ON THE CLASSIFICATION OF TECTONIC FAULTS

A. A. Belitskii

In recent years considerable attention has been devoted to the development of detailed classifications of tectonic faults \[2, 3, 4, 5, 6\]. Such great interest in this question is quite understandable, if one takes into account the important practical significance of these studies. It must be noted, however, that in the development of such classifications there is as yet no single principle and no unified terminology. Each newly proposed classification in this respect is inevitably original, with all the substantial inconveniences for practical workers that follow from this.

It is generally known that such a classification is necessary for detailed geological studies, i.e., for the exploration and exploitation of mineral deposits. At the same time it is needed first of all for deposits with a complex fault structure, such as, for example, the Prokopyevsk–Kiselevsk and Anzhero-Sudzhensk districts of Kuzbass. Let us imagine a mine field with an enormous number of tectonic faults, oriented in various ways in space (Fig. 1). Under such conditions two tasks usually arise. First, it is necessary to compile geological sections and maps (a map of the outcrops of beds beneath the overburden, a seam map, a structural map, or a projection onto a vertical plane). For this purpose, according to certain features, all tectonic faults must be traced and correlated with one another along strike and dip. Second, in such cases an extremely important task is the search for the displaced wing of the bed. It is enough to say that in the complexly dislocated districts of Kuzbass mine geologists are compelled to spend all their time deciphering tectonic faults, since in the course of exploration, when these faults are very numerous, it is impossible to cut and study all of them in exploratory workings. As for small faults with a displacement amplitude of up to 25 m, they are revealed substantially either only in the course of operational exploration or in the course of development work, and therefore they are usually encountered unexpectedly.

Let us consider briefly which features are the most important in solving the indicated tasks. It is clear that these features should form the basis of a detailed classification of tectonic faults. M. V. Gzovskii \[5\] considers the principal feature on the basis of which tectonic faults should be classified to be the true direction of the relative displacement of the wings of the fault. An additional feature is the geometric relationships of the elements of the occurrence of the fault with the beds cut by it. Against such a proposition there is hardly any ...

one might object. In principle this is correct. But the trouble is that such a formulation, generally speaking a correct one, is interpreted incorrectly by many and, in the final analysis, leads to an underestimation of the “additional criterion”—the geometric relation between the elements of occurrence of the fault and the bed. Meanwhile, in a number of cases a complex fault structure arises with a relatively limited range in the direction of displacement, for example, only thrust-strike-slip faults or only normal-strike-slip faults. Yet among such faults one can often observe a great variety of forms, and we shall not be able to solve the indicated problems if, along with the direction of displacement, we do not analyze in detail the geometric features of these faults. At the same time, we should not be embarrassed by the fact that one and the same fault, cutting beds of different attitudes, will form different forms (Fig. 2). In a concrete case this circumstance does not make matters more difficult but, on the contrary, facilitates the correlation of faults on plans and sections, since then, when the geologist has a clear idea of such phenomena, he will be able without particular difficulty to compile a map or find the displaced limb of a bed. As for names, for the general characterization of such complex faults one may recommend the usual geological four-member classification—thrust fault, normal fault, reverse fault, strike-slip fault; and for characterizing faults on different limbs of folds, one should take names from detailed morphological classifications. The latter is extremely necessary for solving a number of practical problems in the exploration and exploitation of a deposit.

Fig. 2. Tectonic fault 1—1 cuts both limbs of a fold in such a way that on one limb a concordant thrust fault (b) is formed, and on the other—a discordant thrust fault (a).

Thus, in order to compile a geological map of some area with a complex fault structure or to find the displaced limb of a bed, it is necessary to know the conditions under which various forms of tectonic faults are formed. It is known that the form of a tectonic fault is determined by two factors—the direction of displacement of the disrupted parts of the bed relative to one another and the geometric interrelation of the bed and the fault dislocator [4]. It is clear that, when compiling a map of some area with a complex fault structure and with a relatively rectilinear character of the dislocators and beds, one can correlate with one another those faults that are characterized by the same direction of displacement and by the same geometric relations between the dislocator and the bed. In tracing a fault under conditions of different attitudes of beds or of the surface of the dislocator, it is necessary to correlate those faults which, in the direction of displacement, will be more or less identical, while in geometric relation they may be different, but are completely determined for the given relation between the bed and the dislocator. For example, if both limbs of a fold are cut by one fault, then, with the thrust character of displacement, on one limb of the fold a fault of the type of a concordant thrust fault with duplication of the broken parts of the bed is formed (Fig. 2b), while on the other limb there is a discordant thrust fault, with the formation of a gap between the broken parts of the bed (Fig. 2a). Knowing the direction of displacement and the geometric relation between the dislocator and the bed, one can with sufficient accu-

Fig. 1. Seam map of the western limb of the III syncline of the Prokopyevsk district of Kuzbass within the Kalinin Mine.

