Earth and Planetary Physics RESEARCH ARTICLE 4: 317–328, 2020 SOLID EARTH: SEISMOLOGY doi: 10.26464/epp2020026

3-D shear wave velocity structure in the shallow crust of the Tan-Lu fault zone in Lujiang, Anhui, and adjacent areas, and its tectonic implications

Cheng Li1, HuaJian Yao1,2*, Yuan Yang1, Song Luo1, KangDong Wang1, KeSong Wan1, Jian Wen1, and Bin Liu1,2 1School of Earth and Space Sciences, University of Science and Technology of China, Hefei 230026, China; 2Mengcheng National Geophysical Observatory, University of Science and Technology of China, Hefei 230026, China Key Points: ● Ambient noise tomography reveals high-resolution shallow crustal structures of the Tan-Lu fault zone in Lujiang ● Strong velocity anomalies exist in the Hefei basin, the Tan-Lu fault zone, and the Dabie orogenic belt ● The high-speed intrusive rocks may come from the Luzong volcanic rock basin through the fractured fault zone Citation: Li, C., Yao, H. J., Yang, Y., Luo, S., Wang, K. D., Wan, K. S., Wen, J., and Liu, B. (2020). 3-D shear wave velocity structure in the shallow crust of the Tan-Lu fault zone in Lujiang, Anhui, and adjacent areas, and its tectonic implications. Earth Planet. Phys., 4(3), 317–328. http://doi.org/10.26464/epp2020026

Abstract: The Tan-Lu fault zone is a large NNE-trending fault zone in eastern China. Investigations of the structures of the fault zone and its surrounding areas have attracted much attention. In this study, we used dense-array ambient noise tomography to construct a three- dimensional shear wave velocity model of shallow crust in an area about 80km × 70km in Lujiang, Anhui Province, eastern China. For approximately one month we collected continuous ambient noise signals recorded by 90 short-period seismographs in the region, and obtained the short-period Rayleigh wave empirical Green's functions between stations by the cross-correlation method; we also extracted 0.5–8 s fundamental mode Rayleigh wave group velocity and phase velocity dispersion curves. Based on the direct surface wave tomography method, we jointly inverted the group velocity and phase velocity dispersion data of all paths and obtained the 3-D shear wave velocity structure in the depth range of 0–5 km. The results revealed important geological structural features of the study area. In the north region, the sedimentary center of the Hefei Basin — the southwestern part of the Chaohu Lake — shows a significant low-velocity anomaly to a depth of at least 5 km. The southwestern and southeastern regions of the array are the eastern margin of the Dabie orogenic belt and the intrusion area of Luzong volcanic rocks, respectively, and both show obvious high-speed anomalies; the sedimentary area within the Tan-Lu fault zone (about 10 km wide) shows low-velocity anomalies. However, the volcanic rock intrusion area in the fault zone is shown as high velocity. Our shallow crustal imaging results reflect the characteristics of different structures in the study area, especially the high-speed intrusive rocks in the Tan-Lu fault zone, which were probably partially derived from the magmatic activity of Luzong volcanic basin. From the Late Cretaceous to Early Tertiary, the Tan-Lu fault zone was in a period of extensional activity; the special stress environment and the fractured fault zone morphology provided conditions for magma in the Luzong volcanic basin to intrude into the Tan-Lu fault zone in the west. Our 3-D model can also provide important information for deep resource exploration and earthquake strong ground motion simulation. Keywords: Tan-Lu fault zone; Lujiang of Anhui; ambient noise tomography; shallow crust structure; intrusive rocks

1. Introduction of several nearly parallel faults, which are mainly of strike-slip and reverse types. At present the fault zone maintains a steady overall The Tan-Lu fault zone has experienced multiple tectonic move- slip rate of about 2 mm/a (Hou MJ et al., 2006). In many of its re- ments and is the greatest large-scale fault zone in east China; its gions earthquakes are still active. In the history of China the great length within Chinese territory exceeds 2400 km and its width var- Tancheng earthquake occurred in this fault zone in 1668, with a ies from tens to more than 200 km, traversing the North China magnitude as high as 8.5, resulting in severe human casualty and Block, the Yangtze Block, and the Qinling–Qilian–Kunlun Fold Belt economic loss. Based on seismicity, regional geology, and tecton- (Xu JW et al., 1985; Zhu G et al., 2004). The fault zone is composed ic evolution, the fault zone can be divided into three distinct seg-

ments: northern, central, and southern (Wang XF et al., 2000). Correspondence to: H. J. Yao, [email protected] Received 19 AUG 2019; Accepted 23 DEC 2019. The present study region is located near Lujiang County, Anhui Accepted article online 13 MAR 2020. Province, at the south end of the near-NS-trending Tan-Lu fault

©2020 by Earth and Planetary Physics. and to the south of Hefei Basin. To its southwest is the Dabie oro-

