Seismic reflection data in the Central Alberta Foothills near Edson reveal the presence of small passive-roof duplexes in Upper Cretaceous rocks within the otherwise undeformed foreland basin, as much as 40 km northeast of the mountain front monocline. The tops and bottoms of the duplexes are defined by backthrusts and sole thrusts, which follow bedding planes within Upper Cretaceous strata. Overlying the structures is an essentially uncontracted 1.8-km-thick section of Upper Cretaceous and Tertiary rocks, which is passively uplifted over the thickened duplexes. The underlying autochthonous sequence of Mesozoic and Paleozoic rocks exhibits some minor folding but is also uncontracted.

Our interpretation extends both the upper and lower detachments of the widely accepted triangle-zone model more than 30 km farther under the foreland basin than has previously been supposed. The seismic data illustrate relatively clearly the form of the leading edge of the last phase of Rocky Mountain thrusting. We expect that similar features will be observed elsewhere in the Rocky Mountain foothills and, probably, at other mountain fronts worldwide.