The Sierra Nevada, California, is underlain by a large volume of granite (a batholith) that represents the roots of ancient volcanoes formed by subduction beneath the western margin of North America during the Mesozoic, 250 to 80 million years ago. The Sierra Nevada (as a mountain range) ends near Susanville, northern California, but its granites continue into the Great Basin to the east. Here they have not been studied in detail. The first part of this project will detail the exact match between these rocks and the northern Sierra Nevada and will address where specific parts of the batholith cross the northern Great Basin. This project will also address a very controversial set of questions about the landscape of California and Nevada from the Cretaceous (80 million years ago) to present. It has been proposed that the Sierra Nevada as we know it today existed since the Cretaceous and that the region to the east of the Sierra constituted a vast high plateau like the Altiplano of the Andes. The plateau collapsed with the onset of Basin and Range faulting that led to formation of the Great Basin. Data from northwestern Nevada, however, suggests that the crust beneath this part of the batholith was never thick enough to produce such a plateau and that rivers appear to have carried debris eroded from the ancient Sierra into the ocean to the north (a region now covered by extensive young volcanic lava flows). These different ideas will be tested using two methods: 1) Ar/Ar, fission track, and (U-Th)/He thermochronology for arrays of samples from the granites of the batholith, collected with depth beneath the tilted volcanic-granite unconformity in Basin and Range fault blocks, will detail the post-intrusive cooling and the erosional history of the batholith; and 2) age populations of zircons in Late Cretaceous and early Tertiary marine and non-marine sedimentary rocks in California and Oregon will allow determination of where this large volume of batholithic material was eroded and transported to.
This project fills a major gap in our knowledge of Cretaceous to Cenozoic geodynamic evolution large area of North American Cordillera. The Sierra Nevada is a widely studied example of continental arc magmatism yet the northern portion of the batholith, and its recently recognized continuation into northwestern Nevada, has received little attention. The tectonic and magmatic evolution of this area is a key unresolved problem in western North American geologic evolution. The project will engage graduate and undergraduate students in the field and laboratory phases of the research. A project web site, presentations at local schools and community centers, and newspaper articles will continue to engage local communities in the progress and outcomes of the research.
