Effective elastic thickness and mechanical anisotropy of South China and surrounding regions

Tectonophysics 550-553 (2012) 47–56

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Effective elastic thickness and mechanical anisotropy of South China and surrounding regions Xiaolin Mao a, Qin Wang a,⁎, Shaowen Liu b,⁎, Minjie Xu a, Liangshu Wang a a b

State Key Laboratory for Mineral Deposits Research, Department of Earth Sciences, Nanjing University, Nanjing 210093, China Key laboratory of Coast and Island Development, Ministry of Education, Nanjing University, Nanjing 210093, China

a r t i c l e

i n f o

Article history: Received 3 August 2011 Received in revised form 8 May 2012 Accepted 23 May 2012 Available online 2 June 2012 Keywords: Effective elastic thickness Mechanical anisotropy Seismicity South China Oceanic subduction

a b s t r a c t South China and surrounding regions extend from the eastern Tibetan plateau, through the tectonically stable Sichuan basin and the broad Mesozoic magmatic and fold belt, to the trench-arc-basin system in the western Pacific which provide an ideal place to study deformation of the continental lithosphere under long-term magmatism and oceanic subduction. We obtained the effective elastic thickness (Te) and its anisotropy of South China and surrounding regions from the analysis of coherence between topography and satellite gravity using wavelet methods. The Te values of the study area vary from 2 to 75 km, with relatively low Te values (≤30 km) along the tectonic boundaries, the North–South Gravity Lineament (NSGL) and seismic zones, and in the regions with high surface heat flow. The evenly low Te values in the Lower Yangtze region and the Cathaysia block can be attributed to the long-lived subduction of the Paleo-Tethys and the Paleo-Pacific ocean basins beneath the South China block (SCB). The NSGL in the SCB may separate the unmodified (high Te) and thermally weakened (low Te) continental lithosphere due to oceanic subduction. Despite different distances to the tectonic boundaries, earthquakes occur more frequently in regions with Te values of 10–30 km, implying strain concentration in the low-Te regions. A positive correlation between seismic activity and the magnitude of Te anisotropy suggests that a highly anisotropic mechanical structure will promote strain localization and brittle failure in the lithosphere. The poor correlation between the weak axis of Te anisotropy and the dynamic indicators of the present tectonic regime (the shear-wave splitting direction, the maximum horizontal compressive stress direction) confirms that Te anisotropy mainly reflects tectonic inheritance of the continental lithosphere. © 2012 Elsevier B.V. All rights reserved.

1. Introduction Spatial variation of the lithospheric strength is a key to link the distribution of orogens, faults and earthquakes with dynamic forces (e.g., plate boundary forces). Unfortunately, it is very difficult to measure the lithospheric strength directly. As a proxy for the integrated lithospheric strength, the effective elastic thickness of the lithosphere (Te) is defined as the thickness of an ideal elastic plate that floats over viscous fluid and would bend by the same amount as the lithosphere under the same applied loads (e.g., Watts, 2001). The relationship between Te and the flexural rigidity (D) is D¼

ET 3
e 2 12 1−ν

ð1Þ

where E is the Young's modulus and ν is the Poisson's ratio of the lithosphere. Because Te is a measure for the integrated yield strength

⁎ Corresponding authors. Tel.: +86 25 8359 6887; fax: +86 25 8368 6016. E-mail addresses: [email protected] (Q. Wang), [email protected] (S. Liu). 0040-1951/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.tecto.2012.05.019

of a lithosphere that is both brittle and ductile, the lateral variations of Te can be used to investigate the correlation between surface tectonics and lithospheric structure (Pérez-Gussinyé et al., 2009), and to predict the strain concentration and hence locations of brittle failure in the lithosphere (Audet et al., 2007; Lowry and Smith, 1995). For the oceanic lithosphere that is young and can be approximated as a single-layer rheological structure, Te depends on the thermal age of the lithosphere and varies from 2 to 50 km (Burov and Watts, 2006; Watts, 1978). By contrast, the continental lithosphere has a long evolution history, very heterogeneous composition and a multi-layer rheological structure. Te of the continental lithosphere is primarily controlled by temperature, composition and state of stress of the lithosphere, with values from 2 km in some active orogens and rifts to ~100 km in stable cratons (Burov and Diament, 1995; Hyndman et al., 2009; Pérez-Gussinyé et al., 2004). Te anisotropy reflects the azimuthal difference in mechanical strength of the lithosphere and may be influenced by many factors including localized brittle failure of crustal rocks under deviatoric stress (Lowry and Smith, 1995), “frozen” deformation by alignment of olivine in the lithospheric mantle (Kirby and Swain, 2006; Simons et al., 2003), and large-scale tectonic features and faults (Audet and Mareschal, 2004; Burov et al., 1998). Te anisotropy fills the gap between deep

