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<p>Geologists broadly understand the 3-D building of Earth’s continents, but much less so the construction of their ancient, Archean nuclei, the cratons. Definition of terranes that play a pivotal role in reconstructions of Archean tectonic histories must consider the entire subcontinental lithosphere, using relevant observations of structures within both the crust and mantle. Multi-azimuthal receiver function analysis identified a regional Ps seismic discontinuity striking 065° and dipping 7° southeast within the mantle of the Superior craton. This strike roughly parallels that of (1) a previous recognized regional extensional fault striking 057° and dipping 18° southeast within the western Superior crust, (2) the axis of the turbiditic Quetico Basin, and (3) an apparent Moho keel. Here we jointly interpret these seismic features as a detachment or decollement with characteristics of an unconformity, low-angle thrust, and normal fault. The unconformity with quartz arenites characterizes the top of gneissic basement within the North Caribou terrane. Thrusting probably relates to the dominant phase of folding and horizontal shortening strain that occurred during the Kenoran crustal deformation, mineralization and peak metamorphism at 2.72–2.66&nbsp;Ga. This shortening is here related to the oblique convergence and underthrusting of a transitional shelf margin of the North Caribou superterrane and its adjacent oceanic lithosphere beneath the juvenile Abitibi terrane along a ‘flat slab subduction’ trajectory. Metal-rich turbidite sediments deposited on North Caribou terrane margins and its adjoining oceanic crust were underthrust to 80–120&nbsp;km depths, melted and rose within the Abitibi lithosphere as conductive carbonatitic fluids, enriching and concentrating gold and other economically important metals. Similar, unrecognized low-angle mantle lithospheric discontinuities may underlie many cratons globally and play equally significant roles in understanding tectonic and metallogenic histories.</p>