Paleozoic-Triassic "trough-platform" division of Tethys ocean basin and the formation stage of main orogenic system in late Indosinian

In this historical stage, from the old to the new, it is further divided into the following three development periods:

The first is the formation of the early Paleozoic proto-Tethys ocean basin and the division of the pan-Yangtze plate "trough platform"

In the Early Paleozoic, the Xingkai movement, which began in the middle and late Early Cambrian, and the Caledonian tectonic cycle, which ended at the turn of Silurian and Devonian, led to the initiation, development, contraction, closure and northward subduction of the pre-Tethyan ocean basin in the central and northern East Kunlun, forming the Caledonian magmatic island arc orogenic belt. At the same time, the Sino-Korean plate was separated from the Pan-Yangtze plate again, which led to the splitting of the Pan-Yangtze plate on the northwest margin of the south side of the ancient ocean basin subduction plate, and entered the "trough-platform" boundary period between the Songpan-Aba rift in the west and the upper Yangtze block in the east (Yao Dongsheng, 1986). The newly-born Songpan-Ganzi Aba continental margin rift basin area also began to undergo structural decomposition and variation under the action of multi-stage differential block ups and downs in Caledonian cycle.

1. Motianling area

The Chengjiang-Xingkai structural layer, which is composed of platform transitional facies marine caprock from Nanhua to early Cambrian, is covered by parallel unconformity of shallow metamorphic thick dark clastic turbidite mixed with siliceous rocks in the Ordovician Dabao Group (Bailongjiang area) or Silurian Bailongjiang Group (Motianling area) formed in the environment of semi-deep stagnation and reduction rift basin. At the same time, Silurian Bailongjiang Group in Bailongjiang area and Motianling area are related to shallow diabase group (whole rock K-Ar age is 389.7410.93 Ma) and marine altered pillow-shaped spilite, respectively, which were formed in intraplate (non-orogenic) tectonic environment. However, the parallel unconformity contact and "trough-platform" transformation between the shallow metamorphic rock series of the Silurian Bailongjiang Group and the overlying Devonian unreformed sedimentary rock series indicate that the late Caledonian movement from the end of Silurian to devonian period was a land-building movement, which was obviously different from the subduction orogeny that occurred in the north-central part of East Kunlun and North Qinling at the same time.

2. Houlongmenshan-Danba area

The extremely thick and shallow metamorphic silty mudstone, represented by the Tonghua Group of Silurian system, is constructed by bimodal volcanic rocks and formed in the deep-water rift basin environment on the continental margin. Parallel unconformity is superimposed on platform-type shallow metamorphic marine caprocks such as Upper Sinian-Lower Cambrian Qiujiahe Formation+Youfang Formation, and Danba and Qingchuan areas are accompanied by rift-type marine basic volcanic rocks and rheological structure volcanic rocks respectively. Therefore, during the development of Caledonian cycle in the early Paleozoic, the Xingkai Rift movement in the late Early Cambrian led to the uplift of the fault in this block and the general absence of Longwangmiao period in the Lower Cambrian and Middle Upper Cambrian. Then it was blocked by synsedimentary fault in the Late Ordovician and evolved into a continental rift basin, where thick deep-water remote fine-grained clastic shale deposits of Silurian Tonghua Group developed, accompanied by rift-type bimodal marine volcanic eruption, and gradually integrated with the dark carbonaceous slate formation of the overlying Devonian dangerous group. It can be seen that the movement of crustal differential block faults in Caledonian cycle has an overall activity evolution trend from strong to weak or even completely disappeared from early to late.

