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LITHOSPHERIC STRUCTURE AND CONTINENTAL GEODYNAMICS

XU Zhonghuai1) and  SHI Yaolin2)

1) Institute of Geophysics, China Seismological Bureau, Beijing 100081, China

2) Center for Earth System Science, The Graduate School of The Chinese Academy of Sciences,

Beijing 100039, China

During the last 4 years since 1999 22nd IUGG general assembly, lithospheric structure and continental geodynamics of China continent is an active research field for Chinese geophysicists. Research interests are mainly in seismic wave velocity structure of the lithosphere and its present-day deformation.

 

I. REGIONAL LITHOSPHERIC STRUCTURE

 

Moho depth map for East Asia  Since 70 s of 20th century to 2000, about 5000 km DSS profiles, including more than 2000 km reflection profiles, have been carried out in China. Based on the data of these profiles and the published DSS study results for the neighboring countries and sea regions, a map of Moho depth distribution has been compiled (Teng et al., 2002, Fig.1). In the compilation the Moho depth data are sampled every 20 to 50 km space for China mainland region, producing about 3000 datum samples, and every 50 to 100 km for surrounding regions, giving additional 1000 datum samples. Background map scale is 1:25,000,000. Depth resolution of the map reaches to ±2 km. This is the most detailed and updated Moho depth map for East Asia


Pn velocity lateral variation  Wang et al. (2003) tomographically inverted nearly 140,000 Pn arrival times, and obtained lateral variation and anisotropy of Pn velocity in top mantle beneath China continent. The result indicates that high velocity is seen in the region of stable basins around Qingzang (Qinghai-Xizang) Plateau, such as Tarim, Junggar, Turpan-Hami, Qaidam and Sichuan basin, while low velocity is found in tectonically active region, such as western Sichuan and Yunnan, middle part of Qingzang Plateau, Shanxi graben and North China basin. Beneath compressive basins the velocity tends to be high, while beneath extensional basins or grabens it is usually low.

 

 

 

 

Fig.2.  Pn velocity variation with respect to the average 8.05 km/s in and around China. Lines demarcate tectonic blocks.

 

Combilation of heat flow data  Hu et al.(2001) published the third compilation (3rd edition) of heat flow data in China mainland. Their paper presents 450 new data since the 2nd edition. Up to now in China mainland there are 862 heat flow data, among which 816 have been published.

Surface wave study  Many researchers investigated wave velocity structure in crust and upper mantle beneath China continent by means of Rayleigh wave dispersion analysis. They have obtained the following main results:  The upper mantle low velocity zone beneath North China plain is shallow and thick (He et al., 2002), with its top boundary being possibly at 80-90 km (Xu et al., 2000). This supports the idea that this region is under a state of extension.  Velocity in upper mantle beneath Qingzang Plateau is remarkably low. Upper mantle velocity beneath stable Tarim basin and Yangtze platform is relatively high (He et al.,2002; Zhu et al., 2002).  In the boundary region between Burma and Yunnan of China there is a clear low velocity zone in upper mantle, as revealed in the wave period range of 12s to 120s (Zhu et al., 2002).  In southern China the Moho depth is 30-40 km and the crust becomes gradually thin from west to east, and low velocity zone exits in the depth range of 60-150 km, with the depth range being laterally variable (Teng et al., 2001).

Using 12000 waveform records of long period Rayleigh wave Zhu et al. (2002) calculated 4100 dispersion curves along great circle path with a period range of 8s to 250s, and then both the dispersion and waveform data were inverted to obtain a 3-D S wave velocity image for the region of East Asia and marginal seas of western Pacific Ocean (WPO). Their result indicates that, in the depth range from upper crust to 70 km deep, S-wave velocity in western part of the region is low, with the Qingzang Plateau in central part of the low velocity zone, while the velocity in eastern part of East Asia and in the region of marginal seas of WPO is high, and in the Iran-Himalaya-Burma-Indonesia plate collision zone low velocity prevails. In the upper mantle of 85 km to 250 km deep, there is a large low velocity zone in the eastern part of East Asia and in the region of marginal seas of WPO. Zheng et al.(2000) studied the lateral variation of Rayleigh wave group velocity in the sea region to the east of China, and also found that the upper mantle beneath Japan Sea and Okinawa trough shows low velocity.

