Éditeur(s) : HAL CCSD
Résumé : International audience
Mylonite bodies are exhumed extensions at depth of shallow structural level brittle faults. They are characterised by intense strain localisation involving large volumes of middle to lower continental crust. The analysis of shear zone geometry and microstructures is a useful tool to decipher deformation kinematics, though finite strain estimation is still relatively qualitative. Timing of the deformation has also been the focus of numerous studies using the complete spectrum of geochronometers. However, only rare examples provide geochronological data that can be unambiguously interpreted and cooling, recrystallisation and neocrystallisation are difficult to tell from each other. Thus, time-scales over which major mylonite zones develop and remain active under ductile conditions as well as their strain rate often remain poorly documented. We investigated two sections across the South-Armorican Shear Zone (SASZ), a crustal-scale pluri-km thick dextral shear zone active during a single tectonic event of late-Variscan age and associated with syntectonic granite emplacement.. Finite strain profiles on these cross-sections, quantified from average Quartz grain-size in 34 samples, show a strong deformation partitioning towards the center of the SASZ, with a decrease, for each section, from 150µm in slightly deformed zones to less than 7-5 µm in the core of the ultramylonite. Furthermore, toward the core of the SASZ, development of a penetrative shear band network progressively eliminates lenses of less deformed rocks and results in homogenisation of the Quartz grain sizes. In parallel, we performed in-situ UV-laser Ar/Ar dating on the fabric-forming white micas. White micas population can be decomposed into inherited muscovites carried by the folation and newly-formed, syn-tectonic substituted phengites located along the shear bands yielding 2 age groups for each sample. Along the profiles, the difference between these two ages evolves from 10-15 Ma in weakly deformed domains to zero toward the SASZ core. In addition, mylonite-forming phengites yielded the same age irrespectively of their position on the section or of the rock strain.. On the regional point of view, this study brings deformation-ages for the late-orogenic evolution of the South Armorican domain. Strain accumulated along the SASZ in a time-span extending to 300-298 Ma, i.e. contemporaneously with the activity of nearby major extensional shear zones. These results support a new model of crustal-scale shear zone activity, where the whole body of the shear zone is active throughout its history (defined as the largest age difference through the section), i.e. the actively deforming zone does not get progressively narrower. The strain distribution nevertheless evolves with time, with an increasingly large fraction partitioned in the ultramylonitic core.
EGU General Assembly
Vienne, Austria

insu-00839320
https://hal-insu.archives-ouvertes.fr/insu-00839320
BIBCODE : 2010EGUGA..12.9881A

Éditeur(s) : Springer
Résumé : While it is well known that the ocean is one of the most important component of the climate system, with a heat capacity 1,100 times greater than the atmosphere, the ocean is also the primary reservoir for freshwater transport to the atmosphere and largest component of the global water cycle. Two new satellite sensors, the ESA Soil Moisture and Ocean Salinity (SMOS) and the NASA Aquarius SAC-D missions, are now providing the first space-borne measurements of the sea surface salinity (SSS). In this paper, we present examples demonstrating how SMOS-derived SSS data are being used to better characterize key land–ocean and atmosphere–ocean interaction processes that occur within the marine hydrological cycle. In particular, SMOS with its ocean mapping capability provides observations across the world’s largest tropical ocean fresh pool regions, and we discuss from intraseasonal to interannual precipitation impacts as well as large-scale river runoff from the Amazon–Orinoco and Congo rivers and its offshore advection. Synergistic multi-satellite analyses of these new surface salinity data sets combined with sea surface temperature, dynamical height and currents from altimetry, surface wind, ocean color, rainfall estimates, and in situ observations are shown to yield new freshwater budget insight. Finally, SSS observations from the SMOS and Aquarius/SAC-D sensors are combined to examine the response of the upper ocean to tropical cyclone passage including the potential role that a freshwater-induced upper ocean barrier layer may play in modulating surface cooling and enthalpy flux in tropical cyclone track regions.
Surveys In Geophysics (0169-3298) (Springer), 2014-05 , Vol. 35 , N. 3 , P. 681-722

Droits : Springer Science+Business Media Dordrecht 2013
http://archimer.ifremer.fr/doc/00152/26334/24430.pdf
DOI:10.1007/s10712-013-9244-0
http://archimer.ifremer.fr/doc/00152/26334/

