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Orogeny

The process of mountain building. As traditionally used, the term orogeny refers to the development of long, mountainous belts on the continents called orogenic belts or orogens. These include the Appalachian and Cordilleran orogens of North America, the Andean orogen of western South America, the Caledonian orogen of northern Europe and eastern Greenland, and the Alpine-Himalayan orogen that stretches from western Europe to eastern China (Fig. 1). It is important to recognize that these systems represent only the most recent orogenic belts that retain the high-relief characteristic of mountainous regions. In fact, the continents can be viewed as a collage of ancient orogenic belts, most so deeply eroded that no trace of their original mountainous topography remains (Fig. 2). By comparing characteristic rock assemblages from more recent orogens with their deeply eroded counterparts, geologists surmise that the processes responsible for mountain building today extended back through most (if not all) of geologic time and played a major role in the growth of the continents.  See also: Continents, evolution of

Fig. 1  Distribution of major orogenic belts. (After E. M. Moores and R. J. Twiss, Tectonics, W. H. Freeman, 1995)

 

 orog1

 

 

Fig. 2  Distribution of orogenic belts, by time of major orogenic distribution, in the continents. (After B. C. Burchfiel, The continental crust, in E. M. Moores, ed., Shaping the Earth: Tectonics of Continents and Oceans, W. H. Freeman, 1990)

 

 orog2

 

 

The construction of mountain belts is best understood in the context of plate tectonics theory. Earth’s lithosphere is currently fragmented into at least a dozen, more or less rigid plates that are separated by three kinds of boundaries: convergent, divergent, and transform. Plates move away from each other at divergent boundaries. On the continents, these boundaries are marked by rift systems such as those in East Africa; in the ocean basins, they correspond to spreading centers, submarine mountain chains (such as the Mid-Atlantic Ridge) where new oceanic crust is produced to fill the gap left behind by plate divergence. Transform boundaries are zones where plates slide past one another; a familiar example on land is the San Andreas fault system. Orogenic belts form at convergent boundaries, where lithosphere plates collide.  See also: Fault and fault structures; Mid-Oceanic Ridge; Plate tectonics; Rift valley; Transform fault

Convergent plate boundaries

 

There are two basic kinds of convergent plate boundaries, leading to the development of two end-member classes of orogenic belts. Oceanic subduction boundaries are those at which oceanic lithosphere is thrust (subducted) beneath either continental or oceanic lithosphere. The process of subduction leads to partial melting near the plate boundary at depth, which is manifested by volcanic and intrusive igneous activity in the overriding plate. Where the overriding plate consists of oceanic lithosphere, the result is an intraoceanic island arc, such as the Japanese islands. Where the overriding plate is continental, a continental arc is formed. The Andes of western South America is an example.  See also: Marine geology; Oceanic islands; Subduction zones

The second kind of convergent plate boundary forms when an ocean basin between two continental masses has been completely consumed at an oceanic subduction boundary and the continents collide. Continent collisional orogeny has resulted in some of the most dramatic mountain ranges on Earth. A good example is the Himalayan orogen, which began forming roughly 50 million years ago when India collided with the Asian continent. Because the destruction of oceanic lithosphere at subduction boundaries is a prerequisite for continental collision, continent collisional orogens contain deformational features and rock associations developed during arc formation as well as those produced by continental collision, and are thus characterized by complex internal structure.

