Lithospheric electrical structure of South China imaged by magnetotelluric data and its tectonic implications

Accepted Manuscript Lithospheric Electrical Structure of South China Imaged by Magnetotelluric Data and its Tectonic Implications Letian Zhang, Sheng Jin, Wenbo Wei, Gaofeng Ye, Jianen Jing, Hao Dong, Chengliang Xie PII: DOI: Reference:

S1367-9120(14)00491-X http://dx.doi.org/10.1016/j.jseaes.2014.10.034 JAES 2157

To appear in:

Journal of Asian Earth Sciences

Received Date: Revised Date: Accepted Date:

25 May 2014 2 October 2014 15 October 2014

Please cite this article as: Zhang, L., Jin, S., Wei, W., Ye, G., Jing, J., Dong, H., Xie, C., Lithospheric Electrical Structure of South China Imaged by Magnetotelluric Data and its Tectonic Implications, Journal of Asian Earth Sciences (2014), doi: http://dx.doi.org/10.1016/j.jseaes.2014.10.034

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Lithospheric Electrical Structure of South China Imaged by Magnetotelluric Data and its Tectonic Implications

Letian Zhang a,b Sheng Jin a,b,* Wenbo Wei a,b,c Gaofeng Ye a,b Jianen Jing a,b Hao Dong a,b Chengliang Xie a,b

a

School of Geophysics and Information Technology, China University of Geosciences, Beijing 100083, China.

b

Key Laboratory of Geo-detection of Ministry of Education, Beijing 100083, China.

c

State Key Laboratory of Geological Processes and Mineral Resources, Beijing 100083, China.

*Corresponding author: [email protected]

Zhang et al. South-China-MT.

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Abstract The region of South China mainly consists of the Yangtze block in the northwest, the Cathaysia block in the southeast and the Jiangnan orogen in between these two major Precambrian continental blocks. The Yangtze block borders the North China Craton in the north and the eastern margin of the Tibetan Plateau in the west. The Cathaysia block adjoins the Pacific tectonic domain in the east. The study of tectonics in this region is of great significance given its important role in understanding the formation of the Asia continent. Under the auspices of SinoProbe Project, new magnetotelluric (MT) data were collected along a ~1200 km long profile starting from central Sichuan Basin near Suining, extending southeastward, passing through the Yangtze Block, Jiangnan Orogen, and terminating within the western Cathaysia Block near Ganzhou. Based on data analysis results, 2D inversions were conducted on the dataset. Resulting model shows that the lithospheric electrical structure of South China is generally resistive which is consistent with the basic feature of stable Precambrian tectonic setting. The resistive western Yangtze block represents the stable, Archean aged cratonic region of the Yangtze basement. While the electrically conductive eastern Yangtze block is characterized by lithospheric shearing of the strike-slip fault system and extensional process that is probably caused by slab roll-back of a flatly subducted plate. The Jiangshao fault performs as a northwestward dipping conductive layer, which indicates the lithospheric underthrusting of Cathaysia block beneath Yangtze block with its frontal edge reaching the area of Jishou in the upper mantle. To the west of Jiangshao fault, eastern flank of the Xuefengshan Mountain marks the overthrusting frontier of the Yangtze block, as well as its southeastern boundary. To the east of Jiangshao fault, the northwestern boundary of

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the Cathaysia block displays the pattern of wedging tectonics, which is characterized by a conductive layer wedging into the Cathaysia lithosphere at the depth range of Moho.

Keywords: Magnetotellurics; South China; Yangtze block; Cathaysia block; Jiangnan orogen; Lithospheric electrical structure

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1.

Introduction The South China block is an ancient continent block that comprises today's southern

and southeastern regions of China, Indochina, as well as parts of Southeast Asia. On its sea side, the South China block is separated on the east from the Okinawa plate by the Okinawa trough, and on the south by the Sunda plate and the Philippine Sea plate (Bird, 2003). On the land side, the South China block borders the North China craton in the north along the Qinling-Dabie orogen and adjoins the Songpan-Ganze block on the eastern margin of the Tibetan plateau in the west along the Longmenshan fault zone. The region of South China mainly consists of the Yangtze block in the northwest, the Cathaysia block in the southeast and the Jiangnan orogen in between these two major Precambrian continental blocks (Fig. 1). The Yangtze block is generally considered as an Archean aged cratonic block (Zheng et al., 2006). The age of Cathaysia block is still debated due to its poorly exposed basement. However, recent studies suggest that the Cathaysia basement is at least Paleoproterozoic in age (Xu et al., 2007), and is probably of Neoarchean age (Yu et al., 2007). Given its long history of evolution, the South China block shows very complicated structural patterns, and it is still controversial in many detailed aspects. In general, the South China block is considered to be formed by the amalgamation of Yangtze block and Cathaysia block along the Jiangnan orogen. However, the timing, location, kinematics and geodynamics of this process still remain disputed. Li et al. (2002; 1995) suggest that the Yangtze and Cathaysia blocks amalgamated during the Sibao orogeny in Mesoproterozoic (1100~1000Ma) and formed the Jiangnan orogen between the two blocks. Another group of viewpoints suggest that the shortening and subduction

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between Yangtze and Cathaysia plates happened in Neoproterozoic (~800Ma) along the Jiangshao fault in the southeastern margin of the Jiangnan orogen (Shu, 2012; Wang et al., 2007; Yao et al., 2014a; Yao et al., 2014b). Furthermore, a third slab-arc model (Zhao et al., 2011) proposes a northwestward subduction process of the Cathaysia block beneath the Yangtze block and the final assembly of these two blocks at ca. 830Ma. In recent years, a revisited model for the tectonic evolution of South China was proposed by Zhang et al. (2013), which suggested that the amalgamation process started from the west prior to 1000 Ma and migrated eastward with the final amalgamation along the suture at around 830–820 Ma. This model might be able to reconcile the competing models mentioned above. After the early-stage evolution, Yangtze and Cathaysia blocks have experienced several episodes of regional folding and extensional processes during early and late Paleozoic. These events result in the final suturing and collision between the Yangtze and Cathaysia blocks during middle Triassic of Indosinian along the southeastern boundary of the Jiangnan orogen (Wan, 2011). Later on, during Jurassic to Cretaceous, South China experienced a series of magmatism events in its southeast region (Lin et al., 2008). These events could be evidenced by the wide spread NNE-SSW trending granite belts of different ages distributed from the southeast coast of China to the southeastern margin of the Jiangnan orogen (Li et al., 2012b). Such feature is commonly related to the subduction of a Paleo-Pacific oceanic plate beneath the South China continent, and several tectonic models have been proposed by different authors to explain this process, such as the flat-slab subduction model (Li and Li, 2007; Meng et al., 2012), the oblique

