Variations of the effective elastic thickness over China and surroundings and their relation to the lithosphere dynamics

Earth and Planetary Science Letters 363 (2013) 61–72

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Variations of the effective elastic thickness over China and surroundings and their relation to the lithosphere dynamics Bo Chen a, Chao Chen a,c,n, Mikhail K. Kaban b, Jinsong Du a, Qing Liang a, Maik Thomas b a

Institute of Geophysics and Geomatics, China University of Geosciences, Wuhan, China German Research Center for Geosciences (GFZ), Potsdam, Germany c Subsurface Multi-scale Imaging Laboratory, Wuhan, China b

a r t i c l e i n f o

abstract

Article history: Received 26 October 2012 Received in revised form 5 December 2012 Accepted 13 December 2012 Editor: P. Shearer Available online 21 January 2013

The effective elastic thickness (Te) characterizes response of the lithosphere to a long-term tectonic loading. As a proxy of the lithospheric strength, Te can be used to address mechanical behavior and deformation of the blocks with complex geological structure. Here we use the multitaper coherence method to determine spatial variations of Te in China and surroundings based on the topography and Bouguer gravity anomaly data. The results show that the Te values are high (4 70 km) over the cratons, e.g. India craton, the Siberian Craton, North China Block and also variations of these values are significant over these blocks. Low Te corresponds to the young orogens, e.g. Himalayas, Altun Shan– Qilian Shan–Longmenshan, Qinling–Dabie, and Daxing’anling–Taihang Mountains etc. Combined with other data, the lateral variations of Te within the North China and South China Blocks and the Tibetan Plateau indicate that the lithospheric strength in China Mainland depends on both lithospheric structure and mantle dynamics. We guess that during the Mesozoic–Cenozoic period, the strength of the lithosphere might have been significantly altered by the thermodynamic processes associated with the India–Eurasia collision in the southwest and the subduction of the Pacific plate in the east. The Te variations also play a major role in the lithospheric evolution and deformation since the Mesozoic. & 2012 Elsevier B.V. All rights reserved.

Keywords: effective elastic thickness multitaper North China Block South China Block Tibetan Plateau

1. Introduction Mechanical strength is one of the primary factors in controlling deformation processes of the lithosphere in response to the long-term ( 4105 yr) tectonic loading (Burov and Diament, 1995). It is commonly described by the flexural rigidity (D) or effective elastic thickness of the lithosphere (Te) (Watts, 1978). These parameters are related as: D ¼ET3e /[12(1  s2)], where E is the effective Young’s modulus and s is the Poisson’s ratio. Te of the continental lithosphere depends on its composition, structure (e.g. crust–mantle decoupling or coupling) and temperature distribution (Burov and Diament, 1995; Tesauro et al., 2009a, 2009b, 2012). In the past forty years, a lot of studies were focused on Te determinations in various continental regions, and finding a link of this parameter with the lithosphere evolution and dynamics (e.g. Banks et al., 1977; Bechtel et al., 1990; Pe´rezGussinye´ and Watts, 2005). For the first time, Lewis and Dorman (1970) calculated an isostatic response function between the gravity field and topography to yield the compensating density distribution over the conterminous United States assuming a local

n

Corresponding author. Tel./fax: þ 86 27 67883639. E-mail address: [email protected] (C. Chen).

0012-821X/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.epsl.2012.12.022

compensation model. Later, Banks et al. (1977) adopted a regional compensation model to estimate Te and density structure of the United States. McNutt and Parker (1978) have calculated flexural rigidity of the lithosphere in Australia and found it to be considerably different from that one of the United States. Zuber et al. (1989) have also determined Te for separate tectonic units in Australia, which indicate that Te increases with time since last modifications of the lithosphere. Based on the Te variations in Europe, Pe´rez-Gussinye´ and Watts (2005) pointed out that the strength of the continental lithosphere could reflect the processes, which formed plate structure. Recent studies of the Te distribution and its anisotropy on a global scale provided evidence that the preexisting mechanical structure had a significant influence on con¨ tinental deformation and evolution (Audet and Burgmann, 2011). Compared to other regions, only few studies on Te variations over China Mainland were carried out. Wang and Xu (1996) estimated the average Te of the main blocks by analyzing a mechanism of the isostatic compensation beneath China. Braitenberg et al. (2003) and Jordan and Watts (2005) calculated spatial variations of Te over the Tibetan Plateau. In addition, several studies were performed for limited regions and along 2D profiles (e.g. Yuan et al., 2002; Zhao et al., 2004; Fielding and McKenzie, 2012). The Chinese Mainland is not a single giant craton; it comprises a number of the stable blocks (Fig. 1): including the North China

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Siberia craton

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Fig. 1. Topography and geotectonic setting of China and adjacent regions. The bathymetry and topography are from ETOPO2v2 digital data (http://www.ngdc.noaa.gov/ mgg/global/etopo2.html). Thick dashed lines are the boundaries of the plates (AM, Amur; BU, Burma; EU, Eurasia; IN, India; ON, Okinawa; OK, Okhotsk; PA, Pacific; PS, Philippine Sea; SU, Sunda; YA, Yangtze) from model PB2002 (Bird, 2003). White arrows denote vectors of the plate motion with respect to GSRM v1.2 (Kreemer et al., 2003) as calculated with the Plate Motion Calculator (http://www.unavco.org/community_science/science-support/crustal_motion/dxdt/model.html). The medium solid lines with names are the major faults (ATF, Altyn-Tagh fault; EKLF, East-Kunlun fault; HYF, Haiyuan fault; LMSF, Longmenshan fault; RRF, Red River fault; TLF, Tan-Lu fault; XSH-XJF, Xianshuihe-Xiaojiang fault). Thin dashed lines denote the main tectonic units: BH, Bohai Sea; JGB, Junggar basin; QLS, Qilian Shan orogen; HM, Himalayan orogen; SCB, Sichuan basin; SLB, Songliao basin; ECS, East China Sea; SCS, South China Sea; QL-DB, Qinling–Dabie orogen; JS, Japan Sea; YS, Yellow Sea.

