MATERIALS ON THE TECTONICS OF INTRUSIVE BODIES

G. L. Pospelov

PREFACE

The present work is, on the one hand, an exposition of the essence of Hans Cloos’s theory of the tectonics of granites and a critique of it, and, on the other hand, a theoretical study in the field of the prototectonics and, in part, the fracture tectonics of intrusive bodies, carried out on the basis of numerous personal observations and the extensive literature available on this question.

The general character of the topic, which I took up at the suggestion of Academician M. A. Usov, is due to the fact that, in the course of analyzing the tectonics of particular intrusive massifs on which I had occasion to make detailed surveys and thematic excursions—certain districts of Altai, Khakassia, and Martaiga in Siberia, the alkaline massifs of the Kola Peninsula, and certain areas of development of young intrusions in the Caucasus—a number of features became apparent which, when compared, compelled me to reflect on an entire series of fundamental questions in the field of the tectonics of intrusive bodies in general. The importance of such work, supported by a critical analysis of the methodological foundations on which contemporary tectonic schools, in particular the school of H. Cloos, are based, is obvious. Not only in Russian but also in foreign literature there are still few works devoted to this question, although geologists’ interest in it is steadily increasing in connection with the study of the structures of ore fields; but even those works that do exist mostly confine their constructions within the principles of classical mechanics, without attempting to approach the analysis of the internal tectonics of intrusive bodies from the standpoint of the general interaction of the processes that determine the origin and development of intrusive bodies and of the tectonic structures within them. Meanwhile, an approach to this topic precisely from this side opens up, as it turns out, a whole series of major physical problems, an entire realm of complex phenomena almost untouched by research. It leads to the possibility of fundamentally new solutions to a whole series of questions concerning the genesis of the structures of intrusive bodies.

Therefore, the creation of such works, even in the form of materials that facilitate the formulation of new problems in this field, is a very timely and necessary undertaking.

In conclusion, I must thank the Leningrad geologists, Dr. N. A. Eliseev, N. K. Nefedov, and S. D. Pokrovsky, who gave me great assistance in becoming acquainted with the tectonics of the Lovozero alkaline pluton.

I. THE ESSENCE OF HANS CLOOS’S THEORY ON THE TECTONICS OF INTRUSIVES

The principal theory on which modern structural analysis of intrusives is chiefly based is Hans Cloos’s theory of the tectonics of granite massifs, subsequently complicated and modernized by various researchers as a result of its application to the tectonics of intrusives in general, but preserving its fundamental content.

In Russian, an exposition of H. Cloos’s theory and of his methods of structural analysis of intrusives—his principal works in this field are listed in the bibliography—is available in a number of books (14; 3; 8; 13; 18). In addition, there are a number of interesting works by A. A. Polkanov, E. A. Eliseev, A. V. Pэk, and others, in which Cloos’s methods are rationally applied to geological practice, with their accompanying improvement and development. This frees us from the need to give a detailed exposition of Cloos’s theory in the present work. I shall therefore confine myself here to presenting those basic points which define the essence of H. Cloos’s views and which, incidentally, often escape the attention of geologists who use H. Cloos’s method in their practical work.

Hans Cloos was not the first geologist to study the tectonics of intrusives. Oriented structures in intrusives, and the unequal capacity of granite to split along different directions, revealed in connection with the working of dimension stone, attracted the attention of investigators as early as the eighteenth century and since then have repeatedly been the subject of discussion. Likewise, researchers long ago drew attention to the regular arrangement of cracks in intrusives, which was in most cases explained by contraction forces—the reduction in volume of the intrusive during its cooling.

However, H. Cloos was among the first to approach the solution of questions concerning the structures of intrusives not from the standpoint of explaining isolated cases without interrelating them, but from the standpoint of the development of a general intrusive process. Revealing the regularities inherent in all intrusives in general, and consistently developing his views, he created a coherent picture of intrusive tectonics, showing that its various elements are the expression of different phases of the intrusion and formation of the pluton.

Hans Cloos regards the volcanic process as a specific movement of a very plastic material, taking place against the background of the general movement of the masses of the continent. He sees the causes of all these movements in various tectonic pressures: in the hydrostatic pressure that expels the molten mass from the magmatic chamber; in oriented lateral pressure; and in the gravitational pressure of the overlying strata, which shape the magmatic masses that have entered the upper horizons of the earth’s crust and lead to the emergence, during different phases of their formation, of different structures.

In the first tectonic, pre-granite phase, i.e., before the appearance of magma, the movement of the continental masses prepares for it the paths of penetration and the receptacles—cavities. This is expressed, first, in folding, in connection with which concordant intrusion may occur, i.e., the intrusion of magma during the process of folding, accompanied by its penetration, as the most plastic element, into all the developing weakened places and voids, especially in the anticlinal regions, which it fills with considerable perfection (14, 45).

Second, this is expressed in block movements, if the continent has already been consolidated by previous folding and vul—

magmatic activity, which leads to the emergence of a discordant intrusion, often penetrating, as studies by H. Cloos in Germany have shown, along the boundary between more ancient formations and the younger folded formations overlying them (harpoliths).

The movement of liquid magma, occurring against the background of the general movement of the enclosing masses, leads to its differentiation during movement and to the emergence of various flow structures. In the earliest, so-called granite phase (52, 93), embracing phenomena that occur in fairly fluid magma, movements take place that are revealed in the orientation of endogenous schlieren, in the parallel elongation into chains of inclusions of country rocks in the magma, etc. This phenomenon cannot be called simply an expression of fluidity or fluidal movement, since it also represents an expression of the change in the form of the magmatic mass and of its adjustment to some other volume. The movement is similar to the movement of a liquid in a round vessel that is deformed into an oval one (52, 93). In a more viscous phase, with the progressive crystallization of the magma, the stretching of the magmatic mass under the influence of the same continuing oriented pressures (lateral and gravitational), occurring in a direction perpendicular to them, is manifested in the formation of structures oriented perpendicular to the pressures, or of stretching (Streckung—stretching), usually expressed in the parallel arrangement of mica flakes and, more rarely, feldspars and quartz. This is, as it were, an expression of schistosity in a solidifying intrusive body, in which the magma becomes so viscous that no special differentiated movements occur in it, as in the stage of schlieren formation, and it, together with all inclusions, moves by stretching as something whole (56, 123), leading to phenomena of protoclasis. This stretching may continue into the period of solidification and in that case be accompanied by cataclasis, sometimes very considerable, proceeding in the same direction (56, 123). In this process the primary stretching passes into secondary stretching—in gneiss, especially near the contacts, where from the very beginning there existed a sharp contradiction between the moving magma and the immobile walls. As a result of this, the so-called marginal gneisses arise.

Thus, stretching gradually and imperceptibly passes into the post-granite phase, when, owing to the resistance of the granite increased by cooling, the latter, continuing to stretch under the influence of the same oriented forces, begins to split and to form fractures which, because of the commonness of their cause, are in a regular relation to the stretching. These fractures may be filled by vein derivatives of the same magma, but in general they reflect postmagmatic tectonics, no longer connected with the formation of the given massif, but belonging to subsequent tectonic movements, to which the granite reacts simply as a rigid part of the continent, as a “tectonic transformer.”

All structures that arose in the pluton during the phase of its flow are united in the concept of “prototectonics,” or primary tectonics; all fracture structures in it are united in the concept of “fracture tectonics.”

Let us dwell briefly on a more detailed genetic characterization of the prototectonics and fracture tectonics of intrusive bodies.

The entry of magma into a cavity entails, above all, a state of extreme heterogeneity, for into a solid frame there enters a mass of magma which, though already viscous, is nevertheless still quite fluid. To this time of sharp contradiction between solid and liquid belongs the formation of schlieren (56, 119). Biotite flakes, from the accumulations of which chiefly are formed

these schlieren in granites respond to flow movements differently than feldspars and quartz do. At a certain stage of viscosity they tend to arrange themselves in parallel, presenting to the movement the planes of least resistance. In the process of movement the flakes may group themselves into schlieren, all the more perfect the stronger the movement was (56, 130—131). Thus, foliated ribbon schlieren are the result of the differentiation of magma in the course of its movement. Blocks of enclosing rocks, as less mobile parts, are flowed around by schlieren (54, 132).

The contradiction between a more fluid and a less fluid medium, leading to the formation of schlieren, naturally appears most sharply at the boundary between a solid and a fluid mass, i.e., at the contacts, near which the sharpest phenomena of schliericity and, in general, of parallel structure arise, repeating the contours of the contact. Thus there arises the marginal tectonics of plutons (56, 119—120). An expression of the same kind of marginal tectonics is schliericity in veins, which is always arranged parallel to the contacts, irrespective of their direction relative to the general pressure. The contacts of plutons are often steep; accordingly, their marginal tectonics is also steep.

Fig. 1. Tectonics of schlieren and fractures at the contact of the granite pluton in Riesenbirge (after H. Cloos)

However, in the central parts of many plutons the schlieren are gently disposed and, in their totality, form an upward-convex, arc-shaped vault (Fig. 1). Such a gentle disposition of schlieren is not a consequence of the influence of the gentle roof of the intrusion, but is the normal, regionally expressed result of the upward movement of the melt, opposed by gravitational pressure, as a result of which the magmatic masses spread from the core of the massif toward its periphery in a direction perpendicular to the lateral and gravitational pressures (Fig. 2). The influence of the latter also accounts for the gentle disposition of the elements of prototectonics, which in this case do not depend on the form of the pluton.

Fig. 2. Relation of parallel structures to the boundaries of the pluton and to the movement of magma in Riesenbirge. Vertical section. (After H. Cloos.)

Lineation, formed as a result of general stretching perpendicular to the pressures by far-advanced processes of homogenization of magma crystallization, sometimes displays a strikingly uniform direction. Obviously, it depends very little on the structure, position, and boundaries of the pluton, everywhere showing the same position in space (53, 71—79). Lineation, in the opinion of H. Cloos, is not properly a fluidal formation, being connected only with very slight lateral movements; and it is possible that, to a considerable extent, it is the result of molecular movements in place (54, 95). In proof of this H. Cloos cites a number of cases in which lineation in granite-aplite and granite-porphyry veins is arranged at the most varied, often right, angles to the contacts of the vein (Fig. 3, a, b, c) (52, 94—96; 55, 17). Such structures

are not the result of the action of pressure on already solidified magma, since no phenomena of cataclasis or crystalloblastesis are observed here. The presence of small inclusions of the country rocks in the vein, oriented in accordance with its lineation, also argues for this (Fig. 3a). It turns out that the orientation of the lineation in the vein coincides with the orientation of the lineation (schistosity) in the country rocks, the reason for which Kloos sees in the identical orientation of the forces that determined the formation of lineation both in the enclosing rocks and in the veins.

Since the position of lineation, just like that of schlieren, is determined, on the one hand, by lateral pressure (stress), and, on the other hand, by load pressure, it follows that lineation must in general be perpendicular to both acting forces. In this connection, on the basis of his observations, H. Kloos distinguishes two types of lineation. One of them, representing a plane-parallel structure, arises owing to the redistribution in the magma of platy-parallel components (micas, feldspars) and is usually horizontal, i.e., perpendicular to the load pressure. The other represents linear lineation, caused

Fig. 3. a and b—aplite veins in schists; the lineation in the veins (dashes) is arranged in accordance with the schistosity in the schists. c—a granite vein with an aplitic selvage in normal granite; the lineation in the vein (elongated grains of quartz) is arranged in accordance with the lineation in the granite (after H. Kloos).

by the linear arrangement of columnar minerals predominantly in a direction perpendicular to the lateral pressure.

In addition to the term “lineation” Kloos also uses the term “fibrousness” (Faserung), chiefly for cases where the lineation is weakly expressed.

Lineation, or fibrousness, even when weakly expressed, determines the directional divisibility of granite during its artificial splitting, which is used in stoneworking. Such divisibility of granite generally coincides with the natural fractures in the granite, which, according to H. Kloos, form under the influence of the same forces that condition the formation of prototectonics, and therefore stand in regular relation to it. However, whereas prototectonics, arising in the process of magma movement, must, in addition to adaptation to oriented pressures, also reflect the position of contacts, being arranged near them in accordance with them, fracture tectonics follows the directions that characterize the broadest surroundings of the massif, and therefore cuts across the contact boundaries, merging with the fracture tectonics of the surroundings (56, 110), (Fig. 1). Thus, the divergence between fracture tectonics and prototectonics is expressed in the fact that, while in the greater part of the massif they exactly or approximately coincide with one another, at the contacts the prototectonics stands more steeply and is crossed by the fracture tectonics (56, 108).

To what extent is the fracture tectonics of intrusions connected with their contraction during cooling?

If the appearance of fractures were connected with the magma itself, then, as the latter cooled, peripheral fractures and veins parallel to the contacts should have arisen; in practice, however, these are not encountered (56, 127). H. Cloos proposes to explain the absence of typical contraction fractures by the influence of lateral pressure, which closes contraction fractures that do not conform to it “before they form,” allowing only those fractures to develop that correspond to their general regular system (52, 92). Owing to the fractures at the contact, the intrusion is closely connected with the tectonics of the surrounding rocks, because at almost all places along the contact the principal fractures pass from the intrusion into the enclosing rocks, displaying here definite relations to tectonic features older than the intrusion—for example, to slaty cleavage, to fold axes, and so forth (56, 127–128). This indicates that “although the formation of the massif creates fractures anew, the directions of the fractures already exist and are merely utilized” (56, 110).

We shall still have occasion to speak about the systematics of the fracture tectonics of intrusions proposed by H. Cloos, and about the mechanism by which the various fracture systems arise; therefore I omit this independent question here.

II. THE PROBLEM OF THE TECTONICS OF INTRUSIONS IN THE LIGHT OF THE THEORY OF THE SELF-DEVELOPMENT OF NATURAL PROCESSES

The theory of H. Cloos on the tectonics of intrusions, the essence of which I have just set forth without any commentary, is wholly based on—and, in its turn, serves as the foundation for—the so-called theory of passive volcanism, whose adherents constitute the majority among the representatives of the modern magmatic schools. Naturally, before speaking about the strength of the edifice of the Cloosian theory, it is first necessary to see how strong the foundation is on which this edifice has been built, for the strength of the foundation determines first of all the strength and stability of the entire structure.

In my general brief remarks on this subject I shall not undertake a detailed factual critique, leaving that for another occasion, but shall confine myself chiefly to the methodological side of the question, since the validity of any scientific generalization is determined above all by the degree to which it accords with the fundamental laws of dialectical materialism, as its great founders have shown with sufficient irrefutability.

Modern geology possesses numerous facts indicating the presence of some general genetic cause not only for terrestrial volcanism, but also for all the principal kinds of movement in the Earth’s crust. For example, it is known that manifestations of volcanism are tied to the time and place of manifestations of mechanical movements in the Earth’s crust, and not everywhere in the same way, but in accordance with the intensity and character of these movements. Thus, with the places of the most intense movements (the “closing” geosyncline) is associated the most intense—intrusive—type of volcanism, accompanied by the reworking and assimilation of colossal masses in the Earth’s crust; whereas with places of less intense movements (chiefly of the oscillatory type) is associated a less intense—effusive—type of volcanism, characterized by simplified relationships between the magma and the enclosing strata. It is also known that definite epochs of volcanism possess, everywhere, pronounced common features both in the material composition of the rocks formed and in the character of the associated mineral resources (ore and non-ore), and also in the form

volcanism (predominantly deep-seated, or predominantly surficial), although at the same time each volcanic region also possesses individual geological and chemical features. Moreover, it is known that the most powerful epochs of volcanism manifest themselves simultaneously over extensive areas of the earth, although the volcanic epoch itself consists of repeated (multiphase) intrusions of magmas, which manifest themselves differently in different regions. In material terms it has long been established that, despite the diversity of chemical elements in magmas and in the rocks surrounding them, there exist not arbitrary but definite types of igneous rocks and their associations, which, moreover, do not appear everywhere in the same way, but depend on the character of movements of the earth’s crust. For example, in regions where the most intense and complex movements occurred (geosynclines), acidic, granitic rocks are more characteristic, whereas in regions where less intense and complex movements occurred (platforms), basic, basaltic, alkaline rocks, etc., are more characteristic.

Such examples, the enumeration of which could be continued at length, point to the profound internal connection that exists between the general development of the Earth and the manifestations of volcanism as a particular case of this development. It would seem that all these facts should have led theorists to a critical reconsideration of the dominant ideas concerning the development of the Earth and to a deep, comprehensive analysis of those processes whose interaction determines the aggregate movement of the upper zones of the Earth in all its concrete forms. However, theorists prefer to proceed along the old metaphysical path, attempting to derive all geological movements, including volcanism, from existing views of the Earth that presuppose the absence of any general qualitative change in all its matter, and that seek to reduce all the geological development of the Earth essentially to mechanical relations¹.

The specific features of volcanism in relation to other structure-forming processes taking place in the earth’s crust are determined by the fact that in the formation of geological structures here there participate complex supramechanical processes, leading to the creation of high-energy systems—magmas—capable of independent qualitative change and of actively transforming the strata surrounding them. Therefore the central question of the problem of volcanism is the question of the nature of volcanism, as the most complex form of manifestation of the general development of the Earth, i.e., the nature of those processes that condition the formation and intrusion of magmatic materials and their conquest of space.

How, then, do petrologists and tectonists resolve this question?

Modern geotectonics builds its generalizations chiefly on the postulate that the Earth can develop only at the expense of thermal energy inherited from the stellar state, inherited radioactive energy, and external forces—the attraction of the sun and moon, the rotation of the Earth, and so forth. This postulate assumes that if supramechanical interaction exists between the central and external zones of the Earth, expressed in the migration of heat from the center to the periphery, then it is determined only by thermal conductivity, which is so small that in practice one may speak of the supramechanical independence of the development of the upper zones of the Earth from the development of its inner zones. Therefore, since supramechanical geological movements take place in the upper zones of the Earth, such as volcanism and deep metamorphism, these movements possess a certain autarky, a supramechanical independence from the central zones

of the Earth, and unfold at the expense of the energy resources of its upper zones, drawn by them from the inherited store of thermal energy, or by means of radioactive decay, or the conversion of mechanical movements into their thermal equivalent.

But such supra-mechanical causes do not have the necessary connection with the general development of all the Earth’s matter, and depend chiefly on local conditions, having a particular character. Therefore, from such supra-mechanical causes one cannot derive the general laws of volcanism required by the facts, although volcanism is indeed a phenomenon primarily of a supra-mechanical order.

The theoretical resolution of this contradiction is carried out by the old method, tested in metaphysical natural science: volcanism proper is declared to be only the intrusion of magma and its conquest of space, whereas the formation of magma is assigned to the preparatory period of volcanism, when, with the aid of the supra-mechanical causes available to theorists, the initial materials—magmas—are created, as the necessary prerequisite of volcanism, constituting a special problem to be solved separately.

With such a resolution of the question of the role of supra-mechanical movements in the volcanic process, the explanation of the causes of the coincidence of volcanism with mechanical movements in the Earth’s crust is naturally sought in the mechanical interdependence of these movements, i.e. in the mechanical interactions of ready-made magma and its surroundings, accompanied, by virtue of the specific properties of magma (high temperature, the release of active gaseous products, etc.), by certain supra-mechanical phenomena (melting, assimilation, contact metamorphism, etc.), which have no direct relation to the movement of magma and constitute an independent group of processes considered separately. In short, the theoretical generalization of the problem of volcanism proceeded not along the path of revealing the general interaction of the processes of volcanism and tectonic movements in all their complexity and many-sidedness, but along the path of dividing the general process into a series of independent, separately considered processes, along the path of putting forward ideas of primordial materials, in which the interrelations of movements proved to be replaced by the interrelations of bodies, of things.

How is the problem usually solved: tectonics and volcanism?

It is assumed that throughout the whole process of volcanism magma appears as a more or less compact body capable of transmitting mechanical pressure, and by this are determined the principal features of the interaction of magma with its surroundings. Magma either exerts pressure on these surroundings, being distended by its own gases, thereby causing dislocations in them (so-called “active volcanism”), or it receives pressure from the surroundings, moving under its influence (so-called “passive volcanism”).

