In thrust belts where faults are widely spaced, basement influence may be an important factor in determining the ramp locations (Wiltscho … Eastman, 1983), but it does not seem appropriate in deformed belts where there are very large numbers of closely spaced thrust faults, having no local connection to basement. Variations in stratigraphy can also account for ramping. As a glide horizon pinches out or changes facies, the basal slippage plane is transferred to a higher incompetent unit (Gretener, 1972; Harris and Milici, 1977). Only one ramp can be formed by each transfer, leaving the majority of ramps unaccounted for. Thrust sheets frequently occur in stacks, which require that each thrust carry its predecessor, piggy-back, by the the precise amount required for stacking (e.g., Figure 16). It is unlikely that this requirement is fulfilled only by chance. It appears more probable that the starting and stopping position of each thrust sheet is governed by a combination of overburden thicknesses, fluid pressures, and other internal characteristics of the overthrust mass, rather than by underlying influences.
Wiltschko and Eastman (1983) state without elaboration that thrusts propagate downward. A form of downward propagation in stages is demonstrated by the unbalanced cross section model of the Absaroka thrust (Figure 11), which moved first as a bedding plane detachment within the Mesozoic section, and secondly as a bedding plane detachment in the Paleozoic section which climbed up-section to join the earlier thrust, a process involving interstratal slip on a regional scale. This suggests a hypothetical model of thrust emplacement (Figure 32) in which thrust faults propagate by developing minor interstratal slip and generating duplex structures at progressively deeper detachment surfaces, until a lithology and fluid pressure environment is reached that allows large amounts of movement along the basal thrust plane.