러시아ㆍ유라시아

The evolution and specific features of seismogynamics of the Baikal zones are reviewed in the context of interactions between deep deformation waves and the regional structure of the lithospheric mantle. The study is based on a model of the mantle structure with reference to chemical compositions of mantle peridotites from ophiolotic series located in the south-western framing of the Siberian craton (Fig. 1). The chemical zonation of the lithospheric mantle at the regional scale is determined from results of analyses of the heterogeneity of compositions of peridotites (Fig. 2, Table 1) and variations of contents of whole rock major components, such as iron, magnesium and silica (Fig. 3). According to spatial variations of the compositions of peridotites, the mantle has the concentric zonal structure, and the content of SiO2 is regularly decreasing, while concentrations of FeO∑ and MgO are increasing towards the centre of such structure (Fig. 4). This structure belongs to the mantle of the Siberian craton, which deep edge extends beyond the surface contour of the craton and underlies the north-western segment of the Central Asian orogenic belt.
Results of the studies of peridotites of the Baikal region are consistent with modern concepts [Snyder, 2002; O’Reilly, Griffin, 2006; Chen et al., 2009] that suggest that large mantle lenses underlie the Archaean cratons (Fig. 5). The lenses are distinguished by high-density ultrabasic rocks and compose high-velocity roots of cratons which have remained isolated from technic processes. Edges of the mantle lenses may extend a few hundred kilometers beyond the limits of the cratons and underlie orogenic belts that frame the cratons, and this takes place in the south-western segment of the Siberian craton.
The revealed structure of the lithospheric mantle is consistent with independent results of seismic and magmatectonical studies of the region. The Angara geoblock is located above the central part of the mantle lense (Fig 6, A); it is one of four main tectonical units that compose the basement of the Siberian craton [Mironyuk, Zagruzina, 1983]. As evidenced by the zonal composition of the mantle lense, the centre of the lense is highly dense, and this explains the location of a seismic anomaly there (Fig. 6, B) which is determined to a depth of about 50–60 km [Pavlenkova G.A., Pavlenkova N.I., 2006]. The high-velocity root located in this segment of the craton is traced by seismic tomography [Koulakov, Bushenkova, 2010] to a depth of about 600 km (Fig. 7). The southward-stretching edge of the sub-cratonic mantle has played a major role in the evolution of the Central Asian orogenic belt. In the Paleozoic, the position and the configuration of the accretional margin of the Siberian paleocontinent were determined by the hidden boundary of the craton (Fig. 8, A). Along the craton’s boundary, rifting zones of various ages are located, and intrusions are concentrated, which genesis was related to extension settings (Fig. 8, B). The Cenozoic sedimentary basins are located above the hidden edge of the Siberian craton, which gives evidence of involvement of the deep lithospheric structure in the formation of the recent destruction zone. The basin of Lake Baikal is located along the mantle edge of the Siberian craton, and the basin’s crescent shape accentuates the strike of the mantle edge.
In the region under study, the wave nature of seismicity is most evidently manifested by the cyclicity of the strongest earthquakes in the Baikal zone (Table 2). Three seismic cycles are distinguished as follows: (1) at the turn of the 20th century (earthquakes in the period from 1885 to 1931, M=6.6–8.2), (2) the middle of the 20th century (earthquakes from 1950 to 1967, M=6.8–8.1), and (3) at the turn of the 21st century (earthquakes from 1991 to 2012, M=6.3–7.3). While moving in the mantle, the deformation front collapses with the craton’s basement, partially releases its energy to the lithosphere and involves the fragmented edge of the crust overlying the craton’s edge into deformation (Fig. 9, A). This interaction resulted in the formation of the Mongolia-Baikal and the Altai-Baikal seismic sutures whereat all the strong earthquake took place in seismic cycles (1) and (3), respectively (Fig. 9, B). The third, West Amur seismic suture framing the boundary of the Amur plate comprises locations of strong earthquakes that occurred in cycle (2) (Fig. 10). An important specific feature of the Baikal seismic zone is orthogonal migration of earthquakes within seismic sutures. In each of the sutures, epicenters of strong earthquakes (M>6.0) migrated in the transverse direction, which established the orientation of maximum compression during interaction of deformation waves with the mantle structures (Fig. 9, and 10). The less strong seismic events (М<6.0) (Fig. 11) migrated along the seismic sutures. At the western flank of the zone, in the Altai-Baikal and Mongolia-Baikal sutures, latitudinal migration took place in the direction from west to east with account of the trajectory of the deformation wave. In the northern part of the West Amur suture, latitudinal migration was directed from east to west, and its direction was gradually changed to meridional in the southern part, which reflected the anticlockwise rotation of the Amur plate. This conclusion can explain a paradox of counter migration of seismicity in the Baikal zone, which is revealed by S.I. Sherman [Sherman, Zlogodukhova, 2011].
In each of the three seismic/deformation sutures, stresses are released via orthogonal multi-directional migration of earthquakes (Fig. 12), and the sutures are regularly combined to compose a complex structure of the stress field in the Baikal seismic zone. Their positions predetermine locations of the major riftogenic structures, primarily sedimentary basins from Tunka to Ubsunur (Fig. 9, B). The three seismic sutures join and overlap each other in the area of Lake Baikal and thus set up the maximum intensity of deformation in this area. Apparently, each of the deformation sutures corresponds to one of the three basins of Lake Baikal (Fig. 13, A). Their depths are correlated with widths of the sutures, which is explained by ‘weakening’ of the deformation wave in the successive cycles of its interaction with the deep structure of the lithosphere. Seismicity of the Baikal zone and its Cenozoic rifting reflect the character of the stress field generated by interaction between the deep deformation wave and the organization of the lithospheric mantle.