Sichuan basin

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Sichuan basin
Satellite image of the Sichuan Basin

The Sichuan Basin ( Chinese  四川盆地 , Pinyin Sìchuān péndì ), which is also known as the Red Basin ( 紅色 盆地  /  红色 盆地 , hóngsè péndì or 赤色 盆地 , chìsè péndì ) because of its red and purple bottoms , is a sink on the upper reaches of the Yangtze in the east of Sichuan Province and in the west of the government-direct city of Chongqing in southwest China.

geography

The basin is roughly rectangular in shape and extends in a south-west-north-east direction over about 600 km in length and 380 km in width. It has an area of ​​around 200,000 square kilometers and is almost completely bordered by mountains: the Wu Shan in the east, Dalou Shan and Daliang Shan in the south, the Qionglai Shan and Min Shan in the west, the Daba Shan and Micang Shan in the north. These mountains used to be the reason for the seclusion of the area, which was only accessible via a river in the north and one in the south.

The surface area in the basin is 400 to 800 meters above sea level. Numerous rivers flow through it. The largest rivers are Min Jiang , Tuo Jiang , Jialing Jiang and Wu Jiang , all of which have their source in the mountainous west of Sichuan Province. The Chinese name of the province of Sichuan - "Four Rivers" - goes back to them. They flow through the basin and flow into the Yangtze . In the western part of the area lies the Chengdu Plain with its fertile soil.

The basin is an important agricultural production area and harbors rich mineral resources.

climate

Climate diagram of Chengdu
Climate diagram of Chongqing

The climate in the Sichuan Basin is balanced. It has hot, humid summers and mild, also humid winters. Precipitation falls throughout the year, the average annual rainfall is 1,000 mm. Three quarters of the precipitation fall in the months of May to September. The mountains in the north and east shield the basin from continental cold air, while the lower mountain ranges in the south cannot prevent warmer subtropical air masses from entering the basin. Therefore, the mean temperature in January is between 5 and 11 ° C, in July it reaches almost 30 ° C. The climatic conditions allow agricultural activities almost all year round, the growing season is 11 months. These favorable conditions have led to intensive agriculture in the Sichuan Basin for 2,000 years.

geology

The Sichuan Basin is part of the Yangtze microcontinent or kraton , which formed in the late Proterozoic (850 to 700 Ma ). Up until the Middle Triassic , the microcontinent was surrounded by the open sea, while sediments from the shallow sea were deposited on it. The first outlines of the Sichuan Basin took shape when the Yangtze Craton initially collided with the Songpan Ganzi Block on its northwestern side during the Indosinian Orogeny in the early Mesozoic . Afterwards, these two microplates collided with the North China-Korean Block in the north and Tibet in the west, so that the Yangtze Kraton became an integral part of the Eurasian Plate .

Today the basin lies at the southeast end of the Tibet collision orogen , which was created by the collision of the Indian and Asian continental plates. While it has been a deposit area almost continuously since the Proterozoic , especially in the northwest , and absorbed up to 12 km of sediment, in the last 10 million years it has been lifted mainly in the center and on its southeast side, so that up to 5 km of sediment is eroded again in places were. Only in its northwestern part do thick Pliocene and Pleistocene deposits indicate permanent subsidence.

Geological construction

The Sichuan Basin is geologically divided into three parts. In the northwest, the northwestern depression ( Northwestern Depression ) takes up massive sediment series as the fore-depth of the Longmenshan fold belt. The central Sichuan basin ( Central Uplift ) has flat or only gently undulating layers, while the southeastern third ( Southeastern Fold Belt ) is characterized by a multitude of long-lasting folds.

