2. 1. 1 banded structural system
The system is mainly developed in the north, middle and south of Ordos Basin, with Yinshan belt structure system in the north, regional east-west structural belt in the middle and Qinling belt structure system in the south.
2. 1. 1. 1 Yinshan banded structural system
The southern part of Yinshan section of Tianshan-Yinshan banded structural system is developed in the northern margin and the northern margin of Ordos Basin, with a complicated history of structural development. After many tectonic activities in Caledonian, Hercynian, Indosinian, Yanshan and Himalayan periods, tectonic activities controlled the distribution and structural evolution of magmatic activity, dynamic metamorphic belt and sedimentary facies belt in a large range. The main body of the basin is distributed in Yimeng uplift and its southern neighborhood, and the Yellow River fault zone is the dividing line between the basin and Yinshan belt. The overall characteristics of the system are as follows:
(1) basement fault zone
The basement fault zone runs from north to south, as shown in Figure 2-2.
1) Wuyuan North-Hohhot Fault Zone: This fault zone is the contact zone between the walnut area and the Tianshan-Yinshan orogenic belt. It was formed in Proterozoic, with obvious Paleozoic activity and strong Mesozoic-Cenozoic activity. The fault dip angle is n, and the dip angle is 30-80. It is a deep fault zone on the southern edge of Yinshan belt.
2) He Lin-Tuoketuo fault zone: This fault zone is a banded fault zone in the northern margin of Ordos Basin, which is obviously reflected in the gravity and aeromagnetic anomaly map and is the boundary zone of the anomaly area.
3) Hangjinqi South-Dongsheng South Fault: This fault starts from southwest Dengkou in the west, south Hangjinqi in the east, Dongsheng South in the east, and reaches southwest Datong. It is an arc fault slightly convex from east to west to south, generally distributed in east-west direction. This fault is located in Yimeng uplift area and is one of the faults that control lithology. To the north of the fault is the Middle and Lower Archean Jining Group, and to the south is the Upper Archean Wulashan Group (Tangxi, 1993).
4) Zhengyiguan-Pianguan fault: This fault starts from Zongbieli, Inner Mongolia in the west, goes to Zhengyiguan, Otog Banner and Yijinhuoluo Banner in the east longitude, and extends to Pianguan in the direction of NEE. Generally, it is nearly east-west, showing an arc protruding to the south in the basin, and exposed to the ground at the western end (west of Table Mountain), which is called Zhengyiguan fault, and it is a compression-torsion fault with a section inclined to the north. In some parts,
(2) aeromagnetic anomaly of basement
Basement aeromagnetism shows three east-west positive anomaly zones and two negative anomaly zones (Figure 2-3), namely: three positive anomaly zones: Hohhot, Tuoketuo and Natuokexi-Pianguan; There are two negative anomalies: Baotou and Dongsheng.
(3) basement gravity anomaly
Gravity anomaly map of basement (Figure 2-4): positive anomaly area: Hohhot and Togtoh; The negative anomalies are in Baotou and Hangjinqi.
Figure 2- 1 Ordos basin and its surrounding structural system feature map
Figure 2-2 Distribution Characteristics of Faults in Ordos Basin and Its Surrounding Bases
(4) Paleozoic banded system
Paleozoic zonal system is characterized by Yinshan ancient land, Hangjinqi uplift and Hohhot depression. The zonal system controls the deposition of CAMBRIAN-Ordovician, making it distribute in the east-west direction (Figure 2-5).
(5) Late Paleozoic banded system
The zoning system of late Paleozoic has always played an obvious controlling role in Carboniferous-Permian.
(6) Mesozoic and Cenozoic activities
Mesozoic and Cenozoic activities were weak, but the rapid uplift of Yinshan orogenic belt forced the Hetao to sink in Cenozoic, which formed a very obvious fault depression control and developed a thick Cenozoic stratum.
