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Dissertation/Thesis Abstract

Development and application of geochronometric techniques to the study of Sierra Nevada uplift and the dating of authigenic sediments
by Cecil, M. Robinson, Ph.D., The University of Arizona, 2009, 176; 3355007
Abstract (Summary)

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.

Indexing (document details)
Advisor: Ducea, Mihai
Commitee: Chase, Clem, Kapp, Paul, Patchett, Jonathan, Zandt, George
School: The University of Arizona
Department: Geosciences
School Location: United States -- Arizona
Source: DAI-B 70/04, Dissertation Abstracts International
Subjects: Geology, Geochemistry
Keywords: Authigenic sediments, Geochronology, Potassium-calcium dating, Sierra Nevada uplift, Uplift
Publication Number: 3355007
ISBN: 978-1-109-12956-4
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