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Vaalbara
Vaalbara
was an Archean
Archean
supercontinent consisting of the Kaapvaal Craton (now located in eastern South Africa) and the Pilbara Craton (now found in north-western Western Australia). E.S. Cheney derived the name from the last four letters of each craton's name. The two cratons consist of crust dating from 2.7 to 3.6 Gya, which would make Vaalbara
Vaalbara
Earth's earliest supercontinent.[1]

Contents

1 Existence and lifespan 2 Evidence for Vaalbara 3 Origin of life 4 See also 5 References

5.1 Notes 5.2 Sources

Existence and lifespan[edit]

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Quaternary

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Human
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There has been some debate as to when and whether Vaalbara
Vaalbara
existed.[by whom?] An Archaean-Palaeoproterozoic (2.8—2.1 Ga) link between South Africa and Western Australia
Western Australia
was first proposed by Button 1976. He found a wide range of similarities between the Transvaal Basin in South Africa and the Hamersley Basin
Hamersley Basin
in Australia. Button, however, placed Madagascar between Africa
Africa
and Australia and concluded that Gondwana must have had a long stable tectonic history.[2] Similarly, in the reconstruction of Rogers 1993, 1996 the oldest continent is Ur. In Rogers' reconstructions, however, Kaapvaal and Pilbara are placed far apart already in their Gondwana
Gondwana
configuration, a reconstruction contradicted by later orogenic events and incompatible with the Vaalbara
Vaalbara
hypothesis.[3] Cheney 1996, nevertheless, found a three-fold stratigraphic similarity and proposed that the two cratons once formed a continent which he named Vaalbara. This model is supported by the palaeomagnetic data of Zegers, de Wit & White 1998.[4] Reconstructions of the palaeolatitudes of the two cratons at 2.78–2.77 Ga are ambiguous however. In the reconstruction of Wingate 1998 they fail to overlap, but they do in more recent reconstructions, for example Strik et al. 2003.[5] Other scientists dispute the existence of Vaalbara
Vaalbara
and explain similarities between the two cratons as the product of global processes. They point to, for example, thick volcanic deposits on other cratons such as Amazonia, São Francisco, and Karnataka.[6] Zimgarn, another proposed supercraton composed of the Zimbabwe and Yilgarn cratons at 2.41 Ga, is distinct from Vaalbara. Zimgarn should have disintegrated around 2.1–2.0 Ga to reassemble as the Kalahari and West Australian (Yilgarn and Pilbara) cratons around 1.95–1.8 Ga.[7] The Archaean-Palaeoproterozoic Grunehogna Craton in Dronning Maud Land, East Antarctica, formed the eastern part of the Kalahari Craton for at least a billion years. Grunehogna collided with the rest of East Antarctica
East Antarctica
during the Mesoproterozoic assembly of the supercontinent Rodinia
Rodinia
and the Grenville orogeny. The Neoproterozoic Pan-African orogeny
Pan-African orogeny
and the assembly of Gondwana/ Pannotia
Pannotia
produced large shear zones between Grunehogna and Kalahari. During the Jurassic break-up of Gondwana
Gondwana
these shear zones finally separated Grunehogna and the rest of Antarctica
Antarctica
from Africa.[8] In the Annandags Peaks, the only exposed parts of Grunehogna, detrital zircons from several crustal sources have been dated to 3.9–3.0 Ga suggesting intracrustal recycling was an important part in the formation of the first cratons.[9] The Kaapvaal craton is marked by dramatic events such as the intrusion of the Bushveld Complex (2.045 Ga) and the Vredefort impact event (2.025 Ga), and no traces of these events have been found in the Pilbara craton, clearly indicating that the two cratons were separated before 2.05 Ga.[10] Furthermore, geochronological and palaeomagnetic evidence show that the two cratons had a rotational 30° latitudinal separation in the time period of 2.78–2.77 Ga, which indicates they were no longer joined after c. 2.8 billion years ago.[11] Vaalbara
Vaalbara
thus remained stable for 1–0.4 Ga and hence had a life span similar to that of later supercontinents such as Gondwana
Gondwana
and Rodinia.[10] Some palaeomagnetic reconstructions suggest a Palaeoarchaean Proto- Vaalbara
Vaalbara
is possible, although the existence of this 3.6–3.2 Ga continent can't be unequivocally proven.[12] Evidence for Vaalbara[edit] South Africa's Kaapvaal craton and Western Australia's Pilbara craton have similar early Precambrian
Precambrian
cover sequences.[13] Kaapvaal's Barberton granite-greenstone terrane and Pilbara's eastern block show evidence of four large meteorite impacts between 3.2 and 3.5 billion years ago.[14] (Similar greenstone belts are now found at the margins of the Superior craton
Superior craton
of Canada.)[15] The high temperatures created by the impact’s force fused sediments into small glassy spherules.[16] Spherules of 3.5 billion years old exist in South Africa
South Africa
and spherules of a similar age have been found in Western Australia,[16] they are the oldest-known terrestrial impact products.[17] The spherules resemble the glassy chondrules (rounded granules) in carbonaceous chondrites, which are found in carbon-rich meteorites and lunar soils.[16] Remarkably similar lithostratigraphic and chronostratigraphic structural sequences between these two cratons have been noted for the period between 3.5 and 2.7 billion years ago.[18] Paleomagnetic data from two ultramafic complexes in the cratons showed that at 3,870 million years the two cratons could have been part of the same supercontinent.[18] Both the Pilbara and Kaapvaal cratons show extensional faults which were active about 3,470 million years ago during felsic volcanism and coeval with the impact layers.[18] Origin of life[edit] See also: Abiogenesis The Pilbara and Kaapvaal cratons are some of the oldest rocks in the world and they contain well-preserved Archaean microfossils. A series of international drilling projects has revealed traces of microbial life and photosynthesis from the Archaean in both Africa
Africa
and Australia.[19] The oldest widely accepted evidence of photosynthesis by early life forms is molecular fossils found in 2.7 Ga-old shales in the Pilbara Craton. These fossils have been interpreted as traces of eukaryotes and cyanobacteria, though some scientists argue that these biomarkers must have entered these rocks later and date the fossils to 2.15–1.68 Ga.[20] This later time span agrees with estimates based on molecular clocks which dates the eukaryote last common ancestor at 1866–1679 Ma. If the Pilbara fossils are traces of early eukaryotes, they could represent groups that went extinct before modern groups emerged.[21] See also[edit]

