Upper crust structure of eastern A’nyemaqên suture zone: Results of Barkam-Luqu-Gulang deep seismic sounding profile

Barkam-Luqu-Gulang deep seismic sounding profile runs from north of Sichuan Province to south of Gansu Prov- ince. It is located at the northeastern edge of Tibetan Plateau and crosses eastern A’nyemaqên suture zone. The upper crust structures around eastern A’nyemaqên suture zone and its adjacent area are reconstructed based on the arrival times of refracted Pg and Sg waves by using finite difference method, ray tracing inversion, time-term method and travel-time curve analysis. The results show that the depth variation of basement along profile is very strong as indicated by Pg and Sg waves. The basement rose in Zoigê basin and depressed in eastern A’nyemaqên suture zone, and it gradually rose again northward and then depressed. The results also indicate that eastern A’nyemaqên suture zone behaves as inhomogeneous low velocity structures in the upper crust and is inclined to- ward the south. Hoh Sai Hu-Maqên fault, Wudu-Diebu fault and Zhouqu-Liangdang fault are characterized by low velocity distributions with various scales. The distinct variation in basement depth occurred near Hoh Sai Hu-Maqên fault and Zhouqu-Liangdang fault, which are main tectonic boundaries of A’nyemaqên suture zone. Wudu-Diebu fault, located at the depth variation zone of the basement, possibly has the same deep tectonic back- ground with Zhouqu-Liangdang fault. The strongly depressed basement characterized by low velocity distribution and lateral inhomogeneity in A’nyemaqên suture zone implies crushed zone features under pinching action.     

Discovery of the Longriba fault zone in eastern Bayan Har block,China and its tectonic implication

Re-measured GPS data have recently revealed that a broad NE trending dextral shear zone exists in the eastern Bayan Har block about 200 km northwest of the Longmenshan thrust on the eastern margin of the Qinghai-Tibet Plateau. The strain rate along this shear zone may reach up to 4-6 mm/a. Our interpretation of satellite images and field observations indicate that this dextral shear zone corresponds to a newly generated NE trending Longriba fault zone that has been ignored before. The northeast segment of the Longriba fault zone consists of two subparallel N54°±5°E trending branch faults about 30 km apart, and late Quaternary offset landforms are well developed along the strands of these two branch faults. The northern branch fault, the Longriqu fault, has relatively large reverse component, while the southern branch fault, the Maoergai fault, is a pure right-lateral strike slip fault. According to vector synthesizing principle, the average right-lateral strike slip rate along the Longriba fault zone in the late Quaternary is calculated to be 5.4±2.0 mm/a, the vertical slip rate to be 0.7 mm/a, and the rate of crustal shortening to be 0.55 mm/a. The discovery of the Longriba fault zone may provide a new insight into the tectonics and dynamics of the eastern margin of the Qinghai-Tibet Plateau. Taken the Longriba fault zone as a boundary, the Bayan Har block is divided into two sub-blocks: the Ahba sub-block in the west and the Longmenshan sub-block in the east. The shortening and uplifting of the Longmenshan sub-block as a whole reflects that both the Longmenshan thrust and Longriba fault zone are subordinated to a back propagated nappe tectonic system that was formed during the southeastward motion of the Bayan Har block owing to intense resistance of the South China block. This nappe tectonic system has become a boundary tectonic type of an active block supporting crustal deformation along the eastern margin of the Qinghai-Tibet Plateau from late Cenozoic till now. The Longriba fault zone is just an active fault zone newly-generated in late Quaternary along this tectonic system.     