Q distribution  Qβ structure of crust and upper mantle in eastern and western China is obtained by inverting Rayleigh wave attenuation factors varying with periods (Li et al., 2000). Their result indicates that in large part of the Yangtze quasi-platform at the depth of 88 km there is a significant high Qβ zone, while to the east of Yangtze quasi-platform and in middle and eastern part of South China fold system the upper mantle low Qβ zone appears at a depth of 85 km. In west Yunnan folding and faulting system in top mantle there is a low Qβ layer of about 40 km thick, and down to the depth of 95 km an even lower Qβ layer reveals. In western China the upper mantle low Qβ layer exits in many regions, for example, on the path from Gaotai to Lhasa across the Qingzang plateau, at about 84 km deep, there appears low velocity and low Qβ layer.

Gravity inversion for lithosphere lower boundary  Using the technique of separating wavelet transformed wave field Fang et al. (2001) found deep gravity anomaly from Bouguer anomaly observation. Under some known constraints they inverted the deep gravity anomaly in a series expression of spherical harmonics in the region of eastern China and its vicinity, and obtained an image of the lower boundary of lithosphere: its depth varies from 35 km in eastern sea region to 110 km inland, the lithosphere is thicker in South China than in Northeast and North China, and there are two strong gravity gradient zones trending NE-SW and NNE-SSW, respectively.

Yunnan  By inverting teleseismic receiver functions deduced from analyzing the broadband records of 23 digital seismic stations in Yunnan Province Wu et al.(2001a) obtained S-wave velocity structure in the depth range of 0 to 100 km beneath the stations. In northwestern Yunnan the crust thickness is 62 km or so, while in southern Yunnan it is only 32 to 34 km. The thick crust


 Fig.3.  Two vertical profiles of S-wave velocity variation in Yunnan region obtained from receiver function

           inversion Left plot shows positions of the two profiles

thins out from northwest to southeast and is limited within the region bounded by Xiaojiang fault and Yuanjiang fault, with its shape in concordance with the Sichuan-Yunnan rhomb block. In eastern and southern Yunnan, where crust is thin, the Moho discontinuity is clear. In the region with thick and variable crust the Moho usually manifests itself as a zone of high gradient of S-wave velocity. In Fig.3 are given two vertical profiles of S-wave velocity variation obtained from receiver function inversion. From the figure we see that the velocity image shown in western profile (Fig. 3b) is obviously different from that in eastern profile (Fig. 3c).

Altyn Tagh fault zone  In a Sino-France cooperative research during late August 1995 to January 1996, 30 one component short-period seismographs were deployed along a 400 km long profile for doing P wave tomographic study. Shi et al. (1999a) inverted 3883 P-wave arrival times picked up from the seismic records and obtained the deep velocity structure. The result reveals that the Altyn Tagh fault is a 40 km wide low velocity belt, which extends vertically down to a depth of about 150 km. The result also shows that the Altyn Tagh fault cuts through a Tarim-like lithosphere, which previously plunged down to the Qaidam basin.

Northeastern margin of Sino-Korean platform  Lu et al.(2002) tomographically inverted 38,000 P-wave arrival times and reconstructed a 3-D velocity structure of crust and upper mantle in northeastern marginal region of Sino-Korean platform. It is shown that in the crust and upper mantle there is notable lateral velocity inhomogeneity, which extends down to 1200 km depth, and several low velocity zones are seen in the upper and middle crust in the region of Haicheng, Chaoyang, Yixian, south of Dandong and Tangshan,etc..

Antarctic  Based on the waveform data of the Rayleigh wave traveled along Earth's great circle across the station South Pole (SPA) and Scott (SBA), Shu and Jiao (1999) computed the phase velocity dispersion and by inversion obtained the lithosphere S-wave velocity structure in the depth range down to 200 km below the two stations. Their result shows that beneath Trans-Antarctic mountain the crust thickness is 45 km, and there is a clear low velocity layer between 55 and 75 km, indicating possible existence of magma.

Some authors made improvement in the method of investigating lithospheric structure.

Smooth constraint technique in genetic algorithm  Smooth constraint is important in linear inversion, but it is difficult to apply it directly to model parameters in genetic algorithms. If the model parameters are smoothed in iteration, the diversity of models will be greatly suppressed and all the models in population will tend to be identical in a few iterations, so the optimal solution meeting requirement can not be obtained. Wu et al.(2001b) introduced an indirect smooth constraint technique in genetic inversion. In this method the new models produced in iteration are smoothed, and then used as theoretical models in calculating misfit function, but in the process of iteration only the original models are used in order to keep the diversity of models. This technique was shown to be effective in a test of surface wave and receiver function inversion.

Evaluating solution with covariance matrix  After converting nonlinear equations to linear ones in seismic tomography, the inversion problem is reduced to the one of solving ill-posed equations. Following the principle of finding resolution matrix of the solution, Liu and Chang (2000) put forward an evaluation criterion by making use of the covariance matrix of the solution for LSQR algorithm. Making correlation analysis can provide a quantitative evaluation of the solution for those inversion algorithms which can not give the resolution matrix.

