Transactions of the Tomsk Order of the Red Banner of Labor Industrial Institute named after S. M. Kirov

Vol. 62, I

1944

Studies on the Methodology of Geological Research

I. Analysis of the Development of Tectonic Structures

K. V. Radugin

1. INTRODUCTION

As a student of the school of Academician M. A. Usov, I more than once heard his lectures, saturated with ideas on the development of tectonic structures in the exposition of phases of tectogenesis in Western Siberia. He illustrated this thought especially vividly with the plicatogenic disjunctives of Kuzbass, which developed in several phases of tectogenesis. In particular, in recent years he explained such a form as a reverse fault, resembling an unconformably dipping normal fault, as an overturned disjunctive, originally a longitudinal conformable reverse fault. It is evident, therefore, that the method described below arose on prepared ground and by no means accidentally. It first took shape for the author in 1936, during work in the region of the city of Krasnoyarsk, at the time of an attempt to reconstruct certain features of the structures and relief of the Devonian period (1, 27—30).

The question of the development of tectonic structures is, although not new, comparatively little elaborated. Until quite recently the complex history of their formation was almost not analyzed at all, and the entire tectonic history was reduced to several epochs of folding. At the same time, the chief point to which attention was directed was an attempt to distinguish the individuality of each new folding chiefly by its extent.

A major step forward in this respect was M. M. Tetyaev’s recent book, Geotectonics of the USSR. In this work he considers it entirely correct that, in order to see a structure as it was created by a given stage of folding, it is necessary to “remove,” to eliminate, the later distortions—the imprint that was superposed on the structure by subsequent stages of folding. Only for structural constructions of the very latest time is this, according to M. M. Tetyaev, not required.

Naturally, therefore, M. M. Tetyaev considers it necessary to conduct the very study of the historical development of structures only in the reverse historical sequence of directions, i.e., from the youngest structures to the more ancient. However, there is no necessity whatever for this; and now it is necessary to return to the natural course of events not only in presenting the results of research, but also in the analysis itself, in reconstructing the development of structures. After all, for example, in the position of the lower Cambrian footwall, the movements of all Paleozoic phases of tectogenesis were reflected, and to “remove”

them—means turning it into a horizontal position together with the underlying Dockembrian and its structures.

Unfortunately, M. M. Tetyaev does not carry through to the end, to practical realization, his thought that structures must be studied in their historical development. In fact, while recognizing the phases of stages, he in practice tries to reconstruct only their summary result. But even along this path he provides not a single map, not a single section that could concretely show, for some region, the restored structures of one or another stage, still less the phases of tectogenesis. Nor does he provide a method of restoration. Of course, this cannot satisfy us from the standpoint of a genuinely complete historical analysis of the development of tectonic structures, and we therefore propose here another path for a more concrete study of it.

Our method is designed above all for the “rejuvenation” of tectonic structures, i.e., as indicated above, for the restoration of their very “youngest,” very first phase of development, when deposits that had previously been horizontal, or primary forms in general, underwent the first phase of tectogenesis. In this respect our method differs from M. M. Tetyaev’s summary analysis, first, in that we carry our analysis through to each phase of tectogenesis and give a concrete path—a methodology—for such analysis; and, second, this alone is still not enough. It is not enough to restore structures as they first arose in each phase of tectogenesis. One must also restore and show the history of their development. M. M. Tetyaev, too, did not attempt to solve such a task in his Geotectonics of the USSR.

And to solve it means to trace the entire history of the further continuously discontinuous development of the tectonic structures just formed; it means to determine what fate, what changes (now quantitative, now qualitative) one or another “newborn” structure underwent in a whole series of subsequent phases of tectogenesis. Only such a study could fully satisfy the tectonician who seeks not merely to describe structures, but to present them in the historical perspective of development.

Unfortunately, such a study is in most cases impossible because of the fragmentary nature of the very documents of the history of the development of tectonic structures. However, it should be borne in mind that precisely our method, when such documents exist, can provide this history, i.e., can depict the structure not only as it originated, but also as it became at any moment of its continuously discontinuous change.

Before proceeding to a description of the method, the author must, in order to avoid ambiguity, dwell on his understanding of tectonic structures and of the phases of tectogenesis that create them (4). Folding (or, more broadly, a phase of tectogenesis) is not any plastic deformation or plicative dislocation (and so forth), but only those that have matured as a result of a relatively prolonged evolutionary development of the matter of the Earth. It takes place in a comparatively short time as one aspect of a qualitative leap in the development of the Earth, and not continuously and simultaneously with the formation of the entire sedimentary sequence (in the form, for example, of oscillation), i.e., it arises at one and the same time with a sharp change in the quality of some part or of the whole Earth. To prove “folding proceeding for a long time, continuously” means, from the standpoint of such a definition, merely to establish the long-known age-long, relatively slow (in particular, oscillatory) movements, or to discover a new form of them, convergent with folding. And such “folding,” as commensurate in its duration with the processes of accumulation of geosynclinal sequences, obviously belongs to the evolutionary phase of development.

A phase of tectogenesis is a change in the structure of the Earth. But at the same time it is a convenient indication of an exceedingly diverse, complex process (at the stage of ca-

qualitative leap in the life of the Earth. The birth and development of a new structure during phases of tectogenesis may be regarded as a reflection, in form (continuously connected with its content), of that qualitative leap which proceeds simultaneously within this content. This is not a purely mechanical process involving quantitative only, and not qualitative, change. That would be an overly narrow and incorrect understanding of the phase of tectogenesis. In reality, this process is inevitably accompanied by the transformation of mechanical motion into other forms of motion.

Such are flashes of light (a silent discharge?) when certain silicate substances are ruptured; electrical phenomena during the blasting of boreholes in rocks, recorded in the electrical prospecting of mineral deposits—in particular, abrupt jumps of electric current in the ore field at these moments; the appearance in rocks, as a result of tectogenesis, of all kinds of anisotropy (tectonic, purely physical with respect to seismic waves, optical, electrical, etc.); the rapid restructuring of the structure of rocks and minerals, which is rarely not accompanied by physical, physicochemical, and chemical changes in them, and so forth.

All this, and much else, takes place not according to mystical or astral laws, but according to the laws of mechanics—though not only to them and, perhaps, not so much to them as to the regularities of physics (thermodynamics, optics, etc.), chemistry, and others—and at the same time represents the transition of mechanical motion into other forms of motion (of electromagnetic nature, into thermal, luminous, etc.). At the same time, all these phenomena proceed according to their own special, specific, geological laws, which reflect in the development of terrestrial matter the universal laws of the continuous-discontinuous change of nature.

