Geophysical Research Letters

Using shear wave splitting measurements to investigate the upper mantle anisotropy beneath the North China Craton: Distinct variation from east to west

Authors


Abstract

[1] A compilation of shear wave splitting measurements in the North China Craton (NCC) is presented to investigate the upper mantle anisotropy beneath the region. By applying the method of Silver and Chan (1991), the splitting parameters of fast polarization direction and delay time are determined from SKS waveforms for 38 broadband stations. The results reveal the presence of small to large seismic anisotropy in the mantle beneath the NCC. The most striking result is the distinct difference of fast polarization directions between the Eastern Block (EB) and the Central Zone (CZ) of the NCC. The fast polarization directions trend SE beneath the EB, which implies that northwestward mantle flow has played a significant role in the reactivating of the EB during Late Mesozoic to Early Cenozoic, and the boundary between the CZ and the EB was possibly a west limit where the mantle flow was deflected. However, a collision event occurred in Early Proterozoic was also a possible cause for the anisotropy of the lithosphere beneath the CZ, depending on the craton lithosphere beneath the region had not been completely reformed by later tectonic activities.

1. Introduction

[2] The North China Craton (NCC) is the one of the world's oldest Archean craton [Liu et al., 1992]. The debate on evaluation of the NCC was long lasting and continues today. Although there are several hypotheses about the tectonic history of the NCC, a reasonable description about tectonic framework has not been achieved yet [Zhai et al., 2004, and references therein]. Some fundamental knowledge still remains enigmatic, such as the characteristics of upper mantle deformation beneath the craton, and the mechanism of mantle flow during Late Mesozoic to Cenozoic. Therefore, an investigation on upper mantle deformation is of great importance to fill the basic knowledge gap of the evolution of the NCC.

[3] By investigating the upper mantle anisotropy using shear wave splitting observation, we can characterize the lattice preferred orientation (LPO) and anisotropic strength of the mantle minerals [e.g., Silver and Chan, 1991; Vinnik et al., 1989; Savage, 1999]. It is generally accepted that LPO of anisotropic olivine and orthopyroxene crystals in the upper mantle is the primary cause of the observed splitting [Mainprice et al., 2000], and strain-induced LPO is considered to be a valuable indicator of either past or present tectonics [Babuska and Cara, 1991; Kay et al., 1999]. In this paper, we present the first shear wave splitting measurements in the NCC to understand the characteristics of upper mantle deformation beneath the region.

2. Observations

[4] The teleseismic events were recorded during several experiments in the NCC (Figure 1). The first experiment was operated by the North China Interior Structure Project (NCISP) of Chinese Academy of Sciences during November 2000 to September 2004. We selected three profiles from this experiment, including 8 portable broadband stations deployed at Taihang Mountains, 10 stations at the Luxi uplift, and 3 stations at Yan Mountains. Each of them lasted for about one year. The second experiment was operated by the State Seismological Bureau of China. We used data from 14 broadband stations deployed at Taihang Mountains and Yan Mountains during September 2001 to August 2003. In addition, we used the data of Chinese Digital Seismic Network (CDSN) station BJT during January 2000 to December 2002 from the website of Incorporated Research Institutions for Seismology (IRIS), stations HHC and HNS during January 2001 to December 2004 from CDSN website. The dataset consists of about 1000 three-component teleseismic recordings with epicentral distance from 85°–115°. All the data were low-pass filtered at 0.2 Hz. Distribution of the events used are shown in Figure 2.

Figure 1.

Map of the study region and the locations of stations where the shear wave splitting measurements were made. The orientations and length of each bar indicate the fast polarization direction and the splitting delay time, respectively. The thick dashed line denotes the boundary between the Eastern Block and the Central Zone of the NCC. The shade zones represent basins, gray solid lines schematically indicate trending of mountain belts [Zhang, 1997], and the two big gray arrows indicate the extensional direction during Late Mesozoic to Early Cenozoic. The inset is a map with topography on which the APM direction [Wang et al., 2001] is marked as a dark arrow.

Figure 2.

