1. Early deep ductile detachment structure
1) includes the Neoproterozoic Qingbaikou Xilangyan Formation to the Middle Triassic Maanshan Formation, forming a multi-level assemblage with NW-NWW compact recumbent fold and bedding recumbent fold (F 1). Bedding slip shear develops between strata with weak ability and between strata interfaces, such as the contact interface between Xilang Formation and Zhougang Formation, the contact interface between Silurian and Upper Devonian, and the thin limestone and shale of Lower Carboniferous, Permian and Triassic, which often leads to the loss and thinning of strata. These folded deformed bodies and sliding sections are folded again with the main deformation (Figure 2-9).
2) Due to the differences in depth and stratigraphic capacity, the rocks in the above-mentioned times developed different forms and degrees of cleavage (S 1). For example, Neoproterozoic Xilangyan Formation, Zhougangyan Formation, Sujiawan Formation, Doushantuo Formation and Dengying Formation (Plate Ⅱ1) developed strongly and continuously with cleavage (S 1) and bedding (S0) strongly explained; See s1∑ s0; ∑ s0 at the folded wing; See S 1 intersection S0 at the turning end. In Paleozoic limestone, it is characterized by intermittent cleavage (S 1), and it mostly intersects with S0 in different forms (Figure 2- 10, Figure 2-1,Figure 2- 12).
3) Developing extensional lineation and mineral growth lineation (L 1). Such as some volcanic breccia, lumps, pores, etc. Both Xileng Formation and Sujiawan Formation are strongly extensional and deformed, and their directions are nearly perpendicular to the direction of the intersection lines (S0 and S 1) (representing F 1 hub) representing the folding hinge in this period (Figure 2- 13, plate Ⅱ 2). On the cleavage plane (S 1) of magnesium-rich spilite in Xilang Formation, NNE-trending mineral growth lineation (L 1) represented by glaucophane develops, and its formation is related to deep detachment shear. Xu Jiawei (1980) once thought that this lineation was related to the sinistral translation of the Tan-Lu fault zone. Xu Zhiqin (1987) thinks that this is related to the deep detachment shear in Indosinian period. More people misjudge this lineation as "A" lineation and "A" fold because it is consistent with the main fold hinge and is the product of the northward subduction and collision of the Yangtze plate. However, the strike of this lineation is almost consistent with the strike of orogenic belt and main fold hinge, and its associated cleavage plane (S 1) is obviously formed earlier than the main fold deformation.
Figure 2-7 tectonic map, Northeast Margin of Yangtze Block
1- metamorphic basement of Neoarchean North China Block; 2- Metamorphic basement of Neoarchean-Proterozoic Yangtze block; 3- Neoarchean-Paleoproterozoic Dabie-Sulu metamorphic complex; 4- Neoproterozoic Hongan Group, Susong Group and Haizhou Group; 5- Neoproterozoic volcanic rock distribution area (Zhangbaling Group and Suixian Group); 6- Neoproterozoic strata in North China; 7- Yangtze type Early Paleozoic-Mesozoic strata; 8— Axial trajectory and reversal of Indosinian main fold; 9-I North China Block; 10-ⅲ Dabie-Sulu orogenic belt; 11-Ⅲ1North Huaiyang structural belt; 12-Ⅲ 2 Dabie metamorphic complex-ultrahigh pressure metamorphic rock belt; 13-Ⅲ 3 Susong-Zhangbaling structural belt; 14-Ⅱ northern margin of Yangtze block; 15- Fault and serial number: ① Queshan-Gushi-Lu 'an fault; ② Mozitan-Xiaotian fault; ③ Yingshan-Shuihouling fault; ④ Tan-Lu fault zone; ⑤ Xiangshui-Huang Po fault.
4) Various mylonites and structural schists are developed in the rock stratum. Especially mylonitized rocks, mylonite, phyllite and various micro-fabrics (wiredrawing, core mantle, fragmentation, mica fish, etc. ) can be seen everywhere in the rocks dominated by hornfels in Xiaxileng Formation. Mylonized limestone and mylonite zone also appear in limestone, and the degree of upward mylonitization and cleavage gradually weakens, and cleavage rocks only appear in thin limestone and shale with weak ability.
