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Insight into craton evolution: Constraints from shear wave splitting in the North China Craton
Physics of the Earth and Planetary Interiors 168 (2008) 153–162
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Insight into craton evolution: Constraints from shear wave splitting in the North China Craton Liang Zhao ∗ , Tianyu Zheng, Gang Lü Seismological Laboratory (SKL-LE), Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China
a r t i c l e
i n f o
Article history: Received 10 October 2007 Received in revised form 15 May 2008 Accepted 3 June 2008 Keywords: North China Craton Upper mantle Seismic anisotropy Shear wave splitting
a b s t r a c t The multi-episodic tectonic activities from the Precambrian to Cenozoic, including nucleus formation, cratonic amalgamation, and rejuvenation, make the North China Craton (NCC) an ideal natural laboratory for studying craton evolution. Spatial change in the upper deformation records is an important aspect for understanding cratonic formation and rejuvenation. In this study, we performed seismic shear wave splitting analysis using SKS phases from 50 portable stations. Two different methodologies, shear wave splitting measurement and amplitude analysis of transverse/radial components, produced mutually consistent splitting results. These results showed that the seismic anisotropy beneath the Ordos Block can be divided into three subgroups reflecting the tectonic control. Combining these results with those from previous splitting studies in the eastern NCC, we suggest that the Proterozoic amalgamation generated the seismic anisotropy in the boundary zone between the Ordos Block and the Trans-North China Orogen, while the anisotropy in the eastern Trans-North China Orogen and eastern NCC were possibly associated with the lithospheric rejuvenation during the Late Mesozoic to Cenozoic. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Archean cratons are characterized by a cold, thick lithosphere keel. The presence of buoyant and refractory peridotite residues beneath Archean regions, is widely held to be responsible for the inherent stability of Archean cratons (Durrheim and Mooney, 1991; King, 2005). The North China Craton (NCC), however, is an anomaly. In contrast to the long-term stabilization of western NCC after the Precambrian amalgamation, the eastern NCC experienced significant tectonic rejuvenation (lithospheric thinning, in particular) in the Late Mesozoic to Cenozoic. Multiple tectonic events, including nucleus formation, cratonic amalgamation, and rejuvenation, make the NCC an ideal natural laboratory for studying craton evolution. One important feature is that the different fates of the eastern and western NCC actually resulted from cratonic formation and lithospheric rejuvenation with spatial changes. Seismic anisotropy is an important indicator of present or past upper mantle deformation (Silver, 1996; Savage, 1999). For example, in southern Africa, the observed seismic anisotropy was dated back to the Archean mantle deformation (Silver et al., 2001). In central Tibet, the seismic shear wave splitting measurements reflected upper mantle deformation during the Cenozoic (Huang
∗ Corresponding author. Tel.: +86 10 82998430; fax: +86 10 62010846. E-mail address: [email protected] (L. Zhao). 0031-9201/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.pepi.2008.06.003
et al., 2000). Compound geodynamic events might have produced spatial changes in the upper mantle anisotropy in the NCC. Thus, seismic anisotropy is useful for addressing issues associated with craton evolution. Previous studies revealed a distinct east–west change in the upper mantle anisotropy in the eastern NCC (Zhao and Zheng, 2005a; Zhao et al., 2007). Particularly, the shear wave splitting measurements from dense seismic stations indicated complex mantle deformation in the boundary zone in relation to the undulated lithosphere–asthenosphere topography. However, the available seismic stations in the previous studies were limited to the eastern and central blocks of the NCC. In this paper, we present the first analysis of shear wave splitting in the western NCC. The integrated database allows us to provide well-covered splitting results for understanding the spatial changes of craton evolution. 2. Geological background The NCC is traditionally considered the Chinese part of the Sino-Korean Craton, which contains some of the oldest known continental rocks, some as old as 3.8 Ga (Liu et al., 1992; Song et al., 1996). Available geochronological data reveal that most pretectonic gneisses and volcanic rocks formed in the mid-Archean with ages ranging from 3.5 to 2.5 Ga (Zhao et al., 1998, 2001; and references therein). According to lithological, geochemical, geochronological, structural and metamorphic P-T path data, it was proposed that the basement of the NCC can be divided into Eastern
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Fig. 1. Simplified geological map of the North China Craton and the locations of available seismic stations. The hatching zones indicate the three-fold tectonic subdivision of the cratonic basement according to Zhao et al. (2001); the thick gray dashed lines give the positions of two suture zones associated with the two-step collision based upon Faure et al. (2007). The solid triangles represent stations from NCISP-4; thick gray line indicates the location of experiment NCISP-2; open triangles mark those used in previous studies (Zhao and Zheng, 2005a; Zhao et al., 2007). The big arrow gives the average APM direction using HS3-NUVEL 1a model (Gripp and Gordon, 2002); white fence symbols represent thrust belt; dual-headed arrow indicates extensional direction of the Bohai Bay Basin during the Mesozoic to Cenozoic. WB: the Western Block; TNCO: the Trans-North China Orogen; EB: the Eastern Block. The inset is a larger scale map of the North China Craton with three-fold subdivision.
and Western (or “Ordos”) blocks, separated by a major boundary zone called the Trans-North China Orogen (TNCO) (Fig. 1) (Zhao et al., 2001). However, the timing, mechanism and modality of the amalgamation are disputed (Zhao et al., 1998, 2001, 2005; Li et al., 2000; Wilde et al., 2002; Zhai and Liu, 2003; Kusky and Li, 2003; Guo et al., 2005; Polat et al., 2005; Kröner et al., 2005; Faure et al., 2007; Trap et al., 2007). For example, Kusky and Li (2003) and Polat et al. (2005, 2006) proposed a westward-directed subduction of an old ocean between the two blocks, with the final collision occurring at ∼2.5 Ga. Faure et al. (2007) also argued for a westward subduction but proposed a two-step geodynamic evolution to account for formation of the TNCO. Other authors, in contrast, have suggested an eastward-directed subduction with the final collision taking place at ∼1.85 Ga (Zhao et al., 2001; Wilde et al., 2002; Kröner et al., 2005). Low heat flow, a lack of volcanism, and other reactive signatures suggest that the Ordos Block has been tectonically stable since the Proterozoic. In contrast to the long-term stability of the Ordos Block, the eastern NCC has gone from having a thick (∼200 km) Archean or Proterozoic lithosphere to its current much thinner lithosphere (60–80 km) (Griffin et al., 1998; Chen et al., 2006; Sodoudi et al., 2006). Contrasting compositions of mantle xenoliths from Paleozoic kimberlites and Cenozoic basalts suggest that removal of the Archean lithospheric mantle occurred between the Ordovician and the Cenozoic (Menzies et al., 1993; Griffin et al., 1998; Gao et al., 2002; Wu et al., 2005). Associated with the lithospheric rejuvenation, the eastern NCC is believed to have been affected significantly by a NW–SE-trending extensional stress-field, as evidenced by the development of a series of half-graben basins and synkinematic granitic pluton emplacement (e.g., Zhao and Zheng, 2005b; Lin and Wang, 2006).
