CONDUCTIVITY STRUCTURE OF CRUST AND UPPER MANTLE BENEATH THE NORTHERN TIBETAN PLATEAU: RESULTS OF SUPER-WIDE BAND MAGNETOTELLURIC SOUNDING

Abstract To study the conductivity structure of crust and upper mantle as well as the thermal regime of lithosphere beneath the northern Tibetan Plateau, the project INDEPTH (III)-MT completed super-broadband magnetotelluric (MT) sounding along Deqing-Longweicuo (line 500) and Naqu-Germu (line 600) in northern and central Tibet in 1998 and 1999. The result shows that the conductivity structure of crust and upper mantle is quite different across the Kunlun Mountains. North to the mountains the crust and upper mantle is relatively highly resistive. South to the mountains, the conductivity structure of crust and upper mantle is of obvious stratification as described following. The upper crust is dominated by discontinuous high-resistivity bodies with intercalated low-resistivity anomalies. And the electric structure of the upper crust in NS direction looks like complicated with discontinuous and block-shaped features. Meanwhile the intermediate and lower crust are characterized by large-scale high conductivity anomalies, indicated by separated high-conductivity bodies of big sizes and differing shapes with resistivity less than 4Ωm. Beneath the Bangong-Nujiang and Jinshajiang River sutures, the high-conductivity bodies in the crust tend to extend toward the upper mantle, implying existence of a low-resistivity conduit between crust and mantle. Based on the observed electric structure of crust and mantle beneath the northern and central Tibetan Plateau, we infer that there are extensive partial melt and thermal fluids there like southern Tibet. Their origin is associated with thermal exchange between crust and mantle beneath th