geology Geology (). is a branch of natural science concerned with the Earth and other astronomical objects, the rocks of which they are composed, and the processes by which they change over time. Modern geology significantly overlaps all other Earth ...

, orogenic collapse is the thinning and lateral spread of thickened crust. It is a broad term referring to processes which distribute material from regions of high

gravitational potential energy Gravitational energy or gravitational potential energy is the potential energy an object with mass has due to the gravitational potential of its position in a gravitational field. Mathematically, it is the minimum Work (physics), mechanical work t ...

to regions of low gravitational potential energy. Orogenic collapse can begin at any point during an

orogeny Orogeny () is a mountain-mountain formation, building process that takes place at a convergent boundary, convergent plate margin when plate motion compresses the margin. An or develops as the compressed plate crumples and is tectonic uplift, u ...

due to overthickening of the crust. Post-orogenic collapse and post-orogenic extension refer to processes which take place once tectonic forces have been released, and represent a key phase of the Wilson Cycle, between continental collision and rifting.

Description

Orogens (also known as orogenic belts, or more simply

mountain ranges A mountain range or hill range is a series of mountains or hills arranged in a line and connected by high ground. A mountain system or mountain belt is a group of mountain ranges with similarity in form, structure, and alignment that have arise ...

) are sections of thickened crust which are built up as

tectonic plates Plate tectonics (, ) is the scientific theory that the Earth's lithosphere comprises a number of large tectonic plates, which have been slowly moving since 3–4 billion years ago. The model builds on the concept of , an idea developed durin ...

collide. The thickening of the crust marks the start of an orogeny, or "mountain building event." As the orogeny progresses, the orogen may start spreading apart and thinning. Collapse processes can begin either once the orogeny ends as the tectonic forces cease, or during the orogeny if the crust becomes unstable. There are two primary mechanisms at work in an orogenic collapse: excess gravitational potential energy and heat flow into the thickened crust. Overthickened crust can become brittle and begin collapsing and spreading under its own weight. The added weight from the thickened crust also causes it to sink deeper into the mantle, where additional heat can flow into the crust. The added heat softens the rock and makes it flow more easily, which can allow material in deeper sections to move up into thinner areas via

buoyancy Buoyancy (), or upthrust, is the force exerted by a fluid opposing the weight of a partially or fully immersed object (which may be also be a parcel of fluid). In a column of fluid, pressure increases with depth as a result of the weight of t ...

forces, reducing the total thickness. Orogens can also be destroyed by eduction and

erosion Erosion is the action of surface processes (such as Surface runoff, water flow or wind) that removes soil, Rock (geology), rock, or dissolved material from one location on the Earth's crust#Crust, Earth's crust and then sediment transport, tran ...

, but these processes are not necessarily associated with orogenic collapse. It has been argued that extension during orogenic collapse is a more effective mechanism of lowering mountains than erosion.

Models

Fixed-boundary collapse

A fixed-boundary collapse is the breakdown of the brittle upper crust and occurs when crust has overthickened while tectonic forces are still active. Flow in the lower crust may or may not occur when this happens. This can lead to exhumation of buried features.

Free-boundary collapse

Free-boundary collapse occurs when tectonic forces have been released and the thickened crust is free to move. This results in both the extension of the surface crust and flow of the lower crust to thinner regions. The surface expression of the extension can include extensive normal faulting. This type of deformation has been compared to leaving a piece of

Camembert Camembert ( , , ) is a moist, soft, creamy, surface-ripened cow's milk cheese. It was first made in the late 18th century in Camembert, Normandy, in northwest France. It is sometimes compared in look, taste and texture to brie cheese, albe ...

cheese out overnight: as the cheese starts to sag and spread, the rind will eventually crack and split.

Examples

Caledonian orogeny

The Scandinavian Caledonides is an example of an orogeny and mountain chain that reached heights of 8–9 km and then collapsed in the

Devonian The Devonian ( ) is a period (geology), geologic period and system (stratigraphy), system of the Paleozoic era (geology), era during the Phanerozoic eon (geology), eon, spanning 60.3 million years from the end of the preceding Silurian per ...

