Landslides, threshold slopes, and the survival of relict terrain in the wake of the Mendocino Triple Junction

Abstract

Establishing landscape response to uplift is critical for interpreting sediment fluxes, hazard potential, and topographic evolution. We assess how landslides shape terrain in response to a wave of uplift traversing the northern California Coast Ranges (United States) in the wake of the Mendocino Triple Junction. We extracted knickpoints, landslide erosion rates, and topographic metrics across the region modified by Mendocino Triple Junction migration. Landslide erosion rates mapped from aerial imagery are consistent with modeled uplift and exhumation, while hillslope gradient is invariant across the region, suggesting that landslides accommodate uplift, as predicted by the threshold slope model. Landslides are concentrated along steepened channel reaches downstream of knickpoints generated by base-level fall at channel outlets, and limit slope angles and relief. We find evidence that landslide-derived coarse sediment delivery may suppress catchment-wide channel incision and landscape denudation over the time required for the uplift wave to traverse the region. We conclude that a landslide cover effect may provide a mechanism for the survival of relict terrain and orogenic relief in the northern Californian Coast Ranges and elsewhere over millennial time scales.