Bolivian hemorrhagic fever (South American hemorrhagic fever) Bolivian hemorrhagic fever is a natural focal viral disease endemic to the central provinces of Bolivia. characterized by high fever, hemorrhagic

This platform experienced a short-term uplift at the beginning of the Silurian as a result of the manifestation of the Taconian phase of folding in the Appalachian geosyncline. Regression has been replaced by transgression with wide distribution of carbonate deposits and reef formations.

Silurian deposits are represented by limestones and dolomites. There are many reef structures in the Lower Silurian sections, and halogen rocks appear in the Upper Silurian, especially in the east of the platform - anhydrites, gypsum and rock salt.

At the very end of the Silurian, huge salt pools arose in North America. The thickness of the Silurian is measured in several hundred meters. In depressions, it increases, for example, in the Michigan depression - up to 1.5 km.

gondwana

The southern continents in the Silurian still stand above sea level, and the Silurian precipitation is insignificant, but where they are present (along the periphery of Gondwana), they are represented by terrigenous formations.

In the South American part of Gondwana, a restructuring took place at the end of the Ordovician - the beginning of the Silurian, probably caused by the influence of the Caledonian folding. In the Silurian, the area of ​​the sea increased. Depressions of the meridional direction appeared. They accumulated significant thickness (up to 800-1200 m) clastic sediments with subordinate carbonate layers. In the Amazon Basin (latitudinal direction), marine sandy-argillaceous sediments 100 m thick are observed. In the Late Silurian and at the very beginning of the Devonian, uplifts again occurred as a result of Late Caledonian movements.

In the African part of Gondwana, sandy strata at the end of the Ordovician and in the Silurian were replaced by dark clays with graptolites. Carbonate muds appeared in the northern part of the basin. Along the margins of the area of ​​marine accumulation, coastal sands were deposited. The thickness of the Silurian rocks is usually small. On the Arabian Peninsula, the Silurian is represented by a continuous section of sandy-argillaceous formations of considerable thickness. At the end of the Silurian, a regression began everywhere in Africa, which was especially clearly manifested in Arabia.

The Australian part of Gondwana in the Silurian was mostly land.

History of development of geosynclinal belts North Atlantic geosynclinal belt

Grampian geosynclinal region. Grampian geosyncline. A section of the Silurian of Wales, the stratotype locality where the Silurian system was identified, can be seen in Scheme III, col. incl.

The Silurian rests on the Ordovician with a structural unconformity caused by the Taconian orogeny. At the base of the Llandovery lie conglomerates and sandstones, above which are replaced by a sandy-clayey stratum with shell rocks; Pentamerides are numerous (the thickness of Llandovery reaches 1.5 km). Wenlock is lithologically diverse: in some areas of calcareous-argillaceous rocks and

limestones with remains of brachiopods and corals (300-400 m), in others - a thick layer of sandstones and siltstones (thickness -1.2 km). The Ludlov deposits are predominantly carbonate: limestone, calcareous shales, calcareous siltstones. There are numerous stromatoporates, corals, brachiopods (thickness - 0.5 km). There are fossil banks with conchidium knighti. In the upper part of the stage, there is a layer of the so-called bone-bearing breccia, which consists of parts and fragments of the bone cover of armored fish.

The described section of three tiers refers to "shell" formations - shallow-water deposits of considerable thickness containing the indicated fauna.

Another type of section of the same stages is also known - in the form of a thin stratum of graptolitic shales. Clay material in this case was deposited in the deep sea areas. The third type of incision is mixed. It contains rocks of the first and second types.

The uppermost part of the Silurian section in England is distinguished as the Downton Stage (thickness -0.6-0.9 km). These are red and variegated sandy-argillaceous rocks with interlayers of red marls. They contain shells of ostracods and ichthyofauna. Downton is gradually replaced by the lower red-colored Devonian. All this is overlapped with structural unconformity by Middle Devonian conglomerates.

In Wales, the total thickness of the Silurian is 3 km. The deposits are folded and metamorphosed. Caledonian folding manifested itself repeatedly and was accompanied by magmatism.

In the Scandinavian part of the Grampian geosyncline, thick clastic strata accumulated, at first typically marine, and towards the end of the Silurian - continental.

