Table of Content

07 March 2001, Volume 23 Issue 1

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Brief Report

A STUDY ON THE BACK-THRUSTING SYSTEM AT ATUSHI-BAPANSHUIMO IN TARIM BASIN

Qu Guosheng, Chen Jie, Chen Xinfa, Canerot Joseph, Li Yigang, Yin Junping, Li Jun, Peng Qiyu, Yin Jinhui

2001, 23(1):  1-14. 

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Through field geological investigation and integrated interpretation of geophysical exploration data (seismic reflection profiles, MT and gravity data), we have defined the back-thrusting systems of Atushi-Bapanshuimo and their triangle zones in southwest Tarim basin, which are located along the foreland belts of southern Tienshan from the east of Bapanshuimo to the west of Ulukqiate. The back-thrusting systems consist of sub-Tarim basin back-thrusting systems of Xiao Atushi-Bapanshuimo, Uer-Kalataoshan and deformed Tarim Basin back-thrusting systems of Xiao Atushi- Ulukqiate. It is suggested that the back-thrusting systems formed along the multi-thrusting faults in Quaternary in the foreland belts of southern Tienshan where the southwest Tieshan orogenic belts thrust to the Tarim basin. The Tarim basin back-thrusting systems formed in the area where the thickness of sedimentary covers is greater than 10km, which corresponds to the depression of the basement (Maigaiti slope) and the adjacent Kalpintage thin-skinned thrusting systems formed in the area where the thickness of sedimentary covers is less than 10km, which corresponds to the uplift of the basement (Bachu basement uplift). The depth of basement in the transitional zones between the thrusting system of Kalpintage foreland belts and back-thrusting system of Tarim basin is about 10km. The balanced cross-sections show that the top of the convex of tectonic arc in thrusting and back thrusting systems has the largest displacement.

LATE QUATERNARY PALEOEARTHQUAKES AND THEIR RUPTURE FEATURES ALONG THE LINGWU FAULT

Chai Chizhang, Liao Yuhua, Zhang Wenxiao, Xu Wenjun, Shen Xuhui, Tian Qinjian

2001, 23(1):  15-23. 

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Based on geological and geomorphic investigation, the paleoearthquake events along the Lingwu faults have been revealed by digging trenches. The results show that there are five faulting events along the Lingwu faults since 28ka B.P. and they happened in about (27.15±0.778)ka、20.0ka,(13.07±0.06)ka,(10.586±0.05)ka and 6.0ka B.P., respectively. According to synthetic comparison and analyses on the five paleoearthquakes events,it is suggested that the fault has the segmented and combined rupture features. And we have estimated their magnitudes by combining the rupture lengths and the coseismic displacement revealed by the trenches.

SEGMENTATION CHARACTER OF SEIMIC SURFACE RUPTURES OF THE PIEDMONT ACTIVE FAULT OF MT. DAQINGSHAN, INNER MONGOLIA

Jiang Wali, Xiao zhenmin, Wan Huanzhen, Gong Fuhua

2001, 23(1):  24-34. 

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We have made field investigation and trench measurement along the piedmont fault of Daqingshan Mountain, Inner Mongolia, which is one of the most remarkable faults surrounding the Ordos block. The results show that the most recent time of activity is after Middle Holocene for the west section and before Late Holocene for the east section of the fault, respectively. The activity of the middle section of the fault during Holocene is characterized by integration. We have also analyzed the geomorphologic features, paleoearthquakes, surface ruptures caused by the Baotou earthquake of AD 849, and present day seismicity. It is suggested that there is a boundary of the fault near Tuzuq County, west of Huhehaute. The activity in Late Holocene of the fault was intensive on the west side of the boundary and not evident on the east side.

DYNAMIC ANALYSIS ON INTERACTION BETWEEN THE HAIYUAN-GULANG-CHANGMA GREAT EARTHQUAKE IN THE NORTH BOUNDARY OF THE TIBETAN PLATEAU

Fu Zhengxiang, Liu Guiping, Cheng Qifu

2001, 23(1):  35-42. 

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This paper has studied the interaction between the Haiyuan-Gulang-Changma great earthquake on the north boundary of the Tibetan plateau based on the conception of Coulomb static stress change. The results show that the Gulang earthquake occurred in advance of 16～44 years from triggering by Haiyuan event, and the occurrence of Changma earthquake was not affected by Coulomb static stress change resulting from Haiyuan and Gulang earthquake occurrence.

ACTIVITY ANALYSIS OF LIXIAN-LUOJIAPU FAULT ZONE IN THE EAST BOUNDARY OF TIBETAN PLATEAU SINCE THE LATE-PLEISTOCENE

Han Zhujun, Xiang Hongfa, Ran Yongkang

2001, 23(1):  42-48. 

