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Abstract

The profile in Figure 3b is typical of many thrust belts. The surface geology features an irregular spacing and local concentration of faults, which a geologist might interpret as evidence for an underlying zone of weakness or basement buttress. However, the irregular thrust spacing in this model is due entirely to varied slipping of a uniformly spaced set of incipient thrusts (3a). While basement warping and faulting may have a significant influence in localized thrust ramping (Wiltscho and Eastman, 1983), basement warping and faulting may not be applicable to this style of thrusting, which involves large numbers of faults having no immediate connection with basement. In areas such as the southern Alberta foothills, where this structural style predominates (Ollerenshaw, 1978), it seems more likely that thrust ramping in the sedimentary cover is ultimately controlled by the distribution of overburden in the hanging wall sequence (Royse et al., 1975). This overburden may govern the distribution of high fluid pressure at the base of the overthrust mass (Gretener, 1972). Controlling ramp location by differential overburden loading is a feature of thrust emplacement at the bases of deltas and submarine fans (Evamy et al., 1978). Differential loading in those environments is caused by sedimentation (Mandl and Crans, 1981), and is unrelated to the underlying section. Although a more competent section may be involved in thrust and fold belt deformation, differential loading because of faulting is potentially much greater.

Closely spaced incipient thrusts interact with each other. Figure 4 is a set of cross sections parallel to each other along the strike of a thrust belt.

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