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, 川滇菱形块体

Abstract: The tectonic stress field and seismic tectonics in the Sichuan-Yunnan rhombic block and its adjacent areas are investigated in terms of the "plastic-flow network and multi-layered tectonic deformation" model for continental dynamics(for short, called "netlike plastic-flow" model or NPF model). According to the model, it is presented that in consideration of the transition of deformation regime from brittle to ductile with increasing 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 the two layers; 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 central eastern Asia plastic-flow network system(CEAs)developed under the compression of the Himalayan driving boundary spreads over a broad area in central eastern Asia. The Sichuan-Yunnan rhombic block is located in the southeastern part of CEAs and includes 6 right handed and 16 left handed plastic-flow belts. Under the control of these plastic-flow belts the large-scale seismic belts and the seismotectonic zones consisting of seismic faults are formed within the overlying seismogenic layer. Most of the seismic faults in this region intersect the relevant seismotectonic zones at angles within the range of -15°～+30°, being longitudinal type faults. 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. 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 36.0° to 78.5°, which are suitable for strike slip. Most of the right handed seismotectonic zones in this region have matured with the apparent maturities Λ ≥0.8(Λ is the ratio of the total length of the segments within seismic fault to that of the related seismic belt), while the Λ values of most of the left handed zones are considerably less than 0.8, called quasi seismotectonic zone, except the Dali Tonghai and Tengchong Jinghong zones which approach to maturing. It is worth noting that the previously active faults may transit to be quiescent, such as the southeastern segment of the Honghe Fault zone, since they deviate from the plastic-flow belts, whereas the new seismotectonic zones may be developed along the plastic-flow belts, such as the Dali Tonghai and Tengchong Jinghong seismotectonic zones. Thus, some boundaries of the rhombic block, such as the Honghe Fault zone, have been probably replaced by new boundaries associated with the plastic-flow belts, and therefore the contemporary "Sichuan-Yunnan rhombic block" can be considered as a tectonic unit defined by some plastic-flow belts of CEAs as its boundaries.

Key words: plastic-flow network, tectonic stress field, seismic belt, seismotectonic zone, apparent maturity, Sichuan-Yunnan rhombic block