Distinct upper mantle deformation of cratons in response to subduction: Constraintsfrom SKS wave splitting measurements in eastern China

Liang Zhao ⁎, Tianyu Zheng 1, Gang Lu 2

State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing, China

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

Article history:Received 29 October 2011Received in revised form 17 April 2012Accepted 22 April 2012Available online 2 May 2012

Keywords:Seismic anisotropyShear wave splittingEastern ChinaPacific plate subduction

Interaction between the subducting slab, the overriding continental lithosphere andmantle flow are fundamen-tal geodynamic processes of subduction systems. Eastern China is an ideal natural laboratory to investigate thebehavior and evolution of cratonic blocks within a subduction system. In this study, we investigate deformationof the upper mantle beneath eastern China. We present seismic shear wave splitting measurements from threenetworks consisting of over 483 broadband stations, with 157 stations giving a total of 516 results. The splittingparameters 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 di-rections for the North China Craton was subparallel to the direction of subduction, whereas fast directions forSoutheastern China were perpendicular to the direction of subduction. The shear wave splitting measurementswere interpreted according to a high resolution tomographic body-wave velocity model. Combining these twodatasets showed that the predominant geodynamic models for the region (mantle plume, mantle wedge andflat-slab subduction models) are incompatible with the observations presented here. We suggest that theNorth China Craton, Yangtze Craton and the Cathaysia block have undergone different deformational eventsdue to differing mantle flow patterns, and distinct spatial and temporal subduction histories of the Pacific andPhilippine Sea plates.

© 2012 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.

1. Introduction

Subduction is one of the most important processes in Earth sciencedue to its far reaching ability to impart major compositional and physi-cal change in continental lithosphere (e.g., Armstrong, 1981; vonHueneand Scholl, 1991; Stern, 2002; Hacker et al., 2003; Stern, 2011; Strauband Zellmer, 2012). The subducting slab draws cold, relatively denseand hydratedmaterial into areas beneath the continental margin, alter-ing thematerial's stress, thermal conditions and physical properties. As-thenospheric mantle drawn toward the trench by the sinking slabinteracts with water and incompatible elements rising from the sinkingplate, causing the mantle to melt (Stern, 2002). The angle, convergencerate and geometry of the subducting plate are influenced by the inter-play between the down-going slab, its induced mantle flow, and theoverriding continental lithosphere. Causal patterns in induced mantleflow are difficult to discern owing to the diversity in subduction zoneparameters listed above.When a cratonic blockwith a complex tectonic

history becomes the continental margin of a subduction system, thedominant structural grain, and other properties of the craton are alsolikely to influence subduction parameters. The involvement of old, com-plex cratons in subduction zone dynamics therefore poses a number ofquestions that can benefit the overall understanding of convergentprocesses.

Eastern China (Fig. 1), located along the eastern margin of theEurasian plate, is an ideal natural laboratory for investigating feedbackbetween a major craton and nearby subduction zone, namely that ofthe Pacific and Philippine Sea plates during Mesozoic to Cenozoic time.The issue of how subduction affected the upper mantle of easternChina has attracted a great deal of attention in the past two decades. Avariety of models have been proposed to explain interactions betweenthe down-going slab and the mantle beneath the continental litho-sphere.Most of themodels can be classified into three conceptual frame-works (Fig. 2): (1) the mantle plume model which focuses on the effectof asthenospheric upwelling (e.g., Deng et al., 2004), (2) the mantle-wedge model which focuses on the effects of the subducting slab andthe induced mantle flow (e.g., Zhao et al., 2004; Niu, 2005; Maruyamaet al., 2009; Zhao, 2009) and (3) the flat-slab subduction model whichemphasizes interactions between oceanic crust and the continental lith-osphere (e.g., Li and Li, 2007; Zhang et al., 2009). In this third model, Liand Li (2007) posited a ~250–190Ma period of flat-slab subduction insoutheastern South China with subsequent decoupling of the slab fromthe continental lithosphere. Their interpretation was based on analysis

Gondwana Research 23 (2013) 39–53

⁎ Corresponding author at: State Key Laboratory of Lithospheric Evolution, Instituteof Geology and Geophysics, Chinese Academy of Sciences, Beitucheng West Road 19#,P.O.BOX: 9825, Beijing, 100029, China. Tel.: +86 10 82998430; fax: +86 10 62010846.

E-mail addresses: zhaoliang@mail.igcas.ac.cn (L. Zhao), tyzheng@mail.igcas.ac.cn(T. Zheng), ganglu@mail.igcas.ac.cn (G. Lu).

1 Tel.: +86 10 62363458; fax: +86 10 62010846.2 Tel.: +86 10 82998420; fax: +86 10 62010846.

1342-937X/$ – see front matter © 2012 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.doi:10.1016/j.gr.2012.04.007

Contents lists available at SciVerse ScienceDirect

Gondwana Research

j ourna l homepage: www.e lsev ie r .com/ locate /gr

of the spatio-temporal progression of Permian–Cretaceous igneous ac-tivity, and the structural evolution of a Permian–Jurassic foreland thrustand fold belt, and basin system in southern China.

The upper mantle seismic anisotropy is mainly generated by the lat-tice preferred orientation (LPO) of major mantle minerals such as oliv-ine. Teleseismic measurements of seismic anisotropy can helpunderstand the character of past and present deformation patterns inthe upper mantle (e.g., Vinnik et al., 1989; Silver and Chan, 1991;Savage, 1999; Fouch and Rondenay, 2006; Karato et al., 2008; Longand Silver, 2009a, 2009b). Beneath the continent, both the lithosphereand asthenosphere can contribute seismic anisotropy (e.g., Silver,1996; Savage, 1999, Fouch and Rondenay, 2006; Long and Silver,2009b). On one hand, past deformation was usually recorded by thefrozen-in anisotropy in the lithosphere (e.g., Silver, 1996). On theother hand, present deformation can develop anisotropy in the as-thenosphere more closely related to the local direction of absoluteplate motion (APM) in the hotspot reference frame (e.g., Vinnik et al.,1989; Fouch and Rondenay, 2006). In this study, we investigated theupper mantle anisotropy of eastern China using shear wave splitting

analysis of datasets collected from two portable networks, and perma-nent stations of the Chinese National Seismic Network in southeastChina.We combined the available splittingmeasurementswith a tomo-graphic model, and interpreted observations from the North China Cra-ton, Yangtze Craton and the Cathaysia block to determine mantle flowpatterns and the apparent spatio-temporal heterogeneities betweenthe Pacific and Philippine Sea subduction systems.

2. Background

2.1. Geological setting

We use the term “eastern China” to refer to the Chinese part ofEurasia's eastern margin that is tectonically involved in the Pacific andPhilippine Sea subduction systems. Eastern China is tectonically com-posed of the North China Craton (north), and the South China Block(south; Fig. 1). Along the eastern margin of eastern China, the Pacificand Philippine Sea plates are subducting beneath the Eurasian plate.Along its northern margin, eastern China is welded to the Siberian

96˚ 100˚ 104˚ 108˚ 112˚ 116˚ 120˚ 124˚20˚

24˚

28˚

32˚

36˚

40˚

44˚

48˚

Y C

Ca B

N C CW N C C E N C C

SCISP−1(61 stations)

NCISP−8(55 stations)

22 mm/yr

22 mm/yr

22 mm/yr

Ordos

Sichuan

N C B

X F

M

Q L M

T L

FK L F

H Y F

L M

F

X J F

R R F

Qinling Dabie Sulu

Siberian Craton

Tibet

Y CCa B

PhilippinePlate

Pac

ific

Pla

te

80 mm/yr

111 mm/yr

N C C

Fig. 1. 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 Struc-ture Project (NCISP) and South China Interior Structure Project (SCISP) portable networks, red triangles represent the Chinese National Seismic Network (CNSN) stations, and whitedots represent stations used in previous studies (Zhao and Zheng 2005, 2007; L. Zhao et al., 2007; Zhao and Xue, 2010; Zhao et al., 2011). Grey solid lines show the boundaries ofmajor tectonic blocks; red lines show the major faults; thin white lines indicate strike-slip or wrench faults in the South China Block (Li, 2000; Wang et al., 2005). The yellow dashedline marks the location of the Pacific plate slab front at 30–60 Ma (Wen and Anderson, 1995). Black arrows represent directions of absolute plate motion (Gripp and Gordon, 2002).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: Xiaojiangfault; RRF: Red River fault; TLF: Tanlu fault; LMF: Longmenshan fault; CaB: Cathaysia Block.

40 L. Zhao et al. / Gondwana Research 23 (2013) 39–53

Craton by the NE trending Xing'an-Mongolian orogenic belt. To thesouthwest, it is separated from the Tibetan Plateau by the NW–SEtrending Longmenshan belt.

The formation of eastern China was completed by the Late Jurassicperiod, following the amalgamation of the North China Craton and theSouth China Block (Zhang, 1997; Meng and Zhang, 2000; Faure et al.,2001). Paleomagnetic data and sedimentary records indicate that con-vergence lasted from the Late Permian to the Middle Jurassic, and cau-sed a ~70° clockwise rotation of the South China Block with respect to

North China (Zhao and Coe, 1987; Meng et al., 2005; Huang et al.,2008). This collision produced the E-W-trending Qinling-Dabie-Suluultrahigh-pressure orogenic belt.

The South China Block can be subdivided into the PrecambrianYangtze Craton to the northwest, and the Cathaysia Block to the south-east. The amalgamation of the Yangtze Craton with the Cathaysia Blockwas completed at ca. 880 Ma (Li et al., 2009).

