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Teleseismic P-wave tomography and the upper mantle structure of the Sulu orogenic belt

(China): implications for Triassic collision and exhumation mechanism

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2016 J. Geophys. Eng. 13 845

(http://iopscience.iop.org/1742-2140/13/6/845)

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845 © 2016 Sinopec Geophysical Research Institute Printed in the UK

1. Introduction

High- to ultrahigh-pressure (HP–UHP) minerals that are found in orogenic belts support a deep subduction model of both oceanic and continental crust (Chopin 2003). Since the discoveries of coesite and diamond in the Dabie and Sulu regions in East Central China (Yang and Smith 1989, Enami and Zhang 1990, Xu et al 1992), the Dabie–Sulu orogenic belt was recognized as one of the largest exposed UHP metamor-phic terranes on the earth. The belt was formed at about 230

Ma due to the Triassic collision between the Yangtze plate (YZP) and the North China plate (NCP) (Ames et al 1996, Cong 1996, Hacker et al 1998, 2000, Liou et al 1998). It was widely accepted that the Dabie orogenic belt was formed by deep subduction of the YZP under the NCP (Jahn et al 1996, Ernst 2005, Ernst et  al 2007), and had ever subducted to a depth of at least 100 km in the upper mantle before it rap-idly exhumed to crustal depths (Xu et al 1992, Cong 1996, Liou et al 1998, 2000, Hacker et al 2000). The Sulu orogen, bounded by the Jiashan–Xiangshui fault (JXF) to the south and the Wulian–Yantai fault (WYF) to the north, comprises a HP greenschist facies unit in the south and an UHP eclogite

Teleseismic P-wave tomography and the upper mantle structure of the Sulu orogenic belt (China): implications for Triassic collision and exhumation mechanism

Miao Peng1, Handong Tan1,3, Mei Jiang2, Zhiqin Xu2, Zhonghai Li1 and Lehong Xu1

1 School of Geophysics and Information Technology, China University of Geosciences, Beijing 100083, People’s Republic of China2 Key Laboratory for Continental Tectonics and Dynamics, Institute of Geology, Chinese Academy of Geological Sciences, Beijing 100037, People’s Republic of China

E-mail: thd@cugb.edu.cn

Received 30 July 2015, revised 12 July 2016Accepted for publication 2 August 2016Published 23 September 2016

AbstractAs the largest ultrahigh-pressure (UHP) metamorphic tectonic unit outcropping in the world, the Dabie–Sulu UHP metamorphic belt is considered to be one of the best areas for studying the continental dynamics. However, their continental collision and exhumation mechanism are still debated. We performed a 3D teleseismic P-wave tomography beneath the Sulu orogen for the purpose of understanding the deep structure. The tomographic results show that there is a prominently near-SN-trending low-velocity zone (LVZ) close to the Tanlu fault (TLF), indicating a slab tear of the subducted Yangtze plate (YZP) during the initial Early Triassic collision. Our results also suggest that both the Yangze crustal slab and the North China lithospheric slab were dragged downwards by the subducted oceanic slab, which constituted a ‘two-sided’ subduction mode. A conceptual geodynamic model is proposed to explain the exhumation of Sulu high- to UHP rocks and imply a polyphase exhumation driven by buoyancy of continental materials at different depth and upward extrusion of crustal partial melting rocks to the surface at the later stage.

Keywords: Sulu ultrahigh-pressure metamorphic belt, seismic tomography, fast marching method, subduction/exhumation

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unit in the north (figure 1), representing a similar metamorphic polarity of the Dabie orogen. It has been displaced about 540 km to the NE along the Tanlu fault (TLF) at present (Liou et al 1998, Hacker et al 2000, Leech and Webb 2013). However, the subduction mode might be different from the Dabie oro-genic belt. For example, the crustal metamorphic basement of NCP might be dragged by deep subduction of the YZP, which forming a two-sided subduction model (Xu 2007).

Various mechanisms and models have been proposed to explain the exhumation of UHP rocks in early research efforts, mainly including buoyancy forces and extension (Platt 1993), lower crustal delamination (e.g. Gao et  al 1998, Kern et  al 1999), buoyancy-driven exhumation of upper crustal slices (e.g. Chemenda et al 1995, 1996), and forced flow in a sub-duction channel (e.g. Cloos 1982, Gerya and Stockhert 2002). On the basis of the Chinese Continental Scientific Drilling (CCSD) project (2001–2005) and multi-disciplinary research (e.g. Yang 2003, Xu 2007), a exhumation mechanism pro-posed by Xu et al (2009) recently has represented a polyphase exhumation mode for continental crust that the Sulu UHP metamorphic terrane is composed of several exhumed tec-tonic slices of the subducted continental Yangtze slab.

