Structural style and evolution of the triangle zone and external Foothills, southwestern Alberta: Implications for thin-skinned thrust-and-fold belt mechanics

A thin-skinned, NNW-striking, structural triangle zone (tectonic wedge) and steep, imbricate, dominantly forelandvergent thrusts characterize the eastern margin of the Rocky Mountain Foothills belt in southwestern Alberta. Recent 1:50,000 scale mapping indicates that the structure of the triangle zone and outer Foothills varies significantly and systematically along strike. These map-scale variations correlate strongly with lateral changes in the structurally involved Cretaceous-Tertiary foreland stratigraphy and the composition of units structurally inserted into the triangle zone, and correlate less strongly with lateral changes in topography. These variations probably reflect the influence of mechanical stratigraphy. and perhaps the associated role of surface processes (landscape evolution) active during structural evolution. On the basis of structural style, the triangle zone and outer Foothills structures are divisible into three regions between the Oldman River (49degrees45' N) and Turner Valley (50degrees40' N), a distance of approximately 80 km. In the southern region, the upper detachment of the triangle zone is localized within the mechanically weak Bearpaw Formation, and the structure is characterized by a series of map-scale, dominantly hinterland-vergent structures in its hanging wall. In the central region, the Bearpaw Formation remains the locus of the upper detachment, but the structure is characterized by an absence of hanging-wall structures, resulting in a simpler "passive roof duplex" geometry. The transition between the central and southern regions is marked by an abrupt but continuous eastward shift in the trace of the upper detachment, coincident with exposure of a south-plunging antiformal stack in its footwall that reflects the subsurface presence of Mississippian carbonates carried on the blind Outwest Thrust. In the northern region, strain associated with the upper detachment becomes distributed over a stratigraphically broad zone (the Longview Deformation Zone), locally over 2 km in width across strike, centred approximately on the northward-thinning Bearpaw shale, The observed structural variations correspond to changes in the lithology of involved units, indicating the importance of "mechanical stratigraphy". However, there is also a correspondence between the change in structural style from the southern to the central region and the topographic expression of Paleocene units (Porcupine Hills Formation) above the upper detachment. This suggests that patterns of syn-deformational erosion, deposition, and lithification may have influenced structural style. "Out-of-sequence" motion is recorded on some faults, as well as reactivation, with deformation apparently accommodated on a number of faults simultaneously.