Challenging the postcollisional paradigm of double vergence in the Alps

Deciphering the mechanisms that control the timing of onset of precollisional orogenic processes during oceanic subduction is key to understanding how precollisional lithospheric architecture may affect mountain building in the upper plate. Europe-Adria convergence triggered subduction of the Alpine Tethys below Adria in the Cretaceous. The occurrence of synsubduction metamorphism and oceanward thrusting in the outermost (northern) edge of Adria is well constrained, whereas the occurrence of coeval crustal shortening in more internal (southern) areas of the upper plate, the Alps retrobelt, remains debated. Here, we present new U-Pb ages from several regional structures of the Alpine retrobelt, revealing that S-vergent thrusting dates back to the Late Cretaceous, 50 m.y. before the Cenozoic Adria-Europe collision. Cretaceous thrusting was synchronous with retro-foredeep turbidite deposition and coeval with N-vergent nappe stacking and high-pressure metamorphism to the north, testifying for a double-vergent precollisional orogenic system. We propose that pre-Alpine inherited structures partitioned strain and localized deformation, promoting forced subduction and strong plate coupling during the early stages of Alpine oblique convergence. This configuration challenges the long-held view of the Southern Alps as a purely Cenozoic postcollisional belt and provides a new framework for understanding orogenic onset at complex plate margins characterized by oblique convergence and microplate interactions.