Block, South China Block, Tibetan Plateau and Tarim block, and several active orogens, e.g. Himalayas, Tien Shan and Qinling– Dabie. Several major processes control its tectonics: the rapid collision of the India–Australia plates with Eurasia in the southwest, the subduction of the Pacific and Philippine Sea plates in the east (Fig. 1), and the continental extension in the Baikal rift in the north (Schellart and Lister, 2005). Due to such complex structure and dynamics, it is necessary to investigate in detail Te distribution for the whole Chinese mainland, in order to better understand evolution and deformation of the lithosphere and to characterize interaction between major blocks. Up to now, several methods have been employed to estimate Te, such as forward modeling of deformations (Karner and Watts, 1983), spectral techniques (i.e. admittance and coherence) based on the cross-spectral analysis of the gravity and topography data (Dorman and Lewis, 1970; Forsyth, 1985), and direct estimations of Te based on the yield strength envelope (Goetze and Evans, 1979; Burov and Diament, 1992; Tesauro et al., 2009a, 2012). The topography and gravity data used in the spectral technique are widely available; therefore this technique is used more often than other methods. The coherence technique, which is based on a spectral analysis of the observed fields, is employed in this study. Used in the pioneer studies (Lewis and Dorman, 1970; Zuber et al., 1989; Bechtel et al., 1990), the periodogram spectral technique may result in a spectral leakage. In order to reduce the leakage, the multitaper technique (Thompson, 1982) was introduced to improve

the coherence method (McKenzie and Fairhead, 1997; Simons et al., 2000). However, even with the multitaper technique the Te values might be underestimated due to a limited size of the analyzing window. Later on, the wavelet technique has been suggested to estimate Te (Stark et al., 2003; Kirby and Swain, 2004). Furthermore, Pe´rez-Gussinye´ et al. (2007, 2009a, 2009b) have applied a window correction to reduce the bias caused by the multitaper method. They suggested merging the Te values evaluated by windows with various sizes to obtain more accurate Te estimations than that by the pure wavelet method. In this study we employ the multitaper Bouguer coherence method (Pe´rez-Gussinye´ et al., 2009a, 2009b) based on the topography and Bouguer gravity data analysis, to estimate Te variations in China and surrounding areas (601E–1451E, 01N–601N). First, the multitaper coherence technique is introduced. Then, we estimate spatial variations of Te for China and surroundings. Finally, we discuss the results combined with the information from geology, GPS, and seismic tomography in order to understand a relation between the lateral variations of Te, and lithospheric structure and dynamics of the main blocks (North China, South China, and Tibetan Plateau).

2. Multitaper Bouguer coherence method The effective elastic thickness can be estimated by analyzing the spectral coherence between the external load (e.g. topography)

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and the lithospheric deflections due to these loads that resulted in the Bouguer gravity anomalies (McKenzie and Bowin, 1976; Watts, 1978). The observed coherence derived from the observed topography and Bouguer gravity anomalies is compared with that predicted for equivalent elastic plate models with a range of Te. The Te value that minimizes the difference between the predicted and observed coherence is the optimal one for the analyzed area. The multitaper technique (Thompson, 1982; Simons et al., 2000) is employed to reduce the frequency leakage in the spectral estimations.

2.1. The observed coherence The observed coherence function g2obs ðkÞ between the topography and Bouguer gravity anomalies in the wave-number domain (k) (McKenzie and Bowin, 1976; Forsyth, 1985) is defined as 2

g2obs ðkÞ ¼

9/BðkÞHn ðkÞS9 , /BðkÞBn ðkÞS/HðkÞHn ðkÞS

ð1Þ

where B and H are the power spectrum of the Bouguer gravity anomaly and topography, respectively; k¼(kx, ky) is the 2-D qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 2 wavenumber, and the modulus is k ¼ 9k9 ¼ kx þ ky ; the angle brackets denote averaging over wavebands; the asterisk denotes complex conjugate.

anomaly can be expressed as ! !   Wi mB mT B ¼ , Hi kB kT H

Taking into account both the initial surface load Hi and the internal load Wi, and having a model with predefined lithospheric constant parameters (e.g., crustal density rc, mantle density rm, depth of the Moho Zm, Young’s modulus E, and Poisson’s ratio s), we can predict the spectral coherence between the surface topography components (i.e. contribution of the surface loading to the topography HT, the contribution of the internal load HB) and the gravity effect of the Moho deformations corresponding to the surface load and internal load (BT, BB). Assuming that the internal load is applied to the Moho, the relationship between the flexural components (HT, HB, BT, BB) and the initial loads (Hi, Wi) in the wave-number domain (Kirby and Swain, 2011) are given by W B ¼ vB W i ,

vB ¼ 1Dr2 =f,

W T ¼ vT H i ,

vT ¼ Dr1 =f,

HB ¼ kB W i ,

kB ¼ Dr2 =f,

HT ¼ kT Hi ,

kT ¼ 1Dr1 =f, 4

ð2Þ

D ¼ ET3e /

where f ¼Dk /g þ rm  rf; Dr1 ¼ rc  rf; Dr2 ¼ rm  rc; [12(1  s2)] is the flexural rigidity; Te is the effective elastic thickness; rf is the density of the overlying fluid (i.e. either rf ¼0 on the continent or rf ¼ rw on the ocean). The observed surface topography H and Moho W represent a sum of the components: H ¼ HT þ HB , W ¼ W T þ W B,

ð3Þ

where the Moho variations W can be derived from the Bouguer gravity anomaly B as W¼

Be9k9zm , 2pGDr2

ð4Þ

where G is the Newtonian gravitational constant. Substituting Eqs. (2) and (4) into (3), the relationship between the initial loads with the topography and the Bouguer gravity

