In the past, simple kinematic classification was not enough to explain the complexity of orogenic belts. The new classification combines lithospheric deformation with plate movement, describes the dynamic characteristics of orogenic belts in depth and detail, and establishes the types of orogenic belts from the origin. For example, kolar Sengor systematically and comprehensively classified orogenic belts, and he divided them into four categories and 20 types according to the process of convergence.
2) Study on the process details of subduction collision of ancient plates and deformation of ancient continental margin.
It is known that the direction and speed of relative movement of plates obviously control the deformation behavior of the boundary area of convergent plates; Collisions between blocks with different sizes and properties lead to orogenic belts with different structural styles and deformation intensities; Usually the butt joint between blocks is uneven and patched together at the same time. Oblique convergence is actually a common form of block docking, which often leads to relative rotation between adjacent blocks, forming divergent thrust and strike-slip structural products; In the collision orogenic belt, the deformation and migration from the internal metamorphic core belt to the external fold thrust belt will last for tens of millions of years; Structurally, the orogenic belt is composed of displaced fragments. These blocks bounded by faults can be regarded as two basic types: one is geological bodies that have not been subducted, such as island arcs, microcontinent and large accretionary complexes, which usually collide and proliferate to continental cores to form original orogenic components; During and after this accretion, the original orogenic components are divided into nappe, strike-slip dual structure and extensional displacement respectively or together, forming the second kind of orogenic chaotic components.
3) Study on crustal thickening and denudation process of orogenic belt.
The crustal thickening and peeling process of orogenic belt is the main tectonophysical process of crustal formation of orogenic belt. According to the theory of early plate tectonics, this process was thickened by the fold thrust of foreland basin and the upwelling of ophiolite slices, and then the rocks were exposed to the surface through denudation, forming today's orogenic landform. However, recent research shows that the thickening process of orogenic belt can be completed by the subduction of rigid continental plate and the subduction of footwall during plate convergence, while the deep rocks are pulled out of the surface at the same time or after the crustal thickening.
In the process of extrusion orogeny, the subduction of rigid continental plate along passive continental margin makes most rocks migrate downward to form roots, rather than upward to form highlands. It was only after the extrusion stopped and the support for the floating roots was removed that the subsequent extension caused the uplift of the orogenic belt.
The shortening of the crust dominated by thrust in the upper wall is still the main factor of crustal thickening, which may be earlier than subduction in time.
High pressure metamorphic rocks are exposed on the surface of many orogenic belts. Studies in recent years show that these rocks used to be at a depth of tens of kilometers to hundreds of kilometers, and there is no continuous middle crustal segment between them and surrounding rocks, showing structural contact. At present, it is believed that these rocks are formed by the action under the plate and the stripping caused by the uplift of the orogenic belt during the extension.
The p-T-t trajectory of metamorphic rocks records the thickening and stripping process of orogenic belts. The p-T-t trajectories of different metamorphic rocks in the same orogenic belt can be different, reflecting their different burial and stripping histories. Considering the movement tracks of these blocks comprehensively, we can reveal the thickening mechanism and stripping mechanism of orogenic belts.
4) Expression and relationship of different deformations in the evolution of orogenic belts.
Besides the thrust nappe structure formed by extrusion, there are also deformation structures caused by strike-slip and tension in the orogenic belt. In recent years, people's understanding of the significance of strike-slip and extension in the formation of orogenic belts has been improved.
Strike-slip not only runs through all stages of orogenic belt evolution, but also causes some strike-slip orogenic belts. Strike-slip and compression often form a compressional orogenic belt, which is characterized by a mid-crust-scale detachment system composed of lateral inferred layers and thrust belts. Based on the study of strike-slip structural styles, the flower-like structure, strike-slip catamaran and pull-apart basin are studied. In recent years, the concepts of compressional structure and structural escape have been further put forward, which enriches and develops the structural research of orogenic belts.
