Abstract The tectonic province of the western Indian Ocean is defined by the East Africa Rift Zone to the west and by the Ninety-East Ridge to the east. The area is bounded to the north by the Arabian Peninsula and to the south by the southern Indian Ocean spreading center. The topography-bathymetry is dominated by the triple-junction Indian Ocean spreading center, the mantle plume extrusions forming the Laccadives-Maldives-Chagos and Mascarene Plateau-Mauritius-Reunion chains of volcanic archipelagos and islands, and the mantle plume extrusion of the Ninety-East Ridge. Initial breakup of ancestral Gondwana, sea floor spreading, and appearance of oceanic crust was preceded by continental sag and development of the Late Carboniferous–Early Jurassic Karoo basins. The first oceanic crust appeared in the Middle Jurassic as the Africa-Arabia plate moved northward relative to the India-Seychelles-Madagascar-Australia-Antarctica plate. This north-south separation continued through the Neocomian. A major jump in the spreading center occurred in earliest Barremian with Antarctica-Australia separating from India-Seychelles-Madagascar. Madagascar separated from India-Seychelles via a transform fault along the east coast of Madagascar. The trans-tensional transform evolved into a spreading center during the middle Cretaceous Barremian-Aptian-Albian as oceanic crust appeared. The mantle plume Rajmahal Traps first appeared in eastern India during the Aptian-Albian, and as the Indian plate continued to migrate northward, evolved into the Ninety-East Ridge. The mantle plume-derived volcanic rocks of the Deccan Traps first appeared in western India near the Cretaceous-Tertiary boundary. The Seychelles began to separate from India in the early Paleocene. By the close of the Paleocene, a broad expanse of oceanic crust separated the Seychelles and western India. The mantle plume formed an extensive oceanic ridge that became the Laccadives-Maldives-Mascarene Plateau. Beginning in the Eocene and continuing through the Oligocene, the ongoing spreading center split the oceanic ridge. North of the spreading center, mantle activity extended the Laccadives-Maldives to include the Oligocene-age Chagos Archipelago, while south of the spreading center, the Mascarene Plateau basalts continued as the Saya de Malha and Nazareth Banks. Mantle plume extrusion continued to the south as the plate moved northward, creating Mauritius Island during the Miocene and Reunion Island during Pliocene-Recent. To the northwest, Red Sea separation of Egypt from Arabia began during the Oligocene. Extension of the Indian Ocean spreading center into the Gulf of Aden between Somalia and Yemen-Oman did not occur until the Miocene. To the north in Ethiopia-Eritrea, the East Africa Rift Zone originated during the early Miocene and has extended southward through Uganda-Kenya-Tanzania-Mozambique into the southern Indian Ocean.

Microcontinents are common in the accreted continental geological record, but relatively rare in modern settings. Many of today's microcontinents are found in the Tasman Sea and Indian Ocean. These include the East Tasman Rise, the Gilbert Seamount Complex, the Seychelles, Elan Bank (Kerguelen Plateau), and possibly fragments of the Lord Howe Rise and Norfolk Ridge, and the Wallaby Plateau. We review their history of formation, and propose that the mechanisms that led to their isolation were mostly plume-related. Tasman Sea continental fragments formed by ridge jumps onto adjacent continental margins after sea-floor spreading in the southern Tasman Sea commenced. The East Tasman Plateau was separated from the Lord Howe Rise at about chron 34 (83 Ma) and the Gilbert Seamount Complex rifted off the South Tasman Rise at roughly 77 Ma, by ridge jumps in opposing directions. Evidence for thermal anomalies under the central Lord Howe Rise, Ross Sea and possible eastern Australian margin explain ridge jumps that led to the isolation of the East Tasman Plateau, Gilbert Seamount and possibly the northern Lord Howe Rise and Dampier Ridge. In the central Indian Ocean, spectacular exposures of granite make the Seychelles a type example of a microcontinent. As in the Tasman Sea, ridge-plume interactions have been responsible for separating a thinned continental sliver from a large continent (India). Elan Bank, aspart of the Kerguelen Plateau, represents another example of a continental fragment in the Indian Ocean. Newly identified M-sequence anomalies in the Ender by Basin, off Antarctica, suggest that this microcontinent was detached from India no earlier than 124 Ma when a northward ridge jump towards the Kerguelen plume may have isolated Elan Bank. This Interpretation implies that a mantle thermal anomaly due to the incipient Kerguelen plume pre-dated the Rajmahal Traps, emplaced at about 118 Ma, by ∼6 million years. An early Kerguelen hot-spot position north of Elan Bank, with subsequent southward migration, is also supported by palaeomagnetic data and mantle convection models. The Wallaby Plateau off Western Australia, located between the Perth and Cuvier Abyssal Plains, may also include slivers of continental crust, but unequivocal evidence is not available, as it has never been drilled. Here it is suggested that most of these microcontinents formed by re-rifting of a young continental margin in the vicinity of a mantle plume stem. The weak inner flank of a rifted margin weakens further when passing over a mantle plume, causing a nearby spreading ridge to jump onto this zone of weakness. This process isolates a passive margin segment, and leaves a narrow passive margin behind. After the onset of subduction and ocean basin destruction, such micro-continents may be accreted again to an active plate margin.