MAIN STRUCTURAL ELEMENTS OF THE EARTH'S CRUST: The largest structural elements earth's crust are continents and oceans.

Within the oceans and continents, smaller structural elements are distinguished, firstly, these are stable structures - platforms that can be both in the oceans and on the continents. They are characterized, as a rule, by a leveled, calm relief, which corresponds to the same position of the surface at depth, only under the continental platforms it is at a depth of 30-50 km, and under the oceans 5-8 km, since the oceanic crust is much thinner than the continental one.

In the oceans, as structural elements, mid-ocean mobile belts are distinguished, represented by mid-ocean ridges with rift zones in their axial part, crossed by transform faults and are currently zones spreading, i.e. expansion of the ocean floor and buildup of newly formed oceanic crust.

On the continents, as structural elements of the highest rank, stable areas are distinguished - platforms and epiplatform orogenic belts that formed in the Neogene-Quaternary time in stable structural elements of the earth's crust after a period of platform development. These belts include modern mountain structures of the Tien Shan, Altai, Sayan, Western and Eastern Transbaikalia, East Africa, etc. also in the Neogene-Quaternary time, they make up epigeosynclinal orogenic belts, such as the Alps, Carpathians, Dinarids, the Caucasus, Kopetdag, Kamchatka, etc.

The structure of the Earth's crust of continents and oceans: The Earth's crust is the outer solid shell of the Earth (geosphere). Below the crust is the mantle, which differs in composition and physical properties - it is denser, contains mainly refractory elements. The crust and mantle are separated by the Mohorovichic boundary, on which there is a sharp increase in seismic wave velocities.

The mass of the earth's crust is estimated at 2.8 1019 tons (of which 21% is oceanic crust and 79% is continental). The bark is only 0.473% total weight Earth.

Oceanic th bark: The oceanic crust consists mainly of basalts. According to the theory of plate tectonics, it continuously forms at mid-ocean ridges, diverges from them, and is absorbed into the mantle in subduction zones (the place where oceanic crust sinks into the mantle). Therefore, the oceanic crust is relatively young. Ocean. the crust has a three-layer structure (sedimentary - 1 km, basalt - 1-3 km, igneous rocks - 3-5 km), its total thickness is 6-7 km.

Continental crust: The continental crust has a three-layer structure. The upper layer is represented by a discontinuous cover of sedimentary rocks, which is widely developed, but rarely has a large thickness. Most of the crust is folded under the upper crust, a layer composed mainly of granites and gneisses, of low density and ancient history. Studies show that most of these rocks were formed very long ago, about 3 billion years ago. Below is the lower crust, consisting of metamorphic rocks - granulites and the like. The average thickness is 35 km.

Chemical composition Earth and earth's crust. Minerals and rocks: definition, principles and classification.

The chemical composition of the Earth: consists mainly of iron (32.1%), oxygen (30.1%), silicon (15.1%), magnesium (13.9%), sulfur (2.9%), nickel (1.8% ), calcium (1.5%) and aluminum (1.4%); the remaining elements account for 1.2%. Due to mass segregation, the interior is believed to be composed of iron (88.8%), small amounts of nickel (5.8%), sulfur (4.5%)

The chemical composition of the earth's crust: The earth's crust is slightly more than 47% oxygen. The most common rock-constituting minerals of the earth's crust almost entirely consist of oxides; the total content of chlorine, sulfur and fluorine in rocks is usually less than 1%. The main oxides are silica (SiO2), alumina (Al2O3), iron oxide (FeO), calcium oxide (CaO), magnesium oxide (MgO), potassium oxide (K2O) and sodium oxide (Na2O). Silica serves mainly as an acid medium and forms silicates; the nature of all major volcanic rocks is associated with it.

Minerals: - natural chemical compounds arising from certain physical and chemical processes. Most minerals are crystalline solids. The crystalline form is due to the structure of the crystal lattice.

According to the prevalence, minerals can be divided into rock-forming - forming the basis of most rocks, accessory - often present in rocks, but rarely making up more than 5% of the rock, rare, the occurrences of which are single or few, and ore, widely represented in ore deposits.

Holy Island of minerals: hardness, crystal morphology, color, luster, transparency, cohesion, density, solubility.

Rocks: a natural collection of minerals of a more or less constant mineralogical composition, forming an independent body in the earth's crust.

By origin, rocks are divided into three groups: igneous(effusive (frozen at depth) and intrusive (volcanic, erupted)), sedimentary and metamorphic(rocks formed in the thickness of the earth's crust as a result of changes in sedimentary and igneous rocks due to changes in physico-chemical conditions). Igneous and metamorphic rocks make up about 90% of the volume of the earth's crust, however, on the modern surface of the continents, their areas of distribution are relatively small. The remaining 10% are sedimentary rocks, which occupy 75% of the earth's surface area.

They testify that on our planet, many hundreds of millions of years ago, both rigid and inactive blocks - platforms and shields, and mobile mountain belts, which are often called geosynclinal, were formed. These include huge, framing the seas and whole. In the XX century. these scientific ideas were supplemented by new data, among which, first of all, the discovery of mid-ocean ridges and oceanic basins should be mentioned.

Platforms are the most stable parts of the earth's crust. Their area is many thousands and even millions of square kilometers. Once they were mobile, but over time they turned into rigid arrays. Platforms usually consist of two floors. The lower floor is built from ancient crystalline rocks, the upper - from younger ones. The rocks of the lower floor are called the foundation of the platform. The protrusions of such a foundation can be observed in , on , in and . Due to their massiveness and rigidity, these protrusions are called - shields. These are the most ancient sites: the age of many reaches 3-4 billion years. During this time, irreversible changes occurred in the rocks, recrystallization, compaction and other metamorphoses.

The upper floor of the platforms is formed by huge strata of sedimentary rocks that have accumulated over hundreds of millions of years. Gentle folds, ruptures, ridges and domes are observed in these strata. Traces of especially large uplifts and subsidences are anteclises and syneclises. its shape resembles a giant hill with an area of ​​60 - 100 thousand km2. The height of such a hill is small - about 300 - 500 m.

The outskirts of the anteclise descend in steps to those surrounding them (from the Greek syn - together and enklisis - inclination). On the outskirts of syneclises and anteclises, individual swells and domes are often found - small tectonic forms. The platforms are primarily characterized by rhythmic fluctuations, which led to a successive change of ups and downs. In the process of these movements, deflections, small folds, and tectonic cracks arose.

The structure of the sedimentary cover on the platforms is complicated by tectonic structures, the appearance of which is not easy to explain. For example, under the northern part of the bottom and under the Caspian lowland there is a huge basin, closed on all sides, with a depth of more than 22 km. In diameter, this basin reaches 2000 km. It is filled with clay, limestone, rock salt and other rocks. The upper 5 - 8 km of sediments are attributed to the Paleozoic age. According to geophysical data, there is no granite-gneiss layer in the center of this depression, and the sedimentary rock mass lies directly on the granulite-basalt layer. Such a structure is more typical for depressions with an oceanic type of the earth's crust; therefore, the Caspian depression is considered a relic of the most ancient Precambrian oceans.

The complete opposite of platforms are orogenic belts - mountain belts that arose on the site of former geosynclines. They, like the platforms, belong to long-term developing tectonic structures, but the speed of the earth's crust in them turned out to be much higher, and the forces of compression and tension created large mountain ranges and depressions on the surface of the Earth. Tectonic stresses in the orogenic belts either increased or sharply decreased, and therefore it is possible to trace both the growth phases of mountain structures and the phases of their destruction.

Lateral compression of crustal blocks in the past often led to the separation of blocks into tectonic plates, each of which had a thickness of 5-10 km. The tectonic plates warped and often moved one on top of the other. As a result, older rocks were pushed over younger rocks. Large thrusts, measured in tens of kilometers, scientists call shariazh. There are especially many of them in, and, but charyazhs are also found on platforms where the displacement of the plates of the earth's crust led to the formation of folds and shafts, for example, in the Zhiguli mountains.

The bottom of the seas and oceans has long remained an unexplored area of ​​the Earth. Only in the first half of the XX century. mid-ocean ridges were discovered, which were subsequently discovered in all the oceans of the planet. They had a different structure and age. The results of deep-sea drilling also contributed to the study of the structure of mid-ocean ridges. The axial zones of mid-ocean ridges, together with rift basins, are displaced by hundreds and thousands of kilometers. These displacements most often occur along large faults (so-called transform faults) that were formed in different geological epochs.

Earth's crust makes up the uppermost shell of the solid Earth and covers the planet with an almost continuous layer, changing its thickness from 0 in some areas of mid-ocean ridges and oceanic faults to 70-75 km under high mountain structures (Khain, Lomize, 1995). The thickness of the crust on the continents, determined by the increase in the velocity of the passage of longitudinal seismic waves up to 8-8.2 km/s ( Mohorovicic border, or moho border), reaches 30-75 km, and in oceanic depressions 5-15 km. The first type of earth's crust was named oceanic,second- continental.

oceanic crust occupies 56% of the earth's surface and has a small thickness - 5-6 km. Three layers are distinguished in its structure (Khain and Lomize, 1995).

