Approaches to the study of deformations in permafrost soils

Approaches to the study of deformations in permafrost soils

Introduction

Most of the frozen strata forming the cryolithozone retains its primary cryogenic structure unchanged from formation to destruction. This makes it possible to analyze the conditions of ice formation and its accompanying processes quite convincingly. Based on this analysis, all genetic classifications of underground ice have been constructed and ideas about the paleogeographic conditions of their formations in certain territories have been formulated. At the same time, there are numerous local cryogenic formations with traces of dynamic processes deforming the primary cryogenic structure of the host frozen rocks.  A distinctive feature of these formations is the presence of numerous plastic and discontinuous deformations, as well as signs of structural restructuring of the primary cryogenic structure. They are widespread in frozen rocks of all ages and all genetic types. Nevertheless, the study of deformation zones and structural changes in the formed frozen rocks, as well as the understanding of their role and significance in the overall set of cryogenic processes is clearly insufficient. This is due to a number of reasons. For frozen rocks, the conditions for the occurrence of local zones of increased intra-soil pressure are not fully understood. Ideas about the mechanisms of the implementation of pressure processes are poorly developed. The systematization of deformations of the cryogenic structure and structural changes of ice and frozen rocks has not been carried out. Considering ice as a mineral or a rock, one cannot limit oneself only to the formation of its primary structure. Under certain temperature and baric conditions, the initially formed ice structure undergoes significant changes.  In this paper, a group of dislocated permafrost rocks and metamorphosed underground ice forming local cryogenic geosystems is considered: heave mounds, formation ice, injection ice, re-vein ice, etc. The authors proceed from the assumption that deformations or structural changes of frozen rocks are the result of the interaction of a complex of intra-soil processes. Zones of increased pressure in frozen rocks can be formed as a result of various reasons, including: cryogenic concentration of water and gas, temperature deformations, decomposition of gas hydrates with increasing temperature or pressure relief, gravitational displacement of soils, intake of pressure gases and groundwater from sub-frozen horizons. The pressure can be transmitted through the lithogenic component of the rock, as well as through the liquid and gas components. Pressure redistribution can occur in the form of a volumetric impact of a rock fragment on the host thickness, or through individual large cracks, or in the form of many small cracks (channels) through which liquid or gas fluids permeate the soil massif, leaving the primary structure unchanged. The article considers examples of studying deformations of ice, mineral and gas components of frozen rocks in cryogenic geosystems of various levels. The theoretical basis of the proposed approach is the provisions developed within the framework of frozen soil mechanics (N. A. Tsytovich, S. S. Vyalov, Yu. K. Zaretsky, G. V. Porkhaev, etc.) and structural ice science (P. N. Shumsky, V. I. Solomatin, etc.).

Theoretical aspects of studying deformations of frozen rocks and ice.

