| Summary: | Timing of breakup of the Indian continent from eastern Gondwanaland and evolution of the lithosphere in the Bay of Bengal still remain as ambiguous issues. Geoid and free-air gravity data of Bay of Bengal and Enderby Basin are integrated with shipborne geophysical data to investigate the early evolution of the eastern Indian Ocean. Geoid and gravity data of the Bay of Bengal reveal five N36 degrees W fracture zones (FZs) and five isolated NE-SW structural rises between the Eastern Continental Margin of India (ECMI) and the 85 degrees E Ridge/86 degrees E FZ. The FZs meet the 86 degrees E FZ at an angle of approx. 39 degrees. The rises are associated with low-gravity and geoid anomalies and are oriented nearly orthogonal to the FZs trend. The geoid and gravity data of the western Enderby Basin reveal a major Kerguelen FZ and five N4 degrees E FZs. The FZs discretely converge to the Kerguelen FZ at an angle of approx. 37 degrees. We interpret the FZs identified in Bay of Bengal and western Enderby Basin as conjugate FZs that trace the early Cretaceous rifting of south ECMI from Enderby Land. Structural rises between the FZs of Bay of Bengal may either represent fossil ridge segments, possibly have extinct during the early evolution of the Bay of Bengal lithosphere or may have formed later by the volcanic activity accreted the 85 degrees E Ridge. Two different gravity signatures (short-wavelength high-amplitude negative gravity anomaly and relatively broader low-amplitude negative gravity anomaly) are observed on south and north segments of the ECMI, respectively. The location of continent-ocean boundary (COB) is at relatively far distance (100-200 km) from the coastline on north ECMI than that (50-100 km) on the south segment. On the basis of geoid, gravity, and seismic character and orientation of conjugate FZs in Bay of Bengal and western Enderby Basin, we believe that transform motion occurred between south ECMI and Enderby Land at the time of breakup, which might have facilitated the rifting process in the north between combined north ECMI-Elan Bank and MacRobertson Land and in the south between southwest Sri Lanka and Gunnerus Ridge region of East Antarctica. Approximately during the period between the anomalies M1 and M0 and soon after detachment of the Elan Bank from north ECMI, the rifting process possibly had reorganized in order to establish the process along the entire eastern margins of India and Sri Lanka
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