NEW EVIDENCE FOR THE TUNGSTEN HILLS, NEAR BISHOP, CA, AS A LANDSLIDE DEPOSIT

Several years ago it was proposed that the Tungsten Hills, near Bishop, CA, is a possible landslide deposit. The principal evidence proposed is primarily geomorphic: the hummocky, hackled, and jointed surface of the Tungsten Hills quartz monzonite contrasts sharply with the surface texture of the surrounding bedrock. Mapping of plutonic rocks by Bateman (1965, 1992) suggests a source area in the Sierra Nevada either to the west or to the south. However, new structural and stratigraphic evidence from Miocene basalt flows in and south of the Tungsten Hills provides new evidence for the landslide hypothesis and the location of the source area. The Tungsten Hills are 24 square km in extent and composed primarily of Triassic quartz monzonite; they have hummocky and hackly appearance, with the quartz monzonite heavily cross-jointed. In the northwestern portion of the Tungsten Hills are three basalt flows of Miocene age, all within 1.5 km of each other, each from 200 to 500 square meters in extent, and 1600 to 1800 m in elevation. Phillips et al. (in press) determined an age of 11.7 Ma for these three flows, and that these three basalt flows are the same age as a large flat-lying basalt sheet located on the northern edge of Coyote warp 10.6 km to the south and 2700 m in elevation. Contrasting structural deformation within and between the three basalt flows in the Tungsten Hills, combined with a stratigraphic comparison between these flows and the large basalt sheet on the Coyote warp, is believed to be evidence consistent with the landslide hypothesis. The Tungsten Hills basalt deposits consists of flows, tephra, and lahars. Deformation of the basalt deposits includes faulting and tilting. Because of the deformation, sharp structural discordance exists between outcrops separated by as little as a few meters. At one outcrop, steeply dipping basalt deposits are truncated by a nearly horizontal fault that separates the basalt from the underlying granitic rocks. The strong structural deformation of the Tungsten Hills basalt contrasts with that of the non-deformed basalt of the same age on Coyote warp. A simple explanation for the deformation contrast between the two areas is that the Tungsten Hills are a landslide deposit and the basaltic deformation is a result of the landsliding process.