a rupture in the earth’s crust that occurs during tectonic movements and rock deformation. The walls of faults are formed by the offset rock masses. When a fault is inclined, a distinction is made between the footwall, which underlies the fault, and the hanging wall, which overlies the fault.
Some faults are marked by insignificant relative displacement of the walls (tectonic fractures), and others by significant displacement (fault displacements). Among the latter several different kinds of faults are identified. A strike-slip fault is formed by the horizontal displacement of the walls along a vertical or inclined rupture. An extension fault results from the movement of walls away from one another, and in a normal fault the hanging wall shifts downward. Reverse faults and thrust faults are formed by a displacement of the hanging wall upward; the difference between the two types of faults lies in the size of the angle of dip. An overthrust nappe is formed when a hanging wall is overthrust with great amplitude along a very gentle, a horizontal, or a wavy fault. Combined displacements (for example, combined normal and strike-slip faults) are common.
The size of faults and the amplitude of displacement along them vary. In most cases, tectonic fractures without displacement are not more than a few meters long. Faults with displacement vary from small fractures a few decimeters long to abyssal fractures that pierce the earth’s crust and part of the upper mantle. The amplitude of normal faults reaches several kilometers, whereas strike-slip faults and overthrust nappes have amplitudes of dozens of kilometers (even several hundred, according to some investigators). Different stresses cause the formation of the various types of faults. Reverse faults, thrust faults, and overthrust nappes, which are usually combined with rock folding, form in zones of crustal contractions. In zones of crustal extension normal and extension faults occur. Zones with large numbers of normal faults are called rifts.
Movement along faults may be brief or may continue for prolonged periods of geological time. In the latter case it takes the form of distinct jolts accompanied by earthquakes. The cavities of faults often serve as routes for ascending hydrothermal solutions that form vein rocks.
V. V. BELOUSOV
in geology, a tectonic fracture of the earth’s crust in which the walls of the fracture are shifted horizontally along its strike; the term “fault” may also refer to the displacement process. A distinction is made between a right fault, in which, when viewed in a plane, the relative displacement of the walls is clockwise, and a left fault, in which the movement of the walls is counterclockwise.
Faults range in length from several meters to many hundreds of kilometers, and the amplitude of displacement may vary from a few centimeters to many dozens, and probably hundreds, of kilometers. When the strike changes significantly, a fault may become a normal fault, a thrust fault, or a strike-slip fault. Characteristically pulsational, faulting only involves certain areas of the walls at one time. Displacement may occur along the fault line or in an adjacent zone as wide as several hundred kilometers, causing buckling and the formation of many fractural elements.
Faults are most characteristic of an area of folds. Large faults start forming during the orogenic period and develop as long as several tens of millions of years. The largest and most extensively studied faults are the San Andreas in California, the Talass-Fergana in the Tien-Shan, and the Great Glen of Scotland.