Although the exhumation of the continental crust has been extensively described, understanding the Triassic collision and origin of the TLF is also of critical importance in solving prob-lems related to the subduction and uplift of UHP metamorphic rocks. The sinistral TLF zone, which terminates abruptly at its south end, separates the Dabie and Sulu orogens. The TLF has been explained in terms of an indenter boundary model (e.g. Yin and Nie 1993), a rotated suture line model (e.g. Gilder et al 1999), a post-collisional orocline model (e.g. Wang et al 2003), a syn-exhumation transform fault model (e.g. Okay and Şengör 1992) and as a syn-subduction transform fault or slab tear (e.g. Zhu et al 2009).

Geophysical surveys played a vital role in probing crust and mantle structure and studying dynamics of the Dabie–Sulu orogenic belts in recent years (Schmid et al 2001, Xu et al 2002, Yuan et al 2003, Dong et al 2004, Li et al 2012, He et al 2014, Lü et al 2015). Several deep seismic reflection pro-files across the Sulu UHP metamorphic belt revealed an asym-metric vaulted slab consisting of HP–UHP metamorphic rocks and exhibited a special face-type complex beltoverlaying the Yangtze craton as the ‘deep root’ for orogenesis by the col-lision between the YZP and the NCP (Yang 2002). The results provide tectonic interpretation of the upper crustal structure, but cannot reveal deeper structure in the upper mantle due to limited depth of detection. As an efficient method to reveal the deep structures of the crust and upper mantle, seismic tomography has been applied to the Dabie (Liu et  al 1995, Wang et al 2000, Luo et al 2012) and Sulu orogenic belt (Xu et  al 2001, 2002, Yang 2009, Huang et  al 2011). Although numerous petrological, geotectonic and geochemical studies were extensively discussed in the region, broadband seismic tomographic research was still relatively limited. Existing tomographic images beneath the Sulu orogenic belt showed a crocodile-like high velocity structure, which implied a crustal detached YZP beneath the NCP (Xu et al 2002). It was also believed that the Sulu and Dabie orogenic belt might have

underwent different geotectonic processes after the Triassic collision.

In order to achieve a better understanding of the deep struc-ture and geodynamic mechanism under the Sulu orogenic belt and adjacent areas, a 3D P wave tomographic model down to 220 km depth was determined by using two separate data sets. In this study, we mainly focused on the upper mantle struc-ture and the relationship between the present P-wave velocity structures and subduction/exhumation mechanism.

2. Data and methods

2.1. Seismic data and data processing

The broadband seismic data used for tomography were com-posed of two parts (figure 1): one part was the broadband seismic survey project for the Binzhou–Rizhao profile imple-mented in the Sulu region from 2003 to 2005 by Institute of Geology, Chinese Academy of Geological Sciences (Jiang et al 2007, Liu et al 2009). This profile passed through the Jiaoliao–Korea block (JLKB), the Jiaolai Basin, the WYF and the Sulu UHP metamorphic belt and reached the YZP on the south. The instruments for data acquisition included REFTEK130 three-component seismographs with bandwidth of 40–60 s and CMG-6TD three-component seismographs with bandwidth of 50–60 s. The other seismic data was from the Data Management Centre of China National Seismic Network in Shandong and Jiangsu province during 2012–2014 (Zheng et  al 2009). The

Figure 1. Schematic of locations of broadband seismic stations used in this study. Blue triangles show stations deployed along a profile from Binzhou to Rizhao during 2003–2005; black triangles show stations from the China National seismic Network. NCP: North China plate; YZP: Yangtze plate; JLKB: Jiaoliao–Korea block; TLF: Tanlu fault; JXF: Jiashan–Xiangshui fault; WYF: Wulian–Yantai fault; UHP: ultrahigh-pressure metamorphic belt; HP: high-pressure metamorphic belt.

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network has 35 three-component seismic stations with inter-sta-tion distance of about 25 km. Joint data processing and analysis were carried out for all these seismic data in this study.

