講演情報
[T4-O-14]変形した下部地殻における地震と流体関与プロセスの役割：東南極ナピア岩体の証拠から★2026年度学生優秀発表賞受賞★
*村山 燎1、スリハリ ラクシマナン1、中村 佳博2 (1. 島根大学、2. 産業技術総合研究所)
【ハイライト講演】下部地殻でのシュードタキライト形成は地殻深部での地震発生に関連している．東南極ナピア岩体リーセルラルセン山では，マイロナイトおよび変形したシュードタキライトを伴う剪断帯が発達している．変形シュードタキライト脈の構造・微細組織解析から，流体の流入による角閃岩相含水条件での脆性―塑性変形の痕跡が見出された．これは地殻深部地震の発生条件と類似し，その発生メカニズムの解明に迫る可能性がある． ※ハイライト講演とは．．．
キーワード：
シュードタキライト、マイロナイト、下部地殻地震
Pseudotachylyte, formed by the rapid quenching of melt generated through frictional melting during fault activity, is known to form even in the lower crust (Austrheim and Boundy, 1994). Such lower-crustal pseudotachylyte act as precursors to ductile shear zone (Menegon et al., 2017). Also, such pseudotachylyte have been shown to evolve into mylonite through fluid infiltration into the veins and their surrounding damage zones (Menegon et al., 2017; Michalchuk et al., 2023, 2025).
Napier Complex in East Antarctica exposes lower-crustal rocks composed of ultra-high temperature metamorphic rock that underwent metamorphism approximately 2500 Ma; the presence of pseudotachylyte has also been reported at several localities (McIntyre Island: Motoyoshi, 1996; Tonagh Island: Toyoshima et al., 1999; Mt. Riiser-Larsen: Ishikawa et al., 2000; Murayama et al., 2026; Mt. Sones: Toyoshima, JARE60). Among these, pseudotachylyte has been identified within the Riiser-Larsen Main Shear Zone (RLMSZ) from Mt. Riiser-Larsen, which formed during retrograde metamorphism (Sheraton et al., 1987). However, comprehensive structural and microstructural studies of RLMSZ are still lacking. This study aims to evaluate the relationship between the RLMSZ and high-grade deformation, and to assess seismic activity occurring at depths below the typical brittle-ductile transition zone.
RLMSZ is ca. 200 m wide, composed of swarms of subparallel localized deformation zones (Murayama et al., 2026), and within the shear zone, narrow to moderately wide mylonite zones have developed (from a few centimeters to 2 m in width). Also, deformed pseudotachylytes are present within this zone.
Microstructural observation of the mylonite reveals pyroxene occurring as porphyroclasts, with coexisting clinopyroxene and orthopyroxene. Parts of the rims and tails of these porphyroclasts are composed of minor amphibole. The pyroxene porphyroclasts exhibit necking and dragged lamellae structures.
Most of the deformed pseudotachylyte veins are parallel to the foliation, but some are slightly oblique to the shear zone. This pseudotachylyte possess branching injection vein like structures, and are characterized by a predominance of quartz and plagioclase clasts. The matrix of this pseudotachylyte consists primarily of fine-grained amphibole aligned parallel to the mylonitic foliation.
To constrain the deformation conditions of pyroxene porphyroclasts in the mylonite, crystallographic preferred orientation was analyzed using EBSD. KAM maps reveal that rim values (3.0–4.5°) are higher than core values (~1.0°), whereas tail values (0–0.5°) are lower than both core and rim values.
The microstructures of pyroxene porphyroclasts observed in the mylonites are consistent with those reported in previous studies from other regions (e.g., Raimbourg et al., 2008; Engvik et al., 2020). Furthermore, EBSD analysis reveals higher misorientation values in the tails compared to the rims, indicating the formation of subgrains by recrystallization. These microstructural observations and EBSD results suggest that the early deformation in the study area occurred under anhydrous, high-temperature conditions.
Observations show that parts of the pyroxene porphyroclast rims and tails are composed of amphibole, and that in deformed pseudotachylyte, the amphibole matrix is aligned parallel to the mylonitic foliation. These results indicate that the pseudotachylyte deformed under amphibolite-facies conditions, suggesting that the mylonite shear zone—deformed under anhydrous conditions—provided the structural anisotropy necessary for pseudotachylyte formation. It is also conceivable that earthquakes occurring under the amphibolite-facies regime were accompanied by fluid infiltration (in the middle to lower crust). Also, fluid infiltration promotes ductile overprinting after brittle rupture. RLMSZ records a multi-stage deformation history, such as ductile deformation under anhydrous conditions and repetitive brittle failure-ductile deformation in amphibolite facies.
References
Austrheim and Boundy, 1994, Science.
