Table of Content

20 September 2002, Volume 8 Issue 3

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Article

A New Fast Method for Continuation of Potential Field on an Undulate Terrain

XU Shi-zhe1, DAI Shi-kun2, LAN Xiang1

2002, 8(3):  241-247. 

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Vertically projecting the observation point at a undulate terrain onto a horizontal plane and copying the potential at the terrain to the corresponding point at the horizontal plane,we get a fictitious potential field on a horizontal plane.An FFT is then can applied to calculate the outward normal derivativ a u／ n of the potential u on the horizontal plane.The calculated derivativeis u／ n is used approximately as the outward normal derivative of u／ n of the potential on the terrain.Based on the potential u and the normal derivativeon the terrain,Green’s formula is used to calculate the potential u at an arbitrary point above the terain.When the potential u at a level above the terrain is obtained,an FFT is used again to calculate the potential u at other level above the sources of the potentia1.The other method of continuation of potential field,e.g.,the equivalent source method, requires solving a large set of linear algebraic equations that take a hefty amount of computer memory and expends considerably the computation time.This method does not require solving algebraic equations and is of a high computation speed.The model tests demonstrated that the accuracy of this method is satisfactory.Using this method to continue the practical aeromagnetic anomaly shows a good result.

Lithospheric Textures of the Tan-Lu Fault Zone and their Genetic Analysis

ZHU Guang, SONG Shuan-zhong, NIU Man-lan, LIU Guo-sheng, WANG Yong-sheng

2002, 8(3):  248-256. 

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Five geoscience transects across the Tan-Lu Fault Zone show that crustal textures are obviously different on both sides of the fault zone.The difference is caused by strike-slip movement on the fault zone.Early Cretaceous magmatism during the strike-slip movement indicates that the fault zone put into crust-mantle boundary.These data demonstrate that the fault zone cut through whole crust during the strike-slip movement and the Moho should serve as largescale gentle detachment while decoupling took place between crust and mantle.During extensional activities of Late Cretaceo us to Eogene,intense upwelling of asthenosphere happened under the fault zone,leading to lithospheric necking in pure shear mode1.In compressive activities since Neogene,higher temperature of upper mantle due to shallower asthenosphere made the fault zone became a weaker belt in lithosphere.Therefore,the fault zone experienced stronger thrusting and large-scale eruption of mantle-derived basalt.

Study on Tectonic Evolution of Junggar Basin

CHEN Xin1 2, LU Hua-fu1, SHU Liang-shu1, WANG Hui-min2, ZHANG Guo-qing2

2002, 8(3):  257-267. 

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Junggar Basin is one of the biggest oil and gas bearing basins in China. Junggar Basin lies among the A1taiorogen on south flank of Siberia plate, the Kazakstan plate and the Tianshan fold belt.It is a Late Paleozoic,Mesozoic, and Cenozoic compressional superimposed basin,experienced the effects Hercynian orogeny,indosinian,Yanshannian and Himalayan orogenisms and was resulted in a complex tectonic frame work.Junggar Basin developed on the basis of Junggar terrain. The terrain consists of Precambrin crystal basement which was formed before 800Ma at least.It consists of“dual basement and four-layers structure”. The crust thinning took place in some parts of the basin.So,the evolution falls into two stages i.e.the first stage,the evolution of the Junggar terrane and the second stage,the evolution of the basin. During the whole Lower Paleozoic,the terrane was in the stable evolution of the terrane.The termin Dassed through three stages and came into being the Junggar Basin：(1)the terrane evolution;(2)the oceanic crust comsumption;(3)the accretion and collage of the terrain and the plates.Based on the structure-layers and the progress of the basin research,the fonnation and evolution of the Junggar Basin has divided into six stages：(1)the formation of the terrain;(2)the evolution of the terrain;(3)the collage of the terrain and the plates and the formation of the enbryonic：(4)foreland basin stage; (5)the intacontinental depression stage;(6)the regeneration fore1and basin.The latter three stages are related to the accumulation of petro1oum and nature gas.

Geological and Geophysical Analysis of the Southwestern and Eastern Sub-Basins,South China Sea

DING Wei-wei1 3, CHEN Han-lin1 3, YANG Shu-feng1, CHU Feng-you2 3

2002, 8(3):  268-279. 