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subduction model (Wang et al., 2011), as well as several other models (Hsü et al., 1990; Zhou and Li, 2000). Since Mesozoic, the region of South China has been constrained by three major convergent tectonic domains, which are the westward subduction of the Pacific Plate, the northward subduction of the India Plate beneath the Eurasia Plate, and the convergence between North and South China Blocks. Within this compressional tectonic domain, structures in South China are mainly characterized by shortening, thrusting and decollement (Li et al., 2012a). The study of tectonics in this region is of great significance given its important role in understanding the formation of the Asia continent. However, large-scale geophysical studies in South China are still very limited due to the complex topography and difficult logistics. Under the auspices of SinoProbe Project (Dong et al., 2013; Dong et al., 2011), we have deployed a long magnetotelluric (MT) profile across the major parts of South China (Fig. 1). The profile starts from central Sichuan Basin in the northwest, extends southeastward across the Yangtze Block, the Jiangnan Orogen, and ends within the western Cathaysia Block. MT signals can easily penetrate high-resistivity bodies and are sensitive to low-resistivity anomalies, thus can provide strong constraints on the rheology and thermal state of subsurface structures. MT data has been effectively used in previous tectonic studies of ancient cratonic regions, such as the East-European Craton (Bubnov et al., 2007), the Kaapvaal Craton in southern Africa (Evans et al., 2011), and the Dharwar Craton in southern India (Naganjaneyulu and Santosh, 2012). Although some previous MT studies have been conducted in South China or the western Sichuan Basin (Deng et al., 1990; Jiang et al., 1992; Zhao et al., 2012a), these studies were either mostly based on one-dimensional (1D) inversion and

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interpretation techniques or too far away from the study area focused in this research. In this paper, we present new imaging result of the lithospheric electrical structure across a major portion of South China continent obtained from the recently collected MT dataset. 2.

MT data acquisition MT data were recorded along the profile shown in Fig. 1. The profile is about 1200

km long and comprises 99 Broadband MT (BBMT) stations. MT data were collected with Phoenix MTU-5 instruments. All 4 orthogonal components of the electromagnetic field (Ex, Ey, Hx, Hy) were recorded during data acquisition. The ~1200 km long profile starts from the central Sichuan Basin, near the city of Suining, extends southeastward and passes through several major cities such as Chongqing, Jishou, Shaoyang, Hengyang, and Ganzhou. It crosses a series of mountain ranges and major faults, such as the Huaying Mountain, Qiyue Mountain, Enshi fault, Dayong fault within the Yangtze plate, the Jishou fault, Kaili fault, Jiangshao fault, Xuefeng Mountain within the Jiangnan orogen, and the Tunxi-Yingtan-Anyuan fault within the Cathaysia block. The profile terminates within the western Cathaysia block, to the west of Zhenghe-Dapu fault. Extending direction of this profile is designed to be roughly perpendicular to the average strike direction of major structures as observed at the surface, which is about N40°E. MT stations are designed to be distributed evenly along the profile with an average spacing of ~10 km. With an average recording time of 20 hours, high quality MT data was obtained within an average period range of 0.003-2000 s. Original MT time series data were transformed into frequency domain with FFT, and frequency dependent transfer function elements were computed through standard remote-reference (Gamble et al., 1979), robust (Egbert and Booker, 1986) routines. Three typical MT sounding curves, which represent

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the electrical structures in Yangtze block, Jiangnan Orogen, and Cathaysia block respectively, are shown in Fig. 2. The locations of these three stations are marked in Fig. 1. It needs to be noted that MT data of stations in the southeastern section of the profile (mainly in the Cathaysia block) are of relatively poor quality in long period band (see the sounding curve of station 690 in Fig. 2). However, this region is featured by widely distributed highly resistive granite rocks. Since estimated skin depth is in proportion with the product of period and corresponding apparent resistivity (Jones, 1983), MT data with relatively short period could still reach a penetration depth deep enough in a highly resistive medium. To further estimate the distribution of penetration depth in our dataset, we calculated the Niblett-Bostick penetration depth (Jones, 1983) for all stations under the period of 500s by using the maximum apparent resistivity value. Results are plotted in Fig. 3. Penetration depth for each station is shown as colored squares. This result shows that, under the period of 500s, stations data in the southeast section of the profile can averagely reflect the electrical structure in the depth of 200 km (purple squares). Within the Sichuan Basin at the northwest section of the profile, where shallow sediments are generally conductive, MT data with period of 500s have an average penetration depth of about 100 km (green squares). The estimated penetration depths prove that our dataset is capable to reflect the subsurface electrical structure in a lithospheric scale. In this study, we focus to discuss the electrical structure within the depth range from surface to 100 km. 3.

MT data analysis

3.1 Dimensionality

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The goal of MT data analysis is to develop a resistivity model of the Earth from the measured MT data. This can include the development of 1-D, 2-D or 3-D resistivity models. It is therefore essential to investigate the dimensionality of the MT data and determine if a 1-D, 2-D or 3-D approach is required. Here we adopt the phase tensor method (Caldwell et al., 2004; Moorkamp, 2007) to evaluate the dimensionality of our dataset. The phase tensor is a useful tool in assessing the dimensionality of MT impedance data given that it is insusceptible to galvanic distortions. It is defined as the product of inverse real tensor and imaginary tensor of the impedance tensor, and can be graphically presented as an ellipse defined by three elements: the maximum tensor value Фmax, the minimum tensor value Фmin, and the skew angle β. Фmax and Фmin correspond to the major and minor axes of the ellipse with their directions indicating the two orthogonal electrical principal axes or two possible strike directions. The skew angle β is the parameter used to assess dimensionality of a station data. Generally, a small β value indicates EM fields compatible with 1D or 2D conditions. Fig. 4 shows the phase tensor maps at periods of 0.1s, 1s, 10s, and 100s. We found most of the stations are of relatively small skew angles (< 5°, as shown in blue colors) over different period bands. A few exceptional stations are mainly distributed within the Jiangnan orogen in longer periods (see Fig. 4(c) and 4(d) in 10s and 100s, β > 20°, as shown in red color), which indicate relatively complex subsurface structures in this region. For the stations within the Sichuan Basin in the northwest section of the profile, the shapes of phase tensor ellipses are close to round circles, which suggest that the uniformly distributed sedimentary strata in this region is generally of 1D character.

