1. Fault zone at the eastern foot of Taihang Mountain
Spatial distribution and segmentation characteristics of (1) fault zone
The fault zone at the eastern foot of Taihang Mountain is the structural dividing line between Taihang Mountain uplift area and Bohai Bay basin area, which consists of more than ten NE-NNE trending faults such as Huang Zhuang-Gao Liying, Xu Shui, Baoding-Shijiazhuang, Handan, Dong Tang and Tangxi. It is generally distributed in the NE-NNE direction, with a total length of about 620km (Figure 3- 1). In addition to a series of NE-NNE faults, there are NW-trending to nearly EW-trending faults such as Nankou-Sunhe and East Lei Zi-Laishui in the fault zone at the eastern foot of Taihang Mountain, which separates the fault zone and its activity differences to varying degrees. Roughly bounded by the East Lei Zi-Laishui, Longyao and Anyang South faults, it can be divided into four sections:
Huairou-Laishui Section (North Section): It consists of four faults, namely Babaoshan, Huang Zhuang-Gao Liying, Shunyi-Liangxiang and Nanyuan-Tongxian, and the Beijing sag controlled by them. The fault strike is NNE, with a large dip angle of about 60 ~ 70, which began to develop in Mesozoic, but the activity in Mesozoic and Cenozoic is not strong, and the fault depression in Beijing sag is small.
Laishui-Longyao section (north-north section): Baoding-Shijiazhuang, Xu Shui and Jinxian faults and Baoding, Shijiazhuang, Xu Shui and Jinxian depressions controlled by them. The fault strike is northeast, and the dip angle is very slow, about 20 ~ 40. The Mesozoic began to develop, and the Cenozoic activity was very strong, which was the section with the strongest horizontal tensile activity and vertical differential activity of the fault zone. The horizontal extension fault distance of Cenozoic fault is several kilometers to ten kilometers, and the horizontal extension fault distance of the northern section of Baoding-Shijiazhuang fault is 17 kilometers. The vertical fault distance of the fault is 3 ~ 6 kilometers.
Longyao-Anyang section (south-central) strikes northeast with an inclination of 40 ~ 60, which began to develop in Mesozoic. Its southern section and Linzhang fault jointly controlled the Mesozoic and Cenozoic Handan sag, and its northern section controlled the Cenozoic Renxian sag. Mesozoic and Cenozoic fault activity is not strong, and the horizontal tension fault distance and vertical fault distance are about 1km.
Anyang-Xinxiang section (south section) strikes northeast with a gentle dip angle of about 40 ~ 60. The Mesozoic began to develop, and the Cenozoic activity was strong, with horizontal extension fault and vertical fault of about 2~3km.
(2) Geophysical field and deep structural characteristics of fault zone.
In view of the geophysical field and deep structural characteristics of the fault zone at the eastern foot of Taihang Mountain, Wang Chunyong et al. (1994) studied Lincheng-Julu deep seismic reflection profiling, which straddles the Baoding-Shijiazhuang fault zone and the southern part of Jizhong Depression. The results show that there is a low-angle detachment structure in the middle and upper crust, which is actually a Baoding-Shijiazhuang fault. It extends to the southeast to the depth of 10~ 16km, with a gentle dip angle, basically located in the lower part of the upper crust and the top of the middle crust, with a horizontal extension distance of 50km, showing a large detachment fault (Figure 3- 1). Baoding-Shijiazhuang fault directly controls Shijiazhuang sag. Jinxian fault and Xinhe fault are the main boundary faults between Jinxian sag and Shulu sag, which extend downward in shovel shape and merge into detachment faults, indicating that the formation of these depressions is closely related to the extension and detachment of Baoding-Shijiazhuang fault. The northern segment of Baoding-Shijiazhuang fault, Xu Shui fault and Daxing fault are basically detachment faults developed in the upper crust. It is speculated that the horizontal extension dip angle of the northern segment of Baoding-Shijiazhuang fault can reach 70km. It can be seen that Jizhong Depression is an extensional structural system formed in the upper crust by the extension and detachment of Baoding-Shijiazhuang, Xu Shui and Daxing faults.
