PUBLICATIONS. Journal of Geophysical Research: Solid Earth

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1 PUBLICATIONS Journal of Geophysical Research: Solid Earth RESEARCH ARTICLE Key Points: Three-kilometer Moho offset across western Weihe Graben Eastward extrusion of the Tibetan crust along the Qinling orogenic belt Weihe Fault separates the stable Ordos Plateau and active Qinling Orogen Correspondence to: Y. J. Chen, Citation: Tang, Y., S. Zhou, Y. J. Chen, E. Sandvol, X. Liang, Y. Feng, G. Jin, M. Jiang, and M. Liu (215), Crustal structures across the western Weihe Graben, North China: Implications for extrusion tectonics at the northeast margin of Tibetan Plateau, J. Geophys. Res. Solid Earth, 12, , doi:. Received 25 APR 214 Accepted 1 JUN 215 Accepted article online 12 JUN 215 Published online 14 JUL 215 Crustal structures across the western Weihe Graben, North China: Implications for extrusion tectonics at the northeast margin of Tibetan Plateau Youcai Tang 1,2, Shiyong Zhou 1, Y. John Chen 1, Eric Sandvol 3, Xiaofeng Liang 4, Yongge Feng 1, Ge Jin 1, Mingming Jiang 4, and Mian Liu 3 1 Institute of Theoretical and Applied Geophysics, School of Earth and Space Sciences, Peking University, Beijing, China, 2 State Key Laboratory of Petroleum Resource and Prospecting, and Unconventional Natural Gas Institute, China University of Petroleum, Beijing, China, 3 Department of Geological Sciences, University of Missouri, Columbia, Missouri, USA, 4 Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing, China Abstract The stable Ordos Plateau, extensional Weihe Graben, and Qinling orogenic belt are located at the northeast margin of the Tibetan Plateau. They have been thought to play different roles in the eastward expanding of the Tibetan Plateau. Peking University deployed a linear seismic array across the western end of the Weihe Graben to investigate the crustal structures of the tectonic provinces of this structure. Receiver function analyses revealed low-to-moderate Poisson s ratios and anticorrelations between Poisson s ratios and topography beneath the Qinling Orogen. These features may indicate a tectonic thickening of the felsic upper crust by foldingandthrustingwithintheqinlingorogen.we observed a strong horizontal negative signal at the midcrust beneath the Ordos Plateau which may indicate a low-velocity zone. This observation would suggest the stable cratonic Ordos Plateau had been modified due to the compression between the Tibetan Plateau and the Ordos Plateau. We also observed an abrupt 4 km Moho offset across the Weihe Fault, changing from ~44 km beneath the Ordos Plateau to ~4 km beneath the Qinling Orogen. We conclude that the Weihe Fault is a lithosphere-scale fault/shear zone, which extends into the upper mantle beneath the Weihe Graben. It acts as the major boundary separating the stable Ordos Plateau and the active Qinling Orogen American Geophysical Union. All Rights Reserved. 1. Introduction The Ordos Plateau, Weihe Graben, and Qinling Orogen are located at the northeastern margin of the Tibetan Plateau (Figure 1). They are close to each other in geography but have substantially different deformation regimes due to the differential eastward extrusion of the Tibetan Plateau and contrasting rheologies [Tapponnier and Molnar, 1977; Yin and Harrison, 2; Jiang et al., 211]. Investigations of the differences in crustal structures between the three tectonic provinces could provide better constraints for the mechanism of eastward expansion of the Tibetan Plateau. The origin of the rift system surrounding the Ordos Plateau and associated seismicity is likely related to the differential eastward extrusion of the Tibetan Plateau [Xu and Ma, 1992]. The Ordos Plateau has been a tectonically stable block within the North China Craton. It is nearly aseismic and does not have any significant internal deformation [Zhang et al., 22], although along the edges of this stable block there is western compression and the southeastern extension [C. Wang et al., 214]. A series of extensional grabens, such as the Weihe Graben and the Shanxi Graben, have developed around the Ordos Plateau during the Cenozoic (Figure 1). To the southwest of the Ordos Plateau, a series of parallel NW-SE trending mountain ranges, e.g., Liupanshan Mountain (Figure 1), have been developed due to the compression between the Tibetan Plateau, the Ordos Plateau, and the restraining bend associated with the Haiyuan fault zone [Tapponnier et al., 21]. The Weihe Graben is part of the rift system that surrounds the Ordos Plateau (Figure 1). In contrast to the tectonic quiescence of the Ordos Plateau, the Weihe Graben is characterized by high seismicity and strong internal deformation (Figure 1). It has been tectonically active for the past 5 Ma and has experienced a number of stages of extension, which have resulted in thick sediments with a maximum thickness of 7 km. The latest event was a NNW-SSE extension during the Late Pliocene Quaternary [Mercier et al., 213]. TANG ET AL. MOHO OFFSET AT WESTERN WEIHE GRABEN 57