318 Earth and Planetary Physics doi: 10.26464/epp2020026 genic belt and to its southeast is the Luzong volcanic rock basin (Figure 1). al., 2012), short-period array ambient noise tomography for the The Dabie orogenic belt is a near east-west-oriented collisional shallow crustal structure of the Feidong segment of the Tan-Lu orogen that separates the North China craton and the Yangtze fault zone (Gu N et al., 2019), the controlling effect of fault belt on craton; it was formed under NS compression in the Indosinian- the Hefei Basin (Liu GS et al., 2002, 2006; Lu GM et al., 2002; Song Early Yanshanian. In the 1980s, after the discovery of coesite-bear- CZ et al., 2003), the evolution and geologic features of the Luzong ing eclogite and widespread high pressure and ultra-high pres- volcanic rock basin (Yuan F et al., 2008; Dong SW et al., 2009; Tang sure metamorphic rocks, the Dabie orogenic belt became the JF et al., 2010), and velocity structure tomography of the shallow world’s largest experimental field for studying continent-contin- and deep structures in the middle-lower Yangtze River metallo- ent collision and plate exhumation (Zheng YF et al., 2003; Wang genic belt (Ouyang LB et al., 2015; She YY et al., 2018; Tian XF et YS et al., 2004). The Luzong volcanic rock basin is located in the al., 2018; Luo S et al., 2019). These previous studies have concen- middle-lower Yangtze River depression belt at the northern bor- trated primarily on geochemical topics or characteristics of the re- der of the Yangtze craton. In this basin, volcanic rocks generally sources in some mining areas, and on fine structure exploration derived from the magmatic activities in the Early Cretaceous are and interpretation of the top few hundred meters in a very small widely distributed. These magmas are responsible for the region’s area (e.g., Tao SZ and Liu DL, 2000). Generally speaking, there is a rich metal mineral resources and geothermal resources (Tang JF lack of fine regional upper crust geophysical modeling, which et al., 2010). The Hefei Basin is an area about 20,000 km2 located in hampers in-depth study of structural details inside the tectonic the middle of Anhui Province; it belongs to the southern North block and its evolution. Because our chosen study region is at the China craton. The formation and evolution of Hefei Basin have junction of areas that have received considerable previous re- been controlled primarily by the Dabie orogenic belt and the Tan- search attention, results of this study can provide new details use- Lu fault zone (Lu GM et al., 2002). Our chosen study region is situ- ful in improving understanding of the shallow crustal structure of ated at the junction of these extensively studied areas. the region and the juncture configuration and relationship of neighboring tectonic blocks, and may be of use in exploration of Previous surface geological investigations and studies have fo- deep resources. cused mainly on the following subjects: Dabie orogen (Wang YS et al., 2004, 2018), Hefei Basin (Liu GS et al., 2006), the tectonic Since development of a method for recovering the empirical characteristics and evolution history of different segments of the Green’s functions of surface waves between two stations from the Tan-Lu fault zone on a relatively large scale (Chen et al., 2007; Hou cross-correlation of ambient noise (Shapiro and Campillo, 2004; MJ et al., 2006; Huang Y et al., 2011; Gu QP et al., 2016; Zhang JD Shapiro et al., 2005; Yao HJ et al., 2006; Fang LH et al., 2010), some et al., 2010; Zhao T et al., 2016), surface wave phase velocity and studies have also discussed the generation mechanisms of sur- shear wave velocity models beneath the Dabie orogen (Luo YH et face waves of relatively low frequency (e.g., Stehly et al., 2006;

32°N

HF ZBL

NCC C TLF D’ CH DBO HFB YZC A B E’ LJ F’ D C’ DBO B’ E A’ 31°N TLF F LZB

CJ

116°E 117°E 118°E

Figure 1. The main geological units and station distribution in the study area. The black triangles represent station locations, the blue asterisk represents a borehole location of Shaxi Copper Mine, and the red solid line represents six profiles of AA', BB', CC', DD', EE', and FF', respectively. The main geological units and geographic sites include: HFB for Hefei basin, TLF for Tan-Lu fault zone, DBO for Dabie orogenic belt, LZB for Luzong volcanic basin, ZBL for Zhangbaling uplift, HF for Hefei, LJ for Lujiang, CH for Chaohu Lake, CJ for Yangtze River, NCC for North China craton, YZC for Yangtze Craton.

Li C and Yao HJ et al.: Shallow crustal structure of the Tan-Lu fault zone in Lujiang

Earth and Planetary Physics doi: 10.26464/epp2020026 319

Yang YJ and Ritzwoller, 2008) and high frequency (e.g., Frank et 2018). Then we cross-correlated the data in the same frequency al., 2009; Picozzi et al., 2009). In the meantime, the ambient noise band of each day for all station pairs, and added together the tomography technique has been applied to many geophysical cross-correlation functions of different days of the same station problems of different scales. pair, finally obtaining the cross-correlation functions (CCFs) of that station pair in the frequency band of 0.5–10 s. Figure 2a shows the Because of the dispersion characteristics of surface waves, relat- cross-correlation functions with signal-to-noise ratio (SNR) great- ively long-period surface waves are more sensitive to deep struc- er than 5 by taking one station as the virtual source. The slope tures, and thus can be used to recover the shear wave velocity between the red line and blue line indicates that the group velo- structures of the crust and the uppermost mantle in different re- city of surface waves is about 2–3 km/s. The amplitudes of the left gions (e.g., Yao HJ et al., 2008; Luo YH et al., 2012; Qiao L et al., and right branches of the cross-correlation functions have little 2018). High frequency surface waves decay very fast, but they are difference, indicating that the distribution of noise sources is ba- sensitive to shallow velocity structures and, therefore, can be used sically uniform with azimuth. We calculated the ratios of the max- to recover the shallow crust and near surface velocity structure in imum amplitudes of the positive and negative branches of the su- small areas, such as of a city, of gas storage, or a location of ore perimposed cross-correlation functions and got the relative amp- concentration (Fang HJ et al., 2015; Li C et al., 2016; Liu Y et al., litude distribution of noise sources of that station pair with azi- 2018; Wang JJ et al., 2018). The ambient noise tomography meth- muth. After taking all the stations as the virtual source in the cal- od has the advantages of convenience, safety, low cost, and high culation, the resulted noise source distribution of most station resolution, so it is widely used to explore the interior structure of the earth. 80 (a) The present study utilizes continuous waveform data of a short period dense array deployed in 2015 in the Lujiang region of An- 70 hui Province. We apply the ambient noise cross-correlation meth- 60 od to get the cross-correlation functions of station pairs, then re- trieve the short period Rayleigh wave dispersion curves from the 50 stacked cross-correlation functions, and invert for the fine 3-D shear wave velocity structure of the shallow crust. Finally, we dis- 40 cuss the characteristics of shallow crust structure in each part of 30 the study region, particularly the characteristics and generation mechanisms of volcanic intrusive rocks in the Tan-Lu fault zone Station distance (km) 20 and their tectonic implications. 10

