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This area is located in the distribution area of sedimentary strata in North China. There are old strata exposed in Taihang Mountain area in the west, covered by Paleogene, Neogene and Quaternary in the east, and a small area of Cambrian strata exposed in the southeast corner. Magmatic activities are developed in this area, and the rock masses can be divided into intermediate metamorphic rocks and basic-ultrabasic rocks.
Fig. 1-2-7 schematic diagram of apparent resistivity isoline (ω m) when electric sounding AB/2=7m.
(According to Ma, 1987)
Figure 1-2-8 Schematic Diagram of Structural Distribution in Northern Henan Province
(According to Li Xianzhi 1986)
Figure 1-2-9 Aeromagnetic Anomaly and Predicted Fault Structure Map of Anyang City
(According to Li Xianzhi 1986)
The regional tectonic outline is mainly controlled by the Neocathaysian tectonic system, including Taihang Mountain Uplift, Tangyin Graben, hua county Uplift and Linqing Depression from west to east (Figure 1-2-8). Anyang City is located in the northern section of Tangyin Graben. The structural form is dominated by faults, with simple folds and gentle stratigraphic occurrence, belonging to the anticlinorium of Taihang Mountain. Fault structures are mainly divided into two groups: NNE and NW, followed by EW and NS faults.
Due to different geophysical and geochemical parameters, different geophysical and geochemical methods reflect different geological factors and faults. The interpretation and inference results of various geophysical and geochemical data obtained are as follows.
1. Interpretation result of aeromagnetic anomaly
The regional aeromagnetic anomalies in this area mainly reflect the fluctuation of crystalline basement, the negative anomalies in Anyang-Tangyin area reflect basement depression, and the positive anomalies in the northern area reflect basement uplift. The local anomaly of alternating positive and negative in the southeast reflects the dislocation of the crystalline basement, and the NNE and NWW faults on the edge of Luxi uplift can be inferred by using the gradient zone of the local anomaly (Figure 1-2-9).
2. Interpretation results of gravity anomalies
The high gravity in this area reflects the bedrock uplift in the lower part of Neogene and Paleogene, and the low gravity reflects the bedrock depression in the lower part of Neogene and Paleogene. The synchronous distortion of gravity gradient zone and abnormal isoline reflects the existence of fault. A series of NNE and NW faults can be inferred from the characteristics of gravity anomalies. Through inversion calculation, the fault distance of stratum, the fluctuation of bedrock interface and the occurrence of fault can be obtained (Figure 1-2- 10).
Figure 1-2- 10 Isogram of Bouguer gravity anomaly in Anyang City and its inferred fault structure map.
(According to Li Xianzhi 1986)
The unit is10g.u.
3. Seismic exploration results
According to the existing seismic exploration profile data, NNE and NW faults can be drawn in this area. Seismic exploration is a more accurate and reliable geophysical method, which can be used to test the effect of other methods.
4. Interpretation results of satellite films
Using the hue of satellite photos, especially the hue changes of different colors after color synthesis, the distribution range and boundaries of different strata can be clearly distinguished. Using satellite photos processed by various image data, we can trace two groups of faults in NNE and NNW, in which the NNE fault is vague, the NNW fault is very clear, and it shows the characteristics of crossing and staggering with the NNE fault. Because the two groups of faults are all displayed in Paleogene, Neogene and Quaternary strata, it shows that they all have the nature of neotectonic activity, and the neotectonic activity of NW-trending faults is stronger than that of NE-trending faults.
5. Mercury gas measurement effect
Two groups of mercury gas anomalies were found in this area (Figure 1-2- 1 1), and their distribution ranges were basically consistent with the results inferred by the above methods. The occurrence of mercury gas anomaly on the fault zone indicates that there is new tectonic activity in this area, and the fault passes through Cenozoic strata, which makes mercury gas have a chance to reach the ground.
