RHEOLOGY AND SEISMOTECTONIC REGIME IN THE NORTHERN CENTRAL MEDITERRANEAN

The connection between thermal field and mechanical properties is anal ysed in the northern central Mediterranean region, extending from the Ligurian-Provencal basin to the Adriatic foredeep. As the thermal regi me is still far from equilibrium in most of the tectonic units, transi ent thermal models are used. The temperature-depth distribution is est imated in four areas affected by the volcanic activity, which from the Neogene to the Present shifted from Corsica to the Apenninic are. In the Adriatic foredeep, the thermal effects of the recent thrust-faulti ng phase in the buried sectors of the northern Apennines are taken int o account. The general context consists of convergence involving westw ard subduction of the Adriatic plate. This process caused anti-clockwi se rotation of Corsica and Sardinia, which led to formation of the Lig urian marginal basin, and also resulted in crustal doubling and overth rusting in the northern Apennines and rifting in the northern Tyrrheni an. Seismic activity is focused in the internal and external zones of the Apenninic are, where low surface heat flux is observed, and in the western margin of the Ligurian-Provencal basin. This is a consequence not only of lateral variations in the thermal field but also of the d ifferent tectonic settings. Regional extensional patterns in the shall ow crust, with minimum principal stress axes trending N60 degrees E an d E-W, are observed in the northern and in the southern sectors of the Apenninic are, respectively. A compressional regime at depths greater than 30 km is observed below the northern sector of the are, while to the south a change in the structure of the lithosphere is marked by a decrease in deeper seismic activity. Thrust faults and strike-slip fa ults with a thrust component support a compressional regime along the western margin of the Ligurian basin with maximum principal stress axi s oriented N120 degrees E. Two lithospheric cross-sections across the study region are constructed, based on structural, thermal, gravity, r heological and seismic data. There is clear evidence of the presence o f the subducting slab of the Adriatic plate, corresponding to a thicke ning of the uppermost brittle layer. The crustal seismicity cut-off co rresponds to temperatures of 320-390 degrees C. A brittle layer of con siderable thickness is present in the uppermost mantle beneath Varisca n Corsica and the Adriatic foredeep, with estimated seismic cut-off te mperature of about 550 +/- 50 degrees C.