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In this paper,we will compare and analyse the geologic and geophysic characters of the Eastern and Southwestern Sub-basins of the South China Sea(SCS)and discuss the SCS’s tectonic evolution history. The results of studies suggest that the magnetic anomalies and the gravity anomalies of Eastern Sub-basin are in EW direction but in NE direction in the Southwestern Sub-basin.In both Sub-basins the Bouguer gravity anomalies in the central part are bigger than in the margins. This indicates that below the central part exists high density matter,which could be closely related to the spreading of the basin and the invasion of high density matter from the mantle. A computational process called the Interpolation Cut Methods was used to ana1yse the magnetic anomalies of the two Sub-basins.Results of the analyse indicate that the magnetic anomalies of the Southwestern Sub-basin is mainly affected by the shallow magnetic sources above about 5km in NE-SW direction. Below 5km ,the magnetic anomalies become blur,and the intensity decreases.The magnetic anomalies of the Eastern Sub-basin is not only affected by the shallow magnetic source above 5km ,but also affected by the deep source. They are all in EW direction.The differences between the two Sub-basins reflect the different tectonic mechanisms. Most faults in SCS exist in the conjunctional area of the Circum-Pacific tectonic belt and the Tethys tectonic belt and are effected by both belts.The faults in SCS basin is mostly in NE and near EW directions.The NE direction faults formed the structure frame work of the Southwestern Sub-basin and the western part of the Eastern Sub-basin.The formation of the NE direction faults are same in the mechanism and time as the faults in the Eastern China, which were commonly formed in Yanshanian stage. In the Himalayan stage,because the main tectonic point moved to east,the NE direction faults of the Southeast China formed earlier and was relative1y stable;the EW direction faults formed later and then cut across the NE direction faults. Because the EW direction faults cut very deep and can even reach the lithosphere,they often formed the gushing channels of earth mantle. The South China Sea is located in the conjunctional areas of the Pacific and the Tethys tectonic belts.Its evolution was affected by both the tectonic belts.The SCS was affected by the Pacific tectonics same as the Eastern China.Both SCS and the eastern China were in back—arc extension tectonics of NE-SW direction.These areas were also affected by the Tetheys tectonics.In this stage,the Tetheys oceanic crust finally subducted under the Eurasia plate.Combined with the strong NE-SW extensional stress,extensional stress in NW-SE direction was formed in the SCS.Therefore, in the action of the two stresses,many graben basins in NE direction appeared in the SCS. As the extension were stronger than in other places of the Eastern China,the SCS crust was completely taken up and intruded by deep magma.Basic-ultrabasic rocks in the crust are the shallow magnetic sources in the Southwestern Sub-basin. After Eocene(42-43Ma)we saw the most important change in the Pacific boundaries.Its subduction direction changed from NNW to NWW. The Pacific subducted at the Philippine trench beneath the Eurasia and formed NEE-EW direction rifts between the Zhongsha-Xisha islands and Nansha Islands.According to the reconstruction of the SE Asia,Hall suggested that the opening of the West Philippine sea and the West Sulawei Sea triggerd the Southward subduction of the Proto-S uth China Sea at the North Borneo-Luzon Arcs,and caused the N—S orientation extension of the South Chinese margin and the southward slab-pull force.Simultaneously the Indian began its collision with the Eurasia and led the uplift of the Tibet and c1ose of Tetheys.For the continuous collision ,the mantle moved in large scale on SE direction and the Indochina plateau extruded.The Red River zone started its left-lateral slip nearly more than 500 kms.In the control of the NEE-EW direction faults in the north margin and the NW direction olle in the west,the micro-plate composed of the Zhongsha Is,Xisha Is and the Nansha is broke away from the main continental and moved southward.Finally The sea floor spreading like the oceanic ridge took place.The Eastern sub-basin in EW direction came into being.The pattern of the magnetic anomalies and the structure are all in EW direction.

Discovery and Implications of Granulite Facies Xenoliths from Some Cenozoic Basalts,SE China

YU Jin-hai, ZHAO Lei, XU Xi-sheng

2002, 8(3):  280-292. 

