Earth and Planetary Science Letters 408 (2014) 378–389

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Earth and Planetary Science Letters

www.elsevier.com/locate/epsl

Crustal and uppermost mantle velocity structure and its relationship

with the formation of ore districts in the Middle–Lower Yangtze River

region

Longbin Ouyang a,b, Hongyi Li a,b,∗, Qingtian Lü c, Yingjie Yang d, Xinfu Li b, Guoming Jiang b, Guibin Zhang b, Danian Shi c, Dan Zheng b, Sanjian Sun b, Jing Tan b, Ming Zhou b

a Key Laboratory of Geo-detection, China University of Geosciences, Ministry of Education, Beijing, 100083, Chinab School of Geophysics and Information Technology, China University of Geosciences, Beijing, 100083, Chinac Institute of Mineral Deposits, Chinese Academy of Geological Sciences, Baiwanzhuang Road, Beijing, 100037, Chinad Australian Research Council Centre of Excellence for Core to Crust Fluid Systems/GEMOC, Dept. Earth and Planetary Sciences, Macquarie University, North Ryde, NSW, Australia

a r t i c l e i n f o a b s t r a c t

Article history:Received 3 June 2014Received in revised form 6 October 2014Accepted 9 October 2014Available online 5 November 2014Editor: P. Shearer

Keywords:the Middle–Lower Yangtze River metallogenic belttwo-plane-wave tomographyambient noise tomography

In this study, we conduct ambient noise tomography and teleseismic two-plane-wave tomography to probe the crustal and uppermost mantle structures in the Middle–Lower Yangtze River region. The data used include 14 months (from July 2012 to August 2013) of continuous vertical component ambient noise data and 219 teleseismic earthquakes recorded at 138 broadband seismic stations from Chinese provincial networks and 19 temporary seismic stations deployed by China University of Geosciences (Beijing). First, we apply ambient noise tomography to the collected ambient noise data to generate Rayleigh wave group and phase velocity maps at 5–42 s periods and two-plane-wave tomography method to earthquake data to generate intermediate-to-long period phase velocity maps at 20–143 s periods. We then combine the short-to-intermediate period velocity maps from ambient noise and the intermediate-to-long period phase velocity maps from earthquake data to generate broadband phase velocity maps at periods from 5 to 143 s. By inverting these 5–143 s Rayleigh wave velocity maps, we construct a 3-D shear wave velocity model from the surface to ∼250 km depth in the Middle–Lower Yangtze River region. The 3-D model shows that in the upper crust, the basin regions, including the JiangHan, HeHuai, SuBei, HeFei and NanYang basins, are all featured with low velocities, and the mountain regions with high velocities. In the uppermost mantle, a low-velocity zone at ∼100–200 km depth is observed beneath the Middle–Lower Yangtze River Metallogenic Belt. Moreover, our tomographic results show that the NingWu and NingZhen ore districts are clearly characterized by the strongest low velocity anomaly in the uppermost mantle at ∼70–200 km depth. The depth extent of the low-velocity zone becomes shallower and the amplitude of low velocity anomaly becomes larger from the southwest JiuRui ore district to northeast NingWu ore districts. The change pattern of the low-velocity zone in the depth extent and the amplitude is consistent with the fact that peak ages of magmatic events along the Middle–Lower Yangtze River Metallogenic Belt progressively become younger and younger from 148 Ma in the southwest to 125 Ma in the northeast. The observed low-velocity zone may represent the cooling hot upper mantle which was partially molten in the past resulting from partial melting of the paleo-Pacific plate or of an enriched mantle source induced by the westward subduction of the paleo-Pacific plate. The upwelling of the mantle-derived magmas may result in the formation of these granitic rocks and coeval ores deposits along the Middle–Lower Yangtze River Metallogenic Belt.

© 2014 Elsevier B.V. All rights reserved.

* Corresponding author at: Key Laboratory of Geo-detection, China University of Geosciences, Ministry of Education, Beijing, China. Tel.: +86 10 82321782.

E-mail address: lih@cugb.edu.cn (H. Li).

http://dx.doi.org/10.1016/j.epsl.2014.10.0170012-821X/© 2014 Elsevier B.V. All rights reserved.

1. Introduction

Eastern China is primarily composed of the South China Block (SCB), the North China Craton (NCC), the QinLing–DaBie–SuLu oro-genic belt and the Tancheng–Lujiang fault (TLF) (Zheng et al., 2013).

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Fig. 1. (a) Schematic illustration of the tectonic background in the Middle–Lower Yangtze River region. SBB: SuBei basin; HFB: HeFei basin; HHB: HeHuai basin; NYB: NanYang basin; JHB: JiangHan basin; JSS: Jiangshan–Shaoxing suture; TLF: Tancheng–Lujiang fault; XSF: Xinyan–Shucheng fault; XGF: Xiangfan–Guangji fault; JXF: Jiashan–Xiangshui fault; YCF: Yangxin–Changzhou fault; MLYMB: Middle–Lower Yangtze River Metallogenic Belt; ED: EDongnan; JR: JiuRui; AG: Anqing–Guichi; LZ: LuZong; TL: TongLing; NW: NingWu; NZ: NingZhen. (b) Topographic map showing the stations used in this study. The red triangles are the CEArray seismic stations operated by the China Earthquake Administration and blue squares are temporary stations deployed by the China University of Geosciences (Beijing). The red rectangle in the bottom left of this panel marks the position of the Middle–Lower Yangtze River region. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

