The Negative Gravity Field Over the 85øE Ridge

An isopach map made from seismic reflection and refraction data in the Bay of Bengal shows two prominent N-S trending features in the basement topography. One is the northernmost portion of the Ninetyeast Ridge which is totally buried by sediments north of 10øN. The other buried ridge trends roughly N-S for 1400 km at 85øE to the latitude of Sri Lanka and then curves toward the west. It has basement relief up to 6 km. Two free-air gravity anomaly profiles across the region show a strong gravity low (----60 mGal) over the 85øE Ridge, while the Ninetyeast Ridge shows a gravity high. To understand the negative free-air gravity anomaly over the 85øE Ridge, we model the lithosphere as a thin elastic plate and calculate its flexural and gravitational response to an uneven sediment load. A plausible formation history for a buried ridge consists of at least two major episodes. The first is the formation of the ridge on a lithosphere with a flexural rigidity of D I. At some later time the ridge is buried by an influx of sediments, the lithosphere is cooler, and the flexural rigidity has increased to D 2. The character of the gravity field depends primarily upon the initial and final values of flexural rigidity. These D1 and D2 values are varied to obtain good agreement between the model and observed gravity anomalies. Best fitting models have a 180 times increase in flexural rigidity between ridge formation and sediment burial. An approximate relationship between flexural rigidity and crustal age shows that the 85øE Ridge was formed on relatively young lithosphere, 5-15 m.y. old and that it was buried when the lithosphere was 40-80 m.y. old.