On the circular diagram are plotted the poles of small tectonic faults; the contours on the diagram are the conventional boundaries of fault systems; the line \(150—330^\circ\) on the diagram is the prevailing strike direction of the beds; solid lines are coal seams, and lines with barbs are tectonic faults.

to determine with certainty the form of a tectonic fault. This can also be used in tracing tectonic faults under any conditions.

Since the forms of tectonic faults have such great significance, they must be distinguished according to certain basic, fundamental features that lead to essential differences. For example, from this point of view, can the distinction between upthrusts and thrust faults according to the angle of dip of the fault plane be considered correct, with the boundary between them taken to be a dip angle of the fault plane of 45°? From Fig. 3 it is evident that the difference between these two faults consists only in the fact that in one of them the fault plane dips at an angle of 40°, and in the other at an angle of 50°. In all other respects these are completely identical faults, and no reference to the fact that these names, understood in this way, have become rooted in our literature justifies distinguishing them. The same must also be said of left-dipping and right-dipping oblique upthrusts, distinguished by M. V. Gzovskii [5]. From Fig. 4 it is evident that these two faults have the same form, although the strikes of the beds relative to the strikes of the faults are different.

Fig. 3. Upthrust (a) and thrust fault (b) according to M. V. Gzovskii, although the direction of displacement and the form of the fault have not changed in principle.

Fig. 4. Left-dipping (a) and right-dipping (b) upthrusts according to M. V. Gzovskii.

To establish the forms of faults there exists a completely objective method, well developed in mining geometry [1; 8] and used for constructing detailed classifications by many investigators [4; 6; 7; 9]. First, the intersection of two planes in space is considered—the bed and the fault plane—and, in doing so, fundamentally different intersections are distinguished. For this purpose let us represent the lying wall of the fault in a projection with numerical elevations, and let us consider, as an example, the more general case—the intersection of a bed with a fault plane in oblique faults (Fig. 5). First we shall find the boundary between conformably dipping and discordantly dipping planes. This boundary will be the line ABC—the projection of a vertical plane onto our plan. Next, among the conformably dipping ones, we shall find the boundary between those intersections in which the bed dips more steeply and more gently than the fault plane. For this, let us construct the lying wall of the bed with the dip angle at which the fault plane dips. Then, when the line of intersection is situated on the segment BEK, the fault plane will dip more steeply than the bed, and on the segment EBC it will dip more gently than the bed.

For the present we shall limit ourselves to the cases of intersection considered and now turn to establishing the forms of tectonic faults. For this it is necessary to pass from the analysis of intersections to the analysis of displacement, and in doing so, unlike mining geometrists, we shall take the true direction of displacement. In order that these forms may be compared, we shall find them for all cases of intersection with the same direction of displacement of the hanging walls, for example, up the dip of the fault plane. On the segment BEK

everywhere we obtain such a form, which will be characterized by concordant dip of the bed and of the fault plane, with the fault plane dipping more steeply than the bed; both in transverse and in horizontal sections the disrupted parts of the bed form an overlap (Fig. 6). In the segment \(EBC\), it turns out, the situation is somewhat more complicated. Here, when the fault plane is more gently inclined in comparison with the dip of the bed, the disrupted parts of the bed may form both overlap and gaping. The boundary between these faults will be the line \(BB\), which corresponds to the line of rise of the fault plane. Consequently, in the segment \(EBC\) it is necessary to distinguish the segment \(EBB\), within which faults with overlapping walls are arranged (Fig. 7a), and \(BBC\), where faults with gaping walls are formed (Fig. 7b).

Fig. 5. Diagram of the intersection of a bed and the fault plane of oblique faults.

Fig. 6. Concordant tectonic fault with a steeper dip of the fault plane compared with the bed.

