Seismic anisotropy of the Northeastern Tibetan Plateau from shear wave splitting analysis

Earth and Planetary Science Letters 304 (2011) 147–157

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Earth and Planetary Science Letters j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / e p s l

Seismic anisotropy of the Northeastern Tibetan Plateau from shear wave splitting analysis Yonghua Li ⁎, Qingju Wu, Fengxue Zhang, Qiangqiang Feng, Ruiqing Zhang Institute of Geophysics, Chinese Earthquake Administration, Beijing 100081, China

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Article history: Received 20 July 2010 Received in revised form 5 January 2011 Accepted 27 January 2011 Available online 20 February 2011 Editor: P. Shearer Keywords: seismic anisotropy shear wave splitting Northeastern Tibet Plateau upper mantle deformation

a b s t r a c t We present shear wave splitting results obtained from the analysis of teleseismic SKS, SKKS and PKS phases recorded by 70 permanent seismographic stations located in the Northeastern Tibetan Plateau. We identify a contrast in the splitting pattern complexity beneath different parts of NE Tibet. In the western and northern part, anisotropy observations are well explained by a single layer of anisotropy with a fast anisotropic direction trending NWW–SEE or NW–SE. In Xining and its adjacent area, the anisotropy shows strong azimuthal dependence of splitting parameters that can be modeled by two anisotropic layers. The fast direction for the upper layer lies in the N75–95°E range, which is consistent with the surface movement direction determined from GPS, and could be associated with middle to lower crustal flow. The fast direction in the lower layer is in the N105–125°E range and similar to the direction observed in the western and northern part where only a single layer is required. These NWW–SEE or NW–SE fast feature could be related to the current orogenesis induced from the India–Eurasia collision, or flow in the asthenosphere related to the absolute motion of Eurasia. Comparison between the anisotropy patterns expected from proposed models with our shear wave splitting observation suggests that no unique geodynamic model can reconcile all splitting measurements for such a complex region. © 2011 Elsevier B.V. All rights reserved.

1. Introduction The collision between India and Eurasia around 55–65 Ma resulted in very strong crustal deformation and active structures that extend from the Himalaya into central Asia (e.g. Meyer et al., 1998; Tapponnier et al., 2001; Yin and Harrison, 2000). How the Tibetan Plateau deforms remains a topic of debate. For example, England and McKenzie (1982) proposed that the entire Tibetan lithosphere uniformly shortens and thickens as a thin viscous sheet in response to the India–Eurasia collision, while the block model of Tapponnier et al. (1982, 2001) emphasizes that most deformation of the continents is localized on major block bounding faults. In the block model, the major strike-slip faults in northeastern Tibet, such as Kunlun fault (KLF) and Altyn Tagh fault (ATF) may be lithospheric faults and facilitate eastward extrusion of Tibetan Plateau. Meanwhile there is growing evidence supporting the alternative view that continental deformation is governed by the ductile flow of the middle-to-lower crust that segregates deformation between the upper crust and the mantle lithosphere (e.g. Clark and Royden, 2000; Royden, 1996) (Fig. 1). The low-velocity and high conductivity layer in the middle-

⁎ Corresponding author. E-mail address: [email protected] (Y. Li). 0012-821X/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.epsl.2011.01.026

to-lower crust revealed by seismic and magnetotelluric observations in northeastern Tibet argues for the crust flow model (e.g. Chen et al., 2009; Duan et al., 2007; Liu et al., 2006; Zhang et al., 2008; Zhao et al., 2004; Zhou, 2005). Geological surveys indicate that active crustal shortening occurs dominantly along and north of the Kunlun range, over much of northern Qinghai and Gansu provinces (Meyer et al., 1998). Global positioning system (GPS) measurements and focal mechanism studies reveal that NE Tibet is undergoing significant shortening and deformation (Gan et al., 2007; Xu, 2001; Xu et al., 2008). Moreover, the presence of abundant strike-slip faults indicates that NE Tibet represents a key area of geodynamic interest for the India–Eurasia collision (Meyer et al., 1998). A powerful tool for imaging deformation in the crust and mantle is seismic anisotropy, defined as the dependence of seismic velocities on propagation and polarization direction. The analyses of seismic anisotropy in such a complex deformation region will help to test these proposed models and clarify the dynamical processes of Tibetan evolution (Flesch et al., 2005; Guilbert et al., 1996; Herquel et al., 1995, 1999; Lev et al., 2006; McNamara et al., 1994). For example, the channel flow model that includes a weak middle or lower crustal layer implies that differential motion of upper crust and lithospheric mantle may produce patterns of mantle anisotropy that have a complex relationship to surface structures. In contrast, an expected consequence of strong mechanical

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Fig. 1. Map showing the major tectonic belts and the location of stations (solid circle) used in this study. The abbreviations are Bangong Nujiang suture, BNS; Jinsha River Suture, JSS; Kunlun fault, KLF; Altyn Tagh fault, ATF; and Haiyuna fault, HF. The big black arrow indicates the absolute plate motion (APM) of the Eurasian plate (Gripp and Gordon, 2002).