I am not even speaking of the fact that the content of the phase of tectogenesis, at whose basis lie profound intra-atomic and molecular regroupings of terrestrial matter (and, consequently, its volumetric changes), includes processes of volcanism, which are substantially directed, determined in their form, and even sometimes depend in their essence on structure. Finally, let us emphasize that in the process of the phase of tectogenesis there arise (or undergo a leap in their development) new spatio-historical interrelations of rocks and formations.

2. DESCRIPTION OF THE METHOD

1. The first method of reconstructing transformed tectonic structures may be applied in the case where there is an observed, or else a more or less reliably constructed, section—for example, like that shown in Fig. 2.

It is quite obvious that before the dislocation of the upper formation 1, its beds, and consequently also its footwall, were arranged horizontally or inclined, depending on the position of the basin floor. This latter question is usually easily resolved by an exact determination of facies and, above all, of the depth of the basin. The situation is approximately the same in the case of river and lake deposits, but certain other continental deposits—for example, talus breccias and proluvium—are much more difficult to subject to an exact determination of the primary inclination of the beds; nevertheless, even the original position of a weathering crust with eluvium can in some cases be established from the overlying deposits.

Thus, the question of the initial position of the basal surface of any formation may be resolved independently of the method proposed here, by the already well-trodden paths of facies analysis. Therefore this question is not considered here.

For simplicity, let us first take the case of horizontal bedding of the strata of Formation I before their dislocation. If below Formation I we draw, at an arbitrary distance, a series of lines parallel to its base as it lies (Fig. 1, dashed line), then, after concentrically parallel folding, the lines would be bent in approximately the same way as this basal surface of Formation I (Fig. 2), and, roughly speaking, independently of the structure of the underlying formation. If we then imagine our system of parallel lines as a system of coordinates, it is not difficult, using it, to reconstruct the structure of Formation II that existed within it before the dislocation of Formation I and that is the goal of our restoration (Fig. 3).

For this purpose, taking the axial line of the fold (A) as the initial (zero) ordinate, we may set off, along the corresponding horizons (Fig. 3) (90, 80, 70, 60, 50, 40, 30 m, etc.), the distances measured along the same “horizons” in Fig. 2 from the axial line to the intersections with any structural line that we wish to depict. In other words, the measurement must be made along the curved lines of Fig. 2 from the axial line to points 1, 2, 3, 4, etc. Having then connected the points belonging to one or another structural line of Formation II, we easily obtain a section of the structure of this formation that interests us at a definite moment of its development.

It is self-evident that the distance between the “horizons” is chosen according to the accuracy with which we wish to depict everything: the more delicate and finer the structural pattern, the more closely, naturally, the “horizons” must be drawn. In some cases it will be possible to use additional horizons, like the intermediate contours of a topographic map, which capture one or another detail of the relief.

Of course, if the dislocation of the upper formation did not occur according to the scheme of concentrically parallel folding, or according to the regularity of gentle, quiet, nappe folds, but with the formation of similar forms, or in an even more complex manner, then, strictly speaking, our system of concentrically parallel coordinates must be replaced by another, corresponding to the actual deformation of the lithosphere. In practice, however, this correction apparently does not play an essential role, at least so it seems after our first experiments in restoring ancient structures. If it is advisable to introduce such a correction, this is not difficult to do, taking into account the specifically observed changes in the thicknesses of the beds of the overlying formation: their thinning in the hinges and stretching in the limbs. Having established the corresponding coefficient, one may then use it to introduce a correction into the position of the “horizons” within Formation II as well.

In the further development of our method it will be necessary to take into account the rule, or “law,” of equal areas and volumes, assuming constancy of the density of the rock masses participating in folding. All the more necessary is it to consider changes in density, if this is possible. The author has the following in mind. The first phase of folding experienced by a loose or weakly diagenetized sediment may be accompanied by very strong compaction. True, such rocks as limestone or sandstone with a chemical (calcareous, siliceous, etc.) cement, being fully diagenetized rocks, may not undergo any significant compaction deserving of our attention (in the restoration of folds). This is the usual result of weak wave-like dislocations in the zone of nappe folds, and the like.

The same considerable compaction must also be taken into account for very powerful phases of tectogenesis, when only diagenetized, even if dislocated, rocks (argillites, etc.) undergo regional cleavage under conditions of development of Alpine-type folding, and still more recrystallization during deep metamorphism.

Fig. 1.

Visible labels in the figure: circled I, circled II; depth marks: 90 m, 80 m, 70 m, 60 m, 50 m, 40 m, 30 m.

Fig. 2.

Visible labels in the figure: A, B, circled I, circled II; distance/depth marks: 30 m, 50 m, 70 m, 90.

Fig. 3.

Visible labels in the figure: A, \(\alpha\), circled I, circled II; depth marks: 90 m, 80 m, 70 m, 60 m, 50 m, 40 m, 30 m.

But in other cases we shall have tectonic development of rocks and their structures without a noticeable change in density. In Western Siberia this is true for the overwhelming majority of Paleozoic phases of tectogenesis. It is precisely under this condition that one may make use of the rule of equal areas and volumes.

It consists in the fact that, with constant density, the volume of the deforming masses remains unchanged, and in the absence of horizontal flow of matter the areas of formations and of their parts in vertical sections also remain constant. This rule will make it possible in many cases to correct a structure obtained by applying the method described, and in some cases to introduce fundamental corrections into it or even to reject it completely. The technique for applying the rule, one must suppose, is not complicated. It is clear that in practice, when using constructed and observed sections, an accurate measurement of areas with a planimeter will be required or, perhaps, their calculation with the aid of a curvimeter. In determining volumes, one may use the available methodology for calculating reserves of mineral resources, with its inevitable determination of the volume of the mineral, i.e., again not without determining the areas of sections by the same planimeter.

Further, one may foresee a case in which the formations in contact are sharply different in their mechanical properties, when they participate differently in a single phase of tectogenesis, when disharmonic folding develops. We shall not examine this case now, considering that our method may perhaps not be applicable to it at all. However, let us note that, at least in a certain type of disharmonic folding, satisfactory results in restoring structures can nevertheless be obtained here as well. Such, for example, is the case shown in Fig. 4, in which it is evident that lines “a” and “b,” originally lying parallel to the underlying side 1 of the formation, although crumpled, nevertheless remain in general concentrically parallel to the bedding of formation 1. The same will evidently be observed also with our “horizons,” and if this additional crumpling is insignificant in comparison with the scale (length and amplitude) of the folds of the stratum in question, then one may quite disregard those inaccuracies of construction in restoring structures which result from disharmonic deformation of our “horizons.”

