关键词: 逆走滑运动, 第四纪, 丽江-小金河断裂

Abstract: The Lijiang-Xiaojinhe Fault lies in Northwest Yunnan Plateau, striking N40°E and extending for 360km. It is an important transverse active fault, obliquely cutting the "Sichuan Yunnan Rhombic Block" in southwest China. Interpretation of aerophotos, field investigation, and age determination of this active fault have revealed that the fault is an active reverse sinistral strike slip fault, dipping northwest at high angle. Relocation of basins and analysis of synsedimentary dislocation of the basins provide an estimate of the horizontal dislocation amount along the fault of 7.4～7.6km since Quaternary. In addition to the large scale horizontal movement, the vertical differentiation of relief and landforms on both sides of the fault is also strikingly shown. The relief along the fault strand from Lijiang northeastward to Ninglang and Muli is higher on the northwest side and lower on the southeast side, between which lie a series of steep fault scarps and a topographically steep zone. Comparison of the relief on both sides of the fault shows that the vertical throw of the fault is 500～700m or more. The horizontal and vertical dislocation rates along the Lijiang Xiaojinhe Fault since Quaternary and since middle Pleistocene are estimated preliminary to be 3.7～3.8mm/a and 1.0～1.5mm/a, respectively. In the northeast of Lijiang, especially along the Changping Muzhuda and Baerqiao area in Ninglang, a series of evidence of sinistral horizontal dislocation, such as offset ridges, gullies, proluvial fans, and terraces was found. The amount of horizontal dislocation varies from several meters to hundreds meters. All these may indicate the characteristics of obvious movement along the fault since late Quaternary. The amounts of horizontal dislocation and the related data of age determination indicate that the average dislocation rate since late Pleistocene is in the range of 2.6～ 4.0 mm/a with a mean value of 3.3mm/a; the average dislocation rate since Holocene is in the range of 2.5～5.0mm/a with a mean value of 3.5mm/a. The similarity of slip rates in various stages of Quaternary indicates the relative stability of movement along the fault and the long range persistence of the active block boundary. The compression and overthrust of the shallow layer crust, the existence of deep geophysical gradient zone and zonal area of strong seismicity along the fault, as well as the variation in movement rate of blocks on both sides of the fault reveal the significant shielding effect of the Lijiang Xiaojinhe fault on the southward migration and "flow" of the mass in the middle upper crust along the "Sichuan Yunnan Rhombic Block" after the uplift of Qinghai Xizang Plateau.

Key words: Reverse strike-slip fault, Quaternary period, Lijiang-Xiaojinhe fault