But if one adheres to active volcanism, i.e. to the idea of the self-movement of magma, then, given the vagueness and particular character of the supra-mechanical premises of volcanism, its general laws do not lend themselves to satisfactory explanation; all the more so because the very mode of active movement of magma, as pictured by the proponents of active volcanism, is based on a whole series of unproved assumptions, since it is applied by them not to particular cases but to the principal manifestations of volcanism and tectonics. Therefore most theorists prefer to adhere to the ideas of passive volcanism, deriving the general laws of volcanism and tectonics through the commonality of mechanical forces, supposedly alone determining both kinds of movement, for in the eyes of most geologists the intrusion of magma and the filling of space by it is a process

tectonic, i.e., a process that represents mechanical motion leading to the transformation of structures.

The tendency to reduce almost all geotectonics precisely to mechanical forces is highly characteristic of modern geological thought, the representatives of which, for the most part, prefer not to trouble themselves with penetrating the mysteries of the structure and development of terrestrial matter, devoting themselves instead to exercises, more readily accessible to so-called “common sense,” in swimming amid the shallow-water element of mechanistic reasoning. Here there is no need to proceed from an analysis of one or another complex geological phenomenon as a necessity arising from the essence of the Earth’s internal development. One need only find a source of a mechanical force sufficient in magnitude, even if it has no relation whatever to the Earth’s own development, and the problem may already be considered half solved: the force exists; now, ignoring the origin of this force, it can be used for the production of various geological structures.

It is assumed that the differentiation of a general tectonic movement, caused by a force obtained in some manner, into a series of derivative or relative movements that condition the diversity of geological structures, depends, in accordance with mechanical experiment, on the mechanical properties of the materials being deformed. Mechanical forces and the mechanical properties of different parts of the crust—this is the main thing that determines the development of geological structures on the Earth.

As applied to volcanism, this idea was expressed most vividly by the German theorist S. Bubnov in his semi-philosophical work Basic Problems of Geology.

“Plutogenesis, i.e., the uplift of molten masses and the formation of the space that accommodates them, and orogenesis, i.e., the displacement in space of solid rocks, are only separate manifestations of one and the same process, the difference between which depends on the physical state of the material at each given moment. This conclusion, showing that plutogenesis is only orogenesis of highly mobile, i.e., very plastic, material, naturally should not yet lay claim to absolute certainty in the sense of the logically complete closure of the chain of inferences. However... this ‘working hypothesis,’ in comparison with other assumptions, has far greater probability, being based to an incomparably lesser degree on unproven assumptions that cannot be verified by facts” (p. 76).

Thus, the adherents of “passive volcanism” believe that the plastic properties of magma are the cause of volcanism as a special geological movement, and that the essence of volcanism is the mechanical displacement of ready-made material in space, while the quality of this movement remains unchanged, being from beginning to end a mechanical movement.

Engels’ characterization, given by him to Wiedemann and others, who advocated mechanical theories of the electric current, is highly applicable to such views in geology.

“Exceptional empiricism, which permits itself thought at best only in the form of mathematical calculations, imagines that it operates with indisputable facts. In reality, however, it operates chiefly with traditional notions, obsolete for the most part, products of the thought of its predecessors... These latter are made by it the basis of endless mathematical elaborations, in which, because of the rigor of the mathematical formulae, the hypothetical nature of the premises is easily forgotten. The more skeptical this empiricism is toward the results of contemporary scientific thought, the more credulous it is—

he relates to the results of the thought of his predecessors. Even experimentally established facts are connected by him, little by little, in an inseparable way with their corresponding traditional interpretations.... This empiricism is not capable of representing facts correctly, for into his representation of them there inevitably enters the traditional interpretation of the facts” (Dialectics of Nature, 1932, pp. 184–185).

An example of such a traditional interpretation of facts is the theory of Kloos, who, on the basis of facts concerning the internal tectonics of intrusions, attempted to prove that “the traces of movements in a liquid and solidifying mass correspond quite harmoniously and consistently to mountain-forming movements in the surrounding rocks” (3,76), thereby presenting “irrefutable proofs” of the validity of the theory of passive volcanism and of the triumph of metaphysics.

Meanwhile, the accumulated geological facts, and the achievements of such new sciences as geochemistry, which has opened new paths for a profound analysis of the content of the process of volcanism—all this presents in a different light the interrelations of the separate processes within the general volcanic process. It is becoming ever clearer that the formation, intrusion, and consolidation of magma are not separate processes independent of one another, but are phenomena causally and closely connected with one another by ties that are far from mechanical. In this respect, the various phenomena accompanying the volcanic process—such as palingenesis, anatexis, various kinds of assimilation and contact metamorphism—and, on the other hand, cases of the absence of any significant alteration of rocks in fragments immersed in magma and at contacts: all these contradictory facts begin to appear not simply as isolated cases of the magmatic process. Consideration of the volcanic process in its development compels us to approach the analysis of these “isolated cases” in their interconnection with one another, revealing the development of the general process of volcanism. All these differences among the phenomena that accompany and characterize the volcanic process—beginning with the complex processes of transformation of rocks into magma and the mixing of deep-seated and local materials, and ending with simple injection without contact-metamorphic changes—show us the stadial character of the development of the volcanic process, which, like any other naturally developing process, must have its own main line of evolution.

The numerous disputes about how magma wins for itself a place in space, accompanied by the citation of abundant factual evidence which, in sum, contradicts one another, are based on an understanding of the volcanic process as a process devoid of internal evolutionary development; they are based on an understanding of the geological life of the Earth as the sum of mechanical movements occurring under the influence of a sum of certain mechanical forces.

Modern science concerning volcanism possesses sufficient materials to review, from top to bottom, the old mechanistic conceptions of volcanism, and in particular the mechanistic conceptions of the internal structure of erupted bodies; for the structures, and the dynamic interconnection of structures that arise one from another and follow one after another, are a reflection of the content of the processes that generated them, which arise one from another and follow one after another.

In this respect, a comprehensive study of the prototectonics of intrusions can render an enormous service to the theorist who has devoted his labor to studying the essence of the volcanic process, the character of the movements that condition it, and the interconnection of these movements in the course of the general evolution of the volcanic process, for all these processes cannot fail to be reflected in the structure of the rocks arising from magma.

In fact, the course of the process of crystallization of magma and the character of the structures arising thereby must proceed in interaction with the symmetry of the crystallization medium (16, 58), and the latter must depend on the character of the intrusive process. The well-known rule of interaction between the symmetry of a growing crystal and that of the crystallization medium states that “in the external form of a crystal only those elements of symmetry are preserved which are common to the medium and to the ideal crystalline polyhedron” (43, 166). This influence of the symmetry of the crystallization medium is reflected not only in the form of the crystal, but also in the course of the crystallization process (16, 116), in the orientation of the crystals that arise (79), and so forth. Mathematical analysis shows that any medium consisting of statistically distributed particles may possess a very diverse symmetry, differing from the spherical symmetry of empty Euclidean space (43, 164; 16, 116). This symmetry will be determined, on the one hand, by external causes that create in the given medium a definite field: gravitational, magnetic, a field produced by externally oriented pressure, etc.; on the other hand, the symmetry of the medium will be determined by internal causes conditioned by the internal development of the medium itself: its motion, the energetic processes taking place within it, etc. Consequently, the symmetry of such a medium as magma will be determined, on the one hand, by the external historical setting in which the development of the intrusive process takes place, and, on the other hand, by the character of the development of this intrusive process, by its evolution.

The possibilities that already objectively exist in the field of studying the structures of igneous bodies and of analyzing, with their aid, the essence of volcanic processes have far outstripped those metaphysically limited ideas on which the contemporary leading trends in structural geology still remain (Cloos, Sander, and others). Instead of an all-sided analysis of the interaction of structure-forming processes, all these tectonic schools in fact occupy themselves with trying to explain all tectonic structures, beginning with folds and fractures and ending with the prototectonics of igneous rocks, in a single mechanical mode, setting forth in their arguments magma as simply a plastic material, like clay, as a liquid “in general”; and not as an exceedingly complex energetic system in which, beneath the evolution of external mechanical properties (melt—solid body), there are concealed complex, as yet almost unsolved supramechanical processes, about which we still have only superficial information.

However, the tectonic constructions of H. Cloos contain elements that go beyond the limits of that metaphysical methodology which makes H. Cloos’s theory fundamentally unacceptable. His ideas contain the recognition that the conditions of mechanical movement of magmatic masses change with their development, and consequently the character of the structures arising as a result of this also changes; these structures are considered by him in dynamic relation to one another. In other words, Cloos develops the idea of the tectonic evolution of magma, which favorably distinguishes the views of H. Cloos and his followers from the views of other adherents of the school of passive volcanism, who limit their reasoning to the problem of the displacement of magma in space. And although this evolution is reduced almost entirely to changes in the mechanical properties of magma, i.e., to one side of the many-sided process of the evolution of a volcanic act, with its other aspects forgotten, nevertheless this is already a significant step forward, shifting the doctrine of the structures of igneous bodies from the dead point at which it stood before Cloos. To this should be added at least the following valuable points, which may be placed

to the credit of H. Cloos before theoretical thought in the field of structural geology.

1) The recognition that oriented structures of intrusives arise in the movement of magmatic masses.

2) The advancement of the proposition that, in the process of movement of magmatic masses, phenomena of differentiation may occur as a consequence of this movement.

3) The assertion that, after the solidification of the magma, an intrusive loses its specific properties as a highly mobile system and becomes a “tectonic transformer,” a simple transmitter of mechanical stresses, being in this respect equated with the rocks that enclose it. Under the influence of general mechanical stresses independent of the intrusive, structures of the same type as those in its surroundings arise within it—systems of fractures, the orientation of which is, in the general case, identical with the orientation of the same structural elements in the enclosing rocks, since it is determined by the orientation of the general mechanical stresses.

4) The demonstrative defense of the proposition that certain intrusive bodies arise as the result of the simple filling by magma of a space formed in the country rocks, occurring without significant participation of assimilative phenomena.

5) The development of the proposition that such intrusive bodies may arise in the process of folding, in which case they take a certain part in it and win their place by the plastic pushing apart, by magma, of the enclosing rocks; and that they may arise after folding (in the “fracture phase”), in which case they show a certain independence from folded structures and win their place by the pushing apart, by magma, of blocks of the enclosing rocks dissected by fractures.

All the points enumerated, and the circumstance that H. Cloos was in essence the first to carry out a broad and systematic investigation of the tectonics of intrusives, while developing a special methodology that has found wide practical application, including in industry (the stone industry), constitute H. Cloos’s enormous merit and explain to a considerable degree the great popularity enjoyed by his theory (not to mention his methodology) among the mass of geologists.

Therefore, while presenting an extensive critique of H. Cloos’s theory and attempts at a fundamentally new solution to the problem of the tectonics of intrusives, I at the same time set myself the task of emphasizing and somewhat developing that which is most valuable that Hans Cloos contributed to structural geology.

III. PROTOTECTONICS IN INTRUSIVE BODIES AS A RESULT OF THE PROCESS OF SELF-DEVELOPMENT OF INTRUSIONS

1. THE INFLUENCE OF THE DEVELOPMENT OF THE MAGMATIC PROCESS ON THE INTERNAL SYMMETRY OF MAGMAS

Most geologists who have dealt with the question of the origin of various prototectonic structures understand this origin as a redistribution in space of structural elements as a result of mechanical causes, reducible to two principal factors: the gravitational effect (sinking and floating) and the translational movement of magma, understood as the result of pressure from the side. Thus, prototectonics is understood as a structure that has arisen chiefly under the action of mechanical forces external to the magma, manifesting their action not only in the magma but also outside it.

With such an approach to prototectonics, it follows that the physical and chemical conditions for the emergence and growth of crystals, or for the formation of one or another variety of rock, and the conditions for the emergence of the orientation of crystals or of the orientation of rock varieties in an intrusion, are regarded as two phenomena that are connected with one another only by a mechanical interrelation. In accordance with this, a peculiar “division of labor” is carried out: all processes in which supra-mechanical forms of motion of matter in magma are manifested (the formation of minerals, of non-mixing magmas and rocks) are assigned to the realm of mineralogy and petrology, which in their constructions make use of all the achievements of modern physics and chemistry; on the other hand, all processes leading to the emergence of oriented structures are considered a sphere of manifestation of the mechanical form of motion of matter and are assigned to the realm of tectonics, which in its constructions makes use essentially of Newtonian mechanics alone.

Thus, the magmatic process is artificially divided into the process, immanently inherent in magma, of the formation of minerals and rocks, and the process of formation of various kinds of oriented tectonic structures, which owe their appearance mainly to causes introduced into the magma from without, i.e., independent of magma as a self-developing system. Owing to such an understanding of prototectonics, many tectonists consider it possible, for their purposes, to regard magma simply as a mechanical mixture of components possessing different mechanical properties, and accordingly they conduct their experimental investigations, intended to confirm one or another point of view. In these experiments there appear mixtures of flowing water and pieces of wood floating in it, mixtures of fragments of crystals and viscous liquids, etc., i.e., various mechanical mixtures of components possessing different mechanical properties, which are qualitatively (!) likened to magma. Thanks to this qualitative analogy, the data of the mechanical experiment are transferred almost without alteration to magma. For example, having established that pieces of wood in flowing water tend to arrange themselves in parallel, and having compared the results obtained with a structure in an eruptive rock analogous in external appearance, they make the generalization that both structures are in all cases the result of the mechanical motion of a mixture of solid components and a liquid medium that are in mechanical interrelation with one another, and there they put a full stop.

It is precisely on this principle that H. Cloos’s views on the internal structure of intrusives are based, which we set forth in the preceding chapter.

But can we, even for tectonic purposes, regard magma only as a mechanical mixture of components possessing different mechanical properties, even if they vary quantitatively with time (an increase in the viscosity of the magma)? After all, in the process of the development of magma there occurs not only a quantitative change in the various properties of its constituent parts, but also a qualitative change in them, taking place owing to their interaction. This change of qualities occurs in the supra-mechanical realm, where the most diverse forms of energy manifest themselves, many of which in magma have as yet been little studied, and some, apparently, are still altogether unknown. Here take place the most complex processes of regrouping of molecules and atoms, conditioning the phenomena of pre-crystallization differentiation, still poorly studied; here occur the emergence, growth, and alteration of the crystal lattices of diverse minerals, and the formation of their accumulations—definite types of rocks, etc. If any mechanical spatial displacements occur in magma, they entail spatial

the transfer of other forms of energy, the transfer of an infinite number of charged particles, between which a supermechanical interaction takes place. Since magma is an energetically heterogeneous system, various energy flows and fields may occur in it that go beyond the limits of mechanical motions. Therefore, when it is said that oriented structures are the result only of mechanical types of motion, manifesting themselves with respect to the phenomena of crystal growth, etc., as if independently, and when the process of the emergence of these structures is compared with the distribution of even firewood in flowing water, without attempting to go further, this gives rise to quite legitimate doubts, especially since we already know from physics examples in which the orientation of growing crystals is determined not only by mechanical types of motion.

For example, Gross’s investigations as early as 1920 (79) showed that a solution of sulfur in carbon disulfide, when an electric field was applied, began to move and yielded sulfur crystals only at the anode. In this case the rhombic sulfur crystals that formed, with the $c$ axis, grew along the current. Since then, a whole series of works has been carried out—chiefly with supercooled liquids—to clarify the influence on crystallization from a melt or solution of a number of dielectrics of radio radiation, X-rays, and electric and magnetic fields. These works have shown that all these influences, especially the influence of an electric field, improve crystallization conditions in substances having dipoles. This is explained by the fact that, under the influence of an electric field, the dipoles acquire a common orientation, and thereby the spontaneous nucleation of crystallization centers is improved (16, 113). The same, apparently, also affects the orientation of the crystals that arise, as in Gross’s experiments. It is interesting that G. E. Mukhin and S. S. Urazumovsky, who carried out experiments with salol, found that the salol molecule underwent no chemical changes in the electric field. On the other hand, the field did not cause salol to be freed from traces of contamination and moisture. An electric field of the order of 10,000 V/cm was used in the experiments (16, 118).

No less interesting are the investigations of Gorsky, Kondoguri, and others on the existence of significant energy fields in the boundary layers of flat preparations. The influence of the surface amounts to an orientation of particles extending deep into the preparation. As the distance from the surface increases, the number of oriented particles decreases (16, 118–120). In Kondoguri’s work with sulfur, it was established that a field perpendicular to the plane of the preparation gives a rise of the entire crystallization curve of the system, whereas a parallel field gives a lowering of the curve. This is explained by the fact that the perpendicular field acts in agreement with the field along the surface of the glass covering the preparation, while the parallel field disrupts the orienting action of the surface field (16, 120).

Unfortunately, such investigations are still not numerous, are little developed, and are so far being carried out only with low-melting materials. Nevertheless, they show quite clearly what influence electric, magnetic, and other fields may exert on the course and character of crystallization. Of course, we cannot transfer the data of these experiments entirely to magma; nevertheless, they should prompt us to take the initiative in seeking phenomena of a similar kind in magmatic systems as well, as systems in which high-energy processes are manifested.

Identical results do not yet imply the identity of the causes that produced them—an axiom known to everyone; and if, despite this, almost all tectonic structures are derived from mechanical motions alone, this follows not from the fact that all these structures really are the result of a single cause, but from the fact that tectonics, as a science, is built upon

mainly on the principles of metaphysical thought, and to this day has not outgrown the framework of Newtonian mechanics, within which that metaphysical thought has confined it.

The crystallization of magma embraces the final stage of its development. Naturally, the mechanical forms of motion within it can here play an enormous role in the spatial distribution of the solid components crystallizing from the magma, since crystallizing magma, besides its other qualities, is also a mixture of components possessing different mechanical properties. However, magma is a high-energy system in which the most diverse energetic processes may manifest themselves, in which the simple mechanical motion of masses entails the spatial displacement also of supra-mechanical forms of motion, which may in turn induce various energetic fields, and so on. Since magma is a self-developing system, all the processes taking place within it cannot proceed outside a general interconnection. If magma is moving, then the properties of this moving magma cannot be identical with the properties of magma at rest; this cannot fail to be reflected in the character of the process of its development, in its internal symmetry as a crystallization system, and, consequently, also in the structure of the rocks arising from it. It may be said that this question has scarcely been studied at all; however, this still does not mean that one cannot already speak of the possible role of these complex energetic phenomena manifested in magma—in the differentiation of magmas, in manifestations of prototectonics, and so forth. And in the descriptions that follow we shall try to show that there is a whole series of facts in manifestations of prototectonics that cannot be explained by mechanical causes alone, and that point to a more complex development of intrusions than is assumed in the usual petrological and tectonic conclusions. Of course, in our investigations we cannot yet go beyond the bounds of a simple formulation of the question in a first approximation, but even such work already has a certain sense and significance.

2. PRIMARY BANDING

Prototectonics is most sharply expressed in so-called primarily layered plutons, i.e., plutons within which a sharply expressed primary banding is observed, manifested in the alternation of bands of different mineralogical composition. This phenomenon is especially widespread in basic and alkaline rocks, but it also occurs in granodiorites, appearing there in a similar way. Examples of primarily layered plutons are quite numerous, and many of them, such as, for example, the Bushveld Complex, have an enormous literature behind them.

R. Couts (15) describes the character of primary banding in basic plutonic rocks in the following way:

“Many bodies of basic plutonic rocks possess banding caused by the alternation of mineralogically dissimilar layers. This banding may have any position (direction), but usually it is parallel to the contacts. The thickness of the bands may range from a few millimeters to many meters, and the transitions between them may be gradual or sharp. Many bands split or are locally curved, distorted, and of variable thickness. In some intrusions the platy minerals are arranged parallel to the banding. The textures of adjoining bands differ very little, and crystals usually intergrow across the contacts. A protoclastic structure is encountered rarely.”

This summary by R. Couts sufficiently reflects the principal features of primary banding not only in basic plutons, but also in acid and alkaline plutons.

Hall, in describing the Bushveld complex, distinguishes local banding (bands of varying melanocracy, from several centimeters to several millimeters thick) from regional banding (the repetition, in different horizons, of thick sheets or lenses of highly specialized differentiates). In the plane of banding there is no single linear orientation of the components, whereas plane-parallel arrangement in the plane of banding is found in both prismatic and columnar crystals (70).

In the gabbro-labradorites of Volhynia, according to the descriptions of A. A. Polkanov (29), primary-banded texture is manifested both in changes in the structure of the rock—from coarse-grained to medium-grained and large-grained—and in changes in mineralogical composition (essentially in the alternation of leucocratic and melanocratic rocks). The boundaries of the bands are sharp or diffuse. In the central parts of the pluton, in addition to banded texture, there is developed an irregular schlieren structure, sometimes approaching ellipsoidal, caused by the appearance, in the horizontal plane, of elongated schlieren composed of the same rocks as the layers of the plane-parallel, banded texture. The outlines of the schlieren, like those of the bands, are now sharp, now diffuse. A distinctive plane-parallel texture is formed by porphyritic phenocrysts of plagioclase, which are sometimes redistributed in space, up to the formation of layers of porphyritic rock within a medium-grained rock with a small number of phenocrysts.