2. Data and Method 0 −50 −40 −30 −20 −10 0 10 20 30 40 50 2.1 The Collection of Data Time (s) In order to detect the multi-scale structure of crust and the deep- 2.0 er part of the middle-lower Yangtze River metallogenic belt, the (b) China Earthquake Administration and other institutions carried out the “Yangtze River experiment” with a new type of large 1.5 volume air gun in the Anhui section of the Yangtze River in Octo- ber of 2015 (Chen Y et al., 2017). To complement this experiment, 1.0 we deployed a fan-shaped array consisting of 90 three-compon- ent short-period seismographs in an area about 80km × 70km in Amplitude ratio 0.5 the southern segment of the Tan-Lu fault zone near Lujiang County. The observation instruments included 50 REFTEK130- 0 L22E and 40 CMG-40T-1 short period seismographs. The inter-sta- −2.0 −1.5 −1.0 −0.5 0 0.5 1.0 1.5 2.0 Amplitude ratio tion distance was 2–8 km (Figure 1). We collected the continuous ambient noise signals about one month, from October to Novem- Figure 2. (a) Cross-correlation functions from a virtual “source” ber of 2015, with a sampling frequency of 200 Hz. station to all other stations (arranged according to the inter-station distance). The red line and blue line represent the velocity of 3.5 km/s 2.2 Data Processing and 1.5 km/s respectively. The surface wave signals concentrate We first pre-processed the data (Bensen et al., 2007). The vertical mainly in the velocity window of 1.5–3.5 km/s; (b) the azimuth component data were cut into one-day long pieces, re-sampled to distribution of the relative amplitude of the noise source, in which the 25 Hz, and corrected for instrument response; after de-meaning, radial value of the red dot represents the ratio of the maximum de-trending, and spectral whitening, the data were filtered into amplitude of the wave packet of the positive and negative branches three frequency bands of 0.5–2 s, 2–5 s, and 5–10 s, and were nor- of the cross-correlation function for a single station pair, and the malized in the time domain and superimposed to obtain a nor- azimuth of the red dot shows the azimuth angle of the station pair (from –90° to 90°). malized waveform in a wider frequency band (Zhang YY et al.,

Li C and Yao HJ et al.: Shallow crustal structure of the Tan-Lu fault zone in Lujiang

320 Earth and Planetary Physics doi: 10.26464/epp2020026 pairs is relatively uniform (Figure 2b). 3. Inversion Result Then we adopted the method of Yao HJ et al. (2006, 2011) to ex- 3.1 Data Residual Distribution and Ray Path Distribution tract from these cross-correlation functions the Rayleigh wave After inversion the standard deviation of the surface wave travel group and phase velocity dispersion curves in the period range time residuals decreases from 2 s to 1.2 s (Figure 4a), and the aver- 0.5–8 s, under the conditions that the SNR is greater than 5 and age value of the residuals is also reduced from the initial 0.81 s to the interstation distance is greater than 2 times the wavelength. 0.009 s; the more concentrated distribution of the residuals Figure 3 shows the path number of dispersion curves at different (Figure 4b) indicates that the model is quite well constrained by periods. The number of dispersion data reaches maximum at 3–4 s the data. We calculated the depth sensitive kernels below the and becomes fewer when the period is greater than 5 s and less central grid for the group (Figure 5a) and phase (Figure 5b) velo- than 2 s. cities of three different periods of 1 s, 3 s, and 5 s. The number of dispersion curves reaches its maximum between 3 s and 4 s, and 2.3 Inversion Method the most sensitive depth corresponding to 3 s is approximately We used the direct inversion method of surface wave travel times 3 km. Therefore, we can roughly infer that the resolution is best at based on ray tracing (Fang HJ et al., 2015, Fang HJ and Zhang HJ, a depth of about 3 km. Figure 6 shows the ray path distribution of 2014) to invert for the 3-D shear wave velocity structure of shal- Rayleigh wave phase velocity measurements at the periods of 1 s, low crust in the Lujiang array area. This method bypasses the in- 3 s, 5 s, and 8 s. In the inversion of this study we used both group version of 2-D group and phase velocity distributions of various and phase velocity data. For extracting the phase velocities in the periods and is able to consider the effect on the surface wave high-frequency band, the group velocities were taken as refer- tomography of ray bending in a complex medium. For the for- ence. Therefore, the group and phase velocity data basically over- ward calculation, this method adopts the fast marching tech- lap in the period band, so their ray path distributions are also sim- nique to calculate travel times and ray paths of the surface waves ilar. Because the structure in the study region is quite complex, of various periods (Rawlinson and Sambridge, 2004). the velocity varies rapidly, so the ray paths are obviously bent (Figure 6). Furthermore, it can be seen that in the northern part of In the iterative inversion, we jointly used all the observed group the array, which is close to the Hefei Basin with tremendously and phase travel times of the surface waves of different paths and thick sediments, the seismic waves are strongly attenuated; short frequencies and obtained the 3D shear wave velocity structure of period surface wave signals (e.g., 1 s) of relatively high SNR can- the study region; in the inversion the density and P-wave velocity not be retrieved at many stations, therefore the dispersion data in are derived from the shear velocity by empirical formulas this area are fewer. Near the Dabie orogeny in the southwest, (Brocher, 2005); for the detailed inversion method and procedure however, the bedrock is outcropped, the medium is of high see Fang HJ et al. (2015) and Li C et al. (2016) strength and low attenuation, and the short period surface wave Because the Rayleigh wave phase velocity is mostly sensitive to signals are relatively strong, resulting in a large number of disper- the shear wave structure at the depth about 1/3 of the sion data. wavelength, we calculated the average shear wave velocity at the approximate depth corresponding to each period based on the 3.2 3-D Shear Wave Velocity Structure phase velocity dispersion curve, and by interpolation we ob- We carried out the joint inversion of all group and phase velocity tained a 1-D initial model for inversion (Fang HJ et al., 2015). In the dispersion data and obtained the 3-D shear wave velocity struc- initial model there are 18 grid nodes in the NS and EW directions, ture of the shallow crust from surface to 8 km depth. Figure 7 respectively, and the grid interval is 0.05°; in the depth direction shows the shear wave velocity structures at four depths: 1 km, there are 29 nodes, the interval is 0.2 km from 0 to 4 km below the 2 km, 3 km, and 5 km; Figure 8 shows the shear wave velocity surface, and 0.5 km from 4 to 8 km depth. structures along 6 vertical profiles marked in Figure 1. The tomo-

