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

Divergent plate boundaries are major sites of magmatism and plate creation, yet their role in whole-mantle convection remains poorly constrained because most mid-ocean ridges lie beneath the oceans. When spreading centres are subducted, slab windows open pathways for asthenospheric and potentially deeper mantle upwelling. Here we use 20 years of teleseismic P-to-S receiver function data from 429 broadband stations to image the 410 km and 660 km discontinuities beneath western Canada. Common-conversion-point stacking reveals coherent depressions of both d410 and d660 across a well-documented slab window generated by subduction of the Resurrection/Farallon ridge, in sharp contrast to the adjacent craton. Using mineral-physics constraints on Clapeyron slopes and an ensemble of global and regional tomography models, we infer MTZ temperature anomalies of order 100–300 K and show that these correlate closely with the reconstructed slab-window geometry. A representative P-wave model (DETOX-P01) further indicates a sharp transition from this hot MTZ to a colder, high-velocity body in the mid–lower mantle, consistent with a detached segment of the subducted Resurrection plate embedded in a warmer-than-average deep mantle. Monte Carlo geotherm calculations suggest that the lower mantle beneath the Cordillera is on average ~150–200 K hotter than global reference profiles. Together, these observations suggest that a fossil divergent boundary, expressed as a slab window, provides a conduit for lower-mantle heat into the upper mantle and lithosphere, and that this deep thermal input contributes to the long-term uplift of the Canadian Cordillera.