Recently, a new form of existence of the earth's crust has been established - a system of rift zones developed both within the oceanic and continental crusts, as well as in their transitional parts and occupying an area equal to the continents only within the oceans. For rift zones, sometimes complex specific relationships between the mantle and the crust are revealed, which are often characterized by the absence of the Moho boundary, and the interpretation of their nature has not yet left the area of discourse, including the issue of their typification. This is. in relation to the types of rift systems identified in accordance with the data of M.I. Kuzmin, who calculated in 1982 for igneous rocks of these systems natural geochemical standards:
oceanic rift zones confined to mid-ocean ridges, which form a single system of oceanic uplifts up to 60 thousand km long, with the presence within them in most cases of narrow rift valleys 1-2 km deep (in the East Pacific uplift - the central horst uplift). The basic rocks are formed from primitive tholeiitic magma of shallow generation depths - 15-35 km;
Continental rift zones are grabens genetically associated with faults such as faults, being often confined to the axial parts of large arched uplifts, the thickness of the crust under which decreases to 30 km, and the underlying mantle is often loosened. In the rift valleys, tholeiitic basalts appear, and in the distance, rocks of the alkaline-basalt and bimodal series, as well as alkaline-ultrabasic rocks with carbonatites;
island arcs, consisting of four elements: a deep trench, a sedimentary terrace, a volcanic arc, and a marginal sea. The thickness of the earth's crust is from 20 km or more, magma chambers at a depth of 50-60 km. There is a regular change of low-chromium-nickel tholeiite series to sodic calc-alkaline series, and shoshonitic series volcanics appear in the very rear of the island arcs; active continental margins of the Andean type, which characterize the "crawl" of the continental crust onto the oceanic, as well as island arcs, are accompanied by the Zavaritsky-Benioff seismic focal zone, but with the absence of marginal seas and the development of volcanism within the margin of the continent with an increase in the thickness of the earth's pores up to 60 km, and the lithosphere - up to 200-300 km. Magmatism is caused by both mantle and crustal sources, starting with the formation of rocks of calc-alkaline (rhyolitic) series, changing to rocks of andesitic formation - latite series; 5) active continental margins of the Californian type, in contrast to island arcs and active continental margins of the Andean type, are not accompanied by a deep-water trench, but are characterized by the presence of compression and extension zones that arose as a result of the thrust of the North American continent on the entire system of the mid-ocean ridge. Therefore, there is a simultaneous manifestation of magmatism, which is characteristic of both rift structures (oceanic and continental types) and compression zones (deep seismic focal zones).
The petrogeochemical standards (types) of igneous rocks calculated by M. I. Kuzmin, which are typical for these zones, are of great scientific importance, regardless of the pleitectonic views of their author, including for typifying the nature of Precambrian magmatism. V. M. Kuzmin believes that the features of these geochemical types of igneous rocks are determined not by age, but by geodynamic conditions of formation, therefore, these types can be the basis for reconstruction in place of mobile belts of past active zones comparable to modern ones. An example of such reconstructions is the identification of the Mesozoic Mongol-Okhotsk belt with a rift system of Californian-type active margins. This idea, which denies the existence of geosynclinal systems at least in the Phanerozoic and extends the patterns of rifting rock formation to the distant past of the Earth, is opposed by the idea, also based on the study of the geochemical patterns of magmatism, that island arcs do not indicate the presence of a transitional type crust, and even more so rift structures, but are typical young geosynclines.
Rift zones are called very extended (mantle hundreds and thousands of kilometers long) planetary scale strip-like tectonic zones distributed within continents and oceans, in which deep (mantle) material rises, accompanied by its spread to the sides, which leads to more or less significant transverse tension in the upper floors of the earth's crust. The most important structural expression of the extension process on the Earth’s surface is usually the formation of a deep and relatively narrow (from several kilometers to several tens of kilometers), often stepped graben (symmetrical or asymmetric), limited by normal faults of great depth (the rift itself or “rift valley”), or several (sometimes a whole series) of similar grabens. The bottom of the grabens is also cut by normal faults and extension cracks. The subsidence of the bottom of the grabens relative to their sides, as a rule, outstrips the accumulation of sedimentary material in them, although the latter is in many cases supplemented by their filling with volcanic products, and therefore rifts usually have a distinct direct expression in the relief in the form of linear depressions. For the most part, rifts are framed on both sides, or at least on one side, by asymmetric uplifts (sloping semi-arches, one-sided horsts, and less often horsts), to some extent broken, like grabens, by longitudinal, diagonal, and transverse cracks, faults, and often complicated by secondary narrow grabens. In some cases, uplift also occurs inside the rift, splitting it into two branches. The ratio of the volumes of these uplifts and rift depressions reflects the ratio of uplift and extension scales in one or another rift zone. Some of them, especially oceanic, are characterized by a significant role of transverse shear displacements, in particular, along the zones of the so-called transforming faults.
Rift zones in general and, first of all, axial grabens (rifts) have increased or even very high seismicity, moreover, earthquake sources lie at depths from a few kilometers to 40-50 km, and the stress pattern in the sources is characterized by the predominance of maximum subhorizontally directed extensions, approximately perpendicular to to the axis of the rift zone. Rift zones, with rare exceptions, are characterized by an increased heat flux, the value of which generally increases as one approaches their axis, often reaching 2–3, and sometimes even 4–5 heat flux units. The development of most rift zones is accompanied by manifestations of hydrothermal activity and magmatism and, in particular, by volcanic eruptions fed from subcrustal and, in some continental rift zones, possibly also from intracrustal magma chambers. However, the scale of the magmatic process, the volumes of its products, their composition, and their association with certain stages of rifting and with certain parts of the rift zone vary extremely widely. Along with rift zones, in which magmatic activity accompanied all stages of their development, and its products cover almost their entire area and reach volumes of hundreds of thousands of cubic kilometers, there are rift zones where it manifested itself locally, sporadically, or was completely absent.
