Syntectonic development of a strain-independent steady-state grain size during mylonitization

Abstract

Michibayashi, K., 1993. Syntectonic development of a strain-independent steady-state grain size during mylonitization. Tectonophysics, 222: 151–164. For a distance of greater than 50 km along the Median Tectonic Line, Japan, Syntectonic recrystallization of quartz has resulted in the development of a strain-independent steady-state grain size within the core of the Kashio Shear Zone, that is also independent of rock-type. The mean grain size of quartz in recrystallized aggregates progressively decreases towards the shear zone core, becoming stable at around approximately 37 μm, even though the strain increases within more strongly mylonitized rocks. Some variation of mean grain sizes may result from secondary effects due to hydrothermal activity associated with the Median Tectonic Line. Despite the development of a steady-state mean grain size, individual grains were not stable, as revealed by log-normal grain-size distributions. Consequently, dynamic recrystallization and grain growth were competing processes that achieved a balance and hence resulted in the development of the steady-state grain size. The actual grain size that resulted probably depended on both the water content and strain rate as well as the flow stress. Approaching the centre of the shear zone, mylonitic fabrics (e.g., S-C fabrics, ‘mica-fish’) were established well before a steady-state grain size was reached. A steady-state grain shape, shape orientation and crystallographic fabrics may not be developed until even more mylonitization had occurred. Variable grain sizes result from heterogeneous deformation as a consequence of deformation partitioning even under a stable stress prior to the development of a steady-state grain size. Consequently, it should be determined whether a steady-state grain size has been achieved within a shear zone, before the grain size is used as a paleopiezometer.