CAUSES OF KM-SCALE STRAIN LOCALIZATION AT THE MARGIN OF THE GRENVILLE FRONT AND CENTRAL GNEISS BELT, ONTARIO, CANADA

The presence of shear zones in an orogenic belt influences patterns of heat flow, kinematics, metamorphism, and topographic variance. Such discrete features, often bounding minimally deformed blocks of mid to lower crustal material, indicate strength heterogeneity and strain localization. In order for orogenic-scale shear zones to form, a positive feedback cycle must develop among strain-dependent weakening mechanisms. During this study we look for evidence of five mechanisms: (1) thermal perturbation; (2) incorporating, expelling, or crystallizing of melt; (3) mechanical change due to textural evolution; (4) incorporation or loss of fluids; and (5) mineralogical change due to metamorphic reactions. These heterogeneities could be due to differences in a preexisting or intrinsic property, an external perturbation, or strain induced textural change. The role these five mechanisms play in mid to lower crustal strain localization during orogenesis is not fully understood. This study is one of a multi-step investigation of strain localization in the Grenville Front Tectonic Zone in Ontario, Canada, as a proxy for deep orogenic deformation. This study focuses on a ~4 km wide megacrystic quartz-monzonite unit bounded by shear zones to the east and west. The transformation of igneous textures, such as the aspect ratio of magmatic enclaves, illustrate that the strain gradient of the eastern shear zone is much broader than that of the western shear zone. In both gradients, deformation resulted in significant grain size reduction and development of a mylonitic fabric in the amphibolite facies. This study documents microstructural changes across both gradients in an effort to determine localization mechanisms associated with each. Preliminary results show a general positive correlation between strain and modal percent of mafic minerals with a marked modal increase in amphibole in the shear zones, indicating that metamorphism played a role in localization.