CAUSES OF METER-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 shear zones, often bounding minimally deformed blocks of mid to lower crustal material, indicate strength heterogeneity and strain localization. The location of shear zone development during orogenesis is dependent on the stress field, water content, and temperature and erosion patterns of each orogen. However, in order for strain to localize, a strain-dependent weakening mechanism must form a positive feedback. 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 preexisting property differences, external perturbations, or strain induced textures. The role these mechanisms play in mid to lower crustal strain localization during orogenesis is not fully understood. This study is the first of a multi-step look at strain localization in the Grenville Front Tectonic Zone in Ontario, Canada, as a proxy for deep orogenic deformation. This step focuses on a ~10m shear zone located at the north of Georgian Bay, Ontario. The shear zone propagated through megacrystic monzonite where igneous textures are transformed across a strain gradient. Deformation resulted in significant grain size reduction and development of a mylonitic fabric in the amphibolite facies. Unlike the larger shear zones of the structurally higher Central Gneiss Belt that run NE-SW, this shear zone strikes E-W and most likely represents a late stage Grenvillian event. Our preliminary interpretation is that localization resulted from a combination of reactions and textural change.