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 N70øE. 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

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Linear magnetic anomalies in the Argo Abyssal Plain have been interpreted as having been recorded by seafloor spreading during Late Jurassic to Early Cretaceous Chrons M26 through M16. Ocean Drilling Program Leg 123 drilled at Site 765 in the southern Argo Abyssal Plain, near the base of the northwest Australia margin between anomalies thought to be M25A and M26. However, initial biostratigraphy of sediments overlying basement gave Early Cretaceous ages, ~20 m.y. younger than expected. With this discrepancy as impetus, we re-examined the magnetic lineations in the Argo Abyssal Plain and decided that the best model is still the sequence M26 through Ml6. In addition, we were also able to construct a model that accounts for most of the lineations with the reversal sequence MO through Mil, closer to the basement age predicted by initial biostratigraphic results from the deepest sediments at Site 765. This model proved unsatisfactory because it left a significant portion of the lineations unexplained, requires an unlikely sequence of tectonic events, and disagrees with a reliable Jurassic radiometric age that has been determined from Site 765 basement basalts. Later biostratigraphic studies caused the ages of the oldest sediments at Site 765 to be revised upward, but not enough to eliminate the discrepancy with the basement age inferred from the magnetic lineations. A 5-10 m.y. difference exists between oldest sediments and basement at Site 765, whereas the discrepancy at nearby Site 261 is 3-8 m.y. The probable explanation is that sedimentation on the Jurassic Argo Abyssal Plain was low because the northeast Australian margin was sediment-starved and rugged, allowing little sediment to reach the Argo basin. However, some of the discrepancy may arise from small inaccuracies in the Jurassic geomagnetic polarity reversal time scale or small ridge jumps in the young Argo Abyssal Plain. Our Argo magnetic lineation map implies a relatively simple tectonic history for the basin. Seafloor spreading began shortly before M26 time along the center of the northwest Australian margin and extended east and west through ridge propagation. An initially-segmented Argo spreading center coalesced into fewer, longer spreading segments until ~M21-M19 time when a global plate reorganization caused the ridge to resegment. Spreading began on the western margin of Australia at M10 time in the Early Cretaceous, but does not appear to have been contemporaneous with the observed period of spreading in the Argo basin.