Geology Reference
In-Depth Information
Fig. 1.22.
End-member displacement boundary conditions showing responses related to the mechani-
cal stratigraphy.
Shaded
beds are stiff lithologies,
unshaded
beds are soft lithologies
Fig. 1.23.
Transition from single layer folding to multilayer folding as space between stiff layers de-
creases. The stiff layers are
shaded
Fig. 1.24.
Strain distribution shown by
strain
ellipses
in folded stiff
layers. The trace of the neu-
tral surface of no strain is
dashed
.
a
Buckle fold.
b
Pure
bend (Drucker 1967)
produce circular to sinusoidal folds if enclosed in a sufficient thickness of softer ma-
terial (Fig. 1.13). Stratigraphic sections made up of relatively thin-bedded multilayer
units result in folds that tend to have planar dip-domain styles (Fig. 1.14).
The strain distribution in the stiff layer of a buckle fold (Fig. 1.24a) is approximately
layer-parallel shortening throughout most of the fold. The neutral surface separates
regions of layer-parallel extension from regions of layer-parallel contraction. Only in
a pure bend (Fig. 1.24b) are the areas of extension and contraction about equal and the
neutral surface in the middle of the layer. The strain in thick soft layers between stiff
