Unroofing history of Alabama and Poverty Hills basement blocks, Owens Valley, California, from apatite (U-Th)/He thermochronology

摘要

Most of the 150km-long Owens Valley of east-central California, the westernmost graben of the Basin and Range Province and location of the active right-lateral Owens Valley Fault Zone, is filled with more than 2-3 km of Neogene sediments. Two prominent but structurally enigmatic basement blocks, the Alabama Hills and Poverty Hills, rise from the floor of the southern part of the valley. The late Cenozoic tectonic origin of these basement blocks is not known, but previously published hypotheses include: (1) transpressional uplifts; (2) down-dropped normal fault blocks; and (3) giant landslides from adjacent ranges. We measured apatite (U-Th)/He ages on 15 samples from the Alabama and Poverty Hills to understand the history of shallow crustal exhumation of these structures, and to potentially correlate them to rocks from adjacent ranges. Apatite He ages for the Alabama and Poverty Hills range from approx 63 to 52 and approx 61 to 39 Ma, respectively, with one sample from the Alabama Hills yielding an older weighted mean age of 79.3 + - 2.2 Ma (2a). These ages are similar to those measured at elevations 1.5-3.0km higher in both the adjacent Sierra Nevada and White/Inyo Mountains. Together with a lack of evidence for transpressional structures and the presence of extensive weathering of an ancient inherited surface on top of the Alabama Hills, these results are most consistent with an origin as a down-dropped normal fault block. A structural reconstruction using tilt-corrected southern Sierra Nevada age-elevation correlations requires 2.6 km of vertical and 1.5 km of eastward motion for the Alabama Hills along the Sierra Nevada Frontal Fault. Although we cannot conclusively rule out a landslide origin, our preferred explanation for the Poverty Hills data is that it is a down-dropped normal fault block that was transported from the Inyo Mountains along a right-lateral fault possessing a significant normal component.