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Earth and Planetary Science Letters 267 (2008) 56 – 68 www.elsevier.com/locate/epsl

Distinct lateral variation of lithospheric thickness in the Northeastern North China Craton
Ling Chen ?, Wang Tao, Liang Zhao, Tianyu Zheng
Seismological Laboratory (SKL-LE), Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China Received 22 March 2007; received in revised form 15 November 2007; accepted 19 November 2007 Available online 3 December 2007 Editor: R.D. van der Hilst

Abstract A detailed knowledge of the thickness of the lithosphere in the northeastern North China Craton (NCC) is important for understanding the significant tectonic reactivation of the craton in the Mesozoic and Cenozoic time. We achieve this goal by applying the newly proposed wave equation-based migration technique to the S-receiver functions recently collected in the region. Distinct negative signals are identified below the Moho in all the S-receiver function-migrated images and stacks, which we interpret as representing the S-to-P conversions from the lithosphere– asthenosphere boundary (LAB). The imaged LAB is as shallow as ? 60–70 km in the southeast basin and coastal areas and deepens to no more than 140 km in the northwest mountain ranges and continental interior. These observations indicate widespread lithospheric thinning in the study region in comparison with the N 180-km lithospheric thicknesses typical of most cratonic regions. The revealed topography of the LAB generally agrees with the lateral variation in upper mantle seismic anisotropy previously measured through SKS splitting analysis. In particular, a sharp LAB step of ? 40 km is detected at the triple junction of the basin and mountains, at almost the same place where an abrupt change from NW–SE to NE–SW in fast polarization direction of shear waves was found. These findings suggest a close correlation between the seismic anisotropy and hence deformation of the upper mantle, the lithospheric thickness, and the surface tectonics of the northeastern NCC. While the thinned lithosphere and the NW–SE fast shear wave polarizations in the east areas probably are related to the dominant NW–SE tectonic extension in the late Mesozoic–Cenozoic time, the thicker lithosphere and the NE–SW fast polarization direction in the west mountain ranges may reflect earlier contractional deformations of the region. Synthetic tests indicate that the LAB beneath the northeastern NCC is a well-defined zone 10–20 km thick. Combined with seismic tomography results and geochemical and petrological data, this suggests that complex modification of the lithosphere probably accompanied significant lithospheric thinning during the tectonic reactivation of the old craton. ? 2007 Elsevier B.V. All rights reserved.
Keywords: lithosphere-asthenosphere boundary; northeastern North China Craton; S-receiver functions; lithospheric reactivation

1. Introduction The Archean North China Craton (NCC), located at the eastern margin of the Eurasian continent, is unique among the cratons of the world for its unusual Phanerozoic tectonic activity (Carlson et al., 2005). The NCC as a whole was tectonically stable from its final cratonization ? 1.85 Ga ago (Zhao et al.,
? Corresponding author. State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, No.19 Beituchengxilu, Chaoyang District, Beijing 100029, China. Tel.: +86 10 82998416; fax: +86 10 62010846. E-mail address: lchen@mail.iggcas.ac.cn (L. Chen). 0012-821X/$ - see front matter ? 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.epsl.2007.11.024

2001) until the Jurassic, before its collision with the Yangtze Craton to the south (Yin and Nie, 1993; Zhang, 1997; Faure et al., 2001). In contrast to its western part, which seems to have retained the characteristics of a stable craton (Zhao et al., 2001), the eastern NCC underwent significant tectonic reactivation during the Late Mesozoic and Cenozoic as evidenced by widespread lithospheric extension, high heat flow and voluminous magmatism (Griffin et al., 1998; Menzies and Xu, 1998; Fan et al., 2000; Xu, 2001; Zhou et al., 2002; Wu et al., 2005). Three major tectonic units in the region, i.e., the Bohai Bay Basin (BBB) in the east, the Taihang Mountains (TM) in the west, and the Yan Mountains (YM) in the north (Fig. 1) have

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Fig. 1. Map of the study region showing the locations of NCISP-I and III (filled triangles) and CEA (open inverted triangles) broadband seismic stations, the individual S receiver function stacking blocks (rectangles labeled in italics) and imaging profiles (thick straight lines). The rectangle around each profile marks the area where the S receiver functions are projected onto the profile for imaging. Two NCISP stations (093 in the south and 229 in the north) and one CEA station (MIY) are highlighted in white, and the stacked P and S receiver functions for these stations are compared in Fig. 2. Piercing points at 85-km depth for S-to-P converted phases are shown as dark gray (for NCISP stations) and light gray (for CEA stations) dots. The dark gray circle on profile C-C’ marks the location where a sharp change in lithospheric thickness is observed (see Fig. 3f). The two segments of the same color on profiles D-D’ and E-E’ give the locations where a strong negative signal is detected at ~130-km depth (Fig. 5). Map inset shows the distribution of teleseismic events used. Major tectonic units in the North China Craton are also marked, including the Bohai Bay Basin (BBB), the Taihang Mountains (TM), the Yan Mountains (YM), the Yin Mountains (YinM), the Luxi Uplift (LU), the Tanlu Fault Zone and the Bohai Sea.

developed as a direct consequence of the cratonic reactivation. The formation of the BBB, which is characterized by NW–SE oriented extensional structures, and the N-NE trending TM are generally considered to have been tectonically coupled. Both probably are associated with the NW–SE tectonic extension of the region in the Late Mesozoic to Cenozoic time (Gao et al.,

1998; Gao et al., 2004; Liu et al., 2000). The E–ENE trending YM, on the other hand, have experienced multiple phases of contractional deformation, mostly N–S directed, before the regional extension in the late Mesozoic (Zheng et al., 2000; Davis et al., 2001; Meng, 2003) and this area is still tectonically active (Ming et al., 1995).