coupling between crust and mantle is that the inferred fast S-wave orientation might be more consistent with geological fabric at the surface (Flesch et al., 2005; Lev et al., 2006). Different methods have been used to estimate anisotropy in the Northeastern Tibetan Plateau. The reconciliation and interpretation of those results is, however, still controversial. For example, Pn anisotropy studies throughout the region suggest a NWW or NW fast direction in the northern and western part of NE Tibet and E–W fast directions below the eastern part (Liang et al., 2004; Pei et al., 2007). Since the Pn wave travels horizontally just below the Moho, this technique is unable to provide information on the deeper structure of the lithosphere. Splitting of shear waves measured in this region show primarily fast shear wave polarizations parallel to the strike of major faults and the orogenic belts, indicating that the crust and lithospheric mantle deform coherently (Chang et al., 2008; Herquel et al., 1999). A similar conclusion was obtained from azimuthal anisotropy of Rayleigh wave group velocities by Su et al. (2008), who found that dominant fast directions do not vary with periods and correlate with the strike of the orogenic belts at periods of 20 and 146 s, indicating a possible coherence of anisotropy between the crust and the upper mantle. However, azimuthal variations of Rayleigh wave phase and group velocities (Shapiro et al., 2003; Yi et al., 2010) show the presence of different layers of anisotropy with distinct fast-axis orientations, presenting evidence for flow in the lower crust and mechanical decoupling of the upper crust and mantle in NE Tibet. In this study, we present new shear-wave splitting measurements obtained at permanent stations located in Qinghai and Gansu provinces, China (Fig. 1). We compare these new results with previous ones (e.g. Chang et al., 2008; Herquel et al., 1999; Wang et al., 2008), as well as with Pn phase anisotropy of this region (Liang et al., 2004; Pei et al., 2007), and discuss the possible models for deformation in the region.

Mar 2010. The data recorded by stations in the Qinghai Regional Seismic Network is available from Aug. 2007 to Mar. 2010. Each station is equipped with a three component broadband sensor. More details about these networks can be found in Zheng et al. (2010). In addition, the data recorded by 4 stations (LZH, GTA, GOM and HTG) in the China Digital Seismograph Network (CDSN) and the Global Seismographic Network were also collected and processed. The recording duration is about 20 years for LZH, and about 10 years for GTA, GOM and HTG. Previous studies (Chang et al., 2008; Wang et al., 2008) have examined a subset of this data recorded before 2006 and had only sparse measurements. We have reanalyzed some of these data, and added a substantial amount of new data from additional events and newly installed stations. In this study, teleseismic SKS, SKKS and PKS phases (hereafter we will denote these phases by XKS) are used to determine receiver side seismic polarization anisotropy. The origins and locations of teleseismic events used in this study are taken from the U.S. Geological Survey (USGS) Preliminary Determination of Epicenters. The Phase arrivals are

2. Data and method The broadband seismic data used in the study were recorded by 70 permanent sites in Northeastern Tibet from the regional seismic networks of Gansu and Qinghai province, China. We used data from the Gansu Regional Seismic Network for the period from June 2000 to

Fig. 2. Spatial distribution of earthquakes used in this study. The NE Tibet area is marked with the black star. The event locations are given by circles whose sizes are proportional to event magnitude (Mw) and filling colors represent depths of hypocenters. The epicentral distances of 80° and 146° are also shown.

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computed using the IASP91 Earth reference model (Kennett et al., 1995). In order to observe high signal-to-noise ratio XKS phases, we systematically selected events with magnitudes (Mw) larger than 5.8 occurring at distances larger than 85° (Fig. 2). These events give us a good azimuthal coverage, at least for sites recording for longer time intervals; however, there are few events for analysis from the north-east and south-west. We calculated the two splitting parameters of φ (fast polarization direction) and δt (delay time) using both the rotation–correlation (RC) method (Bowman and Ando, 1987) and the minimum energy method (Silver and Chan, 1991) using the SplitLab package (Wüstefeld et al., 2008). For each selected event, we evaluated XKS splitting over multiple time-frequency windows and chose the window giving us the most robust results, depending on waveform matches between fast and slow components, the rectilinear polarization of the particle motion in the horizontal plane after anisotropy correction, and contour plots with well-defined minima. Wherever possible, we measure splitting from either raw data or a weak bandpass filter (0.02–1 Hz). The strongest filter we applied to the data has a bandwith 0.04–0.5 Hz. In most cases, the arrivals with high S/N ratio give almost apparent splitting results for passbands of 0.04–0.5 s and 0.02–1 s, which is not surprising because both bands include the dominant period of XKS. Fig. 3 displays an example of the application of these methods for an event that arriving from the east. It shows that different methodologies give similar splitting parameters for records with high signal-to-noise ratios. The comparison of results between these two methods cannot only help to provide increased confidence in individual measurements, but also to distinguish null observations from the real splitting cases and allow us to qualify any measurements (Wüstefeld and Bokelmann, 2007). Null measurements occur when the XKS phase is not split. It may be explained either by an absence of anisotropy or by initial shear wave polarization parallel to the fast or slow polarization direction in the anisotropic layer (e.g. Silver and Chan, 1991; Wüstefeld and Bokelmann, 2007). We then assign a quality factor (good, fair or poor) to the shear wave splitting measurements using the method and criteria defined by Wüstefeld and Bokelmann (2007). To avoid misinterpretations due to poor quality data, only measurements rated as ‘good’ or ‘fair’ have been retained in the subsequent analysis and discussion. We obtained both null and non-null results at 58 stations out of a total of 70 stations in this study, and solely nulls have been observed at some stations (ANT, HNT, JFS, JYG, LYT, SNT, HTG and LEH) that provided good quality data. At four stations (DHT, YUS, TTH and ZHQ), we could not obtain any high-quality measurements due to insufficient data or data of poor quality (see Tables A1 and A2, given in the Supplementary materials). In the following we will present only measurements obtained with the minimum energy method, as this method generally produced more stable results over a broader range of analysis windows (e.g. Vecsey et al., 2008). We compare them with the results of the RC method for the quality assignment only.

