ON SMALL TECTONIC FRACTURES FILLED WITH CALCITE, AND THE SEQUENCE OF THEIR FORMATION

A. M. KUZMIN

In the article offered to the reader’s attention, small fractures in rocks of the Cretaceous flysch of the Caucasus, filled with calcite, are considered, as are their positions relative to one another, which make it possible to determine the sequence of their formation and to draw a number of other conclusions.

1. Introduction

Small fractures in very large numbers accompany dislocations of the Earth’s crust of any scale. However, the author has limited himself to consideration of fractures filled with calcite or quartz. Such calcite veinlets were studied in sandy limestones or in quartz–calcite sandstones of the Cretaceous flysch developed in the area of the settlement of Golovinka on the northeastern coast of the Black Sea. Examination of a vast amount of material, represented by semi-rounded platy fragments of carbonate rocks penetrated to varying degrees by calcite veinlets 0.5–10 mm thick, reminded the author of A. A. Belitskii’s assertion that \(N\) fracture systems are formed in one or two stages [2]. It was precisely this assertion that prompted the author to undertake a study of such fractures. The results of these investigations are offered to the reader’s attention.

It should be noted, however, that the object of the investigation was not the fractures themselves as such, nor their nature. The main attention was focused on seeking the possibility of determining the age relationships of calcite or quartz veinlets with one another, taking into account M. V. Lomonosov’s rule that later mineral veins often cut earlier ones and even displace them. A careful study of such veinlets on a large body of material made it possible to correctly understand the relationships among calcite veinlets, to distinguish fracture systems and the sequence of their formation, and thereby to obtain substantial methodological material for judging the tectonic history of the region under study.

2. Description of small tectonic fractures

In connection with this topic, the author examined a small number of bedrock exposures along the Shakhe River and studied more than 10,000 semi-rounded flat fragments of the same rocks on the Black Sea coast and in the valley of the Shakhe River. Fragments with calcite veinlets constitute no less than 50–60% of their total number. Material from bedrock outcrops, in terms of the clarity of the vein pattern, is inferior to the semi-rounded fragments, from which abra-

on the weathering crust, and the marly film and rusty coatings were removed.

In the Golovinka area the flysch deposits are represented by steeply dipping packages of alternating beds of calcareous sandstones or sandy limestones interbedded with marls. In bedrock outcrops the latter are usually strongly destroyed and disintegrate into fine angular rubble. The carbonate interbeds are no more than 20 cm thick, and more often no more than 3–4 cm thick. Fractures filled with calcite in the carbonate beds are, as a rule, clearly expressed. In general, the fracture pattern in the dense rocks of the bedrock outcrops is analogous to the same pattern in subrounded fragments on the seashore.

Therefore, for bedrock exposures one may confine oneself to the description of a single example of investigation (Fig. 1), which, in the character of the calcite veinlets depicted on it, has many counterparts among coastal fragments of carbonate rocks of smaller size. Thus, in one of the gullies entering from the left into the valley of the Shakhe River, a slab of dark-gray sandy limestones, dipping at an angle of about

Fig. 1.

80–85°, was prepared. On its surface four systems of calcite veinlets are clearly visible; of these, the fractures earlier in time of formation are cut and displaced by later ones. The earliest of them are the parallel veinlets of the first fracture system, 1–2 cm thick, which are spaced 80–100 cm apart. Within each block bounded by fractures (1), there are thinner veinlets, 0.5 cm thick, parallel to the first ones, spaced from one another and from the first by an average of 20 cm. Along their extent, some of them cut the visible part of the slab from edge to edge; others, in the form of long and narrow lenses, pinch out and, in the form of lenticles of lens-like shape, cut the slab.

The second fracture system (2), no more than one centimeter thick, is represented by a series of parallel calcite veinlets spaced 60 cm apart. This fracture system (2) displaces fractures (1) filled with calcite. The third system (3), consisting of almost horizontal veinlets 1.5–2 cm thick, displaces the calcite veinlets of the first and second systems. The fourth system of parallel fractures (4), 2–2.5 cm thick, dipping at an angle of 85°, displaces the veinlet-fractures of earlier origin.

