Tectonic structure of Russia. Tectonic structure

STRUCTURE TECTONIC- a set of structural forms of any part of the earth's crust, which determines its geol. structure and due to the dominance of one or another tect. regime. In a broad sense, this term covers various parts of the earth's crust, formed due to many combinations of decomp. structural forms. The most essential signs by which S. is systematized by t. And which are dependent on each other are scale, morphology and genesis. When classifying S. of t. By size, they mean specific, more or less isolated areas of the earth's crust, which differ from adjacent areas by a certain combination of composition, forms of occurrence, and geophysics. parameters of their components; in turn, these differences reflect the specifics of the history of the movements of the earth's crust, or tect. mode typical for individual stages of development of a given site. The generally accepted classification of S. of t. Has not yet been developed; the most common is the following. 1. S. t. I order -, and between them. 2. S. t. II order - [for example, Siberian (ancient), West Siberian (young)], (Altai-Sayan), geosynclinal areas(Kuril-East Kamchatka), within the oceans -, mid-ocean mobile belts... 3.S. t. III order - in the folded areas folded systems (Ural, Tyanyian), middle massifs (Omolonsky), intermontane depressions; on ancient and young platforms -, etc.; within the limits of oceanic depressions, the identification of structures of the third order has just begun (depressions, ridges, swells). Structures of I and II orders are related to deep-seated structures (Argan, Peive); the upper part of the mantle is involved in their structure. Structures of the III order are localized within the sieges, and partially granite-metam. (granite-gneiss) layer of the earth's crust, why can be attributed to S. t. corrvym. Deep structures differ from S. of t. Cows also in that their shape along the sole of the bark often does not coincide with the shape along the roof. Deep structures usually cannot be considered simply bends of the crustal plates and, therefore, there is not only a quantitative, but also a qualitative difference between them and crustal structures. 4. To S. t. IV order and smaller within the platforms include

Plate tectonics (plate tectonics) is a modern geodynamic concept based on the provision of large-scale horizontal displacements relative to integral fragments of the lithosphere (lithospheric plates). Thus, plate tectonics considers the movements and interactions of lithospheric plates.

For the first time, the hypothesis of the horizontal movement of crustal blocks was made by Alfred Wegener in the 1920s within the framework of the hypothesis of "continental drift", but this hypothesis did not receive support at that time. Only in the 1960s, studies of the ocean floor provided conclusive evidence of horizontal plate movements and the processes of expansion of the oceans due to the formation (spreading) of the oceanic crust. The revival of ideas about the predominant role of horizontal movements took place in the framework of the "mobilistic" direction, the development of which led to the development of the modern theory of plate tectonics. The main principles of plate tectonics were formulated in 1967-68 by a group of American geophysicists - W.J. Morgan, C. Le Pichon, J. Oliver, J. Isaacs, L. Sykes in the development of the earlier (1961-62) ideas of American scientists G. Hess and R. Digz on the expansion (spreading) of the ocean floor

Basics of plate tectonics

The fundamentals of plate tectonics can be summarized in several fundamental

1. The upper rocky part of the planet is divided into two shells, significantly different in rheological properties: the rigid and fragile lithosphere and the underlying plastic and mobile asthenosphere.

2. The lithosphere is divided into plates, constantly moving along the surface of the plastic asthenosphere. The lithosphere is divided into 8 large plates, dozens of medium plates, and many small ones. Between the large and medium slabs, there are belts composed of mosaics of small crustal slabs.

Plate boundaries are areas of seismic, tectonic, and magmatic activity; the inner regions of the plates are weakly seismic and are characterized by a weak manifestation of endogenous processes.

More than 90% of the Earth's surface falls on 8 large lithospheric plates:

Australian plate,
Antarctic plate,
African plate,
Eurasian plate,
Hindustan plate,
Pacific plate,
North American Plate,
South American Plate.

Middle plates: Arabian (subcontinent), Caribbean, Philippine, Nazca and Cocos and Juan de Fuca, etc.

Some lithospheric plates are composed exclusively of oceanic crust (for example, the Pacific Plate), others include fragments of both oceanic and continental crust.

3. There are three types of relative displacements of plates: divergence (divergence), convergence (convergence) and shear displacements.

Accordingly, three types of main plate boundaries are distinguished.

Divergent boundaries- boundaries along which the slabs move apart.

