Sense and non-sense of shear reloaded

Abstract

This paper reviews the geometrical and mechanical evolution of selected groups of kinematic indicators that are frequently used for inferring large-scale geodynamics but are prone to ambiguous shear sense interpretations: (i) Flanking folds and shear bands are complex structures because under general shear conditions co-rotating secondary shear zones can experience either syn- or antithetic slip leading to extensional or contractional offsets. On the other hand, counter-rotating secondary shear zones can exhibit either extensional or contractional offsets but always experience synthetic slip. (ii) Boudinaged layers may display both syn- and antithetic offset across the interboudin surfaces. In addition to the flow type, the initial orientation of interboudin surfaces strongly influences the sense of slip. Under coaxial flow conditions and with high-strength contrast between the boudinaged layer and the matrix, domains of layer-oblique parallel interboudin surfaces can form by chance and their local occurrence may hence lead to misinterpretations of shear zone kinematics, in particular if the number of observable boudins is small. If the viscosity contrast between the boudinaged layer and the matrix is low, domino boudins may strongly deform internally during shearing so that, in contrast to flanking structures, antithetic slip along interboudin surface may lead to a marked normal drag, resulting in shear band boudinage geometries that appear to have formed during flow with an opposite shear sense. (iii) Winged inclusions are pinch-and-swell shaped objects rotating into the shear direction. After several revolutions, these structures can resemble σb-type clasts or clasts with stair stepping that formed during shear flow with the opposite shear sense. In this paper, new and published examples of these ambiguous shear sense indicators are discussed and interpreted. We give some guidelines for the correct interpretation of these structures with the caveat that if marker lines and their deflections are not well-preserved these shear sense criteria are not always reliable kinematic indicators.