Moho Depth of Qinghai-Tibet Plateau Revealed by Seismic Probing

INTRODUCTION

Moho, the interface between the crust and the mantle, was discovered by Yugoslavian scientis Mohorovicic in 1909, and then named"Mohorovicic discontinuity"or"Moho"for short.

From the researches of global scientists in the past decades, Moho is well known as a distinct interface in the interior earth.The nature and distribution features of the Moho reflect the crustal evolution and the mutual response between the crust and the mantle And Moho could be very different in different geophysical processes or geological units.Moho is a first-degree velocity discontinuity and distributes very stably in stable blocks, such as craton; in these blocks the seismic reflection characters are very strong.Beneath the orogenic belts, Moho exhibits a strong, unstable velocity gradient and distribution; the seismic reflection characters are very weak, even invisible.Moho, an important part in establishing the three-dimensional structure of the crust or the lithosphere, whether as a chemical discontinuity or a phase change, is the key element to understanding the regional tectonic evolution (Yin, 2001).

Moho depth characterizes the crustal thickness and records the crustal growth and the geophysical process undergone, so it is the key evidence for people to understand the earth's evolution.The propagation and reflection characteristics of seismic waves are very evident on the Moho surface and can be observed by people; seismic probing (active-source and passive-source) is the main method to reveal the Moho depth.

Now, the common viewpoint is that the average thickness of the crust (Moho depth) is thicker; the normal crustal thickness is about 30–40 km.High mountains and plateaus have the thickest crust, which can be 60–70 km thick.The depth of the ocean is much thinner, just about 7 km.The Qinghai-Tibet plateau is the continental area with the highest elevation on the earth, and the deepest Moho is about 80–90 km.The huge crustal thickness is a very important parameter in researching the deep structure and continental geophysics beneath the Qinghai-Tibet plateau.

Since the 1950s, there have been many activesource (deep seismic sounding and deep seismic reflection profiling) and passive-source seismic probings (broadband seismic observation) implemented to reveal the crust-mantle structure.In this article, the authors mainly summarize these three seismic probings to discuss the Moho depth and its distribution characteristics in the Qinghai-Tibet plateau based on the previous summaries (Zhao et al., 2008, 2002; Lu et al., 2006; Li et al., 2004a; Zhao and Wu, 2004; Zeng et al., 1998, 1994; Gao, 1997; Wang et al., 1991) and further discuss the crust-mantle structure of the Qinghai-Tibet plateau and the geodynamic significance implied by these characteristics.

RESEARCH DEGREE AND DATA SUMMARY OF SEISMIC PROBING IN QINGHAI-TIBET PLATEAU

It is well known that the Qinghai-Tibet plateau is the youngest orogenic belt formed by the collision between the Indian and Eurasian continents, and consists of several terranes with different attributes (Fig. 1).From north to south, there are five terranes, including the Qilian terrane, the Kunlun-Qaidam terrane the Songpan-Ganzi-Hoh Xil terrane, the Qiangtang terrane, and the Lhasa terrane (Yin, 2001), and these terranes are separated by several suture belts.

Figure  1.  Seismic probing in Qinghai-Tibet plateau (~2008).Tectonic settings (Yin, 2001) : IYS.Indus-Yarlung suture belt; BNS.Bangong-Nujiang suture belt; JS.Jinsha suture belt; AKMS.Anemaqen-Kunlun-Mutztagh suture belt; SQS.South Qilian suture belt; DHS.Danghe Nanshan suture belt; NQS.North Qilian suture belt; KDS.Kudi suture belt; STDS.South Tibet detachment system; MCT.main cen-tral thrust; MBT.main boundary thrust.1.normal fault; 2.thrust fault; 3.strike-slip fault; 4.suture; ①–㉓.DSS profiles (red lines); 1–9.deep seismic reflection profiles (purple lines); kinds of signal (blue)broadband seismic observations.The detailed information of each profile is listed in Tables 1, 2 and 3.

