Table of Content

20 September 2000, Volume 6 Issue 3

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Article

Late Mesozoic Granitic Magmatism and Its Relation to Metamorphism－Ductile Deformation in the Changle－Nan’ao Fault Zone, Fujian Province

SHU Liang-shu, YU Jin-hai, WANG De-zi

2000, 6(3):  368-378. 

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The Changle-Nan'ao fault is an important tectonic zone with s trongly Late Mesozoic granitic magmatism and metamorphism-ductil e shear deformation in the coastal region of Southeastern China. All Pre-Late Cretaceous se dim entary rocks, volcanic and volcanoclastic rocks and Late Cretaceous CA -type hor nblende biotite granite and granodiorite were matamorphosed and deformed to form various amphibolite-facies metamorphic rocks and quartz -feldspar-facies duct ile sheared rocks characterized by penetrative fabrics that are developed well i n the wallrocks and granitc rocks. In this zone, a NE striking sheared foliatio ns are subvertical, strech ing lineations are subhorizontal, and non-coaxial ductile de formed fabrics occu r widespreadly. Kinematic study suggests that this is a sinistral strike -slip d uctile shear zone with slip-down component partly; Late motion sense of dextral strike-slip ductile shear exists locally. Granitic bodie s are distribut ed in the central part of the fault zone. From the center of shear zone to the o uter part, metamorphic grade decreases from amphibolite facies gneiss to greenschist facies phyllite and slate; metamorphic minerals change from sillima nite to garnet, mica and sericite. Mylonization and ductile shear fabrics are al so weakened from the center of ductile shear zone or of granitic body to the ou ter parts. Many small-scale ductile deformed zones were found in the Changle-Nan'ao zone. Single deformed zones have a wi dth from several meters to dozens meters. Asymmetric ductile shear criteria that can be used to distinguish a sense of shear include σ-type asymmetric porphyro clastic system, feldspar or quartz augen structure, fish-tails of mica, S-C comp osite foliation, dynamical recrystallized quartz aggregates in an oblique direct ion and fibrous chlorite pressure shadows around a large pyrite, and so on. Thi s paper proposes that this zone was a high temperature domain provided by shear heating, and ductile deformed fabrics were formed syntectonically during emp lace ment of granitic magma, which provided the heat for metamorphism. This is confirmed by occurrence of most high temperature minerals near granitoids, and the metamorphism and deformation in wall rocks away from granitic bodies becomes weaker gradually. Ductile deformation took place in t he interval of 100～120 Ma and at the depth of middle crust. Metamorphism, mylon itization and granitic magmatism resulted from a Late Mesozoic collision of Paci fic oceanic plate that subducted toward the continental margins of Eastern Asia along the Japan ese Central Tectonic Line to the longitudinal valley zone in Taiwan. Three phase s of granitic magmatism can be recognized in the zone: the subduction-type or CA-type gra nitoids with low-K2O content about 160～180 Ma, the strike-slip-type foliated and mylo nitized granitoids about 100～120 Ma, and the A-type or exte nsional type granite accompanied by magma mixing or basaltic magma underplating.

Deep Geodynamics of Yanshain Movement

WU Fu-yuan, SUN De-you, ZHANG Guang-liang, REN Xiang-wen

2000, 6(3):  379-388. 

