What was formed as a result of subduction. Collision of plates and subduction zones. A volcanic arc is what

Conditions that determine the structural development of the region

Different tectonic structures develop in different tectogenesis regimes typical for them. The very nature of the regime is determined by the tectonic conditions existing in a given territory at a given period of geological time.

The main indicators of tectonic conditions are:

1) the value of endogenous energy, manifested in a given region;

2) the value of the gravitational imbalance of matter in the lithosphere.

Belousov identified the main conditions that determine the structural development of the region, which include:

1) the permeability of the lithosphere for liquid and gaseous fluids;

2) the form of magmatism, lava composition, lava volume;

3) processes of deformation, metamorphism and granitization;

4) contrast and degree of intensity of tectonic movements;

5) the relationship between the total amplitude of positive and negative vertical movements;

6) the relationship between vertical and horizontal movements.

The world's largest Pacific mobile belt is located on the borders between the oceanic and the mainland hemisphere, its length is approximately 56000 km. It is divided into the western and eastern Pacific mobile belt.

The mainland hemisphere has a more mosaic and complex structure than the oceanic one. It consists of 6 separate continental massifs, separated by 4 oceanic depressions.

Continental massifs form 2 groups: western - New World and eastern - Old World.

New World - North America, South America, Antarctica - they form a belt stretching in the meridian direction.

Old world - Eurasia, Africa, Australia.

The eastern border is separated from the western border by a depression in the Atlantic Ocean. The eastern border tends to be divided into 2 subgroups: Euro-African, Australasian.

The continents are also divided in a latitudinal direction: the northern and southern hemispheres are separated by the Mediterranean geosynclinal belt.

Interaction of lithospheric plates during oncoming traffic, i.e. at convergent boundaries, gives rise to tectonic processes that penetrate deep into the mantle. These processes are complex and varied. On tectonic maps, these processes are expressed by zones of tectonic-magmatic activity, such as island arcs, continental margins of the Andean type, and folded mountain structures.

There are two main types of convergent interaction of lithospheric plates: subduction and collision.

Subduction develops where the continental and oceanic crust or oceanic and oceanic crust converge at the convergent boundary, and with their counter movement the heavier lithospheric plate goes under another and then plunges into the mantle.

Collision- the collision of lithospheric plates, develops where the continental crust converges with the continental, their counter movement is difficult and is compensated by the deformation of the lithosphere, its thickening and the formation of mountain folded systems.

Obduction- movement of fragments of the oceanic crust to the edge of the continental crust. This is extremely rare.

Earthquakes and volcanic eruptions constantly occur on earth in different places. There are such movements that a person does not even feel them. These movements occur constantly, regardless of the territory, season. Mountains grow and shrink, seas grow and dry up. These processes are invisible to the human eye, as they occur slowly, millimeter by millimeter. All this is due to phenomena such as spreading and subduction.

Subduction

So what is it? Subduction is a tectonic process As a result of this process, when plates collide, the densest rocks of which the ocean floor is made move under the light rocks of the continents and islands. At this moment, an incredible amount of energy is released - this is an earthquake. Part of the rocks that have sunk to a great depth, when interacting with magma, begins to melt, after which it splashes to the surface through volcanic vents. This is how volcanoes erupt.

Subduction of lithospheric plates is an integral part of planetary life. It is as important as breathing for a person. It is impossible to stop this process, although many people die every year because of such movements.

Subduction zone

Classification of subduction zones

Subduction zones are classified according to their structure. The types of subduction are divided into four main ones.

• Andean type. This type is characteristic of the Pacific coast on the east side. This is the zone in which the newly formed young crust of the ocean floor at an angle of forty degrees enters under the continental plate at great speed.
• Sunda type. Such a zone is located in places where the ancient massive oceanic lithosphere plunges under the continental one. She goes off at a steep angle. Usually, such a plate goes under the continental, the surface of which is much lower than the ocean level.
• Mariana type. This zone is formed by the interaction of two parts of the oceanic lithosphere or their pushing.
• Japanese type. This is a type of zone where the ocean lithosphere moves under the island ensialic arc.

All these four types are conventionally divided into two groups:

• The East Pacific (this group includes only the Andean type. This group is characterized by the presence of a vast continental margin);
• West Pacific (it contains all the other three types. This group is characterized by the hanging edges of the volcanic arc of the islands).

For each type, where the subduction process takes place, basic structures are characteristic, which necessarily exist in different variations.

Forearc slope and deep water trench

The deep-water trench is characterized by the distance from the center of the trench to the volcanic front. This distance is mainly one hundred to one hundred and fifty kilometers, it is related to the angle at which the subduction zone is inclined. In the most active areas of the continent's outskirts, this distance can reach three hundred and fifty kilometers.

The forearc slope consists of two bases - a terrace and a prism. The prism is the bottom of the slope; it is of a scaly type in structure and structure. The bottom is bordered by the main slope, which comes out to the surface, in contact and interacting with precipitation. The prism is formed by the layering of sediments below. These sediments are superimposed on the oceanic crust and with it go down the slope for about forty kilometers. This forms a prism.

In the area between the prism and the volcanic front, there are large scarps. Terraces are separated by ledges. On the flat areas of such terraces, sedimentation basins are located, on which volcanic and pelagic sediments are deposited. In tropical areas, such terraces may develop reefs, crystalline basement rocks or alien blocks may be exposed.

Volcanic arc - what is this?

This article refers to the term island or volcanic arc. Consider what it is. The tectonically active belt, which coincides with the zones of the largest earthquakes, is designated as a volcanic island arc. It consists of arc-shaped chains of currently active stratovolcanoes. For such volcanoes, an explosive eruption is characteristic. This is due to the large amount of fluid in the island arc magma. Arcs can be double or even triple, and a special shape is a bifurcated arc. The curvature of each arc is different.

Outlying pools

This term denotes a basin or a number of such basins. They are semi-closed and are formed between the mainland and the island arc. Such basins are formed due to the fact that the mainland is torn apart or a large piece is separated from it. Usually in such basins young is formed. This process of crust formation in basins is called back-arc spreading. - this is one of the types of such pools, it is fenced off. In recent years, there is no new information that rifting occurs somewhere; it is usually associated with the fact that the subduction zone is redirected or abruptly jumps to another place.

