Structural layers and tectonic evolution models

According to the analysis of stratigraphic sequence filling sequence, paleontological sequence, magmatic activity and ore-forming events in Qiannan and Gui Zhong depressions, since Mesoproterozoic, the two depressions have experienced Bao Si movement at the end of Mesoproterozoic, Jinning movement at the beginning of Neoproterozoic, Guixi movement at the end of Middle Silurian, Indosinian movement at the end of Middle Triassic and Yanshan movement after Middle Jurassic. At the same time, accompanied by volcanic eruption in the late Mesoproterozoic, Devonian and Permian, it intruded into granite in the early Neoproterozoic, lamprophyres from late Ordovician to Silurian and alkaline basic rocks in Paleogene, and experienced basin formation in South China-Silurian, Devonian-Early Triassic, Meso-Cenozoic and Late Ordovician-Silurian, Middle Triassic-Middle Jurassic and Late Cretaceous.

Among them, the Bao Si movement was the earliest orogenic event in the geological history of this area. Nanyang, Gu Hua, which developed in Mesoproterozoic, was closed, and the deep basement of Qiannan Depression was folded. Later, it was gradually strengthened into a crystalline basement through many orogenic events. Under the influence of Jinning Movement in Neoproterozoic Nanhua Period, South China, which was United with the ancient continent, gradually split, forming a limited ocean basin that was rifted in the northeast of South China in the pre-Tethyan period, and deposited a very thick terrigenous clastic rock series. After the brief Chengjiang movement, the shallow metamorphic basement in southern China was formed, and the limited ocean basin in the continental rift in South China gradually developed into a passive continental margin. From Sinian to Early Paleozoic (Z-O 1), with the global sea level rising, the Yangtze block gradually evolved into a craton, and carbonate platform sediments developed. However, Qiannan, located in the southeastern margin of the Yangtze continent, is a typical passive continental margin rift basin, and the water body gradually deepens from Qiannan to Gui Zhong Depression. The Guangxi movement at the end of Middle Silurian was an important intracontinental orogenic event, which ended the basin deposition in Xuefeng area in the east of Qiannan Depression and entered the stage of uplift (platform) development, and the Lower Paleozoic in Guangxi also folded to form the depression basement. In the Late Paleozoic, due to the expansion of the Paleo-Tethys Ocean, the southern part of the Yangtze block after accretion broke into a broken continental margin again, and two depressions reappeared in the basin environment. During Indosinian period, with the disappearance of Paleo-Tethys Ocean and the retreat of Sino-New Tethys Ocean, the two depressions gradually retreated and ended the marine sedimentary history, and entered the stage of continental basin development, and after Yanshan period, they entered the stage of continental basin transformation as a whole.

According to the development generation and stratigraphic sedimentary filling sequence of the two depression basins, combined with the analysis of the deformation characteristics of their sedimentary caprocks in the later stage, the caprocks in Qiannan depression can be divided into three sets of structural layers: Lower Paleozoic, Upper Paleozoic and Mesozoic, and the caprocks in central Guangxi depression can be divided into two sets of structural layers: Upper Paleozoic and Mesozoic.

Lower Paleozoic structural layers: including Sinian, Cambrian, Ordovician and Silurian. Among them, the Sinian and Lower Paleozoic sedimentary facies belts in Qiannan Depression are distributed in the northeast direction, mainly depositing a set of marine carbonate rocks and clastic rocks; Nanhua, Lower Cambrian and Lower Silurian are plastic layers, while Middle-Upper Cambrian and Ordovician are dry layers, with brittle deformation and developed joints and faults. It is mainly exposed in the east of Qiannan Depression, and the Guiding-Duyun area is characterized by a wide and gentle anticline. The western margin of Xuefeng Mountain is dominated by thrust nappe and fault-related folds. The central and western parts of the depression are characterized by wide and gentle anticlines. Due to many tectonic movements in the central Guangxi depression, such as Yunnan-Guizhou, Duyun and Guangxi, etc. The structural layer is mostly denudation fold metamorphism, which is mainly manifested as a set of medium-thick sandstone, glutenite and thin-layer shallow metamorphic shale, and the deformation is strongly closed fold.