The processes of horizontal stretching of the lithosphere are called rifting... These boundaries are confined to continental rifts and mid-ocean ridges in oceanic basins.

The term "rift" (from the English rift - rupture, crack, gap) is applied to large linear structures of deep origin, formed during the stretching of the earth's crust. In terms of structure, they are graben-like structures.

Rifts can be laid both on the continental and on the oceanic crust, forming a single global system oriented relative to the geoid axis. In this case, the evolution of continental rifts can lead to the rupture of the continuity of the continental crust and the transformation of this rift into an oceanic rift (if the expansion of the rift stops before the stage of rupture of the continental crust, it is filled with sediments, turning into an aulacogen).

The process of sliding plates in zones of oceanic rifts (mid-ocean ridges) is accompanied by the formation of a new oceanic crust due to magmatic basaltic melt coming from the asthenosphere. This process of formation of a new oceanic crust due to the influx of mantle matter is called spreading(from the English spread - to spread, unfold).

The structure of the mid-ocean ridge

In the course of spreading, each extension pulse is accompanied by the inflow of a new portion of mantle melts, which, while solidifying, build up the edges of plates diverging from the MOR axis.

It is in these zones that the formation of a young oceanic crust takes place.

Convergent boundaries- boundaries along which the collision of plates occurs. There can be three main variants of interaction in a collision: "oceanic - oceanic", "oceanic - continental" and "continental - continental" lithosphere. Depending on the nature of the colliding plates, several different processes can take place.

Subduction- the process of shifting the oceanic plate under the continental or other oceanic. Subduction zones are confined to the axial parts of deep-sea trenches, conjugated with island arcs (which are elements of active margins). Subduction boundaries account for about 80% of the length of all convergent boundaries.

When the continental and oceanic plates collide, a natural phenomenon is the underdling of the oceanic (heavier) plate under the edge of the continental; when two oceanic ones collide, the older (that is, the cooler and denser) of them sinks.

Subduction zones have a characteristic structure: their typical elements are a deep-sea trench - a volcanic island arc - a back-arc basin. A deep-sea trench is formed in the bend and underthrust zone of the subducting plate. As it sinks, this plate begins to lose water (which is abundant in the composition of sediments and minerals), the latter, as is known, significantly reduces the melting temperature of rocks, which leads to the formation of melting centers that feed the volcanoes of the island arcs. In the rear of a volcanic arc, some stretching usually occurs, which determines the formation of a back-arc basin. In the zone of the back-arc basin, the tension can be so significant that it leads to rupture of the plate crust and the opening of the basin with the oceanic crust (the so-called back-arc spreading process).

The subsidence of the subducting plate into the mantle is traced by earthquake foci arising at the contact of the plates and inside the subducting plate (colder and therefore more fragile than the surrounding mantle rocks). This seismic focal zone was named Benioff-Zavaritsky zone.

In the subduction zones, the process of the formation of a new continental crust begins.

A much rarer process of interaction of the continental and oceanic plates is the process obduction- thrusting of a part of the oceanic lithosphere onto the edge of the continental plate. It should be emphasized that in the course of this process, the separation of the oceanic plate occurs, and only its upper part - the crust and several kilometers of the upper mantle - is advancing.

In the collision of continental plates, the crust of which is lighter than the material of the mantle, and as a result, is not able to submerge in it, the process takes place collisions... In the course of the collision, the edges of the colliding continental plates are crushed, crumpled, and systems of large thrusts are formed, which leads to the growth of mountain structures with a complex fold-thrust structure. A classic example of such a process is the collision of the Hindustan plate with the Eurasian one, accompanied by the growth of the immense mountain systems of the Himalayas and Tibet.

Collision process model

The collision process replaces the subduction process, completing the closure of the oceanic basin. At the same time, at the beginning of the collision process, when the edges of the continents have already approached, the collision is combined with the process of subduction (the subsidence of the oceanic crust continues under the edge of the continent).

Large-scale regional metamorphism and intrusive granitoid magmatism are typical for collisional processes. These processes lead to the creation of a new continental crust (with its typical granite-gneiss layer).

Transform boundaries- boundaries along which plate shear displacements occur.

