LATE OROGENIC EXTENSION OF THE BETIC CORDILLERA AND THE ALBORAN DOMAIN - A LITHOSPHERIC VIEW

The Betic Cordillera of southern Spain provides a clear example of a collisional orogen that has undergone large-scale extensional collapse while convergent motion of the bounding plates continued. Extension was accommodated by coeval shortening in thin-skinned fold and thrust belts around the periphery of the system, and much of the region has now subsided to form a large marine basin. The thermal and deformational record of these processes is preserved in rocks from the upper mantle, crystalline crust, and sedimentary cover. Upper mantle peridotites record evidence for exhumation in several stages from asthenospheric depths to the surface. Early stages of exhumation probably occurred during Mesozoic rifting. Cooling at midlithospheric depths reflects continental convergence, and subsequent heating indicates loss of most of the underlying lithosphere and ascent of asthenosphere, whilst the final stages of exhumation in early Miocene time reflect extensional collapse. Crustal rocks in the internal zone of the Betic Cordillera were metamorphosed down to 50 km depth and are now exposed beneath major low-angle normal detachment zones that separate them from heavily faulted low-grade rocks above. Cooling ages of associated mylonites indicate that these detachments were active during the early to middle Miocene. Fault-bounded intramontane basins, developed during the early to middle Miocene, contain coarse continental sediments heavily affected by normal fault systems, followed by a less deformed late Miocene marine succession. All of these phenomena can be explained by convective removal of the lithospheric root beneath a Paleogene collisional orogen, leading to large-scale extension followed by thermal subsidence of the center of the system.