The complex geophysical 3D model of the Earth's crust and the upper mantle is created for the Archaean Karelian Craton and the Late Palaeoproterozoic accretionary Svecofennian Orogen of the southeastern Fennoscandian Shield with the use of methods of complex inversion of geophysical data based on stochastic description of interrelations of physical properties of the medium (density, P-wave velocity, and heat generation). To develop the model, we use results of deep seismic studies, gravity and surficial heat flow data on the studied region. Numerical solutions of 3D problems are obtained in the spherical setting with an allowance for the Earth's surface topography. The geophysical model is correlated with the regional geological data on the surface and results of seismic CMP studies along 4B, FIRE-1 and FIRE-3-3A profiles. Based on results of complex geophysical simulation and geological interpretation of the 3D model, the following conclusions are drawn. (1) The nearly horizontal density layering of the continental crust is superimposed on the previously formed geological structure; rock differentiation by density is decreasing with depth; the density layering is controlled by the recent and near-recent state of the crust, but can be disturbed by the latest deformations. (2) Temperature variations at the Moho are partially determined by local variations of heat generation in the mantle, which, in turn, are related to local features of its origin and transformation. (3) The concept of the lower continental crust being a reflectivity zone and the concept of the lower continental crust being a layer of high density and velocity are not equivalent: the lower crust is the deepest, high-density element of near-horizontal layering, whereas the seismic image of the reflectivity zone is primarily related to transformation of the crust as a result of magmatic under- and intraplating under conditions of extension and mantle-plume activity. (4) At certain combinations of crustal thickness and temperature at the level of Moho discontinuity, the crust in a platform region can be transformed into eclogites. In this case, the crust–mantle boundary is determined by quantitative proportions of the rocks that underwent eclogitization or escaped this process and by corresponding density and velocity values. (5) High compaction of rocks in the crust under lithostatic loading cannot be explained by «simple» concepts of metamorphism and/or rock compaction, which are based on laboratory studies of rock samples and mathematical simulations; this is an evidence of the existence of additional, quite strong mechanisms providing for reversible changes of the rocks.

Due to local groundwater seeping and freezing in layers that accumulate over each other and create large ice clusters on the ground surface, specific conditions of energy and mass transfer are created in the atmosphere–soil–lithosphere system. In winter, the vertical temperature distribution curve is significantly deformed due to heat emission from the water layer above the ice cover during its freezing, and a thermocline is thus formed. Deformation of the temperature curve is gradually decreasing in size downward the profile and decays at the interface of frozen and thaw rocks. Values and numbers of temperature deviations from a 'normal' value depend on heat reserves of aufeis water and the number of water seeps/discharges at a given location. The production of the thermocline alters freezing conditions for underlying ground layers and changes the mechanism of ice saturation, thus leading to formation of two-layer ice-ground complexes (IGC). IGCs are drastically different from cryogenic formations in the neighbouring sections of the river valley. Based on genetic characteristics and the ratios of components in the surface and subsurface layers, seven types of aufeis IGCs are distinguished: massive-segregation, cement-basal, layered-segregation, basal-segregation, vacuum-filtration, pressure-injection, and fissure-vein. Annual processes of surface and subsurface icing and ice ablation are accompanied by highly hazardous geodynamic phenomena, such as winter flooding, layered water freezing, soil heaving/pingo, thermokarst and thermal erosion. Combined, these processes lead to rapid and often incidental reconfigurations of the surface and subsurface runoff channels, abrupt uplifting and subsiding of the ground surface, decompaction and 'shaking-up' of seasonally freezing/thawing rocks, thereby producing exceptionally unfavourable conditions for construction and operation of engineering structures.

Formation and development of river networks are heavily influenced by aufeis deposits and processes taking place at the aufeis surfaces, especially in areas of discontinuous and continuous permafrost where an average thickness of the ice cover on rivers ranges from 1.0 to 2.5 m, and the major part of the ice cover is accumulated layer by layer due to freezing of discharged groundwater. In the permafrost zone, the intensity of cryogenic channelling is clearly cyclical, and the cycles depend on accumulation of aufeis ice above the river level during the autumn low-water period. Five stages of cryogenic channelling are distinguished: I – pre-glacial development, II – transgression, III – stabilization, IV – regression, and V – post-glacial development. Each stage is characterised by a specific glaciohydrological regime of runoff channels and their specific shapes, sizes and spatial patterns.

