1. Division of structural units
Based on the analysis of gravity and magnetic anomalies, earthquakes and regional geological data, the structural units of Jiaolai Basin are re-divided into eight secondary structural units, namely Gaomi sag, Zhucheng sag, Laiyang sag, Haiyang sag, Chonglou-Shichang sag, Mouji structural belt, Chaigou uplift and Dayetou uplift. Each structural unit can be divided into several secondary structural units (Figure 3-23).
Figure 3-23 Division of Structural Units in Jiaolai Basin
2. Basic structural characteristics and fault system
Jiaolai basin is a pull-apart fault basin, and its formation and development are controlled by fault activities. The NNE-trending Tanlu fault zone and the NE-trending Wurong fault zone are the main control faults that constitute the basin boundary. The Mouji fault in the basin has caused a structural pattern of east-west division, and the structural evolution of the two areas is obviously different from the present form. The activities of wulong village fault, Pingdu fault, Jiaoxian fault and Baichihe fault in the east-west direction divide the basin into two parts, and the mutual cutting of each group of faults in the basin leads to structural superposition.
Basic structural characteristics of (1) basin
In the vertical direction, Jiaolai basin has two different structural characteristics: basin basement and caprock. The former has strong fold deformation, while the latter is dominated by fault blocks. Fault activity caused the basement to rise and fall greatly, and Cretaceous massive sediments were filled in the half graben and graben inside the basin as caprocks, which was more obvious in the later stage of basin development.
Jiaolai basin is characterized by complex structure and large basement fluctuation. With Mouji fault as the boundary, the basement in the east of the basin is shallow and the basement in the west is deep. The basement in Chaigou and Dayetou areas in the west of the basin is shallow, in which the basement rock series in Dayetou area has been exposed to the surface in a large area, and the basement in Laiyang, Gaomi and Zhucheng areas is deep, with the height difference of basement fluctuation reaching 8000 meters.
Fault block structure is the main structural type in Jiaolai basin. Faults in different levels and directions in the basin development stage and post-basin stage make the basin caprock form many fault blocks with different shapes and scales. The Cretaceous joint structure is also quite developed.
According to the stratigraphic distribution characteristics of Laiyang Formation, Qingshan Formation and Wangshi Formation, the basic structural framework of Jiaolai Basin in Laiyang Period, Qingshan Period and Wangshi Period is different. The main controlled faults in Laiyang basin are Tanlu fault, Wurong fault and Mouji fault, and there are two subsidence centers, which are located in Zhucheng area and Laiyang area respectively. Qingshan basin is mainly controlled by Mouji fault. Wulong village fault, Pingdu fault, Jiaoxian fault, Baichihe fault and so on developed in the basin controlled by the Wang Dynasty, and the basin was characterized by a combination of graben, half graben and horst (Figure 3-24).
(2) Fault system and its relationship with the Tan-Lu fault zone.
1) fault strike and sequence. The fault structures in Jiaolai Basin and its surrounding areas are well developed (Figure 3-25). According to the geophysical characteristics and scale of faults and their control on tectonic units, faults can be divided into four grades:
Figure 3-24 Regional Structural Profile of Jiaolai Basin
Figure 3-25 Distribution Map of Fault Structures in Jiaolai Basin and Its Surrounding Areas
First degree fracture. Including Tanlu fault (Yishu fault), Wurong fault and Mouji fault, the scale is huge, which constitutes the boundary of the basin and controls the formation and evolution of the basin.
Tanlu fault zone: located in the west of Jiaolai basin, also known as Yishu fault zone, it consists of four main faults, and the Changyi-Dadian fault in the east constitutes the western boundary of Jiaolai basin. In Jingzhi area near Changyi-Dadian fault zone, the bottom of Laiyang Formation is exposed from the ground, with huge angular gravel, which is composed of Proterozoic metamorphic rocks, characterized by marginal facies and controlled by Yishu fault zone. Zhucheng is the subsidence center of Jiaolai basin, adjacent to Yishu fault, which is the main control fault developed in Jiaolai basin.
