DOCUMENTATION OF QUATERNARY DEXTRAL ACTIVITY ON THE TUMALO FAULT, SISTERS FAULT ZONE, CENTRAL OREGON

The northwest-trending Sisters fault zone in Deschutes County, Oregon provides an ideal study area to document Quaternary faulting in the region and elucidate the influence of surrounding tectonic domains on crustal deformation. The >45-km-long Tumalo fault is the longest fault strand of the Sisters fault zone. Previous studies note that the sense of slip on the Tumalo fault is largely unknown. Recent studies identify normal and strike-slip components of slip on the fault, but dextral motion is poorly constrained. We document Quaternary dextral activity on the Tumalo fault using remote sensing and field observations which reveal the Tumalo fault has a larger component of dextral strike-slip motion than previously documented. Quaternary basalt flows, pyroclastic deposits, and glacial outwash deposits originating from the Three Sisters volcanoes and the northern flank of the Newberry volcanic center are cut by the Tumalo fault and dextrally offset by approximately 90 ± 24 m. Previously published age constraints of these offset units provide a rough dextral slip rate estimate of 0.1 – 0.5 mm/yr. Direct sampling of these units for Quaternary geochronology will provide a more accurate measure of strike-slip faulting and total slip rates on the Tumalo fault and the Sisters fault zone. Unrecognized significant dextral motion on the Tumalo fault may have implications for the influence of the tectonic domains that surround central Oregon on faulting and seismic hazards in the region.