Flow, deformation, and transport - Physical Processes in Earth and Environmental Sciences - page 176

4.16.2 Size, shape, and orientation

amplitude and the distance between the inflection points
of a fold.
Fold orientation in a 3D space is described by the
orientation of the axial surface and the hinge line. Axial
surface orientation is given by the strike and dip of the
surface, whereas the hinge line is defined by the plunge
(the vertical angle between the line with its horizontal
projection) or the rake (pitch) measured over the axial
surface between the hinge line, which is always located
on the axial surface and a horizontal line located in the
axial surface. There is also a broad nomenclature and fold
classification concerning different kinds of folds according
to their orientation; for example, upright folds are those
having vertical axial surfaces; in these the hinge line can be
horizontal, inclined, or vertical. Folds having horizontal
axial surfaces are called recumbent folds (the hinge line is
always horizontal) and finally, folds having inclined axial
surfaces are called steeply, moderately , or gently inclined
folds depending on the inclination. Inclined folds can have
horizontal or inclined hinge lines. When the dip of the
axial surface and the hinge line are equal in angle and ori-
entation, the folds are called reclined .
Other classifications are based on geometric properties
of the folded surfaces. One of the most commonly used is
the Ramsay classification of folds (Figs 4.119 and 4.120),
which is based on the definition of three geometrical
elements: the dip isogons, the orthogonal thickness,
and the axial trace thickness. To trace the dip isogons, first
the axial trace and a normal line to it are plotted. The
normal is the reference line to define different angles
(

Folds occur over a range of sizes, from several kilometers
to millimeters and are defined in two dimensions by two
components, the wave length (

) and the amplitude ( A ),
in the same way that other wave-like forms are measured.
To accurately establish both components a reference line
is drawn joining all the inflection points, called the
median line, and all the hinge points in both antiforms
and synforms, called enveloping lines. The wave length
is the distance between the hinges of two consecutive
antiforms or synforms, measured in a straight line
parallel to the reference lines (Fig. 4.117). The ampli-
tude is the distance, measured parallel to the axial trace,
between the median line containing the inflection points
and the envelope line containing the hinge points
(Fig. 4.117).
The shape of folds can be described by means of dif-
ferent elements. The cylindricity of a fold that can be
considered an important element in fold descriptions
has been illustrated previously when discussing folds
containing an axis. Other elements are the fold symme-
try or asymmetry, which can be given by the length and
the shape of the limbs (Fig. 4.117). Symmetrical folds
have equally long limbs and the axial surface is a symme-
try plane that divides the fold in two halves identical in
shape but mirror images. Asymmetrical folds have limbs
with different lengths and the axial surface is not a sym-
metry plane; z -folds or clockwise folds and s -folds or
counterclockwise folds can be defined (Fig. 4.118) on
the basis of the limbs' rotation with respect with to a
symmetric position. The asymmetry of s - and z -folds
changes if we look at the folds from one side or the
opposite facing along the axial surface, and so, conven-
tionally, the sense of rotation is defined looking down
the plunge of the hinge line if it is inclined. When the
hinge line is horizontal some geographical reference has
to be included in the description. Other elements to
measure fold shapes are the tightness, the bluntness, and
the aspect ratio. The tightness is defined by the inter-
limb angle ( i , Fig.

, Fig. 4.120).

z- fold

z- folds

s- folds

,
Fig. 4.117). The limb angle is the angle that forms the
tangents at each inflection point of the limbs, and the
fold angle between the normal lines of both tangents to
the limbs. According to these angles, folds can be classi-
fied into acute (when i has a value between 180

4.117) or the fold angle (

z- folds

and 0

and between 0 and 180 ), isoclinal (when i 0 and

180 ), and obtuse ( i from 0 to 180 and

Fig. 4.119 Parasitic folds can be superimposed on larger symmetrical
or asymmetrical folds. Note the change from z - to s -folds at both
sides of the larger fold. The photo shows an example of some folded
layer displaying parasitic folds.

between 180 ). The bluntness describes the

degree of roundness or curvature in the hinge zone or
closure, and the aspect ratio the relation between the

and 360