Crustal and lithospheric structure of Northeast China from S-wave receiver functions

Earth and Planetary Science Letters 401 (2014) 196–205

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Crustal and lithospheric structure of Northeast China from S-wave receiver functions Ruiqing Zhang ∗ , Qingju Wu, Lian Sun, Jing He, Zhanyong Gao Institute of Geophysics, China Earthquake Administration, Beijing 100081, China

a r t i c l e

i n f o

Article history: Received 9 December 2013 Received in revised form 3 June 2014 Accepted 5 June 2014 Available online xxxx Editor: P. Shearer Keywords: lithosphere–asthenosphere boundary Northeast China pure shear model North–South Gravity Lineament

a b s t r a c t Lithospheric thickness is closely associated with lithospheric response to tectonic forces or thermal processes. Analysis of S-wave receiver functions (SRFs) from a dense seismic array has revealed lithospheric thickness variations along a 1200 km long profile that spans all major geological terranes of Northeast China. The SRF images identify the shallowest Moho and lithosphere–asthenosphere boundary (LAB) depths beneath the rifted Songliao Basin, thus spatially correlating well with the thick sediment accumulation. These features can be explained by a pure shear regime operating in the lower crust and upper mantle, suggesting the predominance of roughly symmetric lithosphere stretching from continental rifting. In contrast, thicker lithosphere in western and eastern terranes of NE China indicates the lithosphere in these regions was not strongly affected by the extensional processes during the Late Mesozoic. Flat LAB structure beneath the Erguna and Xing’an terranes along with the overlying relatively deeper Moho beneath the Great Xing’an Range, suggests the North–South Gravity Lineament may not be a trans-lithospheric structure. © 2014 Elsevier B.V. All rights reserved.

1. Introduction Northeast China (NE China) lies within the eastern segment of the Central Asian Orogenic Belt (CAOB), one of the world’s largest accretionary orogens. The CAOB extends from the Uralides to the Pacific margin and is surrounded by Eastern Europe to the west, Siberia to the north, and the North China and Tarim cratons to the south (e.g., Sengör et al., 1993). The eastern CAOB was formed during the Middle Paleozoic collision of the Tuva–Mongolian terrane and Siberian Craton, and a subsequent Late Paleozoic–Early Mesozoic collision between the North China Craton (NCC) and the Siberian platform (e.g., Didenko et al., 1994; Meng et al., 2010; Safonova et al., 2011). NE China is composed of several micro-continental terranes or blocks, which include the Erguna and Xing’an blocks in the west, the Songliao Basin in the centre, and the Jiamusi massif in the east (Wu et al., 1995). The NNE-trending North South Gravity Lineament (NSGL) forms a striking geologic feature in NE China. The lineament exhibits Bouguer anomalies from −100 mGal in the west to −40 mGal in the east in association with a relatively rapid topographic change (Ma, 1989, Fig. 1). It spans the transitional area between the Xing’an block and Songliao Basin, and also

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http://dx.doi.org/10.1016/j.epsl.2014.06.017 0012-821X/© 2014 Elsevier B.V. All rights reserved.

extends thousands of kilometers past the NCC into south China. Some geophysical studies have concluded that, as a pre-existing lithosphere-scale structure, the NSGL separates the NCC into two distinct domains and thus influenced the NCC’s tectonic evolution (Chen et al., 2008; Chen, 2010). The source of the gravity anomaly and its exact depth remain uncertain however (Griffin et al., 1998; Xu, 2007). NE China experienced intensive extensional deformation and magmatism in the Late Mesozoic. Many extensional basins developed in NE China and adjacent Mongolia during that time, including the Erlian, Hailar, Songliao and Eastern Gobi basins (Graham et al., 2001; Meng, 2003). Among these basins, the NNE-trending Songliao Basin, as the largest and only oil-producing basin, developed through extensional graben-faulting from Late Jurassic to Early Cretaceous and further growth faulting in the Middle Cretaceous (Wang et al., 2002). Exhumed Cretaceous metamorphic core complexes reported in areas along the China–Mongolia border (Zheng et al., 1991) and southernmost Mongolia (Webb et al., 1999) demonstrate a relatively rapid extension at that time. The extension is also manifested by Late Mesozoic volcanic rocks, occurring mainly in the Great Xing’an tectonic–magmatic belt along the NNE or NE direction (Wu et al., 1982; Faure and Natlin, 1992), and also appearing in eastern and southern Mongolia and in Korea (Ren et al., 2002). In addition, intrusive rocks are also widespread in NE China, including the Great Xing’an Range with granitoids mainly emplaced during the Early Cretaceous, and the