Wurong fault: it is the boundary fault between Jiaobei block and Jiaonan block, and the strike is NEE-SWW. On the aeromagnetic anomaly map, it shows a dense gradient zone with obvious segmentation. Wurong fault, which controls the basin, is a steep dip fault to the north, with a dip angle of >: 60 and an east-west extension of 140km. The vertical fault distance of the upper wall of the fault is 1~2km, which controls the deposition of Laiyang formation in the basin in the early Cretaceous.
Mouji fault: strike NE, mainly composed of four nearly parallel normal translation faults arranged at equal intervals. The fault passes through Jiaolai basin and divides it into two parts: east and west. The fault was dominated by extension in the early Cretaceous, and left-lateral translation and compression in the late Cretaceous and beyond. The early Cretaceous Laiyang period was obviously extensional, which controlled the deposition of Laiyang Formation in the northern part of Laiyang. During the Qingshan period, the tensile activity increased and the cutting depth increased, which led to magma upwelling and volcanic eruption. Mouyi fault has a large-scale left-lateral translation activity, which dislocates magmatic rocks and faults formed in the early stage, with a horizontal sliding distance of about 60km. At the same time of sinistral translation, the fault zone was extruded and uplifted, and did not accept deposition, and developed a compressive deformation structure characterized by compression-torsion fracture.
Secondary fracture. The secondary fault is a large fault in the basin, which obviously controls the development and evolution of the secondary structural units in the basin and is the boundary fault of the secondary structural units. The strike of the secondary fault is a nearly east-west normal fault, which is controlled by the main fault and is active in multiple stages, mainly in the sedimentary period of Wang Formation.
Pingdu fault: developed in Pingdu area, extending from east to west, about 30 kilometers long. The section is inclined to the south, the upper part is steep and the lower part is shovel-shaped. Pingdu fault constitutes the northern boundary of Jiaolai Basin, bordering Jiaodong Group in the north and Huangxian Formation in the Cretaceous and Paleogene in the south, which mainly controls the deposition of late Cretaceous Wangshi Formation and Laiyang Formation in Pingdu Depression to some extent.
Wulong village Fault: Located in the southern margin of Laiyang Depression, it extends from east to west, and its profile is inclined to the north with an inclination of 60 ~ 70. It is a high-angle normal fault with a plow shape with steep top and gentle bottom. Laiyang sag in the north and Dayetou uplift in the south. The main active period of this fault is Wangshi period, which controls the development of Wangshi Formation in Laiyang Depression.
Jiaoxian fault: located in the north of Chaigou horst, it extends nearly east-west, with a length of 80km, and its main section is inclined to the north, which is a normal fault. The Jiaoxian fault began to move in the late sedimentary period of Laiyang Formation, which led to the separation of Chaigou horst and Gaomi sag, and the activity was strong in the King Period.
Baichihe fault: developed in the northern margin of Zhucheng sag, it extends nearly east-west, with a length of 76 kilometers. The section is inclined to the south and is plow-shaped, with a maximum vertical fault distance of more than 8,000 meters. The fault started to be active in the late Laiyang Formation, which controlled the Laiyang period, and the Wangshi period was its intense active period.
Three-level and four-level faults. The scale of faults No.3 and No.4 in the basin is relatively small, and they are controlled by faults No.1 and No.2, which obviously control the structural zones and depressions in the sag. There are not many such faults exposed on the surface, and nearly 100 faults have been found, mainly concentrated in Gaomi sag and Mouji fault zone, and the main strike is NW, ne, NNE and near EW.
Statistical characteristics of fault strike. Through the statistics of four types of faults, the basic laws are as follows:
The first type of regional large faults are mainly NNE (20 ~ 30) and belong to the Tancheng-Lujiang fault system, which reflects the dominant position of the Tancheng-Lujiang fault system in this area.
The second type of faults has the largest number, in which the dominant direction is NNE ~ NE(20 ~ 45°), which also belongs to the Tan-Lu fault system, and the secondary dominant direction is NW ~ SE(300 ~ 330°), which is basically perpendicular to the direction of the Tan-Lu fault system.
