The occurrence of graphite-bearing fault rocks in the Atotsugawa fault system, Japan: Origins and implications for fault creep

Graphite in fault zones has received little attention even though it is a well-known solid lubricant that could affect frictional properties of faults dramatically. This paper reports the presence of abundant graphite in fault zones of the Atotsugawa fault system, central Japan. Mesoscopic and microscopic observations of fault rocks revealed two processes of carbon enrichment in fault zones. One is a pressure solution process or diffusive mass transfer in general which removes water-soluble minerals such as quartz and carbonates from rocks, resulting in the enrichment of insoluble minerals including carbon. The other process is precipitation of graphite from a high-temperature carbon-rich fluid, forming graphite filling fractures within cataclasitic fault zones. The two processes have led to the concentration, up to 12 wt% of graphite, in the Atotsugawa fault zones, compared to 0 to 3 wt% of carbonaceous materials in the host rocks. This concentration is high enough for graphite to affect frictional properties at wide range of slip rates. The presence of graphite may provide an explanation for the low resistivity, the patterns of microearthquakes and fault creep along the western part of the Atotsugawa fault system. Graphite should receive more attention as a weakening and stabilizing agent of faults. ► Blackish faults along the Atotsugawa fault system contain ∼12 wt% of graphite. ► These graphite-enriched faults are widespread in western part of the fault system. ► Solution transfer process is one of the candidates for graphite enrichment. ► Hydrothermally precipitated graphite veins are also found along the cataclasite. ► Graphite may be responsible for the peculiar seismicity along the fault system.