2 Figure 1. (a) Topography map of the study region. Light blue dots are earthquakes from China Earthquake Data Center catalog during the last 3 years. Three big historical earthquakes (Ms 6.75) are also shown with focus mechanism [Wan et al., 26]. Red triangles are seismic stations. White lines denote active faults. H. F.: Haiyuan Fault, W. F.: Weihe Fault, Q. F. F.: Qinling Front Fault, Liupanshan M.: Liupanshan Mountain belt. The red line denotes the location of a reflection/ refraction profile [Ren et al., 212]. The green box shows the region of Figure 2. The inset shows a larger geological map that indicates the study region by a black box. N.C.: North China Craton, S.C.: South China Craton. The Weihe Fault is the main fault zone in the middle of the Weihe Graben (Figure 1). It has developed in early Proterozoic and has controlled the initiation and development of the Weihe Graben [Peng, 1992]. The fault separates the basement of the Weihe Graben with a high dip angle of 65 8 [Feng et al., 23; Peng, 1992]. The basement north of the Weihe Fault is mainly composed of early Paleozoic and Mesozoic rocks, while the basement south of the Weihe Fault consists mainly of Archaeozoic/Proterozoic metamorphic rocks and Mesozoic granites [Peng, 1992]. The Weihe Fault is also an active seismic zone (Figure 1) [Shi et al., 27] with 7% of the earthquakes in the Weihe Graben associated with the fault [Peng, 1992]. Source mechanisms of three large historical earthquakes occurred on this fault show large extensional components with left-lateral strike-slip components (Figure 1) [Wan et al., 26]. To the south of the Weihe Graben is the Qinling Orogen, which has been developed during the Late Triassic collision between North China Craton and South China Craton [Faure et al., 28; Li et al., 1993; Ping et al., 213].TheQinlingOrogenhasbeenreactivated during the Cenozoic [Ratschbacher et al., 23; Enkelmann et al., 26]. The apparent eastward decrease in the amount of surface uplift, as well as the onset age and the amount of exhumation within the Qinling Orogen, has been thought to be related to the eastward extrusion of the Tibetan Plateau [Enkelmann et al., 26]. These data can be interpreted in terms of two competing models: crustal shortening model [Tapponnier et al., 21] and lower crust flow and/or extrusion model [Clark and Royden, 2]. In the crustal shortening model, rapid uplift and exhumation in the western Qinling have resulted from thrusting and folding in crustal shortening and thickening. The Tibetan Plateau may be growing eastward faster in the western Qinling than in the Ordos Plateau and the Sichuan Basin. Alternatively, the lower crustal flow and/or extrusion model predict/predicts that TANG ET AL. MOHO OFFSET AT WESTERN WEIHE GRABEN 571

3 Figure 2. (a) Distribution of the earthquakes (green dots) used for receiver function calculations. The red star is the center of the seismic array. (b) Piercing points at 4 km depth of the raypath of receiver functions are shown as purple circles. The location of the profile is shown as a green box in Figure 1. Red triangles denote the seismic stations. the uplift and exhumation in the western Qinling are maintained by the dynamic pressure from the mafic lower crust flow and/or extrusion. The difference between the crustal shortening model and lower crust flow and/or extrusion model lies in whether the felsic upper crust or the mafic lower crust dominates the uplift of the Qinling Orogen. Since the mafic lower crust has a higher Vp/ Vs ratio or Poisson s ratio than the upper crust, a study of the relationship of Poisson s ratio and topography may provide a way to discriminate between the two models for the uplift of the Qinling Orogen. In this study, we investigated the crustal structure across the western part of the Weihe Graben from receiver function analyses of earthquake data recorded by a dense linear portable seismic array operated by Peking University. The differences in the crustal structures between the two tectonic provinces surrounding the Ordos Plateau, Weihe Graben, and Qinling Orogen may provide us clues as to the role of the Tibetan plateau growth on the surrounding tectonic provinces. 