Аннотация:Geoelectric strike and resistivity structure of the crust have been estimated from 37
magnetotelluric (MT) data sites along a profile from Roorkee to Gangotri in Uttarakhand
Himalaya. Impedance decomposition schemes based on Bahr’s, Groom Bailey and Phase
tensor were implemented in a MATLAB code for the average strike estimation. Geoelectric
strike direction varies with period as well as in different litho-tectonic units along the profile.
In the period band from 1 to 100 s average geoelectric strike in southern end of the profile
(Indo-Gangetic Plains) is N79W, which is slightly rotated to the north in the Lesser
Himalayan region and becomes N680W whereas it is N810W in the Higher Himalayan region. However, average strike is stabilized to N77W for the entire profile in the long
period band (100-1000 s). Geoelectrical structure of the crust has been obtained along the
profile by 2D inversion of MT data. Major features of 2D resistivity model are: (i) Southern
part of the model is a low resistivity (< 50 Ωm) zone at shallow depth (5-7 km) representing
the loose sediments of the Indo-Gangetic Plains (IGP) whose thickness increases in the south.
(ii) Highly resistive (> 1000 Ωm) layer below the IGP sediments is the basement rock,
representing the resistivity of top of the subducting Indian Plate. (iii) The Main Boundary
Thrust (MBT) and the Main Central Thrust (MCT) zones can be seen in the electrical image.
However the Himalayan Frontal Thrust (HFT) could not be resolved. (iv) A low resistivity (<
10 Ωm ) feature in the MCT zone extending to the depth of 30 km is delineated. This low
resistivity could be due to fluid-filled fractured rock matrix or partial melt zone. Hypocenters
of many earthquakes are concentrated along the boundary of this low resistivity zone and
relatively high resistivity blocks around it. The resulted model support flat-ramp-flat
geometry of the Main Himalayan Thrust along which the Indian Plate is subducting.
