| Citation: | Jing Tan, Hongyi Li, Xinfu Li, Ming Zhou, Longbin Ouyang, Sanjian Sun, Dan Zheng. Radial anisotropy in the crust beneath the northeastern Tibetan Plateau from ambient noise tomography. Journal of Earth Science, 2015, 26(6): 864-871. doi: 10.1007/s12583-015-0543-x
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In the past decades, numerous geoscientists have proposed various models to explain the uplift of Tibetan Plateau. These competing models mainly focus on the mechanism of crustal deformation of the Tibetan Plateau. One of the best places to test these models is the northeastern Tibetan Plateau, which is the leading edge of the entire plateau extending into the continent. In this paper, the study area is bounded by the Tarim Basin in the northwest, the Alanshan Block in the north and the Ordos Block in the east and has a series of parallel NW-SE-trending mountain ranges (Fig. 1). The Qilian orogen was an intraplate orogen developed in the Neoproterozoic continental craton and finally formed during the Late Pleistocene Himalayan movement. The formation of the orogen was synchronized with the uplift of the Tibetan Plateau and both of them were controlled by the collision between the Indian and Eurasian plates (Ge and Liu, 1999). As a transitional zone between the Tibetan Plateau and the Yangtze Craton, the Songpan-Ganzi terrane experienced a strong deformation and fracture dislocation during the uplift of the Tibetan Plateau. This study area is also seismically active.
Seismic anisotropy has become an effective means to explain the mechanism of deformation of crust and uppermost mantle. Recently, different methods have been used to estimate anisotropy in northeastern Tibet, such as Pn wave travel time (Pei et al., 2007; Liang and Song, 2006), shear wave splitting measurements (Chang et al., 2008; Flesch et al., 2005) and surface wave tomography (Chen et al., 2009; Peng et al., 2007; Shapiro et al., 2004). Each method has its own advantages and disadvantages. Pn wave tomography is unable to provide information in deeper lithosphere. Although shear wave splitting has good lateral resolution, it has poor ability on the vertical resolution. Surface wave tomography method generally has good vertical resolution.
The difference between Vsh and Vsv inferred respectively from Love wave and Rayleigh wave dispersion curves is called radial anisotropy. To date, analysis of radial anisotropy has been widely applied to study the mechanism of crustal deformation. Recent intermediate-period surface wave tomography across Tibet indicated a marked radial anisotropy within the middle-to-lower crust beneath the plateau (Shapiro et al., 2004). They suggested that the anisotropy was consistent with a thinning of the middle crust by about 30%. Ambient noise surface wave data from Huang et al. (2010) also found that radial anisotropy with Vsh > Vsv was most prominent in the mid-lower crust of southeastern Tibet and this anisotropy correlated well with LVZs in deep crustal zones. Cheng et al. (2013) also obtained radial anisotropy along two linear seismic arrays across the North China Craton from ambient noise tomography.
In this study, crustal radial anisotropy in the northeastern Tibetan Plateau is measured from Rayleigh and Love wave experimental Green's functions with ambient noise tomography method. The continuous seismic waveform data was recorded by 223 stations from 2007 to 2010 deployed by China Earthquake Administration, ACSENT experiment and NETS experiment. The main purpose of this study is to discuss spatial correlations between the radial anisotropy and crustal LVZs which may provide information for better understanding of the uplift and deformation of the Tibetan Plateau.
Continuous three-component seismic data are collected from 223 broadband stations from the Chinese Provincial Digital Seismic Networks operated by China Earthquake Administration, the temporary ACSENT and NETS experiments between May 2007 and July 2010 (Fig. 1). We follow the similar procedure as described by Bensen et al. (2007) and Li et al.(2012, 2010) to process three-component continuous seismic data. In order to extract Love wave dispersions, we rotate the horizontal-component data, the north and east components, into transverse and radial components by setting the first station as the 'event' and the second station as the 'receiver' before the cross-correlations are computed and stacked. In addition, according to the P wave particle-motion analysis of Niu and Li (2011), we also correct those stations with problems operated by China Earthquake Administration, which include mis-orientation, mis-labeling and polarity reversal.
Then empirical Green's functions of Rayleigh and Love wave are established from the cross correlation between all possible station-pairs. Nearly 5 000 group velocities and 5 800 phase velocities dispersion curves of Rayleigh wave (8–50 s), and 4 700 group velocities and 3 700 phase velocities dispersion curves of Love wave (8–46 s) have been extracted using time-frequency analysis (Herrmann, 1973; Dziewonski et al., 1969). After manually checking each dispersion curve, more than half of the cross-correlations are discarded.
