Orogeny (Greek for "mountain generating") is the process of mountain building, and may be studied as a tectonic structural event, as a geographical event and a chronological event, in that orogenic events cause distinctive structural phenomena and related tectonic activity, affect certain regions of rocks and crust and happen within a time frame.

Orogenic events occur solely as a result of the processes of plate tectonics; the problems which were investigated and resolved by the study of orogenesis contributed greatly to the theory of plate tectonics, coupled with study of flora and fauna, geography and mid ocean ridges in the 1950s and 1960s.

The physical manifestations of orogenesis, the process of orogeny, are orogenic belts or orogens. An orogen is different from a mountain range in that an orogen may be completely eroded away, and only recognizable by studying (old) rocks that bear the traces of the orogeny. Orogens are usually long, thin, arcuate tracts of rocks which have a pronounced linear structure resulting in terranes or blocks of deformed rocks, separated generally by dipping thrust faults. These thrust faults carry relatively thin plates (which are called nappes, and are different to tectonic plates) of rock in from the margins of the compressing orogen to the core, and are intimately associated with folds and the development of metamorphism.

The topographic height of orogenic mountains is related to the principle of isostasy, where the gravitational force of the upthrust mountain range of light, continental crust material is balanced against its buoyancy relative to the dense mantle.

Erosion inevitably takes its course, removing much of the mountains, leaving the core or mountain roots, which may be exhumed by further isostasy events balancing out the loss of elevated mass. This is the final form of the majority of old orogenic belts, being a long arcuate strip of crystalline metamorphic rocks sequentially below younger sediments which are thrust atop them and dip away from the orogenic core.

History

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Orogeny was used by Gressly (1840) and 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 orogenic belts created and contained. His theory essentially held that mountains were created by the squeezing of certain rocks.

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 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, contested hotly by proponents of vertical movements in the crust (similar to tephrotectonics), or convection within the asthenosphere or mantle (geology).

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.

Zwart (1967) drew attention to the metamorphic differences in orogenic belts, proposing three types, modified by Pitcher (1979);

• Hercynotype (back-arc basin type);
  • Shallow, low-pressure metamorphism; thin metamorphic zones
  • Metamorphism dependent on increase in temperature
  • Abundant granite and migmatite
  • Few ophiolites, ultramafic rocks virtually absent
  • very wide orogen with small and slow uplift
  • nappe structures rare
• Alpinotype (ocean trench style);
  • deep, high pressure, thick metamorphic zones
  • metamorphism of many facies, dependent on decrease in pressure
  • few granites or migmatites
  • abundant ophiolites with ultramafic rocks
  • Relatively narrow orogen with large and rapid uplift
  • Nappe structures predominant
• Cordilleran (arc) type;
  • dominated by calc-alkaline igneous rocks,andesites, granite batholiths
  • general lack of migmatites, low geothermal gradient
  • lack of ophiolite and abyssal sedimentary rocks (black shale, chert, etcetera)
  • low-pressure metamorphism, moderate uplift
  • lack of nappes

The advent of plate tectonics has explained the vast majority of orogenic belts and their features. The cooling earth theory (principally advanced by Descartes) is dispensed with, and tephrotectonic style vertical movements have been explained primarily by the process of isostasy.

Some oddities exist, where simple collisional tectonics are modified in a transform plate boundary, such as in New Zealand, or where island arc orogenies, for instance in New Guinea occur away from a continental backstop. Further complications such as Proterozoic continent-continent collisional orogens, explicitly the Musgrave Block in Australia, previously inexplicable (see Dennis, 1982) are being brought to light with the advent of seismic imaging techniques which can resolve the deep crust structure of orogenic belts.

Physiography

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The process of orogeny can take tens of millions of years and build mountains from plains or even the ocean floor. Orogeny can occur due to continental collision or volcanic activity. Frequently, rock formations that undergo orogeny are severely deformed and undergo metamorphism. During orogeny, deeply buried rocks may be pushed to the surface. Sea bottom and near shore material may cover some or all of the orogenic area. If the orogeny is due to two continents colliding, the resulting mountains can be very high (see Himalaya).

Orogeny usually produces long linear structures, known as orogenic belts. Generally, orogenic belts consist of long parallel strips of rock exhibiting similar characteristics along the length of the belt. Orogenic belts are associated with subduction zones, which consume crust, produce volcanoes, and build island arcs. These island arcs may be added to a continent during an orogenic event.

List of orogenies

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North American orogenies

• Caledonian orogeny
  • the Taconic phase in the NE U.S. and Canada during the Ordovician Period.
  • the Acadian phase in the Eastern U.S. during Silurian and Devonian Periods.
• Antler orogeny
  • Ancestral Sierra Nevada western United States.
• Appalachian orogeny, usually seen as the same as the Variscan orogeny in Europe.
  • Appalachian Mountains, is a well studied orogenic belt resulting from a late Paleozoic collision between North America and Africa.
• Grenville orogeny
  • Worldwide during the late Proterozoic, 1300-1000 mya. Associated with the assembly of the supercontinent Rodinia. Formed folded mountains in Eastern North America from Newfoundland to North Carolina, 1100-1000 mya.
• Laramide orogeny
  • Rocky Mountains, western North America, 40-70 Myr ago.
• Nevadan orogeny
  • developed along western North America during the Jurassic Period.
• Ouachita orogeny
  • Ouachita Mountains of Arkansas and Oklahoma is an orogenic belt that dates from the late Paleozoic Era and is most likely a continuation of the Appalachian orogeny west across the Mississippi embayment - Reelfoot Rift zone.
• Penokean orogeny
  • Wisconsin, Minnesota, and Michigan, U. S. A. and southern Ontario, Canada, 1900 Myr ago.
• Sevier orogeny
  • Rocky Mountains, western North America, 140 - 50 million years ago.

