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Continental rifts. Rift zone systems

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 is Baikal

KhovsgolNuur.jpg

Lake Khubsugul is also located in the Baikal Rift region, 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 / Responsible. ed. Doctor of Biological Sciences A. S. Pleshanov; . - Irkutsk: Publishing House of the Institute of Geography SB RAS, 2002. - 133 p.

An excerpt characterizing the Baikal Rift Zone

Natasha quietly closed the door and went with Sonya to the window, not yet understanding what they were saying to her.
“Do you remember,” Sonya said with a frightened and solemn face, “do you 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 Sonya then said something about Prince Andrei, whom she saw lying down.
- Do you remember? – Sonya continued. “I saw it 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 had closed his eyes, and that he was covered with a pink blanket, and that he had folded his hands,” Sonya said, making sure that as she described the details she saw now, that these same details she saw then. She didn’t see anything then, but said that she saw what came into her head; but what she came up with then seemed to her as valid as any other memory. What she said then, 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 said and saw that he was covered with a pink, exactly pink, blanket, and that his eyes were closed.
“Yes, yes, exactly in pink,” said Natasha, who now also seemed to remember what was said in pink, and in this she saw the main unusualness and mystery of the prediction.
– But what does this mean? – Natasha said thoughtfully.
- Oh, I don’t know how extraordinary all this is! - Sonya said, clutching her head.
A few minutes later, Prince Andrei called, and Natasha came in to see him; and Sonya, experiencing an emotion and tenderness she had rarely experienced, remained at the window, pondering the extraordinary nature 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, raising her head from the letter as her niece walked past her. – Sonya, won’t you write to Nikolenka? - said the countess in a quiet, trembling voice, and in the look of her tired eyes, looking through glasses, Sonya read everything that the countess understood in these words. This look expressed pleading, fear of refusal, shame for having to ask, and readiness for irreconcilable hatred in case of refusal.
Sonya went up to the countess and, kneeling down, kissed her hand.
“I’ll write, maman,” she said.
Sonya was softened, excited and touched by everything that happened that day, especially by the mysterious performance of fortune-telling that she just saw. Now that she knew that on the occasion of the renewal of Natasha’s relationship with Prince Andrei, Nikolai could not marry Princess Marya, she joyfully felt the return of that mood of self-sacrifice in which she loved and was accustomed to living. And with tears in her eyes and with the joy of realizing a generous deed, she, interrupted several times by tears that clouded her velvety black eyes, wrote that touching letter, the receipt of which so amazed Nikolai.

At the guardhouse where Pierre was taken, the officer and soldiers who took him treated him with hostility, but at the same time with respect. One could still feel in their attitude towards him doubt about who he was (whether he was 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 the officers and soldiers - it no longer had the meaning that it had for those who took him. And indeed, in this big, fat man in a peasant’s caftan, the guards of the next day no longer saw that living man who so desperately fought with the marauder and with the escort soldiers and said a solemn phrase about saving the child, but saw only the seventeenth of those being held for some reason, by by order of the highest authorities, the captured Russians. If there was anything special about Pierre, it was only his timid, intently thoughtful appearance 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 suspected suspects, since the separate room he occupied was needed by an officer.
All the Russians kept with Pierre were people of the lowest rank. And all of them, recognizing Pierre as a master, shunned him, especially since he spoke French. Pierre heard with sadness the ridicule of himself.
The next evening, Pierre learned that all of these prisoners (and probably himself included) 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 was with the precision and certainty with which defendants are usually treated, supposedly exceeding human weaknesses. where he was? for what purpose? and so on.
These questions, leaving aside the essence of the life matter and excluding the possibility of revealing this essence, like all questions asked in courts, had the goal only of setting up 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 took a groove, and the water could flow wherever it wanted. In addition, Pierre experienced the same thing that a defendant experiences in all courts: bewilderment as to why all these questions were asked of him. He felt that this trick of inserting a groove was used only out of condescension or, as it were, out of politeness. 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 flames]. - Why did he fight with the marauder? Pierre answered, that he was defending a woman, that protecting an insulted woman is the duty of every person, that... He was stopped: this did not go to the point. Why was he in the courtyard of a house on fire, where witnesses saw him? He answered that he was going to see what was happening in Moscow. They stopped him again: they didn’t ask him where he was going, and why was he near the fire? Who was he? They repeated the first question to him, to which he said that he did not want to answer. Again he answered that he could not say that .

How to relate to the poet’s words given above? Is nature really so simple that in fact everything in it is clear and the science of nature is pure delusion, an artificial creation of mysteries, on the solution of which humanity has spent so much vain effort? It would be a mistake to think that Fyodor Ivanovich Tyutchev did not understand what science is and that revealing the secrets of nature is not useful. The whole point is that nature itself, regardless of human consciousness, does not contain, cannot contain, anything mysterious. The subjective concept of mystery arises as a consequence of the imperfect reflection of natural phenomena by human consciousness. Overcoming this imperfection and striving for it constitute the path of development of science.

Riddles, secrets, mysteries of nature for the inquisitive human consciousness - a world full of romance and incomparable attractiveness. And in this sense, nature did not offend Eastern Siberia. She created Baikal as a mystery for us, as a naturally necessary phenomenon in the development of the earth's interior.

The enormity and harsh nature of Lake Baikal were mysterious to the first explorers who came to its shores. This mystery was resolved in their minds by the conviction that Baikal is a sea. New and new discoveries forced us to abandon the recognition of Baikal as a real sea. So a new mystery arose: what is it that is unlike either the sea or the largest lakes then known to science? New discoveries followed. And new mysteries immediately appeared. Already in the post-war period, a new term appeared in the language of scientists, which means little to the general reader - the Baikal rift and the Baikal rift zone.