ð5Þ

where

mB ¼ 2pGDr2 e9k9zm vB , mT ¼ 2pGDr2 e9k9zm vT : The initial loads Hi and Wi can obtained from Eq. (5) based on the power spectrum of the observed topography and Bouguer gravity anomaly, and then the flexural components HT, HB, BT and BB are determined from Eq. (2). When the determinant is not equal to zero, a unique solution of Eq. (5) exists. However, if mBkT  mTkB E0, the coefficient matrix is singular. Thus, when the determinant is less than a small value (e.g. 10  6), a constant loading ratio f0 (i.e. Wi ¼f0Hi) should be employed to calculate the initial loads. Because of the difference in rf, the equivalent topography approach (Stark et al., 2003; Kirby and Swain, 2008) is used to transform the oceanic terrain to equivalent land topography. Based on the assumption that the surface and subsurface loads are statistically uncorrelated (Forsyth, 1985), the predicted coherence g2pre ðkÞ (Lowry and Smith, 1994; Kirby and Swain, 2011) can be written as 2

g2pre ðkÞ ¼ 2.2. Predicted coherence

63

9/BT HnT þ BB HnB S9 : /HT HT þ HB HnB S/BT BnT þBB BnB S n

ð6Þ

The observed coherence is derived from Eq. (1), and then compared with the predicted coherence obtained from Eq. (6) for a given Te. Using an optimization algorithm, the best Te for the study area can be determined, which minimizes the mean-square error e between the observed and predicted coherence. The meansquare error e is weighted by the variances of the observed coherence as vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi uPN u ½g2 ðki Þg2pre ðki Þ2 =s2 ðki Þ e ¼ t i ¼ 1 obs , ð7Þ PN 2 i ¼ 1 1=s ðki Þ where N is the number of wavenumber annuli; ki is the i wavenumber, i¼1, 2, y, N; s(ki) is the jackknifed error estimate of the observed coherence (Thomson and Chave, 1991) and averaged within annular bins. 2.3. Slepian multitaper spectral analysis The 2-D multitaper transform is performed to obtain spatial variations of Te by tapering the row and column of the data with the 1-D Slepian tapers (Simons et al., 2000). Given a resolution parameter NW (usually given 2, 3, or 4), the P ¼(2NW 1)2 independent tapers are obtained (NW¼ 3 and P¼32 in this study). The p Slepian eigencomponent B(k, Sp) of the Bouguer gravity anomaly b(x, y) is Z þ1 Z þ1   B k,Sp ¼ ð8Þ bðx,yÞSp ðx,yÞe2piðkx x þ ky yÞ dxdy, 1

1

where Sp(x, y) represents a set of the Slepian tapers, p¼ 1, 2, y, P. Similarly, the p Slepian eigencomponent of the topography H(k, Sp) can also be determined. Considering the orthogonal property of the Slepian sequences, the cross-power spectrum of the two signals is estimated by a weighted sum of the Slepian eigencomponents (Prieto et al., 2007)   n  PP1 p ¼ 0 dp B k,Sp H k,Sp BðkÞHn ðkÞ ¼ , ð9Þ PP1 p ¼ 0 dp

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where dp (p ¼1, 2, y, P) is the weight, which is given by the product of the eigenvalues of the two corresponding tapers.

layer (Stolk et al., in preparation). Meanwhile, the corresponding thickness of sediments is also corrected by estimating an equivalent thickness with the density as of the normal topography. Other constants are given in Table 1.

3. Data The topography data (Fig. 1) are based on the ETOPO2v2 digital elevation model (http://www.ngdc.noaa.gov/mgg/global/etopo2. html), in which the ocean bathymetry of Smith and Sandwell (1997) is used. Since small-scale variations of the bathymetry have been derived for a predefined isostatic model with a specific Te value (Smith and Sandwell, 1997), the Te estimations in the oceanic areas are not reliable and we do not consider them. The Bouguer gravity anomalies (Fig. 2) are obtained by removing the gravity effect of the topography from the free air gravity anomalies. The free air gravity anomaly data are derived from the EIGEN-6C model (http://icgem.gfz-potsdam.de/ICGEM/modelstab. html) to degree and order 720 at the elevation of 10 km relative to the reference spheroid. The gravity effect of topography is calculated on the basis of a spherical cap volume integral. The gravity effect of topography has been limited to the same degree and order (720) as of the initial free air anomalies to avoid introducing artificial signals in the resulting Bouguer anomalies. Assuming that both the internal load and compensation are associated with Moho variations, the average depth of Moho is used in Eq. (4). It is taken from CRUST2.0 model (Bassin et al., 2000). Anomalous structure of sediments remarkably influences the Bouguer gravity anomaly. Therefore, the gravity effect of sediments is removed from the gravity anomaly based on recent compilations of the thickness and average density of the sedimentary

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Table 1 Parameters used in the flexure modeling. Parameter

Symbol

Value

Units

Crustal density Mantle density Seawater density Gravity acceleration Gravitational constant Young’s modulus Possion’s ratio

rc rm rw

2670 3300 1030 9.78 6.67259e  11 100 0.25

kg m  3 kg m  3 kg m  3 m s2 m3 kg  1 s  2 GPa

g G E

s

4. Results In the multitaper coherence method resolution of the recovered Te depends on the window size. In order to recover Te with different wavelengths, the Te results estimated with three windows of different size are merged to obtain the final Te (Pe´rezGussinye´ et al., 2009b). The window sizes are 400 km (400 km  400 km), 600 km and 800 km. The study area is divided into overlapping windows and then the observed and predicted coherence is obtained by the multitaper method in each window. Next, the Brent’s method (Press et al., 1992) is used to minimize the mean-square error (Eq. (7)). Finally, the center of the window

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Fig. 2. Bouguer gravity anomalies for China and adjacent regions overlain by the plate boundaries and major tectonic units as shown in Fig. 1. The free air gravity anomalies are derived from EIGEN-6C and calculated at the elevation of 10 km relative to the reference spheroid. The Bouguer gravity anomalies are obtained by removing the gravity effects of the topography and sediment from the free air gravity.

B. Chen et al. / Earth and Planetary Science Letters 363 (2013) 61–72

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Fig. 3. The Te of China and surrounding areas from three different windows: (a) 400 km  400 km; (b) 600 km  600 km; (c) 800 km  800 km. Since the estimators for higher Te decrease greatly for a limited window size (Pe´rez-Gussinye´ et al., 2009a), we set the color scale to a maximum of 110 km and the values over 110 km are considered as indeterminately high as suggested by Pe´rez-Gussinye´ et al. (2009a).

