5:15 PM - 6:45 PM
[SCG47-P04] Origin of water in the deep megasplay faults estimated by hydrogen and oxygen isotopic compositions of water in fluid inclusion of quartz veins at the Nobeoka Thrust
Keywords:seismogenic zone, quartz vein, fluid inclusion, hydrogen-oxygen isotopic compositions, fluid flow
Understanding the mechanisms behind large earthquakes at plate boundaries, such as the Nankai Trough, is critical for assessing earthquake disaster risks. The close relationships between water and earthquakes have been noted, with high pore fluid pressures observed along the plate boundary seismogenic zone in the Nankai Trough (Tsuji et al., 2014). To comprehend the origin of high pore fluid pressures, estimating the origin and migration pathways of water around the faults by chemical analysis is essential. However, the plate boundary seismogenic zone is too deep to drill and collect fluid by scientific drilling. Therefore, an alternative approach is needed to obtain chemical information of water in the seismogenic zones. In this study, we focused on fluid inclusions within quartz veins distributed around the Nobeoka Thrust in Miyazaki, Japan. The Nobeoka Thrust is an on-land example of an ancient megasplay fault (Kondo et al., 2005). The fluid inclusions in quartz veins formed around the fault may contain water that contribute to the past high pore fluid pressures. Recent advances in analytical techniques have enabled the analysis of hydrogen and oxygen isotopic compositions (δD-H2O and δ18O-H2O) in water at the μL level. Using these advanced analytical methods, we analyzed the δD-H2O and δ18O-H2O of water within fluid inclusions in in quartz vein around the Nobeoka Thrust, as well as the oxygen isotopic composition of quartz (δ18O-qz), to investigate the origin and migration pathway of water around the fault.
Samples for fluid inclusion analysis were collected from extension veins in the hanging wall and footwall of the fault (Otsubo et al., 2016). The δD-H2O and δ18O-H2O in the fluid inclusions ranged from -52.4 to -98.0‰, and: +6.0 to +29.7‰, respectively. These values were lower δD and higher δ18O values compared to seawater. Comparison of the δD-H2O and δ18O-H2O values of the hanging wall and footwall samples with previously reported values for water and water in rock minerals suggested that the water may originate from the slab or mantle. This implies that water experienced high temperature and pressures migrated along the plate boundary to the splayfault zone, where quartz precipitated in the extension fractures formed by earthquakes, and trapping water as fluid inclusions.
The estimated temperatures based on the oxygen isotopic equilibrium between quartz and water in the footwall ranged from 178 to 224℃, lower than the experienced temperature of the footwall sediments determined by vitrinite reflectance. These temperatures corresponded well with the homogenization temperatures of fluid inclusions reported by previous studies (140-250℃), indicating that the host of veins had already cooled down when tension cracking and vein precipitation took place (Kondo et al., 2005).
The equilibrium temperatures estimated from oxygen isotope fractionation of quartz and water in the hanging wall were 693℃ and 769℃. For one other sample, equilibrium temperatures could not be determined because δ18O-H2O was higher than δ18O-qz, contrary to the ordinal trend for quartz precipitation from water (δ18O-qz > δ18O-H2O). Assuming that fluids with temperatures above 600℃ flowed in, local thermal metamorphism in the rocks surrounding the quartz vein would be expected, but such a phenomenon has not been reported. Therefore, temperatures above 600℃ are generally considered too high, suggesting isotopic disequilibrium. These results suggest isotopic disequilibrium between quartz and water in the hanging wall samples. The observations of rock thin sections suggested the secondary trap of water after the formation of quartz veins, possibly through cracks that developed in quartz crystals after vein formation, causing isotopic disequilibrium in fluid inclusions.
Keywords; Seismogenic zone, quartz vein, fluid inclusion, hydrogen-oxygen isotopic compositions
Samples for fluid inclusion analysis were collected from extension veins in the hanging wall and footwall of the fault (Otsubo et al., 2016). The δD-H2O and δ18O-H2O in the fluid inclusions ranged from -52.4 to -98.0‰, and: +6.0 to +29.7‰, respectively. These values were lower δD and higher δ18O values compared to seawater. Comparison of the δD-H2O and δ18O-H2O values of the hanging wall and footwall samples with previously reported values for water and water in rock minerals suggested that the water may originate from the slab or mantle. This implies that water experienced high temperature and pressures migrated along the plate boundary to the splayfault zone, where quartz precipitated in the extension fractures formed by earthquakes, and trapping water as fluid inclusions.
The estimated temperatures based on the oxygen isotopic equilibrium between quartz and water in the footwall ranged from 178 to 224℃, lower than the experienced temperature of the footwall sediments determined by vitrinite reflectance. These temperatures corresponded well with the homogenization temperatures of fluid inclusions reported by previous studies (140-250℃), indicating that the host of veins had already cooled down when tension cracking and vein precipitation took place (Kondo et al., 2005).
The equilibrium temperatures estimated from oxygen isotope fractionation of quartz and water in the hanging wall were 693℃ and 769℃. For one other sample, equilibrium temperatures could not be determined because δ18O-H2O was higher than δ18O-qz, contrary to the ordinal trend for quartz precipitation from water (δ18O-qz > δ18O-H2O). Assuming that fluids with temperatures above 600℃ flowed in, local thermal metamorphism in the rocks surrounding the quartz vein would be expected, but such a phenomenon has not been reported. Therefore, temperatures above 600℃ are generally considered too high, suggesting isotopic disequilibrium. These results suggest isotopic disequilibrium between quartz and water in the hanging wall samples. The observations of rock thin sections suggested the secondary trap of water after the formation of quartz veins, possibly through cracks that developed in quartz crystals after vein formation, causing isotopic disequilibrium in fluid inclusions.
Keywords; Seismogenic zone, quartz vein, fluid inclusion, hydrogen-oxygen isotopic compositions