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The eastern China was characterized by intensive magmatic ac tivities, structrual deformation, basin formation and mineralization in the Mesozoic, whic h wa s described as the Yanshain Movement Wong (1927). Although a lot of studies have been conducted in the past 70 years on the Yanshain Movem ent, a warmly debate still exists about its geodynamic background, such as the sub duction and back-arc extension of Pacific plate, continental -continental collisi on, sheared strike-slip continental margin, impacting and lithosph eric thinning , etc. A comprehensive study about the mineralogical, petrological and Sr-Nd isotopic features of Cenozoic basalts and their mantle xenoliths has indicated t hat the eastern China in Yanshain Period was characterized by lithospheric delaminat ion, especially the direct contact of asthenospheric mantle with the crust. Most of the pre-existed ancient lithospheric mantle had been delaminat ed, the present lithospheric mantle was formed since the Late Mesozoic. According to the presen t st udy, this lithospheric removal is probably related to the subduction of Pacific Oceanic Plate. The early stage (170 Ma or so) of subduction of eastern oceanic p lat e thickened the lithosphere in the eastern China, which resulted in the mantle d elamination in 160 and 145～120 Ma duo to the gravity instability of thicken ed lithosphere. Therefore, the nature of Yanshain movement is the thinning, even disappearing of lithospheric mantle beneath eastern China. This phenomenon is r are in the world and was reported only in the Basin and Range of western United States. Du ring the period of lithospheric removal, the asthenosphere upwelling and heating , and the underplating of mantle-derived magma in the lower cr ust resulted in th e intensive Late Mesozoic magmatism-tectonism-minera lization of Yanshain Movement in China. 　　This research also suggests that the present lithospheric mantle beneath e astern China is juvenile, and does not correspond to the old ages found in the o verlyi ng crustal rocks. This implies a mantle-crust decoupling, which is inconsistent with the results of present researches on the major Archean Cratons in the world. Th erefore, further investigation should be conducted to study the mechanism of th is process.

Tectonic Units in the Western Dabie Orogenic Belt

XU Bei, WANG Chang-qiu

2000, 6(3):  389-395. 

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From our detail structural investgation in the Xinxian area of the nor thwestern Dabie, seven structural units have been recognized, which include : (1) Mafan fold belt (MFB), (2) Liangting melange belt (LMB), (3) Laoshan fold b elt (LFB), (4) Sujiahe thrust sheet (STS), (5) Xong-Huwan shear belt (XHB), (6) Kafang gneiss dome (KGD), and (7) Pengdian shear belt (PSB)(Fig. 1). 　　The MFB is the northernmost unit and consists of foliated amphibolites, quart zofeldspathic schists, and intercalated leptynites and marbles called the Erlan gping Group. The MFB is pervasively affected by middle-small scale, tight to iso clinal folds. Dominant foliations are S-dipping. Petrologic affinities indicate that the MFB protolith may have originated from a Palaeozoic arc developed along the southern margin of the Sino-Korean craton. The LMB is characterized by a ch a otic block assemblage and penetrative shear deformation. Blocks include marble, chert, gneissic amphibolite, and garnet-plagiogneiss. The me lange matrix is mylo nitic and exhibits mineral assemblages indicating greenschist- facies recrystalli zation. Shear fabrics include S-C structure, mica fish, asymmetric recrystall i zed tails of quartz lenses, together with the south-dipping shea r foliation and small asymmetric fold structures, all indicating a top-to-north shear sense. Thses have been overprinted by a dextral shear deformation. The LFB forms the hanging wall of the LMB. The section reaches a maximum structural thickness of 6 km and is composed of interbedded mica-quartz schists and schistos e amphibolites of th e upper Proterozoic Guishan Formation. Moderate-to tight[ JX*7〗-folds and penetrative south-dipping foliations are prevalent. These folds are gen erally asymmetric, e xhibiting north vergence and E-W striking, subhorizontal fold axes. The STS is b ounded by a folded thrust dipping to the south from the LFB and dipping to the n orth from the XHB. It consists of Devonian unmetamorphosed to lower greenschist facies rocks that are affected by predominately south-vergent th rust faults, inc lined to recumbent folds, and slaty cleavage that all exhibit a top-to-south she ar sense. Structurally underlying the STS, the XHB is defined by a 3～5 km thick zone of mylonitic gneisses that contains sheared eclogite. Mylonitic foliations affecting the XHB generally dip to the north. Two generations of S-C mylonitic f abrics consisting of top-to-north thrust sense o verprinted by subhorizontal dext ral shear have been recognized. With a NWW-SEE strike, the PSB consists of gnei s ses intercalated with sheared eclogite and ultramafic rocks. Top-to-NE shear fab rics defined by mica-fish and S-C structures are largely oblitera ted by steep-dipping foliations, subhorizontal lineation d efined by phengite and amphibole, and boudinage of eclogite, which all clearly indicate a dextral shear sense. The f i nal unit (KGD) consists of gently domed gneiss and intercalated eclogites and oc cupies a triangle area of over 500 km2 bounded by the XHB to the north a nd by the PSB to the southwest. The shallowly inclined foliation within the KGD contrasts markedly with the >45° dip angles that characterize the XHB and PSB. A domal structure is centered approximately on Kafang. 　　The MFB may belong to a Paleozoic southern margin of the Sino-Korean craton. The LMB has been interpreted as the suture position between the Sino-Korean and Yangtze cratons. History of deformation in the western Dabie has been suggested: (1) folding with W-E strike followed by (2) top-to-north shear, (3) later dextral strike-slip faulting and (4) doming event related to exten sion.