Fig. 4.

It is necessary to note that, in the case of nonhorizontality of the originally deposited beds, everything set forth above remains valid; only the “horizons” (coordinates) are arranged parallel to the primary inclination of the basin floor after determining this inclination, or in general the form of the floor, on the basis of studying the facies character of the basal horizon of formation 1.

Plicative dislocations may be conceived, in addition to concentrically parallel and similar folding with their longitudinal bending of the beds, also as plastic deformations without longitudinal bending (Fig. 5), as well as with attenuation or intensification of folding with depth. The last two cases are not considered here, since they are of significance only for constructing sections to a very considerable depth, and not for

ordinary geological sections with a depth of about 1–2 km. The first case, however, should be discussed somewhat further.

In this case the horizontal beds of the upper formation remain horizontal, and it would seem that there is no need to reconstruct the structures of the lower formation; however, generally speaking, this is not so, as is emphasized in Fig. 5. Reconstruction of the structure of the older formation in this case requires knowledge of the coefficient of compression, which can be established from the deformation of crystals, fossils, and, in general, bodies of definite form and structure (Fig. 5). Although the case under consideration (Fig. 5) is of significance, as many will think, only for the saddles and troughs of folds, in reality it apparently has a wider distribution, especially for areas of considerable development of igneous rocks and crystalline schists.

Finally, in the case of the separation of formations from one another and the appearance of interformational bodies, for example laccoliths or veins (for example, in a structure of the Bendigo deposit type), this fact must also be taken into account; moreover, restoration in this case usually does not change depending on whether the bodies appeared before or after the dislocation of the upper formation, and the “horizons” are drawn parallel to the lying side of the interformational body. But if, during intrusion, the upper formation is not uplifted but, on the contrary, the floor of the laccolith subsides and folding occurred after its solidification, then naturally the “horizons” above must be drawn parallel to its hanging side, or, more precisely, parallel to the beds of its roof.

Fig. 5.

2. Instead of the construction described above, the restoration of the structures of ancient phases of tectogenesis can be carried out by another method, namely by means of an elastic rubber film. Having depicted on it the present-day structure (Fig. 2), we can easily, by stretching the film upward (in the places of synclines) or downward (beneath anticlines), give the beds of the upper formation a horizontal position (Fig. 3), fixing this position of the stretched rubber with clamps.

3. The third method of reconstructing the structures of ancient phases of tectogenesis consists in the use of a special instrument, which in its concept represents an analogy to Fedorov’s stage, with its 4 axes of rotation of the object under study. Its first, fairly simple construction, in our work, received the following form.

The purpose of the instrument is that, by rotating into the original, horizontal position the lying side of the younger for-

formation, and together with it also turn back¹ the otherwise oriented strata (and, in general, the bodies and structures) of the underlying formation, and thereby restore their occurrence prior to the dislocation of the upper formation. Approximately the same idea is embodied in A. Burachok’s instrument (On the method of measuring oriented pebbles and oblique bedding. Notes of the All-Union Mineralogical Society, part 62, issue 2, p. 432). But it is too primitive, inconvenient for the precise orientation of ancient structures, and intended for another purpose.

But before carrying out this restoration, it is obviously necessary, with the aid of the instrument, to report the observed occurrence of the beds for both the one and the other formation. This is achieved as follows.

Mark 7 (Fig. 6) is oriented to the north. Circle 1 is set so that the strike azimuth of the beds of the upper formation, corrected for magnetic declination, coincides with mark 7. Then circle 3 is turned to one side or the other, depending on the dip direction of the lying side of formation A, through an angle equal to the dip angle. This rotation is easily carried out with the aid of mark 4, opposite which, in the case of horizontal occurrence, stands the reading \(0^\circ\). Having fixed the position of circle 3 with the spring and clamp, and having released clamp 8, we turn semicircle 9 into a horizontal position, which is verified with the aid of plumb line 10, fastened perpendicular to the surface of semicircle 9 and to its zero meridian.

Fig. 6.

Then we fix the horizontal position of semicircle 9 with clamp 8 and turn circle 12 so that line 1—1, or the horizontal axis of this circle 13, has a strike azimuth equal to the strike azimuth of the bed or of any structure of the underlying formation. The angle of this rotation must be calculated in accordance with the position of circle 1. Now it remains only to incline circle 12 by the corresponding number of degrees in order to obtain the observed position of some surface of formation B. This is achieved with the aid of plumb line 11, and the position of circle 12 is fixed by a clamp at axis 13.

Further, it is clear, it remains to turn our entire system, together with circle 3, about its axis so that opposite mark 4 there is again the reading \(0^\circ\). This will be equivalent to returning formation A to a horizontal position. At the same time, circle 12 will turn into the position that it had before the dislocation of formation A. It remains then to measure this position with a compass and to record the elements of occurrence restored in this way.

Finally, let us note the apparently emerging possibility of applying the Fedorov stage and, in any case, stereographic nets. Since both are known to every geologist, there is no need here to explain how they can be used for such purposes.

3. RECONSTRUCTION OF THE PHASES OF TECTOGENESIS AND THEIR STRUCTURES WHEN THE BEDS OF ADJACENT FORMATIONS ARE PARALLEL

The peculiarity of marine conditions as compared with continental ones is of exceptionally great importance in the doctrine of formations. After its formation and a phase of tectogenesis, a formation of sedimentary rocks usually enters the zone of continental life; under these conditions it loses, through denudation, a considerable part of its thickness (if not all of it), and in the overall result the phase of folding and denudation is separated from younger formations by an angular unconformity or, as is said, by a tectono-denudational break. If, however, marine conditions are preserved after the phase of tectogenesis, then the formations are arranged conformably (in the usual formal sense of the word) with one another, i.e., the footwall of the overlying formation is disposed concentrically-parallel to the beds of the underlying formation. Thus, in the presence of a tectonic leap, a denudational break here, as a rule (if there is no underwater ablation), is not formed and, consequently, the formations have their full thickness, which is the most remarkable and very important property of “conformable” formations. This property will make it possible to estimate the duration of phases of tectogenesis and to put an end to theories of folding, or indeed of the phase of tectogenesis in general, as an evolutionary phase in the development of the earth. But this is a special and very substantial topic, to which another work by the author is devoted.

