17:15 〜 19:15
[SSS10-P23] Deformation mechanisms and stress field induced by seamount subduction: A case study at Funafuseyama Unit, Mino belt
キーワード：沈み込み帯、海山
We focus on the relationship between seamount subduction and earthquakes in subduction zones. Geodetic and seismic observations indicate that large earthquakes are more likely to occur in regions where smooth seafloor subducts, whereas rough seafloor features may promote creep and slow slip (Wang & Bilek, 2014). A widely accepted model proposes that subducting seamounts induce plastic deformation in the overlying plate, creating complex fracture networks (Wang & Bilek, 2014). The role of fluid in this process is also emphasized (Sun et al., 2020; Chesley et al., 2021). However, compared to geophysical observations and modeling, geological studies remain limited, leaving significant gaps in our understanding of metamorphic and deformation processes, stress distribution, and their implications for earthquakes. This study aims to elucidate geological characteristics of an accreted seamount and its surrounding sedimentary facies, such as deformation mechanisms, temperature-pressure conditions, and stress distribution during accretion.
We investigated the Funafuseyama Unit of the Mino Belt in Gifu Prefecture, a Jurassic accretionary complex composed of mélanges with various-sized blocks of Permian chert, limestone, and basalt in a muddy matrix (Wakita, 2000). The limestone of shallow marine origin (Sano, 1988) and the basalt from plume activity (Ichiyama et al., 2008) suggest seamount accretion in this area. This study mainly focuses on Nukumitani as the main body of subducted seamount dominated by basalt, and compares it with Shirakuradani and Tarudani where chert and mélange are dominantly observed. In this study, we first created a route map and sketches of the outcrop at the bottom of basalt body. From samples, the chemical composition of cataclasite was analyzed using µXRF. Thin-section observations and µXRD analyses were performed for mineral identification. Furthermore, the stress inversion analysis (Yamaji, 2000) was also carried out using meso-scale faults and extension veins data collected from the geological survey. The maximum temperature for the muddy matrix was determined by Raman spectroscopy of carbonaceous material.
In Nukumitani, basalt outcrops were continuously observed, which corresponds to an approximately 1.3 km-thick seamount. At the basalt-mudstone boundary, a 48 m-thick shear zone was present with complex deformation structures of chert, limestone, basalt, and mudstone. We found minimal deformation for chert blocks, whereas basalt exhibited significant deformation and formed foliated cataclasite and fragmented structures. According to observations for polished slab and thin sections, pressure solution and cataclasis may accommodate deformation. These findings suggest that deformation during seamount accretion was concentrated at the basalt-mudstone boundary in the macroscopic scale. In a meter or smaller scales, shear strain is more concentrated within basalt and mudstone and less within chert and limestone.
The µXRD results revealed that the basalt in Nukumitani contains albite, oligoclase, chlorite, quartz, with absence of epidote or actinolite. Maximum temperature of muddy matrix indicate approximately 270C. Combining these results suggests that the basalt has experienced pressures less than 1 GPa.
Different stresses were detected in the Nukumitani and Shirakuradani/Tarudani. In Nukumitani, the stresses inferred from the faults have two distinct groups that may correspond to two different stress stages. The stress from the mineral veins show that sigma1 is nearly vertical in both Nukumitani, Shirakuradani/Tarudani, while sigma2 and sigma3 are rotated about 30 degrees clockwise compared to Shirakuradani/Tarudani. These results indicate the variations in deformation stages or the presence of small-scale heterogeneities (less than 1 km), indicating localized stress fields. Additionally, the pore fluid pressure ratio in Nukumitani inferred from veins data is higher than one in Shirakuradani/Tarudani.
We investigated the Funafuseyama Unit of the Mino Belt in Gifu Prefecture, a Jurassic accretionary complex composed of mélanges with various-sized blocks of Permian chert, limestone, and basalt in a muddy matrix (Wakita, 2000). The limestone of shallow marine origin (Sano, 1988) and the basalt from plume activity (Ichiyama et al., 2008) suggest seamount accretion in this area. This study mainly focuses on Nukumitani as the main body of subducted seamount dominated by basalt, and compares it with Shirakuradani and Tarudani where chert and mélange are dominantly observed. In this study, we first created a route map and sketches of the outcrop at the bottom of basalt body. From samples, the chemical composition of cataclasite was analyzed using µXRF. Thin-section observations and µXRD analyses were performed for mineral identification. Furthermore, the stress inversion analysis (Yamaji, 2000) was also carried out using meso-scale faults and extension veins data collected from the geological survey. The maximum temperature for the muddy matrix was determined by Raman spectroscopy of carbonaceous material.
In Nukumitani, basalt outcrops were continuously observed, which corresponds to an approximately 1.3 km-thick seamount. At the basalt-mudstone boundary, a 48 m-thick shear zone was present with complex deformation structures of chert, limestone, basalt, and mudstone. We found minimal deformation for chert blocks, whereas basalt exhibited significant deformation and formed foliated cataclasite and fragmented structures. According to observations for polished slab and thin sections, pressure solution and cataclasis may accommodate deformation. These findings suggest that deformation during seamount accretion was concentrated at the basalt-mudstone boundary in the macroscopic scale. In a meter or smaller scales, shear strain is more concentrated within basalt and mudstone and less within chert and limestone.
The µXRD results revealed that the basalt in Nukumitani contains albite, oligoclase, chlorite, quartz, with absence of epidote or actinolite. Maximum temperature of muddy matrix indicate approximately 270C. Combining these results suggests that the basalt has experienced pressures less than 1 GPa.
Different stresses were detected in the Nukumitani and Shirakuradani/Tarudani. In Nukumitani, the stresses inferred from the faults have two distinct groups that may correspond to two different stress stages. The stress from the mineral veins show that sigma1 is nearly vertical in both Nukumitani, Shirakuradani/Tarudani, while sigma2 and sigma3 are rotated about 30 degrees clockwise compared to Shirakuradani/Tarudani. These results indicate the variations in deformation stages or the presence of small-scale heterogeneities (less than 1 km), indicating localized stress fields. Additionally, the pore fluid pressure ratio in Nukumitani inferred from veins data is higher than one in Shirakuradani/Tarudani.