Alteration and mass transfer inferred from the Hirabayashi GSJ drill penetrating the Nojima Fault, Japan

Citation
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
Citations number
23
Language
INGLESE
art.tipo
Article
Categorie Soggetti
Earth Sciences
Journal title
ISLAND ARC
ISSN journal
1038-4871 → ACNP
Volume
10
Issue
3-4
Year of publication
2001
Pages
401 - 410
Database
ISI
SICI code
1038-4871(200109/12)10:3-4<401:AAMTIF>2.0.ZU;2-M
Abstract
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.