Fig. 1: Study area and illustration of the seismic discontinuities discussed in this study.

a Global map showing the age distribution of the oceanic lithosphere with data from ref. ¹². Our study area (star), the Cascadia subduction zone, has a young and warm slab, in contrast to the old and cold slab in the other two subduction zones (squares; Japan and New Zealand) where evidence for sub-slab melt-rich Lithosphere-Asthenosphere Boundary (LAB) has been reported^1,8. b Regional tectonic setting of our study area and locations of seismic stations used in this study. Dashed lines show depth to the surface of the slab from Slab2.0 model⁵⁹. The maps were generated using Generic Mapping Tools⁶⁰, with topography and bathymetry data from the Global Multi-Resolution Topography Synthesis⁶¹. c Cartoon showing how the LAB beneath the Juan de Fuca plate (plate-LAB, based on ref. ¹⁴) may continue after subduction to give rise to the slab-LAB. Question marks indicate that the origination of melts for the LAB remains under debate, and the depth extent of the slab-LAB requires further investigation.