5 5 (a) Dispersion curve (b) Dispersion curve 800 Data number 1000 Data number 4 4 800 600 3 3 600 400 2 2 Data number 400 Data number Phase velocity (km/s) Group velocity (km/s) 200 1 200 1

0 0 0 0 1 2 3 4 5 6 7 8 9 10 1 2 3 4 5 6 7 8 9 10 Period (s) Period (s)

Figure 3. (a) Group velocity and (b) phase velocity dispersion curves and data numbers. The solid line represents the dispersion curve and the dotted line represents the variation of data number with period.

Li C and Yao HJ et al.: Shallow crustal structure of the Tan-Lu fault zone in Lujiang

Earth and Planetary Physics doi: 10.26464/epp2020026 321

2.5 4000 (a) Last: mean—0.009 s (b) 3500 Original: mean—0.81 s 3000 2.0 2500 2000 1500 1.5 Path number 1000 500 Standard deviation of residuals (s) 1.0 0 1 2 3 4 5 6 −15 −10 −5 0 5 10 Iteration number Travel time residuals (s)

Figure 4. (a) Decrease of the standard deviation of surface wave travel-time residuals with iteration number; (b) distribution of travel-time residuals before and after inversion; blue represents the initial distribution of data residuals; red represents the distribution of data residuals after iterative inversion.

0 0 (a) 1 s (b) 1 s 1 3 s 1 3 s 5 s 5 s 2 2

3 3

4 4 Depth (km) 5 Depth (km) 5

6 6

7 7

8 8 −0.2 0 0.2 0.4 0.6 0.8 1.0 −0.10 −0.05 0 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 Senstivity Senstivity

Figure 5. Depth sensitive kernels to shear wave velocity for Rayleigh wave group velocities (a) and phase velocities (b) of three different periods of 1 s, 3 s, and 5 s. graphic result correlates quite well with the geological and geo- vious. The structure in entire region is quite complex; the lateral morphological background of the study area. The result shows variation of shear wave velocity exceeds 50%, leading to obvious that in the sedimentary basin area (area O) near the Chaohu Lake bending of short period surface wave ray paths (Figure 6). The in the northwestern part of the study region the shear wave velo- profiles in Figure 8b coincide basically with the profiles derived city from surface to 5 km depth is very low (1.5–2.4 km/s), and the from the first arrival travel time tomographic study by Liu ZD et al. velocity increases with depth. To the north of this area is the (2012). The two results also confirm each other: from west to east Chaohu Lake, where the sedimentary layer is very thick. In our the velocity varies as high−low−high−low, and the high velocity study region the maximum thickness of sediments exceeds 4 km anomaly displays an upwelling shape, indicative of the volcanic (Figure 8c). In the southwest the bedrock area (area P) at the east intrusive rock body. margin of the Dabie orogen is obviously a high velocity area where the sediments are very thin, and the velocity increases with 3.3 Model Recovery Test and Travel Time Error Estimation depth from 3.3 km/s to 3.5 km/s. The area to the east of Lujiang In order to test the capability of the data to recover the model, we County (area Q) is a high velocity area with the peak velocity value used the result model to generate theoretical travel time data. Be- of about 3.3 km/s. It corresponds to the Yefushan region, where cause the phase velocity dispersion error caused by the non-uni- the volcanic intrusive rock body is outcropped and the area of the form distribution of noise sources is generally about 1% (Yao HJ high velocity body extends to southeast with the increase of and van der Hilst, 2009), we added 2% random errors to the theor- depth. In Figure 8b the high velocity body appears to intrude into etical travel time data. After inversion we obtained the results the Tan-Lu fault zone and the low velocity sediments fill in the shown in Figure 7(e, f) and Figure 8(g, h). Comparing with Figure 7(a, b) and Figure 8(a, b), we observe that the major struc- fault zone. The area R in Figure 7c shows relatively low velocity, tural bodies in the original models are basically recovered, indicat- corresponding to the small-scale drainage systems and thin sedi- ing that the 3-D velocity model obtained in this study is reliable. ments in this area, such as Huangpi Lake and Luo River. The sedi- mentary thickness this area is less than 2 km (Figure 8a), and with Because unevenness of noise source distribution may cause er-

the increase of depth the sediment effect becomes no longer ob- rors in travel time measurements, we adopted the method of Fro-