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This study presents the petrological and mineralogical characteristics of granulite xenoliths from Cenozoic basalts in Nushan,Guizishan,Xilong and Qilin,Southeastern China.Garnet granulite was first discovered in Nushan Cenozoic basalt.It is significantly different from two-pyroxene granulite in mineral compositions and formation condition.Clinopyroxenes from the garnet granulites have high Al203 and Na20,and orthopyroxenes are rich in Al203. Garnet granulites formed in deep lower crust(> 40 km)and lower geotherm (19～ 25E ／km),suggesting that Nushan was in relatively stable craton at that time.Two granulite samples from Nushan contain the clinopyroxene and orthopyroxene with lower Al203 and Na20 contents.They formed in the tectonic setting with higher geotherm (31～34℃ /km),which probably was an extensional environment in continental margin or rift.Complex granulite facies xenolith types and distinct formation conditions indicate that the lower crust beneath Nushan was constructed by multi-periodic magmatism in different tectonic settings. Lower A1203 and Na20 contents characterize the salites of two-pyroxene granulites from Qilin and Leizhou basalts,Guangdong Province.T-P estimations indicate that these granulites recrystallized at shallower level(23～27km)and high geotherm environment.It is suggested that South China was in the intense extension setting during that time.Granulite xenoliths from Xilong and Xinchang of ZheJiang Province were between those of Nushan and Qilin—Leizhou in mineral composition and recrystallization condition.The rock assemblage and formation condition of Nushan granulite facies xenoliths are similar to those from Archean granulite terrene and Hannuoba granulite xenoliths in North China,and markedly different from those in South China.It is suggested that the Nushan lower crust component should be a part of North China lower crust.In South China many mafic granulites were found from north to south, showing Mesozoic mafic magma underplating was widespread under the coastal region of South China.

Isotopic Dating and Genesis for Fogang Biotite Granites of Conghua Area,Guangdong Province

CHEN Xiao-ming, WANG Ru-cheng, LIU Chang-shi, HU Huan, ZHANG Wen-lan, GAO Jian-feng

2002, 8(3):  293-307. 

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Fogang(main body)biotite granites are a part of large-scaled Fogang granitic complex of east-west trending.They occupy about 85％ of the complex in area.They are composed of about 2～ 6％ biotite,35～ 41％ quartz,10～ 18％ plagioclase (An10—23)and 36～ 48％perthite,corresponding to biotite syenogranite(3a)on the basis of mineral contents by lUGS classification.The biotite has lower Mg／(Mg+Fe )values(0.17-0.21)and the K-feldspar is usually microcline perthite.Fogang biotite granites have an age of 167.5± 7.5Ma(MSWD =2.4),determineted by Rb-Sr whole rock-minera1 isochron method (emplaced during Middle Jurassic period).Similar to S type granites in south China,Fogang granites are markedly rich in silica(Si02：71.74～77.64％)and K20 relative to Na20 (averaging K2O／Na20=1.66),and are moderately peraluminous (averaging A／NKC = 1.056) and calcic-alkaline to weakly subalkaline series(NK／A=0.797 in average).Based on the trace element and rare earth element data,the granites commonly have higher Rb／Sr(1.98～ 41.71),Rb／Nb(11～23),K/Nb(1498-2976)ratios,lower Nb／Ta ratio(5.94 in average),strong to moderate Eu depletion(Eu／Eu* =0.12～0.46),and higher(La／Sm)N values(1.67～5.39).In Sr,Nd,Pb,O,isotopic composition field,Fogang granites have higher Isr (0.7116±0.0023)and εsr (t)values(96～ 103),showing that South China Upper Crust end member occupies about 66～69％ in their source materials in terms of the model calculation. The granites possess higher δ18O‰(+9.3～ +12.5)and 207Pb／204Pb(15.734)and 206Pb／204Pb(18.968～19.049) indicating that they are consistent with Upper Crust Evolution Line in the Pb-tetonic model diagram.The above mentioned data wholely indicate that Fogang biotite granites were formed by partial melting of source materials consisting of mid to lower crust rocks.Some of dark enclaves of dioritic or granodioritic compositions are found in Fogang biotite granites. This suggests that basalt underplating resulted from lithosphere westward subduction of the Palaeo Pacific plate and northward subduction of the Philippine Block,effected upon the formation of Fogang granites.

Geochemistry of Baimajian Granitoids in Dabie Mountains and Its Tectonic Significance

HU Xiong-xing, MA Dong-sheng

2002, 8(3):  308-317. 

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Baimajian granitoids in Dabie Ultra High Pressure(U )belt are mainly composed of monzogranite.Pertrographical and geochemical characteristics indicate that they were emplaced in the post-collision regime(1ate Yanshan period).The granitoids belong to the metaluminous-peraluminous transitional type on Shand diagram (ACNK-ANK diagram (ACNK = 0.97～1.01)),and belong to high-K calc-alkaline series on K2O vs silica diagram.REE pattern diagram shows that they are rich in LREE,with strong negative Eu anomaly(δEu=0.37～0.65,average 0.53).The Nb-Y,Rb-Y+Nb,Rb-Hf-Ta diagrams indicate that the source materials of the Baimajian granitoids were possibly derived from the old island arc,though they cannot exactly reflect the tectonic background of the intrusion activity.Mantle upswelling associaed with extension process during the Yanshanian movement not only created the geotectonic conditions for granitoids formation,but also greatly influenced the fast exhumation of metamorphic rocks in Dabie Mountains.

