On unifying carbonate rheology

We review the results from twenty four experimental works conducted on the rheology of carbonates from the last fifty years to revisit the long-noted discordance in the experimental results from a range of limestones and marbles. Such an exercise is needed to bring together the various datasets generated in the twenty three years since the last major review, as many of them observe relationships that run contrary to existing rheological models. By revisiting the large data set, we find that most low and high stress experimental measurements can be explained by the combined effect of grain size and the molar fraction of magnesium carbonate (XMgCO3). Our results highlight that much of the calcite-dolomite series exists in a continuum of strength that changes with XMgCO3. In contrast to previous findings, we establish that diffusion creep in calcite is sensitive to both grain size and magnesium content, showing that an increase in XMgCO3 acts to weaken a rock. While in dislocation creep we confirm the observation that XMgCO3 has a strengthening effect but extend it beyond synthetic Mgcalcite samples to natural starting materials. Most notably our results suggest that when the composition of a carbonate is factored in then grain size can be shown to have a weakening effect in high temperature creep for fine grained rocks. This is the opposite finding to the currently accepted flow law for high homologous temperature deformation of calcite rocks where a decrease in grain size strengthens a rock. We contextualise these new results by combining them with data from natural shear zones to show that carbonates are much weaker than would be expected from previous flow laws in a crustal section. Ultimately our review provides new pragmatic flow laws for carbonates in the calcite-dolomite series for diffusion and dislocation creep.