Lowermost mantle (D″ layer) structure beneath the Indian Ocean: Insights from modeling of ScS-S and PcP-P residuals

Abstract

• High Compressional (P) and shear (S) wave velocities in the lowermost mantle ( D ″ layer) beneath the Indian Ocean Geoid Low. • Evidence for slabs at the CMB beneath Indian Ocean Geoid Low. • Low R values in the lowermost mantle ( D ″ layer) beneath the western Indian Ocean Geoid Low. In this study, we investigate the seismic structure of the D ″ layer beneath the Indian Ocean by modeling the ScS-S and PcP-P differential travel time residuals corrected for the velocity structure above it. These times, representative of the anomalies in the D ″ layer, vary from −9.58 to 5.06 s for the shear wave (ScS-S) and −4.54 to 3.14 s for the compressional wave (PcP-P). Modeling of the residuals using a grid search approach reveals velocity perturbations in the range of −3.06% to 5.72% for the shear and −4.81% to 5.47% for the compressional waves, in the D ″ layer. Interestingly, the perturbations are positive below the Indian Ocean Geoid Low (IOGL) and negative below the adjoining region. The results clearly reveal presence of high velocity material atop the Core Mantle Boundary (CMB) beneath the IOGL, representing dehydrated Tethyan subducted slabs. Further, the low R SP values in the lowermost mantle beneath the western part of IOGL, calculated using the P and S velocity estimates from this study, mostly lie between 0.01 and 2.7. This implies that the anomalies may be thermal in origin, owing to the heterogeneities resulting from cold lithospheric slabs at the CMB.