Extension also runs through the evolution process of the whole orogenic belt. There are three types of extensional separation: pure shear, simple shear and detachment. The detachment is subdivided into five types: lithospheric wedge, layered detachment, detachment+pure shear (including large lateral detachment and small lateral detachment) and layered detachment+pure shear of lower crust. The scale of its separation may or may not pull out the oceanic crust, so the early extension has a great influence on the later evolution of the orogenic belt. In the process of convergence, a bending and stretching zone of the upper crust parallel to the convergence zone developed in some areas. The physical force produced by the balanced compensation elevation required for crustal thickening and the physical force produced by the retreat of subduction zone can drive the extension of lithosphere and produce extension structures parallel to or perpendicular to orogenic belts. Concepts such as plough fault, detachment fault, metamorphic core complex and magmatic core complex further enrich and develop the study of extensional structures. In particular, the research results on the structure, formation process and dynamic mechanism of metamorphic core complexes can better reflect the research status and progress of extensional structures.
5) Deep structure of orogenic belt
Recent studies have revealed the deep structure of orogenic belts, usually in the form of two-way subduction structural fans and their variants, while the crust and lithosphere under the crust in orogenic belts are in the form of complex grooves and fingers intertwined. Usually, the upper crust has a large-scale detachment structure, and the middle part has a double structure and a crocodile structure, while the lower crust has a strong oblique seismic reflection and a flat seismic reflection; Moho has some ups and downs with mountain roots, and some are relatively flat without mountain roots.
6) Rheological structure of orogenic lithosphere and its dynamic significance.
Rheological properties of lithosphere in orogenic belt are the basis of discussing orogenic belt dynamics. In recent 10 years, the rheological stratification of continental lithosphere and the rheology of lower crust have attracted considerable attention.
The rheological stratification of different tectonic units is different, which mainly depends on the material composition stratification, thermal structure and fluid characteristics in the deep lithosphere. The two most typical rheological stratification profiles obtained from orogenic belts are as follows:
(1) sandwich structure, where the ductile lower crust is sandwiched between the brittle upper crust and the uppermost mantle;
(2) A "four-layer" structure, in which two ductile zones (one is 20-30km and the other is 40-60km) separate two brittle crustal layers and one brittle mantle layer.
In recent years, the rheology of the lower crust has attracted extensive attention, mainly because it strongly affects the properties and evolution of many first-order crustal structures. Many dynamic processes of orogenic belt are related to it, such as:
(1) Subplate action and mantle diapir action of mafic and ultramafic magma;
(2) remelting;
(3) Structural strain related to thickening or thinning.
These effects affect the rheology of the lower crust by changing heat flow and geothermal gradient, pressure-temperature trajectory and local geochemical environment.
7) Crust-mantle cycle of orogenic belt
From the orogenic process, there was a strong exchange of crust-mantle materials during subduction, and the results were recorded in orogenic belts. The collision resulted in strong crust-mantle exchange and recycling in the lithosphere of orogenic belt. The detachment and balance adjustment of the upper mantle intensified the basaltic subduction and granulite facies metamorphism at the crust-mantle boundary. Thrust and subduction make surface rocks and fluids insert into the ground, remelt into granite slurry and invade. All these actions lead to the differentiation, migration and recycling of orogenic components. The crust-mantle cycle in this stage is the main content and characteristic of the study of crust-mantle cycle in orogenic belt. In addition, in some crustal sections, it is found that some lower crust is younger than the upper crust, which indicates that there are important events under the basalt plate.
8) Evolution of metamorphism during orogeny.
In recent years, the research focus of orogenic metamorphism has shifted from the metamorphism in subduction stage to the metamorphic evolution history of thickening-stripping process in collision stage.
The deformation, uplift and stripping of orogenic belt have an important influence on the distribution of metamorphic rocks. The distribution of these metamorphic rocks can indicate the uplift and stripping process of metamorphic rocks.
Metamorphism has different manifestations in different stages of orogeny. Generally speaking, from subduction to uplift, the metamorphic temperature is from high to low.