First, or sedimentary, a layer no thicker than 1 km occurs in the central part of the oceans and reaches a thickness of 10–15 km at their periphery. It is completely absent in the axial zones of mid-ocean ridges. The composition of the layer includes clay, siliceous and carbonate deep-sea pelagic sediments (Fig. 6.1). Carbonate sediments occur no deeper than the critical depth of carbonate accumulation. Closer to the continent, an admixture of detrital material carried from land appears; these are the so-called hemipelagic sediments. The propagation velocity of longitudinal seismic waves here is 2–5 km/s. The age of the sediments of this layer does not exceed 180 Ma.

Second layer in its main upper part (2A) it is composed of basalts with rare and thin layers of pelagic

Rice. 6.1. Section of the lithosphere of the oceans in comparison with the average section of ophiolite allochthons. Below is a model for the formation of the main units of the section in the zone of oceanic spreading (Khain and Lomize, 1995). Symbols: 1 -

pelagic sediments; 2 – outflowing basalts; 3 – complex of parallel dikes (dolerites); 4 – upper (not layered) gabbroids and gabbro-dolerites; 5, 6 - layered complex (cumulates): 5 - gabbroids, 6 - ultramafic rocks; 7 – tectonized peridotites; 8 – basal metamorphic halo; 9 – basaltic magma change I–IV – successive change of crystallization conditions in the chamber with distance from the spreading axis

ical precipitation; basalts often have a characteristic pillow (in cross section) separation (pillow lavas), but there are also covers of massive basalts. In the lower part of the second layer (2B), parallel dolerite dikes are developed. The total thickness of the 2nd layer is 1.5–2 km, and the velocity of longitudinal seismic waves is 4.5–5.5 km/s.

third layer The oceanic crust consists of full-crystalline igneous rocks of basic and subordinately ultrabasic composition. In its upper part, rocks of the gabbro type are usually developed, and the lower part is composed of a "banded complex" consisting of alternating gabbro and ultra-ramafites. The thickness of the 3rd layer is 5 km. Speed longitudinal waves in this layer it reaches 6–7.5 km/s.

It is believed that the rocks of the 2nd and 3rd layers were formed simultaneously with the rocks of the 1st layer.

Oceanic crust, or rather oceanic-type crust, is not limited in its distribution to the bed of the oceans, but is also developed in deep-water basins of marginal seas, such as the Sea of ​​Japan, the South Okhotsk (Kuril) basin of the Sea of ​​\u200b\u200bOkhotsk, the Philippine, Caribbean and many others

seas. In addition, there are serious grounds to suspect that in the deep depressions of the continents and shallow inland and marginal seas of the Barents type, where the thickness of the sedimentary cover is 10-12 km or more, it is underlain by oceanic-type crust; this is evidenced by the velocities of longitudinal seismic waves of the order of 6.5 km/s.

It was said above that the age of the crust of modern oceans (and marginal seas) does not exceed 180 Ma. However, within the folded belts of the continents, we also find a much older, up to the Early Precambrian, crust of the oceanic type, represented by the so-called ophiolite complexes(or just ophiolites). This term belongs to the German geologist G. Steinmann and was proposed by him at the beginning of the 20th century. to designate a characteristic "triad" of rocks commonly found together in the central zones of fold systems, namely serpentinized ultramafic rocks (layer 3 analog), gabbro (layer 2B analog), basalts (layer 2A analog), and radiolarites (layer 1 analog). The essence of this paragenesis of rocks was erroneously interpreted for a long time, in particular, gabbro and ultramafic rocks were considered intrusive and younger than basalts and radiolarites. Only in the 1960s, when the first reliable information about the composition of the oceanic crust was obtained, did it become obvious that ophiolites are the oceanic crust of the geological past. This discovery was of cardinal importance for a correct understanding of the conditions for the origin of the Earth's mobile belts.

Structures of the earth's crust of the oceans

Areas of continuous distribution oceanic crust expressed in the relief of the Earth oceanicdepressions. Within the oceanic basins, two major elements stand out: ocean platforms and oceanic orogenic belts. ocean platforms(or thalassocratons) in the bottom topography look like vast abyssal flat or hilly plains. To oceanic orogenic belts include mid-ocean ridges, having a height above the surrounding plain up to 3 km (in some places they rise in the form of islands above ocean level). Along the axis of the ridge, a zone of rifts is often traced - narrow grabens 12-45 km wide at a depth of up to 3-5 km, indicating the dominance of crustal extension in these areas. They are characterized by high seismicity, a sharply increased heat flow, and a low density of the upper mantle. Geophysical and geological data indicate that the thickness of the sedimentary cover decreases as it approaches the axial zones of the ridges, and the oceanic crust experiences a noticeable uplift.

The next major element of the earth's crust - transition zone between continent and ocean. This is the region of maximum dissection of the earth's surface, where island arcs, characterized by high seismicity and modern andesitic and andesite-basalt volcanism, deep-sea trenches and deep-water basins of marginal seas. The earthquake sources here form a seismic focal zone (the Benioff-Zavaritsky zone), plunging under the continents. The transition zone is the most

pronounced in the western part of the Pacific Ocean. It is characterized by an intermediate type of structure of the earth's crust.

continental crust(Khain, Lomize, 1995) is distributed not only within the continents themselves, i.e., land, with the possible exception of the deepest depressions, but also within the shelf zones of continental margins and individual areas within oceanic microcontinent basins. Nevertheless, the total area of ​​development of the continental crust is smaller than that of the oceanic one, and accounts for 41% of the earth's surface. The average thickness of the continental crust is 35-40 km; it decreases towards the margins of the continents and within microcontinents and increases under mountain structures up to 70-75 km.

Generally, continental crust, like the oceanic one, has a three-layer structure, but the composition of the layers, especially the two lower ones, differs significantly from those observed in the oceanic crust.

1. sediment layer, commonly referred to as a sedimentary cover. Its thickness varies from zero on shields and smaller uplifts of platform foundations and axial zones of folded structures to 10 and even 20 km in platform depressions, frontal and intermountain troughs of mountain belts. True, in these depressions the crust that underlies the sediments and is usually called consolidated may already be closer in character to oceanic than to continental. The composition of the sedimentary layer includes various sedimentary rocks of predominantly continental or shallow marine, less often bathyal (again, within deep depressions) origin, and also, far

not everywhere, covers and sills of basic igneous rocks forming trap fields. The velocity of longitudinal waves in the sedimentary layer is 2.0-5.0 km/s with a maximum for carbonate rocks. The age range of the rocks of the sedimentary cover is up to 1.7 billion years, i.e., an order of magnitude higher than that of the sedimentary layer of modern oceans.

2. Upper layer of consolidated crust protrudes onto the day surface on shields and arrays of platforms and in the axial zones of folded structures; it was penetrated to a depth of 12 km in the Kola well and to a much shallower depth in wells in the Volga-Ural region on the Russian Plate, on the US Midcontinent Plate and on the Baltic Shield in Sweden. A gold mine in South India went through this layer up to 3.2 km, in South Africa - up to 3.8 km. Therefore, the composition of this layer, at least its upper part, is generally well known; the main role in its composition is played by various crystalline schists, gneisses, amphibolites and granites, in connection with which it is often called granite-gneiss. The speed of longitudinal waves in it is 6.0-6.5 km/s. In the basement of young platforms of Riphean-Paleozoic or even Mesozoic age, and partly in the inner zones of young folded structures, the same layer is composed of less strongly metamorphosed (greenschist facies instead of amphibolite) rocks and contains less granites; hence it is often referred to here granite-metamorphic layer, and typical velocities of longitudinal wills in it are of the order of 5.5-6.0 km/s. The thickness of this layer of the crust reaches 15-20 km on platforms and 25-30 km in mountain structures.

3. The lower layer of the consolidated crust. Initially, it was assumed that between the two layers of the consolidated crust there is a clear seismic boundary, which received the name of the Konrad boundary after its discoverer, a German geophysicist. The drilling of the wells just mentioned cast doubt on the existence of such a clear boundary; sometimes, instead of it, seismic reveals not one, but two (K 1 and K 2) boundaries in the crust, which made it possible to distinguish two layers in the lower crust (Fig. 6.2). The composition of the rocks that make up the lower crust, as noted, is not well known, since it has not been reached by boreholes, and is exposed fragmentarily on the surface. Based

Rice. 6.2. Structure and thickness of the continental crust (Khain and Lomize, 1995). BUT - the main types of the section according to seismic data: I-II - ancient platforms (I - shields, II

Syneclises), III - shelves, IV - young orogens. K 1 , K 2 -surfaces of Konrad, M-surface of Mohorovichich, velocities are indicated for longitudinal waves; B - histogram of continental crust thickness distribution; B - generalized strength profile

general considerations, V. V. Belousov came to the conclusion that, on the one hand, rocks that are at a higher stage of metamorphism should prevail in the lower crust and, on the other hand, rocks of a more basic composition than in the upper crust. So he called this layer of bark gra-zero-basic. Belousov's assumption is generally confirmed, although outcrops show that not only basic, but also acidic granulites are involved in the composition of the lower crust. At present, most geophysicists distinguish between the upper and lower crust on a different basis - according to their excellent rheological properties: the upper crust is rigid and brittle, the lower one is plastic. The velocity of longitudinal waves in the lower crust is 6.4-7.7 km/s; belonging to the crust or mantle of the lower part of this layer with velocities of more than 7.0 km/s is often disputable.

