Evolution of an Early Permian extensional detachment fault from synintrusive, mylonitic flow to brittle faulting (Grassi Detachment Fault, Orobic Anticline, southern Alps, Italy)

Abstract Lower Permian volcanic and sedimentary rocks of the Collio Formation in the Orobic Anticline do not rest with a depositional contact on the Variscan basement but are separated from it by the subhorizontal Grassi Detachment Fault, consisting of a cataclasite layer underlain by mylonite. Field relations indicate that both the cataclasite and the mylonite are Early Permian in age. The mylonite formed in a continuous process before, during, and after the intrusion of the Val Biandino Quartz Diorite in the footwall of the detachment fault. Microstructure and quartz texture of the mylonite indicate top-to-the-southeast displacement. Quartz textures of mylonite close to the intrusive bodies are characterized by c-axis single maxima near the Y-direction of the finite strain, indicating prism <a>glide as the dominant gliding system and hence high temperatures (above c. 500 °C) during mylonitization. This is explained by heat advection through the rising quartz diorite melt. During detachment faulting, the footwall of the Grassi Detachment Fault was bowed up to form a metamorphic core complex. The Ponteranica Conglomerate was deposited as a proximal, syntectonic fan-delta on the southeast side of the metamorphic core complex late in its evolution. The unconformity of the Verrucano Lombardo over the Collio Formation and the basement results from erosion of the topography created by detachment faulting, core complex updoming, and block tilting. These results indicate dramatic SE–NW stretching (in present-day coordinates) of the South-Alpine crust during the Early Permian. The return from the thickened, orogenic crust at the end of the Hercynian orogeny to the normal crustal thickness ( c. 30 km) of Late Permian and Early Triassic times was accommodated to a large extent by crustal extension, at least in this part of the southern Alps.