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

Abstract

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. Our tests indicate that the strain weakening in Carrara marble is predominantly caused by grain size refinement through dynamic recrystallization. The development of a crystallographic preferred orientation contributes towards only about one third of the total weakening observed.