NEW CONSTRAINTS ON QUATERNARY SLIP RATES OF THE WASSUK RANGE FAULT SYSTEM, WESTERN NEVADA

The Wassuk Range fault system is among the most active in the Basin and Range province. Late Quaternary slip across the east-dipping, high-angle range-bounding normal fault of the range has produced fault scarps that cut alluvium at many locations along the eastern range front. At the mouth of Penrod Canyon, in the central Wassuk Range, a large, abandoned alluvial fan displays a well-defined escarpment striking approximately N30°E at a right-step in the NNW-striking range-front fault system. This escarpment displays eroded wave-cut benches created by late Pleistocene lake-level highstands, and well-preserved Holocene fault scarps subparallel and proximal to the base of the escarpment suggest that this segment of the range-bounding fault system remains active today.

The abandoned upper alluvial surface displays 40 meters throw relative to the active alluvial surface on the hanging wall of the normal fault. This value is assumed to be the minimum throw across the fault since the alluvial surface was truncated, because past and present alluvial deposition on the hanging wall have buried the correlative upper alluvial surface exposed on the footwall. Cosmogenic nuclide ¹⁰Be concentrations from two large boulders on the abandoned alluvial fan surface yield surface exposure ages of 91 +/- 12 ka and 106 +/- 15 ka. These ages give the maximum age for the latest deposition on the upper surface of the alluvial fan. Based on the measured fault throw and these exposure ages, the time-averaged slip rate along this fault segment is approximately 0.4 mm/year. This Late Pleistocene-Holocene value is the same as an estimate based on previous paleoseismic work for the last 5000 years of fault motion and is slightly lower than the estimated time-averaged slip rate since the Late Pliocene of 0.5 – 0.75 mm/year. These data imply a relatively constant rate of extensional deformation along the range-bounding fault system of the central Wassuk Range since the Late Pliocene.