Page:EB1911 - Volume 10.djvu/219

Rh 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.

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.

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.

The effect of normal faults upon the outcrop may be thus briefly summarized:—a strike fault that hades with the direction of the dip may cause beds to be cut out at the surface on the upthrow side; if it hades against the dip direction it may repeat some of the beds on the upthrow side (figs. 11 and 12). With dip faults the crop is carried forward (down the dip) on the upthrow side. The perpendicular distance between the crop of the bed (dike or vein) on opposite sides of the fault is the “offset.” The offset decreases with increasing angle of dip and increases with increase in the throw of the fault (fig. 13). Faults which run obliquely across the direction of dip, if they hade with the dip of the strata, will produce offset with “gap” between the outcrops; if they hade in the opposite direction to the dip, offset with “overlap” is caused: in the latter case the crop moves forward (down dip) on the denuded upthrow side, in the former it moves backward. The effect of a strike fault of diminishing throw is seen in fig. 14. Faults crossing folded strata cause the outcrops to approach on the upthrow side of a syncline and tend to separate the outcrops of an anticline (figs. 15, 16, 17).

In the majority of cases the upthrown side of a fault has been so reduced by denudation as to leave no sharp upstanding ridge; but examples are known where the upthrown side still