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These distinct tectonic processes should leave significant imprints on the crust and mantle lithosphere beneath different geological domains at the surface. Accurate knowledge of the structure and thickness of the lithosphere is thus important for unraveling the tectonic evolution of the eastern NCC. Numerous studies have focused on the crustal structure of the region (e.g., Gao et al., 1998; Huang and Zhao, 2004; Zheng et al., 2006, 2007). Recent teleseismic P receiver function (P-RF) studies (Zheng et al., 2006, 2007) showed marked structural differences of the crust among the BBB, the TM and the eastern YM areas, suggesting a close correlation between the shallow structure and the Mesozoic–Cenozoic tectonics of the region. However, the structural features of the underlying mantle lithosphere, particularly the transition from the lithosphere to the asthenosphere under most areas of the eastern NCC are still poorly understood. Recent geochemical and petrological work (e.g., Griffin et al., 1998; Xu, 2001; Menzies et al., 1993; Wu et al., 2006) has suggested that during the reactivation process a large portion of the thick cratonic lithosphere in the eastern NCC was removed or at least thermally and chemically altered enough that the region no longer has the kind of “keel” that is a feature of typical Archean cratons. With this reshaping of the lithosphere, the eastern NCC appears to have a much smaller tectonothermal age than a stable Precambrian craton (Ren et al., 1999; An and Shi, 2006). Geophysical evidence for this unusual lithospheric modification, however, is rather scarce. Previous regional seismic tomography studies have revealed a complex and dramatically thinned lithosphere beneath the eastern NCC (Chen et al., 1991; Yuan, 1996; Huang et al., 2003; Zhu et al., 2004). High temperatures in the upper mantle estimated from seismic tomography (An and Shi, 2006) indicate a thermal lithosphere ? 100 km thick across much of the region. The resolutions of these studies are, however, rather low due to the limited data coverage and intrinsic limitation of the methods. To what extent, both laterally and in depth, the cratonic keel has been lost remains a subject of controversy. By applying a newly developed wave equation-based migration technique to the teleseismic P receiver function (P-RF) data from dense seismic station arrays, we recently constructed a finescale lithospheric structural image along a ?300-km E–W profile across the most active segment of the Tanlu Fault Zone at the Luxi Uplift area (Fig. 1, Chen et al., 2006). Our image shows a lithosphere only 60-80 km thick, and localized undulations of the Moho discontinuity. Through detailed waveform modeling, we further presented evidence for a clearly marked lithosphereasthenosphere boundary (LAB) 10 km or less in thickness beneath the area (Chen et al., 2006). However, we will show later in this paper that the LAB cannot be detected coherently through similar P-RF analysis and imaging in some other areas in the northeastern NCC, presumably due to the interference of the crustal multiple reverberations with the P-to-S (Ps) converted phase at the LAB. Therefore, in this study we expand the wave equation migration method for Ps phase to be suitable for S-to-P (Sp) converted phase and employ this method to investigate the structural features of the lithosphere beneath the northeastern NCC using the S receiver function (S-RF) data. S-RFs are much

noisier than P-RFs due to their later arrival times, and also have longer periods than the P-RFs. As a result, the fine structural features of the Moho may not be well-recovered using the SRFs. However, the fact that they are free of multiples enables the identification of Sp conversions at mantle discontinuities. Recent applications have already proven that the S-RF technique works well in detecting the LAB at local to regional scales (e.g., Li et al., 2004; Wittlinger et al., 2004; Kumar et al., 2005; Yuan et al., 2006). Benefiting from the establishment of dense seismic broadband station arrays in the northeastern NCC, we are able to observe distinct depth variations of the LAB through S-RF migration and imaging. 2. Data and method In this study, we used teleseismic waveform data collected by two seismic experiments in the NCC. One was under the Northern China Interior Structure Project (NCISP) conducted by the Chinese Academy of Sciences. The other was carried out by the China Earthquake Administration (CEA). We selected 62 NCISP-I, 51 NCISP-III (two sub-projects of NCISP) and 46 CEA broadband stations (Fig. 1) for lithospheric structural imaging. The NCISP stations of each sub-project were emplaced roughly linearly with an average station spacing of 10 km. The NCISP-I operated from November 2000 to February 2003 focusing on the Tanlu Fault Zone area at the Luxi Uplift (for details, refer to Chen et al., 2006). NCISP-III stations, operated from April 2003 to October 2004, formed a nearly N–S linear array across the YM at the northern margin of the NCC (Zheng et al., 2007). The CEA stations were more spatially scattered across the TM and YM areas but formed a roughly E–W extension with its eastern end close to the NCISPIII line (Fig. 1). We used data collected at these CEA stations during September 2001 to August 2003. Following the synthetic study by Yuan et al. (2006), we set an epicentral distance range of 55°–85° for calculation of S-RFs using S phases. SKS phases were not considered in this study because only a much smaller amount of data was usable within the appropriate distance range of N85° (Yuan et al., 2006). In addition to this distance constraint, we also visually inspect the S wave data and only retained records with clear S phases. We applied a simplified method using a time-domain maximum entropy deconvolution of the vertical component by the radial to construct the S-RFs. This method is similar to that we have used for P-RFs (Chen et al., 2005b, 2006), exchanging only the respective roles of the vertical and radial components. An analogous approach has also been used by Wittlinger et al. (2004) in their study of the lithospheric and upper mantle structure beneath Tibet. Here we adopted a Gaussian parameter of 3 and a water level of 0.001 in the deconvolution, and further applied band-pass filtering with corner frequencies of 0.03 Hz and 0.5 Hz to eliminate high-frequency noise in the resultant SRFs. Careful visual inspection was then conducted to remove the bad traces that are obviously different from the majority. Finally, 809 and 764 S-RFs for the NCISP-I, III stations, and 925 S-RFs for the CEA stations were selected for further structural imaging.

