GENERAL SHEAR STRAIN IN THE RUBY MOUNTAINS-EAST HUMBOLDT RANGE METAMORPHIC CORE COMPLEX

The Ruby Mountains-East Humboldt Range metamorphic core complex, northeast Nevada, records top-west normal-sense exhumation of strongly deformed Proterozoic-Paleozoic passive margin strata and older basement. To characterize the nature, geometry, and kinematics of extension and exhumation, particularly the mylonitized and ductilely attenuated lower plate, we conducted 1:24k-scale mapping of the southwestern East Humboldt Range with integrated structural, geochemical, geochronological, carbon isotope, and microstructural analyses. Bedrock stratigraphy is pervasively intruded by Eocene and Oligocene intrusions (>2/3 volume). A NE-oriented transect across the core complex reveals a relatively intact, but severely attenuated (<20% original stratigraphic thickness) section from the basal Neoproterozoic McCoy Creek Group to Ordovician Eureka Quartzite. New δ¹³C analyses confirmed Paleozoic (~ negative) versus Neoproterozoic (+6-10%) marbles in the map area. Microstructural and EBSD analyses demonstrate that mylonitized quartzites from this traverse deformed primarily via pure-shear with lesser simple shear, and we present a coupled general shear model. Based on piezometry and wet quartzite flow laws, we estimate 10^-13-10^-12 s^-1 strain rates, which suggests overall rapid shearing (~1 Myr). These shear estimates suggest ~15-20 km top-west displacement due to general shear. Direct cross-cutting relationships include mylonitization of ca. 29 Ma granite and cross-cutting 17 Ma subvertical undeformed basalt dikes. Therefore, this system could have deformed either following voluminous Oligocene intrusions or in the earliest stages (pre-17 Ma) of mid-Miocene-to-present extension. Although non-unique, a model of mylonitization following Oligocene intrusions is supported by the (1) local volume of deformed Oligocene intrusions, (2) compatibility with interpretations from other core complexes in Utah-Nevada, (3) published fission-track data, and (4) poorly dated leucogranite that cross-cut mylonitization, and we are unaware of Miocene leucogranites in the region. This work supports the emerging view that metamorphic core complexes may deform due to local thermal anomalies, accommodating ductile stretching of the section without major low-angle normal-sense displacement.