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1 Gondwana Research 23 (23) Contents lists available at SciVerse ScienceDirect Gondwana Research journal homepage: Distinct upper mantle deformation of cratons in response to subduction: Constraints from SKS wave splitting measurements in eastern China Liang Zhao, Tianyu Zheng, Gang Lu 2 State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing, China article info abstract Article history: Received 29 October 2 Received in revised form 7 April 22 Accepted 22 April 22 Available online 2 May 22 Keywords: Seismic anisotropy Shear wave splitting Eastern China Pacific plate subduction Interaction between the subducting slab, the overriding continental lithosphere and mantle flow are fundamental geodynamic processes of subduction systems. Eastern China is an ideal natural laboratory to investigate the behavior and evolution of cratonic blocks within a subduction system. In this study, we investigate deformation of the upper mantle beneath eastern China. We present seismic shear wave splitting measurements from three networks consisting of over 483 broadband stations, with 57 stations giving a total of 56 results. The splitting parameters exhibit complex regional patterns but are relatively coherent within individual tectonic units. Tectonic blocks exhibited distinctive fast directions relative to regional features. The dominant attitude of fast directions for the North China Craton was subparallel to the direction of subduction, whereas fast directions for Southeastern China were perpendicular to the direction of subduction. The shear wave splitting measurements were interpreted according to a high resolution tomographic body-wave velocity model. Combining these two datasets showed that the predominant geodynamic models for the region (mantle plume, mantle wedge and flat-slab subduction models) are incompatible with the observations presented here. We suggest that the North China Craton, Yangtze Craton and the Cathaysia block have undergone different deformational events due to differing mantle flow patterns, and distinct spatial and temporal subduction histories of the Pacific and Philippine Sea plates. 22 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.. Introduction Subduction is one of the most important processes in Earth science due to its far reaching ability to impart major compositional and physical change in continental lithosphere (e.g., Armstrong, 98; von Huene and Scholl, 99; Stern, 22; Hacker et al., 23; Stern, 2; Straub and Zellmer, 22). The subducting slab draws cold, relatively dense and hydrated material into areas beneath the continental margin, altering the material's stress, thermal conditions and physical properties. Asthenospheric mantle drawn toward the trench by the sinking slab interacts with water and incompatible elements rising from the sinking plate, causing the mantle to melt (Stern, 22). The angle, convergence rate and geometry of the subducting plate are influenced by the interplay between the down-going slab, its induced mantle flow, and the overriding continental lithosphere. Causal patterns in induced mantle flow are difficult to discern owing to the diversity in subduction zone parameters listed above. When a cratonic block with a complex tectonic Corresponding author at: State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beitucheng West Road 9#, P.O.BOX: 9825, Beijing, 29, China. Tel.: ; fax: addresses: zhaoliang@mail.igcas.ac.cn (L. Zhao), tyzheng@mail.igcas.ac.cn (T. Zheng), ganglu@mail.igcas.ac.cn (G. Lu). Tel.: ; fax: Tel.: ; fax: history becomes the continental margin of a subduction system, the dominant structural grain, and other properties of the craton are also likely to influence subduction parameters. The involvement of old, complex cratons in subduction zone dynamics therefore poses a number of questions that can benefit the overall understanding of convergent processes. Eastern China (Fig. ), located along the eastern margin of the Eurasian plate, is an ideal natural laboratory for investigating feedback between a major craton and nearby subduction zone, namely that of the Pacific and Philippine Sea plates during Mesozoic to Cenozoic time. The issue of how subduction affected the upper mantle of eastern China has attracted a great deal of attention in the past two decades. A variety of models have been proposed to explain interactions between the down-going slab and the mantle beneath the continental lithosphere. Most of the models can be classified into three conceptual frameworks (Fig. 2): () the mantle plume model which focuses on the effect of asthenospheric upwelling (e.g., Deng et al., 24), (2) the mantlewedge model which focuses on the effects of the subducting slab and the induced mantle flow (e.g., Zhao et al., 24; Niu, 25; Maruyama et al., 29; Zhao, 29) and(3) the flat-slab subduction model which emphasizes interactions between oceanic crust and the continental lithosphere (e.g., Li and Li, 27; Zhang et al., 29). In this third model, Li and Li (27) posited a ~25 9 Ma period of flat-slab subduction in southeastern South China with subsequent decoupling of the slab from the continental lithosphere. Their interpretation was based on analysis X/$ see front matter 22 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved. doi:.6/j.gr

