Orogeny

An orogeny is an event that leads to both structural deformation and compositional differentiation of the Earth's lithosphere (crust and uppermost mantle) at convergent plate margins. An orogen or orogenic belt develops when a continental plate crumples and is uplifted to form one or more mountain ranges; this involves a series of geological processes collectively called orogenesis.[1][2]

Geologic provinces of the world (USGS)

Orogeny is the primary mechanism by which mountains are built on continents. The word "orogeny" (/ɒrˈɔːdʒəni/) comes from Ancient Greek (ὄρος, óros, lit. ''mountain'' + γένεσις, génesis, lit. ''creation, origin'').[3] Although it was used before him, the term was employed by the American geologist G.K. Gilbert in 1890 to describe the process of mountain-building as distinguished from epeirogeny.[4]

Physiography
See also: Subduction, Plate tectonics, and Continental collision
Two processes that can contribute to the formation of orogens. Top: delamination of orogenic roots into the asthenosphere; Bottom: Subduction of lithospheric plate to mantle depths. The two processes lead to differently located metamorphic rocks (bubbles in diagram), providing evidence as to which process actually occurred at convergent plate margins.[5]
Subduction of an oceanic plate beneath a continental plate to form an accretionary orogen. (example: the Andes)
Continental collision of two continental plates to form a collisional orogen. Typically, continental crust is subducted to lithospheric depths for blueschist to eclogite facies metamorphism, and then exhumed along the same subduction channel. (example: the Himalayas)

Orogeny takes place because of convergence of tectonic plates. This may take the form of subduction (where a continent rides forcefully over an oceanic plate to form an accretionary orogeny) or continental collision (convergence of two or more continents to form a collisional orogeny).[6]

Orogeny typically produces orogenic belts, which are elongated regions of deformation bordering continental cratons. Young orogenic belts, in which subduction is still taking place, are characterized by frequent volcanic activity and earthquakes. Older orogenic belts are typically deeply eroded to expose displaced and deformed strata. These are often highly metamorphosed and include vast bodies of intrusive igneous rock called batholiths.[7]

Orogenic belts are associated with subduction zones, which consume crust, thicken lithosphere, produce earthquake and volcanoes, and often build island arcs. These island arcs may be added to a continental margin during an accretionary orogeny. The orogeny may culminate with continental crust from the opposite side of the subducting oceanic plate arriving at the subduction zone. This ends subduction and transforms the accretional orogeny into a collisional orogeny.[8] The collisional orogeny may produce extremely high mountains, as has been taking place in the Himalayas for the last 65 million years.[9]

The processes of orogeny can take tens of millions of years and build mountains from what were once sedimentary basins.[7] Activity along an orogenic belt can be extremely long-lived. For example, much of the basement underlying the United States belongs to the Transcontinental Proterozoic Provinces, which accreted to Laurentia (the ancient heart of North America) over the course of 200 million years in the Paleoproterozoic.[10] The Yavapai and Mazatzal orogenies were peaks of orogenic activity during this time. These were part of an extended period of orogenic activity that included the Picuris orogeny and culminated in the Grenville orogeny, lasting at least 600 million years.[11] A similar sequence of orogenies has taken place on the west coast of North America, beginning in the late Devonian (about 380 million years ago) with the Antler orogeny and continuing with the Sonoma orogeny and Sevier orogeny and culminating with the Laramide orogeny. The Laramide orogeny alone lasted 40 million years, from 75 million to 35 million years ago.[12]

The topographic height of orogenic mountains is related to the principle of isostasy,[13] that is, a balance of the downward gravitational force upon an upthrust mountain range (composed of light, continental crust material) and the buoyant upward forces exerted by the dense underlying mantle.[14] Erosion of overlying strata in orogenic belts, and isostatic adjustment to the removal of this overlying mass of rock, can bring deeply buried strata to the surface. The erosional process is called unroofing and the resulting exposure of formerly deeply buried strata is called exhumation.[15]

An orogenic event may be studied: (a) as a tectonic structural event, (b) as a geographical event, and (c) as a chronological event.

Orogenic events:

  • cause distinctive structural phenomena related to tectonic activity
  • affect rocks and crust in particular regions, and
  • happen within a specific period

Orogen (or "orogenic system")
The Foreland Basin System

In general, there are two main types of orogens at convergent plate margins: (1) accretionary orogens, which are produced by subduction of one oceanic plate beneath one continental plate to result in either continental arc magmatism or the accretion of island arc terranes to continental margins; (2) collisional orogens, which are produced by collision between two continental blocks, with subduction of one continental block beneath the other continental block.

