Quaternary stratigraphy and late Holocene faulting along the base of the eastern escarpment of Steens Mountain, southeastern Oregon

Two late Wisconsin pluvial high lake stands and at least one late Holocene faulting event have been recognized within the Alvord Valley along the base of the eastern range front of Steens Mountain in southeastern Oregon. The timing of the pluviations was estimated by correlation with the established chronology of Lake Lahontan about 80 km to the south. The older lake reached an elevation of 1310 m from approximately 24,000 to 16,000 yr B.P. A spillover and subsequent channel incision occurred during this lake stand at Big Sand Gap in the Tule Springs Rim on the eastern side of the valley. The lake level was lowered to 1280 m. The valley was occupied once again by a lake during a maximum pluviation from 16,000 to 12,000 yr B.P. Large-scale geomorphic evidence of recent tectonic activity along Steens Mountain includes: 1) a steep, rugged range front escarpment accompanied by numerous landslides and knickpoints, 2) numerous geothermal springs, and 3) recently formed fault scarps. A complex zone of high angle faults, the Steens Fault Zone (SFZ), is located along the base of the Central Steens. Numerous youthful appearing fault scarps are present within the SFZ. A segment of the SFZ, the Alvord fault, is expressed as a 19 km-long, north-northeast trending sharp-crested scarp. The scarp is the result of a single movement along the fault, probably as recently as late Holocene time. Evidence for the recent activity of the Alvord fault includes displacement of the 1280 m Lake Alvord shoreline, a youthful scarp in the lake playa that has been subjected to little wave modification, and a basal graben formed during faulting that is preserved in an older alluvial fan deposit. In addition, an analysis of the degraded fault scarp provides further evidence that the most recent faulting event occurred within the last few thousand years. The SFZ is located within the transition between the Basin and Range province and the Columbia River Plateau province. Faults within the SFZ may be related to a north-northeast trending zone of Holocene tectonic activity along the northwest margin of the Black Rock-Carson Sink zone of extension in Nevada.