Pulsative Extensional Activeites of the Tan-Lu Fault Zone

ZHU Guang, LIU Guo-sheng, SONG Chuan-zhong, WANG Dao-xuan

2000, 6(3):  396-404. 

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The Tan-Lu fault zone experienced intense extensional activities during the Late Cretaceous to the Early Paleogene. Thus, faulted basins were developed along the fault zone. The extension started from the earlier Late Cretaceous in the middle and southern segments of the fault zone whereas from the earlier Early Paleogene in the northern segment, showing a northward migration through time. The extensional activities present pulsative and progressive development. It has been identified that six extensional events took place from the Late Cretaceous to Early Paleogene. Correspondingly, depositional response occurred in the fault controlled basins. The extensional activities increased their influence and scope progressively. The extensional migration and pulsative development are related to the contemporaneous Pacific Plate movement.

Tectono-Geochemical Analysis of Jiapigou Gold Belt，Jilin Province

DENG Jun1, SUN Zhong-shi2, YANG Li-qiang1, CHENG Yu-ming3, LIN He-ping2

2000, 6(3):  405-411. 

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Genesis of Jiapigou gold deposits has been argued for a long time. Systematic tectono-geochemical analysis on the gold ore bodies, deposit belt and ore forming elements indicates that subduction and collision of the Paleo-Pacific plate with the North China plate during Mesozoic time resulted in generation of mantle-sourced and crust-sourced magmas and fluids. Mobilization and migration of ore-forming elements in the fluids yielded lateral Au, Cu zoning in regional scale and symmetric Au-Cu-Bi distribution within the one belt. Close relationship of controlling structure assemblage with features of primary halos, and stable isotope fractionation characteristics etc. show that the Jiapigou gold deposits were the products of Mesozoic tectono-magmatic and metallogenetic activity, in which both the mantle-sourced and crust-sourced fluids played an important role and ore forming materials were multiple-sourced.

U-Pb Chronological Study of Zircons from the Huangcaoshan and Xiasuozi Granites in the Western Margin of Yangtze Plate

SHEN Wei-zhou1，LI Hui-min2，XU Shi-jin1, WANG Ru-cheng1

2000, 6(3):  412-416. 

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Idiomorphic grains of zircon from the Huangcaoshan and Xiasuozi granites in the western margin of Yangtze plate were analyzed by using the　206Pb-235U mixed dilution technique. The results show that U-Pb ages of zircons from the Huangcaoshan granite are variable. The apparent-age relationship t207/206>t207/235>t206/238 is evident for these zircons, indicating that the difference in apparent ages for different grains is caused by radiogenic Pb loss in different degrees. In the concordance diagram, the lower intercept age of 786±36 Ma represents the crystallization age of the zircon; the upper intercept age of 195±78 Ma may be interpreted as the age of heat event causing the radiogenic Pb loss. Differing from the Huangcaoshan granite, there is no difference in the apparent ages of zircons from the Xiasuozi granite within the analytical error. In the concordance diagram, the data-points are all plotted on the concordance line, indicating that U-Pb isotopic system remains closed after crystallization of zircons. Therefore, the concordance age of 805±15 Ma may represent the crystallization age of zircons. 　　The above-mentioned results show that the Huangcaoshan and Xiasuozi granites were the products of Jinning period magmatism, and the important components of the Jinning orogenic belt around the Yangzte plate.

