Structural controls on fluid migration through the Rencurel thrust zone, Vercors, French Sub-Alpine Chains

Abstract

Abstract The Vercors, French Sub-Alpine Chains, is part of a classic foreland thrust belt which accommodates the last few kilometres of WNW-directed shortening in the Alps. The Rencurel thrust zone locally emplaces Barremian limestones onto Miocene sandstones. The thrust zone is well exposed allowing detailed structural logging and sample collection to be carried out. Calcite veins occur within gouge zones along minor faults occurring pervasively throughout the fault zone. However, at the base of the exposed fault zone, where a large displacement thrust has developed, the dominant vein fill is ferroan calcite, locally accompanied by bitumen. Cross-cutting relationships indicate that the pervasive set of minor faults developed early in the deformation sequence. The lack of ferroan calcite and bitumen within the early set of faults suggests that they were impermeable during later faulting. Fluid migration during deformation along the large displacement thrust was confined to underneath the impermeable zone of early minor faults. This study indicates that the style and location of deformation changes during the incremental development of a foreland thrust zone. Distributed deformation by mesoscale faulting or folding may be characteristic of deformation which occurs before the localization of displacement onto a major fault zone. Deformation probably occurs episodically with diffusive mass transfer and cataclasis as the dominant deformation mechanisms. Microstructures evolve during progressive deformation and this may induce changes in the permeability along a fault zone through time. These changes in the nature of fault zones during incremental deformation must be considered in order to assess fully the role of faulting in controlling syn-tectonic fluid migration.