The channel network is subject to the maximum transformation in aufeis development stages III and IV, when the transit flow channel is split into several shallow-water branches, producing a complicated plan pattern of the terrain. In the mature aufeis glades, there are sites undergoing various development stages, which gives evidence that aufeis channelling is variable in a wide range in both space and time. With respect to sizes of aufeis glades, river flow capacities and geological, geomorphological, cryo-hydrogeological conditions, aufeis patterns of the channel network are classified into five types as follows: fan-shaped, cone-shaped, treelike, reticular, and longitudinal-insular types. The aufeis channel network is a reliable indicator of intensity of both recent and ancient geodynamic processes in the cryolithozone.

In Siberia and the Far East, the aufeis deposits are much larger, more numerous and more important in terms of morpholithology in comparison with the 'classical' (sedimentary metamorphic) icing structures. The more contrasting is the terrain, the more active are neotectonic movements, the lower is the mean annual air temperature, and the higher is the annual percentage of the territory covered by aufeis ice. The aufeis ratio of the permafrost zone is determined from parameters of over 10000 ice fields and amounts to 0.66 % (50000 km²). In mountains and tablelands, the total area of aufeis deposits amounts to 40000 km², and the number of ice clusters (0.77 km² in average) exceeds 60000. On the rivers up to 500 km long, the aufeis size depends on the stream rank. In all the natural zones, the majority of gigantic aufeis spots produced by groundwater are located in river valleys of ranks 3 and 4. The square area of aufeis deposits of mixed feed, i.e. produced by river water and groundwater, which occupy the entire river channel, yet do not go beyond the floodplain, amounts to 68000 km², i.e. by a factor of 1.7 larger than the area of all the aufeis deposits (taryns). The cumulative channel-forming effect of aufeis phenomena is expressed by an increment in the channel network relative to characteristics of the river segments located upstream and downstream of the aufeis glade. This indicator is well correlated with the aufeis ratios of the river basins, morphostructural and cryo-hydrometeorological conditions of the territory under study. The incremental length of the channel network, ρn per one groundwater aufeis deposit is increased, in average, from 3.5 km in mountains in the southern regions of East Siberia to 23 km in the Verkhoyansk-Kolyma mountain system and Chukotka. The value of ρn is decreased to 2.2 km in the plains and intermountain depressions of the Baikal rift system where the average dimensions of the ice fields are smaller. An average incremental length of the channel network per one large groundwater aufeis deposit amounts to 12.2 km, and the total incremental length in continuous and discontinuous permafrost areas (F=7.6 mln km²) is estimated at 690000 km.

Combined impacts of aufeis and icing processes on underlying rocks and the channel network is a specific (aufeis) form of cryogenic morpholithogenesis that is typical of regions with inclement climate and harsh environment. A more detailed research of these processes is required, including large-scale aerospace surveys, monitoring and observations on special aufeis polygons.

Seismicity migration is studied by a new method based on space-time diagrams and a combination of cluster and regression analyses. Data from the global and Baikal regional earthquake catalogues are analysed with the application of the specially designed geographic information system (GIS) in order to establish parameters and mechanisms of seismicity migration in space and time. We study the migration of seismic events in the following geostructural systems: the Baikal rift zone (BRZ), the area between BRZ and the Indo-Eurasian interplate collision zone, the area between BRZ and the West-Pacific seismic foci Benoiff zone, and two segments of the Middle Atlantic ridge.

