This paper analyzes the reasons why the Sichuan Basin is far from the ocean but has a maritime climate.

Sichuan Basin is one of the four great basin in China, located in the south-central part of the Asian continent, the heart of China and the middle of the eastern edge of China.

Sichuan Basin covers the central and eastern Sichuan Province and most parts of Chongqing, also known as envelope basin and purple basin, with a total area of about 260,000 square kilometers. Surrounded by Qinghai-Tibet Plateau, Daba Mountain, Wushan Mountain, Dalou Mountain and Yunnan-Guizhou Plateau. The surrounding mountains are mostly between 1 1,000m and 3,000m above sea level, with an area of about 1 10,000m2. The middle basin bottom is low, with an altitude of 20000. Generally, there are 2 ~ 5 vertical natural zones from bottom to top in marginal mountainous areas.

Sichuan Basin is mainly divided into three parts: parallel ridges and valleys in eastern Sichuan, hills in central Sichuan and Chengdu Plain in western Sichuan. The main cities in the basin are Chengdu, Mianyang, Luzhou, Nanchong, Zigong, Deyang, Guangyuan, Suining, Neijiang, Leshan, Yibin, Guang 'an, Dazhou, Ya 'an, Bazhong, Meishan and Ziyang in Sichuan Province, Zunyi and Bijie in Guizhou Province, Zhaotong and Chongqing in Yunnan Province. Generally speaking, the climate of the basin belongs to subtropical monsoon humid climate, but most areas of the basin are similar to temperate maritime climate.

The western Sichuan basin is low and flat, with fertile soil, and the Chengdu plain is densely covered with water networks and rich in products. It is a famous land of fish and rice in the Yangtze River basin, with the reputation of "land of abundance", and the eastern part of the basin is low mountains and hills. Fangshan Hill in the middle of the basin, accounting for 62% of the total area, is mainly composed of purple sandstone and shale. The basin is also rich in fossil energy such as coal, oil and natural gas, as well as minerals such as salt, apatite and sulfur.

Geomorphological features

The Sichuan Basin can be clearly divided into two parts: marginal mountain and basin bottom, with an area of about 654.38+10,000 square kilometers and160,000 square kilometers respectively. There are many middle and low mountains in the marginal mountainous areas. The elements of the landscape are obviously transitional, such as the composition of animals and plants permeating the components of Central China, Southwest China, Qinghai-Tibet Plateau and North China respectively. Generally, there are 2 ~ 5 vertical natural zones from bottom to top in marginal mountainous areas. The marginal mountainous areas are the bases of various economic trees and timber forests in Sichuan. There is serious soil erosion in agriculture. There are many hills, low mountains and plains at the bottom of the basin. The surface composition is new and single, sandy mudstone and Quaternary sediments. The climate belongs to the middle subtropical zone, and the heat is much higher than that in the marginal mountainous areas, but the precipitation is not as good as that in the marginal mountainous areas. Vegetation is replaced by artificial secondary forest trees, and the coverage rate is low, and the cultivated land is contiguous. Drought and flood in summer are the biggest disasters in this area.

The internal topography of the basin shows three obvious characteristics, namely, the plain landform in the west of the basin, the hilly landform in the middle of the basin and the landform bounded by Huaying Mountain and Longquan Mountain in the east of the basin.

morphological character

Basin morphology

(1) The western basin plain between Longquan Mountain, Longmen Mountain and Qionglai Mountain is the largest plain in southwest China, covering an area of about 8,000 square kilometers. Because Chengdu is located in the plain, it is called Chengdu Plain. Chengdu Plain is 460-750m above sea level, and the terrain inclines from northwest to southeast. The surface is flat, and the relative height difference is generally not more than 30-50 meters. It is composed of eight rivers, such as Minjiang River, Tuojiang River, Fujiang River and Qingyi River, with fertile soil and dense canals. Dujiangyan gravity irrigation is known as "Tianfu".

(2) The basin hills between Longquan Mountain and Huaying Mountain are relatively low in topography, ranging from 300m to 500m above sea level, with a relative elevation difference of 50m to150m. The terrain is inclined from north to south, and the rock stratum is nearly horizontal. Under the long-term erosion and cutting of running water, stepped Fangshan hills, shallow hills in the south and deep hills in the north are formed. Purple sand shale with different hardness on the hilly surface is easy to weather into purple soil, rich in phosphorus and potassium, with high natural fertility and wide adaptability. It is the main producing area of grain and cash crops in Sichuan Province.

