SYN- AND POST-CONVERGENT DEFORMATION OF HETEROGENEOUS CRUST: NUMERICAL MODELS WITH APPLICATION TO THE WESTERN GRENVILLE OROGEN

Crustal-scale numerical models including systematic lateral changes in 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. The western Grenville orogen 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 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 phase of the orogeny overlapped with ductile extension in the orogenic core. Comparison between numerical model results and crustal-scale cross-sections from the Georgian Bay transect shows close correspondence between observed and predicted geometries and PTt paths. This demonstrates that the models produce geologically realistic results, and provides a context for interpreting the tectonic evolution of the western Grenville orogen. Contrasts between results from homogeneous (Himalayan-style) channel flow models and those with lateral strength variations suggest that the heterogeneity introduced by collision with strong external (cratonic) crust, common during orogenesis, can exert a fundamental control on the style of ductile flow in orogens, with effects that can be distinguished in the geologic record.