2000 Volume 22 Issue 1 Pages 9-21
Fission-track dating method using zircon was performed on three pyroclastic rock specimens of the Torageyama Formation obtained from the N8-AY-2 well drilled in the Akinomiya geothermal area, Northeast Japan. All of the zircons suitable for dating were measured and statistically analyzed in detail, using x2 test, finite mixture model, track length analysis and forward modeling calculation. All of the three samples contained of (20-100 Ma) and slightly old (3-4 Ma) detrital zircons, and essential zircons of 1 Ma. The old detrital zircons should have derived from granitic basements and the slightly old ones from Pliocene igneous rocks. Although the volcanic activity of the Torageyama Formation was previously considered to be 3-6 Ma, the obtained results revealed that it lasted as recently as 1 Ma.