The initiation and early tectonic history of the Paleo-Pacific subduc-tion system in this region are poorly understood partly due to the loss ofinformation inherent in the destruction of ocean crust during subduc-tion. The subduction system's history is mainly inferred from continen-tal records (Sun et al., 2007). On the basis of geological, geochemical andgeophysical analysis of Mesozoic igneous rocks, authors (Zhou and Li,2000; Li and Li, 2007) proposed that eastern China became an activecontinental margin sometime before the Jurassic period. From Late Ju-rassic to Cretaceous, the margin overrode the subducting Pacific plateto the south, and was involved with concurrent oblique subduction ofthe Izanagi plate to the north (Sun et al., 2007 and references therein).Global and regional tomography (Zhao et al., 1994, 2004; Huang andZhao, 2006; Zhao et al., 2009; Li and van der Hilst, 2010) indicate thatmost of the slab material beneath the Western Pacific and East Asiahave stagnated in the mantle transition zone as a result of subductionof the Pacific plate from the east, and of the Philippine Sea plate fromthe south. Wen and Anderson (1995) proposed that the front of thestagnant slab was physically non-uniform, and located beneath east-ern China at ~30–60 Ma (see Fig. 1). At present, the Pacific andPhilippine Sea plates have absolute plate motions of 111 mm/yearand 80 mm/year towards the northwest, respectively (Gripp andGordon, 2002; Fig. 1). Tomographic models (e.g., Huang and Zhao,2006; Li and van der Hilst, 2010), and regional receiver function im-ages of the mantle transition zone (Chen and Ai 2009; Chen, 2010;Xu et al., 2011) illustrate that the Pacific plate's slab front is locatedat about ~105°E, beneath the South China Block. This area is furtherwest than the slab's corresponding position beneath the NorthChina Craton, located at ~118°E.

The Pacific plate subduction system reactivated the upper mantle ofeastern China during Mesozoic to Cenozoic time (e.g., Menzies et al.,1993; Griffin et al., 1998; Wu et al., 2005, Zhu et al., 2011). This reac-tivation manifests tectonically as large-scale extension, metamorphiccore complexes (e.g., Ren et al., 2002; Darby et al., 2004; Wu et al.,2005; Lin et al., 2008) and widespread Mesozoic igneous activities(e.g., Wu et al., 2005; Li and Li, 2007). A geochronological study byWu et al. (2005) indicates that the Early Cretaceous was a period of sig-nificant igneous activity in eastern China. Early Cretaceousmagmatismswere widespread across eastern China, but diminished westward(Ratschbacher et al., 2003). The lithospheric thickness of the NorthChina Craton decreases progressively from west to east (Chen et al.,2008), indicating a possible relationship between upper mantle reac-tivation and subduction of the Pacific plate.

2.2. Previous studies on the upper mantle anisotropy of eastern China

Numerous regional and large-scale shear wave splitting studieshave been conducted in the last 20 years to better understand theupper mantle anisotropy in eastern China.

Previous large scale studies (e.g., Zheng and Gao, 1994; Liu et al.,2001; Luo et al., 2004; D. Zhao et al., 2007; L. Zhao et al., 2007; Changet al., 2009; Huang et al., 2011) have generally used the Chinese Dig-ital Seismic Network, whose stations are relatively sparse (averagespacing>300 km), and thus offer only limited spatial resolution.Using data from 33 permanent stations, D. Zhao et al. (2007), L.Zhao et al. (2007) reported pronounced spatial variations in uppermantle anisotropy beneath eastern China. An extensive study byHuang et al. (2011) analyzed shear-wave splitting parameters from138 permanent stations in continental China. This report showedthat the uniform fast directions of the anisotropy in eastern China

(a) Mantle plume

(b) Mantle wedge

(c) Flat-slab subduction

Fig. 2. Three conceptual models for mantle flow patterns beneath eastern China duringMesozoic to Cenozoic time. On the surface of eachmap, black arrows show the fast polar-ization directions expected for SKS waves. (a) Mantle plume model; white circle and dotindicate a speculated center of the mantle plume above an upwelling column (redshape). (b) Mantle-wedge flowmodel; orange arrow represents the mantle flowmotion.(c) Flat-slab subductionmodel. The expected splitting fast directions remain uncertain forthe flat-slab subduction model (see text).

41L. Zhao et al. / Gondwana Research 23 (2013) 39–53

(WNW–ESE) are generally consistent with the absolute plate motiondirection of the Eurasian plate, suggesting that the anisotropy ismainly located in the asthenosphere. Regional studies using betterseismic station coverage (e.g., Zhao and Zheng, 2005; Liu et al.,2008; Bai et al., 2010; Li and Niu, 2010; Zhao and Xue, 2010; Changet al., 2011; Li et al., 2011; Zhao et al., 2011) show greater spatial var-iations in splitting parameters however.

Previous regional studies have focused on the North China Craton,Northeast China or Tibet (Huang et al., 2000; Zhao and Zheng, 2005;Lev et al., 2006; Zhao and Zheng, 2007; Liu et al., 2008; Wang et al.,2008; Zhao et al., 2008; Bai et al., 2010; Li and Niu, 2010; Zhao andXue, 2010; Chang et al., 2011; Li et al., 2011; Zhao et al., 2011). Forexample, a series of shear wave splitting measurements performedby Zhao and Zheng (2005, 2007), D. Zhao et al. (2007), L. Zhao etal. (2007), Zhao et al. (2008, 2011), and Zhao and Xue (2010) usingdata from 571 broadband stations, revealed relatively complicatedupper mantle patterns beneath the North China Craton and adjacentareas. Li and Niu (2010) measured SKS wave splitting parameters at108 stations deployed throughout northeast China. Based on the ob-servation that splitting times correlated positively to lithosphericthickness, Li and Niu (2010) suggested that lithospheric deformationmay cause the seismic anisotropy observed in northeast China.

In contrast to northern China, the South China Block has not beenstudied at such high seismic resolution. In the absence of a detailedimage of upper mantle anisotropy for this region, upper mantle defor-mation patterns and overall mechanisms for the evolution of easternChina remain uncertain (e.g., Zhao and Zheng, 2007; Huang et al.,2011). In recent years, large-scale portable seismic arrays (e.g., Zhenget al., 2008b) and upgrades to the Chinese National Seismic Network(CNSN) have significantly improved the quality and extent of seismicobservation in the South China Block (Zheng et al., 2010). These

developments provide us with an opportunity to improve the under-standing of geodynamic processes affecting eastern China.

3. Data and analysis

3.1. Data

This study used data recorded at 483 seismic stations distributedacross three different networks (Fig. 1): the North China Interior Struc-ture Project (NCISP-8, 55 stations), the South China Interior StructureProject (SCISP-1, 61 stations) and the Chinese National Seismic Net-work (CNSN, 367 stations). The NCISP-8 network is a dense NW–SEtrending linear array with an average station spacing of 10–15 km,that operated from October, 2008 toMarch, 2010. The SCISP-1 networkhad an average station spacing of 25 km and operated from November,2009 to March, 2011. Both the NCISP and SCISP networks wereequipped with CMG-3ESP or 3T sensors, and REFTEK-72A or 130 data-acquisition systems. Data from the CNSN permanent stations were ac-quired from July, 2007 to May, 2010 (Zheng et al., 2010). A fraction ofthe data acquired by the CNSN stations and used here has been reportedin previous studies (Luo et al., 2004; Zhao and Zheng, 2007;Huang et al.,2011). The bulk of the data used in this study however has not been pre-viously reported or used in other studies.

Shearwave analysis was based on data from65 teleseismic events (5recorded by NCISP-8, 15 by SCISP-1, and 45 by CNSN), occurring at epi-central distances of 85° to 115° from their respective stations (Fig. 3).The events mostly originated in the area of the Tonga trench, which isroughly located in the 90°–180° quadrant of the back azimuth shownin the inset of Fig. 3. Due to the distribution of teleseismic events, rela-tively few datawere available for stations located in the Cathaysia Block.

96˚ 100˚ 104˚ 108˚ 112˚ 116˚ 120˚20˚

24˚

28˚

32˚

36˚

N C C

Y C

Ca B

S C B

T L

F

NCISP−8

SCISP−1

L M

F

X J F

R R F

X F

M

Qinling Dabie SuluCQCHS

GXDHX

GXHCS

SCXCE

YNJIG

YNYOS

ST004

ST024

1 sec

null

Fig. 3. Map showing new SKS splitting results for the study region. Black bars represent results from NCISP-8 and SCISP-1 stations and blue bars represent results from CNSN sta-tions. 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 shearwave 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 aswhite dots with abbreviated labels (see Table S1). The inset shows the study area (blue trapezoid) and locations of seismic events (red dots) analyzed in this study.

42 L. Zhao et al. / Gondwana Research 23 (2013) 39–53

In order to reduce noise, we applied a two-pole Butterworth band-pass filter to the data. The filter used a lower corner frequency of0.02 Hz and upper corner frequency of 0.2 Hz.

Splitting parameter calculations are sensitive to the selection of thetime window over which waveforms are processed. To select the opti-mal time window, we applied an auto-adapted time-windowing tech-nique similar to that of Evans et al. (2006). This method begins withan initial time window selected by visual inspection. The time windowis then adjusted tominimize error in significance testing of the associat-ed splitting parameters (F-test analysis; Silver and Chan, 1991). As ameans of quality control, we calculated signal to noise ratios of the radi-al component (SNRr) for each SKS record as the peak amplitude of theSKS phase relative to the average amplitude of the time window, 8 sprior to the onset of the SKS phase. All SKS records with SNRrb8 wereexcluded from further analysis.

