Quartz veins in the Eastern Tonale mylonite zone (Italian Alps) were deformed in strike-slip shear. Due to the synkinematic emplacement of the Adamello Pluton, a temperature gradient between 280°C and 700°C was effected across this fault zone. The resulting dynamic recrystallization microstructures are characteristic of bulging recrystallization, subgrain rotation recrystallization and grain boundary migration recrystallization. The transitions in recrystallization mechanisms are marked by discrete changes of grain size dependence on temperature. Differential stresses are calculated from the recrystallized grain size data using paleopiezometric relationships. Deformation temperatures are obtained from metamorphic reactions in the deformed host rock. Flow stresses and deformation temperatures are used to determine the strain rate of the Tonale mylonites through integration with several published flow laws yielding an average rate of approximately 10−14s−1 to 10−12s−1. The deformation conditions of the natural fault rocks are compared and correlated with three experimental dislocation creep regimes of quartz of Hirth & Tullis. Linking the microstructures of the naturally and experimentally deformed quartz rocks, a recrystallization mechanism map is presented. This map permits the derivation of temperature and strain rate for mylonitic fault rocks once the recrystallization mechanism is known.

Dynamic recrystallization of quartz: correlation between natural and experimental conditions

https://pangea.stanford.edu/sites/default/files/Warren&Hirth_2006.pdf

Grain size sensitive deformation mechanisms in naturally deformed peridotites

Small-scale convection associated with instabilities at the bottom of the lithospheric plates on the Earth and other terrestrial planets occurs in the stagnant lid regime of temperature-dependent viscosity convection. Systematic numerical simulations of time-dependent, internally heated stagnant lid convection suggest simple scaling relationships for a variety of convective parameters and in a broad range of power law viscosities. Application of these scaling relationships to the Earth's oceanic lithosphere shows that for either diffusion or dislocation viscosity of olivine, convective instabilities occur in the lower part of the lithosphere between 85 and 100 km depth (the rheological sublayer). “Wet” olivine satisfies constraints on the heat flux and mantle temperature better than “dry” olivine, supporting the view that the upper mantle of the Earth is wet. This is also consistent with the fact that the rheological sublayer is located below the Gutenberg discontinuity which was proposed to represent a sharp change in water content. The viscosity of asthenosphere is (3–6)×1018 Pa s, consistent with previous estimates. The velocities of cold plumes are relatively high reaching several meters per year in the dislocation creep regime. A low value of the heat flux in old continental cratons suggests that continental lithosphere might be convectively stable unless it is perturbed by processes associated with plate tectonics and hot plumes. The absence of plate tectonics on other terrestrial planets and the low heat transport efficiency of stagnant lid convection can lead to widespread melting during the thermal evolution of the terrestrial planets. If the terrestrial planets are dry, small-scale convection cannot occur at subsolidus temperatures.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2000JB900197

Scaling of time‐dependent stagnant lid convection: Application to small‐scale convection on Earth and other terrestrial planets

This graduate textbook presents a comprehensive, unified treatment of the materials science of deformation as applied to solid Earth geophysics and geology. The deformation of Earth materials is presented in a systematic way covering elastic, anelastic and viscous deformation. Advanced discussions on relevant debates are also included to bring readers a full picture of science in this interdisciplinary area. This textbook is ideal for graduate courses on the rheology and dynamics of solid Earth, and includes review questions with solutions so readers can monitor their understanding of the material presented. It is also a much-needed reference for geoscientists in many fields including geology, geophysics, geochemistry, materials science, mineralogy and ceramics.

Deformation of Earth Materials: An Introduction to the Rheology of Solid Earth

Synthetic fine-grained anorthite aggregates were deformed at 300 MPa confining pressure in a Paterson-type gas deformation apparatus. Creep tests were performed at temperatures ranging from 1140 to 1480 K, stresses from 30 to 600 MPa, and strain rates between 2×10−6 and 1×10−3 s−1. We prepared samples with water total contents of 0.004 wt % (dry) and 0.07 wt % (wet), respectively. The wet (dry) material contained 120 MPa we found a stress exponent of n = 3 irrespective of the water content, indicating dislocation creep. However, the activation energy of wet samples is 356±9 kJ mol−1, substantially lower than for dry specimens with 648±20 kJ mol−1. The preexponential factor is log A = 2.6 (12.7) MPa−n s−1 for wet (dry) samples. Microstructural observations suggest that grain boundary migration recrystallization is important in accommodating dislocation creep. In the low-stress regime we observed a stress exponent of n = 1, suggesting diffusion creep. The activation energies for dry and wet samples are 467 ±16 and 170 ± 6 kJ mol−1, respectively. Log A is 12.1 MPa−n μmm s−1 for the dry material and 1.7 MPa−n μmm s−1 for wet anorthite. The data show that the strengths of anorthite aggregates decrease with increasing water content in both the dislocation and diffusion creep regimes. A comparison of the creep data of synthetic plagioclase from this study with published data for feldspar, olivine, and quartz indicates a linear relationship between activation energy and log A similar to the suggested compensation law for diffusion in silicates.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2000JB900223

Dislocation and diffusion creep of synthetic anorthite aggregates

A hypothesis is advanced that dynamic recrystallization of Earth materials undergoing solid state flow may represent a balance between grain size reduction and grain growth processes occurring directly in the boundary between the dislocation and diffusion creep fields. Accordingly, the recrystallized grain size (D) and flow stress (σ) at steady state will be related by the equation delineating the field boundary, which in general is temperature dependent. Creep experiments on a metallic rock analogue, Magnox, yielded D=10 1.12 exp[29.3/RT]σ 1.2:3 and demonstrated that D (μm) decreases with increasing σ (MPa) and increasing temperature (T) in a manner which is in agreement with the field boundary hypothesis. If the model applies to rocks, the widely accepted idea that dynamic recrystallization can lead to major rheological weakening in the Earth may not hold. Moreover, empirical D-σ relations, used in paleo-piezometry, will need to be modified to account for temperature effects.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/98GL02690

J. P. J. Reijs

Papers

On dynamic recrystallization during solid state flow: Effects of stress and temperature