Analysis of the geological history of the active Atera Fault, central Japan, based on fault and fracture systems and infilling minerals

Abstract

Understanding the history of faulting is important in assessing the long-term hydrological, geochemical, and rock-mechanical behavior of a fault and related fracture zones. In particular, the characterization of faults and fracture systems is critical for the safe construction and operation of underground facilities such as structures for the storage of liquefied petroleum gas (LPG), high-level radioactive waste (HLW), and CO₂. Here, we studied the Atera Fault, a well-known active fault in central Japan, as an analog to investigate faulting history and fracture systems with infilling minerals. To investigate the structural and mineralogical evolution of the Atera Fault, we carried out detailed mapping of faults and fracture systems in and around the fault, as well as microscopic observations, XRF analysis of fault rocks, and XRD analysis of infilling minerals within fractures. Our analysis of the microtexture and infilling minerals along the fault plane and in adjacent fractures reveals that the Atera Fault records the following three-stage faulting history: Stage I) the formation of cataclasite under relatively high confining pressure; Stage II) the circulation of high-temperature groundwater, resulting in hydrothermal alteration and the growth of infilling minerals within fractures (prehnite, actinolite, sericite, chlorite, and quartz); and Stage III) once the fault had been uplifted to close to the ground surface, the penetration of low-temperature oxidized rainwater to form iron oxyhydroxides in open fractures. Our results suggest that analyses based on faults and fracture systems with infilling minerals are an effective method in assessments of the long-term use of underground storage facilities.