Résumé
The Pyrenean Belt ends against the Gulf of Lion passive margin. The mechanism responsible for dismantling the mountain belt during Oligocene rifting has not yet found a proper explanation. The Late Eocene and Oligocene period is characterized by a first order change in subduction dynamics in the Mediterranean and the subduction zones started to retreat with back-arc basins forming at the expense of mountain belts build earlier. The slab subducting below Provence and Sardinia-Corsica started a fast south-eastward retreat, forming the Liguro-Provençal Basin. This syn-rift period in the Gulf of Lion margin is coeval with exhumation of the eastern Pyrenean basement, while underthrusting of Iberia continued until Early Miocene. Based on interpretation of seismic lines, we propose a tentative model in which the mantle flow related with Apennine slab retreat has (1) exhumed and thinned the continental mantle below the Gulf of Lion and the eastern Pyrenees and (2) exhumed the lower crust, leading to crustal thinning and subsidence of the Gulf of Lion margin. The wide distribution of syn-rift volcanism in the transition between the Gulf of Lion and Valencia Basin is in line with the geometry observed in the margin suggesting ductile deformation of a weak continental crust, typical of volcanic margins. The direction of SKS-waves seismic anisotropy below the Pyrenees and the observed migration of exhumation toward the west fit this simple model. The concentration of syn-rift magmatism and the ductile behaviour of the crust during rifting can be explained by the high heat flow above the slab tear that separates the future Apennines and Maghreb branches of the West Mediterranean subduction zone. Finally, removal of upper mantle, inducing uplift and an increase of potential energy, may explain why thrusting continued in the Pyrenees while rifting was still active nearby along strike.