Geochemistry and regional distribution of ophiolites and associated volcanics in Mianle suture, Qinling-Dabie Mountains

The Mianl黣 tectonic zone has been recently identified as an ophiolitic tectonic m閘ange zone on the southern margin of the Qinling Belt[1—6]. The m?lange zone, stretching from east to west in the Mianxian-Lueyang region, represents a newly recog-nized Mianl黣 ophiolitic complex which is quite dif-ferent from the Shangdan ophiolite cropping out in the area between the North and South Qinling in terms of age and composition[1—6]. This paper is to present re-sults of geochemical studies ca…     

A recent paleoearthquake on Qingfengling seismic fault of Tanlu fault zone

Introduction The Tanlu fault zone lies in the eastern China, which is an important huge active fault with a long history. It has experienced a complex generation and evolution process and affects significantly the regional structure, paleogeography, magma activity, minerogenesis and earthquake activity in the area. With a length of 2 400 km, the fault zone consists of 2-4 or more parallel faults of 10-40 km in width, cutting through different geotectonic elements in the eastern China （FANG et al, 1986）. On July 25 in 1668, an extraordinarily large earthquake of M=8.5 occurred on the Changyi-Dadian fault （F1） that is an embranchment of Tanlu fault zone, resulting in a surface rupture with a total length of 130 km （LI et al, 1994; CHAO et al, 1995）. The paleoseismic study reveals that 3 events with a magnitude equal to 8 occurred on the Changyi-Dadian fault. The recent event occurred 3 500 a ago and the reoccurrence interval is about 3 500 a （LIN and GAO, 1987）. During the Tancheng earthquake （on July 25, 1668）, the Anqiu-Juxian fault was not ruptured, which was a Late Pleistocene active fault （ZHENG et al, 1988; GAO et al, 1988; CHAO et al, 1994） and was doubted as the seismogenic fault of the M=7.0 Anqiu earthquake occurred in 70 BC by certain geologists （CHAO et al, 1994）.     

Recent tectonic stress field zoning in Sichuan-Yunnan region and its dynamic interest

In this paper, we have carefully determined the stress zones in the Sichuan-Yunnan region with reference to the in-situ stress data of hydraulic fracturing and the inverted fault slip data by using the step-by-step convergence method for stress zoning based on focal mechanism solutions. The results indicate that the tectonic stress field in the Sichuan-Yunnan region is divided into 3 stress zones by 2 approximately parallel NNW-trending stress transition belts. The area between the 2 belts is the Sichuan-Yunnan stress zone where the maximum principal stress σ1 is just in the NNW direction. The eastern boundary of Sichuan-Yunnan stress zone （the eastern stress transition belt） is basically consistent with the eastern boundary of Sichuan-Yunnan rhombic block. The western boundary of Sichuan-Yunnan stress zone （the western stress transition belt） is not totally consistent with the western boundary of Sichuan-Yunnan rhombic block. The northern segment of the western stress transition belt extends basically along the Jinshajiang fault and accords with the western boundary of Sichuan-Yunnan rhombic block, while its southern segment does not extend along the southwestern boundary of the rhombic block, i.e., Honghe fault and converge with the eastern stress transition belt, but stretches continuously in the NNW direction and accords with the Yingpanshan fault. We therefore consider that under the combined influence from the northward motion of India Plate, the southeastward shift of east Qinghai-Xizang Plateau and the strong obstruction of South China block, the tectonic stress field in the Sichuan-Yunnan region might not be totally controlled by the previous tectonic frame and new stress transition belt may have possibly formed.     

2-D P-wave velocity structure of lithosphere in the North China tectonic zone: Constraints from the Yancheng-Baotou deep seismic profile