Having undergone two major tectonic events, the Proterozoic amalgamation and the Mesozoic–Cenozoic rejuvenation, the structural grains of the NCC are dominated by NNE to NE trending fault systems. Two fault systems define the major terrane boundaries, which roughly correspond with the limits of a 100–300-kmwide TNCO. Both faults strike N–S in their central and southern parts and turn NE at their northern extensions. However, whether these two faults represent the original fundamental boundaries between the two blocks and the TNCO remains unknown because of their Mesozoic reactivation (Zhao et al., 2001). Re-Os data for Tertiary alkali basalts suggest that the lithospheric mantle beneath the TNCO formed at about 1900 Ma, which approximately matches the age of the major collisional event in the orogen, but is significantly younger than the overlying late Archean crust. This chronology suggests that the original Archean lithosphere of the TNCO may have been replaced at about 1900 Ma in response to the collision between the Eastern and Ordos Blocks (Gao et al., 2002). Based on a study of the basalts and mantle xenoliths from the Eastern Block and the TNCO, Xu et al. (2004) presented geochemical evidence arguing that a progressive lithospheric thinning took place beneath the TNCO during the Cenozoic. 3. Seismic experiment and data This study is based on the analysis of SKS waveforms recorded at 50 temporary stations from a sub-project of the North China Interior Structure Project (NCISP-4). Uniformly equipped with REFTEK data loggers and Güralp CMG-3ESP sensors (50 Hz-30 s), NCISP-4 was operated from September 2005 to September 2006 by Chinese
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Fig. 2. Distribution of the events used in this study. Solid point denotes event used in shear wave splitting measurement (Fig. 4), star denotes that used in T/R value analysis (Fig. 5). Rectangle represents the study area.
Academy of Sciences. This profile was set up in an E–W direction starting in the western TNCO, ending at the western margin of the Ordos Block (Fig. 1), and integrating with a previous experiment called NCISP-2 and some permanent stations from the Capital Zone Project of China to the east (Zhao and Zheng, 2005a). These experi-
ments contributed to the first database for stations widely covering the NCC along an E–W orientation (Fig. 1). Among teleseismic events at distances in the range of 85–115◦ , 20 events (see Fig. 2 for locations) provided useful data for a shear wave splitting analysis. In the following two sections, we applied
Fig. 3. Examples of shear wave splitting measurement for event 2006:223:14:30. (A) For station 277.The upper panel shows records of vertical, radial and transverse displacement. ‘A’ and ‘F’ mark the start and end point of time window for analysis, respectively. Four small boxes in the left panel illustrate normalized fast and slow components and their particle motions; the right panel shows grid search result in the (, ␦t) domain. (B) For station 291.
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Fig. 3. (Continued ).
two methods to analyze the shear wave splitting of the SKS phases: one is a routine method of shear wave splitting measurement, and the other is a statistical analysis of the peak–amplitude ratio between the transverse and radial components of the SKS phases. 4. Shear wave splitting measurements First, we used the method developed by Silver and Chan (1991) to determine the fast polarization direction and delay time ␦t between the fast and slow components, assuming that SKS waves traverse a single homogeneous anisotropic layer. In order to assess the reliability of the estimated splitting parameters, the F-test analysis (Silver and Chan, 1991) was applied. To investigate the frequency dependence of results, we applied various twopole Butterworth band-pass filters to the data before the splitting measurements. All the filters adopted had a fixed lower corner frequency of 0.02 Hz but different upper corner frequencies ranging from 0.2 to 1.0 Hz. This filtering yielded approximately the same splitting parameters, indicating that high frequency scattering did not significantly affect the results. Fig. 3 displays the examples of splitting measurements from stations 277 and 291. Among 50 stations, 48 stations (except stations 287 and 288) yielded 151 valid shear wave splitting and null results. Table 1s (supplementary data) lists all individual splitting measurements with their 2 uncertainty in the –␦t domain. The mapped
view in Fig. 4 shows the individual results, revealing complex spatial changes of the splitting parameters. The splitting results in this study fall into three major subgroups from east to west (except stations 307 and 308) as shown in Fig. 4. At the eastern end of the profile, including stations 258–272, the majority of fast splitting directions were N120–140◦ with an average delay time of 0.8 s. At the central segment, including stations 273–292, the fast directions orient coherently NW–SE similar to those in the eastern end but with an average delay time of 1.8 s. At the western segment of the profile, including stations 293–306, most of the null splitting results or small delay times were obtained. At the westernmost end of the profile, stations 307 and 308 were different, yielding obvious splitting and abruptly varying fast directions. To the west of the profile, strong splitting was also obtained at station YCH from a previous study (Zhao et al., 2007). 5. Amplitude analysis of transverse/radial components In the splitting measurements, some stations gave only a few valid splitting results. Based on visual inspection of the SKS waveforms, this outcome can be attributed to the weak energy of the transverse component. Although the SKS phase can be tracked, it fails to give valid results with reasonable error estimation. To enhance the reliability of the results, we used a statistical method to investigate anisotropy through the analysis of peak–amplitude
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Fig. 4. Individual splitting measurements in this study (black bars and gray cross-bars) and previous studies (blue bars) (Zhao and Zheng, 2005a; Zhao et al., 2007). The orientation of each bar indicates the fast direction; bar length is proportional to the delay time. The gray cross-bar indicates a null measurement, with orientation parallel or perpendicular to the polarization direction of the incoming energy. Dashed lines give the boundaries between major tectonic units. Big arrows give the average APM directions using HS3-NUVEL1a model (Gripp and Gordon, 2002).
ratio between the transverse (T) and radial (R) components (T/R value for brevity hereafter). This method is similar to the multichannel analysis proposed by Chevrot (2000) except that we employed a T/R value directly instead of projecting the transverse components onto the radial components derivatives. 5.1. Methodology We follow the denotation of Kennett (2002) to construct relationships between T/R value and splitting parameters. In terms of the incident polarization azimuth and the fast direction of the anisotropic media, = − , and then at the place where a wave incidents into anisotropic media, the fast and slow components are written as
f = RAW s
Ri Ti
,
Thus, in the receiver-frame, the observed radial and transverse component are represented by
Rr Tr
= RWA EA U RAW
Ri Ti
,
(2)
here Rr and Tr are the recorded radial and transverse components, respectively, and RWA is the transposed matrix of RAW . For an incident wave without transverse energy, such as the SKS phase, it is feasible to calculate the T/R value assuming that the fast direction and incoming radial component are known. A propagation of Ricker wavelet was assumed with a center frequency of 0.2 Hz, the same as the major frequency of our SKS records, through a single-layer anisotropic medium with a delay time of 0.5, 1.0, and 1.5 s respectively. Synthetic calculations showed that the T/R value is a function of delay time and with periodicity of /4 (Fig. 5). The maximum T/R value corresponded to the case of being equal to n/4. The amplitude range of T/R value depends on the value of the delay time, which can be used to constrain the delay time
(1)
where f and s are the fast and slow components, Ri and Ti are the input radial and respectively, the rotation transverse components, cos sin matrix RAW = . − sin cos Consider wave propagation close to vertical and label the two vertical wave slowness qf and qs , it is convenient to introduce the mean slowness q˜ and the wave slowness deviation q, q˜ =
(qf + qs ) , 2
q =
(qf − qs ) . 2
The phase increment for upward propagation through a vertical distance h is then given by
EA U
=e
iω˜qh
e−iω qh 0
0 eiω qh
.