One of the noteworthy points is that calcite

the veins of all four systems are perpendicular to the surface of the slab, parallel to the bedding plane. Incidentally, this rule is strictly maintained also on all the examined semi-rounded flat fragments of rock, on stones used for reinforcing the sides of roadside embankments, the bottoms of ditches, and in bridge abutments and foundations.

In a number of other small outcrops, calcite veins in carbonate rocks, in terms of their density and the nature of their mutual relationships, resemble the patterns of veins that are excellently represented on a vast number of fragments scattered along the seashore and the towpath of the Shakhe River.

In Fig. 2 one of the fragments from the Black Sea beach is shown. In the character of the arrangement of the calcite veins it resembles the fracture pattern shown in Fig. 1. In this case there are only three systems of parallel fractures filled with calcite. The system of fractures (1) is the earliest. The following parallel fractures of system (2) displace fractures (1), and then a third system of calcite veins develops, which displace the veins of the first and second systems.

Fig. 2.

In the following Fig. 3 a small slab 14 cm long and 3 cm thick is shown. It is penetrated by two series of parallel calcite veins. The first of them is represented by a system of parallel fractures 0.5–2.5 mm thick, bent to one side. The calcite veins that filled the fractures have a more complex character: individual fractures, before being filled with calcite, underwent splitting in the same direction. The second system of fractures, 2–3 mm thick, proves to be more sparse in comparison with the first; it displaces the calcite veins (1).

In Fig. 4 a fragment of sandy limestone is shown. Its dimensions are 10 × 17 cm. The direction of layering is shown by an arrow. The rock is cut by two systems of calcite veins 1.5–3 mm thick. The earlier of them is displaced by the later system. Both systems of calcite veins are perpendicular to the bedding.

Fig. 3.

In the following Fig. 5 two systems of calcite-filled fractures 2.5–3.5 mm thick are shown, cutting dark-gray limestone. The first system of fractures (1) and the fractures of the second system (2) form an angle of 130°. The development of fractures of the second system leads to displacement of the earlier ones with the formation, at the junction node, of a “zigzag.” The occurrence of the latter can be explained by the fact that displacement occurred not only along the horizontal system of fractures, but at the same time also along the fractures of the first. The calcites of this latter system bear clear traces of deformation in the form of crumpling and curvature of prismatic crystals. In Fig. 6 an image is given of traces of fracture tectonics on a small slab of sandy limestone, penetrated by systems of fractures filled with ...

![Figure 4 not reproduced]

Fig. 4.

calcite. Analyzing the relationships of the healed fractures present, one can see that each subsequent system of fractures displaces the preceding ones. The amplitude of displacement is generally 2–3 mm. However, the ruptures and displacements are clearly visible even to the naked eye.

Figure 7 shows a system of veining in a relatively large hand specimen, \(24 \times 11 \times 10\) cm. This specimen contains a large number of systems of fractures filled with calcite. In this case as well, all fracture systems cut the rock perpendicular to the plane of its bedding. Studying the relationships of the fractures, one again has to state that in this case too any subsequent fractures, in forming, displace all the preceding ones. When tracing calcite veinlets of one system or another along the dip, and the relationships of fractures of different systems, what is most striking is that newly arising veinlets, as a rule, do not follow the direction of earlier fractures healed with calcite, but make their way along fractures newly created for them immediately beforehand. In this case too, the rocks are cut perpendicular to the bedding.

![Figure 5 not reproduced]

Fig. 5.

In the following Fig. 8, up to 5 systems of fractures are presented, cutting the rock perpendicular to the bedding. The most frequent fractures (1) are intersected by fractures (2). Fractures (3) and (4) are isolated. The latest fracture system (5) first follows parallel to veinlets (1), slides along fractures (4), and then, in the upper left corner, again follows fracture (1). The rock as a whole undergoes complete silicification, with preservation of the pattern created by a series of calcite veinlets. Fig. 9 is a striking—

![Figure 6 not reproduced]

Fig. 6.