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Consequently, we conclude that, on the whole, the dimensionality of the profile is 2D, and 2D inversion procedures are applicable to our dataset. 3.2 Regional strike direction Under a 2D circumstance, MT data can be decomposed into two independent groups, which are the transverse electric (TE) mode data and transverse magnetic (TM) mode data, along two orthogonal directions that are perpendicular and parallel to the strike direction. Then these two groups of data can be inverted respectively or jointly. Thus a regional strike direction along the profile must be determined in advance. In this study, the regional strike direction is chosen by using the multi-site, multifrequency tensor decomposition technique of McNeice and Jones (2001) based on the method of Groom and Bailey (1989). Strike analysis results in four different period bands

are plotted in maps and rose diagrams as shown in Fig. 5. Blue bars on each site indicate the preferred principal axis which presents possible strike directions. Rose diagram in each subfigure shows the statistical result of regional strike direction in the corresponding period band for all stations along the profile. It should be noted that strike directions determined with MT include an inherent 90° ambiguity. Therefore, other information such as the geological structures observed at the surface is needed to determine the correct strike direction. In the study area, most major structures such as mountain ranges and major faults roughly follow a pattern that trend in NNE-SSW directions. This pattern is consistent with our strike analysis result as shown by the blue bars over each period band in Fig. 5. Rose diagrams also indicate a dominant regional strike direction at around N40°E over these four different period bands. According to these results, the regional strike direction was determined as N40°E, and all MT data were decomposed into two

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independent polarized modes (TE and TM) along the regional strike direction. 2D inversions were then conducted on this dataset. 4.

MT data inversion

4.1 2D inversion MT data were inverted with 2D NLCG (Nonlinear conjugate gradients) algorithm of Rodi and Mackie (2001). Before inversion, the selection from different MT data modes (TE and TM) has always been a controversial issue and difficult task. The inversions of single mode data generally lead to quite different resulting models. Ledo (2005) concluded that the TM mode data are more suitable for the interpretation of 3D body parallel to the 2D regional strike direction, and TE mode is more robust for the interpretation of 3D structures normal to the regional strike direction. Generally, the TM mode is more sensitive to shallow resistive structures, and is more stable under the 3D effects caused by conductors. However, TE mode is more sensitive to deep conductive structures, and is relatively robust in the presence of 3D effects produced by resistive features (Berdichevsky, 1999). Consequently, to make sure the full information of subsurface electrical structure is properly extracted from the MT data through inversion, it is suggested that both modes should be taken into inversion. However, given that the TE mode data are more susceptible to 3D effects (Becken et al., 2008), the TE and TM mode data generally cannot be inverted jointly with the same weight. A better solution is to down-weight the TE mode data, especially the TE apparent resistivity, during the inversion by increasing their error floor. Down-weighting the TE apparent resistivity can also reduce the influence of static shift.

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Accordingly, in this study, we jointly invert the TE and TM data with the error floor for TE apparent resistivity and phase set to be 50% and 2.9°, respectively. For the TM mode data, the error floor was set as 10% for apparent resistivity and 1.45° for phase. TE and TM mode data ranged within 0.01-10000s (6 decades) are all taken into inversions. All inversions are started from a 100 Ωm uniform half space, and several regularization parameter τ ranged from 1 to 30 are used for different inversions. We chose the inversion model with τ = 3 as the final model through a trade-off analyses, which corresponds to the corner of an L-curve between overall root-mean-square (r.m.s.) misfits and model norms. This model is shown in Fig. 6. Pseudo sections of observed data and calculated responses for both modes are also plotted therein. Overall RMS misfit for the final model is 2.603. The relatively low RMS misfit value and the similarities between pseudo sections of observed and calculated data indicate a proper fitting and prove the final model is reliable. 4.2 Inversion Results The most distinct feature of the 2D inversion model is the widely distributed resistive regions along the profile (Fig. 6(a)). These resistive regions could be further roughly divided into several resistive blocks. We mark them as resistors R1-R7. There are also two major conductive regions along the profile, which are marked as conductors C1 and C2. Conductors C1 and C2 both extend from the surface to 100 km deep and further discrete the profile into three resistive sections that consist of R1, R2-R4, and R5R7, respectively. Conductor C1 is a sub-vertical column-shaped conductor, while conductor C2 shapes like a northwestward dipping layer with some fluctuations.

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Generally, signals from the natural Electro-Magnetic (EM) field can easily penetrate high-resistivity bodies and are sensitive to conductive anomalies. However, the physics of EM signal diffusion in the Earth means that MT data cannot accurately determine the lower boundary of a conductor. To further constrain the electrical structures beneath conductors C1 and C2, we conducted two groups of forward modeling by calculating the new responses from a set of models modified after the original 2D inversion model and observing the change in RMS misfits to see if the data are sensitive to these modifications. For conductor C1, we would like to know if this deeply extended conductor is of lithospheric scale. Consequently, a set of new models with the lower boundary of C1 constrained at depth of 80 km, 60 km, 40 km, and 20 km were created. New response data are computed through forward modeling and the site-by-site RMS misfit distributions of the original model and these modified models are plotted in Fig. 7. We observed that the changes in the lower boundary of C1 will cause an obvious increase in the RMS misfit values of the stations above conductor C1, especially while the lower boundary is constrained shallower than 60 km (Fig. 7(a)). Besides the site-by-site RMS misfit, the overall RMS misfit also increases as the constrained depth getting shallower (Fig. 7(b)-(f)). These results prove that the MT data are sensitive to the electrical structures beneath conductor C1, and therefore the extending depth of C1 is at least 60 km or even deeper. For conductor C2 we are interested in whether the resistors R5 and R6 beneath C2 are real structures inverted from MT data or artificial structures produced by inversion due to the lack of long period data. We took the similar strategy by gradually wiping out the resistive features of R5 and R6 above 60 km, 80 km and 100 km from the original

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model. Resulting RMS misfit distributions are plotted in Fig. 8. Notable increase in siteby-site RMS misfit values of the stations above resistor R5 and R6, as well as the overall RMS misfit, are both observed as the upper boundary of R5 and R6 is constrained deeper than 60km. We therefore conclude that the resistors R5 and R6 covered by conductor C2 are real structures properly inverted from MT data. 5.