Study on Sequence Stratigraphic Framework and Relative Sea-level Changes of the Permian in Guizhou and Guangxi Areas,South China

MEI Ming-xiang, GAO Jin-han, YI Ding-hong, MENG Qing-fen, LI Dong-hai

2002, 8(3):  318-333. 

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The Permian system in Guizhou and Guangxi provinces of south China is characterized by obviously different depositional facies, such as the coal measures in connective platform and the sponge reefs in its margin as well as in isolated platforms,which formed a specially sedimentary setting of “platform –basin-hill-trough”.The period with the most apparent differences in depositional facies is correspond to the developing period of sponge reefs.The Permian system in researching areas could be divided into three series, which are named as Chuanshanian series. Yangsingian series and Lepingian series respectively. Two obvious unconform ities can be recognized in the Perm ian of these areas,one is related to the main episode of Qiangui epeirogeny in the transitional period between the Chuanshanian epoch and the Yangsingian epoch,another was formed during the main episode of the Dongwu revolution in the transitional period between Yangsingian epoch and Lepingian epoch. These unconformities delegate not only the typical sequence boundaries but also the changing surfaces of sedimentary-basin natures. Several third-order sequences,which are described as SQ19 to SQ25 in this paper could be discerned in Permian system.The Chuanshanian series and the top part of the Carboniferous constitute one third-order sequence(SQ19).In settings of isolated platform and inter-platform ditch is composed by a set of limestones belonging to the middle to upper part of the Maping Fm .. However. in the background of the connective platform ,M aping limestones makes up its transgressive system tract and the coal-measures of the Liangshan Fm .forms its high-stand system tract.The strata of Yangsinian epoch could be divided into four third-order sequences(SQ20-SQ23),whose general charaters could be summarized as follow:transgressive system tract and the early high-stand system tract consist chiefly of limestones of open-platform facies;more dololimestones and limedolostones of restricted--platform facies are developed in late high-stand system tract;in the margin of the connective and the windward parts of isolated platform ,SQ23 is marked by a set of sponge-reef limestones.Two third-order sequences (SQ24 and SQ25)could be discerned in the Lepingian series.The SQ24 is composed by a set of coa1-measures called the Longtan Fm.in the setting of the connective platform and is also constituted by a special set of coal-measures that shan Fm. in the isolated platform but is represented by grainstones of beach facies in the margin of the connective platform.The SQ25 consist of a set of sponge-reef limestones in the margin of the connective platform and the windward part of isolate platform ,which is a set of limestones of open-platform facies called the Changxing Fm .in the connective platform .In the background of inter-platform ditch,both SQ24 and SQ25 are constituted by a set of deep-water sediments called the Linghao Fm..The particular drowning event of carbonate platform occurred in transitional period between the Perm ian and Triassic,lead to the generation of a drowning nonconformities.In term of their geological meanings,sequence boundaries could be grouped into four types,which are tectonic nonconformity,sedimentary nonconformity,drowning nonconform ity and their relative surfaces.

A Study on Present Hydrodynamics and Properties of Fluids in Up-Palaeozoic Reservoir of The Ordos Basin

JIN Ai-min1, LOU Zhang-hua1, ZHU Rong1, ZHANG Wen-zheng2

2002, 8(3):  334-344. 

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The migration and accumulation of oil-gas are closely related to the generation and evolution 0f the hydrodynamics,which controls the formation and distribution of oil-gas fields. Theoretica1 mode1s of the hydrodynamics for petroliferous sedimentary basins could be classified as two types, i.e; syrnmecal and asymmetrical models. With in a sedimentary basins, four different types of local hydrodynamic units can be recognized：① centrifugal flow caused by mudstone compaction at the depression area,② gravity—induced centripetal flow recharged by meteoric water at the basin edge generally,③ discharge area by cross-formational flow,or cross-formational flow-evaporation,as well as④ stagnation area. For the Ordos basin,the hydrodynamics in the Up-Palaeozoic reservoir is characterized by strong asymmetry,gravity-induced centripetal flow in the northern and eastern ascending areas of the basin,centrifugal flow caused by mudstone compaction in the western-southern declining areas of the basin,and cross-formational flow or cross-formational flow-evaporation in the areas between those areas mentioned above.By studying the migrating power,direction and channels of fluids,such as groundwater and nature gas,we can establish the present hydrodynamic model of the Ordos Basin.Groundwater flows from east to west,and nature gas migrates from west to east,which match with the distribution of fluid potential in the basin.Because of the inhomogeneity in the reservoir, distribution of the local hydrodynamic units follows certain patterns in the basin,which make the hydrodynamics complex.The exploration has also proved the formation and distribution of the hydrodynamics in the basin,because many gas fields have been found in the center of basin and groundwater is primarily located in the northern and eastern of Ordos Basin.

