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Physics of the Earth and Planetary Interiors 162 (2007) 73–84

Shear wave splitting in eastern and central China: Implications forupper mantle deformation beneath continental margin

Liang Zhao ∗, Tianyu Zheng 1, Ling Chen 2, Qunshu Tang 2

Seismological Laboratory (SKL-LE), Institute of Geology and Geophysics, Chinese Academy of Sciences, Qijiahuozi,Deshengmenwai, Chaoyang District, P.O. Box 9825, Beijing 100029, China

Received 9 May 2006; received in revised form 22 November 2006; accepted 12 March 2007

bstract

Eastern China, located at the east margin of the Eurasia plate, is an ideal place to study the deformation pattern of upper mantleeneath continental margin. We investigated the upper mantle anisotropy beneath eastern China by applying teleseismic shear waveplitting measurements. We examined shear wave splitting measurements at 33 permanent stations from CDSN and GSN/IRISetworks in eastern and central China, as well as splitting parameters from a previous regional study. The splitting observationsre characterized by apparent diversity of anisotropy pattern in adjacent tectonic domains, including the Southeast China Orogenicelt and the Yangtze Craton, and the eastern and central North China Craton. Based upon the straightforward relationships between

eismic anisotropy and the development of lattice preferred orientation of mineral in upper mantle rocks, we interpret the splittingesults in terms of tectonic fabric within the upper mantle. Our interpretation implies that the upper mantle anisotropy beneath

astern China remained the effects of tectonic rejuvenation occurred during the Late Mesozoic to Cenozoic. We proposed that therere possibly two kinds of mechanisms involved in the upper mantle deformation beneath eastern China, depending on the roles ofhick root of cratonic lithosphere. One mechanism is the deflecting the asthenospheric flow by the thick root of craton and the otherne is the extensional asthenospheric flow causing the craton lost part of its thick root.

2007 Elsevier B.V. All rights reserved.

ave sp

eywords: Eastern China; Upper mantle; Seismic anisotropy; Shear w

. Introduction

Eastern China, located at the eastern margin of the

urasia plate, is an ideal place to study the deformationattern of the upper mantle beneath continental margin.rom the Late Mesozoic to Cenozoic, eastern China went

∗ Corresponding author. Tel.: +86 10 62007354;ax: +86 10 62010846.

E-mail addresses: zhaoliang@mail.igcas.ac.cn (L. Zhao),yzheng@mail.igcas.ac.cn (T. Zheng), lchen@mail.igcas.ac.cnL. Chen), tqsh@mail.igcas.ac.cn (Q. Tang).1 Tel.: +86 10 62363458; fax: +86 10 62010846.2 Tel.: +86 10 62007928; fax: +86 10 62010846.

031-9201/$ – see front matter © 2007 Elsevier B.V. All rights reserved.doi:10.1016/j.pepi.2007.03.004

litting

through multiple episodes of significant tectonic activ-ities characterized by widespread tectonic extension,basin-range extension and volcanism (Liu et al., 1997;Ren et al., 2002). What dominated the tectonic reactiva-tion and how the upper mantle deformed in response tothe reactivation of eastern China during the Late Meso-zoic to Cenozoic are still controversial.

Many models (Uyeda and Miyashiro, 1974; Liu,1987; Fan and Menzies, 1992; Menzies et al., 1993;Griffin et al., 1998; Deng et al., 2004; Xu et al., 2004; Wu

et al., 2005; Niu, 2005) have been proposed to explainthe cause and tectonic mechanism of the Late Mesozoicto Cenozoic tectonic activities, which could be classifiedinto two major groups. The first group proposed an

