Coupled micro-faulting and pressure solution creep overprinted on quartz schist deformed by intracrystalline plasticity during exhumation of the Sambagawa metamorphic rocks, southwest Japan

摘要

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 ℃) 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 microstmctures 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.