LONG-TERM GRAVITATIONAL DEFORMATION OF ROCKS BY MASS ROCK CREEP

Subsurface rocks beneath slopes are deformed to various degrees in various ways by mass rock creep (MRC), when they are subjected to a gravitationally unstable state for a long period. MRC forms folds, faults and numerous kinds of fractures; such deformation is one of the main factors for deteriorating rock masses on slopes and also precursors of many landslides. MRC deformations, which have often been ascribed to tectonic origins, can be distinguished from tectonic ones by their deformational features, landforms and relationships between slope morphology and subsurface structures. MRC folds are flexural-slip folds that have the following mesoscopic to microscopic characteristics: their axial surfaces are not smooth but are jagged and many visible openings or open fractures develop in association with the folds. The jagged axial surfaces become smoother at depth and openings decrease in width with increasing overburden at the deformation site. Macroscopic types of MRC folds change according to the relationship between foliations and slopes; buckling folds form in consequent slopes, drag folds form in a deeper part of obsequent slopes and bending folds form in slopes with steep-dipping foliations (consequent or obsequent) by downslope bowing of foliations. Shear fractures and tension fractures are the main deformational structures formed by MRC in massive rocks. Among shear fractures, those of listric origin are remarkable. Tension fractures, in general, occur either in networks, dividing rocks into fragments that slip and rotate irregularly in relation to each other, or in parallel arrays of steeply dipping fractures. An MRC fault has a shear zone consisting of a pulverized zone with gouge and a phyllitic or brecciated zone. In a densely foliated rock, the phyllitic zone is formed by microscopic slip along foliations; in a sparsely foliated rock, the brecciated zone is formed by random crushing except for the faults made through the shear fractures in massive rocks. Landforms due to MRC are gentle slopes bounded by horseshoe knicklines upslope, convex slopes, ridge-top depressions, multiple ridges, uphill-facing scarps and ridge-crossing depressions.