REVIEW

What controls fold shape?

competency and competency contrast. Ptygmatic folds are an example of intestinal folds that are the result of a stiff layer (e.g. quartz) within a much more ductile unit.

DEFINITIONS

READINGS

 Ch. 11 Folds

LECTURE
 
 

ORIGIN OF FOLDS

FOLIATION

 At higher degrees of flattening, we develop foliation: or a preferred orientation of minerals, parallel to the axial surface (known as axial surface foliation). The origin of this foliation is debatable but similar folds are produced in this environment when folding is very intense. Shortening can be accomplished by dissolution and recrystallization. It is under these conditions that thicknesses parallel to the shortening direction are diminished but thicknesses at right angles are not. In this type of folding the layers are not truly buckled and the layering does not have much mechanical influence on the fold geometry. The intense flattening occurs at P and T where rocks can dissolve and minerals can recrystallize
 
 

SLIDES

 Under these conditions, thicknesses at right angles to the shortening are conserved and similar folds are produced. The geometry of a similar fold is such that an original fold is translated parallel to the fold axis, conserving axis-parallel bed thicknesses.

 In the field, you may see foliations where they cut the beds: intersection lineations (geological structural elements formed from the intersection of planar structures)

 In the field, you may be able to tell the position of the fold you are in by the relationship between the foliation and the fold limb.

 In the filed you will see that the supposed parallelism between the foliation and the axial surface is not exact. Sometimes in alternating bedding with different grain sizes in each bed, the foliation will appear to fan. This fanning or 'refraction' shows that the foliation is not always parallel to the fold axial surface:
 
 

SLIDES