Several features of the Alboran Sea suggest that it may have been a high collisional ridge in Paleogene time that subsequently underwent extensional-collapse, driving radial thrusting around the Gibraltar arc. (1) The basin is underlain by thin (13-20 km) continental crust, has an east-west-trending horst and graben morphology, was the locus of Neogene volcanism, and has subsided 2-4 km since the middle Miocene. (2) Extension and subsidence in the basin coincided in time with outwardly directed thrusting in the surrounding mountain chains. (3) Africa and Europe were converging slowly during this period, so extension must have been driven by internally generated forces. (4) Onshore, rocks metamorphosed at 40 km depth are exposed beneath major low-angle normal faults that separate them from low-grade rocks above. (5) Emplacement of solid bodies of Iherzolite at asthenospheric temperature into the base of the collisional edifice in late Oligocene time suggests detachment of the lithospheric root beneath the collision zone. This would have increased the surface elevation and the potential energy of the system and would have favored extensional collapse of the ridge.