Ural-Mongolian geosynclinal belt

Ural-Tien Shan geosynclinal region stretches from Novaya Zemlya to the southern Tien Shan.

Ural geosyncline. Silurian deposits are widely developed in the Urals. On the western slope of the Urals, carbonate and terrigenous sediments (up to 2 km) accumulated quietly under miogeosynclinal conditions. On the eastern slope, in the eugeosyncline, lavas and tuffs, siliceous shales and limestones accumulate (thickness - 5 km). In the Silurian in the Urals, the main geotectonic structures were laid, which later turned into the existing anticlinoria and synclinoria. The Silurian of the Urals of the western and eastern slopes contains the same fauna, which indicates a single geosynclinal Ural basin in the Silurian. ,; On the territory of the western slope of the Urals and on Novaya Zemlya, miogeosynclinal conditions dominated, so carbonate and carbonate-argillaceous deposits (500-1500 m) with a diverse complex of organic remains accumulated here. Shallow coastal sand and pebble rocks are known on the western outskirts of the Northern Urals (Polyudov Ridge). In the west of the central part of the Urals, on Pai-Khoi and in places on Novaya Zemlya, black clayey graptolite shales are exposed.

Caledonian folding, in contrast to other geosynclines of the Ural-Mongolian belt, is not typical for the Urals; it did not cause structural unconformities, but the ultrabasic and basic intrusions of the central zone are considered Caledonian.

Silurian deposits are widespread in Kazakhstani part of the Ural-Mongolian belt. They are represented by typical geosynclinal formations of considerable thickness with the remains of a rich fauna. Horizons of brachiopod and coral limestones are characteristic.

In the context of Mt. Chingiztau Silurian is represented only by the lower section (see Scheme III, color inc). Silurian deposits (up to 2.5 km) accumulated in eugeosynclinal marine conditions with strong volcanism. Actively manifested Caledonian folding. The most pronounced is the last - Late Caledonian - phase of folding, which led to the retreat of the sea from the territory of the Chingiztau Ridge, to the completion of the first, actually geosynclinal, stage of its development.

tiya. The gently dipping Lower and Middle Devonian effusive rocks and felsic tuffs crowning the section accumulated already in terrestrial conditions. They are usually identified as volcanogenic molasses of the orogenic stage of development. The repeated intrusion of large granitoid intrusions is associated with folding.

Altai-Sayan folded region. Silurian deposits are known in the same place as the Ordovician, but in the west limestones and terrigenous rocks with rich fauna predominate, in the east (Western Sayan, Tuva) the role of coarse clastic rocks with depleted fauna increases. The thickness of the Silurian deposits in the west is 4.5 km, in the east - up to 7.5 km.

In the Silurian section of Western Tuva (see Scheme III, color incl.), the Silurian deposits (Chergak Group) overlie the Ordovician. They have a large thickness (2.5-3 km), consist of sandy-argillaceous rocks with interlayers, packs and lenses of limestones. The highest carbonate content is confined to the middle part of the section. The fauna is rich and varied. These are stromatoporates, tabulates, heliolithids, rugoses, crinoids, bryozoans, brachiopods, trilobites. Many local (endemic) forms. Evidently, in the Silurian there existed a shallow marine basin with small reefs, coral and crinoid thickets, and banks of brachiopods. The endemism of the fauna speaks of the difficult communication with other seas. By the end of the Silurian, the basin gradually shrank, became shallow, its salinity changed, and only euryhaline organisms survived in it.

In the Ordovician, Silurian and early Devonian in Western Tuva, a single huge (10 km) transgressive-regressive Tuva complex was formed with marine deposits in the middle part and red-colored continental rocks in the base and roof. The deposits of the Tuva complex are folded and intruded by small basic and felsic intrusions. The upper part of the section under consideration is composed of thick Lower Devonian terrestrial effusive rocks and red clastic rocks of the Middle Devonian. These are continental deposits of intermountain depressions formed during the regression caused by the Caledonian orogeny. - "In the section of Western Tuva, three structural floors that differ sharply from each other are clearly distinguished: the first is the Lower Cambrian; the second is the Ordovician, Silurian, lower Devonian; the third is the upper part of the Lower Devonian and the Middle Devonian. The floors record different stages of geological development: the first - eugeosynclinal, the third - orogenic, and the second - intermediate (transitional).At the second stage, the subsidence developed on an already consolidated basement, the regime resembled miogeosynclinal.Ore deposits of iron and copper are associated with acidic intrusions.

