PALEOSEISMIC ASSESSMENT OF SLIP RATE AND EARTHQUAKE HISTORY OF THE WARM SPRINGS VALLEY FAULT, WESTERN NEVADA: IMPLICATIONS FOR DISTRIBUTED FAULT RUPTURES

The northern Walker Lane, accommodates ~15% of the motion between the North American and Pacific plates along north-oriented normal faults and a series of northwest-oriented left-stepping en echelon strike slip faults, including the ~70-km-long Warm Springs Valley fault (WSVF). Here, we present results of a multi-faceted study of the WSVF combining paleoseismic trenching, Quaternary geologic and fault mapping, and a slip rate analysis of offset pluvial shorelines.

Mapping observations show the WSVF as 1) a single trace in the Warm Springs Valley, 2) multiple parallel stepping and anastomosing fault strands that form push up mounds, and sag ponds between the Dogskin and Virginia mountains, and 3) a range front fault along the Fort Sage Mountains that steps to the east and cuts pluvial lake shorelines in the Honey Lake basin. Fault scarps in young alluvial deposits are not continuous along strike signifying a distributed rupture pattern. Previous studies of the fault reveal a decreasing slip rate from 1.8-2.4 mm/yr in the late Pleistocene to 0.2 mm/yr in the Holocene. A trench was excavated across a 3-m-high downhill facing scarp on a young alluvial surface in order to constrain an earthquake history and recurrence for the fault. The exposure revealed sandy alluvium on the footwall with a weakly developed carbonate soil juxtaposed against red gravel and colluvial deposits on the hanging wall. Based on stratigraphic relations and soil development, we infer that at least two surface ruptures occurred along the WSVF since the latest Pleistocene. Assessment of event timing is pending OSL and radiocarbon analyses. In the Honey Lake basin, the fault cuts pluvial lake shorelines at 1231 meters in elevation which corresponds to the ~12,500 cal yr B.P. Younger Dryas highstand. This trace of the fault is sub-parallel to the rangefront trace along the Fort Sage Mountains and indicates potential slip partitioning between the two traces. Quantification of the offset is in progress using a digital elevation model generated by drone photogrammetry. The observed youthful offset suggests that the Holocene slip rate may be faster than previously reported and highlights the need for multifaceted approaches to adequately assess slip rates and associated seismic hazards for distributed fault systems.