High-amplitude folding of linear-viscous multilayers

Abstract High-amplitude folding of viscous multilayers during shortening can be analyzed with a theoretical solution motivated by first-order theoretical analysis of folding by Raymond Fletcher and Ronald Smith. The solution method can better match boundary conditions along irregular interfaces than the first-order method, so it increases the range of slopes over which linear-viscous folding theory can be applied. In our method, rather than solving algebraically for a small number of constants in the flow equations, we numerically solve for a large number of constants, the values of which are chosen so that they minimize errors in matching conditions at the interfaces in a least-squares sense. A similar method has been applied to problems of density instability involving a single deformable interface with bonded contacts; however, we extend the method to include shortening parallel to interfaces and many deformable interfaces so that we can deal with problems of multilayer folding. Contacts between the layers can be firmly bonded, slip freely, or slip with viscous resistance. We use the solution to produce high-amplitude folds in single layers embedded in soft media, and in simple repetitive multilayers confined above and below by stiff or soft media. We show that the folding of linear-viscous multilayers can largely reproduce the gross forms of some small folds in the Huasna syncline in the central California Coast Range as well as the Berry-Buffalo syncline in the central Pennsylvania Appalachians. However, the sharp, chevron-like forms in these natural examples are notably missing in the simulations based on linear-viscous theory.