Éditeur(s) : HAL CCSD
Résumé : The effects of climate change on extreme events are difficult to assess because extremes present large variability and consequently, it is difficult to identify significant trends in relation to the lack of instrumental long time series. Here we present a record of these extreme storm events in the French Mediterrannean coast over the past 7000 years based on a long sediment core from lagoonal environment in Gulf of Lions. Using a high resolution multiproxies approach on core associating grain size, faunal to reconstruct Mid to Late Holocene history of backbarrier deposits in relation to landfalling activity. Even if change in lagoon geomorphological setting over the Holocene does not allow to compare storm events in terms of intensity trough time, we have recorded six periods of increase in storm activity at 6200, 5500, 4400, 3200, 1500 yr cal B.P. and over the LIA (450 and 100 yr cal B.P.). These evidences of changes in coastal hydrodynamic, inversely correlated to periods of aridification in Western Mediterranean region, are in phase with those observed over the North Atlantic and correspond to Holocene cooling events. This increase in storm activity during Holocene cold events over Mediterranean region was probably due to thermal gradient increase leading to enhanced lower tropospheric baroclinicity over a large Central Atlantic-European domain. This study demonstrates that temperatures in North Atlantic region influence the severe storm activity and therefore the Mediterranean climate at Holocene timescale.
Les effets du changement climatique sur les événements extrêmes sont difficiles à estimer car ces phénomènes présentent une large variabilité, et par conséquent il est délicat d'identifier des tendances significatives compte tenu de l'absence de longues séries de données instrumentales. Dans cette étude, nous présentons un enregistrement de ces tempêtes extrêmes sur la côte Méditerranéenne française au cours des 7000 dernières années à partir de carottes sédimentaires prélevées dans un système lagunaire du Golfe du Lion. L'utilisation d'une approche multi-traceur à haute résolution avec des analyses granulométriques, faunistiques, géochimiques (XRF), en minéraux argileux associées à des données chronologiques de radiocarbone et de 210Pb, 137Cs ; nous a permis de reconstruire l'histoire, au cours de l'Holocène, des dépôts lagunaires en relation avec l'activité des tempêtes. Bien que les paramètres morphologiques de cet environnement lagunaire aient changé durant l'Holocène, ne permettant pas de comparer ces événements au cours du temps en terme d'intensité, nous enregistrons six périodes d'augmentation de l'activité des tempêtes datées à 6200, 5500, 4400, 3200, 1500 an cal B.P. et durant le Petit Age Glaciaire (de 450 à 100 an cal B.P.). Ces changements de l'activité hydrodynamique côtière, en antiphase avec les périodes d'aridifications en Méditerranée de l'Ouest, sont corrélées avec celles observées dans le bassin Nord Atlantique et correspondent aux événements de refroidissement Holocène. L'augmentation de l'activité des tempêtes dans la cette région de la Méditerranée durant les périodes froides, associée à une diminution des températures au pôle Nord, est probablement liée à l'augmentation du gradient thermique qui, par interaction barocline, favorise les cycles d'évolution des tempêtes dans toute l'Europe et la Méditerranée de l'Ouest. Cette étude démontre ainsi que la température dans la région Nord Atlantique influence l'activité des tempêtes extrêmes et donc du climat Méditerranéen à l'échelle de l'Holocène.
https://tel.archives-ouvertes.fr/tel-00437757

tel-00437757
https://tel.archives-ouvertes.fr/tel-00437757
https://tel.archives-ouvertes.fr/tel-00437757/document
https://tel.archives-ouvertes.fr/tel-00437757/file/Sabatier_these.pdf

Éditeur(s) : HAL CCSD Elsevier
Résumé : Hot collisional orogens are characterized by abundant syn-kinematic granitic magmatism that profoundly affects their tectono-thermal evolutions. Voluminous granitic magmas, emplaced between 360 and 270 Ma, played a visibly important role in the evolution of the Variscan Orogen. In the Limousin region (western Massif Central, France), syntectonic granite plutons are spatially associated with major strike-slip shear zones that merge to the northwest with the South Armorican Shear Zone. This region allowed us to assess the role of magmatism in a hot transpressional orogen. Microstructural data and U/Pb zircon and monazite ages from a mylonitic leucogranite indicate synkinematic emplacement in a dextral transpressional shear zone at 313 +/- 4 Ma. Leucogranites are coeval with cordierite-bearing migmatitic gneisses and vertical lenses of leucosome in strike-slip shear zones. We interpret U/Pb monazite ages of 315 +/- 4 Ma for the gneisses and 316 +/- 2 Ma for the leucosomes as the minimum age of high-grade metamorphism and migmatization respectively. These data suggest a spatial and temporal relationship between transpression, crustal melting, rapid exhumation and magma ascent, and cooling of high-grade metamorphic rocks.;Some granites emplaced in the strike-slip shear zone are bounded at their roof by low dip normal faults that strike N-S, perpendicular to the E-W trend of the belt. The abundant crustal magmatism provided a low-viscosity zone that enhanced Variscan orogenic collapse during continued transpression, inducing the development of normal faults in the transpression zone and thrust faults at the front of the collapsed orogen.
ISSN: 0040-1951

hal-00445260
https://hal.archives-ouvertes.fr/hal-00445260
DOI : 10.1016/j.tecto.2009.03.022

