Tectonophysics 634 (2014) 246–256

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Composition model of the crust beneath the Ordos basin and the Yinshanmountains in China, based on seismic velocity, heat flow and gravity data☆

Yongqian Zhang a,⁎, Jiwen Teng b, Qianshen Wang b, Fuyun Wang c, Qingtian Lü a

a MLR Key Laboratory of Metallogeny and Mineral Assessment, Institute of Mineral Resources, Chinese Academy of Geological Sciences, Beijing 100037, Chinab Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, Chinac Center of Geophysical Exploration, China Earthquake Administration, Zhengzhou 450002, China

☆ Article part of the Special Issue: Advances in seismic i⁎ Corresponding author at: Institute of Mineral Re

Geological Sciences, Beijing 100037, China. Tel.: +86 10 5E-mail addresses: zhangyq@mail.iggcas.ac.cn, zyq_imr

http://dx.doi.org/10.1016/j.tecto.2014.07.0060040-1951/© 2014 Elsevier B.V. All rights reserved.

a b s t r a c t

a r t i c l e i n f o

Available online 12 July 2014

Keywords:Geophysical dataLithologic model of the crustOrdos basinYinshan mountainsChina

In this paper, we have constructed a composition model of the crust beneath the Ordos basin and the Yinshanmountains in north China, with reference to the seismic velocity, heat flow and gravity data measured alongthe 650-km geophysical profile in the direction of N–S from Yanchuan county in the Ordos basin to Mandulatown on the north side of the Yinshan mountains. In order to get this model, we have corrected the crustalphysical parameters measured along the profile into the data under the pressure of 600 MPa and room temper-ature (20 °C), which we can use to compare with the data measured in laboratory under the same pressure andtemperature conditions. Inversion of the geophysical data set to rock compositions indicates that the compositionof the crust along the Yanchuan–Mandula profile reveals difference in the Ordos basin and the Yinshanmountains respectively. In general, the compositionmodel of the crust beneath the Yanchuan–Mandula profile gen-erally varies significantly in vertical direction but slightly in lateral direction. The heterogeneity of the distribution oflithologic composition exists mainly in the coupling zone between the Ordos basin and the Yinshan mountains.

maging of crust and mantle.sources, Chinese Academy of7909032.@163.com (Y. Zhang).

© 2014 Elsevier B.V. All rights reserved.

1. Introduction

The Ordos basin (OB) and the Yinshanmountains (YSM) are locatedin the western part of the North China Craton (NCC), which lies at theeastern margin of the Eurasian continent and contains crustal rocks asold as 3.8 Ga (Liu et al., 1992). In the past 20 years, the crustal structureand evolutions of theNCChave been a focus of geosciences study. For in-stance, observations of low Bouguer gravity anomalies (Wang et al.,2005) and low heat flow (Ma, 1989; Yuan, 1996; Zhai and Liu, 2003)suggest that the western part of NCC, especially the OB, is perhaps stillstable; the receiver function imaging (Tian et al., 2011; Zheng et al.,2007) reveals that the YSM is characterized by a sharp Moho disconti-nuity, a thin-to-normal crust and a lower crust with low velocity andnormal thickness; GPS measurements (Fan et al., 2003) indicate thatthe deformation in the interior of the OB is minor but the motion oftheOB respect to its adjacent block is obvious; and so on. Also, recent re-gional receiver function experiments (Ai et al., 2008; Chen et al., 2008;Zheng et al., 2009), surfacewave tomography (Li et al., 2012) and ambi-ent noise tomography (Tang et al., 2013) demonstrate that there is clearW–E variation of lithosphere disruption in the NCC. However, the crust-al responses to the N–S variation of lithospheric disruption are still un-clear (Zhang et al., 2012, 2013b). In order to understand the crustal

geophysical responses to the N–S variation of lithospheric disruptionof the NCC, we conducted a 650-km geophysical profile in an almostN–S direction along the line of Yanchuan–Mandula in 2005, which ex-tends from the OB to the Inner Mongolian fold system (IMFS) andcrosses the Hetao graben (HTG) and the YSM (Teng et al., 2008, 2010;Tian et al., 2011; Zhang et al., 2011b; 2013a). Along this profile, a seriesof observations and researches have been conducted, including thedeep seismic sounding (DSS), the broad-band teleseismic observationand the gravity measurement, all of which have provided significant in-formation for us to understand the crustal structure and geodynamicsbeneath the OB and the YSM.

One thingwe have to point out is that, most of the geophysical stud-ies cited above focus primarily on the architecture and physical proper-ties of the crust and the lithosphere, while few of them have studied thelithologic composition of the crust in particular. But an estimation of thecomposition of the Earth's crust is important because such a modelis critical for understanding the growth and the evolution of thecontinents (Brown et al., 2003; Holbrook et al., 1999; Rudnick, 1995;Rudnick and Fountain, 1995; Sengor and Burtman, 1993; Zhang et al.,2008; 2009; 2011a; Zhao et al., 2013).Whereas the upper crust is acces-sible to geological sampling and measurements, the deeper portions ofthe crust are relatively inaccessible. To date, the deepest drill hole haspenetrated only 12 km of crust (Kremenetsky and Ovchinikov, 1986).Nevertheless, these deepportions of the crust contain important informa-tion related to the bulk composition as well as the formation of the con-tinental crust. Hence, it is of great significance to know the lithologic

247Y. Zhang et al. / Tectonophysics 634 (2014) 246–256

composition of the deep crust with the help of advanced techniques ofgeophysics. For about 20 years, geophysics has been used to reconstructthe crust–mantle composition model and discuss its formation and evo-lution (Chevrot and Vander Hilst, 2000; Christensen and Mooney, 1995;Holbrook et al., 1999; Rudnick and Fountain, 1995; Sengor and Natal,1996; Zhang et al., 2008, 2009; 2011; Zhao et al., 2013).

