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Table of Content

    06 March 2003, Volume 25 Issue 1
    Brief Report
    RHEOLOGY OF CRUSTAL MEDIA AND A RELATED SEISMOGENIC MODEL
    ZHANG Guo-min, LI Li
    2003, 25(1):  1-10. 
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    Based on the analyses of focal depth distribution in mainland China, we studied the physical features of the crustal media, especially the rheological features of the crustal rocks, and set up a related seismogenic model to reveal the earthquake development environment and explain the mechanism of focal depth distribution. Geothermal results are used to deduce the depth of the brittle-ductile transition zone in the crust of East China and West China, which shows that the depth is about 20~25km in East China and 35~45km in West China. The depths are respectively consistent to the lower limits of the focal depths in East China and West China, which indicates that the continental earthquakes in China terminate at the depth of the brittle-ductile transition zone. According to the deduced results, the brittle-ductile transition zone in the crust would control the distribution of focal depths. A focal model is developed to simulate the continental strong earthquakes, and to explain the mechanism and processes of earthquake development in continental areas.
    DYNAMIC ANALYSIS OF CONTINENTAL MEIZOSEISMAL REGION IN CHINA
    HONG Han-jing, YU Yong, TAO Wei, LIU Pei-xun, ZHENG Xiu-zhen
    2003, 25(1):  11-22. 
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    Based on the analyses of the density of energy released by strong earthquakes in East Asia, the contemporary dynamics of continental deformation is studied. The west China and its vicinity including North China can be considered as the Continental Meizoseismal Region of China. The deformation and stress fields in China and its vicinity are studied by modeling the long-term squeezing of the Indian subcontinent in term of 3-dimensional layered visco-elastic finite element method. The regime of energy distribution in this region is basically similar to the model with Indian's indentation, but has some differences. The super shear zones on both sides of the Tibet Plateau proposed by Dewey et al. (1990) are essentially the deformation twist belts: the stress is concentrated in both belts as compared with that inside and outside the plateau, in spite of a slight difference of stress orientations from the elastic homogeneous model. The vectors of crust motion in east China are leaning towards the east, and the vectors in the east and northeast parts of the Tibet Plateau are significantly different from those in their vicinity. There are three arcuate belts with high seismic energy densities in the eastern Tibet Plateau: Chayu arc in the southeast, Kangding arc in the northeast and Haiyuan arc in the north-northeast. The meisoseismal region in China is developed by three combined dynamic processes: (1) The long-distance squeezing of the Indian subcontinent results in the gradual extending of the deformation starting from both corners of the plateau and ending in the collision zone, which then causes the formation of the deformation twist belts on both sides of the plateau. The thick crust of the Tibet Plateau provides the medium condition for elastic energy accumulation. The creep of the lower crust at higher temperature leads to the concentration of stress in the upper elastic layer. (2) The large-scale eastward motion of the eastern Asia has given rise to the change of the squeezing direction of Indian Plate in the northern Tibet from north-northeast to northeast, and the turning of both the deformation twist belts from north-northeast-trending to northeast-trending. (3) The inhomogeneous extrusion within the Tibet Plateau promotes the formation of three arcuate belts on the eastern side of the Tibet Plateau.
    PRESENT-DAY ACTIVITY AND DEFORMATION OF TECTONIC BLOCKS IN CHINA'S CONTINENT
    HUANG Li-ren, WANG Min
    2003, 25(1):  23-32. 
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    On the basis of carefully pre-processed GPS data observed at reference and basic stations of CMONOC (Crustal Movement Observational Network Of China) in 1998 and 2000, the coordinates in ITRF97 referenced to the epochs of 1998.6808 and 2000.4658 as well as their covariance matrices at 79 GPS stations distributed in the main tectonic blocks over China's continent are derived respectively. Under the tectonic framework of the main tectonic units and the main active blocks of China proposed by MA Xing-yuan et al. and ZHANG Pei-zhen et al., respectively, 20 major tectonic blocks in China's continent are tested and identified one by one by using an extended QUAD (Quasi-Accurate Detection of gross errors) method (HUANG Li-ren, et al., 2002). The adjacent blocks with no significant relative motions to each other are merged into one block. Thereby, the active blocks and their boundaries are identified. A model combining the rigid motion, block uniform strain and local deformation is used to describe the present-day activity and deformation of tectonic blocks in China's continent. The Euler's vector and the entire uniform deformation parameters of the blocks with significant relative motion, as well as the local heterogeneous deformation within the blocks are derived. The pattern and rate of motion on active boundaries are also calculated. Based on the afore-mentioned results, it is pointed out that the crustal motion in China's continent is much stronger in the western part than in the eastern part. In addition, the quantitative comparisons of crustal motion among the main active blocks and boundaries in the western part of China are also given. This result may provide the deformation background for the evaluation of earthquake risk regions.
