Using EBSD crystalline vorticity axes to deduce complex kinematics during the Eo-Alpine deformation of the Plattengneis Shear Zone (Koralpe, SE Austria).

The Koralpe Complex of the Eastern Alps hosts a major crustal-scale shear zone within which the exhumation of Eo-Alpine eclogites and the formation of highly-strained, high-P-T (680-700°C, 12-13 kbar) Plattengneis mylonites was localised. Although no convincing kinematic indicators have been published and macro- and microscopic fabrics record an orthorhombic symmetry in the N-S section parallel to the prominent stretching lineation, a top-north shear sense has been inferred from published quartz EBSD analyses. In this contribution, we present a clear monoclinic fabric perpendicular to the stretching lineation revealing a top-west shear sense. Vorticity axes preserved in the crystal lattice of deformed quartz grains (constrained by EBSD data) are used as a quantitative solution for deciphering the strain history and to establish a newly informed constraint on the Eo-Alpine kinematics of the Koralpe.

Observations of macro- and micro-scale monoclinic fabrics (e.g. feldspar sigma-clasts and tourmaline and garnet delta-clasts) revealed an unequivocal and consistent top-west shear sense, localised around a north-south (N-S) striking vorticity axis (VA_Fsp), perpendicular to previous estimations of kinematics. To resolve the conflict between reported and observed shear sense, our investigation probed for crystalline vorticity axes preserved in quartz grains that experienced crystal plasticity and rotational distortion of the crystal lattice during deformation. The vorticity analysis of quartz EBSD data revealed a bulk crystalline vorticity axis (CVA_Q) striking east-west (E-W), inclined 60-70° to the west. The inclined orientation of CVA_Q is geometrically incompatible with the kinematic configuration of pure-shear dominated general shear necessary to produce the defining structural fabric of the Plattengneis (N-S stretching lineation, L_S; pervasive planar foliation, S₁). This incompatibility, along with the implication of an additional vorticity axis (VA_Fsp), indicates that CVA_Q resembles a compound vorticity axis re-orientated into an inclined position during a two-phase deformation history.

To resolve the two-phase transposition of vorticity axes, we modelled a theoretical solution: a horizontally inclined initial orientation of CVA_Q with subsequent rotation around VA_Fsp using mechanically compatible quartz slip-systems. The initial orientation of CVA_Q (E-W striking) is predicted to form during D₁ by dominant prism<a> slip under nearly plain strain pure-shear conditions. During D₂, CVA_Q is subsequently rotated c. 60-70° around the N-S striking VA_Fsp with a top-W shear sense. Based on the quartz EBSD dataset (CPO and misorientation axes of low angle boundaries (LAB)) we presume a dominance of prism<a> slip during the initial pure-shear deformation in D₁ (under upper amphibolite facies condition). In the second deformation, continuation of prism<a> slip is inhibited by sub-optimal orientation of quartz grains relative to the D₂ stress field; based on the heterogeneous distribution of LAB misorientation axes, we propose that the D₂ rotation of CVA_Q was accommodated by the interaction of multiple, non-dominant and geometrically-necessary slip-systems.

The crystal-scale kinematic analysis revealed a previously unknown poly-phase deformation during the formation of the Plattengneis shear zone with a top-west component in accord with the overall Eo-Alpine kinematics and demonstrated the vast potential of the crystalline vorticity axis analysis method for accurately resolving complex kinematics.