Abstract

A northeast-facing panel of Middle Proterozoic (Purcell Supergroup) strata occurs beneath Cambrian and Devonian strata along the east side of the Rocky Mountain Trench in the Mount Fisher area. Anomalous northeast-trending folds, faults, and cleavage that formed during Cretaceous–Paleocene deformation occur in this panel in the northern part of the area. The dominant structure in the southern part is the northwest-trending Lizard segment of the (Mesozoic) Hosmer nappe, which folds an older north-trending cleavage that probably formed during the East Kootenay Orogeny (1300–1350 Ma). Thickness and facies variations in Purcell strata and changes in the level of erosion beneath the sub-Devonian unconformity imply that many of the important structural boundaries in the Mount Fisher area and also the normal faults along the southern Rocky Mountain Trench follow the locus of older structures that were active in the Middle Proterozoic and the early Paleozoic.The anomalous northeast-trending structures in the Mount Fisher and adjacent areas formed because the underlying Hosmer thrust developed across a major, pre-Devonian, northwest-facing drape fold, the Dibble Creek monocline. Ramps connecting bedding-glide zones were deflected across the monocline, and strata were gravitationally compressed to form northeast-trending folds, faults, and cleavage as they were displaced up the monocline along the Moyie – Dibble Creek fault.

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