DETERMINING SLIP RATES FOR THE WINTER RIM FAULT SYSTEM IN THE SUMMER LAKE BASIN, OREGON

The Summer Lake basin in the northwestern Basin and Range is bounded by the active 59-km-long Winter Rim Fault (WRF) system, which includes Holocene fault scarps that cut and offset Quaternary units. The slip rate and seismic hazard for the WRF system is poorly constrained. The Winter Rim Fault System comprises four segments: Winter Ridge, Ana River, Slide Mountain, and the recently identified Summer Lake fault. The basin is also rimmed by several paleoshorelines that mark stillstands of Pleistocene Lake Chewaucan, a pluvial lake that reached its highstand after the Last Glacial Maximum. These paleoshorelines represent a paleohorizontal datum, and modern deviations from horizontal may be attributable to tectonic deformation.

In order to better constrain the slip rate along the WRF, we (1) mapped and analyzed fault scarps and paleoshorelines, and (2) dated paleoshoreline tufas and measured displacements. Topographic profiles across fault scarps along the length of the WRF were analyzed for surface offset. Seventeen carbonaceous shoreline tufa samples spanning elevations from 1,316 m to 1,406 m were collected in the field August 2015, and 21 samples were dated using radiocarbon geochronology (including some duplicates). Ages of the samples range from 12.85 ± 0.12 ka to 40.97 ± 0.48 ka.

A 14.7 ± 0.34 ka paleoshoreline has been offset by 6 m by the Ana River fault, resulting in a slip rate of 0.41 mm/yr, which compares well with rates derived from paleoseismic trenching. A ca. 13.5 ka paleoshoreline been offset by 65 meters across the Summer Lake fault, producing a slip rate of 0.21mm/yr. Offsets measured along fault scarps range from less than 1 m to 40 m, cutting units with ages ranging from Holocene to Oligocene, producing slip rates that fall within reasonable ranges. These new data allow us to refine the spatial and temporal distribution of slip rates within the Summer Lake Basin.