A Preliminary Study on the Three-dimensional Microstructures of Fault Tectonite in the Pearl River Delta Region

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.