orogeny n : the process of mountain formation (especially by the upward displacement of the earth's crust)
Orogeny (Greek for "mountain generating") is the process of natural 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 differ from 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 and exposing the core or mountain roots (metamorphic rocks brought from tens of km depth to the surface). Such exhumation may be helped by isostatic movements balancing out the buoyancy of the evolving orogen. It is a question of debate to what extent can erosion modify the patterns of tectonic deformacion (see erosion and tectonics). 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.
HistoryBefore 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, for Neoplatonic thought, which influenced early Christian writers, assumed that a perfect Creation would have to have been in the form of a perfect sphere. Such thinking persisted into the eighteenth 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 orogenic belts created and contained. 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, contested hotly 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);
- 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;
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.
PhysiographyThe 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
North American orogenies
- Along western edge of Canadian shield, 2100-1900 mya.
- Hudsonian orogeny or Trans-Hudson orogeny
- Penokean orogeny
- Big Sky orogeny
- Mojave province, south western USA
- mid to south western USA, circa 1750 mya.
- mid to south western USA, circa 1600 mya.
- Grenville orogeny
- Caledonian orogeny
- Appalachian orogeny, usually seen as the same as the Variscan orogeny in Europe.
- Ouachita orogeny
- Antler orogeny
- Innuitian orogeny or Ellesmerian orogeny
- Rocky Mountains, western North America, 270 - 240 million years ago.
- Developed along western North America during the Jurassic Period.
- Rocky Mountains, western North America, 140 - 50 million years ago.
- Rocky Mountains, western North America, 40-70 Myr ago.
- The Caledonian orogeny
- Uralian orogeny
- The Variscan orogeny (also called the Hercynian orogeny)
- The Alpine orogeny, encompassing:
- Ongoing (happening now):
- the Mediterranean Ridge.
- The Aravalli-Delhi Orogen (precambrian)
- The Altaid Orogeny (Paleozoic)
- The Cimmerian and Cathayasian orogenies
- Alpine orogeny, encompassing:
- 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. 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
- 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.)
New Zealand orogenies
- 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.
orogeny in Asturian: Oroxénesis
orogeny in Bengali: গিরিজনি
orogeny in Catalan: Orogènesi
orogeny in Czech: Orogeneze
orogeny in German: Gebirgsbildung
orogeny in Spanish: Orogénesis
orogeny in French: Orogenèse
orogeny in Korean: 조산활동
orogeny in Croatian: Orogen
orogeny in Italian: Orogenesi
orogeny in Hebrew: אורוגנזה
orogeny in Kazakh: Орогенез
orogeny in Lithuanian: Kalnodara
orogeny in Hungarian: Hegységképződés
orogeny in Dutch: Gebergtevorming
orogeny in Japanese: 造山運動
orogeny in Polish: Orogeneza
orogeny in Portuguese: Orogênese
orogeny in Romanian: Orogeneză
orogeny in Russian: Горообразование
orogeny in Slovak: Orogenéza
orogeny in Slovenian: Orogeneza
orogeny in Serbo-Croatian: Orogeneza
orogeny in Finnish: Orogenia
orogeny in Swedish: Orogenes