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E-twinning is the dominant mechanism of plastic deformation of calcite at low temperature (< 300◦ C), and in most limestones, e-twins are, at the crystal scale, the dominant microstructures. Intracrystalline twin lamellae are caused by lattice rotation between rows of atoms along the {01–12}(or {018}) e-plane in response to shear stress. Once a twin is formed, progressive deformation can either create new twins elsewhere or deform the twinned rock portion by other mechanisms. Mechanical twinning accommodates only a very limited amount of the bulk strain accommodated by the rock, so twinning deformation generally co-exists with other deformation mechanisms, such as brittle failure and cataclasis, porosity reduction, pressure solution, or dislocation creep. Recent experimental work has enabled significant progress in the understanding of the initiation and growth of calcite twins and of their controlling factors, as well as of the contribution of twinning to the deformation of calcite grains at various conditions [1–5]. Coevally, inversion techniques that allow for the determination of principal stress or strain orientations and differential stress magnitudes from naturally deformed calcite-bearing rocks have been developed [6–8]. These techniques were widely applied in paleo-tectonic studies, eg, as in [9–13].

Despite significant advances, one pending question about twinning in calcite is related to the existence of a critical resolved shear stress and (if it does exist) to its actual significance. Many paleopiezometric approaches have been built around the concept of a constant critical resolved shear stress for twinning, with a value of~ 10 MPa being …