Between the two extreme types of the earth's crust - oceanic and continental - there are transitional types. One of them - suboceanic crust - It is developed along the continental slopes and foothills and, possibly, underlies the bottom of the basins of some not very deep and wide marginal and inland seas. Suboceanic crust is thinned up to 15-20 km and permeated with dykes and sills of basic igneous rocks.

bark. It was discovered by a deep-water drilling at the entrance to the Gulf of Mexico and exposed on the Red Sea coast. Another type of transitional cortex is subcontinental- is formed when the oceanic crust in ensimatic volcanic arcs turns into a continental one, but does not yet reach full “maturity”, having a lower thickness, less than 25 km, and a lower degree of consolidation, which is reflected in lower seismic wave velocities - no more than 5.0-5.5 km/s in the lower crust.

Some researchers single out as special types two more varieties of oceanic crust, which have already been discussed above; this is, firstly, the oceanic crust of the internal uplifts of the ocean (Iceland, etc.) thickened up to 25-30 km, and, secondly, the oceanic-type crust, “built on” by a thick, up to 15-20 km, sedimentary cover (the Caspian depression and etc.).

The Mohorovichic surface and the composition of the upper manti. The boundary between the crust and the mantle, usually seismically quite clearly expressed by a jump in the velocities of compressional waves from 7.5-7.7 to 7.9-8.2 km / s, is known as the Mohorovichic surface (or simply Moho and even M), by name the Croatian geophysicist who established it. In the oceans, this boundary corresponds to the transition from the banded complex of the 3rd layer with a predominance of gabbroids to continuous serpentinized peridotites (harzburgites, lherzolites), less often dunites, in some places protruding to the bottom surface, and in the rocks of São Paulo in the Atlantic against the coast of Brazil and on about. Zabargad in the Red Sea, towering above the surface

ocean. The tops of the oceanic mantle can be observed in places on land as part of the bottoms of ophiolite complexes. Their thickness in Oman reaches 8 km, and in Papua New Guinea, perhaps even 12 km. They are composed of peridotites, mainly harzburgites (Khain and Lomize, 1995).

The study of inclusions in lavas and kimberlites from pipes shows that even under the continents, the upper mantle is mainly composed of peridotites, both here and under the oceans in the upper part, these are spinel peridotites, and below, garnet ones. But in the continental mantle, according to the same data, in addition to peridotites, eclogites, i.e., deeply metamorphosed basic rocks, are present in a subordinate amount. Eclogites may be metamorphosed relics of oceanic crust dragged into the mantle during subduction of this crust.

The upper part of the mantle is secondarily depleted in a number of components: silica, alkalis, uranium, thorium, rare earths, and other incoherent elements due to the smelting of basalt rocks from the earth's crust from it. This "depleted" ("depleted") mantle extends under the continents to a greater depth (covering all or almost all of its lithospheric part) than under the oceans, giving way to a deeper "non-depleted" mantle. The average primary composition of the mantle should be close to spinel lherzolite or a hypothetical mixture of peridotite and basalt in a ratio of 3: 1, called by the Australian scientist A. E. Ring-wood pyrolite.

At a depth of about 400 km, a rapid increase in the velocity of seismic waves begins; from here to 670 km

erased Golitsyn layer, named after the Russian seismologist B.B. Golitsyn. It is also distinguished as a middle mantle, or mesosphere - transition zone between the upper and lower mantle. The increase in the velocities of elastic oscillations in the Golitsyn layer is explained by an increase in the density of the mantle matter by about 10% due to the transition of some mineral species into others, with a denser packing of atoms: olivine into spinel, pyroxene into garnet.

lower mantle(Khain and Lomize, 1995) starts from a depth of about 670 km. The lower mantle should be composed mainly of perovskite (MgSiO 3) and magnesia-wustite (Fe, Mg)O - products of further alteration of the minerals that make up the middle mantle. The core of the Earth in its outer part, according to seismology, is liquid, and the inner one is solid again. Convection in the outer core generates the Earth's main magnetic field. The composition of the core is accepted by the vast majority of geophysicists as iron. But again, according to experimental data, it is necessary to admit some admixture of nickel, as well as sulfur, or oxygen, or silicon, in order to explain the lower density of the core compared to that determined for pure iron.

According to seismic tomography, core surface is uneven and forms protrusions and depressions with an amplitude of up to 5-6 km. At the boundary of the mantle and core, a transitional layer with the index D "is distinguished (the crust is indicated by the index A, the upper mantle is B, the middle is C, the lower is D, the upper part of the lower mantle is D"). The thickness of layer D" in some places reaches 300 km.

Lithosphere and asthenosphere. Unlike the crust and mantle, distinguished by geological data (by material composition) and seismological data (by the jump in seismic wave velocities at the Mohorovichich boundary), the lithosphere and asthenosphere are purely physical concepts, or rather rheological ones. The initial basis for the allocation of the asthenosphere is a weakened, plastic shell. underlying a more rigid and fragile lithosphere, there was a need to explain the fact of the isostatic balance of the crust, discovered during measurements of gravity at the foot of mountain structures. It was originally expected that such structures, especially as grand as the Himalayas, should create an excess of gravity. However, when in the middle of the XIX century. appropriate measurements were made, it turned out that no such attraction was observed. Consequently, even large irregularities in the relief of the earth's surface are somehow compensated, balanced at depth so that significant deviations from the average values ​​of gravity do not appear at the level of the earth's surface. Thus, the researchers came to the conclusion that there is a general desire of the earth's crust to balance due to the mantle; this phenomenon is called isostasis(Khain, Lomize, 1995) .

There are two ways to implement isostasy. The first is that mountains have roots immersed in the mantle, i.e., isostasy is provided by variations in the thickness of the earth's crust and the lower surface of the latter has a relief that is the opposite of that of the earth's surface; this is the hypothesis of the English astronomer J. Erie

(Fig. 6.3). On a regional scale, it is usually justified, since mountain structures really have a thicker crust, and the maximum thickness of the crust is observed in the highest of them (Himalayas, Andes, Hindu Kush, Tien Shan, etc.). But another mechanism for the realization of isostasy is also possible: areas of elevated relief should be composed of less dense rocks, and areas of low relief, more dense; this is the hypothesis of another English scientist, J. Pratt. In this case, the sole of the earth's crust may even be horizontal. The balance of the continents and oceans is achieved by a combination of both mechanisms - the crust under the oceans and much thinner and noticeably denser than under the continents.

Most of the Earth's surface is in a state close to isostatic equilibrium. The greatest deviations from isostasy - isostatic anomalies - reveal island arcs and associated deep-sea trenches.

In order for the striving for isostatic equilibrium to be effective, i.e., under an additional load, the crust would sink, and when the load was removed, it would rise, it is necessary that a sufficiently plastic layer exist under the crust, capable of flowing from areas of increased geostatic pressure to areas reduced pressure. It was for this layer, originally identified hypothetically, that the American geologist J. Burrell proposed in 1916 the name asthenosphere, what does "weak shell" mean. This assumption was confirmed only much later, in the 60s, when seismic

Rice. 6.3. Schemes of isostatic equilibrium of the earth's crust:

a - by J. Erie, b - according to J. Pratt (Khain, Koronovsky, 1995)

logs (B. Gutenberg) discovered the existence at a certain depth under the crust of a zone of decrease or absence of an increase, natural with an increase in pressure, velocity of seismic waves. Subsequently, another method of establishing the asthenosphere appeared - the method of magnetotelluric sounding, in which the asthenosphere manifests itself as a zone of lower electrical resistance. In addition, seismologists have identified another sign of the asthenosphere - increased attenuation of seismic waves.

The asthenosphere also plays a leading role in the movements of the lithosphere. The flow of asthenospheric matter drags lithospheric plates-plates along with it and causes their horizontal displacements. The rise of the surface of the asthenosphere leads to the rise of the lithosphere, and in the limiting case, to a break in its continuity, the formation of separation and subsidence. The outflow of the asthenosphere also leads to the latter.

Thus, of the two shells that make up the tectonosphere: the asthenosphere is an active element, and the lithosphere is a relatively passive element. Their interaction determines the tectonic and magmatic "life" of the earth's crust.