2. Data and Method Peking University operated a linear seismic array of 15 portable broadband seismic stations (Figure 1) from September 25 to August 26. The array started from the southwestern part of the Ordos Plateau (stations N1 N5), crossed the western end of the Weihe Graben (stations N6 N9), and then extended southward into the Qinling Orogen (stations N1 N15) (Figure 2). Stations N13, N14, and N15 were deployed 3 months later than others and operated only for 9 months. The average station spacing was about 1 km. Each station was equipped with a Guralp CMG-3ESP seismometer andareftek13digital acquisition system. We used P wave receiver functions to investigate the crustal structure along the seismic array. P wave receiver functions are used to isolate the P to S converted arrivals (Ps) at discontinuities beneath seismic stations and are frequently used to constrain the depths of the discontinuities [Ammon, 1991; Cassidy, 1992; Langston, 1977]. Teleseismic events with epicentral distances of 3 to 9 with high signal-to-noise ratios (SNR) (Figure 2) were used to calculate receiver functions. We first rotated the two horizontal components of seismograms to radial and transverse components. Then the waveforms were filtered using a band-pass filter between.5 Hz and 1 Hz. Radial receiver functions were calculated by deconvolving the vertical components from the radial components. The deconvolution was performed using an iterative time-domain method [Ligorria and Ammon, 1999]. A Gaussian low-pass filter was applied to radial receiver function to minimize high-frequency noise. To ensure self-consistency, we subjected each trace of receiver function to two Gaussian filters at 1.2 Hz and 2.4 Hz, respectively. Then receiver functions with low SNR were eliminated by visual inspection. TANG ET AL. MOHO OFFSET AT WESTERN WEIHE GRABEN 572

4 Low frequency RF (1.2 Hz) Ps2 Ps1 Ps (a) High frequency RF (2.4Hz) Ps2 Ps1 Ps (b) Station Number N1 N2 N3 N4 N5 N6 N7 N8 N9 N1 N11 N12 N13 N14 N15 T=5.86s T=5.86s Qinling Orogen Weihe Graben Ordos Plateau Time (s) Time (s) Figure 3. Stacked radial receiver functions at each station. The vertical axis is the station number which increases from north to south (Figure 2b). The Ps phases at Stations N1 N14 are about.74 s earlier than other stations (see the vertical reference line t = 5.86 s). Waveforms are (a) low (1.2 Hz) and (b) high (2.4 Hz) frequency-stacked receiver functions, respectively. RF: receiver function. The number above each trace indicates how many RFs were stacked at that station. Finally, a total of 831 receiver functions with high SNR from 153 earthquakes were retained. The number of high-quality receiver functions at each station varies from 22 to 111 (Figure 3). A 2.5-D Kirchhoff migration method [Dellinger et al., 2; Wilson and Aster, 25] was applied to image the subsurface structures along profile AA (Figure 2). The wavefield u(x, z =,t) recorded at each station was back propagated to all potential scatter points. The amplitudes of radial receiver functions were then scaled and summed onto the scatter points according to the Kirchhoff integral [Dellinger et al., 2; Wilson and Aster, 25]: ux; ð zþ ¼ 1 1 cos θ 2 2π S tr ux; ð z ¼ ; tþdx (1) vr t where v is the root-mean-square (RMS) velocity at the imaging point (x, z), r is the distance between the station and imaging point, t is the lapse time of receiver functions, θ is the incident angle of the raypath at the surface. The weight S tr varies as a function of the angle of raypaths of direct wave and scattering wave sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi X n recorded at the station. RMS ¼ t i V 2 i = Xn is a travel time weighted average velocity from the surface i¼1 t i i¼1 to the imaging point [Allaby and Allaby, 1999], n is the number of the layers above the point, t i and V i are travel time and velocity for S wave in each layer [Wilson and Aster, 25]. We also used a slant stack method to estimate the local average Vp/Vs ratio (κ) and the crust thickness (H) [Zhu and Kanamori, 2]. This method searched the maximum of the stacked receiver function amplitudes in the H κ space. For a homogeneous and isotropic crust, the travel times of Ps, PpPs, and PsPs + PpSs can be expressed as T P ¼ H * sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 1 p 2 ; T S ¼ H * sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 1 p 2 (2) V 2 P T Ps ¼ T S T P ; T PpPs ¼ T S þ T P ; T PpSs ¼ T PsPs ¼ 2*T s (3) V 2 S TANG ET AL. MOHO OFFSET AT WESTERN WEIHE GRABEN 573