2 4 L. Zhao et al. / Gondwana Research 23 (23) of the spatio-temporal progression of Permian Cretaceous igneous activity, and the structural evolution of a Permian Jurassic foreland thrust and fold belt, and basin system in southern China. The upper mantle seismic anisotropy is mainly generated by the lattice preferred orientation (LPO) of major mantle minerals such as olivine. Teleseismic measurements of seismic anisotropy can help understand the character of past and present deformation patterns in the upper mantle (e.g., Vinnik et al., 989; Silver and Chan, 99; Savage, 999; Fouch and Rondenay, 26; Karato et al., 28; Long and Silver, 29a, 29b). Beneath the continent, both the lithosphere and asthenosphere can contribute seismic anisotropy (e.g., Silver, 996; Savage, 999, Fouch and Rondenay, 26; Long and Silver, 29b). On one hand, past deformation was usually recorded by the frozen-in anisotropy in the lithosphere (e.g., Silver, 996). On the other hand, present deformation can develop anisotropy in the asthenosphere more closely related to the local direction of absolute plate motion (APM) in the hotspot reference frame (e.g., Vinnik et al., 989; Fouch and Rondenay, 26). In this study, we investigated the upper mantle anisotropy of eastern China using shear wave splitting analysis of datasets collected from two portable networks, and permanent stations of the Chinese National Seismic Network in southeast China. We combined the available splitting measurements with a tomographic model, and interpreted observations from the North China Craton, Yangtze Craton and the Cathaysia block to determine mantle flow patterns and the apparent spatio-temporal heterogeneities between the Pacific and Philippine Sea subduction systems. 2. Background 2.. Geological setting We use the term eastern China to refer to the Chinese part of Eurasia's eastern margin that is tectonically involved in the Pacific and Philippine Sea subduction systems. Eastern China is tectonically composed of the North China Craton (north), and the South China Block (south; Fig. ). Along the eastern margin of eastern China, the Pacific and Philippine Sea plates are subducting beneath the Eurasian plate. Along its northern margin, eastern China is welded to the Siberian Siberian Craton 48 mm/yr 44 Tibet N C C Y C Ca B 8 mm/yr Philippine Plate Pacific Plate 22 mm/yr 4 Q L M H Y F W N C C Ordos N C C N C B E N C C 36 K L F 22 mm/yr 22 mm/yr Qinling Dabie Sulu T L F 32 L M F Sichuan X F M NCISP 8 (55 stations) 28 X J F Y C SCISP (6 stations) 24 R R F Ca B Fig.. Topographic maps showing the simple tectonics of eastern China and locations of seismic networks used in this study. Blue triangles represent the North China Interior Structure Project (NCISP) and South China Interior Structure Project (SCISP) portable networks, red triangles represent the Chinese National Seismic Network (CNSN) stations, and white dots represent stations used in previous studies (Zhao and Zheng 25, 27; L. Zhao et al., 27; Zhao and Xue, 2; Zhao et al., 2). Grey solid lines show the boundaries of major tectonic blocks; red lines show the major faults; thin white lines indicate strike-slip or wrench faults in the South China Block (Li, 2; Wang et al., 25). The yellow dashed line marks the location of the Pacific plate slab front at 3 6 Ma (Wen and Anderson, 995). Black arrows represent directions of absolute plate motion (Gripp and Gordon, 22). The inset shows topography overlaid by a simplified tectonic map of the region. NCC: North China Craton; NCB: North China Basin; SCB: South China Block; YC: Yangtze Craton; ENCC: eastern NCC; WNCC: western NCC; Qinling-Dabie-Sulu: Qinling-Dabie-Sulu orogenic belt; HYF: Haiyuan fault; KLF: Kunlun fault; QLM: Qilian Mountain; XJF: Xiaojiang fault; RRF: Red River fault; TLF: Tanlu fault; LMF: Longmenshan fault; CaB: Cathaysia Block.

3 L. Zhao et al. / Gondwana Research 23 (23) (a) Mantle plume (b) Mantle wedge (c) Flat-slab subduction Fig. 2. Three conceptual models for mantle flow patterns beneath eastern China during Mesozoic to Cenozoic time. On the surface of each map, black arrows show the fast polarization directions expected for SKS waves. (a) Mantle plume model; white circle and dot indicate a speculated center of the mantle plume above an upwelling column (red shape). (b) Mantle-wedge flow model; orange arrow represents the mantle flow motion. (c) Flat-slab subduction model. The expected splitting fast directions remain uncertain for the flat-slab subduction model (see text). Craton by the NE trending Xing'an-Mongolian orogenic belt. To the southwest, it is separated from the Tibetan Plateau by the NW SE trending Longmenshan belt. The formation of eastern China was completed by the Late Jurassic period, following the amalgamation of the North China Craton and the South China Block (Zhang, 997; Meng and Zhang, 2; Faure et al., 2). Paleomagnetic data and sedimentary records indicate that convergence lasted from the Late Permian to the Middle Jurassic, and caused a ~7 clockwise rotation of the South China Block with respect to North China (Zhao and Coe, 987; Meng et al., 25; Huang et al., 28). This collision produced the E-W-trending Qinling-Dabie-Sulu ultrahigh-pressure orogenic belt. The South China Block can be subdivided into the Precambrian Yangtze Craton to the northwest, and the Cathaysia Block to the southeast. The amalgamation of the Yangtze Craton with the Cathaysia Block was completed at ca. 88 Ma (Li et al., 29). The initiation and early tectonic history of the Paleo-Pacific subduction system in this region are poorly understood partly due to the loss of information inherent in the destruction of ocean crust during subduction. The subduction system's history is mainly inferred from continental records (Sun et al., 27). On the basis of geological, geochemical and geophysical analysis of Mesozoic igneous rocks, authors (Zhou and Li, 2; Li and Li, 27) proposed that eastern China became an active continental margin sometime before the Jurassic period. From Late Jurassic to Cretaceous, the margin overrode the subducting Pacific plate to the south, and was involved with concurrent oblique subduction of the Izanagi plate to the north (Sun et al., 27 and references therein). Global and regional tomography (Zhao et al., 994, 24; Huang and Zhao, 26; Zhao et al., 29; Li and van der Hilst, 2) indicate that most of the slab material beneath the Western Pacific and East Asia have stagnated in the mantle transition zone as a result of subduction of the Pacific plate from the east, and of the Philippine Sea plate from the south. Wen and Anderson (995) proposed that the front of the stagnant slab was physically non-uniform, and located beneath eastern China at ~3 6 Ma (see Fig. ). At present, the Pacific and Philippine Sea plates have absolute plate motions of mm/year and 8 mm/year towards the northwest, respectively (Gripp and Gordon, 22; Fig. ). Tomographic models (e.g., Huang and Zhao, 26; Li and van der