Large-scale duplex structures in the McConnell thrust sheet, Rocky Mountains, Southwest Alberta

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 northwest 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 contrast...

Deep transverse basement structural control of mineral systems in the southeastern Canadian Cordillera1Geological Survey of Canada Contribution 20110294.

“…Palinspastic restoration of the northeastern part of the northern structural corridor is well constrained; estimated shortening is based on balanced cross sections constructed using petroleum industry seismic and well data (McMechan 1998;Fermor 1999). The northeasterly transport direction for the thrust slices is well defined by tear faults, microstructural studies and the matching of surface and subsurface lateral ramps (Fermor 1999;McMechan 2001;Norris 2001). Palinspastic restoration of the points where transverse structures intersect the leading edge of the McConnell and Misty thrust sheets puts the original location of these structures di-rectly above the Red Deer basement zone as outlined by aeromagnetic anomalies.…”

“…oblique to the regional transport direction of the thrust and fold belt, as determined from seismic data, tear faults, lateral ramps, and microstructural studies (Price 1981;Fermor 1999;McMechan 2001;Norris 2001).…”

“…LL, Larch Lake structural zone. (c) Ediacaran (sub-Ediacaran or Cambrian unconformity) and lower Paleozoic (Ediacaranlower Paleozoic) assemblage: active structures based on Foo (1979), McMechan and Price (1982), Mott (1989), Pope (1990), Aitken (1993), Cecile and Norford (1993), Höy (1993), Reesor (1996), andMcMechan (2001). KHR, Kickinghorse Rim.…”

U-Pb geochronology reveals that hydrothermal dolomitization was coeval to the deposition of the Burgess Shale lagerstätte

“…(iii) Cambrian to Triassic strata were deposited on what has historically been considered a passive margin [55][56][57][58][59] , but there is local evidence that episodic basement reactivation gave rise to elevated heat flow throughout the Cambrian [15][16][17][18][19][20][21][23][24][25][26][27]60 . (iv) Jurassic to Paleocene strata were deposited in a foreland basin that developed during the Columbian (Jurassic to Early Cretaceous) and Laramide (Late Cretaceous to Paleocene) orogenies [61][62][63] .…”

Basin scale evolution of zebra textures in fault‐controlled, hydrothermal dolomite bodies: Insights from the Western Canadian Sedimentary Basin

“…The modern structure of the southern Rocky Mountains is dominated by E‐NE verging thrust faults that formed during the Antler, Columbian and Laramide orogenies (McMechan, 2022; Price, 1981; Root, 2001). However, there are several pre‐orogenic structural elements in the southern Rocky Mountains, including NE–SW trending normal and transtensional faults that intersect Middle Cambrian strata (Davies & Smith, 2006; Stacey et al, 2021).…”