SWW measurements are visible and the delay times range from 0.4 s to 1.6 s, with an average of about 1 s. The distribution of these measurements are comparable with previous shear-wave splitting results for this area (Chang et al., 2008; Guilbert et al., 1996; Herquel et al., 1999; Wang et al., 2008). Our results however present more splitting measurements due to the fact that the denser seismic coverage has recently become available. An obviously difference with respect to previous studies (Chang et al., 2008; Herquel et al., 1999; Wang et al., 2008), is that there is regional variability in splitting pattern complexity beneath NE Tibet. Most stations exhibit a rather good coherence of the fast polarization directions and delay times for different events over a wide range of back-azimuths, and the distribution of null events align with the measured fast directions beneath the stations (Figs. 4 and 5), suggesting the splitting observed at those stations is likely originating from a single layer of anisotropy. Thus we can reasonably relate the upper mantle anisotropy to an average anisotropic layer (Fig. 6). Interestingly, the average NW–SE and NWW–SEE fast directions at these stations roughly parallel to the strike of major faults (e.g. KLF) and the orogenic belts (e.g. Qilian and Kunlun), which agrees with observations previously made by various authors in the NE Tibet (Chang et al., 2008; Guilbert et al., 1996; Herquel et al., 1999; McNamara et al., 1994; Wang et al., 2008). At most of the stations near Xinning and Lanzhou, there is a significant variation in fast direction and delay time, for example stations LJS, QSS, TOR, QIL etc. (Figs. 4, 5, 7 and 8). We cannot obtain lots of splitting parameters at few stations near Xining (e.g. DUL) which is due to limited azimuthal distribution of high-quality events, but its show that the Null back azimuths are not related to the observed fast polarization directions (Figs. 4 and 5). All these splitting patterns are consistent with a complex anisotropic structure beneath the stations, which may be explained by the presence of laterally heterogeneous anisotropy (e.g. Alsina and Snieder, 1995; Liao et al., 2007; Savage and Sheehan, 2000), a dipping axis of symmetry (Hartog and Schwartz, 2000) or multiple layers of anisotropy (Silver and Savage, 1994). Thus we do not try to show average splitting parameters for these stations. To discern the possible meaning of the scattering in the present study, we show the distribution of apparent splitting parameters with respect to the back-azimuth of the analyzed events (Figs. 7 and 8). Interestingly, 22 stations near Xingning and Lanzhou exhibit similar azimuthal dependence of splitting parameters, and events with a similar back azimuth give similar apparent splitting observations at these stations. Fig. 7 displays a strong back azimuthal variation of Φ in the range N54°E to N119°E and of δt in the range 0.4 to 1.6 s. The splitting parameters, especially the delay times, appear to exhibit a 90 periodicity as a function of BAZ. Such a uniform azimuthal dependence of the splitting parameters at different stations is not easily explained by lateral variation and inclined axis of symmetry. These imply that the presence of double-layer anisotropy beneath these stations which should be taken into account.

3. Splitting measurement results

3.2. Modeling of two anisotropic layers

3.1. Individual splitting measurements

The presence of multiple layer anisotropy beneath a given station is expected to produce an azimuthal variation of the splitting parameters that are obtained under the assumption of a single anisotropic layer (Rumpker et al., 1999; Silver and Savage, 1994) with a 90° periodicity in the apparent anisotropy in the presence of depthdependent anisotropy. As mentioned in Section 3.1, the scattering and well-developed 90° periodicity variations of splitting parameters observed at these stations near Xining and Lanzhou support a model with two anisotropic layers. We therefore try to constrain the possible geometries of these anisotropic layers beneath stations near Xining and Lanzhou using the scheme proposed by Silver and Savage (1994). Constraining the four parameters of a two-layer model requires high-quality recordings of splitting from many azimuths (e.g. Gao and

A total of 382 sets of well constrained splitting measurements were obtained, among which 242 were classified as good and 140 as fair. In addition, 507 high quality (good + fair) null measurements were identified (Supplemental Tables A1 & A2). The individual splitting measurements that we performed in northeastern Tibet are presented in Fig. 4a at the various stations used to evaluate the coherence of the results. The nulls are plotted in Fig. 4b as crosses parallel and perpendicular to the back-azimuth of the analyzed events. Our observations show that both fast directions and delay times change significantly across the NE Tibet (Fig. 4). NNW–SSE and NW–SE seems to be the most frequent fast direction but also W–E and NEE–

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Fig. 4. (a) Individual splitting measurements plotted at each station; the orientation and length of the bars correspond to the fast direction and delay time, respectively. Black dot represents stations where no splitting is observed. (b) Null measurements plotted at the station location as crosses whose bars parallel and perpendicular to the back-azimuth of the analyzed events. Black dots are stations where no nulls were measured.

