Crustal structure from the Ligurian Sea to the Northern Apennines — a wide angle seismic transect

Abstract

A new wide-angle seismic reflection profile, 250 km long, was shot along a line extending from Parma in the Apennines to the deepest part of the Ligurian Sea. Thirty seismic land stations (LOBS) and 25 ocean bottom seismographs (OBS) recorded 54 shots fired at sea. The resulting 55 record sections allowed the definition of the transition from continental to oceanic crust in a geologically complex, and hitherto poorly understood, region. The subducted continental crust beneath the Northern Apennines has a thickness of 40 km, consisting of 30 km of igneous crust overlain by 10 km of sediments. Two sedimentary layers are identified with v P-velocities in the range from 2.8 to 5.5 km/s. The igneous portion of continental crust thins gradually to approx. 11 km across a 120-km-wide zone extending from the southwestern part of the Apennines (some 20 km inland) into the Ligurian Sea. An easterly-thinning low-velocity body (7.2–7.5 km/s) immediately beneath the base of the crust is interpreted as an underplated unit, emplaced prior to the onset of seafloor spreading. This may have been initiated by magmatic activity along the incipient continental–oceanic boundary. A basement high coincides with the northeastern edge of the Ligurian Sea and is interpreted as part of the Alpine ophiolite belt, indicating strike-slip motion. The two sedimentary layers above the transition zone are typically 5 km thick and have slightly lower velocities than those further to the northeast. A sharp continent–ocean break is recognised in the Ligurian Sea. Oceanic crust is 6–7 km thick and is overlain by a two-layered sedimentary succession approx. 5 km thick. Mantle velocities in the region range from 7.9 km/s beneath the thinned crust to 8.1 km/s beneath the thick crust. The ratio δ c/o of continental to oceanic crustal thickness at the continent–ocean transition is approx. 1.5 and the overall crust stretching factor ( β) at that boundary is approx. 3.5. These values are similar to other continent–ocean boundaries and indicate that the oceanic crust in the Ligurian Sea formed largely by stretching, with a small pull-apart component.