Li C and Yao HJ et al.: Shallow crustal structure of the Tan-Lu fault zone in Lujiang

322 Earth and Planetary Physics doi: 10.26464/epp2020026

31.7 31.7 (a) 1 s (b) 3 s 31.6 31.6

31.5 31.5

31.4 31.4

31.3 31.3 Latitude (°) Latitude (°) 31.2 31.2

31.1 31.1

31.0 31.0 116.7 116.8 116.9 117.0 117.1 117.2 117.3 117.4 117.5 116.7 116.8 116.9 117.0 117.1 117.2 117.3 117.4 117.5 Longitude (°) Longitude (°)

31.7 31.7 (c) 5 s (d) 8 s 31.6 31.6

31.5 31.5

31.4 31.4

31.3 31.3 Latitude (°) Latitude (°) 31.2 31.2

31.1 31.1

31.0 31.0 116.7 116.8 116.9 117.0 117.1 117.2 117.3 117.4 117.5 116.7 116.8 116.9 117.0 117.1 117.2 117.3 117.4 117.5 Longitude (°) Longitude (°)

Figure 6. Ray path distribution for phase velocities at four different periods: (a) 1 s, (b) 3 s, (c) 5 s, and (d) 8 s. Black lines represent ray paths; black triangles represent stations. ment et al. (2010) to estimate the relative travel time error caused lowing we discuss the structural and tectonic characteristics of the by an uneven noise source distribution. From the cross-correla- shallow crust in the west of the Tan-Lu fault zone, within the fault tion functions of all station pairs we get an approximate distribu- zone, and in the eastern part, respectively. tion weight B(θ) of the noise energy variation with azimuth, which ( ) = + + can be expressed by Fourier expansion as B θ B0 B1cosθ 4.1 Shallow Crustal Structure in the East Margin of Dabie ( ) + ( ) + .... B2cos 2θ B3cos 3θ Orogen and Hefei Basin The travel time error caused by an uneven source distribution can The Dabie orogen in the southwestern part of the array has been ′′ B (0) a favored location for investigations of high pressure and ultra- be written as δt = for the positive correlation time part 2 ( ) high pressure metamorphic rocks (Xu SF et al., 1992; Wang YS et ′′ 2tω0B 0 B (180) and δt = for the negative correlation time part, where al., 2004) since the discovery of coesite-bearing eclogite in the 2 ( ) 2tω0B 180 1980 s (Okay et al., 1989). The lithology in this area is dominated δt is travel time error, t is reference time, and ω0 is angular fre- mainly by metamorphic, some magmatic (e.g., Mesozoic granite), quency. and ultra-high-pressure rocks (Wang YS et al., 2004). The imaging According to the above formula, the higher the frequency or the result of Huang Y et al. (2011) for the depth range 2–25 km in this longer the travel time, the smaller the travel time error. We calcu- area using earthquake multi-phase body wave travel time tomo- lated the average value and the standard deviation of the relative graphy, and that of Hu J et al. (2016) using air-gun source body travel time errors along all the paths at period 4 s. The result indic- wave travel time data, detected a high P-wave velocity structure ates that both are within 1.5%, which is far smaller than the velo- in this area. The result of ambient noise tomography of Luo YH et city anomalies (above 30%), suggesting that the travel time error al. (2012) also showed that the phase velocity and shear wave ve- caused by uneven noise source distribution can be neglected in locity in this region are high. Our result indicates that the shallow the inversion. crust of this region is possessed of relatively high shear wave velo- cities, which increase from about 3 km/s at 1 km depth to about 4. Discussion 3.4 km/s at 4 km depth. Our result reveals finer structural charac- Remarkable lateral variation is found in the shallow crustal velo- teristics of the orogen in the shallow crust; i.e., with the increase of city structure of the study region (Figures 7 and 8), which is char- depth, the margin of the high velocity body extends northward, acterized by a variety of distinct structural features such as oro- to the sedimentary basin in the north, from south of Shucheng

gen, fault belt, sedimentary basin, and intrusive rocks. In the fol- and Lujiang in the south (Figures 7 and 8).