Hilst, 2), and regional receiver function images of the mantle transition zone (Chen and Ai 29; Chen, 2; Xu et al., 2) illustrate that the Pacific plate's slab front is located at about ~5 E, beneath the South China Block. This area is further west than the slab's corresponding position beneath the North China Craton, located at ~8 E. The Pacific plate subduction system reactivated the upper mantle of eastern China during Mesozoic to Cenozoic time (e.g., Menzies et al., 993; Griffin et al., 998; Wu et al., 25, Zhu et al., 2). This reactivation manifests tectonically as large-scale extension, metamorphic core complexes (e.g., Ren et al., 22; Darby et al., 24; Wu et al., 25; Lin et al., 28) and widespread Mesozoic igneous activities (e.g., Wu et al., 25; Li and Li, 27). A geochronological study by Wu et al. (25) indicates that the Early Cretaceous was a period of significant igneous activity in eastern China. Early Cretaceous magmatisms were widespread across eastern China, but diminished westward (Ratschbacher et al., 23). The lithospheric thickness of the North China Craton decreases progressively from west to east (Chen et al., 28), indicating a possible relationship between upper mantle reactivation and subduction of the Pacific plate Previous studies on the upper mantle anisotropy of eastern China Numerous regional and large-scale shear wave splitting studies have been conducted in the last 2 years to better understand the upper mantle anisotropy in eastern China. Previous large scale studies (e.g., Zheng and Gao, 994; Liu et al., 2; Luo et al., 24; D. Zhao et al., 27; L. Zhao et al., 27; Chang et al., 29; Huang et al., 2) have generally used the Chinese Digital Seismic Network, whose stations are relatively sparse (average spacing > 3 km), and thus offer only limited spatial resolution. Using data from 33 permanent stations, D. Zhao et al. (27), L. Zhao et al. (27) reported pronounced spatial variations in upper mantle anisotropy beneath eastern China. An extensive study by Huang et al. (2) analyzed shear-wave splitting parameters from 38 permanent stations in continental China. This report showed that the uniform fast directions of the anisotropy in eastern China

4 42 L. Zhao et al. / Gondwana Research 23 (23) (WNW ESE) are generally consistent with the absolute plate motion direction of the Eurasian plate, suggesting that the anisotropy is mainly located in the asthenosphere. Regional studies using better seismic station coverage (e.g., Zhao and Zheng, 25; Liu et al., 28; Bai et al., 2; Li and Niu, 2; Zhao and Xue, 2; Chang et al., 2; Li et al., 2; Zhao et al., 2)showgreaterspatialvariations in splitting parameters however. Previous regional studies have focused on the North China Craton, Northeast China or Tibet (Huang et al., 2; Zhao and Zheng, 25; Lev et al., 26; Zhao and Zheng, 27; Liu et al., 28; Wang et al., 28; Zhao et al., 28; Bai et al., 2; Li and Niu, 2; Zhao and Xue, 2; Chang et al., 2; Li et al., 2; Zhao et al., 2). For example, a series of shear wave splitting measurements performed by Zhao and Zheng (25, 27), D. Zhao et al. (27), L. Zhao et al. (27), Zhao et al. (28, 2), and Zhao and Xue (2) using data from 57 broadband stations, revealed relatively complicated upper mantle patterns beneath the North China Craton and adjacent areas. Li and Niu (2) measured SKS wave splitting parameters at 8 stations deployed throughout northeast China. Based on the observation that splitting times correlated positively to lithospheric thickness, Li and Niu (2) suggested that lithospheric deformation may cause the seismic anisotropy observed in northeast China. In contrast to northern China, the South China Block has not been studied at such high seismic resolution. In the absence of a detailed image of upper mantle anisotropy for this region, upper mantle deformation patterns and overall mechanisms for the evolution of eastern China remain uncertain (e.g., Zhao and Zheng, 27; Huang et al., 2). In recent years, large-scale portable seismic arrays (e.g., Zheng et al., 28b) and upgrades to the Chinese National Seismic Network (CNSN) have significantly improved the quality and extent of seismic observation in the South China Block (Zheng et al., 2). These developments provide us with an opportunity to improve the understanding of geodynamic processes affecting eastern China. 3. Data and analysis 3.. Data This study used data recorded at 483 seismic stations distributed across three different networks (Fig. ): the North China Interior Structure Project (NCISP-8, 55 stations), the South China Interior Structure Project (SCISP-, 6 stations) and the Chinese National Seismic Network (CNSN, 367 stations). The NCISP-8 network is a dense NW SE trending linear array with an average station spacing of 5 km, that operated from October, 28 to March, 2. The SCISP- network had an average station spacing of 25 km and operated from November, 29 to March, 2. Both the NCISP and SCISP networks were equipped with CMG-3ESP or 3T sensors, and REFTEK-72A or 3 dataacquisition systems. Data from the CNSN permanent stations were acquired from July, 27 to May, 2 (Zheng et al., 2). A fraction of the data acquired by the CNSN stations and used here has been reported in previous studies (Luo et al., 24; Zhao and Zheng, 27; Huang et al., 2). The bulk of the data used in this study however has not been previously reported or used in other studies. Shear wave analysis was based on data from 65 teleseismic events (5 recorded by NCISP-8, 5 by SCISP-, and 45 by CNSN), occurring at epicentral distances of 85 to 5 from their respective stations (Fig. 3). The events mostly originated in the area of the Tonga trench, which is roughly located in the 9 8 quadrant of the back azimuth shown in the inset of Fig. 3. Due to the distribution of teleseismic events, relatively few data were available for stations located in the Cathaysia Block. 36 N C C 32 SCXCE L M F S C B CQCHS Qinling Dabie Sulu X F M Y C ST4 T L F NCISP 8 ST24 28 X J F SCISP YNYOS 24 R R F GXHCS GXDHX Ca B YNJIG Fig. 3. Map showing new SKS splitting results for the study region. Black bars represent results from NCISP-8 and SCISP- stations and blue bars represent results from CNSN stations. The orientation of each bar indicates the fast direction and bar length is proportional to the associated delay time. Small grey crosses represent null results, indicating shear wave orientation parallel or perpendicular to the polarization direction of the incoming waveform (the back azimuth of the event). Stations mentioned in the text are marked as white dots with abbreviated labels (see Table S). The inset shows the study area (blue trapezoid) and locations of seismic events (red dots) analyzed in this study. sec null

