Effects of Ninetyeast Ridge Magmatism and Pre India-Eurasia Collision Dynamics on Basement and Crust-lithospheric Structures of the Northeastern Indian Ocean

摘要

The coexistence of multifaceted geodynamical situation in the Northeastern Indian Ocean (Bay of Bengal) lithosphere inspires to investigate the origin, present day orientation and gravitational stability of its basement, crust and lithospheric structures. The origin, evolution and the present day configuration of these structures such as the northward down dipping of the Bay of Bengal basement and its lithosphere, the Moho of the 85oE ridge, the formation of the Ninetyeast ridge median graben and ridge normal faults and their interaction with slow convergence and stress diffusion processes in the north Andaman trench, remain conspicuous. In this paper, we explain the above mentioned processes and the existence of the Bay of Bengal basement features through plausible tectonic scenarios that related with past time dynamics of the Indian Plate prior to the India-Eurasia collision and lithospheric thermal perturbation due to the Ninetyeast Ridge magmatism. Our analysis on the basis of satellite altimeter-derived geoid, gravity, sediment thickness, basement undulation, isostatic and prospecting geoid anomaly; suggests apart from the enhanced Oligocene Himalayan erosion and subsequent increased sediment load, the gravitational stability of the basin load (like formation of ridge graben) was also affected by altering the basin rheology to inelastic, owing to the enhanced thermal perturbation by the ninetyeast ridge magma. This made the Bay of Bengal lithosphere rheologically weak. Pre India-Eurasia collision, fast movement and fast convergence of the Indian plate at erstwhile north Andaman subduction zone made the lithosphere relatively stretchable due to the prevalence of intra-plate extensional stresses. The NE-SW orientation of the Central basin depression and the northward (NE-SW) dipping of the Bay of Bengal (BOB) lithosphere are pointing to such a stretchable lithosphere and its remnants are present at the northeast of the northern Ninetyeast ridge, as observed from the low prospecting geoid anomaly. The presence of such a thermally altered, weak lithosphere as a subducting slab at the north Andaman trench caused the present day observed low convergence rate, lack of seismicity, as well as the cessation of fault rupture followed by the Great Sumatra Andaman earthquake of Mw (9.1) on 26 December 2004. The Ninetyeast ridge oblique collision with the north Andaman trench was presumed around Pleistocene based on the timing of abandonment of the Nicobar fan from the Bengal head fan. The discontinuity of the ridge along the oblique convergence zone of the north Andaman and its isostatic stability are mainly controlled by the subduction zone dynamics, unlike the 85oE ridge.