Seafloor spreading on the Southeast Indian Ridge over the last one millionyears: a test of the Capricorn plate hypothesis

Plate motions in the Indian Ocean are inconsistent with a rigid Indo-Austra lian plate. An equatorial. diffuse boundary dividing the plate into separat e Indian and Australian plates significantly improves the fit of kinematic plate models to the spreading rates, transform azimuths, and earthquake sli p vectors on the spreading center boundaries. An additional boundary, furth er dividing the Australian plate into Australian and Capricorn plates has b een proposed to account for much of the remaining inconsistency and the pat tern of intraplate earthquakes [J.-Y. Royer. R.G. Gordon. Science 277 (1997 ) 1268-1274]. The proposed boundary is similar to 2000 km wide where it int ersects the Southeast Indian Ridge. Several recent geophysical cruises to t he Southeast Indian Ridge, including a cruise within the proposed boundary. provide many new data for investigating the validity of the Capricorn plat e model. These neu observations strongly support the hypothesis that the Ca pricorn plate exists. Statistical tests of the data from the Southeast Indi an Ridge alone are not sufficient to confirm it, but motion about the Rodri guez Triple Junction (RTJ) suggests some nonrigidity in the Antarctica-Aust ralia-Somalia circuit. Inferred deformation with enforced closure about the RTJ leads to an estimate of plate motion consistent with the Capricorn pla te model. However, the diffuse Capricorn-Australia boundary does not extend south of the St. Paul Fracture Zone, 800 km narrower than the previously p roposed boundary. (C) 2001 Elsevier Science B.V. All rights reserved.