliths reconcile the rather high anisotropy evidenced by the horizontally propagating surface waves with the apparent isotropy revealed by the absence of splitting of vertically propagating teleseismic SKS waves recorded by the GEOSCOPE Kerguelen station.
ISSN: 1525-2027

hal-00366222
https://hal.archives-ouvertes.fr/hal-00366222
https://hal.archives-ouvertes.fr/hal-00366222/document
https://hal.archives-ouvertes.fr/hal-00366222/file/bascou_Kerguelen_2007GC001879_hal.pdf
DOI : 10.1029/2007GC001879

Éditeur(s) : Amer Geophysical Union
Résumé : On November 21, 2004, a magnitude 6.3 earthquake occurred offshore, 10 km south of Les Saintes archipelago in Guadeloupe (French West Indies). There were more than 30000 aftershocks recorded in the following two years, most of them at shallow depth near the islands of the archipelago. The main shock and its main aftershock of February 14, 2005 (M-w = 5.8) ruptured a NE-dipping normal fault (Roseau fault), mapped and identified as active from high-resolution bathymetric data a few years before. This fault belongs to an arc-parallel en echelon fault system that follows the inner edge of the northern part of the Lesser Antilles arc, accommodating the sinistral component of oblique convergence between the North American and Caribbean plates. The distribution of aftershocks and damage (destruction and landslides) are consistent with the main fault plane location and attitude. The slip model of the main shock, obtained by inverting jointly global broadband and local strong motion records, is characterized by two main slip zones located 5 to 10 km to the SE and NW of the hypocenter. The main shock is shown to have increased the Coulomb stress at the tips of the ruptured plane by more than 4 bars where most of the aftershocks occurred, implying that failures on fault system were mainly promoted by static stress changes. The earthquake also had an effect on volcanic activity since the Boiling Lake in Dominica drained twice, probably as a result of the extensional strain induced by the earthquake and its main aftershock.
Journal Of Geophysical Research-solid Earth (0148-0227) (Amer Geophysical Union), 2011-10 , Vol. 116 , N. B10301 , P. 1-25

Droits : Copyright 2011 by the American Geophysical Union.
http://archimer.ifremer.fr/doc/00200/31099/29511.pdf
DOI:10.1029/2011JB008310
http://archimer.ifremer.fr/doc/00200/31099/

Éditeur(s) : HAL CCSD Oxford University Press (OUP)
Résumé : International audience
A temporary network of 10 broad-band seismic stations has been installed in French Polynesia for the Polynesian Lithosphere and Upper Mantle Experiment (PLUME). All the seismic stations were installed either on volcanic islands or on atolls of the various archipelagos of French Polynesia in a manner which complements the geographic coverage provided by the regional permanent stations. The primary aim of PLUME is to image the upper mantle structures related to plate motion and hotspot activity. However, because of its proximity to all sites, the ocean is responsible for a high level of noise in the seismic data and we show that these data can also be used to analyse ocean wave activity. The power spectral density (PSD) analyses of the seismic data recorded in French Polynesia show clear peaks in the 0.05– 0.10 Hz band (periods between 10 and 20 s), which corresponds to swell frequencies. Clear peaks in this frequency band are also observed in infrasonic data recorded on Tahiti. Ground motion analysis shows that the swell-related seismic noise (SRSN) is linearly polarized in the horizontal plane and its amplitude decreases rapidly with the distance from the shore. The microseismic and the infrasonic 'noise' amplitudes show very similar variations from station to station and both are strongly correlated with the swell amplitudes predicted by the National Oceanic and Atmospheric Administration (NOAA), wind-forced, 'WaveWatch' models. The swell direction can be estimated from SRSN polarization analysis but this has to be done with care since, for some cases, the ground motions are strongly controlled by the islands' anisometric shapes and by swell refraction processes. We find cases, however, such as Tahiti or roughly circular Tuamotu atolls, where the azimuth of the swell is in good agreement with the seismic estimates. We, therefore, demonstrate that the SRSN and the infrasonic signal observed in French Polynesia can be used in such cases as a proxy for swell amplitude and azimuth. From the continuous analysis of the data recorded in 2003 at the permanent seismic station PPTL in Tahiti, transfer functions have been obtained. This could provide a way to quantify the swell activity during the last two decades and, therefore, assist in the investigation of climate changes.
ISSN: 0956-540X

hal-01388809
http://hal.univ-reunion.fr/hal-01388809
http://hal.univ-reunion.fr/hal-01388809/document
http://hal.univ-reunion.fr/hal-01388809/file/Barruol_swell_GJI2006.pdf
DOI : 10.1111/j.1365-246X.2006.02871.x

