Changes in magnetic and fractal properties of fractured granites near the Nojima Fault, Japan
Changes in magnetic and fractal properties of fractured granites near the Nojima Fault, Japan
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DOI：
10.1111/j.1440-1738.2001.00347.x
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发表时间：
2001-09
期刊：
影响因子：
1.5
通讯作者：
N. Nakamura;H. Nagahama
中科院分区：
文献类型：
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作者：
N. Nakamura;H. Nagahama
摘要磁化率各向异性(AMS)已被用来推断塑性变形岩石的有限应变组构，但对裂隙断层岩磁性的研究很少。描述了从京都大学防灾研究所(DPRI)500m钻探岩心到Nojima断层的破碎花岗岩的磁性和分形性的变化以及面片状断层泥的变化。对破碎花岗岩的分维分析表明，分维(D)沿断层泥带呈线性增加。在弱裂隙花岗岩(D=1.05-1.24)中，AMS的程度与分维呈正相关，表明存在与断裂有关的磁性组构。在强破碎花岗岩(D=1.25-1.50)中，发现了较弱的、接近各向同性的AMS，表明磁性矿物的碎裂作用使AMS消失。在断层泥带内，发现了一种各向同性的AMS组构，磁场强度和磁化率增加了一倍。这些变化反映了新的磁铁矿颗粒的产生，随后的滞后研究证实了这一点，这表明断层泥可能是活动断裂上区域地磁场对比的来源。因此，AMS显然是一种潜在的有用工具，可用于推断裂隙岩石中磁性矿物的破裂结构，并从断层泥的高磁化率对比度中检测活动断层。
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.