Influence of pre-existing faults on damage distribution in carbonate fault zones: the case study of the Roccapreturo Fault, central Apennines

Understanding the origin and distribution of damage within carbonate-hosted fault zones is crucial, yet it remains a complex challenge, which hampers the overall assessment of their mechanical and hydraulic structure. In carbonate-hosted fault zones, shattered to intensely brecciated non-cohesive rocks have been reported. Although their origin has been related to the propagation of multiple seismic ruptures, great uncertainties persist regarding their interpretation and distribution.

The NW-SE striking, approximately 15 km long Roccapreturo Fault, in the central Apennines of Italy, is an intriguing case study where non-cohesive fault rock domains occur within its footwall damage zone. These domains elongate in a NE-SW direction for ~200 meters from the main slip surface.

We employed a multiscale approach to better understand the distribution and origin of the non-cohesive fault rocks. The fault geometry and throw distribution along the main fault segments were characterized through fault-perpendicular geological cross-sections. Virtual outcrop models of key exposures, located in and around an abandoned quarry, were constructed using Structure from Motion-Multiview Stereo photogrammetry. These models utilized photos taken with a Mavic Mini 2 drone. The interpretation of virtual outcrop models, combined with classical fieldwork, allowed us to map the damage and minor fault strands.

The Roccapreturo Fault displaces Cretaceous rocks originally deposited in various depositional environments. Along its strike, from NW to SE, the fault intersects rocks from internal or restricted carbonate platform, margin, and proximal slope to basin depositional environments. Notably, non-cohesive fault rocks are exposed between the margin and proximal slope rocks. This area coincides with the maximum throw of the fault, which is ca. 600 meters, and with the intersection with a system of pre-existing NE-SW-striking steeply dipping faults.

At the outcrop scale, faults exhibit two preferred orientations, parallel and perpendicular to the main slip surfaces of the Roccapreturo Fault, respectively. The former ones show predominant dip-slip kinematics, while the latter ones show both dip-slip and strike-slip kinematics.

We interpret the distribution of non-cohesive fault rocks along the Roccapreturo Fault as influenced by its intersection with the NE-SW fault system, where most of the slip accumulated. Accordingly, the pre-existing NE-SW faults accommodated transtensional slip during latest extensional deformation and coeval rock exhumation from depth. The transition of the Cretaceous depositional environments, which was accommodated by the NE-SW-striking faults, therefore highlights the pivotal role of pre-existing anisotropies in dictating the distribution of damage, particularly of non-cohesive fault rocks, in carbonate hosted faults.