Strain-induced amorphization of graphite in fault zones of the Hidaka metamorphic belt, Hokkaido, Japan Yoshihiro Nakamura a, * , Kiyokazu Oohashi b, 1 , Tsuyoshi Toyoshima c , Madhusoodhan Satish-Kumar c , Junji Akai c a Graduate School of Science & Technology, Niigata University, 2-8050 Ikarashi, Nishi-ku, Niigata 950-2181, Japan b Department of Earth Science, Graduate School of Science, Chiba University, 1-33 Yayoi cho Inage-ku, Chiba 263-8522, Japan c Department of Geology, Faculty of Science, Niigata University, 2-8050 Ikarashi, Nishi-ku, Niigata 950-2181, Japan article info Article history: Received 10 February 2014 Received in revised form 18 October 2014 Accepted 24 October 2014 Available online 6 November 2014 Keywords: Graphite Micro-Raman spectroscopy Amorphization Delamination Pulverization abstract 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 þ 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 d 13 C 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. © 2014 Elsevier Ltd. All rights reserved. 1. Introduction Carbonaceous materials (CM) and graphite are common acces- sory minerals in sedimentary and metasedimentary rocks, and they are often present in fault rocks, associated with other sheet silicate minerals (e.g., Zulauf et al., 1990; Craw, 2002). Sheet silicate min- erals and graphite have one key characteristic in common: the presence of weak bonding along the (001) planes. This weak bonding, such as hydrogen or Van der Waals bonding, potentially results in the low frictional strength of natural fault materials (Morrow et al., 2000; Moore and Lockner, 2004). In fault zones, therefore, it is important to determine the deformation mecha- nisms of minerals with a sheet structure based on microstructural observations. The most common strain-induced defects of sheet silicate minerals are mainly classified under the four headings of polytypic disorder, layer flexures, dislocations, and interlayer delamination, as suggested by HRTEM studies (Kogure and Kameda, 2008; Viti and Collettini, 2009). Viti and Collettini (2009) reported that the microstructure of talc in fault rocks is affected by several strain- induced defects, among which (001) interlayer delamination re- sults in sub-lamellae from 200e300 nm to 10e30 nm thick. On the other hand, graphite also has one of the weakest bondings in crystal structures, and it is well known as a “solid lubricant” in fault zones (Oohashi et al., 2011). Graphite gouge has a markedly low coeffi- cient of friction of around 0.1 in experimental studies, and many researchers have pointed out that graphite is also an important material in fault zones, potentially controlling frictional strength (Morrow et al., 2000; Oohashi et al., 2011). In the case of natural occurrences of graphite in mylonite zones, Kretz (1996) reported that the graphite underwent deformation by cleavage separation and the formation of folds and kink bands. However, most previous studies of microstructures, frictional strength, and deformation mechanisms in natural fault rocks have mainly focused on sheet silicate minerals (e.g., Collettini et al., 2009; Lockner et al., 2011), and similar studies on graphite are limited. * Corresponding author. Tel.: þ81 25 262 6161; fax: þ81 25 262 6194. E-mail address: f14n002a@mail.cc.niigata-u.ac.jp (Y. Nakamura). 1 Present address: Department of Geosphere Sciences, Faculty of Science, Yamaguchi University, 1677-1 Yoshida, Yamaguchi 753-8511, Japan. Contents lists available at ScienceDirect Journal of Structural Geology journal homepage: www.elsevier.com/locate/jsg http://dx.doi.org/10.1016/j.jsg.2014.10.012 0191-8141/© 2014 Elsevier Ltd. All rights reserved. Journal of Structural Geology 72 (2015) 142e161