Integrated estimates of the thickness of the fault damage zone in granitic terrain based on penetrative mesocracks and XRD analyses of quartz

摘要

We estimated the thickness of the fault damage zone in granitoid along the Iinan transect at the Median Tectonic Line (MTL) and along the Magawa transect at the Atotsugawa Fault, southwest Japan, based on the density of penetrative mesocracks, and on analyses of intracrystalline strain in pulverized quartz using X-ray line broadening (β) and the crystallinity index (CI). The mesocrack density and intracrystalline strain show an increase toward the MTL, from ~400 m from the fault. High amounts of intracrystalline strain in quartz, characterized by large β and low CI, reflect the presence of dense, heterogeneous tangles of short dislocations. For the Atotsugawa Fault, in contrast, we observed no increase in intracrystalline strain within quartz, even within highly fractured rock showing high mesocrack density located close to the fault (~ 10 m). This difference in the intensity of intracrystalline strain close to the fault between the MTL and the Atotsugawa Fault suggests that dislocation substructures did not accumulate at shallower levels in the brittle regime, but accumulated at deeper levels. Accordingly, analyses of intracrystalline strain in pulverized quartz, using X-ray line broadening and CI are useful in evaluating the degree of cataclasis of quartz-bearing rocks at deeper levels of the brittle regime.