A NEW LATE PLEISTOCENE-HOLOCENE SLIP RATE ESTIMATE FOR THE WASSUK RANGE NORMAL FAULT, WESTERN NEVADA: THE IMPACT OF TIME-DEPENDENT SLIP VARIABILITY ON SEISMIC HAZARD ASSESSMENT

The 100-km long, east-dipping, NNW-striking Wassuk Range normal fault zone, within the central Walker Lane of western Nevada, is among the most active and well-studied in the Basin and Range province. Previous studies of the fault zone have yielded time-averaged vertical uplift rates for post-Pliocene (0.50 – 0.75mm/yr) and Holocene time (0.7±0.1mm/yr), but a significant gap between these estimates prevents a rigorous analysis of time-dependent slip rates. We present new cosmonuclide exposure ages, landform analysis, shallow seismic refraction data, and gravity data that provide a well-constrained time-averaged rate of slip since Late Pleistocene time.

At the mouth of Penrod Canyon, in the central Wassuk Range, a large, abandoned and alluvium-covered bedrock pediment is truncated by the normal fault system that flanks the east side of the range and displays a fault-related vertical offset of 48 m. The abandoned pediment is the only such exposure along the Wassuk fault system with good exposures of the underlying Mesozoic intrusive units; the alluvium-bedrock contact provides excellent velocity contrast for shallow seismic investigation of the fault system’s buried hanging wall. Cosmonuclide ages from 5 boulders on the inactive, alluvium-covered pediment yield ages of 111.9 ± 5.4 ka, 89.6 ± 4.6 ka, 73.3 ± 2.6 ka, 69.8 ± 3.6 ka, and 57.2 ± 2.8 ka. We interpret the oldest age, which corresponds to the largest boulder, as the best estimate for the minimum exposure age for the alluvial surface. We also completed a 200-m long seismic refraction survey that extended from the base of the primary fault scarp toward the valley. The best-fit tomographic models of these data converged to similar results, suggesting a granitic bedrock surface dipping from about 50-m depth near the escarpment to more than 55-m depth further to the southeast. These data support a vertical throw across the fault system of approximately 98 m, a value that is supported by data from a gravity survey across the escarpment at the same location. Combined, these data suggest a vertical slip rate of 0.9mm/yr, which indicates little time-dependent slip variability along the Wassuk fault zone since the Pliocene. This finding contrasts with other major normal fault systems in the Basin and Range and perhaps simplifies the assessment of seismic hazard for the Wassuk Range.