What are the major tectonic provinces of North America? The continent’s most ancient crust is exposed in the Canadian Shield. South of the Canadian Shield is the interior platform, where Precambrian basement rocks are covered by layers of Paleozoic sedimentary rocks. Around the edges of these provinces are elongated mountain chains. The Appalachian fold belt trends southwest to northeast on the eastern margin of the continent. The coastal plain and continental shelf of the Atlantic Ocean and Gulf of Mexico are parts of a passive continental margin that subsided after rifting during the breakup of Pangaea. The North American Cordillera is a mountainous region running down the western side of North America that contains several distinct tectonic provinces.
What types of tectonic provinces are found worldwide? The types of tectonic provinces seen in North America are found on other continents as well. Continental shields and platforms make up continental cratons, the oldest and most stable parts of continents. Around these cratons are orogens, the youngest of which are found at the active margins of continents, where tectonic deformation continues. The passive margins of continents are zones of crustal extension and sedimentation.
How do continents grow? Two plate tectonic processes, magmatic addition and accretion, add crust to continents. Buoyant silica-rich rocks are produced by magmatic differentiation, primarily in subduction zones, and added to the continental crust by vertical transport. Accretion occurs when preexisting crustal material is attached to existing continental masses by horizontal plate movement in one of four ways: the transfer of buoyant crustal fragments from a subducting plate to a continent on an overriding plate; the closure of a sea separating an island arc from a continent; the transport of crust laterally along continental margins by strike-slip faulting; or the collision and suturing of two continents and their subsequent rifting apart.
How do orogenies modify continents? Horizontal tectonic forces, arising mainly from plate convergence, can produce mountains by folding and faulting. Thrust faulting can stack the upper part of the crust into overthrust sheets tens of kilometers thick, pushing up high mountains. Compression can double the thickness of continental crust, causing the rocks in the lower crust to melt. This melting generates granitic magma, which rises to form extensive batholiths in the upper crust.
What is the Wilson cycle? The Wilson cycle is a sequence of tectonic events that occur during the assembly and breakup of supercontinents and the opening and closing of ocean basins. It has four main phases: rifting during the breakup of a supercontinent; passive margin cooling and sediment accumulation during seafloor spreading and ocean opening; active magmatic addition and accretion during subduction and ocean closure; and orogeny during continent-continent collision. Orogeny is followed by erosion, which thins the crust.
What are the mechanisms of epeirogeny? Epeirogeny is a downward or upward movement of a broad region of crust without folding or faulting. Epeirogenic upward movements can result from glacial rebound, heating of the lithosphere by upwelling mantle material, and possibly uplifting of the lithosphere by a “superplume” in the deep mantle. The cooling of previously heated lithosphere can cause epeirogenic downward movements in the interior of a continent or at the margins of two continents separated by rifting. These movements form thermal subsidence basins that become filled with sediments.
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How have continental cratons survived billions of years of plate tectonic processes? The oldest regions of the cratons, formed in the Archean eon, are underlain by a layer of cool, strong mantle material more than 200 km thick that moves with the continents as they drift. These cratonic keels are probably made up of mantle peridotites that have been depleted of their denser chemical constituents by the extraction of magmas through partial melting. This process lowers the density of the keels and stabilizes them against disruption by plate tectonic processes.