Characteristics of tectonic stress field in each period

According to the division of structural stages in Hunan, the regional stress field can be divided into four structural stages: Wuling-Xuefeng stage, Caledonian stage, Hercynian-Indosinian stage and Yanshan-Himalayan stage.

Characteristics of stress field in Wuling-Xuefeng period: There are mainly two groups of fault systems in Wuling-Xuefeng period in Hunan, which are nearly east-west and northeast (as shown in Figure 4-3). Among them, the nearly east-west Changsha-Taojiang fault is connected with the northeast Xupu-Sanjiang fault, which becomes an important structural boundary in the region, controlling the distribution of two east-west volcanic rocks in Mesoproterozoic and Neoproterozoic, as well as the development of lithofacies paleogeographic characteristics and sedimentary assemblage. The NE-trending Yongshun-Sangzhi, Dayong-Cili, Xupu-Sanjiang and Xinhua-Longsheng faults make the central and western parts of this area form an alternating pattern of horst and graben. The NNE-trending Liling-Chenzhou fault was mainly active in Paleozoic and 82 subsequent periods, but it was produced in Mesoproterozoic.

Figure 4-3 Main Structures and Direction of Maximum Principal Stress in Wuling-Xuefeng Period

Deep fault:1-Sangzhi-Yongshun; 2- Dayong-Phoenix; 3- Xupu-Sanjiang; 4- Xinhua-Longsheng; 5- Jidong-dual card; 6- Liling-Chenzhou; 7- Changsha-Taojiang; 8— Xinhua—Shuangpai fold belt: 9— Shimen—Huarong—Linxiang; 10-Anhua-Ningxiang-Liuyang; 11-baimashan-Longshan-Liling; 12— Huayuan—Dayong; 13-Anhua-Qianyang-Huitong; 14— Pingjiang—Liuyang

Characteristics of Caledonian stress field: In Caledonian, including Sinian, with the clutch between Yangtze block and Cathaysian block, the tectonic stress changed from tensile stress to compressive stress. That is, since Sinian, the Yangtze block and Cathaysian block have been dispersed, and the tectonic stress in this area has changed from compressive stress to tensile stress. From Middle Ordovician to the end of Silurian, it turned into compression; At the same time, the blocking effect of the Yangtze rigid block induced a series of NW-trending and NW-trending structures. The general characteristics of Caledonian tectonic line direction are shown in Figure 4-4.

Characteristics of Hercynian-Indosinian stress field: After Caledonian movement, the southeastern margin of the Yangtze block matured and merged with the South China fold belt to form the southern plate, so the Hercynian-Indosinian structure in Hunan was the product of intraplate activities.

Figure 4-4 Schematic Diagram of Main Structures and Maximum Principal Stress Direction in Hercynian-Indosinian Period

Deep faults: ① Sangzhi-Yongshun; 2 Dayong-Phoenix; ③ Xupu-Sanjiang; ④ Xinning-Wangcheng; 5 dual card-Hengyang; ⑥ Chenzhou-Chaling; 7 Xinhua-Shuangfeng; 8 Lingling-Chenzhou; Fold belt: 1- Shimen-Huarong-Linxiang fold belt; 2- Anhua-Ningxiang-Liuyang fold fault zone; 3- baimashan-Longshan-Liling fold fault zone; 4- Nanling-uplift fold fault zone; 5- Xinning sag belt; 6- Dupanling uplift belt; 7- Daoxian sag belt; 8- Changning-Linwu sag belt; 9- Yanling-Rucheng uplift belt; 10- Chenxi-Jingxian fold depression belt; 1 1- Xianxi Xibu fold depression belt; 12- Liuyang-Hengshan fold depression belt; 13- huangfengqiao-anren fold depression belt; 14- Ningyuan-Jiangyong fold depression belt; 15- Zixing-Rucheng fold belt

The early tectonic movement in Hunan Province was influenced by the Paleo-Tethys rift, and the anticlockwise rotation of the Yangtze block led to the extensional rift activity in the junction zone between the Yangtze plate and the Cathaysian plate, forming an extensional rift basin belt. By the Indosinian period, due to the southward movement of the North China plate and the northward movement and compression of the Indosinian block, the compressive stress in the north-south direction continued to act. At the same time, due to the compression of the paleo-Pacific plate from the southeast, the north-south tectonic line was formed, and the pre-tectonic activities in the northeast and northwest continued, which made the Indosinian paleo-structures in Hercynian show a complex pattern of mutual interleaving, as shown in Figure 4-5. During Hercynian-Indosinian period, the structures with strong activity and basic stereotypes are as follows:

(1) Near east-west structure: Shimen-Huarong Linxiang fold belt, Anhua-Ningxiang-Liuyang fold belt and baimashan-Longshan-Liling fold belt.