2. East-west structural belt in1.1.2 area
Zhongning-Lishi fault zone;
The fault zone is distributed in the east-west direction along the 38 N line. Zhang Fuli et al. (1994) called it Dingbian-Wubao east-west structural belt, and Wang Shuangming et al. (1996) called it 38 N east-west structural belt. In fact, this fault reflects a concealed basement fault zone. The fault zone is concentrated between 37 and 38 20' north latitude, extending from east to west, starting from the west of Zhongwei in the west, passing through Zhongning, Jingbian and Suide, and reaching Lishi, and still extending eastward, with a width of about 100km from north to south and a length of about 300km in the basin.
In the regional magnetic field, the fault zone appears as an east-west magnetic anomaly zone. According to the analysis of magnetic anomaly characteristics and basement lithology, this structural belt was caused by a series of large basement faults, which may have been formed in Archean (Wang Shuangming et al., 1996). However, there are no obvious faults in the seismic profile inside the basin, indicating that there is no strong activity in the later period. According to its obvious control of sedimentary caprocks on the north and south sides, it is speculated that its formation time can be traced back to Paleozoic, and it reflects the activities since Paleozoic in a specific way.
Figure 2-3 Aeromagnetic Anomaly Map of Ordos Basin
(According to Changqing Oilfield 1983)
2. 1. 1.3 Qinling banded tectonic system
As Professor Li Siguang pointed out, the banded tectonic system in Qinling Mountains is an extremely intense and complex compressive tectonic belt formed by long-term repeated tectonic movements. No matter the strike of strata, the distribution of metamorphic zones and magmatic rocks, or the strike of fault zones, compressional zones and small mountain basins, it is obviously east-west.
The Qinling banded structural belt is located in the Qinling area, and the generally described northern boundary ends at about 34 30' north latitude, that is, the Huashan front. However, the author found that some structural features to the north of the line should be related to the banded structural system. There are some east-west thrust faults in Guishan, Laolongshan and Jueshan in Weibei, which were formed before the Late Paleozoic, that is, before the formation of Qilu Island Arc. In Changwu area, it has generally entered the shielding area of "mountain" structure, and some east-west folds and faults are scattered sporadically. According to the data of neighboring provinces, there are some compressional faults near Sanmenxia. There are a series of east-west thrust faults in the south of Jincheng and Yangcheng (south of 35 35 ′ north latitude) in Shanxi Province, and some folds parallel to them are seen, which are considered to belong to the northern branch of Qinling belt structure belt. It can still be seen in Henan province in the east, but it is intercepted by the "mountain" shape of Xinhua Xia system and Qilu in the west, and only appears sporadically in some areas, such as Yuanqu, Pinglu and Zhongtiaoshan. At the same time, the plane profile of magnetic field intensity in Houma and Hejin areas shows an east-west anomaly zone. The Archean compressional zone developed in the south of Luliang Mountain. There are also east-west structures in the western part of Hancheng, which echo the east-west structures in Guaishan and the western part of Changwu, and appear in strips at the same latitude, indicating that there is an inevitable connection between them. It's just that the zonal structural belt becomes weaker as it goes northward, and it becomes unclear due to the concealment, disturbance and transformation of the structural system formed in the later period. However, there is no doubt that these structural features exist as an integral part of the Qinling banded structural belt, which runs through the Wei Fen basin and becomes the basic structural system controlling the basin. In the south of the basin, the early Paleozoic distributed in the east-west direction.
Figure 2-4 Bouguer gravity anomaly map of Ordos Basin
(According to Changqing Oilfield 1983)
The zonal tectonic system in Qinling Mountains is very obvious in basement fault, aeromagnetism, gravity anomaly and sedimentary characteristics.
(1) basement fault zone
Basement faults are Qingyang-Fuxian fault, Longxian-Tongguan fault and Baoji-Luonan fault from north to south. These faults are all reflected in the gravity and magnetic map. The fault was formed in Paleozoic, with strong activity in Mesozoic, and it was a southward reverse fault, and it was reversed into a normal fault in Cenozoic.