Many researchers have noted that cryolithogenesis is not limited to the act of phase transformations of water into ice, but includes various transformations of ice and ice-ground structures after their formation. Therefore, when studying the history of the development of cryogenic rocks, in addition to analyzing the patterns of crystallization of groundwater, the processes of metamorphism of already formed ice should also be considered. P. A. Shumsky for the first time considered various types of deformations of frozen rocks.  He noted that during the formation of injection mounds of heaving, under the influence of increasing pressure of freezing water, the roof of frozen rocks bends. Sometimes cracks form in the roof, which can regenerate over time. At the lateral surface of the re-vein ice, they identified a zone that, under the influence of increasing lateral pressure, crumples into folds and breaks into cracks into a number of blocks experiencing displacement. In his genetic classification, ice rocks were divided into three groups: 1. congelation ice (constitutional ice, including cement ice, segregation ice, injection ice, vein ice); 2. sedimentary ice; 3. metamorphic ice.^[1]. The author stated that congelation and sedimentary ice after its formations are always subject to change (metamorphism) to one degree or another. At the same time, the deformed frozen rocks considered by him were not included in the group of metamorphic ice (only glacial ice was included there).  This significantly impoverished the classification of underground ice, reducing it to three main groups: re-vein, formed as a result of repeated ice filling of frost-breaking cracks; segregation, associated with pulling water to the crystallization layer and injection, associated with freezing embedded under the pressure of water or water-saturated soil. The petrogenetic classification of underground ice rocks proposed by V. I. Solomatin practically repeats the classification of P.A. Shumsky. In it, all the underground ice is divided into three groups. The first group is congelation ice, their structure is determined by the conditions of in-ground crystallization of free and bound water. The second group is metamorphosed ice (vein and segregation–segregation ice exposed to dynamometamorphism). These are ice formations with traces of plastic deformations, ice flows, in the formation of the structure of which recrystallization processes under the influence of external influences play an essential role. The third group is sedimentogenic (buried) ice, the formation of which occurs in subaerial conditions with subsequent overlap and preservation by sedimentary rocks ^{[2, 3]}. It is important that dislocated ice-ground formations with traces of dynamometorphism were separated from injectable ice.  At the same time, the dynamometamorphism of underground ice itself is associated only with re-vein ice, in which the transformation of the structure of elementary veins under the influence of systematically occurring stresses plays a major role ^[3]. Sh.Sh. Hasanov, studying the metamorphism of gas inclusions of re-vein ice (PGL), came to the important conclusion that underground ice, being at negative temperatures, are subjected to significant textural and structural rearrangement and deformations. These processes develop especially intensively when the temperature of frozen rocks increases under load conditions. Under these conditions, a group of intra-ground ice is formed with traces of plastic deformations and ice flow. He suggested that deformed ice soils may be widespread in the permafrost zone and this should be taken into account in the course of cryolithological studies. To do this, it is necessary to develop an adequate model of the processes of deformation of frozen soils, using the experience and achievements of glaciology in solving such problems ^[4]. At the same time, no classification features were proposed to distinguish the group of deformed ice. E. A. Vtyurina and B. I. Vtyurin noted in the fundamental work on ice formation in rocks that underground ice is characterized by various processes of metamorphic ice formation. The main sources of these processes are either the energy of the ice rock itself, or external influence on it, and more often a combination of them. In the classification of metamorphism processes of frozen rocks, dynamometamorphism is isolated into a separate group, but it is not reflected in the genetic classification of types of ice formation in rocks ^[5]. A review of publications shows that many researchers pointed out the importance of the problem of transformation of formed underground ice in theoretical and practical terms, but did not offer directions for its solution. To do this, first of all, it is necessary to find out how stable the state of Stress that causes deformations in frozen rocks and underground ice is associated with many processes: external dynamic influence, temperature fluctuations of frozen rocks, their own weight, the emergence of zones of increased pressure of intra-ground gas, groundwater pressure in the frozen ground massifs of the formed frozen rock, which the conditions transfer it from a stable state to a dynamic one, and often without phase transitions and what kind of stress processes this is accompanied by. Understanding these issues is also important for solving applied problems related to maintaining the stability of engineering structures located in the permafrost zone.  The mechanical processes in frozen rocks are the processes of their deformation and destruction, including the displacement of mineral particles, as well as partial or complete destruction of ice crystals, its recrystallization and even flow. Rheological processes in frozen rocks, unlike other solids and unfrozen rocks, originate and develop under almost any (even very small) loads. The presence of a stress drop in frozen rocks causes the movement of unfrozen water and ice in them from areas of increased to areas of reduced compressive stress or to areas with higher tensile or shear stresses. This is primarily due to the presence of ice inclusions in frozen rocks (in the form of ice-cement or ice layers), for which a load of any (arbitrarily small) magnitude causes plastic flows and reorientation of crystals ^[6]. The adhesion of cementation by ice is due to the connection between ice crystals and mineral particles. This connection is carried out not by direct contact of ice and mineral particles, but through a liquid film enveloping solid particles and ice crystals. This adhesion depends on the ice content, the area of its contact with mineral particles and the temperature of frozen soils. This structural bond is the least stable and under natural conditions continuously changes, in accordance with the temperature fluctuation of the frozen strata. With an increase in temperature, the adhesion of frozen soils decreases, with a decrease, on the contrary, it increases ^[7]. For ice, the magnitude of the critical shear stress at which its plastic flow occurs does not exceed 0.01 MPa. The limit of the long-term strength of ice during shear at a temperature of -0.4 ° C is no more than 0.2 MPa ^[6]. Even at low external pressures, stresses of up to hundreds of kg/cm2 can occur at the contacts of mineral particles and ice crystals. This causes the ice to melt and the resulting water to be squeezed into less stressed areas, where it freezes again. When sufficient stresses are reached, plastic deformations of ice occur and its squeezing from a more stressed zone of frozen rock into a less stressed one, due to the viscoplastic flow of ice, already without phase transitions. At the same time, gas and mineralized waters contained in the frozen rock are also squeezed out ^[7].  The ductile-viscous flow of frozen rocks is especially dangerous when their temperature rises to values close to the temperature of phase transitions. The stress corresponding to the beginning of fluidity for frozen sand at a temperature of -1.6 °C is about 0.2 MPa, and for frozen clay at a temperature of -1.9 °C is about 0.1 MPa ^[6].

During the compression of frozen rock, there is also a significant restructuring of the structure (into a finer-grained) of schlier and pore ice with melting of the sharp edges of ice crystals, as well as significant plastic-viscous flows of ice crystals and their aggregates. If the stresses in the ice do not exceed the limit of its elasticity, then recrystallization is the only way to adapt to forced deformations. Depending on the magnitude of stresses and temperature, recrystallization (sublimation or reduction) can be observed in the cryogenic textures of frozen rock or underground ice. In the presence of significant bending stresses in individual crystals, polygonization occurs, accompanied by disintegration into small crystals with a crystallographic orientation close to that of the primary crystal.    A further increase in the oriented voltage, at which an excess of the body's resistance to brittle fracture is achieved, leads to the crushing of crystals ^[8]. Three main mechanisms of ice deformation should be distinguished: 1 — ice flow with a slow shift parallel to the basic planes of crystals (without changing the ice structure); 2 - violation of the crystal lattice of ice due to molecular decay, recrystallization, intercrystalline shifts, chips; 3 - melting of ice at high operating stresses. A single crystal of ice is the simplest cryogenic system. Single crystals of ice from the point of view of plastic properties have a strongly pronounced anisotropy. The plastic properties of monocrystalline ice strongly depend on the direction of application of the load relative to the reference plane. P.I. Shumsky ^[1] describes the behavior of a single crystal of ice during bending. With a small load, a biaxis appears in the crystal. At the same time, the crystal itself remains within the same boundaries. When the load is eliminated (at temperatures ranging from -5 to -12 ° C), a fairly rapid relaxation occurs and the disappearance of the biaxis. The preservation of biaxiality after the removal of the load takes place only in cases where the bends cause the optical axes to diverge no more than 7-8 ° C. With further deformation, the curved part of the crystal breaks up into a series of blocks with more or less orientation close to each other without visible discontinuity, thus achieving a stable equilibrium that persists even after release from the load. Deformations occurring in an ice crystal can be explained using the theory of dislocations arising after the application of a load. It has been strictly proved that there are dislocations inside the ice that shift along the basic planes when an external load is applied. The dislocation displacement rate at a shear stress of about 0.1 MPa is approximately 0.5 microns/s (2 mm/h). If you increase the voltage, the speed will increase in proportion to it. Thus, a single dislocation moves quite slowly. However, as the deformation develops, the number of dislocations increases and, as a result, their insignificant displacements lead to the fact that macroscopic sliding along the basic planes becomes possible ^[9].

