MAGSAT scalar anomaly map of India and a part of Indian Ocean‐magnetic crust and tectonic correlation

Abstract

Magnetic scalar anomaly maps over India and a part of the Indian ocean are prepared from the MAGSAT data using an average of the observed field in a grid of 1°×1°. The MAGSAT scalar anomaly map thus obtained is characterised by ‘highs’ over the Indian shield and large ‘lows’ over the Himalayas centered over Tibet Plateau. A NE‐SW magnetic trend emanating from NE India and spreading almost over the entire peninsular shield is the single most prominent feature of the map which might be related to the stress system or deepseated structural trends associated with plate motion. The Deccan Traps are reflected by ’nosing’ of the magnetic contours. The Shillong massif is characterised by a separate anomaly which deflects the NE‐SW magnetic trend referred to above. A magnetic crustal map prepared from this MAGSAT scalar anomaly, using the principles of harmonic inversion, yields a thickness of 30 km under the cambay basin and the western part of the Narmada‐Son lineament for a magnetization of 2A/m suggesting a higher subsurface thermal gradient in this area and therefore a tectonically active region.A maximum thickness of 40‐42 km is found under the regions of Aravallis, Singhbhum and south of the Narmada‐Son Lineament, which are Precambrian stable cratons. Major lineaments and faults are reflected as variations in the thickness of the magnetic crust along these lines.The Himalayas are chaaracterised by an average thickness of 35‐36 km with a thickening of 2‐4 km under the Tibet plateau and Shillong massif. The MAGSAT anomaly in the northern part of the Indian ocean can be accounted for by a thickening of 8 km in the magnetic crust.The center of this crustal thickening derived from the MAGSAT anomaly after appropriate correction for the inclination of the earth's magnetic field in this region coincides with the wellknown Indian ocean gravity and geoid lows.