Examination of mineral assemblage and chemical composition in the fracturezone of the Nojima Fault at a depth of 1140 m: Analyses of the HirabayashiNIED drill cores

A 1800 m deep borehole was drilled at Nojima Hirabayashi, Japan to penetrat e through the Nojima Fault, which was activated at the time of the 1995 Hyo go-ken Nanbu earthquake (Kobe earthquake). Three fracture zones were recogn ized 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 analyze d in an interval between depths of 1108 m and 1161 m, focusing on the fract ure 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 t o be greater in the hanging wall than in the footwall. The relative amounts of minerals were estimated qualitatively. In the analyzed interval, not on ly 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 an d 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 el ements were analyzed quantitatively. Concentrations of Al2O3, Fe2O3, MnO, T iO2, and P2O5 all decreased throughout the interval except at some points. H2O+ and CO2 increased throughout the interval. Na2O increased in the regio n outside the central plane, whereas MgO and CaO increased in the hanging w all and decreased in the footwall. SiO2 and K2O decreased in the hanging wa ll 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.