Our research aims to calculate the crustal compositionmodel acrossthe OB and the YSM and thence place further constraints on the crustaldifferences between the two tectonic units. In thefirst part of this paper,we present the crustal P-wave velocity model previously obtained fromthewide-angle reflection/refraction seismic data (Teng et al., 2010), thecrustal densitymodel calculated by forwardmodeling to fit the Bouguergravity anomaly (Zhang, 2010) and the temperature distribution in thecrust modeled with surface heat flow data (Hu et al., 2000;Wang et al.,2001). Further, we examine the properties of the crustal rocks and com-position model beneath the profile by comparing the integrated geo-physical information available with the published data of physicalparameters on specific rock types measured in laboratory. Althoughsuch modeling is non-unique, the seismic velocity, density and heatproduction can still be reasonably correlated with laboratorymeasurements, and provide an important data set for determiningcrustal composition, thus help us obtain a best-fit crustal composi-tion model that may help further understand the crustal structureand interactions of the intra-continental collision zones betweenthe OB and the YSM (Brown et al., 2003; Christensen and Mooney,1995; Rudnick and Fountain, 1995; Santosh, 2010; Santosh et al.,2007a, 2007b, 2012; 2013; Zhang et al., 2008, 2009; 2011a; Zhaoet al., 2013).

2. Brief geological and tectonic setting

The NCC is bounded by the early Paleozoic Qilianshan Orogen andlate Paleozoic Central Asian Orogenic Belt (CAOB) to the west and the

Fig. 1. Geologic and tectonic map of the North China Craton (revised after Santosh, 2010), showcations of ultrahigh-temperature and high pressure orogens. The inserted red rectangle shows

north respectively, and by the Mesozoic Qinling–Dabie and Su-Luultrahigh-pressure metamorphic belt to the south and the east respec-tively (Zhao et al., 2005). The NCC is generally considered to be a collageof two discrete crustal blocks namely, the Eastern Block and theWesternBlock with the intervening Trans-North China Orogen (TNCO) (e.g. Zhaoet al., 2002, 2005; Wilde et al., 2002; Kusky et al., 2007; Santosh et al.,2007; 2010; 2012; 2013; among others). And the Western Block of theNCC is dissected into the Yinshan Block to the north and Ordos Block tothe south by the Inner Mongolian Suture Zone in tectonics (Fig. 1).

The crustal growth and stabilization of the NCC can be correlated tothree major geological events in the Precambrian (Zhai and Santosh,2011): (1) a major phase of continental growth at 2.7 Ga; (2) theamalgamation of micro-blocks and cratonization at 2.5 Ga; and(3) Paleoproterozoic rifting, and subduction–accretion–collision tecton-ics during 2.0–1.82 Ga. The Late Paleoproterozoic tectonic evolution andcontinental growth in the NCC have been addressed in a number of re-cent works (Wilde et al., 2002; Zhao et al., 2005; Kusky et al., 2007;Santosh et al., 2007a, 2007b, 2009; among others). One of the modelsproposes that the basement of the NCC was involved in at least twoPaleoproterozoic collisional events: the first event occurred at 1.95–1.92 Ga forming the Khondalite Belt along which the Yinshan Block inthe north and the Ordos Block in the south amalgamated to form theWestern Block (Zhao et al., 2005; Santosh et al., 2007); the secondcollisional event occurred at ~1.85 Ga, forming the TNCO along whichtheWestern and Eastern Blocks collided to form the coherent basementof the NCC (Zhao et al., 2005;Wilde et al., 2002; Faure et al., 2007). In arecent study, Santosh (2010) synthesized the geologic, geochronologicand geophysical data available from various crustal blocks and interven-ing shear/suture zones in NCC and proposed a double-sided subductionmodel to account for the Paleoproterozoic evolution of this region andthe incorporation of the NCC within the Columbia supercontinent.

Our study area, which covers theWestern Block of NCC, is a theoret-ical area for the research into the geodynamic process and evolution

ing the distribution of themain tectonic subdivisions, proposed subduction zones, and lo-the position of Fig. 2.

248 Y. Zhang et al. / Tectonophysics 634 (2014) 246–256

history of theWestern Block of NCC, especially for the research into theoblique southward subduction and collisional mechanism of the YSMbeneath the OB to the south, along the Inner Mongolian Suture Zone.As is illustrated in Fig. 2, the geophysical profile analyzed in this workextends in the direction of N–S from Yanchuan county to Mandulatown and crosses several major active faults and tectonic units, whichare the OB, the HTG, the YSM, and the IMFS. The OB, which occupies alarge part of the Western Block of NCC, is located to the northeasternmargin of the Tibet plateau and surrounded by mountain belts on allfour sides. The OB is currently expressed as a topographic plateau; how-ever, from mid-Triassic through Cretaceous, it was the locus ofnonmarine sedimentation, with repeated deformation on all margins(Bradley et al., 2004). The HTG marks the northern and eastern partsof the Inner Mongolian Suture Zone (IMSZ) in tectonics, and dissectsthe Western Block of the NCC into the Yinshan Block to the north andOrdos Block to the south. An E–W trending system of ductile shear