 Use of polarization data  Liu et al. (2000) used broadband three-component seismic records of the Beijing station (BJI) and calculated P-wave polarization of teleseismic events. The polarization data were then inverted to obtain the subsurface velocity structure, especially the detail of velocity discontinuities, in the region around the Beijing station. The result shows existence of an obvious low velocity zone in crust to the west of the station, in good agreement with previous studies. This verifies that polarization data could be applied to inversion for velocity structures. Travel time and polarization data can be jointly used to study velocity structure, while polarization data are more suitable for delineating the boundary of velocity anomalies.

 

II.  DEEP SEISMIC SOUNDING SURVEY

Northeastern Qingzang margin profiles  To study the interaction between Qingzang plateau block and Ordos block and investigate the deep process responsible for regional tectonic deformation, the basic research project “Mechanism and Prediction of Continental Strong Earthquakes” carried out a 980 km long Maqin-Lanzhou-Jinbian DSS profile across the northeastern margin of Qingzang Plateau (Li et al., 2002). Result of data interpretation indicates that the crust of Qingzang plateau has a much more complicated structure than that of Ordos block. As going from northeast to southwest, the crust thickness gradually increases due mainly to lower crust getting thicker, and the average crustal velocity gradually decreases. To the west of Zeku there are several low velocity zones in crust, and in the vicinity of Haiyuan there is also a low velocity zone in crust. In the region of Haiyuan and west of Zeku, Pm wave is much more complicated than elsewhere. Result of this DSS profile shows that the northeastern margin of Qingzang Plateau is an intra-continental block contact boundary under compressive deformation. This result does not support the model that the Qingzang Plateau uplifts due to lithospheric subduction from both southern and northern sides. From the Maqin-Lanzhou-Jinbian DSS profile in the northern boundary zone of Qingzang Plateau we see no evidence for southward under-thrusting of lithosphere or crust, but the evidence for crust thickening due to compression.

 


Fig.4.  Two seismic record sections of the Maqin-Lanzhou-Jinbian DSS profile CIn upper right inset is shown the shot position

The Xiji-Zhongwei, southern Ningxia province, seismic reflection/refraction profile, being 248 km long and stretching N-S, was carried out in August to September, 1999. The 2-D processed result of the profile data (Li et al., 2001) shows that in middle part of the northeastward convex arc-belt, north of Haiyuan county, Moho depth is 45 km, shallower than that to the south, 51 km, and to the north, 50 km, of the belt. Low velocity zone exists in upper crust in both south and north side region of the arc-belt. These indicate that the northeastern marginal region of Qingzang plateau has undergone compressive deformation

Dabei DSS profile  Based on the data of Dabei DSS profile Wang et al. (1999) deduced the 2-D crust structure, which shows evidence for collisional orogeny in Dabei region. The 3-D velocity structure of the upper crust reveals a relatively high velocity anomaly in the ultra-high pressure metamorphic (UHPM) belt. Based on Bouguer gravity anomaly the authors also expounded that the crust density in UHPM belt should be lower than that in surroundings. The low density materials is possibly related to the upward return of previously northward subducted Yangtze crust.

Shi et al. (1999b) analyzed data of the DSS profile implemented in March 1997 by a Sino-German cooperation. Tomographic study revealed that the top boundary of a high velocity body might be at the depth of about 1.5 km beneath sea level in Dabei UHPM belt, and the fan-like profile directly reveals the Shuihou-Wuhe fault, a demarcation between the southern and northern Dabei, dipping southwestward at an angle of about 45º at the bottom of upper crust.

West Xinjiang DSS profile  The 1200 km long DSS profile along the Xinjiang Global Geoscience Transaction, from Quanshuigou to Dushanzi, started from West-Kunlun mountain in the south, crossing over Tarim basin and Tianshan mountain, and ended on southern margin of Junggar basin. Totally 973 effective 3-component records were obtained. By comparing the seismic phases appeared on whole profile Li Qiu-Sheng et al. (2001) recognized 6 phases and built up velocity model by travel time inversion and ray tracing modeling. The final model indicates that beneath southern Tarim the Moho dips southward at an angle in concordance with the surface of crystalline basement. From the middle Tarim uplift southward to the front of West-Kunlun Mountain the Moho depth increases from about 40 km to 57 km. Further to the south beneath northern wing of the west Kunlun Mountain the Moho flattens and its depth decreases to 54 km. Based on the observations on basement uplift of the northern slope of west Kunlun, the thickening of lower crust and existence of very thick sedimentation in the foreland depression, Li Qiu-Sheng et al. (2001) inferred that the crust of southern margin of the Tarim basin subducts downward towards the west Kunlun mountain, but with limited subducting distance and depth. In the whole region of Tianshan mountain the average Moho depth is 52 km, and the crust shows variable layer thickness, indicating crust shortening under compression from both northern and southern side.