, forming major extensional structures such as the Nordfjord-Sogn Detachment. The collapse was such that the modern

Scandinavian Mountains The Scandinavian Mountains or the Scandes is a mountain range that runs through the Scandinavian Peninsula. The western sides of the mountains drop precipitously into the North Sea and Norwegian Sea, forming the fjords of Norway, whereas to th ...

do not owe their height to the former orogeny but to other processes that occurred in the

Cenozoic The Cenozoic Era ( ; ) is Earth's current geological era, representing the last 66million years of Earth's history. It is characterized by the dominance of mammals, insects, birds and angiosperms (flowering plants). It is the latest of three g ...

Basin and Range Province

The

Basin and Range Province The Basin and Range Province is a vast United States physiographic region, physiographic region covering much of the inland Western United States and Northern Mexico, northwestern Mexico. It is defined by unique basin and range topography, charac ...

of the Western United States was previously a high plateau within the

American Cordillera The American Cordillera ( ) is a chain of mountain ranges (cordilleras), consisting of an almost continuous sequence of mountain ranges that form the western "backbone" of the Americas. Aconcagua is the highest peak of the chain. It is also the ...

, which has since been extended and thinned. The characteristic

topography Topography is the study of the forms and features of land surfaces. The topography of an area may refer to the landforms and features themselves, or a description or depiction in maps. Topography is a field of geoscience and planetary sci ...

is caused by the crust breaking up into fault blocks as a result of the extension. The cause of the extension is debated, though it is likely related to the transition from a

subduction Subduction is a geological process in which the oceanic lithosphere and some continental lithosphere is recycled into the Earth's mantle at the convergent boundaries between tectonic plates. Where one tectonic plate converges with a second p ...

zone to a transform boundary between the North American and

Pacific The Pacific Ocean is the largest and deepest of Earth's five oceanic divisions. It extends from the Arctic Ocean in the north to the Southern Ocean, or, depending on the definition, to Antarctica in the south, and is bounded by the cont ...

plates, as well as possible mantle upwelling.

Aegean Sea Plate

The Aegean Sea Plate is a section of continental crust which has been thinned, and is considered a high plateau between the

Mediterranean The Mediterranean Sea ( ) is a sea connected to the Atlantic Ocean, surrounded by the Mediterranean basin and almost completely enclosed by land: on the east by the Levant in West Asia, on the north by Anatolia in West Asia and Southern ...

and the

Black Sea The Black Sea is a marginal sea, marginal Mediterranean sea (oceanography), mediterranean sea lying between Europe and Asia, east of the Balkans, south of the East European Plain, west of the Caucasus, and north of Anatolia. It is bound ...

. The northern part of the plate underwent the Aegean orogeny (), followed by crustal extension and thinning due to slab rollback of the African Plate.

Variscan orogeny

The Variscan orogeny was a result of the collision between the

Laurussia Laurasia () was the more northern of two large landmasses that formed part of the Pangaea supercontinent from around (Million years ago, Mya), the other being Gondwana. It separated from Gondwana (beginning in the late Triassic period) during ...

and

Gondwana Gondwana ( ; ) was a large landmass, sometimes referred to as a supercontinent. The remnants of Gondwana make up around two-thirds of today's continental area, including South America, Africa, Antarctica, Australia (continent), Australia, Zea ...

plates during the formation of

Pangaea Pangaea or Pangea ( ) was a supercontinent that existed during the late Paleozoic and early Mesozoic eras. It assembled from the earlier continental units of Gondwana, Euramerica and Siberia during the Carboniferous period approximately 335 mi ...

. This resulted in a high plateau of thickened crust. , the northward subducting slab began retreating southward, resulting in the thickened crust beginning to thin from a combination of gravitational collapse, fault detachment, and softening of the crust due to added heat.

Tibetan Plateau

Although the

Tibetan Plateau The Tibetan Plateau, also known as the Qinghai–Tibet Plateau or Qingzang Plateau, is a vast elevated plateau located at the intersection of Central Asia, Central, South Asia, South, and East Asia. Geographically, it is located to the north of H ...

is in a primarily compressional environment caused by the collision of the Indian and Eurasian plates, it is also experiencing east-west extension which began . The primary cause of this extension is likely gravitational collapse of the plateau from excess gravitational potential energy, as well as possible basal shearing as the Indian plate subducts under Tibet.

References

{{reflist Orogeny