Éditeur(s) : HAL CCSD Oxford University Press (OUP)
Résumé : We present a regional surface waveform tomography of the Pacific upper mantle, obtained using an automated multimode surface waveform inversion technique on fundamental and higher mode Rayleigh waves, to constrain the VSV structure down to ∼400 km depth. We have improved on previous implementations of this technique by robustly accounting for the effects of uncertainties in earthquake source parameters in the tomographic inversion. We have furthermore improved path coverage in the South Pacific region by including Rayleigh wave observations from the French Polynesian Pacific Lithosphere and Upper Mantle Experiment deployment. This improvement has led to imaging of vertical low-velocity structures associated with hotspots within the South Pacific Super-Swell region. We have produced an age-dependent averagecross-sectionforthePacificOceanlithosphereandfoundthattheincreaseinVSV with age is broadly compatible with a half-space cooling model of oceanic lithosphere formation. We cannot confirm evidence for a Pacific-wide reheating event. Our synthetic tests show that detailed interpretation of average VSV trends across the Pacific Ocean may be misleading unless lateral resolution and amplitude recovery are uniform across the region, a condition that is difficult to achieve in such a large oceanic basin with current seismic stations.
ISSN: 0956-540X

hal-00105949
https://hal.archives-ouvertes.fr/hal-00105949
https://hal.archives-ouvertes.fr/hal-00105949/document
https://hal.archives-ouvertes.fr/hal-00105949/file/Maggi_Pacif_tomo_GJI2006.pdf
DOI : 10.1111/j.1365-246X.2006.03037.x

Éditeur(s) : HAL CCSD
Résumé : 1 p.
International audience
The 12th May 2008 Wenchuan earthquake in the Longmen Shan occurred on a large thrust fault largely inherited from an Indosinian structure itself probably controlled by an older structural heritage of the South China block continental margin. Within the whole northeast Tibet region, such a structural inheritance has had a major impact on the Tertiary deformation. It appears of primary importance to assess the pre-Tertiary tectonic evolution of the main blocks involved to understand the actual deformation in the eastern edge of Tibet. Over the past decades, the Proterozoic to Cenozoic tectonic, metamorphic and geochronologic history of the Longmen Shan and Songpan Garzê area have been largely studied. We present a synthesis of the tectonic evolution of the Songpan Garzê fold and thrust belt from Triassic to present. The Songpan-Garzê belt was formed during closure of a wide oceanic basin ?lled with a thick (5 to 15 km) sequence of Triassic ?yschoid sediments. Closure of the basin due to Triassic subduction involved strong shortening, intense folding and faulting of the Triassic series. A large-scale décollement, that presently outcrops along the eastern boundary of the belt (Danba area), allowed the growth of a wide and thick accretionary wedge. It develops in the Paleozoic and Triassic series and separates the accretionary prism from an autochthonous crystalline basement which shares many similarities with the basement of the Yangtze Craton (0.7-0.9 Ga). To the north and northwest, below the thickened Triassic series of the belt, the composition (oceanic or continental) of the basement remains unknown. During the Indosinian orogeny the emplacement of orogenic granites (220 - 150 Ma) was associated to crustal thickening. The isotopic composition of granitoids shows that their magma source were predominantly derived from melting of the proterozoic basement with varying degrees of sedimentary material and negligible mantle source contribution. In the Danba area, the décollement outcrops in a large tertiary antiform with a NNW-SSE axis. It has been exhumed too in the hanging wall of the NE-SW faults of the Tertiary Longmen-Shan belt that marks the present day transition from the Tibetan plateau to the Sichuan basin. These faults have episodically absorbed signi?cant shortening since the Late Triassic. The amount and precise timing of post-triassic deformation are dif?cult to constrain especially because of the dif?culty to isolate the tertiary thermochronological signal from the protracted late Triassic - Cretaceous thermal history. Nonetheless it is generally accepted that Jurassic - Cretaceous tectonism did not modi?ed the general Triassic architecture of eastern Tibet contrarily to the Tertiary deformation. The long-term cooling histories obtained on Mesozoic granites and on the metamorphic series of the Danba dome are very similar showing a very slow and regular cooling during Jurassic andCretaceous, con?rming the absence of major tectonic event between c.a. 150 and 30 Ma. Low temperature thermochronology data indicate that ?nal exhumation and cooling occurred in theTertiary with an acceleration between 10 and 5 Ma along the major tectonic structures. Within the Longmen Shan range, a total denudation of 7 to 10 km is estimated for the late Cenozoic period. Similar amounts of late Tertiary denudation have been estimated along an east-west section across the Xianshuihe fault
Asian climate and tectonics seminar
Utrecht, Netherlands

insu-00610298
https://hal-insu.archives-ouvertes.fr/insu-00610298
DOI : 10.1016/j.jseaes.2010.03.008

