The role of thrust and strike-slip faults in controlling regional-scale paleofluid circulation in fold-and-thrust belts: Insights from the Jura Mountains (eastern France)

We combine structural and microstructural data with stable and clumped isotopes of syntectonic calcite veins or slickenfibers of five thrust and four strike-slip faults to characterize the paleofluid circulation in the Jura foldand-thrust belt, eastern France. Syn-tectonic fluid circulation occurred along high permeability networks of breccias, foliated fault rocks, and discrete fault surfaces. At the regional scale, fluid circulation was dominated by cold meteoric fluids that infiltrated downward along both thrust and strike-slip faults, with various degrees of interaction with the carbonate host rocks. Fault-related mineralizations precipitated from fluids with delta O-18 values between -0.8%o and - 7.8 parts per thousand (V-SMOW) at temperatures between 8 degrees C and 54 degrees C. The delta O-18 values of the paleofluids partly overlap those of modern meteoric waters of the region. The calcite mineralizations yielding the highest calculated delta O-18 paleofluid values are also the warmest ones and those with the heaviest carbon isotope signal, indicating strong host rock buffering and fluid heating at depth. A contribution of deep fluids from PermoTriassic rocks cannot be excluded, but not likely. This could be related to the presence of Middle Jurassic Opalinus Clay Formation, which acted as a regional low-permeability barrier in the sedimentary succession. This can create two different fluid circulation systems characterized by warm and partly basement-derived fluids and by cold meteoric fluids, respectively, below and above the Middle Jurassic Opalinus Clay Formation. Results from this study can be used for modeling fluid circulation at the regional scale in carbonate-dominant foreland fold-and-thrust belts elsewhere in the world.