Regional Geological and Tectonic Environment and Evolutionary Characteristics of the Yangbajing-Gedaxiang Basin,Tibet
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Gedaxiang Basin in Tibet is located in the south-eastern section of Gangdisi structural belt, with a typical geological and tectonic environment and a commonly developed tectonic pattern in plateau area. By studying the structural geological environment of Geda Township area and analyzing its internal geology, it can be seen that: (1) Six large-scale faults developed in the area control the basic geological and tectonic structure of the working area and affect the spatial structure of the area; The fault development direction is mainly north-south, mostly brittle faults at shallow surface level, and a few of them are east-west orientated. Most of the faults in the region are caused by the development of geological and tectonic activities in the Himalayan period, and the ones that developed later are the north-south oriented positive faults formed by the extension movement in the late Himalayan period. (2) There are three periods of tectonic environment evolution in the area of the evolutionary history, since Jurassic: the first period is the late Yanshan period, tectonic activity for the extrusion environment, the formation of north-south fold deformation under the compression environment; The second stage is early to middle mountain stage, which formed large scale EW-trending nappe structure and thrust fault, and the regional stress background is compressional environment. The third stage is the late Alpine stage, forming a series of tectonic systems of grabens and normal faults with SN strike, which were mainly formed by geological activities under the extensional system. The results of this study provide a reference for distinguishing the trace of geological rock group in the tectonic evolution of the Himalayan tectonic period.Keywords:
Thrust fault
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The thrust nappe structure in the Xiangshan-Tianjingshan area, Ningxia, consists of the Tianjingshan northern piedmont fault zone (main detachment fault) and Xiangshan-Tianjingshan thrust sheet (which may further divided into nine second-order thrust sheets). The front of the thrust sheet is composed of a series of nearly parallel listric, imbricate thrust nappe faults and its tectonic deformation is strong, while tectonic deformation inside the thrust sheet weak. The thrust nappe as a whole underwent Yanshanian and Himalayan compression and nappe-thrusting. In the Yanshanian period the principal compressional stress σ1 was oriented in a 44° direction and the nappes was thrust northeastward with a total displacement of 26.6 km. In the Himalayan period, the principal compressional stress σ1 was oriented in a 75° direction and the nappes was thrust from SWW to NEE. Since the late Pleistocene, there also occurred left-lateral strike-slip motion with a total displacement of more than 4 km.
Thrust fault
Clockwise
Principal stress
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Based on an analysis of the available geological data combining ore forming geological conditions with geological characteristics and the large amount of geological information expored the underground works of this deposit in the past few years,the authors have reached the conclusion that the ore controlling structure should belong to the thrust_nappe type,F_1、F_2 faults that are imbricate thrust groups is a GradeⅠfault in the ore district.Under the tectonic stress,F_1、F_2 acted as the principal thrustsliding plane,and the sub grade NNW-trending trust faults and fold called GradeⅡfaults.Between GradeⅡfaults,the sub NNE-trending trust faults called GradeⅢfaults.According to Summary law of thrust-nappe structure,we had achieved good results in practice the overall design of prospecting exploration.
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Thrust fault
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Nearly vertical seismic reflection stacked migration section study has proved that the distant stress field formed by the westward subduction of Pacific plate lead to the detachment of upper crust.In the upper basement of Songliao basin in Binbei area,the thrust-nappe fault system was controlled by middle-distance Mongolia-Okhotsk suture zone and distant westward subduction of Pacific plate together.The fault system shows bidirection and the asymmetric intensity shows in longitudinal and transverse directions.The thickness is 3～4 km and 1～2 km for the westward thrust-nappe fault system and the eastward fault system,respectively.The structure of fault system is complex.According to the distribution of geophysical field along both sides of the Sunwu-Shuangliao fault which controlled the evolution of the basin,the thrust-nappe fault system in the upper basement of Songliao basin was overlapped on the Sunwu-Shuangliao fault.As we know,the Mongolia-Okhotsk suture zone has formed the huge Yinshan-Yanshan thrust-nappe zone more than 1000 km far away from the suture zone.Taking this and the regional tectonic environment of Songliao basin into account,a thrust-nappe fault system should be formed in the south Songliao basin and the Binbei area beyond the Sunwu-Shuangliao fault system.
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Pacific Plate
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we analyzed the characteristics and evolution of thrust nappe structure in Dashuigou area in the middle part of Helan mountain with the methods of geological observations and structural mapping. It consists of Ruqigou-Dashuigou fault and Gaogougoukou fault. Both of them are low angle imbricate thrust nappe fault, and the stress directions are NWW. Based on the comprehensive analysis and the results of previous studies, we concluded that the Dashuigou thrust nappe was formed at the Yanshan-Himalayan stage. Its formation time was consistent with the thrust nappe of the whole Helan Mountain.
