Abstract
Gravitational deformation of mountains (sagging) is evidenced by a series of ridge-top depressions and uphill-facing scarps in the upper reaches of the Abe River, central Japan. The characteristics are present in an area extending from the Oya-kuzure landslide to Mt. Yambushi.
Ridges are underlain by slate, the cleavage of which trends slightly obliquely to the ridge axis. The cleavage dips steeply (from 70° to 90°) in most of the valley bottom outcrops, but dips gently toward the mountain at higher elevations. This orientation indicates that the slate bowed toward the valley at the surface of the slopes. Such bowing likely generated the ridge-top depressions and uphill-facing scarps. Some ridge-top depressions and uphill-facing scarps are parallel to the strike of the slatey cleavage, and some are parallel to the ridge. The depressions and scarps parallel to the ridges branch from those that are parallel to the strike, indicating that those that are parallel to the strike formed first. Thus sagging first started with the separation of rocks along cleavage planes, and then the separation surface propagated parallel to the ridge.
Depositional accumulations in depressions on ridge-tops (_??_10m deep) and at the foot of uphill-facing scarps (several meters deep) exhibited regional tephra and characteristic facies, indicating that the sagging has been developing since the late Pleistocene. Ridge-top depressions existed for at least 20 ka, and the lower uphill-facing scarps formed about 10 ka BP. The edges of ridge-top depressions and uphill-facing scarps at higer elevations are more rounded than those at lower elevations, suggesting that the higher features are older than those at lower elevations, i.e., sagging started near the ridge-top and propagated downslope.