To process the data, we selected teleseismic events from the China Earthquake Data Center (CEDC). Theoretical travel-times were then calculated using the TauP toolkit (Crotwell et  al 1999). We used the GSAC tool (Herrmann 2013) to band-pass filter the waveforms from 0.05 to 2 Hz in order to suppress noise. The first P-wave arrivals were automatically picked for each event using waveform cross-correlation, with the analysis window length of 30 s (−10 s to 20 s with respect to the theoretical first P arrival time). Additional manual adjustments were performed if needed after visual inspection. We finally respectively identified the first arrival of P-wave of the same event obtained at different stations (figure 2).

Several criteria were followed for the arrival time data selection: (1) each event was recorded by more than eight seismic stations; (2) all the teleseismic events were located at an epicentral distance between 30° and 92° from the center of the study area; (3) the travel-time residuals are within ±4.0 s and the relative travel-time residuals are within ±1.0 s. With the above selection criteria, we obtained a total of 16 248 P-wave arrival times from the mobile broadband dataset (352 teleseismic events with magnitude ⩾5.2 from 2003 to 2005) and the fixed station dataset (268 teleseismic events with mag-nitude ⩾5.0 from 2012 to 2014).

2.2. Tomographic method

Seismic tomography is a classic geophysical method used for deep earth probe. Seismic instruments record the seismic waves generated when a natural earthquake occurs, thus the deep structure and rock properties in the earth can be inter-preted. It is an effective method to study lithosphere forma-tion, the state of internal mantle and its dynamic process.

The P-wave teleseismic tomographic method applied in the study is the travel-time inversion algorithms developed by Rawlinson and Urvoy (2006). The method uses an eikonal equation-based fast marching algorithm (FMM) (Sethian and Popovici 1999, Rawlinson and Sambridge 2004, De Kool et al 2006) to solve the forward problem in travel-time calculation, and employs a non-linear iterative inversion scheme to invert multiple parameters, including velocity, interface depth and hypocentral location.

For an elastic medium, the earthquake propagated wave surface satisfies the eikonal equation (Vidale 1988, Qin et al 1992, Hole and Zelt 1995, Qian and Symes 2002):

∇ =Tv x

1x ( ) (1)

where ∇x represents gradient operator, T represents travel time, x is the position vector of a point, and v is the seismic wave velocity at this point; this equation describes the variation of

Figure 2. Teleseismic waveform records of different events at various seismic stations in the study area. (a) The Ms5.6 earthquake occurring in the south (28.15W, 62.33E) of Pakistan at 17 April 2013, 03:15:51, recorded by the China national station network and local station network; (b)–(d) are the waveforms of different events at some stations in the Binzhou–Rizhao profile, Shandong, event (b) is the Ms5.8 earthquake occurring in Tonga Islands (−20.19W, −174.06E) at 5 October 2005, 10:07:21, event (c) is the Ms6.0 earthquake occurring in the east (−7.05W, 146.55E) of New Guinea at 25 October 2005, 19:40:41, and event (d) is the Ms6.4 earthquake occurring in Indian Ocean Ridges (−45.66W, 96.74E) at 29 October 2005, 04:05:53.

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geometry of the wave front at different time points. The FMM solves equation (1) to trace the propagation of the whole wave surface rather than conducts point-to-point ray tracing just through media to calculate the travel-time (Um and Thurber 1987, Sambridge and Kennett 1990).

The inversion procedure is conducted by least-squares approximation to n-dimensional subspace of model space using the iterative subspace method (Kennett et  al 1988, Sambridge 1990). Disturbance δm is composed of a series of (n) M-dimensional base vectors:

∑δ µ µ= ==

m a Aj

n

jj

1 (2)

where A is a projection matrix with M rows and n columns, and the weighting coefficient μj is the length of corresponding base vector aj.

For the derivation and simplification of the above equation, see Rawlinson et al (2001). The perturbation of the model is finally written as:

[ ( ) ]δ ε γ= − +− − − �m A A G C G C A AT Td m

T1 1 1 (3)

where γ is gradient vector; G is the Fréchet matrix for partial derivative calculation in the forward process; and the variables A, γ and G are updated constantly in successive iteration.

