Upper mantle structure of the Northern Eurasia from peaceful nuclear explosion data

Several long-range seismic profiles were carried out in Russia with Peaceful Nuclear Explosions (PNE). The data from 25 PNEs recorded along these profiles were used to compile a 3-D upper mantle velocity model for the central part of the Northern Eurasia. 2-D crust and upper mantle models were also constructed for all profiles using a common methodology for wavefield interpretation. Five basic boundaries were traced over the study area: N1 boundary (velocity level, V = 8.35 km/s; depth interval, D = 60–130 km), N2 (V = 8.4 km/s; D = 100–140 km), L (V = 8.5 km/s; D = 180–240 km) and H (V = 8.6 km/s; D = 300–330 km) and structural maps were compiled for each boundary. Together these boundaries describe a 3-D upper mantle model for northern Eurasia. A map characterised the velocity distribution in the uppermost mantle down to a depth of 60 km is also presented. Mostly horizontal inhomogeneity is observed in the uppermost mantle, and the velocities range from the average 8.0–8.1 km/s to 8.3–8.4 km/s in some blocks of the Siberian Craton. At a depth of 100–200 km, the local high velocity blocks disappear and only three large anomalies are observed: lower velocities in West Siberia and higher velocities in the East-European platform and in the central part of the Siberian Craton. In contrast, the depths to the H boundary are greater beneath the craton and lower beneath in the West Siberian Platform. A correlation between tectonics, geophysical fields and crustal structure is observed. In general, the old and cold cratons have higher velocities in the mantle than the young platforms with higher heat flows.Structural peculiarities of the upper mantle are difficult to describe in form of classical lithosphere–asthenosphere system. The asthenosphere cannot be traced from the seismic data; in contrary the lithosphere is suggested to be rheologically stratified. All the lithospheric boundaries are not simple discontinuities, they are heterogeneous (thin layering) zones which generate multiphase reflections. Many of them may be a result of fluids concentrated at some critical P–T conditions which produce rheologically weak zones. The most visible rheological variations are observed at depths of around 100 and 250 km.     

The types of the earth's crust

For the comprehensive mapping of the deep structure of the Earth's crust a determination of the general types of the crust based on its most essential structural parameters is attempted. Three parameters are considered as most informative, and are accepted as basic for this purpose: thickness of the Earth's curst, thickness of the sedimentary layer, and average velocities of compressional waves in the consolidated parts of the crust. Six types of the crust are recognized. A more detailed division of the crust into sub-types has also been worked out.     

Seismic studies of low-velocity layers and horizontal inhomogeneities within the crust and upper mantle on the territory of the U.S.S.R.

Recent seismological studies of low-velocity layers in the U.S.S.R. have led to the development of new methods of investigation. The most important results obtained are presented in this paper. Several new techniques of record treatment and advanced computer programs make it possible to solve two-dimensional problems of seismic wave propagation in complex media and to outline zones of velocity inversion in the crust and mantle of many regions in the U.S.S.R.Zones of this type seem to occur only locally and are typical of some particular geostructures. Lateral inhomogeneities are also found to be closely related to geological features. Their depths sometimes can reach 300–400 km.     

A three-dimensional model of the Baltic Shield crust from data of deep seismic studies

A 3-D velocity structure of the crust has been constructed for almost the entire Baltic Shield area from data of extensive deep seismic studies on the shield. The construction involved a revision of all primary data (record sections and observed traveltime curves) obtained in this region over 50 years of research. Comparative analysis of wave fields revealed that three reference reflectors traceable throughout the shield area are K1 (a boundary velocity of 6.4–6.5 km/s), K2 (～6.8 km/s), and the mantle surface M (8.0–8.2 km/s). The resulting 3-D velocity structure is represented in the form of structural maps of these surfaces and a velocity distribution scheme in the upper crust. Using this general basic model, seismic cross sections are constructed by means of mathematical modeling along all profiles. They showed that, in addition to the main layers and reflectors above the K1 boundary, a lower velocity layer is traceable almost everywhere and the majority of deep faults flatten out toward this layer. On the whole, lateral variations in the velocity structure of the crust are small up to a depth of 40 km. The variations are most significant in the M topography: its average depth being 40–45 km, two deep (down to 50–60 km) depressions exist in southern Finland and the Baltic region. The origin of this depression filled with high velocity (7.2–7.4 km/s) rocks remains unclear.     

Joint analysis of seismological data by the USSO and DSS stations in the Caucasus region

This paper deals with a procedure of a joint analysis of seismic data from earthquakes and those obtained by DSS. The DSS data are used as a first approximation to construct a two-dimensional model of the medium made up of individual blocks. These models serve as a basis when constructing specific three-dimensional travel-time curves. These travel-time curves are further used for the calculation of hypocenter parameters in a laterally inhomogeneous block medium.The hypocenter field and the travel times obtained are input data for the computation of three-dimensional fields of velocities in earthquake focal zones. Results of applying the proposed procedure to the Caucasus region are presented.     

Seismic models of inner parts of the Euro-Asian continent and its margins

Analogies are drawn between continental and continental margin structures on the basis of seismic data on the crustal structure of Eurasia and its Atlantic margins. Crustal thinning from the inner parts of the continent to its margins is observed to be a general feature common to the formation of deep midland depressions and sedimentary basins of shelf zones. The latter are characterized by crustal thinning and its assimilation. These phenomena cannot be explained solely be sea-floor spreading effects in the process of active rifting and formation of oceanic crust. It appears that the main role in the formation of the margins in played by processes of mantle erosion in connection with heating at continental margins and with the migration of mantle material to the lower part of the crust.     

Generalized geophysical model and dynamic properties of the continental crust

Detailed seismic investigations of the continental crust have produced evidence of definite regularities in the general layering of the consolidated crust despite its high degree of inhomogeneity. Three main layers may be resolved in the inner part of a continent: an upper layer with velocities of 5.8–6.4 km/s and a velocity gradient about 0.04–0.05 s⁻¹, an intermediate layer with velocities of 6.2–6.6 km/s and velocity gradient about zero, and a lower layer with velocities of 6.8–7.2 km/s and a high-velocity gradient of 0.05–0.1 s⁻¹. The intermediate layer is characteristically different not only because of its low average velocity gradient, but also because of its more pronounced horizontal layering, inversion zones, and its higher “transparency” and V_p/V_s ratio. The gravity and magnetic data have shown that basement inhomogeneities disappear at the top of the intermediate layer. Also there are few earthquakes in this layer. These pecularities may be interpreted as the result of partial melting (weakening) of rocks and their possible horizontal mobility inside this layer.Thus, dynamic models of tectonic processes must take into consideration the possible existence of a weak zone in the crust.     

A combined geophysical model of the crust for the south of the European part of the USSR

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Presented at the KAPG Symposium Problems of Interpretation and Construction of Physical Models of Litosphere, Liblice (CSSR), March 6–10, 1978.