    SEISMIC MOMENT TENSOR ELEMENT Mrr AS A SIMPLIFIED REPRESENTATION OF CRUSTAL POTENTIAL ENERGY CHANGE: ITS SPATIAL DISTRIBUTION PATTERN AND RELATION TO THE CONTINENTAL TECTONIC BLOCKS IN CHINA
    WU Zhong-liang, HUANG Jing, ZHANG Dong-ning, ZHOU Gong-wei, ZHANG Guo-min
    2003, 25(1):  33-38. 
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    Spatial distribution of the signs of the seismic moment tensor element Mrr is mapped by using the Harvard CMT catalogue and the catalogue of earthquake focal mechanisms in China. This distribution pattern corresponds to a simplified version of the change of crustal potential energy calculated by Tanimoto and Okamoto (2000), in which the sign of Mrr indicates the state of stress at the seismic source. When Mrr is positive, it represents a compressional state; when Mrr is negative, it represents a tensional state. We define the 'mixed state' for the case that positive and negative Mrr are mixed together, which corresponds to a complex stress state with a significant shear component. This simplification is taken because for many earthquakes the focal depths cannot be precisely determined so that the calculation of crustal potential energy cannot get reliable results; moreover, for most of the regional/local catalogues of focal mechanisms, their completeness cannot be ensured. Comparison between the spatial distribution of Mrr and the dynamic pattern of continental blocks in China shows that the stress state within each tectonic block has significant consistency, while the stress state at the boundary is different from that within the block, implying the existence of the blocks and their relative motions to each other.
    3-D VISCO-ELASTIC FINITE ELEMENT MODEL FOR THE SUBDUCTION OF THE OCEAN PLATES INTO THE EASTERN PART OF CHINA'S CONTINENT
    TAO Wei, HONG Han-jing, LIU Pei-xun, YU Yong, ZHENG Xiu-zhen
    2003, 25(1):  39-51. 
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    In this paper, a 3-D visco-elastic finite element model is used to describe the long-term average movement of China's continent. The boundary conditions of the model are taken in reference to the average plate velocity obtained from geological information, as well as the subduction of the Philippines and Pacific Plates into the continent and the collision between the Indian Plate and China's continent. The results of GPS may reflect the contemporary movement of China's continent. The difference between the contemporary movement and the long-term average crustal movement can then be recognized by the comparison of the modeling result and the GPS result. The two kinds of results show much consistency and little difference. It indicates in one point that each short-term movement of the continent might be a small dynamic adjustment process near the long-term average state, and can be attributed to the continuous adjustment of the continental crust to reach an equilibrium state in response to the movements of the surrounding plates. The modeled stress field shows that the stress is higher in the western and southern parts and lower in the eastern and northern parts, consistent with the stress field obtained by the other studies. The subduction of the Pacific and Philippine plates has led to a complex effect on the eastern part of the continent. In Northeast and North China, the E-W-directed stress is dominated by compression due to the compression of the ocean plate and the obstacle of the block to the north. However, the S-N-directed stress becomes gradually to be extensional, as the S-N-directed displacement becomes greater from north to south. Because of the difference of motion rate between North China and South China, North China is subjected to extensional stress. This is consistent with the results of Shen et al. (2000) and DING Guo-yu (1986). In South China, the S-E-directed compressive stress is predominant, but alternating compressive and extensional stresses are predominant in the vicinity of the eastern boundary of the continent. Three cross sections are cut along the X-direction of the model to observe the stress and displacement on X-Z plane. In contrast