Folding of an isolated finite length power law layer embedded in a
Newtonian viscous matrix is investigated and compared to conventional
folding experiments where the layer is of infinite length or in direct
contact with lateral boundaries. The approach employed is a combination
of the complex potential method for the basic state and the thin plate
approximation for the linear stability analysis and is verified by
finite element models. The resulting theory reveals that the aspect
ratio of a layer has a first-order influence on the development of
folds. The aspect ratio competes with the effective viscosity contrast
for dominant influence on the folding process. If the aspect ratio is
substantially larger than the effective viscosity contrast, the
conventional theories are applicable. In other situations, where the
aspect ratio is smaller than the effective viscosity contrast,
substantial corrections must be taken into account, which lead to a new
folding mode that is mainly characterized by decreasing growth rates
with increasing effective viscosity contrast (relative to the far-field
shortening rate). This new folding mode helps explain the absence of
large wavelength to thickness ratio folds in nature, which may be due to
the limitations of aspect ratios rather than large effective viscosity
contrasts.