Deep crustal structure of the area of intersection between the Shackleton Fracture Zone and the West Scotia Ridge (Drake Passage, Antarctica)

The Shackleton Fracture Zone, which forms the boundary between the Antarcti c and Scotia plates in the Drake Passage, is characterized by a present-day left-lateral motion. The West Scotia Ridge, an extinct spreading centre, f ormed the oceanic crust of the western Scotia Plate and intersects the Shac kleton Fracture Zone in a complex deformed area. Multichannel seismic, grav ity, 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 i ntersection of these two tectonic features. The new data reveal its asymmet rical deep crustal structure, which developed as a consequence of the overp rinting of extensional and contractional deformation events, The main seism ic features of the crust of the Scotia and Antarctic plates are deep dippin g braided reflectors, which may be a consequence of an initial stretching d eformation related to the Shackleton Fracture Zone. At present, this fractu re zone is characterized by thickened oceanic crust, deformed by reverse an d transcurrent faults and locally bounded by areas of crustal thinning, The present morphology of the West Scotia Ridge has the characteristics of slo w spreading centres, with a central valley bounded by two elongated highs. However, its structure reveals that after spreading ended, there was a NW-S E contractional deformation event, with a thrust of about 40 km of slip tha t resulted in asymmetrical crustal thickening. Incipient subduction initiat es subparallel to the strike of the spreading centre, This tectonic event m ay be related to an episode of closure of South America and the Antarctic P eninsula, probably of Pliocene age. We conclude that in oceanic domains, areas with a weak crust (fracture zone s and spreading centres) constitute the most sensitive regions for analysin g the regional tectonic evolution since deformation events are better recor ded there than in normal oceanic crust. (C) 2000 Elsevier Science B.V. All rights reserved.