(2) Near North-South structure: Xinning sag belt, Daoxian sag belt, Changning-Linwu sag belt and Yanling-Rucheng uplift belt.

(3) NE-trending structures: Chenxi-Jingxian fold depression, Jingxian-Chengbu fold depression, Liuyang-Hengshan fold depression, Huang Feng Bridge-Anren fold depression and Zixing.

-Rucheng fold depression belt, etc.

Figure 4-5 Principal stress diagram of Indosinian period

(4) NW-trending structures: Except Xinhua-Shuangfeng and Lingling-Chenzhou faults, other structures are all over the region, and faults are the main ones.

Wan Tianfeng (1993) made use of the 21:200,000 map of eastern China, and obtained quantitative analysis by measuring the folds and joints of the Triassic and its underlying rock series in Indosinian, and obtained the characteristics of the Indosinian tectonic stress field. The results of 1787 times show that the maximum principal stress σ 1. According to the 12 map, 48 anticlines and 54 synclines were measured in Hunan, and the principal stresses σ 1 = 176 ∠ 7, σ 2 = 88 ∠ 4 and σ 3 = 356 ∠ 83 were calculated, as shown in the figure. Among them, σ 1 stress is the main force from the south, which is equivalent to the compressive force of Indosinian plate, and σ2 is the main force from the east, which is equivalent to the compressive force of ancient Pacific plate.

Characteristics of Yanshan-Himalayan stress field: The Yanshan-Himalayan tectonic stress field inherited the previous characteristics, but changed to some extent. During Yanshan period, the crustal movement was strong, and the main stress came from the southeast, which made almost the whole area of Hunan fold and uplift, accompanied by strong multi-stage magmatic activity, and the early stage was characterized by uplift, depression and large faults. The late stage is dominated by large faults. The NE fold belt continued to move, and the activity of the east-west tectonic-magmatic active belt in Nanling reached a climax, forming many NE fold belts such as Sangzhi-Shimen fold belt and Yuanling-Mayang depression belt. During Himalayan period, the main tectonic stress was north-south, which made the crust uplift, tilt and fold continuously (Table 4-3).

Table 4-3 Analysis Results of Structural Stress Direction during Yanshan-Himalayan Period in Hunan Province

(According to Wan Tianfeng's data, 1993)

As can be seen from the table, the maximum principal stress in the Yanshanian period in Hunan is in the southeast direction (120 ∠ 20) and gradually changes to the south-west direction (2 13 ∠ 2). The direction of principal stress in Himalayan period is nearly north-south (4 ∠ 2); After Miocene (< < 0.7Ma), it entered a new period of tectonic activity, and finally formed a generally wide depression and uplift in Hunan, as well as a horseshoe-shaped basin surrounded by mountains in the east, west and south, and facing water in the north.

Through the analysis of the action mode and direction of the corresponding stress field, it can be known that the tectonic system formed by crustal movement during the Mesoproterozoic Wuling tectonic period and the Neoproterozoic Xuefeng tectonic period is mainly east-west structure, followed by northeast and northwest structures, reflecting that the regional tectonic stress field at that time was north-south compression; During the Caledonian tectonic period of early Paleozoic, there were great differences in crustal balance, tectonic development and boundary conditions. The crustal stress in the north-south direction is relatively weakened, and the compression in the north-south direction and the southeast direction is intensified, showing the uplift in the east-west direction. At the same time, a series of northwest structures have appeared in the southeast of Hunan and the border areas of Hunan and Jiangxi. During the Hercynian-Indosinian tectonic period from late Paleozoic to early Mesozoic, the compressive stress in the north-south direction continued, and the compressive stress in the east-west direction appeared at the same time. The former produced Weishan-Hengshan structural belt, while the latter produced a series of north-south structures.

In the early stage of Mesozoic Yanshan structure, the mode and direction of regional tectonic stress changed greatly, and the compressive stress in the north-south direction increased, especially in the central and eastern Hunan. The compressive stress in the east-west direction is transformed into the compressive stress in the northwest-southeast direction, and at the same time, the right-hand torsion in the same direction appears, which makes some structures in the near east-west direction move to the right. During the late Yanshanian-Himalayan tectonic period, the regional tectonic stress field basically inherited the previous model and direction, but the compressive stress in the southeast-northwest direction dominated by the former twisted the later structure to the right, which may be related to the northwest compression and subduction of the Pacific plate. At the same time, a series of NW-trending structural features were formed under the compressive stress of NNE-SW-SW, which may be related to the continental crust collision between Indian plate and Asian plate. To sum up, the way and direction of crustal movement in the province are mainly north-south and east-west pushing or twisting, and other directions and modes of movement are basically induced by this.