UPPER-MANTLE ELECTRICAL-CONDUCTIVITY IN THE HIMALAYAN REGION

The electrical conductivity profile of the Earth at depths of about 50 to 500 km was determined using the quiet ionospheric current variatio ns observed at a line of stations near 75 degrees East longitude. We f ound conductivity values of about 0.06 Sim from 50 to approximately 35 0 km depth with slight relative maxima near 125 and 275 km, interperse d by relative minima near 210 and 330 km. Thereafter, the conductivity increased sharply toward a value of about 0.18 Sim at 500 km with no indication of leveling off. A comparison with regional seismic wave-ve locity models shows good correspondence between high conductivity and low-velocity zones. The conduction by hydrogen-saturated pyroxene is e nvisaged as a possible mechanism for the high conductivity and its var iation in the upper mantle.