The basin is surrounded on all sides by belts of folds and overthrusts. The north-west side of the basin is covered by the rocks of the Longmenshan fold and thrust belt, which moved southeast before the collision between India and Asia ( 龍門山 褶皺 沖 斷 帶  /  龙门山 褶皱 冲 断 带 , Lóngmén shān zhězhòu chōng duàndài , English Longmenshan fold and thrust ) run over at a thrust zone. The fold and thrust belts of the Min Shan and Daba Shan in the northeast run across the other geological structures of the Sichuan Basin. Similar to the Longmenshan fold belt, Proterozoic and Paleozoic, partly metamorphic rocks are pushed onto the mighty series of the Triassic and Jurassic . The southeast side is affected by the folds and thrusts of the Qiaoshan fold belt, which, to the southeast outside the basin, brings ever older layers to the surface through thrusts and scaling. In the southwest, the basin is bounded by the Xichang-Yunnan fold belt, a complex zone that extends over 1,400 km from Qinghai to southern Yunnan . Significant horizontal shifts to the left have taken place on it. Its west side is formed by the Anninghe Fault and the Xianshuihe Fault, which are still active today .

Longmenshan fault zone

USGS map of the 2008 Sichuan earthquake : numerous aftershocks trace the course of the fault zone

The seismically active Longmenshan fault zone , named after the Longmen Shan Mountains , which is considered to be the cause of the disaster of the earthquake on May 12, 2008 , is located on the northwestern edge of the basin. It is part of the Longmenshan fold and thrust belt. The circle Wenchuan , the location of the epicenter of the quake, is located in the Longmenshan earthquake belt ( 龙门山地震带 , Longmen Shan dizhendai ).

The fault zone is regularly the site of smaller and larger earthquakes. The occurrence of a stronger earthquake in the region of the earthquake of 2008 was predicted by Chinese scientists in 2006.

Northwest Depression

The north-western depression is filled by a wedge-shaped sediment pile up to 12 km thick, which thins out considerably to the south-east towards the central basin. The steady and ongoing deposition of weathering debris from the western mountains created an essentially flat plain, especially in the southwestern part of the basin in the plain around Chengdu. Geological profiles show that the depression is crossed from the northwest by the intricately constructed Longmenshan fold and thrust belt. It is made up of thick thrust bodies of granitic rocks from the Proterozoic and sedimentary rocks from the Paleozoic. In the front part of the thrust belt, which borders the Red Basin, the lowest shear surface lies at the level of the Proterozoic or Cambrian . It rises to the southeast into Mesozoic rocks. Rocks that belonged to the Sichuan Basin are also affected here; geologically speaking, this extends a bit below the thrust belt.

The mountain-forming movements that affected the lower Paleozoic of the frontal Longmenshan fold belt and the Mesozoic of the northwestern depression go back to the collision of India with Asia.

Central Sichuan Basin

The central Sichuan Basin is underlain on the surface by rocks of the Jurassic, which dip flat to the northwest towards the northwest depression. The layers are only moderately affected by bends or wrinkles. Typical of the architectural style of this area are wide anticlines , which encompass the entire sequence of layers and whose flanks are locally affected by faults . Rocks from the Silurian to Carboniferous and those of the Chalk were eroded as a result of repeated uplift and erosion, so that the entire sequence of layers is about 6 km thick here.

Southeastern fold area

The south-eastern fold area is characterized by elongated, pointed saddles , which are also visible on the surface, and stretch out as narrow ridges over many kilometers. They are made of sandstones from the Triassic and Jurassic, while the wide and flat hollows in between are made of Jura claystones. The shape and arrangement of the folds is an indication of a shear horizon under the entire fold area. The construction of geological profiles has shown that this must be at the level of the Cambrian slate at a depth of about 7 km. Thrusts branch off from it, ending blindly in the core of the saddles. The wrinkle area is the frontal part of the Qiaoshan wrinkle belt.

Sequence of layers

The Sichuan Basin is characterized by long stretches of flat layers of the Neoproterozoic , Paleozoic and early Mesozoic times, overlaid by mighty sedimentary series from the Triassic to Cretaceous.

