Folds and folding - Introducing Tectonics, Rock Structures and Mountain Belts - page 53

Figure 6.9 Multilayer buckle folding. A. In a
set of buckled layers of approximately equal
thickness and strength, the wavelengths of the
layers will be equal, but the weaker material
between the layers will deform by flow to
take up the space left. B. Three layers, X, Y
and Z, with differing strength and thickness,
experience different degrees of layer-parallel
shortening before buckling takes place; the
minimum amount of shortening is revealed by
unravelling the layers. C. A thinner and weaker
layer between two stronger buckled layers, will
form shorter-wavelength folds that are refolded
by the longer-wavelength folds. D. Folds as in C
after further shortening experience shear strain
in the limbs causing the smaller parasitic folds to
become asymmetric. E. The sense of asymmetry
of parasitic folds can be used to determine the
(unexposed) hinge position of the major folds;
i.e. this must represent the overturned left limb
of an antiform, not the right-hand limb of an
upright antiform.

X

6

Y

buckled
stronger
layers 52 53

Z

A

minimum shortening
in X

weaker
buckled
layer

minimum
shortening
in Y

Z

B

hinge. In reality, of course, folds are
three-dimensional objects that may
vary considerably in the dimension
parallel to their hinge lines, and folds
that maintain their profile shape for
long distances are uncommon.
Figure 6.1 displays three-dimen-
sional variation in a quite spectacular
way. The main anticline in the picture
gradually dies out away from the viewer
by means of a decrease in height and
width, and a parallel anticline to the left
of it is shaped like an upturned boat,
with an oval ground plan. Between
these two structures, and to the right
of the main anticline, there are rather
oddly-shaped synclinal structures.
Folds of this type, which vary in height
along their length such as to plunge
in opposite directions at each end, are
termed periclines and may be either
anticlinal or synclinal. In the extreme
case, an anticlinal pericline where the
dips are radial becomes a dome , and the
synclinal equivalent becomes a basin .

C

hinge this side

unexposed

D E

example that quite complex fold pat-
terns can be achieved by buckle folding,
given some variety in the physical
properties of the layers involved.
The small-scale folds that affect the
thin layers of a multilayer set such as
the central band of Figure 6.9C are often
called parasitic (or satellite ) folds. As
the thicker controlling layers tighten,
these parasitic folds will become asym-
metric due to the shear strain exerted
by the adjacent buckling layers (Figure

6.9D). This feature is useful in deter-
mining which direction the main anti-
formal and synformal hinges are, in
situations where the outcrop is limited
and the fold limb is steeply inclined
or possibly overturned (Figure 6.9E).

Folds in three dimensions
Most of the discussion so far has been
based on considering folds in profile,
where their geometry is described
in a plane perpendicular to the fold