Table of Content

09 September 1981, Volume 3 Issue 3

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Brief Report

THE METAMORPHISM AND GENETIC PROBLEMS OF ECLOGITES FROM MTS, HONGTUSHAN AND QINGLONGSHAN OF THE DONGHAI REGION, JIANGSU PROVINCE

Ying Sihuai, Yu Libao, Yang Zhu'en

1981, 3(3):  17-30. 

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This paper deals with the mineralogical and petrological features of eclogites from Mts. Hongtushan and Qinglongshan of the Donghai region, Jiangsu province. According to the experimental results of related rocks the genetic condition of eclogites can be estimated as T = 890° - 1275℃ and P = 17.25-24.75kb, and at the depth of 50-70km. From the petrochemical characters, the parent rock of eclogites can be considered as tholeiitic basalt. It can be assumed that the evolutionary process of eclogites in studied region started with metamorphism of tholeiitic basalt into basic granulite, and then, the basic granulite further turned into eclogite due to the dynamic process of progressive metamorphism. This process can be expressed by means of 0.61 hypersthene + plagio-clase (An = 61) = Omphacite (Jd = 39) + 0.61 kyanite + quartz. Finally, the eclogites a℃ompanied by the faulting intruded into the supracrustal migmatitic gneisses. In this process the temperature was decreasing from 1080℃±190℃ to 750℃±80℃, and the pressure from 21.0kb±3.75kb to 4.0kb±2.5kb. So that, the sequent retrogressive metamorphism of eclogites was caused by the dynamic effect of faulting. The conditional formula of this dynamic metamorphism can be approximately expressed as P(kb) = 0.0506T(C)-33.65.

DIVISION AND CORRELATION OF THE LATE QUATERNARY STRATIGRAPHY AND DISCUSSION ON THE RECENT TECTONIC MOVEMENT IN THE REGION OF THE LUANHE RIVER DELTA

Peng Gui, Jiao Wenqiang, Li Darning, Li Guiying

1981, 3(3):  31-36. 

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Based on the field investigations and C14 datings of 25 specimens, the problems about the division and correlation of the Late Quaternary stratigraphy and the recent tectonic movement in the region of the Luanhe River Delta were discussed.The studies indicate that the three incised alluvial fans of the Luanhe River were formed during about 18000, 11000 and 6000-3000yrs. B P, respectively. In the northern coast of the Bohai Bay, there was a transgression during the Middle Holocene. The ages of three ancient coastlines are of about 5500, 3700 and 350yrs. respectively. The largest subsidence amplitude is of about 10m(Jianggezhuang). Owing to the difference of the east and west parts of newest delta of Luanhe River in subsidence rate,the course of the Luanhe River was continuously migrating eastward since the Middle Holocene.

EARTHQUAKE FAULTING AND STRESS FIELD

Zang Shaoxian

1981, 3(3):  37-46. 

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The relationship between the earthquake faulting and tectonic stress was considered in this paper. The results of earthquake mechanism were reviewed. It shows that the fault plane obtained from the fault plane solution agrees well with the surface break caused by the earthquakes,but the fault plane solution does not give good explanation for the orientation of stress, and that an earthquake may be a complex rupture process and the onset of P waves just reflects the initial state of this process.The Coulomb criterion for rock failure was applied to investigate the earthquake faulting. It shows that the angle between the faulting plane and the greatest principal stress is less than or equal to π/4 in the homogeneous and isotropic materials.the result obtained from the fault plane solution can be regarded as a special case for them,μ= 0, i. e. no friction exists. When the material contains preexisting faults or weakened zones, the angle may be distributed in a wide range. It depends on the fracture parameters of the material, such as So, S_w, μ, μ_w as well the intermediant principal stress. Five basic processes of the stress changes which may cause an earthquake are discussed in this paper.

A PRELIMINARY STUDY OF THE FREE AIR GRAVITY FIELD IN CHINA CONTINENT

Wei Menghua, Wang Qiming, Shi Zhihong, Yin Xiuhua, Liu Zhanpo, Zhang Yumei

1981, 3(3):  47-60. 