An orogeny produces an orogen, but a (mountain) range-foreland basin system is only produced on passive plate margins. The foreland basin forms ahead of the orogen due mainly to loading and resulting flexure of the lithosphere by the developing mountain belt. A typical foreland basin is subdivided into a wedge-top basin above the active orogenic wedge, the foredeep immediately beyond the active front, a forebulge high of flexural origin and a back-bulge area beyond, although not all of these are present in all foreland-basin systems. The basin migrates with the orogenic front and early deposited foreland basin sediments become progressively involved in folding and thrusting. Sediments deposited in the foreland basin are mainly derived from the erosion of the actively uplifting rocks of the mountain range, although some sediments derive from the foreland. The fill of many such basins shows a change in time from deepwater marine (flysch-style) through shallow water to continental (molasse-style) sediments.[16]

Orogenic cycle
See also: Wilson cycle

Although orogeny involves plate tectonics, the tectonic forces result in a variety of associated phenomena, including crustal deformation, crustal thickening, crustal thinning and crustal melting as well as magmatism, metamorphism and mineralization. What exactly happens in a specific orogen depends upon the strength and rheology of the continental lithosphere, and how these properties change during orogenesis.

In addition to orogeny, the orogen (once formed) is subject to other processes, such as sedimentation and erosion.[2] The sequence of repeated cycles of sedimentation, deposition and erosion, followed by burial and metamorphism, and then by crustal anatexis to form granitic batholiths and tectonic uplift to form mountain chains, is called the orogenic cycle.[17][18] For example, the Caledonian Orogeny refers to a series of tectonic events due to the continental collision of Laurentia with Eastern Avalonia and other former fragments of Gondwana in the Early Paleozoic. The Caledonian Orogen resulted from these events and various others that are part of its peculiar orogenic cycle.[19]

In summary, an orogeny is an episode of deformation, metamorphism and magmatism at convergent plate margins, during which many geological processes play a role at convergent plate margins. Every orogeny has its own orogenic cycle, but composite orogenesis is common at convergent plate margins.

Erosion

Erosion represents a subsequent phase of the orogenic cycle. Erosion inevitably removes much of the mountains, exposing the core or mountain roots (metamorphic rocks brought to the surface from a depth of several kilometres). Isostatic movements may help such exhumation by balancing out the buoyancy of the evolving orogen. Scholars debate about the extent to which erosion modifies the patterns of tectonic deformation (see erosion and tectonics). Thus, the final form of the majority of old orogenic belts is a long arcuate strip of crystalline metamorphic rocks sequentially below younger sediments which are thrust atop them and which dip away from the orogenic core.

An orogen may be almost completely eroded away, and only recognizable by studying (old) rocks that bear traces of orogenesis. Orogens are usually long, thin, arcuate tracts of rock that have a pronounced linear structure resulting in terranes or blocks of deformed rocks, separated generally by suture zones or dipping thrust faults. These thrust faults carry relatively thin slices of rock (which are called nappes or thrust sheets, and differ from tectonic plates) from the core of the shortening orogen out toward the margins, and are intimately associated with folds and the development of metamorphism.[20]

Biology

In the 1950s and 1960s the study of orogeny, coupled with biogeography (the study of the distribution and evolution of flora and fauna),[21] geography and mid ocean ridges, contributed greatly to the theory of plate tectonics. Even at a very early stage, life played a significant role in the continued existence of oceans, by affecting the composition of the atmosphere. The existence of oceans is critical to sea-floor spreading and subduction.[22][23]

Relationship to mountain building
An example of thin-skinned deformation (thrust faulting) of the Sevier Orogeny in Montana. Note the white Madison Limestone repeated, with one example in the foreground (that pinches out with distance) and another to the upper right corner and top of the picture.
Sierra Nevada Mountains (a result of delamination) as seen from the International Space Station.

Mountain formation occurs through a number of mechanisms.[24][25][26]

Mountain complexes result from irregular successions of tectonic responses due to sea-floor spreading, shifting lithosphere plates, transform faults, and colliding, coupled and uncoupled continental margins.

— Peter J Coney[27]

Large modern orogenies often lie on the margins of present-day continents; the Alleghenian (Appalachian), Laramide, and Andean orogenies exemplify this in the Americas. Older inactive orogenies, such as the Algoman, Penokean and Antler, are represented by deformed and metamorphosed rocks with sedimentary basins further inland.

Areas that are rifting apart, such as mid-ocean ridges and the East African Rift, have mountains due to thermal buoyancy related to the hot mantle underneath them; this thermal buoyancy is known as dynamic topography. In strike-slip orogens, such as the San Andreas Fault, restraining bends result in regions of localized crustal shortening and mountain building without a plate-margin-wide orogeny. Hotspot volcanism results in the formation of isolated mountains and mountain chains that look as if they are not necessarily on present tectonic-plate boundaries, but they are essentially the product of plate tectonism.

Regions can also experience uplift as a result of delamination of the orogenic lithosphere, in which an unstable portion of cold lithospheric root drips down into the asthenospheric mantle, decreasing the density of the lithosphere and causing buoyant uplift.[28] An example is the Sierra Nevada in California. This range of fault-block mountains[29] experienced renewed uplift due to abundant magmatism after a delamination of the orogenic root beneath them.[28][30]

Finally, uplift and erosion related to epeirogenesis (large-scale vertical motions of portions of continents without much associated folding, metamorphism, or deformation)[31] can create local topographic highs.

