Date of Award

2011

Level of Access Assigned by Author

Open-Access Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Earth Sciences

Advisor

Scott E. Johnson

Second Committee Member

Christopher C. Gerbi

Third Committee Member

Edward S. Grew

Abstract

This thesis investigates the cause of mass transfer during crenulation cleavage formation and the role that elastically anisotropic minerals and fabric development play in promoting seismic anisotropy in the crust. Crenulation cleavage is the most common fabric in multiplydeformed, phyllosilicate-rich metamorphic rocks. During its formation the originally planar fabric gets crenulated, eventually leading to the differentiation of quartz- and feldspar-rich regions (QFdomains) in the crenulation hinges, and phyllosilicate-rich regions (P-domains) in the crenulation limbs. This differentiation is driven by the dissolution of quartz and feldspar in the P-domains, and the precipitation of those minerals in the QF-domains. Finite element models are created to investigate how the elastic interactions of quartz and muscovite minerals affect the grain-scale stress and strain distributions at different stages of crenulation cleavage development. Gradients in mean stress and volumetric strain develop between the limbs and hinges of the microfolds during fabric formation and are sufficient to drive mass transfer between the two domains. v To study the influence of different microstructural variables on seismic wave speed anisotropy, simplified muscovite-quartz models are created with varying amounts of muscovite, varying quartz and muscovite orientations, and varying spatial distributions. The asymptotic expansion homogenization method coupled with finite element modeling (AEH-FE) is used to calculate bulk stiffness tensors and seismic wave speeds. Muscovite’s abundance and preferred orientation have significant influence of seismic wave speed anisotropy due to the extreme anisotropic elasticity of the mineral. The same method is employed to study the seismic behavior of rocks containing different stages of crenulation cleavage. Mineral orientation maps of rock samples were created, using electron backscatter diffraction, and used as input files for the AEH-FE program. Schists with a planar foliation are highly elastically anisotropic, but a rock with a well developed crenulation cleavage is much less anisotropic. These results imply that regions with larger scale crustal structures, such as folds and shear-zones, can be much more muted in their seismic signal than the schistose samples that make up those structures.

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