Columbia (supercontinent) Ur (continent)

References[edit] Notes[edit]

^ Zegers, de Wit & White 1998, Abstract ^ Button 1976, Synopsis, p. 262; for Button's reconstruction see fig. 20f, p. 286 ^ de Kock, Evans & Beukes 2009, Introduction, pp. 145–146 ^ Zhao et al. 2004, pp. 96–98 ^ Strik et al. 2003, Implications for the Vaalbara
Vaalbara
Hypothesis, pp. 19–20, fig. 11 ^ Nelson, Trendall & Altermann 1999, Independent development of the Pilbara and Kaapvaal cratons — implications, pp. 186–187 ^ Smirnov et al. 2013, Abstract ^ Marschall et al. 2010, Geology of the Grunehogna Craton, pp. 2278–2280 ^ Marschall et al. 2010, Conclusions, p. 2298 ^ a b Zegers, de Wit & White 1998, Discussion, pp. 255–257 ^ Wingate 1998, Abstract ^ Biggin et al. 2011, p. 326 ^ de Kock 2008, p. VII ^ Byerly et al. 2002, Abstract ^ Nitescu, Cruden & Bailey 2006, Fig. 1, p. 2 ^ a b c Erickson 1993, p. 27 ^ Lowe & Byerly 1986, p. 83 ^ a b c Zegers & Ocampo 2003 ^ Philippot et al. 2009, Abstract; Waldbauer et al. 2009, Conclusions, p. 45 ^ Rasmussen et al. 2008, p. 1101 ^ Parfrey et al. 2011, Discussion, p. 13626

Sources[edit]

