Electrical structure of the Himalaya of Central Nepal: high conductivity around the mid-crustal ramp along the MHT

Twelve broadband magnetotelluric (MT) soundings were performed across the Himalaya of Central Nepal in 1996 in order to determine the electrical structure of the crust and its relation to geological structures and active tectonics. The MT impedance tensors were obtained for frequencies between 0.001 and 500 Wt. The 2-D section, derived from joint inversion of TE- and TM mode after RRI and Groom/Bailey decomposition, shows high conductivity in the foreland basin (similar to 30 Omega.m) that contrasts with the resistive Indian basement (> 300 Omega.m) and Lesser Himalaya (> 1000 Omega.m). In addition, our MT sounding reveals a major conductive feature beneath the front of the Higher Himalaya, also characterized by intense microseismic activity, and; the position of a mid-crustal ramp along the major active: thrust fault (MHT). This high conductivity zone probably reflects metamorphic fluids, released during underthrusting of the Indian basement and pervading well connected microcracks induced by interseismic stress build-up, or distributed brittle deformation around the ramp.