Tectonic Theories

Tectonic Theories

 

scientifically grounded theories on the causes of the movements and deformations of the earth’s crust that are responsible for the crust’s structure.

The causes of tectonic deformations are still not fully understood because the primary source of such deformations must be sought in the mantle and we lack exact data on the state and movement of matter below the base of the crust. Furthermore, the opportunities presented by quantitative processing of the data of regional and historical geology have not yet been fully taken advantage of. Such data permit the reconstruction of the course, on a planetary scale, of endogenic processes—for example, the development of transgressions, regressions, uplifts, depressions, folding, and magmatism. Consequently, a great number of tectonic theories have been advanced. In these theories the causes of tectonic development are attributed to very different factors.

All the theories that have been proposed can be divided into the two groups of fixism and mobilism. The fixist theories assume that there has been no change in the relative horizontal positions of the individual crustal blocks throughout geologic history and that vertical tectonic movements have played the principal role in the development of the crust. The mobilist theories assume large horizontal displacements of continental blocks of crust and assign to these displacements the primary role in the development of the crust.

The first attempt at a scientific explanation of the deformation of rock layers was made in the 18th century by A. Werner in his theory of neptunism, which attributed disruptions of the horizontal bedding of layers to subaqueous slumps or cave-ins.

At the same time, J. Hutton advanced the theory of plutonism, which was based on the notion that vertical uplifts predominated in the development of the earth. In the first quarter of the 19th century, this notion was elaborated by the German scientists C. von Buch, A. von Humboldt, and B. Studer. They advanced the hypothesis of “uplift craters,” which explained the formation of folded mountain structures by the rising of magma in volcanic processes and intrusive magmatic processes. This explanation, however, proved inadequate.

In the second half of the 19th century and the early 20th century, the contraction hypothesis received almost universal recognition. Among the scientists who contributed to its development were the Western Europeans L. Elie de Beaumont, A. Heim, E. Suess, and H. Jeffreys and the Russians A. P. Karpinskii, F. N. Chernyshev, I. V. Mushketov, A. P. Pavlov, and K. I. Bogdanovich. The contraction hypothesis was based on the Kant-Laplace cosmogonic hypothesis, which postulated an originally molten earth that subsequently underwent a gradual cooling. According to the contraction hypothesis, tectonic deformations are due to the cooling of the earth and reduction of its radius. The bending of layers into folds was attributed to compressive horizontal forces arising in the crust as the size of the planet decreased. The discovery of the radioactivity of rocks, however, cast doubt on the hypothesis’s basic assumption of an earth undergoing cooling from an originally molten state. It was demonstrated that the thermal energy released during radioactive decay compensates, or perhaps more than compensates, for the earth’s loss of heat. In the first half of the 20th century the contraction hypothesis was supplanted by the theories of abyssal differentiation, subcrustal currents, pulsation, continental drift, and an expanding earth.

The abyssal differentiation theory was proposed by the Dutch scientist R. W. van Bemmelen and the Soviet geologist V. V. Belousov. It is based on the assumption of an initially cold earth. The high temperature of the earth’s interior is attributed to warming as a result of the release of heat in the decay of radioactive elements. The warming causes partial melting of the mantle material and its differentiation, which occurs irregularly. Continental crust forms in zones of maximum accumulation of melted light silicate material. In conformity with the theory of isostasy, the top layers of the crust are uplifted, and elevations form. Thus, vertical movements are considered primary. Folding is regarded as being due to two causes. One cause is gravity tectogen-esis: layers are bent into folds when masses of material slide down the slopes of elevations. The other cause is what Belousov calls abyssal diapirism—that is, the uplift of abyssal masses of crust along the axis of folded structures during regional metamor-phism and granite formation. Folding occurs on the periphery of these structures as a result of the extension and compression of sedimentary strata. The formation of the ocean basins is considered the result of subsidence of the ocean floor without significant extension but with transformation of continental crust into thinner basalt crust. This process of crustal transformation is called oceanization, or basification, and has been investigated by Belousov and S. I. Subbotin.

The subcrustal currents theory was developed by the Austrian tectonics specialist O. Ampferer, the German scientists R. Schwinner and E. Kraus, and the Dutch geophysicist F. Vening Meinesz. It assumes the existence of convection currents in the mantle that draw the crust after them and thereby cause the crust’s deformation. Equal importance is assigned to vertical and horizontal movements of the crust. It has not yet been demonstrated, however, that permanent or long-lasting convection currents exist or can be formed in the mantle.

