Lithoprobe seismic-reflection profiles in the Western Canada Sedimentary Basin reveal a prominent band of dipping reflections that appear to extend into the upper mantle. The NNE strike of these reflections, determined from intersecting profiles, is parallel to the trend of the ca. 1.8 Ga Rimbey domain, an overlying magmatic belt. Receiver-function analysis of teleseismic data recorded at a nearby site shows evidence for a low-velocity zone (LVZ) beneath the Rimbey domain between 13 and 16 km depth. The unusually low S-wave velocity of the LVZ (2.9¿0.2 km/s) suggests a composition similar to serpentinized peridotites that occur in ophiolites. Taken together, our data support a model of Proterozoic subduction beneath the Hearne province. We interpret the dipping reflections observed on the seismic profiles as a shear zone that developed during subduction of a SE-dipping slab, and the LVZ as a fragment of oceanic lithosphere that was obducted to the overriding plate. ¿ American Geophysical Union 1996