Table of Content

20 September 2001, Volume 7 Issue 3

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Article

Some Thoughts on the Seismic Tectonics of Strong Earthquake Occurring Sections in China

LI Ping1, HUANG Guang-si2, YANG Mei-e3

2001, 7(3):  245-256. 

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The term strong earthquake occurring sections(SEOS)is used to specify thee places where strung earthquakes(M≥6)have occurred since Holocene，and will occur repeatedly in the future. The crust of China consists of several plates or blocks. Their boundaries are mostly large fault zones. Quaternary active faults have been forming and developing along the zones under the united effect of India Plate’s collision towards NNE and Pacific Plate’s subduction towards W-Southwest China is an area where SEOS are concentrated. During Q1 and Q2，the Sichuan-Yunnan (Rhombus) Block was pushed to SSE，so that left-lateral strike slipping took place in the Xianshuihe Fault Zone and Xiaojiang Fault Zone. Some fault-basins were formed in the tensional areas nearby these zones. During Q3，due to changed tectonic stress direction，Xiaojiang Fault Zone 1inked with Xianshuihe Fault Zone and formed a huge curve fault Zone，where a lot of SEOS are located. Activation of faults in Q3 has an universa1 significance. Al1 the faults 1ast to the current time，and the faults are called active faults for this reason. SEOS aye 1oeated at specific sections of the active faults. Epicenters in SEOS express characteristics in the tectonophysics and geomorphology. Widely developed graben-horst system ，long-range of remaining fault hills， head-cut rivers，remarkable differential in relief by accending and descending of the operate fault banks in the area where strong earthquakes occurred，are some obvious samples In SEOS，strong earthquakes of similar magnitudes repeated in the same places in certain temporal interva1. The model of the processes is “breaking-healing-breaking again-healing again”. According to the similarity of the map patterns of iso-value of acceleration of Jiji Earthquake (9.21 1999，Taiwan，China)and the iso-intensity for Yuzha Earthquake(1955，Sichuan，China),evident differences of buildings damage between in fault zones and out of fault zones and deformation of young deposition layers，the deformation mechanics of the strong earthquakes can be generalized as follows: 1 SEOS have a three-layer structure: a rupture layer and two complete rock layers as the fault walls located on two sides of the rupture 1ayer. 2 The rupture layer is a place where tectonic stress is concentrated and released. It plays the rule of reflecting seismic wave propagation. 3 Strong earthquakes repeat in certain temporal interval and spatial scope. Determining SEOS and utilizing fully the features mentioned above are very important for engineering construction.

Features of the Thrust Wedge of Deformation Belt in Kuqa Rejuvenation Foreland Basin

LU Hua-fu1, JIA Chen-zhao2， JIA Dong1，CHEN Chu-ming1，LIU Zhi-hong1, WANG Guo-qing, WANG Sheng-li

2001, 7(3):  257-271. 

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Kuqa rejuvenation foreland basin and its thrust wedge of foreland deformation belts are structurally responsed to the India-Eurasia collision in the interior of Asia continent Based on the detailed surface geology reconnaissance and interpretations of many seismic profiles, we conclude that thrust wedge of deformation belt in Kuqa rejuvenation foreland basin is divided into five structural units, including the marginal thrust and hidden structural zone, the Shidike anticline zone, the North linear anticline zone, the Baicheng piggy-back depression and the Qiulitage thrust fronta1 zone．The fauult-related folds aye the basic deformation styles of the rejuvenation foreland thrust wedge, which are classified into nine kinds of fault-related folds, including fault-band fold, fault propagation fold, detachment kink fold, duplex, pop up, hybrid fault-bend/detachment fold, stacked anticline, composed wedge structure and superimposed fault-lend/fault-propagation fold In the northern part of the fiat/ramp thrust wedge, the fault-related folds are mainly of fault-bend folds, fault-propagation folds, and duplexes For the very front of the wedge there exist well developed blind thrusts and detachment kink folds, forming a typically cryptic thrust wedge front edge．Growth strata recorded accurately initial emplacement ages of every structure belt and detailed forming processes．Thrusts and thrust-related folds propagate from the north to the south within the thrust wedge , i.e. Misibulake anticline initiated in the Miocene(23.3Ma), velocity of blind thrust 0.16mm/a; Kalabahe anticline and Kelasu anticline in 16.9Ma, velocity of hidden thrast 0.16mm/a and 0.16mm/a; Dawanqi anticline in 3.6Ma, velocity of hidden thrust 0.82mm/a: Dongqiulitage anticline in 3 Ma, velocity of blind thrust 1.3turn/a; and Yaken anticline in 1.87Ma, velocity of hidden thrust 3mm/a.The crustal shortening velocity which caused the thrusting is slow at about 0.355mm/a in average in the Miocene, then approaches O.82mm/a in middle Pliocene. During the late Pliocene and the Quaternary the velocity gets faster in about one magnitude order, up to 1.29-3mm/a, implicating that an accelerating crustal shorting process occurred in the late Cenozoic in the Kuqa thrust belt and approached a climax in the Pliocene and the Pleistocene.