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
We provide results of a detailed study of the first peridotite xenoliths of proven mantle origin reported from Bezymyanny volcano in the Klyuchevskoy Group, northern Kamchatka arc. The xenoliths are coarse spinel harzburgites made up mainly of Mg-rich olivine as well as subhedral orthopyroxene (opx) and Cr-rich spinel, and also contain fine-grained interstitial pyroxenes, amphibole and feldspar. The samples are unique in preserving the evidence for both initial arc mantle substrate produced by high-degree melt extraction and subsequent enrichment events. We show that the textures, modal and major oxide compositions of the Bezymyanny xenoliths are generally similar to those of spinel harzburgite xenoliths from Avacha volcano in southern Kamchatka. However, coarse opx from the Bezymyanny harzburgites has higher abundances of light and medium rare earth elements and other highly incompatible elements than coarse opx from the Avacha harzburgites. We infer that (1) the sub-arc lithospheric mantle beneath both Avacha and Bezymyanny (and possibly between these volcanoes) consists predominantly of harzburgitic melting residues, which experienced metasomatism by slab-related fluids or low-fraction, fluid-rich melts and (2) the degrees of metasomatism are higher beneath Bezymyanny. By contrast, xenolith suites from Shiveluch and Kharchinsky volcanoes 50-100 km north of the Klyuchevskoy Group include abundant cumulates and products of reaction of mantle rocks with silicate melts at high melt/rock ratios. The high melt flux through the lithospheric mantle beneath Shiveluch and Kharchinsky may be related to the asthenospheric flow around the northern edge of the sinking Pacific plate; lateral propagation of fluids in the mantle wedge south of the plate edge may contribute to metasomatism in the mantle lithosphere beneath the Klyuchevskoy Group volcanoes.
ISSN: 0377-0273

hal-00941258
https://hal.archives-ouvertes.fr/hal-00941258
DOI : 10.1016/j.jvolgeores.2012.12.022

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

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 Oxford University Press (OUP)
Résumé : International audience
A temporary network of 10 broad-band seismic stations has been installed in French Polynesia for the Polynesian Lithosphere and Upper Mantle Experiment (PLUME). All the seismic stations were installed either on volcanic islands or on atolls of the various archipelagos of French Polynesia in a manner which complements the geographic coverage provided by the regional permanent stations. The primary aim of PLUME is to image the upper mantle structures related to plate motion and hotspot activity. However, because of its proximity to all sites, the ocean is responsible for a high level of noise in the seismic data and we show that these data can also be used to analyse ocean wave activity. The power spectral density (PSD) analyses of the seismic data recorded in French Polynesia show clear peaks in the 0.05– 0.10 Hz band (periods between 10 and 20 s), which corresponds to swell frequencies. Clear peaks in this frequency band are also observed in infrasonic data recorded on Tahiti. Ground motion analysis shows that the swell-related seismic noise (SRSN) is linearly polarized in the horizontal plane and its amplitude decreases rapidly with the distance from the shore. The microseismic and the infrasonic 'noise' amplitudes show very similar variations from station to station and both are strongly correlated with the swell amplitudes predicted by the National Oceanic and Atmospheric Administration (NOAA), wind-forced, 'WaveWatch' models. The swell direction can be estimated from SRSN polarization analysis but this has to be done with care since, for some cases, the ground motions are strongly controlled by the islands' anisometric shapes and by swell refraction processes. We find cases, however, such as Tahiti or roughly circular Tuamotu atolls, where the azimuth of the swell is in good agreement with the seismic estimates. We, therefore, demonstrate that the SRSN and the infrasonic signal observed in French Polynesia can be used in such cases as a proxy for swell amplitude and azimuth. From the continuous analysis of the data recorded in 2003 at the permanent seismic station PPTL in Tahiti, transfer functions have been obtained. This could provide a way to quantify the swell activity during the last two decades and, therefore, assist in the investigation of climate changes.
ISSN: 0956-540X

hal-01323691
http://hal.univ-reunion.fr/hal-01323691
http://hal.univ-reunion.fr/hal-01323691/document
http://hal.univ-reunion.fr/hal-01323691/file/Barruol_et_al_2006.pdf
DOI : 10.1111/j.1365-246X.2006.02871.x

