Fluid circulation in the depths of accretionary prisms: an example of the Shimanto Belt, Kyushu, Japan

International audienceAccretionary prisms constitute ideal targets to study fluid circulation and fluid-rock interactions at depths beyond the reach of active margin deep drilling. The highest-grade rocks from the Shimanto Belt on Kyushu were buried under 3-5 kbars at ~ 300°C (Toriumi and Teruya, 1988). They contain abundant quartz veins, formed throughout burial and exhumation and variably affected by brittle and ductile deformation.Cathodoluminescence (CL) reveals the existence of two distinct types of quartz, characterized by a blue and brown color, respectively. CL-blue quartz fills macro-veins (width ≥ 10μm), while CL-brown quartz is present in micro-veins (width ~ 1 − 10μm) and ductilely recrystallized domains. On the basis of microstructures, the fluids associated with the CL-blue and CL-brown quartz are interpreted as “external” and “local”, respectively. Quartz growth rims of alternating CL colors as well as mutually cross-cutting veins show that the two fluids cyclically wetted the host rock.From fluid inclusions analysis, the fluid associated with CL-blue quartz has a salinity similar to seawater, while the fluid associated with CL-brown quartz is less saline. In addition, CL-blue quartz is richer in aluminum than the CL-brown one. In contrast to the salinity/aluminum signature, the δ18O isotopic signature of both quartz types is similar and buffered by host rock. The difference between the preservation of the salinity signature of the fluid and the loss of its δ18O signature is explained by quicker exchange kinetics and larger host rock buffering capacity for isotopic reequilibration.The “local” fluid, associated with CL-brown quartz, reflects the dilution of pore water by the pure water produced by prograde dehydration reactions of clay minerals. The “external” fluid associated with CL-blue quartz is interpreted as seawater or pore water from shallow (depth<1-2 km below seafloor) sediments. We propose that downward percolation of shallow water to depths ~ 10km is a transient process associated with mega-earthquakes

A new method of reconstructing the P-T conditions of fluid circulation in an accretionary prism (Shimanto, Japan) from microthermometry of methane-bearing aqueous inclusions

International audienceIn paleo-accretionary prisms and the shallow metamorphic domains of orogens, circulating fluids trapped in inclusions are commonly composed of a mixture of salt water and methane, producing two types of fluid inclusions: methane-bearing aqueous and methane-rich gaseous fluid inclusions. In such geological settings, where multiple stages of deformation, veining and fluid influx are prevalent, textural relationships between aqueous and gaseous inclusions are often ambiguous, preventing the microthermometric determination of fluid trapping pressure and temperature conditions. To assess the P-T conditions of deep circulating fluids from the Hyuga unit of the Shimanto paleo-accretionary prism on Kyushu, Japan, we have developed a new computational code, applicable to the H2O-CH4-NaCl system, which allows the characterization of CH4-bearing aqueous inclusions using only the temperatures of their phase transitions estimated by microthermometry: Tmi, the melting temperature of ice; Thyd, the melting temperature of gas hydrate and Th,aq, homogenization temperature. This thermodynamic modeling calculates the bulk density and composition of aqueous inclusions, as well as their P-T isochoric paths in a P-T diagram with an estimated precision of approximatively 10 %. We use this computational tool to reconstruct the entrapment P-T conditions of aqueous inclusions in the Hyuga unit, and we show that these aqueous inclusions cannot be cogenetic with methane gaseous inclusions present in the same rocks. As a result, we propose that pulses of a high-pressure, methane-rich fluid transiently percolated through a rock wetted by a lower-pressure aqueous fluid. By coupling microthermometric results with petrological data, we infer that the exhumation of the Hyuga unit from the peak metamorphic conditions was nearly isothermal and ended up under a very hot geothermal gradient. In subduction or collision zones, modeling aqueous fluid inclusions in the ternary H2O-CH4-NaCl system and not simply in the binary H2O-NaCl is necessary, as the addition of even a small amount of methane to the water raises significantly the isochores to higher pressures. Our new code provides therefore the possibility to estimate precisely the pressure conditions of fluids circulating at depth

Apport d'une analyse 4D du réseau de fracturation d'un réservoir pour approcher son histoire cinématique. Exemple de l'anticlinal de Boltaña, Aragon, Espagne

International audienceLes réservoirs carbonatés plissés constituent une part importante des réserves mondiales en hydrocarbures. L’étude du réseau de fractures (géométrie et dynamique de mise en place) permet une approche particulièrement précise de la cinématique de ces structures.L’anticlinal de rampe de Boltaña est situé entre deux bassins sédimentaires de la zone sud pyrénéenne. Il résulte d’une histoire tectono-sédimentaire complexe datée de l’Eocène. Sa disposition suivant un axe N-S contraste fortement avec la tendance générale E-W de la chaine des Pyrénées. Des études récentes montrent d’une part une croissance syn-sédimentaire de la structure, et d’autre part une rotation horaire syn-cinématique d’environ 50° autour d’un axe vertical.Nous présentons ici les résultats d’une analyse cinématique conduite à partir de l’étude spatiale et temporelle du réseau de fractures. Celle-ci montre que la configuration actuelle de la structure de Boltaña résulte de cinq régimes de contraintes successifs. Les quatre premiers sont compressifs, ils entraînent la rotation puis le plissement du domaine de Boltaña et se terminent par un cisaillement général de l’anticlinal entre les bassins voisins. Le dernier régime de contraintes intervient au cours des derniers incréments de rotation et de croissance du pli. Il correspond à une phase extensive traduite par l’apparition d’un système de failles normales à transtensives impliquant les parties Est et Sud du domaine. Nous montrons que tous ces différents régimes pourraient être une décomposition locale d’une contrainte régionale (NNW-SSE) liée à la collision entre la plaque Ibérique et l’Eurasie. Tenant compte des précédents résultats et de notre propre étude, nous proposons une histoire cinématique inédite dans un cadre spatio-temporel précis. Nos conclusions précisent le cadre structural de la zone. Elles mettent en lumière un mode de mise en place original directement lié aux mouvements des nappes chevauchantes de la zone centrale sud pyrénéenne

