TheMadagascar plate orMadagascar block is atectonic plate holding the island ofMadagascar. It was once attached to theGondwanasupercontinent and later theIndo-Australian plate.
Rifting in the Somali Basin began at the end of theCarboniferous 300 million years ago, as a part of theKaroo rift system. The initiation of the Gondwana breakup, andtransform faulting along the Davie Fracture Zone, occurred in theToarcian (about 182 million years ago) following the eruption of the Bouvet (Karoo) mantle plume. At this time, East Gondwana comprising theAntarctic, Madagascar,Indian, andAustralian plates, began to separate from theAfrican plate. East Gondwana then began to break apart about 115–120 million years ago when India began to move northward.[1] Between 84 and 95 million years ago rifting separatedSeychelles and India from Madagascar.
Since its formation, the Madagascar block has moved roughly in conjunction with Africa, and thus there are questions as to whether the Madagascar plate should be still considered a separate plate.[2][3]
Madagascar was formerly located in the central part of the supercontinent Gondwana. It contains part of theEast African Orogen, which formed in theNeoproterozoic toCambrian assembly of the Gondwana. This heavily influenced the geology of central and northern Madagascar.[4]
The entire island can be divided into four tectonic and geologic units:[5] the Antongil block, the Antananarivo block, the Bekily Belt in the south, and the Bemarivo Belt in the far north.
The blocks in the northern part of the island are made up ofArcheancratonic material.[5] The Antongil block has been linked with theDharwar Formation of India, however the Antananarivo block to the west has been too heavily altered to link easily to another continent.[5]
The central part of the island contains metasediments from African and Indian continental shelves. This is the Itremo Group, which also contains intrusions of material from the Antongil block. The Itremo sheet wasfolded in the amalgamation of Madagascar ~700 Mya, and now contains upright folds, divergent reversefaults, and strike-slip faults.[4]
The Madagascar plate experienced two major rifting events during the break-up of Gondwana. First, it separated from Africa about 160 Mya (million years ago), then from the Seychelles and India 66–90 Mya.[6]
The first rifting event, separation fromSomalia and the rest of Africa, caused displacement along the Davie Ridge, in theMozambique Channel to the west of the islands, a now extincttransform. The rifting is also associated with extensive deformation as well asvolcanism in the lateCretaceous andCenozoic (Eocene toMiocene).
The second separation caused volcanism in the southern part of the island as well as further south, such as onMarion Island.[6] The volcanism was so extensive that in the late Cretaceous Madagascar may have been entirely covered inflood basalts from volcanism associated with this second rifting event.[6] It was at this point in the end of the Cretaceous that Madagascar became entirely isolated from any other continent.
Madagascar remains seismically and volcanically active. The most seismically active area is beneath the Ankaratra Plateau in the centre of the island, which experienced magnitude 5.2 and 5.5earthquakes in 1985 and 1991. The Aloatra-Ankay rift to the north of the plateau is also seismically active, as well as the Davie Ridge off the coast, which is an extension of theEast African Rift Zone.[7]
The Ankaratra Plateau contains a major volcanic field withvolcanic cones and extensive flows. It was active from the Neogene to the Quaternary. To the northwest, there are the recently activeComoro Islands, which are hypothesized to be related to a hot spot.[6]
The Madagascar plate now moves mostly in conjunction with the African plate, so some believe it should not be still considered an independent plate.[6]
