SM

X(S

a

ARRA

KROYHCML

1

eCatMl1siNt

neOraa

0d

Physics of the Earth and Planetary Interiors 184 (2011) 186–193

Contents lists available at ScienceDirect

Physics of the Earth and Planetary Interiors

journa l homepage: www.e lsev ier .com/ locate /pepi

tructure of crust and upper mantle beneath the Ordos Block and the Yinshanountains revealed by receiver function analysis

iaobo Tian ∗ , Jiwen Teng, Hongshuang Zhang, Zhongjie Zhang, Yongqian Zhang, Hui Yang, Keke Zhangphd.Editor)tate Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, No. 19, Beitucheng Xilu, Chaoyang, Beijing 100029, China

r t i c l e i n f o

rticle history:eceived 23 January 2010eceived in revised form 22 October 2010ccepted 17 November 2010

eywords:eceiver functionrdos Block

a b s t r a c t

A temporary seismological network of broadband three-component stations has been deployed N–S toinvestigate the crust and upper mantle structure across the Ordos Block and the Yinshan Mountains.P wave receiver functions reveal the Moho depth to be about 41 km beneath the central Ordos Blockand down to 45 km beneath the northern Ordos Block, a slight uplifting to 42–43 km beneath the HetaoGraben, increasing to 47–48 km beneath the Yinshan Mountains and then decreasing to 44 km beneaththe northern Yinshan Mountains along the profile. In the Ordos Block, the crustal Vp/Vs ratio (about 1.80)south to the Hetao Graben differs from that (about 1.75) beneath the center Ordos Block. The crustal

inshan Mountainsetao Grabenrust and upper mantleantle transition zone

ower-crustal ductile flows

Vp/Vs ratio is significantly lower (about 1.65–1.70) beneath the Yinshan Mountains. The P wave receiverfunction migration imaging suggests relatively flat discontinuities at 410 and 660 km, indicating the lackof a strong thermal anomaly beneath this profile at these depths, and a low S wave velocity anomaly inthe upper mantle beneath the Hetao Graben. We suggest that the low S wave velocity anomaly may beattributable to heat and that the thermal softening advances the evolution of the Hetao Graben, while

flows

the lower-crustal ductilecrustal thickening.

. Introduction

The North China Craton (NCC) consists of two major parts (theastern NCC and the western NCC), separated by the Trans-Northhina Orogen (Fig. 1). Previous studies have suggested that therchaic lithospheric basement of the eastern NCC reactivated inhe Late Mesozoic and Cenozoic (e.g., Deng et al., 2004; Fan and

enzies, 1992; Griffin et al., 1998; Xu, 2001). The observations ofow Bouguer gravity anomalies and low heat flow (Ma, 1989; Yuan,996; Zhai and Liu, 2003) suggest that the western NCC is perhapstill stable, especially for the Ordos Block. However, an understand-ng of the nature and evolution of the mantle beneath the westernCC and the history of the NCC is hindered by the lack of data for

he western part.The Western Block consists of the Yinshan Mountains in the

orth and the Ordos Block in the south. GPS measurements (Fant al., 2003) indicate that the deformation in the interior of the

rdos Block is minor but that the motion of the Ordos Block with

espect to the adjacent Block is outstanding. The Graben basinround the Ordos Block is considered to have resulted from anctive mantle convection induced by the westward subduction of

∗ Corresponding author.E-mail address: txb@mail.iggcas.ac.cn (X. Tian).

031-9201/$ – see front matter © 2010 Elsevier B.V. All rights reserved.oi:10.1016/j.pepi.2010.11.007

transfer from the Hetao Graben to the northern Ordos Block, resulting in

© 2010 Elsevier B.V. All rights reserved.

the Pacific plate (e.g., Northrup et al., 1995; Tian et al., 1992) andthe India–Asia collision (Liu et al., 2004). Some studies suggestthat the lithosphere is stretched by the far-field effect of theIndia–Eurasia convergence (e.g., Shen et al., 2000; Xu and Ma,1992; Zhang et al., 1998).

The Hetao Graben, the eastern part of the Yinchuan–HetaoGraben, is about 400 km long and 60–100 km wide on the northernmargin of Ordos. It currently extends NWN–SES (with a backazimuth of 163◦) with an extension rate of ∼2 mm/y (Zhanget al., 1998). Structural sections across the troughs and boreholemeasurements (You, 1985) reveal the asymmetric sedimentarybasin. Since the Oligocene, the Graben has subsided, accompaniedby the uplift of the northern Ordos Block (Zhang et al., 1998).However, the nature of the forces that drove the rock uplift of thelow-relief, high-elevation, tectonically stable Ordos Block near theextensional graben basin remains unclear.

In this study, we use the P wave receiver function (RF) methodto investigate the crust and upper mantle structure with newdata collected from a dense broadband seismic station array inthe northern Ordos Block and the Yinshan Mountains (Fig. 1). We

focus on studying lateral variations in the crustal thickness, thecrustal Vp/Vs ratio, the seismic velocity in the upper mantle, andthe mantle transition zone (MTZ) thickness. Finally, we discuss theimplication of our observations for the uplift of the northern OrdosBlock.