Upper Paleozoic structural layers: including Devonian system, Carboniferous system and Permian system, can be divided into three structural sub-layers, mainly characterized by northwest and nearly east-west rifting sedimentary filling sequence, platform basin deep-water sedimentary area along the fault zone, and carbonate platform sedimentary area in other areas. Among them, the bottom of middle and lower Devonian, lower Carboniferous and upper Permian are weak layers with strong folding deformation. Large sets of limestone and dolomite are developed in Upper Devonian, Upper Carboniferous and Lower Permian, mainly brittle deformation, and cracks and joints are developed. Generally speaking, the platform facies is dominated by brittle deformation, while the platform basin facies is dominated by plastic deformation. The alternating soft and hard strata make the two depressions develop fold deformation patterns related to faults, in which Qiannan sag is characterized by interlayer sliding structure development and Gui Zhong sag is characterized by uplift structure combination development.

Mesozoic-Cenozoic structural layers: mainly including Triassic, Jurassic and Cretaceous, except Triassic, other strata are only sporadically exposed in syncline cores of two depressions. The Triassic is exposed in a large area in the northwest of the two depressions, with strong folds and thrusts, and the Triassic develops typical plastic deformation and fault-related folds (Figure 1-5).

Figure 1-5 structural evolution model of depression

(A) Transformation characteristics since Mesozoic-Cenozoic

Since Mesozoic, the Paleo-Tethys Ocean has gradually closed (that is, Indosinian events occurred); At the end of the Middle Triassic, with the gradual uplift and regression after the formation of the United continent, a foreland basin environment characterized by land-sea interaction and coal-forming was developed in Gui Zhong sag in southern Guizhou in the Late Triassic, and the sedimentary history of marine strata basically ended in the whole region at the end of this period. During Indosinian period, due to the revival of Jiangnan-Xuefeng Mountain structural belt and Dayaoshan structural belt, fold-thrust and gravity-slip structural deformation systems developed in the eastern margin of Qiannan Depression and the eastern and northern margins of Gui Zhong Depression respectively.

During the Early-Middle Jurassic, with the continuous combination of Peyo block and Yangtze block, Yangtze block and North China block in the southwest margin of South China United continent, and the combined influence of the westward subduction of the eastern paleo-Pacific plate, the Yunnan-Guizhou-Guangxi region was generally in a state of convergence of forces in the east, west and north (that is, the early Yanshan event occurred), while the structural belt and orogenic belt formed in the eastern part of Qianzhong Depression revived, folded and thrust, and a large number of granites and rocks were formed around it. As a result, many extensional faults developed in the early stage of the depression edge reversed and gradually evolved into piedmont thrust contraction basins on the edge of the depression, which were filled with coarse-grained molasse-like structures, such as Jurassic basins such as Mucheng, Xiwan in eastern Guangxi and Longjiao-Anletang in southwestern Guizhou, and died out at the end of the middle Jurassic. In the Late Jurassic, with the further convergence of the South China United continent, the eastern crust was sharply thickened, and then the lithosphere collapsed, collapsed and thinned in the Early Cretaceous, and a tensile stress field environment appeared on the surface of the crust (that is, in the middle Yanshan period), thus forming a series of NE-trending rift basins, accompanied by strong volcanic eruption and acidic-intermediate-acidic magma intrusion, leading to the uplift, denudation, strong transformation and destruction of marine strata in the eastern South China. From Late Cretaceous to Eocene (that is, the late Yanshan period), influenced by the merger of the Qinghai-Tibet landmass on the eastern margin of NeoTethys with the landmasses in Sanjiang and Yangtze regions and the eastward compression of the Qinghai-Tibet landmass, the foreland and continental basins developed in the early central and western parts of South China gradually disappeared, and most of them were transformed into intermountain depression-type moral red basins in the late Cretaceous. At the same time, due to the westward movement of crustal thickening and thinning activities caused by Yanshan event, a large number of small intermountain fault basins began to develop in Yunnan, Guizhou and Guangxi during the late Cretaceous-Eocene, and a series of small ne-trending Dauder special Cretaceous red basin deposits developed in the central depression of southern Guizhou and its surrounding areas, characterized by alluvial red molasses deposits filling rivers.