Baikal in the XVII-XVIII centuries. became famous as the fresh sea. In the next century, it became known throughout the world as the deepest completely fresh lake on Earth. In the first half of our century, it gained the glory of being a closed center of biological speciation, in which organisms (endemics) unique to it arose and developed. In the second half of our century, Baikal became famous as the only rift structure in Asia that arose in the very depths of the continent. This is a kind of scientific “career” of Baikal. And what is especially remarkable is that in its last role, in revealing the “secret” of Lake Baikal that did not exist in nature, but which haunted science, earthquakes, volcanic structures, and the very location of the mountains in the south of Eastern Siberia found their natural place.

Let us now recall once again the main features of the structure of the earth’s crust in the Baikal region. Here the ancient Siberian platform and an area of equally ancient folding meet, constituting, as it were, the frame of the platform, or, as is often said, its southern folded frame. The border between these regions has a fairly simple outline, with two “bays” to the south - Irkutsk and Aldan. The Siberian platform has a flat or slightly undulating topography of the surface of the watersheds, but its river valleys are deep, with steep slopes. Hence the other, geographical, name of the platform - the Central Siberian Plateau. Its southern edge is everywhere expressed by a rather sharp ledge - a transition to the mountainous region of the Sayans, the Baikal Mountains and the Stanovoy Highlands. The common feature of all these mountains is the predominance of massive forms over sharp, sharp ones, then the parallelism of the main more or less isolated hills (ridges, chains) to the edge of the Siberian Platform and moderate heights, not exceeding, as a rule, 3000 m above sea level. The further south you go from the northern edge of the mountains, the less influence of this region on the direction of individual large elevations, but still the gentle bend - the transition of the northwestern “Sayan” strikes to the northeastern “Baikal” ones - is preserved in general terms and within Mongolia. Near the line of the plateau-mountain junction, in some places moving away from it into the depths of the mountains, and in others approaching it closely, individual low areas are visible - intramountain (intermountain) depressions, which at first glance seem to be simply greatly expanded sections of river valleys. Convenient, flat places at the bottoms of these depressions, of course, primarily attracted the first settlers to them, the first travelers stopped in them, and the nature surrounding them, first of all, attracted attention. Therefore, the intermountain depressions of this mountainous region have historically turned out to be the primary objects of geological science. One of them, of course, the very first, was the depression of Lake Baikal.

The first travelers, among them the luminaries of the then science (their names are inscribed on the cornice of the Irkutsk Museum of Local Lore), judged these spacious lowlands among the mountain heights differently, but already at the end of the 18th century, some scientists saw in them catastrophic failures caused by deep forces, namely those that make themselves known by private local earthquakes. Opinions have been expressed that the enormous subsidence among the mountains is a consequence of volcanic processes. Many people believed that these were simply the remains of huge ancient river valleys, and I. Chersky believed that the Baikal basin was a slowly deepening and contracting concave fold of the earth’s crust.

In the 19th century similar large intermountain depressions have been well studied in Europe. At that time, naturalists from different countries began to judge many things based on European models. It was found that the typical structure of large intermountain depressions is a graben, that is, a subsidence of a longitudinal section of the earth's crust between two parallel faults. Similar grabens then began to be found in almost all mountainous countries, and their example, prototype, remained the Rhine graben - a subsidence along faults between the Black Forest and Vosges mountain ranges. They began to compare the Baikal depression with it. This was greatly facilitated by the authority of the largest explorer of Siberia, V. A. Obruchev, who believed that the “ancient crown” of Asia throughout its entire space was divided into separate blocks, partly lowered, partly raised, and against such a “structural background” the Baikal depression was only the largest and the youngest.

Further studies showed that the intermountain depressions of the Baikal region and Northern Mongolia form a unified system, as if connected by extended faults in the earth’s crust, forming with their links, i.e., individual depressions, a kind of chain stretching more than 2000 km from the lake. Khubsugul in Mongolia to South Yakutia. Earlier, at the beginning of the 19th century, the noted external similarity of the depressions suggested the idea of the geological relationship of all links in such a chain, the close time and similar method of their formation. At the beginning of this century, the English geologist J. Gregory described a similar, even larger system of similar depressions in East Africa, calling them rift valleys. Another English geologist B. Willis, studying the Dead Sea depression in Palestine, found that the marginal parallel faults that form it are not faults, but reverse faults, or steep thrusts, with which the graben walls seem to compress the central downward strip. He called this structure, in contrast to the rift, a ramp-pom. Soon after this, the ramp model was applied to the Baikal depression. Earlier, at the very beginning of this century, geologist Lvov pointed out the similarity of the Baikal depression with the depression of another deep lake - Tanganyika in Africa. Finally, the geologist Pavlovsky, who also noted the similarity of the Baikal depressions and East Africa, proposed the successful common name “Baikal-type depressions” for all parts of the Baikal system of interstate subsidences.

