The Baikal rift zone is a divergent boundary located in the region of the lake and the Eastern Sayan. Its central part is located under the lake. This is where the earth's crust diverges. The Eurasian Plate is located in the west of the rift, and the Amur Plate limits it in the east, moving from the rift towards Japan at a rate of about 4 mm per year.
General information
As in other divergent zones, the earth's crust of the Baikal Rift is thinning and magma comes very close to the earth's surface. Hot springs are present both at the bottom of the lake and on the surface. However, no signs of volcanic activity have been found in the immediate vicinity of the lake's shoreline. In relatively recent times, volcanism occurred near the lake and is probably associated with the rift zone. These are the volcanic zones of the Udokan plateau, located about 400 km northeast of the upper edge of the lake, the Oka plateau with Kropotkin and Peretolchin volcanoes northwest of the southern tip, a plateau 200 km east of the rift, and the Tunkinskaya depression, located between the lakes and , which is not a submerged part of the rift. In the southwestern part of the Baikal Rift, on the territory of Mongolia, there is Lake Khubsugul.
Some researchers explain the formation of the Baikal rift by the mechanism of a transform fault, others suggest the presence of a mantle plume under Baikal, and others explain the formation of the basin by passive rifting as a result of the collision of the Eurasian plate and Hindustan. There are suggestions that the subsidence of the basin is associated with the formation of vacuum chambers due to the outpouring of basalts on the surface (Quaternary period). The Baikal Rift is active. Earthquakes constantly occur in its vicinity.
Along with the East African Rift, the Baikal Rift is another example of a divergent boundary located within the continental crust.
Application. Baikal Rift
The first geological descriptions of Lake Baikal were carried out as early as the 18th century. So, in 1772, a Russian academician, a German by birth, wrote:
“Baikal seems to be witnessing a great catastrophe; it is immeasurably deep in places, has several cliffs, like pillars, as if carved out of the depths. But in the mountains they do not find, except for unfortunate and weak earthquakes, no other destruction ... no faults, no traces of volcanoes, lavas..
Faults and volcanoes were discovered later, in the next century (their detailed study made it possible to attribute Baikal to rift structures). However, the topic of rifting became seriously interested only in the middle of the 20th century. A significant contribution to the study of the Baikal rift was made by the staff of the Institute of the Earth's Crust of the Siberian Branch of the Russian Academy of Sciences, who formed a scientific school for the study of continental rifting.
Causes of Rifting: Hypotheses
In the early 1970s, there was a wide discussion about the causes of rifting. This dispute also affected the Baikal Rift. Well-known researchers, American Peter Molnar and Frenchman Paul Tapponier, drew attention to the connection between the collision of the Asian and Indian plates with deformation in the interior of Asia. They suggested that this mechanism could lead to "passive" stretching in the Baikal rift zone. This point of view has gained great popularity abroad. Vera Alexandrovna Rogozhina and Vladimir Mikhailovich Kozhevnikov from the Institute of the Earth's Crust, using seismic data, recorded an anomalous decompression at sublithospheric depths under the Baikal Rift, in the so-called upper mantle of the Earth. Therefore, the Russian side defended the point of view about the dominant role of deep thermal processes - that is, "active" rifting. This long-term problem about the "passive" and "active" mechanism of the Baikal rift extension still remains relevant. Although in Lately more and more researchers come to the idea of the simultaneous action of both mechanisms. The author does not impose any definite opinion on the mechanisms of formation of the Baikal rift. Instead, new, and in my subjective opinion, the most important data on tectonics, volcanism, sedimentation, and deep structure are presented. The interpretation of these data often remains ambiguous.
The structure of the Baikal rift
The Baikal Rift System is located in the interior of the continent and separates the northern stable part of the Eurasian Plate from another large stable block called the Amur Microplate. The rift system consists of a series of depressions (the largest of them is Baikal) and uplifts separating them, stretching for more than 1500 km, and also includes fields of late Cenozoic volcanism located at some distance from the depressions and their mountainous framing.
The Baikal Basin consists of two independent basins - the South Baikal and the North Baikal, separated from each other by the Akademisky underwater ridge.
Scientific school for the study of continental rifting at the Institute of the Earth's Crust SB RAS (Irkutsk)
founders scientific school became geologists and, as well as geophysicist Andrey Alekseevich Treskov. They laid the foundations for a systematic study of the Baikal Rift. In his autobiography (April, 1984) N.A. Florensov wrote:
"In my doctoral dissertation, it turned out to be mixed elements of coal geology, young volcanism, and most importantly, elements of the Late Mesozoic and Cenozoic tectonics of the Baikal and Transbaikalia. Earlier ..., I was looking for differences from typical African rifts, but then it turned out that there is a clear similarity between both Fortunately, my mistake turned out to be just with me, and the summary given in the dissertation and then in the monograph ... served as the starting point for extensive and many years of research on rift topics by almost our entire institute ..."
After the departure of Nikolai Alexandrovich, the baton was taken over by his closest colleague and student, an academician.
Nikolai Alexandrovich Florensov was the founder (until 1962 - the Institute of Geology of the East Siberian Branch of the USSR Academy of Sciences) and its first director. During the leadership of Nikolai Alekseevich Logachev (1976-1998), the rift theme brought the Institute wide, including international, fame. Research in this direction is still being carried out by their students and colleagues.
Age of sedimentary strata
The amount of loose sediments in the Baikal Basin is estimated at 75000 km2, which is approximately 70% of the sedimentary deposits in the basins of the entire rift system (Logachev, 2003). The South Baikal depression is considered the most ancient. In the 1970s, Nikolai Alekseevich Logachev and Nikolai Aleksandrovich Florensov suggested that its formation began in the late Eocene - early Oligocene, approximately 30-35 million years ago. Since then, this value has traditionally appeared in most publications about the Baikal Rift. IN last years Nikolai Alekseevich Logachev noted that in fact the depression could be much older.
Determination of the time of the beginning of depression formation is difficult. In order to get an answer to this question, you need to get to the rocks buried under many kilometers of sedimentary strata. Within the framework of the international project "Baikal-drilling", several wells were drilled in the Baikal sediments during the winter periods of 1996-1998. from barges frozen in ice. The longest age record was obtained when drilling sediments on the Akademichesky Ridge, since this section of the Baikal bottom is remote from all coastal sources of material drift and, therefore, is characterized by the lowest sedimentation rate. Sediments at the base of a 585 m long drilled sediment core were determined to be approximately 8.3 Ma (Horiuchi et al., 2004). This is the minimum proven age of Lake Baikal. According to the latest data, the rate of sedimentation in the last 4.5 Ma on the Akademichesky Ridge averaged about 0.04 mm per year, while earlier it averaged about 0.1 mm per year (ibid.). That is, the rate of sedimentation has decreased by more than two times! This is an unexpected result, since traditionally, according to the study of the variability of the sedimentary section of the upland depressions of the Baikal rift, the stages of "slow" Oligocene-Miocene and "fast" Pliocene-Quaternary rifting were distinguished.
In other words, the recorded change in the rate of sedimentation is directly opposite to the expected one. The only explanation for this fact, in my opinion, can be a significant uplift of the Akademichesky submarine ridge at the turn of 5-4 million years ago, which led to its isolation from terrigenous material brought mainly by the Selenga, Barguzin and Upper Angara rivers.