From the end of Eocene to the beginning of Oligocene, the Qinghai-Tibet Plateau gradually formed and moved eastward with the combination of India and Eurasia on the western edge of Sanjiang and South China United continent and the closure of the NeoTethys Ocean (that is, the Himalayan event). The marine strata in the southwest margin of Sanjiang and Yangtze areas began to compress and fold, and gradually spread to Guizhou and Guangxi and the southeast coast, and the Gui Zhong sag in southern Guizhou and its surrounding areas suffered further uplift and erosion. From Miocene to Miocene, with the continuation of the combination of India and Eurasia, the lithosphere of the Qinghai-Tibet block thickened rapidly, and the crust rose sharply, accompanied by large-scale faults and magmatic activities. Affected by the eastward movement of the Qinghai-Tibet landmass, a series of deep and large faults thrust and strike-slip from west to east developed in the early suture zone and its vicinity in the southwest margin of Sanjiang and Yangtze areas, and a series of Neogene fault-depressed dammed lake swamp basins characterized by the development of coal measures strata formed along it. Uplift and denudation are dominant in the central and eastern parts of South China, quasi-leveling is dominant in the southeast coast, and the sediments are characterized by the development of coal-bearing swamp lakes. From the end of Pliocene to the beginning of Pleistocene, due to the convergence of land masses and the further thickening of lithosphere, the whole crust of Qinghai-Tibet Plateau rose strongly, forming the present plateau landform pattern. Affected by this, the surface of the Sanjiang River and the southwest edge of the Yangtze region (Yunnan, Guizhou and Guangxi) gradually turned into a mountainous environment characterized by uplift and erosion and frequent seismic activity, while the southeast coastal areas basically maintained a relatively calm quasi-plain sedimentary environment.

According to the analysis of strata distribution and interlayer contact relationship in Guizhou central depression since Mesozoic and Cenozoic, the upper Triassic-lower Jurassic is generally absent in the southeastern Guizhou and northern Guangxi adjacent to Xuefeng mountain structural belt and the marginal areas of two depressions adjacent to eastern Yunnan and Niushoushan uplift, indicating that the uplift around the depression may be the main provenance supply area of Indosinian foreland basin in the central and western depression. The fact that the upper Triassic-lower Jurassic coal measures strata developed in the central and western depressions are mostly in false conformity contact with their underlying strata proves that these two depressions. After Yanshanian period, it gradually strengthened into strong fold and thrust deformation, especially in the middle and east of the two depressions. Mainly reflected in the fact that the marine strata in the central and eastern parts of the two depressions generally suffered from strong uplift and erosion under the joint action of the subduction of the Pacific plate to the west and the intracontinental transformation of Xuefeng Mountain. It is characterized by unconformity contact between Jurassic-Cretaceous strata and its underlying strata in the axis of a few syncline belts in the middle and east of the two depressions, false integration contact in the central and western regions, and Yanshanian metal mineralization events are mainly concentrated. Since the Himalayan period, the two depressions have been in a state of intense uplift and erosion due to the eastward thrust fold caused by the escape of the Qinghai-Tibet Plateau from west to east and the accompanying strike-slip shear adjustment of the Sanjiang orogenic belt, which is mainly reflected in the deep valleys and karst development of the residual marine strata in the two depressions. Today, there are no Paleogene and Neogene deposits and residues on the surface. Through the calculation and analysis of the homogeneous temperature of Ro and inclusions, it is found that the denuded strata in Jiangnan-Xuefeng and Nanpanjiang areas on the east and west sides of the two depressions since Yanshan event are 5000 ~ 6000 m, and the inner strata in the depressions are 2500 ~ 4500 m, mainly involving upper Devonian and above strata, and some middle Devonian strata are exposed.