The SCB is further divided into the Cathaysia Block (CaB) in the southeast and the Yangtze Craton (YC) in the northwest. The amalgamation between the CaB and the YC took place along the northeast–southwest trending Jiangshan–Shaoxing suture (JSS) during the Jinning Orogeny at about 860–800 Ma in the Neopro-terozoic (Wang et al., 2007). The NCC is made up of two major blocks, the Eastern and the Western Block, which were amal-gamated along the north-trending Trans-North China Orogen at about 1.8 Ga in the Paleoproterozoic (Zhai and Santosh, 2011). The QinLing–DaBie–SuLu orogenic belt is a major Triassic colli-sion zone that resulted from the closure of a paleo-Tethys oceanic arm caused by the northward subduction of the YC beneath the NCC at about 245–210 Ma and also is the most famous high and ultrahigh pressure (HP and UHP) metamorphic zone in the world (Hacker et al., 2006; Zhang et al., 2009). The TLF is the largest strike-slip fault in eastern China, separating the northeastern SuLu orogenic belt from the southwestern DaBie orogenic belt (Wang, 2006). During the Mesozoic, tectonic activities in eastern China were very intensive; especially after Early Jurassic, the subduction of paleo-Pacific plate into the Eurasian plate had resulted in mas-sive and vigorous magmatic activities in the SCB and lithospheric thinning in the eastern NCC (Chen et al., 2006; Li and Li, 2007;Zhu et al., 2012). The study area of this work includes the south-eastern NCC, the QinLing–DaBie–SuLu orogenic belt, the central eastern YC and most parts of CaB. Since the Yangtze River runs through this area, we name the area as the Middle–Lower Yangtze River region (MLYR) (Fig. 1a). There are five large basins in the MLYR, namely, SuBei basin (SBB), JiangHan basin (JHB), NanYang basin (NYB), HeHuai basin (HHB) and HeFei basin (HFB). The SBB, JHB and NYB are rich in oil and gas resources. In the center of the MLYR, there is an important polymetallic metallogenic belt and mineral resource base, where more than 200 kinds of polymetallic deposits have been found (Chang et al., 1991; Pan and Dong, 1999;Mao et al., 2006). The arc-like metallogenic belt, also called the Middle–Lower Yangtze River Metallogenic Belt (hereinafter referred to as MLYMB), is situated in the foreland of the DaBie orogenic belt and confined by three main deep faults, the Xiangfan–Guangji faults (XGF) and the TLF to the northwest, the Yangxin–Changzhou fault (YCF) to the southeast (Fig. 1a). In the narrow MLYMB, there

are seven big ore-concentrated districts, which are EDongnan, JiuRui, Anqing–Guichi, TongLing, LuZong, NingWu and NingZhen ore districts (Fig. 1a). Because of the great scientific significance and economic value of the MLYMB, geologists have intensively studied its tectonic process, magmatic evolution, and metallo-genic process for more than a century (e.g. Chang et al., 1991;Pan and Dong, 1999; Lü et al., 2005; Mao et al., 2011). However, its deep geodynamic process and the origin of magmatic activities are still under debate.

In the past two decades, numerous geochemical, geological and geophysical researches have been done in this area, but most of these researches are based on geologic, geochemical and geochronometric analysis and focused on the ore deposits, miner-alization and magmatic activities (e.g. Chang et al., 1991; Pan and Dong, 1999; Xu et al., 2002; Lü et al., 2005; Wang et al., 2006;Hou et al., 2007; Ling et al., 2009; Mao et al., 2011; T.F. Zhou et al., 2012; X.H. Li et al., 2013). These studies provide detailed descriptions of the regional geological and geochemical charac-teristics of the ore deposits in MLYMB; and various models have been proposed to explain the ore genesis and the formation of related Cretaceous igneous rocks in the MLYMB. These models can be generally sorted into three groups: (1) melting of thick-ened and/or delaminated lower continental crust (Xu et al., 2002;Wang et al., 2006; Hou et al., 2007), (2) subduction of a mid-ocean ridge between the ancient Pacific and Izanagi plates (Ling et al., 2009), and (3) flat-slab subduction of oceanic plate since the early Mesozoic beneath southeastern China and subsequent delamina-tion and foundering of the flat-slab (X.H. Li et al., 2013). Geophysi-cal constraints on the deep crustal and upper mantle structure are critical to assessing these proposed models.

Compared to the large number of geological and geochemical studies carried out in the MLYMB, geophysical studies of the deep crustal and upper mantle structure are relatively limited, since most of geophysical surveys in this region are restricted to very shallow depths for ore prospecting (e.g. Lü et al., 2005, 2011; Liu et al., 2012). Recently, the SinoProbe (Deep Exploration in China) pro-gram have completed five intersecting deep seismic reflection pro-files in LuZong (Dong et al., 2013; Lü et al., 2013) and a multidisci-plinary transect across the NingWu of the MLYMB (Shi et al., 2013;

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Xu et al., 2014; Zhang et al., 2014). However, these geophysical re-searches are either along linear profiles or only focus on single ore district. Therefore, the deep 3-D crustal and upper mantle struc-ture of the whole MLYMB is still poorly understood. Jiang et al.(2013) used teleseismic P-wave tomography to determine the 3-D P-wave velocity structure of the mantle beneath the northeastern MLYMB. Their tomographic results suggested that the lithosphere is delaminated beneath the NingWu ore district. But, the resolution of their model is 1◦ × 1◦ horizontally and 50–100 km vertically. Due to low seismic activity in the MLYR and sparse seismic sta-tion coverage in the past, only a few surface wave tomographic studies have been carried out in this area (e.g. Luo et al., 2012;L. Zhou et al., 2012). Using ambient noise data in their tomogra-phy, Luo et al. (2012) and L. Zhou et al. (2012) are mainly inter-ested in the crustal structure of the DaBie orogenic belt and of south China, respectively. Therefore, in order to better understand the deep geodynamic process beneath the MLYMB, it is essential to construct a high-resolution 3-D crustal and uppermost mantle velocity model in the MLYR, which could help understand the pro-cess of ore-forming and discover new mineralization zones in the future.

