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

Rifts during periods of their active development (rifting) are characterized by seismicity (with shallow earthquake sources) and high heat flow. During the development of rifts, thick strata or can accumulate in them, in which large oils, ores of various metals, etc. are enclosed. to the sides, and the overlying bark is a kind of vault-like bulging. Some researchers consider these processes to be the main reason for the formation of rifts, others believe that the local uplift of the upper mantle and crust only favors the emergence of a rift and predetermines its localization (or even is its consequence), while the main cause of rifting is regional (or even global?) extension. bark. Under especially strong horizontal extension, the ancient continental crust within the rift undergoes a complete rupture, and in this case, a new thin oceanic-type crust is formed between its separated blocks due to the mafic igneous material coming from the upper mantle. This process, inherent in the rifts of the oceans, is called spreading.

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

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

Intra-oceanic rifts (the so-called mid-ocean ridges) have oceanic-type crust both in their axial zones (modern spreading zones) and on their flanks (Fig. 5). Such rift ridges can arise either as a result of further development intercontinental rifts, or within older oceanic regions (for example, in pacific ocean). The scale of horizontal expansion in intraoceanic rifts is the largest (up to a few thousand km). These rifts are characterized by the presence of transverse ruptures (transform faults) crossing them, as if shifting the adjacent segments of these rift zones relative to each other in plan view. All modern intra-oceanic, intercontinental, and also a significant part of intra-continental rifts are directly interconnected on the surface of the Earth and form the rift world system.

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

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. This is how new riddle: 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, which did not exist in nature, but did not give rest to 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 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 local history museum), 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 different countries much began to be judged by European standards. 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 their model, prototype, was the Rhine graben - sinking along faults between the Black Forest and Vosges mountain ranges. They began to compare the Baikal depression with it. This was greatly facilitated by the authority of the largest explorer of Siberia, V. A. Obruchev, who believed that the “ancient crown” of Asia throughout its entire space was broken into separate blocks, partly lowered, partly raised, and against such a “structural background” the Baikal depression was only the largest and the youngest.

Further studies showed that the intermountain depressions of the Baikal region and Northern Mongolia form a 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, as early as the beginning of the 19th century, the noted external similarity of the depressions suggested the idea of ​​the geological relationship of all links in such a chain, of a 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 Baikal depressions and East Africa, proposed for all links of the Pribaikalsky system of inter-semitic subsidence the apt common name "baikal-type depressions".

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 quite deep wells. 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 was a real sensation and became a milestone in the development of the Earth sciences. The basis of the World Rift System is made up of mid-ocean ridges, connected with each other into a single grid, as if entangling the entire Earth. 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 basalt lava outpourings, strong seismicity, and the presence of fragments of ultramafic 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 emerges from 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 strata rocks. It was time to move on to the study of possible means and methods of deep bowels under the entire rift zone. The Institute of Geology and Geophysics of the Siberian Branch of the Academy of Sciences in Novosibirsk, other institutes of the Irkutsk scientific center, many Siberian production 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 basins) 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 extends, 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 joint 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. Width western slope 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 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 only very superficially (both in the direct and indirect sense) we can judge from the materials presented about the origin, causes and mechanism of formation of the Baikal rift zone. 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 “anomalous” mantle contour 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 outlets 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 along with the main work - the study of the seismicity of the Mongolian-Siberian mountain system as one of the important conditions, important components of the natural environment in which we live, work, build. The results of the 1950 Mondinsk, 3957 Muya, and 1959 Middle Baikal earthquakes, together with the observation of traces of ancient, prehistoric earthquakes expressed in relief and data from the current seismic service in Eastern Siberia and Mongolia, as well as historical information about the earthquakes that occurred here, are the most valuable material for the mapping of seismic zoning, a work of national importance 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 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. AT last years the entire BAM route, most of which runs in the rift zone, received a more accurate seismic hazard assessment.

Such scientific tasks as determining the depths of local earthquakes, focal mechanisms, the distribution and density of epicenters, the frequency of earthquakes in time - all this serves as scientific purposes and solving 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 anomalies 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 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 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 under the rift zone is located at a depth of 40-50 km, 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 - a hodograph, that is, a graph of the time of arrival of waves at a recording station versus 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 one or another 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, the beginning of the approach of the anomalous mantle to the Earth's crust can be dated in this region at about 25 million years.

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 recall, 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 material under the crust can continue only until isostatic equilibrium is reached, that is, until the moment when the weight of the arched 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. The tensile forces are supplemented by forces directed along the slopes of the arched uplift - the crust, like any body on an 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 ultrabasic 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 as 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 be stretched 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 substantiated 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.