As evidenced by the obtained results, studying regimes of seismic migration provides for analyses of space-time distribution of seismic energy in the fault-block structure of the lithosphere and facilitates more detailed studies of the origin of deformation waves and mechanisms of the seismotectonic regime of the Earth. Forward (from the equator) and backward (towards the equator) migration of seismic events are established in all the regions under study. It is assumed that this phenomenon may result from regular changes of the polar compression of the Earth due to variations of its rotation regime. Besides, it is revealed that energy clusters of migration are regularly generated, and the regularity may be related to the 11-year cycle of the solar activity which impacts the seismic regime. We discuss the need to study the interference of wave deformations in the lithosphere which are initiated by several external energy sources. It is proposed to consider the regimes of planetary seismicity migration as a reflection of redistribution of endogenic (primarily heat) energy of the Earth during the destruction of its lithospheric shell under the impacts of cosmogenic factors via triggering mechansms. With reference to our positive experiences of applying the proposed concept to BRZ, we consider possibilities of using the seismicity migration data for prediction of earthquakes in the planetary and regional scales.

Tilt measurements have been taken in the underground gallery at Talaya Seismological Station for almost three decades, from March 1985 till 2014. Based on such data, deformation curves were constructed and analysed in the frame of elastic and viscous-elastic models of the geological medium. From estimated annual deformation rates, it became possible to reveal deformation cycles ranging from 3 to 18 years with amplitudes up to 5 arc-seconds (2·10–5). For the bedrock in the Talaya stream valley, the elastic modulus was estimated at 20 GPa. In frame of the Kelvin viscoelastic model, the apparent viscosity of the medium was estimated at 1019 Pa·sec by deformation delay curve for 1989–2014 epoch. Observed vertical rates were used to estimate the size of the studied area (from 0.1 km to 6.0 km). The values estimated in our experimental investigation are used in a wide range of geophysical studies: modelling tectonic, co-seismic and post-seismic processes.

The lower part of lithosphere in collisional orogens may delaminate due to density inversion between the asthenosphere and the cold thickened lithospheric mantle. Generally, standard delamination models have neglected density changes within the crust and the lithospheric mantle, which occur due to phase transitions and compositional variations upon changes of P-T parameters. Our attention is focused on effects of phase and density changes that may be very important and even dominant when compared with the effect of a simple change of the thermal mantle structure. The paper presents the results of numerical modeling for eclogitization of basalts of the lower crust as well as phase composition changes and density of underlying peridotite resulted from tectonic thickening of the lithosphere and its foundering into the asthenosphere. As the thickness of the lower crust increases, the mafic granulite (basalt) passes into eclogite, and density inversion occurs at the accepted crust-mantle boundary (P=20 kbar) because the newly formed eclogite is heavier than the underlying peridotite by 6 % (abyssal peridotite, according to [Boyd, 1989]). The density difference is a potential energy for delamination of the eclogitic portion of the crust. According to the model, P=70 kbar and T=1300 °C correspond to conditions at the lower boundary of the lithosphere. Assuming the temperature adiabatic distribution within the asthenosphere, its value at the given parameters ranges from 1350 °C to 1400 °C. Density inversion at dry conditions occurs with the identical lithospheric and asthenospheric compositions at the expense of the temperature difference at 100 °C with the density difference of only 0.0022 %. Differences of two other asthenospheric compositions (primitive mantle, and lherzolite KH) as compared to the lithosphere (abyssal peridotite) are not compensated for by a higher temperature. The asthenospheric density is higher than that of the lithospheric base. Density inversion occurs if one assumes the presence of the asthenosphereic material in the composition similar to that of the primitive mantle or lherzolite KH in amounts no less than 1.40 and 0.83 wt. %, respectively, of the conventionally neutral fluid. This amount of the fluid seems to be overestimated and thus does not fully correlate with the current estimates of the fluid content in the mantle. Therefore, the most appropriate material for delamination of the thickened lithosphere is only the fluid-bearing asthenosphere which composition corresponds to that of the depleted mantle of middle-ocean ridges (DMM) being the reservoir existing from the Precambrian. In our model, abyssal peridotite is most similar to DMM as compared with other more fertile compositions of the lithosphere. Heat advection due to uplift of fluid-bearing plumes that occurred much time after collisional events may initiate repeated delamination of gravitationally instable parts of the orogenic and cratonic lithosphere.

 

The report presents a chronicle of the XXVI All-Russia youth conference “Lithosphere structure and Geodynamics”, dedicated to the 85th anniversary of academician Nikolai A. Logachev – outstanding geologist, specialist on the continental rifting. The major events are highlighted and a thematic review of the conference papers is given.