The structure of polycrystalline ice includes differently oriented crystals. Inside each crystal grain, if force is applied to it, dislocation movement will occur and sliding along the basic planes will begin to develop. However, the main optical axes, and hence the basic planes of different grains are oriented in different ways, so sliding along the basic planes in each crystalline grain is limited to neighboring grains and cannot develop as freely as in a single crystal. As a result, the plastic properties of polycrystalline ice are strongly dependent on the size of the crystalline grains forming it and on the orientation of their optical axes. The process of deformation of such a structure includes several stages ^[7]. At the first moment after the application of the load, elastic deformation of the crystals occurs, and in the places of the greatest stress concentration caused by packaging defects, crushing zones appear at the joints of differently oriented crystals, stress-free fragments appear due to chipping of the corners and edges of the crystals, microcracks develop. The orientation of cracks at this stage is chaotic. If the load reaches a critical value, crack growth becomes avalanche-like, trunk cracks are formed oriented in the direction of maximum tensile or shear stresses, and the process ends with brittle fracture. Prolonged small loads, acting for a long time, cause a creep process, during which a significant restructuring of the ice structure occurs.  Cracks either do not occur, or are localized and do not determine the macroscopic behavior of ice; the processes of gradual reorientation of crystals that occur uniformly throughout the volume, accompanied by molecular decay and recrystallization with a decrease in their average size, prevail. Due to anisotropy, crystals tend to flow along their basic planes, while the maximum flow velocity will be for crystals oriented in the direction of maximum shear stresses. The development of this flow is hindered by the different orientation of the crystals, which leads to the intersection of the slip bands, and the most unfavorably oriented crystals have the greatest resistance. Stress concentration occurs in the appropriate places, causing cracking, crushing and disintegration of crystals. With prolonged deformation, the processes of molecular decay and crushing lead to a significant decrease in the average size of crystals. At the same time, a recrystallization process takes place in the ice, the centers of which are unstressed fragments and less stressed crystals. In this case, the newly formed crystals are oriented by the basic planes along the direction of the shift. At sufficiently high loads, the process of microcracking is decisive, which determines the transition to a block mechanism of sliding along a system of cracks oriented according to maximum shear stresses. The structure of the main volume of crystals remains unchanged; the reorientation is confined to thin boundary sections. In the scheme proposed by P.A. Shumsky ^[10], seven mechanisms of ice deformation are distinguished, replacing each other depending on the nature and magnitude of the load and the rate of deformation: 1) sliding along the basic planes in individual crystals; 2) small changes in the crystal lattice, an increase in the average grain size in the aggregate and the movement of boundaries between them; 3) significant violations of the crystal lattice, polygonization and incipient recrystallization; 4) intracrystalline movements accompanied by grinding of ice grains and almost complete destruction of the original structure; 5) development of brittle fractures along certain planes; 6) mylonitization; 7) reduction. In I. Solomatin, the ratio of deformations of ice crystals under plastic and brittle deformations is considered (Table 1)

Table 1 Types of deformation of ice crystals ^[2].

The considered materials on the study of deformations of frozen rocks show that by now the ideas that underground ice can undergo significant changes, and the recrystallization of ice during its metamorphism is an important element of cryolithogenesis, have been developed in sufficient detail. At the same time, the degree of study of metamorphism processes in the transformation of underground ice in real conditions, according to E. A. Vtyurina and B. I. Vtyurina, is so insufficient that all considerations on this issue are presumptive ^[5].

Formation of deformation structures in frozen rocks

            The materials discussed above show that in geocryology it is long overdue to assess the significance of deformation processes, as well as the role of deformed frozen rocks and underground ice in the general system of cryolithogenesis. To do this, it is necessary, first of all, to analyze the structural and textural deformations of various kinds of cryogenic formations. Their brief overview is given below.

Volumetric local deformations in permafrost massifs.

When exposed to an external load in frozen soils, a complex of processes leading to a plastic-viscous flow of frozen rocks and ice can begin to develop. These processes are accompanied by a violation of structural bonds and rearrangement of solid particles, resulting in irreversible structural deformations. This type of deformation occurs only when the tangential stresses exceed the forces of internal interaction that cause the equilibrium state of the frozen rock. In the zone of impact of the load, due to compaction, a core of rigidity is formed. When the load reaches the limit value and the greatest tangential stresses occur in the soil mass in the vicinity of the dense core, deformations of the plastically viscous flow begin to develop. At this stage, the compacted core begins to push apart the surrounding less dense soil and penetrate into it ^[7].In natural conditions, these processes are widespread enough. A previously formed ice body or a block of a soil massif can act as a compacted core (Fig. 1). Here is an anticline fold formed by a block of ice and a dislocated ice-ground mass embedded in layered coastal-marine sands and siltstones.  In the central part of the fold lies an elongated monolithic ice block. In the photo of the upper one. The parts of the Yamal crater (Fig. 2) clearly show how a strongly deformed rounded block of gas-saturated ice is pressed into the frozen layered thickness under the influence of a layered ice screw, tearing it with wedge-shaped cracks.

Fig.1. The introduction of a monolithic block of pure ice (1) and a dislocated ice-ground mass (1) into layered coastal-marine sands and siltstones (2) curved into an anticline fold (Yamal Peninsula) ^[11].

Fig.2. The upper part of the Yamal crater. The impact of an ice block on a layered ice-ground massif. Photo by A. Lupachev.

