The SWIR is offset by 200 krn at the Bouvet Fracture Zone, which is marked by a northeast-southwest trending transform valley, as deep as 5000 m below sea level, and characterized by steep walls with an average gradient of 37%. Samples dredged from the walls are mainly basalts, gabbros and peridotites with variable degree of serpentinization, The northern wall is bounded by east-west striking structural highs, that may be related to intense block faulting resulting from complex interaction between the Antarctic and African plate. The short segment of the SWIR north of Bouvet island is characterized by anomalously low depths, probably related to the presence of the Bouvet hot-spot, which induces a generalized swelling in the ocean floor. The Bouvet F. Z. ends abruptly towards the southwest, against the Spiess volcanic system, that constitutes the anomalous westernmost segment of the SWIR.
The Spiess volcanic system (Fig 3) rises up to 320m below sea level and has on its summit an elliptical east-west oriented caldera; the overall volcanic complex is also elliptical but oriented roughly perpendicular to the Bouvet FZ., and is characterized by an intense positive axial magnetic anomaly (Fig. 4). The presence of enriched ferrobasalts LEROEX1982, LEROEX1987 may contribute to the strong axial anomaly. We argue that the Spiess volcanic system is a young (
1 my old) NW-propagating ridge resulting from major magmatic events, that have modified the plate boundary geometry in the riple Junction region LIGI1997.