In the classical version, subduction occurs when two oceanic or oceanic and continental plates collide. However, in recent decades, it has been revealed that during the collision of continental lithospheric plates, one lithospheric plate underneath another also undergoes subduction; this phenomenon is called continental subduction. But at the same time, none of the plates sinks into the mantle due to the low density of the continental crust. As a result, there is a clustering and piling of tectonic plates with the formation of powerful mountain structures. The Himalayas are a classic example.

According to the theory of plate tectonics, the mechanism of subduction (contraction and destruction of the oceanic crust) is compensated by spreading - the mechanism of the formation of young oceanic crust in the mid-oceanic ridges: The volume of oceanic crust absorbed in subduction zones is equal to the volume of the crust originating in the spreading zones. At the same time, in the subduction zones, there is a constant build-up of the continental crust due to accretion, i.e., stripping and intense crushing of the sedimentary cover from the subsiding plate. The warming up of the subsiding crust is also the reason for the widespread development of volcanism along the active continental margins. The most famous in this regard is the Pacific Ring of Fire. The large-scale absorption of the oceanic crust along the periphery of the Pacific Ocean indicates a process of shrinking (closing) of this oldest existing oceanic basins on the planet. Similar processes have taken place in the past. Thus, the ancient Tethys ocean began to shrink from the Mesozoic and by now has ceased to exist with the formation of residual basins, now known as the Mediterranean, Black, Azov, Caspian seas.

The most famous subduction zones are located in the Pacific Ocean: the Japanese Islands, the Kuril Islands, Kamchatka, the Aleutian Islands, the coast of North America, the coast of South America. Also, the subduction zones are the islands of Sumatra and Java in Indonesia, the Antilles in the Caribbean, the South Sandwich Islands, New Zealand, etc.

Subduction zone classifications

There are 4 types of subduction zones according to structural features:

1. Andean
2. Sunda;
3. Mariana;
4. Japanese;

Andean (Andean) type subduction zone- the zone, which is formed where the young oceanic lithosphere at high speed and at a gentle angle (about 35-40º to the horizon) moves under the continent. The lateral structural row from ocean to continent includes: marginal ridge - trench - coastal ridge (sometimes an underwater rise or terrace) - frontal basin (longitudinal valley) - main ridge (volcanic) - rear basin (foothill trough). Typical for the east coast of the Pacific Ocean.

Probe-type subduction zone- the zone where the ancient oceanic lithosphere is pushed up, going to a depth at a steep angle under the thinned continental crust, the surface of which is mainly below the ocean level. The lateral structural series includes: marginal ridge - trench - non-volcanic (outer) island arc - forearc basin (trough) - volcanic (inner) arc - back-arc basin (marginal (marginal sea)). The outer arc is either an accretionary prism or the basement protrusion of the hanging wing of the subduction zone.

Mariana type subduction zone- a zone formed when two sections of the oceanic lithosphere are pushed up. The lateral structural series includes: a marginal ridge - a trench (there is quite a bit of terrigenous material) - a coastal ridge, a non-volcanic arc - a forearc basin (as a frontal one) - an ensimatic volcanic arc - a back-arc basin (or an inter-groove as a rear on a thinned continental or newly formed oceanic bark).
Japanese type subduction zone- the zone of pushing the oceanic lithosphere under the ensialic island arc. The lateral structural series includes: marginal ridge - trench - coastal ridge (sometimes an underwater rise or terrace) - frontal basin (longitudinal valley) - main ridge (volcanic) - back-arc basin (marginal, marginal sea) with newly formed oceanic or suboceanic crust ...

The listed types of subduction zones are often conventionally combined into 2 groups based on morphological characteristics:

• East Pacific - this includes the Andean type zone. The presence of an active continental margin is characteristic.
• Western Pacific - this includes other types of subduction zones. The development of a volcanic island arc at the hanging edge is characteristic.

Basic structural elements

Cross-section subduction zones of the Western Pacific type stand out:

1. deep-sea trough
2. forearc slope

Deepwater Chute

The distance from the axis of the trench to the volcanic front is 100-150 km (depending on the angle of inclination of the subduction zone, the distance reaches 350 km on the active continental margins). This distance corresponds to a slab depth of 100-150 km, where magma begins to form. The width of the volcanic zone is about 50 km, with the total width of the entire zone of tectonic and magmatic activity 200-250 km (on active continental margins up to 400-500 km).

Forearc slope

The forearc slope includes 2 main elements:

1. Accretion prism
2. Pre-arc terrace

Subduction and magmatism

Meaning

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An excerpt characterizing the Subduction Zone

