Interpretation of Gravity Anomalies over the Eighty-Five East Ridge (2017)
Dr. M. M. P. M. Fernando and Prof. D. A. Tantrigoda
Several profiles of the negative gravity anomaly over the submerged 85o E ridge in the Bay of Bengal of the Indian Ocean have been interpreted in two dimension and results are combined together to give a detailed picture of its morphology and thickness of surrounding sediments. Observed negative gravity anomaly has been explained as the combined effect due to the positive mass anomaly caused by the replacement of sediments by high dense igneous rocks of the ridge and the negative mass anomaly caused by replacing the high dense upper mantle material by the oceanic crust which has been bended and sunk into the upper mantle due to the pressure exerted by the ridge. Downward migration of the oceanic crust has been also calculated assuming the oceanic crust is behaving as a thin infinite elastic plate resting on inviscid fluid half space and found closely agree with the results of the gravity study. Both studies indicate that the thickness of the ridge vary from 11 km to 16 km while the oceanic crust has undergone a depression of 9 km to 14 km.
Interpretation of gravity anomalies provides a relatively inexpensive way of revealing the shallow crustal structure of the Earth. A gravity anomaly is a manifestation of mass anomalies inside the Earth. Low dense sediments in sedimentary basins surrounded by high dense country rocks give rise to negative anomalies while high dense igneous intrusion emplaced in an environment of relatively low dense country rocks give rise to positive anomalies. Most of the gravity anomalies that we have observed strictly adhere to this general principle. An apparent exception to this general principle can be observed along the 85oE longitude in the Bengal fan in the Indian Ocean. Existence of a buried ridge known as 85o East Ridge of high dense volcanic material within the sediments in this region has been established by Curray and Moore, 1971, 1974; Moore at. Al 1974. A positive free air anomaly is expected to observe over the buried ridge reflecting high density of the ridge relative to surrounding sediments. However, the satellite gravity anomaly depicted in Fig. 1 shows negative gravity anomaly over the region of the Ridge. The only possible way to have a negative anomaly over the region is depression of the oceanic crust below the ridge due to the weight of the ridge replacing high dense upper mantle material by relative low dense oceanic crust. Negative anomaly caused by this density difference may out weight the positive anomaly resulting from the positive density contrast between the ridge and the sediments giving rise to the resulting negative anomaly. In this study, this hypothesis has been tested by interpreting gravity anomalies over the ridge and also by studying the flexural deflection of the oceanic crust due to the pressure exerted by the ridge.
An isopach map over the 85oE Ridge has been compiled using results of two-dimensional interpretation of seven profiles of satellite gravity anomalies (Fig. 6). As can be seen from this map, sediment thickness over the ridge at the extreme north is very low and about 200 m. It goes up to about 3 km within a short distance of 50 km along the profile.
Isopach map compiled in this study closely agree with the isopach map compiled by Levchenko et al. (1993) based on seismic studies. This ridge may have influenced the sediment distribution around Sri Lanka. As shown in the chapter 4, there is rich crescent shaped sediment distribution, which has a thickness of the order of 2.0 – 3.0 km. The 85oE Ridge may have blocked the free floor of sediments forcing them to get accumulated around Sri Lanka.
Normally most of the oceanic ridges produce positive gravity anomalies. A good example for this is the 90oE ridge situated close to the 85oE ridge. This is because outcropping part of a ridge buried in the low dense sediments gives rise to a positive anomaly due to the positive density contrast between sediments and material of the ridge which has a density more or less equal to that of the oceanic crust. It has been revealed in the gravity interpretation that oceanic crust has sharply downward migrated replacing high dense upper mantle material by low dense material of the ridge. As can be seen from Fig. 5.11 and Fig. 5.13 oceanic crust bends as much as 9-14 km due to the pressure exerted by the ridge producing an intense negative gravity anomaly. This negative anomaly outweighs the positive anomaly due to the part of the ridge buried in the sediments making the resultant anomaly negative.
Downward migration of the oceanic crust below the ridge was also calculated using elastic properties of the oceanic crust approximating the oceanic crust to a very thin plate lying on a viscoelastic fluid. It was also assumed that the region under investigation is in isostatic equilibrium. This study produced results approximately same as that of the gravity interpretation indicating the region is in isostatic equilibrium.