5:15 PM - 6:45 PM
[SSS05-P15] Pseudotachylyte-like materials in fault gouge of the Median Tectonic Line in Shikoku, southwestern Japan
Keywords:Pseudotachylyte, Median Tectonic Line
Pseudotachylyte represents the most robust and established marker of seismic slip in the geological record (Rowe and Griffith, 2015). On the other hand, pseudotachylyte is rare in the rock record compared to the frequency and distribution of earthquakes in active faults (Kirkpatrick and Rowe, 2013). That is one of the big enigmas in fault rock research. In recent years, researchers have focused on preservation bias caused by the rapid destruction and recycling of pseudotachylyte veins in active faults to implicate the crucial topic. Naturally, pseudotachylyte could be heavily unstable in fault damage zones, because many pseudotachylytes are influenced by any brittle to viscous overprinting and infiltration of hydrothermal fluids which can further contribute to obliterating pseudotachylytes primary features. The short preservation of pseudotachylyte is shown by laboratory experiments (e.g., Fondriest et al., 2020)
In this study, we report intriguing pseudotachylyte-like materials in fault gouge in the Median Tectonic Line (MTL) in Japan. The occurrences of the remarkable texture are not completely consistent with the implication of short preservation of pseudotachylytes.
We drilled a borehole to obtain a geological core sample penetrating the MTL in Ehime Prefecture. The total core length is 250 m long. The fault plane of the MTL is within 37.45 m depth. The hanging wall and footwall zones are the Izumi Group in Ryoke Belt and Sanbagawa metamorphic rocks, respectively. The gouges are distributed with c.a. 5 m-width in the Izumi Group.
The pseudotachylyte-like materials in the fault gouge are not discerned by the naked eye. However, many pseudotachylyte-like materials are observed as black patches along shear planes in the fault gouge using optical microscopes. The size of the black patches is from a few micrometers to several millimeters. The black patches are filled with small fragments on backscattered electron (BSE) images. The size of fragments is from a few to several hundreds of micrometers. Most fragments distinctly show embayment shape which is one of the texture evidence to diagnose pseudotachylytes.
Energy dispersive spectroscopy (EDS) performed in the black patches cleared that Si contents of fragments widely vary from 95% to 70%. Al and K contents change depending on Si contents. The original rock of the fault gouge is alteration rocks of sand and mudstone of the Izumi Group (hereafter, Izumi rock). The main rock-forming minerals of the Izumi rock are quarts and feldspars. Si contents of quartz could not have a wide range, because of very strong against weathering. Additionally, the chemical composition of the fragments in the black patches does not correspond to those of other minerals in the Izumi rock. Therefore, given the conspicuous embayment texture and chemical composition, it is likely to be considered that the fragments were reformed by melting in fast slips.
Under conditions where phyllosilicate growth is favored and water is available, the primary glass may incorporate water during devitrification to crystallize hydrous minerals. On the other hand, the growth of phyllosilicate could also play a role as a barrier to percolation of hydrothermal water. Several pseudotachylytes could be preserved due to the encompassment of clay minerals in fault gouge.
In this study, we report intriguing pseudotachylyte-like materials in fault gouge in the Median Tectonic Line (MTL) in Japan. The occurrences of the remarkable texture are not completely consistent with the implication of short preservation of pseudotachylytes.
We drilled a borehole to obtain a geological core sample penetrating the MTL in Ehime Prefecture. The total core length is 250 m long. The fault plane of the MTL is within 37.45 m depth. The hanging wall and footwall zones are the Izumi Group in Ryoke Belt and Sanbagawa metamorphic rocks, respectively. The gouges are distributed with c.a. 5 m-width in the Izumi Group.
The pseudotachylyte-like materials in the fault gouge are not discerned by the naked eye. However, many pseudotachylyte-like materials are observed as black patches along shear planes in the fault gouge using optical microscopes. The size of the black patches is from a few micrometers to several millimeters. The black patches are filled with small fragments on backscattered electron (BSE) images. The size of fragments is from a few to several hundreds of micrometers. Most fragments distinctly show embayment shape which is one of the texture evidence to diagnose pseudotachylytes.
Energy dispersive spectroscopy (EDS) performed in the black patches cleared that Si contents of fragments widely vary from 95% to 70%. Al and K contents change depending on Si contents. The original rock of the fault gouge is alteration rocks of sand and mudstone of the Izumi Group (hereafter, Izumi rock). The main rock-forming minerals of the Izumi rock are quarts and feldspars. Si contents of quartz could not have a wide range, because of very strong against weathering. Additionally, the chemical composition of the fragments in the black patches does not correspond to those of other minerals in the Izumi rock. Therefore, given the conspicuous embayment texture and chemical composition, it is likely to be considered that the fragments were reformed by melting in fast slips.
Under conditions where phyllosilicate growth is favored and water is available, the primary glass may incorporate water during devitrification to crystallize hydrous minerals. On the other hand, the growth of phyllosilicate could also play a role as a barrier to percolation of hydrothermal water. Several pseudotachylytes could be preserved due to the encompassment of clay minerals in fault gouge.