2026 年 60 巻 2 号 p. 31-38
The timing and duration of fault-zone activity remain inadequately constrained, impeding quantitative insight into brittle deformation processes and earthquake-cycle dynamics. Authigenic minerals precipitated within fault zones constitute direct archives of deformation and hydrothermal fluid flow, enabling fault activity to be dated independently of displaced stratigraphic markers. Recent advances in fault geochronology—most notably illite K–Ar/Ar–Ar, hematite (U–Th)/He, and calcite U–Pb—have established robust temporal constraints on deformation and fluid-related events. By contrast, the geochronological potential of silica minerals, particularly quartz and opal, remains insufficiently developed despite their pronounced sensitivity to brittle failure, seismic cycling, and hydrothermal processes. Silica undergoes repeated dissolution–precipitation in hydrothermal systems and can selectively incorporate uranium; however, fundamental constraints on uranium uptake, retention, and system behavior are sparse. While recent in-situ studies demonstrate the feasibility of dating fault-related silica, significant methodological challenges persist. Concurrently, evidence that fault-hosted opal records earthquake-related fluid discharge, and that quartz vein formation is closely coupled to fault-valve behavior, underscores the need for a systematic framework. Here we develop an integrated approach to uranium-series geochronology of quartz and opal, building on the achievements and limitations of calcite-based methods. Focusing on the Japan arc system, where silica veins are widespread and geological, seismic, and geodetic constraints are extensive, we combine microstructural, geochemical, and isotopic analyses to resolve high-resolution temporal records of brittle deformation and fluid migration from shallow to mid-temperature crustal regimes.
