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.