3.2. Method

Our SKS wave analysis followedmethods of Silver and Chan (1991).Themethod assumes shear wave splitting across a single homogeneousanisotropicmedium in order to derive the fast polarization direction (ϕ)and the delay time (δt) between the fast and slow components of thepolarized SKS waves. In a simple case, an anisotropic layer with a hori-zontal axis of symmetry appears as azimuthal anisotropy. For the morecomplex cases of multi-layer anisotropy or anisotropy with a dippingaxis of symmetry, splitting parameters depend on the back azimuth ofthe event. For the convenience of the Discussion section and to addresscertain measurement instabilities, we describe the algorithm for theanalysis using the notation of Kennett (2002) below. In terms of the in-cident polarization azimuth (θ) and the fast shear wave polarization di-rection (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

f tð Þs tð Þ� �

¼ RAWRi tð ÞTi tð Þ� �

; ð1Þ

where f(t) and s(t) are the fast and slow components; Ri(t) and Ti(t)are the incident radial and transverse components, respectively; andthe rotation matrix RAW is given as

RAW ¼ cosψ sinψ− sinψ cosψ

� �:

Assuming wave propagation is close to vertical and assigning waveslowness components qf and qs, the model introduces the mean slow-ness, q̃, and the wave slowness deviation, Δq, as

~q ¼ qf þ qs� �

=2; Δq ¼ qf−qs� �

=2:

The phase increment for upward propagation through a verticaldistance h is then given by

EAU ¼ eiω

~qh e−iωΔqh 00 eiωΔqh

� �:

Within the receiver frame, the observed radial and transversecomponent are thus represented by

R tð ÞT tð Þ� �

¼ RWAEAURAW

Ri tð ÞTi tð Þ� �

; ð2Þ

where R(t) and T(t) are the recorded radial and transverse compo-nents respectively, and RWA is the matrix transposed of RAW,

RWA ¼ cosψ − sinψsinψ cosψ

� �.

For an incident wave having no transverse energy such as the SKSwave, Ri(t)=u(t), and Ti(t)=0. In this case the time domain expres-sions for the observed radial and transverse components are

R tð Þ ¼ u− tð Þ cos2ψþ uþ tð Þ sin2ψ; ð3Þ

T tð Þ ¼ u− tð Þ−uþ tð Þ� �sin2ψ=2; ð4Þ

where u−(t) denotes u(t−δt/2) (earlier), and u+(t) denotes u(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 transversecomponent.

3.3. Some measurements with “figure-8” shaped particle motions

From a statistical point of view, previous teleseismic studies of east-ern China have yielded consistent results (Liu et al., 2001; Luo et al.,2004; Lev et al., 2006; D. Zhao et al., 2007; L. Zhao et al., 2007; Liu etal., 2008; Wang et al., 2008; Chang et al., 2009; Bai et al., 2010; Li andNiu, 2010; Chang et al., 2011; Huang et al., 2011; Li et al., 2011; Zhaoet al., 2011). We note however that some of these studies obtainedfast directions that varied by more than 30° for a given station. For ex-ample, varying results were reported for the permanent station BJT inNorth China in (e.g., Iidaka and Niu, 2001; Zhao and Zheng, 2005; Baiet al., 2010; Li and Niu, 2010; Huang et al., 2011). Excluding the influ-ence of complex upper mantle anisotropy, Vecsey et al. (2008) andLong and van der Hilst (2005) addressed possible causes for the abovediscrepancies using differentmethodologies. We suggest thatmeasure-ment instability could also obscure fast direction calculation for a givenstation.

Eqs. (3) and (4) demonstrate that afixed delay timewill giveR(t) andT(t) as stable functions of angle, ψ. We use the proof by contradiction ap-proach to illustrate that the inverse of Eq. (4) is unstable. Assuming a sta-ble 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 opera-tion is stable. For convenience, we use α to denote 2ψ. The perturbationterms in Eq. (4) canbe decomposed into zero-order andfirst-order terms

T ′ tð Þ ¼ T tð Þ þ δT tð Þu′ tð Þ ¼ u tð Þ þ δu tð Þα′ ¼ α þ δα; δα→0 :

then,

T tð Þ þ δT tð Þ ¼ u− tð Þ−uþ tð Þ þ δu− tð Þ−δuþ tð Þ� �sin α þ δαð Þ=2

¼ u− tð Þ−uþ tð Þ þ δu− tð Þ−δuþ tð Þ� �sinα cos δ α þ cosα sin δ αð Þ=2:

ð4Þ

Given sin δα≈0, we obtain

δα ¼ arccos2 T tð Þ þ δT tð Þ½ �

2T tð Þ þ δu− tð Þ−δuþ tð Þ� �sin α

!: ð5Þ

Here α is a constant. Improper inversion of u(t) and fluctuation inthe term δu−(t)−δu+(t) could cause the angle represented in paren-theses on the right side of Eq. (5) to approach values of 1; i.e., δαcould become quite large. This means that the grid search operationby which ϕ and δt are determined may yield unstable solutions.

43L. Zhao et al. / Gondwana Research 23 (2013) 39–53

Instabilities in the transverse function can thus cause variation in thesplitting parameters for a given station.

The measurement instability described above is a feature of signalsthat exhibit figure-8 shaped particle motion after delay time correction.For SKSwave splittingmeasurements, a good observation should exhibit

a linear particlemotion after delay-time correction. Figure-8 shaped par-ticle motion however is associated with improper search results for theϕ, δt domain. Measurements showing signs of this kind of instabilitywere reported in previous studies (e.g., Fig. 4d in Li and Niu, 2010; Fig.3b in Bai et al., 2010).

−1

0

1

1310 1320 1330

As recorded

1310 1320 1330

delay removed

Time (s)

−1

0

1

−1 0 1

−1

0

1

−1 0 1

−90

−60

−30

0

30

60

90

0 1 2 3 4

Azi

mu

th (

°)

Lag time (s)

Grid Search

−1

0

1

1300 1310 1320 1330 1340Time (s)

TA F

SKSac

S−1

0

1RA F

SKSac

S

Station: YNJIG(a) Event 2007:278:07:17:52:8100, (74.00+−5.50°,0.90+−0.05 s)

−1

0

1

1300 1310

As recorded

1300 1310

delay removed

Time (s)

−1

0

1

−1 0 1

−1

0

1

−1 0 1

−90

−60

−30

0

30

60

90

0 1 2 3 4

Azi

mu

th (

°)

Lag time (s)

Grid Search

−1

0

1

1290 1300 1310 1320Time (s)

TA F

SKSac

−1

0

1RA F

SKSac

(b) Event 2008:185:03:02:37:5600, (−78.00+−1.00°,1.45+−0.10 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 andtransverse 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, δtdomain. The red bars give the optimal result with 2σ-error. The increment of contour line is 10. (a) Event 2009:278:07:17:52 (depth=509 km, baz=117°, distance=90.3°).(b) For event 2008:185:03:02:37 (depth=581 km, baz=115°, distance=90.1°).

44 L. Zhao et al. / Gondwana Research 23 (2013) 39–53

Another example is found within our own data collected from sta-tion YNJIG. The YNJIG measurement exhibits figure-8 shaped particlemotion (Fig. 4a) and gives splitting parameters that are inconsistentwith other measurements (Fig. 4b) from events having similar raypaths.

From a kinematic point of view, we can use a pair of Ricker waveletsto illustrate the possible cause of the figure-8 shaped particle motion.Fig. 5 shows that particle motion is strictly linear when the fast andthe slow components have the same frequency, but assumes a figure-8 shape when the two components have different frequencies. The un-derlying physical cause of the differing frequencies is not well under-stood. To ensure the reliability of the splitting parameters reportedhere, we have excluded this type of suspect measurement from ourdataset.

4. Results

We calculated a total of 516 shear wave splitting measurements in-cluding 42 null measurements made using SKS phase data from 157seismic stations (Table S1). It is noted that the minimum errors of fastdirection and delay time are fixed to be 5° and 0.1 s respectively evenif the F-test analysis could give error values less than these values, be-cause previous forward calculations (Zhao, et al., 2008) show thatchanges of fast direction b5° and delay time b0.1 s is actually out ofthe resolution of the SKS shear wave splitting. Examples of splitting pa-rameters for stations CQCHS and YNYOS are shown in Fig. 6. Fig. 3shows a map of splitting parameters for each station. The dense cover-age of our results allows us to better identify subsurface variationsacross several important tectonic boundaries, including the Tanlu faultzone (TLF), the Xuefeng Mountain (XFM), the Xiaojiang Fault (XJF)and the Longmenshan Fault (LMF; see Fig. 1). Below, we describe re-gional patterns in splitting parameters and compare them with thosereported previous studies.

4.1. New results in the Yangtze Craton and its boundaries

For the Yangtze Craton and its margins, fast directions exhibitedcomplex regional patterns but were relatively coherent within individ-ual tectonic blocks (Fig. 3). The TLF marks the boundary zone betweenthe southern North China Craton and the South China Block. Althoughthe densely spaced NCISP-8 network covered this area, it experiencedonly infrequent teleseismic events (5 events) and thus gave only 13 re-sults for 12 stations. The available fast directions were mainly orientedNW–SE, and the delay times ranged from 1.3 to 2.2 s. At a permanentstation located less than 20 km from the network, Huang et al. (2011)reported one splitting result with a NW–SE fast direction and a delaytime>2 s. Overall, these fast directions are inconsistent with the orien-tation and structural attitudes of surface features in the area.

