Chengdu Plain is developed on the basis of northeast-southwest syncline structure, and consists of eight alluvial fans, such as Minjiang River, Tuojiang River and their tributaries, which originated in the western Sichuan Plateau. The whole plain has loose surface, thick sediments and flat terrain, with an average slope of only 3% ~ 10% and a relative height difference of less than 20m. ?
Chengdu Plain is surrounded by mountains, and its basement is composed of Cretaceous and Paleogene clastic rocks (red beds). The main material of the plain is composed of Quaternary loose deposits. The plain has four distinct seasons, less sunshine, mild climate and abundant rainfall, and belongs to the warm and humid subtropical Pacific southeast monsoon climate zone. Its water system pattern is special and spindle-shaped. Out of the mountain pass, the river is divided into many tributaries, which run to the plain, criss-crossing tributaries and criss-crossing canals. ?
The terrain of Chengdu Plain inclines from northwest to southeast, generally about 600 meters above sea level, with fertile soil. Dujiangyan irrigation project, which is famous since ancient times, has developed agriculture, rich products and dense population. It is an important rice, sugarcane, silk and rapeseed producing area in China, and has been known as "Land of Abundance" since ancient times.
geographical environment
geologic structure
Structurally, Chengdu Plain is located in the southwest of Sichuan Basin, the third subsidence zone of Neocathaysia in China, surrounded by Longmenshan uplift fold belt, Longquanshan fold belt and Wuzhongshan fold belt, and its northern part is limited by Mianyang and Hexingchang fold belt, which has the characteristics of a faulted basin.
● Longmenshan uplift and fold belt
Longmenshan uplift and fold belt belongs to Cathaysian tectonic system. It is a structural belt with strong fold and fault activities, multi-stage compound and huge scale. It consists of a series of NE-trending uplift, depression, simple fold and composite fold, compressive and compressive-torsional faults, including Longmenshan fault zone. The stratum is Proterozoic-Triassic, with multi-stage magmatic rocks distributed. This structural belt is adjacent to Anxian-guanxian fault zone on the western edge of Chengdu Plain.
● Longquanshan fold belt and Wuzhongshan fold fault zone.
The Longquanshan fold belt and the Wuzhongshan fold belt belong to the Neocathaysian system, and the third subsidence belt, the western Sichuan fold belt, is located in the west of Longquan Mountain, inserted into Longmen Mountain in the northeast, crossed Emei Mountain in the southwest and moved to the northeast, and divided into two branches. The east branch will be the Longquanshan fold belt, the west branch will be named the Wuzhongshan fold belt, and the Chengdu fault depression will be in the middle.
Longquanshan fold belt: adjacent to the east side of Chengdu Plain, it consists of a series of northeast 20-30 degrees. Distribution of composition of folds and faults. The stratum is Jurassic-albite, bounded by Longquanshan fault and the eastern edge of Chengdu Plain.
Wuzhongshan fold-fault zone: near the west of Chengdu fault depression, it is also composed of a series of folds, faults and Feilai peaks distributed in the northeast. This stratum consists of Triassic-Tertiary strata.
● Cyclone structure of Mianyang Hexingchang
Hexingchang spiral structure is located in the northeast of Chengdu Plain, between Deyang, Luojiang, Zhongjiang and Jintang. It consists of east-west broom structure and reformed Cathaysian structure. The constituent strata are the old strata before the white matter system. Mianyang ring structure is located in the northern edge of the plain, occupying the area between Mianyang, Yanting, Shehong and Zhongjiang. The formation is from Upper Jurassic to Cretaceous.
● Chengdu (fault depression)
Chengdu Plain is sandwiched by Neocathaysian, Wuzhongshan Fold Belt and Longquanshan Fold Belt, and on the basis of Jurassic-albite subsidence, it falls into a fault basin that accepts huge Quaternary loose accumulation. Its scope starts from Xiushui in Anxian County in the north, goes to Mingshan and Xinjin in the south, starts from Dayi and guanxian in the west, and ends in Longquan Sao and Jintang in Chengdu in the east. Its long axis is 30-40 degrees north latitude. Extending eastward, it is an asymmetric depression basin with steep west and slow east. In this fault basin, a series of depressions and uplifts extending northeast have been formed, and buried faults extending northeast and north-south have been developed. The edge of the fault depression consists of a series of NE-NE folds and faults. Therefore, the fault depression is divided into three parts: the western marginal structure; Eastern marginal structure; Central depression.
① The western marginal structure consists of a series of discontinuous NNE-trending comb folds and NNS-trending buried faults. The east wing of the fold is steep and the west wing is gentle. East wing 20-90, west wing general 10-25. Most structural planes tend to the northwest. From east to west, there are Dengxia-Dayi-Pengxian fault, Yuelai Jinmachang anticline and Juyuan-Guankou fault in turn. The Dengxia-Dayi-Pengxian fault is the boundary between the western marginal structure and the central depression, and the surface of the fault is not exposed. However, according to geophysical exploration, aerial and satellite interpretation and drilling data analysis, the fault is composed of a group of NE-trending faults and N-S-trending secondary faults. According to the artificial seismic data, the fault is located in the stratum above Shaximiao Formation of Jurassic, which tends to the northwest and the northeast as a whole, with a fault distance of 200-400 meters.