A very sharp increase in geological research in the interstate basins of the Baikal region occurred in the 50s in connection with the search for oil and gas. Several fairly deep wells were drilled. The Institute of the Earth's Crust, then simply the Institute of Geology of the USSR Academy of Sciences in Irkutsk, was closely involved in the geology of this entire area. Important results were obtained for the Baikal depression and its closest neighbors. However, the most important thing was that it was at this time that extensive international research on the bottom of the World Ocean was carried out on a new scientific and technical basis and the World Rift System was discovered. This discovery was a real sensation and became a major milestone in the development of Earth sciences. The basis of the World Rift System is made up of mid-ocean ridges, connected to each other into a single grid, as if entangling the entire globe. Mid-ocean ridges gravitate towards the middle (median) parts of the oceans, but not all of them occupy such a middle position: it is best seen in the Atlantic underwater ridge, especially in its northern part. By themselves, these elevations of the ocean floor bear little resemblance to the real ridges that we see on land. These are uplifts with a base width of hundreds to one and a half thousand kilometers and a relative height of up to 3 km. The total length of the system of such ridges exceeds 70,000 km, and the area is equal to the area of all continents. Sharp forms of relief are found only in the summit, ridge parts of the ridges. They are created, firstly, by the stepped nature of the slopes, and secondly, by the presence of deep and narrow axial depressions of fault origin - rift “valleys”. Being uplifts of thin (7-10 km) oceanic crust, underwater ridges are distinguished by high heat flow values (up to 3-10 µcal cm 2 s), strong volcanism with outpourings of basaltic lavas, strong seismicity, and the presence of fragments of ultramafic rocks, indicating the close occurrence of to the surface of the bottom of the mantle material. The postcard and further study of the World Rift System gave impetus to the creation of the spreading hypothesis (expansion, growth of the ocean floor symmetrically in both directions from the median ridges), as well as the hypothesis of enormous - thousands of kilometers during geological history - horizontal movements of lithospheric plates.

One of its branches of the World Rift System emerges from the Indian Ocean onto land, where it continues in the form, firstly, of the huge rift structure of the Red Sea, and secondly, in the form of the East African zone of continental rift basins. As for the Rhine graben and the grabens of the Baikal zone, in a number of ways they turned out to be very close to oceanic rift gorges, although they do not have a direct spatial connection with the World Rift System. It is clear that with their “landscape”, accessibility for comprehensive research, the possibility of direct, visual acquaintance and already quite high geological knowledge, the Rhine, Baikal and long-standing candidates for similar structures of the earth’s crust, the Province of Ridges and Basins in the Western United States, became the subject of special study by the International program.

In 1966, in Irkutsk, within the walls of the Institute of the Earth’s Crust, a visiting session of the Scientific Council for the Study of the Earth’s Crust and Upper Mantle of the USSR Academy of Sciences, chaired by V.V. Belousov, took place. The results of what was done on the Baikal depression and neighboring structures similar to it were summed up. A program of further research has been drawn up. The Baikal section of the named Scientific Council was organized. The study of Baikal as a natural phenomenon caused by deep processes has entered a new stage.

If now the Baikal-type depressions have turned into “rift valleys” or simply into rift depressions, then the question arose about their relationship to the World Rift System. The Baikal rift zone seemed completely isolated, as if “abandoned” into the depths of the Asian mainland, and it was also located on a territory composed of ancient and partly ancient rock strata. It was time to move on to exploring possible means and techniques of the deep subsurface beneath the entire rift zone. The Institute of Geology and Geophysics of the Siberian Branch of the Academy of Sciences in Novosibirsk, other institutes of the Irkutsk Scientific Center, and many Siberian industrial organizations were involved in this Work. Naturally, geophysical work came to the fore. We will talk about them in more detail below.

In Fig. Figure 7 shows a general diagram of the Baikal rift zone. It shows the contours of rift depressions, the distribution fields of Neogene-Quaternary volcanic rocks and the main faults of the earth's crust, expressed in relief, as well as the contour of the Sayan-Baikal arched uplift (highlands) within the isohypsum (line of equal heights) 1500 m above sea level. All these are the main characteristics of the rift zone. The diagram shows that the rift zone in the southern part is closely adjacent to the northern border of the Mongol-Siberian Mountains and thereby to the southern border of the Siberian Platform, and in the northeast it retreats from this border to the south. Volcanic fields gravitate toward the flanks of the rift zone, but the Vitim lava plateau is shifted east of it. Baikal, the main central link of the rift zone, is associated with particularly powerful faults in the earth's crust. Many faults throughout the zone are the result of cracking of the earth’s crust, which occurred in the Neogene and Quaternary periods, right up to the present day. Almost all of the depressions and Lake Baikal, of course, are also more or less asymmetrical; their northern and northwestern sides are shorter and steeper than the southern and southeastern ones.

All rift depressions are filled to varying depths with sediments of riverine and lacustrine-marsh descent. Similar sediments continue to accumulate in them now. The sedimentary strata are best studied along the southern edge of the Baikal basin and in the Tunka depression adjacent to it to the west, which is associated with oil searches and deep drilling in these areas. It was found that the accumulation of terrestrial and aquatic sediments (and, consequently, the origin of rift basins) began in the Upper, and perhaps Middle, Paleogene and continued throughout the Neogene and Quaternary periods, i.e., more than 25 million years. As is usually the case in continental (rather than marine) environments, sediment accumulation occurred unevenly as the rift basins “grew,” that is, deepened and widened. On the western flank of the rift zone, the accumulation of sediments was accompanied by repeated outpourings of basaltic lavas and emissions of pyroclasts, that is, detrital volcanic materials. The composition and structure of such thick lenses of sediments can be judged from Fig. 5. In places, both along the edges and in the middle parts of the rift basins, the sediments are affected by faults and folded into small folds.

Much interesting data on the accumulation of sediments in modern deep-sea Baikal has been obtained in recent decades. They confirmed its “youth” and showed that the mechanism of sediment accumulation in it is similar to that of the sea. By the way, a few words about the depths and topography of the bottom of Lake Baikal.