In the XFM, the fast directions mainly trend NE–SW, parallel to thestrike of the orogenic belt. To the east, fast directions shift to a ENE–WSW orientation, also consistent with change in the strike of the oro-genic belt. Several stations in the XFM region yielded large delaytimes (δt>2.0±0.1 s; Table S1). To the west of the Sichuan Basin,fast directions exhibited pronounced spatial variation. Our resultsfor this region are generally consistent with those reported in Lev etal. (2006) from the portable IRIS-PASSCAL array, but provide greaterdetail 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 directionsare oriented mainly NW–SE. Along the juncture between the LMF andthe 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 deforma-tion is coupled to tectonic movement along the XJF.

To the south of the Yangtze craton, most of the fast directions ex-hibit E–W or ESE–WNW orientations. Lev et al. (2006) reported ashift in fast directions across 26° N latitude (moving north to south;marked as the black line in Fig. 7). Our results based on denser cover-age indicate that the transition zone should be about 40 km to north(boundaries marked in Figs. 7 and 9). Several stations located outsideof the Yangtze Craton gave splitting parameters that were dependenton the event back azimuth, indicating complex anisotropy beneaththese areas (e.g., stations YNJIG and SCXCE, see Fig. 8).

4.2. New results for the Cathaysia Block

Many stations within the Cathaysia Block did not contribute validmeasurements due to the lack of high quality teleseismic events withepicentral distances>85° (Fig. 3). In the eastern Cathaysia Block (eastof 110°E longitude), the majority of fast directions are oriented ENE–WSW,with an average delay time of ~1.1 s. These fast directions are ap-proximately parallel to the strike of the Cathaysia-Yangtze boundarybelt, and the strike of Mesozoic wrench faults in the area (Li, 2000;Wang et al., 2005). In the western part of the Cathaysia Block, the ma-jority of fast directions are oriented in an E–Wdirection. Many stationsgave splitting parameters that were dependent on the back azimuths ofevents (e.g., GXHCS andGXDHX; Fig. 8). These results are in good agree-ment with splitting parameters obtained by previous studies for eventswith back azimuths ranging from ~30° to 130°, and recorded at stationslocated within the eastern Cathaysia Block (D. Zhao et al., 2007; L. Zhaoet al., 2007; Huang et al., 2011). For the southeastern Cathaysia Block, D.Zhao et al. (2007), L. Zhao et al. (2007) and Huang et al. (2011) havereported several measurements with fast directions trending ENE–WSW. These earlier measurements are consistent with those reportedhere for the Cathaysia–Yangtze boundary.

5. Depth and origin of the anisotropy

SKS wave splitting actually reflects the integrated effects of bire-fringence along the wave's ray path. Near-vertical propagation ofSKS waves provides good horizontal resolution but poor vertical

−1.0

−0.5

0.0

0.5

1.0

0 5 10 15 20 25 30

(a) Consistent frequency

8 s8 s

Time (s)

−1.0

−0.5

0.0

0.5

1.0

−1.0 −0.5 0.0 0.5 1.0

Particle Motion

−1.0

−0.5

0.0

0.5

1.0

0 5 10 15 20 25 30

(b) Inconsistent frequency

8 s7.5 s

Time (s)

−1.0

−0.5

0.0

0.5

1.0

−1.0 −0.5 0.0 0.5 1.0

Particle Motion

Fig. 5. Synthetic waveforms generated with 10% white noise to demonstrate aspects offigure-8 shaped particle motion. (a) A robust (stable) measurement. Left panel showsthe fast and slow componentswith consistent frequency; right panel shows linear particlemotion. (b) A suspect measurement. Left panel shows the fast and slow components withinconsistent frequency; right panel shows figure-8 shaped particle motion.

45L. Zhao et al. / Gondwana Research 23 (2013) 39–53

resolution for splitting parameters. The depth of the major subsurfacefeatures generating the observed patterns is an important factor inunderstanding underlying geodynamic processes. Below we discussmethods for resolving features in the vertical dimension.

5.1. Determining crustal contribution

The eastern China crust is estimated to be ~30–45 km thick (Sun andToksöz, 2006; Zheng et al., 2008b; Zhu and Zheng, 2009). Assuming an

average crustal shear wave velocity of 3.6 km/s, a delay time of 1.0 sthat was solely produced by anisotropy within the crust would requirean anisotropic strength of ~9–14%. The average slow-wave delay time is3.55±2.93 ms/km for North China, and 2.5±1.5 ms/km for the Cat-haysia Block (Wu et al., 2007, 2009). This delay time range correspondsto 0.2–2.3% anisotropy. The predictedmaximumdelay time for crust hav-ing a thickness of 45 km is ~0.25 s. Based on comparisonwith themajor-ity of the delay times calculated here (ranging from0.8 to>2.0 s), seismicanisotropy apparently occurs primarily in the upper mantle.

−1

0

1

1280 1290

As recorded

1280 1290

delay removed

Time (s)

−1

0

1

−1 0 1

−1

0

1

−1 0 1

−90

−60

−30

0

30

60

90

0 1 2 3 4

Azi

mu

th (

°)

Lag time (s)

Grid Search

−1

0

1

1270 1280 1290 1300Time (s)

TA FSKSac S−1

0

1RA FSKSac S

(a) STATION: CQCHS

−1

0

1

1300 1310

As recorded

1300 1310

delay removed

Time (s)

−1

0

1

−1 0 1

−1

0

1

−1 0 1

−90

−60

−30

0

30

60

90

0 1 2 3 4

Azi

mu

th (

°)

Lag time (s)

Grid Search

−1

0

1

1290 1300 1310 1320Time (s)

TA FSKSac−1

0

1RA FSKSac

(b) STATION: YNYOS

Fig. 6. Examples of individual shearwave splittingmeasurements at stations (a) CQCHS and (b) YNYOS, for event 2008:185:03:02:37 (Longitude: 179.8°W, latitude: 23.4°S, back-azimuth≈115°, epicentral distance≈90°). See Fig. 4 caption for further details.

46 L. Zhao et al. / Gondwana Research 23 (2013) 39–53

5.2. Assessing the uniform asthenospheric flow model

To investigate the degree to which asthenospheric flow may con-tribute to anisotropic observations, we compared fast directions withAPM. If the plate is coupled to the underlying asthenosphere, the direc-tion of plate motion will also indicate the direction of asthenosphericflow. The predicted APM direction depends on the frame of reference.The hotspot frame HS3-NUVEL1A model (Gripp and Gordon, 2002) forexample predicted a coherent APM direction of ~N 288°±5° in easternChina (Fig. 1), while the GEODVEL 2010 model (DeMets et al., 2010)predicted a coherent direction of ~N 110°±5° for the same region. Acomparison by Huang et al. (2011) demonstrated that fast directionswere consistent with the APM direction (HS3-NUVEL1A) within an an-gular difference of less than 25°. Our results from the denser seismicnetworks however show that the uniform APM direction does notmatch the spatially variable fast directions, regardless of the referenceframe. Fast directions for the XFM (trending NE–SW) and for theCathaysia Block (trending ENE–WSW) calculated here do not matchAPM directions. The assumption of simple, uniform asthenosphereflow is therefore incompatible with the complex splitting patterns ob-served in the study area.

5.3. Contributions from the lithosphere and/or asthenosphere

To distinguish contributions from the lithosphere and/or astheno-sphere to the observed anisotropy,we overlaid SKSwave splitting resultsfrom this study and from previous studies (Iidaka and Niu, 2001; Luo etal., 2004; Zhao and Zheng, 2005, 2007; D. Zhao et al., 2007; L. Zhao et al.,2007; Huang et al., 2008; Liu et al., 2008; Zhao et al., 2008; Zhao and Xue,2010; Huang et al., 2011; Zhao et al., 2011) with a shear wave tomo-graphic model of eastern China (Fig. 9, Zhao et al., in press). The tomo-graphic image is normalized to the vertical average of the shear wavevelocity perturbation relative to the IASPI91 model (Kennett andEngdahl, 1991). We calculated the shear wave velocity perturbationover a 120–300 km depth range so as to capture the subsurface zone inwhich most interactions between the asthenosphere and lithospheregenerally occur (Zhao et al., 2011). Varying the lower limit of the depthrange from 200 km to 300 km however had little effect on the distribu-tion of anomalies. Shear wave velocity anomalies can arise in the uppermantle from both temperature differences and partial melting (e.g.,Cammarano et al., 2003). Theoretical models predict that a 2.5% shearwave velocity anomaly corresponds to a ~200 °C temperature perturba-tion under dry conditions at 200 kmdepth.We assume that the variation

98˚ 100˚ 102˚ 104˚ 106˚

22˚

24˚

26˚

28˚

30˚

S

L M

S

X J F

R R F

MC01

MC02

MC03MC04MC05

MC06MC07 MC08 MC09 MC10

MC11

MC12MC13MC14

MC15

MC16

MC17

MC18MC19

MC21

MC22

MC23

MC24

MC25

KMI

SCXCE

YNJIG

YNYOS

1 sec

null

Fig. 7. Comparison of splitting parameters calculated by this study (blue bars) and a previous study (black bars; Lev et al., 2006,) 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. (2006). Our data suggests this boundary is slightly to the north, as indicatedby the blue line.

