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Abstract

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Data and Method
  5. 3. Structural Image Beneath the Bohai Bay Basin
  6. 4. Discussion and Conclusions
  7. Acknowledgments
  8. References

[1] We apply a receiver function poststack migration method to image the crustal and upper mantle structures beneath the Bohai Bay Basin, eastern China. The thick sediments of the basin appear to have a substantial influence on the migrated image of the deep structure. By using the surface reflected PpPs multiples instead of the commonly used Ps converted phases, a curved Moho is coherently imaged, showing a thinned crust of ∼30 km in the basin area compared with the >40-km thick crust in the west mountain range. By incorporating the sedimentary velocity model into migration, both the 410-km and the 660-km discontinuities present a simple structural feature with relatively flat topography within the basin's interior, although possible local structural complexities may exist at the base of the upper mantle around the boundary areas of the basin. The mantle transition zone is on average 10–15 km thicker than the global average, reflecting a cold lower upper mantle environment that is distinct from the warm shallow upper mantle of the region.

1. Introduction

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Data and Method
  5. 3. Structural Image Beneath the Bohai Bay Basin
  6. 4. Discussion and Conclusions
  7. Acknowledgments
  8. References

[2] The Archean craton of eastern China began an episode of deformation in Mesozoic. Recent work [e.g., Menzies et al., 1993; Griffin et al., 1998; Xu, 2001] has suggested that the uppermost mantle in this region has been removed or at least thermally altered enough that the region no longer has the kind of “keel” that is characteristic of Archean cratons. Studies of the deep structure beneath this region, however, have been hindered due largely to the presence of thick sedimentary basins, in particular the Bohai Bay Basin which occupies a large area of eastern China with a maximum width of about 450 km (Figure 1). Many efforts have been made to investigate the local properties of the sedimentary cover for the purpose of petroleum exploration [e.g., Kang, 1996; Wang et al., 2002] or focus on the basin-scale structure, mostly within the crust [e.g., Zhao and Zheng, 2005; Zheng et al., 2005]. Knowledge about the deep structure of the basin, however, is more limited.

image

Figure 1. Regional map showing locations of 51 NCISP broadband seismic stations (triangles) and two imaging profiles (solid lines) across the Bohai Bay Basin (BBB) and the Taihang Mountain (THM): A-A′ for crustal structure and B-B′ for upper mantle structure. Map inset shows the distribution of the selected teleseismic events within back-azimuth ranges of 110°–150° and 285°–325°. Piercing points at 660-km depths of P-to-S converted phases for the above two groups of data are shown as yellow and light blue dots, respectively. Rectangle marks the area where the receiver functions are stacked for profile B-B′. Two thick dashed lines give the same locations as those shown in Figure 4.

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[3] We employed the newly developed receiver function poststack migration technique of Chen et al. [2005a, 2005b] to study the crust and upper mantle structures beneath the Bohai Bay Basin. To side-step the interference of sediment reverberations with effective signals from crustal discontinuities, we expanded the migration method for Ps converted phase to be suitable for surface reflected PpPs multiples. We also adopted a well established sedimentary velocity model [Zheng et al., 2005] to improve the image quality of the upper mantle transition zone. The subsurface structural images we constructed provide useful information for understanding the crustal and mantle property and the associated tectonic processes of the region.

2. Data and Method

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Data and Method
  5. 3. Structural Image Beneath the Bohai Bay Basin
  6. 4. Discussion and Conclusions
  7. Acknowledgments
  8. References

[4] Teleseismic waveform data used in this study come from 51 NCISP (Northern China Interior Structure Project) portable broadband stations. These stations form a SE-NW trending array starting from the interior of the Bohai Bay Basin and ending at the west Taihang Mountain area with an average spacing of about 10 km (Figure 1). The majority of the stations (32) were located within the basin that is filled by thick sediments. The detailed information about the stations and the NCISP can be found elsewhere [Zhao and Zheng, 2005; Zheng et al., 2005]. We analyzed data from 234 events with body wave magnitude ≥5.5 and epicentral distance between 28° and 92°. 213 of these events fall within the back azimuth range of 110°–150° and the remaining 21 events fall within 285°–325°, roughly centered about the trending direction of the station array (inset in Figure 1). We computed a total of 5766 receiver functions from these data in the same manner as described by Chen et al. [2005b] and further performed band-pass filtering with corner frequencies 0.03 Hz to 1 Hz. Finally we selected 3209 receiver functions after careful visual inspection for imaging.