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anisotropy (e.g., seismic anisotropy) and surficial mechanical anisotropy. Different relationships between the weak axis of Te anisotropy (φe) and dynamic indicators (e.g., shear-wave splitting directions (φs), maximum horizontal compressive stress direction (φh)) have been proposed, which provide valuable information on the evolution of the continental lithosphere. For example, Kirby and Swain (2006) found that φe is nearly perpendicular to φs in Australia, while Audet et al. (2007) obtained φe parallel to both φs and φh in western Canada, suggesting that Te anisotropy is not only determined by the present tectonic regime. According to the lack of preferred angular relationship between Te anisotropy and the dynamic indicators in a worldwide comparison, Audet and Burgmann (2011) proposed that Te anisotropy is mainly inherited from pre-existing tectonic structures. South China and surrounding regions extend from the eastern Tibetan plateau, through the cratonic Sichuan basin and the broad Mesozoic magmatic and fold belt, to the trench-arc-basin system in the western Pacific (Fig. 1). So far our knowledge about Te and Te anisotropy of South China is only from a global study of Audet and Burgmann (2011), which hampers our understanding of the complex deformation in the continental lithosphere under long-term magmatism, oceanic subduction and continental collision. In addition, the relationships of Te and Te anisotropy with other geophysical proxies are not clear yet. It is necessary to perform an integrated study to clarify tectonic implications of Te and Te anisotropy. In this paper, we present high-resolution Te and Te anisotropy of South China and surrounding regions using a wavelet coherence method. The low Te values in eastern part of South China correspond to the high heat flow and widespread Mesozoic and Cenozoic igneous rocks. The correlation between the earthquake distribution, Te and Te anisotropy suggests that the way of the lithospheric stress release strongly depends on Te. Comparison of Te anisotropy with the dynamic indicators of φs and φh confirms that Te anisotropy is mainly controlled by pre-existing tectonic structures.

2. Geological setting The South China block (SCB) was formed by collision of the Yangtze and Cathaysia blocks around 0.8–1 Ga (Li et al., 2002; Zhou and Zhu, 1993). Recent U/Pb and Sm/Nd dating on ophiolitic rocks of the Song Ma suture zone (northern Vietnam) indicates the existence of Paleotethyan lithospheric remnants at 387–313 Ma between South China and Indochina (Vuong et al., in press). The final closure of the Paleo-Tethys ocean and subsequent subduction of the Indochina block beneath the SCB probably occurred in the early-middle Triassic along the Song Ma suture zone, according to the coeval eclogite facies (243–238 Ma) and high-pressure granulite facies metamorphism (233± 5 Ma) in this region (Nakano et al., 2008, 2010), ductile deformation and high-temperature metamorphism (250–240 Ma) in northern Vietnam (Carter et al., 2001; Lepvrier et al., 2004), and syncollisional granites in the Yunkai massif (250–242 Ma) and the Shiwandashan granitoid belt (236–230 Ma) in the southwestern SCB (e.g., Deng et al., 2004; Wang et al., in press; Zhou et al., 2006). In the north, the SCB subducted beneath the North China block in the early Triassic and the syn-collisional exhumation of the continental crust resulted in the widespread high-pressure and ultrahighpressure metamorphic rocks in the Qinling-Dabie-Sulu orogen (e.g., Liou et al., 2009; Xu et al., 2009; Zheng, 2010). By the late Triassic, the South China, North China, Indochina, Simao and Sibumasu blocks have accreted to form the proto-East Asia (e.g., Lepvrier et al., 2004; Metcalfe, 2006). South China is famous for the massive Mesozoic granitoids and volcanic rocks (Fig. 2). The early Mesozoic magmatism (Indosinian episode, T1–T3) in the SCB is characterized by dominant S-type granites and minor calc-alkaline I-type granites (e.g., Li and Li, 2007; Zhou et al., 2006), with coeval volcanic rocks restricted in the western Youjiang basin (Ren et al., 1999). The outcrop area of the Triassic

Fig. 1. Topography/bathymetry (a) and tectonic outline (b) of South China and surrounding regions. The solid black lines refer to faults and block boundaries and the dashed black line indicates a suspicious extension of the Jiangshao fault (modified after Yin, 2010). Both the GPS velocity vectors and the relative motions of surrounding plates are with respect to the stable Eurasia (after Wang et al., 2001). The bold arrows denote the motion directions of the Indian, Pacific and the Philippine Sea plates relative to the Eurasian plate. The seismic data are from IRIS, and the volcanic data are from Global Volcanism Program. Abbreviations: CDT: Chuandian terrane; HHB: Hehuai basin; JHB: Jianghan basin; JSF: Jiangshao fault; LTB: Longmenshan thrust belt; LYR: Lower Yangtze region; MT: Manila Trench; NSGL: North–South Gravity Lineament; OT: Okinawa Trough; PSP: Philippine Sea plate; PT: Philippine Trench; RT: Ryukyu Trench; SBB: Subei basin; SCB: Sichuan basin; SGFB: Songpan-Ganzi fold belt; XXFS: Xianshuihe–Xiaojiang fault system; YJB: Youjiang basin.

