Tectonic Evolution of the Western Grenville Orogen: Insight from Numerical Models for Syn- and Post-Convergent Flow of Heterogeneous Crust

The western Grenville Orogen in Ontario displays across-strike variations in age, tectonic history, and protolith association suggesting variable pre-collision crustal strength. The Laurentian margin and accreted terranes were variably reworked at synorogenic depths of 25-35 km and temperatures of 700-900°C during the Ottawan orogeny. Deformation propagated from juvenile monocyclic rocks in the southeast into polycyclic rocks flanking the craton on the northwest. Thrusting in the Grenville Front Tectonic Zone during the terminal Rigolet orogenic phase overlapped with ductile extension in the orogenic core. Crustal-scale numerical models with variable lower crustal strength are used to investigate deformation of a simplified orogenic system in which a strong craton, flanked by progressively weaker terranes, collides with another continent. Middle and upper crust is laterally homogeneous and subject to melt-weakening. With progressive convergence, thickening, and heating, lower crust becomes decoupled from overlying crust, forming a ductile orogenic infrastructure beneath a stronger superstructure. Collision with strong external crust results in expulsion of weak lower crust from the orogenic core, creating allochthonous ductile nappes overlying a lower crustal indentor. When convergence is stopped, the model orogen undergoes gravitational spreading, leading to ductile thinning in the core and thrusting at the flanks. Discrete normal-sense ductile shear zones develop in melt-weakened middle crust just above the leading edge of the indentor. Comparison between model results and data from the Georgian Bay transect shows close correspondence between observed and predicted crustal architecture and PTt paths. This demonstrates that the models produce geologically realistic results, and offers a context for interpreting the tectonic evolution of the western Grenville orogen. In particular, the results provide an explanation for detachment and transport of the Parry Sound klippe, and suggest that late-stage GFTZ thrusting and coeval extension in the orogenic core reflect gravitational spreading following the end of tectonic convergence.