Folding/fracturing and reactivation relationship on the Boltana anticline, southern Pyrenees, Spain

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Ebullition explosive du méthane et mégaséismes dans les zones de subduction

National audienceL’analyse des inclusions fluides dans les paléo-zones de subductionmontre que jusqu’à 300°C le fluide baignant la roche est un mélanged’eau salée et d’une faible quantité de méthane. La présence dans denombreux exemples de deux types d’inclusions (riche en eau et richeen méthane) implique l’immiscibilité du méthane dans l’eau en profondeur.Par ailleurs, les inclusions riches en méthane du paléoprismed’accrétion Shimanto, au sud-ouest du Japon, enregistrent des fortes etrapides variations de pression de fluide. Une hypothèse est que ces variationsbrutales reflètent l’enregistrement par le fluide d’un événementsismique, qui connecterait des poches de liquide isolées et ferait chuterla pression de fluide. A ce mécanisme d’enregistrement passif desséismes par les fluides, nous proposons d’ajouter un mécanisme actif,par lequel le fluide entretient la rupture sismique. En effet, lorsque lefluide acqueux est saturé en méthane, une petite chute de pression defluide, associée par exemple à un séisme, provoque une ébullition explosivedu méthane susceptible de fracturer la roche encaissante. Cettefracturation et la dilatance qui l’accompagne permettent de propager lachute de pression initiatrice et l’ébullition explosive. L’ébullition explosivede méthane est donc un processus qui potentiellement accompagneet favorise la génèse de mégaséismes en libérant de l’énergie. Afin devalider ce modèle, nous avons étudié la production de méthane par craquagede la matière organique dans des échantillons de carottes de zonesde subduction. Cette analyse montre que (1) la matière organique, bienque présente en faible quantité, est suffisamment abondante pour saturerl’eau dans les pores d’une roche sédimentaire fortement compactée et(2) la caractère sismique/asismique des zones de subduction autour duglobe recouvre, de façon très grossière, une différence entre une « forte »et une « faible » productivité en méthane

Fluid circulations in the depths of accretionary prism: the record of quartz from the Shimanto Belt, Japan

Communication OraleInternational audienceFluids present in the depths of subduction zones play a major role on seismogenesis, although fluid circulations paths and physico-chemical conditions are still largely unknown. Two main reservoirs of water, either in the pores of sediments or bound to hydrous minerals, release large amounts of water in the relatively shallow and deep domains of subduction zones, respectively. The usual model of circulation assumes then a bottom-up circulation driven by fluid pressure gradients. This study aims at reassessing this model, using the record of rocks from a paleo-accretionary prism, the Shimanto Belt in Japan. These rocks, buried to 5kbars and 300˚C (Toriumi and Teruya, Modern Geology, 1988), were affected by pervasive fracturing throughout their history, from burial to exhumation. The quartz filling these fractures and the fluid inclusions that it contains keep the track of the fluid associated with the rock evolution. Using a combined approach of microstructural observations by optical microscopy and cathodoluminescence (CL), and chemical characterization by electron and ion microprobe as well as microthermometry, we show that there are actually two distinct fluids that have cyclically wetted the rock at depth. The first one is an " external " fluid penetrating through macroscopic fractures and precipitating a quartz blue in CL. In contrast, a " local " fluid attended the formation of quartz brown in CL, precipitating in microfractures or associated with ductile recrystallization. The two fluids are also chemically distinct: Both have a salinity close to seawater, but the local fluid is fresher than the external one. In addition, the external fluid is richer in aluminum than the local one. Finally, the external fluid is very slightly depleted in δ 18 O, although the difference is probably not significant and the first-order isotopic signal is a buffering by host rock

Organic matter cracking: A source of fluid overpressure in subducting sediments

International audienceThe pressure of deep fluids in subduction zones is a major control on plate boundary strength and earthquake genesis. The record, by methane-rich fluid inclusions, of large (~ 50–100 MPa) and instantaneous pressure variations in the Shimanto Belt (Japan) points to the presence of large fluid overpressure at depth (300–500 MPa, ~ 250 °C). To further analyze the connection between methane and fluid overpressure, we determined with Rock-Eval the potential for a worldwide selection of deep seafloor sediments to produce methane as a result of organic matter (OM) cracking due to temperature increase during subduction. The principal factor controlling the methanogenesis potential of sediments is OM proportion, while OM nature is only a subordinate factor. In turn, OM proportion is mainly controlled by the organic terrigenous input. Considering a typical sediment from ocean-continent subduction zones, containing 0.5 wt% of type III OM, cracking of OM has two major consequences: (1) Methane is produced in sufficient concentration as to oversaturate the pore-filling water. The deep fluid in accretionary prisms is therefore a mechanical mixture of water-rich and methane-rich phases; (2) CH4 production can generate large fluid overpressure, of the order of several tens of MPa, The conditions for these large overpressure are a low permeability of the upper plate ( 10 km) where OM thermal cracking occurs. At these depths, OM thermal cracking appears as a source of overpressure larger than the last increments of smectite-to-illite reaction. Such large overpressures play potentially a role in facilitating slip along the plate interface. Conversely, the scarcity of earthquakes in ocean-ocean subduction zones such as Marianna or Barbados may be related to the low influx of detrital OM and the limited methane/overpressure generation at depth