47L. Zhao et al. / Gondwana Research 23 (2013) 39–53

in velocity anomalies reflects the contrasting properties of the litho-sphere and asthenosphere, and thus their respective spatial distribu-tions. This kind of division is in good agreement with previous studiesof subsurface structures that used other methodologies (e.g., Chen etal., 2008; Obrebski et al., 2012). For the areas having substantially thickerlithospheric roots (>200 km), such as the Ordos and the Yangtze cratons(grey outlines in Fig. 9), we assume that the observed anisotropic proper-ties belong to the lithosphere rather than the asthenosphere. Precambriantectonic blocks, such as the Precambrian component of the European con-tinent, have been shown to inherit lithospheric anisotropy (e.g., Babuška

and Plomerová, 2006; Plomerová and Babuška, 2010). Anisotropy mayarise from the asthenosphere for thinner areas of the crust such as theCathaysia Block, which has a lithospheric thickness ofb100 km.

Along the profile of the SCISP-1 network in the XFM, high velocityanomalies extend to depths of greater than 200 km (Huang and Zhao,2006; Li and van der Hilst, 2010; Zhao et al., in press). The observedfast directions are consistent with the strike of surface features relatedtoMesozoic and Cenozoic tectonic events. This suggests that anisotropicfeatures in the lithosphere beneath these regions are vestiges past tec-tonic events.

In the western Yangtze Craton, shear wave velocity is relatively highwithin the uppermost mantle and fast directions show complex spatialvariations. This zone represents the junction between the LMF and theXJF. The observed anisotropy may therefore reflect the combined andvariegated effects of past tectonic events.

To the southwest and south of the Yangtze Craton, the fast directionsfrom stations located above prominent low velocity anomalies do notmatch the strike of local faults. Tomographymodels show that the lith-osphere in this area is relatively thin, with an average thicknessofb100 km (Obrebski et al., 2012; Zhao et al., in press). These lines ofevidence suggest that velocity anomalies reflect features in the astheno-sphere. The back azimuth dependence of fast directions observed forsome stations in this region indicates complex anisotropic behaviorthat may be caused by multi-layer anisotropy or anisotropy with a dip-ping axis of symmetry.

The Cathaysia Block consists of lithosphere that is much thinner thanthat of the adjacent Yangtze Craton. The lowvelocity volumebeneath theCathaysia Block extends to depths of 100 to 300 km. The fast directionsare subparallel to the strike of the Cathaysia-Yangtze boundary beltand the strike of the Mesozoic wrench faults in the area (D. Zhao et al.,2007; L. Zhao et al., 2007; Huang et al., 2011). This feature implies a com-bined contribution from vestigial deformation of the lithosphere andpresent deformation of the asthenosphere. At the boundary belt betweenthe Yangtze and Cathaysia blocks, the back azimuth dependence of fastdirections from stations GXDHX, GXHCS (Figs. 7 and 8) indicates a com-plex anisotropic behavior, potentially caused by multi-layer anisotropyor anisotropy with a dipping axis of symmetry.

TheNCISP-8 network in the Sulu orogenic belt traversed a collisionalboundary between the North China Craton and the Yangtze Craton.Data from this region revealed a weak high velocity anomaly extendingto depths of>200 km(Zhao et al., in press). The fast directionswere notparallel to the strike of the orogenic belt formed by the collision of theYangtze and North China cratons. As discussed below, this patternmay reflect a vestigial lithospheric structure, namely the roots of theSouth China Block subducted beneath the North China Craton.

6. Discussion

Shear wave splitting results reported here are in good agreementwith those of previous large-scale (e.g., Luo et al., 2004; D. Zhao et al.,2007; L. Zhao et al., 2007; Huang et al., 2011) and regional studies(e.g., Lev et al., 2006). The station coverage of this study allows for amore detailed interpretation of the subducting slab, continental litho-sphere and mantle flow. Yang et al. (2008) showed that olivine inmantle-derived peridotite xenoliths from eastern China contained rela-tively little water. Given dry conditions in the mantle, shear waves areexpected to split with fast directions subparallel to thedirection ofman-tle flow (Jung and Karato, 2001). The above relationships and assump-tions form the basis for the interpretation of eastern China's tectonicframework given below.

Our data revealed significant contrasts between fast directions, theorientation of structural features, and the overall direction of conver-gence in the study area (Fig. 9). These contrasts varied from region toregion, and were deemed less significant in areas where the observedanisotropy was clearly related to vestigial deformation of the litho-sphere from past tectonic events. The fast directions in the eastern

0

30

60

90

120

150

180

90 120 150 180back−azimuth (°)

ø (

°)

(a) Station: GXDHX

0

1

2

3

90 120 150 180back−azimuth (°)

Del

ay t

ime

(s)

0

30

60

90

120

150

180

90 120 150 180back−azimuth (°)

ø (

°)

(b) Station: GXHCS

0

1

2

3

90 120 150 180back−azimuth (°)

Del

ay t

ime

(s)

0

30

60

90

120

150

180

90 120 150 180back−azimuth (°)

ø (

°)

(c) Station: YNJIG

0

1

2

3

90 120 150 180back−azimuth (°)

Del

ay t

ime

(s)

0

30

60

90

120

150

180

90 120 150 180back−azimuth (°)

ø (

°)

(d) Station: SCXCE

0

1

2

3

90 120 150 180back−azimuth (°)

Del

ay t

ime

(s)

0

1

2

3

90 120 150 180

Fig. 8. Relationship between splitting parameters (fast polarization direction, f, shownleft 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. Therectangle indicates the parameter value; error bars represent 2σ error.

48 L. Zhao et al. / Gondwana Research 23 (2013) 39–53

North China Craton strike subparallel to the direction of subduction,whereas fast directions for the South China Block strike perpendicularto the direction of subduction. This section discusses different geo-dynamic scenarios and historical factors that may contribute to the ob-served anisotropic patterns.

Upper mantle deformation in eastern China possibly reflects somecombination of major tectonic events occurring from Late Proterozoicto Cenozoic time. These include (1) the Late Proterozoic amalgamationof the South China Block along the Yangtze-Cathaysia boundary; (2) col-lision between the North China Craton and the South China Block, com-plete by Late Jurassic time; and (3) spatial variation in mantle flowbeneath the North and South blocks induced by Mesozoic to Cenozoicsubduction. Below we interpret shear wave splitting patterns with re-spect to these events.

6.1. Effects of the Yangtze-Cathaysia amalgamation

The NE–SW trending fast directions observed in XFM are parallel toboth the strike of Proterozoic Yangtze-Cathaysia orogenic features, andthat of Mesozoic intercontinental orogenic features related to Pacific

plate subduction (Li and Li, 2007). Most of the delay times for this re-gion were greater than 1.8±0.15 s (Table S1). Assuming simple trans-verse anisotropy with a horizontal symmetry axis, average velocity of4.0 km/s and constant anisotropic strength of 4% (e.g., Savage, 1999),a 1.8 s delay time corresponds to an anisotropic lithospheric thicknessof 180 km. We therefore interpret the observed anisotropy beneaththe XFM to reflect an area of overlap between the Yangtze-Cathaysiaamalgamation and the Mesozoic orogenic belt.

6.2. Effects of the collision between the South China Block and the NorthChina Craton

Zhao et al. (2011) reported fast directions that were parallel to thestrike of the northern Qinling-Dabie orogenic belt (Fig. 9), and inter-preted them as evidence that the collision between the South ChinaBlock and the North China Craton contributed to upper mantle defor-mation in this area. Their interpretation did not address distinct var-iations in fast directions across the southern Qinling-Dabie Orogenicbelt. Along the North China Craton's southern boundary where itabuts the Yangtze craton, fast directions are not parallel to the strike

100˚ 105˚ 110˚ 115˚ 120˚ 125˚

20˚

25˚

30˚

35˚

40˚

45˚

0−4.8% 4.8%

Average Vs perturbation(120 ~ 300 km)

N C C

S C B

X J F

X F

M

T L

F

L M

F

R R F

Qinling Dabie Sulu

Q L M

H Y F

Y C

Ca B

Splitting1 secnull

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 verticallyaveraged over 120–300 kmdepth; Zhao et al., in press). Black bars show results from Zhao and Zheng (2005, 2007), Zhao et al. (2008, 2011), and Zhao and Xue (2010). Black circles showresults from Iidaka and Niu (2001), Luo et al. (2004), Liu et al. (2008), and Huang et al. (2008, 2011). The red line marks an estimated transition in splitting fast directions between theYangtze Craton and the Cathaysia Block. Thick grey outlines represent areas having high velocity zones that extend to depths of>200 km where the lithosphere might contribute themajority of anisotropy. Solid dark grey arrows represent directions of absolute plate motion (also shown in Fig.1).

49L. Zhao et al. / Gondwana Research 23 (2013) 39–53

of the orogenic belt. Geological studies have suggested that the litho-sphere of the Yangtze Craton subducted beneath the North China Cra-ton (Meng and Zhang, 2000). A northward dipping convergence zonewould create bi-layered anisotropy and/or anisotropy with a dippingaxis of symmetry. This deformational scenario is consistent with theobserved fast directions and the strike of the orogenic belt, but limit-ed event distributions do not allow us to identify more complex an-isotropy patterns in support of the subduction hypothesis (i.e., bi-layered anisotropy, dipping axis of symmetry).