5 L. Zhao et al. / Gondwana Research 23 (23) In order to reduce noise, we applied a two-pole Butterworth bandpass filter to the data. The filter used a lower corner frequency of.2 Hz and upper corner frequency of.2 Hz. Splitting parameter calculations are sensitive to the selection of the time window over which waveforms are processed. To select the optimal time window, we applied an auto-adapted time-windowing technique similar to that of Evans et al. (26). This method begins with an initial time window selected by visual inspection. The time window is then adjusted to minimize error in significance testing of the associated splitting parameters (F-test analysis; Silver and Chan, 99). As a means of quality control, we calculated signal to noise ratios of the radial component (SNRr) for each SKS record as the peak amplitude of the SKS phase relative to the average amplitude of the time window, 8 s prior to the onset of the SKS phase. All SKS records with SNRrb8 were excluded from further analysis Method Our SKS wave analysis followed methods of Silver and Chan (99). The method assumes shear wave splitting across a single homogeneous anisotropic medium in order to derive the fast polarization direction (ϕ) and the delay time (δt) between the fast and slow components of the polarized SKS waves. In a simple case, an anisotropic layer with a horizontal axis of symmetry appears as azimuthal anisotropy. For the more complex cases of multi-layer anisotropy or anisotropy with a dipping axis of symmetry, splitting parameters depend on the back azimuth of the event. For the convenience of the Discussion section and to address certain measurement instabilities, we describe the algorithm for the analysis using the notation of Kennett (22) below. In terms of the incident polarization azimuth (θ) and the fast shear wave polarization direction (ϕ) of anisotropic media, ψ ¼ θ ϕ; For a point where seismic waves transit into anisotropic media, the fast and slow components are written as fðþ t R ¼ R i ðþ t st ðþ AW ; ðþ T i ðþ t where f(t) and s(t) are the fast and slow components; R i (t) and T i (t) are the incident radial and transverse components, respectively; and the rotation matrix R AW is given as R AW ¼ cosψ sinψ : sinψ cosψ Assuming wave propagation is close to vertical and assigning wave slowness components q f and q s, the model introduces the mean slowness, q, and the wave slowness deviation, Δq,as ~q ¼ q f þ q s =2; Δq ¼ q f q s =2: The phase increment for upward propagation through a vertical distance h is then given by E A U ¼ e iω~qh e iωδqh e iωδqh : Within the receiver frame, the observed radial and transverse component are thus represented by Rt ðþ ¼ R Tt ðþ WA E A R i ðþ t UR AW ; ð2þ T i ðþ t where R(t) and T(t) are the recorded radial and transverse components respectively, and R WA is the matrix transposed of R AW, cosψ sinψ R WA ¼. sinψ cosψ For an incident wave having no transverse energy such as the SKS wave, R i (t)=u(t), and T i (t)=. In this case the time domain expressions for the observed radial and transverse components are Rt ðþ¼u ðþcos t ψ þ u þ ðþsin t ψ; ð3þ Tt ðþ¼ u ðþ u t þ ðþ t sin2ψ=2; ð4þ where u (t) denotesu(t δt/2) (earlier), and u + (t) denotesu(t+δt/ 2)(later). The last step of the procedure determines the splitting parameters (ϕ, δt) using grid search of the ϕ, δt domain on the basis of Eqs. (3) and (4). The optimal solutions minimize the energy on the transverse component Some measurements with figure-8 shaped particle motions From a statistical point of view, previous teleseismic studies of eastern China have yielded consistent results (Liu et al., 2; Luo et al., 24; Lev et al., 26; D. Zhao et al., 27; L. Zhao et al., 27; Liu et al., 28; Wang et al., 28; Chang et al., 29; Bai et al., 2; Li and Niu, 2; Chang et al., 2; Huang et al., 2; Li et al., 2; Zhao et al., 2). We note however that some of these studies obtained fast directions that varied by more than 3 for a given station. For example, varying results were reported for the permanent station BJT in North China in (e.g., Iidaka and Niu, 2; Zhao and Zheng, 25; Bai et al., 2; Li and Niu, 2; Huang et al., 2). Excluding the influence of complex upper mantle anisotropy, Vecsey et al. (28) and Long and van der Hilst (25) addressed possible causes for the above discrepancies using different methodologies. We suggest that measurement instability could also obscure fast direction calculation for a given station. Eqs. (3) and (4) demonstrate that a fixed delay time will give R(t)and T(t) as stable functions of angle, ψ.weusetheproofbycontradictionapproach to illustrate that the inverse of Eq. (4) is unstable. Assuming a stable transverse component function, a small perturbation in R(t)and T(t) would be predicted to cause only a slight shift in ψ if the inverse operation is stable. For convenience, we use α to denote 2ψ. The perturbation terms in Eq. (4) can be decomposed into zero-order and first-order terms T ðþ¼ t Tt ðþþδtðþ t u ðþ¼ut t ðþþδuðþ t α ¼ α þ δα; δα : then, Tt ðþþδtðþ¼ t u ðþ u t þ ðþþδu t ðþ δu t þ ðþ t sinðα þ δαþ=2 ¼ u ðþ u t þ ðþþδu t ðþ δu t þ ðþ t ðsin α cos δαþ cos α sin δαþ=2: Given sin δα, we obtain δα ¼ arccos ð4þ 2½Tt ðþþδtðþ tš!: ð5þ 2TðÞþ t δu ðþ δu t þ ðþ t sin α Here α is a constant. Improper inversion of u(t) and fluctuation in the term δu (t) δu + (t) could cause the angle represented in parentheses on the right side of Eq. (5) to approach values of ; i.e., δα could become quite large. This means that the grid search operation by which ϕ and δt are determined may yield unstable solutions.

