Erosional response to northward-propagating crustal thickening in the coastal ranges of the U.S. Pacific Northwest

Abstract

We measured basin-scale erosion rates, using cosmogenic ¹⁰Be concentrations in quartz, from fluvial sediment in rivers draining the coastal mountain ranges of the U.S. Pacific Northwest between 40° and 47° N. Apparent erosion rates are 0.1 to 0.2 mm yr⁻¹ throughout the Oregon Coast Ranges north of 43° N, and increase to the south to 0.6 to 1.1 mm yr⁻¹ in the northern California coast ranges near 40° N. We propose that these observations display the erosional response to northward-migrating crustal thickening associated with subduction of the Mendocino Triple Junction. North-south variations in erosion rate, range elevation, and metrics of landscape relief and steepness are consistent with the hypotheses that i) their primary cause is northward-migrating crustal thickening; ii) erosion rates are strongly controlled by topographic relief and weakly, if at all, controlled by climate; and iii) the dependence of erosion on relief is nonlinear and obeys a threshold-relief relationship.