The enormous depth of Lake Baikal was known, of course, to the first inhabitants of Lake Baikal - the Buryats, Evenks, Kurykans and, perhaps, more ancient peoples who mastered fishing here. Measurements using a simple sea survey were carried out in the last century, more accurate measurements were carried out by Drizhenko’s expedition at the beginning of this century. The work of the Baikal Limnological Station of the Academy of Sciences showed the greatest depth of Baikal not far east of Olkhon Island. It was equal to 1740 m. However, later, already in the 60s, the Limnological Institute undertook special studies of the lake using an echo sounder and compiled the first relief map of the bottom of Lake Baikal. The maximum depth of Baikal found in approximately the same area turned out to be 1620 m. It is currently accepted as the most reliable. And despite, so to speak, some “loss of points,” Baikal remains the world champion among freshwater lakes in terms of its depth.

The map of the bottom relief of the lake as a whole confirmed the assumptions that Baikal consists of three clearly isolated basins, that the deepest is the middle one, that the northwestern underwater slope is very steep and stepped, that the southeastern side is longer and flatter, but has very complex terrain, that the deepest parts of Baikal are like underwater plains, that to the northeast of the northern tip of Olkhon Island, towards approximately the Ushkany Islands, there is an underwater hill called the Academic Ridge, that, finally, the underwater slopes are furrowed in places, as in ocean, deep canyons. Nevertheless, work on studying the lake bottom continued. More and more measurements using echo sounding profiles allowed V.I. Galkin to create a sculptural plaster model of the Baikal depression. Finally, the joint forces of the Limnological Institute and the Institute of Oceanology of the Academy of Sciences carried out even more accurate studies of the Baikal basin, carried out through precision (high-precision) echo sounding, underwater photography and even direct observations from the Pysis underwater vehicles. They fully confirmed the main results of early underwater work, but significantly detailed them. And what’s remarkable is that in the diagram, in the idea, the current structure of the Baikal depression turned out to be exactly the way that geologists in the 50s imagined and depicted it almost intuitively. The width of the western slope of the depression turned out to be only 3-5 km, with steep or sheer cliffs and very narrow areas of individual steps. On the contrary, the width of the eastern slope is much greater (25-30 km), it is very uneven, broken into numerous blocks by both longitudinal and transverse faults. It turned out that lake sediments, including the youngest ones, are affected by faults, which was especially clearly visible at the base of the western slope, that is, in the sphere of influence of the main Obruchevsky fault. It was once again confirmed that the Baikal depression is a sharply asymmetrical rift structure that continues its development.

Everything that has been discussed so far in this chapter constitutes, so to speak, the external geological picture of the Baikal rift zone and its central link - the Baikal rift. Nature has clearly shown us their main features. But we cannot be content with this, since we can only judge very superficially (both in the direct and indirect sense) from the presented materials about the origin, causes and mechanism of formation of the Baikal rift zone. But this zone is a recognized example, a genotype of continental rift zones in general. Let's try, as far as possible, to “go deeper” into the earth’s crust under the rift zone.

Both historically and essentially, the first word in the knowledge of the earth’s crust in the Baikal region belongs to seismology. Back in the 17th century, material about local earthquakes began to accumulate, and it became clear that the Baikal region was an area of high seismicity. In the 1930s, in connection with the search for oil on Lake Baikal, seismic sounding began to be carried out in the South-Eastern Baikal region, using artificial exciters of elastic vibrations in the upper layers of the earth's crust (explosive devices). Seismic sounding for solving general problems of crustal structure acquired a large scope in the 1979s. It was carried out jointly with Novosibirsk academic and Irkutsk production (exploration) scientists. These works showed with great certainty that the earth's crust in the Baikal region is underlain by a layer with reduced density and viscosity, the thickness of which under Baikal is 30-50 km. This so-called asthenospheric (weak) layer in different regions of the Earth lies at different depths - up to 200-300 km and, thus, between it and the bottom of the earth's crust is usually located the upper part of the mantle with normal values of density and viscosity, which makes up the lower rock shell - lithosphere. Work using the DSS method has shown that in the Baikal region the speed in the anomalous layer of longitudinal seismic waves is 7.6-7.8 km/s, and in the underlying “normal” upper mantle - 8.1-8.2 km/s. This difference is the visual basis for judging the reduced viscosity and density of the asthenospheric layer. Further we will see that the relatively shallow depth of the “weak” layer under Baikal can be established by other methods.

To study local earthquakes, the epicenters of which gravitate towards Baikal and the Baikal rift zone as a whole, the Institute of the Earth's Crust organized a whole network (up to 20) of seismic stations. A dense network of stations made it possible to very accurately determine the location of the epicenters of local earthquakes and compile their map, which is constantly being updated with material from new and new earthquakes. It was found that the foci, that is, the places of discharge of accumulated seismic energy and thereby the sources of elastic waves in the Baikal region, are located at a relatively shallow depth - up to 15-20 km. Analysis of stress in many of these centers, starting from southern Baikal and to the eastern flank of the rift zone, showed approximately the same picture: near-horizontal extension, directed across tectonic and orographic lines and approximately parallel to the latter, more or less horizontal compression. In the earthquake foci to the west of Lake Baikal, the compression and expansion vectors seemed to change places. Such a picture, as was known even earlier, is characteristic of earthquake foci that are very seismic in Soviet Central Asia and all of Central Asia. These data are very important for understanding the modern mechanics of the earth’s crust in the Baikal region. In the 60-70s, the work of the Institute of the Earth's Crust established systematic delays of seismic waves arriving from distant earthquakes to stations in the Baikal region. The study of these phenomena showed that under the entire Mongolian-Siberian mountain system there is a huge drop-shaped region of decompressed and, apparently, overheated mantle, the upper boundary of which under Baikal approaches the very base of the earth’s crust. It turned out that the horizontal projection of the contour of the “anomalous” mantle very closely covers the territory of the latest mountain formation with high, and in places - in Western Mongolia - the highest seismicity (up to 11 points), the Baikal rift zone, the area of distribution of hot water outcrops and traces of recent volcanism. That’s how much seismic methods have advanced our knowledge about the structure of the subsoil of the Baikal region and neighboring regions, that’s how much more precise the unique geological position of the Baikal basin has become, and with it the unique lake itself!

