Analysis of Aeromagnetic data of the NE region of the Indian sub-continent.

The NE region of the Indian subcontinent is a very interesting place for geodynamic and tectonic studies due to the collision of the Indian plate with the Himalayas in the North and with Myanmar in the East. Some of these areas are difficult to access due to the high topography and air borne surveys help map the region efficiently. Further, the NE region including areas of Assam and Bangladesh are drained by the Ganges and Brahmaputra rivers that bring thick sediments with them giving rise to the Bengal Fan that has sediments of up to 20 km in some regions and these mask the underlying crust and pose severe restrictions in constructing the geodynamical history of the region. Geopotential data offer the unique opportunity of addressing some of these issues. We utilize available / published aeromagnetic maps over Bangladesh, Meghalaya, Chachar, parts of North Assam, Brahmaputra valley, Manipur and Nagaland in the North Eastern region of India for understanding the complexities of the tectonics of the Indian sub-continent. All available data are reduced to a common altitude to prepare a composite magnetic anomaly map of the region. We also look at the Grace and Bouguer / isostatic gravity maps of the region; the highest isostatic gravity anomaly of the Indian region lies over Meghalaya with a trough in Sylhet (Bangladesh) to its south. The Eocene hinge zone is a 25-km wide northeast-southwest zone that separates the Precambrian platform in the northwest from the geosynclinal basin to the southeast of Bangladesh. We find that the Hinge Zone in Bangladesh and part of the Dauki fault form a strong divide such that the gravity anomalies and aeromagnetic anomalies show high frequency anomalies to the north of this divide and also most of the magnetic sources as seen from the analytic signal are concentrated to the north and the Euler solutions give shallow solutions to the north and deep solutions to the south of this divide. The tilt derivative of the aeromagnetic data clearly defines the edges of the different geological formations. Within the Meghalaya plateau most of the high frequency anomalies are found associated with near surface ultramafic intrusions, mineralized zones etc. and appear to be controlled by the Dudhuni and Kulsi faults. Most of the shallow magnetic sources in the upper Assam and Brahmaputra valley show NW-SE trend and are restricted by the NW-SE Dhansili-Kopili fault. The Eastern Boundary thrust and the Dhubri fault appears to constrain the Grace anomaly highs in the east and west respectively. Through an analysis of the aeromagnetic data we identify extensions of the known fault below thick piles of sediments and presence of same hitherto unknown faults / lineaments. Results of this analysis and its implication for the geodynamic history of the region will be presented.