Lithospheric weakening during "retroforeland" basin formation: Tectonic evolution of the central South Alpine foredeep

Beginning with the Late Cretaceous, convergence between Europe and Adria caused the southward subduction of the European plate beneath Adria. Contractional movements at high crustal levels were partly accommodated by southward thrusting (Southern Alps) causing flexure of the loaded Adriatic plate and the formation of the Late Cretaceous to Late Miocene South Alpine foredeep. With respect to the Alpine subduction system, the South Alpine foredeep is a retroforeland basin. In the Late Paleogene, dip values of the base foredeep varied between 3° and 5°. The shallower dips were found along profiles crossing domains which had undergone extension during previous Late Triassic to Middle Jurassic rifting. With ongoing convergence, the base foredeep along the entire basin steepened and dip values in the Tortonian are of about 7°-8°. The increase in dip goes hand in hand with an increase in the curvature of the loaded plate. This suggests a progressive weakening of the flexed plate. Modeled effective elastic thickness (Te) values derived from the analysis of plate curvatures decrease from 15-20 km in the Late Paleogene to <5 km in the Tortonian. Preconvergence values obtained from modeling studies were even higher, in the 24-27 km range. A practically complete decoupling of the upper/middle crust from its mantle substratum enabled the increase in curvature of the hinge zone. The progressive weakening of the South Alpine lithosphere around the bulge zone is correlated with the its "upper plate" position with respect to the Alpine subduction system.