1) Formation and evolution of orogenic belts and basins.
Orogenic belts and basins are the basic structural units of the continent and the basis for studying continental dynamics. At present, the study of continental orogenic belts and basins is developing in the direction of systematically exploring the mechanism of ups and downs, deep geological processes and dynamic evolution laws. The following aspects are highly valued.
Extension of (1) orogenic belt
In recent 10 years, some scholars have discovered large-scale extensional structures in the world-famous orogenic belts such as Cordillera, Alps and Himalayas, making them one of the hot spots in contemporary geotectonic research. In addition to the extensional structures found in the young orogenic belts mentioned above, Caledonian, Qinling and Meng Xing ancient orogenic belts also show obvious ductile extension and exposure. Most scholars believe that the extensional structure is the result of gravity expansion and extensional collapse after the orogenic belt was formed, and some people think that it is due to the pulsating upwelling of deep magma, which leads to thermal uplift and extension, forming metamorphic core complexes and related detachment faults. This tectonic thermal process is closely related to the uplift of mountains and the peeling of metamorphic cores. In the future, an important research direction of orogenic belt is to treat orogenic belt and related basins as an organic whole, and deeply discuss the temporal and spatial relationship and genetic relationship between extensional and compressional activities and magmatic activities, with special emphasis on the study of composite forms and transformation mechanisms of multiple tectonic systems in orogenic belt.
(2) Formation mechanism and deep process of sedimentary basin.
In recent years, the relationship between deep tectonic process and shallow extensional rifting in sedimentary basins has been paid attention to, and many types of pure shear extensional models and simple shear extensional models have been put forward. The results show that, although the tectonic styles of crustal extension in continental depression belt are different, all active sedimentary basins have high geothermal gradient, which is characterized by deep high temperature rheology and bedding ductile shear, and shallow rifting is closely related to deep bedding ductile extension. However, up to now, no representative basin compressional structural model has been proposed.
(3) Material movement and structural transformation of basin-mountain tectonic system.
Basin and orogenic belt are closely related in spatial development and formation mechanism. The crust of orogenic belt (region) is obviously thickened, and the crust of sedimentary basin (region) is greatly thinned, which shows the mirror symmetry relationship between tectonic landform and continental Moho surface, and its genesis may be related to the differential rheology and underplating of different horizons. The crust-mantle reaction caused by underplating and the crust-mantle mixture produced by partial melting are added to the bottom of the lower crust, and the heat flow material of the lower crust continues to flow from the mantle uplift area to the mantle depression area, continuously transferring the heat softening material at the bottom of the basin to the orogenic belt. In the process of deep material migration, it not only causes shallow basin expansion and rifting and corresponding orogenic belt compression and contraction, but also causes a series of deep geological effects: magma evolves from basic to intermediate acid from mantle uplift source area to mantle depression area; The metamorphic phase changes from high temperature and high pressure to low temperature and low pressure through medium temperature and medium pressure; Structural deformation changes from viscoplastic flow to ductile-brittle shear; The quality defect is caused by the loss of material in the deep part of the active sedimentary basin, while the excess quality is caused by the accumulation of heat flow in the orogenic belt, which eventually leads to the abnormal gravity balance in the active tectonic belt (belt).
(4) Thermal state of orogenic belt and basin
Different tectonic units in the mainland have different thermal states and thermal structures at different stages of evolution, and the thermal activity in the newly-born rift basin is very significant, especially the lower crust affected by underplating and crust-mantle reaction is in a state of overheating and superplastic flow, and a large number of partially molten materials, pore fluids, flaky and lenticular mafic intrusions and ductile shear zones appear, forming a layered stratum with heat advection conduction and lateral creep of heat flow materials. Accordingly, a huge ductile fluid layer with high heat flux or low velocity and high conductivity appears at the bottom of the young active orogenic belt. Under the continuous action of heat conversion and heat dissipation of basin-mountain tectonic system, orogenic belts and basins tend to be stable, aging, eventually flattened, cooled and solidified, forming a stable craton, the geothermal gradient is obviously reduced, the lithosphere structure is relatively uniform, and there is no active low-velocity layer.
2) Layered structure and rheological properties of continental lithosphere.
