On Deciphering the Two Scales of the Regional Bouguer Anomaly of the Deccan Trap and Crust-Mantle Inhomogeneities

Regional Bouguer anomaly over the Deccan Trap (south of Poena) is modelled, constraining it from Deep Seismic Sounding (DSS) studies. A regional, based on the normal Moho obtained from DSS studies along a profile, provides a broad 'hump' over the trap. The residual Bouguer anomaly consists of 'lows' which are modelled in the form of undulations of the Moho (37-41 krn) and Conrad (17-21 km) discontinuities and their rise towards the coast conforming with the DSS studies. These depressions of the Moho and Conrad discontinuities coincide with high plateaus on the surface on either side of river Bhima suggesting them to be caused by local isostanc compensation. The broad regional hump is attributed to the high density inhomgeneities in the upper mantle, approximately at a depth of 130-200 krn which also seem to be reflected in higher P-wave residuals in the teleseismic experiments conducted in this region. It is suggested that this upper mantle inhomgeneity could be due to the erosional unloading of the Deccan traps dod may represent some form of regional compensation. Therefore, two kinds of compensation are basically suggested in this region, namely, local in the form of variations in the level of the lower crust and regional in the form of high density mhomogeneity in the upper mantle. This analysis, therefore, does not support the suggestion of the existence of any sediment-filled rift valley under the traps in this region.