Model interpretation and discussion

5.1 Western Yangtze block The profile presented in this paper crosses three major geologic units in South China, which are, from northwest to southeast, the Yangtze block, the Jiangnan orogen and the Cathaysia block (Fig. 9). From the inversion model, these three units all present as resistive bulks (R1, R2-R4, and R5-R7), which is consistent with the basic feature of a stable Precambrian tectonic setting. These three groups of resistive bulks are separated by two major conductors (C1 and C2). Beneath western Yangtze block, resistor R1 represents the stable, Archean aged basement of the Yangtze craton. The highly resistive electrical structure suggests that the basement is cold and of very low fluid content. In previous large-scale seismic tomography studies, the western region of Yangtze block is commonly imaged as a deeply rooted stable cratonic block featured by fast seismic velocities (Huang and Zhao, 2006; Obrebski et al., 2012; Zhao et al., 2012c; Zhou et al., 2012). The electrical property of Yangtze craton reflected in our model is consistent with these observations. Above R1, the Huayingshan fault marks the boundary between Sichuan Basin and East Sichuan decollement fold belt. In our model, this fault performs as a southeastward dipping conductor which cut into the resistive basement of the Yangtze craton to the

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depth of about 30km. This structure also reflects the northwestward overthrusting of the Huayingshan Mountain, which is consistent with the geological observation on the earth surface (Yan et al., 2003). To the west of the Huayingshan fault, the resistive Yangtze basement is covered by a ~20 km thick conductive layer, which reflects the sedimentary covers within the Eastern Sichuan Basin. To the east of Huayingshan Mountain, there is also a conductive layer above the resistive basement within the East Sichuan decollement fold belt between Huayingshan fault and Qiyueshan fault. This conductive layer is probably generated from the relatively high fluid content in the shallow fractured rocks produced by the thinskinned structure of decollement folds with the lower boundary of this layer marking the detachment plain. Judging from the geometry of this layer, we further infer that there are probably a series of westward overthrusts in this region. 5.2 Eastern Yangtze block and Jiangnan orogen The Qiyueshan fault marks the eastern boundary of the East Sichuan decollement fold belt. To the east of Qiyueshan, the eastern Yangtze block is characterized by a conductive lithosphere represented by conductor C1. It has been tested in the above forward modeling study that the extending depth of conductor C1 is at least 60 km or even deeper. Seismic studies have discovered that the average depth of Moho in the Yangtze block is about 40 km (Deng et al., 2011; Xiong et al., 2009; Zhou et al., 2012). The conductor C1 therefore represents a lithospheric electrical feature in our model. There are three major faults distributed in the region of eastern Yangtze block above conductor C1. From the inversion model, the Qiyueshan fault presents as a southeastward dipping electrical interface between R1 and C1, which is consistent with the geological

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observation from earth surface by Yan et al. (2003), and the Dayong fault is expressed as a northwestward dipping interface dividing C1 and R2. These two faults bound the western and eastern boundaries of the eastern Yangtze block. The Enshi fault locates in the central region between Qiyueshan fault and Dayong fault. There is no distinct electrical feature to determine the extending tendency in subsurface of the Enshi fault. However, given that the geometry of C1 is relatively wide and fractured in its shallower portion, we infer that this pattern is consistent with a “positive flower structure” that is generally formed by strike-slip motions (Xiao et al., 2011). Furthermore, the surface geological structures in this region are also generally characterized by ductile shear zones produced by left-lateral strike-slip motions (Shu, 2012). Consequently, we propose the Enshi fault is a sub-vertical left-lateral strike-slip fault that vertically cut through C1 in its central portion with a minor component of northwestward dipping. The southeastward dipping Qiyueshan fault and northwestward dipping Dayong fault therefore might converge with the Enshi fault around Moho and formed a large scale “positive flower structure” as illustrated in Fig. 9. On the other hand, according to structural geology study (Yan et al., 2003), the Enshi fault also displays some features of normal faulting, which indicate the region of conductor C1 is probably in an extension state. This feature can be correlated with the effect of slab roll-back at the frontal edge of a subducted plate (Shu et al., 2011; Zhao et al., 2012b). Based on the flat-slab subduction model (Li and Li, 2007), the Paleo-Pacific oceanic plate has subducted beneath the South China continent progressively from the southeast coast of Cathaysia block to the region of eastern Yangtze block since late Permian. Seismic tomography studies have also found similar features that support this

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hypothesis (Huang and Zhao, 2006; Zhou et al., 2012). The slab roll-back process within the mantle transition zone might generate convection in the asthenosphere and cause the upwelling of hot mantle fluids, which lead to the extension of lithosphere. However, this depth range is far beyond the penetration depth of our MT data, thus these structures cannot be imaged in our model. Within the Jiangnan orogen, three resistors R2, R3 and R4 are separated by Jishou fault and Kaili fault. From the inversion model, these two faults both present as subvertical interfaces between two bordering resistors. According to the analysis of the Enshi fault above, we infer these two faults are also of the properties of left-lateral strikeslipping. The region confined between Dayong and Jishou faults roughly coincides with the northwest boundary of the Jiangnan orogen. This boundary, however, is suggested to be a collision zone between the northern and southern Yangtze blocks during the Sibao orogeny in late Mesoproterozoic (Wan, 2011). We therefore further infer that the resistive bulk formed by R2, R3 and R4 is probably of the same properties as the cratonic Yangtze basement R1. Resistors R2-R4 might have been part of the ancient Yangtze block, and were separated from the Yangtze basement and intersected by the late-stage left-lateral strike-slipping and normal faulting. Accordingly, we assume that the resistors R1-R4 and conductor C1 all belong to the Yangtze tectonic regime. 5.3 Jiangshao fault and the tectonic relationship between Yangtze block and Cathaysia block In the forward modeling study discussed above, we have testified that the resistors R5 and R6 are both real structures that properly inverted from MT data. Thus the resistive region consists of R5, R6 and R7 should be the electrical representing of the Cathaysia

