Repeated change from crustal shortening to orogen-parallel extension in the Austroalpine units of Graubünden

Abstract

The structural analysis of the Austroalpine units in Graubunden reveals the existence of two orogenic cycles, Cretaceous and Tertiary in age, both including thrusting followed by extensional overprint. Such extensional faulting occurred in the Late Cretaceous and in the Early to Mid-Oligocene. In both episodes, the direction of extension was parallel to the strike of the Alpine chain. Five stages of the tectonic evolution are described. The recognition of two orogenic cycles contradicts the classical view that the Alpine orogeny involves a continuous tectonometamorphic evolution from Cretaceous subduction and high-pressure metamorphism to Tertiary exhumation and Barrow-type metamorphism. Instead, it is postulated that nappe formation related to subduction and exhumation associated with extension occurrred twice during the Alpine orogeny. -from Authors