17:15 〜 19:15
[HDS07-P16] Structural Control by Joints and High-angle Fault Systems Recognized in Deep-seated Landslides of the Shimanto Accretionary Complex in Kii Peninsula
キーワード：四万十帯、断裂系、深層崩壊、紀伊半島
Introduction Numerous deep-seated landslides occurred in the Kii Mountains, where the Shimanto accretionary complex served as the bedrock, during the typhoon heavy rains of 1889 and 2011. The authors have clarified the characteristics of high-angle joint and fault systems and the structural landforms they produce, based on the analysis of lineaments using slope maps and field surveys. This study reports on the features of fracture systems and their control over river channels and landslide occurrences, using major deep-seated landslides in the Totsukawa River basin.
Topography and Geology The survey area spans from the Omine Mountains to the Totsu River basin in the Kii Peninsula, mainly located in the northern part of the Shimanto Belt (Fig.1), which consists of the sediment-dominant accretionary complex, and generally exhibits a geological structure with an ENE-WSW to E-W strike and a northward dip.
Lineaments and Fracture Systems By analyzing slope maps from 5m DEM, more than 3,000 lineaments have been identified. These lineaments were classified into three systems: NE-SW to NNE-SSW and NW-SE to NNW-SSE orthogonal systems, N-S and E-W systems, and ENE-WSW systems. The orthogonal systems are developed throughout the area, while the latter two systems are concentrated around the Omine Mountains. Lineaments are sparse and scattered in the Totsu River basin area, resulting from the thicker cover of soil and detritus and the loosening of bedrock there. Most of the lineaments are faults or joints accompanied by fracture zones ranging from a few millimeters to several tens of centimeters. These fracture systems are clarified to have formed after the mid-Miocene, resulting from fractures in hornfels produced by thermal metamorphism of felsic igneous rocks.
Deep-Seated Landslides and Fracture Systems Representative several deep-seated landslides in Nara Prefecture clearly demonstrate the presence of high-angle fracture systems that intersect the geological structures of the accretionary complex, such as Akadani and Kawarabigawa landslides in Gojo City, and Nigoridani, Nagatonodani, and Kuridaira landslides in Totsukawa Village. These deep-seated landslides can be divided into planar and wedge types based on the cross-sectional shape of the sliding surfaces. In the planar type, the lateral margins are linear and parallel, forming side cliffs where high-angle faults can be observed on site, such as in Nagatonodani and Kawarabigawa. Lineaments can be observed crossing adjacent valleys and ridges along the side cliffs. In contrast, the wedge type, such as Akadani and Nigoridani, exhibits high-angle faults along its central axis, which are parallel to the maximum slope direction of the sliding surfaces. Additionally, the channel directions in the middle reaches of the Totsukawa River and its tributaries are parallel to prominent fracture systems.
Conclusion Observations of microtopography from major deep-seated landslides caused by the typhoon heavy rains of 1889 and 2011 reveal that most of the lateral margins of planar deep-seated landslides are delineated by high-angle fracture systems, and the central axes of wedge-type deep-seated landslides are penetrated by high-angle fracture systems. This confirms that high-angle fracture systems formed after the mid-Miocene strongly regulate the microtopography of deep-seated landslides. Future surveys should focus on the presence of high-angle fracture systems and their role in the preparation process of deep-seated landslides in non-metamorphosed accretionary mountain areas such as the Totsukawa area.
Topography and Geology The survey area spans from the Omine Mountains to the Totsu River basin in the Kii Peninsula, mainly located in the northern part of the Shimanto Belt (Fig.1), which consists of the sediment-dominant accretionary complex, and generally exhibits a geological structure with an ENE-WSW to E-W strike and a northward dip.
Lineaments and Fracture Systems By analyzing slope maps from 5m DEM, more than 3,000 lineaments have been identified. These lineaments were classified into three systems: NE-SW to NNE-SSW and NW-SE to NNW-SSE orthogonal systems, N-S and E-W systems, and ENE-WSW systems. The orthogonal systems are developed throughout the area, while the latter two systems are concentrated around the Omine Mountains. Lineaments are sparse and scattered in the Totsu River basin area, resulting from the thicker cover of soil and detritus and the loosening of bedrock there. Most of the lineaments are faults or joints accompanied by fracture zones ranging from a few millimeters to several tens of centimeters. These fracture systems are clarified to have formed after the mid-Miocene, resulting from fractures in hornfels produced by thermal metamorphism of felsic igneous rocks.
Deep-Seated Landslides and Fracture Systems Representative several deep-seated landslides in Nara Prefecture clearly demonstrate the presence of high-angle fracture systems that intersect the geological structures of the accretionary complex, such as Akadani and Kawarabigawa landslides in Gojo City, and Nigoridani, Nagatonodani, and Kuridaira landslides in Totsukawa Village. These deep-seated landslides can be divided into planar and wedge types based on the cross-sectional shape of the sliding surfaces. In the planar type, the lateral margins are linear and parallel, forming side cliffs where high-angle faults can be observed on site, such as in Nagatonodani and Kawarabigawa. Lineaments can be observed crossing adjacent valleys and ridges along the side cliffs. In contrast, the wedge type, such as Akadani and Nigoridani, exhibits high-angle faults along its central axis, which are parallel to the maximum slope direction of the sliding surfaces. Additionally, the channel directions in the middle reaches of the Totsukawa River and its tributaries are parallel to prominent fracture systems.
Conclusion Observations of microtopography from major deep-seated landslides caused by the typhoon heavy rains of 1889 and 2011 reveal that most of the lateral margins of planar deep-seated landslides are delineated by high-angle fracture systems, and the central axes of wedge-type deep-seated landslides are penetrated by high-angle fracture systems. This confirms that high-angle fracture systems formed after the mid-Miocene strongly regulate the microtopography of deep-seated landslides. Future surveys should focus on the presence of high-angle fracture systems and their role in the preparation process of deep-seated landslides in non-metamorphosed accretionary mountain areas such as the Totsukawa area.