Modern block movement
The rate of expansion of the Baikal Basin remained until recently a subject of serious dispute. The issue was resolved by using satellite systems navigation - GPS. Based on ten-year observations with the help of permanent and temporary GPS points, it was possible to find out that the rate of expansion of stable blocks of the Siberian Platform and the Amur microplate is 4 mm per year. In this case, all deformations are localized along the axial part of the Baikal rift.
deep structure
An important role in understanding rifting is played by studies that allow one to "see" the modern deep structure of the crust and mantle. Based on the data of seismic tomography carried out in the course of the Russian-American experiment in 1992, a velocity section of P-wave propagation was constructed (Mordvinova et al., 2003). It was found that one low-velocity anomaly is located almost under Baikal. However, the second one is located much to the south, under the territory of Mongolia, where there is no crustal extension. A reasonable question arises: what causes a decrease in the velocities of the passage of seismic waves in the mantle - an increased temperature or features of the composition of the substance? The first explanation is usually accepted.
Evolution of the Deep Thermal Regime of the Lithosphere
Partial melts from the mantle of alkaline basaltoids on their way to the surface sometimes capture fragments of surrounding rocks. The finds of such rocks, called xenoliths, are very valuable for understanding the material composition and conditions of "life" of the earth's depths. In the Baikal rift, the largest "harvest" of mantle xenoliths was collected in the eastern part of the Vitim volcanic field by Igor Viktorovich Ashchepkov and his colleagues from the Joint Institute of Geology, Geophysics and Mineralogy of the Siberian Branch of the Russian Academy of Sciences.
It turned out that mantle xenoliths from the Miocene lavas of the Vitim field indicate a large range of pressures, and they originated from great depths, from young Quaternary lavas - a smaller range. This indicates a greater thickness of the lithosphere in the Miocene under the Vitim field, in comparison with the Quaternary time. According to calculations, the thinning of the lithosphere over 13 Ma was approximately 15 km. At the same time, the boundary between the levels of formation of indicator minerals, garnets and spinels, deepened by about 8 km, which, according to experimental data, indicates an increase in temperature.
We note one more interesting feature. Despite the significant thinning of the lithosphere under the Vitim field, there was no significant stretching of the crust. According to drilling data, depressions under lavas filled with sediments do not exceed a few tens of kilometers in width, and a few hundreds of meters in depth.
Volcanism
When determining the age of the volcanic rocks of the Baikal Rift, a complex migration of volcanism was established in the Eastern Sayan and on the Udokan Range. In both areas, volcanism shifted over time along intricate trajectories with a predominant westerly trend, i.e. practically in opposite side from the general movement of the Eurasian lithospheric plate. This likely indicates tectonic control of magma uplift at the junction of compression and extension structures, with a general westward displacement of volcanism consistent with the existence of a relatively immobile hot magma source in the asthenosphere.
In order for a partial melt to appear in the mantle, it is necessary either to raise its temperature, or to reduce the pressure, or to saturate the mantle with volatile components. With passive rifting at a rate of 5 mm per year, as well as with such a thickness of the lithosphere and crust as in the Baikal rift, the pressure in the mantle will never decrease so much that the mantle rocks begin to melt in the absence of volatile components. However, if there are fusible areas in the mantle with water-containing minerals or carbonates, then such areas, even with slight temperature and pressure drops, will pass into the melt.
Characteristically, the distribution of volcanic fields tends neither to rift depressions nor to gravitational minima, areas of potential heat increase. A particularly indicative example is the Dariganga volcanic plateau in Mongolia. Apparently, this indicates that the melting of the mantle of the Baikal Rift and adjacent territories is controlled, first of all, by its composition.
To study the composition of the melting mantle, the isotope ratios of the elements are studied. The ratio of neodymium and strontium isotopes measured in the lavas of the southwestern part of the Baikal Rift, in comparison with the compositions of the lavas of the Khangai Ridge, showed that the mantle melting region can be divided into three parts (arbitrarily designated as components A, B, and C). Component A refers to the region of the sublithospheric mantle (asthenosphere), and the other two components characterize the inhomogeneous lithospheric mantle. Moreover, component B can refer to the deeper parts of the garnet-containing mantle, and component C, to the spinel-containing mantle or the region of the crust-mantle transition.
There are two extreme models of stretching of the lithosphere in the inland areas, called the models of "active" and "passive" rifting. driving force"Active" rifting is the heat source of an ascending mantle flow, commonly referred to as a plume. It is assumed that the region of origin of such plumes can be located at the section of the upper and lower mantle at a depth of 650 km or even at the boundary with the core at a depth of 2700 km.
The main characteristics of "active" rifting are the formation of tectonic basins against the background of a large regional uplift, an increased heat flow, and widespread volcanism. The latter must precede both the formation of a regional uplift and the formation of a depression. The predominant composition of the volcanic rocks of an "active" rift should manifest itself over a large area and not depend on the composition and age of the lithosphere.
In the model of "passive" rifting, the main cause of extension is considered to be tectonic stresses that occur at the boundaries of lithospheric plates at a considerable distance from the extension area. The fixed uplift of the sublithospheric mantle passively follows the thinning of the lithosphere. A characteristic of "passive" rifts is the confinement of all rift structures to the ancient boundaries between lithospheric blocks different ages and weakly manifested volcanism. At the same time, extension precedes volcanism, and volcanic rocks reflect the heterogeneous composition of the lithosphere.
Correlation of tectonic events
Only crustal stresses from the zone of the Indo-Asian collision or local sources of heat in the mantle could not lead to the formation of the Baikal rift. In recent years, the idea of the important role of the interaction of lithospheric plates on the eastern margin of Eurasia has also been discussed.
It is noteworthy that episodes of compression and extension in the collision zones of the Indo-Asian and Pacific-Asian plates are shifted relative to each other in time. If compression acted on the southern margin of Central Asia, then at that time there was an extension regime on its eastern margin. And, conversely, the significant compression that occurred on the eastern margin, the southern margin experienced an episode of relaxation.
Such dynamics of compression and extension could "swing" the inner parts of Central Asia, lead to the displacement of blocks, which, given their geometry, formed zones of compression and extension at the boundaries of these blocks. With such a mechanism, it should be expected that the impulses of the main tectonic events in Central Asia (for example, the rotation impulses of the Amur microplate) will coincide in time with the change in the tectonic regime at the boundaries of the lithospheric plates. Unfortunately, the dating of such pulses is still a difficult task. For the Baikal rift, uplift periods can be estimated from data on the position of dated lavas in the relief. In total, 4 such episodes were identified: 21-19, 16-15, 5-4 and about 0.8 million years ago. It is interesting that the change in the rate of sedimentation on the underwater Akademichesky Ridge, which occurred 5-4 million years ago, coincided with one of such episodes of uplift. As noted earlier, this event may mark the beginning of the stage of "rapid" rifting. At that time, there was an expansion regime in the front of the Indo-Asian collision, and compression on the eastern margin of Central Asia began a little earlier than this episode. Thus, the stage of "rapid" rifting cannot be genetically related to distant tectonic events in the front of the Indo-Asian collision. It is associated either with tectonic events on the eastern border of Asia, or with thermal and/or chemical effects on the lithosphere due to local mantle heat sources.
Conclusion
So what is the Baikal rift after all - “active” or “passive”?
Crustal deformations and extensions are mainly controlled by distant tectonic events occurring at lithospheric plate boundaries. Heating, thinning and melting of the lithosphere are carried out due to deep sources of heat, or due to the existence of light-melting regions in the mantle. This means that the Baikal rift bears the features of both "active" and "passive" rifting. Trying to consider the development of the Baikal rift solely from the standpoint of studying crustal deformations or the evolution of volcanism, or deep geophysics, we find ourselves in the position of blind wise men who study the elephant by touch in a well-known parable. Only the integration of various research areas will allow us to answer which of the mechanisms of rifting prevailed, whether their ratio changed over time, whether the processes of crustal extension and magma formation are related, or are these two independent processes. The need to combine their efforts today is recognized by almost all researchers, which means that someday, when starting an article about the Baikal Rift, it will be possible to say “we know how and why it was formed.”