Fig. 5. Theoretical results showing the relationship between T/R value, delay time and .
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in cases of good azimuthal coverage of the events (e.g., Chevrot, 2000). For a fixed delay time, T/R value is a bimodal function of among domain (0, ), which introduces a /2 nonuniqueness of fast direction. Nevertheless, we can use the splitting parameters obtained from splitting measurements to reduce the nonuniqueness. Our statistic method consists of two-step processes. First, we visually inspected the waveform of SKS for all events one by one. Second, we assigned time-windows of SKS phases for each record and calculated the T/R value. For data to be useful the SKS core phases have to be well-separated from other phases such as S and ScS. 5.2. T/R value analysis Fig. 6 shows the results for all stations which include 391 values with a valid number much larger than that of the splitting measurements. As shown in Fig. 6, the T/R value has a systematical variation along the profile and could be divided into three subgroups from east to west, just like the splitting parameters. This consistency illustrates the robustness of shear wave splitting analyses. To determine splitting parameters (, ␦t) beneath the profile, we used m control points to construct a single-layer starting model represented as x(0 , ␦t0 ) (i = 1, m); here xi is the longitude of the ith control point, and (0 , ␦t0 ) are assigned with average splitting parameters interpolated from the results of splitting measurements. For each event–station pair, we can calculate the theoretical T/R value using Eq. (2) and the square variance between the theoretical values and the recorded ones. Considering the /2 periodicity of T/R, we searched around the starting model within range of (0 ± 22.5◦ , ␦t0 ± 0.5 s). To explain our observation, we tested hundreds of models and obtained an optimal model by adjusting model parameters x(, ␦t) to minimize the average square vari-
ance between the calculated T/R values and the recorded ones. Fig. 7 shows the optimal model and the comparison between the calculated T/R values and the recorded ones (average square variance is equal to 0.138). Because the majority of T/R values of stations at the western end are less than 0.2, the estimated fast directions have large uncertainties beneath this region. However, these findings are in good agreement with the splitting measurements in that most of the null splitting results were observed in this region. 6. Interpretations and discussion The shear wave splitting measurements and T/R value analyses yield mutually consistent results, both indicating that the seismic anisotropy beneath the western NCC can be divided into three subgroups. In contrast, to the east of this profile, a previous study (Zhao and Zheng, 2005a) showed that the fast directions trend NE-NNE with an average delay time of 1.3 s in the eastern TNCO, or NW–SE with an average delay time of 1.2 s in the eastern NCC. The near-vertical incidence angle of the SKS phase leads to a high lateral resolution for the subsurface anisotropic structure. However, the seismic anisotropy recorded at the Earth’s surface represents a vertically integrated effect of anisotropy along the mantle ray path on the receiver side, making the depth resolution rather poor. Although alignments of mantle minerals in some region of the lowermost mantle also contribute (e.g., Niu and Perez, 2004), the lattice-preferred orientation of anisotropic olivine crystals in the upper mantle is believed to be the primary cause for SKS wave splitting (Savage, 1999; Mainprice et al., 2000). Most of studies agree on values ranging from 0.1 to 0.2 s for deformed crust (e.g., Herquel et al., 1995; Barruol and Mainprice, 1993), and a contribution from the lower mantle and transition zone that is typically less than 0.2 s (Savage, 1999). In addition, the observation of abrupt variation of splitting parameters constrains the top of the anisotropic layer
Fig. 6. T/R analysis results of SKS phases from recorded data. The point marks T/R value for an event-station pair with different color distinguishing different back-azimuthal range.
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Fig. 7. (A) The optimal splitting-parameter (, ␦t) model for T/R analysis. The arrow denotes fast direction beneath the profile with respect to north; the solid line represents delay time. Symbol “?” indicates the region where uncertainty of fast direction exists. (B) Comparison between the recorded T/R values (open circles) and calculated ones (solid triangles) from the above model. For the above model, the average square variance is minimized to 0.138.
within a shallow depth based on sensitivity zone analysis (Rümpker and Ryberg, 2000; Zhao et al., submitted for publication). According to previous multidisciplinary studies on the NCC (e.g., Fan and Menzies, 1992; Menzies et al., 1993; Griffin et al., 1998; Zhao et al., 2005; Kusky et al., 2001; Gao et al., 2002; Wilde et al., 2002; Zhai and Liu, 2003; Kusky and Li, 2003; Xu et al., 2004; Zhang et al., 2004; Zhao and Zheng, 2005a; Guo et al., 2005; Polat et al., 2005; Kröner et al., 2005; Lin and Wang, 2006; Faure et al., 2007; Trap et al., 2007), three candidate geodynamic events are expected to explain the shear wave polarization in the NCC: (1) the present-day asthenospheric flow; (2) the lithospheric rejuvenation that occurred during the Mesozoic to Cenozoic; or (3) the cratonic amalgamation between the Ordos and Eastern blocks took place during the Proterozoic. To test these three models, we compared the orientations of our shear wave splitting observations with predictions from the different tectonic events.
6.1. Present-day asthenospheric flow-induced anisotropy The fast directions of stations 264–292 may be consistent with the present-day asthenospheric anisotropy caused by the northwestward absolute plate motion (APM). The HS3-NUVEL1a model (Gripp and Gordon, 2002) predicts a coherent APM direction
of ∼N287◦ (webpage: http://tectonics.rice.edu/hs3.html) that are nearly parallel to the fast directions in the eastern Ordos and Eastern blocks. However, the coherent APM direction does not match the spatial change of fast directions observed in the Ordos Block or the splitting observed in the TNCO. If the plate motion is driven by the underlying asthenosphere, a simple and uniform asthenospheric flow could not explain the splitting observations.
6.2. Spatial range of lithospheric rejuvenation responsible for anisotropy Based on the shear wave splitting results observed in the eastern NCC, Zhao and Zheng (2005a) proposed that the anisotropy beneath the eastern TNCO was possibly generated by a compressional stress field induced by the northwestward asthenospheric flow associated with the lithospheric rejuvenation, or by the Proterozoic amalgamation between the Eastern and Ordos blocks. If the effect of lithospheric rejuvenation extended to the Ordos Block, it is predicted that the fast directions of the Ordos Block should be coherent with those in the TNCO or smoothly change in response to a large scale mantle flow. However, this prediction is inconsistent with the abrupt changes of fast directions observed in the Ordos Block and the TNCO. On the other hand, if the craton
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Fig. 8. Map showing spatial distribution of the lithospheric rejuvenation and cratonic amalgamation. The dual-head arrow marks direction of the lithospheric extension; the gray thrust symbol in the TNCO represents a possible compression related to the lithospheric rejuvenation.
lithosphere beneath the TNCO was reformed after the Proterozoic amalgamation, the anisotropy generated by amalgamation would have been frozen-in, and the fast directions would have remained coherently in the collisional belt including the TNCO and the eastern Ordos Block. This prediction is also inconsistent with the abrupt change of fast directions. These comparisons imply that an effect of lithosphere rejuvenation has possibly occurred beneath the TransNorth Orogen but not extended into the Ordos Block. In addition, the variation of fast direction from the Eastern Block to the TNCO exhibits a heterogeneous pattern during the lithospheric rejuvenation. Fig. 8 displays the possible spatial range of the lithospheric rejuvenation. In the Eastern Block, an extensional event dominates. On the other hand, in the TNCO the anisotropic pattern matches neither the extension in the eastern NCC nor the collision belt in the Ordos Block. If the change in lithospheric topography from the eastern NCC to the TNCO deflected the asthenospheric flow, two kinds of models could explain the observed anisotropy pattern. The first is that the deflected asthenosphere material flowed vertically, which would produce a NE–NNE-striking compression at the boundary zone. The second possibility is that the asthenosphere flowed horizontally at the base of the thick lithosphere, which would generate a NE–NNE trending extension (e.g., Lin and Wang, 2006). A larger scale observation in the TNCO, however, is required to test these two hypotheses in the future.
