The Hangai Dome is an inland uplift area with volcanic activity in the central segment of the CAOB. It is located in the central part of the Asian continent, far from the plate boundary. Currently, the understanding of its formation mechanism and geodynamic background is insufficient. This may be related to the interaction between the crust and the upper mantle. To study this issue, Käufl et al. (2020) collected 3D MT array data from the Hangai Dome and Gobi–Altai Mountain regions of Mongolia (Figure 4A) and, via multiscale 3D inversion, obtained an electrical structure model of the crust and upper mantle of the region (Figure 4B). Using these results, the genetic mechanisms of inland deformation and intraplate volcanic activity in this area are further assessed.

This study deployed an MT array covering a region of 650 × 400 km² in the Hangai and the Gobi–Altai Mountains in central Mongolia, including 97 array stations and 175 densely arranged stations along four profiles (Figure 4A). The traditional MT observation method and a geoelectric station that only measures the electric field were used. The latter can improve the spatial resolution and reduce the cost of data acquisition. With the use of finite element forward and inversion codes, multiscale model parameterization and terrain fitting were realized on the basis of an unstructured hexahedral mesh. The inversion process was divided into four stages, and more elaborate model parameterization and more data were used in each stage. A multiscale 3D resistivity structure model of the lithosphere was obtained, as shown in Figure 4B.

The model reveals that the conductivity structure of the Hangai Dome has the following main characteristics. In the high-resistivity upper crust, several low-resistivity anomalies are observed, which may be related to the late Cenozoic volcanic belts and modern geothermal regions, which are closely related to the deep mantle structure. In addition, some major fault systems, such as the boundaries that divide different geological units and ore-forming areas, also present low resistivity. In the lower crust, some nonuniform low-resistivity anomalies are found, and they may be composed of fluid-rich regions. These fluids may have originated from the dynamic processes of the mantle or deep crust. The upper mantle contains a large-scale low-resistivity area, indicating that the lithosphere–asthenosphere boundary is not uniform. There are some areas of local upwelling, which cause the asthenosphere to rise to a depth of 70 km. This low-resistivity area may be related to deep melt or fluid activity, and it may also be one of the reasons for the uplift of the Hangai Dome and volcanic activity.

This study is the first to reveal the crust‒mantle resistivity structure of the Mongolian Plateau region through 3D MT inversion, which provides new constraints and clues for further understanding the geological and geodynamic processes of this region.

The Beishan Orogenic Belt (BOB) at the southern end of the central segment of the CAOB is bordered by the Outer Mongolia accretion tectonic system in the north and the Kunlun‒Qaidam‒Qilian Basin system in the Tethys tectonic domain in the south. This belt is an ideal place for joint studies of the two tectonic systems. In the BOB, geophysical work is still very limited. At present, the understanding of the deep crust and mantle structure of the BOB is limited to some local areas, and the electrical structure model of the overall tectonic unit of the BOB has not been studied. We used a MT profile arranged in a northeast‒southwest direction from Ejin Banner in the Inner Mongolia Autonomous Region to Jiuquan city in Gansu Province (Figure 5) to conduct profile surveying across each tectonic unit in the entire Beishan area. A total of 52 broadband MT stations were recorded at a spacing of approximately 5 km.

The ModEM code (Egbert and Kelbert, 2012; Kelbert et al., 2014) was used to perform 3D inversion of MT data via the finite memory quasi-Newton algorithm (L-BFGS). The MT data used in the 3D inversion had a frequency range of 0.01–5000 s, with a total of 35 frequency points. The full impedance tensor and vertical magnetic transfer functions (tippers) were used in the inversion, where the error floor of the four elements of the impedance tensor was 5 % × Z x y × Z y x and the error floor of the two components of the tipper data were both 0.05. The initial model and prior model were set to 100 Ωm uniform half-spaces, and the number of meshes for model subdivision was 85(NS) × 160(EW) × 51(Z). The initial regularization parameter is λ = 1000. During the inversion process, whenever the data fitting difference no longer decreases, λ is reduced to one-tenth of the original value. The smoothing factor in each direction was set to 0.2. The 3D inversion converged after 166 iterations, and the root-mean-square (RMS) misfit of the data was 2.187.

