Advances in marine geophysical studies of the Indian Ocean: Contributions from India (2010-2015)

Abstract

Indian scientists carrying marine geophysical research at different Earth Science institutions have made significant advances during the last six years (2012-2015) for understanding evolution of continental margins of India and deep sea regions. The investigations were mostly focused on continental margins of India encompassing deep-water regions, mid-ocean ridges, aseismic ridges and Andaman Sea including the back-arc basin. The studies were aimed at unraveling the tectonic and physical processes associated with evolution of the margins, ocean basins, volcanic structures, etc. In addition, industryrelated research for mapping of energy resources such as gas hydrates and hydrocarbons was also carried out in rifted basins on Eastern Continental Margin of India. The investigations were successfully carried out with integration of several geophysical datasets available at National Institute of Oceanography, India, National Geophysical Data Center, USA and petroleum industries, and by acquiring new geophysical data and geological sampling. In this report we describe major outcomes of the investigations focused on specific geological aspects of the Indian Ocean. Investigations carried out by individual works are also included in the bibliography. Geophysical studies carried out over the Ninetyeast Ridge led to understand how the Kerguelen plume interacted with the spreading centers as well as with the lithosphere during emplacement of volcanic edifices, isostatic compensation mechanisms of the ridge and magma production rates in response to the Indian plate motion. The analyses of seismic data over the 85°E Ridge revealed the internal structure consisting of volcanic plug and several stratigraphic units such as lava-fed deltas and prograding clinoforms. The lava deltas along with intervening erosional surfaces and mass wasting on ridge flanks suggested that the ridge was built by both subaqueous and multiphase submarine volcanism during the late Cretaceous. Study of seismic reflection data of the Eastern Continental Margin of India and adjacent deep-water region revealed two different phases of sediment deposition piled in the Bay of Bengal. The sediment distribution pattern, thickness and the bay architecture unveiled an occurrence of major change in sediment source from the peninsular India to the Bengal Basin at ∼23 Ma as a result of uplift of Himalayas and onset of Asian monsoon system and this eventually led to the commencement and growth of the Bengal Fan. The gas hydrates studies in Krishna-Godavari (KG) offshore basin revealed that the hydrate distribution in the basin is controlled mainly by fault systems. The velocity analysis of sediment strata within the basin suggested occurrence of high concentration of gas hydrate close to the large-scale fault system. Thus, the fault system in KG Basin provides a conducive environment for the migration of methane, which upon entering the hydrate stability zone forms the fracture-filled gas hydrate deposits. Marine geophysical data of both the continental margins of India and western Andaman region were studied for extending the country's limit of the continental shelf beyond the Exclusive Economic Zone. The scientific and technical information extracted from the studies would serve its primary purpose of delineating the outer limits of India's continental shelf beyond 200 nautical miles. The teleseismic studies of the back-arc basin, Andaman Sea revealed that the back-arc spreading fabric is undergoing readjustment to be in equilibrium with the current plate motions. A passive Ocean bottom seismometer (OBS) experiment in the back-arc basin recorded several events including teleseismic, microseismic and local events. Geological and geophysical studies over the Carlsberg and Central Indian Ridges have brought out finer scale segmentation of the ridge system and delineated zones of magmatic and sparsely magmatic sections of the ridges. The water column and deep-tow investigations carried out over the Carlsberg Ridge provided evidences for a prominent plume signatures emanating from a seafloor hydrothermal vent system. A model for the early opening of the Arabian Sea for the period 88 to 56.4 Ma is proposed with the consideration of the Gop and Laxmi Basin spreading centers as two arms of a triple junction and Narmada Rift on the Indian peninsula in the east as third arm of the triple junction. The reconstruction model did provide a new view on dispersals of Madagascar, Seychelles and India, during their early drift period, wherein the Laxmi Ridge and the Laccadive Plateau have been accommodated as intervening continental slivers.