Crustal-scale extension in a convergent orogen : the Sterzing-Steinach mylomte zone in the Eastern Alps

Abstract

The western margin of the Tauera Window (Eastern Alps) is defined by a low angle westward dipping fault zone of potently We disp lacement. Ductile deformation of the fault rocks results in a carpet of mylonites up to 400 metres thick. Evidence from shear criteria and the excision of part of the Cretaceous-Tertiary metamorphic edifice both indicate normal displacements, and relative movement of Austroalpine nappe complex towards the west. The Sterzing-Steinach mylonite zone overprints the Alpine nappe edifice. Movements occurred on the cooling path of the Tauern metamorphism, and may be as recent as Middle Miocene.

The Kinematics and geometry of the mylonite zone constrain two likely t ectonic explanations that are both compatible with secondary thining of a thick orogenic wedge. (1) Ute the Austroalpine nappe pile due to tectonic unroofing of the Tauern window. (2) Continental escape by east-west stretching of the Alpine orogenic wedge in response to continental collision.