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From the 1950s to the present, there have been many geophysical methods used to research the crust-mantle structure of the Qinghai-Tibet plateau, which provide the scientific foundation to discuss the uplift mechanism and geodynamics of the plateau.Among the geophysical methods taken to explore the earth interior, seismic waves are the most effective and accurate way to sketch the deep structure and reveal the Moho depth.Exploration achievements, typically by deep seismic sounding (DSS), deep seismic reflection profiling, and broadband (BB) seismic observations, outline the basic characteristics of the crust-mantle structure of the Qinghai-Tibet plateau and reveal the local Moho depth and distribution features.

In this article, the authors collected all the references about the exploration achievements in the Qinghai-Tibet plateau by these three seismic methods as far as possible.All the sources of the data are listed in Tables 1, 2, and 3; all the exploration profiles and BB seismic stations are shown in Fig. 1.In order to provide comprehensive research and a comparative analysis of the Moho depth and its distribution characteristics, the values of the Moho depth along each profile are shown in Fig. 2 according to the exploration achievements and spatial locations from the collected references.

Table  1.  Profiles of deep seismic sounding

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Table  2.  List of deep seismic reflection profiles across Qinghai-Tibet plateau

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Table  3.  List of the broadband seismic observations across Qinghai-Tibet plateau

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Figure  2.  The Moho depth map of Qinghai-Tibet plateau revealed by seismic probing (up to 2008). Tectonic settings are the same as those in Fig. 1. 1. Normal fault; 2. thrust fault; 3. strike-slip fault; 4. suture belt; 5. Moho depth along the profile revealed by DSS; 6. Moho depth along the profile revealed by deep seismic reflection profiling; 7. Moho depth along the profile revealed by BB seismic observations.

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It should be pointed out that, in previous researches, different experts used different methods to research the Moho depth of the Qinghai-Tibet plateau, such as integrated inversion of gravity and seismic probing (Chen and Ozalaybey, 1998; Lu et al., 1997), active-source deep seismic sounding (Li et al., 2009), broadband seismic observation (Wang W M, et al., 2008; Zeng et al., 1992), and deep seismic reflection profiling (Gao et al., 2006, 2001a, b; Zhao et al., 1997, 1993).These researches as well as other achievements made numerous well-known discoveries.However, forthe broad plateau, only one method of probing cannot cover the whole plate according to the present work degree.Therefore, in order to have a comprehensive and objective understanding of the Moho depth of the plateau, we show all the achievements of these three seismic probings in Fig. 2.Assuredly, the probing accuracy of each method is different and is discriminately sensitive to the nature of the Moho.Considering that only the Moho depth of the Qinghai-Tibet plateau is researched in this article, the achievements by each method have reference value.At the same time, no literature data achieved by each method from the references are elected artificially, and all the achievements are shown in Fig. 2.Obviously, there are some problems with this presentation method because of the different Moho depths probably revealed by different methods.As shown in Fig. 2, the probing results may differ with the different methods or different researcher interpretations in the same place or along the same profile.We don't avoid these contradictions in the Moho depth because deep discussion can lead us to further understand the Moho complexity of the Qinghai-Tibet plateau.We introduce the research features of each method of seismic probing and their research degree, respectively.

DEEP SEISMIC SOUNDING (DSS)

Deep seismic sounding, also called wide-angle reflection and refraction, is a seismic exploration method using low frequency to research the deep structure of the earth, and this method has been adopted worldwide since the 1950s.DSS is an effective method for determining the Moho depth due to the high density of shot points and recording stations.During"the international upper mantle program"in the 1960s, "The Geodynamics Project"in the 1970s, and"the international lithosphere program"in the1980s, DSS was used to research almost all the tectonic units and made lots of achievements.These achievements reflect the change and heterogeneity vertical of the crust and upper mantle with complicated structure, depict the earth interior meticulously, and reveal the different geodynamics of different geological units.

