Effect of pre-existing crystallographic preferred orientation on the rheology of Carrara marble

• Carrara marble was deformed during torsion experiments at high pressure/temperature.
• Experiments were performed in shear-anneal-shear mode (multi-stage).
• The development of a CPO contributes to strain weakening in Carrara marble.
• Annealing can cause localization by decreasing rock strength at large shear strains.
• Strain rate has little effect on the contribution of CPO to strain weakening.

Localized deformation during high temperature plastic flow is frequently attributed to mechanical weakening caused by grain size reduction and, in some cases, by the development of a crystallographic preferred orientation (CPO). This study aims to investigate experimentally the contribution of CPO development to the strain weakening seen in Carrara marble samples during large strain torsion experiments at temperatures in the range 600–800 °C at constant strain rate and confining pressure. The starting material shows little or no CPO. Samples were treated in three steps. First, they were deformed to achieve a well-developed CPO and recrystallized to a finer grain size. Second, the samples were annealed at 727 °C for 5 h to grow the grains to approximately their original size and shape while maintaining a reasonably strong CPO. Finally, the samples were deformed again, under the same conditions as the first step. Re-deformed samples showed a strain weakening lower than during the first step. We infer that this strength difference is caused primarily by the CPO developed during the first deformation event. This implies that CPO development is an important process that contributes to strain weakening during flow of Carrara marble and likely of other rock types as well.