The quantitative analysis of tectonic-thermal events in Jiangnan-Xuefeng area based on fission track data shows that the SILURIAN clastic rocks in this structural belt have experienced at least three regional uplift and cooling events since Indosinian, namely, Indosinian (250-235 Ma) in the early Middle Triassic, Late Jurassic-Early Cretaceous (140-1/kloc-). Taking the thermal history simulation results of apatite fission track of Luo Zhi in Kaili, southwest margin as an example, the time for this set of strata to rise from the maximum buried depth to above the apatite annealing zone is about 1 15Ma, and the uplift amplitude of strata is about1040m. After that, it experienced a slow uplift in ① 1 15 ~ 78ma (middle Yanshanian period), the paleotemperature dropped by 28℃, the stratum uplifted by 1 120m, and the uplift rate was 30.3m/Ma；; ; ② After 78 ~ 45 ma (late Yanshan-early Himalayan) adjustment, the paleotemperature dropped by 2℃; (3) During 45 ~ 30ma (early Himalayan period), it entered a rapid uplift period, and the paleogeothermal temperature of this stratum decreased by 55℃. After intense erosion, the * * * layer rises by 2200m, and the rising rate is as high as146.7m/year. ④ ④ After ④④30Ma (late Himalayan period), it entered the adjustment period again, and the Silurian paleotemperature was close to the present surface temperature.

To sum up, Yanshan period is the main formation period of the present structural framework of Gui Zhong sag in southern Guizhou, and the thrust nappe of Jiangnan-Xuefeng structural belt may be the main dynamic factor of the structural deformation of the two depressions. Among them, the thrust nappe of the western edge of Xuefeng structural belt controls the structural deformation style, distribution and evolution of Qiannan depression, and the strong uplift of the southern edge controls the structural deformation style, distribution and evolution of the decollement structure in the central and northern Gui Zhong depression.

(2) Fault distribution and structural zoning

The main structural framework of the two depressions began to form in Yanshanian period, showing trough-like folds, and mainly developed three groups of structural lines in NNE, NNW and EW.

According to the field geological survey in recent years, it is found that the Cretaceous in the periphery of central Guangxi participated in the deformation and folding of the caprock in the depression. At the front of the northern Hechi-Liucheng fault (Yishan section), the Devonian dolomite limestone in the upper wall of the fault overthrusts the Cretaceous in the lower wall, and the underlying Cretaceous and Triassic are in unconformity contact, and both Cretaceous and Triassic have been involved in strong plastic deformation. Although Cretaceous overthrust overthrusts the Triassic in Yang Na area on the southern margin of the depression, it is found that the deformation of Triassic is strong, while that of Cretaceous is relatively weak. Combined with the scattered distribution of Cretaceous in the whole depression and relatively weak deformation and transformation, it is speculated that the finalization period of the central Guangxi depression may end in Cretaceous, that is, the middle and late Yanshan period.

Similarly, according to the results of field geological survey in the central and western parts of southern Guizhou, it is found that the Upper Cretaceous in the depression is in false integration contact with the Middle Triassic and Upper Permian, and the Upper Cretaceous has been folded and deformed with other sequences as a whole. According to the phenomenon that the Upper Cretaceous is not widely integrated with the Upper Paleozoic and Cambrian-Ordovician discovered in Jiuzhou and Shibing, Ping Huang in the northern part of the sag, it is speculated that the structural framework of Qiannan sag may be later than Guizhong sag, mainly concentrated in the late Yanshan period.

Based on the analysis of geological data and geophysical data of Qiannan and Gui Zhong depressions and their surrounding areas, the anticlines of the two depressions are relatively wide and gentle, and they are box-shaped; Syncline is narrow and steep, strip-shaped, which generally reflects the Jura-type trough-separated fold style. There are four groups of deep and large fault systems with different scales in the two depressions, namely, NNE, NNW, NEW and NEW. Three structural belts are formed in the depressions, which control the main structural patterns and features of the two depressions.

