(Department of Earth Sciences, Sun Yat-sen University, Guangzhou, Guangdong 5 10275)

The tectonic nature and evolution of "Xuefeng ancient land" has been a long-standing debate in China geological circles. The older generation of geologists in China should pay attention to or set foot here and publish articles about the structural nature and evolution of Xuefeng Mountain, including Huang (1945, 1960), Zhang (1959) and Qian Xianglin (6549). In the late 1970s, Mr. Zhu Xia led Chen Huanjiang, Zhang Yuchang and others to personally inspect the geological structure of Xuefeng, and was the first person to propose Xuefeng as a nappe. He explained the deformation pattern and basin evolution in South China by detachment and nappe structure, and pointed out that the Jiangnan ancient land ... was probably pushed to the northwest in this way, but he believed that this activity was the result of the detachment of the continental lithosphere at the bottom of the silica-alumina layer and did not involve the oceanic crust.

In the early 1980s, Mr. Xu Jinghua compared the regional tectonic pattern of the Appalachian Mountains in South China and North America with the method of geotectonic comparison, and thought that they were very similar. He also thinks that South China is a Mesozoic alpine orogenic belt, Banxi Group is migmatite, South China has Triassic ocean, and Jiangnan-Xuefeng ancient land is an alpine remote nappe from Cathaysian ancient land (Xu Jinghua, 1980, 198 1), in which the fold belt from east Sichuan to north Guizhou is equivalent to the valley ridge province, and the late Precambrian deformation of Xuefeng mountain is equivalent to. Metamorphic rocks and plutonic rocks in South China and Southeast China can be compared with the piedmont zone in structure. If Mr. Xu's opinion can be established, it will be of great significance to the regional structure of South China and the second round of oil survey. Therefore, China's geological circles, especially the tectonic circles and petroleum geological circles, invested considerable efforts to test Mr. Xu's views during the Seventh Five-Year Plan and the Eighth Five-Year Plan. During the Seventh Five-Year Plan period, under the personal guidance of Mr. Zhu, we (Zhang Yuchang, Qiu Yuanxi, Ma Wenpu, etc. ) undertook the 54th sub-project of the national key project "Study on the nappe structure of Jiangnan-Xuefeng ancient land". After the research was completed, we received the support of the National Natural Science Foundation and continued to carry out research work in Xuefeng Mountain area for 7 years. Mr. Zhu died in 1990 when we were implementing the fund project. However, even though he has left us, we still feel that Mr. Zhu's academic thought has always been a beacon to illuminate our scientific research and monograph writing. Of course, it should be pointed out that I personally don't know enough about Mr. Zhu's academic thought and have a deep experience. At the beginning of my research work, I was deeply influenced by the Xu Jinghua model. I followed the model for some time, until the practice proved that the model did not conform to the geological reality of Jiangnan-Xuefeng area, and I abandoned it and returned to the correct track guided by Mr. Zhu. This also shows from the opposite side that Zhu's academic thought is correct in understanding Xuefeng Mountain and even Xuefeng Mountain. Here, I will expound the guiding significance of Zhu Xia's academic thought from several aspects of understanding Xuefeng Mountain structure and its oil and gas exploration prospect.

1 is a quasi-in-situ thrust nappe rather than a remote nappe in different places.

1. 1 year deep structure characteristics of lithosphere

The deep lithosphere is 120 ~ 150 km thick and has a layered structure. The upper mantle is composed of sedimentary layers and basal layers, and the lower crust is composed of ancient landmasses. The crust type is continental stable crust. Magnetotelluric sounding reveals that the lithosphere has an uneven layered structure in the vertical direction, the top interface of asthenosphere is shallow (about 100km), and there are two high-resistivity mantle blocks below 100km underground. The obvious high gravity of Xuefeng Mountain indicates the existence of basement uplift and nappe composed of ancient blocks in the deep. The horizontal distance between Xinhua and Lianyuan is only 2 1km, but the gap between the top boundary of the lithosphere is as high as 97km, indicating that there is a subduction zone from east to west in the deep part, but the upper crust is a uplift nappe zone from east to west (Figure 1), and geophysical data fail to provide the existence of the deep part.

