Abstract

Duplex fault zones are well exposed over a 40 km strike length in the far-travelled (40 km displacement) McConnell thrust sheet southwest of Calgary, Alberta. Two levels of duplex faulting, separated by a shared detachment, have formed. The McConnell Thrust forms the floor of the lower (Little Elbow) duplex. In this duplex, faults ramp across 800 m of carbonate section from glide zones in the Cambrian to a glide zone near the base of the Upper Devonian Palliser Formation, and trailing fault slices are folded over leading fault slices to form a folded, bumpy roof thrust. The exposed geometry is consistent with the ramp anticline model of thrust displacement and duplex formation. The roof of the Little Elbow duplex is also the floor of the overlying Elbow multiduplex, formed within carbonates of the 230 m thick Palliser Formation. The Elbow multiduplex consists of over twenty distinct duplexes comprising imbricate fault slices and separated by subsidiary roof and floor thrusts. The smooth, knife-sharp roof thrust of the multiduplex has folded somewhat independently of the floor, and has truncated, most likely by progressive shearing, underlying duplex structures. The duplex appears to have been enclosed within active roof and floor thrusts along its entire length and to have had simultaneous motion on many link faults. The overall geometry of the Elbow multiduplex changes along strike from a tight antiformal pod to a broadly folded sheet as a result of spacial variations in the stacking, folding, and shortening of component duplexes. Shortening within the Elbow multiduplex increases from near zero at its southeast termination to at least 12 km in the northcentral part of the study area.

Both the Elbow multiduplex and the Little Elbow duplex formed by progressive tectonic erosion in the footwall of the “initial” McConnell Thrust. The “initial” McConnell Thrust, now represented by the roof thrust of the Elbow multiduplex, formed before the multiduplex, and eastward translation continued along it until the end of duplex formation. The Little Elbow duplex postdates the initial formation of the Elbow multiduplex and signals the initiation of two levels of duplex faulting and a stratigraphically lower position of the McConnell Thrust. Simultaneous motion on the “initial” McConnell Thrust, the shared detachment between the two duplexes, and the stratigraphically lower position of the McConnell Thrust allowed the simultaneous deformation of the Little Elbow duplex and Elbow multiduplex to occur in the same vertical section. Motion on the “initial” McConnell Thrust ended when the present McConnell Thrust was established as the dominant thrust. The Elbow multiduplex and the Little Elbow duplex end abruptly to the north west at nearly coincident lateral ramps in the hanging wall of the “initial” McConnell Thrust and the present McConnell Thrust, respectively. Strain caused by an abrupt lateral change in fault displacement partitioning at the northwest end of the duplexes resulted in right-lateral displacement along and contrasting structural geometry across the Fullerton Tear.

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