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Zhangguangcai Range with Jurassic granitoids, thus exhibiting by a westward younging trend (Wu et al., 2011). Petrology and geochemical analysis reveal that most of these granitoids are felsic I- and A-types. The identified A-type granites and alkaline units suggest that NE China was not in a compressive regime during the Late Mesozoic (Wu et al., 2005). Tectonic extension is closely connected with deformations of both the crust and upper mantle, and is most likely associated with lithospheric thinning. For example adjacent to NE China, continental rift systems and extensional basins were also widespread in the eastern Archean NCC during the Cretaceous–Paleogene (Ren et al., 2002). Geochemical and petrological studies have suggested that Cretaceous magmatism occurring in the eastern NCC reflects the replacement of a thick, old, cold and refractory lithospheric keel by a thin, young, hot and fertile mantle (e.g., Menzies et al., 1993; Reisberg and Lorand, 1995; Griffin et al., 1998; Gao et al., 2002; Xu, 2007). Studies of entrained mantle xenoliths provide direct evidence of lithospheric thinning in the eastern NCC during the Mesozoic. Lithospheric thinning has been further confirmed by geophysical studies using receiver functions and surface wave tomography (Huang et al., 2003; Chen et al., 2008; Chen, 2010). The similar tectonic extension experienced by NE China and the NCC indicates lithospheric thinning throughout eastern China (Wu et al., 2005; Xu, 2007). Petrological and mineralogical studies of peridotitic xenoliths suggest a more depleted lithospheric mantle in basalt compositions accompanied by younger, overlying continental crust in NE China than that of the NCC. The geochronological decoupling between crust age and mantle composition in NE China has been used to argue for a small degree of lithospheric thinning (Yu et al., 2007) or even a whole-scale loss of the lithospheric mantle (Zhou et al., 2007). So far, the xenoliths found in NE China mostly consist of spinel-bearing lherzolite, along with a few garnet-bearing peridotites. Within NE China, the Songliao Basin appears to experience the greatest lithospheric thinning (e.g., Lin et al., 1997; Ren et al., 2002; Meng, 2003). In contrast to the basins of northern China– Mongolia, the Songliao Basin has experienced significant thermal subsidence following Early Cretaceous initial rifting, receiving up to 6 km of sediment (Ma et al., 1984). Meng (2003) interpreted the distinct subsidence history as evidence that the underlying lithosphere may have thinned to a considerable degree. The rifted Songliao Basin is also characterized both by high heat flow and geothermal gradient, and the latter can reach a maximum of 6.2 ◦ C/100 m (Tian et al., 1992), indicating a relatively hot, shallow upper mantle. Lithospheric thickness can be expressed in a variety of geophysical parameters, including seismic velocities, thermal boundary and electrical conductivity. Direct geophysical constraints on lithospheric thickness in NE China, however, are rather scarce and still highly debated. In seismic tomography, the mantle lithosphere generally refers to the crust and the high velocity zone in the upper mantle. The lithosphere directly overlies the asthenosphere, which is characterized by a distinct low velocity zone in the mantle. A large-scale traveltime tomographic study showed elevated P-wave velocity down to about 200 km depth beneath the Songliao Basin, suggesting a thicker lithosphere (C. Li et al., 2006). The nearly vertical incidence of rays used in bodywave tomography typically reduces its radial resolution. Surfacewave tomography studies have taken the depth of the maximum negative velocity gradient below the high-velocity zone, or the depth at which velocity begins to decease, as proxies for the lithosphere–asthenosphere boundary (LAB). A thinned lithosphere (∼70–100 km) is then inferred beneath the Songliao Basin according to Rayleigh wave inversions from seismic waves (Huang et al., 2003; Li et al., 2013), as well as from ambient noise (Zheng et al., 2011). By comparison, surface-wave tomography can pro-

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vide robust constraints on absolute shear-wave velocity structure at LAB-like depths. The primary limitation of surface-wave tomography is that surface waves alone cannot distinguish a change in mantle velocity that occurs at an instantaneous depth, from that which occurs over tens of kilometers (Fischer et al., 2010). The lithosphere is also technically defined as the subsolidus layer which can only transfer heat by conduction, and which acts as a mechanical barrier to convective upwelling of the asthenosphere. Assuming a mantle temperature of ∼1350 ◦ C as the switch from a conductive to convective (adiabatic) geotherm, the thermal lithosphere is ∼100 km thick throughout Eastern China and does not show much variation (Wang, 2001; An and Shi, 2006). In contrast, electrical structure revealed by west-to-east magnetotelluric (MT) sounding profiles across NE China, showed lateral variations in high-conductivity layers at a depth of 120 km in the west, at depths of only 60 km beneath the Songliao Basin, and at depths reaching 90 km in the east (Liu et al., 2006). Receiver function analysis has been developed as an effective tool for mapping sharp velocity discontinuities in the crust and upper mantle. Previous studies have used P-wave receiver functions (PRFs) to map Moho topography as well as the 410 and 660 km discontinuities beneath NE China (e.g., Ai et al., 2003; Li and Yuan, 2003; Shen and Zhou, 2009; Liu and Niu, 2011; Wei and Chen, 2012; Zhang, 2012), but have not addressed lithospheric structure. Conventional PRF study suffers from interference by crustal multiples that mask LAB conversions. S-wave receiver function (SRF) method however analyze converted phases that arrive earlier than direct phases, and thus avoid later multiple reverberations. Previous studies have demonstrated LAB detection using SRFs (e.g., Kumar et al., 2005; Sodoudi et al., 2006; X.Q. Li et al., 2007; Chen et al., 2008; Miller and Eaton, 2010; Abt et al., 2010; Hu et al., 2011; Xu et al., 2013). Here we use SRFs to investigate the crustal and particularly lithospheric structure beneath NE China. Portable broadband seismic stations have been recently deployed in a relatively densely spaced linear array that traversed NE China from west to east. The volume of data available offers a unique opportunity to use SRFs in investigating the seismic structure of the mantle lithosphere. By examining variations in crustal and lithospheric thickness across the NSGL, and throughout the Songliao Basin and adjacent areas, and their relationships to surface topographic features, we can discuss the possible deep mantle process involved in tectonic continental rifting. 2. Seismic data and methods From April 2009 to September 2011, a portable seismic array consisting of about 60 broadband stations spaced at ∼20 km intervals was deployed in NE China. The stations were equipped with Guralp CMG-3ESPC seismometers and REFTEK-130B recorders, which provided high-quality seismic data. The station arrays followed a linear, west–east trend that traversed all major geologic blocks of NE China and the NSGL (Fig. 1). This deployment is suitable for seismic imaging of the crustal and lithospheric thickness variations at a geologically relevant length-scale. SRFs analyze the S-to-P (Sdp) converted phases resulting from seismic velocity layering. Similar to PRF, the amplitude of the conversion phase depends on the impedance contrast across the interface and the incidence angle. Layer depths are constrained by the delay of the converted phase with respect to the arrival of the direct phase using a reference velocity model. There are many kinds of S-to-P phases that can be used to calculate SRFs, including S, SKS, and ScS phases (Yuan et al., 2006). At epicentral distances of 55◦ –85◦ , the S phase is the first dominant arrival followed by the ScS phase. At greater epicentral distances (85◦ –115◦ ), SKS arrives first, followed by the SKKS phase. The S and SKS phases usually