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Fig. 2. Outcrops of Mesozoic and Cenozoic igneous rocks (modified after Ren et al., 1999; Zhou et al., 2006). Abbreviations are the same as Fig. 1.

granitoids is about 15,000 km2, accounting for ~7% of the total exposed Mesozoic granitoids in South China. After a magmatic quiescence between 205 and 192 Ma, the SCB was subjected to the widespread late Mesozoic magmatism (Yanshanian episode). The Jurassic and Cretaceous granitoids cover an area over 200,000 km 2, composed of predominant I-type granites with less S- and A-type granites and associated with coeval volcanic rocks (Zhou et al., 2006). In addition, the late Mesozoic magmatism produced a giant ~3500-km-long and ~800-km-wide NE-trending magmatic belt in eastern China, indicating the significant influence of the NW–WNW-ward subduction of the Izanagi plate (also inferred as the Paleo-Pacific plate by some authors) (Wu et al., 2005; Xu, 2007; Zhou et al., 2006). The Pacific plate was formed at 175–170 Ma as a small triangle between the Farallon, Phoenix, and Izanagi plates (Müller et al., 2008; Smith, 2007). Following the subduction of Izanagi–Pacific spreading ridge beneath southern Japan at 65–60 Ma, the Pacific plate began to play a critical role in tectonics of East Asia. The transition from the Izanagi plate to the Pacific plate probably triggered a rapid eastward migration of the western Pacific trench system, as indicated by the sharp decrease in Pacific–Eurasia convergence rate from ~120 to 140 mm/y in the late Cretaceous to 30–40 mm/y in the Eocene, widespread extension along the eastern margin of Eurasia between 60 and 40 Ma, and eastward younging of volcanic arcs from late Cretaceous to early Tertiary (Northrup et al., 1995; Ren et al., 2002; Yin, 2010). This extensional event resulted in several back-arc basins and a highly extended (and thus thinned) continental margin in the eastern SCB, i.e., in the Yellow Sea, the East China Sea and the South China Sea. In addition, the South China Sea basin developed the oceanic crust in its central part between 38 and 23 Ma due to the localized intense extension (Briais et al., 1993; Zhou et al., 1995). Since 15 Ma, the seafloor spreading ceased in the eastern margin of Asia and the east–west contractional deformation prevailed. The Eurasian plate started subduction beneath the Philippine Sea plate and collision of the Luzon arc with Asia created the Taiwan orogenic belt (Sibuet et al., 2002; Teng, 1990). The convergence of the South China Sea and the Philippine Sea plate along the Luzon arc, and subduction of the Philippine Sea plate beneath Asia along the Ryukyu Trench form an active seismic zone (Figs. 1 and 8). In the west, due to the Indo-Asia collision, the Longmenshan thrust belt, the Xianshuihe–Xiaojiang

fault system and the Red River fault zone comprise a boundary between the relatively stable SCB and a north-trending seismic zone in the eastern Tibetan plateau, which corresponds to the fast surface velocity in GPS observations (Wang et al., 2001; Yin, 2010; Zhang et al., 2004). It is worthy to note that a NEE-trending Bouguer gravity anomaly gradient, known as the Daxing'anling–Taihangshan or North–South Gravity Lineament (NSGL), extends over 3500 km from NE China to South China. The NSGL is an important boundary of surface topography, magmatism, crustal and lithospheric structure between east and west China (e.g., Ma, 1987; Xu, 2007). Huang and Zhao (2006) found that the NSGL from NE China to the Lower Yangtze region roughly coincides with the western edge of the stagnant Pacific slab in the mantle transition zone. However, in the region south of 27˚N latitude, the Pacific slab rolls back toward the trench side and penetrates directly into the lower mantle, while the subducting Philippine Sea slab and the Eurasian slab may have reached to the mantle transition zone and become stagnant there together with the broken Pacific slab (Huang and Zhao, 2006; Huang et al., 2010). 3. Method and data 3.1. Isotropic Te We use a high space-resolution fan wavelet transform of Kirby and Swain (2011). In their method, the value of the central wavenumber pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi of the Morlet wavelet, |k0|, is set to be π 2= ln16 ≈ 2.668, and a complete 2D Morlet wavelet as follows

ψðxÞ ¼

ik0 x

e

−jk0 j2 2

−e

!

−jxj2 2

e

ð2Þ

where x refers to the space location and ψ refers to the wavelet, is used to replace the 2D Morlet wavelet. The wavelet transform is then applied to the Bouguer gravity anomaly and topography to calculate the autoand cross-spectra at different azimuths and scales. After averaging the auto- and cross-spectra over an azimuthal extent of 180° (90° for the anisotropic case as discussed below), the Bouguer gravity anomaly

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and topography are used to obtain the observed coherence between the two variables from the equation 2

γ ðx; sÞ ¼

   hGH ihGH i  hGG ihHH  i

ð3Þ

where s is the wavelet scale, the angular bracket means average over the azimuthal extent, and * indicates complex conjugation. According to Kirby and Swain (2009), we substitute the coherence with the square of the real part of coherency (SRC) 2

