Seismic reflection data across the Rocky Mountain trench in the southern Canadian Cordillera reveal prominent reflections from Mesoproterozoic (Aldridge Formation) sills that allow a direct stratigraphic correlation across the trench when 10 km of extension is restored. Trun cated reflections at ∼11-15 km depth on the west side of the trench are interpreted as a footwall cutoff of the Lewis thrust in Belt-Purcell strata that lies ∼115 km west of the leading edge of the Lewis thrust, exposed in the Waterton area; retrodeformation of the Lewis sheet requires a minimum of 75 km of transport on the Lewis thrust and 40 km of transport by the development of footwall-domain duplexes. The trench is located above a major change in slope of the top of the North American basement that coincides with a pronounced increase in thickness of lower Belt-Purcell strata in their retrodeformed position. This transition may represent part of an ancient margin that was initially established in Paleoproterozoic time.

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