塑性流动网络控制下川滇菱形块体及邻区构造应力场与地震构造

TECTONIC STRESS FIELD AND SEISMIC TECTONICS CONTROLLED BY PLASTIC-FLOW NETWORK IN THE SICHUAN-YUNNAN RHOMBIC BLOCK AND ITS ADJACENT AREAS

川滇地区位于中东亚塑性流动网络系统东南部 ,研究区内岩石圈下层含右向网带 6条和左向网带 16条 ,受其控制在多震层内形成相应的地震带。多震层和岩石圈下层的构造应力场在总趋势上基本一致 ,进一步证明了下层网状流动对上层的控制。沿网带以不同交角展布的发震断裂组成地震构造带 ,其中多数右向地震构造带已发育成熟 (视成熟度Λ≥ 0 .8) ,而左向带除大理 -通海和腾冲 -景洪两带接近成熟外 ,多数的Λ值显著小于 0 .8。“川滇菱形块体”因塑性流动网络的存在和块体边界的变迁 ,其现今构造和动力学涵义有待探讨。

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.