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Gravity field and deep structure of the Bengal Fan and its surrounding continental margins, northeast Indian Ocean

Publication Date
DOI: 10.1016/0040-1951(91)90370-8
  • Geography


Abstract A revised gravity anomaly map for the northeast Indian Ocean shows that the shelf edge underlying the eastern continental margin of India is a rather narrow but extensively linear gravity low (minimum free-air = −149 mGal). The Bengal Fan seaward of the shelf has a depressed gravity field (average free-air = −20 to −30 mGal) in spite of the enormous thickness of sediments of as much as 10–15 km. The two buried ridges below the Bengal Fan—the 85° East and 90° East Ridges—have a large negative (−75 mgal) and a substantial positive (40 mGal) free-air anomaly, respectively. The Andaman and Burmese arcs lying along the east margin of the Bengal Fan are active subduction areas which have typical bipolar gravity signatures with a maximum amplitude of 300 mGal. Gravity interpretation for three regional traverses across the central and northern parts of the Bengal Fan and their surrounding continental margins suggests that a thickened oceanic crustal wedge juxtaposes the transitional crust under the eastern continental slope of India; the 85° East Ridge, that was created when the Indian Ocean lithosphere was very juvenile, appears to underlie a nearly 10 km thick and 120 km wide oceanic crustal block consisting of the ridge material embedded in the upper lithosphere; while the 90° East Ridge submarine topography/buried load below the Bengal Fan is probably isostatically compensated by a low-density mass acting as a cushion at the base of the crust. The Bengal Fan crust, with its thick sediment layer, is carried down the Andaman subduction zone to a depth of about 27 km where, possibly, phase transition takes place under higher pressure. The maximum sediment thickness at the Andaman-Burmese subduction zone is of the order of 10–12 km. The gravity model predicts a low density zone about 60 km wide below the Andaman-Burmese volcanic arc, penetrating from crustal to subcrustal depths in the overriding Burma plate. A more complex density distribution is however, envisaged for the Andaman volcanic arc that is split by the Neogene back arc spreading ridge. The ocean-continent crustal transition possibly occurs farther east of the volcanic arc; below the Shan plateau margin in Burma or below the Mergui terrace at the Malayan continental margin east of the Andaman Sea.

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