The rift zones of the oceans are characterized by a contrasting strip-like bilaterally symmetrical magnetic field, which, according to the prevailing ideas, is created in the process of rifting and, as it were, imprints its individual stages. However, the magnetic field of continental rift zones largely reflects the features of the structure of their basement and has undergone only some restructuring in the process of rifting. Rift zones are usually, though not always, characterized by gravity minima in the field of Bouguer anomalies, but in the axial parts of some of them narrow maxima are distinguished, caused by the rise of mafic and ultramafic material. However, the shapes, sizes of gravity anomalies, and the nature of the factors causing disturbances can differ significantly. As a rule, rift zones are close to the state of isostatic equilibrium.
The earth's crust in modern rift zones is somewhat thinned compared to adjacent areas, and the upper part of the mantle, at least immediately below the M surface, in many of them is characterized by an anomalously low velocity of longitudinal seismic waves (7.2-7.8 km / s ) and a slightly lower density and viscosity, which is apparently due to the increased thermal regime and, in some cases, the occurrence of selective melting centers in the upper mantle. These lenses or "pillows" of decompacted mantle material are probably protrusions of the roof of the asthenosphere, reaching the bottom of the earth's crust under modern rift zones. Rift zones rarely exist in isolation; as a rule, they form more or less complex combinations. Methods of "docking" neighboring rift zones and the general plan of their grouping can be very diverse, and at the same time they differ significantly in continental and oceanic zones. We call combinations of a number of closely interconnected in space approximately the same age rift zones of a similar or different type rift systems. This term can be applied to any combination of rift zones, regardless of their size, complexity and pattern, but is mainly used in relation to those combinations that are characterized by the presence of differently oriented rift zones, a tree-like pattern or the presence of several semi-isolated branches, not band-like, but close to to the isometric general outline. In cases where rift zones (or their systems), combined with each other, form linearly elongated structures with a length of several or even many thousand kilometers, we call them rift belts (by analogy with geosynclial and geosynclial belts comparable in length and width). orogenic belts). The term rift system is also used to refer to all interconnected rift belts of the Earth, which together form a complex winding and branching network on the surface of our planet. In the latter case, we are talking about the world rift system. The latter, with its main branches, unites most of the Earth's rift belts (and systems). Its main part crosses the oceans, and its fading ends and branches in several regions of the Earth penetrate deep into the continents. However, within the continents (and possibly in the oceans) there are also separate, isolated rift belts and even separate rift zones that are not connected with the world rift system.
1) oceanic, or intra-oceanic, in which both the axial "rift valley" and its framing have a crust close to the oceanic, which is underlain by a ledge of mantle material with anomalously reduced seismic wave propagation velocities and density compared to those typical for the upper part of the mantle;
2) intercontinental, in which the axial part of the rift has a crust close to that of intraoceanic rift zones, its peripheral parts have somewhat thinned and reworked continental crust, and the “shoulders” have a typical continental crust. Intercontinental rift zones, like intracontinental ones, can be formed either on platforms (the Aden and Krasnomorsky rifts) or within a young folded area (the Gulf of California rift);
3) continental or intracontinental, in which both the rift and its "shoulders" have a continental-type crust, but usually somewhat thinned, especially under the rift (from 20 to 30-35 km), fragmented, anomalously heated and underlain by a lens of a somewhat decompacted mantle material.
Mutual transitions observed in nature and close structural connections of intercontinental rifts as a result of a far advanced process of development of intracontinental rifts. At least some part of the width of the intercontinental rift zones (of the order of several tens of kilometers) is apparently due to tensile or tensile-strike-slip deformations of blocks of the continental crust and the protrusion of mantle-derived material between them, while in intracontinental rifts we mainly deal with graben-like subsidence of blocks of the continental crust with an extension amplitude of the order of several kilometers and, far from always, with the filling of opening cracks with dike-like intrusions. In turn, the intercontinental rift zones are structurally closely related to the rift belts of the Indian and Pacific Oceans, in which the process of upwelling of deep material and horizontal expansion proceeds even more intensively. However, it would be imprudent to assume by analogy that all rift zones and oceanic belts represent a further stage in the development of intercontinental rifts and, therefore, arose as a result of an even greater separation of blocks of the continental crust. For example, in relation to the East Pacific Rift Belt, it can be stated with sufficient certainty that it is younger than the Pacific Ocean and arose on the oceanic crust. The fact that the continuation of this rift belt passes almost completely into the North American continent and is superimposed on the Cordillera Mesozoic folded region obviously indicates that the driving mechanism of rifting is associated with such great depths that are no longer affected by differences between oceans and continents, but the specific manifestations of this process on the surface of the Earth differ significantly depending on whether it affects the earth's crust of the oceans, young folded regions, platforms, etc.