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74 L. Zhao et al. / Physics of the Earth

extensional tectonic rejuvenation that dominated theevolution of eastern China during the Late Mesozoicto Cenozoic, and resulted in a dramatic transition fromE–W to NE–SW striking of the major tectonic unitsin eastern China. However, the geodynamic cause andmechanism of tectonic extension are controversial.For example, some authors proposed that the ancientlithospheric mantle was totally or partially replaced byjuvenile material in the Late Mesozoic (Griffin et al.,1998; Wu et al., 2005, and references therein), and sug-gested that the Mesozoic subduction of the Kula-PacificPlate was the main cause, while other authors attributedthe tectonic reactivation to an upwelling mantle plumeor a regional asthenospheric flow (e.g., Liu, 1987; Fanand Menzies, 1992). The second group preferred that thelateral mantle flow induced by the collision and subse-quent penetration of the India plate into the Eurasia platewas responsible for the upper mantle modification inNorth China or eastern China during the Cenozoic (e.g.,Menzies et al., 1993; Niu, 2005, and references therein).

New constraints are needed to test and distinguishbetween these competitive models. Because plate-dynamic processes are always associated with uppermantle deformation, the deformation pattern of the uppermantle in eastern China would be of great importance toimprove our understanding of the controlling factors forthe evolution of this region. In order to address such anissue, we use seismic shear wave splitting observations.Teleseismic shear wave splitting, in particular SKS split-ting, mainly reflects the upper mantle anisotropy (Vinniket al., 1989; Silver and Chan, 1991; Silver, 1996; Savage,1999; Mainprice et al., 2005), and provides informa-tion about mantle strain. Laboratory studies show thatunder moderate to large strains, the a-axis of olivinewill become aligned in the direction of maximum finiteextension (Karato, 1987) or in the direction of mantleflow (Zhang and Karato, 1995). It is generally acceptedthat the lattice preferred orientation (LPO) of anisotropicolivine and orthopyroxene crystals in the upper man-tle is the primary cause of the observed SKS splitting(Nicolas and Christensen, 1987; Silver and Chan, 1991;Ben Ismail and Mainprice, 1998; Mainprice et al., 2000),and strain-induced LPO is considered to be a valuableindicator of either past or present tectonics (Babuska andCara, 1991; Kay et al., 1999). Constraints on anisotropicstructure beneath eastern China therefore have the poten-tial to elucidate upper mantle deformation beneath theregion.

Several shear wave splitting studies have been per-formed in continental China. Zheng and Gao (1994)measured SKS splitting from 8 permanent stations andcompared the tectonic directions with fast polarization

netary Interiors 162 (2007) 73–84

directions; Liu et al. (2001) employed shear wave split-ting on ScS phases at 20 stations and suggested that thefast polarization orientations are subparallel to the tensilestress direction of plate motions; Luo et al. (2004) pre-sented splitting results (including 20 stations in easternand central China) and gave a preliminary interpreta-tion of their data. Many other regional studies focusedon western China where substantial mantle deformationtook place in association with the India–Eurasia con-tinental collision (McNamara et al., 1994; Hirn et al.,1995; Herquel et al., 1995, 1999; Guilber et al., 1996;Huang et al., 2000; Chen et al., 2006; Flesch et al.,2005). The understanding of the upper mantle deforma-tion beneath eastern China and its generating mechanismis still preliminary. In this study, we incorporated datafrom 33 permanent stations with those from a regionalseismic experiment in eastern China (Zhao and Zheng,2005a). The increased number of shear wave splittingobservations provides a picture of seismic anisotropyin the upper mantle beneath eastern China. The split-ting results and its interpretation are crucial to improveour understanding of how eastern China deformed inresponse to the plate-convergent events and intraconti-nental tectonic activities during the Late Mesozoic toCenozoic.

2. Geological background

Continental China (Fig. 1) is a composite terranemade up of several continental blocks including theNorth China Craton (NCC), the South China Block(SCB), the Tarim craton, and the Tibetan Plateau. Theseblocks have been welded together by Phanerozoic oro-genic belts, with ages of amalgamation ranging fromPaleozoic to Cenozoic.

Before the Late Mesozoic, the geodynamic frame-work of China was dominated by the E–W strikingtectonic events (Ren, 1990). During the late Paleozoic,the Precambrian NCC collided with the PrecambrianSiberian Craton to the north, which resulted in the upliftof the E–W-trending Central Asian Fold Belt. During thelate Permian time, the NCC collided with the SCB to thesouth, and the E–W-trending Qinling-Dabie OrogenicBelt was built in that collision (Zhang et al., 2000).