(3) Formation and evolution of fault zone and its influence on Bohai Bay Basin.
The eastern foot fault zone of Taihang Mountain is the western boundary fault of Bohai Bay Basin. Its formation and evolution have an important influence on the western Bohai Bay Basin, and it is the main power source for the formation of the NNE structural line in Jizhong Depression. The evolution of the fault zone at the eastern foot of Taihang Mountain has roughly experienced three stages: formation period (JBOY3 -K 1 initial strike-slip), extension period (K 1-E) and recent period (N-Q). In the early Cretaceous, rifting occurred in the eastern part of North China under the background of relative uplift, forming faults such as Babaoshan, Baoding-Shijiazhuang and Handan at the eastern foot of Taihang Mountain, and a series of NNE-trending fault basins along the eastern side of the fault zone. In the Late Cretaceous, North China generally uplifted and suffered denudation, and most of the fault basins closed and disappeared. In Paleogene, the pre-existing faults re-cracked and turned into normal faults, and at the same time some new faults were formed, which controlled the development of a series of fault basins with different orders. After intense stretching, detachment and mutual infiltration, they further developed into the western boundary of Bohai Bay fault basin. From the end of Paleogene to Quaternary, the fault activity tended to stop, and the basin entered the depression stage from the fault depression stage.
Figure 3- 1 Lithospheric Structure Profile of Bohai Bay Basin
2. Cangdong fault
Spatial distribution characteristics of (1) fault zone
Cangdong fault starts from Ninghe River in Hebei Province in the north, passes through Cangzhou, Nanpi, Wuqiao and Dezhou in Shandong Province in the south, and reaches the vicinity of Linqing, with an overall strike of about NE30 and a total length of about 350km. Cangdong fault is the boundary fault between Huanghua depression and Cangxian uplift in Bohai Bay basin, and it is also an important separation zone between Linqing depression and Hengshui-Xingtai uplift. The formation period, structural form, active nature and strength of the fault zone directly affect the formation and development of adjacent structural units.
(2) Deep structural features of earth objects and fault zones.
In the geophysical field, Bouguer gravity anomaly and residual gravity anomaly gradient zone are displayed in the northern part of Cangdong fault, and the abnormal isoline is parallel to the fault trace, while in the southern part of the fault, there is no obvious gravity anomaly reflection on both sides of the fault, and the fault trace crosses the isoline. The aeromagnetic △T anomaly has a similar situation (Lu Kezheng, 1999). Both Bouguer gravity anomaly and residual gravity anomaly are caused by the change of crustal medium density, and aeromagnetic △T anomaly is caused by the fluctuation of magnetocrystalline basement, which shows that the northern part of Cangdong fault has a deep influence and a large fault amplitude.
On the velocity structure profile of the crust and upper mantle across the northern part of Cangdong fault (Figure 3- 1), the crust of Cangxian uplift and Huanghua depression is divided into upper, middle and lower layers, and the layered interface is continuous without faults. The thickness of the upper crust in Huanghua Depression is relatively small, at 12km, while the thickness of Cangxian Uplift and Chengning Uplift is relatively large, at 16km. The interface between Huanghua Depression and Cangxian Uplift is flexible, and there are low-velocity bodies (wave velocity is 6. 10km/s) at the bottom of the upper crust and the top of the middle crust. Moho surface in Huanghua depression is 2~4km higher than Cangxian uplift and Chengning uplift on both sides. It is worth noting that although the crustal velocity structure of Cangxian uplift and Huanghua depression is similar, the upper crustal velocity structure is obviously different, and Cangdong fault is the boundary of this velocity structure. The velocity structure of the middle and lower crust is laterally continuous. The Cenozoic Cangdong fault is shovel-shaped, with steep upper part and gentle lower part. This section extends to the depth of 10km, and the dip angle is very gentle. It is a large detachment fault developed in the upper crust, and its horizontal dip angle extends for tens of kilometers (Gao Zhanwu, 2000). In Datong-Haixing crust and upper mantle velocity structure profile (Geophysical Exploration Center of State Seismological Bureau, 1987), Cangdong fault also shows similar characteristics.
