Kaapvaal Craton South Africa
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.
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
Map of Africa showing cratons, orogens, rifts, and main geographic elements. The Kalaharai craton in the south comprises the Zimbabwe and Kaapvaal cratons, the Congo craton occupies much of central Africa, and the West African craton is located in northwest Africa. The East African Orogen includes the Arabian-Nubian shield in the north and the Mozambique Belt in the south, whereas the active East African Rift cuts from the Gulf of Aden past Lake Victoria to the Mozambique Channel, and offshore to Madagascar. The Atlas Mountains are located in northwest Africa. (modeled from Alan Goodwin, 1991)
are complexly folded together with migmatitic gneiss, biotite-hornblende tonalitic gneiss, and lenses of 3.3-3.0 billion-year-old quartz monzonite. Several folding and deformation events are recognized from the Ancient Gneiss complex, whose history spans a longer interval of 700 million years, longer than the entire Phanerozoic.
There are six main greenstone belts in the Kaapvaal craton, the most famous of which is the Barberton greenstone belt. Although many studies have attempted to group all of the greenstone sequences of the Kaapvaal craton into the term Swaziland Supergroup, there is little solid geochrono-logic or other evidence that any of these complexly deformed belts are contemporaneous or related to each other, so this usage is not recommended. Other greenstone belts include the Murchison, Sutherland, Amalia, Muldersdrif, and Pietersburg belts. U-Pb (Uranium-Lead) isotopic ages from these belts span the interval from 3.5 to 3.0 billion years ago, a period of 500 million years. The greenstone belts include structurally repeated and complexly folded and metamorphosed sequences of tholeiitic basalts, komatiites, picrites, cherts (or metamorphosed felsic mylonite), felsic lava, clastic sediments, pelites, and carbonates. Possible partial ophiolite sequences have been recognized in some of these greenstone belts, particularly in the Jamestown section of the Barber-ton belt.
one of the long-held myths about the structure of greenstone belts in the Kaapvaal craton is that they represent steep synclinal keels of supracrustal rocks squeezed between diapiric granitoids. Detailed structural studies of the Murchison greenstone belt have established, however, that there is a complete lack of continuity of strata from either side of the supposed syncline of the Murchison belt, and that the structure is much more complex than the pinched-synform model predicts. Downward-facing structures and fault-bounded panels of rocks with opposing directions of younging (the direction toward the younger beds) and indicators of isoclinal folding have been documented, emphasizing that the "stratigraphy" of this and other belts cannot be reconstructed until the geometry of deformation is better understood; early assumptions of a simple synclinal succession are invalid.
Detailed mapping in a number of greenstone belts in the Kaapvaal craton has revealed early thrust faults and associated recumbent nappe-style folds. Most do not have any associated regional metamorphic fabric or axial planar cleavage, making their identification difficult without very detailed structural mapping. In some cases late intrusive rocks have utilized the zone of structural weakness provided by the early thrusts for their intrusion.
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.
The Pietersburg greenstone belt is located north of the Barberton and Murchison belts, near the high-grade Limpopo belt. Greenschist to amphibo-lite facies oceanic-affinity basaltic pillow lavas, gab-bros, peridotites, tuffs, metasedimentary rocks, and banded iron formation are overlain unconformably by a terrestrial clastic sequence deposited during a second deformation event marked by northward-directed thrusting between 2.98 and 2.69 billion years ago. Coarse clastic rocks deposited in inter-montaine basins are imbricated with the oceanic affinity rocks and were carried piggyback on the moving allochthon. Syn-thrusting depositional troughs became tightened into synclinal structures during the evolution of the thrust belt, and within the coarse-clastic section it is possible to find thrusts that cut local unconformities, and unconformities that cut thrusts.
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.
Continue reading here: Witwatersrand Basin
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