PRELIMINARY 1:48,000-SCALE GEOLOGIC MAP OF THE SPOTTED RANGE AND ADJACENT AREAS, SOUTH-CENTRAL NEVADA, USA

Despite considerable progress mapping the geology of the United States over the past century, numerous areas of pronounced interest for tectonic and climate history, resource development, engineering, ecosystem protection, and hazards reduction remain poorly mapped. One such location is the Spotted Range and Ranger Mountains of south-central Nevada, a large area where detailed geologic mapping has long been impossible due to access restrictions. Although 1:100,000 and 1:250,000 scale regional mapping exists over this area, stratigraphic units are highly generalized, particularly for Devonian, Mississippian, and Cenozoic rocks, and structural and stratigraphic boundaries are notional in some areas. We present a preliminary 1:48,000-scale geologic map of the Spotted Range and surrounding areas based on recently released high-resolution aerial imagery. Our mapping of middle Cenozoic sedimentary and volcanic rocks was supported by preliminary field characterization and forthcoming U-Pb zircon and ⁴⁰Ar/³⁹Ar geochronology in the northernmost Spotted Range and southernmost Fallout Hills. This work is part of the U.S. Geological Survey Intermountain West Project, which is developing a seamless digital geologic map of a large portion of the southwestern USA. The new mapping improves prior work with more detailed differentiation of upper Paleozoic lithologies based on regionally accepted lithostratigraphic units rather than generalized chronostratigraphic units, as well as more precise and accurate linework for stratigraphic contacts and structural features. These improvements yield considerably better delineation of normal and strike-slip faults, which will assist with restoring Cenozoic transtensional deformation patterns and identifying potential seismic hazards. Our mapping also provides better control on the surface expression and subsurface structure of the Spotted Range thrust fault system in the Ranger Mountains and southern Spotted Range. Finally, the improved structural mapping may assist with characterizing the Rock Valley fault system as well as the hydrogeologic properties of the Las Vegas basin and the greater Death Valley regional groundwater flow system.