Evidence of large-scale Mesozoic detachments preserved in the basement of the Southern Alps (northern Lago di Como area)

A polyphase metamorphic basement crops out in the northern part of the South Alpine domain of the Alps. It is involved into Alpine tectonics and is locally overprinted by a low-grade Alpine metamorphism. Pre-Alpine and Alpine events must be distinguished in order to reconstruct the tectonic evolution. We investigated Pre- Alpine structures in the basement of the northern Lago di Como area with the aim to identify rift-related faults. The well-known Lugano Val Grande Fault has been taken as example in order to identify other detachments preserved in the studied area. We investigated the tectonic contacts between Triassic carbonates and the Variscan basement in the Domaso-Cortafo Zone, south of the Insubric line. We report for the first time the Sasso Pelo Detachment, characterized by the superposition of carbonatic breccias without basement clasts above basement mylonite. Fault rocks are tectonic breccia, cataclasite, and mylonite. Brittle deformation is concentrated in the hanging wall and ductile deformation in the footwall, its intensity increasing towards the hanging wall. This extensional fault has been folded during Alpine orogeny and partially eroded.