Kaapvaal Kraton

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Depiction of the Kaapvaal craton in southern Africa

The Kaapvaal Craton is in Archean incurred, stable crustal block in southern Africa. Besides the Pilbara craton in Western Australia, it is the only remaining tectonically unchanged crust on earth, which has an archaic age of 3.6 to 2.5 billion years.

The structural development of the African continent is generally characterized by several mountain formation processes, with processed folding tectonics being metamorphically overprinted in larger areas as well as being permeated by numerous deep-seated plutonic intrusions . In this way, the corresponding areas developed into stable continental blocks or cratons , which, due to plate tectonic collision and subsequent orrogenic processes , merged and finally formed the so-called African shield. The complete cratonization of the African continent is given as the lower Paleozoic Era , at which point a total of five orogenic processes of regional proportions took place.


The Kaapvaal craton occupies an area of ​​about 1.2 · 10 6  km 2 and is bounded in the north by the Zimbabwe craton , in the west and in the south by the Proterozoic rock belt. In the east, among other things, Jurassic volcanic rocks are exposed, which are associated with the breakup of Gondwana . Furthermore, the craton can be subdivided into twelve different terrains, which were created during the gradual development from a small continent to the craton in its current size between 3.7 and 2.7 billion years ago. The formation age of the first continental crust of the Kaapvaal craton is between 3.7 and 3.3 billion years, whereby the eastern shield complex in particular was cratonized or stabilized by extensive intrusions of intracontinental granites about 3.0 billion years ago. This northeastern section of the Kaapvaal craton is therefore the oldest shield in southern Africa and is known as the eastern shield or the Witwatersrand Block. The central and western part of the Kaapvaal Kraton, defined as the Kimberley Block, is much younger, around 3.2 to 2.7 billion years old. The old core of the Kaapvaal shield is now represented by the old gneiss complex of the Ngwane gneiss and the Barberton greenstone terrain.

Witwatersrand Block

The eastern Kaapvaal shield was built about 3.2 billion years ago through tectonic convergence as a coherent unit and includes, among other things, the continental crust, which must have formed between 3.7 and 3.3 billion years. The exposed Lochiel granite in the Malolotja nature reserve in Swaziland is interpreted as the marker for the cratonization of the eastern shield, and more recent studies also suggest that the stabilization of the Witwatersrand block in particular took place around 3.08 billion years ago and was itself a crust-mantle coupling in association with the formation of a stable tectosphere also took place from this point in time. The Natal Granite-Greenstone Terrain, 3.4 to 3.2 billion years old and getting younger from north to south, contains sediment packages of shallow marine affinity which were around 3.3 to 3.2 billion years ago Trondhjemite-tonalite magmas or post-tectonic granites about 3.0 billion years ago intruded. The post-tectonic granites in particular indicate the end of the accretion of the Natal terrain with the Kaapvaal shield and made it possible to stabilize the newly formed crust fragments. The extensive crustal melts above the Kaapvaal shield and the expired granitoid - plutonism between 3.1 and 3.08 billion years ago consequently led to the eruption of the volcanic-sedimentary Dominion group, which occurred between 3.09 and 3.07 billion years ago deposited in a continental rift system. The subsequent marine sedimentation age of the Witwatersrand supergroup was defined with the help of detritic zircons at 2.97 billion years, the subsequent eruption age of the Crown Lava Formation is given as 2.91 billion years. The further deposition time of the so-called Central-Rand group could in turn be determined with detritic zirconia ages, according to which it began about 2.89 billion years ago and then with the beginning of the intrusion of the post-Pongola granitoids before 2.82 billion years ago. Years ago. The relatively undeformed volcanic-sedimentary Pongola sequence also overlays discordantly the units of the ancient gneiss complex and the Natal Greenstone terrain. The main rift processes in the central and western area of ​​the Kaapvaal Craton are characterized by the ultramafic to intermediate volcanic rocks of the Klipriviersburg and Platberg groups, which are assigned to the Venterdorp supergroup and given an age of 2.71 billion years.

Kimberley Block

The Kimberley Block occupies the western area of ​​the Kaapvaal Craton, is delimited from the eastern shield of the Witwatersrand Block by the Colesberg Lineament and is characterized by the absence of the Dominion Group and the Witwatersrand Supergroup. It should be noted that the amalgamation of the Witwatersrand and Kimberley blocks is a relatively young process in the development of the Kaapvaal kraton, with a minimum age of 2.71 billion years for this coexistence, determined by the oldest Ventersdorp- Sequence that covers both blocks is assumed. The crystalline basement of the Kimberley Block lies largely under the Phanerozoic cover, which is characterized by neo-archaic sediments and volcanic rocks. Outcrops of the crystalline basement are represented by the Swiss Reneke Dome, the Kraaipan Greenstone Belt and neighboring granitic gneisses, with dated zircons from corresponding gneisses and slates of these surroundings as well as crustal xenolites from Kimberley an age between 3.25 and 2 .79 billion years ago. The Kraaipan Greenstone Belt is composed, among other things, of rhyolithic tuffs, which have a dating age of 2.930 billion years, whereby this age refers to the time when on the one hand Felsic volcanism and on the other also marine chemical and clastic sedimentation within the Kraaipan Greenstone Belt took place. In addition to these processes, extensive calcareous-alkaline plutonic activity occurred during this period, which shows a north-south trend within the Kraaipan-Greenstone Belt and possibly contributed to the geophysical signature of the Colesberg Lineament. In this context, the existence of a volcanic island arc and a backarc basin on the eastern edge of the Kimberley Block is suggested about 2.93 billion years ago. Around 2.88 billion years ago, the tectonic deformed structure of the Kraaipan belt was cut transversely by potassium-rich granitoids, which are represented by the exposed Swiss Reneke granite , with these post-tectonic granites also being the Document the time when the Kimberley block was welded to the Witwatersrand block. In summary, it can be stated that the Kimberley Block, in contrast to the Witwatersrand Block, shows a very extensive magmatism (from 2.93 to about 2.88 billion years), an ubiquitous deformation of the rock packages and a high-grade metamorphosis of the central crustal areas. There is also a relevant difference in the degree of geology over the Colesberg Anomaly, with central crustal areas being exposed in corresponding parts of the Kimberley Block and comparing them to the simultaneous subsidence and deposition on the eastern shield. This comparison is used to model that the Witwatersrand Block is defined as the lower plate being subducted in a subduction zone below the Kimberley Block, defined as the upper plate.

Post-archaic development

The further development of the Kaapvaal craton in the early Proterozoic was dominated by intracontinental extension processes. The Transvaal sedimentary basin was developing at this time, associated sediments of the Transvaal supergroup superimposed discordantly on the rock units of the Ventersdorp group. Moreover, years ago 2.05 billion it came to the spacious intrusion of the Bushveld Complex in the upper Transvaal sediments.


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