② Eastern marginal structure: It consists of a series of box-shaped short-axis anticlines with width between them, with an axial direction of 30 northeast, arranged in a goose shape. These folds are steep in the west, with dip angle of 15-80, even vertical inversion, often accompanied by overthrust faults, and the east wing is gentle, with dip angle of 10-40, with bending phenomenon. From west to east: Su Wharf anticline,
Yanjinggou Anticline, Mumashan-Puxing Syncline, Zonggangshan Xiongpo Anticline, Pujiang-Xinjin Fault, Shuangliu-Chengdu Buried Fault, Mopanshan Fault, Xindu-Guanghan Buried Fault and Mianyuanhe Buried Fault. Pujiang-Xinjin fault, together with Shuangliu, Chengdu, Mopanshan, Xindu and Guanghan faults, is the eastern margin of the fault depression.
Boundary between marginal structure and central depression. The southwest section of Pujiang-Xinjin fault is exposed to the ground, about 30 kilometers long, inclined to the southeast, with an inclination angle of 45-55, and the northeast section is covered by Quaternary. The fault spans Xinjin in the north, with a total length of more than 80 kilometers, and the overall trend is northeast. The fault plane is inclined to the southeast with an inclination of 25-35. The maximum vertical fault distance of Pujiang is 3300 meters, and the horizontal fault distance is 4000-5000 meters, which is not obvious behind Wangjiashan in Shuangliu. According to satellite remote sensing, aerial photo interpretation and geophysical data analysis, the fault extends to the northeast, passes through the western part of Chengdu and may be connected with the Mopanshan fault, extending to the western part of Guanghan.
③ Central depression: it is an oblique depression formed between Dengxia-Dayi-Pengxian fault, Pujiang-Xinjin fault and its concealed extension fault, and its extension direction is controlled by two faults. There are mainly two groups of concealed structures in the depression: NE-trending faults, such as guanxian Qinglong-Tangxian-Pengxian, Huaiyuan-Shiyugaojiao fault, Dayi Xinchang-Danxian Hong Guang fault and Dengxia Sangyuan-Chongqing-Jixian fault; North-South secondary faults: Dengxia Xihe fault, Guanyuan Shangyuanchang-Chongqing Daomingchang fault, etc. Controlled by the structure, deep depressions and relative uplifts are developed in the depressions, including Zhuwa Depression in Pengxian County and Dayi Depression in Chongqing. The west wing of these depressions is steep, the east wing is gentle, and the settlement center is in the west. Relative uplift includes Gaoshanzhen uplift and Juyuan Chongyibao uplift.
topography
Chengdu Plain is surrounded by mountains. Longmen Mountain (Dengxia Mountain) is inclined to the west, with Longmen Mountain in the east, the plateau formed by Deng Bingxun in the south, and Xiushui hilly land in Yi 'an County in the north. Topographically, a closed rhombic basin landscape is formed, which is confronted by the north and the south, gripped by the east and the west, and gradually rises from the center of the plain to the periphery. The interior of the plain is flat, about 200 kilometers long from north to south and nearly 90 kilometers wide from east to west. The ground elevation is 730-460 meters, and it slopes from northwest to southeast, with a ground slope of 3- 1 1‰. Topographically, the plain can be divided into fan-shaped plain and surrounding platform. The former is located in the main body of the plain, called the flat dam area; The latter is intermittently distributed around the plain.
● Alluvial fan group
It is mainly distributed in the piedmont area in the west of the plain, and mainly consists of Ganhezi-Mianyuan River Fan, Shiting River Fan, Yujiang River Fan, Shanminjiang River Fan, Xihe River Fan and Nanjiang Fan from north to south. The terrain of each sector is inclined from northwest to southeast and connected in groups. The leading edge of the fan body staggered in the late Pleistocene, and gradually changed with the floodplain and terrace, and the fan body was in a low-lying area. Among them, the rising river fan is the biggest, from Dujiangyan to Xifu and Yongding in Dan County.
● Ice water accumulation fan
It is formed by the accumulation of ice water and rivers in the late Pleistocene. Widely distributed in the east, south and southeast of the plain, the slope is 2-3‰, slightly inclined to the east and southeast. The surface morphology has the characteristics of "large flatness and small unevenness". When it is distributed along the river, it is the second terrace of the river, with a specific height of 4-17m, but sometimes there is no obvious scarp between it and the first terrace, and it is in the form of a stalk or capsize, which constitutes an interriver block. It looks like a pedestal near the edge of the plain. Terrace distribution is 440-700 meters above sea level.
