Changes in magnetic and fractal properties of fractured granites near the Nojima Fault, Japan

Citation
N. Nakamura et H. Nagahama, Changes in magnetic and fractal properties of fractured granites near the Nojima Fault, Japan, ISL ARC, 10(3-4), 2001, pp. 486-494
Citations number
33
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
486 - 494
Database
ISI
SICI code
1038-4871(200109/12)10:3-4<486:CIMAFP>2.0.ZU;2-Y
Abstract
Anisotropy of magnetic susceptibility (AMS) has been used to infer finite s train fabrics in plastically deformed rocks, but there are few studies of m agnetic properties in fractured fault rocks. Changes in magnetic and fracta l properties of fractured granites from the Disaster Prevention Research In stitute, Kyoto University (DPRI) 500 m drilling core towards the Nojima Fau lt and of the well-foliated fault gouge are described. Fractal analysis of fractured granites shows that the fractal dimension (D) increases linearly toward the gouge zone of the fault. In weakly fractured granites (D = 1.05- 1.24), it was found that the degree of AMS correlates positively with the f ractal dimension, suggesting a fracture-related magnetic fabric due to frac turing. In strongly fractured granites (D = 1.25-1.50), weaker, nearly isot ropic AMS is found, suggesting erasure by the fragmentation of the magnetic minerals. Within the fault gouge zone, an isotropic AMS fabric was found, as well as twofold increases in magnetic intensity and susceptibility. Thes e changes reflect the production of new magnetite grains, subsequently conf irmed by hysteresis studies, which suggests that fault gouge might be regar ded as the source of the regional geomagnetic field contrast along active f aults. Thus, AMS is clearly a potentially useful tool for inferring the fra cturing texture of magnetic minerals in fractured rocks and detecting activ e faults from the high susceptibility contrast of fault gouge.