The role of intragranular stress heterogeneity in transient dislocation-mediated deformation of calcite mylonites

Transient creep of calcite controls the strength evolution of carbonate shear zones during postseismic deformation. However, a lack of information on the dominant microphysical mechanisms of transient creep of calcite hinders the development of constitutive equations. Specifically, for dislocation-mediated deformation, it is unclear whether strain hardening occurs primarily by short-range dislocation interactions and is therefore isotropic or by long-range elastic interactions and is therefore anisotropic. Here, I test whether mylonitic calcite marbles from the mid-crustal shear zone of the Karakoram Fault Zone, NW India, preserve residual stresses indicative of these long-range elastic interactions among dislocations. Previous work demonstrated that the mylonitic fault rocks experienced bulk stresses in the range 40–250 MPa as they were exhumed and cooled from approximately 480°C to 300°C. I analysed the microstructure and micromechanical state of three samples, including undeformed wall rock, protomylonite, and ultramylonite, using electron backscatter diffraction and high-angular resolution electron backscatter diffraction. The undeformed wall rock has a grain size of 130 µm, whereas the protomylonite and ultramylonite have grain sizes of 22 µm and 12 µm, respectively. Densities of geometrically necessary dislocations (GNDs) increase from the wall rock into protomylonite and ultramylonite. In the deformed lithologies, GND densities generally increase with proximity to grain boundaries over distances of 10–15 µm. Residual stresses in the wall rock are below the noise level of the HR-EBSD measurements, with a 99^th percentile of 54 MPa. However, significant heterogeneity in residual stress is present in the protomylonite and ultramylonite, with 99^th percentiles of 325 MPa and 742 MPa respectively. Both the spatial and probability distributions of the residual stresses reveal that they are imparted primarily by dislocations. Autocorrelation of the stress fields indicates that the typical length scale of stress heterogeneity increases from approximately 2 µm in the wall rock to 4 µm in the protomylonite and 7 µm in the ultramylonite. Collectively, these observations demonstrate that dislocations in calcite generate long-range internal stresses that cause elastic interactions. These elastic interactions are typically inferred to manifest as a backstress that counteracts the applied stress and generates a component of anisotropic kinematic hardening. The contribution of this mechanism of transient creep is missing from existing constitutive equations for calcite and should be represented by a backstress that is subtracted from the applied stress and can evolve with strain and time.