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During basin burial, interstitial fluids initially trapped within the sedimentary pile easily move under thermal and pressure gradients. As the main mechanism is linked to fluid overpressure, such fluids play a significant role on frictional mechanics for fault reactivation and sediment deformation. The Lodève Permian Basin (Hérault, France) is an exhumed half-graben with exceptional outcrop conditions providing access to barite-sulfide mineralized systems and hydrocarbon trapped into syn-rift roll-over faults. Architectural studies show a cyclic infilling of fault zone and associated bedding-parallel veins according to three main fluid events during dextral/normal faulting. Contrasting fluid entrapment conditions are deduced from textural analysis, fluid inclusion microthermometry and sulfur isotope geothermometer. We conclude that a polyphase history of trapping occurred during Permian syn-rift formation of the basin. The first stage is characterized by an implosion breccia cemented by silicifications and barite during an abrupt pressure drop within fault zone. This mechanism is linked to the dextral strike-slip motion on faults and leads to a first sealing of the fault zone by basinal fluid mineralization. The second stage consists of a succession of barite ribbons precipitated under overpressure fluctuations, derived from fault-valve action. This corresponds to periodic reactivations of fault planes and bedding-controlled opening localized at sulphide-rich micro-shearing structures showing a normal movement. This process formed the main mineralized ore bodies by the single action of fluid overpressure fluctuations undergoing changes in local stress distribution. The last stage is associated with the formation of dextral strike-slip pull-apart infilled by large barite and contemporaneous hydrocarbons under suprahydrostatic pressure values. This final tectonic activation of fault is linked to late basinal fluids and hydrocarbon migration during which shear stress restoration on the fault plane is faster than fluid pressure build-up. This integrated study shows the interplay action between tectonic stress and fluid overpressure in fault reactivation during basin burial that clearly impact potential economic reservoirs. [ABSTRACT FROM AUTHOR]

The polyphasic tectonic evolution of the northern Pyrenees (southern France) makes the context of foothills a privileged place for deep fluid circulations. Among these, the presence of CO2- and H2S-rich deep brines has been identified in the South-Acquitaine oil & gas fields, trapped within Jurassic and Cretaceous carbonate formations. Expelled from the depths to the margins of sedimentary basins, H2S is likely to form large cavities in the carbonate host rock when it interacts with O2 to form sulfuric acid (H2SO4). Known to be an important "cave-forming agent" in various places around the world, this Sulfuric Acid Speleogenesis (SAS) has never been proven in the Pyrenees through concordant morphological and geochemical data.The exploration of numerous caves along the north-Pyrenean foothill has revealed several evidences of this hypogene speleogenesis, particularly in the Arbailles region (Pyrénées-Atlantiques, France). In this massif, several karsts developed in the Cretaceous and Jurassic carbonates show morphological markers (deep notches, convection niches, replacement pockets, ceiling channels and cupolas) and minerals (gypsum, sulfides, hydrothermal carbonates) compatible with the action of the SAS. These karstic networks are notably located close to thermo-mineral springs containing abnormal concentrations in dissolved sulfates and sulfides.In addition to field observations, petrographic and mineralogical analyses, coupled with isotopes geochemistry (carbon, oxygen and sulfur), allow to reconstruct the paragenetic sequences during dissolution/precipitation phases affecting the karstic systems and to decipher the sulfur cycle involved in the speleogenesis. The comparison of sulfur isotopic signatures between cave sulfates and all the potential sources of sulfur at the scale of the Arbailles basin (Triassic evaporites, pyrites and pyrrhotites in sedimentary formations, dissolved sulfates and H2S from thermo-mineral springs) indicates that the precipitation of gypsum in karst results from the oxidation of H2S, confirming that the SAS has played a role in the development of some cavities. The production of sulfuric acid occurred during the interaction between meteoric waters and ascending H2S which, according to the multiple sulfur isotopes, derived from the thermochemical reduction of the Triassic evaporites at depth. Then, the fabric of H2S-rich carbonate veins affecting the karsts, whose carbon and oxygen isotopic signatures confirm the thermochemical reduction, specifies that the H2S migration started during the Cretaceous extension. A part of this H2S is even still adsorbed in the Jurassic rocks today. However, for the time being, we do not have solid arguments on the true timing of the SAS, which may be either contemporary with the production of H2S during Cretaceous or more recent link to the oxidation by meteoric water of the H2S previously adsorbed in the host rocks.Understanding the functioning of hypogene karsts is crucial, especially concerning the exploration of geothermal resources, but also the engineering/environmental hazards related to the impact of per ascensum fluids in the formation of large vertical void-conduit systems and leakages of H2S.Keywords: Sulfuric Acid Speleogenesis, hypogene karst, Pyrenees, structural geology, fluid-rock interactions, sulfur isotopes. [ABSTRACT FROM AUTHOR]