Baoji-Tongguan fault is distributed along the northern edge of Weihe Graben, and the surface is exposed for many times, tending to es. There is an obvious gravity line dense zone along the fault (Figure 2-4), but it disappears after extending for 30 km. It also shows landforms, satellite photos and magnetism, and hot springs are distributed along the fault. The fault may have been formed in Paleozoic, which may have a certain control effect on the Paleozoic on its north side. The Mesozoic activity was strong, forming an east-west trending reverse fault, and it was only in the Cenozoic that it was reversed into a normal fault with right-handed northeast and left-handed southwest. In addition, there are Baoji North-Tongguan Fault and Baoji-Luonan Fault.
Figure 2-5 Schematic Diagram of Ordos Basin and Early Paleozoic Structure
(modified according to Guo et al. 1994)
(2) aeromagnetic anomaly of basement
The basement aeromagnetic anomaly zone is southwest-Tongguan positive anomaly zone.
(3) gravity anomaly
The gravity anomaly zone includes Tongguan positive anomaly zone and Binxian negative anomaly zone.
(4) Early Paleozoic
The early Paleozoic Cambrian-Ordovician system filled the east-west subsidence zone with thick sediments.
(5) Late Paleozoic
Due to the control of the banded tectonic system, the Late Paleozoic showed an east-west subsidence area, and thick Carboniferous-Permian strata were deposited.
(6) Cenozoic
Due to the rapid uplift of the Cenozoic Qinling orogenic belt, the piedmont fault depression (Weihe fault depression) was formed and thick Cenozoic strata were deposited.
2. 1.2 Cathaysian tectonic system
Cathaysian tectonic system is a "polymorphic" tectonic system in eastern China, which was mainly formed and developed from Paleozoic to Middle Triassic. It plays an important role in controlling the sedimentary formation and lithofacies distribution in Paleozoic (especially in late Paleozoic).
The Archean basement Cathaysian structural system in this area is obvious, which is a set of faults, uplifts and depressions extending northeast, and the depressions have been developed until Permian.
2. 1.2. 1 basement fault zone
The basement fault zone runs from north to south, as shown in Figure 2-2.
1) Dingbian-Yulin fault zone and Wuqi-Datong fault zone.
2) Qingyang-Shuoxian fault zone: this fault zone extends in ne direction and is parallel to Fuxian-Lishi fault zone, which is Proterozoic and Neoarchean faults respectively. There is no fault in the sedimentary caprock in the reflected seismic data, so it is speculated that the fault zone may be mainly active in Neoarchean-Proterozoic.
3) Fuxian-Lishi fault: The fault also extends in NE direction, starting from Qixian County, Shanxi Province in the north, passing through Lishi and Fuxian County, and reaching Yongshou in the south. This fault is clearly reflected on the aeromagnetic map, which is the boundary between positive and negative magnetic fields, and also reflected on the gravity anomaly map (Figure 2-3, Figure 2-4).
It is inferred that the basement on the north and south sides of the fault are Neoarchean and Paleoproterozoic respectively, so it is also one of the faults that control the lithology of the basement and belongs to the super-crust fault. The fault was formed in Proterozoic, and its activity was weak after Proterozoic. It may be the southern boundary of the Proterozoic Shanxi-Shaanxi trough.
2. 1.2.2 aeromagnetic anomaly of basement
The basement aeromagnetic anomaly zones are two positive anomaly zones: Longxian-Yan 'an and Hancheng, and two negative anomaly zones: Wuqi-Yulin and Huangling-Lishi (Figure 2-3).
2. 1.2.3 basement gravity anomaly
The basement gravity anomaly zones are Huachi-Xingxian, Huangling-Lishi and Hancheng-Linfen, with two positive anomaly zones sandwiched by a negative anomaly zone (Figure 2-4).
2. 1.2.4 Mesoproterozoic
The meso-Neoproterozoic structural map shows the characteristics of northeast uplift and depression, indicating the beginning of the ancient northeast structure.
