QUANTIFYING EFFECTIVE VISCOSITIES IN THE PARRY SOUND DOMAIN OF THE GRENVILLE PROVINCE, SOUTHEASTERN ONTARIO

Formation of shear zones decreases the bulk strength of the region in which the shear zones develop. If operative over a sufficiently large region, this strength decrease can have a first-order effect on an orogen. Therefore, in order to develop more accurate models of orogen development, we must better quantify the strength changes that take place as rocks deform within that orogen. We have developed a numerical method utilizing microstructure and experimentally-determined flow laws to determine the effective viscosity of naturally-deformed rocks. This is an improvement upon analytical solutions that must account for the microstructure using a numerical parameter. Nevertheless, where possible, we have compared our numerical method against analytical solutions and found it accurate. We analyzed several quartzofeldspathic samples from the Parry Sound domain, which lies at the southwestern terminus of the Canadian Grenville Province. The domain comprises intact and retrograded granulite that was deformed in both the granulite and, at its margins, amphibolite facies. Granulite facies deformation was distributed, with few sites of strain localization outside of a km-scale domain-marginal shear zone. In places the amphibolite-facies deformation is penetrative, but much of it is concentrated in mm- to m-scale shear zones associated with pegmatites and/or fluid access. Our numerical mechanical analysis yields the following primary observations. (1) Bulk effective viscosity lies between the iso-stress and iso-strain rate bounds and is dependent on microstructure. (2) Bulk effective viscosity is strongly sensitive to temperature and water fugacity, so accurate estimation of these variables is critical for accurate viscosity quantification. (3) The degree to which shear zones are weaker than the original rock is highly variable, ranging from less than a factor of two to orders of magnitude depending on the mechanism of shear zone formation. These results are not surprising, but they reinforce both the challenge and importance of using naturally-deformed rocks to constrain geodynamic models.