The Shackleton Fracture Zone, which forms the boundary between the Antarctic and Scotia plates in the Drake Passage, is characterized by a present-day left-lateral motion. The West Scotia Ridge, an extinct spreading centre, formed the oceanic crust of the western Scotia Plate and intersects the Shackleton Fracture Zone in a complex deformed area. Multichannel seismic, gravity, magnetic and multibeam swath bathymetry data were acquired during the ANTPAC 97/98 cruise with the Spanish vessel B/O HESPERIDES in the area of intersection of these two tectonic features. The new data reveal its asymmetrical deep crustal structure, which developed as a consequence of the overprinting of extensional and contractional deformation events. The main seismic features of the crust of the Scotia and Antarctic plates are deep dipping braided reflectors, which may be a consequence of an initial stretching deformation related to the Shackleton Fracture Zone. At present, this fracture zone is characterized by thickened oceanic crust, deformed by reverse and transcurrent faults and locally bounded by areas of crustal thinning. The present morphology of the West Scotia Ridge has the characteristics of slow spreading centres, with a central valley bounded by two elongated highs. However, its structure reveals that after spreading ended, there was a NW–SE contractional deformation event, with a thrust of about 40 km of slip that resulted in asymmetrical crustal thickening. Incipient subduction initiates subparallel to the strike of the spreading centre. This tectonic event may be related to an episode of closure of South America and the Antarctic Peninsula, probably of Pliocene age. We conclude that in oceanic domains, areas with a weak crust (fracture zones and spreading centres) constitute the most sensitive regions for analysing the regional tectonic evolution since deformation events are better recorded there than in normal oceanic crust.

The Spanish ‘Comisión Interministerial de Ciencia y Tecnología’ (CYCIT) provided financial support through Research Project ANT99-0817. We acknowledge with gratitude the cooperation of the Commander, officers and crew of the B/O HESPERIDES, who made it possible to obtain these data. We are indebted to Dr Ch. Goldfinger, University of Oregon, for facilitating the use of the program FLEDERMAUS to visualize the swath bathymetry and to Dr A. Carbó, for the program Lanzada to calculate gravimetric anomalies.

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