Fold terminology (R.P. - 8.3.2.1, 8.3.2.3)

Folding in layered rocks consists of deformation of strata without faulting. Folds are formed by compression, draping over basement blocks, and compaction. Faulting often accompanies folding and can precede it or follow it. The triaxial stress system for folding is representative for faulting since the two are intimately related.

Folds are manifested as different types according to morphology, origin and type of internal deformation. General fold terminology is descriptive and relates to elements of individual features applicable to all types of folds

1. Anticline is a fold with upward convexity. Beds that make up an anticline dip in opposite directions away from the crest or high point of the fold.

2. Syncline is a concave upward. Beds making up a syncline dip in opposite directions toward the trough, or low point of the fold.

3. The hinge point occurs at the locus of maximum curvature of a fold. The hinge point is applicable equally to anticlines and synclines. Hinge points connected on any particular bedding surface are joined along a hinge line.

4. The locus of all hinge lines of a fold is the hinge surface of the fold.

5. An inflection point occurs where bed curvature in one direction changes to bed curvature in the opposite direction. Inflection occurs where concavity changes to convexity.

6. The limbs or flanks of folds are those portions adjacent to the inflection lines of folds. They converge upon the inflection line

synclines and diverge from it in anticlines.

7. A fold is symmetrical when its shape and mirror image are identical or when the fold is bilaterally symmetrical about the hinge surface. Symmetrical folds are usually represented by a vertical hinge surface.

8. A fold that is not bilaterally symmetrical about the hinge surface is asymmetrical. Its hinge surface is usually inclined.

(Абрамова Р.Н., Квеско Н.Г., Introduction to Continuum Mechanics- учебное пособие, Томск, ТПУ. 2005.)