K. Fujimoto et al., Alteration and mass transfer inferred from the Hirabayashi GSJ drill penetrating the Nojima Fault, Japan, ISL ARC, 10(3-4), 2001, pp. 401-410
Mineralogical and geochemical studies on the fault rocks from the Nojima-Hi
rabayashi borehole, south-west Japan, are performed to clarify the alterati
on and mass transfer in the Nojima Fault Zone at shallow depths. A complete
sequence from the hornblende-biotite granodiorite protolith to the fault c
ore can be observed without serious disorganization by surface weathering.
The parts deeper than 426.2 m are in the fault zone where rocks have suffer
ed fault-related deformation and alteration. Characteristic alteration mine
rals in the fault zone are smectite, zeolites (laumontite, stilbite), and c
arbonate minerals (calcite and siderite). It is inferred that laumontite ve
ins formed at temperatures higher than approximately 100 degreesC during th
e fault activity. A reverse component in the movement of the Nojima Fault i
nfluences the distribution of zeolites. Zeolite is the main sealing mineral
in relatively deep parts, whereas carbonate is the main sealing mineral at
shallower depths. Several shear zones are recognized in the fault zone. In
tense alteration is localized in the gouge zones. Rock chemistry changes in
a different manner between different shear zones in the fault zone. The ma
in shear zone (MSZ), which corresponds to the core of the Nojima Fault, sho
ws increased concentration of most elements except Si, Al, Na, and K. Howev
er, a lower shear zone (LSZ-2), which is characterized by intense alteratio
n rather than cataclastic deformation, shows a decreased concentration of m
ost elements including Ti and Zr. A simple volume change analysis based on
Ti and Zr immobility, commonly used to examine the changes in fault rock ch
emistry, cannot account fully for the different behaviors of Ti and Zr amon
g the two gouge zones.