Frequent landslides controlled by steep cleavage planes: A case of crystalline schist area in the central Shikoku Mountains where metamorphic processes superimposed

Many landslides occur in crystalline schist in the central Shikoku Mountains of Japan. Although landslides are thought to occur frequently in areas with inclined schistosity planes, an area in the southern Shirataki unit of the Sanbagawa metamorphic complex exhibits mesoscopic to microscopic folds (MMFs); the geological structure is horizontal at the mountain scale, but several rapid and catastrophic landslides have occurred over time. Most of these folds are upright, with an east–west strike and nearly horizontal hinge lines, and are associated with prominent cleavage planes parallel or at a steep angle to the axis plane. In this study, the relationships between landslides and MMFs in the southern Oboke area were examined along with detailed surveys of the recent Toyonaga, Iwahara-Tojiyama, and Aruse rockslides. The results showed that many landslides occurred in the north–south direction along cleavage planes. Among the landslides investigated in detail, there were detachment surfaces along cleavage planes, rupture surfaces along both cleavage planes and schistosity planes, and dense fissures that opened along cleavage planes and became rainwater pathways deep into the rock.