L'accrétion océanique au niveau des dorsales lentes est alimentée par un apport magmatique et par l'exhumation de péridotites mantelliques. Une partie de la chaleur dégagée lors du refroidissement et de la cristallisation des magmas sert de moteur à la circulation hydrothermale qui peut transporter de l'eau de mer jusqu'aux roches mantelliques. Une réaction d'oxydoréduction impliquant l'eau de mer, l'olivine et le pyroxène des péridotites peut alors avoir lieu pour produire de la serpentine, de la magnétite et de l'hydrogène. Cette réaction, dite de serpentinisation, contrôle les propriétés physico-chimiques de la lithosphère océanique et est, de fait, incorporée dans les modèles géophysiques avec une cinétique encore mal contrainte. Ce travail de thèse est centré sur la cinétique d'hydratation de l'olivine de San Carlos, une réaction de serpentinisation simple. Un dispositif expérimental de suivi de la cinétique d'hydratation a été mis au point, basé sur la mesure du signal magnétique associé à la magnétite formée. Cette méthode, couplée à une modélisation thermodynamique, permet également de quantifier indirectement la production d'hydrogène. L'influence de la taille initiale des grains l'olivine et de la température sur la cinétique d'hydratation a aussi été étudiée. Une augmentation de la vitesse de serpentinisation dans les premiers temps de la réaction est mise en évidence et expliquée par la mise en place de microtextures également observées dans le milieu naturel (puits de corrosion et fractures) contribuant à un accroissement de la surface réactive. De manière générale, la cinétique de serpentinisation obtenue ici est de un a deux ordres de grandeur plus lente que celle utilisée jusqu'alors dans les modèles géophysiques. En plus de cette étude expérimentale, une réaction d'oxydoréduction dans un contexte naturel a été étudiée au niveau d'une zone réactionnelle entre des serpentinites et des marbres en Corse alpine. Sur la base d'une approche pétrographique et thermodynamique, la présence de wollastonite, CaSiO3, est expliquée par l'atteinte de conditions très réductrices associées à la présence de fluides riches en hydrogène en contexte de subduction. Ces fluides sont interprétés comme résultant de la conservation du potentiel réducteur des serpentinites au cours de la subduction. / Slow-spreading ridges are fed by a magmatic input and the exhumation of mantle peridotites. Part of the magmatic heat is evacuated through the hydrothermal circulation which can transport seawater up to the mantle rocks. A RedOx reaction between seawater and the olivine and pyroxene from the peridotite can then take place, producing serpentine, magnetite and hydrogen. This reaction, so-called serpentinization, controls the physical and chemical properties of the oceanic lithosphere and is, thus, considered in geophysical models even if its kinetics are poorly constrained. Therefore, this study focuses on the kinetics of San Carlos olivine hydratation, a simple serpentinization reaction. An experimental method has been developed which allows monitoring magnetically the kinetics of the reaction of interest through the amount of magnetite that it produces. This method provides also an indirect estimate of the hydrogen production when coupled to thermochemical modeling. The influence of initial grain size and temperature on the hydration kinetics has also been investigated. An increase in the serpentinization rate at the beginning of the reaction is explained by the formation of microtextures also observed in natural samples (fractures and etch pits) which contribute to the generation of new reactive surface area. The inferred kinetics of serpentinization are by one to two orders of magnitude lower than the one commonly input in geophysical models. In addition to this experimental study, a natural case of RedOx reaction involving serpentinites has been studied which deals with a decimetre wide reaction zone between serpentinites and marbles from Alpine Corsica. The presence of wollastonite, CaSiO3, is explained on a petrological and thermodynamical basis as related to highly reducing conditions characterized by the presence of H2-rich fluids in this subduction zone environment. These fluids are interpreted as resulting from the conservation of the reducing potential of serpentinites throughout the subduction process.

