Shear heating-induced strain localization across the scales

Abstract

We investigate the dynamics of thermally activated shear localization in power law viscoelastic materials. A two-dimensional (2D) thermomechanical numerical model is applied that uses experimentally derived flow laws for rock. We consider viscous and viscoelastic rheologies and show that the numerical solutions for shear bands are mesh insensitive and energetically conservative. Deformation under long-term tectonic strain rates (10 −14 s −1 ) yields to shear localization on the scale of kilometres. Although viscous and viscoelastic models exhibit comparable shear zone thickness, the timing of shear localization and stress magnitudes are affected by viscoelasticity. Large values of shear modulus (10 11 Pa) promotes fast stress loading (within 1% strain) and localization (<10% strain), whereas lower values (5 × 10 9 Pa) delays localization (∼15% strain) and reduces the maximum effective stress by a factor two. High stress exponents (up to n = 10) focuses the deformation into narrow shear zones (∼1000 m) while maintaining a relatively high average stress level in the material outside the shear zone (stress drop of ∼60%). Conversely, Newtonian materials produce broad shear zones (∼6× larger than for n = 10) and exhibit a stronger thermal weakening (stress drops of ∼85%). Finally, we evaluate the thickness of shear zones for a wide range of strain rates (from 10 −14 s −1 to 10 10 s −1 ) using both numerical models and an analytical scaling law. Our results suggest that thermally activated shear localization may occur from the scale of kilometre down to the nanometre.

Finite Thickness of Shear Bands in Frictional Viscoplasticity and Implications for Lithosphere Dynamics

“…However, when the shear zone reaches a steady‐state situation, that is, when the width D vp has stabilized, the shear bandwidths are virtually independent of the size of the imperfection, as shown in Figure . This is similar to results obtained with thermo‐mechanical models of strain localization in temperature activated rate‐dependent materials (Lemonds & Needleman, ) using a power law viscous rheology (Duretz et al, ) and a power law viscoelastic rheology (Duretz et al, ). In a steady state, the characteristic shear‐band thickness is essentially independent of the size of the imperfection.…”

Breakup Without Borders: How Continents Speed Up and Slow Down During Rifting

“…Several lithospheric weakening mechanisms are not accounted for such as strain softening, intrusion-related heating, shear heating, and structural softening (Duretz et al, 2015(Duretz et al, , 2016Huismans et al, 2005). Several lithospheric weakening mechanisms are not accounted for such as strain softening, intrusion-related heating, shear heating, and structural softening (Duretz et al, 2015(Duretz et al, , 2016Huismans et al, 2005).…”

Thick-Skinned and Thin-Skinned Tectonics: A Global Perspective

“…References [31,32] postulate that mylonitic shear zones in the crust become continually weaker owing to the rising temperatures within the shear zone. Reference [33][34][35][36][37] argue that faults within crystalline basement become weak because of reaction softening: phyllosilicate-rich mylonites (phyllonites) form from the mechanical breakdown of feldspars, which is followed by a chemical breakdown in the presence of a fluid, or grain-size reduction allowing grain-boundary sliding. Reference [38] report that the crystalline basement in the external parts of the Western Alps deformed in a brittle-ductile regime with distributed shear within spaced shear zones or shear bands that are up to a few hundred meters thick.…”