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The Scotia–Antarctic plate boundary extends along the southern branch of the Scotia Arc, between triple junctions with the former Phoenix plate to the west (57°W) and with the Sandwich plate to the east (30°W). The main mechanism responsible for the present arc configuration is the development of the Scotia and Sandwich plates from 30–35 Ma, related to breakup of the continental connection between South America and the Antarctic Peninsula. The Scotia–Antarctic plate boundary is a very complex tectonic zone, because both oceanic and continental elements are involved. Present-day sinistral transcurrent motion probably began 8 Ma ago. The main active structures that we observed in the area include releasing and restraining bends, with related deep extensional and compressional basins, and probable pull-apart basins. The western sector of the plate boundary crosses fragmented continental crust: the Western South Scotia Ridge, with widespread development of pull-apart basins and releasing bends deeper than 5000 m, filled by asymmetrical sedimentary wedges. The northern border of the South Orkney microcontinent, in the central sector, has oceanic and continental crust in contact along a large thrust zone. Finally, the eastern sector of the South Scotia Ridge is located within Discovery Bank, a piece of continental crust from a former arc. On its southern border, strike-slip and normal faults produce a 5500-m-deep trough that may be interpreted as a pull-apart basin. In the eastern and western South Scotia Ridge, despite extreme continental-crustal thinning, the basins show no development of oceanic crust. This geometry is conditioned by the distinctive rheological behaviour of the crust involved, with the bulk concentration of deformation within the rheologically weaker continental blocks.

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We discuss the results of measurements of the magnetic anomaly field of several seamounts located on the ocean floor and on the bordering continental slopes of the Tyrrhenian Sea. This geomagnetic study of seamounts is based on joint 3-D analyses of the magnetic anomalies and the bathymetry, on measurements of the magnetic properties of rock specimens and on geochronological data. The 3-D analysis of the magnetic anomalies of large volcanoes emplaced in the small oceanic basins of Vavilov and Marsili serves to shed light on the spatial distribution of the magnetized bodies by eliminating the effect of topography. A new magnetic anomaly map of the southern Tyrrhenian Sea is obtained by the merging of shipborne local data, measured on topographic highs, with data of regional type. The study of the magnetic patterns and age of the oceanic seamounts indicates that the volcanic activity mainly developed prior to chron C2 (> 2.02 Ma BP) on the Vavilov and Magnaghi edifices (Vavilov Basin), whereas the volcanism of a big oceanic seamount emplaced in the small basin of Marsili took place mainly during the period of chron C1 (0-0.78 Ma BP).