Figure 3 shows the ice-ground structure  with traces of deformations ^[12]. In the central part, an ice body with numerous dislocations can be distinguished. On the left and right, zones of layered ice ground with traces of plastic deformations, passing into deformed sand layers, adjoin it. In the upper part of the section, a peat horizon lies with a sharp disagreement. The nature of dislocations, plastic deformations and traces of ice and ice-ground flow indicates a long-term gradual effect of the ice rod on the ice-ground thickness that overlaps it and overlying precipitation. The ice rod pierces the overlapping rocks, pushing and deforming them. Subsequent thawing cuts off the protruding apical part of the deformed roof.

Fig. 3. Paragenesis of a subvertically layered ice core and deformed horizontally layered formation ice and overlapping sands in the valley of the Yerkutayakha River in the south of the Yamal Peninsula ^[12]. In all the cases considered, the formation of geodynamic zones with traces of the movement of large ice-ground blocks in the frozen rock massif is observed. In the terminology adopted in geocryology, these formations and the processes that form them are usually defined as injection, although there are no signs of injections of water or water-saturated soils. It is possible that the driving force of these cryogenic formations are pressure processes that occur either during the freezing of aquifers, or the impact of underground gases under high pressure. But even in this case, the frozen strata is deformed under the influence of local blocks of frozen rocks. At the same time, plastic and discontinuous deformations and ice flow are observed, both in ice and ice-ground blocks, and in the host frozen rocks. In most cases, the lower part of the folds cannot be detected. Therefore, the processes of their formation remain unclear. In some cases, local volumetric deformations of frozen rocks do not reach such intensity and manifest themselves in the form of gentle anticlinal folds. Near the Bovanenkovsky GCM, an anticline fold was found deforming a layered ice-ground massif (Fig. 4). It is not pronounced in the relief, since it was "cut off" by thermodenudation processes.

Fig. 4. Deformed layered ice-ground layer near the Bovanenkovsky GCM (photo by G. A. Rzhanitsyn) ^[13]

Deformations of frozen rocks under the influence of local water intrusion or water-saturated soils

Injectable cryogenic formations in frozen rocks are widespread and have been identified as a separate type of ice formation in almost all classifications. In the context of this article, injection processes are of interest primarily as a cause of deformations of previously formed ice. Let's consider them by the example of dislocations of formation ice and overlapping icy loams.These cryogenic formations were studied by us in the lake area. Here, on the Yamal Peninsula. Here, in some areas, there are ice-ground layered formations with traces of intense dynamic processes (Fig. 5), which deform the formation ice and the icy loams overlapping them ^{[14, 15]}.

Fig.5. Local layered ice-ground zones, breaking through the reservoir and deforming clay with mesh cryotextures. Photo. Yu. B. Badu

A complex of paragenetically connected ice formations is observed in the dislocation zones (Fig. 5).  These are strongly deformed, sharp contacts of layered and stratified ice. In the contact layer of the formation ice, the crystals are fragmented, numerous cracks are observed. At the upper contact of the layered ice-ground zone, deformation of the roof, loams and cryotextures overlapping the formation ice are observed. In those places where the ice-ground layers approach the interface of ice and soil at right angles, the deformation is greatest.  Elongated ground inclusions are observed in the deformation zone in the axial seams of vertical slots of mesh cryotextures. The partially destroyed part of the loam is immersed in layered ice soils. Such deformations can be observed in the case when a pressure sufficient for local destruction of the previously formed overlying ice layer occurs in the not yet frozen part of the water-saturated soil massif. Through the cracks formed, the water-saturated soil rises up and rests against a more durable array of permafrost-saturated loam. The effect of injection processes on the structure of previously formed ice is also discussed in the section on heaving mounds (see below).

Deformations of ice structures that occur during the formation of long-term heave mounds

The study of the structure of perennial heave mounds of segregation-injection genesis in the Salekhard region (north of Western Siberia) (Fig. 6, 7, 8, 9, 10), it showed the presence of a large number of various deformations associated with the complex history of the development of these formations. Figure 6 shows the structure of a layer of segregation ice formed in a multi-year bulge of heaving exposed to injection. Large crystals are broken by narrow crack zones, to which gas and ground inclusions are confined. Cleavage cracks are observed in the layers of pure ice. The freezing of embedded water injections is marked by elongated gas bubbles oriented perpendicular to the freezing front. Chains of teardrop-shaped air bubbles are confined to some channels.

Fig.6. The zone of crushing of previously formed ice by injections of water-saturated soil. Bulge of heaving in the area of Salekhard, depth 4.75 m. Photo by V. E. Gagarin.

The pressure exerted on ice crystals leads to the formation of crushing zones along the crystal boundaries and polygonization of large crystals. They are crushed by cracks into smaller crystals with a close orientation of the optical axes (Fig. 7).

Fig. 7. Cracks that break and deform large crystals, crushing zones are observed along the edges of the crystals. Bulge of heaving in the area

G. Salekhard depth 7.5m. Photo by V. E. Gagarin.

With more intense exposure, large crystals are crushed into separate blocks, between which there are crushing zones with smaller crystals, as well as numerous gas and ground inclusions (Fig. 8).

Fig. 8. Crushing of large crystals array 

(photo in polarized light). Bulge of heaving in the area

G. Salekhard, depth 8,6 m. Photo by V. E. Gagarin.

In the ice mass directly adjacent to the injection cracks, narrow (3-5 mm) zones of deformed ice are observed, formed due to stresses arising during injections and their subsequent freezing (Fig. 9).

Fig. 9. Deformation zones in ice adjacent to injection cracks,

dissecting an array of pure coarse-crystalline ice.

Bulge of heaving in the area of Salekhard, depth 9.9 m

Photo by V. E. Gagarin.

