[S-GL23] Geologic structure and tectonic history of East Asia and Japanese Islands

[SGL23-P08] Paleostresses in middle west Kyushu, Japan obtained from minor faults cutting Early Pliocene sedimentary rocks.

*Keigo Kawaguchi¹, Kiyokazu Oohashi¹ (1.Yamaguchi University)

The rift zone from Beppu to Shimabara (Beppu-Shimabara graben, Matsumoto, 1979, 1993) is known to exist in central Kyushu, but the tectonics of this area from the Miocene to the present is not simple. Based on geodetic, seismic, and geological data, Ohashi et al. (2020) explained the Quaternary to present tectonics of central Kyushu using lateral tensile tectonics, and distinguished it from the N-S tensile tectonics that occurred intermittently since the Miocene. However, the geodetic and seismic stress and strain fields cover a spatially extensive area, whereas the stress field obtained from the geology is limited, and the understanding of the Quaternary stress and deformation history in central Kyushu is not sufficient. Kawaguchi (2020) conducted a stress multiple inverse analysis (Yamaji, 1999) using small faults developed in the Pliocene (Hoshiwara Formation) in northern Kumamoto Prefecture to clarify paleo-stress in central Kyushu. As a result, normal stress in the N-S direction and strike-slip stress in the E-W direction were derived. The spatial extent of the stress field is unknown because the distribution area of the Hoshiwara Formation is localized, and the relationship between the two paleostresses obtained remains to be clarified. In this study, we describe small faults in sedimentary rocks of the Pliocene (Maetsue Formation) in the Chikuhi Mountains of southern Fukuoka Prefecture and perform stress multiple inversion analysis to clarify paleostress in a wider area in central and western Kyushu. The results of the stress analysis and the combination of clay minerals in the fault gouge will be used to discuss the pre- and post-stress relationships and the age. The study area is located in Hoshino Village and Yabe Town, Yame City, Fukuoka Prefecture. The Maetsue Formation (Kido, 1981) was deposited between the Early Miocene and the Early Pliocene. We investigated the Kunitake conglomerate and Tashiro pyroxene andesite lava, which are part of the Maetsue Formation. The Tashiro pyroxene andesite lava was dated to 3.39 ± 0.34 Ma by fission track dating by Watanabe et al. (1987). In this study, geological survey, stress multiple inversion analysis, and XRD measurement are conducted. In the geological survey, fault-slip data were obtained from small faults in the outcrop, and fault gouge sampling was performed from faults with widths on the cm order. Stress multiple inverse analysis was performed using the method of Yamaji (1999). XRD measurements were made on the bulk of the whole rock and on clay minerals separated by the water-hydrothermal method. In addition to stereotaxic and ethylene glycol treatments, heat and hydrochloric acid treatments were performed as necessary for the clay mineral measurements. Three stresses were detected in the stress multiple inverse analysis: (1) normal fault-type stress along the base circle, (2) normal fault-type stress in the low-angle NNW-SSE direction, and (3) transverse fault-type stress in the low-angle WNW-ESE direction. Stresses (2) and (3) are in harmony with the results of Kawaguchi (2020). The mineral identification by XRD measurement revealed smectite-rich smectite-illite-chlorite mixed-layer mineral and smectite-chlorite mixed-layer mineral, smectite, illite, and kaolinite. Sawai et al. (1998) reported smectite-chlorite as an alteration mineral combination in the volcanic rocks of this area, and based on K-Ar dating (2.61 ± 0.12 Ma), they place the late Pliocene as the time of hydrothermal alteration. Kido (2012) conducted stress analysis based on the orientation of veins that penetrated into the Tsue Mountains area around 3.3 Ma, and inferred that a north-south tensile stress field from the Late Miocene persisted until this time. Based on these existing studies and the results of this study, it is expected that a transition from normal fault stress to lateral fault stress occurred in the Chikuhi Mountains from the late Pliocene to the early Pleistocene.