Accommodation structures during kink band evolution; quantitative methods applied to Late Variscan deformation of Portugal

摘要

Kink bands are common in rocks with a strong planar mechanical anisotropy, being generally developed in polydeformed areas during the late stages of deformation, in brittle to brittle-ductile conditions. In the Late Variscan deformation events in the Iberian Massif, several kink band structures were developed in association to the NNE-SSW sinistral strike-slip faults, which are induced by the Laurentia and Gondwana E-W dextral collision on Carboniferous times. Within two metric-scale kink bands, several 2nd order structures are generated in response to internal shortening during layering rotation. The present paper describes the internal geometry and kinematics of these type III fixed hinges kinks bands, discussing their deformation mechanisms. In order to quantify the internal shortening within kink bands, a new graphical method, using simple angular parameters, is proposed. This graphical approach applied to the studied kink bands allows their internal shortening to be quantified; it ranges between 12 and 18. The genesis of 2nd order structures within kink bands results from the accommodation of the internal layer distortion during kink bands progressive deformation and is controlled by two main deformation processes: layer parallel shortening and layer parallel slip.