EVALUATION OF THE SLIP HISTORY AND HOLOCENE ACTIVITY OF FAULTS IN THE STRAWBERRY MOUNTAINS, GRANT COUNTY, OREGON

Lidar topographic data for the Strawberry Mountains, OR, has revealed youthful fault scarps that likely result from Holocene-age earthquakes. Scarps have been mapped striking roughly east-west in the north foothills of the Strawberry Mountains (Strawberry Mountains fault, SMF), and along another lineament striking south along the uppermost John Day River 10 km south. SMF scarps are aligned with the bedrock John Day Fault, a reactivated structure that is accommodating distributed extension north of the Basin and Range region. The Strawberry Mountains sit near the nexus of the Basin and Range province to the south and the Blue Mountains province to the north. This project evaluates the timing and size of earthquake-driven displacements along these faults, as well as how they relate to current tectonic stresses. Understanding the history of these fault structures will provide insight to both local and regional tectonic strains. Quaternary slip rates on the fault system will be constrained by cosmogenic radionuclide (CRN) dating. During the Last Glacial Maximum, the Strawberry Mountains contained numerous alpine glaciers. These alpine glaciers deposited large lateral and recessional moraines which are offset 1–3 meters by the SMF. The dominance of mafic Strawberry Volcanics in the study area favors ³He dating of pyroxene within large andesite and basaltic andesite boulders located on the crests of these moraines. Constraining the absolute age of these moraines will provide a long-term slip rate history extending into Pleistocene time, and the first absolute ages of these glacial features. We seek to constrain the age of the most recent earthquake with ¹⁴C dating of organics within scarp-derived colluvium and alluvium ponded against an uphill facing scarp. Data collection and processing is ongoing and will provide insight into the Pleistocene and Holocene behavior of the SMF, as well as important first-steps to better understanding seismic hazard in Eastern Oregon.