Abstract: We have studied the variations of microstructures associated with the motion transformation in the central-southern section of the Red River Fault.Our work includes careful observation of microstructures of the 18 mylonite,12 breccia,10 cataclasite,and 8 sandy conglomerate samples from the fault,some of which are dated by the fission track in apatite(FT)method.Meanwhile,fault slip data are used to infer tectonic stress tensors.The results show that the microstructure of mylonite is indicative of left-slip during the early stage of movement of the Red River Fault.The FT ages of the mylonite and undeformed sandy conglomerate exceed 20Ma,implying a thermal event occurred on the Red River Fault at that time.The microstructure of catalsite and breccia shows the right-slip during the late period of the fault movement.These features and FT ages(9.9～12.7,6.8～8.4,2～4.6Ma BP)suggest that the Red River Fault has experienced three dislocation events since the Miocene.The first is indicated by broken mylonite with compressive microstructure,and fault occurrence and slickenlines on the fault plane plunging both less than 30°,indicative of dominant strike-slip,and an activity age of 9.9～12.7Ma.The second has slickenlines plunging between 35°～45°(42° on average),implying dominant right-lateral slip with normal faulting component,which occurred during 6.8～8.4Ma.And the third event is featured by slickenlines plunging 35°～70°(65° on average),meaning normal faulting with a strike-slip component,which happened probably during 2.0～4.6Ma BP.

Key words: Red River Fault, fault rock, microscopic structure, FT dating

摘要： 通过对红河断裂带中南段18个糜棱岩、12个角砾岩、10个碎裂岩和8个砂砾岩的显微结构详细观察,部分典型样品的磷灰石裂变径迹测年(简称FT测年,以下同),获得了红河断裂带中南段断层活动转换过程中微观特征的变化:糜棱岩的微结构特征显示红河断裂带早期断层活动为左旋走滑,糜棱岩和未变形的砂砾岩FT年龄均>20Ma,表明这期间红河断裂带曾发生一次构造热事件;碎裂岩和角砾岩的微结构特征显示红河断裂带后期断层活动为右旋走滑,微结构特征不同的碎裂岩和角砾岩的FT年龄可分3个时段(9.9～12.7,6.8～8.4,2～4.6MaBP),表明红河断裂带中新世以来曾发生3次断层位错事件。

关键词: 红河断裂带, 断层岩, 显微结构, FT测年