5) In Zhangbaling area on the southern edge of Dabie orogenic belt, passing through Chuzhou, Hexian and Chaohu, there is a common fold shape that is closely recumbent in dry period and non-axis superposition in main period. Due to the development and strength of the main fold and the concealment and transformation of the late slip-nappe thrust structure, this superimposed fold is mistaken for the formation of the late "thrust nappe structure" Through typical outcrop structural anatomy and regional structural tracking, the existence of early folds and the superimposed deformation of main folds can still be determined. The early fold hinge (L 1) and axial plane (S 1) in Chuzhou area were bent and refolded by the main fold (F2) (Figure 2- 14). The early recumbent fold (F 1) was turned by the almost orthogonal main fold (F2) and folded again (Figure 2- 15). In the area from Yinpingshan to Kaicheng Bridge in Chaohu Lake, the inversion wing of the early recumbent fold folds again, forming a series of regional anticline and radial anticline (F2) structures (Figure 2- 16). Obviously, these folds are not "fan folds", let alone "inverted folds" formed by thrust nappe structures (Xiamulin, 1989). From outcrop scale to mapping scale, the existence of the above superimposed folds is the main basis for determining the existence of early fold deformation in this area.
Fig. 2-8 tectonic map in Hexian-Hanshan-Chaohu area, the northern edge of the eastern Yangtze block.
1- four yuan; 2- Cretaceous; 3- Lower Jurassic; 4- Middle Triassic; 5- Upper Devonian-Lower Triassic; 6- Upper Devonian-Carboniferous; 7- Silurian; 8- CAMBRIAN-SILURIAN; 9- CAMBRIAN-Ordovician; 10-CAMBRIAN; 11-Sinian system; 12- South China; 13—— Western Union Formation of Neoproterozoic Zhangbaling Group; 14- Neoproterozoic Feidong Group; 15- Late Archean-Early Proterozoic Kanji complex; 16- Mesozoic granite; 17 —— Measure and predict cracks; 18- sliding thrust fault; 19 —— early Indosinian fold trajectory; 20— the trajectory of main fold axis in Indosinian period; 2 1- anatomical region and number; 22— Anatomical point and number
Figure 2-9 Plan view of detachment fault (F) involved in fold
(Quoted from1:50,000 Chaohu Lake)
SG- Silurian Gaojiabian Formation; SF- Silurian tomb construction; Carboniferous and Hezhou Formation; D3-c 1- Upper Devonian-Lower Carboniferous Wu Tong Formation; CJ- Carboniferous Jinling Formation
Figure 2- 10 Limestone Synclinal Fold of Sujiawan Formation in Chaohu City
Fig. 2- 1 1 Siliceous belt fold of Sinian Doushantuo Formation in Wali, Hexian County
Fig. 2- 12 banded microcrystal limestone folds of upper CAMBRIAN Langjiashan Formation in Cangyin Depression, hanshan county.
Figure 2- 13 Relationship between tensile lineation and F 1 fold hinge of Zhougang Formation in Shijiaji, Chuzhou City
Figure 2- 14 Schematic Diagram of Geological Structure of Yinpingshan-Kaicheng Bridge in Chaohu Lake
1-Cretaceous; 2- Lower Jurassic; 3- Jurassic; 4- Middle Triassic; 5- Triassic; 6- Permian; 7- Upper Devonian to Permian; 8- Upper Devonian to Triassic; 9- Upper Devonian to Carboniferous; 10-Silurian; 11-Ordovician; 12-CAMBRIAN; 13-CAMBRIAN to Ordovician; 14-Sinian; 15— Indosinian linear fold; 16— linear oblique fold in Indosinian main period; 17-Indosinian main back fold; 18— Early Indosinian creeping fold; 19— Feilai peak and structural window; 20— Fault measured and inferred; 21-personal data; 22-anatomical region; 23— Early axis trace of Indosinian; 24— Axis trace of Indosinian main period; 25-the new direction of class change
Fig. 2- 15 F 1/F2 superimposed fold map from Laozi to Tiangushan in Beiyin Mountain, hanshan county.