to the compression of the Indian plate, the subduction of the ocean plates gives rise to the complicated distributions of stress and displacement on the profiles. Although the whole continent, and especially the western part of the continent, is dominated by compressive stress, alternating high, low and high stress regions may occur from west to east in the eastern part of the continent, and extensional stress may to different extent occur in the region from Huanghai sea to Taiwan. Because of the differences of the rheological properties of the media in various layers of the model, stress will gradually concentrate in the high viscocity layers of the model as time goes on. Due to the subduction of the ocean plates, small-scale high stress region with high stress gradient may occur at depth of the lithosphere beneath the eastern boundary of the continent. In addition, some convection circles may occur in the lithosphere beneath the eastern boundary of the continent, but the features of stresses in various quadrants are different due to the complexity of the crust and upper mantle. Further study is needed to test this conclusion. The modeling results in this paper indicate that the subduction of the Pacific and Philippine Plates into the continental lithosphere has very important effect on the orientation and features of the stress field in eastern China's continent. LI Zu-ning et al. (2002) proposed that the ignorance of the effects of the subduction of Pacific and Philippine plates is the main reason that causes the incompatibility of their modeling result to the results of GPS and seismic observations in China's continent. Obviously, a better understanding of the dynamic background of China's continent can be gained only by taking the effects of the Pacific and Philippine Plates into consideration.

    DIFFERENCES OF CRUSTAL STRUCTURES IN NORTHEASTERN EDGE OF TIBET PLATEAU, ORDOS AND TANGSHAN EARTHQUAKE REGION IN NORTH CHINA-RESULTS OF DEEP SEISMIC SOUNDING
    ZHANG Xian-kang, LI Song-lin, WANG Fu-yun, JIA Shi-xu, FANG Sheng-ming
    2003, 25(1):  52-60. 
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    Obvious differences of crustal structures exist in different tectonic blocks of China's continent. These differences can be found mainly in crustal stratification, structural features of the upper and lower crust, degree of crustal heterogeneity, properties of crust-mantle boundaries, distributions of crustal low-velocity layers and the interfaces within the crust, especially the tectonic forms of the Moho. These differences of crustal structures reflect the differences of deformation features and geodynamic processes within the crust of these regions, and may provide some constraints for delineating active tectonic blocks. The descriptions of the degrees of heterogeneities of the active tectonic blocks will help to understand the probabilities of decoupling of these tectonic blocks at different depth levels. The mode of motion of these active blocks is controlled by the behavior of their boundaries and the contacting styles of the blocks. Most of the strong earthquakes in China occur near the boundary belts of these active tectonic blocks. A wide-angle reflection and refraction profile about 1 000km long was deployed in 1999, which crosses through Bayan Har fold belt, Qinling-Qilianshan fold belt, Haiyuan strong earthquake region and Ordos block from southwest to northeast. From the analysis of these data, fine structural models of the crust for the eastern edge of Tibet plateau and Ordos block were established, and the results were compared with those of North China. The differences of crustal structures in Bayan Har block of eastern Kunlun at the northeastern edge of Tibet Plateau, Ordos block and Tangshan earthquake region in North China, as well as their relationships to strong earthquakes are discussed in this paper.
    TECTONIC AND PALEOMAGNETIC EVIDENCE FOR THE CLOCKWISE ROTATION OF THE SICHUAN- YUNNAN RHOMBIC BLOCK
    XU Xi-wei, CHENG Guo-liang, YU Gui-hua, SONG Fang-min, XIANG Hong-fa, ZHANG Lan-feng, Hagai Ron, WANG Yang-long, WEN Xue-ze
    2003, 25(1):  61-70. 