Li C and Yao HJ et al.: Shallow crustal structure of the Tan-Lu fault zone in Lujiang

Earth and Planetary Physics doi: 10.26464/epp2020026 323

km/s km/s 1.0 1.5 2.0 2.5 3.0 3.5 1.0 1.5 2.0 2.5 3.0 3.5

(a) 1 km (b) 2 km 31°36'N 31°36'N CH

31°24'N SC 31°24'N

LJ 31°12'N 31°12'N

TC 31°00'N 31°00'N

116°48'E 117°00'E 117°12'E 117°24'E 116°48'E 117°00'E 117°12'E 117°24'E

km/s km/s 2.0 2.5 3.0 3.5 2.0 2.5 3.0 3.5

(c) 3 km (d) 5 km 31°36'N 31°36'N O 31°24'N 31°24'N P 31°12'N Q 31°12'N

31°00'N R 31°00'N 116°48'E 117°00'E 117°12'E 117°24'E 116°48'E 117°00'E 117°12'E 117°24'E

km/s km/s 1.0 1.5 2.0 2.5 3.0 3.5 2.0 2.5 3.0 3.5

(e) 1 km_recovery (f) 3 km_recovery 31°36'N 31°36'N

31°24'N 31°24'N

31°12'N 31°12'N

31°00'N 31°00'N

116°48'E 117°00'E 117°12'E 117°24'E 116°48'E 117°00'E 117°12'E 117°24'E

Figure 7. Shear wave velocity slices at the depth of (a) 1 km, (b) 2 km, (c) 3 km, and (d) 5 km after inversion; black triangles represent stations and the black lines show the main faults. In (a), CH represents Chaohu Lake, the black circles represent the location of cities (LJ for Lujiang, TC for Tongcheng, SC for Shucheng), the red triangle represents the Yefushan area, and the red pentagon represents the Shaxi area. The O, P, Q and R in (c) represent four different regions: O for the Hefei basin, P for the Dabie orogenic belt, Q for the Yefushan area, R for the sedimentary area of Huangpi Lake and other smaller local drainage systems, and the black arrow represents the direction of the Luzong volcanic basin. (e) and (f) show the results of the model recovery test in Section 3.2 for horizontal slices at depths of 1 km and 3 km, respectively.

Li C and Yao HJ et al.: Shallow crustal structure of the Tan-Lu fault zone in Lujiang

324 Earth and Planetary Physics doi: 10.26464/epp2020026

A A’ B B’ (a) 0.0 (b) 0.0

2.5 2.5 Depth (km) Depth (km)

5.0 116.7542 116.8739 116.9936 117.1133 117.2330 117.3528 117.0924 117.1545 117.2165 117.2786 117.3406 117.4027 Longitude (°) Longitude (°)

C C’ D D’ (c) 0.0 (d) 0.0

2.5 2.5 km/s 3.5 Depth (km) Depth (km)

5.0 5.0 117.1490 117.2134 117.2778 117.3422 117.4066 117.471 31.4512 31.4025 31.3539 31.3053 31.2566 31.2080 3.0 Longitude (°) Longitude (°)

2.5 E E’ F F’ (e) 0.0 (f) 0.0 2.0 2.5 2.5 1.5 Depth (km) Depth (km)

5.0 31.2090 31.1798 31.1507 31.1216 31.0924 31.0633 31.1532 31.1276 31.1020 31.0764 31.0508 31.0252 Latitude (°) Latitude (°)

A A’ B B’ (g) 0.0 (h) 0.0

2.5 2.5 Depth (km) Depth (km) 5.0 5.0 116.7542 116.8739 116.9936 117.1133 117.2330 117.3528 117.0924 117.1545 117.2165 117.2786 117.3406 117.4027 Longitude (°) Longitude (°)

Figure 8. The shear wave velocity structures along six vertical cross-sections shown as the red lines in Figure 1: (a) AA', (b) BB', (c) CC', (d) DD', (e) EE', and (f) FF'. The black upward triangle represents the orogenic belt area, the black inverted triangle represents the area of volcanic intrusive rocks, and the black dotted line represents the location of the two main faults in the Tan-Lu fault zone. (g) and (h) show the results of a model recovery test in Section 3.2 for the AA’ and BB’ cross-sections, respectively.

The Hefei Basin in the northwestern part of the study region is south of Sucheng County and the north of Lujiang County. With controlled primarily by the uplifting of the Dabie Orogen, but its increasing depth the sedimentary area shrinks rapidly towards the eastern part is also controlled by tectonic activity of the foreland Chaohu Lake direction (e.g., Figure 7 and Figure 8a, c, d), indicat- basin of the Tan-Lu fault zone. A set of continental strata with a ing that the migration direction of sedimentation is centered in maximum thickness of 7 km were deposited in the basin (Lu GM the eastern part of Hefei Basin. At the same time, this variation et al., 2002; Liu GS et al., 2006). In the profile, the closer to Chaohu pattern in the low velocity area is coupled with the extension dir- Lake the thicker the sediment layer becomes. In the study area it ection of a high velocity anomaly in the Dabie orogenic area in may be thicker than 5 km (Figure 7 and Figure 8c, d). The basin the south, indicating the area of impact of the Dabie orogen at has been filled by continental clastic sediments since the Jurassic. deeper depths. Alluvial fan facies deposits exist widely in the margin of the basin. In the Chaohu Lake direction, the basin successively received de- 4.2 The Characteristics of Intrusive Rocks in the Tan-Lu posits of river facies, shore-shallow lake facies, and shallow-semi- Fault Zone deep lake facies. After later multiple stages of basin development In this region the lithology of the Tan-Lu fault zone varies, leading and shrinkage, the sedimentary area reached its present area and to different velocity characteristics in different areas. There are shape (Liu GS et al., 2006; Li Z et al., 2001). Our result clearly exhib- both sedimentary clastic rocks left by fault slipping and extension, its the impact range of the sediments at different depths in the and intrusive rocks formed by magmatic activities. The intrusive

study area. In the shallow layer (1 km) the impact may reach to the rock body in the Tan-Lu fault zone is distributed mainly in the area