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The Lixian Luojiapu fault zone shows clear activity since the late Pleistocene. The Holocene strata are directly offset in the Lixian-Luojiapu and Tianshuizhen-Jiezikou segments. Surface scarps are easily to be found and the gullies are sinistrally offset along the fault. Considering the widely distributed earthquake slope slides and liquifaction, it is suggested that the Lixian Luojiapu fault zone is the seismogenic structure of the 1654 Tianshui M8 earthquake. The average horizontal and vertical slip rates are 0.95mm/a and 0.35mm/a, respectively since late Pleistocene. The ratio of vertical slip rate to horizontal slip rate is about 1/3, which is about the same as the ratio between the vertical and horizontal displacement in an event.

CHARACTER OF THERMAL FLUIDS IN UPPER CRUST AND RELATIONSHIP WITH SEISMICITY IN YANQING-HUAILAI BASIN

Che Yongtai, Wang Jihua, Yu Jinzi, Liu Wuzhou, Song Guanyi

2001, 23(1):  49-54. 

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In this paper, some characters of temperature, hydro-chemistry, gas-chemistry and isotope chemistry of thermal fluids in upper crust of the Yanqing-Huailai basin are studied. The genesis of atmospheric water permeating of thermal fluids is determined. The temperature of thermal reservoir and circulation depth of thermal fluids are calculated. And their relationships with seismic and fault activity are discussed.

LAKE DEVELOPMENT AND CLIMATIC CHANGES, DURING THE MIDDLE AND LATE TIMES OF LATE PLEISTOCENE IN THE YANHUAI BASIN

Zheng Rongzhang, Ji Fengju, Li Jianping

2001, 23(1):  55-62. 

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There was lake development during late Pleistocene in the Yanhuai basin. On the basis of the results of sporopollen sample ananlyses for four profiles of the middle and late times of late Pleistocene in the basin, combined with dating of the lacustrine stratum and the stages of lake development, we have restored the paleoclimatic environment during 55～26ka B.P. and 20～8ka B.P., during which the second and the third period lacustrine stratum developed. The lake sediment of the middle and late time of late Pleistocene reflects a wet climatic characteristic of the Yanhuai basin.

PLANE GEOMETRY AND MORDERN ACTIVITY OF THE QINGDAO CANGKOU-WENQUAN FAULT

Luan Guangzhong, Zhang Haiping

2001, 23(1):  61-68. 

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We describe the plane geometry and structural features of the Cangkou Wenquan fault in Qingdao on the basis of field observations. We study the recent activity of this fault by using geomorphy, geology, and dating data. It is suggested that this fault has been becoming stable since Holocene time and is a nonactive fracture of late Quaternary.

PETROLOGICAL STRUCTURE OF THE CRUST AND UPPER MATLE IN YUNNAN, CHINA

Zhou Zhenheng, Xiang Caiying, Yang Kunjie

2001, 23(1):  69-78. 

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Through the comprehensive studies of the metamorphic rocks outcropping on the surface, DSS data, and experimental data of rock seismic velocity under high temperature and high pressure, the rock constituents of the crust and upper mantle for Yunnan Province have been studied. This research indicates that the upper, middle and lower crust in Yunnan is composed of the metamorphic rocks of greenschist (sedimentary layer in the top part), amphibolite and granulite facies, respectively or granitoids, diorites and gabbros corresponding to them respectively, and that there exist the mafic eclogites in the bottom of the crust in some areas; and that the upper mantle is composed of peridotites and the crust-upper mantle transitional zone probably composed of the mafic eclogites and peridotites in some areas such as Lanping-Simao depression and Central Yunnan depression.

A NEW K-Ar、⁴⁰Ar-³⁹Ar ISOCHRON METHOD:⁴⁰Ar/⁴⁰K-³⁶Ar/⁴⁰K and⁴⁰Ar/³⁹Ar-³⁶Ar/³⁹Ar ISOTOPE CORRELATION DIAGRAMS

Li Daming, Li Qi, Wang Yu

2001, 23(1):  79-85. 

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A new K-Ar and ⁴⁰Ar-³⁹Ar isochron of ⁴⁰Ar/⁴⁰K-³⁶Ar/⁴⁰K、⁴⁰Ar/³⁹Ar-³⁶Ar/ ³⁹Ar isotope-correlation is proposed based on theoretical induction. Using examples proves that this kind of isochron can be confirmed each other with other isochrons. In some cases, such as other isochrons are not suitable, the new ⁴⁰Ar/⁴⁰K-³⁶Ar/⁴⁰K and ⁴⁰Ar/³⁹Ar-³⁶Ar/³⁹Ar isochron would indicate its value and significance. However, the difference between these three isochrons and their applicability need to be studied and discussed.

SEISMIC SOUNDING PROFILE AND ITS INTERPRETATION IN THE REGION OF XIJI—ZHONGWEI

Li Songlin, Zhang Xiankang, Ren Qingfang, Zhang Chengke, Shi Jinhu, Zhao Jinren, Liu Baofeng, Pan Suzhen, Zhang Jianshi, Liu Jianjun

2001, 23(1):  86-92. 

