THE WESTERN MEDITERRANEAN EXTENSIONAL BASINS AND THE ALPINE OROGEN

The western Mediterranean late Oligocene-Miocene basins (Alboran, Vale ncia and Provencal basins) are a coherent system of interrelated troug hs. 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-a re opening related to the eastward roll-back of the W-directed Apennin es-Maghrebides subduction zone, similarly to western Pacific back-arc settings. These late Oligocene-early Miocene basins nucleated both wit hin the Betic cordillera (e.g. Alboran sea) and in its foreland (Valen cia and Provencal troughs), The N40-70 degrees direction and shows its structural independence from the orogenic roots. Thus, as the extensi on cross-cuts the orogen and developed also well outside the thrust be lt front, the westernmost basins of the Mediterranean had to develop i ndependently from the Alps-Betics orogen. Therefore, the Alboran exten sion, considered a classic example of a basin generated by the collaps e of an orogen, cannot be ascribed to the detachment or annihilation o f the lithospheric root. In contrast with the eastward migrating exten sional basins, the Betic-Balearic thrust front was migrating westward producing interference or inversion structures.