3. Muli area

The Lower Paleozoic is underdeveloped, lacking a lot, and only the lower Ordovician and lower Silurian strata with limited thickness are preserved. Among them, the lower Ordovician is represented by Rengong Formation and Wachang Formation, the lower part is thick-massive shallow metamorphic sandstone mixed with silty slate formation, and the upper part is shallow metamorphic variegated feldspar sandstone and slate, which are rich in graptolite, brachiopod and gastropod fossils in the middle and late Early Ordovician, with a small amount of basalt layers with thicknesses ranging from 750 to 2000 m; It is in parallel unconformity contact with the stable platform-type crystalline dolomite of the underlying Upper Sinian crystalline construction, during which the Cambrian system is missing. The Lower Silurian is collectively called Mihei Formation, which is mainly composed of carbonaceous siliceous slate, with metamorphic glutenite at the bottom, metamorphic fine sandstone and metamorphic volcanic rocks at the lower part, and siliceous rocks and siliceous limestone at the middle and upper part. It is rich in early Silurian graptolite fossils, generally 130 ~ 206 m thick, and can reach 800m locally, which is similar to the underlying Wachang Formation of Lower Ordovician and the overlying Qiongyi Formation of Lower Carboniferous. To sum up, under the restriction of the multi-act differential block uplift movement of Caledonian cycle, Muli area experienced the development process of "three uplifts and two depressions", which was obviously different from the neighboring Danba-Houlongmen mountain area, that is, from the stable platform in late Sinian to the block uplift in Cambrian, to the shallow-water facies deposition on the sub-stable slope of the early Ordovician continental margin and the associated rift-type basic volcanic activity, and then to the secondary block uplift in the middle and late Ordovician and the early Pleistocene continental margin.

To sum up, during the development of Caledonian cycle in the early Paleozoic, the multi-stage horizontal extension movement caused by the early Xingkai ground fissure movement and the vertical uplift movement of differential faults were the main structural systems that led to the splitting of Songpan-Aba area from the passive continental margin in the northwest of Pan-Yangtze plate, which led to the division of "trough and platform", and then led to the structural decomposition and zoning changes of rift area. In addition, it has a parallel and complementary internal genetic relationship with the trough-arc-basin subduction orogenic system that occurred simultaneously in the north-central part of East Kunlun and North Qinling. In addition, the ancient rift zone and syngenetic (or synsedimentary) ancient fault zone, which are of great significance for division, also laid a basic structural framework for the division of tectonic strata and structural units.

2. Devonian-Middle Permian Paleo-Tethys oceanic basin and continental margin uplift period.

During the Devonian-Middle Permian Variscan development period, the Anima Qing-Mianlue Paleotethys north branch ocean basin and Litang-Batang Jinsha River Paleotethys south branch ocean basin opened and closed almost at the same time, and swooped north and west respectively, forming an island arc-shaped orogenic belt with nearly east-west and anti-S-shaped linear distribution on the subduction zone. At the same time, the Songpan-Aba continental margin, located in the east and south of the south and north branches of the above-mentioned Paleo-Tethys ocean basin, and in the north and west of the Houlongmenshan-Xiaojinhe ancient fault zone, gradually uplifted and evolved into an ancient uplift area of the continental margin under the restriction of the uplift movement of the Variscan differential block fault. It can be seen that the regional crustal movement in the development period of Variscan inherited and developed the characteristics of Caledonian geological structure development, and more obviously showed the dual structural attributes of orogeny and land-making movement going hand in hand.

1. Songpan-Aba area

During this development period, confined to the inverted triangle area between the Qingmian ophiolite melange belt, Ganzi-Litang rift belt and Houlongmenshan-Xiaojinhe ancient fault belt in Anima, the crust continued to rise slowly and the water body changed from deep to shallow under the restriction of the vertical lifting movement of differential blocks. From bottom to top, it is composed of Devonian Guanweiqun+Lower Carboniferous Xianglatai Formation deep-water basin carbonaceous siliceous slate mixed with radiolarian siliceous rocks → Lower Carboniferous Xuebaoding Formation deep-water marine marl-siltstone mixed sedimentary formation → Upper Carboniferous-Lower Permian shelf facies carbonate formation → Middle Permian Sandaoqiao Formation platform tidal flat facies gravelly dolomite limestone formation, which is not integrated in parallel under Lower Triassic Bozigou Formation, and is generally absent during this period. Combined with the obvious thinning of gravel dolomite limestone in Sandaoqiao Formation of Middle Permian around Zoige block, it is the pre-splitting mantle uplift before the large-scale eruption of Emei rift movement that leads to the continuous slow mantle uplift of the continental crust, and then the Songpan-Aba continental margin syngenetic paleouplift and the ancient land with Zoige block as the core are formed.

