EARLY TERTIARY GRAVITY-FIELD RECONSTRUCTIONS OF THE SOUTHWEST PACIFIC

The aim of our study is to chronicle the development of plate boundari es in the Southwest Pacific Ocean during the early Tertiary. This regi on has been the subject of numerous and often conflicting studies that have attempted to construct the history of plate motion and plate bou ndary evolution as the Australia and Pacific plates separated from Ant arctica. Our novel approach entails reconstructing gravity fields from satellite altimeter gravity by first removing anomalies overlying sea floor younger than a selected age, and then rotating the remaining ano malies through appropriate finite rotations. Our reconstructions revea l: (1) an extensional plate boundary (the Iselin rift) existed between West and East Antarctica prior to A24 time; (2) the arrival of the So utheast Indian ridge (SEIR) at the Tasman ridge (prior to A24) led to the extinction of the Iselin rift as well as the conversion of the eas ternmost portion of the Tasman plate boundary (between the SEIR and th e Iselin rift) into a transform fault on the Pacific-Antarctic ridge; and (3) an early (A24 or younger) inception of the Australia-Pacific p late boundary, Our scenario for the opening of the Southwest Pacific O cean can explain the present-day gravity anomalies and magnetic isochr ons observed in the northwest Ross Sea. We find that the East Antarcti c seafloor northeast of the Iselin Bank was generated by spreading on the Tasman ridge prior to A24 time. (C) 1997 Elsevier Science B.V.