FENCEMAKER THRUST AND LOW-ANGLE-NORMAL FAULTS, NORTHWESTERN HUMBOLDT RANGE, NEVADA

New mapping and structural studies along the western front of the Humboldt Range, between Humboldt and Rochester Canyons, suggest that the Jurassic Fencemaker thrust and significant footwall imbricate structures are present and have been reactivated as low-angle normal faults. The anticlinal range exposes volcanic rocks of the Triassic Koipato Group in its core, which are overlain and flanked by Triassic strata of the Star Peak Group (Prida and Congress Canyon Formations; Silberling and Wallace, 1969; Nichols and Silberling, 1977). North of Wright Canyon, slices of ductilely deformed marble tectonite (formerly Natchez Pass Fm) rest discordantly upon sub-greenschist facies strata of the Prida Fm, along a low-angle normal fault originally mapped as the Humboldt City thrust (as noted by Heck (1986)). The marble tectonite is structurally overlain by low-grade siliciclastic strata of the Grass Valley Fm, which is generally regarded as part of the Fencemaker allochthon. Within the core of the range, ductile shear zones imbricate felsic volcanic rocks of the Koipato Gp. We interpret these relations as follows: the Koipato Gp,, Prida Fm, and Congress Canyon Fm (along the range summit) form the footwall of the east-vergent Fencemaker thrust, which emplaced Grass Valley Fm and Auld Lang Syne Gp in adjacent ranges. The marble tectonite is a deep footwall or hangingwall slice that was thrust over the footwall rocks. The antiformal geometry of the footwall, and ductile shear zones in the Koipato reflect deep-level, parautochthonous footwall duplexes. The base of the Grass Valley Fm and the Humboldt City and Standard faults are parts of the Fencemaker thrust zone that were reactivated as top-to-the-west low-angle-normal faults during late Cenozoic extension. These faults subsequently were truncated by high-angle-normal faults along the western margin of the range. The geometry of thrusts, low-angle-normal faults, and high-angle-normal faults has important implications for reconstructing Mesozoic and Paleozoic geology in the region and for both geothermal prospects and epithermal gold mines along the range margin.