PARAMETERIZATION OF STRAIN LOCALIZATION USING GRANITIC SHEAR ZONES OF THE GRENVILLE FRONT TECTONIC ZONE, ONTARIO, CANADA; WHICH FACTORS SHOULD WE CONSIDER?

Identifying the triggers of shear zone formation is increasingly important as geodynamic models become more computationally robust and sophisticated. Many numerical models assume that a single mechanism is sufficient to create a strength heterogeneity that leads to shear zone formation, yet a detailed look at nearly any shear zone indicates that parameterization of weakening is not usually so straightforward. Here, we present chemical and microstructural data from several granitic shear zones of the Ontario segment of the Grenville Front Tectonic Zone (GFTZ) indicating that several mechanisms cooperated to sustain strain localization. We evaluate the role of environmental factors (stress, temperature, fluids) and rock properties (grain size, modal distribution, morphology) and find that feedback loops likely perpetuated strain. For example, a stress heterogeneity led to strain induced grain size reduction, allowing for fluid pumping, which in turn facilitated the growth of hydrous minerals and/or affected the chemistries of the existing minerals, further weakening the area and continuing strain localization. However, whether feedback loops occurred or not, abundant evidence from the strain gradients of granitic shear zones within the GFTZ indicates that several weakening mechanisms were active during strain localization and that a single parameter is insufficient to capture the rheological change.