The preliminary resistivity model (Figure 6) shows that the southwestern part of the Beishan block is relatively stable tectonically, appearing as a high-resistivity rigid block. The central and northeastern parts are relatively active, and conductors are widely distributed from the lower crust to the upper mantle. The low-resistivity characteristics across the crust below the northeastern part of the BOB may be related to the closure of the back arc basin or ocean, indicating the locations of the paleo suture zones formed via subduction–accretion processes during the closure of the Paleo-Asian Ocean. The resistivity model of the study area revealed that the high-resistivity bodies was intersected by low-resistivity stripes.

We used MT data collected in the central area of Inner Mongolia to study the electrical structure of the Bainaimiao segment of the CAOB. The MT data used in this study are shown in Figure 7 and are divided into two profiles. The sounding profiles all showed NNE‒SSW strikes and were nearly parallel. 38 MT stations were laid out along profile Line 1, and 25 MT stations were laid out for Line 2, resulting in a total of 63 MT stations. The two profiles spanned six tectonic units, including the Hegenshan Complex Belt, the Baolidao Arc, the Solonker–Linxi suture zone, the Ondor Sum Complex Belt, the Bainaimiao Arc and the Inner Mongolia axis, in the northeastern CAOB. The large fault zones traversed include the Erenhot fault, the Xilinhot fault, the Linxi fault, the Xar Moron fault, and the Chifeng–Bayan Obo fault.

Using the dimensionality and tectonic strike analysis of the MT data, it is found that most of the data in the study area are two-dimensional. It can be determined that the strike direction of Line one is roughly along NE 95° and the strike direction of Line two is roughly along NE 60°. On this basis, a 2D lithospheric electrical structure model was obtained through 2D inversion via the nonlinear conjugate gradient (NLCG) algorithm (Rodi and Mackie, 2001) (see Figures 8, 9, upper panels). The two profiles were inverted using both the TE and TM mode data. Considering that the TE mode data are more susceptible to 3D distortion effect (Becken et al., 2008; Jones, 1983; Wannamaker et al., 1984), the inversion parameters were set to 100% of the apparent resistivity error floor and 10% of the impedance phase error floor for the TE mode, and 10% of the apparent resistivity error floor and 5% of the impedance phase error floor for the TM mode. Different values of the regularization factor were used for inversion, and after comparison, the value τ = 1 was determined as the best trade-off between model roughness and data misfits. The initial model was a 100 Ω m uniform half-space, and the initial RMS misfit of the data is 13.04 for Line 1 and 12.67 for Line 2. After 200 iterations, the RMS misfit of the data is 1.76 for Line 1 and 1.52 for Line 2. Furthermore, a 3D electrical structure model at the crust and upper mantle scales in the study area was obtained via 3D inversion using the finite memory quasi-Newton algorithm (L-BFGS) (Figures 8, 9, lower panels). The ModEM code (Egbert and Kelbert, 2012; Kelbert et al., 2014) was used for 3D inversion. The frequency range of the MT data is 0.01–10000 s, with a total of 36 frequency points. The full impedance tensor data were used in 3D inversion, and the error floors of the four elements of the impedance tensor were set as 5 % × Z x y × Z y x . The initial model and prior model were set to 100 Ωm uniform half-space, and the mesh size of the model was 128(NS) × 76(EW) × 50(Z). The initial regularization factor is λ = 1000. During the inversion process, whenever the data fitting difference no longer decreases, λ is reduced to one-10th of the original level. The smoothing factor in each direction was set to 0.2. The initial RMS misfit is 15.77, after 124 iterations the RMS misfit is 3.17.