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By integrating surface geology, seismic data, resistivity sections, and drilling data, the structural deformation characteristics of the frontier fault of thrust nappes were delineated in detail. The frontier fault of thrust nappes in northwest Scihuan Basin is a buried thrust fault with partial exposure in the Xiangshuichang—Jiangyou area, forming fault propagation folds in the hanging-wall and without presenting large-scale basin-ward displacement along the gypsum-salt layer of the Triassic Jialingjiang Formation to the Triassic Leikoupo Formation. The southwestern portion of the frontier fault of thrust nappes (southwest of Houba) forms fault bend folds with multiple ramps and flats, giving rise to the Zhongba anticline due to hanging-wall slip along the upper flat of the Jialingjiang Formation. In contrast, the northeastern portion of the frontier fault of thrust nappes (northeast of Houba) presents upward steepening geometry, leading to surface exposure of Cambrian in its hanging-wall. With the frontier fault of thrust nappes as the boundary between the Longmenshan Mountain and the Sichuan Basin, the imbricated structural belt in the hanging-wall thrusted strongly in the Indosinian orogeny and was reactivated in the Himalayan orogeny, while the piedmont buried structural belt in the footwall was formed in the Himalayan orogeny. In the footwall of the frontier fault of thrust nappes, the piedmont buried structural belt has good configuration of source rocks, reservoir rocks and cap rocks, presenting good potential to form large gas reservoirs. In comparison, the hanging-wall of the frontier fault of thrust nappes north of Chonghua has poor condition of oil/gas preservation due to the surface exposure of Triassic and deeper strata, while the fault blocks in the hanging-wall from Chonghua to Wudu, with Jurassic cover and thicker gypsum-salt layer of the Jialingjiang formation, has relative better oil/gas preservation conditions and thus potential of oil/gas accumulation. The frontier fault of thrust nappes is not only the boundary between the Longmenshan Mountain and the Sichuan Basin, but also the boundary of the oil/gas accumulation system in northwestern Sichuan Basin.
Anticline
Thrust fault
Orogeny
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Abstract Near‐vertical seismic reflection stacked migration section study has proved that the distant stress field formed by the westward subduction of Pacific plate led to the detachment of upper crust. In the upper basement of Songliao basin in Binbei area, the thrust‐nappe fault system was controlled by middle‐distance Mongolia–Okhotsk suture zone and distant westward subduction of Pacific plate together. The fault system shows bidirection and asymmetric intensity in longitudinal and transverse directions. The thickness is 3~4 km and 1~2 km for the westward thrust‐nappe fault system and the eastward fault system, respectively. The structure of fault system is complex. According to the distribution of geophysical field along both sides of the Sunwu–Shuangliao fault which controlled the evolution of the basin, the thrust‐nappe fault system in the upper basement of Songliao basin was overlapped on the Sunwu–Shuangliao fault. As we know, the Mongolia–Okhotsk suture zone has formed the huge Yinshan–Yanshan thrust‐nappe zone more than 1000 km far away from the suture zone. Taking this and the regional tectonic environment of Songliao basin into account, a thrust‐nappe fault system should be formed in the south Songliao basin and the Binbei area beyond the Sunwu–Shuangliao fault system.
Thrust fault
Basement
Pacific Plate
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Jizhoushan thrust nappe belt is located in the southwestern part of the Mesozoic Yanshan orogenic zone. The thrust nappe belt lies on the northwestern limb of the Jizhoushan syncline with a NEˉstriking axial and occurs during the late period of the late Jurassic. It is composed of a series of nearly parallel thrust faults like plough shape in across section, forming an imbricate arrangement with steep in the front and gentle in the back. The thrust direction of the nappes is mainly southeastward and the slipping distance is more than 5.8 km. The stress during the thrust changed regularly both in horizontal and longitudinal direction. In the basic belt, the main structure is made of highˉangle thrust faults and complex multilevel folds with compression stress. In the middle belt, the stress turned into shearing, and formed imbricate structure. And the recoil fault formed with the compression increasing in the peak belt. Later, the bigger scale normal fault appeared in the foreland with the compression decreasing.
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Syncline
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Thrust fault
Principal stress
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The tectonic zonation of Longmen Mountain thrust zone is the key problem in the study of Longmen Mountain structure.According to the field measured geologic section and the geological survey as well as the different structural types of the different part within the thrust zone,the northern front range of Longmen Mountain thrust zone is divided into three tectonic sub-belts,Tangwangzhai-Yangtianwo klippe and gliding nappe tectonic subbelt,Louziba-Yanmenba-Majiaoba imbricate thrust subbelt and Kuangshanliang-Tianjingshan-Erlangmiao thrust and anticline subbelt,namely.Each has the different tectonically deformation.The klippe and gliding nappe tectonic subbelt is characterized by structural superimposition and tectonic evolution of nappe and addition gliding nappe.The thrust structure of the imbricate thrust subbelt is well developed,different systems,different formation in the same system as well as the different member in the same formation in the imbricate thrust subbelt thrusts from northwest to southeast,from older to younger in turn and forms the typical imbricate structural type.The thrust faults take on listric inclination westward and converge above the basal thrust plane of the thrust zone.There well developed fault-propagation folds in the structural rock fragments.These folds have almost the same axial strike and axial-plane occurrence as those faults in the imbricate thrust zone,and all fall toward foreland.The thrust anticline subbelt has the dual feature,on the one hand,it shows as brachyanticline on the surface or the near surface,on the other hand,thrust sheets are distributed in the deep crust of earth.In Xiasi-Jinzishan-Houba foreland,the thrusts are of scarcity,small scale and weak deformation in the shallow which mainly shows that monoclinal structure dipps toward southeast.Whereas,buried thrust faults has been well developed in the depth.Thrust zone has the tendency of regular decreasing gradually in its deformation strength,the involved strata gradually becoming new and the deformation levels gradually becoming shallow from front range to foreland,which demonstrates that thrust structures have the fore-expanding feature.Thrust structures are the product of both Later Triassic and Cenozoic era tectonic deformation.The tectonical deformation during Later Triassic is intensive and belongs to the major formation stage of Longmen Mountain foreland thrust zone,the tectonical deformation in Cenozoic era,however,is weak and superposed and changed the tectonical deformation formed in Later Triassic era.
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Thrust fault
Thermochronology
Main Central Thrust
Fission track dating
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