I the role of remote sensing
Remote sensing has the characteristics of macro, reality and image. For outcrop area, remote sensing image can be regarded as a special topographic and geological map reduced to a certain scale. It not only records the geological information of the visible part of the surface, but also supplements some ground information recorded in the near infrared spectrum band. Therefore, the amount of information people observe has expanded. Because of the macroscopic nature of remote sensing, it provides sufficient space for people to observe various geological phenomena, and makes some isolated or discontinuous geological phenomena get a more comprehensive understanding under certain spatial conditions. In particular, we can avoid making one-sided conclusions on some huge geological bodies, because they are beyond people's horizons in field work. The realization and visualization of remote sensing can quickly establish geological concepts in line with objective reality for people, thus improving the ability to solve geological problems.
Second, remote sensing geological characteristics along the large section
(A) the fault structure
The main manifestations of fault structures on remote sensing maps can be summarized as the following five signs:
(1) Linear (banded) anomalies of some shadow structures or tones;
(2) the linear boundary of plane (plate) tone;
(3) boundaries of different geomorphic units;
(4) Mountain dislocation or huge steep slope on the terrain;
(5) The linear combination of linear water system, spring water or inflection point of water system, and the comprehensive characteristics of the above signs. According to the above signs, the geological interpretation of1:500,000 MSS standard false color mosaic (referring to 1: 1 10,000 South 7-band image mosaic and some TM images) is carried out. Interpretation shows that five groups of linear structures are developed along the geological section (Qinzhou-Guilin section), which are NNE, NE, NW, NW and nearly EW, among which 50, 30, 60 NE and 300 NW are the most developed (see table 1 and figure 1). From the relationship analysis of linear structures, the near-EW formation developed first, followed by the NE formation, and the NE and NW formations developed later. In space, two groups of linear structures (NE and NNE) cover the southern and northern parts of the geological profile. The NE-trending formation is mainly distributed in the southern section, and the nearly EW-trending formation is mainly distributed in the middle section of the profile. Judging from the development scale of linear structures on the profile, the NE-trending formation is the highest among the five groups. For example, Fangcheng-Yongfu (2) and Ningxiang-Chongyang, two large faults that obliquely pass through central Guangxi and central Hunan with almost overlapping sections, are second to none in linear structures in the region, regardless of their extension length, scale and cutting depth. From the analysis of1:2 million southern image map, the above-mentioned two NNE linear (fault) structures are actually two sections of a huge fault zone, the southern end of which is located near Fangcheng, Guangxi, passing through Luancheng, Laibin (East), Luzhai and Yongfu to Ningxiang (East), Hunan, and then passing through Wangcheng (West), Xiangyin and Miluo. In Hunan and Hubei, it is characterized by light-colored broadband images with a bandwidth of over 40km, which controls the distribution of topography, water system and large lakes (such as Axe Lake and Liangzi Lake). According to the magnetotelluric sounding data of Dayong-Zixing, Hunan Province, the fault zone has penetrated into the upper mantle, tilted to the southeast, and the upper wall thrust westward. Combined with the linear extension characteristics of the fault on the plane, it is speculated that it is a compression-shear giant fault structure. Various fault structures are developed along the large section, among which there are as many as 8 faults with the length exceeding 100km (see Table 2).
Table 1 main fracture data table (Qinzhou-Guilin)
Figure 1 fracture azimuth histogram
Table 2 Characteristics of Large Faults
(2) Ring or block structure
The annular or blocky tone anomaly or texture structure in remote sensing images is often a comprehensive reflection of geological bodies at a certain depth on the surface or underground. According to the requirements, we have made a preliminary interpretation of the large-scale annular or massive image anomalies on the section, which are described as follows from north to south:
1. Huarong uplift
Located in the south of Jianghan Basin and north of Dongting Lake. On the 7-band map of1:2 million "Southern Elephant". It shows that the light-colored block image with a length of 80km from east to west and a width of 60km from north to south is abnormal and the boundary is blurred. During this period (Shishou-Jianli), the meandering of the Yangtze River generally developed in the east-west direction, and the abnormality of light-colored images was related to modern alluvial deposits. On the 1:500000 standard false color map, there are two obvious secondary circular images, one is in the east, which is NNE-oriented and oval (35km× 20km); One in the west is NW-trending and nearly circular (25km× 22km). Geological data show that the anomaly of the eastern oval image is composed of pre-Sinian system and Yanshanian granite intruded into it, and it is a block sandwiched between NNE faults. The fault on the east side is a part of the western margin of Ningxiang-Chongyang giant mantle-type fault zone, and Huarong uplift is located in the footwall of this large fault zone. According to the image characteristics, the secondary ring image in the west is also a fault block composed of pre-Sinian system, but it is only controlled by NW-trending faults. Therefore, from a macro point of view, Huarong Uplift is a huge normal fault block structure controlled by the intersection of two sets of faults in NE and NW directions, and its internal structure still leaves deep traces of two sets of structural lines. Moreover, it is precisely because of these two groups of large faults that the Paleozoic distribution continuity and structural integrity between Xiangxi and southeastern Hubei were destroyed.