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    The inner part of the Sichuan-Yunnan rhombic block is dissected by the Lijiang-Xiaojinhe Fault, and hence can be subdivided into Northwest Sichuan sub-block in the north and Central Yunnan sub-block in the south. The eastern boundary faults of these two sub-blocks are regularly characterized by left-lateral strike-slip, while the western boundary faults are characterized by right-lateral strike-slip. The slip rate of both the eastern and western boundary faults are significantly different. All these phenomena may indicate the composite movement of these sub-blocks characterized by southeastward horizontal slipping associated with clockwise rotation around a vertical axis during the Cenozoic time. Among them, the horizontal slip rate of the Southwest Sichuan sub-block is 5mm/a, and the angular velocity of clockwise rotation is about 1 4°/Ma, while those of the Central Yunnan sub-block are 3.5mm/a and 1 5°/Ma, respectively. About 90 oriented samples have been collected from Paleogene strata in Yaoan, Dayao, Yongren and Beimajie of Kunming within the Central Yunnan sub-block. The vectors of remanent magnetism of each sample (measured magnetic declination and inclination) have been obtained through alternating field demagnetization and thermal demagnetization. The comparison between the measured magnetic declination and the expected value shows that the accumulated clockwise rotation of the Central Yunnan sub-block of the Sichuan-Yunnan rhombic block since early Miocene has reached up to 30°~48°. The feature represented by the entire rotation of the sub-blocks accompanied by left-lateral slipping along the boundary active faults is consistent with the kinetic model of clockwise rotation of the block in left-lateral strike-slip faulting region.
    FOCAL MECHANISMS, DISPLACEMENT RATE AND MODE OF MOTION OF THE SICHUAN-YUNNAN BLOCK
    CHENG Wan-zheng, DIAO Gui-ling, LÜ Yi-pei, ZHANG Yong-Jiu, LI Gui-fang, CHEN Tian-chang
    2003, 25(1):  71-87. 
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    In this paper, Sichuan-Yunnan and its adjacent areas are divided into Yajiang, Central Yunnan, Sichuan-Qinghai, Central Sichuan, and Myitkyina-West Yunnan blocks. The mode and rates of motion of these blocks and their boundaries are studied separately. The predominant direction of the principal compressive stress axes within the blocks is analyzed in term of focal mechanism solutions of 442 moderate strong earthquakes. The models of focal faulting are determined through the rake angles λ obtained from source parameters of 771 events with magnitudes about 3 or more, and they are evidenced by focal faulting or slip modes derived from the distributions of N axis plunges of P-wave first motion solutions for moderate-strong earthquakes. A comparison is made between the moment tensor rates for moderate-strong earthquake and the calculated annual average slip rates of each seismotectonic zones within the blocks. Based on regular resurveying of across fault short level lines and baselines during the period of 1980-2001 along the border of the Sichuan-Qinghai, Yajiang and Central Yunnan blocks, the annual average rates of horizontal and vertical deformation for each site are analyzed. The motion rates of the Sichuan-Yunnan Block are inhomogeneous. The Sichuan-Qinghai Block moves toward SEE direction, and the predominant direction of the principal compressive stress axis falls in the range of N100°~130°E. The motion rate of this block is smaller than that of the Yajiang block, implying a delayed motion relative to that of the later. As a result, the Xianshuihe Fault between these two blocks displays a strong left lateral displacement. In addition to earthquakes of strike-slip type, the earthquakes of reverse dip-slip type are also predominant in the Sichuan-Qinghai Block. The predominant direction of the principal compressive stress axes in the Yajiang Block is within the range of N160°~170°E, i.e. SSE. The motion rate of this block is larger than that of the Sichuan-Qinghai Block, and the percentage of strike-slip type earthquakes in this block is larger then that in the other blocks. The motion direction of the Central Yunnan Block is nearly the same as that of the Yajiang Block, leaning slightly to the east. The predominant direction of the principal compressive stress axes in this block is in the range of N150°~160°E, and there are various types of earthquake failure including strike-slip, normal dip-slip, and reverse dip-slip. The direction of the principal compressive stress axes in the Central Sichuan Block falls in the range of N110°~140°E, approximately the same as the motion direction of the South China Block. The percentage of strike-slip type events in this block is relatively low, and there is a portion of events belonging to normal dip-slip or oblique dip-slip types. The motion rates at the northeast boundary between the Yajiang and Central Yunnan Blocks are greater than those at the west and south boundaries. The SSE-directed motion of the blocks is associated with right lateral component. Similarly, as the motion rates of the eastern boundary of the Sichuan-Qinghai Block is greater than that of the Longmenshan belt to the southeast of the block, the SSE-directed motion of the Sichuan-Qinghai Block is also associated with right lateral component. Two predominant directions of the principal compressive stress axes in the Myitkyina-West Yunnan Block indicate the NE-directed squeezing and the SE-directed escaping of the block, while the motion rate of the block is larger than that of the Yajiang and Central Yunnan Blocks. This might be attributed to the dynamic mechanism of the formation of collision and squeezing belts due to the direct action of the Asam Wedge while Indian Plate moving northward.
