EVALUATING SPATIAL VARIATIONS IN TOPOGRAPHY AND EROSION RATE ALONG THE CASCADIA FOREARC

Topography in the forearc of convergent margins records the long-term interplay between permanent deformation of the overriding plate and climatically modulated erosion and may provide a window into the geodynamics of plate boundary evolution. Along the Cascadia convergent margin, north-south variations in topographic relief and mean elevation of the Coast Ranges have been attributed to both variations in the rate of differential rock uplift and to differences in the erodibility of bedrock substrate. Here, we present 44 new basin averaged erosion rates derived from 10Be concentrations in modern sediment which provide a measure of erosion rates over the past ~10-100ka. In combination with previously published data (Balco et al., 2013) and quantitative analysis of topography and channel longitudinal profiles, we explore the significance of these variations in topography along the forearc. Our results show that average erosion rates range from ~60-120 m/Myr in the central Oregon Coast Ranges, whereas the Klamath and Siskyiou mountains of southern Oregon and northern California are characterized by erosion rates ranging from ~200-300 m/Myr. Erosion rates co-vary with channel steepness (a measure of channel gradient normalized for differences in drainage basin area), suggesting that high erosion rates in the southern forearc must be sustained by differential uplift of rock. Variations in rock mass quality appear to exert a subsidiary influence on channel steepness. The transition between these regions occurs at approximately 43°N and corresponds to abrupt transitions in the geodetic strain field, the age and character of the downgoing plate, and the architecture of accreted terranes in the overriding plate. We interpret the high topography south of this boundary to reflect a relatively recent increase in rock uplift driven by the northward migration of the Mendocino Triple Junction, although we cannot rule out shortening driven by coupling along the Gorda plate interface.