DISASSEMBLING CALIFORNIA: DOES LITHOLOGY-DEPENDENT BOULDER DELIVERY TO RIVERS INFLUENCE LANDSCAPE RESPONSE TO TECTONICS?

The response of landscapes to tectonic perturbations governs Earth’s sediment mass balance, rates of geochemical cycling, and the spatial and temporal distribution of natural hazards. While bedrock lithology is known to influence landscape response in a general sense, the specific mechanisms by which lithology governs transient landscape adjustment are not well understood. Among other effects, rock type sets the size of sediment delivered to river channels, controlling the efficiency with which rivers respond to tectonic forcing. Here we investigate the Mendocino triple junction region of northern California, where large boulders delivered to rivers by landsliding have been hypothesized to influence the response to recent rock uplift of channels in some rock units but not others.

We analyze patterns of channel steepness, an indicator of erosion resistance, and ask how steepness varies as a function of lithology and mapped landslide proximity. We then map boulder size distributions at landslide toes and apply a numerical model for erosion in boulder-influenced channels to test whether lithologically controlled boulder delivery is a probable cause of steepness variations. We find that channels in two units, a low-grade sedimentary mélange and a schist, are approximately twice as steep as nearby sandstone channels. The mélange is thought to be mechanically weak, suggesting that its greater steepness requires an explanation beyond in situ rock strength. Boulder mapping using satellite imagery reveals that the mélange contains substantially more and larger boulders than the other units, and modeling suggests that observed boulder sizes and concentrations could account for the approximately two-fold greater steepness in mélange relative to sandstone channels. These results suggest a lithologic control over large-scale landscape evolution through rock-type-dependent boulder delivery. Additional constraints on in situ rock erodibility and the spatial variability of erosion rates are required to constrain the influence of the boulder delivery mechanism with greater certainty.