We obtained the 2-D P-wave velocity structure of the lithosphere in the eastern North China Craton, Shanxi fault subsidence zone, and Yinchuan-Hetao fault subsidence zone by ray tracking technology based on six groups of clearly identified crustal phases and one group of lithospheric interface reflection phases from seismic recording sections of 21 shots along the 1300-km-long Yancheng-Baotou deep seismic wide-angle reflection/refraction profile. The results indicate significant differences between the lithospheric structure east and west of the Taihang Mountains, which is a gravity-gradient zone as well as a zone of abrupt change in lithospheric thickness and a separation zone of different rock components. East of the Taihang Mountains, the Mesozoic and Cenozoic lithospheric structure of the North China Craton has undergone strong reformation and destruction, resulting in the lithosphere thickness decreasing to 70–80 km. The North China Basin has a very thick Cenozoic sedimentary cover and the deepest point of crystalline basement is about 7.0 km, with the crustal thickness decreasing to about 31.0 km. The crystalline basement of the Luxi uplift zone is relatively shallow with a depth of 1.0–2.0 km and crustal thickness of 33.0–35.0 km. The Subei Basin has a thicker Cenozoic sedimentary cover and the bottom of its crystalline basement is at about 5.0–6.0 km with a crustal thickness of 31.0–32.0 km. The Tanlu fault is a deep fracture which cuts the lithosphere with a significant velocity structure difference on either side of the fault. The Tanlu fault plays an important role in the lithospheric destruction in the eastern part of the North China Craton. West of the Taihang Mountains, the crustal thickness increases significantly. The crust thickness beneath the Shanxi fault depression zone is about 46 km, and there is a low-velocity structure with a velocity of less than 6.1 km s?? in the upper part of the middle crust. Combined with other geophysical study results, our data shows that the lithospheric destruction at the Shaanxi-Shanxi fault depression zone and the Yinchuan-Hetao rift surrounding the Ordos block is non-uniform. The lithosphere thickness is about 80–90 km in the Datong-Baotou area, 75–137 km at the Dingxiang-Shenmu region, and about 80–120 km in the Anyang-Yichuan area. The non-uniform lithospheric destruction may be related to the ancient tectonic zone surrounding the Ordos block. This zone experienced multi-period tectonic events in the long-term process of its tectonic evolution and was repeatedly transformed and weakened. The weakening level is related to the interactions with the Ordos block. The continental collision between the Cenozoic India and Eurasia plates and N-E thrusting by the Qinghai Tibet Plateau block is causing further reformation and reduction of the lithosphere.     

Deformation Evolution Characteristics Revealed by GPS and Cross-fault Leveling Data before the MS8.0 Wenchuan Earthquake

Based on GPS velocity during 1999-2007, GPS baseline time series on large scale during 1999-2008 and cross-fault leveling data during 1985-2008, the paper makes some analysis and discussion to study and summarize the movement, tectonic deformation and strain accumulation evolution characteristics of the Longmenshan fault and the surrounding area before the M_S8.0 Wenchuan earthquake, as well as the possible physical mechanism late in the seismic cycle of the Wenchuan earthquake. Multiple results indicate that:GPS velocity profiles show that obvious continuous deformation across the eastern Qinghai-Tibetan Plateau before the earthquake was distributed across a zone at least 500km wide, while there was little deformation in Sichuan Basin and Longmenshan fault zone, which means that the eastern Qinghai-Tibetan Plateau provides energy accumulation for locked Longmenshan fault zone continuously. GPS strain rates show that the east-west compression deformation was larger in the northwest of the mid-northern segment of the Longmenshan fault zone, and deformation amplitude decreased gradually from far field to near fault zone, and there was little deformation in fault zone. The east-west compression deformation was significant surrounding the southwestern segment of the Longmenshan fault zone, and strain accumulation rate was larger than that of mid-northern segment. Fault locking indicates nearly whole Longmenshan fault was locked before the earthquake except the source of the earthquake which was weakly locked, and a 20km width patch in southwestern segment between 12km to 22.5km depth was in creeping state. GPS baseline time series in northeast direction on large scale became compressive generally from 2005 in the North-South Seismic Belt, which reflects that relative compression deformation enhances. The cross-fault leveling data show that annual vertical change rate and deformation trend accumulation rate in the Longmenshan fault zone were little, which indicates that vertical activity near the fault was very weak and the fault was tightly locked. According to analyses of GPS and cross-fault leveling data before the Wenchuan earthquake, we consider that the Longmenshan fault is tightly locked from the surface to the deep, and the horizontal and vertical deformation are weak surrounding the fault in relatively small-scale crustal deformation. The process of weak deformation may be slow, and weak deformation area may be larger when large earthquake is coming. Continuous and slow compression deformation across eastern Qinghai-Tibetan Plateau before the earthquake provides dynamic support for strain accumulation in the Longmenshan fault zone in relative large-scale crustal deformation.     

