5:15 PM - 6:45 PM
[SCG46-P25] Exposed dike swarms as geological analogue to subvolcanic deep low-frequency earthquakes (DLFEs) in terms of stress state and moment magnitude
Keywords:lower crust, deep low-frequency earthquakes
Recent geophysical observations suggest magmatic activities might trigger subvolcanic DLFEs (Yukutake et al., 2019). Such crustal fracturing by magmatic intrusions is possibly recorded in high-temperature metamorphic rocks. To comprehend the deep crustal fracturing processes, we estimated the depth, fracturing mode and duration of magma-induced fracturing, along with the potential earthquakes' magnitude.
The study area is located in a collision zone-originated high-temperature metamorphic terrane, Sør Rondane Mountains, East Antarctica. Gneissose felsic granulite is cut by two types of fractures, granitic dikes (10–100 m) and hornblende-bearing hydrothermal veins (~1–50 m) branching from the dikes. Extensional-shear displacements were observed for the dikes. Along the granitic dikes, the felsic granulite transforms into whitish amphibolite-facies hydrous reaction zones, indicating that aqueous fluid expelled from the magma reacted with felsic granulite. Geothermobarometries at the dike-reaction zone boundary suggest that the hydration reaction occurred at 670–750℃, 0.8–0.9 GPa. Applications of reactive transport modeling to the Cl profile across the reaction zones suggests the duration of fluid infiltration was 22–180 h (Nara et al., 2023, JpGU).
The orientations of 91 granitic dikes were measured from a 3D outcrop model reproduced from drone photographs at Hitosashiyubi-one ridge. They mostly orient WNW-ESE in strike and vertical in dip. Paleo-stress inversion by GArcmB (Yamaji, 2016) suggests a constant stress state with subvertical σ2 and subhorizontal σ1 and σ3, and stress ratio φ=(σ2-σ3)/(σ1-σ3)=0.26.
The moment magnitude of the dike intrusion and hydrothermal veins was estimated by geometry and maximum potentially induced moment magnitude (McGarr, 2014), respectively by assuming that the dike sizes represent fracture sizes. Based on the geometry of dikes and the rigidity of the host rock (~30 GPa), the magnitude is estimated as −0.4 to 3.0. On the other hand, the water expelled from the granitic dike was estimated from 9.4 × 10-5 to 78 m3, and the maximum magnitude of induced hydrothermal fracturing is estimated as -1.7 to 2.3.
The above-observed depth, mode and duration of crustal fracturing by a single magmatic intrusion are compatible with those of an episodic subvolcanic DLFEs (Kurihara and Obara, 2021). Magnitudes of a single subvolcanic DLFE which have been detected vary between −1 to 3 worldwide (Kurihara et al., 2019; Ikegaya & Yamamoto, 2021; Hensch et al., 2019) and are compatible to those of the dike intrusions and hydrothermal fracturing observed in this study. These suggest that the dike swarm and its formation processes observed in this study show natural similarities of magmatic intrusions into the lower crust with DLFE observations, which provides petrological evidence of DLFEs.
The study area is located in a collision zone-originated high-temperature metamorphic terrane, Sør Rondane Mountains, East Antarctica. Gneissose felsic granulite is cut by two types of fractures, granitic dikes (10–100 m) and hornblende-bearing hydrothermal veins (~1–50 m) branching from the dikes. Extensional-shear displacements were observed for the dikes. Along the granitic dikes, the felsic granulite transforms into whitish amphibolite-facies hydrous reaction zones, indicating that aqueous fluid expelled from the magma reacted with felsic granulite. Geothermobarometries at the dike-reaction zone boundary suggest that the hydration reaction occurred at 670–750℃, 0.8–0.9 GPa. Applications of reactive transport modeling to the Cl profile across the reaction zones suggests the duration of fluid infiltration was 22–180 h (Nara et al., 2023, JpGU).
The orientations of 91 granitic dikes were measured from a 3D outcrop model reproduced from drone photographs at Hitosashiyubi-one ridge. They mostly orient WNW-ESE in strike and vertical in dip. Paleo-stress inversion by GArcmB (Yamaji, 2016) suggests a constant stress state with subvertical σ2 and subhorizontal σ1 and σ3, and stress ratio φ=(σ2-σ3)/(σ1-σ3)=0.26.
The moment magnitude of the dike intrusion and hydrothermal veins was estimated by geometry and maximum potentially induced moment magnitude (McGarr, 2014), respectively by assuming that the dike sizes represent fracture sizes. Based on the geometry of dikes and the rigidity of the host rock (~30 GPa), the magnitude is estimated as −0.4 to 3.0. On the other hand, the water expelled from the granitic dike was estimated from 9.4 × 10-5 to 78 m3, and the maximum magnitude of induced hydrothermal fracturing is estimated as -1.7 to 2.3.
The above-observed depth, mode and duration of crustal fracturing by a single magmatic intrusion are compatible with those of an episodic subvolcanic DLFEs (Kurihara and Obara, 2021). Magnitudes of a single subvolcanic DLFE which have been detected vary between −1 to 3 worldwide (Kurihara et al., 2019; Ikegaya & Yamamoto, 2021; Hensch et al., 2019) and are compatible to those of the dike intrusions and hydrothermal fracturing observed in this study. These suggest that the dike swarm and its formation processes observed in this study show natural similarities of magmatic intrusions into the lower crust with DLFE observations, which provides petrological evidence of DLFEs.