Folding with thermal–mechanical feedback

Folding of viscous layered rocks is traditionally viewed as an instability arising from viscosity differences between layers. This approach derives from Biot, and for purely viscous materials predicts the growth of single wavelength systems; the dominant wavelength is sensitive to layer thickness, the viscosity ratio between the layers, the amount of shortening and the boundary conditions. This paper presents one alternative theory of folding to that of Biot and addresses the deformation of elastic-viscous materials where the viscosity ratio between layers is small and Biot theory predicts folds will not form. Such ratios are consistent with situations in the mid to lower crust as indicated by experimental data. Folding results from the coupling between temperature dependent viscosity and heat generated by deformation; the result is weakening in the hinges of embryonic folds and subsequent buckling. This process is distinct from the Biot buckling process. The structures that develop resemble natural structures in that folds develop at a range of length scales, hinges undergo strong thickening, and axial plane crenulations form. This approach is grounded in non-equilibrium thermodynamics; the coupling of deformation to fluid flow and chemical reactions is explored as part of a unified framework for rock deformation processes.