(1) Quartz CPOs record consistent transitions with changing temperature. Transitions are likely caused by changes in the active slip system(s), which we attribute to (a) a decrease in grain boundary mobility (and quartz recrystalliza- tion mechanism) due to lower temperature, and (b) finite strain.
(2) Quartz CPOs in the hanging wall of the Moine Thrust show evidence for inheritance of grain orientations from deeper regions, indicating incomplete transitions to the easy slip system at shallower levels.
(3) We postulate that transitions in easy slip system may transiently strengthen quartz aggregates, causing strain to become increasingly parti- tioned into phyllosilicate and polyphase layers. This partitioning could trigger the development of fine-grained polyphase interconnected weak networks, which likely deform by grain size sensitive mechanisms including diffusion- accommodated grain boundary sliding.