Deformation and fluid flow of a major out‐of‐sequence thrust located at seismogenic depth in an accretionary complex: Nobeoka Thrust in the Shimanto Belt, Kyushu, Japan

Abstract

[1] Nobeoka Thrust in Kyushu, southwest Japan, was investigated to understand the relationship between the seismogenic out-of-sequence thrust (OST) and fluid flow in accretionary prisms. The Nobeoka Thrust is a fossilized OST, being active at seismogenic depth. The hanging wall exhibits a penetrative plastic deformation, while a brittle, cataclastic mélange-like occurrence characterizes the footwall, although both of them have same shale and sandstone-dominant protolith. Vitrinite reflectance analyses indicate that the maximum temperatures of the hanging wall and footwall are approximately 320 and 250°C, respectively. This thermal gap across the thrust corresponds to 8.6-14.4 km of displacement assuming a 28-47°C/km geothermal gradient. The brittle damage zone of the thrust is asymmetric: only 2 m for hanging wall side and 100 m for footwall. Three types of mineral veins, quartz, and carbonate are well developed, especially in the damaged footwall: the tension crack-filling vein, the fault-filling vein, and postmélange one. The first is harmonious with fabric, perpendicular to the P surface. Fluid inclusion geothermobarometry indicates the P-T of fluid in the intensively damaged zone of the footwall is $300°C, 230 -250 MPa, higher than that from vitrinite reflectance, which suggests that hydrothermal fluid flow is associated with deformation. The same type vein in the lowest portion of the footwall-damaged zone includes a much lower P-T fluid. This difference suggests that continuous underplating caused the damaged zone to propagate downward with cooling and shallowing, which differs from faults characterized by shear localization and might be unique to aquiferous OST in accretionary complexes. Citation: Kondo, H., G. Kimura, H. Masago, K. Ohmori-Ikehara, Y. Kitamura, E. Ikesawa, A. Sakaguchi, A. Yamaguchi, and S. Okamoto (2005), Deformation and fluid flow of a major out-of-sequence thrust located at seismogenic depth in an accretionary complex: Nobeoka Thrust in the Shimanto Belt, Kyushu, Japan, Tectonics, 24, TC6008,

Generation Depth of the Pseudotachylyte from an Out-of-Sequence Thrust in Accretionary Prism – Geothermobarometric Evidence

“…Deformation fabrics of both of these rocks are consistent with the sense of shear along the Nobeoka thrust (Kondo et al, 2005). Geothermometry using thrust (Kondo et al, 2005). Geothermometry using hrust (Kondo et al, 2005).…”

“…Footwall strata of the Eocene Hyuga group are composed of a shale matrix mélange group are composed of a shale matrix mélange roup are composed of a shale matrix mélange with sandstone and basaltic blocks deformed mainly by brittle mechanisms. Deformation fabrics of both of these rocks are consistent with the sense of shear along the Nobeoka thrust (Kondo et al, 2005). Geothermometry using thrust (Kondo et al, 2005).…”

“…An on-land, fossilized, major OOST thrust exhumed from , fossilized, major OOST thrust exhumed from fossilized, major OOST thrust exhumed from , major OOST thrust exhumed from major OOST thrust exhumed from seismogenic depth can be observed in the early Tertiary Shimanto Belt, Kyushu, Japan (Kondo et al, 2005). The shear zone associated with the thrust is thicker than 100 m and is characterized by the development of crack-fillings and is characterized by the development of crack-fillings and characterized by the development of crack-fillings and crack-seal veins, which suggest that the shear zone was filled by geothermal fluids at depth.…”

“…Geothermometry using thrust (Kondo et al, 2005). Geothermometry using hrust (Kondo et al, 2005). Geothermometry using ., 2005).…”

Deformations and pressure-temperature history of tectonic mélange: Miyama assemblage, the Cretaceous Shimanto Belt, Kii Peninsula, Southwest Japan

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