Classification of multilayered folds based on harmonic analysis: example from central India

The harmony of multilayered folds can be determined objectively by comparing the b3/b1 (ratio of third and first coefficient of harmonic components of the Fourier series) for different surfaces in an individual fold. A plot of the b3/b1 value against surface number provides a simple graphical technique for assessing fold harmony. On such a diagram, a straight line parallel to abscissa denotes a strictly harmonic fold, while other lines and curves denote variation of fold shape from surface to surface. A new scheme of multilayered fold classification based on ‘Index of Non-Harmony (INH)’, which reveals the degree of variation in shape of different surfaces of the fold, is proposed. INH is obtained from standard deviation (σn) of b3/b1 ratios of ‘n’ number of surfaces of a quarter wave sector. On the basis of values of INH ( = 1000 · σn) the multilayered folds are classified as ‘Strictly Harmonic’ (INH = 0), ‘Periharmonic’ (0 < INH ≤ 15), ‘Subharmonic’ (15 < INH ≤ 30), ‘Subnonharmonic’ (30 < INH ≤ 45), ‘Nonharmonic’ (45 < INH ≤ 75) and ‘Strongly Nonharmonic’ (INH > 75). In a case study of polydeformed Precambrian rocks of central India, it is found that the index of nonharmony decreases in folds of later (younger) generations.