Structural study of a shear zone in the Koralm Complex at Hirschegg-Pack (Austroalpine Nappes, Austria)

The Koralm Complex in eastern Austria comprises various gneisses and eclogite lenses which record high-pressure metamorphism of Late Cretaceous age, acquired in an intracontinental subduction zone. The mechanism of exhumation of the Koralm Complex is either slab extraction or wedge extrusion. The present study aims at adding structural evidence to this problem. We studied an WSW-ESE-striking shear zone with SSE-dipping foliation and subhorizontal stretching lineation in the central part of the Koralm Complex. Based on quartz crystallographic preferred orientations (CPO) in gneiss, Krohe (1987) demonstrated sinistral shearing in this shear zone. We investigate microstructure and CPO of samples from the same shear zone using electron backscatter diffraction and photometry. Microstructures indicate grain-boundary migration, subgrain rotation, and static annealing. We find sinistral as well as dextral shear sense. This may either reflect flattening in the shear zone or two phases of deformation with contrasting kinematics. These two possibilities are discussed based on the textural data.

References

Krohe, A., 1987. Kinematics of Cretaceous nappe tectonics in the Austroalpine basement of the Koralpe region (eastern Austria). Tectonophysics 136, 171–196. https://doi.org/10.1016/0040-1951(87)90024-2.