Éditeur(s) : Université de Bretagne Occidentale
Résumé : ln Java island, a number of quaternary volcanoes are Iocated in a back-arc position relative to the Sunda arc volcanie axis. These volcanie centers show an important variability in the petrologic composition of their products (potassic calc-alkali, shoshonitie, alkali potassie and alkali u/trapotassie leucitic series).
L'île de Java comporte, à l'arrière de l'axe volcanique de l'arc de la Sonde, un grand nombre de volcans quaternaires dont les laves présentent une grande diversité pétrologique (séries calco-alcalines potassiques, shoshonitiques, alcalines potassiques, alcalines ultrapotassiques à leucite). Ces édifices arrière-arc, mis en place à la faveur de fractures majeures obliques par rapport à l'axe de l'arc, se situent à des distances très variables de la fosse (profondeurs du plan de Benioff comprises entre 200 et 650 km). Le but de ce travail est la caractérisation géochimique (éléments en trace et isotopes Sr-Nd-Pb) des laves arrière-arc de cette région, leur comparaison avec celles des magmas de l'axe de l'arc, et la discussion de l'origine des variations observées.

Droits : UBO
http://archimer.ifremer.fr/doc/00034/14540/11815.pdf
http://archimer.ifremer.fr/doc/00034/14540/

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 Oxford University Press (OUP)
Résumé : International audience
Island arc picrites and boninites are magnesian magmatic rocks believed to be generated by high degrees of melting of depleted mantle sources fluxed by subduction-derived, volatile-rich components. These magmas can be probes of both the mantle wedge protoliths and subduction components, but are rare among other, usually more evolved, types of arc lavas. Furthermore, many arc picrites and boninites show evidence for late-stage differentiation prior to or during eruption, masking their primary, mantle-derived geochemical signatures. We report textural and chemical data on spinel-hosted melt inclusions of mantle origin in amphibole-bearing websterite veins cross-cutting spinel harzburgite xenoliths from the active andesitic Avacha volcano (south Kamchatka, Russia). The data are used to constrain the composition and origin of melts that formed the websterite veins in the sub-arc lithospheric mantle. The melt inclusions typically contain euhedral orthopyroxene and clinopyroxene and occasionally minor amphibole in silicate glass. The melt inclusions were homogenized using heating stages and gas-mixing furnaces. The homogenized glasses range from subalkaline primitive silica-rich picrite and high-Ca boninite (>15 wt % MgO, 48–54 wt % SiO2) to rhyolite. High-Ca boninite glasses have moderate volatile and low heavy rare earth element contents and elevated Cs, Rb, Ba, U, Sr, and Li abundances, with extremely high U/Th. In turn, the glasses display no negative spikes in the high field strength elements Nb, Ta, Zr, Hf, and Ti. We show that the silica-rich picrite and high-Ca boninite liquids in this study formed by high degrees of melting (>25%), at volatile under-saturation, of hybrid melt-depleted but silica-rich mantle sources at ≥1·5 GPa. The hybrid sources formed in two stages: first, by extraction of ∼15% melt from the convecting mantle to form a refractory protolith, which was subsequently enriched in silica via interaction with subduction-derived components prior to or during remelting in the mantle wedge. The subduction-derived components were enriched in fluid-mobile elements and probably oxidized. Overall, our results suggest that silica-rich picrites and high-Ca boninites can be primary melts in mature subduction zones and differentiate within the mantle wedge and the deep arc crust to form more evolved andesite magmas.
ISSN: 0022-3530

hal-01553676
https://hal.archives-ouvertes.fr/hal-01553676
DOI : 10.1093/petrology/egw066

