Geological-structural map of the Orobic and Porcile thrust junction, central Southern Alps (N Italy)

Abstract

The central Southern Alps retrobelt was built trough a polyphase tectonic evolution that developed in the hangingwall of the SE-directed Alpine Tethys subduction. In the northern sector of the central Southern Alps the earliest stages of shortening resulted in the inversion of structures inherited from the Permian and Triassic rifting phases, together with southward thrusting and large-scale folding. In the San Marco Pass area (N Italy) the Variscan basement overrides the Permian-Triassic sedimentary cover along regional thrusts. Here the E-W trending Orobic Thrust joins the NE-SE Porcile Thrust. Initial thrusting phases formed greenschist facies mylonites in the basement and S-verging folds in the sedimentary covers. The Porcile line, a feature likely inherited from the Permian extension, was re-activated as a steep mylonite shear zone. Fault activity continued at brittle conditions: cataclastic shear zones overprinted the mylonites. During this phase, shortening was preferentially accommodated along the Orobic Thrust. A final transpressive re-activation observed along both thrusts is linked to the Oligocene activity of the Insubric Fault.

Low-angle normal faults record Early Permian extensional tectonics in the Orobic Basin (Southern Alps, N Italy)

“…The central Southern Alps are a fold-and-thrust belt (SCHÖNBORN, 1992;CARMINATI et alii, 1997;ZANCHETTA et alii, 2015) deeply involving a polyphase Variscan basement (MILANO et alii, 1988;SPALLA & GOSSO, 1999) and its upper Palaeozoic to Cenozoic sedimentary cover. A polyphase Alpine deformational history, occurring between the Cretaceous and the uppermost Miocene marks the southward stacking of the thrust sheets (BRACK, 1981;SCHÖNBORN, 1992;FANTONI et alii, 2004;ZANCHETTA et alii, 2011ZANCHETTA et alii, , 2015D'ADDA & ZANCHETTA, 2014), which mainly occurred in brittle conditions with no significant metamorphism (e.g. : CARMINATI & SILETTO, 2005).…”

Metasomatism by Boron-Rich Fluids along Permian Low-Angle Normal Faults (Central Southern Alps, N Italy)

“…During the Alpine orogenic event, significant crustal shortening was accommodated by thrusting and folding at very low-grade metamorphic conditions [40]. The main tectonic transport direction was toward SE with basement units that were stacked southward onto the Permian-Mesozoic cover [8,36,[41][42][43][44]. In particular, in the northern area of the belt, the Orobic-Gallinera Thrust System occurs, stacking the Variscan basement on the Permian-Mesozoic succession of the northern portion of the Orobic Anticlines included in a set of ENE-WSW dextral enéchelon anticlines.…”

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

“…Moving southward, an array of three basement-cored WSW-ENE-trending anticlines, the "Orobic Anticlines" of De Sitter and De Sitter-Koomans (1949), includes the Lower Permian volcanic, volcaniclastic, and siliciclastic sequence, unconformably covered by the Upper Permian to Lower Triassic units (Forcella and Jadoul 2000;Berra and Siletto 2006). During the Alpine orogenic event, crustal shortening was accommodated by S-SE-vergent thrusting and folding in two distinct stages, D 3 and D 4 , respectively, predating and (Brack 1981;Schönborn 1992;Carminati et al 1997;Fantoni et al 2004;D'Adda et al 2011;Zanchetta et al 2011Zanchetta et al , 2015D'Adda and Zanchetta 2015;Mittempergher et al 2021). In the northern area of the belt, the basement and the Permian to Lower Triassic successions crop out between the Orobic-Porcile-Gallinera thrust to the north and the Valtorta-Valcanale Fault to the south.…”