6 44 L. Zhao et al. / Gondwana Research 23 (23) Instabilities in the transverse function can thus cause variation in the splitting parameters for a given station. The measurement instability described above is a feature of signals that exhibit figure-8 shaped particle motion after delay time correction. For SKS wave splitting measurements, a good observation should exhibit a linear particle motion after delay-time correction. Figure-8 shaped particle motion however is associated with improper search results for the ϕ, δt domain. Measurements showing signs of this kind of instability were reported in previous studies (e.g., Fig. 4d in Li and Niu, 2; Fig. 3b in Bai et al., 2). (a) Event 27:278:7:7:52:8, ( ,.9+.5 s) A F S SKSac A F S SKSac As recorded delay removed 3 3 Station: YNJIG Azimuth ( ) R T Grid Search Lag time (s) (b) Event 28:85:3:2:37:56, ( 78.+.,.45+. s) A F SKSac A F SKSac As recorded delay removed Azimuth ( ) R T Grid Search Lag time (s) Fig. 4. An example showing splitting parameter measurements obtained at station YNJIG for two events with close back azimuths. The upper panel shows records of radial and transverse displacement. Both components are normalized to the peak amplitude for each station. The letters A and F mark the start and end point of the time window analyzed, respectively. Four small boxes in the left panel illustrate normalized fast and slow components and their particle motions. The right panel shows grid search results in the ϕ, δt domain. The red bars give the optimal result with 2σ-error. The increment of contour line is. (a) Event 29:278:7:7:52 (depth=59 km, baz=7, distance=9.3 ). (b) For event 28:85:3:2:37 (depth=58 km, baz=5, distance=9. ).

7 L. Zhao et al. / Gondwana Research 23 (23) Another example is found within our own data collected from station YNJIG. The YNJIG measurement exhibits figure-8 shaped particle motion (Fig. 4a) and gives splitting parameters that are inconsistent with other measurements (Fig. 4b) from events having similar ray paths. From a kinematic point of view, we can use a pair of Ricker wavelets to illustrate the possible cause of the figure-8 shaped particle motion. Fig. 5 shows that particle motion is strictly linear when the fast and the slow components have the same frequency, but assumes a figure- 8 shape when the two components have different frequencies. The underlying physical cause of the differing frequencies is not well understood. To ensure the reliability of the splitting parameters reported here, we have excluded this type of suspect measurement from our dataset. 4. Results We calculated a total of 56 shear wave splitting measurements including 42 null measurements made using SKS phase data from 57 seismic stations (Table S). It is noted that the minimum errors of fast direction and delay time are fixed to be 5 and. s respectively even if the F-test analysis could give error values less than these values, because previous forward calculations (Zhao, et al., 28) show that changes of fast direction b5 and delay time b. s is actually out of the resolution of the SKS shear wave splitting. Examples of splitting parameters for stations CQCHS and YNYOS are shown in Fig. 6. Fig. 3 shows a map of splitting parameters for each station. The dense coverage of our results allows us to better identify subsurface variations across several important tectonic boundaries, including the Tanlu fault zone (TLF), the Xuefeng Mountain (XFM), the Xiaojiang Fault (XJF) and the Longmenshan Fault (LMF; see Fig. ). Below, we describe regional patterns in splitting parameters and compare them with those reported previous studies. (a) Consistent frequency s 8 s (b) Inconsistent frequency s 7.5 s Particle Motion Particle Motion Fig. 5. Synthetic waveforms generated with % white noise to demonstrate aspects of figure-8 shaped particle motion. (a) A robust (stable) measurement. Left panel shows the fast and slow components with consistent frequency; right panel shows linear particle motion. (b) A suspect measurement. Left panel shows the fast and slow components with inconsistent frequency; right panel shows figure-8 shaped particle motion. 4.. New results in the Yangtze Craton and its boundaries For the Yangtze Craton and its margins, fast directions exhibited complex regional patterns but were relatively coherent within individual tectonic blocks (Fig. 3). The TLF marks the boundary zone between the southern North China Craton and the South China Block. Although the densely spaced NCISP-8 network covered this area, it experienced only infrequent teleseismic events (5 events) and thus gave only 3 results for 2 stations. The available fast directions were mainly oriented NW SE, and the delay times ranged from.3 to 2.2 s. At a permanent station located less than 2 km from the network, Huang et al. (2) reported one splitting result with a NW SE fast direction and a delay time>2 s. Overall, these fast directions are inconsistent with the orientation and structural attitudes of surface features in the area. In the XFM, the fast directions mainly trend NE SW, parallel to the strike of the orogenic belt. To the east, fast directions shift to a ENE WSW orientation, also consistent with change in the strike of the orogenic belt. Several stations in the XFM region yielded large delay times (δt>2.