Éditeur(s) : HAL CCSD American Association of Petroleum Geologists
Résumé : International audience
We describe the structure, microstructure, and petrophysical properties of fault rocks from two normal fault zones formed in low-porosity turbiditic arkosic sandstones, in deep diagenesis conditions similar to those of deeply buried reservoirs. These fault rocks are characterized by a foliated fabric and quartz-calcite sealed veins, which formation resulted from the combination of the (1) pressure solution of quartz, (2) intense fracturing sealed by quartz and calcite cements, and (3) neoformation of synkinematic white micas derived from the alteration of feldspars and chlorite. Fluid inclusion microthermometry in quartz and calcite cements demonstrates fault activity at temperatures of 195degC to 268degC. Permeability measurements on plugs oriented parallel with the principal axes of the finite strain ellipsoid show that the Y axis (parallel with the foliation and veins) is the direction of highest permeability in the foliated sandstone (10-2 md for Y against 10-3 md for X, Z, and the protolith, measured at a confining pressure of 20 bars). Microstructural observations document the localization of the preferential fluid path between the phyllosilicate particles forming the foliation. Hence, the direction of highest permeability in these fault rocks would be parallel with the fault and subhorizontal, that is, perpendicular to the slickenlines representing the local slip direction on the fault surface. We suggest that a similar relationship between kinematic markers and fault rock permeability anisotropy may be found in other fault zone types (reverse or strike-slip) affecting feldspar-rich lithologies in deep diagenesis conditions.
ISSN: 0149-1423

hal-00903428
https://hal.archives-ouvertes.fr/hal-00903428
DOI : 10.1306/03071312127

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
The origin of plate tectonics is one of the most fundamental issues in earth and planetary sciences. Laboratory experiments indicate that the viscosity of silicate rocks is so strongly temperature-dependent that the entire surface of the Earth should be one immobile rigid plate. The rheology of oceanic lithosphere is, however, still poorly understood, and there exist few constraints on the temperature dependency of viscosity on the field scale. Here we report a new kind of observational constraint based on the geoid along oceanic fracture zones. We identify a large number of conspicuous small-scale geoid anomalies, which cannot be explained by the standard evolution model of oceanic lithosphere, and estimate their source density perturbations using a new Bayesian inversion method. Our results suggest that they are caused most likely by small-scale convection involving temperature perturbations of ∼300 K±100 K∼300 K±100 K. Such thermal contrast requires the activation energy of mantle viscosity to be as low as View the MathML source100±50 kJmol−1 in case of diffusion creep, and View the MathML source225±112 kJmol−1 in case of dislocation creep, substantially reducing the thickness of the stiffest part of oceanic lithosphere. Oceanic lithosphere may thus be broken and bent much more easily than previously thought, facilitating the operation of plate tectonics.
ISSN: 0012-821X

hal-01467827
https://hal.archives-ouvertes.fr/hal-01467827
DOI : 10.1016/j.epsl.2016.05.027

Éditeur(s) : HAL CCSD Springer Verlag
Résumé : A detailed and integrated geologic–hydraulic–geophysical–geochemical study of groundwater flow in the near-vicinity of a borehole drilled into fractured and karstified limestone demonstrates the power of such an integrated approach in localising and characterising preferential groundwater flow pathways and ambient borehole flow. The study, conducted in the vicinity of a 100-m-deep research borehole on the island of Mallorca, achieves such characterisation on a scale from millimetres to tens of metres in scale. More specifically, it adds single packer and open-hole pumping test interpretations to the results obtained during fluid logging, impeller flowmeter testing, borehole imaging, core descriptions and an innovative fracture analysis. This approach allows the delineation of the main flowing features and showed that such features are directly or indirectly linked to karst phenomena. Understanding this complex flow system is critical for an appropriate assessment of groundwater resources and the design of sustainable groundwater production schemes.
ISSN: 1431-2174

hal-00404921
https://hal.archives-ouvertes.fr/hal-00404921
DOI : 10.1007/s10040-006-0086-4

Éditeur(s) : Gauthier-villars
Résumé : The aim of the 20 Nautile dives of the recent Kanaut cruise was the study of the Kane Fracture Zone from its intersection with the Mid-Atlantic Ridge up to 80 km to the west. The dives have been conducted along four massifs located along the southern wall of the fracture valley. The flanks of the massifs consist of slightly tilted peridotites, gabbros and basalts, which have been exposed along normal and strike-slip major faults. No dike complex similar to that observed along the ZF Vema was observed The observed sections show pervasive cataclastic deformation and greenschist-facies metamorphic overprint.
Lors de la récente campagne Kanaut du Nautile (novembre-décembre 1992), 20 plongées ont eu pour objectif I’étude du mur sud de la Zone de fracture Kane, depuis son intersection orientale avec la dorsale médio-Atlantique jusqu’h environ 80 km de celle-ci. Les plongées réalisées SUT quatre massifs successifs bordant la vallée transformante, permettent d’observer des sections comprenant des péndotites, des gabbros et des basaltes, mis 21 l’affleurement B la faveur d’accidents normaux décrochants et d’un léger basculement. Les coupes observées ne montrent pas de complexe filonien épais, comme celui observé le long de la ZF Vema par exemple. L’ensemble de ces séries est fortement cataclasé et métamorphisé dans le facies des schites verts.
Comptes Rendus de l'Academie des Sciences Serie II (1251-8069) (Gauthier-villars), 1993-12 , Vol. 317 , N. 12 , P. 1641-1648

