The Eastern Alps: Result of a two-stage collision process

The present structure and the Late Paleozoic to Recent geological evolution of the Alps are reviewed mainly with respect to the distribution of Alpidic, metamorphic overprints of Cretaceous and Tertiary age and the corresponding ductile structure. According to these data, the Alps as a whole, and the Eastern Alps in particular, are the result of two independent Alpidic collisional orogenies: The Cretaceous orogeny formed the present Austroaipine units sensu lato (extending from bottom to top of the Austroaipine unit s. str., the Meliata unit, and the Upper Juvavic unit) including a very low- to eclogite-grade metamorphic overprint. The Eocene-Oligocene orogeny resulted from an oblique continent-continent collision and overriding of the stable European continental lithosphere by the combined Austroalpine/Adriatic continental microplate. A fundamental difference seen in the present-day structure of the Eastern and Central/ Western Alps resulted as the Austroaipine units with a pronounced remnants of a Oligocene/Neogene relief are mainly exposed in the Eastern Alps, in contrast to the Central/Western Alps with Penninic units, which have been metamorphosed during Oligocene. Exhumation of metamorphic crust, formed during Cretaceous and Tertiary orogenies, arose from several processes including subvertical extrusion due to lithospheric indentation, tectonic unroofing and erosional denudation. Original paleogeographic relationships were destroyed and veiled by late Cretaceous sinistral shear, Oligocene-Miocene sinistral wrenching along ENE-trending faults within eastern Austroaipine units and the subsequent eastward lateral escape of units exposed within the central axis of the Alps. Eastward extrusion was facilitated by the above-mentioned sinistral wrench corridors and the dextral E- to ESE-trending Periadriatic fault system due to the indentation of the rigid Adriatic/Southalpine indenter.