5 Depth (km) Ordos Vel Weihe Vel Qinling Vel Ordos RMS Weihe RMS Qinling RMS Ordos ref Weihe ref Qinling ref Low velocity zone (a) P Velocity (km/s) Amplitude (b) Hz 2.4 Hz N3.5 1 N7 N N3 N7 N Time (s) Time (s) (c) Figure 4. (a) P wave velocity models used in receiver function analyses. They are modified from a wide-angle reflection/ refraction experiment [Ren et al., 212]. Colored dots indicate the RMS velocities (6.46 km/s, 6.35 km/s, and 6.32 km/s for Ordos, Weihe, and Qinling, respectively) that were used in the slant stack process (Figure 5). RMS: root-mean-square, ref: the reference model from the reflection/refraction line, vel: the modified model from the reference model. (b and c) Comparison between observed receiver functions (solid lines) and synthetics (doted lines) from the modified models at low- (1.2 Hz) and high (2.4 Hz)-frequency range, respectively. where p is ray parameter, V P and V S are P and S waves velocities, respectively. The summation of amplitudes for the three phases can be expressed as SH; ð κ Þ ¼ ω 1 RT ð Ps Þþω 2 RT PpPs ω3 RT PpSs þpsps where R(T) is the amplitude of receiver functions at time T, ω 1 =.5, ω 2 =.35, ω 3 =.15 are the weight for the amplitudes of the three phases. The three phases will be stacked in phase if we can get the correct travel times, T Ps, T PpPs, and T PpSs + PsPs. So the point (H, κ) that has the maximum will represent the best estimation of the crust thickness and the average Vp/ Vs ratio. (4) 3. Results Figure 3 shows the stacked radial receiver functions in two different frequency ranges for each station. We first applied a moveout correction to each receiver function by aligning its Moho converted phase (Ps) to that of a reference (ray parameter of.6). Then we stacked the aligned receiver functions from all azimuths to produce one-single receiver function for each station. The number of receiver functions that were used to produce the stacked trace is labeled at the end of each trace. High-frequency receiver functions exhibit more complex waveforms in general, and they show similar trends at lower frequencies. The Ps phases converted at the Moho show three different types, which are associated with the three different tectonic provinces, the Ordos Plateau, the Weihe Graben, and the Qinling Orogen, respectively. The Ps phases at stations within the Qinling Orogen (N1 N15) are about.74 s earlier compared to stations within the Ordos Plateau and the Weihe Graben (N1 N9) (Figure 3). Stations N1 N15 have relatively narrow and simple Ps phases, while stations N1 N5 have wider or double Ps phases TANG ET AL. MOHO OFFSET AT WESTERN WEIHE GRABEN 574

6 k k k k H (km) H (km) Figure 5. Slant stack results at some stations located within the Ordos Plateau and the Qinling Orogen, respectively. The red star denotes the best estimations of the Moho depths (H) and the average ratios of Vp/Vs (κ). Blue crosses are possible estimations from bootstrap tests. at lower frequencies. The wide phases split into two or more peaks for high-frequency receiver functions (Ps1 in Figure 3). In addition to Moho conversions, stations N1 N4 also show a broad negative phase at 3 4 s Stations N6 N9 (located within the Weihe Graben) have anomalous waveforms with long duration of coda, which may be caused by the reverberations within the thick sedimentary layer of the Weihe Graben. The receiver functions also display significant differences in the first pulses of the time series: stations N1 N9 have widening and asymmetric first pulses (Ps2 in Figure 3) at the lower frequency range while stations N1 N15 have thin and simple first pulses. The widening firstpulsesatstationsn1 N9 split into two peaks at the high-frequency range. TANG ET AL. MOHO OFFSET AT WESTERN WEIHE GRABEN 575

7 Depth/km Topography/m Journal of Geophysical Research: Solid Earth Figure 6. Migrated image of receiver functions along profile AA (Figure 2b) with positive and negative polarity energy shown in red and blue, respectively. Topography and Poisson s ratios are shown in the top panel. The red circles and blue circles indicate Poisson s ratios and the Moho depths induced from slant stack of receiver functions, respectively. The error bars indicate the standard deviations from bootstrap tests. The two dashed lines indicate the 4 and 44 km depths. The Moho depths are plotted below the sea level (the elevations are subtracted from the depths). To estimate crustal thickness and Vp/ Vs ratio from receiver functions, a reference velocity model is required. In this study, the reference velocity model was constructed from a P wave model from a nearby NW-SE wide-angle reflection/refraction profile [Ren et al., 212] (red line in Figure 1). The profile was located relatively close to our seismic array (the closest distance is ~5 km) and crossed the same tectonic provinces (Figure 1). Therefore, it could be an appropriate representation of the