Generation of Folds
Most folds in layered rocks can be described geometrically as the products of a finite number of faults. Flow of incompetent rocks can be simulated in two dimensions by modeling normal and thrust fault movement simultaneously (Figure 18). The style of folding depends upon the nature of the rocks involved. In the concentric style of folding (Figure 19a), geometry demands the existence of both lower and upper detachment zones (Dahlstrom, 1969a), each of which is a fault duplex. If these detachments involve incompetent material such as evaporites, folding is accommodated by material flow from synclinal troughs to anticlinal crests in the lower detachment, and from anticlinal crests to synclinal troughs in the upper zone of detachment. Shortening in the folded sequence may match the shortening derived from flow in the detachment zones. However, if the material in the detachment zones is too competent to flow, then the upper and lower detachment zones are filled by thrust duplexes. In this case, the aggregate shortening in the thrust duplexes is much greater than the shortening within the folded sequence.
In Figure 19b, differential horizontal movement (horizontal distance X-X' between the uppermost and lowermost units is several times greater than the shortening of the folded sequence. Each fold in this model resembles the Grease Creek structure (Figure 14) or an inversion of it, and each was built up in the same manner by step thrusting between detachments. For this cross section to be balanced, slip in the upper faulted section may occur in the manner shown by Figure 12.