Print this pageKinematic Evolution of Canadian Rockies Fault-Propagation Folds
RAIT, GEOFFREY J., and JOHN M. DIXON
Fault-propagation folds grow in association with the displacement gradient above propagating thrust tips. There are two contrasting hypotheses for their kinematic evolution: one holds that the anticline forms as a pair of kink bands and grows in amplitude by kink-hinge migration but does not tighten as the fault propagates; the other holds that the anticline forms as a fixed-hinge buckle that tightens as deformation progresses. Determining which is correct in a region can aid when attempting to predict the subsurface geometries of these important reservoir structures.
Folds formed by each process would have contrasting infinitesimal strain histories, which would likely be reflected in their minor structure assemblages. Also, presuming they grew laterally as they propagated forward, there would be contrasts in how their geometries change along strike. We analyzed minor structure assemblages and along-strike geometric changes in four Canadian Front Range folds having 'model' fault-propagation fold geometries to determine by which of the two mechanisms they evolved.
Early structures indicating hinge-normal shortening along bedding are cut by structures indicating flexural flow, which in turn are cut by structures indicating hinge-normal extension. We interpret this sequence as resulting from progressive limb rotation during transport-parallel compression and shortening and subvertical extension. One fold, close to a thrust termination, still had a relatively large amplitude at the point where the thrust died out, suggesting it preceded rather than accompanied thrust propagation. Our results provide evidence that these folds grew by buckling and limb rotation rather than kink-hinge migration.