In the segment \(BCM\) there will be discordant-dip faults. It is known that, in plan, for any relationship between the bed and the fault plane, diagonal discordant-dip faults, when the hanging wall of the bed is displaced along the line of rise of the fault plane, form gaping of the disrupted parts of the bed (Fig. 8a). However, in a transverse-vertical section, under these same conditions, gaping (Fig. 8b) and overlap (Fig. 8c) of the bed may form in discordant-dip faults. Recently a number of authors [5; 6] have considered it necessary to distinguish both of these groups of faults. The boundary between them will be the line \(BL\) (Fig. 5), which—

corresponds to the line of intersection of the fault plane with a plane of the bed perpendicular to it. Consequently, in the section SBL there will be nonconformable faults, in which, in a transverse-vertical section, the broken parts of the bed form a gap, while in the section BLM they form a doubling.

Figure 7. A concordant tectonic fault with a gentler dip of the fault plane as compared with the bed.
a — doubling of the bed is formed; b — a gap in the bed is formed.

Thus, taking into account the various geometric relations of the bed and the fault plane, with the same direction of displacement of the hanging wall of the fault along the strike of the fault plane, one can distinguish, for oblique faults with an inclined dip of the bed and the fault plane, five different forms, which differ from one another in essential features. These forms, if we exclude special cases, exhaust the geometric relations of the bed and the fault plane for oblique faults with displacement along the strike of the fault plane. All of them are actually existing forms. It should be noted, incidentally, that in the classification of M. V. Gzovskii [5] one of these forms is entirely absent. He did not take into account the fact that, in concordant faults in which the fault plane dips more gently than the bed (according to M. V. Gzovskii—lagging faults), both doubling and gaping can be formed with one and the same direction of displacement of the hanging wall of the bed, but with different relations between the elements of occurrence of the bed and of the fault plane¹.

Figure 8. A nonconformable tectonic fault in plan (a) and in transverse-vertical sections:
b — gaping, v — doubling.

Now, having accepted the indicated forms as initial ones, it is necessary to consider how the mutual arrangement of the broken parts of the bed will change depending on the direction of displacement. Some investigators restrict consideration of this question to only three directions of displacement—along the strike line of the fault plane

(reverse faults, thrusts), along the strike line of the dislocator (strike-slip faults), and along the dip line of the dislocator (normal faults). Meanwhile, it is known that such directions occur relatively rarely. In the more general case, the direction of displacement may occupy any inclined position with respect to the elements of attitude of the dislocator. At the same time, the practice of detailed studies of tectonic ruptures has compelled many authors of such classifications to consider the change in the mutual arrangement of the disrupted parts of the bed when they are displaced in the directions \(0^\circ\)—\(90^\circ\), \(90^\circ\)—\(180^\circ\), \(180^\circ\)—\(270^\circ\), \(270^\circ\)—\(0^\circ\). Underestimating this most important question in a detailed morphological classification of tectonic ruptures is inadmissible.

Without a detailed consideration of this question, such a classification gives nothing for the search for the displaced limb of a bed. M. V. Gzovsky’s attempt to solve this question by correlating the elements of a reverse fault or normal fault with the elements of a strike-slip fault (the results of reverse-fault or normal-fault components are added to the results of strike-slip components, or subtracted from them) does not bring clarity to this question. In reality, for example, in concordant reverse strike-slip faults, if the result of the reverse fault is added to the result of the strike-slip fault, the amplitude of duplication will be increased (Fig. 9, BB) in comparison with a “pure” reverse fault (Fig. 9, AA). If, however, they are subtracted, then two cases are possible here: the direction of displacement lies between the line of rise of the dislocator and the line of intersection of the dislocator and the bed; then the amplitude of the bed will decrease, but duplication remains (Fig. 9 VV); if the line of displacement lies in the acute angle between the line of intersection and the strike of the dislocator, then the disrupted parts of the bed form a gap (Fig. 9 GG).

Fig. 9. Change in the form of a rupture (mutual arrangement of the disrupted parts of a bed) with a change in the direction of displacement.

It should be noted that this question is of great importance not only for the search for the displaced limb of a bed, but also for the compilation of a geological map. Cases often occur in which, in adjacent beds, ruptures of identical form are encountered at approximately the same strike (Fig. 10 LL, KK). The question arises: is this one and the same rupture, or different ruptures? It turns out that it is not enough to say “reverse strike-slip fault”; it is also necessary, in the name, to emphasize how the strike-slip element is oriented in the plane of the dislocator. Then, obviously, we shall correlate on the map only those ruptures in which, other conditions being equal, the disrupted parts of the bed have been displaced approximately in one and the same direction. In our case the ruptures LL and KK cannot be correlated with one another, since the opposite directions of displacement indicate that these are different ruptures.

