Examination of mineral assemblage and chemical composition in the fracture zone of the Nojima Fault at a depth of 1140 m: Analyses of the Hirabayashi NIED drill cores

A 1800 m deep borehole was drilled at Nojima Hirabayashi, Japan to penetrate through the Nojima Fault, which was activated at the time of the 1995 Hyogo-ken Nanbu earthquake (Kobe earthquake). Three fracture zones were recognized in cores at approximate depths of 1140 m, 1300 m and 1800 m. The mode of distribution of fault rocks, minerals and chemical elements were analyzed in an interval between depths of 1108 m and 1161 m, focusing on the fracture zone at the depth of 1140 m. Foliated blue-gray fault gouge constituted the central part of the fracture zone. The degree of fracturing appeared to be greater in the hanging wall than in the footwall. The relative amounts of minerals were estimated qualitatively. In the analyzed interval, not only were quartz, orthoclase, plagioclase, biotite and hornblende detected in the parent rock (granodiorite), but also kaolinite, smectite, laumontite, stilbite, calcite, ankerite and siderite, which are related to hydrothermal alteration. In particular, biotite disappeared both in the hanging wall and footwall across the central fault zone; it disappeared over a wider range in the hanging wall than in the footwall. The amounts of major chemical elements were analyzed quantitatively. Concentrations of AlO, FeO, MnO, TiO, and PO all decreased throughout the interval except at some points. HO super(+) and CO increased throughout the interval. NaO increased in the region outside the central plane, whereas MgO and CaO increased in the hanging wall and decreased in the footwall. SiO and KO decreased in the hanging wall and increased in the footwall. These results elucidate the higher degree of fracturing and chemical changes present in the hanging wall of the 1140 m fracture zone than in the footwall.