Γ R ðx; sÞ ¼

ðRehGH  iÞ2 hGG ihHH  i

ð4Þ

elevation and free-air gravity anomaly to the Cartesian coordinate system using the Lambert conic conformal projection, and re-sample into 10 km× 10 km grids. The Bouguer gravity anomaly (Fig. 3) is obtained by performing complete terrain correction to free-air gravity anomaly using the FA2BOUG software of Fullea et al. (2008). We use the crustal structure model of CRUST2.0 (Bassin et al., 2000), and assume that the Poisson's ratio ν = 0.25, the Young's modulus E = 1 × 1011 Pa, the density of the top-crust ρt = 2670 kg m − 3, the sea water density ρw = 1030 kg m − 3, the whole crust density ρc = 2800 kg m − 3, and the mantle density ρm = 3300 kg m − 3. 4. Results 4.1. Spatial variations of Te

where Re denotes that only the real part is used. Using the model of Forsyth (1985) with assumed Te, we perform load deconvolution and get the predicted coherence between different loads. Finally, we use the misfit (L2 norm) between the observed SRC and predicted coherence to find the best-fitting Te. Bathymetry is converted to equivalent topography to avoid two independent analyses (e.g., Stark et al., 2003). In the Te calculation, noise is defined as the internal load without topographic expression but having effects on the final Te values (McKenzie, 2003). To evaluate the influence of noise, Kirby and Swain (2009) use the maximum of normalized squared imaginary part of the coherency (NSIC) between free-air gravity anomaly and topography for three wavelengths that are immediately shorter than the observed Bouguer SRC transition wavelength (where SRC near 0.5). When the maximum NSIC exceeds 0.5, the noise is not ignorable. Here we apply this noise analysis method to our isotropic Te map. 3.2. Anisotropic Te Calculation of Te anisotropy also includes inversion of observed SRC and modeling of predicted coherence, however, the auto- and crossspectra are averaged over an azimuthal extent of 90° at 36 central azimuths (from 0° to 175° with 5° increments) to get the relative observed SRCs. Theoretical coherences on the six central azimuths are from Forsyth's model (1985), which can be adapted to an orthotropic plate model (Kirby and Swain, 2006) assuming load ratio of 1. There are three parameters in the orthotropic plate: the maximum Te (Tmax); the minimum Te (Tmin) at a perpendicular angle; and the direction of Tmin (φe). Unlike Kirby and Swain (2006), we first determine the weakest and strongest axes of the Te anisotropy using the transition wavelengths of the observed SRCs. In this approach the shortest transition wavelength is used to infer the weakest Te anisotropy direction, which is similar with the maximum coherence method used by Audet and Mareschal (2007). Then, we minimize the difference between the theoretical coherences and the observed SRCs on all central azimuths to get the best-fitting Tmax and Tmin. Although our method is more timeconsuming, it allows better constraints on the direction of Te anisotropy, which is the most difficult parameter to estimate. In this study, both Te and Te anisotropy are calculated in the Cartesian coordinate system, and the results are shown in geographical coordinate system using a Mercator projection.

As illustrated in Fig. 4, Te of South China and surrounding regions exhibits a mosaic distribution with values from 2 to 75 km, which are closely related with tectonic boundaries. Te values along the boundaries of the South China block are relatively lower than the surrounding regions. The Yangtze block can be divided into three parts with Te values decreasing from west to east: Te reaches 20–40 km in the Sichuan basin, 15–30 km in the Jianghan basin, and 5–15 km in the Lower Yangtze region. Similar to the Lower Yangtze region, most part of the Cathaysia block has Te values around 5–15 km, but locally Te of ~30 km is observed. To the north of the Qinling-Dabie-Sulu orogenic belt, the southern part of the North China craton is composed of a low-Te region along the NSGL between two relatively high-Te blocks. The NSGL is characterized by the relatively low Te values in the South China block as well. In addition, the Tibetan plateau shows elevated Te values to the west of the seismic zones (seismicity vs Te is shown in Fig. 8a). The correlation between the low Te values and tectonic boundaries is more evident off the continent. Te values are very low for regions under intensive extension, e.g., 2–8 km around the spreading center in the South China Sea and 8–10 km near the Okinawa Trough. Watts (1978) has found that away from the ridge, Te values increase with cooling of the oceanic lithosphere. In this study, there are two blocks with Te of 20–30 km to the west of the Manila Trench in the South China Sea, and a block with Te of 25–35 km to the south of the Ryukyu Trench in the Philippine Sea plate. In addition, Te values decrease sharply under the trenches due to the bending of the lithosphere, e.g. the Manila

3.3. Data Bouguer gravity anomaly and topography/bathymetry are required for the coherence analysis, and the crustal structure is needed in the load deconvolution. We use free air gravity anomaly data from the V18 Gravity Anomaly model, which provides 1′ × 1′ grid on both land and ocean (Sandwell and Smith, 2009). The elevation data are derived from ETOPO1 digital elevation model (DEM) on 1′ × 1′ grid node (Amante and Eakins, 2009). To avoid boundary effects, the original data cover a much larger area than the study area (Fig. 1). We project

Fig. 3. Map of the Bouguer gravity anomaly of South China and surrounding regions. The blue line stands for the North–south Gravity Lineament (NSGL).

