Abstract:We have studied the structural geology and geomorphology of the fault zones in the junction area of the Angara-Lena uplift and the Predbaikalsky trough. We have analyzed faults and folds and reconstructed paleostresses for this junction area named the Irkutsk amphitheatre. Our study shows that syn-fold (Middle Paleozoic) faults include thrusts, reverse faults and strike-slip faults with reverse components, that occurred due to compression from the neighbouring folded region. Recently, contrary to compression, faulting took place under the conditions of extension of the sedimentary cover: most of these recent faults have been classified as normal faults. In the Late Cenozoic, the platform cover was subjected to brittle and partly plicative deformation due to the NW-SE-trending extension that is most clearly observed in the adjacent Baikal rift. Thus, the divergent boundary between the Siberian block of the North Eurasian plate and the Transbaikalia block of the Amur plate is a zone of dynamic influence, which occupies the area considerably exceeding the mountainous region on the Siberian platform. Important factors of faulting are differentiated vertical movements of the blocks comprising the platform. Such vertical movements might have been related to displacements of brine volumes. In the Late Cenozoic basins, movements along separate faults took place in the Late Pleistocene -Holocene.Key words: Siberian platform; Late Cenozoic stress state; active fault; kinematics GEODYNAMICS & TECTONOPHYSICS P U B L I S H E D B Y T H E I N S T I T U T E O F T H E E A R T H ' S C R U S T S I B E R I A N B R A N C H O F R U S S I A N A C A D E M Y O F S C I E N C E S R e c e n t G e o d y n a m i c s RESEARCH ARTICLEReceived: May 12, 2016 Revised: November 7, 2016 Recommended by S.V. Rasskazov (Guest Editor) Accepted: December 15, 2016For citation : Sankov V.A., Parfeevets A.V., Miroshnichenko A.I., Byzov L.M., Lebedeva M.A., Sankov A.V., Dobrynina А.А., Kovalenko S.N., 2017. Late Cenozoic faulting and the stress state in the south-eastern segment of the Siberian platform.

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We consider the seismicity of the Erguna region in northeast China (48°–51° N, 117°–123° E) which is poorly studied from seismological point of view as it is characterized by a low level of seismic activity. We calculate focal parameters (focal mechanisms, scalar seismic moments, moment magnitudes, and hypocentral depths) for seven regional earthquakes with Mw 4.2–4.6 that occurred in 2000–2017 using global seismic data of Rayleigh- and Love-wave amplitude spectra and P-wave first-motion polarities recorded at regional stations. It has been shown that the study earthquakes are of small magnitudes (Mw 4.2–4.6), of various hypocentral depths (3–37 km), and are characterized by different kinematics in their sources (normal and thrust faults, strike slips). The different faulting mechanisms could reflect local stress redistribution in small-scale crustal blocks bordered by local short-length nonconnecting faults. The available geophysical and geological data evidence that the observed features of the seismic process in the Erguna region—low-seismic activity and inhomogeneity of the stress-strain field—are likely to be controlled by the structure of the crust and the upper mantle.

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