2. Motianling area

During this period, the rift ocean basin that formed the end of the Oriental basin has been completely closed and entered the development period of the Variscan stable platform. From bottom to top, a complete set of typical Yangtze platform shelf facies sedimentary assemblages such as Devonian, Carboniferous and Middle and Lower Permian have developed. All departments and series are integrated and have not deteriorated. The bottom conglomerate formed by marine carbonate rocks of Upper Permian Wujiaping Formation is covered by parallel unconformity, but the formation loss is not obvious and the sedimentary discontinuity time is short.

Third, the formation period of NeoTethyan ocean basin, rift system, passive continental margin and late Indosinian main orogenic system from late Middle Permian to Triassic.

This is an epoch-making (generalized) Indosinian tectonic development period with the strongest crustal activity, the widest influence and the most obvious geological performance in Songpan-Aba and its adjacent areas. During this period, the Emei Rift Movement, which started from the late Middle Permian to the late Permian, and the late Indosinian orogeny, which ended in different periods of the late Triassic, formed Litang-Batang intracontinental crustal rift-NeoTethys ocean basin, Songpan-Ganzi continental margin-intracontinental trigeminal rift system and so on. Subsequently, the land-sea (and ocean-land) transformation was fully realized, and a huge late Indosinian orogenic system composed of various types of orogenic belts was formed in the huge Tethys tectonic domain.

1. Ganzi-Litang area

The main performance is that after the closure of the Paleo-Tethys ocean basin in Jinsha River, the Ganzi-Litang belt continued to develop and became an ocean crust rift and a Neo-Tethys ocean basin.

First of all, under the action of Emei Rift in the late Middle Permian to the late Permian, the ocean crust rift belt represented by "Kaer ophiolite formation" was formed in Ganzi-Litang area. Ophiolite assemblage is mainly composed of jointed metamorphic mafic-ultrabasic layered accumulation complex, gabbro-diabase wall group, pillow basalt containing exotic limestone blocks in different periods of Paleozoic (the whole rock age of K-Ar method is 242Ma, which is roughly equivalent to the Late Permian) and deep-water radiolarian silicalite of Middle Permian closely related to basalt. Above it is covered with siliceous rocks at the bottom of the Dangen Formation, which is rich in late Permian-early Triassic or middle Triassic deep-water radiolarian assemblages. To sum up, the "Kal ophiolite Formation" has the characteristics of material composition representing the existence of ocean basins. However, considering that the mineral and chemical composition of basic ultrabasic rocks are mostly iron series, and rare magnesium-based ultrabasic rocks have petrological and petrochemical characteristics, Ganzi-Litang belt was defined as an intercontinental oceanic crust separating Qiangtang-Changdu block and Songpan-Aba block from the late Middle Permian to the late Permian. At the same time, in Yidun-Zhongdian area, rift-type marine basic volcanic rocks, sandshale, crystalline limestone and siliceous rocks, represented by Gundaguan Formation, are formed. What does this group contain? Biofossils of collembola, brachiopod and coral show that the formation time is mainly in the late Permian, and the lower part spans the late middle Permian Maokou Formation, with a small amount of parallel unconformity on the lower Carboniferous Qiongyi Formation and integration under the lower Triassic Dangen Formation, which is the contemporaneous product with the "Kaer Ophiolite Formation". Accordingly, under the action of Emei rift, Ganzi-Litang and Yidun-Zhongdian continental crust rift zones were opened in the late Middle Permian, which was also the day when the Paleo-Tethys ocean basin of Jinsha River was completely closed, and they were closely related in time and space distribution and genesis.

Secondly, in the Early-Middle Triassic, the semi-deep-sea slope-deep-sea basin facies shallow metamorphic clastic turbidites, represented by Danggen Formation and Liyi Formation of Yidun Group, including micrite limestone, radiolarian siliceous rocks, a small amount of metamorphic volcanic rocks and diabase layers, were integrated and covered on the metamorphic volcanic rocks of Gangda Group. Similarly, at the top of the "Kal ophiolite Formation", there is also a set of mottled deep-water radiolarian chert cover with early Triassic radiolarian assemblage. In addition, in Baiyuzengke-Ma Rong and other places in the north of the Yidun belt, important magmatic activities such as pyroxenite and diabase invaded by the Middle and Lower Triassic, and a few rock masses covered by parallel unconformity deposits in the Upper Triassic can also be seen. To sum up, the marine "rift cover" of Middle and Lower Triassic, represented by Yidun Group, which is integrated on the Garze-Litang and Yidun-Zhongdian oceanic crust rift, is marked by the widespread distribution of radioactive siliceous rocks in deep water and the appearance of associated metamorphic volcanic rocks and basic-ultrabasic intrusive rocks, indicating that it was formed on the basis of "rift column" formed on land from late Middle Permian to late Permian.