Thus, the Caledonian epoch of tectogenesis covered the regions of northwestern Kazakhstan, partly the Altai Mountains, the northern Tien Shan and the eastern part of the Altai-Sayan folded region - the Western Sayan and Tuva, where the Caledonides arose.

Mediterranean geosynclinal belt

In the European part of this belt, conditions are preserved that are close to those previously described in the Ordovician. This is still the insular land of the Franco-Czech massif (Moldanub block) and marine conditions to the north and south of it (Prague synclinorium, see diagram III, color incl.). In northern Europe, sandstones, black shales, bituminous limestones (0.5 km thick) accumulate, siliceous shales appear, due to manifestations of underwater volcanic activity. In southern Europe, between the Franco-Bohemian Massif and the Atlas Mountains in Africa, the Silurian is represented by monotonous facies: black shales with graptolites, which are replaced by limestones at the top of the section.

AT Asian geosynclinal region the Silurian is known in Turkey, the Caucasus, in the mountainous structures of Iran, Afghanistan, and the Pamirs.

Here, under eugeosynclinal conditions, thick strata of terrigenous rocks and volcanic rocks of basic and felsic composition, or small terrigenous-carbonate facies accumulated in miogeosynclinal zones (Zagros Himalayas, etc.).

America occupies the position of a watershed between the expanses of the Atlantic and Pacific Oceans.

From the west, it is bounded by folded mountain structures that rise steeply above the deeply submerged bed of the Pacific Ocean. In the east, the continents have abrasive shores. The continental slope is sharply defined and steep, rising at some distance from the coast above the great depths of the Atlantic Ocean.

The vast land masses of the Western Hemisphere - North and South America - are independent, historically unrelated continental structures. However, both continents have much in common. Their wedge-shaped outlines have a southerly direction. The extended part of the land is facing north. The western shores of the continents are bordered by high mountain ranges, and plains predominate in their eastern part. North America in relation to South is located much to the west. The continents are separated by a latitudinal mobile zone, in which the island arcs of the Antilles and the mountain structures of Central America, which have already articulated with the continents, are located. The Antilles-Mexican region, as we noted (Bondarchuk, 1946), is a structural analogue of Indonesia, located between the continents of Asia and Australia.

North American platform. Most of North America has a crystalline Precambrian basement. Precambrian rocks are found in the area of ​​the Canadian Shield. Separate Precambrian blocks protrude in Colorado, the Rocky Mountains, in the provinces of basins and ranges. Most of the N American Platform is covered by a thick sedimentary platform cover. In the north, in some islands of the Arctic archipelago and Greenland, the crystalline basement lies under a thick ice sheet.

The model of the structure of the North American platform, in the light of the data of K. K. Stockwell (1967) and F. B. King (1967), is characterized by such features. The oldest part of the crystalline basement in the Hudson Bay Basin, the central part of the United States and the Arctic Islands is covered by a platform cover. The Canadian shield has a zonal structure of Precambrian folded zones, gradually increasing its boundaries. Paleozoic and subsequent folded structures, building up the platform in the same way, determined the modern features of the tectoorogeny of the North American continent.

Within the territory under consideration, Precambrian folding is distinguished (King, 1967): Kenoran, Hudson, Elson, and Grenville. They deform thick Precambrian strata with a complex composition. The oldest formations of the shield are volcanogenic and sedimentary formations located among gneiss fields and other metamorphic rocks. These formations, as well as the gneisses surrounding them, host numerous gabbro and granite intrusions of different ages. Precambrian fold zones characterize individual provinces.

Kenoran folding is located in the southeast of the shield in the provinces of Upper and Slane, as well as in its northwestern part, bordering younger structures. Its age is 2390 million years.

The leveled surface of the Kenoran folding is overlain by undisturbed strata of the platform cover of the Proterozoic age. The Huronian folding includes Proterozoic deposits and older undivided gneisses and granites. It occupies the northeastern part of the shield, where it is adjacent to the Kenoran folding. In the northwestern part of the Canadian Shield, the Huronian folding is located between the areas of Kenoran folding. In Labrador and the southern edge of the Rocky Mountains, Nain Province, but according to F. B. King, these structures are reworked by later, Olson, folding.

