Is China's Sichuan Basin a crater or a crater?

A basin of its own

It takes a long time for the Earth's geology to change. Today's geography is the result of the activity of the Earth's land plates and mantle over billions of years.

While we marvel at these geological changes today, we still have to remember how they have changed over the years.

A good example is the Sichuan Basin, also known as the Sichuan-Chongqing Basin, which is surrounded by mountains.

East for Wushan, south of the Dalou Mountain, Daliang Mountain, west of the big snow mountain, Minshan, Qiongxia Mountain, north is Daba mountain, Micang mountain.

The elevation of the Sichuan Basin is about 1~3 km, and its geography is almost completely closed.

In terms of water sources, the basin's only drainage channel for the Yangtze River Wu Gorge. Most of the basin is covered by hilly areas, with Chengdu Plain at the western end.

On the formation of the plain, mostly because of alluvial effect.

The Minjiang River and other rivers fan out from the northwest into the basin and are separated from the rest of the basin.

The easternmost part of the Sichuan Basin is marked by folds and valleys, which in turn cause long ridges of vegetation to rise upward at low levels.

This kind of ridge mainly includes Mingyue Mountain, Huaying Mountain, Fangdou Mountain, etc. It is mainly located in Chongqing area.

To the northwest, the basin is close to the Tibetan Plateau.

The Sichuan Basin and the Indian subcontinent extend southwest and are then separated by the Hengduan Mountains.

The Yunnan-Guizhou Plateau is located in the south and southeast of the basin, and the Sichuan Basin has many mountains, which form the unique features of the whole basin.

In the main mountain areas, limestone, granite and basalt are the main rock components.

In addition, Yunnan-Guizhou area is dominated by karst landforms, and there are many karst caves and underground rivers. The main stream of the Yangtze River is covered by a large area of purple red sandstone and mudstone of the Mesozoic era, so it is called the red bed basin.

But because of the mountains around it, the Sichuan Basin often suffers from smog, which is caused by the reverse transfer of temperature to the basin's troposphere, which covers a layer of air moving eastward across the Tibetan Plateau.

So wet and long periods of overcast weather dominate the whole seasonal variation in the Sichuan Basin.

Summers are very hot and humid, winters are occasionally frosty, and summer climate intensity varies widely across the basin, depending on location.

From the perspective of geographical environment and shape features, the Sichuan Basin is very special in the map of China.

Since ancient times, there has been a difficult Shu road, in modern research and investigation there is still a lot of speculation about the Sichuan Basin.

Viewed from above, the Sichuan Basin looks like a crater, but there are no volcanic traces.

Or craters, with little actual evidence of meteorite impacts.

In addition, the Jurassic strata are well preserved in the Sichuan Basin. For example, the Zigong area is rich in preserved dinosaur fossils and remains of other prehistoric animals.

Is it a crater or an impact crater?

It is actually quite simple to know whether the Sichuan Basin belongs to craters and craters.

Now that we know it's a basin, we can do a pretty good job of ruling it out.

First, a few words about the crater, the most visible external feature of a volcano.

Volcanoes occur most often at the bifurcation or confluence of tectonic plates, and are mostly underwater, while volcanoes from divergent tectonic plates also occur.

According to the theory of plate tectonics, the Earth's lithosphere is divided into 16 larger plates and several smaller ones, which are in slow motion.

Beneath this convection, most of the Earth's volcanic activity occurs, where the convergence or divergence of plates causes the lithosphere to be destroyed or new lithosphere to be formed.

In ocean ridges, successive tectonic plates develop into a thinner ocean crust with warmer mantle rock.

The pressure in the rising mantle rock decreases and causes adiabatic expansion and partial melting of the rock, eventually forming volcanoes and creating new ocean crust.

And at converging plate boundaries, usually between oceanic and continental plates, you have subduction zones, where they collide.

Subduction of oceanic plates creates deep oceanic trenches offshore. As a result of subduction, water released lowers the melting temperature of the upper mantle, and eventually the flux melts to produce magma.

But with so much silica in it, the magma is so viscous that it usually doesn't reach the surface, but cools and solidifies deep down.

But when it rises to the surface, it forms a volcano.

