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The left-lateral Altyn Tagh Fault forms the northern boundary of the Tibetan Plateau. The strike-slip rate of the active Altyn Tagh Fault decreases northeastward and reduces close to zero as it passes north of the Qilian Shan. This geometry raises controversies on whether and how the fault terminates or extends further east. To address these controversies, wide-band magnetotelluric data were collected along four profiles across the Altyn Tagh Fault ranging from 135 to 261 km in length. All four profiles are located in the foreland of the Qilian Shan Ranges and are oriented perpendicular to the inferred fault zone that could be the continuation of Altyn Tagh Fault. Both the two-dimensional and three-dimensional electrical resistivity models derived from our magnetotelluric data show that the Hexi Corridor crust is generally of low resistivity, whereas the crust of the Huahai-Jinta basin is, in general, of high resistivity with a local and isolated low-resistivity anomaly within the mid-lower crust. The generally high-resistivity crust of the Huahai-Jinta basin may be rheologically unfavorable for the Altyn Tagh Fault passing through the basin toward the northeast. The entirely different electrical structure between the Hexi Corridor and its northern neighbors indicates the existence of a tectonic boundary that coincides with the Altyn Tagh Fault in the west and reverse faults in the east. The two-dimensional electrical conductivity models suggest that the Altyn Tagh Fault transfers from a single fault in the west to a branching set of mainly dip-slip faults in the east.

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[1] 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.