Droits : Académie des Sciences
http://archimer.ifremer.fr/doc/00183/29470/27886.pdf
http://archimer.ifremer.fr/doc/00183/29470/

Éditeur(s) : HAL CCSD Université des Antilles
Résumé : International audience
Loin d’apparaître comme de simples interfaces, les littoraux de la Caraïbe sont constituésd’enclaves fonctionnelles qui se juxtaposent donnant une mosaïque créole polyéthnique(mosaïque raciale), aux fractures sociales et spatiales marquées (mosaïque sociale et spatiale).Dans ces îles paysannes qui traditionnellement tournent le dos à la mer, la plage a longtempsété perçue par les populations littorales et les pêcheurs comme une continuité de l’île de laplantation pour y habiter et y pêcher. Une marge ouverte à tous dont la population rurale seméfiait pour ne l’aborder que quelques heures dans l’année à Pâques ou à la Pentecôte.2 Tout au long du XX siècle, une véritable subversion du regard s’opère par les classesdominantes, bourgeoises et touristes qui investissent les plages, souvent celles qui n’étaientguère fréquentées. Quitter la plage peu à peu investie par les touristes pour les fonds blancsou la Baignoire de Joséphine constitue une nouvelle marque, une volonté de maintenir unehiérarchie sociale par la distance.3 Depuis quelques années, les plages deviennent des lieux d’hybridation entre les populationslocales et les touristes. Avec la tertiairisation des sociétés, les populations antillaises desclasses moyennes et défavorisées découvrent les plages qui occupent une place importantedans la culture (publicité, télé…) des jeunes générations. Les plages sont investies etdeviennent peu à peu des espaces conviviaux.4 La ségrégation sociospatiale demeure cependant dans les pratiques, les temps et même lesespaces occupés. Si certains îlets et fonds blancs, comme les plages s’ouvrent au plus grandnombre, la piscine privée, le yacht, l’îlet privé, l’enclave protégée permettent toujours deconserver distance et discrétion. De même la capacité financière nécessaire à la pratique decertains sports nautiques permet cette démarcation. Ainsi, touristes, Métropolitains et Antillaisdes classes moyennes ou appartenant à la bourgeoisie, se côtoient et se croisent mais nepratiquent pas forcément la même plage.
ISSN: 1779-0980

hal-01202979
https://hal.archives-ouvertes.fr/hal-01202979
https://hal.archives-ouvertes.fr/hal-01202979/document
https://hal.archives-ouvertes.fr/hal-01202979/file/etudescaribeennes-693-4-la-plage-reflet-des-fractures-sociales-et-ethniques-aux-antilles.pdf
DOI : 10.4000/etudescaribeennes.693

Éditeur(s) : HAL CCSD Springer Verlag
Résumé : International audience
Brecciated and fractured peridotites with a carbonate matrix, referred to as ophicalcites, are common features of mantle rocks exhumed in passive margins and mid-oceanic ridges. Ophicalcites have been found in close association with massive peridotites, which form the numerous ultramafic bodies scattered along the North Pyrenean Zone (NPZ), on the northern flank of the Pyrenean belt. We present the first field, textural and stable isotopic characterization of these rocks. Our observations show that Pyrenean ophicalcites belong to three main types: (1) a wide variety of breccias composed of sorted or unsorted millimeter- to meter-sized clasts of fresh or oxidized ultramafic material, in a fine-grained calcitic matrix; (2) calcitic veins penetrating into fractured serpentine and fresh peridotite; and (3) pervasive substitution of serpentine minerals by calcite. Stable isotopic analyses (O, C) have been conducted on the carbonate matrix, veins and clasts of samples from 12 Pyrenean ultramafic bodies. We show that the Pyrenean ophicalcites are the product of three distinct genetic processes: (1) pervasive ophicalcite resulting from relatively deep and hot hydrothermal activity; (2) ophicalcites in veins resulting from tectonic fracturing and cooler hydrothermal activity; and (3) polymictic breccias resulting from sedimentary processes occurring after the exposure of subcontinental mantle as portions of the floor of basins which opened during the mid-Cretaceous. We highlight a major difference between the eastern and western Pyrenean ophicalcites belonging, respectively, to the sedimentary and to the hydrothermal types. Our data set points to a possible origin of the sedimentary ophicalcites in continental endorheic basins, but a post-depositional evolution by circulation of metamorphic fluids or an origin from relatively warm marine waters cannot be ruled out. Finally, we discuss the significance of such discrepancy in the characteristics of the NPZ ophicalcites in the frame of the variable exhumation history of the peridotites all along the Pyrenean realm.
ISSN: 1437-3254

insu-00944405
https://hal-insu.archives-ouvertes.fr/insu-00944405
https://hal-insu.archives-ouvertes.fr/insu-00944405/document
https://hal-insu.archives-ouvertes.fr/insu-00944405/file/clerc-etal-IJES-Review.pdf
DOI : 10.1007/s00531-013-0927-z

