A Preliminary Study on the Three-dimensional Microstructures of Fault Tectonite in the Pearl River Delta Region
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摘要: 岩石内部成分和结构反映其形成过程的环境条件。本文利用高分辨率X射线微观层析成像（微观CT）技术，对珠江三角洲地区断层构造岩（以断层角砾岩、碎裂岩为主）进行三维成像观测和定量分析，尝试从微观角度给出该地区断裂活动特征的解释。研究珠三角地区8个断层构造岩的样品，除一个样品中仅以孔隙状态呈现外，样品中均可见明显裂隙；样品孔隙度范围在1.4%到13.4%；大量样品中可见代表高密度矿物成分的白色影像，其体积占比范围从0.7%到15.6%；裂隙和白色影像具有很好的相关性。对裂缝大小、延伸方向和延伸尺度等参数进行定量分析，并提取了裂隙体积和裂隙长度的分形维数。样品内部，不同尺度的微裂隙的优势方向具有很好的一致性；定向样品还表明，微米CT样品中裂缝的优势方位与宏观断裂的主要活动方位具有一致性。背散射电子成像及能谱分析表明，断层角砾岩的基质以石英为主，白色影像为铁氧化物，可能是流体在孔隙、裂隙中流动所逐步沉淀形成。断层角砾岩的微观结构显示其形成的力学属性为拉张环境，至少经历了岩石破碎、流水作用形成含铁矿物沉淀、胶结，以及后期的进一步破碎三个发展阶段。Abstract: The compositions and structures of rocks reflect the environmental conditions during their formation. In this paper, three-dimensional high-resolution X-ray microtomographic (micro-CT) images of fault tectonites in the Pearl River Delta region were acquired and quantitatively analyzed to investigate the fault activities in this area from the perspective of the preserved micro-structures. Eight samples were analyzed, which have porosities from 1.4% to 13.4%. High X-ray density minerals, which are bright (in white color) on CT images, can be seen in most of samples. There exists an excellent consistency between the morphology of micro-cracks and the structure of the white area on the CT images. The size, orientation and extensional dimensions of individual pore-structures are characterized. The fractal dimensions of crack volume and crack length are obtained. Not only the predominant orientations of micro-fractures of different sizes in individual samples are consistent, but based on the analysis of two orientated (direction-labelled) samples the orientations of micro-fractures and that of the main active faults at macro-scale are also consistent. Backscattering Electron Diffraction (BSED) measurements show that the matrix of breccia is quartz, while the white area on the CT image comprises mainly iron oxide, which may have been formed by the precipitation from fluids flowing in the pores and cracks. The characteristics of micro-fractures of breccia show that fracturing happened under a tensile environment. The breccia may have experienced at least three stages of evolution:fault movement and the cataclasis of rocks, iron-mineral precipitation and cementation by fluid flow during a quiet period, and later fault activity and cataclasis.
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