Pierre noticed how after each ball that hit, after each loss, the general animation flared up more and more.
As from an advancing thundercloud, more and more often, brighter and brighter, a hidden, flaring fire flashed on the faces of all these people (as if in response to the ongoing) lightning.
Pierre did not look ahead at the battlefield and was not interested in knowing what was going on there: he was all absorbed in contemplation of this more and more flaring fire, which, in the same way (he felt), was kindling in his soul.
At ten o'clock the infantry soldiers, who were in front of the battery in the bushes and along the Kamenka River, retreated. The battery could be seen as they ran back past it, carrying the wounded on their guns. Some general with his retinue entered the mound and, after talking with the colonel, looking angrily at Pierre, went downstairs again, ordering the infantry cover, who was standing behind the battery, to lie down to be less exposed to shots. Following this, in the ranks of the infantry, to the right of the battery, a drum was heard, shouts of command, and from the battery one could see how the ranks of the infantry moved forward.
Pierre looked over the shaft. One face especially caught his eye. It was an officer who, with a pale young face, walked backwards, carrying a lowered sword, and looked around uneasily.
The ranks of infantry soldiers disappeared into the smoke, their drawn-out screams and frequent firing of rifles were heard. A few minutes later, crowds of wounded and stretchers passed from there. The shells began to hit the battery even more often. Several people were lying uncleaned. The soldiers moved more busily and lively near the cannons. Nobody paid any attention to Pierre anymore. Once or twice at him they shouted angrily for being on the road. The senior officer, with a frowning face, with big, quick steps, moved from one weapon to another. The young officer, blushing even more, commanded the soldiers even more diligently. The soldiers fired in, turned, loaded, and did their job with intense panache. They bounced on the move as if on springs.
A thundercloud moved, and the fire that Pierre had watched burned brightly in all faces. He stood beside the senior officer. A young officer ran up to the eldest, with his hand to the shako.
- I have the honor to report, Colonel, there are only eight charges, will you order to continue firing? - he asked.
- Buckshot! - Without answering, the senior officer shouted, looking over the shaft.
Suddenly something happened; the officer gasped and, curled up, sat down on the ground like a bird shot on the fly. Everything became strange, vague and gloomy in Pierre's eyes.
One after another whistled cannonballs and fought at the parapet, at the soldiers, at the cannons. Pierre, who had not heard these sounds before, now only heard these sounds alone. On the side of the battery, on the right, with a shout of "hurray", the soldiers ran not forward, but backward, as it seemed to Pierre.
The cannonball hit the very edge of the rampart in front of which Pierre was standing, poured the earth, and a black ball flashed in his eyes, and at the same instant slapped into something. The militias, who had entered the battery, ran back.
- All buckshot! - shouted the officer.
The non-commissioned officer ran up to the senior officer and in a frightened whisper (as a butler reports to the owner at dinner that there is no more wine required), he said that there were no more charges.
- Robbers, what are they doing! - shouted the officer, turning to Pierre. The senior officer's face was red and sweaty, and his frowning eyes glittered. - Run to the reserves, bring the boxes! He shouted, angrily avoiding Pierre and turning to his soldier.
“I'll go,” said Pierre. The officer, not answering him, took long strides in the other direction.
- Don't shoot ... Wait! He shouted.
The soldier, who was ordered to go for the charges, ran into Pierre.
- Eh, sir, you don't belong here, - he said and ran downstairs. Pierre ran after the soldier, bypassing the place where the young officer was sitting.
One, another, a third core flew over him, hitting in front, from the sides, from behind. Pierre ran downstairs. "Where am I?" - he suddenly remembered, already running up to the green boxes. He hesitated to go back or forward. Suddenly a terrible jolt threw him back to the ground. At the same instant, the brilliance of a large fire illuminated him, and at the same instant there was a deafening thunder, crackling and whistling that rang in his ears.
Pierre, waking up, was sitting on his backside, resting his hands on the ground; the box he was near was not; only burnt green boards and rags were scattered on the scorched grass, and a horse, rubbing the shafts with fragments, galloped away from it, while the other, like Pierre himself, lay on the ground and screeched piercingly, prolongedly.

Pierre, not remembering himself from fear, jumped up and ran back to the battery, as to the only refuge from all the horrors that surrounded him.
While Pierre was entering the trench, he noticed that no shots were heard on the battery, but some people were doing something there. Pierre did not have time to understand what kind of people they were. He saw the senior colonel lying with his back to him on the rampart, as if looking at something below, and he saw one soldier, who he saw, who, bursting forward from the people who were holding his hand, shouted: "Brothers!" - and saw something else strange.
But he had not yet had time to realize that the colonel had been killed, that the one who was shouting "brothers!" there was a prisoner that in his eyes another soldier was stabbed in the back with a bayonet. As soon as he ran into the trench, a thin, yellow, with a sweaty face, a man in a blue uniform, with a sword in his hand, ran up to him, shouting something. Pierre, instinctively defending himself against the push, since they, not seeing, fled against each other, put out his hands and grabbed this man (it was a French officer) with one hand on the shoulder, the other proudly. The officer, releasing his sword, grabbed Pierre by the collar.
For a few seconds, both of them looked with frightened eyes at the faces alien to each other, and both were at a loss as to what they had done and what to do. “Was I taken prisoner or was he taken prisoner by me? - thought each of them. But, obviously, the French officer was more inclined to think that he was taken prisoner, because Pierre's strong hand, moved by involuntary fear, was tightening and tightening its grip on his throat. The Frenchman was about to say something, when suddenly a cannonball whistled low and terribly over their heads, and it seemed to Pierre that the French officer's head had been torn off: so quickly he bent it.
Pierre also bent his head and let go of his hands. Not thinking anymore about who had captured whom, the French ran back to the battery, and Pierre downhill, stumbling over the dead and wounded, who, it seemed to him, were catching him by the legs. But before he had time to go down, dense crowds of fleeing Russian soldiers appeared to meet him, falling, stumbling and shouting, merrily and violently ran to the battery. (This was the attack that Ermolov attributed to himself, saying that only his courage and happiness could have accomplished this feat, and the attack in which he allegedly threw the St. George's crosses on the mound, which were in his pocket.)
The French, who had occupied the battery, fled. Our troops, shouting "Hurray", drove the French so far beyond the battery that it was difficult to stop them.
The prisoners were taken from the battery, including the wounded French general, who was surrounded by officers. Crowds of the wounded, familiar and unfamiliar to Pierre, Russians and French, with faces disfigured by suffering, walked, crawled and rushed on a stretcher from the battery. Pierre entered the mound, where he spent more than an hour, and from the family circle that took him to him, he did not find anyone. There were many dead here, unknown to him. But he recognized some. The young officer was still curled up at the edge of the rampart, in a pool of blood. The red-faced soldier was still twitching, but he was not removed.
Pierre ran downstairs.
"No, now they will leave it, now they will be horrified at what they have done!" Thought Pierre, aimlessly following the crowds of stretchers moving from the battlefield.
But the sun, obscured by smoke, was still high, and in front, and especially to the left of Semyonovsky, something was boiling in the smoke, and the rumble of shots, shooting and cannonade not only did not subside, but intensified to despair, like a man who, straining, screams with the last bit of strength.