X. Tian et al. / Physics of the Earth and Planetary Interiors 184 (2011) 186–193 187

Fig. 1. Regional map showing locations of the broadband seismic station array(green triangles) across the Ordos Block and the Yinshan Mountains. The thickdashed line shows the boundary between the North China Craton (NCC) and theCmNc

2

wtbodYfdstrseuIud

qrcctaAbitRaa

a

entral Asian Orogenic Belt (CAOB). The bottom inset shows the simple tectonicap and the location of the study region in NCC. WNCC, western NCC; TNCO, Trans-orth China Orogen; ENCC, eastern NCC. (For interpretation of the references toolor in the text, the reader is referred to the web version of the article.)

. Data and methods

A 20-element broadband three-component digital seismic arrayas deployed along 110◦E from 36◦N to 43◦N at the Ordos Block,

he Yinshan Mountains, and the Hetao Graben (Fig. 1) from Octo-er 2006 to April 2008, moving three times and with each locationperating for 6 months. Most stations were located along the N–Sirection, almost perpendicular to the tectonic strikes, such as theinshan Mountains and the Hetao Graben. The teleseismic P wave-

orm contains S waves generated by P-to-S conversion at velocityiscontinuities in the crust and upper mantle beneath the seismictations. RFs are the radial waveforms created by deconvolvinghe vertical component from the radial component to isolate theeceiver site effects from other information contained in the tele-eismic P waveforms (Ammon, 1991). RF analysis has become anffective way to study the S wave velocity structure of crust andpper mantle (Wu et al., 2007; Zhang et al., 2010; Zhu et al., 2006).

n this study, we use the N–S RF profile to study the crust andpper mantle structure. The RF method is used to image seismiciscontinuities, such as Moho and 410- and 660-km discontinuities.

To calculate RF, we select teleseismic P waveforms from earth-uakes with magnitudes Mw ≥ 5.3 and in the epicentral distanceange of 30–90◦. After rotation of the three observed Z, N, and Eomponents to Z, R (radial direction), and T (transverse direction)omponents, the radial RF is calculated by the spectrum division ofhe Z component from the R component in the frequency domainnd inverse transformed into the time domain (e.g., Ammon, 1991).water level of 0.001 is introduced to keep frequency division sta-

le. We have also applied a low-pass filter at 0.03–0.75 Hz (formaging the crustal structure) and at 0.03–0.50 Hz (for imaginghe upper mantle structure) to remove high-frequency noise in

F. After removing poorly deconvolved RFs, we obtained a total ofbout 2200 high-quality RFs. Fig. 2 shows the P-to-S piercing pointst 40 km, 410 km, and 660 km of depth.

We produce images of the seismic discontinuities in the crustnd upper mantle along a profile AA′ (in Fig. 2) using common con-

Fig. 2. The locations of piercing points at 40-km, 410-km, and 660-km depths ofP–to-S converted phases. The solid line indicates the location of profile AA′ . The topinset shows the distribution of selected teleseismic events used in this study.

version point (CCP) stacking of the P wave RFs (e.g., Dueker andSheehan, 1998; Wu et al., 2005; Zhu et al., 2006). The procedureconsists of two steps: back projection and stacking. In the first step,we calculate the ray paths of the RFs using a background velocitymodel. The amplitude at each point on the RF is assigned to thecorresponding location on the ray path where the P-to-S conver-sion occurred, using its time delay with respect to the direct P. Thisamplitude represents the velocity change, or more precisely theimpedance change, of the medium at the conversion point. Second,we then divide the volume along the profile into certain size binsand sum all amplitudes in each bin to obtain the average amplitude.During this process, the surface topography is included to correctthe ray path and the delay time of the conversion phases after thedirect P.

3. The crustal structure

Along the profile AA′, a wide-angle seismic reflection and refrac-tion projection (WARR) was carried out during summer 2005 (Tenget al., 2010). Seismic waves excited by shot large chemical explo-sions (∼2 tons TNT for each shot) were recorded by 280 portablegeophones spaced at an average interval of about 2 km. After fittingthe travel times and amplitudes of reflected and refracted phases,Teng et al. (2010) obtained the seismic structure of the crustalP wave velocity model (see Fig. 3). Using the P wave velocity inFig. 3(b) as the reference velocity, the RF migration for the shallow

′

structure (<80 km) is performed along the profile AA , assuming aratio between the P and S wave velocities, Vp/Vs, of 1.74 in crust.During migration, the bin size is set to 10 km (horizontally alongthe profile) by 1 km (vertically). Fig. 4(b) shows the image of thecrust and the topmost part of the upper mantle. At a shallow depth

188 X. Tian et al. / Physics of the Earth and Planetary Interiors 184 (2011) 186–193

Fig. 3. (a) The topography along the profile AA′ . HG, Hetao Graben; YM, Yinshan Mountain. (b) 2D Vp model by WARR (Teng et al., 2010). The bold lines are seismic interfaces,a locitie( ust al

ootT4irsd1

rrwt1c(vact

tFWb

nd the light lines are velocity contours. The numbers in the velocity section are veTeng et al., 2010). The dashed line shows the average Vp (6.23 km/s) of all of the cr

f about 20 km, there are two clear amplitude bands: the shallowerne is positive, and the deeper one is negative, and they are the mul-iple reflections from sedimentary cover with a thickness of ∼4 km.he most significant positive amplitude band in the depth range of0–50 km along the profile indicates the Moho depth, but this result

s different from the Moho depth derived from the WARR inversionesults (Teng et al., 2010) beneath the northern Ordos and the Yin-han Mountains. This difference implies that there might be a largeiscrepancy between the crustal Vp/Vs ratio and the assumption of.74 in this region.