Surface waves have been widely adopted to study the elastic structure of the crust and upper mantle because surface waves propagate along the Earth’s surface and are most sensitive to elas-tic properties of the crust and upper mantle. In the past decade, ambient noise tomography (ANT) based on cross-correlating con-tinuous long time series of ambient noise to extract surface wave dispersion curves has been widely used to image the crustal and uppermost mantle shear velocities of the Earth (e.g. Shapiro et al., 2005; Yao et al., 2006; Yang et al., 2007; Bensen et al., 2008;Lin et al., 2008; Zheng et al., 2008; Sun et al., 2010; Zheng et al., 2011; Li et al., 2012; L. Zhou et al., 2012). ANT can be ap-plied to aseismic areas where a seismic array exists and easily ob-tain short-to-intermediate period dispersion measurements; while, two-plane-wave tomography (TPWT) is suitable for constructing intermediate-to-long period phase velocity maps (Forsyth and Li, 2005; Yang and Forsyth, 2006a). TPWT uses teleseismic events by representing an incoming wavefield from each teleseismic earth-quake as the sum of two plane waves, each with initially unknown amplitude, phase, and propagation direction (Forsyth and Li, 2005). With the growth of regional seismic networks and the availabil-ity of high quality seismic recordings, spectacular applications of TPWT have been reported in recent years (e.g. Li and Burke, 2006;Li and Detrick, 2006; Yang and Forsyth, 2006b; Wagner et al., 2010; Jiang et al., 2011; L. Li et al., 2013).

Combining ambient noise and earthquake data can yield fine structures of both the crust and uppermost mantle (Yao et al., 2008; Yang et al., 2008; Xu et al., 2013). In this paper, surface wave dispersion measurements from ANT and TPWT are combined to generate surface wave phase velocity maps at 5–143 periods and group velocity maps at 5–42 s periods. Then, by inverting these surface wave dispersion maps, a 3-D shear wave velocity model from the surface down to 250 km depth is constructed. In this study, we focus on investigating the deep velocity structure be-neath the MLYR and discussing how it is related with the shallow ore-centralization area as well as their implications for the under-standing of the deep geodynamic process and the mechanism of magmatic activities in the MLYR.

2. Data and method

2.1. Ambient noise tomography

Continuous vertical component ambient noise data from July 2012 to August 2013 recorded by 157 stations including 19 tempo-rary broadband seismic stations deployed by the China University

Fig. 2. (a) Location of the 219 events used in this study, the blue triangle represents the study center, the blue square shows the study region. (b) Numbers of phase (blue solid line) and group (black dashed line) velocity dispersion measurements at periods of 5–42 s measured from the ANT, and numbers of Rayleigh waveforms (red solid line with circles) used at periods of 20–143 s in the TPWT inversions for phase velocities. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

of Geosciences (Beijing), and 138 broadband seismic stations from Chinese provincial networks (CEArray) are used for our ambient noise analysis (Fig. 1b). The procedures of ambient noise data pro-cessing are the same as those described in our previous studies (Li et al., 2009, 2014). In the single station data preparation, raw seis-mograms are cut into a series of one-hour long segments. If a seg-ment has spikes and the peak amplitude of any spike is 10 times larger than the root mean square (RMS) of the whole segment, that segment is discarded. After doing cross-correlation and stack-ing, only the stacked cross-correlations with signal-to-noise (SNR) >8 and interstation distance larger than three wavelengths of the interested surface waves are retained. Then group and phase ve-locity dispersion curves are measured from the cross-correlations by using automatic Frequency Time Analysis (Bensen et al., 2007;Yang et al., 2007). Fig. 2b shows the number of selected group and phase velocity dispersion measurements at different periods. Before tomography is performed, checkerboard tests are carried out to evaluate lateral resolution using the Occam’s technique (Constable et al., 1987; Huang et al., 2003), which minimizes data misfits by considering smoothness constraint. A recent application of this method can be found in Li et al. (2009). Following the same procedure as given by Li et al. (2009), a series of checkerboard tests using the path coverage at different periods are conducted to estimate the resolution. Fig. 3a shows the recovered phase velocity map for a testing checkerboard model with 0.4◦ × 0.4◦ anomaly cells at 29 s period. The checkerboard test suggests that the lateral resolution is about 0.4◦ in most parts of the study area except in the margin where station coverage is sparse.

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Fig. 3. (a) and (b) show ANT and TPWT phase velocity checkerboard test results at periods of 29 s and 83 s with a grid spacings of 0.2◦ and 0.5◦ , the size of alternatively high- and low-velocity cells is 0.4◦ and 1.0◦ , respectively.

