Amazingly perfect creation - man! He can not only see, hear, feel what is next to him or around him, but also mentally imagine what he has never seen. Can dream, can imagine. Let's imagine the oceans and seas ... without water, and for this we look at the physical and geographical map of the ocean floor. We will see that at the bottom along the margins of the oceans there are long and very deep slit-like depressions. These are deep sea trenches. Their length reaches thousands of kilometers, and the bottom is three to six kilometers deeper than the bottom of the adjacent parts of the ocean.

Deep-sea trenches are not found everywhere. They are distributed near the mountainous edges of the continents or along island arcs. Many of you probably know the Kuril-Kamchatka, Philippine, Peruvian, Chilean and other trenches in the Pacific Ocean, the Puerto Rican and South Sandwich trenches in the Atlantic. Deep-sea trenches border the Pacific Ocean on many sides. But they are few in the Indian Ocean. They are almost completely absent along the periphery of the Atlantic Ocean and are completely absent in the Arctic basin. What's the matter here?

Trench - the deepest depressions on our planet. They are located most often near the high mountain ranges of the land. So mountain ranges on land or along the margins of the oceans and deep-sea trenches are actually adjacent to each other. We remind the reader that the highest point on Earth ( Mount Everest or Chomolungma) has a height of 8844 meters ( according to some sources 8882 meters), and the bottom of the deepest Mariana Trench is at a depth of 11022 meters. The difference is 19866 meters! Such an almost twenty-kilometer span has an oscillation of the surface of our planet.

However, Chomolungma is several thousand kilometers away from the Mariana Trench. But at Mount Lullaillaco ( 6723 meters) in the Cordillera and the adjacent Chilean Trench ( 8069 meters) the difference is 14792 meters. This is perhaps the most sharp contrast heights and depths on earth

With geological development, the mountains rise - the gutters deepen, the mountains collapse - the gutters are filled with sediment. Thus, mountain ranges and deep-sea trenches represent a single system. This is " Siamese twins» in geology.

But the nature of the formation of these geological twins is a mystery of mysteries. To this day, scientists cannot find a single answer to it. It was assumed that in the places of the trenches the earth's crust sags under the influence of some unknown forces. Then scientists began to believe that the gutters were formed at the site of deep cracks. Subsequently, scientists learned that trenches form where two lithospheric plates move against each other. Faced, one of them "wins" - it crawls onto the other. But they continue their movement even after the collision, and at a fairly fast, from a geological point of view, speed - about 5 - 10 centimeters per year. Such a fast movement does not allow the edges of the plates to crumple into folds. Therefore, one of the plates must give way to the other. The “winner” in the struggle between these two geological giants is the continental plate: it “crawls” onto the thinner oceanic crust, crushing it under itself. The "defeated" oceanic plate goes into the softened and strongly heated mantle - into the asthenosphere. There it is strongly heated and again passes into a semi-molten substance - magma. According to the calculations of the Soviet scientist O. G. Sorokhotin, about 50 billion tons of the substance of the oceanic crust is submerged in the trenches under the continental plates per year. Consequently, the subsoil “devours” and melts down almost the same amount per year. oceanic crust how much it grows in rift valleys mid-ocean ridges.

The area where one plate is pushed under another is called a subduction zone. The oceanic plate there is strongly bent down. In the place of such a bend, deep and narrow depressions are formed - deep-sea trenches.

Many of you, dear readers, while studying geographical maps, have noticed that island arcs and deep-sea trenches on the maps have a horseshoe shape. You will ask why? Imagine that you are cutting an apple with a knife. They made a small incision and ... stop! Take out the knife. Look at the notch at the top. It has the shape of a semicircle. The earth is round. Plates also have the shape of hemispheres. When one plate rises to another, the place of their collision and subsidence occurs along a plane directed, like the plane of a knife when cutting an apple, not perpendicular to the surface of the sphere ( Earth), but at some angle. This causes the formation of grooves in the form of an arc. This form of them is very clearly visible if you look at the Kuril-Kamchatka region and the Aleutian Islands.

Coming over the oceanic plate continental crust cracks in places of subsidence. A semi-molten substance - magma - rises into cracks from the bowels of the Earth under the influence of a huge compression force. Numerous volcanoes and volcanic mountains form along the edges of the cracked continental plate, often lining up in a long chain. This is how individual mountains or island arcs and mountain ranges with numerous active and extinct volcanoes are formed. Such are the Aleutian, Kuril, Lesser Antilles and other islands, mountain ranges - the Cordillera and others. Such mountain ranges and island arcs with volcanoes surrounding the oceans are called the “ring of fire”.

As is known, trenches mark zones of convergent margins of lithospheric plates on the ocean floor, i.e., they are a morphological expression of the subduction zone of the oceanic crust. The vast majority of deep-sea trenches are located on the periphery of the giant Pacific ring. It is enough to look at Fig. 1.16 to see this. According to A.P. Lisitsyn, the area of ​​the trenches is only 1.1% of the ocean area. Ho, despite this, they together form an independent giant belt of avalanche sedimentation. The average depth of the trenches exceeds 6000 m, which is much greater than the average depth of the Pacific (4280 m), Atlantic (3940 m) and Indian (3960 m) oceans. In total, 34 deep-sea trenches have now been identified in the World Ocean, of which 24 correspond to convergent plate boundaries, and 10 to transform ones (Romansh, Vima, Argo, Celeste, etc. trenches). In the Atlantic Ocean, the Puerto Rico (depth 8742 m) and South Sandwich (8246 m) trenches are known, in the Indian Ocean - only the Sunda (7209 m). We'll look at the Pacific Trench.
On the western edge Pacific Ocean the troughs are closely associated with volcanic arcs, forming a single geodynamic arc-trough system, while the troughs of the eastern margin are directly adjacent to the continental slope of South and North America. Volcanism is recorded here along the Pacific margin of these continents. E. Zeybold and V. Berger note that out of 800 active volcanoes active today, 600 fall on the Pacific ring. In addition, the depth of the trenches in the east of the Pacific Ocean is less than in the west. The trenches of the Pacific Rim, starting off the coast of Alaska, form an almost continuous chain of strongly elongated depressions, stretching mainly in the south and southeast directions to the islands of New Zealand (Fig. 1.16).

In table. 1.5 we have tried to bring together all the main characteristics of the morphography of the trenches of the Pacific Ocean (depth, extent and area, and the numbers of deep-sea drilling stations are also indicated there). Table data. 1.5 convince of the unique characteristics of deep sea trenches. Indeed, the ratio of the average depth of the trench to its length reaches 1:70 (Central American Trench), the length of many trenches exceeds 2000 km, and the Peru-Chile Trench is traced along the western coast South America almost 6000 km. The data on the depth of the gutters are also striking. Three trenches have depths from 5000 to 7000, thirteen - from 7000 to 10,000 m and four - over 10,000 m (Kermadek, Mariana, Tonga and Philippine), and the depth record belongs to the Mariana Trench - 11,022 m (Table 1.5).
Here, however, it should be noted that the depth of the depth - strife. Such significant depths are fixed by oceanologists, for them the depth of the gutter is the bottom mark, counted from water surface ocean. Geologists are interested in a different depth - without taking into account the thickness sea ​​water. Then the depth of the trough should be taken as the difference between the elevations of the base of the oceanic trough and the bottom of the trough itself. In this case, the depths of the trenches will not exceed 2000-3500 m and will be comparable to the heights of the mid-ocean ridges. This fact, in all likelihood, is not accidental and indicates the energy balance (on average) of the spreading and subduction processes.