Based on the discrepancy in Moho depth, as shown between theed dashed line (imaging by RFs with the assumption of a Vp/Vsatio of 1.74) and black dashed line (inversion by WARR) in Fig. 4(b),e estimate the crustal Vp/Vs ratio along the profile. Fig. 4(d) shows

he lateral variations in the crustal Vp/Vs ratio with an average of.732, close to the average Vp/Vs ratio over the mainland Chinarust (about 1.730) (Chen et al., 2010). A normal crustal Vp/Vs ratioabout 1.75) is obtained beneath the center Ordos Block. A highalue of 1.80 is estimated beneath the northern Ordos Block, withnormal (about 1.74) value in the Hetao Graben. To the north, the

rustal Vp/Vs ratio decreases to 1.66 beneath the Yinshan Moun-ains.

We migrate the RFs for the shallower depth (<80 km) again usinghe crustal average Vp/Vs ratio in Fig. 4(d). The Moho depth inig. 4(c) is consistent with the one obtained through inversion byARR (Teng et al., 2010). The Moho is flat at a depth of 41–42 km

eneath the center Ordos Block and then sinks slightly towards the

s in km/s. (c) The north–south variation of crustal average Vp calculated by WARRong the profile.

north beneath the northern Ordos Block. Beneath the Hetao Graben,a 2–3 km uplifting Moho can be observed. Similar uplift of the Mohohas been revealed beneath the Shanxi Rift by seismic refraction pro-filing (Sun et al., 1988; Zhang et al., 1988) and Bouguer gravity data(Xu and Ma, 1992). To the north, the Moho depth increases quicklyand up to 47–48 km beneath the Yinshan Mountains. In the north-ern Yinshan Mountains, the Moho becomes shallower again with adepth of 44 km.

However, there is always a trade-off between crustal thicknessand the Vp/Vs ratio when a single phase, Pms, is used. If the Vp andcrustal thickness are given, the crustal Vp/Vs ratio can be deducedreliably from Pms. There are some deviations of the crustal Vp andthickness ineluctable between the real crustal model and the WARRinversion results, we need to carry out several tests to illustrate thetrade-off between Moho depths and lateral variations in crustal Vpand the Vp/Vs ratio. First, using the Vp model in Fig. 3(b) with avariation of ±2% and a Vp/Vs of 1.74 as the reference velocity, theRFs migration is performed again to image the Moho. Based on thediscrepancy between the Moho depth given by the RF image and byWARR, we estimate the crustal Vp/Vs again. The dashed boundariesin Fig. 4(d) show the effect of the ±2% uncertainties of the crustalVp model on the estimation of the crustal Vp/Vs ratio; the errors

of Vp/Vs are less than about 0.017. Second, using the crustal Vp/Vsratio in Fig. 4(d) with a variation of ±1% and the Vp model in Fig. 3(b)as the reference velocity, the RF migration is performed once moreto image the Moho. The dashed boundaries in Fig. 4(e) show theeffect of the ±1% uncertainties of the crustal Vp/Vs ratio on the

X. Tian et al. / Physics of the Earth and Planetary Interiors 184 (2011) 186–193 189

Fig. 4. Crustal section of the profile AA′ by RFs. (a) The topography along the profile. HG, Hetao Graben; YM, Yinshan Mountains. (b) Ps migration by the Vp model in Fig. 3(b)and average crustal Vp/Vs ratio of 1.74. The red dashed line marks the Moho imaged by Pms. The black dashed line denotes the Moho depth obtained by WARR (Teng et al.,2010). If there are no lateral variations in the crustal Vp/Vs ratio, the Moho depths generated by RFs and WARR should mimic each other. (c) Ps migration by the Vp model inFig. 3(b) and the crustal Vp/Vs ratio estimated from (b). The black dashed line denotes the Moho depth obtained by WARR (Teng et al., 2010). (d) The crustal Vp/Vs ratio. Theb e effeT d the± ext, th

i1

4z

6

old line denotes the values estimated from (b), and the dashed boundaries show thhe bold line denotes the Moho depth migrated by the 2D Vp model in Fig. 3(b) an1% in the crustal Vp/Vs ratio. (For interpretation of the references to color in the t

mage of the Moho depth; these errors of depth are less than aboutkm.

. The structure of the upper mantle and mantle transitionone

The MTZ is bounded by a 410-km discontinuity to the top and a60-km discontinuity to the bottom. The RF method has become a

ct of a variation of ±2% in crustal Vp. (e) The north–south variation of Moho depths.crustal Vp/Vs ratio in (d); the dashed boundaries show the effect of a variation ofe reader is referred to the web version of the article.)

main tool for studying the lateral variations of the depths of 410-and 660-km discontinuities or the MTZ thickness (e.g., Li and Yuan,2003; Niu et al., 2004; Owens et al., 2000).