How to relate to the above words of the poet? Is nature really so simple that in fact everything is clear in it, and the science of nature is a pure delusion, an artificial creation of riddles, on the solution of which mankind has spent so much futile effort? It would be a mistake to think that Fedor Ivanovich Tyutchev did not understand what science is and that revealing the secrets of nature is not useful. The thing is that in itself, independently of human consciousness, nature does not contain, cannot contain anything mysterious. The subjective concept of mystery arises as a result of the imperfection of the reflection of natural phenomena by human consciousness. Overcoming this imperfection, striving for it constitute the path of development of science.
Riddles, secrets, mysteries of nature for an 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 riddle for us, as a natural and necessary phenomenon in the development of the earth's interior.
The immensity and harsh nature of Baikal were mysterious for the first explorers who came to its shores. This mystery was resolved in their minds by the conviction that Baikal is the sea. New and new discoveries forced to abandon the recognition of Baikal as a real sea. So a new riddle arose: what is it, unlike either the sea or the largest lakes then known to science? New discoveries followed. And immediately new mysteries appeared. Already in the post-war period, a new term appeared in the language of scientists, which says little to the general reader - the Baikal rift and the Baikal rift zone.
Baikal in the XVII-XVIII centuries. became famous as a fresh sea. In the next century, it became known to the whole world as the deepest completely fresh lake on Earth. In the first half of our century, the glory of a closed center of biological speciation came to him, in which organisms peculiar to him alone (endemics) 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 mainland. Such is the peculiar scientific "career" of Baikal. And what is especially remarkable - in their last role, in revealing the "secret" of Baikal that did not exist in nature, but haunted science, earthquakes, volcanic structures, and the very location of the mountains in the south found their natural place. Eastern Siberia.
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 join together, forming, as it were, the frame of the platform, or, as it is often said, its southern folded frame. The border between these regions has a rather simple contour, with two "bays" to the south - Irkutsk and Aldan. The Siberian platform has a flat or slightly undulating watershed surface relief, but its river valleys are deep, with steep slopes. Hence another, 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 Sayan Mountains, the Baikal Mountains and the Stanovoy Uplands. A 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, usually not exceeding 3000 m above sea level. The farther south from the northern edge of the mountains, the less the influence of this region on the direction of individual large hills, but still a gentle bend - the transition of the northwestern "Sayan" strikes to the northeastern "Baikal" ones is generally preserved within Mongolia. Near the junction line of the plateau-mountain, in some places moving away from it into the depths of the mountains, and in some places coming close to it, separate lowered areas are visible - intramountain (intermountain) depressions, which at first glance seem to be just greatly expanded segments of river valleys. Convenient flat places in the bottoms of these depressions, of course, first of all attracted the first settlers to them, the first travelers stopped in them, 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 basin 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 in different ways, but already in late XVIII centuries, some scientists have seen in them catastrophic failures caused by deep forces, precisely those that declare themselves by private local earthquakes. Opinions were expressed that the huge subsidence among the mountains is a consequence of volcanic processes. Many believed that these were just the remains of huge ancient river valleys, and I. Chersky believed that the Baikal basin was a slowly deepening and shrinking 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 by European models. It was found that the typical structure of large intermountain depressions is a graben, that is, the subsidence of a longitudinal section of the earth's crust between two parallel faults-dumps. Similar grabens were then found in almost all mountainous countries, and the Rhine graben remained their model, prototype - subsidence along faults between mountain ranges Black Forest and Vosges. 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 broken into separate blocks, partly lowered, partly raised, and against such a “structural background” the Baikal basin was only the largest and the youngest.
Further studies showed that the intermountain depressions of the Baikal region and Northern Mongolia form a single system, as it were, connected by extended faults in the earth's crust, making up with its links, i.e., separate depressions, a kind of chain stretching for more than 2000 km from Lake. Khubsugul in Mongolia to South Yakutia. Earlier, back in early XIX c., the noted external similarity of the depressions suggested the idea of the geological relationship of all the links of such a chain, of the close time and a similar method of their formation. At the beginning of our century, the English geologist J. Gregory described a similar, even more grandiose system of similar depressions in East Africa, calling them rift valleys. Another English geologist B. Willis, while exploring the Dead Sea depression in Palestine, found that the marginal parallel faults that form it are not faults, but reverse faults, or steep overthrusts, with which graben walls, as it were, compress the central lowered strip. Such a structure, in contrast to the rift, he called ram-pom. Shortly thereafter, the ramp model was applied to the Baikal basin. Earlier, at the very beginning of our century, the geologist Lvov pointed out the similarity of the Baikal depression with the depression of another deepest lake - Tanganyika in Africa. Finally, the geologist Pavlovsky, who also noted the similarity of the basins of Baikal and East Africa, proposed for all links of the Pribaikalsky system of inter-height subsidences the apt common name "Baikal-type basins".
A very sharp rise in geological research in the inter-hydro basins of the Baikal region occurred in the 1950s in connection with the search for oil and gas. Several fairly deep wells have been drilled. The Institute of the Earth's Crust, then simply the Institute of Geology of the USSR Academy of Sciences in Irkutsk, came to grips with the geology of this entire region. Important results were obtained on the Baikal depression and its nearest neighbors. However, the most important thing was that it was at this time that extensive international studies of the bottom of the World Ocean were carried out on a new scientific and technical base and the World Rift System was discovered. This discovery came real sensation and it became milestone in the development of the 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 submarine 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 relief forms are found only in the summit, ridge parts of the ranges. They are created, firstly, by the stepped slopes, and secondly, by the presence of deep and narrow axial depressions of fault origin - rift "valleys". Being uplifts of a thin (7-10 km) oceanic crust, underwater ridges are characterized by high heat fluxes (up to 3-10 μcal cm 2 s), strong volcanism with basaltic lava outpourings, strong seismicity, and the presence of fragments of ultrabasic rocks, indicating a close occurrence to the surface of the bottom of the mantle substance. Postcard and further study of the World Rift System served as an impetus for 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 huge - thousands of kilometers over the course of geological history - horizontal displacements of lithospheric plates.
One of its branches, the World Rift System leaves the Indian Ocean on 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 depressions. As for the Rhine graben and the grabens of the Baikal zone, they turned out to be very close to oceanic rift gorges in a number of ways, although they have no direct spatial connection with the World Rift System. It is clear that with its “land”, accessibility for comprehensive research, the possibility of direct, visual acquaintance and already quite high geological knowledge, the Rhine, Baikal and the Province of Ranges and Basins in the Western United States, which has long been a candidate for similar structures of the earth’s crust, have become the subject of a special study by the International program.
In 1966, in Irkutsk, within the walls of the Institute of the Earth's Crust, a traveling session of the Scientific Council for the Study of the Earth's Crust and Upper Mantle of the USSR Academy of Sciences was held under the chairmanship of VV Belousov. The results of what had been done on the Baikal depression and neighboring structures similar to it were summed up. A program for further research has been drawn up. The Baikal section of the aforementioned Scientific Council was organized. The study of Baikal as a natural phenomenon, determined by deep processes, has entered a new stage.
If now the basins of the Baikal type have turned into "rift valleys" or simply into rift basins, then the question arose about their relationship to the World Rift System. The Baikal rift zone seemed to be completely isolated, as if "abandoned" deep into the Asian continent, and it was also located on a territory composed of ancient and, in part, the most ancient rock strata. It was time to move on to the study of possible means and methods of deep bowels under the entire rift zone. This Work involved 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. Naturally, geophysical work came to the fore. We will talk about them in more detail below.
