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| Fig. 1.—Section of clean-cut fault. |
FAULT (Mid. Eng. faute, through the French, from the popular Latin use of fallere, to fail; the original l of the Latin being replaced in English in the 15th century), a failing, mistake or defect.
In geology, the term is given to a plane of dislocation in a portion of the earth’s crust; synonyms used in mining are “trouble,” “throw” and “heave”; the German equivalent is Verwerfung, and the French faille. Faults on a small scale are sometimes sharply-defined planes,[1] as if the rocks had been sliced through and fitted together again after being shifted (fig. 1). In such cases, however, the harder portions of the dislocated rocks will usually be found “slickensided.” More frequently some disturbance has occurred on one or both sides of the fault. Sometimes in a series of strata the beds on the side which has been pushed up are bent down against the fault, while those on the opposite side are bent up (fig. 2). Most commonly the rocks on both sides are considerably broken, jumbled and crumpled, so that the line of fracture is marked by a belt or wall-like mass of fragmentary rock, fault-rock, which may be several yards in breadth. Faults are to be distinguished from joints and fissures by the fact that there must have been a movement of the rock on one side of the fault-plane relatively to that on the other side. The trace of a fault-plane at the surface of the earth is a line (or belt of fault-rock), which in geological mapping is often spoken of as a “fault-line” or “line of fault.” Fig. 3 represents the plan of a simple fault; quite frequently, however, the main fault subdivides at the extremities into a number of minor faults (fig. 4), or the main fault may be accompanied by lateral subordinate faults (fig. 5), some varieties of which have been termed flaws or Blatts.
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| Fig. 2.—Section of strata, bent at a line of fault. |
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| Fig. 3.—Plan of simple fault. |
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| Fig. 4.—Plan of a fault splitting into minor faults. |
“Fault-planes” are sometimes perpendicular to the horizon, but more usually they are inclined at a greater or lesser angle. The angle made by the fault-plane with the vertical is the hade of the fault (if the angle of inclination were measured from the horizon, as in determining the “dip” of strata, this would be expressed as the “dip of the fault”). In figs. 1 and 2 the faults are hading towards the right of the reader. The amount of dislocation as measured along a fault-plane is the displacement of the fault (for an illustration of these terms see fig. 18, where they are applied to a thrust fault); the vertical displacement is the throw (Fr. rejet); the horizontal displacement, which even with vertical movement must arise in all cases where the faults are not perpendicular to the horizon and the strata are not horizontal, is known as the heave. In fig. 6 the displacement is equal to the throw in the fault A; in the fault B the displacement is more than twice as great as in A, while the throw is the same in both; the fault A has no heave, in B it is considerable. The rock on that side of a fault which has dropped relatively to the rock on the other is said to be upon the downthrow side of the fault; conversely, the relatively uplifted portion is the upthrow side. The two fault faces are known as the “hanging-wall” and the “foot-wall.”
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| Fig. 5.—Plan of main fault, with branches. |
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| Fig. 6.—Section of a vertical and inclined fault. |