An important trend in the study of contemporary continental tectonics and dynamics is to re-recognize and evaluate the structure, strength, composition and rheological properties of the continental lithosphere, especially paying attention to the horizontal and vertical heterogeneity of the continental lithosphere. The layered rheological phenomenon of continental lithosphere is very obvious. The results show that: ① the continental lithosphere consists of two ductile layers of 20 ~ 30 km and 40 ~ 60 km, and two brittle crustal layers and one brittle upper mantle layer sandwiched between them; (2) The continental lithosphere is a sandwich structure consisting of a ductile layer in the lower crust and two brittle layers in the upper and lower crust; (3) With the increase of stress, rheological softening and strain localization occur near the rheological transition zone of the upper mantle below the Moho surface of the mainland, and the ductile layer peels down; ④ The continental lithosphere is different from the rigid oceanic lithosphere, with obvious heterogeneity in structure and strength; ⑤ The stratified rheology of continental tectonic belts or tectonic domains with different tectonic activities is quite different, showing the characteristics of zonal geothermal field and changeable stress field.
3) Structural properties and deformation characteristics of continental lower crust.
It is generally believed that the tectonic transformation of the continental-oceanic system is driven by the material movement of the huge heat flow system in the lower mantle. The magmatic activity of the continental-marine system is mantle-friendly, and ultramafic rocks distributed along the mid-ridge of ocean expansion are typical. The magmatic activity of the intracontinental uplift system shows that it is crustal friendly. Although the mixing of basic magma and crust-mantle occurred in the continental extension area, its genesis is still closely related to the underplating of mafic rocks at the bottom of the crust and the related crust-mantle reaction and partial melting. Some people regard the lower crust as the main object of continental dynamics, trying to study the material composition, thermal state, rheological state, deformation and metamorphism of the lower crust, and then explore the internal relationship between the deep continental tectonic process and the shallow continental tectonic activity, and summarize the material movement law of the continental tectonic system.
In recent years, people have made a detailed study on the high-grade metamorphic rocks, deep xenoliths and deep seismic reflection profiling in the lower crust exposed in nature, and found that there is a nearly horizontal seismic fabric in the lower crust in the continental active area, and a fluid-rich extensional ductile shear zone has developed. This fluid layer with strong rheological properties or ductile fluid layer is the product of tectonic thermal activity under the background of mantle upwelling. Therefore, the continental lower crust can be regarded as a tectonic rock assemblage that changes with the transformation of tectonic activities in a certain space-time structure, and its formation age can be very new. Generally speaking, the middle and lower crust and Moho surface in the active zone were formed later than the middle and upper crust, and their related orogenic-basin-making processes are roughly the same. Obviously, the tectonic activity of shallow fault blocks in continental crust is closely related to the laminar flow of deep materials.
4) Regularity of continental seismic activity and its causes.
It is generally believed that earthquakes are the product of tectonic activities, so earthquakes can also be used as a sign to establish structural models and a yardstick to test structural theory. The distribution of earthquakes in Chinese mainland is regular. According to Zongjin Ma's research, continental earthquakes are distributed in a certain depth range in the middle and upper crust, which is called earthquake-prone layer or multi-seismic layer. These seismogenic layers are the medium layers that accumulate earthquakes, and they are cut and dislocated by active seismogenic faults, thus generating earthquakes. Therefore, continental earthquakes are scattered in layers and concentrated locally in bands, which are mainly controlled by near-horizontal seismic forces.
The main factors that control the focal depth and influence seismic activity include: crust thickness, crustal layered structure, development degree of low-speed and high-conductivity layer and heat flow value. Continental earthquakes mainly occur in areas with strong tectonic thermal activity, especially in young orogenic belts and nascent rift basins, and are concentrated along active strike-slip faults, thrust faults or extensional faults. The seismic zone migrates with the development of the structure in a certain space-time structure. Zongjin Ma believes that seismic migration is a wave propagation process, not a continuous extension of seismic faults. Many people realize that continental seismic activity is related to the material movement of ductile fluid layer in the deep crust and the interlayer slip between low-speed layer and high-speed layer, which is difficult to be explained by plate subduction or collision model.
5) Dynamic mechanism of continental tectonic activity
The continental crust has a long history, diverse continental structures and complex structures. In view of the complexity of the continental structure, some scholars believe that the crustal movement of the continent is composed of various power sources, and it is difficult to generalize the continental structure with a unified model.
The continental crust with layered rheology also shows the characteristics of layered stress. The tectonic deformation in the brittle zone of the upper continental crust is mainly controlled by stress, and crushing and frictional sliding are the main deformation mechanisms. It is generally believed that the complex and changeable stress state of the continent is mainly restricted by the deep material movement. The phenomenon of thermal softening in the ductile region of the lower continental crust is very obvious, especially in the active tectonic zone of the low-velocity layer, and the heat flow value is usually very high. The Moho surface temperature above the hot spot can reach 65438 0200℃ in the extension area of the underplating. The thermal action closely related to this not only causes the crust-mantle reaction and partial melting in the hot spot active area, but also causes the vertical growth of the continental crust, and also restricts the rheological state and material flow law of the continental lower crust, which has nonlinear dynamic characteristics.