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OROGENIC BELTS 287

10.1 INTRODUCTION latitude (Fig. 10.1a) and the Himalayan–Tibetan orogen
(Section 10.4) display these latter characteristics. Pro-
cesses that change the strength and rheology of conti-
nental lithosphere during orogenesis commonly include
Orogenic belts are long, commonly arcuate tracts of magmatism, metamorphism, crustal melting, crustal
highly deformed rock that develop during the creation thickening, sedimentation, and erosion.
of mountain ranges on the continents. The process of The gradual accretion of continental fragments,
building an orogen, or orogenesis, occurs at convergent island arcs, and oceanic material onto continental
plate margins and involves intra-plate shortening, margins over millions of years is one of the primary
crustal thickening, and topographic uplift. Ancient mechanisms by which the continents have grown since
orogens, whose topography has been reduced or elimi- Precambrian times (Sections 10.6, 11.4.2, 11.4.3). Most
nated by erosion, mark the location of old, inactive ancient and active orogens record many cycles of accre-
plate margins and, thus, provide important information tion and orogeny where distinctive assemblages of
on past plate movements (e.g. Section 11.4.3). crustal material called terranes (Section 10.6.1) have col-
The processes that control orogenesis vary consid- lided and become attached to the continental margin.
erably depending on the tectonic setting and the type This process is augmented by other mechanisms of
of lithosphere involved in the deformation. Noncolli- continental growth, including magma addition, sedi-
sional or Andean-type orogens (Section 10.2) result from mentation, and the creation and destruction of exten-
ocean–continent convergence where plate motions and sional basins (Section 10.6.3). Orogens that have grown

other factors controlling subduction (Section 9.6) lead significantly by these processes over long periods of
to compression within the overriding plate. Collisional time, often without ocean closure, generally have been
orogens (Sections 10.4, 10.5) develop where a continent termed accretionary orogens. Examples include the
or island arc collides with a continental margin as a Paleozoic Altaids, which form much of northern China
result of subduction. In these latter belts, the thickness and Mongolia (Sengör & Natal’in, 1996); the western
S
and positive buoyancy of the colliding material inhib- Cordillera of North America (Sections 10.6.2, 11.4.3);
its its descent into the mantle and leads to compres- and the Lachlan Orogen of southeast Australia (Section
sion and orogeny. The Himalayan–Tibetan belt and 10.6.3). Pure accretionary orogens may lack evidence of
the European Alps represent orogens that form by a major continent–continent collision and consist of
continent–continent collision following the closure many small terranes and arc–continent collisions that
of a major ocean basin (i.e. Himalaya-type). Another have occurred along the margin of a long-lived ocean.
variety where continental collision is highly oblique In this chapter, examples from South America, Asia,
and did not involve ocean closure occurs in the South- North America, Australia, and the southwest Pacifi c
ern Alps of New Zealand (Sections 8.3.3, 8.6.3). illustrate the diverse characteristics of orogens and the
Orogens that form by arc–continent collision include major mechanisms of orogenesis, including the evolu-
belts in Taiwan and the Timor–Banda arc region in tion of compressional sedimentary basins.
the southwest Pacifi c.
Much of the internal variability displayed by both
collisional and noncollisional orogens can be explained
by differences in the strength and rheology of the conti- 10.2 OCEAN–
nental lithosphere and by processes that infl uence these
properties during orogenesis (Sections 10.2.5, 10.4.6).
For example, where the continental lithosphere is rela- CONTINENT
tively cool and strong, orogens tend to be comparatively
narrow, ranging between 100 and 400 km wide. The CONVERGENCE
Southern Alps of New Zealand (Fig. 8.2a) and the south-
ern Andes near 40°S latitude (Fig. 10.1a) exhibit these
characteristics. Conversely, where the continental litho- 10.2.1 Introduction
sphere is relatively hot and weak, strain tends to delocal-
ize and is distributed across zones that can be over a One of the best-studied examples of an orogen that has
thousand kilometers wide. The central Andes near 20°S formed by ocean–continent convergence lies in the