Crustal and upper mantle structure of northern Tibet imaged with magnetotelluric data

Abstract

Magnetotelluric (MT) data were collected in northern Tibet along the Amdo to Golmud highway during the 1995 and 1999 Project INDEPTH (International Deep Profiling of Tibet and the Himalaya) surveys. Broadband and long period MT data were collected and the TE‐mode, TM‐mode and vertical magnetic field data were inverted to yield a minimum structure, two‐dimensional resistivity model. The model obtained from inverting all responses simultaneously shows that a pervasive midcrustal conductor extends from the Kunlun Shan to the Bangong‐Nuijiang suture. The vertically integrated conductivity (conductance) of this crustal layer is greatest in the northern Qiangtang terrane at latitude 34°N. The electrical resistivity of the upper mantle is constrained by the MT data to be in the range of 10–30 Ωm across the Songpan‐Ganze and Qiangtang terranes. This is lower than would be expected if Asian lithosphere underthrusts northern Tibet as far as the Qiangtang terrane. The MT responses are more consistent with a model in which Asian lithosphere extends as far south as the Kunlun Shan, and the upper mantle beneath the Songpan‐Ganze and Qiangtang terranes is sufficiently hot to contain a small fraction of interconnected partial melt.