Relationship Between Fault-Related Folds and Oil-Gas Traps in Southwestern Margin of the Ordos Basin

XIE Guo-ai1, XIAO Wen-xia2, ZHANG Qing-long1, REN Wen-jun2, GUO Ling-zhi

2001, 7(3):  272-277. 

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Based on the investigation in the field and seismic profiles, we applied the theory of fault-related fold in analyzing the traps of petroleum an d natural gas in the southwestern margin of Ordos Basin. As a result, two types of fault bend folds which are suitable for traps of petroleum and natural gas were recognized, namely: (1)fault-bend folds without fore-kink bands and (2) full structural fault-bend folds. It is demonstrated that the two kinds of fault-bend folds are commonly existing in Sancha-Yanwu district of southwestern Ordos Basin and there are petroleum and natural gas traps in this area. By analyzing the geometry and kinematics of fault-bend folds, the range of traps is estimated. Another important side of using this method is to judge the tectonic stress．The main parts of the area are covered by loess layer and the base rocks are seldom exposed on the surface, especially the lower layers are fewly exposed．The authors made a preliminary analysis for the folds and faults in the basin on the fault-be nd folds theory It offers new ideas for the exploration of petroleum and gas, as well as a new method for the research of basemental geology．

Study of Trace Fossil Assemblage and Paleoenvironment of Middle Ordovician Lashizhong Formation, Ordos Basin

FEI An-wei

2001, 7(3):  278-287. 

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Trace fossils from the Middle Ordovician Lashizhong Formation in Zhuozishan, the northwest margin of Ordos Basin are studied systematically. The Paleodictyon assemblage, including Paleodictyon of majus, P. sp., Megagrapton irregulare, Neonereites biserialia, Planolites vulgaris, Chondrites filiformis, Ch．furcatus and Zoophycos of brianteus, has been recognized for the first time in the studied area．The trace fossil assemblage is characterized by the abundant and spotted trace fossils and the wide spread occurrence of Paleodictyon．The classificatory characteristic of the trace fossils, such as morphology, behavior, preservation, etc., indicate that the Paleodictyon assemblage corresponds to the Nereites ichnofacies representing depositional environment of far-turbidity current in deep sea．During the late period of sedimentation．however, the water depth became shallower as the environment of turbidity current in continental slope rising. The characteristics of sedimentary sequence, sedimentary structure and lithology in the Lashizhong Formation indicate a typical turbidite facies deposits, especially far-turbites facias. According to the palaeocurrent measurement, it is inferred that the directional change of the gravity currents roughly reprepsents the trends of decrease of extensional faulting and of uplift in the Helan aulacogen during the Middle Ordovician period ．

The Basic Model of Wumishan Cyclothems and Their Markov Chain Analysis, Jixian Section of Tianjin

MEI Ming-xiang1, MA Yong-sheng2, GUO Qing-yin1

2001, 7(3):  288-299. 

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The Mesoproterozoic Wumishan Formation in Jixian section of Tianjin is composed of 3 300m thick carbonate strata with a duration of about 100Ma(1 310±20Ma-1 207±lOMa)．In this set of strata, particularly carbonate meter-scale cyclic sequences belonging to peritidal type with approximately symmetrical lithofacies-succession fabric are developed very wel1. The widespread development of superimposed patterns of 1:4 shows that these meter-scale cyclic sequences are genetically related to the short-eccentricity cycles, which are called figuratively Wumishan cyclothems and could represent true sedimentary cycles. Generally, massive and thick-bedded limedolostones and dolomitic limestones of stromatolite biostromes and thrombolte bioherms constitute the central part of Wumishan cyclothems, the bottom and upper part are consisted of tidal flat dolostones, the top part is composed of lagoon-facies dolomitic shales with a cap of paleosol layers. Therefore, the approximately symmetrical lithofacies-succession fabric is formed. Many features such as the clear deepening and shoaling vectors of cyelothems, and all kinds of fresh-water diagenesis marks suggest that the Wumisham cyclothems are the products of autocyclic sedimentation governed by allocyclic high-frequency sea-level changes. The results of Markov chain analysis reaffirmed the objective existence of 1ithofacies-succession fabric of Wumishan cyclothems. The boundaries of Wumaishan cyclothems are marked by instantaneous exposed punctuated surface, which lead to the discrepancy between the cyclothem and the parasequerce of sequence stratigraphy terminology system defined by Vail et al. (1977). It is difficult to judge that the time-span of Milankovitch cycles in Precambrian is certainly equal to those of the Phanerozoic, but the regularly vertical stacking patterns of seventh-order rhythms, sixth-order cyclothems and fifth-order partasequence sets stil1 indicate their consistence with the period duration of Milankovitch cycles in the Phanerozoic.

