Continents are very large areas of stable continental crust. After their initial accretion, they rift and move about the earth but undergo compressional deformation almost entirely on their margins. We start our discussion of continents by identifying the varieties of terranes that come together to create them. Because accretion of terranes requires closure of oceans between them, we continue our discussion by describing two different processes of closure. Then we recognize that assembly merely develops a group of terranes, and they must be “fused” or “welded” together before they can be a coherent continent. This process takes place partly during assembly, but most of it appears to be the result of post-collisional processes that continue for tens to hundreds of millions of years. This fusion develops lower continental crust and subcontinental lithospheric mantle (SCLM), the part of the upper mantle directly underlying continental crust, that have similar, although slightly variable, properties across the entire continent. The lower crust and SCLM are separated by the seismic discontinuity known as the Moho (chapter 1), and we finish this chapter by describing the lower crust and SCLM and variations in the depth of the Moho through time. Many of the blocks involved in continental accretion are “exotic” terranes that formed somewhere away from the continent and became “allochthonous” when they accreted to the continent. They include large continental blocks that collide with each other, small continental fragments that accrete to the margins of existing continents, intraoceanic island arcs, and small amounts of oceanic lithosphere. Terranes formed on the margin of a growing continent are regarded as “autochthonous” terranes. We recognize two of them: continental-margin magmatic arcs and sediments accumulated on passive margins. Collision of large continental blocks causes intense orogeny. We illustrate this process with the collision of the Russian platform and the Siberian plate to form the Urals in the Late Paleozoic (fig. 5.1; Fershtater et al., 1997; Puchkov, 1997; Friberg and Petrov, 1998; Brown and Spadea, 1999). The East European (Russian) platform was formed by the fusion of the Baltic and Ukrainian cratons at ~2 Ga.