Fig. 7

...of a sample of sandy limestone 3.5 cm thick, in which there is a system of parallel fractures 2–5 mm thick, filled with calcite. In addition, there are similar calcite veinlet-fractures that diverge in a bundle, like rays from a certain point. Along these rays there is a displacement, visible to the naked eye, of the earlier veinlets. Such diverging bundles of fractures could have formed as a result of rotation of the rock mass.

In Fig. 10 a new interesting feature of small-scale tectonics is presented. The earlier system of fractures (1) is represented by a series of parallel calcite veinlets, one of which, 12 mm thick, proves to be complex. The inner part of this veinlet is composed of columnar calcite

Fig. 8.          Fig. 9.

with small relict lenses of fragments of the enclosing rock. The heads of the parallel acicular crystals are somewhat deformed as a result of movement of the blocks in the plane of the veinlet. Later, in its selvages, thin calcite veinlets of 2–2.5 mm appear. Then a new system of fractures (2) forms, which, in the form of two diverging bundles, cut and displace the earlier fractures. Such a picture clearly indicates rotation to which the flysch rock masses were subjected.

3. Structure of calcite veinlets

The structure of calcite veinlets filling tectonic fractures can be schematically illustrated by the following figures (Fig. 11). The first of them (Fig. 11, a) presents an example of the structure of a simple

Fig. 10.

rarely filled with ore material. Sometimes the forming calcite veinlets, in the form of small lenses, include fragments of the enclosing rocks (Fig. 11 and Fig. 10).

Alongside veins of simple structure there are more complex veins, built as if of several layers; namely, two- and three-layered ones have been encountered. For example, two-layer calcite veinlets may be of two kinds. Let us imagine the formation of a calcite veinlet of the usual simple structure 1—1 (Fig. 11, д); subsequent extension then ruptures the veinlet along the median seam, with formation of a cavity (crack), on the walls of which the heads of prismatic crystals project. In this cavity, calcium-carbonate solutions promote the formation of new prismatic calcite crystals 2—2, which fill the cavity completely or partly. The process of such opening along a new seam line may be repeated a third and a fourth time.

Another type of formation of a multilayered veinlet is shown in Fig. 11, е. Here, as in the preceding case, there is a veinlet 1—1, filled with parallel prismatic calcite. Later, as a result of extension, a rupture crack develops along one of the selvages of veinlet 1—1. The cavity formed in this way along the crack, with the participation of carbonate waters, is filled with prismatic calcite 2—2. Later, as a result of repeated extension, a gaping cavity may develop along the other selvage of the veinlet. In this case the prismatic calcite crystals, perpendicular to the walls of the cracks, grow simultaneously from the two sides of the cavity and, as a rule, in close contact, form a median seam, clearly distinguishable in section, in the form of a zigzag line. Along this seam a free cavity—a “slot”—may remain (Fig. 11, б); and if the calcite crystals on the walls of the crack grow quite irregularly, then in this latter case cavities are formed into which well-formed calcite or quartz crystals project (Fig. 11, в). Such cavities are not infrequently filled with ore material. Sometimes the calcite veinlets forming in the form of small lenses include fragments of the enclosing rocks.

Fig. 11. Structure of calcite veinlets.

Not infrequently, small displacements occur along the plane of fractures filled with calcite. In this case, along the plane of the “median seam,” the heads of prismatic calcite, under friction, are crushed or undergo plastic deformation. Fig. 11, г shows a two-layer calcite veinlet in which the parallel prismatic crystals 1—1 have undergone plastic deformation: their heads proved to be bent,

sometimes brecciated, while the cavity that subsequently formed during tension was filled with brush-like calcite 2-2.

In Fig. 11, з it is shown that in the selvages of the deformed calcite veinlet 1-1 the cavities that formed are filled with fibrous calcite. Fig. 11, к illustrates that in the selvages of simple and complex (Fig. 11, к) veinlets, sulfides (black cubes) are often deposited together with calcite, with calcite and quartz.

