GEOPHYSICAL EVIDENCE OF A RELICT OCEANIC-CRUST IN THE SOUTHWESTERN SCOTIA SEA

The southwestern part of the Scotia Sea, at the corner of the Shacklet on Fracture Zone with the South Scotia Ridge has been investigated, co mbining marine magnetic profiles, multichannel seismic reflection data , and satellite-derived gravity anomaly data. From the integrated anal ysis of data, we identified the presence of the oldest part of the cru st in this sector, which tentative age is older than anomaly C10 (28.7 Ma). The area is surrounded by structural features clearly imaged by seismic data, which correspond to gravity lows in the satellite-derive d map, and presents a rhomboid-shaped geometry. Along its southern bou ndary, structural features related to convergence and possible incipie nt subduction beneath the continental South Scotia Ridge have been evi denced from the seismic profile. We interpret this area, now located a t the edge of the southwestern Scotia Sea, as a relict of ocean-like c rust formed during an earlier, possibly diffuse and disorganized episo de of spreading at the first onset of the Drake Passage opening. The s uccessive episode of organized seafloor spreading responsible for the opening of the Drake Passage that definitively separated southern Sout h America from the Antarctic Peninsula, instigated ridge-push forces t hat can account for the subduction-related structures found along the western parr of the South Scotia Ridge. This seafloor accretion phase occurred from 27 to about 10 Ma, when spreading slopped in the western Scotia Sea Ridge, as resulted From the identification of the marine m agnetic anomalies.