An iterative nonlinear inversion scheme has been designed for different cases, including velocity, interface depth, and source location parameters (Rawlinson and Urvoy 2006). The crustal velocity structure and Moho interface depth were inverted simultaneously in this study. We adopted the Moho interface inversion because of the lateral variation of the Moho topography in the study region. CRUST1.0 (Laske et al 2013), an acceptable global crustal model, was used to describe the spatial variations of the Moho interface as a priori information.

When interface structure is inverted for, interface nodes are perturbed in depth, which means that vertically, the layer can expand or contract. However, we found that it was difficult to resolve variable Moho topography by only using teleseismic P wave travel-time residuals based on our previous study in Namche Barwa (Peng et al 2016). The capability of the scheme to reshape Moho interface geometry is also limited in this study. A better way is to simultaneously invert multiple refraction and reflection phases, such as pP, sP, ScP, PcP (De Kool et al 2006, Rawlinson and Urvoy 2006). The availability of these subse-quent phases with different incidence angles has a beneficial effect on the interface determination and tomographic imaging.

We calculated relative travel-time residuals (figure 3) for the seismic rays at each station and inverted the deep velocity structure of this area using the selected seismic data including a total of 10 148 travel-time data from 560 teleseismic events at 55 stations. With a volume from 33.2° to 39.0° in latitude, 115.0°–122.0° in longitude, and 220 km deep, we set up 3D grids of the study region along the NS, EW, and vertical directions in a spherical coordinate system, and obtained 12 × 32 × 32 grids, with the smallest mesh being of 20 km × 20 km × 20 km. The teleseismic travel-time was calculated using the AK135 global reference model, and the initial model was modified from the 1D crustal velocity model (table 1) in the study area (Xu et al 2000).

3. Results and discussions

3.1. Resolution test

To evaluate the reliability and resolution capability of these data, checkerboard tests were conducted first (figure 4). The tests use an input model of alternating regions of high and

Figure 3. Observed relative travel-time residuals (in s) versus epicentral distances (in degrees).

Table 1. The initial 1D P-wave velocity model (modified from Xu et al 2000) in this study area.

Depth (km) 0 <3 >3 <15 >15 <25 >25 <35 >35

Velocity (km s−1) 4.5 5.6 5.8 6.1 6.2 6.3 6.4 6.8 7.8

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low velocities. Travel-time data computed from the model with velocity variation of ±0.4 km s–1 and from the same hypocenter and station locations based on observation data were inverted using the inversion scheme. Gaussian noise with a standard deviation of 100 ms was added to the synth-etic teleseismic arrival time residuals. The 20-dimensional subspace inversion procedure was carried out by six itera-tions with a damping value ε = 4 and smoothing value η = 20.

The results of the checkerboard resolution test show a fairly good quality of recovered checkerboard pattern within the deep crust in the vicinity of dense teleseismic rays (figure 4(a), 20 km), whereas the structure in the shallow ground (<10 km) cannot be recovered well (figures 4(b) and (c)); and the struc-ture in the upper mantle with depth less than 200 km is also adequately resolved even beneath the central area where few broadband seismic stations are located (figure 4(a), 90 km, 120 km and 160 km). Furthermore, we also present ray paths on

Figure 4. Checkerboard resolution tests used for the solution robustness examination. (a) Horizontal slices changing with depth, the first picture is the theoretical input model of the slice at 20 km, and others are inversion results at various depths; (b) the horizontal profiles changing with latitude, the first picture is the theoretical input model of the profile at 35.0°N; (c) the horizontal profiles changing with longitude, the first picture is the theoretical input model of the profile at 118.3°E.

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a horizontal slice and four profiles including AA′, BB′, CC′, and DD′ (figure 5). The following ray-path charts show that ray path coverage is sufficient in the upper mantle beneath the central and southeastern region, especially beneath the Sulu orogen. There is insufficient ray path coverage beneath the NCP because few broadband seismic stations are located there. However, in this study we mainly focused on the upper mantle structure beneath the Sulu orogen. Considering some smearing artifacts (profile CC′ and DD′ in figure 5) on the velocity profiles, the interpreta-tion of these structures should be prudent. On the whole, we can conclude that this set of stations and seismic data can achieve a resolution of at least 40 km, so it is completely feasible to use the same mesh and parameters for observed datasets, which can yield reliable tomographic images.

3.2. Tomographic results

The observed datasets are inverted using the same inversion parameters as the synthetic datasets of checkerboard tests. The final solution model reduces the RMS travel-time residual from an initial value of 368 ms to a final value of 132 ms, which corresponds to the data variance reduction from 0.142 to 0.021 s2.