Distribution of the events used in this study. Solid points denote event; rectangle represents the study area.

[5] We used the methodology of Silver and Chan [1991] to determine the fast polarization direction ϕ and delay time δt, assuming that shear waves traverse a single homogeneous anisotropic layer. We analyzed SKS phases, and found the best-fitting ϕ and δt from a grid search over trial ϕ and δt, by minimizing the energy on the reconstructed transverse component. In order to assess how well the estimated splitting parameters, the F test analysis described by Silver and Chan [1991] is applied. To obtain a reliable estimate of splitting parameters, we only retained the unambiguous results that satisfying the following standards: (1) clear SKS arrivals, (2) the particle motion at surface is elliptic, the two horizontal component waveforms similar, and the particle motion linearized by removing the effect of anisotropy [Herquel et al., 1999], and (3) the result of error analysis is reasonable. Figure 3 gives an example of waveform splitting at station QSD. As a result, an electronic auxiliary material (Table 1) lists all shear wave splitting measurements with their 1σ uncertainty determined from the 95% confidence interval in the (ϕ–δt) domain.

Figure 3.

Examples of shear wave splitting analysis at station QSD. (a) Radial and transverse components before and after removing the delay time on the slow component. A and F mark the start and end point of time window for analysis, respectively. (b) Four small boxes showing normalized fast and slow components and their particle motions. (c) Plot of grid search in the (ϕ–δt) domain and error estimate associated (shade zone indicates the 95% probability area).

3. Results

[6] The complete compilation of 92 shear wave splitting measurements is plotted in Figure 1. Small to large values of δt (0.70–1.94 s) are found at all stations, and the fast direction ϕ displays great variations, indicating the presence of complex seismic anisotropy in the upper mantle beneath the stations. The most striking result derived from the observation of Figure 1 is the distinct variation of the fast polarization directions from east to west, which exhibits geological control. Our study area could be divided into two regions based on observed fast polarization direction, where the thick grey dashed line in Figure 1 marks the boundary between them. According to their geographic positions in the North China Craton, we name the two group as the Central Zone and the Eastern Block, respectively.

[7] We have dense station coverage in the Central Zone. Although the observations are obtained from different experiments, the values of ϕ in the Central Zone show good consistency trending NE to NNE, except for stations GAN and XIT with E-W trending fast polarization directions. The permanent stations HNS and HHC in this region have long-term dataset from January 2001 to December 2004. Table 1 (auxiliary material) shows small standard deviation both in delay time and fast azimuths giving the impression of the data quality: ϕ and δt variations do not exceed ±5° and ±0.15s, respectively. Small to large values of delay time δt are found in this region, and the largest delay time 1.94 s belongs to station XIL, which located at the most east of this subdomain.

[8] In the Eastern Block of the NCC, the results derived from some high-quality records allow us to interpret the final results with confidence. The obvious feature in the Eastern Block is the homogeneity of ϕ trending, with an average of N110° and standard deviation of 6°. Mean delay times range between 1.00 to 1.40 s in most cases. It is noteworthy that stations 213 and 214 in the Eastern Block give distinct measurements compared to stations WAX and ZAH in the Central Zone using the data from the same event 2003208 (see Table 1).

4. Discussion

[9] We first test the influence of the present-day mantle flow by comparing the observed fast polarization directions to the absolute plate motion (APM) direction [Silver et al., 2001]. The GPS measurements [Wang et al., 2001] show coherent movement of North China at rates of 2 to 8 mm/year to the direction of N120°–N140° (marked as a big arrow in the inset of Figure 1). Whatever the reference frame taken, the slow plate motion is not expected to generate strong flow in the upper mantle and could hardly explain short-scale ϕ variations of 90° within the region. As discussed next, our results are compatible with the fossil lithospheric structures and tectonic history. Combining with the previous tectonic studies, we propose that two tectonic events could have possibly reorientated the olivine crystals within the uppermost mantle: one is related to a widespread rifting of Eastern China during Late Mesozoic to Early Cenozoic [Ren et al., 2002], and the other is the amalgamation of the NCC during Late Archean to Paleoproterozoic [Zhao et al., 2001].

