Introduction

In some folds the thicknesses of layers remain essentially uniform with the result that they display constant orthogonal thickness. Such folds are parallel. They commonly have gentle to close shapes and are typically developed in well-bedded sedimentary rocks. There are two end-member shapes (see Fig. 14.4):

1. Rounded forms have smoothly curved limbs and broad hinge zones.

2. Angular forms have straight limbs and narrow hinge zones.

We seek ways of reconstructing both types of such folds in profile from field data. Here we consider only the case of horizontal folds, that is, folds whose profile planes are vertical. The methods will not work for plunging folds and the additional steps required to construct their profiles are treated in Chapter 17.

Rounded folds

First, we treat the case of smoothly rounded folds, sometimes referred to as concentric folds.

The requirement of strict constant orthogonal thickness within a folded layer or packet of layers severely limits the states of strain which can exist in parallel folded layers. There are two mechanisms involved: shear and extension.

Some insight into possible states can be obtained by considering simple models. Once again we resort to a deck of thin cards. To produce a fold hold the deck with both hands and press inward on the ends (it helps to give the center of the deck a slight upward nudge with the thumbs).