Gutters also share some common geophysical characteristics; reduced heat flow, a sharp violation of isostasy, slight anomalies of the magnetic field, increased seismic activity, and, finally, the most important geophysical feature - the presence of the Wadati - Zavaritsky - Benioff seismic focal zone (WZB zone), plunging in the region of the trench under the continent. It can be traced to a depth of 700 km. It is with it that all earthquakes recorded on island arcs and active continental margins adjacent to the trenches are associated.
And yet, it is not so much the morphometric characteristics of deep-sea trenches that are unique, but their location in the Pacific Ocean: they seem to trace the places of convergence (convergence) of lithospheric plates on the active margins of the continents. Here, the destruction of the oceanic crust and the growth of the continental crust take place. This process is called subduction. Its mechanism has so far been studied in the most general terms, which will give some right to opponents of plate tectonics to classify subduction as unprovable, purely hypothetical assumptions put forward supposedly in favor of the postulate of the constancy of the Earth's surface area.
Indeed, the subduction models developed to date cannot satisfy specialists, since the number of questions that arise significantly exceeds the capabilities of existing models so far. And the main of these questions concern the behavior of sediments in deep-sea trenches, which morphologically trace the places of convergence of plates. The fact is that opponents of subduction use the nature of the sedimentary filling of trenches as one of the essential arguments against the subduction of the oceanic plate under the continent. They believe that the calm, horizontal occurrence of sediments in the axial parts of all trenches is not consistent with the high-energy process of underthrusting a multi-kilometer oceanic plate. True, the drilling work carried out in the Aleutian, Japanese, Mariana, Central American, Peru-Chile trenches (see Table 1.5) removed a number of questions, but new facts appeared that do not fit into existing models and require evidence-based explanation.
Therefore, we made an attempt to construct a sedimentologically consistent model of subduction, which provided answers to questions related to the sedimentary filling of the trenches. Of course, the sedimentological argumentation of subduction cannot be the main one, but none of the tectono-geophysical models of this process can do without it. By the way, let us note that the main purpose of all subduction models developed to date, both taking into account the sedimentary filling of trenches and neglecting it, is to explain this process in such a way that the model captures the main known characteristics of plate movement and the rheological properties of the lithosphere substance and at the same time time, its resulting (output) indicators did not contradict the morphography of the trenches and the main tectonic elements of their structure.
It is clear that, depending on what goal the researcher sets for himself, he fixes certain characteristics in the model and uses the appropriate mathematical apparatus. Therefore, each of the models (there are now more than 10 of them) reflects only one or two key aspects process of uplift and leaves dissatisfied those researchers who interpret the qualitative side of this phenomenon differently. Proceeding from this, it seems to us that it is most important to understand precisely the qualitative characteristics of subduction, so that all the observed consequences of this process become physically explainable. Then the construction of a formalized model on a quantitative basis will become a technical matter, i.e., it should not cause fundamental difficulties.
All currently known subduction models can be classified as shown in Fig. 1.17. The greatest contribution to the development of these models was made by L.I. Lobkovsky, O. . Sorokhtin, S.A. Ushakov, A.I. Shsmenda and other Russian scientists, and from foreign experts - J. Bodine (J.N. Bodine), D. Cowan (D.S. Cowan), J. Dubois (J. Dubois), G. Hall (G. A. Hall), J. Helwig (J. Helwig), G. M. Jones, D. E. Karig, L. D. Kulm, W. D. Pennington, D. W. Scholl ), W. J. Schwelier, G. F. Sharman, R. M. Siling, T. Tharp, A. Watts , F.By (F. T. Wu) and others. Of course, we are primarily interested in TS models in which one way or another the sedimentation of the trenches is taken into account. These include the so-called "accretion model" and a model in which precipitation plays the role of a kind of "lubrication" between two interacting plates.

