Deep geoelectric structure of the Sikkim Himalayas (NE India) using magnetotelluric studies

Abstract

Broadband (0.001–1000s) magnetotelluric soundings were carried out at 18 locations with a station interval of 5–8km across the Sikkim Himalaya (northern India) along a 120km long traverse from Siliguri in the south to Yumthang in the north. Magnetotelluric transfer functions were computed after robust processing of single site and remote reference sites. The two-dimensional (2D) model derived from the joint inversion of TE and TM mode data shows distinct electrical signatures of the Main Frontal Thrust (MFT), the Main Boundary Thrust (MBT) and the Main Central Thrust (MCT). The MFT and MBT zones are expressed by a conductive feature of about 10–40Ωm indicating, the presence of Siwalik molasse sediments of Gangetic foreland basin. An anomalously high conductive (2–5Ωm) in the crust in the depth range of 3–15km is observed to the north of MBT. This may indicate the presence of Siwalik molasse sediments together with lesser Himalayan sediments with trapped fluids in the fault zone. These sediments may act as a lubricant accommodating underthrusting of continental crust. The nature of the low resistivity associated with the Main Himalayan Thrust in the higher Himalayas (north of the MCT) might indicate presence of metamorphic fluids released due to under thrusting of the Indian plate.