Figure 1 Geological Profile of Xuefeng Mountain and Its Adjacent Area (Fenghuang-Chaling)

1. Lower crust; 2. Granite; 3. Rigid curtain block; 4. Low density anomaly mantle; 5. Moho noodles; 6. Ductile shear zone; 7. Structural weakening zone; 8. asthenosphere; 9. Thrust fault

1.2 surface characteristics

On the surface, sliding along the basement and bedding detachment make the caprock deform strongly, which can be seen everywhere in the field fault flat and slope structures. Comb folds and thrust imbricates are formed in the caprock, and regional detachment deformation layers are typical thin-skinned structures. Feilaifeng structural window, double thrust structure and syncline fold are common in the front belt. Fore-deep basin is developed at the front edge of nappe, and the sedimentary center of foredeep basin moves to the northwest with the overthrust (such as Ma Yuan basin), and recoil structure is developed at the back edge (Figure 2). However, there are no giant concealed folded rocks and long-distance nappes, and the nappe distance is generally more than 5km. For example, the nappe distance revealed by the Ping Huang-Kaili seismic profile in southeastern Guizhou has reached 30km, but according to the drilling data in Jiangnan-Xuefeng area, the maximum nappe distance is only 60km (Jiangxi Bureau of Geology and Mineral Resources). Although the nappe structure has reduced the sedimentary facies belt, it has not destroyed its original sedimentary palaeogeographic pattern (platform facies, slope facies and basin facies in turn from northwest to southeast), and the off-site system and in-situ system of the upper and lower walls of the nappe structure.

Fig. 2 Regional structural profile of Xuefeng Mountain and its adjacent area

ⅰ. Yanshan arc comb fold thrust belt from southeast Sichuan to northwest Hunan; ⅱ. Cili-Baoyong-Baojing Yanshanian arc fault-fold belt; Ⅲ. Mesozoic foredeep basin in Ma Yuan; Ⅳ. Xuefeng Mountain basement fold thrust belt; ⅴ. Arc fold belt of Hunan-India branch

Xu Jinghua once suggested that "several Sinian and Lower Paleozoic strata in southeastern Guizhou are structural windows in the Banxi Group complex" and that "the granite at the waist of Fanjing Mountain is a rootless alien" and "Feilai Peak". However, according to our field investigation, the former is a syncline structure rather than a structural window, and the latter has been confirmed by 654.38+0 ∶ 50,000 regional survey and drilling, and granite can extend all the way to the deep part of Fanjingshan Mountain. Fan Jingshan is not a Feilai peak, but a two-wing anticline structure of Lower Sinian-Cambrian. That is to say, Banxi Group is not a fold zone or a tectonic mixed zone. On the ancient land, a large area of lower Paleozoic marine strata was not covered under Banxi Group, and the Precambrian on Xuefeng ancient land only had a slight NW-trending overthrust at its nappe front, thus slightly covering part of the lower Paleozoic strata, forming a relatively small Feilai peak group and structural window group at its front. Therefore, it can be said that Xu Jinghua's long-range nappe model cannot be established, and the overlapping crustal wedge in his thin plate structural model does not exist.

In the process of completing the national fund research, we found large-scale low-dip extensional detachment faults and gravity sliding structures in Xuefeng Mountain area, most of which were formed in the post-orogenic period, but this still did not exceed the formal mechanism of internal detachment of lithosphere pointed out by Zhu. In fact, Mr. Zhu has already pointed out that the comb folds in the east of Beichuan, Guizhou Province are the products of gravity sliding.

2 is an intracontinental orogenic belt, not an alpine orogenic belt.

