LITHOPROBE seismic reflection data acquired in 1994 within the Trans-Hudson orogen include a 180 km profile that crosses the boundary between a collapsed marginal basin (Kisseynew domain) and an older volcanic-plutonic arc terrane (Lynn Lake belt). The seismic data image a prominent band of northeast-dipping anastomosing reflections that extend to 10 s two-way traveltime from the surface location of a geological boundary where interpreted oceanic crust is preserved, and separate thick crust (16 s) of the Kisseynew domain from the abruptly thinner crust (12 s) of the Lynn Lake belt. Highly reflective lower crust (10–16 s) of the Kisseynew domain dips gently northward and extends laterally for at least 40 km beneath the adjacent Lynn Lake belt in a manner reminiscent of the western Alps. Complementary gravity modeling shows that the thickness of metasedimentary rocks within the Kisseynew domain is 20 ± 3 km. The seismic image is interpreted as portraying a crustal-scale collisional suture that formed as the end result of northward subduction of backarc basin crust (ancestral Kisseynew basin floor), and subsequent contraction and thrusting of the remnant Kisseynew basin fill under the margin of the Lynn Lake belt.