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We have performed 2D processing, calculations and interpretation of the seismic sounding data of a wide-angle reflection and refraction profile from Xiji to Zhongwei. Our purpose is to study the velocity structure of the crust and upper mantle around the arc-shape tectonics near Zhongning, Zhongwei and Tongxin, Ningxia. The results show that the velocity structure of the crust and upper mantle of this region is remarkably not homogeneous. The crustal thickness has big variations. The Moho interface is relatively shallow at the middle of the arc-shape tectonic zone and becomes deeper at the other places.

A DETAILED DETECTION OF THE TANGSHAN ACTIVE FAULT USING SHALLOW SEISMIC SURVEY

Hao Shujian, You Huichuan

2001, 23(1):  91-97. 

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A detailed detection of the Tangshan fault was carried out by using high resolution shallow seismic sounding technigue. Distinct images of the fault structure above 300m depth and especially near the sueface were obtained through the detection. The results show that during the faulting process, the southern segment of the NW-dipping Tangshan fault appeared as a high angle normal fault dissecting the Holocene deposits, while the northern segment of the fault appeared as a reverse fault. This may indicate the segmentation of the activity of the Tangshan fault.

THE CHANGES OF STRESS FIELD IN THE FOCAL REGION AND ITS ADJACENT AREAS BEFORE FOUR MODERATE-STRONG EARTHQUAKES IN NORTH CHINA

Zhou Cuiying, Wang Hualin, Wang Hongwei, Wang Mei, Liu Fenglan, Zhou Yuanfu

2001, 23(1):  98-110. 

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By using the data of mechanism solutions of small earthquakes, we have analysed the temporal and spatial changes of stress field in different tectonic positions of the focal region and its adjacent areas before the Haichang earthquake of magnitude 7.3 in 1975,Tangshan earthquake of magnitude 7.8 in 1976, Heze earthquake of magnitude 5.9 in 1983, and Cangshan earthquake of M_S 5.2 in 1995. It is verified that there were some anomaly changes of stress field around the focal region before earthquakes. For instance, the axes of principal stresses had orientation coincidence in focal area as long as 4 years before the Tangshan earthquake. The P axis azimuths of composite mechanism solutions of small earthquakes concentrated first, and turned then, before the Heze earthquake. The azimuths of P axes deflected and its coincidence increased before the Cangshan earthquake. In the meanwhile, the stress field changes before the Tangshan earthquake may began earlier than 1972. The composite mechanism solutions of small earthquakes recorded by Douhe station, which is in the focal region, is different from that of Changli station, which is outside the focal area. The focal-mechanism solutions and the seismic activity in different stress areas near the focal fault have dynamic differences. To a certain extent, these phenomena indicate that the mechanical state in different tectonic positions of focal areas is not same and small earthquakes are developing in focal areas before strong earthquakes under different tectonic conditions and stress background. This has practical meanings for studying the earthquake preparation process and differences for different earthquakes.

SYNTAXIS FAULTS AT THE NORTHEASTERN CORNER OF THE TIBETAN PLATEAU AND THE ANNULAR TECTONICS IN SOUTHERN ORDOS

Li Jianhua, Shen Xuhui

2001, 23(1):  116-121. 

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The interpretaition of satellite images reveals distinctly the existence of 4 arcuate fault zones (the Maomaoshan-Nanhuashan-Liupanshan,Xiangshan-Tianjingshan,Yantongshan and Niushou~shan Luoshan fault zones ) at the northeastern corner of the Tibetan plateau ,as well as the existence of annular tectonics in southern Ordos. Both the kinetic features of the arcuate structures and the configuration of annular tectonics in southern Ordos are analysed to explore the origin of these fault zones. It is suggested that the subduction of India plate in NNE direction has given rise to the extrusion of the Tibetan plateau in NEE direction. As this NEE-trending extrusion is blocked by the annular tectonics in southern Ordos, 4 arcuate fault zones, converging toward Guyuan and Jingyuan towns and diverging in NWW and NW directions, are fomed at the northeastern corner of the Tibetan plateau. This condition has caused the sinistral strike slip motion along the NWW-and NNW-trending faults,and the dextral strike-slip motion along the NW-or S-N-trending faults at the diverging portion,as well as the compressional thrusting along the faults at the converging portion.

TERTIARY N-S DIRECTION CRUSTAL SHORTENING OF FENGHOUSHAN AREA IN CENTRAL QINGHAI-XIZANG PLATEAU

Liu Shun, Wang Chengshan, Yi Haisheng, Liu Zifei

2001, 23(1):  122-125. 

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Based on newly acquired deformational data of Fenghuoshan group, we have estimated the N-S crustal shortening of Fenghuoshan area in central Qinghai-Xizang plateau .The shortening is greater than 41%, the majority of which occurred in late Oligocene and the minority in late Miocene.