THE 4D STRUCTURE OF THE CORDILLERA-CRATON TRANSITION IN WESTERN CANADA (Invited Presentation)

Western Canada can broadly be divided into two distinct geological regimes: the high-elevation and juvenile Canadian Cordillera orogen in the west and the low-elevation ancient craton in the east. The two regions exhibit a marked contrast in geophysical properties, including surface heat flow, mantle seismic velocity, effective elastic thickness and mantle electrical resistivity. Collectively, these observations indicate an eastward increase in lithosphere thickness from the hot Cordillera (60-70 km thick) to the cooler craton (>200 km thick).

Here, we examine the 3D structure and time-dependent dynamics of the gradient in lithosphere thickness at the Cordillera-craton boundary. Recent high-resolution seismic tomography images show that the transition from thin to thick lithosphere occurs over a horizontal distance of <100 km, approximately coinciding with the Rocky Mountain Trench - Tintina Fault system at the surface. North of ~55N, the western edge of the craton lithosphere is sub-vertical to eastward dipping; south of this, the boundary is sub-vertical to westward dipping, with a clear westward dip (i.e., toward the Cordillera) south of ~52N.

Thermal-mechanical models are used to examine the dynamical evolution of a step-change in lithosphere thickness at the craton margin. The primary controls on the stability of the lithosphere step are the rheology and compositional density of the craton mantle lithosphere. If the craton mantle follows laboratory olivine rheological laws, a high gradient in lithosphere thickness can only be maintained if the craton is dry (thus strong) with moderate chemical depletion; otherwise the craton margin is rapidly eroded through edge-driven convection. Minor gravitational instabilities and/or shearing of the craton lithosphere by regional mantle flow create transient variations in the dip direction of the lithosphere step, suggesting that the observed along-strike changes in the craton margin geometry may be short-lived (<50 Ma) features.