P-wave velocity perturbation images of horizontal depth slices, from 20 to 200 km under the Sulu UHP metamorphic belt and its adjacent areas, are shown in figure 6. The 20 km slice image exhibits an obvious characteristic of high velocity in the northwest and low velocity in the southwest as a whole. It also shows a strong high-velocity perturbation zone with P-wave velocity perturbation >0.2 km s−1 in the middle crust

Figure 5. Seismic ray-path chart of the Sulu UHP metamorphic belt and its adjacent areas. NCP—North China plate; YZP—Yangtze plate; JLKB—Jiaoliao–Korea plate; WYF—Wulian–Yantai fault; TLF—Tanlu fault; JXF—Jiashan–Xiangshui Fault; UHP—ultrahigh-pressure metamorphic belt.

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beneath the north of the NCP. A relatively continuous crustal low-velocity anomaly is distributed in the south segment of the Sulu UHP metamorphic belt, whereas the 40 km slices of the upper mantle at the same location exhibit high-velocity perturbation. It indicates that the materials may be related to partial melting in the crust beneath the Sulu UHP metamor-phic belt. Geodynamic numerical simulation of a two-sided subduction model demonstrates that partial melting occurs in the subduction channel which is a rheologically weak zone between the strong subducting and overriding plates under the ‘bulge’ of the overriding plate. Continued continental sub-duction leads to the extrusion of these partially molten rocks, which finally exhume to the surface forming a dome structure near the suture zone (Li et al 2011).

Various slice images of the upper mantle show that there is a stable and continuous high-velocity zone developed in the NCP on the north of latitude 36°. But we found that there is a little vertical distortion of structure exists beneath this region because of insufficient ray path coverage. Only one local low-velocity anomaly is distributed in the central part of this high-velocity zone, exhibiting a characteristic of southward migrating with the increase in depth. The slices at

various depths of the upper mantle also show that there are low-velocity bodies widely developed in the deep part of the mantle in the southwestern Shandong region, in particular, there is a near-SN-trending strong LVZ in the upper mantle (80–160 km) about 100 km to the west of the TLF, whose maximum negative perturbation of P-wave velocity exceeds 0.3 km s−1. There is a dominant high-velocity zone distributed in the mantle lithosphere in the southeast area along the JXF, which extends at least to 180 km depth and likely corresponds to a remnant Yangtze slab in the mantle.

Both of the AA′ and BB′ vertical sections  start from the NCP in the north, then pass through the TLF, the Sulu UHP metamorphic belt and the WYF, and finally reach the YZP (figure 7). The AA′ profile shows a remnant of the ancient subducted Yangtze slab towards the NW direction, and a SE-trending detached North China slab. The North China lith-osphere subducts southwards, its main body crossed the TLF, and arrived at the LVL beneath the WYF. These high-velocity slabs went deep to the upper mantle and subducted at least to a depth of 160 km. The low-velocity layer (LVL) was well developed between the Yangtze subduction slab and the North China in the upper mantle.

Figure 6. Map views of P-wave tomography of the Sulu UHP metamorphic belt and its adjacent areas. The main suture zones and large active faults are labeled on the 20 km slice: NCP: North China plate; YZP: Yangtze plate; JLKB: Jiaoliao–Korea block; WYF: Wulian–Yantai fault; TLF: Tanlu fault; JXF: Jiashan–Xiangshui Fault.

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Note that the solution from ray-theoretical inversion of body wave data may suffer from along-raypath smears which tend to produce artifact or under-map sub-horizontal layers (Hung et al 2004, Zhou 2011). Considering the well-known disadvantage, here we focus on the impact from smearing effect and illustrate the distortion of the velocity anomalies along the ray paths in the vertical sections. The smears are illustrated in figure 7 (both of AA′ and BB′ section) where a high-velocity anomaly dips southeastwards beneath the YZP due to the smear along the ray path. In contrast, these high-velocity anomalous bodies repre-senting the detached YZP lithosphere are likely to dip north-westwards inferred from the previous tomographic results (Xu et  al 2001, 2002, Yang 2009, Huang et  al 2011). However, we mainly focused on the deep structure in the upper mantle beneath the Sulu orogen. Except there are some smears in the edge and the shallow depth, our checkerboard resolution tests

(figure 4) show good resolution in the depth range of 80–160 km in the upper mantle beneath the central and southeastern region, especially beneath the Sulu orogen of these vertical sections. A relatively low-velocity anomaly in the crust and upper mantle is distributed beneath the Sulu UHP metamor-phic belt and between two high-velocity bodies, which repre-sent the remnant Yangtze Slab and the remnant North China Slab respectively. The feature is reliable according to raypath distribution analysis (figure 5).

