Investigating mantle dynamics and lithospheric evolution in the Southern Canadian Cordillera: Insights from numerical modeling of the Cascadia subduction zone

The subduction of the Juan de Fuca Plate at the Cascadia subduction zone significantly influences the mantle dynamics and the structure of the overlying North American Plate. In southwest Canada, the Cordillera lithosphere is thin (60-70 km) with high surface heat flow, low mantle seismic velocity, and low mantle electrical resistivity for ~500 km inboard of the subduction zone. Magmatism and geological observations suggest that the Cordillera lithosphere has been thin for at least 30 Myr. The eastern limit of thin lithosphere approximately underlies the Rocky Mountain Trench. East of this, the Laurentian Craton is thick (>200 km), and recent seismic data show that the Cordillera Craton boundary is marked by subvertical to west-dipping lithospheric step.

In this study, we investigate the effects of subduction and the lithosphere step on mantle dynamics and the evolution of the Cordillera lithosphere over the last 40 Myr. We use 2D thermal-mechanical models of ocean-continent subduction, where the domain is 3000 km wide and 660 km deep. We first test models where subduction of the Juan de Fuca plate occurs below a 60 km thick continent with no lateral variations, representing the Canadian Cordillera. These models show that if the mantle rheology is based on dry olivine, it has a relatively high viscosity, and the mantle flow field is dominated by subduction-driven corner flow. This results in a slow thickening of the backarc continental mantle lithosphere to nearly 90 km within 40 Myr. If a weaker (more hydrated) olivine rheology is used for the mantle, backarc thickening is inhibited by the development of small-scale convection (SSC). To maintain a ~65 km lithosphere, our models predict that the backarc mantle must be hydrated and weak (viscosity of 10¹⁸ – 10¹⁹ Pa s). In the second set of models, 200 km thick Craton lithosphere is added to the models. The presence of the lithosphere step at the Cordillera-Craton boundary induces edge-driven convection (EDC), which is enhanced for a hydrated mantle or weak craton mantle lithosphere. We find that EDC had only a secondary influence on the Cordillera lithosphere in the arc and central back arc regions, but EDC may be important for maintaining a sharp thermal contrast between the Cordillera and Craton.

In the final set of models, we investigate the effects of subduction termination on mantle dynamics, using the model structure that includes the Craton lithosphere step. After plate convergence ceases, SSC and EDC continue for tens of millions of years, and these slow the cooling and thickening of the continent. However, even with a hydrated mantle, the Cordillera thickens to ~80 km after 40 Myr. This suggests that the central Canadian Cordillera lithosphere (north of the current subduction zone), where subduction terminated in the Eocene, may be somewhat cooler than the modern backarc to the south. Future work will focus on how the slab edge geometry of the Juan de Fuca plate influences mantle flow patterns and lithospheric structure in the Canadian Cordillera.