Figure 2 shows a synthetic example to illustrate how a LVZ in the middle crust affects the Rayleigh and Love wave dispersion curves. As seen in Fig. 2b, phase velocity dispersions (black dot line) and group velocity dispersions (gray dot line) of Rayleigh wave generated from the Vs model with a mid-crustal LVZ show an obvious velocity minimum at periods from 15 to 32 s, but this is not pronounced in the Love wave dispersion curves (Fig. 2c). Figures 2e and 2f show measured Rayleigh and Love wave dispersion curves for different station-pairs across the Songpan-Ganzi terrane and the northwestern Qilian orogen. It is apparent that the dispersion curves measured from the stations-pairs located in the Songpan-Ganzi terrane and northwestern Qilian orogen show a local velocity minimum at periods between 15 and 32 s. Therefore, the local minimum of S wave velocity model in the crust can cause Rayleigh wave dispersion curves to change from positive gradient to negative one. In this study, we focus on discussing spatial correlations between the radial anisotropy and crustal LVZs based on the shear velocity models inverted from Rayleigh and Love wave, respectively.
Following the same procedure by Li et al. (2012), we invert the 3-D shear wave velocity models. First, we measure group velocity and phase velocity dispersions for Rayleigh wave and Love wave at different periods (Figs. 3a and 3b). Then, we use these dispersions to construct group and phase velocity maps of Rayleigh wave (8–44 s) and Love wave (8–36 s). Before inversion, we conduct checkerboard tests to evaluate lateral resolution for tomography. As an example, the lateral resolution at 15 s period (both Rayleigh and Love wave) is about ~0.8° in most parts of the study area (Figs. 3c, 3d, 3e and 3f). Finally, we obtain 1-D Vsv and Vsh models by inverting pure path dispersions at each grid of 0.4 by 0.4. These 1-D profiles are assembled to form a 3-D shear wave velocity model. During the 1-D shear wave velocity inversion, the initial velocities and Moho depths are referred to the previous tomography (Bao et al., 2013; Yang et al., 2012; Karplus et al., 2011; Chen Y et al., 2010; Li et al., 2009; Wang et al., 2007; Chen J H et al., 2005; Xu et al., 2000) and receiver function results (Yue et al., 2012; Pan and Niu, 2011).
We obtain Vsv and Vsh structures by inverting the Rayleigh wave and Love wave phase and group velocities, respectively. Figure 4 shows the Vsh and Vsv structures averaged from depths 0–20, 21–40, and 41–70 km. The obtained Vsh and Vsv velocity models show some differences which can be explained in terms of radial anisotropy. Due to the sparse coverage of Love wave, the Vsh models beneath the Qaidam Basin and the eastern Kunlun Mountain have poor qualities. In Figs. 4a and 4d, relatively low velocities are observed beneath the Qaidam Basin owing to its thick sedimentary cover but high velocities are confined to the non-basin areas, which are more obvious in the Vsv image. Figures 4b and 4e show the Vsv and Vsh models averaged from 21 to 40 km, which are different from the velocity distribution in the upper crust. A prominent feature beneath the Songpan-Ganzi terranes is characterized by a LVZ, and this depth is corresponding to the middle crust in the Tibetan Plateau. However, the velocity structure beneath the Qaidam Basin shows a distinct pattern from mountain regions and features relatively higher velocity. It is obvious that the velocity distribution in the northwestern Qilian orogen is different from that in the southeastern part. The northwestern Qilian orogen is resolved with relatively lower velocity than the southeastern orogen. The Vsh and Vsv averaged from 41 to 70 km are given in Figs. 4c and 4f, and the distribution is similar to that in depth range from 21 to 41 km. The velocity beneath the Qaidam Basin and the southeastern Qilian orogen is high, consistent with their thin crustal thickness. However, the northwestern Qilian orogen and Songpan-Ganzi terrane have an apparent low velocity, indicating a weak crust and deep Moho depth.