    CONTEMPORARY TECTONIC STRESS FIELD AROUND THE ORDOS FAULT BLOCK INFERRED FROM EARTHQUAKE FOCAL MECHANISMS
    FAN Jun-xi, MA Jin, DIAO Gui-ling
    2003, 25(1):  88-99. 
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    This paper discusses mainly the spatial distribution and change of stress orientation at focal depth around the Ordos fault block. The stress orientation are inverted by using the Grid Test Method from more than 3 000 earthquake focal mechanism solutions calculated from earthquakes with magnitude less than 5 recorded by local seismological stations around the Ordos during the years of 1980 to 1999. The region around the Ordos is divided into ten subregions based on neotectonic, geomorphologic and GPS data. The P and T axes being close to horizontal in all subregions implies that the Ordos and its vicinity are dominated by strike-slip motion. The spatial distribution pattern of the principal compressive stress axes around the Ordos Block appears as a fan convex toward the southwest of the Ordos Block. The mean orientation of the P axis is NE-directed in the west, ENE-directed in the north, and nearly E-W-directed in the south of the Ordos Block. In fact, the orientation of P axis in each subregion may vary with time. In 1992 and 1996, the direction of P axis in each subregions was gradually turned toward N75癊 direction. Based on the changes of P axis orientation with time in each subregion, three types of subregions are identified: the first type includes the areas where the orientation of P axis is relatively unchanged, such as the Weihe, Liupanshan and Yingchuan subregions; the second type includes the areas where the change of the orientation of P axis with time is complex, such as the Linfen and Tongxin subregions; the third type includes the areas where the orientation of P axis is sensitive to the earthquakes occurred nearby, such as the Datong and Baotou subregions. In Datong and Baotou subregions, the changes of P axis orientation doesn't simultaneously respond to the occurrence of an earthquake; when the direction of P axis in one subregion turns to NNE direction, then an earthquake will occur in this area. However, during the same period, the orientation of P axis in the other subregions doesn't turn to NNE direction. Another interesting thing is that the inconsistency ratio of the Grid Test Method for Datong subregion decreased before the Datong earthquake in 1989, while the ratio for Baotou subregion decreased before the Baotou earthquake in 1996.
    THE RELATIONSHIP BETWEEN THE SATELLITE INFRARED ANOMALIES BEFORE EARTHQUAKE AND THE SEISMOGENIC FAULT-A Case study on the 2001 Kunlun Earthquake
    CHEN Mei-hua, DENG Zhi-hui, JIA Qing-hua
    2003, 25(1):  100-108. 
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    The Kunlun MS8.1 earthquake took place on Nov. 14, 2001 in eastern Kunlun Mountains, Northwest China. Two methods are developed to extract the information of the satellite infrared anomalies prior to the earthquake. One is pixel-by-pixel analysis by subtracting the average brightness temperature 1 month before the event from the average brightness temperature in the same period in 2 000 pixel by pixel. The obtained image can be divided into two different regions: temperature increase and non-increase regions. The image shows that most regions in north Tibetan Plateau are temperature non-increase, but some temperature increase belts occur along active faults, and especially along the eastern Kunlun Fault. This temperature increase is considered to be earthquake precursor. The second method is based on the analysis of temperature difference inside and outside the fault zone. The inside region is defined as the buffer region created by GIS within 15km radius to the Hoh Sai Hu segment of the Eastern Kunlun Fault. The outside region is the buffer region in 15~30km radius to the inside region. This method is used to calculate the difference of the average brightness temperature for inside region and outside region in each night(0~8am) in 2001. The result shows that the temperature of the inside region is normally about 2℃ lower than that of the outside region. However, one and half months before the earthquake (beginning from Oct.2001),the brightness temperature of the inside region became 1℃ higher than that of the outside region. It returned to normal value after the event. The results indicate that the satellite infrared anomalies correspond spatially to the seismogenic fault. The temperature increase in the seismogenic fault belt is higher than that in the other regions. The analysis of temperature difference inside and outside the fault zone is an effective method for extracting the precursor information from satellite infrared images.