Plastic deformations formed when crystals are pressed into each other are widespread in the structure of primary ice structures. In the center of the photo (Fig. 10), a plastic deformation in the ice is observed, formed under the influence of pressure exerted by a wedge-shaped ice crystal (a dark wedge-shaped crystal). It is pressed into a block with a lighter color, in which a wide range of deformations is observed: polygonization, crushing, plastic changes, breaks inside the crystals and along their boundaries.

Fig. 10. Pressing ice blocks into each other.

Bulge of heaving in the area of Salekhard, depth of 8.4 m

Photo by V. E. Gagarin.

Deformations of frozen rocks under the influence of in-ground gases

According to the currently prevailing ideas, the cryolithozone is a screen that prevents underground gas from escaping to the surface. V.S. Yakushev,

having analyzed the huge material on the possibility of gas filtration in permafrost rocks, comes to the conclusion that the high-ice cover deposits of the cryolithozone (upper 40-50 m) are practically impervious to

gas, even coming under pressure from the depths. For permeable sand layers inside the cryolithozone, the critical value of the ice saturation of the pore space, at which gas filtration is feasible, can be taken as 50% of the pore volume. If the ice saturation of the pore space is greater than this value, then the rock becomes unsuitable for natural gas filtration, if less, one can expect

manifestations of migration and accumulation of natural gases in the free state ^{[16, 17]}. With all the persuasiveness of the arguments supporting this position, we believe that it is not entirely correct. When assessing the possibility of gas filtration, it is necessary to take into account not only their iciness, but also their structure, properties, as well as the ratio of temperatures and pressure inside the permafrost strata. It should be borne in mind that gas filtration is preceded and accompanied by significant intra-soil plastic and discontinuous deformations. Thus, the question of whether filtration occurs or does not occur in permafrost rocks should sound different. What thermobaric conditions provide filtration in specific landscape, geological and climatic conditions? When studying the funnels of gas emission, it was suggested that gas filtration occurs in the icy frozen thickness, under the pressure of gas fluids. In this case, an intensive restructuring of the original cryogenic structure occurs, accompanied by plastic and discontinuous deformations of the frozen substrate ^[18]. A paragenetic relationship is established between gas filtration and plastic deformations, which determines the self-development of the geosystem. Gas under pressure penetrates into the frozen rock, which significantly weakens its strength and causes plastic and rupture deformations. The cracks and dislocations that have appeared accelerate gas filtration ^[19]. A.N. Kurchatova and V.V. Rogov in studying the structure of frozen rocks in the southern part  In the samples of frozen soil and ice of the Tazovsky and Gydan peninsulas, signs of vertical jet migration of gas were found to be widespread, including through rocks traditionally considered impenetrable. Filtration can be carried out due to crack permeability with the development of shear deformations, which are diagnosed by cryogenic crack-type textures with the formation of a system of parallel inclined slots along the sliding planes (Fig. 11) ^{[20, 21]}.

Fig. 11. Shear deformations in ice sheet 1 – boundaries of ice crystals;

2 – the direction of the shift^[20].

In the course of laboratory experiments, it was possible to record gas filtration through ice and frozen soils, as well as to study some of the processes accompanying it. It is established that gas filtration in ice and soils occurs only at certain ratios of pressures and temperatures. At temperatures of -9 ° C in the range of gas pressures of 2-4 kg / cm2, at which experiments were carried out, filtration was not observed. Only when the temperature rose to values close to the region of phase transitions, it began to manifest itself. In ice, filtration occurs in the form of gas channels. Gas channels are chains of bubbles with a diameter of 1-2 mm. Along with the chains of bubbles, there are worm-like channels filled with gas 2 - 3 mm wide . Channels of complex structure have been identified, the lower part of which have a worm-like shape up to 2-3 mm thick, then passes into a crack cutting through the ice and above continue in the form of a chain of gas bubbles about 1 mm in diameter (Fig. 12). It is important to note that the boundaries frozen during the preparation of the sample layers are not an obstacle to the allocated channels, they move from layer to layer without breaks.

Fig. 12. A gas channel crossing the boundaries of the ice layers.

Shooting in passing light. Photo by A.N. Khimenkov ^[22].

During the laboratory study of gas filtration under pressure in ice, plastic deformations of crystals, wave-like boundaries, indentation of crystals into each other, cracks and crushing zones, chains of air inclusions at crystal contacts were observed (Fig. 13).

Fig. 13. Zone of crushing and plastic deformations.

Shooting in polarized light.  Photo by A.N. Khimenkov.

When passing gas under pressure through a sample of frozen soil, gas filtration was also observed. Let's consider this process by the example of gas filtration through a frozen sample with a layered cryotexture. The thickness of the ice slots is up to 1 mm, the thickness of the ground layers is from 1 to 2 – 3 mm. In the zone of layered cryotextures, a large number of branching, curved, subvertically oriented gas channels with a thickness of fractions of millimeters are observed (Fig. 14). The channels break the ice sheets and shift them without destroying their overall orientation. The connection of channels in layered textures with the gas supply zone in the frozen sample is traced. With all the variety of shapes, channels in all frozen samples form a single system that unevenly penetrates the samples from bottom to top.

Fig. 14. Filtration channels breaking through primary layered cryogenic

Textures. Photo in reflected light.

Photo by A.N. Khimenkov ^[23]

The conducted studies allow us to draw some preliminary conclusions. The gas supply under pressure leads to local deformations, through which disparate gas flows in the form of small gas channels (up to a millimeter in diameter) diverge from the center of the gas supply to the edge parts of the sample, spreading throughout the sample array. The movement of gas channels is represented in the form of chaotic oscillations corresponding to the choice of the most weakened zones in the frozen ground (Fig. 15).

