Folds and folding - Introducing Tectonics, Rock Structures and Mountain Belts - page 49

A whole range of possible fold shapes
exists in nature, embodying transi-
tions between these 'end-member'
types. In most folds, the layers differ in
thickness to a greater or lesser degree
and often show a variation between
a more parallel shape in the thicker,
stronger layers, and a more similar
shape in the thinner, weaker layers. As
an example, compare the thick layers
in the lower fold of Figure 6.2B with the
thinner layers in the core of the fold.
Fold sets may be described as
symmetric if the opposite limbs
are of equal length, or asymmet-
ric if unequal (Figure 6.4F).

alternative mechanisms

Figure 6.5 The folding process.
A. Parallel or concentric folding:
once the folds have reached a
semicircular shape, any further
tightening will result in distortion
of the limbs and the folds
will no longer have a parallel
geometry; note that below and
above the parallel-folded layer,
an alternative fold mechanism
must operate. The wavelength
(w) of the parallel folds at this
point equals approximately
twice the thickness (t) of the
folded layer. B. Similar folding:
the fold shape is achieved by
variable flow across the red
layers, which behave passively;
the flow direction is parallel to
the axial plane of the folds.
C. Chevron folding: each layer
is parallel folded and slides
over the layer beneath; gaps
are formed at the hinges, which
may fill with vein material;
however, the overall fold shape
is similar.

6

parallel folding
restricted to
this layer

t

48 49

centres of
curvature

alternative
mechanisms

w ~ 2t

A concentric

folding

shear (flow)
direction

The folding process
Parallel folds
These folds are restricted by their geom-
etry to a depth that is determined by
their wavelength, as shown in Figure
6.5A, and must be replaced by a dif-
ferent mechanism at their centre of
curvature. This means that perfectly
parallel folds (which are actually a
set of arcs of concentric circles) are
restricted to a given thickness of layers.
Consequently, we can assume that it
is the thickness of the (strong) layers
involved in the folding that determines
their wavelength, and by knowing that,
an estimate of the original thickness
of the layers involved can be deduced.
For example, the very large-scale folds
seen in Figure 6.1, which have a wave-
length of several kilometres, must have
affected a particularly strong layer
whose thickness could have been up to
approximately half this amount. Such
folds may rest on a detachment surface,
such as a thrust plane, separating them
from a different set of rocks beneath the
parallel-folded layer, or alternatively,

C chevron folding

B

similar folding

the shortening may be taken up by
a different mode of deformation in
the rock beneath, as in Figure 6.2B.

plane of the fold as in Figure 6.5B. The
process can be visualised by drawing a
set of lines across the edges of a pack of
cards, then shuffling the pack into a fold
shape. The movements here are obvi-
ously parallel to the individual cards,
which themselves are parallel to the
axial surface of the fold. Since the duc-
tility (i.e. relative strength) of a rock will
depend on the ambient temperature, as
explained in Chapter 4, it follows that
similar folds are much more common
in rocks deformed under metamorphic
conditions, for example in shear zones
( see below) where the flow direction is
parallel to the walls of the shear zone.
The scale on which a fold is viewed is
important here, since folds that appear
to be similar on a large scale turn out to

Similar folds
Perfectly similar folds are characteris-
tic of very ductile (i.e. weak) material
where the individual layers behave pas-
sively under compression and accom-
modate themselves to a new shape
determined by the constraints imposed
by the geometry of the surround-
ing stronger material. The process of
deformation here is described as 'flow',
using the analogy of liquid behaviour,
although of course the movement takes
place in the solid state. The direction of
flow is across the individual layers, and
in the ideal case, parallel to the axial