Éditeur(s) : HAL CCSD
Résumé : The Lesser Antilles is an area of high volcanic and earthquake activity, characterized by a 1000 km convergence zone resulting from the Atlantic plate subduction under the Caribbean plate with a slow convergent motion (2 cm/year). With five years of available data, the CDSA data base presents a more homogeneous vision of Lesser Antilles arc seismicity and allows detecting low seismic activity zones, in the north near the Virgin Islands, and in the south between St-Lucia and Grenada. The accuracy improvements of location is used to better study the relationship between tectonic structures and seismicity, to better define the subduction slab geometry, and to analyse the spatial variations of the Gutenberg-Richter law b-value. Concerning the 21 november 2004 Les Saintes earthquake, EMS98 intensities in Les Saintes islands were evaluated from an individual request and hypocenter relocalisations of the Les Saintes earthquake and the biggest aftershock of 14 february 2005 were made using a method of master/slave in order to contrain the seismic source of the mainchock. As seismic hazard assessment depends from strong ground motion generated by earthquakes, the last part of this work concerns modeling accelerometric records of the Les Saintes earthquakes, combining a composite source model with an empirical Green Function technique. In our study, we used Les Saintes aftershocks as Empirical Green Function.
L'arc des Petites Antilles situé sur la bordure nord-est de la plaque Caraïbe, résulte d'un phénomène de subduction, les plaques américaines plongeant sous celle de la Caraïbe avec une vitesse de 2 cm/an. Les Antilles Françaises représentent la région française où le risque sismique est le plus important. Ce travail dont l'objectif est d'améliorer la connaissance de l'aléa sismique dans les Antilles Françaises, repose sur les données du Réseau Accélérométrique Permanent (RAP) et sur celles du « Centre de Données des Antilles Françaises ». L'analyse de ces données permet de mieux contraindre la sismicité de l'arc. Deux zones de très faible sismicité sont mises en évidence : au nord près des Iles Vierges et au sud entre Sainte-Lucie et Grenade. D'autres points sont aussi résolus : imagerie de la subduction le long de l'arc, relations entre la sismicité superficielle et la tectonique active, variations de la pente (b-value) de la loi de Gutenberg-Richter. Le séisme majeur du 21 novembre 2004 Mw=6.3 est au cœur des deux derniers chapitres. Plusieurs aspects y sont présentés : macrosismicité avec l'évaluation d'intensités EMS98 obtenues du dépouillement de formulaires individuels recueillis pour les Iles des Saintes, et relocalisation par la méthode de maître/esclave du choc principal et de sa plus forte réplique du 14 février 2005, de façon à mieux contraindre l'imagerie de la source du choc principal, étape primordiale pour la dernière partie portant sur la modélisation des signaux du choc principal des Saintes. Nous tentons dans cette dernière partie de modéliser les enregistrements de mouvements forts avec une approche semi-empirique en se basant sur un modèle stochastique large-bande et proposons l'utilisation de plusieurs Fonctions de Green Empiriques (FGE) sélectionnées parmi les répliques du séisme des Saintes.
https://tel.archives-ouvertes.fr/tel-00409021

tel-00409021
https://tel.archives-ouvertes.fr/tel-00409021
https://tel.archives-ouvertes.fr/tel-00409021/document
https://tel.archives-ouvertes.fr/tel-00409021/file/these_BengoubouMendy_2008.pdf

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 Elsevier
Résumé : International audience
Studies of paleomagnetic fields during polarity transitions recorded over the past few million years frequently suggest a configurational persistence consistent with long-lived mantle control over the pattern of flux emanating from Earth's fluid core. Fundamental to an understanding of the core-mantle dynamics involved is the question of the spatial-temporal nature of the physical conditions at the base of the mantle that may be responsible. Our analysis of the field during a mid-Cenozoic, reverse-to-normal (R-N) polarity change, recorded in a continuous sequence of lava flows in southeastern Queensland, Australia, provides strong evidence that the time span over which local mantle features retain such an influence is far longer than had thus far been observed. Specifically. the reversal is dominated by two sequential groupings of transitional magnetic vector directions, each associated with a clustering of virtual geomagnetic poles (VGPs) at locations common to several more youthful reversal datasets from about the globe. Our Ar-40/Ar-39 age determinations indicate that these Australian lavas erupted 25.56 +/- 0.48 Ma, strongly suggesting that the characteristic time for invariant control by the mantle over flux emerging from the outer core is at least on order of a few tens of Myr. The availability of transitional paleomagnetic data from still earlier times within the Cenozoic may further delineate this duration and, perhaps, provide a means to track changes in the pattern of long-standing concentrations of magnetic flux at the core-mantle boundary. (C) 2008 Elsevier B.V. All rights reserved.
ISSN: 0031-9201

hal-00411919
https://hal.archives-ouvertes.fr/hal-00411919
DOI : 10.1016/j.pepi.2008.07.012