1- four yuan; 2- Lower Jurassic; 3- Middle Triassic; 4- Middle and Lower Triassic; 5- Lower Triassic; 6- Permian; 7- Carboniferous-Permian; 8- Upper Devonian-Carboniferous; 9- Upper Devonian-Permian; 10-Middle Silurian; 1 1- Lower Silurian; 12- CAMBRIAN-Ordovician; 13-CAMBRIAN; 14- geological boundary; 15- Unconformity interface; 16- failure; 17- sliding thrust fault; 18-Indosinian fold axis; 19 —— early Indosinian fold trajectory; 20— Formation occurrence; 2 1- Formation occurrence inversion; 22— Section line
6) The inverted wing formed by the early folds is considered by Xue Hu (1992) as the expression of "thin skin structure". For example, the stratigraphic sequence exposed in Ningzhen Mountain Range is an inverted superimposed stratigraphic sequence, not a normal stratigraphic sequence. Its folding deformation is not "three-back and two-direction", but three-direction anticline and two anticlines (which can be called directional anticline and anticline, the author's note), that is, the "three-direction and two-back" structure (Figure 2- 17). The author thinks that the detachment interface under it should be an early slip fault interface or a late resurrection, but there should be a normal stratigraphic sequence under the interface. There is a similar situation in Maoshan area of Jiangsu province. In Huangmei area, Hubei province, the exploration data of Huangmei iron mine show that the Wulidun inverted anticline does not exist (Xue Hu, 1992), but is an anticline structure composed of inverted strata (Figure 2- 18), which was later pushed over Jurassic by late structures.
7) This kind of fold structure has been reported in Wu (1994), Wei Xu (1996), (1997) and Chudongru (1998) in the northern part of the Yangtze block in southern Anhui, and they all belong to the deep layers under the early extensional structure system. Its main feature is the development of bedding fold and bedding cleavage (S 1). Chu Dongru (1998) clearly pointed out that the Paleozoic strata in Ningguo, southern Anhui Province experienced two stages of Indosinian deformation. In the early stage, it was a bedding recumbent fold formed under the extensional tectonic system, which was formed by the superposition of the second and third stages (equivalent to the main stage delineated by the author). The early extensional lineation trend was N70°E, which was consistent with the direction of the second stage compression fold hinge (NE).
Fig. 2- 16 anatomical map of superimposed fold structure in hexian county, Hanshan county and north Korea (F 1/F2)
1-Middle Triassic; 2-Nanlinghu Formation; 3- Helongshan Formation; 4- Longtan Formation; 5— Gufeng Formation; 6- Wu Tong Group; 7- graves; 8- Gaojiabian Group; 9-Ordovician; 10-Guanyintai Formation; 1 1- baotaishan formation; 12-Mufushan Formation; 13- Langyashan Formation; 14— Yangliugang Formation; 15— Huanglishu Formation; 16-CAMBRIAN; 17-CAMBRIAN-Ordovician; 18-Sinian; 19- stratum occurrence; 20— inversion of stratum occurrence; 2 1- measured fault; 22— Presumption error; 23- Slip-thrust fault; 24— the second folding track; 25— the first folding trajectory; 26- Refer to Figure 28 for the number and location of drawings.
Figure 2- 17 Structural Schematic Diagram of Longtan-Funiu Mountain in Ningzhen Mountain Range
(quoted by Xue Hu, 1992)
T3-J2- Upper Triassic to Middle Jurassic; C-T2- Carboniferous to Middle Triassic; S-D- Silurian to Devonian; S-T- Silurian to Triassic (in S-T- system)
Fig. 2- 18 Structural Profile of Huangmei Iron Mine Area in Hubei Province
(quoted by Xue Hu, 1992)
S-D- Silurian to Devonian; C- carboniferous; P- Permian
To sum up, in the northeast margin of the Yangtze block, from east to west and from north to south, the early fold deformation caused by the deep ductile detachment shear structure under the early extensional tectonic system and its accompanying structural traces are widespread. Because the research has just begun, and most areas are covered up, there are not many deep data, and the structural details are not very clear. However, through the structural anatomy of some typical areas, according to the lineation vector and the direction of fold overlap, its overall movement direction slides from NE to SW and is separated from thrust.
