The Western Mediterranean extensional basins and the Alpine orogen

The western Mediterranean late Oligocene-Miocene basins (Alboran, Valencia and Provençal basins) are a coherent system of interrelated troughs. In all basins normal faults and thermal subsidence migrated toward the east progressively moving to the Miocene-to-Pleistocene Algerian and Tyrrhenian basins. All those troughs appear elements of the back-arc opening related to the eastward roll-back of the W-directed Apennines-Maghrebides subduction zone, similarly to western Pacific back-arc settings. These late Oligocene-early Miocene basins nucleated both within the Betic Cordillera (e.g. Alboran sea) and in its foreland (Valencia and Provençal troughs). The N40-70° direction of grabens is oblique to the coexisting N60-80°-trending orogen and shows its structural independence from the orogenic roots. Thus, as the extension cross-cuts the orogen and developed also well outside the thrust belt front, the westernmost basins of the Mediterranean had to develop independently from the Alps-Betics orogen. Therefore, the Alboran extension, considered a classic example of a basin generated by the collapse of an orogen, cannot be ascribed to the detachment or annihilation of the lithospheric root. In contrast with the eastward migrating extensional basins, the Betic-Balearic thrust front was migrating westward producing interference or inversion structures.