Engvik et al., 2020, JSG.
Menegon et al., 2017, Geochemistry, Geophysics, Geosystems.
Michalchuk et al., 2023, Journal of Geophysical Research: Solid Earth.
Ishikawa et al., 2000, National Institute of Polar Science, Tokyo.
Michalchuk et al., 2025, Geochemistry, Geophysics, Geosystems.
Motoyoshi, 1996, In Proceedings of the NIPR Symposium on Antarctic Geosciences.
Murayama et al., 2026, JSG.
Raimbourg et al., 2008, Earth Planet Sci.
Sheraton et al., 1987, Bureau of Mineral Resources Bulletin.
Toyoshima et al., 1999, Polar Geosci.
Napier Complex in East Antarctica exposes lower-crustal rocks composed of ultra-high temperature metamorphic rock that underwent metamorphism approximately 2500 Ma; the presence of pseudotachylyte has also been reported at several localities (McIntyre Island: Motoyoshi, 1996; Tonagh Island: Toyoshima et al., 1999; Mt. Riiser-Larsen: Ishikawa et al., 2000; Murayama et al., 2026; Mt. Sones: Toyoshima, JARE60). Among these, pseudotachylyte has been identified within the Riiser-Larsen Main Shear Zone (RLMSZ) from Mt. Riiser-Larsen, which formed during retrograde metamorphism (Sheraton et al., 1987). However, comprehensive structural and microstructural studies of RLMSZ are still lacking. This study aims to evaluate the relationship between the RLMSZ and high-grade deformation, and to assess seismic activity occurring at depths below the typical brittle-ductile transition zone.
RLMSZ is ca. 200 m wide, composed of swarms of subparallel localized deformation zones (Murayama et al., 2026), and within the shear zone, narrow to moderately wide mylonite zones have developed (from a few centimeters to 2 m in width). Also, deformed pseudotachylytes are present within this zone.
Microstructural observation of the mylonite reveals pyroxene occurring as porphyroclasts, with coexisting clinopyroxene and orthopyroxene. Parts of the rims and tails of these porphyroclasts are composed of minor amphibole. The pyroxene porphyroclasts exhibit necking and dragged lamellae structures.
Most of the deformed pseudotachylyte veins are parallel to the foliation, but some are slightly oblique to the shear zone. This pseudotachylyte possess branching injection vein like structures, and are characterized by a predominance of quartz and plagioclase clasts. The matrix of this pseudotachylyte consists primarily of fine-grained amphibole aligned parallel to the mylonitic foliation.
To constrain the deformation conditions of pyroxene porphyroclasts in the mylonite, crystallographic preferred orientation was analyzed using EBSD. KAM maps reveal that rim values (3.0–4.5°) are higher than core values (~1.0°), whereas tail values (0–0.5°) are lower than both core and rim values.
The microstructures of pyroxene porphyroclasts observed in the mylonites are consistent with those reported in previous studies from other regions (e.g., Raimbourg et al., 2008; Engvik et al., 2020). Furthermore, EBSD analysis reveals higher misorientation values in the tails compared to the rims, indicating the formation of subgrains by recrystallization. These microstructural observations and EBSD results suggest that the early deformation in the study area occurred under anhydrous, high-temperature conditions.
Observations show that parts of the pyroxene porphyroclast rims and tails are composed of amphibole, and that in deformed pseudotachylyte, the amphibole matrix is aligned parallel to the mylonitic foliation. These results indicate that the pseudotachylyte deformed under amphibolite-facies conditions, suggesting that the mylonite shear zone—deformed under anhydrous conditions—provided the structural anisotropy necessary for pseudotachylyte formation. It is also conceivable that earthquakes occurring under the amphibolite-facies regime were accompanied by fluid infiltration (in the middle to lower crust). Also, fluid infiltration promotes ductile overprinting after brittle rupture. RLMSZ records a multi-stage deformation history, such as ductile deformation under anhydrous conditions and repetitive brittle failure-ductile deformation in amphibolite facies.
References
Austrheim and Boundy, 1994, Science.
Engvik et al., 2020, JSG.
Menegon et al., 2017, Geochemistry, Geophysics, Geosystems.
Michalchuk et al., 2023, Journal of Geophysical Research: Solid Earth.
Ishikawa et al., 2000, National Institute of Polar Science, Tokyo.
Michalchuk et al., 2025, Geochemistry, Geophysics, Geosystems.
Motoyoshi, 1996, In Proceedings of the NIPR Symposium on Antarctic Geosciences.
Murayama et al., 2026, JSG.
Raimbourg et al., 2008, Earth Planet Sci.
Sheraton et al., 1987, Bureau of Mineral Resources Bulletin.
Toyoshima et al., 1999, Polar Geosci.
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