[HDS12-P03] Gravitational deformation on mountain slope to the northeast of Tokugotoge Pass, Nagano Prefecture, Japan
Keywords:precise DEM, gravitational slope deformation, geologic structural control, rock loosening, geomorphic development
Multiple ridges and linear depressions continue about 12 km along the ridge from Tokugotoge Pass to Mount Chogatake. We examined geomorphic development of mountain slope from Tokugotoge to Tokusawa by means of interpretation on the precise DEM by airborne LIDAR, analysis of sediments in a linear depression and geological reconnaissance.
Micro landforms suggested deep seated gravitational slope deformation (DSGSD) such as linear depressions, uphill-facing scarplets and mid-slope steplets are widely developed in this area. They are unevenly distributed, are widespread on the northwest facing slope and are limited near the main ridge on the southwest facing slope. This tendency indicates that gravitational deformation ware formed under control of geologic structure of the Mino Belt. On the other hand, the micro landforms are developed concordantly with the bending of ridge from E-W to N-S trend, and are also recognized around the branching ridge with NW-SE trend, which means that the gravitational deformation originated after main geomorphic framework ware formed.
Result of geological reconnaissance reveals that the bedding plane of strata trends NE-SW and dips northwestward steeply, however, tends to dip southeast at the foot of slope facing to Azusagawa River. The bedrock is loosened intensely near the ridge top and the foot of slope. Furthermore, as the rock is loosened, the bedding plane dips gently, which suggests buckling or flow deformation of the rock mass. Predominant rock loosening restricted to the tributary valley bottom, though the depth of loose zone reaches over 100 m.
Horseshoe-shaped landslide scarps with a few hundred meters wide and tens of meters relative height are also developed in this area, whereas distinguished features of deep-seated landslide toe are hardly recognized. So it is assumed that the gravitationally deformed mass originated by rock loosening did not form deep-seated landslide body separated from sliding surface, but disintegrated into smaller scale surface rock failures.
Gravitational deformation arose after LGM at the latest because the DSGSD features are also seen on the lower slope to “Dissection front in Postglacial age.” Azusagawa River cut down into deep gorge at the foot of the slope until the damming by Akandana Volcano effusives 12.4 ka. The deformation is inferred to start at the time when the relative height to the ridge was larger than present. Since pyroclastic fall deposit from Yakedake Volcano before 2.3 ka buried ridge top depressions, then the depressions were already formed.
Micro landforms suggested deep seated gravitational slope deformation (DSGSD) such as linear depressions, uphill-facing scarplets and mid-slope steplets are widely developed in this area. They are unevenly distributed, are widespread on the northwest facing slope and are limited near the main ridge on the southwest facing slope. This tendency indicates that gravitational deformation ware formed under control of geologic structure of the Mino Belt. On the other hand, the micro landforms are developed concordantly with the bending of ridge from E-W to N-S trend, and are also recognized around the branching ridge with NW-SE trend, which means that the gravitational deformation originated after main geomorphic framework ware formed.
Result of geological reconnaissance reveals that the bedding plane of strata trends NE-SW and dips northwestward steeply, however, tends to dip southeast at the foot of slope facing to Azusagawa River. The bedrock is loosened intensely near the ridge top and the foot of slope. Furthermore, as the rock is loosened, the bedding plane dips gently, which suggests buckling or flow deformation of the rock mass. Predominant rock loosening restricted to the tributary valley bottom, though the depth of loose zone reaches over 100 m.
Horseshoe-shaped landslide scarps with a few hundred meters wide and tens of meters relative height are also developed in this area, whereas distinguished features of deep-seated landslide toe are hardly recognized. So it is assumed that the gravitationally deformed mass originated by rock loosening did not form deep-seated landslide body separated from sliding surface, but disintegrated into smaller scale surface rock failures.
Gravitational deformation arose after LGM at the latest because the DSGSD features are also seen on the lower slope to “Dissection front in Postglacial age.” Azusagawa River cut down into deep gorge at the foot of the slope until the damming by Akandana Volcano effusives 12.4 ka. The deformation is inferred to start at the time when the relative height to the ridge was larger than present. Since pyroclastic fall deposit from Yakedake Volcano before 2.3 ka buried ridge top depressions, then the depressions were already formed.