Liu, 2009; Li et al., 2010; Silver and Savage, 1994; Walker et al., 2005), which we do not have. In order to increase the data coverage, we group all these splitting observations over 22 stations that exhibit similar scattering. Thus a total of 140 high quality splitting measurements are considered in the modeling of two anisotropic layers with horizontal axis of symmetry. We do not include the null measurements in the modeling. Using the method of Silver and Savage (1994) and for a dominant signal frequency of 0.125 Hz, we compute the apparent splitting back azimuthal variation for each two-layer model, by varying in each layer the fast directions in steps of 1 (from 0° to 180°) and the delay time by steps of 0.1 s (from 0 to 1.6 s, which is our maximum observed value of delay time), providing a total of 8,294,400 models. We then use the statistical technique described by Walker et al. (2005) and Fontaine et al. (2007) to judge the significance of the variance reduction over the best-fitting one-layer model. As mentioned by Walker et al. (2004) the goal of R2adjusted is to estimate the

degree to which two-layer models fit the splitting observations better than a single-layer model with horizontal fast axis. Walker et al. (2005) and Fontaine et al. (2007) consider models that fit the data with R2adjusted N 0.25 to be statistically significant (i.e. the model explains N25% of the variation). We find that the best-fitting model is characterized by φlower = N118°E, δtlower = 0.8 s, φupper = N80°E, δtupper = 0.6 s, R2adjusted = 0.60. The predicted apparent splitting parameters for this model are shown in Fig. 7. It indicates that the two layer model significantly improve the fit to the data. The null back azimuths predicted from our preferred two-layer model are of 16° and 106°, which are consistent with our nulls as they fall within their error bars (Figs. 7 and 8). This model, presented in Fig. 9 is representative of the population of best-fitting models since more than 60% of the best-fitting thousand models have upper layer parameters N75°E ≤ φupper ≤ N95°E and 0.4 s ≤ δtupper ≤ 0.8 s and lower layer parameters N105°E ≤ φlower ≤ N125°E and 0.4 s ≤ δt lower ≤ 0.8 s. The

Fig. 3. Example of a SKS splitting measurement using the SplitLab package at BAM station. (a) The left-hand panel shows the observed seismograms (dashed line, radial component; solid line, transverse component; vertical dashed lines, the phase arrival times computed using IASP91 model; gray zone, calculation window). The right-hand panel presents a stereoplot of the result. The header shows the information of teleseismic event as well as splitting parameters resulting from the three techniques. (b) and (c) display the results from the Rotation–Correlation technique and the minimum energy, respectively. From left-hand side to right-hand side: (1) fast (dashed) and slow (solid) components, corrected for the calculated splitting delay time; (2) corrected radial (dashed) and transverse (solid) components; (3) particle motion before (dashed) and after (solid) correction; (4) contour plot for the maximum value of correlation coefficient and for the energy on transverse component as function of delay time and fast polarization angle.

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Fig. 5. High-quality measurements of φ (top) and δt (bottom) as a function of BAZ from stations GOM, TOR and DUL. Closed symbols represent well constrained observations of splitting, where open symbols represent nulls. Horizontal dashed line illustrates the weighted average values calculated from stacking the splitting observation from all the events. a. Splitting pattern observed at station GOM, located near KLF. The splitting measurements exhibit very little variation with back azimuth, and the distribution of null events align with the measured fast directions b. Splitting pattern observed at station TOR, located near Xining. The measured splitting parameters exhibit significant variation with back azimuth. c. Splitting pattern observed at station DUL, located near Xining. At this station the splitting parameters also exhibit little variation with back azimuth, but the Null back azimuths are not consistent with the observed fast directions.

polarization directions in the upper layer are consistent with the surface movement direction revealed by GPS measurements in this region (Gan et al., 2007), whereas the fast directions in the lower layer are parallel or sub-parallel to the orogenic strike and the absolute plate motion (APM) direction of Eurasia (Gripp and Gordon, 2002)(Fig. 6). This idea is similar to the two-layer models observed by surface wave tomography (Shapiro et al., 2003).

4. Discussion 4.1. Depth distribution of the anisotropy Because the conversion from an S- to P-wave in the outer core removes any splitting due to the source-side of the path, shear wave splitting measurements represent an integral along the mantle ray

Fig. 6. Anisotropy map of Northeastern Tibet presenting the averaged splitting measurements (black bars) together with the best two-layer models as modeled using XKS phase recorded at a group of stations near Xining and Lanzhou (red bars, lower layer; blue bars, upper layer). Previous results (Guilbert et al., 1996; Herquel et al., 1999; Huang et al., 2000; Liu et al., 2008; McNamara et al., 1994) are plotted in purple. The blue arrows are GPS-vectors relative to Eurasia (Gan et al., 2007). The big black arrow indicates the APM of the Eurasian plate (Gripp and Gordon, 2002).