In addition to the LVL beneath the WYF, we also found that there was an SN-trending upper mantle LVZ in the NCP on the west of the TLF based on the 80–180 km tomographic slice images. To trace the extension morphology and occur-rence in the deep part of this LVZ, we drew the CC′ profile perpendicular to this LVZ along 36°N and the DD′ profile cutting the LVZ along 117.8°E (figure 7). The CC′ profile

Figure 7. Vertical cross sections of P-wave tomography under the Sulu UHP metamorphic belt and its adjacent areas. NCP—North China plate; YZP—Yangtze plate; JLKB—Jiaoliao–Korea plate; WYF—Wulian–Yantai fault; TLF—Tanlu fault; JXF—Jiashan–Xiangshui fault; UHP—ultrahigh-pressure metamorphic belt; YZS—Remnant Yangtze slab; NCS—Remnant North China slab; LVZ—low velocity zone.

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shows that this prominent LVZ is about 40 km wide, dipping eastwards and extending from the top of the upper mantle to a depth of 200 km underground. The CC′ profile also exhibits a low-velocity anomalous belt dipping southwards beneath the TLF, which might cut through the Moho interface. But we cannot conclude that it is a deep seated channel for large-scale mantle upwelling due to along-ray path smear of the slow-velocity anomalies. These vertical profiles are recovered well, although a little distortion of structure exists in some regions, i.e. the structure beneath the UHP on the CC′ profile in figure 5, which may be attributed to the vertical smearing. In fact, parts of eastern NCP have experienced lithospheric thinning, which was originally composed of relatively thick Archean or Proterozoic lithosphere that was significantly attenuated to 80 km thickness during Late Mesozoic to Cenozoic time (e.g. Zheng 2008, Zhu et al 2009, Zheng 2012). To the west of the TLF, the obvious LVZ anomalies shown in this paper are con-sistent with previous studies (Zhao et al 2012, Lei 2012). Zhao et al (2012) showed a similar anomalous feature at ~100 km depth resolved from P- and S-wave finite-frequency tomo-graphic slices. Lei (2012) also revealed north–south trending LVZs extending down to the lower mantle under the TLF. Significant lithospheric reactivation mainly extended beneath the central NCP during the Cenozoic from recent receiver func-tion imaging and tomography studies (Chen et al 2006, Huang and Zhao 2006, Zhao et  al 2009). These results suggest the existence of the upwelling of hot material in the asthenosphere under the eastern NCP.

4. Discussions

4.1. Triassic collision between the NCP and the YZP

The Dabie–Sulu orogenic belt is characterized by the occur-rence of eclogite-facies metamorphic rocks due to the Triassic continental collision between the YZP and the NCP in East-Central China (Ames et  al 1996, Cong 1996, Hacker et  al 1998, Liou et al 1998, Ernst et al 2007, Zhang et al 2009, Wu and Zheng 2013). The convergence is related to a series of tectonic processes such as oceanic subduction, terrane accre-tion and continental collision. Continental subduction might follow oceanic subduction during Early Triassic (Gilder et al 1999) owing to the probable pull dragged by the subducted oceanic slab before the prominently event of exhumation of HP–UHP metamorphic rocks in the continental collision zones (e.g. Ames et al 1996, Hacker et al 2000, 2006, Zheng 2012). Numerical models also demonstrate that a thick conti-nental crust can be pulled downwards under the oceanic slab pull and subduct to depths greater than 200 km (Li and Gerya 2009, Li et  al 2011). Although abundant evidence demon-strates that the YZP have experienced northward subduction beneath the NCP along both the Dabie and Sulu orogenic belt (Ames et al 1996, Jahn et al 1996, Hacker et al 2000, 2006, Ernst 2005, Ernst et al 2007), the Triassic collision process and origin of the TLF is still under debate.

