HR: 1330h
AN: T52C-0297    [PDF]
TI: Fluid origin and its over-pressure ratio estimated from mineral veins in an ancient accretionary complexe: Constraints from fluid inclusion analysis
AU: * Hashimoto, Y
EM: hassy@eps.s.u-tokyo.ac.jp
AF: Kochi University, Akebonocyo 2-5-1, Kochi, 780-8520 Japan
AB: In IODP, we target to drill the fault rocks at the seismic front in the subduction plate interface. Fault and fluid system is one of the most interests of the project to understand rock and fault mechanics associated with fluid, heat and material transfer along subduction interface, and formation of biological environment. P-T conditions of fluids have been estimated fluid inclusions in mineral veins observed in ancient accretionary complexes on land. The P-T condition, however, is just of fluid at the time of precipitation of minerals. We have no information about the equality in temperature between fluid and host rocks (in-situ or exotic origin) and over-pressure ratio of fluid. This study suggests a way to estimate the fluid origin (in-situ or exotic) and its overpressure ratio from mineral veins in ancient accretionary complex on-land on the basis of fluid inclusion analysis. The study area is the Miyama assemblage which is the Cretaceous Shimanto complex, Kii Peninsula, SW Japan. The Miyama assemblage is consisted of mainly sandstone and shale-tectonic melange, and minor basalt and chert. Map-scale geologic structure shows the duplex structure. The existence of tectonic m‚lange and duplex structure indicates that the study area records the deformation along subduction interface from underthrusting and underplating. P-T conditions of fluids were estimated by microthermometry of fluid inclusion. The coexistence of the two types of inclusions (methane-rich and water-rich) are observed and suggests that the fluid was a mixture of methane and water, which let an assumption that the water is saturated by methane at the time of fluid trapping. Under the assumption, pressure and temperature of trapping is estimated from one sample. Estimated temperature ranges from ca. 150 degree C to 270 degree C, and pressure ranges from ca. 80MPa to 300MPa. The P-T range corresponds to the seismogenic zone suggested by thermal model. The methane ratio to water is estimated from simply the ratio between numbers of methane inclusion and all inclusion in each sample. A considerable linear-trend is found, that is, methane ratio to water increases with temperature. This trend suggests that the temperature of fluid was equaled to that of host rocks (may be in-situ origin) because the all P-T data from fluid inclusion analysis may represent instantaneous condition. In other words, the methane ratio is an indicator of relative depth. Only one datum is not on the line, which may have exotic origin. In in-situ data, the fluid pressure range between lithostatic pressure and hydrostatic pressure should be shown in fluid pressure vs. the methane ratio space. We can constrain the lithostatic line (an upper limit line may be close to lithostatic line) in the space, and then hydrostatic line (about 1/3 of the slope of the lithostatic line). All data are within the space between the two lines in the fluid pressure vs. the methane ratio space. Therefore, we can estimate overpressure ratio from the lines and pressure data. The on-land observation of accretionary complexes, although the complexes is fossil rocks of seismogenic plate interface, may become more significant after drilling rather than before drilling because the drilling core has much new information but almost one-dimensional information. Other hand, on-land complexes have much wide two or three-dimensional outcrops. In the case of this study, no time-spacio relations between fluid origin, overpressure ratio, and deformation structure is represented. We need more detailed observations and much more fluid inclusion data.
DE: 8000 STRUCTURAL GEOLOGY (New field, replaces single entry 8165)
DE: 8010 Fractures and faults
DE: 8045 Role of fluids
SC: Tectonophysics [T]
MN: 2003 Fall Meeting