Location of the Africa‐Australia‐India Triple Junction and Motion Between the Australian and Indian Plates: Results From an Aeromagnetic Investigation of the Central Indian and Carlsberg Ridges

Abstract

Prior studies have proposed and examined the hypothesis that India and Australia are separate rigid plates separated by a wide, near‐equatorial, E‐W striking, plate boundary. Attempts to place narrow limits on the location of the Africa‐Australia‐India triple junction have been hindered, however, by the lack of useful magnetic profiles crossing the eastern Carlsberg Ridge and northern Central Indian Ridge. Herein we present near‐ridge portions of new profiles from an aeromagnetic survey of the Carlsberg Ridge east of 66°E and of the Central Indian Ridge north of 19°S. These new data are used to estimate 35 new spreading rates averaged from the middle of chron 2A (3.03 Ma) to the present. All other plate motion data along the Central Indian and Carlsberg ridges are also analysed to investigate the present kinematics of the Indian Ocean, especially the motion and boundary between the Indian and Australian plates. Unlike prior efforts, we objectively estimate uncertainties in the strikes of transform faults along the Carlsberg and Central Indian ridges. Indian Ocean plate motion data are unambiguously inconsistent with a model in which India and Australia lie on the same rigid plate, but remain consistent with the existence of distinct and rigid Indian and Australian plates. the plate motion data also remain consistent with closure of the Africa‐Arabia‐India plate motion circuit. the data are consistent with closure of the Africa‐Antarctic‐Australia plate motion circuit and place an upper bound of 7 mm yr−1 on the summed deformation around the Rodriguez triple junction if we have accurately estimated the errors in the data. From data along the Carlsberg and Central Indian Ridges, 95 per cent confidence limits on the location of the Africa‐Australia‐India triple junction are 6.2°S to ∼9°S if the boundary between India and Australia is discrete (i.e. very narrow) where it intersects the Central Indian Ridge. When closure is enforced about the Owen and Rodriguez triple junctions, these limits decrease to 8°S‐9°S, which is centred on the Vema fracture zone and is more than 10 times narrower than we found without the new data. the resulting specific prediction of the location of, and velocity across, a hypothetical narrow boundary between the Indian and Australian plates permits us to test the hypothesis of a narrow boundary. Available data suggest, but do not prove, that no such narrow boundary accommodating the predicted direction of Copyright © 1994, Wiley Blackwell. All rights reserved