André Dumont medallist lecture 2011: Extension in a convergent tectonic setting: a lithospheric view on the Alboran system of SW Europe
Abstract
The Betic Cordillera of southern Spain forms a clear example of a collisional orogen that has undergone large-scale late-orogenic extension while convergent motion of the bounding plates continued. The orogen provides a unique opportunity to study the tectonics of the system at different lithospheric levels. At shallow levels in the crust, fault-bounded intramontane basins, formed during the early to middle Miocene, contain coarse continental sediments heavily affected by normal faulting, followed by a less deformed late Miocene marine succession. Extension was accommodated by coeval shortening in thin-skinned fold-and-thrust belts in the periphery of the system, and much of the region has now subsided to form a large marine basin, the Alboran basin. The thermal and deformational record of these processes is preserved in rocks from deeper crustal levels in the internal zone of the Betic Cordillera. These rocks were metamorphosed down to 50 km depth and are now exposed beneath major low-angle 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. Peridotite massifs in the western Betics emplaced in the early Miocene provide coherent outcrops of subcontinental upper mantle that allow insight in coeval processes in the mantle lithosphere. The 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 mid-lithospheric depths possibly reflects early crustal thickening, followed by extension and subsequent heating. A sudden rise of ambient temperatures in the mantle rocks by about 400 °C suggests loss of most of the underlying lithosphere and ascent of asthenosphere, whilst the final stages of exhumation in early Miocene time reflect extensional collapse. All of these phenomena can be explained by some form of 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. The processes inferred here for the Alboran region are in all likelihood not unique, as many similarities can be identified with the geology of the Tibetan Plateau, but also with domains in the Variscan and the Pan-African orogenic belts where extensional processes and associated LP/HT metamorphism and magmatism can be shown to equally have occurred in a convergent tectonic setting.