An extremely interesting example of a primarily layered pluton is the Lovozero alkaline pluton on the Kola Peninsula, with which I had the good fortune to become personally acquainted in the summer of 1937. This pluton has been worked out structurally in considerable detail by a group of staff members of the Northwestern Branch of Soyuzredmetrazvedka: V. A. Unksov, A. V. Vanilovskaya, A. S. Sakharov, N. K. Nefedov, K. Zelenkov, and a number of others, under the consultation of N. A. Eliseev (11, 16).

According to their investigations, the Lovozero pluton is a distinctive “layered pie,” composed of four principal horizons, each of which represents an independent phase of intrusion, the younger the higher it lies. All these horizons, which in general occur almost horizontally, are composed of diverse nepheline syenites; moreover, the bulk composition of the rocks grouped by intrusive phases shows a decrease in the amount of potash feldspar from phase to phase, with an increase in the amount of aegirine and hornblende (45). Within each phase there is observed an alternation of layers of different petrographic composition, maintained over large areas, i.e., forming a kind of pseudostratification.

This pseudostratification is, in some phases, remarkably persistent, especially in the complex of the second phase, whose section is more or less constant for the entire Lovozero pluton. This allowed the geologists working on it, during mapping, to use the methods employed in geological surveying of sedimentary rocks. Consequently, the detailed geological maps that were compiled turned out, in outward appearance, to be extraordinarily similar to geological maps of areas composed of sedimentary, weakly dislocated complexes.

In addition to such pseudostratification, within some “stratal layers” there is also observed an alternation of bands of melanocratic and leucocratic composition.

Somewhat less widely developed than pseudostratification is trachytoid texture, expressed in the subparallel arrangement of feldspar laths with strongly developed (010) faces, reaching in certain hori-

zones (lujavrites) of high perfection. Nevertheless, linear orientation of feldspars and egerine crystals is absent. Cataclastic phenomena, with the exception of the rocks of the first phase, are likewise not observed.

Near the contacts, and also around large blocks—xenoliths—pseudostratification and trachytoid structure acquire steep angles of dip.

The examples cited are sufficient to form a certain idea of the character of primary banding in plutons. It should be noted that the disposition of primary banding in space is extremely varied. Plutons with a comparatively gentle attitude of primary banding predominate; however, plutons are by no means rare in which it has steep to vertical dips. An excellent example of this kind was described by V. A. Nikolaev (21), who studied the alkaline rocks of the Kaindy River in the Talas Ala-Too; here the primarily differentiated alkaline pluton forms an elongated stock with steep, almost vertical contacts, in which the primary banding is arranged generally in accordance with the elongation of the stock, everywhere having a vertical dip parallel to the contacts.

How does primary banding arise? How, within a single pluton, does the simultaneous formation occur of bands or layers having different mineralogical composition or different texture, or both together, and why does this usually also give rise to a planar—parallel—arrangement of the crystals composing these banded rocks? Why does primary banding repeat the contours of the pluton?

Many geologists have already worked on resolving the question of the origin of primary banding, proposing a whole series of their own explanations, in part accompanying them with experiments using artificially selected mechanical mixtures (15). Here propositions were advanced concerning layer-by-layer injection (lit-par-lit), the squeezing out of residual melt, the assimilation of xenoliths, the pressure of the roof, repeated pressures of a pulsational character, the pulsating release of mineralizers, gravitational phenomena in the magmatic chamber, where the crystallizing magma is in a state of rest, etc. (see summaries 12, 15, 67). We shall not analyze each of these points of view here, but shall note that such explanations encounter such an abundance of contradictory data that each of them, taken by itself, loses the practical value which their authors would like to see in these explanations, although one cannot deny a certain particular applicability of one or another point of view, especially the application of the phenomenon of gravity (12). What, then, is the main cause of the origin of primary banding?

In the process of accumulating and generalizing empirical material, a new opinion arose that the cause responsible for the formation of primary banding in plutons is the flow of the magmatic melt. This opinion, expressed by Adams (45), Harker (69; 71), Grout (67), and others, was subsequently developed by H. Cloos, who built upon it his doctrine of the prototectonics of intrusions. In his works H. Cloos substantiated the proposition that, in the process of movement of a magmatic melt, differentiation of the crystallizing magma may take place as a result of this movement, with the formation in granites of ribbon-like biotite schlieren. The phenomena of differentiation during movement were later studied and elaborated in detail by R. Balk and A. A. Polkanov, who provided remarkable examples of rational structural analysis of intrusions, and by a whole series of other geologists who followed their path. In recent times, extremely interesting work in this respect has been carried out by a team of workers of the Northwestern Division of Redmetrazvedka on the Lovozero pluton, under the consultation of the scientist A. A. Polkanov—N. A. Eliseev, about which I have already spoken above.

One of the very instructive works in the field of prototectonic analysis is the work of A. A. Polkanov, “An Asymmetrical Diabase Dike from the Coast of Kola Fjord” (25). It describes a thick inclined diabase dike, whose asymmetrical structure is reduced to an asymmetrical distribution within it of plagioclase phenocrysts forming several bands on the hanging-wall side of the dike, against the background of the symmetrical structure of the main mass of the diabase. A detailed structural study of the dike showed that the movement of the diabase magma along the tectonic fissure had a pulsational character, i.e., occurred as if in jerks, with temporary stops. The number of bands in the diabase dike gives the number of these leaps, or phases of movement. From the hanging and footwall sides of the dike toward its center the diabase becomes increasingly coarse-grained. However, the increase in grain size from the margins toward the center occurs discontinuously: each new band has, in contact with the preceding band, a coarser-grained structure; moreover, the diabase of the footwall side of each upper band, in comparison with the diabase of the hanging-wall side of the lower band, is more altered, which is an expression of the contact effect of new, hotter portions of magma rising from below upon the already solidified margins of the overlying bands. Each new leap of movement is marked by the fluidal structure of the main mass of the outer margin of the band. The temporary attenuation of movement leads to the gradual disappearance of the fluidal texture.

As has already been said above, the plagioclase phenocrysts form a series of accumulations—bands—on the hanging-wall side of the vein. The number of such bands characterizes the number of leaps—movements of magma along the fissure. These accumulations of plagioclase phenocrysts are elements that arose in the process of magma movement, as a result of the floating up of already formed plagioclase phenocrysts in the not yet solidified mass (the phenomenon of gravitation) and their simultaneous transport together with the moving magma. To confirm his point of view, the author carried out an experiment—he melted the rock of the dike and established that pieces of unmelted plagioclase floated up in the liquid magma. In addition, using Stokes’ and Allen’s formula for determining the fall and rise of spheres in a liquid medium, the author calculated that the larger the radius of the sphere, the greater the velocity of its rise; by this he explained the fact that large plagioclase phenocrysts are situated higher than small ones in the profile of the dike.

Thus, in his work A. A. Polkanov showed that, in the process of movement of a magmatic melt, banded textures can arise as a result of dynamic crystallization differentiation, and that this phenomenon can produce a diversity of rocks even on quite considerable scales. In addition—and this is very important—he showed that the movement of the magma that formed the diabase vein had a pulsational character, i.e., occurred in a leaping manner, which is further evidence of the leaping character of magmatic tectogenesis in general.

R. Balk’s works on the structure of the Adirondack pluton in the USA (46; 47) brilliantly demonstrated the applicability of structural geological analysis of prototectonics to the solution of very complex and tangled petrogenetic questions. In this work R. Balk shows how, in the process of magma movement, by means of crystallization differentiation, norites and gabbro arose from the original magma of approximately dioritic composition on the one hand, and anorthosites, syenites, and normal granites on the other. As a result of friction arising between floating minerals and the liquid magma in the process of their common movement, and also as a result of friction of the minerals against one another, against the walls of the magmatic chamber, and against inclusions, there occurs a slowing of the movement of the minerals, which leads to the beginning of accumulation of solid bodies among the liquid. The dif-

the accumulation that has begun will increase in the presence of motion and with a constant influx of solid floating bodies, and will result in the formation of layers or bodies of irregular shape, but subordinate to the character of the motion of the liquid, in which the solid floating bodies will accumulate in relatively greater quantity than in the surrounding liquid. Naturally, such banding will repeat the outlines of the walls of the magmatic chamber and of inclusions that impede the free movement of the magma. Such streamlined inclusions (blocks) may be the largest crystals or earlier accumulations of minerals and fragments of these accumulations. Similar “block textures” are widely distributed in the Adirondack pluton.

Intruding magma, entering from the feeder channel into a gently dipping, lens-shaped magmatic chamber situated among shales, expanded this chamber by the force of its own pressure, thus winning space for itself. The stresses that arose were reflected in the structure of the rocks being formed in the form of phenomena of protoclase and protoblastesis, accompanied by granulation of the coarser-grained rocks, and also in the formation of the so-called “schlieren-zones”—zones of disschistation that arose after the final recrystallization of the rocks, as a result of movements of blocks around one another, accompanied by the appearance of cracks and zones of crumpling, which were “healed” with the aid of still-unsolidified material.

Fig. 4. Large crystals of nepheline (blocks) in schlieren-banded ijolite-urtite (Khibiny). 1—ijolite schlieren; 2—urtites; 3—aegirine; 4—nepheline. (After N. A. Eliseev.)

Conclusions of a similar kind—that differentiation in magma takes place not in situ, in a static position, but in the process of magma movement—are reached by Bubnov, who carried out a detailed tectono-petrological study of the southern Schwarzwald (50), A. A. Polkanov, who studied a pluton of gabbro-labradorites in Volhynia (29), and several others.

Fig. 5. Direct contact of dense malignites with rischorrites (crosses) (Khibiny). Trachytic texture in the malignites repeats the outline of the contact surface (sketch by E. N. Volodin).

Very instructive are the tectonic studies of recent years carried out in the Khibiny and Lovozero alkaline plutons (11; 12). Here block textures are widely developed, described in detail by N. A. Eliseev (11), especially in the Khibiny, in the areas of development of apatite deposits, where, in addition to crystallization differentiation in the process of magma movement, dynamic liquation differentiation was evidently also widely developed.

An example of a block texture, taken from the work of N. A. Eliseev (11), is shown in Fig. 4. But even more widely developed here than the block textures are the textures of eruptive breccias, consisting of fragments of older rock (xenoliths) and a cement, not yet solidified, derived from a younger intrusion (11). The study chiefly of these eruptive breccias made it possible to establish precisely that the Khibiny and Lovozero plutons are multiphase plutons, i.e., formed in several stages. At the same time, observations of the manifestations of eruptive breccias confirmed still more convincingly that the differentiation of magma and the orientation of its components and differentiates arise in the process of the movement of magma, and not in its stationary state. These observations showed that the banding and trachytoid character of a younger phase repeat the outlines of contacts with an older phase and of xenoliths of older phases and country rocks, as is shown in Fig. 5, taken from the work of N. A. Eliseev (11). As a result of this, the banding and trachytoid character of the younger phase may, in certain cases, truncate the banding and trachytoid character of older phases, as, for example, is shown in Fig. 6, sketched by me on Mt. Strashempakh in the Lovozero pluton.

Fig. 6. Block sketch showing the structural relationships of gneisses (marked by crosses), porphyritic nepheline syenites enveloping the gneisses, and the surrounding lujavrites. Dashes show the direction of trachytoid character (Lovozero).

We have already noted above that near contacts and large xenoliths of enclosing rocks, trachytoid character and banding of various scales change their attitude, adapting themselves to the form of the contact or xenolith, as is shown, for example, in Fig. 13. Such a change in the attitude of the prototectonics sometimes takes place over several tens, and even more, meters up to the contact; sometimes, however, as, for example, in the case shown in Fig. 7, it embraces only a 5–10-meter band adjoining the contact. Cross-cutting vein-type formations and erupting stocks of younger phases have their own banding and trachytoid character, independent of the surrounding rocks and following the boundaries of these bodies; such examples, incidentally, were among the most weighty evidence adduced by H. Cloos to explain banding as the result of differentiation in the process of magma movement.

Fig. 7. Contact of lujavrites with shales (black). Dashes show the trachytoid character adapting itself to the contact (Lovozero).

An extremely curious detail is the corrugation—folding—of the trachytoid character that occurs here and there, manifesting itself like ordinary corrugation in shales. The axes of the small folds, formed by plagioclase laths, are maintained over a rather considerable extent; in places of bending of the small folds, the plagioclase laths are in places bent or broken. By all indications, this corrugation is a simple plastic deformation that took place before the complete crystallization of the rock. I had occasion to observe such corrugation several times in the Lovozero alkaline pluton. There, together with N. A. Eliseev, we also observed an interesting case of the superposition of one [structure] upon another.

onto one another of two streams of one and the same magma (lujavritic). Platy lujavrites are overlain by non-platy lujavrites, fused only slightly near the boundary, as shown in Fig. 8. The boundary is established distinctly. In addition to such regular platy structure at contacts with older rocks, twisting of trachytoid structure and interweaving of streams are observed in places, as shown in Figs. 9 and 10, which I also sketched in the Lovozero pluton.

The examples cited only once again emphasize the role of magma movement in the formation of plane-parallel structures, and also show that, as a result of the manifestation of local pressures, before the rock has finally solidified, the rock may undergo deformation of the folding type.

Thus, primarily layered plutons are, apparently, plutons that arose through an essentially mechanical filling by magma of a space formed in some way within the enclosing mass, with parallel crystallization differentiation of the magma, manifesting itself in the process of its movement. Assimilation phenomena are generally not characteristic of this kind of pluton. Very often contact effects are almost not manifested, or are manifested weakly, as is shown, for example, in Fig. 11.

Fig. 8. Block sketch. Platy lujavrites (below) are overlain by non-platy lujavrites (above). The strokes show the arrangement of feldspar plates (Lovozero).

G. Kloos, describing the granites of Riesenbirge, with their fine alternation of layers of normal composition with layers sharply enriched in biotite, cites the calm concentric arrangement of bands—ribbon-like schlieren—as one of the proofs against magma conquering space for itself by assimilation, since no traces of exchange of materials, even in the form of convection currents, are observed. Therefore G. Kloos believes that a closed front of arched schlieren is incompatible with the existence of a broad, uniform rise of magma in the sense of the concept

Fig. 9. Detail of the behavior of the trachytoid structure of lujavrites at their contact with porphyries. The strokes show the arrangement of plagioclase plates in the lujavrites (Lovozero).

Fig. 10. Interweaving of the flow of trachytoid structure in lujavrites at their contact with gneisses (crosses). Sketch of an outcrop (Lovozero).

batholiths, as interpreted, for example, by Daly (56, 133). H. Cloos believes that a coarse fluidal structure, or a primary parallel-banded structure, is created in the case where magma began to crystallize before reaching its final position. Conversely, if the magma entered its cavity in a completely liquid state, not a fluidal structure but schreckung proper is formed. A similar idea—that primary banding arises in magma that began to crystallize before reaching its place—is expressed by other geologists as well, in particular R. Balk and A. A. Polkanov. N. A. Eliseev, in an article on the geological structures of the Khibiny and Lovozero intrusive massifs, writes that “banding arose as a result of flow phenomena during the filling of the cavity, as a result of whose filling the intrusive massif also arose. It is obvious that at the moment the cavity was being filled the magmatic material consisted of a mixture of solid crystals and liquid and gaseous phases, since only in such a case can one understand the origin of primary oriented or bedded accumulations of individual crystals. If the Lovozero and Khibiny massifs had arisen as a result of the intrusion into a cavity of only liquid magma, crystallizing in a state of rest, as is commonly thought, then there would have been no ‘banding’” (12, 5). As regards the genesis of the Lovozero and Khibiny magmas, N. A. Eliseev considers that “both the structural relationships and the extremely limited scale of assimilation indicate a comparatively low temperature of the intruding magmatic masses. The presence of well-pronounced stratification, as was already stated above, compels one to think that magma formation took place somewhere at great depth, while within the cavity (subsequently the intrusive body) crystallization differentiation took place, leading to the emergence of a primarily stratified intrusive massif at the expense of an initial semiplastic mass, which represented a mixture of solid crystals and liquid and gaseous phases” (11, 17).

Fig. 11. Sketch of an outcrop. Lujaurites overlying gneisses cut by a porphyrite dike. The position of trachytoid structure in the lujaurites is shown by short strokes. All contacts are sharp. Contact metamorphism of the gneisses is scarcely noticeable. (Lovozero).

In general it must be said that ordinarily primarily layered plutons show a considerable, or even sharp, difference in chemical composition from the enclosing rocks.

Magma intruding into a cavity must possess considerable internal mechanical stress in order to push apart the walls of its cavity during intrusion. Such pressure of the magma on the walls of the cavity may exert a certain influence on the structure of the enclosing rocks. A classic example in this respect is provided by the investigations of Balk and Grout (48), who, in studying one small (8 × 5 km) stock, found that the syenites and granites composing the stock, as they approach the contacts, acquire a distinctly gneiss-like appearance, with the marginal gneisses repeating the outlines of the stock. On the other hand, the sedimentary sequence (graywackes and shales), which in the region has a uniform NW strike, having undergone hornfelsing around the stock, at the same time acquires a clearly expressed parallel structure, adapting itself to the outlines of the stock. In the opinion of Balk and Grout, the concordant arrangement of the marginal gneisses and the oriented structures in the hornfelses

occurred in the following way: after the intrusion had been emplaced, the country rocks were heated and for some time remained, together with the marginal part of the massif, in a semi-plastic state. At this time there was a renewed pressure of magma. The hydrostatic pressure, transformed into oriented lateral pressure, led to the formation of marginal gneisses and of hornfels structures conformable with them, surrounding the stock.

A similar adjustment of the banding in hornfelsed rocks to the contact of an intrusive body is described by N. A. Eliseev for the Khibiny pluton (11, 11) and by a number of other authors. I myself have had occasion to observe the same phenomenon at the contact of the Lovozero lujaurites with ancient porphyries. Above we have already cited a more or less detailed description of Balk’s work on the Adirondack pluton, in which he develops the idea of the gradual liquefaction by magma of a more or less plastic envelope.

In addition to such plastic formation of a chamber, the formation of a chamber by block movements along fractures is apparently also possible, an idea developed by H. Cloos. In this case the contacts of the pluton will in general follow the directions of the principal systems of fractures, while xenoliths of country rocks within the pluton will have more or less clear geometric outlines determined by the fracture planes along which the given xenolith was detached. Examples of this kind are cited in abundance by H. Cloos and others, for example by Balk and Grout (49), who studied the banded borders of Harney Peak in Dakota. Similar examples may be found in abundance in the Lovozero pluton. It is possible that some assistance in the formation of a chamber for a pluton, especially a chamber arising along an interformational seam, is provided by external lateral pressure, as H. Cloos believes.

Since the general direction of magma flow is determined mainly by the position of the chamber in space, it must be assumed that the general orientation of the prototectonics of a pluton indicates the general position of the pluton in space; i.e., a gently dipping disposition of the prototectonics speaks of a gently dipping position of the pluton (laccoliths), while a steep disposition of the prototectonics speaks of a steep position of the pluton. As for marginal prototectonics in a pluton, it repeats the outlines of its contacts and, in the general case, will deviate from a certain mean direction characterizing the general position of the pluton in space.

However, the question of the origin of primary-banded tectonics in intrusions is far from exhausted by all that has been said. There is much in it that cannot be explained by a purely mechanical method alone.

First of all, it should be noted that the intensity of the manifestation of primary banding and trachytoid texture is not the same in all components of a primarily stratified pluton. For example, in the same Lovozero pluton, normal lujaurites of phase II have a more perfectly expressed trachytoid texture than do the lujaurites of phase III, which differ little from them petrographically. Likewise, phase II of the pluton is more sharply and completely pseudostratified than phase III. However, even within phase II not all “strato-layers” have a well-expressed trachytoid texture. For example, whereas in the normal lujaurites of phase II the trachytoid texture is always excellently expressed, in the foyaite of phase II, which in average bulk composition has much in common with the normal lujaurites, the trachytoid texture is weakly expressed and not always present.

If we turn our attention to other plutons, we shall encounter the same features there as well. Thus, for example, in the alkaline intrusion of Kaindy, described by B. A. Nikolaev (21), the rocks of the shonkinite series are sharply primarily stratified, whereas in the gabbro-monzonite rocks the primary stratification is poorly expressed.

