Architecture and seismotectonics of a regional low-angle normal fault zone in central Italy

Information from surface geology, subsurface geology (boreholes, seismic reflection, and refraction profiles), and seismicity are used to depict the geometry and the possible seismogenic role of the Altotiberina Fault (AF), a low-angle normal fault in central Italy. The AF extends along the inner Umbria region, for a length of similar to 70 km, with an average dip of similar to 30 degrees and an horizontal displacement up to 5 km. It emerges west of the inner border of the Tiber basin and deepens beneath the Umbria-Marche carbonate fold-and-thrust belt to a depth of 12-14 km. Close to the AF surface trace, low-angle synthetic east dipping normal faults extensively outcrop, whereas high-angle antithetic west dipping normal faults prevail farther east. Integrating geological and seismologic information, it can be stated that the AF behaves as an active extensional fault zone and represents the basal detachment of the west dipping seismogenic normal faults of the Umbria-Marche region. The AF belongs to a regional NE dipping low-angle normal fault system (Etrurian Fault System (EFS)), which extends for similar to 350 km from northwestern Tuscany to southern Umbria. Early preliminary considerations suggest that the EFS may play an important role in controlling active extension and related seismicity in northern central Italy.