Éditeur(s) : HAL CCSD Springer Verlag
Résumé : International audience
During the development of the Variscan orogeny, large amounts of granitic melt were produced, giving rise to the intrusion of granitoids at different structural levels. Despite numerous studies, ages available from previous work on the Cevennes granites remain largely imprecise. In order to better constrain the age and emplacement mode of these granites, we have combined U-Pb dating on monazites and zircons and Ar-40/Ar-39 dating on biotites with petrological observations, major element chemical analysis and SEM zircon imaging on five samples from the Aigoual-St Guiral-Liron and Mont Lozere granitic massifs. The results revealed that granitic intrusions and cooling in Southern Cevennes occurred in a short time span at similar to 306 Ma after the main episode of regional metamorphism. Petrological and chemical data suggest that they result from a mixing between mantle-derived basic magmas (lamprophyres) and lower crust acid magmas. At a regional scale the production of these melts occurred at the end of crustal thickening induced by nappe stacking, at the same time as the late anatectic events recorded further north in the Velay dome and the granulite facies metamorphism recorded in metasedimentary granulite enclaves brought up by Tertiary volcanoes of the Velay area (Bournac).
ISSN: 1437-3254

hal-00412183
https://hal.archives-ouvertes.fr/hal-00412183
DOI : 10.1007/s00531-007-0187-x

Éditeur(s) : HAL CCSD
Résumé : International audience
The Cenozoic weathering of peninsular India has been advocated as a major driver of globalcooling, however, its timing remains virtually unconstrained such that proposed links betweentropical weathering of the Deccan Traps, regional monsoonal intensity and global climate remainsspeculative. To solve this conundrum we present here a revised weathering chronologybased on the identification of lateritic paleosurfaces and their dating using 40Ar/39Ar on supergeneK-rich manganese oxides. We focus on the Indian western passive margin where theWestern Ghats Escarpment (WGE) – a ~1000 m high topographic barrier resulting from thecontinental break up of Greater India – exposes exceptionally preserved weathering profilesfrom the western coastal lowlands to the eastern highland plateaus above the WGE. Four mainlateritic paleosurfaces have been identified: Three in the highland; a high landsurface (Al-Femostly bauxite) at elevation 1200-1000 m asl, an intermediate landsurface covered by a Feduricrustat 1000-900 m asl and a lower pediment landsurface at 850-600 m asl; and one inthe coastal lowland underneath a pediment landsurface at 150-50 m asl. The 40Ar/39Ar agesdocument three major weathering periods over the Cenozoic. The oldest weathering period(W1) is recorded between 53 Ma to 45 Ma both in the highland and the lowland. This correlatesto the Global Eocene climatic Optimum, and allows defining a bauxitic paleolandsurface acrossthe escarpment. Intense bauxitic weathering between ~ 45 and 47 Ma in the lowland indicatethat the WGE was stabilized before ~ 47 Ma on its margin and also implies low denudation rateat the foot of the WGE (<5 m Ma-1) since that time. The second weathering period between37 Ma to 19 Ma (W2) occurred in two distinctive stages (W2a and W2b). From 37 to 26 (W2a),weathering is only recorded in the highland and affect intermediate surfaces. The intensityof chemical weathering decreased sensibly between 32 and 29 Ma – potentially linked to theEarly Oligocene cooling – and resulted in the formation of the lower pediment surface. From~26 Ma to 19 Ma (W2b), only the western coastal lowland weathered while the highland surfacesare mostly dissected. This change and the highland/lowland weathering mitigation after~26 Ma documents a change in climatic patterns over peninsular India that signs the onset ofmodern-like monsoon regime. These results are being interpolated to the Deccan Traps basaltsto quantify their weathering and ultimately estimate of the contribution to CO2 fluxes of thesethree weathering periods and their potential link to Asian Monsoons and global cooling.
Source to Sink: a long term perspective of sediment budgets and sources characterization, Abstracts
Rennes, France

