Deep-seated gravitational slope deformation in Lushan, Taiwan: Transformation from cleavage-controlled to weakened rockmass-controlled deformation

•Deep-seated gravitational slope deformation (DSGSD) in Lushan was investigated.•Transformation from cleavage- to weakened rockmass-controlled DSGSD observed.•Drilling cores, acoustic borehole televiewing, P-wave logging, inclinometers, GPS utilized.•Five geometrical styles and geotechnical zones of deformed rockmasses identified.•Empirical model for Taiwan slate proposed: V=0.6 × log Qc + 4.0.