Table of Content

09 December 1981, Volume 3 Issue 4

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Brief Report

CHARACTERISTICS OF FAULT ROCKS AND THEIR POTENTIAL EVIDENCES FOR SEISMIC EVENTS

Yang Zhuen, Ying Shihuai, Lin Chuanyong, Yu Libao

1981, 3(4):  1-14. 

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It has been found from field observation and laboratory study that the fault rocks on the Beishicheng fault zone in Miyun region, Beijing can be classified into four main types: cataclasite series, mylonite series, pseudotachylite and fault gouge. They have distinct features in mineralogy, deformation and microfabric, representing the different mechanisms of their origin. It is shown by the features and occurrences of the fault rocks in this region that the fault zone had experinced two stage of movement of different mechanical behavior: the ductile shearing in the early stage and the brittle fracturing in the late stage accompanied by seismic events.The existences of pseudotachylite and cataclasite types of fault rocks can be regarded as the potential evidences for seismic events. The former is produced by preferred melting of minerals in country rocks due to the frictional heating on the fault plane during seismic faulting, while the later is a product of brittle fracturing due to rapid release of strain energy. During rapid frictional heating process of seismic faulting, the mafic minerals are the first to be melted, but the felsic minerals are melted at the temperature much higher than the mafic ones, so that the SiO2 contents in the pseudotachylite are much lower than that in the country rocks. That is why the crush breccia and breccia in pseudotachylite consist mainly of felsic materials, and this characteristics are completely different from that of magmatic origin.

ON THE ACTIVITY OF THE TANCHENG-LUJIANG FAULT ZONE IN CHINA

Francis T. wu, Zhang Yuming, Fang Zhongjing, Zhang Shiliang

1981, 3(4):  15-26. 

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This work was made on the basis of data on the interpretation of false color den-site slicer, the fracture profiles, tectonite, fault gouge obtained from the investigations along the middle and southern segments of Tancheng-Lujiang fault zone as well as the studies of Quaternary active faults. Part of data cited here came from previous works on regional geology and geophysics. Some topics, such as the extension, deep geological features, tectonic movement, evolution history and current activity of these segments of the fault zone concerned in which the interest is growing, are dealt with in this paper.From our research results we come to a preliminary idea that the Tancheng-Lujiang fault zone in East China is a complex fault zone which has been undergoing a long tectonic activity and following intense reformation and that it is one of the main fault zones controlling the occurrence of strong earthquakes in East China. The constitution and deformation characteristics of both bottom and cover of this fault zone are sharply different each other on its opposing sides.The oldest trace showing the activity of this fault zone can be found in the Archaean Taishan Group being occurred in the host whithin this zone.The schitosity of Taishan complex along this zone shows an obvious local left-lateral drag which is also found in the Dabie Group (metamorphic series of rocks) along the SE foothills of Mt. Dabieshan. This explains the significant horizontal displacement along this fault zone during the Pre-Sinian period.From the Sinian period up to the Late Triassic period, in the Yangtse block fault running on the eastern side of this fault zone deposited marine sediments, while the north Jiangsu-east Shandong region was uplifting for a long time. From Late Protero-zoic to Early Cenozoic in the vast area west of this fault zone were developed North China type of marine sediments which were followed by the continental sedimentation.In the Mesozoic time the activity of this fault zone only occurred from the Indo-Chinese to the Yanshanian ages with the dominance of the latter. Judging by the met- amorphic series of rocks found on the northern foothills of Dabieshan and Jiaonan County, we can eatimate that the amount of displacement here is 520km in total. In fact it can be regarded as a resultant amount of displacements from lots of tectonic movements in the geological history, partly belonging to the Proterozoic, partly to the Hercy-nian and Indo-Chinese left-lateral strike-slip.The Himalayan movement probably caused the change of Tancheng-Lujiang fault zone activities in nature, from left-lateral displacement into right-lateral with thrusting movement. There ia an evidence that this fault zone was active in Cenozoic and shows a clear lineament on the satellite images with contrast shade. The other evidences are .as follows: the development of fault gouge with new stria and schistose texture and clastic materials. In addition, the geomorphological features and gravity anomaly also .show its tectonic activity during the Cenozoic period.The Cenozoic right-lateral strike-slip along this fault can be followed obviously, particularly on the Malingshan western slope, Tancheng County. And the right-lateral reversed phenomenan along this fault can be observed in the place where the small co-ombes run through the fault with the offset reaching 25m. As a result, the diabase vein formed in the Late Tertiary was offsetted and dragged by this fault. The solution of mechanism of earthquake also reveals the fact that the fault is still subjected to the right-lateral strike slip.The left-lateral, right-lateral as well as tensile and compressive displacements along the fault imply that throughout the long history of the Tanlu fault, the fault has had larger displacement. This may explain why the fault is such a prominent feature. This change of displacement as a function of time along major faults may de characteristic of intraplate deformation.