From then on to the Late Triassic, in Ganzi-Litang belt, different tectonic plates (blocks) of "Universal Ophiolite Formation" and "Kaer Ophiolite Formation" overlapped in parallel, which was marked by the appearance of ophiolite suite and volcanic arc basalt-andesite-rhyolite combination, indicating that they entered the formation period of NeoTethys ocean basin and volcanic island respectively. Later, in the late Noriti of the Late Triassic, the pseudo-conformity covered the "universal ophiolite formation", which restricted the continuous appearance of the continental coarse clastic rocks of the Yingzuniang Formation exposed at the highest point of the Upper Triassic in West Kowloon. The sequence relationship between them and the different strata of the underlying Upper Triassic superimposed by the unconformity of Jurassic land-sea interaction and marine deposition shows that the Litang-Batang NeoTethys ocean basin has been in the Late Triassic. Under the influence of the late Indosinian orogeny, the NeoTethys ocean basin first swooped westward in the Ganzi-Litang deep trench, forming a ophiolite melange belt spliced by structural sheets (or blocks) of different ages, different provenances and different compositions, and its associated high-pressure and low-temperature dynamic deformation-greenschist facies metamorphic rock belt, glaucophane. At the same time, the Yidun-Zhongdian volcanic arc belt located on the west side of the subduction zone also experienced large-scale island-arc plutonic magmatic emplacement during the same tectonic period, represented by the Cuojima-Dongcuo intermediate-acid complex belt. So far, Ganzi-Litang ophiolite melange belt and Yidun-Zhongdian volcano-abyssal island arc belt have basically formed.

2. Songpan-Aba area

The development of Indosinian cycle is mainly characterized by the formation of early continental margin-intracontinental trigeminal rift system, middle passive continental margin and late multistage island arc detachment-thrust orogenic belt.

Trigeminal rift system: mainly formed in the late Middle Permian to the late Permian, it consists of the continental margin rift belt of Houlongmenshan-Xiaojinhe and the continental rift belt of Luhuo-Daofu, and divides Songpan-Aba area into two rift continental uplift structures, namely Marcand in the east and Yajiang in the west. The connection point of the trigeminal rift system is located in Yangliuping section of Kangding-Danba. The "Rift Column" is filled with marine basic volcanic rocks of Dashibao Formation, covered by parallel unconformity on the "Rift Basement" composed of gravelly dolomite crystalline limestone of Sandaoqiao Formation of Middle Permian, and located under the "Rift Cover" of Bozigou Formation of Lower Triassic. According to the lower limestone interlayer, there are a few late maokou? Fossil, belonging to the late middle Permian-late Permian. Marine basic volcanic rocks are stably distributed in Xiaojin reach of continental margin rift zone and connected with Ganzi-Litang intercontinental oceanic crust rift zone centered on Muli. In the north, there are uneven iron ultrabasic rock groups and layered platinum-nickel basic-ultrabasic rock accumulations in the Kangding-Danba rock flow flat section near the junction center of the trigeminal system. Continuing to extend to the northeast near Lixian in the southern section of Houlongmen Mountain, the metamorphic volcanic rocks are gradually extinguished, but in the northern section of Beichuan-Qingchuan Chaba line, the shallow diabase veins of the same period are still intermittently exposed.

