Formation of arcuate orogenic belts in the western Mediterranean region

The Alpine orogen in the western Mediterranean region, consisting of the Rif-Betic belt and the Apennine-Calabrian-Maghrebide belt, is a classic example of an arcuate orogen. It contains fragments of Cretaceous to Oligocene high-pressure/low-temperature (HP/LT) rocks, which were exhumed and dispersed during post-Oligocene extensional deformation and are presently exposed in the soles of metamorphic core complexes. In this paper, we illustrate that the arcuate shape of the orogenic belt was attained during extensional destruction of the earlier HP/LT belt, driven by subduction rollback in a direction oblique or orthogonal to the direction of convergence. Since the Oligocene, subduction of Mesozoic oceanic lithosphere, accompanied by rollback of the subducting slab, led to progressive bending and episodic tearing of the slab. This process resulted in the formation of several slab segments presently recognized in tomographic images beneath the Alboran Sea, North Africa and Italy. The remnant slabs can account for nearly all the volume of oceanic domains that existed in the western Mediterranean during the Oligocene. Subduction rollback led to extension in the overriding plate and to the opening of backarc basins. Extensional tectonism affected the original, relatively non-arcuate HP/LT belt. Allochthonous fragments of the original belt (e.g., Alpine Corsica, Calabria, and the Internal Betic) rotated and drifted as independent units until they were accreted in an arcuate fashion into the continental paleomargins of Africa, Iberia, and Adria. Therefore, the present exposures of HP/LT metamorphic rocks in the western Mediterranean region do not represent sites of continental collisions between major large-scale tectonic plates.