Abstract
Gravitational slope deformation occurs in the Ho-oh Mountains that runs on the south of E-W trending Median Tectonic Line (MTL) in central Shikoku.
The deformation occurs intermittently along the main ridge as long as 20km, and deformational features, such as ridge-top depressions are made in some portions. It is found that the deformation was induced by the tectonic upheaval of the south side of the MTL and in turn it affected the near-surface morphology of the MTL fault. And it also affected gravitational slope deformation. The slopes along the MTL, which consist mainly of pelitic schist, form dip slopes along the MTL. And also it is that supposed ; it became unstable when they rise along the MTL and consequently gravitationally deformed by the downslope movement of the rock. The fault plane of the MTL, which is generally high-angle, is exceptionally and locally low-angle and dips to the south along the foot of the gravitationally deformed slopes. This could be attributable to that the fault, which is high-angle in the depth, occurs south-dipping thrust near the ground surface when the south side rose under the compressional force exerted by the southern slopes which are gravitationally deformed.
The examination of age and structures of lacustrine deposits in a ridge-top depression indicates that the gravitational deformation made a lake 50 thousand years ago, then, the lake breached 45-24 thousand years ago, and the current morphology of the mountains was formed.
