• Article close
• Geodinamika i Tektonofizika close

Introduction. In this publication, shear zones, being traditional objects of tectonophysical studies, are considered in terms of their strain states. This approach differs from a commonly applied one when shear zones are studied with consideration of stress fields. The difference of a stress field and a field of strain for a simple shearing has been already noted by the researchers (Figure 1). As is known, secondary fractures in natural shear zones and in experiments do not always correspond to structures which are theoretically predicted by stress field studies. The problem under investigation in this publication is which combinations of secondary structures are possible/impossible in specific emerging strain fields? Initial concept. The theoretical basis is the well­known scheme of secondary fractures proposed by P. Hancock [1985]. His representation of combinations of structures (Figure 2) is arbitrarily compiled: some of the secondary fractures (such as thrusts and normal faults) can not exist simultaneously as this leads to opposite deformation results (Figure 3). Theoretical consideration of 2D strain in a shear zone. As a priority, all cases of elongation and shortening of the zone are theoretically studied in the constant volume of the zone. In previous studies, the situation was considered with additional compression or tension in the direction perpendicular to the shear zone (Figure 4), but not with elongation or shortening of the shear zone. The analysis of the strain state of the shear zone revealed that development of Riedel shears of R and R′ types (which are paired and identical in the stress field of pure shearing) can lead to opposite results in deformation of the zone. Shear cracks of R type cause elongation of the zone and reduction of the zone’s width (Figure 5). Shear cracks of R′ type can occur with shortening of the zone and increase in its width (Figure 6). Shear cracks of X and P types (which are also paired) demonstrate similar behavior: X cracks occur with lengthening of the zone, while P cracks occur with its shortening. Cracks of Y type, which go parallel to the zone, can be observed in both cases. Influence of increase or reduction of the shear zone’s volume on possible combinations of structures, including tension fractures and stylolithic fractures, is also considered. Combinations of secondary fractures revealed by the theoretical studies are tabulated (Table 1); six cases are distinguished with regard to active, possible and impossible structures. GEODYNAMICS & TECTONOPHYSICS PUBLISHED BY THE INSTITUTE OF THE EARTH’S CRUST SIBERIAN BRANCH OF RUSSIAN ACADEMY OF SCIENCES Tectonophysics Examples of combinations of secondary fractures in experiments and natural structures. Examples of echelon structures are considered in terms of the strain state of shear zones. In experiments, alternations of domains, wherein shear cracks of R and R′ types are developing along shear zone, are interpreted as a combination of domains with elongation and shortening of the medium (along the strike of the zone), while the total length of the zone remains unchanged (Figure 7). It is assumed that variations of widths of zones of influence of faults, that are observed in natural structure, and changes of amplitudes of displacement in seismogenic faults (Figure 8) are related to this phenomenon of alternation of domains wherein shear cracks of R and R′ types are developing, i.e. there is a relation to elongation and shortening of such domains of a fault zone. Structures of terminations of large faults of ‘horse­tail’ and ‘fish­bone’ types are interpreted as domains wherein shear cracks of R and R′ types develop as secondary faults under conditions of lengthening and shortening of the sides of the main fault (Figure 9). It is shown that shatter zones in the basalt detachment of the Vorontsovsky nappe are related to shear cracks of R type; they evidence elongation of the nappe’s body (Figure 10). In the scale of the given outcrop, a number of specific combinations of share cracks of P type and tension fractures are reviewed (Figure 11, 12, 13, 14, and 15). Structures with development of shear cracks of X type are specified; these are synthetic faults in the body of the landslide and echeloned normal faults in sides of regional shear faults in petroliferous structures of the Western Siberia (Figure 16). Theoretical research of zones of simple shearing in a massif which is subject to general deformation of pure shearing. Simple shearing zones, which are located in massifs which are subject to pure shearing, are a target of special theoretical studies. Under such conditions of the massif’s deformation, the length of shear zones in the massif will either increase or decrease, depending on orientations of such zones relative to the axis of shortening (Figure 17). Assumptions of possible combinations of secondary fractures in such shear zones are made. onclusions. It is established that in a shear zone, cracks of R and R′ types can not develop in one domain as they lead to opposite deformation consequences. However, this has not been taken into account when describing shear zones in terms of stress fields. Concerning emerging deformations of a shear zone, it is revealed that cracks of R and X types are paired (in case of zone’s elongation), and cracks R′ and P types are in the opposite pair (case of zone’s shortening). The table of theoretically possible and impossible secondary fractures is compiled for a variety of deformation conditions of a shear zone. The problem of collecting data on stable combinations of echelon secondary structures, that occur in shear zones, and developing a systematic review of such combinations on the basis of concepts of the strain state of the shear zones is put forward. It is proposed to apply changes of shear zone length in modeling of these structures on equivalent materials.

