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Keywords:

  • landslide;
  • rock fall;
  • seismic trigger;
  • cosmogenic nuclide dating

Abstract

Large rock slope failures from near-vertical cliffs are an important geomorphic process driving the evolution of mountainous landscapes, particularly glacially steepened cliffs. The morphology and age of a 2·19 × 106 m3 rock avalanche deposit beneath El Capitan in Yosemite Valley indicates a massive prehistoric failure of a large expanse of the southeast face. Geologic mapping of the deposit and the cliff face constrains the rock avalanche source to an area near the summit of ∼8·5 × 104 m2. The rock mass free fell ∼650 m, reaching a maximum velocity of 100 m s−1, impacted the talus slope and spread across the valley floor, extending 670 m from the base of the cliff. Cosmogenic beryllium-10 exposure ages from boulders in the deposit yield a mean age of 3·6 ± 0·2 ka. The ∼13 kyr time lag between deglaciation and failure suggests that the rock avalanche did not occur as a direct result of glacial debuttressing. The ∼3·6 ka age for the rock avalanche does coincide with estimated late Holocene rupture of the Owens Valley fault and/or White Mountain fault between 3·3 and 3·8 ka. The coincidence of ages, combined with the fact that the most recent (AD 1872) Owens Valley fault rupture triggered numerous large rock falls in Yosemite Valley, suggest that a large magnitude earthquake (≥M7.0) centered in the south-eastern Sierra Nevada may have triggered the rock avalanche. If correct, the extreme hazard posed by rock avalanches in Yosemite Valley remains present and depends on local earthquake recurrence intervals. Published in 2010 by John Wiley & Sons, Ltd.