Fig. 2. Location map of the profile going from south to north between Yanchuan and MandulTriangles and stars mark the geographical positions of seismometers and shot points of theobservations are conducted along the same line with DSS. The major faults are: FCC: Chuanjing–north fault; FWLF: Wulashan piedmont fault; FDQS: Daqingshan fault; FBT: Baotou fault; FDLT: Dtectonic units are as follows: OB, Ordos basin; HTG, Hetao graben; YSM, Yinshan mountains; a

zones between the Paleoproterozoic rock association within HTG andthe Neoarchean TTG (tonalite–trondhjemite–granodiorite) gneissesand granulites within the YSM marks the northern margin of IMSZ(Santosh et al., 2012). The southern margin of the IMSZ is not wellconstrained due to the Phanerozoic sedimentary cover in the Ordosbasin (Santosh et al., 2012). The HTG is considered to have resultedfrom an active mantle convection induced by the westward subductionof the Pacific plate (e.g. Northrup et al., 1995; Tian et al., 1992; Zhanget al., 1998) and the India–Asia collision (Liu et al., 2004; Zhang et al.,1998). Also, some other studies suggest that the lithosphere is stretchedby the far-field effect of the India–Eurasia convergence (e.g. Shen et al.,2000; Xu andMa, 1992; Zhang et al., 1998). As for the YSMbelt, it marksthe northern margin of the NCC. Like other Mesozoic intra-continentalmountain belts of China, the YSM is believed to have been controlledby the Late Paleozoic–Mesozoic tectonic amalgamation of central andeastern Asia (Enkin et al., 1992; Yin and Nie, 1996) and have undergone

a. The inserted map shows the geographic position of this study area in China mainland.deep seismic wide-angle reflection/refraction sounding (DSS) respectively. The gravityChifeng fault; FLS: Langshan piedmont fault; FST: Serteng piedmont fault; FWLN: Wulashanalate fault; FOPN: Ordos platform north fault; and FYF: Yulin–Fugu fault. Abbreviations fornd IMFS, Inner Mongolian fold system.

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strong deformation and magmatism (Davis et al., 1998) during theMesozoic and the Cenozoic period. Modern researches into seismic ac-tivities even imply that the YSM belt is still tectonically active (Minget al., 1995). Specifically, Davis et al. (1998) argued that the Jurassic clo-sure of the Mongol–Okhotsk ocean between the composite Mongolia–North China Block plate and the Siberian Cratonmay have been respon-sible for the synchronous shortening in the Yinshan–Yanshan belt(Bradley et al., 2001). The IMFS lies between the YSM belt and theSolonker suture belt and is confined by the Chuanjing–Chifeng faulton the south and Xilamulunhe fault on the north. During the Paleozoic,the IMFS amalgamated to theNCC as an accretion terrene of the CAOB tothe north (Xiao et al., 2003; Yang et al., 2006). The CAOB records theconvergence and interactions among various types of orogenic compo-nents including the Japan-type, Mariana-type, and Alaska–Aleutian-type arc systems, as well as the active marginal sequences of the SiberiaCraton, which incorporated wide accretionary complexes and accretedarcs and terranes (Xiao and Santosh, 2014). The multiple convergenceand accretions among these various orogenic components generatedhuge orogenic collages in the late Paleozoic and even in the early Trias-sic, involving parallel amalgamation, circum-microcontinent amalgam-ation and oroclinal bending (Xiao and Santosh, 2014).

3. Geophysical dataset

In the framework of the multidisciplinary exploration of the NorthChina Craton, integrated geophysical experiments were carried outalong the 650-km profile extending from Yanchuan county in Sha'anxiprovince at the south end of the profile to Mandula town in InnerMongolia Autonomous Region at the north end of the profile (Fig. 2).

3.1. DSS observation and the velocity structure

The seismological information comes from the records of the 650-km wide-angle seismic reflection/refraction profiling. Within the pro-file, 9 explosive shots were fired at different sites approximately every60 km. For each shot, a series of closely spaced boreholes were drilledand loadedwith 1800–2280 kg of dynamite. A total of 280 portable seis-mometers were used to record the seismographs along the 400-km-long offset profile at intervals of 2 kmon average. Based on the interpre-tation of the registered wide-angle seismic reflection/refraction wavephases, a two-dimensional P-wave velocity model for the whole crustbeneath the profile crossing the OB and the YSM has been constructed(Teng et al., 2010).

The crustal velocity structuralmodel along theprofile is illustrated inFig. 3. The accuracy of the final crustal velocity model depends on the

Fig. 3. P-wave velocity model of the crust along the Yanchuan–Mandula profile (Teng et al., 201plified tectonic settings in the study area aremarkedon the top of theVpmodel for a better undefigure, S means South, and N, North.

correct identification of the various seismic phases, the density of rays,the shot point interval and the receiver density. Seismic velocity deter-minations generally have fewer errors than depth determinations. Forthe seismic data compiled in this profile, the seismic velocities are accu-rate within 3% or ±0.2 km/s. All boundary depths, including the Moho,are accurate within 10% of the stated depth (Mooney and Braile, 1989).

