Over 9700 km of new aeromagnetic data were acquired off the northwest coast of Australia and combined with existing magnetic data to map magnetic isochrons in the eastern Indian Ocean. The isochrons were used to constrain a tectonic model of the evolution of the seafloor in the Argo, Cuvier, and Gascoyne abyssal plains. A complete set of anomalies, from M26 through M16, was found in the Argo Abyssal Plain, trending generally N70¿E. Spreading commenced in the center of the basin at or prior to M26 and propagated outward until at least M24 time. Anomalies M10--M0, recording the separation of Australia and India, were found in the Cuvier and Gascoyne abyssal plains, with a trend of about N30¿E. A significant crustal age discontinuity occurs in the vicinity of the Joey Rise where the two lineation sets converge. Because there appears to be no overlap of isochron ages in the two groups, it is not necessary to postulate that a triple junction existed off northwest Australia as has been previously suggested. At M4--M5 time a 10¿ clockwise change in spreading direction occurred on the Cuvier-Gascoyne spreading system. This event triggered ridge jumps that transferred two pieces of the Indian plate to the Australian plate. Overlapping spreading on the forming and dying ridges, curved fracture zones and lineations, as well as fanned lineation trends, suggest that the ridge jumps occurred by ridge propagation and that the transferred lithospheric blocks behaved as microplates for a brief interval of approximately 1--2 m.y. ¿ American Geophysical Union 1989