Submeridional boundary zone in Asia: seismicity, lithosphere structure, and the distribution of convective flows in the upper mantle

Abstract

The study is focused on the submeridional transregional boundary that stretches as a wide band along 105°E in Central Asia. In modern seismic models, it is traceable to a depth of ~600 km. In the continental area to the west of this boundary, seismic activity is increased. Following the study of the origin of the transregional boundary zone, it becomes possible to assess its contribution to the current geodynamic processes in Asia. This article presents a comprehensive analysis based on comparison of the available data with the results obtained in our study using independent methods. The distribution of earthquakes was analyzed by depth. We revealed a correlation between the characteristics of seismotectonic deformation (STD) reconstructed from earthquake focal mechanisms, the structure of P-velocity anomalies, and the distribution of convection flows in the upper mantle. The pattern of seismic velocity anomalies in the upper mantle was investigated on the basis of the data from the ISC catalogue for the period of 1964–2011. The modeling was carried out for two regional tomographic schemes, using the first arrivals of P-waves from [Koulakov et al., 2002 and PP-phases from [Bushenkova et al., 2002, with the subsequent summation with weight coefficients depending on the distribution of the input data in each scheme. A similar approach was applied in [Koulakov, Bushenkova, 2010 for the territory of Siberia; however, that model only partially covered the submeridional transregional boundary zone and was based on fewer ISC data (until 2001). The parameters of the combined model were used to estimate variations in the lithosphere thickness, which can significantly influence the structure of convection flows in the upper mantle [Chervov et al., 2014; Bushenkova et al., 2014, 2016. The thickness variations were taken into account when setting boundary conditions in the numerical modeling of thermal convection, which followed the algorithm described in [Chervov, Chernykh, 2014. The STD field was reconstructed from the earthquake focal mechanisms (M≥4.6) which occurred in Central Asia in 1976–2017. The analysis shows that the zone, wherein the seismic regime changes, correlates with the band wherein the STD principal axes are turning, the submeridional high/low velocity elongated boundary in the seismotomographic model, as well as with the submeridionally elongated descending convective flow in the upper mantle. Shortening of the STD principal axes is observed in the submeridional direction in the western part and in the sublatitudinal direction in the eastern part of the study area. The directions of the principal axes turn in the 93–105°E zone. It is thus probable that the submeridionally elongated descending convective flow in the upper mantle of this region, which results from the superposition of the lithosphere thickness heterogeneities, is a barrier to propagation of seismically manifested active geodynamic processes caused by lithospheric plates collision.