K-Ar ages of the 'Ishizuchiyama Tertiary System' in western Shikoku, Japan, and their implications.

The 'Ishizuchiyama Tertiary System' consists of the Ishizuchi and Kuma Groups. The former Group lies on the latter one with clino-unconformity. We determined K-Ar biotite ages for two acidic tuff beds from the Kuma Group and the lower formation of the Ishizuchi Group, respectively. The tuffs from the Kuma Group give 16.9 and 15.5 Ma in age, and those from the Ishizuchi Group, 15.1 and 14.9 Ma. This small range of ages limits the timing of deformation associated with the formation of the E-W-trending Tobe thrust in the Kuma Group. The N-S-trending compression tectonics in western Shikoku at ca 15 Ma can be ascribed to an inversion tectonics from the N-S-trending extension tectonics associated with the opening of the Japan Sea, and prevailed in the whole southwest Japan.