Kinematic restoration of the western Tauern Window

The Tauern Window (TW) in the European Alps is one of the largest tectonic windows in the world. Its formation started in the Cretaceous with subduction of the Penninic realm beneath the northern margin of Adria leading to the collision between Europe (Subpenninic) and the Adria margin (Austroalpine). The resulting Penninic and Subpenninic nappe stack was exhumed by ca. 20 km by the approach of the Dolomites Indenter (Eastern Southern Alps) in the Miocene. This last deformation stage resulted in synkinematic N-S shortening of the western TW (ca. 70 km), W-E extension and lateral extrusion towards the east. However, how the Subpenninic core (Venediger Duplex; VD) and the Penninic and Austroalpine nappes (PN and AN, respectively) in the hanging-wall were tectonically stacked, upright folded and emplaced is poorly understood. This study investigates the deformation accommodated by each major tectonic basement unit of the western TW, and contributes to a better understanding of orogenic processes in general.

We kinematically restore the cross-section of [1] along the Brenner Base Tunnel (W of the TRANSALP seismic profile) using the software MOVE^tm (Petroleum Experts), focusing firstly on the VD. We choose area balancing as minimum criteria, because we do not know how much material was transported out of the plane of cross-section by extension and lateral extrusion. We integrate zircon fission-track data (ZFT) as a temporal constraint and test different geothermal gradients. Petrological data are used to define the maximum depth the VD reached at the time of indentation and as marker for the transition from brittle to viscous conditions of the felsic rocks of the VD (lowest temperature for folding). Finally, we reconstruct the hanging-wall nappes above the restored VD, thereby precisely constraining the position of the AN at that time. The surface samples taken from the AN must have reached thermal conditions between the annealing zones of apatite fission-tracks and ZFT (115°C and 180°C, respectively) as only the former system was reset in the Miocene.

We first displace the entire VD down along the Sub-Tauern Ramp below the 300°C isotherm (brittle to viscous transition of felsic rocks). For this, the geothermal gradient of 50°C/km fits well to the petrological data. ZFT ages reveal upright folding of the VD terminated at ca. 17 +/- 2 Ma. Subsequent unfolding of the gneiss cores, while conserving surface area, reveals the model to be extended ca. 70 km to the south (i.e. thus equaling indenter shortening), which means that no material left the plane of cross-section by W-E extension or lateral extrusion. However, the situation for the hanging-wall nappes is different: The total thickness of the northern limbs of the AN and the PN together is twice as much after restoration compared to today. We postulate that the extension on the Brenner Normal Fault mainly caused this tectonic thinning, which is approximately 10 km.   

References

[1] Reiter, F., Freudenthaler, C., Hausmann, H., Ortner, H., Lenhardt, W., & Brandner, R. (2018). Tectonics, 37(12), 4625-4654.