During the Late Mesozoic to Cenozoic, eastern Chinawent through significant tectonic rejuvenations. As aresult, the structure trend of the major tectonic unitschanged from E–W to NE–SW (Ren, 1990; Liu et al.,

1997). Many studies have noted a pronounced changein lithospheric properties from west to east, includinga decrease in continental crustal thickness from ∼50to ∼37 km (Gao et al., 1998, and references therein),

L. Zhao et al. / Physics of the Earth and Planetary Interiors 162 (2007) 73–84 75

Fig. 1. Topography and simplified geological map for eastern and central China, showing major tectonic units (modified from Liu et al., 1997). Tri-a ries betG h faultd c units iS teau.

a2As1NblieCfi

oautt

ngles represent station locations. Gray dashed lines denote the boundaradient Line (NSGL). Thin white lines indicate strike-slip or wrencenote thrust folds in the NCC (Zhang, 1997). Inset: the major tectoniEB, Southeast China Orogenic Belt; TC, Tarim Craton; TP, Tibet Pla

decrease in lithospheric thickness from greater than00 km to about 80 km (Ma, 1987; Deng et al., 2004;n and Shi, 2006; Huang and Zhao, 2006), and a

harp increase in Bouguer gravity anomaly (Liu et al.,997) across a N–S-trending giant linear zone called theorth–South Gradient Line (NSGL, Fig. 1). Separatedy the Central China zone with these sharp changes inithospheric properties, China can be divided tectonicallynto the western part and the eastern part (Fig. 1). In east-rn China, the dense distribution of the Late Mesozoic toenozoic magmatism, the NE or NNE-trending wrench

aulting and extensional basin systems (Tian et al., 1992)mply an extensional stress field at that time.

Geologically, eastern China is presently composedf the NNE-trending Xing’an-Mongolian Orogenic Belt

nd the NCC in the north, the E–W-trending Dabie-Sulultrahigh-pressure collisional belt (eastern segment ofhe Qinling-Dabie Orogenic Belt) in the middle, andhe SCB in the south (Wu et al., 2005). The SCB is

ween major tectonic units. Thick gray line represents the North–Souths in the SCB (Li, 2000; Wang et al., 2005). Thin dashed white linesn continental China. NCC, North China Craton; YC, Yangtze Craton;

classically separated into the Yangtze Craton (YC) tothe northwest and the Southeast China Orogenic Belt(SEB) to the southeast (Charvet et al., 1996; Gilder etal., 1996). The Jiangnan belt striking NE has been usu-ally regarded as the present southern edge of the YC. Thegeology of SEB is characterized by extensive Mesozoicmagmatism, NE-trending strike-slip fault zones with ageof Early Mesozoic (Wang et al., 2005), NNE-trendingwrench fault and formation of extensional basin systemwith age of Late Mesozoic (Li, 2000).

3. Data and methodology

In our study, we used a total of 33 permanent seismicstations among which 8 are from the GSN/IRIS networks

and others are from the CDSN networks. These threecomponent broadband stations were instrumented withSTS2, CTS1 and JCZ1 sensors, respectively. We selecteddata from September 2000 to December 2004. For data to

76 L. Zhao et al. / Physics of the Earth and Planetary Interiors 162 (2007) 73–84

ting stu

ments as ‘good’, ‘fair’ and ‘poor’ (Barruol et al., 1997;Heintz et al., 2003). Measurements satisfying the threecriteria, producing similar waveforms and linearizingparticle motions after correction, were rated as ‘good’,

Fig. 2. Epicenter distribution of the 68 events used in the SKS splitrepresents the study area.

be useful the SKS core phases have to be well-separatedfrom other phases such as S and ScS and have a highsignal-to-noise ratio on the transverse seismogram (Xueand Allen, 2005). Among ∼200 teleseismic events atdistances of 85–115◦ with magnitude equal to or greaterthan 5.5, 68 events provided useful data (Table 1 in Sup-plement data). Fig. 2 shows that the majority of the eventsused lie in Tonga or its adjacent regions. These eventsprovide poor backazimuthal coverage and do not allowthe detection of complex upper mantle structure, such asmultiple layers of anisotropy.