In the axial zones of the mid-ocean ridges, especially in the East Pacific Rise, the roof of the asthenosphere is located at a depth of only 3-4 km, i.e., the lithosphere is limited only to the upper part of the crust. As we move towards the periphery of the oceans, the thickness of the lithosphere increases due to

lower crust, but mainly the upper mantle and can reach 80-100 km. In the central parts of the continents, especially under the shields of ancient platforms, such as the East European or Siberian, the thickness of the lithosphere is measured already 150-200 km or more (in South Africa 350 km); according to some ideas, it can reach 400 km, i.e., here the entire upper mantle above the Golitsyn layer should be part of the lithosphere.

The difficulty of detecting the asthenosphere at depths of more than 150-200 km gave rise to doubts among some researchers about its existence under such areas and led them to an alternative view that the asthenosphere as a continuous shell, i.e., the geosphere, does not exist, but there is a series of disparate "asthenolenses ". We cannot agree with this conclusion, which could be important for geodynamics, since it is these areas that demonstrate a high degree of isostatic balance, because they include the above examples of areas of modern and ancient glaciation - Greenland, etc.

The reason why the asthenosphere is not easy to detect everywhere is obviously the change in its viscosity laterally.

The main structural elements of the earth's crust of the continents

On the continents, two structural elements of the earth's crust are distinguished: platforms and mobile belts (Historical Geology, 1985).

Definition:platform- a stable rigid section of the earth's crust of the continents, which has an isometric shape and a two-story structure (Fig. 6.4). Lower (first) structural floor - crystalline foundation, represented by highly deformed metamorphosed rocks cut through by intrusions. The upper (second) structural floor is gently sloping sedimentary cover, weakly dislocated and non-metamorphosed. The exits to the day surface of the lower structural floor are called shield. The areas of the foundation covered by the sedimentary cover are called stove. The thickness of the sedimentary cover of the plate is a few kilometers.

Example: two shields (Ukrainian and Baltic) and the Russian plate stand out on the East European platform.

Structures of the second floor of the platform (case) there are negative (deflections, syneclises) and positive (anteclises). Syneclises are saucer-shaped, and anteclises are inverted saucers. The thickness of deposits is always greater on the syneclise, and less on the anteclise. The dimensions of these structures in diameter can reach hundreds or a few thousand kilometers, and the fall of layers on the wings is usually a few meters per 1 km. There are two definitions of these structures.

Definition: syneclise - a geological structure, the fall of the layers of which is directed from the periphery to the center. Anteclise - a geological structure, the fall of the layers of which is directed from the center to the periphery.

Definition: syneclise - a geological structure in the core of which younger deposits emerge, and along the edges

Rice. 6.4. Platform structure diagram. 1 - folded foundation; 2 - platform case; 3 Faults (Historical Geology, 1985)

- more ancient. Anteclise is a geological structure, in the core of which there are older deposits, and at the edges - younger ones.

Definition: deflection - an elongated (elongated) geological body, having a concave shape in cross section.

Example: on the Russian Plate of the East European Platform stand out anteclises(Belarusian, Voronezh, Volga-Ural, etc.), syneclises(Moscow, Caspian, etc.) and troughs (Ulyanovsk-Saratov, Pridnestrovsko-Black Sea, etc.).

There is a structure of the lower horizons of the cover - av-lacogen.

Definition: aulacogene is a narrow elongated depression extending through the platform. Aulacogens are located in the lower part of the upper structural stage (sheath) and can be up to hundreds of kilometers long and tens of kilometers wide. Aulacogens are formed under conditions of horizontal extension. Thick strata of sediments accumulate in them, which can be folded into folds and are close in composition to the formations of miogeosynclines. Basalts are present in the lower part of the section.

Example: Pachelma (Ryazan-Saratov) aulacogene, Dnieper-Donetsk aulacogen of the Russian plate.

History of platform development. Three stages can be distinguished in the history of development. First- geosynclinal, on which the formation of the lower (first) structural element (foundation) takes place. Second- aulacogenous, which, depending on the climate, accumulates

red-colored, gray-colored or coal-bearing sediments in aulacogenes. The third- slab, on which sedimentation occurs over a large area and the upper (second) structural floor (slab) is formed.

The process of accumulation of precipitation, as a rule, occurs cyclically. Accumulates first transgressive maritime terrigenous formation, then carbonate formation (transgression maximum, Table 6.1). During regression in an arid climate, a saline red-flowered formation, and in a humid climate - paralytic coal-bearing formation. Precipitation is formed at the end of the sedimentation cycle continental formations. At any time, the stage can be interrupted by the formation of a trap formation.

Table 6.1. Sequence of slab accumulation

formations and their characteristics.

End of table 6.1.

For mobile belts (folded areas) characteristic:

linearity of their contours;

the enormous thickness of the accumulated deposits (up to 15-25 km);

consistency composition and thickness of these deposits along strike folded area and abrupt changes across its stretch;

the presence of peculiar formations- complexes of rocks formed at certain stages of development of these areas ( slate, flysch, spilito-keratophyric, molasse and other formations)

intense effusive and intrusive magmatism (large granite batholith intrusions are especially characteristic);

strong regional metamorphism;

7) strong folding, an abundance of faults, including

thrusts indicating the dominance of compression. Folded regions (belts) arise at the site of geosynclinal regions (belts).

Definition: geosyncline(Fig. 6.5) - a mobile area of ​​the earth's crust, in which thick sedimentary and volcanogenic strata initially accumulated, then they were crushed into complex folds, accompanied by the formation of faults, the introduction of intrusions and metamorphism. There are two stages in the development of the geosyncline.

First stage(properly geosynclinal) characterized by a predominance of subsidence. Great rainfall in the geosyncline is the result of the stretching of the earth's crust and her bending. AT the first half of the firststages sandy-argillaceous and clayey sediments usually accumulate (as a result of metamorphism, they then form black argillaceous shales, released in slate formation) and limestones. The subsidence may be accompanied by ruptures along which mafic magma rises and erupts under subsea conditions. The resulting rocks after metamorphism, together with the accompanying subvolcanic formations, give spilit-keratophyric formation. Simultaneously with it, siliceous rocks and jaspers are usually formed.

oceanic

Rice. 6.5. Scheme of the structure of geosync-

molting in a schematic section through the Sunda Arc in Indonesia (Structural Geology and Plate Tectonics, 1991). Symbols: 1 - sediments and sedimentary rocks; 2 - volcano-

nic breeds; 3 - basement conti-metamorphic rocks

Specified formations accumulate at the same time, but in different areas. Accumulation spilito-keratophyric formations usually occur in the interior of the geosyncline - in eugeosynclines. For eugeo-synclines the formation of thick volcanic sequences, usually basic, and the intrusion of gabbro, diabases, and ultrabasic rocks are characteristic. In the marginal part of the geosyncline, along its border with the platform, there are usually miogeosynclines. Here, mainly terrigenous and carbonate strata accumulate; volcanic rocks are absent, intrusions are not typical.

In the first half of the first stage most of the geosyncline is sea ​​with significantdepths. Evidence is provided by the fine granularity of sediments and the rarity of faunal finds (mainly nekton and plankton).

To middle of the first stage due to different sinking rates in different parts of the geosyncline, sections are formed relative uplift(intrageoantic-linali) and relative subsidence(intrageosyncline-whether). Small plagiogranite intrusions may occur at this time.

In second half of the first stage as a result of the appearance of internal uplifts, the sea becomes shallower in the geosyncline. now this archipelago separated by straits. Due to shallowing, the sea advances on adjacent platforms. Limestones accumulate in the geosyncline, thick sandy-clayey rhythmically built strata, forming flysch for-216

mation; there is an outpouring of lavas of medium composition, composing porphyritic formation.

To end of the first stage intrageosynclines disappear, intrageoanticlinals merge into one central uplift. This is a common inversion; it matches main phase of folding in the geosyncline. Folding is usually accompanied by the intrusion of large synorogenic (simultaneous with folding) granite intrusions. There is a crushing of rocks into folds, often complicated by overthrusts. All this causes regional metamorphism. At the site of intrageosynclines, synclinoria- complex structures of the synclinal type, and in place of the intrageoanticlinals - anticlinoria. The geosyncline "closes", turning into a folded area.

In the structure and development of the geosyncline, a very important role belongs to deep faults - long-lived ruptures that cut through the entire earth's crust and go into the upper mantle. Deep faults determine the contours of geosynclines, their magmatism, the division of the geosyncline into structural-facies zones that differ in the composition of sediments, their thickness, magmatism, and the nature of structures. Inside geosynclines are sometimes distinguished mid arrays, limited by deep faults. These are blocks of more ancient folding, composed of rocks of the base on which the geosyncline was laid. In terms of sediment composition and thickness, the middle massifs are close to platforms, but they are distinguished by strong magmatism and rock folding, mainly along the massif edges.

The second stage of the development of the geosyncline called orogenic and is characterized by a predominance of uplifts. Sedimentation occurs in limited areas along the periphery of the central uplift - in edge deflections, arising along the boundary of the geosyncline and the platform and partially overlapping the platform, as well as in intermountain troughs, sometimes formed inside the central uplift. The source of precipitation is the destruction of the constantly rising central uplift. In the first halfsecond stage this uplift probably has a hilly relief; when it is destroyed, marine, sometimes lagoonal sediments accumulate, forming lower molasse formation. Depending on climatic conditions, this may be coal-bearing paralytic or saline thick. At the same time, the intrusion of large granite intrusions - batholiths - usually occurs.

