Gravity Maps of the Lithospheric Structure Beneath the Indian Ocean

Abstract

The lithospheric structure beneath the Indian Ocean is probably the most complicated, but at the same time, the least understood among world’s oceans. Results of tomographic, geochemical, magnetic and other surveys provide the evidence of its complex geological history. Seismic surveys have been a primary source of information about the lithospheric structure beneath the Indian Ocean, but these experiments are mainly concentrated at locations of a high geophysical interest. Marine gravity data obtained from processing the satellite altimetry measurements, on the other hand, deliver a detailed image of the whole seafloor relief, advancing further the knowledge about its formation, tectonism and volcanism. In this study, we use gravitational, bathymetric, marine sediment and lithospheric density structure data to compile the Bouguer and mantle gravity maps. We then use both gravity maps to interpret the lithospheric structure beneath the Indian Ocean. The Bouguer gravity map reveals major tectonic and volcanic features that are spatially correlated with crustal thickness variations. The mantle gravity map exhibits mainly a thermal signature of the lithospheric mantle. Gravity lows in this gravity map mark distinctively active oceanic divergent tectonic margins along the Central, Southeast and Southwest Indian Ridges including also the Carlsberg Ridge. Gravity lows extend along the Red Sea–Gulf of Aden and East African Rift Systems, confirming a connection between mid-oceanic spreading ridges (in the Indian Ocean) and continental rift systems (in East Africa). The combined interpretation of the Bouguer and mantle gravity maps confirms a non-collisional origin of mountain ranges along continental rift systems in East Africa. The evidence of a southern extension of the East African Rift System and its link with the Southwest Indian Ridge in the mantle gravity map is absent. Similarly, the ongoing breakup of the composite Indo-Australian plate is not manifested. A missing thermal signature in the mantle gravity map at these two locations is explained by the fact that the southern Nubian-Somalian plate boundary (i.e., the Lwandle plate) and the Indo-Australian plate boundary (i.e., the Capricorn plate) are diffuse zones of convergence, characterized by low deformation and seismicity due to very slow rates of relative motions accommodated across these boundaries. The clear manifestation of the thermal signature of intraplate hot spots in the mantle gravity map is also absent. This finding agrees with the evidence from direct heat flow measurements that do not indicate the presence of a significant positive temperature anomaly compared to the oceanic lithosphere of a similar age.