In addition to such cases, which reflect the difference between individual primary layers, numerous examples are known in which, within a single layer, in different parts of it, a finer lamination, the orientation of components, etc., is expressed with varying intensity.

Such cases cannot be explained solely by one difference in the rates of mechanical movement, nor in general solely by means of mechanical movements of various kinds.

Indeed: why are bands—plates—of different petrographic composition formed, sometimes persisting over an enormous extent, as, for example, in Lovozero, where some “strato-layers” of one phase persist throughout the entire intrusive massif, forming something like independent intrusive layers, whereas it may be considered proven that they are of the same age?

If, in order to explain the primary banding of accumulated crystals in horizontal and inclined intrusive bodies, one could invoke the effect of gravity, as in the asymmetric diabase dike described by A. A. Polkanov, then in vertical plutons of the Kain type, described by V. A. Nikolaev, this effect is not applicable at all, either to crystals or to differentiating magmas. Since the crystals are suspended in the magma, then during movement of the magma these crystals, if they lag behind in the movement, should form accumulations—blocks, or spindle-shaped clusters. If, however, they form plates, this means that some orienting influence was present. If this orienting influence is the mechanical orienting influence of contact, manifested through the friction of crystals against the walls and against one another, then this orienting influence presupposes a continuous growth of a layer of crystals from the wall inward into the intrusion, i.e., it presupposes a successive differentiation with a change in composition from the margins toward the center, whereas in reality what occurs is the simultaneous formation of a series of alternating layers of crystals, separated from one another by intervals of magma. Moreover, if only a mechanical orienting influence of contact acted in the magma, then this influence, especially in powerful magmatic currents, owing to phenomena of gravity, convection, and various kinds of vortices, would have had to lose its significance rather quickly. Meanwhile, in such a pluton as the Lovozero, or the Kakortokite pluton in Greenland described by Ussing, very persistent and numerous parallel layers are observed, revealing the breaking up of a large mass of magma into a series of streams, from which layers are formed that differ, in relation to their neighbors, in petrographic composition and differ structurally. Furthermore, the very process of magma intrusion under pressure, the pushing apart of the cavity, in the case of the manifestation only of purely mechanical orienting influences, would have had to lead to a mixing up of the layers that were forming; yet such mixing is the exception.

The situation is no better with trachytoid structure. Here the problem of magma movement is reduced to the regrouping of crystals that arise independently of this movement. Therefore it is assumed that lujaurites, for example, possess a well-expressed trachytoid structure because crystals of feldspar of platy form are concentrated in them. However, this does not remove the question—why do crystals of platy form arise, and why are they more characteristic of lujaurites than, for example, of foyaites? It should be emphasized that in almost all cases where the orientation of crystals in primary-banded rocks is discussed, a plane-parallel structure is indicated, generally consistent with the arrangement of plane-parallel primary layers, and moreover in

in plan-parallel arrangement both platy and columnar crystals take part; at the same time a linear orientation of the components is usually absent, whereas such an orientation of the components ought to have had a very broad development, since primary banding is taken as the result of comparatively intensive mechanical movement—the flow of magma. No less interesting are cases of concordant disposition of trachytoid structure in adjacent coeval layers when the shape of their boundaries is irregular, as is shown, for example, in Fig. 12, sketched by V. A. Unksov in the Lovozero pluton.

Fig. 12. Boundary of coeval “strato-layers”: trachytoid foyaite (thin strokes) and loparite lujavrite (Lovozero). The strokes show the arrangement of feldspar laths (sketch by V. A. Unksov).

Fig. 13. Contact of trachytoid lujavrites with ancient porphyries (crosses). The strokes show the arrangement of feldspar laths (Lovozero).

There, too, I made the sketch reproduced in Fig. 13, where a sharply expressed trachytoid structure in the lujavrites is situated almost “right up against” the contact with ancient porphyries. Similar examples require considerable strain in explaining them solely by mechanical movement.

Thus, all the structural examples listed above, the number of which can be multiplied upon more detailed analysis, indicate that mechanical movement of magma alone is still insufficient to explain such an original phenomenon as the primary stratification of plutons.

In the preceding chapter we stated a priori the proposition that mechanical movements alone would be insufficient to explain the prototectonics of intrusives, that mechanical movements in such an energetically saturated system as magma cannot proceed outside the general interconnection of phenomena. It seems to us that the formation of prototectonics does not occur without the orienting and redistributing effects of certain supramechanical processes accompanying the mechanical movements of magma, or even simply caused by them. It may be that precisely in this domain will be found those planar polarizing influences that promote the emergence of plan-parallel textures, in particular of simultaneous “strato-layers,” which do not become confused with one another and which persist over considerable distances. Clearly, in this case the character of differentiation in the process of movement will also have to be changed, enriching it with a deeper content than the simple mechanical displacement of crystals in space. Apparently, in the domain of supramechanical phenomena one must also seek an explanation for the circumstance that among granodiorite massifs primary stratification is incomparably less widespread and less perfect than primary stratification in basic and alkaline plutons, almost all of which are, to one degree or another, primarily stratified. Alkaline and basic magmas

are systems standing at a higher energy level in comparison with granitic magma, for they contain a larger quantity of strong cations, which is reflected in their crystallization, acquiring an agpaitic character (41), and, apparently, in some way affects the character of the prototectonics of the plutons formed by these magmas.

Thus, primarily layered plutons appear to us chiefly as plutons formed in one of the last stages of the intrusive process, when magma wins a place for itself by essentially mechanical means. However, banding, partly recognized as primary, i.e., having arisen during crystallization of the magma, is also very widely developed in gneissic formations, to which, evidently, the definition given above does not apply. Therefore, to elucidate the nature of this banding in gneisses we shall devote a special chapter, following the chapter on linear structure.

3. LINEAR STRUCTURE

According to H. Cloos, streaking, or linear structure, is the expression of stretching of a magmatic melt in a direction perpendicular to the acting pressure, i.e., it is the expression of schistosity in granite. Streaking arises chiefly when magma becomes viscous, when special differential movements, as at the stage of formation of primary banding, do not occur in it, and the magma, together with all its inclusions, is stretched as a whole (56, 123). As a result of this, streaking is almost independent of the position and structure of the pluton, everywhere revealing the same position in space. This position of streaking in space is determined by the action of lateral pressure and load, to which it is oriented perpendicularly. “If one were to imagine that the load suddenly ceased for a minute, then the molten mass would, under the influence of pressure, have had to deviate vertically upward. If, on the contrary, the load were everywhere equally great, then relative spreading could occur only sideways, i.e., horizontally” (14, 16).

I have deliberately repeated H. Cloos’s principal propositions on streaking, of which we spoke in the chapter setting forth the essence of his theory, in order to emphasize H. Cloos’s idea that streaking is not, properly speaking, a fluidal phenomenon, like primary banding, but is connected only with very slight lateral movements, and that, possibly, in considerable part it is the expression of molecular movements in place (54, 95). The fact that streaking is generally oriented in accord with primary banding is explained by H. Cloos by the sameness of the acting causes, i.e., the pressures shaping the pluton. If, however, we accept that primary banding is, in the main, the result of independent movements of the magma, as we established above, then the origin of streaking, which proves to have been formed as if under the action of causes other than primary banding, becomes unclear.

It must be said that there are incomparably fewer observations on streaking than observations on primary banding, since streaking does not attract the eye so much by its unusualness. Therefore, although streaking is more widely distributed than primary banding, we know less about streaking than we know about primary banding.

Between intrusive massifs with primary banding and massifs with only streaking there are a number of intermediate forms, in which the primary bands or long and frequent ribbon schlieren are replaced by shorter and rarer ribbon schlieren, then by lenticular schlieren and, finally, by streaking alone.

An example of this kind may be the famous granodiorite massif in the Sierra Nevada, the structure of which was described by B. Evans (63). This pluton, formerly regarded as a model of a large batholithic body (its dimensions are 400 × 70 miles), proved to be composed of a whole series of smaller intrusions having slight differences in age¹). In the plutons the prototectonics is fairly well expressed, and it greatly aided B. Evans and his assistants in distinguishing the separate intrusive bodies. These intrusive bodies generally form laccolith-like domes with broad bases. Where denudation has exposed only the domes, as in the Yosemite section, domes or arcs of schlieren occur, as in Riesenberg, described by H. Cloos. In places with deeper denudation sections, as in the Rock-Creek Salient, the prototectonics has a vertical position. The author emphasizes that here the prototectonics—the schlieren and the foliation—produce in their arrangement a whole series of local deflections and eddies; moreover, each intrusion composing the overall pluton has its own flow structure, independent of the regional trend and reflecting local conditions that characterize the arrival of magma into place by partial increments. At the same time the author expresses the idea that such an example of flow signifies free movement, more or less independent of lateral pressure, and considers this to be known proof that magma rises under the influence of its own forces.

I had occasion to observe a rather interesting example of the behavior of foliation in one of the granodiorite massifs in Khakassia, situated in the middle course of the Askiz River. The massif has an elongated form, with a cross-sectional size of 20 km, and represents a more or less simple body in which it was not possible to establish with certainty multiphase filling. Structural mapping carried out on this massif showed that, in this extensive pluton, even in its central parts, foliation is frequently manifested, expressed chiefly in the parallel orientation of the dark-colored components and sometimes in a more or less distinct elongation of quartz grains. The foliation is most sharply manifested in the contact zones of the intrusion, which, in general, have a somewhat more basic character than in the central parts. A schlieren texture is not characteristic and appears only in the contact belt, so that the prototectonics of the intrusive body is determined almost solely by foliation. On the whole, the foliation is oriented parallel to the long axis of the intrusion and in most cases shows steep to vertical dips. In many places a linear strike can be distinguished, dipping westward at an angle of 45–65°. The dip of the contacts, judging from the waviness and the high exposures on the Askiz River, is steep. More detailed structural mapping over an area of 40 sq. km showed that the isolines of foliation generally form wavy shapes and sometimes even large eddies, in which the dip of the foliation in places flattens to 30° (Fig. 14). Observations of the behavior of foliation around xenoliths of older rocks showed that the foliation sometimes reveals clear traces of flow around angular xenoliths, as shown in Fig. 15.

The examples cited show that foliation is not simply a structure of passive adaptation to external pressures, but that it

¹) Apparently, in general there are no especially large continuous batholithic masses such as, for example, Daly depicts. Comparison of numerous facts shows that many complex intrusive bodies must in general have a treelike form, i.e., they must consist of branches of different shape and size somehow interweaving with one another; especially complex interweavings should be expected in multiphase plutons similar to the granodiorite pluton of the Sierra Nevada.

Fig. 14.

arises during the movement of magma, manifesting itself as an independent movement. But this by no means exhausts the question of the origin of foliation. As an example, to some extent indicating a more complex origin of foliation than simple mechanical movement, I should like to cite some of my observations on schlieren, made in the above-mentioned granodiorite massif in Khakassia.

This massif, in the near-contact zones, is in places rich in remnants of hornfels and marble. In addition to remnants, the intrusive body in the near-contact zones is sometimes also rich in small lenticular melanocratic schlieren and angular xenoliths, assimilated to varying degrees. The least assimilated—or, more precisely, contaminated—of these occur in areas immediately adjoining the contact, and consist either of hornblende hornfels or of fine-grained biotite-hornblende diorite-porphyrites, among which a secondary biotite impregnation can sometimes be observed. As the degree of alteration increases, the xenoliths are increasingly transformed into ellipsoidal and lens-like, sharply outlined schlieren, consisting of fresh hornblende-biotite material, small feldspar phenocrysts corroded by amphibole, and abundant magnetite impregnation. A similar phenomenon of replacement of xenoliths, not accompanied by their melting, is known under the name of the phenomenon of contamination, analyzed in great detail by C. R. Nockolds (22; 23). C. R. Nockolds characterizes this interesting process as follows.

Fig. 15. Sketch of a hand specimen. Xenolith of fine-grained diorite (crosses) in medium-grained diorites; the direction of the dashes indicates the orientation of the dark-colored minerals (Askiz).

“One of the most remarkable features of xenoliths consists in the fact that they preserve their outlines even in the event that they are completely altered.” “This preservation of outlines indicates that xenoliths remain solid throughout the entire course of their alteration. There is no true solution here, but the alteration represents one of the forms of molecular replacement; insignificant portions of one of the components pass into solution during a short interval of time and are replaced by new material. For this, free diffusion into the xenolith and back out of the xenolith is necessary” (22, 10). Thus, “contamination in acid magmas is closely connected with the volatiles contained in the magma,” which assist in the mutual reaction.

More careful observation of such schlieren, formed by contamination, showed that where the intrusive body has weakly expressed foliation, the components of the schlieren are arranged more or less disorderly. Where the intrusive body has sharply expressed foliation, the crystals of green hornblende and biotite in the schlieren have an orientation generally parallel to the foliation; moreover, under the microscope it is evident that they form fluidal streams penetrating the primary quartz-feldspar mass of the schlieren-xenolith. Some hornblende crystals cut across the boundary of the schlieren, but in general the schlieren has sharply outlined contours.

The phenomenon described does not lend itself to a simple mechanical explanation and requires a deeper physical substantiation.

Thus, here too, as in the formation of primary banding, certain supra-mechanical orienting influences arise,

exerting an influence not only on already formed crystals, but also on the movement of volatile components, and on the direction of crystal growth in a solid medium—in xenoliths.

For the most part, the examples cited by H. Cloos (Fig. 3), where aplite and granitaplite veins of slight thickness have a lineation situated at the most varied angles to the contacts of the vein, but in accordance with the sharp lineation of the enclosing granites or with the schistosity in the enclosing schists, do not lend themselves to a simple mechanical explanation. I have had occasion to observe phenomena of the same kind in gneiss-granites on the Kola Peninsula; here, at times, veins of aplite-pegmatites of slight thickness, with well-bounded feldspars, without any traces of cataclasis or recrystallization, possess a well-expressed primary lineation caused by elongated accumulations of quartz, which are oriented in accordance with the sharply expressed lineation of the enclosing gneiss-granites, even when the aplite vein is situated across the lineation in the gneiss-granites (Fig. 16). Apparently, here some orienting influence of the structure of the enclosing rock is manifested, which is cut by the thin aplite or granite-porphyry vein; for where the gneissic character in the gneiss-granites is weakly expressed, the primary lineation in the veins is oriented parallel to the strike and dip of the vein, independently of the direction of gneissosity in the gneiss-granites enclosing the vein.

Fig. 16. Aplite veins (hatched) in gneiss-granites. The small strokes show the position of the lineation in the gneiss-granites. In the aplite veins, the most elongated accumulations of quartz grains are contoured (Lovozero).

A very interesting example, shown in Fig. 17, is given by H. Cloos (52, 13). Here a fine-grained porphyritic granite forms a lens-shaped inclusion of tailed form in normal granite. The feldspar phenocrysts in the porphyritic granite are arranged parallel to the fibrousness in the normal granite. Cataclasis is absent. H. Cloos believes that the movement occurred before final solidification.

Fig. 17. Inclusion of fine-grained porphyritic granite (1) in normal granite (2). The small strokes and rectangles show the arrangement of biotite flakes and feldspar phenocrysts (after H. Cloos).

All the examples cited indicate the participation, in the formation of oriented structures, of certain supramechanical movements that still remain to be discovered and studied. Perhaps, in this respect, some support may be provided by the few works that have clarified the influence of electrical and mag-

... fields upon the crystallization conditions of dielectrics, which I mentioned above.

Both primary banding and fibrousness in plutons repeat, near the contact, its contours; moreover, usually (though not always) they are more sharply expressed here than in the center of the pluton. This, apparently, is also explained not only by mechanical causes—the sharp boundary between liquid and solid—but also by the fact that the contact is the boundary between systems characterized by different energy levels; it is the expression of a qualitative leap separating the actively developing system—the magma—from the passively developing system—the enclosing mass. In this respect the contact acts as a surface orienting the direction not only of mechanical movement, but also of other types of movement that may take place in the magma but cannot take place in the country rock. It is possible that the contact may simply induce in the magma certain types of supramechanical orienting fields. The fundamental possibility of phenomena of this kind is confirmed by the investigations of Gorskii and others on the existence, in the boundary layers of flat dielectric preparations, of significant orienting fields, as I mentioned in the first chapter of the present section. Therefore one cannot put a full stop after the assertion that, in the formation of oriented structures, the contact acts only as the boundary between liquid and solid; the influence of the contact on prototectonics must be subjected to additional physical study, going beyond the limits of Newtonian mechanics.

In the considerations set forth above I expressed the idea that fibrousness is formed essentially as a result of the active mechanical movement of magma, accompanied by other types of movement. However, this does not free us from the question of the influence of an external, with respect to the magma, oriented pressure, which may occur in the course of the development of an intrusion and affect the internal symmetry of the magma as a crystallization medium and, consequently, the structures of the rocks arising from it.

Recognition of the intrusive process as a self-developing process does not at all exclude, but presupposes, the influence upon its development of the external historical setting—in particular, of external oriented pressure—which, as we know, exerts an enormous influence on the process of development of passive masses, causing in them corresponding qualitative changes (processes of metamorphism). This influence of external pressure may manifest itself in various ways. First, external pressure may exert its influence indirectly, through the tectonic structures of the enclosing mass, to which, to a certain degree, the spatial development of the intrusive process is subordinated. Second, it may make itself felt by direct action upon the magma as a body possessing higher plastic properties than its surroundings, thereby exerting a certain influence on the movement of the magma and its distribution in space; this should acquire special significance in the final stages of the intrusive process—provided, of course, that this process takes place during a phase of mechanical compression in the earth’s crust.

Third, external mechanical compression must enter as one of the components determining the internal symmetry of the crystallization system—the magma—and in certain cases must play a not unimportant role in the emergence of oriented structures in the rocks crystallizing from it. This is confirmed by a whole series of experimental data on the character of crystallization in solid and very viscous bodies (glasses) under the influence of external oriented pressure. And although on the basis of these experiments it is still impossible to draw the conclusion that gradual crystallization from a liquid system, such as magma is, under the influence ...

oriented pressure, will proceed in the same way as recrystallization in metamorphosed rocks; nevertheless it cannot be denied that oriented pressure may exert a significant influence on the emergence and orientation of prototectonic elements in crystallizing magma, especially when the intrusive act coincides with an epoch of mechanical pressures.

However, a phenomenon of this kind, if it occurs, should be regarded only as a special case among the processes leading to the emergence of prototectonics; moreover, pressure here should be regarded not as the sole cause, but as the leading cause, bringing about the emergence of fibrosity.

In addition to all that has been said, it should be noted that the mechanical movements themselves, since their manifestations depend to a certain degree on the structure of the medium in which they develop, will most readily be expressed along structurally weakened places, i.e., along the same sharply expressed fibrosity. Incidentally, such adaptation to structures depends to a considerable extent on the scale of the movements. For micromovements of the type of secondary gneissosity, such adaptation to structures is evidently a regularity.

Thus, one of the leading roles in the formation of fibrosity, just as in the formation of trachytoid structure and primary banding, belongs to the active movement of the magma itself, taking place during the emergence and growth of crystals.

Apparently, the spatial position of prototectonics in an intrusive body generally characterizes the direction of the spatial displacement of magma, and therefore, to a certain extent, characterizes the position of the intrusive body in space. It is even possible that the very fact of elongation of a magmatic body may also prove to be one of the causes inducing certain orienting fields in the magma. Thus, from the behavior of prototectonics one can, to a certain degree, form an idea of the general structure and disposition of a pluton.

According to the teaching of Cloos, fibrosity is the result of the action of two vectors of forces: stress and gravity, to which it is arranged perpendicularly. If this is so, then determining the position of a pluton in space from fibrosity becomes fundamentally impossible: these two acting forces are manifested in every pluton; consequently, if the values of these forces are equal, fibrosity will have the same position regardless of the position of the pluton in space. Our point of view, which proceeds from the formation of fibrosity mainly owing to the active movement of magma, assumes that steep plutons will in most cases have steep fibrosity, and gentle plutons—gentle fibrosity. Fibrosity at contacts reflects the form of the contacts.