These models, which explain the reaction of sediments to the high-energy process of underthrusting of the oceanic plate, although they give a completely plausible interpretation of this process, still ignore a number of important questions that must be answered in order for the proposed tectono-geophysical models to be considered sedimentologically consistent. The most important of them are the following.
1. How can one explain the fact that sediments in the trough itself always have a horizontal undisturbed occurrence, despite the fact that the plate is actively sinking from the ocean side, and a strongly deformed accretionary prism builds up from the continental slope of the trough?
2. What is the mechanism for the formation of an accretionary prism? Is it the result of a chaotic dumping of sediments torn from a subducting plate, or is its growth influenced by processes occurring on the continental slope itself?
To answer these questions, i.e., to construct a sedimentologically consistent model of subduction, it is necessary to more closely link the proposed tectonic mechanisms of this process with the data of deep-water drilling along profiles through a number of trenches most studied from these positions. This must also be done so that the control of the proposed model by the data of "live" lithology becomes an integral element of the model.
Let us begin the presentation of a sedimentologically consistent model of subduction with a description of the tectonic premises underlying it. It should be noted that any model includes specific assumptions, it relies on them and with their help tries to link them into a single whole. known facts. Our model uses tectonic prerequisites drawn from subduction schemes that have already been tested by physically substantiated calculations.
The first assumption concerns the impulsive (discrete) nature of the underthrust process. It means that the next phase of underthrust is preceded by the accumulation of stresses in the oceanic crust, which, due to the tectonic stratification of the lithosphere and inhomogeneities earth's crust are transmitted from spreading centers with varying intensity and, in any case, are extremely unevenly distributed in the ocean. Guess it has enough deep meaning, since it can be used to explain the change in the petrological properties of the already submerged part of the oceanic plate, which partially predetermines the possibility of the next subduction pulse.
The second assumption assumes multidirectional distribution of stresses directly in the Wadati-Zavaritsky-Benioff (WZB) zone. It appears like this. Experiencing compressive forces at deeper horizons, the zone at the inflection point, which marks the deep-water trench, is subjected to tensile stresses, which leads to the formation of faults both on the inner and outer sides of the trench. These faults separate the submerging parts of the plate into separate segments from the ocean side. (steps); at the next thrust pulse, the segment closest to the axis of the chute is involved in this process. This idea was constructively tested by L.I. Lobkovsky in his kinematic scheme of subduction.
The third assumption refers to discrete oceanward migration of the trough centerline. It is a consequence of the first two assumptions. Special studies have also established that the rate of migration of the axis of the trench depends on the age of the absorbed crust and the slope of the WZB zone.
The fourth assumption assumes an energy balance in time between the processes of accretion of oceanic crust in mid-ocean ridges and its processing at active margins. What this assumption not without reason, indirectly controlled by the equality (on average) of the heights of the mid-ocean ridge and the depths of the trenches corresponding to specific spreading vectors, which we have already noted. As noted by T. Hatherton, the possible balance between spreading and subduction processes provided a reliable physical basis for plate tectonics. Violation of this balance at certain moments leads to an increase in arch rises, a restructuring of the global system of oceanic water circulation and, as a consequence, to global breaks in sedimentation.
If we are looking for the reason for the differences in the depths of the trenches, we must take into account the close correlation between the subduction rate and the age of the absorbed crust (for a fixed value of the tilt angle of the TZB zone). This issue was studied in detail by S. Grillet and J. Dubois on the material of ten convergent systems (Tonga-Kermadek, Kuril, Philippine, Izu-Bonin, New Hebrides, Peru-Chile, Aleutian, Central American, Indonesian and Japanese). In particular, these authors found that the higher the subduction rate, the smaller (on average) the depth of the trough. But the depth of the trench increases with the age of the subducting plate. M.I. Streltsov successfully supplemented this study by establishing that the depth of the trough also depends on the curvature of the volcanic arc: the deepest troughs are associated with arcs of maximum curvature.
Let us now consider in more detail the mechanism of sedimentogenesis in troughs, i.e., let us construct a general sedimentological model of the trough. An analysis of the sections of deep-water drilling wells, on the one hand, and the nature of the tectonic structure of the trenches, on the other hand, allow us to draw the following fairly reliable conclusions.
1. The sedimentary cover is significantly different on the inner (continental) and outer (oceanic) slopes of the trench, and although the tectonic structure of these elements of the structure of the trench is also heterogeneous, the composition of sediments is primarily a function of the actual sedimentological processes on different slopes of the trench: pelagic sedimentogenesis on the outer slope and supssion-flow, superimposed on the pelagic - on the inner.
2. At the base of the inner slope of the trench, accumulation of sediments is often recorded; here they are always more intensely compacted and structurally represent a large lenticular body called an accretionary prism. On the outer slope, the sediments are inclined at a slight angle to the axis of the trough, while on the bottom they lie horizontally.
3. According to geophysics, the sediments at the bottom of the trenches occur in the form of two “layers”: an acoustically transparent lower layer, interpreted as compacted pelagic deposits of the oceanic plate, and an upper one, represented by turbidites that were carried into the trench from the side of the continental slope in the period between two adjacent pushing impulses.
4. The thickness of turbidite deposits at the bottom of the trenches depends on many factors: on the ruggedness of the relief of the continental slope and climate, as if predetermining the rate of denudation of the adjacent land, on the intensity and frequency of earthquakes in the area of ​​the trench, and on many other factors. The duration of plate interaction, i.e., the time of existence of a specific subduction zone, should also play a significant role in increasing the thickness of the turbidite sequence at the bottom of the trench, but only if the trench, as a tectonic structure, had independent meaning in the process of subduction; but since it is only a reaction to this process expressed in the topography of the ocean floor, and besides, its position is not constant in time, this factor does not play a decisive role in the process of accumulation of turbidites at the bottom of the trench. We know that the current position of the trenches marks only the last phase of a long-term underthrust process.
5. Four major facies complexes of sediments are closely associated with deep-sea trenches: fans of the continental slope, turbidites of the bottom and basins on the inner slope, pelagic deposits recorded within all morphological elements of the trench, and, finally, sediments of the accretionary prism.
At present, the sedimentological models of the Aleutian, Peruvian-Chilean and especially the Central American trenches have been developed in sufficient detail. However, these models, unfortunately, are not related to the general mechanism of subduction in these trenches.
M. Underwood and D. Karig, as well as F. Shepard and E. Reimnitz, who studied in detail the morphology of the inner slope of the Central American Trench in the region of the continental margin of Mexico, note that only in this area four large canyons adjoin the inner slope of the trench, of which the most the Rio Balsas (an underwater continuation of the Balsas River) was thoroughly investigated, traced to the very gutter. A clear correlation has been established between the thicknesses of turbidites at the bottom of the trench and at the mouths of large canyons. The thickest sediment cover (up to 1000 m) in the trench is confined to the mouth of the canyons, while in other parts of it, their thickness decreases to several meters. At the mouth of canyons, a sediment fan is always fixed; it is indented by numerous channels - a kind of distribution system of the alluvial cone. The clastic material entering through the canyons is carried by the longitudinal current along the axial line of the trench in the direction of the bottom subsidence. The influence of each canyon on the distribution of precipitation in the central part of the trench is felt even at a distance of 200-300 km from the mouth. Data from deep-water drilling in the Central American Trench confirmed that in different parts of it, the reaction of sediments to the underthrust process is not the same. Thus, in the area of ​​the Guatemalan drilling profile, subduction is not accompanied by accretion of sediments, while wells in the area of ​​the Mexican profile, on the contrary, revealed the presence of an accretionary sedimentary prism at the base of the continental wall of the trench.
Let us now dwell in detail on the main sedimentological paradox of subduction. As is now firmly established by geophysical work and deep-sea drilling, sediments at the bottom of all trenches are represented by turbidites of different lithological composition, which have a horizontal occurrence. The paradox is that these sediments must either be torn off the oceanic plate and accumulate at the base of the continental slope in the form of an accretionary prism (accretionary subduction models), or be absorbed together with a fragment of the oceanic plate in the next phase of underthrust, as follows from the “lubrication model » O.G. Sorokhtin and L.I. Lobkovsky.
The logic of opponents of subduction is therefore simple and fair: since subduction is a high-energy process involving rigid plates tens of kilometers thick, then a thin layer of loose sediments cannot but react to this process. If the sediments at the bottom of the trenches lie horizontally, then subduction does not take place. It must be admitted that earlier attempts to explain this sedimentological paradox were unconvincing. The horizontal occurrence of sediments was explained by their youth, periodic shaking of already accumulated turbidites, after which they were deposited, as it were, anew, etc. There were, of course, more realistic interpretations that considered the dependence of the volume of sediments in the trenches on the ratio of sedimentation and subduction rates.
O.G. Sorokhtin made a simple, but, unfortunately, unconvincing calculation of this process, trying to bring the actual base under his lubrication model, analyzed above. He noted that in most trenches the thickness of the sedimentary cover is insignificant, despite the very high rate of sediment accumulation (several centimeters per 100 years). At such a speed, according to O. G. Sorokhtin, if the “lubrication” mechanism had not worked, the troughs would have been completely covered with sediments in a few tens of millions of years. In reality, this does not happen, although some trenches exist and continue to develop for hundreds of millions of years (Japanese, Peruvian-Chilean).
This calculation is unconvincing for two reasons. First, regardless of the sediment absorption mechanism, the troughs are the most important component of the dynamic system of the subduction zone, and for this reason alone it was impossible to calculate the rate of their filling with sediments as if it were a fixed settling tank. Secondly, the trenches in their modern morphological expression fix only the reaction to the last phase of the underthrust process (see the third assumption of our model), and therefore the time of their existence cannot be identified with the duration of the development of the entire subduction zone, i. especially hundreds of millions of years as the age of the gutter is not necessary. For the same reasons, the similar approach to this problem presented in the article by J. Helwig and G. Hall cannot be considered convincing.
Thus, this paradox cannot be resolved if we rely on already developed subduction schemes, in which the mechanism and velocity characteristics of plate underthrust are not linked to the mechanism and velocity characteristics of sediment accumulation.
Information on the rates of sedimentation in the trenches of the Pacific Ocean, which were estimated from the results of deep-sea drilling, is contained in a multi-volume publication, the materials of which allow us to conclude that, in general, the trenches are indeed characterized by relatively high rates of sediment accumulation: from a few tens to hundreds and even thousands of meters per a million years. These speeds, of course, vary in time even at one drilling point, but in general the order of numbers is preserved.
Let us, however, pay attention to one circumstance that apparently escaped the attention of geologists. The fact is that geologists are used to estimating the rate of precipitation accumulation in Bubnov's units: millimeters in 10w3 (mm/10w3) or meters in 10w6 (m/10w6) years. This approach is caused objective reasons, because geologists have reliable information only about the thickness of the section and much less reliable data about the duration of the corresponding stratigraphic interval. They, of course, represent that the velocity values ​​obtained in this way have a very remote relation precisely to the rate of accumulation of sediments, since they do not take into account either the fact that different lithological types of rocks are formed at different rates, or the fact that within the studied interval of the section can be hidden breaks in the accumulation of precipitation (diastema). If, moreover, we take into account that the sediments of the axial part of the trenches are formed in the injective regime of cyclosedimentogenesis, then in this case this approach to assessing the rate of sediment accumulation cannot be used at all, because, strictly speaking, the entire sequence of turbidites is formed as a superposition of suspension-flow sedimentogenesis on normal pelagic sedimentation: in other words, the thickness of turbidites accumulates, as it were, in a sedimentation pause. Based on numerous factual materials on modern and ancient turbidites, such a mechanism of sedimentogenesis is substantiated in the author's monographs.
When work on plate tectonics appeared and geophysicists published the first data on spreading and subduction rates (measured in centimeters per year), geologists, trying to correlate the known values ​​​​of sedimentation rates with the newly obtained information about the rates of plate movement, still operated with changes in speed in units Bubnov, without making attempts to bring the compared values ​​to a common denominator. It is easy to understand that such an approach gives rise to a number of misunderstandings that hinder the study of the actual role of sedimentological processes in different subduction models and lead to an incorrect assessment of their significance. Let us cite several typical examples to illustrate this point, without repeating the description of the lithological composition of sediments recovered by deep-sea drilling.
