IS EROSION DOMINATED BY LARGE, RARE, ROCK AVALANCHES? EXAMPLES FROM THE EASTERN SIERRA NEVADA AND OWENS VALLEY

It has become increasingly recognized that large rock avalanche masses and slides are a common mode of mass removal from active mountain terrains, with high elevation differentials and steep slopes. This is now recognized to be a commonplace phenomenon on volcanic edifices, and in the oceanic environment generally. While “classic” slide masses have long been recognized and documented, I suggest that these features are perhaps far more common than may be appreciated. Indeed, Herrmanns and Strecker [1999] have documented widespread Quaternary deposits in Argentina between 24 and 28 degrees latitude, while Bishop [1999] has used morphology and surface geology of 11 major rock masses in the Owens Valley of California to propose that these units are rock avalanches derived principally from the steep eastern margin of the Sierra Nevada. These well-known intra-valley masses have been mapped as in-situ bedcrock occurrences. Both authors notice a probable association with active faulting, and propose that these slide masses are earthquake-triggered. In this paper, I have compiled detailed geometric descriptions of the Sierra Nevada avalanche masses from GIS-DEM data, and propose the source area and breakaway zones. It would appear that mass removal is dominated by large-scale rock avalanches in settings with sufficient relief and tectonic activity.