(3) To the east of Huaying Mountain, there is a ridge valley in the east of the basin, which is composed of several belt-shaped anticline mountains and syncline wide valleys running near northeast and southwest. The mountain is steep and narrow, with an altitude of 700m- 1000m, of which Huaying Mountain is the highest peak at the bottom of the basin. After limestone on the top of the mountain is dissolved by rain, it often forms grooves, so most mountains have the characteristics of "one mountain with two ridges and one groove" or "one mountain with three ridges and two grooves". The valley between mountains is wide and gentle, with an altitude of 300-500 meters, in which hills and flat dams crisscross, which is the main area of industrial and agricultural production in parallel ridge and valley areas.

Marginal mountainous area

The marginal mountainous area of Sichuan Basin belongs to a strong uplift and fold belt. The landform is characterized by high altitude and obvious transition, surrounded by a series of Zhongshan and low mountains. Micang Mountain and Daba Mountain in the northern margin of the basin are nearly east-west, which are the southern wings of the famous Qinba Mountain area. Generally, the altitude is between 1, 500 m-2,200m, the mountain is steep and the valley is deep, and the relative height difference can reach 500- 1, 000m. The Dalou Mountain on the southern edge is a part of the majestic Yunnan-Guizhou Plateau. There are Longmen Mountain, Qionglai Mountain and Emei Mountain in the west. The elevations of the ridges are all between 65,438+0,500 meters and ~ 3,000 meters, and the relative height difference can reach 65,438+0,000 meters, which belongs to the first level of Chinese topography. 1. The main peak of Mount Emei is 3,099 meters high, with a relative elevation difference of 2,660 meters from the nearby plain. It is one of the four famous Buddhist mountains in China.

Topographic elements

Daba Mountain, Micang Mountain and Longmen Mountain in the north, Qionglai Mountain and Daliangshan Mountain on the edge of Qinghai-Tibet Plateau in the west, Dalou Mountain in the south and Wushan Mountain in the east, surrounded by mountains and plateaus with an altitude of 2000-3000 meters. Sichuan basin is a typical basin in structure and human landform. It has been a large depression area with relatively stable crust since Sinian. During the Indosinian Movement in the Late Triassic, it became an inland lake basin, but its scope was much larger than that of Sichuan today. Purple sandstone and shale with a thickness of 3000 ~ 4000 meters were accumulated in Mesozoic, so people also called it "red basin" or "purple basin". At the end of Mesozoic, the Sichuan movement made the surrounding areas of the basin fold into mountains, the middle part was relatively depressed, the outline was basically formed, and the strata inside the basin also underwent large-scale deformation. A group of NE-trending folds appeared in the east, which is called the eastern basin fold belt. The dome structure is formed in the middle, which is called the dome belt in the basin; The western part of the basin became a subsidence zone, which laid the foundation for the three geomorphic areas of the basin today. The Cenozoic Himalayan movement caused the surrounding mountains to rise again, the basin to sink again, and Chengdu to sink deeper. At the same time, the Yangtze River flows through Wushan and flows eastward, completing the unified Yangtze River water system and transforming the internal flow basin into the external flow basin. The Sichuan Basin can be clearly divided into two parts: marginal mountain and basin bottom, with an area of about 6,543,800 square kilometers and 6,543,800 square kilometers respectively. Sichuan Basin is the largest outflow basin in China, covering an area of over 260,000 square kilometers.

The hills and plains of Sichuan Basin are staggered, with high terrain in the north and low terrain in the south. Because of the different surface morphology, Huaying Mountain and Longquan Mountain are the boundaries.

The pelvic floor can be divided into three parts:

(1) To the east of Huaying Mountain is a roughly parallel ridge valley in East Sichuan, which is composed of several strip mountains running from northeast to southwest, with an elevation of 700-800 meters. There are many low hills and flat dams in the valley, with an elevation of 200-500 meters. It is a place where agriculture and population in East Sichuan are concentrated.

(2) Fangshan lies between Huaying Mountain and Longquan Mountain. Due to the gentle dip angle of purple sand shale in the area, a large square hill is formed after cutting. The altitude is 350 ~ 450 meters, and the relative height is tens of meters. The local working people used the deep soil layer of Fangshan Mountain to build terraces all the way to the top of the mountain.