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block. Given the series of magmatism events happened in the evolution history of Cathaysia block, these imaged resistive structures are probably the electrical reflection of subsurface granitic plutons and mafic-ultramafic intrusions. In comparison with the Yangtze Block, electrical structure within the Cathaysia block seems more fractured, which is likely due to intense late-stage modifications of the subsurface structures. Most structures within the Cathaysia block are reflected to be distributed within the crust. The only structure that is probably of lithospheric scale is the Tunxi-Yingtan-Anyuan fault. Judging from its geometry, this fault appears to be a strike-slip fault. However, given that the fault locates at the southeast end of the profile, the geometry of this fault might not be very precisely constrained from the inversion model. The Jiangshao fault is generally considered as the suture zone between Yangtze and Cathaysia blocks (Chen and Jahn, 1998; Gilder et al., 1996; Shu, 2012; Yao et al., 2014a; Yao et al., 2014b). However, due to the lack of strong evidences of this suturing process, such as ophiolite suites, the precise location of this suture zone is still not well defined. In our model, the surface location of the northwestward dipping conductive layer C2 roughly coincides with the inferred lineament of the Jiangshao fault. By assuming that the resistors R2-R4 belong to the Yangtze tectonic regime, this structure should be the reflection of a lithospheric underthrusting structure between Yangtze and Cathaysia blocks. The hanging wall comprises resistors R2-R4 of Yangtze affinity, and the foot wall consists of R5 and R6 distributed in the lithospheric mantle of the Cathaysia block. The conductive layer C2 probably consists of materials of the relict oceanic crust between the ancient Yangtze and Cathaysia blocks (Li et al., 2008; Wang et al., 2006).

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The folded shape of C2 indicates that the subduction structure might have experienced late-stage folding and extension processes. As revealed by previous seismic studies, the average Moho depth within the Cathaysia block is ~30 km (Deng et al., 2011; Xiong et al., 2009; Zhou et al., 2012). Considering the average Moho depth in Yangtze block is ~40 km as mentioned above, we suggest there might be a Moho offset across the Jiangshao fault which is represented by conductive layer C2. C2 is dipping northwestward with its frontal edge reaching the area of Jishou, which suggests that the underthrust Cathaysia block has reached this region in the upper mantle. To the west of Jiangshao fault, resistor R4 represents the root of the Xuefenshan Mountain. The eastern flank of the Xuefengshan Mountain marks the overthrusting frontier of the Yangtze block, as well as its southeastern boundary. To the east of Jiangshao fault, the Cathaysia crust is characterized by northwestward overthrusting above the conductive layer C2. Combining the northwestward overthrusting in the crust and underthrusting in its lithospheric mantle, we propose that the tectonic pattern of the northwestern boundary of the Cathaysia block can be summarized as wedging tectonics (Quinlan et al., 1993), which is characterized by a conductive layer C2 between the Yangtze and Cathaysia blocks wedging into the Cathaysia lithosphere at the depth range of Moho. Such tectonic pattern is generally regarded as a typical feature in continent-continent collision zones, and has been revealed by numerous previous studies (Wan, 2011). 6.

Conclusions Through this study, we come up with the following 4 points of conclusions:

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(1) A new ~1200 km long MT profile across the major portion of South China continent was acquired for the study of the dynamics of Yangtze block, Cathaysia block and Jiangnan orogen. Data analysis has shown good 2D characteristics. Accordingly, joint 2D inversions of TE and TM data were conducted on the dataset. (2) 2D inversion result shows that the lithospheric electrical structure of South China is generally resistive, which corresponds to the basic feature of a stable Precambrian tectonic setting. The electrical structure along the profile could be generally described as three resistive bulks (R1: the western Yangtze block, R2-R4: the Jiangnan orogen, and R5-R7: the Cathaysia block) separated by two major conductors (C1: the eastern Yangtze block, and C2: the Jiangshao fault). (3) The resistive western Yangtze block represents the stable, Archean aged cratonic region of the Yangtze basement, while the electrically conductive eastern Yangtze block is characterized by lithospheric shearing of the strike-slip fault system and extensional process that is probably caused by slab roll-back in the mantle transition zone at the frontal edge of a flatly subducted plate. (4) The lithospheric underthrusting structure between Cathaysia block and Yangtze block is revealed. The Jiangshao fault performs as a northwestward dipping conductive layer with its frontal edge reaching the area of Jishou, which suggests that the underthrust Cathaysia block has reached this region in the upper mantle. To the west of Jiangshao fault, eastern flank of the Xuefengshan Mountain marks the overthrusting frontier of the Yangtze block, as well as its southeastern boundary. To the east of Jiangshao fault, the northwestern boundary of the Cathaysia block displays the pattern of wedging tectonics,

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which is characterized by a conductive layer C2 between the Yangtze and Cathaysia blocks wedging into the Cathaysia lithosphere at the depth range of Moho.

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Acknowledgements This work was jointly supported by the grants from Project SinoProbe (SinoProbe-0204),Special Fund for Scientific Research in the Public Interest of the Ministry of Land and Resources (No. 201011043), National Natural Science Foundation of China (No. 41404060), and Fundamental Research Funds for the Central Universities (2652014016). The authors would like to express their gratitude to the anonymous reviewers for their constructive suggestions, as well as Prof. Alan G. Jones and Prof. Martyn J. Unsworth for their excellent codes that assist data processing and figure plotting.