6.3. Spatial variation in mantle flow patterns due to subduction

Recent tomographic studies attribute the low velocity anomaly vol-umes in eastern China to the effects of the Pacific and Philippine Sea sub-duction systems (Huang and Zhao, 2006). Large-scale extension andwidespread volcanism indicate significant reactivation of deformationalfeatures in the lithosphere of eastern China (Wu et al., 2005). If the

Pacific and Philippine Sea plate subduction zones are driving upperman-tle deformation in eastern China, patterns in fast directions for the east-ern North China Craton and the Cathaysia Block will reflect the attitudesof convergent plate motion.

Zhao and Xue (2010) proposed a subduction-induced mantle flowmodel for the eastern North China Craton in which the mantle flow isparallel to the direction of subduction (Fig. 10a). This model howeverfails to account for NE–SW to ENE–WSW trending fast directions inthe Cathaysia Block that strike roughly perpendicular to direction ofsubduction. The upper mantle anisotropy observed beneath the SouthChina Block therefore cannot be explained solely by subduction-related deformation of the sub-lithospheric mantle, and requires an al-ternative geodynamic model.

One such alternative is that the sub-lithospheric mantle bears theimprint of earlier deformational features that have rotated out of theiroriginal alignment with the subduction zone. In this model the an-isotropy reflects convergent deformation with a radial offset that

(a)

(b)

Fig. 10. Schematic diagram explaining upper mantle deformation patterns in eastern China. (a) Geodynamic model modified from Zhao et al. (2011) emphasizing regional mantleconvection 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 direc-tions for the region. Orange sub-surface plumes illustrate regional asthenospheric upwelling deflected at the lithospheric root, and outlining the topography of the lithosphere'slower boundary. The thick light blue arrows represent the horizontal flow-direction. Grey lines outline the margins of cratons with high velocity zones extending to depthsof>200 km, where the lithosphere might contribute the majority of anisotropy. Orange arrows represent the migration of asthenospheric upwelling induced by subduction. Theregional asthenospheric upwelling could deflect the westward thickening lithosphere, generating an anisotropic pattern aligned with the topography of the lithosphere.

50 L. Zhao et al. / Gondwana Research 23 (2013) 39–53

matches the clockwise rotation of the South China Block, relative tothe North China Craton. Paleomagnetic reconstructions by Zhao andCoe (1987) and Huang et al. (2008) have shown that ENE-WSW fastdirections (~N60°–80°) could be restored to a N–S orientation(N170°–190°; parallel to subduction) by a counterclockwise rotationof 70°. Fast directions along the eastern margin of the North ChinaCraton however generally strike in a NW–SE direction (~N110°–140°) and cannot be restored to match the NW orientation of Pacificplate subduction in a straightforward manner. Clockwise rotation ofvestigial deformation is thus inconsistent with the spatial variationobserved among splitting parameters measured for the South ChinaBlock.

As mentioned in the Introduction, three conceptual models (Fig. 2)have been proposed to explain the geodynamic processes affectingeastern China during Mesozoic to Cenozoic time: the mantle plume,mantle-wedge and flat-slab subduction models. We assess each ofthese models for their consistency with the observed anisotropy pat-terns below.

The mantle plume model holds that mantle flow will ascend tothe base of the lithosphere and then flow radially outwards. Underthis scenario, fast directions would exhibit either a vertical axis ofsymmetry (a column; little to no shear-wave splitting) or a radialpattern (the plume; Fig. 2a). Neither of these patterns was observedamong fast directions measured for the Cathaysia Block.

The mantle-wedge model suggests that horizontal mantle flowwould strongly influence the area overlying the subducting slab front.This type of large-scale horizontal flow would lead to extension in theCathaysia Block, and generate anisotropic features with fast directionsparallel to the direction of flow (Fig. 2b). Fast directions for the CathaysiaBlock however strike perpendicular to the direction of subduction. Large-scale extensional features such as rift basins are also absent from thisregion.

Finally, theflat-slab subductionmodel suggests that the down-goingslab (evident as a high velocity anomaly) extends beneath the Cathaysialithosphere. In this scenario, fast directions will exhibit complex pat-terns owing to multi-layer anisotropy and anisotropy with a dippingaxis of symmetry (Fig. 2c). This hypothesis cannot be fully addressedthrough shear wave splitting parameters, which are scarce due to thelimited distribution of teleseismic events. Tomographic observationshowever indicate that the Cathaysia Block does not overlie thickenedlithosphere (Huang and Zhao, 2006; Zhao et al., in press). The flat-slabsubduction model therefore cannot consistently explain processes re-lated to more recent subduction events. Flat-slab subduction mayhave played a more significant role during earlier initiation of the con-vergent system.

Comparison of splitting parameters and tomographic patterns withthe uppermantle deformation predicted by each of the three geodynamicmodels indicates that none of these models provides a consistent expla-nation for observations reported here. We therefore propose a new sce-nario that emphasizes the combined interactions of the subducting slab,mantle and lithospheric root of the craton (Fig. 10b). As shown inFig. 10b, the fast directions reflect the strike of a transitional zonebetween high and low velocity anomalous volumes in the upper mantle.Fast direction patterns suggest interactions of thickened lithospherebeneath the Yangtze craton, and asthenospheric flow caused bysubduction. Back azimuth dependence of fast directions from stationsGXDHX, GXHCS may reflect anisotropy caused by complex mantle flowpatterns.

In the scenario outlined above, the key processes of upper mantledeformation beneath the eastern South China Block, and regional as-thenospheric upwelling are both related to the Pacific and PhilippineSea subduction systems. The presence of buoyant and refractory perido-tite residues beneath the craton may contribute to its inherent stability(King, 2005). Asthenospheric flowwould be deflected by the westwardthickening lithosphere, with the former assuming an anisotropic pat-tern alignedwith the topography of the lithosphere. Compressive stress

recorded by gold lode deposits in the South China Block may reflect theresistant strength of this thickened cratonic lithosphere (Sun et al.,2007). The boundary between the mantle flow and the cratonic litho-sphere may migrate with the subduction zone, also causing migrationof volcanic activity (Li and Li, 2007), orogeny, and intermittent exten-sion and transpression, evident as basin and range type structures.

7. Conclusions

We used SKS shear wave splitting measurements to investigateupper mantle anisotropy beneath eastern China. The splitting parame-ters exhibit complex patterns but are relatively coherent within individ-ual tectonic units. Both the North China and South China blocks exhibitdistinct relationships between fast directions and tectonic/orogenicbelts. In boundary areas between the North China Craton and theYangtze Craton, fast directions trend in a NW–SE direction that is incon-sistent with the strike of known surface features. For the Yangtze Craton,fast directions are roughly parallel to the strike of regional orogenic beltsor and other structural features. The stations in the Cathaysia blockyielded fast directions trending NE–SW or ENE–WSW, roughly perpen-dicular to the direction of subduction.

We combined shear wave splitting measurements with a high-resolution tomographic model of the upper mantle to further interpretanisotropic patterns. Splitting measurements in eastern China may re-flect vestigial deformation in the lithosphere induced by tectonic eventsprior to initiation of the Pacific and the Philippine Sea plate subductionsystems. The overall tectonic history of eastern China suggests that dif-ferences between fast directions and the strikes of the Qinling–Dabieand Sulu orogenic belts may result from subduction of the Yangtze lith-osphere beneath the North China Craton.

Subduction apparently reactivated the upper mantle of easternChina during Mesozoic to Cenozoic time. The seismic observations de-scribed here were not consistent with the existing conceptual modelsof geodynamic processes in the study region (i.e., the mantle plume,mantle-wedge and flat-slab subduction models). We propose an alter-native scenario that emphasizes interactions between the cratonic lith-osphere and subduction-inducedmantle flow. In this scenario, the thicklithospheric roots of the Yangtze craton deflect mantle flow. The ob-served anisotropy therefore reflects mantle material in contact withthe lower boundary of the cratonic lithosphere.

Supplementary data to this article can be found online at http://dx.doi.org/10.1016/j.gr.2012.04.007.

Acknowledgements

We thank Wei Lin, Baochun Huang for their constructive discussion,Mingming Jiang and Kun Wang for help with data collection. We alsothank Editor M. Santosh, guest editors, and two anonymous reviewersfor their constructive reviews. Waveform data are provided by Seismo-logical Laboratory, IGGCAS and Data Management Centre of ChinaNational Seismic Network at Institute of Geophysics, China EarthquakeAdministration. This research was financially supported by the ChineseAcademy of Sciences KZCX2-YW-QN102, Sino-probe and NSFC90914011.

References

Armstrong, R.L., 1981. Radiogenic isotopes: the case for crustal recycling on a near-steady-state no-continental growth Earth. Royal Society of London PhilosophicalTransactions 301, 443–472.

Babuška, V., Plomerová, J., 2006. European mantle lithosphere assembled from rigidmicroplates with inherited seismic anisotropy. Physics of the Earth and PlanetaryInteriors 158, 264–280.

Bai, L., Kawakatsu, H., Morita, Y., 2010. Two anisotropic layers in central orogenic beltof North China Craton. Tectonophysics 494, 138–148.

Cammarano, F., Goes, S., Vacher, P., Giardini, D., 2003. Inferring upper-mantle tempera-tures from seismic velocity. Physics of the Earth and Planetary Interiors 138, 197–222.

Chang, L., Wang, C., Ding, Z., 2009. Seismic anisotropy of upper mantle in eastern China.Science China Earth Sciences 52, 774–783.

