SEISMIC AND GRAVITY-DATA REVEAL TERTIARY INTERPLATE SUBDUCTION IN THEBELLINGSHAUSEN SEA, SOUTHEAST PACIFIC

Satellite gravity data reveal extraordinary lineations in the gravity field of the western Bellingshausen Sea. Major north-south-striking gr avity anomalies west of Peter I Island and between the island and the De Gerlache Seamounts raise questions of the deep crustal structure an d tectonic events in that part of the Antarctic plate. As part of two cruises in the Bellingshausen and Amundsen seas with RV Polarstern in 1994 and 1995, we acquired multichannel seismic records and shipborne gravity data across the gravity anomalies. The combined data set shows strong evidence for a converging event within the oceanic crust, incl uding a subducted crustal segment and possibly accreted sediments on t op of the downgoing basement, West and east of the basement step and d iffraction zone, the seismic sections indicate normally developed ocea nic crust with moderate basement undulations, The sequence of undistur bed sediments on top of the compressional structure suggests a tectoni c event between 50 and 13 Ma, while relative motion between the Antarc tic-Bellingshansen plate and the Phoenix plate had already begun in th e Late Cretaceous. We suggest that this convergent tectonic structure could have developed as an early Tertiary transcurrent plate boundary to accommodate the relative motion between the Antarctic-Bellingshause n plate and the southward migrating and subducting Phoenix plate.