The third type of fault is determined by gravity and magnetic data interpretation, and has three dominant directions, namely NW-SE direction, NE-SW direction and EW direction. NW-SE and NE-SW faults are almost orthogonal, and these two groups of faults are consistent with the second kind of secondary faults.
The fourth type of fault established by seismic profile also has three dominant directions, namely NW-SE, WE and NE-SW, among which NW-SE and WE are the main directions, and NE-SE fault is not well developed.
Law of fault activity and its relationship with Tanlu fault. Statistical analysis of fault strike shows that Jiaolai basin is controlled by three groups of fault structures, and these three groups of faults are obviously controlled by regional large faults.
A. the intersecting relationship between the Tancheng-Lujiang fault zone and the Jiaolai basin fault.
The west boundary of Jiaolai basin is Tanlu fault zone (Yishu fault zone), namely Changyi-Dadian fault and Anqiu-Juxian fault. These two faults are generally regarded as the dividing line between the eastern Shandong block and the western Shandong block, and also control the Yishu rift. From Figure 3-25, we can clearly see the mutual cutting relationship between faults in the western boundary of Jiaolai Basin. In Wulian-Juxian area, the westward inclined Changyi-Dadian fault cut off the volcanic rocks of Qingshan Formation in the southwest margin of the Chonglou-Shichang sag and Zhucheng sag, and at the same time controlled the deposition of Sheng Da Formation in the Anqiu-Juxian rift zone on the upper wall of the fault. However, in Hemengtuan area in the west of Zhucheng, the fault was covered by the laterite cliff section of the late Cretaceous Wang Formation and was cut by the fault in the east-west direction. Jiayue fault is a normal fault inclined to the south, which crosses Anqiu-Juxian Rift Zone and controls the half-graben fault basin in laterite cliff section. According to the field investigation, the jiayue fault extends westward, without crossing the Anqiu-Juxian fault, and ends at the east side of the fault zone.
The above-mentioned fault intersection relationship and its controlled stratigraphic age provide an important basis for the evolution sequence of Jiaolai basin and Tanlu fault zone. Through analysis, Anqiu-Juxian fault may be the most important boundary fault with a long active period, which became the western boundary of Zhucheng sag and Zhonglou-Shichang sag during the sedimentary period of Laiyang Formation in the early Cretaceous. In the Late Cretaceous, many dextral strike-slip activities of faults limited the activity range of east-west faults to jiayue. The active period of Changyi-Dadian fault was mainly in the sedimentary period of Sheng Da Formation in the late Early Cretaceous, with normal faults as the main fault, which became the western boundary fault of Anqiu-Juxian rift zone. The activity of the Baichihe fault in the northern margin of Zhucheng was relatively late, mainly in the Late Cretaceous.
B. Intersection between Mouji fault zone and Jiaolai basin fault.
The Mouji fault zone consists of several NNE parallel faults, the most important of which are Taocun-Dongdoushan fault, Guo Cheng fault, Jimo fault, Zhu Wu fault and Qingdao fault. Taocun-Dongdoushan fault is a typical left-lateral strike-slip fault. Strike-slip movement caused the left-lateral dislocation of Yashan and Yuangezhuang plutons in the late Yanshan period of about 25km, and the early Cretaceous eastward slip movement controlled the western boundary of Guo Cheng sag. The available dating data show that the zircon U-Pb age of Yashan pluton is 138 ~ 158 Ma, and the Rb-Sr age is 164Ma, which is consistent with the intrusive age of Linglong granite pluton in Jiaobei. According to the age of the rock mass, the sinistral strike-slip activity of the Taocun-Dongdoushan fault is later than the intrusion age of the rock mass, but earlier than the formation age of the Guo Cheng sag, which may be between 150 ~ 140Ma. The Taocun-Dongdoushan fault extends southwest and connects with the east-west wulong village fault that controls the southern boundary of Laiyang Depression, forming a strike-slip-extension fault combination. The active period of the fault combination mainly occurred in the sedimentary period of the late Cretaceous Wang Formation, and the Taocun-Dongdoushan fault has the characteristics of dextral strike-slip movement.