The bottom sediments of the Aleutian Trench are of Holocene age, their thickness reaches 2000 and sometimes 3000 m. The rate of subduction of the Pacific Plate under the Aleutian Trench, according to K. Le Pichon et al., is 4-5 cm/year, and according to V. Wakye - even 7 cm / year.
The rate of sedimentation in the trench, if measured in units of Bubnov, is interpreted as anomalously high (“avalanche”, according to A.P. Lisitsyn): 2000-3000 m / 10 in 6 years. If the sedimentation rate is expressed in the same units as the subduction rate, then we get 0.2-0.35 cm/year, and for interglacial periods it is even an order of magnitude lower: 0.02-0.035 cm/year. And yet, the rate of precipitation accumulation in the Aleutian trench (in whatever units we measure them) is very high, R. von Huene rightly notes that the trenches western outskirts The Pacific Ocean, which is characterized by a sedimentary cover of the bottom with a thickness of more than 500 m, was undoubtedly in the zone of influence of high-latitude glaciation of the coasts. Deltas also have a significant impact. major rivers flowing into the ocean in the gutter area.
Thus, what is considered by lithologists as the “avalanche” rate of sedimentation turns out to be almost two orders of magnitude lower than the plate underthrust rates. If these data are correct and if they are correlated with the model of monotonous (frontal) subduction, then it becomes clear that with such an interpretation of the underthrust mechanism, sediments simply would not have time to accumulate, and at least the axial part of the trench should have been completely free of sedimentary cover. Meanwhile, its thickness in the northeastern part of the Aleutian Trench reaches, as we have already noted, 3000 m.
Well 436 was drilled on the outer slope of the Japan Trench. From the borehole section, we will be interested only in a 20-m-thick clay unit recovered at a depth of 360 m. Their age is estimated at 40–50 Ma (from the Middle Miocene to the beginning of the Paleogene). It is easy to calculate that the rate of formation of these deposits was negligible: 0.44 m/106 years (0.000044 cm/year, or 0.5 microns/year). To visualize this figure, it is enough to say that in an ordinary city apartment in winter months(when the windows are closed) such a layer of dust accumulates in a week. It is now clear how clean the deep-water zones of the oceans are from clastic suspensions and how enormous the creative role of geological time is, capable of fixing a 20-m-thick clay thickness in the section after 45 million years at such vanishingly low rates of sedimentation.
Equally low sedimentation rates were noted on the oceanic slope of the Kuril-Kamchatka Trench (well 303), where they range from 0.5 to 16 m/106 years, i.e., from 0.00005 to 0.0016 cm/year. The same order of numbers is preserved for other trenches of the Pacific Rim. An increase in the rate of sediment accumulation on the inner slopes of the trenches up to a few hundred meters per million years, as is easy to understand, does not change the ratio of two velocity characteristics: sediment accumulation and oceanic plate underthrust. In this case, too, they differ by at least two orders of magnitude (the lowest values ​​of the subduction rate, from 4 to 6 cm/year, were noted for the Japanese, Kermadek, Aleutian, and New Hebrides trenches, and the largest, from 7 to 10 cm/year, for the Kuril-Kamchatka , New Guinea, Tonga, Peru-Chile and Central America.In addition, it was found that the rate of convergence of the northern and eastern margins of the Pacific Ocean increased from 10 (from 140 to 80 million years ago) to 15-20 cm / year (between 80 and 45 million years ago), then dropped to 5 cm/year The same trend was noted for the western Pacific Rim.
It might seem that there is a correlation between the lifetime of the subduction zone and the thickness of the sedimentary cover at the bottom of the trenches. However factual material refutes this assumption. Thus, the time of functioning of the New Hebrides subduction zone is only 3 Ma, and the thickness of sediments in the trench is 600 m. . Therefore, it is necessary to look for a new effective mechanism that would link these (and many other) characteristics.
So far, one thing is clear: sediments in the trench can only persist if the sedimentation rate is significantly higher than the subduction rate. In the situation that geologists tried to comprehend, the ratio of these quantities was estimated as directly opposite. This is the essence of the “sedimentological paradox of subduction”.
There is only one way to resolve this paradox: when assessing sedimentation rates, one should not abstract from the genetic type of deposits, because, we repeat, not for all strata, the usual arithmetic procedure used to calculate the sedimentation rate is applicable: the ratio of the thickness of the stratum (in meters) to the stratigraphic volume of time (in million years). Moreover, the author has repeatedly noted that this procedure is completely inapplicable to turbidites, since it will give not just an approximate, but an absolutely incorrect estimate of the rate of precipitation accumulation. Consequently, in order for sediments to be preserved in the axial part of the trenches and, moreover, to have a horizontal occurrence, despite the subduction of the oceanic plate, it is necessary and sufficient that the sedimentation rate be significantly higher than the subduction rate, and this can only be when sedimentation in the trench is realized in the injective mode of cyclosdimentogenesis. A consequence of this peculiar sedimentological theorem is the exceptional youth of the bottom sediments of all deep-sea trenches, the age of which usually does not exceed the Pleistocene. The same mechanism makes it possible to explain the presence of highly carbonate sediments at depths that obviously exceed the critical one for the dissolution of carbonate material.
Before understanding the second of the questions we posed (about the violation of the normal stratigraphic sequence of sediments at the base of the continental slope of the trench), it is necessary to note the following circumstance, which, probably, was thought of by many who tried to analyze the mechanism of subduction. Indeed, if the process of underthrust (from the point of view of kinematics) proceeds similarly in all trenches and if it is accompanied by scraping of sediments from the subducting plate, then accretionary prisms should be fixed at the foot of the inner slopes of all trenches without exception. However, deep sea drilling has not established the presence of such prisms in all trenches. Trying to explain this fact, the French scientist J. Aubouin suggested that there are two types of active margins: margins with a predominance of compressive stresses and active accretion; complete absence sediment accretion. These are the two extreme poles, between which practically all currently known convergent systems can be placed, if we take into account such important characteristics as the tilt angle of the TWZ zone, the age of the oceanic crust, the subduction rate, and the thickness of sediments on the oceanic plate. J. Aubouin believes that the arc-gutter systems are closer to the first type, and the Andean type of margin is closer to the second. However, we repeat, this is nothing more than a rough approximation, because real situations in specific underthrust zones depend on many factors, and therefore a wide variety of relationships can occur in the systems of both the western and eastern margins of the Pacific ring. So, V.E. Hine, even before J. Aubouin singled out these two extreme cases, rightly noted that the Aleutian, Nankai and Sunda profiles only partially confirmed the accretion model, while the profiles through the Mariana and Central American (in the region of Guatemala) trough did not reveal an accretion prism. What conclusions follow from this?
Most likely, sediment prisms (where they undoubtedly exist) are not always the result of only the scraping of sediments from the oceanic plate, especially since the composition of the sediments of these prisms does not correspond to the sediments of the open ocean. In addition, the undoubted absence of such prisms (for example, in the Central American Trench) gives reason not to consider the scraping of sediments as a sedimentologically universal process for subduction, which explicitly follows from the “lubrication model” of O.G. Sorokhtin and L.I. Lobkovsky. In other words, in addition to the accretion of sediments, some more general sedimentological process must manifest itself in convergent systems, leading to the formation of a prism of sediments at the base of the continental slope of the trench.
We have already pointed out that the sediments at the base of the continental slope of the trenches are strongly compacted, folded into a complex system of folds, the age sequence of layers is often disturbed in them, and all these sediments have a clearly turbidite genesis. It is these facts that require a convincing explanation in the first place. In addition, within the accretionary prism (where its presence has been undoubtedly proven), a rejuvenation of sediments down the section towards the trough has been established. This indicates not only that each subsequent plate of sediments torn off from the oceanic plate seems to slip under the previous one, but also about the peculiar kinematics of the underthrust process, according to which the next subduction impulse is accompanied by migration of the axis of the trench towards the ocean with simultaneous expansion of the shelf zone of the continental slope and deflection of its base, which makes it possible in general to realize this mechanism. With more detailed study structures of accretionary prisms (Japanese and Central American trenches), it also turned out that the regularities of the change in the age of individual plates are more complex: in particular, two or three times the appearance of coeval packs among sediments, both younger and older, was established. This fact can no longer be explained by the mechanism of pure accretion. Probably, the leading role here is played by the processes leading to the displacement of partially lithified masses of sediments, which take place directly within the continental slope of the trench. It should also be taken into account that the very mechanism of sediment compaction within the accretionary prism also has its own specifics, which consists, in particular, in the fact that the stress stresses that accompany the subduction process lead to a sharp reduction in the pore space and the squeezing of fluids into the upper sediment horizons, where they serve as a source of carbonate cement. There is a kind of stratification of the prism into differently compacted rock packs, which further contributes to the deformation of rocks into folds, dissected into layers with shale cleavage. A similar phenomenon took place in the Kodiak Formation of Late Cretaceous, Paleocene, and Eocene turbidites exposed in the hall. Alaska between the Aleutian Trench and an active volcanic arc on the Alaska Peninsula. A.P. Lisitsyn notes that the accretionary prism in the area of ​​the Aleutian Trench is broken by faults into separate blocks, and the movement of these blocks corresponds (in the first approximation) to the irregularities of the underlying crust, they seem to “track” all the large irregularities in the topography of the surface of the oceanic plate.
The accretionary prism in the region of the Antilles island arc (Barbados) has been most thoroughly studied, to which two special cruises of the R/V Glomar Challenger (No. 78-A) and Joides Resolution (No. 11) were devoted. The East Caribbean active margin here is expressed by the following structures: o. Barbados, interpreted as a fore-arc ridge, > Tobago depression (inter-arc) > St. Vincent (active volcanic arc) > Grenada depression (rear-arc, marginal) > Mt. Aves (dead volcanic arc). Here, thick sedimentary accumulations of the Orinoco PKV and partially displaced sediments from the Amazon mouth are close to the subduction zone. Deep water wells 670-676 (cruise No. 110) near the front of active deformations confirmed the presence of a powerful accretionary prism here, consisting of overthrust basins of Neogene deep-sea sediments plucked from the weakly deformed Campanian-Oligocene oceanic complex. The shear zone is composed of Upper Oligocene-Lower Miocene mudstones and is inclined to the west. Directly above the shear zone, a series of steeper scaly overthrusts was exposed. The total thickness of the section exposed by drilling is from 310 to 691 m. Siliceous mudstones of the Lower-Middle Eocene occur at its base. Above - clayey sediments, calcareous turbidites, cross-bedded glauconite sandstones of the Middle-Upper Eocene, thin-layered argillites and carbonate rocks of the Oligocene, siliceous radiolarian mudstones, calcareous mudstones and biogenic carbonate sediments of the Lower Miocene-Pleistocene. A characteristic phenomenon here is the lateral migration of fluids both in the body of the accretionary prism (chlorides) and from the oceanic side of the deformation front (methane). We also emphasize that at several levels, a repetition in the section of lithologically the same type and coeval rock units was revealed.
In addition to what is already known about the tectonic structure of the trenches, let us take revenge: within the underwater submerged terrace in the middle part of the inner slope of the Japanese and other trenches, active tectonic processes occurred, indicating, on the one hand, significant horizontal displacements of blocks, and on the other hand, about active vertical movements, which led to a relatively rapid change in the bathymetric conditions of sedimentation. A similar phenomenon was also established in the Peru-Chile Trench, where the rates of vertical block displacements reach 14-22 cm/year.
Detailed geophysical studies of the Japan Trench have shown that its inner and outer sides are a complex system of blocks in contact along faults. These blocks experience shifts of various amplitudes. In this case, the sequence of fault formation, the behavior of crustal blocks at different stages of underthrust, and, most importantly (for our purpose), the reflection of all these processes in the sedimentary cover of a deep-water trench are essential. The position of the Japanese geophysicists Ts. Shiki and 10. Misawa, who believe that since the concept of subduction is basically “extensive and global in nature”, in a model of this scale “sediments and sedimentary bodies can be ignored”, seems extreme.
On the contrary, it is only through the features of the mechanism of filling the basins on the slopes of the trenches and the trenches themselves with sediments that one can understand the subtle details of subduction, which otherwise will simply be overlooked by researchers. Figuratively speaking, precipitation makes it possible to make a cast from the gutter and thereby not only understand its details internal structure, but also more reasonably restore the processes that led to its formation.
The mechanism of accumulation of sediments at the base of the continental slope seems to be as follows. In the initial phase of subduction - when a deep-water trench is formed as a result of the collision of continental and oceanic plates - a break in the continuity of the crust occurs at the base of the continental slope (Fig. 1.18, a); along the fault, the crust sags in the direction of the gutter axis and sediments from the upper step (terrace) slide down (Fig. 1.18, b). At the lower step, stratigraphically inversion occurrence of bed packs (I, 2, 1, 2) will be recorded. In the phase of relatively calm underthrust, when the stresses arising in the subduction zone do not exceed the tensile strength continental lithosphere, sediments accumulate on the inner slope of the trench: from coastal-marine to deep-sea (Fig. 1.18, 6, units 3 and 4), and in the basin on the lower terrace - turbidites.

