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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.

[1] Quartzite samples experimentally sheared at conditions where subgrain rotation and grain boundary migration recrystallization are active show a striking change in the c axis crystallographic preferred orientation (CPO) with increasing shear strain (up to = 8) and degree of recrystallization (up to 100%). We used optical methods to determine the c axis CPO for the bulk samples and for porphyroclasts and recrystallized grains separately and to track the strengths of the CPOs in five different orientation domains with increasing . The c axis pole figure evolves from a broad peripheral maximum indicative of basal hai slip, to an inclined single girdle with two maxima indicative of rhomb hai slip, and finally an elongate single maximum at the girdle center indicative of prism hai slip; throughout this sequence the fabric skeleton rotates with the sense of shear. The bulk preferred c axis orientation results from growth and shrinkage of differently oriented crystallographic domains. Grain boundary density analysis indicates that the size of prism hai recrystallized grains is 1.5 times that of average and 2 times that of rhomb grains, indicating that prism hai slip is significantly easier at our experimental conditions. This inference is supported by autocorrelation function analysis of the various orientation domains and how they evolve with strain. Similar progressive changes in c axis CPO are observed in quartzites naturally sheared at conditions where grain boundary migration recrystallization operates. These results indicate that achievement of microstructural and mechanical steady state requires high strain and complete recrystallization.

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Field studies and seismic data show that semi‐brittle flow of fault rocks probably is the dominant deformation mechanism at the base of the seismogenic zone at the so‐called frictional‐viscous transition. To understand the physical and chemical processes accommodating semi‐brittle flow, we have performed an experimental study on synthetic granitoid fault rocks exploring a broad parameter space (temperature, T = 300, 400, 500, and 600°C, confining pressure, Pc ≈ 300, 500, 1000, and 1500 MPa, shear strain rate, trueγ⋅ ≈ 10−3, 10−4, 10−5, and 10−6 s−1, to finite shear strains, γ = 0–5). The experiments have been carried out using a granular material with grain size smaller than 200 µm with a little H2O added (0.2 wt %). Only two experiments (performed at the fastest strain rates and lowest temperatures) have failed abruptly right after reaching peak strength (τ ~ 1400 MPa). All other samples reach high shear stresses (τ ~ 570–1600 MPa) then weaken slightly (by Δτ ~ 10–190 MPa) and continue to deform at a more or less steady state stress level. Clear temperature dependence and a weak strain rate dependence of the peak as well as steady state stress levels are observed. In order to express this relationship, the strain rate‐stress sensitivity has been fit with a stress exponent, assuming γ̇ ∝ τn and yields high stress exponents (n ≈ 10–140), which decrease with increasing temperature. The microstructures show widespread comminution, strain partitioning, and localization into slip zones. The slip zones contain at first nanocrystalline and partly amorphous material. Later, during continued deformation, fully amorphous material develops in some of the slip zones. Despite the mechanical steady state conditions, the fabrics in the slip zones and outside continue to evolve and do not reach a steady state microstructure below γ = 5. Within the slip zones, the fault rock material progressively transforms from a crystalline solid to an amorphous material. We present and interpret the experimental results both in terms of sliding friction and viscous flow, and we discuss the possible effect that the formation of nanocrystalline and amorphous layers may have on earthquake nucleation.

To understand the processes involved in phase mixing during deformation and the resulting changes in rheological behavior, we conducted torsion experiments on samples of iron‐rich olivine plus orthopyroxene. The experiments were conducted at a temperature, T, of 1200°C and a confining pressure, P, of 300 MPa using a gas‐medium deformation apparatus. Samples composed of olivine plus 26% orthopyroxene were deformed to outer radius shear strains up to γ ≈ 26. In samples deformed to lower strains of γ ≲ 4, elongated olivine and pyroxene grains form a compositional layering. Already by this strain, mixtures of small equant grains of olivine and pyroxene begin to develop and continue to evolve with increasing strain. The ratios of olivine to pyroxene grain size in deformed samples follow the Zener relationship, indicating that pyroxene grains effectively pin the grain boundaries of olivine and inhibit grain growth. Due to the reduction in grain size, the dominant deformation mechanism changes as a function of strain. The microstructural development forming more thoroughly mixed, fine‐grained olivine‐pyroxene aggregates can be explained by the difference in diffusivity among Me (Fe or Mg), O, and Si, with transport of MeO significantly faster than that of SiO2. These mechanical and associated microstructural properties provide important constraints for understanding rheological weakening and strain localization in upper mantle rocks.

Quartzite samples were experimentally deformed with partial to complete dynamic recrystallization by axial compression (strain magnitude of 0.8 to 1.4) and by general shear (strain magnitude of 1.3 to 2.8) in each of the three dislocation creep regimes, and subsequently annealed with complete static recrystallization at the deformation temperature for 120 hours. The c-axis crystallographic preferred orientation (CPO), 3D grain size distribution, grain boundary surface shape, and misorientation density were measured before and after annealing. The effect of annealing on the CPO was minor, but the microstructure was greatly changed. All of the annealed samples were completely recrystallized. The recrystallized grain size increased by a factor of 2 to 5, and was greatest for samples deformed at lowest temperature. The grain boundary lobateness (PARIS factor) and misorientation density were reduced significantly. The CPOs for all the deformed samples were relatively unchanged by annealing, although the strengths are somewhat decreased; for sheared samples the asymmetry was preserved. The results suggest microstructural criteria for recognizing the occurrence of static annealing and for estimating the dynamically recrystallized grain size relevant for paleopiezometry from annealed samples.