LANDSLIDE RESPONSE TO ROCK UPLIFT ALONG THE MENDOCINO TRIPLE JUNCTION, NORTHERN CALIFORNIA

The northern Californian Coastal ranges present a unique opportunity to study landscape response to a wave of rapid uplift related to the northward migration of the Mendocino Triple Junction. Geodynamic modeling in the region posits a double-humped uplift field spanning the Coastal ranges with peaks in uplift rate currently located between the Eel and Van Duzen Rivers and between the Russian and Eel Rivers as supported by evidence of drainage capture. A major research question is whether we can detect a signature of the order of magnitude variation of uplift in landslide activity and document how the channel network communicates this signal to hillslopes.

Using air photos and Worldview imagery, we manually mapped more than 2000 earthflows and debris slides in the Eel and surrounding catchments that span the ~400 km-long region of varying uplift. The velocities of active earthflows were estimated by visually tracking features between images of different dates. We mapped channel steepness from 10m NED DEMs in Topotoolbox 2 and developed a new tool to automatically define knickpoints along the channel network.

Earthflows occur almost exclusively in a band of Franciscan mélange oriented along the MTJ transect whilst debris slides are more evenly distributed by lithology. Landslides, particularly earth flows, are clustered in the Eel catchment around the proposed uplift peaks and are largely absent outside of these zones. Within these areas of high landslide densities, we observe a peak in active earthflows adjacent to a peak in dormant earthflows to the south, suggesting that the signature of earthflow activity remains for a period of time once the uplift peak has passed. Landslide density, mean landslide area, and earthflow velocity all increase rapidly above threshold values of channel steepness and local relief. Near the northern portion of the transect where the zone of rapid uplift is commencing, landslides, particularly earth flows, are concentrated below an elevation band bracketed by abundant knickpoints that we suggest reflects a wave of incision sweeping upstream from the outlet. Further upstream, we suggest that landslide activity above this zone of adjusting topography may reflect a more complex response to passage of the uplift peak that causes tilting of the headwaters followed by a decline in rate of uplift.