Éditeur(s) : HAL CCSD
Résumé : This work focus on the characterization of the source and dynamic of compactional fluids during sedimentary burial, through two complementary examples of late orogenic oil‐field half‐grabens: The exhumed Lodève Permian Basin and a deep buried Jurassic basin in the North Viking Graben (North Sea).Constituting the main part of the thesis, a multi‐disciplinary approach was conducted in the Lodève Basin where Ba‐F‐Cu‐Pb polymetallic mineralized systems are trapped into synrift faults and paleokarsts in the carbonate basement at the hinge point of the rollover. The source, timing and P/T conditions of fluid migration were deduced from the analysis of the microfabric, the fluid inclusions microthermometry, and the isotopic (Sr, S, O, H) and Rare Earth Element (REE) signature. Results are then crossed with a structural and thermal modeling that consolidates the sequence and dynamics of fluid during burial.A similar approach was conducted in the North Viking Graben where fluid markers are restricted to 3D seismic and well core data. Comparable Ba‐Pb‐Zn veins are reported in basin margin, plugging one of the most important siliciclastic hydrocarbon reservoir in the substratum. This analysis provides additional constraints on basinal fluid behavior and allows us to propose a global dynamic model for various compositions of fluids and reservoirs.We conclude to a polyphase fluid sequence history including:(a) In the carbonate basement of the Lodève Basin, karstic paleocanyon incisions and associated cavities coupled to synrift fault, act as major drain for fluids. These structures are early affected by hypogen‐sulfuric karstification in response to the interaction between bacterial oxidation of sulfides entrapped within Lower Permian blackshales and the basement oxidizing aquifer.(b) Disequilibrium compaction initiates overpressure‐driven basinal fluid migration towards basin margins, characterized by temperatures around 150‐180°C and salinities between 9 et 18wt%eq.NaCl. Isotopic (Sr, S, O) and REE analyses reveal that Ba‐M+‐rich mineralizing fluids derived mainly from buried blackshales diagenesis. External fluids coming from the lower crust are also identified that play a key role in fluorite precipitation by the leaching of late hercynian granites (mean temperature of 250°C and salinity > 20wt%eq.NaCl).(c) During the synrift period, fluid overpressure is responsible for the periodic reactivation of fault plane according to seismic‐valve process, bedded‐control shearing and hydraulic brecciation at the basement‐seal interface. These mechanisms induce cyclic polymetallic mineralization by the mixing between in‐situ formation water and deep ascending basinal fluids.(d) Thermogenic fluids expulsion starts with last basinal fluids during late burial stage. Hydrocarbons thus migrate along the same regional pathways up to the rollover crest, where they are partly rerouted by the previous mineralized baffle.(e) In the Lodève basin, post‐rift exhumation of the margins led to the remobilization of synrift deposits by subaerial biochemical processes at the sulfate‐methane transition. The latter results from the interaction between the still active hydrocarbon dysmigration with a playa lake sulfate‐rich aquifer. Secondary low‐temperature barite fronts precipitate then within basement meteoric karsts.In addition to the « source to sink » model of basinal fluids, this work provides new insights on the early plugging of hydrocarbon reservoirs and for the metallogenesis of Mississippi Valley‐Type deposits.
Ce travail porte sur la caractérisation de la source et de la dynamique des fluides de bassin au cours de leur chargement à travers deux exemples complémentaires de demi‐grabens tardi‐orogéniques pétroliers : Le Bassin Permien de Lodève, aujourd’hui à l’affleurement et un bassin jurassique enfouis dans le North Viking Graben (Mer du Nord).Le coeur de la thèse concerne le Bassin de Lodève où, à partir d’une approche pluridisciplinaire intégrée, nous avons caractérisé l’architecture des minéralisations (Ba, F, Cu, Pb) piégées dans un réseau paléokarstique alimenté par les failles syn‐rift, dans le substratum carbonaté à l’apex du roll‐over. La source, le calendrier et les conditions de migration des fluides ont été approchés à partir de l’analyse de la micro‐fabrique, la microthermométrie sur inclusions fluides, les analyses isotopiques (Sr, S, O, H) et de Terres Rares. Les résultats analytiques ont été enfin croisés avec un modèle thermique et structural du bassin qui conforte la séquence et la dynamique du système fluide en cours d’enfouissement.Une démarche similaire, mais plus limitée, a été conduite dans le bassin du North Viking Graben où l’accès aux marqueurs fluides est restreint aux données de sismique 3D et de carottes. Comme à Lodève, les minéralisations Ba‐Pb‐Zn colmatent un réservoir dans le substratum à l’apex du roll‐over. Elles se présentent sous forme de ciments dans des grès ou des fractures. Cette analyse apporte des contraintes complémentaires et permet de proposer un modèle dynamique général avec des variantes en fonction de la nature des fluides et des réservoirs.On retiendra donc la séquence fluide suivante :(a) Dans le cas du bassin de Lodève sur substratum carbonaté, les chemins préférentiels de drainage se développent dans des paléocanyons N‐S couplés à un réseau de fractures et d’endokarsts météoriques. Ces derniers sont élargis en début de rifting par la dissolution hypogène sulfurique produite par l’oxydation bactérienne des sulfates et des pyrites des blackshales, au contact de l’aquifère oxydant du Cambrien.(b) Le déséquilibre de compaction initie la migration des fluides interstitiels en surpression vers les marges avec des températures autour de 150‐180°C et des salinités entre 9 et 18wt%eq.NaCl. Les analyses isotopiques (Sr, S, O) révèlent que la majorité des fluides provient de l’altération diagénétique des blackshales riches en métaux. Des interactions sont également mises en évidence avec des fluides profonds (entre 240°C et 260°C ; salinités > à 20wt%eq.NaCl), qui lessivent les granites tardi‐hercyniens.(c) Pendant le syn‐rift, les conditions de surpression de fluide permettent la réactivation cyclique des failles, les décollements stratigraphiques et la formation de brèches hydrauliques, favorisant la mise en connexion avec les réservoirs superficiels à l’apex du rollover. Le modèle de Sibson ajusté aux fluides de bassins est alors le moteur de la migration verticale.(d) Les fluides thermogéniques commencent à être expulsés avec les derniers fluides de compaction au cours d’un stade plus évolué de l’enfouissement en empruntant les mêmes chemins jusqu’à l’apex du roll over. Ils sont alors partiellement freinés et déviés par les colmatages minéralisés antérieurs.(e) A Lodève, la continentalisation des minéralisations antérieures au cours de l’exhumation post‐rift conduit à leur remaniement partiel au niveau de la transition sulfate‐méthane induite par l’interaction entre une playa évaporitique et la dysmigration des hydrocarbures. Des barytines secondaires de basse température, déprimées en Sr sont alors précipitées de manière synsédimentaire dans des karsts météoriques du socle.Outre l’illustration d’un modèle complet (source to sink) de dynamique des fluides dans un bassin, ce travail apporte de nouvelles contraintes dans l’approche du colmatage des réservoirs à hydrocarbures sur les têtes de blocs basculés et sur la genèse des gîtes miniers de type Mississippi Valley‐Type.
https://hal.archives-ouvertes.fr/tel-01457583

