LATE HOLOCENE DEFORMATION RATES WITHIN THE ALVORD BASIN, NORTWESTERN GREAT BASIN, SOUTHWESTERN OREGON

Vertical uplift over the last 12,000 years is recorded by fault scarps cutting late Pleistocene and Quaternary wave cut terraces within the Alvord Desert of southeast Oregon. The Alvord Desert lies in a north-northeast trending fault bound basin bordered to the west by Steens Mountain, which forms the footwall to an east-facing normal fault system that accommodates at least 3.0 km of stratigraphic throw. The basin lies between the Owyhee and Modoc Plateaus to the east and west, respectively and is the northern continuation of the locus of active extension in the western Great Basin. The Alvord fault system cuts mid-Miocene basalts and Upper Miocene siliceous volcanic and volcanogenic sedimentary rocks and initiated deformation after 7 to 4 Ma. Late Pleistocene and Quaternary alluvial deposits within the basin are incised by wave-cut terraces formed in at least three highstands of paleo-Lake Alvord. The physiographic expression of individual terraces varies with age and topographic morphology of the basin margin and exhibit elevations that vary around the perimeter of the basin. Fault scarps crosscut highstand terraces on at least three active structures and exhibit 27 m of cumulative vertical displacement. The terraces ostensibly correlate with 12,000 to 10,000 year old Lahontan wave-cut terraces exposed approximately 15 km south in the Black Rock Desert of northwestern Nevada and yield an integrated vertical displacement rate of between 2.3 to 2.7 mm/yr. This vertical displacement rate was accommodated by faults dipping between 45° to 60° and corresponds to an integrated horizontal motion of 1.3 to 2.7 mm/yr over the last 12,000 years. Farther south in the western Great Basin, differential horizontal velocities measured geodetically across the Central Nevada Seismic Belt are between 4 to 6 mm/yr. Although differing by a factor of 2 to 3, the geodetic and geologic displacement rates in the western Great Basin and on three faults of the Alvord basin are within the same order of magnitude. At the time-scale of 12,000 years, horizontal displacement rates in the Alvord basin and western Great Basin are essentially steady-state but differ in seismogenic strain release. The western Great Basin is seismically active, but the Alvord basin has experienced only two instrumentally recorded earthquakes.