Table of Content

08 March 2000, Volume 22 Issue 1

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Brief Report

NEW EVIDENCE OF PALEOEARTHQUAKES AND DATE OF THE LATEST EVENT ON THE XIAOXIANGLING MOUNTAIN SEGMENT OF THE ANNINGHE FAULT ZONE

Wen Xueze, Du Pingshan, Long Dexiong

2000, 22(1):  1-8. 

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Surface rupture traces of several large paleoearthquakes are found along the Xiaoxiangling Mountain segment of the Anninghe fault zone. Trenches are excavated across the main fault at two sites to investigate later Holocene paleoearthquakes of this fault segment. The results show that at least 4 paleoseismic events with surface ruptures occurred on the fault segment during the last 2 300 years, suggesting an average recurrence interval of 600 to 700 years. Through carrying out the dendrochronologic calibrations of the ¹⁴C ages of 4 samples taken from micro stratum-units lian over and under the latast event's horizon, and analyzing the probability distributions for the calendric dates from the calibrations, the date of the latest event here is bracketed sometime in the 15th century.Further,combining the fragmentry documented record of an ancient earthquake for a nearby town,it is believed that the latest event occurred possibly in the year of A.D. 1480. This event produced an average left lateral offset as much as 3 meters along the surface fault. Such an amount of mean coseismic offset is comparable with those yielded by large strike-slip earthquakes with moment magnitude 7.5?0.3.

RECONSTRUCTION OF PALEOEARTHOUAKE SEQUENCE USING STRATIGRAPHIC RECORDS FROM DRILL LOGS: A STUDY AT THE XIADIAN FAULT, BEIJING

Xu Xiwei, Ji Fengju, Yu Guihua, Chen Wenbin, Wang Feng, Jiang Wali

2000, 22(1):  9-19. 

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The Xiadian fault is one of the most important concealed active faults in the northern part of the North China Plain. The Sanhe-Pinggu earthquake (M=8) is the latest surface rupturing event on this fault. Two stratigraphic drill logs on both sides of the Xiadian fault are chronologized by using thermoluminescene and ¹⁴C dating methods. Comparison shows a differential sedimentation on both sides of the fault in the last 26 ka B.P. The stratigraphic marks are established for identifying paleoearthquakes in the drill logs and 11 events are recognized. The main results of this study include: (1)The vertical slip of the Xiadian fault decreased since the Last Glacial Maximum in the past 26ka B.P. The vertical slip rate reached 1.2mm/a during the period of 21～26 ka B.P., 0.98mm/a during the period of 10～21 ka and 0.34mm/a in the past 10 ka B.P., respectively. (2)11 paleoearthquakes are identified by the drill logs which indicate an irregular recurrence behaviour in the past 26ka B.P. The paleoearthquakes clustered with a relatively short interval of 900～1 200 years in the Last Inter-Stadial and Last Glacial Maximum (19.3～26 ka B.P.), while several paleoearthquakes recurred quasi-periodically with an interval of 3 770～5 800 years in the Post Glacial. (3)The paleoearthquake recurrence pattern seems to be associated with paleo-climate variation in the Last Glacial. The earthquake clustering occurred in the Last Inter-Stadial and Last Glacial Maximum with a short recurrence interval. The recurrence interval became longer in the Post Glacial.

GEOTECTONIC SETTING, FORMATION AND EVOLUTION OF THE QINGHAI LAKE

Bian Qiantao, Liu Jiaqi, Luo Xiaoquan, Xiao Jule

2000, 22(1):  20-26. 

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The Qinghai Lake area is situated at the juncture of 3 tectonic units and several deep faults, so tectonic deformafion is exfensive there. Fault depression took place in Pliocene and the Qinghai Lake formed in Middle Pleistocene. The Lake basin can be divided into 3 horsts and 3 second-order fault basins. The formation and evolution of the Qinghai Lake had been mainly controlled by removement of south marginal large fault belt of Middle Qilian block, ZhongwulongshanQinghainanshan large fault belt and HeimaheDari large fault belt, the movement of faults with strikes in NWW, NW, NNW, SN and NE, and regional differential uplifting as well. Its formation and evolution had close relationship with uplifting of Qinghai-Tibet plateau, especially with negative inversion structure formed by change of stress field in Middle Pleistocne. Since the formation of Middle Miocene leveling plane, the neotectonism and the formation and evolution process of the Qinghai Lake are characterized by obvious pulsation. Since Middle Miocene, neotectonic activities over Qinghai Lake area can be divided into 5 periods, and the formation and evolution of the Qinghai Lake can be divided into 9 stages.

ON THE CENOZOIC DENUDATION AND UPLIFT OF THE DAQINGSHAN MOUNTAINS,NEI MONGGOL

Cheng Shaoping, Deng Qidong, Yang Guizhi, Ren Dianwei

2000, 22(1):  27-36. 