Fibrosity is incomparably more widespread than primary banding and is apparently encountered in various plutons formed at different stages in the development of the intrusive process, since in them movements of magma may take place during the period of crystallization, accompanied by the emergence of some still other orienting fields. Such movements should be most widespread in the final stages of the intrusive process, accompanied by crystallization of the magma, when, under certain conditions, primary banding arises and all transitions from it to fibrosity occur. For plutons formed at higher-energy stages of development of the intrusive process, one can imagine crystallization under conditions of comparatively immobile magma, with the formation of indistinctly oriented or unoriented structures. However, here too, relatively spo—

…the quiescent state of the magma during crystallization may be repeatedly disturbed by phenomena of pulsating renewals of movement, leading to the multiphase character of intrusions, to the formation of complex intrusive bodies, if the phases at a given level prove to be spatially separated from one another.

A well-known example in this respect may be the complex intrusion in Khakassia of Upper Cambrian age. The plutons composed by the individual phases of this intrusion generally form steep discordant bodies, at times having very complex outlines, which in separate areas—as, for example, on the Ulena—are intricately interwoven with one another, forming comparatively large complex intrusive bodies. However, some large massifs of the same age, such as, for example, the Askiz pluton, do not reveal a multiphase structure, although the constituent rocks include, in general, the same petrographic varieties that are characteristic of the individual phases of the intrusion. At the same time, in the same Askiz pluton steep foliation is widely developed, manifested, it is true, in patches, but throughout the whole body of the pluton. One may think that here there occurred a combination of phases rapidly following one another in one large pluton, which, as it were, passed through all these phases until the moment of its complete crystallization, whereas in other places the individual phases became spatially isolated in the form of comparatively small bodies and, grouping one after another, formed complex plutons of larger dimensions, similar to the way this occurred, for example, in the granodiorite massif of the Sierra Nevada described above.

However, the question of foliation is incomparably more complex than we have portrayed it up to now. The point is that the concept of foliation can be extended to the so-called primary gneisses, or orthogneisses, formed in the process of crystallization of magma, as is supposed, at great depth and under great lateral pressure. Here, too, various banded igneous rocks are formed, which we have already had occasion to mention in the preceding chapter. Therefore we cannot avoid making, concerning gneiss formations, several general remarks of a problem-oriented character, arising from our understanding of the development of the intrusive process.

4. GNEISSES.

Gneisses are convergent formations. The most diverse rocks—sedimentary and igneous—may, by progressive transformation, become gneisses. Solid bodies may pass into gneisses by recrystallization; magmatic bodies—by crystallization under special conditions. In addition, gneisses may arise through assimilation along textural paths, occurring during layer-by-layer injection of magma into the enclosing rocks. In this case it is sometimes impossible to distinguish by what path particular gneisses arose and from what primary material they originated. There is an enormous literature on this subject, and, of course, we do not intend here to analyze this question, which goes beyond the limits of individual remarks concerning the mode of origin of gneissic textures.

In F. Yu. Levinson-Lessing’s petrographic dictionary the following definition of gneisses is given: gneisses are “schistose rocks corresponding in composition to granites: fine-grained, coarse-grained, or porphyroid crystalline schists, chiefly consisting of orthoclase, plagioclase, quartz, and one or several minerals: biotite, muscovite, hornblende, augite. These are metamorphic rocks, often closely associated with granites and mica schists.”

Thus, gneisses are here defined as metamorphic rocks arising through the recrystallization of diverse rocks under the phenomena of dynamothermal metamorphism. Petrology possesses a large number of examples confirming such a definition. In this respect, the example cited in the work of M. A. Usov (39, 156–157) is very instructive: it describes the porphyritic granites of the Kolyvan district in the Altai, which in places have been seized by powerful zones of crumpling of a later age. It is interesting that, while crossing the contours of the pluton, the zones of crumpling do not noticeably distort them, i.e., they do not produce linear displacement. Within these zones of crumpling the granites are transformed into a rock resembling augen gneisses, with a sharply expressed cataclastic structure. Despite the mechanical disintegration, the rock appears on the whole firm, which is due to its later cementation by fine-grained quartz in connection with the intrusion of ore-bearing porphyries.

However, besides such gneisses, about whose metamorphic character there can be no doubt, there are also primary gneisses, which in the same dictionary of F. Yu. Levinson-Lessing appear under the names of granite-gneisses, fluidal gneisses, and so forth. What is the origin of these primary gneisses, in which gneissosity appears as a sharply expressed streaking or fibrousness?

In investigations in the Bavarian Forest, H. Cloos (54) established that gneisses there gradually pass over (in patches) into granites, i.e., they occur at the expense of granites. Further detailed observations on gneiss-granite massifs showed that the structure of the gneiss-granites (gneissosity, banding) is adapted to the structure of the surrounding sedimentary rocks. This, according to H. Cloos, is proof that the gneiss-granites arose not by remelting of sedimentary stratified rocks, but by the intrusion into them of granitic magma during the process of folding, when powerful directed pressures, on the one hand, altered the structure of the content, and on the other hand formed the magmatic bodies and the structures arising in them. In his work translated into Russian (14), H. Cloos formulates the problem of granite-gneisses in the following way.

That which turns granite into gneiss is pressure, and specifically strong and prolonged lateral pressure, which could act both during solidification and continue its action after it, leaving its younger traces of action over older ones. Under normal conditions granites are formed with unoriented granularity, in which lateral movement causes the appearance of cracks $S$, or schistosity in the granites. This schistosity, under elevated conditions, passes into the plane of schistosity of granite-gneiss. What are these elevated conditions?—1) intensified and longer-continuing pressure, 2) increased plasticity, 3) increased temperature, 4) more prolonged movement of the individual parts.

“We know only,” writes H. Cloos, “that these elevated conditions were realized at great depth and that, perhaps, the same pressure which in the upper horizons merely stretches or makes fibrous a solidifying granite, and breaks into blocks and displaces an already solidified one, at great depths calmly and slowly compresses the molten mass into gneiss” (14, 49).

This idea of H. Cloos is developed in detail in his works on the Kola Peninsula by A. A. Polkanov—one of the foremost authorities on gneiss formations. He believes that primary gneissic formations arise during the crystallization of intrusive bodies (batholiths and sheet intrusions) under conditions of abyssal depths (high pressure), during the period of ongoing mountain-building processes (differential pressure) and

under conditions of constant movement by part of the recrystallized magmatic melt (27, 233). The schistose structure characteristic of gneisso-granites, in his opinion, must in some cases be regarded as a palimpsest structure of Sederholm, i.e., a relict structure inherited from an assimilated originally layered sequence, and in other cases—as a structure reflecting the flow or fluidality of the magma, as well as phenomena of crystallization or partial recrystallization under conditions of differential pressure.

The author depicts the formation of banding and gneissosity during the movement of crystallizing magma in the following way.

The incipient crystallization of the more basic components of the rock, with the accompanying flow of the magma, first of all leads to the formation of a fluidal texture. The same movement may also lead to a redistribution of the solid components separated out in the magma, with some participation of the force of gravity. The formation of bands of accumulation of the separated solid components is also facilitated by the fact that the residual magma, becoming enriched in volatile components and freed from solid components, acquires greater ease of movement in comparison with bands overloaded with solid components. Bands of magma of more acid composition, separated out by a mechanical route from the forming rock of more basic composition, cannot be in equilibrium with one another. The former begin to react with the latter, causing a whole series of chemical-mineralogical transformations (phenomena of autometamorphism in the broad sense of this concept). The process of crystallization differentiation, during prolonged movement of the residual magma, leads, through its constant changeability, to a composition close to aplitic magma. Ultimately this difference in composition from the skeleton of the forming rock becomes very great, and the rocks that form acquire a very great morphological similarity to ordinary migmatites; the processes of chemical-mineralogical transformations also become similar. But the newly formed rocks are automigmatites, in contrast to the migmatites described by Sederholm, in which material introduced from outside figures and which arise under the influence of allometamorphism, injection, and assimilation and are allomigmatites.

Thus, a layered, banded, schistose texture is in some cases palimpsest, and in others—a texture of flow or fluidal texture.

What is the role of pressure in their formation? The signs of mechanical deformations are very weak. In the presence of a panallotriomorphic or granoblastic structure, recrystallization of the mineralogical composition in the solid state did not occur, which compels one to recognize the existence of primary blastesis or protoblastesis that took place in the final stage of cooling with the participation of stress. In the main, the role of pressure consists in the fact that it conditioned the stretching of the magmatic mass in a direction perpendicular to it. Manifestations of cataclastic metamorphism took place only at a later time.

How is one to distinguish gneisses formed by metamorphism from gneisses formed by crystallization differentiation in a magma moving under great pressure? It is extremely difficult to answer this question. In a whole series of descriptions of gneiss complexes A. A. Polkanov comes to the possibility of a dual solution. Thus, for example, in describing a variegated complex of pyroxene gneiss-diorites, elongated in the direction of the enclosing gneisses and possessing a prototectonics similar to that described above, the author considers that the crystallization of the rocks of the complex took place in the great majority of cases during slow cooling and with constant movement of the melt and of the solid phase that had separated out, which, however, does not

excludes the possibility of the formation of similar rocks during metamorphism of sedimentary material, for which there is also a series of proofs.

In such cases, chemical and mineralogical analyses of the corresponding formations may provide some material for judgment.

More definite solutions are supplied by structural features. Such, for example, are the features of the microcline granites widespread on the Kola Peninsula (27), which form independent geological bodies of considerable size, as well as small satellite bodies and migmatite fields situated among more ancient formations. The texture is massive, gneissic, and fluidal, conformable to the line of contact, but sometimes disharmonious with the external tectonics—with the schistosity of the surrounding gneisses. The author considers that, in the group of discordant intrusions, the presence of a porphyritic structure indicates conditions of crystallization diametrically opposed to the conditions under which the gneissic facies develops. Fluidality arranged disharmoniously to the structures of the enclosing gneisses is similar to the fluidality that arises in cutting dikes. Gneiss-like varieties develop owing to the parallel arrangement of porphyritic phenocrysts, sometimes owing to elongation of quartz corroding feldspars, but gneissosity is expressed most sharply in varieties with a high biotite content. Cataclasis manifests itself to an extremely insignificant degree.

Thus, two extreme genetic types of gneissic textures are distinguished: gneisses—metamorphic rocks that arose through recrystallization of rocks in the solid state, with local melting under the action of high temperatures and pressure, or of pressure alone (cataclastic gneisses), and gneisses—primary rocks that arose by crystallization of magma under conditions of high pressures, accompanied by mechanical separation of the crystallizing components during the movement of magma. The oriented structures formed in the latter case are, strictly speaking, prototectonics—primary banding and pronounced Streckung, sometimes accompanied by subsequent cataclasis or secondary Streckung, as G. Cloos called it. An intermediate form between these two types is represented by complex gneisses or migmatites (Sederholm), which arose through layered injection of magma into the country rocks, accompanied by phenomena of assimilation, with the formation of a dough-like mass that acquires a gneissic texture partly owing to magmatic movement and partly as a consequence of pressure at great depths. Such regional granitization, with the formation of a dough-like viscous mass possessing a certain magmatic character and capable of movement, was called anatexis by Sederholm. In its extreme development this process results in complete mixing and melting of the rocks, with the formation of new bodies of magma, which are capable of injection and of passing through the cycle of magmatic transformations. This process of regeneration of magma was called palingenesis by Sederholm (37, 319).

In all the points of view cited, the following figure: enclosing rocks and intruding eruptive masses, the difference between which is smoothed out in the phenomena of palingenesis—general remelting, regeneration of magma.

We wish to pose the question of the origin of gneisses in a somewhat different plane.

Most plutons composed of gneiss-granites, assumed to be primary formations, are conformable, concordant with the tectonics of the enclosing sequence and usually have no sharply expressed boundaries (contact lines) with it. As for the spatial distribution of gneissosity within granite-gneiss plutons, here one usually observes—

there is a striking harmony with the stratification, gneissosity, etc., of the various inclusions of the country rocks; moreover, more detailed studies show that the gneissosity of granitic gneisses, recognized as a primary structure, often bearing no traces whatever of cataclasis, forms in space bends like folds of different orders. Thus, for example, A. V. Pek (20) observed in the Urals how gneiss-granites form gentle folds with axes parallel to the general strike of the folding of the enclosing strata. Indications of the same kind are given, for example, by a whole series of geologists who worked on the Kola Peninsula. The gneissosity in the gneiss-granites here is arranged now steeply, now gently, changing directions and forming folds of a certain general strike. I had occasion to observe a similar picture in the gneisses near the Lovozero alkaline pluton on the Kola Peninsula. According to my admittedly few observations, but concentrated over a comparatively small area, the gneissosity in the granitic gneisses here forms, as it were, folds, changing its strike from almost meridional and northeastern to almost latitudinal. Its dip is generally steep, but in places rather gentle. The gneissosity itself in the gneiss-granites is not manifested everywhere in the same way: in places it is intense, in places almost entirely imperceptible. Transitions between oriented and unoriented structures are gradual, but they occur over different, often very short intervals. Among the ordinary granitic gneisses there occur island-like relicts of mica gneisses gathered into folds and permeated by leucocratic granitic material. The transitions to gneiss-granites are gradual. In addition, among the ordinary gneiss-granites there occur relics of schists, in the form of bands of melanocratic biotite material having the form of folds, separated by relatively leucocratic granitic material, which in places possesses clearly expressed gneissosity and in places shows an almost imperceptible orientation of biotite or quartz. One of the sketches of outcrops is presented in Fig. 18. I draw attention to the fact that the closures of the folds formed by the biotite relict bands in the granite are not broken through, as would be expected with a forward movement of the intruding magma. At the same time, the form of the folds does not permit the supposition that the melanocratic bands arose as a result of crystallization differentiation during the movement of the magma.

Here it will be appropriate to recall the detailed studies of Fenner, who studied the modes of origin of gneisses in the highland region of New Jersey (see 37, 316). Fenner found that the gneisses here were formed by lit-par-lit injection. The process of injection apparently proceeded calmly and gradually, and the penetration of the erupted material into the body of the adjoining rock, with accompanying interaction, took place without strong disturbances of the position of the layers or sheeted structure. Preservation of the original position of the rocks into which the intrusion takes place presupposes such a viscosity within the magma as is not consistent with the penetrating capacity of magmatic substances. Therefore it is supposed that the slow

Fig. 18. Relict dark biotite bands among granitic gneisses (Lovozero).

the advance, ahead of the main body of magma, of a more fluid or even volatile part, whose action was a preparatory stage in the impregnation of the country rocks, dissolving their minerals, reacting with them, and causing a general softening or, perhaps, even some liquefaction.

However, even the assumption of such a preparatory action cannot satisfactorily explain why the injecting magma does not tear the closures of folds, as in the case shown in Fig. 24, and does not disturb the relict bands—which, in the case of layer-by-layer injection of magma and the gradual spreading apart and absorption of the injected thickness occurring during the intrusion of the magma, should have been no less widespread than the preservation of the original disposition of the absorbed rocks. Likewise, if the primary banding and gneissosity in gneiss-granites are explained as the result of mechanical differentiation in the process of the mechanical movement of crystallizing magma under pressure, then it is difficult by this method to explain why, for example, gneissosity sometimes forms (and apparently often; few special investigations have been carried out on this question) a semblance of folding, with elements of bedding common to the folding of the enclosing sequence.

The question touched upon is rather specialized and, if it is to be examined in detail, a separate work must be written. Therefore I shall confine myself to what has been said and shall give only a few of my considerations, following from the point of view I have adopted concerning the intrusive process.

It seems to me that primary gneiss formations should be regarded as formations belonging to one of the early stages of the intrusive process, taking place chiefly at great depth—which, however, apparently is not always obligatory, since under certain conditions powerful energetic processes may also unfold at higher horizons. I would characterize this process as metamorphism passing into magma formation, which occurs under anisotropic conditions and is spatially determined mainly by the structure of the sequence undergoing qualitative change. The structural pathways, prepared chiefly by folding that had already arisen earlier, or was still in the process of arising, direct the action of energy flows, which apparently is accompanied by the action of powerful oriented pressure. The process of magma generation does not occur simply as melting. It is a complex and manifold process, including the formation of volatiles, accompanied by their effective action; selective melting, accompanied by local movements of the magmas that arise; various molecular transformations and redistributions occurring in oriented energy fields; and, finally, in its extreme form, the formation of molten masses possessing, at the first stage of development, an internal symmetry inherited from the symmetry of the transformed sequences.

The oriented structures arising from such magma will be, on the one hand, inherited structures—one may say palimpsest structures—and, on the other hand, induced structures, conditioned by the internal symmetry of the magma. In areas where the magma proves to be deprived of oriented internal symmetry, oriented structures may not arise. In particular cases the process indicated may not be accompanied by displacements of large magmatic masses, by the intrusion of magma as a body. However, one should expect that, with a more intense energetic process of magma formation, the process of its general spatial displacement may begin, during which the old symmetry will gradually be lost and replaced by a new symmetry, corresponding to the conditions of the moving magma. As a result of this, it may

it may turn out that the oriented structures formed under the conditions of the magma’s new symmetry will prove discordant with the structures of the enclosing mass.

The gneissic formations described above should be distinguished from the so-called marginal gneisses, of which we have already spoken earlier, from local gneissification, etc.; although similar in outward appearance, they arose in a somewhat different way. Likewise, one must distinguish normal flow structure from primary gneissosity, and the primary banding of intrusives, which arose in the final stage of the intrusive process, from banding in gneisses, although they have many similar external features. In other words, identical results do not yet imply identity of the causes that produced them. Therefore, in order to discover these causes in the field of prototectonics, it is necessary to go beyond the framework of Newtonian mechanics and to bring to bear, in solving such tectonic questions, the modern achievements of physics and chemistry as well. And this is the concern not only of tectonicians, but also of petrologists and mineralogists and physicists. The problems of the geological structures of intrusives must be solved in parallel with petrological problems, and this requires supplementing petrographic mapping with a detailed structural and generally geological analysis of intrusives—work which the majority of geologists-petrologists, unfortunately, still prefer not to undertake.

IV. FRACTURE TECTONICS OF INTRUSIVES AS A MANIFESTATION OF REGIONAL FRACTURE TECTONICS

1. THE RELATIONSHIP OF PROTOTECTONICS TO FRACTURE TECTONICS

The question of the causes of parting, of cleavage in granite, and of its relation to jointing in granite has a fairly extensive literature, the achievements of which are summarized in the brief but interesting article by J. F. Bell, “Investigation of the Flow Cleavage of Granites” (5). The phenomenon of cleavage, i.e., the mechanical anisotropy of granite expressed in the different resistance of the rock to pressure in different directions, was explained by the parallel arrangement of particles in granite—the primary effect of compressive stress, by fractures in feldspars, in quartz, etc. Investigations with the aid of the new statistical methods of structural analysis introduced by Sander (77) showed that cleavage in granite stands in a definite relationship to the regular arrangement of biotite and quartz; moreover, Bell established that all assertions that there are more fractures in feldspars than in quartz, or the reverse, on which some authors had based their conclusions, are untenable. Thus structural analysis confirmed H. Cloos’s view that the parting, or cleavage, of granite is the result of its flow structure, i.e., of the regular arrangement of mineral particles in the granite.

What is the relationship of cleavage to joints in granite? Bell, statistically studying the granites of Büchelberg in the Bavarian Forest, found that there is a definite correlation between cleavage and one or two systems of parting in the granite. The same was established by Maroschek (74) for certain Austrian granites. On the other hand, Osborne (76) showed that cleavage in the Laurentian granites is not connected with parting. Bell himself, in making his summary, indicates that sufficiently clear results have not yet been obtained on these questions.

In the preceding chapter we showed that the prototectonics of intrusives is the expression of a complex process of the formation of plutons, manifesting itself as the self-development of the intrusion, as its self-movement, which occurs in

under the conditions of a definite historical setting, including the conditions of definite oriented pressures and in a known dependence on the structures of the transformed continent that they have caused. On the other hand, fracture tectonics is a passive structure of adaptation of the solidified pluton to the conditions of the general mechanical stresses prevailing in the continent. Hence, among other things, it follows that fracture tectonics in the general case will be a structure superimposed upon prototectonics and to a certain extent independent of it, and that the coincidence of prototectonics and fracture tectonics is a special case in the process of the formation of an intrusion.

As an example, let us cite a detailed structural card for part of the Askyz pluton (Fig. 14), on which it is clearly seen that the fracture tectonics behaves independently of the prototectonics—the fibrousness of the intrusive body—and if one or another system of fractures coincides in individual places with the direction of the prototectonics, then this coincidence is a local special case and is usually never exact. I observed a similar picture in my own, admittedly rather few, observations of gneiss-granites on the Kola Peninsula. It turned out that here aplite veins and fractures rarely coincide with the direction of gneissosity, usually cutting it at various angles, sometimes very acute ones. It is interesting that even in sharply banded gneisses, which themselves, upon weathering, split into slabs bounded by planes of gneissosity, long and even fractures sometimes cut them at an acute angle to the gneissosity.

