Qinling orogenic belt

The Qinling orogenic belt is a tectonic feature that evolved throughout the Proterozoic and Phanerozoic eons due to a variety of tectonic activities. It is a part of the Central China Orogenic Belt, aligned in an east–west orientation across Central China, and spans portions of Shaanxi, Henan and Gansu provinces along the Qinling Mountains which are one of the greatest mountain ranges in China. The first materials involved in the Qinling orogenic belt formed around 2.5 billion years ago, whereas the main morphology of the belt now largely reflects the Triassic collision between the North China Plate and the South China Plate and Cenozoic extension across China. During these 2.5 billion years, various types of rocks have been formed here due to different tectonic processes and chemical reactions between rocks. Therefore, geologists are able to reconstruct the evolution of mountain belt based on evidence preserved in these rocks.

Tectonic evolution

Throughout the long history of the evolution of Qinling orogenic belt, there were several cycles of plate collisions and plate separations together with ocean openings and closures occurred. The process is known as a Wilson Cycle. The Qinling orogenic belt was formed largely because of the movements of North China Block and Yangtze Block of the South China Plate.

The Qingling orogenic belt can be divided into two major regions, the North Qinling Belt and the South Qingling belt, which are located at the boundary of the southern North China Craton and the northern South China Craton respectively. The most interesting thing about the evolution of Qinling orogenic belt is the multiple individual micro-block interactions. The tectonic evolution of the whole Qinling belt was not a single event but a combination of several collisional and extensional events, which mainly includes 4 phases:

# Development of basement of the belt (2.5 billion years to 800 million years ago)
# Evolution of plates and associated ocean basin (800 to 250 million years ago)
# Major belt formation (orogeny) (250 to 140 million years ago)
# Extension and stretching of the belt (140 million years before to the present)

Development of basement of belt (2.5 billion years to 800 million years ago)

At the beginning of the Qinling rock record (around 2.5 billion years ago), the North Qinling belt and South Qinling belt were not initially formed together at the same location at the same time. The South Qinling belt was formed by continental magmatic activities 2.5 billion years ago. Then, magma cooled down and became rocks which contribute to the major basement of the South Qinling belt. On the other hand, the North Qinling belt was formed later. It was first formed 1000 million years ago by magmatic activities which occurred in an oceanic-arc environment.

During the early Neoproterozoic (1000 million years ago), the North Qinling belt and the South Qinling were aligned along the same subducting plate boundary at the Northeastern part of super-continent Rodinia (an extremely large tectonic plate composed of different smaller plates). During subduction, the South China block overrode an oceanic plate was compressed and South Qinling belt first formed on a small scale.

On the other hand, the formation of North Qinling belt was more complicated. It did not initially exist with North China Block, but a part of supercontinent Rodinia. At the subducting plate boundary, it collided with Rodinia and was folded up forming North Qinling Belt.

In addition, at some distances further away from the North Qinling Belt, a continental arc with volcanoes was formed at another subducting plate boundary as well, which is named the Proto-Erlangping arc. An arc can be formed because the subducted lower plate melted in the mantle and rose up to the opposite upper plate while cutting through lines of weakness of plate. As a result, magma eventually reached the top of the plate, then cooled down and solidified into rocks, forming an arc. In the meantime, Proto-Erlangping ocean was created at divergent plate boundary where plates separate, so that the Proto-Erlangping arc was moving away from North Qinling belt.

Evolution of belt and associated ocean (800 to 250 million years ago)

Later on in the middle Neoproterozoic (around 750 million years ago), the supercontinent containing Proto-Erlangping arc, North Qinling Belt and South Qinling Belt was broken up. The two belts were transported to another place together. The oceanic part of South China block was broken and separated into two parts creating the Shangdan ocean. This is because divergent convection magma dominated at that period, when two parts separate, magma rise up from the gap of them creating a larger oceanic plate (as well as an ocean). On the other end of the oceanic plate, it collided with another oceanic plate coincidently. An island arc called 'DanFeng island arc' was formed.

A similar process occurred at the North Qinling belt, North Qinling belt and Rodinia were separated. As a result, a new ocean formed as well. At this time North and South Qinling belt were still aligning next to each other, therefore they shared the same Shangdan ocean. By the time the North Qinling Belt moved away from Rodinia, it was colliding with the Proto-Erlangping arc as well. Therefore, two subduction processes occurred at the same time at the Proto-Erlangping arc.

At the early Cambrian (around 540 million years ago), Gondwana (considered as super-continent by some geologists) started to develop. The North and South Qinling Belt were located at the Northeastern part of it. This was the time when North China Block first met the North Qinling belt, locating at the other end of Proto-Erlangping arc. During that period, the North Qinling belt was no longer located next to South Qinling belt, but facing it. As same as the situation before Cambrian, the North Qingling belt and South Qinling Belt still share the Shangdan ocean. What different was the subducting plate changed from one to another, at the Danfeng island.

At late Cambrian (around 500 million years ago), the North China block migrated closer to North Qinling block. Consequently, the Proto-Erlangping ocean was closed as the whole oceanic plate had subducted to the mantle. This also implies the North Qinling belt collided with Proto-Erlangping belt after the closure of the ocean.

From the late Ordovician to late Silurian (460 to 420 million years), the collided North Qinling belt and Proto-Erlangping belt were moved to a magma spreading centre, which split plates apart by divergent convection magma. This turn out the Erlangping ocean was opened again.

At the early Devonian (around 400 million years ago), South Qinling belt and North China block moved towards each other, while extensive convergent convection magma occurred, although some geologists claimed it happened between 320 and 300 million years before. As a result, Erlangping arc, North Qinling belt, South Qinling belt and North China block all collided together. With all oceans closed up and blocks moving towards each other, rocks were then piled up within blocks. Meanwhile, Mianlue ocean was opened, since South China block and the rest of the block complex were separated by divergent convection magma current. The ocean created is also regarded as back-arc basin.

During the middle Mississippian (around 300 million years ago), the Mianlue ocean stopped spreading. The South China Plate moved towards the Qinling complex and North China Block. The oceanic part of South China Block subducted to the mantle and the ocean started closing.

Major belt formation (250 to 140 million years ago)

At the early Triassic (around 250 million years ago), the South China block finally collided with Qinling complex, and the continent-continent collision occurred. Accordingly, the Mianlue ocean eventually closed completely. Resulting from extremely strong compressional force, all individual blocks were shortened horizontally but thickened vertically. At middle Jurassic (174-163 million years ago), the South China block subducted beneath the South Qinling block and part of the plate broke off to the subduction zone.

However, the collisional event was different from normal one. It is because the South China Plate collided to the North China plate with a relatively rotational motion. Such that the eastern part of Qinling belt was compressed earlier than the western part.

Extension and stretching of belt (140 million years ago to the present)

Starting from 140 million years (Cretaceous) before to the present, the tectonic activities changed from collisional to extensional, which is a process of crustal stretching resulting in crustal thinning. Until the late Cretaceous (83 million years ago), the Qinling complex was affected by a WNW-ESE extension event in west of Qinling. As a result, right-lateral strike slip fault became dominated. At Mid-Eocene to Early Oligocene (45 to 24 million years ago), normal fault dominated the complex back due to extensional event at the northern part. Until the late Oligocene to early Miocene (24 to 14 million years ago), left-lateral strike-slip fault became the major deformation feature in Qinling. However, at the late Miocene (9 million years ago), normal fault replaced left-lateral strike-slip fault because of a NE-SW extensional event caused by rifting until the late Pliocene

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