4. Structure of the junctions of conjugate veinlets

Another complex and difficult question is presented by cases of conjugation of some calcite veinlets with others, i.e., determining the relative time of formation of one system of fractures and the carbonate veinlets filling them in comparison with another. One of the very important points in this case is the finding of convincing facts concerning the true character of the intersection of some veinlets by others. In Fig. 12, а and б, examples are given of the intersection of one veinlet (1) by another (2). In Fig. 12, б, a calcite veinlet approaches another (2), earlier veinlet at an oblique angle and intersects it in a perpendicular direction, i.e., in the direction of orientation of the fibrous crystals of veinlet (2), and on leaving it preserves the same direction as before the intersection; that is, the fracture, approaching the veinlet, changes its direction and intersects it along the line of least resistance. The filling of fractures (2) with calcium carbonate proceeds in such a way that the fibrous calcite crystals in the cutting part of the veinlet are oriented perpendicular to the fracture walls (Fig. 12, б).

In Fig. 12, а, an analogous case is shown of the intersection by a calcite veinlet (1) of another carbonate veinlet (2). The first intersects the second along the line of least resistance, but in the cutting part of veinlet (1) the thin parallel fibrous crystals of calcite are oriented parallel to the walls of the veinlet and parallel to the crystals of the intersected veinlet (2). In this case the intersection area appears especially white and dense against the general background of the calcite veinlets, which outside the place of intersection are usually translucent. In this case the orientation of the new crystals is subordinated to the orientation of the previously formed crystals.

In Fig. 12, в, an example is shown of a clear intersection of veinlet (1) by veinlet (2). In the following Fig. 12, д, it is clearly visible that calcite veinlet (1) intersects and slightly displaces veinlet (2). In Fig. 12, г, there is a conjugation of three calcite veinlets. Of these, the earliest is a complex veinlet with indices 2—1—1—2, which is displaced by veinlet (3). Carbonate veinlet (4) approaches the third at an angle, slips along the seam between veinlets 3 and 4, and at the exit deviates

Fig. 12. Junctions of conjugate calcite veinlets.

to the right. Fig. 12e illustrates a complex junction of calcite veinlets, analogous to the junctions in Fig. 5. The calcite veinlet (1) is broken up into blocks, each of which undergoes a complex displacement along veinlet (1) and simultaneously along fracture (2). Fig. 12zh shows that calcite layers 1, 2, 3, which make up the structure of a complex veinlet, and the fracture that subsequently arose in a direction perpendicular to them are filled by a two-sided layer of fibrous calcite. Later, metasomatic calcite is deposited along the outer margin of this complex veinlet, now on one side and now on both sides.

In a word, conjugation junctions may be constructed in a very complex manner; several systems of fractures may take part in their structure, and for them the following rule holds: as far as possible, they do not follow along already existing fractures.

5. Mechanism of fracture formation

Before turning to the question of the mechanism of fracture formation, let us present the main conclusions that follow from the factual material on the fracturing of calcareous-sandy rocks from the Cretaceous flysch of the area of the settlement of Golovinka.

1. Fractures within a bed that are parallel to bedding are absent.

2. Fractures, filled in some cases with calcite and in others with vein quartz, are generally perpendicular to the bedding and to the schistosity of the rocks.

3. Fractures of one system are parallel to one another and are spaced from one another at more or less equal distances, or at distances that are multiples of some period of recurrence.

4. One system of fractures, relative to another or to others, is inclined at a certain definite angle. In this case one system of fractures, being later, cuts and displaces the earlier one or the earlier systems of fractures.

5. Paired fractures that could be assigned to “shear” fractures are absent.

6. Study of the mutual relationships of fractures among themselves makes it possible to say that successively arising fractures avoid the directions of previous fractures and, as a rule, follow new tectonic directions.

7. However, the structure of calcite and quartz veinlets and veins indicates that the structure of veins is often complicated, and they sometimes acquire a multilayered structure as a result of repetition of processes of extension in one and the same direction.

8. With repetition of processes of compression and extension in one direction, repeated fractures may arise, parallel to one of the previous systems—fractures that had earlier been created under tangential stress of the same direction.

9. Determination of the mutual relationships of fractures among themselves will make it possible to outline concretely the history of the sequence of formation of fracture tectonics.