The DSS methods used for exploring the interior Qinghai-Tibet plateau can be dated back to the 1970s.In 1977, researchers from the Institute of Geophysics, Chinese Academy of Sciences, took this method firstly to research the crust-mantle structure in southern Tibet along"Yadong-Damxung", a 460 km long profile, and achieved the Moho depth (Institute of Geophysics, Chinese Academy of Sciences, 1981).During 1980 to1982, in the program"Formation and Evolution of Himalayan Geological Structure, Crust and Upper Mantle"held by China and France, "SelincuoYa'anduo"and"Paiku Co-Puma Yumco", two WE-trending profiles, the"Gala-Anduo"NS-trending profile, and a non-longitudinal profile across the Himalaya range were carried out in South-Central Tibet During 1986 and 2000, under the implementation of the GGT (global geosciences transect) across the Qinghai-Tibet plateau, including the"YadongGolmud", "Altay-Taiwan", "Golmud-Ejin Banner"and"West Kunlun-Tarim-Tianshan"projects, 4 profiles were carried out.Afterwards, a number of DSS profiles were organized and implemented by the National Natural Science Foundation of China (NSFC), the Ministry of Science and Technology (MST), the Ministry of Land and Resources (MLR), Chinese Academy of Sciences (CAS), and China Seismological Bureau (CSB) under different investigation and research programs.Up to 2008, there have been 23DSS profiles finished across the Qinghai-Tibet plateau or the marginal area, the cumulative length of which is16 000 km.All the DSS profiles are listed in Table 1.

DEEP SEISMIC REFLECTION PROFILING

Deep seismic reflection profiling, also called"near-vertical reflection technique"or COCORP (consortium of continental reflection profiling), is the most effective method to research the fine structure of the lithosphere.It can detect the subtle variations of the Moho and the inner crust.

Deep seismic reflection carried out in the Qinghai-Tibet plateau can be dated back to the reflection-refraction integrated crustal investigation of Qaidam in 1958 (Teng, 1974; Zeng and Gan, 1961), which first revealed the Moho depth of the Qaidam basin.During the Sino-American joint project"INDEPTH, "deep seismic reflection using multiple stacking was firstly carried out to research the southern Qinghai-Tibet plateau (Zhao et al., 1993), and the Moho depth beneath the northern slope of the Himalayan Mountain was found to be about 72–76 km.In1992, the"Golmud-Ejin Banner"GGT program team group carried out a deep seismic reflection experimen in Qilian Mountain, the northern margin of the Qinghai-Tibet plateau (Wu et al., 1995), and found that the Moho became deeper from the Hexi corridor in the north to Qilian Mountain in the south when crossing the range-basin conjunction.During1998–2000, the Lithosphere Research Center (LRC) of Chinese Academy of Geological Sciences (CAGS) finished 2 deep seismic reflection profiles across the"West Kunlun-Tarim"and"Altyn Tagh-Tarim"range-basin conjunction, respectively, and found strong reflectance signatures along the profiles.These achievements provide powerful evidence of collision and deformation in the northern margin of the Qinghai-Tibet plateau (Gao et al., 2001a, b, 2000).During2004 and 2005, in order to cooperate with the project"Strategic Petroleum Target Area Selection", LRC of CAGS implemented the longest deep seismic reflection profile (about 260 km) across Songpan-West Qinling under the joint funding of the MLR, the NSFC, and the China Petrifaction Company and found the shallowest Moho (49.5 km) of the Qinghai-Tibet plateau (Gao et al., 2006).From 2007 to 2008, under the joint funding of the MST, the MLR, and the NSFC, the team group of the LRC of CAGS finished 2 deep seismic reflection profiles, each of which was about50 km.These two profiles jointly crossed the central uplift in the central Qiangtang terrane and can provide available information about the deep Qiangtang basin and the front margin of the bilateral collision zone between India and Eurasia.

At present, the cumulative length of deep seismic reflection profiles is about 1 400 km; detailed information on each deep seismic reflection profile is listed in Table 2.

BROADBAND (BB) SEISMIC OBSERVATION

Recently, with the development of digital observation techniques and mobile observation stations, broadband seismic observation has been accepted widely to research the structure of the crust and upper mantle under the stations.Among the BB seismic observations, receiver function is becoming a convenient and efficient method to research the deeper structures around the world.