±. s; Table S). To the west of the Sichuan Basin, fast directions exhibited pronounced spatial variation. Our results for this region are generally consistent with those reported in Lev et al. (26) from the portable IRIS-PASSCAL array, but provide greater detail especially for the conjunction zone between the XJF and LMF, and along the southwest boundary of the Yangtze Craton (Fig. 7). Along the southern boundary of the Sichuan Basin, the fast directions are oriented mainly NW SE. Along the juncture between the LMF and the XJF, the fast directions change from a N-S to a NW SE orientation, parallel to the strike of the XJF. This implies that subsurface deformation is coupled to tectonic movement along the XJF. To the south of the Yangtze craton, most of the fast directions exhibit E W or ESE WNW orientations. Lev et al. (26) reported a shift in fast directions across 26 N latitude (moving north to south; marked as the black line in Fig. 7). Our results based on denser coverage indicate that the transition zone should be about 4 km to north (boundaries marked in Figs. 7 and 9). Several stations located outside of the Yangtze Craton gave splitting parameters that were dependent on the event back azimuth, indicating complex anisotropy beneath these areas (e.g., stations YNJIG and SCXCE, see Fig. 8) New results for the Cathaysia Block Many stations within the Cathaysia Block did not contribute valid measurements due to the lack of high quality teleseismic events with epicentral distances>85 (Fig. 3). In the eastern Cathaysia Block (east of E longitude), the majority of fast directions are oriented ENE WSW, with an average delay time of ~. s. These fast directions are approximately parallel to the strike of the Cathaysia-Yangtze boundary belt, and the strike of Mesozoic wrench faults in the area (Li, 2; Wang et al., 25). In the western part of the Cathaysia Block, the majority of fast directions are oriented in an E W direction. Many stations gave splitting parameters that were dependent on the back azimuths of events (e.g., GXHCS and GXDHX; Fig. 8). These results are in good agreement with splitting parameters obtained by previous studies for events with back azimuths ranging from ~3 to 3, and recorded at stations located within the eastern Cathaysia Block (D. Zhao et al., 27; L. Zhao et al., 27; Huang et al., 2). For the southeastern Cathaysia Block, D. Zhao et al. (27), L. Zhao et al. (27) and Huang et al. (2) have reported several measurements with fast directions trending ENE WSW. These earlier measurements are consistent with those reported here for the Cathaysia Yangtze boundary. 5. Depth and origin of the anisotropy SKS wave splitting actually reflects the integrated effects of birefringence along the wave's ray path. Near-vertical propagation of SKS waves provides good horizontal resolution but poor vertical

8 46 L. Zhao et al. / Gondwana Research 23 (23) (a) A STATION: CQCHS SKSac F SR A SKSac F S T (b) As recorded delay removed Grid Search Azimuth ( ) Lag time (s) STATION: YNYOS ASKSac F R ASKSac F T As recorded delay removed Azimuth ( ) Grid Search Lag time (s) Fig. 6. Examples of individual shear wave splitting measurements at stations (a) CQCHS and (b) YNYOS, for event 28:85:3:2:37 (Longitude: 79.8 W, latitude: 23.4 S, back-azimuth 5, epicentral distance 9 ). See Fig. 4 caption for further details. resolution for splitting parameters. The depth of the major subsurface features generating the observed patterns is an important factor in understanding underlying geodynamic processes. Below we discuss methods for resolving features in the vertical dimension. 5.. Determining crustal contribution The eastern China crust is estimated to be ~3 45 km thick (Sun and Toksöz, 26; Zheng et al., 28b; Zhu and Zheng, 29). Assuming an average crustal shear wave velocity of 3.6 km/s, a delay time of. s that was solely produced by anisotropy within the crust would require an anisotropic strength of ~9 4%. The average slow-wave delay time is 3.55±2.93 ms/km for North China, and 2.5±.5 ms/km for the Cathaysia Block (Wu et al., 27, 29). This delay time range corresponds to.2 2.3% anisotropy. The predicted maximum delay time for crust having a thickness of 45 km is ~.25 s. Based on comparison with the majority of the delay times calculated here (ranging from.8 to>2. s), seismic anisotropy apparently occurs primarily in the upper mantle.

9 L. Zhao et al. / Gondwana Research 23 (23) MC L M S S MC2 3 MC5 MC4 MC3 SCXCE MC6 MC7 MC8 MC9 MC 28 MC4 MC3 MC2 MC MC6 MC5 YNYOS X J F MC7 26 MC8 MC9 MC2 MC22 R R F MC23 KMI MC25 24 MC24 YNJIG 22 sec null Fig. 7. Comparison of splitting parameters calculated by this study (blue bars) and a previous study (black bars; Lev et al., 26,) for the western Yangtze Craton and eastern Tibet. The black line is a boundary that separates areas with contrasting fast directions identified by Lev et al. (26). Our data suggests this boundary is slightly to the north, as indicated by the blue line Assessing the uniform asthenospheric flow model To investigate the degree to which asthenospheric flow may contribute to anisotropic observations, we compared fast directions with APM. If the plate is coupled to the underlying asthenosphere, the direction of plate motion will also indicate the direction of asthenospheric flow. The predicted APM direction depends on the frame of reference. The hotspot frame HS3-NUVELA model (Gripp and Gordon, 22) for example predicted a coherent APM direction of ~N 288 ±5 in eastern China (Fig. ), while the GEODVEL 2 model (DeMets et al., 2) predicted a coherent direction of ~N ±5 for the same region. A comparison by Huang et al. (2) demonstrated that fast directions were consistent with the APM direction (HS3-NUVELA) within an angular difference of less than 25. Our results from the denser seismic networks however show that the uniform APM direction does not match the spatially variable fast directions, regardless of the reference frame. Fast directions for the XFM (trending NE SW) and for the Cathaysia Block (trending ENE WSW) calculated here do not match APM directions. The assumption of simple, uniform asthenosphere flow is therefore incompatible with the complex splitting patterns observed in the study area Contributions from the lithosphere and/or asthenosphere To distinguish contributions from the lithosphere and/or asthenosphere to the observed anisotropy, we overlaid SKS wave splitting results from this study and from previous studies (Iidaka and Niu, 2; Luo et al., 24; Zhao and Zheng, 25, 27; D. Zhao et al., 27; L. Zhao et al., 27; Huang et al., 28; Liu et al., 28; Zhao et al., 28; Zhao and Xue, 2; Huang et al., 2; Zhao et al., 2) with a shear wave tomographic model of eastern China (Fig. 9, Zhao et al., in press). The tomographic image is normalized to the vertical average of the shear wave velocity perturbation relative to the IASPI9 model (Kennett and Engdahl, 99). We calculated the shear wave velocity perturbation over a 2 3 km depth range so as to capture the subsurface zone in which most interactions between the asthenosphere and lithosphere generally occur (Zhao et al., 2). Varying the lower limit of the depth range from 2 km to 3 km however had little effect on the distribution of anomalies. Shear wave velocity anomalies can arise in the upper mantle from both temperature differences and partial melting (e.g., Cammarano et al., 23). Theoretical models predict that a 2.5% shear wave velocity anomaly corresponds to a ~2 C temperature perturbation under dry conditions at 2 km depth. We assume that the variation