This kind of noncoincidence of fractures with prototectonics sometimes presents geologists with difficulties in determining the nomenclature of fractures according to H. Cloos’s system. Such difficulties arose, for example, for A. V. Pek in his study of the tectonics of the Khibiny complex pluton (34). In the Lovozero pseudostratified pluton it can often be seen that gentle fractures sharply cut the gently situated plane-parallel texture, which likewise caused a number of misunderstandings among its investigators. However, the adherents of the Cloosian point of view themselves acknowledge the conditional nature of the coincidence of prototectonics and fracture tectonics. Thus, for example, in his methodological work A. A. Polkanov (26), citing as an example of the relations of structural elements in an intrusion the block diagram of the tectonics of the Yosemite pluton in California, compiled by Cloos, admits that “if one mentally removes from the given block diagram of the Sierra Nevada pluton all designations relating to the phenomena of the flow of magmatic masses, then it will be perfectly clear that the remaining skeleton of late-magmatic fracture tectonics is to a considerable degree (and sometimes entirely) deprived of the key to understanding the structure of the pluton. Conversely, the absence of the skeleton of fracture tectonics, while of course not giving a complete picture of the evolution of the entire magmatic body, nevertheless, with sufficiently complete observations, does not deprive us of the possibility of understanding the basic features of the primary tectonics of the pluton” (26, 14).

Thus, the principle of the close conjugation of prototectonics and fracture tectonics, advanced by H. Cloos in connection with his syngenetic interpretation of both these structural elements, encounters serious objections of both an empirical and a theoretical nature; therefore it loses its significance as a basic genetic principle, acquiring the character of a special case in the tectonics of intrusives.

2. INTERRELATIONS OF THE FRACTURE TECTONICS OF AN INTRUSIVE BODY AND THE ENCLOSING MASS

In a special chapter we set forth H. Cloos’s views that “the formation of a massif, although it creates fractures anew, nevertheless the directions of the fractures already exist and are only utilized” (56, 110). H. Cloos believes,

that the fracture tectonics of an intrusion is closely connected with the fracture tectonics of the surrounding rocks, and that almost everywhere along the contact the main fractures pass from the granite into the enclosing rock (56, 127).

However, among the great mass of geologists, for some reason, there is a widespread conviction that intrusive massifs possess a certain autarchy of fracture tectonics with respect to their surroundings, and that the arrangement of fracture tectonics in space is closely dependent on the form and position of the pluton in space. In accordance with this, some tectonicists even regard the study of fracture tectonics as the principal method in studying the structure of intrusive massifs, and think that in this way it is possible to establish, for example, the presence within a single pluton of several intrusive phases. Such an opinion—confirmed and developed by no one either theoretically or experimentally—is erroneous and arises from incorrect conceptions of H. Cloos’s views and materials on this question.

One may cite a whole series of examples showing the common character of fracture systems both in intrusions and in the rocks enclosing them. For example, in A. V. Peck’s work on the tectonics of titanomagnetite deposits of the Urals (20), a whole series of rose diagrams is given for fractures measured in granite-gneisses, in granitoids, in quartzites, in limestones, and even in the ore bodies of the Kusinsk deposit, where they were measured by a special method devised by the author; from them it is clearly seen that in all these rocks, diverse in character and age, the orientation of the principal fracture systems is the same. Similar diagrams are also given in the work of engineer V. S. Sverchkov, “Alkaline Granites of the Central Watershed of the Kola Peninsula and the Suites of Ancient Crystalline Rocks Enclosing Them” (35). From these diagrams it likewise follows that the principal fracture systems in the alkaline granites and in the crystalline rocks enclosing them have one and the same direction. Investigators of the Lovozero alkaline pluton note in their works that in the gneisses and in the Paleozoic sedimentary-effusive sequence the same latitudinal and meridional systems of fractures are developed as are characteristic of the Lovozero pluton.

To clarify the question of the interrelations between the fracture tectonics of intrusions and of the enclosing rocks, I carried out a special tectonic survey in the Dashkesan granite pluton in Transcaucasia, described by K. I. Paffengolts (32). This pluton is very convenient in that it is situated among a gently dipping Jurassic sequence, consisting essentially of porphyrites, which it appears to pierce, forming a stock elongated in a latitudinal direction and complicated by small horizontal satellites. Here, over an area of more than 10 km², I made more than 1000 measurements of joints at 110 points distributed over the whole pluton and the rocks surrounding it. The results of the survey are shown on the tectonic scheme presented in Fig. 19. It was not possible to establish elements of prototectonics in the pluton, except for two points at the southern contact, where a weakly expressed vertical fibrosity was encountered; therefore the scheme shows only fracture tectonics, which, as is evident from the figure, is represented by four principal systems of fractures common both to the intrusion and to the enclosing sequence. For greater clarity of representation, two systems are drawn, unlike the other two systems, with heavier lines. The dip of the fractures is 90–75° in both directions. More gently dipping fractures (45–50°) occur more rarely and are arranged in the same directions. There are no regularities in the arrangement of the gently dipping system of fractures (25°–0°). Observations in thick skarn bodies situated at the boundary of two porphyrite formations near their contact with the intrusion,

showed that the systems of fractures manifested in them do not differ from the systems of fractures in the porphyries enclosing them and in the intrusion with which they are genetically connected.

If we turn to multiphase intrusions, we shall see a similar picture there. For example, A. V. Pek’s investigations in the Khibiny pluton (34), now recognized as a multiphase pluton (11; 12), showed

Fig. 19.

the commonality of fracture tectonics for all its varieties. Special observations of fracture tectonics in the multiphase Lovozero pluton showed that in all phases composing this complex pluton, the fracture systems are arranged in parallel, cutting across the phase boundaries (36). My observations of multiphase intrusions in Khakassia likewise showed that the intrusive phases in contact with one another do not differ from each other in the arrangement of the principal fracture systems, which are common both to them and to the rocks enclosing them.

Fractures in the rocks of the continent, more ancient than the intrusion, often delimit large blocks of the enclosing rocks within the intrusion, determine the direction of veins contemporaneous with the main intrusion that develop in places near its contact, and sometimes, as H. Cloos showed, also influence the position of the contact boundaries during the formation of the banded

intrusion by block movements in plutons formed in the last stages of the intrusive process).

Fig. 20 gives a sketch of an exposure of the contact of a granite stock located on the Baksan River (Northern Caucasus), in the area of the Tyrny-Auz molybdenum deposit. The sketch shows that the granites form, near the contact, a series of small veins connected by bridges, which are situated along the principal systems of fractures in the schists at this place. The fractures in the schists are partly intersected by the granite veins, and partly cut through them and, developing in length, penetrate into the intrusion itself, sometimes for a considerable distance, while sometimes they die out immediately beyond the contact or terminate at it. On the other hand, similar fractures develop in the intrusion; some of them do not pass beyond the limits of the intrusion, while others cut its boundary and penetrate into the schists. Thus, here we see continuity in the directions of fractures, the same both in the schists and in the intrusion.

Fig. 20. Sketch of an exposure. Contact of granite with schists. Granite veins follow old fractures in the schists and, in turn, are cut by fractures of the same directions. (Caucasus. Tyrny-Auz).

What is the role of contraction phenomena in the formation of fractures in an intrusion? G. Cloos believes that typical contraction fractures in an intrusion usually do not arise, and indeed, in practice it is very difficult to establish the effect of contraction in the formation of fractures. It is true that one can often encounter the observation that at least one system of fractures develops parallel to the contact; however, in such cases it often turns out that the contact itself is situated approximately in the direction of one of the principal fracture systems. Fractures that would repeat the curving contours of the intrusion (as should be expected of contraction fractures) are encountered very rarely. I, for example, have not had occasion to observe such well-developed fractures. It is possible that the effect of contraction is to a considerable extent obscured, compensated by elastic movements within the cooling rocks and their immediate surroundings. However, it does not follow from this that the influence of the contraction effect on the position and distribution of fractures should be entirely rejected. In particular, one may think that the contraction effect influences the arrangement of small aplite veinlets developing in the near-contact areas of intrusions, which often have curved, irregular forms, whereas the fractures that usually cut them have a persistent strike, as is shown, for example, in Figs. 21a and b, sketched near the northern contact of the Dashkesan pluton. Apparently, to contractional influence should be attributed the strong curvature of fractures in the Dashkesan pluton near its western contact, where, as a consequence of sharp turns of the contact line, one may expect a considerable intersecting influence of the contraction effect. It is interesting that in the enclosing porphyrites near their contact with the granites no curvature of fractures is observed.

(Figs. 22 and 23). In places where curvature is manifested, the margins abound in fractures, which form either irregular curved networks, as shown in Fig. 22, or bundles of curved fractures cutting one another, as shown in Fig. 23. In general, the curvature of fractures in intrusives near contacts is not rare and has been noted by a number of investigators who have studied fracture tectonics. On the other hand, however, it cannot be asserted that such a phenomenon is characteristic only of near-contact areas. Sometimes strong curvature of fractures may also be observed in the central parts of plutons, so that the question of the causes of fracture curvature must be approached with great caution.

Fig. 21. Curved aplite veinlets in granite near the contact. The lines indicate fractures in the granite (Transcaucasia, Dashkesan).

We shall not examine here the special cases in which radial and concentric systems of fractures are formed in igneous bodies, chiefly in volcanic necks; such phenomena require special investigations, since here, apparently, not only contraction forces are manifested, for examples are known in which even in small stock-like intrusive bodies of isometric form ordinary fracture systems appear, without signs of their radial or concentric arrangement. Likewise, we shall not develop here in detail the question of dikes, in which their own fracture systems usually appear, arranged in a definite way with respect to the contact surfaces of the dike. It should be noted, however, that such autarky of fracture systems in dikes with respect to the fracture systems of the enclosing rocks is manifested up to a certain limit of dike thickness. In any case, I have had occasion to observe in Khakassia thick, gently dipping

Fig. 22. Curvature of fractures in granite near the contact (Transcaucasia, Dashkesan).

Fig. 23. Bundles of curved fractures in granite near the contact (Transcaucasia, Dashkesan).

special investigations, for here, apparently, not only contraction forces are manifested, since examples are known when, even in small stock-like intrusive bodies of isometric form, ordinary systems of fractures appear, without indications of their radial or concentric arrangement. Likewise, we shall not develop here in detail the question of dikes, in which their own systems of fractures usually appear, arranged in a definite way with respect to the contact surfaces of the dike. However, it should be noted that such autarky of fracture systems in dikes with respect to the fracture systems of the enclosing rocks is manifested up to a certain limit of dike thickness. In any case, I have had occasion to observe in Khakassia thick, gently dipping

veins of granitaplites (up to 30 meters and more in thickness), in which the fracture systems were the same as in the granites enclosing them.

Dike systems of fractures, distinct from the fracture systems in the enclosing rocks, are due to the specific conditions under which the effect of contraction is manifested in dikes (the proximity of the contacts to one another), and also to the manifestation in dikes of the most sharply expressed shearing stresses, since here, owing to the presence of parallel contacts situated close to one another—surfaces of weakness—the emergence of a couple of forces acting, as in scissors, against one another is most readily effected. Such a case is shown in Fig. 24. Here, in a porphyrite dike cutting gneisses, there are old fractures mineralized with epidote material, characteristic of the dike itself; then fractures entering the dike from the gneisses and cutting it; and, finally, a dense network of parallel oblique fractures, bounded by the limits of the dike, which arose as a result of the manifestation of shearing stresses, shown in the figure by arrows. It is possible that the development of this kind of fractures, arising under various kinds of shear, may, under suitable conditions, take place on rather large scales (such a case is discussed, for example, by A. V. Peck, (20)). Clearly, these fractures should be distinguished from fractures which are usually called jointing fractures in an intrusive body, or diaclases (after Daubrée).

Fig. 24. Porphyrite dike in gneisses. The dotted line shows epidote mineralization in the dike; thick lines are fractures common both to the porphyrites and to the gneisses; oblique lines are fractures caused by shearing stresses, the direction of which is shown by arrows (Lovozero).

Thus, in the general case, the fracture tectonics of intrusions is not manifested independently of the fracture tectonics of the enclosing continent, with which it has common features. However, the fracture tectonics of intrusions has a number of its own characteristic features, determined above all by the homogeneity of the medium—something by which the enclosing continent is usually not distinguished—and by the absence of folded structural forms, which introduce a number of peculiar elements into the fracture tectonics of the continent. As a result, the fracture tectonics of intrusions usually differs from the fracture tectonics of the masses enclosing them by a greater regularity of fracture systems and by their persistence over considerable distances.

From all that has been said it follows that, in the general case, fracture tectonics does not determine the form of an intrusion, being independent of it or only slightly dependent on it, since the fracture tectonics of intrusions is the most regular expression of regional fracture tectonics.

3. FRACTURE SYSTEMS IN INTRUSIONS

H. Cloos counts five principal systems of fractures in intrusions, of which one is flat-lying, while the other four have steep or moderate angles of dip and intersect in plan in such a way that they form angles of 45° with one another (Fig. 19). No other fracture systems in intrusions (apart from special cases of radial and concentric fracturing) have so far been established by anyone, and in all descriptions of fracture tectonics there usually appear two or four systems of steep (with dip angles from 90 to 45°) fractures and one system of gently dipping fractures.

However, the basis of H. Cloos’s systematics of fracture tectonics is the nomenclature of fractures proposed by him: Q, S, L, D, which subdivides fractures according to their genetic features. This nomenclature is adhered to by most geologists engaged in the study of the fracture tectonics of intrusives, but the concrete application of this nomenclature in practice often encounters such a number of difficulties that the naming of fractures assumes a conventional character, depending chiefly on the subjective impressions of the investigator.

Let us first examine the question from the formal side.

Individual systems of fractures are classified by H. Cloos on the basis of the following features (see the block diagram, Fig. 25).

1. Fractures Q. They are situated perpendicular to the foliation, in the direction of the oriented lateral pressure and load pressure. The angle of dip is conditioned by the oblique action of the load. They are filled by aplite veins and by various mineralization. Sometimes—longitudinal displacements. They are expressed most sharply.

Fig. 25. Block diagram showing the systems of fractures and veins in intrusives, and their relations with the prototectonics of a plane-parallel structure (shown by small squares on plane L and by small strokes on planes Q and S) and linear streaking (shown by small strokes on plane L and by dots on plane Q).

1. Fractures S. They are situated along the lineation, perpendicular to the lateral pressure, in the direction of the load pressure. Veins, mineralization, and displacements are absent. They are expressed less perfectly than fractures Q.

2. Fractures L. They are situated in the plane of the plane-parallel structure, perpendicular to the load pressure, in the direction of the oriented pressure. They are often filled by veins and mineralization.

3. Diagonal fractures. They are situated obliquely with respect to the prototectonics and represent Mohr fractures, or circular sections of the ellipsoid of elasticity. They are often filled by veins and mineralization. Displacements occur most often along them.

From what has been presented it is evident that the principal distinguishing features of one or another system are: its relation to the prototectonics, filling by veins and mineralization, the presence of displacements along it, and the perfection of its external expression.

Let us now imagine a steep pluton with a vertical plane-parallel structure and vertical or steeply inclined linear streaking, and let us assume that in this pluton there is a regular spatial conjugation between the prototectonics and the fracture tectonics of the pluton (see the block diagram, Fig. 26). Let us examine the systems of fractures according to their relation to the prototectonics.

Fig. 26. Block diagram showing the interrelations of fractures and veins with the prototectonics in steeply situated intrusives. The designations are the same as in Fig. 25.

1. Steep fractures coinciding with the plane-parallel structure,

should have been called fractures L, but they are vertical, i.e., are situated not perpendicular to the load pressure, but parallel to it, like fractures Q or S. Since, according to H. Cloos, prototectonics and fracture tectonics are the result of one and the same cause—lateral pressure and load pressure—one should expect that a plane-parallel structure, if it is parallel to the load pressure, must be perpendicular to the lateral pressure. In that case the given fracture ought to be called a fracture S, but in it aplite dikes were found (as, for example, this is manifested in the Kainsk alkaline pluton, (26)), which corresponds more to fractures Q. Thus it is left to the investigator’s discretion to decide what to call this fracture—L, Q, or S.

1. Steep fractures, perpendicular to the preceding system of fractures and arranged in the direction of the linear strike, ought to be called fractures S. However, if the lateral pressure is perpendicular to the plane-parallel structure, then, with respect to this pressure, these fractures should be called fractures Q.

2. A gently dipping system of fractures cuts the prototectonics at a right angle and, by this feature, is more appropriate to fractures Q. However, it is situated perpendicular to the load pressure and parallel to the lateral pressure, which is a feature characteristic of fractures L.

3. Fractures oblique with respect to the prototectonics may be called diagonal. However, if the linear strike is absent (as, for example, in the Lovozero pluton), then it is impossible to establish which fractures will be oblique with respect to the prototectonics.

Thus, if one adheres exactly to the criteria given by H. Cloos for determining fractures according to their relation to prototectonics, which proceed from his theory of the identity of the causes determining the behavior both of prototectonics and of fracture tectonics, we shall encounter a whole series of insurmountable difficulties, even of a purely formal character, as soon as we leave the circle of examples relating to the case of an ideal laccolith. Moreover, practice shows that fracture tectonics and prototectonics are not always harmoniously coupled with one another; that fracture tectonics is more consistent in its manifestations than prototectonics; and that in different parts of a pluton one and the same system of fractures may in one case coincide with the prototectonics, and in another cut it at different angles, appearing in the manner of diagonal fractures (for example, Fig. 14). In addition, steeply dipping fractures filled with various vein material, which according to H. Cloos’s systematics can only be fractures Q or diagonal fractures, are often situated not transverse to the prototectonics, but parallel to it. Such is the example of the Kainsk pluton noted by us above; such is the arrangement of veins in the Adirondack pluton, of which we spoke in the chapter on prototectonics (46), etc. Even in gneisses, where one would expect a strict persistence of H. Cloos’s conditions for naming systems of fractures, aplites, pegmatites, and quartz veins often occur in the direction of the schistosity of gneiss-granite masses, as is noted by A. A. Polkanov (25, 278) and others. Such an absence of the proper harmonious coupling between prototectonics and fracture tectonics, in fact acknowledged by a whole series of investigators, only once again emphasizes the untenability of the viewpoint that regards prototectonics and fracture tectonics as the result of the same mechanical pressures, manifested differently in different materials possessing different physical properties (magma—rock).

Let us now examine the reliability of other criteria for determining systems of fractures: the filling of fractures with vein material, the presence along them

displacements and the perfection of their external expression. In doing so, let us assume that the gently dipping system of fractures will be system L. Therefore the question of naming fractures is reduced to the question of how to distinguish the fracture systems Q, S, and the diagonal ones.

Let us take as an example the Lovozero alkaline massif, in which a horizontal, plan-parallel structure is very sharply expressed, but a linear structure is absent. Here, according to the data of investigators of this pluton, there are developed chiefly vertical or steeply inclined systems of meridional and latitudinal fractures, in relation to which the northeastern or northwestern fractures have a subordinate significance. Of the two principal fracture systems, the fractures of latitudinal strike are the more highly developed; they are longer, straighter, and more frequent than the meridional fractures. Vein rocks in the Lovozero massif are not widely distributed, but among the steep fractures they prefer the latitudinal ones. In these same fractures nepheline mineralization is also encountered. Dislocations are absent altogether. On the basis of the features enumerated, the latitudinal fractures were assigned the name Q, and the meridional ones S. In accordance with this it would have to be considered that the Lovozero alkaline massif was formed with the participation of latitudinally oriented pressure; however, regional investigations show that the formation of the Devonian alkaline plutons was accompanied by the action not of latitudinal, but of meridional lateral pressure (27), so that, if one adheres to the genetic classification of fractures, the latitudinal fractures should be called S, and the meridional ones—Q, which, however, does not agree with their morphological features.

