There is considerable debate as to when the Sierra Nevada developed as a significant topographic feature and how it affected the past climate of this region. One view holds that the Sierra Nevada is a young mountain range, with net surface uplift of some 2km in the last 10 million years. In contrast, recent work suggests that the Sierra Nevada has been a long-standing topographic feature. Previous analysis of the hydrogen and oxygen isotope composition of smectite produced by the weathering of volcanic ashes from sections along the east side of the Sierra Nevada shows that, in the central Basin and Range, the oxygen isotopes of smectite increased slightly in the last 16 million years. This result contrasts with that of one section in the southern Basin and Range, which shows a large increase in oxygen isotopes from 14 to 6 million years. These results indicate that the Sierra Nevada has been a long-standing topographic feature at least for the past 16 million years. Moreover, if these isotopic shifts result solely from topographic change, then the Sierra has a spatially varying topographic history, with some 700 m of topographic loss in the northern Sierra and ~2000m of lowering of the southern Sierra in the Late Cenozoic. The data can also be explained by a strengthening of the summer monsoon in the Late Cenozoic that could result in isotopically heavier precipitation, particularly in the southern areas of the Basin and Range.
This study is testing the hypothesis that the Sierra Nevada had a spatial varying topographic history by: 1) collecting isotopic data from the southern Basin and Range in order to separate topographic from climatic effects; 2) constructing a regional isotopic map of the Basin and Range to enhance understanding of the regional topographic and climactic history of this region; 3) sampling over a larger time range to determine what isotopic changes have occurred both in rocks younger than 2 million years and older than 16 million years; and 4) collecting isotopic data from other substrates, particularly fossil mammals, to reduce the uncertainty associated with any one isotopic proxy for paleoprecipitation.
