Early Permian syndepositional tectonics in the Orobic Basin, Southern Alps, Italy

Along the present day central Southern Alps (cSA), evidence of synsedimentary tectonics during the Early Permian are recorded in the Laghi Gemelli Group, which is characterized by the occurrence of abrupt facies variations often associated to coarse-grained deposits. These features are generally attributed to active faults: few works (e. g. Berra et al., 2011) describe local synsedimentary features such as liquefaction or slumping due to seismic shaking, however, further key areas across the Orobic Alps were identified. In particular, we recognized dewatering structures and sedimentary dikes, ball and pillars and small slumps, occurring along hundreds of mesoscopic faults showing meter-scale displacement along high-angle conjugate systems as well as domino-style faults, often accompanied by growth structures. These structures are mainly concentrated in fine-grained sediments of the Pizzo del Diavolo Formation, which was deposited on top of the volcaniclastic succession of the Ca’ Bianca Volcanite and crossed by seismogenic synsedimentary faults. The Permian synsedimentary structures of cSA are mostly associated with high-angle Andersonian normal faults which are combined with low-angle normal faults (LANFs) that developed along the interface between the Permian sedimentary cover and the Variscan basement (Zanchi et al., 2019). LANFs systems are relevant for the Permian hydrothermal circulation, resulting in widespread tourmaline deposition along the fault surfaces and locally in U mineralization. According to our structural analysis, across the central Southern Alps the Permian tectonic setting is characterized by pure extension, dominated by ENE-WSW striking normal faults inverted during the Alpine shortening as high-angle reverse faults (Zanchetta et al., 2015). We point out that an independent tectonic extensional event occurred 80 My after the Permian extension, since the strikes of the Early Permian structure in the considered area are at odds with the Early Jurassic normal faults which generally show a N-S strike and were reactivated as strike-slip faults