The geoid, small-scale convection, and differential travel time anomalies of shear waves in the central Indian Ocean

We have measured 37 differential travel times for the phase pairs SS-S, sSS-sS, ScS2-ScS, ScS2-S, sScS2-sScS, and sScS2-sS. Each pair included a surface bounce point in the central Indian Ocean between 4°N and 21°S and 69° and 85°E. Residuals were calculated relative to both the J-B travel time tables and the PEM-C earth model and were corrected for the elevation of the bounce points. The mean J-B residual associated with bounce points in the Central Indian Basin is -2.8±0.7 s (fast). This contrasts with +4.2 s (slow) reported for the western Pacific by Sipkin and Jordan, and is similar to the -4.0 s found for old continental nucleii. After correcting for known differences between continental and oceanic crustal structure and for differences in measurement technique, we find that travel times in the mantle beneath the Central Indian Basin are midway in character between those beneath pre-Cambrian shields and those beneath the western Pacific ocean basin. The fastest differential travel times occur along a line passing close to the center of the large Indian Ocean geoidal low. There is a clear increase in travel time with distance from the center of the geoid low, suggesting that the source of the geoid anomaly may also be largely responsible for the difference between the Central Indian and Pacific basins. The Chagos-Laccadive Ridge seems to coincide with a transition from the fast differential travel times of the Central Indian Basin to slower differential travel times associated with bounce points beneath young seafloor near active spreading centers. There are also variations within the Central Indian Basin that are not related to surface geological features or to the age of the seafloor. These variations are periodic with a wavelength of about 640 km and with a NNE trend, which agrees approximately with the trend that would be expected for linear, convective rolls in the upper mantle.