Synconvergent ductile flow in variable-strength continental crust: Numerical models with application to the western Grenville orogen

We present results from numerical models for a convergent orogen with laterally variable lower crustal strength, representing a simplified orogenic system in which a strong craton, flanked by progressively weaker terranes, collides with another continent. With progressive convergence, crustal thickening, and thermal relaxation, lower crust becomes decoupled from upper and middle crust, forming a ductile orogenic infrastructure beneath a stronger superstructure. Collision with strong external crust results in uplift and expulsion of ductile nappes from the orogenic core, creating allochthonous terranes overlying a lower crustal indentor. The extent of transport and exhumation of lower crustal nappes over the indentor reflects the amount of convergence and the erosion rate. The western Grenville orogen displays across-strike variations in age, tectonic history, and protolith association, suggesting a systematic variation in precollision crustal strength. The Laurentian craton, margin, and accreted terranes were variably reworked at synorogenic depths of 25-35 km during the Ottawan orogeny. Deformation propagated from younger monocyclic rocks in the southeast into older polycyclic rocks flanking the craton on the northwest. A comparison between numerical model results and crustal-scale cross sections from the Georgian Bay and Montreal-Val d'Or transects shows close correspondence between crustal structure and model geometry. This indicates that the models produce geologically realistic results and provides a context for interpreting the tectonic evolution of the western Grenville orogen. Contrasts between the results of homogeneous channel flow models and the present ductile nappe models suggest that the effects of different styles of ductile flow can be distinguished in the geologic record.