CCP stacking is used to image the 410- and 660-km discontinu-ities along the profile AA′. We use the same procedure as the crustimaging method described above. During migration, the bins areset as 5 km in vertical size, with horizontal size increasing with theimaging depth. Considering the Fresnel Zone, the horizontal size

190 X. Tian et al. / Physics of the Earth and Planetary Interiors 184 (2011) 186–193

Fig. 5. Migrated RF data along profile AA′ with different reference models, (a) IASP91 model (Kennett and Engdahl, 1991), (b) the S wave velocity model of Huang et al. (2009),( del ofs ariati( , ShanC

o(th

rectTaTod

c) the P wave velocity model of Huang and Zhao (2006), (d) the P wave velocity moolid lines mark 410- and 660-km depths. The gray dashed lines show the lateral ve) shows the region of low S wave velocity zone revealed by Zhao et al. (2009). SSGAOB, Central Asian Orogenic Belt.

f the bins is set as one fifth of the depth. The crustal Vp modelinversion by WARR) and average Vp/Vs ratio (estimated above) inhe crust structure are used to remove the crust seismic velocityeterogeneity effects on the deeper image.

The IASP91 model (Kennett and Engdahl, 1991) is used as theeference model of the upper mantle and MTZ, with spherical lay-rs for calculating the ray paths and the delay times between theonversion phases and the direct P. Fig. 5(a) shows the image ofhe upper mantle and MTZ by migrating RFs along the profile AA′.

he overall 410- and 660-km discontinuities approach the globalverages, except for an abrupt deepening of 10–20 km at N38–41◦.he thickness of the MTZ shows a significant variation in the rangef N37–39◦. The image of the 660-km discontinuity is 20–30 kmeeper than the global average in the range of N37–38◦, and there

Tian et al. (2009), and (e) the S wave velocity model of Zhao et al. (2009). Thin grayons of the 410- and 660-discontinuities migrated by RFs. The black dashed line inxi–Shaanxi Graben; OB, Ordos Block; HG, Hetao Graben; YM, Yinshan Mountains;

is a normal 660-discontinuity over the deeper one. Although thetwo-layer model of the 660-discontinuity has been observed inother regions (Ai et al., 2003), the possibility of falsity cannot beexcluded because an improper upper mantle velocity model mightbe used in migration along this profile. Low S velocity anomaliesalong the ray paths increase the travel time of the converted phase,and the RF migration will thus result in a deeper image than isactually the case. We suggest that the lateral depth variation of the410- and 660-km discontinuities might result from the low S wave

velocity above the 410-km discontinuity in the range of N38–41◦.

Some seismic tomography studies have suggested high P wave(or S wave) velocity beneath the Ordos Block and low P wave (or Swave) velocity beneath the Hetao Graben. In the RF studies, thereis always a trade-off between the discontinuity depths and lateral

X. Tian et al. / Physics of the Earth and Plan

Fb

vtvowNtpotkasF

tmeb(tsmInaaHiIba6

imZimvik

ig. 6. The depths of the 410- and 660-discontinuities and the MTZ thickness imagedy different reference models.

elocity variations in the crust and upper mantle. For determininghe topography of a discontinuity accurately, the lateral velocityariations should be taken into account. Huang et al. (2009) carriedut surface wave (Rayleigh wave) tomography to reconstruct the Save velocity structure of crust and upper mantle (0–300 km) in theCC. We used their S wave velocity model and the Vp/Vs ratio from

he IASP91 model to construct a new reference model in the shallowart. The IASP91 model is retained as the deeper part (>300 km)f the reference model. The CCP stacking image in Fig. 5(b) fromhe new reference velocity model indicates that the 410- and 660-m discontinuities are shallower than their nominal depths (410nd 660 km). The 410- and 660-km discontinuities also becomemoother compared to those derived from the IASP91 model inig. 5(a).

Huang and Zhao (2006) and Tian et al. (2009) used P wave arrivalimes to reveal the lateral heterogeneities in the crust and upper

antle under the NCC, respectively. We construct two new ref-rence models by replacing the shallow part of the IASP91 modely the P velocity model of Huang and Zhao (2006) and Tian et al.2009), respectively. The Vp/Vs ratio of the IASP91 model is usedo construct the S wave velocity of the reference models. The CCPtacking images in Fig. 5(c) and (d) from the new reference velocityodels show less significant difference compared to those from the

ASP91 model in Fig. 5(a), specially the 410- and 660-km disconti-uities. By using teleseismic body waves, Zhao et al. (2009) presentn S wave velocity model under the NCC and show low S velocitynomalies (up to 3%) in the upper mantle beneath the Hetao Graben.ere we introduce the S velocity model into the reference veloc-

ty models, and the new CCP stacking image is shown in Fig. 5(e).t is clear that the depths of the 410- and 660-km discontinuitiesecome very flat, with depth variations of less than 5 km, and nonomaly in MTZ thickness can be found; further, two layers of the60-km discontinuity beneath the central Ordos Basin disappear.