We used the methodology of Silver and Chan (1991)to determine the fast polarization direction φ and thedelay time δt between the fast and slow components,assuming that SKS waves traverse a single homoge-neous anisotropic layer. In order to assess how well theestimated splitting parameters, the F-test analysis devel-oped by Silver and Chan (1991) was applied. Our dataprocessing experiences showed that the time-windowingof waveforms could influence the results of splittingmeasurement (e.g., Evans et al., 2006). To search thetime-window producing the best splitting measurement,we applied an optimal time-windowing technique sim-ilar with that of Evans et al. (2006). At the beginning,we assigned a start time-window by visual inspectionbased upon the waveform and a predicted arrival timeof SKS phase from the IASP91 tables (Kennett andEngdahl, 1991). Then we searched around the start time-window (Fig. 3), and chose that having the minimumerror in F-test analysis as the optimal time-windowand the corresponding shear wave splitting parameters

as the preferred results. To investigate the frequencydependence of the results, we applied various two-poleButterworth bandpass filters to the data before the split-ting measurements. All the filters adopted had a fixed

dy. Black solid dots indicate the locations of earthquakes, rectangle

lower corner frequency of 0.02 Hz but different uppercorner frequencies ranging from 0.2 to 1.0 Hz. Thesefiltering yielded approximately the same splitting param-eters, indicating that high frequency scattering did notsignificantly affect the results.

To obtain a reliable estimate of splitting parameters,we only retained the unambiguous results that satisfyingthe following criteria: (1) clear SKS arrivals, (2) similarwaveforms of the two horizontal components and ellip-tic particle motion at the surface which can be linearizedby removing the effect of anisotropy (Herquel et al.,1995), and (3) reasonable result of error analysis. Fig. 4gives examples of waveform splitting at stations CNSand HHC for event 2004:067:11:18:10. With respect tothese three criteria, we sorted the individual measure-

Fig. 3. Searching time ranges for the optimal time-windowing tech-nique. A and F mark the start and end points of the time-window foranalysis. Shaded zones indicate searching regions for the start and endpoints of time windows, respectively.

L. Zhao et al. / Physics of the Earth and Planetary Interiors 162 (2007) 73–84 77

F HHC fs onentsr main.

tOi

4

wtrsrmthm(litasi

ig. 4. Examples of shear wave splitting analysis at stations CNS andmall boxes in the left panel showing normalized fast and slow compespectively. The right panel is the results of grid search in (φ − δt) do

hose with slightly larger time errors were rated as ‘fair’.nly ‘good’ and ‘fair’ measurements were retained for

nterpretation.

. Splitting results

Significant shear wave splitting or typical null resultsere observed at most of the stations, except for sta-

ions LZH, SNY and WHN due to poor signal-to-noiseatios. After careful selection, 175 individual shear waveplitting measurements were retained, including 88 nullesults. Table 2 in Supplement data lists all the individualeasurements with 1σ uncertainties determined from

he 95% confidence interval in the (φ − δt) domain. Weave obtained many null results due to the fact that theajority of events used lie in Tonga or adjacent regions

Fig. 2) with backazimuth either parallel or perpendicu-ar to the average fast direction at certain stations, mainlyn central and northeastern China (Fig. 5). At some sta-

ions such as ENH, KMI, PZH, and so on, null resultss well as non-null results were obtained. For thesetations, in each case of null splitting, the event backaz-muth was either parallel or perpendicular to the average

or event 2004:067:11:18:10. (a) Station CNS; (b) station HHC. Fourand their particle motions, before and after removing the delay time,

fast direction derived from non-null observations. Thecombination of null observations and well-resolved fastdirections from other events at the same stations indicatesthat a single layer of anisotropy dominated the splittingobservations.

