Abstract
In the Sambagawa schist, southwest Japan, while ductile deformation pervasively occurred at D-1 phase during exhumation, low-angle normal faulting was locally intensive at D-2 phase under the conditions of frictional viscous transition of quartz (c. 300 degrees C) during further exhumation into the upper crustal level. Accordingly, the formation of D-2 shear bands was overprinted on type I crossed girdle quartz c-axis fabrics and microstructures formed by intracrystalline plasticity at D-1 phase in some quartz schists. The quartz c-axis fabrics became weak and finally random with increasing shear, accompanied by the decreasing degree of undulation of recrystallized quartz grain boundaries, which resulted from the increasing portion of straight grain boundaries coinciding with the interfaces between newly precipitated quartz and mica. We interpreted these facts as caused by increasing activity of pressure solution: the quartz grains were dissolved mostly at platy quartz-mica interface, and precipitated with random orientation and pinned by mica, thus having led to the obliteration of existing quartz c-axis fabrics. In the sheared quartz schist, the strength became reduced by the enhanced pressure solution creep not only due to the reduction of diffusion path length caused by increasing number of shear bands, but also to enhanced dissolution at the interphase boundaries. (c) 2012 Elsevier Ltd. All rights reserved.