Flow, deformation, and transport - Physical Processes in Earth and Environmental Sciences - page 175

a1

l

Envelope

l

b1

AT

A

AT

A

Median
line

AT

AT

AT

AT

Envelope

i

b2

a2

i

f

f

Fig. 4.117 Fold size and symmetry in (a) symmetrical folds; (b) asymmetrical folds. To define fold size the wavelength ( ) and the amplitude

(A) are defined. The wavelength is measured from two consecutive antiform or synform hinges parallel to the median line. The amplitude
is the distance between the median line and one of the external envelope, measured parallel to the axial trace (AT). a2 and b2 show some com-
ponents to establish fold symmetry or asymmetry.

A

Clockwise asymmetric fold (z-fold)

Counterclockwise asymmetric fold (s-fold)

Fig. 4.118 Asymmetrical folds are defined as clockwise or z -folds and counterclockwise or s -folds. As photo shows Mike and Storm discussing
an example of a z -fold (Scotland, UK).