As shown in Fig. 5, values of δt ranging from 0.70 to2.20 s are found at most of the stations, indicating ubiq-uitous anisotropic structure beneath the stations. The fastdirection φ displays apparent spatial variations, reflect-ing the presence of complex seismic anisotropy in theupper mantle. Fig. 6 shows that the splitting observationsare characterized by apparent diversity of anisotropy pat-tern in adjacent tectonic domains, including the SEB andthe YC in the SCB, and the eastern and central NCC.

4.1. The SCB

We have 16 stations in the SCB. The first order obser-vation in this region is the discrepancy of the fast splitting

direction between the SEB and the YC.

In the SEB, the fast splitting direction φ shows goodconsistency trending NE or ENE. Small to large valuesof delay time are found in this region with an average of

78 L. Zhao et al. / Physics of the Earth and Planetary Interiors 162 (2007) 73–84

orientaresults.tropy.

Fig. 5. Map showing individual shear wave splitting observations. Thethe splitting delay time, respectively. The thin gray crosses denote nullof the incoming waves and represents a possible fast direction of aniso

∼1.3 s. The largest delay time up to 2.0 s is observed atstation GUL located to the east of the boundary betweenthe SEB and the YC. At station WZH, data from threeearthquakes were analyzed. An average fast directionof N59◦E with a delay time of 1.6 s was obtained fortwo of the earthquakes. The waveform from the thirdevent yielded a null result. As the backazimuth of thisearthquake is nearly perpendicular to the fast directionsobtained for the other two earthquakes, it is reasonableto derive that the fast direction of anisotropy is per-pendicular to the polarization direction of the incomingenergy. At station GZH, four null results were obtained.These four earthquakes were at backazimuths of N43◦E,N43◦E, N128◦E and N126◦E, respectively, consistent

with a NE–SW fast direction observed at the nearbystation SZN (∼100 km apart from GZH).

In contrast, in the YC, the fast splitting direction φ

shows a complex pattern. At station GYA, PZH, and

tion and length of each bar indicate the fast polarization direction andEach branch of a cross is parallel or perpendicular to the backazimuth

ENH in the western YC, null results as well as non-nullresults were obtained. Station ENH gives a fast direc-tion trending N 37◦E, approximately perpendicular to thesplitting direction of NW observed at stations GYA andPZH. Seventeen null results were made at station CD2located at the northwestern end of the YC. Consideringthe backazimuthal coverage of the data used, the resultspossibly suggest an apparent horizontal isotropy beneathstation CD2. On the other hand, stations HEF, NJ2, CNSand NNC in the eastern YC, near the boundaries betweenthe YC and the NCC to the north and between the YCand the SEB to the south, show consistent fast directionsof NE with those observed in the SEB.

4.2. The NCC

Shear wave splitting was observed at 8 stations in theNCC. Each station has 3–14 individual measurements.

L. Zhao et al. / Physics of the Earth and Planetary Interiors 162 (2007) 73–84 79

Fig. 6. Available splitting observations and geological features superposed on a map of seismic-thermal lithospheric thickness (lithospheric thicknessd ves thea tensionaC

ToN

ftweuyt1gff2

cH

ata was from Dr. M.J. An, personal communication). White arrow gind Gordon, 2002). Big dual-head arrows represent the possible exenozoic.

he first order feature of the results is the diversity of therientation of fast axes between the eastern and centralCC.In the eastern NCC, splitting results were measured

rom three earthquakes at station TIA. A fast direc-ion of N124◦E ± 2◦ and a delay time of 2.18 ± 0.81 sere obtained from one of the earthquakes. Other two

arthquakes, with azimuth either parallel or perpendic-lar to the fast direction given by the first earthquake,ielded null results. The fast directions of N124◦ at sta-ion TIA is consistent with the previous result (N123◦E,.2 s) observed by Luo et al. (2004). The fast directioniven at TIA is also in good agreement with the NW–SEast directions observed at 14 nearby stations (Fig. 6)rom the previous regional study (Zhao and Zheng,

005a).