BB seismic observations in the Qinghai-Tibet plateau can date back to 1991, when Zeng Rongsheng and Wu Daming deployed 11 BB seismic recorders along the Golmud-Shigatse line and revealed the Moho depth variation along the line (Chen et al., 1992; Zeng et al., 1992).During the Sino-French Project (1992–1994), 108 BB seismic recorders were laid along the Qinghai-Tibet Highway (Jiang et al., 1994) and, later, observations in the northern margin of the Qinghai-Tibet plateau were finished (Xu et al., 2001).The INDEPTH-III"Deqing-Longweicuo"broadband seismic observations crossed the Bangong-Nujiang suture (BNS) and entered the Qiangtang block to deepen the research on the deformation mechanism of the interior structure of the plateau (Li et al., 2006a, b) During 1997 and 1998, researchers from Mainland and Taiwan, China worked together to carry out BB seismic observations in Tarim and northern West Kunlun and found the duplexing of the Moho (Kao et al., 2001).

In this century, there have been a number of broadband seismic observations carried out in the Qinghai-Tibet plateau, including the"Nepal-Tibet"project, the"Hi-climb"project that crossed the Himalayan Mountain (Jiang et al., 2008; Li et al., 2008; Qian et al., 2007; Pelkum et al., 2005), and the"West Sichuan plateau"and"Linzhi-Yongchuan"observations that crossed the Longmenshan fault belt in the eastern margin of Qinghai-Tibet (Wang et al., 2008; Xu et al., 2007).Now broadband seismic observations have crossed all the suture belts that separate the main terrains, a cumulative length of about 10 000 km (Fig. 1).All the detailed information on BB seismic observations is listed in Table 3.

THE MOHO DEPTH AND ITS VARIATIONS IN QINGHAI-TIBET PLATEAU

As shown in Fig. 1, these seismic probings almost covered the entire Qinghai-Tibet plateau and its marginal orogenic belts, such as the Himalayas, the west Kunlun-Altyn-Qilian fault belt, and the Longmenshan orogenic belt as mentioned above.These seismic probings provide abundant information to understand the Moho depth of the Qinghai-Tibet plateau.

The Deepest Moho and the Shallowest Moho

The broadband seismic observation reveals that the deepest Moho in the Qinghai-Tibet plateau is located in the West Kunlun structural knot (90±2 km in thickness) beneath the southern branch of the Altyn fault (Wittlinger et al., 2004).Gao et al., (2006) accomplished a deep seismic reflection profile crossing the West Qinling orogen and the Zoigêbasin, and discovered a strong reflection of the Moho at 16.50 s TWT (two-way travel time), corresponding to a depth of approximately 49.50 km, which is considered to be the shallowest Moho in this region.

The Moho Depth and Its Variation in Each Tectonic Belt in Qinghai-Tibet Plateau

The Himalayan tectonic belt, a geological unit between the Ganges plain and the IYS, consists of three rock slices, including the Lesser Himalaya, the High Himalaya, and the Tethys Himalaya, bordered by the MBT, the MCT, and the STDS from south to north, respectively (Fig. 1).About the Moho depth of this tectonic belt, there is still a controversy (Zhang Z J et al., 2002; Zhao et al., 1993; Hirn et al., 1984).The High Himalaya: according to the non-longitudinal DSS profile, its Moho depth is 55 km (Hirn et al., 1984).Then, the INDEPTH deep seismic reflection profile achieves 22.8–24.4 s TWT and a conversion depth of about 72–76 km (Zhao et al., 1993); the interpretation of the receiver function along"NepalTibet"showed that the Moho depth is gradually deeper, from 45 km in the Lesser Himalaya to 75 km in the Tethys Himalaya (Pelkum et al., 2005).Jiang et al. (2008) thought that the Moho was flat in the whole Himalayan tectonic belt, with a depth of about 45 km using the BB data of"Hi-climb".Wang W M et al. (2008) used the transform function method and thought that the Moho depth of the Lesser Himalaya was 55–61 km, while that of the High Himalaya was65–76 km, and that the deepest Moho of the Himalayan terrain was about 82 km in the Tethys Himalaya.The"Paiku Co-Puma Yumco"DSS profile crossed the Tethys Himalaya and revealed its Moho depth to be70–79 km (Teng et al., 1985; Hirn et al., 1984); the recent interpretation is about 74 km (Zhang Z J et al., 2002).Although different Moho depths are given by different seismic probing methods, the trend outlook of the Moho depth in the Himalayan terrane is gradually deeper from south to north, about 55 to 70–75 km.