The main action of the Battle of Borodino took place in the space of a thousand fathoms between Borodin and Bagration's flushes. (Outside this space, on the one hand, the Russians made a demonstration of Uvarov's cavalry in half a day, on the other hand, behind Utitsa, there was a clash between Poniatovsky and Tuchkov; but these were two separate and weak actions in comparison with what happened in the middle of the battlefield. ) On the field between Borodino and the flushes, near the forest, on an open and visible stretch from both sides, the main action of the battle took place, in the simplest, most ingenious way.
The battle began with cannonade from both sides with several hundred guns.
Then, when the smoke covered the whole field, in this smoke two divisions (from the French side) moved on the right, Desse and Compana, on flushes, and on the left, the regiments of the Viceroy at Borodino.
From the Shevardino redoubt, on which Napoleon stood, the flushes were at a distance of a mile, and Borodino was more than two miles in a straight line, and therefore Napoleon could not see what was happening there, especially since the smoke, merging with fog, hid the whole locality. The soldiers of Dessé's division, aiming at the flush, were visible only until they descended under the ravine that separated them from the flush. As soon as they descended into the ravine, the smoke of cannon and rifle shots on the flashes became so thick that it covered the entire rise of that side of the ravine. Something black flashed through the smoke - probably people, and sometimes the glint of bayonets. But whether they were moving or standing, whether they were French or Russian, it was impossible to see from the Shevardinsky redoubt.
The sun rose brightly and slanted beams right into the face of Napoleon, who was looking out from under his arm at the flush. Smoke spread in front of the flushes, and it seemed that the smoke was moving, then it seemed that the troops were moving. The screams of people were sometimes heard from behind the shots, but it was impossible to know what they were doing there.
Napoleon, standing on the mound, looked into the chimney, and in the small circle of the chimney he saw smoke and people, sometimes his own, sometimes Russians; but where that which he saw was, he did not know when he looked again with a simple eye.
He left the mound and began to walk up and down in front of him.
From time to time he stopped, listened to the shots and peered into the battlefield.
Not only from the place below where he stood, not only from the mound on which some of his generals were now standing, but also from the very flushes, on which there were now together and alternately now Russians, now French, dead, wounded and alive, frightened or the maddened soldiers, it was impossible to understand what was going on in this place. For several hours at this place, amid the incessant firing of rifle and cannon, now there appeared only Russians, now only French, now infantry, now cavalry soldiers; appeared, fell, shot, collided, not knowing what to do with each other, shouted and ran back.
From the battlefield, his sent adjutants and orderlies of his marshals incessantly galloped to Napoleon with reports on the progress of the case; but all these reports were false: both because in the heat of the battle it is impossible to say what is happening at a given moment, and because many adjutapts did not reach the real place of the battle, but transmitted what they heard from others; and also because while the adjutant was passing those two three versts that separated him from Napoleon, circumstances changed and the news he was carrying was already becoming incorrect. So the adjutant rode up from the Viceroy with the news that Borodino was occupied and the bridge on Koloch was in the hands of the French. The adjutant asked Napoleon if he would order the troops to leave? Napoleon ordered to line up on the other side and wait; but not only while Napoleon was giving this order, but even when the adjutant had just driven away from Borodino, the bridge had already been recaptured and burned by the Russians, in the very battle in which Pierre participated at the very beginning of the battle.

... bta Gorda and the Gulf of California. On the Canadian segment, the boundary of the same two plates is the Queen Charlotte Fault - a ridge-arc transform system. The Aleutian subduction zone demonstrates another case, when the curvature of the arc in combination with the direction of subduction plays a decisive role: along the arc from east to west, subduction becomes more and more obliquely oriented and, finally, near the Commander Islands, it turns into a transform displacement

27. Deep structure of subduction zones.

Subduction is a process in which continental and oceanic lithosphere or oceanic and oceanic lithosphere converge at a convergent boundary. With their counter movement, the heavier lithospheric plate (always oceanic) goes under another, and then plunges into the mantle.

By the end of the 50s. G. Stille expressed the idea that the formation of deep-water trenches, accompanying negative gravian anomalies and seismic focal zones going into the mantle, is associated with an oblique subduction of the oceanic crust; at a certain depth, it melts, giving rise to volcanic chains that run parallel to the trench.

By the nature of the interacting areas of the lithosphere, subduction zones are divided into 2 types: marginal-continental zones (Andean, Sunda, and Japanese types) and oceanic zones (Mariana type). The former are formed where the oceanic lithosphere subducts beneath the continent, the latter - during the interaction of two parts of the oceanic lithosphere.

The structure and subduction regime of the marginal continental zones are varied. The longest of them, the Andean (about 8 thousand km), is characterized by gentle subduction of the young oceanic lithosphere, the predominance of compressive stresses and mountain building on the continental wing.

The Sunda arc is distinguished by the absence of such stresses, which makes possible the thinning of the continental crust, the surface of which is mainly below sea level; an older oceanic lithosphere subducts beneath it, sinking to a depth at a steeper angle.

Subduction zones of the Japanese type can also be considered a variety of continental margins, an idea of ​​which is given by the intersection passing through the Japan Trench - Honshu - Sea of ​​Japan. The presence of a marginal sea basin with areas of newly formed oceanic or suboceanic crust is characteristic. Geological, geophysical and paleomagnetic data make it possible to trace the opening of the marginal Sea of ​​Japan as a strip of continental lithosphere was cut off from the Asian margin. Bending gradually, it turned into a Japanese island arc.

During the formation of subduction zones of the oceanic (Mariana) type, the older (and therefore more powerful and heavier) oceanic lithosphere subducts under the younger one, at the edge of which an island arc is formed. Example: the southern Antilles system.

28. Kinematics of subduction, main options. (kind of similar to placement patterns)

The basis is the horizontal sliding of 2 lithospheric plates, as well as the gravitational lowering of one with negative buoyancy on the asthenosphere.

Three main vectors of motion: horizontally directed slip vectors (2) and downward gravitational sinking vector.

According to calculations, the oceanic crust loses its + buoyancy at the age of 10 million liters - the density increases relative to the underlying asthenosphere.

The opposite, offensive displacement of the hinge of the subducting plate is impeded by the submerged part of the plate, anchored in the mantle.

The vectors of the horizontal movement of the lithospheric plates can be oriented both at right angles and at an acute angle to the trough. With oblique subduction along the boundary, longitudinal strike-slip faults develop - the Sunda arc

At high speeds of movement of the upper plate + the place where a relatively light or thickened oceanic lithosphere subducts, the upper plate comes behind the hinge line of the lower plate and overlaps it. A very flat near-surface part of the Benioff zone is formed, characteristically pronounced under the central segment of the Andes.

Subduction orthogonality rule, its explanation and use.

The convergence of lithospheric plates during subduction occurs in a direction that intersects the strike of the jelly at a small angle. (<60 в 80% случаев)

The frictional resistance of subduction is minimal at a relative angle of 90 and increases as the angle decreases to 45; this is seen as a dynamic justification for orthogonality.

