Structural Geology of Stewart Peak Culmination, Idaho-Wyoming Thrust Belt

Abstract

A structural culmination is the highest point along a structural axis or fold system, away from which the folds plunge. Structural culminations are an important and predictable component of foreland fold-thrust belts, and are prime targets for oil and gas accumulations. The Stewart Peak culmination is a doubly plunging anticlinorium in the Absaroka thrust sheet of western Wyoming, located in the northern Salt River Range. Major thrust faults in the Stewart Peak culmination comprise a large-scale duplex zone. The roof fault of the duplex is the Absaroka thrust, the floor fault is the basal or regional decollement, and the intervening Murphy and Firetrail thrusts are trailing-edge imbricate faults in the Darby plate that have locally uplifted and deformed the Absaroka into an asymmetric, breached anticlinorium or culmination. In addition, these major thrust faults subdivide the culmination into fault-bound structural horses, each with a characteristic style of deformation as a function of its stratigraphic and structural level in the duplex. A recent exploratory test of the culmination provided data which suggest that the Absaroka thrust locally cuts downsection in the transport direction. This may be explained by initial folding and breaching of the Firetrail anticline, followed by main motion on the Absaroka thrust which overrode the fold. The available data suggest that the Stewart Peak culmination is the result of (1) polyphase uplift or arching of the Absaroka thrust sheet by the Murphy and Firetrail thrust sheets, (2) lateral imbrication and ramping, and (3) arching of the Precambrian basement complex beneath the northern Salt River Range. In addition, a basic kinematic model of deformation applicable to structural culminations includes emplacement of an allochthonous mass over a parautochthonous substratum, closely followed by, or contemporaneous with, deformation and translation of the footwall section to produce a duplex zone that folds the overlying allochthon. The Stewart Peak culmination bears many similarities in structural style and origin to polyphase folded-fault domes in the Canadian Rocky Mountains. Specifically, the Stewart Peak area is compared to the Savanna Creek and Moose Mountain culminations.