2. 1.2.5 Early Paleozoic
Early Paleozoic indicates that the Cathaysian tectonic system includes Lvliang uplift, Yan 'an sag and Qingyang uplift. In addition, the northeast Suide-Yichuan depression belt and the Wushenqi-Huachi fault uplift belt appeared in Cambrian. The system was still active in the Late Paleozoic, which controlled sedimentation to some extent (Figure 2-5, Figure 2-6).
2. 1.2.6 Late Paleozoic
The late Paleozoic sedimentary facies also showed the characteristics of northeast uplift and depression distribution.
2. 1.3 Neocathaysian tectonic system
The Neocathaysian tectonic system starts from the north of Langshan Mountain in the north, passes through Tianshui and Minshan Mountain in the south, passes through the west section of Longmen Mountain, passes through Wuzhongshan Mountain, obliquely passes through the Sichuan-Yunnan meridional tectonic belt, and reaches Jinping Mountain area on the west side of Mianning. The NE-trending landforms, satellite photos and structural features clearly show the existence of this structural belt. The structural belt is dominated by faults, followed by folds, accompanied by some small fault basins and tectonic dynamic metamorphic belts. According to the regional data, this tectonic belt starts from the Russian Oliynyk uplift belt in the north, passes through the outer Baikal fold belt and the Mongolian Wendur Khan area, and extends to China along the Helan Mountain uplift. After crossing Jinping Mountain in the south, it may intermittently extend to Yanyuan-Lijiang area and be blocked by the anti-S-shaped structure of Qinghai-Tibet, without obvious traces. Although the existing data show that it is small in scale and poor in continuity, it is indeed the most important boundary in the western part of the Neocathaysian system, and it is also an important boundary for structural deformation analysis in the eastern and western parts of China. The depth shows the abrupt change zone of crustal thickness.
According to 1979 Bouguer gravity anomaly map of China compiled by Geophysical Institute of Ministry of Geology and Mineral Resources, China can be divided into two gravity fields (regions) with obviously different characteristics, with Dengkou-Helan Mountain-Liupanshan Mountain-Longmen Mountain-Jinping Mountain-Yulong Snow Mountain as the boundary. The gravity gradient zone is NNE with a width of about 50 ~ 100 km. This huge gravity gradient zone is basically consistent with Helanshan-Wuzhongshan-Jinpingshan structure, which is not only the boundary zone separating the east-west gravity field in China, but also the boundary zone of the east-west tectonic field in Chinese mainland since the Late Paleozoic. Geomorphological, geological and geophysical anomalies are basically the same, but the surface structure reflects the interference and influence of early structural signs and forms in different regions on this structural belt. This structural belt can be divided into Helanshan-Wudu fault uplift belt and Wuzhong-Jinping fault uplift belt from north to south in China.
Figure 2-6 Cambrian Paleostructural Map of Ordos Basin
(Modified by Zhao Zhongyuan, 1996)
1) Helanshan-Wudu fault uplift zone: This fault zone starts from the border between China and Mongolia in Urad Houqi, Inner Mongolia Autonomous Region in the north, passes through Jilantai and Helan Mountain, passes through Huining, Wushan, Tianshui and Lixian, and reaches Zhouqu and Wudu. It is along the Wendur Khan belt in the north and corresponds to the middle section of Wuzhong Mountain in the south, with a total area of about NE25, which is composed of some secondary uplift and fault depression. Wulate Houqi-Langshan fault uplift belt is developed in Yinshan belt, which is composed of late Yanshanian granite body and two NE-trending compression-torsion fault zones, which run through the belt and Alashan arc east wing structural belt. The Helan Mountain fault uplift zone of NE30 in the south, and the piedmont hidden faults on the east and west sides of Helan Mountain, which are obliquely combined in the Helan Mountain meridional structural zone. Geophysical data show that the buried faults on both sides are steep slopes or gravity density gradient zones with a fault distance of 500-3000m and steep profile, which makes Helan Mountain a northeast horst-like uplift, with secondary folds of 20-30 northeast composed of Paleozoic and Mesozoic, and small thrust fault groups and torsion. There are Jilantai fault basin and Yinchuan fault basin on both sides of Helan Mountain, which lack Mesozoic deposits, while Cenozoic deposits reach about 3000 m. The fault basin is the product of pre-Paleogene and still has strong activities. There is strong seismic activity in this area, which is called Yinchuan-Wuzhong seismic activity zone. The Langshan fault zone was formed in the Early Cretaceous-Paleogene, cutting the late Yanshanian rock mass. The NE-trending Paleogene basalt and NE-trending craters are exposed on the east side of the fault zone from Bayin Hang Gai Gobi to today's map, which should be the expression of the activities of the structural belt during its finalization period.