U-Pb Zircon Ages of the Eclogite and Tonalitic Gneiss from the Northern Dabie Mountains, China and Multi-Overgrowths of Metamorphic Zircons

LIU Yi-can1;2, LI Shu-guang1, XU Shu-tong2, LI Hui-min3, JIANG Lai-li2, CHEN Guan-bao2, WU Wei-pin2, SU wen2

2000, 6(3):  417-423. 

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The U-Pb zircon ages of the eclogite and tonalitic gneiss from the Northern Dabie Mountains were presented. The data suggest that: (1) the peak metamorphic age of the eclogite facies from the Raobazhai eclogite is 226～230 Ma; (2) the tonalitic gneiss near Ta'erhe underwent a Triassic metamorphic event; (3) zircon U-Pb ages of 226～230 Ma for the high-or ultrahigh-pressure rocks from the Northern Dabie Mountains are similar to those from the Southern UHP metamorphic belt, implying that most of eclogites or tonalitic gneiss here should be a part of Yangtze subducted continental crust and underwent a Triassic HP or UHP metamorphism; (4) the protolith ages of both eclogite and tonalitic gneiss are Late Proterozoic Era; (5) they are both affected strongly by Early Cretaceous thermal event; (6) the zircons from eclogite and gneiss have experienced two episodes of overgrowths on the margin of old zircons during the Triassic HP or UHP metamorphism and the Early Cretaceous thermal metamorphism. The overgrowth model of zircon under multi-metamorphism is very helpful to interpret the U-Pb zircon data from the high-grade metamorphic rocks.

A PTt- D Paths Study: Textural and Structural Evolution of Ultrahigh-Pressure and High-Pressure Metamorphic Rocks from Dabie Mountains，China

GUO Zhen-yu1, DONG Huo-gen2, GUO Ling-zhi2

2000, 6(3):  424-430. 

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There are abundant mineral textures and deformational structures in different scales within ultrahigh-pressure （UHP） and high-pressure （HP） metamorphic rocks in Dabie Mountains，which were developed during metamorphism. Based on textural-structural analysis and PTt research， we present the PTt-D paths of UHP and HP metamorphic rocks. They show firstly，the mineral textures change from static texture of garnet to quartz pseudomorphs after coesite and radiating fractures； secondly，the linear and planar fabrics change from L>S fabric of eclogites to L-S or S>L fabric of retrograded eclogites；lastly and most importantly，the main meso-scale deformational structures developed from original mineral foliation Se to D1 deformation，and then superimposed D2 deformation. The D1 is pre-amphibolite facies deformation. It formed recumbent fold F1e and boudinage in eclogites. The D2 is amphibolite facies deformation. It formed amphibolite facies foliation Sam， A-type lineation Lam， sheath fold Sh and lineation-parallel isoclinal fold F2am. Structural geometry of F1e and F2am accords with type-Ⅲ interference pattern of superimposed folds of Ramsay. The microstructures of XZ plane on Sam and small-scale structures at UHP metamorphic rocks outcrops show a top-to-NW-NNW shearing sense during amphibolite facies retrogradation accompanied with D2 deformation. The PTt-D paths of UHP and HP metamorphic rocks also reveal the exhumation history of these rocks and the evolution of collisional tectonics of Dabie Mountains. This paper gives some discussions in the following：（1） The passive continental margin of Yangtze craton subducted under Sino-Korean craton with a maximum strength at 232～221Ma. The materials of transitional crust of Yangtze passive continental margin experienced UHP metamorphism at 100 km of asthenospheric depth，and the other shallower structural layers experienced HP metamorphism. In this stage，the static texture of garnet shows that the UHP rocks stayed in their peak metamorphic region for a period of time.（2） By the mechanisms of buoyancy and high flexural rigidity of subducted passive continental margin，the UHP and HP rocks broke up to slabs and returned to the level of the boundary of crust and mantle during 220～207Ma， accompanied with D1 deformation.（3） By the mechanisms of retro-thrust， the UHP and HP rocks returned to upper-middle crust level during 206～178Ma，accompanied with D2 deformation.（4） Since then，the whole orogen was controlled by denudation，accompanied with granite intrusion and movement of Tan-Lu fault in Cretaceous age. The UHP and HP rocks exposed to near surface during 120～110Ma， and orogeny in Dabie Mountains completed.

