Structural pattern and shortening quantification within fixed hinge kink bands; examples from the Late Variscan deformation of Iberia

Abstract

Kink bands are usually related to the late deformation stages in polydeformed areas. In Portugal, the Late Variscan deformation generates in Almograve and Abrantes regions (SW Iberian Massif), brittle to brittle-ductile NNE-SSW sinistral kink bands, which deflects the NNW-SSE regional trend. This deflection induces the counterclockwise rotation of primary layering/foliation in the kink band inner domains, generating a complex and heterogeneous structural pattern. In Almograve, within metric to decametric kink bands, a wide spectrum of structures (mainly folds and brittle to brittle-ductile shear zones) are developed, while in the internal domains of the Abrantes metric kink band, a high strain pattern composed by 2nd order conjugate kink bands were generated. Previous structures allows to calculate internal and external stress fields, showing a variation from the outer to inner domains of kink band. During the development of the type III fixed hinge kink bands, the formation of the internal structures is mostly controlled by layer parallel shortening and layer parallel slip. The inevitable area distortion in the plane orthogonal to the kink band boundaries could be easily estimated using a new graphic method.