Abstract

The Lewis thrust, which has a surface trace of over 300 km, is inferred to terminate within the Rundle thrust sheet at Mount Kidd, 120 km southwest of Calgary. The south face of Mount Kidd is dominated by a spectacular chevron fold pair which has long been considered the manifestation of the surface termination of the Lewis thrust. Previous workers interpreted the Lewis thrust as forming a core high within the anticline of this fold pair. This interpretation dictates that folding takes a minor role in shortening and the solution of “room” problems. However, detailed mapping of the termination has shown that the leading edge of the Lewis thrust remains structurally low beneath the anticline. Within the core of the anticline, complex folding of both competent and incompetent units, with only minor interformational faulting, is seen to be an important mechanism. The major fold pair is observed to die out abruptly to the north within a lateral distance of 3 km. This geometry is unexpected in view of the scale of the folds and is seen to be the direct result of a flanking conical fold (half apical angle = 64°) and the non-coaxial geometry of the major fold pair. The recognition of the flanking structure as noncyclindrical is critical to the structural interpretation. Local decollements aid in room-problem solutions in anticlinal cores because they result in intraformational folding. Based on the fold geometry and the low magnitude of the intragranular twinning strains, it can be inferred that bedding-plane slip was a major factor in the deformation.

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