15. Scattering of gas in a frozen ground sample supplied from a local source ^[23].

Deformations of frozen soils associated with the formation of gas discharge funnels

Gas discharge funnels are new forms of cryogenic formations, first recorded in 2014 on the Yamal Peninsula near the Bovanenkovsky gas field. To date, 17 similar objects have been discovered. In all of them, various kinds of deformations of frozen rocks were found, indicating a smooth increase in stresses in the period preceding the explosion. The most likely reason for the formation of funnels is the pressure effect of underground gas.  Methane was detected in 2 funnels in which the gas composition was analyzed. The methane content is 98.4-99.9% ^[24]. The presence of hydrocarbon gases is also evidenced by the cases of ignition accompanying explosions. Analysis of the cryogenic structure of the funnel walls shows that the explosions were directly preceded by a series of successive stages of development of frozen rocks, each of which differs by an individual set of processes and their inherent formations^[19].  Before a pneumatic explosion forming a funnel, a bulge of heaving develops on the surface. Figure 16 shows a section of such a hillock in the Yamal crater area. The violation of the primary layering is clearly visible. The highest point of the dome-shaped fold is confined to the center of the funnel.

Fig. 16. Dome-shaped deformation of the primary stratification over a gas-saturated ice-ground rod. Screenshot from the video "the mystery of the Yamal crater" on July 26, 2015 ^[25]. 

When studying the funnels of the gas ejection, specific formations were found, which are annular layered structures forming the walls of the crater

(fig. 17, 18) ^[19].  

Fig. 17. Yamal crater, top view. The shading indicates the direction of the subvertical layers in the zone adjacent to the crater walls, a solid line is the lower boundary of the active layer Photo by V. V. Olenchenko,

A study by E.I. Galeeva and co-authors showed that the layering is due to the viscoplastic flow of ice. Shear deformations lead to the formation of folds, secondary layering (cleavage) oriented at an angle of up to 60° to the horizontally lying primary layering ^[26].

Fig.18. The subvertical stratification of the ice of the ring structure bordering

Yamal crater July 2014 ^[27].

These materials indicate that local gas-saturated zones with increased gas pressure were formed in permafrost rocks, under the influence of which icy rocks were squeezed upwards, with the formation of heaving mounds.  After the limit of the long-term strength of the frozen roof was overcome, there was an explosion and the release of fragments of frozen rocks and ice at a distance of up to hundreds of meters ^[19].

Plastic-viscous flow of frozen soils on slopes

N. A. Tsitovich suggested that plastic-viscous flow, similar to those observed in glacial ice, is possible in the massifs of permafrost rocks located on the slopes ^[6]. It was confirmed in the works of Sh. Sh. Hasanov and N. N. Romanovsky, who described these formations for the conditions of slopes composed of high-ice rocks, where a fan-shaped slope of the upper parts of syngenetic re-vein ice is observed (Fig. 19) ^{[28, 29]}.

Fig. 19. Diagram of the deformation of the "ice complex" in the Alasna basin: I - ice complex, II – taberal deposits, III – deposits of thermokarst lakes, IV - landslide deposits, V – alasna deposits; 1 – syngenetic re-vein ice and the dusty sandy loam containing them;  2 – powdery sands with a massive cryogenic texture; 3 – powdery sandy loams with peat interlayers; 4 – sandy loams with malacofauna inclusions; 5 – sandy loams with turf inclusions; 6 – bone remains of mammoth fauna; 7 – MMP boundaries ^[29].

Kurums are formed on the rocky slopes – mobile stone formations ^[30]. The lower parts of some of them represent an ice-ground layer, plastic deformations of which lead to the movement of the kurum down the slope. D. O. Sergeev, who studied the kurums in Transbaikalia, identified kurum-glaciers formed in the lower parts of the slopes as a result of plastic deformations of the ice-ground layer. The surface of these formations is complicated by crescent-shaped shafts. He recorded an average long-term (over 20 years) the speed of movement of the kuromo-glacier is 14 cm/year ^[31].

   A.I. Popov identified a special type of frozen rocks associated with the mixing of flows of subaqueous marine sediments on the underwater slopes of the shelves of the Arctic seas ^[32]. Under these conditions, precipitation accumulates at subzero temperatures. When landslides occur, thixotropic liquefaction occurs in slightly polymerized marine sediments with the release of free water. Freezing of the released water forms ice sluices with traces of plastic deformations. Plicative dislocations can be traced for tens and hundreds of meters along the stretch and up to hundreds of meters vertically. Their sizes are different:  from small ones - from 0.5 – 1 m long and of the same order in height to large ones - 100 - 200 m long or more and several tens of meters high. It should be noted that cryogenic formations are formed in the sediments of the Actic Shelf even without the dynamic impact of landslides ^[33], and landslide processes only deform cryotextures that have already formed.

Deformations of frozen soils and ice associated with the formation of

re-vein ice

Re-vein ice is formed as a result of repeated ice filling that occurs in frozen rocks of frost-breaking cracks. Such ice is characterized by the presence of vertical and inclined banding. The structure of vein ice is allotriomorphic-granular, lamellar and hypidiomorphic-granular. The orientation of the main optical axes is most often chaotic, in rare cases ordered - parallel to the heat flow, which has a subhorizontal direction. The sizes of ice crystals naturally decrease in veins from top to bottom (Fig. 20) ^[34].

Fig. 20. Texture and structure of polygonal-vein ice (photo: Y.V. Tikhonravova, 2018): 1– elementary veins with an axial seam;  2–recrystallized elementary veins ^[34].

