Deep Continental Roots and Cratons
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Review Deep continental roots and cratons https://doi.org/10.1038/s41586-021-03600-5 D. Graham Pearson1 ✉, James M. Scott2, Jingao Liu3, Andrew Schaeffer4, Lawrence Hongliang Wang5, Jeroen van Hunen6, Kristoffer Szilas7, Thomas Chacko1 & Received: 24 July 2020 Peter B. Kelemen8 Accepted: 30 April 2021 Published online: 11 August 2021 The formation and preservation of cratons—the oldest parts of the continents, Check for updates comprising over 60 per cent of the continental landmass—remains an enduring problem. Key to craton development is how and when the thick strong mantle roots that underlie these regions formed and evolved. Peridotite melting residues forming cratonic lithospheric roots mostly originated via relatively low-pressure melting and were subsequently transported to greater depth by thickening produced by lateral accretion and compression. The longest-lived cratons were assembled during Mesoarchean and Palaeoproterozoic times, creating the stable mantle roots 150 to 250 kilometres thick that are critical to preserving Earth’s early continents and central to defning the cratons, although we extend the defnition of cratons to include extensive regions of long-stable Mesoproterozoic crust also underpinned by thick lithospheric roots. The production of widespread thick and strong lithosphere via the process of orogenic thickening, possibly in several cycles, was fundamental to the eventual emergence of extensive continental landmasses—the cratons. The outer highly viscous ‘skin’ of the Earth—the lithosphere—separates lithospheric roots are becoming increasingly recognized as key factors the surface from its interior. Lithosphere definitions have many in the topographical expression of continents3, lithospheric volatile nuances1. Here, we define the lithosphere simply as Earth’s strong outer storage4 and the location of many metal deposits5. thermal boundary layer, through which heat is primarily transferred Understanding the role of lithospheric mantle in the stabilization and by conduction (Box 1). The base of the lithosphere can be defined as subsequent protection of continents requires clarification of the term the point at which a linear extrapolation of this conductive geotherm craton. The original use—kratogen—from the Greek kratos, meaning intersects the mantle isentrope. The cooler temperatures and higher strong6, merely implied a continental terrane displaying long-term sta- viscosities of lithosphere compared to underlying asthenospheric man- bility of hundreds of millions of years, with no age definition. Following tle contribute to it being one of the longest-lived large-scale features Kennedy7, Clifford8 recognized an association of ancient continental of the solid Earth. The mantle portion of the lithosphere, the ‘mantle masses (>1.5 billion years (Gyr) old) with certain mineral deposits, espe- root’, is generally thicker and older beneath continents than oceans2. cially diamonds, gold and platinum, though more recent widespread Given the controversies surrounding the origin of the crust that we use of the term ‘craton’ has become synonymous with Archean regions. walk on and sample readily as geologists, it should not surprise the Yet many such ‘cratons’ have long-lived tectonic histories that belie the reader that the origin of the deeper parts of the solid Earth, such as the image of post-Archean ‘stability’. continental lithospheric mantle, is just as controversial and more diffi- Studies of the Kaapvaal craton (Box 1) generally use a craton defini- cult to constrain. Here, we review some physical and chemical properties tion specifying a region where basement crustal rocks are >2.5 Gyr old of the deep lithospheric roots beneath continents and examine their (for example, see ref. 9), yet major mid-craton disruption and mag- integral role in forming the oldest parts of the continents: the cratons. matic addition affected it in Palaeoproterozoic and Mesoproterozoic We explore how these properties arose in the context of mantle melting times. Studies of the Siberian and Amazonian cratons (Fig. 1) have environments. We examine the melting ages of peridotites forming the followed the broader definition10 of “a segment of continental crust cratonic roots, and their temporal relationship with the overlying crust, that has attained and maintained long-term stability, with tectonic and we use geodynamic models to constrain the origin of the large-scale reworking being confined to its margins”. For the 4.1-million-km2 geological characteristics of cratons, that is, how the cratons were made. Siberian craton, much of the crust surrounding two Archean nuclei was either intensely metamorphosed or formed in Palaeoproterozoic