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Fig. 4. The bias-corrected Te of China and surrounding areas estimated from the three windows: (a) 400 km  400 km; (b) 600 km  600 km; (c) 800 km  800 km.

is moved for 40 km each time, and the calculations are repeated to cover the whole area. The spatial variations of Te determined with three windows are shown in Fig. 3. Although locally the Te values may be different, they demonstrate the same trend: the high Te mainly correspond to the old stable cratons, e.g. the Siberian craton, India craton, Ordos basin, Tarim basin; while the low-Te usually appears in the orogenic areas. It should be noted that the Te values estimated with the 400 km window (Fig. 3a) are generally lower than that by the 600 km and 800 km windows (Fig. 3b and c), although lots of short wavelength variations are recovered, such as the smallscale variations in the orogens. However, the intermediate and long wavelength variations from the 600 km and 800 km windows look more consistent and reliable but without small-scale variations. Clearly, the small window can effectively recover only local variations related to weak zones with low Te, whereas the high Te amplitudes are poorly constrained since the transitional wavelengths are greater than the window size. In contrast, the large window can reliably recover the long wavelengths, while information in the short wavelengths is chiefly lost due to averaging. For each window we apply the correction for Te given by Pe´rez-Gussinye´ et al. (2009a) from 100 modeling tests to fix the downward bias due to windowing. The bias-corrected results from three different windows (Fig. 4) show very similar spatial distributions, particularly for the high-Te stable areas surrounded

by steep boundary changes. Since the high-Te values estimated with the small window are likely underestimated, the errors may be also amplified by the bias-correction, which results in smallscale artifacts (i.e. Fig. 4a). We consider this effect while interpreting the result. In order to optimally recover both the small- and largescale variations, the bias-corrected results estimated with three windows are merged by weighted averaging. The weights are determined using the approach of Pe´rez-Gussinye´ et al. (2009b). The merged map (Fig. 5a) shows that the pattern of the Te variations in China and surrounding areas agrees well with the tectonic provinces. Obviously, the results are also consistent with previous regional findings (e.g. Bechtel et al., 1990; Pe´rezGussinye´ and Watts, 2005) that Te of the old Archean cratons is much larger than that of the younger Phanerozoic orogens. For the Tibetan Plateau and India, on the average our results are also in a good agreement with previous studies (Braitenberg et al., 2003; Yang and Liu, 2002; Jordan and Watts, 2005), but show remarkably more details. Particularly, for the Qaidam basin, our estimates are relatively high (about 50–70 km), which is consistent with the values of 50–60 km from Jordan and Watts (2005) and 60–80 km from Braitenberg et al. (2003). Furthermore, to the west of the Tarim basin, Jordan and Watts (2005) and Yang and Liu (2002) recovered high Te of  50 km and 60–100 km, respectively, which is in a good agreement with our results (60–100 km).

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Our results are generally in a good agreement with the global ¨ Te map derived by Audet and Burgmann, 2011 using the wavelet transform. However the map based on the wavelet approach is smoother than the map based on the multitaper method. The ¨ results of Audet and Burgmann (2011) also show that most areas of China Mainland are dominated by low Te values, and high Te values are found in India (up to 120 km) and Siberian cratons (80–140 km), which are consistent with our estimates (up to 110 km). On the other hand, within the stable Ordos and Sichuan ¨ basins, the Te variations of Audet and Burgmann (2011) are more smooth without localized zones with the average of about 40–60 km, which is lower than our result (up to 110 km).

5. Discussions The lithosphere of China and surroundings is complex, composed of different blocks with various age, composition and rheology. The main blocks bounded by active young orogens are the North China (NCB) and South China Blocks (SCB) in the east,

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and the Tibetan Plateau and Tarim Block in the west (Fig. 1). The orogens are the Central Asian Orogenic Belt (CAOB), the Kunlun– Qilian Shan–Qinling–Dabie orogens, Tethys-Himalayas orogens and some others. Our results (Fig. 5a) show a great variability of Te over China and its surrounding area. We confirm that the cold stable cratonic lithosphere is characterized by high Te (470 km) and consequently by high strength. In contrast, the Te is low (o40 km) in the young Phanerozoic orogens. These orogens may have intrinsically low mechanical strength, because they represent intercontinental active belts involved in repeating tectonic processes with large crustal deformations and lots of active thrust faults. Since spatial variations of Te reflect differences of the integrated strength of the continental lithospheres, Te should relate to a rate of the tectonic deformation and earthquake activity (Tassara et al., 2007; Hyndman et al., 2009). Only few earthquakes occurred within the old blocks with high-Te, while most of the earthquakes with high magnitude are concentrated within the areas with low Te (Fig. 5b and Fig. 6). About 70% of the intracontinent earthquakes occurred in the low Te ( o40 km) regions.

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Fig. 5. (a) The final Te of China and surrounding areas after merging bias-corrected results from three different windows (400 km, 600 km and 800 km). (b) Te map overlain by earthquakes and volcanoes; grey circles represent earthquakes since 1973 to 2012.06.11, from the data base: USGS/NEIC (PDE) (http://earthquake.usgs.gov/ earthquakes/eqarchives/epic/); the green circles are the significant earthquakes before the year 1973 (2150 B.C.–1973 A.D.) from the NGDC Significant Earthquake Database; the dark green ones show the earthquakes with an unknown magnitude; The purple symbols are volcanoes from http://www.ngdc.noaa.gov/hazard/volcano. shtml. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

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Fig. 6. (a) Absolute number of earthquakes (M Z 3) over China Mainland versus Te; (b) number of earthquakes (M Z 3) normalized by the area. In order to reduce a strong impact of the high seismicity near plate boundaries, only the intraplate earthquakes (within the boundary of China Mainland) are considered.

