黄土地区活动断层的显微构造和显微沉积学探测

以发育在黄土地层的陕西省西安市临潼—长安断裂为对象,采用显微构造和显微沉积学分析的方法,从微观的角度探讨黄土地层中的活动断层。观测了该断裂上的7个露头剖面和一条探槽剖面,采集样品42件。在室内对所采集的样品进行了显微构造、超微构造和能谱测定(扫描电镜)以及显微沉积学分析。研究结果表明,野外宏观观测难以辨认的断层迹象,在断层带样品的切片(大薄片)上肉眼即可清楚辨别。显微构造观测发现,断层在微观上可以表现为错动面、较宽的断层带、断层破碎带、变形条带、泥质条带和愈合的张裂隙等。扫描电镜观测和能谱测定显示,断层破碎带或泥质条带与其外围地层具有不同的超微结构,而且成分有所变化,表明断层活动过程中有流体的参与,并导致某些元素的带出和带入。粒度分析结果发现,断层样品相对于外围样品,粒度>0.03mm的颗粒减少,<0.03mm的颗粒和粘土质增加,表明黄土地层的断层活动仍有碎裂作用发生。分形分析结果表明,黄土地层的碎屑颗粒分形特征呈弧形分布,但断层样品更趋于线性分布。此外,在其中一个剖面的样品中,发现了震击物(seismite)的一些微观标志,如球状体构造、负荷构造和流体通道构造等。所有研究结果表明,显微构造和显微沉积学分析是黄土地层中活动断层研究的有效手段。

Microstructural and microsedimentologic observations of active faults in loess area.

Active fault in loess formation usually did not leave macroscopically visible trace.The active fault in loess formation, therefore, can only be identified through the dislocation of a specific marker horizon, such as calcareous concretion bed in loess formation.Once the dislocation of marker horizon cannot be traced, the identification and timing of the fault may evoke much controversy.Therefore, it is necessary to develop a new methodology for the identification of active faults in loess formation.Recently, a lot of data have shown that microscopic analysis might be an effective method for resolving this issue.In this study, an attempt has been made to identify the active fault in loess formation by means of microstructural and microsedimentologic observations.A case study on the Lintong-Chang'an fault developed in loess formation in Xi'an, Shaanxi Province is illustrated in this paper.Along this fault, 7 natural outcrops and 1 trench across the fault were observed, and 42 samples were collected.Microstructural observation by both optical microscopy and Scanning Electron Microscopy (SEM), as well as microsedimentologic analysis have been carried out on the collected samples.The results show that, the macroscopically obscured trace of the fault might easily be recognized in the large thin section cut from the sample collected across the fault plane.Microstructural observation shows that microscopically the fault may appear as a relatively wide micro-fault zone, micro-fault shutter zone, pelitic bands, filled tensile fissures and deformation bands.SEM-EDX analysis of the samples shows that the microstructures and compositions of the materials on both sides of the fault trace are to some extent varied.This may indicate the participation of fluids during the faulting process, causing the migration of elements.Grain-size analysis of the clastic particles of the loess samples collected on and outside the fault shows the reduction in percentage of grains greater than 0.03 mm in diameter and the increase in the percentage of grains less than 0.03 mm in diameter, indicating that even in loess formation the faulting might cause further cracking of grains greater than 0.03 mm in diameter.In other words, cataclasis may occur during the faulting process of loess.particle size distribution (PSD) analysis shows that the PSDs of all the samples show an arcuate shape, but the PSD of the sample collected on the fault shows a shape closer to the linear distribution.Moreover, microscopic indicators comparable to the macroscopic indicators of seismites, such as ball and pillow, load structure, fluid flow, mass flow and the floatation of organics, are observed in the samples collected from the trench.All the results show that microscopic analysis is an effective approach to the identification of obscured active fault and paleo-seismic event in loess formation.