Deep crustal deformation driven by reaction-induced weakening

Deep crustal shear zones, fundamental to the dynamics of terrestrial plate tectonics, exhibit complex processes of initiation and evolution that are yet to be comprehensively quantified across both long and short temporal scales. Conventionally, thermo-mechanical models posit that crustal rock behaviour is dominated by monomineralic aggregates undergoing processes like intracrystalline plastic deformation by dislocation creep. However, high-pressure and temperature conditions in crustal rocks involve minerals with extremely strong mechanical properties, challenging strain localization theories. Drawing on deformation experiments performed at eclogite-facies conditions, our research reveals that strain is efficiently localized through dissolution-precipitation creep, operating at notably lower stresses than dislocation creep. Strain accommodation and mass transfer are episodically accelerated by local transient fluid flow resulting from grain boundary movements, fracturing and densification reactions. Our results illuminate the interconnected thermo-hydro-mechanical-chemical processes underpinning crustal shear zone development, regardless of the plastic strength of mineral phases. We advocate that the inception and progression of subduction plate interfaces are predominantly steered by local transient changes of rheology beyond the seismogenic zone. Such changes are rooted in the chemical disequilibrium and fluid concentration of the slab materials, including sediments and mafic to ultramafic rocks.