松散沉积物中活动断层的微观探测方法

松散沉积物中的活动断层常常看不到明显的宏观形迹,构成所谓的隐性断层。单靠探槽和天然露头的宏观观测,常常难以得到全面的认识。因此,开拓松散沉积物中活动断层探测的新思路和新方法是当前的迫切任务。本研究试图另辟蹊径从微观的角度解决这个问题。为此,观测了北京顺义—前门—良乡断裂、太原交城断裂、乌鲁木齐王家沟断裂和西山断裂以及西安临潼—长安断裂的24条探槽和天然露头剖面,采集原态样品140件,散状样品122件,切制大薄片275件。对所采集的样品开展了显微构造、显微沉积学以及扫描电镜(SEM)测定。研究结果表明,松散沉积物中的断层在微观上是有迹可循的,它们可以表现为变形条带、变形条带的带、滑动面、较宽的断层带、微破碎带、碎屑颗粒集中的带和铁质有机质集中的带等。断层内外样品在碎屑颗粒的棱角度、粒度大小、粒度分布特征以及表面结构特征等均有所变化。观测结果还发现了可以和震击物(seismites)的宏观构造标志相互对比的微观标志。显然,上述显微构造和显微沉积学特征可以作为松散沉积物中宏观形迹不明(隐形)断层及其是否伴有古地震事件的微观判断标志。总之,研究结果表明,微观探测是研究松散沉积物中活动断层具广阔应用前景的有效手段。

Microscopic observation method for active faults in soft sediments.

Active faults in soft sediments usually do not have distinct macroscopic appearance, forming so-called obscured faults.Therefore, active faults in soft sediments cannot be identified thoroughly through macroscopic observations of trenches or natural outcrops across the faults.Obviously, it is necessary to develop a new method for the identification of active faults in soft sediments.An attempt to identify active faults in soft sediments through microscopic observations has been made in this study.For this purpose, a total of 24 trenches and natural outcrops across the Shunyi-Qianmen-Liangxiang Fault in Beijing, Jiaocheng Fault in Taiyuan, Wangjiagou and Xishan Faults in rümqi, and Lintong-Chang'an Fault in Xi'an have been observed.During the observations, 140 samples were collected by using sampling boxes to preserve their original state, from which 275 large thin sections were cut in the lab, and 122 samples of dispersed particles were also collected at the same position.Microstructural, micro-sedimentologic and SEM observations were carried out on all the samples collected.The results show that the obscured faults in soft sediments can be identified distinctly under microscope.Microscopically, they may appear as slip surfaces, deformation bands, zone of deformation bands, relatively wide micro-fault zones, micro-shutter zones, filled tension cracks, zones of concentrated clastic grain and zones of concentrated ferruginous materials or organics.The angularity, grain-size, grain-size distribution and surface textures of the clastic grains from the samples collected on the fault plane and outside are significantly different.Microstructural observations have revealed several microstructures comparable well with macroscopic structures indicative of seismites, such as balls-and-pillows structure, load structure, fluid flow, mass flow, truncation surface and floatation of organics.Obviously, the afore-mentioned microstructural and micro-sedimentologic features can be used as the microscopic indicators of obscured faults in soft sediments, as well as for the indicators of whether the faulting process is accompanied by seismic event.The results of this study, therefore, indicate that microscopic observation is an effective approach to the identification of active faults in soft sediments.