Still more interesting observations on fracture tectonics were made in the Khibiny pluton, adjacent to the Lovozero pluton, where a special party headed by A. V. Pek worked on the investigation of fracture tectonics (33;34). Here more than 8000 measurements of fractures and veins were made, carefully processed and reduced to corresponding diagrams on stereographic nets. The results showed that “features permitting one in the field comparatively easily to distinguish Q rupture fractures from S fractures and Mohr’s diagonal planes are entirely absent here, and all fractures are essentially alike. No displacements or traces of such were found either” (34, 5). Therefore, in interpreting the fracture net, A. V. Pek relies almost exclusively on their relative position and combinations, which in most cases leads him to an ambiguous solution. A. V. Pek notes that in almost all the diagrams he compiled there are 2 pairs of mutually perpendicular fracture systems, one of which represents the Q and S fractures, and the other—Mohr fractures. However, he finds it difficult to say definitely which fractures are Q and S fractures and which are Mohr fractures, since with all of them are associated one or another vein formations syngenetic with the alkaline intrusion, all fractures are expressed more or less equally, and dislocations are absent. The author sees the way out of the situation that has arisen in recognizing the heterogeneity of the dynamic conditions of development of the fracture tectonics of the Khibiny massif. In the conclusion of his article, A. V. Pek, who devoted much time to the study of the fracture tectonics of intrusions and considered this to be the principal method of studying the structure of massifs, makes the valuable admission that the field of this method proves to be limited and that it “is not a universal means permitting all questions of tectonics to be solved.” (34, 49).

If we take as examples the granodiorite plutons—the Askyz pluton (Fig. 14), the Dashkesan pluton (Fig. 19), or the Central pluton, situated in the area of the Central gold mine in Martayga (western Siberia), where I also had occasion to carry out special tectonic investi-

…investigation (30), we shall see that everywhere two principal pairs of mutually perpendicular fracture systems are developed, in which, however, it is very difficult to determine which of them are \(Q\) and \(S\), and which are diagonal. Let us take, for example, the granodiorite massif (a steeply dipping acmolith) of the Central Mine area. Here, as is shown in the block diagram (Fig. 27¹), the aplite and quartz veins are arranged along all four fracture systems; thus, if we wished here to distinguish the \(S\) fractures by their principal feature—the absence of veins in them—we would not find any such fractures. Here we can say only that fractures of NE and latitudinal strike are richer in mineralization and veins, especially long ones, in comparison with fractures of meridional and NW strike. However, it is precisely along the latter fractures that dislocations most often occur, which is not characteristic of \(S\) fractures.

Fig. 27. Block diagram showing the systems of veins in the area of the Central gold-ore deposit (Western Siberia): 1—aplite veins; 2—quartz veins; 3—lamprophyre veins.

If we turn to the Dashkesan pluton (Figs. 19, 28, and 29), we shall see the same picture here. It is impossible here to distinguish \(S\) fractures (and thereby, consequently, to determine the names of the other fracture systems) on the basis of objective materials. The same case is also observed in the Askiz pluton.

Fig. 28. Rose diagrams of fractures in the Dashkesan pluton (Transcaucasia). Compiled on the basis of 160 measurements at 20 points. \(a\)—rose diagram of steep fractures with dips from \(90^\circ\) to \(70^\circ\) in both directions; \(b\)—rose diagram of fractures with dips of \(35–50^\circ\); the dips of the fractures on the diagram are directed clockwise. The scale of diagram \(b\), as compared with diagram \(a\), is enlarged twofold.

If we speak of the sharpness with which fracture systems are manifested, it must be said that in some parts of the plutons some fracture systems are more sharply expressed, in other parts—other systems. Usually in each pair one fracture system is expressed more sharply than the other, perpendicular to it (Fig. 28), so that one may, for example, by dividing the acute angle

between the two most frequent and sharply expressed systems of fractures, to speak of a certain mean direction in which the fractures are more perfect and more numerous than in another, perpendicular direction. But to decide which of the two most highly developed fracture systems is the \(Q\) system, and which is the diagonal system (if the two other systems are counted as \(S\) and the second diagonal), is possible only in a purely conventional way, thereby offending objectivity.

If we try to call upon the dip angles of the fractures for assistance (for in the ideal case, according to H. Cloos, Mohr fractures under the influence of gravity should have a dip of about \(45^\circ\), while fractures \(Q\) and \(S\) should generally dip more steeply), then in the examples cited we must state that all four fracture systems consist, first, of fractures with a steep dip of \(60\)—\(70^\circ\) to one side and to the other, and, second, of fractures with a dip of \(35\)—\(55^\circ\), likewise to one side and to the other. Thus one may say that if, in plan, we have four fracture systems, then in section each fracture system, characterized by a definite strike, will correspond to three systems differing from one another in dip, to which a fourth system—the flat-lying \(L\)—may be added (Figs. 28 and 30), as a result of which the framework of fracture tectonics, forming its skeleton, acquires a symmetrical character. It should be noted, however, that, for example, in many observed cases the systems with intermediate dip angles had a statistically subordinate importance relative to the steeply dipping systems, and moreover were not developed equally in all directions, having predominantly the strike of the least developed steeply dipping fractures (Fig. 28). As a result, it becomes possible to speak of two mutually perpendicular axes, one of which bisects the acute angle between the two most highly developed systems of steeply dipping fractures—

Fig. 29. Rose diagrams of aplite, quartz, and garnet veins in the Dashkesan pluton (Transcaucasia). Compiled on the basis of 55 measurements at different points of the pluton. a—steeply dipping veins; b—veins dipping at an angle of \(35\)—\(50^\circ\); dip directions—clockwise.

Fig. 30. Symmetry of the framework of fracture tectonics. a—four fracture systems in plan. Each system is in turn divided into three systems differing in dip, as shown in section b. As the fourth system, the flat-lying fractures \(L\) are included here.

cracks, while the other bisects the acute angle between the two most developed systems of cracks with medium angles of dip. It would be interesting to verify how regular such a phenomenon proves to be.

Thus, the main thing yielded by the analysis of fracture tectonics according to H. Cloos—namely, the elucidation of the dynamic conditions in which the fracture tectonics of the given intrusive body developed, i.e., the determination of pressure vectors from those features on which H. Cloos’s genetic system of cracks is built—encounters in practice so many difficulties and contradictions that, even on the basis of empirical data alone, we are entitled to express doubt concerning the correctness of this genetic interpretation of fractures by H. Cloos. A critique of the theoretical foundations of the Cloos classification will be carried out by us below.

To all that has been said about systems of cracks, it is necessary to add one further, very substantial complication, which is revealed in detailed observations of cracks. The point is that each system of cracks proves in some cases to consist of two conjugate surfaces of splitting, differing somewhat from one another both in strike and in dip. The difference in strike reaches up to 20°. Such pairs do not appear together in all cases; more often they replace one another, cutting across one another or passing smoothly one into the other, as is shown in the sketch of the outcrop in Fig. 31, made at Lovozero. Here they were first noted by engineer A. S. Sakharov, according to

Fig. 31. Sketch of an outcrop of lujaurites. Pairs of cracks (Lovozero).
Fig. 32. Veinlets of aplite developing along a pair of cracks (Dashkesan).

whose observations both the meridional and latitudinal and L-systems of the Lovozero pluton consist of pairs of cracks that intersect, cut across, and bend one another (36). It should not be thought that these cracks are of different age. In the Dashkesan pluton I have had occasion to observe aplite veinlets developing along both cracks of a pair (Fig. 32). Sometimes these pairs of cracks are reflected in rose diagrams of cracks, especially if the measurements were made over a small area. For example, such pairs stand out rather well in the rose diagram of cracks measured in the Yuksporr adit (Khibiny) by A. V. Pek (34), Fig. 33. In rose diagrams of cracks of the Dashkesan pluton, Fig. 28, such pairs also stand out clearly. Such a pair could be identified with the well-known “Cloos pair,” representing an unproved experimental pair of shear cracks with an angle of intersection decreasing to 0°, which was invoked by H. Cloos to explain the appearance of Q cracks (see the chapter on H. Cloos’s theory). However, such pairs of cracks are characteristic not of one

any one system of fractures, but to all the systems, which argues for a certain genetic commonality of these systems.

In conclusion it must be noted that, in addition to the primary systems of fractures, whose behavior is maintained both at the surface and in deep mine workings (30, 34), there are also secondary fractures, distinguished by H. Cloos (Bannung), which repeat the contours of the relief. A classic example of fractures of this kind, expressed very sharply, I had occasion to observe in one of the granite massifs of the Northern Caucasus. This example is shown in Fig. 34. It must be said that, where the relief is strongly dissected, such fractures may sometimes be very well developed, which must be taken into account in field investigations.

Fig. 33. Rose diagram of fractures measured in the Yuksopor adit (Khibiny) (after A. V. Pek). Pairs of fractures stand out in the diagram.

Fig. 34. Secondary fractures (heavy lines), superposed on primary fractures and repeating the contours of the outcrop surface (Tyrny-Auzs).

4. TIME OF FORMATION OF FRACTURES IN INTRUSIVES

The formation of fractures in an intrusive does not occur during some special phase of compression, but rather as the pluton solidifies, as H. Cloos proposes in his works.

In fact, the formation of fractures in an eruptive rock can be observed at a very early stage of its development. A very interesting example confirming this I had occasion to observe in the Lovozero alkaline massif. Here, in the lujavrites, there is a sharply expressed horizontal—

Fig. 35. Block sketch. Bands of lujavrites with vertical feldspar laths among lujavrites with horizontal feldspar laths (Lovozero).

Fig. 46. Garnet veinlets in the Dashkesan granodiorites.

trachytoid structure caused by the horizontal disposition of the isometric feldspar laths. Thus, in several places I observed the curious phenomenon that, in ordinary lujaurites, there appeared thin, lens-shaped bands of the same lujaurites, oriented in the latitudinal direction, in which the feldspar laths were arranged vertically (Fig. 35). The orientation and dip of these bands coincide with one of the principal systems of fractures in the massif, so that one may think that here we have an example of the very early appearance of local fractures filled by material that rose along them from below before it had yet solidified.

In general, examples are widely known of the development along fractures in intrusions of garnet (Fig. 36), aplite, and vein hydrothermal formations; moreover, veins formed in fractures often show a high degree of perfection of the fracture tectonics which the intrusion had attained by the time of the appearance within it of vein differentiates.

Thus, the fracture tectonics of intrusions is laid down and formed in parallel with the solidification of the rocks composing the intrusions.

5. DEVELOPMENT OF FRACTURES IN INTRUSIONS

In what sequence do fractures arise? Are they formed successively or simultaneously?

Since fractures, according to the views of H. Cloos, arise as the result of the action of stress and load pressure manifested simultaneously, there are no reasons that would entail a succession in the formation of the principal fracture systems. The fact that aplite and quartz veins develop in all the principal fracture systems indicates that, by the time these veins appeared, the fractures already existed. Consequently, we are entitled to state that all the principal systems of fractures making up the framework of the primary fracture tectonics of the massif are formed approximately simultaneously.

The question of differences in the time of origin of fractures usually arises for the investigator when he has before him several types of veins of different ages, intersecting one another. Thus, for example, A. V. Pek finds that in the Khibiny pluton there are eight types of veins of successive ages syngenetic with it. However, the author’s attempt to use the age scheme of the veins to establish the sequence of fracture formation proves unsuccessful, since, for example, vein nepheline syenites turn out to be scattered over the entire area of the circle of the diagram and, consequently, by the time of their emplacement all the fracture systems existed in full. Therefore the author makes the reservation that, apparently, all the fractures are more or less simultaneous, but owing to the dynamic environment changing with time, at a given moment only certain, quite definite systems of this fracture network were accessible to the magmatic material (34, 31–32).

Thus, in the question of the time of fracture formation, two moments should be distinguished: the emergence of the principal systems of fractures forming the framework of fracture tectonics, which apparently takes place simultaneously, and the emergence of conditions favorable to the filling of one or another fracture system by veins. In this connection, as practice shows, the maxima of spatial development of veins of different ages often do not coincide, intersecting one another, which indicates the variable character of the dynamic environment accompanying the development of fractures. The same is indicated by dislocations of different ages that cut one another and are manifested along different fracture systems, as well as by various kinds of crumpling zones, appearing in some directions and not appearing in others. For example, A. A. Polkanov cites a case observed by him in which a younger pyroxenite dike, cutting diabase, is dif-

fragmented, while the diabase is not; or when the main diabase dike remains in “absolute freshness,” whereas its apophysis is strongly fragmented (24, 962), etc.

Thus, questions concerning the origin of fracture systems and their use by veins and dislocations are questions of several different orders. In other words, the simultaneity of the origin of all the principal fracture systems still does not imply an identical character of their development and utilization, which depend on the character of the changing dynamic setting that conditions the formation of fractures and their use by veins and dislocations; whereas the simultaneous establishment of fracture systems is, apparently, a mechanical law, manifesting itself analogously to the law, developed in mechanics, for the formation of a pair of Lüders–Mohr fractures. Moreover, the simultaneity of the origin of fracture systems also does not imply that fractures of all systems will be developed at all points of the pluton. On the contrary, it is usually the case that in some parts of the pluton mainly one pair of systems is developed, and in others another. Sometimes such alternation can be observed not only in space but also in time, though not for the entire pluton, but for one of its parts. Thus, for example, Fig. 37 shows a case in which aplite veins developed along one pair of mutually perpendicular fracture systems; however, the fractures of these systems themselves are not present in this particular place: they are present in an adjacent part, while here fractures of another pair of systems are developed. We repeat that the alternation observed here of fracture systems in time may apply only to this particular area, and for several areas it already loses its force, since in other, neighboring areas the relationships may be reversed.

Fig. 37. Aplite veins cut by systems of fractures of other directions that developed later in this place (Dashkesan).

Fractures are discontinuities of continuity, an interruption of continuity, a definite type of deformation. This does not mean, however, that displacements must occur along all fractures. On the contrary, noticeable displacements along joints (diaclases) are encountered rather rarely, although slickensiding of fracture walls and slickenside grooves are observed more often than noticeable displacements established from veins, schlieren, etc. Very often in plutons one may encounter sharply expressed, very long fractures along which no displacements are observed. Moreover, true zones of crushing can not infrequently be encountered, along which, despite the sharpness of their expression, no dislocations occur (Figs. 38 and 39). In Fig. 39, for example, it is evident that an aplite vein, entering a zone of strong schistosity, undergoes no displacement along the plane of schistosity and at the same time is displaced by dislocators oblique to the schistosity. Interesting examples of intense schistosity without displacements may be observed in the quarries of the apatite mine (Khibiny), where they cut banded apatite ores in the form of crushing zones, accompanied by brecciation, up to a meter and more in thickness, which, however, do not displace the intersected bands of rock. Very many examples of this kind may be cited.

Such crushing zones should be distinguished from zones of coarse schistosity, which constitute a very widespread phenomenon in the fracture tectonics of in-

blocks. These zones are belts of the most intensive development of one or two systems of fractures, which either constitute a local phenomenon or can be traced for considerable distances, as is shown on the schematic tectonic maps in Figs. 14 and 19. It is difficult to determine precisely the width of the belts of coarse jointing; one can say only approximately that it ranges from several meters to several tens of meters.

Apparently, the belts of coarse jointing constitute an element of block folding which, as geological investigations show, is very widespread and should be regarded as one of the necessary elements of the tectonics of granites. A very

Fig. 38. An aplite vein in granodiorites, cut by zones of jointing that produce no noticeable displacements of the vein (Dashkesan).

Fig. 39. An aplite vein in granodiorites in a zone of jointing. Displacements of the vein occur along fractures obliquely cutting the zone of jointing (Dashkesan).

clear example of block folding I had the good fortune to observe in the middle course of the Askiz River (Khakassia), at a place where a red-colored Devonian gently dipping sequence overlies Upper Cambrian granodiorites. Excellent exposure and dissection of the relief make it possible here to see the manifestations of block folding in section. It is clearly visible that the sequence overlying the granodiorites forms large waves with limbs dipping at angles to the horizon of up to \(30^\circ\).

According to the character of their manifestation, fractures in an intrusive body should be divided into two main types: fractures—planes—the usual type, and fractures—zones, or feathery fractures—a rather rare type.

Fractures—planes may be uneven and rough, or even—rough or smooth. The smoothness of fractures is sometimes striking. But still more striking is the fact that these even, as though chopped fractures, appear independently of the structure of the rock. Numerous cases are known in which, even in relatively loose conglomerates, fractures of this kind, without changing their direction or character, cut through both the cement and pebbles of various sizes.

It must be said that roughness or smoothness is not the exclusive privilege of any one system of fractures; but in some cases one can speak of a statistical predominance of smooth fractures within some one system or two systems over the others. Usually smoother are those fractures in whose direction the most widespread belts of coarse jointing are arranged.

Feathery fractures (Fig. 40) are, according to H. Cloos, zones of crushing produced by the opposing movement of two blocks. A. A. Polkanov, who made many observations of feathery fractures in the gneisses of the Kola Peninsula, considers them the initial stage of friction breccias and mylonite belts. At first no visible displacements take place along these feathery fractures, but later they displace the dikes they cut; in such cases the feathery fractures are replaced by belts of complexly crushed rocks (27, 433–434). However, one should not think that feathery fractures arise only where various kinds of zones of brecciation, mylonitization,

Fig. 40. Feathery fractures, often filled with vein material. The arrows at the sides indicate the directions of movement (after A. A. Polkanov).

Fig. 41. Sketch of a hand specimen. Feathery garnet veinlets in granites (Ulen, Khakassia).

and the like have developed, i.e., all kinds of secondary tectonic formations; such feathery fractures are also readily formed at times in the early stages of the development of fracture tectonics and, being filled with garnet, aplite, or quartz, lead to the appearance of distinctive veinlets, shown, for example, in Figs. 41 and 42. However, such formations are relatively rare and do not belong to ordinary joint fractures, which are characterized by even shear planes or uneven cuts.

Fractures do not immediately form as long as we often see them. They grow, merging with one another and cutting one another. The growth of fractures is manifested with particular clarity at the contacts of an intrusive body with the enclosing rocks, or with veins of aplites, lamprophyres, and the like (Fig. 20), where it is evident how fractures, as they develop, pass from one rock into another; how, after being cut by a vein, they resume and cut this vein, creating the impression of younger fractures; how fractures readily resume along the contacts of veins, forming even selvages there, and so forth. Developing in length, fractures pass behind one another, forming coulisse-like systems, or merge with one another, cross one another, or cut one another.

Fig. 42. Feathery veinlets of aplite in granodiorites (Dashkesan).

As is known, the length of fractures is not infinite, and the number of fractures is usually not so great as to split the rock into small pieces; moreover, if we compare ancient intrusives with young ones, we do not see in them any especially great difference in the number of fractures or in their length.

Sometimes, in some Proterozoic gneisses, fracture tectonics looks more perfect and orderly than in a very young intrusion affected by zones of disturbance, in which abundant fractures of all directions arise and it is sometimes quite impossible to assign them to systems. And if there are usually more such areas in ancient intrusions than in young ones, this depends on the fact that ancient intrusions are, for the most part, richer in zones of disturbance than young intrusions—which, I emphasize once again, is not, however, a law.

Thus, if we consider the general case of the development of normal fracture tectonics, not complicated by subsequent zones of disturbance, we shall see that the number of fractures and their length do not depend on the age of the intrusion; that in ancient intrusions fracture tectonics may be expressed just as it is in a young intrusion, examples of which may be provided at least by the granodiorite massifs already cited by me—the Upper Cambrian, the Askyz (Fig. 14), and the Upper Jurassic Dashkesan (Fig. 19). From what has been said it follows that the development of fractures in intrusions has a certain limit, beyond which no increase in the number of fractures or in their average length takes place. Obviously, the existing fractures and the barely perceptible movements along them fully compensate for those mechanical stresses which the intrusions experience under the influence of external mechanical forces. Thus, the number of fractures and their length are not a question of the age of the intrusions, but of the magnitude of the maximum mechanical stresses which they experience in the course of their geological development.

6. INHERITANCE OF THE DIRECTIONS OF FRACTURE SYSTEMS.

In the preceding chapter I already pointed out the fact that the fracture tectonics of old and young intrusions in a number of cases shows no significant differences. G. Cloos himself long ago drew attention to this circumstance, repeatedly indicating the sameness of the directions of fracture systems in intrusions of different ages (52, 88, 103), and expressing this in his generalization: “the formation of a massif, although it creates new fractures, finds the directions of fractures already present and merely makes use of them” (55, 110). The idea of inheritance over a long period of time in the orientation of the components of pressure is even contained in his classification of fractures. Pointing out that, in contrast to fractures $S$, fractures $Q$ are often filled by quartz–feldspar and chlorite veins with limonite and manganese compounds, G. Cloos notes that this indicates not only that these fractures arose during the cooling of the granite as extension fractures, but also that they were partly open later as well, as a result of which groundwater seeped into them and produced secondary alterations and deposits, whereas in fractures $S$ nothing of the kind occurs.

