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Shintaro Yamasaki, Kyoto University, Disaster Prevention Research Institute (Japan)
Masahiro Chigira, Kyoto University, Disaster Prevention Research Institute (Japan)
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Many landslides are known in mountainous areas of pelitic schist, suggesting that non-tectonic faults are made within pelitic schist as sliding surfaces. Non-tectonic faults are thus important in slope development and slope stability, but their formative processes have scarcely been studied. Non-tectonic faults are formed near the ground surface with low confining pressures, so their formative processes and features must be different from tectonic faults formed in the depths. Pelitic schist is formed in the deep crust and is originally very intact, but when it is brought to near the ground surface, it is affected by unloading and chemical weathering and becomes weaker. In order to clarify the features and the formative processes of non-tectonic faults, we observed and analyzed non-tectonic faults in pelitic schist by using high-quality drilled cores obtained from a landslide area with the stiff-foam drilling technique. These cores were essentially continuous ones not being separated into pieces even though they had many fractures in them. We observed presumably non-tectonic faults in many depths in apparently homogenous pelitic schist. This suggests that there are vertical variations of rock properties in the pelitic schist and that nucleation sites of non-tectonic faults could be determined by these variations. We analyzed the structure of the cores with X-ray CT scan and observed the cross sections of paraffin-impregnated samples, and measured hardness and color continuously along the cores to detect local variations in rock properties. X-ray CT images clearly showed the structure of non-tectonic faults, from which we could read the various developing stages of the faults. A non-tectonic fault has a shear zone, which is bounded by a smooth shear surface if it is along the schistosity and by a jagged surface if it crosses the schistosity. From the hanging wall or footwall into the shear zone, we observed a sequential change from schistosity-parallel shear surface, shear lenses, jigsaw-puzzled fragments, and to rock fragments floated in a gouge. In addition, the locations of schistosity-parallel shear surfaces coincided with the locations of pelitic layers, which are rich in graphite and pyrite. Because graphite has a property as a solid lubricant, continuous graphite-rich layers likely become nucleation sites of shearing. Pyrite, which is genetically closely associated with graphite in pelitic schist, also plays an important role to weaken the rock; when it is oxidized it produces sulfuric acid, which reacts with rock forming minerals.
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