Delineation of crustal structure of Mahanadi basin from ground magnetic survey

Abstract

Ground magnetic surveys were conducted over the Mahanadi basin covering an area of 21.000 km'. Both total field and vertical field data were collected at 5 km interval, mainly to study the deeper regional features. Total field anomaly map prepared after correcting for the IGRF and external field variations, shows a combination of NE-SW, E-W, and NW-SE trends. To identify magnetic sources related to different depths, various transformations have been applied to the total field anomaly map. The second vertical derivative and downward continued map showed that the NE-SW and E-W trends are related to ridges and depressions of the coastal basin; implying that these are shallow features. The deeper features evidenced from upward continuation showed NW-SE trends. Thus, the Mahanadi delta is composed of two structural units associated with different tectonic events. These NW-SE trends may possibly be associated with the extension of Mahanadi graben underneath the coastal basin. The study reveals that the shallow NE-SW to E-W trends are superposed on the deeper NW-SE trends formed prior to the break-up of Gondwanaland, and the former trends resulted due to the dismemberment of Gondwanaland. A comparison with DSS studies conducted over Mahanadi delta and Lambert graben of East Antarctica showed that the. deeper features can be correlated with an intracrustal layer related to a velocity of 6.4 km/s-6.6 km/s at an average depth of 6.5 km. seen in both the areas. Analysis of aeromagnetic data over Lambert graben of East Antarctica will help reconfirm our observation.