Synrift sedimentation, Jurassic and Alpine tectonics in the central Ortler nappe (Eastern Alps, Italy)

New evidence for Early Jurassic rifting is presented for the Austroalpine Ortler nappe (Eastern Alps, Italy) in the area between Livigno and Valle di Fraele. Active fault scarps are documented by the presence of megabrecias, conglomerates and calcarenites in the Liassic Allgau Formation and by an angular unconformity that connects eastward to the east-dipping Monte Torraccia normal fault. This normal fault can be followed for a distance of more than 5 km. New findings of ammonites document that carbonate redeposition and tectonic activity along the fault initiated during the Late Hettangian. Two main phases of Alpine deformation can be recognized. The first, late Cretaceous phase is characterized by westnorthwest-directed thrusting, nappe emplacement and mylonite formation along the Quattervals thrust and the Zebru thrust. At the same time sediments of the Ortler nappe were affected by open to isoclinal folding and west-thrusting, accompanied by reactivation of the M. Torraccia Jurassic normal fault that led to the inversion of the Jurassic sedimentary basin. During a later, probably Tertiary deformation phase large scale folds with northwest-southeast oriented fold axes developed. This phase did not essentially alter the geometry produced by the first one, but is responsible for the general north-dip of bedding and nappe contacts in the area.