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To verify our S-RF analysis and imaging scheme and to better understand the respective imaging features of P- and S-RFs, we performed systematic comparisons of the two types of data and their images. The P-RFs from the NCISP-I stations have been constructed and used to image the lithospheric structure beneath the Tanlu Fault Zone area in our previous study (Chen et al., 2006). Here we calculated the P-RFs for the NCISP-III and the CEA stations in the same manner as before. Note that different parameters, including the Gaussian parameter (5) and water level (0.0001) in waveform deconvolution and the upper cutoff frequency (1 Hz) in filtering, were adopted for the P-RFs compared with those used for the S-RFs. In general, we used 5–7 times as many P-RFs as S-RFs for imaging. Arrival times of the Sp and Ps conversion phases relative to the direct S and P waves, and the piercing points of the conversion at depth, were calculated using the average 1D velocity model for eastern NCC as we did for P-RFs (Chen et al., 2006). All the receiver functions were then moveoutcorrected to the case of horizontal slowness p = 0 (vertical incidence as required for migration, see below and Chen et al., 2005a). Fig. 2 shows examples of the P- and S-receiver functions stacked within a dominant back azimuth range of 110°–150° for NCISP-I station 093, NCISP-III station 229

and CEA station MIY (see Fig. 1 for station locations). To make the S-RFs directly comparable with the P-RFs, the polarity of the S-RFs and the time axis have been reversed. The Moho Sp and Ps conversions arrive at about the same time (? 4 s after the S and P principal wave arrivals). In addition to the possible side-lobes of Moho phases, all the S-RFs exhibit significant negative phases at ? 6–10 s, likely representing a downward negative velocity gradient at the uppermost mantle. However, these signals are hardly visible in the P-RFs (except that for station 093 in Fig. 2a), possibly because they fall in the time window of crustal reverberations. We interpret these signals as Sp conversion at the base of the lithosphere (labeled LAB). To gain a general picture of the lithospheric structure of the study region, we applied the wave equation-based poststack migration method to both the S- and P-RFs. The migration method consists of two basic procedures: common conversion point (CCP) stacking and backward wavefield extrapolation (Chen et al., 2005a). In the CCP stacking procedure, CCP binning and stacking were performed on the moveout-corrected (p = 0) receiver functions. Note that moveout correction to p = 0 was required here so that the resultant CCP stacked gather can be used as a good approximation of the zero-offset (zero sourcereceiver distance) data set, similar to the common midpoint (CMP) stacked records of reflected data routinely constructed in reflection seismology. Backward wavefield exploration is a migration process to project the Sp or Ps convertors to their true positions by backward-propagating the zero-offset wavefield observed at the surface (the CCP stacked gather) to the whole space and to the time at which the conversions occur. More details of the method can be found in Chen et al. (2005a). For SRF migration, all the procedures were same as for P-RFs except that moveout correction was performed according to the time advance of the Sp phase with respect to the direct S wave. The corresponding migration velocities for Sp (with p = 0) were exactly the same as those for Ps used in P-RF migration (equation (13) in Chen et al., 2005a). 3. Lithospheric structural image According to the coverage of piercing points (e.g., those at 85-km depth shown as blue and pink dots in Fig. 1), we first constructed the lithospheric structural images along three profiles, two E–W (A–A', C–C') and one N–S (B–B') (Fig. 1), using the three subsets of data individually. We then combined the S-RF data from NCISP-III and CEA stations to image the lithospheric structure along two additional profiles (D–D' trending WNW–ESE and E–E' trending ENE–WSW, Fig. 1) that are sub-perpendicular and sub-parallel respectively to the dominant NE–SW trend of the major tectonic units in the region (Ren, 1990; Liu et al., 1997). However, the density and spatial coverage of the P-RFs are insufficient to allow coherent structural imaging for these two profiles. The migrated images (Figs. 3 and 5) were obtained by adopting the same 1D velocity model used in the calculation of delay times and piercing point locations, and superposing the migrated frequency contents of the stacked receiver functions in a frequency band of 0.03–1 Hz for P and 0.03–0.5 Hz for S-RFs.