Fig. 7

Here we shall examine the method that interests us for reconstructing structures, first for folds developing in the neritic zone of the sea. In doing so, we assume the absence of ablation on the sea floor. Otherwise we obtain conditions close to continental ones, discussed above, i.e. the case of the formation of a clear tectono-denudational break, or else of an angular or obvious unconformity.

Suppose that some bed A (Fig. 7, I), of a definite facies allowing the depth of its formation to be established at 200 m, after a phase of gentle or weak folding took on the form shown in Fig. 7, II and, at the same time, did not emerge from below sea level. By determining, from the changing facies character of the overlying horizon B, the depths of deposition of this bed, we can not only prove the phase of folding between A and B, but also establish from these depths—shown in section and especially in plan, for example with the aid of isobaths—the amplitude, wavelength, and strike of the folds.

Incidentally, let us note that the case described is at the same time an interesting example of false folding of bed B, which merely repeats and drapes folds of bed A that had been created earlier by the ordinary tectonic route. Such false folds, representing not tectonic forms but forms of plasticity copying the relief of the sea floor (even though the latter arose tectonically), and recognizable by the characteristic distribution of facies, can be established, but only presumably, also by the absence-

of regularly associated with the extent of folding of systems of fracturing. However, if the initially wavy bed B is, in addition, subjected to true folding, then such fracturing will appear in it as well; and this feature, generally speaking, can help us only in a very precise analysis of fracturing or in the presence of an intermediate phase of volcanism (for example, when low-temperature veinlets of this phase are cut by high-temperature veinlets of another complex); in the usual case, on the contrary, it will only obscure the actual course of events, especially in inherited folding, when the fracturing will fit the attitude of false folds just as well as that of true ones.

With renewed folding, on the contrary, the fracturing will have a different direction, more or less sharply distinct from the characteristic directions of false folds. But in this case, the presence of the latter can be definitively proved only on the basis of facies analysis, and not by the absence of fracturing harmoniously associated with the false folds, since the folding of loose and even diagenetically altered strata, it is believed, can proceed without the development of fracturing, by means of plastic deformation of the material alone.

Let us note that false, or “sedimentary folds,” after their participation in folding, will give a picture of more intense folding than actually took place; consequently, in the case of gradual or inherited (concordant, according to Krejčí) folding, and in analyzing its development, this must be kept in mind.

In the case where bed B has not undergone folding at all, there is, of course, no need to restore the structure of bed A and of the underlying, according to the strata that mantle it, series. In this case it is necessary only to check how far the observed amplitude of the heights of bed B corresponds to the difference in depths determined by facies analysis.

If, however, the basal horizon B, deposited continuously on bed A, like the overlying series, was crumpled into folds, it would be a gross error to straighten it into a horizontal position in order to restore the structure of the underlying formation, as we did above, since in such a case, with parallelism and concordant attitude of A and B, we would inevitably obtain for series A as well an undislocated structure. Thus, in the present case, if we wish to use our instrument, we cannot return to a horizontal position the basal beds of the overlying (formally concordant) formation in order to obtain the attitude of the underlying stratum unchanged by subsequent dislocations; nor can we use the flank, lying parallel (“concordantly”), of a younger formation for the graphical restoration of sections (the first method), since both in this and in the first case such restoration will be incorrect and will give a horizontal attitude. The use of elements of the attitude of beds or of the flank not of the adjacent, “concordantly” lying formation, but of that which unconformably overlaps the latter, will also not give the required effect.

Therefore there remains only one method for analyzing “sedimentary folds,” which is unthinkable without exact knowledge of the changing depths of formation at each point of bed B (Fig. 7). By using them to restore its original position before dislocation, we thereby also restore the folded structure of A before the formation of the basal bed B. The most convenient method here will be the construction of maps in isobaths, from which it is easy to construct any section of the restored structure.

Passing further to the bathyal and abyssal facies of marine sediments, we emphasize that under these conditions even the establishment, and still more the restoration, of “submarine” phases of tectogenesis is very difficult, yet nevertheless it is not hopeless, even if they are not accompanied, as

described above, with characteristic and regular changes of facies depending on the folded dislocation of the sea floor; the absence of such changes should be observed in the bathyal and abyssal divisions of the sea much more often, and even, as a rule; hence follows all the difficulty of diagnosing “submarine” phases.

Nevertheless, such a restoration is possible. For example, in the case of a change in the strike of folds during renewed¹) folding, the previously formed folds will, in all probability, acquire a sharply brachyanticlinal character or, perhaps, even two systems of undulation superposed one upon the other. In any case, the fracturing of the upper formation will reflect only the second phase of tectogenesis and will be arranged differently from that which was created earlier in the lower formation. The latter, in turn, if it does take on the younger system of fracturing, will not do so in the same form as in the overlying formation, but under the influence of the already existing system of fractures, the development of which along the former seams will relieve the stress and partly forestall the emergence of a new system of fractures in the younger formation.

Of course, manifestations of volcanism after the dislocation of the older formation and before the deposition of the formation that conformably (concordantly) overlaps it may greatly help in separating them and in establishing a tectonic break in the continuous sedimentary mass. In this case the fracture system of the older formation will be healed by emanations, which we shall not see in the younger formation; or else we shall notice there only another, younger system of veinlets with a different sequence and composition. If the intrusion was so active that magma reached the sea floor and pillow lavas formed, then the latter, marking the upper horizon of the lower formation, will in future play a role analogous to the significance of basal conglomerates; only the basal surface of the upper formation must be drawn not along the footwall, but along the hanging wall of these submarine lavas. Thus the latter become a kind of guiding boundary horizon between “concordant” formations, and with their aid it will perhaps be easiest to determine the full thickness of the formations and, consequently, also the duration of the phases of folding²).

Mineral neoformations associated with volcanism, in particular deposits of thermal springs, may be the same guiding boundary horizon, since the ocean floor can easily be imagined as a surface or screen where, owing to a sharp lowering of the temperature of the springs, many of their components will be precipitated, even without “telescoping,” and simultaneously. Of course, of this complex only the parts insoluble in seawater can survive—for example, carbonates under conditions of deposition of organogenic silts, and the like. Specific features acquired thanks to submarine weathering will help us distinguish thermal deposits at the hanging wall of a formation from similar, but intraformational, bedded formations. The latter, moreover, may have a noticeable effect on their hanging wall, which will not be observed in the case of thermal deposits during an interformational break.

Finally, contact metamorphism from an intrusion can conceivably be used to delimit “concordant” formations; however, if it does reach the hanging wall of a formation in large intrusions, then probably only in exceptionally rare cases.

