Fig. 3. Selected 3D incremental strain fields from the onset of strain clustering (marked in Fig. 1c).
Incremental dilation, Δϵd (blue-pink), and shear strain, Δϵs (yellow-green), were calculated from digital volume correlation between successive pairs of tomographic volumes and are shown a parallel to strike (y,z orientation) and b perpendicular to strike (x,z orientation). The lower threshold of 0.0017 was set at four standard deviations from the mean of the error distribution of Δϵs (Supplementary Fig. 6) and the upper threshold shows regions with strain >0.01 (maximum Δϵs and Δϵd were ~0.04; Supplementary Figs. 4 and 5). The thresholds were chosen to visually highlight regions of localised strain. Number labels correspond to those in Figs. 1 and 2, with the strain increment between the numbered tomogram and its subsequent neighbouring tomogram. Strain localisation began at 70% of the peak stress, σP. The yield point here is the same as that shown in Fig. 1c, with the corresponding strain increment shown immediately following yield. The shear zone formed as crack localisation occurred along the critically oriented plane in tomogram 22 (Fig. 2), and developed between strain increments 22 and 31, with patches of high intensity strain moving first up, then across and eventually down the sample, until high intensity dilation almost stopped. We defined coherent sliding as sliding along the whole shear zone, with the onset of coherent sliding at strain increment 31 since the intensity of dilation was significantly less in this increment than in previous increments, and this increment coincided with the point at which the rate of stress reduction slowed down in Fig. 1a. The full time-series of incremental dilation and shear strain from the onset of strain localisation is shown in Supplementary Fig. 4.