Graphite as a fault lubricant

Graphite is a well-known solid lubricant, and has been found in ~14 vol% of fraction from fault zones in a variety of geological settings (e.g. the Atotsugawa fault system, Japan: Oohashi et al., 2011a, submitted; the KTB borehole, Germany: Zulauf et al., 1990; and the Err nappe detachment fault, Switzerland: Manatschal, 1999). However, it received little attention even though friction of graphite gouge shows strikingly low (steady-state friction coefficient ≈0.1) over seven orders of magnitude in slip rate (0.16 μm/s to 1.3 m/s; Oohashi et al., 2011b). Thus the friction experiments were performed on mixed graphite and quartz gouges with different compositions in order to determine the minimum amount of graphite in reducing the frictional strength of faults dramatically, by using a rotary-shear low to high-velocity friction apparatus. Experimental result clearly indicates that the friction coefficient of the mixture gouge decreases with graphite content following a power-law relation irrespective of slip rate; it starts to reduce at the fraction of 5 vol% and reaches to the almost same level of pure graphite gouge at the fraction of more than 20 vol%. This result implies that the 14 vol% of graphite in natural fault rock is enough amount for reduce the shear strength to half of initial. According to the textural observation, slight weakening of 5-8 vol% of graphite mixture is associated with the development of partial connection of graphite matrix, forming a slip localized surface. On the other hand, the formation of through-going connection of diffused graphite-matrix zones along shear planes is most likely to have caused the dramatic weakening of gouge with graphite of more than 20 vol%. The non-linear power-law dependency of friction on graphite content leads to more efficient reduction of fault strength as compared with the previously reported almost linear dependency on the effects of clay minerals (e.g. Shimamoto & Logan, 1981). Hence the result demonstrates the potential importance of graphite as a weakening agent of mature faults as graphite can reduce friction efficiently as compared with other weak clay minerals. Such mechanical properties of graphite may explain the lack of pronounced heat flow in major crustal faults and the long-term fault weakening.