The Slave Province is a small Archaean fragment (600 × 400 km), bounded by Proterozoic mobile belts, in Arctic Canada. It hosts major diamond deposits in Paleozoic to Miocene kimberlites, which now are being mined. Detailed geochemical studies of mantle-derived xenoliths and xenocrysts have defined an unusual two-layered lithospheric mantle beneath the craton: a shallow, ultradepleted (low in basaltic components Ca, Al, Fe), olivine-rich layer interpreted commonly as oceanic or arc-related lithosphere emplaced during early tectonics, and a deeper, less depleted layer, interpreted as a frozen Archaean plume head. We have mapped variations in the relationship between gravity and topography across the Slave Province in terms of the effective elastic thickness (Te). Our results show that the northern part of the craton is characterised by a relatively weak lithosphere (Te < 25 km), probably related to compositional changes due to the intrusion of the Mackenzie Plume (approximately 1270 Ma) that modified the subcontinental lithospheric mantle. Areas of weak lithosphere correlate with areas where heat flow values are relatively high (46?56 mW/m²) within the Slave Province. The strongest lithosphere is found in the eastern part of the craton (Te > 56 km). A N-S zone of low Te along the middle of the craton coincides approximately with the surface expression of the suture between the ancient continental block making up the western part of the craton, and the younger accreted terranes that make up the eastern part. The zone of maximum Te gradient coincides with an area of strongly conductive upper mantle, and with the Nd-isotope line which defines a major crustal boundary at depth. The Te gradient probably marks the deep expression of the major steep suture, and this lithosphere-scale structure has apparently guided kimberlite intrusion over approximately 400 Ma.