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Fig. 7. Plots of the apparent fast polarization direction φ (top) and delay time δt (bottom) as a function of BAZ. The measurements are from 22 stations (detail in text, Figs. 1 and 6). Apparent splitting parameters predicted for the optimum two-layer model (R2adjusted = 0.60; ϕlow = N118°E, δtlow = 0.8 s, ϕupp = N80 E, δtupp = 0.6 s) are plotted for frequencies of 0.125 Hz (solid line). Dotted horizontal lines show the mean splitting parameters. Light gray Vertical bars illustrate the BAZ distribution of nulls.

path on the receiver side. Thus the major limitation of using corerefracted waves to study anisotropy is that there is no direct constraint on the depth of anisotropy. However, the following observations give some insight to the depth distribution of the anisotropy. First, the crust thickness is about 45–55 km in the region (e.g. Duan et al., 2007; Li et al., 2006; Liu et al., 2006; Zhang et al., 2008), but birefringence of Moho P-to-S converted phases indicates average crustal splitting times are in the range of 0.1 s to 0.3 s (Herquel et al., 1995; McNamara et al., 1994), which are much smaller than the average delay time reported in this study. We cannot exclude that the crustal anisotropy beneath northeastern Tibet could be locally high, as suggested by some authors using receiver function analysis (Sherrington et al., 2004; Vergne et al., 2003), however, we expect that our splitting times reflect anisotropy mainly in the upper mantle at most stations. Second, the fast anisotropy directions derived from Rayleigh wave phase and group velocities are different to one another, but all these results show that the anisotropy is strongest at periods of 40 s or so, and detectable at periods ranging from 20 to 140 s, suggesting that the anisotropy is located in the uppermost mantle in this region (Shapiro et al., 2003; Su et al., 2008; Yi et al., 2010). If we assume that the magnitude of the anisotropy is 3%, as suggested by the surface wave observations, then a 150 km thick anisotropic medium would be necessary to accumulate ~ 1.0 s delay time. Third, the computations of Fresnel zones also help to estimate the depth of the anisotropy (Alsina and Snieder, 1995). Differences in splitting characteristics between nearby stations (e.g.

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Fig. 8. The high-quality individual measurements of φ (top) and δt (bottom) as a function of BAZ from QSS, QIL, LJS and TOR stations. Symbols are the same as in Fig. 5. Also include are curve calculated for the double-layer anisotropic parameters given in Fig. 7. It shows that these stations near Xining have a similar azimuthal behavior.

MEY and HJT, separated by ~110 km) suggest that anisotropy occurs above 160 km depth. Since the lithosphere thickness beneath this region is 130–160 km depth (Chen et al., 2009; Feng et al., 2010; Zhang et al., 2007), anisotropy may primarily be located in the lithospheric mantle, with a possible contribution from the asthenospheric mantle. This is consistent with other published work (e.g. Huang et al., 2000; McNamara et al., 1994). 4.2. Impact of major strike-slip faults The Altyn Tagh fault and the Kunlun fault at the northeastern edge of the Tibetan plateau are two major active strike-slip faults that may help accommodate the ongoing collision between the Indian and Eurasian plates (Meyer et al., 1998; Tapponnier et al., 1982, 2001; Yin and Harrison, 2000). A long-standing debate has developed over whether these faults are confined to the upper crust (e.g. Zhao et al., 2006) or cut through the whole lithosphere (e.g. Herquel et al., 1999; Wittlinger et al., 1998). If the fault is a crust scale structure, it has little effect on local anisotropy. Lithosphere-scale faults should be associated with large-scale deformation of the lithospheric mantle, which will result in fault-parallel fast directions in the vicinity of the fault (e.g. Herquel et al., 1999; Lave et al., 1996). At stations (GOM, DAW, DBT and WDT) lying close to KLF, we have shown that anisotropy is characterized by a single-layer structure, whose average fast direction tends to be parallel to the strike of the strike-slip fault (Figs. 4 and 6). Similar results are also reported in previous studies of shear wave splitting analysis (Herquel et al., 1995; Huang et al., 2000; McNamara et al., 1994). The correlation of anisotropy

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Fig. 9. Distribution of the four splitting parameters for the 2500 two-layer models(with misfit reduction R2adjusted N 0.52) included in the 95% confidence region from multi-station two-layer modeling approach. (a) Fast direction in the upper layer; (b) delay time in the upper layer; (c) fast direction in the lower layer; (d) delay time in the lower layer.

