Seven profiles of intense negative gravity anomaly over the South Western, Southern and South Eastern parts of the continental rise of Sri Lanka have been interpreted two dimensionally. Results of the interpretation of profiles were interpolated and a map of sediment thickness has been compiled. As revealed by the interpretation and the map compiled there are three noticeable sediment distributions with thickness varying from approximately 2.5 km to 3.5 km. Sediment thickness map in this study closely agree with that compiled using seismic studies by previous workers. Results of this study may have applications in the oil and gas exploration activities over the region.
Keywords: gravity anomaly, sediment thickness, Bay of Bengal, Indian Ocean, Sri Lanka
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. The 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 that the oceanic crust is behaving as a thin infinite elastic plate resting on inviscid fluid half space and is found to 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.
Satellite free air gravity anomalies over the Indian ocean region 79°E–86°E, 2°S–8°S were obtained from the website http://topex.ucsd.edu and a contour map was compiled. Five profiles of the anomaly have been interpreted in terms of two-dimensional structures in the ocean. Thickness of sediments lying on the oceanic crust determined from the interpretation of gravity profiles were used to compile an isopach map of the region 79°E–86°E, 2°S–8°S. This map in combination with one of the isopach maps compiled by previous workers, provides information regarding the thickness of sediments up to 6° S. According to this map sediment thickness varies from ∼600 m over the middle part of the region to ∼800 m further south, indicating that thinning of sediments in the middle part of the region is only localized. Information provided by this gravity study may be useful in planning detailed seismological studies to delimit the outer edge of the continental margin of Sri Lanka, defined according to the United Nations Convention of the Law of the Sea (UNCLOS).
Foot of the continental slope of Sri Lanka has been determined using satellite bathymetry information downloaded from NOAA database (http://topex.ucsd.edu/) employing the second derivative method. Foot of the continental slope of Sri Lanka is situated at an average distance of about 47 km. Its maximum and minimum distances from the zero height contour are 127 km. and 16 km. Almost 70% of the foot of continental slope occurs in the depth range of 2600 -3800 m.