Then, with a new active impulse of subduction, the axis of the trench shifts towards the ocean and a new fault is formed at the base of the inner slope, along which sediments from the upper terrace slide down (Fig. 1.18, c), and part of the coastal-marine shallow-water accumulations end up on the second terrace. A new portion of still insufficiently compacted sediments slides into the base of the inner slope of the trench, which, in the process of moving down along the uneven relief of the slope, accumulate, crumple into folds, etc. There is another build-up of the prism at the base of the continental slope.
Most trenches on the continental slope have three morphologically pronounced steps - terraces. Consequently, if our scheme is correct, then during the existence of the subduction zone, at least three major structural rearrangements occurred, accompanied by the advancement of the trench towards the ocean and the formation of faults on its inner slope. The final phase of this process is shown in fig. 1.18, d: sediment prism at the base of the continental slope is formed. In it three times (according to this simplified scheme) the stratigraphic sequence of layers is violated.
This process takes place in one way or another, the main thing is that in those cases when it was possible to drill out the base of the continental slope (the Japanese and Central American trenches), it really turned out that the normal stratigraphic sequence of rocks was disturbed here; they are compacted to a much greater extent than the synchronous deposits of the outer slope, and, most importantly, these deposits do not in any way resemble the pelagic sediments of the oceanic slope of the trench. Significant vertical movements also become explainable, as a result of which obviously shallow-water deposits are buried at depths of several thousand meters.
Before proceeding to the model substantiation of the indicator series of sedimentary formations of deep-water trenches, it is necessary to pay attention to one important circumstance that was not previously taken into account by geologists. Meanwhile, it obviously follows from those tectono-geophysical prerequisites of subduction, which are the fundamental characteristics of this process and which we have taken as the basis of our sedimentologically consistent model of subduction. This refers to the fact that modern deep-sea trenches are not sedimentary (accumulative) basins in the strict sense of the word, but represent only a reaction of the earth's crust to the subduction process, morphologically expressed in the topography of the ocean floor. We already know that the subduction of the oceanic crust under the continent is marked by a seismic focal zone, at the inflection point of which the deep-water trench is located; that subduction itself is an impulsive process, and each successive impulse of subduction corresponds to an abrupt migration of the trough axis towards the ocean; that sediments in the trench have time to accumulate only due to the fact that the rate of deposition of turbidites significantly exceeds the rate of subsidence of the oceanic plate, but their main mass goes together with the subducted plate into deeper horizons of the lithosphere or is torn off by a protrusion of the continental plate and is loaded into the base of the continental slope of the trench. It is these circumstances that explain the fact that, despite the long (tens of millions of years) existence of most subduction zones, the age of the sedimentary filling of the bottom of the trenches does not exceed the Pleistocene. Modern trenches, therefore, do not record all stages of subduction in the sedimentary record and, therefore, from the standpoint of sedimentology, they cannot be considered as sedimentary basins. If they are nevertheless considered as such, then the gutters are very peculiar pools: pools with a "leaky" bottom. And only when the subduction process stops, the seismic focal zone is blocked by a continent or microcontinent, the position of the deep-water trench becomes stable, and it begins to be filled by sedimentary complexes as a full-fledged sedimentary basin. It is this phase of its existence that is preserved in the geological record, and it is precisely the series of sedimentary formations formed during this period that can be considered as indicative of deep-sea trenches of subduction zones.
Let's move on to its description. Let us note right away that we are talking about the tectonic-sedimentological substantiation of the classical series of finely rhythmic terrigenous formations: slate formation > flysch > marine molasse. This series (following M. Bertrand) was empirically substantiated by N. B. Vassoevich on the material of the Cretaceous-Paleogene flysch of the Caucasus, by the way, making a noteworthy conclusion: since in this series the deposits of the lower (marine) molasse are the youngest (in a continuous section), then the modern epoch is predominantly the epoch of molass accumulation; a new stage in the formation of flysch has not yet begun, and the old one has long ended. This conclusion turned out to be incorrect.
B.M. Keller confirmed the established N.B. Vassoevich's successive change of sedimentary formations of the flysch series on the material of the Devonian and Carboniferous sections of the Zilair synclinorium to Southern Urals. According to B.M. Keller, in this synclinorium, a siliceous formation was successively formed, slate, which is an alternation of greywacke sandstones and shales with a rudimentary flysch-type cyclicity (sections in the Sakmara river basin), and, finally, deposits of marine molasses. The same regularity was revealed by I.V. Khvorov. In the Eastern Sikhote-Alin, the Lower Cretaceous (Hautherivian-Albeckian) flysch strata are crowned with coarse flysch and marine molasse. In the Anui-Chuy synclinorium Gorny Altai green-violet slate and flyschoid (graywacke-shale) formations are replaced by black shale (slate), followed by sub-flish sequence, then (higher in the section) - lower molasse. This sequence is crowned by sedimentary-volcanogenic deposits of the continental molasse. M.G. Leonov established that older flysch complexes in the Caucasus have been mapped onto the marine molasse of the Late Eocene. In the late Eocene, the Transcaucasian massif slowly migrated northward, as a result of which more and more coarse-grained sediments were recorded in the section, and turbidites became more and more sandy. The same phenomenon, only slightly shifted in time, is observed in the Austrian and Swiss Alps, as well as on the Apennine Peninsula. In particular, the Upper Cretaceous Antola Formation developed in the Northern Apennines is interpreted as a turbidite sequence of facies of a deep-water trench. It shows a distinct coarsening of sediments up the section.
A distinct roughening of turbidite complexes upward along the section is noted in the Dalnsgorsky ore region (Primorye). It is naturally accompanied by a gradual "shallowing" of faunal complexes. A.M. Perestoronin, who studied these deposits, notes that a feature of the section of allochthonous plates is the gradual change (from bottom to top) of deep-water chertous deposits with radiolarians, first silty, and then shallow-water sandstones with Bsrrias-Valanginian flora. A similar trend in the replacement of turbidite complexes has been established in the Zal. Cumberland on about. St. George. It is composed of Late Jurassic - Early Cretaceous turbidites with a total thickness of about 8 km. The lithofacies specificity of this formation is that, up the section, coarsening of the clastic material is recorded within the limits of single cycles and an increase in the thickness of the cycles themselves. The series of flysch > marine molasse > continental molasse of interest to us is also distinguished in the Western Carpathian basin of the Oligocene-Miocene age. In the Western Urals, the Upper Paleozoic flysch complex is divided into three formations successively replacing each other in the section: flysch (C2) > lower molasse (C3-P1) > upper molasse (P2-T). Moreover, finely rhythmic distal turbidites are developed in the lower part of the section.
Thus, the empirically established pattern of successive appearance in the section of more and more coarse-grained differences in the flysch series requires lithogeodynamic substantiation. The model we propose is based on the following assumptions.
1. Of all the variety of modern settings for turbidite accumulation, the geodynamic settings of the marginal parts (and junctions) of lithospheric plates turn out to be geologically significant (the deposits of these zones are stably preserved in the geological record). This is the continental foot of the passive margins of the continents, as well as deep-sea trenches of the active margins. Here the mechanism of avalanche sedimentation is realized. From the point of view of geodynamics, the active margin corresponds to the setting of subduction of the oceanic crust.
2. Sedimentological control of subduction, discussed in detail in previous works of the author, guarantees that the main genetic type of sediments that fill the bottoms of the trenches and terrace basins on their continental slope are turbidites.
3. In all likelihood, successively changing strata, similar in lithological composition and structure of elementary sedimentation cycles, fix not different, although dependent on each other, sedimentation processes, but long stages of development single process cyclogenesis, which is implemented in an injective mode, but due to changes in the depths of the basin and the intensity of the removal of clastic material at different stages of development, it fixes cycles in sections that differ in thickness and grain size of deposits.
4. Installed by N.B. Vassoevich's empirical series does not necessarily have to be as fully expressed as possible. For example, the Triassic-Yurskian slate strata of the Taurian Series of the Crimea, the Upper Cretaceous flysch of the Central and Northwestern Caucasus etc.
The essence of the lithogeodynamic model proposed by us is clearly illustrated in Fig. 1.19, and the vast literature that characterizes the conditions for the generation, movement and discharge of density (turbidity) flows, as well as the composition and structure of the turbidite bodies formed by them, gives the right not to dwell on these issues in detail.

