Opening the window slightly earlier: Oligocene east-directed extension along the East Tauern Detachment System

In the NE Tauern Window of the Eastern Alps, new mapping in Nordrahmen Zone and Glockner nappes reveals significant deformation associated with W-E extension and a component of N-S shortening during the transition from high-pressure metamorphism and nappe stacking to extensional deformation. Kinematic indicators, including winged inclusions, tiling, and climbing pinch-and-swell veins, reveal a clear top-to-E shear sense, and deformation is further highlighted by progressively deformed quartz-calcite-dolomite veins, whose rotation was used to quantify flow parameters. The deformed veins used to quantify the flow parameters related to ductile deformation reveal that pure shear and simple shear contributed relatively equally. Moreover, the sub-horizontal axial planes of DIII fold structures is indicative of vertical flattening, which is signifies vertical shortening during ductile deformation. The ductile deformation is overprinted by E-dipping shear bands and faults, which transition into brittle-ductile faults compatible with incremental strain axes also indicating vertical shortening during top-to-E extension. Raman spectroscopy data show a temperature gradient with higher structural levels exhibiting paleotemperatures <450°C, increasing to >500°C at deeper levels. White mica Ar-Ar analyses in both shear veins and recrystallized fabrics yield Oligocene deformation dates (25-34 Ma). Distributed ductile thinning is a characteristic feature in the footwall of detachment systems, and prompted further investigation up section. At higher crustal levels at the upper limit of the Nordrahmen Zone, the deformation gradient progressively increases towards the newly discovered top-to-E Schuhflicker Detachment, defined by a knife-sharp fault surface of ultramylonites and cataclasites. The hanging wall is defined by slightly deformed quartzites and dolomites of the Lower Austroalpine Unit. The Schuhflicker Detachment developed at mid-crustal levels during the Oligocene, and during progressive exhumation, W-E extension was transferred to the structurally higher Katschberg Fault and Katschberg Shear Zone System during the Miocene. Collectively, these structures form the East Tauern Detachment System, which accommodated tens of kilometers of exhumation of the Tauern Window, facilitating the incipient stages of its exhumation during the Oligocene and subsequent erosion-dominated unroofing in the Miocene.