Gravity anomalies over the Central Indian Ridge between 3?°S and 11?°S, Indian Ocean: Segmentation and crustal structure

摘要

High-resolution shipboard geophysical investigations along the Indian Ocean ridge system are sparse especially over the Carlsberg and Central Indian ridges. In the present study, the shipboard gravity and multibeam bathymetry data acquired over a 750 km long section of the Central Indian Ridge between 3?°S and 11?°S have been analysed to understand the crustal structure and the ridge segmentation pattern. The mantle Bouguer anomalies (MBA) and the residual mantle Bouguer anomalies (RMBA) computed in the study area have shown significant variations along the ridge segments that are separated by transform and non-transform discontinuities. The MBA lows observed over the linear ridge segments bounded by well-defined transform faults are attributed to the thickening of the crust at the middle portions of the ridge segments. The estimates of crustal thickness from the RMBA shows an average of 5.2 km thick crust in the axial part of the ridge segments. The MBA and relative RMBA highs along the two nontransform discontinuities suggests a thinner crust of up to 4.0 km. The most significant MBA and RMBA highs were observed over the Vema transform fault suggesting thin crust of 4 km in the deepest part of the transform fault where bathymetry is more than 6000 m. The identified megamullion structures have relative MBA highs suggesting thinner crust. Besides MBA lows along the ridge axis, significant off-axis MBA lows have been noticed, suggesting off-axis mantle upwelling zones indicative of thickening of the crust. The rift valley morphology varies from the typical V-shaped valley to the shallow valley floor with undulations on the inner valley floor. Segments with shallow rift valley floor have depicted well-defined circular MBA lows with persistent RMBA low, suggesting modulation of the valley floor morphology due to the variations in crustal thickness and the mantle temperature. These are supported by thicker crust and weaker lithospheric mantle.