The rift zones and belts belonging to the three identified categories differ significantly in their size, morphology of structural forms, the scale of volcanism (the largest in the rift zones of the oceans), the chemistry of its products (tholeiitic basalts in rift zones, rocks very diverse in acidity and alkalinity in rift zones). zones of the continents), the magnitude of the heat flow (the highest in oceanic rift zones), the structure of the magnetic field, the plan of stresses in earthquake sources (in continental rift zones, the compressive stress vector is oriented subvertically, and in oceanic zones it is usually subhorizontal and subparallel to the strike of the rift zone), etc. e. Continental rift belts are characterized by such spatial combinations of adjacent rift zones as their bead-shaped, en echelon arrangement, articulated articulation, fan-shaped splitting, the junction of three zones converging at different angles, mutual parallelism, enveloping by two adjacent zones of the separating them relative to thin block, which plays the role of a kind of median massif in the structure of the rift belt. On the contrary, the rift belts of the oceans are characterized by their intersection by numerous transverse or diagonal so-called transforming faults, dividing these belts into separate transverse segments (rift zones), the axes of which seem to be displaced relative to each other.
Types of rift zones on the continents. When distinguishing types among modern continental rift zones, the following main criteria should be taken into account: a) features of the tectonic position, base structure and previous geological history of the area that became the arena of rifting, b) the nature of tectonic structures created in the process of rifting, and the patterns of their formation, c) the role, scale, and features of magmatic processes accompanying rifting, and sometimes preceding it.
Based on the first criterion, rift zones and continental belts can be divided into two main groups: 1) rift belts and platform zones (epiplatform rift belts and zones), in which rifting began after a very long (200-500 million years to more ) stage of platform or close to it development; 2) rift belts and zones of young folded structures (epiorogenic rift belts and zones), where a similar process directly followed the completion of their geosynclinal development, i.e., after the orogenic stage, or even combined with phenomena characteristic of epigeosiclinal orogeny. The epiplatform rift belts are characterized by rift zones with large single axial grabens and subalkaline or alkaline products of accompanying volcanism, often with the participation of carbonatites. On the contrary, epiorogenic rift belts and zones are characterized by combinations of many narrow grabens, horsts, and one-sided blocks, and their volcanic formations belong to the calc-alkaline series.
Most of the modern continental epiplatform rift zones are confined mainly to the protrusions of the folded base of the platforms, i.e., to areas that have experienced a long-term stable uplift, and much less often to areas of development of the platform cover (Levanta, North Sea, and partially Ethiopian rift zones). In most cases, rift zones are superimposed on areas of late Proterozoic (Grenville, Baikal) folding or tectonic-magmatic regeneration, "avoiding" areas of more ancient - Archean or Early Proterozoic consolidation, which serve as the outer "frame" of these rift belts or form inside them a kind of "rigid » middle massifs (Victoria massif in the southern part of the African-Arabian belt). Much less frequently, rift zones occur on the Epipaleozoic platform base (Rhine-Rhone area of the Rhenish-Libyan rift belt). In most cases, young rift structures inherit the strikes of ancient folded and ruptured basement structures or "adapt" to them, forming cranked, zigzag, echelon combinations. Thus, in the process of rifting, the ancient anisotropic basement splits along the weakest directions, just as a log of firewood splits according to the fibrous texture of wood. The weakened basement zones used by the Cenozoic rift structures became active at times (in the Paleozoic or Mesozoic) during a long platform development and served either as zones of increased permeability for magmatic melts and intrusions, in particular ring-type alkaline massifs, or zones of faults and grabens.
Among the epiplatform rift zones, two types are clearly distinguished, which differ significantly in the nature of structures, the relative role of volcanism, and the history of formation. The author called them fissured and dome-volcanic (Milanovsky, 1970):
a) rift zones of the dome-volcanic type (Ethiopian and Kenyan zones of East Africa) are characterized by exceptionally powerful and prolonged terrestrial volcanic activity. It begins in a wide area even before the rift is laid, and subsequently continues within the axial graben and associated secondary grabens and fault zones. The main role is played by eruptions of basic and intermediate lavas and pyroclastolites of strongly alkaline and weakly alkaline series. Acid (high alkalinity) volcanics also play a significant role in the Ethiopian rift zone. The emergence of a rift is preceded by a long growth of a vast gently sloping oval arched uplift, accompanied by powerful eruptions, then a relatively shallow graben is laid in its axial weakened zone, as well as additional grabens and normal faults associated with it - transverse and diagonal on the wings of the arch and fan-shaped diverging on its periclines. The amplitude of horizontal extension in dome-volcanic rift zones is minimal. They are characterized by moderate seismicity. The formation of the dome, characterized by a large gravitational minimum, is apparently associated with the appearance of a lens of decompacted, anomalously heated material and with individual magma chambers in the upper mantle, and the formation of grabens is partly due to subsidence of crustal blocks during the unloading of these chambers during eruptions;
b) slot-type rift zones are characterized by a greater depth of grabens, which can reach 3–4 km (Upper Rhine graben) and even 5–7 km (South Baikal graben). Large gravity minima are associated with the large thickness of loose sediments in the grabens. Often the grabens substitute each other on the link. Marginal uplifts are much narrower than in domed volcanic rifts, are not observed everywhere, often only on one side of the graben, and sometimes are completely absent, and in some cases (the rift zone of the North Sea) the development of rifts occurs against the background of general subsidence. In some places inside the rift zone, dome- and horst-like uplifts arise, reaching in some cases a huge height (up to 4-5 km in the Rwenzori block in the Tanganyika zone). Gravity maxima are associated with internal uplifts, and their protrusion is of an antiisostatic nature. Slit rift zones are characterized by relatively weak, local and episodic manifestations of volcanism or their complete absence. On this basis, weakly volcanic (Tanganyika, Upper Rhine) and non-volcanic zones (the middle segment of the Baikal rift belt) can be distinguished among them. Eruption centers are confined to saddles between clearly arranged grabens, their marginal steps, marginal uplifts, and other uplifted areas. Petrochemically, volcanism is close to dome-volcanic zones, but extremely alkaline series (sodium or potassium) and carbonatites are more often present here. Volcanic activity can manifest itself at different stages of rifting.