West-Kunlun to Tarim DSS profile  Gao et al. (2001) interpreted the data from a 103 km long DSS profile across the junction between West-Kunlun Mountain and Tarim basin, and, for the first time, investigated the detailed velocity structure of the crust and top mantle at the junction between mountain and basin. They found the evidence of strong wave reflection indicating southward dipping of the lower Tarim lithosphere and northward dipping of the lower West-Kunlun mountain lithosphere. This face to face dipping structure implies that the Tarim lithosphere intrudes into underneath of West-Kunlun by compression. The data of this profile also reveals a V-shape basin-mountain coupling contact between the lithosphere of west Kunlun mountain and Tarim basin, representing a kind of intra-continental deformation due to continent-to-continent collision.

Xayar-Burqin DSS profile  Using wavelet analysis Zhao et al. (2001) processed the reflected wave data coming from the crust-mantle transition zone between Tianshan orogenic belt and Junggar basin, and obtained the travel time curve of reflected wave coming from the transition zone and generated by every shot. The result indicates that the crust-mantle transition zone along the profile from Xayar, south of Tianshan, to Burqin, northern margin of Junggar basin, may be divided into 3 sections, namely the northern margin of Tarim basin, the Tianshan orogenic belt and the Junggar basin. In the section of Tianshan orogenic belt the crust-mantle transition zone show very complicated structure, consisting of 7-8 stacked thin layers, with variable thickness of 2-3 km and alternative high and low velocity, and with a total thickness of about 20 km. The other two sections have a relatively simple structure. The different structure of the crust-mantle transition zone beneath Tianshan orogenic belt and Junggar basin offers a important base for building the model of “subducting with intrusion into layers” for the Tianshan orogenic belt.

He et al. (2001) studied the crust structure along the GGT profile from Tianshan (Dushanzi) to West-Kunlun. The inversion result shows that there is a low density layer between middle and lower crust in the region of Tianshan Mountain and Junggar basin, while there is no such layer in Tarim basin. The compression from both north and south side has brought about the crust of Tarim basin having an asymmetric arc structure with a middle ridge sided by steep southern wing and gentle northern wing.

DSS profile in Tengchong volcano region  The Tengchong, Yunnan, DSS profile was implemented in 1999. The 178 km major profile trends north-south and the 85 km auxiliary profile obliquely intersects with the major. Using finite difference inversion and forward travel time fitting Wang Chun-Yong et al. (2002) built the 2-D P-wave velocity structure of the crust. According to their result, in upper crust of the Tengchong geothermal region there are low velocity bodies. The S-wave velocity structure obtained form teleseismic waveform inversion also indicates the existence of low S-wave velocity anomaly.

 

III. GPS OBSERVATION STUDIES

Global Positioning System (GPS) observation in China started in early 90's of 20th century. The networks being successively built up include national, regional and movable observation stations. In the 4 year period these networks have produced about 10 year data, based on which a batch of researches on present-day tectonic movement of China continent comes into being.

Compression shortening and continent escape  Horizontal movement observed by GPS clearly indicates that the Qingzang plateau is presently in a state of shortening deformation due to compression, while the eastern China continent is “escaping” southeastward. Wang Qi et al.(2001) made a unified processing of the data obtained from multiple GPS networks in and around China and gave a map (Fig.5) showing the vectors of motion velocity at every observation site with respect to the stable Eurasia. From this map we can see the existence of a global feature in the deformation of China continent. Their research result indicates that crustal shortening accommodates most of India's penetration into Eurasia. Deformation within the Qingzang Plateau and its margins, the Himalaya, the Altyn Tagh, and the Qilian Mountain, absorbs more than 90% of the relative motion between the India and Eurasia plates. Internal shortening of the Tibetan plateau itself accounts for more than one-third of the total convergence. However, the part of Qingzang plateau south of the Kunlun and Ganzi-Mani faults is moving eastward relative to both India and Eurasia. This movement is accommodated through rotation of material around the eastern Syntaxis. The North China and South China blocks, east of Qingzang Plateau, move coherently east-southeastward at rates of 2 to 8millimeters per year and 6 to 11 millimeters per year, respectively, with respect to the stable Eurasia.