On fig. 7 shows the general scheme 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 isohypse (line of equal heights) 1500 m above sea level. All these are the main characteristics of the rift zone. It can be seen from the diagram that the rift zone in the southern part closely adjoins the northern boundary of the Mongolian-Siberian Mountains and, thus, the southern boundary of the Siberian Platform, while in the northeast it retreats from this boundary to the south. Volcanic fields gravitate towards the flanks of the rift zone, but the Vitim lava plateau is displaced to the east of it. Baikal - the main central link of the rift zone - is associated with especially powerful faults in the earth's crust. Very many faults throughout the zone are the result of cracking of the earth's crust, which occurred in the Neogene and Quaternary period, up to the present day. Almost all of the depressions and 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 basins are filled to a certain depth by sediments of river and lacustrine-marsh descent. Similar precipitation continues to accumulate in them now. Sedimentary strata are best studied along the southern edge of the Baikal basin and in the Tunkinskaya depression adjacent to it to the west, which is associated with oil prospecting and deep drilling in these areas. It has been found that the accumulation of terrestrial and water sediments (and, consequently, the emergence of rift depressions) began as early as the Upper, perhaps the Middle Paleogene and continued throughout the Neogene and Quaternary period, i.e., more than 25 million years. As is usually the case in continental (rather than marine) conditions, the accumulation of sediments occurred unevenly, as the "growth", that is, the deepening and expansion of the rift basins. On the western flank of the rift zone, the accumulation of sediments was accompanied by repeated outpourings of basaltic lavas and ejections of pyroclasts, that is, detrital volcanic materials. The composition and structure of such thick lenses of sediments can be judged from Fig. 5. In some places, both along the edges and in the middle parts of the rift basins, the sediments are affected by faults, crumpled into small folds.
A lot of 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 Baikal bottom.
The enormous depth of Baikal was known, of course, even to the first inhabitants of Baikal - the Buryats, Evenks, Kurykans and, perhaps, the more ancient peoples who mastered fishing here. Measurements using a simple sea lot were carried out in the last century, more accurate measurements were made by the Drizhenko expedition at the beginning of our century. The work of the Baikal Limnological Station of the Academy of Sciences showed the greatest depth of Baikal not far to the 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 with the help of an echo sounder and compiled the first relief map of the Baikal bottom. 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 in its depth among freshwater lakes.
The map of the bottom relief of the lake as a whole confirmed the assumptions that Baikal consists of three clearly separated basins, that the deepest one is medium, that the northwestern underwater slope is very steep and stepped, that the southeastern side is longer and gentler, but has very complex relief, that the deepest parts of Baikal are, as it were, underwater plains, that northeast of the northern tip of Olkhon Island, in the direction approximately towards the Ushkany Islands, an underwater hill, called the Academic Ridge, stretches, that, finally, the underwater slopes furrow in places, as in ocean, deep canyons. Nevertheless, work on the study of the lake bottom continued. More and more measurements on echo sounding profiles allowed V. I. Galkin to create a sculptural plaster model of the Baikal depression. Finally, the combined efforts 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 by means of precision (high-precision) echo sounding, underwater photography, and even direct observations from the Pisis submersibles. They fully confirmed the main results of early underwater work, but significantly detailed them. And what is remarkable, in the scheme, in the idea, the current structure of the Baikal basin turned out to be exactly what 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 platforms of individual steps. On the contrary, the width of the eastern slope is much greater (25-30 km), it is very uneven, divided into numerous blocks by both longitudinal and transverse faults. It turned out that the lacustrine sediments, including the youngest ones, are affected by faults, which was especially clearly seen at the foot of the western slope, that is, in the sphere of influence of the main Obruchev fault. Once again, it was confirmed that the Baikal basin is a sharply asymmetric 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 satisfied with this, since we can only very superficially (both in the direct and indirect sense) judge the origin, causes and mechanism of the formation of the Baikal rift zone from the materials presented. But this zone is a recognized sample, the genotype of continental rift zones in general. Let's try, as far as possible, to "deep" 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 began to accumulate about local earthquakes, and it became clear that the Baikal region is a region of high seismicity. In the 1930s, in connection with the search for oil on Lake Baikal in the South-Eastern Baikal region, seismic sounding began to be carried out using artificial exciters of elastic vibrations in the upper layers of the earth's crust (explosive devices). Seismic sounding for solving general problems of the structure of the crust acquired a large scale in the 1979s. It was carried out jointly with the Novosibirsk academic and Irkutsk industrial (exploratory) personnel of 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 sole of the earth's crust, the upper part of the mantle with normal values \u200b\u200bof density and viscosity, which makes up the bottoms of the stone shell, is usually located - lithosphere. Using the DSS method, it was shown that in the Baikal region the velocity in the anomalous layer of longitudinal seismic waves is 7.6–7.8 km/s, and in the “normal” upper mantle underlying it, it is 8.1–8.2 km/s. This difference is the ocular basis for judging about the reduced viscosity and density of the asthenospheric layer. Later we will see that the relatively shallow depth of the "weak" layer under Baikal is also 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 draw up their map, which is constantly replenished with the material of new and new earthquakes. It was found that the centers, that is, the places of discharge of accumulated seismic energy and thus the sources of elastic waves in the Baikal region, are located at a relatively shallow depth - up to 15-20 km. Analysis of stresses in many of these sources, from southern Baikal to the eastern flank of the rift zone, showed approximately the same pattern: near-horizontal extension directed across tectonic and orographic lines and approximately parallel to the latter, more or less horizontal compression. In the earthquake sources 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 sources that are much more seismic than Soviet Central Asia and all of Central Asia. These data are very great importance for understanding the modern mechanics of the earth's crust in the Baikal region. In the 1960s and 1970s, the work of the Institute of the Earth's Crust established systematic delays of seismic waves coming 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-like region of decompressed and, apparently, overheated mantle, the upper boundary of which under Baikal comes to the very bottom of the earth's crust. At the same time, it turned out that the horizontal projection of the contour of the "anomalous" mantle very closely covers the territory of the latest mountain building, high, and in some places - in Western Mongolia - the highest seismicity (up to 11 points), the Baikal rift zone, the area of hot water outcrops and traces of the latest volcanism. This is how seismic methods have advanced our knowledge of the structure of the bowels of the Baikal region and neighboring regions, this is how much the unique geological position of the Baikal basin, and with it the unique lake itself, has become more precise!
Looking through these lines, readers may think that seismic research at the Institute of the Earth's Crust is carried out only in order to understand the structure of the interior of the surrounding territory and get closer to understanding the formation mechanism of the Baikal rift zone. Yes, they are carried out for this purpose, but only in passing with the main work - the study of the seismicity of the Mongolian-Siberian mountain system as one of important conditions, important components of the natural environment in which we live, work, build. The results of the Mondinsk 1950, Muya 3957, and Middle Baikal 1959 earthquakes, together with the observation of the traces of ancient, prehistoric earthquakes expressed in the relief and the data of the current seismic service in Eastern Siberia and Mongolia, as well as with historical information about the earthquakes that were here, are the most valuable material for compiling a map of seismic zoning, work state significance carried out by the Institute of the Earth's Crust for many years. Such maps, which are based on seismo-statistical material, assessing with varying probability the seismic hazard of individual territories, are compiled on different scales and, according to the corresponding statement, have a normative value. The planning of the placement of new buildings, the types of structures, the types of building materials and the amount of appropriations largely depend on them. We saw above that the region of the central segment of the BAM route in the draft map of the seismic zoning of the territory of the USSR in the 50s was assessed as quite safe, but in fact, as shown by the ISC, it lies in that region of the Baikal rift, the seismicity of which is now, on the basis of 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 seismic hazard assessment.
Such scientific tasks, such as determining the depths of local earthquakes, focal mechanisms, the distribution and density of epicenters, the frequency of earthquakes in time - all this serves both scientific purposes and the solution of quite specific practical problems. The shift of our knowledge in both directions, made in recent years, is very great.
We will return to earthquakes, and now we will briefly talk about conventional geophysical methods and their application in the Baikal area.
