Deformation styles near the leading edge of the fold-and-thrust belt, Pincher Creek, Alberta

Abstract

A 150 km2 area of the Foothills Belt near Pincher Creek was mapped at 1 :20,000 scale. A series of structures previously mapped as vertical transverse faults are reinterpreted as a duplex that dips obliquely to regional trends. Two models are proposed for its structural evolution. Along the leading edge of the Livingstone Thrust sheet, a tear fault occurs along strike of a duplex; it is interpreted to be a steep lateral ramp from which displacement is transferred through the duplex. Another duplex is documented in the Wapiabi Formation. Local structure is affected by a deep southeast-plunging synform that developed east of a Palaeozoic duplex that dies out along strike. The Triangle Zone is less deformed than further north. Tertiary stratigraphy is thrust repeated outboard of the Triangle Zone. A duplex imaged on seismic sections occurs 16 km northeast of the Triangle Zone. Effects of shortening are observed at surface 65 km outboard of the Triangle Zone, along the Oldman River near Lethbridge. Three regional, restorable cross-sections are presented. Displacement occurs along low angle ramps and long flats, contributing to an overall shortening of 65%.