FROM MICROKINEMATICS TO MACRO-INTERPRETATION: VARIATION IN KINEMATIC VORTICITY AND STRAIN LOCALIZATION IN THE RUBY-EAST HUMBOLDT SHEAR ZONE, NORTHEASTERN NEVADA

Here we analyze variations in microstructure and quartz CPOs from the kilometer-scale Ruby Mountains – East Humboldt Range (R-EH) extensional shear zone as a function of strain path and deformational intensity. The R-EH shear zone forms the mylonitic carapace of the R-EH metamorphic core complex and consists of a zone of protomylonitic to mylonitic gneiss extending ~150 km along strike with a maximum thickness >500 m. Thermobarometric and chronometric constraints indicate that mylonitization took place under amphibolite to upper amphibolite conditions during the early stages of late Oligocene to Miocene extensional unroofing. Estimated PT conditions up to 600^oC, 0.35 GPa suggest that mylonitization occurred under hot crustal conditions with a steep geothermal gradient perhaps exceeding 40^oC/km. A suite of quartzite samples collected across the shear zone and parallel to the inferred transport direction reveal dramatically increasing grain sizes with structural depth as the rocks transition from the subgrain rotation field to the rapid grain boundary migration field of dynamic recrystallization. A variety of kinematic indicators such as asymmetric porphyroclast systems and composite fabrics indicate systematic WNW-directed, normal-sense noncoaxial strain concentrated in the upper half of the shear zone. Quartz CPOs measured with SEM-EBSD typically show strong WNW-asymmetric single girdle patterns characterized by strong c-axis maxima at Y or double maxima symmetrical about Y in the YZ-plane; the dominant a-axis maximum most commonly plunges ~20-30^o relative to X in the shear direction. We introduce an approach to strain path estimation based on calculating the cumulative Schmid factor for the most likely active slip systems in all grains as a function of kinematic vorticity (W_k). The preferred W_k value is taken as the value that maximizes the cumulative Schmid factor. Applying this methodology, we estimate that W_k decreases from ~1 (near-simple shear) in the uppermost shear zone to ~0.35 in the lower part of the shear zone, suggesting that the shear zone functions as a “stretching shear zone” that separates the brittle upper crust from a more homogeneously thinning deeper crust. Both the intensity of strain and the contribution from simple shear increase upward toward the shear zone’s upper boundary.