关键词: 通甸-巍山断裂, 中段, 晚第四纪, 河流阶地

Abstract:

The Tongdian-Weishan Fault is a branch of the Honghe Fault zone,which is the boundary between the Yangtze block and the Indo-China block.Due to the limited field conditions,the study of late Quaternary Fault activity on the mid-segment of this fault is lacking and presently the time of its newest faulting and the Quaternary slip rate are not clear.Our field investigations along the fault between the areas of Yangbi county and Xierhe river,by quantitative geomorphologic deformation measurement and dating of young terrace deposits,indicate that the mid-segment of the Tongdian-Weishan Fault is a Holocene dextral strike-slip fault with the component of normal slip and its newest faulting occurred at 2.2ka BP.The terrace T₂,incised during mid-late period of late Pleistocene epoch,was offset by the mid-segment of the Tongdian-Weishan Fault and its right-lateral and vertical displacements are 45m and 6m,respectively.The late-Holocene terrace T1 and the alluvial fan were also displaced with the right-lateral and vertical displacement of 12m and 2m,respectively.The strike-slip rate of the mid-segment of the Tongdian-Weishan Fault is 1.25mm/a since mid-late period of late Pleistocene epoch.The vertical rate becomes obviously stronger from 0.17mm/a of 36～2.2ka BP to 0.91mm/a of 2.2ka BP to the present and this tendency is coherent to the character of vertical movement of Northwest Yunnan during the Holocene epoch.The strike-slip rate of the Tongdian-Weishan Fault is only a quarter of the rate of the Zhongdian Fault and Honghe Fault.It may indicate that the pull-apart basins between the Zhongdian Fault zone and Honghe Fault zone,east of the Tongdian-Weishan Fault,absorbed most of the deformation on the southwestern boundary of Sichuan-Yunnan rhombus block,and the Tongdian-Weishan Fault and its pull-apart basins are tributary to this deformation process.This study on the mid-segment of the Tongdian-Weishan Fault will be beneficial not only to provide the elementary data for seismic hazard assessment of this fault,but also to understand the spatial characteristics of distribution of tectonic deformation on the southwestern boundary of Sichuan-Yunnan rhombus block.

Key words: the Tongdian-Weishan Fault, mid-segment, late Quaternary, river terrace