2.2. Two-plane-wave tomography

The TPWT method can account for the non-planar energy in incoming waves caused by scattering and multipathing outside a study area by representing an incoming wavefield with the inter-ference of two plane waves (Forsyth and Li, 2005). This two-plane-wave assumption is an approximation for a complex teleseismic wavefield. In most cases, it well describes the amplitude and phase variations across a seismic array. Meanwhile, the sensitivities of surface waves to lateral heterogeneities inside a study area are cal-culated using finite-frequency response kernels for both amplitude and phase (Yang and Forsyth, 2006a). More detailed descriptions of the method are given by Forsyth and Li (2005) and Yang and Forsyth (2006a).

We follow the same procedures of data analysis first outlined by Forsyth and Li (2005) and described in detail by Yang and Forsyth (2006b) to process vertical component teleseismic sur-face waves recorded by the 157 stations in 2010–2013 (Fig. 1b). 582 teleseismic events with surface wave magnitude larger than 5.5 and epicentral distances between 30◦ and 120◦ were selected. We first remove the instrument responses, means and trends from seismograms, and then decimate seismic data to 1 sample per second and bandpass filter them with a series of narrow band-pass (10 mHz), zero phase shift, and four-pole Butterworth filters (Fig. S1a). To ensure the quality of data, we visually inspect each individual waveform and keep only those with SNR larger than 5. Finally, 219 events with high SNR Rayleigh waves are retained for tomography.

As seen in Fig. 2a, the azimuthal distribution of the events is generally good except that there are small gaps in the east and west quadrants. Fundamental mode Rayleigh waves are isolated by cutting the filtered seismograms using a boxcar window with a 50 s half cosine taper at each end (Fig. S1a). Fig. 2b displays the number of fundamental mode Rayleigh wave waveform data at different periods. Before performing final tomography, we also carry out the resolution test for the teleseismic Rayleigh waves. The testing velocity model is discretized into a 0.5◦ × 0.5◦ grid, and the size of alternatively varying high- and low-velocity cells is 1◦ × 1◦ . Each cell has a constant velocity anomaly of 5%, above or below the average velocity. Gaussian distributed random noise

with zero mean and one standard deviation of 10% are added to the synthetic data. At last, these synthetic data are used to in-vert for 2-D phase velocity maps. The resulting phase velocity map at 83 s period is shown in Fig. 3b. The checkerboard ve-locity anomalies are mostly resolved in most parts of the study region.

3. Group and phase velocity maps

Rayleigh wave group and phase velocities at different periods are primarily sensitive to shear wave velocities at different depths: the longer the period, the deeper structure the waves sample (Fig. S1b). Group velocity maps from ANT at 8 s, 16 s, 25 s and 33 s periods are shown in Fig. 4, and phase velocity maps from ANT and TPWT at periods of 25 s, 33 s, 50 s, and 83 s are displayed in Fig. 5. These velocity maps are clipped to only show regions with good resolution according to the results of resolution tests shown in Figs. 3a and 3b.

At periods shorter than 20 s, group and phase velocities are most sensitive to shear velocities in the upper crust. In the 8 s and 16 s group velocity anomaly maps (Figs. 4a, 4b), strong low velocity anomalies are observed beneath the major sedimentary basins, such as the JHB, HHB and SBB; while high velocity anoma-lies are imaged in the DaBie–SuLu orogenic belt, the CaB and the Lower YC. Moreover, small scale basins are also clearly depicted with low velocity anomalies, such as the NYB separating the Qin-Ling and DaBie orogenic belts, which have not been reported in any previous tomography studies. We also notice that the MLYMB is generally delineated with low velocity anomalies, distinct from the surrounding high velocity DaBie and Lower YC areas.

At intermediate periods of 20–40 s, the imprint of sedimen-tary layers has gradually diminished and the group and phase velocity maps are mainly affected by the lower crust shear wave velocity and crustal thickness. Because group velocities are more sensitive to shallower structures than phase velocities at each pe-riod, the influence of sedimentary basins can still be seen at the 25 s group velocity anomaly map, though much weaker than at shorter periods, and gradually attenuates at 33 s period (Figs. 4c, 4d). Moreover, it is noticeable that the northeast of the DaBie oro-genic belt and Upper YC are featured with low velocity anomalies.

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Fig. 4. Estimated Rayleigh wave group velocity maps at periods 8, 16, 25, 33 s. Velocities are presented as the perturbation to the average value across the map in percent. Period and average velocity are indicated in the upper left corner of each map.

In the 25 s and 33 s phase velocity maps (Figs. 5a, 5b, 5d), a big patch of low velocity anomalies is observed beneath the NingWu and NingZhen mineralization zones. Meanwhile, the low velocity anomalies at 25 s period beneath the northeast of the DaBie oro-genic belt have also been reported by Luo et al. (2012).

ANT is suitable to generate phase velocity maps at short periods (typically <40 s); while teleseismic-earthquake-based TPWT can generate phase velocities at periods longer than 20 s. Therefore, in the overlapped periods from 20 to 40 s, we compare surface wave dispersion maps from ANT with those from TPWT. The compari-son shows that the two sets of Rayleigh wave phase velocity maps agree with each other very well except at the border region where resolution degrades in both tomography methods. The mean of dif-ferences between TPWT and ANT at 33 s period (Fig. 5f) is about 0.008 km/s and the standard deviation is 0.040 km/s. We average the two surface wave velocities at each geographic point in the overlapping period band of ANT and TPWT to produce combined Rayleigh wave phase velocities.

At long periods >40 s, phase velocity dispersion maps are only obtained from TPWT. The most striking feature at these periods is low velocity anomalies trending northeast in the northern MLYMB (Figs. 5c, 5e). The west part of the study region is generally fea-

tured with high velocity anomalies, and the east part with low velocity anomalies.

