A revised subduction inception model to explain the Late Cretaceous, doubly vergent orogen in the pre-collisional western tethys: evidences from the Northern Apennine.

Orogenic processes can be strongly controlled by inherited structures. The paleogeography and tectonic configuration of the converging margins, might influence the location of subduction initiation, the distribution of deformation between upper and lower plate, the shape of the accretionary prism and of the subsequent orogeny, i.e., the development of single or doubly-vergent orogens, and, as a corollary, the modality of exhumation of metamorphosed units. The alpine belts around the Mediterranean are characterized by complex architectures, that are the result of a long geological evolution, going from subduction to collision, but that are also a heritage of the rifting-related continental margins configuration. The northern Apennine is a segment of these belts originated during Late Cretaceous-Early Oligocene closure of the Ligure-Piemontese ocean, and collision between the Europe and Adria plates. The different configuration of the Adria and Europe continental margins, inherited from an asymmetric rifting, is recorded in the Ligurian Units, that preserve incorporation into the subduction factory of fragments of oceanic domain (Internal Ligurian Units), and portions of the Ocean-Continent Transition Zone (OCTZ) toward Adria (External Ligurian Units). We have reviewed the structural and sedimentological characteristics of the External and Internal Ligurian Units successions, and the ages of the tectono-metamorphic events recorded in these units. These data suggest that at 80 Ma, a doubly-vergent, accretionary prism was growing between the deposition basins of the two groups of units, and feeding both basins with clasts belonging to the ocean realm, the continental crust and the subcontinental mantle. We propose therefore that closure of the Ligure-Piemontese ocean occurred through subduction that nucleated inside the OCTZ, and developed a doubly-vergent prism by accreting continental extensional allochthons and ocean-derived rocks from the OCTZ, and then rocks and sediments from the oceanic realm. This revised location of subduction onset, and the subsequent pre-collisional architecture, inherited from the rifting and spreading phases, allow reconciling most of the controversies on the pre-collisional geodynamic evolution of the Apenninic orogeny