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Displacement of the Lewis thrust sheet in southwestern Canada: New evidence from seismic reflection data

¹ Department of Geology and Geophysics, University of Calgary, Calgary, Alberta T2N 1N4, Canada

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.