The Hurricane Fault is a 250-km long, west dipping, Basin and Range-bounding normal fault in SW Utah and NW Arizona that initiated in the mid-Miocene to Pliocene. It has been primarily active in the Quaternary, with slip rates of 0.2 – 0.6 mm/yr. There are multiple hot springs along its 250-km length and multiple late Tertiary-Quaternary basaltic centers broadly parallel the fault. Possible sources of hot spring fluids include deeply-circulated meteoric water that experienced water-rock exchange at high temperatures (>100 °C) and deep-seated crustal fluids. Aside from the source of modern hot spring fluids and heat, questions about the spatio-temporal history of fluid flow along the Hurricane Fault remain unaddressed. Abundant damage zone veins, cements, and host rock alteration are present, indicative of past fluid flow. Carbonate veining and cementation is a key feature of the Hurricane Fault zone, and is the primary feature exploited to characterize the thermochemical history of fault-related paleofluids. A combination of macroscopic and microscopic carbonate observations, chemical composition, and precipitation temperature of calcite veins was used to determine past water-rock diagenetic interaction and vein evolution in the Hurricane Fault zone. Calcite iv in concretions and veins from the damage zone of the fault shows a wide range of carbon and oxygen stable isotope ratios, with δ13CPDB from -4.5 to 3.8 ‰ and δ18OPDB from -17.7 to -1.1‰. Fluid inclusion microthermometry homogenization temperatures range from 45 to 160 °C, with fluid salinities of 0 to 15 wt% NaCl calculated from melting temperatures. Combining the two datasets, two main fluids that interacted with the fault zone are inferred: (1) basin brines with a δ 18OSMOW of 9.2 ‰ and (2) altered meteoric fluids with a δ 18OSMOW of -11.9 to -8.3 ‰. Calculated dissolved CO2 δ 13CPDB (-8.5 to -1.3 ‰) indicates mixed marine carbonate and organic or magmatic sources. Fault zone diagenesis was caused by meteoric water infiltration and interaction with carbonate-rich rocks, mixed with upwelling basin brines. Fluid-rock interaction is concentrated in the damage zone, where fracture-related permeability was utilized for fluid flow. A distinct mineralization event punctuated this history, associated with basin brines that were chemically influenced by nearby basaltic magmatism. This implies a hydrologic connection between the fault and regional magmatism.

The Siljan Ring in Dalarna, Sweden is the site of the largest meteor impact crater in Europe and has long been a topic of discussion regarding methane production. However, the source of this methane and the timing of production in relation to the impact remain unclear. An outcrop of red Ordovician limestone preserved on the edge of a downfaulted zone encircling the crater’s central plateau is crosscut by fractures surrounded by pale-coloured reduction haloes within which precipitates can be observed. These haloes suggest interaction with a reducing agent mobilised within a fluid flow, of which methane would be a prime candidate. A field study was subsequently undertaken to establish the reaction whereby these haloes were formed, as well as the timing of their formation relative to the Siljan impact based upon petrological, geochemical, and structural data obtained in the field. Results from this study show that a methane-bearing hydrothermal fluid mobilised within the fractures has preserved the original mineralogy of the limestone within the reaction haloes whilst the country rock beyond was being oxidised. Pyrite is shown to be preserved within the pale reaction haloes, whilst its oxidation within the country rock is shown to be the source of the limestone’s distinct red colouring. Fracture and bedding orientation at the study site suggest the hydrothermal event to have been simultaneous with the meteor impact, with the fractures forming part of a wider complex network of impact features. Whilst mobilisation associated with the meteor impact is a likely cause of methane release, the exact source of the methane active at the study site is unclear.

Discrete (mm- to m-scale) mylonitic shear zones in the northeastern Harquahala metamorphic core complex, Arizona, show evidence of fluid-mineral interactions catalyzing deformation and metamorphism. Many contain a deformed central epidote vein with adjacent bleached haloes and flanking paired shear zones that indicate significant fluid-rock interaction during deformation. An integration of structural and geochemical methods was employed to understand timing, metamorphic conditions, and physiochemical processes responsible for producing the discrete shear zones. Field and microstructural evidence suggest the zones initiated on antecedent fractures. Electron backscatter diffraction (EBSD) analyses show a significant coaxial contribution to the shear, and quartz deformation predominately by prism <a> slip, along with some rhomb <a> slip, suggesting amphibolite-facies conditions during shearing. Fourier Transform Infrared spectroscopy analyses of quartz reveal higher water contents within shear zones than within country rocks, indicating fluid infiltration synchronous with shearing. Stable isotope analyses of quartz and feldspar from mylonites are consistent with an igneous or metamorphic fluid origin. Microstructural observations suggest that the zone morphology with epidote veins, bleached haloes, and flanking discrete paired shear zones was developed predominantly from reaction softening mechanisms. The increase in deformation from bleached rock to flanking shear zones is marked by progressive modal increases in biotite and myrmekite, and modal decreases in K-feldspar, and locally epidote and titanite. Myrmekitic textures recrystallized readily and resulted in progressively greater grain size reduction of feldspar, which aided in the progressive alignment and linkage of the biotite grains, which together concentrated the deformation in bands. Volume reduction resulting from some of the metamorphic reactions may have led to a positive feedback cycle among fluid infiltration, metamorphism and deformation. U-Pb isotope analyses of syn-metamorphic titanite yield an age of ~70 Ma, suggesting the shear zones formed during cooling of the Late Cretaceous (75.5±1.3 Ma) Brown’s Canyon pluton, consistent with their top-to-the-southwest sense of shear, rather than during top-to-the-northeast directed Miocene metamorphic core complex exhumation. Petrography, EBSD analyses, and U-Pb dating of titanite from other (non-discrete) mylonites in the area imply most formed synchronously with the discrete shear zone mylonites. Only rare, scattered mylonites show features consistent with metamorphic core complex exhumation. / text

