Eocene to recent development of the South-west Indian Ridge, a consequence of the evolution of the Indian Ocean Triple Junction

Abstract

Summary. The South-west Indian Ridge, the contact between the African and Antarctic plates, lies between the Bouvet Triple Junction in the South Atlantic and the Indian Ocean Triple Junction about 2100 km east of Madagascar. From the vicinity of Prince Edward Island at 40° E it trends north-easterly and it is segmented by a suite of deep north—south gashes terminating on the north-east with two spectacular meridional fracture zones, the ‘Atlantis II’ and the ‘Melville’, at 57° 30′E and 60° 30′E respectively. From there northeast to the Indian Ocean Triple Junction at 25° 30′S, 70° 00′E the ridge trends N75° E; it is characterized by a triangle of rough topography with the triple junction at the eastern apex. From all available data an instantaneous pole of relative motion for Africa/Antarctica was computed; it lies at 8.4° N, 42.4° W, with a rate of 0.15° Myr−1. Since the marked change in the direction and rate of spreading in the Madagascar, Crozet and Central Indian Basins that occurred in the Eocene (44 Ma, Anomaly 19), the poles of relative motion for the African, Indian and Antarctic plates have changed very little. We fixed the Africa/Antarctica and Africa/India poles and computed that for India/Antarctica. We justified this pole by comparisons of predicted isochrons with observed magnetic lineations and determined the tectonic history of the triple junction. Since the Eocene (44 Ma, Anomaly 19), this junction has moved as rapidly east-wards with respect to Africa as Antarctica has moved south. The resultant geometry and slow spreading account for the triangle of rough topography produced by the South-west Indian Ridge east of the Melville Fracture Zone. The triple junction evolved as a stable ridge-ridge-ridge type with the South-east Indian Ridge remaining approximately constant in length. It was not resolved whether this constancy in length is maintained by frequent ridge jumps or by oblique spreading on the South-west and Central Indian Ridges near the triple junction.