4. Shear wave velocity structure

Surface wave group and phase velocities only tell us integrated information about the crust and upper mantle structure. To obtain direct structural information at various depths, we invert group and phase velocities for shear wave velocities. Local dispersion curves at each geographic node of the 0.5◦ ×0.5◦ grid are extracted from the group and phase velocity maps obtained in tomography. Adopting an iterative linearized least-square inversion scheme of surf96 (Herrmann and Ammon, 2004), we invert the local disper-sion curves for the 1-D shear wave velocity profiles. Then, these 1-D profiles are combined together to construct a 3-D shear wave velocity model.

During the inversion, the V p/V s ratio of each layer is fixed, and the P wave velocity is updated accordingly as the S wave velocity changes. Density is calculated based on the Nafe–Drake relation (Nafe and Drake, 1963). Since surface waves have poor resolution on resolving velocity discontinuity and a strong trade-off between the shear wave velocity and Moho depth exists in surface wave

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Fig. 5. (a–b) Rayleigh wave phase velocity maps derived from ANT at periods of 25, 33 s and (c, d, e) TPWT at 50, 33, 83 s. (f) The differences between the phase velocity determined by ANT and TPWT at 33 s period. Anomalies are presented as the percent deviation from the average velocity across the region at the given period.

inversion, the Moho depth is one of the most important parameter in the inversion. The Moho depth used in our inversion is taken from the Crust 1.0 model (Laske et al., 2013), which is a global crustal model with 1◦ × 1◦ grid, and the starting velocity model is set to the AK135 model (Kennett et al., 1995). The layer thickness is 2 km from the surface to the Moho, and depth from the Moho

to 50 km is parameterized as one single layer. Then, from 50 km down to 400 km, the layer thickness is 10 km.

Shear velocity anomaly maps of the resulting 3-D model at different depths are plotted in Fig. 6. These maps reveal ma-jor velocity features, which are similar in their distributions to phase/group velocity anomalies that have been briefly discussed

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Fig. 6. Estimated shear wave velocity maps at depths of 6, 26, 70 and 160 km. Velocities at depth 6 km and 26 km are plotted as the percent deviation relative to the mean velocity across the map (a, b), and at depth 70 km and 160 km relative to the modified AK135 global model (Kennett et al., 1995) (c, d). Depth and reference velocity is indicated in the upper left corner of each map.

in the dispersion maps. At shallow depths (such as 6 km, Fig. 6a), low velocity anomalies are imaged in all the basins, including the HHB, SBB, JHB, HFB and NYB, due to thick sedimentary cover in these areas. Previous studies suggest that the HFB has a ∼7 km thick sedimentary layer (Shi et al., 2013; Xu et al., 2014) and the SBB and HHB are covered with an 11 km thick sedimen-tary layer (Zhu et al., 2006). Low velocity anomalies are also ob-served beneath the MLYMB. High velocity anomalies appear in the DaBie–SuLu orogenic belt, CaB and YC. Previous studies pro-pose that the high velocity beneath the DaBie orogenic belt mainly results from the HP/UHP metamorphic rocks (eclogite and ultra-mafic rocks) which usually have higher seismic wave velocities than other upper crust rocks (Wang et al., 2000; Wang et al., 2005;Luo et al., 2012). Moreover, Mesozoic magmatic rocks are widely distributed along the DaBie–SuLu orogenic belt (Zhao and Zheng, 2009). Recent researches show that the Mesozoic granitoids and mafic rocks with elemental and isotopic features similar to the UHP metaphoric rocks (Zhao and Zheng, 2009) also have a very high seismic velocity. High velocities are also observed in the up-per crust of the CaB and the Lower YC where Jurassic–Cretaceous postorogenic magmatism occurred widely (Li and Li, 2007). The high velocities in the upper crust of the DaBie–SuLu orogenic belt,

CaB, and Lower YC may be attributed to the presence of the Meso-zoic magmatic rocks. At 26 km depth (Fig. 6b), prominent low velocities appear in the DaBie orogenic belt which is opposite to that observed high velocities at shallow depths. These low veloc-ities beneath the eastern DaBie orogenic belt are similar to those Luo et al. (2012) observed and may result from the tectonic col-lapse of orogenic roots in response to lithospheric extension (Zhao and Zheng, 2009). A small patch of high velocities is found around the HHB. At 70 km depths in the uppermost mantle, the most striking feature is prominent low velocity anomalies found beneath the NingZhen and NingWu ore districts. At 160 km depth, instead of the only two ore districts of NingZhen and NingWu, the whole MLYMB is clearly delineated with conspicuous low velocity anoma-lies.

Fig. 7 shows three vertical cross sections of shear wave veloci-ties from the surface down to 250 km depth. Two NW–SE trend-ing profiles A–A′ and B–B′ are approximately orthogonal to the strike of MLYMB; and one profile C–C′ in SW–NE direction crosses the MLYMB (Fig. 6d), nearly parallel to the TLF. The profile A–A′(Fig. 7a) crosses the HHB in the NCC, the NingWu ore district in the MLYMB, and the Lower YC. This profile displays a conspicu-ous low-velocity zone under the NingWu ore-concentration area

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Fig. 7. Vertical cross-sections of shear wave velocity structure (a, b, c) along the three lines A–A′ , B–B′ and C–C′ , and locations of these cross-sections are shown in Fig. 6d. Absolute velocities are presented in the crust and velocity anomalies which are calculated relative to the modified AK135 global model (Kennett et al., 1995)are shown in the mantle. The thin black line is the Moho. Topography is plotted above each profile (black area).

