Faults in tight limestones and dolostones in San Vito lo Capo, Sicily, Italy: Internal architecture and petrophysical properties

摘要

Low-porosity carbonates in San Vito lo Capo underwent two episodes of faulting: extensional faulting in the late Miocene and strike-slip faulting in the Plio-Pleistocene. Limestones and dolostones reacted differently to these faulting events. The first extensional faulting event within the limestone formed localized faults with a single fault core surrounded by a damage zone. The fault rock was either dolomitized as deformation proceeded or resealed by calcite cement after brecciation. The dolomitized fault rocks were reactivated during the following strike-slip faulting, forming polyphase breccias. On the other hand, strain was distributed in dolostones throughout a wider area during the extensional faulting forming a pulverized fault zone. The pulverized rock experienced cataclastic deformation during the following strike-slip faulting forming anastomosing networks of cataclastic shear bands. Fault cores hosted in the limestone appear to have acted as flow conduits until they were cemented or hardened due to evolving cataclasis. The cataclastic shear bands in the dolostone are likely to form baffles to flow, at least on a local scale. The fracture spacing in the damage zone also varies significantly between the lithologies. In particular, damage zones in the limestone have a 5-10 cm fracture spacing whereas fracture spacing is 0.5-3 cm in the dolostone. It is likely that the differing mechanical and chemical properties of the dolostone and limestone were responsible for creating contrasting fault zone architectures.