 

Based on the horizontal movement rates obtained from the observation at 375 GPS stations in recent 10 years, Yang et al. (2002) gave the current horizontal crust strain field in the region of China continent and its surroundings. The result shows: Horizontal strain of the continent is “strong in west and weak in east”, and globally shear strain is larger than the normal strain (in absolute magnitude). Superficial contraction dominates in the region surround Qingzang plateau, while within the plateau superficial expansion prevails. Large shear strain is seen in the region of Himalaya arc, western margin of China and Sichuan-Yunnan rhombic block.  Wang Xiao-Ya et al.(2002) also made a unified analysis on the 10-year GPS measurement data from nationwide and regional networks, as well as from the Asia-Pacific Regional Geodetic Project, and gave a combined and consistent velocity field solution in ITRF97. In order to study present-time intra-plate crustal deformation of China they constructed a new present-time plate motion model named ITRF97VEL, which describes present-time features of global plate motion better than the geological model NNR-NUVEL1A. Based on ITRF97VEL the deformation rates of more than 260 GPS sites were determined. The result shows that the western China is in N-S shortening and E-W extension, the Himalaya block shows a converging rate of about 15 mm/a, and the west Tianshan converges in 9-13 mm/a, and the Lhasa block is in E-W extension in a rate of (20.2±1.2) mm/a. The derived motion of major faults supports the supposition of crust thickening. In eastern China the Northeast China block is most stable, while the North China block is deformable.

Mode of intra-continental deformation  Based on available GPS observations we have been able to preliminarily verify whether the intra-continental movement manifests itself mainly in continuous deformation or principally in block motion. Wang Qi et al.(2002) analyzed 10-year data observed at 229 GPS stations and showed that the intra-continental movement in China manifests itself in a style of block motions. On the basis of the velocities observed at 79 GPS base stations, which were built up by the project Crustal Movement Observation Network of China, Fu et al.(2002) constructed a map of displacement rate of the stations in China with respect to the motion model of the Eurasia plate in ITRF97 frame, and gave a table listing the deduced displacement rate of every station. Furthermore, by fitting the GPS observations they gave positions of the rotation poles and rotation rates of 15 tectonic blocks in the region of China continent. Another GPS research by Jiang et al.(2001) pointed out that the interior of the Alxa block and Gansu-Qinghai block presently shows NWW-SEE extensional deformation.

Present motion of faults  GPS data can be used in investigating present motion of major intra-continental faults. By analyzing GPS data Wang Qi et al. (2002) inferred that the Altyn Tagh fault now shows a strike-slip motion at a rate of (5.1±2.5)mm/a, which is obviously lower than that inferred from geological evidence, and the Longmen Shan fault zone shortens also in a relatively low rate of (6.7±3.0)mm/a. In contrast to the present low motion rate of the Altyn Tagh fault, the Qilian-Haiyuan fault shows significant left-lateral strike-slip movement as indicated by the remarkable difference in the GPS motions of the Alxa block in north and the Gansu-Qinghai block in south (Jiang et al.,2001). Based on theory of elastic dislocation Shen et al.(2002) calculated the surface displacement field generated by the slip of multiple subsurface rectangular faults. By fitting this fault motion model with the 1991-1999 GPS displacements in Sichuan-Yunnan region they deduced the strike-slip and dip-slip movement of major faults around the Sichuan-Yunnan rhombic block, and expected the possibility of predicting the locked sections of faults by inverting GPS observations.

Taiwan collision zone  Using dislocation model of aseismic deformation and the hybrid global inversion, He and Yao (2002) analyzed the 1990-1995 annually GPS data from 89 stations in southern Taiwan, China, and its vicinity. Geological model of the studied region simply consists of six blocks and nineteen fault patches. The result shows that the Philippine Sea plate moves, with respect to the Eurasia plate, at a velocity of (69±2)mm/a in a direction of (317°±2°). About a half of the converging rate is accommodated by the deformation of Longitudinal Valley fault, and the other half spread over the boundaries of western blocks. Both Philippine Sea plate and Central Range of Taiwan move northwestward and push the China continent, while to the east of the Central Range blocks move divergently westward, with the fan-like moving directions in concord with the directions of maximum principal stress.

 

IV.  GEODYNAMICS OF QINGZANG PLATEAU

Depth of Himalaya thrust  How deep is the Himalaya thrust fault along which the India plate subducts ? Zeng et al.(2000) studied this problem by presenting new seismic evidence. The authors analyzed the spatial distribution and focal mechanism of the earthquakes with focal depth of 33 to 120 km in eastern Qingzang plateau. The earthquake study and DSS investigation have offered evidences showing that the thrust faults in Himalaya and southern Xizang extend to a depth of 80 to 100 km, and end there. The authors suggest that the thrust faults underneath MCT, MBT and Yarlung Zangbo suture are closely related to multiple crustal subduction in Himalaya and southern Xizang. The impinging Indian crust is too light to go further down, and retreat of crust subduction is required for continuing northward movement of the Indian plate, so the crust subduction occurred in multiple episodes, each time stopped at a depth of 80 to 100 km.