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several weakly anisotropic areas with higher Te values, e.g., the Sichuan basin and the western part of the North China craton. In the areas with extremely low Te, the Te anisotropy is often high. Interestingly, regions under extensive extension, e.g., the ancient ridge of the South China Sea, show random Te anisotropy directions. Conversely, Te anisotropy directions are well ordered in compressional areas, e.g., the trenchparallel φe is observed in the Manila Trench, the Philippine Trench and the Ryukyu Trench. In addition, φe has a high angular relationship to the coastline, which is similar with the case in Australia (Kirby and Swain, 2006). 5. Discussion 5.1. Noise analysis

Fig. 4. Te map of South China and surrounding regions. Abbreviations are the same as Fig. 1.

Trench, the Philippine Trench and the Ryukyu Trench. However, it is noteworthy the Yellow Sea, the Subei basin and the Hehuai basin have the highest Te of 50–75 km in the study area, although they experienced significant back-arc extension in the Cenozoic and affected by the activation of the Tanlu fault. This discrepancy may be caused by gravitational noise, which will be discussed in Section 5.1. 4.2. Te anisotropy Our Te anisotropy results are obtained on a 40 × 40 km grid by spatially averaging the auto- and cross-spectra of G and H in 40 × 40 km bins to get the observed SRCs. The averaging procedure does not alter the anisotropy directions in regions where the Te anisotropy is well aligned. However, in the regions where the weak axis is poorly aligned, the averaging results may give a mean weak anisotropic structure. As shown in Fig. 5, the black bar indicates the direction of the weakest lithospheric strength (φe). Most part of the studied area presents moderate or strong anisotropy ((Tmax − Tmin)/Tmax >0.3). There are

Fig. 5. Variations of Te anisotropy in South China and surrounding regions, obtained by spatially averaging the auto- and cross-spectra in 40 × 40 km bins.

Fig. 6 illustrates variances of topography and bathymetry and the regions that are likely to be affected by significant noise. As pointed out by Kirby and Swain (2009), significant noise generally appears in subdued regions. Such noise is found in the regions with low topography variance, such as the Yellow Sea, the East China Sea, the Beibu gulf basin and the Zhujiang delta region near the south coastline of South China. However, we find that significant noise also exists in regions with sharp topography, e.g., in the Dabie Mountain, the Xuefeng Mountain (western part of the Jiangnan Neoproterozoic orogenic belt), and the Ryukyu Trench, which are all convergent boundaries. In fact, only the vertical force is taken into account in the isostatic analysis. However, the horizontal tectonic stress may also contribute to the balance of the loads in convergent boundaries. If the internal load is not balanced by topography, coherence between the Bouguer gravity anomaly and the topography will decrease, which gives rise to the significant noise in convergent boundaries. As shown in Fig. 4, except the Yellow Sea and its adjacent basin, Te variations agree well with geological boundaries and tectonic regimes, suggesting that Te values are not significantly affected by noise. The Yellow Sea has the highest Te and smooth topography, but relatively high surface heat flow and remarkable Cenozoic extension. In the global Te map of Audet and Burgmann (2011), the relatively high Te of the Yellow

Fig. 6. Topography/bathymetry variances of South China and surrounding regions, calculated in a moving window of 100 × 100 km. The scale bar shows the magnitudes of the logarithm of the variances. The shaded areas have the maximum of normalized squared imaginary part of the coherency between free-air gravity anomaly and topography for three wavelengths that are immediately shorter than the observed Bouguer SRC transition wavelength (NSIC) >0.5, which implies possible influence of significant noise to the Te results. Abbreviations: BBGB: Beibu gulf basin; ZJD: Zhujiang delta, and others are the same as Fig. 1.

