COSMOGENIC BERYLLIUM-10 EXPOSURE DATING OF THE TILTILL ROCK AVALANCHE IN YOSEMITE NATIONAL PARK

Yosemite National Park is an excellent natural laboratory for studying rock falls and rock avalanches, because in many areas these are the main processes modifying the near vertical slopes of this glaciated landscape. Mass wasting represents a significant hazard in the region. The inventory database of historic rock falls and other mass wasting events in Yosemite is extensive, dating back to 1857 (Stock et al., 2013). However, this record is too short to capture recurrence characteristics and triggering mechanisms of the very largest events, necessitating studies of the geologic record of mass wasting. Rock avalanches are readily dated by cosmogenic nuclide methods due to their instantaneous deposition, and results can be tied to potential triggering events such as seismic activity (e.g. Stock and Urhammer, 2010). Here, we apply cosmogenic beryllium-10 exposure dating to the 2.10 x 10⁶ m³ Tiltill rock avalanche north of Hetch Hetchy Reservoir. Assuming an erosion rate of 0.0006 cm/yr and negligible snow shielding, the ¹⁰Be data yield a mean exposure age of 11,100 +/- 400 year B.P. The age of the Tiltill rock avalanche is similar to earthquakes on the Owens Valley Fault between 10,800 +/- 600 and 10,200 +/- 200 year B.P. (Bacon, 2007) and the Deep Springs Fault, between 10,200 +/- 100 and 11,200 +/- 100 year B.P. (Lee et al., 2001). Given that movement on the Owens Valley fault in 1872 caused a number of rock falls in Yosemite, the coincidence of ages between the Tiltill rock avalanche and paleoseismic events, and the size of the Tiltill rock avalanche, we propose that a large earthquake in Eastern Sierra Nevada may have triggered this event. If seismicity is a dominant process contributing to large rock slope failures in Yosemite, the hazard posed by these events exists and depends on local earthquake recurrence intervals.