Late Cenozoic High-Angle Transtensional and Low-Angle Detachment Faults in the Eastern Mina Deflection, West-Central Nevada

Abstract

The region in west-central Nevada underlain by the Mina deflection, a belt of curved east-northeast and north-northwest high-angle faults linking the Eastern California Shear Zone and southern Walker Lane to the central Walker Lane, records late Cenozoic extension on a low-angle detachment fault system and superposed high-angle faults. The northern Silver Peak and Monte Cristo Ranges, the southern 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 120 km long and 50 km wide, that is characterized by well-developed scarps in alluvium and bedrock. The faults have displacements that reflect transtensional deformation that was initiated between 3 to 5 Ma. These pervasive structures are superposed onto a previously unrecognized, regionally extensive detachment fault system, the Monte Cristo detachment, 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 29-24 Ma, passing upward into rhyolite domes and flows and 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) and basalt flows (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. Displacement on the high-angle faults and detachment system has resulted in the localization and exposure of precious metal deposits in the region.