Orogenic belts are long, commonly arcuate tracts of highly deformed rock that develop during the creation of mountain ranges on the continents. The process of building an orogen, or orogenesis, occurs at convergent plate margins and involves intra-plate shortening, crustal thickening, and topographic uplift. Ancient orogens, whose topography has been reduced or eliminated by erosion, mark the location of old, inactive plate margins and, thus, provide important information on past plate movements (e.g. Section 11.4.3).
The processes that control orogenesis vary considerably depending on the tectonic setting and the type of lithosphere involved in the deformation. Noncolli-sional or Andean-type orogens (Section 10.2) result from ocean-continent convergence where plate motions and other factors controlling subduction (Section 9.6) lead to compression within the overriding plate. Collisional orogens (Sections 10.4, 10.5) develop where a continent or island arc collides with a continental margin as a result of subduction. In these latter belts, the thickness and positive buoyancy of the colliding material inhibits its descent into the mantle and leads to compression and orogeny. The Himalayan-Tibetan belt and the European Alps represent orogens that form by continent-continent collision following the closure of a major ocean basin (i.e. Himalaya-type). Another variety where continental collision is highly oblique and did not involve ocean closure occurs in the Southern Alps of New Zealand (Sections 8.3.3, 8.6.3). Orogens that form by arc-continent collision include belts in Taiwan and the Timor-Banda arc region in the southwest Pacific.
Much of the internal variability displayed by both collisional and noncollisional orogens can be explained by differences in the strength and rheology of the continental lithosphere and by processes that influence these properties during orogenesis (Sections 10.2.5, 10.4.6). For example, where the continental lithosphere is relatively cool and strong, orogens tend to be comparatively narrow, ranging between 100 and 400 km wide. The Southern Alps of New Zealand (Fig. 8.2a) and the southern Andes near 40°S latitude (Fig. 10.1a) exhibit these characteristics. Conversely, where the continental lithosphere is relatively hot and weak, strain tends to delocal-ize and is distributed across zones that can be over a thousand kilometers wide. The central Andes near 20°S
latitude (Fig. 10.1a) and the Himalayan-Tibetan orogen (Section 10.4) display these latter characteristics. Processes that change the strength and rheology of continental lithosphere during orogenesis commonly include magmatism, metamorphism, crustal melting, crustal thickening, sedimentation, and erosion.
The gradual accretion of continental fragments, island arcs, and oceanic material onto continental margins over millions of years is one of the primary mechanisms by which the continents have grown since Precambrian times (Sections 10.6, 11.4.2, 11.4.3). Most ancient and active orogens record many cycles of accretion and orogeny where distinctive assemblages of crustal material called terranes (Section 10.6.1) have collided and become attached to the continental margin. This process is augmented by other mechanisms of continental growth, including magma addition, sedimentation, and the creation and destruction of exten-sional basins (Section 10.6.3). Orogens that have grown significantly by these processes over long periods of time, often without ocean closure, generally have been termed accretionary orogens. Examples include the Paleozoic Altaids, which form much of northern China and Mongolia (^engor & Natal'in, 1996); the western Cordillera of North America (Sections 10.6.2, 11.4.3); and the Lachlan Orogen of southeast Australia (Section 10.6.3). Pure accretionary orogens may lack evidence of a major continent-continent collision and consist of many small terranes and arc-continent collisions that have occurred along the margin of a long-lived ocean.
In this chapter, examples from South America, Asia, North America, Australia, and the southwest Pacific illustrate the diverse characteristics of orogens and the major mechanisms of orogenesis, including the evolution of compressional sedimentary basins.
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