The conclusions just presented, relating to fracturing in sandy limestones, concern in general a particular case in the tectonics of the Caucasus. Nevertheless, the data presented attract special attention and undoubtedly deserve careful study. However, as regards explaining the nature of the fracturing described above from the modern standpoint of our knowledge, there are certain difficulties. Thus, G. D. Azhgirei, after a critical analysis of G. F. Becker’s hypothesis of deformation of rocks, proposes completely abandoning the application of the mathematical apparatus of Becker’s hypothesis for the analysis of deformations of mountain rocks.

rocks, since this analysis, as he asserts, can with equal success be carried out on the basis of the direction of the three principal axes of deformation [1]. On this basis G. V. Azhgirei assumes that, during tectonic movements under “varied conditions,” the simultaneous formation, in the same rocks, of from two to six and more systems of “shear” fractures is possible [1]. Similar assertions, however, had earlier been made by A. A. Belitskii [2], M. B. and N. I. Borodaevskii [6], partly by L. V. Pek [8], and others.

A. V. Pek, in his explanations of fracture tectonics, uses the deformation ellipsoid [8], which, as follows from his latest work [9], proves to differ little from a sphere; i.e., the angle \(2V\) between the shearing planes will be small and the two shearing planes that should arise will merge into one \(BC\) plane, perpendicular to \(A\)—the direction of deformation. This very important and well-founded conclusion follows from the assumption that the ellipsoid is close in volume to a sphere. However, with regard to the question of fracturing, A. V. Pek warns that “the mere presence of displacements of some fractures by others, without sufficiently weighty evidence, does not provide grounds for speaking of their different ages, but at the same time one should not fall into the other extreme and assign all fracture systems to a single tectonic stage” [8]. This view of L. V. Pek is, to a certain extent, also supported by G. D. Azhgirei [1].

The author of the present work has no serious grounds for rejecting the application of the deformation ellipsoid in solving tectonic questions. At the same time it should be remembered that the Earth’s crust and its tectonic structures are in constant motion and development. Here the author has in mind ascending and descending movements, which cause the axes of folds now to plunge, now to rise. Therefore fractures parallel to \(BC\) of the ellipsoid at one position of the fold axes will be intersected by fractures parallel to \(BC\) at another position of the fold axis. Further, the deformation ellipsoid should not be applied mechanically from “strength of materials” to the explanation of natural phenomena. An experiment in the laboratory with the compression of cylinders and prisms lasts minutes; natural experiments last, as a rule, “hundreds of thousands and millions” of years, and therefore fractures parallel to \(AB\), \(AC\), and \(BC\) will predominantly develop, although in individual cases the formation of “diagonal fractures” is not excluded.

Of the factual material set out above, the most important point is that the fractures of each system are spaced from one another at approximately equal distances, or, in other words, these fractures, filled with calcite, are spaced from one another at a distance that is a multiple of some period of recurrence. The second conclusion is that one system of fractures, relative to another, is inclined to one side or the other by a certain constant angle. It must be supposed that the fractures studied owe their origin to stresses arising during plicative dislocations. These stresses, acting for a long time in one direction, determine the appearance of longitudinal-harmonic oscillations. V. V. Golitsyn already believed that the couplings occurring in earthquake foci serve as the source of longitudinal and transverse elastic waves of colossal power [8]. According to modern data, our Earth is a body that continuously undergoes oscillations as a result of an enormous number (many hundreds of thousands) of earthquakes of varying power [5]. Hence, in accordance with harmonic longitudinal and transverse oscillations, the particles composing rocks will, during longitudinal-harmonic oscillations in the mass, experience compression and extension (Fig. 13, a). In places of compression, compaction of the rock and redistribution of matter along the plane of compression by transverse ...