10 48 L. Zhao et al. / Gondwana Research 23 (23) (a) ø ( ) (b) ø ( ) (c) ø ( ) (d) ø ( ) Station: GXDHX in velocity anomalies reflects the contrasting properties of the lithosphere and asthenosphere, and thus their respective spatial distributions. This kind of division is in good agreement with previous studies of subsurface structures that used other methodologies (e.g., Chen et al., 28; Obrebski et al., 22). For the areas having substantially thicker lithospheric roots (>2 km), such as the Ordos and the Yangtze cratons (grey outlines in Fig. 9), we assume that the observed anisotropic properties belong to the lithosphere rather than the asthenosphere. Precambrian tectonic blocks, such as the Precambrian component of the European continent, have been shown to inherit lithospheric anisotropy (e.g., Babuška Delay time (s) 3 2 Station: GXHCS Delay time (s) 3 2 Station: YNJIG Delay time (s) 3 2 Station: SCXCE Delay time (s) Fig. 8. Relationship between splitting parameters (fast polarization direction, f, shown left and the delay time, dt, shown right) versus back azimuth for stations (a) GXDHX; (b) GXHCS; (c) YNJIG; and (d) SCXCE. Station locations are also labeled in Fig. 3. The rectangle indicates the parameter value; error bars represent 2σ error. and Plomerová, 26; Plomerová and Babuška, 2). Anisotropy may arise from the asthenosphere for thinner areas of the crust such as the Cathaysia Block, which has a lithospheric thickness ofb km. Along the profile of the SCISP- network in the XFM, high velocity anomalies extend to depths of greater than 2 km (Huang and Zhao, 26; Li and van der Hilst, 2; Zhao et al., in press). The observed fast directions are consistent with the strike of surface features related to Mesozoic and Cenozoic tectonic events. This suggests that anisotropic features in the lithosphere beneath these regions are vestiges past tectonic events. In the western Yangtze Craton, shear wave velocity is relatively high within the uppermost mantle and fast directions show complex spatial variations. This zone represents the junction between the LMF and the XJF. The observed anisotropy may therefore reflect the combined and variegated effects of past tectonic events. To the southwest and south of the Yangtze Craton, the fast directions from stations located above prominent low velocity anomalies do not match the strike of local faults. Tomography models show that the lithosphere in this area is relatively thin, with an average thickness ofb km (Obrebski et al., 22; Zhao et al., in press). These lines of evidence suggest that velocity anomalies reflect features in the asthenosphere. The back azimuth dependence of fast directions observed for some stations in this region indicates complex anisotropic behavior that may be caused by multi-layer anisotropy or anisotropy with a dipping axis of symmetry. The Cathaysia Block consists of lithosphere that is much thinner than that of the adjacent Yangtze Craton. The low velocity volume beneath the Cathaysia Block extends to depths of to 3 km. The fast directions are subparallel to the strike of the Cathaysia-Yangtze boundary belt and the strike of the Mesozoic wrench faults in the area (D. Zhao et al., 27; L. Zhao et al., 27; Huang et al., 2). This feature implies a combined contribution from vestigial deformation of the lithosphere and present deformation of the asthenosphere. At the boundary belt between the Yangtze and Cathaysia blocks, the back azimuth dependence of fast directions from stations GXDHX, GXHCS (Figs. 7 and 8)indicates a complex anisotropic behavior, potentially caused by multi-layer anisotropy or anisotropy with a dipping axis of symmetry. The NCISP-8 network in the Sulu orogenic belt traversed a collisional boundary between the North China Craton and the Yangtze Craton. Data from this region revealed a weak high velocity anomaly extending to depths of>2 km (Zhao et al., in press). The fast directions were not parallel to the strike of the orogenic belt formed by the collision of the Yangtze and North China cratons. As discussed below, this pattern may reflect a vestigial lithospheric structure, namely the roots of the South China Block subducted beneath the North China Craton. 6. Discussion Shear wave splitting results reported here are in good agreement with those of previous large-scale (e.g., Luo et al., 24; D. Zhao et al., 27; L. Zhao et al., 27; Huang et al., 2) and regional studies (e.g., Lev et al., 26). The station coverage of this study allows for a more detailed interpretation of the subducting slab, continental lithosphere and mantle flow. Yang et al. (28) showed that olivine in mantle-derived peridotite xenoliths from eastern China contained relatively little water. Given dry conditions in the mantle, shear waves are expected to split with fast directions subparallel to the direction of mantle flow (Jung and Karato, 2). The above relationships and assumptions form the basis for the interpretation of eastern China's tectonic framework given below. Our data revealed significant contrasts between fast directions, the orientation of structural features, and the overall direction of convergence in the study area (Fig. 9). These contrasts varied from region to region, and were deemed less significant in areas where the observed anisotropy was clearly related to vestigial deformation of the lithosphere from past tectonic events. The fast directions in the eastern

