Commonly, tectonic windows have very complex structures, exemplified by the Tauern Window (TW) in the European Eastern Alps. Its complex folded nappes were formed during several deformation phases since the early Cenozoic. The final phase (Miocene) was driven by the indentation of the Dolomites indenter (DI), which heavily shaped the structure of the TW, especially its western end. Here, exhumation took place along the west-directed, viscous Brenner normal fault (BNF). However, the structure and geometry of the western TW is not completely deciphered, although it provides insights into its Miocene deformation history. Our aim is to unveil its structure by constructing a 3-D model using the software MOVETM. We compiled and analyzed published geological maps, cross-sections and structural data, and modelled three tectonic horizons and the viscous BNF by applying inter- and extrapolating methods. Our analysis shows that the antiforms of the western TW deformed differently. The almost cylindrical shape of the Tuxer antiform deviates from the non-cylindrical Zillertaler antiform. This variation is linked to the proximity and asymmetry of the DI, suggesting initially uniform structures were deformed by the indenter and the associated reorganization of the lower crust. Comparing fold axes orientation with the dip of the viscous BNF, we found that the latter’s influence zone ranges between 3 and 4 km in its footwall. The Schöberspitzen antiform likely developed pre- or synkinematic to the viscous BNF, as its fold axis orientation correlates with the BNF’s dip.

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 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 (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 (i) the depth of brittle-viscous transition (ca. 300°C) and (ii) pre-indenter depth. First, we displace the VD down along the Sub-Tauern ramp below the 300°C isotherm. To that time, a GG of 50°C/km prevailed. ZFT ages reveal that viscous folding of the VD terminated at ca. 17 ± 2 Ma. Unfolding of the VD, while conserving surface area, yield that the model is extended by ca. 70 km (i.e. thus equaling indenter shortening), which means that the VD was not affected by W-E extension or lateral extrusion. 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 35–50% more than it is today. We interpret this as tectonic thinning, which was mainly caused by the Brenner normal fault. Our model shows that tectonic processes can explain the exhumation of the western TW; erosional denudation seems to play only a minor part.