Annualmeeting:2017 CSDMS meeting-014

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Numerical simulations of transient landscape adjustment along the Mendocino Triple Junction

Nathan Lyons, Tulane University New Orleans Louisiana, United States.
Nicole Gasparini, Tulane University New Orleans Louisiana, United States.

[[Image:|300px|right|link=File:]]The South Fork Eel River (SFER) in the northern California Coast Ranges exhibits characteristics indicative of transient landscape adjustment: stream terraces, knickpoints, and more slowly eroding headwater terrain. A tectonically-induced uplift wave is commonly invoked as the driver of transience in this region. The wave is attributed to the northward migration of the Mendocino Triple Junction (MTJ) where the San Andreas fault, Cascadia subduction zone, and Mendocino fracture zone meet. Nested basin-mean erosion rates calculated from 10Be detrital quartz sand increase downstream along the SFER that roughly coincides with the direction of MTJ migration. This erosion trend is attributed to the proportion of adjusted and unadjusted landscape portions upstream of the locations where the nested 10BE samples were collected. Yet to be determined are the conditions that led to transient erosion. Adjusted and unadjusted landscape portions are separated by a broad knickzone that contains 28% of topographic relief along the mainstem. Knickzone propagation and considerable stream incision is suggested by projection of the upper SFER above the knickzone through the highest flight of strath terraces. These terraces are approximately 80 m above the modern valley floor near the outlet of the SFER. Here we evaluate the pattern of transient landscape characteristics predicted by multiple uplift scenarios using the Landlab modeling framework and constraints provided by previous work in this region. Notably, model outcome when uplift is simulated as a wave is incompatible with the tectonic history of the region and field observations, and the gradient of uplift along modeled streams has an important control on knickpoint generation.