11:00 〜 13:00
[SCG44-P10] 長崎変成岩三重メランジュにおける変形温度及び歪み速度の推定
キーワード：メランジュ、剪断帯、ブロック-イン-マトリックス、スロー地震、長崎変成岩
Deep slow earthquakes, including deep low-frequency tremors, deep very-low-frequency earthquakes, and deep slow slip events, are commonly observed down-dip of the seismologic zone at depths of ~30–50 km in subduction zones and in many cases the top of the boundary between the subducting slab and the wedge mantle. The results of thermal modelling suggests that the temperature in such domains is ~300–500 °C. This domain includes the brittle-ductile transition with brittle deformation promoted by high fluid pressures and high strain rates.
The Nishisonogi metamorphic rock (NMR) in NW Kyushu, SW Japan consist dominantly of pelitic schist (metamorphosed subducted mudstone), and a prominent local metamorphic mélange with a block-in-matrix structure is located on the outcrop scale. It has been proposed that this mélange provides a record of deformation of the exhumed megathrust subduction-related shear zones formed at depths of ~25 km and at temperatures of 440–524 °C (Mori et al., 2019).
In the Mie mélange of NMR, the block-in-matrix structure shows blocks composed of metabasite and pelitic schist in chlorite-actinolite schist (CAS) matrix. Spatial changes in deformation within the structure may provide geological clues to the relation between the rheology of the sheared mélange and the occurrence of deep slow earthquakes.
In the present study, we conducted electron backscatter diffraction (EBSD) analyses of pelitic schist, metabasite and CAS samples from the Mie mélange. The deformation temperatures, estimated from the opening angle and the crystallographic preferred orientation (CPO) for the quartz c-axis, are ~400–500 °C, regardless of the rock types. The deformation temperatures in this study are consistent with the peak metamorphic temperature for the NMR (440-524 °C) estimated from Raman carbonaceous material geothermometer (Mori et al., 2019). This implies that the block-in-matrix structure in the Mie mélange can be useful for the study to examine the geological features formed close to the peak of metamorphic conditions and therefore to the subduction domain relevant to deep slow earthquakes.
We also estimated the strain rate for each sample using the quartz flow law of Hirth et al. (2001). The differential stress for each sample was estimated based on the recrystallized grain size piezometer for quartz proposed by Cross et al. (2017). The estimates of strain rates were ~10-12–10-13/s, for all samples. The estimated strain rates are consistent with those estimated for metabasite of NMR reported by Tulley et al. (2020). However, Ujiie et al. (in a review) reported faster strain rate of CAS matrix than that of pelitic schist blocks in the Nishikashiyama mélange of NMR. In this study, the strain rate of CAS matrix is similar with that of the metabasite block. CAS samples used in this study included abundant albite grains considered to be formed in association with metasomatic fluids affecting the block-in-matrix structure. This heterogeneity within CAS matrix may affect the deformation behavior compared with the schist composed dominantly of chlorite with or without actinolite, resulting in lower strain rates of CAS in the Mie mélange relative to the Nishikashiyama mélange.
We also discuss details of the deformation behavior of the CAS matrix based on the microstructure and the deformation stage of the Mie mélange incorporating information on relationship between shear sense in the block-in-matrix structure and fault movements in NMR derived from larger-scale field studies.
The Nishisonogi metamorphic rock (NMR) in NW Kyushu, SW Japan consist dominantly of pelitic schist (metamorphosed subducted mudstone), and a prominent local metamorphic mélange with a block-in-matrix structure is located on the outcrop scale. It has been proposed that this mélange provides a record of deformation of the exhumed megathrust subduction-related shear zones formed at depths of ~25 km and at temperatures of 440–524 °C (Mori et al., 2019).
In the Mie mélange of NMR, the block-in-matrix structure shows blocks composed of metabasite and pelitic schist in chlorite-actinolite schist (CAS) matrix. Spatial changes in deformation within the structure may provide geological clues to the relation between the rheology of the sheared mélange and the occurrence of deep slow earthquakes.
In the present study, we conducted electron backscatter diffraction (EBSD) analyses of pelitic schist, metabasite and CAS samples from the Mie mélange. The deformation temperatures, estimated from the opening angle and the crystallographic preferred orientation (CPO) for the quartz c-axis, are ~400–500 °C, regardless of the rock types. The deformation temperatures in this study are consistent with the peak metamorphic temperature for the NMR (440-524 °C) estimated from Raman carbonaceous material geothermometer (Mori et al., 2019). This implies that the block-in-matrix structure in the Mie mélange can be useful for the study to examine the geological features formed close to the peak of metamorphic conditions and therefore to the subduction domain relevant to deep slow earthquakes.
We also estimated the strain rate for each sample using the quartz flow law of Hirth et al. (2001). The differential stress for each sample was estimated based on the recrystallized grain size piezometer for quartz proposed by Cross et al. (2017). The estimates of strain rates were ~10-12–10-13/s, for all samples. The estimated strain rates are consistent with those estimated for metabasite of NMR reported by Tulley et al. (2020). However, Ujiie et al. (in a review) reported faster strain rate of CAS matrix than that of pelitic schist blocks in the Nishikashiyama mélange of NMR. In this study, the strain rate of CAS matrix is similar with that of the metabasite block. CAS samples used in this study included abundant albite grains considered to be formed in association with metasomatic fluids affecting the block-in-matrix structure. This heterogeneity within CAS matrix may affect the deformation behavior compared with the schist composed dominantly of chlorite with or without actinolite, resulting in lower strain rates of CAS in the Mie mélange relative to the Nishikashiyama mélange.
We also discuss details of the deformation behavior of the CAS matrix based on the microstructure and the deformation stage of the Mie mélange incorporating information on relationship between shear sense in the block-in-matrix structure and fault movements in NMR derived from larger-scale field studies.