Orogeny

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Orogeny (Greek for "mountain generating") refers to natural mountain building, and may be studied as (a) a tectonic structural event, (b) as a geographical event, and (c) a chronological event. Orogenic events (a) cause distinctive structural phenomena and related tectonic activity, (b) affect certain regions of rocks and crust, and (c) happen within a specific period of time.

Orogenic events occur solely as a result 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 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 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.

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 removes much of the mountains, exposing the core or mountain roots (metamorphic rocks brought to the surface from a depth of several kilometres). Such exhumation may be helped by isostatic movements balancing out the buoyancy of the evolving orogen. There is 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 dip away from the orogenic core.

Contents 1 History 2 Physiography 3 List of orogenies 3.1 North American orogenies 3.2 European orogenies 3.3 Asian orogenies 3.4 South American orogenies 3.5 African orogenies 3.6 Australian orogenies 3.7 Antarctic orogenies 3.8 New Zealand orogenies 4 See also 5 References 6 External links

[edit] History

Before the development of geologic concepts during the 19th century, the presence of 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 and assumed that a perfect Creation would have to have taken the form of a perfect sphere. Such thinking persisted into the 18th century.

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.

H.J. Zwart (1967) drew attention to the metamorphic differences in orogenic belts, proposing three types, modified by W. S. 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.

[edit] Physiography

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.

[edit] List of orogenies

[edit] North American orogenies

• Wopmay orogeny
  • Along western edge of Canadian shield, 2100–1900 Ma.
• Hudsonian orogeny or Trans-Hudson orogeny
  • Extends from Hudson Bay west into Saskatchewan then south through the western Dakotas and Nebraska. Result of the collision of the Superior craton with the Hearne craton and the Wyoming craton during the Proterozoic.
  • Lasted from 2000–1800 Ma.
• Penokean orogeny
  • Wisconsin, Minnesota, and Michigan, U. S. A. and southern Ontario, Canada, 1850-1840 Ma.
• Big Sky orogeny
  • Proterozoic collision between the Hearne craton and the Wyoming craton in southwest Montana, 1770 Ma.
• Ivanpah orogeny
  • Mojave province, south western USA
• Yavapai orogeny
  • mid to south western USA, circa 1750 Ma.
• Mazatzal orogeny
  • mid to south western USA, circa 1600 Ma.
• Grenville orogeny
  • Worldwide during the late Proterozoic, 1300–1000 Ma. Associated with the assembly of the supercontinent Rodinia. Formed folded mountains in Eastern North America from Newfoundland to North Carolina, 1100–1000 Ma.

• Caledonian orogeny
  • the East Greenland Orogen, formed from Cryogenian to Devonian
  • 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.
• 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.
  • Taconic orogeny
  • Acadian orogeny
  • Alleghenian orogeny
• 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.
• Antler orogeny
  • Ancestral Sierra Nevada western United States.
  • Late Devonian - early Mississippian.
• Innuitian orogeny or Ellesmerian orogeny
  • Innuitian Mountains, Canadian Arctic, extending from Ellesmere Island to Melville Island, Mississippian 345 Ma.
• Sonoma orogeny
  • Rocky Mountains, western North America, 270–240 Ma.
• Nevadan orogeny
  • Developed along western North America during the Jurassic Period.
• Sevier orogeny
  • Rocky Mountains, western North America, 140–50 Ma.
• Laramide orogeny
  • Rocky Mountains, western North America, 40–70 Ma.

[edit] European orogenies

• The Precambrian orogeny
  • Formation of the Baltic Shield, which includes southern Norway, nearly whole Sweden, Finland and small part of Russia near the Finnish border.
• The Cadomian Orogeny on the north coast of Armorica in the Ediacaran.
• The Caledonian orogeny
  • Formation of the highlands of western 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 Czech Republic during the Devonian and Carboniferous Periods.
• The Alpine orogeny, encompassing:
  • the Formation of the Alps during the Eocene through Miocene Periods.
  • the Carpathian orogeny building the Carpathian Mountains of eastern Europe during the Jurassic-Cretaceous to Miocene Period.
  • the Hellenic orogeny in Greece and the Aegean area during Eocene through Miocene Periods.
• Ongoing (happening now):
  • the Mediterranean Ridge.

[edit] Asian orogenies

• The Aravalli-Delhi Orogen (precambrian)
• The Altaid Orogeny (Paleozoic)
• 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.

The Dabie-Sulu Orogen (Mesozoic)

[edit] South American orogenies

• Pampean orogeny
• Famatinian orogeny
• Gondwanide orogeny
• Toco orogeny
  • Chilean Coast Range 300–330 Ma.^[1]
• Andean orogeny
  • Andes Mountains, 0–200 Ma.

[edit] African orogenies

• Pan-African orogeny (Neoproterozoic)
• Damaran Orogeny

[edit] 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. 1765 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, Mount 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. 550–535 Ma late Neoproterozoic to Cambrian), 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

[edit] Antarctic orogenies

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

[edit] New Zealand orogenies

• Tuhua Orogeny (370–330 Ma)
• Rangitata Orogeny (142–99 Ma)
• Kaikoura Orogeny (24 Ma–present)

[edit] See also

• Continental collision
• Plate tectonics

[edit] References

• Élie de Beaumont, J.-B., 1852. Notice sur les Systèmes de Montagnes ("Note on Mountain Systems"), Bertrand, Paris, 1543 pp. (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.

• Harms, Brady, Cheney, 2006. Exploring the Proterozoic Big Sky Orogeny in Southwest Montana, 19th annual Keck symposium.

[edit] External links

• Maps of the Acadian and Taconic orogenies
• Antarctic Geology