Thus, it is assumed that tensile forces act throughout the entire dome uplift, 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 damp 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 riddle of the Baikal rift, and with it the riddle of the formation of other rift zones of the Earth, which have so many common features, has been successfully and completely solved? Of course, this cannot be said, which, however, should in no way disappoint us. In fact, 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 physical and chemical 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 the 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.

AT recent times a new form of existence of the earth's crust has been established - a system of rift zones developed both within the oceanic and continental crusts, as well as in their transitional parts and occupying an area equal to the continents only within the oceans. For rift zones, sometimes complex specific relationships between the mantle and the crust are revealed, which are often characterized by the absence of the Moho boundary, and the interpretation of their nature has not yet left the area of ​​discourse, including the issue of their typification. It. It should be borne in mind with regard to the distinguished types of rift systems in accordance with the data of M.I. Kuzmin, who in 1982 calculated natural geochemical standards for igneous rocks of these systems:

oceanic rift zones confined to mid-ocean ridges, which form a single system of oceanic uplifts up to 60 thousand km long, with the presence within them in most cases of narrow rift valleys 1-2 km deep (in the East Pacific uplift - the central horst uplift). The basic rocks are formed from primitive tholeiitic magma of shallow generation depths - 15-35 km;
Continental rift zones are grabens genetically associated with faults such as faults, being often confined to the axial parts of large arched uplifts, the thickness of the crust under which decreases to 30 km, and the underlying mantle is often loosened. In the rift valleys, tholeiitic basalts appear, and in the distance, rocks of the alkaline-basalt and bimodal series, as well as alkaline-ultrabasic rocks with carbonatites;

island arcs consisting of four elements: a deep trench, a sedimentary terrace, a volcanic arc, and a marginal sea. The thickness of the earth's crust is from 20 km or more, magma chambers at a depth of 50-60 km. There is a regular change of low-chromium-nickel tholeiite series to sodic calc-alkaline series, and shoshonitic series volcanics appear in the very rear of the island arcs; active continental margins of the Andean type, which characterize the "crawling" of the continental crust onto the oceanic, as well as island arcs, are accompanied by the Zavaritsky-Benioff seismic focal zone, but with the absence of marginal seas and the development of volcanism within the margin of the continent with an increase in the thickness of the earth's pores up to 60 km, and the lithosphere - up to 200-300 km. Magmatism is caused by both mantle and crustal sources, starting with the formation of rocks of calc-alkaline (rhyolite) series, changing to rocks of andesitic formation - latite series; 5) active continental margins of the Californian type, in contrast to island arcs and active continental margins of the Andean type, are not accompanied by a deep-water trench, but are characterized by the presence of compression and extension zones that arose as a result of the thrust of the North American continent on the entire system of the mid-ocean ridge. Therefore, there is a simultaneous manifestation of magmatism, which is characteristic of both rift structures (oceanic and continental types) and compression zones (deep seismic focal zones).

The petrogeochemical standards (types) of igneous rocks calculated by M. I. Kuzmin, which are typical for these zones, are of great scientific importance, regardless of the pleitectonic views of their author, including for typifying the nature of Precambrian magmatism. V. M. Kuzmin believes that the features of these geochemical types of igneous rocks are determined not by age, but by geodynamic conditions of formation, therefore, these types can be the basis for reconstruction in place of mobile belts of past active zones comparable to modern ones. An example of such reconstructions is the identification of the Mesozoic Mongol-Okhotsk belt with a rift system of Californian-type active margins. This idea, which denies the existence of geosynclinal systems at least in the Phanerozoic and extends the patterns of rifting rock formation to the distant past of the Earth, is opposed by the idea, also based on the study of the geochemical patterns of magmatism, that island arcs do not indicate the presence of a transitional type crust, and even more so rift structures, but are typical young geosynclines.

Along with the East African Rift, the Baikal Rift is another example of a divergent boundary located within the continental crust.

Gallery

Lake Baikal.JPG

The main lake of the rift - Baikal

KhovsgolNuur.jpg

Lake Khubsugul is also located in the area of ​​the Baikal Rift, at its southwestern tip.

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Notes

Literature

• Lyamkin V.F. Ecology and zoogeography of mammals in the intermountain basins of the Baikal Rift Zone / Ed. ed. d.b.n. A. S. Pleshanov; . - Irkutsk: Publishing House of the Institute of Geography SB RAS, 2002. - 133 p.