tel-01457583
https://hal.archives-ouvertes.fr/tel-01457583
https://hal.archives-ouvertes.fr/tel-01457583/document
https://hal.archives-ouvertes.fr/tel-01457583/file/th%C3%A8seLaurent2015V2.pdf

Éditeur(s) : Gauthier-Villars
Résumé : We report here our findings on the chemistry of methane, manganese and chloride dissolved in the interstitial water of the Barbados accretionary complex. The decollement separating the subduction plate from the accretionary wedge is geochemically characterized by high methane and manganese and low chloride concentrations relative to buried seawater. Sandstone layers occurring below the decollement and the compressive structures recording some of the earliest effects of offscraping are also characterized by similar anomalies. A fluid with the same characteristics is also found six kilometres to the east of the deformation front within the oceanic plate. High manganese concentration and low chlorinity also occur in the pore water of the offscraped sediment packages, but without associated anomalous methane concentrations. These results suggest that: Dewatering processes occur below the decollement in association with thermogenic methane production; Fluid with low chlorinity and high methane concentrations circulates mainly along the decollement, its propagation in the oceanic domain and along the main fracture zones being associated with these detachment surfaces; Diagenetic processes essentially control the dissolved manganese concentration and probably affect the methane content of the interstitial waters; The observations made in this paper illustrate advective processes in the front of an accretionary prism.
Oceanologica Acta (0399-1784) (Gauthier-Villars), 1991 , Vol. 14 , N. 1 , P. 33-49

Droits : info:eu-repo/semantics/openAccess
http://archimer.ifremer.fr/doc/00101/21262/18873.pdf
http://archimer.ifremer.fr/doc/00101/21262/

Éditeur(s) : HAL CCSD American Geophysical Union
Résumé : International audience
Claystone caprocks are often the ultimate seal for CO2 underground storage when residual CO2 gas reaches the reservoir top due to buoyancy. Permeability changes of a fractured claystone due to seepage of CO2-enriched brine and water vapor-saturated CO2 gas are investigated. Results show that brine flow induces a large porosity increase (up to 50%) in the vicinity of the fracture due to dissolution of calcite and quartz, while permeability remains unchanged. Conversely, cyclic flows of CO2-brine and CO2-gas increase the fracture aperture abruptly after each gas flow period, producing a progressive decrease of the caprock seal capacity. Aperture increase is controlled by decohesion of the clay framework within a micrometer-scale-thick layer induced by CO2-gas acidification. Results show that hydraulic aperture increases linearly with duration of the preceding CO2-brine flow period, emphasizing the kinetic control of the quartz grains dissolution during the brine flow periods.
ISSN: 0094-8276

hal-00412332
https://hal.archives-ouvertes.fr/hal-00412332
DOI : 10.1029/2008GL034467

