Superplasticity in Geological Materials

The term “superplasticity” is taken as meaning high-strain-rate-sensitivity flow when it is used in geological contexts involving predominantly compressive stress states. Such flow has been observed in calcite and olivine polycrystalline aggregates in laboratory tests and the mechanism deduced to be diffusion-controlled granular flow. From microstructural observations, similar deformation processes are inferred to have occurred in a number of geological situations. On the basis of extrapolation from experiment to geology, superplastic flow might be expected under some geological conditions in calcite and olivine rocks at grain sizes approaching the order of one millimetre, while there are indications that the grain size limit for quartzites may be rather smaller. However, the knowledge of superplasticity in most rocks is still very fragmentary, especially in regard to mechanisms where a fluid phase may be involved, as in porous rocks.

Dynamic recrystallization of quartz: correlation between natural and experimental conditions

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.

Newtonian Dislocation Creep in Quartzites: Implications for the Rheology of the Lower Crust

Mechanical and microstructural evidence indicates that a natural and a synthetic quartzite deformed by Newtonian dislocation (Harper-Dorn) creep at temperatures higher than 1073 K and stresses lower than 300 megapascals. The observation of this creep in these materials suggests that the lower crust may flow like a Newtonian viscous fluid by a dislocation mechanism at stresses much smaller than those previously postulated.