Tectonic Movement


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Tectonic Movement

 

(or diastrophic movement), a mechanical movement of the earth’s crust caused by forces that act in the crust and, primarily, mantle. Tectonic movements result in the deformation of the rocks making up the crust and are usually associated with a change in the chemical composition, phase state (mineral composition), and internal structure of the rocks subjected to deformation. Such movements affect very large areas at one time. Geodetic measurements show that virtually the entire surface of the earth is constantly in motion. The rates of tectonic movements, however, are small, varying from hundredths of a millimeter to a few tens of millimeters per year. It is the accumulation of such movements over a long stretch of geologic time, such as several tens or hundreds of millions of years, that gives rise to large-scale movements of individual segments of the crust.

A commonly used classification of tectonic movements was proposed by the American geologist G. Gilbert in 1890 and was elaborated upon by the German geologist H. Stille in 1919. It divides tectonic movements into the two groups of epeirogenic and orogenic. Epeirogenic movements are expressed in the prolonged uplifting or subsidence of segments of the earth’s surface. Orogenic movements occur in definite zones during orogenic phases; they are responsible for folding and faulting and lead to the formation of mountain structures (seeOROGENY).

Although still employed today, this classification has the shortcoming of combining in the single concept of orogenesis two fundamentally different processes: (1) folding and faulting and (2) mountain building. For this reason, other classifications have been proposed. In a classification advanced by, for example, the Soviet geologists A. P. Karpinskii and M. M. Tetiaev, oscillatory fold- and fault-forming tectonic movements are distinguished from mountain building. In a classification set forth by the German geologist E. Haarmann and the Dutch scientist R. W. van Bemmelen, undation (wave) and undulation (fold) tectonic movements are distinguished from mountain building (seeOSCILLATORY MOVEMENTS OF THE EARTH’S CRUST). It became clear that tectonic movements differ greatly not only in the form they take and in the depth at which they originate but also in the mechanisms and causes of their occurrence.

As long ago as the 18th century, M. V. Lomonosov divided tectonic movements according to a different principle into the two types of slow, or secular, and rapid. Rapid movements are associated with earthquakes and are usually characterized by high speeds that are several orders of magnitude greater than the speeds of slow movements. Displacements of the earth’s surface during earthquakes are only occasionally greater than 10 m. Such displacements, however, occur sporadically; their overall effect is not much greater than the effect of the slow movements.

Tectonic movements are divided into vertical (radial) and horizontal (tangential) movements. This distinction is of fundamental importance, although it is somewhat arbitrary because the movements are interrelated and change into one another (seeHORIZONTAL MOVEMENTS OF THE EARTH’S CRUST). It is more correct, therefore, to speak of tectonic movements with a predominant vertical or horizontal component. Predominant vertical movements cause the uplifting and subsidence of the earth’s surface, including the formation of mountain structures. Such movements are the basic cause of the accumulation of thick sedimentary layers in the oceans and seas and, in part, on land. Horizontal movements are evidenced most strikingly in the formation of faults along which individual crustal blocks are displaced by hundreds or even thousands of kilometers relative to other blocks and in the overthrusting of crustal blocks for hundreds of kilometers. It has been argued that horizontal movements are also reflected in the formation of ocean basins thousands of kilometers wide through the moving apart of blocks of continental crust (seeMOBILISM).

Tectonic movements are distinguished by a definite periodicity or by irregularity, which is expressed in changes in sign and/or rate with time. Relatively short-period vertical movements with frequent changes of sign (reversible movements) are said to be oscillatory. Horizontal movements usually maintain their orientation for a long time and are irreversible. Oscillatory tectonic movements are probably the cause of transgressions and regressions of the sea and the formation of marine and stream terraces. With respect to time of manifestation, a distinction is made between recent and present-day tectonic movements. Recent tectonic movements are directly reflected in the existing relief of the earth and therefore are identified through the use of both geological and geomorphological methods. Current tectonic movements are studied not only by geological and geomorphological methods but also by geodetic methods, such as repeated leveling. Recent and present-day tectonic movements are the subject of investigation of neotectonics.

