A Study On The Deep Structure Of The Northern Section Of Northeast China And North - South China Structural Belt And Its Significance

Continental dynamics is one of the frontier fields in the geological research, which is aimed at studying the composition, structure, evolution and dynamic mechanism of the continental lithosphere. The seismic tomography is the most efficient method to study the deep structure of the continent and its dynamic mechanism at present. Northeast Japan is located in the eastern margin of the Eurasian continent and the western circum-Pacific seismic belt, which belongs to the tectonic system of the subduction plate boundary. The Helan-Chuandian North-South tectonic belt is the classic intracontinental tectonic belt and very important seismic belt in the interior of Chinese Mainland, and its northern segment (also called Helan-Liupan-Ordos western margin tectonic belt) is located in central region of the northern Chinese Mainland in the interior of the Eurasian continent, which belongs to the tectonic system of the interior of the continent. Therefore, we selected the two regions as our study areas and inverted the three-dimensional (3-D) velocity and P-wave anisotropic structures of the crust and uppermost mantle under the study areas using the seismic tomography.We determined the high-resolution3-D velocity structure as well as the anisotropic structure beneath Northeast Japan. Our results show that the hypocenters of the mainshock and three large aftershocks of the2008Iwate-Miyagi earthquake (M7.2) are located in a boundary zone where both seismic velocity and Poisson’s ratio change drastically in a short distance. A zone with pronounced low-velocity and high Poisson’s ratio is revealed in the lower crust and uppermost mantle under the source area and active volcanoes in Northeast Japan. These results indicate that the seismic and volcanic activities are influenced by the ascending arc magma and fluids associated with the dehydration reactions of the subducting Pacific slab and the mantle convection in the mantle wedge under Northeast Japan. The complex images of P-wave anisotropy also reflect the strong heterogeneities under the study area. The obtained P-wave fast-velocity directions (FVDs) in the uppermost mantle (40km depth) are nearly normal to the strike of the trench in most of the study area, which is consistent with a model showing that the olivine a axis aligns with the transport direction induced by the slab-driven corner flow in the mantle wedge. However, the FVDs in the fore-arc are generally parallel to the trench, which may be caused by simple corner flow geometry when the B-type olivine fabric dominates in the fore-arc area.The P-wave velocity and anisotropic structures in the crust and uppermost mantle under the Helan-Liupan-Ordos western margin tectonic belt show that several high-and low-velocity anomalies were revealed in the upper crust, which reflect the significant different surface geological features in the different tectonic units. The prominent low-velocity (low-V) anomalies exist widely in the lower crust beneath the study region and the low-V zones extend to the uppermost mantle in some local areas, suggesting that the lower crust contains higher-temperature materials and fluids related to the northeastward or eastward extension of the northeastern margin of the Qinghai-Tibetan Plateau and the local upper mantle upwelling, which is consistent with the relatively high surface heat-flow values in this region and high-conductivity zones existing in the lower crust under the main part of the study area. The obvious high-V anomalies exist in the lowermost crust and uppermost mantle, which may indicate the residual ancient blocks, such as Qinling microplate and Qilian Block. The major fault zones, especially the large boundary faults of major tectonic units, are located at the transition zones between the low-V and high-V anomalies in the upper crust, and extend to the low-V anomalies in the lower crust, whereas individual faults cut through the low-V anomalies in the lower crust. These results indicate that the velocity anomalies structure beneath this region not only is controlled by the large faults, but also affect the distribution and extension of the large faults in the study area. Most of large historical earthquakes are located in the boundary zones where P-wave velocity changes drastically in a short distance. Beneath the source zones of most of the large historical earthquakes, prominent low-V anomalies are visible in the lower crust and even the uppermost mantle, which may represent geothermal anomalies or fluid reservoirs beneath the source area, finally causing the large intracontinental earthquakes. Significant P-wave azimuthal anisotropy is revealed in the study region, and the pattern of anisotropy in the upper crust is consistent with the surface geologic features. The FVDs in the lower crust are complex and may be caused by the lattice preferred orientation of minerals, which may reflect the lower-crustal ductile flow with varied directions. The pattern of anisotropy in the uppermost mantle is similar to that in the lower crust. However, the FVDs in the high-V anomalies in the uppermost mantle may represent the original fossil anisotropy of the residual ancient blocks. In addition, combining the present results with the previous anisotropic study results, we speculate that double-layer flows, i.e., the lower-crust flow and the upper-mantle flow, coexist in the crust and upper mantle beneath the study region and adjacent areas.In summary, the velocity anomalies structure of the crust and upper mantle as well as the seismic and volcanic activities are associated with the dehydration reactions of the subducting Pacific slab and the mantle convection in the mantle wedge in Northeast Japan. However, the complex velocity anomalies structure of the crust and uppermost mantle and the seismic activities are influenced by the early geological structure, the northeastward or eastward extension of the northeastern margin of the Qinghai-Tibetan Plateau and the local upper mantle upwelling in the Helan-Liupan-Ordos western margin tectonic belt. Therefore, the deep dynamic processes in the margin and interior of the continent are obviously different.