By comparing the depths of the 410- and 660-km discontinu-ties and the MTZ thickness imaged by different reference velocity

odels (see Fig. 6), we suggest that the S wave velocity model ofhao et al. (2009) is more appropriate than other models for remov-

ng the effects of lateral velocity variations in the crust and upper

antle from the discontinuity depths. The IASP91 and two P waveelocity models (Huang and Zhao, 2006; Tian et al., 2009) resultn similar depths and thickness, as characterized by a deep 410-m discontinuity beneath the central and north parts of the profile

etary Interiors 184 (2011) 186–193 191

and a deep 660-km discontinuity and a thicker MTZ beneath thecentral part of the profile. The slight discrepancy in depth and thick-ness is possibly the result of the low resolution of tomography andunderestimation of the S wave velocity anomaly derived from the Pwave velocity model. Using the model of surface wave tomography(Huang et al., 2009), the MTZ thickness is the same as that identifiedusing the IASP91 model, but the 410- and 660-km discontinuitiesbecome shallow. Compared with the IASP91 model, the model ofHuang et al. (2009) shows significantly low velocity anomalies inthe lower crust and in the asthenosphere, which result in the 410-and 660-km discontinuities uplift in Figs. 5(b) and 6. The low Svelocity from surface wave tomography has been suggested to beattributable to the radial anisotropy that propagates the Rayleighwave slower than the Love wave and teleseismic S wave (Fischeret al., 2010; Nettles and Dziewonski, 2008). However, the S wavevelocity model of Zhao et al. (2009) results from the teleseismic Swave, which is horizontally polarized, similar to converted phasesin RF. Using the S wave velocity model of Zhao et al. (2009), the newCCP stacking image shows the flat 410- and 660-km discontinuitiesand the MTZ without distinct lateral variation.

According to the Clapeyron slopes of the olivine (�) to wad-sleyite (�) transition and the spinel phase (ringwoodite) intoperovskite and magnesiowüstite (e.g., Bina and Helffrich, 1994),the MTZ thickness is sensitive to temperature variation; i.e., a10-km anomaly of depth in the 410- and 660-km discontinuitiescorresponds to a temperature variation of 200 K. P- and S-wavetomography (Zhao et al., 2009) has suggested that the source ofthe warm mantle material is at least as deep as the transition zone.Therefore, a possible mantle plume might exist beneath the north-eastern Ordos Block. Along the profile AA′ in Fig. 5(e), however, theobservations of the flat 410- and 660-km discontinuities and thenormal thickness of the MTZ imply no temperature anomaly in theMTZ.

5. Discussion and conclusion

In this section, we combine the results presented in Sections3 and 4 and propose a scheme in Fig. 7 to illustrate the dynamicprocesses under the study area.

A low S wave velocity zone in the upper mantle beneaththe Hetao Graben has been indicated by body wave tomogra-phy (Huang and Zhao, 2006; Zhao et al., 2009) and surface wavetomography (Huang et al., 2009). It has also been supported bythe S wave receiver functions (Chen et al., 2009) showing a sharpchange from the thick lithosphere (>200 km) beneath the stableOrdos Block to the thinned lithosphere (up to 80 km) beneaththe Hetao Graben. After introduction of the low S wave veloc-ity anomaly into the reference velocity model, the RF migrationproduces flat images of the 410- and 660-km discontinuities atthe depths predicted by IASP91 model (see Fig. 5(e)). Thus, theresults of RF migration also indirectly support the finding of alow S wave velocity anomaly in upper mantle beneath the HetaoGraben.

In addition to the partial melting after decompression in theupper mantle, the low S wave velocity zone in the upper man-tle beneath the Hetao Graben may result from high temperature.The subduction of the Pacific slab and its stagnancy in the MTZunder East Asia may produce a big mantle wedge in the uppermantle (Lei and Zhao, 2005; Zhao et al., 2007). Corner flow inthe big mantle wedge and deep slab dehydration may cause the

upwelling of the hot asthenospheric material as an origin of thehot mantle flow beneath Hetao Graben. The significant lateralextrusion of the mantle asthenosphere driven by the Indo-Asiancollision may have partly fueled the widespread asthenosphericupwelling under the NCC (including the Hetao Graben) (Liu et al.,

192 X. Tian et al. / Physics of the Earth and Plan

Fig. 7. The graphic illustrating the dynamic processes under the study area. OB,Ordos Block; HG, Hetao Graben; YM, Yinshan Mountains. The dashed lines presentthe boundary between the upper and lower crust, which were constructed by WAAR(Teng et al., 2010). LAB represents the lithosphere–asthenosphere boundary, and itsdepth is from Chen et al. (2009) and Huang et al. (2009). The region surrounded bythe red dotted line represents the low Vs and hot zone in the upper mantle. Thered arrows represent the upwelling from the hot upper mantle pushing the lowercrust upward beneath the Hetao Graben. The region in yellow denotes the bottomof the lithosphere decreasing in density after heating in the northern Ordos Block.The region in orange represents the lower-crustal ductile flows that thicken thecrust beneath the Ordos Block. The orange also denotes the high Vp/Vs ratio. Thered arrows in the thickened lower crust indicate the direction of ductile flows. ThetrM

2up

inzat(swshudCuedh

Gt2cnFeTptbicc

boundary. Annu. Rev. Earth Planet. Sci. 38, 551–575, doi:10.1146/annurev-

hick sedimentary basin beneath the Hetao Graben is shown in gray. The stripedegion shows the south-directed nappe structure in the upper crust of the Yinshanountains.

004). The hot mantle flow may be a branch from the hotpwelling beneath Datong volcano, 200–300 km east of the N–Srofile.

