Microstructures and physical properties of fault gouges from Amphoe Khao Kho, Changwat Phetchabun

Abstract

Fault gouge is formed due to tectonic forces along the fault and mainly composed of clay mineral. Despite an abundance of fault gouges in Thailand, relatively little is known about their microstructures and crystallographic preferred orientation (CPO) or fabrics of clay minerals, which play an important role in fault behavior. For example, smectite has an ability to absorb water to its structure, causing fault gouge to swell, slip, and reduce the cohesiveness of the fault gouge. This study is thus aimed to study microstructures and CPO of fault gouges and their host rocks from the fault on Highway-12 in Khao Kho area, Phetchabun. A total of 10 samples were collected to investigate mineral composition, volume fractions, and CPO by synchrotron X- ray diffraction. Results show that the CPO of fault gouges are weak and asymmetric with maxima for (001) ranging from 1.19 to 2.36 multiples of random distribution (m.r.d.). while the CPO of host rocks are from weak to quite high ranging from 1.14 to 3.87 m.r.d. Dominant clay minerals include kaolinite, Illite-mica, and Illite-smectite (68%). Despite having high clay content, fault gouges have lower magnitudes of CPO than host rocks possibly due to fault movements that largely disrupt initial CPO. In addition, an amorphous phase is observed in fault gouges, which may have resulted from the comminution of clasts ( crush- origin pseudotachylytes) . To further describe the microstructures, selected areas on the samples were further analyzed with Scanning Electron Microscopy (SEM). The presence of deformed quartz grains and goethite veins indicates deformation and hydrothermal alteration process in the fault gouges, implying aggressive fault movement. The host rocks are moderately deformed due the tectonic forces.