2004 Vol. 10, No. 3

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MESOZOIC N-S EXTENSION IN THE EASTERN ALTYN TAGH RANGE ON THE NORTHERN MARGIN OF THE QINGHAI-TIBET PLATEAU
CHEN Xuan-hua, AN Yin, George E. GEHRELS, Eric S. COWGILL, Marty GROVE, T. Mark HARRISON, WANG Xiao-Feng, YANG Nong, LIU Jian
2004, 10(3): 193-212.
Abstract (151) PDF (868KB)(10)
Abstract:
Lying in the eastern Altyn Tagh Range along the northern margin of the Qinghai-Tibet Plateau, the >300-km long, E-W-trending Lapeiquan fault is a south-dipping normal fault, with its dip angle as low as <30°.The central segment of the fault is a 30~50 m thick ductile shear zone with a well-developed mylonitic fabric and stretching mineral lineation, whereas the eastern and western segments are characterized by cataclastic deformation.The kinematic indicators such as asymmetric boudinage structure, asymmetric folds and minor brittle and ductile faults within the fault zone all indicate a hanging wall-south normal-slip sense of shear.The age of the Lapeiquan fault is constrained by two lines of evidence.First, a sequence of Early-Middle Jurassic sediments is locally present in the hanging wall of the Lapeiquan fault.The clasts of the Jurassic strata, particularly the stromatolite-bearing, cherty limestone and purple quartzite, can be correlated with those in the footwall of the fault.The Early-Middle Jurassic strata were probably deposited in an extensional basin above the Lapeiquan fault.Second, 40Ar 39Ar thermochronological analyses indicate two prominent cooling events in the Lapeiquan footwall.The older event occurred in the latest Triassic-earliest Jurassic between~220 and 187 Ma, while the younger event occurred in the late Early Cretaceous at~100 Ma.Because the 220~187 Ma cooling ages are prevalent in the Lapeiquan footwall, they represent the main stage of faulting.The authors interpret the younger phase of fault motion at~100 Ma to have been related to fault reactivation.The deformation was aided by motion along the south-dipping Qiashikan normal fault that merges with the eastern Lapeiqaun fault. The Mesozoic extension in the Qinghai-Tibet Plateau and other parts of East Asia may be summarized as back-arc extension resulting from northward subduction of the Tethyan oceanic plate and the westward subduction of the Pacific plate.
SEISMIC DANGER DIVISIONS BASED ON HYDROGEOCHEMICAL-TECTONIC CHARACTERISTICS ON THE CONTINENT OF CHINA
ZHANG Chun-shan, ZHAN Li-hai, WU Man-lu, ZHANG Ye-cheng, HU Jing-jiang, GAO Qing-zhao
2004, 10(3): 213-226.
Abstract (108) PDF (441KB)(13)
Abstract:
Seismic danger zones on the continent of China are distinguished by taking the main indices of hydrogeochemical fields of hot groundwater and mineral groundwater, tectonic framework and tectonic activities and regularity of historical seismic activities as the main criteria for the division, combined with distributions of geophysical fields, ground temperature fields and grouping of regional strata.In this paper the continent of China is divided into six seismic danger regions, 23 danger subregions and 30 danger zones and the main hydrogeochemical and tectonic characteristics in various danger subregions and danger zones are dealt with.The coupling between earthquakes and the hydrogeochemical characteristics and hydrochemical field has been determined through studying the hydrogeochemical characteristics of groundwater and the hydrochemical field and analyzing the tectonic characteristics and activities.This work has great significance for the layout of earthquake and hydrological observation wells and prediction of earthquakes.Through long-term monitoring of the change of the chemical composition of groundwater, a new kind of premonitory information for earthquake prediction may be provided.
3-D DENSITY STRUCTURE AND GRAVITY ANOMALY OF THE WEST-CENTRAL SEDIMENTARY BASIN OF TAIWAN
ZHANG Jian, SONG Hai-bin
2004, 10(3): 227-234.
