The Nature and significance of the Barrier Peak Thrust Stack, Rocky Mountain Front Ranges, Alberta, Canada

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Barrier Peak thrust stack is located west of Calgary at the very front of the Rocky Mountains, which are carried at this latitude by the McConnell Thrust fault (MCT) onto the Cretaceous molasse of the Alberta Foothills. The stack is part of the leading edge of the MCT. It represents a westerly dipping imbricate fan repeating formations which are Cambrian to Mississippian in age. In map view, the MCT decollement on the east side of the stack has a nearly linear trace and consists of an assemblage of fault surfaces. The map traces of the roof thrust (Barrier Thrust) and of the Bryant Thrust are warped around the west side of the stack. The map area is subdivided into three zones, from east to west: -the duplex zone, carried by the main sole thrust (McConnell Thrust); -the Barrier Thrust sheet, which overlies the duplex zone; and -the Bryant Thrust sheet that, at the present level of erosion, wraps the Barrier Thrust sheet and its lower duplex. The Bryant thrust sheet is the transition zone between the complicated structure of the study area and the regionally parallel thrust sheets of the Front Ranges further west. The stack is complicated by hinterland dipping duplexes and horses. Horses of Upper Paleozoic and Mesozoic rocks are accreted to the hanging wall of Barrier Thrust in the roof of the duplex. This thrust is a late splay of the MCT that itself picked up horses of various rock ages during its early motion. The horses were either lost from the hanging wall of the MCT or dragged up after detachment from the footwall during the motion of the MCT. The development of horses was followed by the splaying of the Barrier Thrust from the MCT. Duplexing and thrusting of preexisting folds and thrusts also characterize a thrusting process that is out-of-sequence. Along strike, Barrier Thrust displays a succession of hangingwall diverging splays, producing a stacking of the transfer zones. Therefore, tip lines and branch lines in the hangingwall of the Barrier Thrust represent overlapping and overlapped lateral ramps, respectively. Analysis of the microscopic and mesoscopic structures in specific lithostructural units permitted a differentiation between a lower structural level where pressure solution generates the cleavage and the thrust rocks, and an upper structural level where spaced fracture cleavage and brecciated thrust rocks are developed. The interpretation of seismic reflection profiles indicates that the MCT is a gently southwest-dipping plane beneath the study area. Consequently, the footwall structures are not responsible for the structural complications of the study area. Instead, the origin of the thrust stack is interpreted to be related to the initial geometry of the MCT. For more than 150km of its trace north of the study area, the MCT carries Cambrian limestones. South of the study area, the MCT carries the Devonian Palliser Formation in its immediate hanging wall for 40km along its trace. A lateral ramp must have originally linked the two flats, and a complex thrust stack developed as the thrust propagated and the thrust sheet moved. Horses in the duplex xone represent remnants of the frontal ramp of the MCT. The frontal and lateral propagation of the MCT through different structural levels is interpreted to have had a significant role in originating the complicated thrust stack of the study area.
Bibliography: p. 240-251.
Maurel, L. E. (1987). The Nature and significance of the Barrier Peak Thrust Stack, Rocky Mountain Front Ranges, Alberta, Canada (Doctoral thesis, University of Calgary, Calgary, Canada). Retrieved from doi:10.11575/PRISM/15393