Fluid inclusions in pseudotachylytes from the Nojima fault, Japan

Pseudotachylytes (i.e., rocks formed by frictional melting) have been obser ved in the Nojima fault that was penetrated by the Hirabayashi borehole dri lled 1 year after the 1995 Hyogo-ken Nanbu (Kobe) earthquake. These rocks d isplay millimeter-scale banding defined by different pseudotachylyte layers . The nature of unmolten crystal fragments (K-feldspar, albite, calcite, an d/or quartz) allows us to infer a minimum melting temperature of 1200 degre esC. The glass has a 8 +/- 3% volatile content and a higher CaO content tha n that of the parent granodiorite, thus suggesting that pseudotachylyte for mation occurred in an already altered, calcite-bearing and hydrated fault z one. Fluid inclusions have been observed in the glass and are filled with a dense low-salinity (4.3 +/- 1.2 wt % eq. NaCl) CO2-H2O fluid characterized by steep isochoric curves. The intersection between the isochores and the measured 24 degreesC/km geothermal gradient indicates a minimum 15 km depth for the pseudotachylyte formation. Such a depth suggests that an important uplift has brought the studied pseudotachylytes to their present position and that these pseudotachylytes are probably pre-Miocene in age. The calcul ated thermal evolution of a typical millimeter-scale pseudotachylyte layer indicates that cooling does not last more than a few seconds. This indicate s that related seismic processes, such as deceleration of fault movement an d healing of the fault, were both very rapid. The geometry and thermal budg et of the millimeter-thick pseudotachylytes suggest that seismic events of magnitude 6 to 7 are responsible for their formation.