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Sea is also biased by gravitational ‘noise’. Therefore our high Te values of the Yellow Sea may be caused by noise, but the estimation of Te for other regions is reliable. 5.2. Te with geological boundaries and surface heat flow As shown in Fig. 4, Te values are low (b 20 km) along the active boundaries of different geological blocks: the Manila Trench, the Philippine Trench and the Ryukyu Trench in the ocean; the QinlingDabie orogen, the Longmenshan thrust belt, the Xianshuihe–Xiaojiang fault system, the Red River fault, the Nan-Uttaradit and Song Ma suture zones in the continent. This suggests the interaction between different blocks has reduced the strength of the lithosphere near the boundaries, which in turn promotes the stress concentration and brittle failures as indicated by the large number of earthquakes along these boundaries (Fig. 8a). Along the NSGL, we reveal a north-trending low Te zone. Although the NSGL is not considered as an important geological boundary in South China at surface, it should be a deep tectonic boundary related to the oceanic subduction of the Pacific plate (see discussion in Section 5.3). Surface heat flow serves as a proxy for the thermal state of the lithosphere. However, it is strongly affected by the crustal heat production and dynamical effects of subduction, erosion, and hydrologic flow (Mareschal and Jaupart, 2004). Despite some uncertainties in the surface heat flow in Fig. 7, distribution of heat flow correlates well with seismic structure and Te values of the lithosphere, suggesting that the thermal state of a lithosphere has profound influence on its strength. For example, the Sichuan basin in the western Yangtze block are characterized by low heat flow (Hu et al., 2000; Wang, 2001), a normal three-layered crust and a ~ 200-km-thick lithosphere with high seismic velocity (Huang and Zhao, 2006; Xu et al., 2007), and high Te values, suggesting that the Late Archean-Paleoproterozoic nucleus of the Yangtze block remains unaffected. By contrast, the Cathaysia block and the Lower Yangtze region show high heat flow, low velocity anomalies in the middle crust and upper mantle (Huang and Zhao, 2006; Huang et al., 2010; Zhang et al., 2005), and low Te values, indicating a reworked continental lithosphere by long-term magmatism and oceanic subduction. The highest heat flow in the South China Sea and Okinawa Trough is associated with the lowest Te values. 5.3. Te and oceanic subduction From the coastline to the NSGL, the Lower Yangtze region and the Cathaysia block show evenly low Te values (b20 km) over a great area

Fig. 7. Distribution of surface heat flow in South China and surrounding regions (modified from Hu et al., 2000; Wang, 2001).

(Fig. 4). Tomography images reveal different upper mantle structures in the SCB (Huang and Zhao, 2006). In the Lower Yangtze region, the stagnant Pacific slab appears as high-Vp anomalies in the mantle transition zone, above which is a big mantle wedge with relatively low-Vp anomalies. By contrast, the Cathaysia block is characterized by vast low-Vp anomalies from ~1200 to 200 km, indicating an upwelling mantle flow from the lower mantle to upper mantle. Because the Cenozoic extension, thinning of the continental crust and synchronous eruption of basalts concentrated in the eastern margin of the SCB, the low Te values over such a great spatial extent cannot be attributed to the subduction of the Pacific plate or the Philippine plate. The coincidence of low Te values with the broad South China Mesozoic magmatic province and the low-Vp anomalies in the upper mantle implies a link between Te, magmatism and the long-lived oceanic subduction. As mentioned above, the SCB was subjected to the northward subduction of the Paleo-Tethys ocean in the Permian and the subsequent subduction of the Indochina block in the early-middle Triassic (Lepvrier et al., 2004; Nakano et al., 2008, 2010). Therefore the early Triassic granites in the southwestern SCB were regarded as synorogenic granites during the Indochina–South China collision, while the widespread S-type granites were post-collisional granites due to partial melting of a thickened crust (Deng et al., 2004, 2012; Wang et al., in press; Zhou et al., 2006). However, this model can not explain why the Triassic thrust belts extend so far into the interior of the SCB and exhibit the NE-striking orientation (orthogonal to the QinlingDabie orogen and the Song Ma suture zone). Li and Li (2007) proposed a ~ 1300-km-wide South China intracontinental orogen that migrated from the coastal region into the continental interior between ~ 250 Ma and 190 Ma, and attributed it to a flat subduction of an oceanic plateau of ~ 1000-km diameter beneath southeastern South China. Unfortunately, recent compilation on the Triassic igneous rocks does not show any temporal–spatial propagation trend (Wang et al., in press). Compared with the western North America that was subjected to the flat subduction of the Farallon plate in the late Cretaceous (Liu et al., 2008; van der Lee and Nolet, 1997), the predominance of the Triassic S-type granites and the absence of coeval volcanic rocks in the SCB also argues the validity of the flat-slab model. It is interesting to notice that the subduction-related calc-alkaline I-type granite with age of 267–262 Ma (Li et al., 2006) and the shoshonitic intrusions of 272 ± 7 Ma (Xie et al., 2006) in Hainan Island, K-rich syenites with age of 254–242 Ma in western Fujian Province (Wang et al., 2005), and detrital zircons of ~280 Ma in the Cathaysia block, implying the occurrence of an Early Permian continental active margin in southeastern China (Li et al., 2011). In fact, paleogeographic reconstruction suggests that in the Permian the Paleo-Tethys ocean and the Paleo-Pacific ocean simultaneously subducted under the SCB in the western and eastern sides, respectively (Ogg et al., 2008), which may have weakened the overriding continental lithosphere by complex corner flow in the mantle wedge. After closure of the PaleoTethys ocean along the Song Ma suture zone, subduction of the Indochina block beneath the SCB was very rapid and short-lived as indicated by coeval eclogite and granulite facies metamorphism at 243–233 Ma (Nakano et al., 2008, 2010). The north–south compression of the Indochina–South China collision may have been compensated by the simultaneous northward subduction of the SCB beneath the North China craton. Hence syn-orogenic Triassic granites were restricted in the vicinity of the Song Ma suture zone. By contrast, the shallowdipping subduction of the Izanagi plate (the western plate of the Paleo-Pacific ocean) in southeastern SCB was associated with top-tothe-NW overthrusting of the continental crust and produced a thickened crust in the Cathaysia block in the early Triassic, which allowed the formation of widely distributed S-type granites from 248 to 202 Ma in South China. From the Jurassic to Cretaceous, the Izanagi plate subducted NW– WNW-ward under East Asia with moderate dipping angle and created