THE STATISTICAL RESULTS ABOUT THE RELATIONSHIP BETWEEN THE GRAVITATIONAL HIGH AND EARTHQUAKES IN THE NORTH CHINA PLAIN

Tang Yongan, Su Mengru

1981, 3(4):  19-57. 

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In order to determine the earthquake risk region, the relationship between the gravitational high and earthquakes occurring nearby is statistically studied by using the gravity data of the North China Plain area. The statistical results obtained indicate that the gravitational high must satisfy certain conditions in case of occurrence of an earthquake of magnitude M_s≥4 near the anomaly area. It is found that the greatest magnitude and total seismic energy released are highly correlated to the product of the gravitational high area with the square root of its intensity. Most of earthquakes of magnitude M_s≥5 took place near the position with greater form-vectors of gravitational high.According to the statistical results we can evaluate earthquake risk regions in the North China Plain area and indicate the conditions for applying these results. Furthermore, the physical implications of statistical parameters of gravitational high are also discussed herein.

THE TONGBAISHAN-DABIESHAN PAIRED METAMORPHIC ROCK BELT IN HENAN AND HUBEI PROVINCES

Zhang Zhaozhong, Zhang Bingliang, Feng Jinjian, Li Songbin

1981, 3(4):  27-37. 

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According to the metamorphic grades, mineral paragenesis, index minerals and the b₀ values of phengites, the metamorphic rocks formed in Dabie Mountains may be divided, from south to north, into three metamorphic belts having different pressure types, Ⅰ, Ⅱ and Ⅲ. The blet I is situated mainly on the southern edge of the Da-bieshan and has the index minerals such as piedontite, stilpnomelane, glaucophane, chlo-ritoid and types 2M-and 3T-phengites. The mean b0 value of muscovites, 9.049Å. and the accumulative frequency curve, are close to that in Sanbagawa metamorphic belt, Japan. The geothermal gradient appeared in the subduction zones is 11℃/km with pressure of 4-7kb, belonging to a high-pressure type of metamorphic facies series. Belt Ⅱ runs on the northern edge of the belt Ⅰ and has the index minerals such as kyanite, sillimanite and cordierite. The mean b0 value of muscovite, 9.032Å and the accumulative frequency curve are close to that in the Otago belt. The belt Ⅱ belongs to the metamorphic facies of medum-high pressure type. Belt Ⅲ is situated on the northern edge of belt Ⅱ. Here, the index minerals are andalusite, cordierite and sillimanite belonging to the metamorphic facies of high-temperature and low-pressure type.The two suite of ophiolites distributed in eastwestern direction can be found in the belt Ⅰ and Ⅱ on both southern and northern edges of Dabieshan respectively. They are closely accompanied with the metamorphosed volcanic rocks, pelites and silicolites etc.Technically the rock bodies are emplaced by the wall rocks, occationally with the pseudosequence. In the Shueixianqiao area (belt Ⅱ) the basic volcanic rocks from these bodies are oceanic tholeiite. On the northern Dabieshan discovered for the first time melanges.Probably, "Qinling earth's axis" and "Huaiyang shield" don't exist, but the folding zones in Paleozoic era. In essential, the above-stated three metamorphic belts of different pressure types can be ascribed to the two paried metamorphic belts with different ages. The older one (Early Paleozoic) perhaps consists of belts Ⅱ and Ⅲ and the younger one (Late Paleozoic) of belt Ⅰ (a high-pressure metamorphic belt) devoid of a low-pressure one.In the early Paleozoic on the northern edge of Qinling-Dabie Mountains being in present position and on the southern margin of "North China ancient island chain" appeared a deep-sea trench in the EW direction. It is possible that the Dabieshan was formed in tne course of underthrusting. In the Late Paleozoic the "South China ancient island arcs" and the "North China ancient island chains" collided and united together.

THE POLLEN ANALYSIS OF UPPER DEPOSITS FROM G Ⅰ-Ⅴ DRILLING CORE IN NINGHE COUNTY AND THEIR ¹⁴C DATING

Peng Gui, Yan Fuhua

1981, 3(4):  38-38. 

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PRELIMINARY STUDY OF ULTRAMAFIC XENOLITHS FROM CENOZOIC ALKALINE BASALTS IN NORTH CHINA

Liu Ruoxin, Yang Mei'e, Xu Huaiji, Quo Jindi, Wang Wenhu

1981, 3(4):  39-48. 

