Craton

A craton ( /ˈkreɪtɒn/ KRAYT-on, /ˈkrætɒn/ KRAT-on, or /ˈkreɪtən/ KRAY-tən;[1][2][3] from Ancient Greek: κράτος kratos "strength") is an old and stable part of continental lithosphere (the Earth's two topmost layers, the crust and the lithospheric mantle). Having often survived cycles of merging and rifting of continents, cratons are generally found in the interiors of tectonic plates; the exceptions occur where geologically recent rifting events have separated cratons and created passive margins along their edges. Cratons are composed of ancient crystalline basement rocks covered by younger sedimentary rocks. They have a thick crust and deep lithospheric roots extending several hundred kilometres into Earth's mantle. In simpler terms, craton is a layer of the earths crust on that has been almost unchanged through all of history. This crust consists of shields and platforms. Shields are made of basement rock and are visible, while platforms are the portion not visible made up of sedimentary rock.[4]

Cratons contain the oldest continental crust rocks on Earth. They were formed in the Archaean (4 to 2.5 billion years ago) and the Proterozoic (2.5 billion- 538.8 million year ago) geologic eons. Most were formed in the Archaean.[5][6]

## Contents

* 1 Terminology
* 2 Examples
* 3 Structure
* 4 Formation
* 4.1 Root origin
* 4.2 Repeated continental collision model
* 4.3 Molten plume model
* 4.4 Subducting ocean slab model
* 4.5 Impact origin model
* 4.6 Evidence for each model
* 5 Erosion
* 6 See also
* 7 References
* 8 Further reading
* 9 External links

## Terminology

Geologic provinces of the world (USGS)

Shield

Platform

Orogen

Basin

Large igneous province

Extended crust

| Oceanic crust:

0–20 Ma

20–65 Ma

>65 Ma

|

The term craton is used to distinguish the stable portion of the continental crust from regions that are more geologically active and unstable.[7]

Bleeker and Davis (2004) define a craton as "a large, coherent domain of Earth's continental crust that has attained and maintained long-term stability, having undergone little internal deformation, except perhaps near its margins due to interaction with neighbouring terranes."

Scott King (2005) define the Archaean cratons as "relatively flat, stable regions of the crust that have remained undeformed since the Precambrian, forming the ancient cores of the continents."

Cratons are composed of two layers: the cratonic basement of metamorphosed crystalline and metamorphic rocks and the platform, which is a younger, weakly deformed sedimentary cover which overlies this basement. Continental shields are exposed (they crop out at the surface) cratonic basement rocks and are thus dominated by crystalline and metamorphic rocks. Shields and platforms are physiographic terms rather than tectonic entities.[8]

The word craton was first proposed by the Austrian geologist Leopold Kober in 1921 as Kratogen, referring to stable continental platforms, and orogen as a term for mountain or orogenic belts. Later Hans Stille shortened the former term to Kraton, from which craton derives.[9]

## Examples

Examples of cratons are the Dharwar Craton[10] in India, North China Craton,[11] the East European Craton,[12] the Amazonian Craton in South America,[13] the Kaapvaal craton in South Africa,[14] the North American Craton (also called the Laurentia Craton),[15] the Gawler craton in South Australia,[16] the Archean Wyoming Craton, and the Superior Craton in Canada.[17]

## Structure

Cratons have thick lithospheric roots. Mantle tomography shows that cratons are underlain by anomalously cold mantle corresponding to lithosphere more than twice the typical 100 km (60 mi) thickness of mature oceanic or non-cratonic, continental lithosphere. At that depth, craton roots extend into the asthenosphere,[18] and the low-velocity zone seen elsewhere at these depths is weak or absent beneath stable cratons.[19] Craton lithosphere is distinctly different from oceanic lithosphere because cratons have a neutral or positive buoyancy and a low intrinsic density. This low-density offsets density increases from geothermal contraction and prevents the craton from sinking into the deep mantle. The cratonic lithosphere is much older than the oceanic lithosphere—up to 4 billion years versus 180 million years.[20]