S. Bubnov, in his work on the southern Schwarzwald (50), indicates that although the position of the Bauquader granites changes in different places, their orientation remains unchanged. It is the same both for the ancient Variscan granites and for the young granites, in which the directions $S$ and $Q$ coincide.

The same phenomenon is reflected in the identical orientation of the principal fracture systems in intrusions and in the rocks enclosing them, of which we have already spoken in detail in its proper place.

All these numerous facts indicate that the directions of the principal fracture systems are somehow inherited, for otherwise,

in ancient plutons, for example, it would be impossible to determine any definite systems of fractures, since there would be so many of these systems that in all cases they would form among themselves the same interrelations as are observed between fractures in zones of intense disruption, which is not confirmed by reality.

On the basis of what has been said, one might conclude that the directions of the principal components of pressure are persistent through time, and this would to a certain degree be correct, since we know that in geosynclinal zones, for example, there is a certain inheritance not only of the directions of adjoining geosynclines of different ages, but also of the directions of the axes of folds within them.

However, intersections of the orientation maxima of veins and dislocations of different ages, frequently encountered in geological practice, indicate a changing character of the dynamic setting that conditioned the development of fractures, as we already mentioned in the preceding chapter. As a result, a contradiction arises: the uniformity of orientation of the principal systems of fractures in intrusions of different ages and in their enclosing rocks indicates the persistence of the dynamic setting that conditioned the development of fractures, whereas the spatial distribution of veins and dislocations of different ages argues that this dynamic setting is changing. The solution to the difficulty that has arisen should apparently be sought in two directions: either the fractures filled by veins are superimposed upon existing systems of fractures, being independent of them and acquiring new directions when a new direction of the acting forces arises, or these changing acting forces manifest themselves along already existing systems of fractures, determining the character of their development in a given period and their utilization by veins and dislocations.

H. Cloos, in his works, considers vein formations in connection with already existing systems of fractures. However, he points out that the mere presence of fractures is still insufficient for the formation of volcanic veins, because otherwise fractures of all existing directions would have been used. Therefore fractures of the corresponding directions must come to life and open shortly before or during volcanic pressure, which is conditioned by the action of correspondingly oriented forces that change their direction somewhat over time (14, 80—81). The numerous followers of H. Cloos also adhere to the same point of view.

I, too, have had occasion to deal specifically with the question of the distribution of veins and dikes in intrusions. In particular, I carried out thematic work on the tectonics of the Tsentralnoe gold-ore deposit (30), where the complete conjugacy of the fracture tectonics of the pluton and of the ore veins occurring in it, of spessartite dikes cutting the ore veins, and of dislocations offsetting the veins and dikes was revealed. Special investigations in the Upper Cambrian Askiz pluton (Fig. 14) showed here the complete conjugacy with its fracture tectonics of both the ore and aplite veins and the porphyry dikes of Devonian age.^1

All this shows that the systems of fractures accommodating dikes and veins do not arise anew, but are utilized; i.e., rejuvenation occurs, with lengthening and opening of fractures belonging to one or several existing systems. At the same time, however, it should be noted that the orientation of particular systems of fractures is not entirely persistent

^1 To the question of the structure of ore fields occurring in crystalline massifs I have devoted a special article, published in Proceedings of the Scientific Conference on the Study and Development of the Productive Forces of Siberia, vol. II, issued by Tomsk State University in 1940.

rectilinear; strike fractures usually bend, change their direction somewhat, and so on. Small veins, situated along minor fractures, usually readily emphasize such changes. But larger veins appear in individual places as if independently of the local directions of the fractures of a given system, maintaining the general strike; this is quite natural, since a fracture that has grown into a very large fissure, in the course of its growth, increasingly acquires the character of an independent unit, showing a certain independence from local changes.

Thus, in studying fracture tectonics one should be concerned chiefly not with determining the relative age of systems of fractures, as some investigators do, but with determining the relative age of the mineral bodies filling the fractures, and of the various dislocations and displacements along fractures; these two questions are far from being one and the same.

Thus, systems of fractures are inherited, but are applied depending on the character and direction of the mechanical stresses existing in the earth’s crust. On the other hand, since these mechanical stresses change their character and direction over time, corresponding changes should also be expected in the systems of fractures, at least in formations of different ages. Obviously, this question still requires further clarification from the factual side, and also needs special theoretical studies, without which it cannot be resolved.

7. MECHANICAL PRINCIPLES OF FRACTURE FORMATION

Fracture tectonics is an expression of the passive adaptation of solid materials to conditions of oriented pressures. Therefore the question of fracture tectonics is a general question, concerning not only intrusions but also the strata enclosing them. And if we distinguish the fracture tectonics of intrusions, and in general of homogeneous masses similar in their mechanical properties to intrusive formations, from the fracture tectonics of folded masses, this is because in intrusions the regularities of regional fracture formation are most sharply manifested, whereas in folded masses these regularities are often obscured or even give way to the regularities of fracture formation that take place in phenomena of folding.

Systems of fractures similar to those we encounter in intrusions are often excellently expressed in gently lying effusive and sedimentary formations (as, for example, in the porphyries of Dashkesan), in various kinds of crystalline and metamorphic rocks, not inferior in the perfection of their expression to the fracture tectonics of intrusions. Therefore, if we analyze the question of the origin of the four principal systems of fractures, plus a fifth—the gently dipping one—which we usually encounter in intrusions, various crystalline masses, and so forth, it must be emphasized at once that we are dealing not only with the fracture tectonics of intrusions, but with a certain regional fracture tectonics, common to large regions. H. Cloos treats this question in this way when he passes from the fracture tectonics of intrusions to the consideration of fracture tectonics in block mountains, noting here the same systems of fractures as in granite and giving them the same genetic definitions (14, 34–35).

In his reasoning H. Cloos proceeds from the action of a non-rotational stress. Theoretical studies show that in this case the planes of maximum shearing are situated at 45° to the direction of greatest shortening.

Let us consider this question from the point of view of the spatial disposition of the stress ellipsoid, a concept that is now widely used in geological practice. The stress ellipsoid is the name given to a triaxial figure, similar to an optical indicatrix, into which an imaginary sphere is transformed under the action upon it of external pressure, which can be resolved along three principal vectors: greatest, least, and intermediate. The greatest pressure corresponds to the smallest axis of the ellipsoid (the greatest shortening), and, conversely, the least pressure corresponds to the greatest axis of the ellipsoid (the greatest extension). Circular sections of the ellipsoid correspond to shear planes and, in the general case, are situated at \(45^\circ\) to the greatest pressure (Fig. 43).

Fig. 43. Stress ellipsoid in orthogonal projections. The sphere transformed into an ellipsoid is shown by a dashed line. The arrows indicate the direction of the transforming force. \(aa\)—axis of greatest compression; \(bb\)—axis of greatest extension; \(cc\)—intermediate axis; \(DD\)—circular sections of the ellipsoid, along which shearing takes place. Above—vertical sections; below—plan.

Long ago Becker drew attention to the fact that the development of intersecting vertical or steeply inclined cracks is possible only in the case of horizontal non-rotational compressive stresses, if the lateral resistance is less than the vertical one (17, 39). Consequently, if vertical diagonal cracks intersecting in plan are to be explained on the basis of the stress ellipsoid, it must be assumed that the lateral pressure is greater than the gravitational pressure, and that the latter, in turn, is greater than the third pressure vector, situated in the horizontal direction, perpendicular to the greatest stress. In the direction of the third vector there must also lie the greatest axis of the ellipsoid, corresponding to the direction of the linear strike and the \(S\) cracks (Fig. 45). This is the case of the normal relation of forces adopted in all the principal arguments of H. Cloos. However, according to H. Cloos the diagonal cracks ...

intersect not only in plan, but also in section, which H. Cloos explains by the simultaneous action of a load, similar in its manifestation to the action of lateral pressure, i.e., causing the formation of a pair of fractures situated at \(45^\circ\) to the pressure of the load (14, 31), (Fig. 44).

Starting from the ellipsoid of elasticity, the origin of fractures intersecting in section can be conceived from two points of view. First, this will occur in the case where the pressure of the load is greater than the lateral pressure and, as a result, the ellipsoid will be arranged as shown in Fig. 46. Second, it will occur in the case where the pressure of the load is less than both horizontal pressure vectors, i.e., when the greatest axis of the ellipsoid is vertical, as shown in Fig. 43. However, the origin of diagonal fractures intersecting not only in plan, but also in section is regarded by

Fig. 44. Formation of diagonal fractures (after H. Cloos). a—diagonal fractures formed only as a result of horizontal pressure (stress)—\(D\); b—diagonal fractures arising only from the pressure of the load \(C\); c—diagonal fractures arising from the combined action of lateral pressure and the pressure of the load.

Fig. 45. Position of the stress ellipsoid for the case when extension takes place in a horizontal direction, perpendicular to the stress \(D\). Above—section; below—plan. Symbols as in Fig. 43.

H. Cloos as a simultaneous act, which, starting from the idea of the ellipsoid of elasticity, finds no explanation, since in this case the same axes of the ellipsoid, in direction, must simultaneously have different magnitude and sign.

But besides the diagonal planes there are also the \(Q\) and \(S\) planes—longitudinal and transverse to the principal lateral pressure—which experimentally, for the case of non-rotational stress, are not confirmed. Proceeding from the characteristics of \(Q\) fractures (long, even, used by veins and displacements), H. Cloos believes that these fractures arise in the direction of pressure, being analogous to the pair of Luders–Mohr fractures known in technology, but under geological conditions with an angle between the individual direc-

the same pairs, often close to \(0^\circ\). Consequently, he considers that the \(Q\) fractures are shear fractures and at the same time tension fractures, since they are arranged perpendicular to the direction of tension (the strike) and, for this reason, are most readily filled by veins. As for the \(S\) fractures, H. Cloos tends to regard them as a manifestation of schistosity that arose along planes of mechanical anisotropy—along the linear strike. Strictly speaking, he treats the \(L\) fractures in a similar way; in his opinion, they arise perpendicular to the gravitational pressure in the direction of the planar strike. But at the same time the \(L\) fractures are situated in the direction of lateral pressure, which makes it possible to regard them, by analogy with the \(Q\) fractures, on the one hand, as shear fractures (displacements are often observed along \(L\)), and, on the other hand, as tension fractures, since they are often filled with aplite and quartz veins.

Fig. 46. Position of the stress ellipsoid for the case when tension occurs in the horizontal direction, owing to the action of load \(C\). Above—section; below—plan. The designations are the same as in Fig. 43.

If one proceeds from the ellipsoid of elasticity, the formation of tension fractures in a direction perpendicular to its greatest axis is theoretically quite possible (see below); but we have already indicated that the application to H. Cloos’s arguments of the concept of the ellipsoid of elasticity encounters a whole series of obstacles, since it is impossible to determine definitely how this ellipsoid is situated in space. And without resolving this question, H. Cloos’s entire genetic classification of fractures loses its meaning, for the main thing that H. Cloos’s analysis of fracture tectonics gives, in accordance with his classification, is the determination of the vectors of forces that acted on the pluton during its emplacement. The question of the expediency of Cloos’s classification of fractures becomes still more complicated if we pass from the non-rotational stresses, from which H. Cloos proceeds in his arguments, to the case of rotational stresses.

At the present time most tectonists incline to the conviction that in nature pure non-rotational stresses occur rarely, and usually act together with rotational stresses, which sometimes have a dominant significance. Therefore let us consider how fractures form under conditions of rotational stresses, i.e., stresses caused by the action of a couple of forces directed, as in scissors, in opposite directions.

Fig. 47. Stress ellipsoid under shearing forces (shown by arrows). The directions of the acting forces do not coincide with the axes of the ellipsoid. The designations are the same as in Fig. 43.

If in non-rotational stresses the stress and the stress field are in a direct relation to one another, i.e., the axes of the stress ellipsoid coincide

coincide with the vectors of the corresponding forces, then in the case of rotational stresses these relationships become considerably more complicated.

From Fig. 47 it is evident that under rotational stress the axis of greatest shortening does not coincide with the direction of the stress, while the planes of maximum shear are arranged in such a way that one of them almost coincides with the direction of greatest pressure, and the second is inclined to it at an angle close to \(90^\circ\). Thus, depending on whether the compression is rotational or non-rotational in character, or whether it represents a combination of the two, the angle between the planes of maximum shear and the axis of the applied stress may vary from \(90^\circ\) to \(0^\circ\) (17, 28).

At the same time, according to C. Lise, it is often very difficult, or even impossible, to establish whether the ellipsoid of strain is the consequence of rotational or non-rotational stress and, consequently, whether the fracture lies approximately at an angle of \(45^\circ\) to the principal pressure or at an angle considerably smaller or larger. Sometimes, in field observations of fractures, it is possible to determine the relationships between stress and strain, but in the majority of cases this does not appear possible (17, 31). “What we see in rocks,” says C. Lise, “is elongation and shortening, and usually we cannot even say whether to attribute them to the action of rotational or non-rotational stress” (17, 16). This remark also applies wholly to the case of twisting, when mutually intersecting fractures are formed at \(45^\circ\) to the axis of torsion (as in Daubrée’s experiments with a narrow glass plate), for here too these fractures may be subjected to analysis in terms of those limited relationships between stresses and strains which reduce to extension, to compression without rotation, or to pure shortening, or to compression with rotation, or, finally, to shear.

Fig. 48. Cracks in the paraffin layer of a rubber plate produced by shearing. The arrows indicate the direction of movement, and the form of the plate the degree of curvature (after Mead).

Thus, all the principal types of deformation give identical pairs of shear fractures, so that on the basis of the mere presence of these pairs it is still impossible to say as the result of which deformation they arose, and, consequently, it is impossible to establish with certainty the direction and character of the acting forces.

Under experimental conditions, horizontal shearing movement along parallel vertical planes gives rise to four systems of mutually intersecting cracks. The different systems appear in more or less rapid succession, overlapping one another. V. Mead describes the appearance of cracks in the paraffin layer of a rubber plate produced by shearing as follows (75, 512–513, Fig. 48).

“The first disruptions, forming in any one place of the tested rubber plate, usually consist of tension cracks inclined approximately at an angle of \(45^\circ\) to the direction of the shearing movement. They pass at right angles to the direction of maximum elongation and have the appearance of vertical open cracks. After them

directly follow two series of vertical disjunctives with horizontal displacement, one series running parallel to the direction of movement, and the other parallel to the free edges of the rubber plate. The latter represent two directions of absence of curvature, or two planes of shear formed by shearing in which the direction of least resistance lies in the plane of the paraffin layer. The other series of disjunctives (only two of them are visible in Fig. 48¹) are pressure disjunctives, extending approximately at right angles to the tension crack and dipping at about an angle of 45° in both directions. They owe their origin to compression acting at right angles to the direction of maximum extension.

In these experiments we should be especially interested in the tension cracks and the pressure disjunctives (hust fault), situated perpendicular to the principal axes of the ellipsoid of elasticity, i.e., perpendicular to the directions of greatest extension and greatest compression.

It is known that under extension, as under compression, planes of maximum shear develop, situated at 45° to the direction of the tensile force (13, 25—26), i.e., the same Mohr cracks, or the planes of circular sections of the ellipsoid of elasticity, develop. However, these shear cracks practically usually do not appear, because rocks, owing to their slight capacity for extension, break more readily by tension than they are cut along inclined planes. Sometimes, however, cracks form not perpendicular to the tension, but along planes of shear, i.e., at 45° to it. In the experiment cited, the cracks arose perpendicular to the direction of greatest tension, i.e., they developed as fracture cracks.

The pressure disjunctives arose perpendicular to the tension cracks, with a dip of 45° to the vector of greatest compression, i.e., they arose as shear cracks along directions independent of the directions of shear in the stress ellipsoid common to the whole plate being tested. In other words, in this stress ellipsoid, conditioned by rotational stress, there arose, as it were, a second stress ellipsoid caused by non-rotational stress, derived from the rotational stress and situated at 45° to it—in the direction of maximum compression of the principal ellipsoid. Already from this experiment follows the insufficiency of the idea of a single stress ellipsoid for explaining cracks arising here simultaneously. But when we pass from experimental conditions to natural systems of simultaneously arising cracks, this insufficiency presents itself to us with full obviousness. In order to explain, by means of the stress ellipsoid, steep, inclined, and gently dipping cracks that form the framework of fracture tectonics, we shall have to invoke not one stress ellipsoid, but a whole group of stress ellipsoids oriented in different ways. Analysis of this kind will be further complicated by the fact that individual systems of cracks consist, as observations show, not of one group of parallel cracks, but of pairs of them, or, better said, of bundles of cracks intersecting one another at acute angles (see the chapter on systems of cracks in intrusions). However, such an analysis encounters fundamental objections, since the usual vectorial representations of deforming forces, from which the concept of the stress ellipsoid is derived, do not permit the simultaneous application, for explaining cracks, of several stress ellipsoids oriented in different ways. Therefore, we must say that, insofar as the factual material

cannot be satisfactorily explained with the aid of the mechanical concepts at our disposal; evidently, these concepts are inadequate and require further development.

From all that has been said it follows that the mechanical principles of H. Cloos’s classification of fractures, as well as the morphological features of these fractures, prove to be highly confused, as a result of which the whole genetic classification of H. Cloos’s fractures acquires an artificial character, i.e., loses the practical value that is ordinarily expected of it. Evidently this classification must be replaced by some other classification, constructed on new theoretical mechanical premises, which still require development.

CONCLUSION

The intrusive process is one of the most complex and, at the same time, least deciphered geological phenomena. Its analysis is accessible to us on the basis of the study of the material composition of masses that are in one way or another connected with the intrusive act, and of the structures of these masses, which are derived from the content of the magmatic form of tectogenesis. If in the field of the study of material composition quite a great deal has been done, then in the field of the study of structures only the first attempts have been made to approach the subject in a primitive form, proceeding from mechanical principles alone. Thus, the question of the tectonics of intrusions is one of the little-studied and, at the same time, very large questions of modern theoretical and practical geology.

If in the old structural schools, chiefly in the school of H. Cloos, prototectonics and fracture tectonics of intrusions were regarded as different kinds of deformation, as a mechanical reaction to external mechanical forces, whose features are determined by the time-varying mechanical properties of the material being deformed (liquid—solid body), then the new theoretical thought, based on the methodology of dialectical materialism and drawing its ideas from the new broad achievements of geological and natural sciences in general, and from objectively selected factual materials, cannot follow the old mechanistic path. This follows unquestionably not only from a methodological analysis of the essence of the old structural schools, but also from a comparison of the theoretical constructions of these schools not only with new, but even with old materials presented in objective illumination.

New theoretical generalizations must approach the analysis of prototectonics and fracture tectonics of erupted bodies with different measures. If prototectonics is the result and indicator of the self-development of the intrusive process, manifesting itself under conditions of a definite historical setting, one of whose important elements is external oriented mechanical stresses, then fracture tectonics is a manifestation of the mechanical reaction of solid material to external oriented forces, common both to the intrusion and to its broader surroundings. In other words, fracture tectonics and prototectonics are not connected by a commonality of leading causes.

Prototectonics is the key to understanding the complex energetic processes that determine the development of the volcanic phenomenon and are determined by it, and this exceptional property of prototectonics should attract the closest attention of theoreticians. In addition, prototectonics

is the key to distinguishing pulsations and phases within a volcanic act and the key to determining the form of plutons and their position in space, which cannot be determined with the aid of fracture tectonics, contrary to a widespread misconception.

On the other hand, fracture tectonics is the key to understanding the structures of vein deposits, and in this respect it should become the subject of detailed study by geologists working on the deciphering of such structures. In addition, fracture tectonics is a phenomenon whose detailed analysis clearly reveals the inadequacy of existing theoretical concepts in the field of mechanics, and creates fertile ground for new theoretical inquiries in this area.

In short, the problem of the tectonics of intrusions is a problem that is of profound interest to representatives of various branches of the natural sciences and, first and foremost, to geologists, who must take it up closely and on a broad scale.

The mass accumulation of new materials in the field of the prototectonics of igneous bodies and fracture tectonics, the organization of numerous experiments to reveal polarizing processes in crystallizing magma, and the search for new theoretical paths for identifying the general laws of evolution of structures caused by the evolution of the volcanic process—this is what, above all, I should like to call geologists to do in concluding my work, which represents an attempt, albeit a very imperfect one, at a new approach to an old theme.

Tomsk, 1938–1939.

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