Helanshan-Wudu fault-uplift belt extends southward to Tianshui and Wudu areas in Gansu, forming a group of NE 20 ~ 25 fault-uplift fault-depression belts, including Tongwei-Zhouqu fault-depression belt, Longde-Tianshui-Lixian fault-depression belt and Qingshui-Chengxian uplift belt from west to east. These uplift zones are separated by NE-trending fault zones, and internal low-order NE and NE-trending faults are developed, indicating that they are anticlockwise twisted by compression and torsion (Figure 2-7).
Figure 2-7 Distribution Map of Neocathaysia Fault Zone in East Gansu Province
(According to Gansu Bureau of Geology and Mineral Resources 1989)
1- fracture; 2- Concealed fault; triple
2) The Neocathaysian tectonic system in Ordos Basin is very obvious. Since the late Triassic, this area has been controlled by the Neocathaysian tectonic system. The sedimentary bodies of Yanchang Formation are NNE-trending, and the stratum missing line is also NNE-trending.
The Jurassic strata are still NNE, which fully shows the controlling role of the Neocathaysian tectonic system. For example, the distribution direction of the residual thickness of the middle Jurassic Zhiluo Formation and Anding Formation is NNE, and the missing lines of the two groups are also NNE (Figure 2-8 and Figure 2-9).
Cretaceous Neocathaysian tectonic system still controls its deposition. In the Early Cretaceous, the uplift of the Lvliang orogenic belt and the subsidence of the Helanshan orogenic belt resulted in an asymmetric pattern of the Lower Cretaceous, which was thin in the east and thick in the west, but the distribution direction was still NNE. The denudation thickness in the Late Cretaceous further shows that the uplift in the east of the basin is strong, and the uplift in the west is much smaller than that in the east. The maximum erosion thickness in the east is 1600m, and the central part of the basin changes to 1000m, and the erosion thickness in the west is 200 ~ 300 m, and the erosion thickness trend is NNE (Figure 2- 10, Figure 2- 1 1).
Figure 2-8 Isothickness Map of Mesozoic Sediments in Ordos Basin (unit: m)
(According to Tan Zhongfu 1989)
Figure 2-9 Isogram of Residual Thickness of Middle Jurassic Anding Formation in Ordos Basin
Fig. 2- 10 Isothickness Map of Lower Cretaceous Residual Strata in Ordos Basin (unit: m)
(According to Tan Zhongfu 1989)
2. 1.4 meridian tectonic system
The western margin of the basin is 2. 1.4. 1 meridional fault.
The meridional faults in the western margin of the basin are divided into three sections: south, middle and north (Figure 2-2).
