A mechanical model for complex fault patterns induced by evaporites dehydration and cyclic fluid pressure increase

Complex fault patterns, i.e. faults which exhibit a diverse range of strikes, may develop under conditions where a regional tectonic stress field is weak or absent (e.g. polygonal faults). The present paper considers a complex system of synsedimentary faults in the Umbria-Marche Apennines (Italy), geometrically similar to polygonal fault systems, developed during an early Jurassic extensional episode. This particular fault pattern differs from many extensional fault systems as it lacks structures that are developed with a classical bimodal conjugate ‘‘Andersonian’’ geometry. A conceptual and mechanical model is proposed to explain the development and evolution of the complex fault pattern in which it is proposed that faulting is primarily controlled by the development of volumetric instability in Triassic Evaporites due to dehydration processes (anhydritisation) during burial. The lithological architecture of the Triassic Evaporites, comprising interbedded Ca-sulphate layers and dolostones, played a fundamental role in controlling the deformation processes. Cyclic fluid overpressure build-up/release and the coexistence of brittle and brittle-ductile flow processes led to horizontal isotropic and non-plane extensional strain fields within the dehydrating rock mass, which favoured the development of complex deformation patterns. The mechanical model proposed shows that the studied fault pattern developed under a stress field consistent with almost homogeneous stress intensities within the horizontal plane. The data presented show that local strain fields and transient fluid pressure conditions have been dominant over weak regional extensional tectonics. The findings are relevant to many other areas where complex faulting patternsdincluding polygonal faultsdoccur in association with evaporite or clay-rich sedimentary sequences.