    RHEOLOGICAL PARAMETERS OF CRUSTAL ROCKS AND CRUSTAL RHEOLOGY OF NORTH CHINA
    ZHOU Yong-sheng, HE Chang-rong
    2003, 25(1):  109-122. 
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    Rheological parameters and deformation mechanisms of rocks are the basis for estimating crustal strength by using frictional constitutive equations and power law creep equations. In the past 30 years, substantial progress has been made in high-temperature and high-pressure experimental studies, which provide a large numbers of data on rheological parameters of crustal minerals and rocks, as well as new insights into the deformation mechanisms. In this paper, we summarize systematically the existing experimental data, and study crustal rheology of North China by using these data combined with focal depth distributions in this region. The results show that the upper crust as represented by granite and low-grade metamorphic rocks is deformed in brittle faulting and frictional sliding regime, the strength of which is controlled by friction on faults; the middle crust comprising felsic-gneiss, as well as the upper layer of lower crust composed of intermediate granulite behave in plastic flow regime; the lower layer of lower crust consists of dry mafic granulite, which behaves in brittle-plastic flow transition regime. The composition and rheological stratification of the crust in North China may cause decoupling of different crustal layers and provide mechanical conditions for strong earthquake generation. In addition, they may also serve as the bottom boundaries for blocks of different scales.
    IMAGE SCANNING ANALYSIS OF CALICHE MICRO-LAMINATION AND ITS SIGNIFICANCE TO THE DATING METHOD
    ZHENG Wen-jun, GUO Hua, LIU Bai-chi
    2003, 25(1):  123-132. 
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    Caliche on gravel, as an indicator of variation of sedimentary environment, records relatively complete information on environment and climate. It usually appears as alternating bright and dark laminations, and its formation depends mainly on the purity of deposited carbonates, size of crystalloids, and foreign matters etc. This paper demonstrates that the shading image of caliche lamination can be obtained by using an ordinary scanner, and then the characteristic curve of the image can be automatically established by means of computer program. Furthermore, it can be compared with the characteristic curves of deep-sea oxygen isotope and loess-palaeosol sequence, and it can also be compared with grain-size and susceptibility curves of loess, as well as with tectonic events. In this way, the age of morphologic surface represented by the caliche can be estimated, while the effect of climate can be evaluated. A pilot study has indicated the feasibility of this method. It is indicated that this method not only may improve the precision and time-span of dating, but also leads to the applicability of sediment dating methods to the research of palaeo-environment and palaeo-climate. The following conclusions can be drawn from the present study: 1. The shading image dating of caliche micro-lamination widens the age range of ordinary dating methods; 2. The method may provide dating control for the research of palaeo-environment and palaeo-climate, and provides another evidence for the tectonic-climatic cycle; 3. The results of present study reveal that the relation between caliche thickness with time is not simply a constant accumulation relation; 4. As a record of successive sedimentation, caliche lamination is a valuable datable material. The major advantages of calliche dating lay on its simplicity, low-cost, abundant information and high accuracy. It is believed that with continued effort, the optimum resolution of this dating method can be improved to reach thousands or hundreds years. The application of this dating method to two sampling sites yields the following results: The age of T 11 terrace at Heishanxia Gorge of the Yellow River is dated to be 1 660ka B.P., and the age of the same terrace at Majiatan of Lingwu County is dated to be 1730~1780ka.B.P., which are in accord with the preexisting results. These results also support the feasibility of caliche dating method.
    QUATERNARY TECTONIC ACTIVITY IN NORTHEASTERN QINGHAI-XIZANG PLATEAU AS REFLECTED BY RIVER TERRACES ALONG THE MIDDLE-UPPER REACH OF THE YELLOW RIVER
    LIU Bai-chi, LIU Xiao-feng, YUAN Dao-yang, ZHENG Wen-jun, GUO Hua, CAO Juan-juan
    2003, 25(1):  133-145. 
