Vol 50, No 6 (2016)

A comprehensive investigation aimed at determining seismotectonic types of destruction and the stress–strain state of the Earth’s crust in the main seismogenerating structures of the Arctic–Asian seismic belt is conducted for the territory of the northeastern sector of the Russian Arctic region. Based on the degree of activity of geodynamical processes, the regional principles for ranking neotectonic structures are elaborated, and neotectonic zoning is carried out based on the substantiated differentiation of the corresponding classes. Within the limits of the Laptev Sea, Kharaulakh, and Lena-Anabar segments, we analyzed I the structural–tectonic position of the most recent structures, II the deep structure parameters, III the parameters of the active fault system, and IV the parameters of the tectonic stress field, as revealed from tectonophysical analysis of Late Cenozoic fault and fold deformations. Based on the seismological data, the mean seismotectonic deformation tensors are calculated to determine, in combination with geological and geophysical data, the orientations of the principal stress axes and to reveal the structural–tectonic regularity for tectonic regimes of the stress–strain state of the Earth’s crust in the Arctic sector of the boundary between the Eurasian and North American lithospheric plates.

The Belomorian Mobile Belt (BMB) in northern Karelia mostly consists of gently sloping shear zones, whose gneisses and migmatized amphibolites and blastomylonites are typically thinly banded, with their banding consistently dipping north- and northeastward. These gently sloping shear zones were not affected by folding after they were produced and are not cut by Paleoproterozoic metabasite dikes. Intrusive metabasites in the gently sloping shear zones make up relatively small (usually <5 m) equant or elongate bodies and occur as fragments of larger bodies. These fragments are often concentrated in stripes. Metabasites in the gently sloping shear zone are sometimes also found as lenses and tabular bodies of relatively small thickness, which are conformable with the foliation of the host rocks. The gently sloping shear zones cut across older domains of more complicated structure, which suggests that these zones are gently sloping ductile shear zones. Along these zones, the nappes were thrust south- and southwestward, and this process was the last in the origin of major structural features of BMB when the Paleoproterozoic Lapland–Kola orogen was formed. Practically identical age values were obtained for the gently sloping shear zone in the two widely separated Engonozero and Chupa segments of BMB: 1879 ± 21 Ma (⁴⁰Ar/³⁹Ar amphibole age of amphibolite whose protolith was mafic rock) and 1857 ± 13 Ma (Sm–Nd mineral isochron age of garnet amphibolites after gabbronorite). The P–T metamorphic parameters in these gently sloping shear zones are remarkably different from the metamorphic parameters outside these zones: the pressure is 3–4 kbar lower and the temperature is 60–100°C lower. Thrusting-related decompression triggered the transition from the older high-pressure episode of Paleoproterozoic metamorphism to a younger syn-thrusting higher temperature metamorphic episode. The peak metamorphic parameters corresponding to the boundary between the amphibolite and granulite facies were reached only in the central portions of the shear zones: T= 680–760°C, P = 8.0–11.9 kbar. In areas of the most intense migmatization, temperature estimates in the central portions of the shear are as high as 810–830°C. The marginal portions of the shear zones were formed at lower temperatures of 610–630°C. The temperature heterogeneous and rock heating in the gently sloping shear zones may have resulted from flows of high-temperature metamorphic fluid that were focused to the central portions of the zones.

This paper analyses the spatial distributions of gravity sources and density contrast of geological media between the centers of mass of density heterogeneities and surfaces of some layers, which is reflected by the values of parameter μ_z, in the Northeast Asia Region and Australia. Statistical images of complex distributions of density heterogeneities are generally consistent with the shallowest tectonic structures, wave velocity, and electric anomalies. The correlation of μ_z minimums with high heat flow suggests melted magmas beneath the crust base and in the asthenosphere. In the lower crust layer, large μ_z maximums correlate with major continental megaelements, which are divided by extended linear μ_z minimums, high conductivity anomalies, and Vp/Vs maximums marking zones of lower viscosity. Deep boundaries and a thickness of lithospheric plates and asthenosphere lenses have been defined in the Northeast Asia Region. In Australia, two roughly NS-striking low-viscosity zones crossing the continent reflect the deep setting of the Lasseter (in the west) and the Tasman (in the east) lines. These zones are displayed in a wide depth range (15–120 km) and are confined to the Archean–Proterozoic and Proterozoic–Phanerozoic boundaries in the Australian lithosphere. Statistical gravity models derived without external (relatively to gravity) geological and geophysical information prove the multilayered tectonosphere structure of both continents and the similar deep structure features of their margins.