The role of recrystallisation on the deformation behaviour of calcite rocks: large strain torsion experiments on Carrara marble

The rheological, microstructural and textural evolution of Carrara marble was studied during experimental deformation in torsion up to very large shear strains (γ=0.4–50). Experiments were performed at constant angular displacement rates corresponding to shear strain rates of 6×10−5–3×10−3 s−1, a confining pressure of 300 MPa and temperatures of 500, 600 and 727 °C. Microstructures and crystallographic preferred orientations (CPO) were analysed using light microscopy and electron backscatter diffraction (EBSD). At all conditions deformation occurred dominantly by dislocation creep, even up to very large shear strains. After peak stresses (γ≤2), recrystallisation mostly by subgrain rotation caused weakening (5–20%), grain size reduction and a change in the CPO. A monoclinic CPO that formed at small strains at all temperatures evolved into two different large strain CPOs at different temperatures. At large strains, even after complete recrystallisation, the CPO continued to strengthen and a secondary foliation was formed at a steep angle to the shear zone boundary. This secondary foliation was continuously reset by both subgrain rotation and grain boundary migration recrystallisation and eventually overprinted the primary foliation. Misidentifying the primary and secondary foliations could result in misinterpretation of the applied strain field. More generally, information about the amount of strain is lacking for microstructures and CPOs of highly deformed ultramylonites.