This study aims to analyze the internal structure of earthquake sources under the modern concepts in physical mesomechanics, which consider the multilevel process of faulting in the geological medium, taking into account specific features of the subsurface and deep levels of the crust. This article includes two parts that present the study results and discuss the interdisciplinary information on the subject of this study. The first part describes the subsurface crustal level, wherein seismogenic faulting takes place. We present the seismogeological data on the structure and development of the sources of three catastrophic earthquakes (М≥8.0–8.5) that occurred inMongoliain the last century. We discuss the deep drilling (1.0–3.5 km) data on the seismodislocations formed after the recent strong and catastrophic earthquakes in the United States, Taiwan and Japan, including the zone of co-seismic fractures caused by the Tohoku-Oki earthquake (M=9.0, November 11, 2011). In the second part, jointly with specialists in petrology and geochemistry A.V. Travin and V.B. Savelieva, we will analyze the field data on the ages and the physical and chemical characteristics of geomechanical processes that took place at large depths in the fault zones, which are outcropped by the long-term denudation of the upper crustal layer in the study area. In the summary, we will describe our concepts of geomechanical and tribochemical processes taking place in the fault zones during the formation of the earthquake sources. The results of this comprehensive study give grounds to conclude that a multidisciplinary approach is needed to investigate the deep geological and geophysical processes of ‘stick-slip’ on the fault planes with diverse relief features in the zones wherein seismicity is generated. This conclusion is of paramount importance: it concerns the potentials of applying new approaches to forecasting, management and mitigation of seismic engineering risks arising from the hazardous effects of strong earthquakes.

The Bekchiul pluton is located in the Lower Amur region and is a large granitoid body of complex structure within the Zhuravlevka-Amur terrain of the Sikhote-Alin orogenic belt. On the northwestern flank of the Bekchiul pluton there is the Mnogovershinnoe gold-silver deposit. To determine the formation time of this pluton, U/Pb dating was performed on zircons from the second-phase granodiorite of the Bekchiul complex and the third-phase granite. Granodiorite yielded U-Pb age of 73.8±0.4 Ma, and granite – 66.2±0.3 Ma. Some zircon grains found in granite have a U-Pb age of 75.6±0.6 Ma which is close to that of granodiorites. Magmatic pulses of about 76–73 Ma and about 66 Ma are synchronous with the stages of ore formation of the Mnogovershinnoe deposit, corresponding to earlier-obtained K-Ar age of adularia from ore zones. The formation of granitoids and the associated mineralization probably occurred due to subduction of the Izanagi Plate beneath the continental margin.

The present paper contains a review of publications, geological maps and reports on the study of the Pribrezhny volcanic complex of South Kamchatka. The Pribrezhny volcanic complex is comprised of a number of closeset volcanic massifs stretching chain-like along the narrow coastline of the Beregovoy ridge from the Avacha Bay to the Vestnik Bay. The relevance of the work is related to the necessity of revising the geodynamic history of formation of the study area which stems from emergence of some fragmentary, contradictory information about magmatic complexes of South Kamchatka. The paper presents the data on magmatic rocks penetrated in the coastal cliffs of South Kamchatka. Consideration is being given to their formation conditions in the context of a once-popular theory of geosynclines and in that of plate tectonics. Based on the review, several open research problems are formulated concerning the undetermined propagation limits of the Pribrezhny volcanic complex and unknown time intervals of its formation, and the lack of mineralogical and isotopic-geochemical studies of magmatic rocks. A separate research problem is the formation of transitional zone between the continental margin of Kamchatka and the Kuril Island Arc. The paper shows the relevance of the study of the Pribrezhny volcanic complex of South Kamchatka and proposes possible solutions to the research problems. A detailed mapping of the junction zone of Pribrezhny volcanic complex of South Kamchatka and the Kronotsky terrain near the Malko-Petropavlovsk zone of transverse dislocations will make it possible to refine the age and kinematics of their relationship. Determination of rare elements and strontium, neodymium and lead isotopes will provide a possibility to clarify the nature of magmatism as compared to heterochronous volcanic series of South Kamchatka. The data obtained will be needed to study magmogeneration conditions for acid volcanism of South Kamchatka and monogenic volcanism of the Malko-Petropavlovsk zone of transverse dislocations. The paleomagnetic studies, along with dating of rock absolute age, will help to reconstruct the paleolatitude of formation of the Pribrezhny volcanic complex and to restore the geodynamic evolution of formation of South Kamchatka.

This study of groundwaters of the Altai Republic is relevant due to the need to identify earthquake precursors in seismically active zones of the study area. Chemical composition of groundwater is widely known as an indicator of seismic processes, which changes in future earthquake focal zones. In this regard, studying helium contents in groundwater is of particular importance – anomalous concentrations of helium are typical of fault zones, and helium concentration variations in time can be referred to as earthquake precursors. Our study was focused on the distribution of helium in groundwaters of the Altai Republic and aimed to determine and justify positioning of permanent monitoring sites for investigation of earthquake precursors. During the field study, groundwater samples were taken from wells and springs located within the area covered by the established state monitoring network (GONS). Groundwater aquifers and water-bearing zones from Quaternary to Proterozoic ages were sampled. Analysis of the groundwater samples shows spatial variations of helium concentration in the study area. Water samples from fault zones have increased concentrations of helium. The concentration of helium is high in the groundwater from the Chuya artesian basin, i.e. near the epicentral zone of the M 7.5 Altai (Chuya) earthquake of September 27, 2003.

This article presents the first map showing the vertical amplitudes of modern disjunctive dislocations inNorthern Atlantic, based on the estimated phase shifts of reflected waves recorded by high-frequency seismic acoustic surveys. The amplitude distribution pattern is mosaic with alternating areas of compression and extension in the flanks of the Knipovich rift system. The modern structure of the Knipovich Ridge, including two strike-slip faults, represents a local rift in the pull-apart setting. The asymmetry of stresses and the presence of compression in the ridge flanks is evidenced by the distribution of the focal mechanisms of strong earthquakes related to reverse faults. In the southeastern Knipovich Ridge, tectonic activity is marked by the asymmetric pattern of the epicenters of small earthquakes.

The origination and differentiation of rare metalbearing, alkaline granites has attracted extensive interests because of their economic significance.