On the other hand, the fast directions display a goodonsistency of NE or NNE-trending at stations BJT,NS, LYN, HHC, TIY, XAN, and YCH, in the central

absolute plate motion directions using HS3-NUVEL1a model (Grippl (rifting) directions of eastern China during the Late Mesozoic to

NCC. The previous observation by Luo et al. (2004) sug-gested fast splitting directions of ∼N116◦E and N97◦Eat stations TIY and HNS, respectively, which are almostperpendicular to the NNE direction obtained by thisstudy. However, our splitting results in central NCC inthis study are in good agreement with the NE- or NNE-trending observed at 29 adjacent stations (with minimumdistance of <50 km) in the regional study by Zhao andZheng (2005a).

4.3. Other stations outside of the NCC and the SCB

Stations XLT, HIA, CN2, BNX, and MDJ locatedin northeastern China also gave valid splitting results.The sparse distribution of these stations, however, does

not allow an unambiguous interpretation of the observa-tion. In the future work, denser station distribution wouldbe helpful for better constraining the seismic anisotropyproperty in this region.

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80 L. Zhao et al. / Physics of the Earth

5. Discussion

At epicentral distances of 85–120◦, the incidenceangle of the SKS energy at a station is within 15◦ fromvertical. The near vertical incidence angle leads to a highlateral resolution for the subsurface anisotropic struc-ture. However, the seismic anisotropy recorded at theEarth’s surface represents a vertically integrated effectof anisotropy along the mantle ray path on the receiverside, making the depth resolution rather poor. Althoughthere are various layers in the Earth contributing to theSKS splitting times, it is widely accepted that anisotropyobserved in teleseismic shear waves mostly results fromthe LPO of rock-forming mineral in the upper mantle.Most of studies agree to some values ranging from 0.1to 0.2 s for deformed crust (e.g., Herquel et al., 1995;Barruol and Mainprice, 1993), and the contribution fromthe lower mantle and transition zone to be typically lessthan 0.2 s. The majority of the measured SKS splittingtimes in the study region exceed 1.0 s, suggesting thatthe upper mantle is the primary source of the observedseismic anisotropy.

Four models may probably explain the shear wavepolarization in eastern China: (1) frozen-in anisotropygenerated during the last orogeny that has affected east-ern China before the Late Mesozoic; (2) rift-relatedfabric, including or not rift parallel melt pockets; (3)sub-lithospheric mantle flow related to the CenozoicIndia–Eurasia collision; or (4) frozen-in anisotropy gen-erated by sub-lithospheric mantle flow associated withthe lithospheric rejuvenation during the Mesozoic toCenozoic.

The disagreement between the mainly E–W-trendingof the last orogeny and the fast polarization directions ineastern China can be used to rule out the first hypothesis.In addition, it is unlikely that a past history of regionalmagmatism has any long-term splitting imprint, whichimplies that appealing to rift-related fabric to explainanisotropy in eastern China is probably inappropriate.In the following discussion, we test the possibility ofmodels (3) and (4).

5.1. The relation to the absolute plate motion andthe effects of India–Eurasia collision

We test the influence of the sub-lithospheric man-tle flow on upper mantle anisotropy by comparing theobserved fast polarization directions of shear waves to

the absolute plate motion (APM) direction (Vinnik et al.,1992).

