TYPES OF CRUST BENEATH THE LIGURIAN SEA

The structural setting beneath the Ligurian Sea results from several t ectonic events reflected in the nature of the crust. The central-weste rn sector, called the Ligurian basin, is part of the northwestern Medi terranean. It is a marginal basin that was generated in Oligocene-Mioc ene time by subduction of the Adriatic plate beneath the European plat e and by the eastward drift of the Corsica-Sardinia block. The eastern sector belongs to the Tyrrhenian basin system and is characterized by extensional activity which since Tortonian time superimposed an earli er compressional regime. Our effort has been addressed in particular t owards simplifying the complex nature of the crust of the Ligurian bas in by modelling its genesis using uniform extension and sea-floor dept h variation with age. In the rift stage of the basin's evolution, the initial subsidence reaches the isostatic equilibrium level of the asth enosphere by a thinning factor of 3.15. The additional passive process , corresponding to the cooling of the lithosphere since 21 Ma, leads t o a total tectonic subsidence of 3.4 km, representing the boundary of the extended continental crust. For values up to 4.1 km a transitional -type crust is expected, whereas for higher tectonic subsidence values a typical oceanic crust should exist. After setting these constraints , the boundaries of the different crust types have been drawn based on total tectonic subsidence observations deduced from bathymetry and po st-rift sediment thickness. Although there is a general agreement with the previous reconstructions deduced from other experimental data, th e oceanic realm has wider extent and more complex shape. The northernm ost part of this realm shows crust of sub-oceanic type alternating bas ement highs with lower subsidence values. The observed surface heat fl ux is consistent with the predicted geothermal field in the Alpine-Pro vencal continental margin and in the oceanic domain. However, a charac teristic thermal asymmetry is clearly visible astride the basin, due t o the enhanced heat flux of the Corsica margin. Even if the uniform ex tension model accounts well at a regional level for the present baseme nt depth, a remarkable tectonic subsidence excess has been found in th e Alpine-Provencal continental margin. This evidence agrees with the r eprise in compression of the margin; the direction of the greatest pri ncipal stress is N120-degrees-E on average.