Tectonic and crustal understanding of the Lesser Himalayas along the Purnea-Sevoke transect through integrated geophysical studies

Geophysical investigation employing deep electrical resistivity, gravity, and magnetic techniques was carried out along Purnea-Sevoke (NH-31) transect. The basement depth varies from 2917 to 4450 m, indicating huge relief. Three basement faults have been mapped over the transect. The Siwalik floor is quite undulatory in this part of the frontal foredeep region of the Himalayas. The gravity profile along NH-31 from Purnea to Sevoke brought out two basinal structures. Spectral analysis of the said gravity profile brought out the Conrad discontinuity and the basement depth at 18.4 and 3.2, km respectively. 2D gravity modelling along the above transect indicates gradual deepening of the basement towards the NE. This is due to sagging of the crustal block in the foothills of the Himalayas. The magnetic anomaly is of fairly high order (800-1600 nT) in the area, which may be due to the presence of traps below the sediment cover.