(3) Segmentation characteristics of fault zone and its control on adjacent tectonic units.
Cangdong fault is generally a normal fault, but its occurrence, development history and activity intensity are quite different in different sections, showing obvious segmentation, which can be roughly divided into north and south parts.
From Dezhou to Linqing in the south, it is about 100km long. Faults are linearly distributed with a large dip angle, generally 65 ~ 70. The vertical fault distance (drop) of the fault is about 2~3km, and the horizontal extension fault distance (extension) is about 15km, which controls two depressions in Linqing Depression, Dezhou and guanxian, and deposits the Paleogene Kongdian Formation (Ek) with thicknesses of 2,000 m and 4,000 m respectively.
The northern part refers to Dezhou to the north of Ninghe River in Hebei Province, which is the western boundary fault of Huanghua Depression. Broadly bounded by Botou and Tangguantun, it is divided into three sections: south, middle and north. The southern section is located between Botou and Dezhou, and the section is shovel-shaped, inclined to the southeast, with a small dip angle of about 30 ~ 40, a vertical fault distance of about 1 ~ 2km, and a horizontal extension fault distance of about 1km, which controls Nanpi and Dongguang depressions. Sedimentary strata include Kongdian Formation and Shahejie Formation, with a thickness of 2000 ~ 3000m; The middle section is located between Botou and Tangguantun, with flat wave distribution, shovel-like profile, gentle dip angle of 20 ~ 30, vertical fault distance of 6~7km and horizontal extension fault distance of 7~ 10km, which controls the development of Cangdong sag. The sedimentary strata are mainly Kongdian Formation and Shahejie Formation, Dongying Formation is very thin, and Paleogene thickness is 4000 ~ 4000. The northern section is the northern part of Tangguantun, and the fault occurrence changes greatly. Two groups of faults are arranged in a zigzag pattern on the plane, and the profile is in a plane shovel shape, with an inclination of about 30 ~ 40, a vertical fault distance of 3~5km and a horizontal tensile fault distance of 3~4km, which controls the development of Beitang and Banqiao depressions. The strata of Kongdian Formation are relatively small, mainly Shahejie Formation and Dongying Formation, with a thickness of about.
Generally speaking, the Paleogene was the main active period of Cangdong fault, and the pre-existing fault was reactivated to become a normal fault, which directly controlled the western boundary of Huanghua Depression and the related fault basin in Linqing Depression. At this time, the east wall (upper wall) of Cangdong fault descends, forming depressions such as Beitang, Banqiao, Cangdong, Nanpi, Dongguang, Dezhou and guanxian from north to south. Paleogene sedimentary thickness is about 2000 ~ 6000 meters, about 2000 meters in the west. Most of the depressions are semi-graben-type, and the strike is mostly north to east NEE. Since the Neogene, Bohai Bay Basin has entered the depression stage (post-rift stage) from the Paleogene fault depression stage, and the whole basin has sunk, and a unified large-scale depression basin has developed on the Paleogene basin-ridge structure. At this time, the activity of Cangdong fault was very weak, and it was covered by Neogene and Quaternary, becoming a concealed fault.
3. Lanliao Fault
Spatial distribution characteristics of (1) fault
Lanliao fault is an important fault zone in the south of Bohai Bay Basin. It starts from Lankao, Henan Province in the south, extends to the northeast via Fanxian-Liaocheng-Yucheng-Jiyang line, and ends in Guangrao, Shandong Province. It is about 500km long and 5~ 10km wide. It is the boundary fault between Linqing Depression and Southwest Shandong Uplift, and also the first-order basin-controlling fault of Linqing Depression (Figure 3-2), such as Dongpu Depression as a whole.
