Construction and Exhumation of the Sierra Nevada Range: an investigation of geochronology, thermochronology and trace element mineral chemistry

Abstract

This dissertation describes the development of the Sierra Nevada range from its construction as a magmatic arc to its current configuration as a significant tectonic block or microplate in the western North American Cordillera. The Sierra Nevada range is a large coherent physiographic and tectonic block built upon a Mesozoic age batholith that is one of the most well studied extinct continental arc systems in the world. Models of batholith construction, magmatic evolution and subduction mechanics have all been refined as a result of the geologic relationships observed in the Sierra Nevada. However, several important questions related to processes of continental arc systems are still unresolved. Specifically, the complex interplay between orogenesis and erosion in the Sierra Nevada is debated, as is the ‘longevity’ of the range created during Mesozoic and early Cenozoic subduction. The cause and source of voluminous magmas emplaced during magmatic flare-up events (e.g. late-Cretaceous flare-up of the Sierra Nevada) is not yet resolved. It is speculated that crustal thickening results in anatexis of the lower crust, but robust validation of this model in the chemistry of flare-up magmas is needed. This dissertation applies multiple geochronologic, thermochronometric and geochemical techniques to examine these questions. In Chapter 1, we investigate the timing and magnitude of erosion in the central Sierra Nevada range. Interpretation of the data presented here supports previous investigations in the southern and northern Sierra Nevada, which posit high rates of erosion in the late Cretaceous and early Cenozoic. The data presented in Chapter 1 also indicates that the paleotopography of the late Cretaceous range was of significant topographic relief and that the distribution of major paleotopographic features (e.g. river canyons, major interfleuve divides) in the late Cretaceous were similar to those of the modern range. Chapter 2 investigates the cooling trends observed in batholithic rocks from temperatures of 800°C <T<60°C. We interpret these results to indicate a characteristic pattern of post-emplacement cooling that reflects two dominating mechanisms: (1) post-emplacement conductive equilibration of temperatures followed by (2) cooling as a result of exhumation. Chapter 3 investigates the long-term fractionation of magmas during the late-Cretaceous flare-up event by analyzing trace element concentrations in magmatic zircons. We find evidence for increasingly fractionated melts throughout the flare-up event; an observation that supports models of anatexis as a result of crustal thickening. We also find that the late- Cretaceous flare-up may have started earlier in the northern portions of the range and migrated south over a period of 10-15 Ma, thus indicating a temporal and spatial progression not yet recognized.