Les processus physiques et chimiques activés pendant le cycle sismique déterminent l'évolution des propriétés mécaniques des failles, à court terme (pendant un séisme) comme à long terme (la récupération des propretés élastiques des roches de faille après un seisme). L'étude des roches de faille naturelles est un moyen pour identifier les processus actives pendant les diverses phases des cycle séismique. En cette thèse, échantillons prévenants de deux failles séismiques sont étudiés: la Faille de San Andreas (California, USA), une faille séismique active, et la faille de Gole Larghe (Alpes Méridionales, Italie), une faille séismique exhumée. La Faille de San Andreas a été forée jusqu'à 2.7km de profondeur. Les échantillons montrent une superposition de: pression-dissolution - hydrofracturation - pression dissolution. La succession des évents est compatible avec la formation de sacs de fluides dans zones de basse perméabilité dans la faille, ou la pression de fluides augmente à cause de le progressif compactage de le gouge de faille, jusqu'à la nucléation de une rupture. La faille de Gole Larghe est une faille exhumée, qui a préservé des pseudotachylytes (roches fondues par le chaleur de friction pendant une frottement séismique) formées à 9 - 11 km de profondeur il y a 30 millions d'années. Deux argumentes sont traités: (i) l'évolution des microstructures des cataclasites associées à les pseudotachylytes, pour identifier les processus qui peuvent porter à la formation de instabilités frictionnelles pendant les premières phases de croissance de une faille. (ii) L'origine des fluides en failles séismiques et pendant la fusion pour friction. La formation de un système de failles à cataclasites permit la percolation de un fluide aqueux de profondeur. La composition isotopique des pseudotachylytes (calculé sans la component de hydratation) est proche à celle des pseudotachylytes reproduites en expériences du laboratoire (sans fluides). La principale source de fluides pendant la fusion pour friction est donc la déshydratation des minéraux hydraté des roches autour de la faille. / The time recurrence of earthquakes is the result of the feedback between the tectonic loading and the evolution of fault strength during the seicmic cycle. This thesis aims to identify the chemical and physical processes in fault rocks from the modern seismogenic San Andreas Fault (California, USA) and the ancient seismogenic Gole Larghe Fault (Southern Alps, Italy). The San Andreas Fault was drilled to 2.7 km depth, and samples were extracted from the depth of nucleation of repeating microearthquakes. A cyclic recurrence of pressure-solution creep – hydrofracture - pressure solution creep supports the idea that isolated compartments of high fluid pressure might cause the nucleation of small to moderate size earthquakes, associated with the dominant creeping activity in this fault segment. The Gole Larghe Fault Zone was active 30 Ma ago at 9 – 11 km depth. The occurrence of pseudotachylytes witnesses its seismic behavior. Two topics were investigated: (i) The fabric evolution of cataclastic rocks with increasing deformation, to identify the processes potentially leading to the onset of unstable slip at the early stages of fault growth. (ii) The origin of fluids involved in seismic faulting and frictional melting. The formation of a cataclastic fault network allows the ingression of external hydrous fluids, probably of deep origin. The similar isotopic composition of natural pseudotachylytes and pseudotachylytes produced in dry conditions suggests that the fluid source is the dehydration of OH-bearing minerals in the wall rocks induced by coseismic frictional heating.

No description available.

