The Development of Subgrains in Deformed Calcite: Influence of Stress and Temperature

The internal substructure of subgrain walls and dislocations in crystals in naturally deformed rocks is an indicator of the deformation mechanisms that were active in the material and can be used to estimate paleo-stress levels in the crust and upper mantle. In order to relate observations made in the internal parts of crystals in a meaningful way to the strength and rheology of rocks, a thorough understanding is needed of the nature of deformation induced substructures and their dependence on conditions of stress and temperature, amongst others. This requires full characterization of the microstructure rather than applying conventional averaging techniques. This is the aim of our study. We systematically investigated experimentally deformed samples of Carrara marble, uniaxially shortened to a natural strain of 0.45, at differential stresses ranging 15-85 MPa, temperatures ranging 700 to 990°C, and a confining pressure of 300 MPa. The samples were analysed using the electron backscattered diffraction technique (EBSD) to make automated 1000 x 1000 μm orientation maps (step size 1 μm). A size measurement technique was developed for separately measuring the size of recrystallised grains, deformed grains, and within the latter, core and mantle subgrains. We observed a complex microstructure in the calcite samples, consisting of deformed and recrystallised grains with heterogeneously distributed small subgrains at grain boundaries and relatively large subgrains in the core of grains. The size of the subgrains varies with stress, as expected on the basis of previous work on subgrains, but temperature, grain size and misorientation angle were also found to be of influence. Moreover, mantle and core subgrains depend differently on these parameters. Most strikingly, the subgrain sizes for mantle and core show a low sensitivity on stress, deviating from the often quoted linear inverse relationship. Mantle subgrains with high misorientation angles appear more sensitive to stress than low angle subgrains. Temperature does not have a systematic influence on the mantle subgrain size, while the core subgrain size is larger at higher temperatures. All this has the important consequence that conventional paleopiezometric relationships between stress and subgrain size cannot simply be used for any type of subgrains. We present a conceptual model involving stress gradients and variability in slip system activity between core and mantle to account for our observations.