Fig. 2. Stacks of P (0.03–1 Hz) and S (0.03–0.5 Hz) receiver functions (moveout-corrected to p = 0) within the dominant back azimuth range of 110°– 150° for NCISP stations 093 and 229 and CEA station MIY. Time and amplitude axis are reversed for S-RFs. The primary P and S waves are adjusted at same time zero and scaled at same amplitude. Arrows mark the P-to-S (only for station 093) and S-to-P converted phase from the LAB.

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Fig. 3. Comparison between the P-RF migrated images (a-c) and S-RF migrated images (d-f) for profiles A–A' (a, d), B–B' (b, e) and C–C' (c, f). The P-RF images are constructed with a frequency range of 0.03–1.0 Hz, while the S-RF images 0.03–0.5 Hz. White dashed lines denote the Moho estimated from the P-RF images, and black ones give the LAB estimated from P-RF (a, d) or S-RF images (b, c, e, f). Predicted depths of Moho PpPs multiples are marked by black arrows.

3.1. Profile A–A' The P-RFs from the NCISP-I stations have revealed a thinned lithospheric structure beneath the Tanlu Fault Zone area (Fig. 3a, modified from Chen et al., 2006). The S-RF image obtained here displays a similar feature, showing the Moho at around 35 km and an arcuate LAB at the shallow depths (60– 80 km; compare Fig. 3d with a). The depth distributions of the LAB and the Moho roughly agree with those in the P-RF image (black and white dashed lines, respectively, in Fig. 3a and d), although the spatial resolution is relatively low due to the lower frequencies of the S-RFs. This provides evidence for the validity of our S-RF migration scheme in imaging lithospheric structures. With the LAB identified for the entire profile, it is interesting to note that the Moho Sp side lobe interferes strongly with the

LAB Sp phase in the S-RF image (Fig. 3d). In particular, the two signals merge into a negative one at ? 60-km depth in the middle portion of the profile where the LAB is the shallowest. In the eastern part, on the other hand, only the strong LAB Sp is identified at ? 70-km depth. The invisibility of the Moho side lobe may be due to cancelling out between the oppositely polarized side lobes of the LAB Sp and Moho Sp phases. However, such signal interference becomes weaker as the LAB deepens in the western part of the profile where both the LAB Sp and the Moho side lobes are visible. As will be shown below, the LAB and the Moho are sufficiently separated in most parts of the other profiles. The LAB images along these profiles using S-RFs appear not to be significantly influenced by the strong Moho side lobes and therefore probably reflect a real structural feature, although no similar image is obtained from the P-RF data as in the case of profile A–A'.

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Fig. 4. Stacked S receiver functions for the western (lower trace) and eastern (upper trace) parts of profile C–C'. The distance ranges considered in stacking are 150–300 km (western) and 450–600 km (eastern), respectively (same distance definition as in Fig. 3e and f). Arrows mark the S-to-P converted phase from the LAB.

obviously weaker than its eastern counterpart (compare the orange colored portion in the west with that in the east in the depth range of 110–150 km in Fig. 3c). All these effects could be attributed to the strong interference and mutual cancelling out of the Moho PpPs by the LAB Ps. The E–W contrast in the LAB structure is also illustrated directly by the different waveforms of the stacked time-domain S-RFs for the western (distance 150–300 km) and eastern (distance 450–s600 km) parts of the profile (Fig. 4). The Sp phases converted at the LAB are ?8 s and ?12 s, respectively, advanced to the direct S phase for the eastern and western stacks, indicating a ?40-km depth difference on average of the LAB beneath the two parts. 3.4. Profiles D–D' and E–E' Both the Moho and the LAB can be coherently identified in the S-RF images of these two profiles (Fig.5). Despite an apparent south-to-north deepening in profile E–E' (Fig. 5b), the topographic variation of the Moho is barely detectable in the images. The LAB, in contrast, appears strongly variable in depth along both profiles. It dips monotonically from ?70 km in the Bohai Sea area down to ?100 km across the western YM (Fig. 5a). On the other hand, the LAB displays an arcuate shape with the shallowest depth of ? 80-km in the BBB near the western coast of the Bohai Sea. It becomes deeper to both sides, reaching ? 100 km at the BBB–TM boundary in the southwest and the eastern YM in the northeast (Fig. 5b).

3.2. Profile B–B' In contrast to profile A–A', profile B–B' shows apparent differences between the sub-Moho images from the P-and SRFs (compare Fig. 3b with 3e). While no mantle discontinuity can be coherently identified above 150 km depth in the P-RF image (Fig. 3b), a strong mantle phase with negative polarity is continuously detected in the 70–110 km depth range below the negative Moho side-lobe in the S-RF image (Fig. 3e). We interpret this phase as representing the LAB in this profile. It deepens by ? 40-km over a distance of about 400 km from the BBB in the south to the YM area in the north, with the sharpest gradient at the basin-mountain boundary (? 150–200 km distance in Fig. 3e). The crustal structure along the profile has proven to be complex (Zheng et al., 2007, also evidenced in Fig. 3b). The Ps converted phase from the LAB therefore is probably masked by multiple reverberations from crustal discontinuities, resulting in weakening or even absence of the LAB signal in the P-RF image (Fig. 3b). 3.3. Profile C–C' Striking differences of the S-RF image (Fig. 3f) from the P-RF image (Fig. 3c) are also obvious for profile C–C'. Although the Moho has similar depth distributions and exhibits a similar eastwest dipping characteristic using both the P- and S-RFs, the LAB is only visible in the S-RF image. In particular, an abrupt step in LAB depth from ?90 km in the east to ?130 km in the west occurs around the center of the E–W profile (Fig. 3f). Such a pattern of the LAB depth in the S-RF image seems consistent with the general appearance of the P-RF image (Fig. 3c), although the LAB cannot be easily identified in the latter. For instance, weak and disconnected signals of negative polarity are present at the LAB depths in the eastern portion of the P-RF image where the Moho PpPs phase arrives much later than the LAB Ps phase. These negative signals therefore may represent the LAB Ps phase that is free from the influence of strong Moho multiples but probably somewhat contaminated by relatively weak ones from intra-crustal discontinuities. To the west, on the other hand, no distinct negative signal is detectable in the uppermost mantle and at the same time the Moho PpPs-induced artificial image looks

