Detachment anticlines formed above a stratigraphically fixed detachment
horizon and without thinning in the syncline will, in general, require
layer-parallel shortening. A kinematic model for detachment folds is
presented that has planar limbs and is area-balanced but requires
bed-length changes that can be expressed as layer-parallel strain. The
fold grows primarily by limb rotation, not by limb lengthening at
constant dip. The kinematic evolution of the geometry and strain is
examined. Fold geometry is well predicted and the strain estimated in
applications of the model to two map-scale detachment folds; a triangle
zone from the front of the Wyoming thrust belt and a duplex-cored
anticline from the Appalachian Plateau.