Fig. 2- 19 Schematic Diagram of Duck Pit Structure in Ningguo City
(quoted from Chu Dongru, 1998)
1- Lower Cambrian Tanghe Formation; 2-Upper Sinian Piyuancun Formation; 3- Upper Sinian Lantian Formation; 4- early stretching lineation; 5— Early folding hinge; 6— trajectory of main fold axis; 7— Formation occurrence; 8-F2 axial cleavage
2. The linear tight fold belt formed in Indosinian period developed in the main period, from north to south and from east to west. Besides Zhangbaling fold belt, there are Chuzhou fold belt, Chaohu fold belt and Huaining-Susong fold belt on the north bank of the Yangtze River. There are Ning (Nanjing) Zhen (Jiang) fold belt, Fan (Chang) Gui (Chi) fold belt, Maoshan-Liqiao fold belt, Wuxi-Yixing-langxi fold belt, Jingxian-Qingyang fold belt, and Dongzhi-Huangshan fold belt and Ningguo-Jixi fold belt adjacent to Jinning fold belt in southern Anhui. According to its tectonic position, it can be divided into north area, middle area and south area from north to south, and its main deformation characteristics are as follows.
1) northern belt. The fold belt in this area is located in the southern margin of Dabie orogenic belt. Except for Zhangbaling fold belt, which is mainly composed of Neoproterozoic Qingbaikou strata, Chuzhou fold belt is composed of a series of tight anticline and syncline structures from Nanhua to Silurian strata, which belong to similar fold (F2), with NE-NNE in the axial direction, NW in the axial direction, reversed SE in the axial direction, and developed by axial plane cleavage and bedding displacement (S0). In Xilang Formation and Sujiawan Formation, S2∑s 1∑S0 can be seen locally, forming cleavage zones. Thrust failures are common in inverted aircraft wings.
2) Southern belt. The fold belt in this area is located in the northern edge of Jinning fold belt in southern Anhui, mainly distributed in Jingxian-Qingyang, Huangshan-East and Ningguo-Jixi areas, and consists of anticline and syncline structures from Cambrian to Silurian. Axial NE turns in an arc of 45 ~ 60, and the axial plane is steep or slightly inclined to SE. The fold hinge fluctuates between 10 ~ 15, and geNErally tends to ne direction. Interval axial cleavage is seen at the turning end of the fold, and the rock layer at the turning end is not obviously thickened, which belongs to an equal thickness fold. It should also be pointed out that in these fold zones, early intralayer folds and bedding shear phenomena, anticline and radial anticline structures formed by superimposing on the inversion wings of early folds, and folds formed in different directions by changing the pivot orientation or axial plane in the early stage due to late structural deformation are common.
3) Intermediate belt. The fold belts in this area are distributed in Chaohu, Huaining, Susong, Ningzhen, Maoshan, Fanchang and Guichi. It is mainly composed of Upper Paleozoic and Middle Triassic Maanshan Formation. Silurian is mainly exposed in the core of anticline, with a small number of Cambrian and Ordovician. The axis of syncline is mainly from Permian to Lower Triassic, and the Middle Triassic Maanshan Formation appears locally. The axial direction is generally NE or NNE arc, mainly vertical to oblique, and is as thick as similar folds. In the area of Maoshan and Guichi, the north axis is mainly inclined to NW, and the south axis is slightly inclined to SE, forming a famous "thrust fold belt". That is, cleavage (consistent with the axial plane of folds) is distributed in an inverted fan shape, and regional anticline and syncline folds superimposed with early folds are also widely developed in this area, but they have not been paid attention to by predecessors, or are regarded as "overturned folds" formed by "thrust nappe".
