There has been some debate as to when and even if Vaalbara existed.[3][4] AnArchaean–Palaeoproterozoic (2.8–2.1 Ga) link between South Africa and Western Australia was first proposed byAlan Button in 1976.[5] He found a wide range of similarities between theTransvaal Basin in South Africa and theHamersley Basin in Australia.[6] Button, however, placed Madagascar between Africa and Australia and concluded thatGondwana must have had a long stable tectonic history.[7] Similarly, in the reconstruction ofRogers 1993,1996 the oldest continent isUr. In Rogers' reconstructions, however, Kaapvaal and Pilbara are placed far apart already in their Gondwana configuration, a reconstruction contradicted by laterorogenic events and incompatible with the Vaalbara hypothesis.[8]
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 thepalaeomagnetic data ofZegers, de Wit & White 1998.[9]Reconstructions of the palaeolatitudes of the two cratons at 2.78–2.77 Ga are ambiguous however. In the reconstruction ofWingate 1998 they fail to overlap, but they do in more recent reconstructions, for exampleStrik et al. 2003.[10]
Other scientists dispute the existence of Vaalbara and explain similarities between the two cratons as the product of global processes. They point, for example, to thick volcanic deposits on other cratons such asAmazonia,São Francisco, andKarnataka.[11]
Zimgarn, another proposed supercraton composed of theZimbabwe andYilgarn cratons at 2.41 Ga, is distinct from Vaalbara. Zimgarn should have disintegrated around 2.1–2.0 Ga to reassemble as theKalahari and West Australian (Yilgarn and Pilbara) cratons around 1.95–1.8 Ga.[12][13]
The Archaean–PalaeoproterozoicGrunehogna Craton inQueen 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 during the Mesoproterozoic assembly of the supercontinentRodinia and theGrenville orogeny.[14][15] The NeoproterozoicPan-African orogeny and the assembly of Gondwana/Pannotia produced large shear zones between Grunehogna and Kalahari. During the Jurassic break-up of Gondwana, these shear zones finally separated Grunehogna and the rest of Antarctica from Africa.[16]In theAnnandags Peaks in Antarctica, the only exposed parts of Grunehogna,detritalzircons 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.[17]
The Kaapvaal craton is marked by dramatic events such as the intrusion of theBushveld Complex (2.045 Ga) and theVredefort 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.[18] 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.[19]
Vaalbara thus remained stable for 1–0.4 Ga and hence had a life span similar to that of later supercontinents such asGondwana andRodinia.[18]Some palaeomagnetic reconstructions suggest aPalaeoarchaean proto-Vaalbara is possible, although the existence of this 3.6–3.2 Ga continent cannot be proven.[20]
South Africa's Kaapvaal craton and Western Australia's Pilbara craton have similar earlyPrecambrian cover sequences.[21] Kaapvaal's Barbertongranite-greenstoneterrane and Pilbara's eastern block show evidence of four large meteorite impacts between 3.2 and 3.5 billion years ago.[22] Similar greenstone belts are found at the margins of theSuperior Craton of Canada.[23]
The high temperatures created by the impacts' forces fused sediments into small glassy spherules.[24] Spherules of 3.5 billion years old exist in South Africa, and spherules of a similar age have been found in Western Australia;[24] they are the oldest-known terrestrial impact products.[25] The spherules resemble the glassychondrules (rounded granules) in carbonaceouschondrites, which are found in carbon-rich meteorites and lunar soils.[24]
Remarkably similar lithostratigraphic and chronostratigraphic structural sequences between these two cratons have been noted for the period between 3.5 and 2.7 Ga.[26] Paleomagnetic data from twoultramafic complexes in the cratons showed that at 3.87 Ga the two cratons could have been part of the same supercontinent.[26] Both the Pilbara and Kaapvaal cratons show extensional faults which were active about 3.47 Ga during felsicvolcanism and coeval with the impact layers.[26]
The Pilbara and Kaapvaal cratons contain well-preserved Archaean microfossils. Drilling has revealed traces of microbial life and photosynthesis from the Archaean in both Africa and Australia.[27]The oldest widely accepted evidence of photosynthesis by early life forms ismolecular fossils found in 2.7 Ga-old shales in thePilbara Craton. These fossils have been interpreted as traces ofeukaryotes andcyanobacteria, though some scientists argue that these biomarkers must have entered these rocks later and date the fossils to 2.15–1.68 Ga.[28]This later time span agrees with estimates based onmolecular clocks which dates the eukaryotelast common ancestor at 1.8–1.7 Ga. If the Pilbara fossils are traces of early eukaryotes, they could represent groups that went extinct before modern groups emerged.[29]
Cheney, E. S. (1996). "Sequence stratigraphy and plate tectonic significance of the Transvaal succession of southern Africa and its equivalent in Western Australia".Precambrian Research.79 (1–2):3–24.Bibcode:1996PreR...79....3C.doi:10.1016/0301-9268(95)00085-2.
_Vaalbara_ancient_continent.png)
