Late Cenozoic geodynamics and mechanical coupling of crustal and upper mantle deformations in the Mongolia-Siberia mobile area

Comprehensive analysis of the parameters characterizing contemporary and neotectonic deformations of the Earth’s crust and upper mantle developed in the Mongolia-Siberia area is presented. The orientation of the axes of horizontal deformation in the geodetic network from the data of GPS geodesy is accepted as an indicator of current deformations at the Earth’s surface. At the level of the middle crust, this is the orientation of the principal axes of the stress-tensors calculated from the mechanisms of earthquake sources. The orientation of the axes of stress-tensors reconstructed on the basis of structural data is accepted as an indicator of Late Cenozoic deformations in the upper crust. Data on seismic anisotropy of the upper mantle derived from published sources on the results of splitting of shear waves from remote earthquakes serve as indicators of deformation in the mantle. It is shown that the direction of extension (minimum compression) in the studied region coincides with the direction of anisotropy of the upper mantle, the median value of which is 310–320° NW. Seismic anisotropy is interpreted as the ordered orientation of olivine crystals induced by strong deformation owing to the flow of mantle matter. The observed mechanical coupling of the crust and upper mantle of the Mongolia-Siberia mobile area shows that the lithospheric mantle participated in the formation of neotectonic structural elements and makes it possible to ascertain the main processes determining the Late Cenozoic tectogenesis in this territory. One of the main mechanisms driving neotectonic and contemporary deformations in the eastern part of the Mongolia-Siberia area is the long-living and large-scale flow of the upper mantle matter from the northwest to the southeast, which induces both the movement of the northern part of the continent as a whole and the divergence of North Eurasia and the Amur Plate with the formation of the Baikal Rift System. In the western part of the region, deformation of the lithosphere is related to collisional compression, while in the central part, it is due to the dynamic interaction of these two large-scale processes.     

Coupling of the Crustal and upper mantle deformations in the Mongolia-Siberian Mobile Area

The complex analysis of parameters characterizing the modern deformations of the Earth’s crust and upper mantle in the territory of the Mongolia-Siberian Area is made. Directions of principal tension axes of stress-tensors, calculated with the use of earthquake source mechanisms have been taken as parameters of modern deformations at the level of the middle crust; directions of axes of horizontal strains in the geodesic network by the GPS data have been taken as such parameters at the level of the Earth’s surface. The strain parameters for the mantle depths are the data on seismic anisotropy derived from the published sources about the results of studies on splitting of transversal waves from distant earthquakes. Seismic anisotropy is interpreted as the ordered orientation of olivine crystals, which appears with great strains resulting from the flow of the mantle material. It has been shown that directions of extensional strain axes (minimal compression) by geodesic and seismological data coincide with anisotropy directions in the upper mantle in the region whose median value is 310°–320°. The observed mechanical coupling of the crust and the upper mantle of the Mongolia-Siberian Mobile Area shows the participation of the lithospheric mantle in the formation of neotectonical structures and enables us to distinguish the principal processes determining the Late Cenozoic tectogenesis in this territory. One of the leading mechanisms for the neotectonical and modern deformations of the Mongolia-Siberian Region is the large-scale NW-SE material flow in the upper mantle causing both motion of the entire northern part of the continent and divergence of the Eurasia and the Amurian Plate. Lithospheric deformations in the western part of the region are related to collision-induced compression, while those in the central part are caused by interaction of these large-scale tectonic processes.     

Space geodesy in geodynamic studies: Recent movements in the Zeya-Bureya Basin

The Amur-Zeya geodynamic test ground was set up in 2000 to study recent intracontinental crustal deformations. The velocity field calculations for the period of 2000 to 2003 describe three movements scales. The general level is characterized by the vectors of IGS sites in the eastern part of Asia, the BLAG (Blagoveshchensk) site included. The southeast movement of the IRKT (Irkutsk) site of the stable Siberian Platform is indicative of deformations in the northeastern part of the Amur Plate. Measurement data on the regional near-latitudinal profile Blagoveshchensk-Sutara, which crosses the Nizhnyaya Zeya Basin, demonstrate a southwestward displacement of the Badzhal-Bureya-Lesser Khingan block relative to the North China block. The dynamic effect of the convergent boundary between the Amur and Okhotsk Sea plates is assumed to extend inland also involving the Zeya-Bureya Basin area. The measurements on the local geodynamic test site relate the deformations of buildings and constructions in the Settlement of Konstantinovka to the mobility of basement faults in the southern part of the Nizhnyaya Zeya Basin. Aseismic deformations are determined by slow horizontal tectonic movements in the junction zone of NNE-trending structures of the latter basin and near-latitudinal faults of the Khailar-Xunhei Belt.     

Seismotectonic destruction of the Earth’s crust in the zone of interaction of the northeastern side of the Baikal rift and the Aldan-Stanovoy block

The paper presents the modern structural-tectonic pattern and a tectonodynamic model of the zone of interaction of the most seismically active northeastern side of the Baikal rift zone (BRZ) and the conjugate system of seismogenic structures of the Aldan-Stanovoy block, where disastrous events with M ≥ 6.0 have been reported. Regularities in the structural formation of active faults and their kinematics are discussed. The faults form block structures accumulating significant tectonic strain. Motions between large tectonic blocks cause sudden release of the strain, which results in catastrophic events (M ≥ 6.0) with focal mechanisms of definite kinematic type.     