with the eastern Kunlun fault in NE Tibet suggests that the fault has cut the base of the crust and extends through the lithosphere, which is consistent with the results of receiver function and seismic tomography studies (e.g. Vergne et al., 2002; Wittlinger et al., 1996). The strike of ATF is approximately NE–SW, which is almost perpendicular to the fast direction as inferred from shear wave splitting at some stations (SBC, AXX) lying close to ATF (Figs. 4a and 6). We cannot determine any splitting measurements at some stations (HTG and LEH) near ATF, and only observe many nulls for event back azimuths are neither parallel nor normal to the strike of ATF (Fig. 4b). All these observations imply that the fault in this region is confined to the crust, in agreement with passive seismic observations (Zhao et al., 2006). However, this argues against the teleseismic tomographic result of Wittlinger et al. (1998) which revealed a low P velocity anomaly below the ATF down to 140 km and suggested that the fault is a lithospheric fault. The lack of Quaternary basaltic eruptions along the eastern segment of the ATF also suggests that in this region the fault is a crustal scale structure (Yin and Harrison, 2000). However, the shear wave splitting measurements reveal that the fast directions align with the western segment of ATF, which suggests that the associated strain likely extends from the crust down to the lithospheric mantle (Herquel et al., 1999, Herquel and Tapponnier, 2005). The geological surveys show that quaternary basaltic eruptions along the trace of the western Altyn Tagh Fault in the Ashiko Basin and Pulu (Deng, 1998), which also implies that the western Altyn Tagh fault cuts the lithosphere (Yin and Harrison, 2000). Thus the Altyn Tagh fault may be a crustal-scale fault at the eastern end and a lithospheric-scale fault at the western end (Yin and Harrison, 2000), with a boundary near 90°E. 4.3. Evidence for significant crustal anisotropy One of the important results from this study is that anisotropy near Xining can be explained by a two-layer structure (Fig. 6). The lower

layer clearly has the same characteristics as the regional anisotropy, which will be discussed in next section. Thus we will focus on the upper layer in this section. Since a unique solution cannot be determined from the splitting observations alone, in the following sections we will use additional a priori geophysical information and discuss the implication of acceptable families of models. As described in Section 3.2, most of the fast polarization directions for the upper layer range from 75° to 95°, the difference between upper-layer fast orientation of the optimum two-layer model and surface movement direction(Gan et al., 2007) for most stations are less than 20°(except DUL and DLH, see Fig. 6 and Fig. A1). The good correlations for most of these stations imply that it is likely to be produced by processes in the crust. The splitting times of the upper layer vary from 0.4 s to 0.8 s, which suggests a thickness of 40 to 80 km if 4% anisotropy is used and larger or smaller values if smaller or larger anisotropy is assumed respectively. Anisotropy estimated for orogens from shear waves splitting has commonly indicated an olivine fast axis parallel to the strike of the mountains (Silver, 1996). Anisotropy in the upper layer is probably not in the subcrustal lithosphere, since the fast directions in the upper layer are oblique to the strike of the orogens (Fig. 6). Crustal anisotropy therefore appears to be a plausible explanation for the upper layer beneath this region. A similar effect was also observed from analysis of receiver functions (Sherrington et al., 2004; Vergne et al., 2003) and surface wave tomography (Shapiro et al., 2003). Crust anisotropy is commonly attributed to micro-cracks in the upper crust or aligned anisotropic minerals at middle to lower crustal depths (e.g. Rabbel and Mooney, 1996). However, the fast polarizations for the upper layer do not appear to bear any obvious correlation to surface structures that have formed in response to the present-day NE–SW compressional stresses (Heidbach et al., 2010; Xu, 2001; Xu et al., 2008). The upper layer anisotropic signal could alternatively be explained by alignment of material which is induced by northeastern plastic flow (Clark and Royden, 2000). Numerous seismological

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observations reveal that the middle-to-lower crust near Xining is hot and weak (e.g. Duan et al., 2007; Liu et al., 2006; Zhang et al., 2008; Zhou, 2005), which also argue for strong horizontal flow. Such a ductile flow would align crustal minerals such as amphibole, which may play the most important role in mid-lower crustal anisotropy (Tatham et al., 2008), and is capable of producing significant seismic anisotropy with the fast polarization direction being subparallel to the flow direction (Tatham et al., 2008; Gao and Liu, 2009). 4.4. NWW/SEE fast orientations The fast direction for the lower layer near Xining ranges from 105° to 125°. A similar NWW–SEE fast layer is also found for most of the stations located in the interior of tectonic blocks, away from the major faults (Fig. 6). This suggests that the lower layer is located in the same depth range as the anisotropy at the other stations, and that the upper layer is what is most different between the regions. These NWW–SEE fast directions are nearly normal to the maximum compressive stress direction (Heidbach et al., 2010; Xu, 2001; Xu et al., 2008), and are approximately consistent with the APM direction calculated in the HS3-NUVEL-1A reference frame (Gripp and Gordon, 2002) for the Eurasian plate. NNE–SSW or NE–SW compression around northeastern Tibet, associated with the India–Eurasia collision, could cause SEE– NWW fast olivine axes and therefore explain these observations. Therefore, assuming the regionally-coherent orientations of anisotropy reflect a common cause, these observed anisotropy results are more easily explained by NNE–SSW or NE–SW compression and shorting, indicating that the anisotropy is related to the current orogenesis induced by the collision of India with Eurasia. Although the magnitude of corresponding delay times and the estimate of the Fresnel zone do not require the contribution of sublithospheric mantle to the anisotropy signal, we cannot rule out the possibility of a significant contribution from deeper anisotropy (see detail in Section 4.1). An alternative way to explain these results are to consider an APM contribution. The anisotropy in the lower layer maybe associated with asthenospheric dynamic flow related to the present-day absolute plate motion of the Eurasian plate. The HS3NUVEL1A model (Gripp and Gordon, 2002) predicts a plate velocity of 2.2 cm yr−1 with an azimuth of about 77° counter-clockwise from the north in the study area. This hypothesis has been recently proposed by various authors (e.g. Gao and Liu, 2009; Li et al., 2010) for the Tibet plateau and its neighboring regions by shear wave splitting analysis. 4.5. Constraints on geodynamic models The main mechanisms which generate anisotropy are related to the strain filed inside the earth. Combining Pn and shear wave splitting anisotropy, the direction of maximum compressive stress and crustal movement allow us to discuss the feasibility of various families of geodynamic models. The shear wave splitting fast directions correlate well with the directions of Pn fast velocity in the western and northern part of NE Tibet (Liang et al., 2004; Pei et al., 2007), again suggesting a coherent anisotropic pattern with depth (Fig. A2). We also find that these fast directions are nearly orthogonal or oblique to the directions of maximum compressional stress mostly determined from focal mechanisms (Heidbach et al., 2010; Xu, 2001; Xu et al., 2008) and crustal movement determined from GPS (Gan et al., 2007)(Fig. A3). This is consistent with the mode of large-scale pure shear deformation or axial shortening deformation (Tommasi et al., 1999), which implies the crust and upper mantle are mechanically coupled and a ~150 km thick layer of anisotropic material has experienced stress conditions in agreement with the surface measurement, This has been verified by joint analysis of SKS splitting and surface deformation data (Flesch et al., 2005; Wang et al., 2008).

