Pre-collisional development and Cenozoic evolution of the central Southern Alps (N Italy)

The central Southern Alps retrobelt was built trough a polyphase tectonic evolution that developed since the pre-collisional stage in the hangingwall of the E-SE directed Alpine Tethys subduction. Two main pre-Late Eocene deformation phases have been recognized. The first phase (D3a) of Late Cretaceous age was characterized by southward thrusting and large scale folding involving both the Variscan basement and the Permo-Mesozoic cover. A second major deformation phase (D3b) was responsible, during the Early-Middle Eocene, for the growth and southward propagation of the basementcored Orobic Anticlines. The successive compressional phase (D4) occurred after the Adamello intrusion and promoted the development of the foreland belt. In the internal sector of the cSA the D4 deformation resulted in a dextral transpressional to strike-slip reactivation of the Orobic-Porcile-Gallinera thrust system, likely linked to the Oligocene activity along the Insubric Line. Apatite fission track data point out that the present-day exposed cSA retrobelt was already structured and largely exhumed prior of the Adamello intrusion. They also show a post-Oligocene differential exhumation within the cSA belt, with exhumation increasing northward. We suggest that the northward younging direction of exhumation was caused by the progressive indentation of the Adria lithospheric wedge beneath the cSA belt. Transpressive activity along the Insubric Line and major thrust surfaces bounding the northern cSA units, allowed the differential uplifting of the northern sector of the belt.