The Huronian folding on the Canadian Shield is expressed in the provinces of Churchill, Bor, and South. Its age is determined by the early and middle Proterozoic about 1640 million years ago. The Elson folding is considered to be Middle-Late Proterozoic. It ended 1280 million years ago.

Late Proterozoic deposits lie horizontally on the Huronian folded basement.

In the southeast of the Canadian Shield, there is an area of ​​Grenville folding, concentrated mainly in the Grenville province. In the era of the Grenville folding, older structures were reworked. This folding belongs to the Late Proterozoic. It ended about 800 million years ago. On the Huronian folded basement, in some places, a platform cover of the Late Proterozoic age has been preserved.

An important role in the structure of the Canadian Shield is played by intrusions of mafic rocks, mainly gabbro and anorthosites, as well as alkaline syenites. These rocks are considered older than granites. The latter are of different age and are associated with the corresponding phases of folding. The largest intrusions are concentrated in the strata of the Kenoran structural stage. Among the post-orogenic formations, “circular structures” are distinguished, which are considered cryptovolcanic formations. They are rings of highly deformed rocks of the platform cover, and some of them belong to the Precambrian formations. Separate circular structures cut through the Kenoran and Grenville deposits. They contain igneous rocks and volcanic breccias of post-Ordovician age. Gabbro and diabase dikes are also known among the platform formations. Where the crystalline basement is exposed, all these rocks can be traced in relief.

The Precambrian basement of the N American Platform is perfectly aligned. It is strongly dissected by faults into blocks, the different positions of which create a series of depressions and elevations (Nalivkin, Gostintsev, Grossgeim, 1969).

The platform cover of the Canadian Shield is composed of sedimentary and volcanic rocks, their occurrence is horizontal or slightly disturbed. The age of the cover deposits is not the same. In the Upper Lake area, the Keninavan series of platform cover forms a wide syncline. Its layers are broken by normal faults and contain numerous bedded gabbro intrusions. In the western part of the shield and up to the Cordillera, the belt sedimentary series, also of Precambrian age, forms the platform cover. Its anchorage has not been broken.

In the Hudson Bay region, between the shield and the Appalachians, Paleozoic deposits take part in the structure of the shield. They form the lowlands south of the Canadian Shield, the plains of Western Canada, and extend into the Arctic archipelago. Further to the west, the platform cover is composed of Mesozoic and Cenozoic rocks.

In the southwestern part, the N American Platform stretches to the Rocky Mountains. Here it is broken by faults into separate blocks, one of which forms the Colorado Plateau. However, it is possible that this block is an independent island massif, one of the island system of the Cordillera fold zone. The Colorado Plateau is bounded on all sides by the structures of the Rocky Mountains. Only in the southwest does it break off with a steep ledge to the Khila valley.

The surface of the plateau rises to 1800-2600 m above sea level. The highest point - Mount San Francisco (3840 m) - an extinct volcano. The surface of the plateau is strongly denuded. Table ootans mountains and individual laccoliths rise above it. River valleys form grandiose canyons up to 1800 m deep.

The basement of the Colorado Plateau is composed of Precambrian crystalline rocks. They are overlain by a horizontally layered sequence of sedimentary rocks from Paleozoic to Quaternary age.

Of great importance are intrusions of igneous rocks and volcanogenic deposits, and on the outskirts of the plateau - lava flows. Extinct volcanoes and laccoliths are characteristic features of the plateau landscapes.

The Precambrian of the Greenland crystalline shield, according to BF King (1967), has much in common with the structure of the Canadian Shield. It composes several islands covered with a common ice cover.

The Precambrian basement of the North American Platform is bordered by fold systems of different ages located between the craton and the oceans surrounding the mainland. The oldest of the Innuit (Caledonian) systems is located along the Arctic Ocean in Northern Greenland and in the north of the Arctic archipelago. The formations of the East Greenland fold zone are considered syntectonic with Innuitic. In the northeast of Greenland, both branches of the Caledonian are articulated. From here, the East Greenland Fold Zone extends south across Scoresby Bay. Deposits of the Cambrian, Ordovician, very thick Silurian and, in places, Devonian age take part in the structure of the Early Naleozoic folded structure. On the leveling surface of the Caledonides lies a platform cover of Carboniferous, Permian and Mesozoic deposits. In some places, the occurrence of these deposits is disturbed by faults.