Éditeur(s) : HAL CCSD
Résumé : International audience
The geology of the North Pyrenean Zone in the central Pyrenees allows for the observation in the field of theentire section of the Pyrenean rift, from the mantle to the crust and the Mesozoic cover (pre, syn and post rift).The good knowledge we have of the pre-Alpine history of the Pyrenees allows us to properly constrain the Alpinegeological evolution of the pre-Triassic rocks which record both Variscan and Alpine orogenic cycles.The mantle outcrop as kilometric to centimetric fragments of peridotite dispersed within a carbonate metamorphicbreccia. The study of peridotite serpentinisation shows several events of low-temperature serpentinisation, incontact with seawater. In some locallities, we can observe a mixture of fragments of variously serpentinizedperidotites. This suggests a tectonic context where fragments of peridotites from different structural levels weresampled more or less synchronously.The granulitic basement is characterized by a Variscan syndeformational HT event (300-280 Ma). So farwe have not found any trace of a Cretaceous HT event (> 500C). On the other hand, the basement is affectedby a regional metasomatism that began during the Jurassic and became more spatially focused with time until itwas restricted to the Pyrenean rift during the Aptien, Albian and Cenomanian. The talc-chlorite metasomatism(120-95 Ma) shows an evolution from a static toward a syn-deformation hydrothermal event, under a more or lessnormal geothermal gradient. Extensional deformation is recorded by the reworking of several inherited low-angleVariscan tectonic contacts, but also by dispersed high-angle extensional shear zones formed under greenshistconditions.The metamorphic Mesozoic cover of the basement massifs, which constitute the so-called Internal MetamorphicZone, is an allochtonous unit made of lenses of Mesozoic rocks enclosed into the breccia, whichlocally contains peridotite and basement clasts. The Mesozoic metamorphic carbonates show a first phase ofsyn-metamorphic (450-600C, P < 2 kb) ductile deformation, and subsequent phases of folding and fracturing.The datation of neoformed minerals give a 108-85 Ma time span for the metamorphism. We interpret this brecciaas an abandonment breccia which marks the emergence of the main detachment. The basal contact of the Mesozoiccover has a complex 3D geometry traced by Triassic evaporites. It corresponds to a major pre- and synorogenicpolyphased tectonic contact.All these data show a geometrically complex hyper-extended rift where the crust was not stretched under ahigh geothermal gradient but thinned by the tectonic extraction of relatively thin lenses and perhaps cut by highangle low-grade shear zones. The 3D geometry, as well as the strain records and the breccia lithologies stronglysuggest a non-cylindricity for the exhumation process, probably within a transtentional system
Geophysical Research Abstracts
Vienne, Austria

insu-01510371
https://hal-insu.archives-ouvertes.fr/insu-01510371

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
Displacement-length data from dilatant fractures (joints, veins, igneous dikes) and several varieties of deformation bands were analyzed statistically to investigate the applicability of mechanical models proposed for their formation. All 17 datasets are generally consistent with equilibrium or long-term power-law slopes on the displacement-length diagram of either 1.0 or 0.5. Similar to many faults, disaggregation deformation bands are consistent with a power-law scaling relation having a slope of approximately c = 1, implying a linear dependence of maximum displacement and discontinuity length (Dmax = γL). In contrast, dilatant fractures, cataclastic deformation bands, and shear-enhanced compaction bands are consistent with a power-law scaling relation with a slope of approximately c = 0.5, implying a dependence of maximum displacement on the square root of length (Dmax = αL1/2). The scaling relations represent an average, or long-term equilibrium outcome of deformation for conditions such as length-scale, time-scale, temperature, chemistry, and an effectively homogeneous far-field stress field, allowing for variations such as rapid and/or localized behaviors. The displacement-length scaling of these geologic structures follows systematic relationships that provide information on host-rock properties and the physics of fracture and deformation-band propagation.
ISSN: 0191-8141

hal-00818130
https://hal.archives-ouvertes.fr/hal-00818130
DOI : 10.1016/j.jsg.2012.12.005