Biggin, A. J.; de Wit, M. J.; Langereis, C. G.; Zegers, T. E.; Voûte, S.; Dekkers, M. J.; Drost, K. (2011). "Palaeomagnetism of Archaean rocks of the Onverwacht Group, Barberton Greenstone Belt (southern Africa): Evidence for a stable and potentially reversing geomagnetic field at ca. 3.5 Ga". Earth
Earth
and Planetary Science Letters. 302 (3): 314–328. Bibcode:2011E&PSL.302..314B. doi:10.1016/j.epsl.2010.12.024. Retrieved 2016-09-12.  Button, A. (1976). "Transvaal and Hamersley basins—review of basin development and mineral deposits" (PDF). Mineral Science Engineering. University of the Witwatersrand, Economic Geology Research Unit, Information Circular 107. 8 (4): 262–293. OCLC 13791945. Retrieved 2016-09-12.  Byerly, G. R.; Lowe, D. R.; Wooden, J. L.; Xie, X. (2002). "An Archean Impact Layer from the Pilbara and Kaapvaal Cratons". Science. 297 (5585): 1325–1327. Bibcode:2002Sci...297.1325B. doi:10.1126/science.1073934. PMID 12193781. Retrieved 2016-09-12.  Cheney, E. S. (1996). "Sequence stratigraphy and plate tectonic significance of the Transvaal succession of southern Africa
Africa
and its equivalent in Western Australia". Precambrian
Precambrian
Research. 79 (1–2): 3–24. Bibcode:1996PreR...79....3C. doi:10.1016/0301-9268(95)00085-2.  de Kock, M. O. (2008). Paleomagnetism
Paleomagnetism
of Selected Neoarchean-Paleoproterozoic Cover Sequences on the Kaapvaal Craton
Kaapvaal Craton
and Implications for Vaalbara
Vaalbara
(Ph.D.). University of Johannesburg. Retrieved 2016-09-12.  de Kock, M. O.; Evans, D. A. D.; Beukes, N. J. (2009). "Validating the existence of Vaalbara
Vaalbara
in the Neoarchean". Precambrian
Precambrian
Research. 174 (1): 145–154. Bibcode:2009PreR..174..145D. doi:10.1016/j.precamres.2009.07.002. Retrieved 2016-09-12.  Erickson, Jon (1993). Craters, Caverns and Canyons – Delving Beneath the Earth’s Surface. ISBN 0-8160-2590-8.  Lowe, D. R.; Byerly, G. R. (1986). "Early Archean
Archean
silicate spherules of probable impact origin, South Africa
South Africa
and Western Australia". Geology. Geological Society of America. 14 (1): 83–86. Bibcode:1986Geo....14...83L. doi:10.1130/0091-7613(1986)14<83:EASSOP>2.0.CO;2. Retrieved 2016-09-12.  Marschall, H. R.; Hawkesworth, C. J.; Storey, C. D.; Dhuime, B.; Leat, P. T.; Meyer, H. P.; Tamm-Buckle, S. (2010). "The Annandagstoppane Granite, East Antarctica: evidence for Archaean intracrustal recycling in the Kaapvaal–Grunehogna Craton from zircon O and Hf isotopes" (PDF). Journal of Petrology. 51 (11): 2277–2301. Bibcode:2010JPet...51.2277M. doi:10.1093/petrology/egq057. Retrieved May 2016.  Check date values in: access-date= (help) Nelson, D. R.; Trendall, A. F.; Altermann, W. (1999). "Chronological correlations between the Pilbara and Kaapvaal cratons" (PDF). Precambrian
Precambrian
Research. 97 (3): 165–189. Bibcode:1999PreR...97..165N. doi:10.1016/S0301-9268(99)00031-5. Retrieved April 2016.  Check date values in: access-date= (help) Nitescu, B.; Cruden, A. R.; Bailey, R. C. (2006). "Crustal structure and implications for the tectonic evolution of the Archean
Archean
Western Superior craton
Superior craton
from forward and inverse gravity modeling". Tectonics. 25 (TC1009). Bibcode:2006Tecto..25.1009N. doi:10.1029/2004TC001717.  Parfrey, L. W.; Lahr, D. J.; Knoll, A. H.; Katz, L. A. (2011). "Estimating the timing of early eukaryotic diversification with multigene molecular clocks" (PDF). PNAS. 108 (33): 13624–13629. Bibcode:2011PNAS..10813624P. doi:10.1073/pnas.1110633108. PMC 3158185 . PMID 21810989. Retrieved April 2016.  Check date values in: access-date= (help) Philippot, P.; Van Kranendonk, M.; Van Zuilen, M.; Lepot, K.; Rividi, N.; Teitler, Y.; Thomazo, C.; Blanc-Valleron, M.-M.; Rouchy, J.-M.; Grosch, E.; de Wit, M. (2009). "Early traces of life investigations in drilling Archean
