Research On The Active Yuanmou Fault In Yunnan

The Yuanmou fault in China, which is located at southwestern margin of the North South seismic belt, significantly limits the geomorphic evolution of western Central-Yunnan block. Moreover, some problems is still further discussed, such as genetic connection between the Yuanmou fault and some basins developed along the Fault, the relationship of the activity of the Fault and seismic activity. In order to analyzing the strike-slip tectonic activity of southern segment of the North-South earthquake belt and geomorphic deformation zone since the Pliocene, the author studied on the activity of the Yuanmou fault in this period, evolution history, the latest active time, activity mode, slip rate and its temporal and spatial variation, and paleo-earthquake recurrence intervals et al, that are the rather important roles for understanding the relation of the fault activity and strong earthquake, landform deformation, coordinating role in the block movement and risk analysis on the future earthquake.The results indicate that:（1） The activity model of the Yuanmou fault generally shows the thrust in the Early Quaternary（Pliocene- Middle Pleistocene） with sinistral strike-slip motion in the Late Pleistocene and Holocene, partly extrusion or tensile-shear motion;（2） The latest activity time of the Yuanmou fault to the north of Jiangbian was later than 12.83 ± 1.09 ka B.P., and the vertical slip rate and horizontal strike-slip rate of the fault was respectively 0.04⁰.10 mm/yr and 1.70 mm/yr since the Pleistocene, with penetrated the T1 terrace. However, the above three parameters was respectively（6.92±0.70）^7.28±0.82ka B.P., about 0.1 mm/yr and 1-2 mm/yr. From the south of Jiangbian to the north of Luochuan area, with penetrated the T2 terrace and the T1 terrace was undeformed. Recrudescing interval of the fault activity is not less than 7 ka to the south of Jiangbian areas, but is 10,000¹2,000 year or less short to the north of Jiangbian areas. In addition, the vertical slip rate and horizontal strike-slip rate of the fault before the mid-Pleistocene was respectively about 0.29 mm/yr and 1.17 mm/yr from Luochuan to the south, however, the latest activity time was about the latest Pleistocene, and shown the undeformed T2 terrace.（3） The maximum horizontal compressive principal stress is in the northnortheast direction during early period of Pliocene and Early Pleistocene and is in the northnorthwest direction during since late period of Pleistocene. The partial dip-slip events are obviously indicated at Xialeinong-Jiangbian, Huatong-Leshenggu and Yipinglang-Luochuan.（4） Differences of the maximum principal compressive stress of the multi-layer shock on the Yuanmou fault and lower lithosphere and crustal deformation in eastern Qinghai-Tibet Plateau by the magnetotelluric imaging showed that the activity of the Yuanmou fault is related with ductile deformation in the lower crust flows and caused the movement of the upper crust.（5） Variation of the tectonic stress field effectively produced obvious extension in the northern segment of the fault, the sinistral strike-slip with uplift in the middle and overall uplift in the other parts rest. The migration of the block to the southsoutheast direction and deformation in Dayao and Yao’an caused the strain adjustment.