The leading deformation mechanism of vein formation is the annual formation of frost–breaking cracks, forms a complex cryogenic geosystem, including: the vein itself, consisting of many elementary deformed veins and zones of recrystallized ice; a cutting edge along the edges of vertical walls corresponding to the zone of ice flow under pressure, lenses of transparent ice overlapping the vein, the formation of which is associated with migration processes. Directly adjacent to the walls of the vein is a zone of deformed displacing deposits with traces of plastic (Fig. 21, 22, 23) and discontinuous deformations.  The listed set of parageneses may vary depending on the composition of the host sediments, their temperature, and iciness. Some elements may be weakly expressed or completely absent. The leading process of the formation of parageneses of PZHL is the annual frost-breaking cracking, the flow of water into the crack and its freezing. The increasing internal pressure leads to a significant recrystallization of ice, and dynamometamorphism associated, in some cases, with the squeezing of the conjugation ice that filled the frost-breaking cracks ^[2]. At the same time, bands of ice are formed. In other cases, frozen rock adjacent to the vein is squeezed out, which at the same time crumples into folds, and sometimes breaks into cracks into a number of blocks experiencing mutual displacement (Fig. 21, 22) ^[15].

Fig. 21. Squeezing deformations near the lateral contacts of growing ice veins (Yana River). 1 – squeezing rollers; 2 – material squeezed from below and subjected to deformation in the conditions of the active layer; 3 – silted peat; 4 – loam; 5 – layering of sand and siltstone; 6 – cutting edge; 7 – lens of transparent ice, devoid of signs of vertical banding; 8 – turf; 9 – permafrost roof; 10 – a young sprout of an ice vein; 11 – an ice vein ^[35].

Fig. 22. Vertical cross-section of the contact of the PPL with the host frozen rock. On the right - ice, on the left – frozen rock Photo by B.I. Vtyurin ^[1].

Fig. 23. Crumpling of the host rocks at the contact with the re-ice vein. Photo by E.M. Katasonov ^[36].

Discussion

The materials discussed above show that various kinds of violations of the formed cryogenic structure are widespread in frozen rocks. Such violations can be traced both in individual ice crystals and in violations of primary cryotextures. Deformations in frozen rock massifs are not only superimposed structures that change their primary structure. In some cases, they are the main element forming integral local cryogenic geosystems. For example, re-vein ice begins to form with temperature breaks in frozen rock massifs. Repeating annually, this mechanism causes the further growth of vein ice and the development of many deformation parageneses associated with it. The growth of heaving mounds is carried out only with the deformation of the roof, as well as with the development of chipping deformations at the contact of the forming ice core and the host array of frozen rocks. Gas filtration in frozen rocks is impossible without the formation of a series of local ruptures, cracks, and shear deformations. The formation of glaciers, kurums and stone glaciers is impossible without plastic and discontinuous deformations that ensure the displacement of these cryogenic geosystems along the slope. The formation of local ice-ground formations with traces of flow deforming the host frozen rocks is caused by plastic deformations resulting from the appearance of foci of increased pressure in the soil massif.

Let 's list some of the reasons for the formation of deformations in frozen rocks:

- the unevenness of freezing causes the formation of cryogenic pressure and lateral movement of permafrost waters and gases to the area of lower pressures (the formation of seasonal and perennial heave mounds);

- freezing of closed taliks and occurrence of zones of high intra-soil pressure;

- narrowing of the cross-section of the intra-soil water flow during its freezing with a simultaneous increase in pressure ;

- the occurrence of increased pressures under the frozen roof over the zones of ascending intra-ground water flows;

- an increase in temperature and a decrease in the strength properties of icy frozen rocks lying on the slopes (current displacement of frozen rocks along the slope);

- the development of temperature ruptures of frozen rocks causing the formation of re-vein ice, increased pressure occurs in the growing vein array, deforming the ice of the vein itself and the frozen rock near it;

- formation of areas of thawing of frozen soils under surface reservoirs. An area of high-temperature permafrost is formed, embedded in an array of low-temperature frozen rocks. An unstable state similar to the state of slopes occurs at their boundary (intra-soil convective flows may occur);

- decomposition of gas hydrates in the area of thawing of frozen rocks under surface reservoirs. Creation of overpressure frozen rocks in the field of dissociation of gas hydrate-containing rocks;

- formation of cryogenic pressure of biogenic gas in freezing rocks, its lateral redistribution and accumulation in local areas;

- formation of abnormal gas pressures under permafrost rocks due to its intake from the underlying horizons.

With all the variety of causes and manifestations of the development of deformities, some common signs can be traced. When the intra-soil pressure increases to a certain value, no matter what reason it is caused, local shear plastic deformations begin to develop, breaking the frozen massif into separate blocks separated by narrow zones of increased fracturing. This, in turn, leads to a decrease in strength and further development of the process of deformation of the array and redistribution of matter in solid, liquid or gaseous form.

Currently, in geocryology, when analyzing problems related to the identification of the history of the development of frozen rocks, first of all, the processes of ice formation are considered. Based on the study of the structural and textural features of the formed cryotextures, a conclusion is made about the origin of underground ice and the conditions of its formation. The observed deformations of the cryogenic structure are considered as an imposed complementary factor and are not taken into account when isolating the genetic type of ice. Since the bulk of the underground ice falls into the category of segregation, injection or vein ice, the processes that form them are reduced to these three main types. The migration of bound water to the freezing front, the introduction of pressure water into the ice formation zone and the formation of cracks during sudden cooling of frozen rocks. If such an approach can be applied to identify the processes and conditions for the formation of the primary cryogenic structure of frozen rocks, then it is not very informative with regard to processes affecting already formed cryogenic formations. The materials considered show that the analysis of deformations of ice formations may be more productive, followed by the identification of processes and conditions causing them. The second approach is more objective, it is based both on fixing the parameters of the primary structure and on taking into account its changes during deformation. At the same time, it is possible to measure the morphological parameters of the deformations themselves, which eliminates subjectivity in studies of changes in the structure of already formed frozen rocks. Identification of the processes forming deformations is facilitated, since it is possible to use data from the analysis of the structure and morphology of ice formations, ground and gas inclusions, as well as knowledge about the strength and deformation characteristics of frozen rocks. V. I. Solomatin highlighted the sequence of changes in the structural mechanisms of dynamometamorphism of ice, based on taking into account changes in the magnitude and speed of the load. These mechanisms include: plastic mechanisms (curvature of crystal faces and boundaries, twinning, sliding lines), recrystallization (local, volumetric, sublimation, reduction), brittle mechanisms (polygonization, intracrystalline cracks) ^[3]. By extrapolating this conclusion to frozen soils as a whole, it is possible to distinguish a series of changes in frozen rocks reflecting the change of deformation processes against the background of increasing load.

