Laboratory modelling of strain variation across rheological boundaries

Abstract

Laboratory models using viscous and viscoplastic silicones to simulate strain variation across competence contrasts are presented. This provides a test for earlier theoretical modelling of strain refraction in layers with Newtonian viscosity contrast, and a method of examining refraction rules for non-Newtonian materials. The main purpose is to test the theoretical rule that the finite shear strain ratio across a boundary is equivalent to the inverse viscosity ratio. Results for simple-shear experiments confirm this within the errors of viscometry and strain measurement. We investigate whether this rule applies to non-Newtonian materials, which necessarily involves a discussion on the nature of viscosity contrast for non-linear materials such as power-law fluids, and its bearing on competence contrasts in rocks. These models also provide data on strain gradients generated by viscosity boundaries, which was not included in the earlier theoretical analyses. In the simple-shear experiments, the normalized shear strain profiles indicate an approximately linear shear stress gradient from the viscosity boundaries. Using an idealized linear shear strain gradient in a Newtonian matrix approaching a contrasting layer, we can derive expressions to predict the viscosity ratio. This may be a viable method of determining approximate viscosity ratios for more general deformations.