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This is a talk given on December 10th, 2019 at the American Geophysical Union annual meeting in the session: EP22A- Advances in Tectonic Geomorphology: The interplay of Tectonics, Climate, and Surface Processes II.The talk presents initial results from a study of landscape response to transient rock uplift at the Mendocino Triple Junction, with the goal of understanding how lithologically controlled boulder delivery might influence landscape evolution.Please note that this talk and abstract represented work in progress, and include approaches and interpretations that were ultimately abandoned. This work was published as Shobe et al (2020; Geological Society of America Bulletin), and that paper should be viewed as the "version of record" of the work.Abstract:Bedrock lithology influences the size of sediment delivered to rivers, and may thereby determine how efficiently rivers can respond to tectonic and climatic perturbations. The Mendocino triple junction (MTJ) region of northern California is one landscape in which large boulders, delivered by hillslope failures to channels, may significantly alter the pace and course of landscape evolution. One lithology, the Franciscan mélange, hosts channels mantled with earthflow-derived boulders, while other rock units do not. Boulders frequently delivered by earthflows in the mélange have been hypothesized to steepen channels and slow river response to tectonic deformation, while channels in other units may experience little boulder delivery and boulder-induced steepening. In this study, we ask 1) whether channel steepness correlates with lithology across the MTJ landscape as a whole, and 2) if so, whether lithologically controlled boulder delivery is a probable cause. We find that channels tend to be steeper in the mélange relative to non-melange units. Though channels in all units approximately double in steepness with increasing proximity to mapped hillslope failures, absolute steepness values are much higher (~2x) in the mélange compared to the non-mélange units. The power-law relationship between erosion rate and channel steepness observed in the study area, in which channel steepness increases less and less as erosion rates increase, indicates that unrealistically high erosion rates would be required to sustain the steepness of mélange channels adjacent to hillslope failures in the absence of some other mechanism of steepening. This suggests that the mélange channels are steepened disproportionately by hillslope failures relative to non-mélange channels. We map boulders at failure toes in the mélange and non-mélange units, and find that boulder sizes, frequency, and concentrations are greater in mélange channels. Using our data to parameterize a model for channel slope response to the presence of boulders, we find that boulder delivery may be responsible for channel steepening of between 16 and 490% in mélange vs. non-mélange channels. Our results support the hypothesis that earthflow-derived boulders may retard landscape adjustment to tectonics and help preserve relict topography at the MTJ and elsewhere. |
