2-D kinematic restoration of the Western Tauern Window, Eastern Alps using thermochronological and P-T constraints

Abstract

The Penninic and Subpenninic nappe stack of the Tauern Window (TW) in the European Alps was formed by collision between Europe (Subpenninic) and the Adria margin (Austroalpine), and finally exhumed by the northward push of the Southalpine Dolomites indenter in the Miocene. In this study, we kinematically restore a cross-section along the trace of the Brenner Base Tunnel, concentrating mainly on the Subpenninic nappes (Venediger duplex; VD). We integrate zircon fission-track data (ZFT) as a temporal constraint for the termination of viscous deformation and test different geothermal gradients (GG). P-T-t data are used to define (a) the depth of the brittle-viscous transition (ca. 300°C) and (b) pre-indenter depth. We displace the VD down along the Sub-Tauern ramp below the 300°C isotherm. At that time, a GG of ca. 50°C/km prevailed. ZFT data reveal that viscous conditions allowing folding of the VD started to cease slightly earlier than 17 ± 0.6 Ma. Unfolding of the VD, while conserving surface area, yield that the model is extended by ca. 70 km (thus equaling indenter shortening), which means that in the westernmost TW, the VD was not significantly affected by W-E extension. Reconstruction of the hanging-wall nappes (Austroalpine and Penninic nappes) above the restored VD reveals that the total pre-indenter thickness of their northern limbs was 25%–48% greater than today. We interpret this as tectonic thinning, which was mainly caused by the Brenner normal fault.