6.3. Precambrian amalgamation-induced anisotropy Most splitting at stations in the Ordos Block was weak or even undetectable and thus significantly smaller than the global average of 1.0 s (Silver, 1996); however, it is similar to data for the southeastern Kaapvaal Craton (Silver et al., 2001). Considering the thick lithosphere beneath the Ordos Block (Li et al., 2006; Huang and Zhao, 2006), the overall weakness of the Archean Ordos Block can be attributed to weak intrinsic anisotropy analogous to that discovered in the Kaapvaal Craton (Ben-Ismail et al., 2001), or to randomly aligned anisotropy of the upper mantle minerals.
If the lithospheric rejuvenation had not overprinted the upper mantle beneath the Ordos Block, the shear wave splitting beneath the eastern Ordos Block was most possibly caused by the fossil upper mantle anisotropy associated with the Precambrian amalgamation. The strongest anisotropy zone corresponds to the collision belt recognized by seismic imaging performed with the receiver function method (Zheng et al., submitted for publication). However, it is surprising that the NW–SE oriented fast directions in the western TNCO and eastern Ordos are not parallel to the NE- or NNEtrending Proterozoic geological features (Fig. 1). This characteristic differs from several other cratons in which the fast direction is parallel to the Precambrian geological features, suggesting a diversity of Precambrian tectonics. Assuming that a dry-condition collision was responsible for the anisotropic generation, we should expect a fast direction perpendicular to the compressional stress in the western TNCO. This prediction does not fit our observations in the eastern Ordos Block. However, NW–SE trending fast direction might be attributed to the hydration of upper mantle minerals (Jung and Karato, 2001) during the collision. Based on an experimental study by Jung and Karato (2001), the relationship between seismic anisotropy and flow geometry undergoes marked changes when water is added to olivine. When the deformation of olivine occurs under waterrich and modest stress conditions, the fast shear wave polarization is orthogonal or sub-orthogonal to the striking of the collisional boundary. If the amalgamation belt of NCC was wet because of the closure of ancient oceans, our fast directions are consistent with the prediction from a NW–SE oriented amalgamation model. Starting at the west boundary of the TNCO, the observed delay time increases from east to west, reflecting the amplitude variation of the upper mantle deformation caused by cratonic amalgamation (Fig. 8). It also indicates the deep boundary marked by larger delay time located to the west of the surface boundary. We speculate that a westward subduction of the Eastern Block might be responsible for the inconsistency between the surface and deep boundary. This speculation is in good agreement with the receiver function imaging (Zheng et al., submitted for publication) and a
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geodynamic model proposed by Faure et al. (2007) based on surface tectonic observation. 7. Conclusions In this study, we performed shear wave splitting analysis using SKS phases from 50 portable stations. Two different methodologies, shear wave splitting measurement and amplitude analysis of transverse/radial components, produced mutually consistent splitting results. The splitting results show that the seismic anisotropy beneath the Ordos Block can be divided into three subgroups. In the western end, most of the weak anisotropy with T/R values less than 0.2 was observed. In contrast, obvious splitting was recorded in the central and eastern Ordos Block with the majority of fast directions trending NW–SE and the central segment having a larger average delay time than the eastern segment. The splitting results in this study and from previous studies in the eastern NCC (Zhao and Zheng, 2005a; Zhao et al., 2007) indicate complex seismic anisotropy beneath the NCC. Combining the available splitting results, we suggest that the seismic anisotropy in the Eastern Block and the TNCO was possibly generated by lithospheric rejuvenation during the Mesozoic to Cenozoic. The spatial change in fast directions from east to west implies a spatially varying deformation pattern. In contrast, in the eastern Ordos Block, the upper mantle remains the “frozen-in” anisotropy generated by the Precambrian amalgamation between the Eastern and Ordos blocks, while the western Ordos Block has the weak anisotropy characterizing a stable craton. The inconsistency between the fast directions and the geological features suggests that a water-rich collision was possibly involved in the cratonic amalgamation of the NCC after the closure of ancient oceans. Acknowledgements We thank Dr. Wei Lin for helpful discussion, Prof. G.C. Zhao for providing data of the subdivision of NCC. The constructive suggestions by two anonymous reviewers and editor improve the manuscript. We acknowledge the participants of Seismological Laboratory, IGGCAS. This research was financially supported by the National Science Foundation of China (Grant 40504006 and 40434012) and Chinese Academy of Sciences. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.pepi.2008.06.003. References Ben-Ismail, W., Barruol, G., Mainprice, D., 2001. The Kaapvaal craton seismic anisotropy: petrophysical analyses of upper mantle kimberlite nodules. Geophys. Res. Lett. 28, 2497–2500. Barruol, G., Mainprice, D., 1993. A quantitative evaluation of the contribution of crustal rocks to the shear-wave splitting of teleseismic SKS waves. Phys. Earth Planet. Inter. 78, 281–300. Chen, L., Zheng, T.Y., Xu, W.W., 2006. A thinned lithospheric image of the Tanlu Fault Zone, Northeastern China: constructed from wave equation based receiver function migration. J. Geophys. Res. 111, B09312, doi:10.1029/2005JB003974. Chevrot, S., 2000. Multhichannel analysis of shear wave splitting. J. Geophys. Res. 105 (B9), 21579–21590. Durrheim, R.J., Mooney, W.D., 1991. Archean and Proterozoic crustal evolution: evidence from crustal seismology. Geology 19, 606–609. Fan, W.M., Menzies, M.A., 1992. Destruction of aged lower lithosphere and accretion of asthenosphere mantle beneath eastern China. Geotectonica Metallogenia 16, 171–180. Faure, M., Trap, P., Lin, W., Monié, P., Bruguier, O., 2007. Polyorogenic evolution of the Paleoproterozoic Trans-North China Belt, new insights from the LüliangshanHengshan-Wutaishan and Fuping massifs. Episodes 30, 1–12.