Links

• / V. E. Khain // Ankylosis - Bank. - M. : Great Russian Encyclopedia, 2005. - S. 662. - (Great Russian Encyclopedia: [in 35 volumes] / ch. ed. Yu. S. Osipov; 2004-, vol. 2). - ISBN 5-85270-330-3.

An excerpt characterizing the Baikal Rift Zone

Natasha quietly closed the door and went with Sonya to the window, not yet understanding what she was being told.
“Do you remember,” Sonya said with a frightened and solemn face, “remember when I looked for you in the mirror ... In Otradnoye, at Christmas time ... Do you remember what I saw? ..
- Yes Yes! - Natasha said, opening her eyes wide, vaguely remembering that then Sonya said something about Prince Andrei, whom she saw lying.
– Do you remember? Sonya continued. - I saw then and told everyone, both you and Dunyasha. I saw that he was lying on the bed,” she said, making a gesture with her hand with a raised finger at every detail, “and that he closed his eyes, and that he was covered with a pink blanket, and that he folded his hands,” Sonya said, making sure as she described the details she saw now, that these same details she saw then. Then she saw nothing, but said that she saw what came to her mind; but what she thought up then seemed to her just as real as any other memory. What she then said, that he looked back at her and smiled and was covered with something red, she not only remembered, but was firmly convinced that even then she had said and seen that he was covered with a pink, precisely pink blanket, and that his eyes were closed.
“Yes, yes, exactly pink,” said Natasha, who also now seemed to remember what was said in pink, and in this very she saw the main extraordinary and mysteriousness of the prediction.
“But what does that mean? Natasha said thoughtfully.
“Ah, I don’t know how extraordinary all this is! Sonya said, clutching her head.
A few minutes later, Prince Andrei called, and Natasha went in to him; and Sonya, experiencing a feeling of excitement and tenderness rarely experienced by her, remained at the window, pondering the whole unusualness of what had happened.
On this day there was an opportunity to send letters to the army, and the countess wrote a letter to her son.
“Sonya,” said the countess, looking up from her letter as her niece passed her. - Sonya, will you write to Nikolenka? said the countess in a low, trembling voice, and in the look of her tired eyes, peering through glasses, Sonya read everything that the countess meant by these words. This look expressed both prayer, and fear of refusal, and shame at what had to be asked, and readiness for irreconcilable hatred in case of refusal.
Sonya went up to the countess and, kneeling down, kissed her hand.
“I will write, maman,” she said.
Sonya was softened, agitated and touched by everything that happened that day, especially by the mysterious performance of divination that she just saw. Now that she knew that on the occasion of the resumption of relations between Natasha and Prince Andrei, Nikolai could not marry Princess Marya, she gladly felt the return of that mood of self-sacrifice in which she loved and used to live. And with tears in her eyes and with joy in the consciousness of committing a generous deed, she, interrupted several times by tears that clouded her velvety black eyes, wrote that touching letter, the receipt of which so struck Nikolai.

In the guardhouse, where Pierre was taken, the officer and soldiers who took him treated him with hostility, but at the same time respectfully. There was also a sense of doubt in their attitude towards him about who he was (isn't he a very important person), and hostility due to their still fresh personal struggle with him.
But when, on the morning of the next day, the shift came, Pierre felt that for the new guard - for officers and soldiers - he no longer had the meaning that he had for those who took him. And indeed, in this big, fat man in a peasant's caftan, the guards of the other day no longer saw that living person who fought so desperately with the marauder and the escort soldiers and uttered a solemn phrase about saving the child, but they saw only the seventeenth of those held for some reason, according to the order of the higher authorities, taken by the Russians. If there was anything special in Pierre, it was only his untimid, concentrated, thoughtful look and French, in which, surprisingly for the French, he spoke well. Despite the fact that on the same day Pierre was connected with other suspects taken, since the officer needed a separate room that he occupied.
All the Russians kept with Pierre were people of the lowest rank. And all of them, recognizing the gentleman in Pierre, shunned him, especially since he spoke French. Pierre sadly heard ridicule over himself.
The next day, in the evening, Pierre learned that all these detainees (and, probably, including himself) were to be tried for arson. On the third day, Pierre was taken with others to a house where a French general with a white mustache, two colonels and other Frenchmen with scarves on their hands were sitting. Pierre, along with others, was asked questions about who he is with that allegedly exceeding human weaknesses, accuracy and determination with which defendants are usually treated. where was he? for what purpose? etc.
These questions, leaving aside the essence of life's work and excluding the possibility of disclosing this essence, like all questions asked at the courts, aimed only at substituting the groove along which the judges wanted the defendant's answers to flow and lead him to the desired goal, that is, to the accusation. As soon as he began to say something that did not satisfy the purpose of the accusation, they accepted the groove, and the water could flow wherever it wanted. In addition, Pierre experienced the same thing that the defendant experiences in all courts: bewilderment, why did they ask him all these questions. He felt that it was only out of condescension or, as it were, courtesy that this trick of the substituted groove was used. He knew that he was in the power of these people, that only power had brought him here, that only power gave them the right to demand answers to questions, that the only purpose of this meeting was to accuse him. And therefore, since there was power and there was a desire to accuse, there was no need for the trick of questions and trial. It was obvious that all answers had to lead to guilt. When asked what he was doing when they took him, Pierre answered with some tragedy that he was carrying a child to his parents, qu "il avait sauve des flammes [whom he saved from the flame]. - Why did he fight with a marauder? Pierre answered, that he defended a woman, that the protection of an offended woman is the duty of every man, that... He was stopped: it did not go to the point. Why was he in the yard of the house on fire, where witnesses saw him? He answered that he was going to see what was being done in Moscow. They stopped him again: they did not ask him where he was going, but why he was near the fire? Who is he? They repeated the first question to which he said that he did not want to answer. Again he answered that he could not say this .

