Stable isotopic evidence for a Pre‐Middle Miocene rain shadow in the western Basin and Range: Implications for the paleotopography of the Sierra Nevada

Abstract

A recently proposed method for approximating the paleorelief of mountain belts makes use of the predictable relationship between the isotopic depletion of precipitation and the net elevation of an orographic barrier over which an air mass rises. This rain shadow effect often creates desert regions on the lee side of mountain belts in which precipitation is isotopically light. Changes in the isotopic composition of precipitation can be estimated from the isotopic composition of authigenic or pedogenic minerals, which can then be used to infer both the development of relief during orogenesis as well as the stability of a rain shadow formed by developing mountains. The δ18O of smectites formed from the weathering of Middle Miocene to Late Pliocene volcanic ashes currently exposed in the rain shadow of the modern Sierra Nevada of California show no indication of large‐scale Late Cenozoic surface uplift of the Sierra and corresponding regional rain shadow development. Rather smectite isotope data tentatively suggest that elevations may have decreased over this time by as much as 2000 m toward the southern end of the range and 700 m in regions farther north. This suggests that the modern rain shadow cast over the western Basin and Range has been in existence since pre‐Middle Miocene and that the Sierra Nevada have been a prominent orographic barrier since before this time. These interpretations are in accord with several recent studies also suggesting a possible Cenozoic elevation loss of an already developed Sierra Nevada mountain range.