Presented by Phillips, Fred M. Authors: McIntosh, William C., Dunbar, Nelia W..

Key words: uplift, extension, Sierra, Pliocene

In Session 228 T15. Cenozoic Basin and Range Tectonics and Geophysical Constraints II (GSA Geophysics Division; Nevada Seismological Laboratory, University of Nevada, Reno)Thursday, November 16, 2000 PM in Room: B16&17 at 04:10 PM for 15 min .

Abstract: The uplift history of the Sierra Nevada has been vigorously debated since the time of the Whitney Survey in the 1880's. Until recently, the general consensus was that most of the current uplift was subsequent to the mid-Pliocene. However, in the past 5 years new results from modeling, thermochronology, and paleobotany have tended to support the view that the range may have been uplifted during the Mesozoic and been steadily eroded, rather than uplifted, since that time. We have found basaltic scoria mixed with diamicton (apparent glacial till) on a small plateau on the crest of the Sierra Nevada north of Mt. Humpherys, at 4,050 m elevation. There are no basaltic eruptive centers in the vicinity of the deposit. The scoria yielded an 40Ar/39Ar age of 3.700.22 Ma, consistent with the timing of a regional pulse of basaltic volcanism. We sampled and dated the 11 basalt flows and cones closest to the deposit and east of it, in an attempt to identify the source of the scoria. Only three yielded similar ages. Of those, two showed pyroxene trace element geochemistry (determined by electron microprobe analysis) similar to that of the scoria on the crest. The best candidate for the source of the high-elevation scoria is 23 km west of the high-elevation scoria site, in the Owens Valley at 1,500 m elevation, and has an 40Ar/39Ar age of 3.410.19 Ma. The tilting history of the range, inferred from markers on the western slope, indicates that stable surfaces on the crest were probably no more than 1000 m lower than present at ~3.5 Ma. This, coupled with evidence for glacial transport of the scoria, indicates that the source flow was probably higher than 3,500 m at the time of eruption. We speculate that the pulse of basaltic volcanism at ~3.5 Ma accompanied the initiation of basin-and-range extension in the far western Great Basin, and that prior to that time what is now the Owens Valley was an altiplano stretching between the current crests of the Sierra Nevada and White Mountains.