Geochronology is pivotal in decoding the thermal and exhumation histories of mountain belts and adjacent basins, which in turn can provide important constraints on their tectonic and topographic development. This dissertation contains studies that use various geochronometric and thermochronometric techniques to better understand the post-magmatic evolution of Sierra Nevada, California. (U-Th)/He ages in apatite and zircon from Sierran batholithic rocks are used to constrain the Cenozoic exhumation of the northern part of the range. By sampling along a broad, range perpendicular transect and using two thermochronometers, we estimated cooling and exhumation rates both spatially and through time. Zircon and apatite ages determined from the same samples revealed relatively rapid cooling and exhumation rates (0.2–0.8 km/My) from ∼ 90 to 60 Ma, followed by tectonic quiescence and slow exhumation (0.02–0.04 km/My) from the late Paleocene to present. Apatite He ages (as well as zircon ages, to a lesser extent) were found to decrease with elevation and are notably younger in samples collected at the modern range crest. In addition to the thermochronology of basement lithologies, the detrital zircon geochronology of grains from preserved Eocene fluvial sediments in the central and northern Sierra Nevada was performed. U-Pb ages of detrital zircons from the deposits were found to have distributions closely matching age-area estimates of Mesozoic plutons in the Sierra Nevada. (U-Th)/He ages from a subset of the detrital zircons were similar to previously determined zircon He ages in Sierran granitoids, suggesting that Eocene river systems were draining local Sierran catchments and likely had steeper axial gradients than has been proposed. Provenance analysis of the Eocene sediments is used to provide constraints on the paleotopography of the Sierra Nevada and inferred range-wide Cenozoic uplift.
In addition to the Sierra Nevada work, this dissertation also contains studies that focus on the development of the K-Ca system as a geochronometric technique suitable for dating the deposition of sedimentary sequences. We present a new method for measuring Ca isotopic ratios using a multi-collector inductively coupled plasma mass spectrometer (MC-ICP-MS) equipped with a hexapole collision cell. Isobaric argon interferences are minimized via gas phase reactions in the collision cell. The reproducibility of Ca ratio measurements is found to be ∼ 0.02% (RSD), which is comparable to high precision TIMS techniques and an order of magnitude improvement over single collector ICP-MS techniques using a similar reaction cell method. A group of standards and carbonate materials are analyzed to demonstrate the utility of this technique in reliably measuring stable isotopic ratios without chemical purification or isotopic spiking. Relatively small enrichments in radiogenic 40Ca are also shown to be measurable. K-Ca ages of glauconite (a potassic micaceous clay that forms authigenically) and K-rich evaporites are determined in order to evaluate the usefulness of the K-Ca system as a sedimentary geochronometer. K-Ca ages in both glauconite and K-salts are found to be variable and significantly younger than documented depositional ages. Reported ages, however, are thought to be recording important basinal thermal histories and recrystallization events.
|Commitee:||Chase, Clem, Kapp, Paul, Patchett, Jonathan, Zandt, George|
|School:||The University of Arizona|
|School Location:||United States -- Arizona|
|Source:||DAI-B 70/04, Dissertation Abstracts International|
|Keywords:||Authigenic sediments, Geochronology, Potassium-calcium dating, Sierra Nevada uplift, Uplift|
Copyright in each Dissertation and Thesis is retained by the author. All Rights Reserved
The supplemental file or files you are about to download were provided to ProQuest by the author as part of a
dissertation or thesis. The supplemental files are provided "AS IS" without warranty. ProQuest is not responsible for the
content, format or impact on the supplemental file(s) on our system. in some cases, the file type may be unknown or
may be a .exe file. We recommend caution as you open such files.
Copyright of the original materials contained in the supplemental file is retained by the author and your access to the
supplemental files is subject to the ProQuest Terms and Conditions of use.
Depending on the size of the file(s) you are downloading, the system may take some time to download them. Please be