[5] Two study profiles parallel to the trending direction of the station array were selected (Figure 1). Profile A-A′ was for imaging the crustal structure just beneath the station array, B-B′ for imaging the upper mantle, particularly the mantle transition zone structure of the Bohai Bay Basin. Note that the uneven spatial distribution of the earthquake sources results in highly uneven spatial coverage of the data, in particular very sparse sampling in the Taihang Mountain area and further to the west (Figure 1). This limits reliable deep structural imaging there. The structural images of the two profiles were constructed following the wave equation-based poststack depth migration procedure using a one-way phase-screen propagator as described previously [Chen et al., 2005a, 2005b]. Both a 1D and a 2D velocity models were adopted in migration. The 1D model was from the IASP91 radial model [Kennett and Engdahl, 1991] superposed by an average 1D crust model of eastern China [Ai and Zheng, 2003]. The 2D model was developed by further incorporating the 2D sedimentary structure of the Bohai Bay Basin [Zheng et al., 2005] with the 1D model.

[6] For the crustal structure imaging, we also migrate the topside reflected PpPs phases in addition to Ps migration. All the procedures are same as for Ps phases except that moveout correction of receiver functions is performed according to the delay time of PpPs with respect to the direct P wave, and the corresponding migration velocities for PpPs

  • equation image

are used instead of those for Ps [Chen et al., 2005a, equation (13)] in migration. We also perform synthetic testing to simulate the effect of the thick sediments on the upper mantle image. The sedimentary structure derived previously [Zheng et al., 2005] and a curved Moho similar to that revealed in this study (see Figure 2d) are built into the synthetic model (Figure 3d), and three flat discontinuities at 210-, 420- and 680-km depths, respectively (dashed lines in Figures 3e and 3f), are also adopted in calculation.

image

Figure 2. Receiver function migration images for profile A-A′ (a, b) using Ps phases with frequencies of 0.03–1.0 Hz and (c, d) using PpPs multiples with frequencies of 0.03–0.35 Hz. Figures 2a and 2c are constructed based on the 1D average velocity model for eastern China; Figures 2b and 2d are obtained by incorporating the 2D sedimentary structure of the Bohai Bay Basin [Zheng et al., 2005] into migration. Dash lines mark the 35-km depth.

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image

Figure 3. (a) Numbers of receiver functions in the bins (blue traces) and bin widths (red traces) used in the CCP stacking at 410- and 660-km depths; receiver function migration images for profile B-B′ (b) using the 1D average velocity model and (c) taking into account the sedimentary structure [Zheng et al., 2005]; (d) synthetic model; and (e, f) similar to Figures 3b and 3c but for the synthetic model in Figure 3d. All the images are constructed from receiver functions with a frequency range of 0.03–0.3 Hz. Dashed lines mark 420- and 680-km depths in Figures 3b, 3c, 3e, and 3f and also 210-km depth in Figures 3e and 3f.

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3. Structural Image Beneath the Bohai Bay Basin

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Data and Method
  5. 3. Structural Image Beneath the Bohai Bay Basin
  6. 4. Discussion and Conclusions
  7. Acknowledgments
  8. References

[7] The thick low-velocity sedimentary structure significantly affects the crustal Ps image beneath the basin area, whereas only minor variations in the mountain area are detected (Figures 2a and 2b). In addition, the Moho can hardly be traced continuously beneath the basin area mostly due to the substantial interference of the strong sediment reverberations with the Moho Ps signal [Zheng et al., 2006]. In contrast, the PpPs images exhibit a much weaker sensitivity to the basin structure, showing similar image features with and without sedimentary velocity model adopted in migration (Figures 2c and 2d). The Moho can be coherently identified, ranging from ∼44 km at the westernmost end of the profile to ∼30 km in the east basin area with a gradual decrease of depth in the Taihang Mountain region (distance ∼0–200 km in Figures 2c and 2d). Note that the shallow Moho depth estimate within the basin is unlikely to be a result of mapping PpPs phases reflected down from the basin basement instead of surface reverberations. The rough topography of the basement [Zheng et al., 2005] is not expected to construct a coherent and flat PpPs image. Traveltime calculation and the consistency with previous seismic refraction result [Ma et al., 1991] also support a shallow Moho beneath the basin. Image comparison thus indicates that PpPs imaging is more feasible than Ps imaging in studying crustal structures of basin areas that are covered by thick sediments.