era system formation Mightiness (noun) rocks
Cenozoic quaternary   0-380 Gravel, sand, clay
Mesozoic Neogene   0-300 Conglomerates, sandstones
Paleogene   0 - 800 Red-brown siltstones, sandstones
chalk   0-2,000 Red-brown siltstones, sandstones, sandy conglomerates, marl
law Penglai 650 - 1,400 Gray sandstones, red siltstones, conglomeratic sandstones
Suining 340-500 Red siltstones
Shaximiao & Qianfoya 600-2,800 Red and green siltstones, sandstones, dark gray claystones
Ziliujing 200-900 Gray mudstones, marls, shill limestone
Triad Xujiahe 250-3,000 Dark gray mudstones, sandstones, coal
Leikoupo 900-1,700 Limestones, dolomites, anhydrite
Jialingjiang Limestones, dolomites, anhydrite
Faixianguan Red clay and silt stones, micritic limestones
Paleozoic Perm Talung & Wujiaping 200-500 Limestone, claystone, coal
Maokou & Qixia 200-500 Limestone, black clay stones
Carbon   0-500 Dolomites, limestones
Silurian   50 Green claystones, calcareous sandy siltstones
  400-900 Black mudstones, silty sandstones
Ordovician   250-610 Limestone, mudstone, sandstone
Cambrian   220-420 Limestone, claystone
  225-900 Black mudstones, siltstones, sandy mudstones
Proterozoic Sinium   200-1,100 Dolomites with stromatolites, slate
  0-400 Sandy conglomerates, mudstones, andesite, rhyolite, tuff
Pre-Sinium     Slate, marble, phyllite, granite

Natural resources

The rocks of the Silurian , Permian and Triassic have the properties of petroleum host rocks, which have led to the creation of extensive natural gas reserves. The deposits are mainly linked to the fold structures of the central and southeastern basin. Other natural resources in the Sichuan Basin are coal , ores , salt and phosphorus .

Individual evidence

  1. a b c d R. T. Ryder, DD Rice, Sun Zhaocai, Zhang Yigang, Qiu Yunyu, uo Zhengwu: Petroleum geology of the Sichuan basin, China; report on US Geological Survey and Chinese Ministry of Geology and Mineral Resources field investigations and meetings, October 1991 . In: USGS Open-File Report 94-426 . 1994, p. 67.
  2. Country Studies China - Basin: The Sichuan Basin . China Information Gateway. Retrieved on May 13, 2008.  ( Page no longer available , search in web archivesInfo: The link was automatically marked as defective. Please check the link according to the instructions and then remove this notice.@1@ 2Template: Dead Link / www.china-club.de  
  3. Carsten Alexander Henke: The Red Basin , presentation on the seminar: Regional Geography of China, University of Osnabrück 2002  ( page no longer available , search in web archivesInfo: The link was automatically marked as defective. Please check the link according to the instructions and then remove this notice.@1@ 2Template: Toter Link / www.geographie4u.de  
  4. Dong Jia et al .: Longmen Shan fold-thrust belt and its relation to the western Sichuan Basin in central China: New insights from hydrocarbon exploration . In: AAPG Bulletin . 90, No. 9, September 2006, pp. 1425-1447. doi : 10.1306 / 03230605076 .
  5. Ryder 1994, p. 14
  6. Magnitude 7.9 - EASTERN SICHUAN, CHINA ( en ) USGS Earthquake Center. Retrieved May 13, 2008.
  7. Gui-Xi Yi et al .: Study on Fault Sliding Behaviors and Strong-earthquake Risk of the Longmenshan-Minshan Fault Zones from Current Seismicity Parameters ( en ) Archived from the original on January 17, 2012. Retrieved May 13, 2008.
  8. E. Kirby et al .: Figure 2. Simplified geologic map of the Longmen Shan region of the eastern margin of the Tibetan Plateau ( en ) American Geophysical Union. 2002. Retrieved May 18, 2008.
  9. Ryder 1994, p. 11
  10. ^ Chen She Fa et al .: Tectonic transition from the Songpan-Garzê Fold Belt to the Sichuan Basin, south-western China . In: Basin Research . 7, No. 3, September 1995, pp. 235-253. doi : 10.1111 / j.1365-2117.1995.tb00108.x .
  11. Yongsheng Ma et al .: Petroleum Geology of the Northeastern Sichuan Basin and the Characteristics of Puguang Gas Field, China ( en ) Retrieved May 13, 2008.

See also

Web links

Coordinates: 30 ° 30 '  N , 105 ° 30'  E