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Fig. 1. Topographic map of NE China showing major tectonic units, including the Erguna and Xing’an blocks, the Songliao Basin and the Jiamusi massif (from the west to east), also along with Great Xing’an Range (1), Lesser Xing’an Range (2) and Zhangguangcai Range (3). Data interpreted here were collected by portable seismic stations (red triangles) and five permanent stations (white triangles). Small, dark dots show SRF piercing points at 100 km depth. Profile along which we construct the lithospheric image is shown in gray rectangle. The North–South Gravity Lineament (NSGL) with a Bouguer gravity gradient larger than 0.0005 mGal/m (Liu, 2007; Shi, 2012) is shown as the irregular shaded gray area. Straight gray lines to the east represent the Tanlu Fault Zone.

intersect at epicentral distances of 80◦ –90◦ . Within this epicentral distance range, overlapping S and SKS arrivals result in significant waveform complications. In this case, we only used records having time differences of greater than 30 s between these two successive arrivals. This selection criterion is appropriate for a region with potential LAB depths not deeper than 200 km, as implied by previous regional-scale surface tomographic images (Huang et al., 2003; Li et al., 2013). SRFs can better resolve the depth of seismic velocity discontinuities than surface wave inversion studies, but cannot resolve crustal features as well as PRF, due to the longer periods of the waveforms used in SRFs. To compute SRFs, we selected teleseismic events with magnitudes greater than Ms 5.5, and ranging in epicentral distances from 55◦ to 115◦ from records collected by the seismic array and permanent stations. Since S-waves are much nosier than P-waves, selected seismograms were checked to ensure clear arrivals for S, ScS or SKS phases at predicted times. According to their clarity, we divided records into different categories. After SRF calculation, we again used visual inspection to exclude traces that did not clearly show a positive Sp phase from the Moho in a time window of 3–7 s, and whose raw data was originally categorized as poor. These selection criteria assume Moho depths as estimated by previous PRF studies (Liu and Niu, 2011; Wei and Chen, 2012; Zhang et al., 2013). To isolate converted phases from direct phases, N–E components were rotated into R–T space according to theoretical back azimuth. For records with S phases (including ScS), Z–R–T components were further transformed into P–SV–SH waves using the free-surface matrix (Kennett and Engdahl, 1991). For the latter transformation, optimal free-surface velocities can be determined by minimizing the correlation between P and SV components within a limited time window around the S-wave arrival

(Abt et al., 2010), or by minimizing the amplitude of each individual P component at the S-wave arrival time (Yuan et al., 2006; X.Q. Li et al., 2007; Hu et al., 2011). This study used a free-surface S-wave velocity of 3.0 km/s for all stations. For NE China, which encompasses elevational changes of as much as 800 m, this assumption may be simplistic. Fig. 2(a) shows SRFs exhibit a degree of positive energy at time zero, as calculated from stations located in high relief (SM01–SM20), and reversed polarity from stations (SM27–SM41) located in the Songliao Basin. Testing of the sensitivity of SRFs to different rotational strategies (e.g., X.Q. Li et al., 2007) showed that these variations exerted only a weak influence on LAB identification. The methods used should therefore adequately resolve the uppermost mantle structure beneath NE China. We used Z and R components for SKS phases without further transformation. Following rotation, SRFs can be obtained by deconvolving SV components from P components to remove the source-site complication and propagation effect (Yuan et al., 2006). In the process of deconvolution, a multi-taper spectral correlation method was used (Park and Levin, 2000; Helffrich, 2006). The SRFs were then subjected to a 0.03–0.5 Hz bandpass filter. For direct comparison with conventional PRFs, we reversed the time axis and the amplitudes of the SRFs so that converted phases with positive amplitudes indicate an impedance increase. Finally, we selected about 215 highquality events for SRF calculation. The number of SRFs calculated from each individual station varied from about 30 to more than 200. The number of usable traces exceeded 100 for more than half of the stations.

