Deep crustal shear zones driven by reaction-induced weakening and fluid flow

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.

Field studies reveal that mineral reactions are ubiquitous in viscous shear zones, while undeformed rocks can remain largely metastable despite significant changes in P–T and/or fluid conditions. Local dissolution and precipitation processes under deviatoric stresses have long been recognized to promote brittle and viscous strain localization by complex chemo–mechanical processes including pressure solution, diffusive mass transfer, fluid flowand nucleation of fine-grained aggregates. Yet, quantifying the nature and relative contribution of these processes remains hindered by the general lack of experimental investigations on crustal rheology at high – to very high – pressure conditions and thermodynamic disequilibrium.

Drawing on novel deformation experiments performed at eclogite-facies conditions and a compilation of characteristics of exhumed materials from fossil subduction zones worldwide, this presentation demonstrates that inception and progression of crustal shear zones are predominantly steered by local transient changes of rheology from dislocation creep to dissolution–precipitation creep (DPC). Strain accommodation and mass transfer are further accelerated by local transient fluid flow resulting from grain boundary movements, fracturing and densification reactions. Because intergranular fluid-assisted mass transfer is orders of magnitude faster than solid-state diffusion, DPC can indeed explain strain accommodation at relatively high strain rates and low magnitude of differential stress, regardless of the mineral plastic strength. Yet, DPC remains a transient process because both fluid depletion and completion of mineral reactions favor grain growth, reducing in turn the efficiency of intergranular mass transfer.