MANTLE DYNAMICS BELOW SOUTHWESTERN CANADA: SMALL-SCALE CONVECTION AND IMPLICATIONS FOR THE THIN CORDILLERA LITHOSPHERE (Invited Presentation)

The Southern Canadian Cordillera is characterized by an average elevation of ~1.1 km, sporadic volcanism, and thin crust and mantle lithosphere (~35 km and ~30 km, respectively). The origin of the thin lithosphere is debated. One idea is that it is inherited from Cordillera accretion, such that it has been in place since at least 100 Ma. Alternately, it may have been formed through rapid gravitational thinning via delamination in the Eocene. We use 2D numerical models to investigate these two hypotheses and examine the relationship between Cordillera lithosphere structure and the dynamics of the underlying mantle. The models show that even if there was an Eocene delamination event, subsequent conductive cooling would have caused lithosphere thickening over time. We conclude that regardless of when the thin lithosphere formed, the present-day thin lithosphere must be maintained by convection in the underlying mantle. Consistent with earlier work, our models show that mantle wedge corner flow driven by subduction of the adjacent Juan de Fuca Plate is insufficient to preserve the thin lithosphere. Instead, the Cordillera mantle must be undergoing small-scale convection. This requires that the sublithospheric mantle is weak, possibly due to extensive hydration. We also find that convection is affected by the presence of the Laurentian craton at the eastern side of the Cordillera. The cool, thick (>200 km) craton lithosphere induces edge-driven convection, which in turn enhances the thermal contrast between the Cordillera and craton lithospheres. The craton lithosphere step itself is an enigmatic feature, and its long-term preservation requires that the craton mantle lithosphere is strong (dry) with moderate chemical depletion. Our models provide insight into the subsurface dynamics of southwestern Canada. More broadly, they demonstrate that even small changes in the rheological properties of the mantle have a profound influence on lithosphere structure and surface observations such as topography and magmatism.