51L. Zhao et al. / Gondwana Research 23 (2013) 39–53

Chang, L., Wang, C., Ding, Z., 2011. Upper mantle anisotropy in the Ordos Block and itsmargins. Science China Earth Sciences 54, 888–900.

Chen, L., 2010. Concordant structural variations from the surface to the base of theupper mantle in the North China Craton and its tectonic implications. Lithos 120,96–115, http://dx.doi.org/10.1016/j.lithos.2009.12.007.

Chen, L., Ai, Y., 2009. Discontinuity structure of the mantle transition zone beneath theNorth China Craton from receiver function migration. Journal of Geophysical Re-search 114, B06307, http://dx.doi.org/10.1029/2008JB006221.

Chen, L., Wang, T., Zhao, L., Zheng, T., 2008. Distinct lateral variation of lithosphericthickness in the Northeastern NCC. Earth and Planetary Science Letters 267, 56–68.

Darby, B.J., Davis, G.A., Zhang, X., Wu, F., Wilde, S., Yang, J., 2004. The newly discoveredWaziyu metamorphic core complex, Yiwulu Shan, western Liaoning Province,Northwest China. Earth Science Frontiers 11, 145–155.

DeMets, C., Gordon, R.G., Argus, D.F., 2010. Geologically current plate motions. Geo-physical Journal International 181, 1–80.

Deng, J.F., Mo, X.X., Zhao, H.L., Wu, Z.X., Luo, Z.H., Su, G., 2004. A new model for the dy-namic evolution of Chinese lithosphere: continental roots-plume tectonics. EarthScience Review 65, 223–275.

Evans, M.S., Kendall, J.M., Willemann, R.J., 2006. Automated SKS splitting and upper-mantle anisotropy beneath Canadian seismic stations. Geophysical Journal Interna-tional 165, 931–942.

Faure, M., Lin, W., Breton, N.L., 2001. Where is the North China–South China blockboundary in eastern China? Geology 29, 119–122.

Fouch, M.J., Rondenay, S., 2006. Seismic anisotropy beneath stable continental inte-riors. Physics of the Earth and Planetary Interiors 158, 292–320.

Griffin, W.L., Zhang, A., O'Reilly, S.Y., Ryan, C.G., 1998. Phanerozoic evolution of the lith-osphere beneath the Sino-Korean Craton. In: Flower, M., Chung, S.L., Lo, C.H., Lee,T.Y. (Eds.), Mantle dynamics and plate interactions in East Asia. American Geo-physical Union, Washington, DC, pp. 107–126.

Gripp, A.E., Gordon, R.G., 2002. Young tracks of hotspots and current plate velocities.Geophysical Journal International 150, 321–361.

Hacker, B.R., Abers, G.A., Peacock, S.M., 2003. Subduction factory, 1, Theoretical miner-alogy, densities, seismic wave speeds, and H2O contents. Journal of GeophysicalResearch 108, 2029, http://dx.doi.org/10.1029/2001JB001127.

Huang, W.C., Ni, J.F., Tilmann, F., Nelson, D., Guo, J., Zhao, W., Mechie, J., Kind, R., Saul, J.,Rapine, R., Hearn, T.M., 2000. Seismic polarization anisotropy beneath the centralTibetan Plateau. Journal of Geophysical Research 105, 27,979–27,989.

Huang, J., Zhao, D., 2006. High-resolution mantle tomography of China and surround-ing regions. Journal of Geophysical Research 111, B09305, http://dx.doi.org/10.1029/2005JB004066.

Huang, B.C., Zhou, Y.X., Zhu, R.X., 2008. Discussions on Phanerozoic evolution and for-mation of continental China, based on paleomagnetic studies. Earth Science Fron-tiers 15, 348–359.

Huang, Z.C., Wang, L.S., Zhao, D.P., Mi, N., Xu, M.J., 2011. Seismic anisotropy and mantledynamics beneath China. Earth and Planetary Science Letters 306, 105–117.

Iidaka, T., Niu, F.L., 2001. Mantle and crust anisotropy in the eastern China region inferredfrom waveform splitting of SKS and PpSms. Earth Planets Space 53, 159–168.

Jung, H., Karato, S., 2001.Water-induced fabric transitions in olivine. Science 293, 1460–1463.Karato, S., Jung, H., Katayama, I., Skemer, P., 2008. Geodynamic significance of seismic

anisotropy: new insights from laboratory studies. Annual Review of Earth andPlanetary Sciences 36, 59–95.

Kennett, B.L.N., 2002. The seismic wavefield Volume II: interpretation of seismogramson regional and global scales. Cambridge University Press, United Kingdom.

Kennett, B.L.N., Engdahl, E.R., 1991. Traveltimes for global earthquake location andphase identification. Geophysical Journal International 105, 429–465.

King, S.D., 2005. Archean cratons and mantle dynamics. Earth and Planetary ScienceLetters 234, 1–14.

Lev, E., Long, M.D., van der Hilst, R.D., 2006. Seismic anisotropy in eastern Tibet fromshear wave splitting reveals changes in lithospheric deformation. Earth and Plane-tary Science Letters 251, 293–304.

Li, Z.X., Li, X.H., 2007. Formation of the 1300-km-wide intracontinental orogen andpostorogenic magmatic province in Mesozoic South China: A flat-slab subductionmodel. Geology 35, 179–182.

Li, J., Niu, F., 2010. Seismic anisotropy and mantle flow beneath northeast China in-ferred from regional seismic networks. Journal of Geophysical Research 115,B12327, http://dx.doi.org/10.1029/2010JB007470.

Li, C., van der Hilst, R.D., 2010. Structure of the upper mantle and transition zone be-neath Southeast Asia from traveltime tomography. Journal of Geophysical Re-search 115, B07308, http://dx.doi.org/10.1029/2009JB006882.

Li, X.H., 2000. Cretaceous magmatism and lithospheric extension in Southeast China.Journal of Southeast Asian Sciences 18, 293–305.

Li, X.H., Li, W.X., Li, Z.X., Lo, C.H., Wang, J., Ye, M.F., Yang, Y.H., 2009. Amalgamation be-tween the Yangtze and Cathaysia Blocks in South China: constraints from SHRIMPU–Pb zircon ages, geochemistry and Nd–Hf isotopes of the Shuangxiwu volcanicrocks. Precambrian Research 174, 117–128.

Li, Y., Wu, Q., Zhang, F., Feng, Q., Zhang, R., 2011. Seismic anisotropy of the Northeast-ern Tibetan Plateau from shear wave splitting analysis. Earth and Planetary ScienceLetters 304, 147–157.

Lin, W., Wang, Q., Chen, K., 2008. Phanerozoic tectonics of south China block: new in-sights from the polyphase deformation in the Yunkai massif. Tectonics 27, TC6004,http://dx.doi.org/10.1029/2007TC002207.

Liu, X.Q., Zhou, H.L., Li, H., Ji, A.D., 2001. Anisotropy of the upper mantle in Chinesemainland and its vicinity. Acta Seismologica Sinica 23, 337–348 (in Chinese).

Liu, K.H., Gao, S., Gao, Y., Wu, J., 2008. Shear wave splitting and mantle flow associatedwith the deflected Pacific slab beneath northeast Asia. Journal of Geophysical Re-search 113, B01305, http://dx.doi.org/10.1029/2007JB005178.

Long, M., Silver, P.G., 2009a. Mantle flow in subduction systems: the subslab flow fieldand implications for mantle dynamics. Journal of Geophysical Research 114,B10312, http://dx.doi.org/10.1029/2008JB006200.

Long, M., Silver, P.G., 2009b. Shear wave splitting and mantle anisotropy: measure-ments, interpretations, and new directions. Surveys in Geophysics 30, 407–461.

Long, M.D., van der Hilst, R.D., 2005. Estimating shear-wave splitting parameters frombroadband recordings, in Japan: a comparison of three methods. Bulletin of theSeismological Society of America 95, 1346–1358.

Luo, Y., Huang, Z.X., Peng, Y.J., Zheng, Y.J., 2004. A study on SKS wave splitting beneaththe China mainland and adjacent regions. Chinese Journal of Geophysics 47,812–821 (in Chinese).

Maruyama, S., Hasegawa, A., Santosh, M., Kogiso, T., Omori, S., Nakamura, H., Kawai, K.,Zhao, D., 2009. The dynamics of big mantle wedge, magma factory, and metamor-phic–metasomatic factory in subduction zones. Gondwana Research 16, 414–430.

Meng, Q.R., Zhang, G.W., 2000. Geologic framework and tectonic evolution of theQinling orogen, central China. Tectonophysics 323, 183–196.

Meng, Q.R., Wang, E., Hu, J.M., 2005. Mesozoic sedimentary evolution of the northwestSichuan basin: implication for continued clockwise rotation of the South Chinablock. Geological Society of America Bulletin 117, 396–410.

Menzies, M.A., Fan, W., Zhang, M., 1993. Palaeozoic and Cenozoic lithoprobes and theloss of N120 km of Archean lithosphere, Sino-Korean craton, China. In: Prichard,H.M., Alabaster, T., Harris, N.B.W., Neary, C.R. (Eds.), Magmatic Processes andPlate Tectonics, Geological Society, London, Specical Publication, 76, pp. 71–81.

Niu, Y.L., 2005. Generation and evolution of basaltic magmas: some basic concepts anda new view on the origin of Mesozoic–Cenozoic basaltic volcanism in easternChina. Geological Journal of China Universities 11, 9–46.