11 L. Zhao et al. / Gondwana Research 23 (23) Average Vs perturbation (2 ~ 3 km) 4.8% 4.8% 4 H Y F N C C Q L M 35 Qinling Dabie Sulu T L F L M F 3 25 R R F X J F Y C X F M S C B Ca B 2 Splitting sec null Fig. 9. Splitting measurements obtained by this study (blue bars) and by previous studies (grey bars) overlaid on a tomographic image of the study area (shear-wave velocity vertically averaged over 2 3 km depth; Zhao et al., in press). Black bars show results from Zhao and Zheng (25, 27), Zhao et al. (28, 2),andZhao and Xue (2). Black circles show results from Iidaka and Niu (2), Luo et al. (24), Liu et al. (28), andhuang et al. (28, 2). The red line marks an estimated transition in splitting fast directions between the Yangtze Craton and the Cathaysia Block. Thick grey outlines represent areas having high velocity zones that extend to depths of>2 km where the lithosphere might contribute the majority of anisotropy. Solid dark grey arrows represent directions of absolute plate motion (also shown in Fig.). North China Craton strike subparallel to the direction of subduction, whereas fast directions for the South China Block strike perpendicular to the direction of subduction. This section discusses different geodynamic scenarios and historical factors that may contribute to the observed anisotropic patterns. Upper mantle deformation in eastern China possibly reflects some combination of major tectonic events occurring from Late Proterozoic to Cenozoic time. These include () the Late Proterozoic amalgamation of the South China Block along the Yangtze-Cathaysia boundary; (2) collision between the North China Craton and the South China Block, complete by Late Jurassic time; and (3) spatial variation in mantle flow beneath the North and South blocks induced by Mesozoic to Cenozoic subduction. Below we interpret shear wave splitting patterns with respect to these events. 6.. Effects of the Yangtze-Cathaysia amalgamation The NE SW trending fast directions observed in XFM are parallel to both the strike of Proterozoic Yangtze-Cathaysia orogenic features, and that of Mesozoic intercontinental orogenic features related to Pacific plate subduction (Li and Li, 27). Most of the delay times for this region were greater than.8±.5 s (Table S). Assuming simple transverse anisotropy with a horizontal symmetry axis, average velocity of 4. km/s and constant anisotropic strength of 4% (e.g., Savage, 999), a.8 s delay time corresponds to an anisotropic lithospheric thickness of 8 km. We therefore interpret the observed anisotropy beneath the XFM to reflect an area of overlap between the Yangtze-Cathaysia amalgamation and the Mesozoic orogenic belt Effects of the collision between the South China Block and the North China Craton Zhao et al. (2) reported fast directions that were parallel to the strike of the northern Qinling-Dabie orogenic belt (Fig. 9), and interpreted them as evidence that the collision between the South China Block and the North China Craton contributed to upper mantle deformation in this area. Their interpretation did not address distinct variations in fast directions across the southern Qinling-Dabie Orogenic belt. Along the North China Craton's southern boundary where it abuts the Yangtze craton, fast directions are not parallel to the strike

12 5 L. Zhao et al. / Gondwana Research 23 (23) of the orogenic belt. Geological studies have suggested that the lithosphere of the Yangtze Craton subducted beneath the North China Craton (Meng and Zhang, 2). A northward dipping convergence zone would create bi-layered anisotropy and/or anisotropy with a dipping axis of symmetry. This deformational scenario is consistent with the observed fast directions and the strike of the orogenic belt, but limited event distributions do not allow us to identify more complex anisotropy patterns in support of the subduction hypothesis (i.e., bilayered anisotropy, dipping axis of symmetry) Spatial variation in mantle flow patterns due to subduction Recent tomographic studies attribute the low velocity anomaly volumes in eastern China to the effects of the Pacific and Philippine Sea subduction systems (Huang and Zhao, 26). Large-scale extension and widespread volcanism indicate significant reactivation of deformational features in the lithosphere of eastern China (Wu et al., 25). If the Pacific and Philippine Sea plate subduction zones are driving upper mantle deformation in eastern China, patterns in fast directions for the eastern North China Craton and the Cathaysia Block will reflect the attitudes of convergent plate motion. Zhao and Xue (2) proposed a subduction-induced mantle flow model for the eastern North China Craton in which the mantle flow is parallel to the direction of subduction (Fig. a). This model however fails to account for NE SW to ENE WSW trending fast directions in the Cathaysia Block that strike roughly perpendicular to direction of subduction. The upper mantle anisotropy observed beneath the South China Block therefore cannot be explained solely by subductionrelated deformation of the sub-lithospheric mantle, and requires an alternative geodynamic model. One such alternative is that the sub-lithospheric mantle bears the imprint of earlier deformational features that have rotated out of their original alignment with the subduction zone. In this model the anisotropy reflects convergent deformation with a radial offset that (a) (b) Fig.. Schematic diagram explaining upper mantle deformation patterns in eastern China. (a) Geodynamic model modified from Zhao et al. (2) emphasizing regional mantle convection beneath the eastern North China Craton. (b) Geodynamic model for the Cathaysia Block. Dark blue bi-directional arrows on the surface show the generalized fast directions for the region. Orange sub-surface plumes illustrate regional asthenospheric upwelling deflected at the lithospheric root, and outlining the topography of the lithosphere's lower boundary. The thick light blue arrows represent the horizontal flow-direction. Grey lines outline the margins of cratons with high velocity zones extending to depths of>2 km, where the lithosphere might contribute the majority of anisotropy. Orange arrows represent the migration of asthenospheric upwelling induced by subduction. The regional asthenospheric upwelling could deflect the westward thickening lithosphere, generating an anisotropic pattern aligned with the topography of the lithosphere.