Seismic and gravity data reveal Tertiary interplate subduction in the Bellingshausen Sea, southeast Pacific

Abstract

arctic Peninsula during the Cenozoic, the sub- ducting process ceased and the continental mar- gin became a passive one. The age of the south- east Pacific basin determined from available magnetic data suggests that the former conver- gent margin extended from the northernmost Antarctic Peninsula to about long 85°W. Owing to the lack of reliable magnetic data west of long 85°W, a former subduction zone along the margin of western Ellsworth Land and eastern Marie Byrd Land can only be assumed, not ABSTRACT Satellite gravity data reveal extraordinary lineations in the gravity field of the western Bell- ingshausen Sea. Major north-south-striking gravity anomalies west of Peter I Island and be- tween 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 ev- idence 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 compres- sional 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 Creta- ceous. We suggest that this convergent tectonic structure could have developed as an early Ter- tiary transcurrent plate boundary to accommodate the relative motion between the Antarctic- Bellingshausen plate and the southward migrating and subducting Phoenix plate.