The process of formation of slotted zones begins with the initiation of narrow linearly elongated grabens (usually confined to ancient weakened zones), filled with initially fine clastic ("molassoid"), as well as carbonate and chemogenic sediments, which are subsequently replaced by coarser clastic continental molasses. This formation series, as well as geomorphological data, show that the intensive growth of marginal and internal uplifts began later than the occurrence of grabens, and in some places has not yet manifested itself. The concept of the emergence of a rift as a result of the collapse of the dome is not applicable to slotted rift zones. These zones are more seismic than dome-volcanic ones. The amplitude of horizontal stretching in them may be greater than in the latter, but, apparently, it usually does not exceed 5–10 km. In the grabens of slotted rift zones, a significant "leakage" of thermal energy apparently occurs. In some crevice zones, in addition to the sliding one, there is a shear component. In the Levantine zone, the latter, apparently, significantly exceeds the transverse tension, and in some of its sections, the horizontal deformation approaches pure shear.
In rift belts and zones of young folded structures, rifting follows the geosynclinal cycle of development, being a direct continuation of its final orogenic stage. In the process of rifting in these zones, a system of narrow, but very extended (up to many hundreds of kilometers) mutually parallel grabens, separated by narrow horsts or one-sided horsts commensurate with them (the Cordillera rift system), often arises in these zones. The amplitudes of the relative movement of the blocks along the normal inclined faults separating them reach 2-5 km. Along with the general significant horizontal tension, significant shear deformations can occur (for example, the San Andreas shear in California). The formation of rift structures is preceded and accompanied by extremely powerful eruptions of calc-alkaline magma, both acidic and basic. Volcanoes were fed from chambers of different depths located both in the upper mantle (foci of basalt volcanism) and in the crust (foci of liparite-dacitic volcanism). The dispersal of extension and accompanying volcanism within a very wide band with numerous grabens in some epiorogenic rift zones is apparently due to the fact that rifting develops under conditions of a more “warmed up” and “plastic”, and in the upper part - fragmented lithosphere compared to relatively "hard" and "cold" lithosphere of epiplatform rift zones.
Along with the East African Rift, the Baikal Rift is another example of a divergent boundary located within the continental crust.
Gallery
Lake Baikal.JPG
The main lake of the rift - Baikal
KhovsgolNuur.jpg
Lake Khubsugul is also located in the area of the Baikal Rift, at its southwestern tip.
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Notes
Literature
- Lyamkin V.F. Ecology and zoogeography of mammals in the intermountain basins of the Baikal Rift Zone / Ed. ed. d.b.n. A. S. Pleshanov; . - Irkutsk: Publishing House of the Institute of Geography SB RAS, 2002. - 133 p.
Links
- / V. E. Khain // Ankylosis - Bank. - M. : Great Russian Encyclopedia, 2005. - S. 662. - (Great Russian Encyclopedia: [in 35 volumes] / ch. ed. Yu. S. Osipov; 2004-, vol. 2). - ISBN 5-85270-330-3.
An excerpt characterizing the Baikal Rift Zone
Natasha quietly closed the door and went with Sonya to the window, not yet understanding what she was being told.“Do you remember,” Sonya said with a frightened and solemn face, “remember when I looked for you in the mirror ... In Otradnoye, at Christmas time ... Do you remember what I saw? ..
- Yes Yes! - Natasha said, opening her eyes wide, vaguely remembering that then Sonya said something about Prince Andrei, whom she saw lying.
– Do you remember? Sonya continued. - I saw then and told everyone, both you and Dunyasha. I saw that he was lying on the bed,” she said, making a gesture with her hand with a raised finger at every detail, “and that he closed his eyes, and that he was covered with a pink blanket, and that he folded his hands,” Sonya said, making sure as she described the details she saw now, that these same details she saw then. Then she saw nothing, but said that she saw what came to her mind; but what she thought up then seemed to her just as real as any other memory. What she then said, that he looked back at her and smiled and was covered with something red, she not only remembered, but was firmly convinced that even then she had said and seen that he was covered with a pink, precisely pink blanket, and that his eyes were closed.
“Yes, yes, exactly pink,” said Natasha, who also now seemed to remember what was said in pink, and in this very she saw the main unusual and mysterious prediction.
“But what does that mean? Natasha said thoughtfully.
“Ah, I don’t know how extraordinary all this is! Sonya said, clutching her head.
A few minutes later, Prince Andrei called, and Natasha went in to him; and Sonya, experiencing a feeling of excitement and tenderness rarely experienced by her, remained at the window, pondering the whole unusualness of what had happened.
On this day there was an opportunity to send letters to the army, and the countess wrote a letter to her son.
