| Summary: | Success of the International Indian Ocean Expedition Program (IIOE,1960-'65) induced an undying stimulus to all the geoscientists to probe further to unravel the various geodynamic and plate kinematic processes that are responsible for the breakup of Pangaea and eastern Gondwanaland, and the subsequent dispersion of the continents. Inference of the mid-ocean ridge system, plateaus, banks, volcanic islands/seamounts, aseismic ridges, trenches, fracture zones, extinct spreading ridges and the hotspot traces in the Indian Ocean are the testimony of the instinctive urge to know more and more about the lithosphere. Integrated geophysical and geological studies have improved considerably the understanding of late Cretaceous evolution of the northeastern Indian Ocean, the inter-relationship between the Broken Ridge-Kerguelen hotspot and the Ninetyeast Ridge. Analysis of geological and geophysical data acquired by the National Institute of Oceanography revealed new interesting results. They include (1) the morphotectonic evolution of new fracture zone along 75 degrees 45'E, and the southern extension of the Indrani Fracture Zone (79 degrees E FZ) up to 15 degrees S, (2) Identification of Mesozoic anomaly sequence M11 to M0 of 133.5 to 118Ma age in the Bay of Bengal, (3) identification of the Cretaceous magnetic smooth zone and the boundary of the late Cretaceous crust in the distal part of the Bengal Fan, (4) structure and origin of the 85 degrees E Ridge, seismic stratigraphy, the presence of carbonate buildup on the eastern flank of the 85 degrees E Ridge, and (5) the intraplate deformation. Some of the major scientific problems, yet to be resolved unambiguously, in the northeastern Indian Ocean are: (1) precise timing of the early opening of the Northeastern Indian Ocean, (2) confirmation of the identified Mesozoic magnetic anomaly sequence and the extent of magnetic quiet zone, (3) structural evolution of the Ninetyeast and 85 degrees E Ridges, (4) the extent and impact of the intraplate deformation in the Bay of Bengal, and (5) deciphering the evolutionary history of the Bay of Bengal. In adequate of magnetic data and age information of the basement rocks of eastern continental margin of India and the Enderby basin of East Antarctica (which are conjugate margins) constrained severely the hypothetical plate reconstruction models in circulation for the breakup of eastern Gondwanaland. Details of some of the scientific problems associated with the evolution of the northeastern Indian Ocean have been extensively discussed in this paper using the geophysical data acquired by NIO and NGDC.
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