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Fig. 2. An example showing the comparison of S receiver functions from S (a) and ScS (b) waves computed at the same stations. The receiver functions are calculated from waveforms of a select event, and the traces are sorted primarily according to station locations from west to east. For most stations, positive phases can be observed at about 5 s (red dashed line).

3. Results 3.1. ScS receiver functions To use seismic data to its fullest potential, and in contrast to previous studies, which typically use S and SKS phases, we also analyzed Sp phases converted from ScS phases. Relative to those calculated from the S phase, ScS receiver functions are more suitable for mapping a mantle discontinuity just below a given station, owing to their small wave slowness. Using the same waveforms, we compute SRF traces from S and ScS phases by selecting respective time widows 30 s before and 30 s following predicted arrivals. Fig. 2 gives an example comparison between the S and ScS receiver functions. After applying a move-out correction, the traces are sorted primarily by station location along a west-to-east profile. The correction is calculated as a product of the difference in slowness between the S and ScS phases using the 1-D IASP91 model (Kennett and Engdahl, 1991). For the selected event, positive phases can be traced along the profile, from about 5 s at stations in the west, ∼3 s at stations in the Songliao Basin (stations SM27 to SM41), then to ∼5 s in the east (Fig. 2(b)). A delay time of 5 s is reasonable for a continental region with a crustal thickness of about 40 km. We therefore interpret the positive phase as the Moho conversions from the ScS waves. For simplicity, we use the SRFs for all the receiver functions from the S, ScS and SKS phases in the following paper. 3.2. Moho topography To establish constraints on crustal structure using the SRFs, we applied two stacking methods. In the first method, the individual

SRF records have been stacked at each station (Fig. 3). Before stacking, the normalized traces are corrected to a reference slowness value of 6.4 s deg−1 . Single station stacking reveals clearly positive Sp conversions from the Moho, which are automatically labeled according to their maximum positive amplitudes between 3 and 7 s (Fig. 3). For most of the stations, the Moho conversions are marked at about 5 s, same as shown in Fig. 2. The Moho arrivals also exhibit variable arrival times, apparently earlier (at around 3 s) at stations (SM27–SM40) in the Songliao Basin, while slightly delayed at stations SM15–SM23 in the Great Xing’an Range (Fig. 3). Receiver functions from a single station span a range of azimuths and incidence angles, and thus record subsurface refractors at different points (e.g., Zhu, 2000). We applied common conversion point (CCP) stacking techniques (Dueker and Sheehan, 1997) to further explore the lateral variation of Moho depth and sharpness. The SRFs in time are first migrated to depth using the 1-D IASP91 model. A three dimensional parametric geometry is constructed with a CCP bin size of 2 km vertical, and 10 by 10 km horizontal. We stack the SRF amplitudes into CCP bins by projecting the piercing points into the parametric space. Additional smoothing is then applied within a radius of 2 pixel points, making the effective horizontal bin size spacing about 40 km. Our CCP image of Moho topography is shown in Fig. 4. In the CCP cross-section, the most prominent feature is the large positive energy (shown in blue) mainly centered at a depth of around 30–40 km, which can also be traced continuously, thus interpreted as Moho conversions (Fig. 4(a)). The continuous conversions also show a clearly regional difference in terms of their depths, as inferred from single station stacking results. In general, crustal thickness estimates give an average value of 35 km, which is consistent with PRF analysis (Zhang et al., 2013).

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Fig. 3. Summed traces of individual S receiver functions for the portable stations after move-out corrections. Traces are arranged by station locations from west to east. Magenta dots mark Moho conversions as the maximum positive amplitudes in a time window of 3–7 s.

Fig. 4. Migrated S receiver function (SRF) image of the Moho (a), and comparison with the corresponding P receiver function (PRF) image (b, Zhang et al., 2013) along the same transect across NE China from west to east. Black dots in (a) and (b) are the same, marking Moho depths from SRFs automatically picked from the largest positive amplitudes within a 20–50 km depth range beneath each surface location.

Beneath the Songliao Basin Moho conversions are at depths of less than 30 km. Such a thinned crust is reasonable for a rifted basin, which underwent pronounced thermal subsidence after earliest Early Cretaceous rifting. The relatively thin crust beneath the Songliao Basin apparent from SRF analysis was not detected in previous PRF results for the same stations (Fig. 4(b), Zhang et al., 2013). The discrepancy in the crustal thickness estimates are probably not due to differing frequency contents of the respective SRF and PRF datasets, which could generate uncertainties of no more than ∼5 km (Sodoudi et al., 2006). In the PRF image, positive energy is observed beneath the Songliao Basin at a depth range of 0–15 km, significantly broader than that in adjacent regions. The

broader energy may indicate involution of sedimentary multiples. Previous synthetics have shown that PRFs within a time window of 0–5 s are typically dominated by multiples produced by a sedimentary layer over crust (J. Li et al., 2007). Sedimentary multiples can cause difficulty in detecting the Moho converted phases (X.Q. Li et al., 2007), which would bias the crustal thickness estimates in the PRFs. Beneath the Great Xing’an Range the Moho conversions deepen gradually to a depth of more than 40 km. This deepening is less prominent than that observed in the PRF image. Compared with PRF results, our SRF image shows a broader region of positive energy within the estimated range of Moho depth, simply reflecting the lower resolution of the SRF method. On the

