Spectral analysis of the Indian Ocean geoid low provides depth to the large wavelength sources as ¡«1300, ¡«700 and 340 km that are supported from the spectral analysis and the modeling of the corresponding large wavelength regional gravity anomaly with negative density contrasts at these levels. The three levels coincide with the sharp changes in the gradient of the seismic velocities related to the olivine-spinel transformation of successively increasing Fe/Mg ratio as depth increases, known as transition zones. The first two segments are supported from continuous wavelet transform analysis of the large wavelength component of the corresponding gravity field. The low density
rocks in this section appear to be related to the subducted Indian/Tethyan lithosphere that roll back and drifted southwards after subduction as inferred from tomography experiments. The relatively short wavelength sources of the spectrum of the geoid data at depths of 162 and 85 km suggest sources along the lithosphere - asthenosphere boundary (LAB) under the Indian continent and surrounding oceans, respectively. A low viscosity zone has been envisaged below 660 km discontinuity that may correspond to the low density
rocks in this section which is popularly referred to as graveyards of the subducted
rocks under geoid lows. The subducted slab is hydrated due to dehydration from metamorphism that causes upwelling in the mid-to-upper mantle which is likely to set in baby plumes. Presence of fluid may transform part of olivine to serpentine further reducing the bulk density of
rocks in this section. They would make this region buoyant that appears to be responsible for the Central Indian Ocean
Deformation Zone with large scale folding, faulting, seismicity, and high heat flow. The same also appear to be responsible for the rapid drift of the Indian plate.
Short wavelength component of the Indian Ocean geoid low reflects most of the surface/shallow tectonics of the region similar to the gravity anomaly providing an additional data set for this purpose. Kachchh and the Shillong plateau inspite of being in the intra plate region are highly seismogenic that compare almost to the activity along the plate boundaries. Tectonics and residual geoid anomalies of the Indian continent and adjoining regions suggest that the geoid highs of Kachchh and the Shillong plateau along the NW and the NE corners of India are connected to the geoid highs of the plate boundaries through several lineaments and faults that also show geoid highs and trends of seismic activity leading to plate boundaries. Geoid highs indicating high density mafic rocks suggest their connections/extensions to plate boundaries indicating them to be plausibly part of diffused plate boundaries. Besides, both these regions are affected by (i) prominent lithospheric flexure of the Indian plate (ii) large scale mafic intrusions and (iii) interaction of proterozoic mobile belts with effects of present day Himalayan orogeny that make them more vulnerable for seismic activity.
