UNDERSTANDING BENDING IN ROCKS: FOLD ANALYSIS
A geological fold is a bend in rock strata that deflects the surface sideways altering the amount and direction of dip. Fold analysis allows the proper characterisation of the fold.
Several methods exist for obtaining informations from folded surface in deformed rocks. These methods include;
- Wavelength – Amplitude Analysis:
Folds exist over a wide range of scales in nature and their sizes often need to be expressed in an appropriate manner. This method of fold analysis uses mathematical concepts of wavelength and amplitude to properly describe folds. The complexity of this system increases as the fold deviates the simple sinusoidal wave shape. Thus, each fold domain must be analysed as a separate entity having its own amplitude and wavelength.
- To determine the wavelength of individual folds (W)I, measure the distance between the two inflexion lines, defining a fold domain ( note that this will be equal to half the full length of the fold, W/2).
- To determine the Amplitude (A), draw a line parallel to the inflexion line of the fold domain identified in (a) above to touch the extremity of the fold. Measure the distance between these 2 parallel lines, which will represent the amplitude.
It is an important aspect in analysing folds. It gives the degree of tightness of a folded layer and is widely applied in the industry. The Interlimb angle is measured as follows;
- Draw a good outline of the fold and determine the inflexion lines
- Draw tangents through these inflexion lines. Where the tangents intersect, use a protractor to measure the angle. This value will represent the Interlimb angle of the fold domain.
- The fold is classified a gentle (if angle is between 120°-180°), Open (if angle is between 120°-70°), closed (if angle is between 70°-30°), tight (if angle is between 30°- 0°) and Isoclinal if angle is 0°).