Obrebski, M., Allen, R.M., Zhang, F., Pan, J., Wu, Q., Hung, S.-H., 2012. Shear wave tomogra-phy of China using joint inversion of body and surface wave constraints. Journal ofGeophysical Research 117, B01311, http://dx.doi.org/10.1029/2011JB008349.

Plomerová, J., Babuška, V., 2010. Long memory of mantle lithosphere fabric— EuropeanLAB constrained from seismic anisotropy. Lithos 120, 131–143.

Ratschbacher, L., Hacker, B.R., Calvert, A., Grimmer, J.C., Webb, L.E., McWilliams, M.,Ireland, T., Dong, S.W., Hu, J.M., 2003. Tectonics of the Qinling (Central China):tectonostratigraphy, geochronology, and deformation history. Tectonophysics 366,1–53.

Ren, J., Tamaki, K., Li, S., Zhang, J., 2002. Late Mesozoic and Cenozoic rifting and its dy-namic setting in eastern China and adjacent areas. Tectonophysics 344, 175–205.

Savage, M.K., 1999. Seismic anisotropy and mantle deformation: what have we learnedfrom shear wave splitting? Reviews of Geophysics 37, 65–106.

Silver, P.G., 1996. Seismic anisotropy beneath the continents: probing the depths of ge-ology. Annual Review of Earth and Planetary Sciences 24, 385–432.

Silver, P.G., Chan, W.W., 1991. Shear wave splitting and subcontinental mantle defor-mation. Journal of Geophysical Research 96, 16429–16454.

Stern, R.J., 2002. Subduction zones. Reviews of Geophysics 40, 1012, http://dx.doi.org/10.1029/2001RG000108.

Stern, R.J., 2011. Subduction erosion: rates, mechanisms, and its role in arc magmatismand the evolution of the continental crust and mantle. Gondwana Research 20,284–308.

Straub, S.M., Zellmer, G.F., 2012. Volcanic arcs as archives of plate tectonic change.Gondwana Research 21, 495–516.

Sun, Y., Toksöz, M., 2006. Crustal structure of China and surrounding regions from Pwave traveltime tomography. Journal of Geophysical Research 111, B03310,http://dx.doi.org/10.1029/2005JB003962.

Sun, W.D., Ding, X., Hu, Y.H., Li, X.H., 2007. The golden transformation of the Cretaceousplate subduction in the west Pacific. Earth and Planetary Science Letters 262, 533–542.

Vecsey, L., Plomerová, J., Babuška, V., 2008. Shear-wave splitting measurements —

problems and solutions. Tectonophysics 462, 178–196.Vinnik, L.P., Farra, V., Romanowwicz, B., 1989. Azimuthal anisotropy in the earth from

observation of SKS at SCOPE and NARS broadband stations. Bulletin of the Seismo-logical Society of America 79, 1542–1558.

von Huene, R., Scholl, D.W., 1991. Observations at convergent margins concerning sed-iment subduction, sediment erosion, and the growth of continental crust. Reviewsof Geophysics 29, 279–316.

Wang, Q., Li, J.W., Jian, P., Zhao, Z.H., Xiong, X.L., Bao, Z.W., Xü, J.F., Li, C.F., Ma, J.L., 2005.Alkaline syenites in eastern Cathaysia (South China): link to Permian–Triassictranstension. Earth and Planetary Science Letters 230, 339–354.

Wang, C.Y., Flesch, L.M., Silver, P.G., Chang, L.J., Chan, W.W., 2008. Evidence for me-chanically coupled lithosphere in central Asia and resulting implications. Geology36, 363–366.

Wen, L., Anderson, D.L., 1995. The fate of the slabs inferred from seismic tomographyand 130 Ma subduction. Earth and Planetary Science Letters 133, 185–198.

Wu, F.Y., Lin, J.Q., Wilde, S.A., Zhang, X.O., Yang, J.H., 2005. Nature and significance ofthe Early Cretaceous giant igneous event in eastern China. Earth and Planetary Sci-ence Letters 233, 103–119.

Wu, J., Gao, Y., Cai, J.A., et al., 2007. Preliminary study on seismic anisotropy in the crustin southeast of Cathaysia Block. Chinese Journal of Geophysics 50, 1748–1756 (inChinese with English abstract).

Wu, J., Gao, Y., Chen, Y.T., 2009. Shear-wave splitting in the crust beneath the southeastCapital area of North China. Journal of Seismology 13, 277–286.

Xu, W.W., Zheng, T.Y., Zhao, L., 2011. Mantle dynamics of the reactivating NCC: con-straints from the topographies of the 410 km and 660 km discontinuities. ScienceChina Earth Sciences 54, 881–887.

Yang, X.Z., Xia, Q.K., Deloule, E., Dallai, L., Fan, Q., Feng, M., 2008. Water in minerals ofthe continental lithospheric mantle and overlying lower crust: A comparativestudy of peridotite and granulite xenoliths from the North China Craton. ChemicalGeology 256, 33–45.

52 L. Zhao et al. / Gondwana Research 23 (2013) 39–53

Zhang, K.J., 1997. North and South China collision along the eastern and southern NorthChina margins. Tectonophysics 270, 145–156.

Zhang, J.J., Zheng, Y.F., Zhao, Z.F., 2009. Geochemical evidence for interaction betweenoceanic crust and lithospheric mantle in the origin of Cenozoic continental basaltsin east-central China. Lithos 110, 305–326.

Zhao, D., 2009. Multiscale seismic tomography and mantle dynamics. Gondwana Re-search 15, 297–323.

Zhao, X.X., Coe, R.S., 1987. Paleomagnetic constraints on the collision and rotation ofnorth and south China. Nature 327, 141–144.

Zhao, L., Xue, M., 2010. Mantle flow pattern and geodynamic cause of the NCC reac-tivation: evidence from seismic anisotropy. Geochemistry, Geophysics, Geosystems11, Q07010, http://dx.doi.org/10.1029/2010GC003068.

Zhao, L., Zheng, T.Y., 2005. Using shear wave splitting measurements to investigate theupper mantle anisotropy beneath the NCC: Distinct variation from east to west.Geophysical Research Letter 32, L10309, http://dx.doi.org/10.1029/2005GL022585.

Zhao, L., Zheng, T.Y., 2007. Complex upper mantle deformation beneath the NCC: implica-tions for lithospheric thinning. Geophysical Journal International 170, 1095–1099.

Zhao, L., Allen, R.M., Zheng, T.Y., Zhu, R.X., in press. High-resolution body-wave tomog-raphy models of the upper mantle beneath eastern China and the adjacent areas.Geochemistry Geophysics Geosystems, http://dx.doi.org/10.1029/2012GG004119.

Zhao, D., Hasegawa, A., Kanamori, H., 1994. Deep structure of Japan subduction zone asderived from local, regional and teleseismic events. Journal of Geophysical Re-search 99, 22313–22329.

Zhao, D., Lei, J., Tang, R., 2004. Origin of the Changbai intraplate volcanism in NortheastChina: evidence from seismic tomography. Chinese Science Bulletin 49, 1401–1408.

Zhao, D., Maruyama, S., Omori, S., 2007a. Mantle dynamics of Western Pacific and EastAsia: insight from seismic tomography and mineral physics. Gondwana Research11, 120–131.

Zhao, L., Zheng, T.Y., Chen, L., Tang, Q.S., 2007b. Shear wave splitting in eastern China,implications for upper mantle deformation beneath continental margin. Physicsof the Earth and Planetary Interiors 162, 73–84.

Zhao, L., Zheng, T.Y., Lu, G., 2008. Insight into craton evolution: constraints from shearwave splitting in the NCC. Physics of the Earth and Planetary Interiors 168, 153–162.

Zhao, L., Allen, R.M., Zheng, T.Y., Hung, S., 2009. Reactivation of an Archean craton: con-straints from P- and S-wave tomography in North China. Geophysical Research Let-ter 36, L17306, http://dx.doi.org/10.1029/2009GL039781.

Zhao, L., Zheng, T., Lu, G., Ai, Y., 2011. No direct correlation of mantle flow beneath theNorth China Craton to the India-Eurasia collision: constraints from new SKS wavesplitting measurements. Geophysical Journal International 187, 1027–1037.

Zheng, S.H., Gao, Y., 1994. The azimuth anisotropy of lithosphere of continental China.Acta Seismologica Sinica 16, 131–140 (in Chinese).

Zheng, T.Y., Zhao, L., Zhu, R.X., 2008b. Insight into the geodynamics of cratonic reac-tivation from seismic analysis of the crust-mantle boundary. Geophysical ResearchLetter 35, L08303, http://dx.doi.org/10.1029/2008GL033439.

Zheng, X.F., Yao, Z.X., Liang, J.H., Zheng, J., 2010. The role played and opportunities pro-vided by IGP DMC of China National Seismic Network in Wenchuan. Bulletin of theSeismological Society of America 100, 2866–2872.

Zhou, X.M., Li, W.X., 2000. Origin of Late Mesozoic igneous rocks in Southeastern China:implications for lithosphere subduction and underplating of mafic magmas.Tectonophysics 326, 269–287.

Zhu, R.X., Zheng, T.Y., 2009. Destruction geodynamics of the North China craton and itsPaleoproterozoic plate tectonics. Chinese Science Bulletin 54, 1–13.

Zhu, R.X., Chen, L., Wu, F.Y., Liu, J.L., 2011. Timing, scale and mechanism of the destruc-tion of the North China Craton. Science China Earth Sciences 54, 789–797.

53L. Zhao et al. / Gondwana Research 23 (2013) 39–53