Tectonic evolution of a low-angle extensional fault system from restored cross-sections in the Northern Apennines (Italy)

Broad geological and geophysical documentation is available on regional extensional systems driven by low-angle normal faults. However, little information exists about the three-dimensional geometry and the offset distribution of such extensional structures. We present a new set of balanced geological sections across the extensional fault system driven by the Altotiberina low-angle normal fault in the Northern Apennines of Italy. We document this extensional system throughout a large set of surface (field surveys and geological maps) and subsurface data (seismic reflection profiles and boreholes). The subsurface data allowed us to define the fault deep geometry and to obtain its structural contours. The fault geometry is characterized by both along-dip and along-strike irregularities. In cross-section, the fault displays a staircase trajectory with the shallowest part being dome-shaped and flattened to horizontal. This bending could be due to the footwall uplift triggered by a footwall uploading greater than about 115 MPa. The sequential restoration of five geological cross-sections yields a maximum extension of about 10 km accumulated over approximately 3 Ma. The resulting long-term slip-rate is about 3 mm/yr, which is of the same order as the present-day extensional rate measured by GPS (2.5-3.0 mm/yr), suggesting an almost steady state extension over the last 3 Ma. The distribution of the extension values along the fault strike is bell-shaped, as expected for a continuous surface. Copyright © 2011 by the American Geophysical Union.