Geophysical constraints on the Cascadia backarc mantle structure, southwestern Canada

The southern Canadian Cordillera (CC) lies in the backarc region of the Cascadia subduction zone. It is bounded on the west by the Cascade volcanic arc and on the east by the North American Craton (NAC). Geophysical observations show that the CC-NAC boundary is marked by a dramatic contrast in mantle physical properties. The CC mantle is characterized by high surface heat flow, low shear wave velocities (V_S), and low electrical resistivity, suggesting high mantle temperatures. It has been proposed that the high CC temperatures are maintained by vigorous convection of the backarc mantle, which has a low viscosity owing to subduction-related hydration. The eastern limit of the backarc (the NAC) has been proposed to be both cool and dry.

In this study, we examine the mantle structure of CC and NAC using a joint analysis of seismic shear-wave tomography (model SL2013NA) and magnetotelluric measurements of electrical resistivity, as both observations are sensitive to temperature and water content. We find that the average CC temperatures are 1200-1300 ^oC at 75-150km depth, whereas those for the NAC are 800-950 ^oC. In addition, both data sets indicate that the thermal boundary between the CC and NAC lies beneath the Rocky Mountain Trench. The analysis also shows that water content in the upper mantle appears to be similar for both regions. Rather, the observations can be explained by temperature variations in a moderately dry mantle (<= 3000 ppm H/Si). Future work will examine the effects of anisotropy and hydrous minerals on the analysis, and will consider the implications for the CC viscosity structure and mantle dynamics.