Prehistoric, Headwater-Basin-Encompassing Debris-Avalanche Landslides, Northern California Coast Ranges: Do Extreme Events Set Landscape Form?

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) debris avalanches in the northern California Coast ranges set landscape form, but the triggering mechanism of such landslides 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, i.e., zero-order basins. 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, "full basin failures" that literally sculpt headwater basins. The landscape also has multiple smaller avalanche scars that do not encompass zero-order basins, making it easier to identify large zero-order-basin avalanche scars that represent single events. No full-basin failures have occurred historically, but the location and extent of several pre-historic failures is well documented through mapping of bowl- or amphitheater-shaped headwater basins that are unvegetated or partially revegetated. These large debris avalanches invariably scour into bedrock; revegetation of bedrock slopes is a slow process that proceeds over centuries. Prehistoric full basin failures display a range of revegetation extents. An unvegetated avalanche scar that encompasses a zero order basin failed in the nineteenth century, based on trees growing on a coherent slide block that recorded the trauma in their growth rings. Another partially vegetated zero-order 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 on two trees indicate that the debris avalanche likely occurred no more than 1000 years ago but failure could have been several centuries later. Other ¹⁴C ages are pending. Large zero-order basin avalanches likely require extreme seismic or climatic events for slope destabilization, and these large debris avalanches hold promise to elucidate the role that extreme events, including large earthquakes, play as destabilizing triggers.