Abstract.

The aim of our study is to chronicle the development of plate boundaries in the Southwest Pacific Ocean during early Tertiary time. This region has been the subject of numerous and often conflicting studies that have attempted to construct the history of plate motion and plate boundary evolution as the Australia and Pacific plates separated from Antarctica. Our novel approach entails reconstructing gravity fields from satellite altimeter gravity by first removing anomalies overlying seafloor younger than a selected age, and then rotating the remaining anomalies through appropriate finite rotations. Our reconstructions reveal 1) an extensional plate boundary (the Iselin rift) existed between West and East Antarctica prior to A24 time, 2) the arrival of the Southeast 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 easternmost portion of the Tasman plate boundary (between the SEIR and the Iselin rift) into a transform fault on the Pacific-Antarctic ridge, and 3) an early (A24 or younger) inception of the Australia-Pacific plate boundary. Our scenario for the opening of the Southwest Pacific Ocean can explain the present-day gravity anomalies and magnetic isochrons observed in the northwest Ross Sea. We find that East Antarctic seafloor northeast of the Iselin Bank was generated by spreading on the Tasman ridge prior to A24 time.

Reconstruction for Chron 24 (53.35 Ma)

Reconstruction for Chron 21 (47.91 Ma)

Present-day gravity anomalies

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