THE SLIP HISTORY AND HOLOCENE ACTIVITY OF THE STRAWBERRY FAULT IN OREGON'S BLUE MOUNTAINS

Lidar topographic data for the Strawberry Mountains, Oregon has revealed youthful fault scarps that likely result from Holocene-age earthquakes. We have mapped scarps striking roughly east-west across the northern foothills of the Strawberry Mountains (Strawberry Fault, SBF), a subrange of the Blue Mountains, and along another lineament striking south along the uppermost John Day River 10 km to the south. SBF scarps generally align with the bedrock John Day Fault, a reactivated Miocene structure that is accommodating distributed extension north of the Basin and Range region. The Strawberry Mountains sit near the interface of the extensional Basin and Range province to the south and the tectonically stable Blue Mountains province to the north. We constrained Quaternary slip rates on the SBF with ³He cosmogenic radionuclide (CRN) dating of glacial features. During past glacial periods, the Strawberry Mountains contained numerous alpine glaciers which deposited prominent lateral and recessional moraines. The SBF offsets these moraines between 1 m and 19 m. Analysis of CRN data shows two glacial features to be LGM-aged with a peak at ~21.6 ka, and one aged ~113.8 ka. When combined with measured offsets, the slip rate of the SBF is 0.1 – 0.2 mm/y of north-south extension in the Quaternary.

An earthquake history site on the SBF, located in a basin impounded by an uphill-facing scarp above a glacial valley, records evidence for two surface-rupturing events since the LGM and vertical separation of at least 3.3 meters. ¹⁴C dates of detrital charcoal from onlapping stratigraphy and surface-rupture correlative colluvium have bracketed the most recent earthquake to CE 200–1754 as of this writing. Data from this study will fill large gaps in knowledge of active tectonics and seismic hazard in Eastern Oregon and gives insight to the Pleistocene and Holocene behavior of the recently discovered SBF in addition to defining a glacial chronology stretching back ~110,000 years.