GEOMETRY OF SUPERPOSED LOW-ANGLE DETACHMENT AND HIGH-ANGLE FAULTS IN THE WESTERN SILVER PEAK RANGE AND NORTHERN FISH LAKE VALLEY, NEVADA

Structures within the northern Eastern California shear zone (ECSZ) and central Walker Lane (CWL) are misaligned and have been kinematically linked since the mid-Miocene by a series of structural stepovers. Northwest directed extension within the stepover resulted in the formation of the Silver Peak - Lone Mountain (SPLM) extensional complex which transferred displacement via a shallowly northwest-dipping detachment. In the upper plate of the detachment, synextensional basins bounded by listric-normal faults accumulated deposits up to 2.5 km thick. The detachment was active from ~12 to 4 Ma, when displacement ceased and the kinematic mechanism of transfer changed from the detachment system to a curvilinear, high-angle fault system that cross-cut the detachment and upper-plate basins. To disentangle faults related to upper-plate basin formation and younger cross-cutting high-angle faults, we conducted a detailed gravity transect, spanning 26 km from exposures of the SPLM detachment in the Silver Peak Range, northwest across high-angle faults in the northern Fish Lake Valley. We collected 170 gravity measurements along the profile using a Scintrex CG-5 Autograv gravimeter with positioning of 2.5 cm or better provided by two Leica Viva duel frequency GNSS receivers. Measurements were taken with a nominal spacing of 300 meters except over known high-angle faults, where the spacing was reduced to 50 meters for a distance of about 2 km, centered on the fault trace. The gravity data were terrain corrected and reduced to a datum density of 2.67 g/cm³ to produce a complete Bouguer anomaly. Following removal of the regional field in calculation of a residual complete Bouguer anomaly, the data were modeled in 2.75 D using Oasis Montaj GMSYS. Testing alternative geologic cross sections, we employed a 7 layer model, with densities ranging from 2.2 to 2.8 g/cm³. The modeling results indicate that seven down-to-the-west extensional faults mapped in the Silver Peak Range and northern Fish Lake Valley cross-cut both upper- and lower- plate rocks of the SPLM. These faults are related to the current episode of transtensional deformation which initiated ~4 Ma, offset the detachment in a stair-step fashion with a cumulative vertical offset of 1.9 km, and help to transfer displacement from the northern ECSZ into the CWL.