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The Daqingshan Mountains and its adjacent Huhehot fault basin, Nei Monggol, are the ideal area to study denudation and uplift. By means of morphomeric analysis and field observation, this paper provides evidence for arching deformation of four accordant summit levels aged in Oligcene, Miocene, Plicene, and Quaternary stream terrace flights. On the basis of the result of seismic surveying for the Huhehot basin, the average Cenozoic denudation and crustal isostatic uplift amounts are estimated to be 3 310m and 2 910m, respectively, for the Daqingshan Mountains. It is thus evident that the resulting relief lowering due to denudation after crustal isostatic uplift is really slight, only 400m. For the denudation of the Daqingshan Mountains, obviously strenthening with time resulted from the long term climatic change during the Cenozoic era. The crustal isostatic uplift is a principal regional deformation mechanism for the Daqingshan Mountains. First, the crustal isostatic uplift controls the Cenozoic landscape evolution history. Second, the rate of the crustal isostatic uplift of late Quaternary has been quicker from the western part to the eastern of the Daqingshan Mountains. And third, the amount of the Quaternary crustal isostatic uplift of the Daqingshan Mountains is greater than the Ordos'tectonic uplift by one order. A close coupling relation between denudation to lose gravity equilibrium and crustal isostatic uplift to restore gravity equilibrium runs through the Cenozoic land evolution history of the Daqingshan Mountains. The faulting has a different position before and after the establishment of the coupling relation. And the flexural isostatic has accommodated long term local differential movement on the fault zone.

A XENOLITH-DERIVED GEOTHERM FOR THE LOWER CRUST AND UPPER MANTLE BENEATH HANUOBA AREA, HEBEI PROVINCE, CHINA AND ITS GEOLOGIC IMPLICATIONS

Shi Lanbin, Lin Chuanyong, Chen Xiaode, Han Xiuling

2000, 22(1):  37-46. 

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A reliable geotherm has been constructed for the lower crust and upper mantle beneath Hanuoba, Hebei Province, China through the detailed study of xenoliths from basalts. The construted geotherm is higher than the oceanic geotherm and that for old cratonic areas. It is comparable to that for the eastern margin of Australian craton, and seems to indicate the tectonic regime of the studied area. The petrologic section derived from this geotherm indicates that the lower crust of this area consists mainly of granulite facies rocks of mafic compositions. They span from about 27km to 42km depth. The upper mantle consists of lherzolite, and it seems that there is a transition zone between spinel and garnet lherzolites. The crust mantle boundary defined by this geotherm is at 42km depth and coincides well with the seismic Moho. However, the overlapping of the equilibrium temperatures of some granulite and spinel lherzolite xenoliths indicates that granulite, spinel lherzolites and garnet pyroxynite may be interlayered in a zone above the crust mantle boundary. This may explain the layered reflectors observed at the base of lower crust. The lithosphere-asthenosphere boundary can be derived to be at about 95km depth, and the underlying asthenosphere consists of garnet lherzolite incorporated by upper mantle fluids. The obtained results may provide basic constraints for the lithosphere mapping and the construction of seismogenic model of the area.

THE FABRIC OF FAULT GOUGE FROM THE EASTERN LIUPANSHAN PIEDMONT FAULT ZONE AND THEIR IMPLICATION

Zhang Bingliang, Xiang Hongfa, Guo Shunmin, Zhang Wanxia

2000, 22(1):  47-52. 

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The mode of slipping along the eastern Liupanshan piedmont fault zone is analyzed through both macroscopic and microscopic investigations. The results show that (1)this fault zone can be divided into north,middle and south three segments based on the characteristics of the fabric from fault gouge. This is consistent with the result from macroscopic along the fault zone in recent years.(2)local strong deformation, traction fold, Riedel shears angle (R₁) being in the range of 11°～26°, P foliation and randomly preferred orientation of illite mineral that developed within the fault gouge are indication of stick slip and left lateral strike-slip along the north segment.(3)general deformation, Riedel shears being at angle of 11°or less, banded foled, ptygmatic structure and polygemic preferred orientation of illite mineral that developed within the fault gouges are the indicators of creep slip and reverse slip along the middle and south segments.

THE FISSION TRACK EVIDENCE OF HUASHAN BATHOLITH UPLIFTING IN MESOZOIC-CENOZOIC

Wan Jinglin, Li Qi, Wang Yu

2000, 22(1):  53-58. 

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Apatite minerals separated from samples that were collected from elevations of 520m to 1 950m on the Huashan batholith, yielded fission track ages of (20.59?3.6)Ma to (29.26?3.3)Ma. The data suggest an uplift rate of 0 019 to 0 189mm/a. One zircon mean fission track age of (43.45?2.7) Ma also suggests that the Huashan batholith started to uplift in early E 2. We infer that there is a sub normal fault developed in north part of the Huashan batholith.

