Folds and folding - Introducing Tectonics, Rock Structures and Mountain Belts - page 46

Folds and folding
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6

Folds and folding

Folds are beautiful and intriguing struc-
tures that attract the interest and curios-
ity of all geologists - which of us has not
paused on a mountain path to admire
a particularly fine example? They come
in a wide variety of sizes and shapes.
The largest are many kilometres across
and can only be viewed from the air or
on a map (Figure 6.1); the smallest are
microscopic. There is also a bewilder-
ing range of shapes - from rounded to
angular, regular to irregular, and so on.
These geometric characteristics are
referred to by structural geologists as

the fold 'style' and reflect differences
in lithology (i.e. the physical proper-
ties of the folded rock layer), and the
physical conditions (temperature and
pressure) under which the folding took
place. The fold style can vary markedly
between one rock layer and the next,
and even within the same layer - look
at the amazing variety of fold shapes
in Figure 6.2. Before examining these
shape variations, however, and the
reasons for them, it is necessary first to
establish a method for describing folds.

Describing the fold shape
The parts of a fold
The simplest fold shape consists of two
fold limbs separated by a hinge - the
line marking the maximum change in
orientation of the folded layer (Figure
6.3A). The fold hinge (sometimes
referred to as the fold closure ) may
be sharply defined, with relatively
straight limbs, or curved, approach-
ing the surface of a cylinder (compare
Figures 6.2B and C), in which case
the hinge is a zone rather than a well-
defined line. The fold angle , as meas-
ured by the smaller angle between the
two limbs (Figure 6.3B) can be used
to give a rough minimum estimate of
the amount of shortening achieved
by the folding. Most geologists gener-
ally only distinguish between open
folds (fold angle greater than ~90°) and
tight folds (less than ~90°) although
some recognise further categories;
isoclinal folds are a special case of
tight folds where the limbs are paral-
lel. The size of a fold can be described
in terms of its width (or wavelength ),
the distance between two adjacent
anticlinal (or synclinal) hinges, and its
height (or amplitude ) (Figure 6.3C).

Figure 6.1 Large-scale folds from the air, Bighorn
County, Wyoming. The prominent anticline in the
centre right of the picture plunges gently away
from us. To its right is another anticline and to
its left, in the distance, is an anticlinal pericline.
Between these anticlinal structures there are
synclinal areas with less regular outcrop shapes.
© Jim Wark.