During the Paleogene, the subduction of the Faraglion plate took place at more and more acute angles to the Cordillera and the Andean continental margins - the separation of the Juan de Fuca, Cocos, and Nazca plates, which, as a result, subduct almost orthogonally.

If the external influence sharply changes the direction, then the dying off of the previous subduction and the initiation of a new one occurs due to the oriented transform fault.

The rule is used in paleotectonic reconstructions to solve the inverse problem: along the strike of the ancient subduction zone, the most probable direction of convergence of lithospheric plates is determined.

29. Seismic focal zones of the benioff. Their depth, profiles, structures, stresses in the foci.

A vivid manifestation of modern subduction is seismic focal zones - a set of seismic foci, obliquely sinking to depth. Seismic foci are confined to the subducting lithospheric slab and together with it penetrate into the asthenosphere, sometimes completely crossing it. In 1949-1955. H. Benioff from the California Institute of Technology summarizing work on seismic focal zones. Therefore, they were named after him.

Depth of Benioff's zones. Comparing the location of earthquake foci with the results of seismic tomography for the same subduction zone, one can make sure that the subsidence of the lithosphere first, to a certain depth, generates foci of elastic vibrations, and then continues as an aseismic process. This is determined by a decrease in the elastic properties of the subducting lithosphere as it warms up. The depth of the Benioff zones depends mainly on the maturity of the subducting oceanic lithosphere, which increased in thickness and cooled with age.

The second important regulator of the depth of the Benioff zones is the rate of subduction. At high speeds (9-10.5 cm / year), even the lithosphere with an age of 80-40 million years retains its elastic properties down to a depth of about 600 km.

Example: the depth of one of the longest seismic focal zones, the Andean, decreases from 600 km in its central part to 150-100 km on the flanks. Changes occur discretely in accordance with the segmentation of this subduction zone.

The vertical distribution of seismic sources in the Benioff zones is extremely uneven. Their number is maximal at the top of the zone, decreases exponentially to depths of 250-300 km, and then increases, giving a peak in the range from 450 to 600 km.

The direction of the slope of the Benioff zones. Following the weak, all Benioff zones are obliquely oriented. In the marginal-continental systems, including the complexly constructed systems of the Japanese type, the slab always plunges towards the continent, since it is the oceanic lithosphere that subducts. Here, with the convergent interaction of two plates of the oceanic lithosphere, the one that is older, and therefore thicker and heavier, sinks. The corresponding Benioff zone is thus tilted under the younger oceanic lithosphere, wherever it is.

Profile of Benioff zones. The slope of each seismic focal zone changes with depth in accordance with the slab configuration traced by seismic tomography. Small angles of inclination near the surface (35-10 °) increase with depth: at first very slightly, then a distinct bend usually follows, followed by a further gradual increase in inclination, up to almost vertical. The reason for the uneven increase in the steepness of the slab (and seismic focal zone) extending into the mantle and the corresponding inflections of its profile is believed to be the compaction of rocks of the subducting lithosphere due to the phase transition of minerals.

Distribution of benioff zones.

Near the surface- under a deep-sea trench, and often on its oceanic framing, foci are located inside the lithosphere, mainly at its top (extension).

Below, at a depth of 15 km, subduction can be aseismic.

Deeper, where the subducting plate breaks out of contact with the hanging lithospheric wing, and then plunges into the asthenosphere, all foci are found again inside the slab.

Finally, even deeper the Benioff zone continues with a chain of foci in the upper part of the lithosphere, formed during compression along the slope of the slab.

Seismicity over the Benioff zones is mainly determined by the thickness of the lithosphere in the hanging wing, as well as by the distribution and intensity of the heat flow passing through it. In island arcs, seismicity over the Benioff zone, starting at the trench, is traced laterally for 500 km or more. These are mainly shallow sources. Regular distribution of seismic sources, Japanese subduction zone

30. Deep structure of subduction zones according to geophysical data.

Methods of seismic, seismology, gravimetry, magnetometry, magnetotelluric sounding, geothermy, complementing each other, provide direct information about the deep state of matter and the structure of subduction zones, which can be traced with their help down to the lower mantle.

Multichannel seismic profiling makes it possible to obtain structural profiles of subduction zones down to depths of several tens of kilometers at high resolution. On such profiles, the main displacer of the subduction zone can be distinguished, as well as the internal structure of the lithospheric plates on both sides of this displacer.

Using seismic tomography methods, the subducting lithosphere is traced deep into the mantle, since this lithosphere differs from the surrounding rocks by its higher elastic properties (“seismic quality factor”) and velocity characteristics. The profiles show how the subducting plate crosses the main asthenospheric layer. In some zones, including under Kamchatka, it continues obliquely, going into the lower mantle to a depth of 1200 km (Fig. 6.6). In other zones, in particular in Izu-Boninskaya, reaching the surface of the lower mantle (where the viscosity of rocks at a depth of 670 km increases by 10-30 times), the lithosphere bends and then follows horizontally above this surface. On the whole, using seismic tomography methods, it was possible to trace the subducted part of the oceanic lithospheric plates up to 1800 km long, counting from the deep-sea trench. Based on average subduction rates, this is the result of convergent interactions over the past approximately 25 Ma.

Extremely important information is provided by seismological observations of earthquake sources arising in the upper part of subduction zones (at a depth of up to several hundred kilometers) and forming powerful oblique seismic focal zones - the so-called Benioff zones (see 6.1.4).

31. Gravimetric and magnetic anomalies over subduction zones, heat flow distribution.

Gravimetry: sharp anomalies of gravity, elongated along the subduction zone, change in a regular sequence when it is crossed. A positive anomaly up to 40-60 mG is usually traced in front of the deep-sea trench in the ocean, confined to the marginal ridge. It is caused by the elastic anticlinal bending of the oceanic lithosphere at the beginning of the subduction zone. This is followed by an intense negative anomaly (120-200, up to 300 mG), which extends over the deep-water trench, being displaced by several kilometers towards its island-arc side. This anomaly correlates with the tectonic relief of the lithosphere, as well as, in many cases, with an increase in the thickness of the sedimentary complex. On the other side of the deep-sea trench, above the hanging wing of the subduction zone, there is a high positive anomaly (100-300 mGl). Comparison of the observed gravity values ​​with the calculated ones confirms that this gravitational maximum can be caused by oblique subduction of denser rocks in the asthenosphere relative to the cold lithosphere. In island arc systems, the continuation of the gravity profile is usually followed by small positive anomalies over the marginal sea basin.