Guo Cheng fault forms the eastern boundary of Guo Cheng sag, extending northward to join Taocun-Dongdoushan fault, and extending southward may be connected with Jimo fault. Zhuwu fault and Qingdao fault are major boundary faults, which together form the east boundary of the northeast Jiaonan-Weihai orogenic belt. From the perspective of basin control, Zhuwu fault and Qingdao fault are important basin control faults, which control the formation and development of Haiyang sag.
Although the above-mentioned NNE faults inherit the fault morphology of the basement orogenic belt, they all belong to the Tan-Lu fault system in the basin-forming stage, and their southward extension may be related to the NNE Rizhao fault zone in the oblique Jiaonan uplift belt. As can be seen from Figure 3-25, Rizhao fault zone cut off Wulian fault at the southern boundary of Zhucheng sag, indicating left-lateral dislocation, but stopped at the east-west strike-slip fault, indicating that the left-lateral strike-slip movement of the fault occurred between the early Cretaceous and the late Cretaceous. Combined with the sinistral strike-slip movement of the Taocun-Dongdoushan fault zone, we infer that there have been at least two important sinistral strike-slip movements in the Mouyi fault zone: the first occurred before the formation of the Jiaolai basin, and the second occurred between the early Cretaceous and the late Cretaceous.
3. Tectonic evolution of Jiaolai basin
Based on the sedimentary filling history, tectonic stress field evolution and magmatic activity of Jiaolai basin, Jiaolai basin experienced the temporal and spatial superposition of Laiyang fault basin, Qingshan volcanic basin and Late Cretaceous-Paleocene fault basin in early Cretaceous, and the prototype characteristics of the basin formed in each period were different. The Tan-Lu fault zone has an important control or influence on the formation of basin prototypes in each period and their later transformation.
By systematically comparing the sedimentary characteristics, basin-forming stages, main tectonic events and magmatic activity sequences of Yishu rift system, Jiaolai basin and North Yellow Sea basin, the evolution history of Yishu rift system and Jiaolai basin is divided into three basin-forming stages.
(1) Early Cretaceous Laiyang extensional fault depression stage
During the sedimentary period of Laiyang Formation, the distribution and sedimentary characteristics of Jiaolai prototype basin were controversial. This study is based on a comprehensive analysis of the characteristics of stratigraphic deposition, lithofacies palaeogeography, basin structure and deep geophysical field in Jiaolai Basin. It is considered that the prototype basin of Laiyang period is composed of two NNE-trending fault basins, which are called Juxian-Zhucheng fault basin and Zhuwu-Jimo fault basin respectively. The so-called fault-depression trough means that these faults are characterized by long extension and narrow fault-depression, and the profile shape is extremely asymmetric, which is controlled by the basement boundary fault. Juxian-Zhucheng fault depression trough is composed of Chonglou-Shichang fault depression, Juxian fault depression and Zhucheng fault depression, and its western boundary is located in the middle of Anqiu-Juxian fault zone and developed along the northern edge of Jiaonan uplift belt. Zhuwu-Jimo fault depression consists of several independent or semi-independent faults, such as Zhuwu fault depression, Facheng fault depression, Huangyadi-Zhongjing fault depression, Jimo fault depression and so on. It mainly develops along the Mouyi fault zone. The main bodies of these two fault basins are controlled by the faults in the northern margin of Sulu orogenic belt, and are currently concealed in the deep parts of these two fault basins. For example, the Baichihe fault on the northern boundary of Zhucheng sag is the surface extension of this boundary fault. A remarkable feature of the fault depression stage in this period is that volcanic eruption along Juxian-Zhucheng fault depression trough and Zhuwu-Jimo fault depression trough is very active, which constitutes the center of Laiyang volcanic cycle. Strong intermediate-basic and intermediate-acid volcanic eruptions occurred in some areas along the fault trough belt. The Longwangzhuang Formation in Haiyang Depression has developed multilayer volcanic eruption rocks, and a set of extremely thick intermediate-acid volcanic eruption rocks has developed in the Chonglou-Shichang Depression and the upper part of Laiyang Formation (Chengshanhou Formation). According to the sedimentary-volcanic activity characteristics of the fault basin, the fault depression in this period belongs to passive extension, and the main extension direction is NW-SE direction. Extension is mainly related to stress relaxation and structural collapse after collision in Sulu orogenic belt.