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 prospecting 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 for the needs of navigation, it was discovered that the width of Baikal, when determined by the astronomical method and by the method of triangulation, turned out to be different - in the first case it was narrower. The key 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 Lake Baikal, the plumb line deviated towards the 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 influence. Thus, anomalies in the force of gravity in 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 postwar years. All of them were connected with the search for oil in Baikal. From the very beginning, a complex gravitational field was expected here. This was, as it were, hinted at by the complex mountainous relief, the huge bowl of Baikal water, the "irrepressibility" of modern movements of the earth's crust, resulting both from high seismicity and from direct measurements by the repeated leveling method along the same profiles. So, it turned out that at present the Baikal depression continues to descend relative to neighboring ridges at a rate of up to 6 mm/year. The picture of gravitational anomalies was found to be really complex, and 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. The calculations made it possible to estimate the thickness of the Cenozoic sediments in the Baikal basin, 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!
Taking into account the role of water and precipitation in creating the negative anomalies of Baikal, scientists came to the conclusion that rocks of increased density should be located at a great depth below it, and on this basis it was suggested that the earth's crust under the Baikal depression is somewhat thinner than under neighboring ridges, and dense rocks of the upper mantle lie, respectively, 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, as it were, floats on the mantle, forming a kind of pinch or, as metallologists say, a "neck" under Baikal. This assumption has been generally confirmed by the latest data from deep seismic soundings.
In the Baikal rift zone, the magnetic field turned out to be relatively simple. On its general, close to normal background, a series of local elongated anomalies is distinguished. The sources of magnetic anomalies, as shown by calculations, 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 interior of the Irkutsk amphitheater.
Very important data was brought by magneto-telluric sounding in the Baikal region - one of the methods for studying the electrical conductivity of the bowels. The existence of a layer of increased conductivity in the mantle under the Baikal region was shown, the upper boundary of which is located at a depth of 40-50 km under the rift zone, and in the neighboring areas of the platform at a depth of about 100-120 km. As follows from the experiments on silicate rocks (they form the mantle), such an increase in electrical conductivity is achieved at a temperature of about 1200°C. Hence it follows that a layer of this temperature is also 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 outcrops of hot springs here, which all together directly indicate an increased heating of the bowels under the Baikal rift zone.
At the beginning of the book, we already pointed out that the deep heat flow at Baikal is noticeably increased. Special measurements have established that linearly elongated thermal anomalies in the Baikal basin do not cover its entire area, but are concentrated in narrow linear fault zones. The value of the specific heat flux in them is two to three times higher than the average for the continents and reaches 3 μcal cm 2 /s. So, this suggests that under the rift zone there is a powerful deep energy source, discovered in the last decade by seismic methods. Let's go back to it again.
The phenomenon of the anomalous mantle in the south of Eastern Siberia was discovered, or rather, it was suspected due to the systematic delay in the time of arrival of seismic waves excited by earthquakes at the seismic stations of the Baikal region. Readers here have the right to ask: what does the delay of seismic waves mean and does their “schedule” exist? 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 oscillations, 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 the recording station on the distance to the source. A huge number of observations of the velocities of seismic waves at various depths of the Earth during earthquakes around the world and knowledge of the 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 a particular point 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, indicates an anomaly in the medium in some of its volume. Restoring, for example, the graphic course of seismic waves, one can thus approximately imagine the shape and dimensions of the anomalous mantle. It is assumed that the decrease in the velocity of seismic waves is associated with the partial melting of the mantle matter through which the waves pass and, consequently, with a decrease in its average density. And if so, then the masses with reduced density should "float" up through the mantle with normal density. The law of Archimedes works. But the relatively light (less dense) mantle matter, rising up, cannot but carry a large supply of heat captured from great depths. Taking all these assumptions, which do not 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 base of the crust near Baikal, and in the southwest it sinks 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 of a powerful source of thermal energy in the deepest depths of this region of Asia. And since the beginning of the formation of the rift zone coincides with the end of the Paleogene or the beginning of the Neogene, then the beginning of the approach of the anomalous mantle to earth's crust can be dated in this area to about 25 Ma.
It is time to sum up the data presented in this essay and try to imagine how the Baikal rift zone was formed or could have been formed, and, following its model, other continental rift zones.
The starting point is the position that in the thickness of the planet, namely, at the border of the mantle and the earth's core, there is a certain separation of matter in density (reaching at these depths, as we remember, 5.9 g / cm 3) and a slow rise of less dense 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 low-density substance, consisting of a mixture of refractory peridotite and molten (smelted from peridotite) basalt, accumulate under the earth's crust and raise 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 matter accumulated under it. The process of uplift and mountain building with the continued inflow of relatively low-density mantle matter under the crust can continue only until isostatic equilibrium is reached, that is, until the moment when the weight of the dome uplift compensates for the buoyant force. But such an equilibrium “along the vertical” will not yet mean that complete mechanical equilibrium has come in the entire system and the process is over. The fact is that the matter 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 the mantle substance creates, due to viscous friction, tensile forces in the earth's crust under the dome uplift. To the tensile forces are added more forces directed along the slopes of the arched uplift - the crust, like any body on inclined plane, will tend to slide off the slopes of the mantle bulge. On the other hand, tension should lead to the opening of cracks in ancient faults in the Earth's crust and to the formation of new faults, and thus it becomes possible for the substance of the anomalous mantle to penetrate into the cracks of faults, its cooling, crystallization and transformation into ultramafic rocks that fill cracks. However, giving off heat 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 whole process proceeds in a broad front (numerous fault cracks open in the crust, and numerous mantle bodies intrude into them), then, in general, the earth's crust will stretch over the mantle ledge, and, consequently, will be driven away. The surface of the Earth above such a ledge will be a rift depression with all its attributes. The stated hypothesis (its main author is Professor Yu. A. Zorin), as we see, is an interpretation of the established facts within the framework of a general idea. It fits in and is justified by geological data (wide development of faults in the first place), and data on the outer 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 earthquake sources, and data on delay seismic waves under the earth's crust, observations of geophysical fields, in a word, all modern scientific material on the Baikal rift zone. On fig. Figure 7 shows the diagram of the structure of the Baikal rift graphically. In principle, it is suitable for explaining the origin of other continental rifts as well.
So, it is assumed that tensile forces act throughout the entire arch, but they deform the earth's crust where it is especially strongly weakened by cracks, heated by intrusions of the mantle substance. 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 be repeated. The long-term (millions of years) formation of the rift basin probably consists in the alternation of the phases of the appearance of open fractures and the phases of extension without ruptures after the mantle melt has been introduced into the fractures. All this, of course, is not easy, and if only because in the upper, less heated and, therefore, more brittle part of the crust, extension 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 deformable 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, earthquake sources in the Baikal and other rift zones, undoubtedly associated with crustal faults, lie mainly at shallow depths - up to 15-20 km.
There remains one more question. The domed uplift and the rift depression on it are, in a certain sense, opposite phenomena, acting, as it were, towards each other. But the spreading of the mantle substance to the sides under the dome rise should lead to its decrease, and then to destruction. In fact, rift depressions, both on land and in the ocean, are almost invariably associated with extensive arched uplifts. Such is the Baikal rift. Current geophysical measurements show that the ridges around the rift continue to rise and the troughs continue to subside. 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 the restoration of the height of the arched uplift.
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, successfully and fully resolved? Of course, this cannot be said, which, however, should in no way disappoint us. In fact, a drawn model of the Baikal rift can follow from the generalization of geological and geophysical extensive and diverse materials. During its construction, mainly physical data were used, and the process of formation of the arched uplift and the rift depression at its top was drawn only as mechanical deformations. But complex physicochemical processes take place in the earth's crust and upper mantle, the essence and results of which cannot be considered fully understood. After all, we are talking about the still inaccessible and opaque bowels of the planet, and no matter how diverse and sophisticated indirect methods of their knowledge, many difficulties are still far from being overcome.
