5:15 PM - 6:30 PM
[SGL23-P01] Estimation of fluid migration along the Out of Sequence Thrust from quartz vein geometric information
-Estuimation of permeability at Nobeoka Thrust in Kyushu, southwest Japan-
Keywords:Quartz vein, Permeability, Accretionary prism, Permeability tensor, Nankai Trough, Subduction zone
Slow slip earthquake has been attracting attention as a phenomenon that induces a megathrust earthquake along the plate boundary. The existence of fluid as well as the surrounding rock physical properties is an important factor for the slow slip earthquake. Geophysics studies demonstrated that the region storing the fluid and seismogenic zones are well overlapping, however, the heterogeneities of spatial fluid distribution and the processes of the increasing fluid pressure are poorly understood. In order to discuss the fluid migration and increasing fluid pressure along the out-of-sequence thrust based on permeability, numerical analysis was carried out using the geometric information of the quartz veins measured at the outcrop at Nobeoka, southwest Japan.
Footwall consisted of muddy melange with maximum paleo-temperature of 250 °C and hanging wall consisted of sand and mudstone with maximum paleo-temperature of 320 °C locate in the west and east part respectively of the Nobeoka thrust. The depth of the Nobeoka thrust fault is estimated to be 9.1 to 7.0 km from the geothermal gradient(Kondo et al., 2005). In this study, the following a) to c) were carried out at the Nobeoka thrust fault. a) observation of thin sections: The thin sections of the mineral veins took at the outcrop were observed, and the constituent minerals were identified and the mineral veins were classified. b) measuring veins: 8 square grids (A to G in the footwall, H in the hanging wall) with a side of 2.5m to the east-west direction across the Nobeoka thrust were generated and width, length, strike, and dip of the mineral veins in the grid were measured. c) Calculation of permeability by the numerical analysis: The permeability k1, k2, k3 and flow direction are calculated from three-dimensional geometry information of veins measured in b) using the tensor analysis method proposed by Oda (1983).
As a result, the veins mainly consisted of quartz and calcite. The three-dimensional geometric distribution of the veins at the footwall could be classified into two groups, A to D and E to G. From the estimated permeability and its flow direction, it was found that the maximum permeability k1 was almost perpendicular to the fault, and the fluid was easy to flow in this direction. The order of the maximum permeability k1 was 10-9 to 10-10. In the grid from E to G, it is difficult to flow in the direction orthogonal to the fault plane (k3 is in the direction orthogonal to the fault plane), and the magnitude of the permeability was k1 = k2 >> k3. On the other hand, in the A to D grids, permeabilities were almost k1 = k2 = k3. These permeabilities estimated from outcrops are expected to enable us to consider the fluid behavior along the fault when considering the Fault valve model by Sibson (1992) using a specific permeability.
Footwall consisted of muddy melange with maximum paleo-temperature of 250 °C and hanging wall consisted of sand and mudstone with maximum paleo-temperature of 320 °C locate in the west and east part respectively of the Nobeoka thrust. The depth of the Nobeoka thrust fault is estimated to be 9.1 to 7.0 km from the geothermal gradient(Kondo et al., 2005). In this study, the following a) to c) were carried out at the Nobeoka thrust fault. a) observation of thin sections: The thin sections of the mineral veins took at the outcrop were observed, and the constituent minerals were identified and the mineral veins were classified. b) measuring veins: 8 square grids (A to G in the footwall, H in the hanging wall) with a side of 2.5m to the east-west direction across the Nobeoka thrust were generated and width, length, strike, and dip of the mineral veins in the grid were measured. c) Calculation of permeability by the numerical analysis: The permeability k1, k2, k3 and flow direction are calculated from three-dimensional geometry information of veins measured in b) using the tensor analysis method proposed by Oda (1983).
As a result, the veins mainly consisted of quartz and calcite. The three-dimensional geometric distribution of the veins at the footwall could be classified into two groups, A to D and E to G. From the estimated permeability and its flow direction, it was found that the maximum permeability k1 was almost perpendicular to the fault, and the fluid was easy to flow in this direction. The order of the maximum permeability k1 was 10-9 to 10-10. In the grid from E to G, it is difficult to flow in the direction orthogonal to the fault plane (k3 is in the direction orthogonal to the fault plane), and the magnitude of the permeability was k1 = k2 >> k3. On the other hand, in the A to D grids, permeabilities were almost k1 = k2 = k3. These permeabilities estimated from outcrops are expected to enable us to consider the fluid behavior along the fault when considering the Fault valve model by Sibson (1992) using a specific permeability.