Archean
hydrothermal and sedimentary rocks of the Pilbara Craton, Western Australia
Western Australia
and Barberton greenstone belt, South Africa". Comptes Rendus Palevol. 8 (7): 649–663. doi:10.1016/j.crpv.2009.06.006. Retrieved 2016-09-12.  Rasmussen, B.; Fletcher, I. R.; Brocks, J. J.; Kilburn, M. R. (2008). "Reassessing the first appearance of eukaryotes and cyanobacteria" (PDF). Nature. 455 (7216): 1101–1104. Bibcode:2008Natur.455.1101R. doi:10.1038/nature07381. PMID 18948954. Retrieved April 2016.  Check date values in: access-date= (help) Rogers, J. J. (1993). "India and Ur". Geological Society of India. 42 (3): 217–222. Retrieved March 2016. (Subscription required (help)).  Check date values in: access-date= (help) Rogers, J. J. W. (1996). "A history of continents in the past three billion years". Journal of Geology. 104: 91–107, Chicago. Bibcode:1996JG....104...91R. doi:10.1086/629803. JSTOR 30068065.  Smirnov, A. V.; Evans, D. A.; Ernst, R. E.; Söderlund, U.; Li, Z. X. (2013). "Trading partners: tectonic ancestry of southern Africa
Africa
and western Australia, in Archean
Archean
supercratons Vaalbara
Vaalbara
and Zimgarn" (PDF). Precambrian
Precambrian
Research. 224: 11–22. Bibcode:2013PreR..224...11S. doi:10.1016/j.precamres.2012.09.020. Retrieved March 2016.  Check date values in: access-date= (help) Strik, G.; Blake, T. S.; Zegers, T. E.; White, S. H.; Langereis, C. G. (2003). "Palaeomagnetism of flood basalts in the Pilbara Craton, Western Australia: Late Archaean continental drift and the oldest known reversal of the geomagnetic field". Journal of Geophysical Research: Solid Earth. 108 (B12). Bibcode:2003JGRB..108.2551S. doi:10.1029/2003jb002475. Retrieved 2016-09-12.  Waldbauer, J. R.; Sherman, L. S.; Sumner, D. Y.; Summons, R. E. (2009). "Late Archean
Archean
molecular fossils from the Transvaal Supergroup record the antiquity of microbial diversity and aerobiosis". Precambrian
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Research. 169 (1): 28–47. Bibcode:2009PreR..169...28W. doi:10.1016/j.precamres.2008.10.011. Retrieved April 2016.  Check date values in: access-date= (help) Wingate, M. T. D. (1998). "A palaeomagnetic test of the Kaapvaal-Pilbara (Vaalbara) connection at 2.78 Ga". South African Journal of Geology. 101 (4): 257–274. Retrieved 2016-09-12.  Zegers, T. E.; de Wit, M. J.; White, S. H. (1998). "Vaalbara, Earth's oldest assembled continent? A combined. structural, geochronological, and palaeomagnetic test" (PDF). Terra Nova. 10 (5): 250–259. Bibcode:1998TeNov..10..250Z. doi:10.1046/j.1365-3121.1998.00199.x. Retrieved April 2016.  Check date values in: access-date= (help) Zegers, T. E.; Ocampo, A. (2003). Vaalbara
Vaalbara
and Tectonic Effects of a Mega Impact in the Early Archean
Archean
3470 Ma. Third International Conference on Large Meteorite Impacts. Nordlingen, Germany. Retrieved 2016-09-12.  Zhao, G.; Sun, M.; Wilde, S. A.; Li, S. (2004). "A Paleo-Mesoproterozoic supercontinent: assembly, growth and breakup". Earth-Science Reviews. 67 (1): 91–123. Bibcode:2004ESRv...67...91Z. doi:10.1016/j.earscirev.2004.02.003. Retrieved 2016-09-12. 

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