The Baikal rift zone is still largely an unsolved mystery, and if, according to Tyutchev, it is actually very simple, then nature continues to hide this simplicity behind complex fences. And the temptation that Tyutchev wrote about is the desire to know the very simplicity, at least involuntarily in complex and difficult ways.
Abstract on the topic: Baikal as a rift zone
- Introduction
- Physical and geographical characteristics of the region
- Lake Baikal
- Rifts (Baikal Rift)
- Conclusion
- Bibliography
Introduction
Within the framework of this work, Lake Baikal, the main features of the Baikal rift zone are considered, the level of its knowledge is shown, and the most important issues for its further study are outlined. In the history of the study of both the Baikal mountain system and Lake Baikal itself, there were many researchers and scientists who held different positions in their views. The question of the origin of the basin of Lake Baikal (and of the entire region) was the essence of a long-term heated debate that began in the last century. For example, P. A. Kropotkin (1875) believed that the formation of a depression was associated with breaks in the earth's crust. ID Chersky, in turn, considered the genesis of Baikal as a trough of the earth's crust (in the Silurian). At present, the theory (hypothesis) of the “rift” has become widespread. According to this hypothesis, as a result of the compression of the earth's crust, a huge arch rise is formed, and tension, which subsequently replaces compression, causes subsidence of the upper part of the arch along the axis.
The hypothesis about the fault origin of depressions is confirmed by the presence of faults, mylonitization zones, thermal springs, etc. in the region.
This paper considers the main accepted hypothesis of the origin and genesis of Lake Baikal itself - its depression.
The Baikal depression belongs to a certain region called Baikal. Within the framework of this work, the name “Baikal region” is synonymous with the more accurate term (albeit more specialized) “Baikal mountain system”.
Baikal mountain system. Geographically, this is a fairly defined and independent region. It is limited: from the north and west - by the Central Siberian plateau, from the east - by the Aldan highlands and the Stanovoy ridge, from the southeast - by the mountains of the Dzhida country, Western and Eastern Transbaikalia. The area of the Baikal mountain system is 575 thousand km2. For clarity: the area of the Baikal mountain system is larger than the territory of France, the largest Western European state, whose area is 551 thousand km 2, and 14 times larger than the territory of Switzerland (area 41.3 thousand km 2). The term Baikal mountain system was introduced into the literature by E. V. Pavlovsky (1948), who devoted much effort and effort to the study of Eastern Siberia. In the present tense, this term is used very widely (and in this work too). The Baikal mountain system includes the following geographical regions: Western, Eastern, Southern Baikal, North Baikal Highlands, Patom Highlands, Vitim Plateau, Olekma-Vitim Highlands. In the aspect of this work, we will mainly consider the areas adjacent directly to the Baikal lake basin. Within this system, the following main geomorphological regions (the largest) are also distinguished - the Baikal mountain belt, the Vitim plateau, the Chai-Patom mountainous country. Also in this paper, characteristic geological terms will be presented, such as “Baikalides” - a special term denoting igneous geobodies formed during the evolution of the Baikal mountain system, or the Baikal rift zone.
Physical and geographical characteristics of the region
As already mentioned above, the boundaries of the region are determined by the Baikal mountain system. The territory of the region is characterized by a significant elevation above sea level and predominantly mountainous terrain. In terms of the section (through the entire region), there will be a general decrease from east to west. The lowest mark is the level of Lake Baikal (456 m), the highest is the peak of Munku-Sardyk (3491 m). Almost the entire territory is dominated by heavily dissected medium-altitude mountains (hills). Most of the ridges in the region have relatively soft outlines and flat tops leveled by long-term denudation processes. Flat surfaces are found only in tectonic depressions and valleys of large rivers.
On geological structure(especially in the region of the Baikal rift) a great influence was exerted by faults in the earth's crust, which have a predominantly northeasterly direction. The great modern tectonic activity of the Baikal mountain system attracts attention from the point of view of planetary activity. In general, the Baikal mountain system belongs to the young seismically active areas. Tectonic activity manifests itself in the form of slow subsidence and uplift of the shores of Lake Baikal (according to V.V. Lamakin, in some places such displacements reach 30 mm per year), as well as intense earthquakes reaching 8-10 points, for example, the largest earthquake (Saganskoye) from January 11-12, 1862, when part of the delta section of the river went under water. The Selenga area is about 260 km 2 with several villages.
In the area of the Baikal mountain system, permafrost of rocks is quite widely developed, with which many permafrost processes and phenomena are associated: thermokarst, heaving mounds, solifluction, icing, fissure-polygonal landforms, etc.
The climate is sharply continental (little snowy windless frosty winters, hot short summer), i.e. there are very large annual and daily fluctuations in air temperature and an uneven distribution of precipitation over the seasons of the year. In winter, the region is located in the powerful northeastern spur of the Siberian anticyclone; in summer, the polar air mass prevails here. Therefore, during the year there is a large number of sunny days(more than 310).
According to the type of climate, the animal is also developed, vegetable world region. The great diversity and special nature of the distribution of soil and vegetation cover, fauna is determined by the position of the region at the junction of two different natural areas- East Siberian mountain taiga and Central Asian steppe zones. There are also many unique endemic species confined mainly directly to Lake Baikal, the flora and fauna of which are three-quarters endemic, such as the Baikal seal, amphipods, gobies, viviparous golomyanka fish.
In the hydrological aspect, the territory of the region is one of the largest watershed nodes in Eurasia - part of the world watershed between the basins of the Arctic and Pacific Oceans passes here. Moreover, 84% of the runoff from the area belongs to the Northern Arctic Ocean, 0.3% - to drainless areas (for example, the Uldza-Torey Plain), and the remaining 15.7%, respectively, to Pacific Ocean. The upper reaches of many large Siberian rivers are concentrated here - the Lena, the Angara, the Lower and Podkamennaya Tunguska, the Amur, etc. Major transit rivers are only the Selenga (upper in Mongolia) and Argun (upper in China). In general, the Baikal region is much less water-bearing than, for example, neighboring Central Siberia. Although a reservation is possible here - in terms of per capita, the region is relatively well off water resources in relation to economic activity due to low population density (3 people per km 2). The average annual precipitation in Buryatia is 400 mm according to the measured and 525 mm according to the corrected data. The lowest amount of precipitation falls on low-lying areas. The nature of the distribution of precipitation over the territory under consideration is determined by the conditions of atmospheric circulation and the structure of the surface of the basin, i.e. as the level of the relief rises, water resources increase and thermal resources decrease. The Baikal region stands out as one of the richest regions in terms of chemistry and thermal mineral waters. The number of only recorded sources and wells is more than 600. This is due to numerous faults in the earth's crust during the Cenozoic.
There are a large number of lakes in the region, which are mostly confined to negative forms relief - to depressions. In general, there are several types of depressions. The main two types are intramountain (Baikal type) and intermountain (Transbakal type) (according to Florensov, 1960). They differ in the asymmetry of the sides, location, and the amount of accumulated deposits. In the aspect of this work, we are only interested in basin lakes of the Baikal type, the largest of which are Lake Baikal and Lake Khuvsugul in Mongolia (Khuvsuugul-Nuur), which belong to this type. The contours of the Baikal-type depressions on the map are viewed as angular, with many straight sections - short, straight faults. The basins of the Bakal type are filled with loose or weakly cemented deposits of predominantly Quaternary age, the accumulation of which occurred under conditions of continuous subsidence of the bottoms of the depressions (according to one of the most accepted theories). According to the same theory, it is believed that the Baikal lake bath consists of two independent depressions united by a water mirror, i.e. implies a complex structure. The southern basin of Baikal is filled with Middle Jurassic-Lower Cretaceous deposits, and Jurassic and Cretaceous strata are inherited from the Mesozoic (older) depressions. There is also a small amount of Tertiary (Neogene) deposits. Quaternary deposits throughout the Baikal Basin are represented by lacustrine, fluvioglacial, glacial, alluvial, proluvial, and eolian formations. The largest thickness of deposits is observed in the delta of the Selenga River (about 500 m), and together with tertiary deposits, more than 600 m.
