Fbcabeb6_8930_4a01_bdbd_4b468e2f1e3d.pdf

11.1 Geometric description It is hscinating to watch folds 6rm and develop in the

laboratory,and we can learn much about folds and folding by performing controlled physical experiments and numerical simulations. However, modeling must alwaysbe rooted in observations of naturally folded rocks, so geometric analysisof folds formed in different settings and rock types is fundamental.Geometricanaly-

sis is importantnot only in order to understandhow various types of fi)lds f»rm, but also when considering such things as hydrocarbon traps and folded ores in the subsurface. There is a wealth of descriptive expressions in use, because folds come in all shapes and sizes. Hence we

will start this chapter by going through the basic jargon related to folds and fold geometry.

Shape and orientation Folds are best studied in sections perpendicular to the folded layering,or perpendicular to what is defined as the axial surface, as shown in Figure 11.1.Unless indicated, we will assume that this is the section ofobservation

in this chapter. In general,folds are made up of a hinge that connects two usuallydifferentlyoriented limbs. The hinge may be sharp and abrupt, but more commonly the curvature of the hinge is gradual, and a hinge zone is defined. A spectrum of hinge shapes exists, from the pointed hinges of kink bands and chevron folds (sharp

and angular folds) to the well-roundedhinges of concentric folds (Figure 11.2). Classificationof folds relativeto hinge curvature is referred to as bluntness. The shape of folds can also be compared to mathemat-

ical functions, in which case we can apply terms such as amplitude and wavelength. Folds do not necessarily show the regularityof mathematical functions as we know

(a) Kit*

Trace of bisectingsurface (b) Chevron tows

(c)

BWs

(d) Box Axial race

Axhl race

Figure 11.2 (a) Kink band, where the bisecting surface, i.e. the surface dividing the interlimb angle in two, is different from the axial surface. (b) Chevron folds (harmonic). (c) Concentric f)lds, where the arcs are circular. (d) Box folds, showing two sets of axial surfaces.