Absolute and relative plate motions and hypotheses on the origin of five aseismic ridges in the Indian Ocean

Abstract

Paleopositions of the African, Indian and Antarctic plates are used as an independant set of data to test various hypotheses, derived from geophysical data, on the origin, emplacement mechanism and evolution of five aseismic structures of the Western and Southern Indian Ocean (Crozet and Kerguelen plateaus, Marion Dufresne, Léna and Ob seamount chain, Madagascar and Mascarene ridges). Except for a continental fragment of very limited extent bearing the Seychelles Islands, these structures, together with the Madagascar and Mascarene ridges, are probably due to anomalous volcanic episodes at —or near— active plate boundaries, rather than to intraplate emplacement. Their creation processes are therefore mainly related to the relative motions between the Antarctic, African and Indian plates. The volcanic episodes are synchronous with major changes in the relative motions between these plates and thus substantiate the role played by frequent irregularities in the kinematic pattern of the Western and Southern Indian Ocean, such as ridge jumps, asymmetric spreading and rapid variations in spreading velocity or direction. Finally, we think that thermal anomalies, located not far from active spreading boundaries, may have played a role in the physical processes creating the aseismic ridges.