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This paper deals with ultramafic xenoliths from 6 occurrences, found in Neogene and Quaternary alkaline basalts rather than in Paleogene subalkaline basalts. The xenoliths are mainly distributed along the Tancheng-Lujiang fault zone and its both sides in eastern North China and in submeridional gravity gradient belt of western North China, that is, in both eastern and western border belts belonging to a regional inhomogeneous uplift of upper mantle in North China. These ultramafic xenoliths were squeezed up and carried out by alkaline magma along the deep-seated fault from the asthenosphere to the surface. The xenoliths with coarsegrained texture constitute relatively large portion among the studied ultramafic xenoliths from western part of the North China fault block. The value of stress estimated from the size of neoblasts is in a ranges from 250 to 300 bars. And the xenoliths from eastern part of the North China fault block are mainly of porphyroclastic texture. The value of stress from these xenoliths ranges in 300-600 bars. The equilibrating P-T conditions of ultramafic xenoliths obtained from the mineral assemlage of the xenoliths appear in 950-1160℃ and 15-25kb (50-80km), Both the interface M₃ estimated from the converted seismic wave sounding and the highly conducting layer in the upper mantle obtained from magnetotelluric sounding are at a depth of 60-80km which may represent the top of asthenosphere in this region. It follows that the ultramafic xenoliths might be the rocks occurring at the top of mantle's asthenosphere. The equilibrating P-T conditions of the xenoliths from this region show itself as a oceanic type of paleogeotherm, and trend to the geotherm of continental rift. Combining the tectontic development and magmatism coming into being since Cenozoic time in the North China region on the large scale inhomogeneous uplift in the upper mantle and the relatively high geothermal gradient, we conclude that this region is characterized by the block-faulting since Cenozoic time which is similar to the continental rifting in nature.

AN EVIDENCE ON THE LIQUEFACTION OF OLD SOIL IN BEIJING SAN-LI-HE AND ITS SIGNIFICANCE

1981, 3(4):  58-58. 

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DISCUSSION ON THE INDUCED SEISMICITY IN SHEN WO RESERVOIR AREA, LIAONING PROVINCE

Zhong Yizhang, Jiang Yuqian, Han Dianzhong

1981, 3(4):  59-68. 

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An earthquake with magnitude 4.8 occurred on December 22, 1974 in the Shen Wo area, Liaoning province. From the analysis of various data concerned, it was known as a typical reservoir induced earthquake.No earthquake occurred at the Tanghe reservoir, only over 20km far away from the Shen Wo reservoir, after the impounding in 1968. Through the comparision of the various conditions in the two reservoirs,discussions are directed to the cause of the Shen Wo reservoir induced earthquake. Our opinions are that the well-developed faults, the fracture of rock- mass and the good conditions for the water penetration are the most important factors causing the reservoir induced earthquake.

SEISMO-TECTONICS OF THE NORTH CHINA BASIN AND GRAVITY CHANGES

Qiang Zuji, Ma Li

1981, 3(4):  69-75. 

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The earth crust evolution under the North China basin since Cenozoic Era can be roughly divided into two stages: firstly, this basin was progressively extending since early Tertiary, with the result of wide development of normal faults and a series of depressions, where accumulated voluminous Cenozoic sediments. Then the extension rate of the earth crust was gradually decreasing at the end of Late Tertiary and eventually turned to be ENE-ward horizontal compression and vertical movement.The seismic.sounding shows that the Xiadian and Tangshan-Ninghe faults have deeply cut through the crust and the thickness of the crust is 36km at Tangshan and 38km at Xiadian.The gravity variation of the Tangshan earthquake region from March to July,1976 before mainshock was about 200 microgals. The depth of the density interface (or discontinuity) causing the gravity variation of the Tangshan area as conjectured from the Beijing-Leting gravity variation curve is about 36km. It has uplifted about 1m under Tangshan. The amount of density change at Tangshan was estimated from the density interface formula as 2.17?10^-4g/cm³ and -1.46?10^-4g/cm³ at Xiadian. Tha gravity anomalies at Tangshan and Xiadian must be considered together as two integral sides of the same movement entity. It is supposed that the North China basin has been subjected to ENE-ward horizontal compression, which gave rise to the activation of the Tangshan deep fault and the upwelling of upper mantle hot material to increase the density.

HAS ANY RELATION BETWEEN THE EARTHQUAKES AND THE EARTH ROTATION?

Li Yuanjun

1981, 3(4):  76-76. 

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REPORT ON THE"SYMPOSIUM ON QUANZHOU-SHANTOU SEISMIC BELT"

1981, 3(4):  77-78. 

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THE RADIOCARBON AGE DETERMINATIONS(1)

Chen Tiemai, Yuan Sixun, Wang Liangxun, Ma Li, Mong Qingping

1981, 3(4):  79-80. 

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BRIEF REPORT ON THE"SYMPOSIUM ON TANCHENG-LUJIANG FAULT ZONE"(continued from No.2)

Yao Yanzhi, Fang Zhongjing

1981, 3(4):  81-82. 

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