¹) I propose this term instead of the rather odious word “renegade folding.” It may also be called discordant (4,28).

²) When boundary lavas are traced in strike into the area of continental conditions, i.e., into the area of nonparallel bedding of formations, they must pass into a weathering crust, into a basal conglomerate, or in general must lead to a clear boundary of formations with angular unconformity.

The zone of bathyal and abyssal deposits of the modern sea includes the most mobile parts of the lithosphere that participate in folding, in particular the so-called deeps. Thus, now, as in the past, it is precisely under these deep-water conditions, and not in the neritic zone, that individual phases of fold formation have a greater probability not only of taking place but also of ending below sea level, without the formation of land. Consequently, it is precisely under these conditions that the structural relations of “concordant” formations may most often be preserved for us in the geological record.

But, as is known, the depths of the ocean under the conditions of the bathyal and abyssal divisions of the sea, owing to the scarcity and slight differentiation of the benthos, or because of the complete absence of organic remains, are poorly amenable to determination. The same may be said of the lithology of the sediments. Thus, the persistence of deep-water sediments over great vertical distances makes their facies analysis unreliable, and the determination of the depth of deposition very crude.

Therefore, the deeper-water the facies and the greater the chances for the formation of “submarine folds,” the less can we hope to reconstruct their forms by the method discussed above, based on the changing facies character of the sea floor.

Fig. 8.

to reconstruct their forms by the method discussed above, based on the changing facies character of the sea floor. Only for the sediments of the boundary belt between the abyssal and bathyal divisions are conditions created analogous to those of the neritic zone: even with gentle folding, anticlines pass from the abyssal division into the bathyal, while the reverse will occur in synclines.

In the case where facies analysis by the method described is powerless to yield anything, for strata with complete uniformity of facies one must bear in mind even such a method as the method of using submarine landslides.

Let us imagine the origin of a submarine anticline on the floor of a deep sea or ocean (Fig. 8). If, in this case, there is no regular distribution of facies reflecting the elements of the fold, if neither comparative analysis of structures nor the distribution of intrusions, etc., helps, then, in the event of the manifestation of submarine landslides caused by folding, they may render us substantial assistance.

Indeed, submarine landslides under conditions of constant seismic shocks and in the complete absence, at great depths, of marine abrasion are, generally speaking, no longer exogenic but endogenic processes, since the instability of marine sediments in many cases is caused—

is explained not by a primary steep slope of the bottom, inherited from the continental phase¹ of the life of the given surface of the lithosphere, but by tectonic movements.

But even in the case of a primarily inclined bottom, the sliding of the accumulated sediments (despite this inclination) will most likely be caused not by exogenous causes (for example, by undercutting of the base of the slope, which in this case is impossible), but by seismic shocks or else by the twisting of slopes as a result of the development of a fold and other tectonic forms.

It goes without saying that landslides on the slopes of an anticline will move away from its axis and toward the troughs of synclines; moreover, the direction of motion, as is known, in such cases is well determined by the overturning of landslide folds in the direction of movement of the sliding masses. If in this way, in ancient deposits, we can establish this regular connection between the distribution and direction of motion of submarine landslides and definite forms of folds, then naturally we may also speak of the origin or development of these folds at a more or less fixed moment, close to the time of deposition of the sediments that slid. Such a conclusion should preferably be checked in the hinge zone of folds, where overturned landslide folds should naturally be absent, but where other accompanying forms of dislocation may occur (for example, ruptures in the axial zone of anticlines). Of course, the final verification and precise fixation of the phase of tectogenesis must consist in tracing the horizon with tectonic submarine landslides into the area of neritic facies and into the area where angular unconformities appear. It should be emphasized that the picture sketched above of the development of “tectonic landslides” is by no means abstract and merely possible. It has already been observed in reality, for example, in the geological study of the Caucasus. It would be interesting to apply our methodology to them, and also to try to elucidate the nature of submarine landslides in the Carboniferous shales of the city of Tomsk.

4. RESTORATION OF THE EVOLUTIONARY PHASES OF TECTONIC MOVEMENTS

The methodology of deciphering them lies beyond the scope of our article and deserves a separate analysis; but since above we touched upon the phases of folding, it would be wrong to pass over the evolutionary stage or phase of the discontinuous-continuous tectonic process. Therefore we shall briefly note here that analysis of stratigraphic columns, with the facies character of series and horizons well studied, with the depth of their formation known, and the construction on the basis of such analysis of tectonograms, can give us, for a number of points where these columns and sections were taken, the rates of uplift and subsidence of the sea floor (even if only in conventional units). By connecting points with equal rates, we can obtain maps in isoanabases (lines of equal uplift) and isocatabases (lines of one and the same subsidence) for definite epochs of sediment accumulation. Such an analysis can give us a complete picture of slow tectonic movements, commensurable in the time of their course with the time of sediment formation, i.e., very prolonged ones; it can reveal to us their relation to subsequent folding and illuminate the very character of these evolutionary secular movements, usually called indiscriminately epeirogenic (or epirogenic) and oscillatory, but in general very diverse—that is, show whether they are: 1) subsidence (uplift) of the bottom parallel to itself in the case of block movements along disjunctives, 2) or else an analogous movement along disjunctives, but not translational, rather rotational (with me-

…a magnitude of subsidence or uplift that varies from place to place), 3) or else, further, oscillatory movements of different wavelength, with different signs in neighboring places at one and the same moment and with a change of sign at different moments, 4) or, finally, a geosynclinal downwarping with a more or less constant sign of movement, dying out toward the margins of the geosyncline and changing to the opposite sign in the region of the geoanticline, but with this sign preserved throughout the entire evolutionary epoch in each separate place (geoanticlinal uplift or geotumor). All these and other forms of movement of the evolutionary phase of the earth’s self-development, as well as other formations convergent with them but historically representing entirely different formations of a qualitative leap in development (gentle near-platform folds, and also weak phases which in the aggregate produce powerful cycles of tectogenesis), are not distinguished in the ordinary geological literature and are incorrectly combined in the summary concept of “epeirogenic,” “oscillatory,” etc., movements. Without going into this question here, let us note that in its resolution (for ancient epochs) the method proposed will, like facies analysis, play a certain role, as is shown in part below.

This is not difficult to understand. First of all let us note that facies analysis cannot be separated from the task of restoring sea depths by other methods, in particular by the proposed method of restoration of the surface of its floor. This path is considered below, and in known cases (with monotony of lithological composition or when it is impossible to establish facies from it and from fauna) it is the only, and in all probability a new, method of determining facies changes, or more precisely, the depths of one or another ancient sea floor.