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Fig. 5. Migrated S receiver function (SRF) image of the lithosphere–asthenosphere boundary (LAB) using our preferred 2.5 D model, and the P receiver function (PRF) image (b) for comparison. The preferred 2.5 D model is based on the IASP91 velocity model in the crust and shear wave velocity structure in the mantle, extracted from a regional-scale surface-wave tomographic image (Li et al., 2013) for each station. The LAB depth is estimated from SRFs as the median depth among the ten most negative values in the vertical CCP profile between depths of 60–170 km, beneath each surface location as indicated by unfilled diamond markers in (a) through (d). The PRF image is from Zhang (2012). The gray bars show 1σ error bound for the mean LAB depth (black dots, c), estimated from bootstrap analysis of the CCP image (resampling 100 times). Panel (d) compares LAB depths estimates from migration using the 1-D reference model (blue crosses) and the 2.5 D model (unfilled diamonds).

other hand, for regions such as the sedimentary basin, the SRF results provide a higher resolution estimate of crustal thickness than that generated by PRF. To summarize, SRF is a useful complement to PRF in resolving crustal structure. 3.3. LAB topography To map the LAB topography, we constructed a CCP image along the profile shown in Fig. 1. As in Yuan et al. (2010), we used

a combination of the IASP91 velocity model in the crust and shear wave velocity structure in the mantle for each station, extracted from a regional-scale surface-wave tomography model (Li et al., 2013), in depth migrations. Our CCP image (Fig. 5(a)) shows discontinuous, negative phases (shown in magenta) at depths ranging from 100 to 160 km, below the shallow positive Moho conversions. The PRF image does not clearly show potential negative Ps conversions from a LAB-like discontinuity (Fig. 5(b)). Beneath

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the Songliao Basin for example, negative phases are only visible at depths of around 80 km, and there are no other discernible negative phases from 90 km to 150 km depth. The visible negative phases beneath the Songliao Basin can also be continuously traced into other regions. These features follow the overlying Moho conversions closely, appearing as side-lobes of these conversions (Fig. 5(b)). The absence of potential negative phases in the PRF image at expected depths estimated from the SRFs possibly could be attributed to interference from the positive crustal multiples. We interpret the deep negative feature in our SRF image as Sp conversions from the LAB. The LAB depth is specifically identified as the median depth from the ten most negative values of the vertical CCP profile beneath each surface location between depths of 60–170 km. The average magnitude of the LAB phase (from automated selection) is on the order of 0.1 (relative to the parent S phase), only half of the average magnitude for Moho conversions. The LAB value can be best fit by a shear velocity drop of 8% based on a set of SRF synthetic tests (Lekic et al., 2011). Given the shearwave velocity of 4.5 km/s for the lithosphere and a maximum velocity jump across the LAB of 0.3 km/s, the velocity contrast suggested by surface-wave tomography (Li et al., 2013) is about 7%, basically consistent with our inferred velocity drop described above (Fig. 5(a)). As shown in Fig. 5(a), the discontinuous LAB exhibits larger variations at depth than the overlying Moho. In the west beneath the Erguna and the Xing’an block, a relatively smooth LAB is presented with large amplitudes at depths of 140–160 km, deeper than the estimate from magnetotelluric sounding data (∼120 km; Liu et al., 2006). In the central profile, we observed a remarkably shallow LAB ranging from 100–120 km depth, indicating variations in lithospheric thickness of as much as 30–40 km between the Xing’an block and Songliao Basin. The relatively discontinuous transition in lithospheric thickness is particularly abrupt, occurring over a horizontal distance of ∼80 km or less. The inferred lithospheric thickness (100–120 km) beneath the Songliao Basin appears slightly thicker than previous estimates (60–100 km), from Rayleigh wave inversions (Zheng et al., 2011; Li et al., 2013), and also from magnetotelluric sounding data (Liu et al., 2006). Further to the east, the negative LAB conversion weakens, and the LAB depths gradually deepen from 120 to 150 km. The weak energy may result from a locally reduced velocity contrast across the LAB. Many factors can affect the spatial resolution of the LAB image and the accuracy of the estimated LAB depth. First order uncertainties could arise from uneven/sub-optimal data coverage, and to a lesser extent, from the migration model. To assess data coverage uncertainties, we bootstrapped the SRF traces at each station by sampling 100 times with replacement (Efron and Tibshitani, 1986) to get the CCP images. For most study regions, the maximum uncertainty in LAB depths is estimated as <8 km for each CCP bin (Fig. 5(c)). Here the uncertainty likely results from the distribution of seismic sources, which predominantly occur around the southwest Pacific Ocean. Very different LAB depths estimated by the entire and resampled dataset typically occur in marginal areas of our study region or along discontinuous stretches of the LAB. These anomalies could arise from scarcity of LAB conversions with large ray parameters, or may reflect complexities in LAB structure, such as a dipping seismic interface. The uncertainty in LAB depth introduced by migration, using the 1-D reference and our preferred 2.5 D model, is generally less than 4 km (Fig. 5(d)). Our analysis of uncertainties indicates that variations in the LAB depths on the scale of 10 km can be well resolved. 4. Discussion Our migrated SRF image reveals strongly continuous positive conversions from the Moho at varying depths beneath NE China.