velocity model beneath our seismic array (profile AA in Figure 2). We assembled three different velocity models (Figure 4) for the Ordos Plateau, Weihe Graben, and Qinling Orogen, respectively, by modifying the reference model of the reflection/refraction line. Synthetic data (dotted lines) from the modified models match the observed data (solid lines) very well. We used these modified models in the later slant stack and migration analyses. Slant stack results at several typical stations are shown in Figure 5. The RMS velocities at the Moho depths (colored dots in Figure 4) were used as the average P wave velocities for the three tectonic provinces. Stations N1 N4 (located within the Ordos Plateau) have the Moho depth of ~ 45.5 km, while stations N1 N13 (located within the Qinling Orogen) have the Moho depth of ~42. km. The difference is as much as 3.5 km. Stations N1 and N3 have a double peak in the ~3 6 s time window. We can see a second peak in the slant stacks at stations of N1 and N3. Stations N2 and N4 have widening Ps, and we can see only one peak in the slant stacks with wavy contours. The second peaks at stations N1 and N3 may come from the bottom of the low-velocity zone (Figure 4). The Vp/Vs ratios (κ) within the Ordos Plateau are systemically higher than those within the Qinling Orogen (Figure 5). Poisson s ratio σ was calculated from Vp/Vs ratio by using the formula σ =.5 * [1 1/(κ 2 1)]. The average of Poisson s ratios within the Ordos Plateau is ~.277, while it is ~.263 within the Qinling Orogen. The Poisson s ratios within the Qinling Orogen show an apparent anticorrelation with topography (Figure 6). Poisson s ratio decreases northward when the topography increases (green lines in Figure 6). However, the Poisson s ratio within the Ordos Plateau does not vary significantly. A migration image of receiver functions along the profile AA (Figure 2b) is also shown in Figure 6. This image was constructed by using the 2.5-D Kirchhoff migration method [Dellinger et al., 2; Wilson and Aster, 25]. TANG ET AL. MOHO OFFSET AT WESTERN WEIHE GRABEN 576

8 Depth (km) Depth (km) Depth (km) N1 N11 N Latitude Figure 7. Migrated images for the three stations around the Weihe Fault. Positive and negative polarity energy of the receiver functions are shown in red and blue, respectively. The dashed black lines indicate the 4 and 44 km depths, respectively. The migration processes were performed within the three tectonic provinces separately by using their RMS velocities (Figure 4) and then were integrated together to compose one image. We observed an abrupt offset (~4 km) in the Moho depths beneath the Weihe Graben, changing from about ~44 km depth beneath the Ordos Plateau to about ~4 km depth beneath the Qinling Orogen. These migrated Moho depths are consistent with the slant stack results. There is a significant difference in crustal structures between the two sides of the Weihe Graben. The Qinling Orogen has a relatively simple crustal structure, while the Ordos Plateau has a large/strong horizontal negative polarity signal in the middle part of the crust with a velocity increase above the Moho. 4. Discussion Station N9 has the most complicated receiver function (Figure 3) which could make the interpretation of an abrupt Moho offset a bit dubious; therefore, we produced two different migration images: one with N9 included and one without N9. Because N9 has fewer records (only 29 high-quality receiver functions) and consequently has less effect on the final result, and the two images show trivial differences. To avoid ambiguity, the migration image in Figure 6 does not include N9. The migration image in Figure 6 used different velocity profiles for the Ordos Plateau, Weihe Graben, and the Qinling Orogen, which are shown in Figure 4a. To test the effect of different velocity profiles on the Moho offset, we generated a new migration image with a laterally constant velocity model. The new image did not show significant difference except for the Moho depths. The Moho offset was ~.4 km larger than that in Figure 6 when we used a laterally constant velocity profile. Therefore, the Moho offset should not be due to the use of different velocity models. To test the resolution of the observed Moho offset (Figure 6), we generated migration images by using the receiver functions at some stations near the offset (Figure 7). Station N8 to the north of the offset shows that the Moho depth is ~44 km, while Station N11 to the south of the offset shows that the Moho depth is ~4 km. Station N1 located just above the offset shows a difference in Moho depths: receiver functions from azimuths that sample the Ordos Plateau/Weihe Graben