Platforms are relatively stable areas crust... They arise on the site of previously existing fold structures of high mobility, formed when geosynclinal systems are closed, by their successive transformation into tectonically stable areas.
A characteristic feature of the structure of all lithospheric platforms of the Earth is their structure of two tiers or floors.
The lower structural floor is also called the foundation. The foundation is composed of highly dislocated metamorphosed and granitized rocks, pierced by intrusions and tectonic faults.
According to the time of formation of the basement, the platforms are divided into ancient and young.
Ancient platforms that also make up the core modern continents and called cratons, are of Precambrian age and formed mainly by the beginning of the Late Proterozoic. Ancient platforms are divided into 3 types: Laurasian, Gondwana and transitional.
The first type includes the North American (Laurentia), East European and Siberian (Angarida) platforms, formed as a result of the collapse of the supercontinent Laurasia, which in turn was formed after the collapse of the Pangea protocontinent.
To the second: South American, African-Arabian, Hindustan, Australian and Antarctic. The Antarctic platform before the Paleozoic era was divided into Western and Eastern platform, which united only in the Paleozoic era. The African platform in the Archean was divided into protoplatforms Congo (Zaire), Kalahari (South African), Somalia (East African), Madagascar, Arabia, Sudan, Sahara. After the collapse of the supercontinent Pangea, African protoplatforms, with the exception of Arabian and Madagascar, merged. The final unification took place in the Paleozoic era, when the African Plate became the African-Arabian Plate within Gondwana.
The third intermediate type includes small platforms: Sino-Korean (Yellow) and South China (Yangtze), which in different time were both part of Laurasia and part of Gondwana.
Fig. 2 Platforms and geosynclinal belts of the lithosphere
Archean and Early Proterozoic formations are involved in the foundation of ancient platforms. Within the South American and African platforms, part of the formations belongs to the Upper Proterozoic time. Formations are deeply metamorphosed (amphibolite and granulite facies of metamorphism); the main role among them is played by gneisses and crystalline schists, and granites are widespread. Therefore, such a foundation is called granite-gneiss or crystalline.
Young platforms formed in the Paleozoic or Late Cambrian time, they border the ancient platforms. Their area is only 5% of the total area of the continents. The platform foundations are composed of Phanerozoic volcanic sedimentary rocks that have experienced weak (greenschist facies) or even only initial metamorphism. There are blocks of more deeply metamorphosed ancient, Precambrian, rocks. Granites and other intrusive formations, among which ophiolite belts should be noted, play a subordinate role in the composition. In contrast to the foundation of ancient platforms, the foundation of the young is called folded.
Depending on the time of completion of basement deformations, the division of young platforms into Epibaikalian (the most ancient), Epicaledonian, and Epigercynian.
The first type includes the Timan-Pechora and Mizi platforms of European Russia.
The second type includes the West Siberian and East Australian platforms.
The third: the Ural-Siberian, Central Asian and Ciscaucasian platforms.
Between the basement and the sedimentary cover of young platforms, an intermediate layer is often distinguished, which includes formations of two types: sedimentary, molasse, or molasse-volcanic filling of intermontane depressions of the last orogenic stage of development of the mobile belt, preceding the formation of the platform; clastic and clastic-volcanogenic filling of grabens formed at the stage of transition from the orogenic stage to the early platform
The upper structural level or platform cover is composed of unmetamorphosed sedimentary rocks: carbonate and shallow-water sandy-clayey in platform seas; lacustrine, alluvial and boggy in a humid climate on the site of the former seas; aeolian and lagoon in an arid climate. The rocks are horizontal with erosion and unconformity at the base. The thickness of the sedimentary cover is usually 2-4 km.
In some places, the sedimentary layer is absent as a result of uplift or erosion, and the foundation comes to the surface. Such sections of platforms are called shields. On the territory of Russia, the Baltic, Aldan and Anabar shields are known. Within the shields of ancient platforms, three complexes of rocks of the Archean and Lower Proterozoic age are distinguished:
Greenstone belts, represented by thick strata of regularly alternating rocks from ultrabasic and basic volcanics (from basalts and andesites to dacites and rhyolites) to granites. Their length is up to 1000 km with a width of up to 200 km.
Complexes of ortho- and paragneisses, forming in combination with granite massifs fields of granite-gneisses. Gneisses correspond in composition to granites and have a gneiss-like texture.
Granulite (granulite-gneiss) belts, which are understood as metamorphic rocks formed under conditions of medium pressure and high temperatures (750-1000 ° C) and containing quartz, feldspar and garnet.
Areas where the foundation is covered everywhere by a thick sedimentary cover are called slabs. For this reason, most young platforms are sometimes referred to simply as slabs.
The largest elements of the platforms are syneclises: vast depressions or troughs with inclination angles of only a few minutes, which correspond to the first meters per kilometer of movement. As an example, the syneclise can be called Moscow with the center near the city of the same name and the Caspian one within the Caspian lowland. In contrast to syneclises, large platform uplifts are called anteclises. In the European territory of Russia, the Belarusian, Voronezh and Volga-Ural anteclises are known.
Grabens or aulacogens are also large negative elements of the platforms: narrow extended areas, linearly oriented and bounded by deep faults. They are simple and complex. In the latter case, along with troughs, they include uplifts - horsts. Effusive and intrusive magmatism is developed along the aulacogenes, which is associated with the formation of volcanic sheets and explosion pipes. All igneous rocks within the platforms are called traps.
The smaller elements are shafts, domes, etc.
Lithospheric platforms experience vertical oscillatory movements: they rise or fall. Such movements are associated with the transgression and regression of the sea that have repeatedly occurred throughout the entire geological history of the Earth.
In Central Asia, the formation of the mountain belts of Central Asia is associated with the latest tectonic movements of the platforms: the Tien Shan, Altai, Sayan, etc. Such mountains are called revived (epiplatforms or epiplatform orogenic belts or secondary orogens). They are formed during the orrogenetic epoch in areas adjacent to the geosynclinal belts.
Platforms - sedentary large isometric blocks of the earth's crust or a basement of igneous and metamorphic rocks, sedimentary cover, characterized by a relatively low permeability of the earth's crust, low seismicity and volcanism.
Platforms are divided into continental (cratons) and oceanic. Their main difference is:
1) the heterogeneous composition of the second layer of the crust;
2) a large difference in the layer-by-layer and total thickness of the lithosphere;
3) in the unequal internal structure of these platforms;
The sedimentary cover of the platforms is characterized by horizontal or almost horizontal bedding of layers, comparative constancy of their composition, consistency of thickness, and a set of certain platform formations.
Continental platforms represent, as it were, the cores of the continents and occupy large parts of the area of the continents. The continental platforms are composed of typical continental crust, 35-40 km thick. Within the platforms, the thickness of the lithosphere reaches 150-200 km, and in some cases 400 km. A significant part of the platforms is covered with a non-metamorphosed sedimentary cover, 3 - 5 km thick, and in the folds and depressions the thickness can reach 10 - 12 km, and in some cases 25 km. The sedimentary cover can include covers of plateau basalts, and sometimes more acidic volcanics. Where the platforms are not covered by a cover, a basement comes to the surface, composed of metamorphic rocks of varying degrees of metamorphism, as well as intrusive-magmatic rocks, mainly granites.
The platforms have a flat relief (low or plateau). Some parts of the platforms may be covered by the shallow epicontinental sea (White and Azov sea). The platforms are characterized by low modern vertical movements, very weak seismicity, absence of volcanic activity, and a lower heat flow (compared to the average terrestrial).
Continental platforms are divided into ancient and young .
The ancients are the most typical platforms with a Precambrian, mainly Early Precambrian basement and constitute the most ancient central parts of the continents. The ancient platforms include the North American, East European, Siberian, Sino-Korean. These platforms make up northern row of platforms. Next are South American, African, Hindustan, Australian, Antarctic, which occupy south row ... A separate group includes the South China Platform, which Japanese geologists call the Yangtze. The basement of these platforms is dominated by Archean formations. They are followed by the Early Proterozoic, Middle Proterozoic and Upper Proterozoic.
Ancient platforms have a polygonal outline and are separated from adjacent strike-slip structures by forward troughs. These deflections are superimposed on the lowered edges of the platforms, or are directly tectonically overlapped by their thrust peripheral zones. On the periphery of the East European Platform, both types of such relations are observed.
That. main features ancient continental platforms are:
1) a two-story structure (the basement is composed of Precambrian rocks and sedimentary cover);
2) a wide distribution of sedimentary cover of consistent thickness and the same composition;
3) intermittent folding;
4) the absence of a direct inherited connection between the structures of the cover and the folding of the basement.
Young continental platforms occupy a much smaller area of continents (about 5%) and are located mainly along the periphery of continents or between ancient platforms.
The young platforms include the Central European and Western European, East Australian, Patagonian platforms. They are located on the outskirts of continents. The West Siberian Platform refers to the platforms located between the ancient platforms.
The basement of young platforms is mainly composed of volcanic sedimentary rocks of Phanerozoic age, which are weakly metamorphosed. Granites and other intrusive formations play a subordinate role in the composition of the basement, and therefore the basement of young platforms is called not crystalline, but folded. Therefore, the basement of young platforms differs from the basement of the sedimentary cover only in its high degree of dislocation. In this regard, depending on the age of the final folding of the basement of young platforms, all platforms or their parts are subdivided into epicaledonian, epigercyn, epikimmerian.
The sedimentary cover of young platforms is composed of Jurassic or Cretaceous-Quaternary deposits. So, on the Epigercynian platforms, the cover begins with the upper feathers, and on the Epicaledonian platforms, with the upper Devon. Due to the fact that young platforms are to a greater extent covered by a sedimentary cover than the ancient ones, in the literature they are often called slabs.
That. young platforms are characterized by the following features:
1) three-story structure: basement, intermediate complex and sedimentary cover;
2) young platforms are located at the periphery of geosynclinal belts and at the junction of ancient platforms;
3) partial inheritance of the structural plan and the type of base folding in the sedimentary cover;
4) the presence of both discontinuous and linear types of folding.
14. CONSTRUCTION OF PLATFORMS
GENERAL CHARACTERISTICS
It was noted above that with the end of the geosynclinal regime, folded areas or their individual parts turn into platforms, after which their further geological development follows the path characteristic of platform areas.
The platforms are characterized by a two-tiered structure. Their foundation or base is, to one degree or another, folded formations metamorphosed and penetrated by intrusive rocks, which arose during geosynclinal development; the upper layer is the cover of sedimentary rocks accumulated during the platform regime. The sedimentary cover is separated from the basement by a pronounced unconformity, and its constituent rocks, as a rule, are not metamorphosed and slightly disturbed, lie horizontally or almost horizontally.
FORMATION
The following associations of formations are most widespread in the sedimentary cover of the platforms:
1) carbonate and glauconite-carbonate, composed of organogenic and chemogenic limestones, marls with an admixture of glauconite, dolomites and, to a lesser extent, clay rocks. Formed in open seas and lagoons;
2) red and halogen, consisting of red sandstones, mudstones and conglomerates, faciesly replaced by salts, gypsum and dolomites;
3) marine detrital, composed of strata of fine-grained sands, sandstones, clays, less often conglomerates and marls. The sands are characterized by the presence of glauconite;
4) continental, among which the formations of humid plains, arid plains and a complex of glacial formations differ. Among the formations wet low plains the most important are coal-bearing strata, alluvial deposits and weathering crust;
5) trap, represented by a complex complex of stratal intrusions and deposits of basic composition (dolerites, porphyrites, gabbro) enclosed among tuffs, tuffites and sedimentary rocks. Traps are widely developed in the sedimentary cover of the Siberian Platform, where they are dated from the Middle Carboniferous to the Lower Jurassic.
STRUCTURAL DISTRIBUTION OF PLATFORMS
The most consistent and detailed division of platforms into separate structural elements was proposed by NS Shatsky. They distinguish several groups of structures. The largest of them are called shields and slabs. Among them, in turn, subordinate structures can be distinguished: syneclises, anteclises, and aulacogens. Small structures of platforms include individual folds, rolls, flexures, breaks and cracks. Deep faults occupy a special place on the platforms.
Shields parts of platforms are called, the folded base of which is characterized by a relatively high position, due to which there is often no sedimentary cover on the shields or it has an insignificant thickness.
Slabs in contrast to shields, they are negative tectonic structures (lowered), as a result of which their sedimentary cover reaches a significant thickness.
Syneclises are extremely flat troughs with a synclinal structure with a barely noticeable fall of layers on the wings (from fractions of a meter to 2, less often 3-4 m per kilometer). These deflections always occupy a very large area and have different shapes.
Anteclises, unlike syneclises, positive structures are called, which are gentle uplifts in the form of arches. Anteclises and syneclises are closely related to each other; the wings of the syneclises are also the wings of the neighboring anteclises.
Entitled " aulacogens»NS Shatskiy identified narrow, linear depressions on the platforms, limited by large faults and accompanied by subsidence in the basement and deep depressions in the platform cover.
PLATFORM MAGMATISM
Magmatic activity within the platforms, as already indicated, is manifested to a weak degree.
The acidic and alkaline intrusions known on the platforms are small in size and concentrated mainly on their margins.
Magmatic processes are much more widespread on the platforms, leading to the formation of basic rocks, called the "trap formation".
The initial and middle phases of trap magmatism, according to A.P. Lebedev, were mainly effusive. At this time, coverings of basalts and dolerites arose and a significant amount of tuffs accumulated. The final phase is expressed in the formation of stratal deposits (sills), forming multi-storey intrusions and, less often, cutting bodies in the form of veins, dikes, columnar stocks, tubes and sometimes a network of thin irregular veins (stockworks). The time of formation of the trap formation on the platforms is associated with the periods of their general extension.
Weak intrusive activity on platforms is the main feature of their development, which distinguishes platforms from folded areas. It is possible that the transition from the geosynclinal stage to the platform stage is caused mainly by the cessation of the formation of silicic magma.
15. APPLICATION OF GEOPHYSICAL METHODS IN STRUCTURAL GEOLOGY AND IN GEOLOGICAL MAPPING
Geophysical methods are based on the study on the surface of the Earth or near it (in the air, mine workings, wells, on the surface of the water or under water) of various physical fields and phenomena, the distribution or nature of the flow of which reflect the influence of the environment - rocks that make up the thickness of the earth's crust on a particular area of research. The possibilities for solving geological problems by geophysical methods are determined by the fact that rocks, depending on the composition and bedding conditions, are characterized by certain physical properties - density, magnetic, electrical conductivity, elasticity, radioactivity, etc., differing in the numerical values of the corresponding physical constants. One and the same in its physical essence, the field, depending on the properties of the geological environment in which it is observed, will be different in intensity and structure. Thus, by studying the physical fields and identifying the features of their manifestation in a given area, we are able to establish the nature of the influence and features of the spatial distribution of rocks and other geological formations that differ in their physical properties.
In geological mapping and structural-geological studies, observations are carried out in such a way as to reveal the features of the fields (so-called anomalies) caused by contacts, faults, folded structures, intrusions, etc., that is, by those geological objects, the detection and application of which on the map and is the most important stage in the study of the geological structure of the studied territories.
Geophysical methods have a number of specific features, without understanding and taking into account which it is impossible to effectively and fully use the data obtained with their help.
First of all, it should be borne in mind that the clarity and intensity of the manifestation of the observed anomalous effects directly depends on the extent to which the rock that composes a separate geological body or layer differs in physical properties from the rocks that compose the enclosing strata or adjacent layers. These differences can manifest themselves in very different proportions and, as a rule, to varying degrees. Therefore, for a more comprehensive study of the area, more often than one, but a complex of geophysical methods is used, although this complicates and increases the cost of conducting geophysical work.
General regularities in the distribution of physical properties of rocks are already well understood. So, the density of rocks is determined mainly by their mineral composition and porosity. Therefore, igneous and highly metamorphosed rocks are denser, and loose sedimentary rocks are less dense; among igneous rocks, the density increases from acidic varieties (granites) to ultrabasic ones.
The resistivity of rocks is almost independent of the mineral composition and is determined by their porosity, moisture, as well as the mineralization of the water contained in the pores of the rock. Therefore, igneous and metamorphic rocks tend to have a higher resistivity than sedimentary ones. Among sedimentary rocks, carbonate and chemogenic deposits have a higher resistance, and terrigenous ones have a lower resistance. In the latter group of rocks, the resistance decreases with an increase in the content of clay particles and an increase in porosity. Only a small group of ore minerals (mainly sulfide), including graphite, have high electrical conductivity, due to which ore bodies and veins can in some cases be identified by electrical exploration methods as natural conductors.
The magnetic properties of rocks are mainly determined by the presence of ferromagnetic minerals in them - magnetite, ilmenite, hematite, pyrrhotite, which, as a rule, are not rock-forming and are present in rocks as accessories. The most magnetic rocks among the igneous are ultrabasic ones, and among the metamorphic ones - ferruginous quartzites. Sedimentary rocks are generally less magnetic than rocks of the two previous groups, but among them sandy deposits are relatively more magnetic and limestones, marls, and rock salts are the least magnetic.
The radioactivity of rocks depends entirely on the presence of minerals of radioactive elements (and radioactive isotopes) in them. The radioactivity of igneous rocks increases from ultrabasic varieties to acidic, among sedimentary rocks - from carbonate deposits to clayey.
The elastic properties of rocks depend on mechanical bonds between rock particles and increase from loose varieties of sedimentary formations towards igneous rocks, among which ultrabasic varieties have the greatest elasticity.
The clarity and intensity of the observed geophysical fields and anomalies directly depends on geometric factors - the size and depth of the geological objects that create them.
Geological objects of different geological nature (composition of rocks and origin), as well as different in size and depth of occurrence, geological objects can create the same geophysical fields; Consequently, one and the same observed geophysical anomaly can be explained by the presence of bodies that are different both in geological nature and in the size and depth of occurrence of bodies.
By the nature of the results obtained, the interpretation of geophysical observations is usually subdivided into qualitative and quantitative. Qualitative interpretation answers questions about the presence or absence of a particular geological body, estimates of its general configuration, the composition of rocks that make up individual bodies and layers, i.e., questions of establishing the nature of the identified anomalies. Quantitative interpretation involves obtaining quantitative indicators - the location (coordinates) of an object, its size or thickness, depth, occurrence elements, etc.
With a qualitative interpretation, ambiguity manifests itself most of all in determining the geological nature of anomalous bodies; with quantitative interpretation in determining the depth and size of objects.
The complexity of real geological conditions is often so great that in some cases they cannot be quantified due to mathematical difficulties. In these cases, the geological setting is schematized, replacing real, complex in shape and structure geological bodies with bodies of a simpler geometric shape with a uniform distribution of physical parameters (layers and veins are represented - approximated - in the form of parallelepipeds or prisms, ore bodies and intrusives - cylinders, ellipsoids, spheres, etc.).
In the practice of geophysical surveys, cases prevail when the observed geophysical fields reflect the presence of not single, but several geological objects in the geological section.
For the correct use of geophysical research materials, one should strictly adhere to unified methods of graphical representation of geophysical observations. They are presented in the form of graphs and maps, the construction of which is carried out according to the rules common to all geophysical methods.
Observations along a separate profile are depicted in the form of a graph, along the horizontal axis of which the observation points are plotted, and along the vertical axis, the value of the observed value.
To construct a geophysical map, profiles and observation points are plotted on the plan, write out about each value of the observed or calculated value as a result of interpretation, and in the numerical field obtained in this way, lines of equal values of the latter, the so-called isolines, are drawn.
Geophysical methods in geological mapping and structural-geological studies, carried out in close connection with forecasting and prospecting for minerals, allow moving from mapping the surface of bedrock to volume mapping. They give an idea of the deep structure of the studied areas within the depths that are often inaccessible to drilling, or in any case, they make it possible to more rationally determine the locations of deep structural or exploratory wells. In closed areas, they greatly facilitate surveying, and a reasonable combination of a network of geophysical observations with a network of mapping workings and wells can significantly increase the efficiency and economy of work. Finally, in all cases, geophysical methods, involving geophysical fields and physical properties of rocks in the field of research, allow a more comprehensive study of the structure of the earth's crust and increase the total amount of information on the basis of which the geologist comes to the final conclusions presented to him in the form of geological maps and predictive -search estimates.
Disagreements.
Geophysical methods are widely used in the study and mapping of unconformities. However, it should be borne in mind that they mark only those unconformities that are at the same time geophysical boundaries, that is, interfaces of rocks that differ in one or another physical properties. Thus, unconformities are generally recorded as contacts of dissimilar rocks. Whether this contact is normal, consistent with the bedding of rocks, or unconformity, it is usually impossible to establish from geophysical data alone.
The study of the unconformity surfaces that separate the structural levels of the platform areas of the earth's crust can be carried out by gravity prospecting, VES methods, telluric currents, frequency sounding, seismic methods and, in some cases, aeromagnetic survey. The most detailed study is carried out by seismic exploration.
The primary task in this case is to study the relief and depth of the surface of the crystalline or folded basement under the sedimentary cover of the platforms or in individual intermontane depressions. Research of this kind is usually combined with the study of the structure of the basement strata in order to identify individual lithological complexes, intrusive formations and faults, along which the basement is divided into separate tectonic blocks.
Horizontally overlying layers.
With the horizontal bedding of layers using geophysical methods, the following tasks are usually solved:
1) dismemberment of the strata of layers into separate horizons and determination of their thickness;
2) identification and tracking of facies changes in layers. To solve these problems, first of all, VES and seismic exploration methods can be involved, and to estimate the total thickness of a horizontal strata in medium and small-scale surveys - methods of sounding by the formation of the field and telluric field.
Facial changes in individual layers are usually established by changes in resistivity, boundary and formation velocities in the horizontal direction (from point to point of observation).
In cases where the lithological boundaries in the section of the study area, which correspond to the geoelectric and seismic boundaries, traced by electrical sounding and seismic prospecting, do not coincide with the stratigraphic ones, they are shown on maps and sections as some conditional horizons. Subsequent analysis or comparison with structural drilling data establishes the geological confinement of these conditional boundaries of the horizons.
To help tracing individual horizons exposed on the slopes of valleys, ravines, but overlain by deluvial deposits, one can use symmetric or dipole profiling, magnetometry, with a low thickness of deluvium, gamma survey, and if there are bituminized and graphitized layers or coal seams in the section, the method natural field.
Oblique layers.
At small angles of inclination of layers, the tasks solved by geophysical methods are similar to those that are put forward in the study of horizontal strata, and they are solved by the same set of methods using the same technique. Despite the fact that the interpretation of VES curves is carried out according to the palettes of theoretical curves calculated for horizontally lying layers, their application at inclination angles of seams up to 5-10 ° does not cause any noticeable errors. With a further increase in the angles of inclination, the conditions for the use of electrical exploration methods change significantly; the complex of private methods involved changes accordingly. Electroprofiling is becoming the leading method, creating favorable opportunities for the application of the induction method (dipole-inductive profiling), the radio kip method.
In seismic observations, the inclined bedding of the layers only changes the geometry of the paths of propagation of seismic waves, which is automatically reflected in the change in the values of the recorded apparent velocities and, accordingly, the shape of the hodographs. The program for the interpretation of the latter already includes the determination of the angles of inclination of the layers, and therefore, on the obtained seismic-geological section, the seismic-geological boundaries reflect the true picture of the bedding of rocks. However, in contrast to electrical prospecting, the effectiveness of which increases with an increase in the angle of incidence of layers up to vertical bedding, seismic methods can be used at inclination angles of rocks not exceeding 30-40 °.
In case of inclined bedding, it is possible to apply such methods as magnetic prospecting, gamma survey (with a small thickness of Quaternary sediments).
With the enlargement of the survey scale and the increase in the detail of the section dissection, the preference among the methods of electrical exploration should be given to electrical profiling with dipole installations.
It is recommended to use the circular profiling technique with dipole settings to determine the occurrence elements of layers overlain by Quaternary deposits.
Folded bedding forms.
The study of folded structures is one of the main tasks of structural geophysics. Its main deep methods are aimed at their solution - vertical electric sounding, sounding by the formation of the field, telluric field, refracted and reflected waves, gravity prospecting, magnetic prospecting.
When studying folded regions, the concept of the so-called reference horizons is used. A reference horizon is understood to be a formation or stratum of rocks that is well distinguished by one or another physical property, which also has sufficient thickness for a clear manifestation in the corresponding physical field. This horizon should occupy a certain stratigraphic position in the section, be consistent along the strike (along the study area) and take part in the structure of the structures under study so that, based on the data of one method or another, the behavior of this horizon could be used to judge the structures under study. This concept is especially widely used for electrical soundings. The best supporting electric horizons among terrigenous rocks are clays characterized by low resistivity; among carbonate rocks - horizons of gypsum, anhydrite, as well as massive limestone, which have a very high resistance. The surface of the crystalline basement is also taken as the reference horizon.
An important role is played by the nature of the folded structures themselves.
For seismic prospecting, structures with angles of inclination of the wings from 2 to 15 °, and, in any case, not more than 35-40 °, are favorable. For electrical soundings, only shallow structures with wing incidence angles of no more than 5-10 ° are available. A more pronounced structural relief is favorable for gravity prospecting and magnetic prospecting. Under the same conditions, electrical prospecting by the VES method is being replaced by electrical profiling. Therefore, electrical exploration by sounding methods and seismic exploration in the study of folded structures are used in platform areas, in foothill and intermontane troughs, in inner zones of large depressions. Gravity prospecting, magnetic prospecting are used both in platform conditions and in folded areas.
It should be borne in mind that the study of folded structures by means of geophysical methods in the practice of modern geophysical work is carried out in most cases inseparably with the study of unconformities between structural levels and, first of all, together with the study of the relief of a crystalline or folded basement.
Cracks.
The study of cracks in rocks is one of the detailed geological and geophysical studies. But if geological methods for studying fracturing require observations on the exposed surface of rocks, then geophysical methods make it possible to identify the main regularities of the spatial distribution of fractures and to quantitatively estimate the degree of fracturing of rocks, even if they lie at a depth of several tens of meters under Quaternary sediments or layers of other bedrocks. Of course, the detail and accuracy of quantitative estimates decreases with depth.
The main geophysical methods for studying fracturing are circular profiling, circular VES and micromagnetic survey.
Circular profiling and circular VES can be used in areas with horizontally or gently deposited sedimentary rocks, or for the study of individual massifs of igneous and effusive rocks. Their use is due to the occurrence of anisotropy in resistivity in rocks due to fracturing in the case when fractures in fractured rock are spatially oriented mainly in one or more directions. This anisotropy can be revealed if, without changing the position of the center of the measuring installation, the separation line of the latter is located at different azimuths.
Explosive violations.
Fractures are usually noted as contacts and unconformities, since often along their lines different rock complexes with different physical properties are brought into contact.
Fault faults can often be recorded either by a decrease in the resistance of rocks in the crushing zone, or due to a vein or dike formed along the rupture line, which differs in physical properties from the surrounding rocks. Detection of such violations is usually carried out by means of electrical profiling by a symmetric method or by dipole installations, by the method of radio kip, magnetic survey, and at low thickness of Quaternary deposits and by gamma survey. Crushed zones can be mapped by the method of emanation survey, since in some cases they serve as ways of removing radioactive emanations from the depth. The advantage of emanation shooting is its greater depth in comparison with gamma shooting.
Thanks to the improvement of electronic measuring technology, it became possible to use the telluric current method in closed areas with the development of thick strata of Quaternary deposits and the weathering crust for mapping tectonic faults. The latter, as a result of crushing and moistening of rocks, often represent linearly elongated conductive zones.
Study of the form and internal structure grabens and horsts can be carried out using a wide range of methods. Determination of the general nature of the structure itself and its delineation are usually carried out by gravimetric survey, and for relatively small sizes - by electrical profiling. The detailing of the structure of the edge parts is carried out by electrical profiling, magnetic survey, induction, gamma survey, which makes it possible to identify and map fault zones framing the structure, as well as to study the structure of the folded framing itself.
Effusive rocks.
The leading geophysical method for studying the conditions and forms of bedding of effusive rocks is magnetic prospecting. This is explained by the fact that effusive rocks, as a rule, are characterized by high magnetic properties, especially those of basic composition.
Dismemberment of effusive rocks revealed by magnetic survey can be helped by electrical profiling, and sometimes gamma shooting, since with an increase in the basicity of effusive rocks, their gamma activity significantly decreases.
The thickness of the effusive covers can be determined by the VES method, as well as by seismic exploration.
Micromagnetic surveying is also widely used in the study of individual massifs of effusive rocks. By the nature of the “direction roses”, it is possible to distinguish individual textural zones within the same massif, to distinguish effusive rocks belonging to different phases of the magmatic process.
Intrusive rocks.
When studying intrusive rocks by geophysical methods, the following tasks are usually solved: 1) identification and delineation of individual intrusive massifs; 2) determination of the form of underground continuation of the massifs; 3) study of the features of their internal structure.
The identification and delineation of intrusive massifs is carried out mainly by means of magnetic prospecting (air or ground, depending on the size of the sought intrusions and the scale of surveys) and gravity prospecting.
All methods of establishing the shape of intrusive bodies are ultimately approximate, since they are based on approximating intrusives by bodies of the simplest geometric shapes with smooth (flat or curved) lateral surfaces - cylinders, truncated cones, prisms.
There have been a number of attempts to study the shape of the lateral surfaces of intrusive bodies through seismic exploration, by analogy with salt domes. However, less favorable velocity ratios and abrupt dislocation and heterogeneity of the host rocks do not favor the use of seismic observations.
The study of the structural features of the massifs themselves is usually performed by electroprofiling methods, magnetic and micromagnetic surveys, gravity prospecting, gamma and emanation surveys. These methods can be used to identify fault zones (electrical profiling, magnetic survey, emanation survey), dikes of aplites, porphyry granite, lamprophyres and other rocks (gamma survey, magnetic survey, dipole profiling), greisenization zones (gravity survey, magnetic survey, emanation and gamma survey), zones of hydrothermal alteration of rocks of the massif (magnetic prospecting, electrical profiling). Magnetite-enriched skarn development zones are clearly distinguished by magnetic survey. Micromagnetic survey in the near-contact area of intrusions makes it possible in some cases to reveal fluid structures, the establishment of which can help to study the formation processes of the massif and estimate the magnitude of the modern erosional section.
Detailed high-precision magnetic survey in a number of cases makes it possible to reveal shallow pegmatite bodies by weakening the magnetic field. For the same purpose, not without success, the seismoelectric method began to be used.
By means of detailed high-precision magnetic surveys in combination with gamma-ray surveys, in some cases within the same massif it is possible to distinguish its individual parts belonging to different phases of the general tectonic-magmatic cycle, since these phases are often characterized by different compositions of accessory minerals and differences in the ratios of rock-forming minerals. ... As a result, this leads to differences in the magnetization and gamma activity of the array in different parts of it.
Metamorphic rocks.
Mapping and study of the structures and forms of occurrence of metamorphic rocks is carried out by the same geophysical methods and on the same fundamental basis as the structures formed by sedimentary and igneous rocks.
But at the same time, geophysical methods allow solving some specific problems. So, in small- and medium-scale surveys, data on changes in the horizontal direction (over the area) of certain physical parameters - density, resistivity, reservoir velocities, etc., established by geophysical observations, make it possible to judge the nature and features of the manifestation of regional metamorphism ...
During large-scale works by means of magnetic survey and electroprofiling, manifestations of contact metamorphism and ferruginization of rocks are established. Circular survey and micromagnetic surveying methods help study the layering and schistosity of metamorphic strata.
Magnetic and gravimetric surveys successfully map areas of development of ferruginous quartzites, as, for example, in the regions of the Kursk magnetic anomaly, in the Turgai trough.
Depending on the conditions of occurrence of metamorphosed rocks using a complex of different methods, they can be subdivided into separate horizons that differ in physical properties and, therefore, in lithological and petrographic characteristics. So, for example, in the areas of development of various shales, it is possible to distinguish formations of siliceous, calcareous, ferruginous, clayey shales on the basis of their different density, magnetism, resistivity, or gamma activity. These tasks are solved by means of large-scale detailed surveys using the methods of dipole profiling, radio kip, magnetometry, and gamma survey.
16. FIELD GEOLOGICAL RESEARCH
The field period is divided into three successive stages. In the first of them, covering 2-3 weeks in duration, an acquaintance with the area of work and its general overview is made. In the second stage, the bulk of the field work is carried out. In the third, final stage, all field data are linked, additional descriptions of sections are drawn up, and, if possible, a detailed study of the most promising of the identified ore-bearing areas is carried out.
TYPES OF GEOLOGICAL SURVEYS
Depending on the scale, goals and conditions of work, geological survey is carried out using various methods. The most widespread are the following surveys: route, area and instrumental.
Route survey it is used for mapping at scales of 1: 1,000,000 and 1: 500,000. It consists in crossing the work area by routes, most of which are located across the strike of rocks or folded complexes. When mapping intrusive formations, routes should cross both the marginal and central parts of the massifs.
Observations made along the route are plotted on the topographic base, and if aerial photographs are available, on them.
The geological structure of the spaces enclosed between the routes is established by interpolating the data of adjacent routes; decoding of aerial photographs can be of great help in this case.
Route studies are also used in the preparation of reference stratigraphic sections, the study of Quaternary deposits and geomorphological observations. They can be successfully used in a comparative analysis of the tectonic structure of individual regions, both for solving general issues and for studying folds, cuts, cracks, etc.
Areal survey is carried out with detailed geological mapping at a scale of 1: 200,000 - 1: 25,000. The observation points cover the entire survey area, the density of which depends on the degree of complexity of the geological structure, exposure conditions, passability, photogenicity. Observations are also carried out along routes that are planned in advance based on the structure of the area and the conditions of exposure.
Geological boundaries in areal surveys can be precisely established on the ground or their position is determined approximately. Direct geological observations, mine workings and boreholes or aerial photographs are used to determine the exact position of the boundaries. Also, they are carefully tied to local landmarks and fixed on the ground, places of finds of minerals and sampling points with a high content of minerals.
The accuracy of establishing boundaries for a geological survey at a scale of 1: 50,000 should not be less than 200 m and for maps of a scale of 1: 25,000 not less than 100 m. Depending on the validity, geological boundaries are divided into reliable and assumed.
Instrumental shooting used for geological mapping, starting at a scale of 1: 10,000 and larger. It is an areal survey, in which the application of geological objects to a topographic base is carried out instrumental. Instrumental survey methods are very different.
In instrumental surveying, it is necessary to have a sufficient network of natural outcrops or mine workings that reveal bedrocks. The contours of the latter must be precisely indicated on a topographic map. You should carefully study aerial photographs, find and mark with benchmarks all deciphered objects on the ground.
GEOPHYSICAL WORKS
Geological survey work should be preceded by a complex of ground geophysical surveys, as well as aeromagnetic and aerodyometric surveys on the scale of geological surveys and gravimetric surveys on a scale of 1: 200,000.
In addition, to solve specific geological problems and detail previously known geophysical anomalies, before or during field work, seismic, gravity, electrical, and other types of work can be carried out in separate areas, either separately or in various combinations.
STUDY AND DESCRIPTION OF NUDES
An outcrop is that part of the rock in natural conditions that is studied by a geologist. This concept equally includes outcrops on the day surface of rocks of various origins and ages, including the formations of the Quaternary period. Even with continuous exposure, it is necessary to select the most characteristic areas for the study of rocks.