insu-01406455
https://hal-insu.archives-ouvertes.fr/insu-01406455

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
A high-resolution record of paleostorm events along the French Mediterranean coast over the past 7000 years was established from a lagoonal sediment core in the Gulf of Lions. Integrating grain size, faunal analysis, clay mineralogy and geochemistry data with a chronology derived from radiocarbon dating, we recorded seven periods of increased storm activity at 6300-6100, 5650-5400, 4400-4050, 3650-3200, 2800-2600, 1950-1400 and 400-50 cal yr BP (in the Little Ice Age). In contrast, our results show that the Medieval Climate Anomaly (1150-650 cal yr BP) was characterised by low storm activity.;The evidence for high storm activity in the NW Mediterranean Sea is in agreement with the changes in coastal hydrodynamics observed over the Eastern North Atlantic and seems to correspond to Holocene cooling-in the North Atlantic. Periods of low SSTs there may have led to a stronger meridional temperature gradient and a southward migration of the westerlies. We hypothesise that the increase in storm activity during Holocene cold events over the North Atlantic and Mediterranean regions was probably due to an increase in the thermal gradient that led to an enhanced lower tropospheric baroclinicity over a large Central Atlantic-European domain. (C) 2011 University of Washington. Published by Elsevier Inc. All rights reserved.
ISSN: 0033-5894

hal-00760909
https://hal.archives-ouvertes.fr/hal-00760909
DOI : 10.1016/j.yqres.2011.09.002

Éditeur(s) : HAL CCSD Wiley-Blackwell
Résumé : International audience
In the Oman ophiolite, the large scale Makhibiyah shear zone, in Wadi Tayin massif was generated with no or little relative motion between the two adjacent blocks, in contrast with what is reported from otherwise similar shear zones in deep crust and upper mantle. This shear zone is asymmetrical with, along one margin an asthenospheric mantle (similar to 1200 degrees C) and along the adjacent margin, a lithospheric mantle (similar to 1000 degrees C). Within the hotter side and with increasing shear strain, horizontal flow lines smoothly swing towards the shear zone direction before abutting against the wall of the lithosphere side. Profuse mafic melts issued from the hotter mantle are frozen in the shear zone by cooling along this lithospheric wall. Tectonic and magmatic activities are entirely localized within the asthenospheric compartment. Mantle flow lines were rotated, during their channelling along this NW-SE shear zone, in the NW and SE opposite directions. Depending on whether the flow lines are deviated NW or SE, dextral or sinistral shear sense is recorded in the shear band mylonitic peridotites. This demonstrates that the shear zone was not generated by strike-slip motion, a conclusion supported by regional observations.
ISSN: 0954-4879

hal-00411334
https://hal.archives-ouvertes.fr/hal-00411334
DOI : 10.1111/j.1365-3121.2008.00806.x

Éditeur(s) : HAL CCSD
Résumé : International audience
The Kazdağ Massif exposes a metamorphic dome in the Biga Peninsula of northwest Turkey. An extensional origin has been proposed for the dome, limited on both flanks by detachments and/or shear zones. The northern flank is bounded by the extensional Alakeçi Shear Zone (ASZ), whose P-T-t path is still poorly known. We therefore focus on its metamorphic conditions and related temporal history to precise its tectono-metamorphic evolution. The local tectonostratigraphy in structurally ascending order comprises: (i) the high-grade metamorphic core rocks of the Kazdaǧ Massif (gneisses and micaschists intercalated with amphibolites and marbles); (ii) the two kilometer-thick ASZ; (iii) the overlaying unmetamorphosed pre-Cenomanian accretionary Çetmi mélange; and (iv) Neogene sedimentary and volcanic cover rocks. ASZ mylonites were derived from both the core rocks and the mélange lithologies. From the north to the south the mylonitic fabrics in the ASZ depict a top-to-the N-NNE shearing, parallel to the NNE-plunging stretching lineation and NNW-dipping mylonitic foliation. This geometry implies normal sense movement i.e. north-side down-dip extensional displacement along this flank of the Kazdaǧ Massif. The northward transition from ductile to brittle-ductile regime through the ASZ shows that the deformation occurred at decreasing temperatures and degree of metamorphism. The paragenesis in equilibrium within the mylonitic gneisses and schists contains Qtz + Fs + Ms + Bt + Grt ± St ± Sill, with late retrogressive chlorite after biotite and garnet. Four samples of ASZ rocks yielded pressures between 6.9-5.7 kbar and temperatures between 706-587°C. Three samples from the mylonitic rocks supplied in situ isochron 36Ar/40Ar mica ages between 31.2-24.2 Ma, which we interpret to date the cooling of the mylonites following the P-T decrease across the ASZ. The metamorphic and structural results support the extensional character of the ASZ, and sketch transition from sillimanite core gneisses in the deeper structural levels to chlorite schists towards the top of the shear zone. These new data allow to precise the peak P-T conditions and the temporal evolution in the northern flank of the Kazdaǧ Massif, where Late Oligocene extensional exhumation was assisted by NNE-directed ductile-brittle ASZ, which had operated from amphibolite to greenschist facies. At the regional scale, this tectono-metamorphic pattern is similar to those observed on other places of the north Aegean domain.
IOP Conference Series
Austin, TE, United States

hal-00642725
https://hal-brgm.archives-ouvertes.fr/hal-00642725
DOI : 10.1088/1755-1307/2/1/012012

