Evidence for active folding and faulting at the northern Apennines mountain front near Bologna, Italy from high resolution seismic reflection profiling

Abstract

We have acquired and processed an ∼2 km long high-resolution seismic reflection profile across a segment of the Northern Apennine mountain front (Italy), west of the city of Bologna. The profile, constrained by several wells, targets a long-postulated shallow blind or emergent thrust called the Pede-Apenninic Thrust Fault. Despite decades of reflection seismology in this part of the Apennines, a shallow or emergent structure consistent with the surface geology has yet to be definitively identified, a problem likely caused by the topography of the Apennine front and the traditional focus on deep hydrocarbon targets where the first 0.5 km of strata is poorly imaged. Our seismic data show an ∼300 m deep high-resolution picture of the Po foreland as it meets the Apennine mountain front. All imaged reflectors are continuous at the mountain front and are foreland-dipping, showing clear growth relationships; higher-angle reflectors are interpreted as faults. Our interpretation includes a possible hinterland-dipping blind thrust and surface normal faults, which offset late Pleistocene-Holocene deposits as much as 60 m (long-term slip rates of 0.1–0.25 mm/yr) that disrupt, but do not conceal, the growth strata relationships. Vp tomographic imaging also suggests coseismic surface-faulting in Holocene colluvium. These results have implications relevant for the effective data collection and processing techniques for these kinds of shallow active structures as well as a re-evaluation of the seismogenic potential of densely populated cities like Bologna along the Apennine mountain front.