10. Folds and Folding
Ask a structural geologist or any geologist for that matter about their favorite structure and chances are that they will say folds. If you have seen a fold in the field you will have marveled at its appearance. Let's face it, it is pretty unbelievable that "hard rocks" are able to change shape in such a dramatic way (Figure). In simple terms, a fold is a structural feature that is formed when planar surfaces are bent or curved. In fact, if such surfaces are not available (e.g., bedding, cleavage, inclusions) you will not see a fold even though the rock is deformed. Folding represents ductile deformation because there is no mesoscopic evidence for fracturing and the deformation is (heterogeneously) distributed over the entire structure. Some fracturing may occur in places, but processes such as grain-boundary sliding, kinking, dissolution and crystal plasticity are dominant. Looking at a fold from a kinematic perspective, you may realize that the strain in this structure cannot be the same everywhere. We recognize distinct sections in a fold, such as the hinge area and the limbs, the inner arc and the outer arc, which each reflect different strain histories. Whereas much of our discussion in this chapter focuses on mesoscale folds, you will find that folds occur on all scales, from mm-size or smaller microfolds in thin sections (see book cover) to a single large folds that occupy entire mountains. Why do folds exist, how do rocks do it, what does folding mean for regional analysis? Well, these and many other questions were first asked quite some time ago and much of what we know today about folds and folding was established well before the 1980's. The geometry of folds tells us something about, for example, the conditions of stress and strain, which in turn provides critical information about the deformation history of a deformed area. Much of the work in more recent years presents refinements of some of this earlier work, in particular we use more sophisticated numerical and experimental approaches. Yet, the fundamental observations remain essentially in tact, and therefore we will mainly look at some of these first principles of folding in this chapter, and their application in structural analysis.