(2) Fault segmentation and basin control characteristics.
There are some differences in the occurrence, development history and activity intensity of Lanliao fault in different sections, showing obvious segmentation. The Fanxian-Daming fault struck by NWW is divided into north and south parts;
The northern section is located in Fanxian-Liaocheng-Yucheng-Qihe area, about 260km long, also known as Qihe-Guangrao fault. From south to north, the strike of the fault gradually deflects eastward, showing an arc slightly protruding to the north, and the dip angle also changes from NWW to NW, and the dip angle of the fault is steep up and slow down, ranging from 24 to 77. The maximum drop of Tg layer is 5800 meters, the minimum drop is 2350 meters, the maximum horizontal tension fault distance is 4800 meters, and the minimum is 1500 meters (week, 1999). On the plane, some bands are branched, and most of them are divided into two or three branches. Generally, there are the following rules: when a single fault occurs, the cross section is steep, the upward dip angle is steep, reaching 60 ~ 70, and the fault distance of the single fault is also large; On the other hand, at the branch of the fault, the dip angle of the fault section is relatively slow, mostly 30 ~ 50, and the fault distance of each fault is not large, and the fault is stepped to form a fault zone. These fault steps can often form large fault blocks and form two steps. According to the research of Wang Xinwen et al. (1999), the activity intensity of the northern part of Lanliao fault is different in different periods. The activity intensity from Kongdian Formation to Es4 (Ek-Es4) is the highest, and the characteristics of segmented activity are obvious: Houying and Shenxian are roughly divided into north, middle and south sections, which control the formation of three and a half grabens respectively. The northern segment has the highest activity intensity, with the fault drop 1 0,000 ~ 1 0,700 m, and the fault activity rate (Vf value) is between10,865,438+0 ~ 309 m/ma, while the activity intensity in the central and southern regions is relatively low, showing the characteristics of strong in the north and weak in the south. The intensity of fault activity in the third member of Shahejie Formation is very small, and the fault drop and fault activity rate are mostly lower than 500m/Ma and 57m/Ma respectively, which basically does not control the formation of half graben and the fault is not segmented. The fault activity and segmentation of the second member of Shahejie Formation-Dongying period (Es2-Ed period) are enhanced compared with the previous period, forming three and a half grabens, but the overall activity intensity is also small; The fault distance of Ek-Es4 period is mirror image with that of Es2-Ed period, that is, the greater the fault distance of Ek-sE4 period, the smaller the fault distance of sE2-Ed period, and vice versa, which reflects the structural migration characteristics of the northern part of Lanliao fault in different periods.
Lanliao fault is distributed in the south of Fanxian-Lankao area, about 250km long. To the northeast, 15 ~ 20, tending to NWW. Its profile is steep, and the maximum total fault distance of Paleogene is 7000 m. It is a boundary fault separating Dongpu Depression and Southwest Shandong Uplift, and it is also a first-class basin-controlling fault of Dongpu Depression, which controls the formation and evolution of Dongpu Depression. The development degree of the southern part of Lanliao Fault is different in different sections and periods: the fourth phase of Shahejie Formation has begun to be strongly active, and the three semi-grabens in the north, middle and south, namely Pucheng, Qianliyuan and Gegangji, are obviously controlled (Figure 3-2). Gegangji sag is the most developed, with the maximum subsidence range near the fault of 1500m and the maximum activity rate of 338m/Ma. However, compared with the northern part of Lanliao fault in this period, the activity intensity of most sections is much less than that of the northern part (week 1999). The third stage of Shahejie Formation is the most active period in the south of Lanliao Fault, which strongly controls the sag. Therefore, this period is also the main rift period and expansion period of Dongpu sag, and the thickness of filling strata accounts for more than 60% of the thickness of Paleogene, and the distribution range of strata is also expanding rapidly. Baimiao lateral uplift forms two sedimentary centers, north and south, and the maximum vertical fault distance and activity rate of the fault can reach 4300 meters and 620 meters/year respectively. This is in sharp contrast with the characteristics of low activity intensity, non-segmented fault and basically uncontrolled by semi-graben in the northern section of Lanliao fault. The activity intensity of Shaer 2-Shaer 1 fault began to weaken, but compared with the north, the vertical fault distance and activity rate of the fault are still very large, and the maximum can reach 1777m and 355m/Ma (Wang Xinwen 1999) respectively. During this period, the difference of uplift and subsidence amplitude between semi-graben and separated semi-graben was obviously reduced, that is, the segmentation characteristics of fault system activities were obviously weakened, the characteristics of unified fault system began to appear, the number of semi-graben decreased, and the merger trend increased. Generally speaking, the activity of Es3 is the strongest, and the activity of Es2-Es 1 is relatively weak. During Dongying period, the activity intensity in the south of Lanliao fault weakened sharply or basically stopped.
