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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.

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