The southeastern part of the North American Platform is bordered by the Appalachian (Hercynian) folded zone. The formation of this zone was completed in the early Mesozoic. Both sedimentary and igneous formations take part in the structure of the Appalachians. They form a mountainous terrain.

In the southwest, the Ouachita fold region is a continuation of the Appalachians. Its strongly leveled structures are buried over a large area under younger formations. They stretch towards the Pacific Ocean, into Mexico, and can be traced under the Cordillera, lying across the strike of their structures.

From the west, the N American Platform is framed by the Cordillera fold system, stretching from Alaska north to South America, where they are continued by the Andes of Venezuela and Colombia. The Cordilleras were formed on the site of several island arcs, they consist of parts of different ages and structures.

The inner zone of the Cordillera includes older formations, dislocated and penetrated by intrusions in the middle Mesozoic (Nevada orogeny). On the outer margins of the zone, structure formation developed later - in the Late Cretaceous and Paleogene (Laramian folding, orogeny of the Rocky Mountains and British Columbia). In the Tertiary period in the Cordillera mobile zone, folding developed in local basins. At that time, fault tectonics and related volcanism played an important role.

As a result of the outpouring of plateau basalts, large volcanic plateaus arose in the states of Oregon, Washington, British Columbia and Greenland. Their outpouring continued also in the Quaternary period. At that time, volcanic fields were formed in the state of Idaho of southern Mexico, etc., as well as volcanic ridges parallel to the general strike of folding in the Cascade Range, structures stretching along the Pacific coast in Central America from Guatemala to Costa Rica.

Along the Pacific coast and in the western part of the Cordillera, the Pacific Fold Zone stands out. The structures of the Antilles island system are considered to be synchronous to it. Deformations in this zone continue to this day.

The structure of the North American Platform is characterized by the same features as other Precambrian parts of the continental crust. Its formation took place around the centers - the constituent parts of the island arcs. The process of structure formation in North America has naturally developed throughout the geological history. Its structures are spatially fixed and do not have drift layers.

The relief of the platform is characterized by a significant smoothness, large areas of accumulative plains, combined with high-mountainous countries. The brilliance of the country's landscapes is enriched by extremely diverse forms of denudation, represented over large areas and often of enormous size. Their features reflect the influence of climate on the physical geography of the steppe plains, semi-deserts, snow-covered Arctic islands, mountainous countries and subtropics covered with forests.

South American platform. The Precambrian crystalline basement of South America is exposed in the northern half of the mainland. Separate protrusions of it are known in the south in Argentina and Chile. In the northwest and west, the platform is framed by the folded mountain zone of the Andes. The mountains and basement projections are separated by the forward trough. Toward the Atlantic Ocean, the platform forms a steep continental slope and has abrasion shores. The general configuration of the coast of South America fully reflects the configuration of the adjacent part of the Mid-Atlantic Ridge.

In the structure of the South American platform, the Guinean, Central, or West Brazilian, Coastal, or East Brazilian shields are distinguished. Isolated protrusions of the Precambrian in the southern part of the mainland are the Apa, Tebikuari, Uruguayan, Northern Hills of Buenos Aires, the Pampa Blocky Country, the South Mendossa Massif, the Sovero-Patagonian and South Patagonian shields. They are separated by the Amazonian, Parnaibsky, San Franonsky, Paranskaya troughs and the Serra Geral plateau basalts associated with them, the La Plata depressions, or the Chaco-Pampasskaya, Rio Negro, Chubutskaya and Santa Cruz. Thick strata of platform cover occur within their limits.

The Guiana Shield lies in the north of South America between the Orinoco and Amazonian depressions. Its distribution generally corresponds to the Guiana Highlands. The surface of the shield is located within 500-1000 m in the west and 200-500 m above sea level in the east. The highest point - the top of Roranma - 2771 m. The highlands in the south are limited by steep slopes, and in the east - by rocky ridges. At the foot of the slopes is a hilly plain, gradually descending to the Amazonian lowland.

