Young convergent-margin orogens, climate, and crustal thickness—A Late Cretaceous–Paleogene Nevadaplano in the American Southwest?

Abstract

The N-S–trending Andes surmount South American lithosphere above the east-descending Nazca plate. The highest mountains, underlain by ∼70-km-thick crust, cap the Cordillera at 25°S, an extremely arid region. In contrast, the precipitation-drenched fjordland at 45°S is supported by ∼35-km-thick crust. The Cascade and Sierra Nevada ranges in the western United States display comparable N-S trends and latitudinal rainfall patterns. Westerly winds supply abundant moisture to the northwest, but precipitation diminishes southward, producing increased aridity where the Sierra achieves its maximum regional elevation around Mount Whitney. Overthickened, now delaminating, ∼42–55-km-thick crust of the southern Sierra exceeds that of the ∼35-km-thick northern Sierra and the active Cascade arc. Contrasts in orogenic crustal thickness in California are not as marked as in the Andes because Sierran arc construction ceased near the end of Cretaceous time. Geologic, geochemical, and stable isotopic data suggest that a Nevadaplano occupied central + southern Nevada + western Utah in the rain shadow of the Late Cretaceous–Paleogene arc. The ∼40–45-km-thick Colorado Plateau crust lies well downwind from the southern Sierra, and, depending on when it became elevated, it might represent part of a broader highland that collapsed during Neogene Basin and Range extension.