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Wilde, S.A., Zhao, G.C., Sun, M., 2002. Development of the North China Craton during the Late Archean and its final amalgamation at 1.8 Ga: some speculations on its position within a global Paleoproterozoic supercontinent. Gondwana Res. 5, 85–94. Wu, F.Y., Lin, J.Q., Wilde, S.A., Zhang, X.O., Yang, J.H., 2005. Nature and significance of the Early Cretaceous giant igneous event in eastern China. Earth Planet. Sci. Lett. 233, 103–119. Xu, Y.G., Chung, S.L., Ma, J.L., Shi, L.B., 2004. Contrasting Cenozoic lithospheric evolution and architecture in the western and eastern Sino-Korean Craton: constraints from geochemistry of basalts and mantle xenoliths. J. Geol. 112, 593–605. Zhai, M.G., Liu, W.J., 2003. Paleoproterozoic tectonic history of the north China Craton: a review. Precambrian Res. 122, 183–199. Zhang, H.F., Sun, M., Zhou, M.F., Fan, W.M., Zhou, X.H., Zhai, M.G., 2004. Highly heterogeneous late Mesozoic lithospheric mantle beneath the North China Craton: evidence from Sr-Nd-Pb isotopic systematics of mafic igneous rocks. Geol. Mag. 141, 55–62. Zhao, G.C., Wilde, S.A., Cawood, P.A., 1998. Thermal evolution of Archean basement rocks from the eastern part of the North China Craton and its bearing on tectonic setting. Int. Geol. Rev. 40, 706–721.
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Insight into craton evolution: Constraints from shear wave splitting in the North China Craton Liang Zhao ∗ , Tianyu Zheng, Gang Lü Seismological Laboratory (SKL-LE), Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China
a r t i c l e
i n f o
Article history: Received 10 October 2007 Received in revised form 15 May 2008 Accepted 3 June 2008 Keywords: North China Craton Upper mantle Seismic anisotropy Shear wave splitting
a b s t r a c t The multi-episodic tectonic activities from the Precambrian to Cenozoic, including nucleus formation, cratonic amalgamation, and rejuvenation, make the North China Craton (NCC) an ideal natural laboratory for studying craton evolution. Spatial change in the upper deformation records is an important aspect for understanding cratonic formation and rejuvenation. In this study, we performed seismic shear wave splitting analysis using SKS phases from 50 portable stations. Two different methodologies, shear wave splitting measurement and amplitude analysis of transverse/radial components, produced mutually consistent splitting results. These results showed that the seismic anisotropy beneath the Ordos Block can be divided into three subgroups reflecting the tectonic control. Combining these results with those from previous splitting studies in the eastern NCC, we suggest that the Proterozoic amalgamation generated the seismic anisotropy in the boundary zone between the Ordos Block and the Trans-North China Orogen, while the anisotropy in the eastern Trans-North China Orogen and eastern NCC were possibly associated with the lithospheric rejuvenation during the Late Mesozoic to Cenozoic. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Archean cratons are characterized by a cold, thick lithosphere keel. The presence of buoyant and refractory peridotite residues beneath Archean regions, is widely held to be responsible for the inherent stability of Archean cratons (Durrheim and Mooney, 1991; King, 2005). The North China Craton (NCC), however, is an anomaly. In contrast to the long-term stabilization of western NCC after the Precambrian amalgamation, the eastern NCC experienced significant tectonic rejuvenation (lithospheric thinning, in particular) in the Late Mesozoic to Cenozoic. Multiple tectonic events, including nucleus formation, cratonic amalgamation, and rejuvenation, make the NCC an ideal natural laboratory for studying craton evolution. One important feature is that the different fates of the eastern and western NCC actually resulted from cratonic formation and lithospheric rejuvenation with spatial changes. Seismic anisotropy is an important indicator of present or past upper mantle deformation (Silver, 1996; Savage, 1999). For example, in southern Africa, the observed seismic anisotropy was dated back to the Archean mantle deformation (Silver et al., 2001). In central Tibet, the seismic shear wave splitting measurements reflected upper mantle deformation during the Cenozoic (Huang
∗ Corresponding author. Tel.: +86 10 82998430; fax: +86 10 62010846. E-mail address: [email protected] (L. Zhao). 0031-9201/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.pepi.2008.06.003
et al., 2000). Compound geodynamic events might have produced spatial changes in the upper mantle anisotropy in the NCC. Thus, seismic anisotropy is useful for addressing issues associated with craton evolution. Previous studies revealed a distinct east–west change in the upper mantle anisotropy in the eastern NCC (Zhao and Zheng, 2005a; Zhao et al., 2007). Particularly, the shear wave splitting measurements from dense seismic stations indicated complex mantle deformation in the boundary zone in relation to the undulated lithosphere–asthenosphere topography. However, the available seismic stations in the previous studies were limited to the eastern and central blocks of the NCC. In this paper, we present the first analysis of shear wave splitting in the western NCC. The integrated database allows us to provide well-covered splitting results for understanding the spatial changes of craton evolution. 2. Geological background The NCC is traditionally considered the Chinese part of the Sino-Korean Craton, which contains some of the oldest known continental rocks, some as old as 3.8 Ga (Liu et al., 1992; Song et al., 1996). Available geochronological data reveal that most pretectonic gneisses and volcanic rocks formed in the mid-Archean with ages ranging from 3.5 to 2.5 Ga (Zhao et al., 1998, 2001; and references therein). According to lithological, geochemical, geochronological, structural and metamorphic P-T path data, it was proposed that the basement of the NCC can be divided into Eastern
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Fig. 1. Simplified geological map of the North China Craton and the locations of available seismic stations. The hatching zones indicate the three-fold tectonic subdivision of the cratonic basement according to Zhao et al. (2001); the thick gray dashed lines give the positions of two suture zones associated with the two-step collision based upon Faure et al. (2007). The solid triangles represent stations from NCISP-4; thick gray line indicates the location of experiment NCISP-2; open triangles mark those used in previous studies (Zhao and Zheng, 2005a; Zhao et al., 2007). The big arrow gives the average APM direction using HS3-NUVEL 1a model (Gripp and Gordon, 2002); white fence symbols represent thrust belt; dual-headed arrow indicates extensional direction of the Bohai Bay Basin during the Mesozoic to Cenozoic. WB: the Western Block; TNCO: the Trans-North China Orogen; EB: the Eastern Block. The inset is a larger scale map of the North China Craton with three-fold subdivision.
and Western (or “Ordos”) blocks, separated by a major boundary zone called the Trans-North China Orogen (TNCO) (Fig. 1) (Zhao et al., 2001). However, the timing, mechanism and modality of the amalgamation are disputed (Zhao et al., 1998, 2001, 2005; Li et al., 2000; Wilde et al., 2002; Zhai and Liu, 2003; Kusky and Li, 2003; Guo et al., 2005; Polat et al., 2005; Kröner et al., 2005; Faure et al., 2007; Trap et al., 2007). For example, Kusky and Li (2003) and Polat et al. (2005, 2006) proposed a westward-directed subduction of an old ocean between the two blocks, with the final collision occurring at ∼2.5 Ga. Faure et al. (2007) also argued for a westward subduction but proposed a two-step geodynamic evolution to account for formation of the TNCO. Other authors, in contrast, have suggested an eastward-directed subduction with the final collision taking place at ∼1.85 Ga (Zhao et al., 2001; Wilde et al., 2002; Kröner et al., 2005). Low heat flow, a lack of volcanism, and other reactive signatures suggest that the Ordos Block has been tectonically stable since the Proterozoic. In contrast to the long-term stability of the Ordos Block, the eastern NCC has gone from having a thick (∼200 km) Archean or Proterozoic lithosphere to its current much thinner lithosphere (60–80 km) (Griffin et al., 1998; Chen et al., 2006; Sodoudi et al., 2006). Contrasting compositions of mantle xenoliths from Paleozoic kimberlites and Cenozoic basalts suggest that removal of the Archean lithospheric mantle occurred between the Ordovician and the Cenozoic (Menzies et al., 1993; Griffin et al., 1998; Gao et al., 2002; Wu et al., 2005). Associated with the lithospheric rejuvenation, the eastern NCC is believed to have been affected significantly by a NW–SE-trending extensional stress-field, as evidenced by the development of a series of half-graben basins and synkinematic granitic pluton emplacement (e.g., Zhao and Zheng, 2005b; Lin and Wang, 2006).