In subduction zones, the absorption of an oceanic plate is always accompanied by an increase in compressive stresses and leads to an increased heating of the rear parts of these zones, due to which the isostatic rise of the continental margin with a strongly dissected mountainous relief occurs. Moreover, if the process of subduction of the oceanic plate itself occurs impulsively and the next subduction impulse is accompanied by migration of the trough axis towards the ocean, then together with the cessation of subduction, the deep-sea trough is also fixed in its final position, and the decrease in compression stresses and the isostatic floating of the rear parts of the subduction zones also occur in waves - from the continent to the ocean. If we now compare these data with the fact that the structure (morphology) of the adjacent land remains practically unchanged, only the length of the route of movement of density flows and the slope of the bottom of the supply canyons change (the length is maximum, and the slope of the bottom, on the contrary, is minimal in the ascent phase I, and in the final phase III, the ratio of these values ​​changes to the opposite), then the sedimentological aspect of the problem becomes clear: with the continuous development of this process, deposits of finely rhythmic distal turbidites (slate formation) should pass into proximal sandy turbidites (flysch and its various structural and lithological modifications), and ts, in turn, are replaced by cycles of coarser-grained proximal turbidites and fluxoturbidites, better known in our domestic literature like cycles of sea molasses.
It should be noted, by the way, that in the Caucasus this undulating process is recorded not only in a directed change along the section of lithologically different types of flysch, but also in the successive rejuvenation of the tectonic-sedimentary structures that host them. Thus, pre-Late Cretaceous folds are distinctly transformed in the Lok-Karabagh zone, and folds laid down in the Early Pyrenean and younger phases are distinctly transformed in the Adjaro-Trialeti zone. In the area of ​​the Gruzinskaya Block, the folds are even younger. Post-Paleogene are structural transformations deposits in the region of Western Abkhazia and in the North-Western Caucasus.
If we analyze the material on the Caucasian turbidite complexes in more detail, we will inevitably come to the conclusion that the entire lateral series of tectonic units from the edge of the Lesser Caucasian oceanic basin to the North Caucasian plate fits well into the concept of a complex continental margin, which, starting from the Bajocian, showed signs of active subduction mode. At the same time, the axis of active volcanism gradually shifted to the north.
The turbidite complexes formed here must also react to the migration of the axis of the subduction zone. In other words, in subduction paleozones, a lateral row of turbidite formations “adhered” to the continent should be recorded, the age of which becomes older in a direction towards the initiation of the subduction zone. So, in the river basin. Arak (southeastern part of the Lesser Caucasus), turbidite complexes become older from west to east. At the same time, the depth of turbidite accumulation decreases in the same direction. If along the banks of the Hrazdan and Azat rivers the Upper Eocene deposits are represented by moderately deep-water turbidites, then to the east (rivers Apna, Nakhichevanchay, Vorotan, etc.) they are replaced by shallow-water sediments.
It can be concluded that the change of formations in the series slate formation > flysch > molasse fixes not different regimes of cyclogenesis, but only the changes in lithogeodynamic conditions in the source of clastic material that we described, superimposed on the continuous process of sedimentogenesis in the deep-water trench. The deposits of the molasse formation thus complete the complete sedimentological evolution of the trenches.
Interestingly, in the process of deep-water drilling, data were obtained that actually confirm the mechanism of filling the trenches with clastic sediments, which coarsen up the section. Well 298 was drilled in the Nankai trough, which is part of the subduction zone, and within which the Philippine plate is slowly subducting under the Asian one. The well passed 525 m of Quaternary sediments, which are finely rhythmic distal turbidites of terrigenous composition. Based on these materials, for the first time, for the facies of modern deep-sea trenches, an increase in the size of grains of sediments up the section was established. In the light of all information known to date, this fact can be considered characteristic of the sediments of any deep-sea trenches that record the final phase of the underthrust of the oceanic plate. As for the diagnostics of the paleosubduction zones of the geological past, it is even more informative than the textures of the currents and the presence of undoubted turbidites in the section.
We emphasize that if turbidite complexes can form in different structural and morphological settings of the ocean, then the troughs after the termination of subduction are always filled with deposits of turbidites coarsening up the section, fixing a successive change of formations: slate (distal turbidites) > flysch (distal and proximal turbidites) > marine molasse (proximal turbidites and fluxoturbidites). Moreover, it is also important that the reverse sequence is genetically impossible.