show a Moho depth of ~44 km while receiver functions sampling the Qinling Orogen show a Moho depth of ~4 km. These receiver functions recorded at station N1 have almost common raypaths in the uppermost crust; therefore, the difference in Moho depth between the receiver functions that sampled different units should be the real difference in Moho depths between different units. The Ordos Plateau and the Weihe Graben have thick sediments (Figure 4), which could have substantial effect on the receiver functions (Figure 3) and may introduce errors in migration images [Schulte-Pelkum and Ben-Zion, 212]. We made synthetic receiver functions from two different models (Figure 8). Model 1 was TANG ET AL. MOHO OFFSET AT WESTERN WEIHE GRABEN 577

9 1. (a) (c).8 Amplitude Time (s) (b) Depth (km) 2 Amplitude Time (s) P velocity (km/s) Figure 8. Synthetic receiver functions in (a) high-frequency range and (b) low-frequency range generated from (c) two models. Ps: conversions at the Moho, Ps_s: conversions at sedimentary layers. modified from the Qinling model in Figure 4. Model 2 was composed by replacing the near-surface layers in Model 1 by the low-velocity layers modified from the Ordos model in Figure 4. We subjected each receiver function to low-pass Gaussian filters at 1.2 Hz and 2.4 Hz, respectively. Synthetic tests show that near-surface layers with low seismic velocities can produce widening near-zero pulse (low-frequency range) or split pulses (high-frequency range) and can introduce a delay into all subsequent phases in the receiver functions (Figure 8). The time delay of Ps phases introduced by the near-surface low-velocity layers is about.13 s, which is much less than the observed.74 s time difference of Ps phases between the Ordos Plateau and the Qinling Orogen (Figure 3). Consequently, we can largely rule out the possibility that the observed Moho offset was caused by the velocity contrast across the Weihe Fault. The observed Moho offset between the Ordos Plateau and the Qinling Orogen is consistent with the results from other profiles across the central part of the Weihe Graben [C. Wang et al., 214; P. Wang et al., 214; Xu et al., 214; Ren et al., 212] where the graben has been well developed. Ambient noise tomography results show that the crustal thickness beneath the Ordos Plateau is much larger than that beneath the Qinling Orogen [Zheng et al., 21, 212]. Receiver function studies across the central part of the Weihe Graben also show Moho depth changes significantly across the Weihe Fault [C. Wang et al., 214]. Some of the Moho depths beneath the Weihe Graben [P. Wang et al., 214] may correspond to a large fault/shear zone cutting through the Moho. A wide-angle reflection/refraction profile across the central part of the Weihe Graben revealed a sharp change in the Moho depths from 45 km to 35 km beneath the Weihe Fault [Ren et al., 212]. Furthermore, the lower crustal velocity structure changes rapidly from the Ordos Plateau to the Weihe Graben. These features are consistent with our observed Moho offset beneath the Weihe Graben. However, we did not observe a shallowing of the Moho beneath the western end of the Weihe Graben as others observed in the central part of the Weihe Graben [C. Wang et al., 214; P. Wang et al., 214] and in Shanxi Graben [Tang et al., 21]. One possible reason is that the western end of the Weihe Graben is in the early stages of the basin formation, while the central part of the Weihe Graben is more mature. TANG ET AL. MOHO OFFSET AT WESTERN WEIHE GRABEN 578

10 In addition to observed changes in crustal thickness, we observe significant changes in crustal complexity across the Weihe Fault. The crustal structure beneath the Qinling Orogen is simple, while the structure beneath the Ordos Plateau is more complicated. We observed a prominent horizontal negative signal at the midcrust above the Moho beneath the Ordos Plateau. The modified velocity model within the Ordos Plateau requires a low-velocity zone between the depths of 2 3 km (Figure 4a) to match the negative polarity conversions at stations N1 N4 within the Ordos Plateau (shaded region in Figure 3). Though the negative polarity conversions may be multiples for a shallower anomaly, the low-velocity zone is consistent with the results of the reflection/refraction profile which had revealed a low-velocity zone between the depths of 15 and 25 km (Figure 4). Synthetic tests show that sediments may introduce negative pulses after the direct P wave (Figure 8); however, the amplitudes of the negative pulses are much smaller than the Ps pulses. The amplitudes of the negative pulses in the observed receiver functions (Figure 4) are