THE IMPACT OF STRAIN RATE ON FOLDING AND BOUDINAGE UNDER PLANE STRAIN: RESULTS FROM ANALOGUE MODELLING

To demonstrate the impact of varying strain rates on growing folds and boudins under plane strain, we investigated a stiff layer consisting of non-linear viscous Kolb grey plasticine that is embedded in a weak matrix, consisting of non-linear viscous Beck's green plasticine. The strain rates used range from 8 x 10-6 s-1 to 2 x 10-2 s-1. The viscosity ratio between layer and matrix increases with strain rate from 3 to 10. Different runs have been carried out in which the layer (S) was oriented perpendicular to the principal strain axes (X>Y>Z). Our results suggest a considerable influence of the strain rate on the geometry of the deformed stiff layer including its thickness. A striking break in both the degree of layer thickening and the change in arc length of folds and boudins is obvious at a strain rate of 1 x 10-3 s-1, which corresponds to a viscosity ratio of ca. 7. The new results confirm that previous analytical solutions do not hold for low viscosity ratios which correspond to slow strain rates in the present case.