Having undergone two major tectonic events, the Proterozoic amalgamation and the Mesozoic–Cenozoic rejuvenation, the structural grains of the NCC are dominated by NNE to NE trending fault systems. Two fault systems define the major terrane boundaries, which roughly correspond with the limits of a 100–300-kmwide TNCO. Both faults strike N–S in their central and southern parts and turn NE at their northern extensions. However, whether these two faults represent the original fundamental boundaries between the two blocks and the TNCO remains unknown because of their Mesozoic reactivation (Zhao et al., 2001). Re-Os data for Tertiary alkali basalts suggest that the lithospheric mantle beneath the TNCO formed at about 1900 Ma, which approximately matches the age of the major collisional event in the orogen, but is significantly younger than the overlying late Archean crust. This chronology suggests that the original Archean lithosphere of the TNCO may have been replaced at about 1900 Ma in response to the collision between the Eastern and Ordos Blocks (Gao et al., 2002). Based on a study of the basalts and mantle xenoliths from the Eastern Block and the TNCO, Xu et al. (2004) presented geochemical evidence arguing that a progressive lithospheric thinning took place beneath the TNCO during the Cenozoic. 3. Seismic experiment and data This study is based on the analysis of SKS waveforms recorded at 50 temporary stations from a sub-project of the North China Interior Structure Project (NCISP-4). Uniformly equipped with REFTEK data loggers and Güralp CMG-3ESP sensors (50 Hz-30 s), NCISP-4 was operated from September 2005 to September 2006 by Chinese
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Fig. 2. Distribution of the events used in this study. Solid point denotes event used in shear wave splitting measurement (Fig. 4), star denotes that used in T/R value analysis (Fig. 5). Rectangle represents the study area.
Academy of Sciences. This profile was set up in an E–W direction starting in the western TNCO, ending at the western margin of the Ordos Block (Fig. 1), and integrating with a previous experiment called NCISP-2 and some permanent stations from the Capital Zone Project of China to the east (Zhao and Zheng, 2005a). These experi-
ments contributed to the first database for stations widely covering the NCC along an E–W orientation (Fig. 1). Among teleseismic events at distances in the range of 85–115◦ , 20 events (see Fig. 2 for locations) provided useful data for a shear wave splitting analysis. In the following two sections, we applied
Fig. 3. Examples of shear wave splitting measurement for event 2006:223:14:30. (A) For station 277.The upper panel shows records of vertical, radial and transverse displacement. ‘A’ and ‘F’ mark the start and end point of time window for analysis, respectively. Four small boxes in the left panel illustrate normalized fast and slow components and their particle motions; the right panel shows grid search result in the (, ␦t) domain. (B) For station 291.
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Fig. 3. (Continued ).
two methods to analyze the shear wave splitting of the SKS phases: one is a routine method of shear wave splitting measurement, and the other is a statistical analysis of the peak–amplitude ratio between the transverse and radial components of the SKS phases. 4. Shear wave splitting measurements First, we used the method developed by Silver and Chan (1991) to determine the fast polarization direction and delay time ␦t between the fast and slow components, assuming that SKS waves traverse a single homogeneous anisotropic layer. In order to assess the reliability of the estimated splitting parameters, the F-test analysis (Silver and Chan, 1991) was applied. To investigate the frequency dependence of results, we applied various twopole Butterworth band-pass filters to the data before the splitting measurements. All the filters adopted had a fixed lower corner frequency of 0.02 Hz but different upper corner frequencies ranging from 0.2 to 1.0 Hz. This filtering yielded approximately the same splitting parameters, indicating that high frequency scattering did not significantly affect the results. Fig. 3 displays the examples of splitting measurements from stations 277 and 291. Among 50 stations, 48 stations (except stations 287 and 288) yielded 151 valid shear wave splitting and null results. Table 1s (supplementary data) lists all individual splitting measurements with their 2 uncertainty in the –␦t domain. The mapped
view in Fig. 4 shows the individual results, revealing complex spatial changes of the splitting parameters. The splitting results in this study fall into three major subgroups from east to west (except stations 307 and 308) as shown in Fig. 4. At the eastern end of the profile, including stations 258–272, the majority of fast splitting directions were N120–140◦ with an average delay time of 0.8 s. At the central segment, including stations 273–292, the fast directions orient coherently NW–SE similar to those in the eastern end but with an average delay time of 1.8 s. At the western segment of the profile, including stations 293–306, most of the null splitting results or small delay times were obtained. At the westernmost end of the profile, stations 307 and 308 were different, yielding obvious splitting and abruptly varying fast directions. To the west of the profile, strong splitting was also obtained at station YCH from a previous study (Zhao et al., 2007). 5. Amplitude analysis of transverse/radial components In the splitting measurements, some stations gave only a few valid splitting results. Based on visual inspection of the SKS waveforms, this outcome can be attributed to the weak energy of the transverse component. Although the SKS phase can be tracked, it fails to give valid results with reasonable error estimation. To enhance the reliability of the results, we used a statistical method to investigate anisotropy through the analysis of peak–amplitude
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Fig. 4. Individual splitting measurements in this study (black bars and gray cross-bars) and previous studies (blue bars) (Zhao and Zheng, 2005a; Zhao et al., 2007). The orientation of each bar indicates the fast direction; bar length is proportional to the delay time. The gray cross-bar indicates a null measurement, with orientation parallel or perpendicular to the polarization direction of the incoming energy. Dashed lines give the boundaries between major tectonic units. Big arrows give the average APM directions using HS3-NUVEL1a model (Gripp and Gordon, 2002).
ratio between the transverse (T) and radial (R) components (T/R value for brevity hereafter). This method is similar to the multichannel analysis proposed by Chevrot (2000) except that we employed a T/R value directly instead of projecting the transverse components onto the radial components derivatives. 5.1. Methodology We follow the denotation of Kennett (2002) to construct relationships between T/R value and splitting parameters. In terms of the incident polarization azimuth and the fast direction of the anisotropic media, = − , and then at the place where a wave incidents into anisotropic media, the fast and slow components are written as
f = RAW s
Ri Ti
,
Thus, in the receiver-frame, the observed radial and transverse component are represented by
Rr Tr
= RWA EA U RAW
Ri Ti
,
(2)
here Rr and Tr are the recorded radial and transverse components, respectively, and RWA is the transposed matrix of RAW . For an incident wave without transverse energy, such as the SKS phase, it is feasible to calculate the T/R value assuming that the fast direction and incoming radial component are known. A propagation of Ricker wavelet was assumed with a center frequency of 0.2 Hz, the same as the major frequency of our SKS records, through a single-layer anisotropic medium with a delay time of 0.5, 1.0, and 1.5 s respectively. Synthetic calculations showed that the T/R value is a function of delay time and with periodicity of /4 (Fig. 5). The maximum T/R value corresponded to the case of being equal to n/4. The amplitude range of T/R value depends on the value of the delay time, which can be used to constrain the delay time
(1)
where f and s are the fast and slow components, Ri and Ti are the input radial and respectively, the rotation transverse components, cos sin matrix RAW = . − sin cos Consider wave propagation close to vertical and label the two vertical wave slowness qf and qs , it is convenient to introduce the mean slowness q˜ and the wave slowness deviation q, q˜ =
(qf + qs ) , 2
q =
(qf − qs ) . 2
The phase increment for upward propagation through a vertical distance h is then given by
EA U
=e
iω˜qh
e−iω qh 0
0 eiω qh
.
