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.

Abstract The atolls of the Maldive archipelago form the central and largest part of the Chagos-Laccadive atoll chain in the equatorial Indian Ocean (Figure 1). The north-south-trending Chagos-Laccadive chain extends from the southwest coast of India to south of the equator and is composed of low-lying coral atolls. The Maldive archipelago consists of 22 large atolls whose sizes range from a few km to tens of km in diameter (Figure 1). The atolls are arranged in clusters separated by deep channels. The shapes of the atolls vary from circular to elongate in map view. Numerous smaller atolls called "faros" are commonly present within the lagoons of the large atolls and, in places, form the rims of the large atolls. The depth of the lagoons ranges from 31 m to 82 m (Purdy and Bertram, 1993) and tends to increase from north to south. Although the archipelago extends for 867 km from north to south, the island area is only 298 km 2 . Approximately 1200 individual islands exist, but only 200 of them are populated. In the central part of the archipelago, the large atolls are arranged in two parallel north-south-trending chains separated by the Inner Sea (Figure 3). Several large drowned flat-topped banks — such as Fuad Bank between Horseburgh and Ari atolls, with its top submerged in 250 m of water — complete the "broken" segments of the double chain of atolls (Figure 3). The water depth of the Inner Sea ranges from 200 m to 500 m. The combined width of the platform (atolls and the Inner Sea) locally adds up to 130 km.

Abstract We review previous models for the Paleogene tectonic evolution of the Arabian and Eastern Somali basins and present a model based on a new compilation of magnetic and gravity data. Using plate reconstructions, we derive a self-consistent set of isochrons for Chron 27 to Chron 21 (61-46 Ma). The new isochrons account for the development of successive ridge propagation events along the Carlsberg Ridge, leading to an important spreading asymmetry between the conjugate basins. Our model predicts the growth of the outer and inner pseudo-faults associated with the ridge propagation events. The location of outer pseudo-faults appears to remain very stable despite a drastic change in the direction of ridge propagation before Chron 24 (c. 54 Ma). The motion of the Indian plate relative to the Somalian plate is stable in direction through Paleogene time; spreading velocities decrease from 6 to 3 cm a -1 . Our reconstructions also confirm that the Arabia-India plate boundary was located west of the Owen Ridge along the Oman margin during Paleogene time; some compression is predicted at about Chron 21 (47 Ma) between the Indian and Arabian plates.