THE POVERTY HILLS, OWENS VALLEY, EASTERN CALIFORNIA: TRANSPRESSIONAL UPLIFT OR LONG-RUNOUT ROCK AVALANCHE?

The Poverty Hills constitute a 5 sq km outcrop of Paleozoic metasedimentary and Mesozoic plutonic rocks situated near the axis of the alluviated Owens valley. Two models have been advocated for the origin of the hills. The first model suggests that the hills are a transpressional uplift created by movement along the Owens Valley fault. The second model proposes that the hills are a long-runout rock avalanche deposit derived from the nearby mountains. It is argued here that the rock avalanche model best fits the geology.

In the transpression model, the Poverty Hills constitute a bedrock uplift caused by convergence where the Owens Valley fault steps leftward 3 km from a fault segment southeast of the hills to the Fish Springs fault northwest of the hills. Although superficially attractive, a major problem for the model is that offset of a cinder cone cut by the Fish Springs fault indicates pure dip-slip motion. There is thus no structural reason for the existence of the western part of the Poverty Hills under the transpression model.

In the landslide model, the hills are a long-runout rock avalanche deposited on Owens Valley alluvial fill. The main evidence for the model is the fact that much of the the rock comprising the Poverty Hills is brecciated in a manner common to long-runout rock avalanches. Along Tinemaha Road, where many of the best breccia exposures occur, brecciated Paleozoic metasedimentary beds have been distorted and contorted by cataclastic breccia flow. At other outcrops scattered across the hills, jigsaw and crackle breccia textures are common. Another line of evidence for the landslide model is the presence of a closed depression on the upper surface of the hills. This feature might be a remnant of hummocky topography formed during during landslide emplacement. The most likely source of the landslide appears to be the Inyo Mountains and the slide is at least several hundred thousand years old.

Recognition of the Poverty Hills landslide allows re-interpretation of Owens Valley fault zone kinematics. Regional right-lateral shear might transfer from the Owens Valley fault to the White Mountains fault zone near the latitude of the Poverty Hills, leaving the possibility that the entire Big Pine segment of the Owens Valley fault is a pure dip-slip fault.