10:45 AM - 12:15 PM
[SSS13-P11] Characteristics of Calcite and Mineral Filling in the Latest Slip Zone of the Neodani Fault, Central Japan
Keywords:Neodani Fault, Mineral Filling
To study mineral filling process in the fault zone, the fault with a certain time after the fault movement is better than that immediately after the fault. The Neodani Fault was ruptured in the 1891 Nobi Earthquake, and132 years passed since the latest earthquake. This indicates that the Neodani Fault enables to study the mineral filling process of the fault. Two boreholes penetrating the Neodani Fault were drilled by the Nuclear Regulation Authority in 2019. These borehole cores include the latest slip zone that was displaced during the Nobi earthquake. Yatabe et al. (2022) conducted X-ray CT scan, powder X-ray diffraction, and X-ray fluorescence analysis of the fault gouges of the Neodani Fault, and suggested that fault movement causes a decrease in density on the latest slip zone, followed by calcite mineralization and density recovery as time passes. However, the calcite formation in the fault gouge of the latest slip zone has not been confirmed directly, and it is unclear whether the calcite is precipitated recently. The purpose of this study is to study the formation process of calcite in the fault gouges including the latest slip zone by the microscopic observation.
NDFP-1 and NDFD-1 were drilled at Neo-Midori, Motosu City, Gifu Prefecture, Central Japan where a 6 m vertical displacement along the Neodani Fault occurred during the Nobi Earthquake. The length of NDFP-1 is 140.3 m, and penetrates the latest slip zone at a depth of 110.8 m. The length of NDFD-1 is 524.8 m, and penetrates the zone at a depth of 387.7 m. The direction of drilling is southwest, and the two boreholes intersect almost perpendicular to the fault in the horizontal plane. The geology of this area is the Mino Belt, which is Jurassic accretionary complex, and is mainly composed of mélange with mudstone matrix and blocks of sandstone, limestone, chert, and basalt.
The calcite mineralization in the fault gorges was observed using a scanning electron microscope (SEM), and elemental analysis was performed using energy-dispersive X-ray spectroscopy (SEM-EDX). Elemental mapping by SEM-EDX showed that Ca is fragmented within the latest slip zone, whereas it is distributed as veins outside the latest slip zone. Ca-bearing fragments in the latest slip zone show rounded and anhedral in shape. This suggests that the calcite was repeatedly fractured and worn by faulting. In addition, a dark gray matrix exists in a narrow zone of about 20 μm near the fracture . Although the elemental composition of the dark gray matrix is almost similar to the other matrix, Ca is detected only in the dark gray matrix. Therefore, there would be very fine-grained calcite in the dark gray matrix. Outside the latest slip zone, Ca seems to appear as veins, but Ca shows irregular shape, uncontinuous distribution and anhedral in the elemental mapping image with higher magnification.
These results indicate that no calcite mineralization in the latest slip zone after the Nobi Earthquake, and no mineral filling has occurred in the latest slip zone.
NDFP-1 and NDFD-1 were drilled at Neo-Midori, Motosu City, Gifu Prefecture, Central Japan where a 6 m vertical displacement along the Neodani Fault occurred during the Nobi Earthquake. The length of NDFP-1 is 140.3 m, and penetrates the latest slip zone at a depth of 110.8 m. The length of NDFD-1 is 524.8 m, and penetrates the zone at a depth of 387.7 m. The direction of drilling is southwest, and the two boreholes intersect almost perpendicular to the fault in the horizontal plane. The geology of this area is the Mino Belt, which is Jurassic accretionary complex, and is mainly composed of mélange with mudstone matrix and blocks of sandstone, limestone, chert, and basalt.
The calcite mineralization in the fault gorges was observed using a scanning electron microscope (SEM), and elemental analysis was performed using energy-dispersive X-ray spectroscopy (SEM-EDX). Elemental mapping by SEM-EDX showed that Ca is fragmented within the latest slip zone, whereas it is distributed as veins outside the latest slip zone. Ca-bearing fragments in the latest slip zone show rounded and anhedral in shape. This suggests that the calcite was repeatedly fractured and worn by faulting. In addition, a dark gray matrix exists in a narrow zone of about 20 μm near the fracture . Although the elemental composition of the dark gray matrix is almost similar to the other matrix, Ca is detected only in the dark gray matrix. Therefore, there would be very fine-grained calcite in the dark gray matrix. Outside the latest slip zone, Ca seems to appear as veins, but Ca shows irregular shape, uncontinuous distribution and anhedral in the elemental mapping image with higher magnification.
These results indicate that no calcite mineralization in the latest slip zone after the Nobi Earthquake, and no mineral filling has occurred in the latest slip zone.