Orogeny
Orogeny is a mountain-building process that takes place at a convergent plate margin when plate motion compresses the margin. An or develops as the compressed plate crumples and is uplifted to form one or more mountain ranges. This involves a series of geological processes collectively called orogenesis. These include both structural deformation of existing continental crust and the creation of new continental crust through volcanism. Magma rising in the orogen carries less dense material upwards while leaving more dense material behind, resulting in compositional differentiation of Earth's lithosphere. A synorogenic process or event is one that occurs during an orogeny.
The word orogeny comes. Although it was used before him, the American geologist G. K. Gilbert used the term in 1890 to mean the process of mountain-building, as distinguished from epeirogeny.
Tectonics
Orogeny takes place on the convergent margins of continents. The convergence may take the form of subduction or continental collision.Orogeny typically produces orogenic belts or orogens, which are elongated regions of deformation bordering continental cratons. Young orogenic belts, in which subduction is still taking place, are characterized by frequent volcanic activity and earthquakes. Older orogenic belts are typically deeply eroded to expose displaced and deformed strata. These are often highly metamorphosed and include vast bodies of intrusive igneous rock called batholiths.
Subduction zones consume oceanic crust, thicken lithosphere, and produce earthquakes and volcanoes. Not all subduction zones produce orogenic belts; mountain building takes place only when the subduction produces compression in the overriding plate. Whether subduction produces compression depends on such factors as the rate of plate convergence and the degree of coupling between the two plates, while the degree of coupling may in turn rely on such factors as the angle of subduction and rate of sedimentation in the oceanic trench associated with the subduction zone. The Andes Mountains are an example of a noncollisional orogenic belt, and such belts are sometimes called Andean-type orogens.
As subduction continues, island arcs, continental fragments, and oceanic material may gradually accrete onto the continental margin. This is one of the main mechanisms by which continents have grown. An orogen built of crustal fragments accreted over a long period of time, without any indication of a major continent-continent collision, is called an accretionary orogen. The North American Cordillera and the Lachlan Orogen of southeast Australia are examples of accretionary orogens.
The orogeny may culminate with continental crust from the opposite side of the subducting oceanic plate arriving at the subduction zone. This ends subduction and transforms the accretional orogen into a Himalayan-type collisional orogen. The collisional orogeny may produce extremely high mountains, as has been taking place in the Himalayas for the last 65 million years.
The processes of orogeny can take tens of millions of years and build mountains from what were once sedimentary basins. Activity along an orogenic belt can be extremely long-lived. For example, much of the basement underlying the United States belongs to the Transcontinental Proterozoic Provinces, which accreted to Laurentia over the course of 200 million years in the Paleoproterozoic. The Yavapai and Mazatzal orogenies were peaks of orogenic activity during this time. These were part of an extended period of orogenic activity that included the Picuris orogeny and culminated in the Grenville orogeny, lasting at least 600 million years. A similar sequence of orogenies has taken place on the west coast of North America, beginning in the late Devonian with the Antler orogeny and continuing with the Sonoma orogeny and Sevier orogeny and culminating with the Laramide orogeny. The Laramide orogeny alone lasted 40 million years, from 75 million to 35 million years ago.
Intraplate orogeny
Stresses transmitted from plate boundaries can also lead to episodes of intracontinental transpressional orogeny. Examples in Australia include the Neoproterozoic Petermann Orogeny, and the Sprigg Orogeny.Orogens
Orogens show a great range of characteristics,but they may be broadly divided into collisional orogens and noncollisional orogens. Collisional orogens can be further divided by whether the collision is with a second continent or a continental fragment or island arc. Repeated collisions of the latter type, with no evidence of collision with a major continent or closure of an ocean basin, result in an accretionary orogen. Examples of orogens arising from collision of an island arc with a continent include Taiwan and the collision of Australia with the Banda arc. Orogens arising from continent-continent collisions can be divided into those involving ocean closure and those involving glancing collisions with no ocean basin closure.
