WESTWARD MIGRATION OF PACIFIC-OCEAN UPPER-MANTLE INTO THE SOUTHERN-OCEAN REGION BETWEEN AUSTRALIA AND ANTARCTICA

The Southern Ocean south of the Great Australian Eight hosts a globall y anomalous region of mid-ocean ridge, the Australian-Antarctic Discor dance. In addition to its recognized morphological and geophysical ano malies, the discordance represents the current on-axis location of a p roposed isotopic boundary between Indian Ocean and Pacific Ocean upper mantle convective regimes, previously defined from analyses of less t han or equal to 4 Ma mid-ocean ridge basalts (MORE). Pb, Sr, and Nd is otope analyses of similar to 36-66 Ma seafloor dredged from either sid e of the northward extrapolation of the Australian-Antarctic Discordan ce show that greater than or equal to 36 Ma seafloor to the east has a n Indian Ocean MORE 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 migrati on of an arcuate-shaped front of Pacific Ocean upper mantle therefore appears to be a consequence of Australian-Antarctic rifting and Southe rn Ocean opening, suggesting that the current location of the isotopic boundary within the discordance may be entirely coincidental.