NUMERICAL MODELING OF TEMPERATURE EFFECTS ON ELASTO-VISCOUS FOLDING

Classic, Biot-style, single-layer fold models do an excellent job of predicting dominant fold wavelengths during buckling of simple materials. Naturally deformed folds in geologic materials, however, rarely display the regularity and periodicity of such idealized models. To date, no satisfactory theory exists that adequately elucidates the folding process. Recent work suggesting that viscosity contrasts in nature may be too low to generate folding via a competence contrast mechanism indicates even more need for refinement.

One approach is to acknowledge the complexity of the evolving fold system and the necessity of incorporating multiple processes and boundary conditions to produce and govern folding. Of the myriad of possible variables to begin with, temperature was chosen as a primary influence since it directly affects fold system rheology, deformation mechanisms, and metamorphic dynamics. As folds record the history of these operators, if correctly understood, they present a fundamental way of understanding a deforming system.

A temperature-dependent viscosity constitutive equation was implemented into a numerical, finite-difference model in order to analyze its effect on fold development. A Maxwell elasto-viscous layer embedded in a similar matrix was chosen because elastic or viscous behavior alone is not geologically realistic over a broad range of deforming conditions. Initial models were run at constant temperature, while subsequent runs included a “stepped” temperature increase or decrease during shortening. By effectively changing competence contrast, temperature was found to affect whether fold behaviour was elastic- or viscous-dominated, at what bulk shortening fold amplification began, what dominant wavelengths developed, and produced different strain patterns and varied folding mechanisms. While still requiring large competence contrasts to generate high-amplitude folds, temperature appears to exert a primary control on fold evolution. Other mechanisms, such as the anisotropy inherent in layering, foliation development, or on the grain-scale, must be important if large viscosity contrasts are not geologically reasonable.