Detachment versus strike-slip ductile shearing in the Nordrahmen Zone of the Tauern Window

The Tauern Window in the Eastern Alps (Austria) is one of the most prominent tectonic windows, which exposes Subpenninic and Penninic nappes derived from the European margin and Alpine Tethys respectively below the Austroalpine Unit derived from the Adriatic continent. Along the northeastern margin of the window, in the so-called Nordrahmen Zone (NRZ), subvertical W-E striking marble mylonites, graphitic schists and phyllonites with a subhorizontal stretching lineation record intense ductile shear deformation. Previous studies suggested that these structures record the ductile history of a major sinistral strike-slip fault (i.e. the Salzach-Ennstal-Mariazell-Puchberg Fault System), which accommodated the Miocene lateral extrusion of the central parts of the Eastern Alps towards the Pannonian Basin.

In this work, we investigated a N-S section along the Grossarl valley, which demonstrates that the subvertical mylonitic rocks are deformed into upright folds with wavelengths and amplitudes on the order of several hundreds of meters and fold axes that are parallel to the mylonitic W-E trending stretching lineation. Reversal of the apparent strike-slip shear sense in the fold limbs suggests that the mylonites have been folded after shear deformation and that mylonites record top-E shearing when unfolded. Ductile subvertical flattening is recorded by a second fold generation with similar W-E trending fold axis but subhorizontal axial planes forming Type 3 refold structures. Ductile top-E shearing is documented by low-angle E-dipping ductile shear zones, shear bands, SC and SCC’ fabrics and brittle ductile conjugate N-S striking high-angle faults. Shear deformation intensifies towards higher structural levels localizing in ultramylonites and cellular dolomite cataclasites below almost undeformed klippen of quartzites and dolomites (Mt. Schuhflicker and Mt. Saukarkopf), which belong to the Lower Austroalpine Unit. Using Raman Spectroscopy of Carbonaceous Materials, we constrain the temperature of mylonitization between 350°C and 400°C. Comparison with published Ar/Ar ages from the Nordrahmen Zone suggests that mylonitization operated around 30 Ma.

We therefore suggest that the mylonites along the northeastern margin of the Tauern Window are not part of a strike-slip fault system. They actually belong to a major top-E detachment system, which records an early stage of the exhumation of the Tauern Window before deformation localized along the Miocene Katschberg Normal Fault at the eastern margin of the Tauern Window.