The upper mantle density structure beneath the Southwest Indian Ocean based on gravity inversion constrained by seismic data

Abstract:

Hotspot and mid-ocean ridge is the manifestation of magmatic activity and mantle upwelling respectively. The Southwest Indian Ridge (SWIR), as the largest and most completely developed ultraslow spreading ridge, has local abnormal thick oceanic crust, relatively accumulated magma supply and frequent hydrothermal activities. These specific phenomena are probably related to hotspots. Moreover, hotspot-ridge interaction can be the main factor for the formation of oceanic plateaus.

In this study, we use ETOPO1 topographic data, EIGEN-6C4 satellite gravity field model and CRUST1.0 model to calculate the gravity anomaly which can reflect inhomogeneous distribution of upper mantle density in the Southwest Indian Ocean. Then we image the upper mantle density beneath the study area by 3D gravity inversion method and obtain the density structure between depths from 0 to 600 km. The inversion result shows that there exist density anomalies of different tectonic units both laterally and vertically beneath the study area, indicating the inhomogeneous distribution of upper mantle density beneath the Southwest Indian Ocean. Two specific segments of the SWIR have obvious low density anomaly, demonstrating the relatively robust magma supply and the effect of transform faults with large offset. Low density anomaly distributed at different depths beneath hotspots reflects the presence of interaction between hotspots and the SWIR as well as four hotspots originating from different depths. The low density anomaly beneath the Madagascar oceanic plateau indicates the existence of deep high temperature materials and the upwelling of Marion mantle plume is the dominant factor of forming the Madagascar oceanic plateau, meanwhile the tectonic activities of the SWIR also play a role.

The quantitative analysis of upper mantle density structure beneath the Southwest Indian Ocean provides the geophysical reference and evidence to study the magma supply characteristic of SWIR, hotspot-ridge interaction and formation of oceanic plateau.

This research is supported by the China Ocean Mineral Resources Research and Development Association Thirteen Five-Year Major Program under contract No DY135-S1-1-06.