In the structure of the shield, sediments of the Middle and Late Pre-Cambrian age are distinguished. Hornblende and other gneisses, mica schists and granite gneisses are considered the oldest. It is associated with gabbro intrusions, as well as deposits of diabases and andesites. The younger formations of Guiana include ferruginous quartzites, a volcanogenic series of predominantly basaltic and andesitic tuffs. In the British part of Guiana, the volcanic series is composed of layered tuffs, agglomerates, lavas, quartzites, shales, and phyllites. This series is cut through by dolerite and gabbro intrusions. It contains large batholiths of granite.

The most complete Precambrian section has been described in French Guiana (Tugarinov and Voitkevich, 1966). The Cayenne system, composed of amphibolites, quartzites, hornfelses, gneisses, and migmatites with interlayers of crystalline limestones, belongs to the Lower Precambrian. These deposits are highly dislocated. The strike of their structures is variable, most often latitudinal. The Middle Precambrian is represented by the Paramaka system. It includes only intensely metamorphosed sequences of chlorite, micaceous and talc schists interbedded with lavas, including peridotites and granite intrusions. Paramak deposits are folded. The Upper Precambrian of French Guiana is divided into two parts: the lower Bonidoro Series and the upper Oranou Series. The first is dominated by detrital rocks, shales, lavas, and volcanic tuffs, including granite intrusions; the second begins with strata of conglomerates, quartzites and shales lie above. It is also cut by granite intrusions, its folded structures extend in a west-north-west direction. The Oranu Series is intruded by rhyolites, on which the sedimentary-volcanogenic Roranma Series of post-Cambrian age overlies.

Three orogenic belts are distinguished in the structure of the coastal part of the Guiana Shield (Shubert, 1956). The oldest - Gilea - covers the Cayenne system. The sedimentary and igneous rocks composing it are highly metamorphosed. The middle belt - the Guianan - includes the strata of the Paramaca system and the youngest Caribbean - deposits of the Bonidoro and Oranu series.

Thus, the Guiana Shield can be considered as an independent center of formation of the continental crust in the Precambrian. As on other shields, the expansion of the land occurred here sequentially, joining the core, composed of sedimentary-volcanogenic strata of new structural floors of folded zones.

After consolidation, the surface of the Guiana Shield was completely leveled. In the late Mesozoic, mainly in the Cretaceous, a cover of sandstones of continental origin formed on it. The remnants of this sandstone, which survived from denudation, form table uplands and play a significant role in the landscapes of the Guiana Highlands.

In the south, the Guianan shield separates the Amazonian trough from the Brazilian one. It stretches in a latitudinal direction from the Atlantic to the Pacific Ocean, from which it is separated by the folded zone of the Andes. Along the trough flows the greatest rock of the world, the Amazon, which has a tectonic valley (a very convincing example of the unity of the structure and topography of the earth's crust). The Amazonian trough is filled with Paleozoic and younger sediments. This is an inter-island accumulation basin. Its development continues in modern conditions.

The Brazilian shield is the central part of the mainland of South America south of the Amazonian trough. The Paramba-San Francisco meridional depression divides the shield into western, central and eastern, Atlantic, parts. Opies are considered as independent shields. The Paramba-San Frapsis Basin separating them is a relic of the interisland basin. The tectonic valleys of Paramba, San Francisco and the upper Parana are associated with it. In the south, the Parana and Chaco-Pampas depressions adjoin the Brazilian Shield.

The surface of the shield is very uneven and considerably raised. Throughout the shield corresponds to the Brazilian Highlands. This is an undulating plain, located on average at an altitude of 600-800 m above sea level. The crystalline foundation of the shield is broken by numerous faults into blocks that are significantly displaced relative to each other. The position of the blocks creates the orographic appearance of the highlands.

The most elevated part of the Brazilian Highlands is made up of blocky massifs of Pico di Bandeira - 2884 m and the city of Itatnaya - 2821 m above sea level. In the central part of Brazil, the watershed of the Paranaiba - Tacantins rivers rises to 1678 m. In the east, the highlands form the Great Ledge - 1000-1500 m above sea level and break off as a fault ledge to the Atlantic Ocean. Along the right bank of São Francisco, blocky ridges of the Serra do Espinhaço (up to 1800 m) stretch from the northeast to the southwest. In the south of the highlands lies the vast Serra Geral lava plateau, rising to a height of up to 1018 m.

