DOES TOPOGRAPHY ALONG THE CASCADIA FOREARC REFLECT PERMANENT DEFORMATION OF NORTH AMERICA?

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 indicate that interseismic uplift rates within the forearc increase toward the south, reaching several mm/yr inboard of the Gorda plate – in a region where plate interface coupling alone would be expected to produce interseismic subsidence. Although spatial changes in interseismic uplift along the forearc are commonly attributed to the transition from locked to unlocked conditions on the plate interface, a concomitant increase in both topographic relief and erosion rate across the boundary between the central Oregon Coast Ranges and the Klamath mountains suggests that ongoing rock uplift is also a component of this signal. Here we test this hypothesis using a combination of stream profile analysis, fluvial terraces, and previously published measurements watershed-average erosion rates. 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 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, coincident with mapped high-angle faults 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. Dating of fluvial sands with optically-stimulated luminescence atop two terrace treads along the Rogue River suggest incision rates in the past 20-30 ka approaching ~1 mm/yr. These differences in landscape morphology, channel profile steepness, geomorphic character of channels, and incision rates suggest that the western Klamath mountains are actively uplifting relative to the eastern forearc at rates of 300-1000m/Myr. Our results suggest that permanent strain across the southern Cascadia forearc may contribute to elevated interseismic uplift rates in the region.