Post-orogenic collapse

Post-orogenic collapse (also orogenic collapse, post-orogenic extension, or gravitational collapse), is the thinning and lateral spread of thickened crust under its own weight. This can be a result of overthickening of the crust or the release of tectonic forces.[1] A post-orogenic collapse marks the end of an orogeny, and represents a key phase of the Wilson Cycle, in-between continental collision and rifting.[2]

Orogens (also known as orogenic belts, or more simply mountain ranges) are built up as crust thickens while tectonic plates collide, in a process known as orogenesis. Once the tectonic forces cease, or the crust becomes unstable, the orogen may begin spreading apart and thinning, ending the orogeny. Orogens can also be destroyed by eduction and erosion, but these processes are not necessarily associated with orogenic collapse.[1] It has been argued that extension during orogenic collapse is a more effective mechanism of lowering mountains than erosion.[3]

There are two primary mechanics in an orogenic collapse: gravitational potential energy and heat flow into the thickened crust, which extends deeper underground. The added weight from the thickened crust causes the surface to sink deeper into the mantle, increasing the heat flow. Additional heat softens the rock, making it flow more easily, which can allow buoyant forces to move material in deeper sections into thinner sections.[4]

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.[1][4]

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.[1][4] This type of deformation has been compared to leaving a piece of Camembert cheese out overnight - as the cheese starts to sag and spread, the rind will eventually crack and split.[5]

Examples

Caledonian Orogeny
Main article: 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 collapse was such that the modern Scandinavian Mountains do not owe their height to the former orogeny but to other processes that occurred in the Cenozoic.[6][7]

Aegean Sea Plate
Main article: Aegean Sea Plate

The Aegean Sea Plate is a section of continental crust which has been thinned by various processes including orogenic collapse. The northern part of the plate underwent the Aegean Orogeny (c. 70 - 14 Ma), followed by orogenic collapse and crustal extension.[8]

References
  1. Adamuszek, Marta (2013-07-28). "Lecture - Orogenic Collapse".
  2. Dai, Liming; Li, Sanzhong; Li, Zhong-Hai; Somerville, Ian; Liu, Xiaochun (2018-02-09). "Post-orogenic unrooting and collapse". www.mantleplumes.org. Archived from the original on 2021-12-10. Retrieved 2021-12-10.
  3. Dewey, J.F.; Ryan, P.D.; Andersen, T.B. (1993). "Orogenic uplift and collapse, crustal thickness, fabrics and metamorphic phase changes: the role of eclogites". Geological Society, London, Special Publications. 76 (1): 325–343. doi:10.1144/gsl.sp.1993.076.01.16.
  4. Selverstone, Jane (May 2005). "Are the Alps collapsing?". Annual Review of Earth and Planetary Sciences. 33: 113–132 – via ResearchGate.
  5. Nance, Damian (2014-03-24). "What is Orogenic Collapse?". Oxford University Press.
  6. Gabrielsen, Roy H.; Faleide, Jan Inge; Pascal, Christophe; Braathen, Alvar; Nystuen, Johan Petter; Etzelmuller, Bernd; O'Donnel, Sejal (2010). "Latest Caledonian to Present tectonomorphological development of southern Norway". Marine and Petroleum Geology. 27: 709–723. doi:10.1016/j.marpetgeo.2009.06.004.
  7. Green, Paul F.; Lidmar-Bergström, Karna; Japsen, Peter; Bonow, Johan M.; Chalmers, James A. (2013). "Stratigraphic landscape analysis, thermochronology and the episodic development of elevated, passive continental margins". Geological Survey of Denmark and Greenland Bulletin. 30: 18. Archived from the original on 24 September 2015. Retrieved 30 April 2015.
  8. Searle, Michael P.; Lamont, Thomas N. (2020-03-03). "Compressional origin of the Aegean Orogeny, Greece". Geoscience Frontiers (published 2020-08-07). doi:10.1016/j.gsf.2020.07.008 – via ScienceDirect.
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