Deep sea trenches are found predominantly along coastlines surrounding the Pacific Ocean. Of the 30 trenches, only 3 are in the Atlantic and 2 in the Indian Ocean. The trenches are usually narrow and predominantly long depressions with steep slopes, extending to a depth of up to 11 km(Table 33).

The features in the structure of deep faults include the flat surface of their bottom, covered with a layer of clayey silt. Fault explorers have found that their steep slopes are exposed to dense, dehydrated clays and mudstones.

L. A. Zenkevich believes that this nature of outcrops indicates that deep depressions are faults of deep, compacted bottom sedimentary accumulations and that these depressions are a rapidly flowing formation, existing, perhaps, no more than 3-4 million years. The nature of the ultra-abyssal fauna in them also testifies to the same.

The origin of deep-sea faults has no explanation. Thus, the hypothesis of the floating of the continents gives some reason to expect the appearance of such faults, however, in this case one should

expect the appearance of deep cracks only on the side of the continents from which they move away. However, faults are also observed on the other side.

To explain the appearance of deep faults due to the expansion of the globe, a hypothesis is sometimes put forward of the heating of the substance that makes up the globe. However, a decrease in radioactive heat by 5-10 times during the existence of the Earth suggests that there are even fewer grounds for this hypothesis than for the hypothesis of an increase in the globe due to a decrease in the tension of the gravitational field.

As facts allegedly proving a continuous increase in the volume of the Earth, in addition to the presence of deep-sea trenches, the presence of mid-ocean ridges is involved.

An appropriate section was devoted to explaining the reasons for the formation of median ridges. Here it must be said that if deep trenches really require either stretching of the earth's crust, or bending it with a fault, then the formation of a mountain range in the ocean can in no way be connected with stretching. It is possible only with compression or an increase in the volume of the ascending substance. Therefore, to attract the presence of a complex mountain system with a length of over 60 thousand km. km there are no grounds for proving the expanding Earth hypothesis.

A more acceptable explanation of the origin of deep faults - trenches, which can be proposed if we consider them as a consequence of the constantly ongoing subsidence of the earth's crust of the oceans and the upward movement of the earth's crust of the continents. These movements are a consequence of the erosion of the continents and the accumulation of sedimentary rocks at the bottom of the oceans. The upward movement of the continents facilitated by erosion and the downward movement of the coastal margins of the oceans in their opposite movement may cause fractures.

Finally, one more variant of the explanation of the origin of the gutters can be expressed, which suggests itself when considering the photograph shown in Fig. 23. It shows that on the bends coastline grooves are formed that resemble real ones in shape. The crust of the ocean floor, as it were, is repelled from the continent in those places where it protrudes into the ocean with relatively narrow protrusions. Having such observations (and there were quite a lot of them), it is possible to imagine the mechanism of moving away of coastal areas of the crust precisely on bends with a large curvature. However, it was impossible to foresee such an effect prior to the experiment. This version of the explanation of the trenches is consistent with their depth, with an equal thickness of the crust, and well explains their shape and location, and, in addition, convincingly confirms the statements of S. I. Vavilov that experiments not only confirm or refute the idea verified by experience, but also have heuristic properties, revealing unexpected properties and features of the studied objects and phenomena.

Deep-sea trenches and associated marginal ridges are important morphological structures of active ocean margins, stretching for thousands of kilometers along island arcs and the eastern continental rim of the Pacific Ocean. Deep-sea trenches trace the exit to the surface of seismic focal zones, reflecting in relief the boundary between the oceanic and continental segments of the Earth's lithosphere. Ocean trenches are narrow long depressions of the ocean floor, which are the deepest zones of the oceans.

There are two types of ocean trenches:

• 1. Ocean trenches associated with island arcs (Marian, Japanese, Sunda, Kamchatka, etc.;
• 2. Ocean trenches adjacent to the continents (Peruan-Chilean, Central American etc.).

The trenches of the island arcs are usually deeper (the Mariana Trench - 11022 m). At high rates of sedimentation, oceanic trenches can be filled with sediment (southern coast of Chile).

Most of the trenches are arcuate, with their concave side facing the island arc or the continent. In the section, they look like regular asymmetric depressions (Fig. 6.28) with a relatively steep (up to 10 ° or more) slope adjacent to the land and a more gentle (5 °) oceanic slope of the trench. On the outer ocean edge of the trench

Rice. 6.28. The schematic structure of the deep-sea trench shows an external dome-shaped uplift, often rising almost 500 m above regional level adjacent ocean floor.

Gutters, even the deepest ones, have little to no precise V-shape.

The width of the oceanic trenches is about 100 km, the length can reach several thousand kilometers: the Tonga and Kermadec trenches are about 700 km long, the Peru-Chile - 4500 km. The narrow bottom of an oceanic trench ranging in width from a few hundred meters to several kilometers is usually flat and covered with sediment. In section, the sediments look like a wedge. They are represented in the lower part of the wedge by hemi-pelagic and pelagic (prefix hemi - semi) sediments of the oceanic plate, falling towards the land. Above them, they are unconformably overlain by horizontally layered sediments of turbidity flows (turbidites) formed due to erosion of the continent or island arc. The type and volume of sediments, the axial zone of the trench are determined by the ratio between the rates of precipitation and the rate of convergence of plates. Sedimentary wedges in the axial zones of the island arc troughs are thinner than those in the troughs adjacent to the continents. This is due to the limited exposure above the ocean (sea) level of the arc surface, which is the main source of precipitation, compared to the continent.

Ocean trenches near continental margins may consist of a series of structurally isolated small depressions separated by sills. Within their limits, in the presence of a slight inclination of the axis, a channel can form, along which turbidity flows flow. The latter can create alluvial swells and erosion structures in the body of the sedimentary wedge and control the distribution of lithofacies in the trench. In areas with very rapidly sedimentation and low convergence rates (Oregon-Washington Trench) can produce extensive fans moving from the continent towards the ocean over the axial sedimentary clip.

Oceanic trenches are convergent plate margins where an oceanic plate is subducted either under another oceanic plate (under an island arc) or under a continent. The rate of convergence of plates ranges from zero to 100 cm/yr. When the plates collide, one of them, bending, moves under the other, which leads to regular strong earthquakes with foci under the slope of the trench adjacent to the land, the formation of magma chambers and active volcanoes (Fig. 6.29). In this case, the emerging stresses in the subducting plate are realized in two forms:

• 1. An external swell-shaped (dome-shaped) uplift is formed with an average width of up to 200 km and a height of up to 500 m.
• 2. Stepwise normal faults and large structures such as horsts and grabens are formed in the curved oceanic crust on the oceanic slope of the trench.

Rice. 6.29. Kamchatka Deep Trench: 1 - active volcanoes, 2 - deep water trough 3 - isolines 1" hollows of magma chambers

There are no folded deformations in the sedimentary strata at the bottom of the trench. Gently dipping thrusts are formed in the slope of the trench adjacent to the land. The underthrust zone (the Benioff - Vadati - Zavaritsky zone) plunges at a slight angle from the trench axis towards the land. It is within this zone that almost all earthquake sources are concentrated.

In the Central American, Peru-Chile and Yap trenches, young basalts were discovered by boreholes (Fig. 6.30). The intensity of magnetic anomalies of the ocean floor near the trench is usually lower. This is due to the presence of numerous faults and ruptures in the curving oceanic crust.

Rice. 6.30. Tectonic scheme of the Central American sector of the Pacific Ocean, according to Yu.I. Dmitriev (1987): I- deep sea trenches 2 - active volcanoes, 3 - wells that uncovered basalts

The accretionary prism of sediments in the lower part of the slope of the trench is deformed, crumpled into folds, and broken by faults and overthrusts into a series of plates and blocks.

Sometimes an advancing continent or island arc rips away sediment from an axial trough and oceanic plate, forming an accretionary sediment wedge. This accretion process is accompanied by the formation of scaly thrust sheets, chaotic sedimentary bodies, and complex folds. Sedimentary-basalt mélange can form here, containing fragments and large blocks of oceanic crust, sedimentary wedge, and turbidites. This mass of accumulated unconsolidated sediments creates a large negative isostatic gravity anomaly, the axis of which is somewhat shifted to the land relative to the trench axis.

The structure of the cuts. The thickness of sediments above the basalt basement varies greatly. In the Central American trench in the well. 500 V, it is 133.5 m, in the well. 495 - 428 m, while sedimentary strata up to 4 km thick are known in other gutters. At the bottom of the trench, the presence of landslide facies and redeposited sediments is noted. Sedimentary and volcanic-sedimentary rocks are widely developed: volcanomictic siltstones, sandstones, gravelstones, clayey, siliceous-clayey rocks, edaphogenic breccias, and basalts in the outer zones. Basalts are characterized by petrochemical and geochemical characteristics that are transitional between typical oceanic and island-arc varieties (Dmitriev, 1987).

In scaly structures of accretionary prisms, these rocks alternate with gravitational olistostromes and landslide breccias. The fragments contain outliers of the oceanic crust: serpentinized ultramafic rocks and basalts. High pressure metamorphic rocks and low temperatures- glaucophane schists.

