Thrusting and Folding Time and Sequence
Minor thrusts rooted in a major thrust are sometimes interpreted to be imbrications within the hanging wall of an existing overthrust sheet, and are believed to be emplaced in a lower-to-higher sequence (Figure 25a) (Douglas, 1950; Dahlstrom, 1970). Imbrications in the footwall of a major thrust (Figure 25b), however, are believed to be emplaced in a higher-to-lower sequence. Although intuitively it appears that the hanging wall imbrications formed after emplacement of the basal thrust, there is no geometric basis for this assumption. Figure 26 shows two computer-generated models with the same hanging wall and footwall imbrications as in Figure 25. Both models in Figure 26 were generated in a left-to-right sequence by progressive faulting of an undisturbed footwall (Figure 26c). The only difference between them is that the sequence for hanging wall imbrications (Fig. 26a) was made by three minor faults followed by a major one, and the footwall imbrications sequence (Fig. 26b) by one major fault followed by three minor ones.
If hanging wall imbrications formed in a sequence different from that for footwall imbrications, the normal transition along strike from major into minor faults and vice versa (i.e., displacement transfer) would be impossible. If both sets of imbrications are emplaced in the same sequence, faults in Figure 25a can pass along strike into those in Figure 25b through displacement transfer. The term imbrication is itself misleading, since it implies the break up of an originally unbroken thrust sheet following its emplacement. The relationships shown in Figure 25 can be modeled by a left-to-right (higher-to-lower) or a right-toleft (lower-to-higher) sequence of thrust emplacement.
Figures & Tables
Exploration for petroleum requires much data, and the principles with which to interpret them. Because well and seismic data are extremely expensive in fold and thrust belts, application of simply geometric principles can multiply the effectiveness of geological and geophysical interpretations. Geometry of layerd rocks, expressed by maps, cross sections, and seismic profiles, is the basis for interpreting the geological structure. This publication contains 7 chapters covering thrust faults, generation of folds, and thrust and fold time and sequence from the geometric perspective.