Figure 1-2- 1 1 Anyang No.3 comprehensive profile
(According to Li Xianzhi 1986)
N- Neogene; E- Paleogene; Pz2—- Upper Paleozoic; Pz 1- Lower Paleozoic; Ar- taiguyu
According to the inference results of the above geophysical and geochemical exploration methods, combined with the geological and drilling data obtained in the past, it can be clearly concluded that there are two groups of new tectonic active faults in this area, which are NNE-trending and NW-trending, and their characteristics are described as follows (Figure 1-2- 12).
Figure 1-2- 12 Geological map of fault structure and bedrock in Anyang city
(According to Li Xianzhi 1986)
P- Permian; C- carboniferous; O- Ordovician; Artest ancient jade
(1) NE-trending fault
The NNE faults in this area are mainly Dong Tang fault and Qingyangkou fault. After they entered this area from Tangyin, they turned in many places and had relatively developed secondary parallel faults. Dong Tang fault dips to the west, while Qingyangkou fault dips to the east, with dip angle above 70 and fault distance generally greater than1000m. The maximum fault distance of Dong Tang fault near Anyang City can reach 3200m. These two faults are obviously reflected in gravity, mercury gas measurement and seismic exploration results. According to the drilling data, the boreholes to the west of Qingyangkou fault generally penetrate the Neogene and Paleogene within 400 meters and enter the Permian, while the boreholes to the east of the fault often do not penetrate the Cenozoic strata below 1000 meters. ..
(2) NW fault
The NW-trending faults are developed in this area. The faults traced from south to north include Tangyin South Fault, Tangyin North Fault, Guocunji Fault, Anyang South Fault, Anyang North Fault, Xindian Fault and Zhanghe Fault. South Anyang fault and Zhanghe fault tend to the south, and the rest tend to the north, with dip angles generally greater than 70 and fault distance of 200 ~1000 m, accompanied by torsional strike-slip phenomenon, especially in the southeast section of the fault north of Anyang, where the strike-slip movement is more intense, and the translation fault distance of Zhanghe fault can reach 700m m.
Based on the analysis of the above data, the structural form of Anyang is a relatively complete fault basin, with Taihang Mountain uplift in the west, Taihang Mountain piedmont secondary uplift in the north, nose uplift on the edge of Linqing Depression in the north, hua county uplift in the south and Yigou uplift in the south. The interior of the basin is divided into checkerboard structural framework by two groups of faults in NNE and NNW, forming some grabens and secondary fault blocks. For example, Tangyin passes through Anyang to the provincial boundary, and the northern section of Tangyin Graben is composed of Dongtang fault and Qingyangkou fault, which is staggered by Zhanghe River at the provincial boundary. For another example, from Shui Ye to Bai Bi via Anyang, Anyang Graben consists of Anyang South Fault and Anyang North Fault. This graben and Anyang Tangyin Graben intersect and overlap to form Anyang Depression, which constitutes the core of the basin. The partition between grabens is a relatively convex fault block.
Graben and Quaternary depression formed by NW-trending faults in this area are favorable water-rich areas for groundwater. The secondary uplift in front of the mountain and the fault block uplift in the basin are favorable areas for finding hidden coalfields. According to the shape of the basin, the relationship between the basin and the western mountainous area, the characteristics of magmatic activity in the area and the comprehensive analysis of mercury gas anomalies, there may be geothermal or natural gas in the inner edge of the basin.
It is worth noting that although the neotectonic movement in this area is very strong, the developed chessboard structure is not necessarily conducive to the concentrated accumulation or explosive release of geostress. The existence of active faults may strengthen the intensity response to distant earthquakes. Therefore, an objective evaluation of the seismic geological background is conducive to guiding the development of economic construction in this area.
In a word, the geological structure in this area has both neotectonic activities and relatively stable geological background, which strictly controls the distribution of underground resources. Only by carefully understanding and mastering the structural pattern of the faulted basin in this area can we correctly guide the resource development and plan the urban development layout in this area.