in the MLYMB from about 70 km to 200 km. On the west side of the TLF, a relatively weak low-velocity zone at the depth from 100 km to 200 km is also observed between LiXin city and the TLF. The SinoProbe multi-disciplinary transect, which starts from LiXin city in the southern NCC, traverses the TLF and the NingWu ore-concentration area in the MLYMB, and then ends at YiXing city at the northern margin of the YC, overlaps with the middle section of our profile A–A′ . The 3-D P-wave tomography results from the SinoProbe transect also revealed a low-velocity zone at ∼100 km to ∼200 km depth beneath the NingWu ore district in the MLYMB, and on the west side of the TLF (Jiang et al., 2013). The profile B–B′ (Fig. 7b) crosses the NCC, the eastern part of the DaBie oro-genic belt, the Anqing–Guichi ore district in the MLYMB, the Lower YC and the CaB. The most pronounced feature in the profile is an evident low velocity anomaly in the lower crust and uppermost mantle beneath the eastern DaBie orogenic belt. The low veloci-ties in the lower crust at the 25–35 km depth are also observed by Luo et al. (2012). Meanwhile, this profile also shows a flat continu-ous low-velocity zone at the 100–200 km depth beneath the DaBie orogenic belt, the Anqing–Guichi ore district and the Lower YC. The amplitude of the low-velocity zone around the TLF including the eastern DaBie orogenic belt and the Anqing–Guichi ore dis-trict appears much stronger than other parts along the profile. The profile C–C′ (Fig. 7c), which crosses through the Lower YC, the six ore districts (JiuRui, Anqing–Guichi, LuZong, TongLing, NingWu and NingZhen) in the MLYMB, and the SBB in the Lower YC, dis-plays a continuous slightly southwest-dipping low-velocity zone in

the upper mantle. We notice that the low-velocity zone beneath the Lower YC is at the depths between 140 km and 240 km with about −2% anomalies. Beneath the southeast section of the MLYMB (including the JiuRui, Anqing–Guichi, LuZong and TongLing ore dis-tricts) the depth extent of the low-velocity zone becomes shal-lower and the amplitude of low velocity anomaly becomes larger than beneath the Lower YC. The strongest low velocity zone with amplitude of about −4 to −5% is distributed at the depths be-tween 70 km and 200 km beneath the NingWu and NingZhen ore districts, which is much shallower than those beneath the south-west section of the MLYMB.

To verify the ability of our data to resolve such a low-velocity zone in the upper mantle, we perform a resolution test. The testing model is laterally homogeneous except that a stair-stepping low-velocity zone is introduced beneath the MLYMB, which has a 3% velocity reduction from 140 km to 200 km beneath the JiuRui ore district, a 4% velocity reduction at depths of 120 km to 180 km be-neath the Anqing–Guichi, LuZong and TongLing ore districts, and a 5% velocity reduction from 90 km to 160 km beneath the NingWu and NingZhen ore districts. The cross section of the testing model and the recovered model along the same profile C–C′ is plotted together for comparison in Fig. S2. As seen in Fig. S2, the major feature of the low-velocity zone in the upper mantle is recov-ered very well except that the amplitude of the velocity reduction is slightly underestimated and the boundaries of the low-velocity zone are smeared vertically.

5. Discussion

The most striking feature of our 3-D velocity model is the low velocity anomalies observed at the ∼100–200 km depth be-neath the MLYMB (Figs. 6d, 7c) with the strongest low velocity anomaly appearing beneath the NingWu and NingZhen ore dis-tricts at the depth between ∼ 70 km and ∼200 km (Figs. 7a, 7c). The low-velocity zone beneath the NingWu and NingZhen ore districts is also observed in the most recent 3-D P-wave tomog-raphy studies (Jiang et al., 2013; Zheng and Li, 2013). Jiang et al.(2013) interpreted the low-velocity zone as the upwelling astheno-sphere induced by the detached lithosphere and suggested that the asthenosphere upwelling accompanying lithosphere delamination could be related to the subduction of the paleo-Pacific plate and cause large-scale melting which resulted in extensive magmatism and the rich mineralization in the MLYMB in the Mesozoic. Zheng and Li (2013) suggested that the break-off of the subducted YC may have led to the upwelling of the asthenosphere and the for-mation of mineral deposits in the MLYMB. However, Jiang et al.(2013) only concentrated on the northeastern MLYMB and Zheng and Li (2013) used only 11 broadband stations. Thus, their resolu-tion in the Anqing–Guichi and the LuZong ore districts is very poor, which renders it hard to evaluate the spatial distribution of the uppermost mantle low-velocity zone beneath the whole MLYMB from their studies. Our results display a continuous northeast up-dipping low-velocity zone beneath the MLYMB from the JiuRui to the NingZhen ore districts (Fig. 7c). The depth extent of the low-velocity zone becomes shallower and the amplitude of low velocity anomaly becomes larger from the southwest JiuRui ore district to the northeast NingZhen ore district.

