SYNTECTONIC DEVELOPMENT OF A STRAIN-INDEPENDENT STEADY-STATE GRAIN-SIZE DURING MYLONITIZATION

For a distance of greater than 50 km along the Median Tectonic Line, J apan, syntectonic recrystallization of quartz has resulted in the deve lopment of a strain-independent steady-state grain size within the cor e 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 appro ximately 37 mum, 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 Me dian Tectonic Line. Despite the development of a steady-state mean gra in 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 henc e resulted in the development of the steady-state grain size. The actu al grain size that resulted probably depended on both the water conten t and strain rate as well as the flow stress. Approaching the centre o f the shear zone, mylonitic fabrics (e.g., S-C fabrics, 'mica-fish') w ere established well before a steady-state grain size was reached. A s teady-state grain shape, shape orientation and crystallographic fabric s may not be developed until even more mylonitization had occurred. Va riable grain sizes result from heterogeneous deformation as a conseque nce of deformation partitioning even under a stable stress prior to th e 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.