![Orogenic collapse](https://upload.wikimedia.org/wikipedia/commons/e/e9/Orogenic_collapse.svg)
In
geology
Geology () is a branch of natural science concerned with Earth and other astronomical objects, the features or rocks of which it is composed, and the processes by which they change over time. Modern geology significantly overlaps all other Ear ...
, 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 a massive object has in relation to another massive object due to gravity. It is the potential energy associated with the gravitational field, which is released (conv ...
to regions of low gravitational potential energy.
Orogenic collapse can begin at any point during an
orogeny
Orogeny is a mountain building process. An orogeny is an event that takes place at a convergent plate margin when plate motion compresses the margin. An ''orogenic belt'' or ''orogen'' develops as the compressed plate crumples and is uplifted t ...
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
The Wilson Cycle is a model that describes the opening and closing of ocean basins and the subduction and divergence of tectonic plates during the assembly and disassembly of supercontinents. A classic example of the Wilson Cycle is the opening a ...
, between continental collision and rifting.
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 (from the la, label=Late Latin, tectonicus, from the grc, τεκτονικός, lit=pertaining to building) is the generally accepted scientific theory that considers the Earth's lithosphere to comprise a number of large te ...
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 an upward force exerted by a fluid that opposes the weight of a partially or fully immersed object. In a column of fluid, pressure increases with depth as a result of the weight of the overlying fluid. Thus the p ...
forces, reducing the total thickness.
Orogens can also be destroyed by
eduction and
erosion
Erosion is the action of surface processes (such as water flow or wind) that removes soil, rock, or dissolved material from one location on the Earth's crust, and then transports it to another location where it is deposited. Erosion is distin ...
, 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
Erosion is the action of surface processes (such as water flow or wind) that removes soil, rock, or dissolved material from one location on the Earth's crust, and then transports it to another location where it is deposited. Erosion is distin ...
.
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
Extension, extend or extended may refer to:
Mathematics
Logic or set theory
* Axiom of extensionality
* Extensible cardinal
* Extension (model theory)
* Extension (predicate logic), the set of tuples of values that satisfy the predicate
* E ...
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 (, also , ) 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 and taste to brie cheese, albeit with ...
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 geologic period and system of the Paleozoic era, spanning 60.3 million years from the end of the Silurian, million years ago (Mya), to the beginning of the Carboniferous, Mya. It is named after Devon, England, whe ...
, forming major extensional structures such as the
Nordfjord-Sogn Detachment
The Nordfjord—Sogn Detachment (NSD) is a major extensional shear zone up to 6 km in thickness, which extends about 120 km along strike from Nordfjord to Sognefjord, bringing Devonian continental coarse clastic sedimentary rocks into ...
.
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 the ...
do not owe their height to the former orogeny but to other processes that occurred in the
Cenozoic
The Cenozoic ( ; ) is Earth's current geological era, representing the last 66million years of Earth's history. It is characterised by the dominance of mammals, birds and flowering plants, a cooling and drying climate, and the current configura ...
.
Basin and Range Province
The Basin and Range region of the Western United States was previously a high plateau within the
American Cordillera
The American Cordillera is a chain of mountain ranges (cordilleras) that consists of an almost continuous sequence of mountain ranges that form the western "backbone" of North America, Central America and South America, with Aconcagua as the h ...
, 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 land forms 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 exact 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 is recycled into the Earth's mantle at convergent boundaries. Where the oceanic lithosphere of a tectonic plate converges with the less dense lithosphere of a second plate, the ...
zone to a
transform boundary
A transform fault or transform boundary, is a fault along a plate boundary where the motion is predominantly horizontal. It ends abruptly where it connects to another plate boundary, either another transform, a spreading ridge, or a subductio ...
between the
North American
North America is a continent in the Northern Hemisphere and almost entirely within the Western Hemisphere. It is bordered to the north by the Arctic Ocean, to the east by the Atlantic Ocean, to the southeast by South America and the Ca ...
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 definition, to Antarctica) in the south, and is bounded by the continen ...
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 north by Western and Southern Europe and Anatolia, on the south by North Africa, and on the e ...
and the
Black Sea
The Black Sea is a marginal mediterranean sea of the Atlantic Ocean 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 bounded by Bulgaria, Georgia, Roma ...
. The northern part of the plate underwent the Aegean Orogeny (), followed by
crustal extension and thinning due to
slab rollback
Oceanic trenches are prominent long, narrow topographic depressions of the ocean floor. They are typically wide and below the level of the surrounding oceanic floor, but can be thousands of kilometers in length. There are about of oceanic tren ...
of the
African Plate
The African Plate is a major tectonic plate that includes much of the continent of Africa (except for its easternmost part) and the adjacent oceanic crust to the west and south. It is bounded by the North American Plate and South American Plat ...
.
Variscan Orogeny
The Variscan (or Hercynian) 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 ( Mya), the other being Gondwana. It separated from Gondwana (beginning in the late Triassic period) during the breakup of Pan ...
and
Gondwana
Gondwana () was a large landmass, often referred to as a supercontinent, that formed during the late Neoproterozoic (about 550 million years ago) and began to break up during the Jurassic period (about 180 million years ago). The final stages ...
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 approximately 335 million y ...
. This resulted in a high plateau of thickened crust. , the northward subducting slab began retreating southward, resulting in the thickened crust beginning to thin due to a combination of gravitational collapse, fault detachment, and softening of the crust due to added heat.
Tibetan Plateau
Although the Tibetan Plateau is in a primarily compressional environment due to the collision of the
Indian
Indian or Indians may refer to:
Peoples South Asia
* Indian people, people of Indian nationality, or people who have an Indian ancestor
** Non-resident Indian, a citizen of India who has temporarily emigrated to another country
* South Asia ...
and
Eurasian
Eurasia (, ) is the largest continental area on Earth, comprising all of Europe and Asia. Primarily in the Northern Hemisphere, Northern and Eastern Hemispheres, it spans from the British Isles and the Iberian Peninsula in the west to the Ja ...
plates, it is also experiencing east-west extension which began . The primary cause of this extension is likely gravitational collapse of the plateau due to excess gravitational potential energy, as well as possible basal shearing as the Indian plate subducts under Tibet.
See also
*
Extensional tectonics
Extensional tectonics is concerned with the structures formed by, and the tectonic processes associated with, the stretching of a planetary body's crust or lithosphere.
Deformation styles
The types of structure and the geometries formed depend on ...
References
{{reflist
Orogeny