Reconciling post-orogenic faulting, paleostress evolution and structural inheritance in the seismogenic Northern Apennines (Italy): Insights from the Monti Martani Fault System

Structural inheritance plays a significant role upon the evolution of fault systems in different tectonic settings. Both positive reactivation of pre-orogenic extensional faults and negative reactivation of syn-orogenic reverse faults during orogenic cycles have been extensively studied and documented. By contrast, only few studies have addressed the impact of structural inheritance in regions undergoing polyphasic tectonic histories. Here, we present the Monti Martani Fault System (MMFS) case study (Northern Apennines, Italy) as a representative example of a seismically active region where to investigate the role of inherited pre-orogenic structural features upon the post-orogenic tectonic evolution. We collected outcrop scale fault-slip data therefrom to constrain fault geometry and kinematics as inputs to paleostress analysis. Based on data from extensional faults that controlled the Plio-Quaternary evolution of the system, we propose that the MMFS does not consist of a c. 30 km long, L-shaped single normal fault, as previously proposed in the literature, but is instead formed by a set of several shorter NW-SE trending extensional faults arranged in an en-echelon style. Paleostress analysis yielded three distinct extension directions during the Plio-Quaternary post-orogenic extension, oriented NE-SW, NNE-SSW and NW-SE. We relate the first two directions to local orientation fluctuations of the regional stress field interacting with the moderately oblique inherited structural features, and the latter direction to a short-live orogen-parallel extensional event whose geodynamic causes remain unclear. We suggest that the NE-SW regional post-orogenic extension direction controls the orientation of most of the NWSE striking extensional faults, while the morphostructural trend of the Monti Martani Ridge and of its boundaries with the surrounding Plio-Quaternary Medio Tiberino and Terni basins is controlled by the strike of the ~N-S and ~E-W pre-orogenic (Jurassic) inherited structural grain, rather than by the orientation of the post-orogenic extension direction. We also discuss the implications of these observations upon the seismotectonics of the MMFS. Our findings suggest that, in contrast to previous suggestions, the fault system cannot be classified as an active and capable structural feature.