Ce travail a pour but d’étudier les paramètres physico-chimiques qui contrôlent les transferts de matière ainsi que la formation et l’évolution des argiles dans des failles chevauchantes en environnement sédimentaire. Deux failles chevauchantes pyrénéennes de faible grade métamorphique ont été étudiées : la faille de Millaris (cf. Mont Perdu) et le chevauchement du Pic-de-Port-Vieux (cf. Gavarnie). Dans la faille de Millaris, la déformation s’accompagne principalement d’une dissolution de la calcite matricielle par pression-solution induisant un changement de volume de la roche de 20 à 40%. Le chevauchement du Pic-de-Port-Vieux enregistre des modifications importantes au coeur de la faille mais aussi dans la zone d’endommagement. Dans les calcaires du mur du chevauchement, une mylonitisation est associée à une dissolution partielle des dolomites en présence de fluides ne dépassant pas 320-340°C. Dans les pélites du toit du chevauchement, la dissolution de l’hématite par un fluide réducteur entraine un changement de l’état redox de la roche (confirmé par spectroscopie Mössbauer) et la précipitation de chlorite dans des veines syncinématiques. Des chlorites à zonations chimiques oscillatoires présentes dans certaines veines révèlent, en combinant cartographie chimique à la microsonde, mesures de l’état redox par μXANES et thermométrie, des variations cycliques de température d’au moins 50°C au cours de la cristallisation. Un processus de valves sismiques pourrait donc être associé à la mise en place du chevauchement du Pic de Port Vieux. / This work aims to study the physical and chemical parameters that control the mass-transfer and the clays formation and evolution in sedimentary environment thrust faults. Two Pyrenean thrust faults in low metamorphic grade were studied: the Millaris fault (related to Mont Perdu) and the Pic-de-Port-Vieux thrust (related to Gavarnie). In the Millaris fault, the deformation is accompanied mainly by dissolution of the matrix calcite by pressure-solution which induces a volume change of the rock from 20 to 40%. The Pic-de-Port-Vieux thrust records significant changes in the fault core-zone, but in the damaged zone also. In the footwall limestone, a mylonitisation is associated with a partial dissolution of dolomite in the presence of not exceeding 320-340°C fluids. In the hanging-wall pelites, the hematite dissolution by a reducing fluid causes a redox state change of the rock (confirmed by Mössbauer spectroscopy) and chlorite precipitation in synkinematic veins. Oscillatory zoning pattern chlorites located in some shearing veins revealed, by combining chemical mapping microprobe, redox state measurements with μ-XANES and thermometry, cyclic temperature variations of at least 50°C during the crystallization. A seismic valves process could be associated to Pic-de-Port-Vieux thrusting.

No description available.

Mid-ocean ridge hydrothermal systems are known to extend to deep levels of the oceanic crust, including the plutonic section, but little is known about the timing and nature of fluid-rock interactions at these levels. To investigate the temporal and spatial characteristics of hydrothermal alteration in the lower crust, this study investigates a suite of hydrothermally altered (<5 to >20% hydrous alteration) gabbroic rocks recovered from the Hess Deep Rift, where 1.2 Ma fast-spreading East Pacific Rise crust is well-exposed. These samples were altered to amphibole-dominated assemblages with chlorite-rich samples occurring in a restricted region of the field area. Hornfels, indicative of reheated, previously altered rocks, are clustered in the central part of the field area. The entire sample suite has elevated 87Sr/86Sr (mean: 0.70257±0.00007 (2σ), n=16) with respect to fresh rock (0.7024). Bulk rock 87Sr/86Sr is strongly correlated with percentage of hydrous alteration and weakly correlated with bulk rock Sr content. The distribution of Sr in igneous and metamorphic minerals suggests that greenshist-facies alteration assemblages (chlorite, actinolitic amphibole, albitic plagioclase) lose Sr to the fluid while amphibolite-facies secondary assemblages (secondary hornblende, anorthitic plagioclase) take up Sr. The temperature-dependent mobilization of Sr in hydrothermal systems has implications for the 87Sr/86Sr and ultimately fluid/rock ratio calculations based on the assessed 87Sr/86Sr systematics. Considering Sr behaviour, minimum fluid/rock ratios of ~1 were calculated for the plutonic section. Due to the large uncertainty regarding fluid Sr composition at depth and the sensitivity of fluid/rock ratio calculations on this parameter, a model combining the sheeted dike complex and the plutonic section to one hydrothermal system is introduced, yielding a fluid/rock ratio of 0.5. This value may be more realistic since the fluid composition entering and exiting the sheeted dike complex is better constrained. The regional distribution of hornfelsed material with elevated 87Sr/86Sr suggests that fluid ingress into the upper plutonics at Hess Deep occurred on-axis in a dynamic interface of a vertically migrating axial magma chamber (AMC) and the base of the hydrothermal system. / Graduate

