THE ROLE OF STRAIN IN AREA-CONSTANT DETACHMENT FOLDING

The classical constant-area relationship for balancing cross-sections is reformulated to accommodate layer-parallel strain leading to a gene ral equation for the layer-parallel strain in a fold formed above a st ratigraphically 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 mod el are realistic, in the order of 10-30% layer-parallel shortening. Th e 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 W yoming thrust belt, is used to illustrate the applicability of this re formulation. The computed layer-parallel strains in the well-controlle d upper portion of the structure are less than 5% because most of the shortening is accommodated by second-order folds and faults.