NORTH AMERICA/Precambrian Continental Nucleus 11 Figure North America’s first diamond mine, the Ekati Mine of the central Slave Craton Main picture shows the flat barren lands of the central Slave Craton, with several pipe like kimberlite bodies being excavated Inset (upper left) shows a close up of one of the partially excavated pipes Several mm size gem quality diamonds are shown on upper right (photos provided by David Snyder and Grant Lockhart) deformation and modification of the Precambrian crust and lithosphere, which locally extends up to 1000 km inboard of the margin The eastern margin of the North American plate is the slow-spreading mid-Atlantic ridge, which runs north through Iceland and to the east of Greenland into the Arctic basin Hence, Greenland, almost entirely underlain by Precambrian crust, is geologically part of North America and, more specifically, part of its Precambrian core, Laurentia With progressive northward opening of the North Atlantic rift, Greenland started to rift apart from the main part of Laurentia at about 80 Ma, opening up the Labrador Sea This rifting was subsequently aborted in favour of propagation of the mid-Atlantic ridge to east of Greenland, along the present trace of the ridge Here, ridge propagation split off a fragment of Precambrian crust from Laurentia, which remained attached to the Eurasian plate and now underlies parts of Scotland, Ireland, and the adjacent continental shelf This Precambrian sliver, known as the Lewisian Gneiss Complex, is equally part of Laurentia Other Precambrian crustal elements may have rifted and drifted off Laurentia with the opening of a proto-Arctic basin (i.e., prior to the Innutian Orogeny, see Figure 1), but here relationships are presently less well understood The stable Precambrian lithosphere underlying much of North America responds to far-field boundary forces by gentle flexing and arching As a consequence, Precambrian crust in the north-central parts of the continent is gently arched up, giving rise to the physiographic province of the Canadian Shield The latter is characterized by scattered rocky outcrops of generally low relief (Figure 2), although, locally, rift of the Burwash Formation, Slave Craton Turbidites such as these are typical for the uppermost units of Archaean greenstone belts Here they have been folded into regional mushroom interference patterns by two upright fold generations (E) Shallow dipping strata of the ca 2.0 Ga Coronation Supergroup onlapping onto peneplaned Archaean basement (flat foreground), western margin of Slave Craton These strata represent some of the first true passive margin sequences in the geological record (F) Archaean Palaeoproterozoic unconformity, metamorphosed to upper amphibolite facies, along the highly deformed western margin of the Superior Craton Archaean gneissic layering (right) truncated by primary layering and foliation in basal quartzitic schists of the ca 2.0 Ga Ospwagan Group (left) (G) Thompson Mine, exploiting one of the largest komatiite associated nickel deposits in the world, hosted by the highly deformed Palaeoproterozoic Ospwagan Group, along the western margin of the Superior Craton (H) Proterozoic mafic dykes of the 1267 Ma giant radiating Mackenzie swarm, northern Canadian Shield (I) Tilted strata of Proterozoic fluvial sandstones overlying the ca 1.8 Ga crustal collage of Laurentia; 100 m high cliffs along the Brock River, North west Territories Deposits such as these form widespread overlap assemblages as early as 1.75 Ga