Fission track dating of the Cenozoic uplift in Mabian area, southern Sichuan Province, China

The apparent ages of samples are obtained from fission track dating of apatite samples collected from the fault zones in Mabian area, southern Sichuan Province. In addition, thermal history is simulated from the obtained data by applying AFT Solve Program, to acquire the thermal evolution history of the samples. The result shows that tectonically the Mabian area was relatively stable between 25 and 3 Ma, compared to the inner parts and other marginal areas of the Tibetan Plateau. The studied area had little response to the rapid uplift events that occurred for several times in the Tibetan Plateau during 25-3 Ma. The latest thermal event related to the activity of the Lidian fault zone (about 8 Ma ) is later than that of the Ebian fault zone (18-15 Ma ) to the west, indicating to some extent that the evolution of fault activity in the Mabian area has migrated from west to east. The latest extensive tectonic uplift occurred since about 3 Ma. As compared with the Xianshuihe fault zone, the Mabian area is closer to the east- ern margin of the plateau, while the time of fast cooling event in this area is later than that in the southeast segment of the Xianshuihe fault zone (3.6-3.46 Ma ). It appears to support the assumption of episodic uplift and stepwise outward extension of the eastern boundary of the Tibetan Plateau in late Cenozoic.     

New Activity of the Tanlu Fault Zone in the South of Huaihe to the Nvshanhu Segment since the Late Quaternary

Taking the Huaihe to the Nvshanhu segment of the Tanlu (Tancheng-Lujiang) fault zone as the main research target to explore whether there has been new activity since the late Quaternary, and based on the interpretation of remote sensing images and repeated surface investigations, we excavated trenches at the sections where the tectonic landform is significant, identified and recorded the deformation patterns of the fault and analyzed the activity behavior. Samples of new activity and deformation were collected and oriented slices were ground based on the samples'' original state to make the micro structural analysis and demonstration. All of the above research shows very clear linear tectonic geomorphology along the fault, three trenches across the fault zone all revealed new deformation traces since late Quaternary. The latest stratum dislocated by the fault is the late Quaternary and Holocene. The main slip mode is stick slip, as represented typically by fault scarps, wedge accumulation, the faults and the filled cracks and so on. In general, it shows the characteristics of brittle high-speed deformation and belongs to the prehistoric earthquake ruins. The above understanding was confirmed partially by microscopic analysis. In addition, the similarities and differences and the possible reasons for the characteristics of the latest activities of the Tancheng-Lujiang fault zone in the north and south of the Huaihe River regions are also discussed in this paper.     

The slip rate and strong earthquake recurrence interval on the Qianning-Kangding segment of the Xianshuihe fault zone

IntroductionLocated in the western part of Sichuan Province, China, the Xianshuihe fault zone is a notable strong earthquake zone in the eastern Tibetan Plateau. At its northwestern end, the Xianshuihe fault zone overlaps the Ganzi-Yushu fault in a left-stepping pattern. The fault zone extends southeastwards through Luhuo, Daofu, Kangding, and Moxi and disappears near Shimian, with a total length of 400 km. The fault trends N40(-50°W in the north, and N20(-30°W to the south of Kangdi…     

Geophysical evidence for thrusting of crustal materials from orogenic belts over both sides of the Yangtze Block and its geological significance

Based on deep geophysical detections, we have reconstructed the crustal structure from the eastern margin of the Tibetan Plateau to the Jiangnan-Xuefeng orogenic belt. The results suggest that the Yangtze Block was overthrusted by crustal materials in its NW direction from the eastern Tibetan Plateau but in its SE direction from the Jiangnan orogen. These overthrusting effects control the crustal structure from the western Sichuan to the western area of the Jiangnan orogen-Xuefeng orogenic belt. The eastward extruded materials from the eastern Tibetan Plateau were blocked by the rigid basement in the Sichuan Basin, where upper-middle crust was overthrusted whereas the lower crust was underthrusted beneath the Sichuan Basin. The underthrusted unit was absorbed by crustal folding, shortening and thickening in the Yangtze Block, forming the Xiongpo and Longquan Mountains tectonic belts and resulting in the NW-directed thrusting of the Pujiang-Chengdu-Deyang fault, and the western hillsiden fault in the Longquan Mountain. These results provide resolution to the controversy where the eastward extrusion material from the Qinghai-Tibet Plateau had gone. Overall, that Yangtze Block was subjected to thrusting of the crustal materials from the orogenic belts over its both sides. This finding has implications for the study of the intracontinental orogenic mechanism in South China, the reconstruction of tectonic evolutionary history and the kinematics processes during the lateral extrusion of the Tibet Plateau.     