[10] It was reported that NW-SE trending extensional stress field (marked as big gray arrows in Figure 1) dominate rifts during Late Mesozoic to Early Cenozoic [e.g., Ren et al., 2002; Zhao and Zheng, 2005] in the Eastern Block. Our results show that the fast polarization directions trending NW-SE agree with the orientation of extensional structures in this region. This suggests that the lithosphere beneath the Eastern Block retains the history of Late Mesozoic to Early Cenozoic deformation structures. Note that rapid changes occur across the boundary between the Central Zone and the Eastern Block, such as the abrupt change from station XIL to station 213 (∼100 km apart). The observation of abrupt changes constrains the top of the anisotropic layer to be no deeper than about 50∼100 km based on the Fresnel-Zone analysis [Rümpker and Ryberg, 2000; Silver et al., 2001], which suggests that the mantle flow beneath the Eastern Block occurred in a shallow depth. These characteristics imply that: (1) a mantle flow played significant role in the reactivating of the Eastern Block of the NCC during Late Mesozoic to Early Cenozoic; and (2) the boundary between the Central Zone and the Eastern Block possibly defined a west limit where the northwestward mantle flow was deflected. The previous geochemical studies showed that the Eastern Block of the NCC has a thin lithosphere with thickness of ∼80 km [Griffin et al., 1998; Fan et al., 2000; Gao et al., 2002; Wu et al., 2003], however the range of the thinning lithosphere is still in debate. Based on our results, we proposed that the lithosphere beneath the Central Zone was thicker than that beneath the Eastern Block when the extensional event occurred, and the thicker lithosphere root of the Central Zone probably acted as a barrier deflecting the northwestward mantle flow [e.g., Fouch et al., 2000]. In this case, a compressional stress field would be introduced to cause fast polarization directions to orient NE-SW or NNE-SSW that is perpendicular to the direction of compression at the boundary.

[11] However, there is another more possible interpretation for the anisotropy of the lithospheric mantle beneath the Central Zone, depending on that the craton lithosphere had not been completely reformed by later tectonic activities. This interpretation is related to the collision event between the Eastern and Western Blocks of the NCC from the Late Archean to Early Proterozoic that resulted in the final amalgamation of the NCC [Zhao et al., 2001]. In the Central Orogenic Belt (corresponding to the Central Zone defined in this paper) formed after the collision, the structural style of Late Archean to Paleoproterozoic basement is generally characterized by linear belts outlined primarily by a number of NNE-NE trending ductile shear zones (grey solid lines in Figure 1) [e.g., Zhai and Liu, 2003; Zhao et al., 2001]. Based on our results that the fast polarization directions in this region are subparallel to the trending of the Late Archean to Paleoproterozoic orogenic structures, we can infer that the Late Archean to Paleoproterozoic deformation had been possibly preserved as fossil mantle anisotropy [e.g., Barruol et al., 1998] similar to that of the Kaapvaal craton, South Africa [Silver et al., 2001], in the case that the craton lithosphere had not been completely reformed by later tectonic activities. Especially, at the boundary between the Central Zone and the Eastern Block, two relatively large delay times are observed at stations XIL (1.94 s) and HNS (>1.50 s). We can attribute the large delay times to an overlapping effect [Xu et al., 2004] of the retained craton lithosphere [e.g., Bormann et al., 1996] and the deflected mantle flow during Late Mesozoic to Early Cenozoic.

Acknowledgments

[12] We thank Dr. L. X. Wen, Dr. M. Xue and Dr. F. Y. Wu for their constructive discussions, Xiao Ren for help with using the CDSN data. We acknowledge the participants of Seismic Array Probe Laboratory, IGGCAS. Some data used in this work were obtained from State Seismological Bureau of China, the CDSN and IRIS seismography networks. This research was supported by the National Science Foundation of China (Grant 40434012), China Earthquake Data Sharing (2003-DZGX-2004) and Chinese Academy of Sciences. We appreciate the careful and insightful reviews by Dr. A. Zollo, Dr. B. Barruol and an anonymous reviewer.

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