--> Fold-Thrust Interactions in the Canadian Rocky Mountains Revisited — A New Kinematic Model and its Implications for Other Shallow Fold-Thrust Belts

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Fold-Thrust Interactions in the Canadian Rocky Mountains Revisited — A New Kinematic Model and its Implications for Other Shallow Fold-Thrust Belts

Abstract

We show that the ramp-flat models are not the general case in the Rocky Mountains and present an alternative. Most folds in the Rocky Mountains are interpreted as a product of thrusting. Three fold-thrust scenarios are recognized which, in some circumstances, represent subsequent stages in the development of ramps and flats: detachment folds, fault-propagation folds, and fault-bend folds. Another, apparently rarer fold-thrust interaction, the break-thrust fold, comprises an antiform-synform pair with a common limb that is thrusted. Most of the ramp-flat folds are interpreted in seismic profiles but they are rarely actually observed. The subsurface interpretations may be biased by model-driven seismic processing owing to poor footwall imaging, possibly enhanced by unsuited acquisition parameters designed for undeformed Plains strata. We base our interpretation on structures that are either well exposed or, if subsurface data are good, seismic images controlled by wells. The result is a more realistic interpretation of antiform-synform pairs. One of our examples from the Front Ranges, the exposed Mt. Allan syncline in the footwall of the Rundle thrust, gives evidence based on small structures such as cleavage and parasitic folds that folding predated thrusting. This is inconsistent with a ramp-flat model. In another example, the geological subsurface model of the Brazeau thrust zone in the Foothills needs to be revised from a fault-propagation fold to a thrusted anticline-syncline pair. We propose a kinematic interpretation consistent with the anticline-syncline geometry as well as with the general deformational environment. The model is applicable to any fold-thrust belt. A similar kinematic picture has been observed in centrifuge experiments designed to represent a Rockies-type environment. The consequence of the new model for hydrocarbon exploration lies in the footwall geometry: layering there is not automatically flat-lying and undeformed, but dips at various angles and is likely to be overturned. In the future, improved seismic techniques may reveal a higher degree of large-scale folding in the Rocky Mountains Foothills than previously believed.