RECENT MAPPING IN THE SOUTHERN STILLWATER RANGE AND NORTHERN SAND SPRINGS RANGE, NEVADA

Southern Stillwater Range and northern Sand Springs Range exposures in the western Great Basin constrain Mesozoic lithotectonic assemblage boundaries and contain the Cenozoic Basin and Range – Walker Lane boundary. The map area contains Luning-Fencemaker thrust belt (LFTB) folds and thrusts and a unique Mesozoic stratigraphy characteristic of the Sand Springs assemblage and different from the nearby exposures of the Luning assemblage at West Gate, in the Clan Alpine Mtns (Henry et al., 2013), and from the Lovelock assemblage at La Plata, in the S Stillwater Range (John and Silberling, 1994).

This study produced a 1:8000-scale geologic map of the S Stillwater Range and adjoining N Sand Springs Range. Complementary techniques applied include: a) measuring orientations of original bedding, compaction foliations, aligned mineral foliations in granitoid and basalt, metamorphic foliations (S1), fault surfaces, and slickenlines, b) calculating contact orientations from their relations to topography, c) constructing cross-sections, and d) inverting fault data to interpret principal stress directions. Mesozoic map units include metamorphic tectonites of siliciclastic, carbonate, volcanogenic, and intrusive protoliths intruded by Cretaceous granitoids. The overlying Cenozoic succession consists of basaltic lava once interpreted as Jurassic and interbedded ash-flow tuffs, andesite flows, tuffaceous sandstone and felsic lava, a major unconformity, volcanogenic sandstone, and Bunejug Formation vesicular basalt cross-cut by four intrusive units.

Mapping led to several discoveries. First, mapping better documented Sand Springs assemblage protoliths, structures, and timings. Second, granitoid foliations bend to parallel the curving granitoid-tectonite contact while adjacent S1 in tectonites remain uniform, implying that granitoid foliations are primary and that granitoids cross-cut D1 structures. Third, the basal Cenozoic basalt-tuff succession, similar to the La Plata section, is gently dipping despite containing a steeply dipping compaction foliation. Fourth, the E-W gold-silver-bearing Summit King fault records N-S and NW-SE extension. Results imply that La Plata tectonites also are part of the Sand Springs assemblage and that a LFTB thrust separates the overlying Lovelock assemblage.