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On the basis of the measured ground data ten free air gravity maps of China have been constructed from mean free air values in square areas of 1白1? to 10白10? using average filter to eliminate successively the shorter wavelengths. These results indicate a great difference between East China and West China in the anomaly configuration: the anomaly value changes are gently in the East and sharply in the West: the short wavelength anomalies trend in a NE direction in the East and in NW in the West, whereas the long wavelength anomalies trend in a NS direction in the East and in EW in the West. In addition, there are four NW disturbance zones cutting across the NE linear anomalies in the East. From the map 1 (1白1?) can be seen that in the East there exist two prominent nearly parallel positive anomaly zones, one of which extends from the Changbaishan mountains through the Liaoning and Shandong peninsulas into the coastal ranges in the Zhejiang and Fujian provinces, and the other runs from the Dahin-gganling mountains along the Taihangshan mountains through the Qinling mountains into the ranges of western Hubei and Hunan provinces. Between the two zones exist a wide and low gravity anomaly zone associated with the Songliao, North China and Jianghan plains and Hengyang basin. In the West, the anomaly configurations are very complex and have the same NW direction as the positive linear anomalies over the Qilianshan mountains, the eastern Tianshan and the Altayshan. On the map 2 (3白3?) are shown the high-amplitude positive anomalies for almost all the mountain regions and the low-amplitude positive or negative anomalies for all the plains and plateaus, whereas the interior basins always correspond to the large high-amplitude negative anomalies. On the map 3 (5白5?) can be found five dominant large anomalies for the China continent: the positive from 110癊 to the east, the negative over the Sichuan basin and Ordos plateau, the positive over the Qinghai and Xizang region, the negative over the Neimon-ggol and Xinjiang Autonomous regions. Besides, a great negative anomaly appears in the eastern Xizang. As shown on the maps 4 and 5 (7白7? and 9白 9?), the direction of the main linear anomalies turns into a normal NS one in East China and EW in the West. It is suggested that the short wavelength anomalies (1白1?) associated with the tectonic features are produced due to successive tectonic movements causing the non-homogenous mass distribution in the lithosphere. The moderate wavelength anomalies (3白3? and 5白5?) resulted from the broad bent of the lithosphere caused by the current stress from plate motions. Probably, the long-wavelength anomalies (7白7? and 9白9?) are related to the mantle deep-seated mass inhomogeneity, except the effect of the plate motions. The relation of the above-stated phenomena to the tectonic framework, isostasy, seismicity as well as the possible geodynamical process are also discussed.

THE RELATION BETWEEN THE RESERVOIR INDUCED EARTHQUAKES AND GEOLOGIC STRUCTURES

Lee Zuwu

1981, 3(3):  61-69. 

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There are many cases showing that the reservoir induceds earthquake have a very close relation with the geologic structures.This paper discusses the relation of reservoir induced earthquakes with the intensively active geotectonic area, the fractured zone and differential movement among blocks in the light of data got in practical work.As is well known, the reservoir induced earthquakes are closely related with the geotectonic condition. Usually, this kind of shocks occurred in strongly active geotectonic areas, such as Cenozoic geosynclinal folding zone and Diwa region, especially in the arcuate inflection points of an active fault or at the intersection of several groups of structural lines, where the neotectonic differential movement was effected evidently. By means of triangulation and levelling it is suggested that the reservoir induced earthquakes also occurred at the tectonic junction of blocks (horst and graben) which are of a tectonic activity to a certain extent at present. Moreover, it is a portion in which the stress, particularly the tensile or shearing stress, is easily concentrated.

THE OPACITIZATION OF BIOTITE

Wan Hanzhong

1981, 3(3):  70-70. 

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MATHEMATICO-STATISTICAL PREDICTION OF LIQUEFACTION OF SOIL DURING AN EARTHQUAKE

Zhu Shulian

1981, 3(3):  71-82. 

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The Tangshan 1976' earthquake (M = 7.8) has caused the extensive soil liquefaction in the broad Beijing-Tianjin-Tangshan plain region resulting in a severe damage. For this reason we have to estimate whether the liquefaction would happen during the future shocks that should be an important basis for proposing the measures of reduction of seismic hazard. As is well known, the engineering construction is in urgent need of the comprehensive prediction of liquefaction. Usually, the conventional geological or experimental methods could only give the predictable indications in the cases of simple and less factors. This paper statistically predicts the soil liquefaction on the basis of the measured and tested data during the engineering exploration.Using the method of stepwise discrimination analysis of Beyes’ criterion 13 and 15 variances selected from the liquidated and unliquidated localities can be used as correlation calculations in five ways. The variance pattern with the highest accuracy and confidence is chosen to predict the liquefaction at a location to be a VI11 level in intensity. The five kinds of prediction are in a close agreement and can be proved each other. Moreover, the maximum probability posterior of grouped samples is mostly above 0.99 with high confidence. In such a case, the extrapolated results are reliable.