LATE CENOZOIC SUPERPOSITION OF HIGH-ANGLE FAULTS AND LOW-ANGLE DETACHMENT FAULTS IN THE EASTERN MINA DEFLECTION, WEST-CENTRAL NEVADA

The region underlain by the Mina deflection, a belt of curved east-northeast and north-northwest high-angle faults linking the central Walker Lane and Eastern California Shear Zone, in west-central Nevada records late Cenozoic extension on low-angle detachment faults and superposed high-angle faults. The northern Silver Peak and Monte Cristo Ranges, the Cedar Mountains, and Royston Hills of the eastern Mina deflection expose a belt of east-northeast left-oblique faults that curve around deep prismatic basins emerging as north-northwest right-oblique faults. The array of curved faults underlies an east-northeast-trending region 70 km long and 50 km wide, that is characterized by well-developed scarps in alluvium and bedrock with surface displacements consistent with earthquake focal mechanisms. The high-angle structures reflect transtensional deformation along a N65°W axis that was initiated at about 4 Ma. These pervasive structures are superposed onto a previously unrecognized, regionally extensive detachment fault system separating Cenozoic volcanic and sedimentary rocks of the upper plate from underlying Paleozoic-Mesozoic strata and Mesozoic plutons of the lower plate. The Cenozoic rocks constitute two lithologic successions separated by an angular unconformity. The lower sequence is composed of ash-flow tuff, ranging in age from 26-24 Ma, passing upward into rhyolite domes and flows, overlain by andesite lava, lahar, and tuff and minor sedimentary rocks, ranging in age from 22 to 17 Ma. The upper Cenozoic sequence is composed of 15 Ma andesite lava, volcaniclastic sedimentary rocks (13 to 11 Ma), and rhyolite (~7 Ma) overlain by basalt flows and breccia (7 Ma). Across the region, rocks of the lower Cenozoic sequence exhibit shallow to moderate dips into the basal detachment with all units of the sequence found in direct structural contact with the underlying Paleozoic-Mesozoic basement. The shallowly dipping detachment is characterized by a zone of cataclasite up to one hundred meters thick and is warped in broad northwest-trending folds. The upper Cenozoic section, albeit locally folded and offset by younger high-angle faults, seals the detachment, which was active between 17 and 15 Ma.