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  1. Abstract. Viscous flow in ice is often described by the Glen flow law – anon-Newtonian, power-law relationship between stress and strain rate with astress exponent n ∼ 3. The Glen law is attributed tograin-size-insensitive dislocation creep; however, laboratory and fieldstudies demonstrate that deformation in ice can be strongly dependent ongrain size. This has led to the hypothesis that at sufficiently lowstresses, ice flow is controlled by grain boundary sliding, which explicitly incorporates the grain size dependence of ice rheology. Experimental studiesfind that neither dislocation creep (n ∼ 4) nor grain boundarysliding (n ∼ 1.8) have stress exponents that match the value ofn ∼ 3 in the Glen law. Thus, although the Glen law provides anapproximate description of ice flow in glaciers and ice sheets, itsfunctional form is not explained by a single deformation mechanism. Here weseek to understand the origin of the n ∼ 3 dependence of theGlen law by using the “wattmeter” to model grain size evolution in ice.The wattmeter posits that grain size is controlled by a balance between themechanical work required for grain growth and dynamic grain size reduction.Using the wattmeter, we calculate grain size evolution in two end-membercases: (1) a 1-D shear zone and (2) as a function of depth within anice sheet. Calculated grain sizes match bothmore »laboratory data and ice coreobservations for the interior of ice sheets. Finally, we show thatvariations in grain size with deformation conditions result in an effectivestress exponent intermediate between grain boundary sliding and dislocationcreep, which is consistent with a value of n = 3 ± 0.5 over the rangeof strain rates found in most natural systems.« less
  2. Abstract Most exposed middle- and lower-crustal shear zones experienced deformation while cooling. We investigated the effect of the strengthening associated with such cooling on differential stress estimates based on recrystallized grain size. Typical geologic ratios of temperature change per strain unit were applied in Griggs Rig (high pressure-temperature deformation apparatus) general shear experiments on quartzite with cooling rates of 2–10 °C/h from 900 °C to 800 °C, and a shear strain rate of ∼2 × 10−5 s−1. Comparisons between these “cooling-ramp” experiments and control experiments at constant temperatures of 800 °C and 900 °C indicated that recrystallized grain size did not keep pace with evolving stress. Mean recrystallized grain sizes of the cooling-ramp experiments were twice as large as expected from the final stresses of the experiments. The traditional approach to piezometry involves a routine assumption of a steady-state microstructure, and this would underestimate the final stress during the cooling-ramp experiments by ∼40%. Recrystallized grain size in the cooling-ramp experiments is a better indicator of the average stress of the experiments (shear strains ≥3). Due to the temperature sensitivity of recrystallization processes and rock strength, the results may underrepresent the effect of cooling in natural samples. Cooling-ramp experiments produced widermore »and more skewed grain-size distributions than control experiments, suggesting that analyses of grain-size distributions might be used to quantify the degree to which grain size departs from steady-state values due to cooling, and thereby provide more accurate constraints on final stress.« less
  3. Abstract Evidence for coseismic temperature rise that induces dynamic weakening is challenging to directly observe and quantify in natural and experimental fault rocks. Hematite (U-Th)/He (hematite He) thermochronometry may serve as a fault-slip thermometer, sensitive to transient high temperatures associated with earthquakes. We test this hypothesis with hematite deformation experiments at seismic slip rates, using a rotary-shear geometry with an annular ring of silicon carbide (SiC) sliding against a specular hematite slab. Hematite is characterized before and after sliding via textural and hematite He analyses to quantify He loss over variable experimental conditions. Experiments yield slip surfaces localized in an ∼5–30-µm-thick layer of hematite gouge with <300-µm-diameter fault mirror (FM) zones made of sintered nanoparticles. Hematite He analyses of undeformed starting material are compared with those of FM and gouge run products from high-slip-velocity experiments, showing >71% ± 1% (1σ) and 18% ± 3% He loss, respectively. Documented He loss requires short-duration, high temperatures during slip. The spatial heterogeneity and enhanced He loss from FM zones are consistent with asperity flash heating (AFH). Asperities >200–300 µm in diameter, producing temperatures >900 °C for ∼1 ms, can explain observed He loss. Results provide new empirical evidence describing AFH and the rolemore »of coseismic temperature rise in FM formation. Hematite He thermochronometry can detect AFH and thus seismicity on natural FMs and other thin slip surfaces in the upper seismogenic zone of Earth’s crust.« less
  4. A different type of defect, the coherency disclination, is added to disclination types. Disconnections that include disclination content are considered. A criterion is suggested to distinguish disconnections with dislocation content from those with disclination content. Electron microscopy reveals unit disconnections in a low albite grain boundary, defects important in grain boundary sliding. Disconnections of varying step heights are displayed and shown to define both deformed and recovered structures.