Structural Evolution of Deformed Belts by Blind Thrusting
The foreland margin of the southern Canadian Cordillera is formed by blind thrusts merging with an upper detachment (Gordy and Frey, 1975; Jones, 1982). In the normal higher-to-lower sequence of thrusting (Elliott, 1976), each thrust, at the time of its emplacement, cuts undisturbed foot wall at the leading edge of the thrust belt. There is no obvious reason why intermediate leading edges of a deformed belt should behave any differently from the final one, which suggests that an entire thrust and fold belt can be formed by this mechanism (Jones, 1982; 1984a; CharlesworthandGagnon, 1985). In contrast, Butler (1985) proposes that blind thrusting at a foreland margin marks the "last gasp" of tectonic activity, and Morley (1986) relates underthrusting to the rate of decrease of stress at the end of the period of deformation.
Figure 17 shows that a large part of the southern Canadian Cordillera, comprising both the foothills and eastern Rockies, can be modeled with blind thrusts beneath a continuous upper detachment at all stages. Strong evidence for this model lies in the fact that the Belly River Formation, which underlies the upper detachment zone at the outer edge of the foothills, occurs in the footwalls of almost all thrusts exposed in that part of the foothills as well as in the footwalls of the McConnell and Lewis thrusts of the eastern Rockies, regardless of which formations comprise the hanging walls. This consistency shows that successive thrusts emplaced at the outer edge of the foothills flattened along a common upper detachment.