Besides being characterized by the apparent upper mantle low velocity anomalies, the MLYMB is also revealed by other geophys-ical data. The Bouguer gravity anomalies are the manifestation of inhomogeneous density distribution in the interior of the Earth. The Bouguer gravity anomaly map shows that the MLYMB is dis-played as a “nose-shaped” positive Bouguer gravity anomaly zone (Lü et al., 2005). The positive Bouguer gravity anomaly in the MLYMB may mainly reflect the doming relief of the Moho and may be caused by the rise of upper mantle. Deep seismic reflec-

386 L. Ouyang et al. / Earth and Planetary Science Letters 408 (2014) 378–389

Fig. 8. Schematic model for the upper mantle low-velocity zone beneath the MLYMB and its relationship with the rich mineralization and extensive magmatism in the MLYMB.

tion profiling can reveal the crustal deformation by probing the detailed structures of the crust. Deep reflection seismic profiles in the MLYMB reveal a thrust-fold structure in the upper crust and strong layered reflections in the lower crust (Lü et al., 2005;Dong et al., 2013; Lü et al., 2013), which are thought to be most likely caused by crustal extension and mantle uplift. Zhang et al.(2014) constructed a 2-D crustal density model based on gravity data collected along the LiXin–YiXing (Fig. 6d) multidisciplinary profile and revealed a high-density zone in the ductile lower crust beneath the NingWu ore concentration. The formation of high-density zone in the ductile lower crust may be caused by the crystallization of upwelling magma from the upper mantle (Zhang et al., 2014). Receiver function and SKS/SKKS shear-wave splitting results from Shi et al. (2013) reveal belt-parallel seismic azimuthal anisotropy in the lower crust and upper mantle of the MLYMB, and Shi et al. (2013) suggested that the lower crustal and upper man-tle anisotropy beneath the MLYMB are most likely caused by the presence of aligned anisotropic minerals, such as amphibole, bi-otite and olivine, which could be consequences of the upwelling of upper mantle magma and then flowing of magma in the lower crust along the belt under the extensional regime. These results discussed above reveal various geophysical characteristics of the MLYMB and all suggest that the large-scale mineralization and in-tensive magmatic activities in the MLYMB are associated with the upwelling of mantle magma (Dong et al., 2013; Lü et al., 2013;Shi et al., 2013; Xu et al., 2014; Zhang et al., 2014).

Based on a large number of geochemical and isotopic investiga-tions, several different mechanisms have also been proposed to ex-plain the formation of the ore-bearing rocks and related Cretaceous igneous rocks in the MLYMB (Xu et al., 2002; Wang et al., 2006;Hou et al., 2007; Ling et al., 2009; X.H. Li et al., 2013). These models can be classified into two end-member groups: (1) par-tial melting of thickened and/or delamination lower continental crust (Xu et al., 2002; Wang et al., 2006; Hou et al., 2007); (2) par-tial melting of a subducted plate or enriched mantle source which is induced by subduction (Ling et al., 2009; X.H. Li et al., 2013;Yang et al., 2014). The fact that the formation of Late Mesozoic rift basins in the MLYR (SBB, HHB and JHB) were related to a litho-spheric extensional regime does not support the crustal thickening in the MLYMB (Ren et al., 2002). Continental adakitic rocks in the MLYMB are closely associated with Cu-Au mineralization (Xu et al., 2002; Sun et al., 2011). Recently, many researchers found that the continental adakitic rocks in the MLYMB consist of monzo-diorite, granodiorite and quartz monzonite and are characterized with high potassium (e.g. Ling et al., 2009; X.H. Li et al., 2013). They suggested that these continental adakitic rocks are proba-bly formed by partial melting of the subduction oceanic litho-sphere or a subduction-enriched mantle source (Ling et al., 2009;

X.H. Li et al., 2013; Yang et al., 2014). Based on the geochemi-cal features of these continental adakitic rocks and previous geo-physics researches, we infer that the low-velocity zone in the up-per mantle at the ∼100–200 depth beneath the MLYMB observed in this study may represent the cooling hot upper mantle rocks which was partially molten in the past resulting from the partial melting of oceanic plate or from melting of an enriched mantle source induced by the subduction process. Our tomographic re-sults provide strong seismic evidence to support the second group model.

Besides the geochemical features of the ore-bearing rocks and related igneous rocks, the tectonic dynamic history of the MLYR could also provide important constraints to the models of the for-mation of these rocks in the MLYMB. The geodynamic evolution of the MLYR could be divided into three major stages with two major tectonic transitions. In the first stage (from the Cambrian to the Early Triassic), the MLYMB had a relative tectonic quies-cence status and several parallel unconformities are distributed throughout the whole belt (Chang et al., 1991; Mao et al., 2011). In the Middle Triassic, the tectonic regime changed from quies-cence to intense compression, which is induced by the collision between the NCC and YC. The large-scale collision has resulted in the development of extensive networks of folds and faults in or around the MLYMB (Chang et al., 1991). In the third stage (from Late Jurassic to the Early Cretaceous), the MLYMB has experienced a tectonic transformation from the early north–south compres-sional regime to late extensional rift regime, which is probably induced by the subduction of the paleo-Pacific plate into the Eu-roasian plate (Ren et al., 2002; Li and Li, 2007; Zheng et al., 2014). In this stage, the MLYMB is characterized with intensive volcanic–magmatic activities and the TLF, which was initiated at the second stage, was reactivated (Wang, 2006; Mao et al., 2011;X.H. Li et al., 2013). The main ore deposits of the MLYMB were formed during this stage and the episodes of mineralization are coeval with the episodes of volcanic–magmatic activities, which imply that both ores and igneous rocks have identical sources (Mao et al., 2011). The obvious low-velocity zone in the upper mantle beneath the MLYMB may be caused by partial melting of paleo-Pacific plate or melting of an enriched mantle source in-duced by the subduction of the paleo-Pacific plate (Fig. 8). At the same time, eastern China, being a component of the circum-Pacific tectono–magmatic activity zone, was characterized by widespread extension and rifting and anorogenic magmatism (Ren et al., 2002;Li and Li, 2007). The back-arc rifting induced by the subduction of the paleo-Pacific plate led to the formation of several continen-tal rift basins (SBB, JHB, HHB) in the MLYR (Ren et al., 2002). The westward subduction of the paleo-Pacific plate can also account for the low velocities at ∼100–200 km depth beneath the eastern CaB

L. Ouyang et al. / Earth and Planetary Science Letters 408 (2014) 378–389 387

and Lower YC, and the distribution of late Mesozoic magmatism and metallogenic events in the southeastern China.

