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

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