orogeny - Physiography
A period of mountain-building involving intense deformation and subsequent uplift of rocks when crustal plates collide. The plate boundaries define an orogenic belt which forms a fold-mountain chain. Metamorphism and igneous intrusion take place at depth in the orogenic belt. Examples include the Pacific belt, where the continental crust collides with the oceanic crust, and the Himalayas, which resulted from the collision of the Indian and Asian continental plates.
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 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.
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. 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.
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.
Steinmann (1906) recognised different classes of orogenic belts, including the Alpine type orogenic belt, typified by a flysch and molasse geometry to the sediments;
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); 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.
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 usually produces long linear structures, known as orogenic belts.
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