Éditeur(s) : HAL CCSD AGU and the Geochemical Society
Résumé : International audience
Although magmatism may occur during the earliest stages of continental rifting, its role in strain accommodation remains weakly constrained by largely 2-D studies. We analyze seismicity data from a 13 month, 39-station broadband seismic array to determine the role of magma intrusion on state-of-stress and strain localization, and their along-strike variations. Precise earthquake locations using cluster analyses and a new 3-D velocity model reveal lower crustal earthquakes beneath the central basins and along projections of steep border faults that degas CO2. Seismicity forms several disks interpreted as sills at 6-10 km below a monogenetic cone field. The sills overlie a lower crustal magma chamber that may feed eruptions at Oldoinyo Lengai volcano. After determining a new ML scaling relation, we determine a b-value of 0.87±0.03. Focal mechanisms for 65 earthquakes, and 13 from a catalogue prior to our array reveal an along-axis stress rotation of ∼60° in the magmatically active zone. New and prior mechanisms show predominantly normal slip along steep nodal planes, with extension directions ∼N90°E north and south of an active volcanic chain consistent with geodetic data, and ∼N150°E in the volcanic chain. The stress rotation facilitates strain transfer from border fault systems, the locus of early-stage deformation, to the zone of magma intrusion in the central rift. Our seismic, structural, and geochemistry results indicate that frequent lower crustal earthquakes are promoted by elevated pore pressures from volatile degassing along border faults, and hydraulic fracture around the margins of magma bodies. Results indicate that earthquakes are largely driven by stress state around inflating magma bodies.
ISSN: 1525-2027

hal-01667198
https://hal.archives-ouvertes.fr/hal-01667198
DOI : 10.1002/2017GC007027

Éditeur(s) : HAL CCSD American Geophysical Union
Résumé : International audience
The aim of this study is to bring new information on water storage dynamics in karst systems from tiltmeter measurements. Newly developed long-base hydrostatic tiltmeters are installed at two sites on the Larzac plateau (France) in a karst aquifer of similar to 100 km(2) recharge area. Each site is located within a karst cave where two tilt directions are monitored. Significant reversible tilt deformation reaching amplitudes of 10(-6) to 10(-5) rad was observed at both sites following heavy precipitation. Elastic parameters were determined with an experiment in which a tiltmeter site was loaded by up to 25 t of weight at strategic locations at the ground surface. Mechanisms responsible for the observed tilt were then examined by means of finite element modeling. Deformation induced by water pressure changes in fractures is our preferred interpretation as the most plausible mechanism. Within this scope, we used the tiltmeter responses to extract time constants using lumped parameter modeling. Time constants are interpreted to be associated with the filling and emptying of fractures nearby the tiltmeters. These instruments therefore appear as useful tools to study the local dynamics of water infiltration in karst systems.
ISSN: 0043-1397

hal-00534461
https://hal.archives-ouvertes.fr/hal-00534461
DOI : 10.1029/2009WR008082

Éditeur(s) : HAL CCSD Wiley
Résumé : 13 pages, 4 figures, to be published on J. Am. Cer. Soc
International audience
Since crack propagation in oxide materials at low crack velocities is partly determined by chemical corrosion, proper knowledge of the crack tip chemistry is crucial for understanding fracture in these materials. Such knowledge can be obtained only from in situ studies because the processes that occur in the highly confined environment of the crack tip are very different from those that take place at free surfaces, or that can be traced post mortem. We report the occurrence of hydrous liquid condensate between the two fracture surfaces in the vicinity of the tip of tensile cracks in silica. Observations are performed in real-time by means of atomic force microscopy (AFM) at continuously controlled crack velocities in the regime of stress corrosion. Condensate formation and changes in extent and shape are demonstrated for a wide range of macroscopic humidity at different crack speeds. Its liquid character is confirmed by the study of AFM phase-contrast data. It is believed that this evidence of a nanoscale liquid hydrous phase at the crack tip will enable novel insights in the chemistry of failure of oxide materials.
ISSN: 0002-7820

hal-00424984
https://hal.archives-ouvertes.fr/hal-00424984
ARXIV : cond-mat/0509209
DOI : 10.1111/j.1551-2916.2005.00765.x

Éditeur(s) : HAL CCSD Wiley
Résumé : International audience
The way faults control upward fluid flow in nonmagmatic hydrothermal systems in extensional context is still unclear. In the Eastern Pyrénées, an alignment of twenty-nine hot springs (29 ∘ C to 73 ∘ C), along the normal Têt fault, offers the opportunity to study this process. Using an integrated multiscale geological approach including mapping, remote sensing, and macro-and microscopic analyses of fault zones, we show that emergence is always located in crystalline rocks at gneiss-metasediments contacts, mostly in the Têt fault footwall. The hot springs distribution is related to high topographic reliefs, which are associated with fault throw and segmentation. In more detail, emergence localizes either (1) in brittle fault damage zones at the intersection between the Têt fault and subsidiary faults or (2) in ductile faults where dissolution cavities are observed along foliations, allowing juxtaposition of metasediments. Using these observations and 2D simple numerical simulation, we propose a hydrogeological model of upward hydrothermal flow. Meteoric fluids, infiltrated at high elevation in the fault footwall relief, get warmer at depth because of the geothermal gradient. Topography-related hydraulic gradient and buoyancy forces cause hot fluid rise along permeability anisotropies associated with lithological juxtapositions, fracture, and fault zone compositions.
ISSN: 1468-8115

hal-01582968
https://hal-brgm.archives-ouvertes.fr/hal-01582968
https://hal-brgm.archives-ouvertes.fr/hal-01582968/document
https://hal-brgm.archives-ouvertes.fr/hal-01582968/file/Taillefer_et_al_Geofluids_2017.pdf
DOI : 10.1155/2017/8190109

