Small half-graben inferred from a Miocene syn-rift succession in the Kinbusan area,eastern Tottori Prefecture,Japan

Understanding the Miocene tectonics of the southwest Japan Arc is key to elucidating the history of the opening of the Japan Sea, and important advances have been made in the last 10 years, such as clarification of the timing of arc rotation. Syn-rift successions occur in the eastern San'in basins, but they have received little attention because the basic stratigraphy and development of the basins are poorly understood. This paper provides details of a field survey of the Miocene geology of the Kinbusan area in eastern Tottori Prefecture, Japan. The lithofacies suggest alluvial fan and plain environments, which correlate with the syn-rifting horizon of the southwest Japan Arc. A syn-depositional graben, which is newly named the Kinbusan Graben, can be inferred from the geometry of the basin fill, and this graben consists of two sub-basins bounded by two ENE–WSW-trending normal faults. Miocene dikes also trend ENE–WSW, indicating that the graben was formed in response to extension oriented perpendicular to the strike of the faults. The age of the Iwami Formation shows that extension was initiated before arc rotation. Fault-slip data, collected from meso-scale faults in the basin fill, indicate axial compressive stress with the maximum principal stress being vertical. The stress field suggests that the basin fill was accommodated not only in fault-perpendicular extension but also in fault-parallel extensions, and fault-parallel extension was the probable cause of differential subsidence of the basin fill during faulting. The results show that the basic stratigraphy and structures of the eastern San'in basins provide important constraints on the timing and style of deformation in the southwest Japan Arc and the Japan Sea.