Conductivity structure and rheological property of lithosphere in Southern Tibet inferred from super-broadband magnetotelluric sounding

Abstract

To understand deep lithosphere structure beneath the Qinghai-Tibet Plateau more comprehensively and objectively and to explore important scientific issues, such as characteristics of plateau lithospheric deformation, state of strain, thermal structure, plate (or terrane) movement, and crust-mantle rheology, it is necessary to research the variation of crust-mantle electrical structure in the east-west direction in every geological unit. For this purpose, six super-broadband magnetotelluric (MT) sounding profiles have been completed by INDEPTH-MT Project in the Himalayas-Southern Tibet. Based on the imaging results from the six profiles, three-dimensional electrical conductivity structure of the crust and upper mantle has been analyzed for the research area. The result shows that the high-conductivity layers in the middle and lower crust exist widely in Southern Tibet, which extend discontinuously for more than 1000 km in the east-west direction and become thinner, shallower and more resistive toward the big turning of the Yarlung Zangbo River. The discussion on the rheology of lithosphere in Southern Tibet suggests that the mid-lower crust there is of high electrical conductivity, implying the existence of “partial-melt” and “hot fluid” in the thick crust of Tibet, which make the medium hot, soft, and plastic, or even able to flow. Combining the experimental result of petrophysics and the MT data, we estimate the melting percentage of the crustal material to be up to 5%–14%, which would reduce the viscosity of aplite in the crust to meet the flow condition; but for granite, it is likely not enough to cause such a change in rheology.