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| orogeny | |
| plate tectonics | |
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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 Ridge s in the 1950s and 1960s. The physical manifestations of orogenesis, the process of orogeny, are ''orogenic belts''. These are usually long, thin, arcuate tracts of rocks which have a pronounced linear structure resulting in Terrane s or blocks of deformed rocks, separated generally by Dipping Thrust Fault s. These thrust faults carry relatively thin plates (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 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 dogma for geologist until the 1960's. 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) . 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);
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 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 Zone s, which consume Crust , produce volcanoes, and build Island Arc s. These island arcs may be added to a Continent during an orogenic event. LIST OF OROGENIES North America n orogenies
European orogenies
Asian orogenies
South American orogenies
Africa n orogenies Australian orogenies
Antarctic orogenies
SEE ALSO EXTERNAL LINKS REFERENCES 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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