Determining vorticity axes from grain-scale dispersion of crystallographic orientations

Kinematic shear sense indicators are critical tools for tectonic interpretation of shear zones. The appropriate plane in which to interpret shear sense indicators is the vorticity normal surface, which is often inferred using fabric (foliation, lineation) orientation. Strain modeling, however, suggests that such fabrics can be unreliable for determining a kinematic framework. We demonstrate the application of a new quantitative method, crystallographic vorticity axis (CVA) analysis, that utilizes rotation statistics to calculate dispersion axes from crystallographic orientations at the grain scale. We apply CVA analysis to samples from three shear zones that exhibit distinct kinematics and deformation geometries. In all cases, the aggregate of calculated grain-scale dispersion axes yields a preferred axis of crystallographic vorticity at the specimen scale that is coincident with the independently determined bulk vorticity axis. This new method allows the position of vorticity axes to be evaluated independently of foliation and lineation information, and without assumptions about the kinematics of deformation.