Structure of crust and upper mantle  Ding et al.(1999) tomographically inverted over 10,000 arrival time data of local seismic events and constructed a 3-D velocity structure of crust and mantle in eastern Qingzang Plateau. Their result reveals the existence of a notable low velocity zone in the 30 km thick upper crust in southern part of eastern Qingzang plateau, and also a low velocity zone in lower crust of its northern part. The tomographic study on broadband surface wave data by Ding et al.(2001) shows a similar result. This indicates that the southern and northern part of the Qingzang Plateau are in a different stage of the plate collision process.

Using receiver function method to analyze broadband seismic data, Guan et al.(2001) obtained new information on crust and upper mantle structure in the junction region between Tarim Basin and Qingzang Plateau. The crust of Tarim block has a relatively simple structure, with the Moho at depth of 42 km on its southern margin and dipping southward, reaching a depth of 50 km in front of the West-Kunlun Mountain. The receiver function image clearly shows the existence of both southward and northward dipping structure beneath southern margin of Tarim block and west Kunlun Mountain. Such kind of structure may be related to the lithosphere collision.

Numerical modeling  Several numerical modeling studies present new views on uplift mechanism of Qingzang Plateau.

Using finite difference method Li Zu-Ning et al.(2002) made a numerical modeling, in which they modified the continuity equation controlling continental deformation in the England-Mckenzie's (1982) thin viscous sheet model and introduced effect of denudation during the uplift process of the plateau, and also considered effect of material removal at bottom of the thickened mantle lithosphere caused by small-scale mantle convection. This modeling study obtained a result, including the modeled crust thickness, topographic elevation, strain rate and horizontal motion velocity, quite different from that of the pure compressive uplift model. Their result fits observation better than that of other models. They proposed that the material removal caused by small-scale convection may play a principal role in later stage of plateau's uplift.

Fu et al. (2000) modeled the continental lithosphere of East Asia as a continuum in a power law rheology, lying on a relatively soft asthinosphere and being limited in a trapezoid geological frame. The India plate was assumed moving northward in a constant rate and the uplift process of Qingzang Plateau under compression was simulated. In the model the role of denudation on modifying the uplift was also considered. The simulation results in an asymmetric distribution of crustal thickness, which fits the reality well. Besides, Fu et al. (1999) proposed a three stage, i.e., break-down uplift, compression uplift and convection uplift, model for the uplifting process of Qingzang plateau.

Based on the stress-strain constitution of incompressible non-Newtonian viscoelastic rheology He et al.(2002) carried out a 2D finite element simulation on the dynamic relation between lower crust ductile flow and upper crust extension in collisional orogen. Their result indicates that during and after the period of continent collision crust extension appears in some tectonic units. Under lateral plate compression, when the gravitational loading caused by formation of mountain root and surface uplift balances the compression, ductile flow is first facilitated in lower crust of the mountain root. After several Maxwell times the flow will be restricted narrowly in crust thickness transition zone in front of the orogenic belt. This variation of flow pattern leads to the transition of the minimum principal stress near the center of orogen from horizontal compression to horizontal tension. This model can be applied to explaining the N-S extending of the E-W striking and north dipping faults on northern side of Himalaya orogen.

Zhang and Shi (2002) studied the function of gravitational potential in deformation of Qingzang Plateau. Under the compression from both south and north side the whole Qingzang Plateau uplifts, the work done by boundary force raises gravitational potential energy within the plateau. Such high potential would drive the plateau spreading continuously towards a balance state with least potential. Due to the constraints on south and north boundary the high potential energy would mainly lead to the E-W extending of the inner plateau between Himalaya and Kunlun Mountain. Using three different methods the authors estimated the potential energy. The result shows that, given available constraints on rheology, the gravitational potential energy that Qingzang Plateau presently has is sufficient to produce a variety of rates and styles of observed deformation. The highest potential is in the region of Shangxiong-Amdo in Gangdise block, where the topographic elevation is not the highest, but the horizontal extension is the strongest.

 

V. DABIE-SULU ULTRA-HIGH PRESSURE MENTAMORPHIC BELT

Comprehensive geophysical investigation  Dabie-Sulu tectonic belt, east of the central orogen of China, is the largest ultra-high pressure metamorphic (UHPM) belt in the world, and is one of the best geological sites worldwide to study the intra-continental collision/subduction and interaction between crust and mantle. In order to understand its formation and exhumation, China launched the Continental Scientific Drilling Project. During the verification period of the project (1996-2000), comprehensive geophysical investigations were carried out.