In the second half of the stage the rate of uplift of the central uplift increases sharply, which is accompanied by its splits and the collapse of individual sections. This phenomenon is explained by the fact that due to folding, metamorphism, and intrusions, the folded area (no longer a geosyncline!) becomes rigid and reacts with splits to the ongoing uplift. The sea leaves this territory. As a result of the destruction of the central uplift, which at that time was a mountainous country, continental coarse clastic strata accumulate, forming upper molasse formation. Splitting of the crest of the uplift is accompanied by terrestrial volcanism; usually these are felsic lavas, which, together with

subvolcanic formations give porphyry formation. Fissure alkaline and small acid intrusions are associated with it. Thus, as a result of the development of the geosyncline, the thickness of the continental crust increases.

By the end of the second stage, the folded mountainous area that arose at the site of the geosyncline collapses, the territory gradually levels off and becomes a platform. The geosyncline transforms from the area of ​​accumulation of sediments into the area of ​​destruction, from the mobile territory into the inactive rigid leveled territory. Therefore, the range of motion on the platform is small. Usually the sea, even shallow, covers vast areas here. This area no longer experiences such strong subsidence as before, therefore, the thickness of precipitation is much less (on average 2-3 km). The subsidence is repeatedly interrupted, so there are frequent breaks in sedimentation; then weathering crusts can form. There is also no vigorous uplift accompanied by folding. Therefore, the newly formed thin, usually shallow sediments on the platform are not metamorphosed and lie horizontally or slightly obliquely. Igneous rocks are rare and are usually represented by terrestrial outpourings of basaltic lavas.

In addition to the geosynclinal model, there is a model of lithospheric plate tectonics.

Tectonics model lithospheric plates

Plate tectonics(Structural Geology and Plate Tectonics, 1991) is a model that was created to explain the observed pattern of the distribution of deformations and seismicity in the outer shell of the Earth. It is based on extensive geophysical data obtained in the 1950s and 1960s. The theoretical foundations of plate tectonics are based on two premises.

The outermost shell of the earth, called lithosphere, lies directly on the layer called acetenosphere, which is less durable than the lithosphere.

The lithosphere is divided into a number of rigid segments, or plates (Fig. 6.6), which are constantly moving relative to each other and whose surface area is also constantly changing. Most of the tectonic processes with intense energy exchange operate at the boundaries between the plates.

Although the thickness of the lithosphere cannot be measured with great accuracy, researchers agree that within the plates it varies from 70-80 km under the oceans to a maximum value of more than 200 km under some parts of the continents, with an average value of about 100 km. The asthenosphere underlying the lithosphere extends down to a depth of about 700 km (the maximum depth of propagation of sources of deep-focus earthquakes). Its strength increases with depth, and some seismologists believe that its lower limit is

single lines - transform faults (shifts); Speckled areas of the continental crust that are undergoing active faulting (Structural Geology and Plate Tectonics, 1991)

It is located at a depth of 400 km and coincides with a slight change in physical parameters.

Borders between plates are divided into three types:

divergent;

convergent;

transform (with offsets along strike).

At the divergent boundaries of the plates, represented mainly by rifts, a new formation of the lithosphere occurs, which leads to the expansion of the ocean floor (spreading). At convergent plate boundaries, the lithosphere sinks into the asthenosphere, i.e., is absorbed. At transform boundaries, two lithospheric plates slide relative to each other, and the substance of the lithosphere is neither created nor destroyed on them. .

All lithospheric plates are constantly moving relative to each other. It is assumed that the total area of ​​all plates remains unchanged for a significant period of time. At a sufficient distance from the edges of the slabs, the horizontal deformations inside them are insignificant, which makes it possible to consider the slabs as rigid. Since displacements along transform faults occur along their strike, the movement of the plates must be parallel to modern transform faults. Since all this happens on the surface of the sphere, then, in accordance with Euler's theorem, each section of the plate describes a trajectory equivalent to rotation on the spherical surface of the Earth. For the relative movement of each pair of plates at any time, you can determine the axis, or pole of rotation. As you move away from this pole (up to the angular

distance of 90°) spreading rates naturally increase, but the angular velocity for any given pair of plates about their pole of rotation is constant. We also note that geometrically, the poles of rotation are unique for any pair of plates and are in no way connected with the pole of rotation of the Earth as a planet.

Plate tectonics is an effective model of processes occurring in the crust, as it is in good agreement with known observational data, provides an elegant explanation for previously unrelated phenomena, and opens up possibilities for prediction.

Wilson cycle(Structural Geology and Plate Tectonics, 1991). In 1966, Professor Wilson of the University of Toronto published a paper in which he argued that continental drift occurred not only after the early Mesozoic split of Pangea, but also in pre-Pangean times. The cycle of opening and closing of the oceans relative to adjacent continental margins is now called Wilson cycle.

On fig. 6.7 shows a schematic explanation of the basic concept of the Wilson cycle in the framework of ideas about the evolution of lithospheric plates.

Rice. 6.7a represents the beginning of the Wilson cycle– the initial stage of the breakup of the continent and the formation of the accretionary margin of the plate. known to be tough

Rice. 6.7. Scheme of the Wilson cycle of ocean development within the framework of the evolution of lithospheric plates (Structural geology and plate tectonics, 1991)

the lithosphere covers a weaker, partially molten zone of the asthenosphere - the so-called low-velocity layer (Figure 6.7, b) . As the separation of the continents continues, a rift valley develops (Fig. 6.7, 6) and a small ocean (Fig. 6.7, c). These are the stages of early ocean opening in the Wilson cycle.. Suitable examples are the African Rift and the Red Sea. With the continuation of the drift of separated continents, accompanied by symmetrical accretion of the new lithosphere on the margins of the plates, shelf sediments accumulate on the border of the continent with the ocean due to the erosion of the continent. fully formed ocean(Fig. 6.7, d) with a median ridge at the plate boundary and a developed continental shelf is called Atlantic type ocean.

From observations of oceanic trenches, their relationship with seismicity, and reconstruction from the pattern of oceanic magnetic anomalies around the trenches, it is known that the oceanic lithosphere dissects and sinks into the mesosphere. On fig. 6.7, d shown ocean with plate, which has simple margins of increment and absorption of the lithosphere, - this is the initial stage of the closure of the ocean in Wilson cycle. The division of the lithosphere in the vicinity of the continental margin leads to the transformation of the latter into the Orogen Andean type as a result of tectonic and volcanic processes occurring at the absorbing plate boundary. If this division occurs at a considerable distance from the continental margin towards the ocean, then an island arc of the type of the Japanese islands is formed. ocean absorptionlithosphere leads to a change in the geometry of the plates and at the end

ends to complete disappearance of the accretionary margin of the plate(Fig. 6.7, e). During this time, the opposite continental shelf may continue to expand, turning into an Atlantic-type semi-ocean. As the ocean shrinks, the opposite continental margin is eventually involved in the plate absorption regime and participates in the development accretionary orogen of the Andean type. This is the early stage of the collision of two continents (collisions) . At the next stage, due to the buoyancy of the continental lithosphere, the absorption of the plate stops. The lithospheric plate comes off below, under the growing Himalayan-type orogen, and comes final orogenic stageWilson cyclewith mature mountain belt, which is a seam between the newly joined continents. antipode Andean-type accretionary orogen is an Himalayan-type collisional orogen.

The internal structure of the Earth

At present, the overwhelming majority of geologists, geochemists, geophysicists and planetary scientists accept that the Earth has a conventionally spherical structure with fuzzy boundaries of separation (or transition), and the spheres are conventionally mosaic-block. The main spheres are the earth's crust, the three-layer mantle and the two-layer core of the Earth.

Earth's crust

The earth's crust makes up the uppermost shell of the solid earth. Its thickness ranges from 0 in some parts of the mid-ocean ridges and oceanic faults to 70-75 km under the mountain structures of the Andes, the Himalayas and Tibet. The earth's crust has lateral heterogeneity , i.e. the composition and structure of the earth's crust are different under oceans and continents. Based on this, two main types of crust are distinguished - oceanic and continental, and one type of intermediate crust.

● oceanic crust occupies about 56% of the earth's surface on Earth. Its thickness usually does not exceed 5-6 km and is maximum at the foot of the continents. It has three layers in its structure.

First layer represented by sedimentary rocks. These are mainly clayey, siliceous and carbonate deep-sea pelagic sediments, with carbonates disappearing from a certain depth due to dissolution. Closer to the continent, an admixture of detrital material removed from the land (continent) appears. The thickness of precipitation ranges from zero in spreading zones to 10-15 km near the continental foothills (in the perioceanic troughs).

