Fig. 434.—Diagram illustrating common phenomena of a faulted region. (Dana.)
Fig. 435.—Diagram showing a fault, the plane of which forms an open fissure and has been filled with débris from above. (Powell.)
The significance of faults.[223]—Faults afford a valuable indication of the conditions of stress to which a region has been subjected, but some caution must be exercised in their interpretation. Normal faults usually indicate an extension of the surface sufficient to permit the fault-blocks to settle down unequally. Reversed faults usually signify a compression of the surface which requires the blocks to overlap one another more than they did before the faulting. In other words, normal faulting usually implies tensional stress, and reversed faulting compressional stress. It is not difficult to see, however, that in an intensely compressed and folded region there might be cases of normal faulting on the crests of folds where local stretching took place, and that reversed faults might occur even in regions of tension. But such cases must usually be local, and capable of detection and elimination by a study of the phenomena of the surrounding region. These exceptional cases aside, the general inference from prevailing normal faults is that the regions where they occur have undergone stretching, while the inference from the less widely distributed reversed faults is that the surface where they occur has undergone compression.
In view of the current opinion that the crust of the earth has been subjected to great lateral thrust as a result of cooling, it is well to make especial note of the fact that the faults which imply stretching are called normal because they are the more abundant; and that the faults which imply thrust are less common, and are styled reversed. The numerical ratio of normal to reversed faults has never been closely determined, but normal faults very greatly preponderate, and are estimated by some writers to embrace 90-odd per cent. of the whole. The testimony of normal faults is supported by the prevalence of gaping crevices, and of veins which are but crevices that stood open until they were filled by deposition. All these phenomena seem to testify to a stretched condition of the larger part of the surface of the continents. This will again claim attention in the study of Earth Movements.
Fig. 436.—Diagram showing an area of rock with monoclinal structure. One layer notably unlike the others.
Effect of faulting on outcrops.—Faulting may bring about numerous complications in the outcrop of rock formations. In a series of formations having a monoclinal structure ([Fig. 436]), many changes may be introduced. Let it be supposed in the following cases that, after faulting, the surface has been reduced to planeness by erosion. If the fault-plane be parallel to the strike of the beds (ab, [Fig. 436]), and hence a strike fault, the outcrop of a given layer may be duplicated (H, [Fig. 437]), or it may be eliminated altogether ([Fig. 438]). If the fault-plane be parallel to the direction of dip (cd, [Fig. 436]), a dip fault, the layer H will outcrop, as in [Fig. 439], if the downthrow was on the far side, or as in [Fig. 440] if the downthrow was on the opposite side. In both cases the outcrop H is offset, the amount of the offset decreasing with increasing angle of dip and increasing with increasing throw of the fault. If the fault be oblique to the direction of dip and strike (ef, [Fig. 436]), an oblique fault, the outcrop of such a layer as H will have the relations shown in [Fig. 441] if the downthrow was to the left, and that shown in [Fig. 442] if the downthrow was to the right. In the former case, it is said that there is offset with overlap; in the latter, offset with gap. The amount of the overlap and gap, respectively, increases with the increase of throw and hade, and decreases with increase of dip. In all cases the outcrop (after the degradation of the upthrow side) is shifted down dip.
Fig. 437.—Same as [Fig. 436], after (1) displacement by a strike fault and (2) base-leveling. The outcrops of certain beds are repeated.
Fig. 438.—Diagram illustrating how a strike fault in such a structure as that shown in [Fig. 436] may cause the outcrop of certain beds to disappear.