Geophysical observations show that there is a dramatic contrast in mantle properties between the western side of North America and the continental interior. In southwestern Canada, these observations indicate that the Cordillera mountain belt is characterized by a hot, thin lithosphere (60-70 km thick), whereas the Laurentian Craton to the east is much cooler and thicker (200-300 km). The transition in lithosphere thickness occurs approximately below the Rocky Mountain Trench, resulting in a >100 km subvertical lithosphere step. The origin of the thin Cordillera lithosphere and the lithosphere step is unclear. One hypothesis is that this structure is inherited from Cordillera accretion at >100 Ma. Alternately, it may have formed through more recent gravitational thinning of the Cordillera, possibly during a large-scale delamination event in the Eocene. Using geodynamic numerical models, we explore these competing ideas and demonstrate how they may be discriminated using surface observations. The models also show that regardless of how the step formed, the present-day thin lithosphere requires that the Cordillera mantle must be undergoing rapid small-scale convection, possibly associated with the adjacent Cascadia subduction zone. The mantle flow patterns may be influenced by the presence of the lithosphere step, which enhances the Cordillera-Craton thermal contrast; the Craton edge may also be undergoing deformation. The models highlight the range of mantle dynamics that can shape the continental lithosphere and create temporal variations in surface topography and magmatism.

*Refreshments: 10:30 AM - 11:00 AM (Atrium)