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 H imalaya of Central Nepal in 1996 in order to determine the electrical struc ture of the crust and its relation to geological structures and active tect onics. 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 mo de after RRI and Groom/Bailey decomposition, shows high conductivity in the foreland basin (similar to 30 Omega.m) that contrasts with the resistive I ndian basement (> 300 Omega.m) and Lesser Himalaya (> 1000 Omega.m). In add ition, our MT sounding reveals a major conductive feature beneath the front of the Higher Himalaya, also characterized by intense microseismic activit y, and; the position of a mid-crustal ramp along the major active: thrust f ault (MHT). This high conductivity zone probably reflects metamorphic fluid s, released during underthrusting of the Indian basement and pervading well connected microcracks induced by interseismic stress build-up, or distribu ted brittle deformation around the ramp.