times11. Similarly, the Amazonian craton has only a small area of clearly Making cratons and their lithosphere established Archean crust12 within large regions of Palaeoproterozoic Defining a craton crust13. It, along with numerous other cratons (for example, the Rae, Despite lithospheric mantle comprising up to 80% of the thickness Hearne and Gawler cratons; Fig. 1) has been extensively intruded by of continental plates, the origin and evolution of these deep roots felsic plutons in the Palaeoproterozoic and Mesoproterozoic eras, remains contentious. Cratons produce over 90% of the world’s gold and making the restriction of the definition of a craton to geological platinum and almost 100% of its diamonds. The properties of cratonic inactivity since the Archean problematic. 1Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton, Alberta, Canada. 2Department of Geology, University of Otago, Dunedin, New Zealand. 3State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Beijing, China. 4Geological Survey of Canada, Pacific Division, Natural Resources Canada, Sidney, BC, Canada. 5Department of Environmental Analyses, Institute of Energy Technology, Oslo, Norway. 6Department of Earth Sciences, Durham University, Durham, UK. 7Department of Geosciences and Natural Resource Management, University of Copenhagen, Copenhagen, Denmark. 8Lamont-Doherty Earth Observatory, Columbia University, Palisades, USA. ✉e-mail: [email protected] Nature | Vol 596 | 12 August 2021 | 199 Review Box 1 Craton definition and Earth’s mantle lithosphere and crust−mantle relationships Craton definition Cratons are coherent blocks of Precambrian lithosphere, typically exclusive to the Archean eon. Larger composite cratons may stable for time periods in excess of a billion years owing to consist of Archean nuclei surrounded by Palaeoproterozoic to protection by deep (>150 km) lithospheric keels. The majority of Mesoproterozoic crust, all underpinned by thick cool lithospheric Earth’s Archean crust lies within such terranes and can be referred mantle. Supercratons comprise multiple composite cratons to as Archean cratons or nuclei. But the moniker ‘craton’ is not (Fig. 1). a 60º N 80º N Temperature (ºC) 200 600 1,000 1,400 3.5 to 2.6 Ga 2.6 to 0.7 Ga >0.7 Ga 0 0 km Plagioclase Plagioclase MOR adiabat Spinel 1 Spi 90 nel 70 Garn 50 et Spinel 2 Graphite Depressed 50 with Diamond Garnet increasingly 3 Graphite depleted 40 compositions 100 Diamond 4 e (GPa) 35 mW m 150 5 essur Pr Depth (km) 6 –2 200 7 Traditional Slave Actual extent ‘craton’ margin of cratonized lithosphere 8 250 300 Southeast Central North Victoria Bathurst Sverdrup Canada MOR Slave Slave Slave Island Island Basin Basin adiabat b Congo craton Damara belt Atlantic Ocean Cratonized lithosphere G Kheis belt O Proterozoic J L R Limpopo beltZimbabwe nucleus M H 100 Archean S 100 ~2 Ga V Ma 3.5–2.7 Ga 200 km Fo# 91.5–92.5 F N Bushveld 200 km R K Kaapvaal nucleus P 2.6 About 2× vertical JF 2.9 Ga exaggeration Lithosphere thinned Mn D slightly in Mesozoic NL Archean depletion Fo# 92–93 E 3.6 Ga M: Murowa Zimbabwe 1.1 Ga Fo# 92–93 S: Sese O:Orapa Angola Zambia L: Letlhakane NL: N. Lesotho Namibia Seismic Fo# 91.5–92.5 LAB Mn: Monastery R: Roberts Victor Botswana N: Newlands 25º S 100 JF: Jagersfontein Proterozoic depletion K: Kimberley Mantle more P: Premier F: Finsch Swaziland fertile than E: East Griqualand V: Venetia adjacent Archean Ma: Markt South Africa mantle No data H: Hoedkop D: Dokolwayo Lesotho 200 km R: Rietfontein J: Jwaneng 25º G: Gibeon Box 1 figure | Lithosphere definition and the three-dimensional structure geotherms and magma geochemistry. Lithospheric thickness varies between of cratons. a, Schematic depiction of Earth’s mantle lithosphere, defined crust of different ages, though lithospheric thicknesses of up to 200 km here as the outer layer of Earth, where heat is lost by conduction (the are present beneath crust of Proterozoic as well as Archean age24,89,113 (see ‘tectosphere’ of Jordan2,14). Geotherms depict pressure−temperature also Fig. 1b). Lithospheric thickness is much thinner beneath Phanerozoic relations for thermally equilibrated lithosphere, with surface heat flow in continental crust and the oceans. b, Three-dimensional perspective of mW m–2. Depth to the base of the lithosphere is taken as the intersection the lithosphere beneath southern Africa, showing the Archean nuclei and of the conductive geotherm with the typical mid-oceanic ridge (MOR) Palaeoproterozoic−Mesoproterozoic domains of the Kalahari craton, which isentrope. The plagioclase-to-spinel (plag to spl) and spinel-to-garnet comprises the Kaapvaal and Zimbabwe Archean nuclei and intervening phase transitions (spl to gnt) is shown for lherzolites. Adjacent idealized Palaeoproterozoic−Mesoproterozoic regions.