Clay mineralogical approach to the slip-surface formation in the O'Dokoro Landslide, Niigata, Central Japan

Samples were collected form the O'dokoro landslide in Niigata prefecture for the mineralogical analysis. The O'dokoro landslide occurs in the sheared belts, located 5 km west from the Itoigawa-Shizuoka Tectonic Line and is positioned in the serpentinite melange. Lamellar serpentinite sample from the bedrock serpentinite bodies contained less than 20% of clay in the particle-size distribution, and the clay fraction consisted of antigorite with a small amount of talc. No other minerals were detected and no mineralogical difference with the particle-size fraction was noticed. In contrast, the clayey material (the fault clay) sampled from the fault-fractured seams which is expected to have a mineralogical similarity to the slip surface, showed the clay content of 50%. The clay fraction was dominated by vermiculite and chlorite accompanied by smectite, talc and amphiboles. Antigorite was not identified, and progress of vermiculization of chlorite was indicated. Chlorite in the fault clay was regarded as Mg- or MgFe-chlorite and was possibly formed by serpentinization of mafic minerals. Vermiculite was probably transformed from chlorite by faulting. Argillation and the increase in the swellability of minerals were proposed to make favorable the slip-surface formation in the O'dokoro landslide in the sheared belts.