Thus, taking into account the tasks that a detailed morphological classification of tectonic faults must solve, it is necessary that, in developing such a classification, all the examined features of faults be taken into account to a sufficient degree. We did not set ourselves the task of giving a new classification of faults, and considered this question only from the standpoint of principle. We object not at all to placing the direction of displacement in the name of a tectonic fault in the foreground,

Fig. 10. Tectonic faults L and K, identical in form but different in direction of movement, and therefore they cannot be correlated.

to regard the geometric relationship between the bed and the dislocator, as well as the mutual arrangement of the separated parts of the bed, as additional features. But the entire practice of detailed investigations of faults in deposits with a complex fault structure urgently requires that the classification of tectonic faults consider with equal detail the influence of the direction of displacement and of the geometric relationship between the bed and the dislocator on the formation of fault forms.

As for the names, they must, first of all, emphasize the direction of displacement, the geometric relationship between the bed and the dislocator, and also, without fail, the mutual arrangement of the separated parts of the bed. Only such a name will provide the complete characterization of a fault necessary for the successful solution of the tasks considered above.

Undoubtedly one should strive for names that are as simple as possible, but this must not be done at the expense of the detailed characterization of the fault. The simplest names are used in the four-member classification of faults—normal fault, reverse fault, thrust, strike-slip fault. However, these names characterize only one aspect of the fault—the direction of displacement, and even that incompletely, since in reality the direction of displacement may occupy any inclined position relative to the plane of the dislocator. Such a classification solves only the principal aspect of the question, which is probably sufficient for small-scale mapping. For detailed investigations, a more complete characterization of the fault is required. Furthermore, it is very important that the semantic meaning of terms and names correspond as far as possible to the essence of the phenomenon and, in any case, not contradict generally accepted concepts. Thus, for example, A. S. Zabrodin [6], distinguishing faults in which the dislocator dips more steeply or more gently than the bed, assigned them, respectively, the names “steep” and “gentle.” As a result it turned out that a “steep” fault (Fig. 11a) may have a very gentle dislocator, while a “gentle” fault (Fig. 11b) may have a steep dislocator.

It is clear that these names are unfortunate, since they contradict generally accepted geological concepts.

Finally, the classification must be developed in such a way that the basic principles are clearly formulated and the method for forming the various types of faults is set forth. Under these conditions, however complex at first glance a classification of tectonic

Fig. 11. “Steep” reverse fault (a) and “gentle” reverse fault (b) after A. S. Zabrodin

faults may be, every geologist, without memorizing the classification, will be able without particular difficulty to determine the forms of faults himself and to find their names. From this point of view, any posing of the question of the number of fault forms in a classification—something to which mining geometers once attached great importance (1)—seems meaningless to us.

REFERENCES CITED

1. Bauman V. I. On the question of normal-strike faults and other displacements of veins and seams. Transactions of the Mining Institute, vol. I, 1907.

2. Belousov V. V. Fundamentals of Geotectonics. Gosgeoltekhizdat, 1954.

3. Belousov V. V. Tectonic faults, their types and mechanism of formation. Proceedings of the Geophysical Institute of the Academy of Sciences of the USSR, No. 17 (144), 1952.

4. Belitsky A. A. Classification of Tectonic Faults and Geometric Methods of Their Study. Gosgeolizdat, 1953.

5. Gzovsky M. V. Basic questions in the classification of tectonic faults. Collection Soviet Geology, No. 41, 1954.

6. Zabrodin A. S. An experiment in searching for the displaced part of a coal seam in disturbed deposits. Bureau of Technical Information, Ugletekhizdat, part I, 1951; part II, 1952.

7. Molchanov I. A. The geometric method of investigation and its application to the search for the displaced part of a deposit. Proceedings of the Tomsk Industrial Institute named after S. M. Kirov, vol. 60, issue 1, 1939.

8. Ryzhev P. A. Mining Geometry. Ugletekhizdat, 1952.

9. Usov M. A. Forms of disjunctive dislocations in the mines of Kuzbass. Collection on the Geology of Siberia, Tomsk, 1933.

Tomsk Polytechnic Institute
named after S. M. Kirov

1. A detailed analysis of this question is given in another work by the author [4]. ↩