North American geology

North America is made up of three major structural provinces: the North American craton, an ancient Precambrian shield area which occupies the major part of the north of the continent, tapering towards the south in an inverted triangle; the Appalachian fold belt, which abuts the southeast margin of the craton; and the Cordilleran fold belt, which lies along the southwest margin of the craton. The Precambrian rocks of the shield are well exposed in the north of the continent, where they form the Canadian Shield, but to the south, in the central plains of the USA, they are largely covered by later Phanerozoic sediments.

Canadian Shield The Canadian Shield is a vast area of Precambrian rocks, which can be divided into a number of structural provinces, each of which represents a mobile area active during a different part of Precambrian time. The provinces are in fact fragments of complexes which were originally of far greater extent.

The oldest Precambrian rocks (formed more than 3,000 million years ago) occur as small isolated masses within larger areas of younger rocks.

Superior province The Superior province is a huge area of rocks of middle Precambrian age. It is made up of a basement of granites, gneisses, and migmatites, within which are elongate greenstone belts – belts where the rocks have been less highly metamorphosed and contain many primary features. These belts have an east–west trend and contain metamorphosed volcanics and sediments; they represent thick accumulations of sediment in unstable basins of deposition. They carry deposits of gold and other metal ores, and form some of Canada's major mining resources. This cycle of Precambrian activity reached a climax in the Kenoran orogeny, 2,600–2,400 million years ago, during which time there was widespread regional metamorphism and deformation and granites were intruded. After this orogenic phase there was a relatively stable phase, during which an extensive dyke swarm was intruded.

Churchill province The Churchill province, which adjoins the north margin of the Superior province, is made up of rocks formed during a later Precambrian orogenic cycle, the Hudsonian cycle. The Hudsonian zones of mobile belt activity occur to the north (in the Churchill province), northeast (in the Nain province), and south (in the Southern province) of the stable Superior province. The rocks in these belts are volcanics and sediments, and contain thick banded iron formations, one of the main sources of iron ore in North America. Uranium, copper, lead, zinc, and silver are other important ores found in these rocks. Within the mobile belts the rocks have suffered intense deformation and regional metamorphism during the Hudsonian orogeny, 1,800–1,650 million years ago, but the sediments deposited on the foreland outside the mobile belts are often only gently folded. The end of the Hudsonian cycle was followed by the welding of the new and stabilized belts onto the old Superior craton to form a much larger shield area. A thick succession of plateau basalts (the Keweenawan succession) was poured out onto the southern part of the craton 1,300–1,050 million years ago; this succession is up to 15 km/9.3 mi thick, and is characterized by copper mineralization.

Grenville province The Grenville province is the youngest province of the Canadian Shield. It is a clearly defined belt lying in the east of the Canadian Shield and extending southwest through the USA to the Mexican border. The Grenville mobile belt is characterized by a high grade of regional metamorphism, and it is therefore difficult to distinguish the original rock types occurring in the belt. This belt suffered its major metamorphism and folding in the late Precambrian, 1,100–900 million years ago.

Appalachian belt This mobile belt came into existence at the end of the Precambrian, and forms a belt 3,000 km/1,865 mi long adjoining the Precambrian craton of North America, running from Newfoundland to Alabama, and partially superimposed on the earlier Grenville belt. The Appalachian cycle lasted from late Precambrian to Permian times, and sediments were laid down in a typically geosynclinal environment. The earliest basins of deposition were initiated in the eugeosynclinal zone, an area which extends the whole length of the belt and which includes detrital sediments, volcanics, and ultrabasic intrusions. The rocks in this zone are usually metamorphosed. From early Cambrian to mid-Ordovician times deposition took place both in the eugeosyncline and in the miogeosyncline – the latter being an area in the northwest part of the belt, adjacent to the foreland. This zone contains abundant carbonate sediments and is unmetamorphosed. Orogenic movements during the Ordovician raised several massifs in the central and southeast part of the belt, and restricted the later basins of deposition. By the late Devonian, much of the northern Appalachian belt was a mountain system, and deposition continued only in intermontane basins. Further south, stabilization occurred at the end of the Palaeozoic. Coal is of immense economic importance in the Appalachian belt, and is preserved at the margin of the mobile belt and on the adjoining foreland. Large oil reserves occur in mid-Devonian to Carboniferous rocks in the Allegheny Plateau.

During the Palaeozoic, shallow-water sandstones and carbonate rocks were laid down in thin layers on the interior craton; later in the Palaeozoic, as the new Appalachian mountain ranges were rising to the east and south, detrital continental sediments were deposited on the craton.

Cordilleran belt This is an extremely long-lived mobile belt initiated in the late Precambrian and still active at the present day. It extends along the whole west coast of North America from Alaska to California and Guatemala. The earliest sediments were laid down in basins in late Precambrian times, and from the early Palaeozoic onwards geosynclinal sediments accumulated at an active continental margin. As in the Appalachian belt, a eugeosynclinal and miogeosynclinal zone can be distinguished, this time with the miogeosyncline on the northeast side adjoining the craton. The first orogenic disturbances occurred in late Mesozoic and Tertiary times, when there was widespread uplift and intrusion of granites. This uplifted zone continued to form a highland region throughout the Tertiary and Quaternary eras. To the west of the Cordilleran belt is the narrow Pacific belt, where the main orogeny occurred in mid-Tertiary or later times. The Cordillera contains rich mineral deposits, many of which were associated with Tertiary volcanic activity. These include gold, lead, zinc, silver, tin, and copper.

Colorado Plateau The Colorado Plateau and the Basin and Range region, Nevada, represent part of the old craton adjoining the Cordilleran mobile belt which became reactivated in Mesozoic and Tertiary times and was eventually incorporated in the new orogenic belt. The central and southern Cordillera contain huge thicknesses of lavas and pyroclastic rocks laid down in Tertiary to recent times.