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
Fault friction is a parameter that is difficult to assess along fault zones since its determination depends on the knowledge of any factor controlling the state of stress around faults. In brittle homogeneous rocks, a limited number of these factors, such as the shape of the fault surface, the vicinity of fault tips or the remote stress ratio, are crucial to constrain for this determination. In this paper, we propose to analyse a field example in which all these properties are met and where the nature of the slipped structure suggest differences in static friction. We compare the orientations of branching fractures at strike-slip relay zones between en echelon stylolites and en echelon joints both reactivated in shear. The field data are compared with both photoelastic and 3-D numerical models that consider the remote stress conditions and the role of the geometry of the strike-slip segments. Based on field observations, these analyses quantitatively demonstrate the significant role of fault friction on the local stress field orientation and subsequent fracture formation. This work points out that estimations of fault friction based on analyses of fracture patterns or in situ stresses must be accompanied with a thorough investigation of the 3-D fault shape, its segmentation and the remote stress state.
ISSN: 0191-8141

hal-00560999
https://hal.archives-ouvertes.fr/hal-00560999
DOI : 10.1016/j.jsg.2010.01.008

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
At the North-Ecuadorian convergent margin (1°S–1.5°N), the subduction of the rough Nazca oceanic plate leads to tectonic erosion of the upper plate and complex seismogenic behavior of the megathrust. We used three selected pre-stack depth migrated, multi-channel seismic reflection lines collected during the SISTEUR cruise to investigate the margin structure and decipher the impact of the subducted Atacames seamounts on tectonic erosion, interseismic coupling, and seismogenesis in the region of the 1942 Mw7.8 earthquake.This dataset highlights a subducted ∼30×40 km∼30×40 km, double-peak seamount that belongs to the Atacames seamount chain and that is associated with a deep morphologic re-entrant containing mass transport deposits.The seamount subduction uplifted the margin basement by ∼1.6 km and pervasively broke the margin by deep and intense reverse faulting ahead of the seamount, a process that is likely to weaken considerably the margin. In the seamount wake, the basement reverse fault system rotated counter-clockwise. This faulted basement is overlain with slope sediment sliding along listric normal faults that sole out onto the BSR. This superposition of deep tectonic contraction within the basement and shallow gravitational extension deformation within the sediment highlights the key role of gas hydrate on outer slope erosion.In addition to long-term regional basal erosion, the margin basement has thinned locally by an extra 0.8–1 km in response to the subduction of the Atacames seamount chain and hydrofracturing by overpressured fluids at the margin toe. This pervasively and deeply fractured margin segment is associated with a seismically quiet and GPS-modeled low interseismic coupling corridor that terminates downdip near the 1942 epicenter and locked zone. We suggest that the deeply buried double-peak Atacames seamount triggered the 1942 earthquake ahead of its leading flank. This result supports previous studies proposing that subducted seamounts provide unfavorable conditions for locking the updip segment of the plate boundary limiting the updip extent of seismogenic zones, but may favor large subduction earthquakes at greater depths.
ISSN: 0012-821X

hal-01256399
https://hal.archives-ouvertes.fr/hal-01256399
DOI : 10.1016/j.epsl.2015.10.043

Éditeur(s) : HAL CCSD American Geophysical Union
Résumé : International audience
The outstanding exposure of deformation bands in a multilayer sequence of the Orange quarry, Provence, France, motivates a study investigating the development and controls of deformation band geometries by field mapping combined with finite element modeling. Field mapping yields new insights into angular relations between, and evolution of, the deformation bands as well as provides input parameters for the setup and boundary conditions of the numerical simulations using ADELI 2D. In particular, reverse-sense deformation bands are found to have developed within three of the four exposed layers of the multilayer stack with similar orientations to bedding, indicating layer-parallel contraction. Numerical simulations, carried out as a parametric study to investigate the influence on deformation band development and geometry, successfully reproduce the sequential development of deformation bands in the three layers as seen in the field as well as recreate the observed angular geometries of similar to 39 degrees to the maximum compression. This angular relationship is put into context with observations from other field studies and theoretical values to explore controls of deformation band orientations. Furthermore, band localization is impeded in the model by increased layer cohesive strength, demonstrating the importance of host-rock property variations on where localization of bands within the multilayer sequence. Results from both field investigation and numerical simulations are consistent with regional tectonics, where the strata underwent gentle folding due to thrust faulting at depth, and bring further insight into the development and geometry of deformation bands in both extensional and contractional tectonic regimes.
ISSN: 0148-0227

hal-00639452
https://hal.archives-ouvertes.fr/hal-00639452
DOI : 10.1029/2011JB008365