“Sonya,” said the countess, looking up from her letter as her niece passed her. - Sonya, will you write to Nikolenka? said the countess in a low, trembling voice, and in the look of her tired eyes, peering through glasses, Sonya read everything that the countess meant by these words. This look expressed both prayer, and fear of refusal, and shame at what had to be asked, and readiness for irreconcilable hatred in case of refusal.
Sonya went up to the countess and, kneeling down, kissed her hand.
“I will write, maman,” she said.
Sonya was softened, agitated and touched by everything that happened that day, especially by the mysterious performance of divination that she just saw. Now that she knew that on the occasion of the resumption of relations between Natasha and Prince Andrei, Nikolai could not marry Princess Marya, she gladly felt the return of that mood of self-sacrifice in which she loved and used to live. And with tears in her eyes and with joy in the consciousness of committing a generous deed, she, interrupted several times by tears that clouded her velvety black eyes, wrote that touching letter, the receipt of which so struck Nikolai.
In the guardhouse, where Pierre was taken, the officer and soldiers who took him treated him with hostility, but at the same time respectfully. There was also a sense of doubt in their attitude towards him about who he was (isn't he a very important person), and hostility due to their still fresh personal struggle with him.
But when, on the morning of another day, the shift came, Pierre felt that for the new guard - for officers and soldiers - he no longer had the meaning that he had for those who took him. And indeed, in this big, fat man in a peasant's caftan, the guards of the other day no longer saw that living person who fought so desperately with the marauder and the escort soldiers and uttered a solemn phrase about saving the child, but they saw only the seventeenth of those held for some reason, according to the order of the higher authorities, taken by the Russians. If there was anything special in Pierre, it was only his timid, concentrated, thoughtful look and the French language, in which, surprisingly for the French, he spoke well. Despite the fact that on the same day Pierre was connected with other suspects taken, since the officer needed a separate room that he occupied.
All the Russians kept with Pierre were people of the lowest rank. And all of them, recognizing the gentleman in Pierre, shunned him, especially since he spoke French. Pierre sadly heard ridicule over himself.
The next day, in the evening, Pierre learned that all these detainees (and, probably, including himself) were to be tried for arson. On the third day, Pierre was taken with others to a house where a French general with a white mustache, two colonels and other Frenchmen with scarves on their hands were sitting. Pierre, along with others, was asked questions about who he is with that allegedly exceeding human weaknesses, accuracy and definiteness with which defendants are usually treated. where was he? for what purpose? etc.
These questions, leaving aside the essence of life's work and excluding the possibility of disclosing this essence, like all questions asked at the courts, aimed only at substituting the groove along which the judges wanted the defendant's answers to flow and lead him to the desired goal, that is, to the accusation. As soon as he began to say something that did not satisfy the purpose of the accusation, they accepted the groove, and the water could flow wherever it wanted. In addition, Pierre experienced the same thing that the defendant experiences in all courts: bewilderment, why did they ask him all these questions. He felt that it was only out of condescension or, as it were, courtesy that this trick of the substituted groove was used. He knew that he was in the power of these people, that only power had brought him here, that only power gave them the right to demand answers to questions, that the only purpose of this meeting was to accuse him. And therefore, since there was power and there was a desire to accuse, there was no need for the trick of questions and trial. It was obvious that all answers had to lead to guilt. When asked what he was doing when they took him, Pierre answered with some tragedy that he was carrying a child to his parents, qu "il avait sauve des flammes [whom he saved from the flame]. - Why did he fight with a marauder? Pierre answered, that he defended a woman, that the protection of an offended woman is the duty of every man, that... He was stopped: it did not go to the point. Why was he in the yard of the house on fire, where witnesses saw him? He answered that he was going to see what was being done in Moscow. They stopped him again: they did not ask him where he was going, but why he was near the fire? Who is he? They repeated the first question to which he said that he did not want to answer. Again he answered that he could not say this .
Rifts (Baikal Rift)
Rifts as global geotectonic elements are a characteristic structure of the earth's crust extension (according to Artemyev and Artyushkov, 1968; Ushakov et al., 1972). The concept of rifts also includes narrow forms of relief - furrows (“grabens”), not yet compensated by sediments and sediments; large and wide depressions with rather mutually distant sides; dome-shaped, or stretching in the form of ridges, uplift systems complicated by an axial graben (for example, rifts in the central parts of the oceans and in East Africa). It is believed that all this is just different time stages of the formation of rift structures, which are currently found in the oceans and on the continents. Age is determined from deposits and sediments.
The first place among planetary rift systems is occupied by the World Rift System (WSR), which was formed during the Cenozoic and is currently developing, discovered in 1957, which stretches for a length of over 60 thousand km under the waters of the World Ocean, and also enters the continent by a number of its branches. . MSR are wide (up to a thousand kilometers or more) rises, rising above the bottom by 3.5 - 4 kilometers and stretching for thousands of kilometers. Active rift zones are confined to the axial parts of the ridges, consisting of a system of narrow grabens (rift gorges of the Baikal type), framed by rift mountain ranges such as the Baikal, Barguzinsky and other ridges surrounding Baikal.
Other rifts (planetary scale) include rifts confined to continents (except those mentioned above) - for example, the Rhine graben (about 600 km long) or the region considered in the work - the Baikal rift zone (more than 2.5 thousand km long). The modern rift zones of the continents have much in common with the rifts of the MSR mid-ocean ridges. Their occurrence is also associated with the processes of upwelling of deep matter, dome uplift, horizontal stretching of the earth's crust under its pressure, thinning of the crust, and uplift of the Mohorovich surface. Continental rift systems (CRS) also form extended systems branching in plan (similar to MSR), but much less pronounced in relief, so some of their links seem to be isolated.