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Rodi, W., Mackie, R.L., 2001. Nonlinear conjugate gradients algorithm for 2-D magnetotelluric inversion. Geophysics 66, 174-187. Shu, L.S., 2012. An analysis of principal features of tectonic evolution in South China Block (in Chinese with English abstract). Geological Bulletin of China 31, 10351053. Shu, X.-J., Wang, X.-L., Sun, T., Xu, X., Dai, M.-N., 2011. Trace elements, U–Pb ages and Hf isotopes of zircons from Mesozoic granites in the western Nanling Range, South China: Implications for petrogenesis and W–Sn mineralization. Lithos 127, 468-482. Wan, T., 2011. The Tectonics of China: Data, Maps and Evolution. Springer. Wang, F.-Y., Ling, M.-X., Ding, X., Hu, Y.-H., Zhou, J.-B., Yang, X.-Y., Liang, H.-Y., Fan, W.-M., Sun, W., 2011. Mesozoic large magmatic events and mineralization in SE China: oblique subduction of the Pacific plate. International Geology Review 53, 704-726. Wang, X.-L., Zhou, J.-C., Griffin, W.L., Wang, R.-C., Qiu, J.-S., O’Reilly, S.Y., Xu, X., Liu, X.-M., Zhang, G.-L., 2007. Detrital zircon geochronology of Precambrian basement sequences in the Jiangnan orogen: Dating the assembly of the Yangtze and Cathaysia Blocks. Precambrian Research 159, 117-131. Wang, X.-L., Zhou, J.-C., Qiu, J.-S., Zhang, W.-L., Liu, X.-M., Zhang, G.-L., 2006. LAICP-MS U-Pb zircon geochronology of the Neoproterozoic igneous rocks from Northern Guangxi, South China: Implications for tectonic evolution. Precambrian Research 145, 111-130.

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Zhang, C.-L., Li, H.-K., Santosh, M., 2013. Revisiting the tectonic evolution of South China: interaction between the Rodinia superplume and plate subduction? Terra Nova 25, 212-220. Zhao, G., Unsworth, M.J., Zhan, Y., Wang, L., Chen, X., Jones, A.G., Tang, J., Xiao, Q., Wang, J., Cai, J., Li, T., Wang, Y., Zhang, J., 2012a. Crustal structure and rheology of the Longmenshan and Wenchuan Mw 7.9 earthquake epicentral area from magnetotelluric data. Geology 40, 1139-1142. Zhao, J.-H., Zhou, M.-F., Yan, D.-P., Zheng, J.-P., Li, J.-W., 2011. Reappraisal of the ages of Neoproterozoic strata in South China: No connection with the Grenvillian orogeny. Geology 39, 299-302. Zhao, K.-D., Jiang, S.-Y., Yang, S.-Y., Dai, B.-Z., Lu, J.-J., 2012b. Mineral chemistry, trace elements and Sr–Nd–Hf isotope geochemistry and petrogenesis of Cailing and Furong granites and mafic enclaves from the Qitianling batholith in the Shi-Hang zone, South China. Gondwana Research 22, 310-324. Zhao, L., Allen, R.M., Zheng, T., Zhu, R., 2012c. High-resolution body wave tomography models of the upper mantle beneath eastern China and the adjacent areas. Geochem. Geophys. Geosyst. 13, Q06007. Zheng, J., Griffin, W.L., O'Reilly, S.Y., Zhang, M., Pearson, N., Pan, Y., 2006. Widespread Archean basement beneath the Yangtze craton. Geology 34, 417-420. Zhou, L., Xie, J., Shen, W., Zheng, Y., Yang, Y., Shi, H., Ritzwoller, M.H., 2012. The structure of the crust and uppermost mantle beneath South China from ambient noise and earthquake tomography. Geophysical Journal International 189, 1565-1583.

29

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30

Zhang Page 31 Fig. 1. Topography map showing major tectonic structures as well as MT station locations in the survey area. Red dots are MT stations. Inferred location of the Jiangnan orogen is after Shu (2012). Abbreviations are: QDO, Qinling-Dabie Orogen; SGB, Songpan-Ganze Block.

Fig. 2. Typical apparent resistivity and phase curves. Station locations are marked in Fig. 1.

Fig. 3. Niblett-Bostick penetration depth for all stations under the period of 500s calculated by using the maximum apparent resistivity value.

Fig. 4. Phase tensor maps at periods of (a) 0.1s, (b) 1s, (c) 10s, and (d) 100s.

Fig. 5. Strike analysis results at period ranges of (a) 0.01-0.1s, (b) 0.1-1s, (c) 1-10s, and (d) 10-100s.

Fig. 6. (a) 2D TE + TM inversion model and pseudo sections of (b) observed TE data, (c) calculated TE data, (d) observed TM data, and (e) calculated TM data

Fig. 7. (a) RMS distribution along the profile for (b) the original 2D inversion model and several forward modeling models with the extending depth of conductor C1 constrained at the depth of (c) 80 km, (d) 60 km, (e) 40 km, and (f) 20 km.

Zhang et al. South-China-MT.

Zhang Page 32 Fig. 8. (a) RMS distribution along the profile for (b) the original 2D inversion model and several forward modeling models with the upper boundary of resistors R5 and R6 constrained at the depth of (c) 60 km, (d) 80 km, and (e) 100 km.

Fig. 9. Tectonic interpretation of 2D inversion model. Moho depth after Xiong et al. (2009), Deng et al. (2011), and Zhou et al. (2012)

Zhang et al. South-China-MT.

