A NEW HYPOTHESIS FOR THE ORIGIN OF THE POVERTY HILLS, OWENS VALLEY, CALIFORNIA

The Poverty Hills are a 13 km² block of Jurassic plutonic rocks and upper Paleozoic sedimentary rocks rising 300 m above the alluvium of Owens Valley south of Big Pine. Several conflicting hypotheses have been offered to explain the origin of these topographically anomalous hills. These include an origin as (1) a landslide mass derived from the Sierra Nevada to the west; (2) a landslide mass derived from the Inyo Mountains to the east; or (3) a transpressional bedrock uplift related to a left step in the dextral strike-slip Owens Valley fault at the north end of the hills. None of these hypotheses, however, is fully consistent with the results of recent studies that show the following: (a) brecciation and other evidence of landsliding are widespread in the Poverty Hills; (b) rocks of the Poverty Hills match those of the Inyo Mountains much more closely than those of the Sierra Nevada; (c ) dextral movement on the Owens Valley fault is not transferred along a left step at the north end of the Poverty Hills as previously proposed, but instead is transferred along a right step to the front of the White Mountains near the south end of the hills; and (d) the Poverty Hills have evidently been uplifted since deposition of granitic boulders that now rest on high hills underlain by Paleozoic sedimentary rocks. An alternative hypothesis, therefore, is needed.

We suggest the following sequence of events to explain the origin of the Poverty Hills. (1) Prior to late Cenozoic opening of Owens Valley, the rocks that form the Poverty Hills lay 5 km to the southeast at the north end of the Santa Rita Flat pluton in the Inyo Mountains where matching rocks are found. (2) As part of the Sierra Nevada structural block, the Poverty Hills rocks were rifted 8 km northwest as the valley opened. (3) These rocks were then buried by alluvium from the high Sierra. (4) Later the rocks were uplifted as part of a north-trending horst between the Fish Springs and Red Mountain faults where elevated granitic basement is now exposed. (5) Finally, the Poverty Hills rocks were displaced as a landslide mass 3 km eastward from the horst to their present position. This interpretation is consistent with the anomalous position of the hills east of the Fish Springs fault on steep magnetic and gravity gradients that define the boundary between the horst and the deep Owens Valley to the east.