Several geodynamic models have been proposed to explain the Triassic collision process and origin of the TLF. An indenter

Figure 8. Tectonic models of collision between the NCP and the YZP from the Triassic to present (modified after Gilder et al 1999) and a conceptual geodynamic model for the exhumation of Sulu UHP rocks (a) The Early Triassic collision between the YZP and the NCP occurred and resulted in the vertical slab tear; (b) the active continental collision continues deforming the YZB and the UHP metamorphic rocks were uplifted to the upper crust; (c) the Dabie–Sulu orogenic belt was formed and the sinistral TLF now cuts the YZP north of Sulu orogen; (d) a 3D model for describing the Early Triassic collision with slab tear; (e) a conceptual geodynamic model for describing the formation and exhumation of Sulu UHP rocks.

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boundary model proposed by Yin and Nie (1993) suggest that the indented YZB had a reversed L shape with the eastern segment of the Dabie–Sulu orogenic belt extending 500 km farther north than the western segment. The indentation was bounded by the sinistral TLF to the west, and the dextra l Honam shear zone (figure 8(c)) on the Korean Peninsula to the east. Paleomagnetic data show relative rotation of the NCB and YZB and various degrees of rotation of those blocks (Zhao and Coe 1987, Gilder et al 1999), which provides con-straints on the YZB–NCB collision and origin of the TLF. It has also been explained in terms of a tear fault model (e.g. Li 1994) and as a syn-subduction transform fault or slab tear (Zhu et al 2009). But these end-member models seems con-troversial and are not able to explain some new results from field investigations stated by later studies (Xu et al 2005, Xu 2007, Zheng 2008, Zhang et al 2009, Leech and Webb 2013, Zhao et al 2016). For example, the syn-collisional structures showing a larger scale of the Sulu orogen and more intense shortening of the NCP (Faure et al 2003, Zhu et al 2009) are incompatible with the indenter boundary model proposed by Yin and Nie (1993).

We prefer an indentation-induced continent tearing model proposed by Zhao et al (2016) to describe Triassic collision between the YZP and the NCP because it is more consistent with our tomographic result, which presents a large near- SN-trending strong LVZ dipping eastwards close to the TLF, and this ductile shear zone might be related to a tear window during the initial Early Triassic collision (figures 8(a) and (d)). It is implied that the NCP promontory behaved as a rigid indenter (Wan and Zeng 2002, Zhu et al 2009) and led to pre-collisional tearing of the closing Paleo-Tethyan oceanic plate along the lateral boundary. Once the YZP collided with the indenter, the vertical tear in the oceanic plate propagated into the passive YZP (Zhao et al 2016). Furthermore, the counter-clockwise rotation of the NCB relative to the SCB as stated by Zhao and Coe (1987) might also occur during the collision and later exhumation, and thus resulting in left-lateral shear on the TLF and 25° counter-clockwise rotation in Sulu orogen rela-tive to Qinling–Dabie orogenic belt in the present (figure 8(c)).

4.2. Implications for exhumation mechanism of Sulu UHP metamorphic rocks

The remarkable exhumation of UHP metamorphic rocks during early continental collision have been increasingly rec-ognized with at least 20 UHP terranes documented around the world (e.g. Liou et al 2004, Yang et al 2009). The increasing evidence suggests that the Sulu UHP metamorphic rocks rap-idly exhumed to crustal depths (Cong 1996, Liou et al 1998, Hacker et al 2000, Liou et al 2000) after the continental deep subduction and formation of UHP metamorphic rocks during the Triassic. It is convincing that the UHP metamorphism in Sulu took place between 243 ± 4 and 225 ± 2 Ma based on U-Pb dating of coesite-bearing domains of zircon sum-marized by Leech and Webb (2013) as a result of numerous publications since 2005. Subsequently, the UHP metamorphic rocks were uplifted to the upper crust during 230–200 Ma at a rate of 4.0–4.5 km Ma−1 (Xu et al 2009). In this stage,

active continental collision continues deforming the YZB, and results in the northern margin of YZB extending farther north (figure 8(b); Gilder et al 1999).

The tectonic styles of continental subduction can be either ‘one-sided’ or ‘two-sided’ and both of them are able to form HP–UHP metamorphic rocks (Li et al 2011). It is extensively accepted that the slab of the YZP subducts northward and sink into the upper mantle and thus forms the ‘one-sided’ subduc-tion, which is not only described in some geodynmic models (Jahn et al 1996, Ernst et al 2007, Lin et al 2009, Yang et al 2012, Xie et  al 2013), but also revealed by seimic tomog-raphy (Xu et al 2001, 2002, Jiang et al 2007, Yang 2009). For example, tomographic images beneath the Sulu orogenic belt showed a crocodile-like high velocity structure, which implied a crustal detached YZP beneath the NCP (Xu et al 2002).