On the Depth of Emplacement of Late Mesozoic Tholeiitic Magma from Coastal Area of Zhejiang Province

CHEN Rong, ZHOU Jin-cheng

2000, 6(3):  431-436. 

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The early Cretaceous mafic-acidic igneous rocks from the coastal area of Zhejiang Province constitute the typical composite lavas flow and dikes. The basalts and diabases in the early Cretaceous composite lavas flow and dikes are highly enriched in large-ion lithophile elements (LILE). The working of source enrichment alone could not result in such high abundance of LILE and related variation, and the contribution of crustal contamination should be considered. Relationship between ratios of Rb/Y and Nb/Y (with Rb/Nb ratios varying from 0.4 to 9.1) indicates that the tholeiitic magma might interact with the rocks of mid-upper crust. Calculated average melt density of the tholeiitic magma by its chemical composition is 2.678g/cm3 . This is approximately corresponding with density of the mid-upper crust of the coastal area of Zhejiang Province. Assuming that the rising of magma was driven mainly by float force resulted from density difference between magma and surrounding rocks of crust; it would stagnate at the level of neutral float. According to the hydrostatic pressure equilibrium between magma and surrounding crystal rocks, it is estimated that the tholeiitic magma from the mantle source would continue to rise till it was trapped by mid-crust at the depth about 16.3 km below the surface. Meanwhile the magma was contaminated by the crustal materials.

Study on Fluid Interaction of the Sandstone-Type Uranium Deposits in the Mesozoic-Cenozoic Terrestrial Sedimentary Basins

WANG Guo1;2, HUA Ren-min1, QIN Li-feng2

2000, 6(3):  437-446. 

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With rapid development of nuclear power industry in China, it is significant to investigate large economic sandstone-type uranium deposits, which are suitable for in-situ leaching mining. The existence and migration of fluid in the crust played an important role in forming most metal deposits, including sandstone-type uranium deposits which are called hydrogenic uranium deposits. Through studying some typical deposits in the Mesozoic-Cenozoic terrestrial sedimentary basins, this paper elaborates the relationship between fluid development and ore genesis in terrestrial sedimentary basins, and underlines the importance of fluid evolution history analysis. Based on the patterns of ore-forming fluid interaction, four sub-types for sandstone-type uranium deposits are proposed, i.e., basement type, brine-interface type, oxidation zone type and oil-gas type. Fluid mixing and interlayer (secondary) oxidation-reduction are two important ore-forming mechanisms. Especially, formation of sandstone-type uranium deposits of interlayer oxidation zone type is a process of superimposed unceasing reformation, which can exist up to present, until supplementation of interlayer fluids is ceased. 　　Sandstone-type uranium deposits originally occur as roll, tabular or pile-type ore bodies within reducing terrestrial sandstone of Late Silurian or younger ages, particularly Jurassic to Tertiary periods, while some of them also occur within red alluvial molasse formation or marine sandstone. With regard to tectonic setting, they generally occur on the stable slopes of terrestrial intermontane sedimentary basins or the margins of the central massif fold belts. The fossil channel-type sandstone uranium deposits are developed in covering strata of sub-platform districts in the margins of reactivated young platforms. Moreover, they always accompany with oil-gas provinces or regional geothermal fields, such as uranium provinces in Colorado-Wyoming of North America, Argentina of South America, and Ural-Middle Asia-Mongolia sandstone-type uranium ore belt of interlayer oxidation zone type and fossil channel type. Our conclusion shows that ore-forming fluids were originated from basement fissure water and surface water, and that some sandstone-type uranium deposits might be controlled by deep-seated thermal water system, e.g., No.382 deposit in western Yunnan Province. 　　Finally, some discussion is briefly given to the research methods on the ore forming fluid interactions of sandstone-type uranium deposits.