As far as southern Guizhou is concerned, the main structural line in the central and eastern part of the depression is nearly north-south, and the eastern part gradually deflects to the northeast. In the western part of the depression near the Ziyun-Luodian fault zone, it gradually transforms to the northwest-northwest, and a small number of nearly east-west structures can be seen in the southern and northern edges of the depression. The overall formation of the depression is dominated by NNE-SN structural lines, and the southern and northern parts are mostly cut and limited by near EW structural lines, while the central, eastern and western parts are limited by NE and NW structural lines respectively. From the distribution of faults, NNE-NNE faults are mainly concentrated in the central and eastern parts of the depression, which mainly reflect thrust and strike-slip properties, such as Shidongkou fault, Fuquan-Duyun fault, Huishui fault, Guiding fault and Guang Shun fault. Some faults control the sedimentary lithology and lithofacies changes of the early Paleozoic primitive strata in the depression, such as Tongren-Sandu fault. The NW-trending faults are mainly concentrated in the west of Qiannan depression, represented by Ziyun-Luodian fault, and also have thrust and strike-slip properties. Near-east-west faults are mainly distributed in the northern part of the depression, represented by Sugiyama fault and Kaili fault.

As far as central Guangxi is concerned, NNE-NE faults are mainly concentrated in the eastern and eastern margins of the depression, such as Longsheng-Yongfu fault zone, Shoucheng-Tunqiu fault zone, Sanjiang-Rong 'an fault zone and Liuzhou-Laibin fault zone, which have the characteristics of early development and long activity time. Late Paleozoic was a extensional normal fault, which was reversed to a compressive strike-slip fault after Indosinian period, forming a thrust fold structural deformation zone in the eastern part of the depression. The time of fault activity in the east-west direction is similar, mainly concentrated in Yishan in the north and Binyang in the south of the depression, such as Hechi-Liucheng fault zone and Pingxiang-Dali fault zone. The NW-trending faults are concentrated in the western margin of the depression, represented by Nandan-Du 'an-Mashan fault. Devonian began to stretch, Permian gradually strengthened, and then reversed after the late Triassic, which was transformed into a compression-torsion thrust fault. NNE-NE fault systems and tectonic lines are mainly developed in the northern part of the depression, and near EW fault systems and tectonic lines are developed in the central and northern parts, forming the Yishan arc tectonic belt, NNE- near SN fault systems and tectonic lines in the central and eastern parts, NW fault systems and tectonic lines in the western edge, and NNE- near SN fault systems and S-shaped tectonic lines in the central and southern parts. The overall structural line in the depression area is characterized by NNE.

According to the analysis of the mutual cutting and limiting relationship between the three groups of main faults developed in the two depressions, NNE-SN fault system and structural lines are the main faults in the central and eastern part of the depression, which mostly cut EW structural lines and faults, but are limited by NE structural lines and faults, indicating that NE-trending faults formed at the earliest time, and may mostly be extensional faults developed in Caledonian, such as Guiyang-Zhenyuan fault and Liuzhou-Laibin fault. NNE-SN trending faults developed late, generally in Yanshanian period, and most of them were the products of Caledonian fault revival and inversion. NW-trending fault systems and tectonic lines are developed in the western margin and southern part of the depression, which have the function of limiting NNE-SN fault systems and tectonic lines, indicating that the formation time of NW-trending fault systems and tectonic lines is slightly earlier than that of NNE-SN fault systems and tectonic lines, and may be the same as or a little later than that of NE-trending fault systems and tectonic lines. Combined with lithofacies palaeogeography, it is inferred that it is Hercynian-Indosinian period. To sum up, it is not difficult to determine that the NNE-SN fault system of the two depressions formed relatively late, belonging to Yanshan period. The NE-trending fault system formed in the early East of Hugary. The NW-trending and nearly EW-trending are the second, which is Hercynian period.

