SINGLE LAYER BUCKLE FOLDING IN NONLINEAR MATERIALS .2. COMPARISON BETWEEN THEORY AND EXPERIMENT

Results from scale-model experiments on the development of single layer folds from an initial perturbation of known shape are compared with infinitesimal amplitude theories for buckling of non-linear viscous materials. The experiments were performed in pure shear, using paraffin wax as an analogue for the power-law behaviour of common rocks. Effective viscosity ratios of 30 and 8 between layer and matrix were used, with power-law stress exponents of around 3.8 for the matrix and around 3 for the layer. The layer material shows strain softening behaviour. The variation of growth rate with wavelength (the range of wavelengths corresponding to the Fourier series representation for the non-periodic fold shape) was determined for each of the perturbation shapes and viscosity contrasts employed. These growth rate curves closely resemble those calculated from theory, but for short wavelengths and particularly for narrow initial pertubations, observed growth rates tend to be higher than theoretical values. This may reflect the strain softening behaviour of the layer. Bonding of the matrix-layer interface appears to have a much greater effect on the growth rate curve than theoretically predicted, at least for the low to moderate viscosity ratios investigated. Experimental fold shapes are also compared directly with theoretical shapes. The best-fit between theory and observation occurs for values of the viscosity ratio and the layer stress exponent which are very close to the calibrated material properties, providing further experimental evidence that current fold theories are a good approximation to low but finite amplitude, single layer folding in non-linear materials.