Contemporary horizontal movements and seismicity of the south Baikal Basin (Baikal rift system)

The contemporary horizontal movements and deformations in the central and southern parts of the Baikal depression are analyzed, and their relationship with contemporary seismicity is studied. Based on the long-term measurements by the Baikal geodynamical GPS monitoring network, the refined estimate is obtained for the velocity of the divergence of the Siberian and Transbaikalian blocks, which is found to occur in the southeastward direction (130°) at 3.4 ± 0.7 mm per annum. This agrees with the parameters of the long-term extension component estimated from the geological data and with the direction of extension determined from the seismic data. The distribution of the displacement velocity across the strike of the rift, which gradually increases from one block to another, suggests a nonrigid behavior of the continental lithospheric plates at the divergent boundary. About 30% (1.0–1.5 mm per annum) of the total increase in the velocity is accommodated by the Baikal Basin. The strain rate within the trough reaches 3.1 × 10^?8 yr^?1 and decreases on either side across the structure. The character of distribution of the horizontal displacement velocities on the Baikal divergent boundary between the Eurasian and Amurian plates favors the model of passive rifting. The zones of highly contrasting topography and increased seismicity are localized within the area of contemporary deformations, and the seismic moment release rate directly depends on the strain rate. Here, the rate of the seismic moment release rate makes up a few percent of the geodetic moment accumulation rate calculated by the approach suggested by Anderson (1979). Based on the coherence between the graphs of the rates of geodetic moment accumulation and seismic moment release rate by the earthquakes with M ≥ 5.0 during the historical and instrumental observation periods, the contemporary seismic hazard for the South Baikal Basin could be assessed at a level of seismic event with M = 7.5–7.6.     

GPS-measurements of recent crustal deformation in the junction zone of the rift segments in the central Baikal rift system

Based on multiyear measurements of present-day motions in the central area of the Baikal rift system, new data on the kinematics of horizontal motions, relative horizontal deformation rates, and rotation velocities in the area of junction of the South Baikal, North Baikal, and Barguzin rift basins have been obtained. This area is an intricate structure with two transfer zones: Ol’khon–Svyatoi Nos and Ust’-Barguzin.It is shown that crustal blocks are moving southeastward, normally to the structures of transfer zones and at an acute angle to the Baikal Rift strike, which corresponds to the right-lateral strike-slip extensional faulting along the major structure. The average horizontal velocities increase from 3.0 mm yr–1 in the northern South Baikal basin to 6.5 mm yr–1 in the Barguzin basin. The elongation axes prevailing in the study region are mainly of NW–SE direction. The areas of intense deformations are confined to structures with high seismic activity in the South Baikal and, partly, Barguzin basins. This confirms the existence of a present-day zone of the Earth’s crust destruction in the Baikal rift system, which is the most likely source of strong earthquakes in the future. Two zones with rotations in opposite directions are recognized in the rotation velocity field. Clockwise rotation is typical of structures of N–NE strike (Maloe More basin, southern North Baikal basin, Barguzin Ridge rise). Counterclockwise rotation is determined for NE-striking structures (northern South Baikal basin, southern Barguzin basin). In general, the obtained data show an intricate pattern of present-day horizontal dislocations and deformations in the area of junction of NE- and N–NE-striking rift structures. This suggests left- and right-lateral strike-slip faults, respectively, within them.     

Prevalence of Trichinella sp. in polar bears (Ursus maritimus) from northeastern Greenland

The occurrence of infections with Trichinella sp. in polar bears (Ursus maritimus) from northeastern Greenland has been studied by examination of muscle samples, mainly diaphragm, from 38 animals shot during the period 1983-1987. Trichinella larvae were demonstrated in 12 bears (32%) with an average of 9.2 larvae/g muscle tissue. No bears younger than three years old were infected. The prevalence of Trichinella among bears of the age group 3-4 years was 25% and 53% among older animals.     

Geodynamic simulation of ore-bearing geological structural units by the example of the Strel’tsovka uranium ore field

Information on designing a 3D integrated model of the deflected mode (DM) of rock massif near the Strel’tsovka uranium ore field (SUOF) in the southeastern Transbaikal region is presented in the paper. This information is based on the contemporary stresses estimated by geostructural and tectonophysical techniques and by studying the seismotectonic deformation of the Earth’s surface using the data on earthquake source mechanisms and GPS geodesy focused on the recognition of active faults. A combination of the results of geostructural, geophysical, geotectonic, and petrophysical research, as well as original maps of faulting and the arrangement of seismic dislocations and seismotectonic regimes (stress tensors), allowed us to design models of the structure, properties, and rheological links of the medium and to determine the boundary conditions for numerical tectonophysical simulation using the method of terminal elements. The computed 2D and 3D models of the state of the rock massif have been integrated into 3D GIS created on the basis of the ArcGIS 10 platform with an ArcGIS 3D-Analyst module. The simulation results have been corroborated by in situ observations on a regional scale (the Klichka seismodislocation, active from the middle Pliocene to date) and on a local scale (heterogeneously strained rock massif at the Antei uranium deposit). The development of a regional geodynamic model of geological structural units makes it possible to carry out procedures to ensure the safety of mining operations under complex geomechanical conditions that can expose the operating mines and mines under construction, by the Argun Mining and Chemical Production Association (PAO PPGKhO) on a common methodical and geoinformational platform, to the hazards of explosions, as well as to use the simulation results aimed at finding new orebodies to assess the flanks and deep levels of the ore field.