Geothermal observations reveal a decrease in heat flow as the relatively cold lithosphere sinks under the island-arc (or continental) side of the deep-water trench. However, further, with the approach to the belt of active volcanoes, the heat flow increases sharply.

Modern subduction is also expressed in magnetometry data. On the maps of linear magnetic anomalies of the oceanic basin, their tectonic boundaries of riftogenic and subduction nature are clearly distinguished. If in relation to the first linear anomalies of the oceanic crust agree (parallel to them), then the subduction boundaries are intersecting, they cut off the systems of anomalies at any angle, depending on the convergent interaction of lithospheric plates.

When the oceanic lithosphere plunges into a deep-sea trench, the intensity of linear anomalies often decreases several times, which is presumably explained by the demagnetization of rocks due to bending stresses. In other cases, anomalies can be traced to the convergent boundary and even further. In fig. 6.12 shows a map of the magnetic field of one of the segments of the Central American Trench (16-17 ° N). Linear anomalies of the Miocene oceanic crust here are elongated in the SE-NW direction, intersect the axis of the deep-sea trench, and then can be traced under the hanging wing of the subduction zone in a strip about 25 km wide. The oceanic lithosphere sinking to a depth, as it were, shines through the sedimentary complexes of the continental margin crumpled into folds. Even further, where it sinks under the thick granite-gneiss crust, linear anomalies are lost.

32. Magmatism of subduction zones, patterns of its location.

Location: The spatial relationship of powerful belts of modern volcanism with deep-sea trenches, Benioff zones and other manifestations of subduction is quite distinct. On the example of the volcanoes of Japan, it was found that the chains of active volcanoes are located above the mid-water part of the seismic focal zone. Later it became clear that this is a regularity that can be traced in all subduction zones. The depth of the inclined seismofocal zone beneath the volcanoes varies from 60 to 350 km, but the maximum magmatic activity is observed over an interval of 100-200 km. The distance of volcanoes from the trench is inversely related to the slope of the seismic focal zone. The greater the angle of inclination, the closer to the gutter volcanism manifests itself, this pattern is maintained globally. The line limiting the volcanic belt from the side of the trench is called the volcanic front - 120-250 km from the deep-water trench. On the opposite side, the boundary of the volcanic belts is not so sharp. The total width of subduction volcanic belts is from several tens of kilometers to 175-200 km, in some places even somewhat larger.

Deep roots: Since at the corresponding depths the slab moves among the asthenospheric material and seismic sources are inside it, a decrease in seismicity under volcanoes most likely means a decrease in the elastic properties of the subsiding lithosphere during separation of fluids or even partial melting. This magma-generating segment of the subduction zone is an area where magma-generating processes are just beginning to continue above the subducting plate in the mantle wedge and crust all the way to the near-surface magma chambers in the basement of volcanoes. The deep roots of the volcanic belt, marked by a decrease in the velocity and elastic characteristics of the rocks, are clearly traced by seismic tomography up to the surface of the slab.

Specific composition of magmas over subduction zones.

The composition of volcanics is influenced by:

Lateral: potassium, strontium rubidium increases in depth of subduction, Fe / Mg decreases

In the t direction, the tholeiitic trough (tholeiitic basalt, ferruginous dacite) is replaced by calc-alkaline (alumina basalt-rhyolite), in the rear of the arc - shoshonite (shoshonite basalt-trachyte)

ORE: Au, Cr, Ni, Cu- Zn? Pb, Mo - under the arc Sn-Wo-U

(probably the same place ...) 47. Specific composition of magmas over subduction zones.

In the formation of magmas feeding subduction volcanism, matter is involved, which is separated from the submerging oceanic lithosphere, from the rocks of the astemospheric wedge located above it, as well as from the mantle and crustal rocks of the hanging wing lithosphere, which serves as the basement of the volcanic belt. An important specific feature of magma formation during subduction is considered to be the movement of the oceanic crust material, including its sedimentary cover, deep into Maitia, which gives the corresponding geochemical features to mantle magmas. In addition, a large amount of water, which is introduced in this case, radically changes the conditions for the partial melting of peridotites above the subduction zone. Judging by laboratory experiments, direct separation of not only basaltic, but also andesite melts is possible from the "watered" mantle. Despite the diversity of subduction volcanics, which include a wide range of rocks of the tholeiitic, calc-alkaline, and shoshonite series, their geochemical specificity in many cases makes it possible to distinguish these rocks from similar volcanic rocks of a different origin.

33. Subduction accretion and subduction erosion, their geological expression.

The tectonic effect of the interaction of lithospheric plates in different subduction zones, and often on adjacent segments of the same zone, is different. Depending on this, one can distinguish between the regime of subduction accretion, the regime of subduction (tectonic) erosion, and also the neutral regime.

There is also another mechanism for building up the island-arc or continental margin. Part of the sedimentary material that goes to a depth with the ocean plate is also retained, separating from it and laminating from below to the hanging wing of the subduction zone.The resulting scaly structure with multiple repetitions of the same fragments of the stratigraphic section was studied in detail in the Cretaceous accretion belt of Simanto (Japan).

Erosion. The mode of subduction erosion is expressed by shearing of the hanging wing under the action of the advancing lithospheric plate, which carries away the products of destruction to a depth. Along with subduction accretion, this is one of the two main tectonic subduction regimes.

Seismic profiles are an important source of information. In 1986, an interpretation of the relationships revealed by profiling under the island-arc slope of the Japan Trench was carried out. 1st sign of erosion: There is no modern accretionary prism here. Tectonic erosion is evidenced by the structure of the hanging (island-arc) wing. This is a Cretaceous layered series inclined from the trough, which is cut off at depth by a gentle surface of tectonic contact: erosion of the hanging wing occurs from below. The consequence of such erosion is considered to be the subsidence of the island-arc slope established along the cores of boreholes.

