Vol 12, No 5 (2018)

The results of petromagnetic and paleomagnetic studies of the Late Paleoproterozoic nickeliferous mafic–ultramafic rocks of the Kun-Manie ore field located in the southeast of the Aldan–Stanovoy Shield are reported. Our research is focused on intrusions of two morphogenetic types: the Black Giant and Meridional dykes and Kubuk and Iken sills. The petromagnetic data suggest a correlation between the attitude of the studied intrusions and the orientation of the axes of the anisotropy ellipsoid of the initial magnetic susceptibility, which can be used during geological survey, prospecting, and evaluation. Based on the data of our paleomagnetic studies, two stages in the Paleoproterozoic igneous activity were distinguished: the first is related to intrusion of sills, the second, to intrusion of dykes. The orientations of the characteristic component of magnetization isolated in all intrusive bodies of the complex were used to calculate the coordinates of the paleomagnetic pole. The position of the pole corrected for the opening of the Vilyui Rift in the Paleozoic is close to the range between 1700 and 1720 Ma of the apparent polar wander path for the Siberian Craton, which is in a good agreement with the geochronological data available.

Magnetotelluric soundings (MTS) were conducted in a broad frequency range of 10³ kHz to 10^–3 Hz at a total of fifty sounding sites spaced 5 km apart along the profile Spassk-Dalnyi – Zerkal’naya Bay across the strike of the Sikhote-Alin Orogenic Belt (SAOB). As a result of data interpretation, the geoelectric section of the crust and upper mantle has been constructed down to 150 km depth. The section distinguishes the crust with a resistivity higher than 1000 Ohm m and variable thickness between 20 and 50 km that is composed of different resistivity blocks bordered by low resistivity (a few tens to a few hundreds of Ohm m) subvertical and inclined zones. The abnormal low-resistivity zones associated with ore deposits are revealed in the upper crust. The upper mantle structure is nonuniform. The lowest resistivity values are noted in the depth range of 40–80 km beneath the central SAOB. It is underlain by a high-resistivity (350–450 Ohm m) layer stretching westwards from the coast and confined by a high-resistivity (about 500 Ohm m) area beneath the Khanka superterrane in the depth range of 80–120 km. The distinguished structure is associated with the ancient subduction zone. We considered the relationship between the deep structure and the resistivity values of the upper mantle and seismicity. The temperature regime in the upper mantle is evaluated. The geodynamic model of evolution of the area is proposed.

Mantle seismic anisotropy beneath the Amur Plate is studied using the data of ScS waves reflected from the outer core of the Earth from local deep earthquakes in the area of five stations located in Primorye and Priamurye regions. The results of measuring the parameters of the split ScS waves near the stations show the dominance of the polarization azimuths of the fast wave along the eastern directions and are consistent with azimuthal anisotropy and the direction of motion of the Pacific Plate (300°) and Amur Plate (~120°) depending on the epicenter–station direction. In Primorye, in the vicinity of the TEY station, the polarization azimuths of the fast ScS wave dominate in the interval of NE–E directions orthogonally to the lines of the mantle flow along the complex 3D surface of the subsiding Pacific Plate. It is revealed that the delay time of ScS waves increases to 2 s and 3.4 s in the upper mantle and in the mantle transition zone, respectively, as the depth of events increases. The highest degree of anisotropy manifests itself in the upper mantle. In the case of the vertical propagation of waves under conditions of horizontal mantle flow, the difference in the arrival times of the ScS waves is the lowest. The anisotropy in the upper zone of the transition mantle can be related to the wadsleyite texture with the polarization of the fast ScS wave parallel or orthogonally to the motion of the stagnant plate. The anisotropy in the upper part of the lower mantle is associated with the texture of perovskite and periclase with the orientation of the symmetry axis and the polarization of the ScS waves in parallel to the plate subsidence and the subduction direction.

Based on the published GPS data for the Upper Amur region, the parameters of the tensor of strain rates are calculated. In particular, we obtain such parameters of the main rates and main directions of strains, maximum shear strain rates and its directions, dilatancy rate, and second invariant of the strain rate tensor. The obtained results indicate a high tectonic activity in the zone of interaction between the Eurasian and Amurian plates and Stanovoy geoblock. The zones of high-rate aseismic displacements are revealed. It is supposed that NE-trending faults will be activated at the contemporary stage at the boundary between the Eurasian and Amurian plates.

On August 14, 2016, at 11 h 15 min UTC, an earthquake with Mw = 5.8 occurred at ~9 km depth in the central part of Sakhalin, near its western coast. The shaking intensity in the epicentral zone was up to VII on the MSK-64 scale. Based on the aggregated seismotectonic data, we find that the slip occurred on the cutting fault joining two large regional fault zones of submeridional trends, the West Sakhalin and Central Sakhalin ones. By the method of waveform inversion we determine the seismic moment tensor and model the earthquake source. In the source, we reveal subhorizontal NE–SW-directed compressional stresses which agree with the contemporary character of deformations in the central part of the island. The slip type is a NW-dipping reverse fault with an insignificant strike-slip component. On the basis of reconstructed slips in the earthquake source, we calculate the coseismic deformations of the earth’s surface in the epicentral zone.