404 Oceanology, Vol. 38, No. 3, 1998, pp. 404–410. Translated from Okeanologiya, Vol. 38, No. 3, 1998, pp. 445–452. Original Russian Text Copyright © 1998 by Bulychev, Gasperini, Gilod, Zitellini, Kulikov, Lodolo, Ligi, Mazarovich, Sokolov, Shreider. English Translation Copyright © 1998 by åÄàä ç‡Û͇ /Interperiodica Publishing (Russia). INTRODUCTION The Bouvet Ridge 1 is one of the extreme links of the Southwest Indian Mid-Ocean Ridge. At 54°10′S, 2°20′E, it joins, through the Bouvet transform fault, with the Spiess Ridge, which, together with the Amer- ica–Antarctic and the Mid-Atlantic ridges, forms the Bouvet triple junction [5, 7, 9]. The southern boundary of the Bouvet Ridge runs along the Moshesh transform fault at 54°10′S, 4°40′E. Over the ridge, several profiles of bottom topography and anomalous magnetic field are known, which enabled tracing of the axial magnetic anomaly and paleomagnetic anomalies 2 and 2A with a trend azi- muth of 13.5° ± (2°–5°) [6, 9]. Aside from this, the region has been poorly studied up to now. In cruise 18 of R/V Akademik Nikolai Stra- khov , multibeam echo sounding, magnetometric sur- vey, and continuous seismic profiling were carried out [2, 8]. They allowed us to acquire the first detailed information on the bottom topography, magnetic anom- alies, thickness of sediments, and acoustic basement topography of the region. These data, in their turn, gave a basis for three- dimensional calculations of distribution of magnetiza- tion of the inversive magnetic layer, reconstruction of 1 Following [9], we regard the Bouvet Ridge as the portion of the Southwest Indian Ridge enclosed between the Moshesh and Bou- vet fracture zones. This name has not been officially approved by the UNESCO International Subcommission on Geographical Names of Ocean Bottom Features. seafloor geochronology, studies of the spreading regime, and compilation of a tectonic scheme of the Bouvet Ridge. The task of this paper consists in presen- tation of these results. A Canadian-made GSM-19MD magnetometer was used for magnetometric survey with a fish hauled at a distance of 200 m, which is thrice as great as the length of the vessel. Magnetic deviation, measured in a region with a quiet magnetic field, has an amplitude of a few tens of nT. These measurements were taken into account while processing the results of observations, which gave us the possibility of compiling a map of magnetic field with a contour interval of 50 nT. The val- ues of magnetic anomalies were obtained by extracting reference values of the EGRF field from the observed data. Echo sounding, performed with the use of an ELAC sounding set and an ECHOS-625 multibeam echo sounder, enabled us to compile a map of bottom topography with a contour interval of 100 m. Continu- ous seismic profiling was carried out with the help of a set of instrumentation manufactured at the Geological Institute, Russian Academy of Sciences. A linear group of two pneumatic sources with a joint volume of 1 dm 3 was used for seismic wave generation. Reception was implemented by a single-channel seis- mic tow with a working length of 30 m. To calculate the depth of the acoustic basement, the variability of seis- mic waves in the upper layer of unconsolidated sedi- ments was assumed uniform and equaled 2 km/s. The basic survey grid over the area under study consisted of MARINE GEOLOGY Spreading in the Eastern Part of the Southwest Indian Ridge from the Data of Detailed Geomagnetic Studies in the Region of Bouvet Island A. A. Bulychev*, L. Gasperini**, D. A. Gilod*, N. Zitellini**, E. Yu. Kulikov*, E. Lodolo***, M. Ligi**, A. O. Mazarovich****, S. Yu. Sokolov****, and A. A. Shreider***** * Moscow State University, Moscow, Russia ** Institute for Marine Geology, CNR, Bologna, Italy *** Observatory of Experimental Geophysics, Trieste, Italy **** Geological Institute, Russian Academy of Sciences, Moscow, Russia ***** Shirshov Institute of Oceanology, Russian Academy of Sciences, Moscow, Russia Received June 19, 1996 Abstract—Main features of bottom topography, acoustic basement, unconsolidated sediments, anomalous magnetic field, magnetization of the inversive magnetic layer, and geochronology of the mid-ocean Bouvet Ridge are established for the past 2 My. Peculiarities of the temperature behavior at the constructive boundary between the African and Antarctic lithospheric plates are assessed. The axial zone of the Bouvet Ridge is char- acterized by the presence of three recent volcanic centers, suggesting discrete supply of the matter of new oce- anic crust during the Brunhes epoch. During this epoch, spreading was not monotonic; its rate varied from 1.2-1.3 cm/y over the southeastern and northwestern margins of the Bouvet Ridge to 0.8 cm/y in its central part.