14:45 〜 15:00
[SCG58-10] 高温変成岩中に記録された中下部地殻におけるマグマ/水熱破砕の物理条件：火山下の深部低周波地震の痕跡か？
キーワード：花崗岩質岩脈、マグマ貫入、反応輸送モデル、古応力解析、深部低周波地震、高温変成岩
Subvolcanic region is one of the most active seismogenic zone in arc crust[e.g., 1]. Recent geophysical observations have revealed that deep low frequency earthquakes (DLFEs) in the lower crust are coupled with volcanic earthquake in the upper crust, suggesting magmatic/hydrothermal fluid flow triggering the earthquakes[2,3]. Such crustal fracturing and dynamic fluid flow are likely to be recorded in high temperature metamorphic terrane. Here we review the occurrences of magmatic dikes and hydrothermal mineral veins in a high temperature metamorphic terrane[4–8], show their mode of fracturing, duration of fluid flow, stress state and fluid pressure during their activities, and discuss their relation to the observed seismic activities.
The study area is a high temperature metamorphic terrane of Sør Rondane Mountains, East Antarctica. High angle biotite granitic dikes and/or hornblende±biotite veins commonly occur throughout the survey area (Mefjell[5,7], Brattnipene[6,8], and Berrheia[4] area; Fig. 1a–c). The dikes and veins cut the local gneissosity with extensional or extensional-shear displacements (Fig. 1b and c), intrude into the felsic granulite (Mefjell, Brattnipene) or mafic granulite (Berrheia), and forms hydrous reaction zones along the dikes and/or veins (Fig. 1b). The host granulites are composed of plagioclase, K-feldspar, amphibole, and clinopyroxene ±orthopyroxene, ±quartz or ±olivine. Reaction zones are free of pyroxenes and characterized by the replacement of the pyroxenes with amphibole and biotite/phlogopite, representing hydration reactions at 600–750°C and ~0.5 GPa (Mefjell and Berrheia) to 0.6–0.8 GPa (Brattnipene) (i.e., 20–30 km depth).
Within the reaction zones, Cl or F concentrations in apatite, biotite and/or amphibole is high within the dike or veins, and decrease towards the host rock (Fig. 1d). Reactive transport modeling of Cl or F suggest that transport was advection dominant (i.e., Peclet number ≫10), and that the duration of fluid activity was on the order of days to months for the dikes, and hours to weeks for the veins.
Thermodynamic analyses of the reaction zones suggest that the fluid pressure gradient from the dikes to host rock was ~10 MPa/cm during the dike/vein formation (Fig. 1d). Combined with the above duration of fluid activities, it is likely that the maximum duration of high fluid pressure conditions were days to months (dikes) or hours to weeks (veins).
The orientations of the granitic dikes and veins vary in some outcrops (Mefjell) or are relatively constant (Brattnipene), suggesting stress state switching in the former and constant stress state for the latter (Fig. 1e and f). These stress state variations are apparently related to the distance to the magmatic chambers (Fig. 1a), and probably reflect the differences in magmatic fluid pressures.
These observations suggest that the extensional and/or extensional shear fracturing is common during the movement of magmatic fluids in the middle-lower crust. The granitic dike release excess aqueous fluids, raise the local fluid pressure lasting days to months, and subsequently form hornblende±biotite veins within hours to weeks. These depth (20–30 km), temperature (~700°C), duration of high-pressure fluids (hours to months), and the extensional-shear mode of fracturing observed in these dikes/veins are largely comparable to the depth and characteristic duration of the subvolcanic DLFEs[e.g., 2, 9]. The geological observations strongly support non-double couple mechanisms for crustal fracturing in these regions. Repeated changes in apparent P-axis on focal mechanisms would likely suggest intrusions of highly pressurized magma.
[References]
1: Hasegawa et al. (2005) Tectonophysics
2: Yukutake et al. (2019) GRL
3: Mannen et al. (2018) EPS
4: Uno et al. (2017) Lithos
5: Mindaleva et al. (2020) Lithos
6: Mindaleva et al. (2023) GRL in press.
7: Uno et al. (2022) JpGU abstract
8: Nara et al. (2023) JpGU abstract
9: Kurihara and Obara (2021) JGR
The study area is a high temperature metamorphic terrane of Sør Rondane Mountains, East Antarctica. High angle biotite granitic dikes and/or hornblende±biotite veins commonly occur throughout the survey area (Mefjell[5,7], Brattnipene[6,8], and Berrheia[4] area; Fig. 1a–c). The dikes and veins cut the local gneissosity with extensional or extensional-shear displacements (Fig. 1b and c), intrude into the felsic granulite (Mefjell, Brattnipene) or mafic granulite (Berrheia), and forms hydrous reaction zones along the dikes and/or veins (Fig. 1b). The host granulites are composed of plagioclase, K-feldspar, amphibole, and clinopyroxene ±orthopyroxene, ±quartz or ±olivine. Reaction zones are free of pyroxenes and characterized by the replacement of the pyroxenes with amphibole and biotite/phlogopite, representing hydration reactions at 600–750°C and ~0.5 GPa (Mefjell and Berrheia) to 0.6–0.8 GPa (Brattnipene) (i.e., 20–30 km depth).
Within the reaction zones, Cl or F concentrations in apatite, biotite and/or amphibole is high within the dike or veins, and decrease towards the host rock (Fig. 1d). Reactive transport modeling of Cl or F suggest that transport was advection dominant (i.e., Peclet number ≫10), and that the duration of fluid activity was on the order of days to months for the dikes, and hours to weeks for the veins.
Thermodynamic analyses of the reaction zones suggest that the fluid pressure gradient from the dikes to host rock was ~10 MPa/cm during the dike/vein formation (Fig. 1d). Combined with the above duration of fluid activities, it is likely that the maximum duration of high fluid pressure conditions were days to months (dikes) or hours to weeks (veins).
The orientations of the granitic dikes and veins vary in some outcrops (Mefjell) or are relatively constant (Brattnipene), suggesting stress state switching in the former and constant stress state for the latter (Fig. 1e and f). These stress state variations are apparently related to the distance to the magmatic chambers (Fig. 1a), and probably reflect the differences in magmatic fluid pressures.
These observations suggest that the extensional and/or extensional shear fracturing is common during the movement of magmatic fluids in the middle-lower crust. The granitic dike release excess aqueous fluids, raise the local fluid pressure lasting days to months, and subsequently form hornblende±biotite veins within hours to weeks. These depth (20–30 km), temperature (~700°C), duration of high-pressure fluids (hours to months), and the extensional-shear mode of fracturing observed in these dikes/veins are largely comparable to the depth and characteristic duration of the subvolcanic DLFEs[e.g., 2, 9]. The geological observations strongly support non-double couple mechanisms for crustal fracturing in these regions. Repeated changes in apparent P-axis on focal mechanisms would likely suggest intrusions of highly pressurized magma.
[References]
1: Hasegawa et al. (2005) Tectonophysics
2: Yukutake et al. (2019) GRL
3: Mannen et al. (2018) EPS
4: Uno et al. (2017) Lithos
5: Mindaleva et al. (2020) Lithos
6: Mindaleva et al. (2023) GRL in press.
7: Uno et al. (2022) JpGU abstract
8: Nara et al. (2023) JpGU abstract
9: Kurihara and Obara (2021) JGR