Rock fragments (xenoliths) carried up from the mantle by magmas containing peridotite have been delivered to the surface as inclusions in subvolcanic pipes called kimberlites. These inclusions have densities consistent with craton composition and are composed of mantle material residual from high degrees of partial melt. Peridotite is strongly influenced by the inclusion of moisture. Craton peridotite moisture content is unusually low, which leads to much greater strength. It also contains high percentages of low-weight magnesium instead of higher-weight calcium and iron.[21] Peridotites are important for understanding the deep composition and origin of cratons because peridotite nodules are pieces of mantle rock modified by partial melting. Harzburgite peridotites represent the crystalline residues after extraction of melts of compositions like basalt and komatiite.[22]

## Formation

Idealized cross-section of Earth's lithosphere, including the relationship between cratons, shields and platforms (Abbreviations: cb=cratonic basin, LIP=large igneous province, MOR=mid-ocean ridge)

The process by which cratons were formed is called cratonization. Much about this process remains uncertain, with very little consensus in the scientific community.[23] However, the first cratonic landmasses likely formed during the Archean eon. This is indicated by the age of diamonds, which originate in the roots of cratons and are almost always over 2 billion years and often over 3 billion years in age.[20] Rock of the Archean age makes up only 7% of the world's current cratons; even allowing for erosion and destruction of past formations, this suggests that only 5 to 40 per cent of the present continental crust formed during the Archean.[24] The Archeon eon makes approximately around 3% of the Earth's current surface, but all continents on earth contain crust from it.[25] Cratonization likely was completed during the Proterozoic. Subsequent growth of continents was by accretion at continental margins.[20]

### Root origin

The origin of the roots of cratons is still debated.[26][27][21][23] However, the present understanding of cratonization began with the publication in 1978 of a paper by Thomas H. Jordan in Nature. Jordan proposes that cratons formed from a high degree of partial melting of the upper mantle, with 30 to 40 per cent of the source rock entering the melt. Such a high degree of melting was possible because of the high mantle temperatures of the Archean. The extraction of so much magma left behind a solid peridotite residue that was enriched in lightweight magnesium and thus lower in chemical density than the undepleted mantle. This lower chemical density compensated for the effects of thermal contraction as the craton and its roots cooled so that the physical density of the cratonic roots matched that of the surrounding hotter but more chemically dense mantle.[28][20] In addition to cooling the craton roots and lowering their chemical density, the extraction of magma also increased the viscosity and melting temperature of the craton roots and prevented mixing with the surrounding undepleted mantle.[29] The resulting mantle roots have remained stable for billions of years.[27] Jordan suggests that depletion occurred primarily in subduction zones and secondarily as flood basalts.[30]

This model of melt extraction from the upper mantle has held up well with subsequent observations.[31] The properties of mantle xenoliths confirm that the geothermal gradient is much lower beneath continents than oceans.[32] The olivine of craton root xenoliths is extremely dry, which would give the roots a very high viscosity.[33] Rhenium–osmium dating of xenoliths indicates that the oldest melting events took place in the early to middle Archean. Significant cratonization continued into the late Archean, accompanied by voluminous mafic magmatism.[34]

However, melt extraction alone cannot explain all the properties of craton roots. Jordan notes in his paper that this mechanism could be effective for constructing craton roots only down to a depth of 200 kilometers (120 mi). The great depths of craton roots required further explanation.[30] The 30 to 40 per cent partial melting of mantle rock at 4 to 10 GPa pressure produces komatiite magma and a solid residue very close in composition to Archean lithospheric mantle. Still, continental shields do not contain enough komatiite to match the expected depletion. Either much of the komatiite never reached the surface, or other processes aided craton root formation.[34] There are many competing hypotheses of how cratons have been formed.

### Repeated continental collision model

Jordan's model suggests that further cratonization resulted from repeated continental collisions. The thickening of the crust associated with these collisions may have been balanced by craton root thickening according to the principle of isostacy.[30] Jordan likens this model to "kneading" of the cratons, allowing low-density material to move up and higher density to move down, creating stable cratonic roots as deep as 400 km (250 mi).[33]

### Molten plume model

A second model suggests that the surface crust was thickened by a rising plume of molten material from the deep mantle. This would have built up a thick layer of depleted mantle underneath the cratons.

