The Cenozoic Paleoelevation and Paleogeographic History of the Southwestern US Cordillera: a combined Sedimentologic and Isotopic Approach.

Abstract

The Cenozoic paleoelevation history of the Western US Cordillera has far-reaching implications for resolving the tectonic and geodynamic evolution of the region. The observed systematic relationship between elevation and the stable isotopic composition (delta-18O and delta-D) of surface meteoric waters provides an opportunity to construct quantitative paleoelevation histories using authigenic mineral proxies for the isotopic composition of paleo-meteoric waters, but uncertainties and complications inherent to this approach require further study. Part I of this dissertation critically evaluates the stable isotope paleoaltimetry technique through investigation of the dominant environmental controls on modern precipitation and surface water delta-18O distributions. This modern analysis reveals that isotope-elevation relationships vary systematically as a function of physiographic and climatic environment, with reduced delta-18O-elevation gradients characterizing continental interior and orogenic plateau regions. This finding has important implications for future interpretations of paleo-meteoric water proxy records as the physiographic and climatic setting in which proxies formed must be taken into account in order for accurate paleoelevation determinations to be made. Part II of this dissertation builds on the findings and implications of Part I to provide new paleoelevation and paleogeographic constraints on the early Cenozoic western US Cordillera. Standard stable isotope paleoaltimetry techniques in conjunction with zircon U-Pb provenance study of early Cenozoic sedimentary basin systems in the southern Sierra Nevada region provides definitive evidence for near sea level paleoelevations in the southernmost Sierra Nevada ~ 60 million years ago. This paleoelevation requires 1.5 – 2 km of surface uplift since Eocene time, providing additional support for models proposing major Late Cenozoic uplift of the central and southern Sierra Nevada due to loss of dense, mantle lithosphere from below the range. Carbonate clumped isotope paleothermometry studies of lacustrine carbonates indicates that the Cordilleran interior was dominated by a high elevation (> 2.6 km) continental plateau prior to Basin and Range extension. The internal buoyancy forces associated with such a high elevation plateau were thus likely to have acted as a primary driver for the widespread Tertiary extension that created the Basin and Range Province that dominates much of the western US continental interior today.