The Late Pleistocene activity of the eastern part of east Kunlun fault zone and its tectonic significance

The nearly EW-trending East Kunlun fault zone is the north boundary of the Bayan Har block.The activity characteristics and the position of the eastern end of its eastward extension are of great significance to probing into the dynamic mechanism of formation of the east edge of the Tibetan Plateau,and also lay the foundation for seismic risk assessment of the fault zone.The following results are obtained by analysis based on satellite image interpretation of landforms,surface rupture survey,terrace scarp deformation survey,and terrace dating data on the eastern part of the East Kunlun fault zone:(1)the Luocha segment is a Holocene active fault,where a reverse L-shape paleoearthquake surface rupture zone of about 50 km long is located;(2)the Luocha segment is characterized by left-lateral slip movement under the compression-shear condition since the later period of the Late Pleistocene,with a rate of 7.68–9.37 mm/a and a vertical slip rate of 0.7–0.9 mm/a,which are basically in accord with the activity rate of segments on its west side.The results indicate that it is a part of eastward extension of the East Kunlun fault zone;(3)the high-speed linear horizontal slip of the nearly EW-trending East Kunlun fault zone is blocked by the South China block at east,and transforms into the vertical movement of the nearly SN-NNE trending Minjiang fault zone and the Longmenshan fault zone,and the uplift of Longmenshan and Minjiang.The area where transform of the two tectonic systems occurred confines the position of the east end;(4)Luocha segment and Maqu segment constitute the"Maqu seismic gap",so,seismic risk at Maqu segment is higher than that at Luocha segment,which should attract more attention.     

Current Vertical Motion Analysis of Northwestern Margin of Ordos Based on Precise Leveling

Ordos block was squeezed by the Qinghai-Tibetan block and North China block, and the tectonic activity was intense. In the periphery of Ordos block, there was a series of folds zones and compressed faults with complicate structures. This paper used three-phase data of 1980,1990 and 2014 to calculate vertical velocity of Northwestern margin of Ordos and the analytical results indicated that ① the Hetao basin between the rise of Yinshan fault block and Ordos fault block showed relatively subsidence, in which Linhe basin was the most evident and the subsidence rate was about 2-4mm/a. The subsidence rate of Jartai -Yinchuan rift zone on the western margin of Ordos block was about 2mm/a; ② the whole testing zone exhibited the evident inherited movement characterized by mountain rise and basin subsidence; ③the two leveling section through the northern margin fault and Dengkou-Benjing fault showed that the difference between vertical velocities on two sides of the fault was less than 0.5mm/a.     

Aftershock sequence relocation of the 2021 M_S7.4 Maduo Earthquake,Qinghai, China

The 2021 Qinghai Maduo M_S7.4 earthquake was one of the strongest earthquakes that occurred in the Bayan Har block of the Tibetan Plateau during the past 30 years, which spatially filled in the gap of strong earthquake in the eastern section of the northern block boundary. In this study, the aftershock sequence within 8 days after the mainshock was relocated by double difference algorithm. The results show that the total length of the aftershock zone is approximately 170 km; the mainshock epicenter is located in the center of the aftershock zone, indicating a bilateral rupture. The aftershocks are mainly distributed along NWW direction with an overall strike of 285°. The focal depth profiles indicate that the seismogenic fault is nearly vertical and dips to southwest or northeast in different sections, indicating a complex geometry. There is an aftershock gap located to the southeast of the mainshock epicenter with a scale of approximately 20 km. At the eastern end of the aftershock zone, horsetaillike branch faults show the terminal effect of a large strike-slip fault. There is a NW-trending aftershock zone on the north side of the western section, which may be a branch fault triggered by the mainshock. The location of the aftershock sequence is close to the eastern section of the Kunlun Mountain Pass-Jiangcuo(KMPJ) fault. The sequence overlaps well with surface trace of the KMPJ fault. We speculate that the KMPJ fault is the main seismogenic fault of the M_S7.4 Maduo earthquake.     

