3-DIMENSIONAL MAGNETIC AND GRAVITY STUDIES OF THE RODRIGUEZ-TRIPLE-JUNCTION IN THE INDIAN-OCEAN

The Rodriguez Triple Junction (RTJ) is a ridge-ridge-ridge triple junc tion connecting three mid-ocean ridges in the Indian Ocean: the Centra l Indian Ridge (CIR), Southeast Indian Ridge (SEIR), and Southwest Ind ian Ridge (SWIR). We carried out three-dimensional analyses of Sea Bea m bathymetry, magnetic, and gravity anomaly data covering an area of a bout 200 km x 140 km at the triple junction and obtained the magnetiza tion distribution and the Bouguer anomaly field. In the magnetic inver sion result some of the magnetic lineations of the CIR and SEIR are re cognized within the SWIR valley, and the Bouguer anomaly field shows l ittle variation over the SWIR valley in spite of its large topographic relief. These results indicate the tectonic rifting at the SWIR by cr ustal thinning without volcanic activity. A 4-km-wide graben in the we stern CIR flank was found to have extremely high magnetization and low er Bouguer anomaly than the eastern flank of the CIR, Because the CIR and SEIR have similar spreading directions and rates at the triple jun ction, we consider the graben represents a fossil propagating rift of the SEIR, which began to propagate toward the CIR at about 1 Ma and th en became inactive when the SWIR grew to reach the CIR+SEIR spreading axis at about 0.5 Ma. We suggest the above process repeats several tim es and composes part of the triple junction evolution on a longer time scale. We also suggest that the whole process is closely related to fo rmation of the CIR segments at the triple junction since 20 Ma.