single layer situation where
a more competent layer is surrounded by an incompetent
material. The former is free to form buckle folds that have a
parallel character, while the former will show another shape.
Biot (1957) --correspondence
principle between elastic and viscoelastic
viscosity), looks at stress-strain relationships in buckled layer.
Ramberg (1959) -- approaches problem as one of fluid dynamics -
particular function which satisfies both the biharmonic equation of a
stream function and boundary conditions around a buckled layer. Both
develop idea of dominant
wavelength. Only valid for small % of strain, but then have
inheritance principle; wd = 2 t ^3 ( µ1 / 6µ2 ) where wd is dominant
wavelength, t is layer thickness, and µ1 and µ2 are the viscosities
of the dominant layer and surrounding medium respectively. note that
deviatoric stress plays no direct role. viscosity contrast, if < 5
get pure shortening. too high deviatoric stress get brittle failure,
too low pure shortening. see this with ptygmatic folds in some
outcrops; average wad/t ratio of 27 are folds with ratio around 5-7;
possibly due to non-Newtonian viscosity. extrados and intrados of a
beam - features that would indicate buckling.