Timing and west-directed kinematics in the footwall part of the Upper Austroalpine extruding wedge: Insights from the Lower Tauern region (Austria)

The timing of the Cretaceous Eoalpine event in the Upper Austroalpine Unit (Eastern Alps) is well established. An initial phase of collision and nappe stacking characterized by (W)NW-directed kinematics lasted from c. 135 to 95 Ma, followed by the exhumation of metamorphic units via extrusion between thrusts with W-directed kinematics and large-scale normal faults with (E)SE-directed kinematics from c. 95 to 65 Ma. Although large-scale normal faults along the Austroalpine Detachment System are thoroughly documented in the hanging wall of the Upper Austroalpine, deformation within its footwall remains poorly understood.

This study investigates the chronological framework and kinematic evolution of the footwall within the Austroalpine extrusion wedge in the Lower Tauern region, where out-of-sequence thrusting, ductile shear zones, and mylonites occur at nappe boundaries. The local geology comprises basement complexes and partial cover units of the Schladming-Seckau and Koralpe-Wölz nappe systems, which record overprinting metamorphism and deformation from Variscan, Permian, and Eoalpine events.

A major shear zone, the Walchen Shear Zone (WSZ), is situated at the boundary between the Wölz and Ennstal complexes within the Koralpe-Wölz Nappe System, affecting rocks of both the underlying Wölz and overlying Ennstal complexes. The WSZ can be traced at the surface for ~800 m perpendicular to strike, spanning ~450 m into the Wölz and 350 m into the Ennstal complex. The mylonitic schistosity dips moderately to steeply to the N(NW), whereas the mylonitic stretching lineation is predominantly subhorizontal or shallowly dipping to the W(NW) or E(SE). Strain intensity increases within the Wölz Complex toward its hangingwall, and deformation is most intense within the footwall of the Ennstal Complex. Mylonitic deformation is particularly well preserved within light-colored, mica-rich quartzites of the Ennstal Complex near the Walchen copper deposit. Microstructural analysis reveals dynamic recrystallization of quartz and calcite, with porphyroclasts of garnet, feldspar, and ankerite. This indicates that the shear zone was active under greenschist-facies conditions between 300 and 450 °C. Within this structural framework, kinematic indicators, such as clast geometries, quartz c-axis fabrics, C- and C'-type shear bands in phyllosilicate-rich rocks, consistently record a top-to-the-west sense of shear. This ductile to brittle-ductile shearing reflects a phase of orogen-parallel to orogen-oblique tectonic transport.

For Ar-Ar geochronology, six samples of deformed quartz veins containing coarse-grained white mica, alongside two samples of white mica from a mylonitic impure quartzite, were collected along the shear zone from both complexes. Single crystal total fusion dates ranging between 91 and 94 Ma are interpreted as formation or deformation ages, constraining the timing of this W-directed kinematic event to the Late Cretaceous. Ultimately, these insights from the WSZ refine existing geodynamic models of wedge extrusion, bridging a critical gap in our understanding of lateral mass transport during continental collision that is accommodated during footwall deformation, which decisively influenced the final architecture of the Central Austroalpine region.