STRUCTURAL GEOLOGY
Lecture 26
Folding of Rocks
(Analysis of Fold Geometry)

Folds are the second important component of fold-thrust belts and rock deformation in general.  Whereas faults were treated as a general manifestation of brittle deformation within the upper 10 km of the crust, folds are generally regarded as a manifestation of ductile deformation.  In fold-thrust belts folds are necessary to accommodate faulting.  Folds are most commonly marked by bedding, a surface referred to as S0 in structural geology.  Folds are characterized by axial surfaces, S1, and a fold axis (Fig. 26-1).  The plane perpendicular to the fold axis is that surface on which the profile of a fold is projected.  A fold that has a planar axial surface is said to be a plane fold, otherwise it is a nonplanar fold.

(Fig. 26-1)

One ideal fold is cylindrical with any profile the same as any other profile (Fig. 26-2).  Real folds are rarely cylinderical but rather conical in shape.  The ideal fold can be divided into the hinge area where the radius of curvature of the fold is smallest and the limb area where the radius of curvature of the fold is largest.  Figure 26-2 shows a profile or cross section through a set of cylinderical folds constituting a wave train.  The Amplitude and wave length of the wave train are shown.

(Fig. 26-2)

Folds are sometimes overturned as in Fig. 26-3.  In that figure the sandstone is younger than the limestone, yet in the overturned limb the older limestone appears over the younger sandstone.  The direction of younging is said to be downward. The enveloping surface of the folds as well as the crest and trough are indicated.

(Fig. 26-3)

Folds have many shapes in cross section depending on their intralimb angle.  Four general classes of folds based on intralimb angle are:  Gentle, Open, Tight, and Isoclinal.  Other shapes of folds are indicated in Fig. 26-4 as Rounded, Angular, Chevron, and Box.

(Fig. 26-4)

Folds may be classified based on the orientation of their hinge line and axial surface.  Turner and Weiss (1963) have identified seven types of folds including: horizontal normal, plunging normal, horizontal inclined, plunging inclined, Reclined, Vertical, and Recumbent (Fig. 26-5).

(Fig. 26-5)

Ramsay's classification of folds is based on the dip isogon.  A dip isogon is the line joining points of equal dip as shown in Fig. 26-6.  Two common types of folds are the similar and parallel folds.  In similar folds the bedding shape remains the same from bed to bed whereas in parallel folds the bedding thickness remains unaltered from bed to bed.

(Fig. 26-6)

In order to maintain similarity from bed to bed, material within the beds must be moved out of the limbs and into the hinges (Fig. 26-7).

(Fig. 26-7)

In parallel folds the shape of the beds must change from layer to layer in order the maintain constant bed thickness throughout (Fig. 26-8).

(Fig. 26-8)

Folds