Paleoclimatic Implication of Frequency Dependent Magnetic Susceptibility of Red Clay-Loess-Paleosol Sequences in the Lingtai Profile

SUN You-bin1, SUN Dong-huai2, AN Zhi-sheng3

2001, 7(3):  300-306. 

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On the basis of rock magnetic properties, red clay has the similar composition of magnetic minerals as that of loess-palm sols sequences, and the magnetic susceptibility value is well correlated to the content of ultra-fine magnetic minerals Magnetic susceptibility of loess-paleosols has been regarded as a proxy index of summer monsoon intensity. However, there are different opinions about the paleoclimatic implications of magnetic susceptibility of red clay, Here we report the magnetic susceptibility and frequency dependent magnetic susceptibility measured for the late Cenozoic red clay-loess-paleosol sequences of the Lingtai profile in Gansu province. A positive correlation is observed between the magnetic susceptibility and frequency-dependent magnetic susceptibility, indicating that the content of ultra-fine magnetic minerals formed by bio-chemical processes has a great contribution to the increase of magnetic susceptibility and frequency-dependent magnetic susceptibility. Compared with the magnetic susceptibility, the frequency dependent magnetic susceptibility, which indicates the content of ultra-fine magnetic minerals formed under different climatic conditions, can reliably and sensitively record the paleoclimatic variations on different time-scales, and also can be used as a proxy index of pedogenic degree and summer monsoon intensity. The different variability of frequency dependent magnetic susceptibility before and after 2.6MaBP indicates the great change between the cold and warm, dry and humid climatic conditions on the Loess Plateau. Summer and winter monsoon variations show different combinations before and after the occurrence of major North Hemisphere G1aciations.

Petrogenetic-Metallogenetic Background and Time-Space Relationship of the East Qinling Molybdenum Ore Belt, China

ZHANG Zheng-wei1;2, ZHU Bing-quan2, CHANG Xiang-yang2, QIANG Li-zhi3, WEN Ming-xing3

2001, 7(3):  307-315. 

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The porphyry type molybdenum ore belt in east Qinling measures more than 200km long and 20km wide, forming a NWW-trending rectangle. Mineralizations are equidistantly distributed along NE direction like a string of beads. It is a typical crossed area of the geochemical steep-dipping zone and geo physical gradient zone. Along a plunging mantle syncline in E-W plunging direction and from the east to the west are distributed check-shaped fractures which control the synergic crust-mantle grano-porphyry rocks in the Luonan-luanchuan area. Our preliminary interpretation is as follows: The Luonan-Luan chuan area at depth wan a westward plunging mantle syncline with a nearly E-W axis direction. The geochemical boundary was controlled by the strike direction of syncline. The geophysical gradient zone was controlled by the steep slope of the syncline. The crossing effect caused the structural fracture assembly on the NNE direction a long the rigid block margin. These superimposed upon parallel fracture bunches on the ancient continental margin, which were expressed as the check-shaped distribution pattern. Under the influence of crust-mantle interaction the fluid moved up along the structures to the shallower crust and form ed mineral deposits There occurs a series of small masses of early and late Yanshanian granite porphyries, diorite and various dyke rocks of acid-intermediate compositions The main controlling factors of this porphyry molybdenum ore halt were dependent on the character of magmatic activities and conditions of occurrences. The molybdenum mineralization was closely related to the highly acidic, alkali and potash-rich Yanshanian small intrusive bodies. Both the rock-forming and ore-forming processes are structurally controlled in time and space.

