EVIDENCE FOR A 45 km2 SIERRA NEVADA LANDSLIDE, NORTHERN OWENS VALLEY, CALIFORNIA

Based on anomalous geomorphology including hummocky topography, hackly surface texture, and overall low slope gradient, the existence of a 45 km² landslide on the Sierran escarpment south of Bishop, California was proposed several years ago. Other than geomorphic expression, however, evidence for the landslide was sparse. Recently acquired structural data from Neogene basalt flows on and off the landslide mass provides new evidence supporting the landslide hypothesis.

Structurally, the slope area containing the landslide is a relay ramp between two large normal faults and is commonly referred to as the Coyote warp. Slope relief is 1500 m. Important basalt outcrops occur at five locations on the warp. The eastern three of these outcrops occur within 2 km of one another and are on the proposed landslide, whereas the western two outcrops occur just off the proposed landslide.

Basalt deposits at the three outcrops on the proposed landslide are faulted and moderately to steeply tilted down to the north, whereas the two outcrops west of the landslide are eroded but not deformed. Phillips et al. (in press) determined an Ar-Ar age of 3.4 Ma for the basalt of the three outcrops on the landslide and 11.7 Ma for the two basalt outcrops west of it. The contrast in deformation of the basalts on and off the proposed landslide supports the landslide interpretation.

In principal, the deformation of the three eastern basalt outcrops could be from either tectonic activity or landsliding. However, given the degree of deformation of the 3.4 Ma outcrops, it seems unlikely that such strong tectonic deformation could have affected the area of these basalts without also affecting the area of the 11.7 Ma basalts. The more reasonable explanation for the localized deformation is that the younger basalt flows were deformed by landsliding.

Recognition that the landslide interpretation is correct lends support that other landslides previously proposed for the Sierran escarpment bordering Owens Valley are real. For example, the Sierran slope above the town of Big Pine, previously proposed to be the Big Pine landslide, has geomorphic characteristics quite similar to the Bishop landslide. Two other similar features are the Independence landslide west of the town of Independence and the Olancha landslide southwest of the town of Olancha.