It is clear that the Sichuan Basin does not belong to one of the two volcanic tectonic processes.

So for impact craters, this is even simpler.

Violent physical impacts, caused by extraterrestrial bodies or meteors, create huge craters in the Earth's surface.

The ability to impact the explosion and the compression of the gas during the contact acceleration, the shock wave that causes the craters to form further.

One of the hallmarks of an impact crater is that it usually leaves behind rock that has undergone impact metamorphism, such as deformed crystals or melted rock.

The problem is that such material is usually buried deep, so identifying impact craters can be tricky.

However, the impact metamorphism caused by the impact is a good means of identification, and the material left over from the impact can be obtained through material analysis.

But the Sichuan Basin mainly came from the orogeny and the extrusion of the Indian plate, which eventually formed the western margin of the Sichuan Basin.

The folds within the Yangtze plate formed ancient faults and Daba Mountains along the eastern basin, which themselves were the result of vertical pressure between the Yangtze plate and the Eurasian plate.

Studies of the Sichuan Basin by geographers further show that the rigidity of the basin withstood much of the eastern movement of the Tibetan Plateau.

In other words, the Sichuan Basin is said to have been formed because of the many tectonic and orogenic changes in the past, but this also shows that the experience of the Sichuan Basin is far more complex than we thought.

A long and complex evolution

Sichuan Basin is a unique type of superimposed basin whose evolution is controlled by the deformation of its surrounding orogenic belt.

Related studies have shown that the tectonic mechanism of the basin is derived from crustal deformation, rather than from deep lithosphere and mantle activities.

Inside the continent lies an ancient plate that cannot be ignored, the cratonic continental core.

This is an ancient and very stable part of the continental crust. It has been untouched by the convergence and fragmentation of continents and supercontinents over the past 500 million years.

Why have cratons, some of which formed two billion years ago or more, been so resilient over eons of change?

This is because cratonic cores are usually found in the interior of continents and are composed of crystalline substrates of ancient rocks.

This part of the bedrock is mainly composed of low-specific felsic igneous rocks and granites, and the crust within the craton is thick, like roots embedded deep into the mantle, up to 200 kilometers deep.

The Sichuan Basin is an inland sedimentary basin, according to the past research results.

It is because the craton continent plays a stabilizing role in the Sichuan Basin that the basin did not rise in the past orogeny.

The study of the Neoproterozoic rifts in the Sichuan Basin shows that the north margin and north northwest margin of the South China block are composed of a variety of mixed and metamorphic mudstone formations.

The seismic reflection data in the deep part of Sichuan Basin show that there are ancient subduction relics in the lower part of Sichuan Basin.

In the Neoproterozoic, shallow dip subduction occurred on the northwest margin of the South China Block, and then the formation of the Rodinia supercontinent.

Later, under the influence of mantle plume, the Rodinia supercontinent disintegrated and the South China block formed a continental rift valley.

In addition, the offshore platform architecture changed during two stretch and compression events. Related studies show that the northwestern part of the basin may be the result of syntropic extension deposition.

During the Early Cambrian period, the Mianyang to Changning craton had a depression geometry, and the relevant studies suggest that there were four evolutionary stages in this area.

The study of Leshan-Longnushi Paleo-uplift in the middle and Late Paleozoic shows that the Sichuan Basin contained about 2000 m of strata deposited on the low carbonate and continental shelf.

Later, the Garishan orogeny led to the erosion of the Early Paleozoic geological environment in the central and western part of the central Sichuan Basin and formed a larger east-west sloping uplift.

In addition, similar findings have been found in many other geological periods, namely, the formation of land strata is complex, and there is obvious geological environment unconformity between the strata.

The structure between the sedimentary cap and the basement as well as between the sedimentary layers covering the Sichuan Basin is very complex. Several cratonic depressions and the paleo-uplift are highly angled with the basin margin.

Eventually, the cratonic core anchored the Sichuan Basin in a long period of orogeny, and the rising mountains around it forced the area to become a basin.

In addition, the whole process is long and gentle, so the Sichuan Basin connection appears to be quite smooth.

It can be seen that the experience of the Sichuan Basin is complex and changeable, but it is also full of a variety of original geographical features, which make Sichuan a famous tourist attraction in China.