20 NORTH AMERICA/Precambrian Continental Nucleus island arcs or, at other times, as magmatic arcs on the growing margin of Laurentia These long-lived active margin processes led to progressive outward growth of and accretion to Laurentia, culminating with the final collisional history of the ca 1.19–0.98 Ga, high-grade, Grenville Orogen (Figure 1) Significant pulses of outward growth that can be recognized along various parts of the margin are the Yavapai (1.79–1.71 Ga) and Mazatzal (ca 1.70– 1.62 Ga) belts of the south-western United States (Figure 1) The latter is broadly coeval with the Labradorian event of north-eastern Laurentia (Figure 1), which involved accetion of a 1.70– 1.66 Ga outboard arc to the margin of Laurentia at about 1655 Ma The Labradorian Arc and a number of younger arcs (e.g., the Pinwarian Arc) were severely reworked during terminal Grenville collisions The latter involved final suturing of Laurentia to a southeastern landmass (Amazonia?) to form Rodinia During its long-lived Proterozoic active margin evolution, lasting from about 1.8 Ga to 1.0 Ga, Laurentia likely faced a ‘Pacific-type’ ocean to the southeast (present coordinates) Inboard of this margin, several major magmatic events suggest the upwelling of hot mantle underneath the growing continent Similar processes ultimately led to the rifting and breakup of Laurentia (as part of Rodinia), starting at about 780 Ma Important examples of mid-Proterozoic igneous events are several pulses of graniterhyolite magmatism concentrated in south-eastern Laurentia; the intrusion of the 1.3 Ga Nain plutonic suite in Labrador; the 1267 Ma giant Mackenzie dyke swarm; several more localised mafic dyke swarms; and the basalt-rhyolite volcanism of the Keewanawan event in the mid-continent rift (Figure 1) Glossary Nuna Earth’s first true supercontinent that formed by progressive amalgamation of numerous microplates and intervening island arc terranes towards the end of the Palaeoproterozoic era, from about 1.9 Ga to 1.7 Ga Sclavia Late Archaean ancestral landmass of the Slave Craton and allied cratons This landmass or supercraton amalgamated during Late Archaean orogenic activity, cratonized shortly after 2.6 Ga, and experienced rifting and break-up between 2.2 Ga and 2.0 Ga, spawning several microplates, including its type craton, the Slave Craton of the Canadian Shield Existence of this late Archaean supercraton is indicated by the Slave Craton being a cratonic fragment surrounded by rifted margins Superia Late Archaean ancestral landmass of the Superior Craton and allied cratons This landmass or supercraton amalgamated during Late Archaean orogenic activity, cratonized shortly between 2.68 Ga and 2.62 Ga, and experienced rifting and breakup starting at 2.48 Ga, spawning numerous microplates, including its type craton, the Superior Craton of the Canadian Shield Existence of this Late Archaean supercraton is indicated by the Superior craton being a cratonic fragment surrounded by rifted margins Supercraton One of several Late Archaean cratonic landmasses, not necessarily a single supercontinent, that on Palaeoproterozoic break-up spawned the present ensemble of ca 35 Archaean cratons with their rifted margins See Also Earth Structure and Origins Gondwanaland and Gondwana North America: Atlantic Margin; Continental Interior; Northern Appalachians; Northern Cordillera; Southern and Central Appalachians; Southern Cordillera Pangaea Plate Tectonics Precambrian: Overview Solar System: Jupiter, Saturn and Their Moons Further Reading Anderson DL (2002) How many plates? Geology 30: 411 414 Bleeker W (1990) New structural metamorphic constraints on Early Proterozoic oblique collision along the Thomp son nickel belt, northern Manitoba In: Lewry JF and Stauffer MR (eds.) The Early Proterozoic Trans Hudson orogen of North America Geological Association of Canada Special Paper 37, pp 57 74 Bleeker W (2002) Archaean tectonics: a review, with illus trations from the Slave craton In: Fowler CMR, Ebinger CJ, and Hawkesworth CJ (eds.) The early Earth: phys ical, chemical and biological development, Geological Society of London Special Publication 199, pp 151 181 UK: Bath Bleeker W (2003) The late Archean record: a puzzle in ca 35 pieces Lithos 71(2 4): 99 134 Buchan KL and Ernst RE (2004) Diabase dyke swarms and related units in Canada and adjacent regions Geological Survey of Canada Map No 2022A, scale 1:5 000 000, Ottawa, Canada Davies GF (1999) Dynamic Earth: plates, plumes and mantle convection Cambridge: Cambridge University Press de Wit MJ and Ashwal LD (1997) Greenstone Belts Oxford Monographs on Geology and Geophysics 35, Oxford: Oxford University Press Ernst RE and Buchan KL (2001) Large mafic magmatic events through time and links to mantle plume heads In: Ernst RE and Buchan KL (eds.) Mantle plumes: their identification through time, Geological Society of America, Special Paper 352, pp 483 575 Colorado, Boulder