Éditeur(s) : HAL CCSD Oxford University Press (OUP)
Résumé : International audience
The mantle above a subducting slab is the site of complex interactions between deformation, partial melting, fluid migration and magma transport. To constrain these interactions and their effects on olivine deformation, we analyze microstructures, crystal preferred orientations, and water contents of peridotite xenoliths entrained by andesites of the Avacha volcano, southern Kamchatka arc. These xenoliths are refractory spinel harzburgites that have coarse-grained microstructures with widely spaced subgrain boundaries and sinuous grain boundaries in olivine, consistent with deformation by dislocation creep under low deviatoric stress (< 13 MPa) and with a significant contribution from diffusional processes. Analysis of crystal preferred orientations (CPO) indicates dominant activation of high-temperature, low-stress {0kl}[100] slip systems in olivine and of(100)[001] in orthopyroxene. In most samples, coarse opx crystals, elongated parallel to the lineation, enclose small olivine grains in crystallographic continuity with neighbouring crystals, indicating crystallization of orthopyroxene at the expense of olivine as a result of reactive percolation of Si-rich fluids coeval with the high-temperature deformation. Secondary crystallization of interstitial orthopyroxene led locally to development of opx-rich lenses parallel to the foliation, characterized by a decrease in olivine grain size and dispersion of the olivine CPO without changing the dominant slip systems. Half of the samples also show acicular orthopyroxene aggregates with which is associated a fine-grained matrix composed of rounded strain-free olivine, orthopyroxene, spinel, and rare amphibole crystals. This matrix occurs pervasively along grain boundaries or forms millimeter-scale irregular lenses and anastomosing veinlets that crosscut the coarse crystals and their ductile deformation structures. Both acicular orthopyroxene and the fine-grained matrix are interpreted as resulting from reactive transport of H2O-rich fluids under static conditions, probably in the lithospheric mantle. Infrared analyses show that olivine contains 1-8 center dot 6 ppm by weight of water. These low water contents are similar to those observed in spinel peridotites from other subduction zones and probably record both the low solubility of water in olivine at low pressure and dehydration during exhumation of the xenoliths. Water contents in orthopyroxene are highly variable (25-506 ppm H2O), probably recording spatially heterogeneous interaction with fluids or melts and compositional disequilibrium in the studied samples. Change in the dominant percolation mechanism from porous flow to fracturing suggests cooling, consistent with the low temperatures estimated from pyroxene thermometry (< 800-900 degrees C). The Avacha xenoliths therefore record pervasive deformation of a region of the mantle under asthenospheric conditions, followed by its accretion to the base of the lithosphere, probably as a result of cooling of the mantle wedge. Percolation of Si-rich fluids or hydrous melts is recorded at all stages; this probably enhanced diffusion and lowered deviatoric stresses during ductile deformation, but did not change the dominant slip direction in olivine from [100] to [001].
ISSN: 0022-3530

hal-00456053
https://hal.archives-ouvertes.fr/hal-00456053
DOI : 10.1093/petrology/egp085

Éditeur(s) : Elsevier
Résumé : We report on a multidisciplinary study of cold seeps explored in the Central Nile deep-sea fan of the Egyptian margin. Our approach combines in situ seafloor observation, geophysics, sedimentological data, measurement of bottom-water methane anomalies, pore-water and sediment geochemistry, and Th-230/U dating of authigenic carbonates. Two areas were investigated, which correspond to different sedimentary provinces. The lower slope, at similar to 2100 m water depth, indicates deformation of sediments by gravitational processes, exhibiting slope-parallel elongated ridges and seafloor depressions. In contrast, the middle slope, at similar to 1650 m water depth, exhibits a series of debris-flow deposits not remobilized by post-depositional gravity processes. Significant differences exist between fluid-escape structures from the two studied areas. At the lower slope, methane anomalies were detected in bottom-waters above the depressions, whereas the adjacent ridges show a frequent coverage of fractured carbonate pavements associated with chemosynthetic vent communities. Carbonate U/Th age dates (similar to 8 kyr BP), pore-water sulphate and solid phase sediment data suggest that seepage activity at those carbonate ridges has decreased over the recent past. In contrast, large (similar to 1 km(2)) carbonate-paved areas were discovered in the middle slope, with U/Th isotope evidence for ongoing carbonate precipitation during the Late Holocene (since similar to 5 kyr BP at least). Our results suggest that fluid venting is closely related to sediment deformation in the Central Nile margin. It is proposed that slope instability leads to focused fluid flow in the lower slope and exposure of 'fossil' carbonate ridges, whereas pervasive diffuse flow prevails at the unfailed middle slope.
Marine Geology (0025-3227) (Elsevier), 2009-06 , Vol. 261 , N. 1-4 , P. 92-104

Droits : 2009 Elsevier B.V. All rights reserved.
http://archimer.ifremer.fr/doc/2009/publication-6586.pdf
DOI:10.1016/j.margeo.2008.10.008
http://archimer.ifremer.fr/doc/00000/6586/