Tectonic movements in the remote geologic past are established from the distribution of transgressions and regressions of the ocean, the total thickness of accumulated sedimentary deposits and the distribution of their facies, and the distribution of the sources of the detrital material transported to depressions. This approach makes it possible to ascertain the vertical component of the movement of the upper layers of the earth’s crust or the surface of the consolidated basement under the sedimentary cover. The level of the world ocean is used as a reference plane. This level is considered almost constant; but deviations of up to 50–100 m may occur during the melting or forming of glaciers, and deviations of as much as several hundred meters may result from the change in the volume of the ocean basins owing to the growth of the basins and the formation of mid-ocean ridges.

Large horizontal movements, whose existence is not recognized by all scientists, are established from (1) geologic data, through the graphic rectification of folds and the restoration of overthrust rock masses to their original position, and (2) the study of the remanent magnetization of rocks and changes in pa-leoclimate (seePALEOMAGNETISM and PALEOCLIMATOLOGY). It is believed that with an adequate amount of paleomagnetic and geologic data it will be possible to reconstruct the past location of the continental blocks and to determine the rates and directions of subsequent movements, for example, movements that have occurred since the end of the Paleozoic.

The supporters of mobilism determine the speeds of horizontal movements from the widths of newly formed oceans, such as the Atlantic and Indian oceans; from paleomagnetic data indicating changes in latitude and orientation with respect to the meridians; and from the widths of the bands of magnetic anomalies of opposite sign that form during the growth of the ocean floor (the bands are compared with the duration of polarity epochs). In this way, estimated values of 0.1–5 cm/yr are obtained for the rates of the movements. Through the use of geodetic methods, similar figures have been obtained for the rates of present-day horizontal movements in rifts (East Africa), folded regions (Japan and Tadzhikistan), and along strike-slip faults (California). Over the course of millions of years the rates of horizontal movements have changed little, and the directions have remained virtually constant.

Vertical movements, by contrast, have a variable, oscillatory character. Repeated leveling shows that the rate of subsidence or uplift in plains areas is usually no greater than 0.5 cm/yr and the rate of uplift in mountain regions, such as the Caucasus, reaches 2 cm/yr. At the same time, the average rates of vertical tectonic movements determined for large time intervals, for example, tens of millions of years, do not exceed 0.1 cm/yr in mobile belts and 0.01 cm/yr in cratons. This difference in rates measured for small and large time intervals indicates that geologic structures record only the overall result of secular vertical movements representing the sum of oscillations of opposite sign. The similarity of tectonic movements that repeat in the same tectonic structures allows us to speak of the hereditary nature of vertical tectonic movements.

Usually not regarded as tectonic movements are rock movements in the surface zone (extending several tens of meters below the surface) that are caused by disturbances of the rock’s gravitational equilibrium under the action of exogenic geologic processes. Also not considered tectonic movements are periodic uplifts and subsidences of the earth’s surface that are due to the earth’s bodily tides resulting from the attraction of the moon and sun. It is debatable whether processes associated with the restoration of isostasy are tectonic in nature; an example of such a process is the occurrence of uplifting when large ice sheets, such as those of Antarctica and Greenland, are reduced in size. Crustal movements owing to volcanic activity are local in nature.

The causes of tectonic movements have not yet been established, although various theories have been advanced (see). Many scientists, for example, O. Ampferer (1906) and R. Schwinner (1919), believe that deep-seated tectonic movements are due to a system of large convection currents that occur throughout the upper and middle layers of the mantle. The currents are apparently responsible for the moving apart of the crust in the oceans and for compression in folded regions above zones where oncoming currents meet and sink. Other scientists, such as V. V. Belousov (1954), deny the existence of closed convection currents in the mantle and believe that upward vertical crustal movements may result from the upwelling of lighter products of mantle differentiation that have been heated in the lower part of the mantle. The cooling of these masses is regarded as the cause of crustal subsidence. Here, horizontal movements are not considered to be of primary importance; rathe’r, they are viewed as derivative from vertical movements. In explaining the nature of the movements and deformations of the earth’s crust, some investigators assign a definite role to the stresses that arise as a result of changes in the speed of the earth’s rotation. Other scientists, however, consider the stresses to be too small to play an important role.

REFERENCES

Khain, V. E. Obshchaia geotektonika, 2nd ed. Moscow, 1973.
Belousov, V. V. Osnovy geotektoniki. Moscow, 1975.

P. N. KROPOTKIN

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