North America is an old continent. Whereas South America and Africa were not assembled until 0.7 Ga, and the assembly of Eurasia began at 0.3 Ga, most of the North American craton has been coherent since 1.7 Ga. This craton, known as Laurentia, included Greenland and northwest Scotland until their partial separation in the Late Cretaceous. This chapter describes the constituents of Laurentia, their aggregation in the Early Proterozoic (Table 1), and subsequent adventures of the craton until the rifting events at the end of the Proterozoic, which gave the continent approximately its present shape.
Radiogenic isotopic data from the Precambrian shield, and from inliers and subsurface samples on the platform indicate that about 55 percent of the area of the craton separated from the mantle in the Archean and about 45 percent in the Proterozoic (Fig. 1). In this regard, the shield is not representative of the craton as a whole, being strongly biased in favor of Archean crust (Fig. 2). Conversely, most of the Proterozoic crust underlies the Phanerozoic sedimentary veneer of the southern interior platform.
The Archean protocraton of Laurentia is an aggregate of seven former microcontinents (Fig. 1): the familiar Superior, Wyoming, Slave, and Nain (North Atlantic) provinces, and the newly recognized Hearne, Rae, and Burwell provinces (formerly parts of the composite Churchill province). The Rae province includes northern, southwestern, and southeastern prongs. Each province is a Late Archean crustal aggregate and contains variable proportions of Early and/or Middle Archean crust. Early Proterozoic rifting and subsequent collisional deformation govern the dimensions of the provinces.