European orogenies

• The Caledonian orogeny
  • Formation of the highlands of west Norway, Britain and Ireland in the Silurian Period.
• Uralian orogeny
  • Formation of the Ural Mountains, Eurasia, during the Permian Period.
• The Variscan orogeny (also called the Hercynian orogeny)
  • Formation of the mountains of western Iberia, SW Ireland, SW England central France, southern Germany and Czechoslovakia during the Devonian and Carboniferous Periods.
• The Alpine orogeny, encompassing:
  • the Formation of the Alps during the Eocene through Miocene Periods.
  • the Carpathean orogeny building the Carpathian Mountains of east Europe during the Miocene Period.
  • the Hellenic orogeny in Greece and Aegean area during Eocene through Miocene Period.

Asian orogenies

• The Aravalli-Delhi Orogen (precambrian)
• The Cimmerian and Cathayasian orogenies
  • Active through Triassic and Jurassic Periods along south and southeast Asia.
• Alpine orogeny, encompassing:
  • The Himalayan orogeny, forming the Himalaya Mountains, as a result of the ongoing collision of the Indian Plate with the Eurasian Plate.

South American orogenies

• Andean orogeny
  • Andes Mountains, 0-200 Myr ago.

African orogenies

• Pan-African orogeny (Neoproterozoic)

Australian orogenies

• Sleaford Orogeny (2440-2420 Ma), Gawler Craton, South Australia
• Glenburgh Orogeny (c. 2005 - 1920 Ma), Glenburgh Terrane, Western Australia.
• Kimban Orogeny (c. 1845-1700 Ma), Gawler Craton, South Australia
• Yapungku Orogeny (c. 1700 Ma), North Yilgarn craton margin, Western Australia
• Mangaroon Orogeny (c.1680 - 1620 Ma), Gascoyne Complex, Western Australia.
• Kararan Orogeny (1650- Ma), Gawler Craton, South Australia
• Barramundi Orogeny (c. 1600 Ma), MacArthur Basin, northern Australia
• Isan Orogeny, c. 1600 Ma, Mt Isa Block, Queensland
• Olarian Orogeny, Olary Block, South Australia
• Capricorn Orogeny, Gascoyne Complex, Western Australia
• Musgrave Orogeny (c. 1080 Ma), Musgrave Block, Central Australia.
• Edmundian Orogeny (c. 920 - 850 Ma), Gascoyne Complex, Western Australia.
• Petermann Orogeny (c. 510 Ma Permian), Central Australia
• Delamerian Orogeny, South Australia and Victoria, Australia, Ordovician
• Lachlan Orogeny, c. 540 and 440 Ma., Victoria and New South Wales
• Alice Springs orogeny in central Australia, Early Carboniferous
• Hunter-Bowen Orogeny, (c. 260 - 225 Ma)Permian to Triassic, Queensland and New South Wales

Antarctic orogenies

• Napier orogeny (4000 ± 200 Myr ago.)
• Rayner orogeny (~ 3500 Myr ago.)
• Humboldt orogeny (~ 3000 Myr ago.)
• Insel orogeny (2650 ± 150 Myr ago.)
• Early Ruker orogeny (2000 - 1700 Myr ago.)
• Late Ruker / Nimrod orogeny (1000 ± 150 Myr ago.)
• Beardmore orogeny (633 - 620 Myr ago.)
• Ross Orogeny (~ 500 Myr ago.)

See also

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• Continental collision

External links

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• Maps of the Acadian and Taconic orogenies
• Antarctic Geology

References

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L. Elie de Beaumont, 1852. Notice sur les Systèmes de Montagnes lit Note on Mountain Systems, Bertrand, Paris, 1543 p. (English synopsis in Dennis (1982))

Buch, L. Von, 1902. Gesammelte Schriften, Roth & Eck, Berlin.

Dana, James D., 1873. On some results of the Earth's contraction from cooling, including a discussion of the origins of mountains, and the nature of the Earth's interior. American Journal of Science, 5, pp. 423-443.

Dennis, John G., 1982. Orogeny, Benchmark Papers in Geology, Volume 62, Hutchinson Ross Pulishing Company, New York ISBN 0-87933-394-4

Hall, J., 1859. Palaeontology of New York, in New York National Survey No. 3, Part 1, 533 p.

Suess, Eduard, 1875. Die Entstehung Der Alpen lit. The Origin Of The Alps, Braumüller, Vienna, 168 p.

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