ELECTRICAL-PROPERTIES AND DEEP-STRUCTURE OF THE RED RIVER FAULT ZONE IN NORTH VIETNAM FROM MAGNETOTELLURIC SOUNDING RESULTS

A Magnetotelluric survey across the Red River fault zone along a profi le of about 70 km from Yen Bai to the North-West of Hanoi, confirms th e existence of three conductive and anisotropic zones respectively ben eath three strike-slip fault zones of lithospheric scale. It also show s that the Song Hong fault s.s. constitutes a clear frontier between t wo blocks with quite different electrical properties. The region locat ed South-West of this fault in the Indochina block has an anomalously high conductivity at depth, especially in the lower crust where partia l melting should have occurred following the extrusion of the Indochin a block to the South-East.