Abstract
Documentation of a latest Pleistocene/earliest Holocene episode of strath formation and fluvial aggradation in the Oregon Coast Range provides a datum from which long‐term bedrock stream incision rates are determined. Variations in long‐term incision rates probably reflect cumulative differential uplift in the forearc of the Cascadia subduction zone, although factors such as bedrock and climatic controls and isostatic adjustments to erosion obscure the precise relationship between surface uplift and stream incision. Patterns of differential incision are most striking near the latitude of Newport, where a steep gradient divides a region of higher rates (∼0.6–0.9 mm/yr) in the northern Coast Range from a region of lower rates (∼0.1–0.3 mm/yr) in the central Coast Range. The steep incision gradient is nearly coincident with abrupt changes in marine terrace (∼80–125 kyr) uplift rates, the locations of Quaternary faults, and the southern flank of a saddle of low historic (∼40–70 years) uplift. The exact causes of these variable patterns of incision/uplift are unknown. Analogies with uplift patterns in other subduction zones and comparisons with other neotectonic data in the region indicate that patterns of differential incision probably are caused by variations in permanent strain accumulation along the Cascadia subduction zone. Such variations may be related to differences in seismic moment release during individual earthquakes, to changes in plate geometry or rates of wedge accretion, to segmentation of earthquake ruptures, and/or to deformation on active structures in the North American plate and accretionary wedge.