5:15 PM - 6:30 PM
[SSS08-P09] Deformation structures of a gouge layer formed along a fault in synthetic quartz during slip weakening at high slip velocities
Keywords:high-velocity friction, quartz, fault gouge
Frictional weakening of quartz rocks occurs at relatively low slip velocities (>~1 mm/s), which implies that the frictional weakening mechanism of quartz rocks may be unique. It has been suggested that the marked weakening results from the formation and shearing of finely comminuted, amorphous, wet material (Goldsby and Tullis, 2002). In fact, frictionally generated material during shearing of quartz material contains fine-grained hydrated amorphous silica (Hayashi & Tsutsumi, 2010; Rowe et al., 2019; Onoe et al., 2019 JpGU meeting). However, little is known of the deformation structures of the gouge material formed along a fault in quartz material. To get a better understanding of fault zone deformation processes in quartz material, we have done intermediate-velocity friction experiments on synthetic quartz samples and have performed transmission electron microscope (TEM) studies of the fault-zone material.
A rotary-shear frictional testing machine was used for the frictional experiments. Synthetic quartz consisting of a single crystal of quartz was utilized as the experimental sample. Solid cylindrical specimens with diameters of 25 mm were cored from the quartz block and a shallow 5-mm-diameter hole was drilled into the center of one of the end surfaces of the cylinder. Friction experiments were performed at slip velocity v = 104 mm/s and at a normal stress of 1.5 MPa. Fault weakening in quartz samples occurred in association with the formation of a ~20 μm-thick fault gouge layer. After the experiments, a thin section was made from which the samples for TEM studies were prepared with an application of a focused ion beam (FIB) system. TEM observations revealed that the fault-zone material consisted mostly of hydrated amorphous silica domain, with minor occurrence of quartz grains with grain size d < ~0.1 μm inside the domain. The amorphous silica domains are characterized by the development of its finely laminated structure; the amorphous silica domain is composed of a number of stacked ~0.5 μm-thick ultra-thin layers. Each of the ultra-thin layers appears to be composed of a package of submicron-sized grains of amorphous silica. Similar deformation structure has been reported from friction experiments on chert (Tsutsumi et al., 2015 GSJ meeting). The boundaries between each of the stacked ultra-thin unit layers are sharp and planer, which suggest that the shear deformation of the faut-zone material during the slip weakening may have been localized into such sharp layer boundaries.
A rotary-shear frictional testing machine was used for the frictional experiments. Synthetic quartz consisting of a single crystal of quartz was utilized as the experimental sample. Solid cylindrical specimens with diameters of 25 mm were cored from the quartz block and a shallow 5-mm-diameter hole was drilled into the center of one of the end surfaces of the cylinder. Friction experiments were performed at slip velocity v = 104 mm/s and at a normal stress of 1.5 MPa. Fault weakening in quartz samples occurred in association with the formation of a ~20 μm-thick fault gouge layer. After the experiments, a thin section was made from which the samples for TEM studies were prepared with an application of a focused ion beam (FIB) system. TEM observations revealed that the fault-zone material consisted mostly of hydrated amorphous silica domain, with minor occurrence of quartz grains with grain size d < ~0.1 μm inside the domain. The amorphous silica domains are characterized by the development of its finely laminated structure; the amorphous silica domain is composed of a number of stacked ~0.5 μm-thick ultra-thin layers. Each of the ultra-thin layers appears to be composed of a package of submicron-sized grains of amorphous silica. Similar deformation structure has been reported from friction experiments on chert (Tsutsumi et al., 2015 GSJ meeting). The boundaries between each of the stacked ultra-thin unit layers are sharp and planer, which suggest that the shear deformation of the faut-zone material during the slip weakening may have been localized into such sharp layer boundaries.