Fig. 9.

In addition, our method (taking into account the rule of constancy of volumes and other laws of structural geology) is apparently capable of introducing certain corrections into the observed thicknesses, distorted by folding and usually used in this form for tectonograms of evolutionary phases. Such corrections can be made by stretching the limbs and increasing the thicknesses in the hinges as a result of layer-by-layer movement of material from the limbs into the hinges. In restoring the exact but varying depth of formation of near-reef facies, the application of our method may provide examples of the correct solution of one of the difficult problems of the doctrine of facies.

Thus, in Fig. 9 a vertical section is shown of a coral reef narrowing upward, with a very broad base at the beginning of growth. It is quite obvious that the relief of the bottom between points Б and В was level in the first moments of reef growth, and the depth of the sea was not more than 40 m. Subsequently this depth remained the same only in the region of point A, while at points Б and В it became about 700–750 m. The exact value of the depth, this most important factor of changing facies, at any point of dislocated reef and near-reef deposits can obviously be obtained easily by applying our method. True, the near-reef facies here will change little in its lithological expression and will rather be a function not so much of depth as of distance from the reef,

but the paleontological aspect of the facies (the composition of the benthos), on the contrary, must change sharply depending on the relief of the bottom or the depth of the sea.

5. RESTORATION OF PALEOGEOMORPHOLOGICAL ELEMENTS AND ESPECIALLY OF THE BOTTOM RELIEF OF ANCIENT BASINS

This variety of our method received its first application and discussion in the literature in 1936 (in the study of the Krasnoyarsk region), although theoretically it had been developed somewhat earlier (1, 27—8, 30). The idea of the method here, too, is that forms of ancient relief, if they have been dislocated and crumpled into folds, can be restored in exactly the same way as any structure, by the method described above of rotating the surface of interest to us into its original position and thereby restoring its former form. The very presence of any irregularity in the ancient relief of the sea floor or of a continent is recognized from sharp bends in the hanging wall of the formation, while its footwall does not participate in these bends and thereby indicates their atectonic genesis, i.e., their belonging to some form of relief.

Thus, for example, in Fig. 10 the bend A of the boundary between \(D_1\) and \(D_2\) cannot be explained by its transverse folding, since neither the footwall (base) of \(D_1\) nor its beds show such a bend; it is perfectly obvious that even before the deposition of \(D_2\) the Lower Devonian had been eroded, in particular with the formation of valley A. In some cases we may encounter on a geological map a sufficiently indicative transverse section of relief forms. In the general case, however, for their exact restoration, in particular for determining the profile and strike of valley A, it would be necessary to apply our apparatus, i.e., to rotate the sides of the valley from their present position into the original one, using the corresponding measurements for this purpose.

Fig. 10.

A second very interesting field of application of the method is the relief of the bottoms of seas and other water basins. Apart from the paleogeographic significance of this method, which makes it possible to restore ancient volcanic edifices, coral and other islands, moats, and similar elements of the relief of the sea floor, it can be of service in the precise study of ancient facies; this is entirely understandable, since by restoring the bottom relief we thereby determine the change in depth, and, knowing its exact value at least at one point, we can consequently speak just as definitely about all other points of the uneven bottom, or, in other words, about what the facies is in one or another place of it.

In the example given above (Fig. 9) this is obvious, since the hidden unconformity along line I—II, which near the reef structure becomes formally angular, imitates a true visible unconformity connected with a phase of folding.

Thus, one may say that the determination of facies, hitherto based on the study of lithology, bathrological properties, and organic remains, with analogy to modern conditions playing a large role in all this, is placed on a new basis.

Let us give one more example, shown in Fig. 11, representing a diagram of the contact of the red-colored Devonian of the Minusinsk basin with its basement. Such a detail was observed on a large scale near

of the Pechishche deposit of copper ores, where the clams (or clefts) of the Devonian desert reached, at least, many tens of meters in depth, thereby determining a rather low minimal depth of the denudation base (“a” in Fig. 11). In other cases, canyons, weathering pockets, etc., are likewise recognized. A lowering of the ancient erosion base was determined in 1940 in the area of a manganese deposit on the Usa River (in the Tom basin), where this lowering occurred in one of the Cambrian ages. It led to the formation of karst relief in the archaeocyathid limestones of the Gavrilovka (?) Formation—in particular, karst shafts no less than 250 m deep. The subsequent transgression of the sea led to their filling and burial beneath the Usinsk ore formation (carbonate manganese ores). Of course, the depth of the shaft well, and especially the details of their original form and position (before folding), are most conveniently reconstructed with the aid of our method.

Fig. 12 depicts a clear result of submarine abrasion (a), which is given for comparison alongside the usual picture of longitudinal undulation of folds “b” (Fig. 12), with their axes plunging and rising. In this case: 1) the abrasion surface lacks signs of continental weathering or bears features of submarine weathering that are difficult to recognize; 2) it is overlain by marine deposits; 3) it cuts the underlying beds near point A, which is not observed in case “b” (Fig. 12); and 4) the entire structure is devoid of symmetry. The latter, however, is not obligatory, since the submarine valley could have had a direction transverse to the fold and could have appeared on its other limb. Incidentally, from this case, as also from case “b” in Fig. 12, the structure of Fig. 13 is readily distinguished, depicting the result of the superposition of a renewed (discordant) meridional syncline upon a previously formed northeastern anticline. Here the important fact is the oblique, rather than perpendicular, disposition of the bends of the folded structure, which cannot be explained by the normal plunge of the axes of brachyfolds.

Fig. 11.

Fig. 12.

Thus, the examples given show that our method proves very useful in reading and analyzing maps and sections, especially where regular folded structures are sustained (such as the folds of the Donbas). Against the background of this striking persistence, primary geomorphological details of the sea floor are in places clearly manifested and restored by our method (relicts of continental forms characteristic, for example, of the Atlantic, results of submarine abrasion, reef structures, etc.), as well as volcanic structures, swellings above laccoliths (Fig. 14), oceanic moats, and the elusive, diffuse details of the bottom of the Earth’s lithosphere. But it must be said that it is not always needed in order to represent, at least partly or in general outline,

a structural pattern created by one or another phase of tectogenesis; or, for example, in order to solve the question of the direction of folds of one or another phase of tectogenesis.