At first glance, it is difficult to call the rift gorge, buried under a water column of 3-3.5 kilometers, an analogue of Baikal. But the origin of the Baikal and oceanic rift zones is essentially the same.
Lake Khubsugul, located in Mongolia, is called the native "brother" of Baikal, elongated in the form of a sickle for 130 kilometers. Its maximum depth reaches 238 meters. The Khubsgul and Baikal depressions are part of the Baikal rift zone. Many (about 70) rivers flow into Khubsugul, as well as into Baikal, and the only one flows out - Egingol.
By the way, Khubsugul is connected with Baikal through the rivers Egingol and Selenga. Khubsugul is 12 times smaller in area, almost 5 times in length and 7 times in depth smaller than Lake Baikal.
Another clear analogue is located in East Africa, and more precisely in the East African Rift Zone, within which the lakes Nyasa, Tanganyika, Kivu, Mobutu-Sese-Seko (former Lake Albert), Idi-Amin-Dada (former Lake Edward) are located) and others, smaller ones.
The first two lakes are rightly called "sisters" of Baikal. Their parameters are remarkably similar. Only a slightly warmer climate and tropical flora distinguish them from Baikal.
Lake Tanganyika is located in Zaire, Tanzania, Zambia and Burundi at an altitude of 773 meters (almost 320 meters above Lake Baikal). Its length is 650 kilometers. The area is almost 34 thousand square kilometers, against 31.5 thousand km at Baikal. Only in depth does Baikal surpass Lake Tanganyika by 150 meters (1620 and 1470 m).
Lake Nyasa, located in Malawi, Mozambique and Tanzania, is not much inferior to Baikal. Its area is 30.8 thousand square kilometers, and its depth is up to 706 meters.
Due to the fact that these lakes are located in the tropics, the water temperature does not fall below 20-22 degrees. The fauna of lakes Tanganyika and Nyasa is almost 70 percent endemic. Moreover, as in Baikal, many species are similar to the inhabitants of the deep sea.
Typically, the width of continental rifts is about 45-50 km, with a vertical amplitude of subsidence of the rift basement (graben) from 1 to 7 km. Usually, the lowering of the bottom of rift troughs is largely compensated by sedimentation processes; however, a significant part of them is represented by depressions occupied by the waters of seas, lakes, and river valleys.
Most of the KSR have a Cenozoic age of formation. The Baikal rift was formed at the end of the Paleogene.
In cross section, the rift zone is a system of blocks sloping at different angles in steps to the axial part (see Fig. 1). The interfaces are usually steeply dipping faults.
The earth's crust of continental rifts is characterized by a noticeable thinning up to 20-30 km, the rise of the Mohorovich surface and an increase in the thickness of the sedimentary layer, therefore, in the section, the earth's crust has the shape of a doubly convex lens.
Deep seismic sounding methods have established the presence of decompacted mantle rocks under the Rhine, Baikal and Kenya rifts.
Continental rifts are also distinguished by the presence of increased heat flow and negative magnetic field anomalies.
The nature of the displacements in the earthquake sources indicates the horizontal stretching of the earth's crust. For the Rhine graben, this is about 5 km, for the Baikal one, it is an order of magnitude higher.
The most significant difference between modern oceanic rift zones (OZRs) and continental rift zones (CZRs), with many similarities between them, is that the relatively thicker and stronger continental crust, although thinned by stretching (and torn in some places), giving way to basalt volcanism, yet retains its integrity. In contrast to the open depths of the OSR, from which rocks of the upper layers of the mantle, or at least a molten mixture of these rocks with rocks of destruction and assimilation of the old crust, come to the surface of the solid crust, no neoformations of the earth's crust occur in the KZR. Perhaps, this means that modern KZR is only the first stage of the formation of the MSR and that in the epoch of the birth, for example, of the Atlantic Ocean, the matter also began with the formation in the body of Laurasia of the links of the KZR, similar to the earlier stage of the Baikal zone, and then (at the subsequent temporal stages) East African Rift. Thus, with some reservation, Baikal can be called the embryo of the future ocean. According to the rift theory, younger analogues of Baikal also existed on the globe. It is believed that one of them is located on the site of the present Red Sea, along which the Red Sea Rift Zone runs. In the geological time scale, relatively recently, on the site of the Red Sea, there was an extensive freshwater deep-water basin, comparable in area, or even several times larger than Baikal. In this case, the opposite worked.
Two neighboring lithospheric plates African and Indian, conjugated along the zone of the Red Sea rift, began to slowly, at a rate of one or two centimeters per year, move away from each other. Because of this expansion, the area of the lake basin also increased, since all new land areas went under water. And then one day, at the site of the current Bab el-Mandeb Strait, the last piece of land separating the paleolake from the Indian Ocean went under water. The ocean poured through the Gulf of Aden into the paleolake.
It was about nine million years ago. There was a mixing of oceanic and lake waters and a rather rapid salinization of the latter. This caused a massive death of the freshwater lake fauna and its replacement by the marine one. Now the Red Sea has an area of 450 thousand square kilometers, and its depth is slightly more than three kilometers. On the globe it is one of the most salty seas (20-40 percent). Within the Baikal rift zone, in addition to Baikal itself, there are a number of large land basins filled with Quaternary lacustrine-river deposits. Among them are Tunkinskaya, Barguzinskaya, Lower and Upper Angarskaya, Muiskaya, Charskaya...
