Structure and origin of the 85°E Ridge

Abstract

The submerged 85°E Ridge in the Bay of Bengal trends approximately N-S between 19°N and 6°N latitudes. Off the southeast coast of Sri Lanka it takes an arcuate shape and seems to terminate with the northward extension of the Afanasy Nikitin seamounts situated around 2°S latitude. The ridge is characterized by positive magnetic (100-400 nT) and negative free-air gravity (< -60 mGal) anomalies with variable widths of 100-180 km. Magnetic model studies revealed that the rocks of the 85°E Ridge are magnetized with reversed polarity. The well-defined geophysical anomalies and lack of magnetic polarity reversals together with the deep burial nature of the ridge may not favor a hotspot origin. Two alternative processes for the ridge emplacement have been suggested. Ridge emplacement may be (1) due to shearing of the lithosphere caused by stretching and compressional forces associated at the time of major plate reorganization immediately after the evolution of the early Cretaceous crust in the Bay of Bengal, more precisely at MO isochron or during the middle Albian reversals within the Cretaceous long normal polarity (K-T superchron) epoch when the Earth's magnetic polarity changed from normal to reversed polarity, and/or (2) due to sagging followed by deformation produced by the buckling instability of the oceanic plate caused by horizontal compressional forces on the passive continental margin. However, more marine geophysical data are required to support the postulated coincidence of the ridge with a reversed polarity magnetic anomaly and their associated model. Further, the Rajmahal traps (normally polarized) and the 85°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°E Ridge seems to extend into the onshore West Bengal Basin as a subsurface ridge and merges with the reported NNE-SSW trending zone of strong geophysical anomalies east of Rajmahal traps up to 25°N latitude. Copyright 1997 by the American Geophysical Union.