1. External changes have not yet manifested themselves, but internal connections are weakening, polygonization is observed inside crystals, mylonitization at crystal contacts.

2. The primary structure as a whole is preserved, while individual micro-discontinuities of continuity without displacement are observed.

3. Violation of the primary stratification of rocks in the zones of occurrence of elevated pressures.

4. Minor displacements of individual fragments of the cryogenic structure are recorded, but the structure as a whole remains

5. There is local mixing in the form of separate zones, within which there is a complete restructuring of the primary structure.

6. Volumetric flow layered ice-ground structures are formed, within which individual blocks with a primary structure can be observed.

At each stage, structures and processes corresponding to earlier stages can be observed.

The transition of a cryogenic geosystem or its parts from one state to another is accompanied by a complete or partial restructuring of its structural elements. It is possible to understand the patterns of development of a specific cryogenic geosystem only by determining its initial structure, identifying intermediate stages and evaluating the final state as a result of all previous transformations. This is possible because the elements of cryogenic structures contain a complete record of events reflecting both the stages of formation and the stages of restructuring of cryogenic geosystems. Given the role of metamorphosed ice among other types of cryogenic formations, it is necessary to somehow introduce them into existing genetic classifications. The importance of this type of ice was previously noted by many researchers, nevertheless, they are not explicitly expressed when studying the history of the development of frozen rocks. It is absolutely necessary to supplement the existing types of ice with another one - "metamorphosed ice", with its division into subtypes according to the degree of metamorphosis.  This approach would allow us to get out of the terminological uncertainty, in which texture-forming ice is divided into only two main types: segregation and injection. At the same time, all specialists involved in the study of underground ice understand that a significant number of cryogenic formations with traces of dynamic processes are associated not with injections of water or water-saturated soil, but with dynamometamorphism of previously formed cryotextures or intra-ground ice arrays.

Polygenetic and stage-by-stage processes of deformation of frozen rocks necessitates the development of an appropriate research method, which we define as structural deformation analysis. Structural and deformation analysis is based on the identification of the sequence of structural and textural changes in the cryogenic structure and the spatial distribution of these changes when stresses occur in the permafrost thickness.  The development and application of this analysis will make it possible to determine the intensity, direction and sequence of the processes of structural restructuring of frozen rocks and ice, to identify the magnitude of emerging pressures, the rate of their change, to assess the scale and morphology of the deformation zone (plastic and discontinuous)  and structural transformations, mechanisms of their development.  This will make it possible to identify the causes of the emergence of local dynamic cryogenic geosystems and restore the history of their development. The following sequence of structural and deformation analysis can be proposed.

At the first stage, the inhomogeneities of the cryogenic structure are fixed. Primary cryogenic textures are distinguished, spatial changes in their morphology are revealed. Then secondary changes in structural and textural features are determined, neoplasms and their interaction with primary cryogenic formations are analyzed. When identifying repeated transformations of primary and superimposed cryotextures, the number and sequence of transformations are revealed.

At the second stage, the stages of development of the cryogenic structure are distinguished at the local level and in the volume of the geological space available for study. For this purpose, the connections of the primary cryogenic structure with the subsequent superimposed one are analyzed and highlighted, the variability and stability of structural and textural changes are assessed, functional connections between the change of states at the local level and the spatial structural organization of the geosystem as a whole are studied.

At the third stage, groups of processes corresponding to the previously identified stages of cryogenic structure change are identified. At the same time, it is important to determine which process triggers the development of a geodynamic cryogenic geosystem and how great its role is at various stages of its development.

At the fourth stage, a scenario for the development of a geodynamic cryogenic geosystem is being developed, for which: data obtained during the study of the structure of the cryogenic strata and various variants of external influences, as well as internal transformations determining the sequence of stages of the development of the cryogenic structure are being compared; data on mineralogical, granulometric composition of rocks, mineralization of pore waters, physical and mechanical properties, temperature regime , etc .

Conclusions.

Changes in the structure of frozen rocks continue after the formation of their primary structure. Data on deformations of the primary cryogenic structure provide information about the history of the development of cryogenic geosystems that have already been formed.

Deformations in frozen rock massifs are not only violations of their primary structure, they often accompany and even are a necessary element of the formation of certain local cryogenic geosystems.

Stresses causing deformations in frozen rocks and underground ice are associated with many processes: external dynamic influence, temperature fluctuations of frozen rocks, their own weight, the emergence of zones of increased pressure of intra-ground gas, groundwater pressure in freezing ground massifs

The widespread occurrence of metamorphosed ice has previously been noted by many researchers, however, as a classification type, they are not explicitly identified when studying the history of the development of frozen rocks.

It is necessary to develop a classification of textural and structural deformations of the cryogenic structure of frozen rocks, in which a special type should be distinguished - metamorphosed ice formations.

It is necessary to develop methods of structural and deformation analysis that allow establishing links between the observed deformations of the cryogenic structure and the processes occurring in frozen rocks.