Kinematic 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.