Cosmogenic beryllium-10 nuclide exposure age dating of the Tiltill rock avalanche in Yosemite National Park

Yosemite National Park serves as an excellent natural laboratory for studying rock falls and rock avalanches because these are the main processes modifying the nearly vertical slopes of this recently glaciated landscape. Mass wasting represents a significant hazard in the region and the database of previous rock falls and other mass wasting events in Yosemite is extensive, dating back to the mid-1800s. However, this record is too short to capture the recurrence characteristics and triggering mechanisms of the very largest events, necessitating studies of the geologic record of mass wasting. Rock falls and rock avalanches are readily dated by cosmogenic nuclide methods due to their instantaneous formation, and results can be tied to triggering events such as seismic activity (e.g. Stock and Urhammer, 2010). Here, we apply exposure dating to the Holocene Tiltill rock avalanche north of Hetch Hetchy Reservoir. The deposit comprises 2.10 x 106 m3 of rock and debris. Assuming an erosion rate of 0.0006 cm/yr and neglecting snowpack shielding, the data yields 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 and the coincidence of ages between the Tiltill rock avalanche and paleoseismic events, a large earthquake in Eastern Sierra Nevada may have triggered this event. Other trigger events are also possibilities, but only through compilation of a database of large rock avalanches can statistically significant groupings of events begin to demonstrate whether seismic triggering is a dominant process.