[8] With our receiver function poststack migration, no intra-crustal discontinuity is imaged in either the Bohai Bay Basin or the Taihang Mountain area. We suggest this is due to the significant influence of thick sediments and/or limited resolution in imaging. The detailed crustal velocity structures have been studied through waveform inversion of receiver functions [Zheng et al., 2006]. The depth distribution of the Moho in our migrated image (Figures 2b and 2d) agrees quite well with their result, although we adopt an average 1D velocity model of eastern China for the sub-sediment crust in our migration procedure. This demonstrates that, except for the thick sedimentary cover of the basin, the lateral heterogeneity of the crust likely has only a minor effect on the image of deep structures in the study region.

[9] The influence of the sedimentary structure on the migrated image of upper mantle structure is also strong (compare Figure 3b with Figure 3c). An abrupt depression following a gap of the 410-km discontinuity (hereinafter referred to as 410) is found in the image without adopting the sedimentary velocity model in migration (distance ∼320 km in Figure 3b). In contrast, the 410 becomes continuous and nearly horizontal when the sedimentary structure is taken into account (Figure 3c). A similar effect is also found in the eastern central portion of the 660-km discontinuity (hereinafter referred to as 660), where a depression (distance ∼350–550 km in Figure 3b) almost disappears by employing the sedimentary velocity model (Figure 3c). Using the real data coverage (Figure 3a), the synthetic images (Figures 3e and 3f) resemble the data images (Figures 3b and 3c) quite well, further demonstrating the important influence of the laterally heterogeneous sedimentary structure on the deep structural image.

[10] Beneath the interior of the Bohai Bay Basin, both the 410 and 660 present a relatively flat image when the basin structure correction is included (distance ∼280–600 km in Figure 3c). However, significant depression (or bifurcation) of the 660 at both ends of the profile are observed (Figures 3b and 3c), irrespective of whether the basin structure is considered or not in migration. In consideration of the highly non-uniform data coverage along the profile (Figure 3a), lack of data at edges might be responsible for the relatively weak image, even invisibility of the 410 and 660 (Figures 3b and 3c). The observed undulations of the 660 is, however, unlikely a result of insufficient data coverage. Synthetic tests indicate that, under the current data condition, the horizontal feature of the 410 and the 660 can be recovered at edges with apparently reduced image strength (Figures 3e and 3f). Comparing with the topography of the 410, the depression/bifurcation of the 660 at the southeastern end of the profile (distance >600 km) may not be attributed to the incorrect velocity model above the mantle transition zone used in migration. In addition, such an image feature appears robust in the sense of being coherently detectable in bootstrap resampling and imaging. With a different data set and using an alternate imaging approach, Ai and Zheng [2003] also found a lateral depression zone and multiple 660-km discontinuities in the same area [Ai and Zheng, 2003, Figure 3b]. Therefore, the locally complex image of the 660 is likely real. On the other hand, incorrect migration velocities may contribute to the topography of the 660 in the northwestern part of the profile (distance <280 km in Figure 3c) where the 410 image displays a slight downwarp before becoming invisible. If this is the case, the depression of the discontinuities in the mantle transition zone thus requires that the shallow upper mantle below the west Taihang Mountain area is considerably slower than its basin counterpart.

4. Discussion and Conclusions

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Data and Method
  5. 3. Structural Image Beneath the Bohai Bay Basin
  6. 4. Discussion and Conclusions
  7. Acknowledgments
  8. References

[11] The high density of the teleseismic data and the well established thick sedimentary structure of the Bohai Bay Basin enable us to study in detail the crustal and upper mantle structure of the region. The imaged Moho exhibits a significant lateral undulation, from >40 km in the west Taihang Mountain area to ∼30 km in the interior of the east Bohai Bay Basin (Figure 2d). The depth uncertainty is confined to within 2 km from bootstrap testing based on various crustal velocity models, even without considering the thick sedimentary structure (Figure 2c). The thinner crust with small thickness variations in the basin area confirms the previous speculation that the crust might have been thinned at the same time the lithosphere was thinned in eastern China during late Mesozoic to Cenozoic [Gao et al., 1998; Zhang et al., 2004]. This result is consistent with the present-day high surface heat flow of the region [Hu et al., 2000]. Moreover, our receiver function imaging experiences indicate that our newly proposed wave equation-based depth migration technique is not only efficient in structural imaging using Ps converted phases, but also applicable to the topside reflected PpPs multiples. Compared with Ps phases, PpPs multiples are more useful in studying crustal structure, especially the Moho of thick-sediments-covered basin areas.

