Kinematics of the Plattengneis Shear Zone, Koralpe, Eastern Alps. Constraints from Quartz Lattice Preferred Orientations

Large-scale models for the tectonometamorphic evolution of the Koralpe (Eastern Alps) must account for the kinematics of deformation in the regional, intracrustal transport horizon known as the Plattengneis shear zone. Published tectonic models have invoked a range of interpretations that imply different distributions for strain geometry and shear sense, both laterally from north to south and vertically across the flat-lying shear zone. To acquire data suitable for differentiating among such models, this study presents quartz lattice preferred orientations, measured using electron backscatter diffraction, from oriented samples collected specifically to provide systematic vertical and lateral coverage of the Plattengneis shear zone. The vertical position within the shear zone is determined using a 3D model of the shear zone geometry that was made for this study in refinement of earlier work. The results yield information on activated slip systems, deformation temperatures and shear sense as a function of the vertical and lateral position within the shear zone. No systematic correlations were found between the vertical position of samples within the shear zone and their inferred sense of shear, or strain geometry. Deformation temperatures, however, display lateral distributions consistent with the regional metamorphic temperature field-gradient defined by prior petrological studies, implying that deformation of the shear zone took place predominantly at or near peak metamorphic temperatures. The observations, measurements and inferences from this study indicate that deformation of the Plattengneis occurred within the range of ~500 °C to 700 °C in an environment that has largely, but not exclusively, non-coaxial top-to-the-north shear sense. This result is consistent with recently invoked tectonic models for the shear zone.