GRAVITY MODEL CONSTRAINTS ON THE GEOMETRY AND DISPLACEMENT HISTORY OF THE SOUTHERN FISH LAKE VALLEY BASIN, SOUTHWESTERN NEVADA

Right-oblique slip on the NNW-striking Fish Lake Valley fault zone (FLVF) played a major role in the displacement transfer system linking the Eastern California Shear Zone and central Walker Lane over the last 10-12 Ma. Since about 5 Ma, displacement on the FLVF opened southern Fish Lake Valley, which is 8-10 km wide, narrows to closure to the south, and widens to the north as the northern Fish Lake and Little Fish Lake Valleys. The FLVF marks the range front separating southern Fish Lake Valley and the White Mountains to the west and consists of two sub-parallel strands separated by about 1.0-1.5 km with the eastern fault forming scarps up to 100 m high in Quaternary deposits and the western strand marking the bedrock-alluvial contact. To the west, the FLVF is linked to the ENE-striking Deep Springs fault system in the southern White Mountains via a curved array of structures, but the relation between the FLVF and major left-oblique, WNW-striking faults in the Silver Peak and Sylvania Mountains to the east is less well-understood. To determine the subsurface geometry of southern Fish Lake Valley, two Scintrex GC-5 gravimeters were used to collect 672 gravity measurements taken at a spacing of 300 m along 9 transects crossing the valley. Geospatial referencing with a 2.0 cm resolution was accomplished by using dual frequency Leica GNSS receivers. A residual complete Bouguer anomaly was calculated using a reduction density of 2.67 g/cm³ and was inverted for depth in 3D using GM-SYS modeling software. Gravity gradients and depth-to-basement models indicate that southern Fish Lake Valley is segmented into three sub-basins with depths of 1.5 to 1.8 km. The sub-basin morphology changes along strike, with a steep western boundary in the south, a steep eastern boundary in the center, and a steep western margin in the north. The basin segments are bounded by extensions of the WNW-striking fault systems observed in the ranges to the east and coincide with left-steps in the FLVF. Vertical offsets on observed and inferred faults in Fish Lake Valley restored to a pre-extensional datum for the northern, central, and southern basin segments are 5.3 km, 4.4 km, and 4.5 km, respectively. Opening of the southern Fish Lake Valley records interaction of the FLVF and the WNW left-oblique faults that reduced slip on the FLVF by transferring displacement to the east.