Three-dimensional electrical structure and deep dynamics of the Khondalite Belt and adjacent areas in the Western Block of the North China Craton
Introduction
The Khondalite Belt is located in the Western Block of the North China Craton. In addition to the widely exposed Khondalite series, there are small amounts of TTG gneiss, mafic granulites, syntectonic charnockites, and S-type granites (Zhao and Cawood, 2012). The Khondalite Belt is surrounded by the Ordos Block to the south, the Yinshan Block to the north, the Alxa and Qilian blocks to the west, and the Trans-North China Orogen to the east. At present, the widely accepted model is that the Khondalite Belt was formed by the amalgamation of the Ordos and Yinshan blocks, with the eastern section being involved in the collision of the Western and Eastern Blocks. As an important collision belt of the North China Craton, it is characterized by crustal thickening and lithospheric thinning (Zhao and Cawood, 2012, Yin et al., 2011, Santosh et al., 2010, Santosh et al., 2013, Kusky and Li, 2003, Zhai and Santosh, 2011). The spatial extension of the Khondalite Belt has attracted considerable attention, but a lot is still unknown. According to zircon dating, which records Paleoproterozoic collision events, it is believed that the Khondalite Belt extends westward to the Qianlishan area (Zhao and Cawood, 2012). Others have suggested that it extends westward to the Beidashan and Longshoushan areas of the Alxa Block (Zhang et al., 2013), while some believe it extends westward to the South Tarim Orogen (Ge et al., 2013). Unfortunately, there are not enough geological and geophysical research results to enable an understanding of the deep properties of the Khondalite Belt.
As a primary deep detection method, magnetotellurics (MT) examines lithospheric structures from the perspective of rock conductivity. It has been widely used in research addressing the formation and evolution mechanisms of the Tibetan Plateau and thinning of the North China Craton (Wei, 2002, Wei et al., 2003). As the Khondalite Belt has experienced multi-stage collision events, different blocks should have obvious electrical structure differences. Moreover, the lithospheric weak zone in the study area is relatively well developed and thus should be characterized by a low-resistivity anomaly. Research on magmatic rocks has shown that the deep structure may be modified by partial melting, which exhibits a low-resistivity anomaly. MT can effectively identify a low-resistivity layer in the lithosphere (Wei, 2002, Wei et al., 2003). Therefore, this study used the SINOPROBE MT array dataset, collected in the Khondalite Belt and adjacent areas of the Western Block of the North China Craton, to build a 3-D lithospheric resistivity model. Furthermore, the model provided the electrical constraints for scientific issues such as the tectonic evolution, magmatism, and seismogenic mechanisms of the Khondalite Belt.
Section snippets
Geological and geophysical background
Since the Paleoproterozoic collision, the Khondalite Belt has experienced a long period of calm geological history. Until the Mesozoic, the tectonic and magmatic activities of the study area were active in the background to multi-plate convergence (Zhang et al., 2007). Compared to the Eastern Block, where the lithosphere is severely thinned (<100 km) and there is a high surface heat flow (60–80 mW/m2), the Western Block maintains a stable craton in which there may be local activation of the
Data
As shown in Fig. 1, 440 broadband MT and 53 long-period MT sites, collected by Phoenix MTU-5 BBMT and LVIV Lemi-417 long-period MT systems, were selected. Data were provided by the SINOPROBE project (Dong and Li, 2009). The MT sites cover the Khondalite Belt, Yinshan Block, northern Ordos Block, and northern Trans-North China Orogen. The solid black lines in Fig. 1 indicate the block boundaries, and the range bounded by the black solid lines between the Yinshan and Ordos Blocks represents the
Strike and dimensionality analysis
To obtain the study area strike, we performed G–B decomposition on the data (Groom and Bailey, 1989). The strike information for each MT site in the 0.1–1-s, 1–10-s, 10–100 s, and 100–1000-s period bands are displayed in Fig. 3. Each site has two orthogonal strike directions, but only one was the real strike direction. The correct strike can only be determined by other geological and geophysical information since the impedance decomposition has a 90-degree ambiguity. In the 0.1–1-s period band,
Three-dimensional inversion
To obtain a 3-D resistivity model of the study area, we applied the ModEM system, which is based on a non-linear conjugate gradient algorithm (Egbert and Kelbert, 2012). According to the average distance between the MT sites, the current study area was divided into 72 × 145 × 60 cells with a latitudinal and longitudinal node spacing of 10.0 km. Vertically, the thickness of the first layer was 100 m and the increasing step factor was 1.1. To satisfy the infinite boundary condition, with a step
Sensitivity test
The low-resistivity anomalies shown in Fig. 6e (C1, C2, C3, and C4) were well covered by the MT sites. However, for MT signals of the same period, when the resistivity of the underground medium is low, the skin depth is small. Taking this into account, we conducted a forward sensitivity test to verify the existence of the four low-resistivity anomalies. Using the investigation of C1 as an example, for modified model C1#-24, C1 (white rectangle marked with C1 in Fig. 6e) was replaced with a
Electrical structure of crust and upper mantle
To clearly illustrate the relationship between the resistivity and depth, the resistivity values at each depth of the Yinshan Block, Khondalite Belt, and Ordos Block were extracted from the three-dimensional resistivity model, and the change in the resistivity with depth was plotted (gray polygons in Fig. 7), as well as the average resistivity at each depth (black lines in Fig. 7). A crustal model of the North China Craton (Chi and Yan, 1998) indicates that the average thickness of the upper
Conclusion
Based on a 3-D resistivity model of the Khondalite Belt and adjacent areas obtained using a SINOPROBE MT array, a number of features which point to the tectonic evolution of the research areas were observed.
Based on our resistivity model, the boundaries between the Khondalite Belt and Yinshan and Alxa Blocks could be determined. Our results showed that the Khondalite Belt extended westward to the Helanshan and Qianlishan areas.
In the middle and lower crust, the Khondalite Belt, Ordos Block, and
CRediT authorship contribution statement
Gaofeng Ye: Project administration, Writing - original draft, Writing - review & editing. Gongshuai Wang: Writing - original draft. Sheng Jin: Methodology. Wenbo Wei: Funding acquisition. Letian Zhang: Validation. Hao Dong: Visualization. Chengliang Xie: Software. Yaotian Yin: Visualization.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This work was supported by National Key R&D Plan (Grant No. 2017YFC0601406), project SINOPROBE on sub-project SINOPROBE-01, National Natural Science Foundation of China (Grants 41274003, 41674101 and 41974112).
Alan Jones and Gary Egbert are thanked for providing the MTMAP and the ModEM codes respectively.
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