Tectonic significance of anomaly-elevation relationships in northeastern India

Abstract

Northeastern India comprises several geologic units having widely different tectonic histories. These include: 1. (1) the Shillong Plateau. 2. (2) the Naga-Lushai Hills. 3. (3) Eastern Himalaya. 4. (4) the Bengal Basin. 5. (5) Brahmaputra Valley. The nature of the gravity field over these areas has been studied in detail by Mukhopadhyay (1974). The correlation of free-air and Bouguer gravity anomalies with elevation has been studied using spot values as well as averages over 1 2° × 1 2° and 1° × 1° areas. The latter clearly suggest that there are five different geotectonic units in northeastern India having characteristic mass distributions. The relationship of gravity versus elevation suggests that a thicker and denser crust may underlie the Shillong Plateau. The relationship for the Naga—Lushai Hills indicates the influence of normal root formation under these hills. The Folded Belt of Tripura, as well as the Himalayan foothills have a larger than normal mass deficiency for the same elevation. In the Bengal Basin, thickness of sediments alone controls the nature of the gravity anomalies. But over the Brahmaputra Valley, the anomalies are largely controlled by low-density sediments of the valley and to some extent by the root effect of the neighbouring mountains.