Second layer oceanic crust at the top(2A) is composed of basalts with rare and thin layers of pelagic sediments. The basalts are often pillow-shaped (pillow lavas), but there are also covers of massive basalts. In the lower part of the second layer (2B), the basalts contain parallel dolerite dikes. The total thickness of the second layer is about 1.5-2 km. The structure of the first and second layers of the oceanic crust has been well studied with the help of underwater vehicles, dredging and drilling.

third layer oceanic crust consists of full-crystalline igneous rocks of basic and ultrabasic composition. In the upper part, rocks of the gabbro type are developed, and the lower part is composed of a "banded complex" consisting of alternating gabbro and ultramafic rocks. The thickness of the 3rd layer is about 5 km. It was studied based on dredging and observations from underwater vehicles.

The age of the oceanic crust does not exceed 180 million years.

When studying the folded belts of the continents, fragments of rock associations similar to oceanic ones were revealed in them. Mr. Shteiman proposed at the beginning of the 20th century to call them ophiolite complexes(or ophiolites) and consider the "triad" of rocks, consisting of serpentinized ultramafic rocks, gabbro, basalts, and radiolarites, as relics of the oceanic crust. Confirmation of this was obtained only in the 60s of the XX century, after the publication of an article on this topic by A.V. Peive.

● continental crust distributed not only within continents, but also within the shelf zones of continental margins and microcontinents located within ocean basins. total area it makes up about 41% of the earth's surface. The average thickness is 35-40 km. On the shields and platforms of the continents, it varies from 25 to 65 km, and under mountain structures it reaches 70-75 km.

The continental crust has a three-layer structure:

First layer- sedimentary, usually called a sedimentary cover. Its thickness ranges from zero on shields, basement uplifts and in the axial zones of folded structures to 10-20 km in exogonal depressions of platform plates, foredeeps and intermountain troughs. It is composed mainly of sedimentary rocks of continental or shallow marine, less often of bathyal (in deep-water depressions) origin. In this sedimentary layer, covers and forces of igneous rocks are possible, forming trap fields (trap formations). The age range of the rocks of the sedimentary cover is from the Cenozoic to 1.7 billion years. The speed of longitudinal waves is 2.0-5.0 km/s.

Second layer continental crust or upper layer of the consolidated crust comes to the surface on shields, massifs or ledges of platforms and in the axial parts of folded structures. It was discovered on the Baltic (Fennoscandian) shield to a depth of more than 12 km by the Kola superdeep borehole and to a shallower depth in Sweden, on the Russian plate in the Saatly Ural borehole, on a plate in the USA, in the mines of India and South Africa. It is composed of crystalline schists, gneisses, amphibolites, granites and granite gneisses, and is called granite gneiss or granite-metamorphic layer. The thickness of this layer of the crust reaches 15-20 km on platforms and 25-30 km in mountain structures. The speed of longitudinal waves is 5.5-6.5 km/s.

third layer or the lower layer of the consolidated crust was isolated as granulite-mafic layer. Previously, it was assumed that there is a clear seismic boundary between the second and third layers, named after its discoverer. Konrad border (K) . Later, during seismic studies, even up to 2-3 boundaries began to be distinguished To . In addition, drilling data from the Kola SG-3 did not confirm the difference in rock composition at the crossing of the Konrad boundary. Therefore, at present, most geologists and geophysicists distinguish between the upper and lower crust by their different rheological properties: the upper crust is more rigid and brittle, while the lower one is more ductile. However, based on the composition of xenoliths from the explosion pipes, it can be assumed that the "granulite-mafic" layer contains felsic and basic granulites and mafic rocks. On many seismic profiles, the lower crust is characterized by the presence of numerous reflecting areas, which can also probably be considered as the presence of bedded intrusions of igneous rocks (something similar to trap fields). The velocity of longitudinal waves in the lower crust is 6.4-7.7 km/s.

● Transitional bark is a kind of crust between the two extreme types of the earth's crust (oceanic and continental) and can be of two types - suboceanic and subcontinental. Suboceanic crust developed along the continental slopes and foothills and probably underlies the bottom of the basins of not very deep and wide marginal and inland seas. Its thickness does not exceed 15-20 km. It is riddled with dikes and forces of basic igneous rocks. The suboceanic crust was exposed by a borehole at the entrance to the Gulf of Mexico and exposed on the coast of the Red Sea. subcontinental crust It is formed when the oceanic crust in ensimatic volcanic arcs turns into continental, but has not yet reached "maturity". It has a reduced (less than 25 km) thickness and a lower degree of consolidation. The speed of longitudinal waves in the crust of the transitional type is not more than 5.0-5.5 km/s.

● Mohorovichic surface and mantle composition. The boundary between the crust and the mantle is quite clearly defined by a sharp jump in the velocities of longitudinal waves from 7.5-7.7 to 7.9-8.2 km / s, and it is known as the Mohorovichic surface (Moho or M) after the name of the Croatian geophysicist who identified it .

In the oceans, it corresponds to the boundary between the banded complex of the 3rd layer and serpentinized mafic-ultramafic rocks. On the continents, it is located at a depth of 25-65 km and up to 75 km in folded areas. In a number of structures, up to three Moho surfaces are distinguished, the distances between which can reach several kilometers.

Based on the results of studying xenoliths from lavas and kimberlites from explosion pipes, it is assumed that under the continents in the upper mantle, in addition to peridotites, eclogites are present (as relics of the oceanic crust that ended up in the mantle during subduction?).

Upper part of the mantle is the "depleted" ("depleted") mantle. It is depleted in silica, alkalis, uranium, thorium, rare earths and other incoherent elements due to the smelting of basaltic rocks of the earth's crust from it. It covers almost all of its lithospheric part. Deeper, it is replaced by an "undepleted" mantle. The average primary composition of the mantle is close to spinel lherzolite or a hypothetical mixture of peridotite and basalt in a ratio of 3:1, which was named by A.E. Ringwood pyrolite.

Layer of golitsin or middle mantle(mesosphere) - the transition zone between the upper and lower mantle. It extends from a depth of 410 km, where there is a sharp increase in the velocities of longitudinal waves, to a depth of 670 km. The increase in velocities is explained by an increase in the density of the mantle matter by about 10%, due to the transition of mineral species to other species with denser packing: for example, olivine into wadsleyite, and then wadsleyite into ringwoodite with a spinel structure; pyroxene to garnet.

lower mantle starts from a depth of about 670 km and extends to a depth of 2900 km with a layer D at the base (2650-2900 km), i.e. to the core of the Earth. On the basis of experimental data, it is assumed that it should be composed mainly of perovskite (MgSiO 3) and magnesiowustite (Fe,Mg)O, products of further changes in the lower mantle substance with a general increase in the Fe/Mg ratio.

According to the latest seismic tomographic data, a significant inhomogeneity of the mantle was revealed, as well as the presence of a larger number of seismic boundaries (global levels - 410, 520, 670, 900, 1700, 2200 km and intermediate levels - 100, 300, 1000, 2000 km), due to the boundaries of mineral transformations in mantles (Pavlenkova, 2002; Pushcharovsky, 1999, 2001, 2005; etc.).

According to D.Yu. Pushcharovsky (2005) presents the structure of the mantle somewhat differently than the above data according to the traditional model (Khain and Lomize, 1995):

Upper mantle consists of two parts: top part up to 410 km, lower part 410-850 km. Section I is distinguished between the upper and middle mantle - 850-900 km.

Medium mantle: 900-1700 km. Section II - 1700-2200 km.

lower mantle: 2200-2900 km.

● Earth's core according to seismology, it consists of an external liquid part (2900-5146 km) and an internal solid part (5146-6371 km). The composition of the core is accepted by the majority as iron with an admixture of nickel, sulfur, or oxygen or silicon. Convection in the outer core generates the Earth's main magnetic field. It is assumed that at the boundary of the core and lower mantle, plumes , which then rise up in the form of a flow of energy or a high-energy substance, forming igneous rocks in the earth's crust or on its surface.

plume mantle – a narrow upward flow of solid-phase mantle material with a diameter of about 100 km, which originates in a hot, low-density boundary layer located either above the seismic boundary at a depth of 660 km, or near the core-mantle boundary at a depth of 2900 km (A.W. Hofmann, 1997). According to A.F. Grachev (2000) a mantle plume is a manifestation of intraplate magmatic activity caused by processes in the lower mantle, the source of which can be at any depth in the lower mantle, up to the core-mantle boundary (layer "D"). (Unlike hot spot, where the manifestation of intraplate magmatic activity is due to processes in the upper mantle.) Mantle plumes are characteristic of divergent geodynamic regimes. According to J. Morgan (1971), plume processes originate under the continents at the initial stage of rifting (rifting). The manifestation of a mantle plume is associated with the formation of large arched uplifts (up to 2000 km in diameter), in which intense fissure eruptions of Fe-Ti-type basalts with a komatiite trend, moderately enriched in light REE, with acidic differentiates, constituting no more than 5% of the total lava volume, occur. . Isotope ratios 3 He/ 4 He(10 -6)>20; 143Nd/ 144Nd – 0.5126-0/5128; 87 Sr/ 86 Sr - 0.7042-0.7052. The formation of thick (from 3-5 km to 15-18 km) lava sequences of Archean greenstone belts and later riftogenic structures is associated with the mantle plume.