On the whole, the above-mentioned fold belts are inclined to NE, rising and converging to SE, reflecting that the metamorphic basement in this area is inclined to NE on the whole. In the north, from Tongcheng to Susong, Chuzhou and Chaohu folds extend to SW, which are obviously cut off from the southern margin of Dabie Mountain and submerged under the buried hill basin and Dabie Mountain, indicating that Dabie Mountain was strongly uplifted and slipped southward before the Late Cretaceous. The folds in Ningguo, Huangshan, Dongzhi and other places in the south obviously intersect with the Jinning fold belt in the south of Anhui. The former is northeast-oriented, while the latter is nearly east-west, which is characterized by the unconformity between the Neoproterozoic Gegongzhen Formation, Xiuning Formation and Mesoproterozoic Shangxi Group (most of which have been destroyed by ductile slip surface) and the underlying Shangxi Group, showing a bifurcated anticline fold inclined to the northeast, which clearly reflects the superposition of the Indosinian main fold and Jinning fold belt. Due to the above-mentioned early and main fold deformation, the Middle Triassic Madong Anshan Formation was involved in the fold and covered by the lower Jurassic Moshan Formation unconformity (Figure 2-20), indicating that its formation time was from the end of Middle Triassic to the early Jurassic.
Fig. 2-20 North J 1m of Baoshan Reservoir in hanshan county is unconformity on t 1-2- Lower Jurassic Moshan Formation; T 1-2 —— Middle and Lower Triassic
3. Late sliding nappe thrust structure
Slip-nappe thrust structure is very developed in the eastern part of the northeast margin of the Yangtze block, which has great damage or transformation effect on the superimposed deformation formed in the early and main period of Indosinian. Due to the construction of Hening Expressway, the mountain range from Feidong to Chaohu Lake was split, and the original "thrust nappe structure" was fully displayed. Through a large number of detailed investigations, the author confirmed that these overthrust faults have the characteristics of shallow steep and deep gentle, trailing edge sliding and leading edge thrust. The folds and superimposed folds formed in the early and main period of Indosinian continued to slide forward and thrust along the detachment plane, but did not cause a completely reversed stratigraphic sequence in the area.
According to the data of Xu et al. (2000), there are Xiangquan nappe and Bantang nappe in Chaohu area. Xiangquan nappe is located between Longwangjian in hanshan county and Xiangquan in Hexian County, extending in an arc from northeast to south, with a length of about 40 kilometers. It is composed of Zhaoguan-Chiershan thin slice, Qingshan-Shrinkage thin slice, Longwangjian-Lion Mill thin slice, Zhaoguan Feilaifeng, Zhaoguan coal mine structural window and so on. The sheet on the sliding section is an oblique inversion fold composed of Silurian-Middle Triassic strata or a anticline and a radial anticline. The sliding section is often located in the Silurian Gaojiabian shale and overthrusts the Moshan Formation in the early Jurassic (Figure 2-2 1). On the whole, the profile is gently inclined to the northwest, gently inclined to the west and steep to the east, and the profile fluctuates by 20 ~ 50 (Figure 2-22 and Figure 2-23). Generally speaking, it shows that the NW-trending slip to the NW-trending, forming a late-developed thrust structure. Bantang nappe is located in Longgushan, Wuwei County and Bantang, Chaohu City, with an arc extending about 70km from northeast to south. It consists of Bantang Longgushan slice, Scattered Soldiers Caoxieling slice, Huangniubei Feilai peak, structural window, etc. Slices on the sliding section are a series of linear compact oblique inversion folds composed of Dengying Formation of Sinian to Ordovician strata, and a reverse structure composed of inversion strata. The sliding section is often located in Gaojiabian shale and argillaceous limestone of Nanlinghu Formation in Early Triassic. The profile strikes NW, with great fluctuation, gentle west and steep east, and generally slides and overthrusts from NW to SE.