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a ~800-km-wide and NE-striking magmatic belt in eastern China. The late Mesozoic magmatism overprinted on the Triassic magmatism and deformation and further weakened the lithosphere of the SCB. A Jurassic–Cretaceous coastward migration of both extensional and arc-related magmatism in the SCB suggests an eastward migration of subduction zone due to increasing dipping angle of the subducted slab (Zhou et al., 2006). It is expected that the long-lived subduction of the Izanagi plate under the Cathaysia block caused the upwelling of the hot asthenospheric materials from the lower mantle, which corresponds to the voluminous Mesozoic magmatic rocks and the low-Vp anomalies at depth of 200–1200 km. However, the Lower Yangtze region was mainly affected by the corner flow and slab-dehydration of the Izanagi plate in the big mantle wedge, as suggested by the relatively sparse distribution of Mesozoic magmatic rocks and the stagnant Pacific slab in the mantle transition zone. Therefore, the SCB in the Mesozoic provides an example of thermally weakened and tectonically thickened continental lithosphere by protracted arc magmatism and oceanic subduction. The subductionrelated overthrusting of the crust and the subsequent partial melting of a thickened crust highlights the far-field effect of oceanic subduction. Such a thermally rejuvenated continental lithosphere would become easily thinned and fractured by back-arc extension in the Cenozoic.

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The NSGL, presumably reflecting a sharp change in the Moho boundary, may thus separate the unmodified (high Te) and modified (low Te) lithosphere due to subduction. 5.4. Te, Te anisotropy with lithospheric deformation The relationship between the elastic thickness and rigidity of the lithosphere in Eq. (1) indicates that Te can be used to study how the lithosphere responds to tectonic stress. As illustrated in Fig. 8a-c, earthquakes occur more frequently in regions with Te values of 10–30 km, but are relatively rare in regions with lower or higher Te, both for the small and large earthquakes. Spatial variations of Te in the western U.S. Cordillera (Lowry and Smith, 1995) and western Canada (Audet et al., 2007) also show good correlation with earthquake distributions due to localized brittle failure of crustal rocks. In their results, earthquakes mainly occur in regions with Te b30 km, but there are only sparse earthquakes in the regions with very low Te ≤10 km, especially in the western U.S. Cordillera. The classical plate tectonic theory assumes that the rigid plates can transmit tectonic stresses over great horizontal distances without buckling, so that deformation is focused along plate boundaries, as shown in strong seismicity in Taiwan Island, the Longmenshan thrust belt, the

Fig. 8. Correlation between distribution of earthquakes between 1979 and 2011 (M >3 and depth b50 km) with: (a) Te and (d) the magnitude of Te anisotropy. The scale bar of (d) shows the magnitude of Te anisotropy defined by (Tmax − Tmin)/Tmax. Histograms on the right side show earthquake events for M ≥3 (b) and M ≥6 (c) within Te bins of 3 km, and for M ≥3 (e) and M ≥6 (f) within the Te anisotropy magnitude bins of 0.05.

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Xianshuihe–Xiaojiang fault system and the Red River fault. However, widespread intracontinental earthquakes, e.g., in the eastern part of the North China craton, around the Sichuan basin and the Jianghan basin, and in the eastern Cathaysia block, demonstrate that the continental lithosphere is not an ideal rigid body. Here we suggest that strain concentrates in relatively weak parts of the continental lithosphere (Te ≤30 km), while the stronger parts work as a rigid body to transfer tectonic stresses with little deformation. The stress concentration in the low Te regions (Te = 10–30 km) results in earthquakes far away from plate boundaries. However, in the extremely low Te (≤10 km) regions, ductile deformation is predominant and few earthquakes occur, which is especially true for the large earthquakes with M ≥6. We also compared the earthquake distribution with the magnitude of Te anisotropy. As shown in Fig. 8d–f, more earthquakes occur in the regions with high Te anisotropy ((Tmax − Tmin)/Tmax >0.5). Meanwhile seismic activity is nearly absent where the Te anisotropy is b0.15 and >0.85, reflecting that Te anisotropy rarely exceeds the two bounds. There is a good positive correlation between seismic activity and the magnitude of Te anisotropy, suggesting that a highly anisotropic mechanical structure will promote strain localization and brittle failure.