With long-term development, subduction erosion cuts off the elements of the island arc or active continental margin closest to the deep-sea trench, while dying off volcanic belts move closer and closer to the convergent boundary. 2nd

2 mechanisms of erosion:

Basal erosion involves the mechanical effect of a submerging plate on the lower surface of the hanging wing of the subduction zone (see Fig. 6.27, A). Erosion of this wing occurs from below, which leads to a decrease in its thickness and a corresponding lowering.

Frontal erosion is the cutting off of the front edge of the hanging wing by a subducting plate, the capture and subduction of the rocks composing this edge. It is especially noticeable where a dissected tectonic relief - a system of grabens and horsts - is formed on the plunging plate during its bending.

Neutral subduction mode - a mode in which subduction is not accompanied by either accretion or tectonic erosion, this is a rare phenomenon

34. Identification and reconstruction of ancient subduction zones.

The presence of ancient subduction zones can be identified by the presence of an accretionary prism.

Also, subduction zones have specific volcanism. An important feature of magma formation during subduction is the movement of oceanic crust matter, including its sedimentary cover, deep into the mantle, which gives the corresponding geochemical features to mantle magmas. In addition, a large amount of water, which is introduced in this case, radically changes the conditions for the partial melting of peridotites above the subduction zone. Judging by laboratory experiments, direct separation of not only basaltic, but also andesite melts is possible from the "watered" mantle.

Above the subduction zones are anomalous afeoliths.

Ophiolites:

Their anomaly over the subduction zones -

The sedimentary formation of back-arc basins is characteristic - on the one hand, volcanic ash from the magma belt, and on the other, terrigenous continental sediments from the continent. The thickness of the pelagite clays is much greater here than in the ocean.

It is possible to determine the direction of subduction in blue-shale and green-shale formations. Blue shales are formed at lower temperatures and higher pressures.

35. Obduction of the oceanic lithosphere and its supposed mechanisms.

Normal interaction of continental and oceanic lithospheres at convergent boundaries is expressed by subduction. Only in places and for a short time does such a combination of tectonic conditions appear, under which the oceanic lithosphere is uplifted and thrust over the continental margin. At present, this process, apparently, does not occur anywhere, but a relatively recent episode (late Miocene - Pliocene) was established at the junction of the Chilean spreading ridge with the Andean active margin. By the time of thrusting, it was a relatively young, moderately thick and still slightly cooled lithosphere with a relatively low average density and therefore, in accordance with isostasy, high hypsometric position- a necessary condition for obduction.

Obduction, as a rule, is accompanied by the dynamothermal metamorphic impact of hot peridotites composing the lower lithospheric plate on autochthonous rocks.

Obduction mechanisms:

Obduction at the edge of the ocean basin occurs at both active and passive margins. This is a model of obduction upon collision of a spreading ridge with an active continental margin. If the ridge extends approximately parallel to the margin, then in the course of subduction the continental plate will overlap its nearest wing and come into contact with the raised edge of the other wing, which as a result may be pulled over. An example is the takeover of the Chilean Spreading Ridge.

Obduction during the closure of ocean-type basins. The geological conditions of the location of many obducted fragments of the oceanic lithosphere near the deep ophiolite sutures of the Mediterranean-Himalayan and other fold belts make it possible to associate their origin with the closure of small oceanic basins, similar to the Red Sea. If the opening of such basins is directly replaced by their compression, then the high heat flux favors the exfoliation of the lithospheric plates. The high hypsometric position of the young oceanic lithosphere and the shoulders of thinned continental crust submerged below sea level at the edges of such spreading basins contribute to obduction. When the continental framing is completely closed, the structural seam rises, and a slope appears at the bottom of adjacent epicontinental basins, providing further gravitational movement of the obducted plates of the oceanic lithosphere, accompanied by the formation of olistostromes.

36. Areas of collision of the continental lithosphere: relief, structure, movements, volcanism, depth characteristics.

If the continental lithosphere approaches the convergent boundary on both sides, then the relatively light sialic rocks do not sink into the mantle, but enter into active mechanical interaction. Intense compression produces complex structures, crustal thickening and mountain building. In this case, the internal tectonic layering of the lithosphere may appear, when it is divided into plates experiencing horizontal displacement and disharmonious deformations. , on the convergent boundary, instead of subduction, collision develops, i.e., the collision of lithospheric plates - a geodynamic regime, which is currently manifested mainly along the Mediterranean-Himalayan fold belt with a length of thousands of kilometers. The collision, associated movements and deformations are maximal in those sections of this belt, where the projections of the continental plates of Hindustan and Arabia oppose the southern margin of Eurasia. In these places, constrictions (twisting) of the folded belt are formed.

The grandiose construction of the Himalayas and Tibet gives an idea of ​​a more mature and still very active phase of collisional interaction of large continental units. It began in the Paleogene 50-70 million years ago, when the oceanic lithosphere separating the Hindustan subcontinent from the Eurasian margin completely subducted under it. The slope of the subduction zone predetermined the southern vergence of folding and thrusts of the collisional stage. The oncoming movement of Hindustan and Eurasia, the speed of which reached 15-20 cm / year before the collision, continued in the future. At first (before the Oligocene) it took place at a speed of about 10 cm / year, later - 5 cm / year or less, and the total approach after the beginning of the collision exceeds 2000 km.

Mountain building during collision is accompanied by the accumulation of powerful molasses in the forward and intermontane troughs.

Longitudinal displacement of rock masses of the collision belt. During the convergence of lithospheric plates of heterogeneous structure, consisting of continental and oceanic parts, as well as where the continental margin interacts with several different plates and microplates, transitions along the strike from collision zones to subduction zones or vice versa are observed. An example is the continuation of the Timor collisional system of the Sunda subduction considered above. The complex structural pattern characteristic of the Mediterranean-Himalayan belt is explained by the irregular outlines and mutual geometric discrepancy of the continental margins forming this belt: the Eurasian, on the one hand, the African-Arabian and Hindustan, on the other.

The most expressive relationships are at the junction of the collisional Anatolian-Caucasian and subduction Aegean-Cypriot segments, since the intense compression of the fold belt ahead of the front of the Arabian indentor is adjacent there with no less intense and stable extension over the subduction zone.

Collisional deformations at a distance from the convergent boundary. Under favorable geological conditions, collisional deformations are manifested not only in the zone of convergent interaction of lithospheric plates, but also at a distance from it. Thus, under pressure from the collisional orogen of the Alps, the platform cover of the foreland was torn off along plastic rocks of the Saline Triassic, displaced and deformed to form a folded system of the Jura Mountains 50-150 km to the northwest.