Looking through these lines, readers may think that seismic research at the Institute of the Earth's Crust is carried out only to understand the structure of the subsoil of the surrounding territory and to come closer to understanding the mechanism of formation of the Baikal rift zone. Yes, they are carried out for this purpose, but only in conjunction with the main work - the study of seismicity of the Mongol-Siberian mountain system as one of the important conditions, important components of the natural environment in which we live, work, and build. The results of the Mondinsky 1950, Muisky 3957, Srednebaikalsky 1959 earthquakes, together with the observation of traces of ancient, prehistoric earthquakes expressed in the relief and data from the current seismic service in Eastern Siberia and Mongolia, as well as historical information about the earthquakes that happened here - this is the most valuable material for compiling a seismic zoning map, work of national importance carried out by the Institute of the Earth's Crust for many years. Such maps, which are based on seismic statistical material, assessing the seismic hazard of individual territories with varying degrees of probability, are compiled on different scales and, according to the corresponding statement, have normative significance. The planning of the placement of new buildings, types of structures, types of building materials and the size of appropriations largely depend on them. We saw above that the area of the central section of the BAM route in the draft map of seismic zoning of the territory of the USSR in the 50s was assessed as quite safe, but in fact, as the work of the IZK showed, it lies in the area of the Baikal rift, the seismicity of which is now, based on quite objective data, is estimated at 10 points. In recent years, the entire BAM route, most of which runs in the rift zone, has received a more accurate assessment of seismic hazard.

Such scientific tasks as determining the depths of local earthquake sources, focal mechanisms, distribution and density of epicenters, frequency of earthquakes over time - all this serves both scientific purposes and the solution of very specific practical problems. The shift in our knowledge in both directions, made in recent years, is very great.

We will return to earthquakes later, but now we will briefly talk about conventional geophysical methods and their application in the Baikal region.

The essence of geophysical research methods is to identify anomalies in the physical fields of the Earth (magnetic, gravitational, thermal, etc.), that is, deviations observed with the help of special instruments, the values of a particular field from normal values. Geophysical methods also serve the practice of searching for minerals and help to understand the physical processes in the bowels of the Earth. Let's start with the anomalies of the gravitational field in the Baikal region.

Even at the very beginning of our century, during the hydrographic description and compilation of Baikal navigation directions for the needs of navigation, it was discovered that the width of Baikal, when determined by the astronomical method and the triangulation method, turned out to be different - in the first case it was narrower. The solution to such a strange, at first glance, phenomenon was that measurements by astronomical methods do not depend on the direction of gravity, while geodetic measurements directly depend on the position of the plumb line. On the shores of Baikal, the plumb line deviated towards mountain slopes composed of dense - about 2.7 g/cm 3 - crystalline rocks. The huge volume of water in Baikal, whose density is close to 1, also had an impact. Thus, gravity anomalies on Baikal associated with density contrasts were discovered for the first time. In the 1930s, gravimetric work began to be carried out systematically, especially in the post-war years. All of them were related to the search for oil on Lake Baikal. From the very beginning, a complex gravitational field was expected here. This was hinted at by the complex mountainous terrain, the huge bowl of water of Baikal, the “irrepressibility” of modern movements of the earth’s crust, resulting both from high seismicity and from direct measurements using the method of repeated leveling along the same profiles. Thus, it turned out that at present the Baikal depression continues to descend relative to neighboring ridges at a speed of up to 6 mm/year. The picture of gravity anomalies was discovered to be truly complex, and the negative gravity anomalies, according to the general opinion, are created here not only by water, but also by the thickness of loose sediments at the bottom of the lake, the density of which is less than the average density of the earth's crust. Calculations made it possible to estimate the thickness of Cenozoic sediments in the Baikal depression, as well as the depth of the surface of the crystalline basement on which they lie. This depth is up to 6000 m below sea level!

Considering the role of water and sediment in creating the negative anomalies of Lake Baikal, scientists came to the conclusion that at great depths beneath it there should be rocks of increased density, and on this basis it was suggested that the earth’s crust under the Baikal depression is somewhat thinner than under the neighboring ridges, and dense rocks of the upper mantle lie, accordingly, closer to the earth's surface. This means that the “lack” of mass in the upper part of the crust is, as it were, compensated by a deep excess, that is, the depression is approximately isostatically balanced. The earth's crust seems to float on the mantle, forming a certain constriction under Baikal or, as metallurgists say, a “neck”. This assumption has been generally confirmed by recent deep seismic data.

In the Baikal rift zone, the magnetic field turned out to be relatively simple. Against its general, close to normal background, a series of local elongated anomalies were identified. The sources of magnetic anomalies, as calculations have shown, lie in the rift zone in a much thinner layer (18 km) than under the neighboring Siberian platform (33 km). It is believed that the thickness of such a layer is determined by a temperature of about 450°C (the so-called Curie point), above which titanium-magnetite loses its magnetic properties; it turns out that under the rift zone the 450° isotherm lies at almost half the depth than, say, in the inside of the Irkutsk amphitheater.

