Electrical resistivity structure at the northern margin of the Tibetan Plateau and tectonic implications

The ENE–WSW‐striking Altyn Tagh Fault (ATF) and the WNW–ESE‐trending western Qilian Mountains define the northern margin of the Tibetan Plateau. New magnetotelluric data were collected along three profiles crossing the eastern section of the ATF and the southern Qilian Mountains. The basic sounding bandwidth ranged from 0.003 to 7000 s. The transverse electric and transverse magnetic mode data sets were inverted into resistivity sections using a conventional two‐dimensional (2D) inversion code, and data at periods of 0.3333–5464 s were inverted using a commonly employed three‐dimensional (3D) code. The 3D results constrain the interpretation of the 2D models, and the final interpretative resistivity models show that lithospheric structures are spatially variable along the ATF and across southern Qilian. In profiles across the ATF, the main fault is imaged as a vertical resistivity boundary, and the high‐resistivity body in the western profile extends about 15 km deeper than the corresponding body in the eastern profile. Positive flower structures are apparent in the western profile but are only weakly visible in the eastern profile. These observations suggest that the depth of the ATF is spatially variable. Mantle resistivity images indicate a relatively cold and rigid Tarim lithosphere, which is consistent with a geodynamic model of oblique subduction of the Tarim Basin below the ATF. A high‐resistivity body in the crust of southernmost Qilian indicates a huge thick‐skinned structure. Low‐resistivity bodies at the crust–mantle boundary south of the main ATF are presumed to form a weak layer beneath northern Tibet.