Observed dispersion of fundamental mode Rayleigh and Love waves across the Bay of Bengal to WWSSN and Indian seismograph stations suggests increasing crustal thickness northward, from an approximate 15-km-thick oceanic crust south of the latitude of the southern tip of India, to a more continent-like thickness of approximately 25 km at 20°N, and over 35 km at the northernmost part of the Bengal Fan. The dispersion data are not consistent with models which simply add the increasing sedimentary thickness on top of a normal oceanic crust. The sediment thicknesses and crust velocities are controlled by extensive seismic refraction results and are in agreement with clearly observed sedimentary higher mode waves recorded at the Indian seismograph stations, Madras and Vishakapatnam. Several hypotheses might explain this surprising result.

  1. The blanketing effect of the sediments, with consequent temperature rise, has brought about differentiation of basalt to increase the crustal thickness (R. N. Singh, personal communication).

  2. The change in Moho depth represents dynamic isostatic adjustment such as a phase change boundary which is lowered because of temperature (or pressure?) perturbations from the sedimentary blanket, or from pressure perturbations of plate collision.

  3. The collision has underthrust a wedge of low-velocity material beneath the oceanic crust.

  4. The northern Bay of Bengal is actually fortuitiously continental, or continent-like, possibly as a result of the influence of the 90° ridge (especially in the northern part).

If the explanation is (1), (2), or (3), the result is important to our understanding of crust and upper mantle dynamics. It is suggested that further controlled studies be performed to verify the result.

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