The paradox of an extensional basin forming at a site of colliding continents is apparent in the evolution of the Neogene Alboran Sea basin of the westernmost Mediterranean. Processes that may be responsible for such basins forming at sites of continental collision include subduction and the removal of subcrustal lithosphere by either convection (extensional collapse) or delamination and have recently been invoked to explain the formation of the Alboran Sea basin. Decoupling of the thermal boundary layer can reverse the stress field from a state of compression to one of extension. The processes of extensional collapse and delamination involve removal of this layer and would have similar consequences in terms of modifying lithosphere geometry: replacement of the detached thermal boundary layer by hot asthenospheric material, which would produce a high geothermal gradient and in turn cause uplift at the surface. With identical effects, it is difficult to distinguish between the two mechanisms. Using the technique of backstripping, a subsidence history which evidences the coexistence of subsided and uplifted areas in the Alboran Sea basin through the Miocene to the Quaternary, has been reconstructed for the basin. While the deformation front of the Alboran Domain migrated westward, subsidence in the Alboran Sea basin propagated in the opposite sense, east/southeastward. This is best explained as the basin forming above a southeastward-propagating, delaminating subcrustal lithospheric slab. ¿ American Geophysical Union 1995