Study of Effective Migration and Accumulation System of Oil in Linnan Area, Bohai Gulf Basin

LIU Zhong-yun1，TANG Hong-san1，LIN Yu-xiang2

2000, 6(3):  447-455. 

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The working area in this article includes Linnan Sag and its surrounding areas. Oils with industrial value found at present all come from the Es3 Member located in depth of Linnan Sag. The sedimentary environment of the Es3 Member in Linnan Sag is deep to semi-deep lake facies with a condition of weak oxidation to oxidation. The content of oleanane detected in most samples shows that the source of organic matter has an input of high plant and the ratio of high plant to aquatic organism varies with the sedimentary facies of Es3 Member. In the water of south part pyrrophyta is more abundant. 　　The oil in Quti uplift has been subjected to light biodegradation and water washing in depth. Biodegradation and water washing did not affect the oil from Linnan oilfield. Oil from Qian-Guantun Structure is not affected by biodegradation also, but has a very low content of aromatics because of water washing. In Linpan oilfield only oil from the Guantao Formation suffered light biodegradation. 　　Although all oil from Linnan area has a common source, i.e. Es3 Member in Linnan Sag, some differences exist between oils from Linnan oilfiled, Quti oilfield, Linpan oilfiled and Shanghe oilfiled, etc. Therefore these oils can be divided into different origin types. Three types of oil are recognized, i.e. Linpan-Shanghe type, Linnan type and Quti type, according mainly to 4-methyl sterane content and also based on light and medium hydrocarbon composition and other biomarker characters of oils. 　　Two oil source areas，i.e. north part and south part oil source area， are recognized according to the results of comparison of oils and its source rocks and composition analyses of liquid inclusion enclave. The former has two stages of oil formation and expulsion and is the source of oil with lower 4-methyl sterane content and accumulated in Central Uplift. The later has only one phase of oil formation and expulsion and its oil has a high maturation level and a higher 4-methyl sterane content. 　　The system that can form industrial oil accumulation is called effective oil migration and accumulation system in this article. Four such systems are recognized. The system Ⅰ is formed at the end of Oligocene, its oil has a low 4-methyl sterane content and accumulated in Linpan and Shanghe oilfield. Other three systems are all formed during the peak period of oil formation of Es3 Member by the end of Miocene. The system Ⅱ also contributed to Linpan oilfiled with oil having low 4-methyl sterane content, the system Ⅲ formed Linnan oilfield and Qianguantun oil pool with oil having high 4-methyl sterane content, and the system Ⅳ resulted in Quti oilfiled with oil from both north source part and south source part and having medium 4-methyl sterane content. 　　According to the characters of all four systems and current level of exploration in Linnan area, the most favorable exploration area is Quti Uplift.

Simulation of System Dynamics in the Hydrocarbon Expulsion of Coal

HE Guang-yu, XU Shun-li

2000, 6(3):  456-461. 

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Recent studies have indicated that the feedback control is an important factor influencing the results of hydrocarbon expulsion of coal. Based on the principle of system theory and simulation methods, a model for the system dynamics of hydrocarbon expulsion of coal is built, which includes three subsystems of paleo-pressure, saturation of hydrocarbon and process of hydrocarbon expulsion. A program based on the new model is designed and used to study the characteristics of hydrocarbon expulsion of coal in the Rongxintun region of the East Liaohe basin. Results of this study proved that the model could be applied to solve many problems, such as feedback control and non-linear features of hydrocarbon expulsion.

Unusual Erosion Channels in Yellow River Delta: Features and Genesis

ZHONG Jian-hua1，NI Jin-ren2，WU Kong-you1，LI Li1

2000, 6(3):  462-469. 

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The geological environment in Yellow River delta is characterized by unusual hydrologic conditions, fine sediments, semi-arid climate and frequent flow-abruption. Consequently, many peculiar geological phenonena, such as the unusual erosion channels, are developed abundantly in the overwater plain. It is believed that these channels are formed by runoff resulting from rain and/or melt snow and ice under exposure condition. Based on their plane morphologies, the unusual erosion channels may be divided into six types, i.e., linear, dendritic, pit-like or hole-like, planar, net-like and tunnel-like channels. The size of channel is variable. The small channels usually are tens of centimeters in length and a few centimeters in width and depth, while the large one could reach up to tens of meters in length. The channel wall often has slump (deformation) structures, rill marks and silt stalatite-like structures.

