2011 Volume 48 Issue 3 Pages 139-146
One ancient large-scale landslide, as well as recent small-scale and ancient middle-scale landslides in the Ikutahara-Minami landslide prevention area can be classified as hydrothermal alteration zone landslides (HAZLs) based on their bedrock geology, and as hydrothermally altered rockslide, and as debris or earth slide, respectively, based on the landslide body. The large-scale HAZL occurred on the hydrothermally argillized zone composed of the smectite-analcite subzone of early-stage regional hydrothermal alteration and the illite-interstratified illite/smectite mineral subzone of early-stage neutral-type vein-related alteration in the Kitano-oh fossil hydrothermal system. The middle-and small-scale HAZLs occurred on the smectite-clinoptilolite subzone of the early-stage regional hydrothermal alteration and the K-feldspar, illite, and interstratified illite/smectite mineral zones of the early-stage neutral-type vein-related alteration. The bedrock of the K-feldspar and illite zones, in particular, contains characteristic swelling clay minerals such as interstratified illite/smectite minerals and smectite. The HAZL occurrence, therefore, is closely related to the swelling clay minerals-bearing hydrothermal alteration zones, regardless of the size of the landslides.
