Kaapvaal Craton – The oldest Continent Rock formation on the Planet.


Kaapvall Craton is a unique geological feature situated in the southern part of the African Continent. The Archean Kaapvaal craton of southern Africa contains some of the world’s oldest and most intensely studied Archean rocks, yet nearly 86 percent of the craton is covered by younger rocks. The craton covers approximately 363,000 square miles (585,000 km2) near the southern tip of the African continent. The craton is bordered on the north by the highgrade Limpopo mobile belt, initially formed when the Kaapvaal and Zimbabwe cratons collided at 2.6 billion years ago. On its southern and western margins the craton is bordered by the Namaqua-Natal Proterozoic orogens, and it is overlapped on the east by the Lebombo sequence of Jurassic rocks recording the breakup of Gondwana.

The Southern Africa Craton comprises the Zimbabwe Craton to the NORTH and the Kaapvaal Craton to the S, merged across the high-grade gneisses of the Limpopo Belt.

Most of the rocks composing the Archean basement of the Kaapvaal craton are granitoids and gneisses, along with less than 10 percent greenstone belts known locally as the Swaziland Supergroup. The oldest rocks are found in the Ancient Gneiss complex of Swaziland, where a 3.65-3.5 billion-year-old bimodal gneiss suite consisting of interlay-ered tonalite-trondhjemite-granite and amphibolite.

A complex series of tectonic events is responsible for the present structural geometry of the greenstone belts of the Kaapvaal craton. Early regional recumbent folds, thrust faults, inverted stratigraphy, juxtaposition of deep and shallow water facies, nappes, and precursory olistostromes related to the northward tectonic emplacement of the circa 3.5 Ga Bar-berton greenstone collage on gneissic basement have been documented. The thrusts may have been zones of high fluid pressure resulting from hydrothermal circulation systems surrounding igneous intrusions, and are locally intruded by syn-tectonic 3.43-3.44 billion-year-old felsic igneous rocks. Confirmation of thrust-style age relationships comes from recent U-Pb zircon work, which has shown that older (circa 3.482 ± 5 Ga) Komatii Formation rocks lie on top of younger (circa 3.453 ± 6 Ga) Theespruit Formation.

Limpopo River

The granite-greenstone terrane is overlain unconformably by the 3.1 billion-year-old Pongola supergroup that has been proposed to be the oldest well-preserved continental rift sequence in the world. Deposition of these shallow-water tidally influenced sediments was followed by a widespread granite intrusion episode at 3.0 billion years ago. The next major events recorded include the formation of the West Rand Group of the Witwatersrand basin on the cratonward side of an Andean arc around 2.8 billion years ago, then further deposition of the extremely auriferous sands of the Central Rand Group in a collisional foreland basin formed when the Zimbabwe and Kaapvaal cratons collided. This collision led to the formation of a continental extensional rift province in which the Ventersdorp supergroup was deposited at 2.64 billion years ago, with the extension occurring at a high angle to the collision. The latest Archean through Early Protero-zoic history of the Kaapvaal craton is marked by deposition of the 2.6-2.1 billion-year-old Transvaal supergroup in a shallow sea, perhaps related to slow thermal subsidence following Ventersdorp rifting. The center of the Witwatersrand basin is marked by a large circular structure called the Vredefort dome. This structure, several tens of kilometers wide, is associated with shock metamorphic structures, melts, and extremely high-pressure phases of silica, suggesting that it represents a meteorite impact structure.

The Bushveld complex is the world’s largest layered mafic-ultramafic intrusion, located near the northern margin of the Kaapvaal craton. The complex occupies an area of 40,000 square miles (65,000 km2) and intrudes Late Archean-Early Proterozoic rocks of the Transvaal Supergroup. Isotopic studies using a variety of methods have yielded age estimates of 2.02.1 billion years, with some nearby intrusions yielding ages as young as 1.6 billion years. The complex consists of several lobes with a conelike form, and contains numerous repeating cycles of mafic, ultramafic, and lesser felsic rocks. several types of ores are mined from the complex, including chromite, platinum-group metals, cobalt, nickel, copper, and vanadiferous iron ores. Nearly 70 percent of the world’s chrome reserves are located in the Bushveld complex. The mafic phases of the complex include dunite, pyroxenite, harzburgite, norite, anorthosite, gabbro, and diorite. The center of the complex includes felsic rocks, including grano-phyres and granite.

Much of the Kaapvaal craton is covered by rocks of the Karoo basin, including fluvial-deltaic deposits and carbonaceous deposits including coal. The top of the Karoo Sequence includes mafic and felsic lavas that were erupted soon before the breakup of Gond-wana 200 million years ago.

Extracts from:Climate-Policy-Watcher

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