Crustal structure from the Ligurian Sea to the Northern Apennines - a wideangle seismic transect

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 Ligur ian Sea. Thirty seismic land stations (LOBS) and 25 ocean bottom seismograp hs (OBS) recorded 54 shots fired at sea. The resulting 55 record sections a llowed the definition of the transition from continental to oceanic crust i n a geologically complex, and hitherto poorly understood, region. The subdu cted 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. T wo sedimentary layers are identified with upsilon(p)-velocities in the rang e from 2.8 to 5.5 km/s. The igneous portion of continental crust thins grad ually to approx. 11 km across a 120-km-wide zone extending from the southwe stern part of the Apennines (some 20 km inland) into the Ligurian Sea. An e asterly-thinning low-velocity body (7.2-7.5 km/s) immediately beneath the b ase of the crust is interpreted as an underplated unit, emplaced prior to t he onset of seafloor spreading. This may have been initiated by magmatic ac tivity along the incipient continental-oceanic boundary. A basement high co incides with the northeastern edge of the Ligurian Sea and is interpreted a s 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 t he thinned crust to 8.1 km/s beneath the thick crust. The ratio delta(c/o) of continental to oceanic crustal thickness at the continent-ocean transiti on is approx. 1.5 and the overall crust stretching factor (beta) at that bo undary is approx. 3.5. These values are similar to other continent-ocean bo undaries and indicate that the oceanic crust in the Ligurian Sea formed lar gely by stretching, with a small pull-apart component. (C) 1999 Elsevier Sc ience B.V. All rights reserved.