Electromagnetic constraints on the East Tien Shan crust: Analysis of the TM mode

The TM mode of the magnetotelluric field is considered on an N-S profile cr ossing the Junggar basin and East Tien Shan. An interpretation is based in terms of thin sheet (quasi-3D) and 2-D models. The electrical conductance o f the upper layer of the Tien Shan crust is three orders of magnitude small er than that of the sedimentary cover in the Junggar basin, which gives ris e to a regional TM-mode anomaly. The leakage of electric current through th e high-resistivity upper crust leads to dissipation of this anomaly and nor malizes the magnetotelluric sounding curves in the southern part of the pro file. A model approximation revealed a layer at a depth of 30 km with an el ectrical conductance of 250 S, which is two to three times lower than in th e Kyrgyz Tien Shan. The estimates made in this work show that dehydration o f chemically bound water in the deep crust of the Tien Shan can account for the observed electrical conductivity.