1 Izvestiya, Physics of the Solid Earth, Vol. 35, No. 1, 1999, pp. 1–15. Translated from Fizika Zemli, No. 1, 1999, pp. 3–18. Original Russian Text Copyright © 1999 by Trukhin, Bagin, Bagina, Zhilyaeva, Bulychev, Gilod, Liggi, Lodolo, Sciuto, Tomilin, Shreider. English Translation Copyright © 1999 by åÄàä “ç‡Û͇ /Interperiodica” (Russia). INTRODUCTION The Bouvet Ridge [Sclater et al., 1976] is an end segment of the mid-ocean Southwestern Indian Ridge. This segment is bounded by the Bouvet transform on the north (54°10′S, 2°20′E) and by the Moshesh trans- form on the south (54°10′S, 4°40′E). The study area is geodynamically interesting, because it lies at a distance of 200 km from the triple junction of the South American, African, and Antarctic lithospheric plates. The Mid-Atlantic, American Ant- arctic, and Southwestern Indian mid-ocean ridges meet at this point. The comparative analysis of anomalous geophysical fields and seafloor rock properties can pro- vide additional constraints on the history of the triple junction kinematic instability over the last million of years, which has been revealed from the research works conducted during the 18th cruise of r/v “Akademik Nikolai Strakhov” [Bulychev et al., 1997; Bonatti et al., 1997; Trukhin et al., 1998]. The study of bathymetry and an anomalous mag- netic field on several profiles across the Bouvet Ridge outlined the position of the axial magnetic anomaly and paleomagnetic anomalies 2 and 2A, striking at an azi- muth of 13.5° ± (2°–5°) [Sclater et al., 1976; Hayes, 1991]. Multiray echo-sounding, magnetometric, and con- tinuous seismic profiling surveys were conducted dur- ing the 18th cruise of r/v “Akademik Nikolai Strakhov” [Peyve et al., 1994; Mazarovich et al., 1995]. Their results were used for calculation of the three-dimen- sional magnetization distribution of the reversed, mag- netically active layer, reconstruction of the seafloor geochronology, and study of the spreading regime and tectonic structure of the Bouvet Ridge [Bulychev et al., 1998]. Along with the magnetometric studies, numerous samples of seafloor bedrock were dredged (Fig. 1), and this work addresses the study of the anomalous mag- netic field ∆T and magnetic layer responsible for this field, based on model magnetization calculations and magnetomineralogical analysis of seafloor rock sam- ples dredged from the mid-ocean Bouvet Ridge. The structure of paleomagnetic anomalies allows a detailed calibration of the seafloor geochronology, which is essential to the paleogeodynamic reconstruc- tion of the area studied. The observed paleomagnetic anomaly characteristics are controlled by the magne- tomineralogical composition of magmas that accreted the seafloor in this area. Because of conservatism of a thermodynamic system, equilibrium conditions, under which magnetic minerals of a fixed composition crys- Magnetism of the Bouvet Mid-Ocean Ridge, South Atlantic V. I. Trukhin 1 , V. I. Bagin 2 , O. L. Bagina 1 , V. A. Zhilyaeva 1 , A. A. Bulychev 3 , L. A. Gilod 3 , M. Ligi 4 , E. Lodolo 5 , F. Sciuto 6 , E. F. Tomilin 1 , and A. A. Shreider 7 1 Physical Faculty, Moscow State University, Vorob’evy Gory, Moscow, 119899 Russia 2 Schmidt Joint Institute of Physics of the Earth, Russian Academy of Sciences, B. Gruzinskaya ul. 10, Moscow, 123810 Russia 3 Geological Faculty, Moscow State University, Vorob’evy Gory, Moscow, 119899 Russia 4 Institute of Marine Geology, Bologna, Italy 5 Observatory of Experimental Geophysics, Trieste, Italy 6 University of Genoa, Genoa, Italy 7 Shirshov Institute of Oceanology, Russian Academy of Sciences, Nakhimovskii pr. 36, Moscow, 117218 Russia Received June 18, 1998 Abstract—The joint analysis of data from traverse surveys and magnetomineralogical studies of dredged seafloor rocks showed that three neovolcanic spreading centers differing in spreading rate exist in the Bouvet Ridge zone between the Bouvet and Moshesh faults. Both isothermal magnetic and thermomagnetic char- acteristics were measured, and basalts were shown to include both single-phase- and multiphase-oxidized ti- tanomagnetite (TM). Except for ferrobasalts, the calculated values of natural remanent magnetization in other basalts fit reasonably well the measured results, confirming that model parameters of the magnetically active layer were correctly chosen. The calculated Curie points indicate that the Bouvet Ridge basalts have similar compositions, and therefore the entire ridge was accreted from magma chambers at a depth of about 70 km.