Éditeur(s) : HAL CCSD IODP
Résumé : Integrated Ocean Drilling Program (IODP) Expedition 335 (14 April–4 June 2011) will be the fourth ocean cruise of the "Superfast" campaign to drill a deep hole into intact oceanic basement and will return to Ocean Drilling Program (ODP) Hole 1256D to deepen this scientific reference penetration a significant distance into cumulate gabbros. Cores and data recovered during the Superfast 4 expedition will provide hitherto unavailable observations that will test models of the accretion and evolution of the oceanic crust. Site 1256 was specifically located on oceanic crust that formed at a superfast spreading rate (>200 mm/y) to exploit the observed relationship between spreading rate and depth to axial low velocity zones, thought to be magma chambers, seismically imaged at active mid-ocean ridges. This was a deliberate strategy to reduce the drilling distance to gabbroic rocks because thick sequences of lavas and dikes have proved difficult to penetrate in past. ODP Leg 206 (2002) initiated operations at Site 1256, including the installation in Hole 1256D of a reentry cone with 16 inch casing inserted through the 250 m thick sedimentary cover and cemented into basement to facilitate deep drilling. The hole was then cored ~500 m into basement. IODP Expeditions 309 and 312 (2005) successfully completed the first sampling of an intact section of upper oceanic crust from lavas, through the sheeted dikes, and into the upper gabbros. Hole 1256D now penetrates >1500 meters below seafloor (mbsf) and >1250 m subbasement and currently resides in the dike–gabbro transition zone. The first gabbroic rocks were encountered at 1407 mbsf. Below this lies a ~100 m complex zone of fractionated gabbros intruded into contact metamorphosed dikes. Although previous cruises achieved the benchmark objective of reaching gabbro in intact ocean crust, critical scientific questions remain. These include the following: 1. Does the lower crust form by the recrystallization and subsidence of a high-level magma chamber (gabbro glacier), crustal accretion by intrusion of sills throughout the lower crust, or some other mechanism? 2. Is the plutonic crust cooled by conduction or hydrothermal circulation? 3. What is the geological nature of Layer 3 and the Layer 2/3 boundary at Site 1256? 4. What is the magnetic contribution of the lower crust to marine magnetic anomalies? Hole 1256D is poised at a depth where samples that should conclusively address these questions can be obtained, possibly with only a few hundred meters of drilling. Importantly, as of the end of Expedition 312, the hole was clear of debris and open to its full depth. Increased rates of penetration (1.2 m/h) and enhanced core recovery (>35%) in the gabbros indicate that this return to Hole 1256D could deepen the hole >300 m into plutonic rocks, past the transition from dikes to gabbro, and into a region of solely cumulate gabbroic rocks.
https://hal.archives-ouvertes.fr/hal-00564953

hal-00564953
https://hal.archives-ouvertes.fr/hal-00564953
DOI : 10.2204/iodp.sp.335.2010

Éditeur(s) : HAL CCSD Wiley
Résumé : International audience
We report trace element analyses by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) of metal grains from nine different CR chondrites, distinguishing grains from chondrule interior ("interior grains"), chondrule surficial shells ("margin grains"), and the matrix ("isolated grains"). Save for a few anomalous grains, Ninormalized trace element patterns are similar for all three petrographic settings, with largely unfractionated refractory siderophile elements and depleted volatile Au, Cu, Ag, S. All three types of grains are interpreted to derive from a common precursor approximated by the least-melted, fine-grained objects in CR chondrites. This also excludes recondensation of metal vapor as the origin of the bulk of margin grains. The metal precursors were presumably formed by incomplete condensation, with evidence for high-temperature isolation of refractory platinum-group-element (PGE)-rich condensates before mixing with lower temperature PGE-depleted condensates. The rounded shape of the Ni-rich, interior grains shows that they were molten and that they equilibrated with silicates upon slow cooling (1-100 K h 1), largely by oxidation/evaporation of Fe, hence their high Pd content, for example. We propose that Ni-poorer, amoeboid margin grains, often included in the pyroxene-rich periphery common to type I chondrules, result from less intense processing of a rim accreted onto the chondrule subsequent to the melting event recorded by the interior grains. This means either that there were two separate heating events, which formed olivine/ interior grains and pyroxene/margin grains, respectively, between which dust was accreted around the chondrule, or that there was a single high-temperature event, of which the chondrule margin records a late "quenching phase," in which case dust accreted onto chondrules while they were molten. In the latter case, high dust concentrations in the chondrule-forming region (at least three orders of magnitude above minimum mass solar nebula models) are indicated.
ISSN: 1086-9379

hal-01003975
https://hal.archives-ouvertes.fr/hal-01003975
DOI : 10.1111/maps.12212