2. Central Hunan Depression
Central Hunan Depression refers to the central part of Hunan with Shaoyang as the center. On the remote sensing map of 1:200000 TM, this area is a light-colored image area composed of multi-ring and broadband shadow structures in different directions. In the north, the dark blocky images distributed from Shuangfeng to Jin Shiqiao in the east-west direction are different from the light-colored images in northern Hunan, and generally distributed in the northeast. Geological data show that this area is a late Paleozoic sedimentary area above Caledonian fold basement, and Indosinian movement caused strong folds in Upper Paleozoic. Is the dark block image of Paleosinian? Block, which may be an ancient mountain peak in the late Paleozoic sedimentary period. According to the remote sensing data, this kind of fold is limited to the shallow fold of the upper Paleozoic, and it is only related to the macro-control of the east-west relative twist of the folded basement in Caledonian, and has no direct relationship with the local structure in the basement (Caledonian) structural layer. In other words, the Upper Paleozoic structure and Caledonian fold basement have no inheritance, and they belong to two tectonic periods. Even if some pre-Caledonian structures are similar in shape and strike to Indosinian structures, they are the result of later reconstruction.
Due to the east-west twist, a large-scale arc fold belt protruding westward has formed in central Hunan. The north and south sides of the arc fold belt are basically symmetrical, so it was once called Qiyang Mountain Structure. However, from the analysis of remote sensing images, the "backbone" as an important basis for defining gable structure has not been found. So whether this is a gable structure deserves further study. On the contrary, an east-west shear fracture with a length exceeding 160km was newly discovered at the position corresponding to the spine. The fault gradually weakens from east to west, but passes through the arc top. Obviously, this is not a gabled structural component, but a reflection of the basement fault in the caprock. From TM remote sensing map, there are two similar fault structures related to arc structural belt: ① Loudi-Xinning fault; It has obvious light-colored linear characteristics, extending along the direction of NE45 and passing through the south side of Shaoyang City. It controls the landform and part of the water system, and the southern end becomes the boundary of Cretaceous intermountain sedimentary basin, with a total length of more than 200 kilometers. ② lengshuitan-Liuduzhai fault; It has obvious characteristics of light-colored banded image, strike is about NW320, the southeast end is consistent with Devonian-Carboniferous fold axis, and the northwest section is obliquely inserted into Yunfeng Mountain, but the intensity is obviously lower than that of the southeast section, which has turned into a dark linear image with a total length of 160km. Parallel faults also appear in the southwest of Che 'an area. It is speculated that the above-mentioned arc structural belts may be related to the macro-control of these two groups of basement faults.
