2015 GSA Annual Meeting in Baltimore, Maryland, USA (1-4 November 2015)

Paper No. 46-26
Presentation Time: 9:00 AM-5:30 PM


RAHL, Jeffrey M.1, MCGREW, Allen J.2 and WOLFE, Franklin1, (1)Department of Geology, Washington and Lee University, Lexington, VA 24450, (2)Department of Geology, The University of Dayton, 300 College Park, Dayton, OH 45469-2364, rahlj@wlu.edu

Detailed SEM-EBSD investigation of quartzite mylonite from Secret Creek Gorge in the Ruby-East Humboldt extensional shear zone reveals a complex kinematic history recorded by domainal quartz crystallographic preferred orientations (CPOs). Prior work indicates that mylonitic WNW-directed shear occurred in equilibrium with meteoric fluids during cooling from ~630o to 450 oC at shear stresses of 19-32 MPa and strain rates of 10-10 to 10-13 s-1. The analysis includes over 2.4 million observations mapped to a ~6.5 mm x 11 mm area. A kinked single girdle quartz c-axis distribution shows two strong maxima oriented approximately ±25o from Y along the shear plane normal. A modal analysis identifies at least 10 distinctly oriented subpopulations; at least 6 modes share a strong <a> axis maximum inclined ~25oclockwise from the shear direction whereas just two modes show <a> axes parallel to the shear direction as expected for simple shear. Several modes form paired Dauphiné twins, suggesting little distinction between positive and negative forms.

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.