Seismic evidence for lower-mantle heat supply beneath the Cordilleran slab window in western Canada

Divergent plate boundaries drive magmatism and plate spreading, yet their role in whole-mantle convection remains unclear, as most systems are submarine. When spreading centres subduct, slab windows open pathways for asthenospheric and potentially deeper mantle upwelling. Here we use 20 years of teleseismic P-to-S receiver functions from 429 stations to map the 410 and 660 km discontinuities beneath western Canada. Common-conversion-point stacking reveals coherent double depressions across a slab window formed by the subducted Resurrection/Farallon Ridge system, in contrast to the adjacent craton. Mineral physics and seismic tomography constraints indicate mantle transition zone temperature anomalies of 100-300 K that follow the slab-window geometry. Monte Carlo estimates of constrained whole-mantle geotherms further suggest a warmer-than-average lower mantle. Together, these observations suggest that the fossil slab window provides a conduit for lower-mantle heat into the upper mantle and lithosphere, contributing to long-term uplift of the Canadian Cordillera.