The Southern Ocean south of the Great Australian Bight hosts a globally anomalous region of mid-ocean ridge, the Australian-Antarctic Discordance. In addition to its recognized morphological and geophysical anomalies, the discordance represents the current on-axis location of a proposed isotopic boundary between Indian Ocean and Pacific Ocean upper mantle convective regimes, previously defined from analyses of ≤ 4 Ma mid-ocean ridge basalts (MORB). Pb, Sr, and Nd isotope analyses of ∼ 36–66 Ma seafloor dredged from either side of the northward extrapolation of the Australian-Antarctic Discordance show that ≥ 36 Ma seafloor to the east has an Indian Ocean MORB isotopic signature. This indicates that the Indian-Pacific isotopic boundary does not extend directly north of the ridge toward the southern margin of Australia. Progressive westward migration of an arcuate-shaped front of Pacific Ocean upper mantle therefore appears to be a consequence of Australian-Antarctic rifting and Southern Ocean opening, suggesting that the current location of the isotopic boundary within the discordance may be entirely coincidental.