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

Abstract

[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.

Structure and geometry of the Aksay restraining double bend along the Altyn Tagh Fault, northern Tibet, imaged using magnetotelluric method

“…As both the NATF and the SATF are subvertical at Geophysical Research Letters 10.1002/2017GL072581 depth through the Aksay restraining bend, this raises a question of how uplift occurs between these strikeslip faults. This concept is consistent with previous MT imaging studies, showing the ATF forms the root of a flower structure between the Tarim Basin and the Tibetan Plateau [Wittlinger et al, 1998;Zhao et al, 2006;Zhang et al, 2015;Xiao et al, 2011]. This concept is consistent with previous MT imaging studies, showing the ATF forms the root of a flower structure between the Tarim Basin and the Tibetan Plateau [Wittlinger et al, 1998;Zhao et al, 2006;Zhang et al, 2015;Xiao et al, 2011].…”

“…10.1002/2017GL072581 inconsistent data points in the apparent resistivity and phase, the construction of initial models, and the selection criterion for regularization parameter are similar to those of the previous profiles in this area [Xiao et al, 2011[Xiao et al, , 2015. Details for inversions and data fit are included in Figures S3-S5 in the supporting information.…”

“…Based on the MT results, we construct a model reflecting the geometric variation of the Altyn Tagh Fault zone passing through the Aksay bend (as shown in Figure 4e). East of the Aksay bend, model L1 (see Figure 4a) shows that the NATF is very close to the subvertical resistivity discontinuity and taken as the major branch of the Altyn Tagh Fault, which we interpreted as a nearly symmetric flower structure controlling the geometry of high-resistivity bodies extending to depth [Xiao et al, 2011]. In profile L13, the electrical structure between the NATF and SATF is nearly symmetrical (Figure 4c), and there is a relatively low-resistivity body M1 below A1 at mantle level (Figure 3b), which suggests that the Altyn Tagh Fault shear zone at depth may be centered between the NATF and SATF.…”

“…This structure was also imaged by a refraction/wide-angle reflection profile [Zhao et al, 2006 . Our own previous MT profiles along the Altyn Tagh Fault from near Aksay toward its eastern termination [Xiao et al, 2011[Xiao et al, , 2012[Xiao et al, , 2013[Xiao et al, , 2015 showed that the depth of the vertical resistivity contrast related to the ATF decreases eastward and terminates against mainly dip-slip faults at the northern margin of the Hexi Corridor. Our own previous MT profiles along the Altyn Tagh Fault from near Aksay toward its eastern termination [Xiao et al, 2011[Xiao et al, , 2012[Xiao et al, , 2013[Xiao et al, , 2015 showed that the depth of the vertical resistivity contrast related to the ATF decreases eastward and terminates against mainly dip-slip faults at the northern margin of the Hexi Corridor.…”

Deep structure beneath the southwestern section of the Longmenshan fault zone and seimogenetic context of the 4.20 Lushan M S7.0 earthquake

“…Using the apparent resistivity and phase data of the two profiles, 2D inversion was carried out using the standard NLCG method [28]. A static correction was applied to those sites that were likely subject to static shifts [29,30]. The 2D inversion was carried out multiple times with the widely-used variable Tau and the L-curve method, in order to analyze the tradeoff between model roughness and data fit and to account for data uncertainties [17].…”