By Steve Drury First PUBLISHED ON April 6, 2021
Plate tectonics is easily thought of as being dominated by continental drift, the phenomenon that Alfred Wegener recognised just over a century ago. So it is at present, the major continents being separated by spreading oceans. Yet, being placed on a near-spherical planet, continents also move closer to others; eventually to collide and weld together.
Part of Wegener’s concept was that modern continents formed from the breakup of a single large one that he called Pangaea; a supercontinent. The current drifting apart began in earnest around the end of the Triassic Period (~200 Ma), after 200 Ma of Pangaea’s dominance of the planet along with a single large ocean (Panthalassa) covering 70% of the Earth’s surface. Wegener was able to fit Pangaea together partly on the basis of evidence from the continents’ earlier geological history. In particular the refit joined up zones of intense deformation from continent to continent.
Although he did not dwell on their origin, subsequent research has shown these zones were the lines of earlier collisions between older continental blocks, once subduction had removed the intervening oceanic lithosphere; Pangaea had formed from an earlier round of continental drift. Even older collision zones within the pre-Pangaea continental blocks suggested the former existence of previous supercontinents.
Aided by the development of means to divine the position of the magnetic poles relative to differently aged blocks on the continents, Wegener’s basic methods of refitting have resulted in the concept of supercontinent cycles of formation and break-up. It turns out that supercontinents did not form by all earlier continental clanging together at one time. The most likely scenario is that large precursors or ‘megacontinents’ (Eurasia is the current example) formed first, to which lesser entities eventually accreted
A summary of the latest ideas on such global tectonic cycles appeared in the November 2020 issue of Geology (Wang, c. et al. 2020. The role of megacontinents in the supercontinent cycle. Geology, v. 49 p. 402-406; DOI: 10.1130/G47988.1). Chong Wang of the Chinese Academy of Sciences and colleagues from Finland and Canada identify three such cycles of megacontinent formation and the accretion around them of the all-inclusive supercontinents of Columbia, Rodinia and Pangaea since about 1750 Ma (Mesoproterozoic). They also suggestion that a future supercontinent (Amasia) is destined to agglomerate around Eurasia.
The further back in time, the more cryptic are ancient continent-continent collision zone or sutures largely because they have been re-deformed long after they formed. In some cases younger events that involved heating have reset their radiometric ages. The oldest evidence of crustal deformation lies in cratons, where the most productive source of evidence for clumping of older continental masses is the use of palaeomagnetic pole positions. This is not feasible for the dominant metamorphic rocks of old suture zones, but palaeomagnetic measurements from old rocks that have been neither deformed nor metamorphosed offer the possibility of teasing out ancient supercontinents.
Commonly cratons show signs of having been affected by brittle extensional deformation, most obviously as swarms of vertical sheets or dykes of often basaltic igneous rocks. Dykes can be dated readily and do yield reliable palaeomagnetic pole positions. Some cratons have multiple dyke swarms. For example the Archaean Yilgarn Craton of Western Australia, founded on metamorphic and plutonic igneous crust that formed by tectonic accretion between 3.8 to 2.7 Ga, has five of them spanning 1.4 billion years from late-Archaean (2.6 Ga) to Mesoproterozoic (1.2 Ga). Throughout that immense span of time the Yilgarn remained as a single continental block. Also, structural trends end abrubtly at the craton margins, suggesting that it was once part of a larger ‘supercraton’ subsequently pulled apart by extensional tectonics. The eleven known cratons show roughly the same features.
On the strength of new, high quality pole positions from dykes of about the same ages (2.62 and 2.41 Ga) cutting the Yilgarn and Zimbabwe cratons, geoscientists from Australia, China, Germany, Russia and Finland, based at Curtin University in Western Australia, have attempted to analyse all existing Archaean and Palaeoproterozoic pole positions (Liu, Y. et al. 2021. Archean geodynamics: Ephemeral supercontinents or long-lived supercratons. Geology, v. 49 ; DOI: 10.1130/G48575.1).
The Zimbabwe and Yilgarn cratons, though now very far apart, were part of the same supercraton from at least 2.6 Ga ago. Good cases can be made for several other such large entities, but attempting fit them all together as supercontinents by modelling is unconvincing. The modelled fit for the 2.6 Ga datum is very unlike that for 2.4 Ga; in the intervening 200 Ma all the component cratons ould have had to shuffle around dramatically, without the whole supercontinent edifice breaking apart. However, using the data to fit cratons together at two supercratons does seem to work, for the two assemblies remain in the same configurations for both the 2.6 and 2.4 Ga data.
Interestingly, all cratonic components of one of the supercratons show geological evidence of the major 2.4 Ga glaciation, whereas those of the other show no such climatic indicator. Yet the entity with glacial evidence was positioned at low latitudes around 2.4 Ga, the ice-free one spanning mid latitudes. This may imply that the Earth’s axial tilt was far higher than at present.
The persistence of two similar sized continental masses for at least 200 Ma around the end of the Archaean Eon also hints at a different style of tectonics from that with which geologists are familiar. Only palaeomagnetic data from the pre 2.6 Ga Archaean can throw light on that possibility. That requires older, very lightly or unmetamorphosed rocks to provide palaeopole positions. Only two cratons, the Pilbara of Western Australia and the Kaapvaal of South Africa, are suitable. The first yielded the oldest-known pole dated at 3.2 Ga, the oldest from the second is 2.7 Ga. A range of evidence suggests that Pilbara and Kaapvaal cratons were united during at least the late Archaean.
The only answer to the question posed by this item’s title is ‘There probably wasn’t a single supercontinent at the end of the Archaean, but maybe two megacontinents or supercratons’. Lumps of continental lithosphere would move and – given time – collide once more than one lump existed, however the Earth’s tectonics operated …
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Many thanks to Steve Drury for permission to republish his article and to Bernie Bell for sending it into The Orkney News