The Subduction and Continental Collision of the North China and Yangtze Blocks: Magnetotelluric Evidence from the Susong-Anqing Section of Western Anhui, China
1 SinoProbe Center-China Deep Exploration Center, Chinese Academy of Geological Sciences, Beijing 100037，China 2 Hebei GEO Unoversity, Hebei, 050031, China 3 Geological Exploration Technologies Institute of Anhui Province, Hefei, 230031, China 4 China University of Mining and Technology, Xuzhou, 221116, China * Corresponding author: Qingtian Lü E-mail address:
[email protected]. Address: 26 Baiwanzhuang Street, Xicheng, Beijing, China, 100037
Abstract: In order to investigate the subduction and continental collision of the North China and Yangtze blocks, two magnetotelluric profiles were obtained across the Dabie Orogenic Belt, the Lower Yangtze Depression, and the Jiangnan Orogenic Belt in the central section (Susong-Anqing section) of the middle-lower Reaches of the Yangtze River in China. After data processing and inversions, we obtained electrical models of the crust and upper mantle. The prominent feature revealed by the inversion is an extensive, arched conductive layer that extends from the middle-lower crust to the upper mantle. To the southeast of this layer, another wedge-shaped conductor is located beneath the Lower Yangtze Depression and the Jiangnan Orogenic Belt. In addition, several resistors, which are distributed from the lower crust to the upper mantle, were also revealed by these lines. These resistors are beneath the conductive layer and separated by vertical conductive bands. Based on these electrical structures, we identified several major faults, including the Tanlu Fault in the eastern part of the Dabie Orogenic Belt, the Yangtze Deep Fault, and the main thrust fault in the Lower Yangtze Depression, which are middle-upper crustal faults. In addition, a “Crocodile” structure was revealed by the major faults in the depression, which are
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Kun Zhang1, Qingtian Lü*,1, Jiayong Yan1, Lusen, Shao2, Dong Guo3, YaWei Zhang4
connected by a middle-lower crustal detachment and the surrounding resistive strata. Based on the different electrical structures of the three belts and the results of previous studies, we conclude that subsequent to the slab subducting toward the North China Block during
into three stages. In the first stage, the weak layer and the Yangtze Fold Belt formed during subduction of the Yangtze Block beneath the North China Block, and the Dabie Orogenic Belt formed during the collision process. In the second stage, the slab buckled as deep material upwelled, and the “Crocodile” structure formed due to the weak layer. In the third stage, the breakup and sinking of the slab caused the Yangtze Fold Belt to subside. Keywords: magnetotelluric, subduction-collision, middle-lower reaches of the Yangtze River Metallogenic Belt, three-dimensional inversion model
1 Introduction The middle-lower reaches of the Yangtze River Metallogenic Belt (MLRYMB) is an area of mineralization in South China Block (SCB, which contains Yangtze Block and Cathaysia Block) (Mao et al., 2014) with a complex basement structure, which is referred to as a “polybasement with one cover” (Chang et al., 1996). Frequent large-scale magmatism resulted in a variety of diagenetic and mineralization stages (Lü et al., 2015), creating seven major ore districts and more than 200 polymetal deposits in the region (Mao et al., 2006).
In the MLRYMB, many studies (e.g., Xing et al., 1995; Zhang et al., 2003; Wang et al., 2004; Hou et al., 2007; Zhou et al., 2016) of adakitic rocks and A-type granites have agreed that the magmatic material had a mantle source, which interacted with the lower crust during its ascent. It indicates that the preceding and contemporaneous regional tectonic processes influenced the changes in the deep structure and the tectonic evolution of the Lower Yangtze Depression (LYD, tectonic name for the MLRYMB). The occurrence of strong intracontinental deformation and the formation of the nappe structure in the LYD from 175– 123 Ma (Song et al., 2010) was immediately followed by the subduction and continental collision of the North China Block (NCB) and the Yangtze Block (YGB) from 240–155 Ma (Yang and Yu, 2001). Thus, the dynamics mechanism of the YGB (contains LYD and Jiangnan Orogenic Belt) is attributed to continental collision (Jiang et al., 2013), which strongly influenced the Mesozoic tectonic evolution of the LYD through changes in the deep upper
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the Triassic, the subduction-collision process that occurred in the study area can be divided
mantle structure (Yang and Yu, 2001) and changes in the movement of the deep material (Huang et al., 2003; Priestley et al., 2006).
for the tectonic properties (not including the debates on tectonic chronology). (1) After the ancient Tethys Ocean closed in the Indosinian (250-200 Ma), the YGB subducted northward and collided with the NCB, which led to the formation of the Qinling-Dabie-Sulu Orogenic Belt (Ye et al., 2000; Yin and Nie, 1993) and northeast conversional shearing in the northern YGB (Li, 2013). (2) The YGB subducted towards the Dabie Orogenic Belt (DOB), peaking at 240–225 Ma (Li et al., 2000; Zheng et al., 2006), and then, the exhumation of the subducted slab and its subsequent detachment resulted in the uplift of previously formed ultra-highpressure metamorphic (UHP) rocks and the DOB, which reached the crust at 170 Ma (Dong et al., 1994). Then, the detached slab sank into the asthenosphere, which disturbed the upper mantle and produced magmatism (Wang et al., 2003).