Thus, in the case of very gentle dislocations of the wave type and, conversely, very steep folds beneath their cover, straightening out this weakly undulating, almost horizontal cover will introduce no changes into the basic pattern of the geological map, into the structural pattern of the underlying basement, especially if the scale of the map is not large. Therefore, when there is no need for special accuracy, or for small-scale maps, even without the proposed method the basic properties and features of ancient folding are sufficiently evident. At the same time, the distorting influence of later disjunctives on the strike of folds is easy to discard if an accurate map is available. In Fig. 15, the meridional strike of the folds (I—I) is completely evident from the map; because of a disjunctive and poor exposure, it might mistakenly be taken, from fragmentary accurate sections, as northwestern (II—II). Of course, in detail, especially in vertical section, and in special zones (for example, dejective folds), the development of ancient structures may in this case too be very significant.

Fig. 13

Then, in particular cases of inherited (posthumous), or concordant and accordant folding, no restoration of the strikes of ancient phases of folding is needed at all, since this strike remains one and the same. True, in order to form a concrete idea of one or another intensity of folds of ancient phases, in order to carry out their comparative analysis, etc., even in this case one cannot do without the method proposed here.

A very interesting and simple case of restoration is presented by vertically dipping contacts of formations. Indeed, in this case the geological map draws for us vertical sections of individual structures of ancient phases, since the vertical limb, when restored, is turned through 90°, and thus the modern horizontal cut (the geological map) becomes a section of the ancient epoch prior to the dislocation of formation I (Fig. 3). The author can illustrate such a case with his sketch map of the Yelandinsky district 2 (Figs. 1—6).

Then, if, for example, we have an intensely folded belt such as the Urals or the Andes, then whatever its subsequent deformations may have been, its general strike, or the strike of the axes of individual long folds, will remain practically unchanged or will change very little. Of course, the smaller the folds or other elements of structure that we take, the more noticeable their transformations will be, in particular even the change in the strike of the axes. It goes without saying that here only subsequent plicative dislocations are meant, whereas the influence of tangential disjunctives on the change in strike of more ancient structures can be easily taken into account.

Fig. 15.  Fig. 14.

Of course, in this case too the application of our method may provide a controlling analysis of the structure.

All the cases cited above limit the application of the method of historical analysis of tectonic structures, but only because there is no particular need for it. It is another matter when there are insufficient data for restoration. Such a case indeed limits the sphere of application of the method. It sometimes occurs because of a lack of observations. This is a temporary obstacle. But in other cases, when denudation has destroyed the overlying dislocated structures, there is as yet no reason to speak of a broad application of our method in general. Only where the structure of the destroyed strata can be extrapolated and interpolated may one attempt, more or less, to restore also the now transformed structure of the exposed basement of the eroded formation. Something further can still be proposed to assist this extra- and interpolation. One may, for example, take into account features of the basement imposed by subsequent movements (a more or less deep denudational cut, the connection of young fracturing with the extrapolated structure, and the use of it to establish anticlinal and synclinal zones of the young structure in the basement, etc.). But it is still too early to speak of this in detail.

Let us next note the significance of the method, which at present, in the first years of its (and moreover very narrow) application, is not easy to assess correctly—even for the author himself. But it seems to me that, on the path toward historical analysis and synthesis of the development of structures of the lithosphere, it will render very substantial assistance and will help to make a noticeable step forward in this direction. It will be one of the concrete forms of the historical method of investigation, without which it is impossible to rise to the heights of materialist dialectics. With its help we shall be able to understand more deeply the nature of the structures that arose and developed continuously and succeeded one another; we shall be able to establish a series of new regularities of the geotectonic process that correspond more closely to the truth.

What, in particular, can the method of historical analysis of structures provide—a method which, under favorable conditions, gives not guesses but a more or less exact reproduction of them?

With its help, it would seem, such an essential question as the division of plicative dislocations into cycles or stages can above all be better resolved. As is known, this division is carried out in different ways. M. M. Tetyaev, for example, proposes to take as the basis of the division such a feature as the strike of folds. He considers that all phases of one stage are characterized by one strike, and only after a serious rearrangement, which separates different stages of folding, may there arise (but not necessarily) a folded zone of another strike; for example, the young structures of the Alps, which in places run at right angles to the Variscides, although in other places both the Alpine and Hercynian folds run parallel—for example, in the Donbas or in western North America. Using this feature, it would be easy to distribute among cycles all phases of tectogenesis in places with different directions of folds.

Although we do not intend to follow M. M. Tetyaev in his methodology of dividing folding into stages, and even consider it too one-sided and simplified, nevertheless the analysis of fold directions is one of those inevitable paths of investigation which will ultimately lead us to the creation of a correct picture of the course of the geotectonic process. It is only necessary to extend such analysis to each phase of folding, without limiting it to its stages; and for this purpose we propose the above-described method of its analysis: for before carrying out a synthesis, grouping the phases of tectogenesis into cycles or stages, it is necessary to subject the individual phases to deep study both with respect to direction and with respect to in-

intensity, as well as other aspects of geotectogenesis. Such an attempt at a deeper analysis of tectogenesis, with its division into numerous phases, has recently been made by Academician M. A. Usov. The methodological proposals developed in the present work will help to advance further along the same path. In particular, let us emphasize that only with the aid of the new methodology will it be possible to reconstruct the weak phases of tectogenesis, if their gentle structures have been crumpled by subsequent intense folding.

Further, it is known that magmatic bodies (dikes, laccoliths, etc.), after their formation, participate in the general folding. But how? What was their original appearance, how did their form, structure, and position in space, in the structure of the sedimentary mass, etc., change? In resolving these questions, the method of reproducing the initial picture of ancient structures and their development can undoubtedly clarify very much.

Our method will play a very substantial role in the study of a very large group of plicative disjunctives. The idea of their transformation during folding deformations arose, if I am not mistaken, in the study of nappes. In the USSR, it seems that only Academician M. A. Usov took into account and studied the subsequent development of certain concrete disjunctives (for example, in the Kuzbass) through their participation in folding. Thus, he established a new type of disjunctive—a reverse fault—which may erroneously be taken for a normal fault-disjunctive of extension, and which arose from a longitudinal concordant thrust during the twisting, or even overturning, of the fold limb cut by it.

No less interesting is the application of our method to the study of the development of fracturing in folded formations. It seems to me that in this field it will help us to penetrate more deeply not only into the history, but also into the very mechanism¹ of fold formation, and to distinguish in certain difficult cases this process from movements of another kind—from vertical oscillatory movements. It will illuminate the role of fracturing in plicative dislocations, its nature and development, and thus we shall better understand the anatomy of folded structures.