Orogens have a characteristic structure, though this shows considerable variation. A foreland basin forms ahead of the orogen due mainly to loading and resulting flexure of the lithosphere by the developing mountain belt. A typical foreland basin is subdivided into a wedge-top basin above the active orogenic wedge, the foredeep immediately beyond the active front, a forebulge high of flexural origin and a back-bulge area beyond, although not all of these are present in all foreland-basin systems. The basin migrates with the orogenic front and early deposited foreland basin sediments become progressively involved in folding and thrusting. Sediments deposited in the foreland basin are mainly derived from the erosion of the actively uplifting rocks of the mountain range, although some sediments derive from the foreland. The fill of many such basins shows a change in time from deepwater marine through shallow water to continental sediments.
While active orogens are found on the margins of present-day continents, older inactive orogenies, such as the Algoman, Penokean and Antler, are represented by deformed and metamorphosed rocks with sedimentary basins further inland.
Orogenic cycle
Long before the acceptance of plate tectonics, geologists had found evidence within many orogens of repeated cycles of deposition, deformation, crustal thickening and mountain building, and crustal thinning to form new depositional basins. These were named orogenic cycles, and various theories were proposed to explain them. Canadian geologist Tuzo Wilson first put forward a plate tectonic interpretation of orogenic cycles, now known as Wilson cycles. Wilson proposed that orogenic cycles represented the periodic opening and closing of an ocean basin, with each stage of the process leaving its characteristic record on the rocks of the orogen.Continental rifting
The Wilson cycle begins when previously stable continental crust comes under tension from a shift in mantle convection. Continental rifting takes place, which thins the crust and creates basins in which sediments accumulate. As the basins deepen, the ocean invades the rift zone, and as the continental crust rifts completely apart, shallow marine sedimentation gives way to deep marine sedimentation on the thinned marginal crust of the two continents.Seafloor spreading
As the two continents rift apart, seafloor spreading commences along the axis of a new ocean basin. Deep marine sediments continue to accumulate along the thinned continental margins, which are now passive margins.Subduction
At some point, subduction is initiated along one or both of the continental margins of the ocean basin, producing a volcanic arc and possibly an Andean-type orogen along that continental margin. This produces deformation of the continental margins and possibly crustal thickening and mountain building.Mountain building
in orogens is largely a result of crustal thickening. The compressive forces produced by plate convergence result in pervasive deformation of the crust of the continental margin. This takes the form of folding of the ductile deeper crust and thrust faulting in the upper brittle crust.Crustal thickening raises mountains through the principle of isostasy. Isostacy is the balance of the downward gravitational force upon an upthrust mountain range and the buoyant upward forces exerted by the dense underlying mantle.
Portions of orogens can also experience uplift as a result of delamination of the orogenic lithosphere, in which an unstable portion of cold lithospheric root drips down into the asthenospheric mantle, decreasing the density of the lithosphere and causing buoyant uplift. An example is the Sierra Nevada in California. This range of fault-block mountains experienced renewed uplift and abundant magmatism after a delamination of the orogenic root beneath them.
File:Mount Rundle, Banff, Canada.jpg|thumb|Mount Rundle, Banff, Alberta
Mount Rundle on the Trans-Canada Highway between Banff and Canmore provides a classic example of a mountain cut in dipping-layered rocks. Millions of years ago a collision caused an orogeny, forcing horizontal layers of an ancient ocean crust to be thrust up at an angle of 50–60°. That left Rundle with one sweeping, tree-lined smooth face, and one sharp, steep face where the edge of the uplifted layers are exposed.
Although mountain building mostly takes place in orogens, a number of secondary mechanisms are capable of producing substantial mountain ranges. Areas that are rifting apart, such as mid-ocean ridges and the East African Rift, have mountains due to thermal buoyancy related to the hot mantle underneath them; this thermal buoyancy is known as dynamic topography. In strike-slip orogens, such as the San Andreas Fault, restraining bends result in regions of localized crustal shortening and mountain building without a plate-margin-wide orogeny. Hotspot volcanism results in the formation of isolated mountains and mountain chains that look as if they are not necessarily on present tectonic-plate boundaries, but they are essentially the product of plate tectonism. Likewise, uplift and erosion related to epeirogenesis can create local topographic highs.