Slides Occurring in Hydrothermal Alteration Zones

Abstract

The geology in the catchment area of the Nitappugawa River consists primarily of the Upper Miocene Ikutahara Formation and Late Miocene rhyolite, andesite, and basalt dikes, as well as the Quaternary System. The Ikutahara Formation is widely distributed in the area and is composed of tuffaceous conglomerate, sandstone and mudstone, lapilli tuff, tuff breccia, tuff, rhyolite lavas, and andesite lavas. The clastic and volcaniclastic rocks occupy the valley and hillside whereas the rhyolite or andesite lavas cap the peaks of the mountains. The rocks of the Ikutahara Formation and a part of the rhyolite dikes were intensely affected by hydrothermal alteration related to the Ryuo epithermal gold-silver mineralization of the Late Miocene Age. The hydrothermal alteration related to an ancient neutral-type geothermal system is primarily characterized by smectite, zeolite, interstratified illite/smectite, illite, and adularia.Seven ancient slide configurations are distributed on the interstratified illite/smectite and smectite zones. The Yasukuni slide and three slide configurations are closely related to the interstratified illite/smectite zone occurring abundantly in interstratified illite/smectite, which is a swelling clay mineral. The other three slide configurations are closely related to the smectite zone characterized by smectite, which is also a swelling clay mineral.These facts are useful when evaluating contemporary slide hazards and constructing hazard maps of ancient hydrothermal fields.