Use of extension testing to investigate the influence of finite strain on the rheological behaviour of marble

Most mechanical testing of rocks at high temperatures is carried out under axisymmetric triaxial shortening to small (<20%) strains. However, the most significant geological deformations involve much higher strains, especially in plastic shear zones where there has been strain localization. Highly strained rocks commonly are microstructurally modified by dynamic recrystallization, so that the sheared rock is microstructurally quite different from its protolith. High strain deformation in experiments can be induced by direct shear or through torsion, or by taking advantage of the tendency for extensional deformations to result in necking. It is important to be able to separate out the geometrical factors favouring localization from those arising from strain-dependent microstructural changes. Here, we present the results of high strain (>1000%) axisymmetric extension tests on Carrara marble at 700 degrees C and 800 degrees C, in the form of strain-time plots for the evolution of material sections through the sample at different distances from the neck. At 800 degrees C, it is clear that total dynamic recrystallization by grain boundary migration is accompanied by substantial mechanical weakening, but sufficient strain could not be applied to reach the steady-state condition for the recrystallized material. The recrystallization is interpreted to produce weakening because the migration of grain boundaries sweeps grains clear of accumulating dislocation density faster than recovery by dislocation climb. In nature, such mechanical weakening would be expected to favour the stable development of a localized zone of deformation. (C) 1998 Elsevier Science Ltd.