Figure 3-2 Schematic Diagram of Structural Characteristics of Lanliao Fault
(3) the evolution stage of the fault
From the end of late Jurassic to early Cretaceous, left-lateral shear occurred in the eastern continental margin of China, forming the embryonic form of Lanliao fault. From the middle of Early Cretaceous to the beginning of Paleogene, the whole uplift of the fault block area in North China suffered erosion and gradually flattened. In Paleogene, pre-existing faults such as the fault zone at the eastern foot of Taihang Mountain and Lanliao fault were re-cracked, which controlled the development of a series of fault basins with different orders. The splitting strength of Lanliao fault is stronger in the north and weaker in the south.
During the third member of Shahejie Formation, the Lanliao fault has obvious difference faults, and the southern part of the fault is in a strong tensile state. At this time, the horizontal tension tensor and vertical difference of the fault fluctuate greatly. The sedimentary strata in the descending plate are 4000m thick and thin from north to south, forming several deep lakes such as Pucheng, Qianliyuan, Gegangji and Guyang, which become the main oil-generating depressions in this area. A series of underwater fans are distributed along the Lanliao fault in the depression, forming rich deep gas reservoirs (Zhang Yamin, 2000). The activity intensity in the north is very small, which basically does not control the formation of semi-graben.
In Oligocene, with the gradual decrease of regional tensile stress, the vertical difference of Lanliao fault decreased, and the lake basin shrank and became shallow. At the end of Dongying Formation, the whole area uplifted, resulting in unconformity contact between Paleogene and Neogene.
4. Taihang Mountain South-Fengpei Fault
The fault zone is located in Jiyuan-Jiaozuo-Xinxiang-Lankao-Shangqiu-Fengxian-Peixian, with a total length of about 450km. Its eastern end was cut off by the Tanlu fault, which was formerly called Jiyuan-Huangkou fault or Fengqiu-Fengpei fault. The fault zone is the dividing line between two tectonic lines in different directions on the North China Platform (attached figure 1 1). The Wei Fen Graben to the north of the dividing line, the fault at the eastern foot of Taihang Mountain, Cangdong fault and Lanliao fault are all in NE-NNE direction. These fault zones divide and control Jizhong-Tangyin basin belt, Cangxian-Neihuang uplift belt, Huanghua-Linqing-Dongpu basin belt and Shapu basin belt. The Shangdan, Mianlue suture zone and a series of Mesozoic-Cenozoic thrust nappe structures to the south of the boundary line are all in NWW direction, and the structural lines of a series of small and medium-sized basin groups such as Zhoukou, Hefu, Xinyang and Nanxiang controlled by these fault zones are also in NWW- east-west direction. Therefore, the South-Fengpei fault of Taihang Mountain is not only the dividing line between two tectonic lines in different directions of North China Platform, but also the dividing line between Bohai Bay Basin and Southwest Shandong Uplift, South North China Basin Group and Xuhuai Fault Uplift. In the final analysis, it is the dividing line between Yanshan and Qinling-Dabie orogenic belt, so it is of great theoretical and practical significance to study the characteristics and formation mechanism of faults.
