Strain localization in the ductile crust triggers by microfracturing and high pore-fluid pressure

Abstract:

The deformation style beneath seismogenic zones is still debated, mostly concerning the ductile deformation (e.g. localized in a narrow shear zone or broadly distributed). Furthermore, the lower crust rheology remained not fully constrained because of the limited amount of outcrops and the difficulty to reproduce experimental study on polyphase rocks. Fluid-rock interactions appear to be the prevailing parameter controlling this rheology, mainly on high pore-fluid pressure conditions. We investigate in this study the role of high pore-fluid pressure and micro-cracking mechanism on the deformation localization. As a novelty, our approach describes the evolution of porosity within ductile shear zones. We develop a one-dimensional numerical model of grain size evolution coupled with a viscous rheology, where the deformation rate is a combination of grain size sensitive diffusion creep and dislocation creep. We use the Mohr-Coulomb criterion τ=μσ to determine when micro-cracking, and subsequent precipitation, occur. Using the Darcy’s law we compute a pore-fluid pressure depending on the porosity and the fluid flow. Our results indicate that the evolution of porosity could increase the effective pore-fluid pressure. Consequently, the yield strength decreases leading to the activation of the micro-cracking mechanism. The positive feedback between micro-cracking and precipitation enables pore-fluid pressure to reach a near-lithostatic value (λ≈1). The combination of spatially heterogeneous micro-cracking and near-lithostatic pore-fluid pressure induces a strain localization within the brittle-ductile transition of feldspathic lower crust. Such strain localization, including high pore-fluid pressure conditions and low strength, is well-known as a weakening mechanism making deep Earth’s crust a favorable environment to trigger tremors and low-frequency earthquakes.