The structure of the Brazilian shield is very complex and has not yet been sufficiently studied. The stratigraphic subdivision of the sedimentary-metamorphic complexes composing it includes an extremely large number of series and systems, the relationship of which is not unified. Conditionally, in the structure of the crystalline basement, the Precambrian is lower, middle and upper. The most ancient are the Bakoa gneisses, whose age is 2400-2500 million years. Younger formations of the Middle and Upper Precambrian are distinguished in the Minae and Itakolomi series.

The composition of the Minae series is quite variable. In the Barbacena region, it is represented by strata of gneisses and shales; north of Lafayette, the Middle Precambrian includes conglomerates, quartzites, dolomites, iron formations, graphite phyllites, lava flows, and volcanic tuffs. The thickness of the series exceeds 3000 m. It includes intrusions of ultramafic rocks and diorites. The ultramafic rocks are locally transformed into serpentinite and talc schists. The entire sequence has a northeast strike. In its southern part, isoclinal folding is well expressed. Numerous faults are known. The formation of this series is correlated with the Grenville formations of North America.

The Itacolomi Series of the Upper Precambrian of Brazil is composed of sedimentary-metamorphic strata, which include phyllites, itabirites (thin-layered, flyschoid, ferruginous quartzites), dolomites, detrital rocks, talc schists, etc. The thickness of the series is about 3000 m.

The general section of the ancient deposits of the Brazilian Shield ends with clastic sedimentary rocks of the Lavras and Bambum series, the age of which is considered Late Precambrian - Early Paleozoic. Some deposits of the Lavras Series are considered as tillites.

The structure of the Brazilian Shield is not well understood. So far, there are four stages in the history of its structure formation: 2400–2510, 1000–1100, 720–760, and 460–600 Ma (Tugarinov and Voitkevich, 1966). Structural relationships of parts of the shield of different ages are most fully displayed in the state of Mipas Gerais. The central part of the massif here is made up of Bakao gneisses (2400, 2510 Ma), they are bordered by formations aged 1350 Ma, further - sedimentary-metamorphic strata of Rio das Veyjas. From the east and west they are bordered by the formations of the Minae series, and from the south by the massifs of the Itakolomi series.

Thus, the general plan of the structure of the Brazilian Shield is a consistent expansion of the ancient structural centers due to the attachment of folded regions, which is also characteristic of the South American platform. The consolidation of the Brazilian Shield ended in the Late Precambrian. Subsequently, its surface was leveled for a long time and was the arena for the formation of a platform cover. The submeridional depression separating the shield is filled with Paleozoic and Mesozoic deposits. In places, the platform cover on the shield is composed of Triassic continental formations, marine layers of the Turonian and Paleocene age in the northern part and in the center - continental horizontally occurring Eocene strata.

The relief of the Brazilian Shield, as well as other Precambrian massifs, is characterized primarily by the position of the leveling surface deformed by faults and the position of blocks. In exposed places, the surface of the Precambrian basement has the appearance of a hilly or undulating plain, the features of which vary significantly depending on the composition of the exposed rocks. The surface dissected by erosion is characterized by rocky relief. The rivers here are rapids, mountainous.

In places covered by platform cover, the Brazilian Shield has a two-story structure. The lower floor is a crystalline plinth, the upper one is a platform cover. It is characterized by a flat surface of plateaus and plateaus, mesas, remnant elevations, limited steep or gentle slopes, the features of which in each individual case are due to the nature of the deposits exposed by depudation and many climatic factors.

In the southern part of the mainland of South America, the Precambrian formations act as separate, unrelated massifs, which in the past were independent islands. Their structure has been studied very little.

In the structure of the Uruguay crystalline shield, the Lower, Middle, and Upper Precambrian are distinguished. The Lower Precambrian deposits are extended along the La Plata valley and have a sublatitudinal strike. Their composition includes various gneisses and mica schists hosting granite intrusions. The Middle Precambrian - the Minae Formation of Uruguay - includes massive quartzites, lenses of crystalline limestones, talc schists and volcanogenic deposits. The intrusions are represented by alkaline rocks and granitoids. Upper Precambrian rocks are combined into the Otgua series. The latter includes volcanic breccias and folded quartzites. Their structures extend in the meridional and northeast directions.