The boundaries of the lithospheric plates of the Earth

1 – divergent boundaries ( a - mid-ocean ridges, b - continental rifts); 2 – transform boundaries; 3 – convergent boundaries ( a - island arc, b - active continental margins, v - collisional); 4 – direction and speed (cm / year) of plate movement.

4. The volume of the oceanic crust absorbed in the subduction zones is equal to the volume of the crust arising in the spreading zones. This position emphasizes the opinion about the constancy of the volume of the Earth. But this opinion is not the only and definitively proven. It is possible that the volume of the plans changes pulsatingly, or there is a decrease in its decrease due to cooling.

5. The main cause of plate movement is mantle convection. caused by mantle heat-gravity currents.

The source of energy for these currents is the temperature difference between the central regions of the Earth and the temperature of its near-surface parts. In this case, the main part of endogenous heat is released at the boundary of the core and mantle during the process of deep differentiation, which determines the decay of the primary chondrite material, during which the metal part rushes to the center, increasing the core of the planet, and the silicate part is concentrated in the mantle, where it is further differentiated.

The rocks heated in the central zones of the Earth expand, their density decreases, and they rise, giving way to sinking colder and therefore heavier masses, which have already given off part of the heat in the near-surface zones. This process of heat transfer goes on continuously, resulting in the formation of ordered closed convective cells. In this case, in the upper part of the cell, the flow of matter occurs almost in the horizontal plane, and it is this part of the flow that determines the horizontal movement of the matter of the asthenosphere and the plates located on it. In general, the ascending branches of the convective cells are located under the zones of divergent boundaries (MOR and continental rifts), the descending branches - under the zones of convergent boundaries.

Thus, the main reason for the movement of lithospheric plates is "dragging" by convective currents.

In addition, a number of other factors act on the plates. In particular, the surface of the asthenosphere turns out to be somewhat raised above the zones of ascending branches and more lowered in the zones of immersion, which determines the gravitational "sliding" of the lithospheric plate located on an inclined plastic surface. Additionally, there are processes of pulling the heavy cold oceanic lithosphere in the subduction zones into the hot, and as a consequence, less dense asthenosphere, as well as hydraulic wedging by basalts in the MOR zones.

Figure - Forces acting on lithospheric plates.

The main driving forces of plate tectonics are applied to the bottom of the intraplate parts of the lithosphere - the forces of mantle drag FDO under the oceans and FDC under the continents, the magnitude of which depends primarily on the asthenospheric current velocity, and the latter is determined by the viscosity and thickness of the asthenospheric layer. Since under the continents the thickness of the asthenosphere is much less, and the viscosity is much higher than under the oceans, the magnitude of the force FDC almost an order of magnitude inferior to FDO... Under the continents, especially their ancient parts (continental shields), the asthenosphere almost wedges out, so the continents seem to be “stranded”. Since most of the lithospheric plates of the present-day Earth include both oceanic and continental parts, it should be expected that the presence of a continent in the plate should generally “slow down” the movement of the entire plate. This is how it actually happens (the fastest moving are the almost purely oceanic plates of the Pacific, Cocos and Nazca; the slowest are the Eurasian, North American, South American, Antarctic and African, a significant part of which is occupied by continents). Finally, at convergent plate boundaries, where the heavy and cold edges of lithospheric plates (slabs) sink into the mantle, their negative buoyancy creates a force FNB(the index in the designation of strength - from English negative buoyance). The action of the latter leads to the fact that the subducting part of the plate sinks in the asthenosphere and pulls the entire plate along with it, thereby increasing the speed of its movement. Obviously the strength FNB acts sporadically and only in certain geodynamic settings, for example, in cases of the above-described slab collapse through the 670 km section.

Thus, the mechanisms that set the lithospheric plates in motion can be conditionally assigned to the following two groups: 1) associated with the forces of mantle "dragging" ( mantle drag mechanism), applied to any points of the base of the slabs, in Fig. 2.5.5 - forces FDO and FDC; 2) associated with the forces applied to the edges of the plates ( edge-force mechanism), in the figure - forces FRP and FNB... The role of this or that driving mechanism, as well as those or other forces, is assessed individually for each lithospheric plate.

The combination of these processes reflects the general geodynamic process, covering areas from the surface to deep zones of the Earth.