Compressive forces at convergent plate boundaries shorten and thicken the crust, and one important characteristic of orogens is the presence of unusually thick (up to 40–60 mi or 70–80 km) continental crust. Because continental crust is more buoyant than the underlying mantle, regions of thick crust are marked by high elevations of the Earth’s surface and, hence, mountainous terrain. Once a mountain range has developed, its high elevation can be maintained by two mechanisms in addition to its own buoyancy. First, compressional forces related to continuing convergence across the plate boundary serve to buttress the orogen against collapse. Second, some mountain ranges are developed on thick, cold, and therefore strong continental lithosphere, and the lithosphere serves as a rigid support. However, such processes work against the gravitational forces that are constantly acting to destroy mountain ranges through erosion and normal faulting. All mountain ranges ultimately succumb to gravity; the resulting denudation exposes deep levels of orogenic belts and provides geologists with important insights regarding the internal architecture.  See also: Asthenosphere; Earth crust; Lithosphere

Continent collisional orogen

 

The tectonics of the Himalayas can serve as an example of the basic anatomy of a continent collisional orogen (Fig. 3). Prior to collision between India and Asia, the two continental masses were separated by an ocean referred to as Tethys. Most Himalayan geologists are convinced that the southern margin of Asia in late Mesozoic time was an oceanic subduction boundary where Tethyan oceanic lithosphere was thrusting beneath Asia. The principal evidence for this interpretation comes from the existence of a continental arc of appropriate age that was built on Asian lithosphere just north of the Himalayan mountain range (Continental Arc Zone). India and Asia collided when all the intervening oceanic lithosphere had been destroyed at the subduction boundary. The collision zone at the surface of the Earth is marked by remnants of Tethyan oceanic crust (ophiolites) and related rocks exposed in the Indus Suture Zone. All tectonic zones south of the suture are part of the Indian continental lithosphere that was folded and imbricated (overlapped at the margins) during collision. The Tibetan Zone includes Paleozoic to early Tertiary sedimentary rocks originally deposited along the northern margin of the Indian continent. The Greater Himalayan Zone consists of crustal rocks that were deformed, metamorphosed, and partially melted at high temperatures deep within the crust as a consequence of Himalayan orogeny; these rocks form the principal substrate for the high mountains of the Himalayas, including Mount Everest. Farther south, the Lesser Himalayan Zone contains Precambrian to Paleozoic sedimentary and metamorphic rocks that have been deformed at lower temperatures and pressures. As collisional orogeny progressed through Tertiary time, material eroded from the rising Himalayan ranges was transported southward by a variety of ancient river systems and deposited at the southern margin of the orogen to form the Siwalik Zone. These rocks were themselves deformed during the most recent stages of Himalayan orogeny as the locus of surface deformation propagated southward toward the Indian subcontinent.  See also: Geologic time scale

Fig. 3  Himalayan orogen. (a) Simplified tectonic map. (b) Generalized cross section. Fault systems are labeled at their surface exposure. Half-arrows indicate directions of movement on the fault systems.

 

 orog3

 

A simplified geologic cross section through the orogen reveals that all of these tectonic zones are separated by major fault systems (Fig. 3b). Most of these are dominated by thrust faults that accommodated shortening of the continental crust during collision. Geologists have discovered that the boundary between the Tibetan Zone and the Greater Himalayan Zone is marked by normal faults of the South Tibetan detachment system. Unlike thrust faults, normal faults thin and extend the crust, and identification of them in a continent-continent collisional setting like the Himalayan orogen was surprising at first. Many Himalayan geologists have come to believe that the South Tibetan detachment system developed when the range grew high enough so that the buttressing effects of plate convergence were no longer capable of supporting the weight and the orogen began to collapse, spreading out the load laterally over a larger region.

 

Other effects

 

Although mountain belts are the most conspicuous products of orogeny, the effects of plate convergence—especially continent-continent collision—can be found hundreds or even thousands of miles away from the orogenic belt. For example, the Tibetan Plateau, which lies just north of the Himalayan orogen, has an average elevation of about 15,000 ft (5000 m) over an area about the size of France. Development of the plateau was undoubtedly related to India-Asia collision, but the exact age and mechanism of plateau uplift remain controversial. Large-displacement strike-slip faults (where one block of continental crust slides laterally past another) are common in central and southeast Asia, and some geologists hypothesize that they extend from the surface down to at least the base of the crust, segmenting this part of Asia into a series rigid blocks that have extruded eastward as a consequence of the collision. Other geologists, while recognizing the significance of these faults for upper crustal deformation, suggest that the upper and lower crust of Tibet are detached from one another and that the eastward ductile flow of Tibetan lower crust has been an equally important factor in plateau growth.