③ The plain is located to the west of Longquan Mountain. Known as West Sichuan Plain or Chengdu Plain, it covers an area of more than 6,000 square kilometers and is the largest plain in Sichuan Basin and Southwest China, with an altitude of about 600 meters.

geologic feature

Sichuan basin belongs to Sichuan depression of Yangtze paraplatform. Paleozoic was relatively uplifted, lacking Devonian and Carboniferous. Indosinian movement transformed into a large depression, and fold uplift occurred in the late Yanshan period, especially after Himalayan movement. The basin pattern is mainly controlled by NE-SW and NW structural lines, forming a typical rhombic basin. Sichuan Guangyuan, Sichuan Ya 'an and Sichuan Xuyong are the four vertices of the diamond, which are slightly longer on the east and west sides, 380-430 km, and slightly shorter on the north and south sides, 365,438+00-330 km. The connecting line of the four vertices of the diamond is roughly equivalent to the contour line of 650 ~ 750 meters in the basin, and the bottom of the basin and the marginal mountains are also divided by this line. Sichuan Basin was an inland lake basin10.40 billion years ago. Until 66 million years ago, the mountainous areas on the edge of the basin rose rapidly, and the water systems in the middle and upper reaches of the Yangtze River began to communicate. The eastward flow of lake water in the basin laid the present landform. There are many low mountains and Zhongshan mountains on the edge of the basin, and the mountains are steep. Most of the rivers in the mountains on the edge of the basin are V-shaped valleys with a height difference of more than 500 ~ 1 000 meters, and the surface is rugged, so there is a saying in history that it is difficult to get through the Shu Road and get into the sky. Ridges are mostly at an altitude of 2,000-3,000 meters, and can exceed 3,000-4,000 meters in the northwest and west, such as 4,984 meters in Longmen Mountain, 3,099 meters in Emei Mountain and 479 1 meter in Xiaoxiangling Mountain. Limestone before Paleozoic is widely exposed on the surface, followed by slate, schist, crystalline limestone, quartzite, sandstone mudstone and conglomerate, and granite and basalt in some places. Karst landforms such as stone forest, karst cave, underground river and valley can be seen in limestone distribution area. Xingwen County in the southern margin of the basin is called "Shilin Cave Township" (see Xingwen Stone Forest). Famous mountains such as Twelve Peaks in Wushan and Jinfo Mountain are also mainly developed from limestone. Emei Mountain, which is composed of limestone, basalt and granite, and Qingcheng Mountain, which is composed of sandstone, mudstone and conglomerate, is known as "Emei is beautiful on earth" and "Qingcheng is quiet on earth", and is a famous tourist attraction in China. Most of the bottom of the basin is 250 ~ 700 meters above sea level, and the terrain is inclined to the southeast. All rivers in the basin converge from the marginal mountains to the main stream of the Yangtze River at the bottom of the basin, forming a centripetal water system. The surface is covered by a large area of Mesozoic purplish red sandstone and mudstone, so it is called "red bed basin", which is the most concentrated area of Mesozoic continental red beds in China. Sichuan basin is a hilly basin with hills at the bottom, followed by low mountains and plains.

Basin period

Sichuan Basin is a part of Yangtze Platform, which is called Sichuan Platform. It is a relatively stable area, but it has experienced two large-scale sea immersion. The first time started from the Cambrian more than 500 million years ago, and continued to the Silurian more than 370 million years ago, and it continued to sink into an ocean basin. The Silurian Caledonian movement took place, and the rest areas rose to land except the Longmenshan geosyncline in the west. At the end of Carboniferous period, 270 million years ago, due to the second seawater immersion, the basin was once again occupied by the ocean. The Permian sea and land alternated, forming Nankong, Songzao, Tianfu and other coal mines near Chongqing. At the end of Permian, magma erupted in the west of the basin, and basalts formed in Xiaojinding and Qingyinge areas of Emei Mountain at this time.

Lake basin period

Triassic, 654.38+0.9 billion years ago, the Indosinian movement gradually raised the basin edge into a mountain, and the area submerged by seawater gradually rose to land, from a sea basin to a lake basin. At that time, the lake occupied almost all the territory of the present Sichuan Basin and was called "Shu Lake", thus ending the history of seawater immersion. During the long period of 6,543.8 billion years in Mesozoic, the climate in the basin was warm and humid, and ferns, cycads and gymnosperms grew everywhere, which was another coal-forming period. Yongrong Coal Mine was formed in Triassic and Jurassic. From Changshou and Dianjiang in the east, Jiangyou and Qionglai in the west, at the foot of Daba Mountain in the north and Chishui in Guizhou in the south. This is also a region rich in natural gas. During this period, reptiles and dinosaurs ruled for a period of time. Mamenxilong in Hezhou discovered in 1957 is 22 meters long and 3.5 meters high. It is the largest and most complete dinosaur fossil in China and Asia.