The pulsation theory was advanced by the American geologist W. H. Bucher and the Soviet scientists M. A. Usov and V. A. Obruchev. Building on the contraction hypothesis’s notion of the compression of the earth, the theory makes use of an alternation of global epochs of compression and epochs of expansion in order to account for a number of phenomena left unexplained by the earlier hypothesis—for example, magmatism and the transgression and regression of the world ocean.

The expanding earth theory was developed by a number of scientists, including the German geologist O. Hilgenberg, the Hungarian geophysicist L. Egyed, and the American geologist B. Heezen. It attributes the formation of the ocean basins to the moving apart of the continental blocks as the earth’s radius increased through geologic time. The causes of such an expansion, however, remain unclear.

The continental drift, or displacement, theories represented a fundamentally new approach to tectonic processes. Seminal contributions to the drift concept were made by the American geologist F. Taylor and, especially, the German geophysicist A. Wegener. The continental drift theories postulate that the continental blocks may undergo horizontal displacements of thousands of kilometers over subcrustal layers or together with them (owing to subcrustal convection currents in the earth’s mantle). The cause of these movements was initially considered to be forces resulting from the rotation of the earth.

The 1960’s and 1970’s saw a rebirth of interest in mobilist concepts. New findings were made use of in the development of the new global tectonics, or plate tectonics, by a number of scientists, including the Americans H. Hess and R. Dietz. This theory postulates the existence of subcrustal convection currents and draws on paleomagnetic and seismological data. In addition, it takes into consideration the characteristics of magnetic anomalies and the results of ocean floor drilling. According to plate tectonics, the comparatively brittle lithosphere, which rests on the plastic asthenosphere, is divided into rigid plates. The plates are separated from one another by tectonic faults, or sutures, along the axial lines of the earth’s seismic belts. The plates encompass not only the continents but also parts of the ocean floor that are appended to the plates and were formed primarily in the Mesozoic and Cenozoic. Various motions may be exhibited by the plates. For example, they may move apart so as to form first rift zones and then oceans. One plate may plunge under another. An additional example is the horizontal displacement of plates relative to each other. The expansion of the lithosphere in the ocean regions and the formation of new oceanic crust are compensated for by a decrease in the crustal surface when some plates are subducted under others along the peripheries of the oceans, in the island arc regions, and at the base of young folded mountains, as in the Himalayan foredeep. This part of the theory is confirmed by the distribution of stresses in earthquake foci. The bending of layers in such zones of crustal compression is expressed in folding. Geodetic data provide evidence for the moving apart of blocks (as in northeast Africa), for the slipping of blocks past one another along faults at rates of 0.5–3 cm a year (as in California), and for the thrusting of one block over another (as in Tadzhikistan). Rates of horizontal displacement of the same order are obtained from paleomagnetic data, the widths of belts of magnetic anomalies along the mid-ocean ridges, and paleogeographic reconstructions.

Because of its fairly complete and simple explanation of diverse geological, geophysical, and geochemical facts, plate tectonics quickly gained many adherents. The theory has failed, however, to clarify a number of points. Examples are the notion of the propelling force that moves the plates, the nature of the geologic processes in the rift zones of the mid-ocean ridges, the mechanism of the underthrusting and subduction of oceanic crust in the island arc zones, and the causes of tectonic processes within the lithospheric plates, especially in the continental cratons. Attempts are being made to overcome these shortcomings and to explain the formation of mineral deposits from the standpoint of plate tectonics. It will probably be possible to choose among the competing models and to create a general theory of the development of the earth’s crust when sufficient geodetic data are obtained on the relative motion of the continents and more reliable information is available on the composition and structure of the lithosphere, especially under the ocean, and of deeper shells of the earth.

REFERENCES

Belousov, V. V. Osnovy geotektoniki. Moscow, 1975.
Kropotkin, P. N. Evoliutsiia Zemli. Moscow, 1964.
Khain, V. E. Obshchaia geotektonika, 2nd ed. Moscow, 1973.
Problemy global’noi tektoniki: Sb. st. Moscow, 1973.
Novaia global’naia tektonika: Sb. St. Moscow, 1974. (Translated from English.)

P. N. KROPOTKIN

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