Fig. 5. Theoretical results showing the relationship between T/R value, delay time and .
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in cases of good azimuthal coverage of the events (e.g., Chevrot, 2000). For a fixed delay time, T/R value is a bimodal function of among domain (0, ), which introduces a /2 nonuniqueness of fast direction. Nevertheless, we can use the splitting parameters obtained from splitting measurements to reduce the nonuniqueness. Our statistic method consists of two-step processes. First, we visually inspected the waveform of SKS for all events one by one. Second, we assigned time-windows of SKS phases for each record and calculated the T/R value. For data to be useful the SKS core phases have to be well-separated from other phases such as S and ScS. 5.2. T/R value analysis Fig. 6 shows the results for all stations which include 391 values with a valid number much larger than that of the splitting measurements. As shown in Fig. 6, the T/R value has a systematical variation along the profile and could be divided into three subgroups from east to west, just like the splitting parameters. This consistency illustrates the robustness of shear wave splitting analyses. To determine splitting parameters (, ␦t) beneath the profile, we used m control points to construct a single-layer starting model represented as x(0 , ␦t0 ) (i = 1, m); here xi is the longitude of the ith control point, and (0 , ␦t0 ) are assigned with average splitting parameters interpolated from the results of splitting measurements. For each event–station pair, we can calculate the theoretical T/R value using Eq. (2) and the square variance between the theoretical values and the recorded ones. Considering the /2 periodicity of T/R, we searched around the starting model within range of (0 ± 22.5◦ , ␦t0 ± 0.5 s). To explain our observation, we tested hundreds of models and obtained an optimal model by adjusting model parameters x(, ␦t) to minimize the average square vari-
ance between the calculated T/R values and the recorded ones. Fig. 7 shows the optimal model and the comparison between the calculated T/R values and the recorded ones (average square variance is equal to 0.138). Because the majority of T/R values of stations at the western end are less than 0.2, the estimated fast directions have large uncertainties beneath this region. However, these findings are in good agreement with the splitting measurements in that most of the null splitting results were observed in this region. 6. Interpretations and discussion The shear wave splitting measurements and T/R value analyses yield mutually consistent results, both indicating that the seismic anisotropy beneath the western NCC can be divided into three subgroups. In contrast, to the east of this profile, a previous study (Zhao and Zheng, 2005a) showed that the fast directions trend NE-NNE with an average delay time of 1.3 s in the eastern TNCO, or NW–SE with an average delay time of 1.2 s in the eastern NCC. The near-vertical incidence angle of the SKS phase leads to a high lateral resolution for the subsurface anisotropic structure. However, the seismic anisotropy recorded at the Earth’s surface represents a vertically integrated effect of anisotropy along the mantle ray path on the receiver side, making the depth resolution rather poor. Although alignments of mantle minerals in some region of the lowermost mantle also contribute (e.g., Niu and Perez, 2004), the lattice-preferred orientation of anisotropic olivine crystals in the upper mantle is believed to be the primary cause for SKS wave splitting (Savage, 1999; Mainprice et al., 2000). Most of studies agree on values ranging from 0.1 to 0.2 s for deformed crust (e.g., Herquel et al., 1995; Barruol and Mainprice, 1993), and a contribution from the lower mantle and transition zone that is typically less than 0.2 s (Savage, 1999). In addition, the observation of abrupt variation of splitting parameters constrains the top of the anisotropic layer
Fig. 6. T/R analysis results of SKS phases from recorded data. The point marks T/R value for an event-station pair with different color distinguishing different back-azimuthal range.
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Fig. 7. (A) The optimal splitting-parameter (, ␦t) model for T/R analysis. The arrow denotes fast direction beneath the profile with respect to north; the solid line represents delay time. Symbol “?” indicates the region where uncertainty of fast direction exists. (B) Comparison between the recorded T/R values (open circles) and calculated ones (solid triangles) from the above model. For the above model, the average square variance is minimized to 0.138.
within a shallow depth based on sensitivity zone analysis (Rümpker and Ryberg, 2000; Zhao et al., submitted for publication). According to previous multidisciplinary studies on the NCC (e.g., Fan and Menzies, 1992; Menzies et al., 1993; Griffin et al., 1998; Zhao et al., 2005; Kusky et al., 2001; Gao et al., 2002; Wilde et al., 2002; Zhai and Liu, 2003; Kusky and Li, 2003; Xu et al., 2004; Zhang et al., 2004; Zhao and Zheng, 2005a; Guo et al., 2005; Polat et al., 2005; Kröner et al., 2005; Lin and Wang, 2006; Faure et al., 2007; Trap et al., 2007), three candidate geodynamic events are expected to explain the shear wave polarization in the NCC: (1) the present-day asthenospheric flow; (2) the lithospheric rejuvenation that occurred during the Mesozoic to Cenozoic; or (3) the cratonic amalgamation between the Ordos and Eastern blocks took place during the Proterozoic. To test these three models, we compared the orientations of our shear wave splitting observations with predictions from the different tectonic events.
6.1. Present-day asthenospheric flow-induced anisotropy The fast directions of stations 264–292 may be consistent with the present-day asthenospheric anisotropy caused by the northwestward absolute plate motion (APM). The HS3-NUVEL1a model (Gripp and Gordon, 2002) predicts a coherent APM direction
of ∼N287◦ (webpage: http://tectonics.rice.edu/hs3.html) that are nearly parallel to the fast directions in the eastern Ordos and Eastern blocks. However, the coherent APM direction does not match the spatial change of fast directions observed in the Ordos Block or the splitting observed in the TNCO. If the plate motion is driven by the underlying asthenosphere, a simple and uniform asthenospheric flow could not explain the splitting observations.
