STRUCTURE AND ORIGIN OF THE 85-DEGREES-E RIDGE

The submerged 85 degrees E Ridge in the Bay of Bengal trends approxima tely N-S between 19 degrees N and 6 degrees N latitudes. Off the south east coast of Sri Lanka it takes an arcuate shape and seems to termina te with the northward extension of the Afanasy Nikitin seamounts situa ted around 2 degrees S latitude. The ridge is characterized by positiv e magnetic (100-400 nT) and negative free-air gravity (< -60 mGal) ano malies with variable widths of 100-180 km. Magnetic model studies reve aled that the rocks of the 85 degrees E Ridge are magnetized with reve rsed polarity. The well-defined geophysical anomalies and lack of magn etic polarity reversals together with the deep burial nature of the ri dge may not favor a hotspot origin. Two alternative processes for the ridge emplacement have been suggested, Ridge emplacement may be (1) du e to shearing of the lithosphere caused by stretching and compressiona l forces associated at the time of major plate reorganization immediat ely after the evolution of the early Cretaceous crust in the Bay of Be ngal, more precisely at MO isochron or during the middle Albian revers als within the Cretaceous long normal polarity (K-T superchron) epoch when the Earth's magnetic polarity changed from normal to reversed pol arity, and/or (2) due to sagging followed by deformation produced by t he buckling instability of the oceanic plate caused by horizontal comp ressional forces on the passive continental margin. However, more mari ne geophysical data are required to support the postulated coincidence of the ridge with a reversed polarity magnetic anomaly and their asso ciated model. Further, the Rajmahal traps (normally polarized) and the 85 degrees E Ridge (reversely polarized) appear to be associated with two different episodes of eruption that might have been triggered by the Kerguelen mantle plume. The 85 degrees E Ridge seems to extend int o the onshore West Bengal Basin as a subsurface ridge and merges with the reported NNE-SSW trending zone of strong geophysical anomalies eas t of Rajmahal traps up to 25 degrees N latitude.