Mechanical Strength and Nature of Lithospheric Deformation Beneath the Rifted Sedimentary Basins of the East India Passive Margin

Abstract

The study of gravity signatures corresponding to sediment load over the passive margins is helpful to constrain the mechanical properties of continental rift systems as well as the adjacent ocean basins. Seismic reflection, free-air gravity, and drill-well information from the East India Passive Margin (EIPM) are investigated through spectral approach and 2-D process-oriented gravity modelling for quantifying effective elastic plate thickness (Te) of the entire margin, thereby to understand rift mechanisms and sedimentary basins evolution. The stretches of lithosphere within the major structural domains along the margin are quite variable. A narrow necking zone (40 km) in the southern segment, and approximately 70 km hyper-thinned and exhumed domains are inferred in the central segment, while a narrow hyper-thinned domain (30-35 km) is observed in the northern segment of the margin. The southern segment of EIPM, adjacent to Southern Granulite Terrain has low Te values range from 5-10 km, the segment in the central margin, offshore Dharwar Craton has moderate Te values of 15-20 km, and the northern segment adjacent to onshore Eastern Ghats Mobile Belt has relatively high mechanical strength with Te values range from 25-35 km. The pattern of Te values and extent of major structural domains along the margin indicates that the southern, central and northern segments evolved as three different modes of rifting such as transform, hyper-extended and hypo-extended, respectively. In oceanic domain, the lithospheric strength broadly increases with age following the thermal induced cooling plate model (600°C isotherm). The thick sedimentary basin in the Bay of Bengal had attained the high value of elastic thickness (Te > 45 km) in the last 23 Myr. The significant rheological strengthening of the oceanic lithosphere could be due to increase upper mantle density and plate curvature caused by excessive deposition of Bengal Fan sediments in the deep ocean basins and modification of stress state of the lithosphere as a convergent interaction of the Indian plate with SE Asia.

Keywords: East India Passive Margin, elastic plate thickness, flexural deformation, process-oriented gravity modelling, admittance analysis, rifted margin