Rice. 5.1. Global system of modern continental and oceanic rifts, main subduction and collision zones, passive (within plate) continental margins.
Rift zones: Mid-Atlantic (SA), American-Antarctic (Am-A), African-Antarctic (Af-A), Southwestern Indian Ocean (SWZI), Arabian-Indian (A-I), East African (VA) ), Red Sea (Kr), Southeast Indian Ocean (SVI), Australo-Antarctic (Av-A), South Pacific (UT), East Pacific (BT), West Chilean (34), Galapagos (G), California (Cl), Rio Grande - Basins and Ranges (BH), Gorda Juan de Fuca (HF), Nansen-Gakkel (NG, see Fig. 5.3), Momskaya (M), Baikal (B), Rhine (P). Subduction zones: 1 - Tonga-Kermadek; 2 - Novogebridskaya; 3 - Solomon; 4 - New British; 5 - Sunda; 6 - Manila; 7 - Philippine; 8 - Ryukyu; 9 - Mariana; 10 - Izu-Boninskaya; 11 - Japanese; 12 - Kuril-Kamchatskaya; 13 - Aleutian:, 14 - Cascade Mountains; 15 - Central American; 16 - Lesser Antilles; 17 - Andean; 18 - South Antilles (Scotia); 19 - Eolian (Calabrian); 20 - Aegean (Cretan); 21 - Mekran.
a - oceanic rifts (spreading zones) and transform faults; b - continental rifts; c - subduction zones: island-arc and marginal continental double line); d - collision zones; e - passive continental margins; e - transform continental margins (including passive ones); g - vectors of relative movements of lithospheric plates, according to J. Minster, T. Jordan (1978) and K. Chase (1978), with additions; in spreading zones - up to 15-18 cm/year in each direction, in subduction zones - up to 12 cm/year

Rice. 5.2. The geometric correctness of the placement of the global system of modern rifts relative to the axis of rotation of the Earth, according to E.E. Milanovsky, A.M. Nikishin (1988):
1 - Cenozoic rifting axes, mostly active; 2 - oceanic lithosphere of Cenozoic age; 3 - the same, Mesozoic age; 4 - areas with continental lithosphere; 5 - convergent borders
Rice. 5.3. The southeastern end of the Nansen-Gakkel oceanic rift zone and seismically active faults continuing it, separating the Eurasian and North American lithospheric plates. According to L.M. Parfenov et al. (1988). Below - focal mechanisms of seismic sources at this active boundary, according to D. Cook et al. (1986):
1 - spreading zones (NG - Nansen-Gakkel zone); 2 - deep-sea trenches (subduction zones); 3 - transform faults; 4 - reverse faults; 5 - faults and shifts; 6 - zones of scattered rifting; 7 - movement of lithospheric plates and microplates; 8 - focal mechanisms of seismic sources; 9 - land within the Eurasian (a) and North American (b) plates. Lithospheric plates and microplates: EA - Eurasian; SA - North American; T - Pacific; ZB - Transbaikal; Am - Amur; Oh - Sea of ​​Okhotsk