A comparison revealed that the 3D model and the 2D model have relatively good consistency (Figures 8, 9). According to both the 2D and 3D models, the upper mantle of the North China Craton has a low-resistivity anomaly, and the interior of the crust is dominated by high-resistivity anomaly, with some distributed nearly vertical low-resistivity strips. The low-resistivity strips were connected with the conductor in the upper mantle, indicating that the northern margin of the North China Craton may have experienced reactivation. Under the Bainaimiao Arc, two low-resistivity zones run through the crust in opposite directions. This feature is consistent with the tectonic paleogeography model proposed by Chen et al. (2020). It is speculated that this was the result of the closure of a paleo-ocean, with a high-resistivity zone captured in between. The high-resistivity zone runs from the crust to the upper mantle, forming an isolated lithospheric high-resistivity body. This electrical characteristic indicates that the Bainaimiao Arc may be more in line with the characteristics of an isolated arc terrane during its evolution. The crust under the Ondor Sum subduction complex, the Baolidao arc-accretion complex and the Hegenshan ophiolite arc-accretion complex all have high-resistivity anomalies. Under the Linxi fault and the Erdaojing subduction complex, which is generally recognized as the Solonker suture, there are two large-scale low-resistivity bodies with opposite dips that run through the crust in a northeast‒southwest direction, which is in good agreement with the bidirectional subduction model of the Paleo-Asian Ocean (Xiao et al., 2015). The Solonker suture is speculated to be the remnant of the bidirectional subduction closure of the Paleo-Asian Ocean, and the low-resistivity anomaly is caused by sulfur- and carbon-bearing sediments (Ye et al., 2019). Graphite is the most likely carbon-bearing conductive material in this case, suggesting that the specific location of the Solonker suture may be nearly 10 km north of the Linxi fault. There are also low-resistivity zones below the Chagan Obo-Arongqi fault. This finding is consistent with the closure model of the Hegenshan back-arc basin proposed by Zhang et al. (2020), and it is speculated that the low resistivity is related to the closure of the back-arc basin. In comparison, the distribution scale of this low-resistivity body is much smaller than that of the lithospheric low-resistivity body near the Solonker suture zone. The purple patches on the geological map in Figure 10 show the distribution of ophiolites, which have a good relationship with the low-resistivity anomalies in the upper crust and are closely related to the locations of the paleo-suture zones.

Ye et al. (2019) conducted MT data analysis, 2D inversion, and model interpretation on the basis of a broadband to long-period, high-resolution MT profile that is approximately 400 km long and crosses the Solonker suture zone in a northwest‒southeast direction (Figure 11). Using 2D anisotropy inversion and model validation, a 2D resistivity model of the study area was obtained (Figure 12). On the basis of previous study results, the model was interpreted to study the evidence and geometric characteristics of the final subduction closure of the Paleo-Asian Ocean. In the high-resistivity lithosphere, a south-dipping low-resistivity zone (Ds) with a dip angle of ∼45° was found, which cuts through the entire crust and reaches the mantle. The low-resistivity anomaly is inferred to be related to sulfur- and carbon-bearing sediments originating from the southward subduction of the Paleo-Asian Ocean, with a subduction depth of up to 80 km. The low-resistivity zones caused by the subduction of oceanic crust have obvious anisotropy. On the basis of the anisotropy characteristics, it is inferred that the high strain rate caused the fold axis to rotate in the dip direction. According to the position of this low-resistivity anomaly, the Paleo-Asian Ocean was inferred to have closed along the Solonker suture zone. The high-resistivity body (R2) found under the northern margin of the North China Craton was interpreted as a thickened lithosphere caused by the southward subduction of the Paleo-Asian Ocean, whereas the north-dipping low-resistivity zones (T1–T5) found in the crust of the CAOB and the North China Craton to the south of the southward subduction location were interpreted as postcollision thrust faults. The two northward-dipping low-resistivity zones (Dn1 and Dn2) may have been related to the northward subduction of the Paleo-Asian Ocean during the closure of the Paleo-Asian Ocean. These results indicate that the Paleo-Asian Ocean exhibited bidirectional subduction characteristics during the closure process, but the deep geometry could not be well traced because of the limited MT sites distribution near the national border at the northwestern end of the MT profile.

In the northeastern boundary zone of the North China Craton, Dong et al. (2015) completed a MT profile with a length of approximately 900 km in the NW–SE direction. The cross-section starts from the Hegenshan Complex Belt northwest of the CAOB and passes through the Baolidao Arc, the Solonker‒Linxi suture zone, the Ondor Sum Complex Belt, the Bainaimiao Arc, the Inner Mongolia axis, the Yanshan Orogenic Belt, and finally terminates at the Liaohe depression inside the North China Craton (Figure 13). The MT impedance tensor decomposition technique was used to analyze the dimensionality of the MT data and determine the regional geoelectric strike directions. The MT data along the profile generally exhibit good two-dimensional traits. Using the 2D NLCG inversion method, the subsurface 2D resistivity structure along the cross-section was obtained (lower panel of Figure 13). The resistivity structure along the cross-section can be roughly divided into three main electrical zones, which vary from high-resistivity to low-resistivity to high-resistivity. Generally, the resistivity characteristics within the CAOB and North China Craton are quite different, with the former changing violently and the latter changing relatively gently. Large-scale low-resistivity bodies are found in the middle–lower crust beneath the Solonker–Linxi suture zone and the Inner Mongolia axis, which may have been formed by deep fluids and partial melting. On the basis of the deep electrical structure and other geological and geophysical observations of the study area, the final suture location between the Siberian plate and North China plate may be in the area between the Xilinhot fault and the Xar Moron fault. In addition, the thicker high-resistivity body under the Yanshan Orogenic Belt may act as a tectonic barrier, causing the uppermost mantles inside and outside the North China Craton to have different convective modes, thus weakening the influence of the tectonic evolution of the CAOB on the destruction of the eastern North China Craton.