The Luhuo-Daofu intracontinental rift zone extending linearly in NW-SE direction was not closed at the end of the Late Permian, but continued to expand into an oceanic crust rift zone composed of the so-called Runiangu Formation and Gedicun Formation until it was covered by the semi-deep slope turbidite of the Lianghekou Formation in the middle of the Late Triassic. Among them, Runiangu Formation is integrated on the uncompensated marine deposits of Bozigou Formation of Lower Triassic, but under Gedicun Formation or Lianghekou Formation, and is mainly composed of gray-green and purplish-red altered pillow basalt, metamorphic volcanic-pyroclastic melange containing Carboniferous-Permian foreign limestone blocks, radiolarian deep-water siliceous rocks, clastic turbidite and gray breccia. Siliceous rocks produced radiolarian assemblages in deep water of Middle Triassic, while limestone and slate contained conodonts and bivalve fossils in early Late Triassic. The characteristic values of rock geochemistry and the flat matching curve of rare earth elements show that the altered basalt of Runiangu Formation was mainly formed in the mid-ocean ridge-quasi-mid-ocean ridge tectonic environment, indicating that it belongs to the oceanic crust component and has expanded into the oceanic crust rift zone (or the initial ocean basin). Gedicun Formation is a pod-like parallel unconformity intermittently embedded above Runian Ancient Formation and below Lianghekou Formation, which is mainly composed of unclassified, metamorphic sandstone breccia, coarse sandstone with a small amount of slate and Jiwo coal seam. Slate contains mixed bivalve fossils and ancient plant remnants in the early and middle period of Late Triassic. It is a debris flow-slump mixed accumulation layer formed in the rift steep slope structural environment.

Passive continental margin: it is limited to the northeast of Ganzi-Litang Neotethys basin, the northwest of Houlongmenshan-Xiaojinhe rift fault zone, and the inverted triangle formed in the south of the Animaqing-Lueyang subduction ophiolitic melange belt in the late Variscan period, commonly known as Songpan-Aba continental margin-foreland basin. Located at the passive continental margin in the northwest of Pan-Yangtze Plate, it was built under the pre-splitting uplift and rifting action of Emei Rift Movement, forming the interface between the Middle Permian fault block-fault platform Sandaoqiao Formation gravelly dolomite crystalline limestone and Wu Dong land-making tectonic movement, and the middle-upper Permian rift type marine phase-change basalt of Dashibao Formation. In time, it roughly experienced the vertical fluctuation and structural zoning of the differential fault blocks in the early-middle Triassic, and the rift deposits in the early-middle-late Triassic matured to atrophy and later period.

In the Early-Middle Triassic, the Songpan-Aba continental margin rift-foreland basin was divided into two structural belts with obvious sedimentary facies, which were surrounded by the Aba-Nongriba arc syngenetic fault zone and the Minjiang north-south syngenetic fault zone respectively.

(1) Marcand-Yajiang tectonic community

Bozigou Formation of Lower Triassic and Zhagashan Formation of Middle Triassic are the representatives. The former is parallel unconformity superimposed on clastic dolomite crystalline limestone of Sandaoqiao Formation of Middle Permian or basic volcanic rock of Dashibaohai phase transition of Upper Permian. It is a rhythmic interlayer of hungry tuffaceous chlorite sericite and thin crystalline limestone formed in uncompensated platform basin environment, containing common bivalve and conodont fossils in Early Triassic, and its thickness generally ranges from tens of meters to 100 meters. The latter, which is integrated on Bozigou Formation, is a kind of turbidite phase-change sandstone, mixed with thin crystalline limestone and reef limestone, formed in the shallow sea-slope sub-deep sea environment of shelf barrier, and is generally hundreds of meters to 1000 meters thick.