Moho depth ranges from the shallowest beneath the Songliao Basin (less than 30 km) to the deepest beneath the Great Xing’an Range (more than 40 km). For other regions of NE China, where the topography is relatively smooth, the Moho occurs at an average depth of about 35 km. The crustal thickness estimates are generally consistent with those obtained from previous PRF images (e.g., Wei and Chen, 2012; Zhang et al., 2013) as well as from deep seismic sounding data for the Great Xing’an Range (S.L. Li et al., 2006). However, the thinned crust beneath the Songliao Basin is not imaged in the corresponding PRF results (Zhang et al., 2013), and is also less than the estimation (∼32 km) derived from earlier near-vertical seismic reflection data (Yang et al., 2003; S.L. Li et al., 2006). While PRF images can provide accurate estimates of crustal thickness, it cannot resolve the Moho topography beneath the Songliao Basin (Wei and Chen, 2012). Interference from sedimentary multiples is especially problematic in the Songliao Basin due to its thick sediment accumulation. Bouguer anomalies are generally associated with Moho topography. In our study area, the crustal thickness can vary across the NSGL by up to ∼10 km, and accompanies changes in both surface topography and gravity field. The observations can be best explained by the Airy model of isostatic compensation, in which elevation is compensated by variation in crustal thickness. The mafic lower crust and upper mantle are assumed to have density values of 2.8 and 3.3 g/cm3 , respectively. Following the Airy model, a crustal thickness variation of 10 km would produce a Bouguer anomaly of −100 mGal (assuming an infinite sheet). The match between the observed Bouguer anomalies, which increase from −100 mGal in the west of the NSGL to about zero in the Songliao Basin (Ma, 1989), and Airy model rough estimates, suggests that the NSGL may extend through the entire crust. Our results also particularly illustrate a discontinuous LAB, and reveal distinct variations in lithospheric thickness between the Songliao Basin and adjacent areas. To the west, the LAB appears flat at 140–160 km depth beneath the Erguna and Xing’an blocks. If robust, there seems a decoupling between the underlying flat lithosphere and the deeper Moho across the NSGL. Then the flat LAB is truncated, rises to shallower depths of 100–120 km, and exhibits 30–40 km of lithospheric thinning beneath the Songliao basin. Further to the east, a weak LAB deepens from depths of 120 km down to 160 km. Together these observation suggest a rough symmetry within the LAB structure. A geophysical estimate of 100 km for lithospheric thickness beneath NE China however is incompatible with the geochemical xenoliths that indicate a thermal lithospheric layer at shallow depths of 40–60 km (Zhou et al., 2007). The roughly symmetric LAB structure beneath NE China also contrasts with that of adjacent regions such as the NCC. Chen (2010) reported a continuous eastward thinning of the lithosphere beneath the NCC, wherein a thicker lithosphere ranged from around 80 km to more than 200 km in the central and western NCC, but thinned to 60–100 km in the eastern NCC. Another difference between the LAB structures for these two regions is the extent of lithospheric thinning in rifted areas. In the eastern margin of the NCC, the thinnest lithosphere ranges from only 60 to 70 km, far below the ∼100–120 km estimate for the rifted Songliao Basin. It has been proposed that the NSGL likely marks a major change in the nature of the subcontinental lithosphere (Griffin et al., 1998). A sharp 20–40 km change in the LAB depth over a lateral distance of ∼100 km, which typically occurs 100–200 km east of the NSGL, has been used as evidence that the NSGL represents a pre-existing lithosphere-scale structure separating the NCC into two distinct domains (Chen et al., 2008). The decoupling between the Moho and underlying lithosphere across the NSGL in NE China and its contrasting lithospheric

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pattern with that of the NCC, calls into question the nature and deep mantle structure of the NSGL. The relatively flat stretch of lithosphere spans a distance of ∼500 km from the Erguna to the Xing’an block, which could not induced by the smoothing in our CCP stacking with effective horizontal bin sizes of about 40 km. The NSGL meanwhile spans a width of about 100 km, which exceeds the lateral resolution that SRFs can achieve. Following the Fresnel zone width calculation used by Lekic et al. (2011), and given the dominant S-wavelength of ∼20 km (X.Q. Li et al., 2007), spatial resolution may optimally reach ∼80 km in the horizontal dimension (Fresnel zone at 150 km depth). To the east, a pronounced thinning of ∼40 km in lithospheric thickness occurs over a horizontal distance of about 80 km between the Xing’an block and the Songliao Basin. This thinning is still 100 km east of the NSGL. In the case of lithospheric effects on gravity field variation, a thick lithosphere typically causes a positive gravity anomaly (Braitenberg et al., 2000). This is inconsistent with our observations in that the thickened lithosphere beneath the Xing’an block, which is in association with large negative Bouguer anomalies, when compared to the thinned lithosphere beneath the Songliao Basin but with nearly zero Bouguer anomalies. As mentioned above, the linear topographic relationship predicted by the Airy isostatic model can explain well the association between the thickened crust and the large negative Bouguer anomalies observed for the NSGL. Thus, the contribution arising from lithospheric thickening to the variation of the gravity field could not be dominant. Taken all this into account, a non-trans-lithospheric feature of the NSGL is suggested. Considering the low spatial resolution of the LAB image observed in boundaries of regions, it should also be pointed out that the deep mantle structure of the NSGL requires further inquiry and analysis. The NSGL spans over 3500 km from NE China to south China. Analysis of the lithospheric structure across the NSGL in south China would likely help resolve this issue. Deformation of both the crust and mantle lithosphere is directly associated with the formation of a sedimentary basin. Rift basins usually result from two different tectonic mechanisms, referred to as the pure shear model (McKenzie, 1978) and the simple shear regime (Wernicke, 1985). In the pure shear model, the lithosphere extends uniformly to form a symmetric basin. The simple shear regime however predicts asymmetric extension due to detachment between the crust and lower lithosphere. The two models can thus be distinguished based on spatial topographic variations between the crust and the underlying lithosphere, as well as the surface expression of rifting. For the Songliao Basin, the general spatial coincidence between the elevated Moho and lithospheric thinning, combined with the overlaying thick sediment accumulation may indicate lithospheric involvement in extensional processes. The lithospheric stretching can be better explained by the pure shear model, in which both the lower crust and lithosphere are vertically reduced in thickness during an extensional episode. The pure shear model is also in accord with the Cretaceous development of the Songliao Basin, which is characterized by earlystage rifting and late-stage sagging sequences (Xu et al., 2000; Meng, 2003). Thickened lithosphere in adjacent regions flanking the Songliao Basin, however, does not indicate lithospheric involvement in extensional processes affecting NE China during the Late Mesozoic. The variation in crustal and lithospheric thickness between the Songliao Basin and its adjacent areas can provide key constraints on the depth dependence of rheology, as described in Lekic et al. (2011) for rifted regions in Southern California. Assuming thicknesses of 35 km and 150 km for unrifted crust and lithosphere, respectively, and taking the ∼6 km of sedimentary fill as a proxy for the depth of the basement, gives a ratio of ∼1.4–1.7 (δ ) for unrifted to rifted crustal thickness (∼25–30 km) in the Songliao

