The classical constant-area relationship for balancing cross-sections is
reformulated to accommodate layer-parallel strain leading to a general
equation for the layer-parallel strain in a fold formed above a
stratigraphically fixed detachment horizon. The strain required for area
balance can be calculated from a knowledge of the fold shape and the
depth to detachment. The strains predicted from a simple kinematic model
are realistic, in the order of 10-30% layer-parallel shortening. The
strain may be penetrative or occur as small-scale disharmonic folds and
faults. The Tip Top anticline, a fold at the leading edge of the Wyoming
thrust belt, is used to illustrate the applicability of this
reformulation. The computed layer-parallel strains in the
well-controlled upper portion of the structure are less than 5% because
most of the shortening is accommodated by second-order folds and faults.