Li C and Yao HJ et al.: Shallow crustal structure of the Tan-Lu fault zone in Lujiang

Earth and Planetary Physics doi: 10.26464/epp2020026 325 from Yefushan in the northeast to Shaxi in the southeast of Luji- enced multi-stage tectonic activities; different areas in the fault ang County. In our result it is shown as a high velocity anomaly zone exhibit different seismic activities. The Anhui segment of the marked as region Q, and extends further southeastward and con- Tan-Lu fault zone exhibits two main types of active fault: broken nects with the Luzong volcanic rock basin (Figures 7 and 8), indic- type and fault gouge type, and the activeness of the former is re- ating that the two velocity structures are continuous. There is a markably higher than that of the latter. This segment from deep seismic reflection sounding profile near and parallel to pro- Tongcheng to Chaohu Lake is mainly of the broken type, contain- file AA’, and the result indicates that in the Tan-Lu fault of that ing many loose broken zones. The active fault zone is composed area the reflection signal from Moho is weak, while to its east, be- of multiple faults (Liu B et al., 2015). We collected data from earth- neath the northern part of Luzong Basin, the magmatic channels quakes of magnitude 1.5 and greater that have occurred in this can be clearly seen (Gao R et al., 2010), probably indicating the and neighboring areas since the 1990 s (Figure 9). The result source of magma and accompanying metal ores in the study re- shows that small earthquakes occurred densely around Lujiang gion. Various mineral resources related to intrusive rock bodies in County, indicating the characteristics of broken strata in this area. the shallowest crust of depth less than 500 m have long been ex- These broken faults with large pore space may provide the condi- ploited; the Shaxi copper mine is an example. With the develop- tion for intrusive rocks outcropping onto the surface. In addition, ment of deep geophysical prospecting, molybdenum, tungsten, the near-NS compressional stress in the Mesozoic Luzong volcan- and other metal ores have been found within the last several ic rock basin (Dong SW et al., 2009) may also have provided the years at the depth of about 1 km near Yefushan. Without a single possibility for the magma intruding westward into the Tan-Lu exception, these metal mineral resources have all been found fault zone. Therefore, we infer that a part of the magmas origin- within the high velocity anomaly bodies of intrusive magmatic ated in the northern part of Luzong volcanic rock basin migrated rocks and extension areas identified in our result. In recent years upward along the broken faults and intruded into the Tan-Lu fault the mining and petroleum industry departments have been very zone, and outcropped on the broken strata. After multi-stage active in the Hefei Basin and its periphery, and have found that in compression and extension of the fault zone, these processes the areas off the intrusive rock body and its extension it is almost have created the present intrusive rock distribution in this region. impossible to find related mineral resources. Therefore, our ima- The Luzong volcanic rock basin is generally considered to be con- ging result may provide some guiding references for geophysical trolled by the rift-depression in the middle-lower Yangtze River. prospecting even in the deeper part. Therefore, we infer that the intrusive rock bodies in the Tan-Lu In terms of lithology the intrusive rocks in the study area show fault zone are closely related, in terms of stress and source materi- continuity with that in the northern Luzong volcanic basin more al, to the Luzong volcanic rock basin to its southeast in the than ten kilometers to the southeast. We collected data of a bore- middle-lower Yangtze River fault zone. These intrusive rocks came hole in the Shaxi copper mine (red star in Figure 7a) and com- or partly came from the Luzong volcanic rock basin, and are con- pared its lithological characteristics to those of the northern part nected with the Luzong volcanic rock basin at depths. These rela- of the Luzong volcanic basin. Both show that a large quantity of tions are the specific manifestation of large area plate motion in a tuff, trachyte andesite, and basalt are mixed in breccia, sandstone, small-scale region. From Late Cretaceous to Early Tertiary the Tan- and shale (Yuan F et al., 2008; Yang SX et al., 2017). In addition, Lu fault zone was in a period of extensional activity (Xu JW and geological dating studies indicate that the strong Early Mesozoic Zhu G, 1994; Zhu G et al., 1995; Xu JW et al., 1995; Zhu G et al., magmatic activities in the Luzong volcanic rock basin occurred 2004), which was also the time of volcanic intrusive rock develop- between 136–124 Ma (Zhou TF et al., 2008; Yuan F et al., 2008; ment in our study area. The extension of the Tan-Lu fault zone in Tang JF et al., 2010), while according to Xie CL et al. (2008a, b) the this time period was mainly controlled by the rapid high-angle age of the magmatic rock at the borehole of the Shaxi copper subduction of the paleo-Pacific plate in the east beneath the East mine is between 125–93 Ma, closely related to the time of magma Asia continent (Engebretson et al., 1985). Such subduction and activity in the Luzong basin. There are also outcropped rock bod- plate retreat facilitated asthenosphere upwelling beneath the ies in the southern segment of the Zhangbaling uplift to the Tan-Lu fault and surrounding areas, further leading to the exten- north, in the same fault zone separated only by a lake, where the sional activities in the fault zone (Zhu G et al., 2001). Many exten- shear wave velocity is also relatively high (Gu N et al., 2019); but its sional basins appeared in the fault zone, and the faults in the zone lithology and age are very different from that of the intrusive rock became more broken. This time was the late stage of develop- in our study area. The southern segment of the Zhangbaling up- ment of the Luzong volcanic rock basin, the magma intruded lift is composed mainly of about 680Ma-old Neoproterozoic through the broken faults into the Tan-Lu fault zone, and out- Feidong metamorphic complex rock (Zhao T et al., 2014; Zhang cropped on the surface. Finally, since the Late Tertiary the west- DB et al., 1995). The Dabie orogeny, only about ten kilometers to ward compression caused by the back-arc spreading of the west the southwest of this area and also with high shear wave velocity, Pacific plate turned the Tan-Lu fault zone to the reverse type, and is composed primarily of Precambrian metamorphic rocks, mixed the large-scale magmatic activity in the fault zone basically disap- with high pressure and ultrahigh pressure metamorphic rocks peared (Zhu G et al., 2001). formed by the orogeny and some Yanshanian and Mesozoic mag- matic rocks (Wang YS et al., 2018). 5. Conclusions Here the intrusive magmatic rocks are outcropped in a rather spe- Based on the continuous waveform data of the Lujiang array in

