隠岐島前岩脈群と丹後半島岩脈群のK-Ar年代

抄録

Eighteen new K-Ar ages for Neogene dike swarms at Oki-Dozen and Tango Peninsula, western Honshu, were determined. The K-Ar ages of dikes at Oki-Dozen area are within a narrow range of 5.7 and6.3 Ma, which are much younger than previously reported results. Slight shift, from N10-15E to N15-70 W, of regional compressional stress axis might have occurred around 6.2 Ma. The present age data for dike swarms from Tango Peninsula area suggest the existence of at least 3 separate periods of their activities, 15-11 Ma, 6 Ma and 3 Ma. Peak of the Miocene igneous activity was during 15-13 Ma, just after the opening period or Japan Sea. Although the present data indicate that the regional compressional stress axis of this area has been NS direction since 15 Ma, which seems not in accord with the conclusion given by TSUNAKAWA (1986), the difference might be explained by the absence of dike swams at 12-9 Ma and less than 1 Ma in the investigated sites in the present study.