Lake Baikal
Baikal is the deepest flowing lake with a unique bioregime. For comparison: the volume of Baikal exceeds the volume of the Baltic Sea. The area of the drainage area for Baikal is more than 588 thousand km2. Some geographic data on Baikal (morphometric indicators): the volume of water is 23 thousand km 3, the surface area (mirrors) is 31,500 thousand km 2, the average depth is about 730 m, the maximum depth is 1620 m, the greatest width is 95 km, the greatest length – 650 km.
Baikal has low water exchange. The complete change of the water volume is calculated in hundreds of years (more precisely, 332 years). This indicates a great conservation of properties. Baikal ranks first among freshwater lakes of a temperate thermal type.
The deep and surface regions of Baikal waters are distinguished. In the deep region, circulation processes are practically not involved; it can be argued that the waters of the deep region of Baikal do not participate in seasonal circulations. The thickness of the deep area of Baikal is about 1400 m. Invariably stable direct and reverse temperature stratification prevails in it with a very small overall temperature drop (from 3.6 0 С to 3.2 0 С), which again indicates a great conservatism. But according to the latest data, some dependence of the waters of the deep region is still observed - there are various transfers of water masses, which are both permanent (for example, the system of cyclonic currents of Southern Baikal) and temporary (for example, wind and runoff currents and masses) in nature. The movement of waters at different depths was also detected. These very slow circulations cover the waters of the deep region up to 1250 m. Currents are also observed under the ice. The nature of all these currents and phenomena has not yet been fully studied and clarified.
Upper area. Its thickness is 200-250 m. Most of the waters of the upper region participate in the annual cycle of heat exchange and autumn-spring circulations. The same layer is limited by seasonal changes in direct and reverse temperature stratification, as well as seasonal changes in chemical composition and biological factors. The main biomass of Lake Baikal is also concentrated here.
The dynamics and structure of the Baikal water mass is determined not only by the size, but also by the shape of the basin, which is basically tectonic (see above). The most characteristic feature of the Baikal bath should be considered the weak development of shallow water, which is associated with a large average depth of the lake and sharp sides. The uneven bottom relief is also characteristic, which, however, is still far from being fully studied. Up to a depth of 100-200 m, rocky soils, stones, boulders, pebbles, and sands predominate (moreover, the area of sands expands with depth). Then, to the greatest depths, the bottom is lined with silt, which contains many valves of diatoms.
Baikal is a powerful flow regulator and a giant natural reservoir. Nevertheless, changes in the ratios of balance elements cause fluctuations in the lake level. Intra-annual deviations average over a long period of about 82 cm, long-term (for the last 60 years, when observations began) - an amplitude of about 194 cm. In this regard, it is important to take into account the anthropogenic factor, such as the construction of the Irkutsk hydroelectric power station. Its construction caused an increase in the level by 1.2 m, which, of course, led to depressing consequences.
Apparently, the main role in the formation of the Baikal-type depressions is played by bending deformations of the earth's crust, accompanied by faults, and the movement of blocks along the faults acquires the main relief-forming significance. The presence of stepped terraces along the sides of the Baikal bath partly confirms this.
As mentioned above, the main theory for this moment considered the "rift" theory.
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). Under the concept of rifts, narrow relief forms are also suitable - furrows (“grabens”), not yet compensated by sediments and deposits; 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 (on a planetary scale) include rifts confined to continents (other than those mentioned above), for example, the Rhine graben (length about 600 km) or the region considered in the work - the Baikal rift zone (length more than 2.5 thousand km). 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 view (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 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 slanted at different angles, stepwise plunging towards the axial part. 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 opening 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, new formations of the earth's crust do not occur in the KZR. Possibly, this means that the modern KZR is only the first stage of the formation of the MSR and that in the epoch of the birth of, for example, 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 The African and Indian, connected along the Red Sea Rift zone, 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 carved 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).
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 decompressed 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.
Conclusion
So, the rift appearance of the entire Baikal zone, in the morphological and dynamic understanding of this term, is evidenced by: a dense network of longitudinal (mainly) faults that cut through the Precambrian crystalline substrate; the latest activity of these faults, coinciding in time with the period of fault activity in other rift systems the globe; steep (according to geological and seismological data) slopes of fault planes; the most correct assessment of the latter as normal discharges; directly documented, very young, Holocene and modern, vertical fault displacements, indicating the ongoing directed development of the zone; splitting of a single system of faults and depressions into branches, typical for the world's rift zones; depression morphology; uneven manifestations of trachybasaltic volcanism; the arched nature of the common surface, covering the entire set of ridges and subsidence; high level seismicity at normal depth of earthquake sources; finally, the abundance of outcrops of thermal deep waters, apparently associated with the high position of the high temperature layer.
The regional background, on which the Baikal rift zone is superimposed, has been generally studied much better than the zone itself, thanks to systematic geological mapping, and this circumstance determines the direction of future research. Since the rift zone itself is a Neogene-Quaternary formation, the issues of recent and modern tectonics, recent volcanism, seismotectonics, and structural geomorphology should take their rightful place along with geophysical studies of all kinds and special geodetic work (especially in the most seismically active areas of the rift zone). At the same time, even now, within the framework of these general directions, some special questions can be outlined, without answering which a comprehensive study of the Baikal rift zone cannot be significantly expanded. Such a first fundamental question is the history of the accumulation of volcanic-sedimentary formations within the rift zone. If all Neogene-Quaternary formations here should be classified as orogenic (Kheraskov, 1963), then among them it is already possible to distinguish pre-orogenic or early orogenic formations belonging to the interval: Upper Oligocene - Early Pliocene, and synorogenic, which, bearing in mind another generally accepted the specific meaning given to this term should be called riftogenic. The formation of the latter began in the Middle Pliocene and continues to this day. They are closely related to rift structures, being their “fillers” and being formed in direct spatial and genetic connection with rift-forming consedimentary faults.
All these formations, having a complex structure, poor in organic remains and accessible throughout the entire section only by drilling, are still barely affected by the study both in terms of stratigraphy, biostratigraphy, lithology and geochemistry, and in terms of their distribution in rift cavities, as well as in various developmental stages of the latter. They are almost unexplored in terms of relationships with the underlying substrate and with each other in various links of the rift chain; the nature of the inheritance by rift basins of accumulation basins of early orogenic formations is not clear, the reasons for the uneven distribution and combination of volcanic formations with sedimentary deposits are unknown; Finally, these formations have been studied very little in terms of general paleogeographic and paleogeomorphological reconstructions for the Neogene and Anthropogen. Of particular interest will be the formational analysis of the Neogene deposits, which, along with the rift basins proper, fill the troughs of the Vitim plateau and Cis-Baikal region, which remained unaffected by the rifting process. If the possibility of identifying a special type of riftogenic continental and marine formations is clear enough in itself, then their diagnostic signs and development in time, as well as the presence of their analogues in the pre-Tertiary history of the Earth, constitute a special problem.