In the northeastern part of the Baltic Shield, and on the Kola Peninsula in particular, it is assumed that mantle plumes caused the formation of Late Archean tholeiite-basalt and komatiite volcanic rocks of greenstone belts, Late Archean alkali granite and anorthosite magmatism, a complex of Early Proterozoic layered intrusions, and Paleozoic alkali-ultrabasic intrusions (Mitrofanov , 2003).

plume tectonics – mantle jet tectonics related to plate tectonics. This relationship is expressed in the fact that the subducted cold lithosphere plunges to the boundary of the upper and lower mantle (670 km), accumulates there, partially pushing down, and then after 300-400 million years penetrates into the lower mantle, reaching its boundary with the core (2900 km). This causes a change in the nature of convection in the outer core and its interaction with the inner core (the boundary between them is at a depth of about 4200 km) and, in order to compensate for the influx of material from above, the formation of ascending superplumes at the core/mantle boundary. The latter rise to the bottom of the lithosphere, partially experiencing a delay at the boundary of the lower and upper mantle, and in the tectonosphere they split into smaller plumes, with which intraplate magmatism is associated. They also obviously stimulate convection in the asthenosphere, which is responsible for the movement of lithospheric plates. The processes occurring in the core, unlike plate and plume tectonics, are designated by Japanese authors as growth tectonics, meaning the growth of the inner, purely iron-nickel core at the expense of the outer core, replenished by crust-mantle silicate material.

The emergence of mantle plumes, leading to the formation of vast provinces of plateau-basalts, precedes rifting within the continental lithosphere. Further development can follow a complete evolutionary series, including the initiation of triple junctions of continental rifts, subsequent thinning, continental crust rupture, and the beginning of spreading. However, the development of a single plume cannot lead to a rupture of the continental crust. A rupture occurs when a system of plumes is formed on a continent, and then the cleavage process proceeds according to the principle of a crack propagating from one plume to another.

Lithosphere and asthenosphere

Lithosphere consists of the earth's crust and part of the upper mantle. This concept is purely rheological, in contrast to the crust and mantle. It is more rigid and brittle than the more weakened and ductile underlying mantle shell, which has been identified as asthenosphere. The thickness of the lithosphere is from 3-4 km in the axial parts of the mid-ocean ridges to 80-100 km at the periphery of the oceans and 150-200 km or more (up to 400 km?) under the shields of ancient platforms. Deep boundaries (150–200 km or more) between the lithosphere and asthenosphere are determined with great difficulty, or are not detected at all, which is probably due to high isostatic balance and a decrease in the contrast between the lithosphere and asthenosphere in the border zone due to a high geothermal gradient, a decrease in the amount melt in the asthenosphere, etc.

tectonosphere

The sources of tectonic movements and deformations lie not in the lithosphere itself, but in the deeper levels of the Earth. They involve the entire mantle up to the boundary layer with a liquid core. Due to the fact that the sources of movements are also manifested in the more plastic layer of the upper mantle directly underlying the lithosphere - the asthenosphere, the lithosphere and asthenosphere are often combined into one concept - tectonosphere as areas of manifestation of tectonic processes. In the geological sense (according to the material composition), the tectonosphere is divided into the earth's crust and upper mantle to a depth of about 400 km, and in the rheological sense, into the lithosphere and asthenosphere. The boundaries between these divisions, as a rule, do not coincide, and the lithosphere usually includes, in addition to the crust, some part of the upper mantle.

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The largest structural elements of the earth's crust are continents and oceans, characterized by a different structure of the earth's crust. Consequently, these structural elements should be understood in the geological, or rather, even in the geophysical sense, since it is possible to determine the type of structure of the earth's crust only by seismic methods. From this it is clear that not all the space occupied by the waters of the ocean is, in the geophysical sense, an oceanic structure, since vast shelf areas, for example, in the Northern Arctic Ocean, have continental crust. The differences between these two major structural elements are not limited to the type of the earth's crust, but can be traced even deeper, into the upper mantle, which is built differently under the continents than under the oceans, and these differences cover the entire lithosphere, and in some places also the tectonosphere, i.e. traced to depths of about 700 km.

Within the oceans and continents, smaller structural elements are distinguished, firstly, these are stable structures - platforms, which can be both in the oceans and on the continents. They are characterized, as a rule, by a leveled, calm relief, which corresponds to the same position of the surface at depth, only under the continental platforms it is at a depth of 30-50 km, and under the oceans 5-8 km, since the oceanic crust is much thinner than the continental one.

In the oceans, as structural elements, are distinguished mid-ocean mobile belts, represented by mid-ocean ridges with rift zones in their axial part, crossed transform faults and are currently zones spreading, those. expansion of the ocean floor and buildup of newly formed oceanic crust. Consequently, stable platforms (plates) and mobile mid-ocean belts stand out as structures in the oceans.

On the continents, as structural elements of the highest rank, stable areas are distinguished - platforms and epiplatform orogenic belts, formed in the Neogene-Quaternary time in stable structural elements of the earth's crust after a period of platform development. These belts include modern mountain structures of the Tien Shan, Altai, Sayan, Western and Eastern Transbaikalia, East Africa, etc. also in the Neogene-Quaternary time, make up epigeosynclinal orogenic belts, such as the Alps, Carpathians, Dinarides, Caucasus, Kopetdag, Kamchatka, etc.

On the territory of some continents, in the continent-ocean transition zone (in the geophysical sense), there are marginal continental, according to the terminology of V.E. Khaina, mobile geosynclinal belts, representing a complex combination of marginal seas, island arcs and deep-sea trenches. These are belts of high modern tectonic activity, contrast of movements, seismicity and volcanism. In the geological past, intercontinental geosynclinal belts also functioned, for example, the Ural-Okhotsk belt, associated with the ancient Paleo-Asian ocean basin, etc.

The doctrine of geosynclines in 1973 celebrated its centenary since the American geologist D. Dan introduced this concept into geology, and even earlier, in 1857, the American J. Hall also formulated this concept as a whole, showing that mountain-folded structures arose at the site of troughs that were previously filled with various marine sediments. Due to the fact that the general shape of these troughs was synclinal, and the scales of the troughs are very large, they were called geosynclines.

Over the past century, the doctrine of geosynclines has been gaining strength, being developed, detailed, and thanks to the efforts of a large army of geologists from various countries, it has formed into a coherent concept, which is an empirical generalization of a huge amount of factual material, but suffered from one significant drawback: it did not, as V.E. rightly believes, . Khain, geodynamic interpretation of the observed specific patterns of development of individual geosynclines. The concept is currently capable of eliminating this shortcoming. lithospheric plate tectonics, emerged only 25 years ago, but quickly became the leading geotectonic theory. From the point of view of this theory, geosynclinal belts arise at the boundaries of the interaction of various lithospheric plates. Consider the main structural elements of the earth's crust in more detail.

ancient platforms are stable blocks of the earth's crust, formed in the late Archean or early Proterozoic. Them distinguishing feature- Two storey building. lower floor, or foundation it is composed of folded, deeply metamorphosed rock strata, cut through by granite intrusions, with a wide development of gneiss and granite-gneiss domes or ovals - a specific form of metamorphogenic folding (Fig. 16.1). The foundations of the platforms were formed over a long period of time in the Archean and early Proterozoic and subsequently underwent very strong erosion and denudation, as a result of which rocks that had previously occurred at great depths were exposed. The area of ​​ancient platforms on the continents approaches 40% and they are characterized by angular outlines with extended rectilinear boundaries - a consequence of marginal seams (deep faults). The folded areas and systems are either thrust over the platforms or border on them through the foredeeps, which, in turn, are thrust by the folded orogens. The boundaries of the ancient platforms sharply unconformably intersect their internal structures, which indicates their secondary nature as a result of the split of the Pangea-1 supercontinent that arose at the end of the Early Proterozoic.

Upper platform floor presented case, or cover, gently lying with a sharp angular unconformity on the basement of non-metamorphosed deposits - marine, continental and volcanogenic. The surface between the cover and the base reflects the most important structural unconformity within the platforms. The structure of the platform cover turns out to be complex, and grabens, graben-like troughs appear on many platforms at the early stages of its formation. aulacogens(from the Greek "avlos" - furrow, ditch; "gene" - born, i.e. born by a ditch), as N.S. first called them. Shatsky. Aulacogens most often formed in the Late Proterozoic (Riphean) and formed extended systems in the basement body. The thickness of continental and, more rarely, marine deposits in aulacogens reaches 5–7 km, and deep faults that bounded aulacogens contributed to the manifestation of alkaline, basic, and ultrabasic magmatism, as well as platform-specific trap magmatism with continental tholeiitic basalts, sills, and dikes. This lower structural stage of the platform cover, corresponding to the aulacogenous stage of development, is replaced by a continuous cover of platform deposits, most often starting from the Vendian time.