### Subducting ocean slab model

A third model suggests that successive slabs of subducting oceanic lithosphere became lodged beneath a proto-craton, underplating the craton with chemically depleted rock.[33][21][26]

### Impact origin model

A fourth theory presented in a 2015 publication suggests that the origin of the cratons is similar to crustal plateaus observed on Venus, which may have been created by large asteroid impacts.[23] In this model, large impacts on the Earth's early lithosphere penetrated deep into the mantle and created enormous lava ponds.[23] The paper suggests these lava ponds cooled to form the craton's root.[23]

### Evidence for each model

The chemistry of xenoliths[31] and seismic tomography both favor the two accretional models over the plume model.[33][35] However, other geochemical evidence favors mantle plumes.[36][37][38] Tomography shows two layers in the craton roots beneath North America. One is found at depths shallower than 150 km (93 mi) and may be Archean, while the second is found at depths from 180 to 240 km (110 to 150 mi) and may be younger. The second layer may be a less depleted thermal boundary layer that stagnated against the depleted "lid" formed by the first layer.[39] The impact origin model does not require plumes or accretion; this model is, however, not incompatible with either.[23]

All these proposed mechanisms rely on buoyant, viscous material separating from a denser residue due to mantle flow, and it is possible that more than one mechanism contributed to craton root formation.[34][23]

## Erosion

The long-term erosion of cratons has been labelled the "cratonic regime". It involves processes of pediplanation and etchplanation that lead to the formation of flattish surfaces known as peneplains.[40] While the process of etchplanation is associated to humid climate and pediplanation with arid and semi-arid climate, shifting climate over geological time leads to the formation of so-called polygenetic peneplains of mixed origin. Another result of the longevity of cratons is that they may alternate between periods of high and low relative sea levels. High relative sea level leads to increased oceanicity, while the opposite leads to increased inland conditions.[40]

Many cratons have had subdued topographies since Precambrian times. For example, the Yilgarn craton of Western Australia was flattish already by Middle Proterozoic times[40] and the Baltic Shield had been eroded into a subdued terrain already during the Late Mesoproterozoic when the rapakivi granites intruded.[41][42]