One of these depressions - Muiskaya, or Muisko-Kuandinskaya - is located on the territory of Buryatia and the Chita region. Along its sides at an altitude of 850-860 meters above sea level (300-350 meters above the floodplain of the Muya and Vitim rivers), a clear line can be traced in sections. At this height, terrace-like ledges, composed of well-rounded lacustrine gravel-pebble and sand deposits, are sometimes leaning against the slopes of the mountains. The level of the lake experienced periodic fluctuations. Sometimes the water rose to a height of 1000-1100 meters above sea level and possibly even higher. In this case, the lake stretched for 260-265 kilometers with a width of up to 50-55 kilometers. The depth of the lake reached, and possibly exceeded 500-1000 meters.
Today, the Muya depression is separated by low bridges from the Chara and Upper Tokk depressions. From time to time, water, apparently, covered these bridges, and then a vast water basin arose, stretching more than 500 kilometers in length. Over time, the Vitim River laid a new channel for itself through the South and North Muya Ranges and the paleo lake was drained. In its place, sandy deposits remained, and near the slopes of the mountains - gravel-pebble and boulder-pebble deposits, now washed by the waters of the Muya, Vitim and their tributaries.
Thus, a significant segment of the Baikal-Amur Mainline is laid along the bottom of former large lakes - ancient analogues of Baikal. And these lakes existed relatively recently - several tens of thousands of years ago.
In the study of rift structures, much has not yet been clarified and has not been studied. Is rifting a process inherent only to the Meso-Cenozoic eras? Did this process arise only in the next 100-150 million years of the life of the Earth, or should the transformation of its face be attributed to it in earlier epochs as well? These questions have not yet been clearly answered.
In general, even such geoobjects as the Dnieper-Donetsk depression, the central part of the Moscow syneclise are considered ancient rift zones (Gordasnikov, Trotsky, 1966) etc.
The processes of rifting should be considered as one of the characteristic features of the development of the earth's crust, which took place throughout the history of its life. They are caused by horizontal stretching of the earth's crust, leading to vertical subsidence. Blocks of the earth's crust and the rise of the mantle substance to the day surface.
There is a certain staging in the development of rift zones. At the first stage, a dome-shaped or linearly extended uplift is formed in the Earth's crust due to the leakage of decompacted mantle matter, then, due to extension, graben troughs form in their most elevated parts. At subsequent stages, rift zones can serve as axial parts of larger subsidences, or, in the case of change from extension to compression, they degenerate into folded uplifted structures of the geosynclinal type.
The distribution of rift zones is not strictly linear. Their individual parts (elements) are mutually displaced in the transverse direction along the transform faults.
The study of modern and ancient rift zones in the ocean and on the continents will provide a clear understanding of the structure and geological history of these large geological planetary structures, as well as the oil and gas potential of many kilometers of sedimentary rocks that fill many rift depressions. Lake Baikal, as a relatively young rift zone, in its further study can provide even more extensive material for a deeper understanding of the essence of geological and magmatic processes in the area of rift zones.
Rice. 5.1. Global system of modern continental and oceanic rifts, main subduction and collision zones, passive (within plate) continental margins.
Rift zones: Mid-Atlantic (SA), American-Antarctic (Am-A), African-Antarctic (Af-A), Southwestern Indian Ocean (SWZI), Arabian-Indian (A-I), East African (VA) ), Red Sea (Kr), Southeast Indian Ocean (SVI), Australo-Antarctic (Av-A), South Pacific (UT), East Pacific (BT), West Chilean (34), Galapagos (G), California (Cl), Rio Grande - Basins and Ranges (BH), Gorda Juan de Fuca (HF), Nansen-Gakkel (NG, see Fig. 5.3), Momskaya (M), Baikal (B), Rhine (P). Subduction zones: 1 - Tonga-Kermadek; 2 - Novogebridskaya; 3 - Solomon; 4 - New British; 5 - Sunda; 6 - Manila; 7 - Philippine; 8 - Ryukyu; 9 - Mariana; 10 - Izu-Boninskaya; 11 - Japanese; 12 - Kuril-Kamchatskaya; 13 - Aleutian:, 14 - Cascade Mountains; 15 - Central American; 16 - Lesser Antilles; 17 - Andean; 18 - South Antilles (Scotia); 19 - Eolian (Calabrian); 20 - Aegean (Cretan); 21 - Mekran.
