Fig. 2: Numerical simulation of the formation hydrated shear zones in the lower crust.

The simulation, using a grid resolution of 4m², started with an initially homogeneous, dry, lower crust (at 650 °C) with a single embedded wet-plagioclase-dominated inclusion (inset in A). When imposing a constant, sinistral shear strain rate of 5 × 10⁻¹⁵ s⁻¹, brittle shear zones develop, followed by hydrated, weak, shear zones. Shown are snapshots that illustrate A effective viscosity, B second invariant of strain rate tensor, C second invariant of plastic strain, D second invariant of the deviatoric stress tensor, E dynamic pressure, and F detail of dynamic pressure within the shear zones. Complex shear zone patterns emerge that have significant variations in stresses and dynamic pressures and may explain the coexistence of high and lower pressure assemblages in the same rocks. See Supplementary Movies S1–4 (Supplementary Notes 3) for animation and (Fig. S1, Supplementary Notes 2; Fig. S7, Supplementary Notes 4) for the temporal evolution for a range of imposed strain rates.