Evidence of Early Permian extension during the post-Variscan evolution of the central Southern Alps (N Italy)

Abstract

In the central Southern Alps (N Italy) some well-preserved Permian extensional structures, that exceptionally escaped the Alpine deformation, have been recently identified. Their analysis offers important insights for the interpretation of the post-Variscan tectonics affecting the Southern Alps during the Permian. We describe here a previously unknown fault system related to the development of the Early Permian Orobic Basin, where large exposures of the Variscan basement preserve their original tectonic contacts with the overlying Lower Permian cover. The fault system consists of Low-Angle Normal Faults (LANFs) accompanied by High-Angle Normal faults (HANFs), these last entirely developed within the volcanic, volcaniclastic and terrigenous deposits. The studied structures occurring in the upper part of the Gerola Valley, share several features with Early Permian normal faults already recognized in other areas of the central Southern Alps. The low-angle normal fault planes are characterized by a continuous layer of coarse-grained fault breccias, locally impregnated by tourmalinites. The fault rocks have been invariably observed along the fault surface at the basement-cover contact all across the study area. This newly identified fault system with its characteristic combination of low- and high-angle normal faults suggests a tectonic regime characterized by pure extension in the central Southern Alps, rather than by a transtensional regime during the Early Permian. The provided data give new insights into the Early Permian geodynamic scenario, which is discussed in light of the transition from the Pangea B to the Pangea A configuration.