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However, the splitting results near Xining are not correlated with Pn anisotropy results (Liang et al., 2004; Pei et al., 2007). The discrepancy of shear wave splitting and Pn anisotropy, together with strong azimuthal dependence of splitting parameters suggest vertical variations in anisotropy within the upper mantle. Our detailed analysis shows that most apparent splitting parameters for a group of stations near Xining can be satisfied by a two-layer anisotropic model with horizontal symmetry axes. These fast polarization directions for the upper layer are consistent with the GPS velocity vector in this area (Gan et al., 2007), and can be associated with the flow of middle-to-lower crustal minerals. The fast directions for the lower layer are similar to the regional anisotropy and parallel or sub-parallel to the orogenic strike and the absolute plate motion (APM) direction of Eurasia (Gripp and Gordon, 2002), suggesting that the upper mantle near Xining is deforming in a manner that is distinct from the crust. This observation is consistent with the channel flow model that includes a low-viscosity middle-lower crustal layer, implying significant mechanical decoupling between the crust and mantle. In this scenario, such a channelize flow would be located only in the vicinity of Xining. This is different to the surface wave results (Shapiro et al., 2003; Su et al., 2008; Yi et al., 2010), but they do not have sufficient resolution to confirm the details of the channel flow hypothesis. Interesting, we note that the anisotropy pattern near Xining is not unique in Tibet and its adjacent area, and a similar two-layer structures are also reported in southern Tibet (Gao and Liu, 2009) and Tienshan (Li et al., 2010). Abrupt changes in anisotropy directions are observed across the lithospheric-scale strike-slip faults in this region, like the KLF. A similar sharp swing of fast polarization occurs at stations located along other major surface structures in Tibet (e.g., BNS, ITS and JSS; Herquel et al., 1999; Huang et al., 2000; Liao et al., 2007; McNamara et al., 1994). Thus the contributions of the eastward extrusion of the Tibetan Plateau continental blocks in response to the collision of India–Eurasia cannot be negligible. 5. Conclusion We determine shear wave splitting parameters of teleseismic SKS, SKKS and PKS phases recorded at 70 permanent seismographic stations located in the Northeastern Tibetan Plateau. The large number of new shear wave splitting anisotropy measurements from the dense network of stations in this region provides constraints on the geodynamic process in the region of the Tibet plateau. Our analyses present evidence for the presence of regional differences in splitting pattern complexity beneath NE Tibet. Our observations show that anisotropy observations for most of the stations are well explained by a single layer of anisotropy with a fast anisotropic direction trending NEE–SWW and NE–SW, in agreement with previous measurements. A remarkable result of this study is the observation of back azimuthal variations of splitting parameters at some stations. Twenty-six stations near Xingning show very similar azimuthal dependence of splitting parameters. We can fit the fast directions and delay times measured at these stations with a twolayer model. The fast directions in the upper layer and in the lower layer are likely to be oriented in the range N75–95°E and N105–125°E, respectively. The lower layers show similar fast direction to what is observed in stations that require a single layer and could be related to the current orogenesis induced from the India–Eurasia collision, or flow in the asthenosphere related to the absolute motion of Eurasia (Gripp and Gordon, 2002). The top layer could be associated with the flow of middle-to-lower crustal minerals. A comparison of shear wave splitting observations, Pn anisotropy as well as crust stress indicators suggest that, for large part of this area, the crust and mantle deform coherently. Close to major surface structures, fast directions align with the strike, whereas fast direction beneath most stations in the interior of tectonic blocks are parallel or sub-parallel to the mountain belts and the APM direction of Eurasia