A Time Series Model with Finite Element Method for Rational Exploration of Fracture-Karst Water Resources in Xuzhou City

Wu Jian-feng, ZHU Xue-yu, QIAN Jia-zhong

2000, 6(3):  470-478. 

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Xuzhou is an industrial city which extracts the largest amount of groundwater for water supply in Jiangsu province. Uncontrolled extraction from the fracture-karst aquifer has caused many environmental geologic problems, such as regional cones of depression, specific capacity decreasing in wells, groundwater quality deterioration, and karst collapse. With the development of the urban size and local economy, the water demand for the city will increase sharply. Therefore, rational utilization of groundwater resources is necessary and urgent for the Xuzhou City. 　　The research area covers five well-field districts. In this study, the five well fields are simulated as a unity. First, porous media model is applied to modeling the fracture-karst flow based upon a simplified conceptual model maintaining the key features of the research area. Then, Finite Element Method (FEM) is used to solve the corresponding mathematical model. The numerical simulation strictly follows the national standards of P. R. C. (GB/T 14497-93) Requirements for the Work of Groundwater Resources Management Model to guarantee the accuracy of study and increases the reliability. The results of modeling demonstrated that the theory of porous media model is suitable for the regions of fracture-karst in North China. In the previous studies, the average rainfalls or the frequency combination of precipitation were used to evaluate groundwater resources, which usually yield a result too conservative to exactly appraise groundwater resources. Therefore, in this study we employed a nonlinear time series model for precipitation in the research area, and apply the model to forecast the rainfalls in the cause of evaluating groundwater resources. We believe this measure could make a more accurate prediction. 　　According to the results of this research, which are compared with the actual statistical data of the current exploitation, the current extraction conditions of Xuzhou City are irrational. 　　Such irrationality lies not in that the current exploitation is over allowable groundwater yield on the whole, but that there is no scientific distribution of exploitation in each well field. 　　If the administrator adjust the existing exploitation conditions in accordance with our research results, the environmental geologic problems will be gradually resolved, and the present contradiction between water supply and water demand in the research area will not be further sharpened. This paper shall be of profound significance to solve the contradiction between water supply and water demand in the regions similar to the research area. Of course, the efficiency using the method of combining the time series model and FEM in the course of long-term groundwater resource evaluation should be verified in the further work.

A Study on the Feasibility of Ground Freezing Method in Nanjing Metro Engineering

JIANG Huang1, L1 Xiao-zhao2, YOU Sheng-wu3

2000, 6(3):  479-486. 

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The authors summarized the application of artificial ground freezing method (AGFM) to geotechnical engineering at home and abroad. 　　Although AGFM has been used in mining shaft construction for over 100 years at abroad and over 40 years in China, its application in other geotechnical engineering is still in its beginning stage. 　　Nanjing Metro will be built in a complicated and adverse geoenvironment, facing a series problems as the alternation of rockmass with soft soils, surface water, pore water in ancient buried river, karst water and fault water etc. Alternation of rockmass with soils makes the excavation hard in Shield Method and is difficult proof water and support in current NATM. Therefore, the authors suggest AGFM, and discuss its feasibility, design method and technical key points taking Zhu-Gu section (from Zhujiang road station to Gulou station) as an example. 　　Considering the unfavorable factors, which will be encountered in urban underground freezing engineering, we suggest that higher safety coefficient should be used in the ice wall design according to Lamé's formular. 　　The authors also believe that the following environmental problems arising from AGFM deserve particular attention: weakening of mechanical properties of soil in freezing-thawing cycle, freezing expansion and thawing collapse, additive stress on subject structure arising from the freezing-thawing process and frost defect to concrete of underground structure. From engineering analogy, it can be concluded that freezing expansion and thawing collapse in the Zhu-Gu section can be limited by building protection of effective engineering countermeasures.