(2) Zoige structural community

It is the eastward extension of the northern stratigraphic regionalization facies belt of Bayankala structure in Qinghai Province. The distribution range is roughly equivalent to the so-called "Zoige Block". Almost all exposed bedrock is shallow metamorphic marine Triassic, which is called lower sandstone overburden group, middle sandstone overburden group and upper sandstone overburden group+top sandstone overburden group, representing lower, middle and upper Triassic strata respectively (lithostratigraphy of Qinghai Province, 1997). Among them, the marine strata of the Lower-Middle Triassic are different from the southern Jinchuan Group, and the main features are as follows: ① The lower sandstone clapboard formation of the Lower Triassic is in angular unconformity contact with the metamorphic clastic rock-carbonate strata of the lower-Middle Permian Marzheng Formation, which means that the early-Middle Permian Anima Qing-Mianluogutethys ocean basin subducted northward. In the north of Bayankala, south of the subduction complex belt—(2) A set of extremely thick Triassic complex detrital turbidite flysch formation was continuously deposited in the foreland fault basin with continuous and rapid subsidence. According to the regional investigation report of1∶ 250,000 in Neijiuzhi and Donggaicuona Lake Sheet (2003-2005), the Lower Triassic "Lower Sandstone Partition Group" contains the Early Triassic ammonite fauna and sporopollen assemblage, with a thickness of 4,600-5,300 m; The Middle Triassic "Middle Sandstone Interlayer Formation" contains a variety of animal fossils, such as ammonites, brachiopods, bivalves and sporopollen assemblages, which are common in the Middle Triassic and are generally about 2500 meters thick. (3) The clastic materials of turbidite have the characteristics of terrigenous and volcanic multi-sources, and sometimes there are intermediate-acid volcanic lava-pyroclastic sedimentary rocks such as metamorphic An Ning gray sandstone, a small amount of metamorphic andesite lava and subvolcanic dacite cryptoexplosive breccia. ④ There are discontinuous outcrops in the "medium sand slate interlayer construction" in Maduo-Gande, Qinghai Province and Maqu-Zoige Heihe Pasture, Gansu Province, which contain foreign Carboniferous-Middle Permian limestone blocks and gravels with different numbers, sizes and shapes, mixed with multi-component clastic matrix deposits, or called "wild flysch beds". To sum up, the Lower-Middle Triassic strata, represented by the "Lower Sandstone Cladding Formation" and "Middle Sandstone Interlayer Formation" of Bayankala Group, are the semi-deep water slope facies-deep water trough basin facies composite detrital turbidite flysch formation rapidly accumulated in the foreland rift flexion under the conditions of continuous large-scale subsidence of the crust and sufficient supply of terrigenous materials and submarine volcanic eruption materials, and are also the early Aba foreland. It was not until the Late Triassic that the subsidence center gradually moved southward from Bayankala-Zoige facies area in the north to Marcand facies area and Yajiang facies area.

The early Late Triassic (Kanian) was the biggest expansion period of Ganzi-Litang Neotethys basin and Luhuo-Daofu oceanic crust rift zone, and also the heyday of Songpan-Ganzi rift basin, which formed the transgression sequence of Kanian Zagunao Formation+Jurassic+xinduqiao Formation and its huge turbidite formation. At the same time, a subsidence center was formed in Jinchuan facies area, with the dark slate of xinduqiao Formation as the largest flooding surface, and the low-speed condensed layer as the representative. Then, limited to the Luhuo-Dawu rift zone and Yajiang stratigraphic zone, it is integrated on the xinduqiao Formation. The upward shallowing and thickening regressive sequence represented by the Noyian Lianghekou Formation and Yajiang Formation and the turbidity flysch formation formed by it are closed into Ganzi-Litang NeoTethys ocean basin and Luhuo-Dawu rift zone, and the Songpan-Ganzi rift area is closed from north to south and from east. This shows that in the middle of Late Triassic (Nori period), Jinchuan community has ended its transgression and returned to land, and the settlement center moved westward to Yajiang community. By the end of Late Triassic (Tianrui period), the marine sedimentary history of Songpan-Ganzi rift basin had completely ended, and marked by the unconformity of coal-bearing pyroclastic rocks in intermontane basin of Nianbao Formation of Lower Jurassic, it covered the structural interfaces of different layers of Upper Triassic, forming a magnificent late Indosinian folded orogenic belt.

