TheIndo-Australian plate is or was amajor tectonic plate. It is in the process of separation into two or three plates, and may be currently separated into more than one plate. It contains thecontinent of Australia, its surroundingocean and extends north-west to include theIndian subcontinent and the adjacent waters.
It was formed by the fusion of the thenIndian and the thenAustralian plates approximately 43 million years ago.[1] The fusion happened when themid-ocean ridge in theIndian Ocean, which separated the two plates, ceased spreading.[2]
Australia-New Guinea (Mainland Australia,New Guinea, andTasmania), theIndian subcontinent, andZealandia (New Caledonia,New Zealand, andNorfolk Island) are all fragments of the ancientsupercontinent ofGondwana. As theocean floor broke apart, these land masses fragmented from one another, and for a time these centers were thought to be dormant and fused into a single plate. However, research in the early 21st century indicates plate separation of the Indo-Australian plate may have already occurred.[3][4][5][6]
The eastern side of the plate is theconvergent boundary with thePacific plate. The Pacific plate sinks below the Australian plate and forms theKermadec Trench and the island arcs ofTonga andKermadec. New Zealand is situated along the southeastern boundary of the plate, which with New Caledonia makes up the southern and northern ends of the ancient landmass ofZealandia, which separated from Australia 85 million years ago. The central part of Zealandia sank under the sea.
The southern margin of the plate forms adivergent boundary with theAntarctic plate. The western side is subdivided by theIndian plate that borders theArabian plate to the north and theAfrican plate to the south. The northern margin of the Indian plate forms a convergent boundary with theEurasian plate, which constitutes the active orogenic process of theHimalayas and the Hindukush mountains.
The northeast side of the Australian plate forms a subduction boundary with the Eurasian plate in theIndian Ocean between the borders ofBangladesh andBurma and to the southwest of the Indonesian islands ofSumatra andBorneo. Along the northernNinety East Ridge under the Indian Ocean there appears to be a weakness zone where the Indian and Australian plates are going different ways.[4][6] The subsidence boundary throughIndonesia is reflected in theWallace line.
The eastern part (Australian plate) is moving northward at the rate of 5.6 cm (2.2 in) per year while the western part (Indian plate) is moving only at the rate of 3.7 cm (1.5 in) per year due to the impediment of the Himalayas. In terms of the middle of India and Australia's landmasses, Australia is moving northward at 3 cm (1.2 in) per year relative to India.[4] This differential movement has resulted in the compression of the former plate near its centre at Sumatra and the possible division into the separate Indian and Australian plates again.[7][6]
A third plate, known as theCapricorn plate, may also be separating off the western side of the Indian plate as part of the continued breakup of the Indo-Australian plate.[8]
There is good evidence that the Indo-Australian plate is in the process of separation into new plates.[9]: 4648 Recent studies and evidence from seismic events such as the2012 Indian Ocean earthquakes, suggest that the Indo-Australian plate may have already broken up into two or three separate plates due primarily to stresses induced by the collision of the Indo-Australian plate with Eurasia along what later became theHimalayas,[10][11] and that theIndian plate andAustralian plate may have been separate since at least3 million years ago.[3]
Contemporary models suggest at present there is a deformation zone between the Indian and Australian plates, with both earthquake andglobal satellite navigation system data indicating that India and Australia are not moving on the same vectors northward.[4][6] In due course, some expect a well defined localized boundary to reform between the Indian and Australian plates.[5] Studies show theNinety East Ridge has active faulting along its whole length so that while the simplest explanation is that the Indian and Australian plates have already separated here, it remains possible that only the Capricorn plate has separated from them.[9]: 4667