[12] Possibly due to the interference of sediment reverberations or because of the high complexity of the real structure, we cannot obtain a clear image at depths of ∼150 km–350 km. However, the structure of the upper mantle transition zone is well imaged (Figure 3c). The 410 is sharp and consistent, and displays a simple near horizontal feature around 422-km depth. The 660 also displays small undulations about the average depth of 683 km beneath the major part of the Bohai Bay Basin, although some local structural complexity and possible depressions are present at the northwestern basin-mountain boundary and the southeastern coastal area. For most parts of both discontinuities, depth errors of <5 km are obtained in a same way as that for the Moho but with sedimentary structure correction. The smooth topography of both discontinuities imaged here and observed previously [e.g., Ai and Zheng, 2003, Figure 3c] within the basin area may be either attributed to a relatively smooth temperature distribution or indicative of the insignificance of the combined influence of temperature and chemical components, such as water and/or iron [Smyth and Frost, 2002; Van der Meijde et al., 2003], on the topography of upper mantle discontinuities. Despite the significant variations at both edges of the profile, the thickness of the mantle transition zone also appears to have only small variations of ≤5 km in the basin's interior where the thickness is determined most reliably (marked by thick dashed lines in Figures 1 and 4) . The transition zone beneath the basin is on average ∼10–15 km thicker than the global average (Figure 4). According to mineral physics studies on phase transformations in the olivine system [Ito and Takahashi, 1989], this implies a generally cold mantle environment consistent with the cooling of a flat–lying slab at the bottom of the upper mantle under eastern China as imaged by tomography [Fukao et al., 2001; Zhao, 2004]. Mineral physics calculations also suggest that at low temperatures multiple phase transformations occur, due to the co-existence of non-olivine components such as garnet with olivine, at most distinct depths around the bottom of the upper mantle [Vacher et al., 1998]. This low-temperature regime of mineral phase changes has been invoked to interpret the presence of multiple 660-km discontinuities at subduction zones [e.g., Simmons and Gurrola, 2000; Ai et al., 2003], and is plausibly responsible for that observed near the southeastern coastal area of the study region. In contrast, geochemical data [Xu et al., 1998; Xu, 2001] and surface heat flow measurements [Hu et al., 2000] suggested a rather warm present-day upper mantle, likely associated with the Mesozoic-Cenozoic lithosphere reactivation and thinning in this region. Such a discrepancy indicate that the shallow and deep upper mantle in the study region probably have not reached a thermal equilibrium, and the Cenozoic lithospheric process and magmatism in the eastern China continent might have originated within the shallow upper mantle, possibly no deeper than the middle of the transition zone.

image

Figure 4. Thickness of the mantle transition zone beneath the study region. Solid line represents the average thickness, gray dashed lines gives the uncertain range from bootstrap testing. Two vertical dashed lines mark the portion of the Bohai Bay Basin where the depths of both the 410- and 660-km discontinuities can be determined reliably (also marked in Figure 1). Thin dotted line denotes the global average thickness of the mantle transition zone (IASP91) [Kennett and Engdahl, 1991].

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Acknowledgments

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Data and Method
  5. 3. Structural Image Beneath the Bohai Bay Basin
  6. 4. Discussion and Conclusions
  7. Acknowledgments
  8. References

[13] We acknowledge the participants of the Seismic Array Laboratory of IGGCAS for collecting the data. We are grateful to Lianxing Wen for providing his 2D P-SV hybrid code that was used in our synthetic seismogram calculation. We wish to thank Gary Pavlis, an anonymous reviewer and the Associate Editor for thoughtful reviews. This research is supported by the National Science Foundation of China (grants 40434012 and 40374015), the National Development and Reform Commission of China (grant 2004-1138), and the Chinese Academy of Sciences.

References

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Data and Method
  5. 3. Structural Image Beneath the Bohai Bay Basin
  6. 4. Discussion and Conclusions
  7. Acknowledgments
  8. References
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