The Lhasa terrane is bordered by the IYS to the south and the BNS to the north.The"YadongDamxung"DSS profile revealed that the average Moho depth is 70 km (Institute of Geophysics, CAS, 1981); the non-longitudinal profile"Gala-Anduo"held by China and France during 1980 and 1982showed that the Moho depth is at least 70 km (Xiong et al., 1985), while Huang et al. (1992) took it to be 56km.The"Selincuo-Ya'anduo"DSS profile revealed that the Moho depth is 70–75 km (Zhang et al., 2001; Liu and Teng, 1990).The"Cuoqin-Sangehu"DSS non-longitudinal profile crossed the western Lhasa terrain and revealed that the Moho depth is 75–78 km (Xiong and Liu, 1997).The"Zhangmu-Cuoqin"DSS profile showed the Moho depth of 78–73 km in the western Lhasa terrane (Liu H B, personal communicat i o n).T h e I N D E P T H-I I I p r o f i l e"D e q i n gLongweicuo"showed that the Moho depth in this terrain was about 65 km (Ross et al., 2004; Zhao et al., 2001).Zeng et al. (1994) thought that the Moho depth of the Lhasa terrain was about 80–84 km using the method of BB seismic observation.Wang W M et al. (2008) thought the Moho depth of Lhasa is deeper than that of the Himalayan block, and the Moho depth was about 82 km.According to"Hi-climb"BB seismic observation datum, Li et al. (2008) thought that the average Moho depth of the Lhasa terrain was about 70 km.But in the eastern Lhasa terrane, the Moho depth is about 60 km, apparently shallower than the western and middle Lhasa terrain.Wang C Y et al. (2008) got a Moho depth of 70–72 km using the method of receiver function.According to these data, the average Moho depth of the Lhasa terrane is about70–80 km and deeper in the west than in the east.

The Qiangtang terrane is located between the BNS and the JS, and is the most controversial among the terranes (Li C et al., 2007, 2006, 1995; Weislogel, 2006; Zhang K J et al., 2006; Kappa et al., 2003; Huang et al., 2001; Kappa, 2001; Zhang K J, 2001).Because it is located in the hinterland of Tibet, the harsh natural environment and awful traffic condition limit deep geophysical exploration in this area.Hitherto, there have been several profiles across this area.According to the BB seismic observations held by China and America in 1991, the average Moho depth of the Qiangtang terrane is about 70 km (Wu et al., 1999; Chen et al., 1992).The"Cuoqin-Sangehu"DSS profile revealed the Moho depth to be 65–68 km (Xiong and Liu, 1997), while the INDEPTH-III"Deqing-Longweicuo"DSS profile revealed that the Moho depth around the area of Bange and Shuanghu is less than 65 km (Rolf et al., 2004; Zhao et al., 2001) The"Tuotuohe-Golmud"DSS profile revealed that the Moho depth in northern Qiangtang was 68 km (Li Q S, 2004b; Lu et al., 1990).INDEPTH-III BB seismic stations crossed central-southern Qiangtang and revealed a Moho depth of about 59–67 km (Li Y H et al., 2006b).The"Linzhi-Yongchuan"BB profile revealed that the Moho depth in the eastern Qiangtang terrane was 65 km (Wang C Y et al., 2008).INDEPTH-III deep seismic profile revealed the Moho depth of Duoma area was about 65 km (Ross et al., 2004), QT07, QT08 also revealed the Moho depth around Shuanghu area was about 66 km (Lu et al., 2009).From recent probing, it seems that the Moho depth of the Qiangtang terrane varies from 60 to 67km, and becomes progressively shallower from west to east.