Fig. 5. S-RF migrated images for profiles D–D' (a) and E–E' (b) that were obtained with the same frequency contents as Fig. 3d–f. Estimated LAB from the image is marked as a black dashed line.

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Note that there are some signals of negative polarity sporadically present below the imaged LAB, especially the strong ones at ?135 km in both profiles (Fig. 5). Their locations roughly coincide with the abrupt change of the LAB that occurs in profile C–C' (Fig. 1). Considering the relatively high dispersion of the S-RF piercing points and the large bin lengths perpendicular to the profiles adopted in this study (160–200 km to ensure sufficient data in each bin, see Fig. 1), these observations probably indicate the presence of small-scale variation and local structural complexity of the continental lithosphere in the study region. 3.5. Other areas In areas outside the black rectangles within which the above migrated images were constructed, for instance in the six rectangular blocks outlined in red in Fig. 1, the data coverage or density of S-RF piercing points are insufficient to allow reliable wave equation-based migration. To gain general information on lithospheric thickness in these areas, we directly mapped the individually stacked S-RFs from time to depth. For each of the six blocks, a negative phase is distinctly identified below the strong positive Moho phase (Fig. 6). It appears at different depths from 70–80 km in the southeast to 90–100 km in the northwest, generally in agreement with the estimated LAB depths from adjacent imaging profiles (Figs. 3 and 5). We thus

regard this phase as the Sp phase from the LAB. Note that there are double negative phases below the Moho at the N2 and M2 blocks (the first and third traces in Fig. 6). In this case, we take the deeper ones as the LAB phases because of their large amplitudes and because with such a choice the lateral variation of the LAB depth appears smoother. Moveover, a strong discontinuity with a negative velocity gradient is unlikely to lie in the asthenosphere, and thus is more likely to represent the LAB. 4. Resolution of the LAB images The spatial resolution of the LAB image and the accuracy of the estimated LAB depth could be affected by many factors including the uncertainty in lithospheric velocity structure, the data coverage and frequencies, and the local structural complexity of the study region (Chen et al., 2006, 2005b). In this section we performed synthetic forward modeling to evaluate the error in the LAB depth determination and assess the spatial resolution of the migrated S-RF images. We calculated the synthetic seismograms by a 2D hybrid method (Wen and Helmberger, 1998). Given that the dominant periods of our S-RFs peak at ?4.5 s, we adopted a value of 4.5 s in calculation. We constructed the S-RFs and the corresponding migrated images from the synthetics in the same way as for the real data, and then compared the images to both the true model and the data images.

Fig. 6. Depth converted S-RF stacks for the six rectangular blocks shown in Fig. 1. The LAB phases with negative amplitudes are marked. The numbers of the S receiver functions involved in stacking are given at the bottom of the plot.

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We focused on two types of lithosphere models. One contains a slightly dipping Moho and a more oblique LAB with the same inclination (Fig. 7a) to simulate the general southeast-tonorthwest deepening of both the Moho and the LAB in the data images (e.g., Fig. 5a for profile E–E'). The other is characterized by a 40-km LAB step (Fig. 8a) that is set to occur within various distance ranges to constrain the sharpness of the similar LAB structural feature in the data image of profile C–C'

(Fig. 3f). For both models, the sharpness of the Moho and the LAB were set to be uniform over the whole transverse range. The velocity drops at the LAB were selected such that the amplitudes of the Sp converted phase in all the cases were comparable to those in the corresponding data images. Table 1 lists all the model parameters adopted in the calculations. The synthetic S receiver functions used in the imaging were picked according to the data coverage and density of the real data for

Fig. 7. Synthetic model constructed to simulate the general southeast-to-northwest deepening of the LAB; The solid and dashed lines represents the Moho and the LAB, respectively; (b-e) S-RF migrated images based on the real data distribution of profile E-E’ for the synthetic model with different sharpnesses of the Moho and the LAB (see Table 1 for model parameters). (f) Waveform comparisons of the migrated real S receiver functions for profile E-E’ (solid black lines) and synthetics at two transverse locations marked in both (d) and Fig. 5a.

