Controls of folding on different scales in multilayered rocks

Folding at different scales is ubiquitous in orogenic belts, and small-scale folds are commonly used to constrain the geometry of larger-scale folds. Analyses of viscous multilayers of N equally-thick stiff layers show that the rate of fold amplification and the dominant wavelength to layer thickness increase with N. This suggests that large-scale multilayer folds would initiate earlier than small-scale minor folds in a single layer or a few layers. How then do the latter amplify at a faster rate than the former to become minor folds on the limbs of larger-scale folds? To answer this question, analytical models of fold initiation in multilayers that contain stiff layers with different thicknesses and viscosities were studied. The models comprise five alternating stiff/soft viscous layers, with a thinner or thicker central stiff layer, in a soft confinement by either viscous half spaces or finite layers against rigid frictionless platens. One or two maxima may occur in the amplification rate - wavelength spectrum: if two, the stronger buckling instability may be either that of the multilayer or a single layer. Early initiation of single-layer folds in a central thin layer is favoured if the multilayer is narrowly confined, and if the layer is significantly stiffer than the adjacent stiff layers. Folds then develop on two scales, creating potential 'minor' and 'major' folds. In models with a thicker central layer, amplification rate decreases as the layer thickness increases. Unusually thick competent layers in a confined multilayer do not act as 'control units' that enhance folding; instead they impede folding.