Satellite gravity data reveal extraordinary lineations in the gravity field of the western Bellingshausen Sea. Major north-south–striking gravity anomalies west of Peter I Island and between the island and the De Gerlache Seamounts raise questions of the deep crustal structure and 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, including a subducted crustal segment and possibly accreted sediments on top of the downgoing basement. West and east of the basement step and diffraction zone, the seismic sections indicate normally developed oceanic crust with moderate basement undulations. The sequence of undisturbed sediments on top of the compressional structure suggests a tectonic event between 50 and 13 Ma, while relative motion between the Antarctic-Bellingshausen plate and the Phoenix plate had already begun in the 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-Bellingshausen plate and the southward migrating and subducting Phoenix plate.