DETAILED SEM-EBSD ANALYSIS OF A QUARTZITE MYLONITE FROM THE RUBY-EAST HUMBOLDT EXTENSIONAL SHEAR ZONE, ELKO COUNTY, NEVADA
For each mode, Schmid factors have been calculated to assess the potential activity of known quartz slip systems. Although the critical resolved shear stress to activate each system is unknown, the analysis indicates that most grains are best-oriented for slip on either r<a> or z<a>. Other systems well-oriented for slip vary between modes, but most commonly include prism<a+c>, prism<a>, or π’<a>. The divergence of the dominant crystallographic slip direction (the a-axis) from the bulk shear plane together with the potential activation of antithetic slip systems suggests a departure from end-member simple shear into the subsimple shear regime.
CPO mapping reveals significant spatial partitioning into ~1.0-1.5 mm thick domains parallel to foliation. The modes with shear-parallel a-axes are clustered together and have c-axes in the northern hemisphere; in contrast, ribbon grains are mostly restricted to domains associated with southern c-axis maxima and show the dominant a-axis alignment ~25o clockwise to the shear direction. These spatial asymmetries suggest a triclinic aspect to the deformation, posing a challenge to strain compatibility. Future work will focus on modeling and comparing crystal-plastic strain in the contrasting domains to better resolve the strain path.