ABSTRACT: Styles of Deformation in the Rocky Mountain Foothills and Front Ranges, Alberta, Canada

LANGENBERG, WILLEM, Alberta Research Council, Edmonton, Alberta, Canada

Two contrasting styles of deformation and related fold-thrust interaction can be distinguished in the thin-skinned fold and thrust belt forming the Alberta Foothills and Front Ranges. Paleozoic

carbonates form thick, competent beams that are cut by east-verging thrust faults with staircase trajectories (ramps and flats). Shortening of the strata may be as great as 50%. Decollement horizons in overlying incompetent units (such as the Fernie shales) can form roof thrusts of duplexes in the carbonates. Strata overlying the decollement horizons show fault-bend folds. This geometry is generally emphasized in the description of the structure of the Canadian Rocky Mountains.

In contrast, the overlying Mesozoic and Cenozoic strata (consisting of flysch and molasse type sediments) show a different type of fold-thrust interaction. These rocks are generally interlayered successions of sandstones and shales, which predominantly show detachment folds, as well as minor fault-bend folds above ramps in the carbonates. Folds are generally cylindrical and of the chevron variety. At their tapering ends, the folds are conical. Although the geometries of the folds fit the fault-propagation fold model, the general absence of thrust faults terminating in the cores of anticlines indicates that the majority of folds are detachment folds. The prominent process appears to be buckling (flexural-slip). A few folds near thrust-fault tips are typical fault-propagation folds.

These two contrasting styles of deformation can be reconciled by the notion that thrust-ramps in competent units (such as the Paleozoic carbonates) are localized by a buckling instability in the layered succession in an early stage of buckle-folding. Thrust faults in the overlying Mesozoic and Cenozoic units form in a later stage of the buckle-folding process, whereby these faults ramp through the forelimbs of tight folds (fault-propagation folds). Consequently, the prominent process in these layered strata is buckling, whereby thrust faults initiate at buckling instabilities. Buckle folds can be modified by fault-bend folding at a later stage, after thrust faults have formed. This process has been observed in plasticine and silicone putty analog scale models deformed in a centrifuge nd is an expression of the folding-first theory in the study of fold and thrust belts.

AAPG Search and Discovery Article #91012©1992 AAPG Annual Meeting, Calgary, Alberta, Canada, June 22-25, 1992 (2009)