## See also

* List of shields and cratons
* Cratonic sequence

## References

1. ↑ "Definition of craton in North American English". http://www.oxforddictionaries.com/us/definition/american_english/craton.
2. ↑ "Definition of craton in British and Commonwealth English". http://www.oxforddictionaries.com/definition/english/craton.
3. ↑ Macquarie Dictionary (5th ed.). Sydney: Macquarie Dictionary Publishers Pty Ltd. 2009.
4. ↑ "Craton | Structure, Shield, Platform | Britannica" (in en). Encyclopedia Britannica. https://www.britannica.com/science/craton.
5. ↑ Frost and Mueller (2024)
6. ↑ Craton, Shield and Platform Explained in Geology (2052) GeologyPoint.com [1]
7. ↑ Jackson, Julia A., ed (1997). "craton". Glossary of geology. (Fourth ed.). Alexandria, Virginia: American Geological Institute. ISBN 0-922152-34-9.
8. ↑ Bleeker and Davis (2004)
9. ↑ Şengör, A.M.C. (2003). The Large-wavelength Deformations of the Lithosphere: Materials for a history of the evolution of thought from the earliest times to plate tectonics. Geological Society of America memoir. 196. p. 331.
10. ↑ Ratheesh-Kumar, R.T.; Windley, B.F.; Xiao, W.J.; Jia, X-L.; Mohanty, D.P.; Zeba-Nezrin, F.K. (October 2019). "Early growth of the Indian lithosphere: implications from the assembly of the Dharwar Craton and adjacent granulite blocks, southern India". Precambrian Research 336. doi:10.1016/j.precamres.2019.105491.
11. ↑ Kusky, T. M.; Windley, B. F.; Zhai, M.-G. (2007). "Tectonic evolution of the North China Block: from orogen to craton to orogen". Geological Society, London, Special Publications 280 (1): 1–34. doi:10.1144/sp280.1. Bibcode: 2007GSLSP.280....1K.
12. ↑ Artemieva, Irina M (August 2003). "Lithospheric structure, composition, and thermal regime of the East European Craton: implications for the subsidence of the Russian platform". Earth and Planetary Science Letters 213 (3–4): 431–446. doi:10.1016/S0012-821X(03)00327-3. Bibcode: 2003E&PSL.213..431A. https://curis.ku.dk/ws/files/49250885/2003_EPSL_EEC_Art.PDF.
13. ↑ Cordani, U.G.; Teixeira, W.; D'Agrella-Filho, M.S.; Trindade, R.I. (June 2009). "The position of the Amazonian Craton in supercontinents". Gondwana Research 15 (3–4): 396–407. doi:10.1016/j.gr.2008.12.005. Bibcode: 2009GondR..15..396C.
14. ↑ Nguuri, T. K.; Gore, J.; James, D. E.; Webb, S. J.; Wright, C.; Zengeni, T. G.; Gwavava, O.; Snoke, J. A. (1 July 2001). "Crustal structure beneath southern Africa and its implications for the formation and evolution of the Kaapvaal and Zimbabwe cratons". Geophysical Research Letters 28 (13): 2501–2504. doi:10.1029/2000GL012587.
15. ↑ Hoffman, P F (May 1988). "United Plates of America, The Birth of a Craton: Early Proterozoic Assembly and Growth of Laurentia". Annual Review of Earth and Planetary Sciences 16 (1): 543–603. doi:10.1146/annurev.ea.16.050188.002551. Bibcode: 1988AREPS..16..543H.
16. ↑ Hand, M.; Reid, A.; Jagodzinski, L. (1 December 2007). "Tectonic Framework and Evolution of the Gawler craton, Southern Australia". Economic Geology 102 (8): 1377–1395. doi:10.2113/gsecongeo.102.8.1377. Bibcode: 2007EcGeo.102.1377H.
17. ↑ Zhu, Rixiang; Zhao, Guochun; Xiao, Wenjiao; Chen, Ling; Tang, Yanjie (2021-09). "Origin, Accretion, and Reworking of Continents" (in en). Reviews of Geophysics 59 (3). doi:10.1029/2019RG000689. ISSN 8755-1209. https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2019RG000689.
18. ↑ Petit, Charles (18 December 2010). "Continental Hearts – Science News". Science News (Society for Science & the Public) 178 (13): 24. doi:10.1002/scin.5591781325. ISSN 0036-8423. http://www.sciencenews.org/view/feature/id/66927/title/Continental_Hearts.
19. ↑ Kearey, P.; Klepeis, K.A.; Vine, F.J. (2009). Global tectonics. (3rd ed.). Oxford: Wiley-Blackwell. p. 349. ISBN 978-1-4051-0777-8.
20. ↑ 20.0 20.1 20.2 20.3 Petit 2010, p. 25.
21. ↑ 21.0 21.1 21.2 Petit 2010, pp. 25–26.
22. ↑ Philpotts, Anthony R.; Ague, Jay J. (2009). Principles of igneous and metamorphic petrology (2nd ed.). Cambridge, UK: Cambridge University Press. pp. 373, 602–603. ISBN 978-0-521-88006-0.