To explain this thought, I shall give an example. Let us take two plastic dislocations: one tangential, the other radial, externally indistinguishable from each other in their form (Fig. 16). How, beneath this convergent form, are we to recognize the essence, the character of the movement? A number of indications, developed by the author into a special methodology, can help resolve this question: thus, for example, the wrinkling of horizontal surfaces of separateness and the crenulation of beds in the locks of folds, then the asymmetry and overturning of folds, which not only establish tangential movement but even give its vector; the deformation of crystals, fossils², and other definite primary structural features of rocks—i.e., precisely such deformation with upward stretching as testifies to tangential stress, whereas under vertical movements deformations must, generally speaking, have a different character; finally, slickensides and bending of drag, which in known cases also establish the character of plicative dislocations,

Fig. 16.

since they are associated with differential interstratal and other movements, including movements along plicatogenic disjunctives.

But in addition to the examples given above, analysis of fracturing by means of the method set forth here can answer the question that interests us. Indeed, in Fig. 17, in case “a,” where a detail of vertical oscillatory movements under the influence of a pair of forces is shown, the dip of the vertical fracturing does not change¹ with rotation of the limb, and therefore the angle between the bed and the fracturing of this limb changes from $\alpha$ to $\alpha_1$; conversely, in case “b,” with tangential movements accompanied by rotation of the limb, the dip angle of the fracturing changes while the angle $\beta$ of their mutual intersection remains unchanged. Where, then, are positions I and II to be established? Obviously, first of all where, in one case (I), our limb is situated in the hinge of the fold of the subsequent phase. There, at a definite dip angle $\gamma$ (as in case I “a” and “b” in Fig. 17), this angle did not change (Fig. 18), and consequently the position of the fracturing created in the first phase of folding did not change. Here, therefore, we can fix the original position of the fracturing.

Fig. 17.

And what it became we shall evidently see beneath the limb of the fold of the second phase of folding (Fig. 17, II). With such a course of analysis it is clear that we cannot dispense with our method of rotating structures into their initial position.

In other words, our method makes it possible to carry out a comparative tectonic analysis: to trace how the development of a structure (a fold, a disjunctive, etc.), in particular fracturing, changes depending on the position of this structure in the hinge or limb of a fold of the first phase of folding, depending on whether it falls into the hinge or the limb of a fold of the subsequent phase, and so on—in general, depending on the tectonic setting changing with time. Moreover, we can now take into account the influence of this changing setting not only on the structural development of magmatic bodies (sills, dikes, etc.) depending on their position in one or another major tectonic zone, and also in one or another limb or hinge of a fold; but, conversely, we shall be able to establish how the course of development of structures changes in the area of a major magmatic body before and after its formation. This is what it means to consider tectonic processes in their self-development, in their complex historical connection with one another, in their mutual connection and mutual influence upon one another. At the same time, of course, it is necessary to remember the influence on development exerted by previously created and inherited structures, the complication introduced into the course of the process by the various properties of rocks and by the unevenness of the process itself, the uneven distribution of forces, for example, in the appearance of disharmonic structures.

Fig. 18.

¹ It should be borne in mind that the same will also be true for tangential plastic dislocations without longitudinal bending.

If our method is called upon to play a certain role in the study of already existing structures, then the second aspect of the method is hardly of lesser importance: its reverse application, i.e., not the deciphering of complex observed structures by “removing” one or another phase of dislocations, but, on the contrary, the exact reproduction of the effect that the formation of an imagined fold will have upon one or another structure in the subsequent phase of tectogenesis. One of the remarkable and fundamental aspects of scientific thought in the mature stage of the development of the sciences consists in the possibility of foreseeing the course of processes according to correctly established regularities. Let us recall, if only, the prediction of the exact position, for a definite time, of a new satellite of a planet, later discovered in the sky by telescope. To a certain extent, here too there appears the possibility of predicting the more or less exact behavior of structures under the influence of subsequent dislocations of one direction or another, of one intensity or another.

Fig. 19.

Let us explain this idea in the following way. If, as shown in Fig. 19, we subject a definite, completed structure—for example, a meridional syncline—to transverse (I) or oblique compression (II and III), with the formation in the new phase of a syncline of latitudinal (I), northeastern (II), or northwestern (III) direction, then, with the aid of the method proposed here, one can predict what horizontal sections of the folds of the first phase will be obtained. These sections, shown in the horizontal row Б (Fig. 19) and repeated in Fig. 19, IV, prove to be quite characteristic in different cases. Thus, for combination I they are symmetrical, while for II and III they are asymmetrical; moreover, the arrangement of the acute and obtuse angles ($\alpha$ and $\beta$) makes it possible to determine correctly the NE or NW strike of the fold of the preceding phase. Without taking these features into account, we might arbitrarily and blindly combine the various sections (a–e) of folds with one another and obtain the most varied possible strikes for them. Of course, the application of our instrument, given a sufficient number of measurements¹, can give an exhaustive answer in each concrete case to the question of the strike of the folds, or which variant of them to choose from among the possible ones. But by applying it, as described above, to cases provided for in advance, we can even pred—

Fig. 20.

indicate, by these or other details of the structures, the correct solution to our problem.

One more example, the last. With the aid of the proposed method it is possible to analyze the question of the nature of brachyfolding. It is known that the waviness of the axes of folds, and consequently also brachyfolds, may arise as the result of a single phase of folding, as a consequence of stretching or pulling apart along the axes of folds of rocks which, despite such elongation, cannot move their ends sideways because of the resistance of neighboring rocks. Thus this elongation is effected by transforming a straight axis into a wavy curved line. It is also known that the plunge of the axis in such a case is, first, less than the plunge of the limbs, and, second, in general usually does not exceed 10–15°.

But since renewed (discordant) folding is sometimes encountered, it too may produce or intensify an existing waviness of the axes; in such a case, first, the axes may be inclined, even steeply, up to 90°, and, second, the rises and plunges of the axes in neighboring folds are situated at the given place regularly, in one direction, as the result of the superposition of later folds of a definite strike (for example, as shown in Fig. 20). Exactly what effect this superposition of renewed (regenerated) folding will have on one structure or another can, once again, be seen more or less clearly by applying our method.

1. From this it is clear that our method imposes new, increased demands on the quality of geological maps, requiring the maximum fixation of structures, especially near surfaces of angular unconformity. ↩↩↩↩

2. Together with the deformation of fibrous minerals of veins, the author proposes this method for use in the diagnosis of disjunctives. ↩