Introduction

The Antarctic plate is almost completely surrounded by divergent margins and meets the south American and African plates in the south Atlantic (Fig. 1a and b) at the Bouvet Triple Junction (BTJ). The BTJ was studied by JOHNSON1973 and FORSYTH1975, who estimated its location at about 54:50'S, 00:40'W, based on ocean floor topography, and the distribution of earthquake epicenters, respectively. SCLATER1976 and Apotria1985 studied the evolution in space and time of the BTJ, and suggested that its configuration has been of Ridge-Transform-Transform (RFF)-type during the last 20 m.y., except for episodic shifts to a Ridge-Ridge-Ridge (RRR)-type configuration.

The Mid Atlantic Ridge (MAR), the American/Antarctic Ridge (AAR) and the South-West Indian Ridge (SWIR) converge in the Triple Junction. They are among the slowest ridges on Earth, with half-spreading rates of 1.6, 0.9 and 0.83 cm/yr, respectively SCLATER1976. The island of Bouvet, located close to the Triple Junction, has been considered to be the surficial expression of a mantle plume MORGAN1972 that may have influenced the topography and composition of the westernmost SWIR and easternmost AAR LEROEX1987. The geology of the southernmost MAR segment may also have been influenced by plumes, such as the hypothetical Shona plume and/or the Discovery plume LEROEX1987, DOUGLASS1995, SMALL1995.

Two expeditions were carried out in 1994 and 1996 to the Bouvet Region by the Istituto di Geologia Marina of the Italian Research Council (CNR) and the Moscow's Geology Institute of the Russian Academy of Science, under the sponsorship of the Italian Antarctic Research Program (Fig. 2a). The main objectives of this work were to define the structural, morphological and petrographical differences of the three divergent margins that meet in the Bouvet region, and to determine the evolution in space and time of the Triple Junction.

Our first 1994 cruise to the Bouvet region was carried out with the R/V Akademik Strakhov; the second took place in 1996 with the R/V Gelendzhik, We present here a report on both cruises, including some preliminary results obtained by on-board data processing and analysis of high resolution swath bathymetry, heat flow measurements, magnetometric, gravimetric and seismic reflection profiles. Rock samples were also dredged in both our cruises (Fig. 2b).

Figure 2: