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tectonics(tek-tonn -iks) Large-scale movements of the crust of a planet or satellite, such as those that give rise to mountain building, faulting, and folding.
(or geotectonics), a branch of geology that studies the structure of the earth’s crust and its changes under the influence of mechanical tectonic movements and deformations associated with the development of the earth as a whole (see and ). Tectonics investigates the present-day structure of the crust—that is, the location and mode of occurrence of various rocks in the crust—and the regular combinations of structural elements of different orders, ranging from small folds and fractures to the continents and oceans. In addition, tectonics seeks to ascertain the history and conditions of formation of the crust (see).
Tectonics is closely connected with such other branches of geology as stratigraphy, petrology, lithology, paleogeography, and the study of useful minerals.
Basic lines of study and methods of investigation. Several lines of scientific inquiry may be distinguished within tectonics.
General tectonics (obshchaia tektonika), which is also called morphological tectonics and structural geology, studies the various types of structural elements of the lithosphere and deals primarily with small- and medium-scale crustal elements. Regional tectonics investigates the present-day distribution of such structural forms within individual regions of the crust or the lithosphere as a whole. Regional tectonics also deals with problems of tectonic subdivision on the basis of data obtained from geologic surveys and through various geophysical methods (primarily seismological methods). The largest structures are rooted in the upper mantle and are called abyssal; examples are the continental cratons and the oceanic, geosynclinal, and orogenic mobile belts. Abyssal structures contrast with crustal structures, which are localized in the crust.
Historical tectonics studies the history of tectonic movements and the history of the formation of individual structural elements of the crust and the crustal structure as a whole. In addition, it seeks to determine the chief phases of development and to identify general patterns of development (see). Historical tectonics uses the following methods of historical-tectonic or paleotectonic analysis: (1) The analysis of facies and thicknesses, that is, the study of the distribution, by area and section, of different types of sedimentary rocks (facies) and the study of changes in thickness. (2) Formation analysis, which involves the investigation of the arrangement, in area and in time (by section), of the formations of sedimentary, volcanic, intrusive magmatic, and metamorphic rocks formed in a definite tectonic setting. In most cases each formation corresponds to a definite phase in the development of the basic types of large structural elements of the crust. (3) The volumetric method, which involves the determination and comparison of the volumes of large complexes of rocks of different origin that accumulated in different phases of crustal development. (4) The analysis of interruptions and unconformities in sedimentary and metamorphic rock sequences where the interruptions or unconformities mark phases of increased activity of tectonic movements and reorganization of the structural plan of large regions of the crust.
The findings of regional and historical tectonics are used to make tectonic maps, which usually show the distribution of fold systems and cratons of different ages.
Genetic tectonics (also known as theoretical tectonics in Russian usage) generalizes the patterns of development established for the crust and individual crustal structures by regional and historical tectonics in order to create an overall theory of the development of the structure of the crust. This branch of tectonics also studies the causes of tectonic movements and the mechanism of formation of particular types of tectonic disturbances and structural elements of the crust. Various methods are used in such study; the most important is structural analysis, which reconstructs the sequence and conditions of formation of disturbances, such as folds, joints, and faults. Depending on the scale of the investigation, structural analyses may be divided into detailed, regional, and global analyses. A fourth type, microstructural and petrostructural analysis is based on the study of the orientation of rock-forming minerals and other linear elements of rock texture (seePETROTECTONICS). The final objective of structural analysis is to reconstruct the stress fields responsible for the creation of the particular structural forms.
The method of comparative tectonics involves the comparative study of as many structural elements of a single class as possible in order to identify their typomorphic characteristics and to establish the sequence of development.
The experimental method is becoming increasingly important in the study of the origin of structures of various types. Based on the principle of similitude, the method carries out the physical simulation of, primarily, medium- and small-sized structural forms. The development of the new area of tectonics known as tectonophysics, which applies the laws of solid-state physics and rheology to the investigation of the physical conditions under which tectonic structures were formed, has contributed to progress in solving the problems of genetic tectonics. Tectonophysics uses physicomathematical models in studying formation of such structures.
The branch of tectonics called neotectonics investigates tectonic movements that have occurred in recent times—that is, during the Neogene and Quaternary—and studies the structures created by these movements. Because such recent movements have played the primary role in shaping the present-day topography of the earth’s surface, they are studied primarily by geomorphologic methods. Present-day tectonic movements are studied primarily through the use of instruments and geodetic methods. Tectonics and seismology come together in the field of seismotectonics, which studies tectonic conditions under which earthquakes occur.
Tectonics is of great practical importance because it permits exploration and prospecting for useful minerals to be carried out in an efficient manner. For example, the shape of ore bodies and coal seams is often determined by the configuration of folds and the location of faults. Ore veins may be associated with systems of tectonic fractures. Oil and gas pools may be associated with anticlines and domes. The general location of, for example, ore belts and coal basins is related to the distribution of large structural elements of the earth’s crust. Findings on the structure of the upper layers of the crust and the intensity of recent tectonic movements are taken into account in building various kinds of civil engineering structures, such as canals and hydroelectric power stations.
Primary stages of development and present state of the field. It was known in ancient times that the surface of the earth is not at rest but is subject to uplifting and subsidence. During the Renaissance, Leonardo da Vinci and other scientists concluded that the discovery of fossilized sea shells at considerable elevations above sea level was the result of uplifting of the land. In the 17th century, N. Steno showed that layers of sedimentary rock were originally deposited horizontally and that their inclination and folding resulted from subsequent disturbances. In the second half of the 18th century, M. V. Lomonosov and J. Hutton recognized vertical movements—that is, uplifts and subsidences—as the main forces in the development of the crust. This idea was developed further in the 19th century by the German scientists C. L. von Buch, A. von Humboldt, and B. Studer; they formulated the first scientific tectonic hypothesis concerning “uplift craters.”
As a result of the development of the mining industry, work began in the mid-19th century on the systematization of fold and fault disturbances of the crust. The first results of this work were summarized in a compendium of structural terms published by the Swiss geologist A. Heim and the French scientist E. de Marjerie in 1888. At the same time, a more detailed study of the construction of folded structures on the basis of geologic mapping showed that the hypothesis of uplift craters was unsatisfactory and led to its replacement by the contraction hypothesis, which was set forth by, among others, L. Elie de Beaumont in 1852. The uneven distribution of folded zones of different ages on the earth’s surface was soon explained in the theory of geosynclines advanced by the American geologist J. Hall in 1859, the American geologist J. Dana in 1873, and the French geologist M. Bertrand in 1887. According to this theory, such zones form at the sites of large depressions filled with thick strata of marine sediments. In 1900, the French geologist E. Haug compared the geosynclines to the modern oceans and distinguished between the geosynclines and the continental areas, which later came to be called platforms (E. Seuss and A. D. Arkhangel’skii), or cratons (L. Kober and H. Stille). The Russian scientist N. A. Golovkinskii, A. P. Karpinskii, and A. P. Pavlov made important contributions to the theory of cratons and of crustal movements and deformations within cratons.
The new geological findings of the late 19th and early 20th centuries cast serious doubt on the contraction hypothesis, which did not give satisfactory explanations for such phenomena as large horizontal displacements of the crust (seeOVERTHRUST NAPPE), uplifting, subsidence, and magmatism. New models of the development of the earth appeared, but none of them gained general recognition. The pulsation theory attempted to overcome the weaknesses of the contraction hypothesis by introducing the idea of an alternation of compression and expansion in earth history. The theory was set forth by W. Bucher and received support in 1940 from the Soviet geologists M. A. Usov and V. A. Obruchev. The expanding earth theory was advanced by the German scientist O. Hilgenberg in 1933 and counted among its adherents the Hungarian geophysicist L. Egyed. The Austrian geologist O. Ampferer suggested in 1906 that subcrustal convection currents in the mantle are the source of tectonic deformations of the crust. This notion found some acceptance among other investigators. In the 1960’s, a number of scientists, such as the Dutch geologist R. van Bemmelen and the Soviet geologist V. V. Belousov, came to discern the source in the abyssal differentiation of the matter of the earth; in their view, the differentiation was stimulated by the heating of the matter as a result of the decay of radioactive elements.
A fundamentally different hypothesis was proposed by the German geophysicist A. Wegener in 1912. The Wegener hypothesis, which is often called the continental drift theory, was the first theory to allow for large horizontal displacements of the blocks of continental crust. The formation of the oceans was accounted for by the movement of these blocks away from one another without a change in the volume of the earth (in contrast to the expanding earth theory). The continental drift theory initiated a new school of thought in theoretical tectonics. This school, called mobilism, is opposed to fixism, which does not allow for any significant horizontal displacements of crustal blocks.
Many scientists have contributed to the investigation of the tectonics of the individual continents and to the establishment of general patterns of the makeup and development of their primary structural elements (geosynclines, orogens, and cratons). These scientists include such Soviet geologists as A. D. Arkhangel’skii, N. S. Shatskii, A. V. Peive, A. L. Ianshin, M. V. Muratov, A. A. Bogdanov, V. E. Khain, and P. N. Kropotkin. Foreign scientists who have made important contributions include the German geologists H. Stille and S. Bubnov (von Bubnoff), the American geologist G. M. Kay, and the French geologist J. Auboin.
The first courses on geotectonics in the USSR were given in the early 1920’s at the Moscow Geological Prospecting Institute and the Leningrad Institute of Mines. The publication of M. M. Tetiaev’s Fundamentals of Geotectonics in 1934 and Belousov’s General Geotectonics in 1948 contributed greatly to the acceptance of tectonics as an independent scientific discipline. After the publication in 1956 of a tectonic map of the USSR under the editorship of N. S. Shatskii, similar methods were used to produce international tectonic maps of Europe, Africa, and North America and a tectonic map of Australia (see TECTONIC MAP).
The Soviet scientists V. A. Obruchev, N. I. Nikolaev, and S. S. Shul’ts did pioneering work in neotectonics. Advances in Precambrian geology and geochronology permitted E. V. Pavlovskii and others to identify characteristics of the early stages of development of the earth’s crust.
A new stage in the development of tectonics began in the 1960’s as a result of great advances made in the geophysical study of the structure of the crust and upper mantle. The existence in the mantle of the layer of reduced viscosity known as the asthenosphere was confirmed. The investigation of the oceans brought about the discovery of the worldwide system of mid-ocean ridges, the rifts complicating the system, and the banded magnetic anomalies stretching along the ridges. A method was developed to determine the orientation of the magnetic field in past geologic ages (seePALEOMAGNETISM), and the phenomenon of reversals of polarity of the earth’s magnetic field was discovered. A method was developed for determining stresses in earthquake foci.
The new findings led to a resurgence of mobilist concepts (seeNEW GLOBAL TECTONICS) and aroused a new debate between the mobilist and fixist schools. New versions of mobilist ideas were advanced by Peive and others. The theory of the abyssal differentiation of the earth’s matter underwent further development by Belousov and van Bemmelen; the former took a purely fixist point of view, and the latter worked from a moderately mobilist standpoint.
Research is conducted on tectonics at a number of institutions in the USSR—for example, at the Geological Institute and Institute of Lithosphere Physics of the Academy of Sciences of the USSR, at the Institute of Tectonics and Geophysics of the Siberian Division of the Academy of Sciences of the USSR, at the geological institutes of the branches of the Academy of Sciences of the USSR and the academies of sciences of the Union republics, at universities, and at the scientific research institutes of the Ministry of Geology of the USSR (such as the All-Union Geological Institute) and the Ministry of the Petroleum Industry. All research is coordinated by the Interdepartmental Tectonics Committee, which since 1965 has published the journal Geotektonika (Geotectonics).
International work in tectonics is carried on by the Commission for Structural Geology and by the Subcommission for the International Tectonic Map of the World, which is headed by, among others, the Soviet scientists Peive and Khain. The subcommission has published an international tectonic map of Europe on a scale of 1:2,500,000 and international tectonic maps of Africa and North America on a scale of 1:5,000,000. It is preparing an international tectonic map of the world on a scale of 1:15,000,000. In addition, international research has been carried on within the framework of the Geodynamics Project (seeINTERNATIONAL PROJECT ON THE UPPER MANTLE OF THE EARTH) and the International Program for Geological Correlation. Problems in tectonics are also discussed at sessions of the International Geological Congress.
REFERENCESBelousov, V. V. Osnovy geotektoniki, Moscow, 1975.
Goguel, J. Osnovy tektoniki. Moscow, 1969. (Translated from French.)
Problemy global’noi tektoniki. Moscow, 1973. (Collection of articles.)
Kosygin, Iu. A. Osnovy tektoniki. Moscow, 1974.
Khain, V. E. Obshchaia geotektonika, 2nd ed. Moscow, 1973.
Khain, V. E. Regional’naia geotektonika. Moscow, 1971.
Novaia global’naia tektonika. Moscow, 1974. (Translated from English.)
Dennis, J. G. Structural Geology. New York, 1972.
Hills, E. S. Elements of Structural Geology, 2nd ed. London, 1972.
Mattauer, M. Les Déformations des matériaux de l’écorce terrestre. Paris, 1973.
Lehrbuch der allgemeinen Geologie, vol. 2. Edited by R. Brinkmann. Stuttgart, 1972.
V. E. KHAIN