4. Lithology and alteration
The core lithology is mostly Cretaceous granodiorite (Nojima granodiorite (Mizuno et al., 1990)) with some porphyry dikes. The rocks above 426m depth are nearly intact granodiorite. The borehole enters into the fault zone at 426m depth and the rocks are affected by the fault even at the bottom of the borehole. We categorize four types of fault rocks, which are weakly altered and deformed granodiorite, fault breccia, fault gouge and cataclasite, based on degree of alteration and pulverization (Tanaka et al., 1999). The polished surfaces of the typical samples are shown in Figure 5.
Characteristic alteration minerals in the fault zone are smectite, zeolites (laumontite, stilbite) and carbonate minerals (calcite, siderite, dolomite), though chlorite may be a relic of the deutelic alteration. Presence of laumontite indicates the paleohydrothermal activity in the fault zone as laumontite is usually formed at about 100ｼC. Calcite is considered to be the most recent sealing material in the shallow level of the Nojima fault. Ignition loss, of volatile contents such as H20 and C02 in the rocks, is an indicator of the degree of alteration. The profile shows that the intense alteration and mass transfer are localized near the coaxial zone (Fig. 4).
Figure 4. Composite column showing Lithology, distribution of minerals and ignition loss (LOI) in the whole rock chemistry. Minerals shown in black represent primary minerals and those in red represent secondary minerals.
Figure 5. Photographs of polished surfaces of typical rocks from the borehole. We found three types of fault gouges which were formed at different stages and environments. The texture indicates the repeated cycles of fracturing and sealing in the coaxial zone.