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[MIS10-P01] Geomorphic evolution of landslides around Heikubo-ike Pond in the Southern Japanese Alps, Yamanashi Prefecture, Japan
Keywords:Landslide, Granite, Southern Japanese Alps
The geology of the Southern Japanese Alps (SJA) is generally composed of sedimentary and metamorphic rocks, and large-scale landslides and slope failures are dominant in many sites. On the other hand, granitic rocks are distributed in the northern part of the SJA, and large-scale landslides and shallow slope failures are partly developed throughout the area. In this study, we examined a case of large-scale landslide in the granitic rock area in the northern SJA and preliminary reconstructed a geomorphological development.
In the vicinity of Heikubo-ike Pond in Hokuto City, Yamanashi Prefecture, two prominent landslide landforms are present (35°50′41″N 138°15′57″E; Landslides A and B; Figure). For Landslide A (LA), a main body with an area of 2×105 m2 presents just below the headscarp, and microforms such as pressure ridges and tension cracks can be seen on the landslide body. On the other hand, in Landslide B (LB), a main body is smaller compared to the size of its headscarp, and the ground surface of the main body is relatively smooth. Compared to the LA, the main body of LB seems to be dissected well. At the toe of the main body of LA, outcrops exist along the Nagare-gawa River, exposing poorly-sorted sand and gravel layers, and silt layers intercalated with 3 fallout pumice layers. These pumice layers can be divided into two groups, X and Y: Group X contains hornblende and biotite phenocrysts and Group Y is characterized with twopyroxene phenocrysts. Volcanic glass shards of pumice-type are common to both groups. Based on lithology, thickness, and mineral compositions, it is certain that those tephras are of Ontake Volcano origin, 70 km west of the study site. Although On-Pm1 (100 ka) and On-In (93 ka) are major candidates, an identification analysis is under progress.
Based on the above, the development process of landslide features at Heikubo-ike is suggested three stages below. (1) Occurrence of Landslide B: The LB was active at first, and landslide materials moved to the east. Landslide materials were estimated to have moved to the present confluence of Nagare-gawa River and the nameless valley, or ran out further downstream. However, geologic residue is scattered at present, so details are still unknown. (2) Occurrence of Landslide A: Then the LA slid and material moved eastward. However, the main body of LB, which was settled prior to the LA, blocked the material movement of LA. Consequently, the main body of LA could not move for a long distance and was placed near the channel of Nagare-gawa River. At this time, a dammed-lake was temporarily formed, and tephra layers were deposited. (3) Dissection of the Landslide B: The body of LB was subsequently dissected by fluvial erosion and secondary landslides, and shrunk and retreated to the vicinity of the headwaters of the nameless valley. Future identification analysis of the tephras make us possible to reconstruct topographic development of LA and LB with numerical ages.
In the vicinity of Heikubo-ike Pond in Hokuto City, Yamanashi Prefecture, two prominent landslide landforms are present (35°50′41″N 138°15′57″E; Landslides A and B; Figure). For Landslide A (LA), a main body with an area of 2×105 m2 presents just below the headscarp, and microforms such as pressure ridges and tension cracks can be seen on the landslide body. On the other hand, in Landslide B (LB), a main body is smaller compared to the size of its headscarp, and the ground surface of the main body is relatively smooth. Compared to the LA, the main body of LB seems to be dissected well. At the toe of the main body of LA, outcrops exist along the Nagare-gawa River, exposing poorly-sorted sand and gravel layers, and silt layers intercalated with 3 fallout pumice layers. These pumice layers can be divided into two groups, X and Y: Group X contains hornblende and biotite phenocrysts and Group Y is characterized with twopyroxene phenocrysts. Volcanic glass shards of pumice-type are common to both groups. Based on lithology, thickness, and mineral compositions, it is certain that those tephras are of Ontake Volcano origin, 70 km west of the study site. Although On-Pm1 (100 ka) and On-In (93 ka) are major candidates, an identification analysis is under progress.
Based on the above, the development process of landslide features at Heikubo-ike is suggested three stages below. (1) Occurrence of Landslide B: The LB was active at first, and landslide materials moved to the east. Landslide materials were estimated to have moved to the present confluence of Nagare-gawa River and the nameless valley, or ran out further downstream. However, geologic residue is scattered at present, so details are still unknown. (2) Occurrence of Landslide A: Then the LA slid and material moved eastward. However, the main body of LB, which was settled prior to the LA, blocked the material movement of LA. Consequently, the main body of LA could not move for a long distance and was placed near the channel of Nagare-gawa River. At this time, a dammed-lake was temporarily formed, and tephra layers were deposited. (3) Dissection of the Landslide B: The body of LB was subsequently dissected by fluvial erosion and secondary landslides, and shrunk and retreated to the vicinity of the headwaters of the nameless valley. Future identification analysis of the tephras make us possible to reconstruct topographic development of LA and LB with numerical ages.