The Songpan-Ganzi-Hoh Xil terrane is sited between the JS and the AKMS.Both the"TuotuoheGolmud"and the"Gonghe-Yushu"DSS profiles revealed that the Moho depth was 60–70 km (Jiang et al., 2006; Lu et al., 1990); another DSS profile, the"Tangke-Benzilan", crossed the southeast of this orogenic belt and showed the depth to be 62–52 km (Wang C Y et al., 2003b).The"Maqin-LanzhouJingbian"DSS profile revealed a depth of 60–62 km (Li S L et al., 2002).The"Hezuo-Tangke"deep seismic reflection profile crossed the northeastern area of this terrane and discovered the shallowest Moho (49.5km) of the whole plateau in the southern segment of the profile, and the Moho was flat along the profile; the average Moho depth was about 51–54 km (Wang H Y et al., 2007; Gao et al., 2006).The"LinzhiYongchuan"BB seismic observations crossed the southeast Songpan-Ganzi terrane and showed a Moho depth of 57–64 km (Wang C Y et al., 2008).The"West Sichuan plateau"BB seismic profiles also crossed this area and showed that the crustal thickness was 60 km (Xu et al., 2007).These seismic probings show that the Moho depth of this terrane varies widely and is gradually shallower from west to east.

The Kunlun-Qaidam terrane consists of two geological units, including Kunlun and the Qaidam basin.A seismic reflection experiment carried out in 1958revealed that the Moho depth of the Qaidam basin was about 50–52 km (Teng, 1974; Zeng and Gan, 1961).The"Mangya-Ruoqiang"BB profile revealed the Moho depth of the Qaidam basin to be about 50–60km (Shi et al., 2007; Jiang et al., 1999).The"GolmudHuahaizi"DSS profile revealed that the Moho depth of the Qaidam basin changed from 53 km in the south to 59 km in the north, and that of the central Qaidam basin was about 50 km (Zhao J M, unpublished).The M o h o d e p t h a l o n g t h e"D a c h a i d a nRuoqiang-Baicheng"profile showed the apparently fluctuated, and the average beneath the Qaidam basin was about 56 km (Zhao J M et al., 2006).The"Tuotuohe-Golmud"DSS profile showed that the Moho depth of East Kunlun was about 64 km (Li Q S2004b; Lu D Y et al., 1990).

The Qilian terrane is located in the northernmos Qinghai-Tibet plateau and consists of three subterranes: North Qilian, Central Qilian, and South Qilian, from north to south.The"Golmud-Ejin Banner"DSS profile revealed that the Moho depth was 60km at least and deeper (57–70 km) from north to south (Cui et al., 1995).The"Qilian-Huahai"deep seismic reflection profile showed that the average Moho depth of northern Qilian was 50 km (Wu et al., 1995).The trend of the Moho is deeper, from 50–57 km in the north to 60–72 km in the south, from the seismic probing above.

The Moho Offset beneath Main Suture Belt

Now, all the main sutures or fault belts have been crossed by active-source or passive-source seismic profiles.From the finished data, the Moho offset beneath each main suture belt can be finely unraveled.

Indus-Yarlung-Zangbo suture belt (IYS)

Hirn et al. (1984) thought that the offset was 20km from the interpretation of the DSS fan shooting held by China and France, but from the associated results of two WE-trending profiles and the NS-trending non-longitudial"Gala-An'duo"profile, the Moho depth to the north of this belt is about 6-8 km shallower than the south (Yin et al., 1990; Xiong et al.1985).Kind et al. (2002) thought that there was no Moho offset beneath the IYS using migrated receiver function data, but other geophysicists thought that there was a 6 km (Wang et al., 2008) or 20 km (Jiang et al., 2008) Moho offset here.

Bangong-Nujiang suture belt (BNS)

Zeng et al. (1998) thought that there was a 10 km Mohodepth.TheINDEPTH-III"DeqingLongweicuo"DSS profile showed that there was no Moho offset (Rolf et al., 2004; Zhao et al., 2001) Kind et al. (2002) got the same result using the method of receiver function, but other results from receiver function inversion showed about 8 km of the Moho offset beneath the belt (Li Y H et al., 2006a; Wu et al., 2004).