Based on the discrepancies between the Vsh model inverted from Love wave and Vsv model computed from Rayleigh wave, we quantify the radial anisotropy with 2×(Vsh-Vsv)/(Vsh+Vsv)×100%. This formula has been applied successfully to study the radial anisotropy in different areas, such as in the Black Sea (Yanovskaya et al., 1998), southern Tibet (Guo et al., 2012; Huang et al., 2010), North China Craton (Cheng et al., 2013) and Dabie orogenic belt (Luo et al., 2013). A comparison of theoretical and observed dispersion of phase velocity is used to illustrate the radial anisotropy (Fig. 5). We select one grid (34.2°N, 102.2°E) located in Songpan-Ganzi terrane (Fig. 5). In Fig. 5b, one can easily identify the significant discrepancies between the Vsh and Vsv models. Theoretical Love wave dispersion curve (gray dashed lines) computed from the inverted Vsv model and theoretical Rayleigh wave phase velocities (black dashed lines) computed from Vsh profile are shown in Fig. 5a. It is apparent that the Love wave dispersion computed from Vsv profile is different from the observed dispersions (diamonds), however, the observed dispersions can be well fitted with the dispersion (black solid lines) computed from the Vsh model. Meanwhile, the above results can be proved by the theoretical Rayleigh wave phase velocity dispersions (black dashed lines) computed from Vsh profile. This indicates that it is impossible to simultaneously fit both Rayleigh and Love wave dispersion curves using an isotropic model and thus radial anisotropy should be introduced in the inversion.
Figure 6 gives the radial anisotropy patterns at depths of 10, 20 and 30 km. The results show that the anisotropic distribution is characterized by lateral and vertical heterogeneities. From surface to 10 km in Fig. 6a, it is obvious that the negative anisotropy (Vsv > Vsh) is dominant throughout the study area, which could be attributed to the vertical alignment of mineral microfractures in the shallow crust. At the depths of 20 and 30 km (Figs. 6b and 6c), the most significant feature of radial anisotropy is the presence of two contrasting zones: one with positive radial anisotropy (Vsh > Vsv) beneath the Songpan-Ganzi terrane and the northwestern Qilian orogen, and the other with negative radial anisotropy in the eastern Kunlun Mountain. In particular, strong positive radial anisotropy occurs near the low velocity zone. Indeed, in the middle crust of the Songpan-Ganzi terrane and the northwestern Qilian orogen, Vsh is larger than Vsv, which is correlated with the LVZs beneath the Songpan-Ganzi terrane at depths from 15 to 35 km and the northwestern Qilian orogen at depths from 20 to 35 km, respectively (Fig. 4b).
Figure 7 describes the spatial correlations between 3-D Vsv structures and radial anisotropy along the profile AA' across the Songpan-Ganzi terrane, northwestern Qilian orogen, and eastern Kunlun Mountain. Figure 7a exhibits the 3-D Vsv map simultaneously inverted from the phase and group velocity dispersion curves of Rayleigh wave. It is prominent that a mid-crustal low velocity zone (~15–35 km) is revealed beneath the Songpan-Ganzi terrane but it diminishes around the eastern Kunlun Mountain. And the northwestern Qilian orogen displays a much weaker LVZ in the middle crust than the Songpan-Ganzi terrane. The radial anisotropy patterns computed from the difference between the Vsv and Vsh models are plotted in Fig. 7b. In the upper crust, the Vsv > Vsh anisotropy is observed beneath the Songpan-Ganzi terrane and the northwestern Qilian orogen. However, in the middle crust, the Songpan-Ganzi terrane and the northwestern Qilian orogen are resolved with the Vsh > Vsv anisotropy, which has a good correlation with the obtained mid-crustal LVZ plotted in Fig. 7a. With increasing depth, the positive radial anisotropy still persists in the Songpan-Ganzi terrane, whereas the northwestern Qilian orogen and eastern Kunlun Mountain are associated with the negative radial anisotropy.
In this study, in the upper crust, negative radial anisotropy is observed beneath the Songpan-Ganzi terrane (Figs. 6a and 7b), which is bounded by the Longmen Shan in the east and adjacent to the Yangtze Platform. In Late Triassic, it developed into a fold belt during the Indo-China movement and experienced the uplifting geodynamic process following the Tibetan Plateau in the Himalayan movement. The above dynamic process may induce the microcracks in the shallow crust dominated by the vertical movement. In the middle to lower crust, the most prominent feature beneath the Songpan-Ganzi terrane is the positive radial anisotropy with the corresponding low shear wave velocities (Figs. 4b, 4c, 4e, 4f and 7b).The previous seismic tomography results (Li et al., 2012; Yang et al., 2012), petrological study (Wang et al., 2012), heat flow data (Wang, 2001) and magneto-telluric (MT) results (Wang et al., 2009) consistently revealed that the mid-low crust beneath the Songpan-Ganzi terrane was characterized with the low isotropic velocity, high conductivity and weak materials. We speculate that the positive radial anisotropy in the deep crust is probably due to the presence of sub-horizontal alignment of anisotropic minerals, which may be induced by lateral flow in the deep crust of relatively low mechanical strength, following the collision between India and Eurasia. Shapiro et al. (2004) also observed a prominent positive radial anisotropy within the middle-to-lower crust across the Tibet. They proposed a sub-horizontal mica fabric to explain the radial anisotropy and argued that the strong radial anisotropy was consistent with a thinning of the middle crust by about 30% after the collision of Indian Plate and Eurasian Plate. In the southeastern Tibet, Huang et al. (2010) also found evidence for a low shear wave speed and positive radial anisotropy in the mid-lower crust beneath the Songpan-Ganzi terrane.
