Lithoprobe deep seismic reflection data from the northwestern Canadian Shield provide images of structures to the base of the lithosphere between the Archean Slave Province on the east and the Cordillera on the west. Mantle reflections dip eastward from the lower crust to about 100 km depth beneath the ~1.88-1.84 Ga Great Bear magmatic arc and almost certainly represent a subduction surface associated with arc development. Where the mantle reflections flatten into the lower crust, they merge with prominent west dipping crustal reflections thus delineating a lithospheric-scale wedge that formed as a result of Proterozoic plate convergence between the Slave craton on the east and the ~1.84 Ga Fort Simpson terrane on the west. The crust throughout the 725 km survey is highly reflective, and the Moho remains at a constant level between about 11.0 and 12.0 s beneath both Archean craton and Proterozoic accreted rocks; the most significant Moho deflection occurs beneath the ~1.8-0.7 Ga Fort Simpson basin where it appears to rise to about 9.0 s (about 27 km). Within the crust of the Slave Province, east dipping, low-angle reflections near the surface beneath the Yellowknife basin may be shallow detachments, and lower crustal geometry is consistent with low-angle imbrication during Late Archean (~2.65 Ga) tectonism associated with development of the Yellowknife basin volcanic rocks. West of the Slave craton, accreted Proterozoic crust is characterized by gently folded upper crustal layers overlying apparent thrust duplexes above detachment surfaces that flatten near the Moho. These lower crustal rocks were likely inserted as a tectonic wedge above the Moho and beneath the Slave Province during the contractional phase (~1.90-1.88 Ga Calderian orogeny) of the Wopmay orogen. On the western end of the profile, the Fort Simpson extensional basin has up to 20--25 km of Proterozoic (meta) sedimentary rocks and sills(?) that are younger than 1.84 Ga. ¿ 1999 American Geophysical Union