1) South section: Qingtongxia-Guyuan fault: The fault is distributed in the southwest edge of the basin in an inverted "S" shape, extending from Qingtongxia to Baoji via Luoshan East Road, Tanshan, Guyuan, Huating Maxiakou and Longxian Guguan, with a total length of about 480 km. This fault extends northwest from Qingtongxia to the periphery of the basin. On the gravity anomaly map, there is a high gravity gradient zone along the fault (Figure 2-4), which is also clearly shown on the magnetic anomaly map (Figure 2-3), and its existence is also confirmed by a few earthquakes passing through the fault. The location of this fault is also the abrupt change zone of the crustal thickness in China, with the eastern part belonging to the middle-thick crust area in Chinese mainland and the western part belonging to the thick crust area in China. This fault serves as the dividing line between the Cenozoic tensile stress field in eastern China and the compressive stress field in western China (Sun Guofan et al., 1983). It belongs to lithospheric fault or deep basement fault, and it is the boundary fault between North China Craton and Chyi Chin fold belt. Proterozoic began to develop, and it was active from Mesoproterozoic to early Paleozoic, and still active in Mesozoic and Cenozoic (Tang Xiyuan et al., 1993). Before the middle of Late Paleozoic (Late Carboniferous), it was the boundary between the North China Platform and the Qinling Trough. The strata on both sides of the fault developed, and the lithology, thickness, distribution, name and contact relationship of strata in different times were quite different. After the Late Carboniferous, the fault activity tends to be calm, and the difference is not obvious.
Fig. 2- 1 1 contour map of total denudation thickness since the pre-late Cretaceous in Ordos basin
2) Middle section: the eastern boundary fault and basement fault of Yinchuan Graben. From Shizuishan in the north to the southeast of Qingtongxia in the south. It is a normal fault with a west dip, with a small north-south fault distance of 670 ~ 2000 m in the south and 300 ~ 800 m in the north.
3) North section: Dengkou-Shizuishan fault, basement fault, starting from Dengkou south in the north, passing through the foothills of Qianlishan and Zhuoshan East, and reaching Tiekesumiao area in the south, which is staggered into three sections by Qianlishan fault and Zhengyiguan fault.
2. 1.4.2 eastern margin fault of the basin
Lishi fault is a crustal fault, which starts from Tuoketuo East in the north, passes through Pianguan and Lishi, and extends southward to the northern boundary fault of Wei Fen Graben, with a length of about 500 km. The magnetic anomaly characteristics of the fault zone can be distinguished in the south of Pianguan, especially in the south of Xingning-Linxian. The fault is intermittently exposed in the south of Xingxian County, and a series of thrust faults are arranged in a wild line in the north of Lishi, with the profile inclined to the west (Shanxi Bureau of Geology and Mineral Resources, 1989). This fault is the dividing line between Ordos basin and Shanxi block. It is not only large in scale, but also has a long activity time. It has been active since the Hutuo period (Lvliang period) at the latest, and there are ultrabasic rocks-alkaline multi-stage magmatic rocks on both sides of the fault.
2. 1.5 Qilu Helan "mountain" structural system
Qilu Helan "Mountain" structural system is located in the north-central part of China, spanning Xinjiang, Qinghai, Gansu, Ningxia, Shaanxi, Shanxi, Hebei and Beijing, and sandwiched between Tianshan-Yinshan belt structural system and Kunlun-Qinling belt structural system. Its geographical position is between 92 00' ~120 00' east longitude and 34 00' ~ 4100' north latitude, with a length of 2000km from east to west and a width of 900km from north to south, and it belongs to a giant "mountain" structural system (Figure 2-1. The east wing of the forearc passes through the Shanxi uplift belt and is connected and compounded with the NE-trending and NNE-trending structures, and the spine is connected and compounded with the north-south belt of Helan Mountain, showing a meridian structural system, with Yi Shan Shield and A Ning Shield on the east and west sides.
Figure 2- 12 Qilu Helan "Mountain" Structural Belt and Relationship Map
(Modified by Zhou Jiyuan, 1989)
Relationship between "mountain" structure in Qilu Helan and distribution of earthquake epicentre;
B —— the relation diagram between the isointensity lines of several large earthquakes with magnitude 8 or above and the Qilu Helan "Mountain" structural belt.