 

 

Topographic expression

 

Earthquake activity in Bhutan, northern India, Nepal, northern Pakistan, and southern Tibet demonstrates that the Himalayan orogen continues to evolve, and this is principally why the Himalayas include such high mountains. As convergence across an orogen ends, erosion and structural denudation begin to smooth out the topographic gradient produced by orogeny. Rugged and high ranges such as the Himalayas are young and still active; lower ranges with subdued topography, such as the Appalachians, are old and inactive. Some of the oldest orogenic belts on Earth, such as those exposed in northern Canada, have no remaining topographic expression and are identified solely on the basis of geologic mapping of characteristic orogenic zones such as those discussed above for the Himalayas. Mountain ranges such as the Himalayas were formed and eventually eroded away throughout much of Earth’s history; it is the preservation of ancient orogenic belts that provides some of the most reliable evidence that plate tectonic processes have been operative on Earth for at least 2 billion years.

 

Ancient geological processes

 

One of the most important axioms in the earth sciences is that the present is the key to the past (law of uniformitarianism). Earth is a dynamic planet; all oceanic lithosphere that existed prior to about 200 million years ago has been recycled into the mantle. The ocean basins thus contain no record of the first 96% of Earth history, and geologists must rely on the continents for this information. The careful study of modern orogenic belts provides a guide to understanding orogenic processes in ancient orogens, and permits use of the continents as a record of the interactions between lithospheric plates that have long since been destroyed completely at convergent plate boundaries.

Many of the basic geologic features of modern mountain systems such as theHimalayas can be found in ancient orogens, and geologists have established generic names for the different parts of orogens to facilitate comparisons. The Foredeep contains sediments transported from the rising mountain chain; in the Himalayan orogen, it corresponds to the more southerly portions of the Siwalik Zone (Fig. 3). The Foreland Fold and Thrust Belt (northern Siwalik Zone and Lesser Himalayan Zone) is a complexly deformed sequence of sedimentary and crystalline rocks that were scraped off the upper portion of the down-going plate during collision. The Orogenic Core comprises the central part of an orogen; it may include large tracts of crust that experienced deformation at deep structural levels (Greater Himalayan Zone), rock sequences that were deformed at higher structural levels but are now juxtaposed with metamorphic rocks by movement on normal fault systems such as the South Tibetan detachment (Tibetan Zone), and a discontinuous belt of ophiolites and related materials marking the zone of plate convergence sensu stricto (Indus Suture Zone). Ophiolite belts can be viewed as the fossil remnants of oceans that no longer exist. The presence of ophiolite belts of various ages within the continents means that a map of the modern distribution of lithospheric plates provides only a recent snapshot of an ever-changing configuration of lithospheric plates on Earth: mountain ranges such as the Himalayas were formed and eventually eroded away throughout much of Earth’s history. Exactly how far back in geologic time plate tectonic processes and their orogenic consequences can be extrapolated is a matter of debate among geologists, but the bulk of evidence suggests that mountain ranges such as the Himalayas provide useful analogs for even the oldest orogens on Earth. The preservation of ophiolite belts as old as 2 billion years in Precambrian orogenic belts provides direct evidence that plate tectonic processes have been operative on Earth for at least the past 50% of Earth history.  See also: Cordilleran belt; Mountain systems; Ophiolite

Kip Hodges

 

Bibliography

  • K. C. Condie, Plate Tectonics, 4th ed., 1997
  • E. M. Moores (ed.), Shaping the Earth: Tectonics of Continents and Oceans, 1990
  • E. M. Moores and R. J. Twiss, Tectonics, 1995
  • J. T. Wilson (ed.), Continents Adrift and Continents Aground, 1996

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