Date of Award

8-2012

Level of Access

Campus-Only Thesis

Degree Name

Master of Science (MS)

Department

Earth Sciences

Advisor

Scott E. Johnson

Second Committee Member

Christopher C. Gerbi

Third Committee Member

Martin G. Yates

Abstract

This thesis investigates the role of microstructural and rheological heterogeneity and their relative influences on the kinematics of porphyroblasts and bulk strength of porphyroblastic schists. Porphyroblasts are valuable because they commonly record long histories of coupled metamorphism and deformation in their orientations and internal inclusion fabrics. Traditionally, the rotation of porphyroblasts has either been viewed in terms of ideal theoretical behavior or not been considered at all. The principal difference between these applications is the degree of porphyroblast-matrix coupling, primarily affected by strain partitioning due to microstructural or rheological heterogeneity. Furthermore, such heterogeneity also, in part, controls the bulk strength of these aggregates and it is unclear if the presence of large, strong porphyroblasts necessarily requires bulk strengthening. Little is known about how rheological and microstructural heterogeneity affects bulk strength and porphyroblast kinematics and how they relate to one another. In this thesis I explore these relationships using a well preserved natural example and numerical modeling of analogous synthetic aggregates.

Electron backscatter diffraction and petrographic analyses reveal asymmetric microboudinage of staurolite, indicating relative rotation of staurolite porphyroblasts synchronous with bulk non-coaxial strain. Boudinage and relative rotation both require porphyroblast-matrix shear coupling. Based on 2D optical observations, the extent of the coupling appears related to the initial and boudinaged staurolite grain shape and orientation as well as the geometry of heterogeneities such as mica domains or shear bands. My observations also indicate that large magnitudes of the observed rotation occurred out of the shear plane, which could lead to reduced kinematic vorticity measurements if traditional techniques were applied.

I designed 2D finite element numerical models to assess the role of microstructural variation and rheological heterogeneity on the degree of porphyroblast-matrix shear coupling and bulk viscous strength. Model results indicate that the bulk strength of a three-phase system comprising inclusion, weak domain, and matrix is sensitive to the relative proximity of weak and strong domains, particularly at high viscosity contrasts (i.e. ηmatrixweak>10). The threshold for bulk weakening below the matrix strength occurs over a narrow range of weak domain viscosities (ηmatrixweak=2.6-5.5), regardless of the relative abundance and spatial distribution of weak domains. Kinematic decoupling of porphyroblasts occurs at low viscosity contrasts when weak domains are proximal (ηmatrixweak=2-5), but for all other spatial distributions and modal abundances investigated, kinematic decoupling occurs at viscosity contrasts of ηmatrixweak=15-20. These data indicate that bulk weakening due to rheological heterogeneity is not necessarily coincident with kinematic decoupling.

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