2. 1.5. 1 distribution range and composition
The "mountain"-shaped forearc is distributed in Qilian Mountain, Longshou Mountain, Shule Nanshan Mountain, Laji Mountain, Luliang Mountain, Wutai Mountain, Hengshan Mountain and other places. It is an arc-shaped structural belt that crosses east and west and protrudes southward, and is called Qilu Arc for short. The top of the forearc is located near Baoji. To the west of Baoji, the structural line gradually changes from east-west to northwest, and the most obvious structural features are huajialing-Baoji anticline, Tianshui-Wushan fault zone and Lixian-Tongren fault zone. To the east of Baoji, the tectonic line gradually changes from east to west to north-east. The most notable structure is Wei Fen Graben, followed by Tongchuan anticlinorium and Zhongtiaoshan anticlinorium. The arc top is reconnected with the Kunlun-Qinling belt structure system.
The west wing of the forearc extends from Jiuquan, Minle and Lanzhou to Dingxi, which is roughly equivalent to the range of Heli Mountain, Longshou Mountain, Maya Snow Mountain, Halagu Mountain, Laji Mountain and Qilian Mountain. The NW-trending fold belt, fault belt and the trough land sandwiched between them are arranged in an inverted "multi" shape. The east of Lanzhou is mainly composed of Huajialing-Baoji anticline, Tianshui-Wushan fault zone and the western segment of Lixian-Tongren fault zone extending from the top of the arc. Between Lanzhou and Linze, anticlines and troughs are arranged alternately in parallel, accompanied by the same thrust. From north to south, they are Helishan-Longshoushan Fold Belt, Zhangye-Minle Trough, Maya Snow Mountain Anticline, Menyuan Trough, Datong Mountain-Qingshiling Anticline, Xining-Minhe Trough, Sun Moon Mountain-Rajishan Anticline and Xunhua Trough. Linze-Jiuquan is mainly composed of a large anticline and a fault zone connected with the structural belt in the old western region. From north to south, it is: Nanshan anticlinorium, Qilian Mountain main peak-corridor, Heihe River upstream trough, Tuolai pasture trough, Tuolai Mountain anticlinorium, Datong River upstream trough, Shule River upstream trough, Shule Mountain anticlinorium.
The east wing of the forearc extends from Hancheng, Lishi and Ningwu to Datong, which is roughly equivalent to the scope of Luliang Mountain, Wutai Mountain and Hengshan Mountain. A large anticline (or ridge) and a large syncline (or trough) arranged in a zigzag shape from ne direction appear, accompanied by the same thrust. From north to south: Yangyuan anticline, Yangyuan Nanshan fault, Sanggan River trough, Sanggan River south-south anticline, Hunyuan trough, Guangling-Yuxian fault, Hengshan anticline, Baihuashan syncline, Fan Shi trough, Wutaishan-Luliangshan anticline and Taiyuan trough.
The "mountain" ridge of Qilu Helan is distributed in the area from Dengkou, Yinchuan to Pingliang on the north side of the arc. It is a narrow structural belt, with its central part slightly protruding to the west in the direction of SN-, which is collectively called Helan Mountain Fold Belt. It reconnects with Helan Mountain meridional tectonic system and may extend northward to the south of Tianshan-Yinshan belt tectonic system. The spine is composed of SN- trending folds and thrust faults, followed by De Lequi-Qinglong Mountain-Pingliang anticline, large faults on its east side and Yanchi-Huanxian syncline from west to east.
Its shield is separated from the east and west sides of the spine. A Ning (Alashan-Huining) shield is located in the west, which was slightly uplifted in Mesozoic, and only deposited in Mesozoic in some areas, and it was a relatively declining area until Cenozoic. On the east side is the famous Yishan shield, which is a relatively declining basin from Triassic or Jurassic to Paleogene-Neogene. Among them, the occurrence of rock strata is gentle, and only wide and gentle folds or domes appear in the edge zone.