Magnetotelluric sounding in the Baikal region brought very important data - one of the methods for studying the electrical conductivity of the subsoil. It was shown that there is a layer of increased conductivity in the mantle beneath the Baikal region, the upper boundary of which under the rift zone is at a depth of 40-50 km, and in neighboring areas of the platform at a depth of about 100-120 km. As follows from experiments on silicate rocks (the mantle is made up of them), such an increase in electrical conductivity is achieved at a temperature of about 1200°C. It follows that a layer of this temperature is also located much higher, under the rift zone. Let us now recall the numerous traces of very young volcanism in the Baikal region, described above, as well as the numerous outlets of hot springs here, which together directly indicate increased heating of the subsoil under the Baikal rift zone.

At the beginning of the book, we already indicated that the deep heat flow on Lake Baikal is noticeably increased. Special measurements have established that linearly elongated thermal anomalies in the Baikal depression do not cover its entire area, but are concentrated in narrow linear fault zones. The value of the specific heat flow in them is two to three times higher than the average for continents and reaches 3 μcal cm 2 /s. So, this all suggests that under the rift zone there is a powerful deep energy source, discovered in the last decade by seismic methods. Let's return to it again.

The phenomenon of anomalous mantle in the south of Eastern Siberia was discovered, or better yet, was suspected due to a systematic time delay in the arrival of seismic waves excited by earthquakes to seismic stations in the Baikal region. Readers here have the right to ask: what does the delay of seismic waves mean and is there a “schedule” for them? Yes, such a schedule exists for each newly occurring earthquake, and its violation means that on one or another segment of the path of seismic vibrations, their, so to speak, normal speed for given depths has changed in one direction or another. In physical seismology, there is an extremely important concept - hodograph, that is, a graph of the dependence of the time of arrival of waves at a recording station on the distance to the source. The huge number of observations of the velocities of seismic waves at various depths of the Earth during earthquakes all over the world and the knowledge of average velocities in different shells of the planet (the shells themselves and their boundaries were established by seismic methods) made it possible to have a theoretical schedule for the arrival of seismic waves at one point or another on the earth’s surface . The very fact of such a delay cannot but mean changes in the properties of the medium through which the wave passes, that is, it indicates an anomaly of the medium in some volume. By reconstructing, for example, the graphical course of seismic waves, one can thus approximately imagine the shape and size of the anomalous mantle. It is assumed that the decrease in the speed of seismic waves is associated with partial melting of the mantle material through which the waves pass and, consequently, with a decrease in its average density. And if this is so, then masses with reduced density should “float” up through the mantle with normal density. Archimedes' law comes into play. But the relatively light (less dense) substance of the mantle, rising upward, cannot but carry a large supply of heat captured from great depths. Taking all these assumptions, which do not at all contradict physical laws, it turned out to be possible to give a diagram of the anomalous mantle under the rift zone and its environs (Fig. 8). In this form, the anomalous mantle supports the very bottom of the crust under Baikal, and in the southwest it plunges to a depth of 700 km or more (Fig. 9).

So, it turns out that the passage of the rift zone and its main link - Baikal - is associated with the existence in the deepest depths of this region of Asia of a powerful source of thermal energy. And since the beginning of the formation of the rift zone coincides with the end of the Paleogene or the beginning of the Neogene, the beginning of the approach of the anomalous mantle to the earth’s crust can be dated in this area to approximately 25 million years.

It’s time to summarize the data presented in this essay and try to imagine how the Baikal rift zone, and other continental rift zones following its example, was formed or could have been formed.

The starting point is that in the thickness of the planet, namely, at the boundary of the mantle and the earth’s core, a certain separation of matter occurs according to density (reaching at these depths, as we remember, 5.9 g/cm 3) and a slow rise of less dense substances begins masses to the surface of the planet. Over time, having passed through the entire thickness of the mantle, that is, almost 3000 km, portions of a substance of low density, consisting of a mixture of refractory peridotite and molten (melted from peridotite) basalt, accumulate under the earth's crust and lift it, thereby causing the beginning of the process of mountain building on earth's surface. An arched uplift of the crust is formed, the dimensions of which will obviously depend on the volume of deep substance accumulated under it. The process of uplift and mountain building, with the continued influx of relatively low-density mantle matter under the crust, can continue only until isostatic equilibrium is achieved, that is, until the moment when the weight of the arched uplift compensates for the buoyancy force. But such equilibrium “vertically” will not yet mean that complete mechanical equilibrium has occurred in the entire system and the process is completed. The fact is that the substance of the anomalous mantle accumulated under the crust should spread to the sides, obeying the principle of striving for a minimum of gravitational energy. So, for example, a piece of pitch placed on a horizontal plane will inevitably spread to the sides. The spreading of mantle material creates, due to viscous friction, tensile forces in the earth's crust under the arched uplift. To the tensile forces are added forces directed along the slopes of the arched rise - the crust, like any body on an inclined plane, will tend to slide off the slopes of the mantle bulge. On the other hand, stretching should lead to the opening of cracks in ancient faults in the earth’s crust and to the formation of new faults, and thus the possibility arises of the introduction of anomalous mantle material into the cracks of the faults, its cooling, crystallization and transformation into ultramafic rocks that act as cracks. At the same time, giving off heat to the environment, the mantle material will heat the crust in a limited volume adjacent to the fault. In turn, in the heated volume of the crust, the viscosity of the substance will decrease and its ability to stretch will increase. If this entire process proceeds as a Broad front (numerous fault cracks open in the crust, and numerous mantle bodies penetrate into them), then in general the earth’s crust will Stretch over the protrusion of the mantle, and therefore, be driven away. The surface of the Earth above such a protrusion will be a rift basin with all its attributes. The stated hypothesis (its main author is Professor Yu. A. Zorin), as we see, is an interpretation of established facts within the framework of a general idea. It includes and is justified by geological data (the widespread development of faults in the first place), and data on the external relief of the rift zone, and seismicity data, especially the conclusion about the predominance of tensile forces transverse to the structures of the rift zone in the foci of earthquakes, and data on the lag seismic waves under the earth's crust, observations of geophysical fields, in a word, all modern scientific material on the Baikal rift zone. In Fig. Figure 7 shows a diagram of the structure of the Baikal rift graphically. In principle, it is suitable for explaining the origin of other continental rifts.

