Date of Award


Level of Access Assigned by Author

Campus-Only Thesis

Degree Name

Master of Science (MS)


Earth Sciences


Christopher C. Gerbi

Second Committee Member

Peter O. Koons

Third Committee Member

Scott E. Johnson


The megacrystic granitoid units of the Grenville Front Tectonic Zone (GFTZ) in Ontario, Canada, from French River Outlet to the orogenic front, provide an excellent laboratory in which to examine the strength heterogeneities that develop during orogenesis. Recent mapping in the area has constrained the boundaries of a series of kilometer scale shear zones in the GFTZ. Studies of two of the many NNE-SSW trending, steeply dipping shear zones in the GFTZ point to the critical role of strain related weakening processes in their formation. Evidence of strain-related weakening is shown at the outcrop scale in the solid-state transformation of migmatitic features, such as an increase in the aspect ratios of magmatic leucosomes. In both of these gradients, deformation resulted in significant grain size reduction and development of a mylonitic fabric in the amphibolite facies units. Although the units are relatively modally consistent across the zones they do show a slight reduction in hydrous minerals from low to high strain. Petrologic, geochemical, and microstructural analyses of these shear zones show that it is unlikely that any one strain weakening process could account for each of the km- scale strength heterogeneities observed in this region. Instead, we propose that these shear zones form as the result of several interacting processes. Preliminary U-Pb zircon ages show a series of events recorded by all the sheared rocks in the GFTZ at ca. 1.7 Ga and 1.4 Ga. We hypothesize that although the strain weakening of all the shear zones occurred during the Grenville Orogeny, its timing was not recorded, since shear zones did not achieve conditions favorable for zircon growth.