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Fig. 8. (a) Same as Fig. 7a, except that the LAB is built with a 40-km step to simulate the image feature of profile C–C' (Fig. 3f); (b–d) S-RF migrated images for the synthetic model based on the real data distribution for profile C–C'. The 40-km depth change of the LAB is set to occur abruptly (b), or over a 100-km (c) or 200-km distance (d), respectively, centered at the transverse location of 440 km.

the true 2D model in Fig. 7a, resulted in a depth error of ? 8 km for the synthetic LAB image (not shown). With various lithopheric velocity models available for the eastern NCC (e.g., crustal models from Zheng et al., 2006; Zheng et al., 2007; Wang et al., 2000; upper mantle models from Chen et al., 1991; Huang et al., 2003; Zhu et al., 1997; Hearn et al., 2004), we found that the depth uncertainties of the imaged LAB were generally less than 8 km. The uncertainty of a few kilometers in depth estimation due to different models appears to be smaller than or comparable to what can be resolved with the real S-RF data (? 10 km). We also investigated the influence of the structure of the Moho and the LAB on the migrated S-RF image. Our synthetic modeling suggests that the sharpness of the Moho has only minor effects on the LAB image, analogous to what we have found in our previous P-RF study (Chen et al., 2006) and also consistent with other recent RF studies (e.g. Rychert et al., 2005). Note that both the synthetic images (Fig. 7b–d) and the data image (Fig. 5a) show an apparent reduction in the image strength of the LAB at transverse distance of ?300 km, indicating that such a lateral variation may not necessarily reflect a real change in the sharpness of the LAB, but probably results from other factors such as the uneven data coverage. Possible lateral structural variation of the LAB, however, cannot be fully ruled out for other profiles. In addition, with the 4.5-s dominant period and frequencies up to 0.5 Hz of the data, a velocity boundary with a sharpness of ? 10 km might not dramatically broaden the Sp waveform and will therefore be detected by the S-RFs as close to a first order discontinuity (compare Fig. 7c with 7b and see Fig. 7f). While the synthetic waveforms for a sharp LAB (≤10-km wide) are narrower than, or have a similar width to, the real data, a 20-km gradient zone produces waveforms that are slightly broader than the data (Fig. 7f). However, the LAB image and individual waveforms for a ? 40km transition zone are too broad to resemble the real cases (Fig. 7e and f). Our synthetic modeling therefore suggests that the LAB beneath the study region is unlikely to be as thick as 40 km, but may be a gradient zone 10–20 km wide. Note that here we did not aim at stringent waveform modeling but simply intended to provide general constraints on the width of the LAB. The synthetic models adopted in our calculation may oversimplify the real structure above the LAB, which could cause misfits between the synthetics and the data, such as shown in the depth range of 50–80 km in Fig. 7f. However, synthetic tests
Table 1 Model parameters in synthetic modeling Model Parameter Moho LAB Moho (Case 2, 3,4) LAB Case 2 Case 3 Case 4 Moho LAB Case 1 Vs contrast (%) 15.6? 4.0 15.6? 4.0 5.0 10.0 15.6? 4.0 Depth range (km) 0 0 10 10 20 40 10 20

profile E–E' and C–C', respectively, so that the effects of the data coverage could be properly accounted for. The velocity model adopted in migration might be the factor that most directly affects the depth distribution of the underlying discontinuities. Our previous P-RF study has shown that the choice of the lithospheric velocity model is not very important for the imaging of the LAB (Chen et al., 2006). This is also true for S-RF imaging. For instance, using a 1D velocity model, which has depth-integrated velocity errors of ? 8% compared to

Dipping LAB model

LAB step model

?The value is same as that given in PREM (Dziewonski and Anderson, 1981).

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suggest that this does not have significant influence on our conclusion about the LAB width. We further evaluated the lateral resolution of the migrated image, especially for profile C–C' that shows an abrupt increase of more than 40 km in the LAB depth (Fig. 3f). To constrain the lateral distance within which the LAB step occurs, we performed synthetic calculations for the lithosphere model shown in Fig. 8a by setting the distance range of the 40-km LAB step to be 0, 100, and 200 km, respectively. For the abrupt-change case, the two segments of the LAB image appear to overlap laterally (Fig. 8b), probably due to the smoothing effect of S-RF stacking and migration. For the more gradual transition cases (Figs. 8c and d), with the current data coverage, the two LAB segments are imaged further apart than is seen in the data image (Fig. 3f). These observations suggest that the distinct change of the LAB depth detected in the image of profile C–C' may take place over no more than 100 km distance. 5. Map of lithospheric thickness and discussion We have integrated the results of our S-RF analysis and migration (Figs. 3, 5 and 6) to produce a map of the LAB depth

across the northeastern NCC (Fig. 9). It shows that the lithospheric thickness of the region is highly laterally variable. The LAB displays significant topography, dropping from ?60– 70 km in the southeast basin and coastal areas to N100 km in the northwest mountain ranges and continental interior. In particular, the lithosphere appears the thinnest around the Tanlu Fault Zone, which runs NNE along the eastern margin of the NCC, and is apparently thicker away from the fault zone (Fig. 9). Besides the general tendency of SE-to-NW deepening, local-scale variations are obvious in the LAB depth map. The most striking is the abrupt ? 40-km change in LAB depth near the triple junction of the BBB, the TM, and the YM (Fig. 9, see also Fig. 3f), which likely occurs within a lateral distance of several tens of kilometers. Another example comes from the area around the concave boundary between the BBB and the TM where the LAB is unusually shallower than in the surrounding areas (Fig. 9). Lateral variations of the lithospheric thickness may be a direct result of the tectonic evolution, specifically the Mesozoic– Cenozoic reactivation of the NCC. The pattern of variation in the LAB depth observed here generally agrees with previous regional seismological observations (Chen et al., 1991; Huang