Abstract (146) PDF (421KB)(9)
Abstract:
The West-central sedimentary basin is one of the important areas for oil-gas exploration in Taiwan.Based on the data of oil wells and gravity surveys, the Kriging insertion equations and the depth-density empirical formula, the authors calculated the 3-D density structure and residual density structure of the sedimentary strata of the study area.Moreover, according to the Poisson equation and the FEM, the authors also computed the residual gravity anomaly caused by the 3-D residual density structure.The 3-D density structure shows that the Pleistocene to pre-Miocene sedimentary strata are broadly divided into four density layers at 0 to 6 km depths, i.e.the 2.0-2.30 g/cm3, 2.30-2.45 g/cm3, 2.45-2.60 g/cm3 and 2.60-2.70 g/cm3 layers.There is a relatively large sedimentary depression at 2-4 m km depths in the middle and lower parts of the continental margins of the Taichung basin.The density there is 0.05-0.10 g/cm3 lower than that in the surroundings.The gravity calculation indicates that the residual gravity anomaly produced in the sedimentary strata in the area ranges from-20 to +15 mGal, and that the deep depression corresponds with a trap with a low negative anomaly, whose shape and value are consistent with those and the measured gravity anomaly.
EVOLUTIONARY CHARACTERISTICS OF THE BOUNDARY FAULT OF THE WENXI DEPRESSION,SHANDONG,AND ITS CONTROL ON SEDIMENTARY SAND BODIES
ZHOU Xin-gui, SUN Bao-shan, SHAO Zhao-gang, ZHANG Lin-yan
2004, 10(3): 235-244.
Abstract (123) PDF (355KB)(8)
Abstract:
Based on field geological surveys and indoor data processing, the authors have intensively studied the geometric and kinematic characteristics of the major boundary fault (named the Nanliu fault) in the Wenxi depression and the deformation relationship and patterns of the major boundary fault and the secondary structures on the two sides of the major fault, defined the phases of the tectonic deformation and reconstructed the paleotectonic stress field.After their analysis of the basin-controlling effect of the major arcuate boundary, they think that the uneven movements of the boundary fault in different geological periods resulted in changes of the development of sand bodies in the basin with the migration of the subsidence center with time and space.The authors point out that the intersection positions of the arcuate boundary fault and its transverse tensional faults are inlets of ancient and present source materials and sites where sand bodies are developed.
FEATURES OF PLIOCENE-LOWER PLEISTOCENE SEDIMENTARY FACIES AND TECTONIC EVOLUTION IN THE ZANDA BASIN,NGARI AREA,TIBET
ZHU Da-gang, MENG Xian-gang, SHAO Zhao-gang, YANG Chao-bin, SUN Li-qian, WANG Jian-ping, HAN Tong-lin, HAN Jianen, DU JIan-jun, YU Jia, MENG Qing-wei
2004, 10(3): 245-252.
Abstract (135) PDF (499KB)(8)
Abstract:
The Neogene Pliocene-Quaternary lower Pleistocene strata in the Zanda basin, Ngari area, Tibet, may be divided into the lower Pleistocene Qangzü (Qp1x) Formation and the Neogene upper Pliocene Guge Group (N2gg) and the latter may be further divided into the Pliocene Zanda Formation (N22z) and the Pliocene Toling Formation (N21t).The strata belong to graben-type lake-basin deposits and consist predominantly of alluvial-proluvial, paraglacial, glaciolacustrine, lacustrine and fluvial facies.The tectonic evolution of the basin went through five stages, i.e.:the early-rifting stage (N21t), sustained rifting (N22z), lake basin outflow (Qp1x), steady uplift (Qp2f) and rapid strong uplift (Qp3-Qh).The activities of several normal faults on the southern and northern sides of the basin control the distribution pattern of the sedimentary facies:when fault activities were strong, the water bodies of the lake basin enlarged, the water became deep and mainly deep-and semi-deep lake deposits occurred; when fault activities weakened, the water bodies became shallow or dried up, the lake basin was filled up and there mainly occurred fluvial and alluvial-proluvial deposits.Since the beginning of the Quaternary, alpine glacial activities around the Zanda basin have intensified with the rapid uplift of Qinghai-Tibet Plateau, and then paraglacial and glaciolacustrine deposits have developed.
GEOLOGICAL HAZARDS IN THE AREA FOR THE CONSTRUCTION OF PIPELINES IN THE PROJECT OF DIVERSION OF NATURAL GAS FROM THE WESTERN TO THE EASTERN REGION
LI Zhi-yi, YAN Yu-sen, LEI Hai-ying
2004, 10(3): 253-259.
