关键词: 塑性流动网络, 构造应力场, 地震带, 地震构造带, 视成熟度, 华北

Abstract: The tectonic stress field and seismic tectonics in the North China region are investigated in terms of the "plastic-flow network and multi-layered tectonic deformation" model for continental dynamics. It is presented in the model that in consideration of the basic state of the transition of deformation regime from brittle to ductile with increase of the depth, the lithosphere is divided into several tectonic deformation layers, i.e. the ductile lower lithosphere (including the lower crust and lithospheric mantle) with netlike plastic-flow and the brittle upper lithosphere (including seismogenic layer and the shallow crust) with shear and tensile fractures, as well as the transitional layers (i.e. low-velocity / high-conductivity layers) discontinuously distributed between those mentioned above; the long-range transmission of driving force applying at continental plate boundary is carried out mainly by the netlike plastic-flow in the lower lithosphere, which controls intraplate tectonic stress field, tectonic deformation and seismic activities; the plastic-flow network, which is composed of two families of plastic-flow belts (i.e. ductile shear belts) intersecting each other, can be referred to as a "slip-line" network developed under large deformation condition. The previous study indicates that the plastic-flow network system developed under the compression at the Himalayan driving boundary spreads over a broad area in central-eastern Asia. The North China region discussed in this paper is located in the northeast part of this network system. The directions of the maximum compressive stresses in the lower lithosphere can be estimated from the lines bisecting conjugate angles of the network. Comparing these directions with those in the seismogenic layer inferred using focal-mechanism solutions, it is shown that both of them are identical with each other in general tendency, indicating that the stress field in the upper layer is controlled by that in the lower layer; meanwhile, the directions of the stress vectors in the seismogenic layer tend to deviate to some extent from the general tendency owing to the influences of local factors and estimation errors. It is emphasized that large-scale seismic belts are in fact a response to and manifestation of the plastic-flow belts in the lower lithosphere, while the large-scale seismotectonic zones in the seismogenic layer and shallow crust are controlled by the underlying plastic-flow belts. There are 6 left-handed and 6 right-handed plastic-flow belts involved in the region studied and, correspondingly, the seismotectonic zones upon them are progressively developing with different apparent maturity, Λ, i.e. the ratio of the length occupied by seismic faults to the general length of the corresponding segment of the zone. A seismotectonic zone is considered to be "mature" when the Λ-value is greater than or approximate to 0.8, such as the Yuncheng-Datong zone (i.e. Shanxi zone), Luoyang-Sanhe zone (or called Cixian-Xingtai-Sanhe zone) and Zhangjiakou-Penglai zone. On the contrary, the previously-active fault zones may transit to be quiescent since they depart from the plastic-flow belts, such as the Tancheng-Lujiang and Changdong fault zones. The analysis of 6 cases of deep-shallow combinations of seismotectonics indicates that the crossing angles between strikes of seismic faults and shallow faults in the region studied are not greater than 6.3°～8.1°, implying that the former can be inferred roughly from the latter; most of seismic faults cross the related plastic-flow belts with small angles, being the longitudinal-type faults, while a few of seismic faults with large angles, being the lateral-type; the horizontal-sliding angle of seismic faults (i.e. the angle included between the directions of the maximum compressive stress and the normal to fault strike) ranges from 44.2° to 65.7°, which are suitable for strike-slip.

Key words: Plastic-flow network, Tectonic stress field, Seismic belt, Seismotectonic zone, Apparent muturity, North China