Éditeur(s) : HAL CCSD Springer
Résumé : International audience
Thermoelastic behavior of different marble types was analyzed using computational modeling and experimental measurements. Eight marble samples with different composition, grain size, grain boundary geometry, and texture were investigated. Calcitic and dolomitic marbles were considered. The average grain size varies from 75 μm to 1.75 mm; grain boundary geometry differs from nearly equigranular straight grain boundaries to inequigranular-interlobate grain boundaries. Four typical marble texture types were observed by EBSD measurements: weak texture; strong texture; girdle texture and high-temperature texture. These crystallographic orientations were used in conjunction with microstructure-based finite element analysis to compute the thermoelastic responses of marble upon heating. Microstructural response maps highlight regions and conditions in the marble fabric that are susceptible to degradation phenomena. This behavior was compared to the measured thermal expansion behavior, which shows increasing residual strains upon repetitive heating-cooling cycles. The thermal expansion behavior as a function of temperature changes can be classified into four categories: (a) isotropic thermal expansion with small or no residual strain; (b) anisotropic thermal expansion with small or no residual strain; (c) isotropic thermal expansion with a residual strain; and (d) anisotropic thermal expansion with residual strain. Thermal expansion coefficients were calculated for both simulated and experimental data and also modeled from the texture using the MTEX software. Fabric parameters control the amount and directional dependence of the thermal expansion. Marbles with strong texture show higher directional dependence of the thermal expansion coefficients and have smaller microstructural values of the maximum principal stress and strain energy density, the main precursors of microcracking throughout the marble fabric. In contrast, marbles with weak texture show isotropic thermal expansion behavior, have a higher propensity to microcracking, and exhibit higher values of maximum principal stress and strain energy density. Good agreement between the experimental and computational results is observed, demonstrating that microstructure-based finite-element simulations are an excellent tool for elucidating influences of rock fabric on thermoelastic behavior.
ISSN: 1866-6280

hal-00858095
https://hal.archives-ouvertes.fr/hal-00858095
DOI : 10.1007/s12665-013-2406-z

Éditeur(s) : HAL CCSD Oxford University Press (OUP)
Résumé : International audience
Three kilns and a collection of baked bricks from Italian archaeological sites have been studied for archaeointensity determination using the Thellier method as modified by Coe. All sites are dated based on archaeological information and their ages range from 500 to 800 AD and 1500 to 1700 AD. Rock magnetic studies identify magnetite, Ti- magnetite and hematite as the main magnetic minerals, and magnetic susceptibility versus temperature shows a good thermal stability of the samples. The intensity results have been corrected for anisotropy of the thermoremanent magnetization and cooling rate effects. The new data together with 136 previously published results are used to estimate the variation of the Earth's magnetic field over the past three millennia. The time distribution of the Italian absolute intensity data is irregular with the majority of determinations concentrated during the last four centuries, while older periods are very poorly covered. Most of the data come from volcanic rocks and show significant discrepancies. All data have been compared with archaeointensity results from Europe and regional and global models predictions. This work indicates the need to obtain more high quality archaeointensity results, in particular for the periods older than 200 BC and between 200 and 1000 AD, in order to determine a robust Italian intensity secular variation curve that, in combination with directional data, could be used for archaeomagnetic dating.
ISSN: 0956-540X

hal-00452451
https://hal.archives-ouvertes.fr/hal-00452451
DOI : 10.1111/j.1365-246X.2009.04455.x