According to the combination characteristics, deformation characteristics and current tectonic lines of the development generations of the two depression basins, combined with the needs of oil and gas exploration, the structural units of the central Guangxi depression are divided into eight secondary structural units, namely Yishan fault depression, Huanjiang shallow depression, Kurenai Wataru shallow depression, Xiangzhou shallow depression, Mashan fault depression, Luocheng low uplift, Liujiang low uplift and Liucheng slope * * *, and the southern Guizhou depression is divided into * *. At the same time, according to the actual data, Liujiang low uplift in Gui Zhong sag is subdivided into three secondary structural units: Datang anticline, Heshan anticline and Liuzhou anticline, and Changshun sag in Qiannan sag is subdivided into six secondary structural units: Pinghuoba-Huangsi anticline, Kedu syncline, Yashui anticline, Tianba-Xinglong syncline, Changshun-Da Lang anticline and Changyuan syncline.

Figure 1-6 Division of Structural Units in Central Guizhou Depression

(3) Structural deformation and structural style

Jura-type piedmont thrust deformation structure is developed in Qiannan depression, and Gui Zhong depression presents "packing-like" fold deformation style.

The depressions in southern Guizhou and central Guangxi are mainly affected by the closure of the Qinfang trough, the extinction of the foreland basin behind Nanpanjiang arc and the multi-stage orogeny of Xuefeng structural belt in the late stage of transformation and deformation. According to the different structural deformation mechanisms and characteristics of different parts, the two depressions are divided into thrust deformation zone, fold deformation zone and compression-torsion deformation zone.

Thrust deformation zone: According to different deformation mechanisms, it can be subdivided into the thrust nappe deformation zone in front of Xuefeng Mountain in the northeast of Qiannan Depression, the gravity sliding nappe deformation zone in front of Xuefeng Mountain in the north of Gui Zhong Depression, and the thrust nappe deformation zone in front of Dayaoshan Mountain in the east and southeast of Gui Zhong Depression.

The thrust nappe fold deformation zone in front of Xuefeng Mountain in the northeast of Qiannan Depression includes Ping Huang shallow depression, Dushan nose uplift and Guiding fault step. The fault-bend fold style is mainly developed, forming a series of nappe structures and recoil structures, and the stratum deformation is strong.

Yizhou gravity sliding thrust deformation zone in front of Xuefeng Mountain in northern Guangxi Depression includes Luocheng low uplift, the eastern part of Huanjiang shallow depression and Liucheng slope, with a large number of NE-trending faults, and the stratum deformation intensity gradually increases from sliding root zone-middle zone-front zone, and the structural style mainly develops broken folds in sliding middle zone and front zone (Figure 1-7).

Figure 1-7 Taiyangshan-Yang Jialing section in the north of Gui Zhong sag.

The thrust nappe deformation area in front of Dayaoshan in the east and southeast of the central Guangxi depression mainly includes Xiangzhou shallow depression, with a large number of NNE strike-slip faults, strong stratigraphic deformation and structural styles such as imbricate structure and flower structure.

Fold deformation zone: it mainly includes Liujiang low uplift in Gui Zhong depression, north-central Kurenai Wataru shallow depression, west and south of Yishan fault depression, west of Huanjiang shallow depression and southwest of Qiannan depression. It is a fold thrust deformation zone, and its structural style is mainly fault-related folds.

Compressive-torsional deformation zone: including southern Mashan fault uplift and Kurenai Wataru shallow depression, mainly affected by Nandan-Du 'an structural zone. The stratum deformation near the fault zone is strong, but the deformation of the uplift to Mashan fault is weak, and the structural style is mainly fault-related folds (Figure 1-8).

There are a series of nearly north-south thrust faults in Qiannan sag, which run through the basement of the sag. These faults all reflect the characteristics of east-west thrust. The closer the stratum deformation is to Xuefeng uplift, the greater the deformation intensity, indicating that it has the typical characteristics of Jura piedmont depression on the southwest edge of Xuefeng. The main body of the depression develops three structures from east to west, namely orogenic wedge, piedmont thrust belt and piedmont fold belt, which embodies the classic combination style of piedmont trough fold thrust deformation. Among them, the thrust nappe deformation zone in the northeast of the depression mainly develops fault-bend folds, and the combination form is characterized by imbricate fans. The Paleozoic below the deformed stratum is dominant, and the deformation forms of surface and underground folds are relatively uniform, and the basement mostly participates in the deformation of caprock. Fault-spreading folds are mainly developed in the southwest of the depression, and the combination form is characterized by uplift structure. The deformed strata include three sets of structural layers: Lower Paleozoic, Upper Paleozoic and Mesozoic. Due to the vertical interlayer difference of lithology and the lateral phase transformation of lithofacies, the surface and underground structural forms often drift or are inconsistent laterally, and the basement deformation degree involved in the cap rock is relatively limited, thus forming a multi-layer deformation structure.