Although some studies have shown that the collision between the NCB and YGB ended during the Middle–Late Jurassic (Gilder and Courtillot, 1997) or during the Early Cretaceous (Yokoyama et al., 2001), it is generally accepted that it occurred from the Indosinian to the Early Yanshanian (Zhang et al., 2009), and a large number of studies suggest that the collision was a complicated, long-term process. Magnetotelluric (MT) data is very useful in interpreting deep structures and in subduction-collision studies (e.g., Yin et al., 2014; Zhang et al., 2015) because of its lateral resolution and the range of depths which could be observed. Xu et al (2016) investigated the fossil Paleozoic intra–oceanic subduction zone in western Junggar (northwestern China) using the MT inversion model. Yin et al (2017) studied the fossil oceanic subduction beneath the western margin of the Trans-North China Orogen using MT data. Tang et al (2013) studied the deep electrical structure and geological significance of the Tongling Ore District (in southern China) using the MT electrical model. To better understand the deep structure of the LYD and the subduction-collision of the NCB and YGB, we created two MT profiles in the central section (Susong-Anqing section) of the MLRYMB, across the Dabie Orogenic Belt, the Lower Yangtze Depression, and the Jiangnan Orogenic Belt (JNOB) shown in Fig. 1, and we obtained regional deep geo-electrical models. Figure 1 here
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In regards to the subduction-collision of the NCB and YGB, there are two major viewpoints
2 Geologic Setting The Archean-Proterozoic basement in the MLRYR is covered by uniform Sinian strata. The stable continental surface marine carbonate rocks developed during the Sinian-Silurian lack
formed by multiple transitions between uplift and depression (Chang et al., 1996). Oceancontinent sedimentary facies formed in the Middle Triassic, followed by massive folding, uplift, and the deposition of continental facies clastic sedimentary strata in the Middle Jurassic (Chang et al., 1996; Mao et al., 2011). From the Late Jurassic to the Early Cretaceous, massive magmatic events formed the Anqing volcanic basin (Zhou et al., 2011), which was followed by a large-scale mixed magmatic intrusion. In addition, from northwest to the southeast, the northeast striking distributed strata have an age distribution pattern of oldnew-old (Fig. 1), with distributed Precambrian basement on both sides (Dabie and Jiangnan Orogenic Belts) and Paleozoic-Cenozoic sedimentary strata in the middle (LYD).
After the compressional and extensional events of the Sipu Orogeny, multiple uplift and subsidence events occurred from the Paleozoic to the Early Mesozoic (Chang et al., 1996). The change in the tectonic regime from EW-trending continental collision to NE-trending subduction was closely related to the mineralization that occurred from the Middle Jurassic to the Early Cretaceous (Zhang et al., 2009) when a series of upper crustal fold-thrust-nappe structures were formed in a compressional environment. Then, the ramp structure developed with the Yangtze Fault (CJF) and the main thrust fault (MTF) (NW dipping to the west, SE dipping to the east) as its center (Lü et al., 2015). In the later extensional stage, the CJF and MTF were converted into normal faults (Lü et al., 2015).
3 Data Processing and Analysis 3.1 Data processing Broadband MT data was collected by 142 stations (in two lines) in the MLRYR and its adjacent areas with site spacings of 2–5 km using Phoenix MTU-CAN instruments. The time series data contained two orthogonal lateral electrical components of the electromagnetic signal (Ex and Ey; x is north; y is east) and three orthogonal magnetic components (Hx, Hy, and Hz). The data were converted into the frequency domain and the impedance data were
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Lower-Middle Devonian and Lower Carboniferous strata, and multiple unconformities were
calculated using remote-reference techniques for de-noising and robust estimation (Simpson et al., 2005), which is accurate within a frequency range of 0.0005–320 Hz. The corresponding skin depth (Jones et al., 1983) suggests a probing depth range from the
Despite several deviant data points and a weak static shift in the apparent resistivity (ρ=|Z|2/ωμ) and the impedance phase (P=atan[Zima/Zreal]) curves, the data profiles generally exhibited good quality and coherent features. In addition, a de-noising (Zhang, 2012a) method was used to select the data automatically, which prevented human error and improved the quality. The data not fitted by the 1D inversion were deleted, leaving only the data that could be fitted (for details see Appendix A). On this basis, all of the deviant data was removed to achieve a continuous and smooth curve. Then, we carried out another data selection process manually for the error analysis of the data processing program.
3.2 Data analysis (1) Dimensionality analysis The phase tensor analysis proposed by Caldwell et al. (2004) was used in this paper to determine the dimensionality and strike of the MT data (the computation was provided by Hao Dong, China University of Geosciences, for details see Appendix B). The phase tensors of all of the frequencies and sites for the two MT lines are shown in Figure 2. The current flow direction is shown by phase tensor ellipses, which fill with skew values (|β|) to indicate the data’s dimensionality (the dimensionality can be considered as 1D or 2D when |β|< 3; Zhang et al., 2015; Yin et al., 2017). The major and minor axes of the ellipse and their directions indicate two orthogonal electrical (tectonic) strikes if the ellipse is not a circle.
In Figure 2a, the ellipses of line An show weak polarization and have relatively low skew values in the high-frequency range (>1 Hz), which indicates a shallow crust (about 3) for most of the line, except for the northwestern part, and the direction of the major axes changes to a NE-NW-
reveals the tectonic relationship between the LYD and JNOB at depth.
In Figure 2b, the ellipses in the central-southeastern section of line SP exhibits characteristics similar to those of the northwest-central section of line An. However, the additional information on the LYD and southeastern DOB indicate a northeastern strike (major axes of ellipses) for the suture of the two belts (line distance about 30 km, tectonic boundary between the DOB and LYD) with relatively high skew values, indicating a 3D structure. In addition, the change in tectonics between the DOB and LYD at depth is represented by changes in the skew values of the low frequency data (
