Initiation of Ptygmatic Folding in a Competent Siliceous Layer: An Experimental Study

An experiment was carried out to develop ptygmatic fold in a competent agate layer embedded in an incompetent marble matrix in a triaxial press at a temperature of 650°C. The layer was deformed up to 21 per cent bulk-shortening under an initial confining-stress of 1 kb and at a strain rate of 7.95 x 10^-7sec^-1. The deformed layer exhibits the initial stages of ptygmatic folding. The stress-strain and time-strain relationships have been discussed. Thin-section study of the deformed layer reveals that recrystallisation of cryptocrystalline silica was the dominant mechanism of deformation. The low confining pressure and high strain rate was responsible for the development of tension and shear fractures in the agate layer.