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| Fig. 7.—Reversed fault, Liddesdale. |
The relationship that exists between the hade and the direction of throw has led to the classification of faults into “normal faults,” which hade under the downthrow side, or in other words, those in which the hanging-wall has dropped; and “reversed faults,” which hade beneath the upthrow side, that is to say, the foot-wall exhibits a relative sinking. Normal faults are exemplified in figs. 1, 2, and 6; in the latter the masses A and B are on the downthrow sides, C is upthrown. Fig. 7 represents a small reversed fault. Normal faults are so called because they are more generally prevalent than the other type; they are sometimes designated “drop” or “gravity” faults, but these are misleading expressions and should be discountenanced. Normal faults are regarded as the result of stretching of the crust, hence they have been called “tension” faults as distinguished from reversed faults, which are assumed to be due to pressure. It is needful, however, to exercise great caution in accepting this view except in a restricted and localized sense, for there are many instances in which the two forms are intimately associated (see fig. 8), and a whole complex system of faults may be the result of horizontal (tangential) pressure alone or even of direct vertical uplift. It is often tacitly assumed that most normal and reversed faults are due to simple vertical movements of the fractured crust-blocks; but this is by no means the case. What is actually observed in examining a fault is the apparent direction of motion; but the present position of the dislocated masses is the result of real motion or series of motions, which have taken place along the fault-plane at various angles from horizontal to vertical; frequently it can be shown that these movements have been extremely complicated. The striations and “slickensides” on the faces of a fault indicate only the direction of the last movement.
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| Fig. 8.—Diagram of gently undulating strata cut by a fault,
with alternate throw in opposite directions. |
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| Fig. 9.—Section of strata cut by step faults. |
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| Fig. 11.—Plan of a strike fault. |
A broad monoclinal fold is sometimes observed to pass into a fault of gradually increasing throw; such a fault is occasionally regarded as pivoted at one end. Again, a faulted mass may be on the downthrow side towards one end, and on the upthrow side towards the other, the movement having taken place about an axis approximately normal to the fault-plane, the “pivot” in this case being near the centre. From an example of this kind it is evident that the same fault may at the same time be both “normal” and “reversed” (see fig. 8). When the principal movement along a highly inclined fault-plane has been approximately horizontal, the fault has been variously styled a lateral-shift, transcurrent fault, transverse thrust or a heave fault. The horizontal component in faulting movements is more common than is often supposed.
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| Fig. 12.—Section across the plan, fig. 11. |
A single normal fault of large throw is sometimes replaced by a series of close parallel faults, each throwing a small amount in the same direction; if these subordinate faults occur within a narrow width of ground they are known as distribution faults; if they are more widely separated they are called step faults (fig. 9). Occasionally two normal faults hade towards one another and intersect, and the rock mass between them has been let down; this is described as a trough fault (fig. 10). A fault running parallel to the strike of bedded rocks is a strike fault; one which runs along the direction of the dip is a dip fault; a so-called diagonal fault takes a direction intermediate between these two directions. Although the effects of these types of fault upon the outcrops of strata differ, there are no intrinsic differences between the faults themselves.
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| Fig. 13.—Plan of strata cut by a dip fault. |