3D GRAVITY DEPTH INVERSION AND DISPLACEMENT ESTIMATES FOR THE LIDA VALLEY PULL-APART BASIN, SOUTHWESTERN NEVADA

Displacement transfer from the northern Furnace Creek – Fish Lake Valley fault system in eastern California and southwest Nevada to the central Walker Lane is accomplished by WNW-striking fault systems that together with NNE-striking faults bound several equant-shaped extensional basins in a large structural stepover underlying much of the southern Walker Lane in Nevada. The WNW Sylvania Mountain transfer fault system stretches east from the Furnace Creek – Fish Lake Valley fault for nearly 60 km where strands of the structure bound the Lida Valley basin. Lida Valley is underlain by a rectangular pull-apart basin that has an east-west dimension of 23 km and a north-south extent of 19 km. The basin is bounded by WNW and NNE segments of the curved Mount Dunfee fault system that forms the southern and eastern margins of the basin, respectively. The western boundary is bounded by the NNE-striking ,east-dipping Magruder Mountain fault that merges to the north with the WNW-striking Jackson Mountain fault. Displacement on the bounding faults occurred between about 7 to 3 Ma with WNW structures dominated by sinistral strike-slip and by normal displacement on NNE structures. The internal geometry of the Lida Valley basin was determined by a gravity survey of 550 measurements made at 300 m spacing using two Scintrex CG-5 gravimeters along 6 transects crossing the basin. Geospatial control for the gravity survey with a resolution of 2.0 cm was provided by dual-frequency Leica GNSS receivers. A residual complete Bouguer anomaly using a 2.67 g/cm³ reduction density was inverted in 3D using GM-SYS 3D modeling software for basin-fill densities of 2.2 and 2.4 g/cm³ to yield greatest basin depths of~625m. Lida Valley is underlain at least 10 east- and west-dipping NNE extensional faults the form two sub-basins, of 625 and 610 m depth, separated by a NNE trending high with a depth of 565 m. Two-dimensional and three-dimensional geologic models were used to forward calculate gravity and together with restoration of the basement contact to a pre-extensional datum provide a cumulative total vertical displacement of the faults bounding and within the Lida basin of ~4.8 km. Using fault dips of 60° to 30° horizontal displacements estimates for the pull-apart basin range between 2.7 and 8.7 km, respectively.