The West African Craton (WAC) is one of the five cratons of the Precambrian basement rock of Africa that make up the African Plate, the others being the Kalahari craton, Congo craton, Saharan Metacraton and Tanzania Craton. Cratons themselves are tectonically inactive, but can occur near active margins, with the WAC extending across 14 countries in Western Africa, coming together in the late Precambrian and early Palaeozoic eras to form the African continent. It consists of two Archean centers juxtaposed against multiple Paleoproterozoic domains made of greenstone belts, sedimentary basins, regional granitoid-tonalite-trondhjemite-granodiorite (TTG) plutons, and large shear zones. The craton is overlain by Neoproterozoic and younger sedimentary basins. The boundaries of the WAC are predominantly defined by a combination of geophysics and surface geology, with additional constraints by the geochemistry of the region. At one time, volcanic action around the rim of the craton may have contributed to a major global warming event.
Location and geology
The craton appears to have formed when three Archean cratons fused: Leo-Man-Ghana, Taoudeni and Reguibat. The first two docked around 2.1 Ga (billion years ago), and the Reguibat Craton docked with the craton around 2 Ga. The roots of the combined craton extend to a depth of over 300 km (190 mi) in the sub-continental lithospheric mantle. The WAC stretches from the Little Atlas mountains of Morocco to the Gulf of Guinea, and is bounded by mobile belts of much younger rocks to the north, east and west. The oldest rocks were metamorphosed 2.9 to 2.5 billion years ago. In the Sahara it is mostly covered by more recent sediments from the Phanerozoic Eon. Further south, younger volcanic and sedimentary rocks outcrop in Ghana, Ivory Coast, and Sierra Leone, surrounded by even younger sediments laid down in the Precambrian. The WAC is made of two distinct regions north and south of each other; the Reguibat shield and the Man shield respectively. Both of these regions are mainly made of rocks that are either Archean or Paleoproterozoic in age, with western Archean nuclei, and rock types are separated by major shear zones. The fold belts surrounding the WAC were folded and metamorphosed during the Pan-African and/or the Variscan orogenies. The WAC underlies the modern countries of Morocco, Algeria, Mauritania, Senegal, The Gambia, Guinea Bissau, Guinea, Mali, Burkina Faso, Sierra Leone, Liberia, Ivory Coast, Ghana, Togo and Benin.
Metamorphism and evolution The metamorphic record of the craton is characteristic of Paleoproterozoic plate tectonics. A definitive evolution of the area has not been determined as a result of conflicting interpretation of the relationship between low-grade greenstone belts and high-grade gneissic terranes dominated by TTG suites. The three major, widely accepted tectonothermal events for the WAC; the 3.5 to 2.9 Ga Pre-Leonean and Leonean Orogeny, the 2.9 to 2.8 Ga Liberian Orogeny, and the 2.15-1.8 Ga Eburnean Orogeny. A definitive answer has strong implications on the geodynamic processes controlling the craton stabilization and maturation after the Archean-Proterozoic transition. Limited geochronological data indicate a prolonged period of metamorphic overprint, lasting approximately 70-million-years, with the support of Sm-Nd garnet-whole-rock isochron age data and U-Pb and Pb-Pb crystallization ages of zircon, monazite, and titanite. Overprinting relationships in the area indicate copper mineralization is associated with the first deformation event in the WAC, with gold mineralization occurring during subsequent deformation events via reactivation of magmatic and hydrothermal fluids.
Constraints The metamorphic rock of the WAC include, but are not limited to, high-grade amphibolite facies amphibolite, gneiss, paragneiss schist, calc-silicate rock, and migmatites. Portions of the region have also been metamorphosed to the greenschist facies, generally termed a greenstone belt in an Archean terrane. The timing of the facies is constrained by the in-situ U-Pb dating, with garnet composition providing constraints for prograde evolution at the blueschist-amphibolite facies transition. The tectonic environment is constrained by a combination of geophysics, surface geology, geochemistry, and metallogenesis.
Wanderings
The Earth formed about 4.54 billion years ago. As it cooled, the lithosphere, consisting of the crust and the rigid uppermost part of the mantle, solidified. The lithosphere rides on the asthenosphere, which is also solid but can flow like a liquid on geological time scales. The lithosphere is broken up into tectonic plates, which slowly move in relation to one another at speeds of 50–100 mm annually, colliding, combining into continents, splitting and drifting apart to form new continental configurations. It is difficult to reconstruct the early wanderings of the West African Craton, but around 1.13–1.071 billion years ago it seems to have been one of the cratons that came together to form Rodinia, a supercontinent. At that time, the Congo Craton lay to the west of the Amazonian Craton, and the West African Craton lay to the south where both were rotated about 180° and retain this relative configuration. Around 750 million years ago Rodinia rifted apart into three continents: Proto-Laurasia, the Congo craton and Proto-Gondwana. The West African Craton may then have combined with other cratons to form Pannotia, a hypothetical supercontinent that existed from the Pan-African orogeny about 600 million years ago to the end of the Precambrian about 539 million years ago. Later it became part of Gondwana, and later still part of Pangaea, the supercontinent that existed during the Paleozoic and Mesozoic eras about between 335 and 175 million years ago, before North and South America separated from Eurasia and Africa and the continents started to drift towards current configurations.
Snowball Earth
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