Yang et al.(1999a) introduced geological settings and result of DSS investigation in northern part of the belt: The UHPM rock slices south to the Wuliang-Qingdao fault dip northward and are  stacked with each other, showing their exhumation in Triassic Period. The UHPM rock slices of about 12 km thickness contain a lot of coesite-bearing eclogite and show a wave velocity as high as 6.8 to 7.3 km/s. Many wedge-shaped reflectors exist around Moho, indicating the intra-continental collision. In Shimen site high-velocity reflector is buried in the depth range of 5 to 7 km.

Yang et al.(1999b) summarized the results of deep seismic reflection and MT surveys along the 139 km Tangchen-Lianshui profile across the Sulu UHPM: The Sulu UHPM and HPM terrane is a rock-slice with high velocity and resistivity, dipping northwestward and bearing heavy deformation. The Yangtze craton, appearing as a bent plate with normal velocity and relatively low resistivity, was subducting northward beneath Sulu UHPM terrane, containing a lot of wedge reflectors related to the collision between Sulu and Yangtze. The Jiashan-Xiangshui fault zone contains two faults dipping steeply southward, corresponding to a low-resistivity zone reaching the uppermost mantle. Both the old Moho of the Indosinian subducted Yangtze crust and the new Moho formed after Indosinian Movement have strong and clear reflectors, indicating strong magmatic activities and interaction between crust and mantle during Mesozoic Period, which contributed to formation of the new Moho. In upper crust under Sulu UHPM belt exist a group of thrusts, which may be related to exhumation of the UHPM belt.

In addition to seismic reflection survey, MT, magnetic, gravity and radioactivity surveys along two regional profiles of total length 126 km have been carried out in Sulu UHPM belt. Yang et al.(1999c) introduced the achievement of these non-seismic surveys, which produced results similar to that from the seismic reflection investigation. Besides, non-seismic surveys have discovered a new fault striking NNW, named Junan-Haizhou fault, which is of significance in disclosing the regional tectonic evolution.

Yu et al.(2002) collected 212 oriented and 35 non-oriented rock samples at 25 sample sites in 4 places of the target region for the China Continental Scientific Drilling (CCSD), and measured elastic wave velocity, density, magnetic capacity and natural remnant magnetization of the rock samples.

Mechanism of subduction and exhumation  A basic question in mechanics of UHPM formation is why the low density continental crust could overcome buoyancy and subduct into the high density mantle more than 100 km deep. Shi and Fan (2001) touched this question by making a 3D finite element modeling test. Based on the test result they proposed that the subducting oceanic lithosphere can drag adjacent continental sliver with a width up to 150 km down to UHPN depth. Based on integrated study of geology, geochemistry, isotopic dating and latest findings of continent subduction at northern South Island in New Zealand, Fan and Shi (2001) proposed a model on the evolution of UHPM rock. The model includes 4 stages: plate subduction with wedge accretion, plate subduction with corner flow driven by the slab, break-off of the slab and buoyant uplifting of the wedge to normal Moho depth, and final exhumation with upper crust extension. Based on this model the authors made a 2D finite element modeling of Newtonian viscous fluid flow to simulate thermo-tectonic evolution of the Dabie-Sulu UHPM belt , and traced trajectories of rocks at different locations and their corresponding P-T-t paths. The calculated P-T-t paths and the UHPM spatial distribution are in good agreement with observations.

Based mainly on geophysical data newly obtained Yang and Yu (2001) proposed a revised 5 stage model for the evolution of Dabie-Sulu UHPM belt, and argued the sophistication of the revolution.

 

VI.  SEDIMENTARY BASIN

South China Sea   Based on McKenzie's (1978) uniformly stretching model and using the approach similar to that used in seismic geohistory analysis, Ding Zhong-Yi et al.(1999) toke the observed seismic reflection data as the stratigraphy data employed in the correction of decompaction. By means of decompaction and backstripping technique the stratigraphic burial history and thermal subsidence history for some profiles of the basin were obtained. Distribution of the stretching factor was then determined in terms of a comparison of the thermal subsidence history with the McKenzie's theoretical curves. The result shows that the Yinggehai Basin is a basin coinciding with the McKenzie's model, and that two identified spreading events in South China Sea played a significant role in basin formation.

Cao et al. (2001) carried out a partitioned waveform inversion of long period surface wave record, and obtained a 3-D S-wave velocity structure from surface to 430 km depth in 2°×2°mesh for South China Sea and its adjacent region. The result indicates that there is significant difference in velocity, lithosphere and asthenosphere structure between South China Sea and its adjacent region. The top boundary of the asthenosphere beneath the South China Sea basin is shallow, at about 65 km depth, while its thickness is as large as about 250 km.