Jinsha River suture belt (JS)

The"Tuotuohe-Golmud"and"Gonghe-Yushu"DSS profiles crossed this belt and showed there was no Moho offset (Jiang et al., 2006; Li Q S, 2004b; Lu et al., 1990).The"Benzilan-Tangke"DSS profile crossed the Xianshuihe fault belt, the eastern segment of the JS, and there was about 4 km in the Moho offset (Wang et al., 2003), but the result from BB seismic observations showed no Moho offset.

Anemaqen-Mutztagh-Kunlun suture belt (AMKS)

The results from BB seismic observations revealed a 15–20 km Moho offset beneath the middle belt (Zhu and Helmberger, 1998), but no Moho offset was revealed by the"Maerkang-Luqu-Gulang"DSS profile (Zhang X K et al., 2008).

DISCUSSION: THE MOHO DEPTH DISTRIBUTION CHARACTERISTICS AND IMPLIED GEODYNAMICS

The Crustal Outline of Qinghai-Tibet Plateau

The seismic probing mentioned above shows that the Moho beneath the Qinghai-Tibet plateau has a complex shape, and its depths are very different.The whole outline is that the Moho is deeper in the west and shallower in the east.The deepest Moho is about40 km deeper than the shallowest Moho.

Separated by the Qiangtang terrane, the Moho depth of the Tethys Himalayan and the Lhasa terrane to the south is deeper than that of the SongpanGanzi-Hoh Xil terrane and the Qaidam basin to the north.The Moho of the Qaidam basin is shallower from 50–52 km in the middle to 60 km on the side, causing an apparent Moho uplift beneath central Qaidam.The Moho of the Songpan-Ganzi terrane is the shallowest in the Qinghai-Tibet plateau, and there is no apparent crustal thickening across the West Qinling orogenic belt.Beneath the Qiangtang terrane, the hinterland of the Qinghai-Tibet plateau, the Moho seems shallower than the adjacent terranes.Another feature is that a Moho offset exists beneath all the main suture belts.

Geodynamics Implied by Distribution Characteristics of the Moho Depth

The 40 km difference between the deepest and the shallowest Moho is almost the thickness of a normal continental crust.Now, the common view is that the age of initiation of the collision between the India plate and the Eurasia plate is from west to east diachroneity, which means that the initial collision age in the west is earlier than the east (Rowley, 1998; Qayyum et al., 1997; Beck et al., 1995).The trend of shallower Moho from west to east may be related to this wedge collision progress; the earlier initiation age in the west means longer compression and more crustal shortening, which induces more crustal thickening in the west.At the same time, maybe the eastward mass flow beneath the Qinghai-Tibet plateau is caused by this huge thickness difference between the west and the east.

Now, the bilateral subduction between the India plate and the Eurasia plate (Gao and Wu, 1995; Wu et al., 1991) has been proven by more and more deep geophysical evidence (Zheng et al., 2007; Pelkum et al., 2005; Xu et al., 2004; Kind et al., 2002; Gao et al., 2001a; Kao et al., 2001; Hauck et al., 1998; Zeng et al., 1998; Nelson et al., 1996; Makovsky et al., 1996; Zhao et al., 1993).The Qiangtang terrane is in the hinterland of the Qinghai-Tibet plateau, but its shallower Moho compared with the adjacent terranes may reflect the crustal collapse caused by the bilateral collision.The tomography research by Zheng et al. (2007) showed that the real front margin of the bilateral subduction between the Indian and Eurasian continents is beneath the Qiangtang terrane.Subsequently, the convergent thickening lithosphere in the Qiangtang terrane delaminated (Kind et al., 2002), and the upwelling upper mantle was mixed with previous crustal material; then the new Moho formed.The widely spread Cenozoic lava may be the surface geological response to this deep tectonic activity.

The Moho of the Tethys Himalaya and the Lhasa terrane to the south of the Qiangtang terrane can be 80km deep, which may reflect the double crustal thickness caused by the northward subduction of the India plate.At the same time, this northward subduction is the active force for the thickening of the QinghaiTibet plateau that persists to the present.The Moho of the Songpan terrane and West Qinling orogenic belt in the northeast plateau is flat, which shows that the lower crust beneath this area is experiencing a strong extensional process.