The Tectonic origin of the Bay of Bengal and Bangladesh

We are able to decipher the tectonic evolution of the Bay of Bengal, a puzzle which has not been satisfactorily solved in the past, and we are also able to shed new light on origin of the buried 85°E Ridge. We do so by incorporating a number of disparate items into a unified solution. These items are the marine magnetic anomalies in the Western Basin of the Bay of Bengal, the Rajmahal and Sylhet traps, and Deep Seismic Sounding lines in India, a prominent magnetic anomaly doublet and seismic Seaward Dipping Reflectors in Bangladesh, and a new precise gravity map of the Bay of Bengal. We identify seafloor‐spreading magnetic anomalies ranging in age from 132 Ma (M12n) to 120 Ma (M0) in the Western Basin. These anomalies are “one sided”; the conjugate anomalies lie in the Western Enderby Basin, off East Antarctica. The direction of spreading was approximately NW‐SE, and the half‐spreading rates varied from 2.5 to 4.0 cm/yr. With the arrival of the Kerguelen plume around M0 time, seafloor spreading was reorganized and a new spreading axis opened at or close to the line joining the Rajmahal and Sylhet traps. The prominent magnetic anomaly doublet connecting the Rajmahal and Sylhet traps indicates that these traps are not individual eruptions at about 118 Ma, but rather, together, define the new line of opening. Spreading started at this line, and subsequently, India changed direction from west to north. The new oceanic crust, thus generated, underlies Bangladesh and the Eastern Basin of the Bay of Bengal and is younger than 118 Ma. The western boundary of the new ocean floor is a transform fault, which was generated by the spreading axis jump. This transform fault appears as the 85°E Ridge, and further north, on land, as a negative free‐air gravity anomaly strip. A unique feature of the northern boundary of the new oceanic crust is that due to the later deposition of enormous sediments derived from the Himalayan orogeny, it lies onshore Bangladesh, in contrast to most continent‐ocean boundaries in the world, which lie offshore.

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