17:15 〜 19:15
[SCG45-P05] Damage Asymmetry of the Median Tectonic Line in Central Shikoku, SW Japan
キーワード：中央構造線、断層破砕帯、X-CT画像、応力場
The Architecture of a fault zone contains the history and mechanisms of fault development. A brittle fault zone generally consists of the fault core and fault damage zone (Shipton et al., 2006). The fault core accommodates the main displacement along fault zone. The fault damage zone is identified as a product of fractured rocks around the fault core during fault slip (Caine et al., 1996). The fault damage zone plays a crucial role in fluid flow because damaged rocks tend to be more permeable than intact rocks (Lockner et al., 2009). The fault damage zones are occasionally known to have asymmetric structures due to several factors such as lithology and deformation history between the hanging wall and footwall, dynamic stress asymmetry during a propagating rupture and fault geometry. Savage et al. (2021) suggested that the asymmetric fault damage zone of the Pāpaku fault resulted from folding and fracturing in the hanging wall, more ductile deformation in the footwall, and/or stress asymmetry around the fault created during fault slip. In a bi-material fault zone juxtaposing stiff crystalline rocks and compliant porous rocks, the fault damage zone often exhibits an asymmetric structure with more damage in the crystalline rocks (e.g., Aben et al., 2017). These studies mainly focus on pulverization in response to fracturing under high shot velocity (>5 m/s). However, the Median Tectonic Line (MTL), which juxtaposes sedimentary rocks in the hanging wall with metamorphic rocks in the footwall in Japan, is known to experience different slip behaviors at different crustal depths (e.g., Shigematsu et al., 2017). In such long-lived faults, the fault zone structure and stress field would be more complex. Therefore, we examine the width of the fault damage zone and stress conditions between hanging wall and footwall of the MTL to understand what factors control the characteristics of the fault damage zone of the MTL.
We obtained a 125-meter-long geological core sample that penetrates the inactive zone of the MTL in central Shikoku, SW Japan. The lithology of the core sample consists of alternating sandstone and mudstone from the Cretaceous Izumi Group in the hanging wall and pelitic schist and psammitic schist with tuffaceous rock lenses from the Sanbagawa metamorphic rocks in the footwall. The fault core consists of ~ 6 cm thick fault gouge originating from mudstone and an altered dike.
First, open fractures were automatically detected using the Image Trace Tool in Adobe illustratorTM for X-CT images of the drilling cores. The width of the fault damage zone was determined by auto-tracing fracture density at each 1 m depth. As a result, the fault damage zone of the MTL shows an asymmetric pattern, measuring approximately 22.4 m in the hanging wall and about 2.0 m in the footwall. This observation aligns with the characteristics of the fault damage zone estimated from geophysical data such as X-CT values, rock densities and seismic wave velocities.
Next, stress states were estimated from paleostress analysis based on fault slip data and vein orientations (Sato 2006; Yamaji 2016). The stress analysis suggests that the footwall experienced a simple stress field, while the hanging wall experienced more complex stress fields. In the footwall, a normal-faulting regime with a NW-SE σ3 axis, estimated from fault slip data is consistent with the stress state estimated from vein orientations. Fluid inclusion analyses suggest that the veins formed at temperatures above approximately 250 °C (Fukunari et al., 2011). Therefore, these stress fields in the footwall are interpreted as paleostress fields at a depth of ~10km. In the hanging wall, not only a normal-faulting regime with a NW-SE σ3 axis during vein formation but also strike-slip- and reverse-faulting regimes were detected. These compressive stress fields detected only in the hanging wall are considered to be the latter stress fields experienced after the vein formation. The difference in stress fields between the hanging wall and footwall may be related to the asymmetric fault damage zone estimated from fracture density. The asymmetric pattern with more damage in the hanging wall in this study is different from the damage asymmetry reported in the bi-material fault zone (e.g., Aben et al., 2017). This may suggest that other factors, including differences in stress state, are dominant in the damage asymmetry of the MTL rather than pulverization at high strain rates.
We obtained a 125-meter-long geological core sample that penetrates the inactive zone of the MTL in central Shikoku, SW Japan. The lithology of the core sample consists of alternating sandstone and mudstone from the Cretaceous Izumi Group in the hanging wall and pelitic schist and psammitic schist with tuffaceous rock lenses from the Sanbagawa metamorphic rocks in the footwall. The fault core consists of ~ 6 cm thick fault gouge originating from mudstone and an altered dike.
First, open fractures were automatically detected using the Image Trace Tool in Adobe illustratorTM for X-CT images of the drilling cores. The width of the fault damage zone was determined by auto-tracing fracture density at each 1 m depth. As a result, the fault damage zone of the MTL shows an asymmetric pattern, measuring approximately 22.4 m in the hanging wall and about 2.0 m in the footwall. This observation aligns with the characteristics of the fault damage zone estimated from geophysical data such as X-CT values, rock densities and seismic wave velocities.
Next, stress states were estimated from paleostress analysis based on fault slip data and vein orientations (Sato 2006; Yamaji 2016). The stress analysis suggests that the footwall experienced a simple stress field, while the hanging wall experienced more complex stress fields. In the footwall, a normal-faulting regime with a NW-SE σ3 axis, estimated from fault slip data is consistent with the stress state estimated from vein orientations. Fluid inclusion analyses suggest that the veins formed at temperatures above approximately 250 °C (Fukunari et al., 2011). Therefore, these stress fields in the footwall are interpreted as paleostress fields at a depth of ~10km. In the hanging wall, not only a normal-faulting regime with a NW-SE σ3 axis during vein formation but also strike-slip- and reverse-faulting regimes were detected. These compressive stress fields detected only in the hanging wall are considered to be the latter stress fields experienced after the vein formation. The difference in stress fields between the hanging wall and footwall may be related to the asymmetric fault damage zone estimated from fracture density. The asymmetric pattern with more damage in the hanging wall in this study is different from the damage asymmetry reported in the bi-material fault zone (e.g., Aben et al., 2017). This may suggest that other factors, including differences in stress state, are dominant in the damage asymmetry of the MTL rather than pulverization at high strain rates.