The second is extremely important question- establishment of the actual distal and lateral boundaries of the Baikal rift zone. If the edges of the zone across its strike are more or less, although not quite exactly, defined (there are signs of their expansion in modern era), then the attenuation regions of the western and eastern ends of the zone are not clearly defined. It is possible that the western flank of the zone rests against the ancient Sangilen massif (southeastern Tuva and the adjacent Western Kosogolye) and that the Darkhat intermountain depression serves as the last outpost of the rift here. This is also likely because, to the west and directly to the south of the Sangilen massif, there is an area of depressions of the Central Asian type, which differs sharply from the rift type (Baikal) itself, is exceptionally seismically active and has a different mechanism of recent mountain building than the Baikal one, but with stress fields in the Earth’s crust, similar to those in the western flank of the Baikal zone.
Bibliography
- Academy of Sciences of the USSR. Institute of Geography of Siberia and Far East. natural conditions and natural resources of the USSR. Cisbaikalia and Transbaikalia. - M.: "Science", 1965.
- Academy of Sciences of the USSR. Siberian branch. Scientific Council on Tectonics of Siberia. Tectonics of Siberia. Volume VII. Tectonics of Transbaikalia and some general questions of geological structures. - M .: "Nauka", 1976.
- Salop L. I. Geology of the Baikal mountain system. Volume I. Stratigraphy. - M .: "Nedra", 1964.
- Salop L. I. Geology of the Baikal mountain system. Volume II. Magmatism, tectonics, history of geological development. - M .: "Nedra", 1967.
- Shagzhiev K. Sh., Raldin B. L. and others. Buryatia: Natural resources. - Ulan-Ude: Publishing House of the Buryat State University, 1997.
- Florensov N.A. Baikal Rift Zone and Some Problems of Its Study - In: Baikal Rift. M., "Nauka", 1968.
In several areas of the earth's surface, mid-ocean ridges come close to the margins of the continents. In some places they fade at the junction with the mainland margin, while in others they “break open” the margin of the mainland and even penetrate deep into it. Yes, branches. East Pacific Rise - Cocos Ridges And Carnegie, Chile Rise - do not show a clear continuation on the continent.
Gakkel Ridge - the northernmost link of the planetary system of mid-ocean ridges - loses its geomorphological severity as it approaches the underwater margin of Asia and is not morphologically traceable on the shelf. Attempts to trace the continuation of the rift zones of the mid-ocean ridges in Yakutia have not led to convincing results.
Articulation of the East Pacific Rise and the western margin of North America.Rift zone of the East Pacific Rise, according to American authors, continues in the western part of the USA and Canada. Narrow gulf of california graben considered as a large rift valley or rift zone. From the top of the bay to the north, the rift system branches out. One branch is widely known San Andreas fault system defines the tectonics and recent geological structure of coastal California. . The San Andreas fault zone proper (its northern segment: - San Benito Fault near Cape Mendocino again goes into the ocean. With its further oceanic continuation, the extreme links of the system of mid-ocean ridges are connected - underwater Gorda Ranges, Juan de Fuca, Explorer. Another branch is developed entirely within the mainland. She covers Rifts Utah and their further continuation - rift system of the Rocky Mountains, traced to the border of Alaska.
The development of faults associated with the rift zones of western North America proceeded more or less in accordance with the main strikes of the Mesozoic structures that form the main part of the mountain structures of this region of the North American continent. Rifting "updated" the ancient structures, emphasized their expressiveness in the relief, but did not cause any significant restructuring of the general structural plan of the territory.
Articulation of the Mid-Atlantic Ridge and Iceland.
Mid-Atlantic Ridge between the Kolbeinsey ranges And Reykjanes crosses Iceland. In the light of modern data, Iceland is a marginal continental mass, in the middle part significantly transformed by rifting. In the relief of the island, this zone is expressed in the form of a large tectonic depression, complicated by a series of rift gorges and mountain ridges separating them, ridges composed of lavas frozen during fissure eruptions, gaping tectonic cracks and large volcanoes (more than 20 active).
According to modern data, the section of the earth's crust in the Iceland region is similar to the section of the continental crust, but differs in a very thick "basalt" layer (seismic velocities 6.6 - 7.0 km/s), the presence of a layer of increased density (up to 7.5 km/s ), deep occurrence of the surface of Mohorovichich (up to 50 km) and a strongly reduced "granite" layer.
Aden Rift.
The junction of the system of mid-ocean ridges with the African-Arabian continental platform has been studied the most. Arabian Indian Range after crossing it owen fault zone experiences a strong shift to the north (approximately 250–300 km). West of the fault zone is traced Aden Rift. Morphologically it is expressed Gulf of Aden.
The bottom relief of the bay is strongly dissected. The shelf is practically non-existent, except for a very narrow coastal shoal along the mainly Arabian coast. The steep sides of the parting at a depth of 1000 - 2000 m are replaced by the bottom of the bay depression. Its relief is characterized by alternating rift valleys and northeast-trending ridges. The deepest depression is located at the entrance to the bay. This Alula-Fartak depression with a depth of 5360 m. The thickness of sediments in the depression is small, but in some places it reaches 500 m; on the surface, these are mainly foraminiferal silts. The crests of rift ridges are flattened and often do not have sediments. Basalts and diabases are exposed here.
The bottom of the bay is characterized by a high degree of seismicity. Especially many earthquake epicenters occur in rift valleys and their transverse faults. All earthquake sources are located at a depth of no more than 60 km. It was found that at a depth of 3–4 km the roof of the “basalt layer” occurs, which is underlain by the surface of Mohorovichich at a depth of 8–10 km. The upper part of the section, as was partly shown by subsequent deep-water drilling data, is expressed by sedimentary and second seismic layers. The absence of a "granite" layer in the section of the earth's crust of the Gulf of Aden is explained by the spreading of the continental masses of the Arabian Peninsula and Africa and the formation of a new oceanic crust during the formation of a juvenile and highly active mid-ocean ridge.
Red Sea Rift.
At the western end of the Gulf of Aden, a branching of the rift zone occurs. There is a vast volcanic area afar, contoured by a series of faults, having the form of a triangle filled with lava fields and strata of young Quaternary effusives. South of Afar extends Ethiopian rift - the northernmost link in the vast and complex system of the East African Rifts. The modern and Quaternary volcanism of East Africa is associated with this system, it includes the deepest rift lakes Tanganyika, Nyasa, Rudolf, Albert.
To the north-north-west of the Afar region stretches Red Sea Rift, expressed in relief as a depression of the Red Sea. In contrast to the Gulf of Aden, the Red Sea has a well-developed coastal shoal, which at a depth of 100–200 m gives way to a clearly defined ledge, morphologically similar to the continental slope ledge. Due to the numerous coral buildings, the coastal shallow has a dissected relief.
Most of the bottom of the Red Sea basin lies in the depth range from 500 to 2000 m. Numerous individual seamounts, islands and underwater ridges rise above the undulating bottom plain, in some places a series of steps parallel to the margins of the sea can be clearly seen. A narrow deep groove runs along the axis of the basin, which is considered as the median rift valley of the Red Sea. Its maximum depth is 3040 m. Powerful outcrops of juvenile waters with temperatures up to 56.5°C and salinity up to 257 ‰ have been discovered in several depressions in the valley. The bottom of the depressions is composed of cemented sediments with very high concentrations of various metals (copper, zinc, tin, silver, gold, iron, manganese, mercury).
The data of geophysical and geochemical studies indicate the absence of a "granite" layer within the axial furrow of the Red Sea. This, as well as the stepping of the bottom of the main basin of the Red Sea, is associated with the expansion of the rift and the "drift" of Arabia and the adjacent part of the African Platform. A granite layer was found on the shelf and on the steps of the bottom of the main basin closest to the mainland. Thus, the separation in the place of the Red Sea is much less than in the Gulf of Aden.
In the northern part of the Red Sea, the rift zone branches again, forming a short (up to 300 km) Suez Rift, corresponding to the bay of the same name, and Gulf of Aqaba Rift, which continues to the north in the form of a graben Dead Sea And Levantine rifts.
