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.

Keywords:

• central Southern Alps
• alpine deformation
• structural analysis
• fault slip data

Acknowledgements

Financial support for field work and analyses were from the Doctoral School of Sciences of the University of Milano Bicocca and from FAR of A. Zanchi. We warmly thank A. Zanchi also for thoughtful discussions on the tectonic evolution of the Southern Alps and for suggestions on an early version of the paper. Constructive reviews by Eugenio Carminati and Niko Froitzheim helped the authors to better clarify some geological aspects of the work. The final version of the geological map greatly benefited from suggestions and observations by Chris Orton.