Types of crust beneath the Ligurian Sea

ABSTRACT The structural setting beneath the Ligurian Sea resuJts from several tectonic events reflected in the nature of the crust. The central‐western sector, called the Ligurian basin, is part of the northwestern Mediterranean. It is a marginal basin that was generated in Oligocene‐Miocene time by subduction of the Adriatic plate beneath the European plate 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 earlier compressional regime. Our effort has been addressed in particular towards simplifying the complex nature of the crust of the Ligurian basin by modelling its genesis using uniform extension and sea‐floor depth variation with age. In the rift stage of the basin's evolution, the initial subsidence reaches the isostatic equilibrium level of the asthenosphere by a thinning factor of 3.15. The additional passive process, corresponding to the cooling of the lithosphere since 21 Ma, leads to 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 post‐rift sediment thickness. Although there is a general agreement with the previous reconstructions deduced from other experimental data, the oceanic realm has wider extent and more complex shape. The northernmost part of this realm shows crust of sub‐oceanic type altemating basement highs with lower subsidence values. The observed surface heat flux is consistent with the predicted geothermal held in the Alpine‐Provençal continental margin and in the oceanic domain. However, a characteristic thermal asymmetry is clearly visible astride the basin, due to the enhanced heat flux of the Corsica margin. Even if the uniform extension model accounts well at a regional level for the present basement depth, a remarkable tectonic subsidence excess has been found in the Alpine‐Provençal continental margin. This evidence agrees with the reprise in compression of the margin; the direction of the greatest principal stress is N120°E on average.