The hot upwelling beneath the Hetao Graben has played anmportant role in the evolution of the Graben and the uplift of theorthern Ordos Block. The Hetao Graben is located on the sutureone (about 2.0–1.9 Ga, Zhao et al. (2005)) between the Yinshannd Ordos Blocks. Extension of the continental lithosphere is foundo follow preferentially preexisting weak zones like suture zonesvan Wijk et al., 2008). Since the Oligocene, the Hetao Graben hasubsided as a result of an active mantle convection induced by theestward subduction of the Pacific plate and the India–Asia colli-

ion (Zhang et al., 1998). Thermal perturbation and re-equilibrationave been proposed as a general mechanism for driving rockplift within plate interiors, particularly in regions of thicker, moreepleted lithosphere adjacent to zones of extension, such as theolorado Plateau (Roy et al., 2009). Like the Colorado Plateau, theplift of the northern Ordos Block since the late Cretaceous (Dengt al., 1999) may have resulted from the hot upwelling, whichecreases the density in the adjacent lithospheric root because ofigh temperature (Fig. 7).

On the top of the hot upwelling, the lower crust of the Hetaoraben has been warmed and the viscosity reduced, a finding that

he low Pn velocity results can support (Liang et al., 2004; Pei et al.,007). Because of the upward push by the upwelling, the lowerrust with the low viscosity creeps from the Hetao Graben to theorthern Ordos Block. The lower-crustal ductile flows (orange inig. 7) facilitate thickening of the lower crust beneath the north-rn Ordos Block and benefit uplift of the topography in this region.his kind of scenario has been suggested for the Basin and Rangerovince (Liu and Shen, 1998), as has a lower-crustal flow belowhe Taihang Mountains just east to the Shanxi–Shaanxi Graben,

ased on P wave tomography (Chang et al., 2007). The thicken-

ng lower crust, which has more mafic composition than the upperrust, and the lower-crustal flow, which is low viscosity, togetherontribute to increase the Vp/Vs ratio beneath the northern Ordos

etary Interiors 184 (2011) 186–193

Block. Beneath the Hetao Graben, thick sedimentary basin and athin lower crust result in a low crustal average Vp (<6.0 km/s, inFig. 3(c)) and a normal crustal average Vp/Vs ratio.

According to the chronological results, a major contraction inthe Yinshan Mountains is suggested to have occurred in the Mid-dle to Late Jurassic and earlier Early Cretaceous periods (Davis et al.,2001). Its origin and relationship are a real puzzle for Mesozoic east-ern Asian plate tectonics. Some studies (Davis et al., 2001; Menget al., 2003; Yin and Nie, 1996) support that shrinkage in the Yin-shan Mountains reflects Mesozoic intraplate shortening related tonorth–south plate convergence in eastern Asia. The thick uppercrust, based on WARR (Teng et al., 2010), suggests a south-directednappe structure denoting a “thin-skinned” tectonic beneath theYinshan Mountains. With more felsic composition in the uppercrust than in the lower crust, the thickening upper crust willdecrease the crustal average Vp/Vs ratio. The low crustal averageVp/Vs ratio (see Fig. 3(e)) supports that the uplifting of the YinshanMountains results from a thickening upper crust in the Mesozoic.

Acknowledgements

The authors are grateful to Fuyun Wang, Peng Dai, and YanhangZhu for help with the field work. We are grateful to ZhongxianHuang, Jinli Huang, Liang Zhao, and You Tian for providing theirvelocity models. We thank Editor Keke Zhang and two anonymousreviewers for their constructive comments and suggestions, whichimproved the manuscript. Gratitude goes to Qingren Meng, AiminDu, Xiao Xu, Jingbo Wang, Ling Bai, and Xiaoyu Guo for helpfuldiscussions that improved the original manuscript. This researchis supported by grants from the Chinese National Natural ScienceFoundation (Nos. 40674048 to J. Teng, 40974025 to X. Tian, and40721003 to Z. Zhang). Figures were made using GMT software(Wessel and Smith, 1995).

References

Ai, Y., Zheng, T., Xu, W., He, Y., Dong, D., 2003. A complex 660 km discontinuitybeneath northeast China. Earth Planet. Sci. Lett. 212, 63–71, doi:10.1016/S0012-821X(03)00266-8.

Ammon, C.J., 1991. The isolation of receiver effects from teleseismic P waveforms.Bull. Seismol. Soc. Am. 81, 2504–2510.

Bina, C., Helffrich, G., 1994. Phase transition Clapeyron slopes and transition zoneseismic discontinuity topography. J. Geophys. Res. 99 (B8), 15,853–15,860,doi:10.1029/94JB00462.

Chang, X., Liu, Y., He, J., Sun, H., 2007. Lower velocities beneath the TaihangMountains, Northeastern China. Bull. Seismol. Soc. Am. 97. 4, 1364–1369,doi:10.1785/0120060210.

Chen, L., Cheng, C., Wei, Z., 2009. Seismic evidence for significant lateral variationsin lithospheric thickness beneath the central and western North China Craton.Earth Planet. Sci. Lett. 286, 171–183, doi:10.1016/j.epsl.2009.06.022.

Chen, Y., Niu, F., Liu, R., Huang, Z., Tkalcic, H., Sun, L., Chan, W., 2010. Crustal structurebeneather China from receiver function analysis. J. Geophys. Res. 115, B03307,doi:10.1029/2009JB006386.

Davis, G.A., Wang, C., Zheng, Y., Zhang, J., Zhang, C., Gehrels, G.E., 2001. The enigmaticYinshan fold-and-thrust belt of northern China: new views on its intraplatecontractional styles. Geology 26, 43–46.