Figure

Guilin

t ul Fa n ua Longyan

Zh

en

gh

B QDO

Ganzhou

104˚E

108˚E

112˚E

116˚E

Fa ul t

SG

Cathaysia Block

u

26˚N

690

-A

Hengyang

Jiangnan Orogen

Guiyang

ny

XuShaoyang

ult

ao Fa

sh Jiang

ap

e ef

ult

n

ha

s ng

a

li F

584

Nanchang

Changsha

eD

Jishou

Kai

Tibetan Plateau Kunming

ho

Jis

an

D

ult

a uF

gt

o ay

Yangtze Block

lt

au

F ng

in

in 432 ay u H Chongqing

Wuhan

i-Y

an

h gs

nx

Sichuan Basin

Qi

30˚N

Suining

Tu

Chengdu

yu es ha n En sh iF au lt

Lo ng m

en

sh

an

SGB

App. Rho (ohm-m)

Figure 10

3

102

10

1

RhoXY

180

Phase (deg)

Station 690

Station 584

Station 432

RhoXY

RhoYX

RhoXY

RhoYX

RhoYX

90 0 -90 -180

PhaseXY -2 10

10

-1

PhaseXY

PhaseYX 0 10

10

1

10

2

3 10

-2 10

10

-1

PhaseXY

PhaseYX 0 10

10

1

Period (sec)

10

2

3 10

-2 10

10

-1

PhaseYX 0 10

10

1

10

2

3 10

Figure 260 30˚

200 140 80 20

26˚

Depth (km) 104˚

108˚

112˚

116˚

Figure

(a) 0.1s

30˚

(b) 1s

1.92864146E−02

−2.9311037

0.16745575

0.25746062 −0.20801617 0.11859054 0.17401773 −3.36058042E−03

2.6267011 −0.85527217 −3.7847958 1.1720880

−0.19400944 −0.25328824

0.17366618 2.9332483

−2.1626847

−3.3321187 −0.34894720 1.1696824

−13.780168 3.2973995

4.8432183 0.32051829 −5.1172953

40

12.547780 2.2127066 1.3326221

−1.1233642 −2.9997029

0.77844787 4.6579957

−1.8738326 0.13000682 0.35566553 −0.19431439

0.59335673 −6.7487411 −6.6404462 0.74910355

−1.5877960 −0.18804219 −0.22426404

−3.6883998 −3.8073082 1.4962941

−21.403482

−18.393564

−1.4694539 −0.21292049

−4.9936433 −9.44575593E−02

−5.0263128 −7.7939548

−7.6985908 −10.854685 6.95488229E−02

−1.2646927

10.496898 13.164899 88.637985 −1.5146272

1.8639504 16.920555 −38.886662 2.55114455E−02

−3.9995446

−7.4605041

1.9171993 0.80773705

1.7508851 −5.3706694

−23.735901

12.560384 1.58922132E−02 −7.4667215

−3.3480334 −13.510159

−2.1529987

2.3479559 4.6625848

3.2156453 −1.3151976 0.12534875

1.9804798 4.3118901

1.6227131 5.23044010.41598502 −0.98056728

2.3226378 15.4088720.13547845 2.2651627

−0.26976797

7.2296414

−2.0037775 −0.26625803 1.9515083 −1.1523737 −3.4240057 0.52412415 7.9218602

−1.6314677 7.2082729 2.8828542 4.4169655 −3.3530064 −12.377735 −11.303626 −2.2602463 10.027490

