A Revised Subduction Inception Model to Explain the Late Cretaceous, Double-Vergent Orogen in the Precollisional Western Tethys: Evidence From the Northern Apennines

The Meso-Cenozoic alpine belts of the Mediterranean area are characterized by complex architectures, result of a complex subduction and collision evolution that preserve also a legacy of the rifting-related configuration of the continental margins. The Northern Apennines is a segment of these belts originated during closure of the Ligure-Piemontese ocean and collision between the Europe and Adria plates. The different configuration of the Adria and Europe margins, inherited from asymmetric rifting, is recorded in the Ligurian Units that preserve incorporation into the subduction factory of fragments of the oceanic domain (Internal Ligurian Units) and portions of the Ocean-Continent Transition Zone (OCTZ) toward Adria (External Ligurian Units). We provide here unpublished data on the stratigraphy and sedimentology of these units, together with a review of what is already established in literature. Both data sets combined testify that at 80 Ma, an accretionary prism was growing between the deposition basins of the two groups of units and fed both basins with clasts from the ocean realm, the continental crust, and the subcontinental mantle. We propose that closure of the Ligure-Piemontese ocean occurred through subduction that nucleated at the transition from the oceanic plate and the thinned Adria margin and developed a double-vergent prism by accreting oceanic material and continental extensional allochthons from the OCTZ. We believe the revised site of subduction initiation, and the precollisional architecture, inherited from the rifting and spreading phases, allows reconciling most of the discrepancies between the various interpretations proposed in literature for the precollisional evolution of the Apennines.