Model (3) supposes that anisotropy beneath easternChina was possibly generated by the asthenospheric flow

netary Interiors 162 (2007) 73–84

caused by India–Eurasia collision. On the one hand,if India–Eurasia collision imparted stresses throughthe lithosphere to drive tectonic escape, the relevantmotion vector for anisotropy generation is the direc-tion of basal shear-induced by the relative motion of theextruded lithosphere above the fixed mantle (G. Helf-frich, personal communication). APM determined foreastern China trends ∼N288◦E using HS3-NUVEL1amodel (Gripp and Gordon, 2002, calculated at web-site http://sps.unavco.org). It would be expected toresult in coherent fast polarization direction φ in east-ern China. On the other hand, if tectonic extrusionis driven by basal tractions on the lithosphere due tomantle flow, then APM has no relevance because thismantle flow presupposes there are local flow patternsthat are not related to the mantle that is fixed to thehot spots (G. Helffrich, personal communication). Inthe SEB and the central NCC, the NE–SW-trendingfast directions are nearly perpendicular to the APM,and the coherent asthenospheric flow could hardlyexplain the short-scale φ variations of ∼90◦ within theNCC and the complexity of anisotropy in the SCBwhatever the reference frame is taken. These factsdo not favor a model of upper mantle deformationinduced by simple asthenospheric flow beneath easternChina.

5.2. The relation to lithosphere rejuvenation duringthe Mesozoic to Cenozoic

Although the geodynamic regime was in debate(Gilder et al., 1996; Wang et al., 2005, and referencetherein), it is unambiguous that the SEB contains widelydistributed NE-trending strike-slip fault zones with agesof early Mesozoic, and NE- or NNE-trending wrenchfault with ages of Late Mesozoic to Cenozoic (Li, 2000,and reference therein). These regional structures werecommonly interpreted in terms of a series of sinistralshear zones. In this region, there is a conspicuous corre-lation between the orientation of the fast split shear wavepolarization plane and the structural trend of the strike-slip fault zones. This correlation may be regarded as aline of evidence for the observed shear wave splittingresults being from anisotropy frozen within the litho-sphere since the Mesozoic sinistral shear. Consideringthe Late Mesozoic to Cenozoic lithospheric rejuvenationwas the last significant event that overprinted the uppermantle beneath the SEB, we suggest that the frozen-

in anisotropy was generated during the Late Mesozoicto Cenozoic lithospheric rejuvenation. However, theeffect from the early Mesozoic transtension cannot beexcluded.

and Planetary Interiors 162 (2007) 73–84 81

mvraNZtssrm

5

nNnetbdpbfletpoNtt

Hsese2otldeipmffcr

Fig. 7. A cartoon showing two mechanisms involved in the upper man-tle deformation beneath the continental margin. (a) The mechanism ofdeflecting the asthenospheric flow by the thick root of a craton. (b) Theother mechanism for a craton having lost its thick root. The shaded zone

L. Zhao et al. / Physics of the Earth

The eastern NCC is believed to have been affectedore significantly by the extensional lithospheric reju-

enation, as evidenced by the development of a series ofifting basins elongated N–NE (Ren et al., 2002), suchs the Bohai Bay basin developed in response to theW–SE-trending extensional strain (Tian et al., 1992;heng et al., 2005; Zhao and Zheng, 2005b). Based on

he facts that the fast directions are parallel to the exten-ional strain in this region, Zhao and Zheng (2005a)uggested that the lithosphere beneath the eastern NCCetained a “fossil” anisotropy after the extensional upperantle deformation.

.3. The possible mechanisms

It is very interesting that different mechanisms domi-ated the upper mantle deformation beneath the easternCC and the eastern SCB, the two neighboring conti-ental margins that went through approximately similarxtensional lithospheric rejuvenation. Considering thathe boundary conditions were the major differencesetween these two neighboring continental marginsuring the lithospheric rejuvenation, we propose twoossible mechanisms (Fig. 7) based on the interactionetween the lithospheric root of craton and the mantleow (Bormann et al., 1996; Barruol et al., 1997; Foucht al., 2000) during the late Mesozoic to Cenozoic. Inhe SEB, the NE–SW-trending frozen-in anisotropy wasossibly caused by an along-margin mantle flow obliquer perpendicular to extensional direction. In the easternCC, on the other hand, the NW–SE-trending fast direc-

ions was produced by a frozen-in anisotropy parallel tohe tectonic extension.

