An analysis of the gravity field in Northeastern India

Abstract

Northeastern India comprises several major tectonic units including the Shillong Plateau, the Upper and Lower Brahmaputra (Assam) Valleys, the Northeastern Himalaya, the Naga Hills and the Bengal Basin. The area lies approximately between latitude 23–28°N, and longitude 88–96°E. A revised Bouguer anomaly map of the area with nearly 400 new observations is presented. A Pratt-Hayford isostatic anomaly map using all available information is also given. The whole area shows a large variation in gravity anomalies, Bouguer anomalies show a variation from +40 mGal over the Shillong Plateau to −250 mGal over the northwestern part of the Upper Assam Valley. Isostatic anomalies also show a variation, from +100 mGal over the Shillong Plateau to −125 mGal in the Assam Valley. A study of the Bouguer anomaly map shows that the gravity field is considerably influenced by low-density sediments overlying the Assam Valley as well as the Bengal Basin. A geological correction for these sediments was computed for a few selected profiles for which geological information was available from seismic and bore-hole data along the Upper Assam Valley and the Bengal Basin. The magnitude of the geological correction was found to be of the order of 50 to 100 mGal for the Assam Valley and 40 to 130 mGal for the Bengal Basin. Models for the crust and mantle underlying the Shillong Plateau, the Upper Assam Valley and the Bengal Basin were constructed considering the nature of geologically corrected Bouguer anomalies as well as isostatic anomalies. Gravity data suggest that the crust underlying the Shillong Plateau is probably denser as well as thicker than normal for its elevation. The Assam Valley may overlie a crust which is thicker than normal for its topography, and the crystalline solid crust underlying a large thickness of sediments of Bengal Basin could be denser as well as thinner than the normal continental crust.