Fig. 9. Map of the LAB depth for the northeastern North China Craton. A Gaussian cap with a radius of 50 km and a maximum effective radius of 120 km is used in depth interpolation. Short black bars give the SKS splitting results from Zhao and Zheng (2005) with the orientation and length of each bar representing the fast polarization direction and splitting delay time, respectively. Labels for the major tectonic units are same as Fig. 1. Thick dashed lines mark the boundaries between the BBB, the TM, the YM and the LU. Thin gray solid lines schematically indicate the trends of the TM and the YM (Zhang, 1997).

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et al., 2003; Zhu et al., 2004) as well as geochemical, geothermal and petrographic data (Fan et al., 2000; Xu, 2001; Zheng et al., 2001; Xu et al., 2004) but with unprecedented detail. Even considering the uncertainties, the estimated LAB depths across the whole study region never reach 150 km, a value considerably smaller than those commonly observed in other Archean cratons (175–400 km, Artemieva and Mooney, 2001 and references therein; Godey et al., 2004; Deen et al., 2006). In particular, the Tanlu Fault Zone area in the eastern NCC, where a Paleozoic lithospheric thickness of N 180 km has been documented by studies of xenoliths in kimberlites (Griffin et al., 1998; Menzies et al., 1993), appears to have the shallowest LAB (60–70 km) at present. These observations, on the one hand, indicate that the lithosphere of the study region, over to the western edge of the TM, might have been widely affected and thinned at some time since its formation in the Archean. On the other hand, the data also provide direct seismological evidence for the significant lithospheric modification and thinning during the Phanerozoic reactivation process beneath at least some areas in the eastern NCC. Moreover, the substantially thinned lithosphere imaged along the whole segment of the Tanlu Fault Zone from the S-RF data strengthens and expands our P-RF imaging results. It confirms previous suggestions that this fault zone extends deep into the lithosphere mantle and might have acted as a major channel for asthenosphere upwelling accompanying the tectonic extension and lithospheric reactivation in the Mesozoic–Cenozoic time (Xu, 2001; Yuan, 1996; Chen et al., 2006; Zheng et al., 2001). There is good correspondence between the lithospheric thickness estimated using S-RF data (this paper) and the upper mantle seismic anisotropy measured through SKS splitting analysis (Zhao and Zheng, 2005) in the study region. As shown in Fig. 9, WNW–ESE trending fast polarization of shear waves is generally observed in areas with apparently thinner lithosphere, whereas in areas underlain by relatively thicker lithosphere the fast polarizations are oriented either NNE–SSW or NE–SW. In particular, the sharp LAB step revealed here appears almost exactly where Zhao and Zheng (2005) found an abrupt change (90°, see Fig. 9) in the fast polarization direction of shear waves. Distinct SKS splitting behavior is also observed in the southern part of the area around the concave boundary between the BBB and the TM, where the lithosphere is thinner than in surrounding areas (Fig. 9). The thin lithosphere and the WNW–ESE oriented fast polarization in the eastern coastal areas of the study region seem to be closely associated with the dominant lithospheric extension (E–W or NW–SE) during the Mesozoic–Cenozoic lithospheric reactivation in the eastern NCC (Tian et al., 1992; Ren et al., 2002). The thicker lithosphere and the NNE–SSW or NE–SW fast polarization in the western TM and YM, on the other hand, may retain the history of compressive deformation in the region. Based on their SKS splitting observations, Zhao and Zheng (2005) argued that the observed WNW–ESE fast polarization direction might have been induced by a northwestward mantle flow traveling beneath the thinned lithosphere in the eastern areas during the Mesozoic–Cenozoic reactivation. The thicker lithosphere in the western and northern areas, on the other hand, probably acted as a barrier deflecting the mantle flow and

