CRYSTALLOGRAPHIC FABRICS OF QUARTZ MYLONITES FROM THE RUBY-EAST HUMBOLT CORE COMPLEX, NEVADA

The Ruby Mountains-East Humboldt Range-Wood Hills metamorphic core complex in northeastern Nevada offers an unusual opportunity to trace the evolution and development of a 1 km thick crustal-scale extensional mylonitic shear zone. An east-to-west transect from the Wood Hills through Clover Hill to the East Humboldt Range exposes progressively deeper structural levels of the mylonitic lower-plate of the Oligocene to earliest Miocene Mary’s River fault system.

Quartz deformation mechanisms have been inferred from the crystallographic-preferred orientations (CPOs) determined by electron backscatter diffraction (EBSD) for eight quartz-rich samples from the Mary’s River fault system. All of the samples have strong quartz crystallographic preferred orientations consistent with previously published reports. C-axis maxima range from 5-6 times uniform distribution in granitoid samples to 19-24 times uniform distribution in mylonitic quartzite. Though somewhat weaker, quartz CPOs from the mylonitic orthogneisses are consistent with those from nearby quartzites; they also serve to bracket the age of deformation between the early Oligocene age of the granitoids and earliest Miocene 40Ar/39Ar biotite cooling ages. The nature of the quartz CPOs changes systematically down the dip of the Mary’s River fault system. The structurally highest Wood Hills mylonites have quartz CPOs with c-axes distributed about an asymmetric girdle. This CPO is interpreted to be the result of quartz deformation with a significant contribution by basal <a> slip. Mylonites from structurally lower Clover Hill outcrops have a quartz CPO with quartz c-axes forming an asymmetric girdle centered on Y, a fabric interpreted to have formed by a combination of rhomb <a> and prism <a> slip. Mylonites from the structurally deepest outcrops from the East Humboldt Range have quartz CPOs characterized by c-axis maxima parallel to Y, suggesting deformation dominated by prism <a> slip. This transition from basal <a> to rhomb <a> to prism <a> slip results from deformation at progressively higher temperatures towards the structurally deeper parts of the Mary’s River fault system.