On the development of zones of reverse shearing in mylonitic rocks

Most shear zones contain a minor proportion of domains (recognizable from thin-section to outcrop scale) which show an opposing shear sense relative to the overall kinematic framework. In this paper, we discuss how these universal features result from the evolving deformation/strain partitioning that generally accompanies progressive general shear, principally in low metamorphic grade shear zones. Because the rheologic properties and, consequently, the contributions of the different deformation mechanisms change continuously with progressive strain, an individual domain with a predominant deformation mechanism much faster or slower than the average may depart from the general kinematics, enhancing the viscosity contrast between adjacent layers and causing locally reverse shear in the domain boundaries. Care is needed, therefore, in interpreting opposing shear senses in the same shear zone as indicating superimposed deformation events.