a change in the mode of bedding, volume, internal structure, or relative position of a body of rock under the action of deep-seated forces that give rise to conditions of unidirectional or multidirectional extension, compression, or displacement (see). Tectonic deformations are evidenced most clearly in sedimentary, igneous, and meta-morphic rocks in the form of various folded and faulted disruptions of the rocks’ initially horizontal bedding. In magmatic rocks and in schists, tectonic deformations lead to reorientation or recrystallization of the constituent minerals (seePETROTECTONICS and ).
The classification of tectonic deformations is based primarily on the study of bedded rock formations. A distinction is made between folded and faulted tectonic deformations (seeFOLDING and FAULT). Folding results from the plastic deformation of rocks, which occurs when the tectonic stresses exceed the rocks’ elastic limit. Faulting occurs as a consequence of the fracture of rocks, which occurs when the tectonic stresses exceed the rocks’ ultimate strength.
Individual regions of the earth’s crust that differ in their geologic history and structure are characterized by a certain combination of tectonic deformations making up a particular tectonic regime. The external appearance, type, and size of tectonic deformations depend on a number of factors, including the physical properties of the rocks and the direction, intensity, and duration of active tectonic forces. The mechanism and causes of tectonic deformations are an important part of the general problem of the development of the earth’s crust and the occurrence of earthquakes (seeSEISMOLOGY and ).
In the mid-20th century, various questions of the occurrence of tectonic deformations are being investigated not only through the detailed study of natural objects but also, to an increasing degree, through experimental research using models based on the physical theory of similitude (see) and through the study of geochemical processes occurring in the crust and interior shells of the earth. (See also).
REFERENCESBelousov, V. V. Strukturnaia geologiia, 2nd ed. Moscow, 1971.
Khain, V. E. Obshchaia geotektonika, 2nd ed. Moscow, 1973.
V. N. SHOLPO