In the Tethys tectonic domain, Indosinian orogeny can be roughly divided into early, middle and late episodes, or I, II and III episodes, which occurred in the late Middle Permian (Latin period), the middle Late Triassic (Nori period) and the late Late Late Triassic (Ruiti period) respectively. Moreover, the Indosinian orogeny has the evolution law of migration from north (East Kunlun-West Qinling Lingnan section) to south (Songpan-Ganzi area) and from east (Songpan Marcand area) to west (Yajiang area) in time and space. The folded orogenic belt in Songpan-Ganzi rift foreland basin was mainly laid by episodic orogeny in Indosinian period. The orogenic process is roughly as follows: during the late Indosinian orogeny, under the collision-contraction-compression tectonic system between the North China block and the upper Yangtze block, the East Kunlun-West Qinling orogenic belt underwent a large-scale north-south thrust, and at the same time, a relatively passive intracontinental subduction-deep tectonic wedge occurred from south to north, forming the Songpan-Aba intracontinental thrust orogenic belt. The late Indosinian orogenic belt is accompanied by low-temperature dynamic metamorphism of greenschist-low greenschist facies area, ductile shear deformation metamorphism controlled by unconformity interface between folded basement and marine caprock, and emplacement of deep-melting intermediate-acid magma controlled by strong diapir emplacement.

3. South area of West Qinling

During the development of Indosinian cycle, the Tazang tectonic melange belt and the Bailongjiang-Motianling horst-type peninsula carbonate platform were formed.

(1) Tazang structural melange belt

It is distributed in Jiuzhaigou County, located in the middle part of the Qing-Mianlue ophiolite melange belt in Anima. Its present structural form is a "structural window", which is partly hidden under the thrust nappe of Xiqingshan and Motianling, and partly exposed. After stripping off the nappe formed in the late period, the Tazang belt is actually a rift structural belt superimposed on the pre-Animaqing-Mianlue late-generation west ophiolite melange belt. Rift zone mainly consists of Late Devonian, Carboniferous, Permian, Early Triassic to Early Late Triassic, Carboniferous-Permian? Rock fragments or blocks of different ages, different lithology, different sizes and shapes, as well as rift-type marine lepidolite volcanic rocks, radiolarian siliceous rocks and clastic turbidites are formed by structural splicing in the form of mixed deposits, volcanoes and structures. Among them, the whole rock K-Ar age values of thinned tourmaline volcanic rocks obtained at present can be divided into two groups: one group is 277Ma, which is roughly equivalent to the theoretical age values of Early Permian and Middle Permian; The other group is 23 1 ~ 224 Ma, which is roughly equivalent to the geochronology from the late Middle Triassic to the early Late Triassic. However, the widely distributed turbidite layer in the early Late Triassic covered the rift-type structural melange belt in the form of "rift cover". To sum up, combined with the comprehensive analysis of the regional geological and structural background of South Qinling, it is preliminarily considered that the Tazang belt may have originated in the Emei Rift Period of the Middle-Late Permian, formed in the Early-Middle Triassic, and closed and deformed in the Early-Indo-China orogeny of the Late Triassic. Geological events related to main orogeny include low-temperature dynamic metamorphism in low greenschist facies area and small-scale acid magma emplacement activities such as crust-source reformed granite porphyry.

(2) Carbonate platform of Bailong River "horst peninsula".

From Late Permian to Early Late Triassic, the structure was relocated to the northern part of the pre-Animaqing-Mianlue late Paleozoic ophiolite melange belt, which is part of the southern margin of the late Paleozoic upper Yangtze platform carbonate land in the south-central part of East Kunlun-West Qinling. During this period, the stable Yangtze platform carbonate rocks, represented by Upper Permian Wujiaping Formation+Changxing Formation, Lower Triassic Zhalishan Formation+Maresongduo Formation and Middle Triassic Guojiashan Formation in Miko Wu, were formed on the middle Permian Maokoudian carbonate rocks in a parallel unconformity manner, with a cumulative thickness of about 3,000 m, during which the cold dock terrace strata at the top of the middle Permian may be missing, and the middle Triassic Latin terrace and above strata may be missing. It can be seen that in the late Middle Triassic (Latin period), Bailong River ended its marine sedimentary history and uplifted into land. Under the collision and compression orogeny from north to south in the Latin period of Middle Permian, it first disintegrated from the southern platform of West Qinling, forming a horst-type "thrust structure" with three main boundaries: thrust fault, mid-belt recoil fault and deep basement detachment shear layer, which spans nearly east and west. Therefore, it is actually a horst-type peninsular carbonate platform extending in the east-west direction, in which the northern margin of the upper Yangtze platform is inserted westward and protrudes from the Indosinian rift basin.