Deng, J.F., Mo, X.X., Zhao, H.L., Wu, Z.X., Luo, Z.H., Su, S.G., 2004. A new model for thedynamic evolution of Chinese lithosphere: “continental roots-plume tectonics”.Earth Sci. Rev. 65, 223–275, doi:10.1016/j.earscirev.2003.08.001.

Deng, Q., Cheng, S., Min, W., Yang, G., Ren, D., 1999. Tectonic activity and dynamicof the Ordos block. Journal of Geomechanics (in Chinese) 5 (3), 13–21.

Dueker, K.G., Sheehan, A.F., 1998. Mantle discontinuity structure beneath the Col-orado Rocky Mountains and High Plains. J. Geophys. Res. 103, 7153–7169.

Fan, W., Menzies, M.A., 1992. Destruction of aged lower lithosphere and accretionof asthenosphere mantle beneath eastern China. Geotecton. Met. 16, 171–180.

Fan, J., Ma, J., Gan, W., 2003. Movement of Ordos Block and alternation of activityalong its boundaries. Sci. China (Series D) 46, 168–180, doi:10.1360/03dz0013.

Fischer, K.M., Ford, H.A., Abt, D.L., Rychert, C.A., 2010. The lithosphere-asthenosphere

earth-040809-152438.Griffin, W.L., Zhang, A., O’Reilly, S.Y., Ryan, C.G., 1998. Phanerozoic evolution of

the lithosphere beneath the Sino-Korean craton. In: Flower M.F.J. et al. (eds.),in: Mantle Dynamics and Plate Interactions in East Asia. AGU, Washington DC,Geodyn. Ser. 27, 107–126.

d Plan

H

H

K

L

L

L

L

L

M

M

N

N

N

O

P

R

S

S

T

X. Tian et al. / Physics of the Earth an

uang, J., Zhao, D., 2006. High-resolution mantle tomography of China and sur-rounding regions. J. Geophys. Res. 111, B09305, doi:10.1029/2005JB004066.

uang, Z., Li, H., Zheng, Y., Peng, Y., 2009. The lithosphere of North China Cra-ton from surface wave tomography. Earth Planet. Sci. Lett. 288, 164–173,doi:10.1016/j.epsl.2009.09.019.

ennett, B.L.N., Engdahl, E.R., 1991. Traveltimes for global earthquake location andphase identification. Geophys. J. Int. 105 (2), 429–465.

ei, J., Zhao, D., 2005. P-wave tomography and origin of the Changbai intraplatevolcano in Northeast Asia. Tectonophysics 397, 281–295.

i, X., Yuan, X., 2003. Receiver functions in northeast China – implications for slabpenetration into the lower mantle in northwest Pacific subduction zone. EarthPlanet. Sci. Lett. 16, 679–691, doi:10.1016/S0012-821X(03)00555-7.

iang, C., Song, X., Huang, J., 2004. Tomographic inversion of Pn travel times in China.J. Geophys. Res. 109, B11304, doi:10.1029/2003JB002789.

iu, M., Cui, X., Liu, F., 2004. Cenozoic rifting and volcanism in eastern China: amantle dynamic link to the Indo-Asian collision? Tectonophysics 393, 29–42,doi:10.1016/j.tecto.2004.07.029.

iu, M., Shen, Y., 1998. Sierra Nevada uplift: A ductile link to mantle upwelling underthe Basin and Range province. Geology 26, 299–302.

a, X.Y. (Ed.), 1989. Lithospheric Dynamic Atlas of China. Cartographic PublishingHouse, Beijing.

eng, Q.R., Hu, J.R., Jin, J.Q., Zhang, Y., Xu, D.F., 2003. Tectonics of late Mesozoic widesupracrustal extensional basin system in the China–Mongolia border region.Basin Res. 15 (3), 397–415.

ettles, M., Dziewonski, A.M., 2008. Radially anisotropic shear velocity structureof the upper mantle globally and beneath North America. J. Geophys. Res. 113,B02303, doi:10.1029/2006JB004819.

iu, F.L., Levander, A., Cooper, C.M., Lee, C.T.A., Lenardic, A., James, D.E., 2004. Seis-mic constraints on the depth and composition of the mantle keel beneath theKaapvaal craton. Earth Planet. Sci. Lett. 224, 337–346.

orthrup, C., Royden, L., Burchfiel, B., 1995. Motion of the Pacific plate relative toEurasia and its potential relation to Cenozoic extension along the eastern marginof Eurasia. Geology 23, 719–722.

wens, T.J., Nyblade, A.A., Gurrola, H., Langston, C.A., 2000. Mantle transi-tion zone structure beneath Tanzania. Geophys. Res. Lett. 27, 827–830,doi:10.1029/1999GL005429.

ei, S., Zhao, J., Sun, Y., Xu, Z., Wang, S., Liu, H., Rowe, C.A., Toksöz, M.N., Gao, X., 2007.Upper mantle seismic velocities and anisotropy in China determined through Pnand Sn tomography. J. Geophys. Res. 112, B05312, doi:10.1029/2006JB004409.

oy, M., Jordan, T.H., Pederson, J., 2009. Colorado Plateau magmatism anduplift by warming of heterogeneous lithosphere. Nature 459, 978–984,doi:10.1038/nature08052.

hen, Z., Zhao, C., Yin, A., Li, Y., Jackson, D.D., Fang, P., Dong, D., 2000. Contempo-rary crustal deformation in east Asia constrained by Global Positioning Systemmeasurements. J. Geophys. Res. 105 (B3), 5721–5734.