(2) Early Cretaceous Qingshan continental rifting stage.
From the middle and late Early Cretaceous, the paleo-Pacific plate went deep into the East Asian continent, and the dynamic process of the lithosphere in eastern China changed greatly, and the lithosphere thinned and strengthened, and the edge of the East Asian continent entered the stage of intracontinental rift evolution. Yishu rift system was formed in Shandong section of Tanlu fault zone, and Jiaolai basin formed in the early stage evolved into a volcanic basin, with volcanic eruption centers located in Juxian-Zhucheng fault depression and Zhuwu-Jimo fault depression. This rifting process can be divided into two stages: in the early stage, volcanic basins were developed marked by large-scale intermediate-acid volcanic eruption; In the late stage, large-scale volcanic activity stopped, mainly extensional fault depression. Yishu fault basin began to form and develop, and a set of poorly sorted and rounded glutenite was deposited in the fault basin, which contained a large number of volcanic eruption rocks. In Jiaonan-Weihai orogenic belt, large-scale complex granitoids (Laoshan granitoids with isotopic age of 123~ 1 13Ma) invaded one after another. There are not only granite intrusions in Jiaobei Uplift, but also a group of faults and fracture planes parallel to the Tanlu fault zone. Different types of dikes develop along the faults and fracture planes, forming a dense dike zone (isotopic age: 126~ 120Ma), which has become an enrichment area of gold deposits in Jiaodong area.
(3) The development stage of the late Cretaceous-Paleocene cable-stayed fault depression.
In the Late Cretaceous, the regional tectonic stress field changed greatly, mainly extending in the north-south direction. On the basis of early Cretaceous fault basin and volcanic basin, the dustpan fault basin was superimposed, which controlled the deposition of late Cretaceous Wangshi Formation and Paleocene Jingankou Formation. The fault basins formed in this period are mainly concentrated between Yishu fault zone and Mouji fault zone, that is, the outline of Jiaolai basin is basically formed now. Along the Yishu fault zone itself, pull-apart fault basins are developed in some areas, such as Weibei sag and Laizhou Bay sag. Small pull-apart basins are also developed along the Mouyi fault zone, such as the Guo Cheng sag and the fault depression zone in the middle of the North Yellow Sea. It is worth pointing out that there is no large-scale granitic magma intrusion in this development stage of the fault basin, but there is intermediate-basic volcanic activity. Volcanic rocks are dominated by basalt and olivine basalt (isotopic age is 73Ma), and there are kimberlites in western Shandong, indicating that strike-slip extension affected the upper mantle during this period.
During the Cretaceous extension, alkaline granite magma intrusion was accompanied by volcanism. Granite intrusion is mainly concentrated in the early Cretaceous, and there are mainly two magmatic intrusion periods: guojialing granite intrusion period, with the intrusion age of 130~ 126Ma, which is the same as the extension period of Laiyang period in the early Cretaceous; Granite intrusive rocks in Laoshan period, intrusion age 123~ 1 13Ma. These granites are alkaline, anhydrous and non-orogenic, and were remelted with crustal materials in the continental rift stage of Qingshan period in the early Cretaceous. Volcanic activity runs through the whole Cretaceous extension history, in which bimodal volcanism is the strongest in the middle and late Early Cretaceous, and the age is concentrated in 123~ 108Ma, which is the peak of continental rifting. However, in the Late Cretaceous, basic and ultrabasic rocks dominated, and the intensity of volcanic activity was small, with an age of 73Ma.