Minerageny. Oil and gas fields in weakly lithified strata. Deposits of antimony and mercury in paleoanalogues, in metasomatites along host rocks (jasperoids and listvenites) in zones of tectonic faults.

test questions

• 1. Determine the position of deep-sea trenches in the structure of the Earth.
• 2. Name the morphometric and structural features deep sea trenches.
• 3. Describe the structure and composition of rock associations that fill deep-water trenches.

General characteristics of oceanic deep-sea trenches

Scientists call the deep-sea trench an extremely deep and elongated depression on the ocean floor, formed by subsidence of the oceanic thin crust under a more powerful continental area, and during the oncoming movement of tectonic plates. In fact, deep-water trenches today are large geosynclinal areas in all tectonic characteristics.

It is for these reasons that the regions of deep-sea trenches have become the epicenters of large and destructive earthquakes, and there are many active volcanoes at their bottom. There are depressions of this origin in all oceans, the deepest of them are located on the periphery of the Pacific Ocean. The deepest of the tectonic oceanic depressions is the so-called Mariana depression, its depth, according to the estimates of the expedition of the Soviet ship Vityaz, is 11022 m.

Mariana Trench

The deepest oceanic trench on the planet is the Mariana Trench, which stretches for 1.5 thousand km in Pacific waters next to the Mariana Trench. volcanic islands. The trough cavity has a clear V-shaped transverse profile and steep slopes. At the bottom, a flat bottom is visible, divided into separate closed sections. The pressure at the bottom of the basin is 1100 times higher than that in the surface layers of the ocean. There is a deepest point in the basin, it is an eternally dark, gloomy and inhospitable area called the "Challenger Abyss". It is located 320 km southwest of Guam, its coordinates are 11o22, s. sh., 142o35, c. d.

For the first time, the mysterious depths of the Mariana Trench were discovered and preliminary measured in 1875 from the board of the English ship Challenger. The studies were carried out with the help of a special deep-water lot, a preliminary depth of 8367 m was established. However, when re-measured, the lot showed a depth of 8184 m. Modern echo sounder measurements in 1951 from the board of the eponymous scientific vessel Challenger showed a mark of 10,863 m.

The following studies of the depth of the depression were carried out in 1957 on the 25th voyage of the Soviet scientific vessel "Vityaz" under the leadership of A.D. Dobrovolsky. They gave results on a depth measurement - 11,023 m. A serious obstacle in measuring such deep-sea depressions is the fact that the average speed of sound in water layers is directly due to the physical properties of this water.

It is no secret for scientists that these properties of ocean water are completely different at different depths. Therefore, the entire water column had to be conditionally divided into several horizons with different temperature and barometric indicators. Therefore, when measuring ultra-deep places in the ocean, certain corrections should be made to the readings of the echo sounder, taking into account these indicators. The expeditions of 1995, 2009, 2011 differed insignificantly in terms of the assessment of the depth of the depression, but one thing is clear that its depth exceeds the height of the highest peak on land, Everest.

In 2010, an expedition of scientists from the University of New Hampshire (USA) set off for the Mariana Islands. With the help of the latest equipment and a multi-beam echo sounder at the bottom of an area of ​​400 thousand square meters. m discovered mountains. At the site of direct contact between the Pacific and modest in size and young Philippine plates, scientists discovered 4 ridges with heights of more than 2.5 thousand meters.

According to ocean scientists, the earth's crust in the depths of the Mariana Islands has complex structure. Ridges in these transcendental depths were formed 180 million years ago with constant contact of plates. With its massive edge, the Pacific oceanic plate descends under the edge of the Philippine, forming a folded region.

Championship in the descent to the very bottom of the gutter Mariana Islands owned by Don Walsh and Jacques Picard. They made a heroic dive in 1960 on the Trieste bathyscaphe. They saw here some forms of life, deep-sea mollusks and very unusual fish. A remarkable result of this dive was the adoption by nuclear countries of a document on the impossibility of burying toxic and radioactive waste in the Mariana Trench.

Unmanned underwater vehicles also descended to the bottom here, in 1995 the Japanese deep-sea probe "Kaiko" descended to a record depth at that time - 10,911 m. Later, in 2009, a deep-sea vehicle with the name "Nerei" descended here. The third among the inhabitants of the planet, the remarkable director D. Cameron descended into the dark inhospitable depths in a single dive on the Dipsy Challenger submersible. He filmed in 3D, using a manipulator to collect soil and rock samples at the deepest point of the Challenger Abyss trough.

A constant temperature in the bottom part of the gutter +1o C, +4o C is maintained by “black smokers” located at depths of about 1.6 km, geothermal springs with water rich in mineral compounds and a temperature of +450oC. In the expedition of 2012, near the serpentine geothermal springs at the bottom, rich in methane and light hydrogen, colonies of deep-sea mollusks were found.

On the way to the abyss of the depths of the trench, 414 m from the surface, there is an active underwater volcano Daikoku, in its area a rare phenomenon on the planet was discovered - a whole lake of pure molten sulfur, which boils at a temperature of + 187 ° C. Astronomers discovered a similar phenomenon only in space on Jupiter's moon Io.

Trench Tonga

Along the periphery of the Pacific Ocean, in addition to the Mariana Trench, there are 12 more deep-sea trenches, which, according to geologists, make up the seismic zone, the so-called Pacific Ring of Fire. The second deepest on the planet and the deepest in the waters of the Southern Hemisphere is the Tonga Trench. Its length is 860 km and the maximum depth is 10,882 m.

The Tonga depression is located at the foot of the Tonga underwater ridge from the Samoa archipelago and the Karmalek trench. The Tonga depression is unique, first of all, for the maximum speed of the earth's crust on the planet, which is 25.4 cm annually. Accurate data on the movement of plates in the Tonga region were obtained after observations of the small island of Nyautoputanu.

Today, the lost landing stage of the famous Apollo 13 lunar module is located in the Tonga depression at a depth of 6 thousand meters; it was “dropped” when the device returned to Earth in 1970. It is extremely difficult to get the stage from such depths. Considering that one of the plutonium energy sources containing radioactive plutonium-238 fell into the cavity with it, the descent into the depths of Tonga can be very problematic.

Philippine Trench

The Philippine oceanic depression is the third deepest on the planet, its mark is 10,540 m. It stretches for 1320 km from the large island of Luzon to the Moluccas near east coast the Philippine Islands of the same name. The trench was formed during the collision of the basalt marine Philippine plate and the predominantly granite Eurasian plate, moving towards each other at a speed of 16 cm/year.

The earth's crust is deeply flexed here, and parts of the plates are melted in the mantle matter of the planet at a depth of 60-100 km. Such immersion of parts of the plates to great depths, followed by their melting in the mantle, forms a subduction zone here. In 1927, the German research vessel "Emden" discovered the deepest depression in the Philippine Trench, which was called, respectively, the "Emden Depth", its mark is 10,400 m. m, the depression was renamed the "Depth of Galatea".

Puerto Rico Trench

There are three deep-water trenches in the Atlantic Ocean, Puerto Rico, Yuzhnosandwich and Romansh, their depths are noticeably more modest than the Pacific trenches. The deepest among the Atlantic trenches is the Puerto Rico trench with a mark of 8,742 m. It is located on the very border of the Atlantic and caribbean, the region is seismically very active.

Recent studies of the basin have shown that its depth is actively and constantly increasing. This happens with the subsidence of its southern wall, which is part of the North American plate. In the depths of the Puerto Rico depression at around 7,900 m, during research, a large mud volcano was found, which is known for its strong eruption in 2004, then hot water and mud rose high above the ocean surface.

sunda trench

In the Indian Ocean there are two deep-sea trenches, the Sunda, which is often called the Yavan, and the East Indian. In terms of depth, the Sunda is the leader deep sea trench, stretching for 3 thousand km along the southern tip of the Sunda Islands of the same name and a mark of 7729 m near the island of Bali. The Sunda oceanic basin begins with a shallow trough near Myanmar, continues and noticeably narrows near the Indonesian island of Java.

The slopes of the Sunda Trench are asymmetric and very steep, the northern island slope of them is noticeably steeper and higher, it is strongly dissected by submarine canyons, extensive steps and high ledges are distinguished on it. The bottom of the trough in the Java region looks like a group of depressions, which are separated from each other by high thresholds. The deepest parts are composed of volcanic and marine terrigenous sediments up to 3 km thick. Formed by the "leakage" of the Australian tectonic plate under the tectonic structure of the Sunda, the Sunda Trench was discovered by the expedition of the research vessel Planet in 1906.