The overall Moho depth increases from 41 km beneath the OB to47 km beneath the YSM, and uplifts for about 5 km beneath the HTG.The whole crust can be divided into two layers, the upper crust withsedimentary cover and the lower crust. The thickness of the sedimenta-ry cover varies between 0 and 8 km with a P-wave velocity of 2.4–5.6 km/s. A good agreement between the near surface velocities andshallow geological formations is generally observed. Relatively high ve-locities coincidewith zoneswhere the basement is near the ground sur-face, and low velocities are clearly associated with the presence ofsedimentary basins. The bottom of the upper crust reaches down to adepth of 27 km beneath the YSM area but varies slightly around18 km beneath the Ordos basin. The P-wave velocity values vary from5.6 km/s to 6.3 km/s in the upper crust. In addition, there is a low-velocity layer with a P-wave velocity of 6.2–6.3 km/s and a thicknessof 5–7 km between the depth of 20 km and 26 km under the YSM.The thickness of the lower crust varies between 20 km beneath theYSM and 25 km beneath the OB. Below the upper crust, the P-wave ve-locity increases abruptly to 6.4 km/s beneath the OB and 6.6 km/sbeneath the YSM and subsequently takes values of 6.8–7.0 km/s nearthe Moho discontinuity.

In addition, in the previous study by Teng et al. (2010), the crustalvelocity model was divided into 8 blocks in lateral direction basedon the extending velocity isolines and the surface geologic tectonics,with 7 boundaries (F1–F7) between each two blocks. In our study, wefollowed such a model.

3.2. Gravity measurement and the density structure

We carried out the gravity measurement along the Yanchuan–Mandula profile in 2006 by deploying two Lacoste–Romberg gravime-ters (No. 596 and No. 1149) with a measurement accuracy of0.082 mGal and restricted the spacing between consecutive measure-ment points to under 1000 m. In total, 673 points along the profilewere measured. Having obtained the raw gravity data, we applied thefollowing reductions: (1) Earth Tide Reduction, (2) Normal reduction,(3) Height reduction, (4) Topographicmass reduction (assuming densi-ty of 2.67 g/cm3) and (5) Terrain reduction (the distance range for thereduction is 0 to 166.7 km from each measurement point). With these

0). The dashed blue lines are block boundaries interpreted by Teng et al. (2010). The sim-rstanding. Abbreviations for tectonic units are the same as those in Fig. 2. In addition, in this

250 Y. Zhang et al. / Tectonophysics 634 (2014) 246–256

reductions incorporated, we estimated the Bouguer gravity anomalyalong the profile with rms accuracy of 0.5 mGal (Fig. 4a).

The Bouguer gravity anomaly values along the Yanchuan–Mandulaprofile vary between −180 mGal in the HTG area and −120 mGal inthe YSM area, and reveal quite different characteristics on the twosides of the HTG (Zhang., 2010). In the OB area on the southern side ofthe HTG, the Bouguer gravity anomaly values vary slightly between−155 mGal and −130 mGal. In the HTG area, the Bouguer gravityanomaly data is as low as −180 mGal, the lowest value along thisprofile. On the north of the HTG, the Bouguer gravity anomaly data ischaracterized by an initial decrease from −120 mGal to −160 mGalfollowed by a small variation between −160 mGal and −140 mGal(Fig. 4a).

Different mathematical correlations between seismic P-wave veloc-ity and density have been tested by many laboratory and field worksabout the elastic properties of crustal rocks (Christensen and Mooney,1995; Deng et al., 2013; Feng et al., 1986; Ludwig et al., 1970; Nafeand Drake, 1957), and Deng et al. (2013) found a new velocity–densitylaw for Yangtze block of South China. In this study, in order to obtain thedensity model of the crust along this profile, we constructed the initialdensity model by converting the P-wave velocity values into densityvalues using the empirical linear velocity–density relationship of Eq. 1,which is obtained from the statistical results of the velocity and density

Fig. 4.Gravity observations and density model of the crust along the Yanchuan–Mandula profilethe profile. (b) Differences between the measured Bouguer gravity anomalies and the calcu0.98 mGal. (c) Two-dimensional density model of the crust along the Yanchuan–Mandula profigravity anomaly was obtained as in Fig. 4a. Abbreviations are the same as those in Fig. 2 and F

parameter values in crust and confirmed as amore valid relationship forthe determination of crustal density in North China (Feng et al., 1986).

ρ ¼2:78þ 0:56 vp−6:0

� �:::::::: vp≤6:0

� �3:07þ 0:29 vp−7:0

� �::::::: 6:0bvp≤7:5

� �3:22þ 0:20 vp−7:5

� �:::::::: vp≥7:5

� � :

8>>><>>>:

ð1Þ

We then derived the crustal densitymodel iteratively until a suitablefitting model was obtained between the observed and the calculatedgravity anomalies. In our calculation, the mismatch between the calcu-lated gravity anomaly from our final density model and the observedgravity anomaly is less than 1.0 mGal in the OB area and 1.5 mGal inthe YSM area. And the final standard deviation between the calculatedand observed gravity anomalies along the whole profile is 0.98 mGal(Fig. 4b). In the final density model (Fig. 4c), there are big variationsfor the density values, which is between 2.32 g/cm3 and 2.64 g/cm3 inthe shallow sedimentary cover of different tectonic units. In thecrystalline crust, the upper crust is mainly characterized by densityvalues of 2.70–2.85 g/cm3, and the lower crust exhibits density valuesof 2.9–3.0 g/cm3. The embedded low-velocity layer at the bottom ofthe upper crust under the YSM has a density of 2.84 g/cm3.

. (a) Measured (blue line) and calculated (red line) Bouguer gravity anomaly curves alonglated anomalies from the density model (the lower plot), and the standard deviation isle after inversion until expected fitting between the calculated and themeasured Bouguerig. 3.

Fig. 5. Two-dimensional pressure model of the crust along the Yanchuan–Mandula profile. The pressure distribution in crust is calculated based on the density model (Fig. 4c) and therelation between density and pressure (Eq. 2). Abbreviations are the same as those in Fig. 2 and Fig. 3.