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Basin, and a ratio of δ = 1.4 for unrifted to rifted mantle lithosphere thickness. The lithospheric stretching thus falls slightly below estimates derived from subsidence modeling for the Songliao Basin (δ = 1.6–1.9; Lin et al., 1997). Our comparable stretching estimates of crust and lithosphere would indicate similar degrees of stretching. However, inaccurate assumptions about unrifted crust and lithospheric thickness may introduce uncertainty in our stretching estimates. For the Songliao Basin, other mechanisms may induce lithospheric thinning during extensional tectonism. Based on relatively high heat flow, averaging about 2.24 HFU and a geothermal gradient of 3.11–4.8 ◦ C/100 m with a maximum of 6.2 ◦ C/100 m, measured in the Songliao Basin (Tian et al., 1992), some studies have stressed the predominance of an actively upwelling of asthenosphere or mantle plume, which would heat, deform and thin the lower lithosphere (Liu et al., 1996; Ren et al., 2002). This thermal mechanism seems unlikely from our comparable stretching in crust and lithosphere and the previous P-wave tomography studies. The P-wave tomography image does not show a significant low velocity anomaly in the mantle, even down to depths of about 200 km (C. Li et al., 2006). Some geochemical studies argue for another potential mechanism, in which lithospheric thinning and extension in the Songliao Basin may result from partial melting of depleted mantle. This mechanism could be induced by a removal of an antecedent thickened mantle root (Song et al., 2010; Wu et al., 2011). Antecedent lithospheric thickening has been interpreted from volcanic rocks, which are characterized with low contents of Ti and Ta and enrichment in K, Pb, signatures of an orogenic nature origin (Zhao et al., 1998). If thinning was primarily limited to the lowermost lithosphere or peeling of the mantle lithosphere along the Moho, this mechanism also cannot consistently explain the comparable degrees of stretching in the crust and sub-Moho (mantle) lithosphere inferred for the Songliao Basin. Thus, additional tectonic mechanisms may have acted. Lithospheric removal cannot be ruled out, however, since prior removal would not preclude subsequent extensional thinning. At this point we cannot determine whether several different mechanisms occurred separately or acted together. 5. Conclusion SRF analysis of recently acquired broadband seismic data provided crustal and lithospheric thickness estimates for the Songliao Basin and adjacent regions in NE China. Our migrated images show two distinct signals with large amplitudes. The strong shallow positive Sp phase from the Moho, is continuous and occurs at an average depth of around 35 km. The Moho deepens to depths of more than 40 km beneath the Great Xing’an Range, in contrast to the shallower, less than 30 km depth beneath the Songliao Basin. Our results indicate a thinner crust beneath the Songliao Basin than that estimated from previous PRF studies. We interpret the deeper negative phase as the lithosphere– asthenosphere boundary (LAB). This discontinuous feature appears strong and flat at depths of 140–160 km beneath the Erguna and Xing’an blocks. The LAB then abruptly appears at relatively shallower depths of approximately 100–120 km beneath the Songliao Basin. Further to the east, the LAB conversion becomes a relatively weak feature that occurs at gradually increasing depths of 120–160 km. Lack of coincidence between the flat LAB and a relatively deep overlying Moho beneath the Great Xing’an Range suggests that the NSGL may be a sharp crustal boundary, rather than a trans-lithospheric structure. The image results also show a rough LAB symmetry beneath NE China, which contrasts with that of adjacent regions such as the North China Craton (NCC). For the Songliao Basin, the spatial coupling of thinning between the crust and underlying lithosphere,

