Structural evolution of the Northern Tyrrhenian Sea from new seismic data

We present the results of the deep penetrating LISA seismic survey suppleme nted by several industrial oil surveys. The LISA cruise shows the deep part of the crust and the Moho reflections are seen at 7.5-8 s TWTT on several seismic lines on the Tuscan (eastern) margin of the North Tyrrhenian Sea. T he Moho depth has been previously evaluated by refraction data at about 22- 25 km and this depth corresponds to the reflections observed on our seismic profiles. Two kinds of dipping reflectors are identified in the crust. The western dipping events are interpreted as traces of thrusts with an Apenni nic vergence, the eastern dipping events are probably extensional detachmen ts Faults. These faults are seen deep in the trust and may be shear zone in the ductile crust. The extensional faults and the thrusts have been previo usly identified on the industrial lines but these lines are processed at a depth of 4s TWTT and consequently the deep reflections are not imaged. The industrial lines and the correlations with the exploratory wells helps to e xtend the results of the LISA cruise in the entire region. The compilation of all the seismic data allow us to present the first depth to basement map of this region. The major feature of the study area is the Corsica basin t hat is more than 8.5 km deep. The deep part of this huge basin is probably filled by Oligocene-early Miocene deposited in an extensional regime. Howev er this extension persisted during the Miocene. The steep faults that bound s the basin toward the west are imaged and we present a model in ramp, and flat system, that explains the particular geometry of the Corsica margin. T he eastern flank of the Corsica basin which is bounded by the Pianosa ridge , is completely different. This region has been intruded by granitic plugs during the Messinian and the early Pliocene and these intrusions ar-e coeva l with horst and graben formation on the Tuscan margin. Before this extensi on, Pianosa ridge and Elba island were deformed by a compression related to the formation of the Apennines during the Miocene. Our observations on the North Tyrrhenian Sea are set in the tectonic framework of the Western Medi terranean. A first extension occurred during the Oligocene-early Miocene in the Provencal basin: east of Corsica and Sardinia. The eastern basins are bounded by strike-slip faults and are located on the top of deformed tecton ic prism. The compressional deformation is related to the middle Miocene ap ennines mountain building. Then compression migrated to the east and the in ner Apennines belt collapsed during the late Miocene and the early Pliocene . This late extension, associated with a high heat flow and a shallow Moho, is well illustrated by the LISA survey in the North Tyrrhenian Sea. (C) 19 99 Elsevier Science Ltd. All rights reserved.