caused the fast shear waves to orient NNE–SSW or NE–SW, perpendicular to that observed in the eastern areas. Our S-RF imaging result therefore corroborates their speculation that a substantial variation in lithospheric thickness might have been responsible for the observed rapid variation of upper mantle seismic anisotropy. Moreover, it was reported that, before the widespread late Mesozoic regional extension, several phases of N–S directed compressive deformation occurred at the northern boundary of the NCC that induced the formation of the E–W trending YM and the Yin Mountains (YinM, Fig. 1) to the west (Meng, 2003; Davis et al., 1998; Davis, 2003). The thickest lithosphere is located below the surface inflexion from the NNE oriented structure in the TM area to the E–ENE oriented structure in the YM area and the upper mantle anisotropy pattern also shows a coincident change there (Fig. 9). These observations lead us to further propose that the tectonic imprint of the earlier multiple N–S compressional deformations may be still preserved in the present-day lithosphere of the northeastern NCC, particularly in the western areas where the influence of the late Mesozoic–Cenozoic lithospheric extension probably was weak. In addition to the significant lateral variations in lithospheric thickness, the northeastern NCC is also characterized by a relatively sharp lithosphere–asthenosphere transition at present. In our previous P-RF study (Chen et al., 2006), we found a sharp LAB (3–7% drop in S-wave velocity over a depth range of 10 km or less) at 60–80 km depth beneath the Tanlu Fault Zone area in the eastern NCC (profile A–A'). Beneath other areas of the northeastern NCC, the LAB cannot be identified without ambiguity using the P-RF data, but is coherently detected in the migrated S-RF images (Figs. 3 and 5) and S-RF stacks (Fig. 6). Due to the longer periods and the relatively sparser data coverage and hence lower spatial resolution of the S-RFs, we are unable to put stringent constraints on the nature of the LAB as we have done based on the P-RFs. Nevertheless, most of the S-RF images and stacked S-RFs show significant LAB signals, suggesting that in general the LAB of the study region is quite sharp. Comparison of the data images (Figs. 3 and 5) with the synthetic modeling results (Figs. 7 and 8) further indicates that the lithosphere–asthenosphere transition beneath the northeastern NCC may be narrow, probably on the order of 10–20 km in depth, with a S-wave velocity drop of several percent. This is not in apparent conflict with our P-RF result for the Tanlu Fault Zone area in the southeastern part of the study region (Chen et al., 2006). The strong and sharp LAB imaged using both the P-and SRF data in the northeastern NCC is consistent with recent tomographic results that show a distinct low velocity zone (LVZ) beneath the NCC (Huang et al., 2003; Huang and Zhao, 2006). However, this feature is different from that of most stable continental regions, including the Archean Ordos Basin in the western NCC and the Proterozoic Tarim platform to the west of the NCC, where the LVZ is difficult or impossible to detect seismically (e.g., Huang et al., 2003; Lerner-Lam and Jordan, 1987; Gaherty et al., 1999; Freybourger et al., 2001; Li et al., 2006; Larson et al., 2006). It is, however, analogous to what has been observed beneath oceanic and off-craton areas (e.g.,

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Gaherty et al., 1999; Grand and Helmberger, 1984; Simon et al., 2003). Petrologic and geochemical studies on mantle xenoliths (Griffin et al., 1998; Menzies and Xu, 1998; Fan et al., 2000; Xu, 2001) also indicate a relatively fertile present-day lithosphere beneath large parts of the eastern NCC, in marked contrast to its cratonic nature before Phanerozoic time. In combination with these observations, the structural features of the LAB revealed in our P-and S-RF studies therefore suggest that the Mesozoic–Cenozoic lithospheric reactivation of the region may not be a purely mechanic thinning process, but probably involved complex modification of the physical and chemical properties of the lithosphere. 6. Conclusions By expanding the wave equation P receiver function migration method to the S receiver functions and applying the new imaging technique to the recently collected high-quality broadband seismic data, we have been able to image the lithospheric structure beneath the northeastern NCC in detail. The lithospheric thickness of the region is highly laterally variable, from 60–70 km in the eastern Tanlu Fault Zone area and BBB to 100–130 km in the western TM and the northern YM. The present-day lithosphere is quite thin compared with that of typical cratonic regions; this indicates that the whole study region might have experienced significant lithospheric thinning through its long tectonic evolution history. In particular, the lithosphere appears to be thinnest around the Tanlu Fault Zone, in large contrast to the Paleozoic lithosphere of N 180 km thick beneath this area. This observation suggests that at least some areas of the eastern NCC experienced significant lithospheric modification and thinning mainly during the Mesozoic and Cenozoic time. The detailed lithospheric image further confirms that the Tanlu Fault Zone played an important role in the Phanerozoic tectonic modification of the eastern NCC. The lateral variations in lithospheric thickness estimated here generally coincide with changes in the direction of upper mantle seismic anisotropy previously derived from SKS splitting measurements. Thin lithosphere with WNW–ESE fast polarization directions of shear waves in the southeastern areas contrasts with relatively thick lithosphere with NE–SW fast polarization directions in the northwestern region. In addition, localized sharp changes in both the LAB depth and fast shear wave polarization direction occur at the boundaries of major tectonic units, especially at the triple junction of the BBB, the TM and the YM. Such a good correspondence suggests that the presentday lithospheric thickness and seismic anisotropy pattern of the upper mantle are mutually correlated. The two types of thickness-anisotropy correlation probably reflect the deformation history of the widespread Late Mesozoic–Cenozoic tectonic extension and the earlier multiple compressions, respectively, in the northeastern NCC. Moreover, the imaged LAB together with synthetic modeling results indicate a sharp lithosphereasthenosphere transition (10–20 km thick) beneath the study region, a characteristic that obviously deviates from typical Archean cratons. Our RF observations therefore provide seismological evidence for the substantial modification of the litho-

sphere during the tectonic reactivation of the NCC, as previously suggested by geochemical and petrological studies. Acknowledgment We thank the Seismic Array Laboratory of the Institute of Geology and Geophysics, Chinese Academy of Sciences, and the China Earthquake Administration for providing the waveform data. Thoughtful and constructive reviews from William L. Griffin and an anonymous reviewer significantly improved the manuscript. Lianxing Wen provided the 2D P-SV hybrid code that was used in this study for synthetic calculation. This research is supported by the National Science Foundation of China (Grants 40674029 and 40434012) and the Chinese Academy of Sciences. References
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