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basin demonstrate that it is underlain by a strong lithosphere. The average crustal thickness of the Ordos basin is about 42 km (Liu et al., 2006). However, Te is more than 100 km in the northern part, which is much larger than the crustal thickness. This indicates that Te of this stable basin is related not only to the crustal strength, but also to the thick craton-like lithospheric root, which was previously found by the mantle tomography (Huang and Zhao, 2006; Li and van der Hilst, 2010). This indicates that the lithosphere in the Ordos basin extends down to at least 200–300 km. In the middle of the NCB, we found a low-Te region (about 10–40 km) along the Yanshan–Taihang orogen, starting from the Daxing’anling, across the Taihang Mountains, and further to the south propagating into the Wuling Mountain. This orogenic belt is characterized by the prominent North–South Gravity Lineament in eastern China (Fig. 2). Geological studies (e.g. Kusky, 2011) have shown that the Taihang Mountains area was formed as an old orogen following the collision of the WNCC and ENCC during Late Archean–Paleoproterozoic, and suffered later deformation at 1.85 Ga. Due to inheritance of mechanical properties of the ¨ lithosphere (Armstrong and Watts, 2001; Audet and Burgmann, 2011), the Taihang orogen inherited weakness of the suture zones. Compared with the WNCC, the Te values vary dramatically in the ENCC (Fig. 7). The relatively high-Te values ( 470 km) appear over the eastern of Bohai Bay basin, the northern part of the Shandong Peninsula, Northern Yellow Sea ( 70 km) and the eastern Hehuai basin (80–110 km). The relatively low Te values correspond to the Luxi Uplift and the western part of the Bohai Bay basin. Particularly to the west of the Lankao–Liaocheng– Yanshan fault, Te decreases significantly from 60 km to 20–40 km. Thus, the Te gradient zone, which coincides with one of the strongest seismic belts in China, is clearly visible in Figs. 5b and 7. This may indicate that the strong Archean lithosphere is weakened in the ENCC. Throughout the Phanerozoic, the WNCC was

Furthermore, the areas with large Te gradient, usually associated with the cratonic margins, also coincide with active seismic zones. Due to the effect of both interacting geodynamic processes and complicated geological structure, the Te pattern exhibits large spatial variations within major blocks in China Mainland. In the following parts we discuss in detail the Te variations for the major blocks, i.e., the NCB, the SCB, and the Tibet Plateau, and analyze their relation to the surface tectonics and mantle dynamics. 5.1. North China Block The North China Block is one of the oldest Archean cratons, where outcrops are as old as 3.8 Ga. The block is bounded by the late Paleozoic CAOB to the north, the Mesozoic Qinling–Dabie orogenic belt and Su–Lu UHP metamorphic belt to the south and east, and the early Paleozoic Qilian Orogen to the west (Zhao et al., 2005). The craton consists of two old major blocks (Fig. 7), the Eastern Block (ENCC), which generally corresponds to the Bohai Bay, Liaohe and Hehuai basins, and the Western Block (WNCC) located in the Ordos basin. The two blocks are separated by the intervening late Archean–Paleoproterozoic Taihang Orogenic Belt (Zhao et al., 2005; Kusky, 2011). The craton nucleus of the NCB is represented by Archean–Paleoproterozoic rocks (Zhao et al., 2005). As shown in Fig. 7, the spatial variations of Te coincide well with major tectonic units. The high-Te mainly occurs in the Ordos basin ( 4100 km) of the WNCC, and in the Bohai Bay and Hehuai basins ( 480 km) of the ENCC. The low-Te is observed along the Yanshan–Taihang orogen (10–40 km) in the middle of the NCB. In the WNCC, the conspicuously high-Te ( 4100 km) occurs in the central and northern parts of the Ordos basin and further northward including the Yinshan mountains. However, in the southern part of the Ordos basin, Te decreases to approximately 30–60 km. Generally high Te values over most parts of the Ordos

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Fig. 7. Te over the major tectonic units of the North China Block/Craton including WNCC, Western North China Block; ENCC, Eastern North China Block; CNCC (or TNCO), Central Taihang Orogen; CAOB, Central Asian Orogen Belt; Y-H Rift, Yinchuan–Hetao Rift; S–S Rift, Shaanxi–Shanxi rift systems; BBB, Bohai Bay Basin; HHB, Hehuai Basin; Luxi, Luxi Uplife; Su–Lu, Su–Lu orogen; BH, Bohai Sea; NYS, Northern Yellow Sea; SYS, Southern Yellow Sea. Bold lines are major faults (Li et al., 2011): LLYF, Lankao– Liaocheng–Yanshan Fault; CF, Cangdong Fault; ETF, Eastern Taihangshan Fault; WQYF, Wulian–Qingdao–Yantai Fault.

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Fig. 8. Te over the major tectonic units of the South China Block. YJB, Youjiang basin; JHB, Jianghan Basin; YKB, Yunkai block; HLA, Huangling anticline; NB, the North Bay; SBB, Southern Jiangsu Basin; SYSB, Southern Yellow Sea Basin. The bold lines are major faults (Wang et al., 2007; Li et al., 2011): XJF, Xiaojiang Fault; JSF, Jiangshan– Shaoxing Fault; HGF, Heyuan–Guangfeng Fault; ZDF, Zhenghe–Dapu Fault; CNF, Changle–Nanao Fault; ALF, Anhua–Luocheng Fault; HYSF, Huayingshan Fault. The thin white lines are the boundaries of the provinces in China.

relatively stable; while the ENCC was tectonically active, with en echelon extensional basins, large-scale magmatism, strong seismic activity and increased heat flow ( 464 mW/m2) (Menzies et al., 2007). Compared with the WNCC, the overall lower-Te values over the ENCC suggest that the lithosphere is totally destabilized (Gao et al., 2002; Menzies et al., 2007), but the destruction is obviously heterogeneous. In this region, the high-Te is high in the Hehuai basin (70–110 km), and still thicker than the average crust (o32 km) (Menzies et al., 2007). This indicates that parts of the lithosphere may still preserve Archean mantle relics beneath the high-Te zone. It is apparent that the lithospheric strength and Te variations over the NCB are likely influenced by structure and dynamics of the upper mantle. Latest tomography models (Huang and Zhao, 2006; Li and van der Hilst, 2010) show that fragments of the subducted Pacific slab, which are preserved at the depths 410–660 km, stretch across the Bohai, westward to the Taihang orogen, and are associated with the widespread low-velocity zone (100–140 km thick) in the upper mantle. Dehydration of the subducted oceanic slabs in the mantle may lead to increase in the water content of the upper mantle resulting in partial melting of the lithosphere. This possibly enables a rapid small-scale convection in the upper mantle due to reduction of the viscosity, and induces abundant magmatic and tectonic activities. The strength of the continental lithosphere might be remarkably reduced by a high volatile content and warming of the mantle due to the convection with a subsequent destruction of the lithospheric mantle. Thus, devolatilization and thermal dynamic processes might be the dominating factors in reducing the lithospheric strength of the ENCC, and further weakening and reactivating the old Taihang orogen.