Between Uruguay and the Brazilian Shield, a vast territory is occupied by the Serra Geral volcanic plateau, structurally connected with the La Plata depression. The plateau has a flat, slightly dissected surface.

Crystalline massifs in the central part of South America stand out along Paraguay - the Ana and Tebikuari horsts. In the south of the mainland, the Precambrian protrusions are concentrated in the west and are adjacent to the mobile Pacific zone. In Patagonia, they form separate shields separated by large depressions. In the Precambrian of the central part of Argentina, phyllites and greywackes are known, crumpled into folds. Their age is considered late Precambrian. In the ridges of Catamarca, La Rioya, San Luis, metamorphic strata contain granite batholiths. The gneisses of the hills of Buenos Aires host diorite intrusions.

There is still very little data on the features of the relief of the Precambrian massifs in the southern part of the South American Platform.

From the west, South America is bordered by a grandiose shaft of the South American Cordillera, which separates the platform from the Pacific Ocean. Between the platform and the folded mountain system, a foothill trough is stretched, made mainly by Cenozoic deposits. The structure of the Cordillera is complex and combines parts of different ages. The model of the cross section of the Cordillera folded zone from east to west consists of the following structural elements:

1) a platform steeply plunging to the west;

2) the forward trough of the Andes;

3) Eastern Cordillera, composed of sedimentary deposits of Paleozoic age, crumpled into folds. At the outer edge, this fold system contains isolated massifs of Precambrian schist, including granite intrusions;

4) Western Cordillera, composed of marine sediments of the Mesozoic age and younger volcanogenic formations. Their volcanic cones form the highest peaks - Chimborazo 6310 m, Cotopaxi 5943 m. In the structure of the mountains, a batholith elongated along the strike of the mountains stands out;

5) remains, or, more precisely, islands, mainly of Hercynian structures. The entire mountain range rises steeply above the adjacent deep depressions of the Pacific Ocean floor.

There are four phases in the formation of the structure of the South American Cordillera. The main folds and faults were laid in the chalk. Thrusts were formed, volcanic activity became more active. Structural formation reached its greatest strength in the early Oligocene, when the Eastern Cordillera was formed. Volcanic activity began in the Andes and continues to this day. A new intensification of movements occurred in the Miocene. Then there were many faults and normal faults, accompanied by numerous intrusions. Intrusive rocks of this age are especially common in the foothills of the Andes. Later, a leveling surface was developed in the Andes. The last phase of mountain building occurred in the Pleistocene. As a result of the general arched uplift, the modern Andes were formed. The uplift was accompanied by grandiose faults and block movements, which created the modern topography of the mountains (King, 1967).

The structure of the South American Cordillera, as rightly stated by W. Oppenheim (Oppenheim, 1948), is the final result of the development of the Late Mesozoic island arc composed of igneous rocks. The islands were separated from the mainland by a geosynclinal trough, and from the ocean by a deep depression. This structure emerged in the Cretaceous, during the first phase of orogeny in the Andes. Since then, the western structural boundary of the mainland has changed little. At the beginning of the Cenozoic, the islands, in the structure of which volcanic rocks took part, gradually united into one mountain shaft. The adjacent geosyncline was filled with terrigenous masses and limestones of marine origin. Accumulation continued until the Middle Oligocene. In the middle of the Cenozoic, the Eastern Cordillera took shape. The succession of mountain uplift is reflected in leveling surfaces and river terraces, indicating periodic rejuvenation of valley erosion.

Structural and geomorphological analysis shows that the mainland of South America has a heterogeneous structure. Its main components - the Guiana and Brazilian shields and the Amazonian trough that separates them - are the most ancient parts of the mainland. They are characterized by a sublatitudinal extension. The southern part of the mainland unites structures of different ages, the main elements of which are paleotectonic island systems, in the east - crystalline massifs of the southern part of the East Brazilian, Coastal and Uruguayan shields, in the west - the blocky country of the Pampa, the North and South Patagonian shields, etc. Between the eastern and the western systems, the La Plata depression in the south has the same importance in the structure of the mainland as the Amazonian depression in the north. With the formation in the Cenozoic of the complex folded system of the South American Cordillera at the site of the island arcs, the final configuration and orography of South America was determined.