3. Central Guangxi Depression
Guizhong Depression refers to the contiguous distribution area of Upper Paleozoic in the northern part of Binyang-Guixian and the southern part of Liucheng-Luocheng, which is a large block syncline in structure. Seen from the remote sensing map, this area is a huge composite ring (block) image. The ring (block) image is composed of light stripe shadow structure. The ring body consists of two secondary ring images with orange peel shadow structure. The large-scale ring image is peach-shaped and rectangular, with the long axis running about 170km and the short axis 135km, which is temporarily called Guangxi Central. Carboniferous-Permian exposed on the ground is a wide syncline or a large block syncline with far-reaching geological background, which is the reflection of the large block dominated by negative movement in Caledonian period of pre-Devonian. The secondary circular image is rectangular, arranged east and west, and runs north and south. The west is bigger than the east. The annular body of Heshan in the west, with the major axis 120km and the minor axis 60km, is a low anticline zone in a large massive syncline, which tends to tilt from north to south. Upper Devonian limestone is exposed at the core of the northern anticline structure. The eastern part is called Xiangzhou Ring, with a major axis of 95km and a minor axis of 30km. The middle and lower Devonian sand and shale are exposed on the ground, so the image is light. The ring body inclines from southeast to northwest, and the uplift amplitude is higher than that of Heshan block. There is a north-south compressive zone between the two sub-blocks, which is located on the Liuzhou-Laibin line, that is, the backbone of Guangxi mountain structure. The mountain structure in Guangxi is well known, and its forearc is very eye-catching on the remote sensing map. Characterized by light-colored zones, it is distributed in Youjiang, Nanning, Yining, Hengxian, yujiang county, Lifu and other places. It is an annular negative landform, and most of it is within the observation range of the profile, which constitutes the outer ring of the above-mentioned block in central Guangxi. Between the inner and outer rings are some short-axis folds of different sizes, lower Paleozoic blocks and a small amount of rock masses. Obviously, the formation of the mountain structure in Guangxi is inseparable from the southward movement of the central Guangxi block, or because of the strong southward push of the central Guangxi block, a huge mountain structure system in Guangxi has been formed. Therefore, it is considered that the Heshan block and Xiangzhou block located in the middle of Guangxi are inseparable parts of the gable structure system. Zeheshan block is the west shield of gable structure, and Xiangzhou block is the east shield of gable structure, both of which are relatively stable parts of gable structure. It should be noted that there is a small but famous Yishan-like structure on the west side of the tail of Guangxi-like structural system. From the analysis of remote sensing image characteristics, Yishan mountain structure was formed in the late stage of the development of Guangxi mountain structure or after its formation. From the structural analysis of the diagram, it is quite characteristic that "the back waves push the front waves". Therefore, there may be some imbricate thrust faults or nappe structures with different sizes in the forearcs of mountainous and Yishan structures in Guangxi, but this needs to be verified and determined in field work.
4. Qinzhou sag
Qinzhou sag is a Mesozoic sedimentary basin controlled by NE-trending faults on the background of Lower Paleozoic folds, so the distribution direction of the basin is strictly controlled by faults. Because it is close to the bay, it is inevitably affected by transgression and sedimentation, so it appears as a light image on the remote sensing map. Combined with regional geological data, the NE-trending faults controlling Mesozoic sedimentation are mainly shear. Macroscopically, the shear fault is an integral part of the southwest section of Qinzhou-Lingshan giant northeast shear fault zone, which has the characteristics of left-handed torsion.
Three. East-west structural belt
From the analysis of remote sensing image characteristics along the large section, the large section spans at least three east-west structural zones:
(1) Shuangfeng-Jin Shiqiao east-west structural belt: the transverse wall is located in the middle of Hunan, which is characterized by the dome anticline of Sinian system (probably the ancient peak of late Paleozoic) and the right oblique line invaded by granite in the early Yanshan period, and divides the central Hunan depression into two parts.
(2) The east-west structural belt of Luzhai-Liu Shan-Yishan: it is distributed in northern Guangxi and can extend eastward to Lifu area. It consists of upper Paleozoic fold belt and east-west fault. Due to the influence of complex block structure, this east-west structural belt extends in waves. The western section is a front arc with a mountain structure. The south side of this belt is the distribution area of giant structures in central Guangxi.
(3) Guixian-Nanning east-west structural belt: distributed in southern Guangxi. It consists of lower Paleozoic folds, faults and Mesozoic granite intrusions. The western section may extend to the north of Vietnam, while the eastern section is cut by the Qinzhou-Lingshan northeast shear fault zone, which belongs to a part of Nanling east-west structural belt. It obviously controls the fold structure of Upper Paleozoic.
Four. Problems and suggestions
This paper is a shallow discussion on the macro-geological characteristics along the large section. Due to the lack of complete remote sensing mosaic along the profile, and the poor quality of MSS remote sensing image (printed map compiled by Guangxi Bureau of Surveying and Mapping) in the southern section (resources-south of Qinzhou), the information provided is not comprehensive and clear, and the interpretation results are only for reference in comprehensive geological research of large profile. For the further study of geological characteristics of large section, it is suggested to use TM remote sensing data with high ground resolution to make mosaic map of 1: 1 10,000, and then make systematic geological interpretation, or at least try it in key areas. Submit it to the relevant parties for consideration.
refer to
Li Siguang. Introduction to geomechanics. Beijing: Science Press, 1973.