High temperature flow and dynamic recrystallization in carrara marble

Abstract

Specimens of Carrara marble have been experimentally deformed at temperatures between 600° and 1050° and at strain rates between 10 −2 and 10 −6 sec −1. No single empirical flow law could be found for the whole range of experimental conditions covered. Instead it was found that three deformation regimes, each with its characteristic microstructural imprint, can be established. Above 1000 bar differential stress a relatively low strain rate sensitivity of the flow stress is observed and twinning is predominant (regime 1). Regime 2 extends down to 200 bar flow stress and exhibits the unusually high stress exponent n = 7–8 in the power law creep equation earlier found by Heard and Raleigh (1972) in Yule marble. The original grains exhibit a “core and mantle” structure. Only in regime 3 below 200 bar differential stress does one find lower values for n of around 4, and now a mosaic of equi-axed subgrains occupies entire grains. Extensive recrystallization and grain boundary migration was found at the higher temperatures. The grain size produced by dynamic in-situ recrystallization was found to be inversely proportional to the applied flow stress. Scanning electron microscopy observations on split cylinders suggests substantial amounts of grain boundary sliding in the low Stress region. The unusually high dislocation densities at low flow stresses as measured under the transmission electron microscope are interpreted to arise through strains induced during cooling under pressure after the deformation experiment. In view of the fact that the different calcite rocks so far investigated under elevated temperatures and low flow stresses exhibit rather different flow laws, caution is indicated when extrapolating such empirical flow laws to geological conditions. The size of dynamically recrystallized grains may directly lead to a paleostress estimate at the time of recrystallization. A subsequent work softening effect through a change in mechanism induced by syntectonic recrystallization may play an important role in the formation of shear zones and mylonite layers.