Award Date

1-1-2007

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Geoscience

First Committee Member

Terry Spell

Second Committee Member

Michael Wells

Number of Pages

210

Abstract

The four chapters in this dissertation consist of projects that utilized 40Ar/39Ar thermochronometry. Chapters 1 and 2 are from a study of the Sisters shear zone on Stewart Island, New Zealand. In these studies, thermal histories obtained using 40Ar/39Ar thermochronometry were combined with field and microstructural observations collected from deformed rocks. These data indicate extensional deformation along the Sisters shear zone was the youngest event related to the breakup of the paleo-Gondwana margin. The Sisters shear zone is related to formation of the Great South Basin and thinning of the Campbell Plateau. The shear zone is also spatially and kinematically linked to the Pacific-Antarctic spreading ridge indicating the shear zone was involved in the separation of New Zealand from West Antarctica. Comparison of timing constraints from Stewart Island with those from other studies and locations indicates the breakup of the Gondwana margin was likely the result of two distinct extensional events; Chapter 3 consists of a 40Ar/39Ar laboratory experiment dealing with the biotite and muscovite micas. Artificial samples of mixed mica populations were analyzed using the vacuum furnace step-heating method. These samples were prepared and analyzed to test the possibility of recovering original ages of individual mica populations from natural samples consisting of multiple generations. The results indicate this is not likely in the vacuum furnace. Additionally, the results indicate that the compositional controls on argon retentivity in nature may also be active during furnace heating in the laboratory; Chapter 4 shows the results of a 40Ar/39Ar study of plutonic rocks that have cross cutting relationships with structures of the Clark Mountains thrust complex in southern California. These data indicate the earliest episode of crustal shortening occurred pre-155 Ma. The Pachalka thrust at ∼444 Ma was previously considered the oldest deformation episode in the region. Diorite-granodioritic magmatism at ∼155 Ma was followed closely by felsic magmatism of the Ivanpah granite (>149 Ma). The ductile Morning Star Mine thrust, which likely correlates to the Keaney-Mollusk Mine thrust cuts the Ivanpah granite.

Keywords

Age Spectra; Argon; Argon-argon Dating; Artificially; Constraints; Mesozoic; Micas; Mixed; New Zealand; Southwest; Spectra; Tectonism; Thermochronological; Thermochronology

Controlled Subject

Geology

File Format

pdf

File Size

8069.12 KB

Degree Grantor

University of Nevada, Las Vegas

Language

English

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