Despite various physio-chemical activities and complexity involved during the formation of fault gouge, it has been commonly observed that the microstructural characteristics indirectly indicates the major tectonic deformation and its consequences in the sense of geometrical analysis. This study presents the statistical investigation of internal structures embedded in fault gouges by using 3D x-ray computed tomographic images. First, the slicing plane method (SPM) quantitatively evaluates the anisotropic variation in x-ray attenuation numbers across imaginary slicing planes. The star volume method (SVD) allows inferring the preferred orientation of grains in gouges. The results of three directionally cored fault gouge specimens suggest that the maximum coefficient of variation from SPM is perpendicular to the fault plane implying that the dominant layered structures is parallel to the fault plane. The SVD reveals that the orientation of grains exists within 30 degrees of the fault plane. The observations made in this study are consistent with those in previous studies, further highlighting the feasibility of image-based analyses to understand the microstructural features of gouges.

1. Introduction

Fault zone, consisting of undeformed host rock, damaged host rock and main gouge zone, has been widely investigated to understand tectonic movement of faults. In particular, main gouge zone which is the inner element of fault zone includes the dominant brittle process of accommodating shear strain (Engelder, 1974; Chester and Logan, 1986). There were attempts to infer the sense of tectonic movements from the gouge that is the result of the movement. Anisotropy of Magnetic Susceptibility (AMS) principal directions can be utilized as a measure of quantifying the finite-strain of magnetic mineral in fault gouge (Borradaile and Henry, 1997). Shape Preferred Orientation (SPO) is the observation on the grain alignment inside gouge (Cladouhos, 1999). The relation of phyllosilicate orientation and clay fabric intensity to shear strain has been investigated (Haines, 2013). This study suggests that by using SPM embedded anisotropic features of the bulk regime of gouge can be determined in 3D without use of selecting certain objects, for example, grains or minerals. To compare the result of SPM with the analysis based on the alignment of constituents in gouge, we employed SVD that evaluates the continuity of volume for a given direction, which is an equivalent of the alignment of grains.

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