Gravity Analysis to Determine Geometry of Superposed Low-Angle Detachment and High-Angle Faults in the Western Silver Peak Range and Northern Fish Lake Valley, Nevada




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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 dual 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 2D using Oasis Montaj GMSYS. Testing alternative geologic cross sections, we employed a 7 layer model, with densities ranging from 1.8 to 2.8 g/cm³. The modeling results indicate that eight 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 2.2 km, and currently help to transfer displacement from the northern ECSZ into the CWL.



Geology, Structural, Gravity, Faults (Geology), Fish Lake (Nev.)