Structural and metamorphic evolution of western part of the Tauern Window, Eastern Alps

Abstract:

The Tauern Window in the Eastern Alps represent a tectonic window, where Penninic continental units and overlying Piemontais oceanic units crop out from below the Austroalpine crystalline nappes. The window is formed by Subpenninic nappe system composed of Variscan basement (Zentralgneiss) with Mesozoic cover sequences overlain by Penninic nappes. The studied nappes were previously recognized as Lower and Upper Schieferhülle and their P-T conditions of up to blueschist facies were determined by Selverstone (1988, 1993).

Our detailed structural and petrological study focused mainly on tectono-metamorphic evolution of different nappes. The Zentralgneiss cover sequences consist mainly of schists, amphibolites and quartzites with originally subhorizontal – gently westward dipping fabric. Dominant fabric was later deformed during deformation stages D2 and D3 that are preserved in the form of folds and cleavages. The later one mainly in the W part of studied area. The Penninic nappes are composed of deformed greenschists, micaschists and marbles, which are together folded by large-scale open folds during D2.

The metamorphic PT conditions were reconstructed by using the phase equilibrium modelling and chemical composition/zoning of garnets, which are mostly synkinematic to the formation of the main deformation fabric. The compositional zoning in garnets revealed an overall prograde PT evolution with PT increase up to 3.5 kbar and 100°C associated with the main deformation event.

The structural and petrological record show the relation of nappe evolution and unroofing of the complex such as described in Jeřábek et al. (2012) from West Carpathians. The E-W stretching and prograde metamorphic evolution is associated with burial, while exhumation is associated with formation of subhorizontal cleavage and dip-slip kinematics towards W contrary to previously published studies.