Corona Texture，Garnet Formation and P-T Evolution of Gabbroic Cumulate from Haiyangsuo Area Eastern Shandong Province，China

NI Zhi-yao1;2，WANG Ren-min2，YUAN Jian-ping2

2001, 7(3):  316-328. 

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The granite gneiss of the Late Proterozoic Jinning stage is the main exposed rock in Haiyangsuo area，eastern Shandong Province，China. These are some lenses and structural blocks of ultramafic rocks(serpentinite)，metamorphic gabbro，plagioclase amphibolite and metamorphic silicalite with varying dimensions and proportions in the granite gneiss. The strong deformation and metamorphism took place in these structural blocks，but the Yaodao accumulative gabbros，that formed through the mineral accumulation under the low pressure and isobaric cooling condition，have the distinct gabbro texture and rhythmic structure，and consist of troctolite，olivine gabbro and amphibole gabbro. Multistage metamorphic reaction textures were preserved in the troctolite and olivine gabbro. There are two different mineral assemblages for the corona texture in the accumulative gabbros. The one is between plagioclase and olivine. It contains the mineral assemblage of orthopyroxene, spinel，amphibole and garnet; The another is between plagioclase and ilmenite. It is only in the olivine gabbro. The mineral assemblage is more simple and contains garnet，amphibole and rutile or garnet and rutile. The former may be divided into three generations on the basis of petrographic evidence. The early mineral assemblage is characterized by orthopyroxene，dinopyroxene and spinel that formed through a reaction between olivine and plagioclase at temperature of about 760℃—876℃ and pressure of 0.44—0.60GPa; the intermediate mineral assemblage is marked by appearance of tschermakite and disappearance of clinopyroxene via the reaction between plagioclase and clinopyroxene subsequently at temperature of about 660℃— 700℃ and pressure of O.40—0.5O GPa: the late mineral assemblage is characterized by formation of garnet and pargasite or pargsitic hornblende resulted from the reaction between tschermakite and plagioclase at temperature of 650℃—660℃ and pressure of 0.83—0.92 GPa The latter is only one generation and marked by the formation of garnet by the reaction between ilmenite and plagioclase at temperature of about 620℃—630℃ and pressure of 0.71—0.86GPa. The temperature and pressure of metamorphism process indicate that the early metamorphism is isobaric cooling and the late metamorphism is isothermal boosting for gabbroic cumulates after magmatism of isobaric cooling This special P-T evolution may reflect the characteristics of cool emplacement and deep subduction of accumulative gabbros.

The Metamorphic Rocks Associated with the Shibanjing-Xiaohuangshan Ophiolite from the Inner Mongolia Autonomous Region and Its Evolution History

ZHOU Guo-qing1, CHEN Xiao-ming1, ZHAO Jian-xin1;2

2001, 7(3):  329-344. 

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Both the andalusite- cordierite hornfels typical of thermal metamorphism and the bluegreen hoorblende schists typical of middle-high pressure regional metamorphism were found within the Shibanjing- Xiaohuangshan ophiolite belt, Inner Mongolia. Superimposed on the hornfelses are the later-stage middle-high pressure metamorphic minerals such as chloritoid and stilpnomenae. Our detailed metamorphism sequence and topological studies indicate that metamorphic rocks at Shibanjing and Xiaohuangshan are part of a tectonic melange. They experienced magmatically-induced thermal metamorphism, followed by a medium-high pressure metamorphism, which is characteristic of an anti-clockwise PT trajectery with maximum T=620℃ and maximum P=0.65Gpa.

Prelim inary Study on the Concentrations of Gold and Other Ore Elements in Crude Oil and Oil-Field Water of the Jiyang Depression

HUA Ren-min1, YANG Shen-biao2, LI Bao-li2, DONG Dong2, ZHANG Shou-peng2 CHEN Jian-hua2, LI Zai-duo2, QIU Li-wen1, CAO Hu-1an1

2001, 7(3):  345-350. 

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49 crude oil samples from different oil fields of the Jiyang Depression were collected and analyzed for their gold and silver concentrations. Result shows that they contain relatively high Au concentrations than those from most other oi1 basins in China Except the highest value of 220.3×10^-9 the average Au concentration of 48 oil samples is 3.60×10^-9. Heavy oil contains higher Au concentrations than light oil. Most gold-rich samples are from the reservoirs in upper Tertiary sandstone and Archean gneiss, of which the latter is the most important source rock for well-known gold mining district of East Shandong peninsula, and the former is the direct erosive product of the latter. Further study reveals that gold mostly occurs in the bituminite and colloidal components of the crude oil. These components contain more S, N , O , Cl and other atoms, which may cause stable gold-bearing complexes in crude oil. Concentrations of gold and some other ore dements in oil, field water samples from the Jiyang Depression aye also determined. Relatively high gold concentration and certain correlation between gold and other ore elements, i.e. As, Hg and Pb, are revealed. The partitioning relationship of gold in rock, oil and water is unknown, whereas this preliminary study shows that oil-field water might bring out some gold from oil in the water-oil separation process.