20

−11.594300 5.6241488 −1.3456357 −1.0600483 −1.4394121 −0.76664090

−1.5238180 3.6515317 0.78277498 13.819706

0.20194416

5.9190145 4.5168042−2.5512338 −3.3072300

4.0147438−2.4173508 −2.7580512

−2.5367014 4.1788716

−1.7234807 6.6155720

1.6850886 0.36055803

2.5417531 −3.4122791

6.1173978

13.938784

−9.6963186

−10.442445 −1.22895017E−02 −2.0519843

4.3210921 1.6419052

−0.66309977 −6.12827204E−02

−3.3903291 −6.89965189E−02 −4.2136130 −2.1361210

−1.7639155 1.9155478

4.8743596

14.173257

−5.6367941 −0.72547281

26˚

−2.5264080 2.2659836

1.8103544 −5.7359419

5.3376646 −0.36317071 −0.33592057 −5.5697384

−1.5206212 −0.24668139

−4.3926930 4.9284267

5.0327582 4.7153211

6.9341683 9.6716595

11.551400 7.8655210

4.21137244.4436383 −4.0109968 3.8207731 −4.4556522

0.744469052.5350797 −5.1690760 2.8014009 −1.8878967 −6.2463083 −7.5051432

−0.36460096 3.6656621 −0.13126007 0.58825195

−0.83467400 2.0094867 0.73230928 11.087102

−9.6323566 7.1426926

10 (c) 10s 30˚

5

(d) 100s

−0.14289311

2

−2.8795002

0.27448535

−2.9577997

2.1638210 1.0412365 0.46442899 1.5817671

−3.3158679 −2.8947649 −2.7642817 −3.0342331

1.1855090 −0.75147063

−2.4711692 −2.9491417

1.7207688

−2.3945427

−4.5907512 1.7776242

2.9058897 0.71920389

9.1548853 2.4861400 4.0132618

10.196898 0.77764910 −0.72658348

−5.91889769E−02 3.6424108

0.85019642 −6.9215121

10.387658 0.33653760 −6.7051554 14.691814

−2.6851993 −4.5982218 −5.6285748 −2.9355094

−6.1082320 −9.7532187

−1.3731089 −10.107108

−12.929278

5.5174599

0.57416099 0.35195577

10.205444 6.1373134

−1.9948426 −14.494929

3.8055909 4.7500281

6.6763010

80.757217

6.01147348E−03 24.782955 27.061949 19.852821

17.684958 11.875873 −6.8320861 16.005926

1.3194420

1.2154847

−5.4242530

0.90361810

0.30996248

−37.679302

−2.2326574

−83.765121

2.2489753

14.394636

0.29605758

−15.041654

−14.666550 −5.0756898

−5.9945946 −1.5936433

−4.6230841 5.9049563 16.548611 −1.4122398

−3.3851721 1.9055939 4.5050364 −2.4391160

14.464087

−5.0634875

1.8947949 52.906311 −14.880983 −21.955891 −9.1617260 −9.8308916 3.0568504

0.91710234 −4.9959607 −1.3519046 −3.0067720 −5.1650538 −4.0711570 −8.7634315

−20.707069

−31.520823

2.5813398 79.372093 1.1621758 2.2342112

−6.4303460 −57.741421 −5.6040945 −17.090261

76.944725

22.181248

−4.7072673 −6.8163562 −0.62856764

−13.794231 −6.4172792 −3.9689484

−61.132969 −5.9557033 4.2972155 −6.1496840

7.6206765 −14.523499 5.2124243

6.13167174E−02

16.110010

14.028260

4.2363276

2.9884541 2.9467258

7.3729367 5.8858724

−4.1828299 −0.48274627

2.1051245 7.7100272

2.2246401 6.0560217

2.0618968 −1.5928353

11.454183

−10.939035 −8.4050369

26˚

3.0988796 3.1579750

0.14618374 −4.3858433

4.3871965 3.8772326

5.4519901 −2.8830140

4.2018824 3.0913391

3.4585965 1.8052690

53.581532 10.803289

−2.5877285 −1.8042930 −5.6342063 −9.0323629 −8.15710202E−02

1.6474346−1.2565730 −1.3723094 4.8225374 −1.0319664

−16.560324 −6.03087582E−02

−2.6530895 −5.4136629

0.44141191 −4.3416142

−1.9158058 −1.8657589

6.0342288 −5.8381696

104˚

108˚

112˚

−77.417900

−7.2342439 10.889277

−3.1078124 −2.1075699

116˚

−11.318576 3.7503545

104˚

108˚

112˚

116˚

skew angle β(°)

0

Figure

(b) 0.1-1s

(a) 0.01-0.1s 30˚N

N 26˚N

N 100 150

100 150

(d) 10-100s

(c) 1-10s 30˚N

N

N 26˚N

60 100

60 100

104˚E

108˚E

112˚E

116˚E

104˚E

108˚E

112˚E

116˚E

Figure

-50

R1

C1

-100 300

400

500

600

R3

R2

R4

800

1000

10 10 10 10

Period (s)

10 10

10 10 10 10 10 10

738

716

700

682

670

1100

1200

1300

10 1

1400

Resistivity (Ohm.m)

Distance (Km) (b) Observed TE data

100

(a) RMS=2.603

R6

900

1000

R7

C2 R5

700

654

638

620

606

596

582

552

516

488

466

436

412

386

362

0

(c) Calculated TE data

Apparent Resistivity (Ohm.m) 1000

-2 -1 0

100

1 2

10

3

1 Phase (Degrees)

-2 -1

90

0

70

1

50

2

30

3

500

1000

1500

(d) Observed TM data 10 10 10 10 10

Period (s)

Depth (Km)

344

SE

10

10 10 10 10 10 10

500

1000

1500

10 Apparent Resistivity (Ohm.m) 1000

(e) Calculated TM data

-2 -1 0

100

1

10

2 3

1

-2

Phase (Degrees)

-1

90

0

70

1

50

2

30

3

10

500

1000

1500 Distance (Km)

500

1000

1500

Figure

8

model (b) model (c) model (d) model (e) model (f)

RMS

6 4

(a)

2

738

716

700

682

670

654

638

620

606

596

582

552

516

488

466

436

412

386

362

344

0

0 -50

C1

-100

(b) RMS=2.603

0 -50 (c) RMS=2.626

Depth (m)

-100 0

Resistivity (Ohm.m)

1000 100

-50 (d) RMS=2.631

-100 0

10 1

-50 (e) RMS=2.673

-100 0 -50

(f) RMS=2.713

-100 300

400

500

600

700

800

900

Distance (Km)

1000

1100

1200

1300

1400

Figure

RMS

6

model (b) model (c) model (d) model (e)

4

(a)

2

738

716

700

682

670

654

638

620

606

596

582

552

516

488

466

436

412

386

362

344

0

0 -50

C2 R6

R5

-100

(b) RMS=2.603

0

1000

-50 Depth (Km)

Resistivity (Ohm.m)

(c) RMS=2.618

-100 0

100 10

-50 (d) RMS=2.651

-100 0 -50

(e) RMS=2.687

-100 300

400

500

600

700

800

900

Distance (Km)

1000

1100

1200

1300

1400

1

Depth (Km)

-50

Fa u

an Ji

Tu

an

nx

gs

ha

o

sh ng fe

Xu e

Ka i

li

Fa u Fa lt ul t

ul t sh Ji

ay

on

ou

g

Fa

ul t D

Fa hi

Basin|——————————|——————|——————|—————|———|————|——————-|————————————————————————————|———|

|——————————————Yangtze

0

En s

Q

H

|Sichuan

iy

ua

ue

yi

sh

ng

an

sh

an

lt

i-Y in g Fa tan ul -A t n

yu

an

Figure

Suining

Moho

Block————————————|———Jiangnan Orogen———|——————————Cathaysia Block———————————|

Chongqing

R1

C1

-100 300

400

Shaoyang Hengyang

Jishou

500

600

R2

R3

R4

800

Distance (Km)

900

1000

R7

R6

1000

Resistivity (Ohm.m)

100

C2 R5

700

Ganzhou

SE

1100

10

1200

1300

1400

1

Depth (Km)

-50

Fa u

an Ji

Tu

an

nx

gs

ha

o

sh ng fe

Xu e

Ka i

li

Fa u Fa lt ul t

ul t sh Ji

ay

on

ou

g

Fa

ul t D

Fa hi

Basin|——————————|——————|——————|—————|———|————|——————-|————————————————————————————|———|

|——————————————Yangtze

0

En s

Q

H

|Sichuan

iy

ua

ue

yi

sh

ng

an

sh

an

lt

i-Y in g Fa tan ul -A t n

yu

an

Graphical Abstract (for review)

Suining

Moho

Block————————————|———Jiangnan Orogen———|——————————Cathaysia Block———————————|

Chongqing

R1

C1

-100 300

400

Shaoyang Hengyang

Jishou

500

600

R2

R3

R4

800

Distance (Km)

900

1000

R7

R6

1000

Resistivity (Ohm.m)

100

C2 R5

700

Ganzhou

SE

1100

10

1200

1300

1400

1

Highlights l

New magnetotelluric data were collected along a 1200km long profile in South China.

l

2D inversion shows that the lithosphere of South China is generally resistive.

l

Cratonic western Yangtze is resistive, while eastern Yangtze is conductive.

l

Lithospheric underthrusting of Cathaysia block beneath Yangtze block is revealed.

l

Northwestern boundary of the Cathaysia block displays pattern of wedging tectonics.