ESR analysis of black fault gouge zones in Taiwan Chelungpu fault drilling project Hole B cores

In Hole B drill cores collected by Taiwan Chelungpu-fault Drilling Project (TCDP), there exist three main fault gouge zones around 1136m, 1194m and 1243m in depth (Hirono et al., 2007), and the 1136m fault gouge zone may have been active in the 1999 Chi-Chi Earthquake (Lin et al., 2008). These fault gouge zones respectively have a black zone with high magnetic susceptibility, which may have been produced due to thermal decomposition of iron-bearing minerals into ferrimagnetic ones by frictional heating (Mishima et al., 2006; Hirono et al., 2007). The inorganic carbon content in the black gouge zones has decreased as compared with the surrounding rocks (Ikehara et al., 2007). This means that carbonate minerals such as calcite in the black gouge zones may have been decomposed by frictional heating. We thus have carried out ESR analysis of the black gouge zones to estimate the degree of frictional heat generation along the Chelungpu fault.Besides the 1194m gouge zone, the black gouge zones show a lower ferrimagnetic resonance (FMR) signal (g=2.1) of maghemite (gamma-Fe2O3), which is produced by thermal dehydration of lepidocrocite (gamma-FeOOH) formed under more oxidative condition, than the surrounding rocks. Step-by-step heating experiments (5 minutes) indicate that the 1136m black gouge zone may have been subjected to heating around 350 degree C at the most, although the temperature depends on the duration of heating. On the other hand, the transformation temperature of maghemite into hematite (alpha-Fe2O3) in a hydrothermal fluid may strikingly decrease (Swaddle & Oltmann, 1980), so that hydrothermal fluids may have passed through the black gouge zone after the production of maghemite. Furthermore, the intensity of Mn2+ ion signal, which is derived from a divalent manganese ion isolated in a crystal such as calcite (CaCO3) under more reductive condition, has been almost annihilated in the black fault zones, whereas Fe3+ ion signal that was isolated in a crystal under more oxidative condition almost remains unchanged in the fault gouge zones. Since calcite can be dissolved in weak acid water containing CO2 (CaCO3+CO2+H2O->Ca2++2HCO3-), the decrease of inorganic carbon content may be subjected to the dissolution of carbonate minerals due to the passing of hydrothermal fluids containing CO2 through the black gouge zones. In this case, Mn2+ ions flowed out of crystals form manganese oxides (MnO) with oxygen ions, and furthermore Mn3O4, which is black and paramagnetic under normal temperature, may be produced by further oxidation during frictional heating. Mn3O4 may be responsible for the blackening of the fault gouge zones with low FMR signal intensity.