5.2. South China Block The South China Block is separated from the NCB by the Qinling–Dabie–Sulu orogenic belt to the north, and connects to the Tibetan Plateau with the Longmenshan–Ailaoshan to the west and southwest. The SCB consists of the Yangtze Block with Archean basement (Zheng et al., 2006), and the Cathayisa Block with basement of Neoproterozoic to early Paleozoic metamorphic rocks (Yu et al., 2008). Two blocks are linked along the Shaoxing– Jiangshan fault by the early Neoproterozoic (  900 Ma) subduction or collision event (Wang et al., 2007; Shu et al., 2011). The Yangtze block consists of the upper, middle and lower Yangtze blocks from west to east. Over the SCB, the Te values are generally lower than that over the NCB. Fig. 8 shows that low Te (10–20 km) prevails over the southeastern part of the craton, e.g. the Jiangshan–Shaoxing suture zone, Xuefeng Mountain, Nanling Mountain region, and Huangling anticline. The relatively high Te ( 460 km) is observed in the eastern Sichuan Basin and the Northern Jiangsu–South Yellow Sea basin (SB-SYSB). In the western SCB, the relatively high Te values mainly appear in the eastern Sichuan basin and the northern Guizhou. The Sichuan Basin is the major tectonic unit in the Yangtze Block. It consists of the Archean basement with the crustal thickness of about 41–47 km, heat flow 50–55 mW/m2 (Hu et al., 2000; Tao and Shen, 2008) and low tectonic and seismic activity. Te seems to be heterogeneous in the Sichuan basin. In the middle of the basin, there exists a low-Te zone (10–40 km) along the west of the Huanyingshan Fault. Along the eastern part of the fault, the relatively high Te ( 460 km) locates in the middle and east parts of the Sichuan basin, stretched to Chongqing, the northern

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from the surrounding strong convergence stresses, resulting in the broad fold belts and wide range of magmatic activity presently observed. Further east in the SCB, it appears remarkable high Te (60–100 km) in the SB-SYSB. This high-Te zone extends across the Su–Lu orogen; it is connected with the maximum in the Hehuai basin. The SB-SYSB belongs to the lower Yangtze Block, which is a Mesozoic–Cenozoic continental basin with the preSinian cratonic basement (Lee et al., 2006) experienced an extensive Cenozoic basaltic volcanism (Lu et al., 2013). The magnetic anomaly inversion (Li et al., 2009) showed that the Curie isotherm depth is evidently deeper in the SB-SYSB than in the surrounding areas, and this basin is a stable unit. Based on the study of peridotite xenoliths, Lu et al. (2013) pointed out that the lower Yangtze block had a thick Archean lithosphere, which had been reworked to 60–100 km during the late Mesozoic–Cenozoic. Comparing with the overall low Te of the other parts of the SCB, origin of the maximum in the SB-SYSB remains unclear. However, it is interesting to note that most of the earthquakes occurred not in the central part of the high Te zone but within the sharp Te gradient zones surrounding it (Fig. 8).

Guizhou and the northwestern part of Hunan. This relatively high-Te zone coincides well with the fast velocity anomaly at a depth of  300 km (Li and van der Hilst, 2010). Thus, these results indicate that the Sichuan is likely not a uniform rigid craton; and the old cratonic nucleus of the Yangtze Block may persist in the eastern Sichuan Basin, northern Guizhou, Chongqing and northwestern Hunan. Over most areas of the middle-eastern SCB, Te is very low (  10–20 km) to the east of the Wuling and Xuefeng mountains, especially along the Shaoxing–Jiangshan suture zone, the eastern Hunan, and the northern Guangdong, where Te is extremely low. Since the Mesozoic, the SCB was affected by the dynamic processes in the Pacific to the east, and the southwest collision of the India–Australian plate with Eurasia (Li et al., 2011). It experienced multiple tectonic activities and intracontinent deformations, resulting in the broad (  1300-km-wide) complex Intracontinental orogen and the wide (  1000 km) Jurassic–Cretaceous postorogenic magmatism in the middle and eastern part of the block (Li and Li, 2007). Li and Li (2007) pointed out that these complex deformations and magmatism are caused by the multiple effects from a flat-slab subduction of the old oceanic lithosphere into the upper mantle beneath the SCB since the earliest Permian. The traveltime tomography also reveals (Li and van der Hilst, 2010) some fast-velocity structures within the transition zone beneath the eastern SCB indicating ancient subducted fragments. In the upper mantle, the low-velocity anomalies are also widespread. Based on the above observations, we suggest that the lithospheric strength of the SCB (particularly in the eastern part) has been reduced by the underlying thermodynamic processes resulted from the subduction of the old oceanic lithosphere. If so, the weakened lithosphere may experience deformation

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5.3. Tibetan Plateau The Tibetan Plateau is the largest and highest plateau on the Earth with the average crustal thickness of  70 km (Li et al., 2008). The plateau is bounded by West Kunlun Shan–Altun Shan–Qilian Shan to the north, Longmenshan to the east, and Himalayas to the southwest. The Tibetan Plateau was formed due to the Cenozoic collision of India with Eurasia about 55 Ma ago (Tapponnier et al., 2001) in the southwest, and blocked by the

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Fig. 9. Te and the major tectonic boundaries of Tibetan Plateau modified from Tapponnier et al. (2001). The bold lines depict major faults and sutures. The purple triangles represent volcanoes. WKLF, West Kunlun Fault; XSF, Xianshuihe Fault; XJF, Xiaojiang Fault; MFT, Main Frontal Thrust; ITS, Indus–Tsangpo Suture; BNS, Banggong–Nuijang Suture; JRS, Jinsha River Suture; SHS, Shuanghu Suture; LSB, Lhasa block; QTT, Qiangtang terrane; SGT, Songpan–Ganzi terrane; HM, Himalayan terrane; TCV, Tengchong volcano. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