When describing sedimentary rocks, the composition is established, which is reflected in the definition of the name of the rock; the color, texture, inclusions, thickness, fracturing, characteristics of weathered and fresh surfaces, the transition to the overlying and underlying layers are indicated. The thickness of each of the layers and their total outcrop thickness are determined. The elements of bedding of rocks, the direction of the most pronounced cracks are established.
The selection of samples from the rocks described should be treated with great care. Each sample taken must be representative enough with fresh surfaces. The average sample size should not exceed the palm area.
Igneous outcrops are described somewhat differently. Observations should be made from the contacts of the intrusive body to its central parts closely monitoring changes in the composition, structure and texture of rocks. It is very important to establish the orientation of the surfaces of intrusive bodies. Studying cracks can help a lot in this. Contacts of magmatic bodies with host rocks can be either intrusive or transgressive. At intrusive contacts in the host rocks, near-contact changes caused by the action of magma are observed; at transgressive contact, intrusive rocks bear traces of weathering and destruction, and sedimentary deposits overlying their eroded surface in the lower basal layer contain fragments of underlying intrusive formations.
Samples from intrusive rocks are selected so that they give an idea of the structure of both the main part of intrusive bodies and the structure of their endo- and exocontact zones. When describing intrusive massifs, their sizes should be indicated, and for veins and dikes - their thickness, directions of strike and fall.
The description of effusive formations - solidified lavas and tuffs - is close to the order of description of sedimentary rocks. When characterizing solidified lavas, special attention should be paid to the characterization of the structure and texture and the shape of the parting.
When studying folds, it is recommended to start with the characteristics of the rocks in which they are developed; the following describes: the structure of the castle and the wings, indicating the angles of their inclination, the extension of the axis and the direction of immersion of the hinges are measured. The morphological type of fold, its height and size of the wings are determined.
When describing discontinuities with displacements, the elements of occurrence of the displacer are given; composition of rocks and conditions of their occurrence on the wings. To determine the direction of movement of the fracture wings, the structure of the fault is carefully studied: grooves and friction mirrors, tectonic breccias, deformations of rocks adjacent to the fault.
One should strive to establish the amplitudes of displacement along the displacement, as well as the type of discontinuity. It should be noted that displacers of fractures with displacements of hundreds of meters can have friction breccias with a thickness of tens or more meters. Among the rubbed debris, large blocks can often be found - cut off from the rocks that make up the fracture wings.
Based on the results of geological survey work, a geological report and a set of geological maps are drawn up, including a map of the factual material, a geological map with geological sections and a stratigraphic column, maps of minerals, tectonic, geomorphological, hydrogeological maps, a map of Quaternary deposits.
LITERATURE
Azhgirey G. D. Structural Geology. Ed. Moscow State University, 1966.
Belousov V.V., Structural Geology. Ed. Moscow State University, 1971.
Buyalov NI Practical guide to structural geology and geological mapping. Gostoptekhizdat, 1955.
District map, textbook " Structural geology and geological mapping", Corresponding volume ...
The program of entrance examinations to the magistracy in the direction 05.04.01 Geology The program was discussed at a meeting of the department
ProgramA.E. Structural geology and geo-mapping. - M .: Nedra, 1991. Loshchinin V.P., Galyanina N.P. Structural geology and geological mapping: textbook ... 2000.238 p. Mikhailov A.E. Structural geology and geological mapping... - M .: Nedra, 1993. ...
Tomsk State University
Working programmThe most important are: “ Geology mineral deposits "," Structural geology and geological mapping"," Physics ". Competence ... fields and deposits. Structurally-geological early stage performances geology in the XVI - XVIII ...
The most important tools for understanding the modern structure and history of the formation and development of the geological environment are, in addition to geological drilling, geological
Document... (electronics, automation, cybernetics, astronautics) and geological (geology, planetology, geochemistry, geotectonics, etc.) sciences ... regionalization, geological mapping, studying the deep structure of the earth's crust, solving such structural tasks, ...
The following structural elements are distinguished:
1. Geosynclinal regions(or folding zones). The name of the structure comes from the Greek words: geo - Earth and sinklino - lean. These are tectonically mobile vast areas of the earth's crust stretched for tens, hundreds and thousands of kilometers. The formation of geosynclinal regions begins with a long trough of the deep ocean floor between continents or along the junction of the ocean floor with the mainland. Under the weight of the accumulation of marine sediments, the trough approaches the upper mantle (asthenosphere). This is accompanied by the formation of cracks and faults, along which the trough penetrates from the mantle into the earth's crust. These intrusions contribute to the transformation of the trough in the earth's crust, their metamorphization and the formation of ore deposits. Then the folding process begins, accompanied by the rise of individual sections of the deflection. The rise leads to the formation of a row. The process ends with the formation of powerful folded areas. Mountainous countries correspond to geosynclinal regions. Thus, the original deflections are transformed into folded mountain structures. The earth's crust in them becomes especially powerful and complexly dissected.
With the extinction of mountain building Mountain country under the influence of exogenous processes, it gradually collapses and turns first into a peneplain (almost), and then into a plain. Over time, geosynclinal areas transform into a platform.
2.Platforms(French plate-forme - flat form). These are vast, inactive areas of the earth's crust (they only commit). The platforms create a solid frame for the earth's crust. They have a two-tiered structure. The upper layer (cover) is composed of calmly lying sedimentary rocks, lying horizontally or crumpled into gentle folds by subsequent movements of the earth's crust. These sedimentary rocks can be marine and continental type, which indicates the slow vertical vibrations that the platform makes. The thickness of the sedimentary cover is relatively small - 3-4 km.
Under the cover is the lower tier of the platform, called the foundation. It is strongly folded into folds in previous geological periods, has various disseminations of magma and consists of folded metamorphosed rocks. The platform foundation is a remnant of the geosynclinal area. Sometimes part of the platform basement rises to the level of sedimentary rocks of the platform cover or above these loose deposits. Such a platform structure is called a shield (South American (Brazilian), Chinese, Indo-Chinese, African-Arabian,;
b) young platforms... At these platforms, not only Precambrian, but also Paleozoic rocks (the result of the Caledonian and Hercynian folding) are crumpled into folds - the platform.
c) There are platforms that have not yet taken shape completely and represent the transition from the geosynclinal stage to the platform stage. They have not yet had time to form a platform cover over the folded foundation. Such platforms are simply called areas of Mesozoic folding.
