Old and stable part of the continental lithosphere
Cratons of South America and Africa during theTriassic Period when the two continents were joined as part of thePangea supercontinent
Acraton (/ˈkreɪtɒn/KRAYT-on,/ˈkrætɒn/KRAT-on, or/ˈkreɪtən/KRAY-tən;[1][2][3] fromAncient Greek:κράτοςkratos "strength") is an old and stable part of continentallithosphere (the Earth's two topmost layers, thecrust and thelithospheric mantle). Having often survived cycles of merging andrifting of continents, cratons are generally found in the interiors oftectonic plates; the exceptions occur where geologically recent rifting events have separated cratons and createdpassive margins along their edges. Cratons are composed of ancient crystallinebasement rocks covered by youngersedimentary rocks. They have a thick crust and deep lithospheric roots extending several hundred kilometres into Earth's mantle.
Cratons contain the oldestcontinental crust rocks on Earth. They were formed in theArchaean (4 to 2.5 billion years ago) and theProterozoic (2.5 billion- 538.8 million year ago)geologic eons. Most were formed in the Archaean.[4][5]
The termcraton is used to distinguish the stable portion of thecontinental crust from regions that are more geologically active and unstable.[6]
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 neighbouringterranes."
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 cratonicbasement ofmetamorphosedcrystalline andmetamorphic rocks and theplatform, which is a younger, weakly deformedsedimentary cover which overlies this basement. Continentalshields are exposed (theycrop out at the surface) cratonic basement rocks and are thus dominated by crystalline and metamorphic rocks. Shields and platforms arephysiographic terms rather thantectonic entities.[7]
The wordcraton was first proposed by the Austrian geologistLeopold Kober in 1921 asKratogen, referring to stable continental platforms, andorogen as a term formountain ororogenic belts. LaterHans Stille shortened the former term toKraton, from whichcraton derives.[8]
Cratons have thick lithospheric roots. Mantletomography shows that cratons are underlain by anomalously cold mantle corresponding tolithosphere 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 theasthenosphere,[16] and the low-velocity zone seen elsewhere at these depths is weak or absent beneath stable cratons.[17] 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 fromgeothermal 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.[18]
Rock fragments (xenoliths) carried up from the mantle bymagmas containingperidotite have been delivered to the surface asinclusions insubvolcanic pipes calledkimberlites. 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-weightmagnesium instead of higher-weight calcium and iron.[19] 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 likebasalt andkomatiite.[20]
The process by which cratons were formed is calledcratonization. Much about this process remains uncertain, with very little consensus in the scientific community.[21] However, the first cratonic landmasses likely formed during theArchean eon. This is indicated by the age ofdiamonds, which originate in the roots of cratons and are almost always over 2 billion years and often over 3 billion years in age.[18] 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.[22] Cratonization likely was completed during theProterozoic. Subsequent growth of continents was byaccretion at continental margins.[18]
The origin of the roots of cratons is still debated.[23][24][19][21] However, the present understanding of cratonization began with the publication in 1978 of a paper byThomas H. Jordan inNature. 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.[25][18] 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.[26] The resulting mantle roots have remained stable for billions of years.[24] Jordan suggests that depletion occurred primarily insubduction zones and secondarily asflood basalts.[27]
This model of melt extraction from the upper mantle has held up well with subsequent observations.[28] The properties of mantle xenoliths confirm that thegeothermal gradient is much lower beneath continents than oceans.[29] Theolivine of craton root xenoliths is extremely dry, which would give the roots a very high viscosity.[30]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 voluminousmafic magmatism.[31]
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.[27] The 30 to 40 per cent partial melting of mantle rock at 4 to 10GPa pressure produceskomatiite 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.[31] There are many competing hypotheses of how cratons have been formed.
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 ofisostacy.[27] 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).[30]
A second model suggests that the surface crust was thickened by arising plume of molten material from the deep mantle. This would have built up a thick layer of depleted mantle underneath the cratons.
A third model suggests that successiveslabs of subducting oceanic lithosphere became lodged beneath a proto-craton,underplating the craton with chemically depleted rock.[30][19][23]
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.[21] In this model, large impacts on the Earth's early lithosphere penetrated deep into the mantle and created enormous lava ponds.[21] The paper suggests these lava ponds cooled to form the craton's root.[21]
The chemistry of xenoliths[28] and seismic tomography both favor the two accretional models over the plume model.[30][32] However, other geochemical evidence favors mantle plumes.[33][34][35] 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.[36] The impact origin model does not require plumes or accretion; this model is, however, not incompatible with either.[21]
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.[31][21]
The long-termerosion of cratons has been labelled the "cratonic regime". It involves processes ofpediplanation andetchplanation that lead to the formation of flattish surfaces known aspeneplains.[37] While the process of etchplanation is associated to humid climate and pediplanation with arid and semi-arid climate, shifting climate overgeological 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 relativesea levels. High relative sea level leads to increased oceanicity, while the opposite leads to increasedinland conditions.[37]
^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. Vol. 196. p. 331.
^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 105491.doi:10.1016/j.precamres.2019.105491.S2CID210295037.
^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.Bibcode:2007GSLSP.280....1K.doi:10.1144/sp280.1.S2CID129902429.
^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.Bibcode:2009GondR..15..396C.doi:10.1016/j.gr.2008.12.005.
^Philpotts, Anthony R.; Ague, Jay J. (2009).Principles of igneous and metamorphic petrology (2nd ed.). Cambridge, UK: Cambridge University Press. pp. 373,602–603.ISBN978-0-521-88006-0.
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