Different from southern Guizhou, central Guangxi has a relatively complex structural framework, with the main body characterized by the development of trough folds, and the anticline is relatively wide and flat in section and box-shaped; Syncline is relatively narrow, steep and trough-shaped. Field investigation shows that the deformation of weak platform facies strata is strong, while the deformation of platform facies strata is relatively weak because of its high hardness. Therefore, the stratigraphic deformation model of the central Guangxi depression is closely related to the paleogeographic model of the prototype basin depression. Considering the generational succession of the central Guangxi depression basin and the unique paleogeographic pattern of "basin-surrounding platform" in the Late Paleozoic, the upper Paleozoic structural layers in the depression naturally combined in plane and profile, forming a "sandwich" strip-like medium structure with alternating soft and hard. When the depression is stressed on three sides and squeezed laterally, the contemporaneous faults that controlled the basin (ditch) deposition in the depression in the early stage are easy to reverse and uplift, forming uplift structures involved in the basement and related folds of the corresponding faults, resulting in strong deformation of the basin (ditch) area, forming closed trough-shaped folds, and relatively weak deformation of the platform area, forming relatively wide and gentle box-shaped folds. After deformation and superposition, the multi-directional platform basin (ditch) is spliced into a multi-directional superimposed trough fold combination pattern on the plane, thus forming a "filled" deformation combination pattern around the platform basin on the section (figure 1-9), which constitutes the most distinctive structural deformation pattern in the central Guangxi depression.

Fig. 1-8 structural deformation zoning map of Gui Zhong sag in southern Guizhou.

The southwestern Qiannan Depression and the central and western Gui Zhong Depression are relatively stable tectonic activities with weak deformation.

According to the analysis of multi-directional balanced profiles of the two depressions, there are obvious differences in the deformation shortening rates of sedimentary caprocks of different structural units in Qiannan Depression, which generally show a decreasing trend from east to west, specifically, Anshun Depression 13.3% in the central and western regions, Changshun Depression 17.5% in the central and eastern regions, and 2 1. 1% in the central and eastern regions.

The shortening rate of caprock in different sedimentary areas in central Guangxi depression is different, and the shortening rate of platform basin (ditch) facies area developed in the early stage is the largest, such as the shortening rate along Nandan-Du 'an fault zone in the west, accounting for 33.5%; Hechi-Yishan fault zone in the north and Pingxiang-Dali fault zone in the south take the second place, accounting for 365,438 0.7% and 365,438 0.6% respectively. However, the overall shortening rate of relatively stable regional caprocks in platform facies deposition is small, such as the shortening rate of Liujiang low uplift below 15%. It can be seen that the tectonic deformation and transformation of the platform-basin facies area in the central Guangxi depression are relatively strong.

Figure 1-9 Composite Deformation Model of Upper Paleozoic "Sandwich" Deformation Sequence and "Filling" Structure in Gui Zhong Depression

Through the analysis of fault development degree, average dip angle of strata and weighted average angle between folds of geological profile in the two depressions, it is found that there are a series of NW-trending fault systems in four structural units around the southern edge of Xuefeng Mountain in the northern part of Gui Zhong Depression, such as Liucheng Slope, Luocheng Low Uplift, Huanjiang Shallow Depression and Yizhou Fault Depression. The fault densities are 0.39/100km2, 0.727/100km2, 0.784/100km2 and 0.863/100km2 respectively. Compared with other structural units in Gui Zhong sag, the fault scale is larger and its position is higher. The dip angles of strata observed in the field of each structural unit are 20.4,14,18.8 and 39.6 respectively, and the average fold wing angles are135.8,143.3 and130.6 respectively. It shows that Luocheng, Huanjiang and Liucheng, which are located in the southern edge of Xuefeng Mountain uplift, have relatively slow dip angle and relatively weak fold deformation, and are generally located in the root zone and middle zone superimposed by gravity sliding on the southern edge of Xuefeng Mountain. They are characterized by a large number of strike-slip fault zones and the overall deviation of oil and gas preservation conditions.

