Deciphering the presence of axial-planar veins in tectonites

Veins and dikes are often oriented subparallel to the axial surfaces of folds in the adjacent layered or foliated rocks. This implies an awkward situation, since veins would lay in planes close-to-parallel to the maximum stretching axis. A series of geometric models have been conceived in order to gain insight into the possible mechanisms for their formation. The models are based on the analysis of a varied selection of field structures and on the review of similar structures in the existing literature. A first categorization consists on distinguishing between axial-planar veins achieved by either progressive or polyphase deformation. Five models of axial-planar veins resulting from progressive deformation are described and discussed:(1) fold-related veins associated with the standard folding mechanisms, (2) fracture arrays localized along the short limbs of folds (asymmetric fold-related veins), (3) folds associated with rotation of extension veins (vein-related folds), (4) high strain and transposition of early veins, and (5) high strain and late veins parallel to axial planar foliations (axial planar foliation-related veins). The axial planar geometry is achieved through variable amounts of progressive rotational strain, except in model 5, in which the co-planarity is acquired at the time of vein intrusion. The possibility for axial-planar veins to have developed in two distinct phases in the context of polyphase or polyorogenic tectonics has also been explored and discussed. This study contributes to a better understanding of the intriguing interplays between deformation, metamorphic and magmatic processes in orogenic belts.