un, W.C., Zhu, Z.P., Zhang, L., Song, Y.J., Zhang, C.K., Zhong, Y.J., 1988. Explorationof the crust and upper mantle in north China. In: Development in the Researchof Deep Structures of China’s Continent. Geological Publishing House, Beijing,

China, pp. 19–37, (in Chinese).

eng, J.W., Wang, F.Y., Zhao, W.Z., Zhang, Y.Q., Zhang, X.K., Yan, Y.F., Zhao, J.R., Li,M., Yang, H., Zhang, H.S., Ruan, X.M., 2010. Velocity structure of layered blockand deep dynamic process in the lithosphere beneath the Yinshan orogenicbelt and Ordos Basin. Chin. J. Geophys. 53 (1), 67–85, doi:10.3969/j.issn.0001-5733.2010.01.008 (in Chinese).

etary Interiors 184 (2011) 186–193 193

Tian, Z., Han, P., Xu, K., 1992. The mesozoic–cenozoic east China rift system. Tectono-physics 208, 341–363.

Tian, Y, Zhao, D., Sun, R., Teng, J., 2009. Seismic imaging of the crust and uppermantle beneath the North China Craton. Phys. Earth Planet. Int. 172, 169–182,doi:10.1016/j.pepi.2008.09.002.

van Wijk, J., van Hunen, J., Goes, S., 2008. Small-scale convection during con-tinental rifting: evidence from the Rio Grande rift. Geology 36, 575–578,doi:10.1130/G24691A.1.

Wessel, P., Smith, W., 1995. New Version of the Generic Mapping Tools Released.Eos Trans. AGU 76 (47), p. 579.

Wu, Q.J., Zeng, R.S., Zhao, W.J., 2005. The upper mantle structure of the TibetanPlateau and its implication for the continent–continent collision. Sci. China (Ser.D) 48, 1158–1164.

Wu, Q., Li, Y., Zhang, R., Zeng, R., 2007. Wavelet modelling of broad-band receiverfunctions. Geophys. J. Int. 170, 534–544, doi:10.1111/j.1365-246X.2007.03467.

Xu, Y.G., 2001. Thermotectonic destruction of the Archean lithospheric keel beneatheastern China: evidence, timing, and mechanism. Phys. Chem. Earth Part A 26,747–757, doi:10.1016/S1464-1895(01)00124-7.

Xu, X., Ma, X., 1992. Geodynamics of the Shanxi rift system, China. Tectonophysics208, 325–340.

Yin, A., Nie, S.Y., 1996. A phanerozoic palinspastic reconstruction of Chinaand its neighboring regions. In: Yin, A., Harrison, M. (Eds.), The TectonicEvolution of Asia. Cambridge University Press, Cambridge, UK, pp. 442–484.

You, H., 1985. The tectonic and formation of Hetao Basins. In: Crustal Motion 1-Continent Rifts and Deep Process. Seismological Press, Beijing, pp. 88–97, (inChinese).

Yuan, X, 1996. Velocity structure of the Qinling lithosphere and mushroom cloudmodel. Sci. China (Ser. D) 39, 235–244.

Zhai, M.G., Liu, W.J., 2003. Paleoproterozoic tectonic history of the NorthChina Craton: a review. Precambrian Res. 122, 183–199, doi:10.1016/S0301-9268(02)00211-5.

Zhang, S., Chen, X., Ding, Y., Zheng, C., Wu, L., Zhao, J., 1988. Interpretation ofMenyuan-Pingliang-Weinan DSS profile in west China (in Chinese). In: Devel-opment in the Research of Deep Structures of China’s Continent. GeologicalPublishing House, Beijing, China, pp. 61–67.

Zhang, Y.Q., Mercier, J.L., Vergély, P., 1998. Extension in the Graben systems aroundthe Ordos (China), and its contribution to the extrusion tectonics of south Chinawith respect to Gobi-Mongolia. Tectonophysics 285, 41–75, doi:10.1016/S0040-1951(97)00170-4.

Zhang, Z., Yuan, X., Chen, Y., Tian, X., Kind, R., Li, X., Teng, J., 2010. Seismic signatureof the collision between the east Tibetan escape flow and the Sichuan Basin.Earth Planet. Sci. Lett. 292, 254–264, doi:10.1016/j.epsl.2010.01.046.

Zhao, G., Sun, M., Wilde, S.A., Li, S., 2005. Late Archean to Paleoproterozoic evolutionof the North China Craton: key issues revisited. Precambrian Res. 136, 177–202.

Zhao, D., Maruyama, S., Omori, S., 2007. Mantle dynamics of western Pacific and eastAsia: insight from seismic tomography and mineral physics. Gondwana Res. 11,120–131, doi:10.1016/j.gr.2006.06.006.

Zhao, L., Allen, R.M., Zheng, T., Hung, S.H., 2009. Reactivation of an Archean craton:constraints from P- and S-wave tomography in North China. Geophys. Res. Lett.36 (L17306), doi:10.1029/2009GL039781.

Zhu, L., Mitchell, B.J., Akyol, N., Cemen, I., Kekovali, K., 2006. Crustal thickness vari-ations in the Aegean region and implications for the extension of continentalcrust. J. Geophys. Res. 111, B01301, doi:10.1029/2005JB003770.