Xiangzhou shallow depression in the east of Gui Zhong depression is similar to the northern structural unit, with developed caprock faults, strong folding deformation intensity and relatively poor oil and gas preservation conditions. The fault density is 0.466/100km2, the fold wing angle is 127.38, and the average dip angle is 37.6. The strike-slip activity influenced by Longsheng-Yongfu fault is obvious, and the typical thrust deformation pattern of Qin Fang folded orogenic belt is developed, which belongs to the piedmont thrust deformation area of Dayaoshan front.

Although the fault density and fold strength are not great, the southwest Huanjiang Shallow Depression, the western end of Yishan Fault Depression, the southern part of Mashan Fault Uplift, and the northwestern and southern parts of Kurenai Wataru Shallow Depression in the west of Gui Zhong Depression have long been at the front of the Nandan-Duan deep fault compression-torsion deformation zone, with obvious later compression and strong deformation along the fault strata. Devonian, the main target layer in the caprock, has been involved in thrust fold deformation, and the exposed surface in some places has formed thrust deformation pattern. The fault line is contained in the thrust deformation area of the structural belt, and the formation shortening rate in some places (the Jiuwei section of Nandan-Du 'an fault) has reached 33.5%, and the oil and gas preservation conditions are generally poor.

Compared with other structural units, the Liujiang low uplift in the middle of Gui Zhong sag, the northern part of Kurenai Wataru shallow sag and the central and northern part of Mashan fault uplift have relatively flat strata, weak structural deformation, and small dip angle, fracture development density and cap shortening rate. In addition, Liujiang low uplift and the northern part of Kurenai Wataru shallow depression have been in the structural transformation zone for a long time, which are less affected by orogenic belts, with relatively weak deformation and good oil and gas preservation conditions. Among them, the northern part of Mashan fault convex is mainly platform facies deposition in the prototype stage, with relatively high machinability and relatively weak deformation and transformation in the later stage. In addition, the prototype period is surrounded by "platform trench facies", and the oil source conditions are sufficient, so it should be included in the favorable oil and gas exploration area.

According to the statistical data analysis of regional survey data in Qiannan sag, the fault densities of Ping Huang shallow sag, Guiding fault terrace and Dushan nose uplift in the front of Xuefeng uplift in the east of the sag are relatively high, which are 0.89/100km2, 0.54/100km2 and 0.76/100km2 respectively, which are densely developed areas of faults in the sag. The average wing angles of folds are 128.26, 12 1.86 and 108.88, respectively, which is the area with strong fold deformation in Qiannan depression. The data of formation shortening rate show that the guiding fault terrace and Dushan Bilong are 2 1. 1% and 35.3% respectively, which are higher than those in Changshun sag and Anshun sag. The statistics of average dip angle of strata also show basically the same conclusion. Combined with the analysis of drilling, seismic and MT exploration data, the eastern part of the depression is dominated by thrust deformation structures, with relatively many fault blocks, and thrust deformation is the main one in general. Anshun sag and Changshun sag in the southwest of the sag are far away from Xuefeng structural belt, and the influence of piedmont thrust fault is relatively weakened, and the fault density is reduced to 0.67/100km2 and 0.46/ 100 km2 relative to the east of the sag. The average dip angle, shortening rate and folding wing angle are 16.7, 20. 1, 13.3%, 17.5% and 126.98,13/kloc-respectively.

To sum up, it can be seen that the central and western parts of Gui Zhong sag and the southwest part of Qiannan sag are areas with relatively weak structural deformation and good oil and gas preservation zones.