Continental basin period

At the end of Cretaceous 70 million years ago, another strong crustal movement "Yanshan Movement" took place. The mountains around the basin are constantly uplifting, and at the same time, many large faults are produced, such as Longmenshan fault in the west and Huayingshan fault in the east, which divides the basin into three parts. Shuhu Lake has shrunk to only 20,000 square kilometers. The closed basin topography and sharply reduced water surface make the climate gradually dry and hot, and the sediments change from marine phase and land-sea alternating phase to continental phase. A large number of weathered, eroded and denuded materials have accumulated thousands of meters thick in the basin, forming purple sand, mud and shale. Gymnosperms decreased and dinosaurs became extinct. Under drought conditions, the inland lake was strongly evaporated, the concentration increased, and the salt accumulated continuously, forming a salt lake. Later, the sediments were buried and preserved in the stratum, and after a long geological process, the strata were formed. Zigong area is a famous well salt producing area. The Neogene was influenced by Himalayan orogeny more than 20 million years ago. Two or three million years ago, in the Quaternary, the earth's crust underwent another tectonic movement. As a result, Sichuan basin has experienced the changes of sea basin, lake basin and land basin from internal flow basin to external flow basin, from closed internal flow area to external flow area, and from accumulation to erosion. Quaternary is an era when glaciers are widely distributed, and a large number of glaciers are developed in the northwest mountainous area of the basin. After the melting of glaciers, a large number of sediments were carried by Minjiang River and Tuojiang River and accumulated in Qianshu Lake in the west, eventually forming Chengdu Plain. Sichuan basin is rich in landforms, with plain 7%, hills 52% and low mountains 4 1%.

Climate type

The terrain of Sichuan basin is closed, and the temperature is higher than other areas at the same latitude. The average temperature in Leng Yue is 5 ~ 8℃, which is 2 ~ 4℃ higher than that in Shanghai, Hubei and Guizhou at the same latitude. Extreme minimum temperature -6 ~-2℃. Frost and snow are scarce, and the frost-free period is 280 ~ 350 days throughout the year. In winter, there is constant frost and snow in Wuhan at the same latitude, and Changning, whose basin is located in the Yangtze River basin, is frost-free all the year round. Summer in all parts of the river basin begins at the end of May and lasts for more than 4 to 5 months. The temperature is high in the east and low in the west. The summer temperature in the west is 3-4 degrees lower than that in Shanghai, Wuhan and other cities at the same latitude, but the humidity is unbearable. The average temperature is around 25℃, with high temperature in the southeast and low basin edge. The temperature is as high as 26 ~ 29℃ in the hottest month, and nearly 30℃ in the Yangtze River basin. The sunny, hot and high temperature weather in midsummer caused a severe summer drought in the southeast of the basin. The annual average temperature is 16 ~ 18℃. The active accumulated temperature above 10℃ is 4500 ~ 6000℃ and lasts for 8 ~ 9 months, belonging to the middle subtropical zone. The accumulated temperature in the southeast Yangtze River basin exceeds 6000℃, which is equivalent to the tropical climate of South Asia south of Nanling in China. The temperature in the basin is high in the east and low in the west, high in the south and low in the north, high in the bottom and low in the edge, and the isotherm distribution is concentric.

The temperature in the mountainous area on the edge of the basin has vertical distribution characteristics. The elevation of Emei Mountain and Jinfo Mountain rises by 100 m, and the temperature drops by 0.55℃ and 0.6 1℃ respectively. The average annual temperature at the top of Mount Emei is only 3℃, and the active accumulated temperature above 10℃ is 586℃, which is equivalent to cold temperate zone and sub-cold zone in climate.

Precipitation condition

The annual precipitation in Sichuan Basin is 65,438+0,000 ~ 65,438+0,300 mm, and the precipitation in the mountainous areas around the basin is very rich. For example, the annual precipitation in the western mountainous area between Leshan and Ya 'an is 1.500 ~ 1.800mm, which is a prominent rainy area in China and is called "West China Rain Curtain". However, winter drought, spring drought, summer waterlogging and autumn Mao Mao rain are unevenly distributed in the year, and 70 ~ 75% of the rainfall is concentrated in 6 ~ 10. The maximum daily precipitation can reach 300 ~ 500mm. "Night rain" has been famous since ancient times, and it accounts for more than 60 ~ 70% of the total rainfall. The fog in the basin is big and wet, and the clouds are low and cloudy. Emei Mountain and Jinfo Mountain are the foggy areas in China, and the annual relative humidity is also the highest in China. The annual sunshine in the basin is only 900 ~ 1300 hours, and the annual solar radiation is 370 ~ 420 KJ/cm2, both of which are the lowest in China, so it is called "the tree spring barks at the sun".