DOES STRESS VARY IN RHEOLOGICALLY HETEROGENEOUS SHEAR ZONES? INSIGHT FROM THE MYLONITES OF SECRET PASS, RUBY-EAST HUMBOLDT METAMORPHIC CORE COMPLEX, NEVADA

Lithologic heterogeneity can lead to important rheological contrasts in plastically deforming metasedimentary rocks. Theory shows pressure (mean stress) variations can develop within mechanically weak shear zones bounded by more competent units. A natural example of this phenomenon comes from the rheologically heterogeneous lithology of the Basin and Range metamorphic core complexes, where ductile shear zones involve mechanically weak marble bounded by competent quartzite. Ductile deformation during extensional exhumation of the Ruby-East Humboldt Metamorphic Core Complex was localized in Neoproterozoic-lower Paleozoic quartzite, calc-silicate and marble strata. The mylonite zone is well exposed in the northern Ruby Mountains at Secret Pass, where Prospect Mountains Quartzite is in low-angle fault contact with lower Paleozoic strata of the Horse Creek assemblage. We expand upon previous studies that explored the microstructural evolution of this shear zone by investigating how paleo-stresses during dynamic recrystallization of quartz varies in quartzite versus calc-silicate formations. EBSD analysis of quartz and calcite shows mean recrystallized grain size is highest in the Prospect Mountain quartz mylonites, and decreases within the calc-silicate and pure marbles of the Horse Creek assemblage. Paleopiezometry of these phases indicate a corresponding increase in differential stress during dynamic recrystallization from quartzite at the boundary to quartzite within the marble shear zone. In summary, rheologically weak marbles experienced a higher differential stress than adjacent, more competent quartzites during dynamic recrystallization. Furthermore, quartzite inclusions within the weak marble experience a yet higher stress than the marble host and the bounding quartzite. We propose this phenomenon may apply more broadly to mechanically heterogeneous tectonic units within the Ruby-East Humboldt and Snake Range core complexes, such as pelites and marbles versus quartzites, which implies pressure estimates from these diverse assemblages may not represent assumed lithostatic values. Further investigation on this topic will help reconcile the long-standing incompatibility between field-based structural restorations and petrology-based estimates of paleo-depth.