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
We report chlorine (Cl) and fluorine (F) abundances in minerals, interstitial glasses, and melt inclusions in 12 andesite-hosted, spinel harzburgite xenoliths and crosscutting pyroxenite veins exhumed from the sub-arc lithospheric mantle beneath Avacha volcano in the Kamchatka Arc (NE Russia). The data are used to calculate equilibrium mineral-melt partition coefficients (Dmineral/meltDmineral/melt) for Cl and F relevant to subduction-zone processes and unravel the history of volatile depletion and enrichment mechanisms in an arc setting.Chlorine is ∼100 times more incompatible in pyroxenes (View the MathML sourceDClmineral/melt = 0.005–0.008 [±0.002–0.003]) than F (View the MathML sourceDFmineral/melt = 0.50–0.57 [±0.21–0.24]), which indicates that partial melting of mantle sources leads to strong depletions in Cl relative to F in the residues. The data set in this study suggests a strong control of melt composition on View the MathML sourceDCl,Fpyroxene/melt, in particular H2O contents and Al/(Al + Si), which is in line with recent experiments. Fluorine is compatible in Ca-amphibole in the ‘wet’ sub-arc mantle (View the MathML sourceDFamphibole/melt = 3.5–3.7 [±1.5]) but not Cl (View the MathML sourceDClamphibole/melt = 0.03–0.05 [±0.01–0.03]), indicating that amphibole may fractionate F from Cl in the mantle wedge. The inter-mineral partition coefficients for Cl and F in this study are consistent amongst different harzburgite samples, whether they contain glass or not. In particular, disseminated amphibole hosts much of the Cl and F bulk rock budgets of spinel harzburgites (View the MathML sourceDClamphibole/pyroxene up to 14 and View the MathML sourceDFamphibole/pyroxene up to 40). Chlorine and fluorine are variably enriched (up to 1500 ppm Cl and 750 ppm F) in the parental arc picrite and boninite melts of primitive pyroxenite veins (and related melt inclusions) crosscutting spinel harzburgites.Based on the data in this study, the main inferences on the behaviour of Cl and F during melting and metasomatic processes in the sub-arc mantle are as follow:(i) Melting models show that most depleted mantle protoliths of intra-oceanic arc sources can have extremely low Cl/F (0.002–0.007) before being overprinted by subduction-derived components.(ii) Chlorine has a higher percolation distance in the mantle than F. Even for small fluid or melt volumes, Cl and F signatures of partial melting are overprinted by those of pervasive percolation, which increases Cl/F in percolating agents and bulk peridotites during chromatographic interaction and/or amphibole-forming metasomatic reactions. These processes ultimately control the bulk Cl and F compositions of the residual mantle lithosphere beneath arcs, and likely in other tectonic settings.(iii) Fluxed melting models suggest that Cl enrichment in arc picrite and boninite melts in this study, and in many arc melt inclusions reported in the literature, could be related to the infiltration of high Cl/F fluids derived from subducted serpentinite or altered crust in mantle wedge sources. However, these high Cl/F signatures should be re-evaluated with new models in light of the possible overprint of pervasive percolation effects in the mantle. The breakdown of amphibole (and/or mica) in the deep metasomatised mantle at higher pressure and temperature conditions than in the slab may explain, at least in part, the positive correlations between F abundances and Cl/F in primitive arc melt inclusions and slab depth.
ISSN: 0016-7037

hal-01553610
https://hal.archives-ouvertes.fr/hal-01553610
DOI : 10.1016/j.gca.2016.10.035

Éditeur(s) : HAL CCSD Elsevier
Résumé : While most rocks from active volcanoes display ((210)pb/Ra-226) activity ratios lower than or close to the equilibrium value of 1, several other have ratios much higher than 1. Transfer of Rn-222 by magmatic volatiles has often been advocated to explain both Pb-210 deficits and excesses. We develop here a model to account for Pb-210 excesses through Rn-222 accumulation. We show that large Pb-210 excesses can be readily obtained, even for moderate ratios of degassing magma over accumulating magma, in a closed-system model where the Rn-222 atoms produced by decay of Ra-226 in the degassing magma are continuously extracted by a gas phase. For a given duration of volatile transfer, relative Pb-210 excesses are expected to be much larger than Pb-210 deficits. This model is applied to samples from Santorini (Aegean arc) and Surtsey (Iceland) both of which show high ((210)pb/Ra-226) ratios of 6.7 and 2.3 respectively at the time of eruption. The agreement with field data suggests that Rn-222 accumulation leading to Pb-210 excesses might be a rather common process in zoned magma chambers as well as in individual lava flows, and that it can be described by a closed-system model.
ISSN: 0012-821X

hal-00496398
https://hal.archives-ouvertes.fr/hal-00496398
DOI : 10.1016/j.epsl.2010.02.048

