Seismic imaging beneath Cascadia shows shallow mantle flow patterns guide lower mantle upwellings

The mantle transition zone plays an important role in modulating material transport between the upper mantle and the lower mantle. Constraining this transport is essential for understanding geochemical reservoirs, hydration cycles, and the evolution of the Earth. Slabs and hotspots are assumed to be the dominant locations of transport. However, the degree of material transport in other areas is debated. We apply P-to-S receiver functions to an amphibious dataset from Cascadia to image the mantle transition zone discontinuities beneath mid-ocean ridges, a hotspot, and a subduction zone. We find a mantle transition zone thinned by 10 ± 6 km beneath the ridges and by 8 ± 4 km behind the slab, closely resembling the 660 discontinuity topography. Depressions on the 410 discontinuity are smaller, 5 ± 2 km on average, focused in the north and the south and accompanied by supra-410 discontinuity melt phases. The depressions occur away from locations of uplifted 660 discontinuity, but near slow seismic velocity anomalies imaged in the upper mantle. This suggests lower mantle upwellings occur beneath ridges and slabs but stall in the transition zone, with upper mantle convection determining upward material transport from the transition zone. Therefore, upper mantle dynamics play a larger role in determining transfer than typically assumed.