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    The middle-upper reach of the Yellow River is a flowing water system passing through the whole "Qaidam-Qilianshan active block". We systematically investigated the river terraces of the Yellow River of about 1 800km length from Gonghe of Qinghai to Shizuishan of Ningxia, measured about 73 cross sections of terraces and obtained 88 age data in total. In addition, we have compiled the vertical profiles of river terraces on this river segment, and carried out the comprehensive analyses of the number, heights, ages and deformation features of the terraces. The following conclusion can be drawn from the obtained results: 1.These drainage areas can be divided into several secondary active blocks; they are Gonghe, Lanzhou, Haiyuan, Weining basin (Alashan) and Yinchuan basin (Ordos) secondary blocks, each of which is bounded by regional active faults and is independent of the others. The amplitudes and rates of uplift of the terraces in each block are significantly different, while those in the southern block are larger than those in the northern. For example, the relative elevation above river level of terrace planes of 1.60Ma age from south to north are: 400m in Gonghe block, 105~200m in Lanzhou block, 200~265m in Haiyuan block, 135~147m in Weining basin block, and -842m in Yinchuan basin block, respectively. 2. The interiors of the secondary active blocks are to some extent rigid, so the number, altitudes and ages of the terraces within the blocks are about the same, while the vertical profiles are relatively continuous and smooth. However, folding deformation is also obvious in local zones due to thrusting and other reasons. 3. Vertical profiles of river terraces reflect that the uplift amplitude of the Qaidam-Qilianshan active block since 1.6Ma B.P. is greater than that during 3.6~1.6Ma B.P. 4. Vertical profiles of terraces from Gonghe to Shizuishan segment of the Yellow River indicate that the rate of crustal uplift during 0.15~0.20 Ma B.P. is 2~6 times as high as that in the other periods, and the rate in the past 10 000 years is 4~10 times as high as that in the other periods, or 2~5 times even after deducting half of the amount that was caused by climatic effect. It indicates that there have been at least two intensive tectonic movements since 1.6 Ma B.P. These two events are comparable to the "Gonghe Movement" and "the Latest Movement" proposed by Li Ji-jun et al (2001).
    PRIMARY STUDY ON QUATERNARY TECTONIC EVENTS BASED ON VARIATION OF FAULT ACTIVITY IN WEIHE BASIN
    TIAN Qin-jian, SHEN Xu-hui, FENG Xi-jie, WEI Kai-bo
    2003, 25(1):  146-154. 
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    Recognition of major tectonic events and tectonic cycles in Quaternary is an important topic that attracts more and more attention of many geologists. Recently, the research in this aspect has focused mainly on Quaternary sedimentation and geomorphologic evolution, while little attention has been paid to the study of fault evolution, which is directly related to tectonic cycle or tectonic event. This is mainly because of the lacking of geological evidence and the limitation of dating technique. Nevertheless, significant progress has been made in loess study, especially the characteristics and dating of loess-paleosoil sequences, which provide a time scale for regional correlation and timing of loess deposits. In this paper, an attempt has been made to discuss the migration and variation of Quaternary activity of fault zone through the analysis of loess deposition along the zone by using this time scale. The main purpose of this study is to provide direct evidence for the division of tectonic cycles in Quaternary. The present study deals mainly with the Lintong-Chang'an Fault and the Lishan mountain front fault on the southern margin of Weihe basin, as well as the Kouzhen-Guanshan Fault on the northern margin of the basin. The loess deposits along these fault zones have been studied in detail, while the main unconformities in loess sequences were identified in the geological sections across the fault zones and dated by using the loess-paleosoil time scale. The results show that tectonic unconformity presents broadly along the Lintong-Chang'an Fault and the Lishan mountain front fault, appearing as discordant contact of the S 8 paleosoil layer with the underlying strata. The underlying strata are offset significantly by the fault, but the overlying strata of S 8 are offset inconsiderably. Along the Kouzhen-Guanshan Fault zone, the S 1 paleosoil layer discordantly contacts with the underlying strata, which are significantly offset by the fault, but the overlying strata of S 1 layer are inconsiderably offset. In term of the loess-paleosoil time scale, the following conclusion can be drawn from the result of this study: The activity of the Lintong-Chan'an Fault zone on the southern margin of the Weihe basin was markedly changed at 800~900ka B.P. At the same time, the migration of activity occurred along the Lishan mountain front, while strong activity started to occur along the Weinan Yuan front Fault and the whole Weinan Yuan began to be uplifted. At 120ka B.P., the activity of the Kouzhen-Guanshan Fault zone began weakened. Thus, the variation of fault activity in this area may indicate two major tectonic events in mid-late Quaternary. This result may provide basic material for regional correlation of Quaternary tectonic events and for the research of the tectonic manifestation of tectonic events.
