Recumbent folds: Key structural elements in orogenic belts

This review has two main parts. The first of them presents existing ideas and data related to recumbent folds, reviewing aspects such as the physical conditions of the development of these folds, the strain inside the folded layers, the kinematic mechanisms of their formation, the role of gravitational forces, the tectonic context of their development and the structures associated with them. In the second part, the above ideas are discussed and possible mechanisms for the development of these folds are presented. It is proposed that initial perturbations of the layers are essential to give rise to the asymmetry of recumbent folds. These perturbations may be nonplanarities of the layering or may be linked to the existence of a core or basement of competent rock that hinders the normal propagation of the deformation. This could explain why many large recumbent folds have a root zone. Deformation with an important component of simple shear is a general condition for the formation of recumbent folds. In areas with very low grade metamorphism, competent layers often play an active role during the deformation and undergo buckling with the development of an overturned fold limb, which can be stretched and thinned to finally produce a pair of recumbent folds separated by a thrust. In areas with low or medium metamorphism, buckling under a simple shear regime is probably the most important mechanism for producing large folds with gentle or moderately dipping axial surfaces; subsequent kinematic amplification by coaxial strain components with vertical maximum shortening is important for the formation of recumbent folds. These components involve a sub-horizontal stretching that can cause a problem of strain compatibility and give rise to a basal thrust. In areas deformed under high P and T conditions, recumbent folds can develop by flow perturbations and kinematic amplification of folds; this is probably a common mechanism in ductile shear zones.