Determination of the history of fault activity using pseudotachylyte from the Median Tectonic Line and from the Atotsugawa Fault

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Fission-Track (FT) ages for zircon in granitic rocks along the MTL and Atotsugawa Fault were determined. The FT ages for the MTL yield about 60 Ma, suggesting the oldest brittle deformation along the MTL, whereas those for the Atotsugawa Fault yields two bimodal ages ; 60+Ma and about 45Ma, even though the protolith age is Jurassic. The older age suggests the rejuvenation of concealed younger granitoid and younger age suggest the formation of the PT. On the basis of these data and previous ages of fault gouges, major faults in granitic crust of SW Japan were probably initiated from early Paleogene.