Mount Rundle, Banff, Alberta.

Mount Rundle on the Trans-Canada Highway between Banff and Canmore provides a classic example of a mountain cut in dipping-layered rocks. Millions of years ago a collision caused an orogeny, forcing horizontal layers of an ancient ocean crust to be thrust up at an angle of 50–60°. That left Rundle with one sweeping, tree-lined smooth face, and one sharp, steep face where the edge of the uplifted layers are exposed.[32]

History of the concept

Before the development of geologic concepts during the 19th century, the presence of marine fossils in mountains was explained in Christian contexts as a result of the Biblical Deluge. This was an extension of Neoplatonic thought, which influenced early Christian writers.

The 13th-century Dominican scholar Albert the Great posited that, as erosion was known to occur, there must be some process whereby new mountains and other land-forms were thrust up, or else there would eventually be no land; he suggested that marine fossils in mountainsides must once have been at the sea-floor. Orogeny was used by Amanz Gressly (1840) and Jules Thurmann (1854) as orogenic in terms of the creation of mountain elevations, as the term mountain building was still used to describe the processes. Elie de Beaumont (1852) used the evocative "Jaws of a Vise" theory to explain orogeny, but was more concerned with the height rather than the implicit structures created by and contained in orogenic belts. His theory essentially held that mountains were created by the squeezing of certain rocks. Eduard Suess (1875) recognised the importance of horizontal movement of rocks. The concept of a precursor geosyncline or initial downward warping of the solid earth (Hall, 1859) prompted James Dwight Dana (1873) to include the concept of compression in the theories surrounding mountain-building. With hindsight, we can discount Dana's conjecture that this contraction was due to the cooling of the Earth (aka the cooling Earth theory). The cooling Earth theory was the chief paradigm for most geologists until the 1960s. It was, in the context of orogeny, fiercely contested by proponents of vertical movements in the crust (similar to tephrotectonics), or convection within the asthenosphere or mantle.

Gustav Steinmann (1906) recognised different classes of orogenic belts, including the Alpine type orogenic belt, typified by a flysch and molasse geometry to the sediments; ophiolite sequences, tholeiitic basalts, and a nappe style fold structure.

In terms of recognising orogeny as an event, Leopold von Buch (1855) recognised that orogenies could be placed in time by bracketing between the youngest deformed rock and the oldest undeformed rock, a principle which is still in use today, though commonly investigated by geochronology using radiometric dating.

Based on available observations from the metamorphic differences in orogenic belts of Europe and North America, H. J. Zwart (1967)[33] proposed three types of orogens in relationship to tectonic setting and style: Cordillerotype, Alpinotype, and Hercynotype. His proposal was revised by W. S. Pitcher in 1979[34] in terms of the relationship to granite occurrences. Cawood et al. (2009)[35] categorized orogenic belts into three types: accretionary, collisional, and intracratonic. Notice that both accretionary and collisional orogens developed in converging plate margins. In contrast, Hercynotype orogens generally show similar features to intracratonic, intracontinental, extensional, and ultrahot orogens, all of which developed in continental detachment systems at converged plate margins.

  1. Accretionary orogens, which were produced by subduction of one oceanic plate beneath one continental plate for arc volcanism. They are dominated by calc-alkaline igneous rocks and high-T/low-P metamorphic facies series at high thermal gradients of >30 °C/km. There is a general lack of ophiolites, migmatites and abyssal sediments. Typical examples are all circum-Pacific orogens containing continental arcs.
  2. Collisional orogens, which were produced by subduction of one continental block beneath the other continental block with the absence of arc volcanism. They are typified by the occurrence of blueschist to eclogite facies metamorphic zones, indicating high-P/low-T metamorphism at low thermal gradients of <10 °C/km. Orogenic peridotites are present but volumetrically minor, and syn-collisional granites and migmatites are also rare or of only minor extent. Typical examples are the Alps-Himalaya orogens in the southern margin of Eurasian continent and the Dabie-Sulu orogens in east-central China.

See also
  • Biogeography – The study of the distribution of species and ecosystems in geographic space and through geological time
  • Fault mechanics – A field of study that investigates the behavior of geologic faults
  • Fold mountains – Mountains formed by compressive crumpling of the layers of rock
  • Guyot – An isolated, flat-topped underwater volcano mountain
  • List of orogenies – Known mountain building events of the Earth's history
  • Mantle convection – The slow moving motion of Earth's solid mantle caused by convection currents carrying heat from the planet's interior to its surface
  • Tectonic uplift – The portion of the total geologic uplift of the mean earth surface that is not attributable to an isostatic response to unloading
  • Epeirogenic movement – Upheavals or depressions of land exhibiting long wavelengths and little folding

Literature

References
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Sources
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