Figure 2-2 1 Schematic Diagram of Structural Window in Yan Jia, hanshan county
Z-∑- Sinian-Cambrian; S1g-Lower Silurian Gaojiabian Formation; J 1m——Lower Jurassic Moshan Formation
Figure 2-22 Section of Longwangjian Slip-nappe Thrust Structure in hanshan county
(quoted by Xu et al., 2000)
S1g-Lower Silurian Gaojiabian Formation; ∈3-o 1- Upper Cambrian-Lower Ordovician Guanyintai Formation; ∈2p- Middle Cambrian Paotaishan Formation; ∈ 1m —— Lower Cambrian Mufushan Formation; Upper Sinian Z3D-Dengying Formation
Figure 2-23 Slip-thrust Structure Profile of Qimen Station in hanshan county
(Xu et al., 2000)
Electronic higher education system; K1-Lower Cretaceous; T- Triassic system; C- carboniferous; D3-C- Upper Devonian-Lower Carboniferous Wu Tong Formation; SF- Silurian tomb construction;
SG 1-3- Silurian Gaojiabian Formation; O- Ordovician; ∈3-o 1- Upper Cambrian-Lower Ordovician Guanyinshan Formation; ∑-Cambrian; Sinian system
Li Zikun et al. (1986) described nappe structures such as Zhangjiashan (north of Hongzhen in Huaining County), Hexi Mountain (Susong), Huolonggang (Wuhu) and Jiulianshan (Beaver Bridge in Xuanzhou City) except Xiangquan and Yinpingshan nappes. There are a series of arc thrust fault segments protruding toward the SE direction in the southeast edge of Dabie Mountain in Hexi, which are gentle in the west and steep in the east, and tend to NW, and the NW-trending SE-trending sliding thrust fault is above the Upper Jurassic. Huolonggang nappe structure, the profile is inclined to SE, steep in the west and gentle in the east, and the dip angle is15 ~ 30, and it slides from SE to NW. The SW direction of Maoshan nappe at Jiulianshan nappe structure tends to SE, and it slows down to the depth of SE, showing a sliding thrust from SE to NW. According to the analysis of the above data, the sliding thrust structure occurred from late Jurassic to early Cretaceous, forming a sliding thrust "thrust belt" from the south of Xuanzhou (Liqiao) to Guichi.
In Jiangsu, Xue Hu (1992) described the thin-skinned structures in Suzhou, Yixing, Ningzhenshan and Maoshan. That is, the folded sheet composed of inverted or superimposed stratigraphic sequence, with the weak working strata as the detachment interface, napped on Mesozoic strata or magmatic rocks, forming a series of flying peaks and structural windows. So Ningzhen and Maoshan are Wugen Mountain. Xu Xuesi et al. (200 1) made a deep study on the Maoshan nappe structure, and thought that it was composed of three foreign rocks (P2-T 1, C-P, S-D) and three lubricating layers (P2, C 1, S 1). The foreign rock sheets are superimposed upward along the slip fault interface, and then slide from SE to NW in turn, generally superimposed on the original system J2-K 1 stratum, and new folds and fractures appear in the underlying stratum in the later period.
In addition, in the middle section (Susong to Wuhan) and the west section (Huangpi to the north of Hubei Shennongjia) of the northern margin of the Yangtze block, according to the geological records of Anhui and Hubei provinces, a series of mainline folds are still developed in the adjacent areas of the Qinling orogenic belt and the Dabie orogenic belt, which are consistent with the main direction of the orogenic belt. For example, a series of S-shaped twisted linear folds and southward thrust fragments appear in the northern part of Shennongjia, and the thrust nappe structural system is arranged in imbricate shape. Shennongjia, Dahongshan and its south area have simple fold deformation and are open dome structures. From Wuhan to Susong, it is also a linear fold in the direction of NWW—NEE, which is cut off by the Dabie Mountain orogenic belt with its bending protruding south, or concealed under the mountain protruding south of Dabie Mountain. Its fold form is closed, syncline and inverted to the south in the south of Wuhan. Local folds are bent into an "S" shape. The fold from Huangshi to Susong is complex in shape, mainly in the form of closed lines, accompanied by thrust faults in the same direction, and a series of structural windows appear. The fold turns to EW-NE direction, and the local twist is S-shaped, and it turns to NE or NW direction. The folds in the above areas were mainly formed in the Late Triassic, and the fold hinge was bent in an "S" shape, which may be caused by the superimposed deformation of multiple folds, but it needs detailed structural investigation to determine.