23. ↑ 23.0 23.1 23.2 23.3 23.4 23.5 23.6 Hansen, Vicki L. (24 August 2015). "Impact origin of Archean cratons". Lithosphere 7 (5): 563–578. doi:10.1130/L371.1. Bibcode: 2015Lsphe...7..563H.
24. ↑ Stanley, Steven M. (1999). Earth System History. New York: W.H. Freeman and Company. pp. 297–302. ISBN 0-7167-2882-6.
25. ↑ "article" (in en). https://www.geosociety.org/GSA/GSA/GSAToday/science/G541A/article.aspx.
26. ↑ 26.0 26.1 Lee, C. (2006). "Geochemical/Petrologic constraints on the origin of cratonic mantle". Archean Geodynamics and Environments. Geophysical Monograph Series. 164. p. 89. doi:10.1029/164GM08. ISBN 978-0-87590-429-0. Bibcode: 2006GMS...164...89L. https://static1.squarespace.com/static/54b9bb6fe4b07b4a7d145b55/t/54c85e17e4b0278ebafdc6e9/1422417431292/24-2006LeeAGUcraton.pdf. Retrieved 20 November 2021.
27. ↑ 27.0 27.1 Miller, Meghan S.; Eaton, David W. (September 2010). "Formation of cratonic mantle keels by arc accretion: Evidence from S receiver functions: Formation of cratonic mantle keels". Geophysical Research Letters 37 (18): n/a. doi:10.1029/2010GL044366.
28. ↑ Jordan, Thomas H. (August 1978). "Composition and development of the continental tectosphere". Nature 274 (5671): 544–548. doi:10.1038/274544a0. Bibcode: 1978Natur.274..544J.
29. ↑ Jordan 1978, p. 546.
30. ↑ 30.0 30.1 30.2 Jordan 1978, p. 547.
31. ↑ 31.0 31.1 Lee 2006.
32. ↑ Jordan 1978, p. 544.
33. ↑ 33.0 33.1 33.2 33.3 Petit 2010, p. 26.
34. ↑ 34.0 34.1 34.2 Kearey, Klepeis & Vine 2009, p. 351.
35. ↑ Miller & Eaton 2010.
36. ↑ Tomlinson, Kirsty Y.; Condie, Kent C. (2001). "Archean mantle plumes: Evidence from greenstone belt geochemistry". Mantle Plumes: Their Identification Through Time. doi:10.1130/0-8137-2352-3.341. ISBN 978-0-8137-2352-5. https://www.researchgate.net/publication/245540739. Retrieved 21 November 2021.
37. ↑ Ernst, Richard E.; Buchan, Kenneth L.; Campbell, Ian H. (February 2005). "Frontiers in large igneous province research". Lithos 79 (3–4): 271–297. doi:10.1016/j.lithos.2004.09.004. Bibcode: 2005Litho..79..271E.
38. ↑ Kearey, Klepeis & Vine 2009, p. 352.
39. ↑ Yuan, Huaiyu; Romanowicz, Barbara (August 2010). "Lithospheric layering in the North American craton". Nature 466 (7310): 1063–1068. doi:10.1038/nature09332. PMID 20740006. Bibcode: 2010Natur.466.1063Y.
40. ↑ 40.0 40.1 40.2 Fairbridge, Rhodes W.; Finkl Jr., Charles W. (1980). "Cratonic erosion unconformities and peneplains". The Journal of Geology 88 (1): 69–86. doi:10.1086/628474. Bibcode: 1980JG.....88...69F.
41. ↑ Lindberg, Johan (April 4, 2016). "berggrund och ytformer" (in sv). http://uppslagsverket.fi/sv/sok/view-103684-BerggrundOchYtformer.
42. ↑ Lundmark, Anders Mattias; Lamminen, Jarkko (2016). "The provenance and setting of the Mesoproterozoic Dala Sandstone, western Sweden, and paleogeographic implications for southwestern Fennoscandia". Precambrian Research 275: 197–208. doi:10.1016/j.precamres.2016.01.003. Bibcode: 2016PreR..275..197L.

## Further reading

* Dayton, Gene (2006). Geological Evolution of Australia. http://courseprofile.cqu.edu.au/CourseProfile/deliverPDF.jsp?docId=33717&courseCode=GEOG11023. Sr. Lecturer, Geography, School of Humanities, Central Queensland University, Australia.
* Understanding Earth (Sixth ed.), W. H. Freeman, 4 February 2010, ISBN 978-1429219518
* Hamilton, Warren B. (August 1998). "Archean magmatism and deformation were not products of plate tectonics". Precambrian Research 91 (1–2): 143–179. doi:10.1016/S0301-9268(98)00042-4. Bibcode: 1998PreR...91..143H.
* Hamilton, Warren B. (1999). "How did the Archean Earth Lose Heat?". Department of Geophysics, Colorado School of Mines, Journal of Conference Abstracts 4 (1). http://www.the-conference.com/JConfAbs/4/140.html. . Symposium A08, Early Evolution of the Continental Crust.

## External links

* Smithsonian. "The Dynamic Earth @ National Museum of Natural History". Smithsonian National Museum of Natural History. http://www.mnh.si.edu/earth/text/4_1_3_1.html.

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