The reason why Xuefeng Mountain nappe is so different from Appalachian remote nappe is ultimately determined by their respective regional geological background and geodynamic process. Xuefeng Mountain has been an intracontinental orogenic belt since at least Caledonian period, which has experienced intracontinental rifting in the southeast margin of Yangtze since Jinning period, intracontinental subduction in Caledonian orogenic curtain, soft collision between micro-blocks, continental crust uplift and extension detachment after Caledonian fold inversion, and intracontinental subduction and compression in Late Paleozoic Paleotethys transgression and Indosinian period, Yanshan period and Himalayan period. This is completely different from the Appalachian Mountains, which were formed by the collision between the Eurasian plate and the American plate. There was no ocean in Triassic in South China, and there was no alpine orogenic belt in South China due to the closure of Triassic ocean. The intracontinental overlapping mountain system caused by multi-stage and multi-level layered sliding structures (thrust nappe and extensional sliding) commonly developed in Xuefeng Mountain and even in South China is formed by multi-cycle orogeny, which is the product of intracontinental subduction, basement detachment and caprock detachment dominated by deep mantle undercurrent under the continental crust background. The Alps orogenic belt was formed by continental collision. In Mesozoic and Cenozoic, South China experienced a series of intracontinental deformation due to the mutual extrusion of the eastern and western fronts such as Tethys-India plate, Pacific plate and Paleo-Asia plate and the expansion of continental margin. Therefore, it is not an alpine orogenic belt in continental collision, but an intracontinental orogenic belt and an intraplate superimposed deformation belt, and their regional geological background and geodynamic mechanism are completely different (Figure 3).

Fig. 3 Comparison between alpine orogenic belt (A) and Xuefeng intracontinental orogenic belt (B)

(1) Thin plate tectonic model of alpine orogenic belt (Xu Jinghua, 1980): 1. Subduction wedge with sedimentary cover; 2. Miscellaneous jumble; 3. Super crustal wedge

(b) intracontinental orogenic belt: 1. Continental crust and upper crust; 2. The lower crust of continental crust

It should be emphasized that our understanding benefits from Mr. Zhu's T (environment) -S (action) -M (reaction) scheme, as well as the structural evolution thought and multi-cycle activity theory of the two systems. We use soft collision to explain the collision between micro-landmasses and intracontinental orogeny, which is inspired by Mr. Zhu's view that the opening and closing of micro-landmasses is soft collision under Paleozoic trough system, and the plate system did not collide hard until Mesozoic. Zhu Cheng systematically analyzed the relationship between deep structure and surface structure, surrounding structural environment and intraplate (intracontinental) structural deformation, and established a four-dimensional historical development model of Xuefeng Mountain structure based on the theory of multi-cycle activity.

The study of "Xuefeng ancient land" provides a basis for oil and gas evaluation and analysis.

The main focus of the second round of petroleum survey is to find out the oil and gas prospect in the distribution area of Paleozoic marine strata on land in China, which is also one of the main problems to be solved in the 54th national key scientific and technological project.

If the Xu Jinghua model can be established, the Precambrian metamorphic rocks in Jiangnan-Xuefeng area will cover a large area of Paleozoic oil-bearing marine strata, which will be an important prospect for the second oil survey. The falsification of Xu Jinghua's long-distance nap mode also denied this distant scenic spot. Of course, due to the existence of NW nappe in Jiangnan-Xuefeng area, some Paleozoic oil-bearing basins may be covered in front of it, which still needs due attention. However, the discovery of oil and gas traces in Majiang area on the northwest edge of "Xuefeng ancient land" proves that it was once an oil and gas accumulation zone in Xuefeng area-southeast edge of Yangtze ancient land margin. The Xuefeng Mountain area and its adjacent areas have experienced the marine sediments of Xuefeng Period, Caledonian Period and Hercynian-Indosinian Period since the Bosi Period, and have been in the marine environment of continental margin and even intraplate (Late Paleozoic) for a long time. The thickness of marine sediments is more than 10,000 meters, and there are many good oil-generating facies zones, and the glutenite facies is a favorable oil and gas reservoir facies zone. The deformation of Mesozoic, especially Xuefeng Mountain and it, is superimposed on this Paleozoic continental margin basin with oil and gas prospect. Of course, the prospective evaluation of these two petroliferous basin systems should be treated separately. As Mr. Zhu once emphasized, the basins of two generations and two systems can be juxtaposed or superimposed with each other, and there is a complex relationship between sedimentation and structure.

Thereby constructively or destructively affecting the distribution and redistribution of oil and gas. This paper only talks about how to make a negative prospect evaluation of Majiang ancient oil and gas accumulation zone under the guidance of Mr. Zhu's above academic thoughts.