251Y. Zhang et al. / Tectonophysics 634 (2014) 246–256

3.3. Pressure model

The velocity of crustalmedia is affected by the pressure environmentaround it. Generally, there is direct correlation between the pressurefield and the velocity field. The pressure in crust includes the pressurefrom surrounding rock, the tectonic super-pressure, the interstitialpressure and some other types of pressure that resulted from thephase transition or other tectonic activities. And the pressure modelwe calculate here mainly refers to the pressure from the surroundingrocks, which is considered as themain pressure in crust interior. The re-lation between depth and pressure from surrounding rock is as Eq. (2)

P zð Þ ¼ gZ

ρ zð Þdz ð2Þ

where g is the acceleration of gravity (9.8m/s2), and z is the depth ofthe calculated mass point.

In the previous studies, most scientists took the average densityvalue of 2.83 g/cm3 in the calculation of P, but we have used the actualdensity value obtained in our research, so as to get a more reliablepressure model of the crust along the Yanchuan–Mandula profile. Thepressure model we obtained is illustrated in Fig. 5, in which we can

Fig. 6. Topography (plot a) andheatflow(b) along theYanchuan–Mandula profile. The red trianis the result of interpolation. Abbreviations are the same as those in Fig. 2 and Fig. 3.

notice that the pressure values are not all the same across crust due todensity variation in crust.

3.4. Heat flow and the temperature model

Temperature environment is different in crust interior, and the tem-perature values not only increase with depth, but also vary in lateral di-rection. For the same type of rocks in other similar environments withdifferent temperatures, the velocity decreases with the increase of tem-perature. Hu et al. (2000) and Wang et al. (2001) compiled the heatflow map for the continental area of China using data of 862 observa-tions from different sites in China (Hu et al., 2000) and over 1500 dataadopted from the global heat flow data set (Wang et al., 2001), whichexhibits the overall variation of heat flow pattern in China mainlandand makes it possible for us to construct the crustal geothermal field.According to the heat flow map of China mainland (Hu et al., 2000;Wang, 2001), the observed surface heat flow values along theYanchuan–Mandula profile vary between 48 mW/m2 and 68 mw/m2,with an average value of 62 mW/m2 for the OB and 51 mW/m2 for theYSM area (Fig. 6). By applying a two-dimensional numerical solutionof the steady-state heat conduction equation (Eq. 3), we calculated

gles in plot b are surface heat flowmeasurements (Hu et al., 2000) and the dashed blue line

Fig. 7. Two-dimensional geothermalmodel of the crust along the Yanchuan–Mandula profile obtained by solving the steady-state heat conduction equation. Abbreviations are the same asthose in Fig. 2 and Fig. 3.

252 Y. Zhang et al. / Tectonophysics 634 (2014) 246–256

the temperature field down to a depth of 50 kmalong the profile using afinite element model.

∂∂x K

∂T∂x

� �þ ∂∂z K

∂T∂z

� �¼ A x; zð Þ ð3Þ

where K stands for thermal conductivity, A stands for heat produc-tion, and T is temperature.

The solution was constrained by surface heat flow, and assumedthermal conductivity K as 3.0, 2.8, and 2.6 W/(mK) in the upper, middleand lower crusts respectively, and heat production A as 2.80, 1.25, 0.83,and 0.4 μW/m2 on the ground surface, in the upper, middle and lowercrusts respectively (Zang et al., 2002). The uncertainties affecting thetemperature field are calculated based on the uncertainties of the sur-face heat flow measurements and are usually estimated to be 15%(Pasquale et al., 1990). As is illustrated in Fig. 7, the temperature valuesvary slightly in the upper 10 kmof the crust but significantly in the deepcrust. The temperature beneath the OB is higher than that beneath theYSM, and temperature near the Moho discontinuity varies laterallyfrom 700 °C beneath the OB to 600 °C beneath the YSM.

4. Composition model of the crust

Numerous approaches to physical properties have been developedeither for identifying lithology and fluid distribution, or for constructingcrustal and mantle composition models (Brown et al., 2003; Chevrotand Vander Hilst, 2000; Christensen and Mooney, 1995; Zhang et al,

Fig. 8. Cell model of the crust beneath the Yanchuan–Mandula profile. The crustal model is dividet al. (2010) (Fig. 3), and 9 layers in vertical directionwith a depth interval of 5 km. Each cell is idk indicates the vertical location of the cell in the crust. Abbreviations are the same as those in F

2008, 2009; 2011a; Zhao et al., 2013). In this study, the composition ofthe crust is derived by comparing the elastic wave velocities anddensities measured along the Yanchuan–Mandula profile with thosefor different rock samples derived from laboratory measurements(Christensen and Mooney, 1995; Zhang et al., 2008, 2009; 2011; Zhaoet al., 2013). The key point is to converse the data obtained in differenttemperature and pressure environments along the profile into the datacomparable to those measured in laboratory.

Velocity values generally become higher with the increase of pres-sure and decrease of temperature. The relation between velocity andthe pressure-temperature field is nonlinear in the shallow part of thecrust, but this relation is near to linear in crystalline crust under thedepth of 5 km, as the pores are almost closed under such a high pres-sure. Hence, we can make pressure and temperature correction to thevelocity data using the published linear thermal and pressure gradients(Eqs. 4 and 5) (Christensen, 1979).

