Transition from near‐surface thrusting to Intrabasement Decollement, Schlinig Thrust, eastern Alps

Abstract

West directed thrusting along the Schlinig thrust amounts to a minimum displacement of 45 km. A discrete plane of brittle thrusting is observed in the frontal part, where the Ötztal unit overrides the sediments of the Engadine Dolomites. Both shortening of the more ductile Lower and Middle Triassic and very substantial stretching by domino‐type normal faults in the brittle Upper Triassic dolomites are contemporaneous with, and caused by, thrusting. Extension is related to extrusion of parts of the sediments in the footwall of the Schlinig thrust. A relatively rapid transition into an intrabasement shear zone, which is as much as 2‐km‐thick, occurs farther to the east in a region where a paleotemperature of 300°C is indicated for Cretaceous metamorphism. Still farther to the east, localized shearing is transformed into a wider area affected by large‐scale folding. This second transition corresponds to estimated temperatures in excess of 550°C. Since the changes in structural style and deformation mechanisms toward the hinterland go hand in hand with increasing metamorphic grade, we regard thrusting as contemporaneous with metamorphism. This view is supported by geochronological data. The age of thrusting and metamorphism (90 Ma) indicates very rapid heating after the deposition of the youngest sediments (Cenomanian) involved in Cretaceous deformation. Our data suggest that localized detachment along a discrete thrust plane and within broader mylonite zones cannot always be traced indefinitely to greater depth. The transformation of localized shearing into large scale folding indicates a fundamentally different deformational style for lower crustal levels.