The structure and dynamics of the uppermost mantle of southwestern Canada from a joint analysis of geophysical observations

Geophysical imaging reveals significant changes in mantle properties from the Southern Canadian Cordillera to the Laurentian Craton in southwestern Canada. We examine mantle structure using shear wave velocity (V-S) from seismic tomography and electrical resistivity from magnetotellurics. Independent analyses of V-S and resistivity are poorly constrained because of the number of free parameters. To overcome these limitations, we conduct a joint analysis of V-S and resistivity to quantify temperature and olivine water content at 75-150 km depth, corresponding to the cordillera asthenosphere and craton mantle lithosphere. For the cordillera, there is a trade-off between temperature and water content; the observations are consistent with either warm, hydrated, and melt-free mantle (similar to 1,240 degrees C; similar to 1,600 ppm H/Si in olivine) or hotter, less hydrated mantle (similar to 1,370 degrees C; similar to 600 ppm H/Si) with some melt at 75 km depth. In contrast, the craton mantle lithosphere is similar to 350 degrees C cooler and drier (<300 ppm H/Si). Temperatures depend strongly on the seismic attenuation model. If this is known, the temperature uncertainty is <100 degrees C. There is significant uncertainty in olivine water content (>500 ppm H/Si), owing to observation uncertainties, the resistivity model, and mantle composition. Our results indicate that the cordillera asthenosphere has a low viscosity (10(19)-10(21) Pa s) and is susceptible to small-scale convection. Approximately below the Rocky Mountain Trench, there is a subvertical eastward increase in lithosphere thickness. The cratonic mantle lithosphere viscosity is 10(22)-10(24) Pa s and the western edge of the craton may be unstable, suggesting that the present-day geometry is a transient feature.