Occurrence characteristics of (1) fault zone
The fault zone is generally distributed in NWW direction and consists of two main faults, north and south, and each main fault is composed of a series of faults with different occurrences. From west to east, the faults strike east-west or NEE (Taihang Mountain South Fault and Mangshan North Fault), NWW (Fengqiu Fault-Yifeng Fault or Longhusi-Shangqiu Fault), near east-west (Fengyu Fault) and NEE (Hubei Fault), and the fault zone is divided by NE-NNE and NW faults.
(2) Geophysical characteristics of fault zone
There is a geophysical variation zone centered on Zhengzhou in the geological profile from Suizhou, Hubei Province to Harqin Banner, Inner Mongolia (State Seismological Bureau, 1992) and the geological profile from Fengxian, Shanghai to Alashan, Inner Mongolia (State Seismological Bureau, 199 1). An important feature of the change zone is the uplift of Moho, with a depth of 30 kilometers at the top, which belongs to the highest point of North China continent. Moho faces down to the east, west, south and north. Another feature of the variation zone is that the top surface of the asthenosphere is arched, and the buried depth of the top surface of the asthenosphere is only 70km below the fault in the southern part of Taihang Mountain. This geophysical variation zone may be the deep reason for the formation of Nanfeng fault zone in Taihang Mountain. The seismic velocity structure and the results of magnetotelluric sounding also show that the fault at the southern foot of Taihang Mountain is a large shovel-like fault with a gentle dip to the south, and the fault reaches Moho surface, which is an important crustal fault.
(3) Formation, evolution and geodynamic mechanism of the fault zone.
At present, there is great controversy about the formation period of this fault zone. It is generally believed that there were faults near the east-west direction in this area before Paleogene, but the faults in this period were only intermittently distributed in some fault basins and did not run through the whole area. In Paleogene, a strong synsedimentary fault depression occurred and was widely distributed, which made the faults in the late Early Cretaceous reactivate and connect with each other, forming the Nanfeng sawtooth fault zone unified in Taihang Mountain. Under the control of the fault zone, Paleogene fault basins in Jiyuan, Zhongmou, Quan Min, Huangkou and Hu Si were formed. There are also a series of Paleogene extensional faults in the north of Tongguan-Lushan-Fuyang-Huaiyang fault zone, which together with Taihang Mountain South-Fengpei fault form an east-west fault zone with a width of 150km.
Nanfengpei fault in Taihang Mountain is the product of superposition of different tectonic settings. South North China and Yanshan area were dominated by north-south compressional nappe during Indosinian and early and middle Yanshan period. In Paleogene, due to the destruction of the North China Craton, a regional extensional environment was created, and a Paleogene extensional fault zone was formed with Taihang Mountain South-Fengpei fault as the northern boundary and Tongguan-Lushan-Fuyang-Bengbu fault zone as the southern boundary, with a width of 150km.
In addition, there are many large faults near the Tanlu fault, such as Mouji fault, Hayes fault and Jiashan-Xiangshui fault, and their formation and evolution also play an important role in controlling the development of Mesozoic and Cenozoic basins in the study area.
Based on the study of the geological, geophysical and active characteristics of the large-scale fault zone near the Tanlu fault zone, combined with the evolution characteristics of the thrust nappe structure in the Qinling-Dabie orogenic belt (Zhang Guowei et al., 1995), it is shown that the migration of the fault structure in the study area is very obvious in Mesozoic and Cenozoic. In the early Indosinian-Yanshan period, the strength of thrust nappe structure in Qinling-Dabie orogenic belt showed a trend of strong in the south and weak in the north, strong in the east and weak in the west. From the late Yanshan period to the early Himalayan period, it is the opposite, showing the structural characteristics of strong north and weak south, strong west and weak east; During Indosinian-Yanshan period, the faults in Yanshan orogenic belt had obvious deflection and migration laws from east to west, from northeast to northeast, and the movement of Tanlu fault zone and its control over the basin had migration laws from south to north from morning till night.