Previous studies (e.g., Ma, 1987; An and Shi, 2006;uang and Zhao, 2006) have shown that the litho-

phere mantle beneath some regions in eastern Chinaxtends to the depth of ∼200 km, thicker than theurrounding domains. Fig. 6 shows that around theasternmost YC with thick lithosphere (An and Shi,006) the observed fast directions are parallel or slightlyblique to the boundary of YC. This lets us proposehe first mechanism (Fig. 7a) that the topography of theithosphere–asthenosphere boundary of the region mayeflect the mantle flow (Bormann et al., 1996; Barruolt al., 1997; Fouch et al., 2000; Heintz et al., 2003) dur-ng the late Mesozoic to Cenozoic, and its anisotropicroperty was then frozen-in after being deflected. Thisechanism may explain the rift, the migration of wrench

aults, or/and the reactivity of pre-existing strike-slipaults in the SEB. This is a common case because the oldraton of low density and high viscosity would inherentlyesist the mantle modification.

indicates the initially thick craton root before the lithospheric thinning.Big arrow represents the direction of asthenospheric flow; dual-headarrow indicates the fast direction of “frozen-in” anisotropy.

In contrast, the second mechanism (Fig. 7b) is thatpart of the cratonic root was replaced by the juvenileasthenospheric flow. This mechanism is counter-intuitivebecause the old craton would not be easily modified dueto its buoyancy and strength. However, the eastern NCCis unique among Archean cratons in having lost a signif-icant proportion of its lithospheric root (Wu et al., 2005,and reference therein), from a thick (∼200 km) Archeanor Proterozoic lithosphere (Menzies et al., 1993; Griffinet al., 1998; Xu et al., 2004) to a presently much thinnerlithosphere (60–80 km) (Xu et al., 2004; Hu et al., 2000;Chen et al., 2006; An and Shi, 2006; Huang and Zhao,2006). The timing of lithospheric thinning was estimatedbetween Ordovician and Cenozoic (Wu et al., 2005). Theeastern NCC is characterized by a seismic anisotropy

pattern with the fast direction aligned with the mantleflow direction (Fig. 6) during the Late Mesozoic to Ceno-zoic rejuvenation. Although the geodynamic processand mechanism of how the craton lost its lithospheric

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82 L. Zhao et al. / Physics of the Earth

root is currently still under debate (Zhang, 2005), theconsistent NW–SE fast polarization direction of shearwaves indicate that the regional NW–SE-trending man-tle flow is expected to have been the most dominantfactor affecting the upper mantle deformation accom-panying the lithospheric thinning of the eastern NCC.The observed short-scale variation in fast polarizationdirection of shear waves between the eastern and cen-tral NCC possibly reflects a sharp change in lithosphericproperties and mantle flow direction (Zhao and Zheng,2005a; Zheng et al., 2007).

6. Conclusions

Shear wave splitting measurements using SKS phaseshave been performed at 33 permanent stations in easternChina, among which valid results are obtained for 30stations. The splitting results in this study combined withthose from a regional study (Zhao and Zheng, 2005a)show diversity of anisotropy pattern in different cratonsor tectonic domains of eastern China. We interpreted thesplitting results in terms of tectonic fabric within theupper mantle. Our SKS splitting observations suggestthat the Late Mesozoic–Cenozoic tectonic rejuvenationmight have played a dominant role in the upper mantledeformation of eastern China.

Acknowledgements

We thank Dr. M. Xue for helpful discussion, Dr.Yumei He and Gang Lu for help with processing data.The CDSN data were obtained from DMC of ChinaEarthquake Administration; the GSN data were obtainedfrom DMC of IRIS. We acknowledge the participantsof Seismological Laboratory, IGGCAS. Critical andconstructive comments from Dr. G. Helffrich and twoanonymous reviewers significantly improved the qual-ity of this paper. This research was financially supportedby the National Science Foundation of China (Grants40434012 and 40504006) and Chinese Academy of Sci-ences.

Appendix A. Supplementary data

Supplementary data associated with this article canbe found, in the online version, at doi:10.1016/j.pepi.2007.03.004.

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