THERMAL PROCESSES OF SELSMOGENE ZONE REVEALED BY THERMOCHRONOLOGY

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(1) FT's were significantly shortened nearby a fault both in the University 500m borehole at the Toshima site and Geological Survey of Japan borehole at the Hirabayashi site. This suggests that rocks along the fault were reheated into the zircon FT' partial annealing zone, sometime after the initial cooling of the Ryoke granitic rocks.(2) Zircon FT dating revealed that the reheating event took place at about 2 Ma for the University 500m borehole and at about 40 Ma for the Geological Survey of Japan borehole. The widths of reheated zones are about 3 m for the former and about 20 m for the latter. In conjunction with the deformation/alteration data of borehole rock, it is likely that the reheating was caused by the transfer and/or dispersion of hot fluids.(3) Laboratory hydrothermal heating experiment revealed that the annealing kinetics is indistinguishable between atmospheric and hydrothermal conditions.(4) A similar widespread zircon FT annealing was found for hanging wall of the Mozumi fault system.(5) In the upper boundary fault of the Okitsu melange of the Shimanto accretionary complex. FT's were significantly annealed for over 3 km distant away from the fault plane.