TECTONIC ROLE OF ACTIVE FAULTING IN CENTRAL OREGON

Geologic and geodetic studies in California indicate that about 1 cm yr-1 of right-lateral shear occurs across what has been referred to as the Eastern California Shear Zone. Northwest trending zones of dextral, sinistral, and normal faults splay eastward from the San Andreas system, continuing through the Mojave Desert, east of the Sierra Nevada, and northward along the Central Nevada and Walker Lane fault zones. Aerial photography, field investigations, and fault studies in southern and central Oregon, compiled with a comprehensive analysis of previous studies nearby, indicate that latest Pleistocene and Holocene fault activity is concentrated along four zones that stretch northward into the Cascade volcanic arc and across the northwestern edge of the Basin and Range Province. The Oregon zones appear to continue the activity in eastern California and northwestern Nevada northward and provide a connection to seismically active zones in southern and central Washington. Several techniques are applied to fault data from the Oregon zones in an attempt to estimate the overall direction and rate of motion across them. The orientations and styles of faults younger than middle Tertiary are used with models of oblique rifting to estimate that the motion of western Oregon is approximately N60-degrees +/- 20-degrees-W, relative to North America. Summation of geologic moment tensors from faults with latest Pleistocene and Holocene slip yields a direction approximately N90-degrees +/- 30-degrees-W at a rate of approximately 0.5 mm yr-1. This result is a minimum since many fault scarps have not been preserved or recognized, and additional deformation is recorded as folding and tilting. Crustal strain associated with slip during 76 of the largest crustal earthquakes in the past 120 years located along this broad zone from northern California and Nevada, across Oregon, to Washington and Vancouver Island, indicates motions at rates of 3 +/- 1 mm yr-1 in a direction N55-degrees +/- 10-degrees-W. Although the motion across central Oregon is much slower, its similarity in style with regions to the north and south suggests that the regional averages are meaningful. Oregon fault zones, taken together, may accommodate as much as 6 mm yr-1 oriented approximately N60-degrees to 70-degrees-W. A tectonic model of fault activity reveals that this proposed shear zone through Nevada, Oregon, and Washington can account for 10% to 20% of the total Pacific-North American transform motion and much of the lateral component of relative motion between the Juan de Fuca and North American plates.