Fold-thrust structures: where have all the buckles gone?

The margins to evolving orogenic belts experience near layer parallel contraction that can evolve into fold and thrust belts. Developing cross section scale understanding of these systems necessitates structural interpretation. However, over the past several decades a false distinction has arisen between some forms of so-called fault-related folding and buckle folding. We investigate the origins of this confusion and seek to develop unified approaches for interpreting fold and thrust belts that incorporate deformation arising both from the amplification of buckling instabilities and from localized shear failures (thrust faults). Discussions are illustrated using short case studies from the Bolivian Subandean chain (Incahuasi anticline), the Canadian Cordillera (Livingstone anticlinorium) and Subalpine chains of France and Switzerland. Only fault-bend folding is purely fault-related and other forms, such as fault propagation and detachment folds all involve components of buckling. Better integration of understanding of buckling processes, the geometries and structural evolutions that they generate, may help to understand how deformation is distributed within fold and thrust belts. It may also reduce the current biases engendered by adopting a narrow range of idealized geometries when constructing cross-sections and evaluating structural evolution in these systems.