So, it is assumed that tensile forces act throughout the arched uplift, but they deform the earth’s crust where it is especially strongly weakened by cracks and heated by intrusions of mantle material. After cooling of the crust, its plastic, that is, without faults, stretching can be replaced by the formation of a new fault in the thin part of the crust, and then the whole process will repeat. The long-term (millions of years) formation of the rift basin probably consists of alternating phases of the appearance of open cracks and phases of extension without ruptures after the introduction of mantle melt into the cracks. All this, of course, does not proceed easily, if only because in the upper, less heated and, therefore, more fragile part of the crust, stretching should be complicated by the formation of new faults that do not go to depth and attenuate in the region of a more heated and plastically deformed crust. This means that such faults (unlike others - deep and superdeep, separating, for example, entire lithospheric blocks or plates) will “work” only in the upper part of the crust. Indeed, the foci of earthquakes in the Baikal and other rift zones, undoubtedly associated with crustal faults, lie predominantly at shallow depths - up to 15-20 km.

One more question remains. The arched rise and the rift depression on it are, in a certain sense, opposite phenomena, acting as if towards each other. But the spreading of mantle matter to the sides under the arched rise should lead to its decrease, and then to its destruction. In fact, rift basins both on land and in the ocean are almost invariably associated with extensive arched uplifts. Such is the Baikal Rift. Modern geophysical measurements show that the ridges around the rift continue to rise and the troughs continue to fall. How can this be explained from the point of view of the mechanism of rkft formation in the form in which it is presented by us? Obviously, the whole point here is the constant influx of anomalous mantle matter under the earth’s crust and thus restoration of the height of the arched rise.

Well, can we now say that the mystery of the Baikal rift, and with it the mystery of the formation of other rift zones of the Earth, which have so many common features, has been successfully and completely solved? Of course, this cannot be said, which, however, should in no way disappoint us. In fact, from the generalization of geological and geophysical vast and varied materials, a drawn model of the Baikal rift can follow. When constructing it, mainly physical data were used, and the process of formation of the arched rise and the rift depression at its top was depicted only as mechanical deformations. But complex physical and chemical processes occur in the earth’s crust and upper mantle, the essence and results of which cannot be considered fully studied. After all, we are talking about the still inaccessible and opaque depths of the planet, and no matter how diverse and sophisticated the indirect methods of understanding them are, many difficulties are still far from being overcome.

The Baikal rift zone remains largely an unsolved mystery, and if, according to Tyutchev, it is in fact very simple, then nature continues to hide this simplicity behind complex barriers. And the temptation that Tyutchev wrote about is the desire to cognize simplicity itself, even if involuntarily in complex and difficult ways.

RIFT (a. rift; n. Rift; f. rift; i. rift), rift zone, is a large strip-like (in plan) zone of horizontal extension of the earth’s crust, expressed in its upper part in the form of one or several close linear grabens and conjugate with them block structures, limited and complicated mainly by longitudinal faults such as inclined faults and thrust faults. The length of the rift is many hundreds or more than a thousand km, the width is usually tens of km. In relief, rifts are usually expressed as narrow and deep elongated basins or ditches with relatively steep slopes.

Rifts during periods of their active development (rifting) are characterized by seismicity (with shallow earthquake foci) and high heat flow. During the development of rifts, they can accumulate thick strata or , which contain large oils, ores of various metals, etc. The anomalously heated and low-viscosity upper part of the mantle under developing rifts usually experiences uplift (the so-called mantle diapir) and some spreading to the sides, and the overlying bark shows some arch-like bulging. Some researchers consider these processes to be the main cause of rift formation, others believe that local uplifting of the upper mantle and crust only favors the emergence of a rift and predetermines its localization (or even is its consequence), while the main cause of rifting is regional (or even global?) stretching bark. With particularly strong horizontal stretching, the ancient continental crust within the rift undergoes complete rupture and between its separated blocks, in this case, due to the igneous material of basic composition coming from the upper mantle, a new thin crust of the oceanic type is formed. This process, characteristic of ocean rifts, is called spreading.

Based on the nature of the deep structure of the crust in rifts and their framing zones, the main categories of rifts are distinguished - intracontinental, intercontinental, pericontinental and intraoceanic (Fig.).

Intracontinental rifts have continental-type crust that is thinner compared to the surrounding areas. Among them, according to the characteristics of the tectonic position, rifts of ancient platforms (epiplatform or intracratonic) of the dome-volcanic type (for example, Kenyan, Ethiopian, Fig. 1) and weakly or non-volcanic crevice type (for example, Baikal, Tanganyika) (Fig. 2) are distinguished. as well as rifts and rift systems of mobile belts, which periodically arise and then transform during their geosynclinal development and are mainly formed during the post-geosynclinal stages of their evolution (for example, the rift system of the Basins and Ranges in the Cordillera, Fig. 3). The scale of extension in intracontinental rifts is the smallest compared to their other categories (several km to the first tens of km). If the continental crust in the rift zone undergoes complete rupture, intracontinental rifts turn into intercontinental rifts (rifts of the Red Sea, Gulf of Aden, and California; Fig. 4).

