ACTIVE SHORTENING AND DIFFERENTIAL UPLIFT ACROSS THE SOUTHERN CASCADIA FOREARC: GEOMORPHIC EVIDENCE FOR PLEISTOCENE DISPLACEMENT ALONG THE ILLINOIS RIVER VALLEY FAULT

The degree to which high topography within the southern Cascadia forearc reflects ongoing mountain building along the North American plate boundary remains uncertain. Interseismic uplift rates reach several mm/yr inboard of the Gorda plate (south of ~43°), and this region is characterized by elevated topography in the western Siskiyou and Klamath mountains. Geomorphic analyses reveal systematic spatial patterns in channel steepness (a measure of channel gradient normalized for contributing basin area) that delineate a western block characterized by high steepness and high local relief from the eastern forearc characterized by lower relief and gentler channels. The boundary between these two domains is a sharp, linear mountain front west of the towns of Cave Junction and Selma, OR that is coincident with a NNE-striking system of steeply west-dipping reverse faults.

As Doug Yule recognized, this fault system, the Illinois River Valley fault (Yule, 1996), separates gentle, alluviated valleys to the east from steep, incised canyons to the west. Here, we employ a combination of stream profile analysis, optically stimulated luminescence (OSL) dating of fluvial terrace deposits, and measurements of watershed-average erosion rates to test Yule’s hypothesis that this fault system accommodates differential uplift across the forearc. Analysis of river profiles reveals significant knickzones along both the Illinois and Rogue rivers localized along the physiographic transition that are not readily explained by models of transient incision. OSL dating of strath terrace deposits west of the fault system yield ages ranging from 20-30 ka to >300 ka; incision rates range from ~0.3 - 1 mm/yr. In contrast, OSL ages from fluvial deposits east of the range front suggest almost no incision into bedrock over the past ~100 ka. We argue that differences in relief, channel steepness, and channel incision require ongoing differential uplift across the forearc at rates of 300-1000m/Myr. These results confirm Yule’s intuition that Quaternary faults within the western Klamath terranes may be responsible for much of the elevated topography. Moreover, results suggest that permanent strain across the southern Cascadia forearc can explain as much as 10-30% of interseismic uplift in the region.