Abstract

Although geomorphic observations suggest that the Sierra Nevada has tilted so that the crest has risen 1–2 km since late Miocene time, deuterium and oxygen-18 isotope concentrations in Cenozoic geologic materials decrease eastward across California and Nevada similarly to those in modern, orographically induced precipitation, as if little change in Sierra Nevadan elevations has occurred since Eocene time. Orographic precipitation, however, depends on the amount of moisture in the atmosphere, which in turn can be much larger in warm air, as in Eocene or Oligocene time and in summer, than in the cooler air characteristic of present-day, dominantly winter, precipitation. Moreover, the integrated rainout of vapor, and hence presumably in stable isotope concentrations in the remaining vapor, depends largely on the difference in heights traversed by air masses, not slopes of mountain ranges. Thus, if due simply to orographically induced rainout, both Eocene and Oligocene variations in deuterium isotopes across the Sierra Nevada and Miocene–Quaternary differences in deuterium and oxygen isotopes between the Great Valley of California and the Basin and Range place only weak constraints on the slope or past elevations of the Sierra Nevada. They do not necessarily contradict the inference that the crest of the Sierra Nevada has risen 1000 m or more since late Miocene time.

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