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rigid Tarim block to the north and the NCB and SCB to the east. Since the plateau interior experienced long-term strong horizontal compression, it is the most active region of the intra-continental tectonic deformation and seismic activity (Fig. 5b). The Tibetan Plateau mainly consists of the Himalayan terrane, the Lhasa block, the Qiangtang, Songpan–Ganzi and East Kunlun–Qaidam terranes separated by the Tsangpo, Banggong–Nujiang, Jinsha River and Ayimaqin–Kunlun suture zones from south to north. A remarkable feature in the Tibetan Plateau (Fig. 9) is the low Te zone (10–40 km) along the plateau margin. This zone further extends along the Altun Shan and Qilian Shan to the north, Longmenshan and Ailaoshan to the south, and the Himalayan orogen to the southwest. These relatively narrow orogens separate the Tibetan Plateau and the surrounding rigid cratons. Due to the inherited weak suture zones, young thermal age and repeated heating during the orogeny, the marginal orogens are characterized by extremely low Te, and absorb most of the deformations and stresses from the convergence of the India plate with Eurasia. Located between the rigid cratons, the orogenic lithosphere experiences strong deformation and horizontal shortening with active seismicity (Figs. 5b and 9). In the plateau interior, the relative high-Te is mainly observed in the Qaidam basin (50–70 km), the middle-west of the Lasha block and the eastern part of Himalayan terrane. Relatively low Te (10–30 km) are prevailing in the middle and eastern parts of the Tibetan Plateau, namely, northern and eastern Lhasa block, Qiangtang terrane, the western and middle parts of Songpan– Ganzi terrane, and along the East-Kunlun fault. It is also visible a broad low-Te zone ( 10–40 km) in the southeastern Tibetan Plateau. This minimum is connected with the low Te zone of the Longmenshan orogens in the north, and southward extends along the Red River Fault, across the Tengchong volcano to the southwest of SCB. This low-Te region generally correlates with the area of high heat flow (475 mW/m2) (Tao and Shen, 2008), low-velocities at a depth of the middle–lower crust (Xu et al., 2007; Yao et al., 2008) and active earthquakes. The widespread low in the middle and southeast of the Tibetan Plateau indicates that the lithosphere strength is much lower in these regions. The strain-rates inferred from GPS observations are consistent with our results. They show that the surface material within the plateau moves roughly eastward with the velocity increasing toward the east (Zhang et al., 2004). Also, P-wave (Li et al., 2006, 2008) and surface-wave (Yao et al., 2008) tomography found wide low-velocity zones in the crust and upper mantle beneath the eastern and southeastern Tibetan Plateau, which likely correspond to low mechanical strength. Moreover, magnetotelluric data in the eastern and southeast Tibetan Plateau (Bai et al., 2010; Rippe and Unsworth, 2010) suggest that there exist high conductivity layers beneath the Lhasa block and Qiangtang terrane indicating hot and fluid-rich middle–lower crust. Based on laboratory measurements by Rosenberg and Handy (2005), partial melt (5–10%) could reduce the crustal strength by one order of magnitude. In addition, possible mechanical decoupling between the crust and mantle with the accompanied ductile flow may principally reduce the lithosphere strength (Cloetingh et al., 2005; Tesauro et al., 2011, 2012). These factors are likely responsible for the weaker lithosphere in the middle, eastern and southeast Tibetan Plateau. Large intracontinental deformations likely occur in the weak zones characterized by low lithospheric strength. Our results reveal the low lithospheric strength in the suture zones and in the interiors of the middle, eastern and southeastern parts of the Tibetan Plateau. For this reason, we suggest that the deformation of the Tibetan lithosphere might be coherently distributed within the Plateau, particularly in the middle, eastern and southeastern parts. This result is also in agreement with the GPS data and shear wave splitting studies (Zhang et al., 2004; Sol et al., 2007).

6. Conclusions Base on the topography and gravity data from the new combined satellite-terrestrial model, Te variations over China and surrounding areas have been estimated. Generally, Te is high (480 km) in the cold stable cratonic blocks, such as the Siberian Craton, India Craton, the North China Block (NCB), the Songliao and Tarim basins, however significant lateral changes also exist within these structures. Low Te ( o40 km) generally correspond to the young Phanerozoic orogens, e.g. Qilian Shan, Himalayas, Qinling–Dabie, Taihang Mountains and some others. Comparison of the Te spatial variations and distribution of earthquakes indicates that most of the earthquakes are situated in the low Te areas or steep Te gradient zones, whereas the stable areas with high Te are characterized by a lack of seismicity. This suggests that the stable tectonic provinces with high Te effectively resist deformation, while the weak lithosphere and areas with steep change of Te are prone to accumulate and then release tectonic stresses causing earthquakes. In the NCB, high Te mainly appear in the Ordos ( 4100 km), the eastern Bohai Bay basin ( 70 km) and Hehuai basin (480 km). Low Te is observed along the Yanshan–Taihang Mountain orogens (10–40 km) in the middle of the NCB. The effective elastic thickness of the Ordos craton with the lithospheric root of 200– 300 km is much higher than the average crustal thickness (  42 km). This suggests that the high Te in the old stable craton largely relates to a strong and thick lithospheric mantle. Compared with the stable West Block with remarkably higher Te, the Te values within the East Block are lower and vary dramatically. We suggest that the originally strong Archean lithosphere of the eastern NCB has been weakened due to subduction of the ocean plate during the Phanerozoic. Nevertheless, the localized high Te zones indicate that the Archean mantle relics are likely preserved in some parts. The high Te zone within the South China block (SCB) is observed in the Northern Jiangsu–South Yellow Sea basin (60– 100 km). Relatively high Te are also found in the eastern Sichuan Basin and northern Guizhou in the west of the Yangzte Block (460 km), which is in agreement with the tomography results indicating that the Sichuan basin is not a uniform rigid craton. We suppose that the old cratonic nucleus of the Yangtze Block may exist in the eastern Sichuan Basin, northern Guizhou, Chongqing and northwestern Hunan. Our results show that the Te values over the SCB are generally lower than that in the NCB. Low Te ( o20 km) prevail over most areas of the middle and eastern parts of the SCB. Combining with the tomography models, we propose that the lithospheric strength of the SCB (especially in the eastern part) has been reduced by the processes related to the subduction of the old oceanic lithosphere. In turn, the weakened lithosphere tends to deform under the surrounding strong convergence stresses, leading to the broad fold belts and a wide range of magmatic activity observed today. Within the Tibetan Plateau, the Te values along the blocks are general lower in the middle, southeastern parts (  10–40 km) and along the weak suture zones between the blocks. The prevailing low Te values of the Tibetan Plateau indicate that these parts are characterized by a reduced strength compared to surrounding rigid blocks. The weakness might be associated with ductile crustal flow, hot upper mantle and high thermal regime. The variations of Te over the NCB, SCB, and Tibet Plateau together with the geological, GPS, and seismic tomography data indicate that the lithospheric strength of China Mainland is a result of various interacting tectonic processes, which continuously modified and reworked the lithosphere structure. In particularly, hydration and thermal dynamic processes associated with the subduction of the ancient Pacific plate under Eurasia, and the

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rapid convergence of India and Eurasia in the southwest during Mesozoic–Cenozoic are the dominant processes, which modified (mainly weaken) the lithosphere of the China Mainland.