Using the seismic data detected by ocean bottom seismographs deployed in Xisha trough of South China Sea in a Sino-German cooperation, Qiu et al.(2000) studied P-wave velocity structure of crust by means of seismic phase analysis and travel time modeling. The result shows that the Cenozoic sedimentation has a thickness of 1 to 4 km, below which P-wave velocity increases downward from 5.5 km/s to 6.8 km/s. No high velocity layer is seen in lower crust. The crust thickness is 25 km on northern and southern side of Xisha trough and thins to 8 km in central part, where the Moho bulges upward and shows a sharp velocity contrast between its upper and lower layer. This crust structure characterizes the crust of Cenozoic rift, with symmetric structure on both southern and northern side of the trough.

Zhang Jian et al.(2000) noticed that heat flow in northern South China Sea region gradually increases in NW-SE direction from continent shelf, through continent slope, to central sea basin, and this variation pattern generally matches with gradual change of Moho depth, which becomes shallower, and crust thinner, from north to south. Using heat flow measurements and the model of Earth's gravitation field they calculated thermal structure and rheology of the lithosphere of northern South China Sea. According to their result, in the continental margin zone of northern South China Sea, the temperature in upper crust is 150 to 300 lower than that in lower crust, while viscosity of the former is 2 to 3 orders higher than that of the later, indicating a highly brittle upper crust and significantly ductile lower crust. Zhang Jian et al.(2001) calculated possible convection velocity field in upper mantle beneath this region, and proposed that expansion of the central sea basin is attributed to the southeastward moving of mantle material and local mantle convection.

Liaohe basin  He et al.(1999) put forward a pure shear model suitable for modeling multiply stretched basins, and applied it to modeling Cenozoic multiple tectono-thermal evolution of the Liaohe basin, northeastern China. This model takes into consideration the inheritance of multiple stretching, non-uniformity of stretch and variability of stretch rate. Using this model the amount of denudation can be calculated. In another study, He (2002) modeled the effects of lithospheric rheology on tectono-thermal evolution of stretching basins. It is found that the strength contrast of the basin lithosphere to the stable lithosphere outside the basin strongly influences its thermal-mechanical history. The weaker the strength of the basin lithosphere, the deeper the basin subsidence and the higher the heat flow peak. Besides, between the end of stretching and beginning of thermal subsidence there is a transition stage with large basin uplift and sustained high heat flow.

Bohai basin  Based on heat flow measurements and inversion of apatite and vitrinite reflentance data Hu et al.(1999) reconstructed thermal history of the Bohai basin, North China. This basin is characterized by lower present-day heat flow varying between 50 and 75 mW/m2 with a background value of 63.6 mW/m2, and experienced a period with a much higher heat flow of 70 to 90 mW/m2 prior to about 25 Ma. The lower present and higher past heat flow corresponding to structural subsidence stage, as well as typical rift subsidence style, support the viewpoint that Bohai basin is an intraplate rift basin.

Junggar basin  Based on temperature observations in 196 wells of 1000~5000 m depth and 90 measurements of rock thermal conductivity in Junggar basin, Wang She-Jiao et al.(2000) calculated 35 terrestrial heat flow values. The result shows that Junggar basin is a relatively “cold basin” now, with lower temperature and lower heat flow than normal. Mean temperature and mean heat flow are about 21.2ºC/km and 42.3 mW/m2, respectively. Heat flow in uplift region is high, while in depression is low.

 

VII. PLATE DYNAMICS

Zang and Ning (2002b) studied the interaction between Philippine Sea plate and Eurasia plate. By analyzing the data of geology, GPS survey, earthquake spatial distribution and focal mechanisms, the authors showed that the interaction on different sections of the boundary between the two plates is different: The compression of Philippine Sea plate to the Eurasia is strong at Nankai trough, while no compression acts at Ryukyu trench, where extension may exists. The collision of the two plates at Taiwan island induces strong push to southeastern China continent. In the region of Philippine Islands there is a complex deformation zone, so the interaction between the two plates is weakened and there is little influence of motion of the Philippine Sea plate on the deformation of South China Sea lithosphere.

Sun et al. (2002) studied mantle unsteady flows in an incompressible and isoviscous 4D spherical shell by using algorithms of the parallel Lagrange multiplier dissonant decomposition method (LMDDM) and the parallel multiplier discontinuous deformation analysis (LMDDA). The velocity, pressure, temperature and stress field in the lithosphere of Asia continent and the forces acted on it induced by mantle flow were calculated on a parallel computer.

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