∂Vp

∂T ¼ 4:5� 10−4 km=sð Þ=�C ð4Þ

∂Vp

∂P ¼ 2:2� 10−4 km=sð Þ=MPa: ð5Þ

With reference to the crustal velocity model and the block bound-aries suggested by Teng et al. (2010) (Fig. 3), we divided the structuremodel of the crust along the Yanchuan–Mandula profile into 8 blocksin the lateral direction and 9 layers in the vertical direction with depth

ed into 8 blocks in lateral direction according to the block boundaries interpreted by Tengentified as nk (such as 11, 12… 79, 89),where n indicates the lateral location of the cell andig. 2 and Fig. 3.

253Y. Zhang et al. / Tectonophysics 634 (2014) 246–256

intervals of 5 km. Then a 72-cell matrix was formed in the structuralmodel of the crust beneath the Yanchuan–Mandula profile (Fig. 8),and the cells were numbered as 11, 12 …… nk ……89 as shown inFig. 8, where n represents the lateral location and k represents the ver-tical location of each cell. Then we applied temperature and pressurecorrection to the measured velocity values in each cell, and correctedthem into the data under pressure of 600 MPa and temperature of20 °C (Table 1).

In order to investigate the lithologic composition of the crust be-neath the profile, we compare the velocity and density parameters ofthe crust beneath the profile with the data measured in laboratory(Christensen and Mooney, 1995). The correlation ellipses in gray inFig. 9 correspond to the P-wave velocity and density values of the 29types of common rocks measured in laboratory, and the center andlength of the axes of the ellipses correspond to the average values anddeviations of the respective rock materials of the crust beneath theprofile. The coordinates of the numbers in Fig. 9 indicate the average

Table 1Pressure, temperature, original velocity and corrected velocity in different blocks of the crust b

Dep 1 2 3 4

100–255 km 255–310 km 310–410 km 410–4

5 km Pre 127.0 127.4 127.4 127Tem 118 99 99 97Ovp 5.90 6.03 6.10 6Cvp 6.03 6.16 6.23 6Ivp 0.13 0.13 0.13 0

10 km Pre 260.0 262.2 262.6 263Tem 212 196 196 185Ovp 6.13 6.20 6.21 6Cvp 6.27 6.34 6.35 6Ivp 0.14 0.14 0.14 0

15 km Pre 395.0 399.4 399.8 400Tem 305 300 300 275Ovp 6.25 6.30 6.29 6Cvp 6.41 6.45 6.44 6Ivp 0.16 0.15 0.15 0

20 km Pre 532.0 542.4 542.4 543Tem 388 375 375 353Ovp 6.40 6.43 6.45 6Cvp 6.56 6.58 6.60 6Ivp 0.16 0.15 0.15 0

25 km Pre 675.0 686.5 686.0 687Tem 481 440 440 415Ovp 6.48 6.50 6.55 6Cvp 6.65 6.65 6.70 6Ivp 0.17 0.15 0.15 0

30 km Pre 819.0 830.6 831.0 830Tem 550 510 510 480Ovp 6.62 6.61 6.62 6Cvp 6.79 6.76 6.77 6Ivp 0.17 0.15 0.15 0

35 km Pre 964.0 976.1 977.1 975Tem 612 580 580 543Ovp 6.69 6.68 6.67 6Cvp 6.85 6.83 6.82 6Ivp 0.16 0.15 0.15 0

40 km Pre 1111.0 1123.1 1124.1 1121Tem 673 620 620 594Ovp 6.80 6.82 6.80 6Cvp 6.96 6.96 6.94 6Ivp 0.16 0.14 0.14 0

45 km Pre 1268.0 1279.9 1278.4 1273Tem 734 682 682 646Ovp 8.05 8.05 8.10 8Cvp 8.20 8.18 8.23 8Ivp 0.15 0.13 0.13 0

Dep: depth (km), Pre: pressure (MPa), Tem: temperature (°C), Ovp: original P-wave velocity (kcrement of the P-wave velocity after the pressure and temperature correction (km/s).Numbers on top of the table indicate the lateral location of each cell.Numbers on left of the table indicate the vertical location of each cell with depth interval of 5

values of the P-wave velocities and densities of the crust beneath theprofile, and the numbers themselves represent different blocks in thecrust interiors. Thus, the comparison pattern can help to determinethe solutions representing distinct rock types at different crustal depths,thereafter the best crustal compositional model for the Yanchuan–Mandula transect was derived in Fig. 10.

The low velocities and densities of the upper 5 km of the profile arelikely to be the result of cracks and fluid, and therefore cannot be con-sidered reliable for estimating composition (Brown et al., 2003; Zhanget al., 2008). We categorize all the results into 7 groups (red circles inFig. 9), the same specific lithology for each group. The possible rocktypes for all these 7 groups can be summarized as follows: I. granite–gneiss (GGN) and phyllite (PHY); II. phyllite (PHY) and biolite (tonalite)gneiss (BGN); III. prehnite–pumpellyite facies basalt (BPP) and micaquartz schist (QSC); IV. zeolite facies basalt (BZE); V. diabase (DIA);VI. diabase (DIA) and greenschist facies basalt (BGR); andVII. greenschist facies basalt (BGR), mafic granulite (MGR), and

eneath the profile.

