Three‐dimensional magnetic and gravity studies of the Rodriguez Triple Junction in the Indian Ocean

Abstract

The Rodriguez Triple Junction (RTJ) is a ridge‐ridge‐ridge triple junction connecting three mid‐ocean ridges in the Indian Ocean: the Central Indian Ridge (CIR), Southeast Indian Ridge (SEIR), and Southwest Indian Ridge (SWIR). We carried out three‐dimensional analyses of Sea Beam bathymetry, magnetic, and gravity anomaly data covering an area of about 200 km × 140 km at the triple junction and obtained the magnetization distribution and the Bouguer anomaly field. In the magnetic inversion result some of the magnetic lineations of the CIR and SEIR are recognized within the SWIR valley, and the Bouguer anomaly field shows little variation over the SWIR valley in spite of its large topographic relief. These results indicate the tectonic rifting at the SWIR by crustal thinning without volcanic activity. A 4‐km‐wide graben in the western CIR flank was found to have extremely high magnetization and lower Bouguer anomaly than the eastern flank of the CIR. Because the CIR and SEIR have similar spreading directions and rates at the triple junction, we consider the graben represents a fossil propagating rift of the SEIR, which began to propagate toward the CIR at about 1 Ma and then 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 times and composes part of the triple junction evolution on a longer timescale. We also suggest that the whole process is closely related to formation of the CIR segments at the triple junction since 20 Ma.