Éditeur(s) : HAL CCSD
Résumé : Poster
The Lesser Caucasus in Armenia is an active volcanic and tectonic zone which resulted from the collision of the Arabian and the Eurasian plates since Neogene times. The During Quaternary, Lesser Caucasus was uplifted (0.3 mm/yr; Mitchell and Westaway, 1999) and experienced extensional tectonics times. Large lakes developed in graben structures. The diatomitic sequences of the Shamb paleo-lake (South Armenia) offer a rare opportunity to give new insights of Western Asia paleo-climate. Based on macroflora analysis, Bruch and Gabrielyan (2002) proposed a cooling and drying general climate trend through Pleistocene times in relation with a general uplift of the chain. Several questions have to be answer for this poorly investigated region. Did the climate record humid glacials and arid interglacials as suggested northward in Kazakhstan? What are the vegetation and climate responses to orbital parameters and to the monsoon? Moreover the lesser Caucasus is known as the entrance way used by the first hominids in Eurasia during Pleistocene time. How was the environment at this time? We present an integrated palynological, 40Ar/39Ar isotopic and magnetostratigraphic study for the most complete section (Joannin et al., in press). 40Ar/39Ar dating of two volcaniclastic layers provided ages of 1.24 ± 0.03 and 1.16 ± 0.02 Ma (2. Magnetostratigraphic data show that the entire Shamb section is of reversed polarity which correlates with part of the Matuyama period (1.785-1.070 Ma). Pollen assemblages and macroremains diversity revealed an alternation of glacial and interglacial phases that are compared with climate changes inferred from the global isotopic curve. The Shamb section ranges from approximately 1.300 to 1.080 Ma in age (marine isotopic stages 40 to 31). The vegetation of the Lesser Caucasus developed in a mosaic pattern in a Pleistocene continental, mostly arid climate, similar to the present-day climate. The vegetation changes record a dominant climate response to the obliquity orbital parameter. The influence of precession could not be established from the Shamb data. Pollen and macroflora both indicate that glacial periods were cold and dry and that interglacials were warm with local humidity. The early Pleistocene climatic model for Western Asia is thus similar to the climatic model for the Mediterranean area. Bruch, A., Gabrielyan, I.G., 2002. Quantitative data of the Neogene climatic development in Armenia and Nakhichevan. Acta Universitatis Carolinae – Geologica 46, 41–48. Joannin, S., Cornée, J.J., Münch, P., Fornari, M., Vasiliev, J., Krijgsman, W., Nahapetyan, S., Gabrielyan, Y., Ollivier, V., Roiron, P., Chataîgnier, C. In press. Early Pleistocene climatic cycles in continental deposits of the Lesser Caucasus of Armenia inferred from palynology, magnetostratigraphy, and 40Ar/39Ar dating. Earth and Planetary Science Letters. Mitchell, J., Westaway, R., 1999. Chronology of Neogene and Quaternary uplift and magmatism in the Caucasus: constraints from K-Ar dating of volcanism in Armenia. Tectonophysics 304, 157–186.
EGU Eurasian Climate and Tectonics
Vienne, Austria

hal-00481799
https://hal.archives-ouvertes.fr/hal-00481799

Éditeur(s) : HAL CCSD Wiley
Résumé : International audience
Facts -The capacity to exercise is markedly reduced in hot environments. -High humidity imposes further physiological strain on the body. -The combination of exercise and hot environments increases the risk of heat illness and heatstroke. Recommendations for players -Fluid intake should be sufficient to restrict body mass loss to o2% of the pre-exercise mass. -A minimum of 1-2 weeks of acclimatization should be undertaken before playing in a hot environment. Recommendations for organizers -Perform pre-participation screenings before competing in the heat. -Watch at-risk individual (e.g. sickle cell trait, vascular disease) and observe signs and symptoms if WBGT > 28°C. -Add additional fluid breaks and foresee medical coverage if WBGT > 30°C. -Reconsider event if WBGT > 32°C. Future research should investigate -The optimal cooling strategies prior and during sporting activities, i.e., halftime in football games. -The influence of acclimatization on football performance. -The impact of heat on decision making in players and match officials. -The impact of heat on spectators from cooler regions.
ISSN: 0905-7188

hal-00706256
https://hal.univ-antilles.fr/hal-00706256

Éditeur(s) : HAL CCSD
Résumé : 1 page
The main aim of this study, co-funded by BRGM, Regional Council of Guadeloupe, European Union, ADEME and UAG, was to examine the silica deposition mechanisms and estimate their deposition kinetics at acidic and basic pH conditions, during the cooling from 260 to 25°C of separated waters, discharged from production wells, located in the Bouillante geothermal field. The acquisition of this type of information is very useful not only for the geothermal exploitation but especially, if production fluids are re-injected in the ground, as this is envisaged in the Bouillante geothermal field. The different on site experiments showed that, in these conditions, the silica deposition was always initiated by a silica polymerization mechanism and the formation of a white colloidal gel, which slowed down the silica precipitation. In agreement with the literature data, this polymerization mechanism is characterized by an order 2 kinetic law, strongly dependant on pH. The determination of the specific surface area by specific mercury porosimetry for this gel (250 m2/g) in the UAG laboratories, allowed estimating the corresponding kinetic constant at 25°C. Other experiments at different final temperatures (50, 75 and 100°C) are necessary in order to define this constant as a function of the temperature. The silica precipitation under amorphous silica form was observed much later, after the polymerization stage, and only in particular conditions (long time of atmospheric exposure, water stagnation...). The pH of the Bouillante geothermal fluid is estimated to be close to 5.3±0.3, at 260°C, in the reservoir conditions. If this parameter, very influent on the kinetics of silica deposition, can be preserved at values less than 7 during the re-injection of the production fluids in the ground at relatively high temperatures (150-160°C), it is strongly probable that no chemical treatment is necessary to avoid silica precipitation.
19ème Conférence Géologique de la Caraïbe - 2011
Gosier, Guadeloupe, France

hal-00575943
https://hal-brgm.archives-ouvertes.fr/hal-00575943