BUCKLING AND LOCALIZED SLIP IN FOLD-THRUST STRUCTURES: EXPANDING THE SOLUTION SET

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, the range of structural geometries used as reference models to assist in interpretation has been greatly narrowed. As a consequence, risks of mis-interpreting structural geometry, especially in the subsurface, have been under-estimated, leading to repeated drilling surprises in exploration. Community-wide focusing on the idealized models constitutes a form of confirmation bias, a limitation that is further restricted by selective study of outcropping systems. Specifically, a false distinction has arisen between some forms of so-called “fault-related folding” and buckle folding. Many studies of fold-thrust belts consider folding to be a consequence of thrust geometry and faulting processes, implying that layer buckling is a rare process in fold-and-thrust belts. However, a spectrum of folding and faulting styles co-exist in stratigraphic multilayers. Therefore, we 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). Only fault-bend folding is purely “fault-related” and other forms, such as fault-propagation and detachment folds all involve components of layer 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. Discussions are illustrated using case studies from the Bolivian Subandean chain (Incahuasi anticline), the Canadian Cordillera (Livingstone anticlinorium) and Subalpine chains of France and Switzerland.