Numerical models of Cretaceous continental collision and slab breakoff dynamics in western Canada

Abstract

ABSTRACT The North American Cordillera is generally interpreted as a result of the long-lived, east-dipping subduction at the western margin of the North American plate. However, the east-dipping subduction seems problematic for explaining some of the geological features in the Cordillera such as large volume back-arc magmatism. Recent studies suggested that westward subduction of a now-consumed oceanic plate during the Cretaceous could explain these debated geological features. The evidence includes petrological and geochemical variations in magmatism, the presence of ophiolite that indicates tectonic sutures between the Cordillera and Craton, and seismic tomography images showing high-velocity bodies within the underlying convecting mantle that are interpreted as slab remnants from the westward subduction. Here we use 2-D upper mantle-scale numerical models to investigate the dynamics associated with westward subduction and Cordillera-Craton collision. The models demonstrate the controls on slab breakoff (remnant) following collision including: (1) oceanic and continental mantle lithosphere strength, (2) variations in density (eclogitization of continental lower crust and cratonic mantle lithosphere density), and (3) convergence rate. Our preferred model has a relatively weak mantle lithosphere, eclogitization of the lower continental crust, cratonic mantle lithosphere density of 3250 kg/m3, and a convergence rate of 5 cm/yr. It shows that collision and slab breakoff result in an ~2 km increase in surface elevation of the Cordilleran region west of the suture as the dense oceanic plate detaches. The surface also shows a foreland geometry that extends >1000 km east of the suture with ~4 km of subsidence relative to the adjacent Cordillera.