Modern tectonic activity is distributed extremely unevenly and is concentrated mainly on the boundaries of lithospheric plates. The two main types of these boundaries (see Chap. 3.1 also correspond to the main geodynamic settings. Rifting develops on divergent boundaries, to which this chapter is devoted, but here we will consider the activity of transform boundaries, since they are associated primarily with the rift zones of the oceans. plates is expressed by subduction, obduction and collision (see Chap. 6).Information about relatively weak, but important in their geological consequences, within-plate tectonic processes will be given in Chapter 7.

term rift valley(eng., rift - crevice) J. Gregory at the end of the last century identified the grabens of East Africa limited by faults, which are formed under conditions of extension. Subsequently, B. Willis contrasted them with ramps - grabens, sandwiched between oncoming reverse faults. The concept, which at first had mainly a structural content, later, especially in recent decades, was enriched with ideas about the geological conditions and probable deep mechanisms of the formation of these linear extension zones, about characteristic igneous and sedimentary formations and, thus, was filled with genetic content. A modern understanding of rifting was taking shape, which a quarter of a century ago was included in the concept of plate tectonics as one of its most important elements. It turned out that most of the rift zones (in their new, broad sense) are located in the oceans, but there rifts, as structures controlled by faults, are of subordinate importance, and the main way to implement tensile stresses is pulling apart.

5.1. Global system of rift zones

Most of today's rift zones are interconnected, forming a global system stretching across continents and oceans (Figure 5.1). The realization of the unity of this system, which engulfed the entire globe, prompted researchers to look for planetary-scale mechanisms of tectogenesis and contributed to the birth of a “new global tectonics”, as the concept of lithospheric plate tectonics was called in the late 60s.

In the system of the Earth's rift zones, most of it (about 60 thousand km) is located in the oceans, where it is expressed by mid-ocean ridges (see Fig. 5.1), their list is given in Ch. 10. These ridges continue one another, and in several places are interconnected by “triple junctions”: at the junction of the West Chilean and Galapagos ridges with the East Pacific, in the south Atlantic Ocean and in the central part of the Indian. Crossing the boundary with passive continental margins, oceanic rifts continue to be continental. Such a transition is traced south of the triple junction of the Aden and Red Sea oceanic rifts with the Afar Valley rift: along it, from north to south, the oceanic crust wedges out and the continental East African zone begins. In the Arctic Basin, the oceanic Gakkel Ridge continues with continental rifts on the shelf of the Laptev Sea, and then with a complex neotectonic zone, including the Momsky rift (see Fig. 5.3).

Where mid-ocean ridges approach an active continental margin, they can be absorbed into a subduction zone. So, at the Andean margin, the Galapagos and West Chilean ridges end. Other relationships are demonstrated by the East Pacific Rise, above which the Rio Grande continental rift formed on the overthrust of the North American Plate. In a similar way oceanic structures of the Gulf of California (representing, apparently, an offshoot of the main rift zone) are continued by the continental system of Basins and Ranges.

The dying off of rift zones along strike has the character of gradual attenuation or is associated with a transform fault, as, for example, at the end of the Juan de Fuca and American-Antarctic ridges. For the Red Sea rift, the Levantian shift serves as the end.

Covering almost the entire planet, the system of Cenozoic rift zones exhibits geometric regularity and is oriented in a certain way relative to the axis of rotation of the geoid (Fig. 5.2). Rift zones form an almost complete ring around South Pole at latitudes of 40-60 ° and depart from this ring meridionally with an interval of about 90 ° by three belts fading to the north: East Pacific, Atlantic and Indian Ocean. As shown by E.E. Milanovsky and A.M. Nikishin (1988), perhaps, with some conventionality, the fourth, Western Pacific belt, which can be traced as a set of back-arc manifestations of rifting, is marked in the corresponding place. The normal development of the rift belt here was suppressed by intense western displacement and subduction of the Pacific Plate.

Under all four belts, to depths of a few hundreds of kilometers, tomography reveals negative velocity anomalies and increased attenuation of seismic waves, which is explained by the upward current of the heated mantle matter (see Fig. 2.1). The regularity in the placement of rift zones is combined with a global asymmetry both between the polar regions and relative to the Pacific hemisphere.

The orientation of the stretch vectors in the rift zones is also regular; The latter are maximum in the equatorial regions, decreasing along the ridges both in the northern and southern directions.

Outside the global system, there are only a few of the large rifts. This is the system Western Europe(including the Rhine graben), as well as the Baikal (Fig. 5.3) and Fengwei (Shanxi) systems, confined to northeast-trending faults, the activity of which is believed to be supported by the collision of the continental plates of Eurasia and Hindustan.