Thermal Experiments on Lime-Shale from the Upper Proterozoic Xiamaling Formation at Xiahuayuan,Zhangjiakou

FANG Jie1 2, LIU Bao-quan2, GUO Shu-zhi2, GU Lian-xing1, LIU Jing-wang2

2002, 8(3):  345-355. 

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Organic matter of most Middle-Upper Proterozoic hydrocarbon sources in northern China has highly evolved to high-maturity or over-maturity stage(Condensate-wet-Methane zone).Shale and lime-shale samples of lower or median maturity(Rb=0.38～0.6％)were selected from the Upper Proterozoic Xiamaling Formation at Xiahuayuan,Zhangjiakou.Simu1ation experiments have been pertormed to study the thermal evolution of the Middle-Upper Proterozoic hydrocarbon sources and to establish a generation mode1.Simulation experiments indicate that simulate temperature of the low-maturity stage is below 275℃ (quite R0 <0.8％),the maturity stage between 275-350℃ (quite R0 0.8～1.32％),High-maturity stage between 350～410℃ (quite R0 1.32-2.0％),and over-maturity stage exceeding 410℃ (quite R0 >2.0％). The higher the simulation temperature,the heavier the carbon isotope( δ13C) of the outcoming hydrocarbon,indicating an effect of temperature on δ13C. Dissolvable aroma-hydrocarbons tend to aggregate at the over-maturity stage,resulting in the reduction of aroma-hydrocarbons and the increase of bitumen.Many characteristics of dissolvable organic matter are reversed at high-maturity and over-maturity stages,such as the appearance of double peaks of normal paraffins, and the predominance of odd-carbon over dual-carbon hydrocarbons.Once Rb exceeds 0.8 1％ as temperature rises,bitumen,litumen neflectance(Rb)will persist to the greater than the vitrinite reflectance at the same temperatures .Compared with water-free runs,water-present experiments will produce liguid hydrocarbons in higher quantities and with more heavy components,indicating that pyrolysis tooks place more intensely during water-free experiments and conditions of water-present runs corresponds better than natusal environments.

A Study on Geological Characteristics and Geodynamic Origin of Jiyang Movement,Bohai Bay Basin

SHI Bu-qing1, ZHENG Feng-yun2, ZHOU Yao-qi3, WU Zhi-ping3, GU Qin2

2002, 8(3):  356-364. 

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The Jiyang movement was a very important geologic event during the Cenozoic evolution of Jiyang Depression,Bohai Bay Basin.It occurred from the late deposition stage of Sha4 member to early Sha3 of the Eogene.The stratigraphic unconformity between these two members can be observed in a broad area.According to the results of paleontologic analysis,the Daleotopography in this area is characterized by uplifting of highland and deepening of depression,which suggest that the Jiyang movement have a process of block faulting and tilting with large amplitude. The geochemistry of volcanic rocks in Sha4 an d Sha3 member indicates a deep background of enhanced stretching extent of lithosphere,enlarged upwelling amplitude of asthenosphere,intensified of magrrmtism and rising thermal flux during the Jiyang movement,which further suggest an obvious transformation of thermal constitution in lithosphere. Therefore,origin of the Jiyang movement could not be interprated as regional uplifting,and probably is related to deep processes.From the continental geodynamics point of view ,the deformation of crust is the response of deep lithosphere process,and the effective elastic thickness (Te)of lithosphere dynamically controls crust deformation.According to the modem observation of continental geophysics,the accretion of Te could minimize the absolute value of lithosphere deformation and makes the spatial topography variation smooth gradually.Similarly,the decrease of Te could enlarge the absolute value of lithosphere deformation and obviously affect the topography.The mechanism is the change of lithosphere deformation style from elastic to plastic,and the response to sediment load changes from flexural to local isostasy,which is the result of smaller Te caused by transformation of thermal constitution in lithosphere.Hence the variation of Te is an important factor for the paleotopography evolution of basins. After theoretical modeling,this paper confirmed the dynamic origin of Jiyang movement is the decrease of Te,and its amplitude should be larger than 8km ,which caused an altitude difference about 1 000m .The factors causing the decrease of Te include the continuous heating of thermal abnormality in asthenosphere,flowing of the plastic lower crust and displacement of block faulting,etc.