Strain-induced amorphization of graphite in fault zones of the Hidaka metamorphic belt, Hokkaido, Japan

We report here on two distinct processes of deformation affecting graphite in fault zones of the Hidaka metamorphic belt, Hokkaido, Japan. One process involves the micrometer-scale delamination (MMD) in the stacking of graphite, and the other is the nanometer-scale delamination (NMD) in the stacking plus pulverization (P) of carbon sheets to less than 10 nm. Graphite in mylonites mainly glide along the (002) planes, and they exhibit layer separation, kink banding, and bending structures produced by MMD processes, whereas NMD + P processes induce a significant size reduction and amorphization of the graphitic structures in cataclasites, ultracataclasites, and pseudotachylytes. The strain-induced amorphization of graphite, as inferred from XRD and micro-Raman spectroscopy, corresponds well with nano-to micro-scale deformation structures observed under the HRTEM, suggesting a systematic increase of edge planes by interlayer delamination and pulverization of carbon sheets. In addition, the carbon isotopic compositions of the graphite in fault rocks have negative δ13C signatures, indicating a sedimentary organic origin; such graphite has a tendency to accumulate within the shear bands. Our data suggest that the graphitic structures record information on both the peak metamorphic temperatures and the deformation processes, and they show the progressive amorphization with increasing brittle deformation.