The western margin of Xuefeng ancient land is the ore-controlling distribution area of mercury, gold and antimony, which is spatially overlapped with Majiang ancient oil and gas accumulation zone and is also the overlapping compound area of two generations of basins. The "primary" source bed of mercury mine is a argillaceous deposit (Bo Xinchun, 1983) similar to the source bed or argillaceous deposit rich in organic iron sulfide, and the medium of ore-bearing hydrothermal solution is the atmosphere. In a weakly acidic to weakly alkaline reducing environment, hot brine reacts with hydrogen sulfide in a closed cycle or waste oil and water carried under the mountain to precipitate lead and zinc sulfides (Liu Baojun et al., 1993). These mercury, gold, lead-zinc deposits have deposited various tensile faults, roof fracture structures and interlayer fracture zones formed since the Eastern Glaciation, which have become the location space for their mineralization. This clearly shows that the ancient oil and gas reservoirs and ancient oil and gas accumulation zones in these stratabound mercury, gold and antimony deposits have been destroyed, and the original Majiang ancient oil and gas accumulation zone has no oil and gas prospect.

Mr. Zhu has always emphasized the existence of verification and falsification in scientific research. Our scientific research work in Xuefeng Mountain not only falsifies Xu Jinghua's remote push mode, but also confirms a series of incisive academic thoughts of Mr. Zhu Xia.

4 conclusion

The above is just my superficial understanding of Mr. Zhu's academic thoughts on the structural research of Xuefeng Mountain in combination with the structural characteristics of Xuefeng Mountain. Zhu is a scholar with profound knowledge and profound thoughts. Needless to say, the oil geologists in his place got the teaching and enlightenment from him, which will be inexhaustible for life, and even I, who only had the opportunity to attend classes in the last few years of his life, benefited a lot. His influence on us is often subtle in many ways. For example, Zhu's basin analysis holism is instructive for us to compile the outline of the book "Structural Nature and Evolution of Xuefeng Mountain". Although this monograph is not a book of basin analysis, it is a book of orogenic belts, but it is not only in space (surface and deep), but also in time. In this paper, the structural characteristics and geodynamic process of Xuefeng Mountain orogenic belt are discussed not only from the interior of Xuefeng Mountain orogenic belt, but also from its tectonic background with surrounding areas. A better understanding of the structure of Xuefeng Mountain as a whole is undoubtedly inspired by Mr. Zhu's holism.

Mr. Zhu not only gives us profound enlightenment in academic thought, but also has charming personality charm. I haven't had much contact with Mr. Zhu, but two things have moved me so far.

First, in the mid-1980s, Yan Yuejin, Mr. Zhu's first doctoral student, set the thesis work area in South China. He asked me to help him guide doctoral students to work in the field in western Guangdong and Hainan through Chen Huanjiang, so I became a doctoral tutor and went to Yunkaishan in western Guangdong and Shilu in Hainan according to what Mr. Zhu wanted for doctoral research. Later, Yan Yuejin went abroad halfway, and his doctoral thesis was interrupted. Mr. Zhu feels sorry for this. When his second doctoral student defended his thesis in Zhou Zuyi, he arranged for me to take part in Zhou Zuyi's doctoral thesis, even though the funds were very difficult. Although I declined politely considering the funds of the inviter, it can be seen from this incident that Teacher Zhu respects the work of the younger generation.

Second, at the end of the Seventh Five-Year Plan, I made a report on the project research work at the report meeting and asked Mr. Zhu for advice. Mr. Zhu was silent. Looking back now, he had some reservations about my conclusion that I followed the Xu Jinghua model. However, when he gave an academic report in Wuxi 54 Project, it was also his last academic report before his death, which confirmed the fact that I found lying folds in Xuefeng Mountain area (it was later proved that it was not a giant lying folded rock mat, but a small and medium-sized lying fold), showing a great scholar's respect for the labor and facts of others.

It is precisely because Mr. Zhu has left us too much spiritual wealth in life and academics. On the occasion of the anniversary of Zhu's death 10, I would like to commemorate Zhu's great life with this article.