Prehistoric, Headwater-Basin-Encompassing Debris-Avalanches, Northern California Coast Ranges: Temporal Association With Plate Boundary Earthquakes

Date: 4/24/2019

Time: 04:45 PM

Room: Vashon

What role do extreme events, climatic or seismic, play in the formation of high elevation landscapes in the northern California Coast Ranges? Exceptionally large (0.2-0.7 km²) prehistoric debris avalanches in the northern California Coast ranges set landscape form, but the triggering mechanism is unclear. Within high-elevation (>1500 m) headwater regions underlain by competent rock types, the main slope process is periodic debris avalanching that occurs in morphologically distinct headwater bowls. Because the scars appear to be single aged based on the extent and character of revegetation, avalanche failures appear to have occurred all at once encompassing the entire headwater basin. Hence, the failures sculpt headwater basins. Although no headwater-basin failures have occurred historically, we map the location and extent of four pre-historic debris avalanches as indicated by headwater basin scars that are unvegetated or partially revegetated. These avalanches scour into bedrock, and revegetation of bedrock slopes is a slow process that proceeds over centuries. One unvegetated avalanche scar that encompasses a headwater basin failed in the interval CE 1871-1904, based on trees growing on a coherent slide block that recorded the trauma in their growth rings. Another partially revegetated debris-avalanche-headwater-basin scar can be dated using an alluvial fill sourced from the avalanche. The alluvial fill extends 1.5 km down channel from the avalanche source and buried riparian trees in growth position; ¹⁴C age determinations indicate one of these trees was buried and killed within the interval CE 937-1071. Large headwater basin avalanches likely require extreme seismic or climatic events for slope destabilization. For the above two headwater-basin-encompassing debris avalanches for which we can constrain the time of failure to a defined time range, the time of failure -- in both cases -- overlaps with the time of a Cascadia plate boundary earthquake.

Presenting Author: Harvey M. Kelsey

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