cial segment of the Tan-Lu fault. The Tan-Lu fault zone has experi- Anhui Province, we used the direct surface wave tomography

Li C and Yao HJ et al.: Shallow crustal structure of the Tan-Lu fault zone in Lujiang

326 Earth and Planetary Physics doi: 10.26464/epp2020026

33°N

TLF

32°N

HF ZBL

HFB CH LJ 31°N DBO LZB CJ

30°N

115°E 116°E 117°E 118°E 119°E

Figure 9. Distribution of earthquakes (black crosses) in this region and adjacent areas since the 1990s, with magnitudes greater than 1.5. The red box corresponds to the Tanlu fault zone from Chaohu to Tongcheng. method to determine a 3-D shear wave velocity model of the shal- of China (project 41790464) and the China Postdoctoral Fund low crust in the depth range of 0–5 km for the Lujiang segment of (BH2080000099). the Tan-Lu fault zone and its neighboring areas. The result reveals the spatial distribution of some anomalous structures, including References the low-velocity Hefei Basin and nearby sedimentary areas, and Bensen, G. D., Ritzwoller, M. H., Barmin, M. P., Levshin, A. L., Lin, F., Moschetti, M. the junction of the high-velocity eastern margin of the Dabie oro- P., Shapiro, N. M., and Yang, Y. (2007). Processing seismic ambient noise gen with the intrusive rocks in the Tan-Lu fault zone, which correl- data to obtain reliable broad-band surface wave dispersion measurements. ates with the geology and landform of the area. The result shows Geophys. J. Int., 169(3), 1239–1260. https://doi.org/10.1111/j.1365- that the sedimentation center of Hefei Basin is toward Chaohu 246X.2007.03374.x Brocher, T. M. (2005). Empirical relations between elastic wavespeeds and Lake. The high velocity rock distribution in the shallow crust is in density in the Earth’s crust. Bull. Seismol. Soc. Am., 95(6), 2081–2092. the eastern margin of the Dabie orogen. We also reveal the struc- https://doi.org/10.1785/0120050077 tural characteristics of the deep connection of the intrusive rocks Chen, L., Zheng, T. Y., and Xu, W. W. (2007). A thinned lithospheric image of the in the Tan-Lu fault zone with the Luzong volcanic rock basin to its Tanlu Fault Zone, eastern China: Constructed from wave equation based southeast. These intrusive rocks have come partly from the receiver function migration. J. Geophys. Res. Solid Earth, 111(B9), B09312. magma in the Luzong volcanic rock basin, which upwelled along https://doi.org/10.1029/2005JB003974 broken faults due to the extension of the Tan-Lu fault zone and by Chen, Y., Wang, B. S., and Yao, H. J. (2017). Seismic airgun exploration of continental crust structures. Sci. China Earth Sci., 60(10), 1739–1751. the aid of stress inside the Luzong basin, and finally cooled and https://doi.org/10.1007/s11430-016-9096-6 outcropped on the surface, providing a possible explanation of Dong, S. W., Gao, R., Lv, Q. T., Zhang, J. S., Zhang, R. H., Xue, H. M., Wu, C. L., Lu, Z. the rich mineral resources in the south area of the Tan-Lu fault W., and Ma, L. C. (2009). Deep structure and ore-forming in Lujiang- zone, and thus a possible guide for deep resources prospecting. Zongyang ore concentrated area. Acta Geosci. Sin. (in Chinese), 30(3), This may also imply that the Tan-Lu fault zone and the middle- 279–284. https://doi.org/10.3321/j.issn:1006-3021.2009.03.001 lower Yangtze River fault-depression zone are closely related in Engebretson, D. C., Cox, A., and Gordon, R. G. (1985). Relative motions between terms of stress and source material. In the meantime, this model oceanic and continental plates in the Pacific basin. Special Paper 206, Boulder, Colo.: Geological Society of America, 1-59. also provides a relatively precise 3-D velocity model for the calcu- Fang, H. J., and Zhang, H. J. (2014). Wavelet-based double-difference seismic lation of earthquake strong motion in this area. tomography with sparsity regularization. Geophys. J. Int., 199(2), 944–955. https://doi.org/10.1093/gji/ggu305 Acknowledgment Fang, H. J., Yao, H. J., Zhang, H. J., Huang, Y. C., and van der Hilst, R. D. (2015). Direct inversion of surface wave dispersion for three-dimensional shallow We appreciate the comments from two anonymous reviewers, crustal structure based on ray tracing: methodology and application. which helped to improve the original manuscript. This study is Geophys. J. Int., 201(3), 1251–1263. https://doi.org/10.1093/gji/ggv080

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Li C and Yao HJ et al.: Shallow crustal structure of the Tan-Lu fault zone in Lujiang