LITHOSPHERE STRUCTURE AND MANTLE DYNAMICS AT THE WESTERN EDGE OF THE LAURENTIAN CRATON (Invited Presentation)

The Laurentian Craton forms the ancient core of the North American landmass. In western Canada, the Craton is located immediately east of the juvenile terranes that make up the Cordilleran mountain belt. Geophysical observations, including seismic tomography, magnetotellurics and surface heat flow, indicate that the Craton lithosphere is cool and thick (>200 km), whereas the Cordilleran lithosphere is much hotter and thinner (50-70 km). Recent high-resolution seismic tomography studies show that the transition in lithosphere thickness occurs over a horizontal distance of 50-100 km below the Rocky Mountain Trench – Tintina Fault system. Furthermore, the geometry of the lithosphere step varies from subvertical-to-east-dipping north of 54°N to west-dipping further south.

Here, we use two-dimensional numerical models to investigate the factors that control the lithosphere structure at the Cordillera-Craton boundary. Models start with a vertical step in lithosphere thickness. Similar to earlier studies, the models show that such a structure is gravitationally unstable, and if the mantle is hydrated, the step undergoes erosion through either lateral spreading or downwelling, depending on its composition. In both cases, the resulting Craton margin has a low-angle dip toward the Craton interior. A subvertical lithosphere step persists for 50 Ma or longer only if the lower part of the Craton lithosphere is both dry and chemically depleted. However, even under these conditions, the Craton boundary geometry is modified on timescales of 10-50 Ma through shearing by regional mantle flow. Conversion of the numerical models to seismic velocity demonstrates that the evolving structure of the craton margin may be resolvable in tomography studies. We conclude that the present-day geometry of the western edge of the Laurentian craton reflects its initial structure and ongoing modification by internal gravitational instability and flow of the surrounding mantle.