According to the geochronological data from the Late Mesozoic Cu–Au–Mo-related magmatic rocks, magnetite–apatite-related vol-canic rocks and A-type granitoids, Yang et al. (2011) found that the peak ages of magmatic events along the MLYMB are successively younger from the southwest to northeast. In the early stage of sub-duction, magmatic activity mainly occurred in the JiuRui ore dis-trict. The U–Pb zircon ages for the Cu–Au–Mo-related magmatism in the JiuRui ore district range from 148 to 138 Ma with a peak at 148–142 Ma (Li et al., 2010; Yang et al., 2011). As the subduction slab changed from amphibolites facies to eclogite facies, its den-sity increased and it became less buoyant (Yang et al., 2014). The subduction angle increased and shortened the arc-trench distance, resulting in a shift of the magmatic activity from west to east into the TongLing and Anqing–Guichi ore districts (Yang et al., 2014). So the peak age of Cu–Au–Mo-related magmatic event in the TongLing ore district is between 142 and 138 Ma which is slightly younger than those in the JiuRui ore district (Li et al., 2010; Yang et al., 2011). At last, the back-arc extension caused pressure release in the crust and lithospheric mantle and gave rise to magmatism in the NingWu and NingZhen ore districts (Yang et al., 2014). The magnetite–apatite-related volcanic rocks and A-type granitoids in the NingWu ore district, which formed in 136–125 Ma, be-longs to this younger generation of magmatism (Yang et al., 2011;Mao et al., 2011). Our results which show that the depth extent of the low-velocity zone becomes shallower and the amplitude of low velocity anomaly becomes larger from the southwest JiuRui ore district to northeast NingWu ore districts in the MLYMB are con-sistent with the peak ages of magmatic events along the MLYMB which progressively become younger and younger from 148 Ma in the southwest to 125 Ma in the northeast.

On the basis of our surface wave tomographic results in the uppermost mantle of the MLYR and previous research results, we propose the following process to explain the ore genesis and the formation of the related igneous rocks in the MLYMB. From late Jurassic to the early Cretaceous, partial melting of paleo-Pacific plate or melting of an enriched mantle source induced by the westward subduction of the paleo-Pacific plate occurred beneath the MLYMB. The low-velocity zone beneath the MLYMB is proba-bly the principal area of formation of magma in the past (Fig. 8). The upwelling of the mantle-derived magmas may result in the formation of these granitic rocks and coeval ores deposits along the MLYMB.

6. Conclusions

We apply ANT and TPWT to 14 months of continuous ambi-ent noise data and 219 teleseismic earthquake data recorded at 157 seismic stations to generate surface wave dispersion maps at periods from 5 to 143 s. Based on the obtained dispersion maps, we construct a 3-D shear wave velocity model from the surface to ∼250 km depth in the MLYR. The 3-D model shows that, in the upper crust, the distribution of shear velocities is closely correlated with the surficial geologic features with basin regions featured with low velocities and mountain regions with high velocities. In the upper mantle, the NingWu and NingZhen ore districts are clearly characterized with the strongest low ve-locity anomaly at ∼70–200 km depth, and the depth extent of the low-velocity zone becomes shallower and the amplitude of low velocity anomaly becomes larger from the southwest JiuRui ore district to the northeast NingWu ore district. The low-velocity zone beneath the MLYMB are probably the principal area of up-per mantle magma reservoir in the past resulting from the partial melting of paleo-Pacific plate or of an enriched mantle source in-duced by the westward subduction of the paleo-Pacific in the past.

Therefore, we consider that the rich mineralization and extensive magmatism in the MLYMB developed from southeast to northwest may mainly result from the subduction of the paleo-Pacific plate under the Eurasian plate in Mesozoic.

Acknowledgements

We are grateful to the China Earthquake Network Center of China Earthquake Administration and China University of Geo-sciences (Beijing) for providing the waveform data at CEArray and temporary stations. We would like to thank Prof. Peter Shearer and two anonymous reviewers for providing critical and construc-tive comments. The manuscript benefited from valuable discus-sions with Profs. Da Zhang and Shaofeng Liu in China University of Geosciences (Beijing). This work was co-supported by Ministry of Finance under Grant No. SinoProbe-03, China Geological Surveyunder Grant No. 1212011220244, National Natural Science Founda-tion of China under Grant No. 41374057, the Program for New Cen-tury Excellent Talents in University (NCET) under Grant No. NCET-12-0948 and the Fundamental Research Funds for the Central Uni-versities. This is contribution 507 from the ARC Centre of Excel-lence for Core to Crust Fluid Systems (http://www.ccfs.mq.edu.au) and 991 in the GEMOC Key Centre (http://www.gemoc.mq.edu.au).

Appendix A. Supplementary material

Supplementary material related to this article can be found on-line at http://dx.doi.org/10.1016/j.epsl.2014.10.017.

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