THERMOMECHANICAL ANALYSIS OF AN EXTENSIONAL SHEAR ZONE USING MICROSTRUCTURAL ANALYSIS, THERMOMETRY, AND GEOCHRONOLOGY

At the latitude of the Thor-Odin dome, British Columbia, the 1 km thick, east-dipping ductile Columbia River detachment defines the eastern margin of the Shuswap metamorphic core complex, separates Tertiary migmatites from upper crust, and localizes deformation in a thick quartzite layer that contains abundant synkinematic white mica. Therefore, this extensional detachment offers a unique opportunity to study the mechanics of shearing and the propagation and localization of strain. We conducted microstructural analysis of the quartzite mylonite, evaluated the temperature of deformation using stable isotope thermometry based on quartz-muscovite oxygen isotope exchange, and dated the microstructures using argon geochronology in the top 200 m of this detachment zone. Quartz deformation took place under the dislocation regimes 1 and 2 of Hirth and Tullis (1992) involving the development of strong crystallographic fabrics, deformation lamellae, subgrain rotation, and limited grain boundary migration; this high-stress microstructure persists over the whole section of mylonite. In contrast, deformation temperatures decrease from the top of the detachment downward from >500°C to ~400°C. Argon geochronology of synkinematic white mica in 5 samples distributed over the top 200 m of mylonite shows ages decreasing systematically from 49.0 Ma to 47.9 Ma. The coupled geochronology and thermometry data suggest that deformation migrated downward from the contact with the hanging wall and that the preserved footwall microstructures where progressively “frozen“ under decreasing temperature; this high temperature gradient requires heat loss that was likely accommodated by convective fluid flow. A possible explanation for the constant high-stress microstructure is that flow stress reached a critical threshold for viscous flow and was buffered by the interplay of strain rate and temperature; high strain rates during early high-temperature deformation produced similar microstructures as lower strain rates later in the deformation history, as cooling by exhumation and convective fluid flow reduced deformation temperatures.