oscillations, while the tensile phenomenon that replaces it leads to rupture and the formation of sites where fractures are initiated. In Theory of Elasticity [10], M. M. Filonenko-Borodich, on the basis of an analysis of Lamé’s equation as applied to longitudinal and transverse oscillations in an unbounded elastic medium, comes to the conclusion that longitudinal-harmonic oscillations cause the appearance of compression zones (nodes), which alternate with zones of tension (antinodes), the distance between nodes or between antinodes being equal to the wavelength (Fig. 13, a and 13, b). This can be confirmed by our experiments on the distribution of matter under standing-wave conditions. The experiments were carried out by Kundt’s method in tubes. Instead of club-moss spores, finely ground powders—

Fig. 13. Diagram of longitudinal-harmonic oscillations:
a — compression nodes and tensile antinodes under longitudinal-harmonic oscillations; b — compression nodes and tensile antinodes under longitudinal- and transverse-harmonic oscillations.

—of magnetite, apatite, orthoclase, mica, actinolite, and acicular aragonite were used. The experimental data showed that the greatest compression occurred at the nodes of the standing wave; in these nodes, at the same time, a distinct movement of particles away from the wave axis in the plane of the node was observed.

After the stress had fallen off (been released), compression in these places was replaced by tension, with the formation not only of tectonically weakened sites but also of fractures, the walls of which diverged with the formation of a cavity. According to A. G. Betekhtin, such rupture fractures are created “instantaneously,” with the formation of a cavity—fracture—void with greatly reduced pressure [4]. This latter circumstance causes vigorous filtration, from the rock itself, of intergranular and hygroscopic water, acidic calcium carbonate salts, carbon dioxide, and subsequent deposition on the cavity walls of prismatic calcite crystals. Study of the structure of a number of calcite veinlets has shown that these veinlets are often parallel-banded (Fig. 11), and the prismatic calcite grains composing them prove to be distinctly zonal, parallel to the faces {1011}, which is probably closely connected with the periodic intermittent growth of calcium-carbonate crystals. Similar phenomena of banded-zonal structure can be seen in veinlets of quartz, aragonite, gypsum, asbestos, actinolite, epidote, garnet, and many others.

other minerals filling fractures, and other forms of cavities. Only what has just been said is in contradiction with A. G. Betekhtin’s assertion that, in the cases described, fractures open slowly and intermittently. The filling of fractures may occur not only through the filtration of material from the host rocks of the vein, but also as a result of the movement of solutions within the cavity of the vein.

As for the inclination of one system of fractures relative to another, this phenomenon can be explained, on the basis of the geological setting, as follows. If \(A\), \(B\), \(C\) are the directions corresponding to the principal axes of the deformation ellipsoid, then the axis \(A\) (horizontal) is parallel to the direction of action of the tangential force. The axes \(B\) and \(C\) lie in a plane perpendicular to \(A\). If the deformation ellipsoid according to A. V. Paek is close to a sphere [9], then the slip planes, with a very small angle between them (probably less than \(20\text{--}10^\circ\)), will be represented by a single fracture coinciding with the plane \(BC\). In this case, it must be assumed that fractures parallel to \(BC\) and \(AB\) are more probable. Usually an important point of historical significance is overlooked, namely: the unevenness of the general movements of the Earth’s crust over time, or, in particular, the subsidence or uplift of fold axes, the uneven uplift and subsidence of blocks while the direction of action of the tangential forces themselves remains relatively constant or constantly intermittent. Hence, naturally, fractures of one system in rocks, formed at one stage, later, at the next stage—for example, during the plunging of the fold axis—change their relative position; for earlier, newly formed fractures parallel to \(BC\) and perpendicular to \(A\) will now prove to be somewhat inclined relative to their former position. Newly arising fractures parallel to \(BC\) of the ellipsoid will be perpendicular to the direction \(A\) and will intersect the earlier fractures, etc. Such a nature of the fractures makes it possible to explain why subsequent fractures do not repeat the earlier fracture directions, even in cases of microfracture formation.

Thus, the assumption that the direction of the tangential forces and the position of the deformation ellipsoid remain constant, while secular oscillations of the Earth’s crust are the cause of uneven uplifts and subsidences of blocks and fold axes, in our particular case makes it possible to understand the nature of the fractures, the relative position of some fracture systems with respect to others, and to establish the sequence of their formation.

References

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