Le bassin de Jaca (Pyrénées espagnoles) est un exemple classique de bassins d’avant pays, où les grandes lignes du remplissage sédimentaire, ainsi que la chronologie des failles ont été très étudiées. Il reste toutefois à mieux comprendre la paléo-hydrologie et l’histoire thermique du bassin, de manière à proposer un modèle de circulation des fluides pendant sa mise en place et sa déformation (Paléocène-Oligocène). Pour ce faire, ce travail propose d’analyser la répartition de la fracturation, d’étudier les conditions de formation des veines syn-tectoniques et de caractériser la maturité de la matière organique sur l’ensemble du paléobassin d’avant-pays de Jaca, des zones internes vers les zones externes.L’analyse pétrographique, géochimique et microthermométrique des veines montre que la grande majorité des fluides minéralisateurs sont à l’équilibre isotopique et thermique avec l’encaissant. Dans le détail, nous avons identifié 2 événements principaux de formation de veines dans la zone interne du bassin (Sierras Interiores), que nous proposons d’associer au fonctionnement des failles majeures dans le socle. Nous suggérons que les fluides circulent le long des niveaux de décollements et sont expulsés sur de courtes distances (< 10 km), au travers des réseaux de fractures, vers le bassin d’avant-pays. Le reste du bassin enregistre principalement des fluides locaux, parfois associés à l’infiltration d’eau météorique. L’analyse des températures d’enfouissement (50°C à 250°C), qui inclut des données de Δ47, montre une organisation N-S relativement homogène depuis les Sierras Interiores (fenêtre à gaz) jusqu’aux Sierras Exteriores (immature), avec des anomalies longitudinales marquées. Les modélisations thermiques 1D sur 9 puits virtuels suggèrent que les températures maximales vers les Sierras Interiores peuvent résulter d’un enfouissement sédimentaire, dont une grande partie est érodée actuellement. Nous proposons que ces parties érodées correspondent à des dépôts tardi-orogéniques conglomératiques déposés à proximité de la zone axiale. Les données suggèrent une répartition non homogène de ces dépôts selon un axe E-W, impliquant des transferts sédimentaires plus complexes qu’habituellement discutés. Au vu de nos résultats et des précédentes études, le modèle paléohydrologique et thermique du bassin de Jaca, et à plus grande échelle, de la chaîne plissée sud-pyrénéenne, est compartimenté à la fois dans l’espace et dans le temps, en lien avec à la propagation latérale et frontale de la déformation, qui contrôle l’ouverture du système. Le modèle paléohydrologique et thermique de la chaîne plissée sud-pyrénéenne constitue donc un potentiel analogue aux chaînes plissées dont le raccourcissement résulte d’une convergence oblique. / The Jaca basin (Spanish Pyrenees) is a classical example of a foreland basin, where the sedimentary filling and the calendar of thrust activation have been extensively studied. It remains to understand the paleohydrology and the thermal history of the basin, so as to provide a fluid flow model related to its formation and deformation (Paleoecene-Oligocene). To do this, this work proposes to analyze the distribution of fracturing, to study the conditions of formation of syn-tectonic veins and to characterize the maturity of organic matter throughout the Jaca foreland basin, from hinterland to external areas.Petrographical, geochemical and microthermometric analysis of veins show that the vast majority of mineralizing fluids are at the isotopic and thermal equilibrium with the host-rock. In detail, we identified two main events of vein precipitation in the inner part of the basin (Sierras Interiores), probably related to major basement thrust activations. We suggest that fluids flow along decollement levels and are expelled over short distances (<10 km), through fracture networks towards the foreland basin. The other part of the basin mainly record local fluids, sometimes associated with the infiltration of meteoric water. Analysis of burial temperatures (50 °C to 250 °C), which includes Δ47 data, shows a relatively homogeneous N-S organization from the Sierras Interiores (gas window) to Sierras Exteriores (immature), with strong longitudinal anomalies. Thermal 1D modelling of 9 virtual wells suggest that the maximum temperatures of Sierras Interiores result from sedimentary accumulation, whose a large amount is now eroded. We propose that this eroded thickness corresponds to late-orogenic conglomeratic deposits near the axial zone. The data suggest an inhomogeneous distribution of the deposits along an E-W axis, involving more complex sedimentary transfers than usually discussed. Given our results and previous studies, the paleohydrological and thermal model of the Jaca basin, and on a larger scale, of the South Pyrenean fold and thrust belt, is compartmentalized both in space and in time, in response to the propagation of and oblique deformational front, which controls the opening of the system. The paleohydrological and thermal model of the South Pyrenean fold and thrust belt is therefore a potential analogue to fold and thrust belt including shortening due to an oblique convergence.