Wet and dry faults at thrust fronts as indicators of paleofluid flow in the carbonates of the Geneva Basin (Switzerland, France)

Thrust fronts are common targets for geothermal exploration in foreland basins because they contain fractured reservoirs made of a floor thrust and a frontal anticline, eventually crossed by high-angle transfer faults, and minor thrust splays. Although regional fluid flow is also controlled by petrophysical properties and the geothermal gradient, the tectonic contribution is best understood through outcrop analogs. We investigate tectonic controls on paleo-circulation across the carbonate-rich thrust front systems surrounding Geneva to better understand present-day hydrothermal pathways. Structural mapping and mesostructural analysis identify high-angle “wet” (mineralized) and “dry” (non-mineralized) faults, revealing where paleofluids circulated and sealed voids. Fault geometry defines a hierarchical network. First-order faults (>2.5 km long), mainly NW-oriented tear faults, accommodated thrust front curvature and acted as primary conduits for mineralizing fluids, except where E-W and WNW faults intercept the thrust front. Second-order faults, which are 0.4–2.5 km long, are less continuous and rarely mineralized. This suggests limited fluid migration in such gregarious faults. The fabric of mineralizations ranges from syn-kinematic slickenfibers to post-kinematic vadose cements, depending on fault orientation, kinematics, offset, and segmentation. The presence of rebrecciated fabrics and multiple generations of syn-thrusting veins and polygenetic calcite in NW-striking faults at thrust fronts suggests that these faults represent fossil examples of transient builds-up of paleofluid overpressure and deformation. Conversely, W- to WNW-tear faults mostly represent alternated pressure-solution and discrete faulting. These findings provide a framework for identifying permeable zones in carbonate thrust fronts, offering practical guidance for assessing geothermal exploration in deformed foreland basins.