EVALUATING THE CRUSTAL CONVEYOR: AN ANALYSIS OF TERRACES AND CHANNEL PROFILES ALONG THE SOUTH FORK EEL RIVER, NORTHERN CALIFORNIA

The South Fork of the Eel River flows north-northwest, constrained by the structural grain of the accreted Franciscan complex and the modern, right-lateral transpression of the San Andreas Fault system. Vertical crustal motions are dominated by a wave of uplift that is hypothesized to follow in the wake of the northward migrating Mendocino triple junction. This model, termed the Mendocino Crustal Conveyor (MCC), provides significant insight into the Neogene and Quaternary geologic and geomorphic evolution of the northern Coast Ranges. In this paper, we evaluate whether the topographic implications of MCC tectonic model are reflected in (a) the present longitudinal profiles of the S. F. Eel River and its tributaries, (b) the distribution of knickpoints within the drainage network and (c) fluvial terraces above the mainstem channel.

Analysis of ~500 longitudinal stream profiles within the S. F. Eel River reveals a landscape in disequilibrium with present base level. Tributary profiles, extracted from a 10 meter USGS DEM, contain numerous, well defined knickpoints that define the boundary between the incised lower catchment and relict upper basins. A large knickpoint on the mainstem isolates the upper quarter of the basin as an elevated, relict landscape. In this region, cosmogenic nuclide-derived denudation rates measured in detrital quartz are a factor of two lower than long-term fluvial incision rates in the same reach.

Tributary and trunk knickpoint elevations align well with the maximum elevations of mainstem fluvial terraces observed in the 10 m DEM. Fluvial terrace height above the mainstem varies systematically over ~160 km. In the lower reaches, the highest terraces are ~50 meters above the mainstem. Heights increase steadily upstream, reaching a maximum of 125 m just below the mainstem knickpoint. Upstream of this, terrace heights rapidly decrease. The pattern of terrace heights increasing to the south could be attributed to a relatively higher surface uplift rate in the southern portion of the basin or simply an artifact of poorer preservation of high terrace levels in the lower portion of the basin. Detrital sediment samples collected along the mainstem within tributaries have been analyzed using cosmogenic exposure dating to explore differing erosion rates in the relict and incising portions of the basin.