Shields and slabs stand out among the largest structural elements of the platforms. Shield - this is a protrusion on the surface of the platform foundation, which throughout the entire platform stage of development had a tendency to rise. Plate - part of the platform covered by a sediment cover and tending to sag. Smaller structural elements are distinguished within the plates. First of all, these are syneclises - vast flat depressions, under which the foundation is bent, and anteclises - gentle vaults with a raised foundation and a relatively thinned cover.

Along the edges of the platforms, where they border on folded belts, deep depressions often form, called pericratonic(i.e. on the edge of the craton, or platform). Quite often, anteclises and syneclises are complicated by smaller secondary structures: arches, depressions, ramparts. The latter arise above the zones of deep faults, the wings of which experience multidirectional movements and in the platform cover are expressed by narrow outcrops of ancient deposits of the cover from under the younger ones. The angles of inclination of the wings of the shafts do not exceed the first degrees. Often found flexures - bendings of cover layers without breaking their continuity and maintaining the parallelism of the wings, arising above the fault zones in the foundation during the movement of its blocks. All platform structures are very gentle and in most cases it is not possible to directly measure their wing slopes.

The composition of the deposits of the platform cover is diverse, but most often sedimentary rocks predominate - marine and continental, forming sustained layers and strata over a large area. Carbonate formations are very characteristic, for example, white writing chalk, organogenic limestones typical of a humid climate and dolomites with sulfate sediments formed in arid climatic conditions. Continental detrital formations are widely developed, usually confined to the base of large complexes corresponding to certain stages in the development of the platform cover. They are often replaced by evaporite or coal-bearing paralytic formations and terrigenous - sandy with phosphorites, clayey-sandy, sometimes variegated. Carbonate formations usually mark the "zenith" of the development of the complex, and then you can observe the change of formations in the reverse order. Ice sheet deposits are typical for many platforms.

The platform cover in the process of formation repeatedly underwent a structural restructuring, timed to coincide with the boundaries of major geotectonic cycles: Baikal, Caledonian, Hercynian, Alpine and others. Platform sections that experienced maximum subsidence, as a rule, are adjacent to that mobile area or system bordering on the platform, which was actively developing at that time.

The platforms are also characterized by specific magmatism, which manifests itself at the moments of their tectonomagmatic activation. The most typical trap formation, uniting volcanic products - lavas and tuffs and intrusions, composed of tholeiitic basalts of the continental type with a somewhat increased content of potassium oxide in relation to the oceanic content of potassium oxide, but still not exceeding 1-1.5%. The volume of products of the trap formation can reach 1-2 million km 3, as, for example, on the Siberian platform. Highly importance has an alkaline-ultrabasic (kimberlite) formation containing diamonds in the products of explosion pipes (Siberian platform, South Africa).

In addition to ancient platforms, young ones are also distinguished, although they are more often called slabs formed either on the Baikal, Caledonian, or Hercynian basement, which is distinguished by a greater dislocation of the cover, a lower degree of metamorphism of the basement rocks, and a significant inheritance of the cover structures from the basement structures. Examples of such platforms (plates) are: epibaikalian Timan-Pechora, epihercynian Scythian, epipaleozoic West Siberian, etc.

Mobile geosynclinal belts are an extremely important structural element of the earth's crust, usually located in the transition zone from the continent to the ocean and in the process of evolution forming a powerful continental crust. The meaning of the evolution of the geosyncline lies in the formation of a trough in the earth's crust under conditions of tectonic extension. This process is accompanied by underwater volcanic eruptions and the accumulation of deep-sea terrigenous and siliceous deposits. Then partial uplifts arise, the structure of the trough becomes more complicated, and graywacke sandstones are formed due to the erosion of uplifts composed of basic volcanic rocks. The distribution of facies becomes more capricious, reef structures and carbonate strata appear, and volcanism becomes more differentiated. Finally, uplifts grow, a kind of inversion of troughs occurs, granite intrusions are introduced, and all deposits are crushed into folds. At the site of the geosyncline, a mountain uplift arises, in front of which forward troughs grow, filled molasses. - coarse-clastic products of the destruction of mountains, and in the latter, terrestrial volcanism develops, supplying products of medium and acidic composition - andesites, dacites, rhyolites. Subsequently, the mountain-folded structure is eroded, as the rate of uplifts decreases, and the orogen turns into a peneplainized plain. Takova general idea geosynclinal cycle of development.

Rice. 16.2. Schematic section through the mid-ocean ridge (according to T. Zhuteau, with simplification)

Advances in the study of the oceans led in the 60s of our century to the creation of a new global geotectonic theory - lithospheric plate tectonics, which made it possible on an actualistic basis to recreate the history of the development of mobile geosynclinal regions and the movement of continental plates. The essence of this theory lies in the identification of large lithospheric plates, the boundaries of which are marked by modern seismicity belts, and in the interaction of plates through their movement and rotation. In the oceans, there is an increase, expansion of the oceanic crust through its new formation in the rift zones of the mid-ocean ridges (Fig. 16. 2). Since the radius of the Earth does not change significantly, the newly formed crust should be absorbed and go under the continental, i.e. going on her subduction(immersion).

These areas are marked by powerful volcanic activity, seismicity, the presence of island arcs, marginal seas, and deep-water trenches, as, for example, on the eastern periphery of Eurasia. All these processes mark active continental margin those. the zone of interaction between the oceanic and continental crust. On the contrary, those sections of the continents that form a single lithospheric plate with part of the oceans, such as, for example, along the western and eastern margins of the Atlantic, are called passive continental margin and are devoid of all the features listed above, but are characterized by a thick layer of sedimentary rocks above the continental slope (Fig. 16.3). Similarities between volcanogenic and sedimentary rocks early stages development of geosynclines, the so-called ophiolite association, with a section of the oceanic-type crust suggested that the latter were laid on the oceanic crust and further development of the oceanic basin led first to its expansion and then to its closure with the formation of volcanic island arcs, deep-sea trenches and the formation of a thick continental crust. This is seen as the essence of the geosynclinal process.

Thus, thanks to new tectonic ideas, the doctrine of geosynclines acquires, as it were, a "second wind", which makes it possible to reconstruct the geodynamic situation of their evolution on the basis of actualistic methods. Based on what has been said, geosynclinal belt,(marginal or intercontinental) is understood as a mobile belt thousands of kilometers long, laid down at the boundary of lithospheric plates, characterized by a long-term manifestation of various volcanism, active sedimentation and, at the final stages of development, turning into a mountain-folded structure with a thick continental crust. An example of such global belts are: intercontinental - Ural-Okhotsk Paleozoic; Mediterranean Alpine; Atlantic Paleozoic; marginal continental - Pacific Mesozoic-Cenozoic and others. Geosynclinal belts are divided into geosynclinal areas - large segments of belts that differ in the history of development, structure and are separated from each other by deep transverse faults, pinches, etc. In turn, within the regions can be distinguished geosynclinal systems, separated by rigid blocks of the earth's crust - middle arrays or microcontinents structures that, during the subsidence of the surrounding areas, remained stable, relatively elevated, and on which a thin cover accumulated. As a rule, these massifs are fragments of the primary ancient platform, which was crushed during the formation of a mobile geosynclinal belt.

At the end of the 30s of our century, G. Stille and M. Kay subdivided geosynclines into eu- and miogeosynclines. Eugeosinklinal ("complete, real, geosyncline") they called the zone of the mobile belt, which is more internal in relation to the ocean, and was distinguished by especially powerful volcanism, early (or initial) underwater, basic composition; the presence of ultrabasic intrusive (in their opinion) rocks; intense folding and powerful metamorphism. At the same time, the miogeosyncline (“not a true geosyncline”) was characterized by an external position (relative to the ocean), contacted the platform, was laid on a continental-type crust, deposits in it were less metamorphosed, volcanism was also poorly developed or completely absent, and folding occurred later than in the eugeosyncline. Such a division of geosynclinal regions into eu- and miogeosynclinal regions is well expressed in the Urals, the Appalachians, the North American Cordilleras, and other folded regions.

Played an important role ophiolite rock association, widespread in various eugeosynclines. The lower part of the section of such an association consists of ultrabasic, often serpentinized rocks - harzburgites, dunites; above is the so-called layered or cumulative complex of gabbroids and amphibolites; even higher - a complex of parallel dikes, replaced by pillow tholeiitic basalts overlain by siliceous schists (Fig. 16.4). This sequence is close to the section of the oceanic crust. The importance of this similarity cannot be overestimated. The ophiolite association in folded areas, which usually occurs in cover plates, is a relic, traces of a former marine basin (not necessarily an ocean!) with oceanic-type crust. It does not follow from this that the ocean is identified with the geosynclinal belt. The oceanic-type crust could be located only in its center, and along the periphery it was a complex system island arcs, marginal seas, deep-sea trenches, etc., and the oceanic-type crust itself could be in marginal seas. The subsequent shrinkage of the ocean space led to the narrowing of the mobile belt by several times. The oceanic crust at the base of the eugeosynclinal zones can be both ancient and newly formed, formed during the splitting and separation of continental masses.