Extensional fracture network attribute distribution in faulted thick sandstone strata: Compione Fault, Northern Apennines, Italy

Intensely fractured rock volumes characterize fault damage zones in cohesive rocks and typically developed from multiple deformation stages associated with fault propagation and linkage. Despite many discussed implications for fluid flow, the connectivity of fault damage zones, which is fundamental to better understand their hydrogeological behavior, has been rarely quantified. In this paper, we report on results from a study performed to characterize the fracture pattern in thick sandstone strata in the footwall of the Compione active extensional fault zone. Fracture network attributes were studied in cross-sectional vertical exposures by the window sampling method, which allows to describe fracture topology, intensity, frequency, and orientations. Our data indicate that fracture network topology in the damage zone is dominated by Y- and X-nodes (Y>55% and 20% < X<35% of nodes) and by C–C branches (>85% of branches) in 5 out of 6 structural sectors going away from the fault core. Fractures are mainly composed by passively rotated and sheared, early conjugate subsidiary extensional faults, overprinted by late high-angle conjugate extensional faults, with fracture intensity logarithmically increasing towards the fault core. This study is a first attempt to quantify cross-fault connectivity of fault-related fractures using circular windows on vertical exposures.