Influence of strain history on the mechanical and micro-fabric evolution of calcite rocks: insights from torsion experiments

Different types of deformation experiments were conducted on specimens of Carrara marble at a temperature of 1000 K, 300 MPa confining pressure, and 3 × 10−4s−1 shear strain rate. Microstructural examinations were carried out using light and electron microscopy. Fabric analysis was performed through electron backscattered diffraction (EBSD) measurements. The stress-strain relationships from the experiments indicate that the mechanical response of Carrara marble depends on the deformation history. After a first ductile deformation event the marble becomes permanently softened and is able to undergo plastic strain at relatively lower stresses compared to previously undeformed marble. After shear strain reversal of γ = 2 or less the original microstructure is restored; in highly re-strained samples (γ = ±3,4 and 5) dynamic recrystallization mechanisms create an evident foliation whose angle in respect to the shear plane defines a shear sense criterion which is in agreement with the reversed sense of shearing. After a minimum shear strain reversal of γ = 3 the pre-existing foliation is totally overprinted. The fabric produced during strain reversal experiments shows features, which are comparable with those of single-stage experiments; but the strength of the crystallographic preferred orientation (CPO) is significantly lower, indicating complex microstructural interactions during plastic reactivation.