Magnetovariational Sounding in South Vietnam according to the Dalat Observatory Data

Abstract

Abstract —This paper describes a three-dimensional inversion of magnetovariational tippers calculated for the Dalat observatory in South Vietnam on the basis of the geomagnetic variation records obtained from the INTERMAGNET network. The frequency dependences of the tippers are inverted using the ModEM software, which makes it possible to construct a geoelectric model of the Dalat zone in a 300 × 300 × 150-km spatial region (x, y, and z axes, respectively) with its center located at the observatory. The resulting model of the geoelectric section contains both surface and deep conductive blocks located at the top and bottom of the Earth’s crust and in the upper mantle. The central part of the upper local conducting block with a resistivity of 0.5–1.0 Ohm⋅m is located near the observatory at a depth of 12–14 km. A more massive conductive block is located to the east of the observatory in the coastal region. Its central part with a resistivity of 0.3 Ohm⋅m is located at a depth of 24–28 km and extends under the continent along the coast for about 80 km. The roots of this block are visible to depths of ~100 km in the South China Sea shelf region. It is assumed that the high electrical conductivity of these blocks is provided by melting in the presence of aqueous fluids and granite series rocks, which are widely distributed in the Dalat zone. The crystallization depths of the samples of these granites estimated from the geothermobarometry data are close to the depth intervals of conductive blocks in the model section, which was also observed in the interpretation of magnetotelluric soundings in Southern Tibet, where granitoid rocks are widespread.