10:45 〜 12:15
[SCG45-P09] Geological constrains on fluid pressure ratio during mélange formation in a shallow transition zone along a subduction plate interface
キーワード：流体圧比、引張鉱物脈、メランジュ
Fluid pressure along décollement has a significant effect on slip behaviors in subduction zone. Previous studies suggested that fluid pressure affects to shear strength and frictional stability. High fluid pressure ratio is expected for slow earthquakes on the basis of the characteristics of slow earthquake with small stress drop and sub-zero of a-b. There are, however, few examples of quantitative constraints on fluid pressure ratios along décollement, except for these in the shallow portions of subduction zone (< 2.5 km) geologically. Therefore, the purpose of this study is to constrain fluid pressure ratio around a shallow transition zone during mélange formation in the Yokonami mélange, the Cretaceous Shimanto Belt, SW Japan.
The study area is the Yokonami mélange, which is bounded by the Goshikinohama fault from a coherent unit in the north. The Yokonami mélange is composed mainly of sandstone and black shale, with minor amounts of acidic tuffs, red shales, limestones, cherts, and basalts. Fluid inclusion microthermometry for shear veins revealed fluid temperature of about 175-225 °C and pressures ranging about 143-215 MPa. In this study, we focus on extension veins in the Yokonami mélange and, classified the veins into two. One is developing only in the mélange blocks (type 1 vein) and the other is developing in the matrix surrounding the blocks (type 2 vein).
We used mixed Bingham statistics to estimate the paleo stress state for the type 1 veins and type 2 veins. The latter was used for the examination of block rotations by the consistency of their paleo stresses. In addition, we applied the fluid inclusion microthermometry to estimate fluid temperature and pressure in type 1 veins.
The results of stress analysis and outcrop observations indicated that the mélange blocks were not rotated, and type 1 veins was formed under a normal fault stress regime, whereas type 2 veins showed a normal and reverse fault stress regime. Fluid inclusion microthermometry for the type 1 veins indicated fluid temperature and pressures as about 175-203.5 °C of and about 171.2-217.9 MPa.
The less variation in temperature respect to the range of pressure variation suggests that type 1 veins were formed at a constant depth with variation in fluid pressures because geothermal gradient must be a constant during the type 1 vein formation. In addition, type 1 veins and shear veins show the overlapping area in temperature conditions, suggesting that Yokonami mélange located at the shallow ductile-brittle transition zone. We constrained the rock tensile strength and the formation depth to be about 11.7 MPa and about 7.9 km, using the rock failure theory and the fluid pressure variations. Furthermore, we calculated fluid pressure ratio and maximum differential stress to be about 0.83-1.05 about 46.8 MPa. Our result indicates the mechanical weakness in footwall below décollement at the shallow transition zone. The results indicate that high fluid pressure ratio can be kept in the deeper portion similarly to that in previous study for shallower depth in the present subduction zone. The high fluid pressure ratio may imply mélange has potential to cause slow slips in shallow transition zone along a subduction plate interface.
The study area is the Yokonami mélange, which is bounded by the Goshikinohama fault from a coherent unit in the north. The Yokonami mélange is composed mainly of sandstone and black shale, with minor amounts of acidic tuffs, red shales, limestones, cherts, and basalts. Fluid inclusion microthermometry for shear veins revealed fluid temperature of about 175-225 °C and pressures ranging about 143-215 MPa. In this study, we focus on extension veins in the Yokonami mélange and, classified the veins into two. One is developing only in the mélange blocks (type 1 vein) and the other is developing in the matrix surrounding the blocks (type 2 vein).
We used mixed Bingham statistics to estimate the paleo stress state for the type 1 veins and type 2 veins. The latter was used for the examination of block rotations by the consistency of their paleo stresses. In addition, we applied the fluid inclusion microthermometry to estimate fluid temperature and pressure in type 1 veins.
The results of stress analysis and outcrop observations indicated that the mélange blocks were not rotated, and type 1 veins was formed under a normal fault stress regime, whereas type 2 veins showed a normal and reverse fault stress regime. Fluid inclusion microthermometry for the type 1 veins indicated fluid temperature and pressures as about 175-203.5 °C of and about 171.2-217.9 MPa.
The less variation in temperature respect to the range of pressure variation suggests that type 1 veins were formed at a constant depth with variation in fluid pressures because geothermal gradient must be a constant during the type 1 vein formation. In addition, type 1 veins and shear veins show the overlapping area in temperature conditions, suggesting that Yokonami mélange located at the shallow ductile-brittle transition zone. We constrained the rock tensile strength and the formation depth to be about 11.7 MPa and about 7.9 km, using the rock failure theory and the fluid pressure variations. Furthermore, we calculated fluid pressure ratio and maximum differential stress to be about 0.83-1.05 about 46.8 MPa. Our result indicates the mechanical weakness in footwall below décollement at the shallow transition zone. The results indicate that high fluid pressure ratio can be kept in the deeper portion similarly to that in previous study for shallower depth in the present subduction zone. The high fluid pressure ratio may imply mélange has potential to cause slow slips in shallow transition zone along a subduction plate interface.