This dataset is supplemental to the paper Wallis et al. (2020) and contains data derived from syn-chrotron X-ray diffraction, electron backscatter diffraction (EBSD), high-angular resolution electron backscatter diffraction (HR-EBSD), and scanning transmission electron microscopy (STEM). The da-taset consists primarily of measurements of the effect of annealing on stress heterogeneity meas-ured by X-ray diffraction; maps of lattice orientation measured by EBSD; maps of lattice rotations, densities of geometrically necessary dislocations (GNDs), and heterogeneity in residual stress measured by HR-EBSD; and images of dislocations obtained by STEM. Data are provided as 66 tab delimited text files organised and labelled by the figure in which they first appear within Wallis et al. (2020). Table 1 of the data description file presents an overview of the datasets and Table 2 provides a description of each data file. Data types are also indicated in the file names.
The largest magnitude earthquakes nucleate at depths near the base of the seismogenic zone, near the transition from velocity weakening frictional slip to velocity strengthening ductile flow. However, the mechanisms controlling this transition, and relevant to earthquake nucleation, remain poorly understood. Here we present data from experiments investigating the effect of slip rate on the mechanical properties and microstructure development of simulated calcite fault gouge sheared at ~550°C, close to the transition from (unstable) velocity weakening to (stable) velocity strengthening behaviour, reported by Verberne et al. (2015).
We conducted experiments at a constant effective normal stress (σneff) of 50 MPa, as well as σneff-stepping tests employing 20 MPa ≤ σneff ≤ 140 MPa, at constant sliding velocities (v) of 0.1, 1, 10, or 100 µm/s. Samples sheared at v ≥ 1 µm/s showed a microstructure characterized by a single, 30 to 40 μm wide boundary shear, as well as a linear correlation of shear strength (τ) with σneff. Remarkably, electron backscatter diffraction mapping of polygonal shear band grains demonstrated a crystallographic preferred orientation. By contrast, samples sheared at 0.1 µm/s showed a microstructure characterized by homogeneous deformation and plastic flow, as well as a flattening-off of the τ-σneff curve. Our results point to a strain rate dependent frictional-to-viscous transition in simulated calcite fault gouge, and have important implications for the processes controlling earthquake nucleation at the base of the seismogenic zone.