Lithospheric thickness and the backarc-craton lithosphere step in southwestern Canada imaged by a 3-D magnetotelluric study

A 3-D electrical resistivity model extending to a depth of 400 km in southwestern Canada and the northwestern United States was created by inverting long-period magnetotelluric (MT) data collected at 331 locations. Detailed analyses of this resistivity model, sensitivity tests including synthetic 3-D MT inversions, and comparisons with seismic velocity models were used to investigate the depth and geometry of the lithosphere-asthenosphere boundary (LAB) and the transition in mantle structure from the Canadian Cordillera to the North American craton. Resistivity increases from west to east, indicating an eastward increase in LAB depth. The model also shows that the LAB dips northward in southern-to-central Alberta, with lithospheric thickness increasing from ~150 km (at 49–50N) to 250 km or more (at 53–54N). A conceptual model was created, including a backarc-craton lithosphere step (BCLS) located beneath the southern Omineca belt, somewhere between the western boundary of the Kootenay Arc (WKA) and the 87Sr/86Sr = 0.706 isopleth. This is 90–200 km west of the southern Rocky Mountain Trench, where earlier studies placed the western edge of the thick craton based on surface heat flow, seismic velocities, and mantle xenoliths. This difference in location could reflect limitations in the resolution of the various methods or complex present-day mantle dynamics at the subvertical lithosphere step.