Abstract: The faults developed in unconsolidated soft sediments are often macroscopically obscured,so that their occurrence and mode of motion are difficult to be identified only by macroscopic observation.Recently,a lot of data have shown that microscopic analysis is an effective approach to the identification of obscured faults and paleo-earthquake event in soft sediments.Our purpose,therefore,is focused mainly on the exploration of the micro-structural and micro-sedimentological indicators of obscured faults and paleo-seismic events in soft sediments.A case study on a trench dug across the Shunyi-Qianmen-Liangxiang Fault at Shunyi,Beijing is presented in this paper.Microstructural observation reveals that the obscured fault in soft sediments can be traced distinctly under microscopic scale.Microscopically,the fault can be identified by its various appearances,such as distinct slip surface,micro-rupture zone,deformation bands,deformation band shear zone,band of clay infillings,and band of concentration and orientation of clay minerals or clastic particles.Micro-sedimentological analy～sis shows that the disturbance of micro-bedding,as well as the changes of angularity,size and distribution pattern of clastic particles from the beds outside and inside the fault can be taken as the micro-sedimentological indicators of obscured fault in soft sedi～ments.The obtained high D value(～2.5)of the particle size distribution(PSD)is believed to represent sudden seismic faulting event.Our microscopic observations have also revealed that the macroscopic indicators of seismites,such as balls and pillows,loading structures,mass flow,liquid flow and floatation of organics,are also reflected in microscopic scale.We believe that these microscopic indicators of seismite can also be used as the indicators of paleo-seismic event in soft sediments.All the results show that microscopic analysis is an effective method for identifying obscured fault and paleo-seismic event in soft sediments.

Key words: soft sediments, active fault, micro-structural analysis, micro-sedimentological analysis, Shunyi-Qianmen-Liangxiang fault

摘要： 松散沉积物中的断层常常未留下明显的宏观形迹,单靠肉眼观测往往难以确定断层是否存在,即使判断出有断层亦常难以确定其活动性质和活动方式.大量资料表明,微观分析是探讨松散沉积物中断层和古地震事件的行之有效的方法.文中通过对采自顺义-前门-良乡断裂的顺义油库探槽样品的微观分析,探寻松散沉积物中宏观形迹不明断层的显微沉积学和显微构造标志,并据此判断是否伴随古地震事件.研究结果表明,松散沉积物中的断层在微观上表现为明显的滑移错动面、微破碎带、变形条带、泥质充填条带、黏土矿物集中和定向的带、铁质物集中的带以及碎屑颗粒集中和定向的带等.显微沉积学研究则表明,断层及其附近的地层中显微纹层常发生扰动现象,断层内样品碎屑颗粒粒度分布的粗尾段降低而细尾端增高,棱角状颗粒含量明显增加;扫描电镜分析表明这些碎屑颗粒有新的破裂产生,而分形研究获得了断层内样品较高的分维值(D约为2.5).上述特征可以作为松散沉积物中宏观形迹不明断层的微观判断标志.此外,本研究还发现了与震击物的一些宏观标志,如球状体或枕状体、负荷构造、流体流、固体物质流以及有机物的悬浮等非常相似的微观特征.显然,它们可以作为判断古地震事件的微观标志.上述结果表明,微观分析是探讨松散沉积物中形迹不明断层和古地震事件的有效手段.

关键词: 松散沉积物, 活动断层, 显微构造分析, 显微沉积学分析, 顺义-前门-良乡断裂