as large as the Ps pulses, so we conclude that these negative pulses indicate a low-velocity zone rather than multiples from shallow sediments. We suggest that this strong horizontal negative signal is caused by a low-velocity zone (LVZ) in the middle crust beneath the Ordos block. The LVZ appears to terminate at 3 km depth in the lower crust where an increase in velocity was inferred above the Moho. The existence of a LVZ in the crust may indicate the stable cratonic Ordos Plateau had been modified due to the compression between the Tibetan Plateau and the Ordos Plateau [Tapponnier et al., 21]. Other studies also suggest that there are large changes in crustal properties across the Weihe Fault. A seismic amplitude tomography study concluded that the Qinling Orogen showed high attenuation, while the Ordos Plateau showed little attenuation [Hearn et al., 28]. Ambient noise tomography revealed large velocity contrasts in the lower crust across the Weihe Fault [Bao et al., 213]. Shear wave splitting has shown that the anisotropy is weak north to the Weihe Fault but increases sharply southward up to the Weihe Fault [Huang et al., 28]. GPS measurements have shown a discontinuity in horizontal crustal displacement across the Weihe Fault [Dai et al., 24], and the vertical deformation across the fault near Baoji (Figure 1) reached 13 mm [Peng, 1992]. Given these observations, we conclude that Weihe Fault is a lithosphere-scale fault/shear zone, which cuts through the Moho producing the ~3 km offset imaged in this study (Figure 6). It may act as a boundary of the different responses between the Ordos Plateau and the Qinling Orogen to the eastward extrusion of the Tibetan Plateau [Xu and Ma, 1992]. The Poisson s ratios within the Qinling Orogen range from.258 to.269, which are relatively low compared to the value of ~.277 within the Ordos Plateau. These features are consistent with previous receiver function results [Pan and Niu, 211; C. Wang et al., 214] and refraction study [Liu et al., 26] in the central Weihe Graben. In general, mafic lower crustal rocks have higher Poisson s ratios than felsic upper crustal rocks. Low-to-moderate Poisson s ratios and the anticorrelation of the Poisson s ratio and topography within the Qinling Orogen (Figure 6) may indicate tectonic thickening of the felsic upper crust by folding and thrusting, which is the dominant mechanism within the Qinling Orogen since the Cenozoic. This is consistent with ambient noise tomography results that the Qinling Orogen has very thick upper crust and very thin lower crust [Zheng et al., 21]. The observation of thickening of the upper crust prefers the crustal shortening model rather than lower crust flow and/or extrusion model for the uplift of the Qinling Orogen. 5. Conclusions We investigated crustal structures along a linear seismic array across the western end of the Weihe Graben and its surrounding areas using receiver functions. The results revealed the following features of the crust beneath the study area: 1. Low-to-moderate Poisson s ratios and anticorrelations between Poisson s ratios and topography within the Qinling Orogen indicate significant tectonic thickening of the felsic upper crust by folding and thrusting. This observation is consistent with a model of eastward expansion of the Tibetan crust along the Qinling orogenic belt which produced the highest elevation in the western Qinling. 2. We observed a strong horizontal negative signal at the midcrust beneath the Ordos Plateau which we interpreted as a LVZ in the midcrust and lower crust. The existence of a LVZ in the crust may indicate TANG ET AL. MOHO OFFSET AT WESTERN WEIHE GRABEN 579

11 the stable cratonic Ordos Plateau had been modified due to the compression between the Tibetan Plateau and the Ordos Plateau. 3. We observed a sharp 3 km offset in the Moho depths beneath Weihe Fault across the western part of the Weihe Graben. The receiver functions also revealed different styles of the crustal structures across the Weihe Graben. We conclude that Weihe Fault may be a lithosphere-scale fault/shear zone, which extends into the upper mantle beneath the Weihe Graben. It acts as the major boundary separating the differential deformational regimes between the stable Ordos Plateau and the active Qinling Orogen. Acknowledgments Continuous seismic data used in this study are available at Peking University. Please contact Yongshun John Chen (johnyc@pku.edu.cn) if you are interested in the data. We would like to thank Qingliang Wang at the Second Monitoring and Application Center, CEA, Xi an, China for providing help in the field work. 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