Our tomographic images, however, not only show a high-velocity body in the upper mantle beneath the Sulu orogenic belt, which is likely to correspond to a remnant lithosphere mantle of the YZP, but also reveal a high-velocity slab dipping southwards in the upper mantle of the North China (figure 7, AA′ and BB′), and thus forming a bidirectional subduction mode. This interpretation of the ‘two-sided’ style is consistent with the fact that the Sulu HP–UHP terrane is composed of two basements proposed by Xu et al (2009): an early-middle Proterozoic (⩾2.4 Ga) basement in the north belonging to the NCB and a Neoproterozoic (700–800 Ma) basement in the south belonging to the YZB based on the CCSD deep drilling and many field observation.

Models of HP–UHP rocks exhumation in the Dabie–Sulu orogen mainly include lower crustal delamination (e.g. Gao et  al 1998, Kern et  al 1999), forced flow in a rheologically weak subduction channel, with materials exhumed from dif-ferent depths (Burov et al 2001, Gerya et al 2002, Stöckhert and Gerya 2005) and syn-collisional exhumation of a buoyant-driven crustal slab (Chemenda et al 1995, 1996, Li and Gerya 2009). Based on our tomographic evidence and the previous regional structural geology, petrology and geochronology (e.g. Yang 2003, Xu et al 2005, Xu 2007), we proposed a conceptual geodynamic model to explain the formation and exhumation of Sulu HP–UHP rocks (figure 8(e)). First, both the North China and Yangze subducting slab were composed of a rigid upper crust, a ductile lower crust and a stiff lithospheric upper mantle (Wang et al 2005). During the Triassic continental collision, the dominant Yangze lithospheric slab subducted northward and collided with the NCB. Second, both the Yangze crustal slab and the North China lithospheric slab were dragged down-wards by the subducted oceanic slab oceanic crust and HP–UHP rocks formed during this period. When pressure exceeds the yield strength of the upper crust due to the increasing of depth of subduction, fracture of the Yangze lithospheric slab may occur in the upper mantle. Consequently, during the exhu-mation stage (230 Ma–200 Ma), these UHP slices were rap-idly thrust over a normal, UHP-free middle lower crust along a series of shear zones driven by buoyancy of continental mat-erials. It was believed that rather than detachment between the whole subducting crust and the underlying lithosphere, there was crustal detachment at different layers, forming the poly-phase exhumation mode for continetal crust (Xu et al 2005, Xu

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2007). At the later period, upward extrusion of crustal partial melting rocks from the plume might uplift high-temperature HP–UHP complexes to the surface.

5. Conclusions

Tomographic results show that a ductile shear zone is charac-terized by a conspicuous near-SN-trending LVZ close to the TLF. The YZP collided with the rigid indenter of the NCP, resulting in the vertical tear in the oceanic plate and propa-gating into the passive YZP. The counter-clockwise rotation of the NCB might also occur after the collision and formed left-lateral shear on the TLF and the present Sulu orogen. Our results also represent a ‘two-sided’ subduction style, which is consistent with the fact that the Sulu HP–UHP terrane is com-posed of two basements proposed by Xu et al (2009).

The formation and exhumation of Sulu UHP rocks can be explained by three possible scenarios: (1) the dominant YZP collided with the rigid NCP promontory during the Triassic continental collision and caused pre-collisional tearing of the closing Paleo-Tethyan oceanic plate along the lateral boundary; (2) both the Yangze crustal slab and the North China lithospheric slab were dragged downwards by the subducted oceanic slab, leading to the formation of UHP rocks at dif-ferent depth; and (3) the UHP slices were rapidly exhumed to crustal depths along a series of shear zones driven by buoyancy of continental materials, and finally uplifted toward the surface by the upward extrusion of crustal partial melting rocks.

Acknowledgments

This study was jointly funded by National Natural Science Foundation of China (41374078) and Open Foundation under State Key Laboratory (z1301-a17). The authors also appre-ciate the Data Management Center of China Seismograph Network for providing waveform data for this study.

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