Acknowledgments The authors thank Marta Perez-Gussinye for providing the Multitaper coherence codes for this study. We thank Donald Forsyth and an anonymous reviewer for constructive reviews. We also thank Wei Fan, Yongdong Li and Yadan Mao for their helpful comments. This research is supported by the Program of International Science and Technology (S&T) Cooperation (Grant no. 2010DFA24580). All projected figures are drawn using GMT (Wessel and Smith, 1991). References Armstrong, G.D., Watts, A.B., 2001. Spatial variations in Te in the southern Appalachians, eastern United States. J. Geophys. Res. 106, 22009–22026. ¨ Audet, P., Burgmann, R., 2011. Dominant role of tectonic inheritance in supercontinent cycles. Nat. Geosci. 4, 184–187, http://dx.doi.org/10.1038/NGEO1080. Bai, D.H., Unsworth, M.J., Meju, M.A., Ma, X.B., Teng, J.W., Kong, X.R., Sun, Y., Sun, J., Wang, L.F., Jiang, C.S., Zhao, C.P., Xiao, P.F., Liu, M., 2010. Crustal deformation of the eastern Tibetan plateau revealed by magnetotelluric imaging. Nat. Geosci. 11, 1–5, http://dx.doi.org/10.1038/NGEO830. Banks, R.J., Parker, R.L., Huestis, S.P., 1977. Isostatic compensation on a continental scale: local versus regional mechanisms. Geophys. J. R. Astron. Soc. 51, 431–452. Bassin, C., Laske, G., Masters, G., 2000. The current limits of resolution for surface wave tomography in North America. EOS Trans. AGU 81, F897. Bechtel, T.D., Forsyth, D.W., Sharpton, V.L., Grieve, R.A.F., 1990. Variations in effective elastic thickness of the North American lithosphere. Nature 343, 636–638. Bird, P., 2003. An updated digital model of plate boundaries. Geochem. Geophys. Geosyst. 4 (3), 1027, http://dx.doi.org/10.1029/2001GC000252. Braitenberg, C., Wang, Y., Fang, J., Hsu, H., 2003. Spatial variations of flexure parameters over the Tibet–Quinghai plateau. Earth Planet. Sci. Lett. 205, 211–224. Burov, E.B., Diament, M., 1992. Flexure of the continental lithosphere with multilayered rheology. Geophys. J. Int. 109, 449–468. Burov, E.B., Diament, M., 1995. The effective elastic thickness (Te) of continental lithosphere: What does it really mean? J. Geophys. Res. 100 (B3), 3905–3927, http://dx.doi.org/10.1029/94JB02770. Cloetingh, S., Ziegler, P., Beekman, F., Andriessen, P., Matenco, L., Bada, G., Garcia-Castellanos, D., Hardebol, N., De´zes, P., Sokoutis, D., 2005. Lithospheric memory, state of stress and rheology: neotectonic controls on Europe’s intraplate continental topography. Quat. Sci. Rev. 24 (3–4), 241–304. Dorman, L.M., Lewis, B.T.R., 1970. Experimental isostasy, 1, theory of the determination of the earth’s isostatic response to a concentrated load. J. Geophys. Res. 75, 3357–3365. Fielding, E.J., McKenzie, D., 2012. Lithospheric flexure in the Sichuan Basin and Longmen Shan at the eastern edge of Tibet. Geophys. Res. Lett. 39, L09311, http://dx.doi.org/10.1029/2012GL051680. Forsyth, D.W., 1985. Subsurface loading estimates of the flexural rigidity of continental lithosphere. J. Geophys. Res. 90, 12623–12632. Gao, S., Rudnick, R.L., Carlson, R.W., McDonough, W.F., Liu, Y., 2002. Re–Os evidence for replacement of ancient mantle lithosphere beneath the North China Craton. Earth Planet. Sci. Lett. 198, 307–322. Goetze, C., Evans, B., 1979. Stress and temperature in the bending lithosphere as constrained by experimental rock mechanics. Geophys. J. R. Astron. Soc. 59, 463–478. Hyndman, R.D., Currie, C.A., Mazzotti, S.P., Frederiksen, A., 2009. Temperature control of continental lithosphere elastic thickness, Te vs. Vs. Earth Planet. Sci. Lett. 277, 539–548. Hu, S.B., He, L.J., Wang, J.Y., 2000. Heat flow and thermal regimes in the continental area of China: a new data set. Earth Planet. Sci. Lett. 179 (2), 407–419. Huang, J., Zhao, D., 2006. High-resolution mantle tomography of China and surrounding regions. J. Geophys. Res. 111 (B9), B09305, http://dx.doi.org/ 10.1029/2005JB004066. Jordan, T.A., Watts, A.B., 2005. Gravity anomalies, flexure and the elastic thickness structure of the India–Eurasia collisional system. Earth Planet. Sci. Lett. 236, 732–750. Karner, G.D., Watts, A.B., 1983. Gravity anomalies and flexure of the lithosphere at mountain ranges. J. Geophys. Res. 88, 10449–10477. Kirby, J.F., Swain, C.J., 2004. Global and local isostatic coherence from the wavelet transform. Geophys. Res. Lett. 31 (24), L24608, http://dx.doi.org/10.1029/ 2004GL021569. Kirby, J.F., Swain, C.J., 2008. An accuracy assessment of the fan wavelet coherence method for elastic thickness estimation. Geochem. Geophys. Geosyst. 9,

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