ABSTRACT

⁴⁰Ar-³⁹Ar analyses were performed for two volcanic rocks recovered from the flank of the Okushiri Ridge, located at the northeastern part of the Japan Basin in the Japan Sea. A basalt (TB3-1), collected by a dredging gives a good plateau age of 14.2 ± 0.3 Ma, whereas a basaltic andesite (353-2a) recovered by a dive with the submersible SHINKAI 2000 gives a much older plateau age of 32.7 ± 0.6 Ma. The former age is similar to those obtained for some volcanic rocks from the seamounts in the Yamato Basin, but the latter is definitely older than those obtained for ODP samples. Since the latter sample was recovered with a subrounded surface, a possibility of exotic origin cannot be ruled out. Present results are not inconsistent with an inference that the Okushiri Ridge was formed during the Japan Sea evolution.