Structural geometry and kinematic evolution of the central Canadian Rocky Mountain Foothills fold-and-thrust belt: Complex kinematic relationships controlled by detachment utilization

Balanced regional cross sections based on surface, seismic, and subsurface data show that the thin-skinned fold-and-thrust belt in the Rocky Mountain Foothills of the Kakwa area of the central Canadian Rockies consists of a lower buried thrust belt developed in Paleozoic and Triassic strata and an upper exposed faulted fold belt of Jurassic to Cretaceous strata. Changes in fold wavelength, amplitude, and geometry with stratigraphic level indicate that multiple detachments were utilized in the upper faulted fold belt. Exposed folds are chevron or box shaped. Most appear to be detachment or fault propagation folds formed by fault-to-fold displacement transfer. Geometric and kinematic relationships in the upper faulted fold belt vary from thrust faults congruently folded by underlying folds (early fault) to folds abruptly truncated by thrust faults (late fault). In contrast, folding of thrust sheets in the buried thrust belt is consistent with in-sequence deformation for all faults except one. A sequential restoration of the balanced regional cross section shows that the variable kinematic relationships observed in the upper faulted fold belt can be explained by changes in the detachment level utilized by successive faults as they climbed out of the buried thrust belt. Chevron-folded thrust faults indicate a younger fault with associated fault displacement transfer folds formed in the footwall of an older fault. These folded thrusts formed by in-sequence faulting and utilization of successively higher detachment levels. Late faults that truncate preexisting folds required out-of-sequence (hinterlandward) utilization of a higher detachment level, and they illustrate another mechanism by which critical taper is maintained in a fold-and-thrust belt.