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(Gripp and Gordon, 2002). All these indicate that NE Tibet is not only affected by eastward extrusion of rigid blocks but is also deforming internally. Meanwhile, our identification of significant decoupling between the curst and mantle near Xining provide evidence in favor of the crust flow model. Comparison between the anisotropy patterns expected from proposed models with the observed shear wave splitting suggests that no unique model will explain all shear wave splitting observations for such a complex region. Supplementary materials related to this article can be found online at doi:10.1016/j.epsl.2011.01.026. Acknowledgments We warmly thank Andreas Wüstefeld for his useful help in the use of SplitLab software. We wish to thank Kristoffer T. Walker for providing us with the forward codes of the two-layer anisotropic model. We thank Dr. Richard Allen and Mei Xue for proofreading the manuscript. Thanks to Data Management Center of China National Seismic Network at Institute of Geophysics, the China Earthquake networks Center and IRIS Data Management Center for the accessibility and the quality of their waveform data. This study was supported by the NSF of China (No. 40704011, 41074067, 90814013 and 40974061). References Alsina, D., Snieder, R., 1995. Small-scale sublithospheric continental mantle deformation: constraints from SKS splitting observations. Geophys. J. Int. 123, 431–448. Bowman, J.R., Ando, M., 1987. Shear-wave splitting in the upper-mantle wedge above the Tonga subduction zone. Geophys. J. R. Astron. Soc. 88, 25–41. Chang, L.J., Wang, C.Y., Ding, Z.F., Zhou, M.D., Yang, J.S., Xu, Z.Q., Jiang, X.D., Zheng, X.F., 2008. Seismic anisotropy of upper mantle in the northeastern margin of the Tibet Plateau. Chin. J. Geophys. 51 (2), 431–438. Chen, Y., Badal, J., Zhang, Z.J., 2009. Radial anisotropy in the crust and upper mantle beneath the Qinghai-Tibet Plateau and surrounding regions. J. Asian Earth Sci. 36, 289–302. Clark, M.K., Royden, L.H., 2000. Topographic ooze: building the eastern margin of Tibet by lower crustal flow. Geology 28, 703–706. Deng, W.M., 1998. Cenozoic Intraplate Volcanic Rocks in the Northern Qinghai–Xizang Plateau. Geologic Press, Beijing. Duan, Y.H., Zhang, X.K., Liu, Z., Xu, Z.F., Wang, F.Y., Pan, J.S., Liang, G.J., 2007. Crustal structure using receiver function in the east part of Anymaqn suture belt. Acta Seismol. Sin. 29 (5), 513–522. England, P., McKenzie, D., 1982. A thin viscous sheet for continental deformation. Geophys. J. R. Astron. Soc. 70, 295–321. Feng, M., Van der Lee, S., An, M.J., Zhao, Y., 2010. Lithospheric thickness, thinning, subduction, and interaction with the asthenosphere beneath China from the joint inversion of seismic S-wave train fits and Rayleigh-wave dispersion curves. Lithos 120, 116–130. Flesch, L.M., Holt, W.E., Silver, P.G., Stephenson, M., Wang, C.Y., Chan, W.W., 2005. Constraining the extent of crust–mantle coupling in Central Asia using GPS, geologic, and shear-wave splitting data. Earth Planet. Sci. Lett. 238, 248–268. Fontaine, F.R., Barruol, G., Tommasi, A., Bokelmann, G.H.R., 2007. Uppermantle flow beneath French Polynesia from shear-wave splitting. Geophys. J. Int. 170, 1262–1288. Gan, W., Zhang, P., Shen, Z.K., Niu, Z., Wang, M., Wan, Y., Zhou, D., Cheng, J., 2007. Present-day crustal motion within the Tibetan Plateau inferred from GPS measurements. J. Geophys. Res. 112, B08416, doi:10.1029/2005JB004120. Gao, S.S., Liu, K.H., 2009. Significant seismic anisotropy beneath the southern Lhasa Terrane, Tibetan Plateau. Geochem. Geophys. Geosyst. 10 (Q02008), doi:10.1029/ 2008GC002227. Gripp, A.E., Gordon, R.G., 2002. Young tracks of hotspots and current plate velocities. Geophys. J. Int. 150, 321–361. Guilbert, J., Poupinet, G., Jiang, M., 1996. A study of azimuthal P residuals and shearwave splitting across the Kunlun range (Northern Tibetan plateau). Phys. Earth Planet. Inter. 95, 167–174. Hartog, R., Schwartz, Y.S., 2000. Subduction-induced strain in the upper mantle east of the Mendocino triple junction, California. J. Geophys. Res. 105, 7909–7930. Heidbach, O., Tingay, M., Barth, A., Reinecker, J., Kurfeß, D., Müller, B., 2010. Global crustal stress pattern based on the World Stress Map database release 2008. Tectonophysics 482, 3–15. Herquel, G., Tapponnier, P., 2005. Seismic anisotropy in western Tibet. Geophys. Res. Lett. 32, L17306, doi:10.1029/2005GL023561. Herquel, G., Wittlinger, G., Guilbert, J., 1995. Anisotropy and crustal thickness of northern-Tibet. New constraints for tectonic modeling. Geophys. Res. Lett. 22 (14), 1925–1928. Herquel, G., Tapponnier, P., Wittlinger, G., Jiang, M., Shi, D., 1999. Teleseismic shear wave splitting and lithospheric anisotropy beneath and across the Altyn Tagh fault. Geophys. Res. Lett. 26, 3225–3228.

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