Abstract (136) PDF (230KB)(14)
Abstract:
The physical geographical and geological environmental conditions in the area for the construction of pipelines in the project of diversion of natural gas from the western to the eastern region are complex and variable.The geological hazards in the area show the regional distribution pattern.With the Mu Us desert and the eastern foothills of the Taihang Mountains as the boundaries, the whole area may be divided into the west section, central section and east section.In the west section, the main hazards are eolian erosion and sand-burying, corrosion by saline soils and expansion of salt, as well as mud flows and flood erosion.Landslides, collapses, mudflows, caving-in in mined-up areas and water creep and collapsing of loess when wetted are the main hazards in the central section.In the east section, the hazards are mainly ground subsidence, ground cracks and expansion and shrinkage of expanded soils.The central section is the most dangerous part of the pipeline project area where there are the most types of geological hazards and the most serious hazards;so the monitoring and early-warning system should be set up in this section.
HIGHWAY GEOLOGICAL HAZARDS OCCURRING ON MULTI-STAGE ACCUMULATIONS OF DEBRIS FLOWS——A CASE STUDY OF THE BIETUO SECTION OF THE WEST APPROACH OF THE ERLANG MOUNTAIN TUNNEL,SICHUAN-TIBET HIGHWAY
LI Yu-shu, LI Tian-bin, GAO Guang-yun, CHENG Gang
2004, 10(3): 260-267.
Abstract (218) PDF (290KB)(8)
Abstract:
The main communication lines in western China are mostly located in mountainous areas, where in history there exist multi-stage accumulations of debris flows which have preferable stability and structural strength in the natural state.How to make rational use of this kind of accumulation and avoid occurrence of engineering geological hazards in highway construction has practical significance.In this paper the multi-stage accumulation of debris flows in the Bietuo section of the west approach of the Erlang Mountain tunnel along the Sichuan-Tibet Highway is selected to study its development, distribution and evolutionary trend, and then the highway engineering geological hazards such as landslides, uneven settlement of the roadbed and deformation of bridge foundation occurring in this section are analyzed and evaluated.At last the authors give some important advices for highway construction on multi-stage accumulations of debris flows.
RELATION BETWEEN FRACTURE-SCALE FRACTAL DISTRIBUTION AND ITS DAMAGE EVOLUTION
XE Zhi-bin, XIE He-ping
2004, 10(3): 268-275.
Abstract (157) PDF (364KB)(8)
Abstract:
The data provided by the related rock fracture test were reviewed, analyzed and calculated using the methods of fractal geometry and damage fracture mechanics.The results show that the rock damage evolution has better statistical self-similarity, that the fractal dimension of the fracture-scale distribution can well describe the degree of rock damage and that the fractal dimension (DL) of fracturescale distribution increases with the linear increase of the load (σ) in the process of the damage evolution of rock fracture.For the fractal dimension of the fracture-scale distribution, the damage evolution is a process of increase in dimension.The empirical relation between the fractal dimension (DL) and the damage variable (ω) was gained according to the calculated results.
GEOCHEMICAL CHARACTERISTICS AND GEOLOGICAL SIGNIFICANCE OF INTERMEDIATE AND INTERMEDIATE-ACID VOLCANIC ROCKS IN THE NORTHERN SECTOR OF THE DA HINGGAN VOLCANIC ERUPTION ZONE
ZHAO Shu-yue, HAN Yan-dong, ZHU Chun-yan, GHO Kui-cheng, WANG Jian-min, LIU Bao-shan
2004, 10(3): 276-287.
Abstract (134) PDF (533KB)(8)
Abstract:
Petrological and geochemical studies of intermediate-acid volcanic rocks of the Late Jurassic Jixiangfeng Formation in the northern sector of the Da Hinggan Mountains show that there exist high-Sr and low-Y type volcanic rocks in the area, which are so-called adakitic rocks in the literature.The authors think that the magmas of the intermediate-acid volcanic rocks were derived from basaltic magmas of the lower crust and are mixed magmas produced by the increase of geothermal gradients due to diapiric injection of basaltic magmas and partial melting of intermediate-basic metamorphic igneous rocks in the low er crust, and that they resulted from a series of tectonomagmatic activities that took place when the closing of the Mongolian-Okhotsk sea trough and continent-continent collision caused crustal thickening and brought about the formation of the Hinggan-Mongolian orogenic belt in the process of oblique subduction of the Paleo-Pacific plate beneath the Siberian plate.The tectonic setting in which they formed was affected and constrained by the Paleo-Asiatic tectonic domain in the setting of the peri-Pacific domain, rather than only resulted from the activity of the peri-Pacific tectonic mobile belt.The acid volcanic rocks of the Early Cretaceous Shangkuli Formation belong to continental crust anatexis type ones (S-type volcanic rocks) and may have formed by compression and migration of the Indian-Australia plate toward the NNE and delamination of the lower crust caused by the lithospheric extension in eastern China.