A. I. Miroshnichenko

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.

Diagnostics of the preseismogenic state of heterogeneous media according to deformation monitoring data

A new approach to diagnostics of the preseismogenic state of the lithosphere within the focal area was considered by the example of spectral analysis of the monitoring data on ice cover deformations in Lake Baikal.

Coupling of late cenozoic faulting of the Siberian Platform margin and Baikal Rifting

The relationship between the Late Cenozoic tectonic deformations at the Siberian Platform (SP) margin and the adjacent Baikal Rift System is considered. This study was aimed at estimation of the stress state and conditions of neotectonic reactivation of faults in the sedimentary cover of the eastern Irkutsk Amphitheater (the Angara‐Lena Uplift and Fore-Baikal Trough coupling zone). On the basis of geological, structural, and geomorphic methods, the Late Cenozoic fault assemblages were investigated and the related stress field was reconstructed to provide evidence for interrelated deformation of the Baikal Rift and the SP margin. The dynamic interaction of the SP block with the surrounding fold systems at the neotectonic stage and the evolution of the Late Cenozoic faulting remain poorly studied. A.G. Zolotarev developed the concept of fore-rift troughs [5 among others]. The brittle fracturing of the upper Pleistocene sediments was documented in the Fore-Sayan Foredeep [1, 8]. The existence of recent seismic activity of the platform has been proven in [2]. Seminsky et al. [8 and others] attempted to demonstrate the interrelation between the seismic activity and deformations in the Baikal Rift. However, these issues remain insufficiently studied because of short-term instrumental measurements and the absence of seismic station network on the platform. The northeastern part of the Irkutsk Amphitheater of the SP is characterized by medium-intensity neotectonic uplifting with an amplitude of 800 m relative to the initial surface [4]. Such values have been estimated for the Angara‐Lena Uplift. The Fore-Baikal Trough, which extends on the eastern side along the SP margin, ascended less intensely (amplitude of vertical movements was not greater than 200 m). The difference in the present-day hypsometric position of these structures suggests a rather high velocity gradient of neotectonic vertical movements in their coupling zone, providing prerequisites for differential motions along fault zones. The schematic map of faults in the Fore-Baikal Trough and Angara‐Lena Uplift coupling zone (Fig. 1) is based on the interpretation of satellite images and aerial photographs, analysis of a digital 3D model of topography, results of field geostructural and geomorphic observations, and compilation of the State Geological Mapping data. The faults in the study territory make up a rather dense network in comparison with inner sectors of the SP, probably owing to the closeness with the Sayan‐ Baikal mobile region. Paleozoic folds and faults significantly controlled the development of local neotectonic structures. The northern part of the territory is dominated by submeridional faults that reflect the general strike of older linear structures in the study region (Kirenga fold zone [3]). The NE-trending faults inherit structures of the Zhigalovo‐Tulukmur fold zone. The neotectonic reactivation stage is characterized by nonuniform manifestation over the study area (Fig. 1). Judging from the density of neotectonic faults, the northern Fore-Baikal Trough and the zone of its coupling with the Angara‐Lena Uplift were most active. The maximums of fault density are confined to the Khanda (southern end), Baldakhin’ya, and Novoselovo basins. The platformal sedimentary cover in the northern Angara‐Lena Uplift is marked by minimum faulting.