Rupture behavior and deformation localization of the Kunlunshan earthquake (M_w7.8) and their tectonic implications

Earthquake surface rupture is the result of transformation from crustal elastic strain accumulation to permanent tectonic deformation. The surface rupture zone produced by the 2001 Kunlunshan earth- quake (Mw7.8) on the Kusaihu segment of the Kunlun fault extends over 426 km. It consists of three relatively independent surface rupture sections: the western strike-slip section, the middle transten- sional section and the eastern strike-slip section. Hence this implies that the Kunlunshan earthquake is composed of three earthquake rupturing events, i.e. the Mw=6.8, Mw=6.2 and Mw≤7.8 events, respec- tively. The Mw≤7.8 earthquake, along the eastern section, is the main shock of the Kunlunshan earth- quake, further decomposed into four rupturing subevents. Field measurements indicate that the width of a single surface break on different sections ranges from several meters to 15 m, with a maximum value of less than 30 m. The width of the surface rupture zone that consists of en echelon breaks de- pends on its geometric structures, especially the stepover width of the secondary surface rupture zones in en echelon, displaying a basic feature of deformation localization. Consistency between the Quaternary geologic slip rate, the GPS-monitored strain rate and the localization of the surface rup- tures of the 2001 Kunlunshan earthquake may indicate that the tectonic deformation between the Ba- yan Har block and Qilian-Qaidam block in the northern Tibetan Plateau is characterized by strike-slip faulting along the limited width of the Kunlun fault, while the blocks themselves on both sides of the Kunlun fault are characterized by block motion. The localization of earthquake surface rupture zone is of great significance to determine the width of the fault-surface-rupture hazard zone, along which direct destruction will be caused by co-seismic surface rupturing along a strike-slip fault, that should be considered before the major engineering project, residental buildings and life line construction.     

Implication of fault interaction to seismic hazard assessment in Sichuan-Yunnan provinces of southeastern China

Coulomb stress changes associated with the strong earthquakes that occurred since 1904 in Sichuan and Yunnan provinces of China are investigated. The study area comprises the most active seismic fault zones in the Chinese mainland and suffers from both strong and frequent events. The tectonic regime of this rhombic-shaped area is affected by the eastern extrusion of the Tibetan highland due to the collision of Eurasian Plate against the Indian lithospheric block along the Himalayan convergent zone. This movement is accommodated on major strike-slip intraplate fault zones that strike in an E-W direction. The gradual 90° clockwise rotation of the faults in the study area contributes to the complexity of the stress field. The seismic hazard assessment in this region is attempted by calculating the change of the Coulomb Failure Function (?CFF) arising from both the coseismic slip of strong events (MS≥6.5) and the stress built-up by continuous tectonic loading on major regional faults. At every step of the stress evolutionary model an examination of possible triggering of each next strong event is made and the model finally puts in evidence the fault segments that apt to fail in an impending strong event, thus providing fu-ture seismic hazard evaluation.     