5 6 7 8

60 km 460–510 km 510–580 km 580–650 km 650–740 km

.4 122.5 132.3 127.4 127.495 92 90 82

.15 4.90 6.21 6.10 6.00

.28 5.03 6.33 6.22 6.12

.13 0.13 0.12 0.12 0.12

.1 254.8 270.0 262.2 259.7173 167 162 149

.25 6.22 6.26 6.20 6.16

.38 6.35 6.38 6.32 6.28

.13 0.13 0.12 0.12 0.12

.8 392.0 408.7 401.8 399.8255 250 245 223

.31 6.30 6.30 6.30 6.30

.45 6.44 6.43 6.43 6.42

.14 0.14 0.13 0.13 0.12

.9 533.1 548.8 541.0 540.5330 322 315 283

.40 6.35 6.35 6.20 6.35

.54 6.49 6.48 6.33 6.47

.14 0.14 0.13 0.13 0.12

.0 675.7 689.4 680.6 681.1390 383 375 341

.48 6.40 6.40 6.20 6.42

.62 6.53 6.53 6.33 6.53

.14 0.13 0.13 0.13 0.11

.1 819.8 832.5 822.7 824.2450 442 435 400

.53 6.58 6.62 6.61 6.52

.67 6.71 6.74 6.73 6.63

.14 0.13 0.12 0.12 0.11

.1 964.8 976.1 965.8 967.8505 496 486 444

.57 6.65 6.65 6.65 6.64

.70 6.77 6.77 6.76 6.74

.13 0.12 0.12 0.11 0.10

.1 1111.3 1121.6 1110.8 1112.8568 554 540 496

.63 6.70 6.68 6.69 6.78

.76 6.82 6.79 6.80 6.87

.13 0.12 0.11 0.11 0.09

.0 1260.8 1268.6 1257.8 1262.2610 597 585 536

.05 8.10 6.83 6.90 8.00

.17 8.20 6.93 6.99 8.07

.12 0.10 0.10 0.09 0.07

m/s), Cvp: P-wave velocity after pressure and temperature correction (km/s), and Ivp: in-

km.

Fig. 9. Compressional seismic wave velocity versus density for different types of rocksunder the pressure of 600 MPa and room temperature of 20 °C. The diagrams showthe correlation ellipses for Vp and mass density. The comparisons refer to temperature–pressure-corrected values for various rock types, and abbreviations are those defined byChristensen and Mooney (1995). Trial values concerning various rock types allow todraw relatively large- or small-sized clusters (gray ellipses), depending on their respectivedeviations. The numbers projected on the map denote the values (as in Table 1) ofdifferent blocks in the crust (Fig. 8) along the Yanchuan–Mandula profile. All the projec-tion results are categorized into 7 groups (red ellipses) with the same specific petrologyfor each group correspond to the distinct rock types quoted in Fig. 10.

254 Y. Zhang et al. / Tectonophysics 634 (2014) 246–256

amphibolites (AMP). In Section 5.1 and Section 5.2, we will discuss thecomposition model of the crystalline crust beneath the different partsof the Yanchuan–Mandula profile.

5. Discussion

5.1. Uncertainties of the results

By comparing the distribution ranges of the densities and P-wavevelocities of the crust model along our studied profile with those datameasured in laboratory (Christensen andMooney, 1995),wefind it pos-sible to reduce the non-uniqueness in deducting the rock types and pro-vide a crustal composition model along the profile. However, due to theuncertainties included in all the steps of the geophysical dataset de-scribed in Section 3, when the data about seismic velocities and mass

Fig. 10. Composition model of the crust beneath the Yanchuan–Mandula profile. This model isprofile and the data measured in laboratory. The different types of lithologies are labeled as fo(BGN); III, prehnite–pumpellyite facies basalt (BPP), mica quartz schist (QSC); IV, zeolite facgreenschist facies basalt (BGR), mafic granulite (MGR), and amphibolites (AMP). Abbreviation

densities are considered for testing with the respective values for com-mon crustal rock types, the non-uniqueness of the compositionmodel isan aspect to be taken into account (Brown et al., 2003; Zhang et al.,2008; 2009; 2011a; Zhao et al., 2013). Hence,wemust admit that the in-terpretation of crustal composition model obtained in this study canonly reveal afirst-order variation in the crustal properties along the pro-file (Zhao et al., 2013).

5.2. Crustal composition model beneath the Yanchuan–Mandula profile

With the correlated temperature–pressure correction, the P-wavevelocity and densitymodels clearly indicate the composition differencesbetween the OB and the YSM area (Fig. 10). The composition model ofthe crystalline crust along the Yanchuan–Mandula profile is layeredwith different possible lithologic compositions in different depths ofthe crust. The upper crust is composed of two lithologic layers, whilethe layer between 5 and 10 km is mainly composed of granite–gneiss,phyllite and biolite (tonalite) gneiss. The existence of granite and biolitegneiss is expected for a continental structure (Rudnick, 1995; Rudnickand Fountain, 1995; Zhang et al., 2008). Phyllite is a low-grade meta-morphic rock of pelitic composition. And from a geological perspective,this type of sedimentary rock can be regarded as a possible solution be-sides an orthogneiss. The layer under 10 km in the upper crust is mainlycomposed of prehnite–pumpellyite facies basalt and mica quartz schist.The zeolite facies basalt composition of block IV exists in the upper 5 kmof themiddle crust of OB but disappears under the YSM.However, basaltis an extrusive igneous rock which is not likely to be the main phase inthemiddle crust in an area that does not appear to have been extensive-ly affected by volcanism.We therefore are inclined to gabbro and a rockof mafic composition and granular texture (Zhang et al., 2008). In thelower part of the middle crust and the lower crust, the compositioncontent is similar beneath the OB and the YSM, with diabase andgreenschist facies basalt in the lower part of the middle crust andupper part of the lower crust, and greenschist facies basalt, mafic gran-ulite and amphibolites in the bottom of the lower crust, except for theirdepth differences.

