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In the previous models, the deeper parts of thrust belts were discussed and the shallower section removed by erosion was ignored. Do such thrusts reach the surface and become erosion thrusts? Do they steepen upward and die out in some shallower part of the section, as indicated by Boyer and Elliott (1982)? If so, how is their slip accommodated? There is good evidence that thrust faults do not reach the erosional surface, but are blind, merging with a preexisting, overlying bedding-plane thrust or upper detachment zone forming a duplex structure (Thompson, 1981; Jones, 1982, 1984, 1985; Charlesworth and Gagnon, 1985).

Figure 6 shows how a fault duplex structure develops by emplacement of a deeper thrust beneath an existing bedding-plane thrust. In this duplex, displacement is progressively transferred from the lower to the upper detachment, which folds in response to the intervening faulting. The presence of an upper detachment means that each thrust fault beneath it generates a ramp anticline where it flattens along the detachment surface. In this way, folds are generated above blind thrusts and may also be generated above blind normal faults (Gibbs, 1984; Jones, 1985).

Dahlstrom (1970) uses the terms roof thrust and floor thrust for the upper and lower detachments, respectively. However, the term detachment is preferable because detachments are not necessarily discrete faults, but may be zones of complex deformation. Also, upper and lower detachments have been described in a similar context for fold duplex structures (Dahlstrom, 1969a).

Boyer and Elliott (1982) state that the roof thrust of a duplex is initiated as a major thrust.

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