The subduction of the Indian continental lithosphere under the Asian continent caused the uplift of the Tibet

under the Asian continent caused the uplift of the Tibet Plateau, resulting in the formation of a thickened continental crust twice of the normal value and the crustal shortening of at least 1500 km. Therefore, many models have been proposed to explain the shortening and material transportation mechanism of the Tibetan Plateau, such as channel flow, subduction, and so on. Moreover, the heat generated by radioactive thermogenesis in the thickened crust of the Tibet Plateau makes it possible for partial melting which is the main explanation for the anomaly observed by the geophysical method in the mid-lower crust. However, geophysical observations gave diverse melt constraints from several percentages (Li et al., 2020) to above 50% (Xie et al., 2017) in the same area which might lead to not only different rheological properties but also distinct deformation mechanisms. Thus, research (Hashim et al., 2013; Chen et al., 2018) suggests that these constraints need to be re-evaluated. To discuss this condition in the western Tibet (Fig. 1a), we establish a 3D resistivity model based on magnetotellurics (MT) observations, and discuss the possible mechanisms behind the electrical anomalies and other geophysical and geochemistry result. Here, magnetotelluric data from ‘SINOPROBE’ and ‘INDEPTH’ projects in 79°–81°E (Fig. 1b) are studied, including the 117 broadband and 10 long-period stations. After analysing the dimensionality of the transfer functions, we compared the result with previous seismic studies in the region. We performed 3-D inversion using these data by the ModEM package (Kelbert et al., 2014). After 219 iterations, the RMS of our model reduced from 83 to 1.6. Our preferred model (Fig. 2) is proven to be reliable for crustal scale, with its horizontal reliable zone extending around the stations. Resistivity structure in the shallow area (0.5 km, Fig. 2a) is consistent with surface structure, as the distribution of surface high conductors is related to water-rich sediments. Sensitivity analysis also shows that: (1) there are no connections between the conductor A and B in the upper-mid crust of the Lhasa block; (2) the conductivity of A and B is higher than that of the conductor C in the middle crust of the Qiangtang block; (3) conductor C in Qiangtang block extends throughout the block and may be connected to Lhasa block's conductor B on the east side of the region. MT data are generally believed to have better estimation for the conductance of a certain layer, instead of the actual resistivity. Therefore, it is often better to estimate the fluid fraction (and hence viscosity) by an effective bulk conductivity derived from conductance. Figure 3 shows the integrated conductance of the middle crust (20–50 km) calculated from our preferred resistivity model. The high conductivity zones (HCZs, >0.3 S/m) is located in the middle crust of Lhasa terrane, which can be interpreted as partial melting. However, there are still some observed results challenging this opinion especially in the uppermiddle crust. Thus, we should consider all possible interpretation of the HCZs. Here, we apply modified Archie’s Law (Glover et al., 2000) to estimate the fluid percentage from conductivity in a two-phase medium. In the case of saline fluid, 4% fluid fraction with 1 wt% NaCl or 1% fraction with 5 wt% NaCl can result in the >0.3 S/m conductivity observed. If partial melting with water is present, it will require a full layer of melt (100% melt content) at depth of 20–30 km. In a deeper depth range of 30–60 km, the melt content needs to exceed 40% to reach the conductivity observed. The melt fraction will be comparable to the magma chamber of an active volcano, which is not realistic for the middle crust of the Lhasa Terrane. Our calculations indicate that due to the unique chemical composition, the melt conductivity in the study area may indeed be higher than that of other locations. Still, even with these conditions considered, the high conductivity cannot be properly explained. Meanwhile, higher contents of volatiles do not generally increase electrical conductivity as expected. Therefore, we must continue to investigate the origin of the HCZs. The relative highly resistive body (D) located in the lower crust of the Lhasa terrane matches the previous seismic tomography results, which suggests the anomaly may be related to the subducted India lower crust. The differences of conductivity between Lhasa and Qiangtang blocks indicate that their rheological properties are likely different. The Bankong–Nujiang Suture Zone separates the HCZs located in Lhasa and Qiangtang blocks, which may due to the different movement direction of the crustal On the Origin of Crustal High Conductivity Zone in the Western Tibet Plateau