Intraoceanic rifts (so-called mid-ocean ridges) have oceanic-type crust both in their axial zones (zones of modern spreading) and on their flanks (Fig. 5). Such rift ridges can arise either as a result of the further development of intercontinental rifts, or within older oceanic areas (for example, in the Pacific Ocean). The scale of horizontal expansion in intraoceanic rifts is the largest (up to a few thousand km). These rifts are characterized by the presence of transverse faults (transform faults) intersecting them, as if displacing neighboring segments of these rift zones relative to each other in plan. All modern intraoceanic, intercontinental, as well as a significant part of intracontinental rifts are directly connected to each other on the surface of the Earth and form the world rift system.

Pericontinental rifts and rift systems, characteristic of the margins of the Indian Ocean, have a highly thinned continental crust, which replaces the oceanic crust towards the interior of the ocean (Fig. 6). Pericontinental rift zones and systems formed in the early stages of the evolution of secondary ocean basins. Intercontinental and intraoceanic rifts arose at least from the middle of the Mesozoic, and possibly in earlier eras. Intracontinental rifts within ancient platforms have been formed since the Proterozoic and subsequently often experienced regeneration (so-called). Rift-like linear zones of extension, which were later subjected to compression, arose already in (greenstone belts).

Rifting (rifting)– geotectonic processes leading to the formation of rifts (rift – cleft, gorge). These can be: 1 - differential movements of blocks - during the uplift of the marginal parts of large blocks along ancient faults, blocks appear that lag behind these blocks in their movement and create rift zones; 2 – tension zones that arise during horizontal multidirectional movement of blocks; 3 – zones of extension and subsidence over large arcogenic (uplifting) structures; 4 – extension zones formed at the initial stages of the splitting of lithospheric plates on the continental (controlled by faults) or oceanic crust (controlled by spreading) above rising plumes.

All variants of the mechanism of continental rifting provide for local thinning of the crust under the influence of tensile stresses with the manifestation of: a system of normal and gentle symmetrical and asymmetrical (relative to the axial part of the structure) faults; graben systems above the top of a large arch (mantle diapir or arcogen); accompanying intense magmatism (Fig. 7.18). Oceanic rifting from the standpoint of plate tectonics is also called spreading. It is based on spreading through magmatic wedging, which can develop as a continuation of continental rifting. At the same time, the modern rift zones of the Pacific and Indian Oceans were formed on the oceanic lithosphere in connection with the restructuring of plate movement and the death of earlier rift zones.

Rift structure (rift) (from the English rift - cleft, gorge) - a slot-like or ditch-like structure of deep origin linearly extending for several hundred km (often >1000 km). Width R.s. from 5 km to 400 km. R.s. stand out. – intracontinental (East African, Baikal, etc.), intercontinental (Red Sea, etc.) and intraoceanic or mid-oceanic(Atlantic, Pacific, etc.). They are characterized by conditions of extension (pulling apart), intense magmatism (intrusive and effusive) and “suppressed” sedimentogenesis.

Inland rifts are a system of grabens bounded by normal faults. The bottom of the grabens is occupied by lakes or filled with coarse sediments. Igneous manifestations are known both inside and outside grabens (in the sides). These are alkaline and alkali-olivine basalts (with mantle marks), plateau basalts (similar to traps), carbonatites, felsic volcanics, etc. Midoceanic rifts are confined to mid-ocean ridges (MOR) and form a single global system with a length of about 80 thousand km. They have a highly dissected topography with a relative elevation of up to 2 km. They form a small amount of deep-sea sediments, pillow lavas of basalts and swarms of dikes.

Within the Kola region, the Pechenga-Imandra-Varzuga structure is classified as intracontinental paleorift structures of Early Proterozoic age. A number of researchers believe that it experienced an oceanic stage in Ludicovian (i.e., it developed as a mid-ocean rift).

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 crust, as well as in their transitional parts and occupying only within the oceans an area equal to the continents. For rift zones, sometimes complex specific relationships between the mantle and 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 realm of discourse, including the issue of their typification. This. It is necessary to keep in mind with regard to the distinguished types of rift systems in accordance with the data of M.I. Kuzmin, who calculated natural geochemical standards for the igneous rocks of these systems in 1982:

oceanic rift zones, confined to mid-ocean ridges, forming a single system of oceanic rises 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 Rise - the central horst rise). 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 normal 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 decompressed. Tholeiitic basalts appear in the rift valleys, and in the distance - rocks of the alkali-basaltic and bimodal series, as well as alkaline-ultra-basic rocks with carbonatites;

island arcs consisting of four elements: a deep-sea trench, a sedimentary terrace, a volcanic arc and a marginal sea. The thickness of the earth's crust is 20 km or more, magma chambers at a depth of 50-60 km. There is a natural change from low-chromium-nickel tholeiitic series to sodic calc-alkaline series, and in the very rear of the island arcs volcanics of the shoshonite series appear; active continental margins of the Andean type, characterizing the “creep” of the continental crust onto the oceanic one, like island arcs, are accompanied by the Zavaritsky-Benioff seismofocal zone, but with the absence of marginal seas and the development of volcanism within the continental margin with an increase in the thickness of the earth's pores 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 the calc-alkaline (rhyolite) series, giving way to rocks of the andesite formation - the 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-sea 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 onto the entire system of the mid-ocean ridge. Therefore, there is a simultaneous manifestation of magmatism, characteristic of both rift structures (oceanic and continental types) and compression zones (deep seismic focal zones).

The petrogeochemical standards (types) of igneous rocks, characteristic of these zones, calculated by M.I. Kuzmin are of great scientific importance, regardless of the playtectonic views of their author, including for typing 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 the geodynamic conditions of formation, therefore these types can be the basis for the 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 active margins of the Californian type. 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 geochemical patterns of magmatism, that island arcs do not indicate the presence of a transitional type of crust, much less rift structures, but are typical young geosynclines.