IMPLICATIONS OF AN ABRUPT CRATON EDGE: CONDITIONS FOR EXTRUSION OF THE CRATONIC LITHOSPHERE

Mobile belts surrounding cratons are thought to have shielded them from erosion contributing to their longevity, but can lithospheric instabilities within the mobile belt induce cratonic deformation? In the southern Canadian Cordillera, a profound east-west transition from cold, thick lithosphere beneath the North American craton to hot, thin lithosphere in the Cordilleran backarc environment is inferred to be roughly coincident with the physiographically defined Rocky Mountain trench and the reconstructed Laurentian margin. This step-like boundary also juxtaposes domains of contrasting rheologic strength, heatflow and buoyancy. Geodynamic models of this transition have shown vigorous edge driven convection can erode the craton boundary. However, recent work has shown that, in some cases, this lateral boundary can result in the extrusion of cratonic mantle lithosphere towards the mobile belt. Seismic tomographic images seem to support this and shows cratonic mantle lithosphere dipping toward the mobile belt. There is compelling evidence that autochthonous mantle lithosphere beneath the former Proterozoic passive margin underwent some form of convective removal to create the present-day Cordilleran backarc (mobile belt). This process has been geodynamically modeled and tied to heatflow and uplift observations. Any proposed mechanism for mantle lithosphere thinning would reduce the integrated strength of the unstable mantle lithosphere and change its resistance to lateral flow. Here we build on the concept of lateral density contrasts driving flow using visco-plastic thermo-mechanical numerical models to evaluate the implications of the removal of the mobile belt mantle lithosphere on the cratonic keel. We find that when the cratonic keel is buoyant, instabilities in the adjacent lithosphere allow the craton to advance.