The Qilian orogen, as the transitional zone between the Tibetan Plateau and Yangtze Craton, has experienced three tectonic stages: (1) in Early Paleozoic Era, craton cracked into the aulacogen; (2) at the end of Middle Triassic, the collision of the Qiangtang Plate caused the indosinian intraplate folding, however, without the occurrence of strong orogenic belt; (3) from Mesozoic Era to Tertiary Period, the northern Qilian orogen stayed in the peneplain stage. Finally, the orogen was formed in the late Pleistocene Himalayan movement (Ge and Liu, 1999). In the upper crust, we attribute the negative radial anisotropy beneath the northwestern Qilian orogen to these dynamic processes which induces a preferred vertical alignment for microcracks in the shallow crust. In the middle crust, the radial anisotropy with Vsh > Vsv, is probably related to the low shear wave speed. Radial anisotropy in the mid-to-lower crust may be induced by a preferred alignment of mica crystals (Nishizawa and Yoshitno, 2001; Weiss et al., 1999). The mid-crustal radial anisotropy may thus suggest sub-horizontal alignment of mica in the mechanically weak zones of the deep crust beneath the northwest Qilian orogen. The preferred horizontal alignment of anisotropic minerals may be related to partial melting that could be initiated by strong crustal shortening in this area. In addition, it is clear that the radial anisotropy with Vsh > Vsv beneath the northwestern Qilian orogen is weaker than the Songpan-Ganzi terrane, which also corresponds to a relatively weaker LVZ beneath the northwestern Qilian orogen than the Songpan-Ganzi terrane. The seismic tomography results (Bao et al., 2013; Yang et al., 2012) also demonstrated 20 km thickness of low velocity layer existed in the middle crust of the northwestern Qilian orogen and it gradually diminished toward the southeast. Receiver function results (Yue et al., 2012) exhibited that there was a 10 km Moho gap between the northwestern Qilian and southeastern Qilian Mountains. As a result, the LVZs observed beneath the northwestern Qilian orogen are caused by both the low velocities in the mid-crust and relatively deep Moho. However, in the lower crust, the northwestern Qilian orogen is associated with the negative radial anisotropy, which is probably the result of vertically aligned seismic anisotropic minerals caused by the crustal thickening.
In summary, we obtain 3-D radial anisotropy and shear wave velocity models in the northeastern Tibetan Plateau by analyzing Rayleigh wave and Love wave Green's functions from seismic ambient noise data between 2007 and 2010 recorded at 223 stations.
(1) In the upper crust, the negative radial anisotropy is dominant throughout the study area which probably reflects fossil microcracks induced by the uplift, folding and erosion geodynamic processes.
(2) With the increase of depth, the pattern of radial anisotropy changes from predominantly negative to positive. A prominent radial anisotropy with Vsh > Vsv has been detected within the middle crust of the Songpan-Ganzi terrane and the northwestern Qilian orogen. The observed positive anisotropy generally has a good correlation with the low shear wave velocity.
(3) At depths deeper than 40 km, there exists a positive radial anisotropy beneath the Songpan-Ganzi terrane which is probably due to sub-horizontally alignment of anisotropic minerals induced by the mid-crustal lateral flow. However, the northwestern Qilian orogen is associated with a negative radial anisotropy, which could be the result of vertically aligned seismic anisotropic minerals caused by the crustal thickening.
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Jing Tan, Hongyi Li, Xinfu Li, Ming Zhou, Longbin Ouyang, Sanjian Sun, Dan Zheng. Radial anisotropy in the crust beneath the northeastern Tibetan Plateau from ambient noise tomography. Journal of Earth Science, 2015, 26(6): 864-871. doi: 10.1007/s12583-015-0543-x
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