A new estimate of the conditions for transition from basal 〈 a〉 to prism [ c] slip in naturally deformed quartz

Abstract

Quartz c-axis fabrics of naturally deformed quartz in metacherts from the Ryoke metamorphic belt in the Yanai district, southwestern Japan, suggest that the prism [ c], prism 〈 a〉 and basal 〈 a〉 slip systems were activated during the plastic deformation under high-temperature metamorphic conditions. With increasing temperature, quartz c-axis fabric patterns change from girdle type, through X-maximum with a faint crossed girdle, to X-maximum. The dominant orientation of subgrain boundaries in the quartz also changes from prism to basal with increasing temperature, closely correlated with the change in quartz c-axis fabric. The fabric transition is also accompanied by an abrupt increase of recrystallized quartz grain size. Moreover, observations with transmission electron microscopy (TEM) clearly identify the dominant activation of [ c] slip through the presence of basal subgrain boundaries and free dislocations with the [0001] Burgers vector in the quartz from the metachert samples exhibiting X-maximum c-axis fabric. Based on these observations it is concluded that the mechanism switch from basal 〈 a〉 to prism [ c] slip systems occurred with increasing temperature in the metacherts. The transition temperature is petrologically estimated to be ca. 550–600°C at the natural strain rate.