a - oceanic rifts (spreading zones) and transform faults; b - continental rifts; c - subduction zones: island-arc and marginal continental double line); d - collision zones; e - passive continental margins; e - transform continental margins (including passive ones); g - vectors of relative movements of lithospheric plates, according to J. Minster, T. Jordan (1978) and K. Chase (1978), with additions; in spreading zones - up to 15-18 cm/year in each direction, in subduction zones - up to 12 cm/year 
Rice. 5.2. The geometric correctness of the placement of the global system of modern rifts relative to the axis of rotation of the Earth, according to E.E. Milanovsky, A.M. Nikishin (1988):
1 - Cenozoic rifting axes, mostly active; 2 - oceanic lithosphere of Cenozoic age; 3 - the same, Mesozoic age; 4 - areas with continental lithosphere; 5 - convergent borders 
Rice. 5.3. The southeastern end of the Nansen-Gakkel oceanic rift zone and seismically active faults continuing it, separating the Eurasian and North American lithospheric plates. According to L.M. Parfenov et al. (1988). Below - focal mechanisms of seismic sources at this active boundary, according to D. Cook et al. (1986):
1 - spreading zones (NG - Nansen-Gakkel zone); 2 - deep-sea trenches (subduction zones); 3 - transform faults; 4 - reverse faults; 5 - faults and shifts; 6 - zones of scattered rifting; 7 - movement of lithospheric plates and microplates; 8 - focal mechanisms of seismic sources; 9 - land within the Eurasian (a) and North American (b) plates. Lithospheric plates and microplates: EA - Eurasian; SA - North American; T - Pacific; ZB - Transbaikal; Am - Amur; Oh - Sea of Okhotsk
Modern tectonic activity is distributed extremely unevenly and is concentrated mainly on the boundaries of lithospheric plates. The two main types of these boundaries (see Chap. 3.1 also correspond to the main geodynamic settings. Rifting develops on divergent boundaries, to which this chapter is devoted, but here we will consider the activity of transform boundaries, since they are associated primarily with the rift zones of the oceans. plates is expressed by subduction, obduction and collision (see Chap. 6).Information about relatively weak, but important in their geological consequences, within-plate tectonic processes will be given in Chapter 7.
term rift valley(eng., rift - crevice) J. Gregory at the end of the last century identified the grabens of East Africa limited by faults, which are formed under conditions of extension. Subsequently, B. Willis contrasted them with ramps - grabens, sandwiched between oncoming reverse faults. The concept, which at first had mainly a structural content, later, especially in recent decades, was enriched with ideas about the geological conditions and probable deep mechanisms of the formation of these linear extension zones, about characteristic igneous and sedimentary formations and, thus, was filled with genetic content. A modern understanding of rifting was taking shape, which a quarter of a century ago was included in the concept of plate tectonics as one of its most important elements. It turned out that most of the rift zones (in their new, broad sense) are located in the oceans, but there rifts, as structures controlled by faults, are of subordinate importance, and the main way to implement tensile stresses is pulling apart.
5.1. Global system of rift zones
Most of today's rift zones are interconnected, forming a global system stretching across continents and oceans (Figure 5.1). The awareness of the unity of this system, which engulfed the entire globe, prompted researchers to look for planetary-scale mechanisms of tectogenesis and contributed to the birth of a “new global tectonics”, as the concept of lithospheric plate tectonics was called in the late 60s.
In the system of the Earth's rift zones, most of it (about 60 thousand km) is located in the oceans, where it is expressed by mid-ocean ridges (see Fig. 5.1), their list is given in Ch. 10. These ridges continue one another, and in several places are interconnected by "triple junctions": at the junction of the West Chilean and Galapagos ridges with the East Pacific, in the south of the Atlantic Ocean and in the central part of the Indian. Crossing the boundary with passive continental margins, oceanic rifts continue to be continental. Such a transition is traced south of the triple junction of the Aden and Red Sea oceanic rifts with the Afar Valley rift: along it, from north to south, the oceanic crust wedges out and the continental East African zone begins. In the Arctic Basin, the oceanic Gakkel Ridge continues with continental rifts on the shelf of the Laptev Sea, and then with a complex neotectonic zone, including the Momsky rift (see Fig. 5.3).
Where mid-ocean ridges approach an active continental margin, they can be absorbed into a subduction zone. So, at the Andean margin, the Galapagos and West Chilean ridges end. Other relationships are demonstrated by the East Pacific Rise, above which the Rio Grande continental rift formed on the overthrust North American Plate. Similarly, the oceanic structures of the Gulf of California (apparently an offshoot of the main rift zone) are continued by the continental system of Basins and Ranges.
The dying off of rift zones along strike has the character of gradual attenuation or is associated with a transform fault, as, for example, at the end of the Juan de Fuca and American-Antarctic ridges. For the Red Sea rift, the Levantian shift serves as the end.
Covering almost the entire planet, the system of Cenozoic rift zones exhibits geometric regularity and is oriented in a certain way relative to the axis of rotation of the geoid (Fig. 5.2). Rift zones form an almost complete ring around the South Pole at latitudes of 40-60° and depart from this ring meridionally with an interval of about 90° by three belts fading to the north: East Pacific, Atlantic and Indian Ocean. As shown by E.E. Milanovsky and A.M. Nikishin (1988), perhaps, with some conventionality, the fourth, Western Pacific belt, which can be traced as a set of back-arc manifestations of rifting, is marked in the corresponding place. The normal development of the rift belt here was suppressed by intense western displacement and subduction of the Pacific Plate.
Under all four belts, to depths of a few hundreds of kilometers, tomography reveals negative velocity anomalies and increased attenuation of seismic waves, which is explained by the upward current of the heated mantle matter (see Fig. 2.1). The regularity in the placement of rift zones is combined with global asymmetry both between the polar regions and relative to the Pacific hemisphere.
The orientation of the stretch vectors in the rift zones is also regular; The latter are maximum in the equatorial regions, decreasing along the ridges both in the northern and southern directions.
Outside the global system, there are only a few of the large rifts. These are the system of Western Europe (including the Rhine graben), as well as the Baikal (Fig. 5.3) and Fengwei (Shanxi) systems, confined to northeast-trending faults, the activity of which is believed to be supported by the collision of the continental plates of Eurasia and Hindustan.
