GEOMORPHIC EVIDENCE FOR DIFFERENTIAL ROCK UPLIFT ACROSS THE SOUTHERN CASCADIA FOREARC

Deconvolving the accumulation of permanent strain in the overriding plate of subduction zones from elastic strain associated with the seismic cycle remains a first order challenge along convergent margins. In central and southern Cascadia, geodetic observations suggest that interseismic uplift rates increase toward the south, reaching several mm/yr inboard of the Gorda plate. Although much of this signal appears to be associated with the proximity of the trench to the coastline, a concomitant increase in topographic relief and erosion rate across the boundary between the central Oregon Coast Ranges and the Klamath mountains suggests that ongoing rock uplift may contribute to this signal. Here we test this hypothesis using a combination of stream profile analysis, mapping and observations of fluvial terraces, and existing determinations of erosion rate from the Rogue River watershed. Our results 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 (up to 1.5 km) from an eastern portion characterized by lower relief and gentler channels. The boundary between these two domains is a sharp, linear mountain front that trends NNE and is approximately coincident with a bedrock fault zone within the Western Klamath Terrane. East of this structure, fluvial networks are characterized by, gentle alluviated valleys, whereas western channels are decorated with flights of strath terraces and perched gravels attesting to recent incision. Longitudinal profiles of the trunk streams of the Rogue and Illinois Rivers exhibit a broad steepening across this transition that is manifest as a convex reach. These differences in landscape morphology, channel profile steepness, and geomorphic character of channels suggest that the western Klamath mountains are actively uplifting relative to the eastern forearc. Differential rock uplift appears to be accommodated by reactivation of bedrock faults within the Western Klamath terrane. Current work aims to test this hypothesis using fluvial terraces, but preliminary results suggest that permanent strain associated with active deformation within the southern Cascadia forearc may contribute geodetically measured uplift rates.