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along with the overlying thick sediment accumulation reflects stretching according to the pure shear model rather than the simple shear regime. For other regions of NE China, similar thicker lithosphere indicates that these regions were not significantly affected by extensional processes. Our study provides important constraints for resolving different interpretations for lithospheric thinning beneath the Songliao Basin. Acknowledgements Seismic data were provided by the Data Management Centre of China, National Seismic Network at the Institute of Geophysics, China Earthquake Administration. This work was supported by the NSF of China (Grant Nos. 90814013 and 40974061), the international cooperation project of the Ministry of Science and Technology of China (2011DFB20210), and the Special Funds for Sciences and Technology Research of Public Welfare Trades (Sinoprobe02-03). We sincerely thank two anonymous reviewers whose suggestions significantly improved the manuscript. We would also like to thank H.Y. Yuan and L.P. Zhu for providing the deconvolution and CCP codes, respectively. References Abt, D.L., Fischer, K.M., French, S.W., Ford, H.A., Yuan, H.Y., Romanowicz, B., 2010. North American lithospheric discontinuity structure imaged by Ps and Sp receiver functions. J. Geophys. Res. 115, B09301. http://dx.doi.org/10.1029/ 2009jb006914. Ai, Y.S., Zheng, T.Y., Xu, W.W., He, Y.M., Dong, D., 2003. A complex 660 km discontinuity beneath Northeast China. Earth Planet. Sci. Lett. 212, 63–71. An, M.J., Shi, Y.L., 2006. Lithospheric thickness of the Chinese Continent. Phys. Earth Planet. Inter. 159, 257–266. Braitenberg, C., Zadro, M., Fang, J., Wang, Y., Hsu, H.T., 2000. The gravity and isostatic Moho undulations in Qinghai–Tibet plateau. J. Geodyn. 30, 489–505. Chen, L., 2010. Concordant structural variations from the surface to the base of the upper mantle in the North China Craton and its tectonic implication. Lithos 120, 96–115. Chen, L., Wang, T., Zhao, L., Zheng, T.Y., 2008. Distinct lateral variation of lithospheric thickness in the northeastern North China Craton. Earth Planet. Sci. Lett. 267, 56–68. Didenko, A.N., Mossakovskii, A.A., Pecherskii, D.M., Ruzhentsev, S.V., Samygin, S.G., Kheraskova, T.N., 1994. Geodynamics of the Central-Asian Paleozoic oceans. Russ. Geol. Geophys. 35 (7–8), 48–61. Dueker, K.G., Sheehan, A.F., 1997. Mantle discontinuity structure from midpoint stacks of converted P and S waves across the Yellowstone hotspot track. J. Geophys. Res. 102 (B4), 8313–8327. http://dx.doi.org/10.1029/96JB03857. Efron, B., Tibshitani, R., 1986. Bootstrap methods for standard errors, confidence intervals, and other measures of statistical accuracy. Stat. Sci. 1, 54–77. http:// dx.doi.org/10.1214/ss/1177013815. Faure, M., Natlin, B., 1992. The geodynamic evolution of the Eastern Eurasian margin in Mesozoic times. Tectonophysics 208, 397–411. Fischer, K.M., Ford, H.A., Abt, D.L., Rychert, C.A., 2010. The lithosphere–asthenosphere boundary. Annu. Rev. Earth Planet. Sci. 38, 551–575. http://dx.doi.org/ 10.1146/annurev-earth-040809-152438. Gao, S., Rudnick, R.L., Carlson, R.W., McDonough, W.F., Liu, Y.S., 2002. Re–Os evidence for replacement of ancient mantle lithosphere beneath the North China Craton. Earth Planet. Sci. Lett. 198, 307–322. Graham, S.A., Hendrix, M.S., Johnson, C.L., Badamgarav, D., Badarch, G., Amory, J., Porte, M., Barsbold, R., Webb, L.E., Hacker, B.R., 2001. Sedimentary record and tectonic implications of Mesozoic rifting in southern Mongolia. Geol. Soc. Am. Bull. 113, 1560–1579. Griffin, W.L., Zhang, A.D., O’Reilly, S.Y., Ryan, C.G., 1998. Phanerozoic evolution of the lithosphere beneath the Sino–Korean Craton. In: Flower, M.F.J., Chung, A.L., Lo, C.H., Lee, T.Y. (Eds.), Mantle Dynamics and Plate Interactions in East Asia. In: Am. Geophys. Union, Geodyn. Ser., vol. 27, pp. 107–126. Helffrich, G., 2006. Extended-time multitaper frequency domain cross-correlation receiver-function estimation. Bull. Seismol. Soc. Am. 96 (1), 344–347. http:// dx.doi.org/10.1785/0120050098. Hu, J.F., Xu, X.Q., Yang, H.Y., Wen, L.M., Li, G.Q., 2011. S receiver function analysis of the crustal and lithospheric structures beneath eastern Tibet. Earth Planet. Sci. Lett. 306 (1–2), 77–85. Huang, Z.X., Su, W., Peng, Y.J., Zheng, Y., Li, H.Y., 2003. Rayleigh wave tomography of China and adjacent regions. J. Geophys. Res. 108. http://dx.doi.org/ 10.1029/2001JB001696. Kennett, B.L.N., Engdahl, E.R., 1991. Travel times for global earthquake location and phase identification. Geophys. J. Int. 105, 429–465.

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