Résumé : La gestion du foncier martiniquais est un sujet complexe, sensible et à forte densité politique. La superficie de la Martinique est de 1 100km2, soit l’une des plus petites régions françaises, avec la plus forte densité de population, soumise à de multirisques (cyclonique, sismique, volcanique, technologique, inondation, pollution de l’air, des sols et de l’eau, tsunami…).La triangulation urbanistique composée d’une multiplicité d’acteurs, de textes et de réglementations et d’un millefeuille institutionnel (6 niveaux d’administration : commune, intercommunalité, département, région, Etat, Europe) annihile toute responsabilité et obère un développement endogène de ce micro territoire.Un juste équilibre entre développement économique, social, culturel et environnemental doit être trouvé en conciliant les objectifs économiques et sociaux liés au développement insulaire.La remarquable biodiversité de la Martinique constitue un atout touristique indéniable, menacé par de fortes pressions anthropiques foncières.Cette recherche démontre la pertinence de la réglementation applicable au foncier à la Martinique et l’influence du développement durable sur cette réglementation. La thèse propose des solutions pérennes à mettre en œuvre pour pallier les incohérences juridiques révélées par l’étude.
Management of land in Martinique is a complex, sensitive and highly political topic.The total area of the island is 1 100 km2 (square meters), e.g, one of the smallest French areas, with the highest concentration of population, submitted to multirisk desasters (cyclonic, seismic, volcanic, technological, flood, air, ground and water pollutions, tsunami…).The urban triangulation made up of a multiplicity of actors, texts and regulations, as well as a multi-level institutional cream cake (6 levels of administration: the city, the intercity, the Department, the Regional Council, the State and Europe), destroys all kind of responsibility and obstruct an endogenous development of this micro territory.A fair balance between economic, social, cultural and environmental development is necessary, by reconciling with the economic and social goals related to the insular development.The astonishing biodiversity of the island of Martinique represents an obvious tourist asset, threatened by strong land anthropic pressures.This research study highlights the relevance of the regulation applicable to the land in Martinique as well as the influence of sustainable development on this regulation. This thesis suggests perennial solutions to implement, in order to mitigate the legal inconsistencies pointed out by the study.

http://www.theses.fr/2011AGUY0503/document

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) : Elsevier Science Bv
Résumé : The late Quaternary paleoceanography and paleoenvironment in the Okinawa Trough, East China Sea, have been well reconstructed over the last decade, while in contrast the provenance of terrigenous sediments that have accumulated there remains enigmatic. In this study, rare earth elements (REE) were used to investigate provenance changes in sediments from Core DGKS9604, taken from the middle Okinawa Trough. Discrimination plots based on REE fractionation parameters suggest that the cored sediments have variable provenances over the last 30 ka, with the lower part (ca. 31-8.2 ka) ultimately originating mostly from the Changjiang (Yangtze River) and the upper part (7.1-0 ka) primarily from Taiwan. During the Last Glacial Maximum and early deglacial period, sea level was low and the main stream of the Kuroshio Current was deflected to the east of the Ryukyu Islands. As a result the Changjiang-derived sediments might have dominated sedimentation of the middle Okinawa Trough. However, since about 7 ka the main stream of the Kuroshio Current strengthened in the area of the trough, as sea level approximated the modern position. This caused near-bottom transport of fine-grained sediments from the continental margin to the trough to become weak and instead, Taiwan-derived terrigenous sediments dominated in the middle trough. The changing provenances of terrigenous sediments into the middle Okinawa Trough are closely related to the evolution of oceanic circulation and sea level in the East China Sea. Two tephra layers in the core have distinct REE compositions and correlate well with two volcanic eruptions at 7.6 and 25.8 ka in southern Japan. (C) 2010 Published by Elsevier B.V.
Marine Geology (0025-3227) (Elsevier Science Bv), 2010-09 , Vol. 275 , N. 1-4 , P. 212-220

Droits : 2010 Published by Elsevier B.V.
http://archimer.ifremer.fr/doc/00013/12459/9402.pdf
DOI:10.1016/j.margeo.2010.06.002
http://archimer.ifremer.fr/doc/00013/12459/