Mantle convection and geodynamic processes

Currently, a two-cell mantle convection with closed cells (according to the model of through-mantle convection) or separate convection in the upper and lower mantle with accumulation of slabs under subduction zones (according to a two-tiered model) is developing in the Earth's mantle. The probable poles of the uplift of mantle matter are located in northeastern Africa (approximately under the junction zone of the African, Somali, and Arabian plates) and in the area of ​​Easter Island (under the middle ridge of the Pacific Ocean - the East Pacific Uplift).

The equator of the subsidence of mantle material runs along an approximately continuous chain of convergent plate boundaries along the periphery of the Pacific and eastern Indian Oceans.

The current regime of mantle convection, which began about 200 million years ago with the disintegration of Pangea and gave rise to modern oceans, will in the future be replaced by a single-cell regime (according to the model of through-mantle convection) or (according to an alternative model) convection will become through the mantle due to the collapse of slabs through the 670 km section. This, possibly, will lead to the collision of continents and the formation of a new supercontinent, the fifth in the history of the Earth.

6. Displacements of plates obey the laws of spherical geometry and can be described on the basis of Euler's theorem. Euler's Rotation Theorem states that any rotation in three-dimensional space has an axis. Thus, rotation can be described by three parameters: the coordinates of the rotation axis (for example, its latitude and longitude) and the rotation angle. Based on this position, the position of the continents in past geological eras can be reconstructed. Analysis of the movements of the continents led to the conclusion that every 400-600 million years they unite into a single supercontinent, which undergoes further disintegration. As a result of the split of such a supercontinent Pangea, which occurred 200-150 million years ago, the modern continents were formed.

Some evidence of the reality of the mechanism of plate tectonics

Aging of the oceanic crust age with distance from the spreading axes(see figure). An increase in the thickness and stratigraphic completeness of the sedimentary layer is noted in the same direction.

Figure - Map of the age of the rocks of the oceanic floor of the North Atlantic (after W. Pitman and M. Talvani, 1972). Areas of the ocean floor of different age intervals are highlighted in different colors; the numbers indicate the age in millions of years.

Geophysical data.

Figure - Tomographic profile through the Hellenic Trench, Crete and the Aegean Sea. Gray circles are earthquake hypocenters. Blue color shows a plate of a plunging cold mantle, red - a hot mantle (according to V. Spekman, 1989)

Remains of the huge Faralon plate, which disappeared in the subduction zone under the North and South America, recorded as slabs of the "cold" mantle (section across North America, along S-waves). By Grand, Van der Hilst, Widiyantoro, 1997, GSA Today, v. 7, No. 4, 1-7

​Linear magnetic anomalies in the oceans were discovered in the 1950s during the geophysical study of the Pacific Ocean. This discovery allowed Hess and Diez in 1968 to formulate the theory of ocean floor spreading, which grew into the theory of plate tectonics. They have become one of the strongest proofs of the theory's correctness.

Figure - Formation of strip magnetic anomalies during spreading.

The reason for the origin of strip magnetic anomalies is the process of the birth of the oceanic crust in the spreading zones of mid-ocean ridges, the erupted basalts, when they cool below the Curie point in the Earth's magnetic field, acquire remanent magnetization. The direction of magnetization coincides with the direction of the Earth's magnetic field, however, due to periodic inversions of the Earth's magnetic field, the erupted basalts form stripes with different directions of magnetization: direct (coincides with the modern direction of the magnetic field) and reverse.

Figure - Diagram of the formation of the strip structure of the magnetoactive layer and magnetic anomalies of the ocean (Vine - Matthews model).

The tectonic analysis of the territory begins and ends with the compilation of a tectonic map, which is a graphical model of the structure and evolution of a part of the western zone. Depending on the scale of the tect. maps can be global (1: 45000000 - 1: 15,000,000), overview (1: 10,000,000 - 1: 2,500,000), regional small-scale (1: 500000), regional medium- and large-scale (1: 200000 - 1: 50,000). Cards can be of general and special purpose. General tectonic maps equally contain data on the modern tectonic structure of the western zone. and the history of its formation. Specialized tect maps include selective data on the structural features of the area of ​​the fault map, isohypsum, maps of ring structures, or reflect the structural characteristics of the area at a particular time interval or at a certain point in geological history (paleotectonic maps). Example: General overview maps - "Tectonic map of the USSR 1: 4000000" under the leadership of Shatsky. Overview maps of specialized content - "Paleotectonic maps 1: 75000000 - 1: 5000000"

