Geo-Thermometric Evidence for Fluid Thermal Pressurization to Hydro-Fracturing With Implosion Along a Seismogenic Out-of-Sequence Thrust, the Nobeoka Thrust in the Shimanto Belt

Recently geological evidence of frictional melting has been reported from ancient accretionary prisms uplifted from seismogenic depths of subduction zones (Ikesawa et al.,2003; Kitamura et al.,2005; Rowe et al.,2004; Mukoyoshi et al.,2006; Okamoto et al.,2006). Under hydrous conditions, however, frictional melting has been considered to be prevented and other mechanisms (e.g thermal pressurization of pore fluid; Mase and Smith,1987) were thought to contribute to the dynamic weakening for unstable slip of earthquakes. Questions are 1) role of fluid during faulting, 2) how it is preserved as geologic tracer, and 3) relationship between frictional melting and fluids. We reported before a new exposure of pseudotachylyte (PS) from a fossilized out-of-sequence thrust in an exhumed accretionary prism (Okamoto et al.,2006). The PS-bearing fault is located right above the Nobeoka Thrust (NT) fault core. The hanging wall unit of NT is composed of phyllite. The maximum temperature of phyllite are approximately 320°C (Kondo et al.,2005). Fault jog and asymmetric cracks along the fault are filled by implosion breccia (Sibson,1986) encircled by carbonate matrix. The PS cuts implosion breccia in the slip part, but pinches out in the dilation jog. Consequently, these observations show that implosion breccia and PS formed at simultaneous slip event. In and adjacent to this fault, we found two kinds of mineral veins. Quartz vein: quartz vein in hanging wall of the NT are distinguished into 2 types (Q1 and Q2). Q1 is cut by PS and asymmetric tension cracks but cut the foliations of host rock. Grains show idiomorphic crystals in Q1. Q1 veins are found along faults parallel to the fault core of the NT. Q2 veins are observed intra-folial veins or oblique veins to the foliation. The Q2 veins shows undulatory extinction and grain-size reduction due to plastic deformation prior to the brittle cracking for Q1 veins. Thus, it is clear that Q1 veins were precipitated later stage than Q2 veins, and before the slip event. On the basis of such a geological observation, we analyzed fluid inclusions in Q1 vein to reveal thermal history of the fluid. Calcite vein: calcites fill the matrix of implosion breccia in fault dilation jogs and asymmetric tension cracks. The asymmetric cracks cut Q1. These jogs and cracks are related to the rupture associated with slip along the fault as reported in Okamoto et al., (2006). In this study, we analyzed fluid inclusions to estimate P-T condition during the vein precipitation. Homogenization temperatures of fluid in Q1 veins range from 180°C to 296°C corresponding to entrapment pressures range from 102MPa to 272MPa. A histogram of the homogenization temperature shows an unimodal peak from 230°C to 240°C. Homogenization temperature of fluids in calcite veins range from 154°C to 362°C and present bimodal peaks. Primary peak is from 230°C to 260°C, and secondary peak is from 310°C to 320°C. This fact suggests that fluid inclusions in calcite veins are trapped at higher temperatures than quartz vein. Taking account cross-cutting relationships among quarts veins, calcite veins and fault slip, the revealed difference in trapped temperature between the quartz and calcite suggests that thermal pressurization of interstitial fluid-hydro-fracturing and implosion and rapid precipitation of carbonates. A several tens of degree temperature increase is enough to obtain super- lithostatic fluid pressure and hydro-fracturing.