A STUDY OF THE RECENT TECTONIC ACTIVITY IN JIANGSUSHANDONG-ANHUI AREA

In this paper two vertical deformation maps have been drawn, based on the accurate leveling data of 1950-1980, in Jiangsu-Shandong-Anhui area. Along combining with neotectonic, geological basis and crustal deformation data, an analysis of the recent tectonic activity in the studied area has been made,and some conclusions have been drawn as follows: (1) The Tancheng-Lujiang fault zone across the studied area is the key tectonic element in recent tectonic activity, and the crustal deform...更多ation is controlled by the fault zone. (2) A study of the two deformation maps shows that the section of the Tancheng-Lujiang fault zone in this area is a creeping section, but both ends of the section are locked parts. The stress concentration ought to be accumulated in these parts. (3) Difference between the two maps suggests that the activity character of the fault zone is right lateral slip during 1950 to 1970, but reverse activity has occurred since 1982 to 1983. This suggests that the most intense earthquake in East 还原 【Abstract】 In this paper two vertical deformation maps have been drawn, based on the accurate leveling data of 1950-1980, in Jiangsu-Shandong-Anhui area. Along combining with neotectonic, geological basis and crustal deformation data, an analysis of the recent tectonic activity in the studied area has been made,and some conclusions have been drawn as follows: (1) The Tancheng-Lujiang fault zone across the studied area is the key tectonic element in recent tectonic activity, and the crustal deformation is controlled by the fault zone. (2) A study of the two deformation maps shows that the section of the Tancheng-Lujiang fault zone in this area is a creeping section, but both ends of the section are locked parts. The stress concentration ought to be accumulated in these parts. (3) Difference between the two maps suggests that the activity character of the fault zone is right lateral slip during 1950 to 1970, but reverse activity has occurred since 1982 to 1983. This suggests that the most intense     

Shear wave velocity structure of the crust and upper mantle in Southeastern Tibet and its geodynamic implications

Southeastern Tibet, which has complex topography and strong tectonic activity, is an important area for studying the subsurface deformation of the Tibetan Plateau. Through the two-station method on 10-year teleseismic Rayleigh wave data from 132 permanent stations in the southeastern Tibetan Plateau, which incorporates ambient noise data, we obtain the interstation phase velocity dispersion data in the period range of 5–150s. Then, we invert for the shear wave velocity of the crust and upper mantle through the direct 3-D inversion method. We find two low-velocity belts in the mid-lower crust. One belt is mainly in the SongPan-GangZi block and northwestern part of the Chuan-Dian diamond block, whereas the other belt is mainly in the Xiaojiang fault zone and its eastern part, the Yunnan-Guizhou Plateau. The low-velocity belt in the Xiaojiang fault zone is likely caused by plastic deformation or partial melting of felsic rocks due to crustal thickening. Moreover, the significant positive radial anisotropy(VSHVSV) around the Xiaojiang fault zone further enhances the amplitude of low velocity anomaly in our VSVmodel.This crustal low-velocity zone also extends southward across the Red River fault and farther to northern Vietnam, which may be closely related to heat sources in the upper mantle. The two low-velocity belts are separated by a high-velocity zone near the Anninghe-Zemuhe fault system, which is exactly in the inner and intermediate zones of the Emeishan large igneous province(ELIP). We find an obvious high-velocity body situated in the crust of the inner zone of the ELIP, which may represent maficultramafic material that remained in the crust when the ELIP formed. In the upper mantle, there is a large-scale low-velocity anomaly in the Indochina and South China blocks south of the Red River fault. The low-velocity anomaly gradually extends northward along the Xiaojiang fault zone into the Yangtze Craton as depth increases. Through our velocity model, we think that southeastern Tibet is undergoing three different tectonic modes at the same time:(1) the upper crust is rigid, and as a result, the tectonic mode is mainly rigid block extrusion controlled by large strike-slip faults;(2) the viscoplastic materials in the middlelower crust, separated by rigid materials related to the ELIP, migrate plastically southward under the control of the regional stress field and fault systems; and(3) the upper mantle south of the Red River fault is mainly controlled by large-scale asthenospheric upwelling and may be closely related to lithospheric delamination and the eastward subduction and retreat of the Indian plate beneath Burma.     

Bouguer gravity study of middle section of Tanlu fault

Although Tanlu fault is one of the most important tectonic fault zones and active earthquake belts in eastern China, little is known about its deep structure. In this study, we use the existing Bouguer gravity data to study the middle section of the Tanlu fault zone, which is also known as the Yishu fault zone. Our gravity inversion results indicate that the Moho has an abrupt offset in depth at the Tanlu fault zone and it has a relatively smooth variation away from the fault zone. The crustal structures on both sides are different from each other. Sediment is thin on the west side with an average thickness of less than 5 km, while it is as thick as 6 km on the east side. The thinnest sediment （3-4 km） is at the fault zone. Moho depth increases from 33 to 34 km on east side and from 36 to 38 km on west side. Tanlu fault zone is shown as a wide zone of linear gradient in the Bouguer gravity anomaly.