Qilu Helan "mountain" reflex arc is distributed in Jiuquan, Yumen, Subei and Annanba in the west wing, which is an arc structural belt protruding northward. The arc top is located in the north of Qiaowan, and consists of folds and thrusts extending in an arc from NE to NWW. From north to south: Liuyuan fault zone and Yufeng Mountain-Liuyuan-Houhongquan syncline, Qiaowan-Dunhuang fault zone and Anxi-Dunhuang uplift zone, Yumen Town-Jiayuguan North trough, Subei-Jiayuguan fault zone and Changma-Yumen trough. The northern part of the reflex arc extends to the Tianshan-Yinshan banded structural belt, its east and west wings are obliquely connected with the banded structural system to the south, and the middle part is reconnected with the Tianshan-Yinshan banded structural system. The reflex arc spines in the west wing are distributed in Yanchi Bay, Crescent Lake and some areas to the south. It is mainly composed of nearly north-south thrust faults and folds. It is not clear that it disappears northward in the south of Shule Nanshan fault zone, and there may be traces in the north of Wulan Daban Mountain.
The east wing reflex arc is distributed in Datong, Xuanhua, Chengde to Qinhuangdao and other places, which is an arc-shaped structural belt protruding northward. The top of the arc is near Chengde, which consists of folds, troughs and thrust faults, and extends in an arc from NEE to NWW. The outer arcs are Zhuolu-Huailai trough, Langshan-Jiuxian fault and Dagucheng-Yongning fault, Yanqing trough, Luanping anticline and Wumishan fault, Longhua anticline and Chengde syncline, Qinglong-Qinhuangdao anticline and Qian 'an-Changli anticline. The inner arc includes Luanxian fold belt, Xiaotangshan-Xiacangshan fold belt and Beijing Xishan fold belt. The northern part of the reflex arc extends to the south side of the Tianshan-Yinshan banded structural system, and its east and west wings are obliquely connected with the banded structural system, and the middle part is reconnected with the banded structural system. According to geophysical data, the composition of reflex arc can still be found in Bohai Sea and Lushun University.
2. 1.5.2 Adult age
According to the available data, the "mountain" shaped arc structural belt in Qilu Helan began to sprout in Carboniferous, and the spine began to appear in Late Triassic. During the Indosinian Movement at the end of Late Triassic, the basic outline of "mountain" was generally formed, the forearc and ridge continued to uplift, the Yishan shield continued to sink, the A Ning shield rose slightly, and a series of anti-polygonal troughs appeared in the west wing of the forearc, which provided a place for Jurassic sedimentation. During the early and middle Yanshanian period from the end of late Jurassic to the end of early Cretaceous, the arc-shaped structural belt and ridge matured one after another, and finally the whole Qilu Helan "mountain" structural system was finalized. In the late Cretaceous, the "mountain" structural system was uplifted as a whole. From Paleogene to Neogene, the trough of the west wing of the pre-arc, A Ning shield and Yishan shield was re-deposited. Recently, the "mountain" tectonic system is still active, and it is one of the most important active tectonic systems in north-central China.
The composite part of Qilu Helan Mountain and other tectonic systems is often the enrichment zone of sedimentary minerals and endogenetic metal minerals. The front of Helan spine is a Paleozoic subsidence area, which is favorable for oil generation, and the back is a compression fold zone, which is favorable for oil and gas accumulation. The structural environment in the south-central part of Dongdun-Ordos basin is stable, which is conducive to oil and gas generation, gentle strata and two groups of torsional fractures, and is also conducive to oil and gas migration and accumulation, which is more conducive to oil and gas preservation. The "mountain" shape of Qilu Helan is also one of the important seismogenic structures in north-central China. Natural earthquakes are quite frequent and intense. Earthquakes of magnitude 7 ~ 8 are banded along the forearc and the reflex arc, which is a strong earthquake active zone (Figure 2- 12a, b).
In addition to the above five types of structural systems, two groups of chessboard faults are widely developed in the middle of the basin (equivalent to the 38 north latitude zone): one group is composed of east-west faults and SN- trending faults, and the other group is composed of northwest faults and northeast faults. From the vertical analysis of faults, their nature seems to be twisted, and their oil and gas control effect is of concern. In addition, there may be a torsion structure in the southwest of the basin between the forearc and the shield.