Abstract: In recent years,the project of active fault prospecting in major cities has been widely carried out in China. However,active faults are usually developed in soft sediments,and sometimes do not leave any macroscopically observable trace. Therefore,even though fault-crossing trench is excavated,the key problems concerning the upward or downward propagation and the property of these obscured faults,as well as the mode of faulting and the time of the last faulting event can not be solved by macroscopic observation alone. In this paper,a microscopic observation method for resolving these problems is introduced through a case study of the active fault prospecting in Urumqi,Xinjiang Uygur Autonomous Region. Macroscopic observations have been carried out on 13 trenches and natural outcrops,and 53 samples have been systematically collected. Detailed micro-structural investigations have been carried out on 106 thin sections cut from the samples,while the grain-size analysis,particle size distribution(PSD)analysis and the statistic measurement of the angularity of the clastic grains from the samples have also been carried out. The problems concerning the mode of faulting,the occurrence of obscured fault,and whether the fault has cut through the overlying strata are resolved through the discovery of microscopic appearances of the faults,the finding of microstructures indicative of seismites,as well as the variation of angularity,grain-size and PSD of the clastic grains of the samples collected on and outside the fault. The results show that microscopic observation is an effective approach to the identification of obscured active faults in soft sediments,and it is widely applicable to the project of active fault prospecting in major cities currently carried out in China.

Key words: soft sediments, active faults, microstructures, micro-sedimentology, seismites, Urumqi

摘要： 目前中国正在广泛开展城市活动断层的探测。但是,这些断层常发育在松散沉积物中,没有明显的宏观形迹。因此,即便开挖了探槽,也往往难以解决这些隐性断层(即宏观形迹不明断层)的延伸情况、是否穿过上覆地层,以及断层的活动性质和方式等关键问题。通过观测乌鲁木齐市的西山和王家沟断裂的13个探槽和天然露头剖面,系统采集样品53件。在室内进行了显微构造、碎屑颗粒棱角度和粒度观测、以及粒度分布分析等。通过显微构造观测所揭示的断层微观表现形式、震击物的微观构造标志以及样品中碎屑颗粒棱角度、粒度大小和分形特征的变化等,解决了宏观探测无法查明的是否存在隐性断层以及有关断层活动方式、断层是否穿过上覆地层等疑难问题。结果表明,微观观测是松散沉积物中宏观形迹不明的活动断层探测的有效手段,它在城市活断层探测工程中具有广阔的应用前景。

关键词: 松散沉积物, 活动断层, 显微构造, 显微沉积学, 震击物, 乌鲁木齐