● Floodplain and the first terrace
Floodlines and beaches are distributed in the center and banks of the river in tongue, strip and crescent shape, 0-2 meters above the water surface, often without obvious scarps, and the beach surface is inclined to the center of the river, and the composition is gray gravel layer. The first terrace is distributed in the middle and lower reaches of rivers in the southeast and east of the plain, and in the rivers of Deng Ming Plateau. It is distributed intermittently along the river and embedded in the fan-shaped plain of ice water, with a specific height of 2-4m. The terrace is flat and slightly inclined to the river bed or downstream. Among the rivers, the terrace of Rise River is the most developed, which is several kilometers wide and dozens of kilometers long. It consists of Holocene gravel layers. Mindeng Plateau or some piedmont rivers are yellow-brown due to the accumulation of the first and second glacial periods or the mixing of weathered red bed parent rocks.
Low moraine ice platform
It consists of ice debt and ice water accumulation in the Middle Pleistocene and the Lower Pleistocene. The fault surrounds the plain to form a platform, which gently inclines from west to east. The average slope of Dengming terrace is ‰, and there are three terraces on the plain, with different slopes.
Climatic characteristics
Chengdu Plain has four distinct seasons, less sunshine, mild climate and abundant rainfall, and belongs to a warm and humid subtropical Pacific southeast monsoon climate zone. In the piedmont area of Longmen Mountain in the northwest of the plain, the temperature is low, the precipitation is abundant, the evaporation is slightly low, the rainfall in the east and southeast is slightly less, and the temperature is slightly higher.
The temperature in Chengdu Plain has not changed much. The annual average temperature is 16. 1℃, and the annual maximum temperature generally appears in July and August. According to the data for many years, the monthly average maximum temperature does not exceed 26℃, and the monthly average minimum temperature is generally not lower than 4℃. So freezing is extremely rare, there is no freezing of soil and groundwater.
Abundant rainfall is one of the climatic characteristics of Chengdu Plain. Longmen Mountain is located on the west side of the plain, which has a significant impact on atmospheric rainfall. The water vapor coming from the east is blocked by the Longmen Mountain barrier, which forms topographic precipitation, resulting in a rainy belt in Ya 'an-Dujiangyan-Anxian, with an annual average rainfall of 1.200- 1.600 mm, and the rainfall decreases to the southeast. Rainfall in Wenjiang, Danxian, Xinfan and Guanghan areas 1 0,000. Rainfall is unevenly distributed in seasons. From June to September, a large amount of precipitation was generated under the control of warm and humid air masses in tropical oceans. The total rainfall in four months was 753.7 mm, accounting for 76% of the total precipitation in the whole year. Winter is controlled by dry and cold air mass in Chinese mainland, with dry climate and scarce precipitation.
The average annual relative humidity in Chengdu Plain is 82. 1%, the average annual evaporation is 994 mm, the highest is115/mm, and the lowest is 960 mm ... The precipitation in summer and autumn greatly exceeds the evaporation, and the precipitation in winter and spring is less than the evaporation. Geographically, there is an increasing trend from the western mountainous area to the foot of Longquan Mountain. The seasonal and regional changes of precipitation and evaporation in this area have caused the seasonal and regional differences of groundwater recharge conditions.
Water system condition
The west side of Chengdu Plain is the entrance of surface water system, which has two major water systems: Jue River and Tuojiang River. After entering the plain, Shanminjiang River and Tuojiang River diverge in a fan shape, meet at the foot of Longquan Mountain on the east side of the plain, and flow out of the plain at Jintang, Su Wharf and Xinjin. There are many rivers in the plain, with an average of one river every 2.SKm. The main rivers are:
Rising water system: Shanminjiang River enters the plain at the weir of Dujiang River, and is divided into two rivers by world-famous water conservancy projects. Neijiang is divided into Puyang River, Zouma River, Baitiao River and Jiang 'an River. Waijiang River is divided into Jinma River, Mayang River and Shahe River. In addition, the Jinwen River, Xiejiang River, Nanjiang River and Pujiang River, which developed in the foothills of Longmen Mountain, were brought into the main stream of Jinma River, outside the Xinjin outflow area.
Tuojiang River System: It is mainly composed of Mianyuan River, Shiting River and Yujiang River. After entering the plain, Sanjiang is divided into fan-shaped streams, and Mawei River and Wading River developed in front of the mountain are equal to Jintang outflow area.
Dense canal system is an important feature of plain water system distribution. Most of the major canals in this area are formed by slightly improved natural rivers. On the basis of main canal, branch canal, bucket canal, agricultural canal and capillary canal are as dense as cobwebs, forming the trend of water network in plain area.
soil types
The soil types in Chengdu Plain are mainly paddy soil and purple soil, and the formed soil includes zonal yellow soil and yellow brown soil, non-zonal tidal soil and paddy soil, and lithologic soil such as lime (rock) soil and purple soil. The cultivated land in this area has deep soil layer and moderate texture.