As for the HTG located between the OB and the YSM, it has thethickest sediments and thinnest crustal thickness along theYanchuan–Mandula profile. Similar to the crustal velocity distributionof the profile, the compositional structure here also shows couplingcharacteristics between the OB and the YSM. The upper part of theupper crust here is mainly composed of granite–gneiss and phyllite,while the OB on its southern side consists mainly of phyllite and biolite(tonalite) gneiss and the YSM on its northern side has the same compo-sition with the deeper part of the upper crust, both of which consist ofprehnite–pumpellyite facies basalt and mica quartz schist. The blockIV of zeolite facies basalt in top of the lower crust ends beneath the

based on the comparison results of P-wave velocities and density data obtained along thellows: I, granite–gneiss (GGN), phyllite (PHY); II, phyllite (PHY), biolite (tonalite) gneissies basalt (BZE); V, diabase (DIA); VI, diabase (DIA), greenschist facies basalt (BGR); VII,s for tectonic units are the same as those in Fig. 2 and Fig. 3.

255Y. Zhang et al. / Tectonophysics 634 (2014) 246–256

HTG. In the depth of lower part of themiddle crust and upper part of thelower crust, the HTG has the thickest existence of diabase and/or lack ofblock VI (diabase and greenschist facies basalt).

5.3. Possible reasons and geodynamics for the differences in lithologybetween the OB and the YSM

During the past decades, quite a number of geophysical researcheshave been conducted focusing on the crustal structure and tectonic evo-lution of the North China Craton and its neighboring areas (Bradleyet al., 2001; Davis et al., 1998; Fan et al., 2003; Teng et al., 2008, 2010;Tian et al., 2011; Wang et al., 2005; Xiao et al., 2003; Zhang et al.,2010, 2011b; 2013a; Zheng et al., 2007) because of the complex geolog-ical structure here. All these researches show that the OB and the YSMhave different crustal structures and experienced different tectonic evo-lution processes, which has led to the distinguishable lithology in thetwo domains. Besides, difference in temperature and pressure can alsoaffect the lithology in the OB and the YSM (Zhang et al., 2008; 2009;2011a; Zhao et al., 2013). According to the crustal composition modelwe obtained beneath the Yanchuan–Mandula profile and some otherstudies, we are inclined to consider that theNorth China plate has expe-rienced widespread intra-continental contractional deformation (Daviset al., 1998), including the intra-continental collision between the OB inthe northern part of the NCC and the YSM lying on the northern edge ofthe NCC. And this intra-continental collision resulted in both the crustalthickening and uplift of the YSM and the obvious lithologic differencesbetween the OB and the YSM. In the subsequent extension environ-ment, active mantle convection induced by the westward subductionof the Pacific plate and the India–Asia collision (Zhang et al., 1998)may cause the upwelling of the hot asthenosphericmaterial as an originof the hot mantle flow beneath the HTG and has played an importantrole in the formation and evolution of the graben. This may be the rea-sons and geodynamics for the formation of the crustal structure patternand the lithologic composition model beneath the OB and the YSM.

6. Concluding remarks

In this study,we review our knowledge of the deep continental crustfrom both geophysical-based and sample-based studies to study thecomposition model of the crust. By comparing the P-wave velocityand density data of the crust obtained from integrated geophysicaldata and those data measured in laboratory, we derived the composi-tion model of the crust along the Yanchuan–Mandula profile. It isworth our attention that the lateral and vertical resolution obtainedfor the composition of the crust is highly dependent on the geophysicaldata sets (seismic wave velocities, gravity anomalies, mass densities,heatflow and temperatures) and laboratorymeasurements for differentrock samples (Zhang et al., 2008). The compositionmodel we discussedin this paper focuses only on the crystalline crust, which is considereddeeper than 5 km, as the shallow parts of the crust are possibly the re-sult of cracks and fluid and therefore cannot be considered reliable forestimating composition.

The compositionmodel of the crust beneath the Yanchuan–Mandulaprofile generally varies strongly in vertical direction but slightly in later-al direction. The difference of the distribution of lithology compositionexists mainly in the coupling zone between the OB and the YSM. Thecomposition of the upper crust is somewhat similar along this profile,and the GGN, PHY, BGN, BPP and QSC provide a best-fit lithology forthe upper crust; the lower crust is mainly composed of DIA, BGR, MGRand AMP. BZE may exist in the upper 5 km (depth between 17 and22 km) of the lower crust beneath the OB, but is not found in the crustunder the YSM. Our research result suggests that the crustal thickeningand uplift in the YSMmay have resulted from the intra-continental col-lision between the OB and the YSM, and the HTG is formed during thesubsequent extension process caused by the active mantle convection.

Acknowledgments

The authors offer their sincere thanks to many people for their con-tribution to this work, especially the observers and technical expertswho participated in the fieldwork and data processing to produce ourexcellent data set. This research is supported by grants from the ChineseNational Natural Science Foundation (No. 41204063, No. 90914012, No.40434009 andNo. 40930418) and theMinistry of Land and Resources ofChina under the Project SinoProbe-03-02.We thankDr. Liu Y.S. for help-ing us process the heatflowdata.We are especially thankful to Prof. JoséBadal, Prof.M. Santosh, Prof. Hans Thybo and two anonymous reviewersfor their helpful comments and constructive suggestions, which helpedus improve substantially the early manuscript.

Appendix A. Supplementary data

Supplementary data to this article can be found online at http://dx.doi.org/10.1016/j.tecto.2014.07.006.

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