Reaction‐Gated Transient Permeability in Ductile Shear Zones

Abstract Ductile shear zones act as transient pathways for crustal fluids, yet the conditions under which their connected porosity becomes sufficient to conduct are not well constrained. We present a porosity balance in which connected porosity evolves under three competing processes: creation by creep cavitation, creation by metamorphic reaction, and thermally activated closure. The model is constrained by existing experimental data, and permeability is linked to porosity through a percolation law with a sharp threshold. Within this constrained model, creep cavitation alone produces a steady‐state porosity orders of magnitude below the threshold at natural strain rates. A net reaction‐driven porosity source resolves this gap, and the critical net porosity‐production rate required to reach the threshold is geologically reasonable. Ductile shear‐zone permeability is therefore likely transient and reaction‐gated, with deformation alone unable to sustain connectivity at natural rates.