Résumé
The role of NW-SE transfer zones during the opening of the Northwestern Mediterranean Basin has long been evidenced, mostly from seismic-reflection and magnetic data. Each of these features corresponds to transverse basement steps and/or fractures controlling the progressive deepening from the Valencia Basin toward the center of the Liguro-Provençal Basin. The most important one, the North-Balearic-Fracture-Zone, accounts for the counterclockwise rotation of the Corsica-Sardinia block and separates the Valencia continental basin from the Liguro-Provençal oceanic basin. Another transfer zone, the Central Fracture Zone is the limit of the NE part of the Valencia Basin floored with volcanic basement. Further north, the Catalan Fracture Zone corresponds to a large scale transfer zone that limits between the eastern Pyrenees and the thinned crust of the Gulf of Lion. New observations on deep seismic lines show in detail the extent of a 1–3 km thick volcanic Province around the North-Balearic-Fracture-Zone. It includes the numerous volcanoes of the Valencia Basin mostly superimposed on the transverse fractures zones. The transfer zones are also characterized by narrow syn-rift grabens interpreted as transtensional pull-apart basins along the transfer faults. The transfer zones are associated at depth with domes uplifting the Moho, thinning locally the crust down to 3 km and exhuming deeper material. Deep reflections and thinning at the base of the crust draw extensional shear-zones and crustal boudins that, together with the large amount of syn-rift volcanism in the area, suggests a ductile behavior during early stage of extension. This ductile behavior has then evolved toward more brittle deformation and deposition of syn-rift sediments in asymmetric basins. The volcanic Province disappears in the distal prolongation of the Catalan Fracture Zone that we observe and characterize for the first time. Here a thin crust with flat top consists of westward-dipping reflections in the upper part, rooted in reflective layers. This crust passes laterally to the type II exhumed domain of the Liguro-Provençal Basin and to the oceanic domain to the south. Three crustal domains can thus be distinguished in the transition between the Valencia and Liguro-Provençal Basins illustrating the complex 3D deformation of the area.