Extensive areas of platforms covered with thick strata (10 to 16 km) of sedimentary rocks are called plates. For example, the West Siberian plate, the Polish-German plate. Plates in geological history were formed later than ancient platforms. In the relief, the platforms and the plains correspond.
The surface of the platform foundation is not always even, it forms gentle deflections () and uplifts (). The troughs and uplifts are covered with a sedimentary cover of varying thickness.
3.Edge deflections.
Foredeeps are often located between geosynclines and platforms. They were formed as a result of the rise of mountains on the border with the platform. Foredeeps are complex in structure, reaching 15-17 km in depth, and their length is often equal to the length of the mountain structure. They are usually filled with sediment that builds up here as a result. These rocks slide down the mountain slopes and accumulate in the foredeep. So, for example, in the foredeep located between and, mined (East Ural deposit), potassium salts (Solikamsk - the most large deposit potassium salts in Russia).
Model of structure and development platform structures z.k.
Platforms are large blocks of the lithosphere consolidated by folding, metamorphism and intrusions. Distinguish between ancient and young platforms (plates).
Ancient platforms develop on the Precambrian basement. They form the cores of modern continents and are framed by younger platforms or folded structures. The ancients are separated from adjacent young platforms by subvertical deep faults, and from folded structures - either by marginal troughs or by thrust faults along which these structures tectonically overlap the platform edges.
Ancient platforms are characterized by a high degree of metamorphism of the basement rocks, a long break between the basement and the plate cover, the difference in the structural plans of the cover and the basement, asymmetry and low heat flux.
Examples of ancient platforms: East European, young: West Siberian, Turan.
The basement of young platforms is less crystalline, the rocks are less metamorphosed, contain less granites, and differ from the sedimentary cover by their intense dislocation. In relief, they are usually expressed by plains or lowlands. The structure of the sedimentary cover shows a great degree of continuity from the structure of the basement.
Platform structure
In a vertical structure, the platform consists of the following elements:
1) crystalline basement (lower structural floor) –AR + PR1.
2) proto-platform cover (top PR, R1-3 to V1)
3) slab cover (upper structural floor V2-KZ)
1.crystalline basement - in the Archean part of the section, 2 types of structures are distinguished:
Granite-gneiss domes are isometric or oval gray-gneiss complexes composed of the most ancient Early Archean crust.
Greenstone troughs (belts) (AR1 + AR2) are composed of relatively weakly metamorphosed, mainly basic, greenstone volcanics and partly sedimentary rocks.
The second type of mobile belts, characteristic of the late Archean, early Proterozoic, is the protogeosynclinal.
2. The formation of the protocell covers the period from R1 to the first half of the Vendian. During this period, the Archean-Early Proterozoic crust experienced local elongation, destruction and the formation of riftogenic-aulacogenic depressions.
3. The upper floor of the platform is represented by structural elements of different order. Structures of the first order - shields, slabs, zones of pericratonic subsidence; second order - anteclises, syneclises, aulacogens; of the third order - vaults, depressions, ramparts, local uplifts.
Christ. Shields - represent the outcrops of the crystal foundation to the surface. They lack the sedimentary cover or contain it in a reduced form.
Slabs - the space between the crystalline shields, filled with a sedimentary cover.
Zones of pericratonic subsidence - marginal, submerged areas of the platform, transitional to geosynclinal zones.
Anteclises are areas of a raised basement, composed of a cover of reduced thickness with numerous interruptions and a coarser composition of sediments.
Syneclises - correspond to areas of relative subsidence of the basement, characterized by greater thickness, completeness of the section.
Aulacogens are large linear troughs in the body of the platform, limited by faults (faults) and filled with thick strata of sedimentary rocks. Sometimes also volcanic rocks.
The surface of the platform foundation corresponds to the sheared denudation of the surface of the folded belt - the orogen. Thus, the platforms follow the orogens in the evolutionary series of large elements of the earth's crust and lithosphere. However, the real platform regime is established on the area of the former mobile belt not immediately, but with the onset of the stage of accumulation of the plate cover. And before that, during the “preplate” stage, the platform undergoes 2 preparatory stages, at which they are distinguished by an even higher mobility - the cratonization stage and the aulacogenic stage.
The first stage of cratonization over most of the area of ancient platforms corresponds in time to the first half of the Middle Proterozoic, i.e. early Riphean. Intense magmatism occurs, and layered plutons are introduced. The platform is uplifting, has a high heat flow, and experiences folding and metamorphism. As a result of the manifestation of magmatism and folding, the primary crust thickens from 8 to 35 km. At this stage, the platforms form their lower structural floor.
2. the next, aulacogenic stage on most of the ancient platforms corresponds to the Middle and Late Riphean and may also include the Early Vendian. It manifested itself in an environment of local extension associated with the uplift of the platform foundation. Grabens, troughs and aulacogenes are formed, in which terrigenous-sedimentary rocks accumulate. Trap magmatism appears. At this stage, the platform forms a protoplatform case.
On young platforms, the preplate stage is greatly shortened in time, the cratonization stage is not pronounced, and the aulacogenic stage is manifested by the formation of rifts directly superimposed on dying orogens in accordance with their strike.
3. slab stage. It begins with the Vendian and covers the entire Phanerozoic. At this stage, a sedimentary cover is formed. The platform develops in a passive gravitational lowering mode. The sedimentation process is regulated by the development of geosynclinal belts adjacent to the platform. According to the Karpinsky rule, the transgressive accumulation of sediments on the platform coincides with the subsidence regime of the geosyncline adjacent to the platform. And the regressive type of sedimentation coincides with the uplift of the geosyncline adjacent to the platform.
4. stage of tectonic-magmatic activation. It manifests itself at the time or after the accumulation of the sedimentary cover. It is associated with the manifestation of active fault tectonics and magmatism. The manifestation of this stage is indicated by the presence of rift structures. This stage is not common on all platforms. And those platforms on which it is manifested are called activated. m
