Deformation mechanisms and fluid behavior in a shallow, brittle fault zoneduring coseismic and interseismic periods: Results from drill core penetrating the Nojima Fault, Japan

Citation
H. Tanaka et al., Deformation mechanisms and fluid behavior in a shallow, brittle fault zoneduring coseismic and interseismic periods: Results from drill core penetrating the Nojima Fault, Japan, ISL ARC, 10(3-4), 2001, pp. 381-391
Citations number
36
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
381 - 391
Database
ISI
SICI code
1038-4871(200109/12)10:3-4<381:DMAFBI>2.0.ZU;2-C
Abstract
This paper describes the results of petrographical and meso- to microstruct ural observations of brittle fault rocks in cores obtained by drilling thro ugh the Nojima Fault at a drilling depth of 389.52 m. The zonation of defor mation and alteration in the central zone of the fault is clearly seen in c ores of granite from the hanging wall, in the following order: (i) host roc k, which is characterized by some intragranular microcracks and in situ alt eration of mafic minerals and feldspars; (ii) weakly deformed and altered r ocks, which are characterized by transgranular cracks and the dissolution o f mafic minerals, and by the precipitation of zeolites and iron hydroxide m aterials; (iii) random fabric fault breccia, which is characterized by frag mentation, by anastomosing networks of transgranular cracks, and by the pre cipitation of zeolites and iron hydroxide materials; and (iv) fault gouge, which is characterized by the precipitation of smectite and localized catac lastic flow. This zonation implies that the fault has been weakened gradual ly by fluid-related fracturing over time. In the footwall, a gouge layer me asuring only 15 mm thick is present just below the surface of the Nojima Fa ult. These observations are the basis for a model of fluid behavior along t he Nojima Fault. The model invokes the percolation of meteoric fluids throu gh cracks in the hanging wall fault zone during interseismic periods, resul ting in chemical reactions in the fault gouge layer to form smectite. The l ow permeability clay-rich gouge layer sealed the footwall. The fault gouge was brecciated during coseismic or postseismic periods, breaking the seal a nd allowing fluids to readily flow into the footwall, thus causing a slight alteration. Chemical reactions between fluids and the fault breccia and go uge generated new fault gouge, which resealed the footwall, resulting in a low fluid condition in the footwall during interseismic periods.