    CHARACTERISTICS OF THE MODERN ACTIVITY OF THE RESHUI-RIYUESHAN FAULT ZONE IN QINGHAI PROVINCE
    YUAN Dao-yang, LIU Xiao-long, ZHANG Pei-zhen, LIU Bai-chi
    2003, 25(1):  155-165. 
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    The Reshui-Riyueshan active fault zone lies within the central part of the Qaidam-Qilianshan active block on the northeastern margin of the Qinghai-Tibet Plateau. It is a NNW-trending right-lateral strike-slip active fault zone with reverse components characterized by clear linear features and strong activities. It initiates to the north of Datong River, and extends southward through the Reshui coal mine, running along the eastern side of the NNW-oriented Datongshan and Riyueshan upheaval area, and obliquely connected with Lajishan active fault zone at the pass of Riyueshan Mountain. The fault zone is generally striking N35°W, having a total length of about 183km and a relatively simple geometrical structure. It consists of 4 discontinuous secondary en echelon faults, which are Datong River (F1-1), Reshui (F1-2), Haiyan (F1-3) and Riyueshan (F1-4) Faults, respectively. Extensional zones or pull-apart small basins, such as Ketu basin, were formed at the step-over of these secondary faults. The wide of the step-over is about 2~3km. The Reshui-Riyueshan active fault zone was a compressive reverse fault zone in the early period, and caused the strong compressive deformation of late Cenozoic strata on both side of the fault. Since late Quaternary, the fault zone has become a right lateral strike-slip reverse fault, which has given rise to the formation of micro-morphology represented by right-laterally offset ridges, valleys and terraces. The larger offset may reach up to hundreds meters or more, while the smaller offsets are just about several meters. The right-lateral offset of the first level terrace is about 8~11m, while that of the second level terrace is about 35m. At the same time, there are many fault cliffs and fault scarps developed along the fault zone. The height of the fault scarp at the first level terrace is about 0.5~1m, and the highest one is about 2.8m; at the second level terrace and pluvial mesa, the height of the scarp is about 2.5~3m,with the highest value of about 4~5m. According to the 14C age of the first level terrace of about 3 000a, the horizontal slip rate along the Reshui-Riyueshan Fault zone since late Holocene is estimated to be about 3.16 mm/a, and the vertical slip rate is about 0.83 mm/a. According to the TL age of the second level terrace of about 23.8±1.2ka, the horizontal slip rate along the fault zone since late Pleistocene is estimated to be about 1.47mm/a, and the vertical slip rate is about 0.10~0.21mm/a. Comparatively speaking, the higher the terrace, the stronger the erosion in later period, and hence the larger the uncertainty of the measured offsets. On the contrary, the lower the terrace, the smaller the erosion, and hence the smaller the uncertainty of the measured offset. It is believed, therefore, that the Holocene horizontal slip rate of about 3.16 mm/a, and vertical slip rate of about 0.83 mm/a for the fault zone calculated from the first level terrace are more reliable.
    DISCOVERY OF SURFACE RUPTURE ZONE PRODUCED BY GUANGUANLING EARTHQUAKE AT THE JUNCTURE OF NINGXIA, INNER MONGOLIA AND GANSU PROVINCE
    CHAI Chi-zhang, JIAO De-cheng, LIAO Yu-hua, ZHANG Si-yuan, DU Peng, SHEN Wei-hua
    2003, 25(1):  167-168. 
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