Mountain building and the shaping of continents
Figure 6.1 outlines five areas selected to demonstrate mountain-building phenomena. Area ‘A’, the southwest Pacific, is a modern laboratory where a variety of ongoing plate-tectonic processes can be observed that will ultimately result in an orogenic system along what is now the eastern margin of Asia. Features of particular interest include the accretion of volcanic island arcs onto the margins of continents, as seen in the environs of Taiwan and Timor; the amalgamation of volcanic island arcs, as observed in the Philippine archipelago; A-subduction, as imaged seismically along the north margin of Borneo; and the slivering and dispersion of continental fragments, as viewed in the Banda Sea.
Area ‘B’, the Himalaya and the Tibet Plateau, illustrates head-on continent—continent collision where a large ocean basin previously existed in the intervening area. Area ‘C’, southern Europe and northern Africa, likewise exhibits continent—continent collision, but here only small basins, floored both by thinned continental and oceanic crust, existed between the continents, and a component of sinistral shear that contributes to the architectural style of the various mountain ranges is important (Pyrenees, Alps, and Apennines). Area ‘D’, the Cordillera of western North America, is the type area for illustrating the disposition of suspect terranes along a continental margin that has faced an open ocean since about 560 Ma. Mountain building in this region reflects complex relations among the processes of accretion, dispersion, and intracontinental shortening (A-subduction). Area ‘E’, the Andes of South America from Colombia to central Chile, comprises two markedly contrasting areas; the northern sector is illustrative of accretion tectonics primarily as a result of the obduction of oceanic crust, whereas the southern sector represents a collision margin with an anomalous absence of crustal accretion. In northern Chile, for example, the Andes constitute a continental-margin volcanic arc. The thick crust is primarily the result of intracrustal thrust tectonics, possibly associated with ridge push from the Atlantic side. To the west, within the subduction zone that defines the plate boundary with the Pacific plate, one discovers a history of tectonic subsidence and crustal erosion.
The salient features of each of the five regions are highlighted in order to underscore the principal styles of crustal thickening that result in mountain-building phenomena. To illustrate these styles, a host of crosssectional renderings are presented. The reader should be cautioned that varying degrees of speculation are built into each section. A growing body of deep seismic data is now becoming available to constrain these notions further, but probably several decades will pass before geologists agree better on the origin of all these mountain systems.
KeywordsContinental Crust Subduction Zone Oceanic Crust North American Continent Mountain Building
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