5:15 PM - 6:45 PM
[SGL18-P05] High driving pore fluid pressure ratio estimated from quartz veins developed in Toishi-type siliceous claystone in the Tamba Belt.
Keywords:subduction zone, plate boundary, underplating, pore fluid pressure, Tamba Belt
It is generally accepted that high pore fluid pressure has a significant role to control the activity and seismicity of plate boundary faults in subduction zones. (e.g., Kimura et al., 2012). However, the cause of high pore fluid pressure has not been well understood especially at the depths of underplating accretion. This study investigated mineral veins in underplated pelagic sediments of onland accretionary complex to find the trace of high pore fluid pressure.
In the Type I Suite of the Tamba Belt, which was accreted to the southwest Japan arc in Jurassic, it is well known that the plate boundary fault was developed in the horizon of Toishi-type siliceous claystone. Toishi-type siliceous claystone was underplated along with other pelagic rocks such as chert (e.g., Kimura, 2000). Yamaguchi et al. (2016) proposed a hypothesis that the preferred development of plate boundary fault in the horizon of Toishi-type siliceous claystone was attributed to high pore fluid pressure caused by the overlying chert acting as an impermeable layer. In this study, we investigated the occurrence of mineral veins along basal faults of Toishi-type siliceous claystone to test the above hypothesis. Furthermore, we conducted paleo stress analysis and estimated driving pore fluid pressure ratio p*=(p-σ3)/(σ1-σ3) from the orientations of mineral veins. The study was carried out at Ashimi Valley in Kyoto City, where Kimura (2000) reported that thrust sheets preserving oceanic plate stratigraphy are repeated.
As a result of our investigation, we found that quartz veins were mostly well-developed around the basal faults of Toishi-type siliceous claystone. Next, we conducted paleo stress analysis by fitting mixed Bingham distribution to the poles of quartz veins (Yamaji and Sato, 2011) at four localities. Consequently, stresses with low stress ratios Φ=(σ2-σ3)/(σ1-σ3) (0.20–0.40) and σ1 axes nearly parallel to the foliations were detected. After correcting the later deformations, we obtained stresses with nearly horizontal EW to WNW-ESE trending σ1 axes and nearly vertical σ2 or σ3 axes. The corrected trends of σ1 axes coincide with the plate convergence vector in the Late Jurassic (Liu et al., 2017). This fact and the concentration of quartz veins at the basal faults suggest that they were developed during the underplating.
Subsequently, we estimated DPIs (driving pressure indices) based on the detected paleo stresses. DPI is the representative value of driving pore fluid pressure ratio defined as the 95th percentile point of the normal stress values acting on mineral veins (Faye et al., 2018). The estimated DPI values were 0.60–0.95. Considering the low stress ratios and the nearly vertical σ2 or σ3 axes after the correction of later deformations, fluid pressure should have surpassed the vertical stress σzz when the quartz veins were developed. In other words, quartz veins were developed under high pore fluid pressure with fluid pressure ratio p/σzz exceeding 1. Moreover, the estimated DPI values were higher than those estimated from mineral veins around a plate boundary fault in the Shimanto Belt (Hosokawa and Hashimoto, 2022). The thickness of chert beds overlying plate boundary faults in the Shimanto Belt is considered to have been smaller than those in the Tamba Belt. The difference between the driving pore fluid pressure ratio estimated from mineral veins around plate boundary faults in the Tamba Belt and Shimanto Belt supports the hypothesis that chert acted as impermeable layers.
In the Type I Suite of the Tamba Belt, which was accreted to the southwest Japan arc in Jurassic, it is well known that the plate boundary fault was developed in the horizon of Toishi-type siliceous claystone. Toishi-type siliceous claystone was underplated along with other pelagic rocks such as chert (e.g., Kimura, 2000). Yamaguchi et al. (2016) proposed a hypothesis that the preferred development of plate boundary fault in the horizon of Toishi-type siliceous claystone was attributed to high pore fluid pressure caused by the overlying chert acting as an impermeable layer. In this study, we investigated the occurrence of mineral veins along basal faults of Toishi-type siliceous claystone to test the above hypothesis. Furthermore, we conducted paleo stress analysis and estimated driving pore fluid pressure ratio p*=(p-σ3)/(σ1-σ3) from the orientations of mineral veins. The study was carried out at Ashimi Valley in Kyoto City, where Kimura (2000) reported that thrust sheets preserving oceanic plate stratigraphy are repeated.
As a result of our investigation, we found that quartz veins were mostly well-developed around the basal faults of Toishi-type siliceous claystone. Next, we conducted paleo stress analysis by fitting mixed Bingham distribution to the poles of quartz veins (Yamaji and Sato, 2011) at four localities. Consequently, stresses with low stress ratios Φ=(σ2-σ3)/(σ1-σ3) (0.20–0.40) and σ1 axes nearly parallel to the foliations were detected. After correcting the later deformations, we obtained stresses with nearly horizontal EW to WNW-ESE trending σ1 axes and nearly vertical σ2 or σ3 axes. The corrected trends of σ1 axes coincide with the plate convergence vector in the Late Jurassic (Liu et al., 2017). This fact and the concentration of quartz veins at the basal faults suggest that they were developed during the underplating.
Subsequently, we estimated DPIs (driving pressure indices) based on the detected paleo stresses. DPI is the representative value of driving pore fluid pressure ratio defined as the 95th percentile point of the normal stress values acting on mineral veins (Faye et al., 2018). The estimated DPI values were 0.60–0.95. Considering the low stress ratios and the nearly vertical σ2 or σ3 axes after the correction of later deformations, fluid pressure should have surpassed the vertical stress σzz when the quartz veins were developed. In other words, quartz veins were developed under high pore fluid pressure with fluid pressure ratio p/σzz exceeding 1. Moreover, the estimated DPI values were higher than those estimated from mineral veins around a plate boundary fault in the Shimanto Belt (Hosokawa and Hashimoto, 2022). The thickness of chert beds overlying plate boundary faults in the Shimanto Belt is considered to have been smaller than those in the Tamba Belt. The difference between the driving pore fluid pressure ratio estimated from mineral veins around plate boundary faults in the Tamba Belt and Shimanto Belt supports the hypothesis that chert acted as impermeable layers.