Collapse of collisional orogens. In the development of collisional mountain structures, the stage of compression, thickening and isostatic uplift of the earth's crust naturally follows the stage of its extension, thinning, and corresponding subsidence (orogenic collapse). In the Alps, where modern extension is manifested seismologically, it was found that in the central zones of the orogen it began 20 Ma ago and coexisted for a long time with fold-thrust compression deformations at the periphery of the mountain structure.

About hot spots, in a heap:

The linearity of volcanic structures and the regular aging of the Imperial Ridge in the Pacific Ocean led W. Morgan (Morgan W.J.) in 1971 to create a hot spot model as a relatively stationary and long-lived thermal anomaly in the mantle. It is the source of magmas enriched in trace elements and feeds the volcanoes of oceanic islands and inland continents. On the earth's surface, a hot spot is reflected in anomalously high volcanic activity, present or past. Ideally, this is a chain of modern and ancient volcanoes, the age of which gradually becomes older in one direction (trail of a hot spot, plume), which is associated with the "burning" of a moving lithospheric plate. When the plate moves away from the hot spot, the volcano ceases to be active, dies off and, together with the plate, moves away from the hot spot. A classic example of a hotspot trail would be a chain of volcanoes that stretches in the Pacific Ocean from the Obruchev Rise with superimposed seamounts, composes the Imperial Ridge and traces to the Hawaiian archipelago with active volcanoes (for example, Mauna Loa). At the same time, this original idea began to be applied to any volcanic structures in the World Ocean, which, in the opinion of the author of this manual, is not unequivocally proven.

Hot spots and mantle plumes

In the 1970s, J. Wilson and J. Morgan proposed the hypothesis Hot spots and "Mantle jets (plumes)"... Based on observations on the Hawaiian and Imperial Ridges in the Pacific Ocean. The first of them is a chain of islands with extinct volcanoes, ending in the southeast with active volcanoes of the Hawaiian islands. At the beginning, it joins with a chain of underwater volcanic uplands known as the Imperial Ridge. Thus, we see a picture of the regular migration in time and in space of volcanic centers. Wilson and Morgan explained this picture by the fact that under Fr. Hawaii is currently operating a hot mantle jet that pierces the asthenosphere and lithosphere and occupies a stationary position. The Pacific plate moved above this hot spot, first in the northwest (Imperial Ridge), and then, since 42 Ma, in the west-northwest direction, while the hot jet “pierced” it and created new volcanoes.

There are about 40 hotspots in the oceans and continents, and nearly all are associated with volcanic activity. Alkaline-basaltic magma originating from the undepleted mantle is characteristic, which indicates the deep position of the “roots” of the hot spots. If we proceed from their stationarity, then it is possible to determine not the relative, but the "absolute" movements of the lithospheric plates, measured in relation to the hot spots anchored in the mantle.

There is also the concept of superplumes, with which the processes of crushing and disintegration of supercontinents are associated.

39. But I'm not sure.

There are two main ways of occurrence and opening of rift zones. The concept of active rifting is based on the traditional concept of the primacy of the ascending

In 1951, in his work on the tectonics of the Alps, Amstutz used the word subduction to denote the conditions that formed the most complex nodular structure of the Alps. After that, for 20 years, this term was almost never used by anyone. In the modern plate tectonic understanding, the term subduction has been used since 1969. Classical plate tectonic subduction provides for the presence of at least one side of the oceanic lithosphere, which is opposed to continental subduction (continent-continent collision).

Subduction boundaries are highly seismic boundaries (almost always expressed in the relief by deep-sea trenches), the most powerful tremors are confined to them.

Subduction troughs are called troughs in geology, everything else is troughs.

Why can't subduction simply be called a lithospheric underthrust or thrust? This is due to the more complex kinematics of the subduction process: most often both plates have opposite motion, less often there is immobility of one of the plates (most often the upper one).

Geographic location of subduction zones.

1. Most of the subduction zones are located on the edge of the Pacific Ocean (with the exception of some zones). This stemmed from the fact that at the beginning of the Mesozoic, at the late stage of development of Pangea, there was an annular subduction zone around it: it began in Australia, covered Pangea almost completely to the south of Northern Eurasia, and turned inside the ring along the southern edge of Northern Eurasia.

2. Purely geographically subduction zones in the Atlantic - in the Lesser Antilles and Southern Antilles (Scotia arc). But these subduction zones are not primary: earlier, the Scotia arc went along the western border of the Andes (i.e., in the Pacific Ocean), and then protruded into the Atlantic Ocean and was cut off from the Pacific Ocean by a later subduction zone. The same thing happened with Lesser Antilles.

3. From the Pacific Ocean to Gibraltar (from the southeast to the northwest) - the tail from the Pacific Rim:

· The Sunda subduction zone is the most active at the present time, causing tsunamis and earthquakes. The oceanic lithosphere of the complex Indo-Australian plate moves under the thinned continental lithosphere of the Eurasian unit.

· Collisional border of Tibet - the complex Indo-Australian plate joins with its Eurasian continental part.

· Makran subduction zone (south of Pakistan) - oceanic part of the Indo-Australian plate and the Eurasian plate.

· Collision of Zagros.

· Subduction zone of the Eastern Mediterranean (the Aegean Sea - its back-arc basin).

· Collision of Greece-Apennines - the continental Adriatic massif collides with Eurasia.

· Ionian subduction zone (Calabrian island arc).

· Gibraltar Subduction Zone - The Atlantic lithosphere subducts eastward beneath the continent.

Thus, a "dotted" structure of this area of ​​distribution of subduction boundaries is observed.

In the framework of the long-lived subduction belt, there are deaths and jumps of subduction zones. Only in one section of the Pacific Rim there is a subduction zone, which has not changed since its formation - almost throughout the Andes (except for Ecuadorian and Colombian).

If the subduction zone combines the continental and oceanic lithosphere, then the subduction goes under the continent. In the intraoceanic situation, the oceanic lithosphere is of different ages (the subduction zone of the New Hybrids, Tonga-Kermadec): the older lithosphere will submerge under the younger one, because it is colder, denser.