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

Abstract

Abstract Anisotropy of magnetic susceptibility (AMS) has been used to infer finite strain fabrics in plastically deformed rocks, but there are few studies of magnetic properties in fractured fault rocks. Changes in magnetic and fractal properties of fractured granites from the Disaster Prevention Research Institute, Kyoto University (DPRI) 500 m drilling core towards the Nojima Fault 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 fractal dimension, suggesting a fracture‐related magnetic fabric due to fracturing. In strongly fractured granites (D = 1.25–1.50), weaker, nearly isotropic 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. These changes reflect the production of new magnetite grains, subsequently confirmed by hysteresis studies, which suggests that fault gouge might be regarded as the source of the regional geomagnetic field contrast along active faults. Thus, AMS is clearly a potentially useful tool for inferring the fracturing texture of magnetic minerals in fractured rocks and detecting active faults from the high susceptibility contrast of fault gouge.