40Ar-39Ar Dating for Volcanic Rocks of Qingshan Formation in Jiaolai Basin，Eastern Shandong Province: A Case Study of the Fenlingshan Volcanic Apparatus in Wulian County

QiU Jian-sheng1;2，WANG De-zi2，LO Qing-Hua3，LIU Hong2

2001, 7(3):  351-355. 

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Based on a case Study of the Fenlingshan volcanic apparatus, the formation ages of volcanic rocks of the Qingshan Formation in the Jiaolai Basin have been determined accurately by employing the highly precise 40Ar-39Ar dating technique． Investigation results demonstrate that the age of intermediate to alkaline potash-rich volcanic rocks of the first cycle of the Qingshan Formation is 109.9±0.6 Ma: and the age of the acidic rhyolitic volcanic rocks of the second cycle of the Qingshan Formation is 108.2±0.6Ma．It is thus evidenced that volcanic rocks of the Qingshan Formation in the Jiaolai volcanic Basin are products of Early Cretaceous magmatism. The ages of volcanic rocks of the Qingshan Formation in Shandong province display a progressively younger tendency eastwards，suggesting that the Mesozoic subduction of the Izanagi plate towards the Eurasian plate is the main dynamic factor that controlled the volcanism in this region.

SIMS U-Pb Dating and CL Images of Zircons from Garnet Peridotite of Bixiling

WU Yuan-bao1, CHEN Dao-gong1, CHENG Hao1, Etienne Deloule2, XIA Qun-ke1

2001, 7(3):  356-362. 

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SIMS U-Pb dating and CL images have been done for zircons of garnet peridotite from Bixiling, southern Dabie. The C1 images reveal that most zircons have core-rim texture. The cores show igneous zircons characteristics. The rims may be recrystallized during ultra-high pressure metamorphism. Thirteen points were dated within seven zircons. A1l the points have The th/U ratios of 0.85～2.13, and ten of them ≥1.5. These suggest that most zircons come from the garnet peridotite, for it has positive Th anomaly in primitive mantle normalized pattern. Except for 4 points, the other nine analyses define a chord intersecting the concordia curve at 824Ma and 254Ma respectively. According to the zircons’ CL images, the upper intercepted age represents the magmatic protolith age, the lower intercepted age defines the metamorphic event. The protolith of Bixiling garnet peridotite-eclogite massif was formed at 824 ± 39Ma, and experienced UHP-metamorphism at 254±38Ma. Some xenocryst zircons have been found. Their ages are older than that of protolith of the Bixiling massif. These zircons may come from the crustal rock that was contaminated during the magmas rising.

Study on Oxygen Isotopic Compositions of Tertiary Basalts in Jiyang Depressional Basin，Shandong Province

JIN Ying1, SHEN Wei-zhou1;2, ZHAO Lian-ze1, ZHAO Ming1, KONG Qing-you1 ,CAI Yuan-feng1

2001, 7(3):  363-368. 

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Oxygen isotopic compositions of 29 Tertiary basalt samples from the Jiyang depressional basin are reported in this paper. The δ18O values of these basalts mostly range from 6.3‰ to 12.2‰. The Late Tertiary basalts with weak alteration haveδ18O values of 6.3‰0～7.9‰, which are similar to those of Late Tertiary basalts from central and eastern Shandong and are essentially in the range of normal continental basalts. However, the Early Tertiary basalts with evident alteration haveδ18O values of 6.7‰～12.2‰, more than 90% samples of which have δ18O values greater than 7.5‰. The obvious change ofδ18O values for Early Tertiary basalts not only occurs in the samples of different stratigraphic units, such as Shasan Member(Es3), Shasi Member (Es4)and Kongdian Formation(Ek), but also in the samples of the S31TIe stratigraphic unit collected from the same drill hole. Combined with the petrological and petrochemical data, it is clear that the increase ofδ18O values for the Early Tertiary basalts in different degrees was mainly caused by meteoric-hydrothermal alteration (carbonatization) at a lower temperature.