FURTHER THINKING ON THE TECTONIC THERMAL-UP WELLING MODEL

Ma Zongjin, Du Pinren

2000, 22(1):  59-64. 

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We have proposed the tectonic thermal-upwelling model which is a dynamic model to explain the kinematic features of both global and local tectonics. It stresses that in a relative sense there are soft and hard layers within the stratified structures of the lithosphere. The hard layers are characterized by rich vertical fractures along which the material of the soft layer can up flow. The varying open and close fractures control the upwelling of the soft materials. Based on the recent observations of the lithosphere, we develop the tectonic thermal-upwelling model into the strati-block tectonic thermal-upwelling model. This paper presents the key points and basis of this model and discusses some tectonic phenomena and some difficult problems which can not be explained by the classic plate tectonics theory.

EXPERIMENTAL STUDY ON ALTERNATE SLIP OF INTERSECTING FAULTS AND BLOCK MOVEMENT

Ma Jin, Ma Shengli, Liu Liqiang, Liu Tianchang, Wu Xiuquan

2000, 22(1):  65-73. 

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Unstable events taking place alternately along two intersecting faults is proved by physical modeling. Two intersecting faults promote and restrain each other during deformation. One fault may make the other be locked for strain accumulation and may trigger slip along it. Slip rate, net slip and the numbers of unstable events depend on the orientation of the fault with respect to the principal stress axes. The slips along different fault segments show both translation and rotation of the fault block surrounded by them in the experiment. The affected area of unstable event is different for events occurred on different parts of the sample and it is wider for the event along the fault with high normal stress. Small precursory events usually occur before strong instability involving the intersecting faults. In fact, the movement of the fault block in the deformation field controls the alternate slips of intersecting faults.

STRUCTURE OF MOHO IN NORTH CHINA—3-D INVERSION FROM DEEP SEISMIC SOUNDING DATA

Yang Zhuoxin, Zhao Jinren, Zhang Xiankang, Yang Jian, Wu Tao

2000, 22(1):  74-80. 

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In this paper, the method to reconstruct 3-D crustal interfaces using traveltime of wide-angle reflected waves is presented. We have treated the PmP reflected wave data from deep seismic sounding profiles carried out during recent years in North China using this technique, and obtained the 3-D Moho structure of this region. The spatial distribution of deep faults in the crust are determined. The results show that the depth of Moho increases from SE to NW in general and presents undulation on this background. The rift basins corresponds to the upper mantle uplift. In the study area, some deep crustal faults extend to Moho, one of which is the NE⁴/trending crustal thickness abrupt variation belt and the other is the NWW-trending crustal thickness variation belt, which are two main deep structural belts. In combination with the distribution feature of strong and moderate earthquakes in the region, the structure background of seismic activities is also discussed.

GEOLOGICAL INTERPRETATION OF S WAVE DATA ALONG THE WENAN-YUXIAN-CAHAYOUZHONGQI PROFILE IN NORTH CHINA

Liu Baofeng, Zhang Xiankang, Zhang Chengke, Song Songyan, Zhou Xuesong

2000, 22(1):  81-88. 

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In this paper,the structure of twodimensional crustmantle velocity and ratio of wave velocity γ is obtained by interpreting S wave data of depth seismic sounding profile alone Wenan—Yuxian—Cahayouzhongqi. The upper crust and middle crust is mainly made of granite, and all is brittle, the γ value for ratio of velocity is about 1.72 for the lower crust it is 1.78; and for the top of upper mantle it is 1.82. The result shows that the lower crust and the top of upper mantle have plastic features. In terms of the features of wave sets and lateral changes of γ, a few deep faults are inferred in this region. Combining with seismicity of this region, it is suggested that the occurrence of earthquakes is related to the structure as well as the character of rocks.

LAMINAR FLOW IN THE LOWER CONTINENTAL CRUST AND ITS SIGNIFICANCE FOR CONTINENTAL DYNAMICS

Li Dewei, Ji Yunlong

2000, 22(1):  89-96. 

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The geological and geophysical data show that in the lower crust of young continental tectonic units may occur pervasive, non-seismic, subhorizontal and ductile flow related to thermal softening. The laminar flow drives fault-block movement of the upper crust, results in the formation of orogenic belts and sedimentary basins, and causes coupling between ductile lower crust and brittle upper crust/mantle. The low viscous material in the lower crust flows from mantle upwelling area under basin to mantle depression area under orogen driven by shear stress and gravity related to underplating of mantle magmas that provide heat and material to the lower continental crust. During the process of laminar flow, the variation of thermal field and velocity field may occur in the lower crustal flow channel.