Sediment Thickness Map of the Indian Region 79oE - 86oE, 2oS - 8oS based on a Satellite Gravity Interpretation (2013)
Dr. M. M. P. M. FERNANDO and Prof. D A. TANTRIGODA
Abstract
Satellite free air gravity anomalies over the Indian Ocean region 79oE – 86oE, 2oS – 8oS 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 79oE-86oE, 2oS-8oS. This map in combination with one of the isopach maps compiled by previous workers, provides information regarding the thickness of sediments up to 6o 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 a 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).
Introduction:
United Nations Convention on the Law of the Sea (UNCLOS), which was ratified in 1982, provides the criteria for demarcating maritime boundaries of coastal states (United Nations, 1983, 1993 ). These criteria have been incorporated in Article 76 of the convention and in the Annex II of the final act. Sri Lanka is eligible to claim its maritime zones using the provisions of Annex II in view of its very special nature of the continental shelf and distribution of sediments over its continental margin. The UNCLOS considers the continental margin, which is the interface between the continents and oceans as a submerged part of the land area and therefore belongs to the relevant coastal state. According to Annex II, a maritime state can delimit its outer edge of the continental margin up to a limit at which thickness of oceanic sediments is ~1 km. Maps compiled by Levchenko et al.(1993 ) and Curray et al.(2003) provide data on sediment thickness in the central Indian Ocean upto 6o S. As evident from these maps, Sri Lanka may have the possibility of extending its outer limits of the continental margins slightly below 6oS over the region 79oE – 86oE, 2oS – 8oS according to the Annex II of the Convention. An interpretation of gravity anomalies was carried out to determine the sediment thickness over this region and a preliminary map of sediment thickness has been compiled. The United Nations Commission on the Limits of the Continental Shelf has recommended reflection seismology as the most reliable method for this delimitation process (United Nations, 1999). It is obvious that much careful planning to necessary before embarking on an expensive marine seismic study. The preliminary map of sediment thickness compiled in the present study, based on an interpretation of satellite gravity anomalies would be useful in planning the detailed seismological work needed for this purpose.
Discussion :
A two dimensional interpretation of five profiles of the free air gravity anomalies close to Afanasy-Nikiten seamount chain have been carried out to estimate the thickness of sediments beyond 6oS of the equator. In the interpretation, the gravity effect of the seamounts were calculated considering them as three-dimensional bodies (Cordell and Henderson, 1968) as they are situated close to the surface of the water where the observed and calculated gravity anomalies are compared. As can be seen from the profiles of observed and calculated anomalies, it has been attempted to model only the large-scale features of the causative body. A much better agreement between observed and calculated anomalies could have been obtained if the effect due to small scale variations, especially at the upper surface of the sediments, had been incorporated. A contour map of the sediment thickness has been compiled incorporating the results of the gravity interpretation with the isopach map of Levchenko et al. (1993) (Fig. 3). As can be seen from this map there is smooth continuation of contours from one map to the other, illustrating consistency of gravity results of the present study with those of the seismic study of Levchenko et al.(1993).
The combined isopach map shows the sediments becoming thinner in the region south of the first member of Afanasy-Nikiten seamount chain and west of the second member of the same chain. Thickness goes down as much as 600 m over this region and goes upto 800 m when going further southwards. Thinning of the sediment over this area can be qualitatively explained in the following manner. Sediments flowing along various channels of the seabed are being blocked by the first member of the seamount chain. The second member of the chain further constrained the flowing of sediments to the region under consideration from the east. Sediments can flow down to this region only from western side and through the gap between the two seamounts. This could have been the reason as to why the thickness of sediments over the region is low. However thickness of sediments recovers back to 800 m around 8o S. Therefore thinning of sediments discussed above may be considered as a local feature and Sri Lanka may extend its outer edge of the continental margin upto 8o based on the satellite gravity interpretation. However, the Commission on the Limits of the Continental Shelf has recommended multi-channel seismics as the main source of information that can be used in delimitation of the outer edge of the continental margins of a coastal state. Therefore it is suggested that region south of the Afanasy-Nikiten sea mount should be further investigated using a multichannel seismic survey to delimit the outer edge of the continental margin of Sri Lanka.
Journal of the Geological Society of India, Vol.82, October 2013, pp.430-434.