4. General structural features of the ancient platforms of Laurasia.

East European, North American, Siberian and Chinese platforms x-Xia basement having an Early Precambrian age. These platforms are surrounded by movable (folded) belts that separate them and simultaneously solder them. Within these belts, blocks from the continental Early Precambrian crust are widespread - the middle massifs that were previously part of these platforms. There are many common features in the composition and structure of the covers of the Laurasian group platforms, expressed in the total number of storeys, the similarity of the composition of deposits at individual stratigraphic levels (R-Riphean, PZ2-Middle Paleozoic, PZ3-T-Upper Paleozoic-Triassic, J-K-Jurassic-Cretaceous)

5. Name the surface structures that transcend the boundaries of the Eurasian plate. The western boundary of the Eurasian plate runs along the MOR: Azores - Reykjanes ridge - further along the Gakkel ridge - through Chukotka and Kamchatka, along the fault zone to the junction of the Kuril-Kamchatka and Aleutian trenches. Further, the border stretches to the south along the Kuril-Kamchatka trench - Nansey - Philippine deep-water trench, skirting in the south along the Sunda trench. Further, the border runs along the periphery of the Hindustan platform, further in the north-west along the Zagros ridge, to the west through the Cretan Trench - Gibraltar and goes to the Azores.

6. The content of the regional text of the map and methods of depicting the elements of the text of the page

Differences in the scale of maps, specificity of regions, elements of specialization in the content are the reasons for the diversity of regional map tekt. Nevertheless, the legends of the largest number of regional maps are drawn up in the image and likeness of the legends of the survey map tekt. Tekt zoning and the internal structure of regions are depicted on maps using color coloring or line icons. Coloring is used to express the basic principle of regionalization. Various colors, their shades, the degree of intensity correspond to the regions differing in age of the main folding, structural number of storeys, material characteristics of the sections, and the degree of deformation of strata of the same age. Lithospheric plates and their border zones are shown in different colors. Line designations are used to depict different types of boundaries of structural zones and individual forms, discontinuities, out-of-scale folded structures, material complexes. Bar marks can be black or colored. The color scheme of the map is supplemented with letter designations - indices that make the map easier to read.

7. General structural features of the platforms of the Gondwana group. In the structure of the basement of the African-Arabian, South American, Hindustan, Australian, and Antarctic platforms, metamorphic Riphean complexes that unite the Archean-Lower Proterozoic blocks are of significant importance. The Upper Archean formations are known in the section of the protoplatform cover of the Gondwana group, which suggests early cratonization processes in the series of platforms of the Gondwana group. The platform cover is slightly developed on almost all platforms. In contrast to the platforms of the northern group, the boundaries of the southern platforms over a large area coincide with the boundaries of the continents. As a result, they are in direct contact with deep-sea depressions. In the Upper Paleozoic, rifting processes actively proceeded on the platforms of the southern row, which led to the accumulation of continental coastal-marine sediments in grabens. The uplift of some areas at the beginning of the Upper Paleozoic contributed to the deposition of glacial formations. In the Mesozoic, large areas were covered by trap magmatism with the introduction of ultrabasic intrusions of increased alkalinity. In the newest stage, most platforms are also characterized by high mobility.

8. Types of oceanic structures... About 250 million sq. km is occupied by oceanic deep-water plains, depressions and intraoceanic uplifts separating them. The depressions of the oceans differ sharply from the continental massifs in that the surface of the earth's crust within them is lowered by 4-5 km relative to the continents, and the thickness of the earth's crust is reduced by 5-7 times. The differences in the structure of the earth's crust of continents and oceans are that in most of the oceans the "granite-gneiss" layer has not been established. The ocean floor differs sharply in the nature of seismicity. Areas of high seismic activity and seismic areas can be distinguished.

The first are extended zones occupied by MOR systems, stretching across all oceans. They are characterized by intense volcanism, increased heat flow, sharply dissected relief with systems of longitudinal and transverse grooves and ledges, shallow bedding of the mantle surface.

The latter are expressed in the relief by large oceanic basins, plains, plateaus, as well as underwater ridges bounded by fault-type ledges and intraoceanic swell-like ridges. Within the regions, there are underwater plateaus and uplifts with continental crust (microcontinents). By analogy with the structural continents, they are called thalassocratons.