5.5. Te anisotropy and tectonic inheritance Te, interpreted as the integral strength of the lithosphere, is often related to the “jelly sandwich” lithospheric model (Burov and Watts, 2006; Watts and Burov, 2003). This model assumes that the lithospheric strength resides in the upper crust and the lithospheric mantle. Te anisotropy can be derived from azimuthal differences of the strength introduced in either or both the upper crust and the lithospheric mantle. In stable cratons, most of the strength resides in the lithospheric mantle, and Te anisotropy may mainly relate to the alignment of olivine crystals. However, Te anisotropy in other regions should relate to both the shallow source (crust) and the deep source (mantle). Dynamic indicators, such as φh, which is usually obtained from earthquake focal mechanisms and represents the stress state in the upper crust, and φs, which represents the deep stress state from seismic anisotropy measurements, are used to compare with the Te anisotropy to seek the anisotropy source (see Audet and Burgmann, 2011 and references therein). It is worthy to emphasize that Te anisotropy can be acquired not only from the current tectonic regime, but also from earlier tectonic events.

Fig. 9. Angular differences between the weakest direction of Te (φe, red bars) in 10× 10 km grid and (a) directions of maximum horizontal compressive stress (φh, blue bars) and (c) fast directions of shear-wave splitting (φs, dark green bars). Stress data are from the World Stress Map (Heidbach et al., 2010), and shear-wave splitting data are from Rau et al. (2000), Flesch et al. (2005), Lev et al. (2006), Chang et al. (2008), Bai et al. (2009), and Huang et al. (2011). Histograms on the right side show the statistical angular differences for (b) |φe − φh|, and (d) |φe − φs|. The red lines represent the average counts.

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As discussed above, South China and surrounding regions are tectonically active and show low Te values. Comparison of our φe results with φh and φs is shown in Fig. 9. The lack of clear relationships between φe and φh, φs (Fig. 9b and d) suggests that Te anisotropy in the study area is not controlled by the current tectonic regime, but may be mainly inherited from earlier tectonic events. Therefore Te anisotropy can be regarded as a structural property that can survive for a long time. Due to its resistance to later tectonic events, Te anisotropy reflects tectonic inheritance of the continental lithosphere. In addition, as discussed in Section 4.2, in the regions with extremely low Te values, Te anisotropy seems to be closely related to the mantle flow directions. For example, lateral flow along the trench with trench-parallel φe in the Manila Trench, Philippine Trench and Ryukyu Trench. We suggest that contribution to Te anisotropy from the tectonic inheritance may decrease with decreasing Te values. 6. Conclusions Except the stable Sichuan basin, South China and surrounding regions show low Te values and high Te anisotropy, which can be attributed to the lasting tectonic activities and magmatism since the early Triassic. Relatively high Te values in the Yellow Sea may be caused by gravitational noise, but Te results in other areas are reliable. The noise is not only limited to the regions with subdued topography, but also exists in the convergent boundaries with sharp topography, implying the involvement of horizontal forces in the load balance process. The Te values are b20 km along the active boundaries of different geological blocks. The evenly low Te values in the Lower Yangtze region and the Cathaysia block coincide with the broad South China Mesozoic magmatic and fold belt, and low-Vp anomalies in the upper mantle. The NSGL in the SCB represents the western boundary of a thermally weakened continental lithosphere by long-lived oceanic subduction. The lithospheric deformation is closely related to the Te and Te anisotropy variations. Earthquakes are prone to occur in areas with Te values of 10–30 km, suggesting that the stress is concentrated in regions with low Te values (Te ≤30 km). The extremely weak lithosphere (Te ≤10 km) may release stress by ductile deformation, while the strong lithosphere (Te >30 km) could transfer the stress effectively. The poor correlation between Te anisotropy and the dynamic indicators (the shear-wave splitting directions, the maximum horizontal compressive stress direction) demonstrates that Te anisotropy mainly reflects tectonic inheritance of the continental lithosphere. Acknowledgment This research is supported by the National Basic Research Program (2012CB214703) and the National Science Foundation of China (Nos. 41172182, 40504013 and 40634021). We are grateful for the constructive comments and suggestion from the two anonymous reviewers. Most of the figures were made by GMT (Wessel and Smith, 1998). References Amante, C., Eakins, B.W., 2009. ETOPO1 1 Arc-Minute Global Relief Model: Procedures, Data Sources and Analysis. NOAA Technical Memorandum NESDIS NGDC-24. 19 pp. Audet, P., Burgmann, R., 2011. Dominant role of tectonic inheritance in supercontinent cycles. Nature Geoscience 4, 184–187. Audet, P., Mareschal, J.-C., 2004. Anisotropy of the flexural response of the lithosphere in the Canadian Shield. Geophysical Research Letters 31, L20601. Audet, P., Mareschal, J.-C., 2007. Wavelet analysis of the coherence between Bouguer gravity and topography: application to the elastic thickness anisotropy in the Canadian Shield. Geophysical Journal International 168, 287–298. Audet, P., Jellinek, A.M., Uno, H., 2007. Mechanical controls on the deformation of continents at convergent margins. Earth and Planetary Science Letters 264, 151–166. Bai, L., Iidaka, T., Kawakatsu, H., Morita, Y., Dzung, N.Q., 2009. Upper mantle anisotropy beneath Indochina block and adjacent regions from shear-wave splitting analysis

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