6.2. Spatial range of lithospheric rejuvenation responsible for anisotropy Based on the shear wave splitting results observed in the eastern NCC, Zhao and Zheng (2005a) proposed that the anisotropy beneath the eastern TNCO was possibly generated by a compressional stress field induced by the northwestward asthenospheric flow associated with the lithospheric rejuvenation, or by the Proterozoic amalgamation between the Eastern and Ordos blocks. If the effect of lithospheric rejuvenation extended to the Ordos Block, it is predicted that the fast directions of the Ordos Block should be coherent with those in the TNCO or smoothly change in response to a large scale mantle flow. However, this prediction is inconsistent with the abrupt changes of fast directions observed in the Ordos Block and the TNCO. On the other hand, if the craton
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Fig. 8. Map showing spatial distribution of the lithospheric rejuvenation and cratonic amalgamation. The dual-head arrow marks direction of the lithospheric extension; the gray thrust symbol in the TNCO represents a possible compression related to the lithospheric rejuvenation.
lithosphere beneath the TNCO was reformed after the Proterozoic amalgamation, the anisotropy generated by amalgamation would have been frozen-in, and the fast directions would have remained coherently in the collisional belt including the TNCO and the eastern Ordos Block. This prediction is also inconsistent with the abrupt change of fast directions. These comparisons imply that an effect of lithosphere rejuvenation has possibly occurred beneath the TransNorth Orogen but not extended into the Ordos Block. In addition, the variation of fast direction from the Eastern Block to the TNCO exhibits a heterogeneous pattern during the lithospheric rejuvenation. Fig. 8 displays the possible spatial range of the lithospheric rejuvenation. In the Eastern Block, an extensional event dominates. On the other hand, in the TNCO the anisotropic pattern matches neither the extension in the eastern NCC nor the collision belt in the Ordos Block. If the change in lithospheric topography from the eastern NCC to the TNCO deflected the asthenospheric flow, two kinds of models could explain the observed anisotropy pattern. The first is that the deflected asthenosphere material flowed vertically, which would produce a NE–NNE-striking compression at the boundary zone. The second possibility is that the asthenosphere flowed horizontally at the base of the thick lithosphere, which would generate a NE–NNE trending extension (e.g., Lin and Wang, 2006). A larger scale observation in the TNCO, however, is required to test these two hypotheses in the future.
6.3. Precambrian amalgamation-induced anisotropy Most splitting at stations in the Ordos Block was weak or even undetectable and thus significantly smaller than the global average of 1.0 s (Silver, 1996); however, it is similar to data for the southeastern Kaapvaal Craton (Silver et al., 2001). Considering the thick lithosphere beneath the Ordos Block (Li et al., 2006; Huang and Zhao, 2006), the overall weakness of the Archean Ordos Block can be attributed to weak intrinsic anisotropy analogous to that discovered in the Kaapvaal Craton (Ben-Ismail et al., 2001), or to randomly aligned anisotropy of the upper mantle minerals.
If the lithospheric rejuvenation had not overprinted the upper mantle beneath the Ordos Block, the shear wave splitting beneath the eastern Ordos Block was most possibly caused by the fossil upper mantle anisotropy associated with the Precambrian amalgamation. The strongest anisotropy zone corresponds to the collision belt recognized by seismic imaging performed with the receiver function method (Zheng et al., submitted for publication). However, it is surprising that the NW–SE oriented fast directions in the western TNCO and eastern Ordos are not parallel to the NE- or NNEtrending Proterozoic geological features (Fig. 1). This characteristic differs from several other cratons in which the fast direction is parallel to the Precambrian geological features, suggesting a diversity of Precambrian tectonics. Assuming that a dry-condition collision was responsible for the anisotropic generation, we should expect a fast direction perpendicular to the compressional stress in the western TNCO. This prediction does not fit our observations in the eastern Ordos Block. However, NW–SE trending fast direction might be attributed to the hydration of upper mantle minerals (Jung and Karato, 2001) during the collision. Based on an experimental study by Jung and Karato (2001), the relationship between seismic anisotropy and flow geometry undergoes marked changes when water is added to olivine. When the deformation of olivine occurs under waterrich and modest stress conditions, the fast shear wave polarization is orthogonal or sub-orthogonal to the striking of the collisional boundary. If the amalgamation belt of NCC was wet because of the closure of ancient oceans, our fast directions are consistent with the prediction from a NW–SE oriented amalgamation model. Starting at the west boundary of the TNCO, the observed delay time increases from east to west, reflecting the amplitude variation of the upper mantle deformation caused by cratonic amalgamation (Fig. 8). It also indicates the deep boundary marked by larger delay time located to the west of the surface boundary. We speculate that a westward subduction of the Eastern Block might be responsible for the inconsistency between the surface and deep boundary. This speculation is in good agreement with the receiver function imaging (Zheng et al., submitted for publication) and a
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geodynamic model proposed by Faure et al. (2007) based on surface tectonic observation. 7. Conclusions In this study, we performed shear wave splitting analysis using SKS phases from 50 portable stations. Two different methodologies, shear wave splitting measurement and amplitude analysis of transverse/radial components, produced mutually consistent splitting results. The splitting results show that the seismic anisotropy beneath the Ordos Block can be divided into three subgroups. In the western end, most of the weak anisotropy with T/R values less than 0.2 was observed. In contrast, obvious splitting was recorded in the central and eastern Ordos Block with the majority of fast directions trending NW–SE and the central segment having a larger average delay time than the eastern segment. The splitting results in this study and from previous studies in the eastern NCC (Zhao and Zheng, 2005a; Zhao et al., 2007) indicate complex seismic anisotropy beneath the NCC. Combining the available splitting results, we suggest that the seismic anisotropy in the Eastern Block and the TNCO was possibly generated by lithospheric rejuvenation during the Mesozoic to Cenozoic. The spatial change in fast directions from east to west implies a spatially varying deformation pattern. In contrast, in the eastern Ordos Block, the upper mantle remains the “frozen-in” anisotropy generated by the Precambrian amalgamation between the Eastern and Ordos blocks, while the western Ordos Block has the weak anisotropy characterizing a stable craton. The inconsistency between the fast directions and the geological features suggests that a water-rich collision was possibly involved in the cratonic amalgamation of the NCC after the closure of ancient oceans. Acknowledgements We thank Dr. Wei Lin for helpful discussion, Prof. G.C. Zhao for providing data of the subdivision of NCC. The constructive suggestions by two anonymous reviewers and editor improve the manuscript. We acknowledge the participants of Seismological Laboratory, IGGCAS. This research was financially supported by the National Science Foundation of China (Grant 40504006 and 40434012) and Chinese Academy of Sciences. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.pepi.2008.06.003. References Ben-Ismail, W., Barruol, G., Mainprice, D., 2001. The Kaapvaal craton seismic anisotropy: petrophysical analyses of upper mantle kimberlite nodules. Geophys. Res. Lett. 28, 2497–2500. Barruol, G., Mainprice, D., 1993. A quantitative evaluation of the contribution of crustal rocks to the shear-wave splitting of teleseismic SKS waves. Phys. Earth Planet. Inter. 78, 281–300. Chen, L., Zheng, T.Y., Xu, W.W., 2006. A thinned lithospheric image of the Tanlu Fault Zone, Northeastern China: constructed from wave equation based receiver function migration. J. Geophys. Res. 111, B09312, doi:10.1029/2005JB003974. Chevrot, S., 2000. Multhichannel analysis of shear wave splitting. J. Geophys. Res. 105 (B9), 21579–21590. Durrheim, R.J., Mooney, W.D., 1991. Archean and Proterozoic crustal evolution: evidence from crustal seismology. Geology 19, 606–609. Fan, W.M., Menzies, M.A., 1992. Destruction of aged lower lithosphere and accretion of asthenosphere mantle beneath eastern China. Geotectonica Metallogenia 16, 171–180. Faure, M., Trap, P., Lin, W., Monié, P., Bruguier, O., 2007. Polyorogenic evolution of the Paleoproterozoic Trans-North China Belt, new insights from the LüliangshanHengshan-Wutaishan and Fuping massifs. Episodes 30, 1–12.
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