Liang et al. (2015) used MT profile data across the Great Khingan range from the eastern segment of the CAOB to study the lithospheric electrical structure (Figure 14). The purpose of this study is to reveal the lithospheric structure of the CAOB from an electrical perspective and to provide new evidence and constraints for the exploration of its tectonic evolution process as well as a new basis for deep resource surveys in the study area. After standardized data processing, analysis and inversion, a reliable deep electrical structure model was finally obtained (lower panel of Figure 14). The results reveal a large area of low-resistivity anomaly in the middle and lower crust of the study area, which reflects the weak nature of the crust and may have formed in a late extensional environment. The low-resistivity anomaly of the lower crust and the formation of regional mineral resources may be related to the upwelling of hot mantle material. The uplift of mantle material may also be an important driving force for the late tectonic extension of the study area.

The Songliao block in northeastern China is located in the superposition of multiple tectonic regimes. Its formation and evolution were affected by the closure of the Paleo-Asian Ocean, the closure of the Mongol-Okhotsk Ocean, and the subduction of the Paleo-Pacific plate (Figure 15A). Deep seismic reflection models revealed that there was a tilting mantle reflector under the Songliao block. This was speculated to be the result of the superposition, mutual subduction and compression of different tectonic regimes in the deep part of the Songliao block, followed by strong extension in the later stage. Despite the superposition and transformation of the later tectonic domains, residual traces of the preexisting tectonic regime can still be observed in the lithospheric structure. However, the relationship between the current lithospheric structure and the tectonic regime of the Songliao block is still unclear. One of the key problems is the unclear understanding of the lithospheric structure beneath the Songliao block.

To address the above problems, Wang et al. (2022) conducted a MT study (Figure 15) by establishing a 3D lithospheric electrical structure model in the northern Songliao Basin (Figure 16). They revealed a heterogeneous lithosphere in the northern Songliao Basin. The lithosphere contains multiple low-resistivity anomalies (∼10 Ω m), and the conductivities of the upper crust (<10 km), middle and lower crust (15–35 km), and upper mantle (>45 km) are discussed. Among them, the resistivity anomalies in the upper crust are mostly distributed in the NE and NW directions, and orthogonal low-resistivity anomalies appear at the intersection locations. There is good correspondence between the electrical structure and the surface geological characteristics, and the low-resistivity anomaly identified is located at the boundary of the known orthogonal network fault system and under the rift basin. The low-resistivity anomaly is the result of temperature-controlled partial melting. On the basis of petrologic and seismic shear wave velocity studies (Zhan et al., 2020) of the study area, the melt proportion is calculated to be approximately 5%. Compared with deep seismic reflection profiles (Fu et al., 2019), these features are distributed near locations where the Moho offset is present, indicating that magma and magmatic fluids ascend through the Moho. Adjacent to the junction of the central depression area and the northern plunge area, the low-resistivity anomaly in the deep upper mantle is interpreted as asthenospheric upwelling near the center of the Songliao block, which represents the thinnest lithosphere in the Songliao Basin. On the basis of the 3D resistivity structure and comprehensive tectonic, geological and other geophysical characteristics, the following interpretations are proposed for the present tectonic mechanism of the lithosphere (Figure 17). Owing to the closure of the Pleo-Asian Ocean and the bidirectional convergence between the Mongol–Okhotsk Ocean and the Paleo-Pacific Ocean, collision, convergence and suturing of the Songliao Block and surrounding blocks occurred. These processes generated weak zones in the lithosphere at the edge of the Songliao Block, providing the structural base for the subsequent upward migration of fluids and melts. The fluids and melts were caused by water-bearing upwelling under the subduction of the Paleo-Pacific system. Therefore, even though the Paleo-Pacific tectonic system directly affects the current status of the lithospheric structure below the Songliao block, it is also closely related to the complex tectonic history and superimposed tectonic systems of the area.