The Dogtooth Duplex, a model for the structural development of the northern Purcell Mountains

Abstract

Structural analysis based on recent detailed mapping of part of the Dogtooth Range of the northern Purcell Mountains confirms and refines a previous interpretation of a duplex structure, which, after its formation, was carried passively in the hanging wall of the out-of-sequence Purcell Thrust. Several parameters, such as thrust spacing, thrust displacement, and fold shape, place geometric constraints on the Dogtooth Duplex. Thrust spacing/displacement ratios indicate that the older structures of the western Dogtooth Range can be approximated by an antiformal stack model, whereas the younger structures of the eastern Dogtooth Range fit a hinterland-dipping duplex. The sole thrust of the Dogtooth Duplex is inferred to follow incompetent rocks in or below the basal part of the Hadrynian Windermere Supergroup. The roof thrust is inferred to follow shaley rocks, probably in the Middle Cambrian Chancellor Formation. From balanced, restored cross sections, total shortening within the duplex is estimated at 18–23 km or 50% of the initial width of the deformed strata. This shortening is interpreted to have been transferred during an early stage of deformation from the sole thrust of the Dogtooth Duplex to an upper detachment in Middle Cambrian strata of the Porcupine Creek Anticlinorium in the western Rocky Mountains. While subsequent shortening was partly accommodated within the Porcupine Creek Anticlinorium and partly transferred to the structures of the Main and Front ranges, the Dogtooth Duplex was carried passively on top of a "parautochthonous" wedge.